Adsorptive selenite removal from water using iron-coated GAC adsorbents

Removal of selenite from aqueous phase using iron-coated granular activated carbons (GAC) was investigated in this study. Five different types of GAC were used for iron coating by oxidizing ferrous chloride with sodium hypochlorite and the iron-coated GAC (Fe-GAC) were tested for selenite removal. Nitrogen adsorption–desorption analyses indicated that Brunauer–Emmett–Teller (BET) specific surface area, pore size, and pore volume decreased with the iron coating. The Darco 12 × 20 GAC was shown to be the most effective adsorbent among the five tested GACs after iron coating. Among the different concentrations used for iron coating, the Darco 12 × 20 GAC coated with 0.1 M ferrous chloride achieved the highest selenite removal (97.3%). High removal efficiency of selenite occurred in a wide range of pH (i.e., 2–8), but the efficiency decreased when pH was higher than 8. Adsorption kinetics showed that selenite removal efficiency reached more than 90% after 6-h adsorption for initial selenium concentration of 2 mg/L and equilibrium was obtained after 48 h. A pseudo-second-order kinetic model was found to characterize the adsorption kinetics well for all the initial selenium concentrations and temperatures tested (R2 ≥ 0.9969). Three temperatures (25, 35, 45 °C) were used to examine temperature effect on the adsorption behavior of the Fe-GAC with initial selenium concentration of 1 mg/L. Activation energy was calculated to be 30.42 kJ/mol. Adsorption isotherms for initial selenium concentration of 2 mg/L at various temperatures and ionic strengths were developed and the data generally fit the Langmuir model well (R2 ≥ 0.994). The adsorption capacity reached as high as 2.50 mg-Se/g-adsorbent at equilibrium for initial concentration of 2 mg/L at 25 °C. The Gibbs free energy was determined to be negative, indicating the spontaneous nature of the adsorption reaction. Oxyanion competitive adsorption showed that sulfate (0.1–5 mM) barely affected selenite adsorption. Other anions (phosphate, silicate and carbonate) impact selenite adsorption to various degrees with phosphate completely excluded selenite adsorption at 5 mM. The possible adsorption mechanisms were discussed.