Effects of lead contaminants on engineering properties of Iranian marl soil from the microstructural perspective

•Pore fluid concentration strongly affect the geotechnical properties of marl soils.•High retention ability of lead contaminants by marl soil is due to their high carbonate content.•Marl soils can retain lead contaminant up to 200 cmol/kg soil. Most of the industries in the south of Iran and north of the Persian Gulf are built on marl bedrocks and these industries are major sources of heavy metal pollutants. Therefore, the engineering properties of marl soils contaminated with heavy metals (particularly lead ions) are worthy of thorough investigation. This was accomplished by artificial contamination of marl soil with varying lead concentrations and measurment of some geotechnical (granularity, Atterberg limits, compressive strength, and permeability) and geo-environmental (CEC, pH, and contaminant retention) characteristics. In addition, with the aid of X-ray diffraction (XRD) patterns and scanning electron microscopy (SEM) images the changes in mineralogy and microstructural behavior of lead-contaminated soil were investigated. According to the findings of the present study, marl soil can hold about 200 cmol/kg-soil of lead concentration. As seen by SEM images, the lead precipitation changed the soil's geotechnical properties by altering the structure and texture of soil (notably flocculation). At 100 cmol/kg-soil lead concentration, the compressive strength was reduced by about 85%, while the hydraulic conductivity increased up to 170 times. Adding lead to the marl soil decreased the intensity of X-ray diffraction peaks for the main clay minerals and created new peaks associated with precipitated cerussite, shannonite, and leadhillite.