Photocatalytic reduction of Se(VI) in aqueous solutions in UV/TiO₂ system: Importance of optimum ratio of reactants on TiO₂ surface

This study investigates the effect of pH, initial concentrations of formic acid and selenate (Se(VI)) ions on the UV/TiO₂ reduction of Se(VI) ions. The adsorption of Se(VI) and formate ions onto TiO₂ surface was essential before the Se(VI) ions could be photoreduced to elemental selenium (Se). The elemental Se was further reduced to hydrogen selenide (H₂ Se) once the Se(VI) ions were exhausted from the solution. An interesting finding arisen from the current investigation is that the optimum reduction rate of Se(VI) was closely correlated to the molar adsorption ratio of 3:1 of formate-to-selenate on the TiO₂ surface. This ratio also corresponded to the stoichiometry of the overall reaction which involved 3 mol of formate ions reacting with 1 mol of Se(VI). It was postulated that this ratio provided the highest efficiency in capturing the photogenerated holes and electrons, hence resulting in optimum Se(VI) photoreduction. The optimum molar adsorption ratio could be achieved by manipulating the initial pH, initial solute concentration and the order of which the solutes were adsorbed.