Inhibitory effects of quinoid redox mediators on a denitrifying culture

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.

Mechanism of quinone mediators modified polyurethane foam for enhanced nitrobenzene reduction and denitrification

The nitrobenzene (NB) reduction and denitrification performance of the immobilized biofilm (I-BF) reactors based on 9,10-anthraquinone-2-sulfonyl chloride (ASC) modified polyurethane foam (PUF-ASC) carriers were investigated. Experiments demonstrated that the quinone mediators enhanced NB reduction and denitrification performance. The NB reduction rates increased by 1.46, while the NO3--N removal rates increased by 1.55 times in the PUF-0.1ASC system. The quinone mediators promote extracellular polymeric substances (EPS) secretion. Electrochemical tests indicated that quinone mediators enhanced the electron transfer of biofilm systems. NADH generation was accelerated and microbial electron transport system activity (ETSA) was promoted. The abundance of genera with electrochemical activity, NB degradation and denitrification ability (Pseudomonas sp., Diaphorobate sp., and Acinetobacter sp.) increased. Metabolic pathways relating to NO3--N and NB reduction were uploaded. In conclusion, electron acquisition by NO3--N and NB was facilitated, bacterial community structure and metabolic pathways were affected by the quinone mediators.

The combined enhancement of RL, nZVI and AQDS on the microbial anaerobic-aerobic degradation of PAHs in soil

Polycyclic aromatic hydrocarbons (PAHs) are ubiquitous persistent organic pollutants in soil, which have carcinogenic, teratogenic and mutagenic hazards. The effects of rhamnolipid (RL), nano zero-valent iron (nZVI), and anthraquinone-2,6-disulfonic acid (AQDS) on the degradation of PAHs in soil were studied. It was found that the treatment of 5 mg·kg^-1RL + 1% nZVI +0.2 mmol·kg^-1AQDS had the highest degradation rate. The degradation rate of total PAHs and HMW-PAHs was 72.81% and 79.47% respectively after 90 days. High-throughput sequencing showed that in RL + nZVI + AQDS enhanced soil, Clostridium, Geobacter, Anaeromyxobacter and Sphingomonas were the dominant species for anaerobic degradation of PAHs. Rhodococcus, Nocardioides, and Microvirga are the dominant species for aerobic degradation of PAHs. The activities of methyltransferase, dehydrogenase and catechol 1,2-dioxygenase in the anaerobic-aerobic degradation process of PAHs were consistent with the degradation process of PAHs, indicating the role of these enzymes in the degradation of PAHs. RL, nZVI, and AQDS combined enhanced microbial anaerobic-aerobic degradation has great application potential in remediation of PAHs-contaminated soil.

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.

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.

Enhancement of anaerobic degradation of petroleum hydrocarbons by electron intermediate: Performance and mechanism

A quinone-respiring strain capable of degrading multitudinous petroleum hydrocarbons was isolated by selective medium and identified as Bacillus sp. (named as C8). Maximum 76.7% of total petroleum hydrocarbons (TPH) were degraded by the biosurfactant-mediated C8 with the aid of nitrate and electron intermediate (anthraquinone-2,6-disulphonate, AQDS). The quantitative real-time PCR results of several intracellular key functional genes suggested that AQDS could participate in the transformation of intermediates and accelerate the electron transfer in the degradation of TPH and nitrate, thereby eliminating the accumulation of nitrite and increasing the degradation efficiency of TPH. A strengthening mechanism, which promoted electron transport in the anaerobic denitrification degradation of petroleum hydrocarbons by quinone-respiring strain with the aid of electron intermediate, was proposed. The influencing factors were evaluated by using response surface methodology, and the TPH removal was positively related to temperature but negatively to pH.