Speciation Analysis of Iodine-129 in Seawater by Coprecipitation and Accelerator Mass Spectrometry

摘要

As a long-lived radioisotope of iodine, ~（129）I is produced by both natural reactions and human nuclearactivities. Because of the ubiquity and high water-solubility, ~（129）I can be used as a kind of oceanographic tracer.Base on the different cyclic processes of iodine species, speciation analysis of ~（129）I in seawater can provideuseful information on the transportation of water masses. A simple and rapid coprecipitation methodcombined with accelerator mass spectrometry (AMS) measurement was improved for inorganic speciationanalysis of ~（129）I in seawater. Iodide was successfully separated from seawater just by adding a certain amountof carrier (0.6 mg) and 100 mg/L Ag~+ with the separation efficiency high to 95%. When the concentrationdifferences of ~（129）I~- and ~（129）IO_3~- are huge, slight crossover during the separation process can introducesignificant analytical error. In order to remove the remaining I~- in supernatant seawater in last step, AgI iscoprecipitated with Ag_2SO_3, AgCl, and AgBr after adding 0.6 mg carrier, 100 mg/L Ag~+ and 0.3 mmol/LNa_2SO_3 at pH about 4.0, which decrease the crossover between ~（129）I~- and ~（129）IO_3~- to 0.05%. Iodate in thesupernatant was converted to iodide by Na_2SO_3 at pH 1-2 and then separated by coprecipitation, with the91% separation efficiency. ~（129）I of total inorganic iodine was analyzed by the same procedure as for iodate.Ag_2SO_3，AgCl and AgBr in the coprecipitate was removed by washing with 3 mol/L HNO_3 and dilutedNH4OH, and the AgI precipitate was obtained for ~（129）I AMS measurement. Two seawater samplescollected from the Indian Ocean were analyzed by this improved method, and the results showed thatthe concentration of ~（129）I~- is significantly high than the concentration of ~（129）IO_3~- in seawater. Three seawatersamples at different depths of the Pacific Ocean were used to analyze ~（129）I of total inorganic iodine by thismethod and solvent extraction and back extraction method, and the results showed no significantdifference (p=0.05 for t-test) between two methods.