Reduction of pentavalent and trivalent arsenic by TiO2-photocatalysis: an innovative way for arsenic removal

I. K. LEVY; M. MIZRAHI; G. RUANO; G. ZAMPIERI; F.G. REQUEJO; M. LITTER

Advanced Oxidation Technologies. Sustainable solutions for environmental treatments

CRC Press/Balkema, Taylor & Francis Group

Año: 2013; p. 20 - 40

Reduction of As(V) and As(III) species by TiO2-HP under anoxic conditions can be an innovative way for arsenic removal from water. As(III) (0.525 mM) can be photocatalytically reduced at pH 3 in the presence and in the absence of MeOH, while HP reduction of As(V) takes place only in the presence of MeOH. As(0) and AsH3 are the products of this reduction. The reductive HP process at pH 3 is very efficient for As (V or III) removal at the lowest concentrations (e.g., 1 mg L−1, a common value found in natural polluted groundwaters) because it allows attaining As levels lower than those established by regulations in drinking water (< 10 mg L−1). As(0) formation would be a definite way of immobilizing As(V) and As(III) species present in aqueous media. However, it is important to remark that extremely toxic AsH3 is formed in these very anoxic HP systems. Known as a very toxic compound, its possible evolution to the air has to be cautioned, and use of a fume hood is mandatory. Some strategies to remove or contain AsH3 are envisaged: 1) adsorption on suitable catalysts as those developed elsewhere (Quinn et al., 2006; Seredych et al., 2010); 2) treatment by a second TiO2 photocatalytic oxidative step in the gas phase using supported TiO2; 3) optimization of a simpler and less reducing system, where nitrogen is bubbled only before irradiation. In addition, other electron donors, such as citric acid or ethanol, less toxic than MeOH, can be proposed for As(V) removal or to accelerate As(III) reduction. This will be the subject of future research. Regarding the applicability of the treatment to reduce As levels in drinking water, it is not possible to judge at the present stage of knowledge its feasibility. Experiments with natural waters (at circumneutral pH values), containing possible inhibitors such as dissolved organic matter (DOM), bicarbonate/carbonate ions, etc., which could affect the chemistry of the redox reactions in solution and at the water/photocatalyst interface should be taken into account. DOM can act as a UV filter, decreasing the efficiency of the method.