The exact manner in which nonionic compounds sorb to solid NOM has long been debated. We employed NMR spin probes to characterize sorption specificity to a soft coal (Beulah-Zap lignite), type II kerogen (IL-6), Pahokee peat, Amherst humic acid, and a rubbery polystyrene-poly(vinyl methyl ether) (PS-PVME) blend. All sorbents were found to be homogeneous on the scale of a few nanometers, except that Beulah and IL-6 contain a minor (CH2)n domain, and Pahokee contains minor carbohydrate and nonpolar alkyl domains. One approach was to sorb stable paramagnetic organic molecules that cause relaxation of carbon atoms located within a 0.5 nm radius of the spin, causing relaxation of that signal in the spectrum. Paramagnetic nitroxyl relaxation probes (TEMPO and HTEMPO) showed little or no preferential sorption based on NOM functional group type. A second approach was to sorb carbonyl-13C-benzophenone-d10 and observe the two-dimensional 1H-13C heteronuclear correlation NMR spectra. Deuteration of the benzophenone eliminates trivial intramolecular 1H-13C correlations and permits correlation of benzophenone 13C=O exclusively with nearby protons in the organic matrix. Benzophenone was found to interact with both aromatic rings and alkyl segments in all samples, but preferentially with aromatic rings in PS-PVME and Beulah. In IL-6 benzophenone preferentially locates near alkyl-substituted edges of aromatic rings. Specific interactions of the sorbate and the presence of domains in the sorbent evidently influence benzophenone sorption strength and selectivity.

Sorption selectivity in natural organic matter studied with NMR spin and relaxation probes

248th National Meeting (2014)

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