Removal of cadmium using electrochemically oxidized activated carbon

A wood-based activated carbon, AUG WHK, was oxidized electrochemically to enhance its metal binding capacity and subsequently studied for the removal of cadmium ions from aqueous solution. Treated adsorbents were characterized by N-2 adsorption at 77 K before and after oxidation, and a quantitative determination of weak-acid surface groups was carried out by direct titration. The BET surface area decreased considerably after oxidation; however, the total amount of oxygen-containing surface groups was 3.36 times higher compared to the untreated adsorbent. Batch equilibrium tests indicated that the oxidized material has 16.5 times higher sorption capacity for cadmium than conventional activated carbon. Equilibrium isotherms were determined at pH 4, 5 and 6 and showed that there was a slight increase in cadmium uptake with increase in pH. The experimental data were fitted by Langmuir and Freundlich isotherms and it was found that the Freundlich isotherm fitted better in all the cases. Overall, the results indicated a rapid adsorption rate with over 96% fractional uptake of metal occurring in the first 12 minutes. Small-scale mini-column experiments were performed and indicated that breakthrough occurred after about 140 bed volumes (BV) of feed solution, containing 1 mM Cd at pH 6, was passed at 10BVh(-1) for oxidized WHK, whereas breakthrough was instantaneous for the unoxidized sample.