UV irradiation and UV-H2O2 advanced oxidation of the roxarsone and nitarsone organoarsenicals

Abstract

Roxarsone (ROX) and nitarsone (NIT) are used as additives in animal feeding operations and have been detected in animal manure, agricultural retention ponds, and adjacent surface waters. This work investigates treatment of organoarsenicals using UV-based treatment processes, namely UV irradiation at 253.7 nm and the UV-H2O2 advanced oxidation process. The apparent molar absorptivity was mapped for ROX and NIT across pH and wavelength. For UV irradiation at 253.7 nm, the fluence-based pseudo-first order rate constant (kp′) and effective quantum yield (Φ) for ROX were 8.10–29.7 × 10−5 cm2/mJ and 2.34–8.37 × 10−3 mol/E, respectively; the corresponding constants were slightly lower for NIT. The observed rate constants are higher during advanced oxidation (e.g., kp,ROX′=3.92(±0.19)–217(±48) × 10−4 cm2/mJ). Second order rate constants for organoarsenical transformation by hydroxyl radicals were determined to be 3.40(±0.45) × 109 and 8.28(±0.49) × 108 M−1s−1 for ROX and NIT, respectively. Solution pH and nitrate concentration did not significantly impact ROX transformation during advanced oxidation; however, bicarbonate and dissolved organic matter from chicken litter reduced ROX transformation through hydroxyl radical scavenging. Inorganic arsenic was the predominant transformation product of ROX during UV-H2O2 treatment.