Production of magnesium-rich solutions by acid leaching of chrysotile: A precursor to field-scale deployment of microbially enabled carbonate mineral precipitation

• Low sulfuric acid concentrations result in stoichiometric chrysotile dissolution
• Moderate sulfuric acid concentrations dissolve 84% of the Mg found in chrysotile
• High acid concentrations produce less Mg than the moderate concentrations
• Mg leached from chrysotile can be a reactant for carbonate mineral formation
• Acid leaching can be optimized for Mg release and biologically-mediated carbonation

Carbonate minerals are one of the primary carbon sinks under investigation for sequestering anthropogenic carbon dioxide (CO2). Ultramafic rock has the potential to act as a magnesium source for carbonate precipitation reactions utilizing atmospheric CO2. This study characterizes the release of magnesium from chrysotile tailings from the Clinton Creek Asbestos Mine (Yukon, Canada) by means of a 42-day leaching experiment using sulfuric acid. Low acid concentrations (targeting 8.33% and 16.67% dissolution of chrysotile) resulted in stoichiometric, dissolution. Moderate (33.33% and 66.67% dissolution) and high (100% dissolution) acid concentrations resulted in non-stoichiometric dissolution of chrysotile. Here, the concentration ratio of Mg:Si in solution was greater than expected for moderate acid concentrations and lower than expected for high acid concentrations. The moderate acid reaction system demonstrated that as much as 84% of the magnesium found in chrysotile can be released into solution via chemical weathering. However, at high acid concentrations, Mg values were in fact lower than the moderate acid reactivity, presumably resulting from adsorption of soluble magnesium to amorphous silica, a byproduct of extensive magnesium leaching from chrysotile fibers. The production of magnesium-rich solutions in this experiment demonstrates that a highly concentrated starting material for magnesium carbonate precipitation reactions can be produced from chrysotile-bearing tailings, providing ultra-mafic hosted mines with an important resource for developing technological strategies for reducing their net carbon emissions. This process represents a win-win scenario for the management of chrysotile-bearing mine wastes, in which a hazardous material is transformed into a geologically stable mineral sink for ‘carbon dioxide storage’.