Photocatalytic decomposition of acephate in irradiated TiO2 suspensions

In the present study, the photocatalytic degradation of acephate (O,S-dimethyl acetyl phosphoramidothioate ((CH3O)(CH3S)P(O)NHCOCH3)) in aqueous TiO2 suspensions is extensively investigated, pertaining to the concentration of photocatalyst and substrate on degradation rate of acephate. It is found that the acephate can be degradated and mineralized. The high-degradation rate is obtained with 4 g/L concentration of TiO2. Moreover, Langmuir-Hinshelowood rate expression is employed for the degradation of acephate with adsorption constant and rate constant, i.e., 2.0 L/mmol and 0.6 mmol/(min L), respectively. The main target is to identify the products by a number of analytical techniques, such as HPLC, IC, ESR and GC-MS. Under acidic condition, the primary products are phosphorothioic acid, O,O′,S-trimethyl ester (CH3O(CH3S)P(O)OCH3) and phosphoramidothioic acid, O,S-dimethyl ester (CH3O(CH3S)P(O)NH2), etc. It indicates that the decomposition of acephate begin from the destruction of C-N and P-N bonds. Subsequently, the P-S, P-O, P-C bonds may be oxidized gradually or simultaneously, and the final products such as CO2, H3PO4, were formed. About 100% sulfur atoms are transformed into SO42− in 180 min, however; only 3% nitrogen atoms and 2% phosphorus atoms were transformed into NO3− and PO43−.