The dynamic nature of mineral organic matter associations in adsorption experiments

Mineral-organic matter associations (MOAs) are predominant in soil environments and encompass a range of soil minerals and organic matter. This study characterizes changes in the structure and chemical composition of MOAs in the presence of metal ions of high (Cu) and low (Ni) affinity towards organic matter. It shows that MOAs are highly dynamic in terms of size, composition, and structure during their interactions with Cu- and Ni-bearing solutions. MOAs are synthesized using montmorillonite and ferrihydrite as the mineral components and polymerized fulvic acid (FA) as the organic component. Adsorption isotherms and Total Organic Carbon (TOC) analysis indicate that desorbed FA complexes both metal ions at lower metal ion concentrations and that the polymerization of Cu- and Ni-FA complexes occurs at higher metal concentrations. Structural characterization of MOAs using electron microscopy, spectroscopy, and X-ray diffraction indicates that Cu induces greater structural and compositional changes than Ni, owing to its higher affinity for polar organic functional groups. Here, the formation of Cu-carboxylate complexes in ferrihydrite-FA-OA increases with the concentrations of Cu, and the interaction of Cu with montmorillonite-FA-OA follows a 2-step intercalation with the formation of higher polymerized Cu-FA complexes at higher Cu concentrations. Additionally, the formation and polymerization of Cu-FA complexes increased the number of polar functional groups, thereby increasing the number of binding sites. Future adsorption studies involving MOAs and metal ions with a high affinity for organic functional groups need to consider that neither the surface area, composition, nor the number of binding sites in MOAs remain constant as the metal ion concentration in solution increases.