Investigation of the impact of heavy metal lead on the geotechnical and environmental properties of marl soils with a special focus on desorption behavior in industrial areas

• Marl soil adsorbs lead up to 100 cmol/kg • Lead exposure increased permeability and reduced Atterberg limits • XRD and SEM revealed new lead phases • Desorption decreases as lead concentration increases • Acidic conditions cause partial lead release from soil The expansion of industrial activities has led to the release of heavy metals, such as lead, into the environment through industrial wastewater and leachates, resulting in soil contamination. Marl soils, which are widely distributed in regions like southern Iran, are characterized by their high carbonate content and the presence of clay minerals such as sepiolite and palygorskite. These components give marl soils unique geotechnical and environmental properties, including a high capacity for contaminant retention. However, marl soils also exhibit a significant potential for pollutant desorption, which poses environmental risks. This study investigates the interaction of lead contamination with marl soils, focusing on adsorption and desorption behaviors and their impact on engineering properties. A series of experiments, including Atterberg limits, permeability coefficient, adsorption, X-ray diffraction (XRD), Toxicity Characteristic Leaching Procedure (TCLP), scanning electron microscopy (SEM), pH, and electrical conductivity (EC) tests, were conducted to evaluate the effects of lead contamination. The results demonstrated that marl soil can fully adsorb lead up to a concentration of 100 cmol/kg-soil. XRD and SEM analyses revealed the formation of new phases, such as cerussite (PbCO₃), shannonite (Pb₂O(CO₃)), and leadhillite (Pb₄(SO₄)(CO₃)₂(OH)₂), indicating complex adsorption mechanisms involving carbonate precipitation and cation exchange. SEM images showed a transition from fibrous and needle-like structures to aggregated and granular forms with increasing lead concentration, which directly influenced the increase in permeability (from 3.5 × 10⁻⁷ cm/s to 4.97 × 10⁻⁶ cm/s) and the reduction in Atterberg limits (liquid limit decreased from 40% to 25%, and plastic limit decreased from 21% to 14%). TCLP tests showed that lead concentrations remained within the permissible limit for industrial environments (200 ppm) at soil concentrations of 50 cmol/kg or lower. However, the percentage of desorbed lead was higher at lower concentrations and gradually decreased as the concentration increased, due to a shift in the dominant mechanism from surface adsorption and carbonate precipitation to cation exchange and microstructural entrapment. Despite its high adsorption capacity, marl soil releases a portion of the adsorbed lead under acidic conditions, emphasizing the importance of understanding desorption behavior and controlling pH to manage lead mobility in contaminated sites. The findings of this study provide critical insights into the adsorption and desorption mechanisms of lead in marl soils, offering valuable data for improving contamination management in industrial areas and landfills.