Adsorptive Removal of Parts per Million Level Selenate Using Iron-Coated GAC Adsorbents

Selenate removal by adsorption using iron-coated granular activated carbons (Fe-GACs) is reported in this study. Adsorption kinetics and equilibrium experiments with initial selenium concentration of 1 mg/L were conducted under three different ionic strengths to study selenate adsorption behavior. Selenate adsorption reached equilibrium within 48 h with more than 85% of the equilibrium capacities being obtained within the first 6 h. High removal efficiency (i.e., >75%) was achieved for pH range of 2–5. Acid-base titration experiments showed point of zero charge (pHpzc) at pH 7.5 for the tested Fe-GAC. Pseudo-second-order kinetic model characterized selenate adsorption kinetics well (R2 = 0.999) and the rate constant decreased with ionic strength. Adsorption capacity decreased significantly with increasing ionic strength, which was not observed in selenite adsorption with the same adsorbent. Competitive adsorption with other four oxyanions (SiO32?, SO42?, PO43?, and CO32?) showed that selenate removal efficiency was reduced to various degrees in the presence of each individual anion. Competitive adsorption of binary adsorbates (selenite and selenate) showed a decreasing trend of selenite adsorption capacity with decreasing ionic strength, indicating stronger competition of selenate against selenite under the low ionic strengths. The Sheindorf-Rebuhn-Sheintuch multiadsorbate competitive adsorption model was applied to quantify the binary competitive adsorption between selenate and selenite.