Mineral Carbonation Employing Ultramafic Mine Waste

Monday, 15 December 2014: 5:15 PM
Gordon Southam1, Jenine McCutcheon2, Ian M Power3, Anna L Harrison3, Siobhan A Wilson4 and Gregory Martin Dipple3, (1)University of Queensland, School of Earth Sciences, St Lucia, QLD, Australia, (2)University of Queensland, School of Earth Sciences, St Lucia, Australia, (3)University of British Columbia, Vancouver, BC, Canada, (4)Monash University, Melbourne, VIC, Australia
Carbonate minerals are an important, stable carbon sink being investigated as a strategy to sequester CO2 produced by human activity. A natural playa (Atlin, BC, CAN) that has demonstrated the ability to microbially-accelerate hydromagnesite formation was used as an experimental model. Growth of microbial mats from Atlin, in a 10 m long flow-through bioreactor catalysed hydromagnesite precipitation under ‘natural’ conditions. To enhance mineral carbonation, chrysotile from the Clinton Creek Asbestos Mine (YT, CAN) was used as a target substrate for sulphuric acid leaching, releasing as much as 94% of the magnesium into solution via chemical weathering. This magnesium-rich ‘feedstock’ was used to examine the ability of the microbialites to enhance carbonate mineral precipitation using only atmospheric CO2 as the carbon source. The phototrophic consortium catalysed the precipitation of platy hydromagnesite [Mg5(CO3)4(OH)2·4H2O] accompanied by magnesite [MgCO3], aragonite [CaCO3], and minor dypingite [Mg5(CO3)4(OH)2·5H2O]. Scanning Electron Microscopy-Energy Dispersive Spectroscopy indicated that cell exteriors and extracellular polymeric substances (EPS) served as nucleation sites for carbonate precipitation. In many cases, entire cyanobacteria filaments were entombed in magnesium carbonate coatings, which appeared to contain a framework of EPS. Cell coatings were composed of small crystals, which intuitively resulted from rapid crystal nucleation. Excess nutrient addition generated eutrophic conditions in the bioreactor, resulting in the growth of a pellicle that sealed the bioreactor contents from the atmosphere. The resulting anaerobic conditions induced fermentation and subsequent acid generation, which in turn caused a drop in pH to circumneutral values and a reduction in carbonate precipitation. Monitoring of the water chemistry conditions indicated that a high pH (> 9.4), and relatively high concentrations of magnesium (> 3000 ppm), compared with the natural wetland (up to 1000 ppm), and dissolved inorganic carbon (> 20 mM C) were ideal for carbonate precipitation. Under optimum nutrient and magnesium inputs, a mass balance calculation using water chemistry data and hydromagnesite as the sole mineral product resulted in a carbon sequestration rate of 61 t C/ha/year.