Nitrate photocatalytic reduction on TiO2: Metal loaded, synthesis and anions effect

Herein we report the study of the photocatalytic reduction of nitrate by TiO2 in presence of inorganic anions and for TiO2 loaded with Ag+, Cu2+, Ni2+, and Fe3+ by following the wet impregnation method. All photocatalytic assays were performed in solution containing formic acid as scavenger of the photogenerated holes (hvb+). The observed dependence of the photocatalytic nitrate reduction on the concentration of HCOOH suggests this specie is oxidized at hvb+ forming CO2− radicals that, in turn, reduces NO3− to N2. The photocatalytic assays in presence of H2PO4−, SO42−, HCO3−, F− and Cl− indicated the surface anionization leading to hvb+ blocking and, as consequence, to the decrease of HCOOH oxidation. Indeed, impedimetric data indicated the adsorbed anions on TiO2 are mainly responsible for hindering the interaction between the electron donor (HCOOH) and the photocatalyst. Among the metal ions used for loading, only Ag+ was efficient in storage the photogenerated electrons (ecb−) with a performance improvement of 15.5 % in respect to the neat TiO2 catalyst. Experiments carried out with groundwater samples showed an activity loss that was assigned to the catalyst poisoning by other ions, although with selectivity to N2 close to 100 %.