Effect of oxidants in the photocatalytic degradation of DEET under simulated solar irradiation in aqueous TiO2 suspensions

Recently, considerable efforts have been devoted to overcome a major limitation in semiconductor photocatalysis, the recombination of photogenerated e^--h⁺ pairs which leads to reduced quantum yields. Considering the impacts of various parameters on the photocatalytic degradation efficiency, our attention has been mainly focused on the improvement of the TiO₂ mediated photocatalysis for the degradation of a representative emerging micro-pollutant, DEET (N,N-dimethyl-m-toluamide), a widespread insect repellent. The efficiency of TiO₂ photocatalysis to degrade DEET was investigated in the presence of hydrogen peroxide and persulfate ions as oxidants-sacrificial electron acceptors under simulated solar irradiation (SSL). The degradation rates were found to be strongly influenced by the addition of oxidants. Higher degradation rates were observed in the presence of oxidants with the following order: S₂O₈ ^2- > H₂O_2. Τhe effect of solution pH in the range of 3–10 was investigated and the photodegradation rate was found to increase along with decreasing pH. Scavenging experiments indicated that that sulfate radicals were predominant species at acidic pH while ^•OH radicals were principally responsible for DEET degradation in alkaline media using SSL/TiO₂/S₂O₈^2-, the most efficient process.