Dynamic hydrogen-bond-driven thorium ion traps in sodium alginate/tannic acid beads

The development of efficient and selective adsorbents for capturing thorium (Th(IV)) ions from wastewater is imperative in the environmental remediation and resource recovery fields. However, conventional adsorbents suffer from low affinity for Th(IV) ions, complex synthesis routes, and restricted exposure of active sites. Herein, a low-cost sodium alginate (SA)/tannic acid (TA) composite biomass beads were synthesized via a green and scalable CaCl₂-induced gelation method. In addition to coordination with Ca(II) ions, TA was also immobilized in the beads via hydrogen-bond interactions with SA. Moreover, this specific dynamic hydrogen-bond interaction among the beads enabled TA to form an optimized adsorption configuration together with SA, thereby facilitating the strong and selective binding of Th(IV) ions. The composite beads showed the adsorption equilibrium time at 120 min and attained the uptake capacity of 497.4 mg g^-1, outperforming most of reported biomass-based adsorbents. Moreover, the beads showed good selectivity toward Th(IV) ions against competing ions and satisfying reusability of > 90% removal efficiency after 12 adsorption-desorption cycles. Notably, the SA/TA beads could realize the column separation of Th(IV) ions from mixed ion solution with a dynamic uptake of 111.8 mg g^-1 and display the separation factors above 100 from binary ion dynamic separation models. More importantly, continuous column adsorption-elution experiments showed the potential of SA/TA beads to recover thorium resource from wastewater with economically feasible ThO₂ products (USD 4.41 per kilogram). Overall, this work provides a sustainable and feasible strategy for selective and effective Th(IV) ion separation from water, with broad implications for wastewater treatment and radioactive resource recovery.