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 CO32 − 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 29.30 μg/g to 285.39 μg/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 KD (defined as [B/Ca]CaCO3/[B(OH)4−/HCO3−]seawater) shows a general decrease with pH, and KD × 1000 ranges from 2.13 to 1.27.