In situ chemical reduction of aquifer sediments: enhancement of reactive iron phases and TCE dechlorination

In situ chemical reduction of aquifer sediments is currently being used for chromate and TCE remediation by forming a permeable reactive barrier. The chemical and physical processes that occur during abiotic reduction of natural sediments during flow by sodium dithionite were investigated. In different aquifer sediments, 10-22% of amorphous and crystalline FeIII-oxides were dissolved/reduced, which produced primarily adsorbed FeII, and some siderite. Sediment oxidation showed predominantly one FeII phase, with a second phase being oxidized more slowly. The sediment reduction rate (3.3 h batch half-life) was chemically controlled (58 kJ mol(-1)), with some additional diffusion control during reduction in sediment columns (8.0 h half-life). It was necessary to maintain neutral to high pH to maintain reduction efficiency and prevent iron mobilization, as reduction generated H+. Sequential extractions on reduced sediment showed that adsorbed ferrous iron controlled TCE reactivity. The mass and rate of field-scale reduction of aquifer sediments were generally predicted with laboratory data using a single reduction reaction.     

Effects of carbonate precipitates on long-term performance of granular iron for reductive dechlorination of TCE

Long-term column experiments were conducted to evaluate the effects of secondary carbonate minerals on permeability and reactivity of commercial granular iron treating trichloroethene (TCE). The results showed that carbonate precipitates caused a decrease in reactivity of the iron, and spatially and temporally varying reactivity loss resulted in migration of mineral precipitation fronts, as well as profiles of TCE, pH, alkalinity, calcium, and dissolved iron. In the columns receiving solutions of dissolved calcium carbonate, porosity gradually decreased in proportion to the source concentrations, as carbonate minerals accumulated. However, the rate of porosity loss slowed over time because of the declining reactivity of the iron. Thus, secondary minerals are not likely to accumulate to the extent that there is a substantial reduction in hydraulic conductivity. The reactivity of the iron was found to decrease as an exponential function of the carbonate mineral volume fraction. This changing reactivity of iron should be incorporated into predictive models for improved designs of iron permeable reactive barriers (PRBs).     

Effect of TCE concentration and dissolved groundwater solutes on NZVI-promoted TCE dechlorination and H2 evolution

Nanoscale zero-valent iron (NZVI) is used to remediate contaminated groundwater plumes and contaminant source zones. The target contaminant concentration and groundwater solutes (NO3-, Cl-, HCO3-, SO4(2-), and HPO4(2-)) should affect the NZVI longevity and reactivity with target contaminants, but these effects are not well understood. This study evaluates the effect of trichloroethylene (TCE) concentration and common dissolved groundwater solutes on the rates of NZVI-promoted TCE dechlorination and H2 evolution in batch reactors. Both model systems and real groundwater are evaluated. The TCE reaction rate constant was unaffected by TCE concentration for [TCE] < or = 0.46 mM and decreased by less than a factor of 2 for further increases in TCE concentration up to water saturation (8.4 mM). For [TCE] > or = 0.46 mM, acetylene formation increased, and the total amount of H2 evolved at the end of the particle reactive lifetime decreased with increasing [TCE], indicating a higher Fe0 utilization efficiency for TCE dechlorination. Common groundwater anions (5mN) had a minor effect on H2 evolution but inhibited TCE reduction up to 7-fold in increasing order of Cl- < SO4(2-) < HCO3- < HPO4(2). This order is consistent with their affinity to form complexes with iron oxide. Nitrate, a NZVI-reducible groundwater solute, present at 0.2 and 1 mN did not affect the rate of TCE reduction but increased acetylene production and decreased H2 evolution. NO3- present at > 3 mM slowed TCE dechlorination due to surface passivation. NO3- present at 5 mM stopped TCE dechlorination and H2 evolution after 3 days. Dissolved solutes accounted for the observed decrease of NZVI reactivity for TCE dechlorination in natural groundwater when the total organic content was small (< 1 mg/L).     

Congener-specific dechlorination of dissolved PCBs by microscale and nanoscale zerovalent iron in a water/methanol solution

Polychlorinated biphenyl (PCB)-contaminated sediments remain a significantthreatto humans and aquatic ecosystems. Dredging and disposal is costly, so viable in situ technologies to dechlorinate PCBs are needed. This study demonstrates that nanoscale zerovalent iron (ZVI) dechlorinates PCBs to lower-chlorinated products under ambient conditions, provides insight into structure-activity relationships between PCB isomers, and compares the reactivity of nanoscale ZVI to that of palladized microscale ZVI. Six PCB congeners were studied (22', 34', 234, 22'35', 22'45', and 33'44') to compare the initial rate of dechlorination of each and to monitor the order in which chlorines are removed. Using 200 g/L of nanoscale ZVI in a 30% MeOH/water mixture, observed surface-area-normalized pseudo-first-order PCB dechlorination rate constants ranged from 1 x 10(-6) to 5.5 x 10(-4) L yr(-1) m(-2) depending on the PCB congener tested. Using 200 g/L of palladized (0.05 wt %) microscale ZVI, surface-area-normalized pseudo-first-order PCB dechlorination rate constants were significantly faster and ranged from 3.8 x 10(-2) to 1.7 x 10(-1) L yr(-1) m(-2), but these rates were not sustainable. For nanoscale ZVI, nonorthosubstituted congeners had faster initial dechlorination rates than orthosubstituted congeners in the same homologue group. Chlorines in the para and meta position were predominantly removed over chlorines in the ortho position, which suggests that more-toxic coplanar PCB congeners are not likely to form from less-toxic noncoplanar, orthosubstituted congeners. Complete dechlorination was not observed over the course of the experiments. PCB dechlorination is rapid enough that nanoscale ZVI may offer novel in situ remedial alternatives for PCB-contaminated sediments.     

Influence of ferric iron on complete dechlorination of trichloroethylene (TCE) to ethene: Fe(III) reduction does not always inhibit complete dechlorination

The effects of Fe(III) reduction on TCE, cis-DCE, and VC dechlorination were studied in both contaminated aquifer material and enrichment cultures. The results from sediment batch experiments demonstrated that Fe(III) reduction did not inhibit complete dechlorination. TCE was reduced concurrently with Fe(III) in the first 40 days of the incubations. While all incubations (plus and minus Fe(III)) generated approximately the same mass of ethene within the experimental time frame, Fe(III) speciation (ferrihydrite versus Fe(III)-NTA) had an impact on daughter product distribution and dechlorination kinetics. 16S rRNA gene clone library sequencing identified Dehalococcoides and Geobacteraceae as dominant populations, which included G. lovleyi like organisms. Quantitative PCR targeting 16S rRNA genes and Reductive Dehalogenase genes (tceA, bvcA, vcrA) indicated that Dehalococcoides and Geobacteraceae were enriched concurrently in the TCE-degrading, Fe(III)-reducing sediments. Enrichment cultures demonstrated that soluble Fe(III) had a greater impact on cis-DCE and VC reduction than solid-phase Fe(III). Geobacteraceae and Dehalococcoides were also coenriched in the liquid cultures, and the Dehalococcoides abundance in the presence of Fe(III) was not significantly different from those in the cultures without Fe(III). Hydrogen reached steady-state concentrations most amenable to complete dechlorination very quickly when Fe(III) was present in the culture, suggesting that Fe(III) reduction may actually help dechlorination. This was contrasted to hydrogen levels in nitrate-amended enrichments, in which hydrogen concentration was too low for any chlororespiration.     

Pd-catalyzed TCE dechlorination in water: effect of [H2](aq) and H2-utilizing competitive solutes on the TCE dechlorination rate and product distribution

The aqueous-phase H2 concentration ([H2](aq)) and the presence of H2-utilizing competitive solutes affect TCE dechlorination efficiency in Pd-based in-well treatment reactors. The effect of [H2](aq) and H2-utilizing competing solutes (cis-DCE, trans-DCE, 1,1-DCE, dissolved oxygen (DO), nitrite, nitrate) on the TCE transformation rate and product distribution were evaluated using 100 mg/L of a powdered Pd-on-Al2O3 catalysts in batch reactors or 1.0 g of a 1.6-mm Pd-on-gamma-Al2O3 catalyst in column reactors. The TCE dechlorination rate constant decreased by 55% from 0.034 +/- 0.006 to 0.015 +/- 0.001 min-1 when the [H2](aq) decreased from 1000 to 100 microM and decreased sharply to 0.0007 +/- 0.0003 min-1 when the [H2](aq) decreased from 100 to 10 microM. Production of reactive chlorinated intermediates and C4-C6 radical coupling products increased with decreasing [H2](aq). At an [H2](aq) of 10 microM (P/Po = 0.01), DCE isomers and vinyl chloride accounted for as much as 9.8% of the TCE transformed at their maximum but disappeared thereafter, and C4-C6 radical coupling products accounted for as much as 18% of TCE transformed. The TCE transformation rate was unaffected by the presence of cis-DCE (202 microM), trans-DCE (89 microM), and 1,1-DCE (91 microM), indicating that these compounds do not compete with TCE for catalyst active sites. DO is twice as reactive as TCE but had no effect on TCE conversion in the column below a concentration of 370 microM (11.8 mg/L), indicating that DO and TCE will not compete for active catalyst sites at typical groundwater DO concentrations. TCE conversion in the column was reduced by as much as a factor of 10 at influent DO levels greater than 450 mM (14.3 mg/L) because the [H2](aq) fell below 100 microM due to H2 utilized in DO conversion. Nitrite reacts 2-5 times slower than TCE and reduced TCE conversion by less than 4% at a concentration of 6630 microM (305 mg/L). Nitrate was not reactive and did not effect TCE conversion at a concentration of 1290 microM (80 mg/L).     

Correlation of 2-chlorobiphenyl dechlorination by Fe/Pd with iron corrosion at different pH

The rate of 2-chlorobiphenyl dechlorination by palladized iron (Fe/ Pd) decreased with increasing pH until pH > 12.5. Iron corrosion potential (Ec) and current (jc), obtained from polarization curves of a rotating disk electrode of iron, followed the Tafel equation at pH < or = 5.5 and pH > or = 9.5. The pH dependence of the dechlorination rate constant (k1) suggests four pH regimes. In the low pH regime (3-5.5), /Ec/ and je decreased with increasing pH and k1 was linearly correlated to /Ec/ and jc0.5. The correlation between k1 and jc0.5 indicates direct involvement of active hydrogen species (on the Pd surface) in PCB dechlorination. In the mid pH regime (5.5-9.5), no significant effect of pH was evident on the values of k1, je, and Ec, a combined result of limiting anodic oxidation of iron to an intermediate product (iron hydroxide) and a proton-independent overall reaction. Both /Ec/ and jc increased significantly as pH increased from 9.5 to 14. A cleartrough of the k1 values in solutions of pH between 12 and 13 and the mismatch between the kinetic and corrosion data suggest two pH regimes (9.5-12.5 and 12.5-14) of different corrosion mechanisms.     

Carbothermal synthesis of carbon-supported nanoscale zero-valent iron particles for the remediation of hexavalent chromium

Nanoscale, zero-valent iron is a promising reagent for in situ reduction of a variety of subsurface contaminants, but its utility in full-scale remediation projects is limited by material costs. Iron nanoparticles (20-100 nm diameter) supported on carbon (C-Fe0) were synthesized by reacting iron salts, adsorbed or impregnated from aqueous solutions onto 80 m2/g carbon black, at 600-800 degrees C under Ar. Similar products were obtained by heating the reactants under air in a covered alumina crucible. X-ray powder diffraction patterns show that Fe3O4 particles are formed at 300-500 degrees C in the initial stage of the reaction and that these particles are reduced to a mixture of alpha- and gamma-Fe nanoparticles above 600 degrees C. When C-Fe0 was combined with carboxymethylcellulose in a 5:1 weight ratio in water, the resulting material had similar transport properties to previously optimized nanoiron/polyanion suspensions in water-saturated sand columns. At a 10:3 Fe/Cr mole ratio, C-Fe0 reduced a 10 ppm Cr(VI) solution to approximately 1 ppm within three days. The surface area normalized first-order Cr removal rate was 1.2 h(-1) m(-2) under these conditions. These results demonstrate that reactive nanoiron with good transport properties in water-saturated porous media can be made in a scalable process from inexpensive starting materials by carbothermal reduction.     

原位聚合SiO2/PSSS纳米复合粒子的制备及其性能

以γ-（甲基丙烯酰氧）丙基三甲氧基硅烷（KH-570）对溶胶凝胶制备得到的纳米SiO2进行表面接枝改性,然后通过原位聚合制备二氧化硅/聚对苯乙烯磺酸钠（SiO2/PSSS）纳米复合粒子。研究了对苯乙烯磺酸钠（SSS）和过硫酸铵（APS）用量对SiO2/PSSS纳米复合粒子粒径的影响,探讨了SiO2/PSSS纳米复合粒子在水相中的分散稳定性。结果表明当SSS占SiO2质量分数的25%,引发剂APS占SSS质量分数的1.5%时,得到SiO2/PSSS纳米复合粒子的粒径为150nm;红外光谱（FTIR）和透射电镜（TEM）测试结果表明纳米SiO2表面成功包覆了PSSS,热失重（TGA）分析表明SiO2表面包覆聚合物的量约占其总重的27.5%,SiO2/PSSS纳米复合粒子在水相中具有良好的分散稳定性。     

Transport of non-newtonian suspensions of highly concentrated micro- and nanoscale iron particles in porous media: a modeling approach

The use of zerovalent iron micro- and nanoparticles (MZVI and NZVI) for groundwater remediation is hindered by colloidal instability, causing aggregation (for NZVI) and sedimentation (for MZVI) of the particles. Transportability of MZVI and NZVI in porous media was previously shown to be significantly increased if viscous shear-thinning fluids (xanthan gum solutions) are used as carrier fluids. In this work, a novel modeling approach is proposed and applied for the simulation of 1D flow and transport of highly concentrated (20 g/L) non-newtonian suspensions of MZVI and NZVI, amended with xanthan gum (3 g/L). The coupled model is able to simulate the flow of a shear thinning fluid including the variable apparent viscosity arising from changes in xanthan and suspended iron particle concentrations. The transport of iron particles is modeled using a dual-site approach accounting for straining and physicochemical deposition/release phenomena. A general formulation for reversible deposition is herein proposed, that includes all commonly applied dynamics (linear attachment, blocking, ripening). Clogging of the porous medium due to deposition of iron particles is modeled by tying porosity and permeability to deposited iron particles. The numerical model proved to adequately fit the transport tests conducted using both MZVI and NZVI and can develop into a powerful tool for the design and the implementation of full scale zerovalent iron applications.     

Oxidative degradation of the carbothioate herbicide, molinate, using nanoscale zero-valent iron

Degradation of the carbothiolate herbicide, molinate, has been investigated in oxic solutions containing nanoscale zero-valent iron particles and found to be effectively degraded by an oxidative pathway. Both ferrous iron and superoxide (or, at pH < 4.8, hydroperoxy) radicals appearto be generated on corrosion of the zero-valent iron with resultant production of strongly oxidizing entities capable of degrading the trace contaminant.     

Aggregation and sedimentation of aqueous nanoscale zerovalent iron dispersions

Nanoscale zerovalent iron (NZVI) rapidly transforms many environmental contaminants to benign products and is a promising in-situ remediation agent. To be effective, NZVI should form stable dispersions in water such that it can be delivered in water-saturated porous media to the contaminated area. Limited mobility of NZVI has been reported, however, attributed to its rapid aggregation. This study uses dynamic light scattering to investigate the rapid aggregation of NZVI from single nanoparticles to micrometer size aggregates, and optical microscopy and sedimentation measurements to estimate the size of interconnected fractal aggregates formed. The rate of aggregation increased with increasing particle concentration and increasing saturation magnetization (i.e., the maximum intrinsic magnet moment) of the particles. During diffusion limited aggregation the primary particles (average radius = 20 nm) aggregate to micrometer-size aggregates in only 10 min, with average hydrodynamic radii ranging from 125 nm to 1.2 microm at a particle concentration of 2 mg/L (volume fraction(phi= 3.2 x 10(-7)) and 60 mg/L (phi = 9.5 x 10(-6)), respectively. Subsequently, these aggregates assemble themselves into fractal, chain-like clusters. At an initial concentration of just 60 mg/L, cluster sizes reach 20-70 microm in 30 min and rapidly sedimented from solution. Parallel experiments conducted with magnetite and hematite, coupled with extended DLVO theory and multiple regression analysis confirm that magnetic attractive forces between particles increase the rate of NZVI aggregation as compared to nonmagnetic particles.     

Rapid dechlorination of polychlorinated dibenzo-p-dioxins by bimetallic and nanosized zerovalent iron

Polychlorinated dibenzo-p-dioxins and furans (PCDD/Fs), especially the 2,3,7,8-substituted congeners, are extremely toxic, persistent, and recalcitrant to remediation. Dechlorination of PCDD/Fs by zerovalent iron (ZVI) is thermodynamically feasible, but useful rates of reaction have not been previously reported. Here we show that ZVI (both micro- and nanosized ZVI, without palladization) dechlorinates PCDD congeners with four or more chlorines in aqueous systems, but the reaction is too slow to achieve complete dechlorination within a practical period of time. In contrast, palladized nanosized ZVI (Pd/nFe) rapidly dechlorinates PCDDs, including the mono- to tetra-chlorinated congeners. The rate of 1,2,3,4-tetrachloro dibenzo-p-dioxin (1,2,3,4-TeCDD) degradation using Pd/nFe was about 3 orders of magnitude faster than 1,23,4-TeCDD degradation using unpalladized ZVI. The distribution of products obtained from dechlorination of 1,2,3,4-TeCDD suggests that palladization shifts the pathways of contaminant degradation toward a greater role of H atom transfer rather than electron transfer.     

Influences of amphiphiles on dechlorination of a trichlorobenzene by nanoscale Pd/Fe: adsorption, reaction kinetics, and interfacial Interactions

The influences of amphiphiles on the catalytic dechlorination of 1,2,4-trichlorobenzene (124TCB) bythe nanoscale Pd/Fe particles were comprehensively examined. The fresh and reacted Pd/ Fe particles were characterized with XRD, TEM, SEM, FTIR spectrometry, and goniometry. Adsorption of amphiphiles on the Pd/Fe particles, iron dissolution, and H2 evolution in the Pd/Fe-water system were quantified to expound the influences of the various amphiphiles on the dechlorination process. The Langmuir-Hinshelwood model is used to elucidate the dechlorination kinetics, and it provides insight into the influence of amphiphiles on 124TCB partitioning to the interfacial layer and the resulting dechlorination rates. The rate constants increased by a factor of 1.5--2.5 with the presence of cationic cetyltrimethylammonium bromide (CTAB). In the anionic sodium deodecyl sulfate(SDS) or nonionic nonylphenol ethoxylate (NPE) and octylphenolpoly (ethylene glycol ether)x (TX-100) surfactant solutions, the 124TCB dechlorination rates were slightly increased over those observed in ultrapure water. However, when concentrations of the surfactants were above their CMCs, the dechlorination rates decreased. The findings also showed that DPC (dodecylpyridinium chloride) and NOM (natural organic matter) might be the competitive H2 acceptors to 124TCB, and they significantly retarded its catalytic dechlorination by the Pd/Fe particles. CTAB at a concentration below the CMC appeared to be the most benign to the 124TCB dechlorination.     

Time trends in sources and dechlorination pathways of dioxins in agrochemically contaminated sediments

Although polychlorinated dibenzo-p-dioxins and dibezofurans (PCDD/Fs) are considered recalcitrant toward biotic and abiotic degradation processes, laboratory studies indicated lateral dechlorination pathways (removal of 2,3,7,8-substituted chlorines) as possible natural remediation strategies under highly reducing conditions prevailing in contaminated sediments. Previous principal component analysis (PCA) of PCDD/Fs in Japanese sediments left unidentified a factor characterized by penta- to octa- homologues fully chlorinated at 1,2,6,9-positions (1,2,6,9-pattern). In the present study, we reexamined PCDD/Fs in sediment cores from urban (Tokyo Bay) and remote (Lake Shinji) areas of Japan using positive matrix factorization (PMF) and revealed a lateral dechlorination fingerprint exhibiting the 1,2,6,9-pattern. Relative molar concentrations of putative lateral dechlorination products linearly increased with sediment depth, suggesting that decades of reaction resulted in the accumulation of hepta- and hexa- chlorinated lateral dechlorination products in the bottom sediment layers. Times required for in situ formation of dechlorination products were estimated to be at least 27.8 +/- 17.9 year(mole %)(-1) in Lake Shinji and 4.7 +/- 0.5 year(mole %)(-1) in Tokyo Bay (both for the formation of 1,2,3,4,6,7,9-HpCDD) and are significantly longer than the dechlorination pathways observed in the laboratory.     

Dehalogenation of polybrominated diphenyl ethers and polychlorinated biphenyl by bimetallic, impregnated, and nanoscale zerovalent iron

Nanoscale zerovalent iron particles (nZVI), bimetallic nanoparticles (nZVI/Pd), and nZVI/Pd impregnated activated carbon (nZVI/Pd-AC) composite particles were synthesized and investigated for their effectiveness to remove polybrominated diphenyl ethers (PBDEs) and/or polychlorinated biphenyls (PCBs). Palladization of nZVI promoted the dehalogenation kinetics for mono- to tri-BDEs and 2,3,4-trichlorobiphenyl (PCB 21). Compared to nZVI, the iron-normalized rate constants for nZVI/Pd were about 2-, 3-, and 4-orders of magnitude greater for tri-, di-, and mono-BDEs, respectively, with diphenyl ether as a main reaction product. The reaction kinetics and pathways suggest an H-atom transfer mechanism. The reaction pathways with nZVI/Pd favor preferential removal of para-halogens on PBDEs and PCBs. X-ray fluorescence mapping of nZVI/Pd-AC showed that Pd mainly deposits on the outer part of particles, while Fe was present throughout the activated carbon particles. While BDE 21 was sorbed onto activated carbon composites quickly, debromination was slower compared to reaction with freely dispersed nZVI/Pd. Our XPS and chemical data suggest about 7% of the total iron within the activated carbon was zerovalent, which shows the difficulty with in-situ synthesis of a significant fraction of zerovalent iron in the microporous material. Related factors that likely hinder the reaction with nZVI/Pd-AC are the heterogeneous distribution of nZVI and Pd on activated carbon and/or immobilization of hydrophobic organic contaminants at the adsorption sites thereby inhibiting contact with nZVI.     

Impact of protein pre-treatment conditions on the iron encapsulation efficiency of whey protein cold-set gel particles

This paper investigates the possibility for iron fortification of food using protein gel particles in which iron is entrapped using cold-set gelation. The aim is to optimize the iron encapsulation efficiency of whey protein by giving the whey protein different heat treatment prior to gelation with iron. The effect of the heat treatment conditions (mild-intermediate-severe) on the iron-induced cold-set gelation process was studied to optimize the gel strength in relation to the iron concentration. Rheology was used to study the protein gel formation, and the stability of the gel particles and iron encapsulation efficiency was determined by measuring the protein and iron content at different pH. Both the iron concentration and the heat treatment conditions appear to affect the gel formation process and gel strength of the iron-induced cold-set gels. With the protein gel particles being stable at a broad pH range, the release of iron from the particles was studied as a function of time. The low release of iron at neutral pH indicated good encapsulation efficiency and capability of whey protein to keep iron bound. At low pH the release of iron increased, as is desired for bio-accessibility. In addition to differences in gel strength, the most relevant result caused by the pre-treatment of the whey protein is revealed in the amount of iron that can be entrapped per protein. It is shown that the amount of iron can be increased going from mild to severe heat treatment conditions. This suggests that the concept of using whey protein particles with iron can effectively be used to fortify food products with iron for human consumption.     

Effect of particle age (Fe0 content) and solution pH on NZVI reactivity: H2 evolution and TCE dechlorination

Subsurface injection of nanoscale zerovalent iron (NZVI) has been used for the in situ remediation of chlorinated solvent plumes and DNAPL source zones. Due to the cost of materials and placement,the efficacy of this approach depends on the NZVI reactivity and longevity, selectivity for the target contaminant relative to nonspecific corrosion to yield H2, and access to the Fe0 in the particles. Both the reaction pH and the age of the particles (i.e., Fe0 content) could affect NZVI reactivity and longevity. Here, the rates of H2 evolution and trichloroethene (TCE) reduction are measured over the lifetime of the particles and at solution pH ranging from 6.5 to 8.9. Crystalline reactive nanoscale iron particles (RNIP) with different initial Fe0 weight percent (48%, 36%, 34%, 27%, and 9.6%) but similar specific surface area were studied. At the equilibrium pH for a Fe(OH)2/H2O system (pH = 8.9), RNIP exhibited first-order decay for Fe0 corrosion (H2 evolution) with respect to Fe0 content with a Fe0 half-life time of 90-180 days. A stable surface area-normalized TCE reduction rate constant 1.0 x 10(-3)L x hr(-1) x m(-2) was observed after 20 days and remained constant for 160 days, while the Fe0 content of the particles decreased by half, suggesting that TCE reduction is zero-order with respect to the Fe0 content of the particle. Solution pH affected H2 evolution and TCE reduction to a different extent. Decreasing pH from 8.9 to 6.5 increased the H2 evolution rate constant 27 fold from 0.008 to 0.22 day(-1), but the TCE dechlorination rate constant only doubled. The dissimilarities between the reaction orders of H2 evolution and TCE dechlorination with respect to both Fe0 content and H+ concentration suggest that different rate controlling steps are involved for the reduction reactions.