Sorption of Polyphenolics (Tannins) to Natural Soils.

Tannins enter soil systems via rainfall through the leaf canopy, leaf litter decomposition, and root exudation and decomposition.  For tannins released into soils, the relative importance of sorption to soil; chemical reactions with soil minerals; and biological decomposition is unknown.  Determining the sorption kinetics and capacity of soils for polyphenolics is an important facet of understanding the fate and effects of tannins in natural systems.  We added aqueous solutions of chemically well-defined polyphenolics to samples of pasture soil (Ultisol), centrifuged, and determined the specific polyphenolics by HPLC with UV detection.  We found that all of the phenolics tested follow the Freundlich isotherm, but the capacity of soil to bind tannin and the kinetic of binding varies for different tannins.  We found that the binding capacity differences depend on structural features such as hydrophopicity.  For example, up to 11 mg of epigallcatechin gallate (EGCG) is bound per g of soil but only 4 mg of methyl gallate is bound per g soil.  EGCG is more hydrophobic (Kow 6.0) than methyl gallate (Kow 1.4), so the higher capacity of the soil for the EGCG suggests nonpolar interactions play a role in sorption.  Kinetics of binding to soil can also range from relatively slow for phenolics such as EGCG, which reached equilibrium after about 400 min of shaking; to relatively fast for other compounds such as methyl gallate, which reached equilibrium within 150 min of shaking.  The next step in our study will be to develop conditions to remove bound tannin from soil.  The combined results from sorption and desorption studies will improve our understanding of how tannins influence soil chemical processes.