Photocatalytic degradation of monocrotophos and quinalphos using solar-activated S-doped TiO2

The present work demonstrates the degradation of two persistent and highly toxic organophosphorus pesticides, i.e. monocrotophos (MCP) and quinalphos (QP), which are an inevitable part of Indian agricultural practices. The major emphasis lies in using the naturally and easily available, inexpensive sunlight for water purification. Thus, TiO2 was modified into sulphur-doped TiO2 (S–TiO2) to make it active under sunlight via wet impregnation method. Various catalysts were synthesized by varying the dopant ratio and characterized using FESEM, EDS, XRD, UV-DRS and TEM. With doping, a significant decrease in band gap energy from 3.2 to 2.9 eV was noticed. FESEM revealed that the particles are spherical shaped and slightly agglomerated. Particle size was found to decrease from 33 nm for P25 to 26.82 nm for doped TiO2. According to the various results obtained, 0.7:1 (S:Ti) dopant ratio catalyst was concluded to give optimum results. Further, the degradation studies were carried out under sunlight by varying and optimizing various parameters, i.e. dopant concentration, calcination temperature, catalyst loading, pH and initial concentration. Maximum degradation of 95.36% and 98.09% for MCP (25 mg/L) and QP (20 mg/L) is obtained, respectively, with 0.7:1 (S:Ti) as optimum dopant ratio. Kinetics was studied using Langmuir–Hinshelwood–Hougen–Watson (LHHW) model and was found to obey pseudo-first-order kinetics. The mineralization of pesticides into simpler compounds was confirmed by COD analysis and degradation pathway is also proposed for both the pollutants.