Effect of bentonite initial pH on selectivity of heavy metals in single and composite systems

Usually, the produced leachate at landfills contains several types of heavy metal contaminants. These heavy metal contaminants have been applied to the soil liner of landfills in combination with each other or sequentially. Accordingly, heavy metals are in competition with each other for sorption and retention by soil liner of landfills. In general, clayey soils, such as bentonite that is commonly used to prevent contaminant transportation in landfills, have different sorption preferential, which is called selectivity. On the other hand, there is a probability of changing the pH of soil liners of landfills by acidic conditions of leachates. This change in pH, affects the retention mechanisms of heavy metals by clay soils due to the change in physical and geo-environmental properties of the soil. In spite of several researches which have been conducted on interaction mechanism of bentonite and heavy metals, there is a lack of study on selectivity of heavy metals and bentonite in single and composite systems. The main objective of this research is to study and to compare the effect of initial pH of bentonite on its selectivity order in single species and composite species of heavy metals. To achieve this objective, the pH of the bentonite samples was changed and was stabilized, at different pHs by addition of concentrated hydrochloric acid to the bentonite suspension. Then, the interaction of these bentonite samples, which had different initial pHs, with lead and zinc in single and composite systems has been studied. This interaction has been investigated from theoretical and experimental point of view. The research results show that, the selectivity of heavy metals by bentonite samples with acidic pH of 2 and 4 in single and composite systems are relatively similar. However, the selectivity percent by natural bentonite (pH:10.5) in composite system relative to the single system was increased. Furthermore, the quantity of retention of lead and zinc in bentonite sample with initial pH of 2 in comparison with retention capability of natural bentonite (initial pH of 10.5) has decreased 60 and 70 percent, respectively. The noticeable reduction in heavy metal retention by acidic bentonite provides a major challenge in acceptance of bentonite as an appropriate material for contaminant adsorption. As the pH of bentonite decreases to acidic conditions, the impact of selectivity adsorption of bentonite at the presence of lead and zinc ions decreases which means a better correspondence with double layer theory. Furthermore, the pattern of lead and zinc retention by bentonite samples with initial pH of 4 and 10.5 in conjunction with the peak intensity variations of montmorillonite in bentonite XRD pattern in those initial pHs show that the largest retention capability of bentonite for zinc retention occurs at 100 cmol/kg-soil. However, the largest retention capability of bentonite for lead retention occurs at 200 cmol/kg-soil. Finally, based on the results of this research, from microstructural point of view the lead ions due to the more interaction with clay double layer have more influence upon microstructural properties of montmorillonite. Such an interaction may affect the extent of contaminant transport in soil.