Is the extraction of thorium onto MnO2-coated filter cartridges uniform?

Open-File Report 2004-1421




Large-volume (i.e., 102 - 103 L) seawater samples are traditionally required to study the partitioning of particle-reactive radionuclides between solution and size-fractionated particulate matter. One of the most frequently used methods to preconcentrate the short-lived isotopes of Th (234Th and 228Th) from such large volumes of water involves the effective extraction of Th onto two MnO2-coated polypropylene cartridges. Determination of dissolved Th activities assumes that the two MnO2-coated filter cartridges extract Th uniformly (same extraction efficiency), but this assumption has not been rigorously validated. Any variability in the extraction efficiency of the two cartridges connected in series will directly introduce an error in the determination of final dissolved Th activity. In this article, we evaluated the variability in the extraction efficiency of MnO2-coated filter cartridges that were prepared under varying conditions.

Thorium-234-spiked seawater was filtered in series through a manifold consisting of six MnO2-coated cartridges. From the activities of 234Th retained in each cartridge, the relative (calculated from the activities in two successive cartridges) and absolute (ratio of 234Th activity retained to the activity entered) extraction efficiency for each of the cartridges was calculated. At a constant flow rate and constant KMnO4 saturation, the absolute extraction efficiency varied by -40% (from 54.1 to 93.8%) within the first two filter cartridges, and over 50% (from 32.3 to 89.3%) on all six MnO2 cartridges. Our results confirm that a uniform extraction efficiency using two filters connected in series is rarely achieved. Using the average extraction efficiency of all cartridges, we propose a new approach that assumes a constant extraction efficiency. This method will reduce the error introduced by the assumption of uniform extraction efficiency in the determination of dissolved Th activities.

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Is the extraction of thorium onto MnO2-coated filter cartridges uniform?
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Open-File Report
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U.S. Geological Survey
33 p.