Interaction mechanism of uranium(VI) with chitosan hydrogel: Insights from the perspective of adsorbent and adsorbate

Chitosan-based hydrogels have been used as adsorption materials and offered remarkable prospects, due to their abundant, biodegradable, inexpensive features and rich amino and hydroxyl groups. In this work, the structure of chitosan (CS) hydrogel and its adsorption performance toward uranyl species (UO₂²⁺ and UO₂(CO₃)₃⁴⁻) were comprehensively investigated on the basis of various characterizations and theoretical simulation. Systematic experiments show that CS hydrogel bears excellent adsorption performance for low-concentration UO₂(CO₃)₃⁴⁻ solution in column adsorption experiment and the removal efficiency is >99 %. The outer-sphere complexation adsorption has been postulated for the case of UO₂(CO₃)₃⁴⁻, where intermolecular hydrogen bonds dominate in the adsorption. Differently, the inner-sphere one is attributed for capturing UO₂²⁺ species, which is coordinated by C[dbnd]N, -NH₂ and -OH groups of CS hydrogel. Since the 3D network structure of CS hydrogel is prone to couple with water, it is difficult for U(VI) to enter inside the hydrogels, which reduces the adsorption performance. However, this phenomenon has been effectively weakening by using column adsorption experiment, which accomplished the exchange between water and U(VI). These detailed investigation toward adsorption mechanism might open a doorway to gain deeper insights on the design and application of efficient adsorption materials for uranium extraction.