Model-based evaluation on simultaneous nitrate and arsenite removal in a membrane biofilm reactor

Heavy metal reduction coupled to methane oxidation：Mechanisms, recent advances and future perspectives

Methane emission has contributed greatly to the global warming and climate change, and the pollution of heavy metals is an important concern due to their toxicity and environmental persistence. Recently, multiple heavy metals have been demonstrated to be electron acceptors for methane oxidation, which offers a potential for simultaneous methane emission mitigation and heavy metal detoxification. This review provides a comprehensive discussion of heavy metals reduction coupled to methane oxidation, and identifies knowledge gaps and opportunities for future research. The functional microorganisms and possible mechanisms are detailed in groups under aerobic, hypoxic and anaerobic conditions. The potential application and major environmental significances for global methane mitigation, the elements cycle and heavy metals detoxification are also discussed. The future research opportunities are also discussed to provide insights for further research and efficient practical application.

Complex effects of pH and organic shocks on arsenic oxidation and removal by manganese-oxidizing aerobic granular sludge in sequencing batch reactors

Biological technologies are efficient and economical methods for removing toxic arsenic (As) from organic wastewaters. In this study, four sequencing batch reactors of manganese-oxidizing aerobic granular sludge (Mn-AGS) were operated in duplicate and imposed with acidic pH and high organic shocks. Batch experiments with different initial conditions were conducted to investigate the effects of pH and organic load on As(III) oxidation and removal. The results indicate that acidic pH shocks (influent pH decreased to 4.0/3.0) unexpectedly increased the As removal efficiency from 23.4-38.2% to 64.7-72.5%. The effects of high organic shocks were very complicated, as the results of the shocks were opposite twice. According to the results of the batch experiments, it was estimated that the suitable pH range for high performance was 5.0-8.5 in reaction liquid. Although acidic pH shocks initially inhibited As(III) oxidation and removal, they largely extended the reaction time of the suitable pH range and finally improved the As removal efficiency. There were many negative and positive factors affecting the As removal during the high organic shocks, leading to the unstable responses. Moreover, the microbial community was not largely changed by pH or organic shocks, and genus Hydrogenophaga (∼8%) might be responsible for the microbial As(III) oxidation. Finally, several operation strategies were proposed to obtain high performance, such as liquid pH control and aeration improvement.

Model-based assessment of chromate reduction and nitrate effect in a methane-based membrane biofilm reactor

Chromate contamination can pose a high risk to both the environment and public health. Previous studies have shown that CH₄-based membrane biofilm reactor (MBfR) is a promising method for chromate removal. In this study, we developed a multispecies biofilm model to study chromate reduction and its interaction with nitrate reduction in a CH₄-based MBfR. The model-simulated results were consistent with the experimental data reported in the literature. The model showed that the presence of nitrate in the influent promoted the growth of heterotrophs, while suppressing methanotrophs and chromate reducers. Moreover, it indicated that a biofilm thickness of 150 μm and an influent dissolved oxygen concentration of 0.5 mg O₂/L could improve the reactor performance by increasing the chromate removal efficiency under the simulated conditions.

Acetylacetone as an efficient electron shuttle for concerted redox conversion of arsenite and nitrate in the opposite direction

The redox conversion of arsenite and nitrate has direct effects on their potential environment risks. Due to the similar reduction potentials, there are few technologies that can simultaneously oxidize arsenite and reduce nitrate in one process. Here, we demonstrate that a diketone-mediated photochemical process could efficiently do this. A combined experimental and theoretical investigation was conducted to elucidate the mechanisms behind the redox conversion in the UV/acetylacetone (AA) process. Our key finding is that UV irradiation significantly changed the redox potential of AA. The excited AA, ³(AA)*, acted as a semiquinone radical-like electron shuttle. For arsenite oxidation, the efficiency of ³(AA)* was 1-2 orders of magnitude higher than those of quinone-type electron shuttles, whereas the consumption of AA was 2-4 orders of magnitude less than those of benzonquinones. The oxidation of arsenite and reduction of nitrate could be both accelerated when they existed together in UV/AA process. The results indicate that small diketones are some neglected but potent electron shuttles of great application potential in regulating aquatic redox reactions with the combination of UV irradiation.