Towards a testing protocol to assess CO2 sequestration potential in mine waste Carroll, Kate Jessica
Carbon sequestration via fixation of CO₂ into carbonate minerals, a process known as carbon mineralization, has the potential to offset greenhouse gas emissions from mining operations through the precipitation of stable carbonates from dissolved ultramafic mine tailings. Under certain chemical conditions, dissolution rates of Mg-bearing tailings minerals control the overall rate of carbon mineralization. A testing protocol is proposed to assess the CO₂ sequestration potential of mine tailings by investigating the capability to extract individual rate parameters of brucite and serpentine from dissolution of a brucite-serpentine mixture. Brucite and serpentine solids were well-characterized prior to experimentation and were selected to broadly mimic mineralogy and surface area of ultramafic tailings at the Mount Keith Nickel Mine in Western Australia. Two brucite, two serpentine, and one mixed brucite-serpentine dissolution experiment was carried out in a continuously-stirred flow-through reactor at circumneutral pH (~8) or acidic pH (~1.2) and with a residence time of 10 or 100 min. Mg concentration decayed from early, elevated transients reflecting both brucite and serpentine dissolution; short-lived plateaus in pH reflected only brucite behaviour; and long-term Si concentration reflected saturation of an amorphous silica phase. Mineral surface area adjustments in PHREEQC geochemical models were used to fit short-lived plateaus in pH and aqueous Mg concentration reflecting brief steady-state, and the data were used to constrain dissolution rates of brucite and serpentine, respectively. A long-lived steady-state regime could not be inferred for brucite or serpentine dissolution, yet rate parameters extracted from the mixed experiment agreed well with those of pure minerals and with Mount Keith tailings surface area data. A dichotomy exists between optimization of experimental design for rate extraction and simulation of real tailings chemistry for extrapolation to natural scenarios. Both options merit investigation, but the relative implications for assessment of tailings carbonation potential requires further research.
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