Competitive Adsorption of Quaternary Metal Ions, Ni[sup.2+], Mn[sup.2+], Cr[sup.6+], and Cd[sup.2+], on Acid-Treated Activated Carbon

This paper examined the competitive removal of metal ions from quaternary aqueous solutions containing Ni[sup.2+], Mn[sup.2+], Cr[sup.6+], and Cd[sup.2+] using adsorption on both acid-modified and unmodified activated carbon. Activated carbon (AC) was oxidized with nitric acid, both in granular (AGC) and powder (APC) forms, and tested for the competitive adsorption of Ni[sup.2+], Mn[sup.2+], Cr[sup.6+,] and Cd[sup.2+] from an aqueous solution. Surface oxidation led to a reduction in BET surface area and HK pore width and an increase in the intensities of hydroxyl and carboxyl functional groups for both AGC and APC compared to unmodified activated carbon, AC, as indicated with BET and FTIR analyses. The adsorption capacity of all four metal ions on AC was in the order Ni[sup.2+] > Cd[sup.2+] > Cr[sup.6+] > Mn[sup.2+], while it was altered for the two oxidized AGC and APC carbons to be Cr[sup.6+] > Ni[sup.2+] > Cd[sup.2+] > Mn[sup.2+]. Acid treatment resulted in high selectivity for Cr[sup.6+] over all other available ions with a 100% removal efficiency, while it decreased for Ni[sup.2+], Cd[sup.2+,] and Mn[sup.2+] compared to AC. This improvement in Cr[sup.6+] adsorption is due to its higher ionic potential and smaller size, which results in a faster diffusion and stronger adsorption to the acidic groups located at the pore edges. Therefore, it will repel and hinder other ions from accessing the activated carbon pores. Modeling of the adsorption isotherms with DKR was better than both Freundlich and Langmuir for the competitive ions. DKR showed strong attraction for both Ni[sup.2+] and Cd[sup.2+] by ion exchange on the AC surface, as indicated by their apparent adsorption energy (E) values. Cr[sup.6+] adsorption was found to be by physical adsorption on AC and by ion exchange on both AGC and APC. Mn[sup.2+] ions had a very weak attraction to all types of tested activated carbons in the presence of other ions.