Equilibrium adsorption study of the adsorptive removal of Cd2+ and Cr6+ using activated carbon.

The performance of activated carbon (AC) with respect to characterization, adsorption kinetics, thermodynamics, and isotherms was addressed in this study. The effects of initial concentration, pH, contact time, ion strength, and temperature on removal efficiency were also studied. The adsorption isotherms of Cd2+ and Cr6+ on activated carbon can fit the Langmuir model well, and correlation coefficients were above 0.99, all higher than the Freundlich and Temkin models. The maximum adsorption quantities of Cd2+ and Cr6+ were 19.380 and 19.305 mg g-1 at 25 °C, respectively. The adsorption capacities of Cd2+ and Cr6+ are clearly pH dependent. The kinetics of the removal of Cd2+ and Cr6+ was in agreement with a pseudo-second-order model, and the adsorption efficiency of Cd2+ is higher than that of Cr6+. The thermodynamic results showed that increased temperature is favorable to adsorption. The speciation on activated carbon was mainly residual Cd2+ and Cr6+, and the potential ecological risk of Cd2+ is higher than that of Cr6+. The adsorptions of Cd2+ and Cr6+ on activated carbon were dominated by chelation and ion exchange, respectively.