Development of a separation method using UTEVA and DGA resins for the quantification of plutonium and americium in brown algae by alpha spectrometry

Introduction: Decades of nuclear activities have polluted marine environments with transuranium actinides. Background: Brown algae have frequently been used as marine bioindicators of radioactivity, but current literature describing the sequential separation and determination of Pu and Am from brown algae matrices is sparse, and does not make use of recent extraction chromatographic resins such as DGA. Aim: The aim of this project was to develop such a method, paying particular attention to achieving a high recovery at ultratrace (< ppm) levels. Method: The samples were pretreated with acid leeching, and preconcentrated with iron-coprecipitation. UTEVA and DGA columns were used to extract Pu and Am respectively, separating them from elements that could interfere during source preparation and analysis. Sources were prepared by electrodeposition, and quantified by alpha spectrometry. Results: Seven different sets of conditions were evaluated for source preparation with electrodeposition. An improvement over the starting method was obtained with an ammonium oxalate (0.3 M) - HCl (0.3 M) electrolyte, a current of 0.6 A, and time of 75 minutes, 114% ± 18% and 70% ± 10% (n = 7) for Pu and Am respectively. A final gross recovery with UTEVA of 102% ± 14% (n = 4) was obtained with only tracers, and 54% ± 6% (n = 4) with seaweed samples. Net recoveries of Am on DGA ranged from 9--84\% with only tracers, and for brown algae samples a gross recovery of 51% was obtained. The concentration of 239+240Pu from Fucus serratus collected in 2018 was estimated to 15.8 mBq/kg DW, significantly lower than what was measured from the same site in the 1990s. Both 238Pu and 241Am was found to be below the minimum detectable activity of approximately 0.1 mBq/kg DW. Conclusion: A selective separation method with satisfactory recoveries was developed for Pu, with a high precision in the estimate of 239+240Pu. The Am separation method on DGA was selective, but it will be possible to improve the repeatability of the recovery. A basis for further lowering of the minimum detectable activities for both Pu and Am was established.