Application of Fe-biochar composites for selenium (Se+6) removal from aqueous solution and effect of the presence of competing anions under environmentally relevant conditions

This study's aim was to compare biochar and steam activated biochar functionalized with iron for removal of selenium as selenate from solutions also containing nitrate and sulfate. The Fe-biochar composites were made impregnating iron (ferric nitrate) onto regular biochar (RB) and steam activated biochar (SAB), forming the Fe-biochar composites FeRB and FeSAB. Iron oxyhydroxide deposits were observed on the surface of FeRB using Raman spectroscopy analysis, but not on the FeSAB surface. Unmodified biochar samples did not remove selenium, as Se(VI), from solution, whereas FeRB and FeSAB recovered 8.3 mg-Se⋅g-composite−1 and 5.9 mg-Se⋅g-composite−1, respectively. Higher Se uptake was achieved at higher Fe-loads and lower acetone:biochar ratio, to a maximum of 17.3 mg-Se⋅g-composite−1 (FeRB). Washing after Fe-impregnation using deionized water diminished nitrate and Fe-leaching from the Fe-biochar composites while removing selenium. Se(VI) and sulfate uptake were observed when washed composites were tested in the presence of possible competing ions at environmentally relevant concentrations ([Se]t=0 = 1.01 ± 0.03 mg⋅L−1; [N–NO3-]t=0 = 40.2 ± 0.2 mg⋅L−1; and [SO42−]t=0 = 496 ± 25 mg⋅L−1). One of the possible mechanisms of removal might be the complexation of Se with the iron oxyhydroxide deposits (goethite and hematite) found on the FeRB surface.