Photocatalytic removal of nickel from aqueous solutions using ultraviolet-irradiated TiO2

The photocatalytic removal of Ni(II) using ultraviolet (UV)-irradiated TiO2 suspensions is described. The photocatalytic reaction is shown to be electron-mediated, and occurs both via direct electron transfer from the photoexcited TiO2 particles to Ni(II), and an indirect route as described by previous authors. The latter requires the presence of an organic additive (acetate, formate, or methanol) that generates an intermediate radical reductant via an initial hole-mediated reaction. The effects of inorganic anions (chloride, sulfate) on the rate of Ni(II) removal are described as are those of other variables including TiO2 dose and solution pH. The direct electron-transfer route is shown to be kinetically sluggish, although it can be made fast by the addition of facile hole acceptors. The dark adsorption of Ni(II) on the TiO2 (anatase) surface is also quantified in a chloride medium. The present data are discussed within the framework of previous studies on the UV/TiO2 photocatalytic deposition of Ni(II) and other metal ions.