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Carbon. Vol. 30, No. 5, pp. 767-771, 1992 OQO8-6223/92 $5.00 + .oOPrinted in Great Britain. Copyright 0 1992 Pergamon Press Ltd.

BEHAVIOR OF PERTECHNETATE ION ADSORPTION FROMAQUEOUS SOLUTIONS SHOWN BY ACTIVATED CARBONS

DERIVED FROM DIFFERENT SOURCESKATSUO ITOand AKIRA YACHIDATE*Research Institute of Mineral Dressing and Metallurgy, Tohoku University, Katahira-2, Aoba-ku.Sendai 980, Japan

(Receiv ed I Ocioher 199 I ; accepted in revised.fbrm 2 January I 99 1)Abstract-Pertechnetate ion (TcO,) adsorption from aqueous solutions is compared between activatedcarbons derived from different sources, by reference to the distribution coefficient Kd of Tc between adsor-bent and solution. Adsorption runs made in systems containing HCI and HN03, with pH parametricallyvaried by NaOH addition, proved Kd o rise with lowering pH in the acidic region in the case of activatedcarbon samples derived from coconut shell, from oil pitch, and from saw dust, but not in the case ofthosederived from coal or bone. In certain systems containing high concentrations of acidic radical, the rise ofKd n the acidic region was followed by a decline toward higher acidities. Also, in a certain range of acidicradical concentration, when the adsorption was left to equilibrate beyond several hours, the TcOk onceadsorbed tended to desorb again, displaced by a coexisting other anion, which caused Kd o relapse withfurther progress ofequilibration time. The desorption behavior differed with the kind ofactivated carbon,and with the kind and concentration of the acid contained in the system. Pretreatment of the adsorbentwith acid devoid of Tc04 resulted in lowering of the adsorption performance.Key Words-Activated carbon, carbon sources, pertechnetate ion. adsorption, desorption by displace-ment.

1. INTRODUCIIONTechnetium-99 is counted among isotopes present-ing the promise of useful applications in the near fu-ture, and this calls for the establishment of a practicalmethod for extracting Tc from its source, which ishigh-level liquid waste (HLLW) discharged from nu-clear installations[ I]. For practical application, thebeta-decay half-life of 99Tc amounting to 2 10,000 ycould be found inexpediently long, but this nuclidecan be converted by nuclear reaction into other iso-topes of much shorter life[2].

A requirement of importance equal to extractingTc for use is to prevent TcO; from leaching out intothe underground water in contact with solidifiedHLLW that has been disposed of after solidificationin glass, since the security of this practice woulddepend on the availability of a reliable means ofprotecting against uncontrolled release of this long-lived nuclide into the surrounding subterraneanenvironment.Such protection can be ensured by adsorbing ontoactivated carbon the TcO; contained in the HLLWbefore solidification, and also after disposal ofthe so-lidified HLLW, by collecting the TcO, that leachesinto the soil.Activated carbon can be derived from various or-ganic sources, and the choice of source material willinfluence the capacity of the product-activated car-bon-to serve its purpose of adsorbing TcO;.

*Subsequently transferred to Nissho lwai Corporation,Tokyo.

The present study is aimed at obtaining basic in-formation to serve in selecting the activated carbon tobe used, and in optimizing the conditions for the sep-aration of Tc from HLLW, and for the collection ofTc that has leached into underground water.

2. EXPERIMENTAL2.1 Preparat i on of 9SmT~

Technetium-95m was produced by95Mo(d,2n)95mTc reaction, using a cyclotron of theCyclotron and Radioisotope Center, Tohoku Uni-versity. Carrier-free 95mT~0; was extracted from te-traphenylarsonium chloride-chloroform solution.This 95mT~O; was stripped into 8 M HNOX; the re-sulting solution was divested of HNOj by evapora-tion[3], and the residue was dissolved in water. Theexperiment was performed in batches, on smallquantities ofthis solution.

The radioactivity of 95mT~ was determined bymeans of a Capintec Inc. CRC- 12 radioisotope cali-brator.2.2 M ateri als

Five samples of carbonaceous materials-listed inthe upper half of Table l-were examined for theiradsorbability of TcO;. They were of the commer-cially marketed grade that is commonly used as fuel.The material was ground and sieved to 0.59-2.00mm size, then decanted with distilled water, followedby drying overnight at 120C.

The activated carbon taken up for study consistedof five samples derived from different sources (listedin the lower half of Table 1). They were of the com-

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768 K. ITO and A. YACHIDATETable 1. Surface area and pore size of activated carbons, and distribution coefficient Kd of TcO; on car-bonaceous materials and activated carbons

Source Particle size Surface Pore sizeSample material (mm) area (m*/g) (nm)

Coke A coal 0.495-1.981 - -oke B Oil pitch 0.495-1.981 - -Coal 0.495-I .98 I - -Anthmcite 0.495-l .98 1 - -Charcoal 0.495-1.981 - -ctivated carbon A Coconut shell 0.59-2.00 952 1.5Activated carbon B Oil pitch 0.35-0.70 810 1.5Activated carbon C Coal 0.5 i-2.00 880 2.3Activated carbon D Saw dust -0.04 1150 2.5Activated carbon E Bone 0.04-0.07 81 12.4

50 ml TcO; solution containing 0.16 M (H,Na)N03, 2 g carbonaceous material.

pH Kd

8.7.9 L37.3 014

;*:2.5

918 3640.410.5 4478.0 2089.1 738.2 16

mercially marketed grade commonly used for water the activated carbons compared with the carbona-treatment. ceous materials.2.3 Procedure

A known quantity of the adsorbent sample wasequilibrated with solutions of various acids and oftheir sodium salts containing IO-l2 M (mol.dm-3,TcO;. Series of runs were performed in a system con-taining the prescribed acid in the prescribed totalacidic radical concentration, and the system pH wasparamet~cally varied by the addition of NaOH. For~uilibmtion, the solution and activated carbon sam-ple were placed in a polyethylene bottle, capped, andshaken at a speed of 50 strokes min- for 24 h at 25C.The pH of the supernatant was measured, and 2 mlof this solution was then submitted to y radioactivitycounting with an NaI(T1) scintillation detector.

These first adsorption equilibrium experimentson activated carbon were carried out in systems con-taining solutions of HCl and HN03 and their sodiumsalts. Figures la and 1b compare the four activatedcarbon samples. In the case of activated carbon A(open square symbols in Fig. la), the acidic region to-ward the left is marked by a rise in &with decreasing

The surface area of activated carbon was deter-mined by means of a MICROMERITICS ASAP-2400 Automatic Surface Area and Pore Size Ana-lyzer, after degassing for 1 h at 200C.

3. RESULTS AND DISCUSSIONThe distribution coefficient Kd of Tc between ad-sorbent and solution is calculated using the formula

Q VKd=T----*-Q mwhere Q is the fraction of Tc adsorbed, V the volumeof liquid phase, and m the weight of adsorbent.In the figures that follow, the results obtained forKd are plotted against pH varied by NaOH addition,or against time elapsed in equilibration. The acid andthe total acidic radical concentration in the systemadopted for each series of runs are indicated in the rel-evant figure legends.The adsorption experiments were first carried outfor comparing the performance between the differentcarbonaceous materials and activated carbons usedas adsorbent. As indicated in Table 1, jncompamblybetter TcO; adsorption ~~ormance is provided by

jiD I 2 4 6 8 10 12 14P

Fig. I. Distribution coefficient &of Tc compared betweenactivated carbon samples derived from different sources andin systems containing different acids adjusted to differentacidic radical concentrations-related to pH of solution ad-justed by NaOH addition. 50 ml TeOr solution, 0.1 g acti-vated carbon: (a) 1M (H,Na)CI system: 0: activated carbonA, a: activated carbon B, a: activated carbon C, A: activatedcarbon D, 0: ctivated carbon E, (b} 1 M (H,Na)N03 system:0: activated carbon A, A: activated carbon 8, Ed activatedcarbon C, 0: activated carbon E.

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Pertechnetate ion adsorption from aqueous solutions 769pH. This behavior is attributable to quickened re-moval of TcO; in the acidic region[4], with the ad-sorption onto activated carbon supplemented byTcO: reaction with oxonium ions[5,6,7] generatedby interaction of H+ with the functional groups ofac-tivated carbon. Imm~iately adjoining this acidic re-gion toward the right is a stretch around neutral pH7 that forms a level stage, created by the balance ofcompetition for adsorption onto activated carbon be-tween TcO: and the relevant anion; this is followedfurther toward the right by the basic region of declin-ing &, where TcO: adsorption starts to be outdonein its com~tition with OH-.

The rise of Kdwith increasing acidity noted abovefor activated carbon A has been similarly observed inother runs made with activated carbons B and D(open and closed triangles), but in the case of acti-vated carbons C and E (closed and open circles), theplots failed to rise in this region. With activated car-bon D, the closed triangle plots are seen to attain inthe acidic region a level that closely approaches theopen squares of activated carbon A. The irrelevantlylow Kd alues for activated carbons indicated in TableI can be imputed to a difference in the conditionsunder which the data were obtained, including theratio of liquid volume to quantity of adsorbent.

The data of Fig. la were obtained in a I M (H,Na)Cl system; in Fig. lb a similar difference in ad-sorption behavior between the different activatedcarbon samples is also noted in a 1 M (H, Na)N03system.

The foregoing findings would indicate that theactivated carbons A and B would be superior to

lo2 -y

1111111,IIII II543201 234567891011121314CHbJQJt PHFig. 2. Distribution coefficient Kd of Tc on activated car-bon A in systems containing different acids adjusted to dif-ferent acidic radical concent~tions-relate to acidic con-centration and to pH of solution adjusted by NaOHaddition. 50 ml TcOL solution, 0.1 g activated carbon A: (a)HCI system: a: I M (H, Na)CI solution, o: 2 M (H, Na)CIsolution, A: 3 M (H, Na)CI solution;(b) HNOx system: o: IM (H, Na)NOs, 0: 2 M (H, Na)NOj, CI: 3 M (H, Na)N03,

A: 4 M (H, Na)Nq.

activated carbon C for adsorbing Tc from HLLWsolutions.

With systems containing 3 M (H, Na)Cl (triangu-lar plots in Fig. 2a) and 2 M or higher (H, Na)NO, (ahplots except square, for 1 M (H, Na)NO,), a drop inK, value is seen to occur with increasing acidity. Inthe instance of 2 M (H, Na) NO3 fopen circle plots inFig. Zb), Kd distinctly falls beyond a peak that is at-tained at around pH 1, which would thus be indicatedas the optimum acidity for adsorbing Tc from a nitricacid system.

In practice, HLLW normally takes the form of anHNOX solution of around 3 M con~ntmtion[8]. Thisconcentration could be lowered down to the aboveoptimum level of pH 1 by denitration to decomposepart of the NO;, or else by partially neutralizing theHN03.

In underground water, a variety of mineral ionsare present in minute amounts, but such ions wouldappear to exert no influence on the Tc adsorption be-havior of activated carbon[4]. The present experi-ment has been conducted on systems containinglarge amounts of acids, and their presence shouldhave to be taken into account in comparing the ad-sorption ~rfo~ance between different activatedcarbon samples. A simple test should be required todistinguish Tc adsorption between activated carbonsamples.

The variation of Kdwith progress of equilibrationtime is shown in Fig. 3 for Tc adsorption on differentactivated carbon samples. The plots for 0.5 MH,SO,---from a previous study[4]-reveal a conspic-uous relapse of Kd beyond 4 h, which can be attrib-uted to the adsorbed TcO; being displaced by themore strongly adsorbed and more profusely presentsulfate anions[9], deriving from the sulfuric acid thatwas added.

Compared with activated carbon C, activated car-bons A and B are seen to have produced higher peak

1oa- Py

102 -

loos 10"Time f hFig. 3. Time dependence ofdistribution coefficient KdofTcadsorbed by various activated carbons in 0.5 M f&SO,, so-lution. IO ml TcO; solution, 0. I g activated carbon A: A:activated carbon A, O: activated carbon B, 0: activatedcarbon C.

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770 K.I~oand A. YACHIDATEvalues of I&, which occur at around 4 h of equilibra-tion: at 100 h, the difference in Kd between the acti-vated carbon samples is seen to have diminished to asmall range. Beyond 100 , Kd as tended to rise againin the case of activated carbons A and B, but not withactivated carbon C. The distinctive divergence of thecurves in Fig. 3 between this activated carbon C andthe other two samples cannot be attributed solely tothe disparities in surface area and in pore size that areindicated in Table I.

This freedom from relapse of Tc adsorption per-formance that characterizes activated carbon C couldbe turned to account by its application in prolongedservice calling for stable performance, such as for thecollection of Tc leaching out of solidified HLLWstored underground.For activated carbon A, the relapse of K-attrib-uted to desorption of TcOi, displaced by other ani-ons deriving from the added acid-has also beennoted on adsorption curves obtained in systems of0.1 M HNO,, 0.5 M HCl, and with HCIO, solutionsof acidity above 0.1 M (Figs. 4a-c). On the other

hand, runs made in a phosphoric acid system (Fig.4d) have presented no indication of such relapsethrough 24 h of equilibration; it is only beyond 30 hthat a small drop in Kd can be discerned.The foregoing phenomenon of TcO; desorption

by displacement was observed only with acidic solu-tions of pH below 1.The effect of pretreating the adsorbent with acid

devoid of TcO; is revealed in Fig. 5, which pre-sents-as in Fig. 4-the time dependence of &, foractivated carbon A in systems containing eitherHNOj or HzS04.

In the case of the H2S04 system, pretreatment ofthe activated carbon A with 10 M HZSOd solution isseen to have impaired adsorption performance, as ev-idenced by the closed plots in Fig. Sa. Pa~i~ularly inthe case of adsorption from 0.5 M H2S04 system(closed symbols), which was a case noted in Fig. 4a tobe subject to TcO; desorption by displacement, therelapse of Kd ue to this phenomenon is seen to havebeen distinctly accentuated and prolonged by the pre-treatment.

L I I I I 1 10.51 10 30 051 10 102Time/h

c

Fig. 4. Time dependence of distribution coefficient Kd f Tc adsorbed by activated carbon A in systemscontaining HN03, HCI, HC104, and H3P0, adjusted to different acidic concentrations. 10 ml TcO; so-lution, 0.1 g activated carbon A: (a) 0:0.05 HN03 solution, l 0. I M HNOJ solution, q :0.5 M HNO,solution, A: 1M HN03 solution; (b) o: 0. I M HCI solution, o: 0.5 M HCl solution, A: I M HCI solution,CI: M HCI solution, V: 10M HCI solution; (c)o: 0.01 M HCIO, solution, A 0.1 M HCIO, solution, OI0.5M HC104 solution, i: 1 M HCIO, sotution; (d) o: 0.1 M HJPOd solution, IX 0.5 M H,POa solution, A: 1M H3POh solution, 7: 1.5M H3P04 solution, V: 2 M H3POI solution, 0 : 4 M H$O, solution.

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Pertechnetate ion adsorption from aqueous solutions 111

I I I10 0.5 1Time/h

I I

10 10 lo3

Fig. 5. Effect of pretreating adsorbent with acid devoid of TcOr on time dependence of dist~bution co-efficient Kd of Tc adsorbed by activated carbon A in systems containing HN03 and H2S04adjusted todifferent acidic concentrations. 10 ml TcO; solution, 0.1 g activated carbon A: Open symbois= no pre-treatment; closed symbols=pretreated with 10 M Hz!304 diagram (a)), and 10 M HNO, (diagram (b)),both devoid of TcO;. (a) AA: Adsorption from 0.1 M HrS04, m: Adsorption from 0.5 M H$SOa.(b) AA: Adsorption from 0.05 M HN03, m Adsorption from 0. I M HNOs.With the HNQ system also, pretreatment with 10

M acid altered the adsorption performance, but asseen in Fig. Sb, the symptom of desorption by dis-placement-discernible in the plots for 0.1 M HN03(open circles)-is not accentuated but conversely ef-faced by the pretreatment, and instead, an overalllowering of the iu, curve is noted (closed circles), aswas the case with 0.1 M H,SO, (closed triangles ofFig. Sa) and with 0.05 M HN03 (closed triangles ofFig. 5b), both of which were not subject to desorptionby displacement.

4. CONCLUSIONActivated carbon derived from coconut shell or

from oil pitch would appear to be the most suitablefor adsorbing Tc from high-level liquid waste. Acti-vated carbon derived from coal could, on the otherhand, be considered to excel others for prolonged ser-vice, for its stable performance with little variationdue to desorption of TcO; from displacement byother anions. This kind of activated carbon shouldtherefore be suitable for use in backfill material forpits bored for disposal of high-level liquid waste.

The TcO; adsorption behavior of activated car-bon will vary with lapse of equilibrium time, as wellas with the kind and concentration of the mineralacids contained in the system.A~~non,led~~rnmr-The authors wish to thank Y. Onoderaof the Government Industrial Research Institute, Tohoku,

for his collaboration in determining the specific surface areaand pore size of activated carbons. Grateful acknowledge-ment is also due to Prof. K. Akiba of the Research Instituteof Mineral Dressing and Metallurgy, Tohoku University,for his valuable discussions provided throughout the courseof this study. M. Yoshida kindly edited the present text forgrammatical conformity.

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3.4.5.6.7.

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REFERENCESF. Luykx, Technetium Discharges into the Environ-ment, In Technetium in the Environment (Edited by G.Desmet and C. Myttenaere), pp. 2 I-28. Elsvier AppliedSci., London and New York (1984).CURE: Clean Use of Reactor Energy, WHC-EP-0268,Westinghouse Hanford Company, Richland, Washing-ton (1990).K. Ito and T. Kanno, J. Nucl. Sci. Technol. 23,37 (1986).K. Ito and K. Akiba, J: Rudioanul. Nwl. Chm. Art icles152,381(1991}.H. P. Boehm and M. Volt, Cu~~~~~ ,227 (1970).V. A. Garten and D. E. Weiss, Ausi. J. Chem., 10, 309(1957).1-I. Yoshida and K. Kameoka, In Carbon MuterialsfitrHigh Technologyin Japanese) (Edited by K. Kobayashiand H. Honda) pp. 165-187. Kogyo Cyosakai, Tokyo( 1987).S. Tachimori, S. Sakurai, and S. Usuda, Fr~)~~~~~j~~~?ft~?~~Third ~nti~r~at~~~na~ ot@ence on ~~11~1~~~ud Reptwwssing and WasreMunugemrnf, April 16, 199 1, Sendai,Japan, pp. 6 I I-6 16. Atomic Energy Sot. Japan, Tokyo(1991).K. Bratzler, Adsorption von Gasen und Dtimnpfkn in La-boraforirrm ~md Technik, Theodor Steinkopff, Dresden(1944) (Japanese Edition, pp. 55-58).