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Adsorption of tetranitronitrosylruthenate on activated charcoal
Annals ~ Nuclear Enere,y. Vol. 6, pp. 399 to 404 Pergamon Press Ltd 1979. Printed in Great Britain
ADSORPTION OF TETRANITRONITROSYLRUTHENATE ON ACTIVATED CHARCOAL E. AKATSU,C. YONEZAWAand K. MOTOJIMA Nuclear Engineering School, Tokai Research Establishment, Japan Atomic Energy Research Institute, Tokai-mura, Ibaraki-ken, Japan (Received in revised form 14 March 1979)
Abstract--Sodium tetranitronitrosylruthenate Na2[RuNO(NO2),OH]2H20, labelled with l°6Ru, was prepared and its properties and adsorption behavior on activated charcoal were studied. The ruthenate was well adsorbed on activated charcoal of Tsurumicoal HC-30 (8-28 mesh). The absorbability was increased by addition of the organic cation of zephiramine or methylene blue at pH 4-8. The adsorbability was lowered by the introduction of dodecylbenzenesulfonate or Emulgen 220, but raised by further addition of zephiramine. The adsorbed ruthenate was not easily desorbed with water.
INTRODUCTION Great complexity of problems are involved in the treatment of highly radioactive fission products. The chemical behavior of ruthenium is especially complex in the system encountered in the reprocessing of the spent fuel, since the chemical species of ruthenium are many, similar to each other and change into another species in the aqueous nitrate system. The species are mainly nitro- and nitrato-complexes of nitrosylruthenium, and minute amounts of them are found in the effluent and must be eliminated before it is introduced into the environment. On the other hand, Motojima et al. 0978) presented an effective method for the removal of cobalt from the waste water of a nuclear reactor facility by adsorption on the activated charcoal with 8-quinolinol. In this work, a similar adsorption was applied to tetranitronitrosylruthenate. Only tetranitro-complexes were studied in detail here in order to clarify step by step the adsorption behavior of many kinds of complexes of nitrosylruthenium. Basic information obtained here must be useful in technological application for decontamination of ruthenium in the effluent of reprocessing plants. The ruthenate was first studied because it is the most stable species among nitro- and nitrato-com-
* ICHa(CH2)t 3N(CH3)2CH2 Q ] t I(CH3)2 N . ~ / / N
~
CI.
N(CH3)2 ]CI.
~:CH3(CH2h 50(CH2CH20).H.
plexes of nitrosylruthenium, and because it was not coprecipitated with ferric hydroxide. The nitrocomplex of nitrosylruthenium was known to be stable in typical plant conditions and poorly eliminated upon treatment of effluent by chemical coagulation or chemical precipitation (Brown et al., 1958; Gardner et aL, 1960), but the nitrocomplex was a dinitrocomplex. On the other hand, an identification of the tetranitrocomplex is not easy because the properties of the tetranitrocomplex are not well known. Thus the properties of the tetranitrocomplex were also investigated. EXPERIMENTAL Materials
Sodium tetranitronitrosylruthenate, Na2[RuNO(NO2),OH]2H20, was prepared from ruthenium trichloride trihydrate and sodium nitrite by the method of Fletcher et al. (1955). The prepared ruthenate was labelled with l°6Ru imported from the Radiochemical Centre, England. The specific activity of the labelled compound was about 0.11mCig -1 ruthenium. A stock solution was prepared by dissolving the labelled ruthenate in redistilled water. Coconut charcoal of Tsurumicoal HC-30 (8 28 mesh) was cordially supplied from Tsurumicoal Co. and used as received. The other reagents used were organic reagents of zephiramine(benzyltetradecyldimethylammonium chloride),* methylene bluet, sodium diethyldithiocarbamate, 8-quinolinol, Emulgen 220,:~ sodium dodecylbenzenesulfonate, methylisopropyl ketone (MIPK) and dibutyl cellosolve (DBC); also sodium hydroxide and nitric acid in JIS special grade. 399
400
E. AKATSU,C. YONEZAWAand K. MOTOJIMA
They were commercially obtained and used without purification.
Measurement and analyses Purified ruthenate was offered for chemical analyses and investigations of chemical properties before the adsorption experiment. Ruthenium and sodium were determined by activation analysis and nitrogen by spectrophotometry, after conversion of nitrogen into ammonium salt (JAERI, 1971; Horwitz, 1975). The absorption spectrum in the visible region was measured with an aqueous solution of the ruthenate. Infrared absorption spectra were measured for samples prepared by coating the powdered ruthenates on thin polyethylene film. Thermogravimetric and differential thermal analyses were done with a heating rate of 20°C min- 1. Paper chromatography was also done using the method of Wain et al. (1960), with MIPK and DBC equilibrated with 3 M nitric acid. The paper used was Top No. 53.
Determination of distribution ratio The batch method was used in the adsorption experiment. Redistilled water of 50.00 + 0.05 ml was pipetted into a 300 ml Erlenmeyer flask with stopper. The stock solution of the ruthenate below 0.1 ml was added to the water to make the initial concentration of ruthenium usually below 23 pg ml- 1. If necessary, an organic reagent given above was added as 0.5 ml of 0.5 wt Vo aqueous solution, and/or dilute sodium hydroxide or nitric acid solution below 0.5 ml was also added. After mixing well, 2.00 _+ 0.01 ml was pipetted out and reserved as an original solution. Then the weighed amount of activated charcoal, usually 100 _+ 1 mg, was put into the flask. The flask was shaken by shaker with 30 strokes rain-1. At the appropriate time, 3.00 _+ 0.01 ml of the aqueous phase was pipetted out and filtered through No. 5c filter paper. Filtration was done for all the remaining aqueous phase at the last sampling. From the filtrate 2.00 + 0.01 ml was taken and its radioactivity was measured by a NaI(TI) scintillation counter as well as the radioactivity of the original solution. The remaining filtrate at the last sampling was used to measure the pH. The distribution ratio, Ka, was calculated using Radioactivity adsorbed on 1 g activated charcoal Ka = Radioactivity remaining in 1 ml aqueous phase The desorption experiment was similar to the adsorption experiment. Activated charcoal adsorbing the ruthenate was filtered through No. 5c filter paper,
washed with 5 ml water and put into a fresh Edenmeyer flask with redistilled water of 50.00 + 0.05 ml. Shaking was continued and sampling, filtration and measurement of radioactivity were the same as the adsorption experiment. In all the cases in the desorption experiment no radioactivity was confirmed in washings. All the experiments were duplicated and done at ambient temperature. The experimental errors were calculated from the count rate of the radioactivity only. RESULTS AND DISCUSSION
The properties of the prepared ruthenate The absorption spectrum in the visible region showed no peak at 35(~900 nm and a continuous increase in optical density towards the ultraviolet. The shape of spectrum was similar to that of the dinitrocomplex (Brown, 1960), but the molar extinction coefficient was smaller than that of the dinitrocomplex. Coprecipitation with ferric hydroxide was 0.4%. This meant that the ruthenate could be kept in the aqueous phase in the floculation process of effluent treatment and be introduced into the environment. Ry values of tetranitronitrosylruthenate were 0.8 and 0 with MIPK and DBC, respectively, and the values were the same for samples having stood for 1 week, 1 month and 4.5 months. The values did not agree with those of Matsumura (1970) and the value given by Brown (1960). However, the ruthenate prepared in the present study had a chemical composition of R u : N a : N = 1:2.3 _+ 0.4:5.2 _+ 0.81, and no compound is known that is stable in water and has such a composition except for tetranitronitrosylruthenate. The other known complexes having R u ' N = 1:5 are [RuNO(NO3hH20]- and [RuNO(NO3),OH] 2(Fletcher et al., 1959; Scargill et al., 1965). These immediately decompose in aqueous solution without concentrated nitrate, and their Rs values in paper chromatography are different from the prepared ruthenate (Gardner et al., 1960; Jenkins et aL, 1956). Infrared absorption was found at 1902cm -1 for both the prepared ruthenate and its dehydrated form. The absorption agreed well with the value of 1905 cm- 1 given by Brown (1960) as specific with NO stretching in the tetranitronitrosylruthenate. Differential thermal and thermogravimetric analyses showed that the endothermic decrease of weight near 105°C exactly corresponded to the difference of weight between the dihydrate of ruthenate, Na2[RuNO(NO2),OH]2H20, and the anhydrous ruthenate, Na2[RuNO(NO2hOH], and that the anhydrous ruthenate was stable below 200°C. Thus the prepared
Adsorption of tetranitronitrosylruthenate on activated charcoal
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ruthenate in the present study was identified as tetranitronitrosylruthenate dihydrate. The dihydrate and the anhydrous ruthenate showed no visible change in air for over 1 month, but the dihydrate gradually effloresced to anhydrous ruthenate in a desiccator with calcium chloride.
Adsorptionbehaviorin water Distribution ratios, determined with various amounts of activated charcoal in water, are shown in Fig. 1, where the distribution ratios are plotted against shaking time. Two kinds of adsorption were observed: one was fast adsorption on the surface of activated charcoal and the other was slow adsorption by migration into the pores of the activated charcoal. The former was naturally larger, with activated charcoal of smaller grain size; this is seen in Fig. 1 as the result obtained with 200-280 mesh activated charcoal. In the latter adsorption an equilibrium was not attained within 4000 rain shaking time, and its reaction rate was proportional to the amount of activated charcoal. In the preliminary experiment, the distribution ratios of the former adsorption were practically the same at room temperature with the initial concentration of ruthenium 15-60/~gml ~, but the rate of the latter adsorption depended on temperature and ruthenium concentration. That is, the slope of the curve of the distribution ratio vs shaking time decreased with increasing ruthenium concentration and with falling temperature. However, the distribution ratios were practically the same within experimental error at ambient temperature with the initial concentration of ruthenium below 25 #g ml- a in the aqueous AN.E,
6 78-
(
401
phases of redistilled water, with pH 1.5 and 10. Thus the distribution ratios were usually determined with an initial concentration of ruthenium of about 20/~gml-~ in this study. In addition, the effect of temperature was not a problem in practical purposes because of the high distribution ratios of the ruthenate. In other words, the percentage of adsorption of ruthenium was almost 100 in all cases.
Effect of additionof organicreagent The effluent from reprocessing plants could contain detergents, therefore the effects of typical detergents were surveyed. The selected detergents were sodium dodecylbenzenesulfonate and Emulgen 220. Effects of 8-quinolinol were also studied, since 8-quinolinol was used in the analysis of ruthenium (Hashitani 1969). Sodium diethyldithiocarbamate was also tested, since ruthenium is known to form many kinds of thiocomplexes and to be precipitated as a sulfide in analyses of ruthenium (Walsh 1963; Wyatt 1961). Diethyldithiocarbamate is already used in the treatment of the liquid waste from nuclear reactor facilities in the removal of radioactive nuclides of cobalt, and the properties of the reagent were shown to be suitable for practical decontamination of the effluent (Motojima 1977). Tht results obtained with the reagents above are shown "n Fig. 2. Addition of these reagents lowered the distribution ratio of the ruthenate. These facts suggested that the ruthenate did not react with 8-quinolinol, diethyldithiocarbamate or dodecylbenzenesulfonate at the experimental conditions. These reagents and Emulgen 220 are adsorbable on activated charcoal themselves, hence a competition adsorption could occur and result in a fall of the distribution ratio. Motojima (1978) found the increasing adsorbability of cobalt by introducing a small amount of 8-quinolinol into the activated charcoal-aqueous solution system. In the present study, the experimental results showed that tetranitronitrosylruthenate was too stable to react and form a complex with 8-quinolinol. Therefore, cationic species of organic compound were expected to react directly with the ruthenate anion, [RuNO(NO2)4OH] 2or H[RuNO(NO2)4OH]-. As is shown in Fig. 2, the addition of methylene blue or zephiramine was effective in increasing the adsorbability of the ruthenate. This fact suggested that the ruthenate was stable enough to keep its anionic form in the aqueous phase and was adsorbed as the neutral species formed with cations of methylene blue or zephiramine. The stability of the ruthenate was confirmed by paper chromatography over one month in the aqueous solutions.
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402
E. AKATSU, C. YONEZAWAand K. MOTOJIMA
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lO
1000 ?000 3000 4000 Shaking time (rain]
Fig. 2. Effect of addition of organic reagent (MB: methylene blue, Z: zephiramine, NR: no reagent, Ox; 8-quinolinol, E: Emulgen220, DDTC: diethyldithiocarbamate, DBS: dodecylbenzenesulfonate).
When both zephiramine and sodium dodecylbenzenesulfonate were added in the concentrations of 1.36 x l0 -2 and 1.49 × l0 -2 M each, the determined distribution ratio of 0.16 × 10-3M ruthenium was higher than that obtained in the addition of dodecylbenzenesulfonate only. After 2000 minutes of shaking. the distribution ratio was increased from 330 to 1800. At the pH of the aqueous phase of distilled water, chemical species of the ruthenate were mostly H[RuNO(NO2)4OH]-, as is given below, and the ruthenate could make more adsorbable species by reacting with the cation of zephiramine, but could not react with dodecylbenzenesulfonate.
Effect of pH Distribution ratios at various pHs are shown in Fig. 3. The distribution ratios were also determined with methylene blue or zephiramine in acidic and in alkaline media, as is seen in Fig. 4 in the example of zephiramine. The result with methylene blue was the same as with zephiramine. From these results it was found that the pH of the aqueous phase had an effect on the adsorbability of the ruthenate. The mechanism of adsorption was considered to be extremely complex and was not studied in this work, however these results could be related with the dissociation of free acid of the ruthenate, H2[RuNO(NO2)4OH]. The values of pKI and pK2 of H2[RuNO(NO2)4OH] were found, from the potentiometric titration curve given by Fletcher
:
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1000 2000 5000 4 0 0 0 Shaking time (rain)
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et al.
(1955), to be 1.83 and 7.00. From these values, percentages of H2[RuNO(NO2)4OH], H[RuNO(NO2)4OH1- and [ R u N O ( N O 2 ) 4 O H ] 2were calculated at various pHs, as is shown in Fig. 5. On the other hand, addition of methylene blue or zephiramine was effective in alkaline media but not in acidic media, as is shown above. This adsorption behavior was understandable when the following is considered. In acidic media, the neutral species of free acid of the ruthenate was more abundant. If the neutral species was mainly adsorbed on activated charcoal, the organic cations of methylene blue or zephiramine could not be effective in increasing the adsorption. In alkaline media of pH 10, the chemical species was almost dissociated [-RuNO(NO2)4OH] 2-, which was stable in the aqueous phase. Therefore,
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Fig. 4. Effect of zephirarnine in alkaline and acidic solu-
tions (Z: zephiramine).
Adsorption of tetranitronitrosylruthenate on activated charcoal
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Fig. 5. (a) Relative abundance of ruthenate species (upper) (A: H2[RuNO(NO2)4OH], B: H[RuNO(NO2)4OH]-, C: [RuNO(NO2)4OH]2-). A + B + C = 0.1 M. (b) Distribution ratio of ruthenate at 2000 min shaking vs pH (lower). adsorbability could be lower than in acidic media, and addition of methylene blue or zephiramine increased the adsorbability to form neutral and adsorbable species. In the region of pH 4-7, the results observed were more complex. Distribution ratios were read (Fig. 3) at 2000 min shaking time, and were plotted against pH in Fig. 5. The curve obtained showed a maximum at pH 5. It was interesting that the shape of curve agreed with that of the variation of relative abundance of H[RuNO(NO2)4OH]- in Fig. 5. On the other hand, addition of methylene blue or zephiramine increased the distribution ratio of the ruthenate at pH 7, as Fig. 2 shows. These facts demonstrated the higher adsorbability of neutral species formed by H[RuNO(NO2)4OH]- and cations of zephiramine or methylene blue over that of the monovalent anionic species of the ruthenate. However, the distribution ratio at pH 4-7 was too high to deny the strong adsorbability of the anion of H[RuNO(NO2)4OH]-. The reason why the monovalent anion was so strongly adsorbable was not clear, but similar adsorption was found in the activated charcoal-nitric acid system (Akatsu et al., 1965). The adsorption behavior of pertechnetate also agreed with the change of amount of TcO~- species. Pertechnetic acid has a pK of - 1 . 5 (Cobble, 1964), and species of the free acid HTcO4 increased with increasing acidity, but the high adsorbability of technetium decreased with increasing acidity. Chromate was also found to be similarly
403
adsorbed. The chemical species of chromate changed due to the pH of the aqueous phase: CrzOT:-, CrO 2-, HCrOg and H2CrO4. The adsorbable range of pH was near to the pH region where HCrO£ and CrO 2predominate (Sillen et al., 1971). In alkaline media, where CrO 2- predominates, chromate was not adsorbable. Thus, the monovalent anion seemed to be adsorbable. If monovalent anionic species could be adsorbable, the monovalent anionic species formed by a [RuNO(NO2)4OH] 2- and a monovalent cation of zephiramine might be also adsorbable when zephiramine was added to the ruthenate solution in alkaline media. In addition, many of the adsorbable elements on activated charcoal were anionic, or possibly could form anionic or neutral chlorocomplexes. They were iodide, iron(Ill), gold, palladium(II), platinum, iridium and osmium (Akatsu et al., 1965; Ikeda et al., 1961). The ruthenate in alkaline media was also adsorbable, although distribution ratios were about 100. These facts suggested a possible chemical interaction of activated charcoal and these anions. The surface of activated charcoal was known to have functional groups of phenol, carboxyi, 1,4-quinone form and nitro and nitrogen in ring form (Studebaker et al., 1956). However, these groups are not considered to react simply with the inorganic anions given above. Further discussions need the elemental analyses of the used activated charcoal and the precise determination of temperature dependence of distribution ratios to estimate the heat of adsorption and distinguish between chemical or physical adsorption. Matsumura and Ishiyama (1970) also reported the adsorption of ruthenium on charcoal. They used powdered charcoal and, naturally, an equilibrium was attained to 30 min shaking. They also reported good adsorption of tetranitronitrosylruthenate at pH 2-9, adjusted by hydrochloric acid and sodium hydroxide. These results almost agree with the present authors' data. Desorption
When the distribution ratio of desorption was measured by changing the aqueous phase with a fresh one, jumping values were found. This fact meant that the ruthenate adsorbed was not easily desorbed. Consequently, the higher initial concentration of ruthenium, about 60/~g ml- 1, must be, used in the desorption experiment to measure the radioactivity of the desorbed ruthenate in the aqueous phase. Similar behavior was shown by the ruthenate adsorbed with zephiramine, methylene blue and those adsorbed from acidic and alkaline solution. The distribution ratios of desorption are summarized in Table 1. The slow
404
E. AKATSU, C. YONEZAWAand K. MOTOJIMA Table 1. Distribution ratios of desorption Adsorption
Desorption
[Ru]i
Reagent
pH*
Reagent
pH*
52 15 28 15 54 18 19
None HNO 3 HNO 3 NaOH Zephiramine Zephiramine Methylene blue
6.6 1.7 1.7 10 6.4 5.8 5.0
None None NaOH None None None None
6.1 3.3 8.0 6.5 5.4 7.4 7.4
Kat 3 4 3 1 7 8 3
× × × × × x
104 105 105 105 104 104
×
104
[Ru]~ = initial conc. of Ru in/~g ml- 1 * Measured after the last sampling was done at 300(0000 min shaking. t The radioactivity of the aqueous phase was near to the background in all cases.
rate of adsorption and high distribution ratio of desorption suggest a type of chemical adsorption. CONCLUSION Tetranitronitrosylruthenate was shown to be stable in the aqueous phase in the range pH 1.5-11, and not to be coprecipitated with iron(III) hydroxide. Hence the ruthenate could be retained in the supernarant fluid after treatment of chemical precipitation and introduced into the environment. O n the other hand, tetranitronitrosylruthenate was highly adsorbable on activated charcoal, and the adsorbability was increased by addition of cationic species such as methylene blue or zephiramine in neutral or alkaline media. The adsorbed ruthenate was difficult to desorb with water. This could be useful in the treatment of the effluent from the reprocessing plant for the removal of tetranitronitrosylruthenate. Acknowledgement--The authors thank H. Kamiyama and K. Shimooka for their contribution to the measurement of infrared adsorption spectra and differential thermal and thermogravimetric analyses, respectively.
REFERENCES
Akatsu E., Ono R., Tsukuechi K. and Uchiyama H. (1965) J. nucl. Sci. Technol. 2, 141. Brown P. G. M., Fletcher J. M., Hardy C. J., Kennedy J., Scargill D., Wain A. G. and Woodhead J. L. (1958) Proc. 2nd Int. Conf. Peaceful Uses of Atom. Energy~ Geneva, P/31, Vol. 17, p. 118, United Nations, Geneva. Brown P. G. M. (1960) J. inorg, nucl. Chem. 13, 73.
Cobble J. W., Koltoff I. M., Elving P. J. and Sandell E. B. (Eds) (1964) Treatise on Analytical Chemistry, Part II, Vol. 6, p. 407, Interscience, New York. JAERI (1971) The Committee on Analytical Chemistry of Nuclear Fuels and Reactor Materials, JAERI-4053, p. 43. Fletcher J. M. (1958) J. inorg, nucl. Chem. 8, 277. Fletcher J. M., Jenkins I. L., Lever F. M., Powell A. R. and Todd R. (1955) J. inorg, nucl. Chem. 1, 378. Fletcher J. M., Brown P. G. M., Gardner E. R., Hardy C. J., Wain A. G. and Woodhead J. L. (1959) J. inorg. nucl. Chem. 12, 154. Gardner E. R. and Brown P. G. M. (1960) AERE R-3551. Hashitani H., Katsuyama K. and Motojima K. (1969) Talanta 16, 1553. Horwitz W., Senzel A., Reynolds H. and Park D. L. (Eds) (1975) Official Method of Analysis of the Association of Official Analytical Chemists (12th edn), p. 17, The Assoc. of Official Analytical Chemists, Washington, D.C. Ikeda N. and Mitsubayashi T. (1961) Radioisotopes 10, 245. Jenkins I. L. and Wain A. G. (1956) J. inorg, nucL Chem. 3, 28. Matsumura T. and Ishiyama T. (1970) A. Rep. Radiat. Center, Osaka, I1, p. 44. Motojima K., Katsuyama K. and Yamazaki Y. (1977) Ann. nucl, Energy 4, 449. Motojima K., Tachikawa E., Kamiyama H. and Imahashi T. (1978) Ann. nucl. Energy 5, 5. Scargill D., Lyon C. E., Large N. R. and Fletcher J. M. (1965) J. inorg, nucl. Chem. 27, 161. Sillen L. G. and Martell A. E. (1971) Stability Constants of Metal-ion Complexes, Suppl. No. 1, The Chemical Society, London. Studebaker M. L and Huffman E. W. D. (1956) Industrial Engng Chem. 48, 162. Wain A. G., Brown P. G. M. and Fletcher J. M. (1960) J. inorg, nucl. Chem. 12, 346. Walsh T. J. and Hausman E. A. (1963) Treatise on Analytical Chemistry, Part II, Vol. 8, p. 379, Interscience, New York. Wyatt E. I. and Rickard R. R. (1961) NAS-NS-3029.
ADSORPTION OF TETRANITRONITROSYLRUTHENATE ON ACTIVATED CHARCOAL E. AKATSU,C. YONEZAWAand K. MOTOJIMA Nuclear Engineering School, Tokai Research Establishment, Japan Atomic Energy Research Institute, Tokai-mura, Ibaraki-ken, Japan (Received in revised form 14 March 1979)
Abstract--Sodium tetranitronitrosylruthenate Na2[RuNO(NO2),OH]2H20, labelled with l°6Ru, was prepared and its properties and adsorption behavior on activated charcoal were studied. The ruthenate was well adsorbed on activated charcoal of Tsurumicoal HC-30 (8-28 mesh). The absorbability was increased by addition of the organic cation of zephiramine or methylene blue at pH 4-8. The adsorbability was lowered by the introduction of dodecylbenzenesulfonate or Emulgen 220, but raised by further addition of zephiramine. The adsorbed ruthenate was not easily desorbed with water.
INTRODUCTION Great complexity of problems are involved in the treatment of highly radioactive fission products. The chemical behavior of ruthenium is especially complex in the system encountered in the reprocessing of the spent fuel, since the chemical species of ruthenium are many, similar to each other and change into another species in the aqueous nitrate system. The species are mainly nitro- and nitrato-complexes of nitrosylruthenium, and minute amounts of them are found in the effluent and must be eliminated before it is introduced into the environment. On the other hand, Motojima et al. 0978) presented an effective method for the removal of cobalt from the waste water of a nuclear reactor facility by adsorption on the activated charcoal with 8-quinolinol. In this work, a similar adsorption was applied to tetranitronitrosylruthenate. Only tetranitro-complexes were studied in detail here in order to clarify step by step the adsorption behavior of many kinds of complexes of nitrosylruthenium. Basic information obtained here must be useful in technological application for decontamination of ruthenium in the effluent of reprocessing plants. The ruthenate was first studied because it is the most stable species among nitro- and nitrato-com-
* ICHa(CH2)t 3N(CH3)2CH2 Q ] t I(CH3)2 N . ~ / / N
~
CI.
N(CH3)2 ]CI.
~:CH3(CH2h 50(CH2CH20).H.
plexes of nitrosylruthenium, and because it was not coprecipitated with ferric hydroxide. The nitrocomplex of nitrosylruthenium was known to be stable in typical plant conditions and poorly eliminated upon treatment of effluent by chemical coagulation or chemical precipitation (Brown et al., 1958; Gardner et aL, 1960), but the nitrocomplex was a dinitrocomplex. On the other hand, an identification of the tetranitrocomplex is not easy because the properties of the tetranitrocomplex are not well known. Thus the properties of the tetranitrocomplex were also investigated. EXPERIMENTAL Materials
Sodium tetranitronitrosylruthenate, Na2[RuNO(NO2),OH]2H20, was prepared from ruthenium trichloride trihydrate and sodium nitrite by the method of Fletcher et al. (1955). The prepared ruthenate was labelled with l°6Ru imported from the Radiochemical Centre, England. The specific activity of the labelled compound was about 0.11mCig -1 ruthenium. A stock solution was prepared by dissolving the labelled ruthenate in redistilled water. Coconut charcoal of Tsurumicoal HC-30 (8 28 mesh) was cordially supplied from Tsurumicoal Co. and used as received. The other reagents used were organic reagents of zephiramine(benzyltetradecyldimethylammonium chloride),* methylene bluet, sodium diethyldithiocarbamate, 8-quinolinol, Emulgen 220,:~ sodium dodecylbenzenesulfonate, methylisopropyl ketone (MIPK) and dibutyl cellosolve (DBC); also sodium hydroxide and nitric acid in JIS special grade. 399
400
E. AKATSU,C. YONEZAWAand K. MOTOJIMA
They were commercially obtained and used without purification.
Measurement and analyses Purified ruthenate was offered for chemical analyses and investigations of chemical properties before the adsorption experiment. Ruthenium and sodium were determined by activation analysis and nitrogen by spectrophotometry, after conversion of nitrogen into ammonium salt (JAERI, 1971; Horwitz, 1975). The absorption spectrum in the visible region was measured with an aqueous solution of the ruthenate. Infrared absorption spectra were measured for samples prepared by coating the powdered ruthenates on thin polyethylene film. Thermogravimetric and differential thermal analyses were done with a heating rate of 20°C min- 1. Paper chromatography was also done using the method of Wain et al. (1960), with MIPK and DBC equilibrated with 3 M nitric acid. The paper used was Top No. 53.
Determination of distribution ratio The batch method was used in the adsorption experiment. Redistilled water of 50.00 + 0.05 ml was pipetted into a 300 ml Erlenmeyer flask with stopper. The stock solution of the ruthenate below 0.1 ml was added to the water to make the initial concentration of ruthenium usually below 23 pg ml- 1. If necessary, an organic reagent given above was added as 0.5 ml of 0.5 wt Vo aqueous solution, and/or dilute sodium hydroxide or nitric acid solution below 0.5 ml was also added. After mixing well, 2.00 _+ 0.01 ml was pipetted out and reserved as an original solution. Then the weighed amount of activated charcoal, usually 100 _+ 1 mg, was put into the flask. The flask was shaken by shaker with 30 strokes rain-1. At the appropriate time, 3.00 _+ 0.01 ml of the aqueous phase was pipetted out and filtered through No. 5c filter paper. Filtration was done for all the remaining aqueous phase at the last sampling. From the filtrate 2.00 + 0.01 ml was taken and its radioactivity was measured by a NaI(TI) scintillation counter as well as the radioactivity of the original solution. The remaining filtrate at the last sampling was used to measure the pH. The distribution ratio, Ka, was calculated using Radioactivity adsorbed on 1 g activated charcoal Ka = Radioactivity remaining in 1 ml aqueous phase The desorption experiment was similar to the adsorption experiment. Activated charcoal adsorbing the ruthenate was filtered through No. 5c filter paper,
washed with 5 ml water and put into a fresh Edenmeyer flask with redistilled water of 50.00 + 0.05 ml. Shaking was continued and sampling, filtration and measurement of radioactivity were the same as the adsorption experiment. In all the cases in the desorption experiment no radioactivity was confirmed in washings. All the experiments were duplicated and done at ambient temperature. The experimental errors were calculated from the count rate of the radioactivity only. RESULTS AND DISCUSSION
The properties of the prepared ruthenate The absorption spectrum in the visible region showed no peak at 35(~900 nm and a continuous increase in optical density towards the ultraviolet. The shape of spectrum was similar to that of the dinitrocomplex (Brown, 1960), but the molar extinction coefficient was smaller than that of the dinitrocomplex. Coprecipitation with ferric hydroxide was 0.4%. This meant that the ruthenate could be kept in the aqueous phase in the floculation process of effluent treatment and be introduced into the environment. Ry values of tetranitronitrosylruthenate were 0.8 and 0 with MIPK and DBC, respectively, and the values were the same for samples having stood for 1 week, 1 month and 4.5 months. The values did not agree with those of Matsumura (1970) and the value given by Brown (1960). However, the ruthenate prepared in the present study had a chemical composition of R u : N a : N = 1:2.3 _+ 0.4:5.2 _+ 0.81, and no compound is known that is stable in water and has such a composition except for tetranitronitrosylruthenate. The other known complexes having R u ' N = 1:5 are [RuNO(NO3hH20]- and [RuNO(NO3),OH] 2(Fletcher et al., 1959; Scargill et al., 1965). These immediately decompose in aqueous solution without concentrated nitrate, and their Rs values in paper chromatography are different from the prepared ruthenate (Gardner et al., 1960; Jenkins et aL, 1956). Infrared absorption was found at 1902cm -1 for both the prepared ruthenate and its dehydrated form. The absorption agreed well with the value of 1905 cm- 1 given by Brown (1960) as specific with NO stretching in the tetranitronitrosylruthenate. Differential thermal and thermogravimetric analyses showed that the endothermic decrease of weight near 105°C exactly corresponded to the difference of weight between the dihydrate of ruthenate, Na2[RuNO(NO2),OH]2H20, and the anhydrous ruthenate, Na2[RuNO(NO2hOH], and that the anhydrous ruthenate was stable below 200°C. Thus the prepared
Adsorption of tetranitronitrosylruthenate on activated charcoal
I
"-~" :]: L L-
I/~ I I00mg
I00rng T 2oo 27o ¢ i -
me'ih'''''' ¢;
~
t~"
/t
(~
~-60mg
0
I
1
i
I
1000 2000 5000 4000 Shaking time (mini Fig.1. Distributionratiooftetranitrocomplexin water(30,
60, 80 and 100 mg are weights of charcoal added to 300 ml aq. phase at the beginning).
ruthenate in the present study was identified as tetranitronitrosylruthenate dihydrate. The dihydrate and the anhydrous ruthenate showed no visible change in air for over 1 month, but the dihydrate gradually effloresced to anhydrous ruthenate in a desiccator with calcium chloride.
Adsorptionbehaviorin water Distribution ratios, determined with various amounts of activated charcoal in water, are shown in Fig. 1, where the distribution ratios are plotted against shaking time. Two kinds of adsorption were observed: one was fast adsorption on the surface of activated charcoal and the other was slow adsorption by migration into the pores of the activated charcoal. The former was naturally larger, with activated charcoal of smaller grain size; this is seen in Fig. 1 as the result obtained with 200-280 mesh activated charcoal. In the latter adsorption an equilibrium was not attained within 4000 rain shaking time, and its reaction rate was proportional to the amount of activated charcoal. In the preliminary experiment, the distribution ratios of the former adsorption were practically the same at room temperature with the initial concentration of ruthenium 15-60/~gml ~, but the rate of the latter adsorption depended on temperature and ruthenium concentration. That is, the slope of the curve of the distribution ratio vs shaking time decreased with increasing ruthenium concentration and with falling temperature. However, the distribution ratios were practically the same within experimental error at ambient temperature with the initial concentration of ruthenium below 25 #g ml- a in the aqueous AN.E,
6 78-
(
401
phases of redistilled water, with pH 1.5 and 10. Thus the distribution ratios were usually determined with an initial concentration of ruthenium of about 20/~gml-~ in this study. In addition, the effect of temperature was not a problem in practical purposes because of the high distribution ratios of the ruthenate. In other words, the percentage of adsorption of ruthenium was almost 100 in all cases.
Effect of additionof organicreagent The effluent from reprocessing plants could contain detergents, therefore the effects of typical detergents were surveyed. The selected detergents were sodium dodecylbenzenesulfonate and Emulgen 220. Effects of 8-quinolinol were also studied, since 8-quinolinol was used in the analysis of ruthenium (Hashitani 1969). Sodium diethyldithiocarbamate was also tested, since ruthenium is known to form many kinds of thiocomplexes and to be precipitated as a sulfide in analyses of ruthenium (Walsh 1963; Wyatt 1961). Diethyldithiocarbamate is already used in the treatment of the liquid waste from nuclear reactor facilities in the removal of radioactive nuclides of cobalt, and the properties of the reagent were shown to be suitable for practical decontamination of the effluent (Motojima 1977). Tht results obtained with the reagents above are shown "n Fig. 2. Addition of these reagents lowered the distribution ratio of the ruthenate. These facts suggested that the ruthenate did not react with 8-quinolinol, diethyldithiocarbamate or dodecylbenzenesulfonate at the experimental conditions. These reagents and Emulgen 220 are adsorbable on activated charcoal themselves, hence a competition adsorption could occur and result in a fall of the distribution ratio. Motojima (1978) found the increasing adsorbability of cobalt by introducing a small amount of 8-quinolinol into the activated charcoal-aqueous solution system. In the present study, the experimental results showed that tetranitronitrosylruthenate was too stable to react and form a complex with 8-quinolinol. Therefore, cationic species of organic compound were expected to react directly with the ruthenate anion, [RuNO(NO2)4OH] 2or H[RuNO(NO2)4OH]-. As is shown in Fig. 2, the addition of methylene blue or zephiramine was effective in increasing the adsorbability of the ruthenate. This fact suggested that the ruthenate was stable enough to keep its anionic form in the aqueous phase and was adsorbed as the neutral species formed with cations of methylene blue or zephiramine. The stability of the ruthenate was confirmed by paper chromatography over one month in the aqueous solutions.
et al.,
et al.,
et al.,
et al.
et al.,
402
E. AKATSU, C. YONEZAWAand K. MOTOJIMA
106
pH40
~,
lO 5
NR _-~ 10 5
E
- ~ 10 4
10 4
-~ '-
/,Y 103
~
,y~,,8"""
£_..
/
,E
~,~___,-•
"" ~ " D l- ~Z,".-'" "[,*'.'~ DSS ~ , ~ . , ~ ~ ' D DTC
o,~, ~-0
5 _o
lo
co -.1:=:
~ 10
5~ cD
0
lO
1000 ?000 3000 4000 Shaking time (rain]
Fig. 2. Effect of addition of organic reagent (MB: methylene blue, Z: zephiramine, NR: no reagent, Ox; 8-quinolinol, E: Emulgen220, DDTC: diethyldithiocarbamate, DBS: dodecylbenzenesulfonate).
When both zephiramine and sodium dodecylbenzenesulfonate were added in the concentrations of 1.36 x l0 -2 and 1.49 × l0 -2 M each, the determined distribution ratio of 0.16 × 10-3M ruthenium was higher than that obtained in the addition of dodecylbenzenesulfonate only. After 2000 minutes of shaking. the distribution ratio was increased from 330 to 1800. At the pH of the aqueous phase of distilled water, chemical species of the ruthenate were mostly H[RuNO(NO2)4OH]-, as is given below, and the ruthenate could make more adsorbable species by reacting with the cation of zephiramine, but could not react with dodecylbenzenesulfonate.
Effect of pH Distribution ratios at various pHs are shown in Fig. 3. The distribution ratios were also determined with methylene blue or zephiramine in acidic and in alkaline media, as is seen in Fig. 4 in the example of zephiramine. The result with methylene blue was the same as with zephiramine. From these results it was found that the pH of the aqueous phase had an effect on the adsorbability of the ruthenate. The mechanism of adsorption was considered to be extremely complex and was not studied in this work, however these results could be related with the dissociation of free acid of the ruthenate, H2[RuNO(NO2)4OH]. The values of pKI and pK2 of H2[RuNO(NO2)4OH] were found, from the potentiometric titration curve given by Fletcher
:
¢ ~ ~
0
pill.5 " _ I--nu- / o .
,/pHZI [3_-' ~" " - ~ H
2
1000 2000 5000 4 0 0 0 Shaking time (rain)
o
Fig. 3. Effect of pH (adjusted by NaOH or HNO3).
et al.
(1955), to be 1.83 and 7.00. From these values, percentages of H2[RuNO(NO2)4OH], H[RuNO(NO2)4OH1- and [ R u N O ( N O 2 ) 4 O H ] 2were calculated at various pHs, as is shown in Fig. 5. On the other hand, addition of methylene blue or zephiramine was effective in alkaline media but not in acidic media, as is shown above. This adsorption behavior was understandable when the following is considered. In acidic media, the neutral species of free acid of the ruthenate was more abundant. If the neutral species was mainly adsorbed on activated charcoal, the organic cations of methylene blue or zephiramine could not be effective in increasing the adsorption. In alkaline media of pH 10, the chemical species was almost dissociated [-RuNO(NO2)4OH] 2-, which was stable in the aqueous phase. Therefore,
- r 1041"" [
E
[
•
,of
S
..Q
5
i~~----
2
pHI 5
.
F
.
pHllI,;:)-
l o l-
........
I~ ~''"
0
~ _
.........
, "I'pHIO
......... I
I
I000 2000 3000 4000 Shaking time [rain)
Fig. 4. Effect of zephirarnine in alkaline and acidic solu-
tions (Z: zephiramine).
Adsorption of tetranitronitrosylruthenate on activated charcoal
O100!i B 0) ¢..) ~o ~o
80 60 40 20
0
~
~q
P--,
I
~_. 10 4 E
"5" tO 3 10 2
0
I
I
I
2
4
6 pH
8"-
8
10
Fig. 5. (a) Relative abundance of ruthenate species (upper) (A: H2[RuNO(NO2)4OH], B: H[RuNO(NO2)4OH]-, C: [RuNO(NO2)4OH]2-). A + B + C = 0.1 M. (b) Distribution ratio of ruthenate at 2000 min shaking vs pH (lower). adsorbability could be lower than in acidic media, and addition of methylene blue or zephiramine increased the adsorbability to form neutral and adsorbable species. In the region of pH 4-7, the results observed were more complex. Distribution ratios were read (Fig. 3) at 2000 min shaking time, and were plotted against pH in Fig. 5. The curve obtained showed a maximum at pH 5. It was interesting that the shape of curve agreed with that of the variation of relative abundance of H[RuNO(NO2)4OH]- in Fig. 5. On the other hand, addition of methylene blue or zephiramine increased the distribution ratio of the ruthenate at pH 7, as Fig. 2 shows. These facts demonstrated the higher adsorbability of neutral species formed by H[RuNO(NO2)4OH]- and cations of zephiramine or methylene blue over that of the monovalent anionic species of the ruthenate. However, the distribution ratio at pH 4-7 was too high to deny the strong adsorbability of the anion of H[RuNO(NO2)4OH]-. The reason why the monovalent anion was so strongly adsorbable was not clear, but similar adsorption was found in the activated charcoal-nitric acid system (Akatsu et al., 1965). The adsorption behavior of pertechnetate also agreed with the change of amount of TcO~- species. Pertechnetic acid has a pK of - 1 . 5 (Cobble, 1964), and species of the free acid HTcO4 increased with increasing acidity, but the high adsorbability of technetium decreased with increasing acidity. Chromate was also found to be similarly
403
adsorbed. The chemical species of chromate changed due to the pH of the aqueous phase: CrzOT:-, CrO 2-, HCrOg and H2CrO4. The adsorbable range of pH was near to the pH region where HCrO£ and CrO 2predominate (Sillen et al., 1971). In alkaline media, where CrO 2- predominates, chromate was not adsorbable. Thus, the monovalent anion seemed to be adsorbable. If monovalent anionic species could be adsorbable, the monovalent anionic species formed by a [RuNO(NO2)4OH] 2- and a monovalent cation of zephiramine might be also adsorbable when zephiramine was added to the ruthenate solution in alkaline media. In addition, many of the adsorbable elements on activated charcoal were anionic, or possibly could form anionic or neutral chlorocomplexes. They were iodide, iron(Ill), gold, palladium(II), platinum, iridium and osmium (Akatsu et al., 1965; Ikeda et al., 1961). The ruthenate in alkaline media was also adsorbable, although distribution ratios were about 100. These facts suggested a possible chemical interaction of activated charcoal and these anions. The surface of activated charcoal was known to have functional groups of phenol, carboxyi, 1,4-quinone form and nitro and nitrogen in ring form (Studebaker et al., 1956). However, these groups are not considered to react simply with the inorganic anions given above. Further discussions need the elemental analyses of the used activated charcoal and the precise determination of temperature dependence of distribution ratios to estimate the heat of adsorption and distinguish between chemical or physical adsorption. Matsumura and Ishiyama (1970) also reported the adsorption of ruthenium on charcoal. They used powdered charcoal and, naturally, an equilibrium was attained to 30 min shaking. They also reported good adsorption of tetranitronitrosylruthenate at pH 2-9, adjusted by hydrochloric acid and sodium hydroxide. These results almost agree with the present authors' data. Desorption
When the distribution ratio of desorption was measured by changing the aqueous phase with a fresh one, jumping values were found. This fact meant that the ruthenate adsorbed was not easily desorbed. Consequently, the higher initial concentration of ruthenium, about 60/~g ml- 1, must be, used in the desorption experiment to measure the radioactivity of the desorbed ruthenate in the aqueous phase. Similar behavior was shown by the ruthenate adsorbed with zephiramine, methylene blue and those adsorbed from acidic and alkaline solution. The distribution ratios of desorption are summarized in Table 1. The slow
404
E. AKATSU, C. YONEZAWAand K. MOTOJIMA Table 1. Distribution ratios of desorption Adsorption
Desorption
[Ru]i
Reagent
pH*
Reagent
pH*
52 15 28 15 54 18 19
None HNO 3 HNO 3 NaOH Zephiramine Zephiramine Methylene blue
6.6 1.7 1.7 10 6.4 5.8 5.0
None None NaOH None None None None
6.1 3.3 8.0 6.5 5.4 7.4 7.4
Kat 3 4 3 1 7 8 3
× × × × × x
104 105 105 105 104 104
×
104
[Ru]~ = initial conc. of Ru in/~g ml- 1 * Measured after the last sampling was done at 300(0000 min shaking. t The radioactivity of the aqueous phase was near to the background in all cases.
rate of adsorption and high distribution ratio of desorption suggest a type of chemical adsorption. CONCLUSION Tetranitronitrosylruthenate was shown to be stable in the aqueous phase in the range pH 1.5-11, and not to be coprecipitated with iron(III) hydroxide. Hence the ruthenate could be retained in the supernarant fluid after treatment of chemical precipitation and introduced into the environment. O n the other hand, tetranitronitrosylruthenate was highly adsorbable on activated charcoal, and the adsorbability was increased by addition of cationic species such as methylene blue or zephiramine in neutral or alkaline media. The adsorbed ruthenate was difficult to desorb with water. This could be useful in the treatment of the effluent from the reprocessing plant for the removal of tetranitronitrosylruthenate. Acknowledgement--The authors thank H. Kamiyama and K. Shimooka for their contribution to the measurement of infrared adsorption spectra and differential thermal and thermogravimetric analyses, respectively.
REFERENCES
Akatsu E., Ono R., Tsukuechi K. and Uchiyama H. (1965) J. nucl. Sci. Technol. 2, 141. Brown P. G. M., Fletcher J. M., Hardy C. J., Kennedy J., Scargill D., Wain A. G. and Woodhead J. L. (1958) Proc. 2nd Int. Conf. Peaceful Uses of Atom. Energy~ Geneva, P/31, Vol. 17, p. 118, United Nations, Geneva. Brown P. G. M. (1960) J. inorg, nucl. Chem. 13, 73.
Cobble J. W., Koltoff I. M., Elving P. J. and Sandell E. B. (Eds) (1964) Treatise on Analytical Chemistry, Part II, Vol. 6, p. 407, Interscience, New York. JAERI (1971) The Committee on Analytical Chemistry of Nuclear Fuels and Reactor Materials, JAERI-4053, p. 43. Fletcher J. M. (1958) J. inorg, nucl. Chem. 8, 277. Fletcher J. M., Jenkins I. L., Lever F. M., Powell A. R. and Todd R. (1955) J. inorg, nucl. Chem. 1, 378. Fletcher J. M., Brown P. G. M., Gardner E. R., Hardy C. J., Wain A. G. and Woodhead J. L. (1959) J. inorg. nucl. Chem. 12, 154. Gardner E. R. and Brown P. G. M. (1960) AERE R-3551. Hashitani H., Katsuyama K. and Motojima K. (1969) Talanta 16, 1553. Horwitz W., Senzel A., Reynolds H. and Park D. L. (Eds) (1975) Official Method of Analysis of the Association of Official Analytical Chemists (12th edn), p. 17, The Assoc. of Official Analytical Chemists, Washington, D.C. Ikeda N. and Mitsubayashi T. (1961) Radioisotopes 10, 245. Jenkins I. L. and Wain A. G. (1956) J. inorg, nucL Chem. 3, 28. Matsumura T. and Ishiyama T. (1970) A. Rep. Radiat. Center, Osaka, I1, p. 44. Motojima K., Katsuyama K. and Yamazaki Y. (1977) Ann. nucl, Energy 4, 449. Motojima K., Tachikawa E., Kamiyama H. and Imahashi T. (1978) Ann. nucl. Energy 5, 5. Scargill D., Lyon C. E., Large N. R. and Fletcher J. M. (1965) J. inorg, nucl. Chem. 27, 161. Sillen L. G. and Martell A. E. (1971) Stability Constants of Metal-ion Complexes, Suppl. No. 1, The Chemical Society, London. Studebaker M. L and Huffman E. W. D. (1956) Industrial Engng Chem. 48, 162. Wain A. G., Brown P. G. M. and Fletcher J. M. (1960) J. inorg, nucl. Chem. 12, 346. Walsh T. J. and Hausman E. A. (1963) Treatise on Analytical Chemistry, Part II, Vol. 8, p. 379, Interscience, New York. Wyatt E. I. and Rickard R. R. (1961) NAS-NS-3029.