Kinetic assessment of the potassium ferrate(VI) oxidation of antibacterial drug sulfamethoxazole

Sulfamethoxazole (SMX), a worldwide-applied antibacterial drug, was recently found in surface waters and in secondary wastewater effluents, which may result in ecotoxical effects in the environment. Herein, removal of SMX by environmentally-friendly oxidant, potassium ferrate(VI) (K2FeO4), is sought by studying the kinetics of the reaction between Fe(VI) and SMX as a function of pH (6.93-9.50) and temperature (15-45 degrees C). The rate law for the oxidation of SMX by Fe(VI) is first-order with respect to each reactant. The observed second-order rate constant decreased non-linearly from 1.33 +/- 0.08 x 10(3) M-1 s(-1) to 1.33 +/- 0.10 x 10(0) M-1 s(-1) with an increase of pH from 7.00 to 9.50. This is related to protonation of Fe(VI) (HFeO4- double left right arrow H+ + FeO42-; pK(a,HFeO4) = 7.23) and sulfamethoxazole (SH double left right arrow H+ + S-; pK(a,SH) = 5.7). The estimated rate constants were k(11) (HFeO4- + SH) = 3.0 x 10(4) M-1 s(-1), k(12) (HFeO4- + S-) = 1.7 x 10(2) M-1 (FeO42- + SH) = 1.2 x 10(0) M-1 s(-1). The energy of activation at pH 7.0 was found to be 1.86 +/- 0.04 kJ mol(-1). If excess potassium ferrate(VI) concentration (10 mu M) is used than the SMX in water, the half-life of the reaction using a rate constant obtained in our study would be approximately 2 min at pH 7. The reaction rates are pH dependent; thus, so are the half-lives of the reactions. The results suggest that K2FeO4 has the potential to serve as an oxidative treatment chemical for removing SMX in water. (c) 2005 Published by Elsevier Ltd.