Liquid purification or separation – Processes – Ion exchange or selective sorption
Reexamination Certificate
2002-10-11
2004-11-30
Hopkins, Robert A. (Department: 1724)
Liquid purification or separation
Processes
Ion exchange or selective sorption
C210S688000, C210S263000, C210S287000, C427S215000, C428S403000
Reexamination Certificate
active
06824690
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention (Technical Field)
The present invention relates to modifying zeolite, montmorillonite minerals, activated carbon and other materials in order to increase their affinity for arsenic species, and their use in selectively removing arsenic species from an aqueous medium. The preferred modifiers are water soluble zirconium-containing chemicals, e.g., zirconyl chloride, zirconium acetate, and zirconyl nitrate.
2. Background Art
Note that the following discussion refers to a number of publications by author(s) and year of publication, and that due to recent publication dates certain publications are not to be considered as prior art vis-a-vis the present invention. Discussion of such publications herein is given for more complete background and is not to be construed as an admission that such publications are prior art for patentability determination purposes.
Arsenic has long been known as a highly toxic element. In order to reduce the public health risks from arsenic in drinking water, the United States Environmental Protection Agency (EPA) recently set the arsenic standard for drinking water at 10 ppb down from an original 50 ppb, emphasizing the need for more efficient and cost-effective means for water treatment to remove trace arsenic.
Arsenic exists in two soluble and dangerous oxidation states, As
3+
, which is known as arsenite, and As
5+
, which is known as arsenate. Both forms are toxic and exist in groundwater, although arsenite is the more lethal and the more difficult to remove.
Higher levels of arsenic are generally to be found in ground water rather than in surface water sources of drinking water. The western states of the United States, e.g., New Mexico, have a great number of water systems with arsenic levels greater than 10 ppb. Therefore, to meet the new As standard for water, it is imperative to seek other cost-effective, easy-to-use technologies that could effectively remove As from water.
Conventionally various techniques have been examined to remove As from water, such as precipitation (e.g., salts of iron, aluminum, or copper) and coagulation and filtration processes. However, these conventional methods are generally unable to successfully remove the As to lower levels due to the affinity and solubility limitation of the resultant products. The procedures are also time-consuming and expensive, and so not cost-effective.
One method for removing arsenic species from an aqueous medium is the use of an alumina sorbent, as discussed in U.S. Pat. No. 5,556,545, to Volchek, et al. However, the method has some inherent limitations, requiring regeneration and conditioning of the sorbent. Therefore, this regeneration process creates a hazardous solution. Furthermore, the regeneration process results in loss of the sorbent, thus increasing the cost of using activated alumina as a method for removing arsenic from an aqueous medium.
Another method to remove arsenic species from an aqueous medium is ion exchange. One of the disadvantages of this process is that the ion-exchangers utilized are mostly synthetic resins and hence are very expensive. See, e.g., Japanese Patent Application Publication No. H06-304573, to Masafumi, et al. Furthermore, few resins are selective in arsenic removal. A variety of anions such as sulfates compete for the ion-exchange sites in the resin. In general, ion-exchange is not a feasible method of removing arsenic from an aqueous medium if the medium contains a high level of dissolved solids or sulfate concentrations.
Another method for removing arsenic species from an aqueous medium is through the use of a membrane process. A membrane process involves passing the aqueous medium through the membrane to filter the selected material. However, membrane processes are costly as a method for removing arsenic species from an aqueous medium.
A recently disclosed method is the use of zirconium-impregnated resin as disclosed in U.S. Pat. No. 6,077,809, to Suzuki, et al. The adsorbent material consists of a porous carrier material such as crosslinked polyacrylate resin beads and a crystalline hydrous zirconium oxide impregnating the pores of the carrier in the monoclinic or cubic crystal form. The adsorbent material is prepared by soaking the carrier material with an alcohol solution of a zirconium compound such as zirconium oxychloride to impregnate the pores with the zirconium compound, followed by hydrolysis of the zirconium compound with an aqueous alkaline solution to convert the same into zirconium hydroxide and subjecting the carrier material impregnated with zirconium hydroxide to a hydrothermal reaction under specific conditions to convert the zirconium hydroxide into crystalline hydrous zirconium oxide which has a monoclinic or cubic crystal form depending on the acidity or alkalinity of the aqueous medium employed in the hydrothermal treatment. This method is not only complicated and very expensive but also involves harsh synthesis conditions. Additionally, the regeneration process creates hazardous solutions.
Another recently disclosed method is the use of pure zirconium hydroxide as a paste in water filters, as disclosed in U.S. Pat. No. 6,383,395, to Clarke, et al. The media includes a material selected from zirconium hydroxide, titanium hydroxide, hafnium hydroxide and combinations thereof. The media is preferably in powder form when used to treat water. The media needs to be regenerated repeatedly in order to reduce the cost, while it creates hazardous solutions that need to be disposed of at a cost. Because the media used is in the form of a paste, it does not have high hydraulic permeability and requires use at high pressure. This significantly limits the use of the material to small, high-pressure systems.
Zirconium-loaded activated charcoal has been used in analytical procedures as an adsorbent material for preconcentrating inorganic compounds of As(V), Se(IV), Se(VI), and Hg(II) in aqueous solutions.
The present invention employs materials comprising zirconium bound to zeolite, montmorillonite, activated carbon, fly ash, and like other materials (e.g., cellulose acetate or a cation-exchangeable clay mineral other than montmorillonite, or mesostructured materials (e.g., silica)) for removing arsenic species from an aqueous medium. Zirconium is reported to be environmentally benign, having low biotoxicity and is relatively inexpensive. Both zeolite and bentonite minerals are naturally occurring, inexpensive minerals. They are ubiquitous in the western states of the United States of America.
Zeolites and smectite contain a net negative charge due to isomorphous substitution in the aluminosilicate layers. In nature, this charge is neutralized by inorganic cations such as Na
+
or Ca
2+
on the clay interlayers and external surfaces. Hydration of these cations in the presence of water initiates a separation of the smectic clay layers causing a swelling of the clay. Zeolites have a generally three-dimensional open framework with channels that accommodate water molecules and cations.
Both zeolite and bentonite (mainly montmorillonite) clay minerals are used widely in the construction of liners for hazardous waste landfills, slurry walls, industrial waste treatment lagoons, sewage lagoons, and tank forms. Activated carbon has been used widely in water treatment due to its high surface area, low cost, and sorptive properties for many compounds.
Other background materials include U.S. Pat. No. 4,046,687, to Schulze; P. Bermejo-Barrera, et al., “A Comparison of different chemical modifiers for the direct determination of arsenic in sea water by electrothermal atomic absorption spectrometry”.
Fresenius Journal Of Analytical Chemistry
355(#2):174-179 (1996); Y. L. Chen, et al., “Determination of arsenic(v) and arsenic(iii) species in environmental-samples by coprecipitation with zirconium hydroxide and pre-atomization atomic-absorption spectrometry”,
Journal Of Analytical Atomic Spectrometry
8(#2):379-381 (1993); E. Farfantorres, et al., “Pillared clays: preparati
Moore Robert C.
Zhao Hongting
Hopkins Robert A.
Myers Jeffrey D.
Sandia Corporation
Watson Robert D.
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