Partitioning of sodium into calcium carbonates synthesized at 10–40 °C: Influence of organic ligands and temperature

Abstract

Calcium carbonates (calcite and aragonite) were synthesized at 10–40 °C to understand the partitioning behavior of sodium during the mineralization of marine organisms. The role of organic ligands (acetic acid) in sodium partitioning into calcium carbonates was also investigated. The partitioning coefficient of sodium, defined as Kd = (Na/Ca)CaCO3/(Na/Ca)solvent, was higher for aragonite than for calcite. The Kd values associated with organic-free solvents decreased with increasing temperature. In contrast, for solvents containing acetic acid, Kd increased with increasing temperature. The K-edge X-ray absorption near edge structure (XANES) measurement of Na suggests that Na substitutes the Ca sites in CaCO3 crystals, regardless of the solvent containing organic ligands. The partitioning behavior associated with the solvent containing acetic acid seems to represent the natural mineralization process in the calcification fluid, which contains a substantial amount of carboxylate. The temperature dependence of Kd was especially clear for aragonite produced from the solvent containing acetic acid. Our findings may help to understand the selection and regulation of crystal polymorphs by marine species in different localities and in different geological era.