Mechanisms of removal of heavy metals and arsenic from water by TiO2-heterogeneous photocatalysis

Abstract

This article is an overview of recent work performed in our laboratory on TiO2 heterogeneous photocatalysis of aqueous systems containing toxic forms of chromium, arsenic, lead, uranium and mercury. The cases of chromium and arsenic are treated in profundity. Photocatalytic treatments can convert the ionic species into their metallic solid form and deposit them over the semiconductor surface, or transform them into less toxic soluble species. When a transformation to the zerovalent state is possible, this allows the recovery of the metal from the waters, with an important economical return. Three types of mechanisms can be considered, 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. Fundamentals of oxidative or reductive heterogeneous photocatalysis are described, with special emphasis on the role of organic donors present in the medium acting as synergists. This indicates the possibility of simultaneous treatment of pollutants of different chemical structure and properties present in the same medium.