Adsorptive removal of Cd(II) ions from aqueous solutions by rice husk ash

This study deals with the adsorption characteristics of Cd(II) onto rice husk ash, low valued agricultural by‐product obtained during the rice processing from rice mill. The influences of pH, contact time, initial metal ion concentration, adsorbent concentration on the selectivity and sensitivity of the removal process were investigated. The optimum pH for adsorption was found to be 5 for Cd(II), and equilibrium was achieved within 2 h of contact time. Maximum adsorption of Cd(II) was obtained at an adsorbent dosage of 7.5 g/L, which may be considered as optimum adsorbent dosage level. Kinetics data were best described by pseudo‐second order model. The value of effective diffusion coefficient was of the order of 10 −10 m 2 /s. The equilibrium adsorption data were better fitted to Freundlich isotherm models. The adsorption capacity ( q max ) of rice husk ash for Cd(II) ions in terms of monolayer adsorption was 35.84 mg/g. The thermodynamic equilibrium constant and the Gibbs free energy were determined for and results indicated the spontaneous nature of the adsorption process. The adsorption energy calculated using Dubinin‐Radushkevich isotherm shows that adsorption of Cd(II) is chemical in nature.