Thioctic acid–functionalized silica for selective palladium adsorption from chloride–based solutions

Selective recovery of Pd from chloride-rich hydrometallurgical leachates, such as those derived from automotive catalytic converters, remains challenging due to the chemical similarity of PGMs and the stability of their chloro-complexes. Here, we report a green, selective sorbent (TA@Si) prepared by covalently grafting the naturally occurring disulfide ligand, thioctic acid, onto mesoporous silica via silane chemistry. In multi-metal batch experiments, TA@Si exhibited excellent selectivity for Pd over Pt, Rh, and base metals (Fe, Zn, Ce) across a wide acidity range (0.05–10 M HCl). The process was endothermic and entropy-driven, remaining thermodynamically favorable under strongly acidic conditions, and adsorption followed pseudo-second-order kinetics, best described by Sips and Temkin isotherm models, reaching a maximum capacity of (49.5 ± 12.4) mg·g −1 at 0.2 M HCl. XPS, Raman spectroscopy and SEM-EDS analyses indicate that Pd(II) coordinates to disulfide-derived sulfur sites of TA, forming stable surface-anchored Pd-S/Cl complexes. Adsorbed Pd (along with minor Pt) was quantitatively recovered using thiourea in dilute HCl, and the sorbent maintained performance over at least five adsorption-desorption cycles. Fixed-bed column studies demonstrated that this selectivity translates to continuous operation, with delayed Pd breakthrough, minimal co-adsorption of competing metals, and efficient Pd recovery through continuous thiourea-assisted desorption. This work introduces an acid-stable thioctic-acid-functionalized silica adsorbent for selective Pd recovery from chloride-rich PGM leachate simulants, demonstrating the combined advantages of TA-disulfide coordination chemistry, broad HCl tolerance, multi-metal Pd selectivity, regenerability, and fixed-bed compatibility.