Liquid purification or separation – Processes – Ion exchange or selective sorption
Reexamination Certificate
2000-01-04
2002-05-07
Hruskoci, Peter A. (Department: 1724)
Liquid purification or separation
Processes
Ion exchange or selective sorption
C210S684000, C210S688000, C210S226000, C210S228000, C210S266000, C210S282000, C210S484000, C210S493100, C210S911000, C210S913000, C210S915000, C210S679000
Reexamination Certificate
active
06383395
ABSTRACT:
FIELD OF THE INVENTION
The present invention is directed to the removal of species from aqueous solutions. In particular, the present invention is directed to the removal of species including an arsenic [III] component from groundwater in purification for drinking.
BACKGROUND OF THE INVENTION
Ion exchange beds are used to remove toxic ions from solutions. In general, ion exchange beds use columns of polymeric material with suitable ion exchange sites, such as sulphonic acid groups or quaternary ammonium salts, which are grafted onto a polymer matrix. The polymeric material is often in the form of beads, which are usually about the size of a grain of rice. The ion exchange process, such as the exchange of chloride ions on the surface of the beads for nitrate ions in an aqueous solution, occurs mainly at the surface of the bead. Consequently, the capacity of an ion exchange column is a function of the number of beads and the available ion exchange sites on the surface of individual beads. Very little of the ion exchange capacity on the inside of the bead is utilized. As a consequence, the capacity of an ion exchange column is typically much smaller than the total number of chemical reactions the ion exchange media could theoretically undergo.
The rate of the ion exchange reaction is further slowed as the reaction depends on the diffusion of the toxic ions into and out of the resin. The concentration of the toxic ions will be lower at the surface of the resin than in the bulk of the liquid. Each section of the column may be considered as an area where an equilibrium is set up between the leaving ion and the entering ion at their relative concentrations. As an aqueous solution moves up or down the ion exchange column and is exposed to clean resin surfaces, the ion exchange reaction is more rapid. This process is analogous to distillation and the concept of theoretical plates. Consequently, when ion exchange columns are used to remove toxic ions from liquid solutions, those skilled in the art favor longer columns to increase the time the aqueous solution is in contact with the ion exchange media.
In order to allow the aqueous solution containing the toxic ions to flow through the column and come into contact with as many ion exchange sites on the resin as possible, those skilled in the art further prefer the ion exchange media to have good hydraulic permeability throughout. Consequently, when zirconia has been used as a component in ion exchange media, those skilled in the art have modified the zirconia, ice. by creating bead like particles using polymeric compounds, to increase the hydraulic permeability of the column. While these modifications to the zirconia increase the ion exchange media's hydraulic permeability, they limit the media's capacity as fewer ion exchange sites are accessible to the toxic ions in the aqueous solution.
Arsenic is a toxin that may be found in various types of water from commercial effluents to naturally occurring groundwater. The presence of arsenic in water creates difficulties in the removal processes as well as in disposal of the media or regeneration solution. Recently, the presence of arsenic in groundwater has lead to a crisis in the Bengal Basin. As described in an article from C&EN, pp. 128-132, Dec. 6, 1999, the United Nations International Children's Emergency Fund (UNICEF) installed millions of wells in Bangladesh villages in the 1970's in an attempt to provide the frequently flood ravaged country with safe drinking water. The program was an early success, drastically reducing instances of cholera and other diseases in the country. Unfortunately, the groundwater was contaminated and people began to suffer from arsenic poisoning.
The World Health Organization sets the standard of acceptable arsenic levels in drinking water at 50 parts per billion (ppb). New laws may set this standard even lower, to levels not greater than 10 ppb. In view of poor testing methods and without the means to achieve even the current standards of safe arsenic levels in Bangladesh, the tragedy continues in that region.
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.
The Environmental Protection Agency (EPA) and others believe that arsenite predominates in aquifers due to a lack of oxygen and oxidizing species that can convert the arsenite to arsenate. This is not the case with surface waters where oxygen is plentiful and there are other ions such as ferric ions which can complete the oxidation. It has been believed that arsenite cannot be as easily removed as the arsenate ion because arsenite is only very slightly ionized in water. The ionization constant for arsenite is only 5×10
−10
compared to the first ionization constant of arsenate, 5.6×10
−3
. Some processes chemically oxidize the arsenite to arsenate to facilitate its removal, but this creates an additional step in the removal process.
There have been various proposals for removing arsenic from drinking water including precipitation with iron or copper and attempts to immobilize the arsenic with biological agents. These approaches have various problems, not the least of which is the difficulty of removing arsenic to the very low levels that are required for safe drinking, on the order of parts per billion. Other difficulties are that other competing ions for ion exchange sites or ions unnecessarily precipitated, are on the order of parts per million, a thousand times more material than arsenic species. Large amounts of non-toxic and beneficial ions are removed along with the very much smaller amounts of arsenic. Accordingly, the removed material is bulkier than necessary, toxic and a disposal problem.
There is a need for an efficient, economical, and high capacity water treatment method and apparatus for removing toxic ions from aqueous solutions and from groundwater in particular. This process should be capable of removing arsenic to levels not greater than 10 ppb. The media should be capable of being regenerated repeatedly with little loss in capacity, to reduce costs and provide for continuous use of wells. The system must be safe and prevent loss of removed arsenic during operation. There must be a minimum amount of waste and the device should be suitable for use in low pressure applications such as wells. Finally, the system should be easy to use and inexpensive to obtain and operate. A step change in technology is required to satisfy all of these needs.
SUMMARY OF THE INVENTION
The present invention advantageously satisfies the above needs. Arsenite and arsenate may be removed to safe levels not greater than 10 ppb and even to 1 ppb levels, using a ceramic media of the present invention. The inventive media is nontoxic, insoluble and chemically and biologically stable. The media has a very high affinity not only for ionic arsenate species, but also for the soluble and nonionic arsenite species. In view of its stability, the inventive media may be used over and over again. Little or no loss in capacity has been observed and the process of stripping arsenic from the ceramic creates a minimum secondary waste.
Arsenite removal occurs in an anomalous way in the inventive media. The notoriously difficult-to-remove arsenite is much more easily removed at a high capacity compared to arsenate. This is surprising in that arsenite is poorly ionized. When the media reaches full capacity the outlet concentration may be equal to the input concentration with no spiking that is observed with ion exchange systems. The inventive media is not strictly an ion exchange material in the normal sense of the term, though it may be used to remove certain ionic species. The inventive media behaves differently than an ion exchange media since it also has the ability to remove nonionic species very well. Removal of arsenite and arsenate is achieved using benign c
Butler Clive J.
Clarke Richard J.
Clarke Stephen R.
Mohanta Sam
Murdock Roderick
Hruskoci Peter A.
Luxfer Group Limited
Watts Hoffman Fisher & Heinke
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