Abstract

Carbon dioxide (CO₂) mineralization of ultramafic mine tailings offers permanent storage of this greenhouse gas (GHG) within stable carbonate minerals, thereby reducing the net GHG emissions of mining.¹ Brucite [Mg(OH)₂], a minor yet reactive phase (0.1–13 wt.%) of ultramafic tailings, is found associated with serpentine from asbestos (e.g., Woodsreef Chrysotile Mine, Australia),² nickel (e.g., Dumont Nickel Mine, Canada),³ and kimberlite (e.g., Gahcho Kué Mine, Canada)⁴ deposits. Yet, under ambient conditions, brucite carbonation is limited by CO₂ supply. Here, we propose coupling inorganic-organic carbon cycling to accelerate brucite carbonation by amending ultramafic tailings with waste organics. Laboratory experiments aimed to understand the effects of tailings grain size and organic content on ambient brucite carbonation. CO₂ gas concentrations, total inorganic carbon (TIC), and stable carbon isotopes were measured. Column experiments (~20 cm height, 6.7 cm diameter; 10 weeks long) involved layering waste organics and brucite-bearing tailings (2 wt.%) of different grain sizes (<63, 63–125, and 125–250 µm). These columns were compared to controls of organics and tailings only. Average carbonation rates for amended columns (110 kg CO₂/t tailings/yr) were ~5 times faster than tailings controls (20.9 kg CO₂/t tailings/yr). Brucite consumption ranged from 40–68% (TIC; +0.089–0.193%), largely reacting within the first 3 weeks and was not significantly influenced by tailings particle size. These values are comparable to those produced in cylindrical tests that used compacted tailings exposed to CO₂-generating organics (64–84% brucite consumption), which resulted in unconfined compressive strengths of 0.4–6.9 MPa, a notable co-benefit of brucite carbonation.⁵ Carbonation rates likely depend on the CO₂ concentration that can be generated,⁶ which may depend on the ratio of organics to tailings. Reacted tailings had ¹³C compositions (-2.83 > δ¹³C > -11.56‰ VPDB) that were depleted compared to the initial tailings, suggesting incorporation of respired CO₂. Incomplete brucite carbonation may occur due to surface passivation, which can relate to poor water availability.⁵ Our study demonstrates that amending ultramafic tailings with waste organics is an effective approach at accelerating brucite carbonation. This approach results in CO₂ avoidance rather than removal of atmospheric CO₂; however, it is a passive system without the need for expensive direct air capture units for concentrating atmospheric CO₂. 

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