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Molecular-Level Investigation of Monoaromatic Compound Sorption to Suspended Soil Particles by Deuterium Nuclear Magnetic Resonance

Zhu, Dongqiang, Herbert, Bruce E., Schlautman, Mark A.
Journal of environmental quality 2003 v.32 no.1 pp. 232-239
benzene, clay, deuterium, fulvic acids, hydrogen bonding, hydrophobicity, methylene chloride, nuclear magnetic resonance spectroscopy, pH, sodium hydroxide, soil, solutes, sorption, toluene
Molecular-level sorption behavior of monoaromatic compounds in suspensions of water-dispersable clay components was studied by measuring ²H nuclear magnetic resonance (NMR) spin–spin relaxation times (T ₂). In general, decreased T ₂ values indicate stronger solute–sorbent interactions and increased sorption of the solute. A decreasing trend for T ₂ values in the order benzene > fluorobenzene > toluene (-C₆D₅ moiety) was observed, which was probably caused by the hydrophobic effect. The T ₂ values for benzene and the -C₆D₅ moiety of toluene increased with increasing pH, whereas the trend with pH was much weaker and less consistent for fluorobenzene and the methyl group of toluene. Conversely, no clear relationship was found between T ₂ values and pH for dichloromethane. These contrasting results cannot be explained by the pH-dependent self-assembly and hydrophobicity of humics. Instead, directed specific forces, including hydrogen bonding, cation–π interactions, and aromatic–aromatic interactions, are proposed between the benzene ring of monoaromatic solutes and soil organic matter (SOM). Substituents of benzene affect these interactions by varying the π electron density. When the soil fraction was treated with NaOH to remove humic and fulvic acids, T ₂ values for the different monoaromatic solutes were surprisingly lower compared with those for the untreated soil fraction. This result is probably caused by the increased ratio of solutes adsorbed to “hard” or “glassy” SOM components, which leads to less mobile sorbed solute molecules, after removing NaOH-extractable humics that contain more “soft” or “rubbery” SOM components.