Quantifying the effect of grain size on weathering of basaltic powders: Implications for negative emission technologies via soil carbon sequestration

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Weathering of basaltic powders was studied experimentally at 35 °C in dilute solutions of oxalic acid and carbonic acid to assess the effect of grain size and reactive surface area for materials under consideration for carbon dioxide reduction (CDR) by enhanced rock weathering (ERW). The basalts chosen for this study (with their mineralogical compositions) are the Blue Ridge (BR) meta-basalt (chlorite > epidote > plagioclase > actinolite) and Pioneer Valley (PV) basalt (plagioclase > augite > quartz > chlorite). Powders of BR and PV basalts were sieved into <45 μ m, 45–150 μ m, and >150 μm fractions, and experiments were performed in open-system reactors designed to simulate a 1 mm thick layer of basalt added to agricultural soil in the humid tropics. Weathering rate was assessed by measuring the flux of base cations leached from silicate minerals and results indicate that silt-dominated basaltic powder (<45 μ m) weathers at approximately double the rate of sand dominated (150–500 μ m) basaltic powder, both for the BR and PV basalts. This study estimates CDR rates between 2.8 and 6.8 t CO2/ha/yr across the range of grain size fractions analyzed. Etched primary mineral grains (e.g. plagioclase, augite, actinolite) with depleted base cations observed by SEM-EDS provide morphological and stoichiometric evidence of dissolution, as do the presence of frayed chlorite grains that contain adsorbed Ca and are compositionally intermediate to end-member chlorite and smectite. Small amounts of micron-scale calcite were also observed as a precipitate on mineral surfaces, likely a consequence of localized saturation of Ca and HCO3 in the matrix of the weathering powders. The results of this study help to constrain differences in weathering flux as a function of grain size, with important implications for effectiveness of CDR via ERW.