Rapid OSDA-Free Synthesis of Al-Pair-Rich CHA Zeolite via SAPO-56 Seeding for Highly Efficient Pb²⁺ and Cd²⁺ Removal

Industrial wastewater containing toxic heavy metals, particularly Pb²⁺ and Cd²⁺, poses severe risks to ecosystems and human health. Low-silica CHA zeolites are promising adsorbents due to their high surface area and abundant cation-exchange sites, while increased framework Al pairing further enhances affinity for divalent metal ions. However, the rapid and environmentally benign synthesis of Al-pair-rich CHA zeolites remains challenging. Herein, a low-silica CHA zeolite with a high Al-pair content (up to 86%) was synthesized within 110 min by introducing SAPO-56 seeds. The process is organic structure-directing agent (OSDA)-free, fluoride-free and requires no aging step, representing one of the fastest hydrothermal routes for CHA zeolites. Seed dosage strongly influenced crystallization behavior and physicochemical properties. The CHA sample prepared with 20 wt % SAPO-56 seeds (CHA-20) exhibited a high Al-pair density and enhanced surface area. Its sodium form (Na-CHA-20) showed outstanding Pb²⁺ and Cd²⁺ removal efficiencies (>99%) at 100 mg/L, even at very low solid-to-liquid ratios. Na-CHA-20 operated effectively over a wide pH range (3-8), maintained high selectivity in the presence of competing cations, and achieved equilibrium within 2.5 min. High adsorption capacities of 625 mg/g for Pb²⁺ and 198 mg/g for Cd²⁺ were obtained, highlighting strong potential for wastewater treatment applications.