Liquid purification or separation – Processes – Ion exchange or selective sorption
Reexamination Certificate
2001-02-06
2003-10-14
Cintins, Ivars (Department: 1724)
Liquid purification or separation
Processes
Ion exchange or selective sorption
Reexamination Certificate
active
06632367
ABSTRACT:
BACKGROUND OF THE INVENTION
(1) Field of the Invention
The present invention relates to the separation of heavy isotopes of hydrogen, and in particular to a process and apparatus for separating tritium oxide (HTO) from contaminated water by passing the contaminated water through a molecular separation material containing hydration sites, i.e., sites with multiple associated waters of hydration, to selectively replace the waters of hydration with water molecules that include the heavy hydrogen isotopes.
(2) Description of the Prior Art
Nuclear power plants must routinely deal with the replacement and disposal of contaminated water that is laden with heavy isotopes of hydrogen, namely deuterium oxides, tritium oxides and deuterium-tritium oxides. Tritium in particular is radioactive having a half-life of about twelve and one half years emitting beta particles to form helium.
Periodically, the contaminated water from nuclear reactors must be replaced. It has become industry practice to dispose of the old contaminated water by simply discharging it as allowed by their permits and licenses over ground areas. This is stressful to the environment as the deuterium oxides and tritium oxides are now known to have contaminated ground water sources. There appears to be no effective and economically practical means for otherwise disposing of this contaminated water so this practice of disposal continues.
Tritium is a radioisotope of hydrogen which decays to helium
3
by beta emission with a half life of 12.3 years. Tritium in the environment exists mainly as tritiated water (HTO) for example as in wastewater discharged from nuclear facilities. A tritium concentration of 1 &mgr;Ci/L in water, is equivalent to a tritium-to-hydrogen atom ratio of 3.12×10
−13
. Deuterium, another isotope of hydrogen with an atomic weight of 2.0, is also present in water at a natural concentration of about 150 ppm or at a deuterium to hydrogen atom ratio of 6.75×10
−4
.
Tritium is typically found in dilute form as HTO in the oxidized state. The US EPA has set a drinking water standard of 0.02 &mgr;Curies/L for HTO. Tritium is used in nuclear weapons systems and is a frequent by-product of nuclear processes. It is estimated that there are 20,000 Curies of tritium at the US DOE's Hanford site and more at the Savannah River Site (Jeppson 1999, Miller 1999). It has also been estimated that greater than 90% of the total dose from radionuclides to downstream water users is HTO as contaminated aquifers have reached the Columbia and Savannah Rivers. These rivers remain well below the drinking water standard but HTO in these streams is a concern.
A typical nuclear power plant produces 500 Curies per reactor per year and again it has been established that over 90% of the dose entering the environment from these plants is HTO. As with the US DOE sites, lakes and rivers to which these power plants discharge remain below the drinking water standard.
Heretofore, there have not been practical processes for separation of HTO from light water at these low concentrations (parts per billion to parts per trillion or even lower). In high concentrations, distillation and cryogenic processes are practiced. Electrolysis followed by gaseous separation processes is also practical at high concentrations.
Selective adsorption is a common method to attack removal of trace quantities in liquid and gas streams. In this process, the selective site is the hydration region surrounding a cation, typically aluminum +3, anchored to a substrate. Most salts have hydrates and aluminum sulfate is reported to have up to 18 waters of hydration (Lide, 1996). All of these potential hydration sites are probably not accessible in a supported site configuration. It has also been reported that some cations have two spheres of hydration sites with the closer sphere having a higher heat of hydration. In addition to having a high total hydration capacity it is necessary for the system to be selective for heavier water isotopes.
SUMMARY OF THE INVENTION
In accordance with the present invention, a process and related apparatus is described for separating tritium oxide (HTO, T
2
O), i.e., heavy water, from tritiated water. The process and apparatus may also be used in separating deuterium oxide (HDO, D
2
O) and, deuterium-tritium oxides from waste water. As used herein, water molecules of the formula H
2
O will be referred to as light water molecules, or simply water molecules, while water molecules in which the hydrogen atoms contain one or two neutrons will be referred to as isotope water molecules, or isotope molecules.
In the described process, a portion of the isotope water molecules are removed from contaminated water, i.e., water containing a small amount of isotope water molecules, through selective adsorption by contacting the contaminated water with a molecular separation material containing sites carrying multiple associated waters of hydration. In the process, isotope water molecules present in the contaminated water selectively replace a portion of the waters of hydration associated with the hydration sites. The molecular separation material can then be separated from the water, reducing the percentage of isotope molecules in the water. After separation, the molecular separation material can be regenerated by removing the isotope molecules, and again used to separate isotope molecules.
Generally, the separation material of the present invention is comprised of either an organic or inorganic high surface area support medium having a plurality of hydration sites, i.e., sites with associated waters of hydration. The effectiveness of the separation material is determined by the number of hydration sites exposed to the contaminated water, and to the number of waters of hydration at each site.
The support structure or medium may be, for example, a polymer, such as polystyrene/divinyl benzene (PSDVB), or polyacrylic/divinyl benzene (PADVB). These polymers are commonly used as supports in ion exchange resins in the preparation of ion exchange resins. The polymer may be functionalized, e.g., sulfonated or phosphonated to provide the sites for attachment of metal or other ions with the required associated waters of hydration. The metal ions may be attached by reaction of the sulfonated or phosphonated resin with a salt, e.g., a sulfate or nitrate salt, in which the metal to be used for the site is the cation. The procedure was applied for RSO
3
H with 2%, 4% and 12% cross-linking, 60 mesh, 100-200 mesh and 200-400 mesh screened for size.
In a preferred embodiment, Al
2
(SO
4
)
3
is reacted with the resin to provide the following structure:
or structures with only one or two SO
3
linkages to the resin.
The relative effectiveness of various cations was measured for selective adsorption of tritiated water at low concentration (125 &mgr;Ci/L) in light water. The substrate was a commercial polyacrylic/divinylbenzene resin loaded with the cations as sulfates at one normal concentration and subsequently rinsed with deionized water until the rinse conductance indicated no change. The cations are ranked in the table below by relative adsorption of HTO per unit wet volume of adsorbate.
TABLE 1
Cation Effects on Selective Adsorption of Tritium
Theoretical Number
Relative HTO
of Hydration Sites
Adsorption
Cation
(as sulfates)
(on Resin)
Al + 3
18
10
Na + 1
10
9.5
Zn + 2
7
7.7
It is important to note that the present invention involves the replacement or substitution of the waters of hydration associated with the site, and not the replacement of the cation or anion as is normally practiced in using this type of resin. Thus, while the resins employed are referred to in some instances as ion exchange resins, since this is the purpose for which they are commonly employed, their function in the present invention is the molecular exchange of isotope water molecules with the associated light water molecules. If one begins with a dry resin, the tritiated water will be preferentially adsorbed and held.
Preferably, each hydration s
Collins Gabriel B.
Furlong Louis E.
Stockinger Siegfried L.
,MacCord Mason PLLC
Cintins Ivars
Molecular Separations, Inc.
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