RU2541449C1 - Method for determining thorium-234 concentration in seawater - Google Patents

Abstract

FIELD: ecology.

SUBSTANCE: invention refers to radiation ecology and biogeochemistry, and aims at Th concentrating from seawater and determining its concentration. The method for thorium-234 concentration in seawater is implemented in series connected absorbers containing manganese dioxide that is followed by direct radiometric measurements of absorbed 234Th as shown by its primary β-emission. Each absorber works in the radially accurate regimen, which is generated by placing the disk absorber between diaphragms. The analysed water sample is supplied into the central portion of the absorber by means of a diaphragm with a central opening, then migrates to the periphery of the sorbed surface with using the diaphragm with peripheral openings.

EFFECT: varying the sedimentation process rate in the marine reservoirs.

Description

The invention relates to the field of radiation ecology and biogeochemistry, is intended to concentrate ²³⁴ Th from sea water and determine its content, which can be used to measure the speed of sedimentation processes in sea water bodies.

Thorium-234, formed during the decay of uranium-238, is widely used as a natural radio tracer to measure the speed of sedimentation processes in the marine environment [1]. Unlike ²³⁸ U, it exhibits sorption reactivity and accumulates with suspended matter to high levels. As a result of this, and also because of the large differences in the half-lives of ²³⁸ U (4.5 billion years) and ²³⁴ Th (24.1 days), the gravitational removal of biogenic and lithogenic suspension from the upper layer of the water column causes a noticeable deviation of the thorium-234 content from the equilibrium with uranium-238. This allows us to estimate the intensity of sedimentation transport of various elements in the marine environment [2-4].

In modern oceanology, two main methods are used to concentrate ²³⁴ Th from sea water for subsequent determination of its content by radiometric methods: flow sorption and adsorption coprecipitation using, in both cases, manganese dioxide as a highly efficient sorbent material [5].

The method of coprecipitation of thorium-234 with manganese dioxide is based on the formation of a fine suspension of MnO ₂ microcrystals directly in the treated sea water sample due to the chemical reaction between potassium permanganate and manganese dichloride, after which the suspension together with thorium-234 adsorbed on it is filtered off on membrane filters, and the resulting thin-layer preparations undergo direct radiometry of ²³⁴ Th β-radiation [5]. This makes it possible to determine the content of ²³⁴ Th in relatively small volumes of seawater (20 or less liters), which is the main advantage of this method. However, during coprecipitation, there is no possibility of direct control of the efficiency of thorium extraction in each sample, which often leads to unsatisfactory reproducibility of the results [5]. The methods proposed by the authors for such control by atomic adsorption measurements of the amount of manganese dioxide remaining in the coprecipitation tanks [6], or by adding tracers of a radiochemical yield, for example, ²²⁹ Th or ²³⁰ Th [7], are indirect, or require additional radiochemical processing the precipitation obtained and the parallel use of not only β- but also α-detectors (and sometimes mass spectrometers) to determine the activity of these tracers [7].

Closest to the claimed method is a flow sorption method (see Buesseler K.O., Cochran JK, Bacon M.R., Livingston HD, Casso SA, Hirschberg D., Hartman MC, Fleer AP Determination of thorium isotopes in seawater by non -destructive and radiochemical procedures // Deep-Sea Research. - 1992. - Vol. 39, No. 7/8. - P. 1103-1114). The known method consists in pumping water through two series-connected adsorbers made of fibrous material impregnated with microcrystals of manganese dioxide. The activity of adsorbed ²³⁴ Th is determined by direct radiometry by its gamma radiation without preliminary radiochemical extraction from the sorbing substance, which is the main advantage of this method, along with the ability to control the efficiency of thorium sorption by the difference in its activity in the first and second adsorber. However, it should be borne in mind that for thorium-234 the main is not gamma, but beta radiation. Moreover, the yield of ²³⁴ Th gamma rays with energies of 63.3 and 92.6 keV is only 3.8 and 5.4%, respectively. This determines the need for processing large volumes of sea water (> 1000 l) for reliable detection of thorium-234 by its gamma radiation, which significantly limits the performance of this method, since sampling water of this volume, especially from great depths, is a rather labor-intensive operation that requires the use of special samplers, including autonomous submersible pumps [2, 5]. In this case, the time required for processing each sample can be very significant, since the speed of pumping water through adsorbers is usually no more than 3-5 l / min [2]. In addition, this method requires sufficiently large adsorbers with an effective volume of 500 or more cm ³ [2]. Such an amount of a sorbing substance cannot be subjected to direct radiometry in most modern γ-detectors without its preliminary compaction by ashing or pressing under high pressure [2, 5].

The basis of the invention. A method for determining the concentration of thorium-234 in seawater is tasked with improving technology, to increase the efficiency of extraction of thorium-234 and the productivity of the method.

The problem is achieved in that the concentration of thorium-234 dissolved in seawater is performed in series-connected adsorbers containing manganese dioxide and direct radiometry of adsorbed ²³⁴ Th by its main β-radiation is carried out.

For the practical implementation of this approach, it is necessary to take into account that β radiation has a significantly lower penetrating power compared to γ quanta. In this regard, for direct β-radiometric determination of the content of thorium ²³⁴ Th in adsorbers, their shape and size should be chosen in such a way as to minimize the effect of β-particles self-absorption in the thickness of the sorbent material and ensure, at the same time, a sufficiently high efficiency of sorption extraction thorium from sea water. To solve the first problem, adsorbers in the form of thin-layer disks are most acceptable, which ensures the optimal geometry for counting β particles of thorium-234 accumulated in them using gas-discharge, gas-flow, and liquid-scintillation detectors, which are most often used in oceanological practice. However, the disk shape of the adsorbers is not optimal for ensuring high efficiency of sorption in the flow mode due to the short residence time of the recoverable element in a thin-layer sorbent material. To eliminate this drawback, it is proposed to use disk adsorbers not in a direct-flow, but in a radially accurate mode, in which the processed sample does not reach the entire surface of the disk adsorber, but only in its central part and then flows to the periphery of the sorbing layer, significantly increasing the contact time with it dissolved thorium. Structurally, this is achieved by using the inlet and outlet diaphragms, the first of which has an opening in the central part for supplying water, and the second has slots at the edges to drain the treated filtrate (Fig. 1).

The invention is illustrated by illustrations. In FIG. 1 presents a Scheme for determining the concentration of thorium-234 (C, Bq / l) in sea water; FIG. 2 - Installation for the extraction of thorium-234 from sea water.

To implement the method, the authors improved the disk adsorber in order to carry out sorption in a radially accurate mode. The adsorber contains (see Fig. 1) the upper part of the housing 1, the inlet diaphragm with a central hole 2, a disk adsorber 3 impregnated with manganese dioxide; the output diaphragm 4 with slots along the periphery, the lower part of the housing 5. The white dotted arrows indicate the direction of the radial water flow through the disk adsorber.

An example implementation of the method.

Surface water samples from the water area of the Sevastopol Bay with a volume of 20 l were taken in plastic containers. To extract thorium-234 from sea water, an apparatus was used (see Fig. 2), containing a peristaltic pump, prefilter 1 and adsorbers 2, 3 connected in series. The arrows indicate the direction of water movement. In the foreground: a disk sorbent impregnated with MnO ₂ (in the cup on the left) and the polypropylene base of the sorbent before impregnation (in the cup on the right), used as a prefilter. In the lower part of the insert, the adsorber body is shown disassembled. Using a peristaltic pump, water was pumped through a disk prefilter to separate the suspended thorium fraction, then through two series-connected disk adsorbers impregnated with manganese dioxide, designed to concentrate the dissolved thorium. The prefilter and adsorbers had the same diameter (25 mm) and thickness (3-4 mm), and were made of pressed polypropylene fiber material with a density of 0.33 gsm ³ . The nominal pore size of this material was about 0.5 μm. For the impregnation of adsorbers with manganese dioxide, they were kept in a saturated solution of potassium permanganate, after adding a 25% aqueous solution of ammonia to it as an alkaline buffer. For better impregnation of the fibrous material of the adsorbers, the solution was heated to 30-40 ° C and kept for 2-3 hours. After this was added a concentrated aqueous solution of manganese dichloride based on: 1 volume part of MnCl ₂ in 2 parts KMnO ₄ . In this case, in the entire thickness of the polypropylene disk, the formation of microcrystals of manganese dioxide occurred by the reaction:

After the disks had a dark brown color (the color of manganese dioxide), they were washed with distilled water, kept at room temperature until they were drip-dry and placed in Plexiglas cases of adsorbers consisting of two cone-shaped parts with a threaded connection (see Fig. 1). Each part of the housing is provided with nozzles for supplying water, or drainage of the filtrate. The tightness of the threaded connection is achieved by a sealing ring made of chemically resistant rubber. A diaphragm with a central hole is fixed in the input part of the casing, and a diaphragm with peripheral slots is fixed in the output part of the casing. A disk adsorber is placed between these diaphragms.

After pumping the entire sample volume, the prefilter and adsorbers were removed from the housings and transferred to 20 ml plastic bottles for liquid scintillation spectrometry. 1-2 ml of 6M hydrochloric acid was added to vials with adsorbers and heated to 50-60 ° C. Under such conditions, manganese dioxide was completely transformed into dissolved manganese chloride, and thorium was desorbed. Then, 15 ml of Optiphase-III scintillation fluid (UK) was added to the vials and thorium-234 content was measured using a QUANTULUS-1220 liquid scintillation analyzer (LKB Wallac, Finland). The efficiency of sorption of thorium by disk adsorbers was calculated by the formula:

where R _A and Rb is the activity of ²³⁴ Th in the first and second adsorbers (Bq). The initial concentration of thorium-234 in sea water (Bq / l) was determined by the equation:

where V is the sample volume (l).

Testing of the proposed Method for determining the concentration of thorium-234 in sea water was carried out in the Sevastopol Bay in different seasons of 2009. This made it possible to measure ²³⁴ Th at different levels of suspended matter due to the seasonal dynamics of the development of phytoplankton and the supply of terrigenous suspension with coastal runoff. Tests have shown that when using disk adsorbers of the indicated sizes, a rather high (> 65%) degree of thorium-234 extraction from 20 liter samples of the Black Sea surface water is achieved at an initial concentration of 1.6 · 10 ^-3 to 2.2 · 10 ^-2 Bq / l, i.e. over the entire range of values of the content of ²³⁴ Th known for this reservoir.

The advantages of the proposed method are that using relatively small sample volumes (about 20 l), the efficiency of thorium-234 extraction can be controlled without the use of expensive tracers of radiochemical output and additional measuring equipment.

Information sources:

1. U.S. GOFS. Sediment trap technology and sampling // U.S. Global Ocean Flux Study. Planning Report No. 10 of the Working Group on Sediment Trap Technology and Sampling / Eds. G. Knauer, V. Asper. - Woods Hole (USA): WHOI, 1989 .-- 94 p.

2. Buesseler K.O., Cochran J.K., Bacon M.P., Livingston H. D., Casso S.A., Hirschberg D., Hartman M.C., Fleer A.P. Determination of thorium isotopes in seawater by nondestructive and radiochemical procedures // Deep-Sea Research. - 1992. - Vol. 39, No. 7 / 8.-P. 1103-1114.

3. Gulin SB Seasonal changes of ²³⁴ Th scavenging in surface water across the western Black Sea: an implication of the cyclonic circulation patterns // Journal of Environmental Radioactivity. - 2000. - Vol. 51, No. 3. - P. 7-19.

4. Waples JT, Benitez-Nelson C, Savoye N., Rutgers van der Loeff M, Baskaran M, Gustafsson 0. An introduction to the application and future use of ²³⁴ Th in aquatic systems // Marine Chemistry. - 2006. - Vol. 100. - P. 166-189.

5. Rutgers van der Loeff M., Sarin MM, Baskaran M., Benitez-Nelson C, Buesseler KO, Charette M., Dai M., Gustafsson O., Masque P., Morris PJ, Orlandini K., Rodriguez y Baena Α., Savoye N., Schmidt S., Turnewitsch R., V6ge 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.

6. Buesseler K.O., Benitez-Nelson C, Rutgers van der Loeff M, Andrews J., Ball L., Crossin G., Charette M.A. An intercomparison of small- and large-volume techniques for thorium-234 in seawater // Marine Chemistry. - 2001. - Vol. 74. - P. 15-28.

7. Pike S.M., Buesseler K.O., Andrews J., Savoye N. Quantification of Th-234 recovery in small volume seawater samples by inductively coupled plasma-mass spectrometry // Journal of Radioanalytical and Nuclear Chemistry. - 2005. - Vol. 263 - P. 355-360.

Claims (1)

A method for determining the concentration of thorium-234 in sea water, including pumping a test sample of water through two series-connected disk adsorbers impregnated with manganese dioxide, as well as monitoring the efficiency of extraction of thorium-234, characterized in that they carry out radiometric determination of thorium-234 content by its basic ß -radiation in adsorbers, with each adsorber operating in a radially accurate mode, which is ensured by placing a disk adsorber between the diaphragms with slots, and we study I water sample is supplied to the central part via an adsorber diaphragm with a central hole and is then fed to the periphery of the sorbent layer by a diaphragm with peripheral slots.

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