TiO2-photocatalytic transformation of Cr(VI) in the presence of EDTA: Comparison of different commercial photocatalysts and studies by Time Resolved Microwave Conductivity

M.I. LITTER

Conferencia; Fourth Latin-American Congress of Photocatalysis, Photoelectrochemistry and Photobiology - LACP3 2016; 2016

Presence of heavy metals and metalloids in water represents nowadays one of the most important environmental problems because the yearly total toxicity of mobilized metals overcomes the total toxicity of anthropogenically generated radioactive and 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 to natural sources, anthropic activities introduce hundreds of billions of tons per year of heavy metals (including arsenic) in the terrestrial ambient. At the same time, the accumulation of metals in effluents represents significant economical losses.Removal of organic and microbiological pollutants from waters has been thoroughfully studied; however, less attention has been paid to the transformation of toxic metals and metalloids in species of lower toxicity or more easily isolated Metals in their various oxidation states have infinite lifetimes, and chemical or biological treatments present severe restrictions or are economically prohibitive. Removal of heavy metals has been carried out, generally, by precipitation, electrolysis, chemical oxidation, adsorption, chelation, all of them presenting several drawbacks. From the beginning of the development of heterogeneous photocatalysis, transformation and deposition of metals ? principally the most noble, expensive and toxic ones ? was visualized as one of the potential applications of the process. Photocatalytic treatments can convert the ionic species into their metallic solid form and deposit them over the semiconductor surface, or transform them in less toxic soluble species. At the end of the process, metallic species can be extracted by mechanical or chemical procedures. When a transformation to the zerovalent state is possible, this allows the recovery of the metal from the waters, with an important economical return [1-3].Three types of mechanisms can be considered for the photocatalytic removal of metal and metalloid ions from water, all of them taking place through successive thermodynamically allowed monoelectronic electron transfer steps until stable species are formed: (a) direct reduction by photogenerated electrons; (b) indirect reduction by intermediates generated by hole or hydroxyl radical oxidation of electron donors present in the media (reducing radicals); (c) oxidative removal by holes or hydroxyl radicals. This presentation 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 will be treated in profundity [1-4].