Synthesis of MOR Zeolite from Natural Stellerite and Clinoptilolite for Efficient Pb²⁺ and Cd²⁺ Removal from Aqueous Solutions

Zeolites are effective in removing heavy metal cations from water, particularly Pb2+ and Cd2+. Among them, mordenite (MOR) is paid special attention for its high adsorption capacity and fast kinetics. Synthesizing MOR from natural minerals offers a cost-effective solution for removing Pb2+ and Cd2+. However, the high-temperature activation required for natural minerals has limited their application. Herein, we developed a safe, facile, and energy-efficient method for activating natural stellerite (STI) and clinoptilolite (HEU) at 200 °C, producing OSDA-free MOR zeolites, designated as S-MOR and C-MOR, respectively. Adsorption tests showed that at solid-to-liquid ratios of 1/2000 and 1/500, both S-MOR and C-MOR removed over 99% of Pb2+ and Cd2+ from solutions containing 100 mg·L–1 of each metal. Remarkably, both materials maintained removal efficiencies above 80%, even with competing ions at concentrations 100–1000 times higher. Furthermore, the resulting MOR zeolites exhibited a broad working pH range (4–7), high maximum adsorption capacities (375.94 mg·g–1 for Pb2+ and 197.63 mg·g–1 for Cd2+), and significant distribution coefficients (4.8 × 106 mL·g–1 for Pb2+ and 1.2 × 106 mL·g–1 for Cd2+). Adsorption kinetics were notably fast, achieving equilibrium within 45 min for Pb2+ and less than 5 min for Cd2+, outperforming most known adsorbents.