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An experimental analysis of enhanced rock weathering with Costa Rican basaltic powders

Year: 2024
Type: dissertation
Abstract: Enhanced rock weathering (ERW) via basalt applications to farmland has shown great potential as an effective negative emission technology (NET) to reduce the effects of climate change by sequestering carbon out of the atmosphere. Alongside offsetting emissions, ERW also poses dual benefits toward improving soil health. The tropics have been deemed an ideal location for the application of powdered basalts given (1) favorable kinetics, i.e. high weathering rates in the tropics, and (2) the presence of leached lateritic soils that would benefit from mineral-derived nutrients.The weathering of two Costa Rican basalts of the Nicoya complex from Herradura and Rinc贸n, and a basaltic-andesite beach sand from Playa Zapote (Osa Peninsula) were studied experimentally in the lab and coupled with a field trial application. The experimental lab portion (bench top) consisted of open-system reactors at 35 oC in dilute solutions of oxalic acid and carbonic acid to assess the effect of grain size and mineralogy for considerations relating to carbon dioxide reduction (CDR) via ERW. The field trial portion consisted of leaving the various rock powders exposed to tropical conditions for a 7-month period. The mineralogical compositions of the materials chosen are as follows: Herradura (1a) pyroxene > feldspar > phyllosilicate > zeolite > iron oxide > calcite; Rinc贸n (2) feldspar > pyroxene > phyllosilicate > zeolite > iron oxide; Playa Zapote (3) pyroxene > feldspar > quartz > phyllosilicate > iron oxide > calcite. The basalts were crushed by hand with additional assistance from a rock crusher then the basalts and beach sediment were sieved into <80 饾渿m, 80-170 饾渿m, and 170-550 饾渿m size fractions and emplaced in fine mesh bags simulating a 1 mm thick layer of basalt added to agricultural soil in the humid tropics. The bench top weathering rate was determined by measuring the flux of base cations leached from silicate minerals. Results indicate that Playa Zapote (sample 3) weathered at double to triple the rate of the basaltic powders used in the thesis. Specifically, CDR rates for Playa Zapote (sample 3) ranged from 4.1 to 6.0 tons CO2 / ha / yr whereas rates for Herradura (sample 1a) and Rinc贸n (sample 2) ranged from 1.3 to 2.65 tons CO2 / ha / yr across the various grain sizes accessed in this study. The benchtop experiment鈥檚 weathering rates and CDR estimates did not show a linear relationship with decreasing grain size as had been the case with previous studies. Mineralogy, specifically the presence or lack thereof of zeolites, and textural differences proved to be important factors towards measurements of weathering rates. Etched primary mineral grains observed by SEM-EDS and XRD diffractograms that show weathering of some minerals provide morphological and stoichiometric evidence of dissolution. Micron-scale calcite crystals were also observed as a precipitate on the Playa Zapote mineral surfaces, indicating a localized saturation of Ca and HCO3- in the matrix of the weathering powders which could lead to an underestimate of ERW and CDR potential. The results of this thesis document the relative merits of the different igneous source rocks used for ERW and CDR applications; this thesis also accesses the limitations to determining CDR when zeolites are present.
Source: Figshare
Language: English
Open Access status: green
OpenAlex ID: W7166932219
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