A research team from Hainan University and Northumbria University has developed a novel polyelectrolyte composite hydrogel that addresses long-standing challenges in uranium extraction from high-salinity marine environments. The study, published in Engineering, presents a polyvinylphosphonic acid (PVPA) and polyamidoxime (PAO) composite hydrogel designed specifically for uranium recovery from concentrated seawater brines generated by solar saltworks and desalination operations.

Uranium extraction from seawater represents a promising strategy to alleviate global uranium scarcity, as the ocean contains an estimated 4.5 billion tons of uranium, approximately 1000 times the amount found in terrestrial ores. However, conventional polyamidoxime hydrogels exhibit salt-induced shrinkage under high-salinity conditions, compromising functional group accessibility and adsorption efficiency. Concentrated seawater brine, a byproduct of salt production and desalination, contains 2–10 times more uranium than natural seawater, yet its complex physicochemical environment poses significant technical challenges.

The researchers developed the PVPA–PAO hydrogel by physically cross-linking the network through electrostatic interactions between oppositely charged polyelectrolyte segments. Under high-salinity conditions, cations and anions accumulate via diffusion around the positively charged amidoxime and negatively charged phosphonic acid groups, weakening interchain electrostatic attractions. This anti-polyelectrolyte effect promotes hydrogel swelling, significantly improving the exposure of binding sites and uranyl ion uptake.

After 24 days of immersion in concentrated natural seawater derived from solar saltworks, the PVPA–PAO hydrogel achieved a uranium adsorption capacity of 43.89 mg/g, significantly surpassing that of previously reported PAO hydrogels (∼10 mg/g). The hydrogel also exhibited a high antibacterial rate of 99.94%, and its adsorption capacity in open seawater decreased by only 6.29% compared to filtered seawater.

The anti-polyelectrolyte effect arises when ions from the surrounding medium diffuse into the network and interact with the charged functional groups on the side chains, increasing interchain spacing and manifesting as swelling at the macroscopic level. Quantum chemical studies using density functional theory revealed that the uranium center is coordinated to two oxygen atoms from the phosphonic acid groups and to one oxygen and one nitrogen atom from the amidoxime groups.

The PVPA–PAO hydrogel demonstrated excellent mechanical properties with a fracture stress of ∼352 kPa and a fracture strain of ∼1158%, superior to the PAO hydrogel’s ∼217 kPa and ∼793%. After salt–alkali pretreatment, the PVPA–PAO sample exhibited the most significant swelling, with its area increasing more than six-fold compared with the initial state. The hydrogel maintained structural integrity and mechanical stability even under harsh conditions up to 5 mol/L NaCl.

The research team noted that integrating uranium extraction technologies with existing industrial processes such as solar saltworks and seawater desalination offers a promising route to enhance extraction efficiency and reduce operational costs. The work reflects a broader shift toward the synergistic development and utilization of multiple seawater-derived resources, enhancing resource utilization efficiency and added value.

The paper “Anti-Polyelectrolyte-Effect Hydrogel Unlocks Efficient Uranium Extraction from Concentrated Seawater,” is authored by Hui Wang, Feng Gao, Taohong Xu, Peng Liu, Zhanhu Guo, Guanbing Zhou, Yihui Yuan, Ning Wang. Full text of the open access paper: https://doi.org/10.1016/j.eng.2025.11.024. For more information about Engineering, visit the website at https://www.sciencedirect.com/journal/engineering.