Removal of Heavy Metal Ions by Photoelectrodeposition in Aqueous Solutions

In order to remove hazardous heavy metal ions dissolved in an aqueous environment, the photoelectrodeposition effect on the removal of Pb(II), Cu(II), Cd(II), and Zn(II) ions has been investigated for the (CeO₂)_1-X(Ln₂O₃)_X (Ln = La, Sm, Gd) photocatalyst oxide particles combined with fine α-FeOOH particles. This effect is also compared with titanium-based oxides. The mixture of 1 mol%α-FeOOH and these photocatalyst oxides showed a high photoelectrodeposition ability particularly for Pb(II) ions under photo irradiation. The removal efficiency of Pb(II) ions was high under the UV irradiation and it decrased with increasing the wavelength of light toward the visible region. This result indicates that the coexistence of fine α-FeOOH particles with (CeO₂)_1-X(Ln₂O₃)_X particles contributes to an efficient separation of holes and electrons generated by the photo irradiation. The high-resolution scanning electron microscopic observation of the samples revaled that fine α-FeOOH particles supported on the surface of the photocatalyst oxide particles are effective for the charge separation between holes and electrons. This effect is caused by the redox reaction occurring for the transfer of electrons to the surface of α-FeOOH-(CeO₂)_1-X(Ln₂O₃)_X particles. These composite particles were also found to be effective for the removal of other heavy ions at low concentrations. The removal efficiency of the heavy metal ions is determined by the standard electrode potential of elements involved.