On the use of MnO₂ cartridges for the plutonium determination in seawater

To analyze plutonium (Pu) in open ocean waters can be challenging due to the low seawater concentrations. In this study we compared two techniques for Pu determination, one in-situ MnO₂ cartridge system and the more commonly used MnO₂ precipitation technique. During the pre-pilot GEOTRACES cruise ANT XXX-1 (2005) we tested MnO₂ cartridges for the pre-concentration of Pu from seawater at 19 sampling stations on a transect in the southeastern Atlantic Ocean between Vigo (Spain) and Cape Town (South Africa). Our in-situ sampling setup consisted of one particle cartridge followed by three MnO₂ cartridges in a series. Through the system we pumped between 956 and 2700 l of surface seawater with a flow rate between 1.6 and 5.2 l/min. We found that the adsorption efficiency of a single MnO₂ cartridge to adsorb Pu was rather constant and on average a 58 ± 7%. The adsorption efficiency was also found to be independent of seawater: temperature in the range of 18.3-29.2 °C, salinity range 34.2-37.1‰, and conductivity in the range of 46.8-58.4 mS/cm. In parallel with the in-situ sampling, discrete surface water samples between 259 and 281 l were taken and Pu was pre-concentrated using the MnO₂ precipitation method. We find a good agreement between the Pu concentrations determined with the two different techniques. The in-situ pre-concentration technique requires more radiochemical work in the laboratory but has the advantage that large seawater volumes can be sampled without the necessity for radiochemical processing on-board the ship. The much larger volumes sampled with the in-situ technique compared with the precipitation technique, enables accurate determination of Pu-isotopic ratios with a low relative standard deviation. We have shown in this study that in-situ MnO₂ cartridge technique can be used in a reliable way for the determination of dissolved Pu seawater concentration in open ocean waters.