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

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 (center dot OH) dominated the oxidation of As( III) for both types of dissolved organic matter (DOM), though singlet oxygen (O-1(2)) also played a role in the DOMrs system. Since acidic conditions are favorable for the formation of center dot 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 mu M). This study is of great significance for understanding the removal/transformation behavior of pollutants in paddy fields that receive straw return. (C) 2020 Elsevier B.V. All rights reserved.

Automated summary

This study investigated the photooxidation of As(III) in aqueous solution by straw-derived dissolved organic matter (S-DOM). The results showed that DOM derived from wheat straw and rape straw could efficiently oxidize As(III) under irradiation, with hydroxyl radical as the dominant oxidant. The oxidation efficiency of As(III) was inhibited in the presence of NO3- while enhanced in the presence of Fe(III).