Method for determining thorium-234 concentration in seawater

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.


The invention relates to the field of radiation ecology and biogeochemistry, intended for concentration234Th from seawater and the definition of its content that can be used to measure the velocity of sedimentation processes in marine waters.

Thorium-234, which is formed by the decay of uranium-238, is widely used as a natural radiotracer to measure the velocity of the sedimentation processes in the marine environment [1]. Unlike238U, he manifests sorption reactivity and accumulated suspended matter to higher levels. As a result, and also due to the large differences in half-lives of238U (4.5 billion years) and234Th (24.1 d), gravitational removal of biogenic and lithogenic sediment from the upper layer of the water column causes a noticeable deviation of the content of thorium-234 from the equilibrium with uranium-238. This allows us to estimate the rate of sedimentation of the transfer of the various elements in the marine environment [2-4].

In modern Oceanography apply two basic methods of concentration234Th from seawater for subsequent determination of its content radiometric methods: flow-through sorption adsorption and coprecipitation using, in both cases, manganese dioxide as a highly absorptive material [5].

Method coprecipitation Toria manganese dioxide based on the formation of finely dispersed suspension of microcrystals Mno 2directly in the treated sample of sea water due to chemical reaction between potassium permanganate and douglasites manganese, after which a suspension together with adsorbed on the thorium-234 filtered on membrane filters, and the resulting thin-layer preparations are direct radiometry β-radiation234Th [5]. This allows you to define the content of the234Th relatively small amounts of sea water (20 or less liters), which is the main advantage of this method. However, the coprecipitation no direct control efficiency of extraction of thorium in each sample, which often leads to poor reproducibility of the results obtained [5]. Authors ' methods of such control, atomic-absorption measurements of the amount of manganese dioxide remaining in the tanks for co-precipitation [6], or by adding samples radiochemical tracer yield, for example,229Th or230Th [7], are indirect, or require additional radiochemical processing of the received precipitation and concurrent use not only β-, and α-detectors (and sometimes mass spectrometers) to determine the activity of these tracers [7].

Closest to the claimed method is a method flow-through sorption (see Buesseler K. O., Cochran J. K., Bacon M. P., Livingston, H. D., S. A. Casso, Hirchberg D., Hartman, M. C., A. P. Fleer 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 involves pumping water through two serially connected adsorber, made of fibrous material impregnated with micro-crystals of manganese dioxide. The activity of adsorbed234Th is determined by the method of direct counting in his gamma radiation without prior radiochemical extraction of the sorbent, which is the main advantage of this method, along with the possibility of monitoring the efficiency of the sorption of thorium on the difference in activity in the first and second adsorber. It should, however, note that for thorium-234 main is gamma, and beta radiation. Moreover, the output γ-ray234Th energy 63.3 and 92.6 Kev is only 3.8 and 5.4%, respectively. This determines the necessity of handling large volumes of sea water (> 1000 l) for reliable detection of thorium-234 in his gamma radiation, which significantly limits the performance of this method, since the sampling of water supplies, especially from great depths is a time - consuming operation, requiring the use of special samplers, including Autonomous submersible pumps [2, 5]. The time required to process each sample, can be quite value is positive, because the rate of pumping of water through the adsorbers is usually no more than 3-5 l/min [2]. Furthermore, this method requires relatively large adsorbers with an effective volume of 500 cm3[2]. This amount of sorbent cannot be subjected to direct radiometry in most modern γ-detectors without compaction by ashing or pressing under high pressure [2, 5].

The basis of the invention a Method of determining the concentration of thorium-234 in seawater tasked by improving technology, enhance the efficiency of extraction of thorium-234 and the performance of the method.

This object is achieved in that the concentration of thorium-234, dissolved in seawater, perform in the series connected adsorbers containing manganese dioxide and carry out direct radiometry adsorbed234Th his primary β-radiation.

For the practical implementation of this approach must be considered that β-radiation has far less penetrating power compared to γ-quanta. In this regard, for direct β-radiometric determination of thorium234Th in the adsorbers, their shape and dimensions must be chosen in such a way as to minimize the effect of self-absorption of β-particles in the thickness of Sorbi the existing material and to ensure that thus, a sufficiently high efficiency of sorption of thorium from sea water. To solve the first problem, the most acceptable are the adsorbers in the form of thin disks, which provides optimal geometry accounts β-particles accumulated in them thorium-234 using gas discharge, gas flow and liquid-scintillation detectors, the most commonly used in Oceanographic practice. However, the disk shape of the adsorbers is not optimal to ensure high efficiency of sorption in flow regime due to the small residence time of the recoverable element in a thin layer of sorbent material. To eliminate this disadvantage it is proposed to use the disk adsorbers are not straight and radially precise mode in which the processed sample arrives not on the entire surface of the disk adsorber, and only in its Central part and then flows to the periphery of the absorbing layer, greatly increasing the contact time with it dissolved thorium. Structurally, this is achieved by using the input and output apertures, the first of which has a hole in the Central part of the water, and the second slits along the edges to discharge treated leachate (Fig. 1).

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

To implement the method, the authors have improved disk adsorber for the implementation of sorption in accurate radial mode. The adsorber contains (see Fig. 1) the upper part of the housing 1, the input aperture with a Central hole 2, disc adsorber 3, impregnated with manganese dioxide; output aperture 4 with the slots on the periphery of the lower part of the housing 5. White dotted arrows show the direction of radial water flow through the disk canister.

An example implementation of the method.

Samples of surface waters from the waters of the Sevastopol Bay with a capacity of 20 l were collected in plastic containers. For the extraction of thorium-234 in seawater was used by the installation (see Fig. 2) containing the peristaltic pump, connected in series prefilter adsorbers 1 and 2, 3. The arrows indicate the direction of water movement. In the foreground: disk sorbent impregnated Mno2(in the Cup on the left) and the polypropylene-based sorbent before impregnation (Cup on the right), used as prefilter. At the bottom of the insert is shown the body of the absorber disassembled. Using a peristaltic pump water was pumped through the disk prefilter for separation of suspended fraction of thorium, then through two successively the United States is x disk adsorber, impregnated with manganese dioxide intended for concentration of dissolved thorium. Prefilter and adsorbers have the same diameter (25 mm) and thick (3-4 mm), and were made of extruded polypropylene fiber material density of 0.33 Pol3. The nominal pore size of this material was about 0.5 μm. For impregnation of adsorbers, manganese dioxide them maintained in a saturated solution of potassium permanganate, pre-adding 25% aqueous ammonia solution as an alkaline buffer. For better impregnation of the fibrous material of the adsorbers, the solution was heated to 30-40°C. and maintained for 2-3 hours. After this was added concentrated aqueous solution douglasthe manganese rate of 1 volume part nl22 parts KMPO4. At the same time, the thickness of the polypropylene disk was accompanied by the formation of micro-crystals of manganese dioxide by the reaction:

After purchasing the disk dark brown (the color of the manganese dioxide), they were washed with distilled water, kept at room temperature for up to drip-dry and were placed in a Plexiglas housing adsorbers, consisting of two cone-shaped parts with threaded connection (see Fig. 1). Each part of the body is provided with nozzles for the evaluation of the Yes water or drainage of the filtrate. The tightness of the threaded connection is achieved by a sealing ring made of chemically resistant rubber. In the input part of the body is fixed to the diaphragm with a Central hole, and in the output panel aperture with a peripheral slots. Between the two diaphragms placed the disk canister.

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

where RAand Rb - activity234Th in the first and second adsorbers (Bq). The initial concentration of thorium-234 in seawater (Bq/l) was determined by the equation:

where V - volume of sample (l).

Testing of the proposed Method of determining the concentration of thorium-234 in Mor is some water was performed in the waters of the Sevastopol Bay in different seasons 2009 This gave the opportunity to carry out measurements234Th at different levels of suspended matter due to seasonal dynamics of phytoplankton and receipt of terrigenous sediment to the coastal runoff. Tests showed that when using disk adsorbers of these dimensions is quite high (>65 %) the degree of extraction of thorium-234 from 20-liter samples of surface waters of the Black sea when the initial concentration of 1.6·10-3up to 2.2·10-2Bq/l, i.e., in the whole range of values of the content of the234Th, known for this reservoir.

The advantages of the proposed method lies in the fact that by using a relatively small sample volumes (about 20 l) you can control the efficiency of extraction of thorium-234 without the use of expensive radiochemical tracer yield and additional instrumentation.

Sources of information:

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., S. A. Casso, D. Hirschberg, M. C. Hartman, A. P. Fleer 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 S. B. Seasonal changes of234Th scavenging in surface water across the western Black Sea: an Memorandum of the cyclonic circulation patterns // Journal of Environmental Radioactivity. - 2000. - Vol. 51, No. 3. - P. 7-19.

4. Waples, J. T., Bnitez-Nelson C, Savoye N., Rutgers van der Loeff M, Baskaran M, Gustafsson 0. An introduction to the application and future use of234Th in aquatic systems // Marine Chemistry. - 2006. - Vol. 100. - P. 166-189.

5. Rutgers van der Loeff M., Sarin M. M., Baskaran, M., Benitez-Nelson C, Buesseler K. O., Charette M., Dai, M., Gustafsson O., P. Masque, P. J. Morris, K. Orlandini, Rodriguez y Baena Α., Savoye N., Schmidt, S., R. Turnewitsch, V6ge I., Waples, J. T. A review of present techniques and methodological advances in analyzing234Th in aquatic systems // Marine Chemistry. - 2006. - Vol. 100. - P. 190-212.

6. Buesseler K. O., Benitez-Nelson C, Rutgers van der Loeff M, J. Andrews, L. Ball, G. Crossin, M. A. Charette 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 to 360 above.

The method of determining the concentration of thorium-234 in seawater, comprising pumping a measured sample of water through two serially connected disk adsorbers, impregnated with manganese dioxide, and the control efficiency of extraction of thorium-234, characterized in that exercise radiometric determination of thorium-234 by default ß-radiation in the adsorbers, each adsorber operates in accurate radial mode, which provides by placing the disk adsorber between the diaphragms with slits, and the sample water flows in the Central part of the adsorber using a diaphragm with a Central hole, then it is served to the periphery soubirou the it layer by using a diaphragm with a peripheral slots.


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