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WILLIAM J. LACY and WALLACE DE LAGUNA Oak Ridge National laboratory, Union Carbide Nuclear Co., Oak Ridge, Tenn.
Decontaminating Radioactive Water C o M h r o u METHODS for purifying water, such as coagulation, settling, and filtration can remove 60 to 85% of radioactive contamination from mixed fission products. Adding an absorbent such as clay can increase this percentage up Subsequent clarification by to 98. chemical coagulation is difficult. Certain long-chain high molecular weight polymers act as coagulants (7, 2 ) . The hydrophilic groups (nonionized) are adsorbed on the particle surface and the accompanying ionized groups keep the polymer in an extended position, thus causing the particles to bridge and coagulate. In this work, polymer coagulants used with several types of ferruginous, calcareous, and glaconitic shale were slurried in tap water to remove the radioactive nuclides by adsorption. The radioactive nuclides used as contaminants were : strontium-90-yttrium90 ; cesium-137"barium-137 ; ruthenium-106-rhodium-106 ; cerium-144praseodymium-144; and a fission product mixture, MFP. This mixed contaminant obtained from a neutronirradiated uranium slug was removed from the graphite reactor, allowed to cool 150 days, and dissolved in nitric acid. The mixture was used as nitrates in nitric acid, and the other radioactive nuclides as chlorides in hydrochloric acid. Two polyelectrolyte coagulants used were Lytron 886 and Separan 2610. Lytron 886 is a partial calcium carboxyl salt containing a high molecular weight polymer. O n dissociation it forms highly charged, adsorptive polyanions, and unlike Separan 2610, lime must be used as a flocculent aid. They are ofien used as a 1 to 1.5% stock solution,
Table I.
Type Shale
which forms good flocs when added to solutions ranging from strongly acid to strongly basic. After dispersal by violent agitation, these solutions can be reflocculated by adding more polymer. Test procedures were identical except for Lytron 886; 100 p.p.m. of lime was added to each beaker before the polymer. To 2.5 liters of Oak Ridge tap. water a quantity of radionuclides was added to give 3000 to 6000 counts/min./ml. counted at 1070 geometry. Shale samples were pulverized and graded using standard U. S. sieves and the particles passed a t 50-mesh sieve but were retained on a 70-mesh. Five-hundred-milliliter portions of tap water with a radioisotope of fission product mixture were placed in five 1-liter beakers containing 2.5, 5, 10, and 20 grams of a shale. The beakers were slurried at a high speed (about 250 r.p.m.) for 30 minutes, then 25-ml. aliquots were removed and filtered. These filtrates, 1 ml. each, were placed in three aluminum dishes, dried under infrared heat lamps, and counted using a 1.8 mg./sq. cm. mica end-window G-M tube and 64 scaler. The beakers were then reslurried and enough polymer was added to the first three to give 6 p.p.m., and to the fourth (containing 20 grams of shale) to give 10 p.p.m. of coagulant. During formation, coagulation was aided by slowly stirring the mixture of floc and clay slurry for 5 minutes. After treated water had settled for 15 minutes, samples of the supernatant were removed and prepared for counting. In all experiments per cent removal by filtration was only slightly higher than in unfiltered samples. The two coagulants gave almost the
Radionuclides Are Removed by Coagulation of Clay Slurry
Pr144 Yw
Dark, weathere 100 Glauconitic 98.6 Argillaceous 97.9 Light, weathered 93.4 Ferruginous 99.4 Calcareous 95.7 Limestone 99.0
94.8 93.4 93.3 86.3 82.8 80.7 65.5
BaI37 99.7 98.4 98.5 94.5 97.4 97.8 95.0
RhIo6 M F P
Prlt4
Y g O Ba137 RhlOG h f F P
97.4 97.2 95.1 70.5 87.5 87.1 86.4
100 98.4 99.5 88.8 91.7 96.6 98.2
94.1 88.9 91.6 86.0 94.3 85.9 82.9
92.3 95.6 92.0 86.4 89.2 91.0 79.2
98.7 96.2 95.4 94.7 95.4 96.2 93.1
98.0 93.5 95.2 75.0 95.4 85.0 96.3
95.9 92.7 92.5 79.0 92.3 91.0 89.5
of shale equals 20 grams in 500-ml. water, 30-min. 3O-Inill. contact time, 5-min. reslurrying a Dosage of nith polymer added, and 15-min. settling.
same per cent removal of radionuclides or fission product mixture (Table I). The radioisotope removal based partly on the exchange properties of the shale is due to the three dimensional structure of the silicate group associated with its various cations. As some surface atoms are ionized, the shale particles carry a negative ionic charge; the positive ions occupy a position in the solution adjacent to the negative charges. This often accounts for failure of the small particles to coagulate when two small particles approach each other. These ionic charge groups can be changed by adding material like the high molecular weight polyelectrolytes. Because of their newness, polyelectrolytes are not used extensively in largescale water coagulation. One application is the military problem of water clarification in the field. Polyelectrolytes have many advantages over the common chemical coagulants : 1. 2. 3. very 4.
Lower dosage of material required Wide effective p H range Good results on either low or high turbid water Excellent settling and filtration 5. Low filter cake moisture content
Conclusions Certain long-chain high molecular weight polymers are efficient agents for the coagulation and clarification of water containing large concentrations of powdered shale. Various types of shale will remove radionuclides from water, the effectiveness depending upon the chemical properties of the nuclides and the physical-chemical properties of the shale. When used as coagulants, maximum possible removal of radioactive material was obtained by slurrying the clay for 30 minutes, followed by addition of the coagulant, 5 minutes of rapid mixing, and 15 minutes of settling.
Literature Cited (1) Johnson, C. E., IND.ENG. CHEM.48,
1080 (1956).
(2) Michaels, A. S., Zbid., 46, 1485 (1954).
Division of Water, Sewage, and Sanitation Chemistry, 131st 13ist Meeting, ACS, Miami, Fla., April 1957. Work performed under a Corps of Engineers Reskarch Research project. VOL. 50,
NO.
8
AUGUST 1958
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