Structure-Reactivity Relationships in the Adsorption and Degradation of Substituted Phenylarsonic Acids on Birnessite (δ-MnO2)

Phenylarsonic acid compounds could be oxidized by manganese oxides in surface soils, resulting in quick release of inorganic arsenic. This study investigated the structure-reactivity relationships in the adsorption and oxidative degradation of six substituted phenylarsonic acids on the surface of a major type of manganese oxides, birnessite (delta-MnO2), using batch experiments conducted under acidic to neutral conditions. The initial adsorption rates of the substituted phenylarsonic acids on delta-MnO2 decreased in the order of phenylarsonic acid (PAA) > 4-aminophenylarsonic acid (p-ASA) approximate to 2-aminophenylarsonic acid (2-APAA) > 4-hydroxyphenylarsonic acid (4-HPAA) > 2-nitrophenylarsonic acid (2-NPAA) > 4-hydroxy-3-nitrophenylarsonic acid (ROX), which could be attributed to steric hindrance of the substituents and the hydrophobicity of these compounds. The oxidation rates of these structural analogues by delta-MnO2 decreased in the order of p-ASA approximate to 2-APAA > 4-HPAA > ROX, while 2-NPAA and PAA were nonreactive because of the lack of electron-donating substituents on their aromatic rings. The redox reactivity of these compounds agrees well with the electron density at C-1, which is determined by the types and position of the substituents on the aromatic ring. Although cleavage of the arsonic acid group from the aromatic ring was the predominant transformation pathway, a range of adduct products also formed through cross-coupling of the radicals and radical substitution. The contribution of radical coupling and substitution in overall degradation decreased in the order of p-ASA > 2-APAA > 4-HPAA > ROX, which results from the varying reactivity and steric hindrance of the substituents. These insights could help better understand and predict the fate of substituted phenylarsonic acids in manganese oxide-rich surface soils and the associated environmental risk of arsenic pollution.