Precipitation Of Mg Carbonates Research Articles

Carbonates’ precipitation by halophilic bacteria as a potential biosignature for the search for life on Mars

The mechanisms of Mg-carbonate precipitation in the Earth's modern sedimentary environments has not yet been completely elucidated. However, it is known that the microbial activity is significant to facilitate or induce mineral formation. The organic EPS matrix secreted by microorganisms provide an ideal physicochemical environment to the mineral nucleation. Sediments and water samples were collected at Lagoa Vermelha, Araruama (RJ), Brazil. This lagoon is characterized of being a hypersaline lake, where we can find a range of microorganisms known as halophilic and halotolerant extremophiles that have adaptation strategies to compensate for the harmful effects of high salinity, such as EPS biosynthesis, which is one of the most common protection mechanisms in bacteria, helping to maintain the integrity of their cells’membrane. In the present project, the bacterial capability of bioprecipitation using bacterial isolates from a hypersaline lagoon was investigated by combining microbiological, microscopic and geochemical analytical techniques. The isolates were evaluated for their ability to produce bioprecipitates using this multi-technique approach that includes Scanning Electron Microscopy (SEM) with EDS, XRD and Raman spectroscopy. It was possible to characterize the carbonates formed by the bacteria isolated. Finally, these carbonates could represent a potential target for astrobiological studies of potential potential biosignatures for the search for life beyond Earth.

CO2 Mineralization Using Brine Discharged from a Seawater Desalination Plant

CO2 mineralization is a method of sequestering CO2 in the form of carbonated minerals. Brine discharged from seawater desalination is a potential source of Mg and Ca, which can precipitate CO2 as forms of their carbonate minerals. The concentration of Mg and Ca in brine are twice those in the seawater influent to desalination process. This study used a cycle for CO2 mineralization that involves an increase in the pH of the brine, followed by CO2 bubbling, and, finally, filtration. To the best of our knowledge, this is the first time that non-synthesized brine from a seawater desalination plant has been used for CO2 mineralization. The resulting precipitates were CaCO3 (calcite), Mg5(CO3)4(OH)2·4H2O (hydromagnesite), and NaCl (halite) with these materials being identified by X-ray Diffraction (XRD), Fourier transform infrared (FTIR) and thermo gravimetric-differentail thermal Analysis (TGA)-DTA. Despite the presence of Ca with Mg in brine being unfavorable for the precipitation of Mg carbonate, Mg reacted with CO2 to form hydromagnesite at a yield of 86%. Most of the Ca formed calcite, at 99% yield. This study empirically demonstrates that brine from seawater desalination plants can be used for CO2 mineralization.

The effect of divalent ions on the elasticity and pore collapse of chalk evaluated from compressional wave velocity and low-field Nuclear Magnetic Resonance (NMR)

Abstract The effects of divalent ions on the elasticity and the pore collapse of chalk were studied through rock-mechanical testing and low-field Nuclear Magnetic Resonance (NMR) measurements. Chalk samples saturated with deionized water and brines containing sodium, magnesium, calcium and sulfate ions were subjected to petrophysical experiments, rock mechanical testing and low-field NMR spectroscopy. Petrophysical characterization involving ultrasonic elastic wave velocities in unconfined conditions, porosity and permeability measurements, specific surface and carbonate content determination and backscatter electron microscopy of the materials were conducted prior to the experiments. The iso-frame model was used to predict the bulk moduli in dry and saturated conditions from the compressional modulus of water-saturated rocks. The effective stress coefficient, as introduced by Biot, was also determined from density and ultrasonic velocities measured on core plugs. Low-field NMR spectroscopy was used in addition to the mechanical testing to prove any changes observed after the saturation related to the surface-to-volume ratio of the pore space in each of the samples or to surface relaxivity. Backscatter-electron (BSE) images were recorded in order to identify the texture of the core plugs under investigation. The experimental results revealed that both elasticity and pore collapse are influenced by the presence of divalent ions in distinct ways. Compressional wave velocities indicate that saturation with water rich in magnesium and calcium ions softens the contact among the mineral grains. Pore collapse strength is deteriorating after the saturation of chalk with water rich in divalent ions. The presence of calcium and sulfate ions in the saturating fluid results in pore collapse at lower stresses than in the case when samples are saturated with deionized water or sodium chloride solution. Low field NMR spectrometry revealed precipitation of crystals in the pore space of chalk saturated with Mg-rich brine. The precipitation of Mg-carbonates was not used to explain the deteriorating pore collapse strength and effects on the elasticity after the saturation since none of the other plugs saturated with divalent ions (Ca2+ and SO 4 2 + ) experienced it.

Direct Nanoscale Observations of CO2 Sequestration during Brucite [Mg(OH)2] Dissolution

The dissolution and carbonation of brucite on (001) cleavage surfaces was investigated in a series of in situ and ex situ atomic force microscopy (AFM) experiments at varying pH (2-12), temperature (23-40 °C), aqueous NaHCO(3) concentration (10(-5)-1 M), and PCO(2) (0-1 atm). Dissolution rates increased with decreasing pH and increasing NaHCO(3) concentration. Simultaneously with dissolution of brucite, the growth of a Mg-carbonate phase (probably dypingite) was directly observed. In NaHCO(3) solutions (pH 7.2-9.3,), precipitation of Mg-carbonates was limited. Enhanced precipitation was, however, observed in acidified NaHCO(3) solutions (pH 5, DIC ≈ 25.5 mM) and in solutions that were equilibrated under a CO(2) atmosphere (pH 4, DIC ≈ 25.2 mM). Nucleation predominantly occurred in areas of high dissolution such as deep step edges suggesting that the carbonation reaction is locally diffusion-transport controlled. More extensive particle growth was also observed after ex situ experiments lasting for several hours. This AFM study contributes to an improved understanding of the mechanism of aqueous brucite carbonation at low temperature and pressure conditions and has implications for carbonation reactions in general.

Sequestration of CO2 after reaction with alkaline earth metal oxides CaO and MgO

Abstract The uptake of CO2 by CaO and MgO in aqueous solution has been studied in batch experiments at different temperatures (15–75 °C), stirring rates (300–600 rpm) and solid liquid ratios (0.0125–0.125 mol L−1) in order to identify the kinetic controls and limitations. Both minerals are constituents in many combustion residues and their carbonation is considered as an option for CO2 sequestration. Generally the uptake of CO2 by metal (hydr)oxide suspension increased with higher turbulence due to better gas/water exchange and higher temperature due to enhanced rates of mineral dissolution and carbonate precipitation. The reaction of CaO and MgO with CO2 in solution occurred at distinctly different pH values. While in the CaO systems a pH between 12.8 (0.1 mol L−1) and 11.7 (0.0125 mol L−1) was established, suspensions with MgO only reached maximum pH values of ∼10.3 even though the mineral surface area was larger in these assays (140 m2 L−1 for MgO compared to 7.5 m2 L−1 for CaO). This difference in reaction pH affected the transfer of CO2 from the gas into the liquid phase as well as dissolution rates of the minerals themselves. Maximum CO2 uptake rates were 0.027 mmol L−1 s−1 for the CaO containing experiments at pH 12.8. Dissolution of CaO was fast and calcite precipitation occurred instantaneously at all tested temperatures and concentrations. Thus, the CO2 dissolution step was limiting the overall carbonation reaction. Under otherwise similar conditions maximum CO2 uptake rates were lower for the MgO system. Magnesium carbonate precipitation was slower than calcite precipitation and depended strongly on the different boundary conditions, in particular temperature and the degree of supersaturation. Temperatures greater than 50 °C were required for assays with low dissolved Mg2+ and CO 3 2 - concentrations in order to precipitate Mg carbonate in the form of hydromagnesite (Mg5(CO3)4(OH)2·4H2O). Below 50 °C the use of high concentrations of Mg and C species were necessary to initialise precipitation of Mg carbonate as nesquehonite (MgCO3·3H2O). This has implications for the application of mineral carbonation in an aqueous system as a means of C capture and storage. Calcium-rich materials can be utilized efficiently and almost independently of reaction regime if an optimum supply of CO2 is provided. In contrast specific conditions are required to use the CO2 binding potential of alkaline Mg minerals, as T > 50 °C or high concentrations are required for Mg carbonate precipitation.