Conversion of Waste Red Mud into LTA Zeolite Adsorbent for Highly Selective Cs ⁺ /Sr ²⁺ Removal: Governing Effects of Inorganic Acid Type and Hydrothermal Temperature on Zeolite Phase Evolution

Sustainable utilization of industrial solid waste and the recovery of value-added resources are critical challenges in environmental engineering. Red mud, a major byproduct of the Bayer process, contains abundant silicon and aluminum species and is a promising precursor for zeolite synthesis. In this study, LTA-type zeolites were synthesized without additional Si/Al sources using red mud acid-leaching solutions prepared with various inorganic acids. Optimal synthesis conditions for LTA-type zeolite were identified by adjusting NaOH concentration and hydrothermal conditions, and the effects of coexisting anions on LTA phase transformation and the resulting Cs + and Sr 2+ adsorption performance were evaluated. At low NaOH concentrations, nonzeolitic phases were observed, whereas 3–4 N NaOH provided suitable conditions for LTA crystallization. Hydrothermal temperature also significantly influenced phase selection, with pure LTA obtained at 60–90 °C, whereas higher temperatures promoted GIS-type frameworks. Under identical alkalinity and temperature conditions, LTA initially formed in all leachate systems, regardless of anion species. However, the transformation from metastable LTA to thermodynamically stable sodalite proceeded at markedly different rates depending on anion species, in the order HNO 3 > H 2 SO 4 > HCl. This phase transformation directly affected the adsorption of Cs + and Sr 2+ . The HCl-derived system maintained the LTA phase for up to 24 h, and the resulting LTA phase exhibited maximum adsorption uptakes of 1.84 mmol g –1 for Cs + and 1.58 mmol g –1 for Sr 2+ . In addition, the LTA phase demonstrated selective removal of Sr 2+ and Cs + in the presence of competing cations, following the selectivity sequence Sr 2+ > Cs + ≥ Ca 2+ > K + > Na + > Mg 2+ . In contrast, accelerated sodalite formation in the HNO 3 -derived system led to a decrease in adsorption efficiency because of its lower cation exchange capacity. These results demonstrate that coexisting anions act as kinetic modifiers governing phase transformation behavior and functional performance.