A new way to use solid-state carbon-13 nuclear magnetic resonance spectroscopy to study the sorption of organic compounds to soil organic matter

Several solid-state 13C nuclear magnetic resonance (NMR) techniques were used to characterize soil organic matter spiked with 13C-labeled organic compounds spanning a range of hydrophobicities (benzoic acid, benzophenone, naphthalene, phenanthrene, and palmitic acid). The chemical shifts of NMR resonances of the sorbed species were shifted by up to 3 ppm relative to those of the neat compounds. Sorption also resulted in increased resonance linewidth for the compounds containing a single 13C label, indicating the presence of a range of different chemical environments at the sites of sorption. On the other hand, sorption decreased the linewidth of the resonance of naphthalene, which was uniformly 13C-labeled. This was attributed to the removal of intermolecular 13C-13C dipolar coupling. Heterogeneity of the organic matter was demonstrated using the spectral editing technique proton spin relaxation editing (PSRE), which enabled the identification and quantification of charcoal-rich domains characterized by rapid rates of proton spin-lattice relaxation in the static frame (T1H), and humic domains characterized by slow rates of T1H relaxation. Furthermore it was demonstrated that the sorbed 13C-labeled molecules "inherit" the T1H "signature" of the organic matrix in their immediate vicinity. Thus PSRE on the spiked soils enabled evaluation of the relative affinity of the two domain types for the sorbate molecules. The charcoal-rich domains were shown to have a twofold to tenfold greater affinity for the organic compounds, with greater differences found for the more hydrophobic compounds.