Highly stable and magnetically separable alginate/Fe₃O₄ composite for the removal of strontium (Sr) from seawater

In this study, a highly stable alginate/Fe₃O₄ composite was synthesized, and systematically investigated for the practical application of strontium (Sr) removal in complex media, such as seawater and radioactive wastewater. To overcome the drawbacks of the use of alginate microspheres, high contents of alginic acid and Fe₃O₄ were used to provide a more rigid structure with little swelling and facile separation, respectively. The synthesized composite was optimized for particle sizes of <400 μm and 1% content of Fe₃O₄. The alginate/Fe₃O₄ composite showed excellent Sr uptake (≈400.0 mg/g) and exhibited outstanding selectivity for Sr among various cations (Na, Mg, Ca and K). However, in diluted Sr condition (50 mg/L), Ca significantly affected Sr adsorption, resulting in a decrease of K_d value from 3.7 to 2.4 at the 0.01 M Ca. The alginate/Fe₃O₄ composite could be completely regenerated using 0.1 M HCl and CaCl₂. In real seawater spiked with 50 mg/L of Sr, the alginate/Fe₃O₄ composite showed 12.5 mg/g of Sr uptake, despite the highly concentrated ions in seawater. The adsorption experiment for radio-active ⁹⁰Sr revealed a removal efficiency of 67% in real seawater, demonstrating the reliability of the alginate/Fe₃O₄ composite.