Interaction of Natural Heulandite–Clinoptilolite Zeolite with Real Acid Mine Drainage: Metal Removal, pH Evolution, and Mineralogical–Microchemical Transformations

Acid mine drainage (AMD) is characterized by low pH and elevated dissolved Cu and Pb concentrations, requiring effective treatment while minimizing secondary waste generation. This study investigates the interaction of natural heulandite–clinoptilolite zeolite with real AMD, focusing on metal removal, solution chemistry, and post-treatment mineralogical and microchemical transformations. Batch experiments used 3.5, 7.5, and 15.0 g of zeolite per 100 mL of AMD, with Cu and Pb removal monitored for 360 min. Initial AMD pH was 3.6 and increased rapidly, stabilizing at approximately 4.3, 5.2, and 6.2 with increasing zeolite dosage. Cu and Pb were rapidly removed, with near-complete removal of Cu and Pb at the highest dosage within 45 min. Kinetic analysis showed that the pseudo-second-order model generally provided the best empirical fit, while Weber–Morris analysis indicated that intraparticle diffusion was not the sole rate-controlling mechanism. X-ray diffraction showed that the principal heulandite–clinoptilolite crystalline phases remained identifiable after treatment, although reflections compatible with Cu-exchanged clinoptilolite and Ca–SO4-bearing phases were also detected. SEM-EDS revealed localized Cu-rich regions containing up to 52.09 wt.% Cu and localized Fe-rich domains in the post-treatment material. Although Pb was efficiently removed from solution, it was not detected in the analyzed post-treatment crystalline or localized solid phases. Overall, natural heulandite–clinoptilolite substantially modified AMD chemistry and removed dissolved Cu and Pb while the principal zeolitic crystalline phases remained identifiable after treatment and localized chemical transformations were detected.