Natural Calcite-Rich Limestone for Lead(II) Immobilization from Mining-Impacted Waters: Insights into the Removal Mechanism

Using naturally available and low-cost materials for the removal of toxic metals from contaminated waters has long been recognized as a promising and sustainable approach. This study evaluates the potential of calcite-rich natural limestone from the "Šuplja Stijena" stone deposit (Pljevlja, Montenegro) for Pb(II) removal from aqueous solutions and acid mine drainage. Batch experiments demonstrated that Pb(II) removal is strongly influenced by the solid-to-liquid ratio, particle size, pH, and initial metal concentration. The highest experimental uptake reached 310 mg/g at an initial Pb(II) concentration of 900 mg/L, with a removal efficiency of 86%. Kinetic analysis revealed two distinct removal stages, indicating that a single adsorption mechanism cannot adequately describe the overall process. Time-resolved ex-situ Fourier transform infrared and X-ray diffraction analyses showed that the transition between these kinetic stages is accompanied by the structural evolution of the sorbent, involving initial surface interactions followed by partial calcite dissolution, Ca2+/Pb2+ ion exchange and progressive cerussite (PbCO3) formation. Although the Langmuir and Freundlich models yielded high correlation coefficients, the obtained parameters lacked physical significance, suggesting that classical equilibrium adsorption isotherms provide only a limited description of this complex removal process. Experiments performed with real acid mine drainage further confirmed the applicability of the investigated limestone under environmentally relevant conditions. These findings demonstrate that locally available calcite-rich limestone overburden is an effective and inexpensive material for Pb(II) removal while providing mechanistic insight into the coupled processes governing metal immobilization.