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.