Photocatalytic reduction of Pb(II) over TiO2: New insights on the effect of different electron donors

Experiments of Pb(II) photocatalytic removal under nitrogen over TiO2 (Degussa P-25) were performed in the presence of electron donors such as methanol, ethanol, 2-propanol, citric acid (Cit) and formic acid (FA) in order to study their effects on the removal efficiencies. Formic acid and 2-propanol showed the highest effect, but the first one was considered a better additive because it can be used at a concentration five-fold lower and it does not introduce toxic degradation products in the system. In the case of formic acid addition at low concentration, redissolution of Pb was observed when the electron donor was completely consumed. Product formation was dependent on the reaction conditions and on the nature of electron donor, but the main identified product in all cases was zerovalent Pb, in the form of colloidal particles, as demonstrated by XRD and XPS analysis. Mechanisms involving reducing radicals were proposed to explain Pb(II) removal and the comparative efficiency of the donors.