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TiO2-assisted photocatalysis was employed for the removal of aqueous phase selenite and selenate species in conjunction with EDTA as a hole (h+) scavenger. Findings from the binary selenite/EDTA and selenate/EDTA systems showed high selenite and selenate removal at pH 4 and pH 6, with faster removal kinetics noted for the selenite species compared with the selenate species that showed a gradual change over the reaction course. The noted removal of selenite and selenate was attributed to their reduction by the conduction band electrons (e-). The effect of pH studies indicated high selenite, selenate, and EDTA removal in the acidic pH range, with the following specific trend: pH 4 > pH 6 > pH 12. Different from the EDTA studies, the use of thiocyanate alone did not initiate reduction of selenium oxyanions, and hence, its role as a hole scavenger in the present systems was not evident. However, the addition of EDTA to respective selenite/selenate/thiocyanate system at pH 4 did yield near complete removal of selenite and selenate species. The marginal role of thiocyanate as a hole scavenger was attributed to its negligible adsorption onto TiO2 surface. Furthermore, at pH 4 and within 3 h reaction time, enhanced selenate removal was noted with an increase in its initial concentration from 20 to 100 ppm, with near complete selenate removal noted for both cases. In general, findings from the present work indicate that both selenite and selenate can be successfully removed from the aqueous phase employing the TiO2-mediated photocatalysis and h(+)-scavenging agent EDTA.

Selenium release into aqueous environment because of increased anthropogenic activities and its adverse effects onto human and animal health have initiated stringent selenium drinking water and wastewater discharge regulations. Though typically selenite (SeO 3 2À ) and selenate (SeO 4 2À ) account for most selenium species in selenium contaminated water bodies however selenocyanate (SeCN À ) is another dominant selenium form in specific industrial effluents including those from petroleum refineries, mining, and power plants using fossil fuels. The present study investigated the potential of solar photocatalytic degradation (SPCD) process employing titanium dioxide (TiO 2 ) as photocatalyst and ethylenediaminetetraacetic acid (EDTA) as a hole scavenging agent, for the removal of selenite, selenate, and selenocyanate species from aqueous phase. The experimental results indicated pH 4 to be the optimum for highest selenium removal. Furthermore, solar energized photocatalytic removal of selenite, selenate, and selenocyanate increased with an increase in both initial selenium and initial EDTA concentrations. Though selenite and selenate can be reduced directly to elemental selenium, selenocyanate is removed via a two steps process, i.e., SeCNcomplex destruction followed by stepwise oxidation to selenite/selenate and then reduction to elemental selenium. Furthermore modeling results for selenocyanate removal using the response surface methodology and Box-Behnken design method are also presented. In general, solar photocatalysis is a promising technology that can successfully remove the aforementioned selenium species from aqueous phase with a careful adjustment of process variables.

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