Anchoring Pentadentate Bis-Thiosemicarbazones on Silica-Coated Fe₃O₄ Nanoparticles for Pd(II) Adsorption

Thiosemicarbazone (TSC) functionalized magnetic nanoparticles (NPs) are good candidates for the recovery of metal ions due to the large surface area of NPs in combination with the easy functionalization of TSC ligands for both metal binding and anchoring on the NP surface. Four TSC ligands, the parent 2,6-diacetylpyridine-bis(4-N-2-pyridyl)-TSC (BTSC), the 4-tert-butyl (tertBTSC), the 4-methyl-carboxylate (ESBTSC), and the 4-carboxylic acid-derivative (OHBTSC), were synthesized. OHBTSC was covalently bound to (3-aminopropyl)triethoxysilane (APTES) functionalized silica-coated Fe3O4 to form the Fe3O4@SiO2–APTES–BTSC adsorbent, which was studied for the removal of Pd(II) ions from aqueous solution. Single crystal X-ray diffraction showed a nonsymmetric Sthiolate^Nimine^Npyridine^Namide binding motif for the complex [Pd(BTSC)] in line with the DFT (density functional theory)-calculated structure, both serving as models for the adsorbent–Pd binding. Powder XRD patterns showed decreasing crystallinity starting from the initial Fe3O4, upon coating with SiO2 and APTES and subsequent functionalization with OHBTSC. Both scanning electron microscopy (SEM)/energy dispersive X-ray (EDX) and high-resolution X-ray photoelectron spectroscopy (XPS) show C, N, and S for the TSC along with Fe, O, and Si for the NP. Pd is found for Pd-loaded adsorbent together with minor amounts of Cl. Adsorption capacities of up to 793 mg/g in the pH range from 2 to 8 were achieved with the best fit of the equilibrium adsorption data to Freundlich isotherms.