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.     

TCE dechlorination rates, pathways, and efficiency of nanoscale iron particles with different properties

Nanoscale Fe0 particles are a promising technology for in situ remediation of trichloroethene (TCE) plumes and TCE-DNAPL source areas, butthe physical and chemical properties controlling their reactivity are not yet understood. Here, the TCE reaction rates, pathways, and efficiency of two nanoscale Fe0 particles are measured in batch reactors: particles synthesized from sodium borohydride reduction of ferrous iron (Fe/B) and commercially available particles (RNIP). Reactivity was determined under iron-limited (high [TCE]) and excess iron (low [TCE]) conditions and with and without added H2. Particle efficiency, defined as the fraction of the Fe0 in the particles that is used to dechlorinate TCE, was determined under iron-limited conditions. Both particles had a core/shell structure and similar specific surface areas (approximately 30 m2/g). Using excess iron, Fe/B transformed TCE into ethane (80%) and C3-C6 coupling products (20%). The measured surface area normalized pseudo-first-order rate constant for Fe/B (1.4 x 10(-)2 L.h(-1).m(-2) is approximately 4-fold higher than for RNIP (3.1 x 10-(3) L.h(-1).m(-2). All the Fe0 in Fe/B was accessible for TCE dechlorination, and 92 +/- 0.7% of the Fe0 was used to reduce TCE. For Fe/B, H2 evolved from reduction of water (H+) was subsequently used for TCE dechlorination, and adding H2 to the reactor increased both the dechlorination rate and the mass of TCE reduced, indicating that a catalytic pathway exists. RNIP yielded unsaturated products (acetylene and ethene). Nearly half (46%) of the Fe0 in RNIP was unavailable for TCE dechlorination over the course of the experiment and remained in the particles. Adding H2 did not change the reaction rate or efficiency of RNIP. Despite this, the mass of TCE dechlorinated per mass of Fe0 added was similar for both particles due to the less saturated products formed from RNIP. The oxide shell composition and the boron content are the most likely causes for the differences between the particle types.     

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).     

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.     

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).     

Combined removal of chlorinated ethenes and heavy metals by zerovalent iron in batch and continuous flow column systems

The combined removal of chlorinated ethenes and heavy metals from a simulated groundwater matrix by zerovalent iron (ZVI) was investigated. In batch, Ni (5-100 mg L(-1)) enhanced trichloroethene (TCE, 10 mg L(-1)) reduction by ZVI (100 g L(-1)) due to catalytic hydrodechlorination by bimetallic Fe0/Ni0. Cr(VI) or Zn (5-100 mg L(-1)) lowered TCE degradation rates by a factor of 2 to 13. Cr(VI) (100 mg L(-1)) in combination with Zn or Ni (50-100 mg L(-1)) inhibited TCE degradation. Addition of 20% H2(g) in the headspace, or of Zn (50-100 mg L(-1)), enhanced TCE removal in the presence of Ni and Cr(VI). Sorption of Zn to ZVI alleviated the Cr(VI) induced inhibition of bimetallic Fe0/Ni0 apparently due to release of protons necessary for TCE hydrodechlorination. In continuous ZVI columns treating tetrachloroethene (PCE, 1-2 mg L(-1)) and TCE (10 mg L(-1)), and a mixture of the metals Cr(VI), Zn(II), and Ni(II) (5 mg (L-1)), the PCE removal efficiency decreased from 100% to 90% in columns operated without heavy metals. The PCE degradation efficiency remained above 99% in columns receiving heavy metals as long as Ni was present. The findings of this study indicate the feasibility and limitations of the combined treatment of mixtures of organic and inorganic pollutants by ZVI.     

Abiotic reductive dechlorination of chlorinated ethylenes by iron-bearing soil minerals. 2. Green rust

Abiotic reductive dechlorination of chlorinated ethylenes by the sulfate form of green rust (GR(SO4)) was examined in batch reactors. Dechlorination kinetics were described by a modified Langmuir-Hinshelwood model. The rate constant for reductive dechlorination of chlorinated ethylenes at reactive GR(SO4) surfaces was in the range of 0.592 (+/-4.4%) to 1.59 (+/-6.3%) day(-1). The specific reductive capacity of GR(SO4) for target organics was in the range of 9.86 (+/-10.1%) to 18.0 (+/-4.3%) microM/g and sorption coefficient was in the range of 0.53 (+/-2.4%) to 1.22 (+/-4.3%) mM(-1). Surface area-normalized pseudo-first-order initial rate constants for chlorinated ethylenes by GR(SO4) were 3.4 to 8.2 times greater than those by pyrite. Chlorinated ethylenes were mainly transformed to acetylene, and no detectable amounts of chlorinated intermediates were observed. The rate constants for the reductive dechlorination of trichloroethylene (TCE) increased as pH increased (6.8 to 10.1) but were independent of solid concentration and initial TCE concentration. Magnetite and/or maghemite were produced by the oxidation of GR(SO4) by TCE. These findings are relevant to the understanding of the role of abiotic reductive dechlorination during natural attenuation in environments that contain GR(SO4).     

Effects of the nonionic surfactant tween 80 on microbial reductive dechlorination of chlorinated ethenes

Recent field studies have indicated synergistic effects of coupling microbial reductive dechlorination with physicochemical remediation (e.g., surfactant flushing) of dense nonaqueous phase liquid (DNAPL) source zones. This study explored chlorinated ethene (e.g., tetrachloroethene [PCE]) dechlorination in the presence of 50-5000 mg/L Tween 80, a nonionic surfactant employed in source zone remediation. Tween 80 did not inhibit dechlorination by four pure PCE-to-cis-1,2-dichloroethene (cis-DCE) or PCE-to-trichloroethene (TCE) dechlorinating cultures. In contrast, cis-DCE-dechlorinating Dehalococcoides isolates (strain BAV1 and strain FL2) failed to dechlorinate in the presence of Tween 80. Bio-Dechlor INOCULUM (BDI), a PCE-to-ethene dechlorinating consortium, produced cis-DCE in the presence of Tween 80, further suggesting that Tween 80 inhibits dechlorination by Dehalococcoides organisms. Quantitative real-time PCR analysis applied to BDI revealed that the number of Dehalococcoides cells decayed exponentially (R(2) = 0.85) according to the Chick-Watson disinfection model (pseudo first-order decay rate of 0.13+/-0.02 day(-1)) from an initial value of 6.6 +/-1.5 x 10(8) to 1.3+/-0.8 x 10(5) per mL of culture after 58 days of exposure to 250 mg/L Tween 80. Although Tween 80 exposure prevented ethene formation and reduced Dehalococcoides cell numbers, Dehalococcoides organisms remained viable, and dechlorination activity pist cis-DCE was recovered following the removal of Tween 80. These findings suggest that sequential Tween 80 flushing followed by microbial reductive dechlorination is a promising strategy for remediation of chlorinated ethene-impacted source zones.     

Distinguishing abiotic and biotic transformation of tetrachloroethylene and trichloroethylene by stable carbon isotope fractionation

Significant carbon isotope fractionation was observed during FeS-mediated reductive dechlorination of tetrachloroethylene (PCE) and trichloroethylene (TCE). Bulk enrichment factors (E(bulk)) for PCE were -30.2 +/- 4.3 per thousand (pH 7), -29.54 +/- 0.83 per thousand (pH 8), and -24.6 +/- 1.1 per thousand (pH 9). For TCE, E(bulk) values were -33.4 +/- 1.5 per thousand (pH 8) and -27.9 +/- 1.3 per thousand (pH 9). A smaller magnitude of carbon isotope fractionation resulted from microbial reductive dechlorination by two isolated pure cultures (Desulfuromonas michiganensis strain BB1 (BB1) and Sulfurospirillum multivorans (Sm)) and a bacterial consortium (BioDechlor INOCULUM (BDI)). The E(bulk) values for biological PCE microbial dechlorination were -1.39 +/- 0.21 per thousand (BB1), -1.33 +/- 0.13 per thousand (Sm), and -7.12 +/- 0.72 per thousand (BDI), while those for TCE were -4.07 +/- 0.48 per thousand (BB1), -12.8 +/- 1.6 per thousand (Sm), and -15.27 +/- 0.79 per thousand (BDI). Reactions were investigated by calculation of the apparent kinetic isotope effect for carbon (AKIEc), and the results suggest that differences in isotope fractionation for abiotic and microbial dechlorination resulted from the differences in rate-limiting steps during the dechlorination reaction. Measurement of more negative E(bulk) values at sites contaminated with PCE and TCE may suggest the occurrence of abiotic reductive dechlorination by FeS.     

Reductive dechlorination of tetrachloroethene to trans-dichloroethene and cis-dichloroethene by PCB-dechlorinating bacterium DF-1

Polychlorinated biphenyls (PCBs) and chlorinated ethenes (CEs) are known to pollute sediment, soil, and groundwater. The anaerobic dechlorination of these compounds is an integral part of their biodegradation in polluted environments. We report for the first time the dechlorination of tetrachloroethene (PCE) and trichloroethene (TCE) by bacterium DF-1. This PCB and chlorobenzene dechlorinating bacterium dechlorinated PCE to TCE, which was then converted into trans-1,2-dichloroethene (trans-DCE) and cis-1,2-dichloroethene (cis-DCE). The ratio of trans-DCE to cis-DCE produced by the culture had a range of 1.2-1.7. Bacterium DF-1 has been enriched in co-culture with a desulfovibrio-like microorganism. PCR-denaturing gradient gel electrophoresis (PCR-DGGE) analysis of the 16S rRNA genes of the co-culture demonstrated that DF-1 was enriched during the dechlorination of PCE, PCB, and chlorobenzene. DF-1 was not detected in the absence of PCE dechlorination and the desulfovibrio-like organism, isolated in pure culture, did not dechlorinate PCE. This is the first identification of a microorganism capable of producing high amounts of trans-DCE from PCE and indicates that microorganisms such as DF-1 are a possible biological source of trans-DCE in the environment.