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SOIL CHEMISTRY

Competitive Adsorption of 2-Ketogluconate and Inorganic Ligands onto Gibbsite and Kaolinite

Biosystems Engineering and Soil Science Dep., 2506 E.J. Chapman Dr., Univ. of Tennessee, Knoxville, TN 37996-4531

^* Corresponding author (messington{at}utk.edu).

The low-molecular-mass organic acid anion 2-ketogluconate (kG) is produced via microbial activity in rhizosphere soils. One of the mechanisms by which this organic ligand may influence the chemistry of soil systems is through adsorption by constant-potential minerals. This study examined the adsorption of kG onto gibbsite and kaolinite in the presence or absence of PO₄, AsO₄, and SO₄ as a function of pH and ionic strength. The adsorption edge studies were performed in the pH 3 to 10 range and at ambient (20–22°C) temperatures. The adsorption of kG is a function of solution pH (decreasing with increasing pH) and independent of solution ionic strength, supporting the conclusion that kG is adsorbed by ligand exchange mechanisms. The adsorption of kG was decreased at all pH values in the presence of PO₄ and AsO₄, and was not significantly affected by the presence of SO₄ at pH values >5. The decrease in kG adsorption in the presence of AsO₄ and PO₄ is further evidence that kG is adsorbed via specific retention mechanisms. The addition of kG to gibbsite containing preadsorbed PO₄ did not result in PO₄ displacement, regardless of the concentration of kG. Ligand adsorption was modeled using the adsorption edge data and the 1-pK basic Stern surface complexation model. The kG adsorption data was described by the formation of two inner-sphere surface species: mononuclear monodentate and binuclear bidentate. The chemical models and associated intrinsic equilibrium constants developed to describe ligand adsorption in single-adsorbate gibbsite systems were used to predict ligand retention in the kaolinite and binary-adsorbate systems. In this manner, the competitive adsorption of all ligands as a function of pH was adequately described. The findings of this study indicate that kG is specifically retained by common soil minerals and may impact the availability of PO₄ and other specifically retained ligands in the rhizosphere.

Abbreviations: kG, 2-ketogluconate • LMMOA, low-molecular-mass organic acid • WSOS/DF, weighted sum of squares of residuals divided by the degrees of freedom