Ultramafic mine tailings, which contain high concentrations of Ni and Cr as well as economically valuable metals such as Co, Cu, Zn, and Fe, are highly reactive and likely to transform rapidly into hydrated magnesium carbonate (HMC) minerals (i.e. nesquehonite, dypingite, hydromagnesite) upon reaction with aqueous solutions. This process not only traps atmospheric CO2 but also concentrates critical metals limiting the dispersion of toxic elements in the environment. However, the mechanisms by which metals are incorporated into hydrated Mg carbonates, and the effects of these metals on the stability and growth/transformation kinetics of these phases have not yet been quantitatively assessed. This study investigates copper coprecipitation with HMC during brucite (Mg(OH)2) carbonation over one month at 25, 40, and 60°C, with Cu concentrations ranging from 0.02 to 157 mmol/L (1-10,000 mg/L). Microscopic and spectroscopic analyses of reaction products revealed Cu removal via co-precipitation, with 95% of aqueous Cu removed within one month, even at high concentrations. Temperature influenced HMC crystallization and Cu removal efficiency, while elevated Cu levels promoted the formation of Cu-rich phases such as hydrous carbonates (e.g., malachite and mcguinnessite) and oxy/hydroxides/chlorides. These findings provide insight into Cu immobilization mechanisms in hydrous Mg-carbonates, with implications for critical metal recovery and CO₂ sequestration as well as remediation of waste water. Future work should optimize reaction parameters and analytical techniques to enhance the scalability of these processes for environmental and industrial applications.

The study of natural analogues of CO2 mineral sequestration combined with the experimental quantification of carbonation reactions constitutes a fundamental approach to understand the spatial and structural distribution of carbonated bodies and the time scales by which large amounts of CO2 can be stored in solid form into geologic formations. To better quantify the carbonation rates of ultramafic rocks and study the evolution of dissolved Mg isotope composition during their interaction with CO2-rich fluids, a series of batch carbonation experiments using a partially serpentinized harzburgite from the Semail ophiolite (Oman) was conducted at 90-180 degrees C and at constant CO2 partial pressures (similar to 15-20 bar). The yield of the carbonation reaction increased from similar to 0 at 90 degrees C to a maximum of 31 mol % at 150 degrees C, decreasing to 12 mol % at 180 degrees C over a period of one month. Magnesites containing 3-9 wt% of Fe and silica polymorphs (SiO2(am) and chalcedony) were the main reaction products, with a fraction of secondary Mg-silicates that increased with increasing temperature, significantly reducing the carbonation extent at 180 degrees C. The aqueous fluid became progressively enriched in heavy isotopes with the progress of the carbonation reaction. The apparent Mg isotope fractionations between the rock and bulk fluid (Delta Mg-26 = delta Mg-26(solid) - delta Mg-26(fluid)) varied from -1.6 parts per thousand at 120 degrees C to -0.9 parts per thousand at 180 degrees C, consistent with the preferential uptake of Mg-24 by carbonate minerals and the decrease of isotope fractionation with increasing temperature. The average magnesite isotope compositions (-1.6 parts per thousand <= delta Mg-26 <= -0.3 parts per thousand) derived from mass-balance calculations were found to be within the range of delta Mg-26 values reported for Oman listvenites, suggesting that the carbonation processes in this geological unit took place within the temperature range considered in this study (similar to 120-180 degrees C). Comparison of experimental results with observations of a well-studied natural analogue provides new indications on the optimum conditions for the development of CO2 sequestration methods in peridotites, or ex situ carbonation techniques using ultramafic minerals. The observed evolution of the Mg isotopic composition of the fluid also suggests that the use of Mg-isotopes could be an effective tool to monitor the spatial and temporal progress of carbonation reactions during field-scale CO2 storage operations in ultramafic rocks.

Mineral carbonation has the potential to permanently sequester CO2 in stable carbonate minerals, mimicking the natural precipitation of Ca- or Mg-rich carbonates. These minerals are found in ultramafic rocks and mine wastes, which also contain metals that are crucial resources for renewable energy technologies. This study aims to better characterize the mechanisms and quantify the potential of cyanobacterium (Gloeocapsa sp.) for hydrous Mg carbonate (HMC) precipitation sequestering both CO2 and heavy metals such as Zn. The results show that Zn removal mechanisms depend on solution pH and bacterial biomass, primarily involving the co-precipitation of Zn with newly formed HMC. Under abiotic conditions, up to 100 % of the initial dissolved Zn (at 1 mg L-1) can be co-precipitated with HMC after 7 days at pH 9.5 to 11. During cyanobacterial growth, up to 87 % of the initial Zn (at 5 mg L-1) can be removed from the aqueous solution through incorporation into live cells and co-precipitation with biotically induced HMC over 8 days of growth. However, at higher concentrations (>5 mg L-1), Zn becomes toxic to cyanobacteria, inhibiting their development. Additionally, sizable fraction of aqueous Zn (up to 40 %) was removed through adsorption onto cell surfaces and incorporation in cyanobacterial biomass. Overall, this study demonstrates that cyanobacterially-induced HMC precipitation can efficiently sequester both Zn and inorganic carbon, and thus atmospheric CO2, from aqueous solutions, in the context of ultramafic waste remediation.

The zinc boron complex formation was studied as a function of temperature (25, 50 and 70 °C) in boric acid solutions of various concentration (0.25, 0.50 and 0.68 mol·kg −1 ). pH was monitored during zinc ion addition by galvanostatic dissolution of a zinc metal electrode, in a solution of boric acid. The determination of the complex formation showed the importance of an accurate model of the polyborate speciation, recalculated for this work based on the previous literature data mainly potentiometric measurements completed by Raman spectroscopy and Ab Initio calculations. Modelling of our experimental results, considering various scenarios of boric acid speciation, was performed using R and PhreeqC, suggesting the formation of an aqueous triborate-zinc (II) complex, ZnB_3O_3(OH)_4(aq)^+, according to the reaction: Zn^2++3B(OH)_3⇌ZnB_3O_3(OH)_4(aq)^++2H_2O+H^+ . The nature and structure of this aqueous complex disagrees with the results reported previously in the literature. Three formation constants of the triborate-zinc (II) complex were determined at 25, 50 and 70 °C as log_10K_ZnB = − 4.73 ± 0.10, − 4.21 ± 0.16 and − 4.94 ± 0.12, respectively. The evolution of zinc boron complex formation as a function of temperature (between 25 and 70 °C) provides information on the effect of the polyborate predominance in the solution on the complexation of zinc.

The element release rates from naturally altered basalts were measured in batch experiments at 27 degrees C, and in mixed-flow experiments at 25 degrees C in reactive fluids initially consisting of pH 3 or 4 aqueous solutions. The basalts reacted in this study consist of 1) surface weathered basalts ranging in age up to 13 million years, and 2) hydrothermally altered basalts from zeolite to actinolite alteration zones (temperatures of similar to 50 to 300 degrees C). The in situ pH in the batch experiments increased over time with the final pH of the fluids ranging from to 4 to 8. Most experiments exhibited a preferential release of Ca and Mg compared to Si from the dissolving basalts, both initially and over the long-term. This behavior is attributed to the combination of an initial rapid cation-proton exchange reaction and the relatively fast dissolution rates of Ca-bearing minerals. The BET surface area normalized Si release rates for all of the altered basalts are within one order of magnitude of each other, and from one to two orders of magnitude lower than corresponding rates from fresh basaltic glass and fresh crystalline basalts. This is interpreted to stem from 1) the fast removal of reactive basaltic glass and olivine from fresh basalts during the natural alteration of the rocks, 2) the similarity of the dissolution rates of the remaining albitic plagioclase, pyroxenes, epidote, and zeolites, and 3) the high surface areas of some less reactive secondary minerals such as clays. Overall, the results suggest that altered basalts will be effective in consuming CO2 during both natural and engineered processes, though at a somewhat slower rate than unaltered basalts.

Within a global warming trend, invasive cyanobacteria, abundant in tropical and temperate regions, can migrate northward and colonize thermokarst lakes in permafrost-affected territories. For a better understanding of the cyanobacterial proliferation mechanism in those lakes, we performed laboratory growth of typical invasive cyanobacteria, Microcystis aeruginosa, onto various organic-rich solutions representative of permafrost peatlands. Aqueous leachates of lichen, moss and peat were the most favorable substrates for massive growth. The growth in the presence of all organic substrates produced an increase in solution pH by two units and a sizable (30-50%) decrease in the concentration of dissolved organic carbon. The observed increase in the dissolved organic carbon aromaticity degree likely reflected preferential cyanobacterial uptake of aliphatic, optically transparent organic substances. Cyanobacterial growth over a bloom period can create a carbon sink (uptake of 2.5 and 8.3 g C-CO2 m-2 d-1) that can offset the net heterotrophic status of thermokarst lakes in permafrost peatlands, thus switching the lake status from a C source to a C sink. Therefore, predictions of future carbon exchanges with the atmosphere in surface waters of permafrost peatlands require explicit accounting for the possibility of invasive cyanobacterial growth.

Due to their widespread distribution in the Earth's crust, it seems likely that altered basalts could be targeted for CO2 storage via subsurface carbon mineralization. To assess the potential efficiency of this approach, the steady state release rates of major elements from a suite of altered basalts have been measured at pH 3, 120 degrees C, and far from equilibrium conditions. The altered basalt samples have chemical compositions close to that of fresh basalt, but their mineralogy varies due to their alteration either at the Earth's surface or hydrothermal conditions at temperatures up to 250 degrees C. The studied altered basalts contain variable amounts of primary plagioclase and pyroxene, and substantial secondary phases including quartz, zeolites, epidote, chlorites and clay minerals. Despite their differing mineralogy, the steady-state element release rates of all the altered basalts are similar to each other when normalized to geometric surface area. These rates, however, are one to three orders of magnitude slower than corresponding release rates of basaltic glass and fresh crystalline basalt, depending on the element and on whether the rates are normalized to initial BET or geometric surface area. If present in small amounts in the altered basalts, calcite dissolves rapidly in the acidic reactive fluids, and does not contribute to the measured steady-state calcium release rates. Taken together, the results of this study indicate that altered basalt formations can provide sufficient divalent cations for subsurface carbon mineralization. As the element release rates of these altered basalts are lower than those of basaltic glass or fresh crystalline basalt, efforts to carbonate subsurface altered basalts may be best targeted at systems having temperatures in excess of 100 degrees C to compensate for the lower reactivity of these rocks.

Carbon trapping in ultramafic (UM) and basaltic basements is one of the options explored to mitigate industrial CO2 emissions in the Earth’s atmosphere. UM rocks and basalts comprise silicates rich in divalent cations (Mg, Ca, Fe) that are dissolved to form carbonates when in contact with CO2-rich fluids, thus trapping CO2 over geological time scales. UM rocks have the highest concentrations in divalent cations and thus they have the highest potential for carbon trapping by CO2-mineralization. Nevertheless, because of their low permeability, UM basements have been overlooked for possible in situ CO2 storage in favor of basaltic basements. Recent research shows that CO2-mineralization is active and efficient in UM basements, and that it is associated to potential benefits, such as the production of H2. However, the hydrodynamic, physical and chemical mechanisms driving CO2-mineralization whilst sustaining fluid flow are still poorly understood and numerous scientific and technological challenges remain before implementing industrial CO2 geological storage in UM basements. Here we present an overview of our recent results on CO2-mineralization in UM rocks combining (i) laboratory experiments, and (ii) field studies of carbonated UM basements with a focus on the Semail ophiolite (Sultanate of Oman), in relation to the recently completed ICDP (International Continental Scientific Program) Oman Drilling Project.

Bacterially-induced sequestration of atmospheric CO2 is at the forefront of geomicrobiological research due to high potential of this process in the mitigation of climate warming. Cyanobacteria have been known to form stromatolites since the Precambrian and could be used to enhance this process by sequestering carbon via the biomineralization of Mg and Ca carbonates. Currently, olivine (MgSiO4) is considered as one of the most efficient silicate minerals suitable for CO2 capture in the form of secondary Mg carbonates. However, the role of dissolved Si on the efficiency of biomineralization is not sufficiently well understood. The present study intended to reproduce in the laboratory the processes of biomineralization by Synechococcus sp. cyanobacteria extracted from modern stromatolites in a carbonate- and Mg-bearing medium containing various Si concentrations, in order to characterize the rates and stoichiometry of reactions as well as mineralogical nature of precipitates. The results demonstrated the dominant role of cyanobacterial metabolism in the precipitation of carbonate minerals by increasing the pH of the medium via photosynthesis and providing a template in the form of cell walls and their EPS for mineral nucleation. Transmission electron microscopy and other microscopic and spectroscopic observations and analyses identified magnesium carbonates and silicates, such as nesquehonite (MgCO3·3H2O) and/or hydromagnesite (Mg5(CO3)4(OH)2·4(H2O) together with amorphous analogue of sepiolite (Mg4Si6O15(OH)2·6H2O) as dominant precipitated minerals. Apparent inorganic C precipitation rates were not affected by the concentration of Mg and Si in the initial solution. However, the carbon sequestration potential was 20–40% higher in the presence of Si. Overall, the experimental approach developed in this study allows efficient reproduction of combined Mg hydroxy‑carbonate and hydrous silicate precipitation under cyanobacterial activity and helps to constrain optimal conditions of cyanobacteria-induced CO2 sequestration.

The biomineralization of CO2 , in the form of carbonate minerals, is considered as one of the efficient solutions of atmospheric CO2 removal, allowing stable and sustainable storage of this greenhouse gas. Cyanobacteria are among the most powerful microorganisms capable of precipitating carbonate minerals, both in the present and in the past. In the modern environments, high Si concentration during geoengineering biomineralization could occur due to dissolution of Mg-bearing primary silicates such as olivine. However, most of experimental studies aimed to understand the formation of these carbonates were performed in Si-poor solutions. Thus, experimental characterizations of the nature, rate, and stoichiometry of precipitated minerals in Si-rich solutions in the presence of bacteria are lacking. The present study attempted to reproduce, in controlled laboratory experiments, the processes of biomineralization in a carbonate- and Mg-bearing medium having high Si concentrations (2-4 mM, which is below the saturation with respect to amorphous silica). These experiments have been carried out in the presence of three contrasting cyanobacteria: Synechococcus sp., Chroococcidiopsis sp. and Aphanothece clathrata in order to characterize the rate of formation, stoichiometry and mineralogical nature of precipitates. The results demonstrated significant role of cyanobacteria in the precipitation of carbonate and silicate minerals by increasing the pH of the medium during photosynthesis. Magnesium precipitation rates measured between 50 and 150 h of reaction time ranged from 0.05 to 0.5 mmol h-1 gdry1 and decreased (Synechococcus sp. and Chroococcidiopsis sp.) or increased (A. clathrata) with an increase in the Si:Mg ratio in solution. The abiotic instantaneous rates of Mg and Si removal from alkaline solutions were similar to those in the presence of cyanobacteria at the same pH value suggesting that photosynthetically induced pH rise was the main factor of mineral formation. The transmission electron microscopy (TEM) and spectroscopic observations and associated analyses identified an amorphous magnesium silicate together with hydrous Mg carbonates (hydromagnesite). The formation of carbonate solid phase at high Mg: Si ratios indicated the potential for the removal of inorganic carbon at pH > 10. The difference in the degree of C removal between different species was primarily linked to different degree of pH rise during photosynthesis. Taken together, the results obtained in this study allowed an efficient reproduction of combined magnesium hydroxo-carbonates and hydrous silicates precipitation under cyanobacterial activity, suitable for geoengineering of biologically controlled CO2 sequestration in Si-Mg-carbonate-bearing solutions.

The subsurface carbonation of basaltic rocks may be a favorable carbon storage option in a number of parts of the world. One great advantage of subsurface carbonation is that it provides safe, long-term storage with no risk of CO2 leakage back to the surface. In addition, subsurface mineral carbonation could be applied in areas where more conventional storage, such as in saline aquifers, is not possible. Examples include large flood basalt provinces, and the oceanic crust. This study is motivated to assess the carbonation potential of altered basaltic rocks, which are far more common than fresh basalts. Towards this goal, dissolution experiments were performed in batch reactors at 27 °C and element release rates were measured on a suite of altered basalts ranging from young surface basalt to basalts hydrothermally altered to the epidote facies. Our results suggest that altered basalts dissolve 0.5 to 2 orders of magnitude slower than basaltic glass and fresh crystalline basalt. Ca and Mg were preferentially released both at the beginning of the reaction and at steady state. Results suggest that altered basalts are suitable for subsurface carbonation but targeting reservoirs having temperatures of ~100 °C or greater would compensate for their slower reactivity compared to fresh basalts.

To date, one of the safest long-term CO2 storage solutions is through carbon mineralization in mafic (or ultramafic) rocks containing high proportions of Mg, Ca, and Fe, which can react with dissolved CO2 to form carbonate-bearing minerals, ensuring its stability over time. The challenges still to be faced by this approach include i) the scalability to a worldwide scenario; ii) the adaptability to local geological contexts; and iii) the standardization of its application at industrial levels. A number of experimental and theoretical studies have been carried out to face these challenges, especially in relation to high temperature scenarios, but few have developed to date a consistent experimental procedure able to determine the in situ carbon mineralization potential in low-temperature geological settings that would, if effective, enhance industrial confidence in CCS/CCUS technologies, and possibly in its future applications. Within this context, we provide an overview of the experimental studies that have been conducted over the last 20 years, with an emphasis on the ongoing research aimed at improving the knowledge of the conditions and elementary processes that control the sequestration potential of mafic and ultramafic reservoirs. The results of these studies should direct the advancement of experimental and analytical protocols and will help the development and successful application of future CCUS actions.

Carbonate mineral precipitation in the presence of cyanobacteria is at the forefront of scientific research due to its importance for understanding paleoenvironments of mineral formation and for optimizing conditions of mineralogical CO2 sequestration via biological pathway. Stromatolites are among the oldest known biological formations, and they provide insight into early Earth environments and climates. It is therefore essential to understand the processes governing their formation. Numerous field studies were carried out to characterize these bio-formed rocks and their way of formation showing that various parameters could be involved in the processes of formation of carbonate rocks. Thus, reproducing natural environments under laboratory-controlled conditions is an efficient approach to better understand the role of each parameter. The present chapter aims to present the results of various laboratory studies on the biomineralization of Ca, Ca-Mg, and Mg carbonates, via analyzing and discussing mechanisms of mineral formation; providing examples of several case studies; assessing, based on available information, the stoichiometry of inorganic carbon removal in the form of carbonate minerals and organic carbon sequestered in the form of bacterial biomass; and finally recommending future research directions in this actively developing field of science.

The isotopic composition of carbonate minerals (e.g., C, O) is widely used as a record of conditions at the time of mineral formation. These isotopic compositions may also be used to trace the efficacy of engineered CO 2 storage in carbon dioxide removal (CDR) strategies. However, the robust interpretation of the isotopic composition of minerals requires that the mechanisms of isotope fractionation are known. In addition, the isotopic composition of the mineral at the time of formation must be preserved over long timescales. Textural evidence of recrystallization can be used to indicate lack of preservation of isotopic compositions, but experiments have revealed that certain minerals may be subject to isotope exchange with their surrounding fluid without any obvious morphological changes [1,2]. We have explored the behavior of Mg, C, and O isotopes between fluids and various carbonate minerals both at chemical equilibrium and during mineral phase transformations. A series of experiments examining Mg-carbonate mineral phase transformations revealed that Mg and C isotopic compositions of the minerals were substantially altered during transformation from a less to more stable hydrated Mg-carbonate. Similarly, a series of experiments that exposed calcite to a fluid at chemical equilibrium but spiked with 13 C and 18 O revealed isotopic exchange between the fluid and solid at 25°C over 4-7 months. The experiments revealed that up to ~0.3% of the C in the solid was exchanged, though the rate and extent of isotope exchange was strongly dependent on the calcite-liquid surface area. Scanning electron microscopy revealed no significant changes in calcite morphology. The results of these experiments confirm not only that isotope exchange in calcite can occur without any visible evidence of recrystallization, but that elements can continuously exchange between fluid and solid without net dissolution or precipitation occurring. Together, these studies provide insights into mechanisms of isotope exchange between fluids and carbonates with important implications for the application of isotopic compositions of carbonate minerals to understanding past environmental conditions and tracing efficacy of CDR.

Assessment of the microbial impact on mineral dissolution is crucial for a predictive understanding of basic (Ca, Mg bearing) silicate weathering and the associated CO2 consumption, bioerosion, and CO2 storage in basaltic rocks. However, there are controversies about the mechanism of microbial effect, which ranges from inhibiting via nil to accelerating. Here we studied diopside interaction with the heterotrophic bacterium Pseudomonas reactants and the soil fungus Chaetomium brasiliense using a combination of mixed-flow and batch reactors and in situ (AFM) and ex situ (SEM) microscopy. The results provide new nano-level insights into the degree to which microorganisms modify silicate dissolution. Taking into account negligible effects of organic ligands on diopside dissolution as reported earlier, we conclude that the microbial effect on Ca-Mg silicates is weak and the acceleration of dissolution of "basic" silicate rocks in the presence of soil biota is solely due to pH decrease in porewaters.

The alkaline playas near Atlin, British Columbia, Canada are likely one of the few surface environments on Earth where contemporaneous formation of hydromagnesite and magnesite occurs at temperatures that do not exceed 15 degrees C. This environment offers a unique opportunity to examine the impact of different formation mechanisms on Mg isotope compositions of Mg-carbonate minerals at low temperature. In this study, we report the Mg isotope composition of ultramafic bedrock, Mg-carbonate sediments, and both surface and ground waters in this geological setting. The composition of hydromagnesite suggests a Rayleigh-type distillation effect on the fluid Mg isotope ratios in unsaturated sediment above the water table. Through this mechanism of formation, hydromagnesite is progressively depleted in Mg-24 obtaining delta Mg-26 values as high as +1.14 degrees% near the sediment surface. In contrast, magnesite formation is characterized by enrichment of the solid phase in Mg-24. The apparent Mg isotope fractionation factor during magnesite formation at similar to 10 degrees C ranges between -0.7 +/- 0.1%o and -1.8 +/- 0.1%o. The distinct Mg isotope composition of hydromagnesite in comparison to magnesite supports magnesite formation occurring by precipitation from the fluid, or dissolution-reprecipitation, rather than solid-phase transformation from a hydrous Mg-carbonate precursor. Overall, the results provide insights on low temperature Mg-carbonate mineral formation that has implications for long-term storage of CO2.

Despite the importance of soil and surface waters freezing in permafrost landscapes, the behaviour of dissolved organic carbon (DOC), nutrients and metals during periodic freeze-thaw cycles (FTC) remains poorly known. The on-going climate warming is likely to increase the frequency of FTC in continental aquatic settings, which could modify the chemical composition of waters. In this study, we conducted 9 repetitive cycles of overnight freezing (−20 °C) and 5 h thawing (4 °C) in the laboratory using representative 0.22 μm-filtered waters from NE European permafrost peatland: leachates of vegetation and soil, and natural surface waters (depression, thermokarst lake and river). Only minor (<5%–15%) changes of DOC concentrations, SUVA254 and molecular weight were observed in all leachates and the depression water. In contrast, several trace elements (Fe, Al, P, Mn, As, and REE) exhibited sizable variations during FTC (>10%). The leachates and the depression water were enriched in trace elements, whereas the thermokarst lake and the river demonstrated a decrease in concentration of Fe (−39 and −94%, respectively), Al (−9 and −85%), and Mn (−10 and −79%) during FTC. Overall, the observations demonstrated an increase in aliphatic low molecular weight organic matter (OM), and the precipitation of Fe, Al hydroxides and organo-mineral particles. Therefore, enhanced of frequency of FTC can favour the release of metals and toxicants from acidic OM-rich surface waters and maintain stable OM-metals-colloids in large lakes and rivers, thus regulating aquatic transport of DOC and metals from soils to the Arctic Ocean.