Impacts of aqueous carbonate accumulation rate, magnesium and polyaspartic acid on calcium carbonate formation (6–40 °C)

Abstract

The formation of anhydrous CaCO3 polymorphs is very common and widespread in low-temperature inorganic and biogenic environments. An increase in aqueous carbonate concentration frequently induces CaCO3 formation in nature. Nevertheless, no systematic experiment has assessed the impact of the continuous accumulation of aqueous carbonate ions on CaCO3 formation. Also, the coupled influences of temperature, inorganic and organic additives, and rates of aqueous carbonate accumulation and supersaturation to induce CaCO3 polymorph formation have been poorly investigated. The latter two factors comprise reaction times that are required for nucleation and subsequently affect the precipitation kinetics through ongoing CaCO3 formation. To address these issues, CaCO3 formation was experimentally studied at a variety of aqueous carbonate accumulation rates (10−1.5≤ACAR≤101.8μmolh−1l−1) between 6 and 40°C using a CO2 diffusion technique at conditions that were analogous to sea water (pH8.3; [Ca2+]=10mmoll−1). The impacts of an inorganic and organic constituent on CaCO3 formation were investigated using Mg2+ and polyaspartic acid (Pasp) at concentrations up to 55mmoll−1 and 3.4mgl−1, respectively.The experimental data clearly reveal that the time for CaCO3 nucleation depends strongly on ACAR, T, [Mg2+] (>0.5mmoll−1), and [Pasp] (>0.1mgl−1) but not on the formation of different CaCO3 polymorphs. Elevated ACAR results in earlier CaCO3 formation and causes Mg2+ to have less of an impact on the retardation of nucleation. The ion activity product (IAP) required for CaCO3 nucleation is positively correlated to the subsequent CaCO3 precipitation rates but is independent of whether aragonite or calcite formation is induced. The type of CaCO3 polymorph from the primary nuclei is controlled by the influences of Mg2+ and Pasp, which are coupled to ACAR and T. Elevated ACAR, T, and [Mg2+] promote the formation of aragonite, and high [Mg2+] levels suppress the formation of vaterite. In contrast, [Pasp]>0.02mgl−1 inhibits aragonite and favours vaterite formation. Thus, if both Mg2+ and Pasp are present, the formation of calcite is favoured. The precipitation rates for ongoing calcite and aragonite formation are nearly the same at analogous conditions and particularly for a given IAP. According to our experimental results, the combined impacts of ACAR, T, [Mg2+], and [Pasp] must be considered to identify distinct conditions of CaCO3 polymorph formation and precipitation kinetics.