Effect of different carbon materials as electron shuttles in the anaerobic biotransformation of nitroanilines

Mechanism of biochar functional groups in the catalytic reduction of tetrachloroethylene by sulfides

In recent years, biochar has become of considerable interest for environmental applications, it can be used as a catalyst for sulfides reduction of perchloroethylene, but the crucial role of biochar properties played in catalyzing dechlorination remained ambiguous investigation. To pinpoint the critical functional groups, the modified biochars were respectively produced by HNO₃, KOH and H₂O₂ with similar dimensional structures but different functional groups. Combined with the adsorption and catalytic results of different biochars, the acid-modified biochar had the best catalytic performance (99.9% removal) due to the outstanding specific surface area and ample functional groups. According to characterization and DFT results, carboxyl and pyridine nitrogen exhibited a positive correlation with the catalytic rate, indicating that their contribution to catalytic performance. Customizing biochar with specific functional groups removed depth demonstrated that the carboxyl was essential component. Further, alkaline condition was conducive to catalytic reduction, while tetrachloroethylene cannot be reduced under acidic conditions, because HS^- and S^2- mainly existed in alkaline environment and the sulfur-containing nucleophilic structure formed with biochar was more stable under this condition. Overall, this study opens new perspectives for in situ remediation by biochar in chlorinated olefin polluted anoxic environment and promotes our insight of modifying for biochar catalyst design.

Progress and prospects of applying carbon-based materials (and nanomaterials) to accelerate anaerobic bioprocesses for the removal of micropollutants

Carbon-based materials (CBM), including activated carbon (AC), activated fibres (ACF), biochar (BC), nanotubes (CNT), carbon xenogels (CX) and graphene nanosheets (GNS), possess unique properties such as high surface area, sorption and catalytic characteristics, making them very versatile for many applications in environmental remediation. They are powerful redox mediators (RM) in anaerobic processes, accelerating the rates and extending the level of the reduction of pollutants and, consequently, affecting positively the global efficiency of their partial or total removal. The extraordinary conductive properties of CBM, and the possibility of tailoring their surface to address specific pollutants, make them promising as catalysts in the treatment of effluents containing diverse pollutants. CBM can be combined with magnetic nanoparticles (MNM) assembling catalytic and magnetic properties in a single composite (C@MNM), allowing their recovery and reuse after the treatment process. Furthermore, these composites have demonstrated extraordinary catalytic properties. Evaluation of the toxicological and environmental impact of direct and indirect exposure to nanomaterials is an important issue that must be considered when nanomaterials are applied. Though the chemical composition, size and physical characteristics may contribute to toxicological effects, the potential toxic impact of using CBM is not completely clear and is not always assessed. This review gives an overview of the current research on the application of CBM and C@MNM in bioremediation and on the possible environmental impact and toxicity.

Bioremediation of Perfluoroalkyl Substances (PFAS) by Anaerobic Digestion: Effect of PFAS on Different Trophic Groups and Methane Production Accelerated by Carbon Materials

Per- and polyfluoroalkyl substances (PFAS) are recalcitrant pollutants which tend to persist in soils and aquatic environments and their remediation is among the most challenging with respect to organic pollutants. Anaerobic digestion (AD) supplemented with low amounts of carbon materials (CM), acting as electron drivers, has proved to be an efficient process for the removal of organic compounds from wastewater. This work explores the impact of PFAS on different trophic groups in anaerobic communities, and the effect of carbon nanotubes (CNT), activated carbon (AC), and oxidized AC (AC-HNO₃), as electron shuttles on the anaerobic bioremoval of these compounds, based on CH₄ production. The inhibition of the specific methanogenic activity (SMA) exerted by perfluorooctanoic acid (PFOA) and perfluorooctane sulfonate (PFOS), at a concentration of 0.1 mg L⁻¹, was below 10% for acetoclastic and below 15%, for acetogenic communities. Hydrogenotrophic methanogens were not affected by the presence of PFAS. All CM reduced the negative impact of PFAS on the CH₄ production rate, but AC was the best. Moreover, the methanization percentage (MP) of sewage sludge (SS) increased 41% in the presence of PFOS (1.2 g L⁻¹) and AC. In addition, AC fostered an increase of 11% in the MP of SS+PFOS, relative to the condition without AC. AC promoted detoxification of PFOA- and PFOS-treated samples by 51% and 35%, respectively, as assessed by Vibrio fischeri assays, demonstrating the advantage of bringing AD and CM together for PFAS remediation.

Enhanced Microbial Ferrihydrite Reduction by Pyrogenic Carbon: Impact of Graphitic Structures

Electron-shuttling agents such as pyrogenic carbon (PC) can mediate long-distance electron transfer and play numerous key roles in aquatic and soil biogeochemical processes. The electron-shuttling capacity of PC relies on both the surface oxygen-containing functional groups and bulk graphitic structures. Although the impacts of oxygen-containing functional groups on the electron-shuttling performance of PC are well studied, there remains insufficient understanding on the function of graphitic structures. Here, we studied the functions of PC in mediating microbial (Shewanella oneidensis MR-1) reduction of ferrihydrite, a classic and geochemically important soil redox process. The results show that PC enhanced microbial ferrihydrite reduction by 20-115% and the reduction rates increased with PC pyrolysis temperature increasing from 500 to 900 °C. For PC prepared at low temperature (500-600 °C), the electron-shuttling capacity of PC is mainly attributed to its oxygen-containing functional groups, as indicated by a 50-60% decline in the ferrihydrite reduction rate when PC was reduced under a H₂ atmosphere to remove surface oxygen-containing functional groups. In stark contrast, for PC prepared at higher temperature (700-900 °C), the formation of PC graphitic structures was enhanced, as suggested by the higher electrical conductivity; accordingly, the graphitic structure exhibits greater importance in shuttling electrons, as demonstrated by a minor decline (10-18%) in the ferrihydrite reduction rate after H₂ treatment of PC. This study provides new insights into the nonlinear and combined role of graphitic structures and oxygen-containing functional groups of PC in mediating electron transfer, where the pyrolysis temperature of PC acts as a key factor in determining the electron-shuttling pathways.

Anaerobic bacterial responses to carbonaceous materials and implications for contaminant transformation: Cellular, metabolic, and community level findings

Carbonaceous materials (CM) enhance the abundance and activity of bacteria capable of persistent organic (micro)pollutant (POP) degradation. This review synthesizes anaerobic bacterial responses to minimally modified CM in non-fuel cell bioremediation applications at three stages: attachment, metabolism, and biofilm genetic composition. Established relationships between biological behavior and CM surface properties are identified, but temporal relationships are not well understood, making it difficult to connect substratum properties and "pioneer" bacteria with mature microorganism-CM systems. Stark differences in laboratory methodology at each temporal stage results in observational, but not causative, linkages as system complexity increases. This review is the first to critically examine relationships between material and cellular properties with respect to time. The work highlights critical knowledge gaps that must be addressed to accurately predict microorganism-CM behavior and to tailor CM properties for optimized microbial activity, critical frontiers in establishing this approach as an effective bioremediation strategy.

Abiotic reductive removal of organic contaminants catalyzed by carbon materials: A short review

Since the observation that carbon materials can facilitate electron transfer between reactants, there is growing literature on the abiotic reductive removal of organic contaminants catalyzed by them. Most of the interest in these processes arises from the participation of carbon materials in the natural transformation of contaminants and the possibility of developing new strategies for environmental treatment and remediation. The combinations of various carbon materials and reductants have been investigated for the reduction of nitro-organic compounds, halogenated organics, and azo dyes. The reduction rates of a certain compound in carbon-reductant systems vary with the surface properties of carbon materials, although there are controversial conclusions on the properties governing the catalytic performance. This review scrutinizes the contributions of quinone moieties, electron conductivity, and other carbon properties to the activity of carbon materials. It also discusses the contaminant-dependent reduction pathways, that is, electron transfer through conductive carbon and intermediates formed during the reaction, along with possibly additional activation of contaminant molecules by carbon. Moreover, modification strategies to improve the catalytic activity for reduction are summarized. Future research needs are proposed to advance the understanding of reaction mechanisms and improve the practical utility of carbon material for water treatment. PRACTITIONER POINTS: Reduction rates of contaminants in carbon-reductant systems and modification strategies for carbon materials are summarized. Mechanisms for the catalytic activity of carbon materials are discussed. Research needs for new insights into carbon-catalyzed reduction are proposed.

Effects of iron-carbon materials on microbial-catalyzed reductive dechlorination of polychlorinated biphenyls in Taihu Lake sediment microcosms: Enhanced chlorine removal, detoxification and shifts of microbial community

Nano zero-valent iron particles (nZVI, 0.09 wt%), micro zero-valent iron particles (mZVI, 0.09 wt%), granular activated carbon (GAC, 3.03 wt%), GAC supported nZVI (nZVI/GAC, 3.12 wt%) and nZVI&GAC (nZVI 0.09 wt%, GAC 3.03 wt%) were evaluated for their effects on polychlorinated biphenyls (PCBs) anaerobic reductive dechlorination, detoxification, as well as microbial community structure in Taihu Lake (China) sediment microcosms. The results showed that all of these five materials could stimulate PCBs reductive dechlorination, especially for dioxin-like PCB congeners, and nZVI&GAC had the best removal effect on PCBs. The reduction of total PCBs increased from 13.5% to 33.2%. H₂ generated by zero-valent iron corrosion was utilized by organohalide-respiring bacteria (OHRB) to enhance the dechlorination of PCBs predominantly via meta chlorine removal in the short term. The addition of ZVI had little impact on the total bacterial abundance and the microbial community structure. The adsorption of GAC and potential bioremediation properties of attached biofilm could promote the long-term removal of PCBs. GAC, nZVI/GAC, nZVI&GAC had different influences on the microbial structure. These findings provide insights into the biostimulation technique for in situ remediations of PCBs contaminated sediments.

Simulating the synergy of electron donors and different redox mediators on the anaerobic decolorization of azo dyes: Can AQDS-chitosan globules replace the traditional redox mediators?

During anaerobic treatment of azo dye wastewater, the decolorization efficiency is low and dissolved redox mediators (RMs) added to the system are easy lost. In order to solve these issues, immobilized RMs have been a hot area of research. In this study a novel immobilized RM material, disodium anthraquinone-2,6-disulfonate (AQDS)-chitosan globules, which is natural, highly efficient and environmentally friendly, was prepared. Compared with natural immobilized RMs (activated carbon) and dissolved RMs (AQDS), it can be considered that it has a significant strengthening effect on the anaerobic biological degradation and decolorization of azo dye wastewater. An electron donor (ED, glucose) or RM (AQDS solution) was dosed into an anaerobic reactor to determine the enhancing effect and appropriate concentration for the decolorization treatment. The results indicate that a certain concentration of ED or RM [300 mg/L (1.667 mmol/L) glucose or 200 μmol/L AQDS solution] can improve effectively the anaerobic biological degradation and decolorization effect of azo dye wastewater. While by adding both 300 mg/L (1.667 mmol/L) glucose and 300 μmol/L AQDS (the concentrations were the initial reactive concentrations) together the decolorization efficiency was improved further. At the same time, the synergy of ED (glucose) and RM (AQDS solution) on the anaerobic decolorization of azo dye was simulated by the central combination design. A mathematical model for the decolorization efficiency has been established. According to this model, the hydraulic retention time of the best decolorization speed and efficiency has been obtained.

Detoxification of Ciprofloxacin in an Anaerobic Bioprocess Supplemented with Magnetic Carbon Nanotubes: Contribution of Adsorption and Biodegradation Mechanisms

In anaerobic bioreactors, the electrons produced during the oxidation of organic matter can potentially be used for the biological reduction of pharmaceuticals in wastewaters. Common electron transfer limitations benefit from the acceleration of reactions through utilization of redox mediators (RM). This work explores the potential of carbon nanomaterials (CNM) as RM on the anaerobic removal of ciprofloxacin (CIP). Pristine and tailored carbon nanotubes (CNT) were first tested for chemical reduction of CIP, and pristine CNT was found as the best material, so it was further utilized in biological anaerobic assays with anaerobic granular sludge (GS). In addition, magnetic CNT were prepared and also tested in biological assays, as they are easier to be recovered and reused. In biological tests with CNM, approximately 99% CIP removal was achieved, and the reaction rates increased ≈1.5-fold relatively to the control without CNM. In these experiments, CIP adsorption onto GS and CNM was above 90%. Despite, after applying three successive cycles of CIP addition, the catalytic properties of magnetic CNT were maintained while adsorption decreased to 29 ± 3.2%, as the result of CNM overload by CIP. The results suggest the combined occurrence of different mechanisms for CIP removal: adsorption on GS and/or CNM, and biological reduction or oxidation, which can be accelerated by the presence of CNM. After biological treatment with CNM, toxicity towards Vibrio fischeri was evaluated, resulting in ≈ 46% detoxification of CIP solution, showing the advantages of combining biological treatment with CNM for CIP removal.

The reduction of nitrobenzene by extracellular electron transfer facilitated by Fe-bearing biochar derived from sewage sludge

In this work, the incorporation of Fe-bearing sludge-derived biochar greatly enhanced both biotic and abiotic reduction of nitrobenzene (NB) to aniline, which was attributed to the concomitant microbial dissimilatory iron reduction. Biogenic Fe(II) produced by Geobacter sulfurreducens dominated the anaerobic reduction of NB following the pseudo-first-order kinetic. Besides, the increase of pyrolysis temperature from 600 to 900 ℃ to generate biochar resulted in an accelerated removal rate of NB in Geobacter-biochar combined system. The morphology and structural characterization of biochar with G. sulfurreducens confirmed the formation of conductive bacteria-biochar aggregates. Electrochemical measurements suggested the presence of graphitized domains and quinone-like moieties in biochar as redox-active centers, which might play an important role in accelerating electron transfer for microbial dissimilatory iron reduction and NB degradation. This study provides a feasible way of using Fe-bearing sludge as a valuable feedstock for biochar generation and its application with electrochemically active bacteria for the bioremediation of nitroaromatic compounds-polluted wastewater.

Tailoring Carbon Nanotubes to Enhance their Efficiency as Electron Shuttle on the Biological Removal of Acid Orange 10 Under Anaerobic Conditions

Anaerobic treatments have been described for the biodegradation of pollutants. However, the reactions proceed slowly due to the recalcitrant nature of these compounds. Carbon nanomaterials (CNM) intermediate in, and favor, the electron transfer, accelerating the anaerobic reduction of pollutants, which act as final electron acceptors. In the present work, different carbon nanotubes (CNT) with modified surface chemistry, namely CNT oxidized with HNO₃ (CNT_HNO₃) and CNT doped with nitrogen in a ball milling process (CNT_N_MB) were prepared using commercial CNT as a starting material. The new CNM were tested as redox mediators (RM), 0.1 g L^-1, in the biological reduction of the azo dye, Acid Orange 10 (AO10), with an anaerobic granular sludge, over 48 h of reaction. Methane production was also assessed to verify the microorganism's activity and the CNM's effect on the methanogenic activity. An improvement in the biological removal of AO10 occurred with all CNM (above 90%), when compared with the control without CNM (only 32.4 ± 0.3%). The best results were obtained with CNT_N_MB, which achieved 98.2 ± 0.1% biological AO10 removal, and an 11-fold reduction rate increase. In order to confer magnetic properties to the CNM, tailored CNT were impregnated with 2% of iron-samples: CNT@2%Fe, CNT@2%Fe_N_MB, and CNT@2%Fe_HNO₃. The better performance of the CNT doped with nitrogen was confirmed with CNT@2%Fe_N_MB, and the magnetic character facilitated its recovery after treatment, and did not affect its good catalytic properties. No dye removal was observed in the abiotic assays, so the removal was not due to adsorption on the CNM. Furthermore, the microorganism's viability was maintained during the assay and methane production was not affected by the presence of the CNM. Despite the toxic character of the aromatic amines formed, detoxification was observed after the biological process with thermally treated CNT.

Influence of electrostatic field and conductive material on the direct interspecies electron transfer for methane production

The influence of electrostatic field on the direct interspecies electron transfer (DIET) pathways for methane production was investigated in a batch bioelectrochemical anaerobic digester (BEAD). The ultimate methane production and methane yield in the BEAD reactor saturated to 925 ± 29 mL/L and 309.9 ± 9.6 mL CH₄/g COD, respectively, which were much higher than 616 ± 3 mL/L and 205.4 ± 205.4 mL CH₄/g COD in the anaerobic digester (AD). In the cyclic voltammogram (CV) for bulk solution, the oxidation peak current was 0.52 mA in the BEAD reactor, which was higher than 0.24 mA of AD reactor. This shows that the oxidizing ability of microorganisms was greatly improved in the BEAD reactor. Anaerolineaceae, a well-known electroactive bacterial family, was well enriched in the BEAD reactor. It indicates that the electrostatic field can enrich the electroactive bacteria and activate the DIET pathways for methane production. Moreover, the conductive material (activated carbon) further improved the performance of BEAD reactor, implies that the conductivities of bulk solution is one of the important parameters for the DIET pathways.

Effects of graphene oxide and reduced graphene oxide on acetoclastic, hydrogenotrophic and methylotrophic methanogenesis

This study describes the effects of graphene oxide (GO) and reduced graphene oxide (rGO) on the acetoclastic, hydrogenotrophic and methylotrophic pathways of methanogenesis by an anaerobic consortium. The results showed that GO negatively affected the hydrogenotrophic and acetoclastic pathways at a concentration of 300 mg/L, causing a decrease of ~ 38% on the maximum specific methanogenic activity (MMA) with respect to the controls lacking GO. However, the presence of rGO (300 mg/L) promoted an improvement of the MMA (> 45%) achieved with all substrates, except for the hydrogenotrophic pathway, which was relatively insensitive to rGO. The presence of either rGO or GO enhanced the methylotrophic pathway and resulted in an increase of the MMA of up to 55%. X-ray photoelectron spectroscopy (XPS) analysis revealed that GO underwent microbial reduction during the incubation period. Electrons derived from substrates oxidation were deviated from methanogenesis towards the reduction of GO, which may explain the MMA decreased observed in the presence of GO. Furthermore, XPS evidence indicated that the extent of GO reduction depended on the metabolic pathway triggered by a given substrate.

Biofilm microbial composition changes due to different surface chemical modifications of activated carbon cloths in the biotransformation of 4-nitrophenol

Activated carbon cloths (ACCs) were used as biofilms supports in the anaerobic biotransformation of 4-nitrophenol (4NP). As received ACC material (AW) was oxidized with HNO₃ (OX) and then functionalized with anthraquinone-2,6-disulfonate (AQ). The three ACCs were packed in hybrid UASB reactors and seeded with anaerobic granular sludge for biotransformation experiments. The results indicated that ACC-packed bioreactors improved the biotransformation of 4NP by twofold as compared to the control reactor without support materials. However, the biotransformation effciency of AW, OX and AQ was very similar (59%), indicating the role of ACC as biofilm support and not as redox mediator. After 4NP biotransformation several physicochemical and biological changes were observed like (1) the point of zero charge (pH_PZC) shift from acidic values (AW = 5.0, OX = 3.4, AQ = 3.1) to neutral values (pH_PZC = 7.6 on average), (2) increase in the concentration of acidic and basic surface functional groups over ACC materials and the amount of supported biomass on ACCs due to biofilm formation, and (3) enrichment of exoelectrogenic microorganisms belonging to the genera Geobacter over carbonyl-rich ACC surface as revealed by 16S rRNA amplicon sequencing. Overall, the results suggest that chemical modifications of ACCs changed the microbial composition of the biofilm, but the higher concentration of carbonyl groups on ACC did not affect the biotransformation of 4NP.

Methane Production and Conductive Materials: A Critical Review

Conductive materials (CM) have been extensively reported to enhance methane production in anaerobic digestion processes. The occurrence of direct interspecies electron transfer (DIET) in microbial communities, as an alternative or complementary to indirect electron transfer (via hydrogen or formate), is the main explanation given to justify the improvement of methane production. Not disregarding that DIET can be promoted in the presence of certain CM, it surely does not explain all the reported observations. In fact, in methanogenic environments DIET was only unequivocally demonstrated in cocultures of Geobacter metallireducens with Methanosaeta harundinacea or Methanosarcina barkeri and frequently Geobacter sp. are not detected in improved methane production driven systems. Furthermore, conductive carbon nanotubes were shown to accelerate the activity of methanogens growing in pure cultures, where DIET is not expected to occur, and hydrogenotrophic activity is ubiquitous in full-scale anaerobic digesters treating for example brewery wastewaters, indicating that interspecies hydrogen transfer is an important electron transfer mechanism in those systems. This paper presents an overview of the effect of several iron-based and carbon-based CM in bioengineered systems, focusing on the improvement in methane production and in microbial communities' changes. Control assays, as fundamental elements to support major conclusions in reported experiments, are critically revised and discussed.

Current state of in situ subsurface remediation by activated carbon-based amendments

The last decade has seen a growing interest in applying activated carbon (AC)-based amendments for in situ subsurface remediation of organic contaminants such as chlorinated solvents and petroleum hydrocarbons. This remedial technology has been promoted by several major AC-based product vendors on the market. These products involve impregnation or co-application of chemical or biological additives to facilitate various contaminant degradation processes in conjunction with contaminant adsorption. During field applications, rapid contaminant removal and limited rebound after emplacement have often been reported and considered as two major advantages for this remedial technology. Nevertheless, questions remain to be answered regarding its true effectiveness and longevity given the lack of subsequent field characterizations and evidence of the degradation process, especially biodegradation. Additional uncertainties reside in how subsurface heterogeneity may affect the design, implementation and performance monitoring of this technology. In light of these uncertainties, this review presents an independent analysis that focuses on both the scientific and practical aspects of AC-based remedial technology for in situ subsurface remediation by gathering and synthesizing the scientific knowledge and practical lessons from a broad range of contaminant removal processes involving adsorption and/or degradation. The analysis showed that the scientific soundness of combining adsorption and degradation proposed for all the AC-based products is well supported by the literature on ex situ treatment. However, the in situ effectiveness might be affected by additional factors, such as geological heterogeneity, amendment transport and distribution, and total contaminant mass, which require more thorough and quantitative evaluation. Overall, the technology may provide a viable tool in addressing major remediation challenges encountered in current practice, such as mitigation of back diffusion from residual sources in low permeability zones and treatment of low concentration plumes.

Activity and Reactivity of Pyrogenic Carbonaceous Matter toward Organic Compounds

Pyrogenic carbonaceous matter (PCM) includes environmental black carbon (fossil fuel soot, biomass char), engineered carbons (biochar, activated carbon), and related materials like graphene and nanotubes. These materials contact organic pollutants due to their widespread presence in the environment or through their use in various engineering applications. This review covers recent advances in our understanding of adsorption and chemical reactions mediated by PCM and the links between these processes. It also covers adsorptive processes previously receiving little attention and ignored in models such as steric constraints, physicochemical effects of confinement in nanopores, π interactions of aromatic compounds with polyaromatic surfaces, and very strong hydrogen bonding of ionizable compounds with surface functional groups. Although previous research has regarded carbons merely as passive sorbents, recent studies show that PCM can promote chemical reactions of sorbed contaminants at ordinary temperature, including long-range electron conduction between molecules and between microbes and molecules, local redox reactions between molecules, and hydrolysis. PCM may itself contain redox-active functional groups that are capable of oxidizing or reducing organic compounds and of generating reactive oxygen species (ROS) from oxygen, peroxides, or ozone. Amorphous carbons contain persistent free radicals that may play a role in observed redox reactions and ROS generation. Reactions mediated by PCM can impact the biogeochemical fate of pollutants and lead to useful strategies for remediation.

Inhibitory effect and mechanism of azo dyes on anaerobic methanogenic wastewater treatment: Can redox mediator remediate the inhibition?

Inhibitory effect of azo dyes on anaerobic methanogenic wastewater treatment (AMWT) has been studied mainly focusing on biological toxicity in the batch test with simulated sole co-substrate. Detailed information on inhibitory effect and mechanism of azo dyes during the long-term operation with real complex co-substrate is limited. Moreover, whether redox mediator (RM) could remediate the inhibition is still unclear in previous studies, especially under the complex scenario. In this study, the real textile wastewater with alternative concentrations of azo dyes (0-600 mg/L) were used to operate a lab-scale high-rate anaerobic methanogenic bioreactor for 127 days, and 50 μM anthraquinone-2-sulfonate (AQS) as RM was added at the last period of operation. Azo dyes with concentration of 600 mg/L could cause significant inhibition on overall (decolorizing and methanogenic) performance of AMWT. Specific methanogenic activity assays showed that acetoclastic methanogens was more susceptible to high concentration azo dyes than hydrogenotrophic methanogens. The spatial distribution of extracellular polymeric substance in the anaerobic granular sludge (AGS) showed that the high biological toxicity of azo dyes was mainly attributed to enrichment effect in tightly bound-EPS (TB-EPS). The channels of AGS was clogged by azo dyes, which was evidenced by the hard release of aromatic amines in EPSs as well as decreased porosity of AGS and scanning electron microscope images. Meanwhile, the settling ability, particle size and strength of AGS all deteriorated after azo dyes concentration exceeded 450 mg/L. The dosing of AQS could mostly remediate overall performance of the bioreactor even if the recovery of acetoclastic methanogens was slow. However, except for the porosity with a part of recovery, physical characteristics of AGS hardly recovered, and washout of sludge from the bioreactor was still happening. It suggested that additional attention should be paid to prevent sludge from washout if RM was practically used to remediate the anaerobic reactor inhibited by azo dyes.

Perspectives on carbon materials as powerful catalysts in continuous anaerobic bioreactors

The catalytic effect of commercial microporous activated carbon (AC) and macroporous carbon nanotubes (CNT) is investigated in reductive bioreactions in continuous high rate anaerobic reactors, using the azo dye Acid Orange 10 (AO10) as model compound as electron acceptor and a mixture of VFA as electron donor. Size and concentration of carbon materials (CM) and hydraulic retention time (HRT) are assessed. CM increased the biological reduction rate of AO10, resulting in significantly higher colour removal, as compared to the control reactors. The highest efficiency, 98%, was achieved with a CNT diameter (d) lower than 0.25 mm, at a CNT concentration of 0.12 g per g of volatile solids (VS), a HRT of 10 h and resulted in a chemical oxygen demand (COD) removal of 85%. Reducing the HRT to 5 h, colour and COD removal in CM-mediated bioreactors were above 90% and 80%, respectively. In the control reactor, thought similar COD removal was achieved, AO10 decolourisation was just approximately 20%, demonstrating the ability of CM to significantly accelerate the reduction reactions in continuous bioreactors. AO10 reduction to the correspondent aromatic amines was proved by high performance liquid chromatography (HPLC). Colour decrease in the reactor treating a real effluent with CNT was the double comparatively to the reactor operated without CNT. The presence of AC in the reactor did not affect the microbial diversity, as compared to the control reactor, evidencing that the efficient reduction of AO10 was mainly due to AC rather than attributed to changes in the composition of the microbial communities.

Redox activity of lignite and its accelerating effects on the chemical reduction of azo dye by sulfide

Redox active lignite can act as a mediator to accelerate azo dye decolorization through enhancing electron transfer from sulfide to the azo dye.