Last advances on TiO2-photocatalytic removal of chromium, uranium and arsenic

Metals and metalloids produce dramatic effects on health, and most conventional treatments for metals and metalloids present restrictions or are economically prohibitive. TiO2-heterogeneous photocatalysis is a convenient technology for the transformation of metals or metalloids in water, converting the pollutant into a less toxic soluble species. In this article, last advances on TiO2-photocatalytic removal of toxic metal and metalloid ionic species from water, particularly chromium, uranium and arsenic, are briefly reviewed. Emphasis on the mechanisms is done, considering three types, all of them taking place through successive thermodynamically allowed monoelectronic electron transfer steps: (a) direct reduction by photogenerated electrons; (b) indirect reduction by intermediates generated by hole or hydroxyl radical oxidation of electron donors (reducing radicals); (c) oxidative removal by holes or hydroxyl radicals. The fundamental studies point out advantages for a successful application of the technology, indicating that more research should be done towards the transfer to industrial applications.