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

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 K_d 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 K_d 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 (F_Po/Pb) varied with salinity for different mineral components while the F_Bi/Pb was less affected by mineral components, salinity and pH. The in-situ experimental results showed that the F_Bi/Pb increased with increase in Chl-a concentration. Considering the much shorter half-life of ²¹⁰Bi (5.0 d) and the high K_d 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.