Removal of Cr(VI) from aqueous solution using activated carbon modified with nitric acid

The adsorption processes of Cr(VI) from aqueous solution onto a granular activated carbon (GAC) and a modified activated carbon (MAC) with nitric acid have been investigated. The surface characteristics of these two activated carbons were measured. The results showed that the MAC has a lower surface area, lower pHIEP and more oxygen functional groups compared with the GAC. Adsorption parameters such as adsorbent dosage, pH of solution, contact time and temperature of the adsorption onto the GAC or MAC have been investigated in a batch adsorption experiment in order to obtain the optimum conditions for the Cr(VI) adsorption process. Experimental equilibrium data have been obtained and correlated with Freundlich and Langmuir isotherms for the determination of the adsorption potential. The results have shown that the Langmuir isotherm model better fits the experimental data compared with the Freundlich isotherm for both MAC and GAC. It was found that the MAC has a higher Cr(VI) adsorption capability, with 16.1 mg g−1, compared with GAC, which has a capacity of 6.40 mg g−1. Furthermore, the separation factor RL for the Langmuir isotherm indicated that the adsorption process is most favourable for the MAC. The breakthrough curve for the adsorption column test was obtained in a continuous adsorption bed experiment, and it would need, under continuous adsorption conditions, a minimum amount of 4.39 g l−1 for MAC, whereas under batch conditions, 2 g l−1 is enough for MAC to achieve the same removal of Cr(VI) with an initial concentration of 25 mg l−1.