Single-step synthesis of prominently selective and easily regenerable POSS functionalized with high loadings of sulfur and carboxylic acids

Abstract

The growing demand for platinum group metals (PGMs) such as platinum (Pt) and palladium (Pd) necessitates novel approaches for efficient and selective separation and regeneration. Polyhedral oligomeric silsesquioxane (POSS)-based materials offer promising potential for retrieving PGMs from water, especially under raucous conditions. In this study, we present the first-ever development of a novel highly loaded carboxylic acid functionalized sulfur-containing POSS (POSS–S–COOH) through a one-step method. POSS–S–COOH demonstrated exceptional adsorption capabilities, with the highest adsorption capabilities for PtCl₆²⁻ and PdCl₄²⁻ measured at 597.2 ± 20.2 mg g⁻¹ and 692.6 ± 25.1 mg g⁻¹, respectively, in highly acidic water media. Remarkably, the adsorbent exhibited high selectivity toward PdCl₄²⁻ and PtCl₆²⁻ in the presence of various co-existing and common metal ions, such as Ni(II), Zn(II), Mg(II), Cu(II), Pb(II), Cd(II), Fe(III), and Co(II). Furthermore, it demonstrated proficient recovery capabilities for both PtCl₆²⁻ and PdCl₄²⁻ at pH 10, without the need for harsh desorbents such as thiourea, marking a significant achievement in this field. The pseudo-second-order as well as intraparticle diffusion kinetic and Langmuir isotherm data fitting indicated strong interactions resulting from electrostatic, H-bonding, and chelate bonding among the active functional sites of POSS–S–COOH and PtCl₆²⁻/PdCl₄²⁻. Thermodynamic analyses revealed spontaneous and endothermic adsorption of PtCl₆²⁻ and PdCl₄²⁻ by POSS–S–COOH. Density functional theory (DFT) calculations supported these findings, demonstrating high binding energy and low energy gaps, contributing to the proficient adsorption of PtCl₆²⁻/PdCl₄²⁻ by POSS–S–COOH. Furthermore, POSS–S–COOH displayed excellent acid-resistance at pH 1, making it a promising adsorbent for potential industrial applications. This novel approach opens up new avenues for highly selective and easily regenerable POSS–S–COOH for the retrieval of precious metals from highly acidic water media.