Detoxification mechanisms of arsenic in nitrification in water systems

Abstract:

Via chemoautotrophic bacteria which are featured by slow growth and high sensitivity to heavy metals, nitrification has been one of the most prevalent biological processes for removing nitrogen species from wastewater. Arsenic, generally existing in two oxidation states of arsenite [AsO₂⁻, As(Ⅲ)] and arsenate [AsO₄³⁻, As(Ⅴ)], is highly toxic, particularly the As(Ⅲ). However, high concentration of As(Ⅲ) (about 400 mg/L) shows no obvious toxicity to nitrifying bacteria in the nitrification system. Through in-depth analysis, we found that the microbial oxidation of As(Ⅲ) was in close relationship with the nitrification. Chemoautotrophic As(III)-oxidizing bacteria can not only directly oxidize As(Ⅲ) under aerobic conditions, but also use NO₂⁻ or NO₃⁻ as electron acceptors to oxidize As(Ⅲ) under anoxic conditions. As typical chemoautotrophic bacteria, nitrifying bacteria may oxidize As(Ⅲ) with O₂, NO₂⁻, and NO₃⁻ as electron acceptors in the nitrification system. In this review, we summarized the detoxification mechanisms of As(Ⅲ) in the nitrification system, such as the EPS adsorption, As(Ⅲ) oxidation, As(Ⅴ) reduction and efflux, arsenic methylation and antioxidant defense. These detoxification mechanisms enhance the resistance to As(Ⅲ) in nitrifying bacteria and play an important role in arsenic biotransformation and geochemical cycling.