Speciation
Analysis of <sup>129</sup>I in Seawater
by Carrier-Free AgI–AgCl Coprecipitation and Accelerator Mass
Spectrometric Measurement
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Abstract
A rapid
and simple method was developed for speciation analysis
of <sup>129</sup>I in seawater by selective coprecipitation of carrier-free
iodide and accelerator mass spectrometry (AMS) measurement of <sup>129</sup>I. Iodide was separated from seawater and other species
of iodine by coprecipitation of AgI with Ag<sub>2</sub>SO<sub>3</sub>, AgCl, and AgBr by addition of only 100 mg/L Ag<sup>+</sup> and
0.3 mmol/L NaHSO<sub>3</sub> at pH 4.2–5.5. The separation
efficiency of iodide was more than 95%, and crossover between <sup>129</sup>IO<sub>3</sub><sup>–</sup> and <sup>129</sup>I<sup>–</sup> fractions is less than 3%. Iodate and total inorganic
iodine were converted to iodide by use of NaHSO<sub>3</sub> at pH
1–2 and then separated by the same method as for iodide. Ag<sub>2</sub>SO<sub>3</sub> in the coprecipitate was removed by washing
with 3 mol/L HNO<sub>3</sub> and the excess AgCl and AgBr was removed
by use of diluted NH<sub>3</sub>, and finally a 1–3 mg precipitate
was obtained for AMS measurement of <sup>129</sup>I. The recovery
of iodine species in the entire procedure is higher than 70%. Six
seawater samples collected from the Norwegian Sea were analyzed by
this method as well as a conventional anion-exchange chromatographic
method; the results from the two methods show no significant difference
(<i>p</i> = 0.05). Because only one separation step and
fewer chemicals are involved in the procedure, this method is suitable
for operation on board sampling vessels, as it avoids the transport
of samples to the laboratory and storage for a longer time before
analysis, therefore significantly improving the analytical capacity
and reliability of speciation analysis of <sup>129</sup>I. This improvement
can stimulate oceanographic tracer studies of <sup>129</sup>I