Anaerobic mineralization of pentachlorophenol (PCP) by combining PCP-dechlorinating and phenol-degrading cultures

A comprehensive review of chlorophenols: Fate, toxicology and its treatment

Chlorophenols represent one of the most abundant families of toxic pollutants emerging from various industrial manufacturing units. The toxicity of these chloroderivatives is proportional to the number and position of chlorine atoms on the benzene ring. In the aquatic environment, these pollutants accumulate in the tissues of living organisms, primarily in fishes, inducing mortality at an early embryonic stage. Contemplating the behaviour of such xenobiotics and their prevalence in different environmental components, it is crucial to understand the methods used to remove/degrade the chlorophenol from contaminated environment. The current review describes the different treatment methods and their mechanism towards the degradation of these pollutants. Both abiotic and biotic methods are investigated for the removal of chlorophenols. Chlorophenols are either degraded through photochemical reactions in the natural environment, or microbes, the most diverse communities on earth, perform various metabolic functions to detoxify the environment. Biological treatment is a slow process because of the more complex and stable structure of pollutants. Advanced Oxidation Processes are effective in degrading such organics with enhanced rate and efficiency. Based on their ability to generate hydroxyl radicals, source of energy, catalyst type, etc., different processes such as sonication, ozonation, photocatalysis, and Fenton's process are discussed for the treatment or remediation efficiency towards the degradation of chlorophenols. The review entails both advantages and limitations of treatment methods. The study also focuses on reclamation of chlorophenol-contaminated sites. Different remediation methods are discussed to restore the degraded ecosystem back in its natural condition.

Classic Pentachlorophenol Hydroxylating Phenylalanine 4-Monooxygenase from Indigenous Bacillus tropicus Strain AOA-CPS1: Cloning, Overexpression, Purification, Characterization and Structural Homology Modelling

The metabolically promiscuous pentachlorophenol (PCP) hydroxylating Phe4MO (represented as CpsB) was detected, amplified (from the genome of Bacillus tropicus strain AOA-CPS1), cloned, overexpressed, purified and characterized here. The 1.755-kb gene cloned in the pET15b vector expressed a ≅ 64 kDa monomeric protein which was purified to homogeneity by single-step affinity chromatography, with a total yield of 82.1%. The optimum temperature and pH of the enzyme were found to be 30 °C and 7.0, respectively. CpsB showed functional stability between pH 6.0-7.5 and temperature 25-30 °C. The enzyme-substrate reaction kinetic studies showed the allosteric nature of the enzyme and followed pre-steady state using NADH as a co-substrate with apparent vmax, Km, kcat and kcat/Km values of 0.465 μM.s-1, 140 μM, 0.099 s-1 and 7.07 × 10-4 μM-1.s-1, respectively, for the substrate PCP. The in-gel trypsin digestion experiments and bioinformatic tools confirmed that the reported enzyme is a Phe4MO with multiple putative conserved domains and metal ion-binding site. Though Phe4MO has been reported to have a diverse catalytic function, here we report, for the first time, that it functions as a PCP dehalogenase or PCP-4-monooxygenase by hydroxylating PCP. Hence, the use of this enzyme may be further explored in the bioremediation of PCP and other related xenobiotics.

Challenges and opportunities for the biodegradation of chlorophenols: Aerobic, anaerobic and bioelectrochemical processes

Chlorophenols (CPs) are highly toxic and refractory contaminants which widely exist in various environments and cause serious harm to human and environment health and safety. This review provides comprehensive information on typical CPs biodegradation technologies, the most green and benign ones for CPs removal. The known aerobic and anaerobic degradative bacteria, functional enzymes, and metabolic pathways of CPs as well as several improving methods and critical parameters affecting the overall degradation efficiency are systematically summarized and clarified. The challenges for CPs mineralization are also discussed, mainly including the dechlorination of polychlorophenols (poly-CPs) under aerobic condition and the ring-cleavage of monochlorophenols (MCPs) under anaerobic condition. The coupling of functional materials and degraders as well as the operation of sequential anaerobic-aerobic bioreactors and bioelectrochemical system (BES) are promising strategies to overcome some current limitations. Future perspective and research gaps in this field are also proposed, including the further understanding of microbial information and the specific role of materials in CPs biodegradation, the potential application of innovative biotechnologies and new operating modes to optimize and maximize the function of the system, and the scale-up of bioreactors towards the efficient biodegradation of CPs.

Alkylphenols and Chlorophenols Remediation in Vertical Flow Constructed Wetlands: Removal Efficiency and Microbial Community Response

This study aims to investigate the effect of two different groups of phenolic compounds (the alkylphenols nonylphenol (NP) and octylphenol (OP), and the chlorophenol pentachlorophenol (PCP)) on constructed wetlands (CWs) performance, including on organic matter, nutrients and contaminants removal efficiency, and on microbial community structure in the plant bed substrate. CWs were assembled at lab scale simulating a vertical flow configuration and irrigated along eight weeks with Ribeira de Joane (an urban stream) water not doped (control) or doped with a mixture of NP and OP or with PCP (at a 100 μg·L−1 concentration each). The presence of the phenolic contaminants did not interfere in the removal of organic matter or nutrients in CWs in the long term. Removals of NP and OP were >99%, whereas PCP removals varied between 87% and 98%, mainly due to biodegradation. Microbial richness, diversity and dominance in CWs substrate were generally not affected by phenolic compounds, with only PCP decreasing diversity. Microbial community structure, however, showed that there was an adaptation of the microbial community to the presence of each contaminant, with several specialist genera being enriched following exposure. The three more abundant specialist genera were Methylotenera and Methylophilus (methylophilaceae family) and Hyphomicrobium (hyphomicrobiaceae family) when the systems were exposed to a mixture of NP and OP. When exposed to PCP, the three more abundant genera were Denitromonas (Rhodocyclaceae family), Xenococcus_PCC_7305 (Xenococcaceae family) and Rhodocyclaceae_uncultured (Rhodocyclaceae family). To increase CWs efficiency in the elimination of phenolic compounds, namely PCP which was not totally removed, strategies to stimulate (namely biostimulation) or increase (namely bioaugmentation) the presence of these bacteria should be explore. This study clearly shows the potential of vertical flow CWs for the removal of phenolic compounds, a still little explored subject, contributing to promote the use of CWs as nature-based solutions to remediate water contaminated with different families of persistent and/or emergent contaminants.

Microbial community response to the toxic effect of pentachlorophenol in paddy soil amended with an electron donor and shuttle

Understanding the degradation of pentachlorophenol (PCP) by indigenous microorganisms stimulated by an electron donor and shuttle in paddy soil, and the influences of PCP/electron donor/shuttle on the native microbial community are important for biodegradation and ecological and environmental safety. Previous studies focused on the kinetics and the microbial actions of PCP degradation, however, the effects of toxic and antimicrobial PCP and electron donor/shuttle on the microbial community diversity and composition in paddy soil are poorly understood. In this study, the effects of PCP, an electron donor (lactate), and the electron shuttle (anthraquinone-2, 6-disulfonate, AQDS) on the microbial community in paddy soil were investigated. The results showed that the presence of PCP reduced the microbial diversity compared to the control during PCP degradation, while increased the microbial diversity was observed in response to lactate and AQDS. The addition of PCP stimulated the microorganisms involved in PCP dechlorination, including Clostridium, Desulfitobacterium, Pandoraea, and unclassified Veillonellaceae, which were dormant in raw soil without PCP stress. In all of the treatments with PCP, the addition of lactate or AQDS enhanced PCP dechlorination by stimulating the growth of functional groups involved in PCP dechlorination and by changing the microbial community during dechlorination process. The microbial community tended to be uniform after complete PCP degradation (28 days). However, when lactate and AQDS were present simultaneously in PCP-contaminated soil, lactate acted as a carbon source or electron donor to promote the activities of microbial community, and AQDS changed the redox potential because of the production of reduced AQDS. These findings enhance our understanding of the effect of PCP and a biostimulation method for PCP biodegradation in soil ecosystems at the microbial community level, and suggest the appropriate selection of an electron donor/shuttle for accelerating the bioremediation of PCP-contaminated soils.

Nitrate supply and sulfate-reducing suppression facilitate the removal of pentachlorophenol in a flooded mangrove soil

An anaerobic incubation was launched with varying nitrate (1, 5, 10 and 20 mM exogenous NaNO₃) and molybdate (20 mM Na₂MoO₄, a sulfate-reducing inhibitor) additions to investigate the characteristics of PCP dechlorination, as well as the reduction of natural co-occurring electron acceptors, including NO₃^-, Fe(III) and SO₄^2-, and the responses of microbial community structures under a unique reductive mangrove soil. Regardless of exogenous addition, nitrate was rapidly eliminated in the first 12 days. The reduction process of Fe(III) was inhibited, while that of SO₄^2- reduction depended on addition concentration as compared to the control. PCP was mainly degraded from orth-position, forming the only intermediate 2,3,4,5-TeCP by anaerobic microbes, with the highest PCP removal rate of average 21.9% achieved in 1 and 5 mM NaNO₃ as well as 20 mM Na₂MoO₄ treatments and the lowest of 7.5% in 20 mM NaNO₃ treatment. The effects of nitrate on PCP dechlorination depended on addition concentration, while molybdate promoted PCP attenuation significantly. Analyses of the Illumina sequencing data and the relative abundance of dominant microorganisms indicated that the core functional groups regulated PCP removal at genera level likely included Bacillus, Pesudomonas, Dethiobacter, Desulfoporosinus and Desulfovbrio in the nitrate treatments; while that was likely Sedimentibacter and Geosporobacter_Thermotalea in the molybdate treatment. Nitrate supplement but not over supplement, or addition of molybdate are suggested as alternative strategies for better remediation in the nitrate-deficient and sulfur-accumulated soil ecosystem contaminated by PCP, through regulating the growth of core functional groups and thereby coordinating the interaction between dechlorination and its coupled soil redox processes due to shifts of more available electrons to dechlorination. Our results broadened the knowledge regarding microbial PCP degradation and their interactions with natural soil redox processes under anaerobic soil ecosystems.

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.

Inhibitory Effects of Sulfate and Nitrate Reduction on Reductive Dechlorination of PCP in a Flooded Paddy Soil

Pentachlorophenol (PCP) is highly toxic and persistent in soils. Bioreduction of PCP often co-occurs with varying concentrations of sulfate and nitrate in flooded paddy soils where each can act as an electron acceptor. Anaerobic soil microcosms were constructed to evaluate the influence of sulfate and nitrate amendments and their redox processes. Microcosms with varying sulfate and nitrate concentrations demonstrated an inhibitory effect on reductive dechlorination of PCP compared to an untreated control. Compared to nitrate, sulfate exhibited a more significant impact on PCP dechlorination, as evidenced by a lower maximum reaction rate and a longer time to reach the maximum reaction rate. Dechlorination of PCP was initiated at the ortho-position, and then at the para- and meta-positions to form 3-CP as the final product in all microcosms. Deep sequencing of microbial communities in the microcosms revealed a strong variation in bacterial taxon among treatments. Specialized microbial groups, such as the genus of Desulfovibrio responding to the addition of sulfate, had a potential to mediate the competitive microbial dechlorination of PCP. Our results provide an insight into the competitive microbial-mediated reductive dechlorination of PCP in natural flooded soil or sediment environments.

Typical Soil Redox Processes in Pentachlorophenol Polluted Soil Following Biochar Addition

Reductive dechlorination is the primary pathway for environmental removal of pentachlorophenol (PCP) in soil under anaerobic condition. This process has been verified to be coupled with other soil redox processes of typical biogenic elements such as carbon, iron and sulfur. Meanwhile, biochar has received increasing interest in its potential for remediation of contaminated soil, with the effect seldom investigated under anaerobic environment. In this study, a 120-day anaerobic incubation experiment was conducted to investigate the effects of biochar on soil redox processes and thereby the reductive dechlorination of PCP under anaerobic condition. Biochar addition (1%, w/w) enhanced the dissimilatory iron reduction and sulfate reduction while simultaneously decreased the PCP reduction significantly. Instead, the production of methane was not affected by biochar. Interestingly, however, PCP reduction was promoted by biochar when microbial sulfate reduction was suppressed by addition of typical inhibitor molybdate. Together with Illumina sequencing data regarding analysis of soil bacteria and archaea responses, our results suggest that under anaerobic condition, the main competition mechanisms of these typical soil redox processes on the reductive dechlorination of PCP may be different in the presence of biochar. In particularly, the effect of biochar on sulfate reduction process is mainly through promoting the growth of sulfate reducer (Desulfobulbaceae and Desulfobacteraceae) but not as an electron shuttle. With the supplementary addition of molybdate, biochar application is suggested as an improved strategy for a better remediation results by coordinating the interaction between dechlorination and its coupled soil redox processes, with minimum production of toxic sulfur reducing substances and relatively small emission of greenhouse gas (CH4) while maximum removal of PCP.

The dechlorination of pentachlorophenol under a sulfate and iron reduction co-occurring anaerobic environment

An anaerobic soil slurry incubation experiment was conducted by controlling different Fe/S mole ratios (1/3, 1/2, 1/1, 2/1, 3/1, 8/1 and the control without sulfate) through the addition of sodium sulfate, to investigate the effect of sulfate and iron reduction on the reductive dechlorination of pentachlorophenol (PCP). Two sequential incubation periods were carried out with the stage I incubation conducted under a low electron donor concentration (0.5 mM lactate) and stage II incubation conducted under increased electron donor supply with lactate at 20 mM. During stage I, the production of Fe(II) occurred markedly while sulfate reduction and PCP dechlorination rate were low, with the highest dechlorination rates of PCP only 11.0% among all treatments at the end of stage I incubation. During stage II, both PCP dechlorination and sulfate reduction were greatly enhanced in all treatments, while the concentration of Fe(II) changed slightly. The rate of PCP dechlorination decreased (from 87.7% to 34.2%) with the increase of sulfate concentration (from Fe/S mole ratio of 8/1 to 1/3). Our study suggested that the presence of a certain amount of sulfate might facilitate PCP dechlorination in the range of Fe/S mole ratios greater than 1 when compared with the control without SO₄^2-. With the investigation of the dechlorination of PCP under the Fe-S-PCP coexisting condition with different Fe/S mole ratios, our study may provide improved strategy for optimizing the remediation of flooded soils and sediments polluted by PCP.

Penta- and 2,4,6-tri-chlorophenol biodegradation during municipal solid waste anaerobic digestion

In this study isotopic tracing using (13)C labelled pentachlorophenol (PCP) and 2,4,6-trichlorophenol (2,4,6-TCP) is proposed as a tool to distinguish the loss of PCP and 2,4,6-TCP due to biodegradation from other physical processes. This isotopic approach was applied to accurately assess in situ PCP and 2,4,6-TCP degradation under methanogenic conditions in several microcosms made up of household waste. These microcosms were incubated in anaerobic conditions at 35°C (mesophilic) and 55°C (thermophilic) without agitation. The volume of biogas produced (CH4 and CO2), was followed for a period of 130 days. At this stage of stable methanogenesis, (13)C6-PCP and (13)C6-2,4,6-TCP were introduced anaerobically in microcosms and its monitoring at mesophilic and thermophilic conditions was performed in parallel by gas chromatography mass spectrometry (GC-MS) and gas chromatography isotope-ratio mass spectrometry (GC-IRMS). This study proved the almost total dechlorination of bioavailable PCP and 2,4,6-TCP into 4-CP at 35°C. Nevertheless, high rate adsorption in particular materials of the two compounds was observed. Furthermore, Carbon-13 Nuclear Magnetic Resonance ((13)C-NMR) Spectroscopy analysis of (13)C labelled 2,4,6-TCP mesophilic incubations showed the partial mineralization of 4-CP at 35°C to acetate and then to HCO(3-). Consequently, NMR results confirm the biogas isotopic results indicating the mineralization of (13)C labelled 2,4,6-TCP into (13)C (CH4 and CO2). Concerning (13)C labelled PCP mesophilic incubations, the isotopic composition of the biogas still natural until the day 262. In contrast, no dechlorination was observed at 55°C. Thus PCP and 2,4,6-TCP were persistent in thermophilic conditions.

Dynamics of the microbial community and Fe(III)-reducing and dechlorinating microorganisms in response to pentachlorophenol transformation in paddy soil

Soil microorganisms play crucial roles in the fates of pollutants, and understanding the behaviour of these microorganisms is critical for the bioremediation of PCP-contaminated soil. However, shifts remain unclear in the community structure and Fe(III)-reducing and dechlorinating microorganisms during PCP transformation processes, especially during the stages from the lag to the dechlorination phase and from the dechlorination to the stationary phase. Here, a set of lab-scale experiments was performed to investigate the microbial community dynamics accompanying PCP transformation in paddy soil. 19μM of PCP was biotransformed completely in 10days for all treatments. T-RFLP analysis of the microbial community confirmed that Veillonellaceae and Clostridium sensu stricto were the dominant groups during PCP transformation, and the structures of the microbial communities changed due to the degree of biotransformation and the addition of lactate and AQDS. However, similar temporal dynamics of the microbial communities were obtained among all treatments. Furthermore, as revealed by quantitative PCR, the dynamics of Fe(III)-reducing and dechlorinating microorganisms, including Geobacter sp., Shewanella sp., and Dehalobacter sp., were consistent with the transformation kinetics of PCP, suggesting the critical roles played by these microorganisms in PCP transformation. These findings are valuable for making predictions of and proposing methods for the microbial detoxification of residual organochlorine pesticides in paddy soil.

The Performance of Four Different Mineral Liners on the Transportation of Chlorinated Phenolic Compounds to Groundwater in Landfills

The aim of this study was to investigate the efficiency of four different mineral liners (clay, bentonite, kaoline, and zeolite) which could be utilized to prevent the transport of phenolic compounds to groundwater through alternative liner systems. Four laboratory-scale HDPE reactors with 80 cm height and 40 cm inner diameter were operated for a period of 180 days. Results indicated that the transport of mono- or dichlorophenols is significantly prevented by the liner systems used, while the transport of highly chlorinated phenolic compounds cannot be prevented by the landfill liner system effectively. Highly chlorinated phenolic compounds in groundwater can be found in higher concentrations than the leachate, as a result of the degradation and transformation of these compounds. Thus, the analysis of highly chlorinated phenolic compounds such as 2,4,6-TCP, 2,3,6-TCP, 3,4,5-TCP, and PCP is of great significance for the studies to be conducted on the contamination of groundwater around landfills.

Spatial Abundance and Distribution of Potential Microbes and Functional Genes Associated with Anaerobic Mineralization of Pentachlorophenol in a Cylindrical Reactor

Functional interplays of microbial activity, genetic diversity and contaminant transformation are poorly understood in reactors for mineralizing halogenated aromatics anaerobically. Here, we investigated abundance and distribution of potential microbes and functional genes associated with pentachlorophenol (PCP) anaerobic mineralization in a continuous-flow cylindrical reactor (15 cm in length). PCP dechlorination and the metabolite (phenol) were observed at segments 0–8 cm from inlet, where key microbes, including potential reductive dechlorinators (Dehalobacter, Sulfurospirillum, Desulfitobacterium and Desulfovibrio spp.) and phenol degraders (Cryptanaerobacter and Syntrophus spp.), as well as putative functional genes, including putative chlorophenol reductive dehalogenase (cprA) and benzoyl-CoA reductase (bamB), were highly enriched simultaneously. Five types of putative cprAs, three types of putative bamBs and seven types of putative nitrogenase reductase (nifHs) were determined, with their copy numbers decreased gradually from inlet to outlet. Distribution of chemicals, bacteria and putative genes confirmed PCP dechlorination and phenol degradation accomplished in segments 0–5 cm and 0–8 cm, respectively, contributing to a high PCP mineralization rate of 3.86 μM d⁻¹. Through long-term incubation, dechlorination, phenol degradation and nitrogen fixation bacteria coexisted and functioned simultaneously near inlet (0–8 cm), verified the feasibility of anaerobic mineralization of halogenated aromatics in the compact reactor containing multiple functional microbes.

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.

Anaerobic mineralization of 2,4,6-tribromophenol to CO2 by a synthetic microbial community comprising Clostridium, Dehalobacter, and Desulfatiglans

Anaerobic mineralization of 2,4,6-tribromophenol (2,4,6-TBP) was achieved by a synthetic anaerobe community comprising a highly enriched culture of Dehalobacter sp. phylotype FTH1 acting as a reductive debrominator; Clostridium sp. strain Ma13 acting as a hydrogen supplier via glucose fermentation; and a novel 4-chlorophenol-degrading anaerobe, Desulfatiglans parachlorophenolica strain DS. 2,4,6-TBP was debrominated to phenol by the combined action of Ma13 and FTH1, then mineralized into CO2 by sequential introduction of DS, confirmed using [ring-(14)C(U)] phenol. The optimum concentrations of glucose, SO4(2-), and inoculum densities were 0.5 or 2.5mM, 1.0 or 2.5mM, and the densities equivalent to 10(4)copiesmL(-1) of the 16S rRNA genes, respectively. This resulted in the complete mineralization of 23μM 2,4,6-TBP within 35days (0.58μmolL(-1)d(-1)). Thus, using a synthetic microbial community of isolates or highly enriched cultures would be an efficient, optimizable, low-cost strategy for anaerobic bioremediation of halogenated aromatics.

Insoluble Fe-humic acid complex as a solid-phase electron mediator for microbial reductive dechlorination

We report that the insoluble Fe-HA complex, which was synthesized with both commercial Aldrich humic acid (HA) and natural HA, functions as a solid-phase electron mediator (EM) for the anaerobic microbial dechlorination of pentachlorophenol. Spectroscopic characterizations and sequential Fe extraction demonstrated that the Fe-HA complex was predominated with Na4P2O7-labile Fe (represented as the organically bound Fe fraction) and poorly ordered Fe fraction (the fraction left in the residue after the sequential extraction), which were associated with different possible binding processes with carboxylate and phenolic groups. The change in the electron-mediating activity caused by Fe extraction indicated that the electron-mediating function of the Fe-HA complex is attributable to the Na4P2O7-labile Fe fraction. The Fe-HA complex also accelerated the microbial reduction of Fe(III) oxide, which suggested the presence of multiple electron-mediating functions in the complex. The electron shuttle assay showed that the Fe-HA complex had an electron-accepting capacity of 0.82 mequiv g(-1) dry Fe-HA complex. The presence of redox-active moieties in the Fe-HA complex was verified by cyclic voltammetry analysis of the sample after electrical reduction, with a redox potential estimated at 0.02 V (vs a standard hydrogen electrode).

Bacterial degradation of chlorophenols and their derivatives

Chlorophenols (CPs) and their derivatives are persistent environmental pollutants which are used in the manufacture of dyes, drugs, pesticides and other industrial products. CPs, which include monochlorophenols, polychlorophenols, chloronitrophenols, chloroaminophenols and chloromethylphenols, are highly toxic to living beings due to their carcinogenic, mutagenic and cytotoxic properties. Several physico-chemical and biological methods have been used for removal of CPs from the environment. Bacterial degradation has been considered a cost-effective and eco-friendly method of removing CPs from the environment. Several bacteria that use CPs as their sole carbon and energy sources have been isolated and characterized. Additionally, the metabolic pathways for degradation of CPs have been studied in bacteria and the genes and enzymes involved in the degradation of various CPs have been identified and characterized. This review describes the biochemical and genetic basis of the degradation of CPs and their derivatives.

A humin-dependent Dehalobacter species is involved in reductive debromination of tetrabromobisphenol A

Tetrabromobisphenol A (TBBPA) is the most widely used brominated flame retardant on the market. It has been detected in various environmental samples, and a growing body of evidence has demonstrated its toxic effects on living organisms. In this study, we report the enrichment and phylogenetic identification of bacteria that debrominate TBBPA to bisphenol A in the presence of humin. Incubation experiments indicated that humin was required for this debromination activity. Of the five compounds examined for inclusion in the TBBPA-debrominating culture, formate was the optimal electron donor. A 16S rRNA gene library showed that the culture was dominated by three known dehalogenator genera: Dehalobacter, Geobacter, and Sulfurospirillum. Further investigation indicated that Dehalobacter was responsible for the debromination of TBBPA. PCR-denaturing gradient gel electrophoresis analysis showed that Dehalobacter grew in the culture by utilizing TBBPA. Moreover, the copy number of the Dehalobacter 16S rRNA genes increased by about two orders of magnitude in the cultures without the addition of TBBPA, whereas it increased by approximately four orders of magnitude when TBBPA was present. The incubation experiments showed that Dehalobacter was reliant on humin for its debromination activity, indicating a new type of metabolism in Dehalobacter that is linked to humin respiration.

Anaerobic conversion of chlorobenzene and benzene to CH4 and CO2 in bioaugmented microcosms

Chlorobenzene is a widespread groundwater contaminant found at many industrial sites. Reductive dechlorination of chlorobenzene requires input of electron donor and results in problematic accumulation of benzene, which is more toxic than chlorobenzene. We hypothesized that coupling a culture capable of reductive dechlorination of chlorobenzene to benzene with a second benzene-degrading methanogenic culture would completely detoxify chlorobenzene. To this end, active chlorobenzene-dechlorinating microcosms that were producing benzene were inoculated with a previously described enriched methanogenic benzene-degrading consortium. The combination resulted in the transformation of chlorobenzene via benzene to the nontoxic degradation products, CO2 and CH4. Sustainable degradation of chlorobenzene and benzene was observed in the microcosms and was further confirmed by shifts in the carbon isotopic ratios of chlorobenzene and benzene during degradation. Moreover, we could show that benzene derived electrons fueled chlorobenzene dechlorination removing the need to provide exogenous electron donor. The results have promising implications for sustainable bioremediation of sites contaminated with chlorinated benzenes and benzene.

Long-term anaerobic mineralization of pentachlorophenol in a continuous-flow system using only lactate as an external nutrient

A simple anaerobic upflow column system (15 cm long, 5 cm inner diameter) for complete pentachlorophenol (PCP) mineralization has been established using a microbial consortium requiring only lactate as the external nutrient. With lactate as an electron donor, PCP was dechlorinated to 3-chlorophenol (3-CP) and phenol. The degradation of 3-CP and phenol proceeded without an external electron acceptor, indicating fermentative or syntrophic characteristics. A tracer experiment using (14)C-U-ring-labeled PCP confirmed the conversion of PCP into CO(2) (54.1%), CH(4) (48.1%), and biomass (0.6%). The nitrogen required for degradation was supplied by N(2)-fixation, evidenced from the nitrogen balance and an acetylene reduction assay. A 16S rRNA gene library analysis showed that bottom of the upflow column harbored the potential dechlorinators, Dehalobacter and Desulfitobacterium, and the phenol/3-CP fermentative or syntrophic degraders, Cryptanaerobacter and Syntrophus. The nitrogen-fixing facultative anaerobes, Rhizobiales, were detected in the top of the upflow column, with other possible nitrogen-fixers at both bottom and top of the upflow column. The mineralization rate reached 1.96 μmoles L(-1) d(-1) for 50 μM of the initial PCP concentration: one of the highest efficiencies reported. This compact anaerobic mineralization system requiring no external supply of an electron acceptor would be useful for the remediation of chlorinated aromatic compounds under anaerobic conditions.

Changing redox potential by controlling soil moisture and addition of inorganic oxidants to dissipate pentachlorophenol in different soils

The potential for dissipation of pentachlorophenol (PCP) was investigated in soils from four different sites in China. These were an umbraqualf (Soil 1), a Plinthudult (Soil 2), a Haplustalf (Soil 3) and an Argiustoll (Soil 4) which were either flooded, to produce anaerobic conditions, or incubated aerobically at 60% water-holding capacity (WHC). The dissipation of PCP in Soil 1 at 60% WHC was higher than under flooded condition, while the opposite occurred in the other three soils. Under flooded conditions, the redox potential decreased significantly in Soil 1 and Soil 4, where sulphate reduction was occurred and the dissipation of PCP was statistically significant (about 96% and 98%, respectively) at the end of incubation. After addition of inorganic oxidants, dissipation of PCP was significantly inhibited by FeCl(3), while Na(2)SO(4) and NaNO(3) had different effects, depending upon the soil type.

Humin as an electron mediator for microbial reductive dehalogenation

We report that humins extracted as the solid fractions from paddy soils or sediment are involved in extracellular electron transfer, coupled with microbial reductive dehalogenation of pentachlorophenol (PCP), by serving as both electron acceptor and electron donor. In our system, humin is requisite for the dechlorination of PCP, and this activity cannot be maintained when humin is replaced with soluble humic substances or related compounds, including 0.1 M NaOH-extracted humic acid from soil, Aldrich humic acid, and anthraquinone-2,6-disulfonate. The function of humins is stable against treatments with H(2)O(2) (30%, 30 min), HCl (0.1 M, 48 h), NH(2)OH · HCl (0.1 M, 48 h), NaBH(4) (0.1 M, 15 h), and heat (121 °C, 30 min). Cyclic voltammograms indicated that humin harbors redox-active moieties, and electron spin resonance suggested that quinone moieties within humin are the redox-active centers. Fourier-transform infrared and nuclear magnetic resonance analyses verified the presence of the aryl carbonyl carbon group in humin. Although the proportion of redox-active carbon is very small, the potential electron-mediating ability is not negligible. The finding that humin, in solid form, is redox active has important implications for in situ bioremediation, given the wide distribution of humin and the diversity and ubiquity of humic substance-utilizing microorganisms.

Migration behavior of landfill leachate contaminants through alternative composite liners

Four identical pilot-scale landfill reactors with different alternative composite liners were simultaneously operated for a period of about 540 days to investigate and to simulate the migration behaviors of phenolic compounds (phenol, 2-CP, 2-MP, 3-MP, 4-MP, 2-NP, 4-NP, 2,4-DNP, 2,4-DCP, 2,6-DCP, 2,4,5-TCP, 2,4,6-TCP, 2,3,4,6-TeCP, PCP) and heavy metals (Pb, Cu, Zn, Cr, Cd, Ni) from landfill leachate to the groundwater. Alternative landfill liners of four reactors consist of R1: Compacted clay liner (10 cm+10 cm, k=10(-8)m/sn), R2: Geomembrane (2 mm HDPE)+compacted clay liner (10 cm+10 cm, k=10⁻⁸ m/sn), R3: Geomembrane (2 mm HDPE)+compacted clay liner (10 cm, k=10⁻⁸ m/sn)+bentonite liner (2 cm)+compacted clay liner (10 cm, k=10⁻⁸ m/sn), and R4: Geomembrane (2 mm HDPE)+compacted clay liner (10 cm, k=10⁻⁸ m/sn)+zeolite liner (2 cm)+compacted clay liner (10 cm, k=10⁻⁸ m/sn). Wastes representing Istanbul municipal solid wastes were disposed in the reactors. To represent bioreactor landfills, reactors were operated by leachate recirculation. To monitor and control anaerobic degradation in the reactors, variations of conventional parameters (pH, alkalinity, chloride, conductivity, COD, TOC, TKN, ammonia and alcaly metals) were also investigated in landfill leachate samples. The results of this study showed that about 35-50% of migration of organic contaminants (phenolic compounds) and 55-100% of migration of inorganic contaminants (heavy metals) to the model groundwater could be effectively reduced with the use of bentonite and zeolite materials in landfill liner systems. Although leachate contaminants can reach to the groundwater in trace concentrations, findings of this study concluded that the release of these compounds from landfill leachate to the groundwater may potentially be of an important environmental concern based on the experimental findings.