Comparison of the divalent heavy metals (Pb, Cu and Cd) adsorption behavior by montmorillonite-KSF and their calcium- and sodium-forms

In this work, the sorption behaviors of Pb(II), Cu(II) and Cd(II) by commercial montmorillonite-KSF (MT), sodium (Na-MT) and calcium (Ca-MT) forms are discussed considering, the structural characteristics of each clay material, the contact time, the initial concentration of the heavy metal ions in solution, the dosage of the adsorbents and the initial pH value. The characterization of all sorbents was performed using X-ray powder diffraction (XRD), scanning electron microscopy (SEM) and energy dispersive X-ray spectroscopy (EDS). The pH(PZC) measurements were also determined. The contact between the metal ions solutions and clay materials were done in a batch system. Mathematical models (pseudo-first and pseudo-second-order) were employed to describe the three metals kinetic adsorption by MT, Na-MT, and Ca-MT as well as Langmuir and Freundlich models to analyze the adsorption isotherms. It was found that the inorganic salt treatments of MT promote an increase in Al/Mg ratio and pH(PZC) value of the unmodified clay, and hence, increased the sorption capacity. The pseudo-second-order kinetic model best describes the sorption behavior (R-2 = 1) and the k(2) diminished when the MT has been modified by sodium and calcium ions. The variation of kinetic rate constant k(2) (g/mg min(-1)) as a function of the adsorbent mass presents an exponential behavior for all clay materials. The initial pH of the metal solutions affects on the time to reach sorption equilibrium, and the percentage of the removal of metal ions in this condition. The mechanism involved on the sorption of Pb(II), Cu(II) and Cd(II) by MT, Na-MT and Ca-MT are ion exchange and precipitation on the surface. The experimental data of the isotherms fitted to the Linear and Langmuir isotherms and the K-d parameter of Na-MT for Pb2+ and Cu2+ is higher than Ca-MT and MT, whereas the K-L was lower in the case of Cd2+ sorption. (C) 2017 Published by Elsevier Ltd.