Investigation And Environmental Tracing Of ¹²⁹I And Uranium Isotopes In Terrestrial Vegetation Samples From Inner Mongolia-Northeast China

¹²⁹I,as one of the main radionuclides released from nuclear activities,because of its unique space-time source,volatile and active chemical character,is widely used in nuclear environmental safety and environmental tracer research.Uranium is a natural radioactive heavy metal element,which may be released to the environment for local radioactive contamination in industrial production.In addition,²³⁵U is the main raw material of nuclear energy production,²³⁵U/²³⁸U isotope ratio may also be affected by different nuclear activities?nuclear weapon test,nuclear fuel cycle,nuclear energy utilization?.In this paper,a method of low level uranium isotope analysis for plant samples was established,which was successfully applied to the analysis of uranium isotopes in terrestrial plants from Inner Mongolia to Northeast China.In addition,the levels of ¹²⁹I and ¹²⁷I in plant samples from the same regions were measured by using the ultra-trace ¹²⁹I analysis method in vegetation samples.By analysing the levels and sources of ¹²⁹I and uranium isotopes in plants,to assess the different effects of nuclear activities and other industries on the environmental and human radioactivity levels in the study region,and to combine their different chemical properties and environmental behaviors,to analyze its environmental tracer significance.The main conclusions are as follows:1.This work developed an analytical method for determination of low-level uranium isotopes in vegetation samples.Dry ashing method was employed to decompose organic matters of plant.The sample ash was further digested using multiple acids.Uranium in the prepared sample solution was separated and purified by an extraction chromatography using UTEVA resin.The detection limits of the method for²³⁸U,²³⁵U,²³⁴U are 3.05 pg/g,0.34 pg/g and 0.04 pg/g,respectively.The detection limits for ²³⁸U and ²³⁵U are 10 times better than the reported values.The determination of ²³⁴U in vegetation samples by ICP-MS has not yet been reported before.2.The ¹²⁷I concentration in terrestrial plants was 0.14-4.99?g/g,the average was0.47?g/g,and the geometric average was 0.41?g/g,which is consistent with ¹²⁷I concentration in most terrestrial plants.The range of ¹²⁹I concentration is 0.59×10⁷-16.72×10⁷ atoms/g,the average is 3.93×10⁷ atoms/g,the geometric average is 2.89×10⁷atoms/g.The ¹²⁹I/¹²⁷I atoms ratio range from 2.87×10^-9 to 87.01×10^-9,with the geometric average of 1.53×10^-8.The level of ¹²⁹I in the study area is higher than three orders of magnitude in the marine and terrestrial environment of prenuclear period,which indicates that ¹²⁹I of plants in this area were mainly anthropogenic sources.Mainly from the global atmospheric deposition,including the regional deposition of¹²⁹I released into the troposphere by the atmospheric nuclear tests located in Lop Nor of China,and Semipalatinsk of the former Soviet Union,and the re-emission and deposition after long-distance transport of ¹²⁹I from the European nuclear fuel reprocessing plant is the most mainly source in study regions.3.By analysing the spatial distribution of ¹²⁹I in terrestrial plants in Inner Mongolia and Northeast China,we studied the transport path and sedimentation model of atmospheric pollutants in northern China,especially in the north-west,Inner Mongolia and northeast regions.The effects of climate factors,such as topography,wind direction and precipitation,on the transport and deposition of atmospheric pollutants were demonstrated.The results show that ¹²⁹I is an excellent tracer for atmospheric environmental tracing.4.The ²³⁸U concentration in terrestrial plants from the study area varied greatly,range from 1.51 to 267.23 ng/g,with a geometric average of 15.74 ng/g.Samples with a higher ²³⁸U concentration?>100 ng/g?are mainly distributed in the mountainous areas with rich mineral resources,such as Inner Mongolia,Shaanxi,Heilongjiang and so on.The distribution of these high-uranium plants basically coincides with the distribution of coal mining in China.Which indicated that the source of uranium in plants in these high value areas may be related to the wide dispersion of uranium-bearing fly ash produced by local coal mining.5.The ²³⁵U/²³⁸U isotope ratio?0.0072-0.0074?in plants in the central and western regions of Inner Mongolia is higher than the natural ratio,indicating that enriched uranium contamination may exist in this area,which may come from the early atmospheric nuclear weapon tests in China and the former Soviet Union,and the nuclear fuel cycle facilities in Lanzhou and Inner Mongolia.But those enriched uranium has little contribution and does not cause the increase of the local uranium cincentration,and would not effect on the local radioactivity level.6.The variations of ²³⁴U/²³⁸U isotope ratio in vegetations from different regions are related to the natural isotope fractionation.The ²³⁴U/²³⁸U isotope ratio can be used as an index of uranium exploration and trace the distribution of high uranium matrix in the environment.