Microstructural Investigation of Lead Ion Interaction with Sodium and Calcium Bentonite under Various Pore Fluid Conditions

This study investigates the microstructural interaction between the heavy metal contaminant lead and two types of bentonite: sodium bentonite and calcium bentonite. Calcium bentonite samples were prepared through homoionic treatment using calcium chloride followed by six stages of deionized water washing. Subsequently, these samples were modified with different concentrations of sodium chloride to assess the influence of monovalent and divalent cations in the pore fluid on the retention capacity of lead.
The treated bentonite samples were exposed to various concentrations of lead nitrate, and atomic absorption spectroscopy (AAS) and X-ray diffraction (XRD) analyses were performed. The results revealed that the calcium bentonite sample, subjected to six washes and modified with 100 cmol/kg-soil NaCl, exhibited the highest retention of lead, reaching complete retention up to the cation exchange capacity (CEC). At higher lead concentrations, this sample maintained superior performance compared to other configurations.
XRD results indicated that treated calcium bentonite displayed enhanced peak intensities, reflecting a more dispersed and homogeneous microstructure due to the presence of sodium ions. These structural characteristics allowed for better lead ion accommodation between clay platelets. Additionally, lead contamination led to a decrease in the plasticity index of the samples, although all remained within the CH (high plasticity clay) classification range.
Overall, the findings highlight the importance of both the type and concentration of cations in the pore fluid in optimizing the use of bentonite in engineered barriers for the containment of heavy metal pollutants in geoenvironmental applications.