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[HSC06-01] Geochemical reaction modeling of air–water–mineral interactions in enhanced weathering using mafic rocks in Japan
Keywords:Negative emission technology,, Enhanced rock weathering, Mafic rock, Carbonate mineral, Geochemical reaction modeling
Enhanced rock weathering (ERW) is a negative emission technology using weathering of mafic (and ultramafic) rocks scattered on the Earth’s surface. As well as in CO2 geological storage (CGS), CO2 will be chemically trapped as dissolved carbonate species (solubility trap) and carbonate minerals (mineral trap) in ERW. Beerling et al. (2020) estimated the amount of carbon dioxide removal (CDR) when alkali basalt and tholeiite powders (~10 μm) are scattered at a rate of 40 t/ha into a 15 cm soil layer using 1D reactive transport simulation and suggested the CDR potentials when applied worldwide. However, mafic rocks generally show various compositions. Before beginning the actual ERW projects, rock samples will be selected specifically, and we need to estimate the CDR potential by modeling site by site. In this context, this work aims to assess the CDR potential of Japanese mafic rock samples by applying the geochemical reaction modeling to the Japanese cases.
In this work, mineral composition data of each Japanese rock sample (at Hokkaido, Iki in Nagasaki, Sado in Niigata, and Hachijo in Tokyo) were obtained by point-counting mineral particles in the thin sections. The sample from Hokkaido is peridotite, that from Iki is alkali basalt, that from Sado is olivine-rich basalt, and that from Hachijo is plagioclase-rich basalt. Geochemical reaction simulations of the above rocks (including alkali basalt and tholeiite in Beerling et al. (2020)) were carried out with The Geochemist’s Workbench software as well as the PHREEQC simulations in Beerling et al. (2020). In the soil layers mixed with the rocks, 20 vol% is water-saturated pore space, and ~1 vol% is the rocks. 1-year weathering with 1200 mm rainfall were predicted using dissolution kinetics of primary minerals and equilibrium theory of secondary mineral formations.
For alkali basalt and tholeiite given as examples in Beerling et al. (2020), the modeling results in this work were similar to those in Beerling et al. (2020). Dissolutions of plagioclase and olivine were observed significantly, while clinopyroxene and K-feldspar showed little change in a year. The same trend was shown in the modeling of the Japanese rock samples, indicating significant dissolution of the Hachijo basalt and the Hokkaido peridotite. The amount of CDR was the sum of the mineral trap in the 15 cm soil layer and the solubility trap in the outflow from the bottom of the soil layer. In this work, the amounts of CDR for the alkali basalt and the tholeiite were ~0.1 t-CO2/t-rock/yr, that for the Iki basalt was 0.08 t-CO2/t-rock/yr, those for the Hachijo and Sado basalt were ~0.12 t-CO2/t-rock/yr, and that for the Hokkaido peridotite was 0.45 t-CO2/t-rock/yr. Since ERW does not fix CO2 in an isolated space such as reservoirs in CGS, CO2 should be trapped by mineralization in the surface soil layer, rather than solubility trap. The mineral traps were estimated in all samples, and the amount of mineralization is significant in the Hokkaido peridotite (assuming hydromagnesite formation, 0.10 t-CO2/t-rock/yr) and the Hachijo basalt (assuming calcite formation, 0.06 t-CO2/t-rock/yr). Therefore, for the other rock samples, it is necessary to develop a method to enhance mineralization, rather than simply scattering the rock on the ground. To improve the ERW modeling, future work should appropriately select mineral species of primary phases and secondary phases, add formation kinetics of the secondary phases, set accurate surface area of the mineral particles, and consider intermittent rainfalls and evaporation events in a year. Since carbonate minerals (e.g., calcite) often form slowly even when the solution is saturated with respect to the carbonates, the model will be more accurate by calculating the critical supersaturation and/or considering the metastable precursors. An environmental impact assessment will also be necessary, showing the behaviors of hazardous elements (e.g., Ni and Cr) derived from the rocks.
In this work, mineral composition data of each Japanese rock sample (at Hokkaido, Iki in Nagasaki, Sado in Niigata, and Hachijo in Tokyo) were obtained by point-counting mineral particles in the thin sections. The sample from Hokkaido is peridotite, that from Iki is alkali basalt, that from Sado is olivine-rich basalt, and that from Hachijo is plagioclase-rich basalt. Geochemical reaction simulations of the above rocks (including alkali basalt and tholeiite in Beerling et al. (2020)) were carried out with The Geochemist’s Workbench software as well as the PHREEQC simulations in Beerling et al. (2020). In the soil layers mixed with the rocks, 20 vol% is water-saturated pore space, and ~1 vol% is the rocks. 1-year weathering with 1200 mm rainfall were predicted using dissolution kinetics of primary minerals and equilibrium theory of secondary mineral formations.
For alkali basalt and tholeiite given as examples in Beerling et al. (2020), the modeling results in this work were similar to those in Beerling et al. (2020). Dissolutions of plagioclase and olivine were observed significantly, while clinopyroxene and K-feldspar showed little change in a year. The same trend was shown in the modeling of the Japanese rock samples, indicating significant dissolution of the Hachijo basalt and the Hokkaido peridotite. The amount of CDR was the sum of the mineral trap in the 15 cm soil layer and the solubility trap in the outflow from the bottom of the soil layer. In this work, the amounts of CDR for the alkali basalt and the tholeiite were ~0.1 t-CO2/t-rock/yr, that for the Iki basalt was 0.08 t-CO2/t-rock/yr, those for the Hachijo and Sado basalt were ~0.12 t-CO2/t-rock/yr, and that for the Hokkaido peridotite was 0.45 t-CO2/t-rock/yr. Since ERW does not fix CO2 in an isolated space such as reservoirs in CGS, CO2 should be trapped by mineralization in the surface soil layer, rather than solubility trap. The mineral traps were estimated in all samples, and the amount of mineralization is significant in the Hokkaido peridotite (assuming hydromagnesite formation, 0.10 t-CO2/t-rock/yr) and the Hachijo basalt (assuming calcite formation, 0.06 t-CO2/t-rock/yr). Therefore, for the other rock samples, it is necessary to develop a method to enhance mineralization, rather than simply scattering the rock on the ground. To improve the ERW modeling, future work should appropriately select mineral species of primary phases and secondary phases, add formation kinetics of the secondary phases, set accurate surface area of the mineral particles, and consider intermittent rainfalls and evaporation events in a year. Since carbonate minerals (e.g., calcite) often form slowly even when the solution is saturated with respect to the carbonates, the model will be more accurate by calculating the critical supersaturation and/or considering the metastable precursors. An environmental impact assessment will also be necessary, showing the behaviors of hazardous elements (e.g., Ni and Cr) derived from the rocks.