WO2006023779A2 - Metal extraction in liquid or supercritical-fluid solvents - Google Patents

Abstract

A method for separating metals from metal-containing materials by extraction without generating large quantities of liquid waste is disclosed. Also disclosed is an extractant composition for use with this method. The method comprises exposing a metal-containing material to a solvent, such as supercritical carbon dioxide, an acid-base complex, and a chelating agent that is not a component of the acid-base complex. The metal is released into the solvent by a combination of oxidation by an oxidizing agent in the acid-base complex and chelation by the chelating agent. The oxidizing agent in the acid-base complex is solubilized by a solubilizing agent. The disclosed method and composition have many applications and are particularly well suited for the extraction of transition metals, including, but not limited to, platinum group metals, nom a metals and coinage metals. Applications include the recovery of metals from scrap materials and the planarization of semiconductor structures.

Description

METAL EXTRACTION IN LIQUID OR SUPERCRITICAL-FLUID SOLVENTS

CROSS REFERENCE TO RELATED APPLICATIONS This application claims the benefit of U.S. Provisional Application No.

60/603,350, filed August 20, 2004, and U.S. Provisional Application No. 60/658,331, filed March 2, 2005, which are incorporated herein by reference.

FIELD This disclosure relates to the extraction of metals with chelating agents and acid- base complexes in liquid or supercritical fluid solvents.

BACKGROUND

The separation of metals from metal-containing materials has substantial practical significance. For example, the separation of metals from industrial waste helps to detoxify the waste and can recoup valuable metals for reuse. Some conventional separation processes involve two primary steps. In a first step, the metal is extracted from the metal-containing material by an initial extraction process. In a second step, the metal is separated from the materials used for or generated by the initial extraction process. Dissolution of solids with an acid followed by solvent extraction is an example of a widely used technique for extracting metals from solid materials. This technique, however, typically generates large amounts of hazardous waste, including spent solvent. Disposal of this waste can be expensive and can have adverse environmental consequences. A more environmentally benign approach to metal separation uses supercritical fluids comprising one or more chelating agents. Methods based on this approach have been shown to separate metals without the use of either organic solvents or aqueous solutions. Furthermore, the chelating agents and supercritical fluids often can be regenerated and reused multiple times. Various features of supercritical fluid extraction of metals are disclosed in U.S. Patent Nos. 5,356,538, 5,606,724, 5,730,874, 5,770,085, 5,792,357, 5,840,193, 5,965,025, 6,132,491, 6,187,911, and U.S. Published Patent Application No. 2003/0183043 ("the Wai patent documents"), each of which is incorporated herein by reference.

The Wai patent documents reflect the development of supercritical fluid extraction technology since 1991. U.S. Patent No. 5,356,538 ("the '538 patent") discloses "exposing [a metal-containing] solid or liquid to a supercritical fluid solvent containing a fluorinated chelating agent, for a sufficient period of time to form [metal- containing complexes] between the agent and species that are solubilized in the supercritical fluid solvent." See the '538 patent at column 15, lines 43-46. The '538 patent explains that "the chelating agent is fluorinated to enhance [the] solubility of the [metal-containing complexes] in supercritical carbon dioxide." See the '538 patent at column 3, lines 31-33. The '538 patent also states that "a modifier may be added to the supercritical fluid to improve the solvent characteristics of the supercritical fluid." See the '538 patent at column 7, lines 23-25.

A variety of chelating agents are proposed by the Wai patent documents. For example, U.S. Patent No. 5,606,724 ("the '724 patent") states that the chelating agent can be "selected from the group consisting of β-diketones, halogenated β-diketones, phosphinic acids, halogenated phosphinic acids, carboxylic acids, halogenated carboxylic acids, and mixtures thereof." See the '724 patent at column 2, lines 54-58. The '724 patent also states that, at least for the extraction of metal from metal oxides, the "chelating agents generally should be sufficiently acidic to donate a proton to the metal oxide, thereby rendering the metal available to form [metal-containing complexes] with the chelating agent." See the '724 patent at column 2, lines 43-46. Treating copper-containing substrates with "a 'dry' homogeneous solution in supercritical carbon dioxide that contain[s] an oxidant, ethyl peroxydicarbonate (EPDC), and a commercially available β-diketone chelating agent" has been disclosed. Bessel, C. A.; Denison, G. M.; Desimone, J. M.; DeYoung, J.; Gross, S.; Schauer C. K.; Visintin, P. M. Etchant Solutions for the Removal of Cu(O) in Supercritical CO₂- Based "Dry" Chemical Mechanical Planarization Process for Device Fabrication. J. Am. Chem. Soc. 2003, 125(17), 4980 ("Bessel"). The method disclosed by Bessel is not well suited for most commercial applications. In part, this is because the method uses the oxidant EPDC, which is highly explosive. On a large-scale, this method is too dangerous to be of any practical value. Extraction of uranium using an acid-base complex comprising triburylphosphate

(TBP) and nitric acid is disclosed in Samsonov, M. D.; Wai, C. M.; Lee, S. C; Kulyako, Y.; Smart, N. G. Dissolution of Uranium Dioxide in Supercritical Fluid Carbon Dioxide. Chem. Commun. 2001, 1868 ("Samsonov"), which is incorporated herein by reference. Samsonov discloses that "[u]ranium dioxide can be dissolved in supercritical CO₂ with a CO₂-philic TBP-HNO₃ [pair] to form a highly soluble UO₂(NO₃)₂'2TBP complex" in a method that "requires no water or organic solvents." See Samsonov at 1868.

The inventors of the present disclosure have discovered that an acid-base complex, such as TBP-HNO₃, while suitable for the extraction of lanthanides and actinides, is limited in its ability to extract other metals. For example, an acid-base complex alone is not well suited for the separation of most transition metals, such as platinum group metals, noble metals and coinage metals.

SUMMARY Disclosed herein are embodiments of a method for separating metals from metal-containing materials by extraction processes. Also disclosed are embodiments of an extractant composition that can be used with the disclosed method. The method can comprise, for example, treating a metal-containing material with a solvent, a chelating agent and an acid-base complex. The acid-base complex can comprise an oxidizing agent that is capable of oxidizing the metal and a solubilizing agent that is capable of solubilizing the oxidizing agent in a solvent, such as liquid or supercritical fluid carbon dioxide.

Some embodiments of the disclosed method are performed at high pressures and/or low temperatures to maintain the solvent in liquid or supercritical fluid form. The solvents, for example, can be gases at room temperature and atmospheric pressure. Using these solvents to replace the liquid solvents used in many conventional metal extraction processes can make the disclosed method more environmentally benign than conventional metal extraction processes. Liquid or supercritical carbon dioxide is an example of a suitable solvent for use with the disclosed method. The oxidizing agent typically is the acid component of the acid-base complex, while the solubilizing agent typically is the base component of the acid-base complex. In some embodiments, the solubility of the oxidizing agent in supercritical carbon dioxide is less than about 0.1 moles per liter at 50 ⁰C and 100 arm and the solubility of the acid-base complex in supercritical carbon dioxide is greater than about 0.5 moles per liter at 50 ⁰C and 100 atm. One example of a suitable acid-base complex comprises nitric acid as the oxidizing agent and TBP as the solubilizing agent.

In some embodiments of the disclosed method, the metal in the metal-containing material is first oxidized by the acid-base complex. After being oxidized, the metal can form metal-containing complexes with the chelating agent. This solubilizes the metal -A-

and allows it to be separated from the metal-containing material. The chelating agent can be, for example, a β-diketone. To improve its solubility in non-polar solvents, such as supercritical carbon dioxide, the chelating agent can be fluorinated. In some embodiments, the chelating agent is a fluorinated β-diketone, such as hexafluoroacetylacetone.

If present, coordinated water molecules on the metal-containing complexes can reduce the solubility of the metal-containing complexes in non-polar solvents, such as supercritical carbon dioxide. In some embodiments of the disclosed method, this effect is mitigated by exposing the metal-containing complexes to a second, different chelating agent for a period of time sufficient for the second chelating agent to displace the coordinated water molecules. This can be, for example, a period sufficient for the two chelating agents to form adducts with the metal. The solubilizing agent originally paired with the oxidizing agent also can serve to displace coordinated water molecules from the metal-containing complexes. For example, TBP released from TBP-HNO₃ after oxidation of the metal can displace coordinated water molecules on the metal- containing complexes.

The extracted metal can be recovered while the metal-containing complexes are still within the solvent or after the metal-containing complexes have been separated from the solvent. The metal-containing complexes can be separated from the solvent, for example, by converting the solvent into its gas form while the metal-containing complexes remain in liquid form. This can be done by reducing the pressure of the solvent and/or by increasing the temperature of the solvent.

Embodiments of the disclosed method are particularly effective at extracting transition metals, including, but not limited to, platinum group metals, noble metals, coinage metals and oxides and sulfides of these metals. Most transition metals cannot be effectively extracted by an acid-base complex, such as TBP-HNO₃, without the addition of a separate chelating agent. Moreover, even metals that can be adequately extracted by an acid-base complex without the addition of a separate chelating agent, such as lanthanides and actinides, often can be extracted more efficiently by the addition of a separate chelating agent.

The disclosed metal separation method has many practical applications. Some embodiments can be used to remove metals, such as copper, from the surface of semiconductor structures. These embodiments can be used, for example, in chemical mechanical planarization processes in conjunction with mechanical polishing by a porous pad. In addition, some disclosed embodiments are useful for removing metals, such as iron, from nanostructures, such as nanostructures comprising at least one carbon nanotube.

BRIEF DESCRIPTION OF THE DRA WINGS

FIG. 1 is a phase diagram for carbon dioxide.

FIG. 2 is a schematic illustration of one embodiment of an apparatus used to perform disclosed extractions.

FIG. 3 A is a scanning electron microscope cross section of a silicon wafer coated with a layer of copper before extraction.

FIG. 3B is a scanning electron microscope cross section of a silicon wafer coated with a layer of copper after extraction at 40⁰C and 200 arm for 30 seconds.

FIG. 4A is a photograph through the quartz window of a view cell in which copper is being extracted with supercritical carbon dioxide, one minute into the extraction process.

FIG. 4B is a photograph through the quartz window of a view cell in which copper is being extracted with supercritical carbon dioxide, 30 minutes into the extraction process.

FIG. 5 A is a photograph through the quartz window of a view cell in which gold is being extracted with supercritical carbon dioxide, one minute into the extraction process.

FIG. 5B is a photograph through the quartz window of a view cell in which gold is being extracted with supercritical carbon dioxide, 35 minutes into the extraction process. FIG. 6 is a neutron activation analysis gamma spectrum of a trap solution after supercritical fluid extraction of gold.

FIG. 7 A is an energy dispersive X-ray spectrum of a gold pin surface before supercritical fluid extraction.

FIG. 7B is an energy dispersive X-ray spectrum of a gold pin surface after supercritical fluid extraction.

FIG. 8 A is a ¹⁹F nuclear magnetic resonance spectra of a Pd(Ma)₂ standard.

FIG. 8B is a ¹⁹F nuclear magnetic resonance spectra of a Hhfa standard.

FIG. 8C is a ¹⁹F nuclear magnetic resonance spectra of the trap solution after supercritical fluid extraction of palladium shot. FIG. 9 A is a ¹⁹F nuclear magnetic resonance spectra of the trap solution after supercritical fluid extraction of palladium shot using TBP(HNO₃)₀.₇(H₂O)₀.₇.

FIG. 9B is a ¹⁹F nuclear magnetic resonance spectra of the trap solution after supercritical fluid extraction of palladium shot using TBP(HNO₃) i.o(H₂0)o.₄ with an extraction time of 2 hours.

FIG. 9C is a ¹⁹F nuclear magnetic resonance spectra of the trap solution after supercritical fluid extraction of palladium shot using TBP(HNOa)Lo(H₂O)O₁₄ with an extraction time of 4 hours.

FIG. 10 is a neutron activation analysis gamma spectrum of a trap solution after supercritical fluid extraction of palladium.

FIG. 11 is a graph showing extraction efficiency for the extraction of lanthanum, europium and praseodymium from soil samples spiked with La₂O₃, Pr₂O₃, and Eu₂O₃ using TBP(HNO₃)Lo(H₂O)O_-4.

FIG. 12 is a graph showing extraction efficiency for the extraction of lanthanum, europium and praseodymium from soil samples spiked with La₂O₃, Pr₂O₃, and Eu₂O₃ using TBP(HNO₃) _LO(H₂0)_O.₄ and thenoyltrifluoroacetone.

DETAILED DESCRIPTION

Throughout this disclosure, the singular terms "a," "an," and "the" include plural referents unless the context clearly indicates otherwise. Similarly, the word "or" is intended to include "and" unless the context clearly indicates otherwise.

The following terms maybe abbreviated in this disclosure as follows: acetylacetone(AA), atmosphere (atm), boiling point (BP), centimeter (cm), central processing unit (CPU), chemical mechanical planarization (CMP), critical pressure (P_c), critical temperature (Tc), energy dispersive X-ray spectrometer (EDS), ethyl peroxydicarbonate (EPDC), half life (XVi), hexafluoroacetylacetone (Hhfa), inductively coupled plasma-atomic emission spectrometry (ICP-AES), inner diameter (ID), kiloelectronvolt (keV), megahertz (MHz), microgram (μg), microliter (μL), milliliter (mL), millimeter (mm), molar (M), nanometers (nm), neutron activation analysis (NAA), newton (N), nuclear magnetic resonance (NMR), scanning electron microscope (SEM), supercritical (sc), supercritical fluid extraction (SFE), thenoyltrifluoroacetone (TTA), trifluoroacetylacetone (TFA), tributylphosphate (TBP), trioctylphosphine oxide (TOPO), tributylphosphine oxide (TBPO) and triple point (T_P).

Described herein are embodiments of a method for extracting metals in a solvent, such as supercritical carbon dioxide, and embodiments of a composition for use with the disclosed method. The disclosed method is a substantial improvement over conventional methods. Some conventional methods, including those disclosed in Samsonov and the '043 application, disclose the oxidation of metals in conjunction with the chelation of metals. These methods primarily are directed to the extraction of uranium. Oxidation in this context was motivated by the fact that uranium dioxide in the +4 oxidation state does not form stable complexes with commonly known chelating agents. Thus, Samsonov and the '043 application disclose the use of an oxidizing agent to convert uranium in uranium dioxide to the +6 oxidation state, which does form stable complexes with a number of chelating agents that are soluble in supercritical carbon dioxide. As with uranium, many metals, including most transition metals, typically must be oxidized before they can be complexed with a proper chelating agent to form stable metal-containing complexes that are sufficiently soluble in non-polar solvents, such as supercritical carbon dioxide. Samsonov and the '043 application disclose oxidation and chelation by a single acid-base complex. See Samsonov at 1868 and the '043 application at paragraph 20. This method is adequate for oxidizing and complexing certain metals, including many lanthanides and actinides, such as uranium, but it is not adequate for oxidizing many other metals. The metals that can be oxidized and chelated by a single acid-base complex are metals that can bond to large numbers of ligands. For example, uranium and lanthanum can form stable complexes with TBP-HNO₃, such as UO₂(NO₃)₂ ^»2TBP and La(NO₃)₃ ^#TBP, respectively. Many other metals, however, cannot bond to large numbers of ligands and will not form stable complexes with TBP-HNO₃. Among these metals are most transition metals, such as platinum group metals, noble metals and coinage metals.

The efficiency of extraction processes directed to metals that cannot bond to large numbers of ligands can be improved by the addition of a chelating agent that is not a component of the acid-base complex. By way of theory, the metal first is oxidized by the oxidizing agent in the acid-base complex and then is chelated by the separate chelating agent. Embodiments of the disclosed method that use an acid-base complex and a chelating agent are capable of efficiently extracting many metals that cannot be efficiently extracted by conventional methods.

Bessel discloses using a β-diketone chelating agent and EPDC for extracting copper. Bessel, however, does not disclose an oxidizing agent that is bound within an acid-base complex, such as TBP-HNO₃. Binding the oxidizing agent within an acid- base complex is a significant improvement over Bessel. Because supercritical carbon dioxide is non-polar and most effective oxidizing agents are polar, very few effective oxidizing agents are soluble in supercritical carbon dioxide without being associated with another compound. The oxidizing agent disclosed by Bessel is one of the few oxidizing agents that is soluble in supercritical carbon dioxide without being associated with another compound, but it also is highly explosive and not well suited for commercial applications.

After oxidizing the metal, organic oxidizing agents, such as EPDC, break up into organic fragments that can be difficult to separate from the metal being recovered. In contrast, oxidizing agents paired in acid-base complexes typically break down into water, gases and other easily separable products. For example, after reducing copper, nitric acid typically is converted into water and nitrogen dioxide.

Acid-base complexes, such as TBP-HNO₃, are superior oxidizing agents for extracting most transition metals, such as platinum group metals, noble metals, and coinage metals. Before oxidation, these metals can be in a variety of forms, such as zero-valent, oxide or sulfide form. Nitric acid is conventionally used for metal dissolution and extraction, but alone it is not soluble in supercritical carbon dioxide. In contrast, when nitric acid is bound to a CO₂-philic solubilizing agent such as TBP, the resulting acid-base complex is highly soluble in supercritical carbon dioxide. TBP therefore serves as a carrier for introducing the acid into the supercritical carbon dioxide phase during metal extractions.

As discussed above, embodiments of the disclosed method use an oxidizing agent that is solubilized by incorporation into an acid-base complex and at least one chelating agent that is not a component of the acid-base complex. Surprisingly, it has been discovered that the acid-base complex and the chelating agent have a synergistic effect on the extraction process. In some embodiments, such as in the extraction of lanthanides and actinides, the oxidized metal can be bound by the acid-base complex or by the separate chelating agent. With most metals, however, only the chelating agent is capable of binding to the metal to form stable complexes in the solvent.

Extraction processes incorporating both an acid-base complex and at least one separate chelating agent potentially can be many times faster than conventional extraction processes. The increased extraction rate is especially pronounced with the extraction of transition metals such as platinum group metals, noble metals and coinage metals. For example, high extraction rates were observed with TBP-HNO₃ as the acid- base complex and a fluorinated β-diketone as the chelating agent. The examples below indicate that in some implementations, the extraction rate is very fast initially and then slows once the bulk of the metal has been extracted. For example, in one trial about 50% of the copper in a copper film was extracted after 30 seconds, about 98% was extracted after 2 minutes and substantially all of the copper was extracted after 4 minutes. Some embodiments of the disclosed method can dissolve copper at a rate greater than about 2 nm per second, typically greater than about 2.2 nm per second, and even more typically greater than about 2.4 nm per second. By way of theory, it is possible that in some embodiments of the disclosed method, the solubilizing agent in the acid-base complex serves a duel role. In addition to solubilizing the acid so that the acid can oxidize the metal and thereby promote chelation, certain solubilizing agents are capable of increasing the solubility of the metal-containing complexes after formation of the metal-containing complexes. For example, the solubilizing agent in an acid-base complex may replace coordinated water molecules on the metal-containing complexes, which increases the solubility of the metal-containing complexes in non-polar solvents, such as supercritical carbon dioxide. TBP is an effective solubilizing agent for solubilizing both the acid and the metal- containing complexes.

A. Metals

Embodiments of the present method are suitable for extracting or purifying many different types of metals from metal-containing materials. Metal-containing materials include mixtures of metal and extraneous materials as well as metal- containing compounds, such as metal oxides and metal sulfides. In general, metals are elements that form positive ions in solution and produce oxides that form hydroxides rather than acids with water. More specifically, metals include all elements other than metalloids and non-metals. The metalloids are boron, silicon, germanium, arsenic, antimony, tellurium, and polonium. The non-metals are hydrogen, carbon, nitrogen, oxygen, fluorine, phosphorus, sulfur, chlorine, selenium, bromine, iodine, and astatine.

Throughout this disclosure, the term "metals" shall refer to both metals and metalloids.

The genus of metals includes many species, including, without limitation, alkali metals, alkali-earth metals, transition metals, noble metals, coinage metals, rare metals, rare-earth metals, transuranic metals, light metals, heavy metals and radioactive metals. The extraction rates of metals that form stable complexes with acid-base complexes, such as TBP-HNO₃, can be improved by some embodiments of the disclosed method. But disclosed embodiments are particularly well suited for the extraction of metals that do not form stable complexes with acid-base complexes, such as TBP-HNO₃. For example, some embodiments of the disclosed method are well suited for the extraction of metals other than lanthanides and actinides. Some embodiments of the disclosed method are especially well suited for extracting transition metals, including the sub- genuses of noble metals, platinum group metals and coinage metals. Before oxidation by the oxidizing agent, these metals can be in a variety of forms, such as zero-valent, oxide and sulfide form.

There is some overlap among the sub-genuses of transition metals. Noble metals, in general, are metals that are resistant to oxidation. The noble metals are gold, silver, palladium, platinum, rhodium, iridium, and osmium. The platinum group metals are platinum, palladium, iridium, rhodium, ruthenium and osmium. The coinage metals are copper, gold, nickel, silver and platinum.

B. Solvents

In embodiments of the disclosed method, the separation of metals occurs in a solvent that can be a fluid and/or a supercritical fluid. In some embodiments the solvent is a gas at room temperature and atmospheric pressure. These solvents are useful, in part, because they can be separated easily from the metal-containing complexes by decreasing their pressure and/or increasing their temperature.

A compound exists as a supercritical fluid when it is at a temperature and pressure above a critical temperature and pressure characteristic of the compound. Materials in a supercritical state exhibit properties of both a gas and a liquid.

Supercritical fluids typically are able to act as solvents, like subcritical liquids, while also exhibiting the improved penetration power of gases. This makes supercritical fluids a preferred class of solvents for the extraction of metals.

Suitable solvents include, but are not limited to, carbon dioxide, nitrogen, ethylene, propane, and propylene. Carbon dioxide is a preferred solvent for both subcritical and supercritical fluid extractions because of its moderate chemical constants and its inertness. Carbon dioxide has a critical temperature (Tc) of 31 ⁰C and a critical pressure (P_c) of 73 atm. Supercritical carbon dioxide is non-explosive and thoroughly safe for extractions. Carbon dioxide also is a preferred solvent because it is abundantly available and relatively inexpensive.

FIG. 1 is a phase diagram for carbon dioxide, which shows the conditions necessary to produce either subcritical liquid carbon dioxide or supercritical carbon dioxide. Certain conditions above the critical point produce a supercritical carbon dioxide fluid solvent useful for metal extraction processes. Representative conditions can be found in the examples below.

As an alternative to supercritical carbon dioxide, liquid carbon dioxide is suitable for some embodiments of the disclosed method. At room temperature carbon dioxide becomes a liquid above 5.1 arm. Depending on the pressure, liquid carbon dioxide has a density comparable to or slightly greater than the density of supercritical carbon dioxide. Thus, the solvation power of liquid carbon dioxide is comparable to or slightly greater than that of supercritical carbon dioxide. Liquid carbon dioxide is able to dissolve metal-containing complexes, but liquid carbon dioxide does not have the "gas-like" properties of supercritical carbon dioxide. Liquid carbon dioxide has a high viscosity, a low diffusivity, and consequently a poor penetration power compared to supercritical carbon dioxide. The extraction efficiency of liquid carbon dioxide may depend on the applied pressure. In addition, it may be possible to improve the extraction efficiency of liquid carbon dioxide by applying agitation. The liquid and supercritical fluid solvents used in embodiments of the disclosed method may be used individually or in combination. Examples of suitable solvents, and their critical temperatures and pressures, are shown in Table 1.

C. Solubility Modifiers

In some embodiments of the disclosed method, a modifier can be added to the solvent to vary the characteristics thereof. For example, a modifier can be added to the solvent to enhance the solubility of a particular complexed metal. Some useful modifiers are low-to-medium boiling point alcohols and esters, such as lower alkyl alcohols and esters. As used herein, the term "lower alkyl" refers to compounds having ten or fewer carbon atoms, and includes both straight-chain and branched-chain compounds and all stereoisomers. Typical modifiers can be selected from the group consisting of methanol, ethanol, ethyl acetate, and combinations thereof. The modifiers are added to the solvent in an amount sufficient to vary the characteristics thereof. This can be an amount, for example, between about 0.1% and about 20.0% by weight. The modifiers contemplated for use with embodiments of the disclosed method most typically are not supercritical fluids at the disclosed operating conditions. Rather, the modifiers simply are dissolved in the liquid and/or supercritical fluid solvents to improve their solvent properties.

In one embodiment of the disclosed method, a modifier is combined with the solvent prior to introducing the solvent into an extraction vessel. Alternatively, the solvent and the modifier can be added to the extraction vessel separately. D. Chelating Agents

A partial list of chelating agents useful for solubilizing metals in non-polar solvents is provided in Table 2. The list is for illustration only. Other chelating agents, whether now known or hereafter discovered, useful for forming metal-containing complexes also may be used to practice embodiments of the disclosed method.

Beneficial factors to consider in the selection of chelating agents include, but are not limited to, high stability constants of the metal-containing complexes formed, fast complexation kinetics, good solubility in the solvent for both the chelating agent and the metal-containing complexes formed, and sufficient specificity to allow selective extraction of a metal or a group of metal ions.

Table 2: Metal Chelating Agents

Oxygen Donating Chelating Agents cupferron chloranilic acid and related reagents β-diketones and related reagents

N-benzoyl-N-phenylhydroxylamine and related reagents crown ethers and calixarenes dibutylcellosolve (C₄H₉OC₂H₄OC₄H₉) octanol-2 and related reagents methyl isobutyl ketone and related reagents

Nitrogen Donating Chelating Agents α-dioximes diaminobenzidine and related reagents porphyrins and related reagents

Oxygen and Nitrogen Donating Chelating Agents

8-hydroxyquinoline nitrosonaphthols and nitrosophenols diphenylcarbazide and diphenylcarbazone tri-alkyl amines, such as (C_nH_2n+Q₃N and related reagents

Phosphate and Phosphine Oxide Chelating Agents tri-n-alkylphosphine oxide, alkyl groups = C₄H₉, C₈H₁₇, etc. n-tributyl phosphate and related reagents bis(2,4,4-trimethylpentyl)phosphinic acid (Cyanex 272) and related reagents

Sulfur Donating Chelating Agents dithizone and related reagents bismuthiol II thioxine (C₉H₇NS*2H₂O) bis(2,4,4-trimethylpentyl)-monothiophosphinic acid (Cyanex 302) and related reagents

Bis(2,4,4-trimethylpentyl)-dithiophosphinic acid (Cyanex 301) and related reagents dioctyl sulfide [(C₈Hn)₂S] dioctyl sulfoxide [(C₈Hn)₂SO] thiourea derivatives and related reagents

For extracting metals directly from metal oxides or sulfides, the chelating agent preferably is a protic chelating agent with enough acidity to break the chemical bonds in the metal oxides or sulfides. Once these bonds are broken, the metal can, in some embodiments, form metal-containing complexes with the chelating agent. Without limiting the scope of the disclosure, chelating agents that are particularly useful for forming metal-containing complexes with the metal of metal oxides and sulfides include β-diketones and related reagents, phosphinic acids and related reagents, and carboxylic acids and related reagents.

Mixtures of chelating agents can be useful in some disclosed embodiments. Without limiting the present disclosure, the initially-formed, metal-containing complexes can include at least one water molecule complexed with the metal and chelating agent. The presence of at least one water molecule increases the polarity of the metal-containing complexes. In some embodiments, the at least one water molecule can be displaced by exposing the initially-formed, metal-containing complexes to a separate, different chelating agent. Removing the at least one water molecule can substantially increase the solubility of the metal-containing complexes in the extraction medium. Metals having at least two different classes of chelating agents coupled thereto are referred to herein as "adducts."

A person of ordinary skill in the art will realize that the selection of a chelating agent, or mixture of chelating agents, will depend on a number of factors, including the ability of the chelating agent(s) to form metal-containing complexes with a particular metal, the availability of the chelating agent(s), the toxicity of the chelating agent(s), etc. A variety of chelating agents also can be selected for use as the second, different chelating agent for displacing metal-bound substances, such as water. In some embodiments, basic chelating agents are preferred for use as the second, different chelating agent for displacing polar compounds, such as water. For example, phosphorous-containing chelating agents, such as phosphates and phosphinic acids, are highly effective at displacing water molecules from the initially-formed, metal- containing complexes. Without limitation, chelating agents that may be useful for practicing embodiments of the disclosed method include β-diketones, phosphine oxides (such as trialkylphosphine oxides, triarylphosphine oxides, and alkylarylphosphine oxides), phosphinic acids, carboxylic acids, phosphates (such as trialkylphosphates, triarylphosphates, and alkylarylphosphates), crown ethers, phosphine sulfides, phosphorothioic acids, thiophosphinic acids, halogenated analogs of these chelating agents, and mixtures of these chelating agents. Some of the useful chelating agents have lower alkyl functional groups. Alkyl-substituted chelating agents with chain lengths of about eight carbons, especially branched-chain alkyl groups, are characterized by high solubilities in supercritical carbon dioxide. Some of the chelating agents useful for practicing embodiments of the present method are discussed in detail below.

1. β-diketones

The carbon atoms of a ketone are assigned Greek letters to designate positions along the carbon chain relative to the carbonyl carbon. The first carbon adjacent the carbonyl carbon is designated α, the second such carbon is designated β, and so on. A β-diketone has at least two ketone carbonyls wherein one ketone carbonyl is located on a carbon β to the other ketone carbonyl. The β-diketones that can be used as chelating agents in embodiments of the present method generally satisfy the following Formula 1 :

Formula 1

wherein Ri and R₂ typically are selected independently from the group consisting of aliphatic groups, including functionalized aliphatic groups, particularly alkyl groups and even more particularly lower alkyl groups, such as halogenated lower alkyl groups, aryl groups, halogenated aryl groups, thenoyl groups, and mixtures thereof. As used herein, a "halogenated lower alkyl group," such as a fluorinated ethyl group, has at least one of the hydrogen atoms present on the alkyl group replaced with a halogen atom, such as a fluorine atom. The term "halogenated lower alkyl group" also can refer to compounds wherein all or any number of the hydrogen atoms have been replaced with halogen atoms, such as fluorine atoms.

In some suitable β-diketones, Ri and R₂ of Formula 1 are selected independently from the group consisting of methyl groups, fluorinated methyl groups, trifluoromethyl groups, ethyl groups, fluorinated ethyl groups, pentafluoroethyl groups, propyl groups, fluorinated propyl groups, heptafluoropropyl groups, butyl groups, fluorinated butyl groups, and nonafluorobutyl groups. Specific examples of suitable β-diketones include, without limitation, acetylacetone, dibutyldiacetate, trifluoroacetylacetone, hexafluoroacetylacetone, thenoyltrifluoroacetylacetone, and heptafluoro- butanoylpivaroylmethane. For some embodiments, the preferred β-diketones are hexafluoroacetylacetone and dibutyldiacetate. The chelating agent can be halogenated to enhance its solubility and/or the solubility of the metal-containing complexes in supercritical carbon dioxide. For some embodiments, the preferred chelating agents are halogenated. Some halogenated, metal- containing complexes, particularly fluorinated, metal-containing complexes, are characterized by solubilities in supercritical carbon dioxide that are enhanced by two to three orders of magnitude relative to the solubilities of the corresponding non- halogenated, metal-containing complexes in supercritical carbon dioxide. For illustrative purposes only, and without limiting the present disclosure, a suitable fluorinated chelating agent is shown below:

Hexafluoroacetylacetone

Most types of β-diketones are commercially available. They can be purchased, for example, from Sigma-Aldrich (St. Louis, Missouri).

In general, β-diketones form stable complexes with metals and hence are useful complexing agents for extracting metals from metal-containing materials. For example, the fluorinated β-diketones shown in Table 3 can be used with supercritical carbon dioxide solvents for the extraction of metal ions. All of the β-diketones shown in Table 3, except thenoyltrifluoroacetone, are liquids at room temperature and atmospheric pressure.

Table 3: Fluorinated β-diketones Used for the Extraction of Metal Ions Using Supercritical Carbon Dioxide

β-diketones exist in at least two tautomeric forms, the "keto" tautomer and the "enol" tautomer. Tautomerism is a type of isomerism in which migration of a hydrogen atom results in two or more different structures called tautomers. By way of theory, β- diketones react with metal ions to form metal-containing complexes either through the enol tautomer or through an enolate anion, which is a negatively charged enol tautomer. The following equilibrium illustrates the tautomeric forms of a β-diketone:

In some applications, such as the extraction of metals from complex matrixes, the presence of a small amount of water can significantly increase the extraction efficiency. Without limiting the present disclosure to one theory of operation, water molecules may facilitate the release of the metal from the metal-containing material. One skilled in the art will realize that the amount of water used during the extraction process may vary. In some embodiments, extraction efficiency may be sufficiently increased by adding about 1 μL of water per 1 μg of metal ions.

2. Phosphinic Acids

As used herein, "phosphinic acid" refers to an organic derivative of hypophosphorous acid [HP(OH)₂]. The phosphinic acid chelating agents that are useful for practicing embodiments of the disclosed method generally satisfy the following Formula 2:

O

Ra "OH

R₄ Formula 2

wherein R₃ and R₄ are selected independently from the group consisting of aliphatic groups, including functionalized aliphatic groups, particularly alkyl groups and even more particularly lower alkyl groups, such as halogenated lower alkyl groups, aryl groups, halogenated aryl groups, thenoyl groups, and mixtures thereof. In some embodiments, R₃ and R₄ preferably are lower alkyl or fluorinated lower alkyl groups. One example, without limitation, of a suitable phosphinic acid chelating agent is bis(2,4,4-trimethylpentyl)phosphinic acid (CYANEX 272)as shown below:

Bis(2,4,4-trimethylpentyl)phosphinic acid (CYANEX 272)

Thiophosphinic acids also are useful for performing extractions. Examples, without limitation, of thiophosphinic acids include bis(2,4,4-trimethylpentyl)~ dithiophosphinic acid (CYANEX 301), bis(2,4,4-trimethylpentyl)-monothiophosphinic acid (CYANEX 302), and analogs thereof. The structures of CYANEX 301 and CYANEX 302 are shown as follows:

Bis(2,4,4-trimethylpentyl)-dithiophosphinic acid (CYANEX 301)

Bis(2,4,4-trimethylpentyl)-monothiophosphinic acid (CYANEX 302)

3. Carboxylic Acids

The carboxylic acids generally useful for practicing embodiments of the disclosed method typically satisfy the following Formula 3:

Formula 3

wherein R₅ generally is selected from the group consisting of aliphatic groups, including functionalized aliphatic groups, particularly alkyl groups and even more particularly lower alkyl groups, such as halogenated lower alkyl groups, aryl groups, halogenated aryl groups, thenoyl groups, and mixtures thereof. R₅ also can be functionalized, such as with functional groups including without limitation, hydroxyl, carbonyl and amine groups. Examples, without limitation, of carboxylic acids that satisfy Formula 3 include methanoic acid (also referred to as formic acid), ethanoic acid, propanoic acid, butanoic acid, pentanoic acid, hexanoic acid, heptanoic acid, octanoic acid, nonionic acid, decanoic acid, and branched analogs of these compounds. Some unsaturated carboxylic acids, such as acrylic acid and methacrylic acid, as well as cyclic carboxylic acids, such as cyclohexanecarboxylic acid, also can be used as suitable chelating agents for the extraction of some metals. In addition, some halogenated forms of alkyl, unsaturated alkyl, and cyclic carboxylic acids can be used as suitable chelating agents for the extraction of some metals. Examples, without limitation, of aryl carboxylic acids suitable as chelating agents include benzoic acid and phenylacetic acid, as well as halogenated derivatives thereof.

4. Phosphates

The phosphates generally useful for practicing embodiments of the disclosed method typically satisfy the following Formula 4:

O

R₆O^ I OR₈ OR₇

Formula 4 wherein R₆ - R₈ are selected independently from the group consisting of aliphatic groups, including functionalized aliphatic groups, particularly alkyl groups and even more particularly lower alkyl groups, such as halogenated lower alkyl groups, aryl groups, halogenated aryl groups, thenoyl groups, and mixtures thereof.

5. Crown Ethers

Crown ethers generally useful for practicing embodiments of the disclosed method typically satisfy the following Formula 5:

Formula 5

wherein x is OH or NHOH and (X) is a dibenzo crown ether of the formula dibenzo [13+3m ] -crown- [4+m] ether, m is an integer from 0 to about 5, n is an integer from 0 to about 6, and Rg is hydrogen or a lipophilic hydrocarbyl group having from 1 to about 18 carbon atoms. For example, (X) can be represented as:

In some embodiments, Rg can be selected from the group consisting of alkyl groups, cycloalkyl groups, alkenyl groups, and aryl groups.

In some embodiments, the crown ether is represented by the following Formula 6:

wherein X is OH or NHOH; Rio and R₁₁ are selected independently from the group consisting of aliphatic groups, including functionalized aliphatic groups, particularly alkyl groups and even more particularly lower alkyl groups, such as halogenated lower alkyl groups, aryl groups, halogenated aryl groups, thenoyl groups, and mixtures thereof; and Ri₂ and R₁₃ are selected independently from the group consisting of hydrogen and halogens.

6. Calixarene Crown Ethers

The fluorinated calixarene crown ethers, also known as molecular baskets, generally useful for practicing embodiments of the disclosed method typically satisfy the following Formula 7:

Formula 7

In Formula 7, R, R' and n are defined by any row in Table 4.

7. Thiourea and Derivatives

The thiourea derivatives generally useful for practicing embodiments of the disclosed method typically satisfy the following Formula 8:

Formula 8 In Formula 8, when R = R' = H, the chemical structure is thiourea. In some thiourea derivatives, R and R' are selected from alkyl groups, phenyl groups and their fluorinated derivatives.

E. Acid-Base Complexes

Some disclosed acid-base complexes comprise an oxidizing agent and a solubilizing agent. In general, the oxidizing agent is capable of oxidizing the metal to be extracted and the solubilizing agent is capable of solubilizing the oxidizing agent in a solvent, such as a non-polar solvent (e.g. supercritical carbon dioxide). As described in the examples below, an oxidizing agent, such as HNO₃, can be combined with a CO₂- philic solubilizing agent, such as TBP, to form an acid-base complex that is soluble in non-polar solvents, such as supercritical carbon dioxide. Without the presence of a solubilizing agent, many oxidizing agents, such as HNO₃, are insoluble in supercritical carbon dioxide.

The oxidizing agent typically is the acid component of the acid-base complex. Suitable oxidizing agents include Lewis acids, Brønsted-Lowry acids, mineral acids, and combinations thereof. Specific examples include, but are not limited to, nitric acid and sulfuric acid. In some embodiments, the oxidizing agent is a compound that, after oxidizing the metal, is converted into products that are easily separable from the metal being extracted. For example, in some embodiments, the oxidizing agent is a non¬ organic acid.

The solubilizing agent typically is the base component of the acid-base complex. Suitable solubilizing agents to be paired with the oxidizing agents include Lewis bases, Brønsted-Lowry bases, and combinations thereof. Solubilizing agents that are well suited for use with the disclosed method include phosphates, such as R₃PO, (RO)R₂PO and (RO)₂RPO, in which R is an alkyl group, such as a butyl group or an octyl group. Examples of useful solubilizing agents include, but are not limited to, tri- alkylphosphates, such as tri-butylphosphate (TBP), tri-alkylphosphine oxides, such as tri-octylphosphine oxide (TOPO) and tri-butylphosphine oxide (TBPO), other Lewis bases soluble in supercritical carbon dioxide and combinations thereof.

TBP-HNO₃ can be prepared, for example, by mixing TBP with a concentrated nitric acid solution. Nitric acid dissolves in the TBP phase forming a Lewis acid-base complex of the general formula TBP(HNO₃)_x(H₂O)_y, which is separable from the remaining aqueous phase. The x and y values depend on the relative amount of TBP and nitric acid used in the preparation. TBP-HNO₃ complexes of different x and y values have been characterized by conventional titration methods as well as by proton nuclear magnetic resonance (NMR) spectroscopy. Data for different complexes are shown in the examples below. The complex is soluble in super critical carbon dioxide and becomes miscible with super critical carbon dioxide at high pressures.

F. Extraction Procedures In some embodiments of the disclosed method, the acid-base complex is formed prior to the extraction process. Once formed, the acid-base complex is placed in a reaction vessel along with the metal-containing material and a chelating agent. In some embodiments, a separate, different chelating agent also is added at this time. The solvent is introduced into the vessel and the vessel then is pressurized to, such as to a pressure greater than the critical pressure of the solvent. As described in the '043 application, the extraction efficiency of at least some extractions can be improved by subjecting the contents of the reaction vessel to ultrasonic vibrations.

After chelation, the metal-containing complexes then are separated from the solvent. This can be done, for example, by reducing the pressure of the solvent thereby converting the solvent from a liquid or supercritical fluid into a gas. Any reduction of the pressure of the solvent can facilitate precipitation of metals and/or metal-containing complexes suspended in the solvent. In some embodiments, the pressure can be reduced to approximately atmospheric pressure, and the solvent expanded into a collection container. The gaseous solvent can be reused, such as by recycling it back through the extraction process. Before or after separation from the solvent, the elemental metal can be separated from the metal-containing complexes by any number of known methods, including treatment with concentrated nitric acid. The reduction of β-diketone complexes of palladium, copper, silver and other metals dissolved in supercritical carbon dioxide to their elemental state in the presence of hydrogen fluoride has been reported. Ye, X. R.; Wai, C. M.; Zhang, D. Q.; Kranov, Y.; Mcllroy, D. K; Lin, Y.; Engelhard, M. Immersion Deposition of Metal Films on Silicon and Germanium Substrates in Supercritical Carbon Dioxide. Chem. Mater. 2003, 75, 83 ("the Wai and Zhang article"). In the Wai and Zhang article, the metals formed thin films on silicon surfaces after separation from β-diketone complexes. The reduction of the β-diketone complexes of these metals was attributed to the following reaction (M = metal):

4HF + Si⁰ + 2M(Ma)₂ -> SiF₄ + 4Hhfa + 2 M⁰

It also may be possible to reduce metal β-diketone complexes in solvents, such as supercritical carbon dioxide, by adding a reducing agent such as H₂ or NaBH₃CN to the fluid phase. This was reported in Blackburn, J. M.; Long, D. P.; Cabanas, A.; Watkins, J. J. Deposition of Conformal Copper and Nickel Films from Supercritical Carbon Dioxide. Science 2001, 294(5), 141 and Blackburn, J. M.; Long, D. P.;

Watkins, J. J. Reactive Deposition of Conformal Palladium Films from Supercritical Carbon Dioxide Solution. Chem. Mater. 2000, 12, 2625. Copper and palladium films can be formed by hydrogen reduction of Cu(hfa)₂ or Pd(hfa)₂ at moderate temperatures in supercritical carbon dioxide. Similar processes for the separation of other metals from metal-containing complexes are known in the art.

G. Applications

The disclosed metal separation method has many practical applications. For example, some embodiments of the disclosed method can be used to recover valuable metals from waste materials, such as gold from abandoned electronics and platinum and palladium from used catalysts. In addition, some embodiments of the disclosed method can be used to remove metal from the surfaces of semiconductor structures, such as in CMP processes in conjunction with mechanical polishing by a porous pad. The removal of metal, particularly copper, is an important part of chemical mechanical planarization (CMP) processes used in the manufacture of semiconductor devices.

Some embodiments of the disclosed metal separation method also can be used to remove contaminants from nanostructures, such as carbon nanotubes. For example, some embodiments of the disclosed method can be used to remove metal catalysts from nanostructures. Metal catalysts are commonly used in the synthesis of nanostructures. These catalysts typically become contaminants after the synthesis processes are complete. Iron, for example, often is used in the synthesis of carbon nanotubes. Experiments have demonstrated that some embodiments of the disclosed method are well suited for removing iron from carbon nanotubes. These and other embodiments also can be used in connection with other nanostructures and/or other metal contaminants. EXAMPLES

The following examples are provided to illustrate certain particular embodiments of the disclosure. Additional embodiments not limited to the particular features described are consistent with the following examples. Among other features of the present disclosure, the following examples illustrate that metals (such as copper, gold, palladium and platinum) in different structures (such as films, coatings, and solid granules) can be dissolved in solvents, such as supercritical carbon dioxide, comprising an oxidizing agent, such as HNO₃, and a chelating agent, such as a fluorinated β-diketone. The HNO₃, for example, can be introduced via an acid-base complex comprising a CO₂-ρhilic solubilizing agent, such as TBP. Other CO₂-insoluble oxidizing agents also can be introduced into non-polar solvents, such as supercritical carbon dioxide, by the same principle. It will be evident that the rapid supercritical fluid dissolution process described in the following examples can be applied to the extraction of a variety of metals for a variety of useful purposes.

Example 1: Preparing the Materials and Equipment TBP was purchased from Alfa Aesar (Ward Hill, MA). Nitric acid [69.5% (w/w)] was obtained from Fisher Chemical (Fair Lawn, NJ). The complexes of TBP(HN0₃)₀.₇(H₂0)o.₇ and TBP(HNO₃)_Lo(H₂O)_0-4 were prepared by the methods described in Enokida, Y.; El-Fatah, S. A.; Wai, C. M. Ultrasound Enhanced Dissolution OfUO₂ in Supercritical CO₂ Containing a CO₂-Philic TBP-HNO₃ Complexant. Ind. Eng. Chem. Res. 2002, 41, 2282 and Enokida, Y.; Tomika, O.; Lee, S. C; Rustenholtz, A.; Wai; C. M. Characterization of a Tri-n-butyl Phosphate-Nitric Acid Complex: a CO₂-soluble Extractant for Dissolution of Uranium Dioxide. Ind. Eng. Chem. Res. 2003, 42, 5037 ("the Enokida references"), which are incorporated herein by reference. For example, in some trials the TBP(HN0₃)o.₇(H₂0)o.₇ was prepared by mixing 5 mL of TBP with 0.82 mL of concentrated HNO₃ (15.5 M). The solubility of the TBP(HN0₃)o.₇(H₂0)o.₇ was about 1.7 mole% in supercritical carbon dioxide at 40 ⁰C and 110 arm. Instrument grade carbon dioxide (purity 99.99%) was obtained from Oxarc

(Spokane, WA). Palladium shot (1 mm diameter), copper shot (3 mm diameter), Hhfa, Pd(hfa)₂ and Cu(Ma)₂ were purchased from Sigma- Aldrich (St. Louis, Missouri). De- ionized water (Millipore Milli-Q system, Bedford, MA) was used for the preparation of all aqueous solutions. The surface morphology of the metal films was examined by an Amray 1830 scanning electron microscope (SEM). An energy dispersive X-ray spectrometer (EDS) was used to measure the metal film composition. A Brucker 300 MHz nuclear magnetic resonance (NMR) spectrometer was used to identify metal- containing complexes dissolved in the carbon dioxide phase.

Silicon wafers (approximately one millimeter thick and coated with copper layers) were obtained from Micron Technology (Boise, Idaho). CPU processors with gold connectors and copper strips (0.8 x 2 cm²), including IBM and Intel PENTIUM^® processors, were cut from circuit boards. The size of the gold pin connectors from the CPU processors was about 0.4 cm (length) x 480 μm (diameter) with an average of 34 μm of pure gold coating, as determined by SEM imaging. Under the gold coating, the connectors comprised a Co, Ni and Fe alloy. EDS data showed that the surface layer of the copper strips also contained a small amount of gold.

The SFE procedures described in Examples 2-5 were performed with an apparatus similar to the apparatus illustrated in FIG. 2. The apparatus comprised a liquid CO₂ tank 10, a high-pressure syringe pump 12, a high-pressure view cell 14 with a quartz window 16 (20-mL volume and 5-cm path length), a stirring and heating plate 18, a thermocouple 20, a pressure transducer 22 and a collection vial 24. Detailed descriptions of the view-cell 14 are given in Wang, S; Koh, M.; Wai, C. M. Nuclear Laundry Using Supercritical Fluid Solutions. Ind. Chem. Eng. Res. 2004, 43(7), 1580 (the "Wang reference"), which is incorporated herein by reference.

All supercritical fluid extraction experiments were performed at 40⁰C and a pressure of 150 or 200 atm. The supercritical fluid extraction apparatus and general procedures were similar to those described in the Enokida references and in the Wang reference. Carbon dioxide was supplied with an Isco, Model 260D syringe pump

(Lincoln, NB). The extraction vessel was either a 14 mL stainless-steel vessel or a 6.2 mL stainless-steel vessel comprising a 5.2 mL porous internal cell and a 1 mL conduit underneath the internal cell connecting it to an outlet valve. During the extractions, the extraction vessel was placed in an oven to maintain a desired temperature. The flow rate of the supercritical carbon dioxide was controlled by the Isco pump. At the oven exit, stainless steel tubing (316 SS, 1/16 inch O.D. and 0.030 inch LD.) with a length of 20 cm was used as a pressure restrictor.

Non-destructive instrumental neutron activation analysis (NAA) was used to determine the concentration of certain metals in the solid and liquid samples. Some samples and standards, including the gold samples and standards, were irradiated for 3 minutes in a one megawatt TRIGA nuclear reactor at a steady neutron flux of 6 x 10¹² Ncm^sec^"1. After being cooled for 18 hours, the neutron-activated samples and standards were counted individually on a Ge(Li) detector.

Example 2: Copper Dissolution

A piece of silicon wafer (1.5 x 1.5 cm) coated with copper was placed in a small glass beaker (0.7 cm diameter). Hhfa (0.20 mL) was placed in another beaker (2 cm diameter). Both beakers then were placed in a stainless steel reactor (14 mL volume, preheated to 40⁰C) containing 1.5 mL of TBP(HNO₃)₀.₇(H₂O)₀.₇. The reaction cell was pressurized to 200 arm. The Hhfa in the beaker and the TBP(HN0₃)o.₇(H₂0)₀.₇ in the reaction cell each were agitated with a mini stirring bar. A 2-liter plastic container was used as a collection vessel at the outlet of the reaction cell. The lid of the plastic container was perforated to allow for CO₂ expansion after the dissolution reactions. Individual reaction times were set at 0.25, 0.5, 1, 2, and 4 minutes. After reaction, the stainless steel cell was depressurized by fully opening the outlet valve, and the silicon wafer then was removed from the system, rinsed with hexane, and dried by a nitrogen gas stream. The SEM images of the wafer before and after 30 seconds of supercritical carbon dioxide extraction are shown in FIG. 3. The thickness of the copper film on the original silicon wafer was about 128 nm (FIG. 3A). After the supercritical carbon dioxide treatment, the thickness of the copper film was reduced to about 49 nm (FIG. 3B). FIGS. 3 A and 3B show that the dissolution of copper using TBP(HNO₃)₀.₇(H₂O)₀.₇ and Hhfa in supercritical carbon dioxide was particularly rapid when compared to the dissolution of copper by known supercritical fluid extraction processes. When the same dissolution process was conducted using liquid carbon dioxide (at 24⁰C and 150 atm), the speed of dissolution was slower by about one order of magnitude.

In a separate trial, in which only pure TBP and Hhfa were used, no copper removal was observed from the silicon wafer under the same experimental conditions. By way of theory, the HNO₃ carried by the TBP into the supercritical carbon dioxide phase apparently is important for oxidation of the copper, and oxidation of the copper is important for the subsequent complexation and dissolution of the copper in the supercritical carbon dioxide phase. The contents of the view cell 1 and 30 minutes into the extraction can be seen in FIG. 4A and FIG. 4B, respectively. The color of the supercritical carbon dioxide solution became deep green when the copper dissolution trials were conducted with copper strips and copper shot (FIG. 4B). In contrast, the distinctive green color of Cu(hfa)₂ could not be detected by the naked eye when the trials were performed on silicon wafers because the wafers only contained a small amount of copper. Ultra violet- visible spectra of the supercritical fluid phase were consistent with a Cu(Ma)₂ complex solution. Because of its paramagnetic properties, Cu(Ma)₂ does not show a measurable NMR spectrum.

Example 3: Gold Dissolution

Gold-coated connector pins cut from a CPU processor (6 pieces with a total weight of about 20 mg) were placed in a high-pressure view cell preheated to 40⁰C. TBP(HN0₃)o.₇(H₂0)o.₇ (6 mL) and Hhfa (0.5 mL) were added to the reactor and carbon dioxide was introduced into the reactor until the pressure reached 150 arm. The supercritical carbon dioxide solution was agitated with a magnetic stirrer. After about one minute, the solution turned light yellow (FIG. 5A) and then became orange red after about 35 minutes (FIG. 5B). Fe(hfa)₃ in supercritical carbon dioxide is red, so the observed color change indicated that the gold coated on the surface of the pins was dissolved first followed by the dissolution of the exposed iron and other metals from the interior of the pins.

Additional gold extraction trials were performed in a specially designed stainless steel vessel. This vessel was fitted with a porous stainless steel cup with a conduit underneath the cup on the bottom of the vessel. The cup served as a filter to remove particulates greater than about 20 μm in size. In one trial, a total of 80 pins from a computer processor were loaded into the cup followed by 6 mL of TBP(HN0₃)o.₇(H₂0)o.₇ and 0.5 mL of Hhfa. The system was preheated to 40⁰C followed by pressurization to 200 arm. A magnetic stir bar was used to agitate the solution in the cup and then the system was left static for 2 hours. After 2 hours, the supercritical fluid phase was released slowly through the conduit underneath the cup and collected in a small container. A known amount of the trap solution was placed in a polyethylene vial (2/5 dram) and heat-sealed for neutron activation analysis. After depressurizing the system, the solution collected in the trapping vial was examined by NAA. The samples and the standards were irradiated for 3 minutes in a one megawatt TRIGA nuclear reactor at a steady neutron flux of 6 x 10¹² Ncm^sec^"1. The irradiated samples and standards were counted individually 3 hours post irradiation on a Ge(Li) detector and counted again 18 hours post irradiation. The γ spectrum obtained 3 hours after irradiation showed the major ¹⁹⁸Au (tVz = 2.69 days) peak at 411.8 keV as shown in FIG. 6. The 846.8 keV peak was assigned to ⁵⁶Mn (tY₂ = 2.58 hours). When the sample was counted 18 hours after irradiation, the 846.8 keV peak was significantly reduced. This decrease was consistent with the decay Of⁵⁶Mn. The ⁵⁶Mn most likely came from the stainless steel extraction cell system. A 511 keV positron annihilation peak also was also observed, which could have resulted from the decay of other high energy γ rays from ⁵⁶Mn (1810 and 2113 keV).

The NAA indicated that 619 μg of gold was extracted with a counting error of 0.69%. Before supercritical fluid extraction, the original connectors showed only gold peaks in the EDS spectrum (FIG. 7A), indicating that the exteriors of the connectors were composed primarily of pure gold. After the supercritical fluid extraction, the gold peaks became smaller and other peaks, including those of iron, nickel, and cobalt were observed (FIG. 7B). The interior of the connector pins apparently was composed of these metals. These results clearly illustrate that gold in CPU connectors can be rapidly dissolved in supercritical carbon dioxide using a mixture of Hhfa and TBP(HN0₃)o.₇(H₂0)o.₇. When the same dissolution process was conducted using liquid carbon dioxide (at 24⁰C and 150 arm), the speed of dissolution of gold was slower by a factor of between 5 and 10. In another trial, in which the dissolution of gold-coated connector pins was carried out using the same components at room temperature and ambient pressure, the dissolution rate was one to two orders of magnitude slower than the dissolution rate achieved with supercritical carbon dioxide. By way of theory, the high diffusivity of supercritical carbon dioxide probably is an important factor in facilitating the oxidation and transport of metal species into the supercritical carbon dioxide fluid phase.

In an additional set of trials, the quantities of extracted metal obtained with Hhfa and TBP(FIN0₃)o.₇(H₂0)o.₇ were compared to the quantities of extracted metal obtained without Hhfa, without TBP(HNO₃)₀.₇(H₂O)₀.₇ and without HNO₃. A control trial also was performed without the gold connector pins. In each trial except for the control, 80 gold connector pins were extracted at 40⁰C and 150 atm for 2 hours. The quantities of extracted metal were determined by ICP-AES, except for gold, which was analyzed by NAA. The results of these trials are summarized in Table 5. Trial 1 was performed with TBP(HN0₃)o.₇(H₂0)o.₇ and Hhfa. Trial 2 was performed with TBP(HN0₃)o.₇(H₂0)o.₇ and without Hhfa. Trial 3 was the control and was performed with TBP(HN0₃)o.₇ (H₂O)_0-7 and Hhfa, but without the gold pins. Trial 4 was performed with Hhfa and without TBP(HN0₃)o.₇(H₂0)₀.₇. Finally, trial 5 was performed with TBP and Hhfa and without HNO₃.

Table 5: Dissolution of Gold Pins using TBP(HN0₃)o.₇(H₂0)o.₇ and Hhfa

The results in Table 5 indicate that significant quantities of gold and other metals can be extracted using TBP(HN0₃)o.₇(H₂0)o.₇ and Hhfa. Significantly less extraction was observed when Hhfa was not used. No extraction was observed without TBP(HNO₃)_O-7(H₂O)_0-7. Similarly, no extraction was observed without HNO₃. The control trial without the gold pins showed trace quantities of metal, possibly from the reaction system.

Example 4: Palladium Dissolution

Dissolution of palladium shot in supercritical carbon dioxide was performed using TBP(HN0₃)i.₀(H₂0)o.₄ and Hhfa at 40⁰C and 150 atm. FIG. 8 shows ¹⁹F NMR spectra for Pd(Ma)₂ (FIG. 8A), Hhfa (FIG. 8B), and a trap solution obtained after the palladium dissolution (FIG. 8C). The palladium dissolved in supercritical carbon dioxide using TBP(HNO₃) _L0(H₂O)_0-4 and Hhfa showed a light yellow color. The ¹⁹F NMR spectrum of the trapped solution confirmed that the extracted palladium was Pd(hfa)₂. For comparison, dissolution of palladium shot in supercritical carbon dioxide also was performed using TBP(HNO₃)₀.₇(H₂O)₀.₇ and Hhfa at 40⁰C and 150 atm. The ¹⁹F NMR spectra in FIG. 9 show that a greater amount of palladium was converted to Pd(Ma)₂ using TBP(HNO₃)_U(H₂O)₀^ (FIG. 9A) than was converted using TBP(HN0₃)o.₇(H₂0)o.₇ (FIG. 9B). FIG. 9 also shows that the amount of Pd(hfa)₂ increased and the amount of Hhfa decreased when the extraction time was extended from 2 hours to 4 hours.

For comparison to palladium shot, palladium films (100 nm thickness) on silicon wafers also were tested. TBP(HNO₃)_L0(H₂O)_C4 was used as the acid-base complex. After a 30 second extraction at 40 ⁰C and 200 atm, EDS results showed that only 1% of the palladium metal remained on the silicon wafer surface. No palladium was found on the silicon wafer surface after four minutes.

TBP has a UV absorbance around 230 nm with an absorption shoulder at 280 nm. Hhfa, as a chelating agent, has an absorption band centered at 280 nm. Since a Pd(hfa)₂ standard solution in a TBP matrix has a broad absorption band around 230-450 nm, it can be difficult to identify individual peaks and to quantify the Pd(hfa)₂ complex. The ¹⁹F NMR spectrum of the trapped solution, however, confirmed the extracted palladium was in the chemical form of Pd(hfa)₂ with the chemical valence of +2, consistent with the Pd(Ma)₂ standard. Another trial was carried out under the following conditions: 2 grains of palladium shot (3 mm diameter), a cell with a volume of 35.3 mL, 6 mL of TBP(HN0₃)i.o(H₂0)o.₄, 0.5 mL of Hhfa, and 6 hours of SFE at 40⁰C and 200 atm. After SFE, the trap solution was back extracted using concentrated HCl (5 mL) and H₂O (2 mL). One mL of the aqueous phase was spiked into a small plastic vial, heat sealed and prepared for neutron activation analysis (NAA). The palladium sample and palladium standard solutions, including an atomic absorption standard and a neutron activation standard, were irradiated for 20 minutes in a one megawatt TRIGA nuclear reactor. FIG. 10 is a NAA gamma spectrum for the trap solution. ¹⁰⁹Pd was found to emit a γ ray at 88 keV with a half-life of 13.46 hours. Quantitative analysis of the sample indicated that a total of 3.98 mg of palladium was extracted from the palladium shot.

Dissolution of the palladium shot also was verified by ¹⁹F NMR. The ¹⁹F NMR spectra of Pd(Ma)₂ and Hhfa standards showed peaks at -72.99 ppm and -76.48 ppm, respectively. The NMR spectrum of the trap solution indicated that the extracted Pd-hfa was in the form of Pd(hfa)₂. The NMR spectrum for the trap solution also showed peaks at -72.99 ppm and -76.48 ppm, indicating Pd(hfa)₂ and Hhfa, respectively.

In additional trials, elemental palladium was extracted from different media, including activated carbon and alumina. The samples first were dried in an oven for several hours at 80 ⁰C to remove moisture. Two samples (10 - 20 mg) were weighed, with one to serve as a standard and the other to be the sample. The extraction was performed with 6 mL OfTBP(HNO₃)_L0(H₂O)_C4 and 0.3 mL of Hhfa. The temperature of the system was maintained at 50 ⁰C and the pressure was elevated to 200 ami. The reaction time was 3 hours. After SFE, the palladium samples were transferred to plastic vials. The palladium samples and standards were heat sealed in plastic vials for neutron activation analysis. The samples and standards were irradiated in a one megawatt nuclear reactor at a steady flux of 6 x 10¹²NCm^-2SeC^"1 for 5 min. The samples and standards then were cooled for 2 hours and counted individually by a Ge(Li) detector for 5 minutes. The extraction efficiencies for these trials are shown in Table 6. The results show that palladium can be extracted from a variety of different matrices by the disclosed method.

Table 6: Extraction of Palladium from Pd/Carbon and Pd/Al₂O₃ Catalysts

Example 5: Platinum Dissolution

Dissolution of platinum metal in supercritical carbon dioxide was performed using a mixture of TBP-nitric acid and TBP-hydrochloric acid as the oxidizing solution and Hhfa as the chelating agent. Another successful trial was performed using the same oxidizing agent, but TTA instead of Hhfa as the chelating agent. The oxidizing solution was prepared by mixing 3 mL of TBP with 0.7 mL of concentrated nitric acid and 2.1 mL of concentrated hydrochloric acid. In one trial, the oxidizing solution was placed in contact with a piece of platinum foil (about 0.8 cm radius) in supercritical carbon dioxide at 50⁰C and 200 atm in the presence of Hhfa. After 3 hours of reaction, the trap solution was removed and treated with 5 mL of concentrated HCl to separate the platinum. About 0.37 mg of platinum was recovered.

TBP-hydrochloric acid can be corrosive to stainless steel reaction vessels, such as the reaction vessel used in the described trials. A special supercritical CO₂ reaction vessel that can resist corrosion of hydrochloric acid may result in better dissolution of platinum metal using this supercritical fluid metal extraction method.

Example 6: Dissolution of Lanthanide Oxides This example shows that, while lanthanides and actinides can be oxidized and chelated by a single acid-base complex, the addition of a separate chelating agent can improve the extraction efficiency. In this example, La₂O₃, Pr₂O₃, and Eu₂O₃ were the target compounds and TBP(HNO₃) i.o(H₂0)o.₄ was used as the extractant.

A TBP(HNO₃)Lo(H₂O)_C4 complex was prepared by mixing 5 mL of TBP with 1.3 mL of concentrated nitric acid (15.5 M) in a Pyrex flask with stirring. After 30 minutes of mixing, the mixture was allowed to undergo phase separation. The organic phase containing the TBP-HNO₃ complex was removed by a pipette.

A high-pressure supercritical fluid extraction system equipped with a porous stainless steel cup was used for the extractions. The three lanthanide oxides (La₂O₃, Pr₂O₃, and Eu₂O₃) were introduced into soil collected from a rural location north of Moscow, Idaho. For each trial, 300-500 mg of soil was spiked with 10% by weight of each of the three lanthanide oxides. The soil samples were placed in the porous stainless steel cup along with 5 mL OfTBP(HNO₃)_L0(H₂O)_C4. Supercritical carbon dioxide then was fed into the extraction cell from a port on the top of the cell. After being introduced into the cell, the supercritical carbon dioxide flowed through the porous stainless steel cup and was collected from a port at the bottom of the cell. The spiked soil samples were treated with 40 minutes of static extraction followed by 60 minutes of dynamic flushing. Lanthanide concentrations in the soil before and after the supercritical fluid extraction were measured by NAA. To perform the NAA analysis, the soil samples were placed in polyethylene vials, heat sealed, and irradiated in a one megawatt TRIGA nuclear reactor with a steady flux of 6 x 10¹² Ncm^"2sec^'!. Neutron irradiation and counting were done at the Nuclear Radiation Center, Washington State University.

Figure 11 and Table 7 show the percent extraction of each lanthanide oxide by supercritical carbon dioxide at 40⁰C and different pressures (100, 150, and 200 arm) with TBP(HNO₃)_L0(H₂O)C₄ as an extractant.

Table 7: Extraction of Lanthanide Oxides with TBP/HNO₃

As shown in Figure 11 and Table 7, europium, praseodymium and lanthanum were removed from the soil at extraction efficiencies of about 90-92%, 78-86% and 71-79%, respectively. The high extraction efficiency for Eu³⁺ suggests that Am³⁺ also can be extracted effectively under the same conditions.

To test the effect of the addition of a separate chelating agent on the extraction efficiency of the lanthanides, additional trials were performed using a mixture of TTA and TBP-HNO₃. The results are shown in Figure 12 and Table 8.

Table 8: Extraction of Lanthanide Oxides with TBP/HNO₃/TTA

As shown in Figure 12 and Table 8, the percent extraction of each lanthanide oxide increased by about 5-10% and the differences between them became smaller. For example, europium, praseodymium and lanthanum were extracted at about 90-98%, 82- 84% and 80-85%, respectively.

To further verify the results, an additional trial was performed with 100 mg samples of the spiked soil treated with 20 minutes of static extraction followed by 40 minutes of dynamic flushing. The extractions were performed at 40 ⁰C and 100 arm. The percent extractions achieved with and without TTA are shown in Table 9. Table 9: Extraction of Lanthanide Oxides with TBP/HNO₃ or TBP/HNO₃/TTA

These results and the other results described in this example show that the addition of a β-diketone, such as TTA, can enhance the extraction efficiency of lanthanide oxides.

OTHER EMBODIMENTS

Other embodiments of the invention will be apparent to those of ordinary skill in the art from a consideration of this specification, or practice of the invention disclosed herein. It is intended that the specification and examples be considered as exemplary only, with the true scope and spirit of the invention being indicated by the following claims.

Claims

WE CLAM:

1. A method for extracting a metal, comprising: exposing a metal-containing material comprising the metal to a liquid or supercritical-fluid solvent, an acid-base complex comprising an oxidizing agent and a solubilizing agent, and a chelating agent that is not a component of the acid-base complex; and extracting for an extraction time sufficient for the chelating agent to form metal- containing complexes with at least a portion of the metal.

2. The method of claim 1, where the metal is not a lanthanide or actinide.

3. The method of claim 1 , where the metal is a transition metal.

4. The method of claim 1 , where the metal is a transition metal oxide.

5. The method of claim 1, where the metal is a transition metal sulfide.

6. The method of claim 1 , where the metal is a zero-valent transition metal.

7. The method of claim 1, where the metal is a noble metal.

8. The method of claim 1, where the metal is a platinum group metal.

9. The method of claim 1, where the metal is a coinage metal.

10. The method of claim 1, where the solvent is a gas at room temperature and atmospheric pressure.

11. The method of claim 1, where the solvent is a supercritical-fluid solvent.

12. The method of claim 1, where the solvent is carbon dioxide.

13. The method of claim 1, where the solvent is supercritical carbon dioxide.

14. The method of claim 1 , where the oxidizing agent is selected from the group consisting of mineral acids, and combinations thereof.

15. The method of claim 1, where the oxidizing agent is selected from the group consisting of nitric acid, sulfuric acid, and combinations thereof.

16. The method of claim 1, where the oxidizing agent is nitric acid.

17. The method of claim 1, where the oxidizing agent is selected to break down into volatile and/or soluble products after oxidizing the metal.

18. The method of claim 1 , where the oxidizing agent is selected to break down into compounds that are gases at room temperature and atmospheric pressure and/or water after oxidizing the metal.

19. The method of claim 1, where the solubilizing agent is an alkyl phosphate.

20. The method of claim 1 , where the solubilizing agent is selected from the group consisting of tri-alkylphosphates, tri-alkylphosphine oxides, and combinations thereof.

21. The method of claim 1 , where the solubilizing agent is tributylphosphate.

22. The method of claim 1, where the solubilizing agent is trioctylphosphine oxide.

23. The method of claim 1, where the solubilizing agent is soluble in supercritical carbon dioxide.

24. The method of claim 1, where the chelating agent is a β-diketone.

25. The method of claim 1, where the chelating agent is fluorinated.

26. The method of claim 1 , where the chelating agent is a fluorinated β- diketone.

27. The method of claim 1 , where the chelating agent is hexafluoroacetylacetone.

28. The method of claim 1, where the oxidizing agent is nitric acid and the solubilizing agent is tributylphosphate.

29. The method of claim 1 , where the acid-base complex has the formula TBP(HNθ₃)_x(H₂O)_y, in which x is greater than or equal to about 0.7 and y is less than or equal to about 0.7.

30. The method of claim 1, where the acid-base complex has the formula TBP(HNO₃)χ(H₂O)_y, in which x is about 1.0 and y is about 0.4.

31. The method of claim 1 , where the metal-containing material is exposed to a mixture comprising the solvent, the acid-base complex, and the chelating agent, the mixture being substantially non-aqueous, with the exception of coordinated water molecules, if present, on the acid-base complex, the chelating agent, or the metal- containing complexes, or any combination thereof.

32. The method of claim 1, where the acid-base complex is a first acid-base complex, the oxidizing agent is a first oxidizing agent, the solubilizing agent is a first solubilizing agent, and further comprising exposing the metal-containing material to a second acid-base complex comprising a second oxidizing agent and a second solubilizing agent, where the first oxidizing agent is nitric acid and the second oxidizing agent is hydrochloric acid.

33. The method of claim 1, where the solvent is supercritical carbon dioxide, the solubility of the oxidizing agent in supercritical carbon dioxide is less than about 0.1 moles per liter at 50 ⁰C and 100 atm, and the solubility of the acid-base complex in supercritical carbon dioxide is greater than about 0.5 moles per liter at 50 ⁰C and 100 atm.

34. The method of claim 1 , further comprising separating the metal- containing complexes from the solvent by reducing the pressure of the solvent.

35. The method of claim 1 , further comprising separating the metal- containing complexes from the solvent by increasing the temperature of the solvent.

36. The method of claim 1, where the metal-containing material comprises a copper film and the copper film is dissolved at a rate greater than about 2 nm per second.

37. The method of claim 1, where the metal-containing material comprises a palladium film and the amount of palladium in the metal-containing material is reduced by 99% in less than about 30 seconds.

38. The method of claim 1, where the metal-containing material, the solvent, the acid-base complex and the chelating agent form a reaction mixture, and further comprising exposing the reaction mixture to ultrasonic vibrations during at least a portion of the extraction time .

39. The method of claim 1, where the metal-containing material, the solvent, the acid-base complex and the chelating agent form a reaction mixture and further comprising filtering the reaction mixture during at least a portion of the extraction time.

40. The method of claim 1, further comprising recycling the solvent, solubilizing agent, chelating agent, or any combination thereof.

41. The method of claim 1 , where the chelating agent is a first chelating agent, and further comprising exposing the metal-containing material to a second chelating agent that is not a component of the acid-base complex for a period of time sufficient for the first chelating agent, the second chelating agent, and the metal to form adducts.

42. The method of claim 41 , where the first chelating agent and the second chelating agent are different.

43. The method of claim 1, where the chelating agent is a first chelating agent, and further comprising exposing the metal-containing material to a second chelating agent that is not a component of the acid-base complex for a period of time sufficient for the second chelating agent to displace one or more coordinated water molecules on the metal-containing complexes.

44. The method of claim 43, where the first chelating agent and the second chelating agent are different.

45. A method for extracting a metal, comprising: exposing a metal-containing material comprising the metal to a liquid or supercritical-fluid solvent, an acid-base complex comprising an oxidizing agent and a solubilizing agent, and a chelating agent that is not a component of the acid-base complex; and extracting for an extraction time sufficient for the chelating agent to form metal- containing complexes with at least a portion of the metal, where the metal is not a lanthanide or actinide, the solvent is a gas at room temperature and atmospheric pressure, the oxidizing agent is selected from the group consisting of mineral acids, and combinations thereof, the solubilizing agent is an alkyl phosphate, and the chelating agent is a β-diketone.

46. The method of claim 45, where the metal is a noble metal, platinum group metal or coinage metal.

47. The method of claim 45, where the solvent is carbon dioxide, the solubilizing agent is tributylphosphate, and the oxidizing agent is nitric acid.

48. The method of claim 45, where the chelating agent is hexafluoroacetylacetone.

49. A method for removing a metal from a surface of a semiconductor structure, comprising: exposing the surface of the semiconductor structure to a liquid or supercritical- fluid solvent, an acid-base complex comprising an oxidizing agent and a solubilizing agent, and a chelating agent that is not a component of the acid-base complex; and extracting for an extraction time sufficient for the chelating agent to form metal- containing complexes with at least a portion of the metal.

50. The method of claim 49, where the metal is copper.

51. The method of claim 49, where the solvent is a gas at room temperature and atmospheric pressure.

52. The method of claim 49, where the solvent is carbon dioxide, the solubilizing agent is tributylphosphate, and the oxidizing agent is nitric acid.

53. The method of claim 49, further comprising contacting the surface of the semiconductor structure with a porous pad.

54. A method for extracting platinum, comprising: exposing a platinum -containing material comprising platinum to a liquid or supercritical-fluid solvent, a first acid-base complex comprising nitric acid and a first solubilizing agent, a second acid-base complex comprising hydrochloric acid and a second solubilizing agent, and a chelating agent that is not a component of the first acid- base complex or the second acid-base complex; and extracting for an extraction time sufficient for the chelating agent to form platinum-containing complexes with at least a portion of the platinum.

55. The method of claim 54, where the solvent is a gas at room temperature and atmospheric pressure.

56. The method of claim 54, where the first solubilizing agent, the second solubilizing agent, or both is/are tributylphosphate.

57. The method of claim 54, where the chelating agent is a β-diketone.

58. A method for removing metal from a nanostructure, comprising: exposing the nanostructure to a liquid or supercritical-fluid solvent, an acid-base complex comprising an oxidizing agent and a solubilizing agent, and a chelating agent that is not a component of the acid-base complex; and extracting for an extraction time sufficient for the chelating agent to form metal- containing complexes with at least a portion of the metal.

59. The method of claim 58, where the metal is iron.

60. The method of claim 58, where the nanostructure comprises at least one carbon nanotube.

61. The method of claim 58, where the solvent is a gas at room temperature and atmospheric pressure.

62. The method of claim 58, where the solvent is carbon dioxide, the solubilizing agent is tributylphosphate, and the oxidizing agent is nitric acid.

63. An extractant composition, comprising: a liquid or supercritical-fluid solvent; an acid-base complex comprising an oxidizing agent and a solubilizing agent; and a chelating agent that is not a component of the acid-base complex

64. The extractant composition of claim 63, where the solvent is a gas at room temperature and atmospheric pressure.

65. The extractant composition of claim 63, where the solvent is a supercritical-fluid solvent.

66. The extractant composition of claim 63, where the solvent is carbon dioxide.

67. The extractant composition of claim 63, where the solvent is supercritical carbon dioxide.

68. The extractant composition of claim 63, where the oxidizing agent is selected from the group consisting of mineral acids, and combinations thereof.

69. The extractant composition of claim 63, where the oxidizing agent is selected from the group consisting of nitric acid, sulfuric acid, and combinations thereof.

70. The extractant composition of claim 63, where the oxidizing agent is nitric acid.

71. The extractant composition of claim 63 , where the oxidizing agent is selected to break down into volatile and/or soluble products after oxidizing a metal.

72. The extractant composition of claim 63, where the oxidizing agent is selected to break down into compounds that are gases at room temperature and atmospheric pressure and/or water after oxidizing a metal.

73. The extractant composition of claim 63, where the solubilizing agent is an alkyl phosphate.

74. The extractant composition of claim 63, where the solubilizing agent is selected from the group consisting of tri-alkylphosphates, tri-alkylphosphine oxides, and combinations thereof.

75. The extractant composition of claim 63, where the solubilizing agent is tributylphosphate.

76. The extractant composition of claim 63, where the solubilizing agent is trioctylphosphine oxide.

77. The extractant composition of claim 63, where the solubilizing agent is soluble in supercritical carbon dioxide.

78. The extractant composition of claim 63, where the chelating agent is a β- diketone.

79. The extractant composition of claim 63, where the chelating agent is fluorinated.

80. The extractant composition of claim 63, where the chelating agent is a fluorinated β-diketone.

81. The extractant composition of claim 63, where the chelating agent is hexafluoroacetylacetone.

82. The extractant composition of claim 63, where the oxidizing agent is nitric acid and the solubilizing agent is tributylphosphate.

83. The extractant composition of claim 63, where the acid-base complex has the formula TBP(HNO₃)_x(H₂O)_y, in which x is greater than or equal to about 0.7 and y is less than or equal to about 0.7.

84. The extractant composition of claim 63, where the acid-base complex has the formula TBP(HNO₃)_x(H₂O)_y, in which x is about 1.0 and y is about 0.4.

85. The extractant composition of claim 63, where the acid-base complex is a first acid-base complex, the oxidizing agent is a first oxidizing agent, the solubilizing agent is a first solubilizing agent, and further comprising a second acid-base complex comprising a second oxidizing agent and a second solubilizing agent, where the first oxidizing agent is nitric acid and the second oxidizing agent is hydrochloric acid.

86. The extractant composition of claim 63, where the chelating agent is a first chelating agent and further comprising a second chelating agent that is not a component of the acid-base complex.