Grantee Research Project Results
2000 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, 2000 through January 31, 2001
Project Amount: $276,944
RFA: Exploratory Research - Environmental Chemistry (1997) RFA Text | Recipients Lists
Research Category: Water , Land and Waste Management , Air , Safer Chemicals
Objective:
In recent decades, phosphonate functional groups (RPO32-) have appeared in a growing number of manufactured chemicals. Phosphonate-based chelating agents are used to prevent the formation of undesirable precipitates and protect others from dissolution. Others alter the reactivity of dissolved metal ions. Phosphonate-containing compounds capable of eliciting a biological response are used as agricultural chemicals, pharmaceuticals, and sterilants. The objective of this project is to explore molecular-level phenomena that affect the partitioning and reactivity of phosphonate-containing chemicals in heterogeneous environmental media.Progress Summary:
Analytical Methods. The ion-pair HPLC method of Nowack (Journal of Chromatography A 1997;773(1-2):139-146) was used to identify and quantify HEDP, NTMP, EDTMP, and DTMP. An ion chromatographic method (with conductivity detection after chemical suppression) was developed for three NTMP breakdown products (IDMP, methyl-IDMP, and FIDMP). Capillary electrophoresis (CE) techniques also have been developed for identifying and quantifying phosphonate-containing compounds. Phenylphosphonic acid, benzylphosphonic acid, and ethyl benzylphosphonic acid, along with the analogous phosphoric acid esters phenyl phosphate and diphenyl phosphate, are all chromophores in the UV spectral range, and hence can be directly determined using UV-detection in anion mode (the CE electrolyte contains 5 mM TTAB electroosmotic flow modifier and 10 mM phosphate buffer at pH 7.0). Non-chromophores such as methylenephosphonic acid, phosphonoformic acid, phosphonoacetic acid, IDMP, and HEDP can be detected by placing Cu(II) in the CE electrolyte. Because the molar absorptivity of Cu(II)-phosphonate complexes at 254 nm is different from that of Cu(II)-electrolyte buffer complexes, each peak can be detected. 5 mM CuSO4 and 5 mM TTAB are used in the electrolyte, along with either a 5 mM phosphate buffer (pH 2.0) or a 5 mM hexamine buffer (pH 5.2).Metal Ion-Phosphonate Coordination Reactions. 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 that are thermodynamically more stable than the parent complex. Ligands possessing at least one phosphonate group capture Cr(III) more rapidly than ligands possessing only carboxylate functional groups. In contaminated waters where Cr(III) speciation is under kinetic control, complexation by phosphonates may therefore be especially important.
The systemic fungicide fosetyl-Al is sold as a neutral 1:3 complex with the toxic metal ion aluminum. Using NMR, coordination equilibria between the phosphonate monoester and dissolved aluminum are being explored.
Adsorption of Phosphonate, Phosphonate Ester, and Phosphate Ester Compounds at the Mineral/Water Interface. Adsorption arises from a combination of near-range bonding interactions and medium- to long-range electrostatic interactions. Near-range bonds are typically strengthened by increasing basicity (proportional to pKa) of the coordinating Lewis Base group of the adsorbate. Medium- to long-range electrostatic interactions are a function of the protonation level (and hence charge) of both the adsorbent and adsorbate.
Phenyl phosphate, phenylphosphonic acid, and benzylphosphonic acid all exist as dianions above pH 7.5, and adsorb quite substantially onto TiO2 and Al2O3. Diphenyl phosphate and ethyl benzylphosphonic acid, which are monoanions at this pH, adsorb to a much lesser extent. Electrostatic interactions are believed to be responsible for the two groupings just described. Small differences within each group, however, are due to differences in near-range bond strength. Ethyl benzylphosphonate adsorbs more strongly onto TiO2 than diphenyl phosphate, while phenyl phosphate adsorbs more strongly onto Al2O3 than phenylphosphonic acid and benzylphosphonic acid. Based on these observations, it should be possible to predict the adsorption behavior of other phosphonate-containing compounds based upon their molecular charge and basicity.
Detailed Study of the Adsorption of (Amino)phosphonates onto Goethite. 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 (methylenephosphonic acid), two hydroxyphosphonates (hydroxymethylphosphonic acid and HEDP), and five aminophosphonates (aminomethylphosphonic acid, IDMP, NTMP, EDTMP, and DTPMP). 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 effects of metal ions on phosphonate adsorption (and phosphonates on metal ion adsorption) were examined using 10 M concentrations of Fe(III), and Cu(II), and varying concentrations of Ca(II) and Zn(II) (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 M Cu(II) at pHs below the adsorption edge, but decrease adsorption at higher pHs, owing to formation of Cu(II)-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. Mn(II) within this complex reacts with O2 far more rapidly than Mn2+(aq); oxidation of NTMP-complexed Mn(II) 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 Ca(II), Cu(II), and Zn(II) considerably slow down the reaction by competing with Mn(II) for NTMP. EDTMP and DTPMP also are susceptible towards Mn(II)-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 an 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 Mn(II) + 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 Co(II)I 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.
Future Activities:
The final 12 months of the project will be used to complete experiments and manuscripts, primarily in the following areas: (1) reaction of (amino)phosphonates and carboxylate/phosphonates with CoOOH and MnOOH; (2) structure-property relationships pertaining to the adsorption of phosphonate-containing compounds onto mineral surfaces; and (3) rates and equilibria of metal ion-phosphonate complex formation in solution.Journal Articles:
No journal articles submitted with this report: View all 19 publications for this projectSupplemental Keywords:
environmental chemistry, oxidation, heavy metals, intermediates, adsorption, dissolution, chelating agents, complex formation, pesticides, phosphonates, capillary electrophoresis, agricultural chemicals, chromium, manganese, iron, cobalt., Scientific Discipline, Air, Environmental Chemistry, Chemistry, Engineering, Chemistry, & Physics, Biology, environmentally conscious manufacturing, ligand exchange, chemical composition, pollutant transport, toxic metals, phosphonates, environmental engineering, chemical kineticsRelevant 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.