Phenylarsenic compounds, primarily p-arsenilic acid and roxarsone, used to be widely used as feed additives in industrial farming of swine and chicken. Although these feed additives have been banned in the Europe and United States, they are still used extensively in many countries around the world, including China. Most of the phenylarsenic compounds fed to the farmed animals are excreted in unchanged forms into animal waste, which is subsequently applied as an organic fertilizer after composting. While literature reported that the degradation of phenylarsenic compounds by aerobic and anaerobic processes are rather slow, our study demonstrated that these compounds could undergo abiotic degradation in soil and aquatic environment very quickly. Manganese oxide, which is abundant in red soils in southern China, was found to mediate the degradation of p-arsenilic acid with fast rates under acidic conditions. Arsenite and arsenate were released from oxidation of p-arsenilic acid, while an arsenic-bearing azo compound was also formed through radical-radical self-coupling during the manganese oxide mediated oxidation. The phenylarsenic compounds entering surface water were found to undergo photodegradation quickly. The photodegradation of these compounds occurs via singlet-oxygen-based self-sensitized photolysis, while direct photolysis has little contribution. The -AsO(OH)2 group could be completely cleaved from the aromatic ring of phenylarsenic compounds during photodegradation, and eventually oxidized to arsenate by the singlet oxygen. Overall, the ready production of arsenite and arsenate from abiotic transformations of phenylarsenic compounds in soil and aquatic environment indicate that animal waste containing these feed additives could be an important source of arsenic pollution and their discharge from animal farms to the environment deserves significant attention.

Abiotic processes controlling the fate of phenylarsenic compounds in soil and aquatic environment

2019 Spring National Meeting

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