Sulphur and Molybdenum Incorporation at the Calcite-Water Interface: Insights from Ab Initio Molecular Dynamics

Sulphur and molybdenum trace impurities in speleothems (stalagmites and stalactites) can provide long and continuous records of volcanic activity, which are important for past climatic and environmental reconstructions. However, the chemistry governing the incorporation of the trace-element bearing species into the calcium carbonate phases forming speleothems is not well understood. Our previous work has shown that substitution as tetrahedral oxyanions [XO₄]^2- (X=S, Mo) replacing [CO₃]^2- in CaCO₃ bulk phases (except perhaps for vaterite) is thermodynamically unfavourable with respect to the formation of competing phases, due to the larger size and different shape of the [XO₄]^2- tetrahedral anions in comparison with the flat [CO₃]^2- anions, which implied that most of the incorporation would happen at the surface rather than the bulk of the mineral. Here we present an ab initio molecular dynamics study exploring the incorporation of these impurities at the mineral-water interface. We show that the oxyanions substitution at the aqueous calcite (10.4) surface is clearly favoured over bulk incorporation, due to the lower structural strain on the calcium carbonate solid. Incorporation at surface step sites is even more favourable for both oxyanions, thanks to the additional interface space afforded by the surface line defect to accommodate the tetrahedral anion. Differences between sulphate and molybdate substitution can be mostly explained by the size of the anions. The molybdate oxyanion is more difficult to incorporate in the calcite bulk than the smaller sulphate oxyanion. However, when molybdate is substituted at the surface, the elastic cost is avoided because the oxyanion protrudes out of the surface and gains stability via the interaction with water at the interface, which in balance results in more favourable surface substitution for molybdate than for sulphate. The detailed molecular-level insights provided by our calculations will be useful to understand the chemical basis of S- and Mo-based speleothem records.