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

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 microsphere (SPCM), was used to selectively capture Pd(II) from acidic solution. SPCM exhibited high efficiency for the adsorption separation of Pd(II) from 0.5 M to 6 M HNO3 solution. The adsorption kinetic 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 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. SPCM 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 SPCM from HNO3 solution is much higher than those of the previously reported sorbents, exhibiting a high economic viability of SPCM 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. Characterizations and theoretical calculations revealed that the adsorption of Pd(II) on SPCM sorbent is dominated by the coordination of [Pd(NO3)2] with O/S containing groups and part of Pd(II) is reduced to Pd(0). The excellent adsorption performance of SPCM provides a feasible and low-cost strategy for the selective recovery of Pd(II) from acidic wastewater.