Stability of calcium carbonate and magnesium carbonate in rainwater and nitric acid solutions

Carbonation of magnesium and calcium silicates has emerged as an interesting option for long term storage of captured CO2. However, carbonated minerals are not stable in acidic environments. This study was conducted to determine if synthetically carbonated minerals dissolve in acidic rain and release CO2. Synthetic magnesium and calcium carbonates were leached in nitric acid solutions of various acidities, as well as rainwater, and the stability of the minerals was investigated with various methods. The experimental study was complemented with thermodynamic equilibrium calculations using Gibbs energy minimization software (HSC 4.0). The leaching of base ions from the two carbonate minerals was found to behave similarly and depend mainly upon the acidity of the solution. The fraction of Mg and Ca dissolved after several days of stabilization in separate solutions with initial pH 1 was 9% for both carbonates, while the fraction of dissolved minerals in a solution with initial pH > 2 was less than 1%. FT-IR analyses of the reactor atmosphere revealed that CO2 gas was more rapidly released from calcium carbonate than from magnesium carbonate. However, only 1.5% of the CO2 stored in the calcium carbonate was released as gas at pH 1 against 0.0% for magnesium carbonate. No notable CO2 release occurred when leaching magnesium and calcium carbonates in solutions of pH 2. The solid residue analyses showed that the fixed CO2 content of the carbonates that had been exposed to nitric acid was even higher than before the treatment.