After carbon dioxide (CO2) is removed from the atmosphere or scrubbed from an industrial source (see CDR vs. CCS), it must be securely prevented from re-entering the atmosphere. One promising method to achieve this is converting the carbon dioxide into carbonate minerals through carbon mineralization. Engineered carbon mineralization emulates and accelerates natural rock weathering, whereby calcium (Ca) and magnesium (Mg) silicate minerals react with carbon dioxide dissolved in water to form calcium and magnesium carbonate minerals that are stable on geologic timescales. Carbonate minerals are so durable that they can only release CO2 if intensely heated (to > 500 ºC for most relevant minerals) or immersed in acid. The enormous quantities of energy that would be necessary to decompose carbonates assures their effective permanence.1
Engineered carbon mineralization of Ca/Mg-rich silicate rock can proceed through three general approaches:
- Surfical: CO2-reactive rock is mined, ground into small particles, and spread over large areas (farmland, mine sites, or beaches) and naturally absorbs CO2 over time
- In situ: CO2 is injected into underground CO2-reactive rocks, thereby converting CO2 into stone
- Ex situ: rocks are mined and reacted with concentrated CO2 in reactors to produce environmentally benign carbonate minerals that can be safely disposed of, or used in industries such as construction.
Whether and how carbon mineralization may be applied depend on geographic factors including rock chemical composition, crystal structure, permeability, and geologic setting.
Surgical mineralization methods disperse minerals, with particle size, over large expanses of land for the minerals to draw down ambient carbon dioxide.2 Some groups are pairing surficial weathering with agriculture lands and forests. Compared to the other types of mineralization, surface weathering can be difficult to quantify overtime because of the ever-changing nature of the chemistry of natural systems (see Monitoring, Verification, and Reporting).
In situ mineralization projects circulate CO2 rich fluid underground for the nearby minerals to absorb the CO2 and form carbonates.3 A well-known example of in situ mineralization is the CarbFix technology in Icelandic basalt. Experts are currently using geospatial analysis methods to find prime opportunities to co-locate in situ carbon mineralization with CO2 point sources or regions with abundant, cheap, clean energy for direct air capture.
Ex situ carbon mineralization can be done with silicate rocks and even alkaline industrial byproducts.4 Calcium and magnesium are found in many other materials that are often considered the 'wastes' of industrial processes: mining wastes, steel slag, air pollution control residue, fly ash, and many other industrial wastes are abundant in magnesium and/or calcium and can be used as feedstocks for ex situ mineralization processes.
At the Clean Energy Conversions Lab, we research carbon mineralization as a method of carbon storage in two projects: the first project loops Mg and Ca oxides that are highly reactive with CO2 to conduct direct air capture;5 the second leaches calcium and magnesium from mining wastes from carbonates.6 Our research into carbon mineralization extends beyond the lab as we consider how economics, mapping, life cycle assessments, and US policy can help deploy carbon mineralization on a broad scale.
- Sandalow, D.; Aines, R.; Friedmann, J.; Kelemen, P.; McCormick, C.; Power, I.; Schmidt, B.; Wilson, S. Carbon Mineralization Roadmap, Columbia University, 2021
- Campbell, J. S.; Fonteinis, S.; Furey, V.; Hawrot, O.; Pike, D.; Aeschlimann, S.; Maesano, C. N.; Reginato, P. L.; Goodwin, D. R.; Looger, L. L.; Boyden, E. S.; Renforth, P. "Geochemical Negative Emissions Technologies: Part I. Review," Front. Clim. 2022, 4.
- Snæbjörnsdóttir, S. O.; Sigfússon, B.; Marieni, C.; Goldberg, D.; Gislason, S. R.; Oelkers, E. H. "Carbon Dioxide Storage through Mineral Carbonation," Nat. Rev. Earth Environ. 2020, 1(2), 90-102. https://doi.org/10.1038/s43017-019-0011-8.
- Huijgen, W. J. J.; Comans, R. N. J.; Witkamp, G.-J. "Cost Evaluation of CO2 Sequestration by Aqueous Mineral Carbonation," Energy Convers. Manag. 2007, 48 (7), 1923-1935. https://doi.org/10.1016/j.enconman.2007.01.035.
- McQueen, N.; Kelemen, P.; Dipple, G.; Renforth, P.; Wilcox, J. "Ambient Weathering of Magnesium Oxide for CO2 Removal from Air," Nat. Commun. 2020, 11 (1), 3299. https://doi.org/10.1038/s41467-020-16510-3.
- Woodall, C. M.; Lu, X.; Dipple, G.; Wilcox, J. "Carbon Mineralization with North American PGM Mine Tailings – Characterization and Reactivity Analysis," Minerals 2021, 11 (8). https://doi.org/10.3390/min11080844.