Polyphosphazene-based hyper crosslinked polymers for efficient uranium ion removal from nuclear wastewater†

This study focuses on the removal of uranium ions from nuclear wastewater by fabricating inorganic–organic hybrid cyclic and linear polyphosphazene based polymers. Synthesized HCP-A and HCP-B had BET surface areas of 497.06 m² g⁻¹ and 410.75 m² g⁻¹, respectively, while the pore size distribution (PSD) was in the range of 1–20 nm. The maximum removal efficiency of uranium by HCP-A and HCP-B for a lab prepared sample was found to be 97.6% and 95.2%, respectively, at pH 6, a contact period of 80 minutes, an adsorbent weight of 0.6 g, and a temperature of 25 °C, while for a nuclear wastewater sample, it was 83.9% and 79.8%, respectively. Lone pair–cation interactions, metal ligand complexation, hydrogen bonding, cation–pi interactions and electrostatic interactions were responsible for adsorption. The point of zero charge (PZC) for both HCPs was at pH 4.6. The optimal uranium uptake capacities of HCP-A and HCP-B were found to be 714.28 mg g⁻¹ and 555.56 mg g⁻¹, respectively. The Freundlich model was the best match for uranium adsorption by both HCPs, with R² values of 0.9775 and 0.9931, respectively. Adsorption kinetics study exhibited that it fitted a pseudo 2nd order kinetic model with R² values of 0.9446 for HCP-A and 0.9882 for HCP-B. The uranium uptake process was found to be spontaneous and exothermic in nature. For HCP-A and HCP-B, a Gibbs free energy (ΔG) of −1.516 kJ mol⁻¹ and −0.27 kJ mol⁻¹, enthalpy change (ΔH) of −41.59 kJ mol⁻¹ and −40.65 kJ mol⁻¹, and entropy change (ΔS) of −0.134 kJ mol⁻¹ K⁻¹ and −0.136 kJ mol⁻¹ K⁻¹, respectively, were observed. The reusability of HCPs with a minor decrease (2% and 1%) in their adsorption capability suggests that they can be used in industrial level applications.