The adsorption of Th and Pa on different particle types in dependence of the provenance of natural seawater

The adsorption of Th and Pa on different particle types in dependence of the provenance of natural seawaterW.GEIBERT1 AND R.USBECK11Alfred Wegener Institute for Polar and Marine Research, Bremerhaven, Germany (wgeibert@awi-bremerhaven.de)We performed experiments in order to investigate the adsorption of Th and Pa on different particles in recently sampled seawater. Water from three different locations was 0.2 µm filtered. Then, four different particle types were added to the respective seawater samples at a concentration of 0.5 mg/l, and neutralized Th and Pa spikes were added. Additionally, one sample was run without particles added. Th and Pa distribution was monitored in increasing time intervals for several days.Spike recoveries ranged from 40 to 100 % for Th and from 50 to 100 % for Pa (depending on particle type). Our recoveries are higher than previously observed in experiments with artificial seawater. The low losses to the container walls allowed the determination of distribution coefficients (Kd) for different particle types. Th on MnO2 was found to have a Kd of >3*107 in all water types, Pa on the same particle type >2*107. On smectite, KdTh was found to range between 4 and 10*106 towards the end of the experiments. KdPa in the same samples was 1.2 to 1.8*106. On biogenic opal particles, KdTh was about 1*106 in two of three samples. In the water sample from the Subtropical Gyre (Argentine Basin), its Kd was found to be twice as high (2*106) at the start of the experiment, and up to 8*106 at the end. For Pa on opal, Kds of 0.5*106 resulted in general, with two somewhat higher (Kd=1*106) exceptions towards the end of the experiments. The experiment with CaCO3 (non-biogenic) resulted in very variable Kds depending on the water type used for both Th and Pa.One of the most interesting results was the large percentage of Th and Pa found in the particulate fraction when no particles were added. This must be attributed to the spontaneous formation of particles from the colloidal phase