ANALYTICAL SCIENCES VOL. ? SUPPLEMENT 1991 1321

RADIOACTIVE NUCLIDES IN THE ENVIRONMENT

BORIS MYASOEDOV, ALEXANDER NOVIKOV

Vemadsky Institute of Geochemistry and Analytical Kosygin str. 19, Moscow, USSR

Chemistry, USSR Academy of Sciences,

Abstract The data on preconcentration of a number of radionuclides (90Sr, 238-240pu, 241&) from natural waters by coprecipitation, ultrafiltration with watersoluble complex forming polymers, dyalisis and membrane extraction are presented. It has been shown that the precipitation as well as ultrafiltration using polyphosphates, polyalcohols and polyoxines are the most appropriate for the group preconcentration. The selective preconcentration can be performed by ion exchange (137Cs - ferrocyanides) , membrane extraction ( 90Sr crown ethers) and by ultrafiltration (238-240pu polyphosphortungstates). An additional deep purification from macrocomponents or other interfering nucludes, if necessary, can be achieved by extraction-chromatographic methods. The data on analysis of soil, water and biological objects from Chernobyl NPP obtained by abovementioned methods are presented. Among the possible sources of continuous impact of radionuclides in biosphere highly radioactive wastes are the most valuable. The preliminary isolation of long-lived transplutonium elements is very important. In the work for this purpose the solvent extraction of TPE (III) by bidentate organophosphorus reagents from strong acid, salt-containing media, extraction from alkaline solution by variuos alkylpyrocatechols and alkylderivatives of amine-alcohols as well as isolation of TPE in "unusual" oxidation states have been used.

words environment, radionuclides, determination

The clarifying of the penetration sources and study of the migration dynamics of hazardous radionuclides is one of the most important problems in environmental monitoring. It demands continual routine analysis of water, soil and various biological samples.

Meanwhile, inspite of considerable progress in the development of physical methods of determination, a preliminary isolation, separation and concentration of radionuclides remain an essential and necessary part of chemical analysis. This point can be illustrated easily on the example of determination of Pu in environmental samples. Several methods are known to be the most effective for this purpose when there is no need in separation of species to be detected from other interfering comp nents: o -spectrometry (Detection Limit (DL) is 4 108 atoms), fission-track detection (DL i 210~ atoms) neutron activation analysis ( DL is 41011 atoms) luminescence technique (3 1012) and the laser fluorescence spectroscopy (DL is 610 atoms). All the methods concerned are distinguished for their high sensitivity, but an applict4 ~n of

alpha-spectrometry requires the purification from macrocomponents and some radioisotopes ( Am); using the fission-track technique assumes a careful removal of all fission materials prior to analysis; for neutron ctivatioj analysis it is necessary to ensure the purification factors from U and Th not less than 10 - 10 7 , ' the luminescence technique can not be successfully applied without the preconcentration and removal of the interfering Np.

Moreover, when determining the Pu content in surface waters or in the subsurface water layers the concentrational stage is indispensable whichever of aforementioned methods is used.

In analysis of natural waters it is important to combine a concentration stage and investigation of a size speciation of the radioactive pollutants. The most convenient way to do this may consist in performing consecutive filtration, ultrafiltration and hyperf filtration either of untreated solutions or in the presence of complex forming substances possessing selective properties in respect to interaction with certain radionuclides. In some cases it may be expedient to employ a more traditional sorption technique or a membrane extraction instead of filtration and a dialysis instead of ultrafiltration with complex forming substances, Table I shows the data on size speciation of a number of radioactive pollutants in the bog water samples taken from the so-called "rust-colored" forest near the Prypyat' river in vicinity of the Chernobyl Nuclear Power Plant (NPP) .

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One can see that the major part of plutonium and radioruthenium is attached to colloid particles, whereas more than 90% of radiocesium has been found to be in a dissolved form. Radionuclides of strontium and rare earth elements are distributed more uniformly between the size fractions investigated.

In most cases the analysis of natural waters involves a preliminary preconcentration. Evaporation of bulky samples of water doesn't ensure good results in view of the slowness of the process and significant loss of the microcomponents to be determined. An application of the sorption or ultrafiltration technique (using water soluble polymers) for this purpose is considered to be more effective. These two techniques are similar as for their chemical nature, the main difference refers to heterogeneous character of the sorption process, while the ultrafiltration can be considered as a homogeneous one.

On some occasions the filtration technique is found to be quite effective for the concentration of radionuclides. This technique is used for the determination of radiostrontium in sea water, when this element is concentrated by coprecipitation with the alkali-earth metals carbonates.

When necessary to ensure a higher extent of concentration one of the most convenient approach may involve ultrafiltration of radioactive contaminants with watersoluble polymers possessing the complex forming properties. Fig. l shows data on the relative concentration of transuranium elements (TUB) by means of ultrafiltration on the Amicon-10 membrane in the presence of poly (ethyleneimine) oxine. Here R denotes the extent of retention which can be expressed as Cz/Cz-1 and Z means the preconcentration factor.

As follows from the data presented this process is quite effective for the group concentration of TUE from the neutral media followed by selective rinsing of the concentrate with acidic solutions or complexon containing solutions and determination of individual radionuclides. The technique is especially convenient for determination of ~. -radioactive nuclides in river water. It has been shown that the loss of Pu and transplutonium elements (TPU) does not exceed 10% for the concentration extent up to 500, the ~c -spectrometric determination of pollutants being performed directly on the surface of membrane after completing the filtration proceduret~~

In some cases it is possible to perform rather selective isolation of individual nuclides from natural waters. For instance, U (VI) is fairly good retained by poly (phosphates) , whereas Pu (IV) isretained by poly (phosphates) and poly (phosphortungstates) and so on. Synthesis of the major part of, these substances is quite expensive and most of them are not commercially available in industrial scale.

A new impulse to further development of this method has been given by an opportunity to use complexing reagents, i.e. a joint application of inexpensive, commercially available polymers (poly (ethyleneimine), polyacrylic acid, etc.) and some additives, responsible for the increase in extent of retention of radionuclides on a membrane. It has been shown that an addition of some complexones (ethylenediaminetetraacetic , diethylenetriaminepentaacetic acids ) into solution significantly increases an extent of retention of radioactive species by poly (ethyleneinune) . This fact

Table water.

1. Distribution of radionuclides (in percentages) between colloidal fractions of

Fig. 1. U(VI),

Variation of retention Am(III) for Z=10 and

pH

of of

0

Pu(IV), POX vs.

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can be explamcd uy the formation of iiiixed compounds consisting of a weatc ease anu do anionic complex of metal. Poly (ethyleneimine) -DTPA especially strongly retains radiocerium, uranium, plutonium, and americium. The loss of Am in case of filtration of solutions with pH values from 3 to 8 does not exceed 5-6%, the concentration factor being equal to 103.

The lesser extent of preconcentration is needed when carrying out the determination of radionuclides in the terrestrial biogeocenosises. In this case the classification of the biogeocenosis components and description of the soil type is followed by calcination of soil samples at 500-600°C and, further, by extraction of radionuclides by mineral acids (HNO3, HCI, HF) or by their mixtures. The subsequent procedures of deep purification of radioactive elements are, as a rule, complicated and multistage. Meanwhile, the growing interest of society to ecological problems requires an application of the convenient, rapid and reliable approaches to analysis of environmental samples. In this connection one of the main tasks among radiochemists engaged in environmental control consists now in developing of the simple, express and at the same time effective methods of isolation and purification of the separate radionuclides. It is highly desirable also to perform selective isolation of a wide variety of radionuclides from the soil leachates without of an additional adjustment of the acidity and salt composition of the solutions investigated.

As an illustration to this new approach we would like to describe here an original method of the radiostrontium and plutonium determination in the soil and agricultural samples, which has been developed in our laboratory. After an acidic leaching of a sample with 7.5 M HN03 followed by lowering of acidity of leachate down to 1.5-2,0 M, the solution is eluted through the combined column filled with LEVEXTREL (dicyclohexyl-18-crown-6 and tri-n-octylmethyl ammonium nitrate) . Then, after rinsing the column, the desorption of Pu is carried out by oxalic acid, while strontium is desorbed by hot water. Further, Pu is co-precipitated with LaF and Sr with SrCO and finally their content is determined through . -spectrometric and -radiometric counting of3 the corresponding precipitates retained on nuclear filters. It has been shown that in the process of chromatographic purification described the removal of more than 98 % of macro- and

13crocompo 24nts from a soil leachate is achieved, the separation factors for Sr and Pu from Cs and Am as main interfering components having been found to be more than 10 . This technique allows to use the chromatographic column several times with a visual control over the fraction of a spent sorbent. Table 2 shows data on determination of radiostrontium in various environmental samples (soil, mushrooms, milk, agricultural production) taken in vicinity of Chernobyl NPP.

As it is clear from data presented the technique proposed is distinguished for good reproducibility of the results and short time of analysis, which is 4-6 times less than for usually applied techniques based on precipitation the radionuclides concernedCxl. On the other hand, besides the abovementioned advantages of the chromatographic method proposed, its application in some cases may be restricted due to a limited capacity of sorbents and extractants in respect to an isolated element.

In some cases, for example, when isolating radiostrontium from concentrated sea water, it is more convenient to apply membrane extraction with dicyclohexyl-l8-crown-6. The polypropylen-supported membranes having pore diameters of 0.5 micrometers and porosity 50% show good working characteristics for one month and provide the high extent of purification of Sr from other interfering p -emitters ( 203 in respect to 734,137Cs ) as well as from various macrocomponents.

When necessary to treat large volumes of waste waters it is better to organize the extraction stage as semi countercurrent process using two centrifugal extraction apparatus or to use the principle of the emulsion membrane extraction. But the most promising way for removal of

TABLE 2. Results of determination of radiostrontium in some biogeozenosis components by newly developed chromatographic technique as compared to usually applied precipitation method.

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radioactive contents from waste waters, on our opinion, may consist in using for this purpose of LEVEXTREL or water soluble polymers mainly due to better possibility to provide a reliable disposal of radioactive wastes. The disposal of highly radioactive wastes is considered to become one of the most hazardous source of ongoing inflow of radioactive elements into biosphere. It is commonly accepted now to perform a preliminary isolation of TPE from these wastes in order to diminish their biological impact on man and nature. Depending on the chemical composition and acidity value of the waste solutions a number of extractants such as bidentate neutral organophosphorus reagents, alkylpyrocatechols and alkylderivatives of amine-alcohols have been successfully applied for this purpose.

Among all presently known types of extractants neutral organophosphorus compounds (NOPC) are the most effective extractants capable of extracting TPE in any oxidation states from acid solutions of complex compositions. Wide possibilities of varying structures of these compounds enables us to change both extractive power and selectivity of these reagents together with their compatibility with solvents. Thus, the substituents at phosphor atoms have been shown to be responsible, mainly, for efficiency of an extractant, and those at nitrogen atom for solubility. Introduction of hard fragments that fix spatial distribution of functional groups in a molecule makes it possible to change the selectivity of a reagent. Reagents with methylene bridge between phosphor atoms, or vinylene bridge which fixes phosphor groups in cis-position, and reagents with three P=O groups are of great extractive power in respect to TPE. Reagents of the second group are also of quite good extractive power, and unlike alkylenediphosphine dioxides are well dissolved in most of conventional diluents. Together with many advantages, dioxides and carbamoyl have substantial drawbacks that one can not overcome even by endeavor changing their structures, e.g. they practically do not dissolve in alyphatic diluents. One means to eliminate these drawbacks to add solubilizers, such as TBP, to solutions of bidentate reagents, that are well compatible with many organic solvents. Addition of TBP not only results in eliminating the third phase, but also provides a non-additive increase in distribution coefficients of TPE (synergistic effect). Addition of T

BP allowed application of aliphatic hydrocarbons widely used in radiochemical industries, as solvents for carbamoyl.

The most interesting feature of these reagents is so-called "abnormal arylic effect". It lies in the fact, against to the rule known for monodentate analogs, extractive power of bidentate NOPC increases essentially (by several orders in some cases) when replacing alkyl substituents at phosphor atoms by the more electronegative arylic ones. The nature of this phenomenon is not completely clear yet. The "abnormal arylic effect" may be caused by: strengthening of a complex by delocalization of electron density from phenyl rings to the cycle formed with a metal and by formation of a system of conjugated bounds in a cycle; increase in the solvation factor accompanied by an increase in complex distribution ratio due to formation of "hard" hydrophobic shell formed by phenyl rings; peculiarities in conformation behavior of aryl-substituted reagents. A conformation isomer is possible to exist with spatial distribution of P=0 groups favorable for cycle closure stabilized in some cases by stacking-interaction of phenyl rings. Owing to this effect aryl-substituted bidentate NOPC turned gave the most promises to isolation of TUE due to high extractive power in respect to only these elements, unlike to acids for which the "abnormal arylic effect" was not observed. Besides, different intensities of this effect in extractions of different metals provide some selectivity of TUE isolations, e.g.in respect to alkali and alkali-earth metals, iron, nickel and some other metals. The high extractive power of aryl-substituted bidentate NOPC gives grounds to their use to isolate and concentrate TPE from solutions with high contents of nitric acid and salts, waste solutions coming from processing of fuel elements of water-water atomic reactors (see table 3) [3, 4J.

When studying possibilities to isolate TPE from various media an unusually high efficiency of extraction from perchloric acid has been established by aryl-substituted carbamois caused by a substantial amplification of the "abnormal arylic effect" in these media (see Fig. 2). The efficiency of isolation is so high that addition of very small amounts of perchloric acid to solutions of other acids or complexing agents results in a formidable increase in the efficiencies of isolations of TPE , rare earths, U (VI) , Pu (IV) and makes it possible to concentrate these elements from various objects, first of all from environmental ones for subsequent analyses [5].

Table 3. Preconcentration of Am (III) methylene diphosphin dioxide (4Ph) oxide (Ph2Bu2) solutions.

from HNO3 and diphenyl

and nitrates solutions by tetraphenyl (dibutyl carbamoyl methyl phosphin)

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Fig. 2. Effect of HC104 on

(a) and U(VI) from dichloroethane.

extraction

H3P04 (b) of

by

Am (III)

carbamoyl

from HNO3

solutions in

Use of alkaline solutions may be promising for concentrating TUE. Extraction of metals from such solutions practically was not studied previously, although in some cases such solutions are intensively used. We have shown that extraction of TUE and many other elements is possible in the presence of complexing reagents by extractants of different classes such as quaternary ammonium bases, amines, alkylpyrocatechols, alkylated aminoalcoholes and -diketones. Some of the studied extractants, especially phenol derivatives, are of high extractive power in respect to TPE. Using these one can effectively isolate TPE from solutions with high concentrations of sodium hydroxide and carbonates of alkali metals (5-6 M). The efficiency of isolation of TUE by extraction depends also on the nature of complexing ligands capable of sustaining elements in dissolved forms. Under various conditions the elements can be extracted from alkaline and carbonate solutions as ion associates, the anion part of which may contain either hydroxo complexes of the metals or their compounds with a complexing agent, or intracomplex coordination-saturated and hydrated compounds. Extraction from alkaline solutions is remarkable for an unusual consequence of metal isolations, when extraction of trivalent metals is higher than that of tetra-, penta-, and hexavalent ones. The selectivity of studied extractants in respect to the trivalent elements provided a solution to a number of practical problems on separation and concentration of close-properties elements for the purpose of their determination [6].

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(1990). 3. B.F.Myasoedov, M.K.Chmutova, N.E.Kochetkova et al, Solv. Extr. and Ion Exch., 4, 61-81

(x986). 4. M.K.Chmutova, N.E.Kochetkova, B.F.Myasoedov, J. Inorg. Nucl. Chem., 42, 897-903 (1980). 5. M.N.Litvina, M.K.Chmutova, N.P.Nesterova, in Abstracts of International Conference "Actinides-

89", Moscow, 1989. p. 310. 6. Z.K.Karalova, B.F.Myasoedov, Zh. Anal. Khim., 39, 119-125 (1985).