Removal of aqueous phase selenium using artificial and solar radiation advanced oxidation processe

TiO2 assisted photocatalysis using artificial and solar radiation and solar photo-Fenton processes were employed for the removal of aqueous phase selenite and selenate species in conjunction with EDTA as a hole scavenger. For the UV-lamp light assisted photocatalysis, faster removal was noted at pH 4 and pH 6 for the binary selenite/EDTA system compared to selenate/EDTA system that showed a gradual change over the reaction course. The effect-of-pH studies indicated high selenite, selenate, and EDTA removal in the acidic pH range, with following specific trend: pH 4 > pH 6 > pH 12. Furthermore thiocyanate alone did not initiate photo-reduction of selenium oxyanions. However addition of EDTA species to respective selenite/selenate/thiocyanate system at pH 4 did yield near complete removal of selenite and selenate (via the reduction route). The marginal role of thiocyanate as a hole scavenger to expedite selenite/selenate reduction, was attributed to its negligible adsorption on to TiO2 surface as compared to EDTA which indeed showed adsorption especially at the acidic pH values. Furthermore selenate reduction increased with an increase in its initial concentration (at pH 4) with near complete removal transpiring within 3 h. Solar light assisted photocatalysis also showed high efficiency for the removal of selenite, selenate and EDTA at pH 4 and pH 6 with a specific trend pH 4 > pH 6 > pH 8. Selenite again showed faster removal compared to selenate species. Though solar energized photo-Fenton showed comparatively lower efficiency, removal of selenite was promising with near complete removal at optimum pH. Initial EDTA concentration had the most significant effect among the parameters studied. Among the three processes studied, UV-lamp assisted photocatalysis had the highest efficiency comparatively followed by solar light assisted photocatalysis. In general, findings from the present work indicate that both TiO2 mediated photocatalysis and photo-Fenton processes in conjunction with EDTA as an h + scavenging agent are efficient techniques for selenium ions removal.