2011 Annual Science Report
University of Wisconsin Reporting | SEP 2010 – AUG 2011
Project 5D: A Computational Study of Mg2+ Dehydration in Aqueous Solution in the Presence of Sulfide - Insights Into Dolomite Formation
Microbial activity has been invoked frequently in the literature to explain the formation of massive sedimentary dolomite at near-Earth’s surface temperatures in the ancient geological rock record and, more generally, for the formation of mixed cation carbonates. Carbonates have been found on Mars, and may serve as a biosignature, if the mechanisms of formation can be elucidated. Sulfide, the product of bacterial sulfate reduction, has been proposed previously to promote dolomite formation by facilitating desolvation of Mg2+ in 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. We, therefore, conducted ab initio reaction path ensemble (RPE) and Molecular Dynamics simulations to determine the energy barrier for removal of a single water molecule from the first solvation shell compared to that for hydrated Mg2+ in the presence of HS- in the second coordination shell of Mg2+. We found that HS- had little, if any, effect on lowering the Mg2+ dehydration barrier in aqueous solution. Empirical observations of HS- promoted dolomite formation may then suggest a potential role for HS- in promoting Mg2+ desolvation at a solid precursor phase. Our project addresses NASA Astrobiology Institute’s (NAI) Roadmap goals of recognizing and preserving biosignatures and NASA’s Strategic Goal of advancing scientific knowledge of the origin and evolution of the Earth’s biosphere and the potential for life elsewhere.
Massive layers of sedimentary dolomite formed at Earth’s surface or near-surface temperatures (< ~ 50 °C) are found in strata of all ages from the Precambrian to the Cenozoic. Modern dolomite formation is observed only in specific environments, such as highly saline lakes, deep-sea sediments and supersaturated saline lagoons, and only in minor quantities, but laboratory synthesis at low temperatures have been difficult. More generally, the synthesis of mixed cation carbonates ((Fe, Ca, Mg)CO3)) at Earth’s near-surface temperatures is difficult inorganically, although such phases are known geologically as fine-grained early diagenetic phases in sedimentary rocks, as cements in oil reservoir rocks, and in the Martain meteorite ALH84001. Microbial involvement in the formation of mixed-cation carbonates has been proposed in the literature, with a role for sulfate-reducing bacteria, which remove SO42- an inhibitor of dolomite formation. Thus, the occurrence of dolomite or other mixed-cation carbonates may serve as a biosignature on Earth and on other solid worlds such as Mars. Based on the proposed promoting role of HS- in Mg2+ desolvation, Zhang et al. synthesized high-magnesium calcite and disordered dolomite successfully at room temperature by mixing NaHCO3 + Na2S, and the effects of increasing pH were properly accounted for. The authors suggested that the low dielectric constant of aqueous sulfide somehow promoted Mg2+ dehydration, but no detailed mechanism was provided. Zhang et al.'s experimental work serves as a direct motivation for our present computational study.
Chemical intuition, based on considerations of charge density, chemical hardness and softness, or molecular electronic structures, does not suggest any particular characteristics of HS- that would render it an effective promoter. In the present work, therefore, we examined the purported role of sulfide in facilitating water dissociation from the Mg2+ center. We used a combination of computational methods ranging from classical molecular dynamics (MD) simulations to ab initio quantum mechanical calculations with model clusters (Evans et al., 2008). In the “real world”, such a promotion effect may occur in the aqueous bulk solution or at the solution-solid interface. In the present work, as a first step, we study the mechanism in bulk solution; future work will address the mechanism at the solution-solid interface.
We conducted ab initio reaction path ensemble (RPE) study for hydrated Mg2+ to determine the energy penalty for removal of a single water molecule from the first solvation shell compared to that for hydrated Mg2+ in the presence of HS- in the second coordination shell of Mg2+. We used large cluster models, where up to the second layer of Mg2+ hydration and the first solvation-shell of HS- are included, in order to address the solvent effect and specific hydrogen-bond interactions from water beyond the first-solvation shell. We found that HS- has little, if any, effect on lowering the Mg2+ dehydration barrier in aqueous solution. We speculated, then, that empirical observations of HS- promoted dolomite formation may indicate a potential role for HS- in promoting Mg2+ desolvation at a solid precursor phase that ultimately leads to a proto-dolomite phase.