Factors affecting antimonate bioreduction by Dechloromonas sp. AR-2 and Propionivibrio sp. AR-3.

The microbial reduction of antimonate (Sb(v)) to antimonite (Sb(iii)), which forms insoluble Sb compounds, is a promising approach to remove antimony (Sb) from wastewater. Among the bacterial strains capable of reducing Sb(v) via anaerobic respiration that have been isolated to date, Dechloromonas sp. AR-2 and Propionivibrio sp. AR-3 are promising agents because they can grow aerobically and reduce Sb(v) under both anaerobic and microaerobic conditions. In this study, the effects of temperature, pH, electron donors, and coexisting electron acceptors on Sb(v) reduction and Sb removal by strains AR-2 and AR-3 were investigated to assess the usefulness of the strains in practical Sb treatment scenarios. Efficient Sb(v) reduction and removal by the two strains occurred over a relatively wide temperature range (15-35 °C) and neutral pH (6-7). In contrast, the carbon sources usable by these strains as electron donors for Sb respiration were limited to simple fatty acids such as acetate and lactate. Although strain AR-2 used nitrate and AR-3 used nitrate and arsenate as electron acceptors for anaerobic respiration in addition to Sb(v), the co-presence of other electron acceptors did not inhibit Sb(v) reduction. These results suggest that strains AR-2 and AR-3 can be potentially used in the practical treatment of Sb(v)-containing wastewater. SUPPLEMENTARY INFORMATION: The online version contains supplementary material available at 10.1007/s13205-021-02703-0.

Isolation and Characterization of Facultative-Anaerobic Antimonate-Reducing Bacteria.

Microbial antimonate (Sb(V)) reduction is a promising approach to remove Sb(V) from wastewater. However, current knowledge regarding microbial Sb(V) reduction is limited to strictly anaerobic conditions. This study was the first to isolate three facultative-anaerobic Sb(V)-reducing bacterial strains from the sludge collected from a wastewater treatment facility in an antimony products plant. Two of the isolated strains, designated Dechloromonas sp. AR-2 and Propionivibrio sp. AR-3, were characterized based on their Sb(V)-reducing abilities. When cultivated under anaerobic conditions with Sb(V) and acetate as the electron acceptor and donor, respectively, both strains could efficiently reduce 5.0 mM Sb(V), removing most of it from the water phase within 7 d. Along with Sb(V) reduction by the strains, white precipitates, which were likely amorphous Sb(OH)3 solids, were formed with a minor generation of soluble antimonite. Additionally, respiratory Sb(V) reduction by both strains occurred not only under anaerobic but also microaerobic conditions. It was suggested that Sb(V) reduction and the growth abilities of the strains under microaerobic conditions presented a substantial advantage of the use of strains AR-2 and AR-3 for practical applications to Sb(V)-containing wastewater treatment.

Bimolecular Rate Constants for FAD-Dependent Glucose Dehydrogenase from Aspergillus terreus and Organic Electron Acceptors.

The flavin adenine dinucleotide-dependent glucose dehydrogenase (FAD-GDH) from Aspergillus species require suitable redox mediators to transfer electrons from the enzyme to the electrode surface for the application of bioelectrical devices. Although several mediators for FAD-GDH are already in use, they are still far from optimum in view of potential, kinetics, sustainability, and cost-effectiveness. Herein, we investigated the efficiency of various phenothiazines and quinones in the electrochemical oxidation of FAD-GDH from Aspergillus terreus. At pH 7.0, the logarithm of the bimolecular oxidation rate constants appeared to depend on the redox potentials of all the mediators tested. Notably, the rate constant of each molecule for FAD-GDH was approximately 2.5 orders of magnitude higher than that for glucose oxidase from Aspergillus sp. The results suggest that the electron transfer kinetics is mainly determined by the formal potential of the mediator, the driving force of electron transfer, and the electron transfer distance between the redox active site of the mediator and the FAD, affected by the steric or chemical interactions. Higher k2 values were found for ortho-quinones than for para-quinones in the reactions with FAD-GDH and glucose oxidase, which was likely due to less steric hindrance in the active site in the case of the ortho-quinones.