US20170151548A1

US20170151548A1 - Sorption And Separation of Various Materials By Graphene Oxides

Info

Publication number: US20170151548A1
Application number: US15/338,407
Authority: US
Inventors: James M. Tour; Alexander Slesarev; Dmitry V. Kosynkin; Anna Y. Romanchuk; Stepan N. Kalmykov
Original assignee: Zonko LLC
Current assignee: Zonko LLC
Priority date: 2011-02-25
Filing date: 2016-10-30
Publication date: 2017-06-01
Also published as: WO2012170086A1; JP2014512258A; US9511346B2; CA2828361A1; EP2678102A1; US20140081067A1

Abstract

Methods of sorption of various materials from an environment are disclosed herein. Embodiments of the materials include radioactive elements chlorates, perchlorates, organohalogens, and combinations thereof. Other embodiments pertain to methods of sorption of cationic radionuclides. Compositions produced by such methods are also disclosed herein. Embodiments of the methods may include contacting graphene oxides with the environment and sorption of the materials to the graphene oxides. In some embodiments, the sorption is relatively rapid in comparison to known sorbents; even in the presence of relatively higher concentrations of complexing agents. In some embodiments, the methods further include separating the graphene oxides that sorbed materials from the environment. Yet other embodiments may include desorbing the materials from the graphene oxides that sorbed the materials, and compositions therefrom.

Description

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a continuation of co-pending U.S. Utility patent application Ser. No. 14/001,255 entitled “Sorption and Separation of Various Materials By Graphene Oxides” filed on Nov. 26, 2013 which claims priority to International PCT Application Serial Number PCT/US2012/026766 filed on Feb. 27, 2012 and having the same title as the previously-mentioned co-pending application Ser. No. 14/001,255 and which claims priority to U.S. Provisional Patent Application Ser. No. 61/446,535, filed on Feb. 25, 2011, The entirety of the above-identified applications are each incorporated herein by reference.

STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH

This invention was made with government support under the U.S. Air Force Office of Scientific Research Grant No: FA9550-09-1-0581 and the U.S. Navy Office of Naval Research Grant No: N000014-09-1-1066, both awarded by the U.S. Department of Defense. The government has certain rights in the invention.

BACKGROUND

Current methods of purifying various environmental contaminants (including radioactive elements and halogenated compounds) have numerous limitations in terms of efficacy, costs, and efficiency. Therefore, a need exists for the development of improved methods for purifying such contaminants from various environments.

BRIEF SUMMARY

In some embodiments, the present invention provides methods of sorption of various materials from an environment. Such methods generally include contacting graphene oxides with the environment. This in turn leads to the relatively rapid rate or rates of sorption of the materials to the graphene oxides. In some embodiments, rapid sorption of the materials to graphene oxide occurs even in the presence of relatively high concentrations of complexing agents. Complexing agents are agents that may compete with graphene oxide, or otherwise prevent or interfere with graphene oxide sorbing materials from the environment. In some embodiments, the materials sorbed from the environment comprise at least one of radioactive elements, chlorates, perchlorates, organohalogens, and combinations thereof. In some embodiments the complexing agents may be one or more of Na⁺, Ca²⁺, NO₃ ⁻, CH₃COO⁻, C₂O₄ ²⁻, SO₄ ²⁻, Cl⁻, CO₃ ²⁻, and combinations thereof.
In some embodiments, the methods of the present invention also include a step of separating the graphene oxides from the environment after the sorption of the materials to the graphene oxides. In various embodiments, the separation step may occur by centrifugation, ultra-centrifugation, filtration, ultra-filtration, precipitation, electrophoresis, reverse osmosis, sedimentation, incubation, treatment with acids, treatment with bases, treatment with chelating agents, and combinations of such methods.
Various methods may also be used to contact graphene oxides with the environment. In some embodiments, the contacting occurs by mixing the graphene oxides with the environment. In some embodiments, the contacting occurs by flowing the environment through a structure that contains the graphene oxides (e.g., a column).
The sorption of materials to graphene oxides may also occur by various methods. In some embodiments, the sorption includes an absorption interaction of the materials in an environment to the graphene oxides. In some embodiments, the sorption includes an ionic interaction between the materials in an environment and the graphene oxides. In some embodiments, the sorption includes an adsorption interaction between the materials in an environment and the graphene oxides. In some embodiments, the sorption includes a physisorption interaction between the materials in an environment and the graphene oxides. In some embodiments, the sorption includes a chemisorption interaction between the materials in an environment and the graphene oxides. In some embodiments, the sorption includes a covalent bonding interaction between the materials in an environment and the graphene oxides. In some embodiments, the sorption includes a non-covalent bonding interaction between the materials in an environment and the graphene oxides. In some embodiments, the sorption includes a hydrogen bonding interaction between the materials in an environment and the graphene oxides. In some embodiments, the sorption includes a van der Waals interaction between the materials in an environment and the graphene oxides. The aforementioned interactions are non-limiting and herein referred to as sorption.
Various graphene oxides may also be utilized in the methods of the present invention. For instance, the graphene oxides may be at least one of functionalized graphene oxides, pristine graphene oxides, doped graphene oxides, reduced graphene oxides, functionalized graphene oxide nanoribbons, pristine graphene oxide nanoribbons, doped graphene oxide nanoribbons, reduced graphene oxide nanoribbons, stacked graphene oxides, graphite oxides, and combinations thereof. In some embodiments, the graphene oxides may be functionalized with functional groups that comprise at least one of carboxyl groups, esters, amides, thiols, hydroxyl groups, carbonyl groups, aryl groups, epoxy groups, phenol groups, phosphonic acids, amine groups, polymers and combinations thereof. In some embodiments, the graphene oxides may be functionalized with polymers, such as polyethylene glycols, polyvinyl alcohols, poly(ethylene imines), poly(acrylic acids), polyamines and combinations thereof.
Furthermore, the methods of the present invention may be utilized to purify various materials from various environments. For instance, in some embodiments, the environment is an aqueous solution, such as contaminated water. In some embodiments, the environment is an atmospheric environment, such as air. In some embodiments, the environment is a solution comprising nuclear fission products.
In some embodiments, the materials to be purified include radioactive elements. In some embodiments, the radioactive elements comprise at least one of metals, salts, metal salts, radionuclides, actinides, lanthanides, and combinations thereof. In more specific embodiments, the radioactive elements in the environment include radionuclides, such as thallium, iridium, fluorine, americium, neptunium, gadolinium, bismuth, uranium, thorium, plutonium, niobium, barium, cadmium, cobalt, europium, manganese, sodium, zinc, technetium, strontium, carbon, polonium, cesium, potassium, radium, lead, actinides, lanthanides and combinations thereof. In some embodiments, the radionuclide is actinide.
In some embodiments, the materials to be purified include chlorates, such as ammonium chlorate, barium chlorate, cesium chlorate, fluorine chlorate, lithium chlorate, magnesium chlorate, potassium chlorate, rubidium chlorate, silver chlorate, sodium chlorate, and combinations thereof. In some embodiments, the materials to be purified include perchlorates, such as ammonium perchlorate, barium perchlorate, cesium perchlorate, fluorine perchlorate, lithium perchlorate, magnesium perchlorate, perchloric acid, potassium perchlorate, rubidium perchlorate, silver perchlorate, sodium perchlorate, and combinations thereof. In additional embodiments, the materials to be purified include organohalogens, such as polychlorinated biphenyls (PCB) and halogenated flame retardants.
In more specific embodiments, the methods of the present invention are used for the sorption of actinides from a solution comprising nuclear fission products. Such methods may also include a step of separating the graphene oxides from the solution comprising nuclear fission products after the sorption step.
The methods of the present invention provide various advantages, including the effective separation of various radioactive elements from various environments. For instance, in some embodiments, the methods of the present invention may be used to reduce radioactive elements in a solution by at least about 70%.
The methods of the present invention also provide various applications. For instance, in some embodiments, the methods of the present invention may be used for waste water treatment and environmental remediation applications.

BRIEF DESCRIPTION OF THE FIGURES

FIGS. 1A-1C show data relating to the removal of various radionuclides by graphene oxide. FIG. 1A shows the kinetics of U(VI), Am(III), Th(IV) and Pu(IV) sorption onto graphene oxide, indicating that steady state conditions are reached within 5 minutes. FIG. 1B shows pH-sorption edges for Th(IV), U(VI), Pu(IV) and Am(III). FIG. 1C shows pH-sorption edges for Sr(II), Tc(VII), and Np(V) at steady state. With respect to the data shown in FIGS. 1A-1C, the concentrations are listed in Example 10.

FIG. 2A shows sorption isotherms for U(VI) in 0.01 M NaClO₄. FIG. 2B shows sorption isotherms for Sr(II) in 0.01 M NaClO₄. FIG. 2C shows sorption isotherms for Am(III) in 0.01 M NaClO₄. Isotherms were fitted with both Langmuir (solid line) and Freundlich (dashed line) formalism. Parameters of fitting and sorption capacity (Q_max) are shown in Table 1 (Example 3). The concentrations of the elements for sorption, and of the graphene oxides are listed in Example 10.

FIGS. 3A and 3B show sorption efficiencies of different materials that are compared along with the coagulation properties of graphene oxide solutions. FIG. 3A shows the removal of U(VI) from simulated liquid nuclear waste (see Table 2) by graphene oxide and also by some routinely used sorbents at equal mass concentrations. FIG. 3B show the removal of Pu(IV) from simulated liquid nuclear waste (see Table 2) by graphene oxide and also by some routinely used sorbents at equal mass concentrations. FIG. 3C shows coagulation of graphene oxide in a simulated nuclear waste solution. The left panel of FIG. 3C shows the initial graphene oxide suspension (labeled as 1), and the coagulated graphene oxide (labeled as 2). The mid-right panel of FIG. 3C shows a scanning transmission electron microscope (STEM) image of the coagulated graphene oxide (labeled as 2) and the corresponding EDX spectrum on the far right. The highlighted section in the STEM image in FIG. 3C shows the formation of nanoparticulate aggregates containing cations from simulated nuclear waste solution (Si and P are trace contaminants in graphene oxide from its preparation, as described in Example 5).

FIG. 4A shows micrographs and analyses of Pu/graphene oxide coagulates. The left side panel of FIG. 4A is a STEM image of Pu(IV) on graphene oxide. The right side panels of FIG. 4A show the EDX spectra corresponding to the different areas labeled 1, 2 and 3 as indicated on the STEM image. Pu-containing particles in graphene oxide could be observed by Z contrast of the STEM image in FIG. 4A and as shown in the EDX spectra of the highlighted regions of the STEM image labeled 1, 2 and 3. FIG. 4B is a high-resolution transmission electron microscopy (HRTEM) image of PuO_2+x.nH₂O nanoparticles together with the FFT of individual nanoparticles from the highlighted region, indicating the cubic structure typical for PuO_2+x.

DETAILED DESCRIPTION

It is to be understood that both the foregoing general description and the following detailed description are exemplary and explanatory only, and are not restrictive of the invention, as claimed. In this application, the use of the singular includes the plural, the word “a” or “an” means “at least one”, and the use of “or” means “and/or”, unless specifically stated otherwise. Furthermore, the use of the term “including”, as well as other forms, such as “includes” and “included”, is not limiting. Also, terms such as “element” or “component” encompass both elements or components comprising one unit and elements or components that comprise more than one unit unless specifically stated otherwise. Ranges of values stated herein include all subranges and such ranges and subranges are included and disclosed herein.
The section headings used herein are for organizational purposes only and are not to be construed as limiting the subject matter described. All documents, or portions of documents, cited in this application, including, but not limited to, patents, patent applications, articles, books, and treatises, are hereby expressly incorporated herein by reference in their entirety for any purpose. In the event that one or more of the incorporated literature and similar materials defines a term in a manner that contradicts the definition of that term in this application, this application controls.
The nuclear industry generates large amounts of radioactive wastewater that must be effectively treated before it is discharged into the environment. The toxic radioactive elements (such as radionuclides) in the wastewater cannot be effectively removed by current drinking water purification techniques. Furthermore, separation techniques involving chromatographic methods can be time-consuming and expensive. Moreover, various industries generate large amounts of halogenated by-products, such as chlorates, perchlorates, and organohalogens (e.g., polychlorinated biphenyls and halogenated flame retardants). As a result, new methods are needed for the relatively rapid and efficient removal of radioactive and halogenated elements from various solutions and environments, especially for waste water treatment and environmental remediation applications wherein relatively higher concentrations of complexing agents may be present. The present invention addresses these needs.
In some embodiments, the present invention provides methods of sorption of various materials from an environment. In some embodiments, the materials to be removed from an environment include at least one of radioactive elements, chlorates, perchlorates, organohalogens, and combinations thereof.
Such methods generally include contacting graphene oxides with the environment. This in turn leads to the sorption of the materials to the graphene oxides. In some embodiments, the methods of the present invention also include a step of separating the graphene oxides from the environment after the sorption of the radioactive elements to the graphene oxides.
As set forth in more detail below, the methods of the present invention have numerous variations. For instance, the methods of the present invention may involve the purification of various materials from various environments by the use of various graphene oxides.

Materials

The methods of the present invention may be utilized to purify various materials from various environments. In some embodiments, the materials to be purified include, without limitation, radioactive elements, chlorates, perchlorates, organohalogens, and combinations thereof. In more specific embodiments, the materials to be purified include, without limitation, polycyclic aromatics, chlorinated and brominated dibenzodioxins and dibenzofurans, chlorinated biphenyls, lindane, dichlorodiphenyltrichloroethane (DDT) and other similar hydrophobic xenobiotics.

Radioactive Elements

In some embodiments, the radioactive elements to be purified from an environment are metals, salts, metal salts, radionuclides, actinides, lanthanides, and combinations thereof. In more specific embodiments, the radioactive elements in the solutions include radionuclides, such as thallium, iridium, fluorine, americium, neptunium, gadolinium, bismuth, uranium, thorium, plutonium, niobium, barium, cadmium, cobalt, europium, manganese, sodium, zinc, technetium, strontium, carbon, polonium, cesium, potassium, radium, lead, actinides, lanthanides and combinations thereof.
In more specific embodiments, the radioactive elements to be purified from various environments include, without limitation, americium(III), actinide(III), actinide(IV), thallium(IV), plutonium(IV), neptunium(V), uranium(VI), strontium(II), technetium(VII), and combinations thereof.
In more specific embodiments, the radioactive elements include, without limitation, thallium-201, iridium-192, fluorine-18, americium-241, americium-243, neptunium-237, Gd-153, niobium-93, barium-133, cadmium-109, cobalt-57, cobalt-60, europium-152, manganese-54, sodium-22, zinc-65, technetium-99, strontium-90, thallium-204, carbon-14, polonium 210, cesium-137, and combinations thereof.
In some embodiments, the radioactive elements may be naturally occurring radioactive materials (NORMs). In some embodiments, NORMs generally comprise uranium and its isotopes. In some embodiments, NORMs may also comprise thorium and its isotopes. In further embodiments, NORMs may also include potassium, lead, and polonium. NORMs are not only considered in the context of their natural abundance and distribution, but also in view of human activities that increase potential for exposure to them. In some cases, those human activities may also serve to concentrate the radionuclides present, thereby resulting in what is called technologically enhanced naturally occurring radioactive materials (TENORMs).
NORMs or TENORMs can be produced by oil and gas production and refining; mineral production; coal mining and combustion; metal mining and smelting; fertilizer production; production of mineral based building materials, including granite, stone, gypsum, and concrete; and beneficiation of mineral sands, including rare earth minerals, titanium and zirconium. NORMs and TENORMs are also present in drinking water supplies, particularly in Western US and Canada. See, e.g., http://world-nuclear.org/info/inf30.html.
While uranium and thorium-based elements are the main isotopes of NORMs and TENORMs, K-40, Po-210, Ra-226, Ra-228, and Pb-210 can also be present. NORMs and TENORMs can also include actinides and lanthanides in general. Thus, some embodiments of the present invention addresses the need for the capture and clean-up of NORMs and TENORMs.
In more specific embodiments, the radioactive elements to be purified include actinides. In some embodiments, the actinides are in a solution that contains nuclear fission products.

Chlorates and Perchlorates

The methods of the present invention may also be utilized to purify various chlorates and perchlorates. Non-limiting examples of chlorates include ammonium chlorate, barium chlorate, cesium chlorate, fluorine chlorate, lithium chlorate, magnesium chlorate, potassium chlorate, rubidium chlorate, silver chlorate, sodium chlorate, and combinations thereof. Non-limiting examples of perchlorates include ammonium perchlorate, barium perchlorate, cesium perchlorate, fluorine perchlorate, lithium perchlorate, magnesium perchlorate, perchloric acid, potassium perchlorate, rubidium perchlorate, silver perchlorate, sodium perchlorate, and combinations thereof. In some embodiments, perchlorates may have similar sorption profiles to various radioactive elements, such as pertechnetate (Tc(VII)).

Organohalogens

The methods of the present invention may also be utilized to purify various organohalogens. Organohalogens generally refer to organic compounds that include one or more halogen groups. In some embodiments, the organohalogen is an organochloride. In some embodiments, the organohalogen is polychlorinated biphenyl (PCB). In some embodiments, the organohalogen is a halogenated flame retardant. In further embodiments, the organohalogens include, without limitation, chloromethanes, dichloromethanes, trichloromethanes, tetrachloromethanes, bromomethanes, bromoalkanes, bromochloromethanes, iodoalkanes, iodomethanes, organofluorines, organochlorines, acyclic organohalogens, cyclic organohalogens, and combinations thereof.

Environments

In the present invention, materials may be purified from various types of environments. In some embodiments, the environment is an atmospheric environment, such as air. In some embodiments, the environment is a solution, such as an aqueous solution. Non-limiting examples of aqueous solutions include water, such as radioactive water, contaminated water, and waste water. In some embodiments, the solution includes nuclear fission products. In some embodiments, the solution to be purified is a non-aqueous solution, such as a solution containing benzenes, toluenes, dichloromethane, and other non-aqueous solvents.

Graphene Oxides

“Functionalized graphene oxide,” as used herein, refers to, for example, graphene oxide that has been derivatized with a plurality of functional groups. “Chemically converted graphene” refers to, for example, graphene produced by reduction of graphene oxide. Reduction of graphene oxide to chemically converted graphene removes at least a portion of the oxygen functionalities present in graphene oxide.
Various graphene oxides may be utilized in the methods of the present invention. Suitable graphene oxides include, without limitation, functionalized graphene oxides, pristine graphene oxides, doped graphene oxides, reduced graphene oxides, chemically converted graphene, functionalized graphene oxide nanoribbons, pristine graphene oxide nanoribbons, doped graphene oxide nanoribbons, reduced graphene oxide nanoribbons, stacked graphene oxides, graphite oxides, and combinations thereof.
In some embodiments, the graphene oxides may be covalently or non-covalently functionalized with various functional groups, such as carboxyl groups, hydroxyl groups. carbonyl groups, aryl groups, epoxy groups, phenol groups, phosphonic acids (e.g., RPO(OH)₂, where R is a carbon group linked to the graphene scaffold), amine groups, esters, ether-based functional groups, polymers and combinations thereof.
In some embodiments, functionalized graphene oxides may be made from graphene oxide that has been derivatized with a plurality of functional groups using a derivitazing agent. In some embodiments, the derivatizing agent is a diazonium species. In some embodiments, the derivatizing agent is an aryl diazonium species. In some embodiments, the diazonium species may be a pre-formed diazonium salt. In other embodiments, the diazonium species may be a diazonium salt that is formed in situ. A diazonium species may be formed in situ, for example, by treating an amine with an organic nitrite such as, for example, isoamyl nitrite.
In some embodiments of functionalized graphene oxide, carboxylic acids, hydroxyl groups, carbonyl groups, and epoxides comprising the graphene oxides (see, e.g., Table 3) may be chemically transformed by the derivatizing agents in forming the functionalized graphene oxide. In some embodiments, functionalized graphene oxides generally contain a plurality of functional groups attached to the graphene oxide through a covalent bond. Chemical bonding of the functional groups may occur to the edge of the graphene oxide, to the basal plane of the graphene oxide, or to both the edge and the basal plane of the graphene oxide.
In some embodiments, the graphene oxides may be functionalized with polymers, such as polyethylene glycols, polyamines, polyesters, polyvinyl alcohols, poly(ethylene imines), poly(acrylic acids), and combinations thereof. Examples of suitable polyethylene glycol functional groups include, without limitation, triethylene glycol di(p-toluenesulfonate), polyethylene glycol methyl ether tosylate, and the like. In some embodiments, polyethylene glycol functional groups on graphene oxides can be further hydrolyzed to remove most or all of any tosylate groups in order to afford terminal hydroxyl groups.
The graphene oxides of the present invention may also have various arrangements. For instance, in various embodiments, the graphene oxides of the present invention may be in stacked form. In some embodiments, the stacked graphene oxides may contain from about 2 layers to about 50 layers of graphene oxide. In some embodiments, the graphene oxides of the present invention may form a single sheet.
In some embodiments, the graphene oxides of the present invention may also include one or more layers of graphene along with the graphene oxides. Such graphenes may include, without limitation, pristine graphenes, doped graphenes, chemically converted graphenes, functionalized graphenes and combinations thereof.
In further embodiments, the graphene oxides may be graphene oxides derived from exfoliated graphite, graphene nanoflakes, or split carbon nanotubes (such as multi-walled carbon nanotubes). In more specific embodiments, the graphene oxides of the present invention may be derived from split carbon nanotubes. In various embodiments, the split carbon nanotubes may be derived from single-walled carbon nanotubes, multi-walled carbon nanotubes, double-walled carbon nanotubes, ultrashort carbon nanotubes, pristine carbon nanotubes, functionalized carbon nanotubes, and combinations thereof. In more specific embodiments, the graphene oxides of the present invention are derived from split multi-walled carbon nanotubes.
In addition, various methods may be used to split carbon nanotubes. In some embodiments, carbon nanotubes may be split by potassium or sodium metals. In some embodiments, the split carbon nanotubes may then be functionalized by various functional groups, such as alkyl groups. Additional variations of such embodiments are described in U.S. Provisional Application No. 61/534,553 entitled “One Pot Synthesis of Functionalized Graphene Oxide and Polymer/Graphene Oxide Nanocomposites.” Also see Higginbotham et al., “Low-Defect Graphene Oxide Oxides from Multiwalled Carbon Nanotubes,” ACS Nano 2010, 4, 2059-2069. Also see Applicants' co-pending U.S. patent application Ser. No. 12/544,057 entitled “Methods for Preparation of Graphene Oxides From Carbon Nanotubes and Compositions, Thin Composites and Devices Derived Therefrom.” Also see Kosynkin et al., “Highly Conductive Graphene Oxides by Longitudinal Splitting of Carbon Nanotubes Using Potassium Vapor,” ACS Nano 2011, 5, 968-974.
In various embodiments, the graphene oxides may be doped with various additives. In some embodiments, the additives may be one or more heteroatoms of B, N, O, Al, Au, P, Si or S.
In more specific embodiments, the doped additives may include, without limitation, melamine, carboranes, aminoboranes, phosphines, aluminum hydroxides, silanes, polysilanes, polysiloxanes, sulfides, thiols, and combinations thereof. In more specific embodiments, the graphene oxides may be HNO₃doped and/or AuCl₃doped.
In various embodiments, the graphene oxides of the present invention may also be dissolved or suspended in one or more solvents before being contacted with environments containing radioactive elements. Examples of suitable solvents include, without limitation, acetone, 2-butanone, dichlorobenzene, ortho-dichlorobenzene, chlorobenzene, chlorosulfonic acid, dimethyl formamide, N-methyl pyrrolidone, 1,2-dimethoxyethane, water, alcohol and combinations thereof.
In further embodiments, the graphene oxides of the present invention may also be associated with a surfactant before being associated with various environments. Suitable surfactants include, without limitation, sodium dodecyl sulfate (SDS), sodium dodecylbenzene sulfonate, Triton X-100, chlorosulfonic acid, and the like.
The graphene oxides of the present invention may have various properties. For instance, in some embodiments, the graphene oxides of the present invention have an aspect ratio in length-to-width greater than or equal to 2, greater than 10, or greater than 100. In some embodiments, the graphene oxides have an aspect ratio greater than 1000. In further embodiments, the graphene oxides of the present invention have an aspect ratio in length-to-width greater less than or equal to 2.
Furthermore, the graphene oxides of the present invention can come in the form of variable sized sheets. Such sheets may have lengths or diameters that range from about a few nanometers to a few hundred microns to several centimeters. In more specific embodiments, the graphene oxides may have lengths or diameters that range from about 1 nanometers to about 3 centimeters.
The graphene oxides of the present invention are generally hydrophilic and can be coagulated upon addition of cations or surfactants. In some embodiments, the graphene oxides have a ratio of total oxygen functionality to graphitic sp²carbon in the range of 2.6:1 and 4.0:1 (for example, as in Table 3 further below Examples 6-7). All individual values and subranges within the range of 2.6:1 and 4.0:1 are included herein and disclosed herein.
In other embodiments, the surface oxidation of the graphene oxides can be gradually and systematically reduced to modify their properties. Such reduction can occur through the addition of reducing agents (e.g., hydrazine, sodium borohydride, acid or base with heat) or thermolysis (with or without H₂being present). In some embodiments, graphene oxide may be reduced with at least one reducing agent to form chemically converted graphene. In some embodiments, the at least one reducing agent for forming chemically converted graphene from graphene oxide may be, for example, hydrazines, iodide, phosphines, phosphites, sulfides, sulfites, hydrosulfites, borohydrides, cyanoborohydrides, aluminum hydrides, boranes, hydroxylamine, diamine, dissolving metal reductions, hydrogen, and combinations thereof. In some embodiments, the at least one reducing agent may be hydrogen. In some embodiments, the graphene oxide may be first reduced with hydrazine or hydrazine hydrate and thereafter reduced with a second, more powerful reducing agent such as, for example, hydrogen. The second reduction may further restore the sp²structure of pristine graphene sheets over that obtained in the first reduction. In various embodiments, reduction of the graphene oxide with hydrogen may involve annealing the graphene oxide in the presence of hydrogen. In some embodiments, annealing may include an inert gas.
Hydrazine, for example, removes ketone and hydroxyl groups from graphene oxide but leaves behind edge carboxylic acid groups in the chemically converted graphene. The residual carboxylic acid groups may disrupt the π-conjugated network of the graphene sheet and lower the conductivity of the chemically converted graphene relative to that ultimately obtainable by their removal. Hydrogen may be more efficient than hydrazine at removing oxygen-containing functional groups from the graphene oxide, since this reagent removes even carboxylic acid groups in addition to carbonyl and hydroxyl functionalities. In some embodiments, borane (BH₃) may be used to reduce the graphene oxide. Borane is particularly effective at reducing carboxylic acids to alcohols, and the alcohols can be further removed with hydrogen and heat in a second reduction step. In some embodiments, the chemically converted graphene may be further reacted with a derivatizing agent to form a functionalized, chemically converted graphene.
The graphene oxides of the present invention may also have various forms. For instance, in some embodiments, the graphene oxides of the present invention may be associated with various composites. In some embodiments, such composites may include organic materials, such as synthetic polymers, natural fibers, nonwoven materials, and the like. In some embodiments, the graphene oxide composites may include inorganic materials, such as porous carbons, asbestos, Celite, diatomaceous earth, and the like. In further embodiments, the graphene oxides of the present invention may be in isolated and pure forms.
In more specific embodiments of the present invention, the graphene oxides are derived from the direct oxidation of graphite. In some embodiments, the oxidation of graphite could be through chemical methods, electrochemical methods or combinations of chemical methods and electrochemical methods that may occur simultaneously or sequentially in either order. In some embodiments, graphene oxides are derived by the chemical oxidation of graphite. Examples of methods of oxidizing graphite are disclosed in Applicants' prior work. See, e.g., Marcano, et al., “Improved Synthesis of Graphene Oxide” ACS Nano 2010, 4, 4806-4814. Also see U.S. Provisional Patent Application Nos. 61/180,505 and 61/185,640. Also see WO 20111016889, and corresponding U.S. Patent Application Publication No. US 2012/0129736 published May 24, 2012. (Although stated elsewhere, each reference is hereby incorporated by reference in its entirety). In some embodiments, the graphene oxide may have a relatively higher level of oxidation, hydrophilicity and degree of dispersability in relatively more polar solvents. In some embodiments, graphene oxides readily disperse in water.
In some embodiments, making graphene oxides may comprise providing a graphite source, providing a solution containing at least one acid solvent, at least one oxidant (oxidizing agent) and at least one protecting agent, mixing the graphite source with the solution, and oxidizing the graphite source with the at least one oxidant (oxidizing agent) in the presence of the at least one protecting agent to form graphene oxide.
In some embodiments of making the graphene oxides, and without being bound by theory or mechanism, addition of what may be conceptually thought of as a protecting agent operable for protecting alcohols or diols may be included in the reaction mixture. In some embodiments of making graphene oxides, such a protecting agent would be, for example, phosphoric acid. Phosphoric acid may be included in the reaction mixture to prepare graphene oxides as provided in Examples 5-6, and Example 8. As demonstrated in those examples, regardless of the mechanism or manner of protection, excessive basal plane oxidation in the graphene sheets is precluded, while the overall level of oxidation is increased relative to that of other methods for forming graphene oxide from bulk graphite. See, e.g., Table 3 and compare Examples 6 and 8 with Example 7.
In various embodiments, the at least one oxidant may be an oxidant such as, for example, permanganate, ferrate, osmate, ruthenate, chlorate, chlorite, nitrate, osmium tetroxide, ruthenium tetroxide, lead dioxide, and combinations thereof. For any of the referenced oxidants that are cations or anions, any counteranion suitable for forming a salt of the oxidant cation or anion may be used in practicing the methods of the present disclosure. However, one of ordinary skill in the art will recognize that certain salts may be more advantageous than others in such properties as, for example, their solubility and stability. In some embodiments, the at least one oxidant is potassium permanganate. In general, the at least one oxidant of the present disclosure is an oxidant that mediates a cis-oxidation of double bonds.
In some embodiments, making the graphene oxides may further include at least one acid solvent. The at least one acid solvent may be, for example, oleum (fuming sulfuric acid), sulfuric acid, chlorosulfonic acid, fluorosulfonic acid, trifluoromethanesulfonic acid, and combinations thereof. In some embodiments, the at least one acid solvent may be sulfuric acid. In some embodiments, the at least one acid solvent is sulfuric acid and the at least one oxidant is potassium permanganate. In various embodiments, oleum may have a free sulfur trioxide concentration ranging from about 0.1% to about 20%. In various embodiments, sulfuric acid may have a concentration greater than about 90% (v/v). Although Examples 5-8 employ potassium permanganate as the at least one oxidant and sulfuric acid as the at least one acid solvent, one of ordinary skill in the art will recognize that many different combinations of oxidants, acid solvents and protecting agents may be used to achieve a similar result in preparing graphene oxides in preparing graphene oxide while operating within the spirit and scope of the present disclosure.
In various embodiments, without meaning to be bound by any particular theory or mechanism, the at least one protecting agent of the present method is operable for protecting vicinal diols. In some embodiments, the at least one protecting agent is operable for protecting vicinal diols in the presence of at least one acid solvent.
In some embodiments and without intending to be bound by any theory or mechanism, the at least one protecting agent is a non-oxidizing acid. In some embodiments, the at least one protecting agent is an anhydride or mixed anhydride that is convertible to a non-oxidizing acid operable for serving as a protecting agent. Such protecting agents are operable for protecting vicinal diols in the presence of a strong acid solvent such as, for example, fuming sulfuric acid, sulfuric acid, chlorosulfonic acid, fluorosulfonic acid and trifluoromethanesulfonic acid. Illustrative protecting agents useful in any of the various embodiments of the present disclosure include, for example, trifluoroacetic acid, phosphoric acid, orthophosphoric acid; metaphosphoric acid; polyphosphoric acid, boric acid, trifluoroacetic anhydride; phosphoric anhydride, orthophosphoric anhydride; metaphosphoric anhydride, polyphosphoric anhydride; boric anhydride; mixed anhydrides of trifluoroacetic acid, phosphoric acid, orthophosphoric acid, metaphosphoric acid, polyphosphoric acid, and boric acid; and combinations thereof. In some embodiments, the at least one protecting agent may be, for example, phosphoric acid, boric acid, trifluoroacetic acid, and combinations thereof. Although Example 5-6 and Example 8 each utilize phosphoric acid as an illustrative protecting agent, similar results have been obtained using trifluoroacetic acid and boric acid as the protecting agent. In some embodiments, a salt of any of the aforesaid protecting agents may be used in the various methods presented herein.
In various embodiments, oxidizing the graphite source takes place at a temperature between about −50° C. and about 200° C. In some embodiments, oxidizing takes place at a temperature between about 0° C. and about 100° C. In some embodiments, oxidizing takes place at a temperature between about 300° C. and about 85° C. In some embodiments, oxidizing takes place at a temperature between about 30° C. and about 50° C. In some embodiments, oxidizing takes place at a temperature between about 25° C. and about 70° C. In some embodiments, oxidizing takes place at a temperature of less than about 50° C. In some embodiments, oxidizing takes place at a temperature of less than about 30° C.
In general, reaction times may vary as a function of the reaction temperature and as a function of the particle size of the starting graphite source. In various embodiments, reaction times may vary from about 1 hour to about 200 hours. In other embodiments, reaction times may vary from about 1 hour to about 24 hours. In other embodiments, reaction times may vary from about 1 hour to about 12 hours. In still other embodiments, reaction times may vary from about 1 hour to about 6 hours.
At the aforesaid temperatures, a high recovery of graphene oxide having a flake dimension approximating that of the starting graphite flakes and only a small amount of mellitic acid and other low molecular weight byproducts are observed. Operation at these temperatures is advantageous to minimize decomposition of the oxidant, particularly when the oxidant is KMnO₄. In strongly acidic media, permanganate slowly decomposes to Mn(IV) species that are incapable of oxidizing graphite to graphene oxide. Hence, the temperature is kept as low as possible to provide for essentially complete conversion of graphite to graphene oxide at an acceptable rate using only a moderate excess of KMnO₄. In the theoretical limit of infinite size graphite crystals, for each gram of graphite being oxidized, 4.40 grams of KMnO₄are required for stoichiometric equivalence. Losses of KMnO₄to decomposition, formation of carboxylic acid groups and other basal plane edge functionality, and hole formation in the basal plane make the addition of oxidant somewhat above the theoretical amount desirable.
In some embodiments of making graphene oxides, about 0.01 to about 10 grams of KMnO₄per gram of graphite (0.002 to about 2.3 equiv. KMnO₄) may be used.
In embodiments having sub-stoichiometric quantities of KMnO₄or any other oxidant, a co-oxidant may also be included to re-oxidize the primary oxidant and make the reaction proceed to completion. Illustrative co-oxidants include, for example, oxygen and N-methylmorpholine N-oxide (NMO). In some embodiments of the present disclosure, about 6 grams of KMnO₄per gram of graphite (1.4 equiv. KMnO₄) may be us to obtain graphene oxide having properties that are different than previously known forms of graphene oxide.
In other various embodiments, methods of making graphene oxides include providing a graphite source, providing a solution containing at least one acid solvent, at least one oxidant and at least one protecting agent, mixing the graphite source with the solution, and oxidizing the graphite source with the at least one oxidant in the presence of the at least one protecting agent to form graphene oxide. The at least one protecting agent is operable for protecting vicinal diols.
In still other various embodiments, methods of making graphene oxide include providing a graphite source, providing a solution containing at least one acid solvent, potassium permanganate and at least one protecting agent, mixing the graphite source with the solution, and oxidizing the graphite source with the potassium permanganate in the presence of the at least one protecting agent to form graphene oxide. The at least one acid solvent may be, for example, oleum, sulfuric acid, fluorosulfonic acid, trifluoromethanesulfonic acid, and combinations thereof. The at least one protecting agent may be, for example, trifluoroacetic acid; phosphoric acid; or orthophosphoric acid, metaphosphoric acid; polyphosphoric acid; boric acid; trifluoroaceitc anhydride; phosphoric anhydride; orthophosphoric anhydride; metaphosphoric anhydride; polyphosphoric anhydride; boric anhydride; mixed anhydrides of trifluoroacetic acid, phosphoric acid, orthophosphoric acid, metaphosphoric acid, polyphosphoric acid, and boric acid; and combinations thereof.
In some embodiments of making graphene oxides, methods of the present disclosure further include isolating the graphene oxide. Isolating the graphene oxide may take place by, for example, centrifugation or filtration. In some embodiments, a poor solvent (e.g. ether) may be added to a solution of graphene oxide to induce precipitation. In some embodiments, the methods further include washing the graphene oxide after isolating the graphene oxide. For example, in some embodiments, the graphene oxide may be washed with solvents including hydrochloric acid, water, acetone, or alcohols to remove small molecule byproducts. In other embodiments, the graphene oxide may be washed with solutions of bases. Washing with solutions of bases such as, for example, sodium hydroxide, sodium carbonate or sodium bicarbonate afford the sodium salt of carboxylates or other acidic functional groups (e.g., hydroxyl groups) on the graphene oxide. Similarly, other basic salts of metal cations such as, for example, potassium, cesium, calcium, magnesium and barium can be used.
Some embodiments of making graphene oxides may further include purifying graphene oxide. However, in alternate embodiments, the graphene oxide may be used in an unpurified state. One of ordinary skill in the art will recognize that various applications for the graphene oxide product may require different levels of purity that might necessitate further purification. Illustrative impurities that may remain in unpurified graphene oxide include, for example, residual inorganic salts and low molecular weight organic compounds.

Contacting Graphene Oxides and Environments

Various methods may also be used to contact graphene oxides with various environments that contain materials to be purified. In some embodiments, the contacting occurs by incubating the graphene oxides with the environment (e.g., an atmospheric environment). In some embodiments, the contacting occurs by mixing the graphene oxides with the environment (e.g., an aqueous solution). The mixing may occur by conventional methods, such as agitation, sonication, and the like.
In some embodiments, the contacting of graphene oxides and an environment containing materials to be purified can also occur by flowing the environment through a structure that contains the graphene oxides. In some embodiments, the structure may be a column or a sheet that contains immobilized graphene oxides.
Additional methods of contacting graphene oxides with various environments can also be envisioned. Generally, such contacting results in the sorption of radioactive elements in the environment to the graphene oxides.

Sorption of Materials to Graphene Oxides

The sorption of materials to graphene oxides may also occur by various methods. In some embodiments, the sorption includes an absorption of the materials in an environment to the graphene oxides. In some embodiments, the sorption includes an adsorption of the materials in an environment to the graphene oxides. In some embodiments, the sorption includes an ionic interaction between the materials in the environment and the graphene oxides. In some embodiments, the sorption includes an adsorption interaction between the materials in an environment and the graphene oxides. In some embodiments, the sorption includes a physisorption interaction between the materials in an environment and the graphene oxides. In some embodiments, the sorption includes a chemisorption interaction between the materials in an environment and the graphene oxides. In some embodiments, the sorption includes a covalent bonding interaction between the materials in an environment and the graphene oxides. In some embodiments, the sorption includes a non-covalent bonding interaction between the materials in an environment and the graphene oxides. In some embodiments, the sorption includes a hydrogen bonding interaction between the materials in an environment and the graphene oxides. In some embodiments, the sorption includes a van der Waals interaction between the materials in an environment and the graphene oxides. The aforementioned interactions are non-limiting and herein referred to as sorption.
The sorption of materials to graphene oxides may also have various effects. For instance, in some embodiments, the sorption leads to the formation of graphene oxide-cation colloids. In some embodiments, the sorption may lead to coagulation or precipitation.
Separation of Graphene Oxides from Environments
In some embodiments, the methods of the present invention also include a step of separating the graphene oxides from an environment containing various materials. The separation step generally occurs after the sorption of the materials to the graphene oxides. Various methods may be used for such separation steps. In some embodiments, the separation step may occur by centrifugation, ultra-centrifugation, filtration, ultra-filtration, precipitation, electrophoresis, sedimentation, reverse osmosis, treatment with acids, treatment with bases, treatment with chelating agents such as EDTA, and combinations of such methods.
In some embodiments, the separation step involves ultra-filtration. In more specific embodiments, the ultra-filtration step may lead to the removal of formed graphene oxide-cation colloids from an environment. In additional embodiments, the graphene oxide-cation colloids may also be removed by ultra-filtration, high speed centrifugation, sedimentation, reverse osmosis, or other suitable separation procedures.
In some embodiments, the separation step involves precipitation. In some embodiments, the precipitation can be initiated by adding one or more polymers to an environment. For instance, in some embodiments, cationic polyelectrolytes, such as poly(ethylene imine), may be added to a solution. In some embodiments, the solution may contain graphene oxide-cation coagulants that precipitate upon the addition of polymers. The precipitated coagulants may then be removed by filtration, centrifugation or other methods.
The separation step may have various effects. In some embodiments, the separation step may lead to a reduction of the radioactive elements in the environment by at least about 70%.
In some embodiments, the separated graphene oxides may then be processed further in order to dissociate the sorbed materials (such as radioactive elements). For instance, in some embodiments, the materials may be dissociated from the graphene oxides by changing the pH or temperature of a solution. The dissociated graphene oxides may then be reused.

Advantages and Applications

The methods of the present invention provide numerous applications. For instance, the methods of the present invention can be used as filters or sorbents for removal of radioactive elements and halogenated compounds from various sources. The methods of the present invention can also be used for nuclear waste treatment, and remediation of contaminated groundwater. For instance, the methods of the present invention can be used as components of ultra-filtration and reverse osmosis technologies in waste water treatment. The methods of the present invention may also be used to mitigate environmental radionuclide contamination. The methods of the present invention may also be used to separate various human-made radionuclides from aqueous solutions of various compositions. In some embodiments, graphene oxides may also be used as components of reactive barriers at contaminated sites.
In some embodiments, the methods of the present invention can be used for the sorption and separation of actinides from nuclear fission products. A classic method for this separation involves using the PUREX process. However, the PUREX process involves multiple extraction and preferential solubility steps and valence adjustments. Thus, by using graphene oxides in accordance with the methods of the present invention, one could mitigate the need for subsequent valence adjustments, extraction into organic phase, valence return, back extraction to the aqueous phase, and sometimes oxalate conversion to oxides.
To the best of Applicants' understanding, graphene oxides were not used previously for the sorption or separation of radioactive elements or halogenated compounds. Furthermore, effective separation of actinides from aqueous solutions and nuclear wastes containing strong complexing agents (such as in nuclear fission products) were not previously reported.
In addition, the use of graphene oxides to purify radioactive elements and halogenated compounds from various environments provides various advantages. To begin with, graphene oxides are comprised of two-dimensional materials that consist of single atomic planes. Therefore, graphene oxides provide a high surface area and a low specific mass, especially when compared to other potential sorbents. This in turn provides graphene oxides with optimal sorption kinetics for radioactive elements.
In addition, graphene oxides are hydrophilic materials with oxygen-containing functionalities that form stable complexes with many radioactive elements, including actinides and lanthanide cations. These attributes can help lead to rapid macroscopic aggregation and precipitation of the formed complexes from various solutions, including water. This process can be further facilitated by the use of additional agents, such as surfactants and polymers.
Moreover, graphene oxides can be readily produced in mass quantities. Furthermore, the sorbed radioactive elements and halogenated compounds that are appended to graphene oxides can be liberated from the graphene oxides upon the lowering of the pH in the solution. This in turn provides for reversible leaching and sorption of radioactive elements.

ADDITIONAL EMBODIMENTS

Reference will now be made to more specific embodiments of the present disclosure and experimental results that provide support for such embodiments. However, Applicants note that the disclosure below is for exemplary purposes only and is not intended to limit the scope of the claimed invention in any way.
The Examples below pertain to the use of graphene oxides for effective radionuclide removal. In particular, we show the efficacy of graphene oxide for rapid removal of some of the most toxic and radioactively long-lived human-made radionuclides from contaminated water, even in acidic solutions (pH<2). The interaction of graphene oxides with cations, including Am(III), Th(IV), Pu(IV), Np(V), U(VI) and typical fission products Sr(II) and Tc(VII) were studied, along with their sorption kinetics. Cation/graphene oxide coagulation occurs with the formation of nanoparticle aggregates on the graphene oxide surface, facilitating their removal. Graphene oxide is far more effective in removal of transuranium elements from simulated nuclear waste solutions than other routinely used sorbents such as bentonite clays, iron oxide and activated carbon. These results point toward a simple methodology to mollify the severity of nuclear waste contamination that has been spawned by humankind, thereby leading to effective measures for environmental remediation.
Treatment of aqueous waste effluents and contaminated groundwater containing human-made radionuclides, among which the transuranic elements are the most toxic, is an essential task in the clean-up of nuclear legacy sites. The recent accident that included radionuclide release to the environment at the Fukushima Daiichi nuclear power plant in Japan and the contamination of the water used for cooling its reactor cores, underscores the need for effective treatment methods of radionuclide-contaminated water. Such technologies should be inexpensive, swift, effective and environmentally friendly. Graphene oxide has been known for more than a century, but has attracted attention in the last decade due to its conversion to graphene.
The colloidal properties of graphene oxide make it a promising material in rheology and colloidal chemistry. The amphiphilic graphene oxide produces stable suspensions when dispersed in liquids and shows excellent sorption capacities. Previously, it was shown that graphene oxide enables effective removal of Cu²⁺, arsenate, and organic solvents. As a result of oxygen functionalization, the graphene oxide surface contains epoxy, hydroxyl and carboxyl groups (see, e.g., Table 3) that are responsible for interaction with cations and anions. In this work, the application of graphene oxide for the effective removal of a variety of radionuclides from aqueous solutions is described. Kinetics of sorption, pH sorption edges and sorption capacity were studied in the batch sorption mode to illustrate the performance of graphene oxide in sequestering radionuclides from solution. Examples 1-4 described below employed the graphene oxide of Example 5. Preparation of the graphene oxide of Example 5, and also several other examples of graphene oxide and chemically converted graphene oxide follow thereafter.

Example 1. Kinetics of Radionuclide Removal

The kinetics of radionuclide removal by graphene oxide are presented in FIG. 1A, indicating that near steady state conditions were achieved within 5 minutes even at very low graphene oxide concentrations (<0.1 g/L by carbon). Without being bound by theory, it is envisioned that the fast sorption kinetics are likely due to graphene oxide's highly accessible surface area and lack of internal surfaces that usually contribute to the slow kinetics of diffusion in cation-sorbent interaction. This fast kinetics are of importance for practical applications of graphene oxide for removal of cationic impurities, including Th(IV), U(VI), Pu(IV) and Am(III).

Example 2. Radionuclide Removal as a Function of pH

FIGS. 1B and 1C show pH sorption edges for Sr(II), Tc(VII), Np(V), Th(IV), U(VI), Pu(IV) and Am(III). All of the radionuclides demonstrate typical S-shaped pH-edges for cations, except for Tc, which exists as the pertechnetate anion, TcO₄ ⁻. This explains its sorption at low pH when the graphene oxide surface is protonated and positively charged. For Lewis “hard” cations such as the actinides Th(IV), Pu(IV) and Am(III), the sorption is high, even from acidic solutions with pH<2. For these cations, the sorption from neutral pH solutions was nearly quantitative, a result that is indicative of the prospects of its application in remediation of contaminated natural waters.

Example 3. Graphene Oxide's Radionuclide Sorption Capacities

Graphene oxide demonstrates high sorption capacity towards U(VI), Sr(II) and Am(III) cations, as determined from sorption isotherms shown in FIG. 2. Even with a graphene oxide concentration of only 0.038 g/L, the saturation limit is not reached. The values for sorption capacity presented in FIG. 2 are calculated from experimental data using Langmuir formalism and Freundlich formalism. The experimental results are summarized in Table 1.

TABLE 1

Parameters for sorption of U(VI), Sr(II) and Am(III) on graphene oxide

Langmuir formalism

Freundlich formalism

	Q_max,μmol/g	K_L,L/μmol	R²	K_F,mol^n-1Lⁿ/g	n	R²

U(VI), pH = 3.5	97 ± 19	0.046 ± 0.014	0.98	0.004 ± 0.002	0.33 ± 0.04	0.95
U(VI), pH = 5	116 ± 5	0.035 ± 0.064	0.97	0.078 ± 0.026	0.68 ± 0.03	0.99
Sr(II), pH = 6.5	272 ± 35	0.026 ± 0.007	0.95	0.034 ± 0.017	0.55 ± 0.05	0.96
Am(III), pH = 3.5	2 ± 1	7.034 ± 3.125	0.97	0.05 ± 0.03	0.67 ± 0.05	0.99

The above-mentioned results were obtained, despite the fact that the graphene oxide surface was far from saturation. While it is difficult to directly compare the sorption performance of different sorbents towards radionuclides since it is dependent on the precise experimental conditions, the sorption capacity of graphene oxide is much higher than that of activated carbon, bentonite clay and Fe(III) oxide, but close to the value determined for oxidized carbon nanotubes (CNTs).
However, sorption rates for oxidized CNTs are much slower than those of graphene oxide since much of the CNT surfaces are internal or inaccessible due to bundling, and the CNTs have been investigated for only a limited number of radionuclides and never with a large host of competing counterions (referred to as complexing agents herein). Moreover, CNT synthesis and subsequent oxidation is far more expensive than synthesis of graphene oxide, the latter coming from the one-pot treatment of graphite, thereby rendering graphene oxide more suitable for large-scale clean-up operations.

Example 4. Removal of Radionuclides from Simulated Nuclear Waste

The removal of radionuclides from waste solutions was tested using simulated liquid nuclear wastes that contains U and Pu salts together with Na, Ca and various complexing substances such as carbonate, sulfate, acetate, and citrate that could potentially complicate sorption of radionuclides. See Table 2 for the complete list. Among the commonly used scavengers for cationic radionuclides (such as bentonite clays, granulated activated carbon and Fe(III) oxide), graphene oxide demonstrates the highest sorption ability towards actinides that form strong complexes in solutions with sulfate, citrate, carbonate and acetate. The comparison of the sorption of different sorbents towards U(VI) and Pu(IV) are presented in FIGS. 3A-3B. Remarkably, even for Pu(IV) that forms strong complexes with such complexing agents in solution, the sorption onto graphene oxide was as high as 80%.

TABLE 2

Composition of Simulated Nuclear Waste at pH 7.5

	Concentration, M

	Na⁺	1.500
	Ca²⁺	0.005
	NO³⁻	0.806
	CH₃COO⁻	0.339
	C₂O₄ ²⁻	0.159
	SO₄ ²⁻	0.014
	Cl⁻	0.010
	CO₃ ²⁻	0.005

Upon the interaction of simulated nuclear waste solution with graphene oxide, coagulation occurred that resulted in visual changes of the suspension (FIG. 3C). A scanning transmission electron microscope (STEM) image of the graphene oxide coagulate with cations and a corresponding energy-dispersive X-ray (EDX) spectrum are presented in FIG. 3C. This is in agreement with the earlier observations that addition of such cations results in coagulation of graphene oxide. The most important observation for application of graphene oxide for radionuclides removal is that, despite the presence of high concentrations of cations that could compete with Pu for sorption sites, the sorption of Pu remains high. Thus, it is envisioned that the coagulation of graphene oxide suspension resulting from cations and radionuclides would enable the effective removal of radionuclides by filtration, reverse osmosis or sedimentation.
The interaction of Pu(VI) with graphene oxide results in its stabilization as Pu(IV) and formation of nanoparticulate PuO_2+x.nH₂O on the graphene oxide (FIG. 4A). Such stabilization could be explained by much higher sorption affinity of Pu(IV) towards the surfaces compared with Pu(V). The high resolution transition electron micrograph (HRTEM) image and EDX show that Pu is concentrated in aggregates of crystalline nanoparticles with an average size of 2 nm (FIG. 4B). The crystal structure of nanoparticles corresponds to cubic Fm3m lattice with d-spacing typical for PuO₂as studied by FFT. The reduction to Pu(IV) is also supported by solvent extraction after Pu-leaching from graphene oxide at pH 0.7. Upon acid leaching (see Example 5), only ˜10% of Pu(IV) was desorbed from graphene oxide after 15 minutes, indicating that Pu(IV) nanoparticles are kinetically stable. This is in concert with the earlier published data that Pu(IV) is minimally leached from sorbents and more kinetically stable than Pu(V) or Pu(VI).
The experimental data verifies the ability of graphene oxide to effectively sorb most toxic radionuclides from various solutions. Graphene oxide is found to be much more effective compared with bentonite days, activated carbon and Fe(III) oxide in actinide removal from liquid nuclear wastes. Graphene oxide containing radionuclide could be easily coagulated and precipitated. The simplicity of industrial scale-up of graphene oxide, its high sorption capacity, and it ability to coagulate with cations makes it a promising new material for responsible radionuclide containment and removal.

Experimental Protocols

Example 5: Synthesis of Graphene Oxide Used in Examples 1-4 Above

Graphene oxide Example 5 was prepared using the improved Hummer's method. Marcano et al., ACS Nano 2010. 4:4806-4814. Large-flake graphite (10.00 g, Sigma-Aldrich, CAS 7782-42-5, LOT 332461-2.5 KG, Batch #13802EH) was suspended in a 9:1 mixture of sulfuric and phosphoric acids (400 mL). Next, potassium permanganate (50.00 g, 0.3159 mol) was added in small portions over a period of 24 hours. After 5 days, the suspension was quenched with ice (1 kg) and the residual permanganate was reduced with H₂O₂(30% aqueous, ˜3 mL) until the suspension became yellow. The product was isolated by centrifugation at 319 g for 90 minutes (Sorvall T1, ThermoFisher Scientific) and subsequently washed with 10% HCl and water. The yellow-brown water suspension (190.0 g) was isolated, corresponding to 10 g of dry product. Gravimetric analysis and XPS shows 81% mass fraction of carbon in the dry product.

Example 6: Synthesis of Graphene Oxide in the Presence of a Protecting Agent

A 9:1 mixture of conc. H₂SO₄:H₃PO₄(360:40 mL) was added to a mixture of graphite flakes (3.0 g, 1 wt. equiv) and KMNO₄(18.0 g, 6 wt. equiv), producing a slight exotherm to 35-40° C. The reaction was then heated to 50° C. and stirred for 12 h. The reaction was cooled to RT and poured on to ice (˜400 mL) along with 30% H₂O₂(3 mL). For work up, the mixture was sifted through a metal U.S. Standard testing sieve (W. S. Tyler, 300 μm) and then filtered through polyester fiber (Carpenter Co.). The filtrate was centrifuged (4000 rpm for 4 h), and the supernatant was decanted away. The remaining solid material was then washed in succession with 200 mL of water, 200 mL of 30% HCl, and twice with 200 mL of ethanol. For each wash the mixture was sifted through the U.S. Standard testing sieve and then filtered through polyester fiber. In each case, the filtrate was centrifuged (4000 rpm for 4 h), and the supernatant was decanted away. The material remaining after the multiple wash process was coagulated with 200 mL of ether, and the resulting suspension was filtered over a PTFE membrane with a 0.45 μm pore size. The solid obtained on the filter was vacuum dried overnight at room temperature. The yield was 5.8 g of a solid having a color similar to that of peanut butter.
The yield of hydrophobic, under-oxidized graphite oxide removed during the first passage through the U.S. Standard testing sieve was 0.7 g. Visual observation of the hydrophobic, under-oxidized graphite oxide showed the amount of recovered solid was significantly less than that obtained by Hummers' Method (Example 7 below) or a modification of Hummers' Method (Example 8, further below).

Example 7. Synthesis of Graphene Oxide Via Hummers' Method

Concentrated H₂SO₄(69 mL) was added to a mixture of graphite flakes (3.0 g, 1 wt. equiv) and NaNO₃(1.5 g, 0.5 wt equiv), and the mixture was cooled to 0° C. KMNO₄(9.0 g, 3 wt. equiv) was added slowly in portions to keep the reaction temperature below 20° C. The reaction was warmed to 35° C. and stirred for 30 min., at which time water (138 mL) was added slowly, producing a large exotherm to 98° C. External heating was introduced to maintain the reaction temperature at 98° C. for 15 min, and the reaction was cooled using a water bath for 10 min. Additional water (420 mL) and 30% H₂O₂(3 mL) were then added, producing another exotherm. After air cooling, the mixture was purified as described for Example 6. The yield was 1.2 g of a black solid. The yield of hydrophobic, under-oxidized graphite oxide removed during the first passage through the U.S. Standard testing sieve was 6.7 g.

Example 8: Synthesis of Graphene Oxide Via a Modification of Hummers' Method

Graphene oxide was also synthesized by a modification of Hummers' Method by including additional KMNO₄in the reaction mixture. Concentrated H₂SO₄(69 mL) was added to a mixture of graphite flakes (3.0 g, 1 wt. equiv) and NaNO₃(1.5 g, 0.5 wt. equiv), and the mixture was cooled using an ice bath to 0° C. KMnO₄(9.0 g, 3 wt. equiv) was added slowly in portions to keep the reaction temperature below 20° C. The reaction was warmed to 35° C. and stirred for 7 h. Additional KMnO₄(9.0 g, 3 wt. equiv) was added in one portion, and the reaction was stirred for 12 h at 35° C. The reaction mixture was cooled to room temperature and poured on to ice (˜400 mL) along with 30% H₂O₂(3 mL). The mixture was then purified as described for Example 6. The yield was 4.2 g of a black solid. The yield of hydrophobic, under-oxidized graphite oxide removed during the first passage through the U.S. Standard testing sieve was 3.9 g.

Solid State ¹³C NMR Analysis of Graphene Oxide Examples 6-8

Solid state ¹³C NMR spectra for the graphene oxides of Examples 6-8 were obtained at 50.3 MHz, with 12 kHz magic angle spinning, a 90° ¹³C pulse, 41 ms FID and 20 second relaxation delay. In the ¹³C NMR spectra, signals near 190 ppm were assigned to carboxylates, signals near 164 ppm were collectively assigned to ketone, ester and lactol carbonyl groups, signals near 131 ppm were assigned to graphitic sp²carbons and signals near 101 ppm were assigned to sp³carbons of lactols. The signals near 70 ppm were assigned to alcohols, and the upfield shoulder of this peak was assigned to epoxides. Integral ratios are summarized in Table 3 below. Table 3 also contains calculated integral ratios for alcohol/epoxide:sp²graphitic carbon and total oxygen containing functionality: sp²graphitic carbon as a measure of the degree of oxidation.

TABLE 3

Summary of Integral Ratios in Solid State 13C NMR Analysis of Examples 6-8

Graphene
Oxide	sp³					Alcohol/	Total Oxygen
Example	Graphitic	Alcohol/			Lactol	Epoxide:Graphitic	Functionality:Graphitic
No.	Carbon	Epoxide	Carboxylate	Carbonyl	sp³	sp2 Ratio	sp²Ratio

6	20	67	4	4	5	3.4:1	4.0:1
7	32	59	3	2	4	1.8:1	2.1:1
8	28	63	3	2	4	2.3:1	2.6:1

Solid state ¹³C NMR indicated that the graphene oxide of Example 6 was more completely oxidized than that of either Example 7 or Example 8. The simplest measure of the degree of oxidation is the ratio of the alcohol/epoxide peak integration to that of the graphitic sp²carbons. A pristine graphene plane having no edge functionalization would have a ratio of zero, since all carbons would be of the sp²type. Upon oxidation to form graphene oxide, the number of sp²carbons in the graphene plane decreases and oxygen-containing functionalities correspondingly increase to produce a non-zero ratio. Higher ratios are therefore indicative of a greater degree of oxidation. As shown in Table 3, the graphene oxide of Example 6 was more oxidized than that of either Example 7 or Example 8, as evidenced by its greater alcohol/epoxide:graphitic sp²carbon ratio and total oxygen functionality:graphitic sp²carbon ratio.

Example 9. Reduction of Graphene Oxide

In some cases, hydrazine hydrate reduction was followed by annealing at 300° C. in H₂. In general, hydrazine hydrate reduction was conducted by dispersing 100 mg of the graphene oxide material in 100 ml of deionized water and stirring for 30 minutes. Thereafter, 1.00 ml of hydrazine hydrate was added. The mixture was then heated for 45 minutes at 95° C. using a water bath. A black solid precipitated from the reaction mixture. The product was isolated by filtration on a 20 μm PTFE filter and as washed thereafter three times each with deionized water and methanol.

Example 10. Sorption Experiments Using the Graphene Oxides of Example 5

Sorption experiments using the graphene oxides of Example 5 were carried out in plastic vials for which sorption onto the vial walls was negligible under the experimental conditions. In the sorption experiments, radionuclides nitrates were added to graphene oxide suspension. Next, the pH was measured by a glass combined pH electrode (In Lab Expert Pro, Mettler Toledo) and adjusted by addition of small amounts of dilute HClO₄or NaOH. After equilibration, the graphene oxide suspension was centrifuged at 40000 g for 20 minutes (Allegra 64R, Beckman Coulter) to separate radionuclides sorbed onto the graphene oxide. The sorption was calculated from the difference between the initial activity of the radionuclides and that measured after equilibration. The initial total concentration of radionuclides in the kinetic experiments and pH-dependence tests was 2.15×10⁻⁷M for ²³³U(VI), 1.17×10⁻⁸M for ²³⁹Pu(IV), 5.89×10⁻¹⁴M for ²³⁴Th(IV), 3.94×10⁻¹⁰M for ²⁴¹Am(III), 3.94×10⁻¹⁰M for ²³⁹Np(V), 3.94×10⁻¹⁰M for ⁹⁵Tc(VII) and 1.24×10⁻⁷M for ⁹⁰Sr(II). The concentration of the graphene oxide suspension was 0.077 g/L in 0.01 M NaClO₄. In all cases, the total concentration of cations was much less than the solubility limit, and the graphene oxide/radionuclide ratio corresponded to a very high under-saturation of graphene oxide sorption sites.
To measure the sorption capacity of graphene oxide towards different radionuclides, the sorption isotherms were obtained using 0.038 g/L graphene oxide suspension in 0.01 M NaClO₄. The concentration of the cations was varied at constant pH values.
To demonstrate the performance of graphene oxide compared with other routinely used sorbents for radionuclide removal, experiments were conducted with simulated nuclear wastes containing high concentrations of complexing agents. The concentrations of actinides was equal to 8×10⁻⁸M for ²³³U(VI) and 3×10⁻⁹M for ²³⁹Pu(IV).
For HRTEM examination, the graphene oxide containing Pu samples were prepared such that the Pu(VI) at a total concentration of 1.14×10⁻⁵M was added to the graphene oxide suspensions having a concentration of 0.28 g/L at pH 4.8. After 18 hours, 99% of Pu was sorbed onto the graphene oxide. The precipitated material was deposited onto a carbon-coated TEM grid and analyzed using HRTEM (JEOL-2100F) at an accelerating voltage of 200 kV. EDX analysis was performed with a JED-2300 analyzer. For Pu leaching tests, concentrated HClO₄was added to the suspensions to make them pH 0.7. After 15 minutes, the concentration of Pu in solution was measured.
Without further elaboration, it is believed that one skilled in the art can, using the description herein, utilize the present invention to its fullest extent. The embodiments described herein are to be construed as illustrative and not as constraining the remainder of the disclosure in any way whatsoever. While the preferred embodiments have been shown and described, many variations and modifications thereof can be made by one skilled in the art without departing from the spirit and teachings of the invention. Accordingly, the scope of protection is not limited by the description set out above, but is only limited by the claims, including all equivalents of the subject matter of the claims. The disclosures of all patents, patent applications and publications cited herein are hereby incorporated herein by reference, to the extent that they provide procedural or other details consistent with and supplementary to those set forth herein.

Claims

1. A method of radionuclide sequestration comprising: contacting substantially hydrophilic graphene oxides with a solution comprising a total initial concentration of one or more cationic radionuclides and a total initial concentration of one or more complexing agents, and

reducing the total initial concentration of the one or more cationic radionuclides in the solution by at least forty percent by sorption of the one or more cationic radionuclides to at least a portion of the substantially hydrophilic graphene oxides.

2. The method of claim 1 wherein the total initial concentration of the one or more cationic radionuclides is equal to or less than 2.15×10⁻⁷M.

3. The method of claim 1, wherein the solution comprises an aqueous solution.

4. The method of claim 1, wherein the contacting comprises mixing the substantially hydrophilic graphene oxides with the solution.

5. The method of claim 1, wherein the sorption comprises absorption.

6. The method of claim 1, wherein the substantially hydrophilic graphene oxides are selected from the group consisting of functionalized graphene oxides, chemically converted graphene, pristine graphene oxides, doped graphene oxides, reduced graphene oxides, functionalized graphene oxide nanoribbons, pristine graphene oxide nanoribbons, doped graphene oxide nanoribbons, reduced graphene oxide nanoribbons, stacked graphene oxides, graphite oxides, and combinations thereof.

7. The method of claim 1, wherein the one or more cationic radionuclides is selected from cations of the group consisting of thallium, iridium, fluorine, americium, neptunium, gadolinium, bismuth, uranium, thorium, plutonium, niobium, barium, cadmium, cobalt, europium, manganese, sodium, zinc, technetium, strontium, polonium, cesium, potassium, radium, lead, actinides, lanthanides and combinations thereof.

8. The method of claim 1, further comprising adjusting the pH of the solution such that the reduction of the total initial concentration of the one or more cationic radionuclides occurs in about twenty minutes or less after contacting the substantially hydrophilic graphene oxides with the solution.

9. The method of claim 1, wherein the total initial concentration of the one or more complexing agents in the solution is in the range of 2.4×10⁴times and 2.6×10¹³times the total initial concentration of the one or more cationic radionuclides.

10. The method of claim 1, wherein the one or more complexing agents is selected from the group consisting of Na⁺, Ca²⁺, NO₃ ⁻, CH₃COO⁻, C₂O₄ ²⁻, SO₄ ²⁻, Cl⁻, CO₃ ²⁻, and combinations thereof.

11. The method of claim 1, wherein the substantially hydrophilic graphene oxides have a ratio of total oxygen functionality to graphitic sp²carbon in the range of 2.6:1 and 4.0:1.

12. The method of claim 1, further comprising separating the graphene oxides from the solution.

13. The method of claim 12, wherein the separating comprises at least one of the following: centrifugation, ultra-centrifugation, filtration, ultra-filtration, precipitation, electrophoresis, reverse osmosis, sedimentation, incubation, treatment with acids, treatment with bases, treatment with chelating agents, and combinations thereof.

14. The method of claim 12, wherein the separating comprises precipitating the graphene oxides from the solution by the addition of a polymer to the solution.

15. The method of claim 1, wherein the one or more cationic radionuclides is a cationic actinide and the solution is comprised of nuclear fission products.

16. The method of claim 15, further comprising separating the graphene oxides from the solution after the sorption.

17. A method of radionuclide sequestration comprising: contacting substantially hydrophilic graphene oxides with a solution comprising nuclear fission products comprised of actinides, and reducing the concentration of the actinides in the solution by sorption of at least a portion of the actinides to the substantially hydrophilic graphene oxides.

18. A method of radionuclide sequestration comprising: contacting substantially hydrophilic graphene oxides with a solution comprising one or more cationic radionuclides having a total initial cationic radionuclide concentration of 2.15×10⁻⁷M or less,

the solution is additionally comprised of one or more complexing agents selected from the group consisting of Na⁺, Ca²⁺, NO₃ ⁻, CH₃COO⁻, C₂O₄ ²⁻, Cl⁻, SO₄ ²⁻, Cl⁻, CO₃ ²⁻, and combinations thereof; and

sorbing the one or more cationic radionuclides to at least a portion of the substantially hydrophilic graphene oxides.

19. The method of claim 18, further comprising reducing the total initial concentration of the one more cationic radionuclides by at least forty percent in about twenty minutes or less after contacting the substantially hydrophilic graphene oxides.

20. The method of claim 18, further comprising desorbing one or more of the radionuclides from the graphene oxides that sorbed the cationic radionuclides and separating the desorbed radionuclides from the graphene oxides to produce radionuclide-desorbed graphene oxides.

21. The method of claim 20, further comprising repeating the method of claim 20 at least once wherein the graphene oxides comprise the radionuclide-desorbed graphene oxides.

22. A composition comprising radionuclide-sorbed graphene oxides prepared by the process of contacting substantially hydrophilic graphene oxides with a solution comprising one or more cationic radionuclides and one or more complexing agents,

sorbing at least forty percent of the total initial concentration of the one or more cationic radionuclides to at least a portion of the substantially hydrophilic graphene oxides thereby producing radionuclide-sorbed graphene oxides,

separating the radionuclide-sorbed graphene oxides from the solution.

23. A radionuclide composition prepared by the process of contacting substantially hydrophilic graphene oxides with a solution comprising one or more cationic radionuclides and one or more complexing agents,

separating the radionuclide-sorbed graphene oxides from the solution, and

desorbing one or more of the radionuclides from the radionuclide-sorbed graphene oxides.