Titanium dioxide mediated solar photocatalytic degradation of thiram in aqueous solution: Kinetics and mineralization

The photocatalyzed degradation conditions of thiram in aqueous titanium dioxide-suspended solution were optimized under sunlight illumination. The effect of various factors, such as photocatalyst loading, initial substrate concentration, temperature, pH, sunlight intensity and illumination time on the photocatalytic degradation of thiram was investigated. The primary photocatalytic decomposition reaction followed a pseudo-first-order kinetic law according to the Langmuir–Hinshelwood model. During the photocatalytic degradation, the first-order rate constant kobs was 0.10 min−1, which was approximately 12 times that observed in direct photolysis. The half-lives (t1/2) and the activation energy (Ea) were 6.7 min and 33 kJ mol−1, respectively. Carbon dioxide and sulfate, nitrate and ammonium ions were detected as the end products. The stoichiometric transformation of organic sulfur to sulfate ions was observed at relatively shot illumination time (2 h), whereas the complete mineralization of organic carbon and nitrogen atoms took place at delayed illumination time. Dimethylamine and monomethylamine were identified as the intermediate products. The photocatalytic degradation mechanisms were proposed on the basis of the experimental results with molecular orbital (MO) simulation of frontier electron density. The solar photocatalytic degradation treatment for the wastewater including thiram is simple, easy handling and low cost. Therefore, since the artificial lamp devices, for example Hg-Xe lamp, are particularly expensive in the local and poor areas, the proposed technique seems to become a very powerful method for the treatment of wastewater including thiram in those areas.