Metal removal from heavy metal-enriched plants by deep eutectic solvents and its mechanism investigation

The use of deep eutectic solvents (DESs) has been demonstrated as an effective method for treating heavy metal-enriched biomass. However, the variations in the mechanisms of biomass deconstruction observed across different types of DESs, coupled with the challenges associated with ’biomass recalcitrance’, have made it difficult to design the efficient DESs. This study aimed to investigate the impact of different types of DESs on the removal efficiency of Cd from phytoremediation biomass and to understand the mechanism of Cd during the treatment using chemical quantitative analysis, micro-imaging and micro-spectroscopy technologies. The findings revealed that acidic DES exhibited superior removal efficiency compared to neutral and alkaline DES. Following treatment with ChCl:LA at 90 ℃ for 1 h, only 7.4% of the original Cd content remained in pulp. Moreover, an increase in the treatment temperature from 25 ℃ to 120 ℃ resulted in a reduction in residual Cd in the pulp from 60.5% to 6.6%. Multiscale analysis demonstrated that Cd in poplar primarily accumulates in the compound middle lamellae and secondary wall. During pretreatment, a portion of the Cd initially dispersed into the liquid phase alongside the dissolution of components such as hemicellulose. The subsequent deformation of the cell wall facilitated the penetration of DES, which enabled the formation of complexes between the heavy metals and the solvent through coordination and proton exchange. These complexes were then transferred to the liquid phase. The findings provide valuable scientific insights for the design of solvents for the safe disposal of phytoremediation biomass in a sustainable way.