Preferential Sites In Soil Organic Matter Probed with Stable Nitroxyl Free Radicals.

A long-standing question in sorption research with respect to soil organic matter is whether preferential sorption exists on the basis of functional group identity and/or domain character (e.g., aliphatic vs aromatic). Knowing the answer would help refine models predicting sorption, hence bioavailability. The existing literature is mixed. To address this issue we studied the 13C-CP/TOSS NMR spectra of stable nitroxyl free radicals as spin probes-- 2,2,6,6-tetramethylpiperidine-1-oxyl (TEMPO), and 4-hydroxy-2,2,6,6-tetramethylpiperidine-1-oxyl (HTEMPO) --sorbed to Pahokee peat soil, Beulah-Zap lignite, a soft coal, and a polystyrene-poly(vinylmethylether), PS-PVME, a polymer blend. Due to strong electron-nuclear spin interactions, these radicals induce relaxation, hence broadening and diminution of the C signals, within a radius of about 1 nm of the sorbed molecule. Titration of the solid with spin probe, therefore, reveals preferential sorption by the concentration rate of change of signal suppression. TEMPO and HTEMPO are nonionic under all conditions and interact through van der Waals and weak H-bonding forces. Polydimethylsilane was used as an inert internal standard. We find little to no selectivity of probes to SOM or PS-PVME on the basis of functional group chemistry (polymethylene, polar and nonpolar aromatic, polar aliphatic). This contradicts previous findings based on macroscopic data of functional group correlations with sorption coefficients. Furthermore, there seems to be no preference for the small amount of black carbon present in the samples (centered at 128 ppm). These results imply that site selectivity may be due to factors other than functional group composition, such as configuration of SOM macromolecules. This work demonstrates for the first time the use of molecular probes to study sorption specifity.