Effect of formate and methanol on photoreduction/removal of toxic cadmium ions using TiO2 semiconductor as photocatalyst

Photoreduction/removal of cadmium was studied at pH 7 using TiO₂ Degussa as photocatalyst, and either formate or methanol as hole scavengers. In the absence of organic additives, approximately 60% of cadmium was found to be removed from the solution by adsorption. Addition of formate resulted in the photoreduction of cadmium to its metallic form. No cadmium reduction was observed when methanol was added as the hole scavenger. Zeta potential measurements of the catalyst suspensions and studies on the extent of organic additive adsorption and mineralisation were used to prove the photoreduction process. It was found that the adsorption of both cadmium and the organic hole scavenger is crucial for the photoreduction of cadmium. It is postulated that to be an effective hole scavenger the organic additive needs to be easily photooxidised under an anoxic environment. In addition, the presence of formate radicals could be responsible for the Cd photoreduction.