KETOGLUCONATE: SOLUTION CHEMISTRY, ADSORPTION-DESORPTION, AND IMPACT ON MINERAL SOLUBILITY

• Sponsoring Institution: National Institute of Food and Agriculture
• Project Status: COMPLETE
• Funding Source: NRI COMPETITIVE GRANT
• Reporting Frequency: Annual
• Accession No.: 0185605
• Grant No.: 2001-35107-10165
• Proposal No.: 2000-00537
• Project Start Date: Dec 15, 2000
• Project End Date: Dec 31, 2004
• Grant Year: 2001
• Program Code: [(N/A)]- (N/A)

Recipient Organization
UNIVERSITY OF TENNESSEE
2621 MORGAN CIR
KNOXVILLE,TN 37996-4540

Performing Department
BIOSYSTEMS ENGINEERING AND SOIL SCIENCE

Non Technical Summary
Low-molecular-weight organic acid anions, such as 2-ketogluconate, are plant root and microbial exudates that are common to the soil solution. Literature and our preliminary studies indicate that this compound has the potential to increase metal solubility and mobility, either through metal cation complexation or by hindering mineral crystallization and growth. However, despite the apparent prevalence and potential impact of 2-ketogluconate on soil solution and solid-phase chemistry, there exists no critically evaluated information on its metal-complexation chemistry, its ability to enhance mineral solubility by inhibiting crystallization, or its adsorption-desorption behavior. The objectives of this proposed research seek to elucidate the chemistry of 2-ketogluconate and to enhance our ability to predict the fate and behavior of components in the soil. A technique will be developed for the analysis of 2-ketogluconate in simple and soil systems. The acid dissociation constant will be determined, as will the ion association constants for the formation of 2-ketogluconate complexes with various trace (Cu, Cd, Pb) and major (K, Ca, Mg, Al, Fe) metals. The mechanisms by which 2-ketogluconate hinders the precipitation of Al and Fe(III) (hydroxy-) oxides and phosphates will be identified. The adsorption-desorption behavior of 2-ketogluconate on common soil minerals in the presence and absence of competing organic and inorganic ligands will be investigated.

Animal Health Component

10%

Research Effort Categories

Basic

90%

Applied

10%

Developmental

(N/A)

Classification

Knowledge Area (KA)	Subject of Investigation (SOI)	Field of Science (FOS)	Percent
102	0110	2000	60%
102	0110	2040	40%

Knowledge Area
102 - Soil, Plant, Water, Nutrient Relationships;

Subject Of Investigation
0110 - Soil;

Field Of Science
2040 - Mineralogy; 2000 - Chemistry;

Keywords

organic acids

root exudates

keto acids

soil chemistry

desorption

solubility

mineral absorption

oxides

ion absorption

soil properties

metal ions

metal compounds

potassium

calcium

magnesium

aluminum

iron

copper

cadmium

lead

hydroxides

soil mineralogy

Goals / Objectives
Develop an analytical methodology to quantify 2-ketogluconate in complex matrices and critically evaluate the ionization constant for the dissociation of 2-ketogluconic acid. Determine ion association constants for the formation of aqueous complexes that contain 2-ketogluconate and the metals: K, Ca, Mg, Al, Fe(III), Cu, Cd, and Pb. Establish the impact of 2-ketogluconate on crystalline and amorphous Al(III) and Fe(III) (hydroxy-) oxide and phosphate mineral solubility and identify the mechanisms responsible (i.e., complexation versus inhibition of crystal formation). Examine the adsorption-desorption chemistry of 2-ketogluconate by metal oxides as a function of pH, ionic strength, and the presence and concentrations of competing inorganic and organic ligands.

Project Methods
An analytical method for the detection of 2-ketogluconate in soils and simple systems will be developed using ion chromatography columns that have a proven suitability for organic acid anions. Variable parameters include: eluent composition, flow rate, and isocratic versus gradient elution. Replicate potentiometric titrations of 2-ketogluconate in several ionic strength environments will be performed to critically and unambiguously determine the ionization constant. The potentiometric (acid-base) titrations will also be employed to investigate the influence of 2-ketogluconate on Al(III) and Fe(III) aqueous speciation. Three different ionic strength systems will be examined. The complexation of K, Ca, Mg, Cd, Cu, and Pb by 2-ketogluconate will be investigated using an ion selective electrode procedure. Potentiometric titrations will be performed in CO2-free systems at fixed pH, and in three different ionic media. The equilibrium solubility of amorphous and crystalline phases of Al and Fe (hydroxy) oxides (gibbsite and goethite) and phosphates (varascite and strengite) will be examined. Mineral solubilities will be examined as a function of undersaturated or supersaturated initial conditions, ionic strength, pH, and in the presence and absence of 2-ketogluconate. The solids will be subjected to x-ray diffraction and scanning electron microscopy to characterize their crystallinity and morphology after equilibrium is attained. The adsorption of 2-ketogluconate by gibbsite, goethite, kaolinite, quartz, and amorphous Al and Fe hydroxides will be examined. Adsorption and desorption isotherms will be generated, as will adsorption envelopes. Adsorption will be characterized as a function of time, pH, ionic strength, 2-ketogluconate concentration, and type and concentration of competing ligands (e.g., orthophosphate, citrate, malate, tartarate, and oxalate). Desorption isotherms will also be determined for selected systems through a minimum of four cycles.

Progress 12/15/00 to 12/31/04

Outputs
The objectives of this research were to elucidate the chemistry of 2-ketogluconate (KG), a microbial byproduct of gluconic acid (a root exudate and monosaccharide), and to enhance our ability to predict the fate and behavior of substances in soil systems. A chromatographic technique was developed for the analysis of KG in aqueous solutions with a method detection limit of 0.1 mg/L. Potentiometric titrations indicated that KG is a relatively strong diprotic weak acid, with pKa1 of the protonated keto group of 1.7, and pKa2 of the carboxyl group of 2.93. The ion association constants for the formation of KG complexes with Ca, Cu, and Al ions illustrated the significant influence of the organic ligand on the aqueous chemistry of these metal cations. Ketogluconate was found to form an outer sphere complex with divalent Ca; log K = 0.9 for Ca(2+) + KG(-) = CaKG(+). Ketogluconate also forms inner sphere and bidentate complexes with divalent Cu (log K = -2.75 for Cu(2+) + KG(-) = CuKG(0) + H(+)) and divalent AlOH ions (log K = -4.56 for Al(3+) + H2O = AlOHKG(0) + 2H(+)). Apparently, the carboxyl and the alcohol group (which is ortho to the keto group on the KG molecule) participate in metal complexation. Ketogluconate may also hinder the precipitation of Al and Fe(III) (hydroxy-) oxides and other soil minerals by forming strong surface complexes. The adsorption-desorption behavior of KG indicates the rapid saturation of surface functional groups and the formation of specific surface complexes by processes that are highly hysteretic. Surface complexation modeling (constant capacitance model) suggests ligand exchange at pH values below the zero point of charge of the adsorbent, and water bridging through adsorbed metals at high pH values.

Impacts
2-Ketogluconate (KG) is found in the rhizosphere at significant concentration levels, particularly in manure-amended soil and in soils that contain low microbial- and plant-available phosphorus. This organic compound has been shown to enhance the solubility of various minerals and the concentrations of various trace and major elements in soil solutions. Rigorously obtained, quantitative information indicate that KG is a naturally occurring chelating agent that is strongly adsorbed by soil minerals. Thus, the ability of KG to enhance the solubility of nutrient-bearing minerals is related to both its soil solution chemistry and adsorption characteristics. The innate ability of KG to effectively solubilize soil minerals indicates that it significantly influences nutrient cycling, and it is an important component of the rhizosphere

Publications

• No publications reported this period

Progress 01/01/02 to 12/31/02

Outputs
The objectives of this research are to elucidate the chemistry of 2-ketogluconate, a microbial byproduct of root exudates, and to enhance our ability to predict the fate and behavior of substances in soil systems. A technique will be developed for the analysis of 2-ketogluconate in soil solutions. The acid dissociation constant will be determined, as will the ion association constants for the formation of 2-ketogluconate complexes with various trace and major metals. The mechanisms by which 2-ketogluconate hinders the precipitation of Al and Fe(III) (hydroxy-) oxides and phosphates will be identified. The adsorption-desorption behavior of 2-ketogluconate on common soil minerals in the presence and absence of competing ligands will be investigated and surface complexation modeling will be employed to aid in the identification of adsorption mechanisms. Efforts during the first year of study have focused on the determination of ion association constants for the formation of aqueous metal-ketogluconate complexes. Potentiometric titrations indicate that 2-ketogluconate is a relatively strong weak acid, with a pKa of 2.71. Typically, ligands having such low pKa values do not tend to form stable aqueous complexes with metal cations. However, data accumulated this reporting year indicate that 2-ketogluconate forms a significant Ca-ketogluconate complex, with an ion association constant of log K = 1.5. Ketogluconate also forms a strong complex with the divalent Cu ion, with an ion association complex of log K > 3. A comparison of the acid-base and complexation chemistry of the chemically similar ketocarboxylic acid, pyruvate, with the measured values for ketogluconate, indicates that the two compounds behave similarly in aqueous environments. The data also indicate that ketogluconate will significantly influence the behavior of several trace metal cations in the soil environment.

Impacts
2-Ketogluconate is found in the rhizosphere at significant concentration levels, particularly in manure-amended soil and in soils that contain low plant-available phosphorus. This organic compound has been shown to enhance the solubility of various minerals and the concentrations of various trace and major elements in soil solutions. Yet, there exists no rigorously obtained, quantitative information that would allow for the consideration of this compound in the geochemical characterization of soil solution chemistry. Due to its significance in soil systems, it is important that the chemistry of this organic acid be delineated.

Publications

• No publications reported this period

Progress 01/01/01 to 12/31/01

Outputs
The objectives of this research are to elucidate the chemistry of 2-ketogluconate, a microbial byproduct of root exudates, and to enhance our ability to predict the fate and behavior of substances in soil systems. A technique will be developed for the analysis of 2-ketogluconate in soil solutions. The acid dissociation constant will be determined, as will the ion association constants for the formation of 2-ketogluconate complexes with various trace and major metals. The mechanisms by which 2-ketogluconate hinders the precipitation of Al and Fe(III) (hydroxy-) oxides and phosphates will be identified. The adsorption-desorption behavior of 2-ketogluconate on common soil minerals in the presence and absence of competing ligands will be investigated and surface complexation modeling will be employed to aid in the identification of adsorption mechanisms. Efforts during the first year of study have focused on the identification of a suitable analytical technique and on the characterization of the acid-base properties of 2-ketogluconate. Analytical methodologies were investigated to detect and quantify 2-ketogluconate in aqueous solutions. Initially, isocratic ion chromatography procedures, using the standard analytical column, eluent, and operating conditions for anion (fluoride, chloride, nitrate, nitrite, bromide, sulfate, and phosphate) determinations were modified. Organic compound concentrations less than 1 mg/L (5 micro-molar) were detectable; however, operating procedures that would result in the isolation of 2-ketogluconate from fluoride and other low-molecular-mass organic acids could not be resolved. The developed ion chromatographic procedure is suitable for 2-ketogluconate analysis in matrices containing chloride, but the procedure is not acceptable for chemically complex systems (e.g., soil solutions and systems containing fluoride ion). Currently, high-pressure liquid chromatographic techniques for 2-ketogluconate isolation and analysis are under development.

Impacts
2-Ketogluconate is found in the rhizosphere at significant concentration levels, particularly in manure-amended soil and in soils that contain low plant-available phosphorus. This organic compound has been shown to enhance the solubility of various minerals and the concentrations of various trace and major elements in soil solutions. Yet, there exists no rigorously obtained, quantitative information that would allow for the consideration of this compound in the geochemical characterization of soil solution chemistry. Due to its significance in soil systems, it is important that the chemistry of this organic acid be delineated.

Publications

• No publications reported this period