The complexity of biosorption treatments for oxyanions in a multi-element mine effluent

Selenium (Se) is a contaminant in effluents from coal mines and coal-fired power stations, where it is encountered as the oxyanion selenate (SeO42−, hereafter SeVI). SeVI can be removed from solution with Fe-treated biosorbents, but the efficacy of these treatments in effluents with multiple contaminants is unclear. This study investigates the interactions between SeVI and the oxyanions SO42− and NO3−. We produce a sustainable biosorbent, Gracilaria Modified Biochar (GMB), by treating a waste product generated after the commercial extraction of agar from cultivated seaweeds with ferric chloride (FeCl3) and converting it to biochar through pyrolysis. We then test interactions between SeVI and competing oxyanions in mock solutions and a real-world mine effluent with high concentrations of SO42− and NO3−. GMB immediately removed 98% of the SeVI from the mock solution, but only 3% from the mine effluent with the same initial SeVI concentration. Notably, 83-89% of the SeVI was removed by GMB when concentrations of SeVI and SO42− were less than or equimolar in mock solutions. Higher concentrations of SO42− reduced the uptake of SeVI. There was no interference from NO3− on the biosorption of SeVI. GMB is a successful biosorbent for SeVI, however, high concentrations of SO42− will compromise the biosorption of SeVI.