Article

Major Anion Effects on the Kinetics and Reactivity of Granular Iron in Glass-Encased Magnet Batch Reactor Experiments

March 2005
Environmental Science and Technology 39(6):1868-74

March 2005
39(6):1868-74

DOI:10.1021/es040413q

Source
PubMed

Authors:

J.F. Devlin

University of Kansas

The relative effects of sulfate (SO4(2-)), chloride (Cl-), nitrate (NO3-), and bicarbonate (HCO3-) (8 mM ionic strength solutions, adjusted to pH 10) on the reactivity of Master Builders iron was investigated using a low-abrasion batch reactor with a glass-encased magnet (GEM). Reactivity of the granular iron surface was assessed by measuring the reduction rate of 4-chloronitrobenzene (4ClNB) as a function of initial 4CINB concentration and anion type. Relative to a similarly prepared perchlorate (ClO4-) solution, in which perchlorate was assumed not to interact with the iron surface, nitrate and bicarbonate inhibited the reduction of the probe compound (4ClNB). Chloride and sulfate enhanced reactivity. Thus, the anions were ranked SO4(2-) > Cl- > or = ClO4- > NO3- > HCO3 (from most enhanced to most inhibited) in their influence on granular iron reactivity toward 4ClNB. Kinetic studies of 4CINB were conducted under conditions that caused the iron surface to saturate with the reacting compound (saturation kinetic studies). These experiments, conducted in the various anion solutions indicated above, showed that the gains in reactivity that occurred in the presence of Cl- and SO4(2-) were due to either increased surface reactivity or sorption capacity. The losses in reactivity that occurred in the presence of NO3- were due to decreases in one or both of these same two factors. However, reactivity declines in the presence of CO3(2-) appear to have been due, in large part, to a reduced affinity of 4ClNB for the iron surface.

Metallic Iron for Environmental Remediation: The Fallacy of the Electron Efficiency Concept

Article

Full-text available

Oct 2021

The suitability of remediation systems using metallic iron (Fe0) has been extensively discussed during the past 3 decades. It has been established that aqueous Fe0 oxidative dissolution is not caused by the presence of any contaminant. Instead, the reductive transformation of contaminants is a consequence of Fe0 oxidation. Yet researchers are still maintaining that electrons from the metal body are involved in the process of contaminant reduction. According to the electron efficiency concept, electrons from Fe0 should be redistributed to: i) contaminants of concern (COCs), ii) natural reducing agents (e.g., H2O, O2), and/or iii) reducible co-contaminants (e.g. NO3-). The electron efficiency is defined as the fraction of electrons from Fe0 oxidation which is utilized for the reductive transformations of COCs. This concept is in frontal contradiction with the view that Fe0 is not directly involved in the process of contaminant reduction. This communication recalls the universality of the concept that reductive processes observed in remediation Fe0/H2O systems are mediated by primary (e.g., FeII, H/H2) and secondary (e.g., Fe3O4, green rusts) products of aqueous iron corrosion. The critical evaluation of the electron efficiency concept suggests that it should be abandoned. Instead, research efforts should be directed towards tackling the real challenges for the design of sustainable Fe0-based water treatment systems based on fundamental mechanisms of iron corrosion.

Metallic Iron for Environmental Remediation: Starting an Overdue Progress in Knowledge

Article

Full-text available

Feb 2020

A critical survey of the abundant literature on environmental remediation and water treatment using metallic iron (Fe 0) as reactive agent raises two major concerns: (i) the peculiar properties of the used materials are not properly considered and characterized, and, (ii) the literature review in individual publications is very selective, thereby excluding some fundamental principles. Fe 0 specimens for water treatment are typically small in size. Before the advent of this technology and it application for environmental remediation, such small Fe 0 particles have never been allowed to freely corrode for the long-term spanning several years. As concerning the selective literature review, the root cause is that Fe 0 was considered as a (strong) reducing agent under environmental conditions. Subsequent interpretation of research results was mainly directed at supporting this mistaken view. The net result is that, within three decades, the Fe 0 research community has developed itself to a sort of modern knowledge system. This communication is a further attempt to bring Fe 0 research back to the highway of mainstream corrosion science, where the fundamentals of Fe 0 technology are rooted. The inherent errors of selected approaches, currently considered as countermeasures to address the inherent limitations of the Fe 0 technology are demonstrated. The misuse of the terms "reactivity", and "efficiency", and adsorption kinetics and isotherm models for Fe 0 systems is also elucidated. The immense importance of Fe 0 /H 2 O systems in

Nonparametric link functions with shape constraints in stochastic degradation processes: Application to emerging contaminants

Article

Full-text available

Jun 2019

Effective oxidation-based elimination of emerging contaminants (ECs) requires a good understanding of the effects of treatment conditions, such as the kinds and dosages of reagents, on the EC degradation rate. Due to limited knowledge on the complex reaction mechanism and the multiple covariates to represent the treatment conditions, it is generally hard to parametrically quantify the relation between these covariates and the degradation rate. On the other hand, qualitative analysis based on chemical mechanisms often provides shape information of the covariate–rate relation, such as monotonicity and local concavity in each coordinate. Based on the chemical kinetics, we use stationary stochastic processes for parametric modeling of log-transformed EC degradation under each combination of the treatment conditions. The tensor-product Bernstein bases are then used to approximate the covariate–rate relation. The shape information is naturally translated to constraints on the coefficients of the bases functions. Likelihood-based inference procedures are developed for both point and interval estimation in the proposed models. Simulation results show that the use of shape information significantly improves the accuracy of estimates. The proposed method is successfully applied to EC degradation data collected from real experiments.

Enhanced Degradation of Insecticide Chlorpyrifos by Iron Salts and Potassium Persulfate during Zerovalent Iron Treatment in Aqueous Solution

Article

Full-text available

Dec 2018

Degradation of the insecticide O,O-diethyl O-3,5,6-trichloro-2-pyridyl phosphorothioate (chlorpyrifos) in aqueous solution was investigated using iron salts and potassium persulfate during ZVI treatment through a series of batch experiments. The degradation rate of chlorpyrifos increased with increases in the concentrations of iron salts and potassium persulfate in the aqueous system. Ferric chloride was found to be the most effective iron salt for the ZVI-mediated degradation of chlorpyrifos in aqueous solution. Further, the iron salts tested could be arranged in the following order in terms of their effectiveness: FeCl3 > Fe2(SO4)3 >Fe(NO3)3. The persulfate-ZVI system could significantly degrade chlorpyrifos present in the aqueous medium. This revealed that chlorpyrifos degradation by treatment with Fe⁰ was promoted on adding ferric chloride and potassium persulfate. The kinetics of the degradation of chlorpyrifos by persulfate-amended Fe⁰ was higher than that for iron-salt-amended Fe⁰. This suggests that using a sequential Fe⁰ reduction-ferric chloride or Fe⁰ reduction-persulfate process may be an effective strategy to enhance the removal of chlorpyrifos in contaminated water.

Effect of field site hydrogeochemical conditions on the corrosion of milled zerovalent iron particles and their dechlorination efficiency

Article

Oct 2017
SCI TOTAL ENVIRON

Milled zerovalent iron (milled ZVI) particles have been recognized as a promising agent for groundwater remediation because of (1) their high reactivity with chlorinated aliphatic hydrocarbons, organochlorine pesticides, organic dyes, and a number of inorganic contaminants, and (2) a possible greater persistance than the more extensively investigated nanoscale zerovalent iron. We have used laboratory-scale batch degradation experiments to investigate the effect that hydrogeochemical conditions have on the corrosion of milled ZVI and on its ability to degrade trichloroethene (TCE). The observed pseudo first-order degradation rate constants indicated that the degradation of TCE by milled ZVI is affected by groundwater chemistry. The apparent corrosion rates of milled ZVI particles were of the same order of magnitude for hydrogeochemical conditions representative for two contaminated field sites (133-140mmolkg⁻¹ day⁻¹, indicating a milled ZVI life-time of 128-135days). Sulfate enhances milled ZVI reactivity by removing passivating iron oxides and hydroxides from the Fe⁰ surface, thus increasing the number of reactive sites available. The organic matter content of 1.69% in the aquifer material tends to suppress the formation of iron corrosion precipitates. Results from scanning electron microscopy, X-ray diffraction, and iron K-edge X-ray adsorption spectroscopy suggest that the corrosion mechanisms involve the partial dissolution of particles followed by the formation and surface precipitation of magnetite and/or maghemite. Numerical corrosion modeling revealed that fitting iron corrosion rates and hydrogen inhibitory terms to hydrogen and pH measurements in batch reactors can reduce the life-time of milled ZVI particles by a factor of 1.2 to 1.7.

Reductive Removal of Pertechnetate and Chromate by Zero Valent Iron under Variable Ionic Strength Conditions

Article

Dec 2022
J HAZARD MATER

Radioactive technetium-99 (Tc) present in waste streams and subsurface plumes at legacy nuclear reprocessing sites worldwide poses potential risks to human health and environment. This research comparatively evaluated efficiency of zero-valent iron (ZVI) toward reductive removal of Tc(VII) in presence of Cr(VI) from NaCl and Na2SO4 electrolyte solutions under ambient atmospheric conditions. In both electrolytes, anticorrosive Cr(VI) suppressed oxidation of ZVI at elevated concentrations resulting in the delay of initiation of Tc(VII) reduction to Tc(IV). In the absence of Cr(VI), no delay was observed in the analogous systems. At low ionic strength (IS), retarded ZVI oxidation inhibited Tc(VII) reduction. Higher IS favored reduction of both Tc(VII) and Cr(VI), which followed second-order reaction rates in both electrolytes attributed to the more efficient iron oxidation as evident from solids characterization studies. Magnetite was the primary iron oxide phase, and its higher fraction in the SO4²⁻ solutions facilitated reductive removal of Tc(VII) and Cr(VI). In the Cl⁻ matrix, Cr(VI) promoted further oxidation of magnetite as well as formation of chromite diminishing overall reductive capacity of this system and resulting in less effective removal of Tc(VII) compared to the SO4²⁻ solutions.

Reductive Removal of Pertechnetate and Chromate by Zero Valent Iron Under Variable Ionic Strength Conditions

Article

Jan 2022

Common Anions Affected Removal of Carbon Tetrachloride in Groundwater Using Granular Sponge Zerovalent Iron

Article

Full-text available

Mar 2020
WATER AIR SOIL POLL

Significant attention has been devoted over the past two decades to research and field applications of zerovalent iron (ZVI) technologies for groundwater remediation. The uncertainty of ZVI effectiveness under complex subsurface environment will affect the application of ZVI remedial techniques. The effects of groundwater common anions Cl−, SO42−, and HCO3− on CCl4 degradation by sponge ZVI were investigated through batch experiments. The surface structure and composition of ZVI before and after reaction were determined by SEM-EDS, X-ray diffraction, and X-ray photoelectron spectroscopy. Cl−, SO42−, and HCO3− promoted the degradation of CCl4 with the order of HCO3− > SO42− > Cl−. HCO3− enhanced the effect of ZVI on CCl4 degradation as a buffer and an oxidant providing cathodic reaction; SO42− dissolved the hydroxide on the ZVI surface to promote the degradation; and Cl− accelerated the degradation rate by pitting corrosion on the ZVI surface. After reaction, the iron oxides on ZVI surface were FeOOH and Fe2O3 in Cl−, SO42− system and the FeOOH was the only iron oxide in HCO3− system. The results suggest that the performance of ZVI will be affected by the composition of field groundwater.

Effects of common groundwater ions on the transformation and reactivity of sulfidized nanoscale zerovalent iron

Article

Full-text available

Feb 2020
CHEMOSPHERE

Sulfidized nanoscale zerovalent iron (S-nZVI) is an Fe-based reactant widely studied for its potential use for groundwater remediation. S-nZVI reactivity has been widely investigated testing various contaminants in various water matrices, but studies on S-nZVI corrosion behaviour and reactivity upon exposure to complex groundwater chemistries are limited. Here, we show that anoxic aging of S-nZVI for 7 days in the absence and presence of key groundwater solutes (i.e., Cl-, SO42-, Mg2+, Ca2+, HCO3-, CO32-, NO3-, or HPO42-) impacts Fe0 corrosion extent, corrosion product and reduction rates with trichloroethene (TCE). White rust was the dominant corrosion product in ultrapure water and in SO42-, Cl-, Mg2+ or Ca2+ solutions; green rust and/or chukanovite formed in HCO3- and CO32- solutions; magnetite, formed in NO3- solutions and vivianite in HPO42- solutions. The aged S-nZVI materials expectedly showed lower reactivities with TCE compared to unaged S-nZVI, with reaction rates mainly controlled by ion concentration, Fe0 corrosion extent, type(s) of corrosion product, and solution pH. Comparison of these results to observations in two types of groundwaters, one from a carbonate-rich aquifer and one from a marine intruded aquifer, showed that S-nZVI corrosion products are likely controlled by the dominant GW solutes, while reactivity with TCE is generally lower than expected, due to the multitude of ion effects. Overall, these results highlight that S-nZVI corrosion behaviour in GW can be manifold, with varied impact on its reactivity. Thus, testing of S-nZVI stability and reactivity under expected field conditions is key to understand its longevity in remediation applications.

Surface Carbon Influences on the Reductive Transformation of TCE in the Presence of Granular Iron

Article

Dec 2017
J HAZARD MATER

To gain insight into the processes of transformations in zero-valent iron systems, electrolytic iron (EI) has been used as a surrogate for the commercial products actually used in barriers. This substitution facilitates mechanistic studies, but may not be fully representative of all the relevant processes at work in groundwater remediation. To address this concern, the kinetic iron model (KIM) was used to investigate sorption and reactivity differences between EI and Connelly brand GI, using TCE as a probe compound. It was observed that retardation factors (Rapp) for GI varied non-linearly with influent concentrations to the columns (Co), and declined significantly as GI aged. In contrast, Rapp values for EI were small and insensitive to Co, and changed minimally with iron aging. Moreover, although declines in the rate constants (k) and increases in the sorption coefficients were observed for both iron types, they were most pronounced in the case of EI. SEM scans of the EI surface before and after aging (90 days) established the appearance of carbon on the older surface. This work provides evidence that iron with a higher surface carbon content outperforms pure iron, suggesting that the carbon is actively involved in promoting TCE reduction.

The comparison of Se(IV) and Se(VI) sequestration by nanoscale zero-valent iron in aqueous solutions: The roles of solution chemistry

Article

May 2017

The sequestration of Se(IV) and Se(VI) by nanoscale zero-valent iron (NZVI) particles were compared under different solution conditions. Firstly, the comparison was conducted at three pH values (4.0, 6.0 and 8.0) in deionized water. Generally, the removal of Se(IV)/Se(VI) by NZVI was more rapid under acidic conditions and the removal efficiency of Se(IV) was much higher than that of Se(VI). Moreover, the pH variation exhibited much larger influence on the sequestration of Se(VI) than that of Se(IV) by NZVI. The spectroscopic analysis showed that both the Se(IV) and Se(VI) were reduced to Se⁰ and Se²⁻, while NZVI was transformed into iron (hydr)oxides. When the selenium-NZVI reactions occurred in synthetic groundwater, all the reaction systems were inhibited in varying degrees. The individual effects of humic acid (HA) and typical inorganic ions were also examined. It seems that HA could substantially hinder the sequestration of Se(IV) compared with that in deionized water, while sulfate (SO4²⁻) and bicarbonate (HCO3⁻) inhibited the Se(VI) removal significantly. Notably, the presence of cations (i.e., Na⁺ or Ca²⁺) ions did not cause obvious interference to the Se(IV)/Se(VI) removal by NZVI, while the presence of Ca²⁺ could alleviate the adverse effect of HA on Se(IV) removal to some degree.

Renewable cellulose aerogel embedded with nano-HFO for preferable phosphate capture from aqueous solution

Article

Full-text available

Nov 2022
ENVIRON SCI POLLUT R

Excess phosphate in water can cause eutrophication, which must be addressed. Despite many efforts devoted to the adsorptive removal of phosphate from water, the development of new adsorbents with high adsorption capacity is highly desirable. Herein, a novel nanocomposite was proposed for phosphate removal by confining hydrated ferric oxide (HFO) nanoparticles into a cellulose aerogel (CA) network named as HFO@CA. Benefiting from the characteristics of the low density and porous structure of CA, the internal surface of the nanocomposite is more accessible and thus improves the utilization of the HFO nanoparticles. Batch adsorption experiments were carried out to evaluate the phosphate uptake by the prepared adsorbent. The maximum adsorption capacity of HFO@CA occurs at near-acidic pH. With increasing temperature, the composite adsorbent is more favorable for phosphate adsorption. Moreover, the hybrid aerogel exhibited fast kinetic behavior for phosphate removal, which could be accurately depicted by pseudo-second-order model. HFO@CA shows excellent adsorption selectivity in solutions containing competitive anions at higher levels. In addition, five cycles of the phosphate adsorption experiments without obvious capacity loss indicated that HFO@CA has great regenerability. These results demonstrate that HFO@CA has a wide field of application with good prospects in phosphate removal from wastewater, which also provides a new strategy to prepare adsorbents with excellent performance using renewable cellulose resources. Graphical Abstract

Metallic Iron for Water Remediation: Plenty of Room for Collaboration and Convergence to Advance the Science

Article

Full-text available

May 2022

Scientific collaboration among various geographically scattered research groups on the broad topic of "metallic iron (Fe0) for water remediation" has evolved greatly over the past three decades. This collaboration has involved different kinds of research partners including researchers from the same organization, domestic researchers also from non-academic organizations as well as international partners. The present analysis of recent publications of some few leading scientists shows that after a decade of frank collaboration in search of ways to improve the efficiency of Fe0/H2O systems, the research community has divided itself into two schools of thought since about 2007. Since then, progress in knowledge has stagnated. The first school maintains that Fe0 is a reducing agent for some relevant contaminants. The second school argues that Fe0 in-situ generates flocculants (iron hydroxides) for contaminant scavenging, and reducing species (e.g. FeII , H2 , Fe3O4) but reductive transformation is not a relevant contaminant removal mechanism. The problem encountered in assessing the validity of the views of both schools arises from the quantitative dominance of the supporters of the first school who mostly ignore the second school in their presentations. The net result is that the various derivations of the original Fe0 remediation technology may be collectively flawed by the same thinking mistake. While recognizing that the whole research community strives for the success of a very promising, but yet to established technology, annual review articles are suggested as an ingredient for successful collaboration.

Removal of microcontaminants by zero-valent iron solar processes at natural pH: Water matrix and oxidant agents effect

Article

Jan 2022
SCI TOTAL ENVIRON

This work deals with microcontaminants (MCs) removal by natural solar zero-valent iron (ZVI) process at natural pH in actual matrices. Commercial ZVI microspheres were selected as ZVI source and hydrogen peroxide and persulfate were used as oxidant agents. The experimental plan comprised the evaluation of sulphates and carbonates/bicarbonates effect on process performance, the possibility of adding an iron chelate (EDDS) to take advantage of leached iron and the treatment of MCs in actual MWWTP secondary effluent. The presence of sulphates and EDDS addition did not lead to significant changes in the process efficiency, while the carbonates naturally present in natural water (458 mg/L) diminished the treatment time need to reach the decontamination goal. Finally, the treatment of a MCs mixture consisting of Atrazine, Carbendazim, Imidacloprid, and Thiamethoxam in the range of μg/L in actual MWWTP secondary effluent by solar/msZVI/H2O2 and solar/msZVI/S2O8²⁻ obtained 7 and 22% of total removal after 180 min, respectively, which indicated a moderate competitiveness of these processes with respect to other advanced oxidation processes.

Investigating the Fe(0)/H2O systems using the methylene blue method: Validity, applications, and future directions

Article

Full-text available

Nov 2021

An innovative approach to characterize the reactivity of metallic iron (Fe0) for aqueous contaminant removal has been in use for a decade: The methylene blue method (MB method). The approach considers the differential adsorptive affinity of methylene blue (MB) for sand and iron oxides. The MB method characterizes MB discoloration by sand as it is progressively coated by in-situ generated iron corrosion products (FeCPs) to deduce the extent of iron corrosion. The MB method is a semi-quantitative tool that has successfully clarified some contradicting reports on the Fe 0 /H2O system. Moreover, it has the potential to serve as a powerful tool for routine tests in the Fe 0 remediation industry, including quality assurance and quality control (QA/QC). However, MB is widely used as a 'molecular probe' to characterize the Fe 0 /H2O system, for instance for wastewater treatment. Thus, there is scope to avoid confusion created by the multiple uses of MB in Fe 0 /H2O systems. The present communication aims at filling this gap by presenting the science of the MB method, and its application and limitations. It is concluded that the MB method is very suitable for Fe 0 material screening and optimization of operational designs. However, the MB method only provides semi-quantitative information, but gives no data on the solid-phase characterization of solid Fe 0 and its reaction products. In other words, further comprehensive investigations with microscopic and spectroscopic surface and solid-state analyses are needed to complement results from the MB method.

Investigation on the efficient separation and recovery of Se(IV) and Se(VI) from wastewater using Fe–OOH–bent

Article

Full-text available

Feb 2021

Decontamination of the toxic selenium compound, selenite (Se(IV)) and selenate (Se(VI)), from wastewater is imperative for environmental protection. Efficient approaches to remove Se(IV) and Se(VI) are in urgent needs. In this work, an accessible adsorbent Fe–OOH–bent was prepared and applied for the removal of Se(IV) and Se(VI) from wastewater. The batch experimental results demonstrate that Fe–OOH–bent exhibits high adsorption capacities of 5.01 × 10 ⁻⁴ and 2.28 × 10 ⁻⁴ mol/g for Se(IV) and Se(VI) respectively, which are higher than most of the reported bentonite based materials, especially in the case of Se(VI). Moreover, the Fe–OOH–bent displayed superior selectivity towards Se(IV) and Se(VI) even in the presence of excess competitive anions (Cl ⁻ , HCO 3 ⁻ , NO 3 ⁻ , SO 4 ²⁻ and PO 4 ³⁻ ) and HA with concentrations of 1000 times higher than Se(IV) and Se(VI). By evaluating the adsorption ratio of Se(IV) and Se(VI), the reusability of Fe–OOH–bent was great through five adsorption-desorption cycles. For practical application, the column experiments were performed with simulated wastewater samples. The breakthrough and eluting curves of Se(IV) and Se(VI) were investigated through the columns packed with Fe–OOH–bent, and the results show that Se(IV) and Se(VI) can be successfully separated and recovered using 0.1 mol/L Na 2 SO 4 (pH = 9.0) and 0.1 mol/L Na 3 PO 4 (pH = 9.0), respectively. Our work provides a new approach for fractional separation as well as the recovery of Se(IV) and Se(VI) from wastewater.

Groundwater geochemical constituents controlling the reductive dechlorination of TCE by nZVI: Evidence from diverse anaerobic corrosion mechanisms of nZVI

Article

Jul 2020
CHEMOSPHERE

The corrosion mechanisms of nanoscale zero-valent iron (nZVI) vary with different geochemical constituents, which affect the reductive dechlorination process of trichloroethylene (TCE). In this study, the effect of nZVI anaerobic corrosion on the reductive dechlorination of TCE with different groundwater geochemical constituents (Ca²⁺-SO4²⁻, Ca²⁺-HCO3⁻, Na⁺-NO3⁻) was investigated. Microscopic characterization by X-ray diffraction (XRD) and transmission electron microscopy (TEM) combined with pH, oxidation-reduction potential (ORP) and dissolved Fe²⁺ in solutions to illustrate the corrosion mechanism of nZVI. In the four systems including ultrapure water (UPW), the reduction of TCE conformed to pseudo-first-order kinetics, the generation of Cl⁻ accorded with zero-order kinetics, and multi-step reaction kinetics was used to fit the generation and degradation of chlorinated byproducts (Dichloroethylene, DCEs). Compared with UPW system, the dissolution corrosion of Ca²⁺-HCO3⁻ and Ca²⁺-SO4²⁻ promoted the reductive dechlorination of TCE (kobs, TCE = 0.658±0.010 & 0.245±0.028 d⁻¹ and kobs, Cl- = 41.682±1.016 & 20.623±1.923 μM⋅d⁻¹ for Ca²⁺-HCO3⁻ & Ca²⁺-SO4²⁻, respectively) and the degradation of DCEs (0.444±0.036 & 0.244±0.040 μM⋅d⁻¹ for Ca²⁺-HCO3⁻ & Ca²⁺-SO4²⁻, respectively); redox-active NO3⁻ competed for electrons and passivated the surface of nZVI, which limited the reductive dechlorination of TCE (kobs, TCE = 0.111±0.025 d⁻¹ & kobs, Cl- = 14.943±0.664 μM⋅d⁻¹) and the degradation of DCEs (0.078±0.018 μM⋅d⁻¹), and the passivation layer promoted the adsorption of TCE. This study from the perspective of nZVI corrosion provides a theoretical basis for the long-term application of nZVI technology in the remediation of TCE-contaminated sites with different groundwater geochemical types.

4-Chlorophenol degradation using ultrasound/peroxymonosulfate/nanoscale zero valent iron: Reusability, identification of degradation intermediates and potential application for real wastewater

Article

Feb 2018
CHEMOSPHERE

In this study, nanoscale-zero valent iron (nZVI) was synthesized and its function was assessed in ultrasound (US)/peroxymonosulfate (PMS)/nZVI process to degrade 4-chlorphenol (4-CP). The influential operation parameters of US/PMS/nZVI were evaluated on 4-CP degradation. 95% of 4-CP was degraded during 30 min under the conditions of pH = 3.0, nZVI = 0.4 g/L, PMS = 1.25 mM, US power = 200 W. The rate constants of 4-CP degradation for US/PMS/nZVI, PMS/nZVI, US/PMS and US/nZVI were 0.1159, 0.03, 0.0134 and 0.0088 min-1respectively. Simultaneous application of US and nZVI synergistically increased 4-CP degradation and PMS activation. nZVI was compared with Fe2+, Fe3+and micro-ZVI and their results indicated high performance of nZVI compared to others. Reusability of nZVI was examined in four cycles. nZVI exhibited that reusability was acceptable in three runs. The results of effect of anions showed that phosphate had significant inhibitory effect on 4-CP degradation in US/PMS/nZVI process. The scavenging experiments indicated that hydroxyl radical had more contribution compared to sulfate radical. Intermediates of 4-CP degradation were identified including five aromatic compounds. Reaction pathway of 4-CP degradation was proposed. Finally, the performance of US/PMS/nZVI process was evaluated on real petrochemical wastewater. The results showed that US/PMS/nZVI can be a suitable pretreatment for biological treatment.

Nitrate removal by combined heterotrophic and autotrophic denitrification processes: Impact of coexistent ions

Article

Dec 2017
BIORESOURCE TECHNOL

In this current study, sawdust and zero-valent iron (Fe⁰) were used as co-electron donors to evaluate the effects of coexistent ions on the combined heterotrophic and autotrophic denitrification (HAD) processes. The results showed that HCO3⁻ and SO4²⁻ drastically enhanced nitrate removal. The promotion effect derived from both biological and chemical process by HCO3⁻ and chemical process by SO4²⁻. However, Ca²⁺ ions would remarkably increase nitrate removal due to promoting the electron transfer and the metabolic activities of bacteria, whereas the Cu²⁺ ions inhibited the biological process due to the deleterious effect on bacteria. Meanwhile, Fe²⁺ and Fe³⁺ ions exhibited inhibition effect firstly because of their toxicity to bacteria and promotion subsequently due to their enhancement on Fe⁰ chemical denitrification. Moreover, byproducts such as nitrite, ammonium, dissolved organic carbon (DOC), etc. were also influenced by common ions.

Catalytic effect of iron on the tolerance of thin-film composite polyamide reverse osmosis membranes to hydrogen peroxide

Article

Nov 2017
J MEMBRANE SCI

Hydrogen peroxide (H2O2) is potentially an attractive alternative to chlorine-based (hypochlorous acid and monochloramine) antifouling agents in reverse osmosis (RO) because H2O2 does not form toxic disinfection byproducts and is tolerated by polyamide (PA) membranes up to high concentrations. However, aqueous H2O2 solutions are corrosive and iron corrosion products activate H2O2 to reactive oxygen species (ROS) that can degrade the PA separation layer. The impact of iron oxides on membrane stability was studied in the presence of H2O2 in two different systems: a corrosion-resistant system (constructed with plastic components) and an all-steel system. Tests in the all-steel system were conducted under enhanced corroding conditions and in deoionized (DI) water. H2O2 concentrations were 2.0 mM (68 mg/L) or 10 mM (340 mg/L). Corrosion was enhanced by adding 10 mM Cl⁻ or suppressed by adding phosphate buffer. Membrane performance was evaluated by determining salt rejection and the water flux. Under corrosion-suppressed conditions, membranes were stable during the 8-d test. In the all-steel testing system containing 10 mM Cl⁻ ion as corrosion promoter, the membrane tolerance was significantly diminished. In DI water, corrosion was relatively slow but degradation of the membranes was noticeable. Kinetic data of pCBA degradation indicated that membrane damage was caused by ·OH radicals. Quenching of the ·OH radical by methanol, and X-ray photoelectron spectroscopy (XPS) and Scanning Electron Microscopy (SEM) data are consistent with the hypothesis that Fenton reactions caused cleavage of the polyamide cross-linkages.

Trichloroethene removal by separately encapsulated and co-encapsulated bacterial degraders and nanoscale zero-valent iron

Article

Aug 2017
INT BIODETER BIODEGR

Degradation of trichloroethene (TCE) by separately encapsulated and co-encapsulated nanoscale zero-valent iron (nZVI) and bacterial degraders was investigated. Pseudomonas putida F1 and Dehalococcoides species BAV1 were used in the separate encapsulation and co-encapsulation, respectively. Results from batch experiments showed that the encapsulation systems were able to degrade 100% of TCE (10 mg/L to less than a detection limit of 0.2 μg/L) in 3 h. After 3 h, 10 mg/L of TCE was re-dosed and the co-encapsulation system was able to again completely remove TCE. Common TCE degradation by-products, dichloroethene and vinyl chloride, were not detected. The first order model was suitable for describing TCE degradation kinetics. The initial TCE degradation was mainly chemical while after the re-dosing biodegradation dominated due to exhaustion of nZVI caused by nitrate and possibly phosphate and chloride in the test medium. The encapsulation systems can overcome problems associated with limited activity longevity of nZVI under field conditions, residual TCE and chlorinated degradation by-products. The systems can potentially be a technique for in-situ remediation of groundwater.

The use of zero-valent iron (ZVI)-microbe technology for wastewater treatment with special attention to the factors influencing performance: A critical review

Article

May 2017

In recent years, the use of zero-valent iron (ZVI) coupled with microorganisms has attracted much attention for the removal of diverse contaminants from wastewater, and the performance of bio-ZVI systems under diverse conditions is of particular interest. This paper comprehensively reviews the recent developments related to (1) the effects of the iron surface area, operating conditions, coexisting ions and contaminant concentrations on the performance of bio-ZVI systems; (2) the potential mechanisms of the major factors affecting the ZVI-microbe performance; and (3) the effects of ZVI on the characteristics of the microorganisms, including both enhancing and deteriorating effects. All of these factors can have notable impacts, which are contaminant specific and highly dependent on the removal mechanisms of the respective pollutants, on the performance of the bio-ZVI system in terms of contaminant removal. Additionally, the stimulating effects of ZVI on the growth and diversity of microorganisms are reasonable considering the synergistic effects of the combined system on pollutant removal, although inhibitory effects of ZVI on bacterial activity have also been proposed. Based on these findings, further efforts should be made to establish feasible strategies to improve the engineering design and performance of integrated ZVI-microbe systems.

Integrated influence of sulfide modification on the reactivity of nanoscale zero-valent iron towards decabromodiphenyl ether under an electromagnetic field

Article

Apr 2024

Effects of common dissolved anions on the efficiency of Fe0-based remediation systems

Article

Full-text available

Mar 2024
J ENVIRON MANAGE

Quiescent batch experiments were conducted to evaluate the influences of Cl–, F–, HCO3–, HPO42–, and SO42– on the reactivity of metallic iron (Fe0) for water remediation using the methylene blue (MB) method. Strong discoloration of MB indicates high availability of solid iron corrosion products (FeCPs). Tap water was used as an operational reference. Experiments were carried out in graduated test tubes (22 mL) for up to 45 d, using 0.1 g of Fe0 and 0.5 g of sand. Operational parameters investigated were (i) equilibration time (0 to 45 d), (ii) 4 different types of Fe0, (iii) anion concentration (10 values), and (iv) use of MB and Orange II (O-II). The degree of dye discoloration, the pH, and the iron concentration were monitored in each system. Relative to the reference system, HCO3– enhanced the extent of MB discoloration, while Cl–, F–, HPO42–, and SO42– inhibited it. A different behavior was observed for O-II discoloration: in particular, HCO3– inhibited O-II discoloration. The increased MB discoloration in the HCO3– system was justified by considering the availability of FeCPs as contaminant scavengers, pH-increase, and contact time. The addition of any other anion initially delays the availability of FeCPs. Conflicting results in the literature can be attributed to the use of inappropriate experimental conditions. The results indicate that the application of Fe0–based systems for water remediation is a highly site-specific issue which has to include the anion chemistry of the water.

Evaluating the role of salinity in enhanced biogas production from two-stage anaerobic digestion of food waste by zero-valent iron

Article

Dec 2023
J ENVIRON MANAGE

Impact of environmental factors on the removal of chloramphenicol by zero-valent iron and pyrite mixture

Article

Sep 2023
SEP PURIF TECHNOL

Comparison of trichloroethylene dechlorination in seawater by sulfidated nanoscale zero-valent iron particles prepared by two methods

Article

Aug 2023

Abiotic reductive transformation of 3-nitro-1,2,4-triazol-5-one by zero-valent iron

Article

May 2023

The military is switching over to insensitive munitions compounds (IMCs) to avoid unintentional detonations during handling and use of explosives. 3-nitro-1,2,4-triazol-5-one (NTO) is an important component of IMCs. NTO may contaminate the subsurface due to its high aqueous solubility. Thus, there is a need to develop remediation technologies for the treatment of NTO-containing (waste) water. This study demonstrated that zero-valent iron (ZVI) reductively transformed NTO to its daughter product, 3-amino-1,2,4-triazol-5-one. The pseudo first-rate constant (k1) of NTO reduction by micron-sized ZVI at pH 3 was 192.6 h−1. Kinetic degradation experiments performed at different pH values showed that ZVI did not effectively reduce NTO at pH 6 (k1 = 0.6 h−1) or higher. The rapid NTO reduction in acidic conditions may be due to dissolution of iron precipitates on the ZVI surface. Additional experiments were conducted to assess the effectiveness of various depassivating pretreatments with deionized water, acetic acid, hydrochloric acid, or bicarbonate. Treatment with 1 M HCl for 15 min was the most effective depassivation method for a ZVI material containing a thick passivating layer (ca. 880 nm), achieving 84.0% NTO removal after 10 min of reaction. On the other hand, a milder treatment involving washing with a diluted bicarbonate solution (60 mM) was sufficient for a ZVI material that was less passivated (estimated thickness of the passivating layer ≈ 300 nm). This study demonstrates that ZVI treatment is a promising approach for the remediation of NTO-contaminated sites or wastewater and provides critical information to optimize this process.

Understanding the process of phosphate removal in Fe0/H2O systems using the methylene blue method

Article

Full-text available

Apr 2023

The removal mechanisms of contaminants in Fe0/H2O are still poorly understood, and characterized by contradictory findings. Therefore, the present study aims to improve the understanding of the processes involved in phosphate removal in Fe0/H2O system. Herein, the methylene blue method (MB method) is used to trace the dynamics within the investigated systems. The MB method utilizes the differential adsorptive affinity of MB onto sand and sand coated with iron corrosion products to evaluate the degree of Fe0 corrosion in Fe0/H2O systems. The extent of MB discoloration and phosphate removal in various Fe0-based systems was characterized in parallel quiescent batch experiments for two weeks and two months. Parallel experiments with Orange II (O-II) as a model contaminant allowed an improved discussion of the results. The investigated systems were: (i) Fe0 alone, (ii) MnO2 alone, (iii) sand alone, (iv) Fe0 + sand, (v) Fe0 + MnO2, and (vi) Fe0 + sand + MnO2. Additional experiments were conducted to test the influence Fe0 type, Fe0 and sand mass loadings on dye discoloration and phosphate removal in Fe0/sand system. Each system was characterized by: (i) pH value, (ii) Fe concentration, (iii) dye discoloration (MB, O-II), and (iv) phosphate concentration. Results showed that the MB method was capable of tracing the extent of iron corrosion in the various investigated systems and clarified the role of in-situ generated iron corrosion products (FeCPs) in the removal of phosphate. The suitability of mixing sand aggregates with Fe0 for efficient phosphate removal is demonstrated through the MB method. Overall, the appropriateness of the MB method to characterize the dynamics of Fe0/H2O systems is validated.

Biochars-Fe PCu Hybrides Deriving from Solid Waste and Waste Acids for Elimination of Refractory Organic Pollutants from Pharmaceutical Wastewater

Article

Apr 2023

A novel Fe based nanocomposite was fabricated by using solid waste, waste acids, iron ores and copper salts as raw materials. The composites were composed of biochars, iron phosphides, iron carbide and Cu (BC-FexPCu), and applied as pre-treating materials to degrade 3,4,5,6-tetrachloropicolinic acid (TCPA) from saline water. In a wide pH (4–10), temperature (25–65 °C) and salinity (0–25 g L⁻¹ of NaCl) range, TCPA could be eliminated completely within 240 min from saline water with the corresponding mineralization efficiency ranged in 65.4–85.9 %. High concentration of ·OH, O2–· and ¹O2 was continuously detected in BC-FexPCu reaction solution owing to efficient activation of dissolved oxygen by BC-FexPCu. The BC-FexPCu showed good performance in pretreating wastewater from a chloropyridine herbicide production plant. In batch experiment, about 60 % of total organic carbon (TOC) was removed from wastewater within 180 min without the requirement of H2O2. In the fixed bed column test, the elimination efficiency of TOC maintained 30 % with the help of discontinuous aeration. The recycle test suggested that the long-lasting activity of BC-FexPCu could be achieved when the targets were degraded only by reactive oxygen species in the system. Compared to zero-valent iron and commercial Fe/C materials, the advantages of BC-FexPCu include good stability, cost-effectiveness, and less chemical requirements.

Unveiling the Neglected Roles of Chloride and Sulfate in the Removal of Nitro Compounds by Sulfidated Zero-Valent Iron/Ferrous Ion Systems

Article

Mar 2023

Ferrous ions (Fe²⁺) can effectively promote the removal of pollutants by sulfidated zero-valent iron (S-ZVI), but the role of anions coexisting with Fe²⁺ was often ignored. This study systematically compared the performances of S-ZVI/FeCl2 and S-ZVI/FeSO4 systems for chloramphenicol (CAP) and nitrobenzene (NB) removal. The results showed that FeCl2 and sulfidation had a synergistic promoting effect on the reaction, but FeSO4 could not promote the removal of nitro compounds by S-ZVI. Liquid chromatography/mass spectrometry (LC/MS) and UV-vis analysis showed that only the nitro group was reduced during the removal of CAP and NB. The results of batch experiments showed that both NaCl and Na2SO4 could effectively promote the reaction, but the promotion effect of Na2SO4 weakened with the decrease of pH. X-ray photoelectron spectroscopy (XPS) analysis showed that Fe²⁺ could promote the adsorption of SO4²⁻ on the surface of S-ZVI. A possible mechanism was proposed: Fe²⁺ could maintain a low pH level because of hydrolysis, which promoted the adsorption of Cl⁻ and SO4²⁻ on the surface of S-ZVI. Cl⁻ promoted the reaction because of its pitting corrosion, while SO4²⁻ competed with nitro groups for the adsorption sites. Our findings suggest that the coexisting anions are quite essential in the removal of nitro compounds by S-ZVI/Fe²⁺ systems.

Differential and mechanism analysis of sulfate influence on the degradation of 1,1,2- trichloroethane by nano- and micron-size zero-valent iron

Article

Feb 2023
ENVIRON TECHNOL

The in-situ reduction of zero-valent iron (ZVI) is an effective method for removing chlorinated aliphatic hydrocarbons (CAHs) from groundwater. The heterogeneity of environmental conditions is also crucial in affecting dechlorination efficiency. Until now, the effect of Sulfate (SO42-) on ZVI activity has been debated, and the related mechanism research on SO42- behavior during the abiotic reduction process of chlorinated alkanes is still lacking. In this study, the impacts of SO42- concentrations (0, 2, 4, 8, 80 mM) on the degradation of 1,1,2-trichloroethane (1,1,2-TCA) by micron-size ZVI (mZVI) and nano-size ZVI (nZVI) were systematically investigated.For mZVI, Kobs increased by 0.6 (2 mM), 0.5 (4 mM), 1.1 (8 mM), and 1.6 times (80 mM). For nZVI, Kobs decreased by 32% (2 mM), 39% (4 mM), 45% (8 mM), and 9% (80 mM). The results showed that SO42- increased the rate of 1,1,2-TCA degradation by mZVI but weakened the reduction performance of nZVI; however, this inhibition was reduced when the concentration reached 80 mM. SO42- controlled the degradation of 1,1,2-TCA mainly through the formation of different iron-sulfate complexes on the ZVI surface: water-soluble bidentate iron-sulfate complexes formed on the mZVI surface promoted the corrosion of the oxide layer and accelerated the reduction of 1,1,2-TCA, monodentate complexes mainly formed on the nZVI surface inhibited the reduction of 1,1,2-TCA by blocking surface sites. These results demonstrate the proof of concept to assist land managers in the field application of ZVI technology for the remediation of CAHs contaminated sites with different background concentrations of SO42-.

IRON-BASED MATERIALS FOR THE REMEDIATION OF CHLORINATED ETHENES

Thesis

Full-text available

Aug 2016

Yanlai Han

Zero-valent iron (ZVI) is one of the most widely used engineered materials for the remediation of chlorinated ethenes in the subsurface environment. The material has been widely used in various in situ remediation technologies including Fe-permeable reactive barriers (Fe-PRB) and subsurface injection of nanoscale zero-valent iron (nZVI) for contaminant plume attenuation and source zone remediation. However, there are two serious drawbacks when ZVI is used for the remediation of chlorinated ethenes: (i) ZVI tends to undergo rapid passivation which undermines its longevity for remediation applications, and (ii) ZVI has low dechlorination reactivity in the absence of catalyst additives. Bimetallic nanoparticles (BNPs), prepared by doping a small amount of catalytic metals (e.g., Pd or Ni), can significantly enhance the particles’ dechlorination reactivity. However, BNPs suffer rapid deactivation when exposed to groundwater media, and the BNPs deactivation mechanisms are still poorly understood. The first part of this dissertation aims to investigate Ni-Fe BNPs and Pd-Fe BNPs deactivation mechanisms when exposing them to common groundwater solutes. Aging experiments were conducted by pre-immersing fresh prepared BNPs in solutions containing different groundwater solutes for 24 h prior to reacting the particles with trichloroethene (TCE) to assess their dechlorination reactivity. Analyses of reaction kinetics and product distribution and stable carbon isotope fractionation measurements suggest that Pd-Fe BNPs were sensitive to solute-induced deactivation, particularly in solutions containing chloride, bicarbonate, nitrate or sulfite ions. Although Ni-Fe BNPs possess higher electrochemical stability than Pd-Fe BNPs in the aqueous media, strong deactivation was observed in sulfate, nitrate, and phosphate solutions. Multiple modes of BNP deactivation were proposed for the two types of BNPs. To overcome the intrinsic limitations of conventional and bimetallic ZVI materials, the second part of this dissertation aims to develop a new form of ZVI with higher dechlorination reactivity without the use of catalyst additives and a greater resistance to environmental passivation. A surface sulfidation treatment was designed and optimized for laboratory-made nZVI and commercial ZVI. The sulfided nZVI demonstrates remarkable improvements in dechlorination rates for chlorinated ethenes. Aging experiments indicated that sulfided nZVI possesses greater stability and maintains its dechlorination reactivity over long-term aging processes. Applying sulfidation treatment to commercial iron results in more efficient tetrachloroethene (PCE) and TCE degradation. Sulfidation treatment therefore represents a simple yet promising approach to increase the reactivity of ZVI using earth-abundant reagents in place of precious catalyst metals.

Kinetics, products and pathways for the removal of pentachlorophenol (PCP) by sulfidated nanoscale zero-valent iron (S-nZVI)

Article

Jan 2022

Sulfidated nanoscale zero-valent iron (S-nZVI) was applied in the treatment of pentachlorophenol (PCP)-polluted water to explore the reaction pathways and the effects of water chemistry on PCP adsorption and transformation.

Conceptualizing the Fe0/H2O System: A Call for Collaboration to Mark the 30th Anniversary of the Fe0-Based Permeable Reactive Barrier Technology

Article

Full-text available

Sep 2022

Science denial relates to rejecting well-established views that are no longer questioned by scientists within a given community. This expression is frequently connected with climate change, and evolution. In such cases, prevailing views are built on historical facts and consensus. For water remediation using metallic iron (Fe0), the remediation Fe0/H2O system, a consensus on electrochemical contaminant reduction was established during the 1990s and is still prevailing. Arguments against the reductive transformation concept has been regarded for more than a decade as 'science denial'. However, is it the prevailing concept that had denied the science of aqueous iron corrosion? This communication retraces the path used by our research group to question the reductive transformation concept. It is shown that the validity of the following has been questioned: (i) analytical applications of arsenazo III method for the determination of uranium, (ii) molecular diffusion as sole relevant mass transport process in the vicinity of the Fe0 surface in filtration systems, and (iii) volumetric expansive nature of iron corrosion at pH > 4.5. Item (i) questions the capability of Fe0 to serve as electron donor for UVI reduction under environmental conditions. Items (ii) and (iii) are interrelated as the Fe0 surface is permanently shielded by an oxide scale acting as diffusion barrier to dissolved species and conductive barrier to electrons from Fe0. The net result is that no electron transfer from Fe0 to contaminants is possible under environmental conditions. This conclusion refutes the validity of the reductive transformation concept and call for alternatives.

Biochars Loaded Iron Phosphides and Cu Hybrides Deriving from Solid Waste and Waste Acids: An Efficient Catalyst to Eliminate Refractory Organic Pollutants from High-Salinity Pharmaceutical Wastewater

Article

Jan 2022

Metallic iron (Fe0)-based materials for aqueous phosphate removal: A critical review

Article

Full-text available

Apr 2022
J ENVIRON MANAGE

The discharge of excessive phosphate from wastewater sources into the aquatic environment has been identified as a major environmental threat responsible for eutrophication. It has become essential to develop efficient but affordable techniques to remove excess phosphate from wastewater before discharging into freshwater bodies. The use of metallic iron (Fe0) as a reactive agent for aqueous phosphate removal has received a wide attention. Fe0 in-situ generates positively charged iron corrosion products (FeCPs) at pH > 4.5, with high binding affinity for anionic phosphate. This study critically reviews the literature that focuses on the utilization of Fe0-based materials for aqueous phosphate removal. The fundamental science of aqueous iron corrosion and historical background of the application of Fe0 for phosphate removal are elucidated. The main mechanisms for phosphate removal are identified and extensively discussed based on the chemistry of the Fe0/H2O system. This critical evaluation confirms that the removal process is highly influenced by several operational factors including contact time, Fe0 type, influent geochemistry, initial phosphate concentration, mixing conditions, and pH value. The difficulty in comparing independent results owing to diverse experimental conditions is highlighted. Moreover, contemporary research in progress including Fe0/oxidant systems, nano-Fe0 application, Fe0 material selection, desorption studies, and proper design of Fe0-based systems for improved phosphate removal have been discussed. Finally, potential strategies to close the loop in Fe0-based phosphate remediation systems are discussed. This review presents a science-based guide to optimize the efficient design of Fe0-based systems for phosphate removal.

The Role of Nitrate in Simultaneous Removal of Nitrate and Trichloroethylene by Sulfidated Zero-Valent Iron

Article

Mar 2022
SCI TOTAL ENVIRON

Sulfidated zero-valent iron (S-ZVI) is commonly used to degrade trichloroethylene (TCE). The reactivity of S-ZVI is related to not only the properties of S-ZVI but also the geochemical conditions in groundwater, such as coexisted NO3⁻. Therefore, the effect of NO3⁻ on TCE degradation by S-ZVI and its mechanism were systematically studied. 95.17% of TCE was degraded to acetylene, dichloroethene, ethene, ethane and multi‑carbon products via β-elimination by fresh S-ZVI that contained 85.31% Fe⁰ and 14.69% FeS in the presence of NO3⁻, demonstrating that NO3⁻ did not affect the degradation pathway of TCE. While high concentration of NO3⁻ (> 10 mg/L) competed for electrons at the Fe/FeOx interface with degradation products, leading to a continuous rising of acetylene. Moreover, the rapid reduction of NO3⁻ to NH4⁺ (89.79%) at the Fe⁰ interface contributed to the release of 5.08 mM Fe²⁺ from S-ZVI, which promoted the formation of Fe3O4 with excellent electron conduction properties on the surface of S-ZVI. Accordingly, NO3⁻ improved the degradation and electron selectivity of TCE by 51.07% and 2.79 fold, respectively. This study demonstrated that S-ZVI could remediate the contamination of NO3⁻ and TCE simultaneously and the presence of NO3⁻ could effectively enhance the degradation of TCE in groundwater.

High electron transfer rate and efficiency on Fe0 modified by sulfidation and pre-magnetization for carbamazepine degradation by heterogeneous electro-Fenton in wide pH ranges

Article

Jan 2022
CHEM ENG J

The coupling of sulfidation and pre-magnetization was for the first time applied for micron-Fe⁰ modification to obtain pre-S/Fe⁰ particle and tested as a highly active and recyclable catalyst for heterogeneous electro-Fenton (EF) degradation of carbamazepine (CBZ), observing the rate constant of this pre-S/Fe⁰-EF process was enhanced 11.9 times compared to conventional Fe⁰-EF process. Especially it performed well in wide pH ranges (3–9), all achieving the complete removal of CBZ of initial concentration of 5.0 mg L⁻¹ within 60 min at current 50 mA and optimum dose of 56 mg L⁻¹ pre-S/Fe⁰, although the removal rate constant decreased obviously when pH ≥ 5. Density functional theory (DFT) calculations illustrated that electrons were more easily transferred from the inner Fe⁰ to the surface through FeS rather than Fe3O4. Tafel curve proved the enhanced corrosion rate of S/Fe⁰ after pre-magnetization. Electron paramagnetic resonance (EPR) analysis and radical quenching experiments certified the role of •OH, •O2– and ¹O2. Eight consecutive runs experiment and the treatment of other typical organic pollutants proved the excellent stability and wide application of pre-S/Fe⁰-EF process. Besides, the CBZ degradation and mineralization in three real wastewaters by pre-S/Fe⁰-EF process also illustrated better performance and relatively low electric energy consumption when compared with that of Fe⁰-EF process. Overall, this study provides a new approach for improving heterogeneous Fe⁰-based EF process for highly efficient degradation of refractory pollutants in wide pH ranges.

Enhanced As(III) sequestration using nanoscale zerovalent iron modified by combination of loading and sulfidation: Characterizations, performance, kinetics and mechanism

Article

Full-text available

May 2021

Nanoscale zero-valent iron (nZVI) and sulfides have been confirmed to be effective in arsenic sequestration from aqueous solution. In this study, attapulgite supported and sulfide-modified nanoscale zero-valent iron (S-nZVI@ATP) are synthesized to realize the superposition effect of enhanced arsenic sequestration. The results indicated that nZVI clusters were well disaggregated and the BET specific surface area increased from 19.61 m2·g−1 to 46.04 m2·g−1 of S-nZVI@ATP, resulting in an enhanced removal efficiency of arsenic from 51.4% to 65.1% at 20 min. The sulfides in S-nZVI@ATP mainly exist as mackinawite (FeS) and this causes the spherical nanoparticles to exhibit a larger average particle size (94.6 nm) compared to bare nZVI (66.0 nm). In addition, S-nZVI@ATP exhibited a prominent ability for arsenic sequestration over a wide pH range of 3.0–6.0. The presence of anions SO42− and Cl− can enhance the arsenic removal whereas HCO3− inhibited it. The arsenic adsorption by S-nZVI@ATP could be explained by the pseudo-second-order kinetic model and the Langmuir model, with the maximum adsorption capacity of 193.8 mg·g−1. The mechanism of As(III) sequestration by S-nZVI@ATP involved multiple processes, mainly including precipitation conversion from FeS to As2S3, surface-complexation adsorption and co-precipitation. HIGHLIGHTS S-nZVI@ATP was synthesized to superimpose the performance of nZVI and sulfides on arsenic removal.; The distribution of sulfides in S-nZVI@ATP and its role for As(III) removal were investigated.; S-nZVI@ATP showed an enlarged specific surface area and an enhanced arsenic removal efficiency.; The maximum adsorption capacity for arsenic was 193.8 mg·g−1.; The mechanism involved the combined action of Fe(0) core and sulfide shell.;

NDMA Adsorption and Degradation by A New-Type of Ag-MONT Material Carrying Nanoscale Zero-Valent Iron

Article

Dec 2020
CHEMOSPHERE

Nitrosamines, which are emerging nitrogenous disinfection by-products, have raised great concern owing to their carcinogenicity and genotoxicity. Thus, exploring efficient materials to remove nitrosamines from the environment is of vital importance. In this work, NaBH4 was taken as a reducing agent and Ag-based metal organic nanotubes (Ag-MONTs) were impregnated in FeSO4·7H2O to prepare nanoscale zero-valent iron (nZVI) supported on the nanotubes ([email protected]). The new material was then characterized and applied to N-dimethylnitrosamine (NDMA) adsorption and degradation in water. The material had excellent ability to adsorb and degrade NDMA, and the total concentrations of iron and silver remaining in water did not exceed standard limits after 120 min of adsorption. Coexisting substances, such as NO3–, Cl–, CO32–, humic acid, trichloromethane, and trichloronitromethane, did not affect the NDMA removal efficiency of the adsorbent. The NDMA removal efficiency of the new material exceeded 88% even in the presence of SO42– and PO43–. The NDMA degradation mechanism of [email protected] included a catalytic hydrogenation reaction and resulted in dimethylamine as the final degradation product. The [email protected] showed favorable stability and reusability. Taking the results together, the [email protected] proposed in this work are applicable to NDMA adsorption and degradation in water.

Enhancing Fe-C micro-electrolysis by coupling MF with electrolyte solution: Mechanism and application

Article

Feb 2021
SEP PURIF TECHNOL

In this study, Fe-C micro-electrolysis rate enhancement was accomplished by a coupled magnetic field (MF) and Electrolyte solution. The rate of micro-electrolysis of Fe-C materials in pure water solution, NaCl solution, NaH2PO4 solution and Al2(SO4)3 solution in the presence of MF was increased by 1.8–6.8 times, and the removal rate of saline organic wastewater was increased by 1.5–4.7 times. The results show that coupling MF with a electrolyte enhances the process of Fe-C micro-electrolysis, and indicates the possible reaction mechanism. SEM and XRD analysis showed that both the electrolyte solution and MF promoted the corrosion of Fe-C materials and accelerated the dissolution of Fe²⁺. The EDX-mapping results confirmed that MF promotes the accumulation of electrolyte ions on the surface of Fe-C materials. Electrochemical workstation testing showed that the promotion effect of MF in different solutions was different. In addition, MF also enhances the electrolyte activity, reduces the corrosion resistance of Fe-C materials, and improves the local current of Fe-C micro-electrolysis. Moreover, the MF/Fe-C/H2O2 process has good applicability in the treatment of mixed inorganic salt wastewater, and the treatment of saline organic wastewater is more economical and efficient.

Iron-assisted biological wastewater treatment: Synergistic effect between iron and microbes

Article

Aug 2020
BIOTECHNOL ADV

Iron-assisted biological wastewater treatment processes have shown a promising potential in removing various types of contaminants. Synergistic effects between iron and microbes on the contaminant degradation make the role of iron beyond that of a nutritional necessity. Exploration of the synergistic mechanisms and the interactions between iron species and microbes and their metabolic products in bio‑iron systems is therefore of significant importance. Iron, including zero-valent iron, ferrous/ferric ions and iron minerals are all reported to be capable of enhancing specific contaminant removals. Although the main role of different iron species in stimulating biological process may differ between each other, their similar transformation pathways may bring us useful information about bio‑iron systems. In this paper, an overview of iron-assisted biological wastewater treatments, including anaerobic digestion, S and Cl reduction, N and P removal, heavy metal immobilization, aromatic and halogenated hydrocarbon compounds degradation, and sludge granulation is provided. Also, the potential synergistic effects between iron and microbes involved in these processes are explored. Furthermore, the main advantages, limitations, and challenges for the development of iron-assisted treatment processes are envisaged.

Nitrate Reduction Using Iron and Copper Bimetallic Nanoparticles Supported by Chelating Resin: Effect of Solution Chemistry, Mechanism, and Regeneration

Article

Apr 2020

Nanoscale zero-valent iron/copper supported on chelating resin (D-Fe/Cu) was prepared for removing nitrate from water. Batch experiments were carried out to investigate the effect of solution chemistry, including the initial pH of the solution, initial dissolved oxygen (DO), coexisting anions, and humic acid on the removal of nitrate. Results showed that the initial pH and DO had a significant effect on the removal of nitrate. Coexisting anions, including Cl−, SO42−, PO43−, and HCO3−, inhibited nitrate reduction. The removal efficiency of nitrate was 95.51%, and the selectivity of N2 was 52.49% at the end of the reaction. The results of X-ray photoelectron spectroscopy (XPS) analysis before and after the reduction showed that Fe(0) was directly involved in the nitrate reduction reactions. After the reaction with nitrate, Fe(0) was oxidized to Fe(II) or Fe(III), the content of Fe(II) increased, and the amount of Fe(III) decreased. The Fe(II)/Fe(III) cycle existed on the surface of Fe(0) during the reduction process with nitrate. Cu only provided active sites to adsorb hydrogen, without direct participation. The results demonstrated that D-Fe/Cu can be reused, and the removal efficiency of nitrate was 97.41% in the fifth recycle.

Reduction of Chlorendic Acid by Zero-Valent Iron: Kinetics, Products, and Pathways

Article

Sep 2019
J HAZARD MATER

Chlorendic acid (CA) is a recalcitrant groundwater contaminant for which an effective treatment technology does not currently exist. In this study, a series of batch experiments were conducted to investigate the treatment of CA by zero-valent iron (ZVI) under various water chemistry conditions. It was observed that CA was removed by ZVI via both adsorption and degradation, with the degradation rate being proportional to the fraction of CA adsorbed onto ZVI. The rate of CA degradation decreased as pH increased, presumably due to the passivation of ZVI and diminishing CA adsorption. Chloride (Cl-) did not appreciably affect CA adsorption and degradation, while sulfate (SO42-) significantly inhibited both processes because SO42- competed with CA for ZVI adsorptive sites. The rate of CA degradation was significantly accelerated by ZVI-associated Fe(II). Nine byproducts of CA transformation were identified by high-resolution mass spectrometry. The formation and subsequent degradation of these products revealed that the transformation of CA by ZVI occurred via a step-wise reductive dechlorination pathway. Overall, this study suggests that ZVI may be effective at remediating CA-contaminated sites.

Influence of coexisting ions on the electron efficiency of sulfidated zerovalent iron toward Se(VI) removal

Article

Jul 2019
CHEM ENG J

Recently, sulfidation of zerovalent iron (ZVI) has gained increasing attention due to its merits to enhance both the reactivity and electron efficiency (EE) of ZVI. While few studies have been conducted to elucidate the influence of coexisting ions on EE of sulfidated ZVI, which is important for up-scaling such a decontamination strategy. In this study, taking Se(VI) as a probe, the influence of coexisting ions, including Cl⁻, SO4²⁻, PO4³⁻, Ca²⁺, and Mg²⁺, on the removal capacity and EE of ZVI under aerobic conditions was investigated. Results revealed that, compared to unamended ZVI, sulfidated ZVI synthesized by ball-milling with elemental sulfur (S-ZVIbm) could enhance EE of ZVI toward Se(VI). However, this enhancing effect is not very effective and the maximum of EE value obtained in this work is only 8.8%. Chloride with concentration range of 1–20 mM was found to exert little influence on EE of ZVI. While for SO4²⁻ and PO4³⁻, relative to 10 mM NaCl, their presence could depress EE of S-ZVIbm, and this inhibiting effect became more pronounced as their concentration elevated. In contrast, Ca²⁺ or Mg²⁺ hardness ions (0.05–2 mM) increased the EE values of S-ZVIbm by 19.2–83.3%. X-ray absorption fine structure analysis revealed that all of Fe⁰ in S-ZVIbm was exhausted within 24 h with lepidocrocite and magnetite being the primary corrosion products. This suggested that it was the amounts of electrons accepted by Se(VI) rather than that donated by S-ZVIbm accounted for the variation of EE. In general, these co-solutes may impose their influence on EE through affecting Se(VI) adsorption and the subsequent electron transfer from Fe⁰ core to Se(VI) at the water-particle interface.

Zinc addition and its effect on the corrosion behavior of a 30% cold forged Alloy 690 in simulated primary coolant of pressurized water reactors

Article

Apr 2019
J ALLOY COMPD

The effectiveness of zinc on the electrochemical behavior and oxide film formation of a 30% cold forged Alloy 690 exposed to high-temperature borated and lithiated water was investigated. The morphology and chemical composition of the formed oxide films with and without zinc injection were analyzed and compared by TEM–EDS and XPS techniques. Zinc clearly improved the electrochemical corrosion parameters of the alloy. Cathodic polarization improved the passivation of the alloy especially in presence of zinc. It was also found that actual zinc concentrations higher than 10 ppb is needed to observe a significant improvement in the corrosion resistance of the alloy in the pressurized water reactor operating conditions. Finally, further investigations of the effect of cold work on the high–temperature electrochemical performance of Alloy 690 and other iron and nickel based alloys in zinc injected solutions is recommended for future studies.

Enhanced removal of antibiotics from secondary wastewater effluents by novel UV/pre-magnetized Fe0/H2O2 process

Article

Apr 2019
WATER RES

Antibiotics have been frequently detected in the aquatic environment and are of emerging concern due to their adverse effect and potential of inducing antibiotic resistance. In this study, we developed an UV/pre-magnetized Fe⁰/H2O2 process (UV/pre-Fe⁰/H2O2) valid for neutral pH conditions, which could remove sulfamethazine (SMT) completely within only 30 min and enhance 1.8 times of SMT removal. Meanwhile, this process demonstrated outstanding mineralization capability with the TOC removal of 92.1%, while for UV/H2O2 and UV/Fe⁰/H2O2 system it was 53.9% and 72.1%, respectively. Better synergetic effect between UV irradiation and pre-Fe⁰/H2O2 system was observed, and the value of synergetic factor was 6.3 in the presence of both ions and humic acid, which was much higher than that in deionized water (4.4), humic acid (5.5) and ions (1.5). Moreover, the process could efficiently remove various antibiotics (800 μg L⁻¹ oxytetracycline (OTC); 800 μg L⁻¹ tetracycline (TC); 400 μg L⁻¹ sulfadiazine (SD) and 400 μg L⁻¹ SMT) in the secondary wastewater effluent. After optimization of Fe⁰ and H2O2 dosage, these antibiotics could be removed within 10 min (kapp (10³) = 288.6 min⁻¹) with a very low treatment cost of 0.1 USD m⁻³, and the EE/O value was only 1.22 kWh m⁻³. Compared with O3, UV/Fe²⁺/PDS, VUV/UV/Fe²⁺ and other US-based processes, the degradation rates by this process could enhance as high as 22.3 folds while the treatment cost or EE/O value could reduce greatly. Therefore, UV/pre-Fe⁰/H2O2 process is promising and cost-effective for the treatment of antibiotics in secondary wastewater effluents.

Pre-magnetized Fe0 activated persulphate for the degradation of nitrobenzene in groundwater

Article

Nov 2018
SEP PURIF TECHNOL

Nitrobenzene (NB) is toxic and resistant to biodegradation and widespread in surface water and groundwater. The persulfate (PS) system has been employed for the NB degradation and proved to be effective. Zero-valent iron (Fe⁰) has been used for the activation of PS (Fe⁰/PS) recently. However, the process exhibits a significant drawback of slow release of Fe²⁺ from Fe⁰, resulting in low efficiency of the approach as Fe²⁺ is the primary species for the PS activation. In this study, a weak magnetic field was utilized to pre-treat Fe⁰ for PS activation (pre-Fe⁰/PS) based on the magnetic memory of iron. The efficiency of the pre-Fe⁰/PS process was tested on the degradation of NB in simulated groundwater under anoxic conditions. The NB removal in the pre-Fe⁰/PS process was significantly enhanced compared with that in the Fe⁰/PS process. The effect of Fe⁰, PS, and NB concentration on the NB degradation was investigated. The influence of single inorganic ions, anion couples and humic acids as ubiquitous species in groundwater on the NB removal in the pre-Fe⁰/PS and Fe⁰/PS processes were studied and discussed. A column filled with pre-Fe⁰ was designed to test the NB removal in the pre-Fe⁰/PS process in a continuous-flow mode. The experimental results indicated that pre-Fe⁰/PS is a promising approach for the remediation of groundwater with high NB concentration.

Dissolved oxygen and nitrate effects on the reduction and removal of divalent mercury by pumice supported nanoscale zero-valent iron

Article

Aug 2018

A series of batch experiments were conducted to identify the effects of dissolved oxygen (DO) and nitrate on the removal and reduction of Hg(II) by a pumice supported nanoscale zero-valent iron (p-nZVI) composite. After the adsorption and consecutive reduction of Hg(II) in an anoxic solution, zero-valent iron, and ferrous and ferric irons were found on the surface of the p-nZVI with a chain-like structure; while in the oxic solution, a thick ferric shell was found on the surface of the p-nZVI with collapsed chain structures. In Hg(II) sorption isotherm tests, with 25, 250, 1000, and 2500 nM of Hg(II), the Hg(II) sorption capacity of the p-nZVI was 6.1 mg g-1 in the oxic aqueous solution and 1.5 mg g-1 in the anoxic aqueous solution. While the adsorption of Hg(II) was more favorable in the presence of DO, the headspace Hg(0), as well as dissolved Fe(II), was largely increased in the absence of DO. The removal of Hg(II) in oxic and anoxic suspensions was not affected by nitrate levels ranging from 0.08 to 8 mM. In contrast, Hg(0) concentration in headspace increased with an increase of nitrate, which was related to the enhanced dissolved Fe(II) production. The experimental results of this study suggest that the absence of DO and the presence of nitrate in groundwater could significantly increase Hg(0) in groundwater and adjacent atmosphere during the usual remediation process using relevant nZVI techniques. The effective capture and treatment methods of Hg(0) should be developed for better application of p-nZVI-based technologies.

Reduction of Nitrate, Bromate, and Chlorate by Zero Valent Iron (Fe0)

Article

Full-text available

Jan 2003

Paul Westerhoff

Oxo-anions occur in drinking waters, pose potential health risks, and should be controlled. It may be possible to incorporate zero-valent iron (Fe-0) into water treatment processes to remove oxo-anions. Under near neutral pH (similar to7) and aerobic conditions, the three oxo-anions studied (NO3-, BrO3-, ClO3-) were electrochemically reduced by Fe-0 in batch and continuous-flow packed column experiments. Mass balances provided strong evidence that ammonia is the primary reduction by-product from nitrate, chloride from chlorate, and bromide from bromate. Protons were consumed during the reaction, resulting in an increase in pH (i.e., production of hydroxide). Oxo-anion removal rates decreased as follows: BrO3->ClO3->NO3. Differing rates of oxo-anion removal between batch and continuous flow column tests suggested that higher solid (Fe-0) to liquid ratios increase oxo-anion electrochemical reduction, and scaling up of batch kinetic data to larger scale must consider the solid-liquid ratios. The atomic structure (atomic radii, electron orbital configuration, electron affinity) of nitrogen, chlorine, and bromine elements of the oxo-anions, and the bond dissociation energy between these elements and oxygen, were good indicators for the relative rates of reduction by Fe-0.

Applicability of Single-Site Rate Equations for Reactions on Inhomogeneous Surfaces

Article

Full-text available

Feb 2004

Single-site rate laws are broadly applicable to surface mediated chemical kinetics even though most surfaces are strongly inhomogeneous. We defined the best single-site approximation for an inhomogeneous rate law by square error minimization and, using this definition, examined the limits of the applicability of the single-site approximation and the nature of the errors that arise when inhomogeneity is ignored. We found that (i) correlation between kinetic inhomogeneity and sorptive inhomogeneity does not affect the success of the single-site approximation, (ii) many types of inhomogeneity produce similar macroscopic kinetic behavior, (iii) ignoring inhomogeneity in a single kinetic experiment causes rates to be overestimated at high and low concentrations and underestimated at intermediate concentrations, (iv) it is difficult to obtain detailed information about inhomogeneity from a single kinetic experiment, and (v) kinetic experiments over varying concentration ranges may be used to diagnose the existence of inhomogeneity and to parametrize an inhomogeneous rate law.

Influence of Denitrification on the Corrosion of Iron and Stainless Steel Powder

Article

Full-text available

Jan 2000

The purpose of this research was to address the anoxic oxidation of metallic iron and stainless steel powder by nitrate, nitrite, and anaerobic mixed cultures. In sterile batch reactors, both nitrate and nitrite (10 mg N/L) could chemically oxidize metallic iron, with a concomitant reduction to ammonium. Nitrate or nitrite reduction coupled to metal corrosion was not observed in the case of stainless steel powder. Combination of an anaerobic mixed culture and metallic iron led to (cathodically produced) H2 consumption and a complete nitrate or nitrite reduction (mainly to NH4+). This caused a slightly enhanced metal oxidation. In the case of stainless steel, corrosion caused by denitrifying microorganisms was evidenced by data on nitrate/nitrite ions and solubilized iron. Experiments with increasing nitrite concentrations indicated that nitrite in the range of 50 mg of NO2- -N/L inhibited the corrosion processes. Moreover, at concentrations above 140 mg NO2- -N/L, a significant production of nitric oxide (NO) was detected. Differences between iron and stainless steel powder at low concentrations of nitrate or nitrite are most probably due to differences in kinetics: metallic iron exhibited faster chemical than biological reactions as opposed to stainless steel. It is postulated that the inhibitory effect of higher nitrite concentrations could partly be due to the chemical formation of NO and its toxic effect on the microorganisms acting at the steel surface.

Degradation of Carbon Tetrachloride in the Presence of Iron and Sulphur Containing Compounds

Article

Full-text available

Oct 1994
CHEMOSPHERE

The effect of several sulphur compounds: sodium sulphate, sodium sulphide, ferrous sulphide,pyrite and an organosulphonic acid on the kinetics of the iron (Fe °) induced degradation of carbon tetrachloride was examined under aerobic conditions. It was observed that all of the sulphur compounds investigated significantly accelerated the reaction. The mechanisms of the processes studied as well as their possible influence on the efficiency of the iron-induced dehalogenation of pollutants, both in situ and in above-ground treatment are discussed.

Sulfate-Reducing Bacteria in the Zero Valent Iron Permeable Reactive Barrier at Fry Canyon, Utah

Article

Dec 2003

Ryan C. Rowland

This chapter presents a study in which a zero valent iron (ZVI) permeable reactive barrier (PRB) was installed in a shallow, colluvial aquifer contaminated with uranium in Fry Canyon, Utah, in September 1997. Aerobic and anaerobic iron corrosion reactions in the ZVI PRB have created a highly reducing, oxygen-depleted, and hydrogen gas-enriched geochemical environment in the PRB that is favorable for sulfate-reducing bacteria (SRB). Stable sulfur isotope, microbiologic, and geochemical evidence indicates that SRB are active in the ZVI PRB. The stable sulfur isotope and SO42– data from wells in the ZVI PRB and a downgradient well show that sulfur is removed by DSR through a Rayliegh-type distillation process. The enrichment factor computed from the Rayleigh plot is close to the values measured in other field investigations of DSR in groundwater systems. The thermodynamic speciation calculations and stable sulfur isotope data indicate that sulfide precipitation is the only sink for sulfur in the PRB. The distribution of SO42- concentrations indicates that most of the sulfide precipitation is occurring in the first 0.15 m of the PRB.

Chemistry and Microbiology of Permeable Reactive Barriers for in Situ Groundwater Clean-Up

Article

Jan 2000

Permeable reactive barriers (PRBs) are receiving a great deal of attention as an innovative, cost-effective technology for in situ clean up of groundwater contamination. A wide variety of materials are being proposed for use in PRBs, including zero-valent metals (e.g., iron metal), humic materials, oxides, surfactant-modified zeolites (SMZs), and oxygen- and nitrate- releasing compounds. PRB materials remove dissolved groundwater contaminants by immobilization within the barrier or transformation to less harmful products. The primary removal processes include: (1) sorption and precipitation, (2) chemical reaction, and (3) biologically mediated reactions. This article presents an overview of the mechanisms and factors controlling these individual processes and discusses the implications for the feasibility and long-term effectiveness of PRB technologies.

Performance Evaluations at the Moffett Field and Department of Defense Permeable Barrier Sites

Conference Paper

Jan 2000

Competitive Sorption of Simple Organic Acids and Sulfate on Goethite

Article

Mar 1996

Sorption of phthalic acid, chelidamic acid, and sulfate onto goethite (α-FeOOH) was examined in single-sorbate and in sulfate−organic acid binary-sorbate systems to determine the extent of competition between the simple organic acids and sulfate. Sorption characteristics of sulfate and the organic acids were similar and resembled those reported for humic substances onto oxides. Sorption data for all three sorbates over a wide range of conditions (pH, I, sorbate/sorbent ratio) were described quantitatively by the generalized two-layer model with a unique set of surface reactions and equilibrium constants for each sorbate. Sorption affinities of sulfate and the organic acids were comparable, and sulfate effectively competed with the organic acids for surface sites on goethite, particularly at low pH. These results suggest that sulfate can significantly influence the sorption of simple organic acids and humic substances in natural aquatic systems. Predictions of sorption in binary-sorbate systems based on single-sorbate data fits represented competitive sorption data reasonably well over a wide range of conditions. However, there were underpredictions of minor-component sorption in the presence of a major component, which may be explained by sorbate-specific surface site heterogeneity and/or by inaccurate representation of Coulombic effects in the model.

Degradation of Chlorinated Ethenes by Fe(0) – Inhibition Processes and Mineral Precipitation

Article

Feb 2002

Granular zero-valent iron was used for the treatment of groundwater pollution caused by chlorinated ethylenes, mainly TCE, cis-DCE and VC at an industrial site. The rapidly decreasing rates of de-chlorination in the initial phase were attributed to the precipitation of carbonates and the development of hydrogen by anaerobic corrosion. After 70 pore volumes, sulphate was reduced by bacteria. From this point in time, the degradation of TCE was slightly accelerated whereas the de-chlorination rates of the other chlorinated ethylenes decreased only slowly. This relative improvement was assumed to be caused by the uptake of electron-transfer-blocking hydrogen by bacteria. Because the overall trend of the degradation rates is negative we conclude that the inhibitive effect of carbonate precipitation and hydrogen formation cannot be compensated for by the positive influence of the activity of sulphate-reducing bacteria.

Applied Hydrogeology

Book

Jan 2001

C. W. Jr. Fetter

A Method to Assess Analytical Uncertainties Over Large Concentration Ranges With Reference to Volatile Organics in Water

Article

Aug 1996

J.F. Devlin

The uncertainty associated with a volatile organic concentration measurement is a function of variability and bias introduced at the various levels of sample handling; collection, storage and analysis. During the past decade, sampling materials and the development and/or improvement of sampling protocols have been the subject of considerable research activity. As a result, in cases of samples properly handled, the analytical variability can be the dominant source of uncertainty in a given concentration value. Here analytical variability refers to any error that might arise during analysis, including the detector response error and any sample handling errors common to both standards and samples. This can be a particular concern for field analyses by gas chromatography (GC). Well-established statistical methods are available to estimate analytical uncertainty from linear calibration curves, but these methods are poorly suited for the analysis of volatile organics because organic samples frequently require instrument calibration (usually GC) over several orders of magnitude in concentration. If a single linear calibration curve is used to determine sample concentrations and uncertainties, then unrealistically large uncertainties may be assigned to low concentration samples. However, the methods can be adopted for extended concentration range calibration curves by breaking the overall calibration line down into smaller sub-calibration lines that span smaller ranges. These can then be examined and used selectively to determine concentrations with more appropriate uncertainties attached. The method of multiple calibration line analysis described here is suitable for programming with any high-level computer language. It can be used to calculate meaningful analytical uncertainty values for any substance analyzed over a wide range in concentrations (i.e., an order of magnitude or more).

Pathways and Kinetics of Chlorinated Ethylene and Chlorinated Acetylene Reaction with Fe(0) Particles

Article

May 2000

Pathways and kinetics through which chlorinated ethylenes and their daughter products react with Fe(O) particles were investigated through batch experiments. Substantial intra- and interspecies inhibitory effects were observed, requiring the use of a modified Langmuir-Hinshelwood-Hougen-Watson (LHHW) kinetic model in which species compete for a limited number of reactive sites at the particle-water interface. Results indicate that reductive Î²-elimination accounts for 87% of tetrachloroethylene (PCE), 97% of trichloroethylene (TCE), 94% of cis-dichloroethylene (cis-DCE), and 99% of trans-dichloroethylene (Trans-DCE) reaction. Reaction of 1,1-DCE gives rise to ethylene, consistent with a reductive Î±-elimination pathway. For the highly reactive chloro- and dichloro-acetylene intermediates produced from the reductive elimination of TCE and PCE, 100% and 76% of the reaction, respectively, occur via hydrogenolysis to lessen chlorinated acetylenes. The branching ratios for reactions of PCE or TCE (and their daughter products) with iron particles are therefore such that production of vinyl chloride is largely circumvented. Reactivity of the chlorinated ethylenes decreases markedly with increasing halogenation, counter to the trend that might be anticipated if the rate-limiting step were to involve dissociative electron transfer. The authors propose that the reaction of vinyl halides proceeds via a di-Ï-bonded surface-bound intermediate. The reactivity trends and pathways observed in this work explain why lesser-chlorinated ethylenes have only been reported as minor products in prior laboratory and field studies of PCE and TCE reaction with Fe(O).

Performance Evaluation of a Zerovalent Iron Reactive Barrier: Mineralogical Characteristics

Article

Oct 2000

There is a limited amount of information about the effects of mineral precipitates and corrosion on the lifespan and long-term performance of in situ Fe° reactive barriers. The objectives of this paper are (1) to investigate mineral precipitates through an in situ permeable Fe° reactive barrier and (2) to examine the cementation and corrosion of Fe° filings in order to estimate the lifespan of this barrier. This field scale barrier (225-ft long x 2-ft wide x 31-ft deep) has been installed in order to remove uranium from contaminated groundwater at the Y-12 plant site, Oak Ridge, TN. According to XRD and SEM-EDX analysis of core samples recovered from the Fe° portion of the barrier, iron oxyhydroxides were found throughout, while aragonite, siderite, and FeS occurred predominantly in the shallow portion. Additionally, aragonite and FeS were present in up-gradient deeper zone where groundwater first enters the Fe° section of the barrier. After 15 months in the barrier, most of the Fe° filings in the core samples were loose, and a little corrosion of Fe° filings was observed in most of the barrier. However, larger amounts of corrosion (10-150 m thick corrosion rinds) occurred on cemented iron particles where groundwater first enters the barrier. Bicarbonate/carbonate concentrations were high in this section of the barrier. Byproducts of this corrosion, iron oxyhydroxides, were the primary binding material in the cementation. Also, aragonite acted as a binding material to a lesser extent, while amorphous FeS occurred as coatings and infilings. Thin corrosion rinds (2-50 m thick) were also found on the uncemented individual Fe° filings in the same area of the cementation. If corrosion continues, the estimated lifespan of Fe° filings in the more corroded sections is 5 to 10 years, while the Fe° filings in the rest of the barrier perhaps would last longer than 15 years. The mineral precipitates on the Fe° filing surfaces may hinder this corrosion but they may also decrease reactive surfaces. This research shows that precipitation will vary across a single reactive barrier and that greater corrosion and subsequent cementation of the filings may occur where groundwater first enters the Fe° section of the barrier.

Remediating Ground Water with Zero-Valent Metals: Chemical Considerations in Barrier Design

Article

Nov 1997

To gain perspective and insight into the performance of permeable reactive barriers contg. granular iron metal, it is useful to compare the degrdn. kinetics of individual chlorinated solvents over a range of operating conditions. Pseudo 1st-order disappearance rate consts. normalized to iron surface area concn. (kSA) recently have been reported for this purpose. This paper presents the results of further exploratory data anal. showing the extent to which variation in kSA is due to initial halocarbon concn., iron type, and other factors. To aid in preliminary design calcns., representative values of kSA and a reactive transport model have been used to calc. the min. barrier width needed for different groundwater flow velocities and degrees of halocarbon conversion. Complete dechlorination of all degrdn. intermediates requires a wider treatment zone, but the effect is not simply additive because degrdn. occurs by sequential and parallel reaction pathways. [on SciFinder(R)]

The Role of Oxides in Reduction Reactions at the Metal-Water Interface

Article

Jan 1998

The oxide layer that lies at the iron-water interface under environmental conditions can influence the redn. of solutes by acting as a passive film, semiconductor, or coordinating surface. As a passive film, oxides may inhibit reaction by providing a phys. barrier between the underlying metal and dissolved oxidants. Sustained redn. of solutes requires localized defects in the passive film (e.g., pits), or some mechanism for transferring electrons through the oxide. In the semiconductor model, conduction band electrons from the oxide may contribute to solute redn., but electron hopping (resonance tunneling) appears to be more important due to the high population of localized states in oxides formed under environmental conditions. Ultimately, electron transfer to the solute must occur via a precursor complex at the oxide-water interface. For dehalogenation of chlorinated aliph. compds. on an iron oxide surface, a surface complexation model suggests that the outer-sphere precursor complex is weak and

Forum - Keeping up with all that literature: The IronRefs database turns 500

Article

Aug 2002

Paul Tratnyek

The IronRefs database covers published research on remediation applications of 0-valent metals and is accessible at http://cgr.ese.ogi.edu/ironrefs/. The rapid growth of groundwater remediation applications of 0-valent metals is reflected in the rapid growth of this database, which exceeded 500 records in Dec. 2001. This is much more complete coverage of this topic than can be obtained from any of the major com. databases of the scientific literature. Specialized databases such as IronRefs, prepd. by experts in a particular field, are becoming increasingly common on the Web and offer distinctive benefits over traditional options for keeping up with the literature in rapidly growing fields of science and engineering. [on SciFinder(R)]

Investigation of the Long-Term Performance of Zero-Valent Iron for Reductive Dechlorination of Trichloroethylene

Article

Dec 1999

This research investigated the long-term performance of zero-valent iron for mediating the reductive dechlorination of trichloroethylene (TCE). Over a 2-year period, rates of TCE dechlorination in columns packed with iron filings were measured in simulated groundwaters containing either 3 mM CaSO4, 5 mM CaCl2, or 5 mM Ca(NO3)2. At early elapsed times, TCE reaction rates were pseudo-first-order in TCE concentration and were independent of the solution pH. With increasing elapsed time, reaction rates deviated from pseudo-first-order behavior due to reactive site saturation and increased iron surface passivation toward the influent end of each column. The extent of passivation was dependent on both the TCE concentration and the background electrolyte solution. For most of the investigation, TCE reaction rates in 3 mM CaSO4 and 5 mM CaCl2 solutions were statistically identical at the 0.05 confidence level. However, TCE reaction rates in 5 mM Ca(NO3)2 were slower. In columns operated using chloride- and sulfate-containing waters, the effective half-life for TCE dechlorination increased from approximately 400 min after 10 days elapsed to approximately 2500 min after 667 days. The effective TCE half-life in the nitrate-containing water increased from approximately 1500 min after 10 days to approximately 3500 min after 667 days. Measurements of iron corrosion rates in nitrate and chloride solutions showed that nitrate contributed to increased iron surface passivation and decreased rates of iron corrosion. Corrosion current measurements indicated that halocarbon reduction on fresh iron surfaces was cathodically controlled, whereas on aged iron surfaces, iron corrosion was anodically controlled. Anodic control of iron corrosion contributed to the development of reactive site saturation with time and to similar reaction rates for TCE and perchloroethylene. Passivation of the iron surfaces was found to be dependent on the adhering tendency of the corrosion products and not on the overall mass of corrosion products in the columns. The decrease in TCE reaction rates over time can be attributed to anodic control of iron corrosion and not to increasing reactant mass transfer limitations associated with diffusion through porous corrosion products.

Biogeochemical Dynamics in Zero-Valent Iron Columns: Implications for Permeable Reactive Barriers

Article

May 1999

The impact of microbiological and geochemical processes has been a major concern for the long-term performance of permeable reactive barriers containing zero-valent iron (Fe0). To evaluate potential biogeochemical impacts, laboratory studies were performed over a 5-month period using columns containing a diverse microbial community. The conditions chosen for these experiments were designed to simulate high concentrations of bicarbonate (17−33 mM HCO3-) and sulfate (7−20 mM SO42-) containing groundwater regimes. Groundwater chemistry was found to significantly affect corrosion rates of Fe0 filings and resulted in the formation of a suite of mineral precipitates. HCO3- ions in SO42--containing water were particularly corrosive to Fe0, resulting in the formation of ferrous carbonate and enhanced H2 gas generation that stimulated the growth of microbial populations and increased SO42- reduction. Major mineral precipitates identified included lepidocrocite, akaganeite, mackinawite, magnetite/maghemite, goethite, siderite, and amorphous ferrous sulfide. Sulfide was formed as a result of microbial reduction of SO42- that became significant after about 2 months of column operations. This study demonstrates that biogeochemical influences on the performance and reaction of Fe0 may be minimal in the short term (e.g., a few weeks or months), necessitating longer-term operations to observe the effects of biogeochemical reactions on the performance of Fe0 barriers. Although major failures of in-ground treatment barriers have not been problematic to date, the accumulation of iron oxyhydroxides, carbonates, and sulfides from biogeochemical processes could reduce the reactivity and permeability of Fe0 beds, thereby decreasing treatment efficiency.

Combined Zero- and First-Order Kinetic Model of the Degradation of TCE and cis-DCE with Commercial Iron

Article

Oct 1999

The design of a permeable iron wall depends to a great extent on the transformation kinetics of the chlorinated compounds. Therefore these degradation kinetics of TCE and cis-DCE with commercial iron and their dependence on the properties of the compounds and on the experimental conditions were studied in mixed-batch and column experiments. Since our data cannot sufficiently be described by a pseudo-first-order kinetics, we successfully applied an enhanced model accounting for both zero- and first-order kinetics. The fitted kinetic parameters, however, were found to depend on the experimental conditions and compound properties, which is interpreted in terms of different rate-limiting processes. The zero-order rate constant turned out to be twice as high for cis-DCE as for TCE in both experimental systems. Despite its slower transformation without transport control, the first-order rate constant was about 4 times higher for TCE than for cis-DCE in the mixed-batch vials. We attribute this to the lower water solubility and thus higher sorptivity of TCE at the polished iron surface. In the column experiments, transformation without transport control was twice as fast as in the batch experiments for both compounds. cis-DCE was degraded faster than TCE in the zero- and first-order region. At higher influent concentrations, the zero- and first-order rate constant of TCE decreased, which we assume to be due to the buildup of iron oxides, and transport to the reactive sites was found to depend a little on flow velocity. Due to the slow first-order kinetics of both compounds, we assume diffusion within micropores to be rate-limiting in flow-through systems. These variations in the kinetic parameters of the combined zero- and first-order model suggest that transport and sorption to reactive sites contribute to kinetic control of the degradation of chlorinated ethenes in addition to charge-transfer processes.

Kinetics of Nitroaromatic Reduction on Granular Iron in Recirculating Batch Experiments

Article

May 1998

Granular iron has been determined to be a potentially useful reductant for the removal of common organic contaminants from groundwater. This research is aimed at improving our understanding of the processes that control the reactivity and longevity of the iron particles when they are used for groundwater treatment. A suite of nitroaromatic compounds (NACs) including 4-chloronitrobenzene (4ClNB), 4-acetylnitrobenzene (4AcNB), nitrobenzene, 2-methylnitrobenzene (2MeNB), and 2,4,6-trinitrotoluene (TNT) was used to investigate granular iron reactivity in anoxic pH 10, 0.008 M KNO3 solution. Master Builder's brand of granular iron with a surface area of about 1 m2/g was used in all experiments. The NACs were reduced rapidly to anilines that were found to sorb reasonably strongly to the solid particles and to interfere with the reduction of NACs. The granular iron was found to lose reactivity quite rapidly over the first few days of exposure and then more slowly over the next several months. Reactivity loss due to reversibly sorbed products was minimized by flushing the system with background electrolyte between experiments. Competition experiments with binary mixtures of 4ClNB and each one of the other NACs were performed to investigate relative affinities of these compounds for the solid surface. Despite the overall loss in reactivity observed for the granular iron, the relative rate constants in the competition experiments appeared to remain constant in time.

Fe(0)-Supported Autotrophic Denitrification

Article

Jan 1998

Proof of concept was obtained that Fe(0) can stoichio metrically reduce nitrate to ammonium and that cathodic hydrogen [produced during anaerobic Fe(0) corrosion by water] can sustain microbial denitrification to reduce nitrate to more innocuous products (i.e., N2O and N2). Autotrophic, denitrifying growth on Fe(0) was proven through the use of a dual-flask apparatus. Cathodic H2 from a flask containing Fe(0) was allowed to diffuse to another (anoxic) flask containing a pure culture of Paracoccus denitrificans, where denitrification and microbial growth were observed. Nitrate reduction and end product distribution were studied in batch reactors amended with either steel wool or Fe(0) powder. Steel wool, with a smaller specific surface area, was less reactive, and its corrosion did not significantly increase the pH of the solution. This allowed for a greater participation of denitrifiers in the nitrate removal process, which increased nitrate removal rates and transformed a greater portion of the added nitrate to innocuous gases rather than to ammonium. Combining denitrifiers with the more reactive Fe(0) powder did not increase removal rates or decrease the proportion of nitrate reduced to ammonium. This was attributed to a corrosion-induced increase in pH above the tolerance range of the bacteria (pH > 10). Nitrate removal was sustained over 4 months in flow-through columns packed with steel wool and seeded with autotrophic denitrifiers. Increasing the hydraulic retention time from 0.67 to 2.33 days increased the nitrate removal efficiency and decreased the fraction of nitrate reduced to ammonium. The finding that Fe(0) can sustain autotrophic denitrification may have practical applications to treat nitrate-contaminated waters in ex-situ or in-situ reactive filters.

Kinetics of Halogenated Organic Compound Degradation by Iron Metal

Article

Jul 1996

A combination of new and previously reported data on the kinetics of dehalogenation by zero-valent iron (Fe0) has been subjected to an analysis of factors effecting contaminant degradation rates. First-order rate constants (kobs) from both batch and column studies vary widely and without meaningful correlation. However, normalization of these data to iron surface area concentration yields a specific rate constant (kSA) that varies by only 1 order of magnitude for individual halocarbons. Correlation analysis using kSA reveals that dechlorination is generally more rapid at saturated carbon centers than unsaturated carbons and that high degrees of halogenation favor rapid reduction. However, new data and additional analysis will be necessary to obtain reliable quantitative structure−activity relationships. Further generalization of our kinetic model has been obtained by accounting for the concentration and saturation of reactive surface sites, but kSA is still the most appropriate starting point for design calculations. Representative values of kSA have been provided for the common chlorinated solvents.

Reduction of Nitro Aromatic Compounds by Zero-Valent Iron Metal

Article

Dec 1995
ENVIRON SCI TECHNOL

The properties of iron metal that make it useful in remediation of chlorinated solvents may also lead to reduction of other groundwater contaminants such as nitro aromatic compounds (NACs). Nitrobenzene is reduced by iron under anaerobic conditions to aniline with nitrosobenzene as an intermediate product. Coupling products such as azobenzene and azoxybenzene were not detected. First-order reduction rates are similar for nitrobenzene and nitrosobenzene, but aniline appearance occurs more slowly (typical pseudo-first-order rate constants 3.5 × 10-2, 3.4 × 10-2, and 8.8 × 10-3 min-1, respectively, in the presence of 33 g/L acid-washed, 18−20 mesh Fluka iron turnings). The nitro reduction rate increased linearly with concentration of iron surface area, giving a specific reaction rate constant (3.9 ± 0.2 × 10-2 min-1 m-2 L). The minimal effects of solution pH or ring substitution on nitro reduction rates, and the linear correlation between nitrobenzene reduction rate constants and the square-root of mixing rate (rpm), suggest that the observed reaction rates were controlled by mass transfer of the NAC to the metal surface. The decrease in reduction rate for nitrobenzene with increased concentration of dissolved carbonate and with extended exposure of the metal to a particular carbonate buffer indicate that the precipitation of siderite on the metal inhibits nitro reduction.

Statistical analysis of mass-transfer parameters for sorption processes and models

Article

Nov 1990

The determination of mass-transfer parameters for sorption processes involving microporous solids such as activated carbon, ion-exchange resins, and soil particles and aggregates is an important element in the modeling and design of separation and purification processes and in the description of sorption phenomena in natural and environmental systems. Such determinations are typically carried out by fitting appropriate mathematical models to sets of rate data obtained in laboratory or field-scale experiments. An aspect of such determinations that has been largely neglected is the evaluation of statistical confidence. Determination of confidence for sorption parameters is made difficult by the use of nonlinear mathematical models and the existence of multiple, jointly determined parameters. This paper presents a method for determining preliminary estimates of confidence intervals for two jointly determined mass-transfer parameters describing sorption by microporous solids in fixed-bed reactors. An application of the method to examine the statistical significance of dissolved background organic substances on rates of adsorption of a specifically targeted compound, p-dichlorobenzene, from different complex aqueous solutions is presented.

Quinone and iron porphyrin mediated reduction of nitroaromatic compounds in homogeneous aqueous solution

Article

Oct 1990

The kinetics of reductive transformations of a series of monosubstituted nitrobenzenes and nitrophenols have been investigated in aqueous solution containing reduced sulfur species and small concentrations of either a naphthoquinone or an iron porphyrin. Boith the two naphthoquinones and the iron porphyrin used in this study mediated the reduction of the nitro group. In all three cases, the rate of reduction of the nitrobenzenes and of the nitrophenols was strongly pH dependent. Dissociated nitrophenols were reduced ~ 3-4 times more slowly as compared to the nondissociated species. For the substituted nitrobenzenes, the effect of substitution on the reaction rate could be described by a linear free energy relationship (LFER) of the general form log k = aE(h)(1') + b, where k is the second-order rate constant for reaction with the hydroquinone monophenolate or the iron porphyrin, respectively, and E(h)(1') is the one-electron reduction potential of the nitroaromatic compound. Competition between different nitrobenzenes was observed in the case of the iron porphyrin, while no effects were found for the reaction with the hydroquinones. The results of this study form an important base for the evaluation and interpretation of reductive transformation processes of nitroaromatic compounds in the environment.

Hydraulic and Geochemical Performance of a Permeable Reactive Barrier Containing Zero‐Valent Iron, Denver Federal Center

Article

May 1999
GROUND WATER

The hydraulic and geochemical performance of a 366 m long permeable reactive barrier (PRB) at the Denver Federal Center, Denver, Colorado, was evaluated. The funnel and gate system, which was installed in 1996 to intercept and remediate ground water contaminated with chlorinated aliphatic hydrocarbons (CAHs), contained four 12.2 m wide gates filled with zero-valent iron. Ground water mounding on the upgradient side of the PRB resulted in a tenfold increase in the hydraulic gradient and ground water velocity through the gates compared to areas of the aquifer unaffected by the PRB. Water balance calculations for April 1997 indicate that about 75 % of the ground water moving toward the PRB from upgradient areas moved through the gates. The rest of the water either accumulated on the upgradient side of the PRB or bypassed the PRB. Chemical data from monitoring wells screened down-gradient, beneath, and at the ends of the PRB indicate that contaminants had not bypassed the PRB, except in a few isolated areas. Greater than 99 % of the CAH mass entering the gates was retained by the iron. Fifty-one percent of the CAH carbon entering one gate was accounted for in dissolved C1 and C2 hydrocarbons, primarily ethane and ethene, which indicates that CAHs may adsorb to the iron prior to being dehalogenated. Treated water exiting the gates displaced contaminated ground water at a distance of at least 3 m downgradient from the PRB by the end of 1997. Measurements of dissolved inorganic ions in one gate indicate that calcite and siderite precipitation in the gate could reduce gate porosity by about 0.35 % per year. Results from this study indicate that funnel and gate systems containing zero-valent iron can effectively treat ground water contaminated with CAHs. However, the hydrologic impacts of the PRB on the flow system need to be fully understood to prevent contaminants from bypassing the PRB.

Cosolvent Influences on Organic Solute Retardation Factors

Article

Mar 1988
GROUND WATER

Prediction of chemical movement in ground water can be complex. Solute transport calculations need to reflect appropriate mathematical treatment of sorption and retardation by aquifer materials. The presence of cosolvents further complicates solute transport prediction. Results from a series of experiments both simple (small tubes) and complex (soil columns) suggest adequate predictability of solute sorption and attenuation in the presence of cosolvents. The results indicate that simple (small tube) short-term tests under a variety of sorbent concentrations can be used to predict soil column solute transport if appropriate physical properties are accounted for (soil type, organic load, bulk density). The final results indicate that low levels of cosolvents showed significant reproducible sorption/ attenuation reductions but probably are ultimately not important to overall solute fate.

Long-Term Performance of an in Situ “Iron Wall” for Remediation of VOCs

Article

Jan 1998
GROUND WATER

The use of granular iron for in situ degradation of dissolved chlorinated organic compounds is rapidly gaining acceptance as a cost-effective technology for ground water remediation. This paper describes the first field demonstration of the technology, and is of particular importance since it provides the longest available record of performance (five years). A mixture of 22% granular iron and 78% sand was installed as a permeable “wall” across the path of a contaminant plume at Canadian Forces Base, Borden, Ontario. The major contaminants were trichloroethene (TCE, 268 mg/L) and tetrachloroethene (PCE, 58 mg/L). Approximately 90% of the TCE and 86% of the PCE were removed by reductive dechlorination within the wall, with no measurable decrease in performance over the five year duration of the test. Though about 1% of the influent TCE and PCE appeared as dichloroethene isomers as a consequence of the dechlorination of TCE and PCE, these also degraded within the iron-sand mixture. Performance of the field installation was reasonably consistent with the results of laboratory column studies conducted to simulate the field behavior. However, if a more reactive iron material, or a higher percentage of iron had been used, complete removal of the chlorinated compounds might have been achieved. Changes in water chemistry indicated that calcium carbonate was precipitating within the reactive material; however, the trace amount of precipitate detected in core samples collected four years after installation of the wall suggest that the observed performance should persist for at least another five years. The study provides strong evidence that in situ use of granular iron could provide a long-term, low-maintenance cost solution for many ground water contamination problems.

Degradation of TCE with Iron: The Role of Competing Chromate and Nitrate Reduction

Article

May 2000
GROUND WATER

This study evaluates the potential of using granular iron metal for the abiotic removal of the organic ground water pollutant trichloroethene (TCE) in the presence of the common inorganic co-contaminants chromate and nitrate, respectively. Our long-term column experiments indicate a competitive process between TCE dechlorination and reductive transformation of chromate and nitrate, which is reflected in a significantly delayed onset of TCE dechlorination. Delay times and therefore the ranges of the nonreactive flowpaths increased with increasing experimental duration, resulting in a migration of the contaminants through the iron metal treatment zone. The present investigation also indicates that the calculated migration rates of TCE and the added cocontaminants chromate and nitrate are linearly related to the initial content of the cocontaminants. With an average pore water velocity of 0.6 m/d and a surface area concentration of 0.55 m2/mL in the column, the calculated migration rates varled between 0.10 cm/d and 5.86 cm/d. The particular similarity between the values of TCE migration and the migration of the strong oxidants chromate and nitrate and the long-term steady state of the TCE dechlorination in the absence of the chromate and nitrate indicates that these competitive transformations are the driving force for the gradual passivation of the granular iron due to the buildup of an electrically insulating Fe(III)-oxyhydroxide. Based on these passivation processes, general formulae were developed that allow a simplified approximation of breakthrough times for the contaminants TCE, chromate, and nitrate.

In Situ Identification of Carbonate-Containing Green Rust on Iron Electrodes in Solutions Simulating Groundwater

Article

Oct 2000

Open-circuit potential-time and spectral measurements were performed on ironelectrodes in aqueous solutions containing calcium carbonate to simulateground-water, to which an amount of carbon tetrachloride was added. In the case of apreoxidized iron electrode, the injection of the chlorinated aliphatic hydrocarbonresulted in the formation of carbonate-containing green rust. In situ identification,performed by Raman spectroscopy, was based on bands at ca. 433, 509, and1053 cm–1, which were assigned, respectively, to the Fe2+—OH stretching modeof green rust, the Fe3+—OH stretching mode of green rust, and the stretchingvibrations of carbonate ions in the interlayer regions of the green rust. Theassignment of the Fe2+—OH and Fe3+–OH stretching mode bands was confirmedby parallel experiments using D2O solution. The results of the open-circuitpotential-time experiments are in good agreement with literature thermodynamic datafor iron in carbonate-containing aqueous solutions.

Degradation of Carbon Tetrachloride by Iron Metal: Complexation Effects on the Oxide Surface

Article

Mar 1998
J CONTAM HYDROL

Dehalogenation of chlorinated aliphatic contaminants at the surface of zero-valent iron metal (Fe0) is mediated by the thin film of iron (hydr)oxides found on Fe0 under environmental conditions. To evaluate the role this oxide film plays in the reduction of chlorinated methanes, carbon tetrachloride (CCl4) degradation by Fe0 was studied under the influence of various anions, ligands, and initial CCl4 concentrations ([P]o). Over the range of conditions examined in these batch experiments, the reaction kinetics could be characterized by surface-area-normalized rate constants that were pseudo-first order for CCl4 disappearance (kCCl4), and zero order for the appearance of dissolved Fe2+ (kFe2+). The rate of dechlorination exhibits saturation kinetics with respect to [P]o, suggesting that CCl4 is transformed at a limited number of reactive surface sites. Because oxidation of Fe0 by CCl4 is the major corrosion reaction in these systems, kFe2+ also approaches a limiting value at high CCl4 concentrations. The adsorption of borate strongly inhibited reduction of CCl4, but a concomitant addition of chloride partially offset this effect by destabilizing the film. Redox active ligands (catechol and ascorbate), and those that are not redox active (EDTA and acetate), all decreased kCCl4 (and kFe2+). Thus, it appears that the relatively strong complexation of these ligands at the oxide–electrolyte interface blocks the sites where weak interactions with the metal oxide lead to dehalogenation of chlorinated aliphatic compounds.

The Effects of Electron Donor and Granular Iron on Nitrate Transformation Rates in Sediments from a Municipal Water Supply Aquifer

Article

Nov 2000
J CONTAM HYDROL

A municipal water supply well for the town of Baden, located about 10 km southwest of Waterloo, ON, Canada, was forced to close due to unacceptably high concentrations of nitrate in the groundwater. Stimulated in situ denitrification was considered a possible solution to the problem. In advance of a planned field test, the effectiveness of various electron donors (acetate, hydrogen gas, elemental sulphur, thiosulphate, aqueous ferrous iron and pyrite) at stimulating denitrification was compared in microcosm experiments involving sediment from the Baden aquifer. All electron donors tested, with the possible exception of pyrite, stimulated nitrate removal from solution. Acetate was found to be the substance that stimulated the quickest initial removal rates, and denitrification was confirmed as the mechanism using the acetylene block technique. Nitrite accumulation was minimal in most systems, although the local water quality guideline limit of 1.0 mg/l NO2−–N was briefly and temporarily exceeded (maximum value was 1.2 mg/l) in some of the acetate amended microcosms. Granular iron was also considered as an electron donor or abiotic reducing agent, but was found to reduce nitrate predominantly to ammonium, in a neutral pH solution buffered with pyrite. In mixed granular iron aquifer sediment systems, where several electron donors were present (hydrogen, ferrous iron and pyrite) that could have supported denitrification, the abiotic reaction with the granular iron appeared to dominate other transformation pathways, and ammonium was again the major product. Based on the testing completed as part of this project, the aquifer at Baden is considered a good candidate for acetate-stimulated in situ denitrification for the removal of nitrate from the groundwater near the municipal water supply well.

In situ sequential treatment of a mixed contaminant plume

Article

Oct 2000
J CONTAM HYDROL

Groundwater plumes often contain a mixture of contaminants that cannot easily be remediated in situ using a single technology. The purpose of this research was to evaluate an in situ treatment sequence for the control of a mixed organic plume (chlorinated ethenes and petroleum hydrocarbons) within a Funnel-and-Gate. A shallow plume located in the unconfined aquifer at Alameda Point, CA, was found to contain up to 218,000 μg/l of cis-1,2 dichloroethene (cDCE), 16,000 μg/l of vinyl chloride (VC) and <1000 μg/l of 1,1 dichloroethene (1,1 DCE), trans-1,2 dichloroethene (trans-1,2 DCE) and trichloroethene (TCE). Total benzene, toluene, ethylbenzene and xylenes (BTEX) concentrations were <10,000 μg/l. Contaminated groundwater was funneled into a gate, 3.0 m wide, 4.5 m long and 6.0 m deep (keyed into the underlying aquitard) where treatment occurred. The initial gate segment consisted of granular iron, for the reductive dechlorination of the higher chlorinated ethenes. The second segment, the biosparge zone, promoted aerobic biodegradation of petroleum hydrocarbons and any remaining lesser-chlorinated compounds, stimulated by dissolved oxygen (DO) and carbon dioxide (CO2) additions via an in situ sparge system (CO2 was used to neutralize the high pH produced from reactions in the iron wall). Groundwater was drawn through the gate by pumping two wells located at the sealed, downgradient, end. Over a 4-month period an estimated 1350 g of cDCE flowed into the treatment gate and the iron wall removed 1230 g, or 91% of the mass. The influent mass of VC was 572 g and the iron wall removed 535 g, corresponding to 94% mass removal. The other chlorinated ethenes had significantly lower influent masses (3 to 108 g) and the iron wall removed the majority of the mass resulting in >96% mass removal for any of the compounds. In spite of these high removal percentages, laboratory column tests indicated that at these levels of chlorinated contaminants, surface saturation of the iron grains likely contributed to lower than expected reaction rates. In the biosparge zone, mass removal of cDCE appeared to occur predominantly by biodegradation (65%) with volatilization (35%) being an important secondary process. The dominant removal process for VC was volatilization (70%) although significant biodegradation was also indicated (30%). Laboratory microcosm results confirmed the potential for aerobic biodegradation of cDCE and VC. When average influent field concentrations for cDCE and VC were 220,000 and 46,000 μg/l, respectively, the sequential treatment unit removed 99.6% of the total mass and when the influent concentrations decreased to 26,000 and 19,000 μg/l for cDCE and VC, respectively, >99.9% removal within the treatment gate was attained. BTEX compounds were found to be significantly retarded in the iron treatment zone. Although they did eventually break through the granular iron, and into the gravel transition zone, none of these compounds was detected in the biosparge zone. No noticeable interferences between the anaerobic (reductive) and aerobic parts of the system occurred during testing. The results of this experiment show that in situ treatment sequences are viable, although further work is needed to optimize performance.

Geochemical and microbial reactions affecting the long-term performance of in situ ‘iron barriers’

Article

Nov 2000
Adv Environ Res

The in situ application of granular iron (Fe0) has become popular for the destruction of halogenated organic compounds and for the immobilization of specific metals in groundwater. However, a knowledge gap exists concerning the long-term performance of the Fe0-barriers. The corrosion of Fe0 may produce mineral precipitates that alter the system’s hydraulic integrity. For example, data from existing barriers show varying trends in pH, alkalinity, mineral precipitation, and microbial activity. Although the chemical behaviors are site-specific, this paper discusses the concepts involved in developing a generic approach for predicting the trend of aqueous and surface speciation, and the resulting effects on Fe0 treatment systems. Observations from existing Fe barriers are summarized, and the chemical and microbial processes that influence chemical speciation, both in water and on surfaces, are reviewed. A conceptual geochemical model is presented, which illustrates the factors that must be considered in developing a quantitative model that can be used to design monitoring plans for timely detection of clogging in Fe0 reactive barriers. In order to develop quantitative predictive models, field and laboratory research should: (1) assess the extent and rates of media deterioration by analyzing coupled chemical and microbial reactions; and (2) identify the controlling mechanisms for hydraulic alteration within and around Fe0 barriers.

Cosolvent-Enhanced Remediation of Residual Dense Nonaqueous Phase Liquids: Experimental Investigation

Article

Aug 1995

The removal of denser than water nonaqueous phase liquids (DNAPLs) trapped at residual saturation is an important problem at many contaminated ground-water sites. Because pump-and-treat technologies have been ineffective in removing DNAPLs, alternative strategies have been suggested, one of which is enhancing the mobilization and dissolution of DNAPLs by flushing with a cosolvent. Tetrachloroethylene (PCE)/methanol/water systems were studied to evaluate the effect of methanol an the remediation of PCE-contaminated porous media. Experimental measurements of interfacial tension, equilibrium phase composition, and phase density at various methanol/water fractions were combined with other published properties to characterize these systems. In methanol flushing experiments, PCE mobilization, nonequilibrium PCE dissolution, and Row bypassing were allobserved. The results demonstrate that (a) small-scale heterogeneities may lead to locally high residual DNAPL saturations that are more easily mobilized than DNAPL residuals in homogeneous media; (b) mass transfer rate coefficients for PCE/methanol/water systems can be predicted to within 30% using an existing correlation developed for systems with similar NAPL emplacement procedures; and (c) flow bypassing, due to nonuniform distributions of DNAPL residual or dissolution fingering, can occur in even small-scale experiments.

Anaerobic Corrosion of Granular Iron: Measurement and Interpretation of Hydrogen Evolution Rates

Article

Dec 1995

Eric J. Reardon

Anaerobic corrosion of iron metal produces Fe2+, OH-, and H-2(g). Growing interest in the use of granular iron in groundwater remediation demands accurate corrosion rates to assess impacts on groundwater chemical composition. In this study, corrosion rates are measured by monitoring the hydrogen pressure increase in sealed cells containing iron granules and water. The principal interference is hydrogen entry and entrapment by the iron. The entry rate is described by Sievert's law (R = kP(H2)(0.5)), and the rate constant, k, is evaluated by reducing the cell pressure once during a test. For the 10-32 mesh iron used in this study, k initially was 0.015 but decreased to 0.009 mmol kg(-1) d(-1) kPa(-0.5) in 150 d. The corrosion rate in a saline groundwater was 0.7 +/- 0.05 mmol of Fe kg(-1) d(-1) at 25 degrees C-identical under water-saturated or fully-drained conditions. The rate decreased by 50% in 150 d due to alteration product buildup. The first 40-200 h of a corrosion test are characterized by progressively increasing rates of pressure increase. The time before steady-state rates develop depends on the solution composition. Data from this period should be discarded in calculating corrosion rates. Tests on pure sodium salt solutions at identical equivalent concentrations (0.02 equiv/L) show the following anion effect on corrosion rate: HCO3 > SO42- > Cl-. For NaCl solutions, corrosion rates decrease from 0.02 to 3.0 m.

Performance Evaluation of A Permeable Reactive Barrier for Remediation of Dissolved Chlorinated Solvents in Groundwater

Article

Sep 1999
J HAZARD MATER

A pilot-scale permeable reactive barrier (PRB) consisting of granular iron was installed in May 1995 at an industrial facility in New York to evaluate the use of this technology for remediation of chlorinated volatile organic compounds (VOCs) in groundwater. The performance of the barrier was monitored over a 2-year period. Groundwater velocity through the barrier was determined using water level measurements, tracer tests, and in situ velocity measurements. While uncertainty in the measured groundwater velocity hampered interpretation of results, the VOC concentration data from wells in the PRB indicated that VOC degradation rates were similar to those anticipated from laboratory results. Groundwater and core analyses indicated that formation of carbonate precipitates occurred in the upgradient section of the iron zone, however, these precipitates did not appear to adversely affect system performance. There was no indication of microbial fouling of the system over the monitoring period. Based on the observed performance of the pilot, a full-scale iron PRB was installed at the site in December 1997.

Mineral Precipitation and Porosity Losses in Granular Iron Columns

Article

Sep 1999
J HAZARD MATER

As permeable reactive barriers containing zero-valent iron are becoming more widely used to remediate contaminated groundwaters, there remains much uncertainty in predicting their long-term performance. This study focuses on two factors affecting performance and lifetime of the granular iron media: plugging at the treatment zone entrance and precipitation in the bulk iron media. Plugging at the system entrance is due principally to mineral precipitation promoted by dissolved oxygen in the influent groundwater and is an issue in aerobic aquifers or in above-ground canister tests. Designs to minimize plugging in field applications where the groundwater is oxygenated include the use of larger iron particles and admixing sand of comparable size with the iron particles. Beyond the entrance zone, the groundwater in anaerobic and mineral precipitation leads to porosity losses in the bulk iron media, potentially reducing flow through the treatment zone. The nature of the mineral precipitation and the factors that affect extent of mineral precipitation have been examined by a variety of tools, including tracer tests, aqueous inorganic profiles, and surface analytical techniques. At short treatment times, porosity losses as measured by tracer tests are due mainly to Fe(OH)(2) precipitates and possible entrapment of a film of hydrogen gas on the iron surfaces. Over longer treatment times, precipitation of Fe(OH)(2) and FeCO(3) in low carbonate waters and of Fe(OH)(2), FeCO(3) and CaCO(3) in higher carbonate waters begin to dominate porosity losses. The control of pH within the iron media by addition of ferrous sulfide was shown not to reduce significantly calcium and carbonate precipitates, indicating that mineral precipitation is controlled by more than simple carbonate equilibrium considerations.

Chemistry and Microbiology of Permeable Reactive Barriers for In Situ Groundwater Clean up

Article

Feb 2000

Permeable reactive barriers (PRBs) are receiving a great deal of attention as an innovative, cost-effective technology for in situ clean up of groundwater contamination. A wide variety of materials are being proposed for use in PRBs, including zero-valent metals (e.g., iron metal), humic materials, oxides, surfactant-modified zeolites (SMZs), and oxygen- and nitrate-releasing compounds. PRB materials remove dissolved groundwater contaminants by immobilization within the barrier or transformation to less harmful products. The primary removal processes include: (1) sorption and precipitation, (2) chemical reaction, and (3) biologically mediated reactions. This article presents an overview of the mechanisms and factors controlling these individual processes and discusses the implications for the feasibility and long-term effectiveness of PRB technologies.

Multicomponent Reactive Transport in an In Situ Zero-Valent Iron Cell

Article

May 2001

Data collected from a field study of in situ zero-valent iron treatment for TCE were analyzed in the context of coupled transport and reaction processes. The focus of this analysis was to understand the behavior of chemical components, including contaminants, in groundwater transported through the iron cell of a pilot-scale funnel and gate treatment system. A multicomponent reactive transport simulator was used to simultaneously model mobile and nonmobile components undergoing equilibrium and kinetic reactions including TCE degradation, parallel iron dissolution reactions, precipitation of secondary minerals, and complexation reactions. The resulting mechanistic model of coupled processes reproduced solution chemistry behavior observed in the iron cell with a minimum of calibration. These observations included the destruction of TCE and cis-1,2-DCE; increases in pH and hydrocarbons; and decreases in EH, alkalinity, dissolved O2 and CO2, and major ions (i.e., Ca, Mg, Cl, sulfate, nitrate). Mineral precipitation in the iron zone was critical to correctly predicting these behaviors. The dominant precipitation products were ferrous hydroxide, siderite, aragonite, brucite, and iron sulfide. In the first few centimeters of the reactive iron cell, these precipitation products are predicted to account for a 3% increase in mineral volume per year, which could have implications for the longevity of favorable barrier hydraulics and reactivity. The inclusion of transport was key to understanding the interplay between rates of transport and rates of reaction in the field.

Arsenate and Arsenite Removal by Zerovalent Iron: Kinetics, Redox Transformation, and Implications for In Situ Groundwater Remediation

Article

Apr 2001

Batch tests were performed utilizing four zerovalent iron (Fe0) filings (Fisher, Peerless, Master Builders, and Aldrich) to remove As(V) and As(III) from water. One gram of metal was reacted headspace-free at 23 degrees C for up to 5 days in the dark with 41.5 mL of 2 mg L(-1) As(V), or As(III) or As(V) + As(III) (1:1) in 0.01 M NaCl. Arsenic removal on a mass basis followed the order: Fisher > Peerless Master Builders > Aldrich; whereas, on a surface area basis the order became: Fisher > Aldrich > Peerless Master Builders. Arsenic concentration decreased exponentially with time, and was below 0.01 mg L(-1) in 4 days with the exception of Aldrich Fe0. More As(III) was sorbed than As(V) by Peerless Fe0 in the initial As concentration range between 2 and 100 mg L(-1). No As(III) was detected by X-ray photoelectron spectroscopy (XPS) on Peerless Fe0 at 5 days when As(V) was the initial arsenic species in the solution. As(III) was detected by XPS at 30 and 60 days present on Peerless Fe0, when As(V) was the initial arsenic species in the solution. Likewise, As(V) was found on Peerless Fe0 when As(II) was added to the solution. A steady distribution of As(V) (73-76%) and As(III) (22-25%) was achieved at 30 and 60 days on the Peerless Fe0 when either As(V) or As(III) was the initial added species. The presence of both reducing species (Fe0 and Fe2+) and an oxidizing species (MnO2) in Peerless Fe0 is probably responsible for the coexistence of both As(V) and As(III) on Fe0 surfaces. The desorption of As(V) and As(III) by phosphate extraction decreased as the residence time of interaction between the sorbents and arsenic increased from 1 to 60 days. The results suggest that both As(V) and As(III) formed stronger surface complexes or migrated further inside the interior of the sorbent with increasing time.

Influence of Solution Composition and Column Aging on the Reduction of Nitroaromatic Compounds by Zero-Valent Iron

Article

Sep 2001
CHEMOSPHERE

Granular iron is used in reactive permeable barriers for the reductive treatment of organic and inorganic groundwater contaminants. The technology is well established, however, its long-term performance and the importance of the groundwater composition are not yet well understood. Here, the influence of chloride, nitrate, silicate, and Aldrich humic acid on the reactivity of Master Builder iron was studied under anoxic conditions using small packed columns and 2-nitrotoluene (2-NT) as a model contaminant. After initially complete reduction of 2-NT to 2-aminotoluene (2-AT) in the column, possibly under mass-transfer controlled conditions, the reactivity of the iron was found to decrease substantially. In the presence of chloride, this decrease was slowed while exposure to silicate resulted in a very quick loss of iron reactivity. Nitrate was found to interfere strongly with the effect of chloride. These observations are interpreted in terms of corrosion inhibition/promotion and competition. Our results suggest that reactive barrier performance may be strongly affected by the composition of the treated groundwater.

Kinetics of Cr(VI) Reduction by Carbonate Green Rust

Article

Sep 2001

The kinetics of Cr(VI) reduction to Cr(III) by carbonate green rust were studied for a range of reactant concentrations and pH values. Carbonate green rust, [FeII4FeIII2(OH)12][4H2O x CO3], was synthesized by induced hydrolysis (i.e., coprecipitation) of an Fe(ll)/Fe(III) solution held at a constant pH of 8. An average specific surface area of 47 +/- 7 m2 g(-1) was measured for five separate batches of freeze-dried green rust precipitate. Heterogeneous reduction by Fe(II) associated with the carbonate green rust appears to be the dominant pathway controlling Cr(VI) loss from solution. The apparent stoichiometry of the reaction between ferrous iron associated with green rust ([Fe(II)GR]) and Cr(VI) was slightly higherthan the expected 3:1 ratio, possibly due to the presence of other oxidants, such as oxygen, protons, or interlayer carbonate ions. The rate of Cr(VI) reduction was proportional to the green rust surface area concentration, and psuedo-first-order rate coefficients (kobs) ranging from 1.2 x 10(-3) to 11.2 x 10(-3) s(-1) were determined. The effect of pH was small with a 5-fold decrease in rate with increasing pH (from 5.0 to 9.0). At low Cr(VI) concentrations (<200 microM), the rate of reaction was first order with respect to Cr(VI) concentration, whereas, at high Cr(VI) concentrations, rates appearto deviate from first-order kinetics and approach a constant value. Estimated amounts of surface Fe(II) and total Fe(II) suggest that the deviation from first-order kinetics observed at higher Cr(VI) concentrations and the 50-fold decrease in rate observed upon three sequential exposures to Cr(VI) is due to exhaustion of available Fe(II).

Arsenate and Arsenite Removal by Zerovalent Iron: Effects of Phosphate, Silicate, Carbonate, Borate, Sulfate, Chromate, Molybdate, and Nitrate, Relative to Chloride

Article

Dec 2001

Batch tests were performed to evaluate the effects of inorganic anion competition on the kinetics of arsenate (As(V)) and arsenite (As(III)) removal by zerovalent iron (Peerless Fe0) in aqueous solution. The oxyanions underwent either sorption-dominated reactions (phosphate, silicate, carbonate, borate, and sulfate) or reduction-dominated reactions (chromate, molybdate, and nitrate) with Peerless Fe0 in the presence of As(V) or As(III), relative to chloride. Pseudo-first-order rate equations were found to describe satisfactorily both As(V) and As(III) removal kinetics in the presence of each competing anion. Of the oxyanions tested for Peerless Fe0 in the pH range from 7 to 9, phosphate caused the greatest decrease in As removal rate (7.0 x 10(-3) to 18.5 x 10(-3) h(-1)) relative to chloride (34.9 x 10(-3) to 36.2 x 10(-3) h(-1)). Silicate, chromate, and molybdate also caused strong inhibition of As removal, followed by carbonate and nitrate, whereas borate and sulfate only caused slight inhibition to As(III) removal. Present results show that Peerless Fe0 may be an excellent permeable reactive barrier medium for a suite of mixed inorganic contaminants. The anion competing effects should be considered when designing permeable reactive barriers composed of zerovalent iron for field applications to remediate As(V) and As(III).

Kinetics of Nitrate, Nitrite, and Cr(VI) Reduction by Iron Metal

Article

Mar 2002

The kinetics of nitrate, nitrite, and Cr(VI) reduction by three types of iron metal (Fe0) were studied in batch reactors for a range of Fe0 surface area concentrations and solution pH values (5.5-9.0). At pH 7.0, there was only a modest difference (2-4x) in first-order rate coefficients (k(obs)) for each contaminant among the three Fe0 types investigated (Fisher, Peerless, and Connelly). The k(obs) values at pH 7.0 for both nitrite and Cr(VI) reduction were first-order with respect to Fe0 surface area concentration, and average surface area normalized rate coefficients (kSA) of 9.0 x 10(-3) and 2.2 x 10(-1) L m(-2) h(-1) were determined for nitrite and Cr(VI), respectively. Unlike nitrite and Cr(VI), Fe0 surface area concentration had little effect on rates of nitrate reduction (with the exception of Connelly Fe0, which reduced nitrate at slower rates at higher Fe0 surface areas). The rates of nitrate, nitrite, and Cr(VI) reduction by Fisher Fe0 decreased with increasing pH with apparent reaction orders of 0.49 +/- 0.04 for nitrate, 0.61 +/- 0.02 for nitrite, and 0.72 +/- 0.07 for Cr(VI). Buffer type had minimal effects on reduction rates, indicating that pH was primarily responsible for the differences in rate. At high pH values, Cr(VI) reduction ceased after a short time period, and negligible nitrite reduction was observed over 48 h.

An In Situ Study of the Role of Surface Films on the Granular Iron in the Permeable Iron Wall Technology

Article

Apr 2002
J CONTAM HYDROL

Permeable walls of granular iron are a new technology developed for the treatment of groundwater contaminated with dissolved chlorinated solvents. Degradation ofthe chlorinated solvents involves a charge transfer process in which they are reductively dechlorinated, and the iron is oxidized. The iron used in the walls is an impure commercial material that is covered with a passive layer of Fe2O3, formed as a result of a high-temperature oxidation process used in the production of iron. Understanding the behaviour of this layer upon contact with solution is important, because Fe2O3 inhibits mechanisms involved in contaminant reduction, including electron transfer and catalytic hydrogenation. Using a glass column specially designed to allow for in situ Raman spectroscopic and open circuit potential measurements, the passive layer of Fe2O3 was observed to be largely removed from the commercial product, Connelly iron, upon contact with Millipore water and with a solution of Millipore water containing 1.5 mg/l trichloroethylene (TCE). It has been previously shown that Fe2O3 is removed from iron surfaces upon contact with solution by an autoreduction reaction; however, prior to this work, the reaction has not been shown to occur on the impure commercial iron products used in permeable granular iron walls. The rate of removal was sufficiently rapid such that the initial presence of Fe2O3 at the iron surface would have no consequence with respect to the performance of an in situ wall. Subsequent to the removal of Fe2O3 layer, magnetite and green rust formed at the iron surface as a result of corrosion in both the Millipore water and the solution containing TCE. The formation of these two species, rather than higher valency iron oxides and oxyhydroxides, is significant for the technology. The former can interfere with contaminant degradation because they inhibit electron transfer and catalytic hydrogenation. Magnetite and green rust, in contrast, will not inhibit the mechanisms involved in contaminant reduction, and hence their formation is beneficial to the long-term performance of the iron material.

Effects of Carbonate Species on the Kinetics of Dechlorination of 1,1,1-Trichloroethane by Zero-Valent Iron

Article

Nov 2002

The effect of precipitates on the reactivity of iron metal (Fe0) with 1,1,1-trichloroethane (TCA) was studied in batch systems designed to model groundwaters that contain dissolved carbonate species (i.e., C(IV)). At representative concentrations for high-C(IV) groundwaters (approximately 10(-2) M), the pH in batch reactors containing Fe0 was effectively buffered until most of the aqueous C(IV) precipitated. The precipitate was mainly FeCO3 (siderite) but may also have included some carbonate green rust. Exposure of the Fe0 to dissolved C(IV) accelerated reduction of TCA, and the products formed under these conditions consisted mainly of ethane and ethene, with minor amounts of several butenes. The kinetics of TCA reduction were first-order when C(IV)-enhanced corrosion predominated but showed mixed-order kinetics (zero- and first-order) in experiments performed with passivated Fe0 (i.e., before the onset of pitting corrosion and after repassivation by precipitation of FeCO3). All these data were described by fitting a Michaelis-Menten-type kinetic model and approximating the first-order rate constant as the ratio of the maximum reaction rate (Vm) and the concentration of TCA at half of the maximum rate (K(1/2)). The decrease in Vm/K(1/2) with increasing C(IV) exposure time was fit to a heuristic model assuming proportionality between changes in TCA reduction rate and changes in surface coverage with FeCO3.

Longevity of Granular Iron in Groundwater Treatment Processes: Solution Composition Effects on Reduction of Organohalides and Nitroaromatic Compounds

Article

Apr 2003

Although granular iron permeable reactive barriers (PRBs) are increasingly employed to contain subsurface contaminants, information pertaining to system longevity is sparse. The present investigation redresses this situation by examining the long-term effects of carbonate, silica, chloride, and natural organic matter (NOM) on reactivity of Master Builders iron toward organohalides and nitroaromatic contaminants. Six columns were operated for 1100 days (approximately 4500 pore volumes) and five others for 407 days (approximately 1800 pore volumes). Nine were continuously exposed to mixtures of contaminant species, while the other two were only intermittently exposed in order to differentiate deactivation induced by water (and inorganic cosolutes) from that resulting from contaminant reduction. Contaminants investigated were trichloroethylene, 1,2,3-trichloropropane, 1,1-dichloroethane, 2-nitrotoluene, 4-nitroacetophenone, and 4-nitroanisole. Column reactivity declined substantially over the first 300 days and was dependent on the feed solution chemistry. High carbonate concentrations enhanced reactivity slightly within the first 90 days but produced poorer performance over the long term. Both silica and NOM adversely affected reactivity, while chloride evinced a somewhat mixed effect. Observed contrasts in relative reactivities suggest that trichloroethylene, 1,2,3-trichloropropane, and nitroaromatic compounds all react at different types of reactive sites. Our results indicate that differences in groundwater chemistry should be considered in the PRB design process.

Longevity of granular iron in groundwater treatment processes: Changes in solute transport properties over time

Article

Jul 2003
J CONTAM HYDROL

Although progress has been made toward understanding the surface chemistry of granular iron and the mechanisms through which it attenuates groundwater contaminants, potential long-term changes in the solute transport properties of granular iron media have until now received relatively little attention. As part of column investigations of alterations in the reactivity of granular iron, studies using tritiated water (3H(2)O) as a conservative and non-partitioning tracer were periodically conducted to independently isolate transport-related effects on performance from those more directly related to surface reactivity. Hydraulic residence time distributions (HRTDs) within each of six 39-cm columns exposed to bicarbonate solutions were obtained over the course of 1100 days of operation. First moment analyses of the data revealed generally modest increases in mean pore water velocity (v) over time, indicative of decreasing water-filled porosity. Gravimetric measurements provided independent estimates of water-filled porosity that were initially consistent with those obtained from 3H(2)O tracer tests, although at later times, porosities derived from gravimetric measurements deviated from the tracer test results owing to mineral precipitation. The combination of gravimetric measurements and 3H(2)O tracer studies furnished estimates of precipitated mineral mass; depending on the assumed identity of the predominant mineral phase(s), the porosity decrease associated with solute precipitation amounted to 6-24% of the initial porosity. The accumulation of mineral and gas phases led to the formation of regions of immobile water and increased spreading of the tracer pulse. Application of a dual-region transport model to the 3H(2)O breakthrough curves revealed that the immobile water-filled region increased from initially negligible values to amounts ranging between 3% and 14% of the total porosity in later periods of operation. For the aged columns, mobile-immobile mass transfer coefficients (k(mt)) were generally in the range of 0.1-1.0 day(-1) and reflected a slow exchange of 3H(2)O between the two regions. Additional model calculations incorporating sorption and reaction suggest that although changes in HRTD can have an appreciable effect on trichloroethylene (TCE) transformation, the effect is likely to be minor relative to that stemming from passivation of the granular iron surface.

Long-Term Performance of Permeable Reactive Barriers Using Zero-Valent Iron: Geochemical and Microbiological Effects

Article

Jul 2003

Geochemical and microbiological factors that control long-term performance of subsurface permeable reactive barriers were evaluated at the Elizabeth City, North Carolina, and the Denver Federal Center, Colorado, sites. These ground water treatment systems use zero-valent iron filings (Peerless Metal Powders Inc.) to intercept and remediate chlorinated hydrocarbon compounds at the Denver Federal Center (funnel-and-gate system) and overlapping plumes of hexavalent chromium and chlorinated hydrocarbons at Elizabeth City (continuous wall system). Zero-valent iron at both sites is a long-term sink for carbon, sulfur, calcium, silicon, nitrogen, and magnesium. After about four years of operation, the average rates of inorganic carbon (IC) and sulfur (S) accumulation are 0.09 and 0.02 kg/m2/year, respectively, at Elizabeth City where upgradient waters contain <400 mg/L of total dissolved solids (TDS). At the Denver Federal Center site, upgradient ground water contains 1000 to 1200 mg/L TDS and rates of IC and S accumulation are as high as 2.16 and 0.80 kg/m2/year, respectively. At both sites, consistent patterns of spatially variable mineral precipitation and microbial activity are observed. Mineral precipitates and microbial biomass accumulate the fastest near the upgradient aquifer-Fe0 interface. Maximum net reductions in porosity due to the accumulation of sulfur and inorganic carbon precipitates range from 0.032 at Elizabeth City to 0.062 at the Denver Federal Center (gate 2) after about four years. Although pore space has been lost due the accumulation of authigenic components, neither site shows evidence of pervasive pore clogging after four years of operation.

Major Anion Effects on the Kinetics and Reactivity of Granular Iron in Glass-Encased Magnet Batch Reactor Experiments

Abstract

No full-text available

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