Photocatalytic reduction and recovery of mercury by polyoxometalates

Photocatalytic reduction of mercury in aqueous solutions using PW12O40(3-) or SiW12O40(4-) as photocatalysts has been studied as a function of irradiation time, concentration of Hg(II), polyoxometalate, and organic substrate in the presence or absence of dioxygen. The photocatalytic cycle starts with irradiation of polyoxometalate, goes through the oxidation of, for instance, propan-2-ol (used as sacrificial reagent), and closes with the reoxidation of reduced polyoxometalate by Hg2+ ions. Mercury(II) is reduced to mercury(I) and finally to Hg(0) giving a dark-gray deposit, following a staged one-by-one electron process and a first-order kinetics in [Hg2+]. The process is slightly more efficient in the absence of dioxygen, while the increase of either catalyst or propan-2-ol concentration results in the augmentation of the rate of reduction till a certain point where it reaches a plateau. The results show that this method is suitable for a great range of mercury concentration from 20 to 800 ppm achieving almost complete recovery of mercury up to nondetected traces (<50 ppb). In addition, this homogeneous process demonstrates advantages such as the lack of necessity for separation of the zero state metal from the catalyst and ensures that the precipitation of metal will not poison the catalyst or hinder its photocatalytic activity.