Sorption of Pyridine to Suspended Soil Particles Studied by Deuterium Nuclear Magnetic Resonance.

Spin-lattice relaxation times (T[sub 1]) and chemical shifts (δ) of perdeuterated pyridine (α, β, γ deuterium of d[sub 5]-pyridine) in aqueous solution (e.g., water, 0.001 M benzoic acid and phenol) are found to decrease with increasing pH values. This is because the concentration distribution of the two pyridine species, protonated and unprotonated, varies with solution pH. The T[sub 1] values of γ are lower than α and β deuterium in water and methanol compared with n-hexane, indicating more anisotropic molecular movements of pyridine resulting from interactions between amine and the polar solvent. Spin-spin relaxation times (T[sub 2]) and δ of d[sub 5]-pyridine in aqueous suspensions of water-dispersible clay (WDC) soil components are highly pH dependent. The lowest T[sub 2] and the most downfield-shifted δ compared with aqueous solution show that the strongest sorption occurs at the weak acidic condition (pH 6). The downfield shifts of δ observed in WDC suspensions are directly caused by the increased mole ratio of protonated pyridine through sorption. However, no significant changes in δ are observed for organic free minerals (H[sub 2]O[sub 2]-treated WDCs and a standard clay mineral) compared with aqueous solution, indicating interactions with mineral surfaces are negligible in pyridine sorption. A sorption mechanism of cation exchange between protonated pyridine and charged sites of soil organic matter (SOM) is inferred based on the measured δ values. [ABSTRACT FROM AUTHOR]