Evidence of superoxide radical contribution to demineralization of sulfamethoxazole by visible-light-driven Bi2O3/Bi2O2CO3/Sr6Bi2O9 photocatalyst

Photocatalytic degradation of sulfamethoxazole (SMX) was investigated using Bi2O3/Bi2O2CO3/Sr6Bi2O9 (BSO) photocatalyst under visible light (>420 nm) irradiation. The photochemical degradation of SMX followed pseudo-first-order kinetics. The reaction kinetics was determined as a function of initial SMX concentrations (5-20 mg L-1), initial pH (3-11) and BSO concentrations (6-600 mg L-1). Approximately, 90% of SMX (10 mg L-1) degradation and 36% of TOC reduction were achieved at pH 7.0 after 120 min irradiation. The main mineralization products, including NH4+, NO3-, SO42- and CO2, as well as intermediates 3-amino-5-methylisoxazole (AMI), p-benzoquinone (BZQ), and sulfanilic acid (SNA) were detected in aqueous solution. The formation of O-2(center dot-) radical was evidenced by using electron spin resonance and a chemiluminescent probe, luminal. A possible degradation mechanism involving excitation of BSO, followed by charge injection into the BSO conduction band and formation of reactive superoxide radical (O-2(center dot-)) was proposed for the mineralization of SMX. During the reaction, the O-2(center dot-) radical attacks the sulfone moiety and causes the cleavage of the S-N bond, which leads to the formation of two sub-structure ;analogs, AMI and SNA. (C) 2013 Elsevier B.V. All rights reserved.