ABSTRACT

Homogeneous partition coefficients (D) for Ra and Ba in calcite were determined from coprecipitation experiments using the Continuous Addition Method (CAM). The D-values derived for Ra and Ba are: D_Ra = (1.5 ±0.6) ×10^-1 and D_Ba = (1.6 ±1.1) ×10^-2. Although these data were derived at low precipitation rates (R) for calcite (R < 9 nmol mg^-1min^-1), the solutions were significantly oversaturated in calcite. Values of D should, ideally, be derived at equilibrium and hence the effect of oversaturation should be examined. Therefore, a coprecipitation experiment using the Free Drift Method (FDM) with slow degassing was also carried out. This experiment occurs under conditions that are only slightly oversaturated and the rate of coprecipitation was confirmed to be slow compared to dissolution and precipitation at the calcite surface. The resultant heterogeneous partition coefficients showed good agreement with homogeneous partition coefficients derived by CAM. This indicates that the oversaturation in CAM did not affect partitioning of these metals in calcite and that the derived D-values represented equilibrium partitioning between the solution and the solid phase. The derived value of D_Ra is one order of magnitude larger than that of D_Ba despite the larger ionic radius of Ra. This indicates that compatibility of ionic radius is not a dominant parameter for preferential incorporation of Ra and Ba in calcite.