Selenite and Selenate Adsorption on Low-Crystalline Magnesium Oxide Under Alkaline Conditions

Natural geochemical processes and anthropogenic activities produce selenium (Se)-containing wastewater, in which selenium is present as selenous acid (Se(IV)) and selenic acid (Se(VI)). Herein, we evaluated the Se removal performances of magnesium oxide (MgO) with different crystallinities at pH 10.5. Normal MgO and low-crystalline (LC-MgO) were reacted in 1, 2, and 5 mg dm⁻³ Se(IV) or Se(VI) solution for 120 min. The Se concentrations of both the species did not apparently decrease in the normal MgO systems. However, >90% of Se(IV) was successfully removed by LC-MgO within 5 min under all conditions. X-ray diffraction (XRD) analysis indicated that Se(IV) was removed via surface complexation on Mg(OH)₂ formed by the rapid hydration of LC-MgO. The enhancement of Se removability of the LC-MgO system was explained by the rapid hydration of the higher surface area of LC-MgO when compared with that of normal MgO. Majority of the Se(VI) (60–80%) was removed by LC-MgO during the first 20 min; however, it was gradually released back into the solution over time. During hydration, the changing XRD peak patterns and zeta potential of LC-MgO indicated that the increased charge density obtained by the accumulation of Mg²⁺ released via MgO dissolution would cause the temporal uptake of Se(VI) in the interlayer of Mg(OH)₂. Subsequently, the charge density decreased with the hydration of Mg²⁺, indicating that Se(VI) was redissolved. Therefore, LC-MgO is a promising material for treating Se-containing alkaline wastewater. However, short-time operation and low concentration (<1 mg dm⁻³) must be ensured for successful Se(VI) removal.