Reductive Dechlorination Of Hexachlorobenzene Research Articles

Polyphasic characterization of an anaerobic hexachlorobenzene-dechlorinating microbial consortium with a wide dechlorination spectrum for chlorobenzenes

An anaerobic consortium that was capable of reductively dechlorinating hexachlorobenzene (HCB) to benzene was enriched from contaminated sediment. The consortium was capable of dechlorinating all chlorobenzene isomers except 1,4-dichlorobenzene. Singly and doubly flanked chlorines, as well as unflanked meta-substituted chlorines, were dechlorinated, although doubly flanked chlorines were preferred. Formate, acetate and lactate (but not ethanol) could be utilized as optimum electron donors for reductive dechlorination. Alternative electron acceptors, including nitrate and sulfate, completely inhibited HCB degradation, whereas amorphous iron oxide (FeOOH) did not suppress dechlorination activity. No degradation was found in chloramphenicol-treated consortium; however, vancomycin, molybdate, and 2-bromoethanesulfonate did not inhibit HCB dechlorination. The results of inhibitory treatments suggested that the dechlorinators were non-sulfate-reducing gram-negative or vancomycin resistant gram-positive bacteria. In addition to physiological characterization, analyses of 16S rRNA gene library of the consortium and quantitative PCR of 16S rRNA genes suggested that Dehalococcoides sp. was involved in the reductive dechlorination of HCB, and Geobacter sp. may serve as a dechlorinating candidate.

Reductive dechlorination of hexachlorobenzene by Cu/Fe bimetal in the presence of nonionic surfactant

This study investigated the enhancement of nonionic surfactant (TritonX-100, TX-100) on the reductive dechlorination of hexachlorobenzene (HCB) by microscale Cu/Fe bimetal. HCB reduction by Cu/Fe without TX-100, in aqueous TX-100, adsorbed with TX-100 and prepared with TX-100 was compared. Results showed that HCB reduction by Cu/Fe in aqueous TX-100 increased with increasing TX-100 concentration, reached a maximum of 98.6% when aqueous TX-100 concentration attained its critical micelle concentration (CMC), and decreased with the further increase in TX-100 concentration. The mechanism was suggested as the increased HCB mass transport at low TX-100 concentration below CMC and the coverage of reactive sites at high TX-100 concentration above CMC. The greatest reduction was achieved by Cu/Fe in the presence of 0.2 mM TX-100, followed by Cu/Fe adsorbed and prepared with 0.2 mM TX-100. Pentachlorobenzene and tetrachlorobenzene were found as the main dechlorination intermediates. Successive reduction by Cu/Fe in aqueous TX-100 solution could be maintained above 98% for four cycles. It is an alternative to use nonionic surfactants to enhance the reductive dechlorination of hydrophobic polychlorinated aromatic hydrocarbons (PCAH) by Cu/Fe bimetal.

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.

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.

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.

Complete degradation of polychlorinated hydrocarbons by a two-stage biofilm reactor

A two-stage anaerobic-aerobic biofilm reactor successfully degraded a mixture of chlorinated organic compounds to water-soluble metabolic intermediates and carbon dioxide. Reductive dechlorination of hexachlorobenzene (HCB), tetrachloroethylene (PCE), and chloroform (CF) occurred on all tested primary carbon sources such as glucose, methanol, and acetate. However, the extent of dechlorination was maximum when the anaerobic biofilm column was fed acetate as a primary carbon source. HCB, PCE, and CF were dechlorinated to the levels of tri- and dichlorinated products (99, 80, and 32%, respectively) with acetate in the feed. This is important, since these less-chlorinated compounds can be metabolized by the aerobic biofilm. The effluent from the anaerobic biofilm column was fed directly into the aerobic column. After both columns, the total amount transformed into nonvolatile intermediates and carbon dioxide was 94, 96, and 83% for [14C]HCB, [14C]trichloroethylene, and [14C]CF, respectively. This research shows the potential application of this novel two-stage bioreactor system for treating groundwaters and industrial effluents composed of highly chlorinated aliphatic and aromatic hydrocarbons.