Freezing–Thawing Strengthened PVA/CS Hydrogel Microspheres for Efficient Removal of Sr²⁺ Ions from Aqueous Solution

Among the nuclides in radioactive wastewater, strontium (90Sr) is a highly toxic water-soluble fission product, which will be extremely harmful to the ecological environment and consequently bring great risk to human health. Therefore, the removal of the strontium element from the aquatic environment is increasingly attracting the attention of the scientific community. In this paper, using poly(vinyl alcohol) (PVA) and chitosan (CS) as raw materials and glutaraldehyde as a cross-linking agent, a kind of interpenetrating PVA/CS hydrogel microsphere was prepared by a facile reversed-phase emulsion method. On this basis, the mechanical strength and stability of the hydrogel microspheres were further enhanced by a freezing–thawing technique. It was discovered that synthetic hydrogel microspheres can effectively remove Sr2+ ions from radioactive wastewater. The influences of the mass ratio of PVA to CS, initial concentrations of Sr2+, pH value, and adsorption times, along with competitive metal ions and nuclides, on the adsorption performance of the hydrogel microspheres were further systematically investigated. Especially, it was found that the freezing–thawing process significantly improved the adsorption capacity of the hydrogel microspheres for strontium ions, and the maximum adsorption capacity of the PVA/CS hydrogel microspheres for Sr2+ was as high as 205.45 mg/g, which was superior to the results of most similar adsorbents at present. Especially, the PVA/CS hydrogel microspheres still maintained satisfactory adsorption capacity and a high degree of sphericity after seven cycles. Furthermore, through the analysis of kinetics and thermodynamics of the adsorption process, a plausible adsorption mechanism was proposed, which will be helpful for designing and constructing adsorbents to remove radioactive ions from aquatic environments.