A novel and cost-effective synthesis of magnetic zeolite 4A using kaolinite and red mud for Sr(II) removal

A novel and cost-effective approach was successfully developed for synthesizing high-performance zeolite 4A encapsulating magnetic particles (magnetic zeolite 4A) using kaolinite and red mud as raw materials via one-step combined activation-reduction and subsequent hydrothermal synthesis. The concurrent reduction of iron compounds in red mud to magnetic particles mainly composed of Fe₃O₄ during the thermal activation of kaolinite eliminated the need for a distinct red mud reduction process, thereby streamlining the synthesis and maximizing efficiency. The resulting product comprised highly crystalline zeolite 4A encapsulating magnetic particles derived from the red mud, avoiding oxidation and corrosion of the magnetic material in acidic solutions. A series of magnetic zeolite 4A (MZ) were obtained by introducing varying percentages of red mud and kaolinite, and their morphological, structural, magnetic and adsorption properties were investigated. The BET specific surface area of the obtained MZ-20 is 13.6 m²/g. MZ-20 (red mud: kaolinite mass percentage is 20%) displayed the maximum adsorption capacity of 172 mg/g for Sr(II). Within 1 h, 96.4% of the Sr(II) was removed. The adsorption kinetics and isotherms followed the pseudo-second-order and Langmuir models, respectively. Using a mixed 0.1 M NaCl and 0.1 M HCl solution as eluent, the magnetic adsorbent maintained high desorption and regeneration ratios along with stable magnetic properties after four adsorption-desorption cycles. In summary, the MZ-20 adsorbent exhibited high adsorption capacity, recyclability and excellent magnetic properties, making it a decontamination candidate for removing Sr(II) from radioactive wastewater.