US20050217427A1

US20050217427A1 - Method of making and using nanoscale metal

Info

Publication number: US20050217427A1
Application number: US11/139,295
Authority: US
Inventors: Suthan Suthersan; David Vance; Peter Palmer
Original assignee: Arcadis Geraghty and Miller Inc
Current assignee: Arcadis Geraghty and Miller Inc
Priority date: 2000-12-21
Filing date: 2005-05-27
Publication date: 2005-10-06

Abstract

The invention provides methods of producing colloids of nanoscale metallic particles particularly useful in the in situ environmental remediation of chlorinated solvents. The methods include ball milling an elemental metal to form a colloid of nanoscale metallic particles having ideal size and metallurgical properties to enhance the reductive dehalogenation of halogenated hydrocarbons.

Description

CROSS REFERENCE TO RELATED APPLICATIONS

This application is a continuation-in-part of pending U.S. patent application Ser. No. 10/890,066, filed Jul. 12, 2004, which is a continuation of U.S. patent application Ser. No. 10/026,329 filed Dec. 19, 2001, now U.S. Pat. No. 6,777,449, which claims the benefit of U.S. Provisional Patent Application Ser. No. 60/257,917 filed Dec. 21, 2000.

BACKGROUND OF THE INVENTION

Since the mid-1990's, there have been a series of dramatic developments for the in-situ treatment of chlorinated solvents. The approach of the present invention is based on the sequential reduction of chlorinated hydrocarbons to innocuous end products such as methane, ethane or ethene. The process has been recognized in scientific circles but it is now being investigated for environmental applications. The process exploits the use of zero valence state elemental metals to reductively dehalogenate halogenated hydrocarbons. In addition, elemental metals may be used to reduce soluble metals such as chromate to insoluble species (Cr (III)) or stabilize metalloids such as arsenic or selenium.
The most common metal being utilized for this purpose is iron. But other metals including tin, zinc, and palladium have also been shown to be effective. The process may be best described as anaerobic corrosion of the metal by the chlorinated hydrocarbon. In this process, the hydrocarbon is adsorbed directly to the metal surface where the dehalogenation reactions occur. Increasing surface area (by reducing the size of iron particles) increases the effectiveness of the process.
Recent research on iron systems indicates at least three mechanisms at work in the reductive process. First, metallic iron may act as a reductant by supplying electrons directly from the metal surface to the adsorbed halogenated compound. Additionally, metallic iron may act as a catalyst for the reaction of hydrogen with the halogenated hydrocarbon. The hydrogen is produced on the surface of the iron metal as the result of corrosion with water. Also, ferrous iron solubilized from the iron metal in the above reactions may act as a reductant for the dehalogenation of halogenated hydrocarbons.
The rate of the reaction of the metallic iron with halogenated hydrocarbons has been demonstrated to be partially dependent upon the surface area of the metallic iron. As the size of the metallic iron is reduced, surface area goes up as well as chemical reactivity. Initial applications of this technology used iron filings. More recent applications have used iron colloids in the micron size range. The applications of the metallic iron reduction of the present invention incorporate nanoscale colloids. These are colloids that range in size from 1 to 999 nanometers.
A key limitation on the development of the technology is the lack of availability of nanoscale metallic colloids. Research, driven primarily by materials science needs (hi-tech electronic chips or component industry products) has contributed to general technologies designed to produce nanoscale colloids. But the research has generally been in the area of colloids that are composed of ceramic or other non-metallic inorganic materials and not metal colloids. Thus, the production of metallic nanoscale colloids has been pursued through the adaptation of methods of making the non-metallic nanoscale colloids.
The method for the production of nanoscale colloids in the nanoscale range may be divided into two primary approaches. The first of these approaches can be referred to as “Bottom Up” production in which colloids of the appropriate size are produced by assembling individual atoms. Within the “Bottom Up” approach there are a number of potentially applicable methods, including chemical precipitation reactions in aqueous or hydrocarbon solutions that produce metals from soluble salts. An example of these processes is the chemical reduction using sodium borohydride in various soluble metal salts (such as ferrous or ferric chloride for iron) in aqueous suspensions or in various hydrocarbon solvents. These processes may or may not be enhanced with sonofication during the reaction. Each mole of nanoscale iron produced by this process requires at least three moles of reductant to produce, and the best results are obtained with concentrations of reductant in excess of the molar requirements. The reducing reagents are expensive resulting in production costs that are irreducible at a level greater than $100 a pound. Thus, this method is not practical for large scale application in groundwater remediation and for full-scale production in ton quantities.
The “Bottom Up” approach may also include various methods of metal volatilization and subsequent deposition, typically under a vacuum. These methods include Gas Evaporation, Active Hydrogen-Molten Metal Reactions, Sputtering, Vacuum Evaporation to Running Oil Surface and Evaporation Using Direct Electrical Current Heating. These methods cannot be scaled up to produce the necessary amount of elemental nanoscale metal particles for use in environmental remediation procedures in a reasonable time or at a cost-effective price.
The “Top Down” approach includes methods in which the attrition of larger particles of the metal is used to produce colloids of the appropriate size. The “Top Down” approach includes two primary variations of mechanical comminution. The primary variation on mechanical comminution is the mechanical agitation of the desired colloidal metal. Examples of this include ball milling using various types of machinery and rod milling. While by 1999 work in this field was being performed on ceramics or other non-metallic inorganic materials, there was no existing capacity for the production of iron colloids using mechanical attrition. The second and less common of these methods is the mechanical agitation provided by high-speed gas jets.
Thus, there is a need for a method of producing nanoscale metals, particularly nanoscale iron colloids for in situ remediation techniques. The method would require efficient and inexpensive mass production of metal colloids of the proper size. Preferably the production capacity would be at least in the 100 to 1000 kilogram range via continuous or batch operations in time frames of 24 to 48 hours.

SUMMARY OF THE INVENTION

The present invention provides methods of producing nanoscale metal colloids by mechanical attrition. Using these methods, nanoscale colloids may be produced in amounts up to 10 kilograms per 24 to 48 hour production run, with readily available and cost effective scale-up production volumes of one ton or more. These processes use feed materials consisting of iron particles having a mesh size of less than about 325 mesh. Organic solvents are used as suspension fluids. The organic solvent must have a high flash point to prevent explosions and must be non-reactive to the surface of the iron colloid. Typical organic solvents suitable for use in the present invention include dodecane, butyl acetate, and polypropylene glycol ethyl ether acetate. One or more dispersants are also used as surface acting agents to prevent the agglomeration of the colloids during the milling processes. Examples of suitable surface acting agents include SOLSPERSE® 20,000, SOLSPERSE® 24,000, SOLSERSE® 32,600, SOLSEPERSE® 32,500, DISPERBYK® 108, DISPERBYK® 164, and DISPERBYK® 167. These dispersion agents are all polymer chains that have different terminations at each end. One end will have properties that allow the polymer to attach to the surface of the metal colloid, with the other end of the polymer chain attracted to the suspending liquid while extending into the ball mill suspending media. The polymer needs to be long enough to provide for steric or electrostatic hindrance for agglomeration due to Van der Waals forces, but not too long to prevent entanglement and polymer collapse. The polymer length ranges from about one to about ten nanometers. These polymers are chains that may be constructed from monomers that include alkyl groups, alkylaryl groups, alpha-olefins, propylene oxides. The dosage range is from 1 to 20 milligrams (mg) per square meter of colloid surface area, with the typical ideal dose around 2.5 mg per square meter of colloid surface area.
The materials are placed into a high energy ball milling system that is capable of using grinding media as small as 0.2 millimeters (mm). The rate of agitation, time of milling, and temperature of milling parameters are used to control the generation of a nanoscale iron colloid having the desired physical and chemical properties. The types of ball mills that can be employed for this work include attritor mills, planetary mills, conventional rotary ball mills, and vibratory mills. Lower energy milling is used initially to insure proper mixing of the solvent, dispersion and iron components.
Through the manipulation of the colloid size, structure, and crystal structure using the above process it is possible to design colloids for variations in specific contaminant types, concentrations, groundwater or reactor flow velocities, subsurface permeability, and provide some control over the transport properties of the colloids during injection.
A colloid of the nanoscale size has several advantages in application for in-situ groundwater treatment or for use in above-ground treatment reactors. These advantages include higher surface area resulting in greater reaction kinetics. The increase in kinetics in turn, allows for a lower mass loading of iron in the treatment zone or reactor because the residence time required for complete dehalogenation is decreased. The small size and greater reactivity of the colloid allows for the application of the remediation technology through direct in-situ injection into the subsurface where the smaller size allows for advective colloidal transport. Additionally, the greater reactivity of the nano-sized iron allows for much lower overall iron mass requirements in the remediation processes.
To further enhance the physical and chemical character of the colloid, a metallic catalyst may be added to the metallic iron to create a bi-metallic colloid. The catalyst further increases the rates of reaction which lowers the amount of iron colloid that must be used to create an effective reductive dehalogenation treatment zone in the subsurface or in a surface reactor. Metals that may be used as a catalyst with the iron include palladium, platinum, nickel, zinc, tin and combinations thereof.
The enhancement of colloid reactivity by control of the production process results in the creation of a highly reactive colloid with unique properties which make it ideal for in situ injection. These properties include a size and mass that is large enough to minimize the effect of interfacial forces associated with colloids smaller than 50 to 100 nanometers, yet is small enough to avoid the effect of gravitational settling observed on colloids and particles larger than about 600 nanometers. The number of colloidal particles per unit mass is inversely proportional to the cube of the colloid diameter. A 500-nanometer colloid is one order of magnitude larger (10 times) than a 50-nanometer colloid. A gram of 50-nanometer colloid has 1000 times more colloid particles than a gram of 500-nanometer colloid. Since colloid aggregation rates are proportional to the square of the particle number, the 500-nanometer colloid is a million times less susceptible to unwanted aggregation compared to the 50-nanometer colloid. However, one gram of 500-nanometer colloids that has undergone reactivity enhancement by the modification of the metal crystal lattice, manipulation of non-crystalline atom to atom bonds, the addition of secondary metals to act as catalyst, and compounds that aid in the damping of unwanted side reactions will have the same reactivity as one gram of 50-nanometer colloids that rely purely on high surface area effects. These physical parameters all contribute to the production of a highly reactive colloid that also has high mobility in permeable subsurface materials.

BRIEF DESCRIPTION OF THE DRAWINGS

In addition to the novel features and advantages mentioned above, other objects and advantages of the present invention will be readily apparent from the following descriptions of the drawings and preferred embodiments, wherein:
FIG. 1 illustrates one embodiment of the present invention wherein a remediation method is provided. More specifically, the nanoscale metal 26 of the present invention is injected by a pump 20 or other means through an injector 22 into an open bore hole 12, through a screen 16 and into a plume 24 that is located beneath ground level 18 and generally below the water table 14.
FIG. 2 illustrates another remediation method of the present invention 30 for injecting the nanoscale metal 46 of the present invention by pump 40 through an injector 42 into an open bore hole 32 through a screen 36 into a plume 44 that is located beneath ground level 38 and generally beneath the water table 34. Non-contaminated water 48 beneath the water table is also illustrated.
FIG. 3 shows another embodiment of the present invention 50 for injecting nanoscale metal 68 of the present invention through an injector 62, through a screen 52 into a plume 64 that is located generally beneath the ground level 58 and above or near the water table 54 with the use of an auger 66 for mixing the soil and a dust or vapor collector 72 for removing the dust through a removal pipe 70. Non-contaminated water 56 beneath the water table 54 is also illustrated.
FIG. 4 shows another embodiment of a method of the present invention 80 for injecting the nanoscale metal 96 of the present invention with a pump 90, through an injector 92, into a open bore hole 82, through a screen 86 into soil beneath ground level 88 and generally beneath the water table 84 with the use of hydraulic pressure 98 for fracturing the soil 94. The hydraulic pressure 98 is used to fracture the soil 94 and the nanoscale metal 96 is injected either simultaneously or after the hydraulic pressure 98 has been applied.
FIG. 5 is a flow chart that explains the process of producing the nanoscale metal of the present invention.

DETAILED DESCRIPTION OF THE INVENTION

The mechanical agitation processes for grinding metal particles can be divided into two types; those that rely on squashing the metal particles between rotating plates or elements, wherein one or both plates may be rotating; and those that rely on impact to fracture larger metal particles into smaller particles. Particle feed rates, grinding time, unit capacity and the physical/chemical properties of the metal to be ground must all be considered in deciding which method should be used for grinding and, more importantly, which method can be used to produce nanoscale colloids in commercial quantities at a cost that is viable for the use of the material in high quantities such as tons.
Rotary plate grinders are limited in their instantaneous capacity, the type of metals which can be attrited in them, and the duration of grinding that is required to produce the desired size range. In general the clearances associated with rotary plate grinders do not allow for the large scale production of nanoscale colloids in desirably restricted size ranges. Conversely, impact grinders (such as ball or rod mills) have extended time ranges for the grinding duration and much greater instantaneous capacity that is easily scaled up to the treatment of ton quantities. A 500-gallon ball mill can grind a ton or more of iron in about one or two days. Conversely, the capacity of a six foot diameter single element rotary plate/stationary plate grinder is on the order of 12 pounds in two days—and only for metals that are responsive to size reduction by squashing.
A comparison of the properties of iron and magnesium serves to illustrate the physical properties that define the difference between a metal that is best ground by impact versus a metal that is best ground by squashing. Important physical properties for consideration include the coefficient of sliding friction. The lower coefficient of friction allows magnesium particles to move and grind in a rotary plate grinder whereas the higher coefficient of friction for iron will cause that metal to seize and deform in a rotary plate grinder.
The lower coefficient of linear expansion for iron compared to magnesium allows magnesium to stay more brittle in a rotary plate grinder. The more brittle magnesium is able to fracture under the stress in a rotary plate grinder, which is a shearing stress only. The more ductile iron is more readily fractured by forces dominated by impact in conjunction with shear as found in various forms of ball mills. Shear stress alone is not sufficient to produce a nonsocial iron particle.
The number of fragments produced from a fracturing event is directly proportional to the density of the metal. Denser metals will produce more fragments from a given size particle in an impact type of grinder such as a ball mill thereby making these more dense metals suitable for use in the mass production of nonsocial metals for environmental remediation purposes in a ball mill. Thus, iron, with a specific gravity of 7.87 will produce more fragments from a given size particle in a ball mill than will magnesium with a specific gravity of 1.74.

Additionally, factors of hardness, elasticity, and toughness are important in determining grind-ability and grinding methods. Grind-ability refers to the economics of the grinding process and the ability to produce a product in the desired size range. The multiple physical properties of iron and magnesium show that the greater strength of iron requires the addition of impact to the grinding process, versus the lower strength magnesium which can be sufficiently ground by shear only. A comparison of the measures of hardness and elasticity of iron and magnesium are presented in Table 1.

TABLE 1


Comparison of the Physical and Thermal Properties of Iron and
Magnesium

Property	Iron	Magnesium

Coefficient of Sliding Friction	0.95	0.4
Thermal Conductivity (Btu/hr ft 0 F.)	46.4	91.9
Coefficient of Linear Expansion (micro	6.7	14
inches/inch ° F.)
Young's Modulus (pounds/inch squared × 10⁶)	30	6.25
Modulus of Elasticity (pounds/inch	28.5	6.4
squared × 10⁶)
Modulus of Rigidity (Shear Modulus) (pounds/	11.9	2.5
inch squared × 10⁶)
Bulk Modulus (pounds/inch squared × 10⁶)	24.7	6.5
Tensile Strength (1000 pounds per square inch)	40	15
Yield Strength (1000 pounds per square inch)	29	11
Fatigue Limit (1000 pounds per square inch)	24	*
Brinell Hardness Number	167	45
Specific Gravity	7.87	1.74
Melting Temperature (° F.)	2797	1200

* Magnesium, like other metals such as copper and aluminum, does not have a fatigue limit and may fail at any stress or number of cycles

Due to these physical differences, a magnesium nanoscale colloid can be produced only in quantities of scientific interest (not commercially useful) using a rotary element grinder. This type of grinding machine works by squashing the particles between a fixed plate and a rotating element, but cannot reliably mass produce iron nanoparticles, as the machinery is not made with mechanical tolerances small enough to reliably produce nanoparticles that have the physical properties of metals such as iron.
In the nanoscale size range, quantum size effects begin to become apparent. For example, in a colloid containing particles that have a diameter of 10-nanometers, about 30% of the atoms are in the grain boundaries (which are highly reactive and subject to quantum effects). These features may affect the physical/chemical behavior of the colloid during use. These effects fall into two broad categories that reflect on production by “Bottom Up” or “Top Down” methods.
A colloid produced by chemical precipitation or reduction, or through the various vapor deposition methods may be nano-structured. This means that the colloid may have nanoscale crystal domains with sharp boundaries between crystals. The grain boundaries are typically only one atom thick and there is low dislocation density in the crystal structures. The reactivity of a colloid of this type may be controlled primarily through the selection of an appropriate overall colloid size and resulting surface area. Smaller size leads to greater surface area and reactivity while larger size leads to lower surface area and reactivity.
Alternatively, a colloid produced by mechanical attrition may be nano-crystalline. In this case, the crystal domains in the colloid are, relative to the overall colloid size, small. The individual crystal domains are separated by wide amorphous transition regions that exhibit a very high dislocation density. These transition regions may be as large as the crystal domains, but are still termed grain boundaries. The amorphous transition regions may be highly reactive. The size and intensity of dislocation density of the amorphous boundary regions rather than the absolute size of the colloid may dominate the reactivity of the colloid. A relatively large colloid produced by this method may have the same or greater reactivity than a much smaller colloid produced by “Bottom Up” methods.
Controlling the reactivity of the colloid is important in producing metal nanoparticles useful for in-situ remediation applications. In use, the iron particles undergo anaerobic corrosion when they react directly with halogenated solvents or when they react with water to produce hydrogen. As the reactivity of the colloid increases, the hydrogen production rate increases as well. By controlling the rate of hydrogen production, reactive metal colloids may be produced with reactivity that will generate hydrogen at the rate required for the desired dehalogenation processes. The correct rate should be maintained because at excessively higher rates, the iron colloid will be consumed by water without reacting with the halogenated solvents undergoing treatment in the remediation process.
Important factors in the control of the colloid structure using mechanical attrition include the composition of metal, the type and concentration of suspending solvent, the type and concentration of dispersion agent, the size and shape of the colloidal particles and the concentration of the particles in the suspension.
The composition of the metal affects the colloid structure through alloy effects as well as the elemental composition. The structure of a nanoscale material can be categorized based on the dimensional character of its structure. Referring to the internal topological organization (as opposed to literal reference to the number of dimensions the object occupies in space), zero-dimensional atom clusters or cluster assemblies are the product of chemical precipitation. They are typically smooth and rounded with little to no disruption of the internal crystal structure. One- and two-dimensional layers result from vapor or electro deposition processes. Three-dimensional structures are produced by severe plastic deformation (ball milling). These particles are rough in three dimensions and have an internal crystal structure that is disrupted. Thus, a colloid produced by ball milling is physically much more complex than one produced by reductive chemical precipitation. The grain boundaries in nano-materials produced by ball milling have distinctly different inter-grain transport properties than boundaries in macroscopic crystalline materials.
The types of boundary structures in ball milled metals and the impact on transport properties can be summarized by noting that climbing grain boundary dislocations enhance diffusion rates of reactants (CVOCs and water) and reaction products (iron oxides, chlorides, ethane/ethane gas, or hydrogen) through the surface film and the internal colloid structure to active sites by 4 to 5 orders of magnitude. Split disclinations enhance diffusion rates by 10 to 50%. Amorphous zones enhance diffusion rates by tens of percent. These changes within boundary zones are overprinted on the relationship between grain boundaries and the gross crystal structures. Diffusion coefficients to and from active sites in grain boundary regions are 1,000 to 1,000,000 times higher than diffusion coefficients within the undisturbed crystal lattice.
The mechanical treatment of metals introduces defects such as dislocations, new grain boundaries, vacancies and interstitials, and anti-site defects. In addition, a distribution of the stress field is also introduced near grain boundary regions and interfaces in non-deformed crystalline regions. Hydrogen diffusion through dislocations and grain boundaries is faster than in the lattice and therefore, they are the most likely diffusion paths for hydrogen. In addition, such defects may also act as deep traps for hydrogen giving ball milled material higher hydrogen solubility and storage capacity. It has directly been demonstrated that the fraction of the surface area of an iron colloid that is reactive towards TCE is related to the number of defects and abnormalities present on the surface of the iron.
An important factor with regards to the production of a ball milled colloid with a purposeful high degree of enhanced reactive surface area is that the permanent imprinting of a high deformation condition on a nanoscale metal colloid requires that the proportion of a highly deformed grain network be greater than 70% of the total grain area. If this condition is not met, the imprinted grain structure will not be stable and will recrystallise by annealing. An additional important detail is that the minimum grain size obtainable by ball milling is a balance between the defect/dislocation structure introduced by the plastic deformation of the milling and its recovery by thermal processes. The thermal recovery is related to the melting temperature of the metal, which in the case of iron yields a minimum grain size of 10 nanometers.
The present invention provides methods of producing a nanoscale metallic colloid by suspending an elemental metal in a non-aqueous organic liquid, adding a dispersant to the suspension and ball milling the suspension to form a colloid of nanoscale metallic particles.
The suspending solvent may be dodecane, a vegetable oil or vegetable oils, an acetate, a glycol or a combination of these solvents. Soy oil is the preferred vegetable oil solvent. Acetates may include ethyl ether acetate or butyl acetate with butyl acetate being the preferred acetate solvent. Propylene glycol is the preferred glycol solvent. These solvents are used in their pure state with the possible addition of trace amounts of dispersants at typical dosage rates of about 2.5 mg per square meter of colloid surface area. The effects that the type of suspending fluid has on colloid structure are driven predominantly by the viscosity of the fluid. This in turn effects the impact of the grinding media on the colloid being ground by controlling the rate of mechanical energy transfer between the colloid and the media.
Dispersants are any surface acting agent used to prevent agglomeration of the colloids during the milling process. Preferably, the dispersant is made up of polymers suspended in either aromatic hydrocarbons or methoxy propyl acetate for use in solvent based suspension systems. The polymers may include a hydroxyfunctional carboxylic acid ester with iron-affinic groups to facilitate iron attachment and terminal solvation. Other types of polymeric chains can include polyisobutylene, polyethylene, or polystyrene chains. The dispersing agent used can control the colloid structure by controlling the agglomeration of the colloids. As the ball mill media impact the iron colloids being ground, the rate of energy transfer and the effect of the energy transfer is modified by the presence of agglomerated colloids versus free colloids. Free colloids are subject to more direct energy transfer and deformation. Agglomerated colloids experience less deformation.
Suitable commercially-available dispersants include SOLSPERSE® 20,000, SOLSPERSE® 24,000, SOLSERSE® 32,600, SOLSEPERSE® 32,500, SOLSEPERSE® 38,500, DISPERBYK® 108, DISPERBYK® 164, DISPERBYK® 167 and combinations of these dispersants. Preferably, the dispersant used is the SOLSEPERSE® 38,500 product. The dispersants are used in a concentration range of between about 1 mg per square meter to about 20 mg per square meter of colloid surface area. Preferably, the concentration of the dispersant is between about 1 mg per square meter of colloid surface area and about 4 mg per square meter of colloid surface area. More preferably, the concentration of the dispersant is between about 2.5 mg per square meter of colloid surface area.
The average size of the metal nanoparticles in the colloid ranges from about 1 micron to about 1000 microns in the longest dimension and the shape of the particles can include spheres, cubes, flakes, and rods of uniform or irregular surface. The shape of a colloid may affect its settling velocity in use, with spherical colloids have a higher settling velocity than those with other geometries. The colloid shape also controls the amount of colloid surface and structural defects, that contribute to colloid reactivity, are exposed to the surface. Spherical colloids will have the lowest amount of surface area per unit of colloid mass compared to any other shapes. Shapes such as flakes will have a greater amount of surface area (and subsequent reactivity) per unit mass. The concentration of metal particles in the suspending solvent can be between about 10% and about 80% wt./vol., depending upon the specific type of grinding mill used. Batch and continuous-feed attritor mills have an ideal loading concentration of 70% wt./vol. while planetary or conventional rotary ball mills have an ideal loading of 45% wt./vol. The grinding media employed can range in size from about 0.2 mm to about one-half inch and have a shape which is generally spherical but may also be cylindrical.
The milling process and particularly the production of nanoparticles of the desired size can be influenced and at least partially controlled by controlling the milling temperature. This temperature control is particularly helpful during the latter stages of the milling processes. The milling temperature can range between the solidification point of the suspending fluid to the boiling point of the suspending fluid. This temperature range is generally between about −80° C. and about 100° C. Temperature ranges from about 20° C. to about 100° C. are preferable for the production of ductile deformation. Temperatures in the range of −50° C. to −80° C. are preferable for brittle deformation. Ductile deformation produces reactive structures, brittle deformation produces smaller colloids.
The time of milling can range from about two hours to about 48 hours. Preferably the milling is performed over a period of about 8 hours.
Post-production processes may also impact colloid structure and crystal structure. These post-production processes may include annealing, which is a heat treatment used to change the crystal structure of the metal and promote new grain growth while relieving internal stresses in the metal. In the annealing process, the metal is heated to temperatures between about 200° C. and approximately 90% of the melting temperature of the metal being annealed. The melting temperature is typically between about 1030° C. and about 1380° C. for iron alloys including cast iron and carbon steel. The time used to heat the metal nanoparticles in the annealing process is preferably between about one minute and about one day. The duration of the annealing may be between about two minutes and about five days. The cooling time after the heat treatment is complete may be between five seconds and about one week. Additionally, multiple heating and cooling steps may be employed during the anneal.
The colloid may also be mixed with one or more additional metals to form a bimetallic or multi-metallic colloid. Any metal may be used for the bimetallic colloid but preferred metals are iron, palladium, platinum, nickel, zinc and tin. This post production treatment is typically conducted by contacting the metal nanoparticles with other aqueous or organic solutions, for the surface addition of low concentrations of these secondary metals that act as hydrogenation catalysts. For example, to add palladium to the iron colloids, a solution containing hexachloropaladate, preferably as the potassium salt or in the hydrogen form, is added to the colloidal suspension of iron nanoparticles in a quantity sufficient to cover the colloid. The mixture of the colloid and the hexachloropaladate solution is gently stirred or shaken for a period of time between about two and about thirty minutes. Preferably, the reaction is continued with gentle agitation for about 10 minutes. The nanoparticles are then rinsed with water and ethanol. Preferably, the pH of the rinse water is between about pH 5.0 and pH 5.5.
For larger volumes of iron to be treated, the volume of reagents are increased appropriately. It is generally more efficient and safe to keep the colloids produced by these methods in liquid solutions. These liquids may be the suspending agents used during the production of the colloids or water. Preferably, the colloids are kept in the suspending agent. Most preferably, the liquid suspending agent is glycol. The suspending agent or other liquid is then removed shortly prior to application.
The present invention provides a method of producing nanoscale metallic colloids in which an elemental metal is suspended in a non-aqueous organic liquid. The organic liquid is non-reactive to the surface of the elemental metal and has been previously prepared by the addition of an appropriate dispersant before the addition of the elemental metal. This mixture is then ball milled at a rate and for a time sufficient to produce nanoscale metal particles in suspension. The milling rate is primarily dependent upon the type of mill used. For example, using a ball mill with media larger than about one-half inch, a milling rate of about 10 revolutions per minute (rpm) to about 50 rpm is preferred. Similarly, when using an attritor mill with media in the one-eighths inch to three-eights inch range, a milling rate between about 60 rpm and about 350 rpm is preferred. Using a high speed attritor mill with media in the 0.5 mm to 3.0 mm range, a milling rate between about 320 rpm and about 1,700 rpm is preferred. Using a horizontal mill with media in the 0.25 mm to 2.0 mm range, a milling rate between about 800 rpm and about 3,800 rpm is preferred.
Additionally, the present invention provides a method of environmental remediation in which a nanoscale metallic colloid is injected into soil in the presence of a carbohydrate at a flow rate sufficient to move the colloid through the soil. The nanoscale metal particles may be injected by any known method but are preferably injected under pressure (without the use of surfactants) or injected with hydraulic pressure. Pure hydraulic pressure may be mechanically applied or pneumatic pressure may be used. Preferably, the metal particles are injected in nitrogen under pressure.
A nanoscale metallic colloid containing an elemental metal or a bimetallic suspension may used alone or may be suspended in nitrogen gas and may be mixed with a carbohydrate solution (also called a carrier). Alternatively, the carbohydrate solution may be in atomized form and may be injected into the compressed nitrogen gas. The injection may also be achieved with the use of hydraulic pressure in which a slurry of elemental metal or bimetallic colloid and carbohydrate solution are injected. Optionally, sand may also be added to the slurry to act as a proppant.
Any carbohydrate solution that creates an oxygen-scavenging environment may be mixed with the metal. Examples of suitable sources of carbohydrate may include corn syrup, molasses, refined sugars, cheese whey, starch, or grain flours such as wheat or rice. Preferably, corn syrup is the carbohydrate used in suspension with nanoscale metal and injected by pressure into a contaminated sub-soil zone. The carbohydrate helps to prevent elemental iron from corroding due to reactions with oxygen which would have the effect of lessening the remediation capability and active life of the metal.
Examination of the nanoscale metal colloids produced by the high-energy ball milling procedure of the present invention reveals iron particles with considerable volumes of grain boundaries, triple junctions, and other defects in the nanoscale iron particles. These characteristics enhance in situ reaction rates and diffusion characteristics of the iron and they provide a means of overcoming passivation films that develop under field applications. The passivating oxide film is typically broken down by anions present in groundwater, predominantly chloride. This localized chloride dissolution of the passivating oxide film takes place at weak points including grain boundaries, dislocations, or other defects that lead to the exposure of the underlying metal and further reaction. Thus, nanoscale iron produced by the ball milling techniques of the present invention, result in defect sites that remain expressed on the surface of the grown iron passive film, allowing the coated surface to become incompletely passivated so that the metallic iron continues to react at those sites where there are high numbers of defects.
Additionally, this ball milling manufacturing method used to produce the nanoscale iron colloids impacts the chemical composition and the physical structure of the colloid, which in turn further influences the magnitude and nature of the reactivity and the subsequent longevity of the iron colloid in groundwater, wastewater or mixtures of either in the presence of targeted contaminants such as chlorinated solvents, chromate, arsenic or other oxyanionic metal complex. The grain boundaries in nano-materials produced by ball milling have distinctly different inter-grain transport properties than boundaries in macroscopic crystalline materials.
The mechanical treatment of metals using high-energy ball milling introduces defects such as dislocations, new grain boundaries, vacancies and interstitials, and anti-site defects. In addition, a distribution of the stress field is also introduced near grain boundary regions and interfaces in non-deformed crystalline regions. Hydrogen diffusion through dislocations and grain boundaries is faster than in the lattice and therefore, these structures are the most likely diffusion paths for hydrogen. Such defects may also act as deep traps for hydrogen. Hydrogen is a critical component of the treatment of chlorinated volatile organic compounds (CVOCs) for which the nanoscale iron colloids are produced. Thus, the ball milled material having higher hydrogen solubility and storage capacity is ideally suited for use in CVOC environmental remediation techniques.
The fraction of the surface area of an iron colloid that is reactive towards CVOCs is related to the number of defects and abnormalities present on the surface of the iron. Thus the methods of the present invention produce a ball milled colloid with a purposeful high degree of enhanced reactive surface area, which requires that the proportion of highly deformed grain network be greater than 70% of the total grain area, otherwise the imprinted grain structure will not be stable and will recrystallise by annealing.
It should also be noted that the minimum grain size obtainable by ball milling is a balance between the defect/dislocation structure introduced by the plastic deformation of the milling and its recovery by thermal processes. The thermal recovery is related to the melting temperature of the metal, which, in the case of iron, yields a minimum grain size of about 10 nanometers.
The nanoscale metal colloid produced by the high-energy ball milling methods of the present invention has the ideal properties of small size that enhances kinetic reactivity due to surface area per unit weight and due to some molecular distortion of atoms in the surface of a small colloid, enhanced kinetics due to the presence of a second more noble metal in trace quantities on or in the colloid, enhanced kinetics for CVOC reactions due to imprinted structural defects of the colloid, protection from water dissociation, enhanced long-term reactivity through passivation layers due to the presence of zones of structural defects, and possible protection against water dissociation reactions by the addition of appropriate solutes.
The foregoing description of the present invention has been presented for purposes of illustration and description. Furthermore, the description is not intended to limit the invention to the form disclosed herein. Consequently, variations and modifications commensurate with the above teachings, and the skill or knowledge of the relevant art, are within the scope of the present invention. The embodiment described hereinabove is further intended to explain the best mode known for practicing the invention and to enable others skilled in the art to utilize the invention in such, or other, embodiments and with various modifications required by the particular applications or uses of the present invention. It is intended that the appended claims be construed to include alternative embodiments to the extent permitted by the prior art.

Claims

1. A method of producing a nanoscale metallic colloid, comprising:

suspending an elemental metal in a non-aqueous organic liquid;

adding a dispersant to the suspension; and

ball milling the suspension to form a colloid of nanoscale metallic particles.

2. The method of claim 1, wherein the temperature of the suspension during the ball milling is between about −80° C. and about 100° C.

3. The method of claim 1, wherein the temperature of the suspension during the ball milling is between about −80° C. and about −50° C.

4. The method of claim 1, wherein the temperature of the suspension during the ball milling is between about 20° C. and about 100° C.

5. The method of claim 1, wherein the elemental metal is selected from the group consisting of iron, tin, zinc, palladium, platinum, nickel and combinations thereof.

6. The method of claim 1, wherein particles of the elemental metal being processed have a size in the range of about 1 micron to about 1000 microns.

7. The method of claim 1, wherein the concentration of the elemental metal in the non-aqueous organic liquid is between about 10% wt./vol. and about 80% wt./vol.

8. The method of claim 1, wherein the concentration of the elemental metal in the non-aqueous organic liquid is about 70% wt./vol.

9. The method of claim 1, wherein the concentration of the elemental metal in the non-aqueous organic liquid is about 45% wt./vol.

10. The method of claim 1, wherein the non-aqueous organic liquid is selected from the group consisting of dodecane, ethyl ether acetate, butyl acetate, polypropylene glycol, vegetable oil and mixtures thereof.

11. The method of claim 8, wherein the dispersant has a concentration of between about 1 mg and about 20 mg per square meter of colloid surface area.

12. The method of claim 1, wherein the ball milling is conducted at a rate of between about 10 rpm and about 50 rpm.

13. The method of claim 1, wherein the ball milling is conducted for a time period between about two hours and about 48 hours.

14. The method of claim 1, comprising the additional step of annealing the colloid of nanoscale metallic particles.

15. The method of claim 14, wherein the annealing temperature is between about 200° C. and about 90% of the melting temperature of the metal.

16. The method of claim 14, wherein the annealing is conducted for a period of time between about 2 minutes and about 5 days.

17. The method of claim 14, wherein the annealing comprises at least two heat treatments wherein each heat treatment is conducted at a temperature between about 200° C. and about 90% of the melting temperature of the metal.

18. The method of claim 1, further comprising contacting the colloid with an organic liquid containing a second metal different from the elemental metal of the suspending step.

19. A method of producing a nanoscale metallic colloid, comprising:

suspending an elemental metal selected from the group consisting of iron, tin, zinc, palladium, platinum, nickel and combinations thereof having a particle size in the range of between about 1 micron and 1000 microns in an organic liquid selected from the group consisting of dodecane, butyl ethyl ether acetate, polypropylene glycol, vegetable oil and mixtures thereof,

adding a polymer dispersant with a chain length of 1 to 10 nanometers to the suspension; and,

ball milling the suspension at a rate of about 1,000 rpm for about 6 hours.

20. A method of producing a nanoscale metallic colloid, comprising:

suspending an elemental metal selected from the group consisting of iron, tin, zinc, palladium, platinum, nickel and combinations thereof having a particle size in the range of between about 1 micron and 1000 microns in an organic liquid selected from the group consisting of dodecane, butyl ethyl ether acetate, polypropylene glycol, vegetable oil and mixtures thereof;

adding a polymer dispersant with a chain length of 1 to 10 nanometers to the suspension;

ball milling the suspension; and,

annealing the suspension at a temperature of between about 200° C. and about 90% of the melting temperature of the elemental metal for a time period of between about 2 minutes and about five days.