Optimization of Cr(VI) Photoreduction in Water over Ag-Promoted Nb2O5: Effects of Operating Conditions and Reactive Species

Photocatalytic reduction of Cr(VI) using Nb2O5-based catalysts has been reported; however, Ag-promoted Nb2O5 remains scarcely explored. In this context, the present study describes the performance of Ag/Nb2O5 catalysts in removing Cr6+ under various experimental conditions. The catalyst was studied, maintaining the nominal metallic load of 5% Ag. To optimize the process, the following operating conditions were analyzed: pH, catalyst concentration, calcination temperature, additive, air bubbling, photolysis, and additive (hole scavenger). The results obtained indicated that the 5%Ag/Nb2O5 catalyst achieves a higher photoreduction rate than bare Nb2O5. Optimization of process parameters showed that the scavenger inhibits chromium adsorption on the catalytic surface. Optimal conditions observed in the tests were as follows: solution pH of 2, with hole scavenger addition, in the absence of oxygen, using a catalyst concentration of 1 g L−1, and a calcination temperature of 300 °C (resulting in a poorly crystalline Nb2O5 phase). Photolysis in the presence of sodium formate reduced Cr6+; however, the reduction was slower. In the quenching tests, the radical scavengers that most influenced the reduction of Cr6+ were silver nitrate (e− scavenger) and oxalate (h+ scavenger). Oxalate was the only one to reduce the total chromium concentration in the solution.