Mineralization of anthropogenic CO2 via water-gas-rock reaction

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  • October 10, 2018
Creator
  • Westfield, Isaac Thomas
    • Affiliation: College of Arts and Sciences, Department of Marine Sciences
Abstract
  • Atmospheric CO2 has increased 50% since the Industrial Revolution due to anthropogenic combustion of fossil fuels, deforestation, and cement production. Mineralization of fossil-fuel-derived CO2 is one method of reducing emission of anthropogenic CO2. Mineralization via aqueous precipitation (MAP) removes CO2 from anthropogenic waste-streams (e.g., fossil-fuel-fired power plants) and creates carbon-negative mineral byproducts (Mg-Sr-Ca-Na-Fe-carbonates) to supplement/replace carbon-positive building materials (cement, drywall). However, sequestration of anthropogenic CO2 as carbonate minerals requires large sources of alkalinity (to convert CO2 to CO32-) and divalent cations (e.g., Mg2+-Ca2+-Fe2+). Ultramafic and mafic silicate rocks (peridotites, serpentinites, basalts) are globally abundant sources of the divalent cations and alkalinity required for CO2 mineralization. Fifty water-gas-rock batch-reaction experiments were performed on seven igneous rock-types to quantify and model MAP reactions under controlled and natural conditions. Rocks were pulverized (40-180 µm) and reacted with water under pure-CO2 and low-pCO2 (ca. 44 ppm in air) and under 25 and 200°C for two weeks. CO2-sequestration increased and grain size decreased with reaction time, although late-stage flocculation occurred. CO2-sequestration was maximized for rock-types as follows: serpentinite--low-CO2/low-T; dunite, websterite, basalt--low-CO2/high-T; intermediate, peridotite--high-CO2/low-T. Peridotites sequestered the most CO2. CO2-sequestration was modeled as a function of solution pH, total alkalinity (TA), temperature (T), pCO2, dissolved inorganic carbon (DIC), and/or total dissolved solids (TDS) via multiple linear regression. Rock-agnostic models were also generated so CO2-sequestration could be predicted when solution/groundwater chemistry was known but rock type wasn't. Empirical relationships between TDS and TA were established for each rock-type for rapid, field-based assessment. Elemental (Mg/Ca-Sr/Ca-Ba/Ca) and isotopic (87Sr/86Sr-δ18O-δ13C) chemostratigraphy of a well-stratified carbonate vein from the Del Puerto Ophiolite (California) revealed that carbonate-formation waters within the system have transitioned between domination by meteoric waters, marine limestones, and ultramafic rocks. Although these periodic flow-changes are likely caused by carbonate-precipitation-induced fracture-sealing, such fracture sealing did not destroy the permeability of these systems--demonstrating that CO2-sequestration in natural ultramafic deposits is not necessarily self-limiting. These results advance basic understanding of CO2-induced silicate weathering--one of few globally scalable mechanisms of sequestering atmospheric CO2 over human timescales and an important moderator of the global carbon cycle throughout Earth history.
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  • In Copyright
Advisor
  • Ries, Justin
Degree
  • Doctor of Philosophy
Degree granting institution
  • University of North Carolina at Chapel Hill
Graduation year
  • 2013
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