Partition coefficient of Ra in witherite

Abstract

The heterogeneous partition coefficient (λ) for Ra in BaCO₃ (witherite) was determined by coprecipitation using the free drift method. Initially, CO₂ (100 %) was bubbled through a solution equilibrated with witherite. After a small amount of Ra was added into the solution, coprecipitation was induced by a slow increase in pH, caused by degassing CO₂(g) from the solution. Kinetically controlled precipitation was restricted to be slow because of the slow degassing of CO₂(g). The observed low precipitation rate allowed for the equilibrium partitioning of elements during coprecipitation of witherite. The value of λ derived for Ra was (1.3 ± 0.7) × 10⁻¹, a value similar to that obtained from the coprecipitation of Ra in CaCO₃ (calcite). The obtained results indicated that partitioning of Ra in carbonate-based minerals was not dependent on the similarity of its ionic radius with that of the metal in the host carbonate.