Chemical and mineralogical controls on humic acid sorption to clay mineral surfaces

Batch sorption experiments with peat humic acid (PHA) were carried out with two types of clay minerals (kaolinite and montmorillonite) under various solution conditions to determine the influence of solution chemistry (e.g. ionic strength, pH and solution cation) and mineralogy on PHA sorption to mineral surfaces. Consistent with contemporary studies, PHA sorption increased with increasing ionic strength and decreasing pH, and the presence of the background electrolyte Ca2+ largely enhanced sorption in comparison to Na+. An attempt was made to separate the contributions of various modes of interactions to PHA sorption in our experiments, where ligand exchange was estimated to account for approximately 32% of PHA sorption on clay surfaces, van der Waals 22% and cation bridging 41% when Ca2+ was the background electrolyte. Kaolinite displayed higher adsorption for PHA than montmorillonite under the same solution conditions, which is explained by selective sorption of PHA by different clays and different binding mechanisms. 1H High Resolution Magic Angle Spinning (HR-MAS) NMR was employed to examine the chemical nature of mineral-bound PHA. By employing 1H NMR techniques, direct evidence for the selective sorption of CH2 groups and proteins on clay mineral surfaces was obtained. Montmorillonite displayed a higher uptake of aromatics and proteins while more CH2 groups were observed to sorb to kaolinite surfaces.