6 research outputs found

    Kinetics and binding capacity of six soils for structurally defined hydrolyzable and condensed tannins and related phenols

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    We investigated tannin–soil interactions by assessing the kinetics of sorption and sorption capacities, and their relationship to the chemical properties of six polyphenolic compounds and the textures of six soils. We developed a new extraction procedure for recovering tannins from soil samples by successive extraction with solvents of decreasing polarity.Sorption of polyphenolic compounds methyl gallate, catechin, oenothein B, pentagalloyl glucose, epigallocatechin gallate, and procyanidin dimer was determined using six soils with textures ranging from 7% silt–89% sand to 52% silt–22% sand. Sorption kinetics and capacity of the soils were determined using room temperature mixing with high performance liquid chromatography (HPLC) to determine polyphenol concentration. Tannins were extracted from soils loaded with known amounts of the model compounds using solvents with a range of polarities, and establishing recovery by HPLC. Multivariable regression was used to establish relationships between polyphenol properties and sorption, and between soil texture and sorption.Sorption of the polyphenols followed the Langmuir isotherm with unique binding capacity and kinetics of sorption for each compound. Tannin sorption was correlated to molecular weight and polarity. For an Ultisol pasture soil, up to 18.7 mg g−1 soil of the large, hydrophobic compound epigallocatechin gallate was bound compared to only 5.11 mg g−1 soil of the smaller, more polar compound methyl gallate. Kinetics of sorption also varied with sorption reaching equilibrium between 75 and 350 min. Silt and sand composition affected polyphenol sorption in a unique fashion for each polyphenol. Using sequential extraction with solvents ranging from polar (water) to non-polar (hexane), we extracted up to 42% of the material that had been loaded on the soil as a mixture of five polyphenols.This work demonstrates that the likely fate of tannins and related polyphenols from plant sources is rapid sorption, with little likelihood of release of unmodified tannins from the soil by leaching. Tight sorption impedes recovery and analysis of soil tannins, but better methods for extraction may improve our knowledge of tannins in the soil
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