Preparation, characterization, and removal of nitrate from water using vacant polyoxometalate/TiO₂ composites

Due to the rapid development of industry and agriculture, nitrate pollution in groundwater has been continuously increasing. NO₃-N is a chemically stable nitrogen species and is quite difficult to remove. In this study, using heteropoly silicotungstate K₈[α-SiW₁₁O₃₉] and Cu²⁺ as the active components, SiW₁₁ and Cu²⁺ were loaded onto TiO₂ by a sol-gel method to prepare a composite photocatalyst SiW₁₁/TiO₂/Cu. The photocatalytic reduction of dissolved NO₃-N was subsequently performed using SiW₁₁/TiO₂/Cu under UV irradiation, and the influence of different experimental parameters on the photocatalytic performance was investigated. The mechanism of NO₃-N reduction by the composite catalyst was also investigated. Free radicals existing within the system were detected by ESR spectroscopy, and the results indicated that $$ anion free-radicals were generated by the reaction of photogenerated holes and formic acid (HCOOH). At a SiW₁₁/TiO₂/Cu dose of 1.2 g L^-1 and in the presence of HCOOH as a hole scavenger, the proposed composite catalytically reduced NO₃-N anddemonstrated significantadvantages in terms of its photocatalytic activity in comparison with pure TiO₂. In particular, the removal efficiency of NO₃-N and the selectivity of nitrogen achieved a maximum of 96% and 77%, respectively.