Adsorption mechanisms of hexavalent chromium from aqueous solutions on modified activated carbons

Functional groups of the activated carbon play the major role in metals removal from aqueous solutions and, for this reason, different treatments can be used to modify the adsorbent surface improving the adsorption capacity for a particular pollutant. In this research, oxidation with nitric acid, heating under an inert atmosphere, and ammonia treatment were applied to modify the activated carbon surface. The modified adsorbents were used for the removal of hexavalent chromium (Cr(VI)) from aqueous solutions at different concentrations (10-500 mg L^-1), pH 6, and 25 °C. Adsorption mechanisms of Cr(VI) on the activated carbon were proposed based on the surface chemistry, adsorption/reduction, and desorption experiments. Findings demonstrate that acid functional groups of the activated carbon had an important effect on the hexavalent chromium removal. For instance, a high reduction of Cr(VI) to Cr(III) (50%) was obtained by the oxidized adsorbents, whereas the heat treated adsorbents achieved a low reduction (35%), but the ammonia-treated activated carbon achieved the lowest reduction (20%). The heat-treated adsorbent showed the best Cr(VI) adsorption capacity (48 mg g^-1), especially at equilibrium Cr(VI) concentration lower than 200 mg L^-1, and the fastest adsorption kinetics among the studied adsorbents. Furthermore, the highest Cr(VI) desorption (90%) was achieved with 0.1 N NaOH-NaCl solutions. In summary, an anionic/reduction coupled adsorption mechanism of Cr(VI) seems to be feasible, and the heat-treated activated carbon is an interesting option for sequestering Cr(VI) species from aqueous effluents.