Pentachlorophenol dechlorination by an acidogenic sludge

Alkali-thermal activated persulfate treatment of tetrabromobisphenol A in soil: Parameter optimization, mechanism, degradation pathway and toxicity evaluation

The continued accumulation of halogenated organic pollutants in soil posed a potential threat to ecosystems and human health. In this study, tetrabromobisphenol A (TBBPA) was used as a typical representative of halogenated organic pollutants in soil, for alkali-thermal activated persulfate (PS) treatment. The results of response surface methodology (RSM) showed a optimal debromination efficiency of TBBPA was 88.99 % under the optimum reaction conditions. Quenching experiments and electron paramagnetic resonance (EPR) confirmed that SO4-•, HO•, O2-• and 1O2 existed simultaneously in the oxidation process. SO4-• played a major role in the initial stage of the reaction, and O2-• played a major role in the the last stage. Based on density functional theory (DFT) and intermediate products, two degradation pathways were proposed, including debromination reaction and β bond scission. Moreover, the basic physical and chemical properties of the soil were affected to a certain extent, while the soil surface structure, elements and functional group composition rarely changed. In addition, the T.E.S.T. analysis and biotoxicity tests proved that alkali-thermal activated PS can effectively reduce the toxicity of TBBPA-contaminated soil, which is conducive to the subsequent safe secondary utilization of soil.

Alkali-catalyzed hydrothermal oxidation treatment of triclosan in soil: Mechanism, degradation pathway and toxicity evaluation

The continuous accumulation of chlorinated organic pollutants in soil poses a potential threat to ecosystems and human health alike. Alkali-catalyzed hydrothermal oxidation (HTO) can successfully remove chlorinated organic pollutants from water, but it is rarely applied to soil remediation. In this work, we assessed this technique to degrade and detoxify triclosan (TCS) in soil and we determined the underlying mechanisms. The results showed a dechlorination efficiency of TCS (100 mg per kg soil) of 49.03 % after 120 min reaction (H₂O₂/soil ratio 25 mL·g^-1, reaction temperature 180 °C in presence of 1 g·L^-1 NaOH). It was found that soil organic constituents (humic acid, HA) and inorganic minerals (SiO₂, Al₂O₃, and CaCO₃) suppressed the dechlorination degradation of TCS, with HA having the strongest inhibitory effect. During alkali-catalyzed HTO, the TCS molecules were effectively destroyed and humic acid-like or fulvic acid-like organics with oxygen functional groups were generated. Fluorescence spectroscopy analysis showed that hydroxyl radicals (OH) were the dominant reactive species of TCS degradation in soil. On the basis of the Fukui function and the degradation intermediates, two degradation pathways were proposed. One started with cleavage of the ether bond between the benzene rings of TCS, followed by dechlorination and the opening of benzene via oxidation. The other pathway started with direct hydroxylation of the benzene rings of TCS, after which they were opened and dechlorinated through oxidation. Analysis of the soil structure before and after treatment revealed that the soil surface changed from rough to smooth without affecting soil surface elements. Finally, biotoxicity tests proved that alkali-catalyzed HTO effectively reduced the toxicity of TCS-contaminated soil. This study suggests that alkali-catalyzed hydrothermal oxidation provides an environmentally friendly approach for the treatment of soil contaminated with chlorinated organics such as TCS.

Dual-response quadratic model for optimisation of electricity generation and chlorophenol degradation by electro-degradative Bacillus subtilis in microbial fuel cell system

The interactions within microbial, chemical and electronic elements in microbial fuel cell (MFC) system can be crucial for its bio-electrochemical activities and overall performance. Therefore, this study explored polynomial models by response surface methodology (RSM) to better understand interactions among anode pH, cathode pH and inoculum size for optimising MFC system for generation of electricity and degradation of 2,4-dichlorophenol. A statistical central composite design by RSM was used to develop the quadratic model designs. The optimised parameters were determined and evaluated by statistical results and the best MFC systematic outcomes in terms of current generation and chlorophenol degradation. Statistical results revealed that the optimum current density of 106 mA/m² could be achieved at anode pH 7.5, cathode pH 6.3-6.6 and 21-28% for inoculum size. Anode-cathode pHs interaction was found to positively influence the current generation through extracellular electron transfer mechanism. The phenolic degradation was found to have lower response using these three parameter interactions. Only inoculum size-cathode pH interaction appeared to be significant where the optimum predicted phenolic degradation could be attained at pH 7.6 for cathode pH and 29.6% for inoculum size.

Acidification inhibition, biodechlorination, and biotransformation of chlorinated acetaldehydes on acidogenic sludge and microbial community changes

Chlorinated acetaldehydes (CALs) are typical chlorinated organic compounds that posing a great threat to biological wastewater treatment plants. In this study, volatile batch acid (VFA) tests were employed to investigate the acidification inhibition, biodechlorination, and biotransformation of high-strength CALs on hydrolytic acidification. The results indicated that the optimum parameters were 4 g/L sludge, pH = 8, and glucose as an electron donor. Moreover, the acidification inhibition and biodechlorination showed a strongly positive correlation with the degree of chlorination and CAL concentrations. Extracellular polymeric substances (EPS) decreased dramatically, while DNA increased sharply under higher CAL concentrations, which was the result of cell death caused by the toxicity of the CALs. Additionally, the relative toxicities of the CALs were as follows: trichloroacetaldehyde > dichloroacetaldehyde > chloroacetaldehyde. Furthermore, Excitation-Emission-Matrix (EEM) spectra of EPS revealed that aromatic protein-like substances I interacted with CALs to achieve a slight removal of CALs. The detected products revealed that some of the chlorine atoms and aldehyde groups in the CALs were removed by microbes to certain degree. Moreover, microbial community analysis indicated that the dominant phyla were Actinobacteria, Bacteroidetes, and Synergistetes, which had a stronger tolerance to CALs. Notably, biodechlorination was closely related to a remarkable increase in members of the genus Trichococcus.

Reconstruction of microbial community structures as evidences for soil redox coupled reductive dechlorination of PCP in a mangrove soil

The aim was to investigate the influence of pentachlorophenol (PCP) on the soil microbial communities and the coupled mechanism between PCP reductive dechlorination and soil redox under anaerobic condition. Accordingly, a slurry incubation experiment was carried out in which bacterial and archaeal communities were detected by MiSeq amplicon sequencing. The original microbial community balance was gradually disrupted and new microbial structure was reconstructed subsequently through self-regulation and acclimation during PCP transformation, coupling with the changes of soil biogeochemical redox dynamics. The phylum Bacteroidetes predominated during the earlier PCP dechlorination period and then was progressively replaced by Proteobacteria and Firmicutes groups when PCP was mostly transformed into 2,3,4,5-TeCP and 3,4,5-TCP. Heatmap and hierarchical cluster analysis revealed the Clostridium-like, Geobacter-like and Dehalococcoides-like organisms enriched concurrently during PCP reductive dechlorination processes. The relative abundance changes of the redox-active microorganisms, together with their relevance to the corresponding biogeochemical redox processes, showed that PCP dechlorination, Fe(III) and SO₄^2- reduction, as well as methanogenesis were coupled terminal electron accepting processes. The combined analysis of the microbial function, the affinity for substrates (H₂ and acetate) and the sensitivity for PCP toxicity by microorganisms might explain why electron transport chain has changed in soil biogeochemical redox process. Our study offers a comprehensive description of the impact of PCP on the soil microbial community structures, which could be very useful for understanding the regulation of soil nutrient and energy transfer during biogeochemical cycling processes in soils with significant inputs of exogenous pollutants.

Effect of co-substrates on biogas production and anaerobic decomposition of pentachlorophenol

This study aims to examine the effect of different co-substrates on the anaerobic degradation of pentachlorophenol (PCP) with simultaneous production of biogas. Acetate and glucose were added as co-substrates to monitor and compare the methanogenic reaction during PCP degradation. During the experiment, a chemical oxygen demand (COD) removal efficiency of 80% was achieved. Methane (CH₄) production was higher in glucose-fed anaerobic reactors with the highest amount of CH₄ (303.3µL) produced at 200ppm of PCP. Scanning electron microscopy (SEM) demonstrates the high porous structure of anaerobic sludge with uniform channels confirming better mass transfer and high PCP removal. Quantitative real-time PCR (qPCR) revealed that methanogens were the dominating species while some sulfate reducing bacteria (SRBs) were also found in the reactors. The study shows that strategic operation of the anaerobic reactor can be a feasible option for efficient degradation of complex substrates like PCP along with the production of biogas.

Loss of the ssrA genome island led to partial debromination in the PBDE respiring Dehalococcoides mccartyi strain GY50

Polybrominated diphenyl ethers (PBDEs), chemicals commonly used as flame-retardants in consumer products, are emerging persistent organic pollutants that are ubiquitous in the environment. In this study, we report a PBDE-respiring isolate - Dehalococcoides mccartyi strain GY50, which debrominates the most toxic tetra- and penta-BDE congeners (∼1.4 µM) to diphenyl ether within 12 days with hydrogen as the electron donor. The complete genome sequence revealed 26 reductive dehalogenase homologous genes (rdhAs), among which three genes (pbrA1, pbrA2 and pbrA3) were highly expressed during PBDE debromination. After 10 transfers of GY50 with trichloroethene or 2,4,6-trichlorophenol as the electron acceptor instead of PBDEs, the ssrA-specific genome island (ssrA-GI) containing pbrA1 and pbrA2 was deleted from the genome of strain GY50, leading to two variants (strain GY52 with trichloroethene, strain GY55 with 2,4,6-trichlorophenol) with identically impaired debromination capabilities (debromination of penta-/tetra-BDEs ceased at di-BDE 15). Through analysis of Illumina paired-end sequencing data, we identified read pairs that probably came from variants that contain ssrA-GI deletions, indicating their possible presence in the original strain GY50 culture. The two variant strains provide real-time examples on rapid evolution of organohalide-respiring organisms. As PBDE-respiring organisms, GY50-like strains may serve as key players in detoxifying PBDEs in contaminated environments.

Rapid restoration of methanogenesis in an acidified UASB reactor treating 2,4,6-trichlorophenol (TCP)

Anaerobic bioreactors are often used for removal of xenobiotic and highly toxic pollutants from wastewater. Most of the time, the pollutant is so toxic that the stability of the reactor becomes compromised. It is well known that methanogens are one of the most sensitive organisms in the anaerobic consortia and hence the stability of the reactors is highly dependant on methanogenesis. Unfortunately few studies have focused on recovering the methanogenic activity once it has been inhibited by highly toxic pollutants. Here we establish a quick recovery strategy for neutralization of an acidified UASB reactor after failure by intoxication with an excess of TCP in the influent. Once the reactor returned to pH values compatible with methanogenesis, biogas production was re-started after one day and the system was re-acclimated to TCP. Successful removal of TCP from synthetic wastewater was shown for concentrations up to 70mg/L after restoration.

Controlling a toxic shock of pentachlorophenol (PCP) to anaerobic digestion using activated carbon addition

Several powdered and granular activated carbons (PACs and GACs) were tested for adsorption of pentachlorophenol (PCP) in bench-scale anaerobic digestion reactors to control the toxicity of PCP to acetoclastic methanogenesis. Results showed that the adsorption capacities of PAC were reduced by 21-54%, depending on the PAC addition time, in the presence of the methanogenic sludge compared to the controls without sludge. As a preventive measure, PAC at a low dose of 20% (mass ratio to the VSS) added 24 h prior to, or simultaneously with, the addition of PCP could completely eliminate the toxic effects of PCP. At the same dose, PAC also enabled methanogenesis to recover immediately after the sludge had been exposed to PCP for 24h. GAC was not effective in enabling the recovery of methanogenesis due to its slow adsorption kinetics; however, at a dose of 80% it could partially ameliorate the toxic shock of PCP.

Dechlorination of pentachlorophenol (PCP) in aqueous solution on novel Pd-loaded electrode modified with PPy-SDBS composite film

Pentachlorophenol (PCP) is a persistent pollutant and a suspected human carcinogen. It can be found in the air, water, and soil and enters the environment through evaporation from treated wood surfaces, industrial spills, and disposal at uncontrolled hazardous waste sites. Ecotoxicity of PCP necessitates the development of rapid and reliable remediation techniques. Electrocatalytic hydrogenolysis (ECH) has been proven as a promising method for detoxification of halogenated wastes, due to its rapid reaction rate, low apparatus cost, mild reaction conditions, and absence of secondary contaminants. Challenge for the application of ECH is to prepare a Pd-coated cathode with high stability, high catalytic activity, and low Pd loading level. In this work, Pd/polypyrrole-sodium dodecyl benzene sulfonate/meshed Ti (Pd/PPy-SDBS/Ti) electrode was prepared and was characterized by cyclic voltammetry, scanning electron microscopy, X-ray diffraction, and inductively coupled plasma-atomic emission spectrometry. Electrochemically reductive dechlorination of PCP on the Pd/PPy-SDBS/Ti electrode in aqueous solution was investigated. Pd microparticles were uniformly dispersed on PPy-SDBS film which was previously electrodeposited on the meshed Ti supporting electrode. The loading of Pd on the electrode was 0.72 mg cm(-2). Electrocatalytic dechlorination of PCP was performed in a two-compartment cell separated by cation-exchange membrane. The PCP removal on the Pd/PPy-SDBS/Ti electrode could reach 100 % within 70 min with dechlorination current 3 mA when PCP initial concentration was 10 mg L(-1) and initial pH was 2.4. Conversion of PCP on the Pd/PPy-SDBS/Ti electrode followed pseudo-first-order kinetics, and the apparent activation energy was 13.0 kJ mol(-1). The removal of PCP still kept 100 % after 70 min dechlorination when the Pd/PPy-SDBS/Ti cathode was reused ten times. The electrode exhibited promising dechlorination potential with high electrocatalytic activity, good stability, and low cost.

Electrochemical mineralization of pentachlorophenol (PCP) by Ti/SnO2-Sb electrodes

Electrochemical degradation of pentachlorophenol (PCP) in aqueous solution was investigated over Ti/SnO2-Sb electrodes prepared by sol-gel method. X-ray diffraction (XRD), scanning electron microscopy (SEM) and electrochemical measurements were used to characterize the physicochemical properties of the electrodes. The electrochemical degradation of PCP followed pseudo-first-order kinetics. The main influencing factors, including the types of supporting electrolyte (i.e., NaClO4, Na2SO4, Na2SO3, NaNO3, and NaNO2), initial concentrations of PCP (5-1000mgL(-1)), pH values (3.0-11.0), and current densities (5-40mAcm(-2)) were evaluated. The degradation and mineralization ratios of 100mgL(-1) of PCP achieved >99.8% and 83.0% after 30min electrolysis with a 10mmolL(-1) Na2SO4 at a current density of 10mAcm(-2), respectively. The corresponding half-life time (t1/2) was 3.94min. The degradation pathways that were involved in dechlorination, protons generation, and mineralization processes were proposed based on the determination of total organic carbon, chloride, and intermediate products (i.e., low chlorinated phenol and some organic acids). The toxicity of PCP and its intermediates could be reduced effectively by electrolysis. These results showed that electrochemical technique could achieve a significant mineralization rate in a short time (<30min), which provided an efficient way for PCP elimination from wastewater.

Low-temperature anaerobic treatment of low-strength pentachlorophenol-bearing wastewater

The anaerobic treatment of low-strength wastewater bearing pentachlorophenol (PCP) at psychro-mesophilic temperatures has been investigated in an expanded granular sludge bed reactor. Using an upward flow rate of 4 m h(-1), a complete removal of PCP, as well as COD removal and methanization efficiencies higher than 75% and 50%, respectively, were achieved. Methanogenesis and COD consumption were slightly affected by changes in loading rate, temperature (17-28°C) and inlet concentrations of urea and oils. Pentachlorophenol caused an irreversible inhibitory effect over both acetoclastic and hydrogenotrophic methanogens, being the later more resistant to the toxic effect of pentachlorophenol. An auto-inhibition phenomenon was observed at PCP concentrations higher than 10 mg L(-1), which was accurately predicted by a Haldane-like model. The inhibitory effect of PCP over the COD consumption and methane production was modelled by modified pseudo-Monod and Roediger models, respectively.

Effect of sulfate on anaerobic reduction of nitrobenzene with acetate or propionate as an electron donor

Sulfate is frequently found in wastewaters that contain nitrobenzene. To reveal the effect of sulfate on the reductive transformation of nitrobenzene to aniline--with acetate or propionate as potential electron donors in anaerobic systems--an acetate series (R1-R5) and a propionate series (R6-R10) were set up. Each of these was comprised of five laboratory-scale sequence batch reactors. The two series were amended with the same amount of nitrobenzene and electron donor electron equivalents, whereas with increasing sulfate concentrations. Results indicated that the presence of sulfate could depress nitrobenzene reduction. Such depression is linked to the inhibition of nitroreductase activity and/or the shift of electron flow. In the acetate series, although sulfate did not strongly compete with nitrobenzene for electron donors, noncompetitive inhibition of specific nitrobenzene reduction rates by sulfate was observed, with an inhibition constant of 0.40 mM. Propionate, which can produce intermediate H₂ as preferred reducing equivalent, is a more effective primary electron donor for nitrobenzene reduction as compared to acetate. In the propionate series, sulfate was found to be a preferential electron acceptor as compared to nitrobenzene, resulting in a quick depletion of propionate and then a likely termination of H₂-releasing under higher sulfate concentrations (R9 and R10). In such a situation, nitrobenzene reduction slowed down, occurring two-stage zero-order kinetics.

Combined effects of enrichment procedure and non-fermentable or fermentable co-substrate on performance and bacterial community for pentachlorophenol degradation in microbial fuel cells

Combined effects of enrichment procedure and non-fermentable acetate or fermentable glucose on system performance and bacterial community for pentachlorophenol (PCP) degradation in microbial fuel cells (MFCs) were determined in this study. Co-substrate and PCP were added into MFCs either simultaneously or sequentially. Simultaneous addition with glucose (simultaneous-glucose) achieved the shortest acclimation time and the most endurance to heavy PCP shock loads. Species of Alphaproteobacteria (simultaneous-acetate, 33.9%; sequential-acetate, 31.3%), Gammaproteobacteria (simultaneous-glucose, 44.1%) and Firmicutes (sequential-glucose, 31.8%) dominated the complex systems. The genus Sedimentibacter was found to exist in all the cases whereas Spirochaetes were merely developed in simultaneous-acetate and simultaneous-glucose. While Epsilonproteobacteria were only absent from sequential-acetate, simultaneous-glucose benefited to the evolution of Lentisphaerae. These results demonstrate simultaneous-glucose is a strategy for efficient system performance and the microbiological evidence can contribute to improving understanding of and optimizing PCP degradation in MFCs.

Reductive dechlorination and mineralization of pentachlorophenol in biocathode microbial fuel cells

Simultaneous anaerobic and aerobic degradation pathways in two-chamber, tubular microbial fuel cells (MFCs) facilitated pentachlorophenol (PCP) mineralization by a mediator-less biocathode. PCP was degraded at a rate of 0.263 ± 0.05 mg/L-h (51.5 mg/g VSS-h) along with power generation of 2.5 ± 0.03 W/m(3). Operating the biocathode MFC at 50°C improved the PCP degradation rate to 0.523 ± 0.08 mg/L-h (103 mg/g VSS-h) and power production to 5.2 ± 0.03 W/m(3). A pH of 6.0 increased the PCP degradation rate to 0.365 ± 0.02 mg/L-h (71.5mg/g VSS-h), but reduced power. While mediators were not needed, adding anthraquinone-2,6-disulfonate increased power and PCP degradation rates. Dominant bacteria most similar to the anaerobic Desulfobacterium aniline, Actinomycetes and Streptacidiphilus, and aerobic Rhodococcus erythropolis, Amycolatopsis and Gordonia were found on the biocathode. These results demonstrate efficient degradation of PCP in biocathode MFCs and the effects of temperature, pH and mediators.

Degradation of pentachlorophenol with the presence of fermentable and non-fermentable co-substrates in a microbial fuel cell

Pentachlorophenol (PCP) was more rapidly degraded in acetate and glucose-fed microbial fuel cells (MFCs) than in open circuit controls, with removal rates of 0.12 ± 0.01 mg/Lh (14.8 ± 1.0 mg/g-VSS-h) in acetate-fed, and 0.08 ± 0.01 mg/L h (6.9 ± 0.8 mg/g-VSS-h) in glucose-fed MFCs, at an initial PCP concentration of 15 mg/L. A PCP of 15 mg/L had no effect on power generation from acetate but power production was decreased with glucose. Coulombic balances indicate the predominant product was electricity (16.1 ± 0.3%) in PCP-acetate MFCs, and lactate (19.8 ± 3.3%) in PCP-glucose MFCs. Current generation accelerated the removal of PCP and co-substrates, as well as the degradation products in both PCP-acetate and PCP-glucose reactors. While 2,3,4,5-tetrachlorophenol was present in both reactors, tetrachlorohydroquinone was only found in PCP-acetate MFCs. These results demonstrate PCP degradation and power production were affected by current generation and the type of electron donor provided.

Mechanism of reductive decomposition of pentachlorophenol by Ti-doped beta-Bi(2)O(3) under visible light irradiation

The reductive decomposition of pentachlorophenol (PCP) by photocatalysis with Ti-doped beta-Bi(2)O(3) was investigated under visible light (lambda > 420 nm) irradiation. The results indicated that hydroxyl radical (*OH) and singlet oxygen ((1)O(2)) could not be detected with electron spin resonance (ESR) on the photocatalyst under light irradiation. An electron scavenger weakened the photocatalytic activity of the photocatalyst for the decomposition of PCP; however, scavengers of reactive oxygen species (ROS) enhanced the activity. The decomposition intermediates of PCP detected by liquid chromatography-mass spectrometry (LC-MS) and gas chromatography-mass spectrometry (GC-MS) suggested the existence of phenol, cyclohexanone, cyclohexanol, glycol, and propylene. All the evidence suggested that reductive dechlorination was the major route in the decomposition of PCP, during which the photogenerated electron under visible light irradiation acted as reductant. The reliability of the proposed reductive mechanism was further verified by comparing the reduction potential (E(re)) of PCP with the conduction band potential (E(cb)) of the photocatalyst. The decomposition pathway of PCP with electron reduction under visible light irradiation was also investigated.