Pb(II) uptake by kaolinite and montmorillonite in aqueous medium: Influence of acid activation of the clays

The adsorptive interactions of Pb(II) ions with kaolinite, montmorillonite, and their acid activated derivatives in aqueous medium are investigated in this work. The adsorption experiments were carried out under batch process with Pb(II) concentration, amount of clay, pH, time and temperature as the variables. The adsorption was strongly dependent on pH of the medium, and the uptake of Pb(II) increased from pH 1.0 to 6.0 after which the experiments could not be continued due to decreasing solubility of Pb(II). Adsorption was very fast at low coverage and equilibrium was approached within 180 min. The results best fitted the second order kinetic model with the rate constant, k2, in the range of 3.5 × 10−2 to 11.2 × 10−2 g mg−1 min−1. Langmuir and Freundlich isotherms were applied and isotherm coefficients were computed. The Langmuir monolayer capacity of the clay adsorbents was from 11.5 to 31.4 mg g−1. The adsorption process was exothermic (ΔH = −58.9 to −83.4 kJ mol−1) accompanied by decrease in entropy (ΔS = −116.4 to −258.7 J mol−1 K−1) and Gibbs energy (ΔG = −35.8 to −79.6 kJ mol−1). The results are discussed to highlight the influence of acid activation on Pb(II)-adsorption characteristics of the clays.