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Enhanced weathering of mafic and ultramafic rocks has been suggested as a carbon sequestration strategy for the mitigation of climate change. This study was designed to assess the potential drawdown of CO 2 directly from the atmosphere by the enhanced weathering of peridotites and basalts in seawater. Pulverized, and ball milled dunite, harzburgite and olivine basalt were reacted in artificial seawater in batch reactor systems open to the atmosphere for two months. The results demonstrate that the ball-milled dunite and harzburgite changed dramatically the chemical composition of the seawater within a few hours, inducing CO 2 drawdown directly from the atmosphere and ultimately the precipitation of aragonite. In contrast, pulverized but unmilled rocks, and the ball-milled basalt, did not yield any significant changes in seawater composition during the two-month experiments. As much as 10 wt percent aragonite was precipitated during the experiment containing the finest-grained dunite. These results demonstrate that ball milling can substantially enhance the weathering rate of peridotites in marine environments, promoting the permanent storage of CO 2 as environmentally benign carbonate minerals through enhanced weathering. The precipitation of Mg-silicate clay minerals, however, could reduce the efficiency of this carbon sequestration approach over longer timescales. [ABSTRACT FROM AUTHOR]

The distribution and evolution of mineral–water–gas interfacial areas exert a fundamental yet poorly documented control on mineral–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 CO 2 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 [MgCO 3 ·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 CO 2 and reduced the overall extent of reaction. Thus, bulk mineral–fluid reaction rates were reduced at both high and low bulk water contents. [ABSTRACT FROM AUTHOR]

Ultramafic and mafic mine tailings are a valuable feedstock for carbon mineralization that should be used to offset carbon emissions generated by the mining industry. Although passive carbonation is occurring at the abandoned Clinton Creek asbestos mine, and the active Diavik diamond and Mount Keith nickel mines, there remains untapped potential for sequestering CO2 within these mine wastes. There is the potential to accelerate carbonation to create economically viable, large-scale CO2 fixation technologies that can operate at near-surface temperature and atmospheric pressure. We review several relevant acceleration strategies including: bioleaching of magnesium silicates; increasing the supply of CO2 via heterotrophic oxidation of waste organics; and biologically induced carbonate precipitation, as well as enhancing passive carbonation through tailings management practices and use of CO2 point sources. Scenarios for pilot scale projects are proposed with the aim of moving towards carbon-neutral mines. A financial incentive is necessary to encourage the development of these strategies. We recommend the use of a dynamic real options pricing approach, instead of traditional discounted cash-flow approaches, because it reflects the inherent value in managerial flexibility to adapt and capitalize on favorable future opportunities in the highly volatile carbon market. [ABSTRACT FROM AUTHOR]

• The Baptiste nickel deposit is ideal for a CO 2 sequestration demonstration project. • Direct air capture of CO 2 would offset ∼25% of a prospective mine's carbon emissions. • The Baptiste deposit could sequester CO 2 from a proposed liquified natural gas pipeline. • Reaction of CO 2 -rich gases with tailings would offset ∼50% of a mine's emissions. • Potential savings would be $10.5 M/yr under a carbon price of $50/t CO 2 equivalent. The Baptiste deposit is located within the Decar nickel district in British Columbia, Canada and is a promising candidate for a CO 2 sequestration demonstration project. The deposit contains awaruite (nickel-iron alloy) hosted in an ultramafic complex, which is dominated by serpentine [Mg 3 Si 2 O 5 (OH) 4 ; ∼80 wt.%] and contains reactive brucite [Mg(OH) 2 ; 0.6–12.6 wt.%]. Experiments were conducted using metallurgical test samples and pulps from cores with the aim of determining the potential for this deposit to sequester CO 2 via direct air capture of atmospheric CO 2 and carbonation using CO 2 -rich gas. The experimental direct capture rate was 3.5 kg CO 2 /m2/yr and would sequester 17 kt CO 2 /yr based on year-round reaction and when extrapolated to the scale of the proposed tailings facility (5 km2). This rate can be increased by ∼5 times (19 kg CO 2 /m2/yr) when aerating the tailings and would offset CO 2 emissions by 95 kt CO 2 /yr (19–25% offset of projected CO 2 emissions). Experimental carbonation rates with 10% CO 2 gas could achieve offsets of up to 210 kt CO 2 /year (42–53% CO 2 offset) based on 1 h reaction times and consumption of 0.8 wt% brucite if present. Targeting brucite-rich ore zones and optimizing CO 2 delivery would likely lead to greater CO 2 offsets. [ABSTRACT FROM AUTHOR]

Coupled gypsum dissolution-calcium carbonate precipitation experiments were performed in closed system reactors and in the presence of either aqueous 0.1 M Na 2 CO 3 , 0.1 M NaHCO 3 or 0.1 M Na 2 CO 3 + 0.2 M NaOH solutions. Gypsum dissolved immediately at the start of each experiment provoking the precipitation sequentially of vaterite, calcite and trace aragonite. Fine-grained amorphous carbon carbonate may also be present shortly after each experiment began. Each experiment approached equilibrium within 119 h leading to the maximum possible transformation of gypsum to calcite over this time frame. The rapid transformation of gypsum to calcite in these experiments suggests a similar rapid transformation of gypsum or anhydrite into calcite could occur during subsurface carbon storage efforts where evaporites are present. Evaporite deposits could thus potentially be used for carbonation if sufficient alkalinity is available to neutralize the acid liberated by the gypsum carbonation reaction. Due to a negative volume of the gypsum or anhydrite to calcite transformation, however, the carbonation of these minerals will potentially damage the integrity of evaporite caprocks. [ABSTRACT FROM AUTHOR]

Flowback and produced water (FPW) generated by hydraulic fracturing operations is highly saline and contains elevated concentrations of ions including calcium and magnesium. Here, we investigate the use of FPW as a source of calcium and magnesium for carbon dioxide (CO 2) removal and storage in carbonate minerals. We performed pH titration experiments to precipitate calcium and magnesium from three FPW samples from Alberta and Saskatchewan, Canada. Depending on the pH (8.5–12), calcite (CaCO 3) and brucite [Mg(OH) 2 ], with occasional aragonite (CaCO 3) and portlandite [Ca(OH) 2 ] precipitate from FPW within 24 h of exposure to atmospheric p CO 2 at ∼18 °C. Our experiments demonstrate that pH adjustments are an effective means of precipitating calcium and magnesium carbonates and hydroxides from FPW, but that relying solely upon passive removal of CO 2 from air is limiting to carbonation. We estimate that carbonation of all calcium and magnesium from brines produced in conventional and hydraulic fracturing operations in Canada could store up to 1,010 Mt CO 2 annually. Carbonation rates could be improved by using higher p CO 2 gases, such as flue gases from fossil energy generation (via CO 2 capture), gas streams produced by Direct Air Capture plants or using novel looping technologies. [Display omitted] • Waste waters from oil and gas production can be used to bind CO 2 in minerals. • Addition of base precipitates Ca carbonates and Ca and Mg hydroxides. • Ca carbonates precipitated from brines are stable stores for atmospheric CO 2. • Up to 12.2–22.5 g CO 2 /L can be stored in the flowback and produced waters tested. • The carbonation potential of oil and gas brines in Canada alone is 1.010 Gt CO 2 /year. [ABSTRACT FROM AUTHOR]