Liquid purification or separation – Processes – Ion exchange or selective sorption
Reexamination Certificate
1998-11-24
2002-08-20
Cintins, Ivars (Department: 1724)
Liquid purification or separation
Processes
Ion exchange or selective sorption
C210S679000, C210S682000, C210S688000, C502S408000, C521S026000
Reexamination Certificate
active
06436294
ABSTRACT:
BACKGROUND OF THE INVENTION
The invention relates to modifying the metal ion sorption capacity of a medium.
Water from waste streams, ground water, holding ponds, water treatment facilities, storage tanks, rivers, and streams can contain metals such as iron, zinc, cesium, plutonium, strontium, technetium, uranium, and americium. For environmental compliance, it is often desirable or necessary to remove these metals from the water.
A variety of methods have been developed for removing metals from the water in waste streams, ground water, holding ponds, water treatment facilities, storage tanks, rivers, and streams. Some of these methods include passing the water containing through a medium that removes the metal. The medium may be an ion exchange medium that is capable of sorbing the metal ions in the liquid. The medium is often packed in a column and once the medium is saturated with metal ions, the medium and/or column is discarded.
A method for removing metals from the medium involves eluting the metal ions from the medium with a strong acid followed by regenerating the medium with a strong base. These methods, however, do not always perform with the same level of effectiveness for all metals. For example, ion exchange media used for the removal of strontium frequently have a relatively low capacity for strontium due to large excesses of calcium and magnesium, which compete with the strontium for sites on the medium. Large excesses of calcium and magnesium relative to strontium are often present in waste streams and ground water.
A variety of agents can be used to elute metal ions from an ion exchange medium. Nitric acid and hydrochloric acid, for example, are often used to elute strontium from a strontium absorber. Nitric acid and hydrochloric acid, however, tend to cause a gradual increase in back pressure in systems in which they are employed as the eluant and in systems in which the medium is reconditioned.
SUMMARY OF THE INVENTION
The invention features a process for modifying a medium to increase its capacity to sorb (i.e., adsorb, absorb and combinations thereof) metal ions, as well as processes for regenerating the metal ion sorption capacity of a medium that has been exposed to metal ions, as well as the modified media, itself.
In one aspect, the invention features a process for modifying a medium that includes treating a medium having a metal ion sorption capacity with a solution that includes (a) an agent capable of forming a complex with metal ions, and (b) ions selected from the group consisting of sodium ions, potassium ions, magnesium ions or a combination thereof, to create a medium having an increased capacity to sorb metal ions relative to the untreated medium.
In preferred embodiments, the complexing agent is an organic acid (e.g., citric acid) and the ions are sodium ions. In some embodiments, the solution includes sodium azide. In other embodiments, the solution includes an organic acid and sodium hydroxide.
In preferred embodiments, the treating solution has a pH of between about 6 and 10, more preferably a pH of between about 7.5 and 8.5.
In one embodiment, the medium is capable of sorbing strontium ions. In other embodiments the medium is capable of sorbing mercury ions.
In another embodiment, the medium includes a membrane filled with particles, e.g., particles selected from the group consisting of particles of sodium titanate (i.e., sodium titanate, sodium nonatitanate, and combinations thereof), crystalline silico titanate, mixed salts of titanium silicate, sulfonated styrene divinyl benzene, SAMMS self-assembled monolayers on mesoporous supports specific for mercury analytes having a formula SiO
2
—CH
2
CH
2
—SH, and combinations thereof.
In other embodiments, the medium includes sorbed metal ions.
In one embodiment, the process further includes contacting the treated medium with a liquid that includes metal ions such that the metal ions sorb onto the medium. The medium that includes sorbed metal ions can then be treated with an agent capable of forming a complex with metal ions for a period sufficient to elute the metal ions. One example of an agent capable of forming a complex with metal ions is a solution that includes an organic acid, e.g., citric acid and sodium hydroxide.
In one aspect, the invention features a process in which the back pressure produced during the process remains relatively constant during the process. In one embodiment, the process further includes providing a medium that includes sorbed metal ions, prior to treating the medium.
The process is useful for treating a medium (e.g., a solid phase ion exchange medium) that has sorbed metal ions (e.g., heavy metals, rare earth metals, and radioactive elements). The processes can regenerate (i.e., restore or increase) the metal ion sorption capacity of articles that have been previously contacted with a source of metal ions. The process is particularly useful in regenerating the ion sorption capacity of articles that are used to remove metal ion contaminates, and to treat aqueous streams from sources such as ground water, storage tanks, holding ponds, waste water treatment facilities, and nuclear waste storage tanks.
The process of the present invention improves the metal ion sorption capacity of an article relative to its metal ion adsorption capacity without treatment. In another embodiment, the process of the invention improves the metal ion sorption capacity of an article that includes sorbed metal ions. The processes according to the present invention also permit the maintenance of a relatively constant back pressure throughout the process.
Certain preferred processes according to the present invention are particularly well suited and can be optimized for the selective removal and recovery of strontium from a medium.
Other features and advantages of the invention will be apparent from the following description of the preferred embodiments thereof, and from the claims.
DETAILED DESCRIPTION
The process includes treating a medium having a metal ion adsorption capacity with a solution that includes: A) an agent capable of forming a complex with at least one metal ion; and B) ions selected from the group consisting of sodium, potassium, magnesium and combinations thereof, to increase the capacity of the medium to sorb metal ions relative to the untreated medium.
The treating solution is a buffer preferably having a pH in the range of about 5 to about 11, more preferably a pH in the range of about 6 to about 10, most preferably a pH in the range of about 7.5 to about 8.5. The treating solution includes a complexing agent capable of forming a complex with at least one metal ion. Preferred agents are capable of forming complexes with ions of, e.g., heavy metals, rare earth metals, actinides, and combinations thereof.
Examples of useful complexing agents include organic acids having more than one carboxyl group including citric acid, tartaric acid, oxalic acid, succinic acid, malonic acid, and ethylenediaminetetraacetic acid (“EDTA”).
Other useful complexing agents include lactic acid, sulphosalicylates, acetylacetonante, and azides (e.g., sodium azide).
The treating solution also includes ions, e.g., sodium ions, potassium ions, magnesium ions and combinations thereof. The treating solution is brought to the desired pH by the addition of an appropriate amount of buffer adjusting solution, e.g., base, which also provides the ions. Examples of useful bases include metal hydroxides including, e.g., sodium hydroxide, potassium hydroxide, calcium hydroxide, and magnesium hydroxide, and sodium azide. The sodium azide can function as both the complexing agent and a source of sodium ions.
The addition of ions can be used to convert substantially all of the particles in the medium to a single salt form, e.g., the sodium form, such that the medium exhibits an increased propensity to selectively sorb predetermined ions, e.g., cations or anions. Preferably ions are added to convert substantially all of the medium to the sodium salt form. Preferably the medium exhibits a propensity to selective
3M Innovative Properties Company
Bond William J.
Cintins Ivars
Griswold Gary L.
Sprague Robert W.
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