Magnesium Carbonate Precipitation Research Articles

CO2 Absorption and Magnesium Carbonate Precipitation in MgCl2–NH3–NH4Cl Solutions: Implications for Carbon Capture and Storage

CO2 absorption and carbonate precipitation are the two core processes controlling the reaction rate and path of CO2 mineral sequestration. Whereas previous studies have focused on testing reactive crystallization and precipitation kinetics, much less attention has been paid to absorption, the key process determining the removal efficiency of CO2. In this study, adopting a novel wetted wall column reactor, we systematically explore the rates and mechanisms of carbon transformation from CO2 gas to carbonates in MgCl2–NH3–NH4Cl solutions. We find that reactive diffusion in liquid film of the wetted wall column is the rate-limiting step of CO2 absorption when proceeding chiefly through interactions between CO2(aq) and NH3(aq). We further quantified the reaction kinetic constant of the CO2–NH3 reaction. Our results indicate that higher initial concentration of NH4Cl ( ≥ 2 mol · L − 1 ) leads to the precipitation of roguinite [ ( NH 4 ) 2 Mg ( CO 3 ) 2 · 4 H 2 O ], while nesquehonite appears to be the dominant Mg-carbonate without NH4Cl addition. We also noticed dypingite formation via phase transformation in hot water. This study provides new insight into the reaction kinetics of CO2 mineral carbonation that indicates the potential of this technique for future application to industrial-scale CO2 sequestration.

Effect of permanent magnetic field on scale inhibition property of circulating water.

Effect of a permanent magnet field on the scale inhibition property of circulating water was investigated. Orthogonal experiments of L16(45) were performed and analyzed using the range analysis method. The operating parameters included magnetic field intensity, initial concentration of Ca2+ and Mg2+, magnetic treatment time, temperature, and flow velocity. Scale inhibition rate, hardness, relative variation in the proportion of free water molecules, electrical conductivity, and relative variation of molecular energy were chosen as the objectives. In addition, the morphology and the composition of CaCO3 and MgCO3 scale were studied by X-ray diffraction analysis. The optimal conditions were initial concentration of 900 mg/L, magnetic field intensity of 0.5 T, temperature of 303 K, time of 54 h and flow velocity of 0.17 m/s. The nuclear magnetic resonance results demonstrated that the number of hydrogen bonds increased between water molecules and hydrated ions. The magnetic field can promote the increase in the number of hydrogen bonds, which can inhibit the formation of calcium and magnesium carbonate precipitation. Moreover, the ratio of calcite, aragonite and vaterite will be changed at different magnetic field intensities, and the aragonite ratio will reach the peak at the optimum conditions.

Formation of Carbonate Nanoglobules by a Mixed Natural Culture under Hypersaline Conditions

The present study demonstrated formation of Ca and P rich nanoglobules by a mixed natural halophilic population enriched from hypersaline lake sediments in laboratory culture experiments. Nanoglobules consisting of complex mixture of Ca, P, O, and C with minor amount of Mg occurred in the external envelop of bacterial cell in the first week of incubation at various Mg+2/Ca+2 ratios and salinity at 30 °C. Unlike the control experiments (e.g., non-viable cells and without cells), later aggregation and transformation of nanoglobules caused the precipitation of calcium and/or magnesium carbonates in variable amount depending on the Mg+2/Ca+2 ratios of the medium after 37 days of incubation. By showing the nucleation of carbonates on bacterial nanoglobules closely associated with the cell surfaces of mixed natural population this study emphasis that formation of nanoglobules may not be specific to a microbial strain or to activity of a particular microbial group. Formation of carbonate nanoglobules under various conditions (e.g., Mg+2/Ca+2 ratios, salinity) with the same halophilic culture suggest that the although metabolic activity of bacteria have an influence on formation of nanoglobules the mineralogy of nanoglobules may be controlled by the physicochemical conditions of the precipitation solution and the rate of mineral precipitation.

Experimental study on the precipitation of magnesite from thermally activated serpentine for CO2 sequestration

Mineral carbonation has the potential to store billions of tonnes of CO2 safely and permanently. Enhancement of the kinetics of the formation of magnesium carbonate from magnesium-bearing silicate minerals has been the subject of numerous research studies. However, significant progress is yet to be achieved. This is, in part, due to a lack of understanding of the mechanism of the formation of magnesite in the presence of additives and under mineral carbonation conditions. In this work, an in-depth study was performed to investigate the precipitation of magnesium carbonate during single step high pressure, high temperature carbonation of thermally activated serpentine in an aqueous bicarbonate solution. Slurry samples were obtained throughout the duration of the carbonation experiments, enabling analysis of both the aqueous and solid compositions over time, providing insight into the reaction mechanism. Additionally, the effect of operating temperature on the formation of various magnesium carbonate species was examined. TGA-MS, in combination with XRD and SEM, confirmed the formation of hydromagnesite in the absence of carbon dioxide (CO2) during the reactor heat up period, owing to a reaction with the sodium bicarbonate (NaHCO3) carrier solution. Hydromagnesite was transformed to magnesite over time, with the rate of this phase transformation highly dependent on the reaction temperature. At 185°C all hydromagnesite converted to magnesite in a few minutes whereas at 120°C even after 90min hydromagnesite remained in the reactor. PHREEQC thermodynamic software was used to assess the observed formation of carbonate species. The model prediction was consistent with the experimental results obtained in this work.

Mathematical model of precipitation of magnesium carbonate with carbon dioxide from the magnesium hydroxide slurry

A mathematical model is proposed for a precipitation process of magnesium carbonate in a heterogeneous stirred tank reactor. The model includes a description of dissolution of Mg(OH)2, absorption of CO2 and precipitation of MgCO3. The Nernst-Planck equation is used in the dissolution model to maintain the mass balance and electroneutrality. The van Krevelen–Hoftijzer expression is introduced to describe the enhancement effect of reaction between dissolved CO2 and OH− on the mass transfer rate of dissolution and absorption. In the precipitation model, a simplified population balance equation is solved by a moment method for both dissolving and precipitating particles. Unknown precipitation kinetics parameters for Mg(OH)2MgCO3 system are fitted against experimental data and compared with Ca(OH)2CaCO3 system. According to the present analysis, the liquid−solid and gas−liquid mass transfer dominate the global rate of precipitation. The precipitation kinetics and pH have strong influences on the concentration of reactants and the yield of precipitation.

Production of magnesium-rich solutions by acid leaching of chrysotile: A precursor to field-scale deployment of microbially enabled carbonate mineral precipitation

Carbonate minerals are one of the primary carbon sinks under investigation for sequestering anthropogenic carbon dioxide (CO2). Ultramafic rock has the potential to act as a magnesium source for carbonate precipitation reactions utilizing atmospheric CO2. This study characterizes the release of magnesium from chrysotile tailings from the Clinton Creek Asbestos Mine (Yukon, Canada) by means of a 42-day leaching experiment using sulfuric acid. Low acid concentrations (targeting 8.33% and 16.67% dissolution of chrysotile) resulted in stoichiometric, dissolution. Moderate (33.33% and 66.67% dissolution) and high (100% dissolution) acid concentrations resulted in non-stoichiometric dissolution of chrysotile. Here, the concentration ratio of Mg:Si in solution was greater than expected for moderate acid concentrations and lower than expected for high acid concentrations. The moderate acid reaction system demonstrated that as much as 84% of the magnesium found in chrysotile can be released into solution via chemical weathering. However, at high acid concentrations, Mg values were in fact lower than the moderate acid reactivity, presumably resulting from adsorption of soluble magnesium to amorphous silica, a byproduct of extensive magnesium leaching from chrysotile fibers. The production of magnesium-rich solutions in this experiment demonstrates that a highly concentrated starting material for magnesium carbonate precipitation reactions can be produced from chrysotile-bearing tailings, providing ultra-mafic hosted mines with an important resource for developing technological strategies for reducing their net carbon emissions. This process represents a win-win scenario for the management of chrysotile-bearing mine wastes, in which a hazardous material is transformed into a geologically stable mineral sink for ‘carbon dioxide storage’.

Efecto agudo del baño con citrato sobre la alcalemia postdiálisis

Introduction: Correcting metabolic acidosis provoked by renal failure is achieved by supplying bicarbonate during dialysis. To prevent the precipitation of calcium and magnesium carbonate produced in the dialysis fluid (DF) when bicarbonate is added, it is necessary to add an acid (normally acetate), which involves secondary effects. Consequently, citrate is presented as an advantageous alternative to acetate, although its acute effects are not known with precision.Objective: Our objective was to assess the acute effect of using a DF with citrate instead of acetate on the parameters of acid-base balance and of phosphorus-calcium metabolism.Material and methods: We carried out a prospective, cross-over study on 24 patients (15 males and 9 females). All the patients were dialysed using an AK 200 ULTRA-S monitor with SoftPac® DF, prepared with 3 mmol/l of acetate, and with SelectBag Citrate®, with 1 mmol/l of acetate-free citrate. Before and after dialysis we extracted: venous blood gases, calcium (Ca), ionized calcium (Cai), phosphorus (P) and parathyroid hormone (PTH).Results: We found differences (P<.05) when we used a dialysate with citrate (C) compared with using acetate (A) in the post-dialysis values of pH (C: 7.43 [0.04] vs A: 7.47 [0.05]), bicarbonate (C: 24.7 [2.7] vs A: 27.3 [2.1] mmol/L), base excess of extracellular fluid (BEecf) (C: 0.4 [3.1] vs A: 3.7 [2.4] mmol/L), corrected calcium (cCa) (C: 9.8 [0.8] vs A: 10.1 [0.7] mg/dl) and Cai (C: 1.16 [0.05] vs A: 1.27 [0.06] mmol/L). We found no differences in any of the parameters measured before dialysis.Conclusion: Dialysis with citrate achieves better post-dialysis acid-base balance, lowering/avoiding post-dialysis alkalemia and producing a lower increase in corrected calcium (Cac) and Cai. This finding is of special interest for patients with predisposing factors to arrhythmia and patients with respiratory failure, carbon dioxide retention, calcifications or advanced hepatopathy.

Acute effect of citrate bath on postdialysis alkalaemia

IntroductionThe correction of metabolic acidosis caused by renal failure is achieved by adding bicarbonate during dialysis. In order to avoid the precipitation of calcium carbonate and magnesium carbonate that takes place in the dialysis fluid (DF) when adding bicarbonate, it is necessary to add an acid, usually acetate, which is not free of side effects. Thus, citrate appears as an advantageous alternative to acetate, despite the fact that its acute effects are not accurately known. ObjectiveTo assess the acute effect of a dialysis fluid containing citrate instead of acetate on acid-base balance and calcium-phosphorus metabolism parameters. Material and methodsA prospective crossover study was conducted with twenty-four patients (15 male subjects and 9 female subjects). All patients underwent dialysis with AK-200-Ultra-S monitor with SoftPac® dialysis fluid, made with 3 mmol/L of acetate and SelectBag Citrate®, with 1 mmol/L of citrate and free of acetate. The following were measured before and after dialysis: venous blood gas monitoring, calcium (Ca), ionic calcium (Cai), phosphorus (P) and parathyroid hormone (PTH). ResultsDifferences (p<0.05) were found when using the citrate bath (C) compared to acetate (A) in the postdialysis values of: pH, C: 7.43 (0.04) vs. A: 7.47 (0.05); bicarbonate, C: 24.7 (2.7) vs. A: 27.3 (2.1) mmol/L; base excess (BEecf), C: 0.4 (3.1) vs. A: 3.7 (2.4) mmol/L; corrected calcium (Cac), C: 9.8 (0.8) vs. A: 10.1 (0.7) mg/dL; and Cai, C: 1.16 (0.05) vs. A: 1.27 (0.06) mmol/L. No differences were found in either of the parameters measured before dialysis. ConclusionDialysis with citrate provides better control of postdialysis acid-base balance, decreases/avoids postdialysis alkalaemia, and lowers the increase in Cac and Cai. This finding is of special interest in patients with predisposing factors for arrhythmia and patients with respiratory failure, carbon dioxide retention, calcifications and advanced liver disease.

Determining pH at elevated pressure and temperature using in situ ¹³C NMR.

We have developed an approach for determining pH at elevated pressures and temperatures by using (13)C NMR measurements of inorganic carbon species together with a geochemical equilibrium model. The approach can determine in situ pH with precision better than 0.1 pH units at pressures, temperatures, and ionic strengths typical of geologic carbon sequestration systems. A custom-built high pressure NMR probe was used to collect (13)C NMR spectra of (13)C-labeled CO2 reactions with NaOH solutions and Mg(OH)2 suspensions at pressures up to 107 bar and temperatures of 80 °C. The quantitative nature of NMR spectroscopy allows the concentration ratio [CO2]/[HCO3(-)] to be experimentally determined. This ratio is then used with equilibrium constants calculated for the specific pressure and temperature conditions and appropriate activity coefficients for the solutes to calculate the in situ pH. The experimentally determined [CO2]/[HCO3(-)] ratios agree well with the predicted values for experiments performed with three different concentrations of NaOH and equilibration with multiple pressures of CO2. The approach was then applied to experiments with Mg(OH)2 slurries in which the change in pH could track the dissolution of CO2 into solution, rapid initial Mg(OH)2 dissolution, and onset of magnesium carbonate precipitation.

Cementation of loose sand particles based on bio-cement

Loose sand particles could be cemented to sandstone by bio-cement (microbial induced magnesium carbonate). The bio-sandstone was firstly prepared, and then the compressive strength and the porosity of the sandstone cemented by microbial induced magnesium carbonate were tested to characterize the cementation effectiveness. In addition, the formed mineral composition and the microstructure of bio-sandstone were analyzed by X-ray diffraction (XRD) and scanning electron microscopy (SEM), respectively. The experimental results show that the feasibility of binding loose sand particles using microbial induced magnesium carbonate precipitation is available and the acquired compressive strength of bio-sandstone can be excellent at certain ages. Moreover, the compressive strength and the porosity could be improved with the increase of microbial induced magnesium carbonate content. XRD results indicate that the morphology of magnesium carbonate induced by microbe appears as needles and SEM results show that the cementation of loose sand particles to sandstone mainly relies on the microbial induced formation of magnesium carbonate precipitation around individual particles and at particle-particle contacts.

Impacts of diffusive transport on carbonate mineral formation from magnesium silicate-CO2-water reactions.

Reactions of CO2 with magnesium silicate minerals to precipitate magnesium carbonates can result in stable carbon sequestration. This process can be employed in ex situ reactors or during geologic carbon sequestration in magnesium-rich formations. The reaction of aqueous CO2 with the magnesium silicate mineral forsterite was studied in systems with transport controlled by diffusion. The approach integrated bench-scale experiments, an in situ spectroscopic technique, and reactive transport modeling. Experiments were performed using a tube packed with forsterite and open at one end to a CO2-rich solution. The location and amounts of carbonate minerals that formed were determined by postexperiment characterization of the solids. Complementing this ex situ characterization, (13)C NMR spectroscopy tracked the inorganic carbon transport and speciation in situ. The data were compared with the output of reactive transport simulations that accounted for diffusive transport processes, aqueous speciation, and the forsterite dissolution rate. All three approaches found that the onset of magnesium carbonate precipitation was spatially localized about 1 cm from the opening of the forsterite bed. Magnesite was the dominant reaction product. Geochemical gradients that developed in the diffusion-limited zones led to locally supersaturated conditions at specific locations even while the volume-averaged properties of the system remained undersaturated.

Investigation of the Different Carbonate Phases and Their Formation Kinetics during Mg(OH)2 Slurry Carbonation

Carbon mineralization is recognized as one of the safest and most permanent methods for storing anthropogenic CO2. Combining CO2 capture and storage eliminates energy-intensive sorbent regeneration and uncertainties associated with geological sequestration of CO2. Carbonation of Mg(OH)2 in a gas–solid system has been investigated, but both the reaction kinetics and the overall conversion were limited. Derived from silicate minerals or industrial wastes, a Mg(OH)2 slurry can react with dissolved CO2 to form a variety of magnesium carbonate phases with differing amounts of hydroxide and crystallized H2O incorporated within the carbonate crystal. In this study, slurry-phase Mg(OH)2 carbonation was investigated at moderate temperatures and CO2 pressures (up to 200 °C and 15 atm). Mg(OH)2 carbonation in the slurry phase involved heterogeneous dissolution of Mg(OH)2 and CO2, in addition to homogeneous precipitation of magnesium carbonates. The reaction conditions responsible for hydrated and anhydrous carbonate...

Directed precipitation of hydrated and anhydrous magnesium carbonates for carbon storage.

Magnesite is the most desirable phase within the magnesium carbonate family for carbon storage for a number of reasons: magnesium efficiency, omission of additional crystal waters and thermodynamic stability. For large-scale carbonation to be a viable industrial process, magnesite precipitation must be made to occur rapidly and reliably. Unfortunately, the formation of metastable hydrated magnesium carbonate phases (e.g. MgCO3·3H2O and Mg5(CO3)4(OH)2·4H2O) interferes with the production of anhydrous magnesite under a variety of reaction conditions because magnesite crystals are slower to both nucleate and grow compared to the hydrated carbonate phases. Furthermore, the reaction conditions required for the formation of each magnesium carbonate phases have not been well understood with conflicting literature data. In this study, the effects of both magnesite (MgCO3) and inert (Al2O3) seed particles on the precipitation of magnesium carbonates from a Mg(OH)2 slurry were explored. It was interesting that MgCO3 seeding was shown to accelerate anhydrous magnesite growth at temperatures (80-150 °C), where it would normally not form in short time scale. Since the specific surface areas of MgCO3 and Al2O3 seeding particles were similar, this phenomenon was due to the difference in the surface chemistry of two seeding particles. By providing a template with similar chemistry for the growth of magnesite, the precipitation of anhydrous magnesite was demonstrated. The effect of temperature on seeded carbonation was also investigated. A comparison with published MgCO3 precipitation rate laws indicated that the precipitation of magnesite was limited by either CO2 adsorption from the gas phase or the dissolution rate of Mg(OH)2.

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.

Mechanistic Study of Magnesium Carbonate Semibatch Reactive Crystallization with Magnesium Hydroxide and CO2

This work investigates semibatch precipitation of magnesium carbonate at ambient temperature and pressure using Mg(OH)2 and CO2 as starting materials. A thermal analysis method was developed that reflects the dissolution rate of Mg(OH)2 and the formation of magnesium carbonate. The method was successfully used to determine the composition of solids precipitated from the reaction. Concentration profiles of Mg2+ and total carbon over time were determined from the liquid phase. The influence of CO2 flow rate and stirring rate on precipitation was analyzed based on comprehensive information from the liquid and solid phases. A stirring rate of 650 rpm was found to be the optimum speed as the flow rate of CO2 was 1 L/min. Precipitation rate increased with gas flow rate, which indicates that mass transfer of CO2 plays a critical role in this precipitation case. Magnesium carbonate trihydrate (nesquehonite) was formed as a bunch of needle-like primary crystals.

Mass Transfer and Kinetics Study of Heterogeneous Semi‐Batch Precipitation of Magnesium Carbonate

AbstractPrecipitation kinetics and mass transfer of magnesium carbonate (MgCO3) hydrates from a reaction of magnesium hydroxide (Mg(OH)2) and CO2 were analyzed. The effect of CO2 flow rate and mixing intensity on precipitation was investigated under ambient temperature and atmospheric pressure. Raman spectroscopy was used to determine the composition of the solids during semi‐batch crystallization. The obtained spectra revealed the dissolution of Mg(OH)2 and the formation of MgCO3. The precipitation rate increased with higher gas flow rate. The rotation speed of the stirrer had a significant effect on the dissolution of Mg(OH)2. In the researched system, the main driver of the precipitation kinetics was the mass transfer of CO2. Nesquehonite (MgCO3·3H2O), as needle‐like crystals, was precipitated as the main product. Raman spectroscopy can serve as a potential tool to monitor the carbonation precipitation process.

Process optimization for mineral carbonation in aqueous phase

Carbon dioxide sequestration by a pH-swing carbonation process was considered in this work. A multi-step aqueous process is described for the fractional precipitation of magnesium carbonate and other minerals in an aqueous system at room temperature and atmospheric pressure. With the aim to achieve higher purity and deliver more valuable mineral products, the process was split into four steps. The first step consists of Mg leaching from the magnesium silicate in a stirred vessel using 1M HCl at 80°C, followed by a three step precipitation in reactors in sequence to remove Fe(OH)3, then Fe(OH)2 and other divalent ions, and finally MgCO3 nucleation and growth. Hydrated magnesium carbonate [MgCO3∙3H2O, nesquehonite] crystals were confirmed using X-ray diffraction (XRD) as final products. The optimal pH of precipitation reactors based on the maximum solid purity and production was determined by carrying out detailed mass balance. The maximum productivity and highest purity for nesquehonite were found to be dependent on pH values for the two last steps. The results also demonstrated that the process is optimized at pH9 and 10 for the second and third step of precipitation, respectively. The highest carbonation efficiency expressed as the conversion of Mg ions to magnesium carbonate reached 82.5 wt %. The maximum magnesium content in the final product was 99.21 wt % of MgO when the second precipitation reactor pH was equal to 9. This experimental study demonstrates that carbon dioxide sequestration requires at least 3.74 times the weight of ore to provide the Mg for mineral production. This confirms the possibility to use this process route for CO2 mitigation.

Effect of cyanobacteria Synechococcus PCC 7942 on carbonation kinetics of olivine at 20 °C

By accelerating the naturally-occurring carbonation of magnesian silicates, it would be possible to sequester some of the anthropogenic excess of CO2 in more geologically-stable solid magnesium carbonates. Reaction rates can be accelerated by decreasing the particle size, raising the reaction temperature, increasing the pressure, using a catalyst, and hypothetically, by bacterial addition. We aimed here at assessing quantitatively the added value of photosynthetic microbial activity on the efficiency of Mg-silicates carbonation processes. Synechococcus PCC 7942 (freshwater cyanobacteria) was selected for this study. Two magnesian silicate minerals (substrates) were chosen: a synthetic forsterite with nanometer-sized grains and an industrial ultramafic slag (scoria). All tests were performed at 20±1°C in closed and sterile 1L Schott® glass bottle reactors. With the aim to elucidate the interaction between mineral phases and bacteria, we used pH and concentration measurements, scanning and transmission electron microscopy along with Raman spectroscopy. The results show that, at ambient temperature, cyanobacteria Synechococcus can accelerate silicate dissolution (i.e. Mg2+ release) and then magnesium carbonate nucleation and precipitation by adsorption on the produced exopolymeric substances and local pH increase during photosynthesis, respectively.

Experimental Study of Organic Ligand Transport in Supercritical CO2 Fluids and Impacts to Silicate Reactivity

The interactions between water-saturated supercritical carbon dioxide, organics, and minerals are relatively unknown despite being important to carbon sequestration and enhanced hydrocarbon recovery activities. The goals of this study include verification of organic transport through scCO2 and exploration of whether organic ligands can impact carbonate formation. An in situ near-infrared spectroscopic technique was used to probe supercritical CO2 (scCO2)-organic mixtures at 35 °C and 100 bar. Observation of C-H bands in the spectra collected from scCO2 equilibrated with a Ca-acetate solution (1.7 m) provided direct evidence of organic partitioning into the scCO2 phase. A series of high pressure X-ray diffraction experiments at 50 °C and 90 bar were performed to investigate how citrate (0.01-0.5 m) affected the coupled dissolution of forsterite and precipitation of magnesium carbonates in scCO2. In control experiments where no citrate was present, nesquehonite (MgCO3·3H2O) was initially produced as a metastable intermediate that was then converted to magnesite (MgCO3). However, experiments with citrate promoted magnesite, rather than nesquehonite, precipitation, and at the highest concentrations reduced the extent of the carbonation reaction. This paper discusses these unique findings on organic and scCO2 interactions that are not currently being considered in the fate and transport of CO2 in the subsurface.

Biologically enhanced degassing and precipitation of magnesium carbonates derived from bicarbonate solutions

This contribution reports the results of batch and semibatch experiments involving bubbling of nitrogen in aqueous solutions of magnesium bicarbonate, with and without the addition of either carbonic anhydrase (CA) or Scenedesmus alga to the solution. Precipitation of nesquehonite occurred during both an accelerated degassing of CO2 induced by sparging small nitrogen bubbles (representative diameter of 20μm), and during slow degassing engendered by introducing large nitrogen bubbles (representative diameter of 5mm). The response of the system during low rates of degassing closely approached quasi-thermodynamic predictions, which permitted an estimation of the level of supersaturation of nesquehonite, prior to the onset of precipitation. Small bubbles and CA significantly increased rates of degassing and indirectly the production of nesquehonite, as the rate of degassing can limit the precipitation process. The response of the system during rapid rates of degassing, prior to precipitation, was not entirely consistent with quasi-thermodynamic predictions. During precipitation, higher rates of degassing produced similar alkalisation and precipitation trends to that observed for lower rates of degassing. Our results agree with the formation of travertine deposits in nature, where the degassing of solutions enriched with inorganic carbon, and enhanced alkalisation by microorganisms, have been shown to influence carbonate formation. The results demonstrate a catalytic effect of CA on the rate limiting carbonate reactions, increasing CO2 exchange between nitrogen and water, and indirectly accelerating the precipitation of carbonates for a system controlled by rate of degassing. The results of this study have applications to large-scale storage of CO2 by mineralisation.