Photoinduced enhancement of ¹³⁷Cs removal by NiFe Prussian blue analogue-alginate hydrogel

Prussian blue analogues are promising materials for ¹³⁷Cs removal; however, their use is limited by the challenge of separating them from an aqueous solution after ¹³⁷Cs adsorption. This study synthesized a NiFe embedded alginate hydrogel (NiFe-AH) via cross-linking with Ni²⁺. The prepared NiFe-AH exhibited an excellent performance for Cs⁺ adsorption. The adsorption capacity of NiFe-AH under UV (0.891 mmol/g) or visible light irradiation (0.885 mmol/g) increased approximately 1.5 times more than before light irradiation (0.591 mmol/g). Moreover, the volume and weight of NiFe-AH after Cs⁺ adsorption decreased substantially, showing advantage in radioactive nuclide disposal. NiFe-AH exhibited high Cs⁺ adsorption capacity in all pH ranges, even in the presence of competing ions such as Na⁺, K⁺, Ca²⁺, and Mg²⁺. Compared to commercialized Cs⁺ adsorbents (AMP-PAN and KNiFe-PAN), NiFe-AH proved to have a greater Cs⁺ adsorption capacity. Analyses using XPS and XANES indicated that the photoinduced enhancement mechanism of Cs⁺ adsorption on NiFe-AH was attributed to NiFe absorbing the light, resulting in the reduction of Fe(III) to Fe(II) within NiFe. Accordingly, Cs⁺ ions were additionally adsorbed on NiFe. The NiFe-AH showed exceptional ¹³⁷Cs removal in seawater, suggesting that it has strong potential as a practical adsorbent.