Solar Photocatalytic Removal of Selenite, Selenate, and Selenocyanate Species

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 (SeO₃ ²⁻) and selenate (SeO₄ ²⁻) account for most selenium species in selenium contaminated water bodies however selenocyanate (SeCN⁻) 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 (TiO₂) 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., SeCN-complex 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.