Enhanced removal of selenate from mining effluent by H₂O₂/HCl-pretreated zero-valent iron

Direct use of zero-valent iron (ZVI) in reductive removal of selenate (Se(VI)) is inefficient due to the intrinsic passive layer of ZVI. Here we observed that ZVI pretreated with H₂O₂ (P-ZVI-O) performs much better in Se(VI) removal from a mining effluent than other three modes of ZVI alone, acid washing ZVI (P-ZVI-A), and simultaneous addition of H₂O₂ and ZVI (ZVI-O) as well. The P-ZVI-O exhibits exceptionally high Se(VI) removal at a low dosage, wide pH range, with Se dropping down from 93.5 mg/L to <0.4 μg/L after 7-h reaction. Interestingly, the initial pH (2-6) of the mining effluent exerted little influence on the final Se(VI) removal. H₂O₂/HCl pretreatment results in the formation of various reducing corrosion products (e.g. Fe₃O₄, FeO and Fe²⁺), which greatly favors the efficient Se(VI) removal. In addition, surface-bound Fe²⁺ ions participated in the reduction of Se(VI). Combined with the influence of Se valence as well as pH and Fe²⁺ (whether dissolved or surface bound), it is deduced that the P-ZVI-O mode induced efficient Se(VI) removal via the adsorption-reduction and/or co-precipitation. This study demonstrates that H₂O₂/HCl pretreatment of ZVI is a very promising option to enhance the efficiency of reductive removal of Se(VI) from real effluents.