Landfill leachate treatment is a necessary measure for environmental protection and supporting sustainable development. Photocatalytic treatment method using N- or P-type TiO2 nanoparticles has been proposed as an efficient method for the treatment of recalcitrant pollutants. Since main oxidizing agents in the photocatalytic treatment by N- and P-type TiO2 are different, investigating their performance for the photocatalytic treatment of landfill leachate is necessary. Therefore, the purpose of this study is to evaluate the performance of P-type TiO2 nanoparticles for the photocatalytic treatment of landfill leachate under visible light irradiation. Silicon (Si) element was considered as TiO2 dopant, and Si-doped TiO2 nanoparticles were synthesized by sol–gel method. Four effective parameters on the photocatalytic treatment process, i.e., the dopant content of Si (Si wt.%), calcination temperature (T), pH, and exposure time, were considered as independent variables, and temporal normalized concentration of leachate residual COD was considered as dependent variable. Decision tree (M5P) model was applied for modeling the photocatalytic process of landfill leachate treatment using the data of 167 experiments. Based on the results, the optimum value of pH, calcination temperature, and dopant content for Si-doped TiO2 nanoparticles are 6, 500 °C, and 2.5 weight percentage, respectively, and the leachate COD removal efficiency at these conditions is about 85% for leachate with 600 mg/L initial COD. Therefore, Si dopant considerably enhances the photocatalytic activity of TiO2 nanoparticles under visible light irradiation. This photocatalytic process can be efficiently applied for landfill leachate treatment. Finally, the performance of Si-doped TiO2 nanoparticles as P-type TiO2 was compared with the performance of tungsten (W)-doped TiO2 nanoparticles as N-type TiO2 for leachate COD removal. Results showed that both types of nanoparticles have a similar performance for oxidizing the leachate COD, and there is no significant difference between them.
