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DIVISION S-2—SOIL CHEMISTRY

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

^a Dep. of Geology and Geophysics, Texas A&M Univ., College Station, TX 77843
^b Dep. of Agricultural & Biological Engineering, Clemson Univ., Clemson, SC 29634-0357, and Dep. of Environmental Toxicology and the Clemson Institute of Environmental Toxicology, Clemson Univ., Pendleton, SC 29670

^* Corresponding author (Don.Zhu{at}po.state.ct.us).

Spin-lattice relaxation times (T₁) and chemical shifts () of perdeuterated pyridine (, ß, deuterium of d₅–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 T₁ values of are lower than and ß 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 (T₂) and of d₅–pyridine in aqueous suspensions of water-dispersible clay (WDC) soil components are highly pH dependent. The lowest T₂ and the most downfield-shifted compared with aqueous solution show that the strongest sorption occurs at the weak acidic condition (pH 6). The downfield shifts of observed in WDC suspensions are directly caused by the increased mole ratio of protonated pyridine through sorption. However, no significant changes in are observed for organic free minerals (H₂O₂–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 values.

Abbreviations: FTIR, Fourier transform infrared • NHC, nitrogen-heterocyclic compounds • NMR, nuclear magnectic resonance • SOM, soil organic matter • T₁, spin-lattice relaxation times • T₂, spin-spin relaxation times • TCE, trichloroethylene • TOC, total organic C • WDC, water-dispersible clay • , chemical shifts

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