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INDUSTRIAL AND ENGINEERING CHEMISTRY
extremely high. A decontamination factor of 104 would be equivalent to B removal efficiency of 99.99%, 106 to 99.999%, etc. The amounts of radioactivity involved in these tests varied widely, but the minute amounts escaping the system remained practically constant in each test.
Conclusions The efficiency of the nozzle system in removing radioactive particles from the flue gas stream has steadily increased. The increase from 78.6 to 97.0% can be attributed to the higher steam pressures. The increase from 97.0 t o 99.Syo was accomplished by operating the nozzle without its water jets and by improving the action of the second Pease-Anthony scrubber. The efficiencies of the Chemical Warfare Service filter and of the Nash compressor (for particle removal) have been much better than expected. On the basis of these pilot plant results, a full scale incineration process is n o v being designed. The over-all efficiency of the rough washer, Pease-Anthony scrubber, and Nash compressor (combined) is of the same order of magnitude as t h a t obtained from the standard commercial-type steam exhauster
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used in the first pilot plant. This would tend t o support t h e theory t h a t a steam exhauster of that type acts on an impactiondiffusion principle, as do these scrubbers.
Acknowledgment The author wishes to express his thanks to Fred M. Huddleston, Paul ?*I.Hamilton, and Frank C. Mead for their help in making this paper possible.
Literature Cited (1) Endebrock, R. W., Mound Laboratory, Miamisburg, Ohio, un-
published report.
(2) Kleinschmidt, Chem. h M e t . Eng., 46, 487 (1939). (3) Kleinschmidt and Anthony, Trans. Am. SOC.Mech. Engrs., 63,
349 (1941). “Chemical Engineer’s Handbook,” 3rd ed., p. 1031, New York, McGraw-Hill Book Co., 1950. ( 5 ) White, H. E., “Classical and Modern Physics,” p. 465, New York, D. Van Sostrand Co., 1940. (6) Yellott, J. I., Trans. Am. Soc. Mech. Engrs., 56, 417 (1934) (4) Perry, J. H.,
RECEIVED November 24, 1950. Work done under the auspices of the Atomic Energy Commission
Contract No. AT-33-1-GES-53.
Treatment of Radioactive Water by Phosphate Precipitation T h e work was done to check the effectiveness of a calcium phosphate floc in removing radioisotopes from large quantities of water. In almost every case the phosphate was found to be more efficient than either alum or ferric hydroxide. Maximum removals were obtained under conditions of high pH and i n the presence of an excess of phosphate. In general, good removals were obtained for those isotopes which would be precipitated
under the same conditions if they were present in macro quantities. While the data must be considered preliminary in nature, they indicate that coagulation and filtration techniques, if performed under the proper conditions, can be applied to treat liquid wastes containing low levels of radioactivity. The total reduction obtained in the activity of the waste will be dependent on the radioisotopes present.
R. A. Lauderdale OAK RIDGE NATIONAL LABORATORY, OAK RIDGE, TENN.
I
N T H E treatment of large quantities of water containing rela-
with a hydroxide, the floc can be formed in a solution of high
tively low levels of radioactivity, coagulation of hydrous precipitates offers a number of advantages. It is the normal method of treating municipal water supplies and could, therefore, be used in existing equipment in the event of an emergency. I n some instances, adsorption is one of the most efficient methods of removing low concentrations of impurities from solution. Thirdly, minimal, rather than massive, doses of reagents are used-an important factor economically. I n normal practice, thGh floes usually employed are aluminum hydroxide or ferric hydroxide. I n the treatment of radioactive water, however, it was felt t h a t other methods should be tested for use either alone or in conjunction with the normal practice. This report describes work that has been done using calcium phosphate precipitation as a method for treating radioactive wastes. The decision to investigate the use of a calcium phosphate floc was based on a comparison of the insolubility of the phosphates and hydroxides, which showed t h a t the number of highly insoluble phosphate compounds exceeded the number of insoluble hydroxides. This was considered desirable from the standpoint of Hahn’s adsorption and coprecipitation rule, which states that the adsorption and/or coprecipitation of a n ion by a precipitant depends on the surface charge of the adsorbing precipitate, and on the degree of insolubility of the adsorbed compounds in the solvent involved. I n addition to a n expected increased coprecipitation of radioactive ions by a phosphate floc, a s compared
pH, a condition favoring the adsorption of certain ions on the turbidity usually present in natural waters and the formation of radiocolloids. (Unpublished data of the author indicate t h a t natural clays are highly efficient in the adsorption of cesium ions, the efficiency increasing with the p H of the solution.) Furthermore, the phosphate floc has a very large surface area (as do the hydroxides), which favors the surface adsorption of ions and charged particles. The isotopes of immediate interest were uranium fission products and particularly those of fairly long half-life. For the initial tests isotopes were selected which would be representative of the different groups of the periodic chart. It should then be possible to extrapolate the data for a given element to other elements having similar characteristics. Tracer-type runs were made with the isotopes of cerium, strontium, zinc, yttrium, antimony, and tungsten. The phosphate floc was formed by adding the radioisotope and a solution of either potassium dihydrogen phosphate or sodium phosphate to a solution of calcium hydroxide in distilled water. The mixture of phosphate, calcium, and radioisotope was given a flash mix for 5 minutes, followed by slow stirring t o allow the newly formed floc t o grow t o a size which would settle. Following the slow mix, the floc was allowed to settle for 2 hours. The p H of the liquid was adjusted for some tests by the addition of dilute sodium hydroxide. Efficiencies of removal were calculated from
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INDUSTRIAL AND ENGINEERING CHEMISTRY I
100,
I
Figure 1.
I
I
I
I
1539
I
Strontium Removed
Figure 2.
For different ratios of sodium phosphate to calcium hydroxide
Strontium Removed as Function of Ratio of Sodium Phosphate to Calcium Hydroxide pH 2 11.3
counting data obtained from samples taken of the water before and after treatment. All samples were counted with a n end-window type of counter, with the exception of Zn" samples which were counted in a gasflow proportional type of counter. Data have been obtained in the tracer runs for the removal of Ce144, Sr80, Ygl, Zns6, Sb1Z4, and WISE. Maximum removals which were found for each of these isotopes are given in Table I.
Table I. Isotope
Isotopes Removed by Phosphate Precipitation Coagulant
Coagulant MdLite; 200 120 100 240
% Removal
100 100 120 50 200
The figures listed in Table I are for a one-stage batch treatment. One two-stage treatment was carried out with Ce14*as the active isotope, with the floc being formed in a composite of the treated liquid from the first stage. The reduction in the activity in the water was approximately 87.5% for the second stage. These data may be compared with similar data obtained using alum or ferric hydroxide as the precipitant (Table 11). It was found that the phosphate treatment in' practically every case produced a greater concentration of radioactivity in the floc. The data are expressed in terms of decontamination factors, because this method shows more effectively the differences in the efficiencies of the three floes.
The results given in Table I1 should not be taken as absolute values, for as yet no study has been made of the variables involved in precipitating with alum and ferric chloride. The data shown were obtained during a series of standardized tests de-. signed to give only an indication of what might be expected if these two coagulating agents were used. The results obtained with the isotopes of Ce144,Zn6J, and YO1 indicated that essentially complete removals could be obtained by phosphate precipitation. The conditions of precipitation had little effect on the removal of these isotopes. A decided variation in the removal of Sb1Z4and Sr80 was found if the conditions of precipitation were varied. For this reason Sr8Qwas selected as the isotope to use in a study of the variables involved (pH and ratio of phosphate to calcium). A series of tests was made with the ratio of phosphate to calcium constant and with p H as the variable. Six such tests were made using ratios of 0.43, 0.86, 2.16, 4.32, 12.90, and 25.8 ing. of sodium phosphate to 1.0 mg. of calcium hydroxide. I n each case the p H was adjusted with nitric acid or sodium hydroxide. These data plotted in Figure 1 show that the pH has a marked effect on the per cent of strontium removed until a p H of approximately 11.3 is reached. At p H values above 11.3 little
Table 11. Decontamination Factors Obtained with Phosphate and Hydroxide Precipitation Isotope
Alum FeCla KHzPOa 20 1.1 1000 11.1 22.7 500 1.1 1.02 5.3 initial, counts per minute per ml. Deoontamination factor final, counts per minute per ml. '
Nad'O4
....
1000 45.5
INDUSTRIAL AND ENGINEERING CHEMISTRY
1540
Table 111. Treatment of Mixed Fission Products by Flocculation Test I
I1 I11 IV
Initial, Counts/Min./ M1. 14,050 13,300 13,475 1,295
Stage Treatment 1 220 p.p.m. clay 220 p.p.m. NaSP04 2 100 p.p.m. Ca(0H)a 1 220 p.p.m. NasPOa 50 p.p.m. carbon 2 100 p.p.m. Ca(0H)z 1 200 p.p.:n. fuller's earth 2 125p.p.m.NaaPOa 1 50 p.p.m. carbon 100 p.p.m. Feci8 2 200 p.p.m. clay 200 p.p.m. KasPOa
pH 11.5
Over-all Decontamination Factor 28
11 5
53 17
11.5 11.5 8.3
33
11.5 8.0
31 3.7
11.5
F7
Method of Separation Centrifuged Centrifuged Centrifuged
4
Centrifuged Centrifuged
Table IV. Treatment of Mixed Fission Products by Phosphate Precipitation Test
VI
Stage 1
VI1
1 2
VI11
I
Treat :ne n t 100 p.p.m. clay 100 p.p.m. KaaPOi 100 p.p.m. clay 100 p.p.m. NaaPOa 100 p.p.m. Ca(OH)* 100 p.p.m. NaxPO, 100 p.p.m. clay 100 p.p.ni. IiasP01
Initial, Counts/ Min./MI. 4800
Final Count& hlin./hIl. 31
Decontamination Factor 165
21.50
27
80
18
119
12
248
2935
effect due to pH was observed. I n Figure 2 are shown data obtained by maintaining the p H of each sample above 11.3 and varying the ratio of sodium phosphate t o calcium hydroxide between the values of 0.43 to 1 and 25.8 to 1. The per cent of strontium removed increased rapidly as the ratio of phosphate to calcium was increased. A sharp break occurred a t a ratio of 2.2 t o 1, which corresponds to a 46% excess of phosphate. Above the ratio of 2.2 to 1 only a slight increase in the removal of strontium was noted. ' As strontium is one of the major constituents in a fission product mixture, these data indicated that maximum efficiency could be obtained in the treatment of fission product mixtures if an excess of phosphate were used a t a pH above 11.3. Consequently, the later tests made with mixed fission products were carried out under those conditions. The fission misture used colltained the fission products that would be found after approximately months, decay time, except iodine, which had beell removed, -4partial analysis o f the mixture showed the follom-ing percentage of isotopes: Trivalent rare earths Cerium Strontium Barium Ruthenium Cesium Total
43.5 27.0 17.4 5.1 2.9
1 1 __ 97.0
Tlitl iemaining 3% would contsiii tiaces of a large number of other isotopes with a low fission yield. The tracer runs which had been made indicated that essentially all the rare earths would he removed, and about 95% of the strontium. Barium would be expected to act like strontium. The ruthenium and cesium would not be expected to precipitate and, hence, these two would be removed by adsorption on the surface of the floc, adsorption on an inert material such as clay or carbon, 01 occlusion as a radiocolloid. The procedure used in the tests vith mixed fission products was essentially the same as in the tracer experiments, except that tap water waF wed for dilution instead of distilled water, in order t o
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give a solution which should be more nearly representative of an actual waste. In the tests in which a two-stage treatment was used, carbon, ferric chloride, and a kaolinitic clay were sometimes used in addition to the phosphate. Typical data for this series of experiments are shown in Table 111. On the basis of these and similar data it was concluded that p H was one of the most important factors in the treatment of radioactive wastes, that carbon and fuller's earth were no more effective than clay in supplementing the phosphate treatment, and that a calcium phosphate floc was superior t o ferric hydroxide and to alum. I n the remainder of the tests, the following procedure m-as used. The pH of the waste v a s raised with sodium hydroxide to between 11.5 and 12.0. After a short stirring time clay was added in quantities up to 200 p.p.m. After approximately 10 minutes' mixing the sodium phosphate was added to produce the calcium phosphate floc. It was not necessary to add calcium, because it was present as a natural constituent of the water used for dilution. After 20 to 30 minutes of slow stirring the floc was allowed to settle and the liquid was filtered through a sand filter made of a 6/*-inch glass tube filled to a depth of 21 inches with a commercial grade of sand.
A portion of the data obtained using this procedure is shown in Table IV. The variations in the decontamination factors listFd are believed to be due to differences in the settling and filtering characteristics of the floc. The decontamination factors listed above can be applied only to the particular mixture of isotopes used in these tests. Any change in the composition of the mixture may be reflected in residual radioactivity left in solution-for example, if cesium were present in greater quantities, the efficiency of the precipitation process would probably be lowered. If the mixture were composed entirely of the rare earth isotopes, the efficiency would probably be higher.
Conclusions
In summary, the follo'ving have Obtained by using a calcium phosphate flocculation to decontaminate water. Essentially all the cerium, yttrium, and zinc were removed from solution by phosphate precipitation* The data can probably be extended to include other elements of similar chemical properties. Approximately 10% Of the tungsten, 67% Of t'he antimony, and 95y0 of the strontium were removed by the phosphate treat,ment. Efficiencies of renioval great'er than 99%, or decontamination factors greater than 100, have been obtained with a mixture of fission products by using clay in conjunction x i t h the phosphate, followed by sand filtration. The efficiency of the process when used with a mixture of radioactive isotopes will depend on the composition of the mixture. The greatest concentrations of activity in the floc were obtained under conditions of high pH and with an excess of phosphate. Acknowledgment The author wishes to acknowledge the assistance of those who helped in the work reported in this paper. These include 0. R. Placak, U. S. Public Health Service; J. P. Byrom, National Research Council fellowship student; E. F. Gloyna, ORNL research participant from the University of Texas; and T. W. Brockett and A. H. Emmons, Oak Ridge National Laboratory. REChIvED November 24, 1950. Work done under Contract TV-7405-Eng-26 f o r the Atomic Energy Commission
