Grantee Research Project Results
1999 Progress Report: Formation of Metal-Phosphonate Complexes and Their Subsequent Chemical Reactions with Mineral Surfaces
EPA Grant Number: R826376Title: Formation of Metal-Phosphonate Complexes and Their Subsequent Chemical Reactions with Mineral Surfaces
Investigators: Stone, Alan T.
Institution: The Johns Hopkins University
EPA Project Officer: Chung, Serena
Project Period: February 1, 1998 through January 31, 2001
Project Period Covered by this Report: February 1, 1999 through January 31, 2000
Project Amount: $276,944
RFA: Exploratory Research - Environmental Chemistry (1997) RFA Text | Recipients Lists
Research Category: Water , Air , Safer Chemicals , Land and Waste Management
Objective:
Synthetic phosphonate chelating agents are brought into contact with toxic +II and +III metal ions during their use in scale/corrosion inhibition, metal finishing, ore recovery, oil drilling, industrial cleansing, pulp, paper, and textile dyeing, and agricultural production. The objectives of this project are to (i) explore processes that generate toxic metal ion-phosphonate complexes; (ii) study their speciation and sorption behavior in aqueous environmental media; and (iii) explore pathways and rates of complex dissociation and ligand breakdown.
Progress Summary:
Analytical Methods. Prior to the present study, Nowack (Journal of Chromatography A 1997;773:139-146) reported on an ion-pair HPLC method for identifying and quantifying strong phosphonate-containing chelating agents at sub-micromolar concentrations. The present study uses this method, along with two additional analytical methods. Oxidative breakdown products are analyzed using ion chromatography (with conductivity detection after chemical suppression). Weak phosphonate-containing chelating agents are identified and quantified using capillary electrophoresis. The analysis is performed in anion-mode with a capillary electrolyte that contains 10 mM phosphate buffer (pH 2), 5 mM CuSO4, and 5 mM TTAB electroosmotic flow modified. Complexation of phosphonate ions with CuII within the capillary generates a chromophore which can be detected at 254 nm.
Adsorption of Phosphonates onto the Goethite-Water Interface. The adsorption of phosphonate chelating agents onto FeOOH (goethite) is far stronger and extends to higher pH values than corresponding carboxylate chelating agents. Langmuir adsorption isotherms and pH adsorption isotherms have been measured for one phosphonate, two hydroxyphosphonates, and five aminophosphonates. At pH 7.2, the maximum extent of adsorption decreases as the number of phosphonate groups increases from one to five. Adsorption is modeled using a 2-pK constant capacitance model that postulates formation of a 1:1 surface complex involving one surface site and one phosphonate group.
The Influence of Metal Ions on the Adsorption of Phosphonates onto Goethite. The effects of metal ions on phosphonate adsorption (and phosphonates on metal ion adsorption) were examined using 10 µM concentrations of FeIII, and CuII, and varying concentrations of CaII and ZnII (up to 1.0 mM). Metal ions exerted an effect on phosphonate adsorption only when concentrations were in the mM range; ternary surface complex formation and adsorption onto precipitated (hydr)oxides of Zn are believed to be responsible. Polyphosphonates increase the adsorption of 10 µM CuII at pHs below the adsorption edge, but decrease adsorption at higher pHs, owing to formation of CuII-polyphosphonate complexes in solution.
Degradation of Nitrilotris(methylenephosphonic Acid) and Related (Amino)Phosphonate Chelating Agents in the Presence of Manganese and Molecular Oxygen. Following the lead of Steber and Wierich (Chemosphere 1987;16:163-178), non-photochemical degradation of NTMP in surface water samples has been confirmed. A survey of common water constituents has identified manganese as the causative agent. Mn2+ forms a complex with NTMP. MnII within this complex reacts with O2 far more rapidly than Mn2+(aq); oxidation of NTMP-complexed MnII has been observed at pHs as low as 3.5. The MnIII-NTMP complex then undergoes intramolecular electron transfer, generating NTMP breakdown products and Mn2+. Other metal cations such as CaII, CuII, and ZnII considerably slow down the reaction by competing with MnII for NTMP. EDTMP and DTPMP also are susceptible towards MnII-catalyzed autoxidation, while HEDP and IDMP are not.
As the autoxidation reaction takes place, the sum (NTMP + IDMP + FIDMP) exhibits mass balance. Production of orthophosphate equals the sum (IDMP + FIDMP). This observation is best explained by one-electron abstraction from the nitrogen, followed by cleavage of a nitrogen-carbon bond, yielding a carbon-centered methylene radical and orthophosphate. FIDMP is generated by interception of the carbon-centered methylene radical by O2, acceptance of a H. atom, and subsequent dehydration. In parallel, the carbon-centered methylene radical can be oxidized to the imminium cation, which rapidly hydrolyzes to yield IDMP and formaldehyde.
Oxidation of NTMP by MnOOH (manganite). Oxidation of NTMP by MnIIIOOH (manganite) generates the same breakdown products as the MnII + NTMP + O2 reaction. The heterogeneous reaction consumes NTMP at lower rates, however, and changes product yields. A lag period is observed, which can be shortened or eliminated by adding Mn2+ to the reaction medium. The presence or absence of O2 also affects reaction rates and product yields. Reaction with MnIIIOOH and other MnIII,IV-containing minerals may represent an important sink for phosphonates in soils and sediments.
Oxidation of NTMP, BPMG, PMIDA, and NTA by CoOOH (heterogenite). CoIIIOOH (heterogenite) is a useful surrogate for the many possible adsorbed and precipitated CoIII species found in contaminated soils and sediments. The reduction potential for the CoOOH/Co2+(aq) half-reaction is similar in magnitude to that of the MnOOH/Mn2+(aq) half-reaction. Despite this fact, electron-transfer reactions involving CoIII (d6, low spin) are typically slower than those involving MnIII (d4, high spin, Jahn-Teller distorted).
An experiment was performed in which 50, 100, and 150 micromolar concentrations of chelating agent were added to suspensions containing 425 micromolar CoOOH (10 mM acetate buffer, pH 4.5). NTA, which contains three carboxylate groups, solubilized equimolar amounts of cobalt primarily via ligand-assisted dissolution. A series of structural analogs is available in which carboxylate groups are systematically replaced by phosphonate groups; PMIDA contains one, BPMG contains two, and NTMP contains three phosphonate groups. The amount of cobalt solubilized increases in proportion to the number of phosphonate groups; reductive dissolution is the predominant solubilization mechanism.
Capture of CrIII by Phosphonate-Containing Chelating Agents. CrIII(IDA)2- was synthesized following the procedure of Weyh and Hamm (Inorganic Chemistry 1968;7:2431-2435) and brought into contact with the carboxylate ligands NTA and EDTA, the mixed phosphonate-carboxylate ligands BPMG and PMIDA, and the phosphonate ligand EDTMP. All five added ligands form complexes which are thermodynamically more stable than the parent complex. Ligands possessing at lease one phosphonate group capture CrIII more rapidly than ligands possessing only carboxylate functional groups. In contaminated waters where CrIII speciation is under kinetic control, complexation by phosphonates may therefore be especially important.
Future Activities:
Our adsorption work has raised important questions regarding the participation of phosphonate moieties in the surface complex formation. To further develop our predictive capacity, the adsorption of a series of substituted phenylphosphonic acids onto FeIII (hydr)oxides will be examined. It is expected that the connection between basicity (pKa) and extent of adsorption will be different from that of analogous carboxylic acids.
Mixed phosphonate/carboxylate chelating agents are widely employed in commerce, but their adsorption behavior and susceptibility towards thermal degradation have not been explored. Our work will focus upon commercial compounds with unusual biological activity, such as those with virostatic properties (e.g., phosphonoacetate and phosphonoformate) and those with herbicidal properties (e.g., glyphosate and Glyphosine).
To date, all compounds susceptible towards Mn2+-catalyzed autoxidation possess at least one amine moiety and at least three phosphonate moieties. A series of experiments is needed to determine whether phosphonates that do not possess amine groups (e.g., 3-phosphonobutane-1,2,4-tricarboxylic acid, used in water treatment) or phosphonates with fewer than three phosphonate groups (e.g., 2-aminoethylaminomethylphosphonic acid and N-ethyliminobis-(methylenephosphonic acid)) also are susceptible towards autoxidation. For all the chelating agents that have been examined so far (NTMP, EDTMP, DTPMP), MnII oxidation to MnIII by O2 is followed by oxidation of the coordinated chelating agent. For some of the commercial compounds that we plan to test, MnII oxidation to MnIII by O2 may still occur, but the coordinated chelating agent may be resistant to subsequent oxidation. Hence, the presence of the chelating agent may yield soluble MnIII, which would have important environmental consequences.
Journal Articles on this Report : 3 Displayed | Download in RIS Format
| Other project views: | All 19 publications | 4 publications in selected types | All 4 journal articles |
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Nowack B, Stone AT. Adsorption of phosphonates onto the goethite-water interface. Journal of Colloid and Interface Science 1999;214(1):20-30. |
R826376 (1998) R826376 (1999) R826376 (Final) |
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Nowack B, Stone AT. The influence of metal ions on the adsorption of phosphonates onto goethite. Environmental Science and Technology 1999;33:3627-3633. |
R826376 (1999) R826376 (Final) |
not available |
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Nowack B, Stone AT. Degradation of nitrilotris (methylenephosphonic acid) and related (amino) phosphonate chelating agents in the presence of manganese and molecular oxygen. Environmental Science and Technology 2000;34(22):4759-4765. |
R826376 (1999) R826376 (Final) |
not available |
Supplemental Keywords:
chemicals, toxics, metals, heavy metals, organics, intermediates, oxidation, redox reactions, coordination chemistry, iron, chromium, iminodiacetic acid, nitrilotriacetic acid, ethylenediaminetetraacetic acid, hydroxymethylphosphonic acid, nitrilotris(methylphosphonic acid), ethylenedinitrilotetrakis(methylenephosphonic acid), diethylenetrinitrilopentakis(methylenephosphonic acid), N-(phosphonomethyl)glycine, ?(phosphonomethyl)iminodiacetic acid, N,N-bis(phosphonomethyl)glycine., Air, Scientific Discipline, Chemistry, Biology, Environmental Chemistry, Engineering, Chemistry, & Physics, environmentally conscious manufacturing, environmental engineering, chemical composition, glyphosate, chemical kinetics, ligand exchange, pollutant transport, toxic metalsRelevant Websites:
http://www.jhu.edu:80/~dogee/stone.html
Progress and Final Reports:
Original AbstractThe perspectives, information and conclusions conveyed in research project abstracts, progress reports, final reports, journal abstracts and journal publications convey the viewpoints of the principal investigator and may not represent the views and policies of ORD and EPA. Conclusions drawn by the principal investigators have not been reviewed by the Agency.