Ultramafic Mine Tailings Research Articles

Reactive Transport Modeling of Natural Carbon Sequestration in Ultramafic Mine Tailings

Atmospheric CO2 is naturally sequestered in ultramafic mine tailings as a result of the weathering of serpentine minerals [Mg3Si2O5(OH)4] and brucite [Mg(OH)2], and subsequent mineralization of CO2 in hydrated magnesium carbonate minerals, such as hydromagnesite [Mg5(CO3)4(OH)2·4H2O]. Understanding the CO2 trapping mechanisms is key to evaluating the capacity of such tailings for carbon sequestration. Natural CO2 sequestration in subaerially exposed ultramafic tailings at a mine site near Mount Keith, Australia is assessed with a process‐based reactive transport model. The model formulation includes unsaturated flow, equations accounting for energy balance and vapor diffusion, fully coupled with solute transport, gas diffusion, and geochemical reactions. Atmospheric boundary conditions accounting for the effect of climate variations are also included. Kinetic dissolution of serpentine, dissolution‐precipitation of brucite and primary carbonates—calcite (CaCO3), dolomite [MgCa(CO3)2], magnesite (MgCO3), as well as the formation of hydromagnesite, halite (NaCl), gypsum (CaSO4·2H2O), blödite [Na2Mg(SO4)2·4H2O], and epsomite [MgSO4·7H2O]—are considered. Simulation results are consistent with field observations and mineralogical data from tailings that weathered for 10 yr. Precipitation of hydromagnesite is both predicted and observed, and is mainly controlled by the dissolution of serpentine (the source of Mg) and equilibrium with CO2 ingressing from the atmosphere. The predicted rate for CO2 entrapment in these tailings ranges between 0.6 and 1 kg m−2 yr−1. However, modeling results suggest that this rate is sensitive to CO2 ingress through the mineral waste and may be enhanced by several mechanisms, including atmospheric pumping.

Microbially mediated mineral carbonation: roles of phototrophy and heterotrophy.

Ultramafic mine tailings from the Diavik Diamond Mine, Canada and the Mount Keith Nickel Mine, Western Australia are valuable feedstocks for sequestering CO₂ via mineral carbonation. In microcosm experiments, tailings were leached using various dilute acids to produce subsaline solutions at circumneutral pH that were inoculated with a phototrophic consortium that is able to induce carbonate precipitation. Geochemical modeling of the experimental solutions indicates that up to 2.5% and 16.7% of the annual emissions for Diavik and Mount Keith mines, respectively, could be sequestered as carbonate minerals and phototrophic biomass. CO₂ sequestration rates are mainly limited by cation availability and the uptake of CO₂. Abundant carbonate mineral precipitation occurred when heterotrophic oxidation of acetate acted as an alternative pathway for CO₂ delivery. These experiments highlight the importance of heterotrophy in producing sufficient DIC concentrations while phototrophy causes alkalinization of waters and produces biomass (fatty acids = 7.6 wt.%), a potential feedstock for biofuel production. Tailings storage facilities could be redesigned to promote CO₂ sequestration by directing leachate waters from tailings piles into specially designed ponds where carbonate precipitation would be mediated by both chemical and biological processes, thereby storing carbon in stable carbonate minerals and potentially valuable biomass.