Adsorptive fractionation of extracellular polymeric substances on iron oxyhydroxides: impact on copper binding

Abstract

The fractionation of extracellular polymeric substances (EPSs) on mineral surfaces alters their composition and reactivity, thereby influencing heavy metal behavior in the environment. However, the molecular-level mechanisms governing how this process affects metal-binding properties remain unclear. Hence, this study combined multispectral techniques and chemometrics to investigate the adsorptive fractionation of EPSs on minerals and its impact on heavy metal binding. Excitation–emission matrix coupled with parallel factor analysis (EEM-PARAFAC) demonstrated that aromatic and protein-like components were preferentially adsorbed on hematite (Hema) and goethite (Goet). Two-dimensional correlation spectroscopy (2D-COS) further indicated that protein-like substances were adsorbed prior to fulvic-like substances, following the sequence: tyrosine-like > tryptophan-like > fulvic-like substances. Notably, the fractionation process altered the binding affinity and order of Cu²⁺ to EPS components. Adsorption onto Hema reduced the copper-binding affinity of EPSs, whereas adsorption onto Goet enhanced it. This difference may be attributed to changes in carboxyl and polysaccharide groups within the EPS. This study elucidates the influence of EPS fractionation on mineral surfaces regarding heavy metal binding at the molecular level. These findings enhance our understanding of the biogeochemical behavior of heavy metals in the presence of mineral–organic composites across aquatic and terrestrial ecosystems, providing a theoretical foundation for environmental remediation.