Enhancing Product Selectivity During Selenium Reduction Using Noble Metal-TiO2 Heterogeneous Photocatalysts

Selenium (Se) is a naturally occurring non-metal, which is essential for all living organisms including humans in trace amounts. Of all essential elements, Se has one of the narrowest therapeutic windows between dietary deficiency (< 40 µg/day) and toxicity (> 400 µg/day) (Fordyce, 2013), which makes it important to carefully control Se exposures to humans and aquatic life. Se can enter surface waterways through a variety of sources, including agricultural runoff, mining, industrial production, coal-powered thermal electric generation, and other anthropogenic activities (Holmes and Gu, 2016; Santos et al., 2015). The World Health Organization (WHO) and the U.S. Environmental Protection Agency (U.S. EPA) recognize the dangers of Se and have mandated maximum acceptable levels for Se in water of 10 µg/L and 1.5 µg/L, respectively (World Health Organization, 2011; U.S. EPA Office of Water, 2016). Se exists in many forms, both organic and inorganic, but the high solubility and bioavailability of inorganic species such as selenite (SeO32-) and selenate (SeO42-) makes these oxyanions the primary focus of Se removal from water. Many methods of removal have been attempted for selenate impacted water including biological reduction (Lai et al., 2014; Mal et al., 2017) and wetland remediation (Mooney and Murray-Gulde, 2008), but the advanced operating complexity, high start-up costs and large footprint requirement have limited the practical application of these techniques. Alternatively, Se adsorptive techniques have been explored which rely on the affinity of selenite and selenate to the surface of designed adsorbents such as ferrihydrite (iron (III) oxyhydroxide), hematite, goethite, activated alumina or various ion exchange resins (Ippolito et al., 2009; Rovira et al., 2008). However, the weak complexation between selenate and mineral surfaces through an outer-sphere adsorption complex makes adsorptive techniques much less effective (Jordan et al., 2013). Photocatalytic techniques have been shown to remove both selenite and selenate with great efficiency (Nakajima et al., 2013; Nguyen et al., 2005; T.T.Y. Tan et al., 2003a). Herein, we synthesize a variety of noble metal (Au, Ag, Pt and Pd) deposited TiO2 photocatalysts for the reduction of selenate from water. By modifying TiO2 with different metallic co-catalysts that have controllable electron affinities, we are able to tune the Se product selectivity towards favourable Se products. The final fate of Se during treatment is a key factor in determining the successful adaptation of a water treatment approach.