Hyper-crosslinked tetraphenylborate as a regenerable sorbent for Cs⁺ sequestration in aqueous media through cation-π interactions

Practical adsorbents that could efficiently collect radioactive Cesium (Cs⁺) are critically important in achieving proper management and treatment measures for nuclear wastes. Herein, a hyper-crosslinked tetraphenylborate-based adsorbent (TPB-X) was prepared by reacting TPB anions as Cs⁺ binding sites with dimethoxymethane (DMM) as crosslinker. The most efficient TPB-X synthesis was attained at 1:4 TPB/DMM mole ratio with sorbent yield of 81.75%. Various techniques such as FTIR, TGA-DTG, N₂ adsorption/desorption and SEM-EDS reveal that TPB-X is a water-insoluble, thermally stable and highly porous granular sorbent. Its hierarchical pore structure explains its very high BET surface area (1030 m² g^-1). Sequestration of Cs⁺ by TPB-X involves its exchange with H⁺ followed by its binding with the phenyl rings of TPB through cation-π interactions. The Cs⁺ adsorption in TPB-X is endothermic and spontaneous, which adheres to the Hill isotherm model (q_m = 140.58 mg g^-1) and follows pseudo-second order kinetics (k₂ = 0.063 g mg^-1 h^-1). Calculations from the density functional theory reveal that the binding of TPB anion is strongest for Cs⁺. Thus, TPB-X was able to selectively capture Cs⁺ in simulated surface water containing Na⁺, K⁺, Mg²⁺, and Ca²⁺ and in HLLW containing Na⁺, Rb⁺, Sr²⁺, and Ba²⁺. Hyper-crosslinking was found beneficial in rendering TPB-X reusable as the sorbent was easily retrieved from the feed after Cs⁺ capture and was able to withstand the acid treatment for its regeneration. TPB-X exhibited consistent performance with no sign of chemical or physical deterioration. TPB-X offers a practical approach in handling Cs⁺ contaminated streams as it can be repeatedly used to enrich Cs⁺ in smaller volume of media, which can then be purified for Cs⁺ reuse or stored for long-term natural Cs⁺ decay process.