Photooxidation mechanism of As(III) by straw-derived dissolved organic matter

Highlights: • Straw-derived dissolved organic matter (S-DOM) exhibited spectroscopic features. • Hydroxyl radicals dominated the photooxidation of As(III) mediated by S-DOM. • The oxidation efficiency of As(III) was inhibited in the presence of NO₃⁻ while enhanced in the presence of Fe(III). Straw return-to-field is a common agronomic practice that would affect the physicochemical characteristics of the paddy soil and overlying water, but few studies have focused on the possible impacts of straw return on the conversion of pollutants. In this study, the photooxidation of As(III) in aqueous solution by straw-derived dissolved organic matter (S-DOM) was investigated. The results showed that dissolved organic matter derived from wheat straw (DOMws) and rape straw (DOMrs) exhibited good spectroscopic features and could efficiently oxidize As(III) under irradiation at pH 5.0, with the k_obs values of As(III) oxidation being 0.15 h⁻¹ and 0.17 h⁻¹ for DOMws and DOMrs, respectively. Quenching studies indicated that hydroxyl radical (OH) dominated the oxidation of As(III) for both types of dissolved organic matter (DOM), though singlet oxygen (¹O₂) also played a role in the DOMrs system. Since acidic conditions are favorable for the formation of OH, As(III) oxidation decreased with an increase of pH value. Additionally, the oxidation efficiency of As(III) was inhibited in the presence of NO₃⁻ (0.2–2 mM) while enhanced in the presence of Fe(III) (5–50 μM). This study is of great significance for understanding the removal/transformation behavior of pollutants in paddy fields that receive straw return.