Reactivity of the calcite–water-interface, from molecular scale processes to geochemical engineering

Abstract

Surface reactions on calcite play an important role in geochemical and environmental systems, as well as many areas of industry. In this review, we present investigations of calcite that were performed in the frame of the joint research project “RECAWA” (reactivity of calcite–water-interfaces: molecular process understanding for technical applications). As indicated by the project title, work within the project comprised a large range of length scales. The molecular scale structure of the calcite (104)–water-interface is refined based on surface diffraction data. Structural details are related to surface charging phenomena, and a simplified basic stern surface complexation model is proposed. As an example for trace metal interactions with calcite surfaces we review and present new spectroscopic and macroscopic experimental results on Selenium interactions with calcite. Results demonstrate that selenate (SeO42−) shows no significant interaction with calcite at our experimental conditions, while selenite (SeO32−) adsorbs at the calcite surface and can be incorporated into the calcite structure. Atomistic calculations are used to assess the thermodynamics of sulfate (SO42−), selenate (SeO42−), and selenite (SeO32−) partitioning in calcite and aragonite. The results show that incorporation of these oxo-anions into the calcite structure is so highly endothermic that incorporation is practically impossible at bulk equilibrium and standard conditions. This indicates that entrapment processes are involved when coprecipitation is observed experimentally. The relevance of nano-scale surface features is addressed in an investigation of calcite growth and precipitation in the presence of phosphonates, demonstrating the influence of phosphonates on the morphology of growth spirals and macroscopic growth rates. It is investigated how physical properties of limestone containing cement suspensions may influence the workability of the cement suspensions and thus the efficacy of limestone in industrial applications. The largest scale is reached in iron filtration experiments in a water-purification-pilot-plant using limestone as filter material, which appeared to be highly effective for removing iron from drinking water. Investigations presented cover a whole series of methods to study the calcite–water-interface. Many calcite related topics are addressed, demonstrating how broad the field of calcite–water-interface research is and how manifold the applications are, for which calcite–water-interface phenomena are of major relevance.