Decontamination of Radioactivelv Contaminated Water by Slurrying with Clay J
J
WILLIAM J. LACY Engineering Research and Development Laboratories, Fort Belvoir, Va.
I
The clay in the Oak Ridge area is composed principally of montmorillonite [(AI or hfg)(8i80,0)(OH)41 XH,O)] and kaolinite [A14(Si4010)(OH)3 and Akl(SirO~)(OH)l~j. The clay used in this test was analyzed by the Geochemistry and Petrology Branch, Geological Survey, U. S,Department of Interior, and found to be the montmorillonite type. The base exchange capacity of this clay was 29 meq. of exchangeable cations per 100 grams dry weight of clay (105’ (2.).
X T H E event of an atomic disaster, water supplies may become contaminated with radioactive materials. The level of radioactivity to be expected is dependent upon many conditions, including the type of bomb (atomic, radiological, or hydrogen), the type of burst (air, surface, underground, underwater), the kind of water (ai. pertaining to induced activity), and atmospheric conditions. The air burst is the most likely use of the bomb for which the contamination of water would be a t a low level (probably less than 10-2 microcurie per ml.). However, even with an air burst, extenuating circumstances such as atmoepheric precipitation (rain or snow) could give rise to considerable contamination. Therefore, all nuclear weapons must be regarded as potentially capable of contaminating water supplies. The dissolved or suspended radioactive material in water could be a source of alpha, beta, and/or gamma radiation. Ingestion of large amounts of radioactive material could cause physiological damage. Morgan and Straub ( 7 ) have presented a fornul la for estimating the emergency maximum permissible concentration (MPC) values of radioactive contamination in air and water following a nuclear explosion. They estimated that the emergency value of maximum permissible concentration for the radioactive fission products in microcuries per milliliter is given approximately by the equation
MPC =
RADIOISOTOPES
Oak Ridge tap water was used in all tests. A chemical analysis of a “grab” sample of this water is given in Tahle I.
98 2 94 110
The following radioactive materials were used as contaminants: ruthenium-106-rhodium-106; strontium-90-yttrium-90; zirconium-95-niobium-95; cerium-141, -144-praseodymium-144; iodine-131; barium-140-lanthanum-140; and four fission product mixtures known as NFP-1) MFP-2, MFP-3, and MFP-4. The ruthenium-106-rhodium-106, strontium-90-yttrium-90, cerium-141, -144-praseodymium-144, and barium-140-lanthanum140 were obtained as the chlorides in hydrochloric acid solution with a radiochemical purity greater than 95%. Zirconium-95niobium-95 was obtained as the oxalate complex in oxalic acid solution. Radioiodine131 was obtained as the iodide in rreak basic sodium sulfite solution having a radiochemical purity greater than 99%. MFP-1 mas a mixed fission product contaminant consisting of 44y0 trivalent rare earths, 27% cerium, 17y0strontium, 5y0 barium, 3% ruthenium, 1% cesium, and 3% traces of a large number of other radioisotopes. MFP-2 was a mixed fission product contaminant consisting of 50% cesium, 16% ruthenium, 10% trivalent rare earths, 10% strontium, 5% cerium, 5% barium, and 4% traces of a large number of other radioisotopes. RIFP-3 was a fission product mixture composed of 20% rare earths, 20% niobium, 15% zirconium, 13% yttrium, 12% ruthenium-rhodium, 12% strontium, and 8% traces of a large number of other radioactive fission fragments. RIFP-4 was a fission product mixture consisting of 30% cerium-144-praseodymium144, 22% promethium-147, 22% strontium-90-yttrium-90, 18% cesium-137-barium-137, 6% ruthenium-106-rhodium-106, and 2% traces of a large number of other radioisotopes. The fission product mixtures were nitrates in strong nitric acid solution. All the radioactive contaminants were obtained from the Operations Division of the Oak Ridge National Laboratory ( 1 ) . A stock solution was made by dissolving the radioactive material to be used in tap water. After mixing, the p H of this “spiked” solution was taken using a Beckman Model G glass electrode pH meter. Then 1-ml. initial samples of the solution were taken, placed in a stainless steel counting dish, dried under infrared lamps, and counted using a Geiger-Muller (G-RI) mica end-window tube (1.8 mg. per sq. em. thick), filled -n-ith helium plus alcohol vapor. This tube was connected to a 64 scaler. The counting results were corrected for background and coincidence loss. Difference between the initial and final count rate represented the removal. The initial concentration of radioactivity in the spiked solution was in the range of 5 X 10-3 to 5 x 10-2 pc. per ml. (assuming 10% counting efficiency).
25 5 6
CONCENTRATION OF CLAY
Kt-1.2
foi 30 minutes to 3 years following the explosion. If time is given in days, K = for drinking water contaminated with any material emitting alpha, beta, or gamma radiation. Many adsorbents have been used for decontaminating water, with varying degrees of efficiency. One such adsorbent of interest because of its effectiveness and low cost is clay. The removal of radioactive contaminants from water by clay has been reported by Straub, Morton, and Placak (8). This report presents jar test data pertaining to the decontamination of radioactively contaminated water by the use of clay indigenous to the Oak Ridge, Tenn., area, as well as the effect on removal of radioactive material of varying the concentration of clay, hydrogen ions, radioactive contaminants, and calcium ions. Also studied was the ease of removal of different nuclides and mixtures of various fission products.
~ v A L Y S I ?O F G R a B TABLE I. CHEMICAL TAPWATER
Chemical Constituent Methyl orange alkalinity (as CaCOd Phenolphthalein alkalinity (as CaCOd Soap hardness (as CaCOa) Dissolved solids Nonvolatile solids Calcium Magnesium Sodium Silicon dioxide PH Except pH
SAMPLE O F O A K
RIDGE
Concentration. P.P.M.a
75
7 7.9
Five hundred milliliters of this stock solution were then added to each of four 1-liter beakers containing quantities of the clay to give concentrations of 1000, 2000, 3000, and 4000 p.p.m. of clay 1061
1062
INDUSTRIAL AND ENGINEERING CHEMISTRY
A% stock solution was made by dissolving the radioactive material to be used (MFP-3) in distilled water. After mixing, 500 ml. of this spiked solution x a s added to each 1-liter beaker containing enough calcium hydroxide or calcium chloride t o give the desired concentration of calcium ions. After the calcium hydroxide or calcium chloride had been dissolved, the pH of t,he solution was adjusted t o the range 7.0 t o 7.9) using either hydrochloric acid 01' sodium hydroxide. Initial samples were then taken for counting and 0.5 gramof clay was adtled t o each beaker. The mist>urex a s slurried for 90 minutes at R, constant speed (250 r.p.m.). Samples xere taken every 15 minutes and filtered, and an aliquot portion was placed in a counting dieh, dried, and counted. By this procedure it %-as pofisible to evaluate the effect of calcium ion concentration on removal of mixed fission products. I n order to ascertain tho effectsof various chemical forms of calcium, both calcium hydroxide and calcium chloride u-ere used.
Figure 1. Jar Test Equipment for Decontamination of Radioactively Contaminated Water by Slurrying with Clay
(Figure 1). The slurry was stirred a t a constant, speed of approximately 250 r.p.m. for 90 minutes. Samples were taken from each beaker every 15 minutes and filtered through filter paper. An aliquot portion of the filtrate vias placed in a counting dish. dried, and counted, using t,he same Geiger-LIuller tube and scaler used for counting the stock solution. By this procedure, it was possible to evaluate the efficiency of the clay at variable concentrations and for different contact times. I n order to ascertain what, if any, portion of the radioactive material was removed by adsorption on the filt,er paper alone, a duplicate sample mas taken a t 90 minutes for each of the tests using 1000 p.p.m. clay. This sample was centrifuged and an aliquot part of the supernatant liquid placed in a counting dish, dried. and counted using the procedure described. pN EFFECT
I n an iiivest'igation of the eflect of hydrogen ion ooiiceiitration, MFP-3 was selected as the radioactive coiitamiriaiit. ii stock solution of the contaminant, in tap water \\-as prepared in the nianner described. Then 500 mi. of this solution were adtled to each glass 1-liter beaker. The pH was adjusted t o the desired hydrogen ion concentration upiiig a solution of either hydrochloric acid or sodium hydroxide. Enough clay was added to give a concentration of 1000 p.p.m. and the test procedure of stirring, sampling, and counting folloived.
Vol. 46, No. 5
RESULTS
Summarized data for the test' are given in Table 11. Detailed data for two tests on two of the more important contaminantP, zirconium-95-niobium-95 and MFP-1, are given in Table 111. The data on effect of pH on removal of mixed fisilion product,r; by clay slurry are given on Table IV. Table V reports the results of the investigation on the effect of calcium ion concentration on per cent removal of LIFP-3, whrvi 1000 p.p.m. of clay ivas used as the slurry agent. The p H values given in Tahle I1 are riot necessarily optimum for the particular radioact,ive materials reported. However, test results reported indicated that variations of 1 or 2 pH units on either side of the neutral point did not, afi'ect the efficiency of removal. The results obtained a t 90 minutes' cont,act time are plotted in Figure 2. The plot s h o w per cent removal versus clay dosage for the various radioactive contaminants.
D E c o x r A n m . I T I o y OF RADIOACTIVELY COSTAMIXATED m A T E R BY SLURRYING WITII CLAY
TABLE 11. COYCENTRATION O F ACTIVITY
In order to detect any effect the initial concentration of radioactivity may have on removal, experiments Tvere made a t t,hree levels of activity: (1) I o n (487 counts per minute per ml.), (2) moderate (4820 counts per minute per ml.)> and (3) high (45,000 counts per minute per ml.). These three concentrations of radioact,ive contaminants cover the expected range of Contamination immediately aft,er a bomb blast near a large wat,er supply ( 8 , 3). The best estimated concentration of radioactivity that can be expected in a large water supply due to induced activity, fall-out, and other factors is about pc. per ml. or 2220 counts per minute per ml. (assuming 10% counting efficiency). The tap Lvater, a t the activity level to be investigated, waE added to each beaker, the p H was t'aken, and the various concentrations of clay material were added. I n this particular investigation, t,he concentrations of clay used were 430, 900, 1800, and 2250 p.p.m. The radioactive contaminant used in this series of te& was fission product mixture hIFP-4. I n order to obtain comparable results, the foregoing t,est procedure was followed. CONCENTRATION O F CALCIUM IOXS
T o note t'he effect of calcium ion concentration on removal efficiency, an additional investigation was made. The test procedure varied only slight,ly from that used previously.
Contaminant
Initial iicti~ity,~ C./Min./ 311. pH
Clay Concentration, P.P.31. 1 O O O b 1000 2000 3000 4000 Per Cent Removal
Ru13B-R11106 Zr05.Nb55
1,380 5.2 50.9 50.5 59.3 61.5 12,100 7.5 97.9 98.0 99.1 99.4 5i.O 3,G70 7.7 8 3 . 3 83.4 89.1 92.9 95.0 I131 3,360 7.5 3.9 4.9 5.0 3.4 3.4 ce141,u4-pr144 99.7 99.8 99.9 9%9 4,150 8.0 99,6 Bal40-Lal40 3,340 7.8 87.8 88.8 92.0 9 4 . 3 97.1 M F P-1 10,900 8 . 8 8 2 . 2 8 2 . 0 82.9 88.3 90.3 MFP-2 2,110 9.0 68.7 70.0 70.9 72.8 73.3 MFP-3 3,290 7.7 79.2 79.0 82.1 83.6 84.9 a Uncorrected for counting efficiency (approximately 10%). All per cent removal figures based upon SO-minute filtered samples, except this column, which is f o r centrifuged samples. gr50.1-90
Contaminant
Initial Clay Activity." Concn., in./ P.P.M. hll.
15
Slurry Time, Winutee Per 30 Cent45Removal 60
1000 12,100 9 3 . 5 9 4 . 6 95.8 97.0 98.0 98.8 2000 1 2 , 0 0 0 9 6 . 7 97.9 3000 12,100 97.1 98.4 98.7 99.1 4000 12,000 9 8 . 5 9 8 . 8 99.2 99.4 MFP-1 1000 10,700 6 1 . 3 70.6 76.6 79.9 2000 10,900 64.7 73.7 78.5 81.4 3000 10,700 67.4 79.4 82.6 85.0 4000 11,300 7 1 . 6 7 0 . 9 87.4 89.1 a Uncorrected for counting efficiency (approximately 10%) ZrQS-NbBb
~~90
97.9 90.1 99.4 99.6 82.0 82.9 86.3 90.3
May 1954
INDUSTRIAL AND ENGINEERING CHEMISTRY
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removal decreases more slowly. I n the range of p H 5 to 11, the removal is essentially constant. Figure 4 is a plot of the per cent removal of MFP-4 obtained a t 90 minutes’ contact time and for three levels of radioactivity versus concentration of clay in parts per million. The results shown indicate that the initial concentration of a fission product mixture does affect the per cent removal, under test conditions, to some extent. The moderate concentration of activity yielded the highest per cent removal, while the highest concentration gave the lowest per cent removal. Figure 5 shows the effect of calcium concentration on the removal of fission product mixture MFP-3. The removals are
TABLE Iv. EFFECTOF pH ON REMOVAL O F hfIXED FISHOV PRODUCTS BY CLAYSLURRY (Radioactive contaminant M F P - 3 ; concentration of clay 1000 p p In ) Slurry Time, Minutes 60 75 90 BOa 15 30 45 PH Per Cent Removal I. 3
49.2 64.6 64.0 64.2 80.3
5
7
9 I1
Initial activity 2730 e./min./ml. 50.9 66.6 66.7 67.5 80.1
50.1 72.6 72.9 73.5 81.1
51.6 74.0 74.9 76.3 81.7
51.4 75.2 78.2 79.5 82.2
*
52.9 76.2 78.7 80.4 82.6
55.3 75.9 78.6 80.5 83.4
11. Initial activity 3400 c./min./ml. 49.8 51.3 52.3 52.9 54.9 55.2 j4.0 06.4 68.5 74.3 76.1 75.9 76.0 76.2 65.5 69.0 75.7 76.7 78.7 78.8 78.5 70.6 79.1 79.2 79.5 74.1 80.8 81.0 10 76.6 77.1 79.4 79.1 78.8 79.2 78.9 a All per cent removal figures based upon filtered samples, except this column which is for centrifuged samples. b Uncorrected for counting efficiency (approximately 10%). ,1 8 7 8
v.
TABLE EFFECTOF CALCIUM 10s CONCENTRATIOS O S R m f o V A L OF MIXED FISSION PRODKCTS BY CLAY SLURRY
Figure 2. Decontamination of Radioactively Contaminated Water by Slurrying with Clay 90-minute contact time
(Radioactive contaminant MFP-3; concentration of clay 1000 p.p.m.) Ca, P.P.hl.
15
pH
30
Slurry Time, Minutes 45 60 75 Per Cent Removal
I. Initial activity 3590 c./min./ml.* 20
90
-
90“
Ca added as Ca(0H)Z 75.5 76.0 77.2 76.6 78.8 78.2 79.9 79.2 78.6 78.9 785 78.0 78.3 78.9 79.2 79.3 78.4 78.8 79.1 78.5 Ca added a8 CaClz 74.7 75.2 75.3 71.3 71.9 75.0 24.1 13.7 75.3 75.3 75.8 756 71.0 73.1 74.7 75.2 76.4 72.9 73.2 70.0 71.2 74.4 75.0 75.0 69.7 72.4 73.5 73.7 removal figures based upon filtered samples, except this for centrifuved samples. for counting efficiency (approximately 10%).
7.20 7.35 7.45 7.70 7.90 11. Initial
71.9 72.8 72.6 72.2 72.9 activity
72.8 74.9 73.9 28.0 75.2 (7.7 74.3 78.2 75.0 78.1 2680 o./min./inl.
It is seen from Table I1 and Figure 2 that clay slurried with con40 60 taminated water was very effective for removing certain radio100 200 active materials, particularly cerium-141,-144-praseodyrnium144, zirconium-95-niobium, barium-140-lanthanum-140, stron40 7.05 tium-90-yttrium-90, MFP-1, and MFP 3. Clay was leas effec7.15 80 tive for MFP-2 and ruthenium-106-rhodium-106, and very poor 120 7.00 150 7.20 for iodine-131. a All per cent Increased concentrations of clay increased the per cent recolumn which is Uncorrected movals, but not to an appreciable degree. It appears that 1000 p.p.m. is an adequate dose for batch treatment and that higher quantities are wasteful of clay. Increased contact times proved to be of value for difficult-toremove r a d i o a c t i v e m a t e r i a1 s . For easy-to-remove materials, such as zirconium-95-niobium-95, 15 minutes’ contact time was almost as effective as 90 minutes’ contact time. The results of the hydrogen ion concentration study are plotted in Figure 3, which shows per cent removal versus pH for the particular clay concentration and MFP-3 solution. It can be seen from Table I V and Figure 3 that PH at pH values lower than 5 the Figure 3. Effect of pH on Removal of MFP-3 by Clay Slurry efficiency of the clay decreases rapidly until pH 4, where the 1000 p.p.m.
INDUSTRIAL AND ENGINEERING CHEMISTRY
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Vol. 46,No. 5
The exchange affinities for most' ionic systems, in aqueous media, folloir- the lyotropic series:
< S a < I< < Rb < Cs 1Ig < Ca < Sr < Ra Sc < Y < E u < Sm < S c i < P r Li
91
CONCENTRATION OF CLAY I N Pp.m.
Figure 4. Effect of Initial Concentration of Radioactbit? on Removal of hlixed Fission Products by Clay Slurrj 90-minute contact time
similar when eithei calcium hydioxide or calcium chloride is used as the source of calcium ions. The plot of the calciuni chloride additive was slightly lower than the cuive obtained with calcium hydroxide. T h k may be due to the slightly Ion-er pH values and the necessary addition of other ions to adjust the pH. Hoviever, as the resulte indicate, calcium ions do not appieciably influence the removal of MFP-3 within the test range studied. DISCUSSION
Clays are colloidal silicates of closely packed oxygen atoms and silicon tetroxide tetrahedra linked together by sharing corners, edges, and faces in such a fashion that large, complex units result. The structural differences betxyeen clays detelmine the degree of preferential adsorption or ionic dissociation which gives rise to the exchange capacities. According to Kaufman (C), the ion exchange capacities of typical clays are, in terms of milliequivalents of cations per 100 grams of clay: 60-100 25-30 20-40 3-16
~Iontmorillonite Attapulgite-montmorillonite group Illite Kaolinite
