Silicoaluminophosphate-seeded Al-pair-enriched low-silica CHA zeolites for enhanced Sr2+ capture†

Abstract

Radioactive strontium-90 (⁹⁰Sr²⁺) in wastewater poses a significant threat to both the environment and living organisms. Conventional treatment strategies, such as ion-exchange resins followed by cement solidification, can still carry the risk of leakage under certain conditions. Low-silica zeolites have demonstrated strong cation sorption capabilities, with CHA zeolites showing particular promise for nuclear wastewater treatment. However, synthesizing low-silica CHA zeolites with Si/Al ratios around 2 typically requires fluorides or complex crystallization processes. In this study, we present a one-step, fluoride-free synthesis method for low-silica CHA zeolites using the silicoaluminophosphate (SAPO) zeolite SAPO-35 as the seed. The SAPO-seeded synthesis method enhances the formation of Al-pairs within the CHA framework by releasing partially connected Si and Al species from the SAPO seed. This significantly improves the zeolite's capability to capture the divalent Sr²⁺. The resulting zeolite exhibits a 10% higher Sr²⁺ sorption capacity per ion-exchange site compared to CHA zeolites synthesized without the SAPO seed. The synthesized zeolite exhibits exceptional Sr²⁺ removal efficiency across dosages of 1/50–1/500 g mL⁻¹ and the pH range of 3–12. At temperatures of 25 °C, 60 °C, and 80 °C, the sorption capacities reach 112 mg g⁻¹, 144 mg g⁻¹, and 186 mg g⁻¹, respectively. This work highlights the potential of SAPO-seeded synthesis as a practical and scalable approach for producing Al-pair-enriched, low-silica CHA zeolites, indicating the high effectiveness for removing ⁹⁰Sr²⁺ from nuclear wastewater and offering a promising solution for radioactive wastewater management.