Assessment of clay mineral selectivity for adsorption of aliphatic/aromatic humic acid fraction

Understanding the interactions of humic acids with clay minerals is important for modeling the geochemical fate and transport of nutrients and pollutants in soils and natural waters. This study assesses the selectivity of the clay minerals kaolinite and montmorillonite for the adsorption of the leonardite humic acid (LHA) fractions. The whole LHA was fractionated by ultrafiltration into five fractions of different molecular size (F1, > 0.2 mu m; F2, 0.2 mu m-300,000 Da; F3, 300,000-50,000 Da; F4, 50,000-10,000 Da; F5, 10,000-1000 Da). Characterization by solid-state C-13 nuclear magnetic resonance spectroscopy (NMR) indicated that the fraction >0.2 mu m contains primarily aliphatic carbon, while the smaller molecules of the fraction <10,000 Da consists of predominantly aromatic carbon species. The adsorption characteristics of humic acid fractions onto kaolinite and montmorillonite at pH = 6 and 0.01 mol dm(-3) NaCl ionic strength, were also evaluated by solid-state C-13 NMR and UV-visible spectroscopy. UV-visible and C-13 NMR spectra demonstrated preferential sorption by the clay minerals for different carbon types; kaolinite preferentially sorbed the aromatic carbon types, while montmorillonite preferentially sorbed the aliphatic carbon at the first point of LHA adsorption (low coverage). However, this preferential effect wasn't pronounced at the complete surface coverage (high coverage).