Calixarene-Containing Extraction Mixture for the Combined Extraction

Chapter 8

Calixarene-Containing Extraction Mixture for the Combined Extraction of Cs, Sr, Pu, and Am from Alkaline Radioactive Wastes 1

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Igor V. Smirnov , Andrey Yu. Shadrin , Vasily A. Babain , Mikhail V. Logunov , Marina K. Chmutova , and Vitaly I. Kal'tchenko 2

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1Khlopin Radium Institute, St.-Petersburg 194021, Russia "Mayak" PA, Ozersk, Russia 3Vernadsky Institute of Geochemistry and Analytical Chemistry, Moscow 117975, Russia 4Institute of Organic Chemistry, Kiev 252660, Ukraine

Downloaded by COLUMBIA UNIV on June 29, 2012 | http://pubs.acs.org Publication Date: July 20, 2000 | doi: 10.1021/bk-2000-0757.ch008

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The extraction mixture containing tert-butyl-calix[6]arene, 4-alkyl2-di-(2-hydroxyethyl)aminomethylphenol and solubilizing additive in dodecane, was proposed for processing of alkaline high-level radioactive wastes. This extraction mixture extracts effectively cesium, strontium, rare-earth elements, americium and plutonium from basic solutions containing up to 8 M sodium salts and 0.1-4 M free alkaline. Acidic solutions could be used for the subsequent stripping of these elements from organic phase. The extraction mixture was tested at "Mayak" PA (Chelyabinsk region). More than 99% of Cs and Sr and about 90% of gross alpha -activity were removed from actual alkaline H L W during the batch test.

in the "cold war" years, great amounts of radioactive wastes were accumulated at the American and Russian facilities for weapon plutonium production. High-level liquid alkaline wastes containing considerable quantities of long-lived radionuclides are most hazardous among them. The separation of these radionuclides from non* The greater part of the work reviewed in the present paper was financed by the European Commission in the frame-work of the Project INCO-COPERNICUS (Contract IC15-CT98-0208).

© 2000 American Chemical Society

In Calixarenes for Separations; Lumetta, G., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 2000.

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volative compounds would considerably reduce the volume of high-level glass to be disposed off. For processing of Hanford high-level waste ( H L W ) British Nuclear Fuels pic (BNFL) has proposed the multistage process involving strontium precipitation on a carrier, coprecipitation of actinides with iron, cesium removal by inorganic sorbent and technetium sorption on ion-exchange resin [1]. In our opinion, the more simple and cheap variant for processing of these wastes could be based on the combined extraction of all long-lived radionuclides in a single extraction cycle. It is known that cesium is readily extracted from alkaline media by calixarenes [2], while rare-earth elements and actinides are extracted by 4-alkyl-2-di-(2-hydroxyethyl)aminomethylphenol (DEAP) [3]. The object of our work was to check the possibilities for simultaneous recovery of the most long-lived radionuclides from alkaline media by one extractant.

Extraction from simulated solutions Preliminary tests have shown that die mixtures of tert-butyl-calix[6]arene and D E A P extract Cs, Sr, R E and actinides from alkaline media. Some results are presented in Table I.

Table I. Extraction of Cs, Sr, Tc, Am from alkaline media by solution of 0.01 M tert-butyl-calix[6]arene and 0.01 M DEAP in 20% POR in dodecane

NaN0 , M 3

NaOH, M

Des

0.1

12.9 11.3 6.5 5.8 4.0 3.8 2.4 2.2 2.2 0.95 0.85 0.92 0.30 0.39 0.51

0 0.1 1.0 0.1 2.0 0.1 4.0 0.1 Saturated (~8) 2

D

S r

3.1 3.3 1.7

DA™

0.4 2.4 0.5

D

T c

0.5 2.3 2.3

3.4

2.4

1.3

2.4 4.3 3.2

0.49 0.55 0.35

5.6

It is seen from Table I that the proposed extraction mixture recovers all longlived radionuclides over a wide range of concentration of nitrate and sodium hydroxide.


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It should be noted that the main impediment to the use of tert-buryl-calix[6]arene as extractant is its low solubility in conventional solvents. The mixtures of paraffines and organophosphorus compounds like T B P and POR (different-radical phosphine oxide) turn out to be good solvents. A n added advantage of POR as solubilizer of calixarene is that it extracts technetium, one of the most hazardous components of alkaline H L W , from alkaline media. The good results obtained for simulated solutions have made possible to determine the conditions and to perform test on actual alkaline H L W of the "Mayak" PA.

Static test on actual HLW The H L W used for the experiment was taken from the storage tank AD-5701/4 and contained about 3 M sodium hydroxide and 3 M sodium nitrate. The chemical and radiochemical composition of feed solution is given more fully in Tables II and III. The feed solution was used without any dilution and preparation. The extractant contained 0.02 M tert-butyl-calix[6]arene and 0.02 M D E A P in a mixture of 40%vol. POR and 60% dodecane. The experiment involved four-fold successive treatment (contacting) of feed solution with fresh portions of extractant at slightly increasing volume ratio of phases O/A being equal to 3:3, 3:2.8, 3:2.6, 3:2.4, respectively. The first extract was contacted three times with fresh portions of the stripping agent containing 30 g/L of oxalic acid and 10 g/L of acetohydroxamic acid at constant phase ratio 0 / A = l : l . In all cases the phases were stirred for 15 min. Time of phase separation (settling) was 510 min. Two to six independent analyses were performed for each component in all the samples. The main experimental data are presented in Tables II and III. The analysis of these data shows that the proposed extraction mixture is highly efficient for recovery of cesium and strontium from alkaline H L W in 4 contacts. The purification coefficients of rafflnate are 2-3 orders of magnitude for Cs and 1-2 orders of magnitude for Sr. Uranium and plutonium are extracted rather well (the purification coefficients lie within 14-27). As a whole, the alpha-emitters are removed by one order of magnitude, which is less than expected. The stripping efficiency of cesium, strontium and trivalent alpha-nuclides is rather high. Stripping of plutonium and uranium is not sufficient. Since cesium-137 constitutes the major portion of radioactivity in feed solution, which may be a hindrance to the determination of other components, a portion of feed sample was treated by 0.12 M solution of chlorinated cobalt dicarbollyde (ChCoDiC) in metanitrobenzotrifluoride at phase ratio 0 / A = 5 : l . Under these conditions, ChCoDiC extracts more than 99% of cesium, without any practical extraction of other nuclides. The analysis of the treated and untreated feed solutions was carried out. Figure 1 presents a schematic diagram of the test and conventional symbols used to represent the various solutions. The γ-, α-spectral and chemical analyses were performed at the Central Plant Laboratory of "Mayak" P A by using the standard procedures.


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Table II. Radiochemical composition of feed and resultant solutions Compo nent

Feed

Raf-

Content of components in solutions, Bq/L Extracts Strips

3.4E9

2.1E8

Recycle solvent 0 1.8E6

2.8E6 n.d.

Calixarene-Containing Extraction Mixture for the Combined Extraction

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