Effects of lead and copper heavy metal contaminants on the geotechnical and mineralogical properties of bentonite, considering pH grade

Bentonite clay is frequently utilized in combination with sand as an effective barrier material for sealing and preventing the migration of contaminants in geo-environmental projects. This study investigates the impact of lead and copper nitrate contamination under varying pH levels and exposure durations on the geotechnical and mineralogical properties of bentonite, as well as the strength behavior of a sand-bentonite mixture with 20% bentonite content. To this end, changes in the pH and Atterberg limits of bentonite contaminated with the aforementioned pollutants were examined at initial pH values of 3, 4, and 6 over exposure periods of 0, 7, 14, and 28 days. X-ray diffraction (XRD) analysis was performed on contaminated bentonite samples to explore the correlation between the altered geotechnical behavior of the bentonite [the primary reactive component of the soil mixture] and its mineralogical changes. The results indicated that the presence of heavy metal contaminants lowered the soil pH, thereby increasing its acidity. Furthermore, as the soil acidity increased, the plasticity index (PI) decreased, with lead exerting a more pronounced effect on this reduction. Unconfined compressive strength (UCS) tests revealed that samples exhibited lower strength in more acidic environments; however, both the type of heavy metal and the resulting acidity levels influenced the magnitude of strength reduction. Finally, XRD analysis demonstrated that heavy metal contamination can alter the mineralogical composition of bentonite, with lead showing a significantly more substantial impact compared to copper.