Abstract
Mafic igneous rocks, such as basalt, are composed of abundant calcium- and magnesium-rich silicate minerals widely proposed to be suitable for scalable carbon dioxide removal (CDR) by enhanced rock weathering (ERW). Here, we report a detailed characterization of the mineralogy, chemistry, particle size and surface area of six mined basalts being used in large-scale ERW field trials. We use 1-D reactive transport modelling (RTM) of soil profile processes to simulate inorganic CDR potential via cation flux (Mg2+, Ca2+, K+ and Na+) and assess the release of the essential plant nutrients phosphorus (P) and potassium (K) for a typical clay-loam agricultural soil. The basalts are primarily composed of pyroxene and plagioclase feldspar (up to 71 wt%), with accessory olivine, quartz, glass and alkali feldspar. Mean crushed particle size varies by a factor of 10, owing to differences in the mining operations and grinding processes. RTM simulations, based on measured mineral composition and N2-gas BET specific surface area (SSA), yielded potential CDR values of between c. 1.3 and 8.5 t CO2 ha−1 after 15 years following a baseline application of 50 t ha−1 basalt. The RTM results are comparative for the range of inputs that are described and should be considered illustrative for an agricultural soil. Nevertheless, they indicate that increasing the surface area for slow-weathering basalts through energy intensive grinding prior to field application in an ERW context may not be warranted in terms of additional CDR gains. We developed a function to convert CDR based on widely available and easily measured rock chemistry measures to more realistic determinations based on mineralogy. When applied to a chemistry dataset for >1300 basalt analyses from 25 large igneous provinces, we simulated cumulative CDR potentials of up to c. 8.5 t CO2 ha−1 after 30 years of weathering, assuming a single application of basalt with a SSA of 1 m2 g−1. Our RTM simulations suggest that ERW with basalt releases sufficient phosphorus (P) to substitute for typical arable crop P-fertiliser usage in Europe and the USA offering potential to reduce demand for expensive rock-derived P.
Highlights
The UNFCCC Paris Agreement aims to limit human-caused climate warming to less than 1.5 ◦C above pre-industrial temperatures
The chemistry and mineralogy of the basalts from the six sites (Table 1) were compared using Total Alkaline Silica (TAS) (Bas et al, 1986) and Quartz, Alkali feldspar, Plagioclase feldspar and Feldspathoid (QAPF) (Streckeisen, 1974) plots (Fig. 1a and b respectively). Comparisons with both TAS and QAPF classifications confirm that the basalts are generally typical of Large Igneous Provinces (LIPs) (25 locations, of 1354 data points) which host large (>1000 km3) reserves of basalt (Bryan et al, 2010)
Under the QAPF scheme, the diverse mineralogy of the six basalts result in Hillhouse and Cragmill being classed as basalt/andesite and Tichum, Oregon and Tawau classed as phonolitic tephrite, latite and dacite, respectively (Fig. 1b)
Summary
The UNFCCC Paris Agreement aims to limit human-caused climate warming to less than 1.5 ◦C above pre-industrial temperatures. During chemical weathering of silicate rocks, dissolved atmospheric CO2 forms aqueous species that accelerate the dissolution of silicate phases (minerals and glasses) with proton consumption to generate bicarbonate (Eq (1)). This bicarbonate is transported to the oceans via runoff where it is stored in a stable form for approximately ~100,000 years (Renforth and Henderson, 2017): CaMgSi2O6 + 4CO2(g) + 6H2O→ Mg2+ + Ca2+ + 2H4SiO4 + 4HCO−3 (1). If crop biomass is not returned to fields following harvest, major cations taken up from the soil (Ca2+, Mg2+, K+) during growth will be unavailable to contribute to CDR via alkalinity production in soil pore waters (Banwart et al, 2009)
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