Phenylarsonic acid compounds have been widely used as feed additives in chicken and swine farming. Their transformation in soil environment, particularly in the presence of birnessite (MnO2), could result in the quick releases of inorganic arsenic. The degradation of six strtucturally similar compounds, namely, p-arsanilic acid (p-ASA), roxarsone (ROX), phenylarsonic acid (PAA), o-aminophenylarsonic acid (2-APAA), 4-hydroxyphenylarsonic acid (4-HPAA), and 2-nitrophenylarsonic acid (2-NPAA), on the surface of MnO2, was investigated to elucidate the structure-reactivity relationship in the MnO2-mediated degradation of phenylarsonic acid compounds. Overall, the degradation rates of these compounds decreased in the order of p-ASA &gt; 2-APAA &gt; 4-HPAA &gt; ROX &gt;&gt; 2-NPAA ≈ PAA. The varying reactivity of these compounds on the surface of birnessite is attributed to the difference in electron, steric and even hydrophobicity caused by different substituents. While arsenate was detected as a major degradation product, full arsenic mineralization did not occur for these phenylarsonic acid compounds in MnO2-mediated degradation. Self-coupling of the phenylarsonic acid radicals resulted in the formation of dimerized products, and the importance of this transformation pathway decreased in the order of p-ASA &gt; 2-APAA &gt; 4-HPAA &gt; ROX, which is directly related to the presence of amino group and hydroxyl group at para or ortho positions on the aromatic ring. Degradation of the phenylarsonic acid compounds was inhibited by the presence of dissolved metal ions, common anions, and humic acid, which could be attributed to their competitive adsorption on the reactive surface sites of MnO2. These insights could help better understand and predict the fate of phenylarsonic acid compounds released into farmland soils with animal waste.

Kinetics, mechanisms, and pathways of oxidative degradation of phenylarsonic compounds by birnessite (δ-MnO2)

2019 Spring National Meeting

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