The diversity of mineral feedstocks and environments for carbon dioxide removal (CDR) presents challenges for measuring rates and verifying carbon storage, which are necessary for the broad deployment of CDR technologies. Here, we present laboratory and field research on 1) CO2 mineralization of mine wastes, 2) enhanced rock weathering (ERW) in agricultural soils, and 3) mineral oxide looping. Alkaline mine wastes from nickel, asbestos, and diamond mines can be used for CDR and reduce the CO2 emissions from mine operations. As part of De Beers’ Project CarbonVault, we demonstrate how geochemical modelling can monitor passive carbonation and management practices that can accelerate mineral dissolution, the main rate limitation for the carbonation of Mg- and Ca-silicates. Outside the mining environment, ERW allows for long reaction periods for rock powders dispersed over large areas, thereby circumventing the main rate limitation. However, accurate carbon accounting is hindered due to slow reaction rates and spatial and temporal variabilities in soil carbon content in large open systems. CO2 fluxes and water chemistry determined CDR rates during year-long field experiments in Central Ontario soils. Although ERW is often considered a negative emissions technology, CO2 fluxes were almost always positive due to soil respiration, suggesting that ERW is predominately a CO2 avoidance strategy. Magnesium oxide (MgO), formed from the calcination of mined carbonate rock, offers far greater CO2 capture rates. In experiments, we show that MgO carbonation is limited by CO2 supply and would require areas of 80–455 km2 to sequester 1 Mt CO2/yr passively. These rates are orders of magnitude faster than ERW using naturally occurring rocks, yet CO2 released during calcination must be stored. In our CDR research of various mineral feedstocks within different environments, the rates we determine and the limitations we identify contribute to designing effective CDR technologies.

CO2 mineralization and enhanced weathering in mining, agriculture, and oxide looping

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