Impacts of dissolved phosphorus and soil-mineral-fluid interactions on CO2 removal through enhanced weathering of wollastonite in soils

The weathering of silicate minerals removes carbon dioxide (CO2) from the atmosphere over geologic timescales and is also investigated as an engineered strategy to mitigate climate change on decadal timescales. “Enhanced rock weathering” (ERW) is a carbon dioxide removal strategy that involves spreading of pulverized, highly reactive silicate rock at the Earth's surface such as within agricultural and natural soils. The rate and efficacy of ERW in agricultural soils to remove CO2 is difficult to quantify owing to the complex geochemical environment including biological-mineral-fluid-atmosphere interactions and human interventions that occur. Column experiments containing wollastonite [CaSiO3] and soils were conducted to investigate the geochemical processes that remove CO2 during ERW, and to determine the potential impact of P-bearing fertilizers on CO2 removal. Columns containing wollastonite, soil, or 8:1 soil-wollastonite mixtures were irrigated with de-ionized water or a 1 × 10−4 M K3PO4 solution. Analysis of column effluent concentrations indicated that P was rapidly removed from solution and retained within the columns, with little difference in weathering rate between columns irrigated with water and those receiving the K3PO4 solution. Bulk wollastonite weathering rates were significantly enhanced in the presence of soils compared to columns with wollastonite alone. Cation concentrations in effluent from columns with soil-wollastonite mixtures were strongly impacted by adsorption to soil surfaces, complicating efforts to estimate CO2 capture. Based on wollastonite weathering rates estimated by Si release, a maximum of 255 kg CO2 t−1 amendment yr−1 was captured in the soil-wollastonite mixture columns. The majority of CO2 stored in the soil-bearing columns was most likely soil-generated rather than removed directly from the atmosphere. The results of this study indicate that adsorption to soils may limit the ability of P to enhance wollastonite weathering, that silicate weathering behaviour within soils differs substantially from that of pure mineral phases, and that CO2 budgets must be carefully constrained to assess the source of stored CO2. Moreover, leachate concentrations of cations and HCO3− may not serve as accurate indicators of the extent of silicate weathering, at least over the short term, due to the numerous processes that control their concentrations in soils, such as sorption to soil components.