Chapter 2. Reduction of pentavalent and trivalent arsenic by TiO2-photocatalysis

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

Advanced Oxidation Technologies

CRC Press

Lugar: Boca Raton, Florida; Año: 2014; p. 23 - 42

Arsenic in drinking water constitutes nowadays a serious problem, affecting the health of several million people all over the world. Only in the Gangetic delta regions of Bangladesh and West Bengal in India, it has emerged as an environmental health catastrophe with more than 100 million people estimated to be at risk (Naidu, 2012); in Latin America, 14 million people could be affected (Litter et al., 2012). Ingestion of more than 100 mg of the element causes acute poisoning, but ingestion of small amounts of As for a long period leads to the occurrence of arsenicosis or Chronic Regional Endemic Hydroarsenicism (hidroarsenicismo cronico regional endemic°, HACRE, in Spanish), responsible for skin alterations and cancer (Bundschuh et al., 2010; 2012; Figueiredo etal., 2010; Litter etal., 2008; 2010; 2012; Morgada et al., 2008, 2010a). The World Health Organization (2011) recommends 10 p. g as the maximum allowable As concentration in drinking water, value taken by most national regulatory agencies. Arsenic pollution in water can be originated in anthropic activities (mining, use of biocides, wood preservers) but most pollution is natural, coming from dissolution of minerals in surface or groundwaters, or volcanic processes (Bundschuh et al., 2010; Litter et al., 2010). Predominant As forms in natural ground-and surface waters (at neutral pH) are arsenate (As(V), as H2AsCt4 and HAs0,2,- ) and arsenite (As(111), as neutral As(OH)3). Methods for As removal from waters are urgent, but they should take into account that while As(V) can be easily removed by conventional treatments such as ion exchange or adsorption techniques, As(111) removal is more difficult due to its nonionic form in aqueous solutions at pH < 9 (Litter et al., 2010). Recently, we focused on the study of the conversion of As(111) and As(V) to MO) for immobilization of As dissolved in water by TiO2 photocatalysis (Levy et al., 2012). Transformation to As(V) makes easier the application of conventional technologies as ion exchange and adsorption. However, new emerging techniques should be investigated to offer low-cost solutions to the arsenic problem, especially for low-income populations, as mentioned before (Litter etal., 2008; 2010; Morgada et al, 2008). Figure 2.1 presents the Latimer diagram for stable As species, and it shows that reduction of As(V) into As(111) and even into As(0) is thermodynamically possible with mild reductants; further reduction to arsine is more difficult. These processes are even harder at basic pH. Heterogeneous photocatalysis (HP) is one of the most studied advanced oxidation processes for water and air treatment. In HP, after the incidence of photons of adequate energy on semicon-ductor particles, electrons in the conduction band (G) and holes in the valence band (14;,) are produced, which lead to redox reactions with species present in solution, close to the interface or adsorbed onto the photocatalyst surface (Hoffmann et al, 1995; Litter, 1999; 2009).