Binding Mechanism Between Fulvic Acid and Heavy Metals: Integrated Interpretation of Binding Experiments, Fraction Characterizations, and Models

In this study, nine fulvic acid fractions (FAs) were obtained by a progressively extracting method from lake sediment and used as adsorbent for binding metal ions. The characterization indicated that former FA fractions had stronger polarity, more O-bearing functional groups, and less aliphaticity. And the results of EEM-PARAFAC indicated that there were four components in FAs, i.e., two humic-like components and two fulvic-like ones. The sorption experiments showed sorption capacity of selected metal ions onto FAs decreased with increasing extractions, and the analysis of binding stability for selected metal ions onto FAs were ranked as FAs-Cu>FAs-Pb>FAs-Cd. For all extracted FAs, the principal component analysis for sorption capacity showed that O-bearing functional groups and molecular weight were dominant factors for FAs binding metal ions. Furthermore, the characterization results of nine FAs pre- and post-adsorption indicated that the phenolic groups of FAs had a greater contribution to bind Pb 2+ over than Cd 2+ and Cu 2+ , while carboxyl and N-bearing groups had a stronger affinity for Cu 2+ than others. The study has the potential significance to insight the FA and metal ions cycling processes in lakes. Graphical Abstract