Strontium removal in seawater by means of composite magnetic nanoparticles derived from industrial sludge

Novel composite magnetic nanoparticles (CuFe₂O₄) were synthesized from industrial sludge by acid leaching, chemical exchange, and ferrite processes in the laboratory. For the first time, these products were applied to investigate the Sr adsorption kinetics and the related thermodynamics in seawater. Rapidly enhanced Sr adsorption was observed when the solution pH changed from 2.61 to 10.25. The maximum adsorption capacity was 23.04 mg·g^-1 at 318 K (pH 10.25). Sr adsorption decreased with the increase of the ionic strength from 0.01 to 0.5 mol·L^-1 at pH 2.61-pH 10.25, indicating that the outer-sphere mechanism was involved in the Sr adsorption at the pH interval. This reaction is spontaneous and endothermic, as indicated by the negative change in the standard free energy (ΔG° = -5.68, -6.45, and -7.23 kJ·mol^-1 at 298, 308, and 318 K, respectively) and positive ΔH° value (2.11 kJ·mol^-1). The positive ΔS° (9.38 mol^-1·K^-1) further confirms that the randomness increased at the solid-solution interface during adsorption. These new results indicate that the composite magnetic nanoparticles can be used for the removal of radiogenic 90Sr nuclide in seawater that was released after the 3/11 earthquake offshore of Japan.