Equilibrium adsorption study of the adsorptive removal of Cd 2+ and Cr 6+ using activated carbon

Abstract

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.