Date of Award
Master of Science
Environmental and Soil Sciences
Michael E. Essington
Mark A. Radosevich, Neal S. Eash
In rhizosphere soil, the low-molecular mass organic acid (LMMOA) anions 2-ketogluconate (kG) is produced via microbial activity and exists in significant and sustained concentrations. One of the mechanisms in which this LMMOA anion may influence the chemistry of soil systems is through adsorption by constant-potential minerals. This study examines the adsorption of kG onto gibbsite, kaolinite and goethite in the presence of absence of phosphate (PO4), arsenate (AsO4) and sulfate (SO4) as a function of pH and ionic strength. The adsorption of kG by gibbsite, goethite, and kaolinite is a function of solution pH and independent of solution ionic strength. The adsorption data supports 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 above 6. 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. However, the addition of PO4 to gibbsite containing preadsorbed kG resulted in the displacement of preadsorbed kG. These results indicate that kG is not held as strongly as PO4 to gibbsite surfaces, and that the ability of PO4 to displace adsorbed kG is greater than the ability of kG to displace adsorbed PO4. The adsorption of kG, PO4, AsO4, and SO4 to gibbsite was modeled using the adsorption edge data and the CD-MUSIC surface complexation model. The kG adsorption data in both the 0.001 M and 0.01 M NaCl gibbsite systems were described by the formation of two monodentate-mononuclear inner-sphere complexes: ≡AlkG1/2- and ≡AlkGH-13/2-. Phosphate adsorption by gibbsite was modeled by the formation of ≡AlOPO3H3/2- and ≡AlOPO3H21/2- in the low ionic strength systems (0.001 M NaCl), and by ≡AlOPO35/2- and ≡AlOPO3H21/2- in the high ionic strength systems (0.01 M NaCl). Arsenate adsorption by gibbsite in both ionic strengths was modeled using the ≡AlOAsO35/2- and ≡AlOAsO3H21/2- inner-sphere surface complexes. Sulfate adsorption was described by the formation of the outer-sphere ≡AlOH21/2+--SO42- species. The adsorption of kG by goethite in both the 0.001 M and 0.01 M NaCl systems was best described by the formation of the monodentate-mononuclear and bidentate-binuclear inner-sphere surface complexes: ≡FekG1/2- and ≡Fe2kGH-11. Both PO4 and AsO4 were completely adsorbed by goethite; therefore, chemical adsorption models could not be derived. However, SO4adsorption by goethite was described with the ≡FeOH21/2+--SO42- species. The chemical models and associated intrinsic equilibrium constants developed for ligand adsorption from the single ligand systems were employed to predict ligand retention in the kaolinite and binary ligand systems. In general, particularly at pH values greater than 7, the predicted adsorption behavior did not adequately predict the experimental adsorption data in the gibbsite or kaolinite systems. This finding suggests that the surface complexation reactions derived for pH >7 systems may have been incorrect. The adsorption behavior Kg establishes the potential for this ligand to significantly impact rhizosphere chemistry. Ketogluconate is specifically retained by common soil minerals and may impact the phytoavailability of PO4 and other specifically-retained ligands in the rhizosphere.
Anderson, Robert Maxwell III, "Adsorption of 2-Ketogluconate by Gibbsite, Goethite, and Kaolinite. " Master's Thesis, University of Tennessee, 2006.