Strategizing Carbon-Neutral Mines: A Case for Pilot Projects

Influence of Iron Substitution and Solution Composition on Brucite Carbonation

Carbon neutral or negative mining can potentially be achieved by integrating carbon mineralization processes into the mine design, operations, and closure plans. Brucite [Mg(OH)2] is a highly reactive mineral present in some ultramafic mine tailings with the potential to be rapidly carbonated and can contain significant amounts of ferrous iron [Fe(II)] substituted for Mg; however, the influence of this substitution on carbon mineralization reaction products and efficiency has not been thoroughly constrained. To better assess the efficiency of carbon storage in brucite-bearing tailings, we performed carbonation experiments using synthetic Fe(II)-substituted brucite (0, 6, 23, and 44 mol % Fe) slurries in oxic and anoxic conditions with 10% CO2. Additionally, the carbonation process was evaluated using different background electrolytes (NaCl, Na2SO4, and Na4SiO4). Our results indicate that carbonation efficiency decreases with increasing Fe(II) substitution. In oxic conditions, precipitation of ferrihydrite [Fe10IIIO14(OH)2] and layered double hydroxides {e.g., pyroaurite [Mg6Fe2III(OH)16CO3·4H2O]} limited carbonation efficiency. Carbonation in anoxic environments led to the formation of Fe(II)-substituted nesquehonite (MgCO3·3H2O) and dypingite [Mg5(CO3)4(OH)2·∼5H2O], as well as chukanovite [Fe2IICO3(OH)2] in the case of 23 and 44 mol % Fe(II)-brucite carbonation. Carbonation efficiencies were consistent between chloride- and sulfate-rich solutions but declined in the presence of dissolved Si due to the formation of amorphous SiO2·nH2O and Fe-Mg silicates. Overall, our results indicate that carbonation efficiency and the long-term fate of stored CO2 may depend on the amount of substituted Fe(II) in both feedstock minerals and carbonate products.

The Mining Industry's Role in Enhanced Weathering and Mineralization for CO2 Removal

Enhanced weathering and mineralization (EWM) aim to remove carbon dioxide (CO2) from the atmosphere by accelerating the reaction of this greenhouse gas with alkaline minerals. This suite of geochemical negative emissions technologies has the potential to achieve CO2 removal rates of >1 gigatonne per year, yet will require gigatonnes of suitable rock. As a supplier of rock powder, the mining industry will be at the epicenter of the global implementation of EWM. Certain alkaline mine wastes sequester CO2 under conventional mining conditions, which should be quantified across the industry. Furthermore, mines are ideal locations for testing acceleration strategies since tailings impoundments are contained and highly monitored. While some environmentally benign mine wastes may be repurposed for off-site use─reducing costs and risks associated with their storage─numerous new mines will be needed to supply rock powders to reach the gigatonne scale. Large-scale EWM pilots with mining companies are required to progress technology readiness, including carbon verification approaches. With its knowledge of geological formations and ore processing, the mining industry can play an essential role in extracting the most reactive rocks with the greatest CO2 removal capacities, creating supply chains, and participating in life-cycle assessments. The motivations for mining companies to develop EWM include reputational benefits and carbon offsets needed to achieve carbon neutrality.

Morphology and stability of mineralized carbon influenced by magnesium ions

Ex situ mineralization of CO₂ is a promising technology that employs Ca- and Mg-rich industrial wastes but it simultaneously produces end products. Although Mg is a major mineralization source, it can adversely impact carbonate precipitation and crystal stability during co-precipitation in combination with Ca²⁺. In this study, the effects of Mg²⁺ ions on the mineralization process and its products were investigated using precipitates formed at different aqueous concentrations of Mg²⁺. The final phases of the precipitates were quantitatively evaluated at the end of each process. The alterations undergone by the calcite crystals, which constituted the dominant carbonate phase in each experiment, were analyzed using a sophisticated crystallographic approach. Aragonite was detected at high Mg²⁺ concentrations (Mg²⁺/Ca²⁺ ratio of 2.00), although brucite was the sole phase of the Mg crystal. The increase in Mg²⁺ ion concentration induced the formation of an amorphous solid. The results revealed that a drastic transformation of the calcite lattice occurred when the ratio of Mg²⁺/Ca²⁺ exceeded 1.00, agreeing with the shifts observed in the calcite structure upon comparing the precipitates formed at the Mg²⁺/Ca²⁺ ratios of 1.00 and 2.00, wherein microstrain and crystallite sizes changed from 0.040 and 55.33 nm to 0.1533 and 12.35 nm, respectively. At a Mg²⁺/Ca²⁺ ratio of 2.00, 6.51% of the Ca²⁺ ions in the calcite structure were substituted by Mg²⁺, increasing the surface energy of the crystal and the solubility of the carbonate. Therefore, Mg²⁺ is a potential hindrance that can impede the precipitation of carbonates and increase instability at certain concentrations.

Geochemical carbon dioxide removal potential of Spain

Many countries have made pledges to reduce CO₂ emissions over the upcoming decades to meet the Paris Agreement targets of limiting warming to no >1.5 °C, aiming for net zero by mid-century. To achieve national reduction targets, there is a further need for CO₂ removal (CDR) approaches on a scale of millions of tonnes, necessitating a better understanding of feasible methods. One approach that is gaining attention is geochemical CDR, encompassing (1) in-situ injection of CO₂-rich gases into Ca and Mg-rich rocks for geological storage by mineral carbonation, (2) ex-situ ocean alkalinity enhancement, enhanced weathering and mineral carbonation of alkaline-rich materials, and (3) electrochemical separation processes. In this context, Spain may host a notionally high geochemical CDR capacity thanks to its varied geological setting, including extensive mafic-ultramafic and carbonate rocks. However, pilot schemes and large-scale strategies for CDR implementation are presently absent in-country, partly due to gaps in current knowledge and lack of attention paid by regulatory bodies. Here, we identify possible materials, localities and avenues for future geochemical CDR research and implementation strategies within Spain. This study highlights the kilotonne to million tonne scale CDR options for Spain over the rest of the century, with attention paid to chemically and mineralogically appropriate materials, suitable implementation sites and potential strategies that could be followed. Mafic, ultramafic and carbonate rocks, mine tailings, fly ashes, slag by-products, desalination brines and ceramic wastes hosted and produced in Spain are of key interest, with industrial, agricultural and coastal areas providing opportunities to launch pilot schemes. Though there are obstacles to reaching the maximum CDR potential, this study helps to identify focused targets that will facilitate overcoming such barriers. The CDR potential of Spain warrants dedicated investigations to achieve the highest possible CDR to make valuable contributions to national reduction targets.

Molecular-scale mechanisms of CO2 mineralization in nanoscale interfacial water films

The calamitous impacts of unabated carbon emission from fossil-fuel-burning energy infrastructure call for accelerated development of large-scale CO₂ capture, utilization and storage technologies that are underpinned by a fundamental understanding of the chemical processes at a molecular level. In the subsurface, rocks rich in divalent metals can react with CO₂, permanently sequestering it in the form of stable metal carbonate minerals, with the CO₂-H₂O composition of the post-injection pore fluid acting as a primary control variable. In this Review, we discuss mechanistic reaction pathways for aqueous-mediated carbonation with carbon mineralization occurring in nanoscale adsorbed water films. In the extreme of pores filled with a CO₂-dominant fluid, carbonation reactions are confined to angstrom to nanometre-thick water films coating mineral surfaces, which enable metal cation release, transport, nucleation and crystallization of metal carbonate minerals. Although seemingly counterintuitive, laboratory studies have demonstrated facile carbonation rates in these low-water environments, for which a better mechanistic understanding has come to light in recent years. The overarching objective of this Review is to delineate the unique underlying molecular-scale reaction mechanisms that govern CO₂ mineralization in these reactive and dynamic quasi-2D interfaces. We highlight the importance of understanding unique properties in thin water films, such as how water dielectric properties, and consequently ion solvation and hydration behaviour, can change under nanoconfinement. We conclude by identifying important frontiers for future work and opportunities to exploit these fundamental chemical insights for decarbonization technologies in the twenty-first century.

The utilization of alkaline wastes in passive carbon capture and sequestration: Promises, challenges and environmental aspects

Alkaline wastes have been the focus of many studies as they act as CO₂ sinks and have the potential to offset emissions from mining and steelmaking industries. Passive carbonation of alkaline wastes mimics natural silicate weathering and provides a promising alternative pathway for CO₂ capture and storage as carbonates, requiring marginal human intervention when compared to ex-situ carbonation. This review summarizes the extant research that has investigated the passive carbonation of alkaline wastes, namely ironmaking and steelmaking slag, mine tailings and demolition wastes, over the past two decades. Here we report different factors that affect passive carbonation to address challenges that this process faces and to identify possible solutions. We identify avenues for future research such as investigating how passive carbonation affects the surrounding environment through interaction with the biosphere and the hydrosphere. Future research should also consider economic analyses to provide investors with an in-depth understanding of passive carbonation techniques. Based on the reviewed materials, we conclude that passive carbonation can be an important contributor to climate change mitigation strategies, and its potential can be intensified by applying simple waste management practices.

Kinetics-informed global assessment of mine tailings for CO2 removal

Chemically reactive mine tailings are a potential resource for drawing down carbon dioxide out of the atmosphere in mineral weathering schemes. Such carbon dioxide removal (CDR) systems, applied on a large scale, could help to meet internationally agreed targets for minimising climate change, but crucially we need to identify what materials could react fast enough to provide CDR at relevant climate change mitigation timescales. This study focuses on a range of silicate-dominated tailings, calculating their CDR potential from their chemical composition (specific capacity), estimated global production rates, and the speed of weathering under different reaction conditions. Tailings containing high abundances of olivine, serpentine and diopside show the highest CDR potential due to their favourable kinetics. We conclude that the most suitable tailings for CDR purposes are those associated with olivine dunites, diamond kimberlites, asbestos and talc serpentinites, Ni sulphides, and PGM layered mafic intrusions. We estimate the average annual global CDR potential of tailings weathered over the 70-year period 2030-2100 to be ~93 (unimproved conditions) to 465 (improved conditions) Mt/year. Results indicate that at least 30 countries possess tailings materials that, under improved conditions, may offer a route for CDR which is not currently utilised within the mining industry. By 2100, the total cumulative CDR could reach some 33 GtCO₂, of which more than 60% is contributed by PGM tailings produced in Southern Africa, Russia, and North America. The global CDR potential could be increased by utilization of historic tailings and implementing measures to further enhance chemical reaction rates. If practical considerations can be addressed and enhanced weathering rates can be achieved, then CDR from suitable tailings could contribute significantly to national offset goals and global targets. More research is needed to establish the potential and practicality of this technology, including measurements of the mineral weathering kinetics under various conditions.

Characterization of Gold Mining Waste for Carbon Sequestration and Utilization as Supplementary Cementitious Material

This study aims to identify the potential of gold mining waste for CO2 sequestration and its utilization for carbon storage in cementitious material. Samples of mine waste were identified from a gold mine for mineralogical and chemical composition analysis using X-ray diffractogram and scanning electron microscopy with energy-dispersive X-ray. Mine waste was utilized in a brick-making process as supplementary cementitious material and as an agent for CO2 capture and storage in bricks. Carbonation curing was incorporated in brick fabrication to estimate CO2 uptake of the brick product. Results indicated that the mine wastes were composed of silicate minerals essential for mineral carbonation such as muscovite and illite (major) and chlorite-serpentine, aerinite, albite and stilpnomelane (moderate/minor phases). The mine wastes were identified as belonging to the highly pozzolanic category, which has a great role in improving the strength properties of brick products. Carbonated minerals served as an additional binder that increased the strength of the product. CO2 uptake of the product was between 0.24% and 0.57% for bricks containing 40–60% of gold mine waste, corresponding to 7.2–17.1 g CO2/brick. Greater performance in terms of compressive strength and water adsorption was observed for bricks with 3 h carbonation curing. The carbonation product was evidenced by strong peaks of calcite and reduced peaks for calcium hydroxide from XRD analysis and was supported by a densified and crystalline microstructure of materials. It has been demonstrated that gold mine waste is a potential feedstock for mineral carbonation, and its utilization for permanent carbon storage in brick making is in line with the concept of CCUS for environmental sustainability.

Carbonation, Cementation, and Stabilization of Ultramafic Mine Tailings

Tailings dam failures can cause devastation to the environment, loss of human life, and require expensive remediation. A promising approach for de-risking brucite-bearing ultramafic tailings is in situ cementation via carbon dioxide (CO₂) mineralization, which also sequesters this greenhouse gas within carbonate minerals. In cylindrical test experiments, brucite [Mg(OH)₂] carbonation was accelerated by coupling organic and inorganic carbon cycling. Waste organics generated CO₂ concentrations similar to that of flue gas (up to 19%). The abundance of brucite (2-10 wt %) had the greatest influence on tailings cementation as evidenced by the increase in total inorganic carbon (TIC; +0.17-0.84%). Brucite consumption ranged from 64-84% of its initial abundance and was mainly influenced by water availability. Higher moisture contents (e.g., 80% saturation) and finer grain sizes (e.g., clay-silt) that allowed for a better distribution of water resulted in greater brucite carbonation. Furthermore, pore clogging and surface passivation by Mg-carbonates may have slowed brucite carbonation over the 10 weeks. Unconfined compressive strengths ranged from 0.4-6.9 MPa and would be sufficient in most scenarios to adequately stabilize tailings. Our study demonstrates the potential for stabilizing brucite-bearing mine tailings through in situ cementation while sequestering CO₂.

Aqueous mineral carbonation of ultramafic material: a pre-requisite to integrate into mineral extraction and tailings management operation

Ex situ aqueous mineral carbonation of ultramafic mining waste is an evolving technology for the CO₂ sequestration from small- to medium-scale emitters. The mineral ores or mine wastes of associated ultramafic mineralogy are a suitable feedstock for mineral carbonation. The aqueous mineral carbonation at ambient temperature is motivating and attractive from an energy-saving perspective. This study has investigated the CO₂ sequestration potential of a locally available ultramafic material generated from a nickel ore mine with a futuristic scope of integrating the method into an ongoing mineral extraction and/or tailing management operation. The mineral characterization and experimental results indicate that the tested material has CO₂ sequestration potential and underwent carbonation at ambient temperature. The carbonate conversion efficiencies obtained for Ca and Mg from the dissolved ionic forms at optimum conditions are 60% and 25%, respectively. The material was able to sequestrate about 0.12 gCO₂ per g solid at this efficiency. Aragonite and hydromagnesite are the major products that evolved out from the aqueous carbonation. Based on the mineral carbonation results, the novel concept of integrating the evolved method to existing mineral extraction and/or tailings management operation is discussed.

Emerging CO2-Mineralization Technologies for Co-Utilization of Industrial Solid Waste and Carbon Resources in China

CO2 mineralization (aka mineral carbonation) is a promising method for the chemical sequestration of CO2 via reaction with oxides of alkaline or alkaline-earth metals to form carbonates. It has documented advantages over similar technological solutions to climate change. The huge amount of industrial solid waste, as a serious environmental issue confronted by China, can provide additional alkalinity sources for CO2 mineralization. In this study, we present an overview of the latest advances in the emerging technologies of CO2-mineralization via industrial solid waste in China, from the perspective of both theoretical and practical considerations. We summarize the types of industrial solid waste that are used (mainly coal fly ash, steel slag, phosphogypsum, and blast furnace slag) and the technological options available in the literature, with an emphasis on the discussion of the involved process-intensification methods and valuable chemicals produced. Furthermore, we illustrate the current status of pertinent policies, and research and development activities in China. Finally, we identify the current knowledge gaps, particularly in understanding the overall sustainability performance of these CO2-mineralization technologies, and indicate that the technical, economic, and environmental challenges of promoting and commercializing these technologies for the co-utilization of industrial solid waste and carbon resources call for, amongst other things, more joint efforts by chemists, chemical engineers, and environmental scientists, and more feedback from the energy and industrial sectors.

Integrated leaching–carbonation kinetic model on CO2 mineralization of alkaline solid wastes in a high-gravity rotating packed bed

Ca²⁺ leaching and carbonation kinetics associated with liquid side mass transfer for CO₂ mineralization of alkaline solid wastes in a RPB were determined.

Atmospheric Carbon Capture Performance of Legacy Iron and Steel Waste

Legacy iron (Fe) and steel wastes have been identified as a significant source of silicate minerals, which can undergo carbonation reactions and thus sequester carbon dioxide (CO₂). In reactor experiments, i.e., at elevated temperatures, pressures, or CO₂ concentrations, these wastes have high silicate to carbonate conversion rates. However, what is less understood is whether a more "passive" approach to carbonation can work, i.e., whether a traditional slag emplacement method (heaped and then buried) promotes or hinders CO₂ sequestration. In this paper, the results of characterization of material retrieved from a first of its kind drilling program on a historical blast furnace slag heap at Consett, U.K., are reported. The mineralogy of the slag material was near uniform, consisting mainly of melilite group minerals with only minor amounts of carbonate minerals detected. Further analysis established that total carbon levels were on average only 0.4% while average calcium (Ca) levels exceeded 30%. It was calculated that only ∼3% of the CO₂ sequestration potential of the >30 Mt slag heap has been utilized. It is suggested that limited water and gas interaction and the mineralogy and particle size of the slag are the main factors that have hindered carbonation reactions in the slag heap.

Alkaline Mineral Soil Amendment: A Climate Change ‘Stabilization Wedge’?

Extreme climate change due to heat-trapping gases, especially carbon dioxide, necessitates its mitigation. In this context, the carbon dioxide sequestration technology of enhanced weathering has for years been investigated, with a possible implementation strategy via alkaline mineral soil amendment being more recently proposed. Candidate materials for enhanced weathering include calcium and magnesium silicates, most notably those belonging to the olivine, pyroxene and serpentine groups of minerals, given their reactivity with CO2 and global availability. When these finely crushed silicate rocks are applied to the soil, the alkaline earth metal cations released during mineral weathering gradually react with carbonate anions and results in the formation of pedogenic carbonates, which, over time, and under the right conditions, can accumulate in the soil. This review paper critically reviews the available literature on alkaline mineral soil amendments and its potential to sequester enough CO2 to be considered a climate change ‘stabilization wedge’. Firstly, evidence of how agricultural soil can serve as a carbon sink in discussed, based on the observed accumulation of inorganic carbon in alkaline mineral-amended soils. Secondly, the impact of alkaline minerals on agricultural soil and crops, and the factors determining the rate of the weathering process are assessed. Lastly, the CO2 sequestration potential via alkaline mineral soil amendment is quantified according to an idealized shrinking core model, which shows that it has the potential to serve as a climate change stabilization wedge.

Carbon Sequestration in Biogenic Magnesite and Other Magnesium Carbonate Minerals

The stability and longevity of carbonate minerals make them an ideal sink for surplus atmospheric carbon dioxide. Biogenic magnesium carbonate mineral precipitation from the magnesium-rich tailings generated by many mining operations could offset net mining greenhouse gas emissions, while simultaneously giving value to mine waste products. In this investigation, cyanobacteria in a wetland bioreactor enabled the precipitation of magnesite (MgCO₃), hydromagnesite [Mg₅(CO₃)₄(OH)₂·4H₂O], and dypingite [Mg₅(CO₃)₄(OH)₂·5H₂O] from a synthetic wastewater comparable in chemistry to what is produced by acid leaching of ultramafic mine tailings. These precipitates occurred as micrometer-scale mineral grains and microcrystalline carbonate coatings that entombed filamentous cyanobacteria. This provides the first laboratory demonstration of low temperature, biogenic magnesite precipitation for carbon sequestration purposes. These findings demonstrate the importance of extracellular polymeric substances in microbially enabled carbonate mineral nucleation. Fluid composition was monitored to determine carbon sequestration rates. The results demonstrate that up to 238 t of CO₂ could be stored per hectare of wetland/year if this method of carbon dioxide sequestration was implemented at an ultramafic mine tailing storage facility. The abundance of tailings available for carbonation and the anticipated global implementation of carbon pricing make this method of mineral carbonation worth further investigation.

Sequestering Atmospheric CO2 Inorganically: A Solution for Malaysia’s CO2 Emission

Malaysia is anticipating an increase of 68.86% in CO2 emission in 2020, compared with the 2000 baseline, reaching 285.73 million tonnes. A major contributor to Malaysia’s CO2 emissions is coal-fired electricity power plants, responsible for 43.4% of the overall emissions. Malaysia’s forest soil offers organic sequestration of 15 tonnes of CO2 ha−1·year−1. Unlike organic CO2 sequestration in soil, inorganic sequestration of CO2 through mineral carbonation, once formed, is considered as a permanent sink. Inorganic CO2 sequestration in Malaysia has not been extensively studied, and the country’s potential for using the technique for atmospheric CO2 removal is undefined. In addition, Malaysia produces a significant amount of solid waste annually and, of that, demolition concrete waste, basalt quarry fine, and fly and bottom ashes are calcium-rich materials suitable for inorganic CO2 sequestration. This project introduces a potential solution for sequestering atmospheric CO2 inorganically for Malaysia. If lands associated to future developments in Malaysia are designed for inorganic CO2 sequestration using demolition concrete waste, basalt quarry fine, and fly and bottom ashes, 597,465 tonnes of CO2 can be captured annually adding a potential annual economic benefit of €4,700,000.

Microstructural characteristics of naturally formed hardpan capping sulfidic copper-lead-zinc tailings

A massive and dense textured layer (ca. 35-50 cm thick) of hardpan was uncovered at the top layer, which capped the unweathered sulfidic Cu-Pb-Zn tailings in depth and physically supported gravelly soil root zones sustaining native vegetation for more than a decade. For the purpose of understanding functional roles of the hardpan layer in the cover profile, the present study has characterized the microstructures of the hardpan profile at different depth compared with the tailings underneath the hardpans. A suit of microspectroscopic technologies was deployed to examine the hardpan samples, including field emission-scanning electron microscopy coupled with energy dispersive spectroscopy (FE-SEM-EDS), X-ray diffraction (XRD) and synchrotron-based X-ray absorption fine structure spectroscopy (XAFS). The XRD and Fe K-edge XAFS analysis revealed that pyrite in the tailings had been largely oxidised, while goethite and ferrihydrite had extensively accumulated in the hardpan. The percentage of Fe-phyllosilicates (e.g., biotite and illite) decreased within the hardpan profile compared to the unweathered tailings beneath the hardpan. The FE-SEM-EDS analysis showed that the fine-grained Ca-sulfate (possibly gypsum) evaporites appeared as platelet-shaped that deposited around pyrite, dolomite, and crystalline gypsum particles, while Fe-Si gels exhibited a needle-like texture that aggregated minerals together and produced contiguous coating on pyrite surfaces. These microstructural findings suggest that the weathering of pyrite and Fe-phyllosilicates coupled with dolomite dissolution may have contributed to the formation of Ca-sulfate/gypsum evaporites and Fe-Si gels. These findings have among the first to uncover the microstructure of hardpan formed at the top layer of sulfidic Cu-Pb-Zn tailings, which physically capped the unweathered tailings in depth and supported root zones and native vegetation under semi-arid climatic conditions.

Microbially Accelerated Carbonate Mineral Precipitation as a Strategy for in Situ Carbon Sequestration and Rehabilitation of Asbestos Mine Sites

A microbially accelerated process for the precipitation of carbonate minerals was implemented in a sample of serpentinite mine tailings collected from the abandoned Woodsreef Asbestos Mine in New South Wales, Australia as a strategy to sequester atmospheric CO2 while also stabilizing the tailings. Tailings were leached using sulfuric acid in reaction columns and subsequently inoculated with an alkalinity-generating cyanobacteria-dominated microbial consortium that was enriched from pit waters at the Woodsreef Mine. Leaching conditions that dissolved 14% of the magnesium from the serpentinite tailings while maintaining circumneutral pH (1800 ppm, pH 6.3) were employed in the experiment. The mineralogy, water chemistry, and microbial colonization of the columns were characterized following the experiment. Micro-X-ray diffraction was used to identify carbonate precipitates as dypingite [Mg5(CO3)4(OH)2·5H2O] and hydromagnesite [Mg5(CO3)4(OH)2·4H2O] with minor nesquehonite (MgCO3·3H2O). Scanning electron microscopy revealed that carbonate mineral precipitates form directly on the filamentous cyanobacteria. These findings demonstrate the ability of these organisms to generate localized supersaturating microenvironments of high concentrations of adsorbed magnesium and photosynthetically generated carbonate ions while also acting as nucleation sites for carbonate precipitation. This study is the first step toward implementing in situ carbon sequestration in serpentinite mine tailings via microbial carbonate precipitation reactions.

Rapid Removal of Atmospheric CO2 by Urban Soils

The measured calcium carbonate content of soils to a depth of 100 mm at a large urban development site has increased over 18 months at a rate that corresponds to the sequestration of 85 t of CO2/ha (8.5 kg of CO2 m(-2)) annually. This is a consequence of rapid weathering of calcium silicate and hydroxide minerals derived from the demolition of concrete structures, which releases Ca that combines with CO2 ultimately derived from the atmosphere, precipitating as calcite. Stable isotope data confirm an atmospheric origin for carbonate carbon, and 14C dating indicates the predominance of modern carbon in the pedogenic calcite. Trial pits show that carbonation extends to depths of ≥1 m. Work at other sites shows that the occurrence of pedogenic carbonates is widespread in artificially created urban soils containing Ca and Mg silicate minerals. Appropriate management of fewer than 12000 ha of urban land to maximize calcite precipitation has the potential to remove 1 million t of CO2 from the atmosphere annually. The maximal global potential is estimated to be approximately 700-1200 Mt of CO2 per year (representing 2.0-3.7% of total emissions from fossil fuel combustion) based on current rates of production of industry-derived Ca- and Mg-bearing materials.

A greenhouse-scale photosynthetic microbial bioreactor for carbon sequestration in magnesium carbonate minerals

A cyanobacteria dominated consortium collected from an alkaline wetland located near Atlin, British Columbia, Canada accelerated the precipitation of platy hydromagnesite [Mg5(CO3)4(OH)2·4H2O] in a linear flow-through experimental model wetland. The concentration of magnesium decreased rapidly within 2 m of the inflow point of the 10-m-long (∼1.5 m(2)) bioreactor. The change in water chemistry was monitored over two months along the length of the channel. Carbonate mineralization was associated with extra-cellular polymeric substances in the nutrient-rich upstream portion of the bioreactor, while the lower part of the system, which lacked essential nutrients, did not exhibit any hydromagnesite precipitation. A mass balance calculation using the water chemistry data produced a carbon sequestration rate of 33.34 t of C/ha per year. Amendment of the nutrient deficiency would intuitively allow for increased carbonation activity. Optimization of this process will have application as a sustainable mining practice by mediating magnesium carbonate precipitation in ultramafic mine tailings storage facilities.