1. Introduction
The mineralization of carbon dioxide (CO2) has the benefits of unlimited raw material supplement and longer-term storage carbonate materials, the output values of which are expected to reach $1 trillion per year by 2030.1 Therein, the anhydrous form of magnesium carbonate, magnesite (MgCO3), is widely used in food and fertilizers, in the manufacture of refractory materials, as a valuable construction material due to its fire-retardant properties, and in the production of eco-cements.2 Mining of MgCO3 exceeds 25 Mt year-1 with deposits concentrated in Russia, China, and Korea;3 hence worldwide usages are accompanied by transport costs. Conversely, magnesium ion (Mg2+) sources are globally widespread and plenty (accessible Mg silicate deposits estimated at 100 000 Gt),4 with MgCO3 able to be locally produced worldwide via mineral carbonation of Mg silicate.5 However, such CO2 mineralization into MgCO3 is limited by the slow rates of magnesite precipitation from solution.5 Its production is an energy-intensive process due to the high temperatures (T = 120-600 °C) required to prevent the formation of hydrated Mg carbonate phases such as nesquehonite (MgCO3·3H2O) and hydromagnesite (Mg5(CO3)4(OH)2·4H2O).6 Albeit of commercial use, these phases dominate industrial outputs and perpetuate the absence of local production of magnesite. The high temperature necessary to promote the direct precipitation of anhydrous MgCO3, the increased solid mass and volume of nesquehonite and hydromagnesite generated per mole of CO2 sequestered, as well as their inferior mechanical and structural properties, negatively impact on the cost, profitability, and thus industrial viability of Mg-mediated CO2 mineralization.7 The culprit slow precipitation rate of MgCO3 has long been ascribed to the very strong Mg2+···H2O interaction (hydration free energy of Mg2+ is −439 kcal mol-1),8 which raises the barrier of Mg2+ dehydration.9
Encouragingly, the solvation environment in which the mineral crystallization occurs may influence the Mg2+ dehydration process. In this regard, McKenzie et al. proposed that bisulfide delivered by sulfate reducing bacteria in sedimentary environments, although dilute, could catalyze the formation of natural dolomite (CaMg(CO3)2).10 Hence, to accelerate the synthesis of anhydrous MgCO3 under standard conditions, efforts have focused on the addition of salts,11,12 complexing compounds,13 alcohols,14 and microorganisms.15 However, there is a lack of understanding of additive identity and concentration, with few comprehensive studies resolving the effects of solution additives on the fundamental processes controlling the process of Mg2+ dehydration. We therefore looked to help bolster knowledge on the formation of Mg carbonates from aqueous solutions.
Bạn đang xem: A Database of Solution Additives Promoting Mg2+ Dehydration and the Onset of MgCO3 Nucleation
By providing a fundamental understanding of how the presence of solution additives can influence the rate-determining Mg2+ dehydration step, the composition of the solution may be rationally tuned to accelerate the kinetics of the early stages of MgCO3 nucleation and growth. In our recent study on the mechanism of Mg2+ dehydration, we have shown that Mg(H2O)62+ is the only stable coordination state in pure water.16 However, solution additive anions such as fluoride, carboxylate, and bisulfide may stabilize undercoordinated configurations and subsequent incorporation into the lattice of magnesium carbonates, which could potentially promote low-temperature crystallization.16 Following these findings, herein we present a comprehensive computational investigation of the influence of thirty solution additives on the hydration properties of Mg2+ to determine which anions accelerate its dehydration as a function of the molecular size and functional groups of the additives. Table 1 reports the thirty solution additive ions (Xn-, n = 1-3) considered in this study: ones that are naturally abundant in groundwater such as chloride (Cl-), fluoride (F-), sulfate (SO42-), nitrate (NO3-), phosphates (HnPO43-n, n = 0-2), silicate (SiO32-), and (bi)carbonate (H)CO3-.17 Also ions that have been deemed important in promoting the formation of anhydrous forms of Mg carbonates include bisulfide (HS-) and carboxylic acids (HCOO- and CH3COO-).10,13 Further, molecular ions containing multiple functional groups that may act cooperatively to promote Mg2+ dehydration such as taurate (C2H6NSO3-), aspartate (C4H6NO42-), oxalate (C2O42-), salicylate (C7H5O3-), citrate (C6H5O73-), tartrate (C4H4O62-), malate (C4H4O52-), and aminophenolate (C6H4ONH2-) have been included. Peptides and alcohol molecules considered responsible for facilitating Mg2+ dehydration such as glycinate (C2H4NO2-), glutamate (C5H8NO4-), aspartate (C4H6NO42-), and isopropyl alcohol ionic (C3H7O2-)14,18−20 were also considered. Finally, the hexafluorosilicate ion (SiF62-) is produced on large scales in volcanoes21 and has been speculated to accelerate natural MgCO3 formation.22 Such a computational database may be used to identify conditions of solution compositions catalyzing the low-temperature CO2 conversion into MgCO3.
We have used a combination of classical molecular dynamics (MD) and enhanced sampling metadynamics (MetaD) to characterize the ability of the solution additive ions (Table 1) to promote Mg2+ dehydration based on the following two well-defined molecular level criteria: formation of (1) solvent-shared ion pairs or (2) contact ion pairs with Mg2+, with either being less stable than Mg2+···CO32- (i.e., so as not to retard formation of the latter). These pairs can effectively stabilize undercoordinated hydrated Mg2+ states with a vacant coordination site (i.e., five-coordinated Mg2+) to which CO32- can bind, initiating the MgCO3 nucleation and/or incorporation of Mg2+ into the growing crystal lattice. Subsequently, we have conducted unbiased classical MD simulations of MgCO3 aggregation in the presence of selected additives to monitor the effect of solution composition, the dynamics of formation, and the structure of prenucleation clusters.
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