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The effect of pH on calcium carbonate polymorphs precipitated from aqueous solutions at 0oC

Yubin Hu 1Gernot Nehrke 1MariĆ«tte Wolthers 2,3Gerhard Dieckmann 1Dieter Wolf-Gladrow 1

1. Alfred Wegener Institute, Am Handelshafen 12, Bremerhaven 27570, Germany
2. University College London, Department of Chemistry (UCL), Gordon Street, London WC1HOAJ, United Kingdom
3. Department of Earth Sciences-Geochemistry, Utrecht University, 3508 TA, Utrecht 80021, Netherlands

Abstract

Calcium carbonate (CaCO3) has six different phases whose formation is affected strongly by precipitation conditions. Among them, pH is considered to be one of dominating factors controlling polymorphism during calcium carbonate precipitation. Furthermore, the Ca/CO3 ratio in solution was recently shown to strongly affect CaCO3 growth rate (e.g. Nehrke et al., 2007). However, the change of solution pH is at the same time associated with a change in hydrogen ion concentration (H+) and the Ca/CO3 ratio in solution, making it difficult to separate pH and solution stoichiometry effects. The objective of this study is to investigate whether the H+ or Ca/CO3 ratio has a control on calcium carbonate polymorphism.
Experiments were carried out at 0oC and two different pH conditions (pH = 12.6 and 9.0). Under each pH condition, different concentrations of CaCl2 and NaHCO3 were applied to achieve ratios of 1:1 and 10:1 for Ca/CO3. CaCl2 and NaHCO3 solutions were pumped into different pH solutions with or without phosphate at different pumping rates. Results showed that, at high pH (pH = 12.6), only ikaite formed, independent of PO4, pumping rate or the ratio of Ca/CO3. At low pH (pH = 9.0), the precipitate was vaterite in the absence of PO4 and ikiate in the presence of PO4 regardless of the ratio of Ca/CO3. These results indicate that at low temperature, the H+ concentration determined which polymorph was precipitated (ikaite or vaterite) rather than Ca/CO3 ratio.

 

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Presentation: Poster at 15th Summer School on Crystal Growth - ISSCG-15, by Yubin Hu
See On-line Journal of 15th Summer School on Crystal Growth - ISSCG-15

Submitted: 2013-04-15 15:47
Revised:   2013-04-15 15:50