Trichloroethylene Degradation Rate Research Articles

Carboxymethyl cellulose stabilization induced changes in particle characteristics and dechlorination efficiency of sulfidated nanoscale zero-valent iron

Polymer stabilization, exemplified by carboxymethyl cellulose (CMC), has demonstrated effectiveness in enhancing the transport of nanoscale zero-valent iron (nZVI). And, sulfidation is recognized for enhancing the reactivity and selectivity of nZVI in dechlorination processes. The influence of polymer stabilization on sulfidated nZVI (S-nZVI) with various sulfur precursors remains unclear. In this study, CMC-stabilized S-nZVI (CMC-S-nZVI) was synthesized using three distinct sulfur precursors (S2−, S2O42−, and S2O32−) through one-step approach. The antioxidant properties of CMC significantly elevated the concentration of reduced sulfur species (S2−) on CMC-S-nZVIs, marking a 3.1–7.0-fold increase compared to S-nZVIs. The rate of trichloroethylene degradation (km) by CMC-S-nZVIs was observed to be 2.2–9.0 times higher than that achieved by their non-stabilized counterparts. Among the three CMC-S-nZVIs, CMC-S-nZVINa2S exhibited the highest km. Interesting, while the electron efficiency of CMC-S-nZVIs surged by 7.9–12 times relative to nZVI, it experienced a reduction of 7.0–34% when compared with S-nZVIs. This phenomenon is attributed to the increased hydrophilicity of S-nZVI particles due to CMC stabilization, which inadvertently promotes the hydrogen evolution reaction (HER). In conclusion, the findings of this study underscores the impact of CMC stabilization on the properties and dechlorination performance of S-nZVI sulfidated using different sulfur precursors, offering guidance for engineering CMC-S-nZVIs with desirable properties for contaminated groundwater remediation.

Oxalate-modified microscale zero-valent iron for trichloroethylene elimination by adsorption enhancement and accelerating electron transfer

Trichloroethylene (TCE) with trace concentrations is often detected in soils and groundwater, posing potential damages to public health. The elimination of TCE can be achieved through reductive dechlorination using zero-valent iron (ZVI). However, ZVI usually suffers from the presence of passive iron (hydro)oxides layer and low electron transfer rate, thus leading to the unsatisfactory reactivity. Herein, we fabricated oxalated ZVI (Ox-ZVIbm) by mechanical ball-milling of micro-scale ZVI and H2C2O4·2H2O to modify the ZVI surface composition. To be specific, the modification of the iron oxide shell by oxalic acid facilitated the generation of unsaturated coordination Fe(II), enhancing TCE adsorption. Furthermore, the formed FeC2O4 on the iron oxide shell improved electron transfer efficiency, contributing to the enhanced TCE reductive dechlorination. Impressively, the rate of TCE degradation by Ox-ZVIbm was 10-fold higher than that of ZVIbm without oxalate modification. Moreover, Ox-ZVIbm samples were filled in a laboratory Permeable Reactive Barriers (PRB) column to treat actual underground wastewater containing TCE pollutants. The effluent concentration of TCE maintained steadily below 0.21 mg/L for over 10 days, complying with the National Groundwater Class IV standard (GBT 14848–2017). This marks a significant step toward practical groundwater treatment.

Enhanced Biogenic Sulfidation of Zero-Valent Iron in Columns: Implications for Promoting Dechlorination in Permeable Reactive Barriers.

Biogenic sulfidation of zero-valent iron (ZVI) using sulfate reducing bacteria (SRB) has shown enhanced dechlorination rates comparable to those produced by chemical sulfidation. However, controlling and sustaining biogenic sulfidation to enhance in situ dechlorination are poorly understood. Detailed interactions between SRB and ZVI were examined for 4 months in column experiments under enhanced biogenic sulfidation conditions. SRB proliferation and changes in ZVI surface properties were characterized along the flow paths. The results show that ZVI can stimulate SRB activity by removing excessive free sulfide (S2-), in addition to lowering reduction potential. ZVI also hinders downgradient movement of SRB via electrostatic repulsion, restricting SRB presence near the upgradient interface. Dissolved organic carbon (e.g., >2.2 mM) was essential for intense biogenic sulfidation in ZVI columns. The presence of SRB in the upgradient zone appeared to promote the formation of iron polysulfides. Biogenic FeSx deposition increased the S content on ZVI surfaces ∼3-fold, corresponding to 3-fold and 2-fold improvements in the trichloroethylene degradation rate and electron efficiency in batch tests. Elucidation of SRB and ZVI interactions enhances sustained sulfidation in ZVI permeable reactive barrier.

Enhanced visible-light activation of peroxymonosulfate by novel Bi2O2SO4/Bi,N,S-TiO2 heterojunction toward efficient trichloroethylene degradation: Insight into function of Bi2O2SO4

The photocatalytic performance of Bi2O2SO4 (BOS) has not been widely noticed. In this work, a novel heterojunction of Bi2O2SO4/Bi, N, S-TiO2 (BOS/BNST) was synthesized. The heterojunction exhibited high efficiency for trichloroethylene (TCE) degradation. Degradation rate of TCE could be over 99 % in 1 h, when concentration of TCE was 7 mg/L, and reaction rate constant could be 0.06573 min−1. The main reasons for the enhancement are depicted as follows: (1) TiO2 was doped with Bi, N and S, which enhanced the visible light response; (2) The combination of BOS and BNST improves the separation efficiency of photo-induced carriers; (3) Loading of BOS prompts PMS adsorption; (4) The Bi4+ and Bi3+ cycling in BOS/BNST facilitated PMS activation. This paper introduces a novel photocatalyst which exhibited excellent photoactivity, and provides a new perspective for the research of PMS activation under visible light.

Peroxymonosulfate activation by Ni-Fe (hydr)oxides through radical and nonradical pathways for efficient trichloroethylene degradation

Peroxymonosulfate (PMS)-based advanced oxidation processes have become promising water treatment technologies in recent years. The redox cycle between transitional metals in heterogeneous catalysis is essential for continuous activation of PMS and efficient removal of pollutants. In this work, we prepared Fe3O4 using precipitation method and subsequently added Ni aqueous solution to synthesize Ni-Fe (hydr)oxides. Ni-Fe (hydr)oxides show efficient PMS activation ability, causing the degradation of 96.8 % trichloroethylene (TCE) after 40 min with the dosage of Fe, Ni and PMS at 0.2, 0.1 and 0.2 mM, respectively. The catalyst concentration normalized TCE degradation rate constant of Ni-Fe (hydr)oxides/PMS system reaches 1.48 L g−1 min−1. Nonradical pathway is dominant at the initial stage of the reaction which generates high-valent metals, SO5•− and indirectly produces 1O2. 1O2 and dissolved O2 together participate in the O2 cycling, •OH generation, and electron transfer involving the reduction of trivalent metals. Divalent metals react with PMS to generate strong oxidative •OH and SO4•− through radical pathway in later stage. TCE is degraded and dechlorinated by multiple reactive species in Ni-Fe (hydr)oxides/PMS system. The optimum Ni/Fe ratio of catalysts under the studied conditions was 0.50. Ni-Fe (hydr)oxides show high reactivity and stability in real water matrices, demonstrating Ni-Fe (hydr)oxides are promising catalysts for PMS activation to degrade pollutants in contaminated water.

Synergistic action of ferrate and biochar in the removal of trichloroethylene from water: Little biochar addition, large ferrate activity improvement

Ferrate (Fe(VI)) is strongly considered an efficient green oxidant for degrading organic pollutants in groundwater. However, overdosage is often used to achieve the desired efficiency, resulting in adverse effects on the groundwater quality and the ecological environment and high costs. This study aimed to enhance the removal of trichloroethylene (TCE) using Fe(VI) by investigating the synergistic action of a ferrate-biochar system. 1 h into the reaction, the TCE degradation rate had increased from 18.7% to 96.0% and the K2FeO4 concentration had increased from 1 mM to 25 mM. The pH strongly influenced the TCE degradation by K2FeO4 by affecting the stability, species, and consequent oxidation capacity of K2FeO4. Co-existing metal cations in the solution could enhance the K2FeO4 self-decay by facilitating the electron transfer process. Biochar addition significantly enhanced TCE removal by K2FeO4 (up to 4.4 times) compared with the combined removal via individual oxidation and adsorption systems. Significant enhancement comparable to that of a high 16 mM Fe(VI) dosage was obtained with 666 mg/L biochar and 5 mM Fe(VI). The enhancement mechanisms by which biochar addition enhanced the TCE removal by K2FeO4 included the oxidation process by prior to TCE adsorption onto biochar, an enhanced electron transfer of Fe(VI) reduction by the biochar functional group, and an enhanced adsorption capacity of biochar owing to its peroxidation by Fe(VI). Thus, compared to individual and excessive chemical oxidation, the ferrate-biochar composite system provides a promising strategy for improving the TCE removal efficiency with lower oxidant consumption and less negative ecological impacts.

A comparative study on the physicochemical properties, reactivity and long-term performance of sulfidized nanoscale zerovalent iron synthesized with different kinds of sulfur precursors and procedures in simulated groundwater

There are plentiful ways to synthesize sulfidized nanoscale zerovalent iron (S-nZVI), and this study investigated the influence of sulfur reagents (Na2S, Na2S2O3, Na2S2O4) and sulfidation sequence (co-sulfidation and post-sulfidation method) on the physicochemical properties, reactivity, and long-term performance of S-nZVI in simulated groundwater. The results suggested that the co-sulfidized nZVI (S-nZVIco) has higher reactivity (∼2-fold) than S-nZVIpost due to the stronger electron transfer capacity, deriving from the higher content of Fe0 and reductive sulfur species. However, during aging, the reactivity of S-nZVIco would be lost more rapidly than S-nZVIpost, due to the faster corrosion of Fe0 and more oxidation of reductive sulfur species. S-nZVIpost has the superior long-term performance with the degradation rate of trichloroethylene (TCE) remained at 30%∼60% even after 90 d of aging. Sulfur precursors can control the selectivity of S-nZVI by affecting the sulfur speciation on the particle surface. The proportion of reductive sulfur species on S-nZVIpost synthesized by Na2S was higher than S-nZVIpost synthesized by Na2S2O3 or Na2S2O4, resulting in a higher selectivity of the former S-nZVIpost than the latter S-nZVIpost. In addition, sulfidation procedures and sulfur precursors did not affect the degradation pathway of TCE. Nevertheless, the degradation product distribution can be affected by the different physicochemical transformation of various types of S-nZVI with the aging time. These results indicated that sulfur reagents and sulfidation procedures have crucial effects on the reactivity and long-term performance of S-nZVI, which can be designed for the specific application scenarios.

Microbiome Reengineering by Heat Selection for Rapid Biodegradation of Trichloroethylene with Minimal Vinyl Chloride Formation

Trichloroethylene (TCE) was a widely used industrial solvent but is now regarded as a notorious groundwater contaminant. Both physicochemical and biological methods have been applied to remediate groundwater contaminated by TCE. For medium to low level of TCE contamination, bioremediation could be more cost-effective. However, bioremediation approaches suffer from slow degradation rates and accumulation of vinyl chloride (VC). In addition, bioaugmentation is often highly encouraged but may introduce foreign genes and increase the pace of microbial evolution. In this study, a microbiome reengineering strategy by heat selection is applied to solve these problems. Out of eight heat-treated mixed cultures, two showed a much-improved TCE degradation rate, more than 70 times higher than the untreated. The biodegradation half-life (t1/2) of TCE was 0.0627 d or shorter. No VC was detected by a gas chromatography equipped with flame ionization detector (GC-FID) and only a minimal amount by a GC-mass spectrometer (GC-MS). Ethene achieved a fairly good mass balance. Polymerase chain reaction-denaturing gradient gel electrophoresis (PCR-DGGE) and next-generation sequencing (NGS) results showed that the heating process did not kill most bacteria, but Dehalococcoides were either not present or very scarce. Acetoanaerobium and Methanosarcina could be the most important species in this reductive dechlorination process. Kinetic study results showed that the maximum specific TCE degradation rate was approximately 1,271 nmole/min/mg cell protein, which are two orders of magnitude higher than that of the mixed cultures reported in literature. These results suggest that apart from biostimulation and bioaugmentation, microbiome reengineering could be a promising approach for rapid bioremediation of TCE-contaminated aquifers.

Electrochemical Oxidation of Perfluorooctanesulfonate by Magnéli Phase Ti4O7 Electrode in the Presence of Trichloroethylene

This study examined the degradation of perfluorooctanesulfonate (PFOS) in electrochemical oxidation (EO) processes in the presence of trichloroethylene (TCE). The EO experiment was performed in a gas-tight reactor using Magnéli phase titanium suboxide (Ti4O7) as the anode. The experimental data demonstrated that 75% of PFOS (2 μM) was degraded at 10 mA/cm2 current density in 30 min without TCE present in the solution, while the presence of 76 μM TCE apparently inhibited the degradation of PFOS, reducing its removal down to 53%. Defluorination ratio suggested that PFOS was significantly mineralized upon EO treatment, and it appeared to be not influenced by the presence of TCE. The respective pseudo-first order rate constants (kobs) of PFOS removal were 0.0471 and 0.0254 min-1 in the absence and presence of TCE. The degradation rates of both PFOS and TCE increased with current density rising from 2.5 to 20 mA/cm2. In the presence of TCE, chloride, chlorate, and perchlorate were formed that accounted for 79.7 %, 5.53%, and 1.51% of the total chlorine at 60 min. This work illustrates the promise of the Magnéli phase Ti4O7 electrode based electrochemical oxidation technology for degrading per- and polyfluoroalkyl substances (PFASs) and co-contaminants in groundwaters.

Batch degradation of trichloroethylene using oleaginous Rhodococcus opacus in a two-phase partitioning bioreactor and kinetic study

In the present study, we present the biodegradation of trichloroethylene (TCE) for the first time using pure bacterial strain of Rhodococcus opacus PD630. TCE utilized as the sole carbon substrate by R. opacus in single phase system and was reached to highest specific growth rate of 0.015 h−1 at the concentration of 75 mg L−1. To further enhance the degradation performance even at high concentration (1000 mg L−1), two-phase partitioning bioreactor (TPPB) was developed. Silicone oil (20%, v/v) was selected as non-aqueous phase solvent to achieve high metabolic activity of bacteria. Furthermore, the experimental results of TCE degradation and specific TCE utilization rate were fitted to various kinetic models. Our experimental results clearly revealed that TPPB system having R. opacus as biological source and silicone oil as solvent was highly efficient and successful for removing high concentrated TCE compared with single phase system.

Degradation of trichloroethylene by photoelectrochemically activated persulfate

Chlorine-containing organic compounds were discharged informally as a result of untreated industrial wastewater, which caused groundwater pollution. In this study, titanium dioxide nanotube arrays (TNAs) were modified with copper oxide to photoelectrochemical (PEC) active persulfate to degrade trichloroethylene (TCE). The SEM results show copper nano-particles with a cubic shape were successfully deposited on the surface of TNAs. The results of UV–vis analysis indicate the absorption wavelengths red-shift to 550–600 nm for better light utilization. CuO/TNAs were dominated by the anatase phase after sintering at 450 °C with significant visible light response. The chemical contents for the surface of CuO/TNAs are 23.7, 53.4, 18.4 and 4.4% for C, O, Ti and Cu, respectively. The photocurrent of CuO/TNAs is 1.89 times higher than that of TNAs-93 cm^2–1hr under 100 W Hg-lamp illuminations. This demonstrates the efficiency of light utilization of TNAs was improved by the modification with copper nanoparticles. The degradation rate of TCE in the anodic chamber is more effective than that in the cathodic chamber because of the synergistic effect of hydroxyl and sulfate radicals. The mechanism of TCE degradation via persulfate in the PEC system was proposed and discussed in detail.

Simulated solar light-driven photocatalytic degradation of trichloroethylene in water using BiOBr promoted by sulfite addition

BackgroundPhotodegradation of trichloroethylene (TCE) in aqueous solution under simulated solar light irradiation was studied under different experimental conditions to determine the reaction mechanism and kinetics that control TCE degradation using bismuth oxybromide (BiOBr) in the presence of sulfite. Photocatalysts were synthesized to be more responsive to visible light under simulated solar light and particular attention was focused on the reactive specie formed by reaction of the sulfite on the surface of BiOBr under simulated sunlight.ResultDegradation rate of TCE was greatly enhanced by the presence of sulfite, and the enhancement increased with sulfite dose to a maximum that was retained at higher sulfite doses. Degradation rate of TCE was also affected by other factors, such as initial TCE concentration, BiOBr dose, and solution pH. In addition, the cycling performance of BiOBr was examined, and the amount of TCE degraded was almost constant over increasing cycle numbers when initial sulfite concentration was high enough to maintain a suitable sulfite concentration throughout the experiment. When TCE was degraded by BiOBr in the presence of sulfite under simulated sunlight irradiation, the major by-product measured was the non-hazardous chloride ion, and dechlorination efficiency was about 58%.ConclusionThis study extended the use of a potential effective photocatalyst (BiOBr) to a semi-volatile organic contaminant (TCE), not limited to mainly focus on organic dyes, and evaluated the use of sulfite as a hole scavenger in order to enhance the degradation of TCE without needing to manipulate the structure of BiOBr. The active species being responsible for TCE degradation in BiOBr/TCE/sulfite system under simulated solar light was the sulfite radical (SO3·−), and the photocatalytic activity of BiOBr did not decrease over a number of treatment cycles when SIV dose was sufficient.

Cometabolic degradation of toluene and TCE contaminated wastewater in a bench-scale sequencing batch reactor inoculated with immobilized Pseudomonas putida F1

Abstract Toluene and Trichloroethylene (TCE) are two of the most common organic contaminates at hazardous sites and contaminated water. In this study, a sequencing batch reactor (SBR) inoculated with PVA/alginate-immobilized Pseudomonas putida F1 was used for biological treatment of toluene and TCE contaminated wastewater. Moreover, a mathematical model was developed to simulate the dynamic behavior of a SBR treating competitive contaminates. Results showed high removal efficiencies of both toluene and TCE in the effluents. For the duration of filling time, an instantaneous fill was the optimum for moderate amount of toluene and TCE. Toluene concentration affected TCE degradation. The lower the toluene presence, the higher the removal efficiency of both toluene and TCE. Interestingly, the presence of 20 mg/L iron, or 1 mg/L nickel stimulated the degradation rates of both toluene and TCE. However, the degradation rate was reduced when both 20 mg/L iron and 1 mg/L nickel were presented simultaneously. The model developed in this research was able to simulate the kinetic behaviors of toluene and TCE removal in the SBR successfully.

Aerobic biodegradation of trichloroethylene by consortium microorganism from turkey litter compost

Trichloroethylene (TCE) is one of the most widely used solvent and have been poses a serious threats to the environment and human health. This work investigated to study the efficient biodegradation of synthetic TCE in a batch reactor using turkey litter compost in the concentration range of 25–500 mg L−1. The physical and biological properties are ensures that the compost has a great extent in biodegradation of hydrophobic compounds. Maximum TCE degradation rate was found to be 0.41 mg L-1 h-1 and more than 90% of removal efficiency was obtained up to the concentration of 300 mg L−1. High performance liquid chromatography was used to identify the major acid metabolites and confirmed the reported biochemical pathway; TCE was first hydrolyzed to dichloroacetyl chloride and then subsequently degraded to dichloroacetic acid and trichloroacetic acid and ultimately converted to carbon di-oxide and water. Furthermore, the degradation of TCE and their metabolites are well fitted with Aiba substrate inhibition and Monod model respectively. Based on biochemical test and 16S rDNA sequencing, Pseudomonas guguanensis NR135725 is an identified predominant strain in the TCE degraded culture. It is therefore concluded that a novel aerobic isolate involved in the degradation of TCE and their metabolites; in addition to offers a great potential application to use in continuous removal of chlorinated contaminants from industrial waste streams by effectively.

Hydroxyl radical and non-hydroxyl radical pathways for trichloroethylene and perchloroethylene degradation in catalyzed H2O2 propagation systems

Catalyzed H2O2 propagations (CHP) is characterized by the most robust reactivity of any of the in situ chemical oxidation (ISCO) technologies because it generates the strong oxidant hydroxyl radical along with nucleophiles + reductants, such as superoxide radical. The most common groundwater contaminants, trichloroethylene (TCE) and perchloroethylene (PCE), were used as model contaminants in evaluating the effect of hydrogen peroxide (H2O2) dosage on contaminant destruction kinetics. Both TCE and PCE degradation rates increased with H2O2 dosages up to 0.1 M, and then decreased with higher H2O2 dosages. Parallel reactions conducted with the addition of the hydroxyl radical scavenger 2-propanol and the hydroxyl radical-specific probe nitrobenzene confirmed that hydroxyl radical is primarily responsible for TCE and PCE degradation; however, 5–20% of their degradation was attributed to a non-hydroxyl radical mechanism. Reactions conducted with the superoxide probe tetranitromethane showed that superoxide generation rates increased with increasing H2O2 doses. These results were confirmed by electron spin resonance spectroscopy. Therefore, the non-hydroxyl radical pathway for TCE and PCE degradation at H2O2 ≥0.5 M was likely via nucleophilic attack by superoxide. The results of this research demonstrate that contaminants present in the aqueous phase that are reactive with hydroxyl radical require only low doses of H2O2 (≤0.1 M), but subsurface systems contaminated with species not reactive with hydroxyl radical (e.g., carbon tetrachloride) require H2O2 concentrations ≥0.5 M.

Enhancement in trichloroethylene dechlorination by mixed particles of iron–iron disulfide or iron–iron sulfide

A column test was carried out to clarify the effect of the mixed particles of iron (Fe) and iron disulfide (FeS2) or iron sulfide (FeS) on the trichloroethylene (TCE) degradation rate and compared with a reported batch test. The TCE degradation rate of the mixed particles of Fe and FeS2 was faster than the laminar packing of Fe and FeS2. The TCE degradation rate proceeded at the first-order reaction and had a peak value at the mass ratio of Fe/FeS2 = 7/3 and Fe/FeS = 3/7, and they showed the same trend as the batch test. The rate constant of Fe/FeS2 and Fe/FeS based on unit surface area remained constant for the mixing ratio of Fe to FeS2 or FeS, and 6.1 and 1.3 times larger than single component of Fe, respectively. The surface chemical reaction of TCE on the Fe/FeS2 and Fe/FeS particles was the rate determining step and the mass transfer steps did not affect the reaction rate. Chloride ion, ethylene and ethane were detected from the treated solution and off-gas due to the TCE degradation, and the pH value increased with the increase in the TCE degradation rate until pH ≒ 10.

Dechlorinating performance of Dehalococcoides spp. mixed culture enhanced by tourmaline

Dehalococcoides spp. were extensively studied and applied to in-situ trichloroethylene (TCE) remediation since it is the only genus that can reduce TCE to harmless ethene completely. However, this technology was hindered because of the requirement of electron donor (i.e. hydrogen or fermentable organic substrate). Considering the spontaneous electric field and in-situ hydrogen production capacity of tourmaline, this mineral was used as an environmental-friendly bio-promoter for dechlorinating mixed culture containing Dehalococcoides spp. in this work. Research results showed that biodegradation of TCE and the intermediates were both significantly improved by tourmaline. The first-order TCE degradation rate coefficient increased from 0.0125 h−1 for the tourmaline-free system to 0.0306 h−1 for the system combined with 5 g L−1 tourmaline, and ethene production increased by 36%. The outstanding TCE-degrading ability in the tourmaline-bacteria system without the addition of fermentative electron donor (i.e. methanol) indicated that tourmaline could also produce electron donor to support dechlorinating bacteria. Tourmaline could have direct electric biostimulatory effect and indirect enhanced effect associated with water-derived H2 production in the electric field of tourmaline. Meanwhile, PCR–DGGE analysis exhibited that tourmaline could accelerate the succession of a bacterial, dechlorinating community. The distinctive effects of tourmaline on bacteria were related to its stable electric properties. Therefore, tourmaline could be continuously used in the bioremediation. The present study provided a safe, convenient and persistent alternative to the commonly used enhancement approaches for anaerobic reductive dechlorination process.

Removal of trichloroethylene by biochar supported nanoscale zero-valent iron in aqueous solution

Abstract Nanoscale zero-valent iron (NZVI) has been widely used for the degradation of trichloroethylene (TCE) in contaminated water, however, it is suffering from being easy to aggregate and having low adsorption capacity for TCE. In order to overcome the shortcomings of NZVI, modified NZVI particles with biochar (NZVI/BC) were applied in this study for removal and degradation of TCE, given the high stability and adsorption capacity of BC. The effects of pyrolysis temperature of BC, mass ratio of NZVI/BC and solution pH on the removal and degradation efficiency of TCE were studied. The different pyrolysis temperatures of BC resulted in the differences on the surface areas, aromaticity and noncarbonized fractions, which determined the sorption capacity of TCE. Compared with pure NZVI, the NZVI/BC at different mass ratios could increase the removal efficiency of TCE to 99%, which was attributed to the higher adsorption capacity of BC for TCE. Besides, the yield of final products (ethane, ethylene and acetylene) differed at different mass ratios of NZVI/BC. Generally, the main product was ethylene in all reactions and the yield of acetylene and ethane were relatively low. Solution pH had little effect on the total removal of TCE but significantly influenced the yield of final products. The yield of ethylene decreased with the increasing pH. The results indicate that solution pH could not affect the sorption of TCE but influenced the degradation rate of TCE by NZVI/BC.

Enhanced Fenton-like degradation of TCE in sand suspensions with magnetite by NTA/EDTA at circumneutral pH.

The present study investigated the degradation of trichloroethylene (TCE) in sand suspensions by Fenton-like reaction with magnetite (Fe3O4) in the presence of various chelators at circumneutral pH. The results showed that ethylenediaminetetraacetic acid (EDTA) and nitrilotriacetic acid (NTA) greatly improved the rate of TCE degradation, while [S,S]-ethylenediaminedisuccinic acid (s,s-EDDS), malonate, citrate, and phytic acid (IP6) have minimal effects on TCE degradation. Quenching tests suggested that TCE was mainly degraded by hydroxyl radical (HO·) attack, with about 90% inhibition on TCE degradation by the addition of HO· scavenger 2-propanol. The presence of 0.1-0.5% Fe3O4/sand (w/w) contributed to 40% increase in TCE degradation rates. In particular, the use of chelators can avoid high concentrations of H2O2 required for the Fenton-like reaction with Fe3O4, and moreover improve the stoichiometric efficiencies of TCE degradation to H2O2 consumption. The suitable concentrations of chelators (EDTA and NTA) and H2O2 were suggested to be 0.5 and 20mM, respectively. Under the given conditions, degradation rate constants of TCE were obtained at 0.360h-1 with EDTA and 0.526h-1 with NTA, respectively. Enhanced degradation of TCE and decreased usage of H2O2 in this investigation suggested that Fenton-like reaction of Fe3O4 together with NTA (or EDTA) may be a promising process for remediation of TCE-contaminated groundwater.

Nitrate reduction and its effects on trichloroethylene degradation by granular iron

Laboratory column experiments and reactive transport modeling were performed to evaluate the reduction of nitrate and its effects on trichloroethylene (TCE) degradation by granular iron. In addition to determining degradation kinetics of TCE in the presence of nitrate, the columns used in this study were equipped with electrodes which allowed for in situ measurements of corrosion potentials of the iron material. Together with Raman spectroscopic measurements the mechanisms of decline in iron reactivity were examined. The experimental results showed that the presence of nitrate resulted in an increase in corrosion potential and the formation of thermodynamically stable passive films on the iron surface which impaired iron reactivity. The extent of the decline in iron reactivity was proportional to the nitrate concentration. Consequently, significant decreases in TCE and nitrate degradation rates and migration of degradation profiles for both compounds occurred. Furthermore, the TCE degradation kinetics deviated from the pseudo-first-order model. The results of reactive transport modeling, which related the amount of a passivating iron oxide, hematite (α-Fe2O3), to the reactivity of iron, were generally consistent with the patterns of migration of TCE and nitrate profiles observed in the column experiments. More encouragingly, the simulations successfully demonstrated the differences in performances of three columns without changing model parameters other than concentrations of nitrate in the influent. This study could be valuable in the design of iron permeable reactive barriers (PRBs) or in the development of effective maintenance procedures for PRBs treating TCE-contaminated groundwater with elevated nitrate concentrations.