Tuesday, November 14, 2006

Probing the Role of Polarity in SOM Interactions.

Robert L. Cook, Louisiana State Univ, 636 Choppin Hall, Baton Rouge, LA 70803 and Baoshan Xing, Amherst, MA 01003, United States of America.

Soil organic matter (SOM) is well recognized to affect soil fertility and sorption of xenobiotic compounds (e.g. agricultural chemicals) to soils.  However, our molecular level understanding of the chemical moieties within the SOM that play important roles in these two areas is very limited.  We have taken two approaches in order to address these issues.  The first is a top-down approach in which a model peat soil is exfoliated by a series of different solvent systems of varying polarity.  The second is a bottom-up approach in which model organic materials are chemically modified to change their polarity. The exfoliation studies showed that an aqueous solution of completely water miscible organic solvents (acetic acid, acetone, acetonitrile, DMSO, and methanol) at a concentration of 4.6 mM exfoliated a substantial amount of SOM.  The study also showed that there was a kinetic effect in terms of the moieties being exfoliated from the SOM.  It was also found that all solvent systems, except for the acetonitrile, exfoliated almost identical amounts and types (in terms of chemical make-up) of organic matter, as evidenced by fluorescence measurements.  The acetonitrile solvent system exfoliated more organic matter, which was more humified in nature, as measured by fluorescence, FTIR, and NMR. Common biopolymers (lignin, chitin, cellulose, cuticular materials) were chemically altered to change their structure and polarity. Some of these biopolymers are precursors of SOM and are identified in soil humic substances. Chemical treatments significantly modified the structures and polarity of biopolymers, as shown by NMR, FTIR, and elemental analysis. These modifications changed the sorption behavior of hydrophobic organic compounds, such as phenanthrene, and hence expected to affect pesticide sorption and efficacy in soils as well as other physical and chemical properties of SOM.