Measuring 210Po in natural waters: A comparison of three methods with similar results

Oceanographers have long used ²¹⁰Po/²¹⁰Pb disequilibria to quantify flux of particulate organic matter from the surface ocean. However, recent evidence by Roca-Martí et al. (2021) found that one of two commonly used methods to concentrate polonium from a natural water sample – coprecipitation with Fe(OH)₃ – resulted in significantly lower ²¹⁰Po activities than the other method – coprecipitation with Co-APDC. In this study, we employed the two methods used by Roca-Martí et al. (2021) and our own modified Fe(OH)₃ technique to measure ²¹⁰Po in freshwater samples collected from lake, harbor, river, and bog sources. We found a non-significant bias of ≤0.5 dpm ²¹⁰Po 100 L⁻¹ between all three methods. We found no evidence that ²⁰⁹Po was not as fully equilibrated with natural ²¹⁰Po in 15 min as it was after 12 h of equilibration. And finally, we found no indication that organic complexation of dissolved ²¹⁰Po was interfering with polonium recovery. Our results do not negate the findings by Roca-Martí et al. (2021). The behavior of ²¹⁰Po in marine systems may indeed be different. However, we recommend that future studies also examine how the collection of iron precipitate and the composition and concentration of particulate organic matter might influence the recovery of absorbed ²¹⁰Po from an unfiltered water sample.