Естественные радионуклиды ²³⁸U и ²³²Th в раковинах моллюсков Каспийского моря

Abstract

The accumulation of radionuclides by Caspian mollusks under conditions of natural and anthropogenic environmental dynamics determines the relevance of the presented study. The aim of the work was to study the concentration features of ²³⁸U and ²³²Th, and to assess the accumulation ability on concentration factor (Cf) of Cardiidae bivalves in the Middle and Northern Caspian. According to the results of sampling in 2012-2016. The content of uranium and thorium in the shell samples was determined by the radiochemical method spectrophotometrically with arsenazo-III using Specol, ²³⁸U at l = 670 nm, ²³²Th at l = 665 nm, with sequential chromatographic separation of uranium and thorium from solution using EDE-10P anionite. A direct positive correlation of uranium with C_org and CaCO₃ has been established. It is revealed that the values of the ratio U/С_орг for Caspian bivalves ranges from 7 to 45·10^-5. It has been shown that the accumulation of ²³⁸U in shells during the growth of Caspian mollusks is influenced by biotic factors: species specificity (mollusks of the Dagestan coast aged 6-7 years are arranged in a decreasing series according to the activity concentration of ²³⁸U their shells Didacna pr. novocaspia > D. pr. protracta >Monodacna caspia >D. trigonoides 53.0; 45.4; 34.0 and 8.82 Bq·kg-1, respectively; mollusks of the Northern Caspian Sea, Didacna pr. novocaspia > Didacna pr. protracta > Didacna pr. submedia, these indicators are 31.5, 22.3 and 7.56 Bq·kg^-1, respectively); age (in young mollusks, uranium accumulation is 2 times higher); the presence of an aragonite shell structure. An indirect effect of abiotic factors is noted, such as the release of hydrothermal waters along tectonic faults at the bottom of the Caspian Sea, with increased seismic activity. In general, the increase in natural radionuclides in the waters of the Caspian Sea is a consequence of the aridization of the climate and the closeness of the reservoir. Thorium accumulation in shellfish shells is more strongly influenced by its supply through the food chain, the thorium content of the soil and the release of reservoir waters during oil production. It has been determined that Caspian bivalves are characterized by Cf of the order of magnitude equel for ²³⁸U n × 10¹ and ²³²Th – n × 10³, as a result of which these species enhance the biogenic flow of uranium and thorium from seawater, which contributes to the self-purification of the waters of the Caspian Sea from radioisotopes. Regional peculiarities in the accumulation ability of mollusks of the Caspian Sea have been revealed: in the shells of adult individuals. The average value of Cf ²³⁸U was 77 and Cf ²³²Th – 1870. There was a tendency to increased accumulation of uranium and thorium in the Derbent area in the shells of the mollusk D. pr. novocaspia to levels of 53.0 ± 0.7 and 11.0 ± 0.09 Bq·kg^-1, with Cf ²³⁸U = 140 and Cf ²³²Th = 3857. The obtained value of ²³⁸U activity concentration (А_т) = 29 Bq·kg^-1 in Didacna and Monodacna shells was close obtained in 1975. In the shells of Caspian mollusks, the calculated value of ²³²Th А_т was 5.3 Bq·kg^-1, which was 8 times lower than those on the coast of Azerbaijan, in the area of the oil and gas fields of the Northern Caspian Sea, the average modern data and the maximum permissible concentration of thorium.

Keywords: accumulation, natural radionuclides 238U, 232Th, shells of bivalves, Caspian Sea