Decontamination of Radioactively Contaminated Water by Slurrying with Clay

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  • Decontamination of Radioactivelv J

    Contaminated Water by Slurrying with Clay

    J

    WILLIAM J. LACY Engineering Research and Development Laboratories, Fort Belvoir, Va.

    X T H E event of an atomic disaster, water supplies may I become contaminated with radioactive materials. The level of radioactivity to be expected is dependent upon many condi- tions, including the type of bomb (atomic, radiological, or hydrogen), the type of burst (air, surface, underground, under- water), 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 atmoe- pheric 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. In- gestion of large amounts of radioactive material could cause physio- logical damage. Morgan and Straub ( 7 ) have presented a for- nul la for estimating the emergency maximum permissible con- centration (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 = 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 pre- sents jar test data pertaining to the decontamination of radio- actively contaminated water by the use of clay indigenous to the Oak Ridge, Tenn., area, as well as the effect on removal of radio- active 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.

    TABLE I. CHEMICAL ~ v A L Y S I ? O F G R a B SAMPLE OF O A K RIDGE TAP WATER

    Concen- tration.

    Chemical Constituent P .P .M.a

    Methyl orange alkalinity (as CaCOd 98 Phenolphthalein alkalinity (as CaCOd 2 Soap hardness (as CaCOa) 94 Dissolved solids 110 Nonvolatile solids 75 Calcium 25 Magnesium 5 Sodium Silicon dioxide P H

    6 7 7.9

    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.).

    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.

    The following radioactive materials were used as contaminants: ruthenium-106-rhodium-106; strontium-90-yttrium-90; zir- conium-95-niobium-95; cerium-141, -144-praseodymium-144; iodine-131; barium-140-lanthanum-140; and four fission prod- uct mixtures known as NFP-1) MFP-2, MFP-3, and MFP-4. The ruthenium-106-rhodium-106, strontium-90-yttrium-90, ce- rium-141, -144-praseodymium-144, and barium-140-lanthanum- 140 were obtained as the chlorides in hydrochloric acid solution with a radiochemical purity greater than 95%. Zirconium-95- niobium-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, 17y0 strontium, 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% ruthen- ium-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-praseodymium- 144, 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 Opera- tions Division of the Oak Ridge National Laboratory (1) .

    A stock solution was made by dissolving the radioactive mate- rial 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 coinci- dence loss. Difference between the initial and final count rate represented the removal. The initial concentration of radio- activity 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).

    CONCENTRATION OF CLAY

    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

    Except pH

    1061

  • 1062 I N D U S T R I A L A N D E N G I N E E R I N G C H E M I S T R Y Vol. 46, No. 5

    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, i t was possible to evaluate the efficiency of the clay at variable concentrations and for different contact times. In 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

    In 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 hydro- chloric 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.

    COYCENTRATION OF ACTIVITY

    In order to detect any effect the initial concentration of radio- activity may have on removal, experiments Tvere made a t t,hree levels of activity: (1) Ion (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 radio- act,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 pH was t'aken, and the various concen- trations of clay material were added. In this particular investi- gation, 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 The test pro- efficiency, an additional investigation was made.

    cedure varied only slight,ly from that used previously.

    A% stock solution was made by dis- solving the radioactive material to be used (MFP-3) in distilled water. After mixing, 500 ml. of this spiked solution xas added to each 1-liter beaker con- taining enough calcium hydroxide or calcium chloride t o give the desired con- centration of calcium ions. After the calcium hydroxide or calcium chloride had been dissolved, the pH of t,he solu- tion was adjusted t o the range 7.0 to 7.9) using either hydroch