Electrochemical stimulation of microbial reductive dechlorination of pentachlorophenol using solid-state redox mediator (humin) immobilization

Effects of humic electron mediators on reductive dechlorination of polychlorinated biphenyl by consortia enriched from terrestrial and marine environments

Humic electron mediators can facilitate the reductive dehalogenation of organohalogenated compounds by accelerating electron transfer. To investigate the effect of humic electron mediators on the microbial anaerobic reductive dechlorination of Polychlorinated biphenyls (PCBs), three types of humic electron mediators, humin (HM), humic acid (HA), and anthraquinone-2,6-disulfonic acid (AQDS, HA analogs), were added to PCB dechlorination cultures enriched from different sources in terrestrial and marine environments (T and M cultures). The results showed that meta- and para-site dechlorination occurred in the M culture, while only meta-site dechlorination occurred in the T culture. The dechlorination process N and the dechlorination process H or H' are presented in both cultures. HM enhanced PCB dechlorination metabolic activity in both cultures mainly by promoting meta-site dechlorination. HA showed a weak promoting effect on the M culture by promoting para-chlorine removal but inhibited the dechlorination metabolism of the terrestrial-origin culture, inhibiting meta-chlorine removal. AQDS showed inhibitory effects on both cultures by inhibiting the microbial removal of meta-chlorine. High-throughput sequencing and qPCR results suggest that HM is not a carbon source for the potential dechlorinating metabolism of Dehalococcoides but may promote reductive dechlorination by changing the community structure, and AQDS may inhibit anaerobic reductive dechlorination of PCBs by inhibiting the growth of Dehalococcoides. This study provides insights into the mechanism of enhancing PCB microbial dechlorination mediated by humic substances and plays a significant role in extending the application prospects of PCBs bioremediation technology.

Current advances of chlorinated organics degradation by bioelectrochemical systems: a review

Chlorinated organic compounds (COCs) are typical refractory organic compounds, having high biological toxicity. These compounds are a type of pervasive pollutants that can be present in polluted soil, air, and various types of waterways, such as groundwater, rivers, and lakes, posing a significant threat to the ecological environment and human health. Bioelectrochemical systems (BESs) are an effective strategy for the degradation of bio-refractory compounds. BESs improve the waste treatment efficiency through the application of weak electrical stimulation. This review discusses the processes of BESs configurations and degradation performances in different environmental media including wastewater, soil, waste gas and groundwater. In addition, the degradation mechanisms and performance-enhancing additives are summarized. The future challenges and perspectives on the development of BES for COCs removal are briefly discussed.

Fulvic acid more facilitated the soil electron transfer than humic acid

Humus substances (HSs) participate in extracellular electron transfer (EET), which is unclear in heterogeneous soil. Here, a microbial electrochemical system (MES) was constructed to determine the effect of HSs, including humic acid, humin and fulvic acid, on soil electron transfer. The results showed that fulvic acid led to the optimal electron transfer efficiency in soil, as evidenced by the highest accumulated charges and removal of total petroleum hydrocarbons after 140 days, with increases of 161% and 30%, respectively, compared with those of the control. However, the performance of MES with the addition of humic acid and humin was comparable to that of the control. Fulvic acid amendment enhanced the carboxyl content and oxidative state of dissolved organic matter, endowing a better electron transfer capacity. Additionally, the presence of fulvic acid induced an increase in the abundance of electroactive bacteria and organic degraders, extracellular polymeric substances and functional enzymes such as cytochrome c and NADH synthesis, and the expression of m tr C gene, which is responsible for EET enhancement in soil. Overall, this study reveals the mechanism by which HSs stimulate soil electron transfer at the physicochemical and biological levels and provides basic support for the application of bioelectrochemical technology in soil.

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.

Reciprocal Interactions of Abiotic and Biotic Dechlorination of Chloroethenes in Soil

Chloroethenes (CEs) as common organic pollutants in soil could be attenuated via abiotic and biotic dechlorination. Nonetheless, information on the key catalyzing matter and their reciprocal interactions remains scarce. In this study, FeS was identified as a major catalyzing matter in soil for the abiotic dechlorination of CEs, and acetylene could be employed as an indicator of the FeS-mediated abiotic CE-dechlorination. Organohalide-respiring bacteria (OHRB)-mediated dechlorination enhanced abiotic CEs-to-acetylene potential by providing dichloroethenes (DCEs) and trichloroethene (TCE) since chlorination extent determined CEs-to-acetylene potential with an order of trans-DCE > cis-DCE > TCE > tetrachloroethene/PCE. In contrast, FeS was shown to inhibit OHRB-mediated dechlorination, inhibition of which could be alleviated by the addition of soil humic substances. Moreover, sulfate-reducing bacteria and fermenting microorganisms affected FeS-mediated abiotic dechlorination by re-generation of FeS and providing short chain fatty acids, respectively. A new scenario was proposed to elucidate major abiotic and biotic processes and their reciprocal interactions in determining the fate of CEs in soil. Our results may guide the sustainable management of CE-contaminated sites by providing insights into interactions of the abiotic and biotic dechlorination in soil.

Photo-aging promotes the inhibitory effect of polystyrene microplastics on microbial reductive dechlorination of a polychlorinated biphenyl mixture (Aroclor 1260)

Polychlorinated biphenyls (PCBs) and microplastics (MPs) commonly co-exist in various environments. MPs inevitably start aging once they enter environment. In this study, the effect of photo-aged polystyrene MPs on microbial PCB dechlorination was investigated. After a UV aging treatment, the proportion of oxygen-containing groups in MPs increased. Photo-aging promoted the inhibitory effect of MPs on microbial reductive dechlorination of PCBs, mainly attributed to the inhibition of meta-chlorine removal. The inhibitory effects on hydrogenase and adenosine triphosphatase activity by MPs increased with increasing aging degree, which may be attributed to electron transfer chain inhibition. PERMANOVA showed significant differences in microbial community structure between culturing systems with and without MPs (p < 0.05). Co-occurrence network showed a simpler structure and higher proportion of negative correlation in the presence of MPs, especially for biofilms, resulting in increased potential for competition among bacteria. MP addition altered microbial community diversity, structure, interactions, and assembly processes, which was more deterministic in biofilms than in suspension cultures, especially regarding the bins of Dehalococcoides. This study sheds light on the microbial reductive dechlorination metabolisms and mechanisms where PCBs and MPs co-exist and provides theoretical guidance for in situ application of PCB bioremediation technology.

Complete pentachlorophenol biodegradation in a dual-working electrode bioelectrochemical system: Performance and functional microorganism identification

Bioelectrochemical system (BES) can effectively promote the reductive dechlorination of chlorophenols (CPs). However, the complete degradation of CPs with sequential dechlorination and mineralization processes has rarely achieved from the BES. Here, a dual-working electrode BES was constructed and applied for the complete degradation of pentachlorophenol (PCP). Combined with DNA-stable isotope probing (DNA-SIP), the biofilms attached on the anodic and cathodic electrode in the BES were analyzed to explore the dechlorinating and mineralizing microorganisms. Results showed that PCP removal efficiency in the dual-working BES (84% for 21 days) was 4.1 and 4.7 times higher than those of conventional BESs with a single anodic or cathodic working electrode, respectively. Based on DNA-SIP and high-throughput sequencing analysis, the cathodic working electrode harbored the potential dechlorinators (Comamonas, Pseudomonas, Methylobacillus, and Dechlorosoma), and the anodic working enriched the potential intermediate mineralizing bacteria (Comamonas, Stenotrophomonas, and Geobacter), indicating that PCP could be completely degraded under the synergetic effect of these functional microorganisms. Besides, the potential autotrophic functional bacteria that might be involved in the PCP dechlorination were also identified by SIP labeled with ¹³C-NaHCO₃. Our results proved that the dual-working BES could accelerate the complete degradation of PCP and enrich separately the functional microbial consortium for the PCP dechlorination and mineralization, which has broad potential for bioelectrochemical techniques in the treatment of wastewater contaminated with CPs or other halogenated organic compounds.

The Emergence of Extracellular Electron Mediating Functionality in Rice Straw-Artificial Soil Mixture during Humification

This study aimed to elucidate the origin of extracellular electron mediating (EEM) functionality and redox-active center(s) in humic substances, where they are ubiquitously distributed. Here, we show the emergence of EEM functionality during the humification of rice straw in artificial soil (kaolin and sand) with a matric potential of -100 cm at 20 °C for one year. We used the dechlorination activity of an EEM material-dependent pentachlorophenol-dechlorinating anaerobic microbial consortium as an index of the EEM functionality. Although rice straw and its mixture with artificial soil did not initially have EEM functionality, it emerged after one month of humification and increased until six months after which the functionality was maintained for one year. Chemical and electrochemical characterizations demonstrated that the emergence and increase in EEM functionality were correlated with the degradation of rice straw, formation of quinone structures, a decrease in aromatic structures, an increase in nitrogenous and aliphatic structures, and specific electric capacitance during humification. The newly formed quinone structure was suggested as a potential redox-active center for the EEM functionality. These findings provide novel insights into the dynamic changes in EEM functionality during the humification of organic materials.

Humin and biochar accelerated microbial reductive dechlorination of 2,4,6-trichlorophenol under weak electrical stimulation

The extracellular electron transfer (EET) is regarded as one of the crucial factors that limit the application of the bioelectrochemical system (BES). In this study, two different solid-phase redox mediators (RMs), biochar (1.2 g/L, T-B) and humin (1.2 g/L, T-H) were used for boosting the microorganisms accessing the electrons required for 2,4,6-TCP dechlorination under weak electrical stimulation (-0.278 V vs. Standard hydrogen electrode). BES with dissolved RM anthraquinone-2,6-disulfonate (AQDS 0.5 mmol/L, T-A) was used as a comparison. The results showed that dechlorination of 2,4,6-TCP could be greatly accelerated by biochar (1.78 d^-1) and humin (1.50 d^-1) than AQDS (0.24 d^-1) and no RM control (T-M, 0.27 d^-1). Moreover, phenol became the predominant dechlorination product in T-H (78.5 %) and T-B (63.0 %) instead of 4-CP in T-M (67.1 %) and T-A (89.8 %). Pseudomonas, Sulfurospirillum, Desulfuromonas, Dehalobacter, Anaeromyxobacter, and Dechloromonas belonging to Proteobacteria or Firmicutes rather than Chloroflexi might be responsible for the dechlorination activity. Notably, different RMs tended to stimulate distinct electroactive bacteria. Pseudomonas was the most abundant microorganism in T-M (41.92 %) and T-A (17.24 %), while Rhodobacter was most prevalent in T-H (20.04 %) and Azonexus was predominant in T-B (48.48 %). This study is essential in advancing the understanding of EET in BES for microbial degradation of organohalide contaminants under weak electrical stimulation.

Effect of microplastics on microbial dechlorination of a polychlorinated biphenyl mixture (Aroclor 1260)

Microplastics (MPs) and polychlorinated biphenyls (PCBs) generally coexist in the environment, posing risks to public health and the environment. This study investigated the effect of different MPs on the microbial anaerobic reductive dechlorination of Aroclor 1260, a commercial PCB mixture. MP exposure inhibited microbial reductive dechlorination of PCBs, with inhibition rates of 39.43%, 23.97%, and 17.53% by polyethylene (PE), polypropylene (PP), and polystyrene (PS), respectively. The dechlorination rate decreased from 1.63 μM Cl^- d^-1 to 0.99-1.34 μM Cl^- d^-1 after MP amendment. Chlorine removal in the meta-position of PCBs was primarily inhibited by MPs, with no changes in the final PCB dechlorination metabolites. The microbial community compositions in MP biofilms were not significantly different (P > 0.05) from those in suspension culture, although possessing greater Dehalococcoides abundance (0.52-0.81% in MP biofilms; 0.03-0.12% in suspension culture). The co-occurrence network analysis revealed that the presence of MPs attenuated microbial synergistic interactions in the dechlorinating culture systems, which may contribute to the inhibitory effect on microbial PCB dechlorination. These findings are important for comprehensively understanding microbial dechlorination behavior and the environmental fate of PCBs in environments with co-existing PCBs and MPs and for guiding the application of in situ PCB bioremediation.

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.

Humin: No longer inactive natural organic matter

The discovery of the function of humin (HM), an insoluble fraction of humic substances (HSs), as an extracellular electron mediator (EEM) in 2012 has provided insight into the role of HM in nature and its potential for in situ bioremediation of pollutants. The EEM function is thought to enable the energy network of various microorganisms using HM. Recently, a number of studies on the application of HM as EEM in anaerobic microbial cultures have been conducted. Even so, there is a need for developing a holistic view of HM EEM function. In this paper, we summarize all the available information on the properties of HM EEM function, its applications, possible redox-active structures, and the interaction between HM and microbial cells. We also suggest scopes for future HM research.

Humin Assists Reductive Acetogenesis in Absence of Other External Electron Donor

The utilization of extracellular electron transfer by microorganism is highly engaging for remediation of toxic pollutants under “energy-starved” conditions. Humin, an organo-mineral complex of soil, has been instrumental as an external electron mediator for suitable electron donors in the remediative works of reductive dehalogenation, denitrification, and so forth. Here, we report, for the first time, that humin assists microbial acetogenesis as the extracellular electron donor using the electron acceptor CO2. Humin was obtained from Kamajima paddy soil, Japan. The anaerobic acetogenic consortium in mineral medium containing CO2/HCO3− as the inorganic carbon source used suspended humin as the energy source under mesophilic dark conditions. Retardation of acetogenesis under the CO2-deficient conditions demonstrated that humin did not function as the organic carbon source but as electron donor in the CO2-reducing acetogenesis. The consortium with humin also achieved anaerobic dechlorination with limited methanogenic activity. Total electron-donating capacity of humin was estimated at about 87 µeeq/g-humin. The metagenomic sequencing of 16S rRNA genes showed the predominance of Firmicutes (71.8 ± 2.5%) in the consortium, and Lachnospiraceae and Ruminococcaceae were considered as the CO2-reducing acetogens in the consortium. Thus, microbial fixation of CO2 using humin introduces new insight to the holistic approach for sustainable treatment of contaminants in environment.

Accelerated microbial reductive dechlorination of 2,4,6-trichlorophenol by weak electrical stimulation

Microbial reductive dechlorination of chlorinated aromatics frequently suffers from the long dechlorination period and the generation of toxic metabolites. Biocathode bioelectrochemical systems were verified to be effective in the degradation of various refractory pollutants. However, the electrochemical and microbial related working mechanisms for bio-dechlorination by electro-stimulation remain poorly understood. In this study, we reported the significantly improved 2,4,6-trichlorophenol dechlorination activity through the weak electro-stimulation (cathode potential of -0.36 V vs. SHE), as evidenced by the 3.1 times higher dechlorination rate and the complete dechlorination ability with phenol as the end dechlorination product. The high reductive dechlorination rate (20.8 μM/d) could be maintained by utilizing electrode as an effective electron donor (coulombic efficiency of 82.3 ± 4.8%). Cyclic voltammetry analysis of the cathodic biofilm gave the direct evidences of the cathodic respiration with the improved and positive-shifted reduction peaks of 2,4,6-TCP, 2,4-DCP and 4-CP. The optimal 2,4,6-TCP reductive dechlorination rate (24.2 μM/d) was obtained when a small amount of lactate (2 mM) was added, and the generation of H₂ and CH₄ were accompanied due to the biological fermentation and methanogenesis. The electrical stimulation significantly altered the cathodic biofilm structure and composition with some potential dechlorinators (like Acetobacterium) predominated. The microbial interactions in the ecological network of cathodic biofilm were more simplified than the planktonic community. However, some potential dechlorinators (Acetobacterium, Desulfovibrio, etc.) shared more positive interactions. The co-existence and possible cooperative relationships between potential dechlorinators and fermenters (Sedimentibacter, etc.) were revealed. Meanwhile, the competitive interrelations between potential dechlorinators and methanogens (Methanomassiliicoccus) were found. In the network of plankton, the fermenters and methanogens possessed the more positive interrelations. Electro-stimulation at the cathodic potential of -0.36 V selectively enhanced the dechlorination function, while it showed little influence on either fermentation or methanogenesis process. The study gave suggestions for the enhanced bioremediation of chlorinated aromatics, in views of the electro-stimulation capacity, efficiency and microbial interrelations related microbial mechanism.

Azo dye biotransformation mediated by AQS immobilized on activated carbon cloth in the presence of microbial inhibitors

In this work, anthraquinone-2-sulfonate (AQS) was covalently immobilized onto activated carbon cloth (ACC), to be used as redox mediator for the reductive decolorization of reactive red 2 (RR2) by an anaerobic consortium. The immobilization of AQS improved the capacity of ACC to transfer electrons, evidenced by an increment of 3.29-fold in the extent of RR2 decolorization in absence of inhibitors, compared to incubations lacking AQS. Experiments conducted in the presence of vancomycin, an inhibitor of acidogenic bacteria, and with 2-bromoethane sulfonic acid (BES), an inhibitor of methanogenic archaea, revealed that acidogenic bacteria are the main responsible for RR2 biotransformation mediated by immobilized AQS. Nonetheless, the results also suggest that some methanogens are able to maintain their capacity to use immobilized AQS as an electron acceptor to sustain the decolorization process, even in the presence of BES.

Redox characteristics of humins and their coupling with potential PCB dechlorinators in southern Yellow Sea sediments

Natural attenuation of polychlorinated biphenyls (PCBs) by indigenous bacteria is an effective remediation strategy for polluted marine sediments. This study investigated the relationships between PCB concentrations in sediment pore water, humin electron transfer capacity, and potential PCB dechlorinators at eight sediment sampling sites in the southern Yellow Sea, China, with differential PCB contamination. Station A2 showed the highest PCB concentration (453.16 ng L^-1 for seven indicator PCBs), especially of less chlorinated PCB congeners (≤5 Cl atoms), humin redox activity, and Dehalococcoides abundance (p < 0.05). Statistical analyses revealed a highly positive correlation between Dehalococcoides abundance and PCB concentration (r = 0.836, p < 0.05) and the electron shuttling ability of humins (r = 0.952, p < 0.01), whereas this was not observed for total bacteria and other potential PCB dechlorinators, e.g., Dehalobacter and Dehalogenimonas. Based on these results, Dehalococcoides might play an important role in the in situ reductive dechlorination of PCBs involving humins in marine sediments, and the natural microbial PCB attenuation capacity at station A2 was high. Chemical characterizations, electrochemical properties, and Fourier transform infrared analysis suggested that humins at station A2 had the highest electron transfer capacity. Furthermore, quinones are likely to be the functional groups that shuttle electrons during PCB dechlorination. Overall, this study provides a useful foundation for evaluating the natural microbial attenuation potential and fates of PCBs in marine sediments and for determining the role of humins as redox mediators in in situ PCB dechlorination by putative indigenous dechlorinators.

Anaerobic Dechlorination by a Humin-Dependent Pentachlorophenol-Dechlorinating Consortium under Autotrophic Conditions Induced by Homoacetogenesis

Anoxic aquifers suffer from energy limitations due to the unavailability of organic substrates, as dictated by hydrogen (H2) for various electron-accepting processes. This deficiency often results in the accumulation of persistent organic pollutants, where bioremediation using organic compounds often leads to secondary contamination. This study involves the reductive dechlorination of pentachlorophenol (PCP) by dechlorinators that do not use H2 directly, but rather through a reduced state of humin-a solid-phase humic substance-as the extracellular electron donor, which requires an organic donor such as formate, lactate, etc. This shortcoming was addressed by the development of an anaerobic mixed culture that was capable of reductively dechlorinating PCP using humin under autotrophic conditions induced by homoacetogenesis. Here, H2 was used for carbon-dioxide fixation to acetate; the acetate produced was used for the reduction of humin; and consequently used for dechlorination through reduced humin. The 16SrRNA gene sequencing analysis showed Dehalobacter and Dehalobacterium as the possible dechlorinators, while Clostridium and Oxobacter were identified as the homoacetogens. Thus, this work contributes to the development of an anaerobic consortium that balanced H2 dependency, where efficiency of humin reduction extends the applicability of anaerobic microbial remediation in aquifers through autotrophy, syntrophy, and reductive dechlorination.

The microbial community responsible for dechlorination and benzene ring opening during anaerobic degradation of 2,4,6‑trichlorophenol

This study describes the dechlorination ability of acclimated biomass, the high-throughput sequencing of the 16S ribosomal RNA (rRNA) gene of such microorganisms, and the analysis of their community structure in relation to special functions. Two types of acclimated biomass (AB-1 and AB-2) were obtained via different acclimated treatment processes and were used to degrade 2,4,6‑trichlorophenol. The degradation pathway and characteristics of trichlorophenol degradation were different between the two groups. AB-1 degraded trichlorophenol only to 4-chlorophenol. AB-2 completely dechlorinated trichlorophenol and opened the benzene ring. The 16S rRNA high-throughput sequencing method was employed to examine the microbial diversity. It was found that the microbial richness and diversity of AB-1 were higher than those of AB-2. Firmicutes and Bacteroidetes were 2.7-fold and 4.3-fold more abundant, respectively, in AB-1 than in AB-2. Dechlorination bacteria in AB-1 mainly included Desulfobulbus, Desulfovibrio, Dechloromonas, and Geobacter. The above-mentioned bacteria were less abundant in AB-2, but the abundance of Desulfomicrobium was twofold higher in AB-2 than in AB-1. The two types of acclimated biomass contained different hydrogen (H₂)-producing bacteria. AB-2 showed higher abundance and diversity of hydrogen-producing bacteria. There was no Ignavibacteriae in AB-1, whereas its abundance in AB-2 was 8.4%. In this biomass, Ignavibacteriae was responsible for opening of the benzene ring. This study indicates that the abundance and diversity of microorganisms are not necessarily beneficial to the formation of a functional dechlorinating community. The H₂-producing bacteria (which showed greater abundance and diversity) and Ignavibacterium were assumed to be core functional populations that gave AB-2 stronger dechlorination and phenol-degradation abilities. Control of lower oxidation reduction potential (E_h) and higher temperatures by means of fresh aerobic activated sludge as the starting microbial group, caused rapid complete dechlorination of 2,4,6‑trichlorophenol and benzene ring opening.

Characterization of biochars derived from different materials and their effects on microbial dechlorination of pentachlorophenol in a consortium

The properties of biochars derived from different raw materials (rice husk, bamboo, caragana, and garbage) and their effects on the microbial reductive dechlorination of pentachlorophenol (PCP) were investigated to understand how biochars influence the biotransformation of environmental pollutants. The results indicated that only caragana-derived biochar showed stable electron transfer activity for PCP dechlorination. Electro(chemical) analyses revealed that caragana biochar had the highest electrical conductivity (EC) (2.22 × 10⁶ μS cm⁻¹), while those of the other biochars were <1500 μS cm⁻¹. The electron transfer capacities were within the ranges of 61.63–155.83 μmol e⁻ g⁻¹. Cyclic voltammetry analysis suggested that there were no obvious redox peaks for the biochars, while the Fourier transform infrared analysis showed similar transmission spectra with variable absorption intensity; this suggested that all biochars possessed similar structures and functional group classes and the enhancement of PCP dechlorination was not attributable to the redox reaction. Overall, the beneficial effects of caragana biochar on PCP dechlorination depended on the EC rather than the redox functional groups, possibly because high EC values enabled the highest electron transfer, and thus resulted in the greatest promotion of reductive dechlorination activity.

The properties of four biochars and their effects on the microbial reductive dechlorination of PCP were investigated.

Humin as an External Electron Mediator for Microbial Pentachlorophenol Dechlorination: Exploration of Redox Active Structures Influenced by Isolation Methods

Humin (HM) has been reported to function as an external electron mediator (EEM) in various microbial reducing reactions. In this study, the effect of isolation methods on EEM functionality and the chemical/electrochemical structures of HM were examined based on the correlation between dechlorination rates in the anaerobic HM-dependent pentachlorophenol (PCP)-dechlorinating consortium and the chemical/electrochemical structures of HM. A lack of PCP dechlorination activity suggested no EEM function in the HM samples prepared as a soluble fraction in dimethyl sulfoxide and sulfuric acid (which did not contain any electric capacitance). Other HM samples exhibited EEM functionality as shown by the dechlorination activity ranging from 0.55 to 3.48 (µmol Cl⁻) L⁻¹d⁻¹. The comparison of dechlorination activity with chemical structural characteristics suggested that HM with EEM functionalities had predominantly aliphatic and carbohydrate carbons with the partial structures C=O, O=C–N, and O=C–O. EEM functionality positively correlated with the proportion of O=C–N and O=C–O, suggesting an association between peptidoglycan structure and EEM functionality. The lack of detection of a quinone structure in one HM sample with EEM functionality and a negative correlation with aromatic or C=C carbon suggested that the mechanism containing quinone structures is a minor component for the functionality of EEM.

Humic substances, their microbial interactions and effects on biological transformations of organic pollutants in water and soil: A review

Depicted as large polymers by the traditional model, humic substances (HS) tend to be considered resistant to biodegradation. However, HS should be regarded as supramolecular associations of rather small molecules. There is evidence that they can be degraded not only by aerobic but also by anaerobic bacteria. HS presence alters biological transformations of organic pollutants in water and soil. HS, including humin, have a great potential for an application in aerobic and anaerobic wastewater treatment as well as in bioremediation. Black carbon materials, including char (biochar) and activated carbon (AC), long recognized effective sorbents, have been recently discovered to act as effective redox mediators (RM), which may significantly accelerate degradation of organic pollutants in a way similar to HS. Humic-like coating on the biochar surface has been identified. Explanation of mechanisms and possibility of applications of black carbon materials have only started to be explored. Results of many original and review papers, presented and discussed in this article, show an enormous potential for an interesting, multidisciplinary research as well as for a development of new, green technologies for biological wastewater treatment and bioremediation. Future research areas have been suggested.

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.

Effect of long-term fertilization on humic redox mediators in multiple microbial redox reactions

This study investigated the effects of different long-term fertilizations on humic substances (HSs), humic acids (HAs) and humins, functioning as redox mediators for various microbial redox biotransformations, including 2,2',4,4',5,5'- hexachlorobiphenyl (PCB₁₅₃) dechlorination, dissimilatory iron reduction, and nitrate reduction, and their electron-mediating natures. The redox activity of HSs for various microbial redox metabolisms was substantially enhanced by long-term application of organic fertilizer (pig manure). As a redox mediator, only humin extracted from soils with organic fertilizer amendment (OF-HM) maintained microbial PCB₁₅₃ dechlorination activity (1.03 μM PCB₁₅₃ removal), and corresponding HA (OF-HA) most effectively enhanced iron reduction and nitrate reduction by Shewanella putrefaciens. Electrochemical analysis confirmed the enhancement of their electron transfer capacity and redox properties. Fourier transform infrared analysis showed that C=C and C=O bonds, and carboxylic or phenolic groups in HSs might be the redox functional groups affected by fertilization. This research enhances our understanding of the influence of anthropogenic fertility on the biogeochemical cycling of elements and in situ remediation ability in agroecosystems through microorganisms' metabolisms.

Energy generation through bioelectrochemical degradation of pentachlorophenol in microbial fuel cell

Bio-electrochemical degradation of pentachlorophenol was carried out in single as well as dual chambered microbial fuel cell (MFC) with simultaneous production of electricity. The maximum cell potential was recorded to be 787 and 1021 mV in single and dual chambered systems respectively. The results presented nearly 66 and 89% COD removal in single and dual chambered systems with corresponding power densities of 872.7 and 1468.85 mW m⁻² respectively. The highest coulombic efficiency for single and dual chambered counterparts was found to be 33.9% and 58.55%. GC-MS data revealed that pentachlorophenol was more effectively degraded under aerobic conditions in dual-chambered MFC. Real-time polymerase chain reaction showed the dominance of exoelectrogenic Geobacter in the two reactor systems with a slightly higher concentration in the dual-chambered system. The findings of this work suggested that the aerobic treatment of pentachlorophenol in cathodic compartment of dual chambered MFC is better than its anaerobic treatment in single chambered MFC in terms of chemical oxygen demand (COD) removal and output power density.

Bio-electrochemical degradation of pentachlorophenol was carried out in single as well as dual chambered microbial fuel cell (MFC) with simultaneous production of electricity.

New combination of xylanolytic bacteria isolated from the lignocellulose degradation microbial consortium XDC-2 with enhanced xylanase activity

Three bacterial strains with extracellular xylanase activity were isolated from the microbial consortium XDC-2. The aerobic strain A7, belonging to Bacillus sp., was combined with the anaerobe Clostridium sp. strain AA3 and/or Bacteroides sp. strain AA4 to obtain an efficient natural xylanolytic complex enzyme. The synthetic microbial community M1 consisting of strains Bacillus and Clostridium showed enhanced extracellular xylanase activity and production, and higher lignocelluloses degradation capability than any of the pure cultures and other synthetic microbial communities. Neither corn straw degradation nor extracellular xylanase activity was enhanced in the other synthetic microbial communities, Bacillus, Bacteroides with or without Clostridium. Quantitative polymerase chain reaction showed that the aerobic strain Bacillus enabled the growth of the anaerobic strain Clostridium, but not that of the anaerobic strain Bacteroides. These findings suggest that strains Bacillus and Clostridium can coexist well and have a positive synergistic interaction for extracellular xylanase secretion and lignocellulose degradation.

Cathodic bacterial community structure applying the different co-substrates for reductive decolorization of Alizarin Yellow R

Selective enrichment of cathodic bacterial community was investigated during reductive decolorization of AYR fedding with glucose or acetate as co-substrates in biocathode. A clear distinction of phylotype structures were observed between glucose-fed and acetate-fed biocathodes. In glucose-fed biocathode, Citrobacter (29.2%), Enterococcus (14.7%) and Alkaliflexus (9.2%) were predominant, and while, in acetate-fed biocathode, Acinetobacter (17.8%) and Achromobacter (6.4%) were dominant. Some electroactive or reductive decolorization genera, like Pseudomonas, Delftia and Dechloromonas were commonly enriched. Both of the higher AYR decolorization rate (k(AYR)=0.46) and p-phenylenediamine (PPD) generation rate (k(PPD)=0.38) were obtained fed with glucose than acetate (k(AYR)=0.18; k(PPD)=0.16). The electrochemical behavior analysis represented a total resistance in glucose-fed condition was about 73.2% lower than acetate-fed condition. The different co-substrate types, resulted in alteration of structure, richness and composition of bacterial communities, which significantly impacted the performances and electrochemical behaviors during reductive decolorization of azo dyes in biocathode.

Relative contribution of set cathode potential and external mass transport on TCE dechlorination in a continuous-flow bioelectrochemical reactor

Microbial bioelectrochemical systems, which use solid-state cathodes to drive the reductive degradation of contaminants such as the chlorinated hydrocarbons, are recently attracting considerable attention for bioremediation applications. So far, most of the published research has focused on analyzing the influence of key (bio)electrochemical factors influencing contaminant degradation, such as the cathode potential, whereas only few studies have examined the potential impact of mass transport phenomena on process performance. Here we analyzed the performance of a flow-through bioelectrochemical reactor, continuously fed with a synthetic groundwater containing trichloroethene at three different linear fluid velocities (from 0.3 m d(-1) to 1.7 m d(-1)) and three different set cathode potentials (from -250 mV to -450 mV vs. the standard hydrogen electrode). The obtained results demonstrated that, in the range of fluid velocities which are characteristics for natural groundwater systems, mass transport phenomena may strongly influence the rate and extent of reductive dechlorination. Nonetheless, the relative importance of mass transport largely depends on the applied cathode potential which, in turn, controls the intrinsic kinetics of biological reactions and the underlying electron transfer mechanisms.

Characterization of humins from different natural sources and the effect on microbial reductive dechlorination of pentachlorophenol

Humins have been reported to function as an electron mediator for microbial reducing reactions. However, the physicochemical properties and the functional moieties of humins from different natural sources have been poorly characterized. In this study, humins extracted from seven types of soil and from a river sediment were examined on the effect on microbial reductive dechlorination of pentachlorophenol (PCP) and characterized polyphasically. All humins facilitated microbial reductive dechlorination of PCP as electron mediators using formate as carbon source, with different dechlorination rates ranging from 0.99 to 7.63 (μmol Cl-) L(-1) d(-1). The highest rates were observed in humins with high carbon contents, extracted from Andisols containing allophone as major clay. Yields of the humins and the elemental compositions varied among sources. Fourier transform infrared analysis showed that all the humins exhibited similar spectra with different absorbance intensity; these data are indicative of their similar structures and identical classes of functional groups. The electron spin resonance spectra of humins prepared at different pH showed typical changes for the semiquinone-type radicals, suggestive of quinone moieties for the redox activity of the humins. Cyclic voltammetry analysis confirmed the presence of redox-active moieties in all the humins, with the estimated redox potentials in the range of -0.30 to -0.13 V (versus a standard hydrogen electrode), falling into the range of standard redox potential between the oxidation of formate as electron donor and the initial dechlorination of PCP as electron acceptor.

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.

Graphene oxide as electron shuttle for increased redox conversion of contaminants under methanogenic and sulfate-reducing conditions

Graphene oxide (GO) is reported for the first time as electron shuttle to increase the redox conversion of the azo compound, reactive red 2 (RR2, 0.5mM), and the nitroaromatic, 3-chloronitrobenzene (3CNB, 0.5mM). GO (5mgL(-1)) increased 10-fold and 7.6-fold the reduction rate of RR2 and 3CNB, respectively, in abiotic incubations with sulfide (2.6mM) as electron donor. GO also increased by 2-fold and 3.6-fold, the microbial reduction rate of RR2 by anaerobic sludge under methanogenic and sulfate-reducing conditions, respectively. Deep characterization of GO showed that it has a proper size distribution (predominantly between 450 and 700nm) and redox potential (+50.8mV) to promote the reduction of RR2 and 3CNB. Further analysis revealed that biogenic sulfide plays a major role on the GO-mediated reduction of RR2. GO is proposed as an electron shuttle to accelerate the redox conversion of recalcitrant pollutants, such as nitro-benzenes and azo dyes.