Kinetics of benzenesulfonates elimination by UV and UV/H2O2

The kinetics of elimination of eight benzenesulfonate derivatives (BS) have been studied in aqueous solution. Experiments were performed at 25 °C, at pH=6.0, in a batch photochemical reactor in the absence and the presence of hydrogen peroxide. For all substrates the degradation was shown to be effective using UV alone (253.7 nm) and the coupling H2O2/UV (generating hydroxyl radicals). The quantum yields for direct photolysis were evaluated to be in the range of 0.03–0.05 for BS except for nitro derivatives for which values about 100 times lower were obtained. The degradation is usually faster in the presence of hydrogen peroxide, but the efficiency depends on hydrogen peroxide starting concentration. The second order rate constants of reaction between BS and HO radicals, kBS, have been determined. Values ranging between 0.7 and 12.6×109 M−1 s−1 according to the substituent on the benzene ring were obtained by working with different hydrogen peroxide starting concentration. Hammett-type correlation was obtained for the reaction of HO radicals with BS log kBS=9.56−0.87∑σ. The optimal hydrogen peroxide concentration leading to the highest rate of BS degradation was calculated. It mainly depends on the second order rate constant between HO radicals and BS.