Quantification of boron incorporation into synthetic calcite under controlled pH and temperature conditions using a differential solubility technique

Due to the greater thermodynamic stability of CaCO₃ relative to Li₂CO₃, synthetic CaCO₃ was precipitated from artificial seawater by the release of CO₃^2- from Li₂CO₃ and Ca²⁺ under different pH conditions. The pH of the solution was controlled by slowly bubbling high-purity NH₃ or CO₂ gas at 25±0.5°C, assuming that the pH at which nucleation takes place is the same as the average pH of the solution. This method is referred to as the "differential solubility technique". The precipitated CaCO₃ is dominated by crystalline calcite, as identified by scanning electron microscopy, X-ray diffraction, and Fourier transform infrared spectrometry. The boron concentrations in the precipitated calcite increased from 29.30μg/g to 285.39μg/g when the artificial solution pH values increased from 7.40±0.03 to 8.80±0.03, indicating that pH has a significant effect on the incorporation of boron into synthetic calcite. Our results support the hypothesis that boron uptake in carbonates takes place predominantly by incorporation of the charged borate species B(OH)₄^-. The partition coefficient K_D (defined as [B/Ca]_CaCO3/[B(OH)₄^-/HCO₃^-]_seawater) shows a general decrease with pH, and K_D×1000 ranges from 2.13 to 1.27.