Stability and inter-conversion of synthetic dawsonites in aqueous media

摘要

Previous works have opened the debate on the importance of dawsonite in CO_2 sequestration by mineral trapping, although there is a lack of systematic studies assessing its long-term stability. With an open eye to this application, we have systematically investigated the stability of synthetic dawsonites (MAlCO_3(OH)_2, M=NH4+, Na~+, and K~+) in a variety of aqueous media (NO3-, Cl~-, SO42-, CO32-, and OH~-) at ambient pressure and 323K. The three compositions dissolved under strong acid (pH 2) and basic (pH 14) conditions. In water (pH 7), dawsonites were not stable, evolving into boehmite (NH_4-form) and bayerite (Na and K-forms). The most interesting transformations occurred upon contacting the synthesized dawsonites with ammonium, potassium, and sodium carbonate solutions. NH_4-dawsonite was converted into Na-dawsonite and K-dawsonite in Na_2CO_3 and K_2CO_3 (pH 12), respectively. Na-dawsonite transformed into NH_4-dawsonite in (NH_4)_2CO_3 (pH 10). The NH_4-dawsonite and Na-dawsonite can be successively inter-converted in the appropriate carbonate solution. These transformations are thought to follow a dissolution-precipitation mechanism. In the pH range of 10-12, K-dawsonite was the most stable among the three synthesized samples. Besides structural changes, the treatment of dawsonites in carbonate solutions caused remarkable morphology and porosity alterations of the product samples. Our study provides an improved understanding of the chemistry of dawsonite-type compounds. In particular, the narrow pH range at which they are stable poses serious constraints in their potential use for geochemical fixation of CO_2.