The Dolomite Problem

Dolomite, CaMg(CO3)2, is an ancient form of carbonate constituting a wide variety of geological formations. Despite its abundant deposits in geologic record, a longstanding mystery termed ‘the Dolomite Problem’ surrounds the formation of this mineral under present Earth conditions. Its otherwise common components (calcium, magnesium and carbonate ions) are not known to come readily into its characteristic layered, alternating organization except via microbial mediation, observed only in a few unique geologic sites. To date, chemical precipitation of dolomite at ambient temperatures has not been successful particularly due to the kinetic barrier of strong ion pairs formed by calcium and magnesium with sulfate ions in solution.

The coastal sabkhas of western Abu Dhabi, alongside other locations like Lagoa Vermelha and Brejo do Espinho (Brazil), the Coorong (Australia) and Qinghai Lake (China) were reported to exhibit modern dolomite formation, driven by the sulfate-reducing activity of the bacterial population in their sediments. The highly evaporative, hypersaline and sulfur-rich environment of the identified sites accommodate the specialized sulfate-reducing bacteria (SRB) hypothesized to overcome the kinetic barrier through a directed process of cation-sulfate pair dissociation and subsequent sulfate reduction into sulfide.

Figure 1: Western Abu Dhabi Coastal Sabkha

 

Biofilms as Microbial Support Systems

Microbial communities in the environment are characterized by their surface-attached living arrangement commonly known as biofilms. A biofilm community comprises both living cellular material and extracellular material such as the exopolymeric substances (EPS). Given the tendency of environmental microbes to develop biofilms as a means of survival, we have focused our research on the relationship between microbial community structures and in situ dolomite biomineralization. Our work at BEEL has so far included characterization of EPS generated by microbial communities of the local sabkhas, and direct examination of biofilm-biomineral progression with Scanning Electron Microscopy (SEM) techniques.

Implications & Intrigue of Biomineralization

This study is an opportunity to approach a geological problem with an appreciation of the significant role of microbial populations and their living processes in shaping the Earth’s surface. Abu Dhabi’s coastal sabkhas offer invaluable insight into the rare process of dolomite biomineralization which, while curiously confounding, also makes for a novel strategy for carbon capture and storage (CCS) and materials engineering.

As a carbonate mineral, dolomite exhibits a higher degree of stability compared to the more commonly occurring and less enigmatic calcite (calcium carbonate, or limestone). There is therefore considerable interest in exploring its biomineralization in CCS as an inorganic carbon sink as well as a capping process for saline formations used for carbon sequestration. In underground CO2 reservoirs, such processes can minimize fugitive emissions of sequestered CO2 into overlying groundwater aquifers and thus support safe implementation of CCS that is protective of Abu Dhabi’s groundwater resources. In addition to this, the areas of materials fabrication and structural restoration find tremendous appeal and intrigue in capturing the ingenuity of inorganic self-assembly at ambient temperatures and pressures. A better understanding of this process, which occurs much more widely in nature than previously recognized, would offer invaluable support to the development of sustainable technologies. One important milestone among many could be the achievement of high-strength and/or high-value material specifications independently from the classic industrial 'heat, beat and treat‘ design with its intensive requisite of energy, pressure and chemical input.