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Mineralization of gaseous carbon dioxide into solid carbonates using alkaline industrial residues such as coal fly ash has a dual advantage of reducing the carbon dioxide footprint of coal power plants and improving ash utilization. However, the slow mineral carbonation rate under atmospheric conditions is a major challenge, especially when using natural minerals or industrial residues for direct air capture (DAC) of CO 2 . In this study, using coal fly ash samples and concentrated alkali carbonate aqueous solutions as a recyclable solvent, we show the feasibility of coupling mineral carbonation with DAC under atmospheric conditions. Findings show that carbonation efficiency is best under alkaline conditions, achieving as high as ~80% conversion to calcium carbonates within 1 h in a 1.9 M sodium carbonate solution. Based on the experimental results, a process coupling DAC and mineral carbonation that operates entirely under ambient conditions is proposed. Here, the techno-economic and life cycle assessments for the proposed process project a levelized cost of $\$116$–133/t-CO 2 -sequestered (US $\$2019$) and process carbon emissions (GWP) in the range of 0.03–0.25 t-CO 2 e/t-CO 2 -sequestered. Considering the low cost, simplicity, and gigaton-scale sequestration potential, we believe that DAC based on alkaline industrial residue carbonation can be considered a “low-hanging fruit” in the pursuit of negative emissions to combat climate change.
Ex-situ carbon mineralization is a process by which CO2 is reacted with alkaline silicate minerals and rocks to produce stable carbonate materials, which can be used for other industrial processes. Arizona, U.S.A., hosts abundant surficial mafic rocks in three young volcanic fields, Geronimo-San Bernardino, San Francisco, and Springerville, and other distributed locations throughout the state. We created a Mafic Rock Resource Inventory (MRRI) that categorizes geochemical, physical, and textural characteristics of a diverse suite of surficial mafic rock samples and provide a benchmark reaction dataset parameterizing the temperature, pressure, and pH conditions best suited ex-situ mineralization in different rock types. MRRI data is publicly available online via a map-viewer. We establish two reaction condition sets, varied in temperature, pressure, and pH, where crystal-rich and glassy rocks reach maximum reaction extent and different carbonate phases are formed. Systematic ex-situ mineralization experiments on 21 diverse rock types show trends in geochemical, mineralogical, and reactivity behavior and establish maximum effective capture capacity. From this, scoria cones in three Arizona volcanic fields have a ~62 Gt effective CO₂ storage capacity with one of the fields having a ~42 Gt storage capacity in lava flows. Reactivity results have applications to alkaline mafic rock resources exposed globally, including producing additional effective storage capacity estimates and scaled commercialization of mafic rock ex-situ mineralization, should reaction extents be improved through advances in mineralization techniques. MRRI data were used to create a Direct Air Capture to Mineralization (DACM) systems model, technoeconomic analysis, and life-cycle assessment. These documents are presented as three appendices.
Magnesium oxide (MgO) is considered as a potential sorbent for direct air capture of carbon dioxide in a looping process. Previous research on mineral looping for carbon capture from flue gas using MgO has shown deactivation of the sorbent with repeated cycles but repeated cycling for carbonation at ambient conditions has not been yet investigated. Here, we tested three cycles of carbonation for MgO nano-scale powders with different starting surface area. We find that carbonation efficiency is higher at higher surface area. No decrease in carbonation efficiency was observed with cycling, instead we find that carbonation efficiency and surface area are stable or even increasing with cycling. Based on our experimental data, we hypothesize that the carbonation in presence of relative humidity leads to first hydration of MgO, leading to formation of brucite (Mg(OH) 2 ). This formation of Mg(OH) 2 is a volume-increasing reaction, which leads to fracturing of MgO particles and results in an increase in surface area. We observed formation of amorphous and crystalline hydrated carbonates. The crystalline phase observed was nesquehonite (MgCO 3 ⋅ 3H 2 O). Our results show that MgO does not show any deactivation with repeated cycling for carbon capture at ambient conditions and in the presence of humidity. These findings therefore indicate that MgO is a suitable candidate as sorbent material for direct air capture of carbon dioxide.
The University of Illinois at Urbana-Champaign (UIUC) led this project to produce a front-end engineering design (FEED) study of an advanced Direct Air Capture and Utilization System (DACUS) system that can remove a minimum of 5,000 tonnes/yr net of carbon dioxide from air (based on cradle-to-gate LCA) and utilizing the CO 2 to produce low carbon intensity ready mix concrete. The designed system, if built, would be larger than any currently existing Direct Air Capture (DAC) collector in the U.S. Such carbon capture technologies are critical to meeting the goals of the DOE’s program to accelerate climate-critical technology. In addition to the power sector, industrial facilities for the manufacture of steel and cement/concrete are among the major sources of anthropogenic CO 2 . DAC is a promising new technology for reducing CO 2 , a potent greenhouse gas, in the atmosphere but is expensive, in part due to the energy required to adsorb and desorb captured CO 2 during cycles. By integrating CarbonCapture Inc. (CCI) DAC modules at United States Steel's Gary Works (USS) and utilizing the site's waste heat, energy, and location this project evaluates the feasibility of utilizing the captured CO 2 and the logistics of transportation. CarbonCapture Inc. has developed an innovative DAC system using novel adsorbents to cost-effectively capture CO 2 . The captured, liquified gas will be trucked to ready-mix concrete plants within the region, the closest of which is approximately 3.5 miles away, where CarbonCure will inject it into concrete during the mixing process at the facilities. The carbon dioxide reacts with concrete, mineralizing into calcium carbonate (CaCO 3 ), permanently locking the greenhouse gas into the matrix of the building material. This FEED study demonstrated a full CO 2 value chain for DACUS from industrial facilities. It also provided a means for Visage Energy Corp. (Visage) to assess the impact of this holistic approach on job creation, regional economic impact, and environmental justice. The project team also included Sargent & Lundy (S&L) to provide the constructability review and costing of the integration of the DAC with the steel plant. Ecotek Engineering USA, LLC designed the outside battery limit (OSBL) infrastructure to connect the DAC and the plant. Activities performed during the project included: (1) Project Management Plan; (2) Technology Maturation Plan (TMP); (3) Initial Workforce Readiness Plan; (4) Workforce Readiness Plan; (5) Front-End Engineering Design (FEED) Study; (6) Project Design Basis; (7) Hazards and Operability (HAZOP) Study; (8) Constructability Review; (9) Project Cost Assessment; (10) Logistics Analysis of CO 2 Transportation to the Utilization Site; (11) Business Case Analysis; (12) Life Cycle Analysis (LCA); (13) Environmental Health and Safety (EH&S) Analysis; (14) Environmental Justice Analysis; and (15) Economic Revitalization and Job Creation Outcomes Analysis. This report provides a summary of the outcomes and results of the project, which was performed between Oct. 1, 2022, through Sept. 30, 2024.
Mineralization by MgO is an attractive potential strategy for direct air capture (DAC) of CO 2 due to its tendency to form carbonate phases upon exposure to water and CO 2 . Hydration of MgO during this process is typically assumed to not be rate limiting, even at ambient temperatures. However, surface passivation by hydrated phases likely reduces the CO 2 capture capacity. Here, we examine the initial hydration reactions that occur on MgO(100) surfaces to determine whether they could potentially impact CO 2 uptake. We first used atomic force microscopy (AFM) to explore changes in reaction layers in water (pH = 6 and 12) and MgO-saturated solution (pH = 11) and found the reaction layers on MgO are heterogeneous and nonuniform. To determine how relative humidity (R.H.) affects reactivity, we reacted samples at room temperature in nominally dry N 2 (~11–12% R.H.) for up to 12 h, in humid (>95% R.H.) N 2 for 5, 10, and 15 min, and in air at 33 and 75% R.H. for 8 days. X-ray reflectivity and electron microscopy analysis of the samples reveal that hydrated phases form rapidly upon exposure to humid air, but the growth of the hydrated reaction layer slows after its initial formation. Reaction layer thickness is strongly correlated with R.H., with denser reaction layers forming in 75% R.H. compared with 33% R.H. or nominally dry N 2 . The reaction layers are likely amorphous or poorly crystalline based on grazing incidence X-ray diffraction measurements. After exposure to 75% R.H. in air for 8 days, the reaction layer increases in density as compared to the sample reacted in humid N 2 for 5–15 min. Further, this may represent an initial step toward the crystallization of the reaction layer. Overall, high R.H. favors the formation of a hydrated, disordered layer on MgO. Based on our results, DAC in a location with a higher R.H. will be favorable, but growth may slow significantly from initial rates even on short timescales, presumably due to surface passivation.
MgO (periclase) is a promising material for direct air capture of CO 2 using a mineral looping process, but it is unknown how impurities in the environment will affect the CO 2 uptake and hence process economics. Here, we investigated the effects of dissolved iron on the extents of MgO hydroxylation and subsequent carbonation reactions to determine if this has a beneficial or detrimental effect. On single-crystal MgO, dissolved iron prevented hydration of MgO to Mg(OH) 2 (brucite) and instead formed a shell of lepidocrocite (γ-FeOOH). This did not passivate the MgO as dissolution below the shell was observed. During hydroxylation of MgO powders in the presence of dissolved iron, formation of brucite containing Fe(II) was observed. In addition, formation of nanoscale iron oxides containing Fe(III) was observed using magnetometry and Mössbauer spectroscopy. Subsequent carbonation experiments showed increased carbonation of MgO hydroxylated in the presence of iron. Our results indicate that the presence of dissolved solute impurities during hydroxylation may be beneficial for carbonation of hydroxylated MgO.
Basic research to identify and understand the fundamental principles governing carbon dioxide removal processes of capture, conversion, and storage is essential for achieving zero carbon emissions by 2050. In March 2022, the Office of Basic Energy Sciences (BES)—in coordination with the US Department of Energy (DOE) technology Offices of Energy Efficiency and Renewable Energy, Fossil Energy and Carbon Management, and Nuclear Energy—held a roundtable titled, “Foundational Science for Carbon Dioxide Removal Technologies,” to discuss the scientific and technical barriers for CO 2 capture, conversion, and storage. Five priority research opportunities were identified to address these scientific and technical challenges and accelerate progress toward the realization of zero carbon emissions. The PROs are as follows: 1) Master Interfacial Processes of CO2 Transport and Reactivity Across Multiple Length and Time Scales; 2) Create Materials that Simultaneously Exhibit Multiple Properties for CO 2 Capture and Release or Conversion; 3) Discover Unconventional Pathways for Energy-Efficient CO 2 Capture, Release, and Conversion; 4) Control Multiphase Interactions Required for CO 2 Conversion into Minerals and Materials; and 5) Achieve Predictive Understanding of Coupled Processes in Complex Subsurface Geologic Systems for Secure Carbon Storage.
Overview The Carbon Management Projects (CONNECT) Toolkit is an online exploratory visualization tool developed by the U.S. Department of Energy's (DOE) Office of Fossil Energy and Carbon Management (FECM) with support from other federal agencies such as the U.S. Environmental Protection Agency (EPA) and the U.S. Department of Transportation (DOT). It provides a single point of access to authoritative information on federal agency investment in a portfolio of research, development, and demonstration (RD&D) projects that have been publicly announced to advance technologies for point source carbon capture, carbon dioxide removal, transport, storage, and conversion, collectively referred to as carbon management. The RD&D programs covered in this tool are authorized by annual congressional appropriations ("Base Program") and the 2021 Infrastructure Investment and Jobs Act (IIJA). The tool also incorporates public information on other federal initiatives, such as the Regional Clean Hydrogen Hubs, and public information released by other government agencies, such as the Environmental Protection Agency's (EPA) and Primacy States’ Underground Injection Control Class VI permits and EPA’s facility level greenhouse gas (GHG) emissions. Developed in a geographic information system, the tool organizes carbon management projects into five groups based on the primary technology that a project aims to advance, each visually represented as a digital layer ("carbon management project layer"). Only federally funded projects are included, which can be awarded projects that are completed or ongoing, or projects that have been selected but are currently under negotiation. Project information can be viewed in the map or in the attribute table below it when turned on. In the map view, each project is displayed at either its host site (for field work), where available, or its performer site (project lead's location, further explained in the table below). Host sites and performer sites are represented in distinct icons. Several reference layers offer additional public information on infrastructural and natural resource environment for carbon management. These reference layers, combined with multiple geographical basemaps, enable users to visualize the carbon management project layers in context. Carbon management project information will be updated monthly based on feedback and information availability. Carbon management project layers Point Source Carbon Capture (PSC) This layer contains DOE-funded projects focused on capturing carbon dioxide (CO2) from power plants or industrial facilities. Carbon Dioxide Removal (CDR) This layer contains DOE-funded projects focused on capturing CO2 from the atmosphere, including direct air capture (DAC) and DAC hubs, direct ocean capture, enhanced mineralization, and biomass carbon removal and storage. For projects with multiple host sites, each of the sites are displayed individually with the project cost and cost sharing information representing the total for the entire project. Carbon Transport This layer contains DOE- and DOT-funded projects focused on CO2 transport. The Transport Research and Development sublayer contains projects that do not involve physical infrastructure; the Proposed Transport Corridor sublayer contains projects for which either a route for the transport infrastructure has been proposed or a general area for the transport infrastructure has been identified. Carbon Storage This layer contains DOE-funded key projects focused on CO2 storage. For projects with multiple field-work sites, each of the sites are displayed individually on the map with the project cost and cost sharing information representing the overall total for the entire project. Carbon Conversion This layer contains DOE-funded projects focused on converting CO2 into economically valuable products. Reference layers The following layers provide additional information in the geographic proximity of carbon management projects. Users should reference the original sources for more details (weblinks provided below and in pop-up windows on the map). Regional Clean Hydrogen Hub and Facility These layers illustrate the approximate areas of the Regional Clean Hydrogen Hubs announced by DOE's Office of Clean Energy Demonstrations (OCED) and the approximate locations of individual facilities that constitute the hubs (see "Where are the H2Hubs located?" on the webpage linked above). EPA Facility Level GHG Emissions (direct emitter) This layer shows direct CO2 emissions from stationary sources in 2022, using data extracted from EPA's Facility Level Information on GreenHouse gases Tool (FLIGHT). Captured and injected CO2 are not deducted from direct emitters’ total emissions. Contact EPA for additional details. Underground Injection Control Class VI permit/permit application This layer shows the locations of CO2 injection wells that are granted or in the process of applying for an Underground Injection Control Class VI permit by EPA or a Primacy State (currently Louisiana, North Dakota, and Wyoming). The URLs for the permits or permit applications are provided in the pop-up windows associated with the well locations. Contact EPA for additional details. Carbon Storage Resource This layer contains information on prospective CO2 storage resources in saline formations and oil and gas reservoirs provided by the National Carbon Sequestration Database and Geographic Information System (NATCARB) spatial database. Contact NETL for additional details. Existing CO2 pipeline This layer shows active CO2 pipelines based on information digitized from the map issued by the Pipeline and Hazardous Materials Safety Administration (PHMSA). Contact PHMSA for additional details.
Brucite [Mg(OH) 2 ] is a promising sorbent for carbon dioxide removal (CDR) due to its availability and low calcination temperatures. However, natural and synthetic brucites tend to contain metal impurities, such as iron or manganese, and how these impurities affect the interfacial chemical reactivity is uncertain. Here, in this study, the impact of low concentrations of iron and manganese impurities on the carbonation efficiency of Mg(OH) 2 was examined. Mg(OH) 2 with small amounts (1–5 mol %) of Fe and Mn was synthesized. The increasing substitution of Fe into Mg(OH) 2 was accompanied by the oxidation of Fe. The phase transformation sequence during the carbonation was found to be brucite [Mg(OH) 2 ] → amorphous magnesium carbonate (MgCO 3 ·nH 2 O) → nesquehonite (MgCO 3 ·3H 2 O), regardless of impurity concentration. Both the Fe- and Mn-doped Mg(OH) 2 samples were more reactive than endmember Mg(OH) 2 , possibly due to their higher surface areas and lower stabilities. During carbonation, 3 mol % Fe- and Mn-doped Mg(OH) 2 showed the highest reactivity. The variance in reactivity for Mn-doped Mg(OH) 2 was less than that of Fe-doped Mg(OH) 2 . These results suggest that natural or industrial waste Mg(OH) 2 with less than 5 mol % Fe and Mn impurities may be targeted as more effective CDR sorbents than endmember Mg(OH) 2 .
Magnesium oxide (MgO) is a promising sorbent for direct air capture (DAC) of carbon dioxide. Iron (Fe) is a common impurity in naturally occurring MgO and minerals used to produce MgO, yet a molecular-scale understanding of Fe-doping effects on carbonation is lacking. Here, in this study, we observed reduced carbonation performance in Fe-doped MgO experimentally. The energetics of adsorbing a (bi)carbonate ion on pristine and Fe-doped MgO(001) surfaces were further investigated using ab initio and machine learning potential molecular dynamics coupled with metadynamics simulations. Both pristine and Fe-doped surfaces exhibited a basic (OH – ) hydration layer, where the (bi)carbonate ion adsorption is thermodynamically favorable. However, the dissolution of surface Fe had smaller energy barriers and was more favorable than Mg. Leached Fe likely neutralized the near-surface basicity, yielding reduced reactivity on Fe-doped MgO. Our observations offer critical insights for material selection and emphasize the importance of evaluating the geologic origin of earth materials used for DAC.
Direct air capture (DAC) may be feasible to remove carbon dioxide (CO 2 ) from the atmosphere at the gigaton scale, holding promise to become a major contributor to climate change mitigation. Mineral looping using magnesium oxide (MgO) is potentially an economical, efficient, and sustainable pathway to gigaton-scale DAC. The hydroxylation and carbonation of MgO determine the efficiency of the looping process, but their rates and mechanisms remain uncertain. Here, in this work, MgO single crystals were reacted in air or CO 2 at varying humidities and characterized by X-ray scattering, microscopy, and vibrational spectroscopy. Results show that the hydroxylation formed a brucite (Mg(OH) 2 )-like layer immediately after crystal cleaving. Concurrently, the carbonation formed hydrated magnesium carbonate phases, including barringtonite (MgCO 3 ·2H 2 O) and nesquehonite (MgCO 3 ·2H 2 O), in the layer. Rapid initial growth of the layer is also manifested in short-range bending/warping of nanocrystallites, resulting in multiple orientations of the same phases on the surface. The layer growth slowed down over time, indicating surface passivation. The formation of barringtonite and nesquehonite with 1:1 CO 3 /Mg ratio indicates an efficient carbonation when compared to other magnesium carbonate phases of lower ratio. Our results are essential for understanding surface passivation mechanisms and tackling the passivation issue of mineral looping DAC technology.
The National Energy Technology Laboratory (NETL) and Carbon To Stone (CTS), Inc. (Participant) will collaborate on a techno-economic assessment (TEA) and life cycle assessment (LCA) of CTS’s carbon dioxide removal (CDR) technology under an awarded project from the Department of Energy’s Direct Air Capture Pre-Commercialization Technology Prize. CTS has developed a technological platform that captures carbon dioxide (CO2) from air and mineralizes it into durable carbonates using alkaline industrial residues such as steel slag and mine tailings. In addition, CTS’s technology uses electrochemistry to recover value-added energy critical metals (ECMs) from these residues. NETL has expertise in CDR TEA and LCA development. The collaboration between CTS and NETL aims to quantify the performance, cost and lifecycle impacts of CTS’s technology and produce LCA and TEA models that can accelerate development and deployment of this technology.
Negative emissions technologies will be essential for limiting anthropogenic global temperature increases to 2 °C in the later years of the 21st century. Carbonic anhydrase (CA) metalloenzymes catalyze the otherwise slow conversion of CO 2 into carbonic acid (H 2 CO 3 ), suggesting their utility in the rapid hydration and downstream capture of dissolved CO 2 in aqueous media for a variety of CO 2 capture methods, such as thermal and pH swings and mineralization. The possibility of driving the rapid capture of CO 2 by catalyzing the CO 2 hydration bottleneck carries real potential for realizing efficient direct air capture (DAC) and direct ocean capture (DOC) systems. However, scaled application of CAs will be dependent on some way of economically sourcing the enzymes at volumes relevant to scaled DAC/DOC operations. In this perspective, we consider the prospect of catalyzing CO 2 hydration using a CA that is bound to the outer membrane of a cyanobacterial host, engineered constructs we call CyCAMs. Coupling extracellular CA expression to a marine, photosynthetic cyanobacterial host results in a CO 2 hydration catalyst that is continuously refreshed during the bacterial growth cycle, without the need for enzyme purification or harvesting steps. In the case of marine cyanobacteria, sunlight and the solutes native to seawater provide the respective energy, nutrients and sources of inorganic carbon essential to maintaining the bacterial population. We consider the potential impact of CA-mediated CO 2 hydration on a variety of DAC processes, with CO 2 mineralization explored as a primary use case.
As the United States transitions toward a clean energy economy, an opportunity exists for redeveloping the more than 17,000 mine land sites located across the nation with clean energy technologies, which have a combined potential for generating more than 85 GW of clean electricity. This report provides an overview of the potential of demonstrating and deploying clean energy projects on current and former mine land. Clean energy project refers to a project that demonstrates one or more of the following technologies: solar; microgrids; geothermal; direct air capture; fossil-fueled electricity generation with carbon capture, utilization, and sequestration; energy storage, including pumped storage hydropower and compressed air energy storage; and advanced nuclear technologies. The report discusses the following technologies and their potential for creating jobs and generating tax revenue that would result in direct and indirect benefits to the local economy: Solar photovoltaics (PV) is being developed on current and former mine land in various parts of the world, including the United States. This approach is attractive because it requires limited infrastructure investment and would utilize the bare surfaces of mines and tailing ponds. Solar resource availability may be greater in the southern regions, including the Interior and Appalachian Basins and the southwestern United States. However, since some mine land sites include areas of significant change in elevation, the deployment of PV on mine land may require sophisticated planning to account for shading and irradiance, or may require regrading of the areas. PV does not create significant environmental risks and generally does not face public resistance; Geothermal systems are often spatially and genetically associated with ore deposits, and in some cases, they have been discovered while in search for epithermal mineral resources. Numerous diverse geothermal applications have been employed at mine land around the world, including power generation, mineral extraction from geothermal brines, process heating, direct use for other mining operations, and direct use for non-mining operations and subsurface energy storage, including geothermal heat pumps. Case studies highlighting these applications provide key lessons relating to identifying drivers and barriers to geothermal resource deployment and can be used to create screening tools for identifying the types and locations of mine land most amenable to utilizing geothermal resources; Carbon capture, utilization, and sequestration technologies include direct air capture (DAC) and enhanced weathering. DAC technologies include air contactors, regeneration systems, and CO 2 compression systems. Captured CO 2 can be converted to valuable feedstocks or possibly injected into abandoned subsurface mines where it would be absorbed by alkaline rock waste and mine tailings or by the porous minerals along the walls of the mine. DAC systems can be coupled with energy sources such as wind, solar, grid, or geothermal. Many DAC systems require a source of water or steam; however, some are expected to be net producers of water. Local impacts of DAC systems are expected to be low, and are related to land footprint, material disposal, and upstream impacts of energy and material production; Compressed air energy storage is an established energy storage technology in salt caverns. It has the potential for implementation in underground mines by pressurizing and storing a large amount of air using electrical compressors when excess electricity is available. When a need for discharge emerges, the air is used to spin turbines and produce the necessary volume of electricity. Abandoned or unused mine openings, including shafts, adits, access tunnels, and mined workings of any orientation, offer potential for vast amounts of compressed air energy storage if the site characteristics meet operational requirements; Pumped hydropower storage can be implemented in surface and subsurface mines. In surface mine applications, both reservoirs may be located in a mine pit or artificial reservoirs made of excavated materials. In subsurface mines, the lower reservoir may be implemented by waterproofing and flooding mine shafts and tunnels. The water is then pumped from the lower reservoir to the upper reservoir during periods of low load and high production, and it is discharged through the turbines during periods of peak demand. The potential environmental damages associated with acidity of mine water or the presence of toxic chemicals incentivizes the development of closed-loop technologies, in which water circulates inside the pumped hydropower facility without being discharged into the external water basins; Advanced nuclear energy technologies include small modular reactors, which can be deployed locally to produce electricity and heat. Such units require seismic stability and a supply of cooling water, but population constraints may exist in some areas. Therefore, remote mine land could represent an optimal location for siting advanced nuclear energy technologies.
Interfaces can actively control the nucleation kinetics, orientations, and polymorphs of calcium carbonate (CaCO 3 ). Prior studies have revealed that CaCO 3 formation can be affected by the interplay between chemical functional moieties on solid–liquid or air–liquid interfaces as well as CaCO 3 ’s precursors and facets. Yet little is known about the roles of a liquid–liquid interface, specifically an oil–liquid interface, in directing CaCO 3 mineralization which are common in natural and engineered systems. Here, in this study, by using in situ X-ray scattering techniques to locate a meniscus formed between water and a representative oil, isooctane, we successfully monitored CaCO 3 formation at the pliable isooctane–water interface and systematically investigated the pivotal roles of the interface in the formation of CaCO 3 (i.e., particle size, its spatial distribution with respect to the interface, and its mineral phase). Different from bulk solution, ∼5 nm CaCO 3 nanoparticles form at the isooctane–water interface. They stably exist for a long time (36 h), which can result from interface-stabilized dehydrated prenucleation clusters of CaCO 3 . There is a clear tendency for enhanced amounts and faster crystallization of CaCO 3 at locations closer to isooctane, which is attributed to a higher pH and an easier dehydration environment created by the interface and oil. Our study provides insights into CaCO 3 nucleation at an oil–water interface, which can deepen our understanding of pliable interfaces interacting with CaCO 3 and benefit mineral scaling control during energy-related subsurface operation.
Manganese oxides are ubiquitous in soils and sediments, and their formation and transformation processes affect the migration and transformation of heavy metals and organic pollutants. Most previous work has been focused the oxidation of Mn(II) on Mn(IV) oxide surface, but it remains poorly understood how Mn(II) interacts with manganite under oxic conditions. Here, the interactions between manganite and dissolved Mn(II) within pH 5.0–9.0 under oxic conditions were systematically studied. The results indicated that adsorbed Mn(II) was directly oxidized to manganite on the surface, and the oxidation rate increased with increasing pH and oxygen partial pressure. Electrons are likely transferred in bulk manganite during the oxidation process of Mn(II). Although manganite induced Mn(II) oxidation and the epitaxial growth of bulk particles, there was no obvious change in mineral phase. Dissolved oxygen accelerated the growth of manganite in Mn(II) solution. Compared with the absence of manganite, the presence of manganite inhibited the formation of hausmannite from Mn(II) oxidation by oxygen in air. Furthermore, this study reveals the catalytic oxidation process and mechanism of Mn(II) on manganite surface, and improves the understanding of manganite crystal growth in supergene environments.