A Database of Solution Additives Promoting Mg 2+ Dehydration and the Onset of MgCO 3 Nucleation.

Formed via aqueous carbonation of Mg 2+ ions, the crystallization of magnesite (MgCO 3 ) is a promising route to carbon capture and reuse, albeit limited by the slow precipitation of MgCO 3 . Although magnesite is naturally abundant, forming at low temperature conditions, its industrial production is an energy-intensive process due to the temperatures required to prevent the formation of hydrated phases. The principal difficulty in aqueous conditions arises from the very strong Mg 2+ ···H 2 O interaction, with high barriers to Mg 2+ dehydration. Using atomistic simulations, we have investigated the influence of 30 additive anions (X n - , n = 1-3), ranging from simple halides to more complex molecules, on the first two steps of MgCO 3 aggregation from solution, as follows: Mg 2+ dehydration and subsequent prenucleative Mg 2+ ···CO 3 2- pairing. We have computed the thermodynamic stabilities of solvent shared ion pairs (Mg 2+ ···H 2 O···X n - ) and contact ion pairs (Mg 2+ ···X n - ) to reveal the propensity of solution additives to inhibit or promote Mg 2+ ···CO 3 2- formation. We have determined the stabilization of undercoordinated hydrated Mg 2+ states with a vacant coordination site to which CO 3 2- can bind, subsequently initiating MgCO 3 nucleation or Mg 2+ incorporation into the crystal lattice. Extensive molecular dynamics simulations of electrolyte solutions containing Na 2 CO 3 with different sources of Mg 2+ (i.e., MgCl 2 , MgSO 4 , and Mg(CH 3 COO) 2 ) further show that the degree of dehydration of Mg 2+ and the structure of prenucleation MgCO 3 clusters change depending on the counterion identity. Through a fundamental understanding of the role of solution additives in the mechanism of Mg 2+ dehydration, our results help to rationalize previously reported experimental observation of the effect of solvation environments on the growth of magnesite. This understanding may contribute to identifying the solution composition and conditions that could promote the low-temperature CO 2 conversion into MgCO 3 at industrially relevant scales.