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 (qm = 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.

Aqueous Synthesis of 14-15-Membered Crown Ethers with Mixed O, N and S Heteroatoms: Experimental and Theoretical Binding Studies with Platinum-Group Metals.

A series of 14-15-membered O, N, and S-containing crown ethers (CEs) was synthesized by cyclization of bis-epoxides with aryl-N or S dinucleophiles using triethylamine as a catalyst and LiCl as a metal template in water. The catalyst dosage, and metal template type and dosage were critical in achieving yields of 56-93 %. Liquid-liquid extraction (LLE) was performed to evaluate the CE complexation with Pd2+ and Pt2+ . Among the CEs, a dioxa-dithia dibenzo CE exhibited the highest Pd2+ selectivity even in the presence of other platinum-group metals (PGMs). Complementary DFT studies reveal that this CE has the most compatible cavity dimension (ØCE =1.58 Å) with Pd2+ (ØPd 2+ =1.56 Å) forming a square-planar S4 geometry. Binding-energy calculations showed the Pd2+ complex has the least energy requirement for structural reorientation during complexation. Overall results highlight the importance of CE cavity dimension and presence of S heteroatoms for the structural design of CEs selective towards PGMs such as Pd2+ .

Aerosol Cross-Linked Crown Ether Diols Melded with Poly(vinyl alcohol) as Specialized Microfibrous Li+ Adsorbents.

Crown ether (CE)-based Li+ adsorbent microfibers (MFs) were successfully fabricated through a combined use of CE diols, electrospinning, and aerosol cross-linking. The 14- to 16-membered CEs, with varied ring subunits and cavity dimensions, have two hydroxyl groups for covalent attachments to poly(vinyl alcohol) (PVA) as the chosen matrix. The CE diols were blended with PVA and transformed into microfibers via electrospinning, a highly effective technique in minimizing CE loss during MF fabrication. Subsequent aerosol glutaraldehyde (GA) cross-linking of the electrospun CE/PVA MFs stabilized the adsorbents in water. The aerosol technique is highly effective in cross-linking the MFs at short time (5 h) with minimal volume requirement of GA solution (2.4 mL g-1 MF). GA cross-linking alleviated CE leakage from the fibers as the CEs were securely attached with PVA through covalent CE-GA-PVA linkages. Three types of CE/PVA MFs were fabricated and characterized through Fourier transform infrared-attenuated total reflection, 13C cross-polarization magic angle spinning NMR, field emission scanning electron microscope, N2 adsorption/desorption, and universal testing machine. The MFs exhibited pseudo-second-order rate and Langmuir-type Li+ adsorption. At their saturated states, the MFs were able to use 90-99% CEs for 1:1 Li+ complexation, suggesting favorability of their microfibrous structures for CE accessibility to Li+. The MFs were highly Li+-selective in seawater. Neopentyl-bearing CE was most effective in blocking larger monovalents Na+ and K+, whereas the dibenzo CE was best in discriminating divalents Mg2+ and Ca2+. Experimental selectivity trends concur with the reaction enthalpies from density functional theory calculations, confirming the influence of CE structures and cavity dimensions in their "size-match" Li+ selectivity.