The Elovich equation as a predictor of lead and cadmium sorption rates on contaminant barrier minerals

• The Elovich equation adequately describes the adsorption/desorption of Pb and Cd.
• Nature of solution and clays are important factors on sorption/desorption.
• The adsorption and removal efficiency of lead and cadmium is low at high pH.

Contaminant sorption reactions in soils are generally heterogeneous, multi-steps solid–liquid reactions that compliment transport processes. For contaminant barrier materials, reaction heterogeneity is promoted by variability in the mineralogy and grain size distribution of materials required in mix proportions to satisfy various design functions. The adsorption/desorption of contaminants in barrier systems is necessary for performance assessments. In this paper, the sorption/desorption rates of Pb2+ and Cd2+ on Na-montmorillonite ((OH)4Si8Al4O20nH2ONa) and kaolinite ((OH)8Si4Al4O10) were investigated by varying nature of aqueous contaminants and clay. Heavy metals including Pb2+ and Cd2+ were adsorbed/desorbed on the clays at pH levels of 2, 3, 4, and 7 for 48 h. Sorption/desorption of these heavy metals was found to be influenced by point of zero charge, cation exchange capacity, particle size, pH of the system, complexation of metals, and hydrated ionic radius of metals. As a result, Elovich equation was found to adequately explain the kinetics of Pb2+ and Cd2+ on two clay minerals with the squared correlation coefficients (R2) that range from 0.70 to 0.99. At pH 2, removal rate were 36.3% for Pb2+ and 64.5% for Cd2+ from Na-montmorillonite while 67.1% for Pb2+ and 75.0% Cd2+ from kaolinite, respectively. The results obtained from the Elovich equation could be useful for assessing the sorption equilibrium for the metals potentially attained in soil barrier systems.