Introduction
Groundwater contamination caused by mining activities represents one of the most critical environmental challenges in semi-arid and arid regions, particularly in countries such as Iran, where groundwater serves as the primary source for domestic, agricultural, and industrial use. Mining wastes, especially those generated from lead and zinc extraction, often contain elevated concentrations of potentially toxic heavy metals, including zinc, cadmium, lead, manganese, and cobalt. When exposed to water, these wastes produce leachate capable of migrating through soil layers and contaminating underlying aquifers. Due to the high mobility, persistence, and bioaccumulative nature of heavy metals, even small-scale leakage from mining waste disposal sites can result in long-term and often irreversible environmental impacts.
Engineered liner systems are widely recognized as one of the most effective measures for controlling leachate migration and protecting groundwater resources in waste disposal facilities. Compacted clay liners, in particular, are commonly used because of their low permeability, natural availability, and environmental compatibility. However, native clay soils often fail to meet both hydraulic and environmental performance requirements simultaneously, especially regarding sufficiently low permeability and adequate heavy-metal adsorption capacity. As a result, soil improvement techniques using chemical stabilizers and nanomaterials have gained increasing attention in recent years.
This study focuses on improving the performance of locally available clay soil used as a liner material at the Angouran lead–zinc mine waste disposal site in Zanjan Province, Iran. The main objective is to evaluate and compare the effects of lime, a conventional stabilizing agent, and nano-silica, a modern nanomaterial additive, on the geotechnical properties and heavy-metal removal efficiency of clay liners under realistic leachate exposure conditions.
Materials and Methods
Clay soil samples were collected from the mining waste landfill area of the Angouran mine. After drying and initial preparation, the soil was amended with different proportions of lime (2, 4, and 6 wt%) and nano-silica (0.5, 0.8, and 1 wt%). These amendment levels were selected based on previous studies and preliminary tests to ensure both practical feasibility and effective soil modification.
A comprehensive experimental program was conducted to assess changes in the geotechnical behavior of the soil. Atterberg limits tests were performed to evaluate variations in plasticity, while standard Proctor compaction tests were used to determine optimal moisture content and maximum dry density. Unconfined compressive strength (UCS) tests were carried out to assess mechanical stability, and falling-head permeability tests were conducted in accordance with ASTM standards to evaluate hydraulic conductivity.
To simulate real environmental conditions, actual mine leachate was collected from the Angouran waste disposal site and used as the permeating fluid in selected permeability and adsorption tests. Column experiments were designed to investigate heavy-metal adsorption behavior under dynamic flow conditions rather than conventional batch tests. Both untreated and treated clay samples were compacted into cylindrical columns and exposed to controlled leachate flow. Effluent samples were collected at regular time intervals, and concentrations of cadmium (Cd), lead (Pb), and zinc (Zn) were measured.
The adsorption kinetics of heavy metals were analyzed using pseudo-first-order, pseudo-second-order, and intraparticle diffusion models. These models were applied to identify the dominant mechanisms governing metal retention and to establish relationships between soil modification, hydraulic behavior, and adsorption performance.
Results and Discussion
The results of the Atterberg limits tests indicated that lime treatment significantly reduced the plasticity index of the clay soil, ultimately eliminating plastic behavior at higher lime contents. This effect is attributed to cation exchange and pozzolanic reactions between lime and clay minerals, leading to the formation of cementitious compounds and an improved soil structure. In contrast, nano-silica addition caused a moderate increase in liquid and plastic limits but resulted in a denser soil structure due to the filling of micro-voids by nano-sized particles.
Compaction test results showed that nano-silica-amended soils achieved a higher maximum dry density compared to untreated and lime-treated samples, reflecting reduced porosity and improved particle packing. Permeability tests demonstrated that the natural clay exhibited a hydraulic conductivity on the order of 10^{-8} cm/s. The addition of 0.8% nano-silica reduced this value to approximately 1.8 \times 10^{-9} cm/s, meeting the typical requirements for engineered landfill liners. Although lime treatment produced a slightly greater reduction in permeability at higher dosages, its performance under leachate exposure was less consistent.
Heavy-metal adsorption analyses revealed that nano-silica-amended clay exhibited substantially higher removal efficiency compared to both untreated and lime-treated soils. Kinetic modeling showed that the pseudo-second-order model provided the best fit for all metals studied, with coefficients of determination (R^2) exceeding 0.99. This finding indicates that chemisorption is the dominant mechanism controlling metal uptake. The formation of strong surface bonds between metal ions and silanol (OH–Si) functional groups played a crucial role in enhancing adsorption capacity.
Among the metals investigated, cadmium exhibited the highest adsorption capacity, reaching up to 99 mg/g in nano-silica-treated samples. Zinc and lead also showed significant removal, although their adsorption mechanisms differed. Smaller ions such as nickel and cobalt were primarily retained through ion-exchange processes, while heavier metals such as cadmium and lead were immobilized mainly through chemical precipitation and surface complexation. These results are consistent with previous international studies and confirm the superior environmental performance of nano-silica-modified clay liners.
Conclusion
The findings of this study demonstrate that the incorporation of nano-silica, particularly at a dosage of 0.8 wt%, significantly enhances the overall performance of clay liners used in mining waste disposal facilities. This modification improves mechanical stability, reduces hydraulic conductivity to acceptable levels, and substantially increases the capacity of the soil to retain toxic heavy metals from mine leachate.
Compared to lime stabilization, nano-silica provides a more balanced improvement by simultaneously addressing geotechnical and environmental requirements. As a result, nano-silica-amended clay liners represent a promising and sustainable solution for mitigating groundwater contamination in mining regions. The use of locally available clay combined with a relatively small amount of nano-silica offers both economic and environmental advantages.
Future research should focus on field-scale implementation and long-term performance monitoring under varying climatic and chemical conditions to further validate the durability and practical applicability of nano-silica-modified clay liners in real mining environments. Overall, the results highlight nano-silica–amended clay liners as a practical, low-cost, and environmentally sound solution that can be readily adopted in mining waste management strategies to safeguard groundwater resources in semi-arid regions.