Alkali-activated magnetic red mud for Sr²⁺ adsorption and microwave vitrification into stable iron phosphate glass

Alkali-fused red mud (RM) was modified into a magnetic adsorbent (MRM) for Sr²⁺ and subsequently immobilized by microwave vitrification into a durable iron phosphate glass (IPG) in this study. The effects of alkali fusion-activation on the phase composition, chemical state, and magnetic properties of RM were systematically investigated. Optimal alkali fusion-activation conditions were identified as an NaOH-to-RM mass ratio of 0.3, a fusion temperature of 600 °C, and a holding time of 1.5 h. Under these conditions, MRM achieved a maximum saturation magnetization of 10.26 emu·g^-1. Adsorption kinetics and isotherm analyses indicated a monolayer chemisorption mechanism with a maximum Sr²⁺ capacity of 45.84 mg g^-1 via ion exchange. During vitrification, Sr was homogeneously incorporated into the IPG matrix without enrichment. The vitrified products exhibited excellent chemical durability, with a normalized Sr²⁺ leaching rate as low as 10^-6 g m^-2·d^-1 after 42 d. This study demonstrated that magnetic RM, coupled with microwave vitrification, provides an efficient and durable strategy for treating of Sr²⁺-containing radioactive wastewater.