Analytical Methods for Ruthenium-106 in Marine Samples

IWASHIMA, K.

Department of Radiological Health, Institute of Public Health, Tokyo, 108

(Received, May 8, 1972)

ABSTRACT

Analyses of ruthenium-106 in marine environmental samples, which are useful for monitoring within a control context, have been developed. The quantity and the quality of the samples applied for the analyses are as follows : two l of sea water filtered through a millipore filter of 0.45 E.cm pore size ; two g (wet weight) of bottom sediments prepared by sieving in sea water ; or the edible part of marine organisms equivalent to ca. 1 g ash. In order to equilibrate 106Ru in the samples with ruthenium carrier and to dissolve the samples completely, marine organisms and sediments are ashed at 400•[500•Ž, and then fused with a mixture of potassium hydroxide and potassium nitrate. Sea water is heated in the presence of an oxidant in an alkaline medium.

Ruthenium is extracted with carbon tetrachloride as ruthenium tetroxide, and then back-extracted with sodium hydroxide solution containing a reducing agent. Hydrous ruthenium oxides are precipitated from the extract and subjected to ƒÀ-activity measurement with a low background

gas-flow counter by use of a 40 mg/cm2 aluminum absorber. The loss of ruthenium throughout ashing and chemical procedures was found to be negligibly small by a tracer experiment. Chemi

cal yields were 89•}5 % and the ratio of chemical and radiochemical yields was 0.98•}0.03. Sen sitivity of the method (3ƒÐ) is 0.2 pCi and the whole chemical procedure takes 3•`4 hours. De

contamination factors for the other activities were: > 3 •~ 104 for 54Mn and 59Fe, 3 •~ 103 for 60Co, 85Sr and 131I , 2•~103 for 137Cs, 1•~103 for 65Zn and 95Zr-95Nb, and approximately 102 for radio nuclides of thorium and uranium series. A rapid ƒÁ-spectrometric technique for 106Rh has also

been developed in which 106Ru was coprecipitated with cobalt sulfide from sea water.

INTRODUCTION

The first reprocessing plant of nuclear fuels in Japan is expected to be in ope

岩 島 清:国 立 公 衆 衛 生 院 放 射 線 衛 生 学 部 港 区 白金 台4-6-1,〒108 ration in 1973. The reprocessing produces low level liquid waste containing a va riety of fission products and other radioactive nuclides which are removed in part before the discharge into water areas. The chemistry of ruthenium in solution is quite complicated and the complete removal can not be effected easily.1.2) The principal chemical forms of ruthenium in nitric acid solutions of irradiated fuel are nitrosyl complexes such as nitrato and nitro complexes of nitrosylruthenium (RuNO), free RuNO3+ ion, and Ru-O-Ru complexes.') These complexes are persistently pre sent in the various subsequent reprocessing stages and are detected in rinsing waters, extraction solvents etc. used in the reprocessing.3) Ruthenium-106, especially because of its long half-life (1.0y), is found abun dantly in the radioactive effluents from nuclear fuel reprocessing plants, being a critical radionuclide for some waste disposal operations,4'5) so that this radionuclide is one of the most important nuclides in the context of controlling marine radio active contamination.1-8) For example, the waste discharged daily from the above mentioned reprocessing plant in Japan is planned to contain 1 Ci total f3-activity of which 0.2 Ci is '(16Ru when the cooling time is set longer than 150 days and 1 ton of fuel is treated daily.9) Preoperational researches on the effects of waste re lease into sea were coordinated by the Japan Atomic Energy Safety Research As sociation and the first report of the Evaluation Committee pointed out that the critical radionuclide would be 1°6Ru and the critical pathways through shellfish such as clams and oyster which are popularly consumed in this country. Thus, a simple method of 106Ru determination in the marine environmental samples is urgently needed for wide use in a control context. Few methods for radiochemical determination of 106Ru in marine environmental samples have been reported. Furthermore, in most cases preliminary treatments of samples have not sufficiently been examined. Considering the situation that our knowledge on the chemical and physical states of radioruthenium in the marine environment is quite limited,"' 14) the following points should be examined carefully : how to treat the suspended materials in sea water, how to preserve water samples or how to extract ruthenium completely from organisms and sediments. A method proposed by the WHO/FAO/IAEA Meeting15) is very tedious and requires much skillfulness. Tsuruga'6.17) proposed a method of fusing ash of organisms with the mixture of potassium hydroxide, potassium carbonate and potassium nitrate, pre cipitating ruthenium as sulfide, and reducing it to metallic state. However no in formation on the radiochemical purity was given, and the procedure takes a long time. Two methods have been reported on the separation of ruthenium. One is re precipitation as sulfide from an acid solution (3 M hydrochloric acid-0.1 M hydrofluoric acid)18'19) and another is distillation of ruthenium as tetroxide.2°-22> One of the disad vantages in the former is incomplete extraction with the acid solution. In the latter a disadvantage is a difference in the distillation rate between ruthenium in samples and that added as the carrier. Furthermore, the distillation technique is very com plicated. In the case of treating bottom sediments, there has been completely lack ing in the consideration on the removal of naturally occurring radionuclides of thorium and uranium series. 14.23.24) Preliminary concentration of ruthenium from sea water for the radiochemical analysis was effected by coprecipitation with magnesium hydroxide and subsequent extraction with carbon tetrachloride,25) or by coprecipitation with iron (III) hydroxide and subsequent distillation or extraction .26> These methods are rather complicated. The sulfide precipitation seems the most promising method of the preliminary con centration for the r-spectrometric determination. However, the effect of chemical form of ruthenium on the recovery in this sulfide method has not been examined. 27,28) In the case of comparatively high levels of radioruthenium, simple procedures such as drying of organisms or sediments and evaporation to dryness of sea water are sufficient for subsequent r-ray spectrometry, 29-31>while, in the case of the low levels, it should be mentioned that chemical procedures are required in some stages of the determination of radioruthenium. Scope of the method of analysis In view of lacking in information about the relation between chemical forms of radioruthenium and its recovery and about radiochemical purity of the final pre cipitate, this study also tries to establish a simple and reliable method in the routine determinations in a control context. In considering a scheme of analysis, it is essential to have some ideas on the required lower limit of detection for a radionuclide concerned in order to determine sample size to be analyzed and to select instruments of radiations measurements to be used. On the basis of the ICRP recommendations on the dose-limit to general public, the minimum consumption rates for important groups of marine foodstuffs and the concentration factors in these organisms, the required lower limits of detec tion in environmental samples can be derived. An example of such limits for a variety of fission products and induced radionuclides was established by a panel organized by the International Atomic Energy Agency (IAEA).32> In Table 1 are shown the data for lo6Ru derived from the literature. 32~ It is possible to estimate

Table 1. Required lower limit of detection of 106Ru in sea water in waste dis posal operations

* ICRP value for the public . * Lower limit is defined as 1 % of the concentration of sea water in equilibrium with or ganisms contaminated at level compatible with permissible ICRP value. The lowest concentration among those derived from three types of organisms. the concentration of 106Ru in sea water in equilibrium with organisms contaminated at level compatible with the permissible ICRP value, and the required lower limit of detection is defined as 1 % of this concentration. The size of various samples are determined in the following way. Sea water: By the use of a low-background gas-flow counter with a 47r type detector, (3-ray from 106Rh can be measured by a counting efficiency of 66.4 through Al-absober (40 mg/cm2), and when the activity is measured for 100 min., 0.32 pCi of 106Rh can be determined with a statistical error of ±0.10 pCi,, or 0.4± 0.13 pCi in original sample can be determined when the chemical yield is 80 %. Consequently, the lower limit of detection of 0.2 pCi/l can be covered by taking 2 liters of sea water sample for the analysis. Marine organisms : The lower limit of detection for edible sea organisms can be derived from 1 % of the permissible daily intake of 106Ru (2.2 x 104 pCi) divided by the maximum consumption of each organism (1,000g for fish, 500g for seaweed and 100g for shellfish). The limits are 0.2, 0.4 and 2.2 pCi/g fresh of fish, seaweed and shellfish, respectively. Practically, a procedure with one gram ash is thought to be desirable for the sample preparation and the radioactivity measurement. Sediments : The concentration factors in sediment have been reported as 11,000 for silt,33) or larger than 103.34) When 103 is taken for the sake of safety, the re quired lower limit for sediment which is in equilibrium with the activity of sea water (0.2 pCi/l) is 0.2 pCi/g. Thus, the required smallest amount of sediment is 2 g.

MATERIALS AND METHODS Radionuclides and reagents Radionuclides used are as follows : 106Ru: RuCl3 (8 M HCl solution) C. F., 1311: NaI (0.05 % sodium thiosulfate solution), 137Cs: CsC1 (0.01 M HCl solution) C. F., 95Zr-95Nb : Oxalate (0.1 M oxalic acid solution) C. F., 54Mn : MnCl2 (0.1 M HCl solution) C. F., 59Fe : FeC13 (0.1 M HCl solution) 8.9 Ci/g, 15Zn : ZnCl2 (0.01 M HCl solution) 6 Ci/g, 60Co : CoC12 (0.01 M HCl solution) 1 Ci/g, 85Sr : Sr(N03)2 (0.1 M HNO3 solution) 3.4 Ci/g. Ruthenium carrier solution : Weigh exactly ammonium aquopentachlororuthe nate (III), (NH4)2[Ru(H2O)C15], (1.6445 g) was dissolved in 0.01 M hydrochloric acid to make up 100 ml. 5 mg Ru/ml. 106Ru standard solution : A standard sample 106RuC13 (1.5 M HC1 solution, 1.77 mCi/g) standardized at the Electric-Technical Laboratory, Agency of Industrial Science and Technology was diluted. Buffer solution : 3 M acetic acid-sodium acetate solution was used. Sodium hypochlorite solution (6-10 %) : Chlorine content was titrated with 0.1 N sodium thiosulfate solution immediately before use. Carbon tetrachloride : When reducing contaminants are present, it is purified by the method proposed by Surasiti and Sandel1.35) Thorium ore : 0.1 % Th+0.004 % U, NBL (New Brunswick Laboratory, AEC, U. S. A.) Analysed Sample No. 80. Preparation of tracer solutions Carrier-free 106Ru in hydrochloric acid solution (8 M) was obtained from the Radiochemical Centre, Amersham, England. The 106Ru was purified by distillation in the form of ruthenium tetroxide before the preparation of tracer solutions.36 Radiochemical purity was examined by r-spectrometry with a solid-state detector Ge (Li). No photo-peak was detected other than 0.513, 0.624 and 1.05 MeV which characterize 106Rh in radioactive equilibrium with '°6Ru. The method of preparation of radioactive ruthenium complexes was mainly based on those proposed by Fletcher et al .37> and Sinitsyn et al .311)and no carrier was added in the course of the preparation. Hydrochloric acid solution of chloro complexes of 116Ru and nitrosyl-106Ru : The method proposed by Miki et al.39> was used. The final residue was dissolved in 0.1 M hydrochloric acid. Nitric acid solution of nitrato complexes of nitrosyl-106Ru : The hydrochloric acid solution of chloro complexes of nitrosyl-1o6Ru was evaporated to dryness and the residue was treated three times by fuming nitric acid and evaporated to dry ness. The final residue was dissolved in 0.1 M nitric acid. Solution of nitro complexes of nitrosyl-106Ru : The solution of chloro complexes of nitrosyl-lo6Ru was evaporated to dryness and the residue was treated with 5 % sodium nitrite solution and evaporated to dryness. The final residue was dissolved in water. The procedure of evaporation to dryness was conducted on a hot bath in any case. Preparation of contaminated samples Shellfish : A mussel (Mitilus edlis) was reared in sea water, to which chloro complexes of 106Ru or nitrato complexes of nitrosyl-106Ru were added, in a 10 1 acrylate container with aeration. Contaminated mussels were washed with natural sea water to remove loosely adhered surface contamination and then dissected into 7 parts : gill, mantle, viscera, gonad, muscle, byssus and shell. Sediment: A minimum volume of 0.1 M hydrochloric acid solution of chloro complexes of nitrosyl-106Ru was added to sea water in a 51 poly-ethylene container in order to make the change of pH negligibly small and the sea water was stored for a half year. Then, 450 cm3 wet bottom sediment obtained from coastal Pacific area was put into the sea water. After standing for another half year, the con taminated sediment was separated by sieving in sea water to make fractions of different particle size. The chemical composition of the used sediment had been analysed by Japan Analytical Chemistry Research Institute to be : SiO2, 40.66 ; Ti02, 0.74; A1203, 15.35; FeO, 2.35; Fe2O3, 5.81; CaO, 1.81; MgO, 2.40; MnO, 0.06; Na20, 4.22; K20, 1.25;H20+, 2.69; H2O-, 8.47; P2O5, 0.26; Ignition loss, 25.24%. Sea water : A minimum volume of the tracer solutions was added to natural sea water. Ses water sample distributed for the purpose of intercalibration by the International Laboratory of Marine Radioactivity, IAEA, Monaco in December 1970 was also used for checking. Apparatuses Equipments for radiation measurements: For tracer experiments, a well-type scintillation detector DDS-14342 in couple with a counter EAG-31103C type (Toshiba Co. Ltd.) was used. Low level n-counting was conducted by a low-background 4 it gas-flow counter (Japan Radiation and Medical Electronics, Inc.). Purity of tracers was determined by r-spectrometry by the use of a Ge (Li) detector (23.8 cm3 type 8101-20, Ortec Inc.) in couple with a type 450 preamplifier and a 1024 channel pulse hight analyzer (Nuclear Data Inc.). Ordinary r-spectrometric determinations were made by an anti-Compton Ge (Li) r-ray spectrometer gated with NaI (Tl) detector .4o) Spectrophotometer : Chemical yields of ruthenium were determined with Shima zu-Bausch & Lomb grating spectrophotometer, " Spectronic 20 ", with standard test tubes (outer diam. 12.7 mm, inner diam. 10.6 mm).

RESULTS AND DISCUSSION Preparation of sea water sample It has been reported that the proportion of 106Ru removed by filtration to that present in sea water is different by different areas of the sea 33,41>and this can be attributable to different patterns of suspended materials13) and plankton42) as well as different chemical states of ruthenium in sea water.10 13) The concentration factor is defined as a ratio of radionuclide concentration in an organism (or sediment) to that in sea water. The concentration factors tabulated in Table 132) were determined on the basis of sea water filtered through a mem brane filter of 1.2 ,um pore diameter, and the filtration through that of 0.1 µm pore size could remove only 5%9 more of the activity.41) The filtration is essential in connection with the concentration factor shown in Table 1. Moreover it is so to obtain reproducible results and to interpret them. For convenience and practicability, the pore size of 0.45/ -Lm was prefered in the case of coastal waters in which a comparatively large amount of suspended ma terials was present, and this size of filter was used throughout this experiment. Loss of 106Ru in sea-water during the storage was examined by tracer experi ments. Tracer solutions of 106Ru were added to sea water previously filtered and the pH was adjusted to 8.2 by the addition of sodium carbonate solution. After 3 days of standing (it is reported that 106Ru behaves differently due to the variation of chemical form during two days after the addition12)) sea water was filtered again and aliquots of the filtrate were adjusted to different pH by the addition of minimum quantities of hydrochloric acid. Radioactivities of the sea water solutions were mea sured and the values were taken as the initial references. After another 2 or 12 days of standing, an aliquot each of the solutions was taken after shaking for one minute, and its radioactivity was measured. The results are summarized in Table Table 2. Percentages of 106Ru remaining in sea water when stored in poly-ethylene bottle.

2 for different chemical forms of 106Ru originally added to sea water. In the case of the chloro complexes of 106Ru and the nitro complexes nitrosyl 106Ru, the loss of 106Ru was found to be irrespective of the pH, but in the case of the chloro complexes of nitrosyl 106Ru, the activities retained at pH 8.2 were 89.2 and 74.3% after 2 and 12 days of standing, respectively. The results give infor mation for the sample preservation that the pH of sea water should be adjusted to 1.5-2.0 by the addition of an acid. The loss of the activities during the storage may. be caused by adsorption of 106Ru onto the wall of container (poly-ethylene) . In order to check this point, another tracer experiment was conducted on sea water containing the chloro com plexes of nitrosyl 106Ru (pH 8.2). After sea water had stood in a 300 ml poly-eth ylene container for 2, 4 or 12 days, an aliquot was taken after shaking for the mea surement of radioactivity. The remaining sea water was discarded and the con tainer was washed with 20 ml of distilled water. Radioactivity still remained pro bably on the wall, was washed with 20 ml of hydrochloric acid solution of different concentrations. The results shown in Table 3 indicates that 106Ru adsorbed on the wall can be removed almost completely by the use of 20 ml of 3 M hydrochloric acid even after preservation of sea water for 12 days. Loss of 106Ru on ashing Strohal et al.43> have made an experiment on a mollusk which was artificially contaminated with 106Ruby rearing in sea water, and found only 89 % of the radio activity remained at the temperature as low as 110°C and 70 % at 450°C with a re producibility of ±9 %. On the contrary, an atomic energy authority report44) has shown that the loss of ruthenium on ashing such materials as seaweed at tempera ture of 450-500°C was not more than 5%'. In order to check this controversy, a tracer experiment on a mussel reared in contaminated sea water was conducted on the similar conditions to those of Strohal et al. Fresh organs or tissues of contaminated mussel (Mytilus edlis) were sliced, and put into a pyrex glass container (¢ 5 cm, depth 2 cm) and measured of their initial radioactivities. Then, the samples were dried at 110°C for 3 and 5 hrs., and then covered with pyrex watch glass and ashed 450-500°C for 3 and 18 hrs. At each stage of the treatment, the radioactivity was measured. Temperature rose from Table 3. Recoveries (go) of 106Ru* by washing the container

N.D.: not detected * Chemical species of "'Ru added: RuNO chloro complexes . ** Figures in parenthesis indicate recoveries by second washings .

Table 4. Loss of 1°6Ru on ashing Mussel (Percent of radioactivity retained)

* Sample I and II were , reared in sea water containing "'Ru chloro complexes Land 106RuNO nitrato complexes, respectively. room temperature to 450°C during 3 hrs. A correction for the change in sample geometry due to the reduction in the volume by ashing was made by dissolving the final ash with a hydrochloric acid solution and filling up to the same volume as the fresh sample. The results are shown in Table 4. Irrespective of the chemical form of 106Ru added to sea water, 96±2 % of the radioactivity of 106Ru incorporated in the mussel remained on drying at 110°C, and 92±1 % on ashing at 450-500°C for 18 hrs. The losses on ashing and alkali fusion of seaweed samples contaminated with 106Ru in vitro were also examined , and it was found that the over-all losses were within -+-5c116. Those results suggest that in a routine determination of a great number of samples for the purpose of moni Table 5. Loss of 106Ru on ignition (450-500°C) of bottom sediment (percent of radioactivity retained)

toring, marine organisms can be dried at 110°C and dry-ashed at 450-500°C. The loss on ignition of bottom sediment was examined similarly. As the results in Table 5 indicate, the losses are slight or negligibly small and ignition time of 3 hrs. at 450-500°C suffices the complete decomposition of organic materials in the sediment. Recovery of ruthenium in the radiochemical separation One gram ash of edible part of shellfish, 2 g of airdried bottom sediment, or 2 1 of sea water, which were artifically contaminated with 106Ru, were taken and ana lysed by the recommended radiochemical method as described in the following sec tion in order to check the chemical and radiochemical yields. The chemical yield was determined by a spectrophotometry, while the radiochemical yield by r-counting of the solution of the final precipitate of hydrous ruthenium oxides.* 45) The results shown in Table 6 indicate a good coincidence between the chemical and radiochemical yields, suggesting the attainment of equilibration between 106Ru and the carrier added. The over-all average yield was 89%. Since conditions of oxidation of ruthenium with a mixture of potassium peroxodisulfate and sodium periodate in an alkaline solution greatly affects the

* In this paper , " hydrous ruthenium oxides " indicates the oxide obtained by the reduction of ruthenium in higher oxidation states such as Ru0,2 in alkaline medium. Ref. 45 de scribes as follows. The addition of alkali hydroxide to platinum metal halide or nitrate solutions precipitates hydrous oxides. They are soluble in acids when freshly precipitated. A black solid, probably Ru203, nH2O, obtained from Ru(III) chloride solutions, is readily oxidized by air, probably to RuO2, nH2O, which can also be obtained by reduction of RuO, or RuO,2 solutions. Table 6. Recovery of ruthenium (%) in the radiochemical separation

chemical yield,46) the effects of the concentration of alkali and heating time were determined on a 1/10 scale (200 ml) of sea water sample. It was found that the heat ing without boiling for 2 hrs. at the concentration of higher than 0.3m hydroxide is important to avoid the loss of ruthenium; boiling with the supply of air resulted in the loss of 10 %o of ruthenium by vapourizing in 2 hrs. even at the concentration of 0.3m potassium hydroxide. Heating almost to boiling with a watch-glass cover, ruthenium can be retained completely in the solution. Contamination by other radionuclides Contaminations by the other radionuclides in the final hydrous ruthenium oxides precipitate were determined. Each of the following nuclides, 85Sr, 95Zr-95Nb, 1111and 137Cs (these are fission products) , 54Mn, 59Fe, 60Co and 65Zn (These are induced radio nuclides) is added to 1 g ash of seaweed together with 10 mg each of ruthenium, and the chemical separation of ruthenium was conducted according to the recom mended method 1. The final hydrous ruthenium oxides precipitate was dissolved in 6 M hydrochloric acid and its r-ac Table 7. Decontamination factors for radio tivity was measured. The results nuclides other than 116Ru. shown in Table 7 indicate the least decontamination factor 1 x 103 for 95Zr-95Nb and 65Zn , the largest one >3X101 for 54Mn and "Fe, and those for other radionuclides in between. In order to check possible con taminations with natural radionu clides of the uranium and thorium series in marine environment, espe cially in the bottom sediments, 5 g of thorium ore (see Table 8) were Table 8. Decontamination of radionuclides of mixed with 10 mg of ruthenium and thorium and uranium series. analysed by the recommended meth od. The precipitate of hydrous ru thenium oxides obtained was assayed, after a week from the separation, by an anti-Compton Ge(Li) r-ray N.D.: not detected spectrometer gated with NaI(Tl) de tector and decontamination factors were determined. The peaks of 909 KeV (abundance 30%) due to 228Ac, and 583.14 KeV (abun dance 30%) due to 208T1 were chosen for representing the thorium series nuclides and the peak of 609.4 KeV (abundance 43%) due to 214Bi for the uranium series. The results are shown in Table 8. By assuming values three times the standard as the detection limit, the decontamination factors for 208T1 and 228Ac were com puted as 1.4 x 102 and 1.2 x 102, respectively. Since 212Bi behaves similarly to 214Bi, the decontamination factor for the former was considered in the order of 102. The contamination of uranium and thorium should be investigated, as these elements exist in a significant amount in marine environment such as sea water or sediment. 14) The average concentration of uranium in sea water is 3.3X10-6 g/l ranging from 2.0 to 4.5x10-6 g/1,23>and the ranges of uranium and thorium in the sediment are 0.2-4.O x 10-6 and 1 -30 x 10-6 g/g,24) respectively. Consequently, when the decontamination factors of uranium and thorium are in the order of 102, the con tamination of them can be lowered to the order of the tenth of the detection limit for 106Ru according to the recommended method. This method was applied to a sediment sample obtained at a point about 500 m from the shore (water depth 7 m) off the site of a nuclear pile of the St. Paul's University, Takeyama, Yokosuka City. The results of analysis shown in Table 9 indicate that the concentrations of uranium and thorium were 3 and 10 x 10-6 g,/'g, respectively, in the ranges appeared in literature 24) and no detectable amount of other radionuclides than 106Ru was found. The precipitate of hydrous ruthenium Table 9. Thorium and uranium contents of bottom sediment and 106Ru determination

N.D.: not detected * Anti-Compton Ge (Li) r-ray spectrometer was used . 50 g sediment was used for Ge (Li) 1-ray spectrometry. ** 20 g sediment was used for radiochemical separation beforehand the r-ray spectrometry, while 2 g for n-counting. oxides was also measured of its R-activity, and both f3 and r-determinations were in a good accordance. These results justify the applicability of the recommended procedure. Preliminary concentration of 106Rufrom sea water for r-spectrometric determination47> Frequently, sea water can be determined directly by r-spectrometry when the level is high, while a preliminary concentration is required for low activity samples. Among the non-specific collectors for fission products and other radionuclides in sea water, sulfides are probably the most promising for radionuclides except alkaline and alkaline earth nuclides. The homogeneous precipitation by the use of thio acetamide orginally developed by Swift and Butler 48) would provide a convenient means. Sodd et al.27> applied such technique for the gross activity measurement in sea water. Chakravarti et al.28) analyzed a number of radionuclides including ruthenium in Pacific waters ranging from 10 to 50 liters in volume after precipitat ing the sulfides at pH 11 by bubbling through the hydrogen sulfide. The present study was designed to provide a preliminary collection method for radioruthenium in 5-10 liters of sea water for r-spectrometry with a detector of NaI (TI) or Ge(Li). A quick, simple and efficient collection of the various complexes of ruthenium was aimed at, and homogeneous precipitation of sulfides by the use of thioacetamide was applied. A preliminary study showed that Co2+ was the best of metal ions which make sparingly soluble sulfides in an alkaline medium. Sea water was filtered through " Millipore " filter with 0.45 um pore size. To 100 ml aliquot of filtered sea water 1 ml of 1o6Ru tracer solution and 1 ml of Co2+ solution were added. The solution was heated on a hot water bath for 10 min. and than 1 M ammonia water or 1 M sodium hydroxide solution was added to adjust the pH to predetermined values. Then, 7 ml of 4 % (w/v) solution of thioacetamide was added and the solution was heated on a hot bath for a given period of time. The sulfide precipitate formed was filtered off and the filtrate was made up to 200 ml by the addition of a diluted hydrochloric acid solution. The radioactivity 'of an aliquot (2 ml) was measured (a cpm). 1 ml of the 106Ru tracer solution was made up to 200 ml by the addition of a diluted hydrochloric acid solution and radioactivity of 2 ml aliquot was measured for comparison (b cpm). The per cent removal of 106Ru was calculated by the equation : Per cent removal = (1 alb) x 100 The radioactivities were measured by a scintillation counter for the period of time long enough to make the statistical error less than one per cent . The effect of pH of the final solution, when the precipitate was completed , on the per cent removal is shown in Fig. 1. The amount of C02+ was 1.0 mg per 100 ml of sea water, and the heating time for the precipitation was one hour. The removal of the complexes was almost complete at the pH higher than 9.0. At the pH of 9.9, the precipitate of magnesium hydroxide began to form and the filtration was more time-consuming. Chakravarti et al. used a higher pH 11.6 for sulfide precipitation with ruthenium carrier and probably a large amount of magnesium hydroxide must have formed since they needed one night to settle the precipitate . In this study, the precipitate of cobalt sulfide at pH 9.5 is easily filterable and the filteration requires only fiveminutes per 100 ml solution when a filter paper (Toyo No. 5 B) is used under gravity. Under the condition of 1.0 mg Co2+ per 100 ml of sea water and pH 9.3 for the precipitation, the effect of heating time after the addition of thioacetamide was examined for the chloro complexes of 106Ru. The per cent removal of the radioactivity ranged only between 97.5 and 98.5 when heating time was varied be tween 30 and 120 minutes. The val ues are a little but significantly higher than the per cent removal 93 obtained for the chloro complexes in the experiment of the pH effect (Fig. 1). The only difference between the two experiments is the period for which the tracer solution was stood after the preparation. The tracer solution was prepared imme Fig. 1. The effect of pH on the recoveries of diately before the experiment of the io6Ru complexes in the Co2+ -thioaceta pH effect, while for the latter ex mide method. periment one month had elapsed since the preparation. The differ ence in the per cent removal might be attributed to a change in the rela tive composition of ruthenium com plexes by aging. 49) The effect of the amount of co balt ion carrier on the recovery was examined under an optimum condi tion (pH: 9.3-9.5, heating time : one hour). The results are shown in Fig. 2. One milligram quantity of Co2+ was required for the complete re covery of ruthenium complexes under consideration and there seemed no further improvement by the addition of more quantity of co balt ion. It has been reported that the Fig. 2. The effect of the amount of cobalt ion carrier on the recoveries of 106Ru nitro complexes of nitrosylruthenium complexes in the CoZ+-thioacetamide cannot be effectively carried down method. by the sulfide or hydroxide precipi tation,' but the present procedure carries this form of the complexes as effective as the other forms. An aged solution of the chloro complexes of 106Ru showed a recovery of 99 , in contrast to only 95.5 % in the case of a fresh solution, the reason of which was stated before. When sodium hydroxide is added to adjust pH of the solution instead of adding ammonia water, careful adjustment of pH was required and colloidal precipitate of the sulfide was formed. Since the filteration of this precipitate is time-consuming, ammonia water is preferable to sodium hydroxide. The time required for treating 5 liters of sea water was two hours for the pre cipitation and twenty minutes for filtration by a 47 mm " Millipore " filter with 0.45 ,um pore size under suction. The present study was designed originally to provide an effective collection method for radioruthenium in sea water, however, in view of the fact that metal sulfides can carry fission products and corrosion products other than alkaline and the alkaline earth nuclides, the present method may be effectively used in the moni toring of these radionuclides in sea water. Recoveries of these radionuclides were found : 0.3% for 85Sr and 137Cs, 96 for 95Zr-95Nb, 99 % for 59Fe, 60Co and 65Zn. Recommended Methods The following analytical methods for 106Ru in marine environmental samples are recommended on the basis of the examinations described above. Several useful methods for analysing ruthenium, which have been reported hitherto, applied in the recomended methods.

1. Radiochemical Analysis Shellfish and Seaweed (1) Remove adhered sediment by washing quickly with water. Blot with filter paper to remove adhered water and weigh to obtain fresh weight. (2) Dry at 110°C completely and put into a porcelain dish. Heat with a burner to char until no gases evolve any more (About 400°C). Care should be taken not to burn. (3) Put the dish into an electric furnace and ash at 450-500°C. After cooling weigh to obtain ash content. (4) Weigh 1 g ash into a nickel crucible and add 2 g of potassium nitrate, potassi um hydroxide and 2 ml of ruthenium carrier solution (10 mg Ru). Mix well, dry by an infra-red lamp and then fuse for one hour at 500°C in a muffle furnace. (5) After cooling, add about 20 ml of water in several portions to the crucible and heat. Transfer the ingredient into a 50 ml centrifugal tube. (6) Add 5 ml of 10 % sodium hypochlorite solution, mix and stand for a half hour. (7) Centrifuge for 2 min. at 3,000 rpm and transfer the supernatant to a 100 ml separatory funnel (A). (8) Add one drop each of 3m sodium hydroxide and 109/0 sodium hypochlorite solution and 10 ml of water to the residue in the centrifugal tube, mix well and then centrifuge for 2 min. at 3,000 rpm. Transfer the supernatant to the separatory funnel (A) and combine the solutions. (9) Add 30 ml of carbon tetrachloride, 5 ml of 6m hydrochloric acid and 10 ml of 3m acetic acid-sodium acetate buffer solution to the separatory funnel (A). Shake the funnel for one minute and then transfer the organic phase to another 100 ml separatory funnel (B). (10) Add 10 ml carbon tetrachloride to the aqueous phase in the funnel (A), shake for one minute and then combine the organic phase with that in the funnel (B). (11) Add 20 ml of water containing one drop of 10 % sodium hypochlorite solution to the organic phase in the funnel (B) and shake for one minute. Transfer the organic phase to another separatory funnel (C). Discard the aqueous phase. (12) Add 20 ml of 3 M sodium hydroxide to which 3 drops of 1 M sodium hydro gensulfite were added and shake for one minute. When the colour remaines in the organic phase, add another drop of 1 M sodium hydrogensulfite and shake for another minute. When a black precipitate forms in the aqueous phase, add one drop of 10 sodium hypochlorite solution and shake to dissolve the precipitate. (13) Transfer the aqueous phase to a 50 ml measuring flask, fill up with water and mix well. Take 1 ml aliquot and obtain the chemical yield of ruthenium by the procedure described under Yield Determination. (14) Transfer the remainder of the solution in the measuring flask to a beaker and wash the flask twice with water. Add 2 ml of ethanol , mix well and heat on a water bath for a while to precipitate hydrous ruthenium oxides . (15) Filter the precipitate through a filter paper (Toyo No. 5c) by suction, wash successively with 5 ml of water containing one drop of 10 M sodium hydroxide , with 5 ml of ethanol, then with 5 ml of ethyl ether. (16) Dry the precipitate with filter paper by the use of a vacuum desiccator, mount, covered with a 40 mg/cm2 Al absorber, and measure the radioactivity with a low background n-gas flow counter (N _-4-AN cpm). Bottom sediments : (1) Collect sediments together with sea water and sieve the sediment in sea water to obtain three fractions : coarse sand (2-0.2 mm) , fine sand (0.2-0.02 mm), and silt and clay (<0.02 mm). (2) Dry at 110°C completely and weigh after cooling. (3) Take 2 g sample in a nickel crucible, add 2 ml ruthenium carrier solution (10 mg Ru), mix well, dry by an infra-red lamp and then ignite at 450-500°C for one hour in an electric furnace. (4) After cooling, add 4 g each of potassium hydroxide and potassium nitrate, and mix well. Fuse for one hour at 500°C in a muffle furnace . (5) After cooling, add about 30 ml of water in several portions to the crucible and heat. Transfer the ingredient into a 50 ml centrifugal tube. (6) Add 8 ml of 10 % sodium hypochlorite solution, mix and stand for a half hour. (7) Centrifuge for 5 min. at 3,000 rpm and transfer the supernatant to a 150 ml separatory funnel (A). (8) Add one drop each of 3 M sodium hydroxide and 10 % sodium hypochlorite so lution and 10 ml of water to the residue in the centrifugal tube, mix well and then centrifuge for 2 min. at 3,000 rpm. Transfer the supernatant to the separatory fun nel (A) and combine the solutions. (9) Add 30 ml of carbon tetrachloride, 10 ml of 6 M hydrochloric acid and 10 ml of 3 M acetic acid-sodium acetate buffer solution to the separatory funnel (A). Shake the funnel for one minute and then transfer the organic phase to another 100 ml separatory funnel (B). (10) Add 15 ml carbon tetrachloride to the aqueous phase and after that follow the procedures from step (10) described under Shellfish and Seaweed. Sea Water : (1) After collection of 2 1 sea water, immediately filter through a millipore filter of 0.45 µm in pore size. Take the filtrate in a poly-ethylene bottle and add 2 ml concentrated hydrochloric acid to make the pH approximately 1.5, then transfer the sea water sample to laboratory. (2) Pour sea water into a 3 l beaker, wash the poly-ethylene bottle with 150 ml of 3m hydrochloric acid and combine the washing with sea water. (3) Add 2 ml of ruthenium carrier solution (10 mg Ru) and heat the solution al most to boiling with occasional stirring. (4) After cooling, add 65 g potassium hydroxide, 20 g potassium peroxodisulfate and 10g potassium periodate. (5) Cover the beaker with a watch glass and heat the solution at 90-95°C (do not boil) for 2 hrs. Wash the watch glass with 10 % sodium hypochlorite solution and combine the washing with the solution. (6) Cool the solution with ice-water and transfer to a 31 separatory funnel (A). Add 200 ml carbon tetrachloride, adjust the pH to 4-5 by the addition of 6 M nitric acid and shake for 5 min. (7) Transfer the organic phase to another separatory funnel (B). Add 50 ml carbon tetrachloride to the aqueous phase in the funnel (A), shake for one minute and then combine the organic phase with that in the funnel (B). Discard the aqueous phase. (8) Add 150 ml of water containing 15 drops of 10 % sodium hypochlorite solution to the organic phase in the funnel (B) and shake for one minute. Transfer the or ganic phase to another separatory funnel (C). Discard the aqueous phase. (9) Add 25 ml of 3 M sodium hydroxide to which several drops of 1 M sodium hy drogensulfite were added, and shake for one minute. Transfer the organic phase to another separatory funnel (D) and take the aqueous phase in a 100 ml measuring flask. (10) Repeat the first half procedure of step (9) on the organic phase in the funnel (D). Discard the orgnic phase and combine the aqueous phase with the solution in the measuring flask. When a black precipitate forms in the aqueous solution, add one drop of 10% sodium hypochlorite and shake to dissolve the precipitate. (11) Fill up with water to 100 ml and mix well. Take 2 ml aliquot of the solution and obtain the chemical yield of ruthenium by the procedure described under Yield Determination. (12) Transfer the remainder of the solution in the measuring flask to a beaker and follow the procedures from step (14) described under Shellfish and Seaweed. Yield Determination : (1) Take 2 ml of ruthenium carrier solution (10 mg Ru) in a 50 ml measuring flask. Add about 20 ml of water, then add 10m sodium hydroxide until black precipitate forms and 10 drops excessively. (2) Add dropwise 10 % sodium hypochlorite solution to dissolve the precipitate, fill up to 50 ml with water and mix well. Take 0.50, 0.75 and 1.00 ml of this solu tion separately in 50 ml beakers. (3) Add 10 ml of 3m sodium hydroxide and then 10 ml concentrated hydrochloric acid, mix well, covered with a watch glass and heat for 15 min. on a water bath, respectively. Then, quickly cool the solution. (4) Transfer the solution into a 25 ml measuring flask, wash the beaker with a small amount of water, combine the washing and fill up to 25 ml with water. (5) Measure the absorbance of the solution at 485 nm5o.51) against 4m hydrochlo ric acid within 2 hrs after the preparation. (6) Draw a working curve for calibration by taking the absorbances against the ruthenium concentrations. (7) Take 1/50 th aliquot of the solution in the measuring flask (1 ml for step (13) under Shellfish and Seaweed, or 2 ml for step (11) under Seawater) in a 50 ml beaker . After that, follow the procedures described in steps (3) through (5) to obtain the absorbance. (8) Calculate the chemical yield of ruthenium (Y %) on the basis of the calibra tion curve obtained by step (6). Standardization* (1) Take 2 ml of ruthenium carrier solution (10 mg Ru) in a 50 ml beaker and add 1 ml of 106Ru standard solution (a dpm) and about 10 ml of water. (2) Add 10m sodium hydroxide until black precipitate forms and then 10 drops excessively. (3) Add 10%' sodium hypochlorite solution dropwise to dissolve the precipitate, stand for a half hour, add 2 ml ethanol and then heat for a half hour on a hot-water bath to precipitate hydrous ruthenium oxides. (4) Filter the precipitate and measure the radioactivity (b cpm) as described in steps (15) and (16) under Shellfish and Seaweed. (5) Calculate the counting efficiency (E %) by the equation : E = b/a x 100 Calculation (1) Calculate the activity of 106Ru (pCi/g or 1) in the sample by the equation :

0.693 t 106Ru= (N±pN)e 365 'M'1 100 100 50 1 E -Y-49-2.22 where N±pN : Measured activity (cpm) t : Days between sampling and measurement M : Sample weight (g) or volume (1) E : Counting efficiency (% ) Y : Chemical yield (%9)

2. Rapid r-Spectral Method for Sea Water. (1) After collection of 5 1 sea water, immediately filter through a millipore filter of 0.45 µm pore size. Take the filtrate in a poly-ethylene bottle and add 5 ml con centrated hydrochloric acid to make the pH approximately 1.5, then transport the sea water sample to laboratory. (2) Pour sea water into a 5 1 beaker, wash the poly-ethylene bottle with about 400 ml of 3m hydrochloric acid and combine the washing with sea water. (3) Add 5 ml of cobalt carrier solution (CoClz, 10 mg Co/ml), heat to boil and then

Corrections for self-absorption and chemical yield are unnecessary, because of the high p-ray energy of io6Rh (2-3.53 MeV) and the almost quantitative precipitation of hydrous ruthenium oxides from alkaline solution. adjust the pH to about 9.7 by the addition of 7 M ammonia water. (4) Heat at 90-95°C for 2 hrs. on a hot plate after the addition of 30 ml of 4 9/0 (w/v) thioacetamide solution. (5) Filter the precipitate through a millipore filter of 0.45 µm pore size and 47 mm in diameter by suction, wash with 10 ml of water containing one drop of 1 M am monia water, and dry. (6) Measure the peak activity at 0.62 MeV by the use of a low-background Ge (Li) -r-ray spectrometer .

Application of the Methods in the Intercalibration Programme of IAEA. The International Atomic Energy Agency has implemented a programme for intercalibration of analytical methods through the distribution of marine radioac tivity reference samples. In December 1970, two samples of sea water were dis patched to the Institute of Public Health from Laboratory of Marine Radioactivity, IAEA, Monaco. The samples were naturally contaminated with 106Ru and other radionuclides. They were consisted of two : one (SW-I--1) was lower and another (SW-I-2) was higher in activity levels. The radiochemical methods recommended in this report were applied to both samples, SW-I-1 and SW-I-2. Since 1 litre each of sea water was used for analysis, the analytical scale was halved except the amount of ruthenium carrier. The rapid r-spectroscopy was applied to SW-I-2 only and a Ge (Li) detector (23.8 cm' Ortec 810-20) was used inside a 10 cm iron shield in couple with a 1024 channel pulse height analyser. The results are summarized in Table 10 toghether with pertinent information supplied by the Monaco Laboratory recently. The analytical data ob tained by the present work are within the ranges of highest frequency and in good

Table 10. Analytical results of 106Ru in SW-I-1 and SW-I-2 sea water sam ples distributed by IAEA for intercalibration.

* Personal communication from Dr . R. Fukai. accordance with the mean values.

ACKNOWLEDGEMENT The author expresses his gratitude to Dr . N. Yamagata, Head of Department of Radiological Health, the Institute of Public Health , for his continuing guidance and encouragement during the course of this work. Thanks are also due to Dr . T. Koyanagi, Marine Radioecological Research Station, National Institute of Radiologi cal Sciences, for preparation of radioactive contaminated organisms , and to Dr. M. Okano, the Institute of Physical and Chemical Research , for measurements of radi oactivity by anti-Compton r-ray spectrometer . The author takes this opportunity to express his thank to Prof . H. Okuno, St. Paul's University, for his suggestion that the chemistry of ruthenium would be an interesting subject of study. Thanks are also due to Profs . T. Ishimori and K. Mizumachi for their discussions and encouragements .

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