The Application of ¹H High-Resolution Magic-Angle Spinning NMR for the Study of Clay−Organic Associations in Natural and Synthetic Complexes

Abstract

The preferential sorption of model compounds to calcium-exchanged montmorillonite surfaces was investigated using 1H high-resolution magic-angle spinning (HR-MAS) and liquid-state NMR. Synthetic mixtures, representing the major structural categories abundant in natural organic matter (NOM), and two soil extracts were sorbed to montmorillonite. The NMR spectra indicate that, of the organic components observable by 1H HR-MAS NMR, aliphatic components preferentially sorb to the clay surface, while carbohydrates and amino acids mainly remain in the supernatant. These results may help explain the highly aliphatic nature of organic matter associated with clay fractions in natural soils and sediments. Investigations using the synthetic mixtures demonstrate a specific interaction between the clay surface and the polar region in 1-palmitoyl-3-stearoyl-rac-glycerol. Similar observations were obtained with natural soil extracts. The results presented have important implications for understanding the role of organoclay complexes in natural processes, and provides preliminary evidence that HR-MAS NMR is a powerful analytical technique for the investigation of organoclay complex structure and conformation.

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

“…The 1 H NMR results (Fig. 3) indicated that aliphatic moieties (0.8-2.2 ppm regions), aromatic moieties (6.5-8 ppm regions) [14,24] and a combination of amino acids, polysaccharides, aromatic methoxyl, or other moieties such as esters and ethers (3-5.5 ppm regions) [14,25] were the dominant fractions in the equilibrium solutions, while the strong signals at 2.50 ppm and 3.35 ppm were attributed to DMSO-d 6 and H 2 O [25,26], respectively. Fe was detected at 1.72 and 2.13 mg/L in the equilibrium solutions after FA and HA reacted with vermiculite, and the slightly broader peaks for unbound FA/HA after adsorption on vermiculite (Fig.…”

HR-MAS NMR Spectroscopy in Material Science

“…[74][75][76][77][78][79] These NMR studies show that at this interface fatty acids, aliphatic esters and alcohols are the prominent species, and that aromatic functional groups are protected by hydrophobic regions and are not directly accessible to the penetrating water, [74] while surface polymethylene groups may control the sorption properties of organo-clay complexes. [76,80] These types of studies have been extended to three-dimensional (3D) HMQC-TOCSY to further increase the resolution of the highly overlapping spectra from humic materials. [81] …”

Composition of dissolved organic matter within a lacustrine environment

“…[35] Waxes and cutins derived from plants have been identified in abundance in humic extracts, [25] and are likely to be preserved because of their cross-linked structure and hydrophobicity. [36] Fig. 2c shows the DE 1 H NMR spectrum for the Coole Bay DOM sample.…”