Humic-modified natural and synthetic carbon adsorbents for the removal of Cd(II) from aqueous solutions

Natural (rice husk) and artificial (mesoporous carbon) materials were mechanochemically treated with humic acid to obtain humic-modified sorbents. The obtained sorbents were characterized by elemental analysis, surface area analysis, FTIR-spectroscopy, and acid-base titrations. Although the specific surface area of the carbon-humic material was significantly higher than that for the husk-humic material, the total number of surface functional groups for both materials was almost the same (approximately 2.0 mmol/g). The effect of the modification on Cd(II) sorption was evaluated at different pH values. The equilibrium data fit to the Langmuir model of sorption. The maximum sorption capacity of both modified sorbents increased with increasing pH values from 5 to 8 and reached approximately 41 mg/g at pH 8. Primarily carboxyl groups define the main contribution towards Cd(II) sorption whereas the basic amino groups are involved in the binding of cadmium at pH above 7. The modified sorbents can be reused after Cd(II) desorption with 0.05 M HNO3. The efficiency of Cd removal and recovery remains constant at least in 5 consecutive cycles. The results obtained demonstrate that both humic-modified sorbents have potential for Cd(II) removal and recovery from contaminated wastewater.