Experimental and theoretical study of kinetic and mechanism of hydroxyl radical-mediated degradation of sulfamethazine

Hydroxyl radical ((OH)-O-center dot)-based advanced oxidation technologies (AOTs) is an effective and clean way to remove sulfonamide antibiotics in water at ambient temperature and pressure. In this study, we systematically investigated the degradation kinetics of sulfamethazine (SMT) by(center dot)OH with a combination of experimental and theoretical approaches. The second-order rate constant (k) of SMT with(center dot)OH was experimentally determined to be 5.27 +/- 0.06 x 10(9) M-1 s(-1)at pH 4.5. We also calculated the thermodynamic and kinetic behaviors for the reactions by density functional theory (DFT) using the B3LYP/6-31G*. The results revealed that(center dot)OH addition pathways at the methylene (C4) site on the pyridine ring and the ortho sites (C12 and C14) of the amino group on the benzene ring dominate the reaction, especially C14 site on the benzene ring accounted for 43.95% of SMT degradation kinetics. The theoreticalkvalue which was calculated by conventional transition state theory is 3.96 x 10(9) M-1 s(-1), indicating that experimental observation (5.27 +/- 0.06 x 10(9)) is correct. These results could further help AOTs design in treating sulfonamide during wastewater treatment processes.