Assessment of Lead and Zinc Absorption in Single-component and Binary-component Systems of Contamination by Kaolinite Clay

Nowadays, the environmental pollutions caused by the increase in population and the development of industries threaten the health of the inhabitants of the planet. The most important pollutants of water and soil resources are heavy metals that lead and zinc are the most abundant elements among them. The presence of large amounts of heavy metals in the soil causes groundwater pollution and eventually the occurrence of many dangerous diseases including cancer, digestive disorders, kidney diseases, mental retardation and blood and brain diseases. The movement of water in the surface and underground streams is one of the main causes of the release of heavy metals in the soil which causes transfer of pollutions from the contaminated soil to the surroundings and entrance of these harmful metals into the human diet. The main purpose of this study was to evaluate the selective absorption of lead and zinc by Kaolinite clay in single and binary-component systems. For this purpose, the Kaolinite clay was firstly mixed with the lead and zinc in distilled water at ratio of 1:20 to prepare model contaminated suspensions. Three specimens were prepared for each sample according to the EPA (1983) and EPA (2010) methods. The effect of different pH (i.e. 2 to 12) and pollutants concentrations (i.e. at concentration of 20 Cmol kg-1) on the adsorption capacity of Kaolinite was measured using spectrophotometer. It was found that by decreasing in pH to 2 in both single and binary component pollutant systems (at concentration of 20 Cmol kg-1), the adsorption percent decreased significantly. However, the adsorption of lead in pore fluid was higher than zinc in alkaline and acidic conditions for both binary and single-component systems. The adsorption percent of Pb2+ was not changed in both systems, however, adsorption of Zn2+ was significantly reduced in the binary system-component compared to the single system-component. In addition, the adsorption percent of Zn2+ in alkaline (pH=12) and acidic (pH=2) conditions decreased by about 50 and 20% in the binary system-component compared to the single system-component, respectively. Furthermore, the findings indicated that Langmuir isotherm had the highest consistency in the single and binary-component systems for lead and zinc contaminations at the studied concentration. The results of absorption kinetics in both systems illustrated that the lead absorption rate was decreased 0.15% in binary-component system compared to the single-component system at concentration of 20 Cmol kg-1. In contrary, the zinc absorption rate in the combination system was reduced 87.5% at this concentration. On the other hand, the adsorption kinetics of lead metal at the test concentration for kaolinite clay in both systems had the same constant as the pseudo-first and second order equations because in both systems the amount of lead adsorption did not change. Instead, the rate of adsorption on zinc had a significant change in the binary system compared to the single state. the resulting values were more consistent with the pseudo- second order equation. In general, the ion adsorption of Pb2+under all environmental conditions in the pore fluid was more than Zn2+ in both systems due to the lower value of the first hydrolysis constant (pk1) of lead compared to zinc (i.e. Pb(7.8) > Zn(9.0)). By comparing the values of k1 and k2 in the concentration of pollution in the experiment, it was found that k1 of lead and zinc metals for a concentration of 20 Cmol kg-1 was higher than k2  and this represented the higher reaction rate to the Kaolinite clay for lead compared to zinc.