Investigation Of ¹²⁹I In Vegetation Samples From Northwest Of China

Iodine is an important biophilic element, which is highly enriched in human thyroid via the food chains.129 I, as the only long-lived isotope?t1/2=1.57×107years? of iodine,mainly originates from anthropogenic nuclear activities, and the¹²⁹I/¹²⁷I ratios in the terrestrial organisms have significantly increased since the 1940 s. It is important to investigate transfer of radioactive iodine in ecosystem by determination of129 I in various vegetation samples in order to evaluate the radiological impact of short-lived radioactive iodine to the environment and population due to nuclear activities.Due to its high volatility and active chemical properties, iodine is easy to be lost during chemical separation. In addition, the conncetration of iodine in normal environmental and biological samples is very low, which make an effective separation of iodine from environmental samples a challenage for accurate determination of129 I in various environmental and biological samples with high organic content.This work studied alkaline ashing and combustion methods for decomposition of vegetation samples in order to convert iodine in the samples into inorganic forms and transfer to solution. By investigating the effects of major parameters to the separation and recovery of iodine from vegetation sample, an effective, safe and reliable combustion method has been established for separation of iodine from vegetation samples. This method has been successfully used to investigate the level, distribution and sources of129 I in vegetation samples from northwest of China. The major achievements include:1) Effects of various paramaters on decomposition of vegetation samples and separation of iodine from them using alkaline ashing method have been investigated,such as ashing time, ashing temperature, ratio of Na OH with sample weight, and addition of reductive reagents. It was found that iodine can be effectively separated from vegetation samples by ashing at 600 ? for up to three hours with a ratio of sample to 5M Na OH solution of 5g: 4ml. It was observed that addition of Na HSO3 does not show a significant improvement of iodine recovery. An overall recovery of more than 70% for iodine in whole procedure is obtained.2) An effective, safe and reliable combustion method using a tube furnace for separation of iodine from large size of vegetation samples has been developed for AMS measurement of 129 I in low level biological samples. The results showed that the ignition temperature for most vegetation ranges from 230 to 270?, and increasing the temperature at a speed of 1¹.5?/min in range of 210-300? is critical for safely ignitation and carbonization of vegetation samples. Combustion at temperature up to800? for relative longer time up to 3 hours or 900? for 1 hour are needed to completely release iodine from sample. The iodine released during the combustion can be effective trapped in Na OH soltion, and the optimal concentration of alkali trap solution is0.2 mol/L. Addition of Na HSO3 in trap solution does not significantly affect the recovery of iodine. A direct and simple Ag I precipitation method is emplyed to separate iodine from the trap solution. Washing preciptiate with diluted NH3·H2O is helpful for removing the Ag Cl and Ag Br coprecipitated with Ag I and obtaining a suitable amount of Ag I preciptiate for AMS measurement of 129 I. With this method, a recovery of more than 80% for iodine was obtained in combustion method measured by 125 I tracer. This method can analyse up to 10 grams of vegetation samples, and 4samples in each batch can be treated without cross-contamination. A detection limit of6?10-17 g/g?or 0.4 n Bq/g? for 129 I is obtained. Compared with conventional method,this method is easy in operation with high recovery of iodine, simple processing and less cross contamination. 5 different species of vegetation were analyzed using both methods presented in this work, the results agree each other very well.3) The ¹²⁹I/¹²⁷I atomic ratios in vegetation samples in northwest of China,except for Yumen, Gansu province, did not show significant variation. The ¹²⁹I/¹²⁷I atomic ratios in Yumen?average value: 2.40×10-6? is almost three orders of magnitude higher than the average in northwest China?average value: 6.69×10-9?. It indicates that some human nuclear activities might contribute to the 129 I level in Yumen besides global atmospheric fallout.