A computational study of Mg2+ dehydration in aqueous solution in the presence of HS- and other monovalent anions - Insights to dolomite formation

Massive sedimentary dolomite formed at near-Earth's surface temperatures is abundant in the ancient geological rock record compared to modern deposition. Extensive experimental work to synthesize dolomite at low temperature and to reveal the formation mechanism has been attempted previously. Sulfide, the product of bacterial sulfate reduction, has been proposed in the literature to play an active role in promoting dolomite formation by facilitating desolvation of Mg2+ in the bulk solution and, thus, incorporation into the dolomite crystal structure. Chemical intuition, however, does not suggest any particular characteristic of HS- that would render it an efficient promoter of Mg2+ desolvation in solution. In order to examine the previously proposed hypothesis, we conduct an ab initio reaction path ensemble (RPE) study along a dissociative mechanism to determine the energy penalty of removing a first-shell water molecule around Mg2+ compared to Mg2+ with HS- located in the second coordination shell. The solvent effect and specific hydrogen-bond interactions from water beyond the first-solvation shell are addressed using large cluster models, where up to the second layer of Mg2+ hydration and the first solvation-shell of HS- are included. Within the context our modeling approach, we find that HS- has little, if any, effect on lowering the Mg2+ dehydration barrier in aqueous solution. Alternative mechanisms must then be invoked to explain the apparent promotional effect of HS- on Mg2+ dehydration kinetics. (C) 2012 Elsevier Ltd. All rights reserved.