Integrated investigations on the adsorption mechanisms of fulvic and humic acids on three clay minerals

Adsorption behaviors of fulvic acid (FA) and humic acid (HA) on kaolinite, smectite and vermiculite were investigated. To explore the adsorption mechanism, characterization of both the adsorption FA/HA-clay complexes and suspensions was conducted by utilizing multiple analytical techniques including liquidstate H-1 nuclear magnetic resonance spectroscopy, high performance size exclusion chromatography, UV-vis spectroscopy, Fourier transform infrared spectroscopy and X-ray photoelectron spectroscopy. Both FA and HA underwent fractionation during the adsorption due to different affinity for the functional moieties of FA/HA on the minerals. More HA was adsorbed than FA on kaolinite and smectite mainly via hydrophobic interaction. Electron transfer from aromatic units of FA to iron cation induced more FA than HA adsorbed onto vermiculite at higher FA/HA concentrations (>20 mg C/L). Specific surface area and pore volume analyses indicated HA with larger particle size was prone to accumulate on the external mineral surfaces, while FA was easier than HA to block pores of the minerals. The increased pH in clay suspensions after FA/HA adsorption suggested that ligand exchange occurred and FA/HA-clay complexes formed, particularly for the 2:1 type minerals of smectite and vermiculite with the increase of pH at 0.41 and 0.62 units, respectively. Furthermore, the increase of the equilibrium pH or the decrease of the ionic strength led to the reduction of FA/HA adsorption on all the three minerals. Due to rich in iron cation, more carboxyl and hydroxyl functional groups were facilitated for the ligand exchange and cation-bridging on vermiculite, and thus improved the adsorption capacity. The results of this study will improve our understanding of the roles of mineral interfacial properties, characteristics of FA and HA in the adsorption of FA/HA on clay minerals. (C) 2012 Elsevier B.V. All rights reserved.