Adsorption of Se(IV) and Se(VI) species by iron oxy-hydroxides: Effect of positive surface charge density

Batch and continuous mode experiments were used to determine the influence of physic-chemicals characteristics of iron oxy-hydroxides (FeOOHs) on selenium adsorption. Batch experiments and continuous flow rapid small-scale column tests (RSSCTs) at pH 7 and NSF (National Sanitation Foundation) water matrix, showed that the adsorption capacity of FeOOHs for Se(IV) is strongly related to positive surface charge density (PSCD), and gradually increases when synthesis pH is lowered. The highest PSCD value of 3.25 mmol [OH^-]/g was observed at synthesis pH 2.5 (FeOOH/2.5) and the lowest, 0.45 mmol [OH^-]/g, was observed at synthesis pH 9 (FeOOH/9). A thermodynamic study verified the endothermic (ΔΗ° 21.4 kJ/mol) chemisorption of Se(IV) by the qualified FeOOH/2.5. EXAFS data showed that Se(IV) is involved in three types of surface complexes: bidentate mononuclear edge-sharing (¹E) and two types of binuclear inner-sphere (²C) linkage between the SeO₃^2- pyramids, and Fe(O,OH)₆ octahedra. The FeOOHs were evaluated by their adsorption capacity (Q₁₀) at residual concentrations equal to the EU drinking water regulation limit of 10 μg/L, e.g. in conditions implemented in full-scale water treatment plants. The qualified FeOOH/2.5 was found to be the most effective for Se(IV) adsorption with a Q₁₀ value 4.3 mg Se(IV)/g. In contrast, the Q₁₀ value for Se(VI) was almost three orders of magnitude lower (10 μg Se(VI)/g) than that for Se(IV). Finally, regeneration experiments showed that FeOOHs reuse for Se(IV) removal is economically feasible and the recovery of selenium by precipitation as elemental Se contributes to green chemistry.