Journal
CHEMICAL GEOLOGY
Volume 421, Issue -, Pages 65-80Publisher
ELSEVIER
DOI: 10.1016/j.chemgeo.2015.12.005
Keywords
Unsaturated zone; Mineral carbonation; Water-limited reaction; Surface passivation; Reactive surface area; Stable carbon isotopes; CO2 sequestration
Categories
Funding
- Carbon Management Canada National Centre of Excellence grant [C390]
- Natural Sciences and Engineering Research Council of Canada (NSERC)
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The distribution and evolution of mineral-water-gas interfacial areas exert a fundamental yet poorly documented control onmineral-fluid reactions in the unsaturated zone. Here, we explore the impact of changing mineral reactive surface area, water content, and gas distribution on the reaction of brucite [Mg(OH)(2)] with CO2 gas to form hydrated Mg-carbonate minerals in partially water saturated meter-scale column experiments. Brucite surface area, which is inferred to exert a direct control on mineral dissolution rates, demonstrates a complex evolution including roughening, fracturing and passivation that is inconsistent with conventional models of geometric evolution. Mineral-fluid reaction in the interior of single brucite grains maintains surface area at near-constant values despite the decreasing volumetric brucite content, until solid carbonate precipitates passivate brucite surfaces. The evolution of reactive surface area during passivation also does not follow simple geometric processes. A porous amorphous carbonate phase permits ready access to brucite surfaces for reaction until recrystallization of the amorphous carbonate into bladed, low-porosity nesquehonite [MgCO3 center dot 3H(2)O] abruptly quenches reaction. The varied water content of the experiments illustrates that the extent of mineral-gas reaction is limited by the abundance of water available to facilitate precipitation of hydrated carbonate minerals. Conversely, at high bulk water saturation, the development of preferential gas flow paths limited the exposure of reactive minerals to CO2 and reduced the overall extent of reaction. Thus, bulk mineral-fluid reaction rates were reduced at both high and low bulk water contents. (C) 2015 Elsevier B.V. All rights reserved.
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