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

M.I. LITTER

Conferencia; Webinar on Catalysis and Chemical Science; 2020

Presence of heavy metals and metalloids in water represents nowadays one of the most important environmental problemsbecause the yearly total toxicity of mobilized metals overcomes the total toxicity of anthropogenically generated radioactiveand organic wastes. Chromium, mercury, arsenic or lead are in the list of priority pollutants of most environmental agencies,and the World Health Organization is establishing more and more exigent MCLs for these chemical species. In addition tonatural sources, anthropic activities introduce hundreds of billions of tons per year of heavy metals (including arsenic) in theterrestrial ambient. At the same time, the accumulation of metals in effluents represents significant economic losses.Removal of organic and microbiological pollutants from waters has been thorough fully studied; however, less attention hasbeen paid to the transformation of toxic metals and metalloids in species of lower toxicity or more easily isolated [1]. Metalsin their various oxidation states have infinite lifetimes, and chemical or biological treatments present severe restrictions orare economically prohibitive. Removal of heavy metals has been carried out, generally, by precipitation, electrolysis, chemicaloxidation, adsorption, chelation, all of them presenting several drawbacks.From the beginning of the development of heterogeneous photocatalysis, transformation and deposition of metals ? principallythe most noble, expensive and toxic ones ? was visualized as one of the potential applications of the process. Photocatalytictreatments can convert the ionic species into their metallic solid form and deposit them over the semiconductor surface, ortransform them in less toxic soluble species. At the end of the process, metallic species can be extracted by mechanical orchemical procedures. When a transformation to the zerovalent state is possible, this allows the recovery of the metal from thewaters, with an important economical return.Three types of mechanisms can be considered for the photocatalytic removal of metal and metalloid ions from water, all ofthem taking place through successive thermodynamically allowed monoelectronic electron transfer steps until stable speciesare formed: (a) direct reduction by photogenerated electrons; (b) indirect reduction by intermediates generated by hole orhydroxyl radical oxidation of electron donors present in the media (reducing radicals); (c) oxidative removal by holes orhydroxyl radicals.This presentation is an overview of recent work performed in our laboratory on TiO2 heterogeneous photocatalysis of aqueoussystems containing toxic forms of chromium, arsenic, lead, uranium and mercury. The cases of chromium and arsenic will betreated in profundity.