Kinetic investigation on UV and UV/H2O2 degradations of pharmaceutical intermediates in aqueous solution

The degradation kinetics of two pharmaceutical intermediates (5-methyl-1,3,4-thiadiazole-2-methylthio (MMTD-Me) and 5-methyl-1,3,4-thiadiazole-2-thiol (MMTD)) have been studied in order to assess the effectiveness and the feasibility of UV processes for the decontamination of water polluted by such intermediates. Experiments were carried out, at 25 degreesC, treating, in a batch reactor, aqueous solutions (1 and 100 mg/l) of both compounds by UV radiations (254 nm) in the presence or absence of hydrogen peroxide. For both substrates, the results showed that: (i) no degradation occurred when H2O2 alone was used; (ii) UV and UV/H2O2 processes were both effective for degrading the substrates; (iii) substrates degradation by photo-oxidation was always faster than by direct photolysis; (iv) during direct photolysis, a lower substrate initial concentration lead to a faster and more efficient degradation. The quantum yields of the photolytic process were experimentally measured for both substrates resulting 14.1 +/- 1.5 and 12.0 +/- 0.7 mmol einstein(-1) for MMTD-Me and MMTD, respectively. Carrying out photo-oxidation experiments using excess of peroxide (i.e., initial substrate concentration of 1 mg/l and H2O2/substrate molar ratios of 5011, 42/1, 34/1 and 23/1), first- and second-order rate constants for MMTD-Me and MMTD degradation were calculated. In particular, the values of these latter resulted (8.3 +/- 0.8) x 108 and (1.6 +/- 0.5) x 10(10) M-1 s(-1), respectively. Our results show that to remove 99% of a few mug/l of the pharmaceutical intermediates with a H2O2 dose of 1 mg/l, 55 and 2.7 min for MMTD-Me and MMTD are necessary, respectively. (C) 2003 Elsevier Science B.V. All rights reserved.