Anthraquinone-2,6-disulfonate Research Articles

Enhanced Extracellular Electron Transfer in Magnetite-Mediated Anaerobic Oxidation of Methane Coupled to Humic Substances Reduction: The Pivotal Role of Membrane-Bound Electron Transfer Proteins.

Humic substances are organic substances prevalent in various natural environments, such as wetlands, which are globally important sources of methane (CH4) emissions. Extracellular electron transfer (EET)-mediated anaerobic oxidation of methane (AOM)-coupled with humic substances reduction plays an important role in the reduction of methane emissions from wetlands, where magnetite is prevalent. However, little is known about the magnetite-mediated EET mechanisms in AOM-coupled humic substances reduction. This study shows that magnetite promotes the reduction of the AOM-coupled humic substances model compound, anthraquinone-2,6-disulfonate (AQDS). 13CH4 labeling experiments further indicated that AOM-coupled AQDS reduction occurred, and acetate was an intermediate product of AOM. Moreover, 13CH313COONa labeling experiments showed that AOM-generated acetate can be continuously reduced to methane in a state of dynamic equilibrium. In the presence of magnetite, the EET capacity of the microbial community increased, and Methanosarcina played a key role in the AOM-coupled AQDS reduction. Pure culture experiments showed that Methanosarcina barkeri can independently perform AOM-coupled AQDS reduction and that magnetite increased its surface protein redox activity. The metatranscriptomic results indicated that magnetite increased the expression of membrane-bound proteins involved in energy metabolism and electron transfer in M. barkeri, thereby increasing the EET capacity. This phenomenon potentially elucidates the rationale as to why magnetite promoted AOM-coupled AQDS reduction.

3D-printed controllable bio-accelerators with sustained release property to boost chromium (VI) inhibited denitrification recovery

Although soluble bio-accelerators have proven effective in mitigating Cr(VI) inhibition within denitrification system, issues persist in immobilizing bio-accelerators and making them slow-release for sustained regulation. In this study, a novel strategy was proposed to fabricate immobilized bio-accelerators with controlled structure, sustained release property by 3D printing technology. Notably, the sustained release of bio-accelerators from 3D-printed bio-accelerators (3DP-B) lasted for at least 144 h. Compared to control group, 3DP-B with basic components (3DP-BB) shortened the recovery time by 1.4 folds, and the COD and NO3--N removal efficiency was 36.5 % and 38.0 % higher than that of natural recovery. Correspondingly, the activity of key enzymes (nitrate reductase, nitrite reductase, nitric oxide reductase, and nitrous oxide reductase), electron transfer system activity and extracellular polymer substances of denitrification biofilm maintained at relatively high levels. Furthermore, introducing 60 mg·L-1 anthraquinone-2,6-disulfonate (AQDS) into the ink showed noticeable superiority on the bio-inhibition release over 1000 mg·L-1 AQDS. The released AQDS facilitated the electron transport capacity by 1.25 times compared with control group. The groundbreaking findings of this study could advance the development of 3D printing technology and utilization of bio-accelerators in the field of wastewater treatment.

Enhancing the nitrogen removal of anammox sludge by setting up novel redox mediators-mediated anammox process

Anaerobic ammonium oxidizing (anammox) bacteria have been proven weak-electroactive. However, the impact of exogenous anthraquinone-2,6-disulfonate (AQDS) on the anammox activity, although it usually plays essential roles in the life activities of many other electroactive microorganisms, is still unknown. Therefore, this study further explored the influences of AQDS on the anammox activity and the interaction mechanism with anammox bacteria, as well as the behaviors of NH4+, NO2−, and NO3−. The results showed that exogenous AQDS increased the ammonium and total nitrogen removal rates by 12.8% and 10.7%, respectively. Interestingly, the conversion from NO2− to NO3− was significantly reduced after adding AQDS, resulting in a 40.1% reduction in NO3− production of anammox process. In this study, we found for the first time that anammox bacteria could not only carry out the conventional anammox process but also perform a weak redox mediator-mediated anammox process, which could achieve the 1:1 consumption of NH4+ and NO2−. The redox mediator-mediated anammox process was related to an endogenous redox mediator (ERM) synthesized and secreted by anammox bacteria, whose redox midpoint potential was around −0.26 V (vs. Ag/AgCl). After adding AQDS, not only the ERM-mediated anammox process was enhanced, but also two novel redox mediator-mediated anammox processes were introduced, including the AQDS-mediated anammox process and ERM-AQDS-mediated anammox process. These three redox mediator-mediated anammox processes significantly improved the nitrogen removal performance of anammox bacteria and reduced energy consumption. These findings will help reduce the dependence of anammox technology on NO2−, reduce the cost of subsequent treatment of NO3−, and provide new visions for optimizing and applying anammox technology.

Enhanced denitrification performance and extracellular electron transfer for eletrotrophic bio-cathode mediated by anthraquinone-2, 6-disulfonate (AQDS) at low temperature

The slow electron transfer rate and low temperature are the bottleneck in biological denitrification process, and quinone electron shuttle (e.g. AQDS) has been confirmed as a viable approach to improve the efficiency of biological denitrification by accelerating electron transport. In this study, we employed the eletrotrophic bio-cathode with cold-tolerant to explore the mechanisms of extracellular electron transfer (EET) mediated by AQDS in simultaneous nitrification and denitrification (SND) for the first time. The results indicated that adding 0.5 mmol/L AQDS increased NH4+-N removal rate and SND efficiency by 18.1% and 17.9%, respectively, and the optimal hydraulic retention time (HRT) was shortened from 5 d to 3 d. The potential mechanism was that AQDS resulted in a 1.47-fold increase in the relative abundance of cathodic electroactive microorganisms, and the relative abundance of functional genes such as cytochrome c, flavin, cold acclimation and translocator increased by about 1.15–300.42%, which accelerated the transfer of extracellular electron and significantly improved the energy metabolism efficiency. In addition, AQDS may act as an electron shuttle to accelerate the electronic transition in long-distance electron transfer. In summary, this study expands our understanding of the role of AQDS in the SND process by bio-cathode under low temperature and provides an insight into the EET of eletrotrophic microorganisms with cold-tolerant.

AQDS-functionalized biochar enhances the bioreduction of Cr(VI) by Shewanella putrefaciens CN32

The bioreduction of toxic chromium(VI) to sparingly soluble chromium(III) represents an environmentally friendly and cost-effective method for remediating Cr contamination. Usually, this bioreduction process is slow and requires the addition of quinone compounds as electron shuttles to enhance the reaction rate. However, the dissolved quinone compounds are susceptible to loss with water flow, thereby limiting their effectiveness. To address this challenge, this study loaded anthraquinone-2,6-disulfonate (AQDS), a typical quinone compound, onto biochar (BC) to create a novel solid-phase electron mediator (BC-AQDS) that can sustainably promote Cr(VI) bioreduction. The experimental results demonstrated that BC-AQDS significantly promoted the bioreduction of Cr(VI), where the reaction rate constant increased by 4.81 times, and the reduction extent increased by 38.31%. X-ray photoelectron spectroscopy and Fourier-Transform Infrared Spectroscopy analysis revealed that AQDS replaced the –OH functional groups on the BC surface to form BC-AQDS. Upon receiving electrons from Shewanella putrefaciens CN32, BC-AQDS was reduced to BC-AH2DS, which subsequently facilitated the reduction of Cr(VI) to Cr(III). This redox cycle between BC-AQDS and BC-AH2DS effectively enhanced the bioreduction rate of Cr(VI). Our study also found that a lower carbonization temperature of BC resulted in a higher surface –OH functional group content, enabling a greater load of AQDS and a more pronounced enhancement effect on the bioreduction of Cr(VI). Additionally, a smaller particle size of BC and a higher dosage of BC-AQDS further contributed to the enhancement of Cr(VI) bioreduction. The preparation of BC-AQDS in this study effectively improve the utilization of quinone compounds and offer a promising approach for enhancing the bioreduction of Cr(VI). It provides a more comprehensive reference for understanding and solving the problem of Cr pollution in groundwater.

The dual effect of disodium anthraquinone-2,6-disulfonate (AQDS) on the Cr(VI) removal by biochar: The enhanced electron transfer and the inhibited adsorption

Due to large specific surface area, abundant surface functional groups, and stable chemical structure, biochar has been widely used in many environmental fields, including the remediation of Cr pollution. Alternatively, electrochemically active organic matter (e-OM), which is prevalent in both natural environments and industrial wastewater, exerts an inevitable influence on the mechanisms underlying Cr(VI) removal by biochar. The synergistic interplay between biochar and e-OM in the context of Cr(VI) remediation remains to be fully elucidated. In this study, disodium anthraquinone-2,6-disulfonate (AQDS) was used as a model for e-OM, characterized by its quinone group's ability to either donate or accept electrons. We found that AQDS sped up the Cr(VI) removal process, but the enhancement effect decreased with the increase in pyrolysis temperature. With the addition of AQDS, the removal amount of Cr(VI) by BC300 and BC600 increased by 160.0% and 49.5%, respectively. AQDS could release more electrons trapped in the lower temperature biochar samples (BC300 and BC600) for Cr(VI) reduction. However, AQDS inhibited the Cr(VI) removal by BC900 due to the adsorption of AQDS on biochar surface. In the presence of the small molecule carbon source lactate, more AQDS was adsorbed onto the biochar surface. This led to an inhibition of the electron transfer between biochar and Cr(VI), resulting in an inhibitory effect. This study has elucidated the electron transfer mechanism involved in the removal of Cr(VI) by biochar, particularly in conjunction with e-OM. Furthermore, it would augment the efficacy of biochar in applications targeting the removal of heavy metals.

Mechanism of extracellular electron transport and reactive oxygen mediated Sb(III) oxidation by Klebsiella aerogenes HC10

Microbial oxidation and the mechanism of Sb(III) are key governing elements in biogeochemical cycling. A novel Sb oxidizing bacterium, Klebsiella aerogenes HC10, was attracted early and revealed that extracellular metabolites were the main fractions driving Sb oxidation. However, linkages between the extracellular metabolite driven Sb oxidation process and mechanism remain elusive. Here, model phenolic and quinone compounds, i.e., anthraquinone-2,6-disulfonate (AQDS) and hydroquinone (HYD), representing extracellular oxidants secreted by K. aerogenes HC10, were chosen to further study the Sb(III) oxidation mechanism. N2 purging and free radical quenching showed that oxygen-induced oxidation accounted for 36.78% of Sb(III) in the metabolite reaction system, while hydroxyl free radicals (·OH) accounted for 15.52%. ·OH and H2O2 are the main driving factors for Sb oxidation. Radical quenching, methanol purification and electron paramagnetic resonance (EPR) analysis revealed that ·OH, superoxide radical (O2•−) and semiquinone (SQ−•) were reactive intermediates of the phenolic induced oxidation process. Phenolic-induced ROS are one of the main oxidants in metabolites. Cyclic voltammetry (CV) showed that electron transfer of quinone also mediated Sb(III) oxidation. Part of Sb(V) was scavenged by the formation of the secondary Sb(V)-bearing mineral mopungite [NaSb(OH)6] in the incubation system. Our study demonstrates the microbial role of oxidation detoxification and mineralization of Sb and provides scientific references for the biochemical remediation of Sb-contaminated soil.

Novel Role of Hematite in Anaerobic Digestion: Manipulating Membrane-Bound Electron Transport Chain by the Construction of Biological Capacitors with Humic Acid.

Humic acid (HA) has attracted much attention for its electron-competitive effect of quinone groups on anaerobic methanogenesis. This study analyzed the biological "capacitor" to determine how it might effectively reduce electron competition. As biological capacitor-producing additives, three semiconductive materials, including magnetite, hematite, and goethite, were selected. The results showed that hematite and magnetite could significantly alleviate the inhibited methanogenesis caused by the HA model compound anthraquinone-2,6-disulfonate (AQDS). The electrons flowing to methane in hematite-AQDS, magnetite-AQDS, control, sole-AQDS, and goethite-AQDS groups accounted for 81.24, 77.12, 75.42, 70.55, and 56.32% of the total produced electrons, respectively. Hematite addition significantly accelerated the methane production rate (18.97%) compared with sole-AQDS. Electrochemical investigation showed that AQDS might have its oxidation potential reduced by adsorbing on hematite, which results in an energy band bending for hematite and the formation of a biological capacitor. The biological capacitor's integrated electric field helps with the transfer of electrons from reduced AQDS to anaerobic consortia via bulk hematite. Metagenomic and metaproteomic sequencing analyses revealed that the ferredoxin and Mph-reducing hydrogenase in hematite addition increased by 7.16 and 21.91%, respectively, compared to sole-AQDS addition. Accordingly, this research suggested that AH2QDS may re-transfer electrons to methanogens via the biological capacitor and the membrane's Mph-reducing hydrogenase, thus lowering the HA electron competition.

Insight into the model quinone compound AQDS mediated the Mn(II) abiotic oxidation and mineral mineralization on hematite surface under oxic and neutral conditions

Oxidation of Mn(II) and the subsequent mineral mineralization can significantly influence the transport and fate of trace metals and organic pollutants in the soil and sediment environments. Abiotic oxidation of Mn(II) on the surface of Fe mineral in the open air is a vital formation pathway of Mn (oxyhydr)oxides; however, the impact of electron shuttle substances at neutral pH has seldomly been reported. Anthraquinone-2, 6-disulfonate (AQDS) is a typical electron shuttle and might affect the abiotic oxidation of Mn(II). In this study, abiotic oxidation of Mn(II) on the surface of a hematite substrate and mineralization of oxidation product in the presence of AQDS (0–1 mM) were investigated at pH 7.5 in the open air. In the Mn(II)-AQDS treatments, most Mn(II) remained in solution, suggesting that AQDS shows little catalytic activity in the abiotic oxidation of Mn(II) without hematite present. By contrast, Mn(II) was rapidly oxidized, and oxidation rates increased from 4.2 × 10−2 to 8.8 × 10−2 h−1 with increasing AQDS concentration from 0 to 1 mM in the hematite-Mn(II)-AQDS treatments. Abiotic oxidation of Mn(II) produced granular-like Mn3O4, fibrous-like β-MnOOH, and rod-like γ-MnOOH, and no other secondary Fe minerals were found. The proposed mechanism of the AQDS-mediated abiotic oxidation of Mn(II) on a hematite surface is the AQDS serving as an electronic medium with semiconductor hematite as a foreign substrate, promoting electron transfer between oxygen and Mn(II) on its surface. Our results revealed that electron mediators impart crucial control on not only the rate but also the compositions of Mn(II) oxidation product. The findings of this study provide new insights into the geochemical functions of electron shuttles on the dynamics of redox-sensitive elements under oxic and neutral conditions and expand the understanding of interfacial heterogeneous nucleation reactions at the molecular scale.

Quinone electron shuttle enhanced ammonium removal performance in anaerobic ammonium oxidation coupled with Fe(III) reduction

Anaerobic ammonium oxidation coupled with Fe(III) reduction (Feammox) is a newly discovered ammonium removal process, but the low ammonium removal efficiency restricts its application in practical wastewater treatment. In this study, iron compounds Fe2O3, Fe(OH)3 and Fe3O4 were added to anaerobic sequencing batch reactors (ASBR) to explore the effect of different iron sources for the Feammox reaction on ammonium removal performance. The removal efficiencies of ammonium in Fe2O3, Fe(OH)3 and Fe3O4 groups were 34.02%, 22.8% and 7.58%, respectively. Additionally, to investigate electron shuttle effect of the quinone compound anthraquinone-2,6-disulfonate (AQDS) between Fe2O3, Fe(OH)3, Fe3O4 and ammonium and whether AQDS can improve the efficiency of Feammox, batch experiments were carried out in 250 mL serum bottles. The ammonium removal efficiency of the AQDS-Fe2O3 group was 48.53%, while those of the AQDS-Fe(OH)3 and AQDS-Fe3O4 groups were 35.63% and 15.46%, respectively. This implied that AQDS enhanced the ammonium removal performance of Feammox, especially accelerating the electron shuttle rate between Fe2O3 and ammonium. X-ray diffraction showed new iron minerals appear in the system. Moreover, modified Boltzmann model was more suitable to describe the removal of NH4+-N and showed a good correlation(R2 >0.99). This study demonstrates the significant influence of AQDS on Feammox process, which provides a theoretical basis for the application of Feammox in practical sewage treatment.

Confocal Fluorescence Microscopy to Probe in Operando electrochemical Conversion

The development of electrochemical devices, such as flow batteries, fuel cells and electrolysers, is critical for the transition to a sustainable economy. Given the critical role of porous electrodes in the performance of these devices, understanding their structure-function relationship is crucial. Although porous electrodes with many different internal structures have been utilized, there is no clear consensus as to what features lead to what aspects of electrode performance. Transport in porous electrodes is commonly approximated by mean-field theories, which assumes a homogenous, Darcy-like permeability in the porous structure. Recent findings have challenged the suitability of these assumptions by revealing extreme heterogeneities in the electrolyte transport, on commercially relevant length scales exceeding the electrode pore size by an order of magnitude. [1]In this study, we introduce a new analytical method for electrochemical flow devices: 4D Confocal Optical Fluorescence Microscopy. This non-invasive in-situ analytical technique distinguishes between oxidized and reduced molecular species by their fluorescence spectra and maps out 3D molecular concentration fields with single-digit micron resolution (Figure 1). 3D movies, correlated with macroscopic chronoamperometry, add time as a fourth dimension. Thus, electrochemical conversion and species advection and diffusion can be monitored at heretofore unprecedented resolution, giving us new routes to understanding local phenomena in porous electrodes. We demonstrate this method by monitoring the reversible electrolytic reduction of 2,7 anthraquinone disulfonate (AQDS) to its corresponding hydroquinone, H2AQDS, in aqueous electrolyte in a transparent electrochemical cell. We monitor changes in the concentration field as a response to the variation of applied potential and current in real time. We correlate the fluorescence intensity of a particular band of wavelengths to the electrolyte state of charge. Furthermore, high sensitivity to non-linear optical effects enables us to capture the reversible formation of a dimer, specific to the AQDS/H2AQDS system, quantitatively in an operating battery.This new high-resolution 4D technique of characterizing operando electrochemical devices is expected to provide insight into the relationship between electrode microstructure and electrochemical performance features.Literature cited:[1] A.A. Wong, S.M. Rubinstein and M.J. Aziz, “Direct visualization of electrochemical reactions and heterogeneous transport within porous electrodes in operando by fluorescence microscopy” Cell Reports Phys. Sci. 2021, 100388; https://doi.org/10.1016/j.xcrp.2021.100388 Figure 1

AQDS Activates Extracellular Synergistic Biodetoxification of Copper and Selenite via Altering the Coordination Environment of Outer-Membrane Proteins.

The biotransformation of heavy metals in the environment is usually affected by co-existing pollutants like selenium (Se), which may lower the ecotoxicity of heavy metals, but the underlying mechanisms remain unclear. Here, we shed light on the pathways of copper (Cu2+) and selenite (SeO32-) synergistic biodetoxification by Shewanella oneidensis MR-1 and illustrate how such processes are affected by anthraquinone-2,6-disulfonate (AQDS), an analogue of humic substances. We observed the formation of copper selenide nanoparticles (Cu2-xSe) from synergistic detoxification of Cu2+ and SeO32- in the periplasm. Interestingly, adding AQDS triggered a fundamental transition from periplasmic to extracellular reaction, enabling 14.7-fold faster Cu2+ biodetoxification (via mediated electron transfer) and 11.4-fold faster SeO32- detoxification (via direct electron transfer). This is mainly attributed to the slightly raised redox potential of the heme center of AQDS-coordinated outer-membrane proteins that accelerates electron efflux from the cells. Our work offers a fundamental understanding of the synergistic detoxification of heavy metals and Se in a complicated environmental matrix and unveils an unexpected role of AQDS beyond electron mediation, which may guide the development of more efficient environmental remediation and resource recovery biotechnologies.

Size and Charge Effects on Organic Flow Battery Crossover Evaluated By Quinone Permeabilities through Nafion

Organic and metalorganic reactants have become promising for long-lifetime flow batteries. Synthetic chemistry unlocks a wide design space to tailor reactant redox potential, solubility, chemical and electrochemical stability, redox kinetics, and transport properties. Minimizing the crossover of reactants through the membrane or separator is one crucial design goal. To that end, this work contributes a systematic evaluation of size- and charge-based effects on small molecule permeability through Nafion. These results inform the design of flow battery electrolytes that improve the transport selectivity of ion exchange membranes.Some recent flow battery designs have included crossover suppression strategies based on size and charge of reactants. One option is to leverage size-exclusion, for example by tethering redox-active moieties to polymer backbones,1,2 or by oligomerizing redox-active monomers.3-5 A charge-based strategy has been employed to decrease viologen crossover: sulfonate6 or phosphonate7 solubilizing groups were attached to the redox active core and paired with a cation exchange membrane, reducing crossover compared to previous iterations of this chemistry. Crossover rates of some organic-based flow battery molecules have been estimated to be very low, but other considerations must be balanced for designing viable battery technology. For example, electrolyte cost and solubility may be in direct tension with a crossover suppression strategy based on increasing redox mediator size.8 Untangling the effects of different membrane-molecule selectivity mechanisms is a valuable step on the path to advancing redox active molecule design.This work evaluates a set of quinones in which size is varied by the number of aromatic rings (e.g. hydroquinone, anthraquinone) and charge number is varied almost independently through sulfonation. Each sulfonate moiety contributes a -1 charge, increasing the magnitude of the molecule charge number with the same sign as the fixed charges in Nafion. Effective size of solvated species is accessed through rotating disk electrode voltammetry: Stokes radii are calculated from measured diffusion coefficients. We found over an order of magnitude permeability reduction per sulfonate, emphasizing the importance of charge-based exclusion for ion exchange membranes. In comparison, size-exclusion effects are less impactful. For example, the Stokes radius of anthraquinone 2,6-disulfonate (AQDS) is twice that of hydroquinone 2,5-disulfonate but their permeabilities fall within the same order of magnitude.1. T. Hagemann, J. Winsberg, M. Grube, I. Nischang, T. Janoschka, N. Martin, M. D. Hager, and U. S. Schubert, Journal of Power Sources, 378, 546 (2018).2. T. Janoschka, N. Martin, U. Martin, C. Friebe, S. Morgenstern, H. Hiller, M. D. Hager, and U. S. Schubert, Nature, 527, 78 (2015).3. M. J. Baran, M. N. Braten, E. C. Montoto, Z. T. Gossage, L. Ma, E. Chenard, J. S. Moore, J. Rodrıguez-Lopez, and B. A. Helms, Chemistry of Materials, 30, 3861 (2018).4. K. H. Hendriks, S. G. Robinson, M. N. Braten, C. S. Sevov, B. A. Helms, M. S. Sigman, S. D. Minteer, and M. S. Sanford, ACS Central Science, 4, 189 (2018).5. S. E. Doris, A. L. Ward, A. Baskin, P. D. Frischmann, N. Gavvalapalli, E. Chenard, C. S. Sevov, D. Prendergast, J. S. Moore, and B. A. Helms, Angewandte Chemie, 129, 1617 (2017).6. C. Debruler, B. Hu, J. Moss, J. Luo, and T. L. Liu, ACS Energy Letters, 3, 663, (2018).7. S. Jin, E. M. Fell, L. Vina-Lopez, Y. Jing, P. W. Michalak, R. G. Gordon, and M. J. Aziz, Advanced Energy Materials, 10, (2020).8. M. L. Perry, J. D. Saraidaridis, and R. M. Darling, Current Opinion in Electrochemistry, 21, 311 (2020).

Quinone Moieties Link the Microbial Respiration of Natural Organic Matter to the Chemical Reduction of Diverse Nitroaromatic Compounds.

Insensitive munitions compounds (IMCs) are emerging nitroaromatic contaminants developed by the military as safer-to-handle alternatives to conventional explosives. Biotransformation of nitroaromatics via microbial respiration has only been reported for a limited number of substrates. Important soil microorganisms can respire natural organic matter (NOM) by reducing its quinone moieties to hydroquinones. Thus, we investigated the NOM respiration combined with the abiotic reduction of nitroaromatics by the hydroquinones formed. First, we established nitroaromatic concentration ranges that were nontoxic to the quinone respiration. Then, an enrichment culture dominated by Geobacter anodireducens could indirectly reduce a broad array of nitroaromatics by first respiring NOM components or the NOM surrogate anthraquinone-2,6-disulfonate (AQDS). Without quinones, no nitroaromatic tested was reduced except for the IMC 3-nitro-1,2,4-triazol-5-one (NTO). Thus, the quinone respiration expanded the spectrum of nitroaromatics susceptible to transformation. The system functioned with very low quinone concentrations because NOM was recycled by the nitroaromatic reduction. A metatranscriptomic analysis demonstrated that the microorganisms obtained energy from quinone or NTO reduction since respiratory genes were upregulated when AQDS or NTO was the electron acceptor. The results indicated microbial NOM respiration sustained by the nitroaromatic-dependent cycling of quinones. This process can be applied as a nitroaromatic remediation strategy, provided that a quinone pool is available for microorganisms.

Coupling methanogenesis with iron reduction by acetotrophic Methanosarcina mazei zm-15.

Application of ferric iron is conventionally considered to inhibit methanogenesis in anoxic environments. Here we show that Methanosarcina mazei zm-15, a strain isolated from the natural wetland of Tibetan plateau, is capable of Fe(III) reduction, which significantly promotes its growth and methanogenesis. We grew Ms. mazei zm-15 in a medium containing acetate supplemented with Fe(III) in ferric citrate or ferrihydrite and to some cultures anthraquinone-2,6-disulfonate (AQDS) was applied as an electron shuttle. The reduction of Fe(III) species occurred immediately. Ferric citrate was more readily reduced than ferrihydrite. The X-ray diffraction spectra analysis showed the formation of magnetite from ferrihydrite and amorphous reduced products from ferrihydrite plus AQDS. The analysis of protein contents revealed that Fe(III) reduction contributed 36%-46% of the cell growth. The growth yield, estimated as protein increment per acetate consumed for Fe(III) reduction, increased by 20- to 30-fold compared with methanogenesis, which is in consistence with the difference in free energy available by Fe(III) reduction relative to methanogenesis. We propose that the outer-surface multiheme c-type cytochrome predicted from Ms. mazei zm-15 genome serves as the terminal reductase with the energy-converting hydrogenase and F420 H2 dehydrogenase involved in electron transport chain for Fe(III) reduction. The findings shed a light to better understand the ecophysiology of Methanosarcina in anaerobic environments.

Arsenic release from microbial reduction of scorodite in the presence of electron shuttle in flooded soil

Scorodite (FeAsO4·H2O) is a common arsenic-bearing (As-bearing) iron mineral in near-surface environments that could immobilize or store As in a bound state. In flooded soils, microbe induced Fe(III) or As(V) reduction can increase the mobility and bioavailability of As. Additionally, humic substances can act as electron shuttles to promote this process. The dynamics of As release and diversity of putative As(V)-reducing bacteria during scorodite reduction have yet to be investigated in detail in flooded soils. Here, the microbial reductive dissolution of scorodite was conducted in an flooded soil in the presence of anthraquinone-2,6-disulfonate (AQDS). Anaeromyxobacter, Dechloromonas, Geothrix, Geobacter, Ideonella, and Zoogloea were found to be the dominant indigenous bacteria during Fe(III) and As(V) reduction. AQDS increased the relative abundance of dominant species, but did not change the diversity and microbial community of the systems with scorodite. Among these bacteria, Geobacter exhibited the greatest increase and was the dominant Fe(III)- and As(V)-reducing bacteria during the incubation with AQDS and scorodite. AQDS promoted both Fe(III) and As(V) reduction, and over 80% of released As(V) was microbially transformed to As(III). The increases in the abundance of arrA gene and putative arrA sequences of Geobacter were higher with AQDS than without AQDS. As a result, the addition of AQDS promoted microbial Fe(III) and As(V) release and reduction from As-bearing iron minerals into the environment. These results contribute to exploration of the transformation of As from As-bearing iron minerals under anaerobic conditions, thus providing insights into the bioremediation of As-contaminated soil.

Effect of anthraquinone-2,6-disulfonate (AQDS) on anaerobic digestion under ammonia stress: Triggering mediated interspecies electron transfer (MIET)

The underlying mechanisms by which humic-like substrates affect anaerobic digestion under ammonia stress are insufficiently understood so far. In this study, anthraquinone-2,6-disulfonate (AQDS), a representative analogue of humic acid, was adopted at a 100 μM concentration as the exogenous additive during anaerobic digestion process along with 5.0 g NH4+-N/L stress. The results showed that AQDS could improve the cumulative CH4 production and the maximum CH4 production rate by 7.3 and 10.8%, respectively, and shorten the methanogenic lag phase by 13.8%. Acetate-related production and methanation were both facilitated, during which the biological rather than the chemical mechanism played a crucial role. The microbial diversity distribution revealed that electroactive Anaerolinea and Methanosaeta were significantly enriched in response to AQDS amendment. Herein, AQDS was presumed to serve as an electron shuttle to trigger a mediated interspecies electron transfer (MIET) network among electroactive consortia, thus accelerating acetate methanation and ameliorating methanogenesis under ammonia stress.

Redox mediator-regulated microbial electrolysis cell to boost coulombic efficiency and degradation activity during gaseous chlorobenzene abatement

Microbial electrolysis cell (MEC), a promising technology, is being developed for volatile organic compounds (VOCs) abatement. One of the critical issues to limit its industrial application is the poor electron transfer. In this work, anthraquinone-2,6-disulfonate (AQDS) and flavin mononucleotide (FMN) are dosed to MECs for the chlorobenzene (CB) removal, which are denoted as MEC-AQDS and MEC-FMN, respectively. The MEC performance is significantly promoted due to that RMs lower down the internal resistance. The CB removal efficiency in MEC-AQDS and MEC-FMN are increased by 40% and 20%, respectively, compared with neat MEC. Moreover, ECmax and Ks of MEC-AQDS are 26% and 75% higher than that of neat MEC, respectively. AQDS is a more suitable RM with 66% and 50% higher in dechlorination efficiency and coulombic efficiency, respectively, compared with FMN. The enhancement of dechlorination process in MEC-AQDS is ascribed to two reasons. The lower resistance accelerates the electron transfer rate during dechlorination; and the enhancing growth of dechlorinating bacterium leads to an abundant excretion of dechlorination enzymes. Additionally, electrochemical analysis and intermediate detection imply that RMs affect the electron transfer pathways but not the degradation mechanism, that is, CB converts into 2-chlorophenol and 3-chlorocatechol successively, then ring-opens and dechlorinates for mineralization.

Effects of biochar/AQDS on As(III)-adsorbed ferrihydrite reduction and arsenic (As) and iron (Fe) transformation: Abiotic and biological conditions

Microbe induced iron (Fe) reduction play an important role in arsenic (As) transformation and the related secondary mineral formation. Meanwhile biochar could react as electron shuttle for this process. Impact of biochar and model electron shuttle anthraquinone-2,6-disulfonate (AQDS) on the chemical/biological iron reduction of As(III)-adsorbed ferrihydrite and the solid-liquid redistribution of As in M1 buffer were studied. Fe reduction results in the release of As adsorbed on ferrihydrite into the solution. Under abiogenic conditions, both biochar and AQDS promoted ferrous production, the chemical oxidation of As(III) and As release. Inoculate with Shewanella oneidensis MR-1, AQDS has greater electronic shuttle function than biochar (with the maximum Fe(II) contents: 154 mg/L > 76.6 mg/L respectively). However, only 12.8 mg/L As was released in the presence of AQDS, which was much lower than that in the presence of biochar (21.6 mg/L), and may be associated with the transformation of As speciation and the formation of secondary minerals. XRD and EDX-SEM confirmed that the As could be fixed by the generated secondary mineral vivianite. The relative contents of vivianite in biological control and AQDS addition were 2.7% and 18.4%, respectively. This study provides information on the transformation and migration of As and Fe with the addition of biochar under anaerobic conditions, which is potential to understand the mechanism of As(III)-contaminated soil remediation.

Quinone-reducing enrichment culture enhanced the direct and mediated biotransformation of azo dye with soluble and immobilized redox mediator

Anthraquinone-2,6-disulfonate (AQDS) was used as an electron acceptor to promote the establishment of a community of quinone-reducing microorganisms from an anaerobic sludge using acetate and glucose as energy sources. The enriched and control (not enriched with AQDS) sludge were used for the direct and mediated biotransformation of congo red. For the mediated reduction, AQDS was used soluble and immobilized on granular activated carbon (GAC). The adsorption capacity of AQDS on GAC was 87.6 mg/g, which improved the electron-accepting capacity of the material 2.26-times compared to the unmodified GAC. The rate of decolorization of congo red with the enriched sludge was 2.94-times higher than the value achieved with the control sludge for the direct biotransformation. For the mediated biotransformation using AQDS immobilized, the rates with the enriched sludge were improved 3.79- and 1.13-times respect to the incubations with the control sludge lacking AQDS and with AQDS immobilized, respectively. The microbial analysis revealed a change in the bacterial composition of the enriched sludge compared to the control sludge. In contrast, the profiles of the archaeal composition remained the same in the two types of sludge. The results presented in this study confirm the potential use of quinone-reducing microorganisms enriched to improve the redox biotransformation of electron-accepting pollutants (e.g. azo dyes) by direct reduction and mediated with soluble and immobilized redox mediators.