水体硝化体系中砷的解毒机制探讨

Detoxification mechanisms of arsenic in nitrification in water systems

摘要:

硝化是目前废水生物脱氮中应用最为广泛的工艺之一，其功能菌为化能自养型细菌，生长缓慢，对重金属十分敏感。砷是一种剧毒的类金属元素，主要以无机形式的亚砷酸盐[AsO₂⁻，As (Ⅲ)]和砷酸盐[AsO₄³⁻，As (Ⅴ)]存在，尤以As (Ⅲ)毒性最强。但研究发现，在硝化体系中，高浓度As (Ⅲ)(约400 mg/L)未对硝化功能微生物产生明显毒性。深入比较发现，As (Ⅲ)的生物氧化与硝化过程具有一定的关联性。化能自养型As (Ⅲ)氧化菌不仅可在有氧条件下将As (Ⅲ)氧化，还可在缺氧条件下以NO₂^–或NO₃^–为电子受体氧化As (Ⅲ)。而硝化细菌也是典型的化能自养菌，且硝化体系内存在氧气及硝化产物NO₂^–、NO₃^–等电子受体，理论上均可接受电子实现As (Ⅲ)的氧化。本文结合硝化反应特性，综述了As (Ⅲ)在硝化体系下的解毒机制，主要为胞外聚合物的保护、As (Ⅲ)的氧化、砷还原和外排、砷甲基化以及抗氧化机制。这些机制赋予了硝化细菌抵抗As (Ⅲ)毒性的能力，并在砷的生物转化和地球化学循环中起到重要作用。

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.