Investigations on the time-series partitioning of 210Pb, 207Bi and 210Po between marine particles and solution under different salinity and pH conditions

Abstract

The long-lived progeny of radon-222, lead-210 (210Pb), polonium-210 (210Po) and bismuth-210 (210Bi), serve as a tool in the study of marine biogeochemical processes. Here, we report our investigations on the partitioning of 210Pb, 210Po and 210Bi (207Bi) between seawater and solids (twelve commonly-occurring mineral particles) under different values of pH, salinity and contact time. The Kd values of 210Pb and 207Bi for MnO2 varied widely between the synthetic and natural minerals, and thus the suitability of using synthetic material for their partitioning studies in marine system needs to be re-assessed. The Kd values of 210Pb in clay minerals were found to be in the order: smectite > illite > kaolinite. The sorption efficiencies (=the particulate activity divided by the sum of particulate and dissolved activity) of 210Pb, 207Bi and 210Po on illite were above 81% when the contact time was >2 h. These radionuclides were more soluble when pH was ≤5, but show more particle-reactive nature for pH of 6 to 8. The fractionation factor between 210Po and 210Pb (FPo/Pb) varied with salinity for different mineral components while the FBi/Pb was less affected by mineral components, salinity and pH. The in-situ experimental results showed that the FBi/Pb increased with increase in Chl-a concentration. Considering the much shorter half-life of 210Bi (5.0 d) and the high Kd values established in this study, we propose that 210Bi/210Pb activity ratio could potentially be used to estimate the particle export from phytoplankton bloom on time scale much shorter than 234Th or 210Po.