Hydroxypicolinic acid tethered on magnetite core-silica shell (HPCA@SiO2@Fe3O4) as an effective and reusable adsorbent for practical Co(II) recovery.

The soaring demand and future supply risk for cobalt (Co) necessitate more efficient adsorbents for its recycling from electronic wastes, as a cheaper and less hazardous option for its production. Herein, a magnetic adsorbent covalently tethered with 5-hydroxypicolinic acid (HPCA) as Co(II) ligand was developed. The magnetic component (Fe3O4) was protected with silica (SiO2), then silanized with chloroalkyl linker and subsequently functionalized with HPCA via SN2 nucleophilic substitution (HPCA@SiO2@Fe3O4). Results from FTIR, TGA, EA, and XPS confirm the successful adsorbent preparation with high HPCA loading of 2.62 mmol g-1. TEM-EDS reveal its imperfect spherical morphology with ligands well-distributed on its surface. HPCA@SiO2@Fe3O4 is hydrophilic, water-dispersible and magnetically retrievable, which is highly convenient for its recovery. The Co(II) capture on HPCA@SiO2@Fe3O4 involves monodentate coordination with carboxylate (COO-) and lone pair acceptance from pyridine (aromatic -N = ) moiety of HPCA, with minor interaction from acidic silanols (Si-O-). The binding occurs at 2 HPCA: 1 Co(II) ratio, that follows the Sips isotherm model with competitive Qmax = 92.35 mg g-1 and pseudo-second order kinetics (k2 = 0.0042 g mg-1 min-1). In a simulated LIB liquid waste, HPCA@SiO2@Fe3O4 preferentially captures Co(II) over Li(I) with αLi(I)Co(II)=166 and Mn(II) with αMn(II)Co(II)=55, which highlights the importance of HPCA for Co(II) recovery. Silica protection of Fe3O4 rendered the adsorbent chemically stable in acidic thiourea solution for its regeneration by preventing the deterioration of the magnetic component. Covalent functionalization averted ligand loss, which allowed HPCA@SiO2@Fe3O4 to deliver consistent and reversible adsorption/desorption performance. Overall results demonstrate the potential of HPCA@SiO2@Fe3O4 as a competitive and practical adsorbent for Co(II) recovery in liquid waste sources.

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.