环境水样中^(129)I分析及其在环境示踪中的应用

Analysis of ^(129)I in environmental water samples and its application on environmental tracing

^(129)I是碘的唯一长寿命放射性同位素(半衰期为1570万年),目前环境中^(129)I主要来源于人类核活动,尤其是欧洲核燃料后处理厂的排放。人工^(129)I的大量释放使环境中^(129)I的水平大幅度升高,具有明显的空间分布规律,并且随排放时间呈现明显的变化趋势。^(129)I是一种理想的环境示踪剂,可应用于核环境安全监测,包括核事故监测和评价、核燃料后处理厂^(129)I排放和环境影响评价以及应用于环境过程示踪研究。陆地环境水样中稳定碘(127I)水平较低,^(129)I含量更是处于超低水平,因此,准确分析其中超低含量^(129)I及其化学形态是研究难点。本文系统地评述了环境水样中^(129)I及其形态的分析方法,^(129)I的水平、分布及其在核设施环境安全监测和稳定碘的地球化学循环示踪领域应用的研究进展。

Background, aim, and scope 129I is the only long-lived radioisotope of iodine （with a half-life of 15.7 million years）. There are two main sources of 129I in the environment, natural and anthropogenic production. 129I is naturally produced through the spallation reaction of atmospheric cosmic rays with Xe in the upper atmosphere, the spontaneous fission of 238U and thermal neutron-induced fission of 235U in the crust, as well as the neutron capture reaction of Te, but this source only accounts for a small part of the 129I inventory in the environment. The main sources of 129I in the present environment are releases from human nuclear activities, including nuclear weapons testing, nuclear reactor operation, nuclear accidents and nuclear fuel reprocessing plants （NFRPs）. Among them, the releases from the NFRPs account for more than 90% of 129I in the environment by now. The releases of anthropogenic 129I have increased the 129I level in the environmental water samples by 3--6 ordersof magnitude compared to the pre-nuclear level, especially in those areas （e.g. Europe, East Asia and North America） close to the point sources such as NFRPs, Fukushima and Chernobyl accident sites. In addition to the significantly spatial variation of 129I level, the records in ice cores, long-term atmospheric depositions and sediment cores have demonstrated variation of 129I level with time： besides a peak value in 1963, increased level was also observed after 1990, which is corresponding to the history of anthropogenic releases of 129I. The iodine concentration in environmental water samples such as atmospheric precipitation, river water, lake water is generally low, and can exist in multi-species. Materials and methods In the environmental water samples, especially in fresh water, organic iodine is an important specie of iodine, which causes a high huge challenge for the accurate determination of ultralow level of 129I in this type of samples. In order to determine the total 129I and speciation analysis of 129I in water samples, organic iodine needs to be converted into inorganic iodine. A typical analytical procedure is presented for determination of 129I and its species including sample pretreatment, iodine separation and concentration, target source preparation and 129I measurements. Results This paper summarizes and critically reviews various methods for determination of 129I and its species in water samples, including decomposition of organic iodine （e.g. UV irradiation, pyrolysis, alkali digestion, ultrasonic alkali digestion, oxidative digestion）, chemical separation of iodine and its species from water （e.g. solvent extraction, ion exchange, direct precipitation, co-precipitation and silver adsorption）, as well as the measurement techniques for 129I （including ）, and X-ray spectrometry and liquid scintillation counting method LSC, neutron activation analysis NAA, accelerator mass spectrometry AMS and inductively coupled plasma mass spectrometry ICP- MS）. Discussion Due to the active properties of iodine, iodine is easy to disperse and migrate in the environment through different media. With the unique sources of anthropogenic 129I and its long half-life, 129I has been widely applied as an envirottmental tracer for geochemical processes of iodine. Conclusions By determination of I291 and its species in the water samples, the level of 129I and its transportation process in the environment can be used to monitor and evaluate environment safety of nuclear facilities. This paper also shows the study progress of 129I as environmental tracer on geochemical cycle of stable iodine and nuclear environmental safety. Recommendations and perspectives With the continuous improvement of 129I analysis methods, especially the development of 129I chemical speciation analysis in freshwater, the 129I tracer would be better used to study the transport and migration of iodine in environmental water bodies, our understanding of the bio-geochemical cycle of iodine will be more and more comprehensive. In addition, to carry on long-term nuclear environmental monitoring can form records of nuclear activity emissions and have a guiding role in assessing emissions history and analyzing the status quo.