Adsorption of Divalent Lead on a Montmorillonite−Illite Type of Clay

Lead(II) was removed from aqueous solution by adsorption on a montmorillonite−illite type of clay (MIC) collected from the Gulbarga district of Karnataka, India. The objectives of the current work were (i) to characterize the clay (adsorbent) and (ii) to perform equilibrium, kinetic, mass-transfer, and thermodynamic studies for the adsorption of Pb(II) on the clay. Batch adsorption equilibrium data were determined with different initial Pb(II) concentrations (100, 150, and 200 ppm) at pH 4 and 37 °C, and the data were tested with isotherm models. The three-parameter Freundlich−Langmuir model gave the best fit to the equilibrium data (R2 = 0.9979). However, as the initial Pb(II) concentration was increased (150 and 200 ppm), multilayer adsorption was observed. The maximum monolayer adsorption capacity of the clay was determined to be ∼52 mg/g. Kinetic studies indicated that the rate of adsorption of Pb(II) on the clay followed a second-order rate mechanism, with decreasing rate constant values of 0.1097, 0.0571, and 0.0022 g/(mg min) as the initial Pb(II) concentration was increased in the order of 100, 150, and 200 ppm, respectively. The value of the Freundlich constant (n) in the range of 2.5−4.6 indicated that MIC was a good adsorbent of divalent lead. At a higher initial Pb(II) concentration (200 ppm), the adsorption process was determined to be film-diffusion-controlled, with a rate of 0.051 min−1. The mean values of the thermodynamic parameters—the change in the free energy (ΔH°), the change in the entropy (ΔS°), and the change in the Gibbs free energy (ΔG°) showed that the adsorption process was endothermic, thermodynamically favorable, and spontaneous. A two-stage adsorber system that has been proposed reduced the clay dose by 8.5%, compared to that of a single-stage adsorption system.