Disulfide-Containing Mesoporous Silica for Selective Palladium Recovery from Complex Chloride Media

Introduction
Selective Pd recovery from chloride-rich leachates remains challenging due to the chemical similarity and stable chloro-complexes of platinum group metals [1]. Conventional adsorbents often show limited selectivity or poor stability in concentrated HCl media. Therefore, selective acid-stable adsorbents are needed for efficient Pd recovery from spent automotive catalytic converters.

Methods
A thioctic acid-functionalized mesoporous silica (TA@Si) adsorbent was synthesized via silane-mediated covalent grafting, confirmed by spectroscopic analyzes. Its adsorption performance was evaluated using multi-metal batch experiments in HCl media (0.05–10 M), supported by kinetic, isotherm, and thermodynamic analyses. Binding mechanisms were investigated using Raman spectroscopy and SEM–EDS.

Results
TA@Si exhibited high selectivity toward Pd over PGMs and base metals across a wide acidity range, with a maximum adsorption capacity of 49.5±12.4 mg·g^-1 at 0.2 M HCl. Adsorption followed pseudo-second-order kinetics and was best described by Sips and Temkin isotherm models, indicating heterogeneous chemisorption. Thermodynamic analysis confirmed an endothermic and entropy-driven process, with Pd adsorption remaining spontaneous under all studied conditions. Spectroscopic results revealed Pd immobilization via mixed Pd–S/Cl coordination without significant disulfide bond cleavage.

Conclusions
TA@Si represents a selective, acid-stable, and regenerable platform for Pd recovery from complex chloride leachates, offering strong potential for application in hydrometallurgical separation processes.

References

[1] H. Narita, R. Kasuya, T. Suzuki, R. Motokawa, M. Tanaka. In: Encyclopedia of Inorganic and Bioinorganic Chemistry, 2nd ed., Wiley, 2020, pp. 1–28.

Acknowledgements

This work was supported by FCT through the project PlatILPlus (2022.04478.PTDC, DOI: 10.54499/2022.04478.PTDC), LSRE-LCM (UID/50020/2025), ALiCE (LA/P/0045/2020, DOI: 10.54499/LA/P/0045/2020), and CICECO (UID/50011/2025, DOI: 10.54499/UID/50011/2025; LA/P/0006/2020, DOI: 10.54499/LA/P/0006/2020), financed through FCT/MCTES (PIDDAC). N. Schaeffer acknowledges ERC support through the DESignSX Starting Grant (Grant Agreement No. 101116461; DOI: 10.3030/101116461). F.H.B. Sosa, F. Braga, and R. Carvalho acknowledge FCT support through CEECIND/07209/2022 (DOI: 10.54499/2022.07209.CEECIND/CP1720/CT0019) and PhD grants 2023.01749.BD (DOI: 10.54499/2023.01749.BD) and 2025.06718.BDANA.