Chemical-physical parameters and microbial community changes induced by electrodes polarization inhibit PCB dechlorination in a marine sediment

Microbial reductive dechlorination of organohalogenated pollutants is often limited by the scarcity of electron donors, that can be overcome with microbial electrochemical technologies (METs). In this study, polarized electrodes buried in marine sediment microcosms were investigated to stimulate PCB reductive dechlorination under potentiostatic (-0.7 V vs Ag/AgCl) and galvanostatic conditions (0.025 mA·cm-2-0.05 mA·cm-2), using graphite rod as cathode and iron plate as sacrificial anode. A single circuit and a novel two antiparallel circuits configuration (2AP) were investigated. Single circuit polarization impacted the sediment pH and redox potential (ORP) proportionally to the intensity of the electrical input and inhibited PCB reductive dechlorination. The effects on the sediment's pH and ORP, along with the inhibition of PCB reductive dechlorination, were mitigated in the 2AP system. Electrodes polarization stimulated sulfate-reduction and promoted the enrichment of bacterial clades potentially involved in sulfate-reduction as well as in sulfur oxidation. This suggested the electrons provided were consumed by competitors of organohalide respiring bacteria and specifically sequestered by sulfur cycling, which may represent the main factor limiting the applicability of METs for stimulating PCB reductive dechlorination in marine sediments.

Advances and challenges in biocathode microbial electrolysis cells for chlorinated organic compounds degradation from electroactive perspectives

Microbial electrolysis cell (MEC) is a promising in-situ strategy for chlorinated organic compound (COC) pollution remediation due to its high efficiency, low energy input, and long-term potential. Reductive dechlorination as the most critical step in COC degradation which takes place primarily in the cathode chamber of MECs is a complex biochemical process driven by the behavior of electrons. However, no information is currently available on the internal mechanism of MEC in dechlorination from the perspective of the whole electron transfer procedure and its dependent electrode materials. This review addresses the underlying mechanism of MEC on the fundamental of the generation (electron donor), transmission (transfer pathway), utilization (functional microbiota) and reception (electron acceptor) of electrons in dechlorination. In addition, the vital role of varied cathode materials involved in the entire electron transfer procedure during COC dechlorination is emphasized. Subsequently, suggestions for future research, including model construction, cathode material modification, and expanding the applicability of MECs to removal gaseous COCs have been proposed. This paper enriches the mechanism of COC degradation by MEC, and thus provides the theoretical support for the scale-up bioreactors for efficient COC removal.

Response of 2,4,6-trichlorophenol-reducing biocathode to burial depth in constructed wetland sediments

Biocathode systems could be used for in-situ bioremediation of chlorophenols (CPs) in constructed wetland (CW) sediments. However, little is known regarding whether or how cathode burial depths affect the dechlorination of CPs in sediments. Here, 2,4,6-trichlorophenol (2,4,6-TCP)-dechlorinating biocathode systems were constructed under a cathode potential of - 0.7 V (vs. a saturated calomel electrode, SCE) at three different cathode burial depths (5, 10, and 15 cm). The 2,4,6-TCP removal efficiency and average transformation rate with the biocathode increased by 21.46-36.86% and 14.63-34.88% compared to those in the non-electrode groups. Deeper cathode burial depths enhanced the 2,4,6-TCP dechlorination performance. Furthermore, the oxidation-reduction potential (ORP) of the sediment decreased with sediment depth and the applied potential created a more favorable redox environment for the enrichment of functional bacteria. Deeper cathode burial depths also promoted the selective enrichment of electro-active and dechlorinating bacteria (e.g., Bacillus and Dehalobacter, respectively). The biocathode thus served as the carrier, electron source, and regulator of functional bacteria to accelerate the transformation of 2,4,6-TCP (2,4,6-TCP → 2,4-dichlorophenol → 4-chlorophenol → phenol) in sediments. These results offer insights into the effects of cathode burial depth on 2,4,6-TCP dechlorination in sediments from a redox environment and microbial community structure standpoint.

Electrostimulated bio-dechlorination of a PCB mixture (Aroclor 1260) in a marine-originated dechlorinating culture

Aroclor 1260, a commercial polychlorinated biphenyl (PCB) mixture, is highly recalcitrant to biotransformation. A negatively polarized cathode (-0.35 V vs. standard hydrogen electrode) was applied for the first time to a marine-origin PCB dechlorinating culture that substantially increased the microbial dechlorination rate of Aroclor 1260 (from 8.6 to 11.6 μM Cl^- d^-1); meta-chlorine removal was stimulated and higher proportions of tetra-CBs (43.2-46.6%), the predominant dechlorination products, were observed compared to the open circuit conditions (23.7-25.1%). The dechlorination rate was further enhanced (14.1 μM Cl^- d^-1) by amendment with humin as a solid-phase redox mediator. After the suspension culture was renewed using an anaerobic medium, dechlorination activity was effectively maintained solely by cathodic biofilms, where cyclic voltammetry results indicated their redox activity. Electric potential had a significant effect on microbial community structure in the cathodic biofilm, where a greater abundance of Dehalococcoides (2.59-3.02%), as potential dechlorinators, was observed compared to that in the suspension culture (0.41-0.55%). Moreover, Dehalococcoides adhering to the cathode showed a higher chlorine removal rate than in the suspension culture. These findings provide insights into the dechlorination mechanism of cathodic biofilms involving Dehalococcoides for PCB mixtures and extend the application prospects of bioremediation to PCB contamination in the natural environment.

Bioremediation of typical chlorinated hydrocarbons by microbial reductive dechlorination and its key players: A review

Chlorinated hydrocarbon contamination in soils and groundwater has a severe negative impact on the human health. Microbial reductive dechlorination is a major degradation pathway of chlorinated hydrocarbon in anaerobic subsurface environments, has been extensively studied. Recent progress on the diversity of the reductive dechlorinators and the key enzymes of chlororespiration has been well reviewed. Here, we present a thorough overview of the studies related to bioremediation of chloroethenes and polychlorinated biphenyls based on enhanced in situ reductive dechlorination. The major part of this review is to provide an up-to-date summary of functional microorganisms which are either detected during in situ biostimulation or applied in bioaugmentation strategies. The applied biostimulants and corresponding reductive dechlorination products are also summarized and the future research needs are finally discussed.

Biogenic iron mineralization of polyferric sulfate by dissimilatory iron reducing bacteria: Effects of medium composition and electric field stimulation

Polyferric sulfate (PFS) is a coagulant widely used for removing contaminants from the aqueous phase; however, PFS destabilizes and recrystallizes in the solid phase in the presence of dissimilatory iron reducing bacteria (DIRB), which has a profound influence on the cycle of Fe and the fate of the associated pollutants. Our objective is to investigate the combined effects of medium composition and electric field stimulation on the biomineralization of PFS. Batch experiments were conducted with PFS and the DIRB Shewanella oneidensis MR-1 under anoxic conditions to examine the microbial reduction of PFS to Fe(II) and its subsequent biotransformation. The high concentration of phosphorous in phosphate buffer solution (PBS) is responsible for slower and less extensive Fe(II) generation compared to the lower concentration of phosphorous in a medium of 1,4-piperazinediethanesulfonic acid (PIPES). The PBS system induces the formation of green rust (SO₄^2-) and vivianite as the major minerals; in contrast, magnetite is the predominant end product in the PIPES system. The application of an anodic potential of 0.2 V significantly stimulates Fe(II) release from PFS, leading to precipitation and transformation of more crystalline minerals in increased quantities. The results demonstrate that Fe(II) catalyzes biomineralization of PFS to a variety of secondary products; this electron transfer process is highly dependent on the rate and magnitude of PFS reduction and the surface reaction with the host compound and adsorbed ions.

Effects of Ferric Oxyhydroxide on Anaerobic Microbial Dechlorination of Polychlorinated Biphenyls in Hudson and Grasse River Sediment Microcosms: Dechlorination Extent, Preferences, Ortho Removal, and Its Enhancement

Microbial reductive dechlorination of polychlorinated biphenyls (PCBs) has been observed in many PCB-impacted sediments. However, this biodegradation is relatively site-specific and can be affected by PCB compositions and sediment geochemical conditions. To better understand the influence of a common competing electron acceptor, ferric oxyhydroxide (FeOOH), on dechlorination, two sediments (Hudson River and Grasse River sediments), and two PCB mixtures (PCB 5/12, 64/71, 105/114, and 149/153/170 in Mixture 1 and PCB 5/12, 64/71, 82/97/99, 144/170 in Mixture 2) were used for this microcosm study. The addition of 40 mmole/kg FeOOH completely inhibited PCB dechlorination in the Hudson sediment, but only moderately inhibited PCB dechlorination in the Grasse sediment with a 3-week longer lag time. The inhibitory effect in the Grasse sediment was mainly due to the loss of unflanked para dechlorination activity. Fe(II) analysis showed that dechlorination started prior to the consumption of Fe(III), which indicates PCB reduction and Fe(III) reduction were able to take place concurrently. Dehalococcoides 16S rRNA genes increased with the commencement of dechlorination in the Grasse sediment, but not in the completely inhibited Hudson sediment. Rare ortho dechlorination pathways were identified in FeOOH-amended Grasse sediment microcosms, dominated by transformations of PCB 25(24-3-CB) to PCB 13(3-4-CB) and PCB 28(24-4-CB) to PCB 15(4-4-CB). The addition of carbon sources (acetate or a fatty acid mixture with acetate, propionate, and butyrate) after 27 weeks of incubation reinitiated dechlorination in FeOOH-amended Hudson sediment microcosms. Also, the addition of carbon sources greatly enhanced ortho dechlorination in FeOOH-amended Grasse microcosms, indicating the utilization of acetate and/or the fatty acid mixture for ortho dechlorination-related microorganisms. A dechlorination pathway analysis approach revealed that para-flanked meta dechlorination was primarily preferred followed by ortho-/double-flanked meta dechlorination and single-/double-flanked para dechlorination in the Grasse sediment.

Time-dependent bacterial community and electrochemical characterizations of cathodic biofilms in the surfactant-amended sediment-based bioelectrochemical reactor with enhanced 2,3,4,5-tetrachlorobiphenyl dechlorination

Applying an electric field to stimulate the microbial reductive dechlorination of polychlorinated biphenyls (PCBs) represents a promising approach for bioremediation of PCB-contaminated sites. This study aimed to demonstrate the biocathodic film-facilitated reduction of PCB 61 in a sediment-based bioelectrochemical reactor (BER) and, more importantly, the characterizations of electrode-microbe interaction from microbial and electrochemical perspectives particularly in a time-dependent manner. The application of a cathodic potential (-0.45 V vs. SHE) significantly improved the rate and extent of PCB 61 dechlorination compared to the open-circuit scenario (without electrical stimulation), and the addition of an external surfactant further increased the dechlorination, with Tween 80 exerting more pronounced effects than rhamnolipid. The bacterial composition of the biofilms and the bioelectrochemical kinetics of the BERs were found to be time-dependent and to vary considerably with the incubation time and slightly with the coexistence of an external surfactant. Excellent correlations were observed between the dechlorination rate and the relative abundance of Dehalogenimonas, Dechloromonas, and Geobacter, the dechlorination rate and the cathodic current density recorded from the chronoamperometry tests, and the dechlorination rate and the charge transfer resistance derived from the electrochemical impedance tests, with respect to the 120 day-operation. After day 120, PCB 61 was resistant to further appreciable reduction, but substantial hydrogen production was detected, and the bacterial community and electrochemical parameters observed on day 180 were not distinctly different from those on day 120.