Combining the Photocatalyst Pt/TiO2 and the Nonphotocatalyst SnPd/Al2O3 for Effective Photocatalytic Purification of Groundwater Polluted with Nitrate

We investigated photocatalytic reduction of NO3- in real groundwater in the presence of the photocatalyst Pt/TiO2 and the nonphotocatalyst SnPd/Al2O3, which were dispersed in the groundwater, under irradiation at lambda > 300 nm, with glucose as a hole scavenger. In this system, photocatalytic H-2 evolution (2H(+) + 2e(-) -> H-2) proceeded over Pt/TiO2, and nonphotocatalytic, that is, conventional catalytic, reduction of NO3- with H-2 (NO3- + 5/2H(2) -> 1/2N(2) 2H(2)O + OH-) occurred over SnPd/Al2O3. NO3- (1.0 mmol dm(-3)) in the groundwater completely and selectively decomposed to N-2 (yield 83%) after 120 h with a 300 W Xe lamp (lambda > 300 nm) over the Pt/TiO2-SnPd/Al2O3 system in combination with photooxidative pretreatment of the groundwater over Pt/TiO2 to decompose organic compounds. The decomposition rate of NO3- in the groundwater was still slower than that in an aqueous NO3- solution even after the pretreatment of the groundwater. The lower photocatalytic performance was due to poisoning of Pt/TiO2 with sulfate and silicate ions and poisoning of SnPd/Al2O3 with polymerized silicate ions. On the other hand, cations, including Na+, K+, Mg2+, and Ca2+, in the groundwater did not affect the photocatalytic and catalytic performances of the system. Sulfate ions adsorbed on the Pt sites on Pt/TiO2, where H-2 evolution occurs, and silicate ions deactivated the oxidation sites on TiO2 by reacting with the surface hydroxyl groups, leading to a decline in the photocatalytic performance of Pt/TiO2.