Dechlorination Of Hexachlorobenzene Research Articles

Phase distribution of hexachlorobenzene in a suspended-growth culture amended with a polysorbate surfactant.

The effect of a non-ionic surfactant on the phase distribution of hexachlorobenzene (HCB) in a suspended-growth culture system was assessed. Tween 60, a polyoxyethylene sorbitan stearate ester surfactant, and an azide-inactivated, mixed, methanogenic, HCB-dechlorinating culture were used in this work. The sorption of HCB on the biomass as well as sorption and aggregation/precipitation of Tween 60 were experimentally quantified. The values of the HCB and Tween 60 distribution parameters were determined and the phase distribution of HCB in the presence of surfactant and biomass was described quantitatively. Both the HCB and surfactant distribution are highly dependent on the total amount of surfactant present in the system. At low initial surfactant concentrations, most of the HCB is associated with the biomass. As the surfactant concentration increases, the effect of surfactant sorption and precipitation diminishes and the higher pool of surfactant micelles shifts the distribution of HCB from the solid phase to the solution phase. The HCB phase distribution in the presence of the surfactant may have a significant effect on HCB bioavailability for reductive dechlorination. The quantitative description of the HCB phase distribution presented here was subsequently used as the basis for the development of a model that describes the bioavailability and reductive dechlorination of HCB in a surfactant/biomass system.

Dehalococcoides ethenogenes strain 195 reductively dechlorinates diverse chlorinated aromatic pollutants.

Dehalococcoides ethenogenes strain 195 dechlorinates tetrachloroethene to vinyl chloride and ethene, and its genome has been found to contain up to 17 putative dehalogenase gene homologues, suggesting diverse dehalogenation ability. We amended pure or mixed cultures containing D. ethenogenes strain 195 with 1,2,3,4-tetrachlorodibenzo-p-dioxin, 2,3,7,8-tetrachlorodibenzo-p-dioxin, 2,3-dichlorodibenzo-p-dioxin, 1,2,3,4-tetrachlorodibenzofuran, 2,3,4,5,6-pentachlorobiphenyl, 1,2,3,4-tetrachloronaphthalene, various chlorobenzenes, or a mixture of 2-, 3-, and 4-chlorophenol to determine the dehalogenation ability. D. ethenogenes strain 195 dechlorinated 1,2,3,4-tetrachlorodibenzo-p-dioxin to a mixture of 1,2,4-trichlorodibenzo-p-dioxin and 1,3-dichlorodibenzo-p-dioxin. 2,3,4,5,6- Pentachlorobiphenyl was dechlorinated to 2,3,4,6- and/or 2,3,5,6-tetrachlorobiphenyl and 2,4,6-trichlorobiphenyl. 1,2,3,4-Tetrachloronaphthalene was dechlorinated primarily to an unidentified dichloronaphthalene congener. The predominant end products from hexachlorobenzene dechlorination were 1,2,3,5-tetrachlorobenzene and 1,3,5-trichlorobenzene. We did not detect dechlorination daughter products from monochlorophenols, 2,3-dichlorodibenzo-p-dioxin or 2,3,7,8- tetrachlorodibenzo-p-dioxin. D. ethenogenes strain 195 has the ability to dechlorinate many different types of chlorinated aromatic compounds, in addition to its known chloroethene respiratory electron acceptors. Remediation of sediments contaminated with aromatic halogenated organic pollutants such as polychlorinated biphenyls and polychlorinated dibenzo-p-dioxins could require billions of dollars in the coming years. Harnessing microorganisms such as Dehalococcoides spp. that detoxify these compounds via removal of halogens may lead to cost-effective biotechnological approaches for remediation.

Stimulation of reductive dechlorination of hexachlorobenzene in soil by inducing the native microbial activity

The reductive dechlorination and behaviour of 14C-hexachlorobenzene (HCB) was investigated in an arable soil. The activity of the native anaerobic microbial communities could be induced by saturating the soil with water. Under these conditions high rates of dechlorination were observed. After 20 weeks of incubation only 1% of the applied 14C-HCB could be detected in the fraction of extractable residues. Additional organic substances, like wheat straw and lucerne straw, however considerably delayed and reduced the dechlorination process in the soil. The decline of HCB was not only caused by dechlorination but also by the formation of non-extractable residues, whereby their amounts varied with time depending on the experimental conditions. Several dechlorination products were detected, indicating the following main HCB transformation pathway: HCB → PCB → 1,2,3,5-TeCB → 1,3,5-TCB → 1,3-DCB, with 1,3,5-TCB as main intermediate dechlorination product. The other TeCB-, TCB- and DCB-isomers were also detected in low amounts, showing the presence of more than one dechlorination pathway. Since the methane production rates were lowest when the dechlorination rates were highest, it can be assumed that methanogenic bacteria were not involved in the dechlorination process of HCB. The established 14C-mass balances show, that with increasing dechlorination and incubation times, the 14C-recoveries decreased.

Dehalorespiration with hexachlorobenzene and pentachlorobenzene by Dehalococcoides sp. strain CBDB1.

The chlororespiring anaerobe Dehalococcoides sp. strain CBDB1 used hexachlorobenzene and pentachlorobenzene as electron acceptors in an energy-conserving process with hydrogen as electron donor. Previous attempts to grow Dehalococcoides sp. strain CBDB1 with hexachlorobenzene or pentachlorobenzene as electron acceptors failed if these compounds were provided as solutions in hexadecane. However, Dehalococcoides sp. strain CBDB1 was able to grow with hexachlorobenzene or pentachlorobenzene when added in crystalline form directly to cultures. Growth of Dehalococcoides sp. strain CBDB1 by dehalorespiration resulted in a growth yield ( Y) of 2.1+/-0.24 g protein/mol Cl(-) released with hexachlorobenzene as electron acceptor; with pentachlorobenzene, the growth yield was 2.9+/-0.15 g/mol Cl(-). Hexachlorobenzene was reductively dechlorinated to pentachlorobenzene, which was converted to a mixture of 1,2,3,5- and 1,2,4,5-tetrachlorobenzene. Formation of 1,2,3,4-tetrachlorobenzene was not detected. The final end-products of hexachlorobenzene and pentachlorobenzene dechlorination were 1,3,5-trichlorobenzene, 1,3- and 1,4-dichlorobenzene, which were formed in a ratio of about 3:2:5. As reported previously, Dehalococcoides sp. strain CBDB1 converted 1,2,3,5-tetrachlorobenzene exclusively to 1,3,5-trichlorobenzene, and 1,2,4,5-tetrachlorobenzene exclusively to 1,2,4-trichlorobenzene. The organism therefore catalyzes two different pathways to dechlorinate highly chlorinated benzenes. In the route leading to 1,3,5-trichlorobenzene, only doubly flanked chlorine substituents were removed, while in the route leading to 1,3-and 1,4-dichlorobenzene via 1,2,4-trichlorobenzene singly flanked chlorine substituents were also removed. Reductive dehalogenase activity measurements using whole cells pregrown with different chlorobenzene congeners as electron acceptors indicated that different reductive dehalogenases might be induced by the different electron acceptors. To our knowledge, this is the first report describing reductive dechlorination of hexachlorobenzene and pentachlorobenzene via dehalorespiration by a pure bacterial culture.

Reductive Dechlorination of Hexachlorobenzene under Various Additions

Microorganisms collected from sediments of Ho-Tsin River in southern Taiwan were used in this study. The ability to dechlorinate hexachlorobenzene (HCB) was induced by enrich- ment incubation in yeast extract amended culture and acclimation with HCB. Addition of lactate or replacement of yeast extract by lactate did not enhance the dechlorination ability. With strong electron capturing capability, denitrifying bacteria resulted in complete inhibition of dechlorination in the mixed culture contained nitrate. In the culture amended with sulfate, sulfate reducing bateria shared electrons and nutrient with HCB-dechlorinating consortium but grabbed more electrons when treated with vancomycin. Results from multi-factors tests indicate that the influences of factors on dechlorination were complicated. Dechlorinating microbes, electron suppliers, sulfate reducing bacteria and denitrifying bacteria, all possibly caused a great effect on dechlorination.

Radiation Induced Catalytic Dechlorination of Hexachlorobenzene on Oxide Surfaces

Radiation induced catalytic dehalogenation of hexachlorobenzene (HCB) adsorbed to alumina (Al2O3), silica (SiO2), titania (TiO2), zirconia (ZrO2), and a commercially available zeolite has been studied using Cobalt-60 (60Co) as a radiation source. Solid-particulate samples were irradiated over a dose range of 0−58 kGy, and the chemical changes were monitored using Fourier transform infrared diffuse reflectance (FTIR-DR) and gas chromatography with electron capture detection (GC-ECD). The extent of HCB degradation on the metal oxides was found to increase dramatically in samples evacuated under vacuum, pointing to the competitive scavenging of conduction band electrons by surface adsorbed species, primarily oxygen. Coadsorbed water diminished HCB conversion on all oxides but to a greater degree on alumina. HCB degradation on metal oxides was found to be highly dependent upon the conduction band energy of the support material, thus confirming the occurrence of ultra-band-gap excitation and charge separation in irradiated oxides. Higher yields of dechlorination products were witnessed in alumina and silica samples. Zeolite, titania, and zirconia were also found to be inefficient in promoting radiation induced catalysis. The absence of oxidation products in the irradiated HCB/oxide samples suggests the inaccessibility of holes to undergo interfacial charge transfer with the organic substrate.

Specificity of Microbial Activities in the Reductive Dechlorination of Chlorinated Benzenes

Anaerobic mixed cultures collected from tropical sediment were found to possess the ability to dechlorinate pentachlorobenzene (QCB) and hexachlorobenzene (HCB) after a long‐term acclimation in HCB. The primary product for HCB and QCB was 1,3,5‐trichlorobenzene (1,3,5‐TCB). During QCB dechlorination, 1,2,3,5‐tetrachlorobenzene (1,2,3,5‐TeCB) is accumulated as the intermediate, but during the HCB dechlorination, QCB and 1,2,3,5‐TeCB are not found. When treated with chloramphenicol, the microorganisms precultured in HCB were found to dechlorinate HCB for only 1 day and dechlorinate QCB for several days. However, microorganisms precultured in QCB could not dechlorinate HCB at all. Results also indicate that HCB is not directly dechlorinated to 1,3,5‐TCB by a single microbial activity but that various microbial systems are responsible for each step of dechlorination from HCB to 1,3,5‐TCB.

Kinetics of the Sequential Microbial Reductive Dechlorination of Hexachlorobenzene

The reductive dechlorination kinetics of hexachlorobenzene and all other chlorinated benzene congeners (CBs) were evaluated using a mixed enrichment culture derived from a contaminated estuarine sediment. The predominant, sequential hexachlorobenzene dechlorination pathway was hexachlorobenzene → pentachlorobenzene → 1,2,3,5-tetrachlorobenzene → 1,3,5-trichlorobenzene → 1,3-dichlorobenzene. Very low levels of 1,2,4,5-tetrachlorobenzene, 1,2,4-trichlorobenzene, and 1,4-dichlorobenzene were also detected. Batch dechlorination assays were performed separately for all CBs with an initial biomass concentration of 330 mg/L as volatile suspended solids, initial concentrations of CBs of 180 nM, and electron donor in excess. The maximum dechlorination rate (k‘) and the half-saturation coefficient (KS) as well as the pseudo-first-order rate constant (kobs) for eight dechlorination reactions were estimated by nonlinear regression of CBs progress curves data based on a Michaelis−Menten-type and the pseudo-first-order...

Radiolytic Reduction of Hexachlorobenzene in Surfactant Solutions: A Steady-State and Pulse Radiolysis Study

Steady-state and pulse radiolysis experiments have been performed to gain insight into the mechanism of hexachlorobenzene (HCB) degradation in nonionic surfactant (Plurafac RA-40) solutions. This understanding is important for the environmental application of radiolysis to remediate soils contaminated with chlorinated aromatic compounds or to treat surfactant solution wastes from soil washing processes. Steady-state experiments showed that, after an applied dose of 50 kGy, reductive dechlorination of HCB to trichlorobenzene occurs under reducing conditions. Under oxidizing conditions at the same dose, reductive dechlorination proceeds more slowly to yield tetrachlorobenzene. Radiolytic experiments on the surfactant alone showed that the reaction rate constant between hydroxyl radicals and RA-40 (1.09 × 109 M-1 s-1) was nearly 2 orders of magnitude higher than that between hydrated electrons and RA-40 (2.0 × 107 M-1 s-1). Reaction kinetics analysis indicates efficient hydroxyl radical scavenging by surfact...

Kinetic study of liquid-phase hydrodechlorination of hexachlorobenzene on Ni/C and 2%PdNi/C

Liquid-phase hydrodechlorination of hexachlorobenzene was kinetically studied in the presence of both nickel (Ni/C) and palladium-promoted nickel (2%PdNi/C) catalysts under different reaction conditions. Molecular hydrogen (at 1 and 20 atm) and sodium borohydride (NaBH4) were used as reducing agents. In the presence of the nickel catalyst, the hydrodechlorination of C6Cl6 occurs via a consecutive mechanism (removal of one chlorine atom from the substrate at each stage), whereas with the 2%PdNi/C catalyst, the transformation of C6Cl6 occurs via both consecutive and multiplet mechanism (with the elimination of several chlorine atoms without desorption of the chloroaromatic substrate from the catalyst surface). The promotion of the nickel catalysts with palladium substantially changes the selectivity of formation of intermediate products of C6C16 dechlorination. The mechanism of hydrodechlorination of hexachlorobenzene was suggested that explained the presence of only certain products of partial dechlorination of hexachlorobenzene in the reaction medium.

Dechlorination of hexachlorobenzene on MWI fly ash

The dechlorination of hexachlorobenzene on activated municipal waste incinerator fly ash was studied within the temperature range of 150 – 350°C in closed systems under nitrogen atmosphere. The following factors were examined influencing the course of dechlorination: hexachlorobenzene loading, temperature and additives (Cu, CuO, MgO, PbO, ZnO, H 2O, methoxyethanol). The mechanism of dechlorination is outlined.

Effect of Tween surfactants on methanogenesis and microbial reductive dechlorination of hexachlorobenzene

AbstractThe effect of nine Tween surfactants (polyoxyethylene sorbitan fatty acid esters) on methanogenesis and the microbial reductive dechlorination of hexachlorobenzene (HCB) was examined using three anaerobic, HCB‐dechlorinating mixed cultures, one glucose fed and two lactate fed, that were derived from contaminated estuarine sediments. The two lactate‐fed cultures differed significantly in their metabolic activity, especially with respect to the rate and extent of HCB reductive dechlorination. The objective of the study was to screen and select biologically compatible surfactants for use in subsequent research on surfactant‐enhanced bioavailability of sorbed‐phase chlorinated organic contaminants. Addition of the Tween surfactants resulted in lower rates of methanogenesis and HCB dechlorination compared with the control (i.e., surfactant‐free culture) for both the glucose‐fed culture and the lactate‐fed culture, which exhibited a high metabolic activity. Tween−61 and −65 exhibited the lowest inhibition of HCB dechlorination at initial surfactant concentrations of 200 and 1,000 mg/L. On the other hand, the Tween surfactants, especially Tween−81 and −85, enhanced the rates of both methanogenesis and HCB dechlorination when the low metabolic activity lactatefed culture was used. The observed rate enhancements may be attributed to bacterial cell membrane modification by the surfactants. Attempts to correlate methanogenesis rates to surfactant properties (such as the hydrophile‐lipophile balance number, fatty acid carbon chain length, and the number of ethylene oxide groups) were not successful. In general, increased surfactant concentrations resulted in lower rate and extent of HCB dechlorination. Although HCB dechlorination was completely inhibited at a surfactant concentration of 1,000 mg/L, the extent of methanogenesis was not affected. Results from this study demonstrate that despite compositional similarities among the Tween surfactants, their effects on biological activities, such as methanogenesis and reductive dechlorination, can differ significantly.

Photosensitized dechlorination of polychlorinated benzenes. 1. Carbazole-photosensitized dechlorination of hexachlorobenzene

Photosensitized dechlorination of hexachlorobenzene (HCBz) in hexane and in acetonitrile (MeCN) solutions as well as in aqueous solutions of poly(sodium styrenesulfonate-co-N-vinylcarbazole) (PSSS-VCz) copolymer has been studied. It was found that the reaction was photosensitized by both molecular and polymeric carbazole (Cz) chromophores. The products have been identified using GC, NMR and MS analyses. The detected products were: pentachlorobenzene, tetrachlorobenzenes, trichlorobenzenes, dichlorobenzenes, chlorobenzene and benzene. It was postulated that dechlorination process is mediated by an electron transfer from the excited carbazole chromophores to the molecule of chlorinated benzene with subsequent heterolytic cleavage of the CCl bond. The photosensitized by Cz dechlorination process is a stepwise reaction. The rate of photosensitized degradation of HCBz in aqueous PSSS-VCz solution was considerably higher than in organic solvents.

Microbial dechlorination of hexachlorobenzene in anaerobic sewage sludge

From this study of the dechlorination of hexachlorobenzene (HCB) in municipal sewage sludge by an anaerobic mixed culture, we determined the dechlorination rate to be 0.29 mg/L/day under the optimal incubation conditions of pH M, 30 C temperature, and a concentration of total solids at 26.8 g/L. In addition, we found that dechlorination did not occur during agitation. Biotransformation occurred along a pathway of HCB → pentachlorobenzene (QCB) → 1,2,3,4tetrachlorobenzene (TecB) + 1,2,3,5-TeCB → 1,2,4-techlorobenzene (TCB) + 1,2,3-TCB + 1,3,5-TCB → 1,2-(dichlorobenzenc (DCB) + 1,4-DCB. Sequential dechlorination was observed within a substrate concentration range of 2–50 mg/L, but slowed down significantly at concentrations of 50 mg/L or higher. High-to-low dechlorination rates under three reducing conditions were identified as methanogenic conditions (0.30 mg/L/day) > sulfate-reducing conditions (0.12 mg/L/day) > denitrifying conditions (0.08 mg/L/day). 01998 Elsevier Science Ltd.

Assessment of metal inhibition of reductive dechlorination of hexachlorobenzene at a superfund site

AbstractLaboratory studies were conducted to identify easily measured parameters that would predict the potential for metal inhibition in sediments co‐contaminated by heavy metals and hexachlorobenzene (HCB) at the Petro Processors Superfund site. Metal inhibition was investigated using HCB‐contaminated sediment and a range of added metal concentrations (1–1,000 mg/kg). Cadmium, Cu, Pb, and Zn all caused inhibition of HCB reductive dechlorination and methane production with inhibiting concentrations of Cd and Pb equivalent to those found in portions of the site. In the second phase of studies, laboratory experiments investigated the relationship between Cd and Pb availability on the kinetics of HCB degradation. Cadmium and Pb affected both dechlorination rates and lag times. In addition, Cd affected the appearance of lower chlorinated benzene metabolites. Inhibition of HCB dechlorination was well correlated to ratios of acid‐volatile sulfide (AVS) to simultaneously extractable metals (SEM) and water‐soluble Cd. Only treatments with a measurable concentration (>6 μg/L) of free, uncomplexed Cd exhibited inhibition. Lead inhibited HCB dechlorination at all concentrations tested (10–1,000 mg/kg) and inhibition did not correlate to either water‐soluble Pb concentrations or AVS/SEM ratios. For both Cd and Pb, methane production was less sensitive to metals than HCB dechlorination, making production of methane a poor indicator of inhibition. Measurement of free, uncomplexed Cd was the best predictor of Cd inhibition while no suitable indicators were found for Pb. These results demonstrate the importance of metal bioavailability on the inhibition of reductive dechlorination.

Microbial hexachlorobenzene dechlorination under three reducing conditions

The potential dechlorination of hexachlorobenzene (HCB) in medium by 1,2,3-trichlorobenzene (TCB)-adapted mixed culture under three reducing conditions was investigated. It was found that strongest to weakest HCB dechlorination occurred in the order of methanogenic conditions > sulfate-reducing conditions > denitrifying conditions. Under denitrifying conditions, no dechlorination was observed during the first 20 days of incubation. Biotransformation occurred in this order: HCB → pentachlorobenzene (PCB) → 1,2,3,5-tetrachlorobenzene (TeCB) → 1,3,5-TCB + 1,2,4-TCB → 1,3-dichlorobenzene (DCB). HCB dechlorination was delayed following treatment with ferric chloride and manganese dioxide, but enhanced by the addition of lactate and pyruvate under methanogenic or sulfate-reducing conditions; the addition of acetate had no significant effect on HCB dechlorination under any of the three reducing conditions. Sequential dechlorination was observed at concentrations of 2–50 mg/L, but a significantly slower rate at the highest concentrations.

Initiation of [ 36 Cl]hexachlorobenzene dechlorination in three different soils under artificially induced anaerobic conditions

The potential for reductive dechlorination of hexachlorobenzene was investigated in samples of three different, naturally oxic soils held under conditions of high oxygen deficiency. The soils were water-saturated and the influence on dechlorination of adding different electron donors, a surfactant and an anaerobic microbial consortium was tested. The influence of supplied electron donors seems to depend on the organic matter content of the soils. Dechlorination in the organic-matter-rich soil from Maulach was not affected by amendment with organic electron donors. A release of about 40% chloride within 140 days was observed for this soil in all biotic-treated assays. By contrast, the organic-matter-poor soil of Eppingen showed no dechlorination in unamended assays. However, when it was supplemented with organic electron donors dechlorination of 2%–37% occurred within 140 days, depending on the type of electron donor. Complex substrate (wheat strawdust), from which carbon is slowly liberated, gave the best results. These two soils had an indigenous dechlorinating anaerobic microflora, whereas the third soil (Rastatt) required inoculation with an anaerobic consortium for dechlorination. The addition of electron donors alone did not cause dechlorination in this sandy soil. The addition of a surfactant (Tween 80) to increase the bioavailability of hexachlorobenzene did not enhance dechlorination. This process was not inhibited by inherent alternative electron acceptors in soil (NO3−, SO42−, Fe3+). The dechlorination did not require methanogenic conditions.

Reductive Dechlorination of Hexachlorobenzene by an Anaerobic Mixed Culture

The potential for reductive dechlorination of hexachlorobenzene (HCB) by a 1,2,3-trichlorobenzene (TCB)-adapted mixed culture was investigated. Optimal dechlorination conditions were assessed at 29 °C ∼ 37 °C and pH 6.1 ∼ 6.9. The observed transformation pathway was HCB → pentachlorobenzene (PCB) → 1,2,3,5-tetrachlorobenzene (TeCB) → 1,3,5-TCB. The dechlorination of HCB was delayed by the addition of ferric chloride and manganese dioxide as electron acceptors, but enhanced by the addition of lactate and pyruvate as electron donors. However, we found that the addition of acetate had no significant effect on HCB dechlorination. Following treatment with bromoethanesulfonic acid (BESA) and vancomycin, it was suggested that the methane-producing bacteria was involved in the dechlorination of CBs.

Reductive dechlorination of chlorobenzenes in surfactant-amended sediment slurries

Microbial anaerobic dechlorination of hexachlorobenzene (HCB) was examined in sediment slurries amended with two classes of nonionic surfactant, polyoxyethylene (POE) sorbitan fatty acid esters (Tweens) and POE alcohols (Brijs). The rationale for surfactant addition was to increase the bioavailability of highly sorbed organic pollutants to degrading microorganisms by enhancing their solubility. The solubility of HCB was initially enhanced via micellar partitioning ; however, primary degradation of most surfactants occurred within 10 d. Dechlorination activity was significantly reduced at POE alcohol concentrations above the critical micelle concentration (cmc), with or without the occurrence of surfactant degradation. Tween 80 decreased HCB dechlorination at concentrations significantly above the cmc (1,500-5,000 mg/L). At concentrations closer to the cmc (300-1,200 mg/L), Tween 80 increased dechlorination rate constants four- to fivefold in acclimated slurries. Additions of Tween 80 at or below the cmc (3, 30, 300, and 911 mg/L) stimulated dechlorination activity in unacclimated slurries that exhibited very little activity in unamended controls. An average of 89% of HCB was dechlorinated after 90 d, compared to 20% in unamended sediments. No effect was observed for POE alcohols at these sub-cmc levels. The lack of a stimulated response for the POE alcohols suggests that Tween 80 may not be acting simply as a source of carbon or energy.

REDUCTIVE DECHLORINATION OF CHLOROBENZENES IN SURFACTANT-AMENDED SEDIMENT SLURRIES

Microbial anaerobic dechlorination of hexachlorobenzene (HCB) was examined in sediment slurries amended with two classes of nonionic surfactant, polyoxyethylene (POE) sorbitan fatty acid esters (Tweens) and POE alcohols (Brijs). The rationale for surfactant addition was to increase the bioavailability of highly sorbed organic pollutants to degrading microorganisms by enhancing their solubility. The solubility of HCB was initially enhanced via micellar partitioning; however, primary degradation of most surfactants occurred within 10 d. Dechlorination activity was significantly reduced at POE alcohol concentrations above the critical micelle concentration (cmc), with or without the occurrence of surfactant degradation. Tween 80 decreased HCB dechlorination at concentrations significantly above the cmc (1,500–5,000 mg/L). At concentrations closer to the cmc (300–1,200 mg/L), Tween 80 increased dechlorination rate constants four- to fivefold in acclimated slurries. Additions of Tween 80 at or below the cmc (3, 30, 300, and 911 mg/L) stimulated dechlorination activity in unacclimated slurries that exhibited very little activity in unamended controls. An average of 89% of HCB was dechlorinated after 90 d, compared to 20% in unamended sediments. No effect was observed for POE alcohols at these sub-cmc levels. The lack of a stimulated response for the POE alcohols suggests that Tween 80 may not be acting simply as a source of carbon or energy.