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Degradation of Carbon Tetrachloride by Iron Metal: Complexation Effects on the Oxide Surface
Abstract
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.
... Cl À Enhancing the performance of ZVI Formation complexes with iron centers to breakdown the protective oxide film; Causing pitting corrosion. (Devlin and Allin, 2005;Hernandez et al., 2004;Johnson et al., 1998) Deteriorating the performance of ZVI Competitive sorption with respect to perchlorate or nitrate reduction. (Hwang et al., 2015;Moore et al., 2003) SO 4 ...
... Firstly, the addition of chloride anions can enforce the breakdown of the protective oxide film coated on ZVI surface. Hard Lewis base ions (such as Cl À , Br À ) were reported to be especially aggressive toward the passivating oxide layers to form strong complexes with iron centers (Johnson et al., 1998). As the oxide layers were broken down by these diffusing anions, more bare metal was exposed and available for water, oxygen or target contaminants. ...
... Sulfate is another common anion associated with an enhancement of the performance of ZVI (Devlin and Allin, 2005;Johnson et al., 1998), although the exact mechanism by which it enhances the performance of ZVI has not been conclusively established. For example, it has been reported that the degradation of CT (Lipczynskakochany et al., 1994), 4ClNB (Bi et al., 2009;Devlin and Allin, 2005), NB (Yin et al., 2012) and the sequestration of As(V) (Biterna et al., 2007) could be accelerated in the presence of SO 4 2À . ...
For successful application of a zero-valent iron (ZVI) system, of particular interest is the performance of ZVI under various conditions. The current review comprehensively summarizes the potential effects of the major influencing factors, such as iron intrinsic characteristics (e.g., surface area, iron impurities and oxide films), operating conditions (e.g., pH, dissolved oxygen, iron dosage, iron pretreatment, mixing conditions and temperature) and solution chemistry (e.g., anions, cations and natural organic matter) on the performance of ZVI reported in literature. It was demonstrated that all of the factors could exert significant effects on the ZVI performance toward contaminants removal, negatively or positively. Depending on the removal mechanisms of the respective contaminants and other environmental conditions, an individual variable may exhibit different effects. On the other hand, many of these influences have not been well understood or otherwise cannot be individually isolated in experimental or natural systems. Thus, more research is required in order to elucidate the exact roles and mechanisms of each factor in affecting the performance of ZVI. Furthermore, based on these understandings, future research may attempt to establish some feasible strategies upon which the deterioration effects can be addressed or the positive effects can be utilized.
... As discussed above, corrosion may lead to the precipitation of solids, which may reduce permeability and/or passivate the reactive material by preventing further access of the aqueous phase to reactive sites. Some research suggests that chloride and sulfate do destabilize passivating films and promote corrosion (Devlin and Allin 2005;Johnson et al. 1998). However, sulfate has also been shown to inhibit arsenic removal by ZVI, as have phosphate, molybdate, chromate, and silicate (Melitas et al. 2002;Su and Puls 2001a;Lackovic et al. 2000). ...
... In the upper right section of the graph, the at-risk PRBs are found, while the not-at-risk PRBs lie below and to the left. This PRB grouping makes sense: higher E H values may lead to more oxidation and thus more potential passivation of iron Johnson et al. 1998;Stumm and Morgan 1996). Chloride has been shown to increase the corrosion of iron (Devlin and Allin 2005;Johnson et al. 1998), which would be expected to improve PRB performance; however, Klausen et al. (2001) showed that the reactivitydiminishing effects of nitrate may outweigh the corrosion-promoting effects of chloride. ...
... This PRB grouping makes sense: higher E H values may lead to more oxidation and thus more potential passivation of iron Johnson et al. 1998;Stumm and Morgan 1996). Chloride has been shown to increase the corrosion of iron (Devlin and Allin 2005;Johnson et al. 1998), which would be expected to improve PRB performance; however, Klausen et al. (2001) showed that the reactivitydiminishing effects of nitrate may outweigh the corrosion-promoting effects of chloride. Although the separation of at-risk and not-at-risk PRBs is not as definitive as in Figure 3.4(a), Figure 3.4(b) still suggests that higher alkalinity and higher NO 3 correlate with being at-risk. ...
Permeable reactive barriers (PRBs) are an in situ technology for remediation of contaminated groundwater. Most employ ZVI as the reactive medium, and although many achieve remediation goals, the performance of others is compromised by the precipitation of naturally-occurring solutes. Therefore, this research aimed to understand the precipitation of solids in these systems. Recent work has suggested the suitability of reduced iron sulfide (FeS) as a reactive material for PRB applications, so this research compared the solids production and hydraulic performance of pure ZVI and FeS-coated sands.
To better understand the factors associated with solids production and the potential for failure, a statistical analysis of data from field PRBs composed of ZVI was conducted. Based on this statistical analysis, a series of column experiments was conducted utilizing a simulated groundwater with high calcium (280 mg/L), total carbonate (420 mg/L), and chloride (405 mg/L) as a base solution, to which oxidants (0, 2, or 8 mg/L dissolved oxygen or 100 mg/L nitrate) were added.
Characterization of the aqueous phase in the ZVI column effluents indicated that both both calcium and carbonate were removed from solution. Production of gas bubbles was also observed. In the ZVI columns, increasing oxidant levels corresponded to higher hydraulic conductivity losses. Yet the spectroscopic analysis of solids produced and mass balances on the aqueous phase could not account for all of the hydraulic conductivity loss. Geochemical modeling of the systems was also used to estimate the potential for solids formation and gas production. Results of this modeling also suggested that hydraulic conductivity losses due to the gas phase may be a crucial component of permeability loss.
In the FeS columns, no calcium or carbonate was removed in the columns, no hydraulic conductivity loss was measured, and no solids were detected on the surface of the solids. Modeling of the FeS and ZVI systems on an equal mass basis indicated the potential for solids formation with FeS is much less than that with ZVI. Based on these hydraulic considerations, FeS may have significant advantages over ZVI for PRB applications.
... The reduction of some alkyl halides with hydrated or complexed ferrous iron is thermodynamically possible but quite slow (Klečka and Gonsior 1984;Doong and Wu 1992). However, surface-bounded Fe 2+ and Fe precipitates are stronger reductants (Johnson et al. 1998;Amonette et al. 2000;Jeong et al. 2013;Bae and Hanna 2015) and can degrade carbon tetrachloride and hexachloroethane (Elsner et al. 2004;Shao and Butler 2007). Green rusts (layered mixed Fe(II)/Fe(III) hydroxide minerals) can also reduce chlorinated ethanes and ethylenes, with a more rapid reduction for highly chlorinated compounds like HCA and PCA for chlorinated ethanes (O'Loughlin and Burris 2004) or PCE and TCE for chlorinated ethylenes (Lee and Batchelor 2002a). ...
... However, this phenomenon is impeded by the diffusion of Fe 2+ resulting from iron corrosion in the opposite direction of the pollutant. As it was reported that shell-bounded Fe 2+ on different iron oxides is a strong reductant with specific redox properties (Johnson et al. 1998;Amonette et al. 2000;Elsner et al. 2004;Silvester et al. 2005;Shao and Butler 2007;Bae and Hanna 2015;Gorski et al. 2016), reduction may occur before the pollutant comes into contact with the surface. The spontaneous electron transfer between Fe 2+ and Fe(III) oxides on the shell, which can be influenced by the presence of surface defect (Gorski and Scherer 2009;Notini et al. 2018), results in an acceleration in the interfacial electron transfer between iron species and the pollutant (Huang and Zhang 2005;Han et al. 2016b). ...
Chlorinated organic compounds (COCs) are common anthropogenic
contaminants encountered in soil and groundwater. COCs were
industrially produced for different applications, such as dry cleaning,
degreasing, or as pesticides. The presence of COCs in the environment is a major concern because of their toxicity and persistence. The most widely used method for their removal is the conventional pump-and-treat approach. However, this technology can hardly achieve a complete remediation because of geological characteristics and the presence of pore space pollution/adsorbed pollution, leading to a residual saturation.
Hence, in addition to the improvement of pump-and-treat systems, In situ chemical processes have been largely developed. These chemical processes involve the injection of chemical reagents for the removal of residual pollution source and/or the treatment of contamination plume. Chemical degradation of COCs can be achieved by oxidative or reductive processes. If chemical oxidation has been first developed for in situ application, chemical reduction is one of the most important emerging remediation techniques for COCs treatment. Due to the electronegative character of chlorine substituents, COCs can effectively be transformed via reductive pathways. Moreover, reductive dechlorination has shown higher efficiency on highly chlorinated compounds.
This chapter focuses on the presentation of the chemical reduction of the most common COCs pollutants, followed by kinetic and mechanistic approaches related to the use of iron-based particles. Developments of in situ chemical reduction technologies aiming to enhance remediation rates are also exposed. Influence of environmental conditions for in situ applications is then developed. Finally, a case study is presented.
... Iron in the sediments was from 49410 (Fig 3c) to 126060 mg kg -1 (Fig 3b) and had positive correlation with organic C, alkaline phosphatase, inorganic and total P (Table1). The higher levels of iron are justifiable, when considered with the highest abundance of iron in planet earth and the lithosphere (Johnson et al., 1997) [24] . Calcium was 3378 (Fig 3c) to 10222.5 mg kg -1 (Fig 3b). ...
... Iron in the sediments was from 49410 (Fig 3c) to 126060 mg kg -1 (Fig 3b) and had positive correlation with organic C, alkaline phosphatase, inorganic and total P (Table1). The higher levels of iron are justifiable, when considered with the highest abundance of iron in planet earth and the lithosphere (Johnson et al., 1997) [24] . Calcium was 3378 (Fig 3c) to 10222.5 mg kg -1 (Fig 3b). ...
... It is well known that there are a wide variety of natural organic matters (NOMs) and ionic components coexisting in the natural waters, which could possibly compete with Se(IV) and Se(VI) for the available adsorption sites [20][21][22]. Moreover, many studies have shown that the presence of NOMs and anions (SO 4 2− , Cl − , HCO 3 − , etc.) could influence the reactivity and surface characteristics of NZVI particles in aqueous systems [23][24][25][26]. However, to date, seldom studies have illustrated the influence of the typical groundwater compositions on the selenium removal by NZVI. ...
... Additionally, previous studies have suggested a trade-off may exist in anion electrolyte (e.g. SO 4 2− , HCO 3 − and Cl − ), which was between decreasing the NZVI reactivity via blocking its reactive sites and increasing its reactivity by accelerating the dissolution of NZVI oxide layer [23,24]. However, the increase in NZVI reactivity by any anions was not observed in this study. ...
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.
... The zero-valent iron-based in-situ reaction zone (ZVI-IRZ), formed by direct injection of finer zero-valent iron (micro/nano-scale) particles into groundwater, is a cost-effective and efficient solution to many challenging and complex groundwater remediation (Ahn et al., 2021;Johnson et al., 1998;Su et al., 2012;Xin et al., 2018). In particular, microscale zero-valent iron (mZVI) has attracted considerable attention in groundwater remediation due to the higher reactivity, longer longevity, lower health risk and lower cost (Velimirovic et al., 2013a;Guan et al., 2015;Sun et al., 2016). ...
S/mZVI is a promising material for groundwater remediation due to its excellent properties. However, the reactivity and electron selectivity toward target contaminant are critical. Thus, this study investigated the effect of complex groundwater chemistries (Milli-Q water, fresh groundwater and saline groundwater) on the reactivity of S/mZVI toward trichloroethylene (TCE), dechlorination pathway, hydrogen evolution kinetic, electron efficiency and aging behaviors. Results showed that sulfidation appreciably increased the reactivity and electron selectivity. The major degradation product of TCE dechlorination by S/mZVI was acetylene, which was consistent with TCE dechlorination by β-elimination. Moreover, reductive β-elimination was still the dominant dechlorination pathway for the application of S/mZVI in three groundwater conditions. However, the rates and the quantities of major products from TCE degradation varied significantly. S/mZVI in saline groundwater can maintain the reactivity toward TCE due to the protection of Fe0 by Fe3O4 deposited on the surface. Thus, the higher TCE removal efficiency and less hydrogen accumulation resulted in the greatest electron efficiency (4.3-79.2%). Overall, S/mZVI was more effective for the application in saline groundwater. This study proved insight into the comprehensive evaluation and implications for the application of S/mZVI based technologies in saline contaminated groundwater.
... The zero-valent iron-based in-situ reaction zone (ZVI-IRZ), formed by direct injection of finer zero-valent iron (micro/nano-scale) particles into groundwater, is a cost-effective and efficient solution to many challenging and complex groundwater remediation (Ahn et al., 2021;Johnson et al., 1998;Su et al., 2012;Xin et al., 2018). In particular, microscale zero-valent iron (mZVI) has attracted considerable attention in groundwater remediation due to the higher reactivity, longer longevity, lower health risk and lower cost (Velimirovic et al., 2013a;Guan et al., 2015;Sun et al., 2016). ...
... The coexisting substances, such as chloride, sulfate, nitrate and organic matter, were investigated, and the results are shown in Fig. 2e. The presence of chloride and sulfate slightly promote the removal of EDTA-chelated Cu, resulting from the enhanced corrosion of the ZVI surface [16]. The corrosion promoters (Cland SO 4 2-) that resulted in more ferrous ion formation would indirectly contribute to replacing EDTA-chelated Cu(II) and activating PDS to generate sulfate radicals. ...
Metal complexes pose a significant challenge to remediate heavy metal-laden wastewater by classical techniques due to their stable chelating structure. The application of zero-valent iron (ZVI) to activate peroxidized sulphate (PDS) has been synergistically (ZVI/PDS) proposed for effective removal of EDTA-chelated Cu from aqueous media. In contrast to ZVI alone, ZVI/PDS process improved the EDTA-chelated Cu or total organic carbon (TOC) removal rate by 6.2 or 7.8 fold, respectively. Factors including Cu/EDTA molar ratio, initial pH, ZVI dosage, and PDS dosage affect the removal efficiency of complexed copper. The formation of Fe ions and sulfate radicals in the solution strengthens the removal effect of zero-valent iron on EDTA complexed Cu. After the reaction, ZVI EDS, XRD, and XPS analyses revealed that Cu was eventually reduced to Cu 0 and immobilized on the ZVI surface. A possible decomposition mechanism followed by a Cu reduction was also proposed. In conclusion, this study presents new perspectives for the removal of complex heavy metals from contaminated water.
... 520 The coordinating surface model is supported by many studies that have used surface complexation modeling to interpret the effects of competing ligands on contaminant reduction rates. 521,522 However, some aspects of this interpretation might require revision to accommodate recent advances in understanding the nature of Fe(II) surface sites on iron minerals, which is described above in section 4.2.4. ...
Iron (Fe) is the fourth most abundant element in the earth's crust and plays important roles in both biological and chemical processes. The redox reactivity of various Fe(II) forms has gained increasing attention over recent decades in the areas of (bio) geochemistry, environmental chemistry and engineering, and material sciences. The goal of this paper is to review these recent advances and the current state of knowledge of Fe(II) redox chemistry in the environment. Specifically, this comprehensive review focuses on the redox reactivity of four types of Fe(II) species including aqueous Fe(II), Fe(II) complexed with ligands, minerals bearing structural Fe(II), and sorbed Fe(II) on mineral oxide surfaces. The formation pathways, factors governing the reactivity, insights into potential mechanisms, reactivity comparison, and characterization techniques are discussed with reference to the most recent breakthroughs in this field where possible. We also cover the roles of these Fe(II) species in environmental applications of zerovalent iron, microbial processes, biogeochemical cycling of carbon and nutrients, and their abiotic oxidation related processes in natural and engineered systems.
... In complex scientific coverage of molecular mechanisms in CCl 4 -induced toxic (Johnson et al. 1998), were 183 identified as reactants for reductive CCl 4 degradation. The half-life of CCl 4 in the 184 aquifer is reported to range from a few days to hundreds of days (Howard 1991). ...
Basic chemistry and water treatment of broad range of oxidants and related radical species are covered in this chapter. A general introduction to oxidants and radicals is followed by detailed sections on chlorine species, advanced oxidation processes, persulfates, and non-consensual radical mechanisms. Further, detailed information on oxidant applicability and activation, oxidant-specific recalcitrant pollutants and commonly formed by-products is provided. To assess the suitability of the specific oxidants for real water conditions, matrix components interferences are discussed. Considerable attention is paid to chemistry of innovative oxidants (persulfates) and to the controversial aspects of superoxide radical anion reactivity with carbon tetrachloride.
... However, the removal rate of 4-NP as a function of initial concentration (Fig. S1) shows they are positively correlated, which suggests removal of 4-NP has features of firstorder kinetics, i.e. it is a transition from zero-to first-order kinetics rather than being a simple zero-order reaction with tailing. Such a behavior is a common observation in heterogeneous reactions and described by a mixed order kinetic model in previous studies (Johnson et al., 1998(Johnson et al., , 1996Wüst et al., 1999). The model was derived based on the hypothesis that saturation of reactive surface sites limits the rate of reduction, in which the sorption, desorption, and reduction of sorbed 4-NP were assumed to be first-order reactions, and their rate constants are k S , k DS , and k, respectively. ...
In this study, Fe2+ addition was employed to overcome the negative effects of humic acid (HA) on contaminant removal by zerovalent iron (ZVI), and its feasibility to improve electron efficiency of ZVI was also tested. HA at high concentrations suppressed the removal of 4-nitrophenol (4-NP) by ZVI, while the addition of 0.25-1.0 mM Fe2+ could greatly mitigate this inhibitory effect and enhance 4-NP reduction. Specifically, with a mixed-order model, global fitting results showed that the addition of Fe2+ increased the rate constant from 0.124 × 10-2-0.219 × 10-2 mM/min to 0.227 × 10-2-0.417 × 10-2 mM/min and shortened lag period from 19.7-47.9 min to 8.0-15.2 min for 4-NP removal. The mechanistic investigation revealed this trend could be explained by the following aspects: i) Fe2+ can facilitate the generation of Fe(II)-containing oxides, which can act as an electron mediator or direct electron donor for 4-NP reduction; ii) the presence of Fe2+ could lead to aggregation of HA particles and accordingly reduced its coverage on ZVI surface. But the results of respike experiments indicate that Fe2+ addition did not show remarkable effect on the electron efficiency of 4-NP by ZVI, which should be associated with that Fe2+ was not able to favor the enrichment of 4-NP on ZVI surface.
... This detrimental influence denied the conjecture that the excessive NaBH 4 would exist in the background solution (pH~8.5) to continuously reduce the Fe 2+ /Ni 2+ which was produced by Fe 0 /Ni 0 in the reaction of CF degradation. Instead, the transformation of NaBH 4 , resulting in the concentration accumulation of borate in the background solution, should be responsible for the inhibitory effect of excessive NaBH 4 on the reactivity of CMC-Fe/Ni (Johnson et al. 1998). ...
The use of stabilizers can prevent the reactivity loss of nanoparticles due to aggregation. In this study, carboxymethylcellulose (CMC) was selected as the stabilizer to synthesize a highly stable CMC-stabilized Fe/Ni colloid (CMC-Fe/Ni) via pre-aggregation stabilization. The reactivity of CMC-Fe/Ni was evaluated via the reaction of chloroform (CF) degradation. The effect of background solution which composition was affected by the preparation of Fe/Ni (Fe/Ni precursors, NaBH4 dosage) and the addition of solute (common ions, sulfur compounds) on the reactivity of CMC-Fe/Ni was also investigated. Additionally, the dried CMC-Fe/Ni was used for characterization in terms of scanning electron microscopy (SEM) and X-ray photoelectron spectroscopy (XPS). The experimental results indicated that CMC stabilization greatly improved the reactivity of Fe/Ni bimetal and CF (10 mg/L) could be completely degraded by CMC-Fe/Ni (0.1 g/L) within 45 min. The use of different Fe/Ni precursors resulting in the variations of background solution seemed to have no obvious influence on the reactivity of CMC-Fe/Ni, whereas the dosage of NaBH4 in background solution showed a negative correlation with the reactivity of CMC-Fe/Ni. Besides, the individual addition of external solutes into background solution all had an adverse effect on the reactivity of CMC-Fe/Ni, of which the poisoning effect of sulfides (Na2S, Na2S2O4) was significant than common ions and sulfite.
... (1) Effects of humic acids on mass transfer Humic acids were demonstrated to inhibit the dechlorination of chlorinated organic compounds including 2,4-dichlorophenol [167], carbon tetrachloride [168], PCE [158,169,170] and p-nitrchlorobenzene [171]. It is deduced that the suppression is caused by the competition between humic acids and target pollutants for the limited adsorption sites on ZVI surface. ...
Soil and sediment contamination by polychlorinated biphenyls (PCBs) is a global health and environmental concern, since PCBs are toxic and recalcitrant. The aim of this thesis is to find a remediation process to PCBs contaminated sediments by using zerovalent iron (ZVI). To begin the studies on remediation, a survey was conducted on sites contaminated by e-waste recycling activities in south China. PCBs contamination was not as severe as previously, whereas the co-existence of PCBs and heavy metals increases the difficulty of remediation. Afterwards, the feasibility PCBs degradation by ZVI in aqueous solutions was investigated. Results confirmed the stepwise dechlorination of PCBs by ZVI and the major pathway with congener specifity and regiospecifity. The relative importance of the influential factors to ZVI degradation of PCBs was in the order of surfactants > humic acid > pH > Ni2+. Sediment decontamination was studied by 1) washing with an aqueous solution of surfactant followed by ZVI dechlorination of PCBs and 2) direct mixing with ZVI and surfactant solution. The second approach gave promising results for remediation
... That is, since TCE particles dissociate, chloride ions are produced. Chloride ions act as an electron donor, releasing electrons to metallic species and generating new reactive sites over the metal surfaces (USEPA, 2005;Johnson et al., 1998;Padmanabhan et al., 2008;Ramamurthy and Eglal, 2014). Zhang et al. (2012) and Kaifas et al. (2014) have indicated a chain dechlorination reaction of TCE into 1,1-dichloroethene (1,1eDCE), cise1,2edichloroethene (ciseDCE), and then vinyl chloride (VC) analyzed using GC/MS. ...
Remediation of dense non-aqueous phase liquid (DNAPL), which consists primarily of chlorinated solvents , is considered a top priority in the field of groundwater decontamination. Downward migration of DNAPL can lead to formation of impermeable strata due to low solubility and high density. Remediation is therefore one of the most complex technical challenges faced by environmental engineers. In the present work, remediation of trichloroethylene (TCE), perchloroethene (PCE), and 1,2-dichloroethene (1,2-DCE) DNAPL-contaminated groundwater was studied by a reductive reaction with poly-ethylenimine (PEI) surface-modified zero-valent iron nanoparticles (PEIenZVI). Compared with fresh nZVI, PEIenZVI exhibited smaller spherical particles of 20e80 nm and a greater surface area of 53.4 m 2 /g. Furthermore, slow desorption of the PEI indicated its potential application as a protective shell layer for efficient delivery of active nZVI to the water/DNAPL interface. Laboratory batch remediation results indicate that both nZVI and PEIenZVI can remove 99% of TCE, PCE, and 1,2-DCE. The rate of reaction for fresh nZVI was higher in the early stage. Comparatively, PEIenZVI had a higher removal rate and efficiency after 2 h. The kinetic studies also revealed that the removal rate for 1,2-DCE was greater than that for TCE and PCE. Additionally, X-ray absorption near edge structure (XANES) and extended X-ray absorption fine structure (EXAFS) spectroscopy studies indicated that the nZVI and PEIenZVI have two central Fe atoms coordinated by primarily FeeO and FeeFe with bond distances of 1.87 Å and 3.05 Å, respectively. Furthermore, after the reductive reaction, nZVI and PEIenZVI were oxidized to Fe 3 O 4 , and bond distance values for the reacted samples were 1.94 Å and 1.96 Å, respectively.
... This suggests that during the experiments aerobic conditions were replaced by anaerobic conditions. TSB does not contain nitrate, sulfate, and other inorganics that can be reduced by nZVI but has chloride and phosphate which have been reported to potentially increase and decrease the reactivity of iron filings through dissolution of the iron oxide layer and formation of a passivating oxide layer, respectively (Agrawal et al., 2002;Devlin and Allin, 2005;Johnson et al., 1998;Su and Puls, 2004). TCE degradation by encapsulated PpF1 predominantly occurred in the first 12 h and was about 60% (Fig. 1c). ...
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.
... 4,23−26 Inner-sphere ET in the form of complexes at the metal surface was proposed based on reactivity trends 7 and computational calculations, 27,28 whereas (OS)-SET transfer has been invoked based on the consideration that metals likely transfer electrons one at a time and that metals in aqueous solution are covered by a passive oxide layer. 4,26,29 OS-SET would also be consistent with the observation that most CE dehalogenation rates correlate with redox potentials derived from LUMOs and one-electron reduction potentials (linear free energy relationships, LFERs). 25,30,31 However, such LFERs may arise owing to different mechanisms 32 so that they cannot uniquely answer the questions of underlying reaction chemistry: which bonds are broken in the chlorinated ethenes, in what order are they broken, and by attack of what reaction partner? ...
Chlorinated ethenes (CEs) such as perchloroethylene, trichloroethylene and dichloroethylene are notorious groundwater contaminants. Although reductive dehalogenation is key to their environmental and engineered degradation, underlying reaction mechanisms remain elusive. Outer-sphere reductive single electron transfer (OS-SET) has been proposed for such different processes as Vitamin B12-dependent biodegradation and zero-valent metal-mediated dehalogenation. Compound-specific isotope effect (13C/12C, 37Cl/35Cl) analysis offers a new opportunity to test these hypotheses. Defined OS-SET model reactants (CO2 radical anions, S2--doped graphene oxide in water) caused strong carbon (epsilonC = -7.9‰ to -11.9‰), but negligible chlorine isotope effects (epsilonCl = -0.12‰ to 0.04‰) in CEs. Greater chlorine isotope effects were observed in CHCl3 (epsilonC = -7.7‰, epsilonCl =-2.6‰), and in CEs when the exergonicity of C-Cl bond cleavage was reduced in an organic solvent (reaction with arene radical anions in glyme). Together, this points to dissociative OS-SET (SET to a σ* orbital concerted with C-Cl breakage) in alkanes compared to stepwise OS-SET (SET to a π* orbital followed by C-Cl cleavage) in ethenes. The non-existent chlorine isotope effects of chlorinated ethenes in all aqueous OS-SET experiments contrast strongly with pronounced Cl isotope fractionation in all natural and engineered reductive dehalogenations reported to date suggesting that OS-SET is an exception rather than the rule in environmental transformations of chlorinated ethenes.
... Third, OC covering on the surface and complexing with Fh can inhibit the accessibility of Fe to microbial reduction (Amstaetter et al., 2012;Shimizu et al., 2013). Especially, inner-sphere coordination between OC and Fe oxide prevents Fh from microbial reduction (Brunschwig et al., 1982;Johnson et al., 1998). Potential blockage of Fh surface reaction sites by OC can also inhibit the microbial reduction (Shimizu et al., 2013;Chen et al., 2015). ...
The dynamics of iron (Fe)-bound organic carbon (OC) during dissimilatory microbial Fe(III) reduction has the potential to play an important role in regulating the biogeochemical cycling of carbon (C) in permanently or transiently anoxic soils and sediments. In this study, we investigated the release and transformation of ferrihydrite (Fh)-bound OC during microbial reduction of Fe by Shewanella putrefaciens strain CN32 under a fixed Fe concentration of 13 mM and varying C/Fe molar ratios. We found that reduction of Fe and reductive release of OC was dependent on the C/Fe molar ratio, with high C/Fe ratio enhancing both reduction of Fe and release of OC. For Fh-OC co-precipitates with C/Fe ratio of 3.7, 54.7% of Fh-bound OC was released to solution phase when 25.1% of Fe was reduced. The presence of OC inhibited the transformation of Fh to more crystalline Fe phases both in the bulk and on the surface. Upon reduction, Fh-bound OC became more concentrated on the surface of Fh-OC co-precipitates, and surface components were enriched with carboxylic functional groups. Reduction increased the lability of Fh-bound OC for Fh-OC co-precipitate with C/Fe ratio of 3.7, and aromatic OC was preferentially retained within the co-precipitates. Our results indicate that microbial reduction altered the quantity and composition of OC released from Fh-OC co-precipitates, depending on the C/Fe ratio and associations between Fe and OC. Assuming higher availability of released OC compared to original Fh-bound OC, reduction of Fh can likely lead to enhanced degradation of OC and result in a shorter residence time for OC in soils and sediments.
... In addition to nitrate, phosphate was also observed to affect As removal by hindering the coprecipitation of As with iron compounds (Kowalski and Søgaard, 2014). Sulfate is another common anion associated with the inhibition of bio-ZVI performance (Wu et al., 2013b;Yin et al., 2012a), which is quite different from the enhancing effects of sulfate on the individual ZVI system (Devlin and Allin, 2005;Johnson et al., 1998). In a bioreactor containing ZVI and H 2 -consuming bacteria, CNB and NB removals were obviously suppressed by SO 4 2¡ (Wu et al., 2013b;Yin et al., 2012a). ...
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.
... On the other hand corrosion products are adsorption sites for Fe II ions from continuously corroding iron. The role of surface bound Fe II for contaminant reduction has been studied (BEHRENDS & VAN CAPPELLEN 2005;HANSEN et al. 1994;JOHNSON et al. 1998;WHITE & PATERSON 1996), and it was found that Fe II adsorbed on mineral surfaces plays an important role in the process of contaminant reduction. ...
... This process favors the hydration of metal ions and increases the ease with which the ions enter solution (Liu et al., 2013). Hernandez et al. (2004) also reported that the addition of chloride accelerated trinitrotoluene degradation during treatment of contaminated water with Fe 0 , and Johnson et al. (1998) observed an almost linear increase in the rate constant for carbon tetrachloride dechlorination by increasing the chloride concentration to 60 mM. ...
Application of microscale zero-valent iron (mZVI) is a promising technology for in-situ contaminated groundwater remediation; however, its longevity is negatively impacted by surface passivation, especially in saline groundwater. In this study, the aging behavior of mZVI particles was investigated in three media (milli-Q water, fresh groundwater and saline groundwater) using batch experiments to evaluate their potential corrosion and passivation performance under different field conditions. The results indicated that mZVI was reactive for 0–7 days of exposure to water and then gradually lost H2-generating capacity over the next hundred days in all of the tested media. In comparison, mZVI in saline groundwater exhibited the fastest corrosion rate during the early phase (0–7 d), followed by the sharpest kinetic constant decline in the latter phases. The SEM-EDS and XPS analyses demonstrated that in the saline groundwater, a thin and compact oxide film was immediately formed on the surface and significantly shielded the iron reactive site. Nevertheless, in fresh groundwater and milli-Q water, a passive layer composed of loosely and unevenly distributed precipitates slowly formed, with abundant reactive sites available to support continuous iron corrosion. These findings provide insight into the molecular-scale mechanism that governs mZVI passivation and provide implications for long-term mZVI application in saline contaminated groundwater.
... The microbiological medium used in this study to cultivate ORB was found to inhibit nZVI reactivity toward TCA when employed without vitamins and l-cysteine. The effects of ionic co-solutes on the reductive capacity of zero-valent iron have been widely studied [30][31][32][33][34][35], and while some metallic cations, chloride and sulphate can enhance electron transfer from Fe 0 [36,37], other ions form inner-sphere complexes with iron oxide and block reactive sites of electron transfer [38,39]. Here, the high concentration of phosphate (1.47 mM) through its strong affinity for iron likely counteracted any positive effects of chloride, sulphate and metallic cations to prevent reduction of TCA by nZVI. ...
... Zero-valent barriers rely not on the oxidation of metallic Fe(0), but on the oxidation of Fe(0) to Fe(II). Ferrous iron is the reactive compound that is oxidized to ferric iron, either from adsorbed Fe(II) or from Fe(II) minerals, such as green rust (Genin et al. 1998), to reductively remediate chlorinated aliphatic contaminants (Balko and Tratnyek 1998;Johnson et al. 1998) or reduction of metals, such as chromate (Blowes et al. 1997;Buerge and Hug 1997). Aqueous Fe(II) can reduce chromate (Eary and Rai 1988), while Fe(II), either as a structural mineral component or adsorbed to an Fe(III)-oxide, clay surface, or zero valent iron surface, is necessary for the dechlorination reactions (Hofstetter et al. 2003). ...
Energetics such as RDX, HMX, and CL-20 exhibit low sorption and natural degradation, resulting in widespread groundwater contamination. Alternatively, TNT exhibits strong sorption and degrades to toxic recalcitrant intermediates. Field scale abiotic, biotic, and coupled abiotic/bioremediation can be more cost effective than pump and treat or sediment removal, but rates of processes in relevant insitu conditions need to be understood.
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.
The solid-solid reaction of microscale zero-valent iron (mZVI) with elemental sulfur (S0) in water can form sulfidated mZVI (S-mZVI) with high reactivity and selectivity. However, the inherent passivation layer of mZVI hinders the sulfidation. In this study, we demonstrate that ionic solutions of Me-chloride (Me: Mg2+, Ca2+, K+, Na+ and Fe2+) can accelerate the sulfidation of mZVI by S0. The S0 with S/Fe molar ratio of 0.1 was fully reacted with mZVI in all solutions to form unevenly distributed FeS species on S-mZVIs as confirmed by SEM-EDX and XANES characterization. The cations depassivated the mZVI surface by driving the proton release from the surface site (FeOH) and resulting in localized acidification. The probe reaction test (tetrachloride dechlorination) and open circuit potential (EOCP) measurement demonstrated that Mg2+ was most efficient in depassivating the mZVI and therefore promoting sulfidation. The decrease of surface proton for hydrogenolysis on the S-mZVI synthesized in MgCl2 solution also inhibited the formation of cis-1,2-dichloroethylene by 14-79% compared to other S-mZVIs during trichloroethylene dechlorination. In addition, the synthesized S-mZVIs exhibited the highest reduction capacity reported so far. These findings provide a theoretical basis for the facile on-site sulfidation of mZVI by S0 with cation-rich natural waters for sustainable remediation of contaminated sites.
The environmental and health hazards created by industrial chemicals and consumer products must be minimized. For safer products to be designed, the relationships between structure and toxicity must be understood at the molecular level. Green chemistry combined with free radical research has the potential to offer innovative solutions to such problems.
Some solutions are "greener then others", and many necessitate significant financial investment. New technology will only be adopted if real benefit can be shown and sometimes adaptation of existing methods is the best option. The efficiency of processes must be assessed, not only in terms of the final yield, but also cost, environmental impact and waste toxicity.
This practical and concise guide showcases the sustainable methods offered by green free radical chemistry and summarizes the fundamental science involved. It discusses the pros and cons of free radical chemistry in aqueous systems for synthetic applications. All transformation steps are covered including initiation, propagation, and termination. Useful background knowledge is combined with examples, including industrial scale processes for pharmaceuticals and fine chemicals.
The book helps chemists to choose appropriate methods for achieving maximum output using a modern, environmentally conscious approach. It shows that, armed with an elementary knowledge of kinetics, an understanding of the mechanistic and technical aspects, and some common sense, it is possible to harness free radicals for use in a broad range of applications.
Streamlining Green Free Radical Chemistry is aimed at chemists, engineers, materials scientists, biochemists and biomedical experts, as well as undergraduate and postgraduate students. It encourages readers to question conventional methods and move towards the "Benign-by-Design" approach of the future. References to further reading are provided at the end of each chapter.
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.
Textile dyes are a significant contributor to aquatic pollution in developing nations, and the current pace of human-caused contamination of worldwide freshwater bodies is frightening. Herein, we report the sonocatalytic decolorization of the methylene blue (MB) dye using a novel synthesis of composite materials of zirconium oxide (ZrO2), calcium oxide (CaO), and magnesium oxide (MgO) nanoparticles (NPs) decorated by phosphorus-doped carbon dots (PCDs). The monoclinic phase of ZrO2 has shown the highest catalytic activity, thus imparting greater decolorization efficiency (DE) in the synthesized metal oxide NPs decorated with PCDs ([email protected]). The sonocatalytic efficiency of [email protected] was evaluated using MB as the analyte molecule. We report DE of up to 99.54% via speckled PCDs decoration and synergistic effect in a short time of 30 min at just 20 kHz frequency. The sonocatalytic activity of [email protected] is AND logic gate supported and was found in order [email protected] (99.54%) > [email protected] (96.47%) > [email protected] (94.68%) > [email protected] (93.40%). However, the sonocatalytic activity of MONPs was only ZCMO (11.21%) > ZMO (9.26%) > ZO (3.79%) > ZCO (2.92%), under identical experimental conditions. The synergistic effect in polymetallic systems opens new pathways to be explored in sonocatalysis. We confirmed the synthesis of PCDs and their irregular decoration on the surface of MONPs using scanning electron microscopy (SEM), and transmission electron microscopy (TEM). The speckled nanocomposites, i.e., [email protected], showed an enhanced and faster decolorization efficiency than the bare MONPs, demonstrating the kinetics studies as a 10¹ fold increase in k values and a 10¹ fold decrease in t1/2 values.
Novelty statement
Novel nanocomposites have been developed for MB dye decolorization using a low frequency of just 20 kHz and possessing 99.54–93.40% DE in only 30 min, thus may aid in industrial dye decolorization and the reuse of wastewater for environmental sustainability.
The objective of this study was to examine the occurrence and characteristics of ciprofloxacin‐resistant (CipR) Escherichia coli isolates on bulk tank milk (BTM) samples (bovine and ovine origins) in Turkey. A total of 91 BTM samples (41.7%, 95% confidence intervals 35.2–48.6%) out of 218 were found to be positive for CipR E. coli isolates (MIC values of ≥4 μg/ml). Analysis of PFGE fingerprint profile for E. coli isolates resulted in the 55 different pulsotypes based on >85% homology. All isolates were resistant to enrofloxacin and nalidixic acid and the resistance rates in bovine and ovine origin isolates were 94.9 and 78.1% for norfloxacin (p < .05) and 27.1 and 34.4% for levofloxacin, respectively. Additionally, resistance to non‐quinolone antibiotics was commonly observed against tetracycline (resistance rates in bovine and ovine = 91.5 and 87.5%, respectively), trimethoprim‐sulfamethoxazole (83.1 and 93.8%, respectively), gentamycin (15.3 and 40.6%, respectively, p < .05) and chloramphenicol (23.7 and 65.6%, respectively, p < .05). The qnrS1 gene (3.1 and 6.8%, respectively) was the most prevalent PMQR genes in isolates from ovine and bovine origins, followed by aac (6′)‐Ib‐cr (0 and 5.1%, respectively) and qnrB19 (0 and 1.7%, respectively). The other resistance genes including tetA, tetB, strA/B, aPozhA1, aadA, aadB, blaCTX‐M, and blaTEM were also identified in various frequencies. The most frequently observed virulence trait was fimH. The low‐level presence of PMQR genes and as well as some virulence traits is an important finding, yet the results of this study are worrisome because quinolone antibiotics are still the drugs of choice for severe infections in humans.
As an aliphatic amino acid, cysteine (CYS) is diffuse in the living cells of plants and animals. However, little is known of its role in the reactivity of nano-sized zero-valent iron (NZVI) in the degradation of pollutants. This study shows that the introduction of CYS to the NZVI system can help improve the efficiency of reduction, with 30% more efficient degradation and a reaction rate constant nine times higher when nitrobenzene (NB) is used as probe compound. The rates of degradation of NB were positively correlated with the range of concentrations of CYS from 0 to 10 mmol/L. The introduction of CYS increased the maximum concentration of Fe(III) by 12 times and that of Fe(II) by four times in this system. A comparison of systems featuring only CYS or Fe(II) showed that the direct reduction of NB was not the main factor influencing its CYS-stimulated removal. The reduction in the concentration of CYS was accompanied by the generation of cystine (CY, the oxidized form of cysteine), and both eventually became stable. The introduction of CY also enhanced NB degradation due to NZVI, accompanied by the regeneration of CYS. This supports the claim that CYS can accelerate electron transfer from NZVI to NB, thus enhancing the efficiency of degradation of NB.
Extracellular polymeric substance (EPS) is secreted by many organisms and makes up a significant constituent of natural organic matter in the environment. However, nothing is known about EPS's role in reduction of pollutants by nano-sized zero valent iron (NZVI). This research showed that the degradation kinetics of nitrobenzene (NB) by NZVI with EPS (0.0272±0.006 min⁻¹) were 2.27 times lower than that without EPS (0.0618±0.006 min⁻¹) in the first cycle, mainly due to competition for reactive sites on the NZVI surface and the complexation of EPS with Fe(II) and Fe(III). In the second and third cycle, the degradation kinetics of NB by NZVI alone decreased obviously, while those in the presence of EPS were preserved or accelerated. Comparative studies with a quinine model compound indicated that EPS did not function as the electron shuttle to transmit electrons effectively. X-ray photoelectron spectroscopy, scanning electron microscopy and X-ray diffraction results suggested that EPS could prevent the oxidation of NZVI and even expose more effective sites on the NZVI surface, thus leading to the preservation or enhancement of NZVI reactivity in the second and third NB degradation cycles. Moreover, we found that EPS also provided colloidal stability to NZVI particles, either by steric mechanisms or electrostatic repulsion. These results indicate that EPS can play an important role in the prolongation of NZVI reactivity during standing application.
A key reaction underlying the charge transport in iron containing oxides, clays, micas is the Fe²⁺-Fe³⁺ exchange reaction between edge-sharing iron octahedra. These reactions facilitate conduction in these minerals by the thermally-activated hopping of small polarons across the lattice. Depending on the mineral and local charge state the small polaron can either encase an electron or hole. The probability for conduction of small polarons depends strongly on the height and adiabicity of the reaction barrier, with larger and more diabatic barriers yielding slow conduction associated with either weak coupling or a large prerequisite rearrangement of the lattice during charge transport. To model these reactions, a first principle electron transfer (ET) method was developed to model the small polaron hopping between the edge-sharing octahedra sites in hematite (e⁻ polaron), goethite (e⁻ polaron), and annite (h⁺ polaron) bulk structures. The ET method is based on electronic structure methods (i.e., plane-wave Density Functional Theory) capable of performing calculations with periodic cells and large size systems efficiently while at the same time being accurate enough to be used in the estimation of the electron-transfer coupling matrix element, VAB, and the electron transfer transmission factor, κel. The calculations confirmed the existence of small polarons in all three minerals, and the reactions were predicted to be strongly adiabatic. It was found that transfer of a hole in the octahedral layer of annite had an adiabatic barrier of 0.311 eV, and the transfer of an extra electron in hematite and goethite had adiabatic barriers of 0.242 eV and 0.232 eV respectively. The electronic coupling parameters, VAB, were found to be 0.188 eV, 0.196 eV, and 0.102 eV respectively for hematite, goethite, and annite. While similar bonding topologies pertain, the findings reveal the importance of subtle differences in local structure.
The purpose of this study was to develop a slow-release technology for in situ groundwater remediation of both hydrophilic (methyl orange dye) and hydrophobic (trichloroethylene) water contaminants. Electrospraying technique was used to fabricate microparticles comprised of biodegradable PLA (polylactic acid) and zero-valent iron nanoparticles (ZVINPs) at a ratio of 10:1. Around 8 wt% ZVINPs was slowly released from the composite microparticles after 60 hrs and 32 hrs of incubation in water, to fully remediate methyl orange (25 mg/L) and trichloroethylene (0.2 vol%) from water, respectively. The results from the sand column study show that released ZVINPs from composite particles could effectively remediate water contaminated with methyl orange solution (25 mg/L) completely over 1.5 months. The usefulness of the slow release remediator was also verified by monitoring the pH and conductivity of the effluents collected from the sand column. The storage stability of encapsulated ZVINPs was vastly improved (>1 month in open air) as compared to bare ZVINPs (1 hr in the open air), and recyclability of the particles was also evaluated (reused upto 4 cycles). These PLA based microparticles fabricated in a single step, can potentially act as slow release reservoir to remediate groundwater contaminants irrespective of their hydrophilicity.
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.
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.
Experiments were conducted to evaluate the potential of zero‐valent iron and sulfate‐reducing bacteria (SRB) for reduction and removal of chromium from synthetic electroplating waste. The zero‐valent iron shows promising results as a reductant of hexavalent chromium (Cr+6) to trivalent chromium (Cr+3), capable of 100% reduction. The required iron concentration was a function of chromium concentration in the waste stream. Removal of Cr+3 by adsorption or precipitation on iron leads to complete removal of chromium from the waste and was a slower process than the reduction of Cr+6. Presence SRB in a completely mixed batch reactor inhibited the reduction of Cr+6. In a fixed‐bed column reactor, SRB enhanced chromium removal and showed promising results for the treatment of wastes with low chromium concentrations. It is proposed that, for waste with high chromium concentration, zero‐valent iron is an efficient reductant and can be used for reduction of Cr+6. For low chromium concentrations, a SRB augmented zero‐valent iron and sand column is capable of removing chromium completely.
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.
An approach for modeling electron transfer in solids and at surfaces of iron - (oxyhydr)oxides and other redox active solids has been developed for electronic structure methods (i.e., plane-wave Density Functional Theory) capable of performing calculations with periodic cells and large system sizes efficiently while at the same time being accurate enough to be used in the estimation of the electron-transfer coupling matrix element, $V_{AB}$, and the electron transfer transmission factor, $\kappa_{el}$. This method is an extension of the valence bond theory electron transfer method for molecules and clusters, implemented by Dupuis and others and used extensively by Rosso and co-workers, in which scaled corresponding orbitals derived from the Bloch states are used to calculate the off-diagonal matrix elements $H_{AB}$ and $S_{AB}$. A key development of the present work is the formulation of algorithms to improve the accuracy of the integration of the exact exchange integral in periodic boundary conditions. This method is demonstrated on model systems for electron small polaron transfer in iron-(oxyhydr)oxides, including bare Fe$^{2+}$--Fe$^{3+}$ ions, and in $[\textrm{Fe}^{3+}(\textrm{OH}_2)_2 (\textrm{OH}^{-})_2)]_n^{n+}$ chains representing the common edge-sharing Fe octahedral motif in these materials.
Firstly, PCP was chosen as a model pollutant, to investigate the oxidation of PCP on the surface of magnetite used as heterogeneous catalyst. Oxidation experiments were carried out under various experimental conditions at neutral pH and correlated with the adsorption behavior. The surface reactivity of magnetite was evaluated by conducting the kinetic study of both H2O2 decomposition and PCP oxidation experiments. The occurrence of the optimum values of H2O2 and magnetite concentrations for the effective degradation of PCP could be explained by the scavenging reactions with H2O2 or iron oxide surface. All batch experiments indicate that Fenton-like oxidation of PCP was controlled by surface mechanism reaction and the species compete with each other for adsorption on a fixed number of surface active sites. The apparent degradation rate was dominated by the rate of intrinsic chemical reactions on the oxide surface rather than the rate of mass transfer. Raman analysis suggested that the sorbed PCP was removed from magnetite surface at the first stage of oxidation reaction. All X-ray powder diffraction (XRD), X-ray photoelectron spectroscopy (XPS), Mössbauer spectroscopy and chemical analyses showed that the magnetite catalyst exhibited low iron leaching, good structural stability and no loss of performance in second reaction cycle. Secondly, Rhodamine B (RhB) was chosen as a model compound pollutant. Two types of iron (II, III) oxides were used as heterogeneous catalysts and characterized by XRD, Mössbauer spectroscopy, BET surface area, particle size and chemical analyses. The catalytic efficiency of iron (II, III) oxide to promote Fenton-like reaction was examined at neutral pH. The adsorption to the catalyst changed significantly with the pH value and the sorption isotherm was fitted using the Langmuir model for both solids. Both sorption and FTIR results indicated that surface complexation reaction may take place in the system. The variation of oxidation efficiency against H2O2 dosage and amount of exposed surface area per unit volume was evaluated and correlated with the adsorption behavior in the absence of oxidant. There is also an optimum amount of H2O2 value for the degradation of RhB. The phenomena could also be explained by the scavenging effect of hydroxyl radical by H2O2 or by iron oxide surface (like the oxidation of PCP). Sorption and decolourization rate of RhB as well as H2O2 decomposition rate were found to be depended on the surface characteristics of iron oxide. The kinetic oxidation experiments showed that structural FeII content strongly affect the reactivity towards H2O2 decomposition and therefore RhB decolourization. Finally, the effect of chelating agent on the heterogeneous Fenton reaction rate of pentachlorophenol in the presence of magnetite was investigated. Six kinds of chelating agents including oxalate, EDTA, CMCD, tartarate, citrate and succinate were chosen. The PCP oxidation rate in this system was significantly improved by using chelating agents at neutral pH. The kinetic rate constant was increased by 5.7, 4, 3.2, 2.4, 2.5 and 1.7 times with oxalate, EDTA, CMCD, tartarate, citrate and succinate, respectively. The enhancement factor of heterogeneous oxidation rate was found to be not correlated with that of dissolved iron dissolution amount. In homogeneous Fenton system (dissolved Fe2+ or Fe3+), EDTA-driven reaction showed the highest oxidation rate, while oxalate seems to be more efficiency in heterogeneous Fenton system (Fe3O4). This observation could be explained by the inactivation of iron surface sites which become unavailable for the interactions with H2O2 to initiate Fenton-like reactions. These results demonstrated that the chelating agent-promoted dissolution of magnetite did not play the key role in determining the efficiency of heterogeneous Fenton oxidation.
Hybrid barriers of zero-valent iron (Fe⁰) filings and hexadecyltrimethylammonium (HDTMA)-bentonite were simulated in columns to remove chromate and trichloroethylene (TCE) via reduction and sorption. The normalized reduction rate constant (and SA) of chromate with the mixture of Fe⁰ filings and HDTMA-bentonite was about twice higher than that with Fe⁰ filings only, suggesting that chromate reduction was enhanced when HDTMA-bentonite was mixed with Fe⁰ filings. For the column of two separate layers of Fe⁰ and HDTMAbentonite, the reduction rate of chromate slightly increased, but not as much as in the column with mixtures of Fe⁰ filings and HDTMA-bentonite. In addition, the sorption rates of both TCE and chromate in the column with mixtures of Fe⁰ filings and HDTMA-bentonite were lower than in the columns of both HDTMA-bentonite only and two separate layers of Fe⁰ and HDTMA-bentonite. Based on the observation from this study, hybrid barriers can be more effective for mixed contaminants. © 2007 by the American Society of Civil Engineers. All Rights Reserved.
The nanoporous materials prepared from iron-iron oxide core-shell nanoparticles are of great interest due to their enhanced possibilities for distribution in the environment, a high rate of chemical reactivity and also the possibility to enhance environmentally friendly reaction paths. However, production of these nanoparticle porous materials by conventional methods is difficult. Therefore, we use a cluster deposition system, which prepares the iron nanoclusters and iron-iron oxide core shell nanoclusters at room temperature. The nanoporous films are synthesized by using the nanoclusters as building blocks. These films are characterized using Transmission Electron Microscopy (TEM), Atomic Force Microscopy (AFM), and the Brunauer-Emmett-Teller (BET) method for surface area determination.
BACKGROUND
Ciprofloxacin (CIP) is an antibiotic largely used to treat bacterial infections and found in sewage treatment plant (STP) effluent. Zero valent iron (ZVI, Fe⁰) technology has great potential for the degradation of residual pharmaceuticals. The effect of some parameters (anaerobic/aerobic, particle size, iron ligands and anions) were evaluated for CIP degradation in distilled water (DW) and finally compared to that obtained in STP effluent.
RESULTS
The smaller ZVI particle (200 mesh) resulted in a lower degradation rate than the larger particle (20 mesh) in both anaerobic and aerobic treatment. This is due mainly to the fast generation of Fe²⁺, hindering the degradation process due to •OH scavenging. A linear increase of CIP degradation rate was observed when the reaction was carried out with increasing EDTA concentrations. The Cl⁻ anions had a positive effect on CIP degradation in the ZVI process. On the other hand, the presence of NO3⁻ resulted in a decrease of degradation rate, both with 20 and 200 mesh particles. CIP could be degraded in two STP effluents mediated by ZVI (20 mesh).
CONCLUSIONS
The ZVI process can be used efficiently for the degradation of CIP in two types of STP effluent (anaerobic treatment or anaerobic/aerobic treatment), revealing a possible applicability of the ZVI process to this type of matrix. © 2017 Society of Chemical Industry
This report is focused on the dechlorination of lindane, a recalcitrant and refractory pollutant, by zero valent iron microparticles (ZVIM) in batch and continuous mode. Experimental variables such as initial lindane concentration, ZVIM dosage and temperature were studied. Batch experiments indicate that the lindane dechlorination is enhanced with the increase of ZVIM dosage and reaction temperature, and is maintained with increasing initial pollutant concentration. Kinetic analyses elucidated that lindane degradation followed a first order reaction for both pollutant and ZVIM concentration. The kinetic model can also accurately predict the results in continuous mode (more realistic conditions), where the high stability of ZVIM has been thoroughly demonstrated. Further studies indicated that co-existence of common ions can i) not affect (SO42-, Na+, Ca2+, Mg+) or ii) promote (HCO3-, Cl-) the lindane dechlorination process. The results implied that ZVIM is a potential approach for in situ remediation of soil and groundwater lindane contamination.
Applications of zerovalent iron (ZVI) for water treatment under aerobic conditions include sequestration of metals (e.g., in acid mine drainage) and decolorization of dyes (in wastewaters from textile manufacturing). The processes responsible for contaminant removal can be a complex mixture of reduction, oxidation, sorption, and co-precipitation processes, which are further complicated by the dynamics of oxygen intrusion, mixing, and oxide precipitation. To better understand such systems, the removal of an azo dye (Orange I) by micron-sized granular ZVI at neutral pH was studied in open (aerobic) stirred batch reactors, by measuring the kinetics of Orange I decolorization and changes in “geochemical” properties (DO, Fe(II), and Eh), with and without two treatments that might improve the long-term performance of this system: sulfidation by pretreatment with sulfide, and magnetization by application of a weak magnetic field (WMF). The results show that the changes in solution chemistry are coupled to the dynamics of oxygen intrusion, which was modeled as analogous to dissolved oxygen sag curves. Both sulfidation and magnetization increased Orange I removal rates 2.4-71.8 fold, but there was little synergistic benefit to applying both enhancements together. Respike experiments showed that the enhancement from magnetization carries over from magnetization to sulfidation, but not the reverse.
IntroductionGeneral Principles of the Degradation of Chlorinated HydrocarbonsDegradation of Environmentally Relevant Chlorinated CompoundsTechnical Applications and Future Aspects
The effectiveness of using ligand-assisted strategies to improve the performance of palladium-doped nanoscale zero-valent iron particles (Pd-nZVI) towards contaminant removal has been investigated previously, however, little attention has been given to either the thermodynamics and kinetics of the Pd-nZVI depassivation process or the effect of the presence of co-existent cations. Results of laboratory investigations using EDTA as the ligand of choice indicate that the presence of Ca(ii) and Mg(ii) ions can significantly improve the ligand-promoted dechlorination efficiency of polychlorinated biphenyls (PCB) with the effect of divalent cations on PCB removal being more significant at higher concentrations of EDTA. The improvement in particle reactivity in the presence of Ca(ii) and Mg(ii) could be attributed to moderate elimination of outer Fe oxide layers induced by the relatively slow release of free EDTA from Ca and Mg-EDTA complexes. The slow release of free EDTA prevented excessive initial loss of Fe oxide surface sites required for PCB sequestration and ensured that sufficient EDTA remained available for the later-time removal of Fe oxide layers that were continuously formed as Fe⁰ was oxidized. A mechanistically-based kinetic model for the ligand-promoted dissolution of Pd-nZVI has been developed with this model enabling quantitative understanding of the relatively complex interplay among Ca(ii) and Mg(ii) ions, EDTA and passivating Fe oxide layers during the contaminant degradation process.
Removal of halogenated hydrocarbons from water by zero-valent iron is discussed. In situ processes based on filtration of groundwater through a permeable barrier formed by granular iron seem to be an advantageous alternative to traditional methods using, e.g., sorption or air stripping. However, only a few full-scale units for the decontamination of groundwater by the in situ "zero-valent" technology have been installed all over the world.
Surface passivation of iron is one of the major problems when permeable reactive barriers are used for in situ remediation of groundwater for a long time. Whether the familiar ions in water would influence the reactivity of nanosized iron particles could have the direct bearing on the application of nanosized iron particles. The influence of familiar ions including Cl-, SO42-, CO32-, HCO3-, Ca2+ and Mg2+ to removal of pentachlorophenol (PCP) by nanosized iron particles synthesized in laboratory is investigated. The results show that Cl- reduces the removal efficiency of PCP, but improves the dechlorinaion efficiency. SO42-, CO32- and HCO3- promote the removal of PCP in the initial 2 h, and then inhibites the removal of PCP obviously. However, these three ions all promote the dechlorination of PCP. Ca2+ and Mg2+ reduce the removal efficiency of PCP, and the performance of the two cations is similar.
For in situ groundwater remediation, polyelectrolyte-modified nanoscale zerovalent iron particles (NZVIs) have to be delivered into the subsurface, where they degrade pollutants such as trichloroethylene (TCE). The effect of groundwater organic and ionic solutes on TCE dechlorination using polyelectrolyte-modified NZVIs is unexplored, but is required for an effective remediation design. This study evaluates the TCE dechlorination rate and reaction by-products using poly(aspartate) (PAP)-modified and bare NZVIs in groundwater samples from actual TCE-contaminated sites in Florida, South Carolina, and Michigan. The effects of groundwater solutes on short- and intermediate-term dechlorination rates were evaluated. An adsorbed PAP layer on the NZVIs appeared to limit the adverse effect of groundwater solutes on the TCE dechlorination rate in the first TCE dechlorination cycle (short-term effect). Presumably, the pre-adsorption of PAP "trains" and the Donnan potential in the adsorbed PAP layer prevented groundwater solutes from further blocking NZVI reactive sites, which appeared to substantially decrease the TCE dechlorination rate of bare NZVIs. In the second and third TCE dechlorination cycles (intermediate-term effect), TCE dechlorination rates using PAP-modified NZVIs increased substantially (~100 and 200%, respectively, from the rate of the first spike). The desorption of PAP from the surface of NZVIs over time due to salt-induced desorption is hypothesized to restore NZVI reactivity with TCE. This study suggests that NZVI surface modification with small, charged macromolecules, such as PAP, helps to restore NZVI reactivity due to gradual PAP desorption in groundwater.
Abstraet--A new iron oxide dissolution method designed to measure the abundance of "free" Fe oxide phases and associated elements in soils and sediments has been tested. The method employs a ternary complex of Ti(III), citrate, and ethylenediaminetetraacetate (EDTA) as a reductant and bicarbonate as a proton acceptor. The Ti(III)-citrate-EDTA-HCO3 method dissolved more synthetic amorphous ferric oxide and goethite, but less synthetic hematite, than the dithionite-citrate-HCO3 method of Mehra and Jackson. The production of acidity by the dissolution indicated that Ti(IV) is hydrolyzed to TiO2 during the extractions. The heated dithionite method dissolved 3-6 times more A1 from kaolinite and nontronite standard clays than room temperature dithionite, and 4-6 times more A1 than the Ti(III)-citrate-EDTA-HCO3 method. Furthermore, the release of Fe from the clay mineral samples consistently and rapidly reached a plateau during multiple extractions by the Ti(III)-citrate-EDTA-HCO3 method, indicating that a well-defined Fe oxide fraction was removed. Fe released by the dithionite method continued to increase with each extraction, suggesting that some release of structural Fe occurred. Tests on two natural sediments and one heavy mineral fraction from the Miocene Cohansey Sand in the New Jersey Coastal Plain suggested that the Ti(III)-citrate-EDTA-HCO3 method removed Fe oxides more effectively and more selectively than the dithionite method. The selectivity of the Ti(III)-citrate-EDTA-HCO3 method is enhanced by rapid extractions at room temperature and low free ligand concentrations.
There are four general pathways of dissolution of reducible metal oxides in acidic aqueous solution: proton-assisted (acid), ligand-promoted acid, reductive, and ligand-promoted reductive dissolution. The presence and reactivity toward the surface of protons, chelating ligands, and reductants dictate the mechanism(s) controlling the dissolution. For the massive reductive dissolution of magnetic by ascorbic acid, the experimental rate law suggests the involvement of surface ≡FeIIIA− complexes. Adsorption isotherms of ascorbic acid onto hematite at pH 3 and 25°C yield a Langmuir-type surface complexation constant Ks = (9.57 × 108M−1). Slow dissolution follows with an empirical rate law R = kobs(≡FeIIIA). It is concluded that the formation and kinetic reactivity of surface complexes determine the rate of dissolution. Dehydroascorbic acid also dissolves magnetite, but at slower rates.Oxalate accelerates the reductive dissolution of hematite by ascorbate even though it competes with ascorbate for surface sites; enhanced detachment of ≡FeII surface species by oxalate complexation may be involved. Autoacceleration of the reductive dissolution by dissolved FeII-carboxylate complexes is observed in EDTA/ascorbic acid mixtures; the rate reaches a maximum at intermediate [EDTA] values, where synergistic effects between EDTA and FeII-EDTA complexes are important. Autoacceleration may also operate in oxalate solutions.
Laboratory batch and column experiments were conducted with Hanford sediments to develop the capability to predict (1) the longevity of dithionite in these systems, (2) its efficiency as a reductant of structural iron, and (3) the longevity and reactivity of the reduced iron with soluble inorganic and organic species. After an initial induction period, the loss of dithionite by disproportionation and oxidation could be described by pseudo-first-order (PFO) kinetics. Other than the initial reaction with ferric iron, the primary factor promoting loss of dithionite in this system was disproportion nation via heterogeneous catalysis at mineral surfaces. The efficiency of the reduction of structural iron was nearly 100% for the first fourth of the ferric iron, but declined exponentially with higher degrees of reduction so that 75% of the ferric iron could be reduced. This decrease in reduction efficiency probably was related to differences in the accessibility of ferric iron in the mineral particles, with iron in clay-sized particles being the most accessible and that in silt- and sand-sized particles less accessible. Flow-through column studies showed that a reduced-sediment barrier created in this manner could maintain a reducing environment.
A review with 32 refs. concerning remediation pollution groundwater with 0-valent metals (corroded iron) is given. Topics discussed include: rediscovering corrosion, the biogeochem. context, sizing an iron wall, variations new and old, and beyond chlorinated solvents. [on SciFinder(R)]
In 1980, the United States Congress passed the Comprehensive Environmental Response, Compensation and Liability Act (CERCLA), requiring cleanup of abandoned hazardous waste sites, and in 1984, amended the Resource Conservation and Recovery Act (RCRA) to require cleanup of contamination at active sites. These legislative actions were prompted by several occurrences of groundwater contamination by hazardous materials; the most highly publicized being the Love Canal incident in Niagara Falls, New York. Thus, within a remarkably short period of time, groundwater remediation emerged as an activity of national priority. A recent and comprehensive report of the U.S. National Academy of Science (NAS, 1994) discusses the history, methods and performance of groundwater remediation efforts in the U.S. over the intervening fifteen years.
Specific features of cementation processes observed in numerous experimental studies are explained on the basis of a recently developed theory. This theory considers the kinetics of the oxidation reduction reactions and the possibility of either migrational or diffusional transport of the noble species. In particular, synergistic and antagonistic effects in simultaneous reductions of noble species, effects associated with anionic promoters, and effects of atmosphere are considered in the present paper.
The passivation characteristics of carbon steel surfaces at 473 K in the presence of certain chelating agents such as NTA DTP A, and HEDTA are described. The relative impacts of these chelants on the passivation process are evaluated by using base metal loss, soluble and insoluble iron concentrations in the mediums SEM studies of the topography of the surface coatings, and electrochemical investigations of the protection afforded by the oxide coating. The results have been compared with the passivation behaviour obtained under simple alkaline pH (LiOH) treatment (without chelating agents) and that previously obtained in the presence of EDTA. It is found that at 473 K, the presence of these complexing agents greatly increases the base metal loss, although a lower base metal loss is observed with LiOH. but the oxide film formed is more protective than the one formed under LiOR or EDTA treatments. The morphology of the coatings formed under complexing conditions has revealed highly developed crystallite faces on the outer surface layer. It is concluded that at 473 K passivation by LiOR is preferred to that given by the chelants.
A review is given of some simplified concepts that will contribute to a better understanding of corrosion fundamentals. The corrosion process involves not only electrochemical reactions but also acid-base reactions, and it is the acid-base nature that diversifies the corrosion phenomena. Anions either catalyze or inhibit the anodic metal dissolution, and the passivation will result from the hydroxide-catalyzed mechanism of metal dissolution. Corrosion precipitates frequently control the selective mass transport in corrosion processes. Anion-selective precipitates accelerate and cation-selective precipitates decelerate corrosion propagation. A bipolar precipitate film, if anodically polarized, undergoes deprotonation and turns into a passive film. The electrochemical stability of passivated metals is determined by the electron energy band structure of the passive film. The passive film of n-type semiconducting oxides appears electrochemically more stable than the passive film of p-type semiconducting oxides.
When ammonium ethylenediaminetetraacetic acid (EDTA) solutions are used to dissolve magnetite (Fe3O4) from utility boilers, iron EDTA is generated. After being oxidized to the ferric [Fe(III)] state, this solution can be used to (1) passivate steel surfaces and (2) remove copper deposits. Using potentiodynamic polarization and rotating disk electrode (RDE) methods, the passivation process is described. The critical variables are (free EDTA), T, [Fe3+], and the flow rate as determined by the Reynolds number of the RDE. An empirical equation relating these four factors has been derived. The generation of ferric EDTA from the ferrous EDTA obtained during the initial cleaning stage using air, air + NaNO2, or H2O2 is also described. The general observation is that passivation is favored by (1) high [Fe3+], (2) high flow rate, (3) low (EDTAfree), and (4) low temperature. Therefore, a strong oxidant such as H2O2 should be used to generate the active Fe3+ species as rapidly as possible.
The adsorption of EDTA on magnetite has been measured as a function of pH and EDTA concentration. The number of anchorage sites of EDTA on magnetite changes from 2 to 4 with increasing pH, while the affinity goes through a maximum in the pH interval 3–4. Adsorption of Fe(II) on EDTA has also been measured, the behavior being similar to that of other hydrolizable ions. The more complex composite systems EDTAFe(II), Fe(III)Fe3O4 are also analyzed and it is concluded that surface and dissolved iron ions compete for EDTA; thus the adsorptivity of the complex ions is less than the adsorptivity of either EDTA or aqueous iron ions. The rates of dissolution of magnetite in EDTA solutions with and without added Fe(II) have been measured and the results interpreted through a much faster rate of phase transfer for Fe(II) as compared to Fe(III). Heterogeneous electron transfer is involved in the dissolution of magnetite by ferrous ions, either exogenous or autogenerated. EDTA is necessary for the reductive pathway, as the larger stability constant of the Fe(III) complex over the Fe(II) complex provides the driving force for the heterogeneous electron transfer.
The dissolution kinetics of most slightly soluble oxides and silicates are controlled by chemical processes at the surface. The reaction controlling steps can be interpreted in terms of a surface coordination model.In dilute acid solutions, in the absence of complex-forming ligands, the dissolution kinetics are controlled by the surface bound protons. The rate of the proton-promoted reaction of δ-Al2O3 is RH = kH(CH3)3 where Ch3 is the proton concentration per unit area on the oxide surface. The mechanism can be described by the attachment of three protons to the reaction site prior to the detachment of an Al species into the solution. The dissolution rate of BeO is proportional to (CH2)2. For δ-Al2O3 at pH ⩽ 3.5 dissolution rate is independent of pH; at this pH maximum surface concentration of protons is reached.The organic ligand-promoted dissolution, RL, is of first order with respect to concentration of surface chelates: where {ML} is the concentration of surface chelates per unit area. Detachable surface complexes result from surface coordination of metal ions of the hydrous oxides with bidentate ligands. Especially efficient are bidentate ligands that form mononuclear surface complexes. The sequence of rate constants shows that five- and six-membered chelate rings (oxalate, catechol, malonate and salicylate) enhance the dissolution reactions to a greater extent than seven-membered rings (phthalate, succinate). Monodentate ligands (benzoate ion), though readily adsorbed, do not enhance dissolution rates. However, they can inhibit dissolution by displacing ligands that catalyze this reaction.
Surface enhanced Raman spectroscopy (SERS) was used to investigate the passive films formed on iron, nickel, chromium and 308 stainless steel in borate buffer solution (pH = 8.4) at low potentials in the passive region. The decay of these passive films, as the potential was scanned in the cathodic direction, was observed via SERS. The passive film on iron at −100 mV(SCE) consisted of an amorphous Fe(OH)2-like species and amorphous Fe3O4 or γ-Fe2O3, which have similar Raman spectra. The passive film on nickel at −100 mV consisted primarily of amorphous β-Ni(OH)2 and some NiO. The passive film on chromium at −100 mV in borate buffer consisted of Cr(OH)3 and another substance that may be similar to Cr(OH)2. The passive film on 308 stainless steel consisted of amorphous Fe(OH)2 and Fe3O4 or γ-Fe2O3, Ni(OH)2, NiO, Cr(OH)3 and the Cr(OH)2-like species. For all metals, as the electrode was cathodically polarized the various constituents of the passive film were concurrently reduced, a result that is inconsistent with passive films composed of discrete layers. In all SERS experiments carbon, a ubiquitous surface contaminant, was detected. The identity of the carbonaceous species changed with potential from predominantly amorphous graphite and CO2 at high potentials to saturated hydrocarbons at low potentials.
To date it does not appear to have been demonstrated in the literature that halogenated ethylenes can undergo reductive {beta}-elimination to alkynes under environmental conditions. The purpose of this paper is to provide experimental evidence that such pathways may be involved in the reaction of chloroethylenes with zero-valent metals as well as to speculate on the significance of the products that may result. Calculations indicate that reductive {beta}-elimination reactions of chloroethylenes are in fact comparable energetically to hydrogenolysis at neutral pH. Experiments were therefore initiated to assess whether {beta}-elimination reactions of chlorinated ethylenes could occur in the presence of two zero-valent metals, Fe and Zn. 76 refs., 3 figs., 1 tab.
Octahedral Fe(III) in the crystal structures of three different smectites was reduced to Fe(II) by actively growing microorganisms indigenous to the clay. The smectites were SWa-1 ferruginous smectite from Grant County, Washington; API 33a, Garfield Nontronite; and API 25, Upton montmorillonite. Bacterial growth was supported by incubating clay suspensions at room temperature in a nutrient broth solution consisting of peptone and beef extract. Some samples were first sterilized (by autoclaving), then seeded with bacteria that had been isolated previously from the SWa-1 sample. The effect of O//2 on microbial reduction of Fe(III) was also tested. Results revealed that, in all three clays, about 0. 30 mmol Fe(III)/g clay was reduced to Fe(II) by bacteria in a 28-day period.
The adsorption and oxidation of catechol and hydroquinone by Fe and Mn oxides has been investigated by Fourier transform infrared spectroscopic (FTIR) analysis of the adsorbed molecules and by the measurement of O//2 consumption by aqueous suspensions of these oxides. A model of surface oxidation by Mn and Fe is presented in which coordination of the organic at the surface is a prerequisite to electron transfer. Oxidation of organics can proceed with or without the uptake of O//2, depending largely on pH, which determines the rate of reoxidation of the reduced metal ions by O//2. The results emphasize the difficulty in interpreting the effects that chemical buffers have on oxidation reactions at oxide surfaces.
Boron adsorption behavior was investigated on various crystalline and x-ray amorphous Al and Fe oxide minerals. Adsorption increased at low pH, exhibited a peak in the pH range 7 to 8, and decreased at high pH. The magnitude of B adsorption was much greater for the x-ray amorphous materials. Since B adsorbs specifically on Al and Fe oxide minerals, the constant capacitance model containing a ligand exchange mechanism was used to describe its adsorption behavior. The constant capacitance model was able to represent B adsorption on all minerals over the entire pH range studied (3-12) using the same set of surface complexation constants. With the exception of amorphous Al oxide, B adsorption on these oxide minerals could be successfully described by optimizing only the B surface complexation constant. Other nonexperimental parameters were held fixed at values identical to those previously used in modeling phosphate, silicate, and selenite adsorption on Al and Fe oxide minerals.
Boron retention by hydroxy iron and aluminum materials was found to be pH dependent with maximum retention occurring in the alkaline range. The hydroxy aluminum materials retained B in amounts that were an order of magnitude greater than the amounts retained by the hydroxy iron materials. Boron retention by these materials was significantly reduced by aging on a steam bath prior to being treated with B.
Iron and aluminum were precipitated from solutions in the presence of B in application of the “mole ratio” method of determining the formulas of complexes. Evidence was obtained indicating Fe(III) or Al(III) and B were precipitated in stoichiometric proportions. Considering these precipitated forms as “solid complexes” with the general formula, MX n , a greater variety of complexes was found in the Al(III) systems than in the Fe(III) systems. In the Fe(III)‐B systems, values of n for the most part were ≤1.0; in the Al(III)‐B systems, values of n were ≥1.0.
Samples of similar constitution to those used in the “mole ratio” study were prepared for each metal at three pH levels. The amount of B combined in the precipitate was determined as a function of aging. The amount of B combined in the precipitate of the pH 6 samples remained relatively unchanged throughout the course of the 42‐day aging period whereas the amount of B combined in the pH 9, 10, and 11 samples decreased with time. Since the observed decreases in the amounts of B combined in the precipitates were limited to the pH 9, 10, and 11 samples, it was concluded that the ready source of hydroxyl ion in these suspensions promoted hydrolysis of the hydroxy iron and aluminum precipitates, resulting in expulsion of B from the precipitates probably as borate ions.
The competitive adsorption of phosphate + arsenate and phosphate + selenite on goethite and gibbsite has been measured. The oxide surface contains sites common to both anions on which competitive adsorption takes place and sites on which only one or the other anion is able to adsorb. The maximum amount of anions adsorbed from a mixture is approximately equal to the sum of the maximum adsorption for each anion in the absence of competition. The number of sites at which competition takes place is almost equal to the number of sites available to the anion with the lower adsorption in the absence of a competitor. Competition is described by a Langmuir-type exchange equation.
The inhibitive effect of an organic cation inhibitor, alkyltrimethylammonium ions on the passive film breakdown of an iron electrode in the borate buffer containing Cl− was investigated by polarization measurement and X-ray photo-electron spectroscopy. The pitting potential of a passivated iron electrode shifted in the positive direction following the addition of the inhibitor, indicating inhibition of the breakdown. Since X-ray photo-electron spectra showed the absence of Cl− in the passive film near the iron substrate, it is concluded that the cation inhibitor suppresses migration of Cl− into the passive film via an electrostatic interaction between the adsorbed Cl− and the cation at the film surface. The inhibitive effect of alkyltrimethylammonium ion increased and then decreased with an increase in the carbon number of an alkyl chain. The increase of the effect is attributed to coverage of the passive film surface with the cation inhibitor to hinder adsorption of Cl− and the decrease is ascribed to lowering of the film thickness and the borate ion content in the film.
This paper will emphasize the use of surface-analytical techniques to study the nature of passive oxide films on Fe. 18O/SIMS has been used to determine the air stability, cathodic removal, growth and breakdown of anodic films formed in borate buffer solution. SIMS has also shown that these passive films contain no incorporated hydroxyl ions, and electron back-scattering Mössbauer spectroscopy (complemented by XPS) has confirmed that the films resemble small particle size crystalline γ-Fe2O3. Results will also be presented on the anodic passivation of Fe in sulfate and perchlorate solutions and on the influence of halide ions on passivation in borate buffer.
The transformation of volatile chlorinated hydrocarbons in aqueous phase containing free ferrous and sulfide ions with and without light irradiation were investigated to evaluate the effect of these reducing ions on the dechlorination of chlorinated hydrocarbons. In the presence of the ferrous ion alone, 84% of the original carbon tetrachloride (CT) was transformed to chloroform within 33 days, and a removal efficiency of 99% was reached when the solution was irradiated by visible light. However, carbon tetrachloride did not appear to be reactive in other media containing sulfide and/or bound ferrous ions. 1,1,1-trichloroethane and tetrachloroethylene were less susceptible than carbon tetrachloride to the reductive dechlorination. No transformation was observed for these two compounds in different types of media in 33 days. Oxidationreduction potential (ORP) measurements showed that carbon tetrachloride could be depleted only when ORP of the environment was below 360 mV (relative to standard hydrogen electrode). This study indicates that free ferrous ion is an active reducing agent for the dechlorination of CT, but has little effect on the transformation of 1,1,1-trichloroethane and tetrachloroethylene, whereas, free sulfide and bound ferrous ions do not appear to have the capability of dechlorination for these heavily chlorinated hydrocarbons.
A method was developed to determine the concentrations of oxalate‐extractable Fe(III) and Fe(II) in sediments. Sediment was extracted in acid ammonium oxalate under N 2 . Iron(II) in the extract was determined with ferrozine. Iron(III) in the extract was reduced with hydroxylamine‐hydrochloride, and total Fe in the extract was determined with ferrozine. Oxalate‐extractable Fe(III) was calculated as the difference between Fe(II) and total Fe in the extract. Iron(II) was not oxidized, and Fe(III) was not reduced during the extraction. For an accurate estimate of oxalate‐extractable Fe(III), fresh samples had to be analyzed, because air‐drying or freeze‐drying the samples oxidized Fe(II) to oxalate‐extractable Fe(III). With a minimal increase in analytical effort, the method yields far more information on Fe geochemistry than the standard aerobic oxalate extraction method.
An examination has been made of the mechanism of breakdown of passive films on iron in borate buffer solution caused by chloride ions. Various electrochemical kinetic criteria were measured. XPS, SIMS, and ISS studies were made of the systems used in the electrochemical work. The rate of breakdown was found to be proportional to and and exponentially dependent on the electrode breakdown potential and field drop in the oxide film. XPS data showed that when chloride ions caused breakdown, the and ratios changed from 2 to 1.5 and 0.5 to 0.1, respectively. SIMS data revealed that heating passive films up to 200°C drove out water from the films and that chloride ions penetrated the whole film thickness on breakdown. ISS data indicated that on changing from a passive to a depassivated film, the ratio changed from 2.07 to 1.5. Discussion of the electrochemical kinetic data shows that it is inconsistent with adsorption‐displacement models, pore models, and chemico‐mechanical models, but is not inconsistent with ion‐exchange processes, point‐defect models, and hydrated polymeric oxide models. Confrontation of the spectroscopic data with the expectations of the latter three models shows some points of agreement with all these models, but the data taken together is most consistent with the hydrated polymeric oxide model.
The anodic oxidation of an iron electrode in a neutral borate electrolyte is studied using a following ellipsometer. The oxide film is found to consist of two layers with different refractive indices: an outer layer of and a thicker inner layer of . A thin layer of is grown on the electrode in the active state, and the electrode passivates when a partial monolayer of is grown at the interface. Passive state anodic oxidation causes simultaneous growth of the two layers, the inner layer growing approximately 80% faster than the outer layer. We conclude that the layer inhibits anodic dissolution of the electrode, and that both the layer and the portion of the layer grown in the passive state act as the electrically limiting barrier across which the overpotential appears. The possibility that the two layers grow in a proportion dictated by the transport numbers of mobile defects (as has been proposed for the anodic oxide of tantalum) is discussed.
Partial table of contents: THE SOLID-SOLUTION INTERFACE. Adsorption Mechanisms in Aquatic Surface Chemistry (J. C. Westall). The Electric Double Layer at the Solid-Solution Interface (R. Parsons). A Two-Phase Model for the Interpretation of Proton and Metal Ion Interaction with Charged Polyelectrolyte Gels and Their Linear Analogs (J. A. Marinsky). The Surface Chemistry of Oxides, Hydroxides, and Oxide Minerals (P. W. Schindler & Werner Stumm). Aspects of Molecular Structure in Surface Complexes Spectroscopic Investigations (H. Motschi).
Effect of chlorides on the passive film formed on iron in borate buffer has been investigated by Auger Electron Spectroscopy (AES) and coulometric methods. AES measurements have not given any evidence for chloride entry into the film. Amount of charges necessary to cathodically reduce the film tended to diminish after exposure to chloride or bromide containing solutions. The results suggest a thinning of the passivating film on iron by halides with subsequent pitting.
Einfluß von Cl−-Ionen auf die Passivschicht auf Eisen
Der Einfluß von Chloriden auf die Passivschicht auf Eisen wurde mittels Auger-Elektronenspektroskopie (AES) und mit coulometrischen Methoden untersucht. Mittels AES konnte kein Eintritt von Chloriden in die Passivschicht nachgewiesen werden. Die für die kathodische Reduktion der Passivschicht verbrauchte Ladungsmenge zeigt nach Einwirkung von chlorid- oder bromidhaltigen Lösungen abnehmende Tendenz. Die Versuchsergebnisse lassen vermuten, daß die Dicke der Passivschicht durch Halidionen verringert wird und als Folge davon Lochkorrosion auftritt.
The title book covers coordination chemistry of the hydrous oxide-water interface; surface charge and the electric double layer; adsorption; chemical weathering phenomena; homogeneous and heterogeneous nucleation and precipitation; particle-particle interaction; carbonate reactivity; redox processes mediated by surfaces; photochemistry; and trace element transport. It can be used as a source book for teaching and for professionals in geochemical and environmental disciplines.
The dissolution of magnetite particles in solutions containing EDTA and FeII was studied as a function of the total concentration of EDTA and FeII; the influence of pH was also studied. The rate shows a Langmuir-type dependence on [FeY2-] when [EDTA]0 ≤ [FeII]0. At constant [EDTA]0, in the range where EDTA is in excess over FeII, the kinetic order on FeII is one; FeII in large excess has no influence on the rate. At constant [FeII]0, the rate of dissolution is maximum when the ratio of [EDTA]0 to [FeII]0 is close to 3. These results are interpreted in terms of fast solution and surface complexation processes followed by slow heterogeneous electron transfer from adsorbed FeY2- to surface >FeIII centers and fast phase transfer of >FeII. The inhibitory effect of excess EDTA results from competitive adsorption of FeY2- and EDTA. The rate increases with decreasing pH up to pH 3.1; at this value a maximum is achieved. The pH dependence of rate is the resultant of several factors, the most important being the influence of pH on the adsorption preequilibrium and the need for adsorbed protons adjacent to the reactive site. The stoichiometry of the dissolution reaction is not constant and the ratio of protons consumed to iron released is sensitive to experimental conditions. In the fatest reactions, this ratio is appreciably lower than the limiting value corresponding to the release of unhydrolyzed FeIII species. The implications of this result are discussed.
A biotic approach for remediating subsurface sediments and groundwater contaminated with carbon tetrachloride (CT) and chromium was evaluated. Cells of the Fe(iii)-reducing bacterium strain BrY were added to sealed, anoxic flasks containing Hanford groundwater, natural subsurface sediments, and either carbon tetrachloride, CT, or oxidized chromium, Cr(VI). With lactate as the electron donor, BrY transformed CT to chloroform (CF), which accumulated to about 1 0 % of the initial concentration of CT. The remainder of the CT was transformed to unidentified, nonvolatile compounds. Transformation of CT by BrY was an indirect process Cells reduced solid phase Fe(ill) to chemically reactive FE(II) that chemically transformed the chlorinated contaminant. Cr(VI), in contrast, was reduced by a direct enzymatic reaction in the presence or absence of Fe(III)-bearing sediments. These results demonstrate that Fe(ill)-reducing bacteria provide potential for transforming CT and for reducing CR(VI) to less toxic Cr(III). Technologies for stimulating indigenous populations of metal-reducing bacteria or for introducing specific metal-reducing bacteria to the subsurface are being investigated.
To address some of the fundamental questions regarding the kinetics of reduction of contaminants by zero-valent iron (Fe0), we have taken advantage of the mass transport control afforded by a polished Fe0 rotating disk electrode (RDE) in an electrochemical cell. The kinetics of carbon tetrachloride (CCl4) dechlorination at an Fe0 RDE were studied in pH 8.4 borate buffer at a potential at which an oxide film would not form. In this system, the cathodic current was essentially independent of electrode rotation rate, and the measured first-order heterogeneous rate constant for the chemical reaction (kct = 2.3 × 10-5 cm s-1) was less than the estimated rate constant for mass transfer to the surface. Thus, for the conditions of this study, the rate of reduction of CCl4 by oxide-free Fe0 appears to be dominated by reaction at the metal−water interface rather than by transport to the metal surface. Activation energies for reduction of CCl4 and hexachloroethane by oxide-covered granular Fe0 (measured in batch systems) also indicate that overall rates are limited by reaction kinetics. Since mass transport rates vary little among the chlorinated solvents, it is likely that variation in kct is primarily responsible for the wide range of dechlorination rates that have been reported for batch and column conditions.
Hydroxypropyl-β-cyclodextrin (HP-β-CD) enhances the solubility of tetrachloroethylene (PCE) in water both in static and in flowing systems. HP-β-CD does not decrease the interfacial tension between PCE and water and, therefore, should not mobilize immiscible-phase PCE in the subsurface. Rates for the reaction of PCE with metallic iron were measured in HP-β-CD solutions under static conditions. In flowing systems, metallic iron removed PCE, and no other chlorinated ethylene species were observed in the column effluent. In several such systems, recycling of the HP-β-CD solution took place following the reaction with iron. No downward mobilization of the PCE pool in the generator column was observed. The solubility enhancement and reaction with metallic iron are consistent with reversible formation of a stoichiometric HP-β-CD/PCE complex. A theoretical treatment of the reaction rates of complexed PCE on an iron surface was developed. This treatment suggests that any material that enhances the solubility of low-solubility organic substances may slow down the rate of reaction in aqueous solution. In many cases, the rate retardation should equal the degree of solubility enhancement. The combination of HP-β-CD and iron metal appears to be a promising groundwater remediation technology.
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.
The adsorption of metal−EDTA complexes onto various oxides (aluminium oxides, crystalline and amorphous iron oxides) was described by the surface complexation model. Data from the literature and from our experiments with several oxides were analyzed by assuming only one type of ternary surface complex for Me2+−EDTA complexes. The surface complexes exhibit an anionic character with decreasing adsorption with increasing pH. This proposed model allows us to describe all available data sets very well. The adsorption of NiEDTA was interpreted by several models (constant capacitance, diffuse layer) onto different oxides (with different pKa values). Log K values within one model agree very well with each other. The pH, where 50% of the complex is adsorbed, increases in the series HFO < lepidocrocite < γ-Al2O3 < goethite < δ-Al2O3 from 6.5 to 8.35 (for 1 μM NiEDTA and 1 mM surface groups). Adsorption of Fe(III)EDTA onto several oxides was studied. Two types of surface complexes, anionic and cationic, were necessary to explain the observed adsorption edge. Fe(III)EDTA is adsorbed over a wide pH range and is the major adsorbed EDTA species at pH greater than 7.
We have studied the surface-mediated reduction of pertechnetate (TcO4- ) in solution by Fe(II)-bearing fracture filling material from a natural fracture in granite, hornblende, and magnetite. The disappearance of technetium from solution was found to follow pseudo first order kinetics, the rate constant being dependent on the specific surface area and Fe(II) content of the solid. Comparison of the rate constants obtained in the experiments with fracture filling material containing chlorite as Fe(II)-bearing mineral, hornblende, and magnetite indicates a strong influence of the binding manner of Fe(II) in the solid phase. Magnetite, with a low band gap (0.1 eV) between the valence and conduction bands was found to be the most efficient reductant, and based on the ionic strength and pH dependence of the rate of TcO4- reduction, it is concluded that sorption of TcO4- on the magnetite surface by a ligand-exchange mechanism is the rate-determining reaction step: >SOH + TcO4- >SOTcO3 + OH-. Oxidative desorption of sorbed/precipitated TcO2(s) into air-saturated groundwater was found to be very slow, most probably due to competing reactions between oxygen and the surface of the Fe(II)-bearing solid.
The reduction of TcO4- to TcO2·nH2O by Fe(II) in slightly acid to basic solution has been investigated in an all-glass reaction vessel with a hydrophobic inner surface. The three-electron reduction process, although thermodynamically feasible, was found to proceed very slowly if at all. Fe(II) sorbed on the wall of untreated reaction vessels or precipitated as Fe(OH)2(s) or FeCO3(s) was found to reduce TcO4-, the observed rates being proportional to the amount of sorbed or precipitated Fe(II). The experimental results are discussed in light of the standard redox potentials and possible reaction paths leading from TcO4- to TcO2·nH2O.
Quantitative aspects of microbial crystalline iron(III) oxide reduction were examined using a dissimilatory iron(III) oxide-reducing bacterium (Shewanella alga strain BrY). The initial rate and long-term extent of reduction of a range of synthetic iron(III) oxides were linearly correlated with oxide surface area. Oxide reduction rates reached an asymptote at cell concentrations in excess of ≈1 × 109/m2 of oxide surface. Experiments with microbially reduced goethite that had been washed with pH 5 sodium acetate to remove adsorbed Fe(II) suggested that formation of a Fe(II) surface phase (adsorbed or precipitated) limited the extent of iron(III) oxide reduction. These results demonstrated explicitly that the rate and extent of microbial iron(III) oxide reduction is controlled by the surface area and site concentration of the solid phase. Strain BrY grew in media with synthetic goethite as the sole electron acceptor. The quantity of cells produced per micromole of goethite reduced (2.5 × 106) was comparable to that determined previously for growth of BrY and other dissimilatory Fe(III)-reducing bacteria coupled to amorphous iron(III) oxide reduction. BrY reduced a substantial fraction (8−18%) of the crystalline iron(III) oxide content of a variety of soil and subsurface materials, and several cultures containing these materials were transferred repeatedly with continued active Fe(III) reduction. These findings indicate that Fe(III)-reducing bacteria may be able to survive and produce significant quantities of Fe(II) in anaerobic soil and subsurface environments where crystalline iron(III) oxides (e.g., goethite) are the dominant forms of Fe(III) available for microbial reduction. Results suggest that the potential for cell growth and Fe(II) generation will be determined by the iron(III) oxide surface site concentration in the soil or sediment matrix.
Reduction of Cr(VI) to Cr(III) is environmentally favorable as the latter species is not toxic to most living organisms and also has a low mobility and bioavailability. Ferrous iron is one possible reductant implicated as a major contributor to the removal of Cr(VI) from suboxic and anoxic waters and soils. Despite the importance of this redox reaction, no mechanistic or kinetic information are available, which are needed to determine the rate of Cr(VI) reduction and to assess the role of oxygen in limiting this reaction. In this study we used a stopped-flow kinetic technique monitored by UV−VIS spectroscopy and an initial rate method to ascertain the rate constant and the rate dependence of each reactant. We observed that the rate of Cr(VI) removal conformed to −d[Cr(VI)]/dt = kcr[Fe(II)]0.6[Cr(VI)]1 where kcr = 56.3 (±3.7) mmol-0.6 min-1 L0.6. Based on this rate expression and that for the oxygenation of Fe(II), Cr(VI) reduction should be unaffected by oxygen except at pH values in excess of 8 even at micromolar concentrations.
The extent of adsorption and the value of the adsorption equilibrium constant (Ksintr) for simple organic ligands is influenced by the identity of the ligand donor groups and other substituents on the aromatic ring. Catechols adsorb onto TiO2 to a significantly greater extent than 2-aminophenols; the adsorption of 1,2-phenylenediamines is negligible. The TiO2 surface has a high ionic contribution to bonding; ligands possessing donor groups with the highest ionic contribution to bonding adsorb to the greatest extent. A covalent contribution to bonding can increase binding to Ti(IV)-containing surfaces, but only when the ionic contribution is already strong. Within each ligand class, substituents alter the competition between protons and surface sites for binding the deprotonated ligand. For this reason, pKa1, pKa2, and log Ksintr are all important in determining the extent of adsorption. Additionally, substituents that impart hydrophobicity also raise the extent of adsorption and the value of log Ksintr.
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.
A review is given of experiments that have contributed to a better understanding of the nature of the passive film on iron and ferrous alloys. The important questions relating to the nature of passive films on iron that are considered are: (1) the thickness—are the films two- or three-dimensional? (2) the composition—do the films contain hydrogen and cations introduced by alloying elements or inhibitors? (3) the structure—are the films crystalline spinel structures or non-crystalline glassy structures? and (4) the electronic properties—are the films insulators or semiconductors?
Paul Schindler''s early work on the acid-base chemistry of oxides was instrumental for the development of the concept of surface complexation. This approach has not only been important in establishing a theory on the adsorption of metal ions and ligands as a function of pH and solution variables, but has become essential in establishing surface speciation (coordinative structural and electronic arrangement at the solidwater interface) which in turn determines surface reactivity. The factors that affect dissolution of Fe(III) (hydr)oxides and inhibition of dissolution are discussed. A few examples for the inhibition of reductive and ligand-promoted dissolution by binuclear complexes of oxoanions (phosphate, borate) and of protonpromoted dissolution by Cr(III) are given.