Utilization of Chrysotile Mine Tailings and Microbially Accelerated Carbonate Mineral Precipitation Reactions As a Strategy for in Situ Carbon Sequestration

Carbonate mineral precipitation was successfully completed in chrysotile mine tailings collected from the abandoned Woodsreef Asbestos Mine in New South Wales, Australia. This was achieved by implementing microbially accelerated mineral precipitation reactions as a method of carbon sequestration. 

A preliminary leaching experiment was first completed to determine the ability of the chrysotile tailings to act as a magnesium source for carbonate precipitation. Tailing samples (5 g) were leached in five, 100 mL sulfuric acid [H₂SO₄] solutions with acid:serpentine ratios of 0.25, 0.5, 1, and 2, with water as a control. Tracking the pH of each leaching system monitored chrysotile dissolution/acid neutralization. The 0.25 system had a final pH of 6.3 and 16 % of the magnesium was leached from the chrysotile, producing 1800 ppm Mg²⁺ in solution. These results were used to design the reaction conditions for the microbial carbonate mineral precipitation trial. Cultures of cyanobacteria enriched from the Woodsreef Mine pit waters and endolithic, carbonate-rich habitats at the mine were capable of producing highly alkaline waters (up to pH 12) when grown in the laboratory. The alkalinity generated by these phototrophic microbes was used to accelerate magnesium carbonate mineral precipitation in acid treated tailings at ambient temperature and pressure. This was achieved by leaching tailings in columns constructed in 60 cc syringes, after which three columns were inoculated with the alkalinity generating cyanobacteria consortium. Two additional columns were not inoculated to act as controls. The microbes were given 4 weeks to grow in the columns, at which point the columns were sampled for characterization of their mineralogy (micro X-ray diffraction), water chemistry (inductively coupled plasma atomic emission spectroscopy), and microbial colonization within the leached tailings (scanning electron microscopy).

Micro X-ray diffraction of the surface material collected from the inoculated columns identified dypingite [Mg₅(CO₃)₄(OH)₂·5H₂O], hydromagnesite [Mg₅(CO₃)₄(OH)₂·4H₂O], nesquehonite [MgCO₃·3H₂O], and chrysotile [Mg₃Si₂O₅·(OH)₄]. When examined using scanning electron microscopy, the carbonate mineral precipitates were observed directly on the filamentous cyanobacteria. This is a result of the microbial cells becoming surrounded by microenvironments containing high concentrations of adsorbed magnesium and photosynthetically generated carbonate ions. When these localized ‘pockets’ of reactants are combined with the ability of the cell exterior and extra-cellular polymeric substances to act as nucleation sites, magnesium carbonate precipitation under ambient conditions becomes possible.

This study demonstrated the ability of microbially enabled carbonate mineral precipitation to act as a method of sequestering carbon dioxide (CO₂) in chrysotile mine tailings, initiating the formation of regolith carbonate. The ability of this process to use atmospheric CO₂ rather than a point source of carbon makes it suitable for utilization at a range of remote abandoned and active mine sites that generate ultramafic (Mg-rich) mine tailings.