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 (SeO4(2-), hereafter Se(VI)). Se(VI) 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 Se(VI) and the oxyanions SO4(2-) 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 Se(VI) and competing oxyanions in mock solutions and a real-world mine effluent with high concentrations of SO4(2-) and NO3(-). GMB immediately removed 98% of the Se(VI) from the mock solution, but only 3% from the mine effluent with the same initial Se(VI) concentration. Notably, 83-89% of the Se(VI) was removed by GMB when concentrations of Se(VI) and SO4(2-) were less than or equimolar in mock solutions. Higher concentrations of SO4(2-) reduced the uptake of Se(VI). There was no interference from NO3(-) on the biosorption of Se(VI). GMB is a successful biosorbent for Se(VI), however, high concentrations of SO4(2-) will compromise the biosorption of Se(VI).