Functionalization of expanded MXene to recover palladium ions by capacitive deionization

This work prepared a sulfur/nitrogen co-doped MXene (S/N-MXene) electrode material via a facile hydrothermal method. The electrode material has been specifically designed to recover palladium(II) from wastewater. Numerous S/N-containing functional groups were introduced into the MXene by S and N co-doping, with the contents optimized for the preferential recovery of Pd(II). These functional groups acted as pillars to expand the interlayer spacing of MXene, facilitating Pd²⁺ diffusion between MXene layers and enhancing accessibility to the anchor sites (–OH groups and heteroatom functional groups). The S/N-MXene electrode exhibited a high recovery capacity of 2.92 mmol g⁻¹, high recovery efficiency of 96.4 % in 15 min and good long-term cycling performance with greater than 80 % capacity retained after 260 cycles. The S/N-MXene electrode showed preferential adsorption of Pd²⁺ from wastewater. The adsorption mechanism was elucidated through density functional theory (DFT) calculations showing that the downward shift of the p-band center of the MXene-OH groups induced by pyrrolic-N and -C=S in S/N-MXene (compared to single doping with S or N) favours enhancement of the soft ion-soft ion interactions. Furthermore, the adsorbed Pd²⁺ on the S/N-MXene electrode could be enriched by simple reversing of the applied voltage to strip the ions from the sorbent, thus avoiding the need to use harsh chemicals for regeneration and minimizing the amount of secondary waste.