JOHN ROSINSKI and C. T. NAGAMOTO Armour Research Foundation of Illinois Institute of Technology, Chicago 16, 111.
Scavenging Radionuclides in Substitute Ocean Water The combination of a permanganate and ferrous salt is potentially useful for precipitating radioactive fission and corrosion products which are released into the sea
IN
MOST EXPERIMENTAL work done on removing radioactive material from water (2, 6, 8) the time of contact between contaminants and coagulants or the time of settling in still water is often a matter of hours. However, this time factor would be completely different if radioactive fission and corrosion products are released into open sea water. The released material would be dispersed by diffusion and sedimentation, but regions of high concentration would occur which would have to be treated. The radionuclides released would be in the form of ions, colloids, and particulate matter. Therefore the scavenger used should adsorb anions and cations and also coagulate colloids and particulate matter. The flocs formed should settle as rapidly as possible under turbulent conditions. Some hydrated metallic oxides have the desired adsorptive properties-e.g., the amorphous gel obtained by hydrolyzing aluminum alcoholates. Different forms of aluminum hydroxide show specific differences in adsorption and ion exchange properties, and also effect of their particle size on surface activity must be taken into account (4, 5, 7). They must be prepared in an exceedingly fine state to participate in heterogeneous coagulation. This can be done by rapid hydrolysis or by rapid chemical reaction which results in the formation of a solid phase. Ferric hydroxide shows similar properties ( 3 ) . Ferric oxide hydrate, freshly precipitated from ferric salt solution, is amorphous and its x-ray diffraction pattern shows no interference. During precipitation it has a strong tendency to carry down with it other substances. In determining adsorptive power, it is essential that the crystalline modification of the compound be known. The differences in energy exhibited by one crystalline modification in its different activity states may be greater than such differences among several crystalline modifications in their normal state. In the work described here, aluminum and ferric alcoholates were selected as one-component systems, and ferric oxide and manganese dioxide hydrates, which were formed by oxidation-reduction of a ferrous salt and permanganate in alkaline
solution, were used as a two-component system. The scavenging system formed by the oxidation-reduction reaction between potassium permanganate and ferrous sulfate removed most suspended radioactive isotopes successfully. Scavenging efficiency depends on the age of the precipitate and is highest for fresh hydroxides. The larger flocs and better settling observed in the dynamic tests indicate the permanganate-ferrous salt system might be used successfully in the field. The mqin difference between the experiments in the laboratory and actual
Table I.
conditions in sea currents is in the concentration of radioactive hydrosols. Concentrations in the laboratory are constant, while those in sea currents continuously change because of diffusion. In the sea, the radioactive hydrosol and the scavenging medium will move with the current and diffuse simultaneously. The number of collisions between particles will diminish because of the continuous dilution of particulate matter. I t is therefore important to enhance flocculation so that all radioactive materials will be removed from the stream in places where high concentrations exist.
Aluminum and Ferric Hydroxides Remove Gamma Radioactivity from Mixed Fission Products in Substitute Ocean Water Conc., P.P.M.
As Coagulant
Coagulant
Alum
Aluminum 8ecbutoxide [in CClal
Oxide
Stirring, Min.
Settling, Min.
Remov a 1 of Gamma, Activity,
1
0 2 35 1200
0 47.3 55.6 64.7
1
0 2 35 1200
0 35.0 66.0 80.0
1
0 2 35 1200
8.8 9.8 30.5 88.0
5
3
7 17 1560
23.9 31.0 74.3
284
1
0 2 35 1200
11.8 11.8 24.2 85.4
None Flocs 11 1
1 5
5
Separan
5
10
1 5 300 1740 5
5.6 53.5 77.0 79.6 35.7
5 5 5 5 240 990
1.1 7.9 11.1 38.9 53.5 66.3
5 15 28 67
34.6 33.2 41.6 73.4
Additive Type P.p.m.
465
50
None
465
50
Flocs I1 1
242
50
None
242
50
Flocs 111
242
50
NazSO4
930 930
100
930
100
100
5
2610
Ferric ethoxide [in CXHSOH]
11 22 43 76
5 9 18 32
None None None Flocs 11 1
5
3 3 3 3
108
46
Flocs 11 1
5
5
VOL. 52, NO. 5
MAY 1960
429
Experimental Procedures Substitute ocean water with heavy metals was used in all experiments ( I ) . A solution of mixed fission products in nitric acid was added to substitute ocean water and the pH was adjusted to 8.2 with sodium hydroxide. Gamma and beta counts per unit time per milliliter were determined by using gamma and beta well-type Baird atomic scintillation detectors, both connected to Packard scalers and a print-out system. The length of counting was varied according to the concentration to minimize error in the net counting rate (a 95% confidence level was attained). The initial concentration of radionuclides in sea water ranged from 3 X lo3 to 2 X lo8 counts per minute (c.p.m.) per milliliter.
The mixed fission products were obtained from the Oak Ridge National Laboratory, Oak Ridge, Tenn. They are the waste from iodine-131 dissolver solution. This waste is from normal uranium which was irradiated for approximately 60 days in the Oak Ridge National Laboratory graphite reactor. Approximately 4397, of the beta activity of these radioactive materials was as follows : % Cerium- 141 Barium-140 Strontium-89 Yttrium-91 Praseodymium-143 Neodymium-147
21.0 10.0 6.0 4.2 1.5 0.3
The remaining 57y0 consisted of relatively short half-life activities. The
Table II. A Manganese Dioxide Hydrate-Ferric Hydroxide System Removes Radioactivity from Mixed Fission Products in Substitute Ocean Water Reactants Permanganate Type P.p.m. Ca(MnO4)z
25
50
pesoa,
p.p.m. 82
164
StirAdditive Type P.p.m. None
ring, Min.
Settling, Min. 5 1440 5 1440
71.9 96.5 66.2 96.3
64.6 88.2
5 1440 5 20 1440
94.8 95.7 96.1 93.5 97.2 97.4
88.9 89.6 89. 7 85.4 86.5 86.7
5
8 20 1440
94.4 94.9 95.5
87.4 88.1 88.9
50
8 20 1440
94.5 95.4 95.9
88.5 89.3 89.5
3 180 300 1200 3 180 300 1200
50.2 50.3 57.1 86.4 53.1 75.3 79.0 86.6
38.1 46.1 55.3 80.8 54.4 71.4 80.2 79.4
240 5
2400 5 20 1200
90.6 86.6 90.0 95.0
84.4 87.4 90.0
1
None
1
20
KMnO4
430
50
164
AgN03
50
164
Pb[N03]2
Removal of Activity, % Gamma Beta 72.2
10
28.8
None
10 25
28.8 72
None None
25
72
Flocs 111
5
5
5 20 1200
90.9 90.2 94.7
86.3 85.5 88.0
25
72
Flocs111
10
5
5 20 1200
88.1 88.9 90.8
84.6 85.9 86.9
25
72
Separan 2610
5
5
5
20 1200
91.1 92.3 90.9
87.2 87.8 86.6
Separan 2610
10
5
5 20 1200
90.5 92.1 93.5
86.1 88.1 89.2
5
5 20 180 1440 5 20 180 1440
90.9 93.8 94.5 95.4 89.4 89.9 93.9 95.0
76.4 80.7 82.1 82.0 88.0 87.3 90.9 91.1
25
72
50
144
None
INDUSTRIAL AND ENGINEERING CHEMISTRY
products contained only a trace o l ruthenium-103 and zirconium-95, because most of these were removed by solids extraction. Barium-140 was not necessarily in equilibrium with its radioactive daughter, lanthanum-140. The beta-to-gamma ratio was about 0.6. Mixed fission products form both solutions and colloidal suspensions in substitute sea water, but in our experiments the hydrosol was not analyzed for the physical state of specific radionuclides, whether ionic or colloidal. Sufficient turbulent motion was used to ensure a uniform dispersion. Experiments were performed under both static and dynamic conditions. For the former, tall, 1000-ml. beakers without spouts containing suspensions of radioactive material in substitute ocean water and equipped with magnetic stirrers were used. Solutions of scavenging agent were added, and the suspensions were stirred for short periods of time and then allowed to settle. Centrifuging was not used to separate the solid phase. Only changes in gross gamma and beta radioactivity were determined. For dynamic conditions, 65 liters of substitute ocean water containing mixed fission products and scavenger was kept in constant motion in a rectangular annulus by means of suspended paddles moving at constant speed. The three speeds available were 3.87, 11.6, and 34.8 cm. per second. The vertical velocity profile for each speed was determined, but sizes of eddies and degrees of turbulence were not. Changes in radioactive concentration were determined for the upper water layer. There is a great deal of difference between the physical and chemical properties of iodine dissolver waste solution and those of actual radioactive contaminants which may be released into the sea from a nuclear reactor. Therefore, experiments with specific radionuclides were performed to obtain better evaluation of the scavenging system. Fission products accumulated in the reactor consist mainly of the following radionuclides, listed in order of the amount by weight formed after a certain period of operation: cesium-137, strontium-90 and -89, cerium-144, niobium-95, zirconium95, yttrium-91, ruthenium-103 and -106, promethium-147, barium-140, lanthanum-140, praseodymium-143, and samarium-l 5 l , Among the corrosion products formed are the following, which were used in this investigation: tantalum-182, cobalt-60, iron-55 and -59. chromium-51, and hafnium-181.
Experimental Results and Discussion One-Component Scavenging Systems. Solutions of alum. of aluminum sec-butoxide in benzene, acetone or carbon tetrachloride, and of ferric ethoxide in ethyl alcohol were used as the
SCAVENGING RADIONUCLIDES one-component systems. Aluminum secbutoxide was chosen because sec-butanol is soluble in water a t the concentrations used. Two flocculating agents, Flocs 111(Hodag Chemical Corp.) and Separan 2610 (Dow Chemical Co.), were used in some experiments, and in one case the molar equivalent of sulfate ions was added to the aluminum alcoholate to give a better basis for comparison with alum. A predetermined amount of sodium hydroxide was added to the solutions of scavenging agent to maintain the desired alkalinity. Flocculation and subsequent removal of mixed fission products with alum, a method which is successful in still waters, did not work in a constantly moving liquid (Table I). In experiments with aluminum hydroxide formed from alum and with that formed from aluminum alcoholate, there was a marked difference in the final product. During hydrolysis of the alum, a gellike aluminum hydroxide (at first probably a hydrogel of amorphous aluminum oxide which later changed into the crystalline hydroxide) was formed, which could be kept in turbulent suspension for long periods of time. During hydrolysis of the alcoholate, on the other hand, an amorphous, strongly surface-active gel was formed as flakes and very small, discrete particles. Some of the particles agglomerated immediately to form flakes which settled at a high rate, giving approximately 9% removal of gamma activity under dynamic conditions; no removal was found with alum under the same conditions and for the same period of time. The role of the alcohol formed in the hydrolysis and present in the solution is not known and was not investigated. Two-Component Scavenging System. The scavenging system consisting of permanganate and a ferrous salt undergoes a rapid reaction in a solution of mixed fission products, and the manganese dioxide hydrate produced coprecipitates with the ferric hydroxide which is formed simultaneously by the oxidation of the ferrous salt. The ferrous salt used throughout this work was ferrous sulfate. Calcium permanganate was used in the preliminary experiments to increase the concentration of alkaline earth group cations and possibly to enhance removal of mainly strontium-89 and barium-140 on calcium carbonate nuclei by formation of substitutional solid solutions. Potassium permanganate was used later. After 24 hours of settling, successful removal of gamma and beta activity was obtained with 25 p.p.m. of calcium permanganate and 82 p.p.m. of ferrous sulfate (Table 11). The same results were attained in only 5 minutes with twice as much of each reagent. In the former experiments, the immediate for-
mation of a fine hydrosol which did not agglomerate rapidly in still water was observed visually and probably accounts for the slower action caused by low settling velocity.
Table
To utilize anions present in sea water as scavengers or additives to enhance settling, silver or lead nitrate was added to form heavy, insoluble silver chloride or a mixture of lead sulfate and basic
111. Manganese Dioxide Hydrate-Ferric Hydroxide System Removes Radioactivity from Specific Radionuclides in Substitute Ocean Water
Radionuclide CS-134
Reactants, P.P.M. KMnO4 FeSOa 10
28.8
50 10
144 288 28.8
Ce-Pr-144
50 100 10
144 288 28.8
Zr-Nb-95
50 100 10
144 288 28.8
50 100
10
144 288 28.8
50
144
100
288
100 Sr-89
Y-91
Ru-103
10
28.8
Pm-147
50 100 10
144 288 28.8
Ba-La-140
50 100 10
144 288 28.8
50
144 288 28.8
100 Ta-182
10 50
100 Cp-60
10
144 288 28.8
Fe-55-59
50 100 10
144 288 28.8
Cr-51
50 100 10
144 288 28.8
50 100
144 288 28.8
Hf-181
10 50 100
144 288
pH Range Initial Final 7.0 8.2 10.0 8.2 8.2 7.0 8.2 10.0 8.2 8.2 7.0 8.2 10.0 8.2 8.2 7.0 8.2 10.0 8.2 8.2 7.0 8.2 10.0 8.2 8.2 8.2 8.2 7.0 8.2 10.0 8.2 8.2 7.0 8.2 10.0 8.2 8.2 7.0 8.2 10.0 8.2 8.2 7.0 8.3 10.0 8.3 8.3 7.0 8.2 10.0 8.2 8.2 7.0 8.4 10.0 8.4 8.4 7.0 8.4 10.0 8.4 8.4 7.0 8.2 10.0 8.2 8.2
6.9 8.0 9.5 7.8 7.4 6.6 7.6 9.7 7.8 7.9 6.8 7.7 9.5 7.8 8.0 6.9 7.8 9.7 7.8 8.0 7.5 8.5 9.4 7.2 7.2 4.4 7.5 7.0 8.0 10.0 7.8 8.0 7.2 8.5 9.7 8.5 7.8 7.0 8.2 9.7 7.8 7.5 6.9 8.2 9.7 8.0 8.3 6.9 8.1 9.8 8.2 8.2 7.2 7.8 9.8 7.1 6.9 7.0 8.3 9.7 8.2 8.0 6.5 7.5 9.6 7.3 8.2
Removal of Activity after Settling, % ’ 2.5 hr. 5 hr. 24 hr. 1.8 1.2 0.0 0.3 0.8 0.4 17.2 16.1 15.9 20.0 52.9 36.7 28.1 90.0 97.7 85.2 21.8 77.2 97.2 98.8 55.2 68.7 85.9 85.5 98.5 30.5 99.2 85.4 82.5 88.7 93.0 92.2 99.3 99.4 99.4 99.7 99.7 39.7 54.5 49.1 80.9 82.9 88.5 91.1 97.4 97.9 99.1 0.1 22.7 99.8 60.0 81.6 85.0 94.2 98.7 93.7 94.8
1.9 0.5 0.0 0.0 0.0 5.9 18.5 17.2 16.2 20.1 62.7 62.2 63.6 92.6 98.4 85.9 72.4 83.4 98.0 99.1 73.3 80.4 91.8 90.4
94.9 97.2 96.7 98.8 98.7
95.2 97.5 99.5 99.3 99.4
VOL. 52, NO. 5
27.8 91.2 89.8 92.7 95.2 95.9 99.5 99.7 99.9 99.8 99.8 46.0 57.7 57.6 81.5 83.4 90.9 96.9 98.2 99.3 99.7 1.3 26.2 99.8 60.7 82.6 89.2 98.5 99.4 95.3 95.7
M A Y 1960
0.4 0.8 0.0 0.0 0.0 7.5 18.4 18.3 14.6 19.5 86.5 89.3 92.5 98.3 99.6 93.9 95.4 93.4 99.3 99.1 96.3 98.5 98.9 96.2 99.5 23.4 99.5 95.2 92.6 94.7 96.3 96.2 99.8 99.5 99.9 99.6 99.9 52.8 61.0 60.1 76.7 80.9 97.0 97.6 98.4 99.1 99.8 34.9 99.9 63.8 83.4 94.2 98.4 99.5 97.6 98.3 98.9 88.6 96.4 99.4 97.1 95.5 97.7 97.2 98.7 98.8
431
100
60I
I
I
I 0.1
I
I AGING TIME,HOURS
I IO
100
Age of the manganese dioxide hydrate-ferric hydroxide system influences removal of gamma radioactivity from mixed fission products in substitute ocean water 0 20 hours 0 3-4 minutes A 5-10 minutes
carbonate. However, these precipitates did not contribute either to the rate of settling or to the scavenging of radioactive materials by manganese-ferric oxide hydrates (Table 11). When either 50 or 25 p.p.m. of potassium permanganate was used with the ferrous sulfate, approximately 95% of the gamma emitters were removed (Table 11). These results compare to those obtained with calcium permanganate. With only 10 p.p.m. of potassium permanganate, percentage removal was less. The use of components of the system singly-i. e., ferric hydroxide formed directly from ferric sulfate and manganese dioxide hydrate precipitated from an alkaline solution of permanganate by hydrogen peroxide-was less effective in scavenging and much slower in settling. Therefore such a procedure was not applicable in this case. Of the specific radionuclides studied, only cesium and strontium could not be removed by the potassium permanganate-ferrous sulfate system (Table 111). This was foreseen a t the beginning of this study; however, strontium was scavenged up to 2070. An entirely different method based on the ion exchange properties of certain chemical compounds is being developed for these elements. I n nearly all the experiments with scavenging systems, the percentage of mixed fission products and specific radionuclides removed depended only slightly upon the scavenger concentration but the rate of removal was proportional to it. When the potassium permanganate-ferrous sulfate system was tested with various additives, no appreciable improvement was observed. A low permanganate concentration (10 p.p.m.) fulfills the requirement for an adsorptive
432
surface for scavenging and higher concentrations (25 to 50 p.p.m.) are required to secure more complete distribution of the hydrosol during its formation and coagulation. The question of using mixed hydroxides precipitated ahead of time and dispersed in the contaminated water at the necessary time directed research toward a study of scavenging efficiency as a function of the age of the precipitate. Hydroxides from the reaction of potassium permanganate and ferrous sulfate were formed in substitute ocean water and the suspensions were added after a predetermined aging time to a solution of mixed fission products. Scavenging decreased with age of the precipitate until approximately 4 hours where it leveled off. I t increased at 25 hours to nearly the value obtained with fresh hydroxides and then it started to drop slowly. The following explanation is suggested for the characteristic peak on the curve found for different settling times. The manganese dioxide-ferric hydroxide mixture is amorphous at the time of precipitation and therefore has high surface activity. In time, a certain amount of rearrangement of particles and of molecules within particles causes a decrease in adsorption efficiency. Later the hydrated oxides start to crystallize into one of the crystalline modifications, and during this change the surface is capable of adsorbing ions very efficiently. In the experiments under dynamic conditions performed in the rectangular annulus, there was a slight variation in experimental procedure. When the coagulant and suspension of mixed fission products had been mixed for a certain period of time, a dynamic equilibrium was established in which a certain
INDUSTRIAL AND ENGINEERING CHEMISTRY
amount of usually small flakes were kept in suspension. At this time Flocs 111 was added and movement was continued. This procedure was adopted to obtain preliminary information on the flocculating action of Flocs 111 under these conditions. It was found that due to improved settling an additional 1Oy0of solids were removed from the suspension in the upper layer. Increase in the size of the flocs and subsequently better settling was observed in all the dynamic tests. Formation of larger flocs during coagulation is influenced by movement in the sol. Collisions caused by movement of the liquid plus those caused by Brownian motion accelerate coagulation. When the movement stopped and the flocs had settled, the liquid was analyzed for the amount of radioactivity removed. Settling of flocs was quite rapid (up to 20 cm. per minute), which indicates that in actual sea currents the separation would be good provided that microturbulence associated with a given sea current was insignificant. The median settling velocity of particles increased with potassium permanganate concentration only up to a certain point-i.e., approximately 50 p.p.m. A t 100 p.p.m., the polydispersed system became more homogeneous. This was observed with both ferrous chloride and ferrous sulfate. With 10 p.p.m. of potassium permanganate, single flakes twice the size (up to 1 cm. in diameter) of those observed with 50 p.p.m. were noted. However under field conditions the optimum over-all performance was around 50 p.p.m. of potassium permanganate. Acknowledgment
The authors wish to thank M. A. Fisher and C. R. McCully for many valuable hours of discussion. literature Cited (1) Am. Soc. Testing Materials, Philadelphia, Pa., D 1141-52. (2) Burbank, N. C., Jr., Lauderdale, R. A., Eliassen, R., E. S . At. Energy Comm., NYO-4440 (Sept. 1,1955). (3) Fricke, R., Klenk, L., Z. Elektrochem. 41, 617-22 (1935). ( 4 ) Fricke, R., Rennenkampf, E. V., Naturwassenschaften 24, 762 (1936). ( 5 ) Huttig, G. F., Peter, A,, Kolloid 2.54, 140-7 (1931). ( 6 ) Joint Committee on Atomic Energy, 85th Congr., Pt. 2, “Removal of Radioactive Fallout from Contaminated Water Supplies,” June 1957. (7) Moscou, L., van der Vlies, G. S., Kolloid Z. 163, No. 1 (March 1959). (8) Oak Ridge National Laboratory, Joint Program of Studies on Decontamination of Radioactive Waters, ORNL-2557; “Radioactive Waste,” TID-4500, 14th ed. (Feb. 9, 1959). RECEIVED for review May 7, 1959 ACCEPTEDJanuary 28, 1960 Work sponsored by U. S. -4tomic Energy Commission under Contract No. AT( 11-1)586.
