Competitive adsorption of 2-ketogluconate and inorganic ligands onto gibbsite and kaolinite

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 degrees 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.