Partition coefficient of 234Th, 233Pa, 210Pb, 210Po, and 7Be
Activity concentrations of 234Th, 233Pa, 210Pb, 210Po, and 7Be were measured by gamma counting the 63.5, 312, 46.5, and 477.6 keV lines, respectively, on a Canberra ultra-highpurity germanium well-type detector. The 210Po activity was analyzed by liquid scintillation counting (Beckman Model 8100 Liquid Scintillation Counter). The filter samples (> 0.2 um particulate phase) were soaked with 1 mol L-1 HCl for 20 min in a counting vial, and the filtrate samples (< 0.2 um dissolved phase) were transferred into counting vials directly. Both filter and filtrate samples were then counted for activities of each radionuclide. All reported activities were decay and geometry corrected. 233Pa was added in equilibrium with 237Np. Under a wide range of environmental conditions, aqueous neptunium speciation is believed to be dominated by the pentavalent cation NpO2+. Np(V) is relative soluble and tends to remain in the water phase, unlike other actinides, such as Pu and Am, which are readily adsorbed by particles in the environment (Atwood 2013). In our absorption experiments, 237Np activities could be found only in the dissolved phase for all samples, supporting the assumption that 237Np would not adsorb onto particles during the time for which decay and in-growth corrections of 233Pa were applied. Traditional partition coefficients (Kd) between dissolved and particulate phases were used to quantify the sorption of radionuclides onto different particles in different experimental systems. Kd was defined here as

Kd = Ap x (Ad x Cp)^-1           (1)

where Ap and Ad represent particulate and dissolved activities (Bq L-1) of radionuclides and Cp is the particle concentration (kg L-1; Honeyman and Santschi 1989; Guo and Santschi 1997).