Grantee Research Project Results
Final Report: Toxic Metal Ion-Synthetic Chelating Agent Interactions in Aqueous Media
EPA Grant Number: R829356Title: Toxic Metal Ion-Synthetic Chelating Agent Interactions in Aqueous Media
Investigators: Stone, Alan T. , Ball, William P.
Institution: The Johns Hopkins University
EPA Project Officer: Hahn, Intaek
Project Period: October 1, 2001 through September 30, 2005 (Extended to September 30, 2006)
Project Amount: $333,057
RFA: Complex Chemical Mixtures (2000) RFA Text | Recipients Lists
Research Category: Hazardous Waste/Remediation , Land and Waste Management , Safer Chemicals
Objective:
This work focused on metal ion-chelating agent mixtures that yield adverse environmental impacts that cannot be anticipated by studying either the metal ion or the chelating agent alone. The objectives of this research project were to: (1) identify ways in which contaminant metal ions are rendered more toxic or otherwise problematic through reaction with naturally occurring or contaminant-derived chelating agents; and (2) identify synthetic chelating agents, which are innocuous as free species but yield toxic or unusually reactive species when they react with naturally-occurring metal ions.
Summary/Accomplishments (Outputs/Outcomes):
Chelating agents are part of many household formulations, including detergents, cleansers, and shampoos. Chelating agents that are not mineralized during waste treatment may enter the environment via wastewater effluent or the land disposal of biosolids. Chelating agent use in large industrial water-based operations is also commonplace. Chelating agents are used to sequester the transition metal ions iron, manganese, and copper during pulp and paper bleaching. Cooling water systems, boiler operations, oil drilling operations, and hydrothermal energy operations use chelating agents for scale and corrosion control.
Glyphosate and a handful of other herbicides possess chelating agent properties. Residential and agricultural uses of such herbicides represent potential nonpoint sources for chelating agent entry into surface waters. Herbicides used for weed control at brownfield sites may also come into direct contact with chromium-contaminated soils.
Chelating agents can yield problematic mixture effects on contact with contaminant-derived metal ions and even with some naturally-occurring metal ions. Our work focuses on two first-row transition metals, chromium and manganese. Manuscripts based on our work will now be summarized:
Dissolution of Amorphous Chromium (Hydr)Oxide by (Amino)Carboxylate Chelating Agents
Chromium-contaminated sediments (e.g., Baltimore Harbor) and soils (e.g., Hudson County, New Jersey) occur at several U.S. locations. In the absence of chelating agents, chromium(VI) is mobile in the environment (as chromate oxyanion), whereas chromium(III) is immobile (either adsorbed or precipitated). Synthetic chelating agent introduction into sediments and soils poses the risk of chromium(III) solubilization.
For chromium(III) solubilization to occur, the following four criteria must be met:
- The chelating agent must adsorb onto chromium-containing surfaces to a significant extent.
- The chelating agent must be able to detach surface-bound chromium(III) atoms.
- The chelating agent must be strong enough to maintain chromium in the dissolved state under the prevalent chemical conditions of the system.
- The chelating agent-chromium(III) complex must not re adsorb to a significant extent.
Adsorption is a ligand exchange reaction involving the inner coordination spheres of surface-bound chromium(III) atoms. Exchange reactions for chromium(III) are expected to be exceedingly slow (i.e., slower than those of manganese(III) and iron(III)) and perhaps more comparable to cobalt(III).
Our experiments began with the synthesis of amorphous chromium hydroxide particles (approximately 10 nanometers in diameter) that resemble those found in sediments and soils. Under carefully controlled pH conditions, extents of chelating agent adsorption and dissolved chromium concentrations were monitored as a function of time. Because of the slowness of exchange reactions involving surface-bound chromium(III), extents of adsorption measured 5 minutes after mixing are as little as one-third of the maximum extent achieved after several days of contact. Dissolution rates are slower still, allowing us to constantly monitor chelating agent adsorption and to calculate rates of dissolution per mole of adsorbed chelating agent.
Carboxylate-containing chelating agents, which predominate in most household and industrial applications, adsorb to a greater extent as the solution pH is decreased. Per mole of adsorbed chelating agent, markedly different dissolution behaviors were revealed. Iminodiacetate, nitrilotriacetate, citrate, and cyclohexaneethylenediaminetetraacetate dissolution reactions all increased in rate under increasingly acidic conditions, whereas trimethylenedinitrilotetraacetate and ethylenediaminetetraacetate dissolution reactions were invariant of pH within the same pH range. Under slightly alkaline conditions (7 < pH < 9), cyclohexaneethylene-diaminetetraacetate, trimethylenedinitrilotetraacetate, and ethylenediaminetetraacetate all yielded a sharp spike in dissolution rate per mole of adsorbed chelating agent. Hence, these last three chelating agents would be much more problematic under high pH conditions.
In the course of our experiments we identified an interesting and potentially quite important synergistic effect. Acetate, which by itself cannot dissolve chromium(III) hydroxides, can nevertheless accelerate dissolution by chelating agents. With iminodiacetate, for example, 5 millimolar acetate caused an eight-fold increase in dissolution rates. To accurately predict dissolution rates in real soils and sediments, it will be necessary to appraise the combined effects of all organic system constituents.
Dissolved Chromium(III) Speciation and Rates of Oxidation by Manganese(III,IV) (Hydr)Oxide: Effect of Aquo/Hydroxo, Iminodiacetate (IDA), and Nitrilotriacetate (NTA) Ligands on Time Course Behavior
The previous section discussed a possible source for dissolved chromium(III). Our experiments now shift to an important sink for dissolved chromium(III), the oxidation to chromate ion by manganese(III,IV) (hydr)oxides found in soils and sediments.
Chromium(III) sulfate salts, when added to solutions with pH greater than 4.0, yield slightly higher rates of chromate ion formation than chromium(III) chloride salts. In both cases, the ratio of dissolved manganese(II) to dissolved chromate ion decreases as the pH increases, indicating greater manganese(II) adsorption. It is important to note that the exact nature of the chromium(III) reactant in such systems (i.e., monomeric hydroxo species, multimeric hydroxo species, and (hydr)oxide colloids) cannot be determined.
It is possible to independently synthesize and purify solutions of specific chromium(III)-chelating agent complexes (i.e., 1:1 complexes with iminodiacetate, nitrilotriacetate, N-(2-hydroxyethyl)ethylene-diaminetriacetate, and ethylenediaminetetraacetate) and 1:2 complexes with iminodiacetate. In our work, capillary electrophoresis was used to confirm the purity of each reactant and to monitor loss of the reactant, production of chromate ion, production and subsequent oxidation of free chelating agent, and production and subsequent oxidation of chelating agent oxidation products. Because concentrations of chromium(III) reactant and chromate ion product can both be monitored, extents of adsorption can be calculated as reactions take place. The level of description that capillary electrophoresis provides is unprecedented.
Oxidation of the 1:1 chromium(III)-nitrilotriacetate complex to chromate ion by manganese(III,IV) (hydr)oxides increases in rate with increasing pH within the range 4 < pH < 9.5. Nitrilotriacetate, released as chromium(III), is converted into chromate ion and is resistant to oxidation under alkaline conditions. As a consequence, concentrations increase throughout the entire time course. Lowering the pH into neutral and acidic range facilitates nitriloacetate oxidation to iminodiacetate, which in turn becomes oxidized to products that we cannot discern by capillary electrophoresis. As a consequence, concentrations of nitrilotriacetate and iminodiacetate increase to a plateau and then decrease. Plateau heights decrease as the pH is lowered.
Dissolved Chromium(III) Speciation and Rates of Oxidation by Manganese(III,IV) (Hydr)Oxide: Factors Affecting Chromium(VI) Production Rates
In a given time- course plot, chemical reactions at intermediate and long time scales are quite complex. Alteration of manganese(III,IV) (hydr)oxide surfaces by chelating agent oxidation takes place and alters surface reactivity. Initial chromate ion production rates are free of this problem and hence will be used to compare the reactivities of different chromium(III) species.
Highest rates of chromium(III) oxidation, regardless of pH, are observed with the sulfate and chloride salts. Chromium(III)-chelating agent complexes where one or more coordination positions around the central chromium(III) ion are occupied by water molecules or hydroxide ions, yield intermediate oxidation rates (e.g., 1:1 complexes with iminodiacetate and nitrilotriacetate ). Complexes where all six coordination positions around the central chromium(III) ion are occupied by the chelating agent yield the lowest oxidation rates. 1:1 complexes with N-(2-hydroxyethyl)ethylene-diaminetriacetate and ethylenediaminetetraacetate and 1:2 complexes with iminodiacetate fall into this low reaction rate category.
Water molecules or hydroxide ions coordinated to the central chromium(III) ion appear to facilitate electron transfer via inner-sphere bond bridging with surface-bound manganese(III,IV) atoms. Thermodynamic factors may be important as well. Although all chelating agents stabilize the chromium(III) relative to chromate ion, this effect increases as the denticity of the chelating agent increases.
The Citric Acid-MnIII,IVO2(s, birnessite) Reaction. Electron Transfer, Complex Formation, and Autocatalytic Feedback
The average abundance of manganese in the earth’s crust is only 1/50th that of iron. Manganese(III,IV) (hydr)oxides become enriched near oxic/anoxic interfaces in soils and sediments, however, owing to the greater solubility and mobility of the +II oxidation state. Manganese(III,IV) (hydr)oxides also accumulate in sand filters of many water supply plants and in cooling water systems drawing on manganese-containing surface waters.
When synthetic chelating agents come into contact with manganese(III,IV) (hydr)oxides, two reactions are possible. With ligand-assisted dissolution, the chelating agent detaches manganese(III) from (hydr)oxide surfaces, yielding highly reactive and toxic manganese(III) complexes. With reductive dissolution, the chelating agent becomes oxidized, and dissolved manganese(II) is released. Because chelating agent oxidation products may possess problematic characteristics not found in the parent compound (e.g., toxicity), identifying them is important.
Citrate is used as a chelating agent in a number of water-based industrial operations, most notably for cleaning printed circuits. Our experiments reveal that citrate is oxidized by birnessite (a phase consisting of 22 percent manganese(III) and 78 percent manganese(IV)) to 3-ketoglutarate and lesser amounts of acetoacetate. When reagents are mixed, nearly negligible citrate loss, 3-ketoglutarate production, acetoacetate production, and dissolved manganese(II) production are observed during the first hour or two, which we characterize as the lag phase. Next, there is an acceleratory phase where rates increase with time, followed by a deceleration phase as reactants become depleted. Hence, the reaction is autocatalytic. Adding dissolved manganese(II) at the onset of reaction causes the lag phase to disappear; the reaction commences with the acceleratory phase. Zinc(II) addition has no such effect. Evidence points to manganese(II) citrate as the autocatalytic species. Once appreciable concentrations of this complex forms, manganese-to-manganese electron transfer can take place, yielding dissolved manganese(III) citrate complexes, which are discernable by capillary electrophoresis. Manganese(III) citrate complexes eventually decompose via intramolecular electron transfer, yielding dissolved manganese(II), which can continue the autocatalytic cycle.
Reaction of MnIII,IV (Hydr)Oxides With Oxalic Acid, Glyoxylic Acid, Phosphonoformic Acid, and Structurally-Related Organic Compounds
It would be desirable to predict the susceptibility of chelating agents towards oxidation by manganese(III,IV) (hydr)oxides based on their functional groups and molecular structure. This way, situations where problematic organic oxidation products may accumulate could be anticipated.
Our findings with the mixed phosphonate/carboxylate chelating agent phosphonoformate are particularly noteworthy. With birnessite, electron transfer yields inorganic carbonate, inorganic orthophosphate, and dissolved manganese(II). No dissolved manganese(III) is observed. The phase manganite consists solely of manganese(III). With manganite, the balance between reductive dissolution and ligand-assisted dissolution depends on pH. At pH 7.0, ligand-assisted dissolution dominates, and dissolved manganese is found entirely in the +III oxidation state. Decreasing the pH to 5.0 raises the importance of reductive dissolution; only one-half of the dissolved manganese can be attributed to the +III oxidation state.
Reductive dissolution converts gloxylic acid into formic acid, pyruvic acid into acetic acid, and 2,3-butanedione into acetic acid. When differences in surface area loading are accounted for, oxalic acid, pyruvic acid, and 2,3-butanedione at pH 5.0 yield virtually the same reductive dissolution rate with birnessite and with manganite. Glyoxylic acid reacts 14-times faster with birnessite than with manganite. Birnessite reacts more slowly than manganite by a factor of 1/16th with oxamic acid, 1/20th with lactic acid, and 1/33rd with dimethyl oxalate. Reaction rates generally increase as the pH is decreased, in part because of increases in the extent of adsorption with decreasing pH.
Reduction of MnO2(Birnessite) by Malonic Acid, Acetoacetic Acid, Acetylacetone, and Structurally-Related Compounds
Oxidation of the chelating agent malonate by the birnessite has been monitored using capillary electrophoresis. The oxidation products formate, oxalate, and inorganic carbonate can all be detected. Formate is resistant to oxidation, so it makes sense that it s concentration builds up as the reaction progresses. Oxalate becomes oxidized but at rates low enough to allow for its concentration to grow to a maximum and then to decline. Tartronate, ketomalonate, and glycolate, all putative reaction intermediates in the oxidation of malonate, were separately reacted with manganese(III,IV) (hydr)oxides. Tartronate and ketomalonate are oxidized at least 380-times more rapidly than malonate, hence, it makes sense that their presence went undetected. The rate of glycolate oxidation is 1.8-times higher than that of malonate. Because glycolate was not observed during malonate oxidation, we can exclude as a possibility the pathway involving glycolate.
The bridging methylene group is a key feature in malonate oxidation. Replacing C-H with C-CH3 gives us methylmalonate, which is oxidized at a slightly lower rate. Replacing both C-H groups in this way, yielding dimethylmalonate, prevents any oxidation reaction from taking place. This observation, combined with pathway information from the preceding paragraph, indicates that the susceptibility of malonate towards oxidation arises either from formation of the more reactive enol tautomer or from the slight acidity of C-H bonds within the methylene linkage. With acetoacetate and acetylacetone, these attributes are more pronounced than with malonate. All three compounds yield oxidation rates that decrease as pH is increased. This decrease is least for acetylacetone, most likely because it s pKa (9.0) is substantially higher than those of acetoacetate (pKa 3.8) and malonate (pKa1 2.9, pKa2 5.7). The higher pKa, in turn, shifts the pH of maximum adsorption to higher pH. Oxidation rates for acetylacetone and five other beta-diketones were compared. Oxidation rates generally increase as the enol content is increased.
Phosphonate- and Carboxylate-Based Chelating Agents that Solubilize (Hydr)Oxide-Bound MnIII
Synthetic chelating agents capable of solubilizing manganese(III) are potentially quite toxic, because manganese(III) is a strong and reactive oxidant. Our experiments revealed that the inorganic chelating agent pyrophosphate and the organic chelating agents methylenediphosphonate and phosphonoacetate yield dissolved manganese(III) complexes without any indication of chelating agent breakdown by oxidation. The phase manganite, which consists solely of manganese(III), can be completely solubilized by sufficient concentrations of these chelating agents. With birnessite, the fraction of manganese in the +III oxidation state (22%) is solubilized, leaving behind the remaining manganese(IV). Effects of pH and chelating agent concentration on reaction rate were explored.
This portion of our research also revealed that 2-phosphonobutane-1,2,4-tricarboxylate, a synthetic chelating agent used for scale and corrosion control in cooling water systems, reacts via both ligand-assisted dissolution and reductive dissolution. The herbicide glyphosate and the industrial chelating agent iminodiphosphonate also react in this manner. Hence, the problematic appearance of dissolved manganese(III) can arise whenever these three chelating agents come into contact with manganese(III,IV) (hydr)oxides.
Journal Articles on this Report : 2 Displayed | Download in RIS Format
Other project views: | All 15 publications | 2 publications in selected types | All 2 journal articles |
---|
Type | Citation | ||
---|---|---|---|
|
Wang Y, Stone AT. Reaction of MnIII,IV (hydr)oxides with oxalic acid, glyoxylic acid, phosphonoformic acid, and structurally-related organic compounds. Geochimica et Cosmochimica Acta 2006;70(17):4477-4490. |
R829356 (Final) |
not available |
|
Wang Y, Stone AT. The citric acid-MnIII,IVO2( birnessite) reaction. Electron transfer, complex formation, and autocatalytic feedback. Geochimica et Cosmochimica Acta 2006;70(17):4463-4476. |
R829356 (Final) |
not available |
Supplemental Keywords:
oxidation-reduction reactions, structure-reactivity relationships, toxic metal ions,, RFA, Scientific Discipline, Waste, Ecosystem Protection/Environmental Exposure & Risk, Environmental Chemistry, chemical mixtures, Fate & Transport, Hazardous Waste, Ecology and Ecosystems, Hazardous, hazardous waste treatment, complex mixtures, contaminated sediments, fate and transport, fate and transport , biodegradation, contaminant biodegradation rates, hazardous organic substances, environmental transport and fate, chemical kinetics, hazardous chemicals, capillary electrophoresis, contaminated soils, analytical modelsProgress 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.