Preliminary L a b o r a t o r y Evaluation o f .
.
Decontaminating Radioactive Ocean Areas through Flocculation An effective decontaminant for sea water could reduce radiation hazards from marine foods and reduce radiation in a contaminated harbor as well T H E ocean, with its continuous movement and thermal gradients, presents difficult problems in decontaminationits high salinity and slightly alkaline p H limit the use of common chemical and precipitation methods. Therefore, a settling floc procedure was selected as a possible method. For simplicity, the amount of decontaminant needed was based on surface area of the ocean. Spread over the surface, it was assumed to carry radioactivity as it settles to the bottom. For planning purposes, a 2.5-mile diameter area was visualized, 5000 tons of decontaminant was set as a limit, and an activity reduction factor of 10 was considered acceptable, possibly to the depth level of the thermocline. These specifications were then used to scale laboratory experiments. Some work has been done in decontaminating fission products by flocculating techniques ( 7 - 4 , but no large-scale decontamination of sea water or portions of the ocean has been attempted, mainly because of the physical magnitude of the ocean.
Procedure Small-volume tests were made in hydrometer cylinders on a number of decontaminating materials. In these initial tests, 0.169 gram of the material, or a combination of two or more materials, was mixed into a slurry with 4 ml. of sea water and then transferred to the top of the hydrometer cylinder which contained 565 ml. of natural sea water with enough mixed fission-product (MFP) activity added to give 10,000 gamma c.p.m. per 5 ml. of solution. A screen baffle was used to prevent turbulence. This weight of material, when spread over the area of the column, is proportional to approximately 1400 tons of material spread over a 2.5-mile diameter area. The floc was allowed to settle for 1 hour. A 5-ml. sample withdrawn from the center of the column was compared with a 5-ml. aliquot of contaminated sea
I
MINORU HONMA and A. E. GREENDALE
U. S. Naval Radiological Defense Laboratory, Son Francisco 24, Calif.
water to determine the percentage decontamination. 'The more promising decontaminating materials were next tested in large columns (8.5-cm. diameter and 120 cm. high). Six liters of natural sea water were mixed with enough individual fission product isotope or M F P solution to give approximately 10,000 gamma c.p.m. for a 5-ml. aliquot. The floc used in the large column was prepared by mixing 0.55 gram of the material (also corresponding to approximately 1400 tons spread over a 2.5-mile diameter area) or combination of materials with 60 ml. of sea water. T h e floc was placed on top of the column over a screen baffle and allowed to settle for 1 hour before 5-ml. samples were taken for gamma counting. Percentage decontamination was again calculated by comparing with the count on a 5-ml. aliquot of untreated contaminated sea water. This technique was adopted as the standard procedure for evaluating decontaminating material. Relative decontaminating efficiencies of differing weights of a sodium orthosilicate aliquot were compared using the standard procedure for decontaminating material evaluatian. Settling rate was followed on silicate flocs by noting the time at which the floc first reached the bottom of a +foot column. Settling time for flocs with particles around to 0.25 inch in diameter was about 5 minutes per 4 feet. In determining the capacity of selected decontaminants, 0.0845 gram of single material or combination of materials was stirred with 283 ml. of radioactive natural sea water. Six-milliliter portions were removed at designated times, centrifuged, and 5 ml. of the supernate were counted. Percentage of activity held by the decontaminating material was determined by comparing the activity of the centrifuged supernate with that of an equal volume of untreated contaminated sea water. Sampling was continued for a maximum of 24 hours,
Results 'Two general surveys were made. The first covered single materials and
the second an equal weight mixture of two materials. I n one case, a six-component system was tested. A number of flocs did not settle completely after 1 hour, so two samples were taken 1 hour after addition of the floc and onc: of these was centrifuged before counting to remove any suspended material. In choosing materials for further testing the greatest importance was given to decontamination by settling alone, as in ocean decontamination a nonsettling material would be of limited value. Other considerations were decontamination capacity, cost of materials, rate of floc formation, floc density, and floc solubility. Results are shown in Table I. The most promising decontaminating materials for MFP-I in sea water were tested with some important gammaemitting fission-product elements to learn how efficient each material or combination of materials was in removing a n individual element from a sea Tvater solution. Some of these were also tested with both contaminants, MFP-I and MFP-11. Table I represents a static situation, that is, the floc was allowed to settle under the force of gravity. To arrive at a decontamination value which would be an optimum for a given material, measurements were made after 1 hour of continuous stirring of 0.0845 gram of material in 283 ml. of sea water contaminated with MFP-I (Table 11).
Discussion The first half of Table I (decontaniination by a single material) shows the superior decontaminating characteristics of sodium silicate. Except for decontamination of the isotopes cesium-137 and iodine-131, it performed better than any of the materials tested. Cement gave the best decontamination of cesium137 (llyc), and iron oxide was the best decontaminant for iodine-131 (135%). For mixed fission-product solutions, sodium silicate was the best decontaminant. One consideration was that a given weight of a mixture of two materials might afford better decontamination VOL. 51, NO. 5
M A Y 1959
697
than the same weight of one material. This premise was tested in the large column under the same experimental conditions. Results are shown in the second half of Table I. It'hen iron oxide was mixed with magnesium oxide, coral, or clay, there was 60 to 80% reduction in the decontaminating efficiency of the iron oxides for iodine-131. I n decontaminating hIFP, binary mixtures with sodium silicate gave decontamination values close to an average for the two components used individually. Thus, no mixture performed as well as sodium silicate
alone. Mixtures of various materials with sodium phosphate, iron oxide, or magnesium oxide gave poorer decontamination of MFP than any of these alone. Iron oxide lost its capability of decontaminating a sea water solution of iodine-131 when sodium silicate was added to the floc. Clays and coral did not perform well in decontaminating cesium-137 or iodine-131. It'ith the decontaminants listed in Table I an attempt was made to remove molybdenum-99 and strontium-85 from sea water. Inactive isotopic carriers were associated with both of the radio-
~~
I.
Table
~~~~~
Selected Decontaminants with Various Contaminants in Sea W a t e Static System
Decontamination of Carrier-Free Contaminant, 70" Ba14O-
Material C, Kb
Clay, montmorillonite, Little Rock, Ark. Clay, Type 140 Coral, Bikini Cement Fez03 MEO Magnesium trisilicate Na3P01 NarSiOd Na2S NarSiO, C, KB
+
Clay, Little Rock Clay, Type 140 Coral, Bikini Fez03 MgO NaaPOd Fez03 Na2S Fez03 Clay, Little Rock Coral, Bikini MgO
+
+
MgO
+
C, KB Magnesium trisilicate
La140
Zr95-
Cs137
Cel44
I131
11
0.2
11
2.5
4.7 7.8 13 5.3 10
0.3
8.2 14 10 11
0
0 0 11
0.7 0.4 3.6 13 1.0 1.6
Nbg5
hPFP-I
6.6 6.7 5.4 5.3 7.5 2.0 6.0 3.7 15 35
7.2 7.2
MFP-I1 13
13 11 15 19 28 3.4
6.0 6.3 5.9 9.7 12 5.4 12 16 22 6.3
18 17 19 18 23 22
17 17 16 18 20 22
5.7 14
13
9.5
3.6 1.6
4.5
11
5.0
1.3
6.5 18 26 1.7
0 0.4 0.4 0.5
7.2 7.8 16 31 7.2
21 21 19 21 20 23
1.2 1.2 0.8 0
26 26 25 25 25 32
15 16
2.3 0.4
16 24
3.3 8.9 10
2.1 0 0.3
1.7 2.6 3.2
2.2 2.6 5.1
2.4 0.7 6.2
6.9 6.0 8.5
10 9.5
6.0 6.5
2.2 2.5
2.6 3.9
1.4 1.2
2.7 3.8
5.8 4.7
8.6 13
0.1 1.0
1.0
7.5 4.7 4.1 0.3
0
0.7 0 1.8
17 17 20 21 24 26
1.7 2.7
13
0
8.5
8.9 7.1
Average of five aliquots from each large column
Single Decontaminants Na4SiOa
Table II. Capacity of Decontaminants" DecontamiCombination nation, 70 Decontaminants
+ coral, Bikini + MgO + clay, montmorillo-
92 91 89
Cement Magnesium trisilicate Coral, Bikini Clay, montmorillonite, Little Rock, Ark.
Na4SiOi NaaSiOa NaiSiO4 nite
88 73 65
MgO C, KB Na3P04 Fez03
Bentonite
70
MgO
NaaPOI
53
+
+ NarSiO4 + Fen03 Na4SiO4 + C NarSiO4 + clay, Type 140 Fez08 + clay, Type 140 FenOs+ coral, Bikini Fen03 + MgO Fer03 + clay, montmorillonite MgO + Magnesium trisilicate
1 hour, MFP-I in sea water, dynamic system.
698
INDUSTRIAL AND ENGINEERING CHEMISTRY
nuclides. Only negligible decontaminations were obtained. The low decontamination (0.5 to 3.3y0 for molybdenum99 and 1 to 7% for strontium-85) is attributed to the presence of the isotopic carrier. Moreover, about 7 p.p.m. of strontium is found in natural sea water, which is also a contributing factor in the case of strontium-85. With MFP-11, a carbonate decontaminant was tested. Initially, a fair decontamination was obtained, but after a few minutes the floc dissolved in the sea water, and the net decontamination was negligible. Hence, solubility of the floc in sea water must be considered in selecting a suitable coagulant. Settling characteristics of the floc also have an effect on the decontamination. A sodium orthosilicate floc prepared with distilled water floats on the surface of sea water and does not settle appreciably, whereas the same material prepared in sea water shows excellent settling qualities. The density difference between the distilled water and the sea water interfered in this case.
Conclusions One of the most serious limitations in scaling data from experimental results up to large volumes is the unknown effect of depth. If the floc can be followed to a depth of a t least 50 feet a more reliable extrapolation can be made to greater depths. Keeping this limitation in mind, the most effective concentration for sodium orthosilicate would be about 1.8 grams for the large column (57 sq. cm.) or about 4500 tons for the 2.5-mile diameter area under consideration. While decontamination of a little better than 70y0 is somewhat less than hoped for, radioactive sea water decontamination might be feasible. Before field evaluations of ocean decontamination are attempted, studies should be made on solution rate of the floc in sea water, the desorption rate of activity from the floc, floc settling rate, rate of transport of floc from one place to another in the sea, as well as on the effect of depth on decontamination.
Decontamination, yo 90 90
89 89 88 84 83 73 71 66 82 57 80
literature Cited (1) Burbank, N. C., Lauderdale, R. A., Eliassen, R., Mass. Inst. Technol., NYO 4440 (Sept. 1 , 1955). (2) Carritt, D. E., Goodgal, S. H., Johns Hopkins Univ., NYO 4591 (October 1953). (3) Lauderdale, R. A., Oak Ridge Natl. Lab. ORNL 932 (Jan. 23, 1951). (4) Lauderdale, R. A., Eliassen, R., Mass. Inst. Technol., NYO 4441 (March 1, 1956). RECEIVED for review June 30, 1958 ACCEPTED November 28, 1958 Division of Water, Sewage, and Sanitation Chemistry, 133rd Meeting, ACS, San Francisco, Calif., April 1958.
