Simultaneous effect of pH, temperature, ionic strength, and initial concentration on the retention of lead on illite

摘要

Sorption of metal ions by clay minerals is a complex process involving different mechanisms, and controlled by different variables that can interact. The aim of this work was to study the retention mechanisms of Pb~(2+) ions on illite. We obtained Pb~(2+) sorption isotherms at constant pH, sorption edges as a function of pH, sorption isotherms at 5, 25 and 45℃ (pH=5.5), and response surfaces of the simultaneous effect of pH, initial concentration, ionic strength, and temperature on the retention of Pb~(2+) ions on illite. Surface topography and atomic composition of illite samples doped with Pb~(2+) were studied by scanning electron microscopy and X-ray fluorescence microanalysis. Below pH 4.5, sorption of Pb~(2+) ions on illite is via ion exchange with H_3O~+ and Na~+ ions that saturated the exchange sites, where the exchange with Na~+ is the main mechanism between pH 3.5 and 4.5. Al~(3+) ions whose concentration in solution increases with decreasing pH can also compete with Pb~(2+) ions. For pH above 5.0, proton stoichiometry was higher than 1.0, and the effect of ionic strength on the amount of Pb~(2+) retained decreased with pH; in fact, at pH 6.8, the amount adsorbed was practically 100% and independent of ionic strength. The pronounced decrease in the intensity of the OH-stretching band for Pb—illite sample at pH 7.0 suggests that Pb~(2+) ions had lost the inner hydration sphere. The analysis of images obtained by SEM from 3 mM Pb~(2+) solutions with and without illite showed particles with the same morphology, which indicate a homogenous lead precipitation from the solution. For pH values higher than 4.5, increase in the temperature favored the retention of Pb~(2+) ions.