Characterization of adsorbed organic matter on mineral surfaces
Humic substances (HS) are the major components of soil organic matter (SOM), which are by far the most abundant organic materials in the environment. Advances on structural characterization of HS and their interaction with clay minerals will provide a more fundamental understanding of HS functions in such important roles as sorption of anthropogenic organic contaminants, stabilization of soil aggregates, and C sequestration. The objectives of this research were to determine any variation in structure and sorption properties of humic acids (HAs) and humins sequentially extracted from a soil and investigate characterization and fractionation of HA and relatively small carboxylic acids upon adsorption on clay minerals.
In a phenanthrene sorption study of sequentially extracted HAs and humins, there were significant chemical and structural differences among the HA fractions and humins, and sorption was greatly affected by chemical structure and composition of humic substances, even from the same soil. A positive trend was observed between the sorption coefficient and the aliphaticity. Humin fractions with the highest aliphatic C contents and the lowest polarity showed the highest sorption capacity and nonlinearity as compared with the HAs. A negative relation was shown between the sorption capacity and polarity of HAs. The aliphatic-rich SOM in this study had less polar moieties, but had relatively high Koc. Therefore, the polarity of SOM is likely one of the important parameters controlling sorption of hydrophobic organic chemicals (HOCs).
Adsorption of SOM onto clay minerals modifies their surfaces and reactivity and strongly influences the fate of organic contaminants and other species in soils and sediments. For investigation of the structural and conformation changes of HA and clay-HA complexes after sequential adsorption by goethite, kaolinite, and montmorillonite, UV-Visible spectroscopy, high performance size exclusion chromatography (HPSEC), Fourier transform infrared (FT-IR) spectroscopy, and solid-state 13C nuclear magnetic resonance (NMR) spectroscopy were employed. Aliphatic rich HA fractions with polar functional moieties readily adsorb to the goethite surface, while aromatic fractions were left in solution. Similar to HA fractionation onto goethite, paraffinic fractions and polar aliphatic fractions sorbed preferentially on montmorillonite, but aromatic functional moieties likely remained in solution. However, paraffinic fractions were observed on kaolinite surfaces although the adsorbed proportion of these fractions was low. Because of high broken edge areas, kaolinite has more adsorption sites for carboxylic and carbonyl rich fractions than montmorillonite. With respect to MW fractionation after sorption, relatively low molecular weight (MW) HA fractions had a greater affinity for the goethite surface from the analysis and inference of the HPSEC chromatograms, which differs from the reported results in the literature. The low MW HA fractions might be intercalated into the interlayer of montmorillonite and high MW HA fractions adsorbed on external surfaces, which results in the adsorption of relatively wide range of MW HA fractions. The binding mode of small HA fractions on the clay minerals may be ligand exchange and/or electrostatic interaction, which gives HA-clay complexes new hydrophobic sorption sites for high MW HA fractions.
To obtain information on the nature of the interactions between SOM and clay mineral surfaces, the adsorption of dicarboxylic acids by kaolinite and montmorillonite at different pH conditions was investigated using in situ attenuated total reflectance Fourier transformed infrared (ATR-FTIR) and ex situ diffuse reflectance infrared Fourier transformed (DRIFT) spectroscopy. The sorption was highly pH dependent and related to the surface characteristics of minerals; adsorption of dicarboxylic acids (succinic acid, glutaric acid, adipic acid, and azelaic acid) was the highest at pH 4 as compared with those at pH 7 and 9, and the sorption capacity of montmorillonite was greater than that of kaolinite. Furthermore, the complexation types, inner- or outer-sphere, depended upon dicarboxylic acid species, pH, mineral surfaces, and solvent conditions. Most samples tend to have outer-sphere adsorption with the mineral surfaces at all tested pHs. However, inner-sphere coordination between the carboxyl groups and mineral surfaces at pH 4 was dominant with freeze-dried complex samples. Therefore, organic acids in an aqueous environment prefer to adsorb onto kaolinite and montmorillonite by outer-sphere complexation, but inner-sphere complexation is favored under dry conditions. These results imply that organic acid binding onto clay minerals under dry conditions is stronger than that under wet conditions. The stable NOM/mineral complexes formed by frequent wetting-drying cycles in nature may resist chemical/microbial degradation of the NOM, which will affect carbon storage in the environment and influence the sorption of organic contaminants.
0768: Environmental science