Highly selective capture of palladium from acidic solution by sulfur-functionalized porous carbon microspheres: performance and mechanism

Abstract

Efficient recovery of palladium (Pd) from waste sources is of paramount importance due to its limited natural reserves and potential environmental hazards. Herein, a carbon sorbent, namely sulfur-functionalized porous carbon microspheres (SPCMs), was used to selectively capture Pd(II) from acidic solution. SPCMs exhibited high efficiency for the adsorption separation of Pd(II) from 0.5 M to 6 M HNO₃ solution. The adsorption kinetics of Pd(II) matched well with the pseudo-second-order model. The adsorption reached equilibrium after 130 minutes and the adsorption capacity of Pd(II) was 79.3 mg g⁻¹ in 1 M HNO₃ solution. The Freundlich isotherm model exhibited a better description of the Pd(II) adsorption, suggesting that the Pd(II) adsorption is a multilayer adsorption process. SPCMs showed a high selectivity for the capture of Pd(II) in simulated acidic wastewater with 26 metal ions, and the selectivity increased with the increase of HNO₃ concentration. The adsorption capacity per US dollar of Pd(II) by SPCMs from HNO₃ solution is much higher than those of previously reported sorbents, exhibiting a high economic viability of SPCMs for Pd(II) capture from acidic solution. The adsorbed Pd(II) could be desorbed using 1.0 M thiourea and 0.1 M HNO₃, and the SPCM sorbent maintained a high adsorption capacity after five adsorption–desorption cycles. Characterization and theoretical calculations revealed that the adsorption of Pd(II) on the SPCM sorbent is dominated by the coordination of [Pd(NO₃)₂] with O/S containing groups and some of the Pd(II) is reduced to Pd(0). The excellent adsorption performance of SPCMs provides a feasible and low-cost strategy for the selective recovery of Pd(II) from acidic wastewater.