Removal of Se(IV) and Se(VI) from water by aluminum-oxide-coated sand

Aluminum-oxide-coated sand (AOCS) was evaluated for the removal of selenite (Se(IV)) and selenate (Se(VI)) from water. Quartz sand was coated at 70°C using 1 M AlCl3 solution aging for 2 d at various coating pH (pH(coating)). The characteristics of the AOCS surface were pH(coating)-dependent. The Al oxide coating was an X-ray noncrystalline, porous compound at low pH(coating), while at high pH(coating), the AOCS could better withstand acid/alkali and the coatings tended to form crystalline boehmite and bayerite. Adsorption of Se(IV) and Se(VI) was more effective using sand coated at low pH(coating) than at high pH(coating). AOCS produced at pH(coating) 5.98 had optimum properties and was employed as the adsorbent for the present adsorption studies. Adsorption experiments of Se(IV) and Se(VI) by AOCS performed as a function of pH, initial concentration, reaction time, and competing ion concentrations were examined. Removal of Se(IV) and Se(IV) increased with decreasing pH but was obviously greater for Se(IV) than Se(VI). In Se(IV) and Se(VI) mixed systems, the adsorption of Se(IV) was evidently inhibited by Se(VI) only at system pH ranging from 3 to 8, and the degree of inhibition was similar at Se(VI) to Se(IV) molar ratios of 1 and 3. However, Se(VI) adsorption significantly decreased with increasing Se(IV) concentration at all system pH. The sequence of foreign anions competing with respect to Se(IV) and Se(VI) adsorption was in the order of SO2−4>HCO−3.