RU2541450C1 - Method for determining thorium-234 concentration in seawater bottom depositions - Google Patents

Abstract

FIELD: ecology.

SUBSTANCE: invention refers to marine radioecology and biogeochemistry facilities. A method for determining the thorium-234 concentration in seawater bottom deposits consists in the fact that a radiochemical yield tracer is natural long half-life α-emitting isotope ²³²Th, initial activity of which is determined in the sub-sample with lead γ-emission -212 if the radioactive Th and Pb balance conditions are satisfied; another sub-sample taken by separating thorium from the respective elements by oxalate deposition, is used for liquid-scintiallation (LS) spectrometric analysis of ²³⁴Th and ²³²Th activities as shown by β- and α-emission; that is followed by calculating thorium radiochemical yield (R) and throrium-234 initial concentration (^234Thyield, Bq/kg) by presented formulas.

EFFECT: invention provides more effective and reliable determination of the ²³⁴Th content.

Description

The invention relates to the field of marine radioecology and biogeochemistry and can be used in basic and applied research to quantify sedimentation rate and sedimentary self-cleaning processes of the marine environment.

The natural radionuclide thorium-234 formed during the decay of uranium-238 is used in marine ecology and biogeochemistry to quantify the sedimentation rate and the processes of sedimentation self-purification of the marine environment in relation to pollutants and eutrophic substances [1-4]. In this case, the ability of thorium-234 to actively accumulate suspended matter and sink with it to the bottom is used, upsetting the radioactive equilibrium with its mother radionuclide ²³⁸ U, which is chemical conservative in the marine environment and is mostly in a dissolved state [5, 6]. As a result, an excess of thorium-234 content (in units of radioactivity) relative to uranium-238 can be observed in the surface layer of bottom sediments, which is a quantitative measure of sedimentation rate [1,5].

The difficulty in determining the activity of ²³⁴ Th in marine sediments is due to the fact that it is a β-emitting radionuclide with a very small contribution of accompanying gamma radiation (about 4 and 5% for γ-quanta with energies of 63.2 and 92.6 keV, respectively). This does not allow the identification of thorium-234 by direct beta radiometry due to the presence of other natural and technogenic β-emitting radionuclides, primarily potassium-40 and strontium-90 [1]. In addition, the relatively short half-life of thorium-234 (T _1/2 = 24.1 days) imposes significant restrictions on the duration of sample processing and determines the need to use the most rapid radioanalytical methods.

Currently, two main methods are used to measure thorium-234 in marine sediments. The first method uses direct measurement of thorium-234 content in bottom sediments by its concomitant gamma radiation [2]. In this case, preliminary radiochemical treatment is not required except for drying and mechanical homogenization of the measured samples, which is an undoubted advantage of this method. However, due to the low yield of γ-quanta of thorium-234, large samples of bottom sediments are used for measurements, which is not always available, for example, when analyzing the content of thorium-234 in thin layers of geological cores. For the same reason, to achieve an acceptable statistical error in determining the activity of thorium-234, γ-spectrometric measurements of each sample are carried out for a long time, often more than one week [1]. This significantly reduces the performance of this method and limits the ability to measure inactive samples.

Closest to the claimed method is a method based on the extraction of ²³⁴ Th with a mixture of strong acids, its radiochemical purification from associated elements using extraction or ion exchange chromatography, followed by determination of the activity of thorium-234 by its β-radiation [2]. Moreover, to assess the radiochemical yield of thorium and its losses during multistage processing, a known amount of artificial thorium isotopes is added to the initial samples of bottom sediments, for example, ²²⁸ Τh, ²²⁹ Τh or ²³⁰ Τh, the content of which in the natural environment is negligible. However, the high cost and low availability of these isotopes is the main disadvantage of this method, along with the lengthy procedure for radiochemical processing of samples and significant losses of thorium at its various stages, which leads to a significant decrease in the sensitivity of the method.

The basis of the invention. A method for determining the concentration of thorium-234 in marine bottom sediments is tasked with increasing the efficiency, productivity and reliability of determining the content of ²³⁴ Th by using natural ²³² Th as a tracer for radiochemical yield and using liquid scintillation α / β spectrometry.

The proposed method for determining the concentration of thorium-234 in marine bottom sediments is based on its isolation by oxalate deposition using a long-lived natural (T _1/2 = 1.4 × 10 ¹⁰ years) α-emitting isotope ²³² Th, which is always present in bottom sediments along with ²³⁴ Th. This eliminates the need for expensive artificial tracers ( ²²⁸ Τh, ²²⁹ Τh, ²³⁰ Τh) and provides high analysis performance, since selective deposition is usually a more rapid method compared to extraction and ion-exchange chromatography. An additional increase in the efficiency and productivity of this method is achieved by the use of liquid scintillation (ZhS) spectrometry, which allows you to simultaneously detect the content of ²³² Th and ²³⁴ Th in the same sample with a maximum, in comparison with other radiometric methods, detection efficiency α- and β- radiation. The initial thorium-232 content in bottom sediments can be measured by the γ-radiation of its daughter lead-212 radionuclide, subject to the radioactive equilibrium condition between ²³² Th and ²¹² Pb. Unlike ²³⁴ Th, lead-212 has a significantly higher yield of γ-quanta (43.5%), and the long half-life of its parent radionuclide ²³² Th allows γ-spectrometric measurements to be performed without time limitation, up to a given level of statistical error. The obtained values of the initial activity of ²³² Th in bottom sediments, measured by γ-radiation of lead-212 ( ²³² Th _{ref [γ]} , Bq), in comparison with its activity determined after radiochemical processing by LC spectrometry ( ²³² Th _{meas [α]} , Bq) are then used to determine the radiochemical yield of thorium (R) and to calculate the initial concentration of thorium-234 ( ²³⁴ Th _ref , Bq / kg) according to the formulas:

- активность тория-234 в исследуемой пробе (Бк), измеренная по его β-излучению с помощью жидкостно-сцинтилляционной спектрометрии; m - масса пробы (кг).Where

- the activity of thorium-234 in the test sample (Bq), measured by its β-radiation using liquid scintillation spectrometry; m is the mass of the sample (kg).

The invention is illustrated by illustrations. In FIG. 1 shows a scheme for the separation of thorium from marine bottom sediments to determine the content of ²³⁴ Th using ²³² Th as a tracer for radiochemical output; FIG. 2 - Energy spectrum of γ-radiation of a sample of marine bottom sediments IAEA-315, measured using an ORTEC GMX-10 germanium detector; FIG. 3 - Spectra of α-radiation of ²³² Th and β-radiation of ²³⁴ Th and ²³⁴ Ra, measured using a QUANTULUS-1220 LC analyzer after the separation of thorium from a sample of marine bottom sediments IAEA-315. The white line shows the spectrum of the radioactive background.

The general scheme of the proposed Method for determining the concentration of thorium-234 in marine bottom sediments is presented in FIG. 1. It includes the following main stages:

1. Drying, grinding, homogenization of samples;

2. Dividing the obtained sample into two sub-samples for γ-spectrometric determination of the initial ²³² Th content from ²¹² Pb radiation and for radiochemical processing followed by LC spectrometric analysis of ²²⁸ Τh, ²²⁹ Τh or ²³⁰ Τh and ²³² Th activity from their β- and α- radiation, respectively;

3. Ashing of the second sub-sample at a temperature of about 450 ° C to remove organics;

4. Dissection of the ashed sediment in boiling concentrated hydrochloric acid to transfer thorium to a dissolved state;

5. Separation of the residual solid phase of bottom sediments by filtration through a paper filter;

6. Oxalate precipitation of thorium in the filtrate by adding a saturated solution of oxalic acid;

7. Filtration of a precipitate of thorium oxalates and related elements through a membrane filter with a pore diameter of 0.2 μm;

8. Calcination of the precipitate obtained at a temperature of about 700 ° C to convert thorium oxalate into sparingly soluble dioxide, and related elements, primarily strontium, into carbonates;

9. Dissolution of the calcined precipitate in 6 M hydrochloric acid to dissolve the carbonates of the associated elements;

10. The separation of thorium dioxide by filtering the resulting solution through a membrane filter with a pore diameter of 0.2 μm;

11. Flushing the filters with distilled water to remove residual hydrochloric acid;

12. Determination of the activity of ²³⁴ Th and ²³² Th on the filter using liquid scintillation α / β spectrometry.

In the course of this radiochemical treatment, thorium is separated from associated natural and technogenic radionuclides that do not form oxalate precipitates ( ²³⁸ U, ⁴⁰ K, ²²⁶ Ra, ¹³⁷ Cs), at stages (6) and (7). The removal of other impurities that interfere with the LS measurements (for example, ⁹⁰ Sr) occurs at stages (8) - (10), as a result of which ThO ₂ particles are retained on the membrane filter, while ⁹⁰ Sr is transferred to the filtrate.

Example.

Laboratory tests of the proposed Method for determining the concentration of thorium-234 in marine bottom sediments were carried out in the Department of Radiation and Chemical Biology of the Institute of Biology of the Southern Seas of the National Academy of Sciences of Ukraine using the standard bottom sediment sample No. IAEA-315 provided by the IAEA for international inalibration of measurements of natural and technogenic radionuclides [7 , 8]. After drying, homogenization, and grinding, this sample was divided into two parts, one of which was kept for 3 weeks in a hermetically sealed container for the accumulation of daughter decay products of thorium-232, including lead-212, to an equilibrium state. Then, the ²¹² Pb content was determined using an ORTEC GMX-10 semiconductor gamma detector (USA) based on an ultrapure germanium crystal (Fig. 2). The second part of the sample was placed in a glass flask for radiochemical processing according to the scheme shown in FIG. 1. At the end of the treatment, membrane filters with a thin-layer precipitate of thorium dioxide were placed in 20-ml Teflon vials for LC radiometry, filled with Optiphase-III scintillation fluid (UK), and the content of ²³⁴ Th and ²³² Th was measured using a QUANTULUS-liquid scintillation spectrometer 1220 (LKB Wallac, Finland) having a shape and duration analyzer for the separate detection of α and β particles. The obtained energy spectra of β-radiation of thorium-234, protactinium-234 (which is a short-lived daughter product of the decay of ²³⁴ Th) and α-radiation of thorium-232 are presented in FIG. 3.

Measurements of the activity of thorium-232, carried out by the emission of lead-212, showed that the initial content of ²³² Th in the studied sample IAEA-315 was 25.4 ± 1.3 Bq / kg dry weight, which corresponds to its passport data (25.6 ± 1.6 Bq / kg, [ 8]). After radiochemical processing and measurement of ²³² Th activity from its α-radiation on a membrane filter, a thorium-232 content of 18.9 ± 0.4 Bq / kg dry weight was obtained in a LC analyzer. The ratio of these two values calculated by the formula (1) showed that the radiochemical yield of thorium in the proposed scheme for its separation is quite high and amounts to 74.4%. Using this value, the thorium-234 content was determined using formula (2), amounting to 17.8 ± 1.4 Bq / kg dry weight, which also corresponds to the passport data of the IAEA-315 sample (17.6 ± 1.2 Bq / kg, [8]).

Thus, laboratory studies have shown that the proposed scheme for the radiochemical extraction of thorium from marine bottom sediments in combination with liquid scintillation α / β spectrometry is highly efficient and provides reliable determination of ²³⁴ Th content using natural ²³² Th as a radiochemical yield tracer.

Information sources:

1. Fuller C. S., A. van Geen, Baskaran M., Anima R. sediment chronology in San Francisco Bay, California, defined by ²¹⁰ Pb, ²³⁴ Th, ¹³⁷ Cs, and ^239,240 Pu // Marine Chemistry. - 1999.-Vol. 64.-P. 7-27.

2. Aller RC, DeMaster DJ Estimates of particle flux and reworking at the deep-sea floor using ²³⁴ Th / ²³⁸ U disequilibrium // Earth and Planetary Science Letters. -1984.-Vol. 67.-P. 308-318.

3. Gulin SB. Radioisotope geochronological reconstruction of pollution and eutrophication of the Black Sea // In the book: Radioecological response of the Black Sea to the Chernobyl accident / Ed. G.G. Polikarpova and V.N. Egorova. - Sevastopol: ECOSI-Hydrophysics, 2008 .-- S. 519-547.

4. Gulin S.B. Recent changes of biogenic carbonate deposition in anoxic sediments of the Black Sea: sedimentary record and climatic implication // Marine Environmental Research. - 2000. - Vol. 49, No. 4. - P. 319-328.

5. Waples JT, Benitez-Nelson C, Savoye N., Rutgers van der Loeff M., Baskaran M., Gustafsson Ö. An introduction to the application and future use of ²³⁴ Th in aquatic systems // Marine Chemistry. - 2006. - Vol. 100. - P. 166-189.

6. Rutgers van der Loeff M., Sarin MM, Baskaran M., Benitez-Nelson C, Buesseler KO, Charette M., Dai M., Gustafsson Ö., Masque P., Morris PJ, Orlandini K., Rodriguez y Baena Α., Savoye N., Schmidt S., Turnewitsch R., Vöge I., Waples JT A review of present techniques and methodological advances in analyzing ²³⁴ Th in aquatic systems // Marine Chemistry. - 2006. - Vol. 100. - P. 190-212.

7. IAEA. Catalog for reference materials and intercomparison exercises 1998/1999. Analytical Quality Control Services. - Vienna (Austria): IAEA. - 1998. - 64 p.

8. IAEA / MEL / 61 Report on the Intercomparison RUN IAEA-315 Radionuclides in marine sediment 1992-1996 / Ed .: S. Ballestra, H. Pettersson, J. Gastaud, P. Parsi and D. Vas (December 1997). Analytical Quality Control Services. - Vienna (Austria): IAEA. -1998.-93 p.

Claims (1)

A method for determining the concentration of thorium-234 in marine bottom sediments, including drying, grinding, homogenization of the test sample, separation from related elements by radiochemical treatment, characterized in that a ²³² Th natural long-lived α-emitting isotope is used as a radiochemical yield tracer, the initial activity of which is determined a portion of the sample of γ-radiation lead 212 when the condition of equilibrium between the radioactive ²³² Th and ²¹² Pb, and the second portion of the sample, separated from the thorium associated elements by oxalate precipitation are used for liquid scintillation (LGL) spectrometric analysis activity ²³⁴ Th and ²³² Th their β- and α-radiation, whereupon calculated thorium radiochemical yield (R) and the initial concentration of thorium-234 ⁽²³⁴ Th _ref Bq / kg) according to the formulas:

Where

- the activity of thorium-234 in the test sample (Bq), measured by its β-radiation using liquid scintillation spectrometry;
m is the mass of the sample (kg).

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