Recovery of strontium (Sr2+) from seawater using a hierarchically structured MnO2/C/Fe3O4 magnetic nanocomposite

Abstract

The recovery of strontium ions (Sr2+) from seawater has attracted much attention as an approach to securing Sr resources to meet increasing industrial demand. In this study, we synthesized a magnetic MnO2 nanocomposite (MnO2/C/Fe3O4) using a simple redox reaction under ambient conditions and applied this nanocomposite to the extraction of Sr2+ from natural seawater. The synthesized nanocomposite exhibited a hierarchical structure of MnO2, carbon, and Fe3O4 with a saturation magnetization of 25 emu/g that enabled effective separation under an external magnetic force. Regardless of the initial Sr2+ concentration, the adsorption of Sr2+ onto MnO2/C/Fe3O4 proceeded rapidly (within <10 min) following a pseudo-second-order kinetic model and agreed well with the Langmuir isotherm model, indicating a maximum adsorption capacity of 42 mg/g. Among the competitive ions, Mg2+ and Ca2+ significantly hindered Sr2+ adsorption onto MnO2/C/Fe3O4, whereas Na+ and K+ had little effect on Sr2+ adsorption. A detailed study of the distribution coefficient (Kd) revealed 11.2-fold and 1.8-fold higher selectivities toward Sr2+ than toward Mg2+ and Ca2+, respectively. The Sr2+ ions adsorbed onto MnO2/C/Fe3O4 were completely recovered through desorption in 0.1 M HCl eluent. Finally, a Sr2+-enriched solution with a concentration of 501 mg/L could be obtained via seawater adsorption and subsequent acid desorption over 8 iterative cycles, demonstrating its effectiveness for practical applications of Sr2+ recovery from seawater.