Photocatalytic degradation of recalcitrant POME waste by using silver doped titania: Photokinetics and scavenging studies

The current paper reports on the photo-degradation of palm oil mill effluent over silver-modified titania (Ag/TiO₂) under visible light irradiation. TiO₂-based photocatalysts with 0.25-5.0wt% of Ag metal loadings were prepared by impregnating TiO₂ with chemically-reduced Ag nanoparticles from the AgNO₃ solution. The XRD characterization suggests that anatase was the predominant phase for all the as-synthesized photocatalysts. In addition, UV-Vis DRS confirmed that the Ag inclusion has extended the absorbance of photocatalysts to the visible light region. Moreover, the band gap energy of TiO₂ was successively reduced to 2.69eV (5.0wt% Ag/TiO₂), a drop from 3.1eV (bare TiO₂). Consequently, the POME degradation efficiency jumped 300-15.03% by photoreaction over 1.0g/L of 0.25wt% Ag/TiO₂ compared to the bare TiO₂. The optimum loading of Ag was found to be 0.5wt%, with corresponding degradation of 19.73%, and can be enhanced further to 26.77% with the optimum catalyst loading of 1.5g/L. Moreover, scavenging study confirmed that the primary reactive species for POME degradation in the current system was OH free radicals. In recycling test, the degradation efficiency of 0.5wt% Ag/TiO₂ dropped to 11% and 13% for second and third cycle, respectively, due to leaching of Ag metal as proven by ICP-MS, post-reaction.