Radiation-induced degradation of sulfamethoxazole in the presence of various inorganic anions

The effect of common inorganic anions (chloride, bicarbonate, carbonate, nitrate, sulfate and phosphate) on the radiation-induced degradation of sulfamethoxazole (SMX) was investigated. In the absence of anions, the removal efficiency of SMX reached 99.6% at dose of 1 kGy. However, the presence of inorganic anions had obvious influence on SMX degradation, which was dependent on their initial concentrations. Nitrate ions had inhibitive effect, which increased with increase of its initial concentration. Chloride ions showed no obvious inhibition at low concentration, while they had significant inhibition at higher concentration. For bicarbonate, carbonate and phosphate ions, they presented inhibition at low concentration, while they had enhancing effect at higher concentration. The promotion of SMX removal efficiency was observed when the initial concentration of bicarbonate, carbonate and phosphate was 50, 20 and 50 mM, respectively. Quenching experiments proved that hydroxyl radicals made a major contribution to SMX degradation in the absence of anions. However, in the presence of 50 mM of carbonate or phosphate ions, carbonate and phosphate radicals played a key role in SMX degradation. In addition, carbonate and phosphate radicals preferentially attacked the amino group and resulted in the formation of different intermediate products compared to hydroxyl radicals. The results suggested that the effect of inorganic anions on the radiation-induced degradation of sulfonamides antibiotics should be considered when radiation technology is used for the treatment of industrial wastewater.