Photolysis of Roxarsone in Aqueous Solutions Containing Goethite under Simulated Sunlight Irradiation: Kinetics, Mechanism, and Degradation Pathways

Land application of animal manure introduces large quantities of ROX to the environment. ROX is highly water-soluble and easily adsorbs on iron hydro(oxides). Photolysis of ROX in goethite (α-FeOOH) suspensions was investigated under simulated sunlight irradiation. The rate of ROX photodegradation in the presence of 2.2 mM α-FeOOH was four times faster than that in pure water. The initial rate equation for ROX photodegradation was determined to be rinit = (2.3 ± 0.4) × 10–4[ROX]0.5[α-FeOOH]1.1[H+]0.1 (μM·h–1) at initial pH values of 2.5–7.0, ROX concentrations of 2.5–10.0 μM, and α-FeOOH dosages of 0.6–2.2 mM. Approximately 6.2% of ROX underwent direct photolysis ([ROX]0 = 5.0 μM, [α-FeOOH]0 = 2.2 mM), while the others were attributed to the indirect photolysis introduced by α-FeOOH. The second-order rate constants determined for the reactions between ROX and •OH and O2•– were 3.07 ± 0.09 × 109 and 2.08 ± 0.03 × 104 M–1·s–1, respectively. Inorganic arsenics were the main arsenic-containing products, most of which were adsorbed on α-FeOOH. Photodegradation pathways of ROX were delineated based on the intermediates detected. The effects of water constituents on photodegradation were evaluated. Results obtained are helpful for better understanding the fate of ROX in aquatic environments.