Experimental Precipitation of Carbonate Minerals: Effect of pH, Supersaturation and Substrate

Abstract

Understanding the controlling factors and elucidating the requirements and conditions necessary for carbon dioxide (CO2) storage by mineral trapping (or carbonation) is of paramount interest for any technical application as a means for carbon dioxide capture and storage (CCS). The effect of pH, supersaturation and substrate has been studied using non-stirred batch reactors at initial constant temperature of 150 oC. These conditions are relevant for mineral trapping. A set of experiments was conducted, where solid carbonate were precipitated from a range of solution compositions. Saturation state and composition of experimental solutions were calculated using the numerical code PHREEQC-2. Two main aqueous inlet solutions containing MgCl2 and Mg(CH3COO)2 with incremental changes in concentration were reacted on feldspar and basalt separately. The substrates used in the experiment, as well as secondary mineral phases that were formed from the experiments were identified using Raman spectroscopy analysis, X-ray diffraction (XRD) analysis, and scanning electron microscope (SEM) images. Results from the experiments show that the growth of secondary carbonates is highly connected to the reactivity of the substrate surface, and that the spatial distribution (number and crystal sizes) of the secondary phases on the surface is highly heterogeneous. Moreover, growth of the magnesian carbonates is highly dynamic with phases being formed and dissolved in a sequence of reactions controlled by the kinetic and thermodynamic stability of the minerals, and by local chemical gradients given by the substrate dissolution. SEM observations also confirmed that acetate was an excellent inhibitor to magnesian carbonate formation in general, with the exception of magnesite at higher pH. This work provides a starting point for future experiments that will more accurately elucidate the solid-solution relationships of carbonates, and provide a template upon which the origin of magnesian carbonates of similar chemistry may be reconciled.