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Kinetics of Trichloroethene Reduction by Zerovalent Iron and Tin: Pretreatment Effect, Apparent Activation Energy, and Intermediate Products
Abstract
The degradation of trichloroethene (TCE) at 2 mg L-1 in headspace free aqueous solution by zerovalent iron (Fe0) and tin (Sn0) was studied in batch tests at 10, 25, 40, and 55 °C and HCl-treated Fe0 and Sn0 at 25 and 55 °C. Surface area normalized pseudo-first-order rate constants (kSA) ranged from 0.44 × 10-3 to 4.3 × 10-3 h-1 m-2 L for Fisher Fe0, 0.029 × 10-3 to 0.27 × 10-3 h-1 m-2 L for Peerless and Master Builders Fe0, and 0.011 × 10-3 to 1.31 × 10-3 h-1 m-2 L for Fisher and Aldrich Sn0. The Aldrich Fe0 was the least reactive with kSA values ranging from 0.0016 × 10-3 to 0.011 × 10-3 h-1 m-2 L. The HCl-washing increased metal surface area and observed rate constant (k) values but generally decreased kSAvalues. The calculated apparent activation energy (Ea) using the Arrhenius law for the four temperature levels ranged from 32.2 to 39.4 kJ mol-1 for the untreated Fe0 metals and 40.5−76.8 kJ mol-1 for the untreated Sn0 metals. Greater temperature effect was observed for Sn0 than for Fe0. Our results indicate that TCE reduction by Fe0 and Sn0 is likely controlled primarily by chemical reaction-limited kinetics rather than by mass transport of the TCE to the metal surface. Both reductive β-elimination reaction and hydrogenolysis reaction are likely involved in the reduction of TCE by both Fe0 and Sn0.
... Regarding the kinetic, Gillham & O'Hannesin (1994) have found out that the dechlorination reaction is a pseudo-first-order reaction with respect to contaminant concentration. Because of sequential dechlorination of major chlorinated organics (MCOs) such as PCE, TCE, 1,1,1-TCA and TCM into the corresponding chlorinated daughter products (CDPs) including cis-DCE, 1,1-DCE, VC, 1,1-DCA and DCM (Vogel & McCarty 1987;Helland et al. 1995;Su & Puls 1999), the following sequential dechlorination models (models 1, 2 and 3), which take account of concurrent production and dechlorination of CAHs, were applied for the computation of dechlorination rate constant (k obs ). The first-order equations for the models 1, 2, and 3 are illustrated in eqs 4 to 8, 10 to 11 and 12, respectively. ...
... The first-order equations for the models 1, 2, and 3 are illustrated in eqs 4 to 8, 10 to 11 and 12, respectively. Since sequential transformations of PCE and TCE into trans-DCE by Fe 0 have not been proven (Su & Puls, 1999), first-order equation solely considering dechlorination of trans-DCE was applied for determining DCE trans k − (equation (9)). Besides, computation of the k obs of 1,2-DCA and DCM was not considered since they have been substantiated to be not treatable by Fe 0 (USEPA, 1998). ...
... As you can see from Fig. 4(a) and (b), excluding the k sa obtained in September 2002, the k sa generally increased in the following order: January, March, August, September, thereby manifesting climatic variation of the barrier's performance on CAHs dechlorination. Probably owing to the low temperature in groundwater in cold season, the Fe 0 reactivity on CAHs dechlorination was comparatively low in January 2003 and March 2000, since the dechlorination reactions of CAHs by Fe 0 have been proven primarily to be reactionlimited kinetic (Su & Puls, 1999). Low temperature in groundwater consequently lowered the rate of oxidation of Fe 0 or release of electron from Fe 0 and thereby adversely affected the rate of dechlorination reactions in cold season. ...
A funnel-and-gate permeable reactive barrier (PRB) packed with zero-valent iron (Fe 0) has been constructed at Vapokon site, Denmark in September 1999 to remediate the groundwater contaminated by chlorinated aliphatic hydrocarbons (CAHs). In order to understand the variation of the performance of the PRB over the past 4 years and identify the major factor(s) affecting the iron barrier's performance, comprehensive monitoring of the behaviour of the Fe 0 PRB was conducted between March 2000 and August 2003. Although there was a continuous decrease in total alkalinity (90.3%), calcium (81.7%) and sulphate (69.2%) ions in the groundwater across the PRB, probably caused by mineral precipitation and resulting in 0.88% porosity loss per year, no noticeable trend of deterioration of the barrier's performance was observed. Virtually, over the past 4 years, about 92.4-97.
... Enhanced reactivity and dechlorination have been observed by some studies following acid pretreatment (Lin and Lo 2005;Liu et al. 2006). In contrast, acid pretreatment has been observed to decrease dechlorination by Cheng and Wu (2000) and also by Su and Puls (1999). The reasons for the apparent discrepancies in ZVI performance on dechlorination reactions as well as the effect of acid pretreatment remain unclear and thus require further investigation and clarification. ...
... Different effects from acid treatment of ZVI have also been reported in previous studies. For example, dechlorination of tetrachloroethylene (TCE) by ZVI could be either enhanced (Lin and Lo 2005;Liu et al. 2006) or inhibited (Cheng and Wu 2000;Su and Puls 1999) by acid washing of the ZVI. ...
... Error bars indicate ± one standard deviation. Total TeCPs, TCPs, DCPs, and MCPs were the sum of all the TeCPs, TCPs, DCPs, and MCPs measured, respectively reason for enhanced dechlorination performance observed with acid-washed ZVI compared to the unwashed ZVI (Agrawal and Tratnyek 1996;Lin and Lo 2005;Su and Puls 1999). However, in this study, acid washing increased the reactivity of ZVI-T but did not change the reactivity of ZVI-H. ...
The dechlorination of chlorinated organic pollutants by zero valent iron (ZVI) is an important water treatment process with a complex dependence on many variables. This complexity means that there are reported inconsistencies in terms of dechlorination with ZVI and the effect of ZVI acid treatment, which are significant and are as yet unexplained. This study aims to decipher some of this complexity by combining Raman spectroscopy with gas chromatography-mass spectrometry (GC-MS) to investigate the influence of the mineralogy of the iron oxide phases on the surface of ZVI on the reductive dechlorination of pentachlorophenol (PCP). Two electrolytic iron samples (ZVI-T and ZVI-H) were found to have quite different PCP dechlorination reactivity in batch reactors under anoxic conditions. Raman analysis of the “as-received” ZVI-T indicated the iron was mainly covered with the ferrous oxide (FeO) wustite, which is non-conducting and led to a low rate of PCP dechlorination. In contrast, the dominant oxide on the “as-received” ZVI-H was magnetite which is conducting and, compared to ZVI-T, the ZVI-H rate of PCP dechlorination was four times faster. Treating the ZVI-H sample with 1 N H2SO4 made small change to the composition of the oxide layers and also minute change to the rate of PCP dechlorination. However, treating the ZVI-T sample with H2SO4 led to the loss of wustite so that magnetite became the dominant oxide and the rate of PCP dechlorination increased to that of the ZVI-H material. In conclusion, this study clearly shows that iron oxide mineralogy can be a contributing factor to apparent inconsistencies in the literature related to ZVI performance towards dechlorination and the effect of acid treatment on ZVI reactivity.
... Two main steps might be involved in the reaction of HBCD with FeS, including diffusion of HBCD to FeS surface and chemical reduction of HBCD on FeS surface. The rate-limiting step determined the E a of the reaction between HBCD and FeS (Su and Puls, 1999). It has been suggested that a diffusion-controlled reaction requires an E a of 15 kJ mol −1 or less, whereas a chemical reduction requires a higher E a (Su and Puls, 1999). ...
... The rate-limiting step determined the E a of the reaction between HBCD and FeS (Su and Puls, 1999). It has been suggested that a diffusion-controlled reaction requires an E a of 15 kJ mol −1 or less, whereas a chemical reduction requires a higher E a (Su and Puls, 1999). The E a of 29.2 kJ mol −1 indicated that the rate-limiting step for the reaction of HBCD with FeS was a surface-mediated reduction. ...
Reactivity of iron sulfide (FeS) towards hexabromocyclododecane (HBCD) was explored under conditions of varying temperature, pH, inorganic ion and dissolved organic matter (DOM) in this study. Results show that the reduction of HBCD by FeS has an activation energy of 29.2 kJ mol⁻¹, suggesting that the rate-limiting step in the reduction was a surface-mediated reaction. The reduction of HBCD by FeS was a highly pH-dependent process. The optimal rate for HBCD reduction by FeS was observed at a pH of 6.2. All the tested inorganic ions suppressed the reduction of HBCD by FeS. XPS analysis confirmed that both Fe(II)-S and bulk S(-II) on FeS surface could be impacted by solution pH and inorganic ions and were responsible for the regulation of HBCD reduction. Some DOMs (i.e., fulvic acid, humic acid, salicylic acid, catechol and sodium dodecyl sulfate) were found to hinder the reduction via competing with HBCD for active sites on FeS surface. However, the presence of 2,2′-bipyridine, triton X-100 and cetyltrimethyl ammonium bromide was able to significantly enhance the rate of HBCD reduction by 5.8, 9.0 and 20 times, respectively. Different factors could influence the reduction efficiency of HBCD diastereoisomers to different extent, but not the reaction orders of HBCD diastereoisomers (α-HBCD < γ-HBCD < β-HBCD). Moreover, FeS could completely remove HBCD diastereoisomers in sediments with multiple factors within 9 d reaction. Our results contribute to give a better understanding on the performance of FeS towards HBCD under real and complex conditions and facilitate the application of FeS in remediation sites.
... Apart from the surface morphology, the impurities (e.g., S, Si, C, O, Cu, Cr, and Mn, etc.) in ZVI samples could stimulate or inhibit the reactivity of ZVI toward contaminants (Butler and Hayes, 2001;Cheng and Wu, 2000;Hassan, 2000;Lipczynska-Kochany et al., 1994;Su and Puls, 1999;Tamara and Butler, 2004). For example, Velimirovic et al. (2013) investigated the impact of carbon, oxygen, and sulfur contents of ZVI on its reactivity toward chlorinated aliphatic hydrocarbons (CAHs). ...
... The surface area has a crucial effect on the number of active surface sites of solid iron, thereby predominantly affect the transformation, adsorption, and co-precipitation processes, irrespective from the nature of the contaminant Noubactep et al., 2005;Ponder et al., 2000). Generally, increasing the surface area of ZVI enhances the rate constants of contaminants removal by ZVI (Agrawal and Tratnyek, 1996;Cope and Benson, 2008;Johnson et al., 1996;Matheson and Tratnyek, 1994;Su and Puls, 1999). ...
Appropriately selecting methods for characterizing the reaction system of zerovalent iron (ZVI) favors its application for water treatment and remediation. Hence, a survey of the available ZVI characterization techniques used in laboratory and field studies are presented in this review for clarifying the characteristic properties, (in-situ) corrosion processes, and corrosion products of ZVI system. The methods are generally classified into four broad categories: morphology characterization techniques, (sub-)surface and bulk analysis mainly via the spectral protocols, along with the (physio)electrochemical alternatives. Moreover, this paper provides a critical review on the scopes and applications of ZVI characterization methodologies from several perspectives including their suitable occasions, availability, (semi-)quantitative/qualitative evaluations, in/ex-situ reaction information, advantages, limitations and challenges, as well as economic and technical remarks. In particular, the characteristic spectroscopic peak locations of typical iron (oxyhydr)oxides are also systematically summarized. In view of the complexity and variety of ZVI system, this review further addresses that different characterization methods should be employed together for better assessing the performance and mechanisms of ZVI-involved systems and thereby facilitating the deployment of ZVI-based installations in real practice.
... A new approach is the use reductive dechlorination which uses catalytic bimetallic nanoparticle systems such as M-Pd where M is one of the following metal ions, (Fe 2+ , Ni 2+ , Cu 2+ Zn 2+ and Co 2+ ) that can form a redox couple (Su and Puls, 1999;Wei, et al., 2006). An example in ethylene conversion, the reaction rate increased by100 times when a nanocatalyst was used and 100,000 times when a second metal such as 1 % wt Fe-Pd was introduced Orellana, et al., 2005;. ...
... eatment. Acid washing leads to a decrease in thickness of the oxide film and an increase in surface area, enhancing the reactivity of ZVI. For example, Su et al. showed that both the specific surface area of iron powder and the apparent rate constant (K obs ) of trichloroethane degradation by ZVI could be significantly increased after acid washing (Su and Robertw. Puls, 1999). However, the surface normalized rate constant (K SA ) may not be impacted by the pretreatment of acid washing for iron powder . In addition, it was found that the positive effect of acid-washing on the enhancement of ZVI reactivity was temporary, as the deposition of oxide would become more severe after this process (Lai and Lo, 2008). ...
Application of zero-valent iron (ZVI) has become one of the most promising innovative technologies for the remediation of environmental pollutants. However, ZVI may suffer from the low intrinsic reactivity toward refractory pollutants, which seriously restricts its practical application in fields. Therefore, strategies have been developing to enhance the reactivity of ZVI. Until now, the most commonly used strategies include pretreatment of ZVI, synthesis of highly-active ZVI-based materials and additional auxiliary measures. In this review, a systematic and comprehensive summary of these commonly used strategies has been conducted for the following purposes: (1) to understand the fundamental mechanisms of the selected approaches; (2) to point out their advantages and shortcomings; (3) to illustrate the main problems of their large-scale application; (4) to forecast the future trend of developing ZVI technologies. Overall, this review is devoted to providing a fundamental understanding on the mechanism for enhancing the reactivity of ZVI and facilitating the practical application of ZVI technologies in fields.
... Considering that diffusion-controlled reactions in solution usually have rather lower apparent activation energies than those limited by chemical reaction kinetics 45 , the apparent activation energies of the aniline coupling reaction were calculated from the Arrhenius plots to investigate the apparent reaction kinetics of h-CoNC and s-CoNC, which have the same atomic dispersed active sites. As indicated in Supplementary Fig. 16, h-CoNC exhibited a larger apparent activation energy (39.1 kJ/mol) than s-CoNC (29.8 kJ/mol), corresponding to easier internal diffusion (smaller internal diffusion blockage) in h-CoNC (the external diffusion effect can be excluded at stirring rates larger than 400 revolutions per minute (rpm)) ( Supplementary Fig. 17) 46 . ...
Single-atom catalysts (SACs) show great promise in various applications due to their maximal atom utilization efficiency. However, the controlled synthesis of SACs with appropriate porous structures remains a challenge that must be overcome to address the diffusion issues in catalysis. Resolving these diffusion issues has become increasingly important because the intrinsic activity of the catalysts is dramatically improved by spatially isolated single-atom sites. Herein, we develop a facile topo-conversion strategy for fabricating hollow mesoporous metal-nitrogen-carbon SACs with enhanced diffusion for catalysis. Several hollow mesoporous metal-nitrogen-carbon SACs, including Co, Ni, Mn and Cu, are successfully fabricated by this strategy. Taking hollow mesoporous cobalt-nitrogen-carbon SACs as a proof-of-concept, diffusion and kinetic experiments demonstrate the enhanced diffusion of hollow mesoporous structures compared to the solid ones, which alleviates the bottleneck of poor mass transport in catalysis, especially involving larger molecules. Impressively, the combination of superior intrinsic activity from Co-N 4 sites and the enhanced diffusion from the hollow mesoporous nanoarchitecture significantly improves the catalytic performance of the oxidative coupling of aniline and its derivatives.
... In this respect, it rarely accounted for the role that the pre-existing (hydr)oxides play in the phase transformation to the final iron sulfide. More commonly, such (hydr)oxide phases are considered "bad" and are thus often purposely removed via acid wash before use (Agrawal and Tratnyek, 1996;Matheson and Tratnyek, 1994;Qin et al., 2019;Su and Puls, 1999). Here, we report that instead of being a "barrier" to the formation of FeS x , the pre-formed surface (hydr)oxide on ZVI acts as an important stepstone to direct the formation of more active S-ZVI materials. ...
The naturally-formed iron (hydr)oxides on the surface of zero valent iron (ZVI) have long been considered as passivation layer and inert phases which significantly reduce the reaction activities when they are employed in environmental remediation. Although it seems there are no direct benefits to keep these passivation layers, here, we show that such phases are necessary intermediates for the transformation to iron sulfides through an anion exchange pathway during sulfidation of ZVI. The pre-formed (hydr)oxides undergo a phase evolution upon aging and specific phases can be effectively trapped, which can be confirmed by a combination of different characterization techniques including scanning electron microscopy (SEM), X-ray photoelectron spectroscopy (XPS), X-ray powder diffraction (XRPD), and X-ray absorption near edge structure (XANES) spectroscopy (XANES). Interestingly, after sulfidation, the resultant samples originated from different (hydr)oxides demonstrate different activities in the Cr(VI) sequestration. The XANES investigation of Fe K edge and Fe L2,3 edge indicates Fe remains the same after sulfidation, suggesting a non-redox, anion exchange reaction pathway for the production of iron sulfides, where O²⁻ anions are directly replaced with S²⁻. Consequently, the structural characteristics of the parent (hydr)oxides are inherited by the as-formed iron sulfides, which make them behave differently because of their different structural natures.
... Their main difference is that β-elimination does not generate vinyl chloride, which is more toxic than the parent compounds. Nevertheless, it should be noted that although earlier studies have reported that the degradation of TCE by nZVI can proceed through both reaction pathways, more recent data suggest that β-elimination is the major route [104]. ...
Background:
The application of zero valent iron nanoparticles (nZVI) to remediate soil and groundwater has gained increased attention within the last decade, primarily due to its high reactivity, cost effectiveness and potential to treat a broad range of contaminants (e.g. chlorinated organic solvents, inorganic anions, or metals).
Objective:
In this paper, the state of the art of the applicability of nanomaterials especially the most frequently used nZVI in soil and groundwater is presented. The purpose of this article is to give an overview of the current knowledge pertaining to the synthesis, employment, limitations, and risk of iron nanoparticles.
Methods:
Therefore, the authors have reviewed and discussed the recent patents and papers related to the developments and approaches made on the synthesis of iron nanoparticles emphasizing the justification of green synthesis methods. The studies related to the effective use of nanoparticles in remediating organic and inorganic contaminants are addressed. The potential limitations, challenges, and risks of this innovative nanoremediation technology are also discussed.
Results:
Studies suggest that nZVI have successfully been applied in nanoremediation, however little is known about the particles’ fate and impacts. Additionally, it has already been proven that synthesis and modification can largely determine the physicochemical and biological properties of the particles.
Conclusion:
This review corroborates the suitability of nanoparticles in the remediation of contaminated media, simultaneously highlighting the work still needed to optimize the syntheses and careful use of such materials, concluding that comprehensive screenings should be performed prior nZVI applications to assess their behavior and impact on the environment and living systems.
... The relationship between Ea and the rate control mode of the system is still in dispute at present. Some researchers have found that 42 kJ⋅mol − 1 is the boundary, the higher Ea tends to be surface controlled, and the lower Ea tends to be diffusion controlled (Su and Puls, 1999;Scherer et al., 1997). Other researchers have also indicated that the surface controlled reaction generally has a higher Ea (>29 kJ⋅mol − 1 ) (Dahm and Brezonik, 1995), while the diffusion controlled reaction has a relatively low Ea (8-21 kJ⋅mol − 1 ) (Lien and Zhang, 2007). ...
Montmorillonite supported nanoscale zero-valent iron (MMT-nZVI) was prepared and proved to be able to induce the heterogenous Fenton process for better removal of 2,3′,4,5-tetrachlorobiphenyl (PCB67) in a long-term polluted soil. PCB67 removal depended highly on the dosages of MMT-nZVI and H2O2, and the initial pH, with the highest removal rate of 76.38% at conditions of H2O2 45.99 g·kg⁻¹, MMT-nZVI 29.88 g·kg⁻¹ and initial pH 3.5 after 80 min of reaction. Furthermore, PCB67 could be removed in a wider pH range (from 3.5 to near neutrality), with a loss of 13.6% in removal rate at neutral pH. With an activation energy of 21.4 kJ·mol⁻¹, the degradation of PCB67 was an endothermic and diffusion-controlled process and followed the pseudo-first-order kinetics. That Fe²⁺ was supplied through aerobic corrosion of MMT-nZVI to activate H2O2 for·OH production was the possible mechanism of PCB67 degradation, leading to complete mineralization of PCB67 through two proposed pathways, with the intermediates of ethylbenzene and 3-hepten-2-one, as well as dibutyl phthalate and butyl acetate respectively.
... During the process, nitrate diffusion, adsorption, chemical reaction, and products diffusion are all included, and the value of activation energy provides information on the limiting step for the reaction 50 . Diffusion requires less energy than a chemical reaction, and the value of the activation energy of a typical diffusion controlled reaction in water ranges from 10-20 kJ/mol 56 . In this study, nitrate reduction in batch experiments using nZVI and nZVI-Pd/NG produced E as of 42.8 and 17.6 kJ/mol over the temperature of 283 K to 313 K. ...
Nitrate reduction by zero-valent iron-based materials has been extensively studied. However, the aggregation of nanoparticles and the preference for unfavored ammonia products limit the application of this technology. To overcome this issue, this study introduced a novel synthesized nanoscale palladized zero-valent iron graphene composite (nZVI-Pd/NG) and explored its nitrate reduction efficiency. A nitrate removal rate of 97.0% was achieved after 120 min of reaction for an initial nitrate concentration of 100 mg N/L. The nitrogen gas selectivity was enhanced from 0.4% to 15.6% at the end point compared to nanoscale zero-valent iron (nZVI) particles under the same conditions. Further analyses revealed that zero-valent metal nanoparticles spread uniformly on the graphene surface, with a thin layer of iron (hydr)oxides dominated by magnetite. The nZVI-Pd/NG exhibited good catalytic activity with the associated activation energy of 17.6 kJ/mol being significantly lower than that with nZVI (42.8 kJ/mol). The acidic condition promoted a higher nZVI utilization rate, with the excess dosage of nZVI-Pd/NG ensuring a high nitrate removal rate for a wide pH range. This study demonstrates an improvement in nitrate reduction efficiency in a nZVI system by combining the exceptional properties of graphene and palladium.
... 1,5 In addition to TCE and PCE, less chlorinated analogues such as cis-dichloroethene (cis-DCE), 1,1-dichloroethene (1,1-DCE), and vinyl chloride (VC) have been frequently detected in the subsurface environment owing to incomplete biodegradation 6 or the formation of intermediates during abiotic transformation. 7,8 These complications have spurred continuous search for effective remediation approaches in the past three decades. ...
Recent studies on the use of controlled sulfur amendment to improve the reactivity and selectivity of zerovalent iron (ZVI) in reductive dechlorination reactions have generated a renewed interest in ZVI-based remediation materials. However, existing studies have focused on the reactions between trichloroethene (TCE) and lab-synthesized ZVI, the applicability of sulfidation to ZVIs with different material characteristics for reductive dechlorination of chloroethenes such as tetrachloroethene (PCE) and cis-dichloroethene (cis-DCE) have not been systematically examined. In this study, four ZVI materials from commercial sources having different size, morphological, and compositional characteristics were subject to various sulfidation treatments and were assessed in batch reactions with PCE, TCE, or cis-DCE. Sulfur-amendment induces modest increases in PCE degradation and steers reaction to a cleaner pathway that has minimum accumulation of partially dechlorinated intermediates. In the case of cis-DCE, bifurcating outcomes were observed that include enhancement effects for two high-purity ZVIs and inhibitory effects for two ZVIs possessing low levels of metal impurities. Further investigations based on controlled metal dosing reveal that the trace metals commonly present in cast iron or recycled metal scraps, such as Cu and Ni, can act as adventitious catalysts for cis-DCE reduction. Sulfidation results in poisoning of these catalytic ingredients and accounts for the adverse effect observed with a subset of ZVIs. Collectively, this study confirms enhanced degradation of highly chlorinated ethenes (PCE and TCE) by sulfidation of ZVIs from diverse origins; nonetheless, the effects of sulfidation can be highly variable for the lesser chlorinated ethenes due to differences in the material characteristics of ZVI and the predominant dechlorination pathways.
... In the case of inorganic contaminants such as U, Cr, and As, contaminant removal may be achieved through reductive precipitation or adsorption (e.g., Cantrell et al., 1995;Blowes et al., 1997Blowes et al., , 2000Powell et al., 1998;Fiedor et al., 1998;Lackovic et al., 2000;Morrison et al., 2001Morrison et al., , 2002. New insight regarding the reactive behavior of zero-valent iron and a more detailed understanding of reaction kinetics and reaction pathways involving zero-valent metals continues to emerge from laboratory studies (e.g., Burris et al., 1995;Agrawal and Tratnyek, 1996;Wüst et al., 1999;Deng et al., 1999;Su and Puls, 1999;Nam and Tratnyek, 2000;Schlicker et al., 2000;Arnold and Roberts, 2000;Farrell et al., 2000;Ruiz et al., 2000;Su and Puls, 2001;Melitas et al., 2001;Scherer et al., 1998;Lien and Wilkin, 2002;Alowitz and Scherer, 2002;Köber et al., 2002). ...
This publication has been produced as part of EPA's strategic long-term research plan. It is published and made available by EPA’s Office of Research and Development (ORD) to assist the user community and to link researchers with their clients. The purpose of this document is to provide detailed performance monitoring data on full-scale Permeable Reactive Barriers (PRBs) installed to treat contaminated ground water at two different sites.
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... These values were calculated based on an Arrhenius-type equation using the natural logarithm to the flux over the feed temperature. The apparent energy of activation for both evaporation of water and percrystallisation of nickel sulphate were both found to be in the range of diffusion-controlled reactions in water (Su and Puls, 1999;Mortimer et al., 2002;Pilling and Seakins, 1999). The apparent activation energy for the flux of water (15 kJ mol −1 ) is comparatively lower than what is found in work investigating pervaporation and membrane distillation processes which is reported to be in the range of 15 to above 50 kJ mol −1 (Feng and Huang, 1996;Sarti et al., 1985;Peng et al., 2005;Huang et al., 2014;Liang et al., 2015;Weschenfelder et al., 2015;Xu et al., 2016;Zhang and Wang, 2016). ...
This research reports on an investigation of the performance of inorganic membranes for use in the percrystallisation of nickel sulphate hydrate. In this novel process, the separation of the solvent (water) and the crystallised solute (nickel sulphate hydrate) occurs continuously in a single-step, avoiding further downstream processing (crystal filtering and drying). The inorganic membranes were synthesised with sucrose solution followed by a post vacuum-assisted impregnation of the coated film on a α-alumina substrate and carbonisation under nitrogen atmosphere. The highest fluxes measured were 22 L m ⁻² h ⁻¹ and 1 kg m ⁻² h ⁻¹ (40 g L ⁻¹ ) for water and nickel respectively. Interestingly, the transport of solution through the membrane also affected the hydration state of the nickel sulphate, as well as the crystal type and shape. High water fluxes delivered pure nickel sulphate heptahydrate with elongated and laminar crystal particles (~200 μm). Lower water fluxes produced both heptahydrate and hexahydrate salts with approximately spherical particles (also ~200 μm). There a number of factors that influence the crystallisation reaction such as the rate of evaporation which affects water availability and the resultant temperature at the permeate side of the membrane. Finally, the activation energy for nickel sulphate crystallisation was estimated to be approximately 16 kJ mol ⁻¹ based on feed solution temperatures.
... Based on the former studies, 6,12 the activation energy for silica dissolution is in the range of [42,96] kJ/mol. Normally, the activation energy of the diffusion and dissolution controlled reaction are below and above 21 kJ/ mol, 45,46 respectively. Hence the silica dissolution process in this study mainly depends on surface reactions. ...
This work aims to study the alkali‐silica reaction (ASR) with incorporated kinetic‐thermodynamic analysis. The model reactant tests were first conducted to study the silica dissolution in the simulated pore solution under different temperatures. Then, the kinetic model for silica dissolution was further improved by considering the influence of effective surface area. The improved kinetic model was also incorporated into the GEMS thermodynamic simulation. The model analysis demonstrated the silica dissolution rate is decreased mainly caused by reduced hydroxyl activity with increased saturation degree. The dissolved silica first reacted with the portlandite phase and formed the Calcium–Silicate–Hydrate (CSH) gel. The ASR gel can only be generated under high silicate ion concentration after the consumption of portlandite.
... Differences in the manufacturing processes of GI lead to differences in grain morphology as well as chemical and physical properties of the iron surface. The chemical nature of GI from various manufacturers has been investigated in previously reported laboratory tests [10][11][12]. These studies have sometimes included electrolytic iron (EI) as the reactive medium, for the purposes of limiting the investigation to a relatively simple metallic surface [13][14][15]. ...
To gain insight into the processes of transformations in zero-valent iron systems, electrolytic iron (EI) has been used as a surrogate for the commercial products actually used in barriers. This substitution facilitates mechanistic studies, but may not be fully representative of all the relevant processes at work in groundwater remediation. To address this concern, the kinetic iron model (KIM) was used to investigate sorption and reactivity differences between EI and Connelly brand GI, using TCE as a probe compound. It was observed that retardation factors (Rapp) for GI varied non-linearly with influent concentrations to the columns (Co), and declined significantly as GI aged. In contrast, Rapp values for EI were small and insensitive to Co, and changed minimally with iron aging. Moreover, although declines in the rate constants (k) and increases in the sorption coefficients were observed for both iron types, they were most pronounced in the case of EI. SEM scans of the EI surface before and after aging (90 days) established the appearance of carbon on the older surface. This work provides evidence that iron with a higher surface carbon content outperforms pure iron, suggesting that the carbon is actively involved in promoting TCE reduction.
We report on the potential of elevated groundwater temperatures and zero-valent iron permeable reactive barriers (ZVI PRBs), for example, through a combination with underground thermal energy storage (UTES), to achieve enhanced remediation of chlorinated hydrocarbon (CHC) contaminated groundwater. Building on earlier findings concerning deionized solutions, we created a database for mineralized groundwater based on temperature dependence of tetrachloroethylene (PCE) degradation using two popular ZVIs (i.e., Gotthart-Maier cast iron [GM] and ISPAT sponge iron [IS]) in column experiments at 25 °C-70 °C to establish a temperature-dependent ZVI PRB dimensioning approach. Scenario analysis revealed that a heated ZVI PRB system in a moderate temperature range up to 40 °C showed the greatest efficiency, with potential material savings of ~55% to 75%, compared to 10 °C, considering manageability and longevity. With a 25 °C-70 °C temperature increase, rate coefficients of PCE degradation increased from 0.4 ± 0.0 h-1 to 2.9 ± 2.2 h-1 (GM) and 0.1 ± 0.1 h-1 to 1.8 ± 0.0 h-1 (IS), while TCE rate coefficients increased from 0.6 ± 0.1 h-1 to 5.1 ± 3.9 h-1 at GM. Activation energies for PCE degradation yielded 32 kJ mol-1 (GM) and 56 kJ mol-1 (IS). Temperature-dependent anaerobic iron corrosion was key in regulating mineral precipitation and passivation of the iron surface as well as porosity reduction due to gas production.
The potential toxic and carcinogenic effects of chlorinated solvents in groundwater on human health and aquatic ecosystems require very effective remediation strategies of contaminated groundwater to achieve the low legal cleanup targets required. The transition zones between aquifers and bottom aquitards occur mainly in prograding alluvial fan geological contexts. Hence, they are very frequent from a hydrogeological point of view. The transition zone consists of numerous thin layers of fine to coarse-grained clastic fragments (e.g., medium sands and gravels), which alternate with fine-grained materials (clays and silts). When the transition zones are affected by DNAPL spills, free-phase pools accumulate on the less conductive layers. Owing to the low overall conductivity of this zone, the pools are very recalcitrant. Little field research has been done on transition zone remediation techniques. Injection of iron microparticles has the disadvantage of the limited accessibility of this reagent to reach the entire source of contamination. Biostimulation of indigenous microorganisms in the medium has the disadvantage that few of the microorganisms are capable of complete biodegradation to total mineralization of the parent contaminant and metabolites. A field pilot test was conducted at a site where a transition zone existed in which DNAPL pools of PCE had accumulated. In particular, the interface with the bottom aquitard was where PCE concentrations were the highest. In this pilot test, a combined strategy using ZVI in microparticles and biostimulation with lactate in the form of lactic acid was conducted. Throughout the test it was found that the interdependence of the coupled biotic and abiotic processes generated synergies between these processes. This resulted in a greater degradation of the PCE and its transformation products. With the combination of the two techniques, the mobilization of the contaminant source of PCE was extremely effective.
Metallic glasses (MGs) as effective catalysts have been extensively studied due to essentially disordered atomic configurations and widely adjustable micro-morphologies. The catalysis performance could be greatly promoted by introducing additional crystalline phases in the amorphous matrix due to the synergistic advantages of the crystalline and amorphous phases. However, the conventional casting and annealing approaches induced amorphous-crystalline (a/c) composites restrict the synergistic and galvanic cells effects because the generated crystalline phases are easily coarsened with meager a/c interfaces. Here, the artificial ultra-fine a/c Fe76Si8B13Nb3 catalyst with spinodal decomposition morphology and extremely high dense a/c interfaces of 2 × 10¹⁶ m⁻² are achieved from MG film precursor with nanoscale phase separation by controllable surface diffusion during deposition and suppressive crystalline coarsening procedures. The designed ultra-fine a/c catalyst exhibits admirable cycling degradation property and extraordinary dye degradation efficiency of 300 times than that of the commercial Fe powder. Especially, the outstanding catalytic performances of a/c composite are achieved without the additional involvement of hydrogen peroxide assistance, which provides an environmental-friendly neutral catalytic condition and avoids the corrosive damage during commercial sewage-treatment. This work provides a distinct perspective to design and regulate catalytic performances by amorphous precursor with pre-existent ultra-fine structures.
The effects of rising groundwater temperatures on zerovalent iron (ZVI)-based remediation techniques will be critical in accelerating chlorinated hydrocarbon (CHC) degradation and side reactions. Therefore, tetrachloroethylene (PCE) degradation with three ZVIs widely used in permeable reactive barriers (Gotthart-Maier cast iron [GM], Peerless cast iron [PL], and ISPAT sponge iron [IS]) was evaluated at 10-70 °C in deionized water. From 10 to 70 °C, PCE degradation half-lives decreased from 25 ± 2 to 0.9 ± 0.1 h (PL), 24 ± 3 to 0.7 ± 0.1 h (GM), and 2.5 ± 0.01 to 0.3 ± 0.005 h (IS). Trichloroethylene (TCE) degradation half-lives at PL and GM decreased from 14.3 ± 3 to 0.2 ± 0.1 h (PL) and 7.6 ± 2 to 0.4 ± 0.1 h (GM). This acceleration of CHC degradation and the stronger shift toward reductive β-elimination reduced the concentration of potentially harmful metabolites with increasing temperatures. PCE and TCE degradation yields an activation energy of 28 (IS), 58 and 40 kJ mol-1 (GM), and 62 and 53 kJ mol-1 (PL). Hydrogen gas production by ZVI corrosion increased by 3 orders of magnitude from 10 to 70 °C, and an increased chance of gas clogging was observed at high temperatures.
A large volume of highly concentrated magnesium salt solution is produced during magnesium flue gas desulfurization. Existing method of concentration recovery is relatively inefficient and uneconomical. To address this problem, a hydrothermal crystallization method was proposed to recover the by-products after oxidation. It was found that 80% of the salt solute was recovered in the form of MgSO4·H2O under the following conditions: an initial magnesium concentration of 6.7%, temperature of 180 °C, and holding time of 90 min. The apparent activation energy during crystallization was 186.7 kJ mol⁻¹, indicating that the rate was controlled by a chemical reaction. In terms of microscopic properties, fewer water molecules within the clusters and stronger associations were crucial for the recovery of kieserite instead of epsomite. The nucleation of the magnesium sulfate salt may be attributed to the hydration inhibition in solvent-separated ion pairs and the transformation to contact-associated ion pairs. More importantly, the recovered products could be reused when calcined at 1150 °C for 60 min. Their chemical reactivity was satisfactory. High salt rejection, good water saving, and lower energy consumption are expected study results.
Modern day worldwide industrial activities have negatively affected the environment due to daily pollutant accumulation. Evidence of this is seen in the heavy metal deposits found in water resources from mining, well drilling, electroplating, and heavy metals production. Mercury and cadmium ions are among the most hazardous heavy metals of significant concern. The removal of these two toxic metals from wastewater requires effective, low-cost techniques, and, therefore, this current study looks at the design and fabrication of SnO2-formaldehyde-chitosan to facilitate the rapid microwave-assisted elimination of Hg(II) and Cd(II) ions. This paper explores the microwave-assisted removal of Hg(II) and Cd(II) ions, a highly efficient process which establishes 1050 μmol g⁻¹ and 600 μmol g⁻¹ capacity values at pH 6, at 10 s and 15 s, respectively. The application of SnO2-formaldehyde-chitosan for the microwave-assisted elimination of Hg(II) and Cd(II) ions from natural water and wastewater is also explored. In this respect, the microwave-assisted removal values of Cd(II) achieved were 95.6%, and 99.2% from tap water and wastewater, respectively. In addition, Hg(II) removal percentages were found to be 94.2% and 98.4% from the same two water samples.
Compared to traditional haber-bosch process for synthetic ammonia, electrochemical reduction of nitrate to ammonia (ERNA) offers a promising and sustainable technology to generate ammonia at ambient temperature. However, the performance of ERNA is impeded by lacking of sound strategy for designing high-performance electrocatalyst. Here, a Cu based electrode with Cu2O/Cu interface was prepared by pulse electrodeposition and electroreduction, achieving the fast rate constant of 0.14 min⁻¹ for reducing nitrate with a high ammonia yield rate of 2.17 mg cm⁻² h⁻¹ (faradaic efficiency: 84.36%) and ammonia selectivity of 94.4% at -0.25 V vs. RHE. Particularly, density functional theory (DFT) calculations for the adsorption energy indicated that Cu2O/Cu interface alleviated adsorption energy of NO2⁻ from -2.02 eV (pure Cu) to -1.59 eV, improving surface diffusion of adsorbed NO2⁻ . And DFT calculations for electronic structure implied that Cu2O/Cu interface upshifted the d band center of Cu (-2.25 eV vs. -2.48 eV of pure Cu), boosting electron transfer to NO3⁻ . Additionally, in-situ infrared spectroscopic analysis confirmed vital intermediates of NO2⁻ and NH2OH on Cu2O/Cu to identify reaction pathway and Gibbs free energy diagram clarified this interface decreased reaction barrier of crucial step (reducing *NO2 to *NO) from 0.31 eV (pure Cu) to -0.77 eV for the enhancement of ERNA. Thereby, these findings demonstrated that Cu2O/Cu interface tunes mobility of NO2⁻ and electron transfer to NO3⁻ on Cu based electrocatalyst is an efficient method to promote performance of ERNA with high efficiency and rate.
Increasing groundwater temperatures caused by global warming, subsurface infrastructure, or heat storage projects may interfere with groundwater remediation techniques using zero-valent iron (ZVI) technology by accelerating anaerobic corrosion. The corrosion behavior of three ZVIs widely used in permeable reactive barriers (PRBs), Peerless cast iron (PL), Gotthart-Maier cast iron (GM), and an ISPAT iron sponge (IS), was investigated at temperatures between 25 and 70 °C in half-open batch reactors by measuring the volume of hydrogen gas generated. Initially, the corrosion rates of all tested ZVIs increased with temperature; at temperatures ≤40 °C, a material-specific steady state is reached, and at temperatures >40 °C, passivation causes a decrease in long-term corrosion rates. The observed corrosion behavior was therefore assumed to be superimposed by accelerating and inhibiting effects, caused by surface precipitates where the fitting of measured corrosion rates by a modeling approach, using the corroded amount of Fe0 to account for passivating minerals, yields intrinsic activation energies (Ea, ZVI) of 81, 90, and 107 kJ mol-1 for IS, GM, and PL, respectively. An increase in H2 production might not be directly transferable to an increase in general ZVI reactivity; however, the results suggest that an increase in chlorinated hydrocarbon degradation rates can be expected for ZVI-PRBs in the immediate vicinity of low-temperature underground thermal energy storages (UTESs) or in the impact areas of high-temperature UTES with temperatures of ≤40 °C.
Nano zero-valent metals (nZVM) widely used as an adsorbent for heavy metal remediation from water. However, the disadvantages of nano zero-valent iron (nZVI) are low stability, rapid passivation, and limited mobility. Therefore, the present study aimed to synthesize stable zero-valent Sn nanoparticles(zero-valent Sn NPS) without any surface protection for water remediation of lead (II). The size of zero-valent Sn NPS were14−39 nm, with high thermal stability. The maximum metal capacity obtained from this experiment was 5300 μmol/g at 20 s under pH 7 via microwave adsorption technique, the kinetic and adsorption isotherm study supported with the Akaike Information Criterion (AIC). Kinetic study shows that zero-valent Sn NPS are undergoing pseudo-second-order, which confirms that the adsorption reaction is a chemisorption reaction. Also, adsorption isotherm data show that the adsorption process obeys the Freundlich model, which confirms the formation of a multilayer of Pb(II) over the nano sorbent surface.
Trichloroethylene (TCE) is a human carcinogen that is commonly found in landfill leachate. Contaminated leachate plumes may be intercepted prior to reaching groundwater and treated in situ using permeable reactive barriers (PRB). This study used a packed column system containing herbal pomace and spruce biochar, previously shown to have TCE adsorptive capabilities. Influent containing raw or autoclaved landfill leachate was used to investigate the potential for environmental micro-organisms to establish a TCE-dechlorinating biofilm on the biochar, in order to prolong the operational life span of the system. TCE removal ≥ 99.7 % was observed by both biochars. No dichloroethylene (DCE) isomers were present in the column effluents, but cis-1,2 DCE was adsorbed to the biochar treating raw landfill leachate, indicating that dechlorination was occurring biologically in these columns. Known microbial species that are individually capable of complete dechlorination of TCE to ethene were not detected by 16S rRNA gene sequencing, but several species capable of partial TCE dechlorination (Desulfitobacterium spp., Sulfurospirillium spp. and Desulfuromonas spp) were present in the biofilms of the columns treating raw landfill leachate. These data demonstrate that biochar from waste material may be capable of supporting a dechlorinating biofilm to promote bioremediation of TCE.
Chlorinated solvents are extensively used in many activities and hence in the past decades impacted a large number of sites. The presence of these contaminants in groundwater is challenging particularly for the management of the vapor intrusion pathway. In this work we examine the potential feasibility of using horizontal permeable reactive barriers (HPRBs) placed in the unsaturated zone to treat chlorinated solvent vapors emitted from groundwater. Zero-valent iron (ZVI) powders, partially saturated with water and characterized by different specific surface areas (SSA), were tested, alone or mixed with sand, in lab-scale batch reactors using TCE as model compound. Depending on the type of iron powder used, a reduction of TCE concentration in the vapor phase from approximately 35% up to 99% was observed after 3 weeks of treatment. The best performance in terms of TCE reduction was obtained using the ZVI characterized by the intermediated values of the specific surface area (SSA). This finding, which is in contrast with the results generally observed in in aqueous solutions, was tentatively attributed to a non-selective higher reactivity of the fine-grained iron samples with water and dissolved oxygen (with a consequent iron passivation) or to the occurrence of a diffusion-limited reaction kinetics. Based on the first-order kinetic degradation rate constants estimated from the experimental data, a horizontal barrier of 1 m containing ZVI or a mixture of ZVI and sand can potentially lead to an attenuation of TCE vapors over 99%.
The nano-sized zero valent iron assisted biochar from hazelnut shell ([email protected]) was prepared and assessed for the feasibility as the binding agent in diffusive gradients in thin-films (DGT) technique. The 1.5% agarose solution containing the optimal [email protected] dose of 15 g L⁻¹ was used to prepare the [email protected] binding gel which owned a high capacity (1010 ± 50 μg disc⁻¹) and a rapid uptake within 30 min. The elution efficiency of phenol from the loaded binding gel was up to 99.3% using the mixture of 1% hydroxylamine hydrochloride and 0.05 mol L⁻¹ HCl. The phenol uptake of [email protected] increased linearly with the increase of deployment time (R² = 0.9938) and was in accord with the theoretical values from DGT equation, while there was no notable interference of the sample matrixes on the phenol uptake of [email protected] in the spiked freshwaters. The good performance of [email protected] was found under a range of pH (4.1–10.2), ionic strength (as pNaNO3) (0.155–4), and dissolved organic matter up to 20 mg L⁻¹. In field, the monitoring of [email protected] was more representative than the results from the grab-sampling with better precision and lower sampling frequency, which can provide reliable information, reduce the cost of human resources, and improve efficiency. These illustrate that the [email protected] is more suitable as the binding agent of DGT for uptake of phenol and [email protected] is an effective tool to monitor in-situ phenol in waters.
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.
دی نیترو تولوئن (DNT) ، متا تولوئن دی آمین (MTD) و ایزومرهای آنها، دلیل عمده آلودگی پساب¬های کارخانجات تولید تولوئن دی ایزوسیانات می¬باشد. در این مطالعه تاثیر فرایند اکسیداسیون پیشرفته با استفاده از سیستم سه جزئی Fe0/Persulfate/H2O2 برای تجزیه DNT و MTD و مشتقات آنها ارزیابی شده است. تاثیر افزایش همزمان سیستم سه جزئی Fe0/Persulfate/H2O2 بر روی حذف COD مورد اندازه گیری قرار گرفت. در طی 32 آنالیز صورت گرفته، تاثیر PH ، غلظت Fe0 ، H2O2 و آمونیوم پرسولفات بر روی پساب پتروشیمی کارون اندازه گیری شد.
Electrochemical transformation of harmful tetrachloroethylene (PCE) is evaluated as a method for management of groundwater plumes to protect the drinking water resource, its consumers and the environment. In contrast to previous work that reported transformation of trichloroethylene, a byproduct of PCE, this work focuses on transformation of PCE in a saturated porous matrix and the influence of design parameters on the removal performance. Design parameters investigated were electrode configuration, catalyst load, electrode spacing, current intensity, orientation of reactor and flow through a porous matrix. A removal of 86% was reached in the fully liquid-filled, horizontally oriented reactor at a current of 120 mA across a cathode → bipolar electrode → anode arrangement with a Darcy velocity of 0.03 cm/min (150 m/yr). The palladium load on the cathode significantly influenced the removal. Enhanced removal was observed with increased electrode spacing. Presence of an inert porous matrix improved PCE removal by 9%-point compared to a completely liquid-filled reactor. Normalization of the data indicated, that a higher charge transfer per contaminant mass is required for removal of low PCE concentrations. No chlorinated intermediates were formed. The results suggest, that PCE can be electrochemically transformed in reactor designs replicating that of a potential field-implementation. Further work is required to better understand the reduction and oxidation processes established and the parameters influencing such. This knowledge is essential for optimization towards testing in complex conditions and variations of contaminated sites.
Sulfide-modified nanoscale zero-valent iron (S-nZVI) is a promising material for removal of organic pollutants from water, but S-nZVI nanoparticles (NPs) easily agglomerate and have poor contact with organic contaminants. Herein, we propose a new S-nZVI/graphene aerogel (S-nZVI/GA) composite which exhibits superior removal capability for trichloroethylene (TCE) from water. Three-dimensional porous graphene aerogel (GA) can improve the efficiency of electron transport, enhance the adsorption of organic pollutants and restrain the agglomeration of the core-shell S-nZVI NPs. The TCE removal rates of FeS, nZVI, GA and S-nZVI were 27.8%, 42%, 63% and 75% in 2 hr, respectively. Furthermore, TCE was completely removed within 50 min by S-nZVI/GA. The TCE removal rate increased with increasing pH and temperature, and TCE removal followed the pseudo-first-order kinetic model. The results demonstrate the great potential of S-nZVI/GA composite as a low-cost, easily separated and superior monolithic adsorbent for removal of organic pollutants.
Polyoxymethylene dimethyl ethers (PODE) was blended in diesel at volume ratios of 0 %, 10 %, 20 %, and 30 % (denoted as P0, P10, P20, and P30). The experimental study was carried on an unmodified YD480Q diesel engine. An engine exhaust particle sizer was introduced to analyze the particulate matter (PM) concentration and particle size distribution of diesel engine emission. The evaporation-oxidation characteristics of the PODE/diesel blends and the effect of the fuel blends on the oxidative activity of PM were investigated by thermogravimetric analysis and Arrhenius theorem. The results showed that blending PODE in diesel improved the evaporation-oxidation characteristics of the fuel and decreased the apparent activation energy of the fuel blends. PODE played a positive role in reducing PM emissions. The particle total number concentration of P30 decreased 28.29 ~ 66.60 % and the particle total volume density decreased 54.16 ~ 80.06 % compared to diesel. The particle size distribution shifted to a smaller particle size as the PODE blending ratio was increased. The mass fraction of the volatile substances (VS) increased and the mass fraction of the dry soot (DS) decreased by employing PODE as a diesel additive. Also, the oxidation activity of VS increased as the PODE blending ratio was increased. The oxidation activity of DS climbed to the peak when the PODE blending ratio was 20 % and then decreased.
A novel two-step reduction process was constructed to removal nitrate selectively in aqueous solution. Nitrate was preliminary reduced into nitrite with low yield of ammonia by micro-scale Cu/Fe bimetal and then the accumulated nitrite was converted into N2 by sodium sulfite. Cu was uniformly dispersed on the surface of Fe by a simple displacement reaction. Role of major iron components and fate of nitrogen species were investigated. The selectivity for N2 was over 90％ and the yield of ammonia was below 10%. In the system, accumulation of nitrite resulted from the lower reduction rate of nitrite by Fe2+ in solution and the Cu/Fe coated by oxidized iron layer than the generation rate of nitrite via the reduction of nitrate. The generation of ammonia was caused by the reduction of nitrate/nitrite by Fe2+ adsorbed on the surface of oxidized iron layer coated on the bimetal at low reaction rate.
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.
This study provides a twenty-two-year record of in situ degradation of chlorinated organic compounds by a granular iron permeable reactive barrier (PRB). Groundwater concentrations of trichloroethene (TCE) entering the PRB were as high as 10,670 µg/L. Treatment efficiency ranged from 81 to >99% and TCE concentrations from <1 µg/L to 165 µg/L were detected within and hydraulically down-gradient of the PRB. After 18 years, effluent TCE concentrations were above the maximum contaminant level (MCL) along segments of the PRB exhibiting upward trending influent TCE. Degradation products included cis-dichloroethene (cis-DCE), vinyl chloride (VC), ethene, ethane, >C4 compounds, and possibly CO2(aq) and methane. Abiotic patterns of TCE degradation were indicated by compound-specific stable isotope data and the distribution of degradation products. δ13C values of methane within and down-gradient of the PRB varied widely from -94‰ to -16‰; these values cover most of the isotopic range encountered in natural methanogenic systems. Methanogenesis is a sink for inorganic carbon in zero-valent iron PRBs that competes with carbonate mineralization and this process is important for understanding pore-space clogging and longevity of iron-based PRBs. The carbon isotope signatures of methane and inorganic carbon were consistent with open-system behavior and 22% molar conversion of CO2(aq) to methane.
Degradation of ornidazole (ONZ) by nanoscale zero-valent iron (nZVI) particles was investigated for the first time in this work. The results showed that ONZ was almost completely degraded within 30 min by 0.1 g L⁻¹ nZVI at pH 5.8 and 25 °C. The effects of the nZVI dose, initial ONZ concentration, pH, and temperature on ONZ removal were systematically investigated, and removal of ONZ was followed by a pseudo-first-order kinetics model. Experimental results demonstrated that higher nZVI doses, lower initial ONZ concentrations, and lower pH levels could increase the pseudo-first-order rate constant (kobs) of ONZ removal. While higher temperatures favored removal, the activation energy results suggested that mass transfer was the limiting step during the removal process. The possible effect of oxygen was ruled out by introducing hydroxyl radical scavengers into the experiment. The variation of ONZ concentrations and total organic carbon (TOC) contents in the solution indicated that adsorption was not the main mechanism. The possibility that precipitation was the main mechanism was also excluded by the results for the change in pH and effect of pH. The characterization of nZVI before and after the reaction indicated that ONZ was reduced on the surface of nZVI, which was the main mechanism. Three intermediates and two final products were detected based on the results of UV-vis and high performance liquid chromatography/mass spectrometry (HPLC-MS) analyses. Dechlorination, nitro reduction, N-denitration, and cleavage were all involved in the entire reaction process, and therefore a complicated potential degradation pathway was proposed.
A sequential chem-bio hybrid process was developed using a novel biochar supported carboxymethyl cellulose-stabilized nanoscale iron sulfide ([email protected]) as a chemical remover and Corynebacterium variabile HRJ4 as a biological agent for trichloroethylene (TCE) degradation. Compared with CMC-FeS, [email protected], bare FeS and biochar600, the [email protected] composite displayed better physiochemical properties (smaller hydrodynamic diameter and higher stability) and demonstrated excellent removal capacity for TCE from aqueous phase. A facultative bacterial strain, Corynebacterium variabile HRJ4, growing well in the presence of [email protected] (added up to 0.25 g L⁻¹), further enhanced TCE removal after chemical treatment. The dechlorination pathway proposed based on the gas chromatography–mass spectrometry (GC-MS) analysis revealed that TCE was dechlorinated to cis-1,2-dichloroethene (cis-DCE) and acetylene via hydrogenolysis and β-elimination, respectively within 12 h by [email protected] Addition of HRJ4 strain into the reaction system effectively enhanced the degradation of the residual TCE, cis-DCE and acetylene to ethylene. Acetylene was the main product in chemical process, whereas ethylene was the main product in biological process as strain HRJ4 could reduce acetylene to ethylene effectively. The results of this study signify the potential application of [email protected]/HRJ4 chem-bio hybrid system for complete degradation of TCE in the anaerobic environment.
The emission tests were performed on a light-duty direct injection diesel engine. A polyoxymethylene dimethyl ethers (PODE) mixture was blended with diesel at a volume ratio of 0, 10, 20 and 30%, denoted as P0, P10, P20 and P30, respectively. The particle size distribution before and after the diesel particulate filter (DPF) was measured to evaluate the DPF filtering efficiency of various modal particles. The oxidation activity of the particles on the DPF intake end plane was analyzed by the Arrhenius method. The regeneration of the DPF was conducted using a non-thermal plasma (NTP) injection system. The results showed that blending PODE with diesel contributed to reducing the particle number concentrations. PODE adversely affected the improvement of the DPF filtering efficiency, especially that of P30. However, the DPF filtering efficiency of all fuels was still higher than 94%. Blending PODE with diesel increased the mass fraction of volatile substances (VS) and decreased the mass fraction of dry soot. Particles of P20 showed a better oxidation activity with lower apparent activation energy. In addition, PODE increased the DPF regeneration effect by NTP technology. The deposit removal mass of the DPF rose to the peak level and then decreased as the PODE blending ratio increased. The better DPF regeneration effect was observed when P20 was employed.
The reduction of trichloroethylene (TCE) in gas phase by different types of granular zero-valent iron (Fe0) was examined in anaerobic batch vapor systems performed at room temperature. Concentrations of TCE and byproducts were determined at discrete time intervals by analysis of the headspace vapors. Depending on the type of iron used, reductions of TCE gas concentration from 35% up to 99% were observed for treatments of 6 weeks. In line with other experimental studies performed with aqueous solutions, the particle size was found to play a key role in the reactivity of the iron. Namely an increase of the TCE removal up to almost 3 times was observed using iron powders with particle size lower than 425 μm compared to iron powders with particle size lower than 850 μm. The manufacturing process of the iron powder was instead found to play only a limited role. Namely, no significant differences were observed in the TCE reduction by Fe0 obtained using an iron powder attained by water atomization and sieving compared to the removal achieved using an iron powder subjected to a further annealing processes to reduce the content of oxides. Conversely, the pretreatment of the iron powder with HCl was found to enhance the reactivity of the iron. In particular, by washing the iron powder of 425 μm with HCl acid 0.1 M the reduction of TCE after 6 weeks of treatment increase from approximately 80% for the as received material to >99% for the pretreated iron powder. We also performed tests at different humidity of the iron observing that not statistical differences were obtained using a water content of 10% or 50% by weight. In all the experiments, the only detectable byproducts of the reactions were C4-C6 alkenes and alkanes that can be attributed to a hydrogenation of the CCl bond.
The kinetics of the catalytic polymerization of pyrene monomer to pyrene pitch in the presence of aluminum trichloride (AlCl3) were studied at temperatures between 330 and 370 °C in a 1-L batch reactor. Both reactants and reaction products were isolated from the bulk pitch using supercritical extraction (SCE), so that they could be quantified. Concentrations of monomer, dimer, and trimer were measured at discrete reaction times, and each was found to exhibit first-order reaction behavior. Analysis of the oligomerization reaction behavior at differing temperatures enabled the calculation of apparent activation energies and pre-exponential factors for each reaction. Relatively low activation energies that decreased with increasing oligomer size were obtained, suggesting that the reactions are mass-transfer-limited. The microkinetic model that was developed can be used to estimate the reaction time for which the formation of liquid crystalline, mesophase-forming pyrene trimer is maximized. Other groups have developed models for bulk mesophase formation, but this is the first time that reaction kinetics have been measured for individual oligomers in a carbonaceous pitch formed from pure polycyclic aromatic hydrocarbons (PAHs) or mixtures thereof.
Column studies were conducted to investigate the influence of benzene or toluene on the dechlorination of perchloroethene (PCE) and trichloroethene (TCE) in columns packed with zerovalent iron (ZVI) in order to simulate a permeable reactive barrier (PRB). Enhancive and inhibitive influences of benzene and toluene, respectively, on PCE and TCE reduction were observed within 10- 80 pore volumes (PV) that flowed through the columns. However, such influences dissipated when the flow-through volume increased above 80 PV. The presence of benzene increased the mean dechlorination kobs of PCE and TCE by 7% and 6%, respectively; in contrast, the presence of toluene decreased the mean dechlorination kobs of PCE and TCE by 21% and 10%, respectively. We presumed that the more competitive adsorption between benzene and toluene in comparison to PCE and TCE on the ZVI particle surface might have caused the disparate influences. With a lower affinity for ZVI, benzene has no substantial influence on PCE and TCE adsorption on the ZVI particle surface. However, toluene has a higher affinity for ZVI and could compete with PCE and TCE by contacting the ZVI particle surface. Moreover, given benzene’s higher polarity, it could also benefit electron transfer from ZVI to PCE and TCE.
Dechlorination of chlorinated aliphatic hydrocarbons (CAHs) by zero-valent iron (Fe⁰) was found to be influenced by the competitive effects exerted by other groundwater contaminants. Laboratory column study of the competitive effects on CAH dechlorination by Fe⁰ indicated that the presence of 1,1,1-trichloroethane (1,1,1-TCA) in the trichloroethylene (TCE)-contaminated groundwater could decrease the normalized dechlorination rate constant (kSA) of TCE from 3.04 × 10⁻² to 2.74 × 10⁻² mL m⁻² hr⁻¹. In a similar fashion, introduction of chloroform (TCM) into the synthetic groundwater containing TCE and 1,1,1-TCA led to a 40 to 54% drop in TCE and 1,1,1-TCA kSA, thus indicating competition among TCE, 1,1,1-TCA and TCM during dechlorination reactions induced by Fe⁰. Activation energy ranging from 34.3 to 53.7 kJ/mol for the simultaneous dechlorination of TCE, 1,1,1-TCA and TCM by Fe⁰ showed that the process of the electron transfer from Fe⁰ to the CAHs is the dominant step limiting the rate of the dechlorination reactions so that the electron released from Fe⁰ is most likely in competition with TCE, 1,1,1-TCA and TCM during the dechlorination reactions. In addition to CAHs, abiotic reduction of hexavalent chromium [Cr(VI)] by Fe⁰ also exerted effects on TCE dechlorination leading to a 31% drop in TCE kSA after the addition of Cr(VI) into the TCE-contaminated groundwater. Groundwater geochemical factors such as alkalinity and contaminant concentration could potentially influence competition among TCE, 1,1,1-TCA, TCM and Cr(VI) during the abiotic reduction of chemical substances by Fe⁰. © 2007 by the American Society of Civil Engineers. All Rights Reserved.
Recently, increasing efforts have been made to explore the applicability and limitations of zero-valent iron(Fe⁰) for the treatment of arsenic-bearing groundwater and wastewater. The experimental studies have demonstrated that arsenate, arsenite, and arsinilic acid are removed effectively from aqueous solution using Fe⁰. Arsenic removal efficacy depends on the type of Fe⁰, surface area and corrosion rate of the Fe⁰, and water chemistry. Enhanced corrosion of Fe⁰ in the presence of dissolved oxygen leads to an increase in surface area of the Fe⁰ from pitting and neoformation of iron oxides and green rusts, which leads to greater arsenic removal. The iron-arsenic precipitates produced in the Fe⁰ system seem to be stable in solution and the precipitation process seems to be irreversible. Toxicity Characteristic Leaching Procedure (TCLP) analysis of the spent media has shown that the arsenic concentration in the leachate is well below the 5 mg/L threshold for arsenic. Virtually all studies have demonstrated that Fe⁰ is an effective sorbent for arsenic, implying that Fe⁰ may be used as a medium in passive chemical permeable reactive barrier (PRB) technologies to immobilize arsenic from groundwater. Possible mechanisms for arsenic removal in the Fe⁰ systems include surface adsorption, precipitation, co-precipitation, and redox transformation. Although experimental results are encouraging so far, there are still various gaps in the knowledge pertaining to detailed mechanisms of arsenic removal, to long-term stability of sequestered arsenic in the Fe⁰ system, and to field-scale application of Fe⁰. The influence of microbial activities in the presence of Fe⁰ on arsenic fate and transport is not well understood. Further studies are needed to improve the technology for both in situ and ex situ arsenic removal from contaminated water. © 2007 by the American Society of Civil Engineers. All Rights Reserved.
This chapter focuses on the heterogeneous catalysis of solution reactions and discusses various steps that can be rate-determining in heterogeneously catalyzed solution reactions. These mechanisms can be distinguished in practice by the resulting kinetic behavior and by other means that are described in the chapter. The chapter also discusses general stoichiometric and thermodynamic aspects and the specific types of catalyzed reaction (substitution, isomerization, and redox) that have been studied. It focuses on catalysis at the solid/liquid interface. In surface-controlled catalyses, the rate-determining step involves the reaction on the surface of an adsorbed reactant or of a derived species. It is convenient to divide surface reactions in solution into four main categories: unimolecular, racemisation and isotopic exchange, bimolecular, and electron transfer. The chapter also describes kinetic and mechanistic studies that have been carried out on the various types of catalyzed solution reactions.
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.
The recent literature on the kinetics of water-rock interactions is reviewed. The data are then extended to provide a quantitative framework for the description of weathering and alteration. The available experimental data on dissolution of silicates verifies quantitatively the usual mineral stability series in sedimentary petrology. The rate of hydration of carbonic acid is shown to be a possible limiting factor in water-rock interactions. The framework is developed to enable use of laboratory dissolution experimental results and thermodynamics to arrive at a rate law applicable up to equilibrium and therefore applicable to natural systems. The kinetic justification for the significance of a water-rock ratio is discussed. With a proper treatment of fluid flow, the equations are applied to the weathering profile leading to the development of bauxites from nepheline syenites.
Flow-through column tests were conducted to investigate the products of degradation of aqueous trichloroethene (TCE) in contact with granular iron metal. The results indicated the degradation process to be pseudo-first-order and the rate constant to be relatively insensitive to the initial concentration of TCE over the range from about 1.3 to 61 mg/L. The principal degradation product was ethene, followed by ethane with substantially smaller amounts of other C1−C4 hydrocarbons. About 3.0−3.5% of the initial TCE appeared as chlorinated degradation products, including the three dichloroethene isomers and vinyl chloride. Although the chloride mass balance was generally between 98 and 102%, a maximum of 73% of the carbon could be accounted for in the identified products. Based on the low concentrations of chlorinated degradation products in the solution phase, it is proposed that most of the TCE remains sorbed to the iron surface until complete dechlorination is achieved.
To gain perspective and insight into the performance of permeable reactive barriers contg. granular iron metal, it is useful to compare the degrdn. kinetics of individual chlorinated solvents over a range of operating conditions. Pseudo 1st-order disappearance rate consts. normalized to iron surface area concn. (kSA) recently have been reported for this purpose. This paper presents the results of further exploratory data anal. showing the extent to which variation in kSA is due to initial halocarbon concn., iron type, and other factors. To aid in preliminary design calcns., representative values of kSA and a reactive transport model have been used to calc. the min. barrier width needed for different groundwater flow velocities and degrees of halocarbon conversion. Complete dechlorination of all degrdn. intermediates requires a wider treatment zone, but the effect is not simply additive because degrdn. occurs by sequential and parallel reaction pathways. [on SciFinder(R)]
A method using ion chromatography (IC) for the analysis of ferrous (Fe ) and ferric (Fe ) ions in soil extracts has been developed. This method uses an ion exchange column with detection at 520 nm after post-column derivatization. Selectivity is achieved by using an anionic chelating agent, pyridine-2,6-dicarboxylic acid, to form anionic complexes with ferric and ferrous ions. Using this method, both ferric and ferrous ions can be analyzed directly and simultaneously. The soil extractions were carried out with both 0.5 M hydrochloric acid (HCl) and 0.36 M oxalate under anaerobic conditions. Ferric and ferrous ions were stable in either 0.5 M HCl solution or deionized water at pH ∼ 1.7 when stored outside of a glove box. Ferrous ion was readily oxidized to ferric ion in aqueous solutions at pH values above 4.0 and in 0.36 M oxalate solutions at all pH values.In addition, we note that ferric ion in 0.5 M HCl was detectably reduced to ferrous ion when stored for 24 hours in a glove box containing a few percent of hydrogen gas. The study indicated that there was no interference in the HPLC analysis from the common cations Ca, Mg and Al.
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.
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 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.
Trichloroethene (TCE) was reduced with zero-valence iron and palladized iron in zero-head-space extractors. Progress of the reaction in these batch studies was monitored with purge-and-trap gas chromatography and a flame ionization detector. When a 5 ppm initial concentration of TCF. reacts with zero-valence iron, approximately 140 ppb of vinyl chloride persists for as long as 73 days. The concentration of vinyl chloride (approximately If) ppb) remaining with palladized iron is approximately an order of magnitude less than when zero-valence iron is the reductant. These data suggest that volatile byproducts may be under-represented in oilier published data regarding reduction with zero-valence metals. These results also demonstrate that the reduction of TCE with palladized iron (0.05 percent palladium) is more than an order of magnitude faster than with zero-valence iron. Wilh a 5:1 solution-to-solid ratio the TCE half-life with zero-valence iron is 7.41 hours. but is only 0.59 hours with the palladized iron.
Laboratory tests were conducted to examine zero-valent iron as an enhancing agent in the dehalogenation of 14 chlorinated methanes, ethanes, and ethenes. All compounds were tested by batch procedures in which 10 g of 100-mesh electrolytic iron was added to 40 ml hypovials. Aqueous solutions of the respective compounds were added to the hypovials, and the decline in concentration was monitored over time. Substantial rates of degradation were observed for all compounds tested with the exception of dichloromethane. The degradation process appeared to be pseudo first-order with respect to the organic compound, with the rate constant appearing to be directly proportional to the surface area to volume ratio and increasing with increasing degree of chlorination. Column tests showed the process to proceed under flow conditions with degradation rates indpendent of velocity and consistent with those measured in the batch tests. When normalized to 1 m2/ml, the t50 values ranged from 0.013 to 20 hr, and were about 5 to 15 orders of magnitude lower than values reported for natural rates of abiotic degradation. The results indicate abiotic reductive dechlorination, with iron serving as the source of electrons; the mechanism is, however, uncertain. Based on the rapid rates of degradation, both in situ and aboveground applications for remediation of contaminated ground water are proposed.
The use of granular iron for in situ degradation of dissolved chlorinated organic compounds is rapidly gaining acceptance as a cost-effective technology for ground water remediation. This paper describes the first field demonstration of the technology, and is of particular importance since it provides the longest available record of performance (five years). A mixture of 22% granular iron and 78% sand was installed as a permeable “wall” across the path of a contaminant plume at Canadian Forces Base, Borden, Ontario. The major contaminants were trichloroethene (TCE, 268 mg/L) and tetrachloroethene (PCE, 58 mg/L). Approximately 90% of the TCE and 86% of the PCE were removed by reductive dechlorination within the wall, with no measurable decrease in performance over the five year duration of the test. Though about 1% of the influent TCE and PCE appeared as dichloroethene isomers as a consequence of the dechlorination of TCE and PCE, these also degraded within the iron-sand mixture. Performance of the field installation was reasonably consistent with the results of laboratory column studies conducted to simulate the field behavior. However, if a more reactive iron material, or a higher percentage of iron had been used, complete removal of the chlorinated compounds might have been achieved. Changes in water chemistry indicated that calcium carbonate was precipitating within the reactive material; however, the trace amount of precipitate detected in core samples collected four years after installation of the wall suggest that the observed performance should persist for at least another five years. The study provides strong evidence that in situ use of granular iron could provide a long-term, low-maintenance cost solution for many ground water contamination problems.
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.
Tetrachloroethylene was transformed by iron powder (4.1g/L) in oxygen-free, HEPES-buffered (pH 7) water at 50°C with a half-life of 20 days. The only products observed were the reactive intermediate, trichloroethylene, and ethene and ethane. 1,1,1-Trichloroethane, 1,1-dichloroethylene, and tetrachloroethylene were transformed by iron at room temperature in both autoclaved buffered water and in two non-autoclaved landfill leachates. The pattern and degree of removal were similar in all cases. Dichloromethane, 1,1-dichloroethane, and 1,4-dichlorobenzene were also tested, but were not removed from any of the systems. If manganese rather than iron was used, the substrates transformed depended upon the aqueous phase. Some biological transformations were seen in Leachate 2, but the activity was reduced by manganese and completely suppressed by iron.
As a possible method for degrading chlorocarbons in contaminated water supplies, the reactions of metallic magnesium, tin, and zinc with CCl4/H2O mixtures have been studied, in the case of Mg, oxidation by water overwhelmed the Mg-CCl4 reaction. However, Sn and Zn were successfully used to degrade CCl4. Major products in the Sn/CCl4/dH(2)O system were CO2, CHCl3, SnO2, and HCl with smaller amounts of CHCl3 and CH2Cl2. In the case of Zn/CCl4/H2O, the major products were ZnCl2, Zn(OH)(2), and CH4 with CHCl3, CH2Cl2 and CH3Cl as intermediate products. Thus, Sn and Zn behave quite differently with the final carbon-containing product, with Zn being CH4 but with Sn being CO2. This is rationalized by the competing reactions of a possible intermediate Cl(3)CMCl, which can be protonated by H2O to give CHCl3 or eliminate CCl2 (which subsequently reacts with water to form CO2 and HCl). Metal surface areas are also important, and the most active metal samples were prepared by a metal vapor-solvent codeposition method (SMAD cryoparticles). However, conventional Zn dust and Sn granules were also effective, only with lower reaction rates.
Sorption and reduction kinetics of trichloroethylene (ICE) and tetrachloroethylene (PCE) with metallic (zero-valent) iron were determined in a closed, well-mixed, anaerobic batch system by measuring aqueous and total system concentrations of the respective chlorinated solvent as a function of time. The reaction orders with respect to TCE and PCE total system concentrations were 2.7 and 1.3, respectively, indicating that the reaction mechanisms are complex. Both compounds exhibited nonlinear sorption behavior and could be fitted by the generalized Langmuir isotherm expression. After accounting for the mass sorbed to the iron, the reduction rates of PCE and TCE are first-order. This indicates that the bulk of sorption is to nonreactive sites. Competitive sorption was observed when both PCE and TCE were present; however, no competition for reaction was detected. The design and study of treatment systems for chlorinated solvents using metallic iron requires consideration of sorption processes.
Reduction of chlorinated solvents by fine-grained iron metal was studied in well-mixed anaerobic batch systems in order to help assess the utility of this reaction in remediation of contaminated groundwater. Iron sequentially dehalogenates carbon tetrachloride via chloroform to methylene chloride. The initial rate of each reaction step was pseudo-first-order in substrate and became substantially slower with each dehalogenation step. Thus, carbon tetrachloride degradation typically occurred in several hours, but no significant reduction of methylene chloride was observed over 1 month. Trichloroethene (TCE) was also dechlorinated by iron, although more slowly than carbon tetrachloride. Increasing the clean surface area of iron greatly increased the rate of carbon tetrachloride dehalogenation, whereas increasing pH decreased the reduction rate slightly. The reduction of chlorinated methanes in batch model systems appears to be coupled with oxidative dissolution (corrosion) of the iron through a largely diffusion-limited surface reaction.
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