Extraction of 129I and 127I via combustion from organic rich samples using 125I as a quantitative tracer

Abstract

Iodine-129 (129I) is a biophilic, naturally occurring radioisotope (half-life: 1.57 × 107 years) that has been released in large quantities by nuclear fuel reprocessing. This iodine has cycled throughout the globe and chiefly the northern hemisphere and can be found in a wide variety of environmental materials, particularly organic rich soil and organic matter. Extracting iodine reliably from solid samples has been done by a variety of methods, however, pyrohydrolysis has been the most widely used. There is a wide variation between existing pyrohydrolysis techniques and this raises questions about the quantitative recovery of iodine from method to method. In order to quantify iodine recovery from pyrohydrolysis we have spiked samples with an iodine-125 radiotracer prior to combustion and trapping in an alkaline solution. Inorganic 125I tracer was used as well as humic acid labeled with 125I to simulate the behavior of 129I and 127I in complex organic substances and extract iodine regardless of how it is partitioned. Using these tracers we explored the effect on recovery of 125I under a variety of combustion parameters. These include carrier gas flow rate and iodine volatilization temperature. We observed that the best recoveries of 125I were at flow rates between 400 and 800 mL/min and most 125I recoveries were above 85%. The experiment to determine the temperature at which iodine volatilizes from the sample showed two distinct trends for the release of iodine. One trend showed that most iodine is released at approximately 525 °C, while the other trend showed that the samples needed to reach 800 °C and remain there for at least an hour. These findings illustrate the usefulness and importance of using a quantitative recovery tracer for every iodine extraction. We then combusted and precipitated several Atlantic Ocean seaweed and standard reference materials for AMS analysis as AgI. The 129I concentration of the seaweed ranged between 4.4–5.5 × 109 atoms/g and the 129I/127I ratio was 2.3–2.9 × 10−9, both of which compare well to published values for Atlantic seaweed. The results for the standard reference materials also agree with specified values indicating that this technique is reliable. By optimizing pyrohydrolysis conditions and testing the recovery of iodine with a 125I tracer it is possible to quantify and maximize recovery from organic samples. This will allow for the investigation of variations in the 129I concentration and 129I/127I ratio with a high degree of precision in complex, organic rich samples.