Effect of pH and Phosphate on Calcium Carbonate Polymorphs Precipitated at near-Freezing Temperature

Yu-Bin Hu,Gernot Nehrke,Dieter A Wolf-Gladrow,Mariëtte Wolthers

doi:10.1021/cg500829p

Abstract

The effects of pH and phosphate on the precipitation of calcium carbonate polymorphs from aqueous solution were investigated. Experiments were carried out at near-freezing temperature and two different pH conditions (pH 13.4 and 9.0). At each pH condition, solutions having different concentrations of CaCl2 and NaHCO3 were mixed to achieve Ca/CO3 ratios of 1:1 and 10:1 at different pumping rates with and without phosphate. Results showed that, at pH 13.4, only ikaite was formed, independent of pumping rate, Ca/CO3 ratio, and phosphate. At pH 9.0, the precipitate was predominantly vaterite in the absence of phosphate and ikaite in the presence of phosphate regardless of the ratio of Ca/CO3. These results indicate that at low temperatures and moderate alkaline conditions (pH 9.0), phosphate can act as a switch between ikaite and vaterite polymorphs. Contrastingly, at higher alkaline conditions (pH 13.4) phosphate is not prerequisite and the high pH in itself is enough to trigger ikaite formation over vaterite.

Highlights

Different polymorphs of calcium carbonate (CaCO3) exist: various amorphous forms, amorphous calcium carbonate (ACC);[1,2] two hydrated forms, calcium carbonate monohydrate (MCC) and calcium carbonate hexahydrate; and three anhydrous polymorphs, vaterite, aragonite, and calcite.The formation of different polymorphs of CaCO3 is strongly affected by precipitation conditions
PH and additives are considered to be two dominating factors controlling polymorphism during calcium carbonate precipitation.[3,4] pH-dependent change of Ca−CO3 binding strength is postulated to play an important role in determining different forms of ACC, which later transform into the particular calcium carbonate polymorphs.[5]
The different polymorphs of calcium carbonate precipitated under two pH conditions in the absence of PO4 might result from the differences in solution saturation state, pH, Ca/CO3 ratio and induction time of vaterite τv, which is the time span from the moment that the ion activity product of Ca2+ and CO32− (IAP) passes through the vaterite solubility until the onset of any calcium carbonate precipitation

Summary

Introduction

Different polymorphs of calcium carbonate (CaCO3) exist: various amorphous forms, amorphous calcium carbonate (ACC);[1,2] two hydrated forms, calcium carbonate monohydrate (MCC) and calcium carbonate hexahydrate (ikaite); and three anhydrous polymorphs, vaterite, aragonite, and calcite.The formation of different polymorphs of CaCO3 is strongly affected by precipitation conditions. PH and additives (e.g., phosphate) are considered to be two dominating factors controlling polymorphism during calcium carbonate precipitation.[3,4] pH-dependent change of Ca−CO3 binding strength is postulated to play an important role in determining different forms of ACC, which later transform into the particular calcium carbonate polymorphs.[5] changes in solution pH will affect the equilibrium concentration of different carbon species (e.g., HCO3− and CO32−). This will affect the Ca/CO3 ratio in solution, making it difficult to separate the effects of pH and solution stoichiometry. The effect of additives on calcium carbonate crystallization has been extensively studied,[4] the mechanism by which additives influence precipitation at near-freezing temperature is not well-known

Results

Discussion

Conclusion