Separable composite-based adsorbents for the removal of Cs(I) and Sr(II) from aqueous matrices

Cesium-137 (¹³⁷Cs) and strontium-90 (⁹⁰Sr), high-yield radionuclides generated during nuclear energy utilization, inevitably enter the environment through wastewater discharges from nuclear power and nuclear industrial activities, posing potential threats to ecosystems and human health. Therefore, the safe and effective removal of ¹³⁷Cs and ⁹⁰Sr from aqueous matrices has become an urgent priority. Various active adsorbents, including clay, zeolites, Prussian blue analogs (PBAs), ammonium phosphomolybdate (AMP), metal sulfides, and crown ethers, have demonstrated effectiveness in removing Cs⁺ and Sr²⁺ from aqueous matrices via adsorption techniques. However, their powdered forms pose significant limitations to practical applications in nuclear wastewater treatment. Increasing attention has been directed toward developing composite adsorbents by immobilizing these active materials onto organic or inorganic substrates. This review systematically summarizes recent advances in separable composite adsorbents formed by integrating active adsorbents (clay, zeolites, PBAs, AMP, sulfides, and crown ethers) and emerging materials (graphene oxide, metal-organic frameworks, and MXene) with organic/inorganic carrier materials, with particular emphasis on their structural characteristics, including bead, membrane, aerogel, and fiber forms. The application performance of these composites in Cs⁺ and Sr²⁺ removal is evaluated in terms of adsorption capacity, equilibrium time, selectivity, and reusability, accompanied by mechanistic analyses of their adsorption behaviors. Furthermore, the possible adsorption mechanisms of Cs⁺ and Sr²⁺ by these composite adsorbents are elucidated, mainly involving electrostatic interactions, surface complexation, ion exchange, and physical interactions. Finally, current challenges and opportunities in the development of composite adsorbents for Cs⁺ and Sr²⁺ removal are discussed, and future research directions are proposed for these advanced materials. This work provides valuable insights into the design and development of high-performance composite adsorbents for the remediation of wastewater containing Cs⁺ and Sr²⁺.