Organic-inorganic carbon cycling: Implications for CO2 mineralization of brucite-bearing mine tailings

• Brucite [Mg(OH)2] carbonation in mine tailings is accelerated by increasing CO2 supply • Column experiments investigated the use of organics as an alternative CO2 source • CO2 sequestration rates were ∼8 × faster using compost compared to controls • Stable carbon isotopes demonstrated the sequestration of organically derived C • Incorporating these processes into mine remediation can lead to reducing net CO2 emissions in mining Enhanced weathering and mineralization of ultramafic tailings permanently stores carbon dioxide (CO 2 ) within stable carbonate minerals. Brucite [Mg(OH) 2 ], a minor phase (up to ∼13 wt.%) of ultramafic tailings, offers substantial potential for CO 2 sequestration, yet its carbonation is limited by CO 2 supply. Here, we propose coupling organic and inorganic carbon cycling to accelerate brucite carbonation by amending ultramafic tailings with waste organics, a strategy often employed for remediation. This study aimed to understand the influence of tailings grain size and organics content on brucite carbonation and tailings cementation. Column experiments (∼20 cm height, 10 weeks) involved layering waste organics and brucite-bearing tailings (2 wt.%) of different grain sizes (<63, 63–125, and 125–250 μm) and were compared to controls of organics and tailings only. Average carbonation rates for amended columns (102 kg CO 2 /t tailings/yr) were ∼8 × faster than tailings controls (13 kg CO 2 /t tailings/yr) that were only exposed to laboratory CO 2 concentrations. Reacted tailings had depleted 13 C compositions (δ 13 C = −10.9‰ to −9.9‰ VPDB) relative to initial tailings (−6.4‰), indicating incorporation of respired CO 2 . Cylindrical tests using compacted mixtures of tailings and organic matter exhibited 16–63% brucite carbonation along with unconfined compressive strengths of 0.08–0.51 MPa, a co-benefit of brucite carbonation. Our study demonstrates that using waste organics significantly accelerates brucite carbonation, which has implications for the remediation of ultramafic tailings. This proposed passive remediation system exploits an inexpensive CO 2 source for rapid carbonation, which can reduce net CO 2 emissions after mine closure.