Light-induced reduction of heavy-metal ions on titanium dioxide dispersions

Abstract

Photoactive semiconductors such as TiO₂ can be applied for wastewater treatment; not only for degradation of organic pollutants, but also for removal of heavy-metal ions. Photoassisted reduction of different, mostly toxic metal ions, such as Hg(II), Pb(II), Bi(III), and Cu(II), has been realized in aqueous systems containing ethanol as a sacrificial electron donor. The efficiency of the photoreduction carried out under the same circumstances is primarily determined by the standard reduction potential of the species containing the metal ion to be removed as demonstrated by the examples of Hg(II), Bi(III), and Pb(II). The relatively lower efficiency for the deposition of Cu may be attributed to a short-circuiting effect. Surfactants of different type also function as potential electron donors in these systems. Negatively charged dodecyl sulfate proved to be much more efficient than ethanol, while cationic cetyltrimethylammonium is rather weak in this respect.