Contribution and composition of extracellular polymeric substances in cadmium removal by two types of biological...

The enrichment efficiency of cadmium (Cd) in biological crusts formed under different environmental conditions varies significantly. However, although extracellular polymeric substances (EPS) are key components in heavy metal removal, the mechanisms by which their content and composition influence the Cd removal capacity of different types of biological crusts remain unclear. Our study revealed distinct differences in EPS composition between mine crusts (MC) and paddy crusts (PC). The extracellular polysaccharide content in MC is 3.02 times higher than that in PC, and the extracellular protein content is 6.56 times higher than that of PC. Therefore, the adsorption capacity of MC for Cd is higher than that of PC in both acidic or neutral environments. The types of monosaccharides and proteins in EPS also varied notably between MC and PC, with the total concentration of monosaccharides in the EPS of MC is higher than that in PC. EPS removal markedly reduced Cd adsorption efficiency in both crust types. While exopolysaccharides in both MC and PC initially adsorb Cd faster, extracellular proteins exhibited greater adsorption capacity due to more available surface sites. Molecular docking simulations further demonstrate that MC extracellular proteins have a higher binding energy with Cd (-3.98 kcal/mol) compared to those in PC (-3.52 kcal/mol). These findings underscored the importance of EPS composition in Cd removal and provided a theoretical basis for optimizing biological crusts to facilitate environmental remediation in metal-contaminated mining and agricultural soils.