Microwave vitrification of Sr-saturated magnetic AMP@SiO₂ for effective strontium removal in radioactive wastewater

The high mobility and long-lived radiotoxicity of ⁹⁰Sr pose persistent challenges in radioactive wastewater management. In this study, a closed-loop adsorption-vitrification strategy was developed to integrate Sr²⁺ capture, magnetic separation, microwave vitrification, and release prediction within a single treatment framework. A magnetic composite adsorbent (AMP@SiO₂, denoted as MAS) was prepared by the crystallization of ammonium molybdophosphate (AMP) within a porous SiO₂ matrix, followed by Fe₃O₄ functionalization, enabling rapid and selective Sr²⁺ uptake. The equilibrium adsorption behavior followed the Langmuir isotherm model, with a maximum adsorption capacity of 38.64 mg g^-1, while the adsorption kinetics conformed to the pseudo-second order model, with a high correlation coefficient (R² = 0.9903). After adsorption, the Sr-loaded adsorbent (Sr-MAS), with B₂O₃ and NH₄H₂PO₄ introduced as glass-formers, was directly converted in situ into iron-phosphate glass via microwave vitrification. The optimized sample (1300-N40), sintered at 1300 °C for 30 min, doped with 5 wt% B₂O₃ and 40 wt% NH₄H₂PO₄ and exhibited a dense and homogeneous amorphous structure, accompanied by high density (2.72 g cm^-3) and excellent chemical durability (normalized Sr²⁺ leaching rate of 4.19 × 10^-6 g m^-2·d^-1 after 42 days). In addition, Gaussian Process Regression (GPR) coupled with 200 bootstrap resampling iterations, was employed to model and predict the medium-term Sr²⁺ release behavior and quantify, the extrapolation uncertainty of the long-term release curves. This study provides a scalable route for the minimization of secondary waste and safe immobilization of radioactive strontium in aqueous radioactive effluents.