Precipitated CaCO3 Research Articles

Mechanical Performance and Environmental Effect of Coal Fly Ash on MICP-Induced Soil Improvement

Coal fly ash (FA) is one of the main byproducts of coal burning. Nearly half of FA cannot be reused or recycled. The potential environmental leaching of trace elements in FA may limit its application. Microbially induced carbonate precipitation (MICP) is a promising technology to improve soil properties. This study was to investigate the effect of fly ash in MICP-stabilized soil on its mechanical and environmental impacts. Two kinds of fly ash were considered: FA1 was Class-F fly ash, FA2 was off-specific fly ash. The 0% (sand only), 3%, 6% and 9% content of FA were introduced to FA-sand mixtures to perform MICP process. Triaxial compression test was performed to evaluate the effect of FA content on the development of strength. The triaxial test results indicated that with 3% addition of FA, the peak deviator stress increased significantly. When MICP-treated sand mixed with 3% FA1, the deviator stress increased to 1,959 kPa compared to that of MICP-treated sand only samples of 800 kPa. The peak deviator stress increased by 154% and 115% when the additions of FA1 were 6% and 9%, respectively. The stress increase was caused by the bonding of precipitated CaCO3 in MICP However, higher content of FA1 (9% or higher) could restrict the activity of bacteria by reducing the void spaces. MICP-treated samples with the addition of FA2 presented a better enhancement in peak stress for its higher CaO content which could lead to additional cementation besides MICP. Leaching tests by toxicity characteristic leaching procedure (TCLP) and sequential extraction tests indicated that there was no potential risk to introduce fly ash into the MICP process during the soil improvement. MICP process resulted to the fraction change of trace metals which could make trace metal more stable. Microscale images at scale of 10 µm, 100 µm and 200 µm have clearly presented the precipitated CaCO3. It showed that large amount of precipitated CaCO3 coated the particle surfaces and filled the void spaces. Small particles were buried and formed aggregates. There was a highly cemented phases produced between soil particle matrix. XRD analysis also confirmed the presence of CaCO3 crystal after the MICP process.

Efficacy of Enzymatically Induced Calcium Carbonate Precipitation in the Retention of Heavy Metal Ions

This study evaluated the efficacy of enzyme induced calcite precipitation (EICP) in restricting the mobility of heavy metals in soils. EICP is an environmentally friendly method that has wide ranging applications in the sustainable development of civil infrastructure. The study examined the desorption of three heavy metals from treated and untreated soils using ethylene diamine tetra-acetic acid (EDTA) and citric acid (C6H8O7) extractants under harsh conditions. Two natural soils spiked with cadmium (Cd), nickel (Ni), and lead (Pb) were studied in this research. The soils were treated with three types of enzyme solutions (ESs) to achieve EICP. A combination of urea of one molarity (M), 0.67 M calcium chloride, and urease enzyme (3 g/L) was mixed in deionized (DI) water to prepare enzyme solution 1 (ES1); non-fat milk powder (4 g/L) was added to ES1 to prepare enzyme solution 2 (ES2); and 0.37 M urea, 0.25 M calcium chloride, 0.85 g/L urease enzyme, and 4 g/L non-fat milk powder were mixed in DI water to prepare enzyme solution 3 (ES3). Ni, Cd, and Pb were added with load ratios of 50 and 100 mg/kg to both untreated and treated soils to study the effect of EICP on desorption rates of the heavy metals from soil. Desorption studies were performed after a curing period of 40 days. The curing period started after the soil samples were spiked with heavy metals. Soils treated with ESs were spiked with heavy metals after a curing period of 21 days and then further cured for 40 days. The amount of CaCO3 precipitated in the soil by the ESs was quantified using a gravimetric acid digestion test, which related the desorption of heavy metals to the amount of precipitated CaCO3. The order of desorption was as follows: Cd > Ni > Pb. It was observed that the average maximum removal efficiency of the untreated soil samples (irrespective of the load ratio and contaminants) was approximately 48% when extracted by EDTA and 46% when extracted by citric acid. The soil samples treated with ES2 exhibited average maximum removal efficiencies of 19% and 10% when extracted by EDTA and citric acid, respectively. It was observed that ES2 precipitated a maximum amount of calcium carbonate (CaCO3) when compared to ES1 and ES3 and retained the maximum amount of heavy metals in the soil by forming a CaCO3 shield on the heavy metals, thus decreasing their mobility. An approximate improvement of 30% in the retention of heavy metal ions was observed in soils treated with ESs when compared to untreated soil samples. Therefore, the study suggests that ESs can be an effective alternative in the remediation of soils contaminated with heavy metal ions.

Surface modification of steel fibers using chemical solutions and their pullout behaviors from ultra-high-performance concrete

To improve the pullout resistance of conventional straight steel fiber from ultra-high-performance concrete (UHPC), its surface was modified using various chemical solutions. Calcium carbonate (CaCO3) particles were precipitated on the surfaces of the steel fiber in an aqueous solution. The morphology and particle size of CaCO3 were controlled by ethylenediaminetetraacetic acid (EDTA) and investigated via scanning electron microscopy; further, its chemical composition was verified through energy-dispersive X-ray spectroscopy. The test results indicate that longer duration of immersion in a solution of CaCl2 and Na2CO3 yield larger CaCO3 particles on the surface owing to Ostwald ripening. The addition of EDTA changes the morphology of the CaCO3 particle and decreases its grain size from 16.2 to 3.1 μm. In order to investigate the effects of the precipitated CaCO3 particles on the fiber pullout resistance, single fiber pullout test was carried out; the modified steel fibers were embedded in UHPC matrix. The CaCO3 particles precipitated on the fiber surface improved the pullout resistance of the steel fiber from the UHPC. The smaller CaCO3 particles were more effective for enhancing the pullout resistance. The average bond stress and pullout work were increased by up to 15% and 37%, respectively, owing to the precipitated CaCO3 particles. The EDTA solution could peel off the surface of steel fiber regularly and render it rough; thus, it greatly increased the pullout resistance (more than 80%) than the plain fiber.

Influence of synthesis conditions on the calcium carbonate microparticle properties obtained by homogeneous and heterogeneous precipitation

CaCO3 microparticles were obtained by chemical precipitation from CaCl2 solution using Na2CO3 or (NH4)2CO3 solutions (heterogeneous precipitation) or carbamide (homogeneous precipitation) as precipitation agents. The effects of various experimental conditions, such as the temperature, duration and reagent ratio on the properties of precipitated CaCO3 were experimentally investigated. It was found that the shape of particles and CaCO3 polymorphs depended on the precipitation agent and process temperature; the size of particles—on the reagent ratio, the precipitation agent and the deposition time. Biphasic calcite-vaterite spherical microparticles with the highest polydispersity coefficient (K = 0.98 with an average particle size ln ≈ 3 μm) were obtained using Na2CO3 solution as a precipitation agent under low temperature (20 °C). Calcite rhombohedral crystals (ln ≈ 3 μm) were obtained using carbamide at high temperature (100 °C). Increasing the temperature of Na2CO3 and (NH4)2CO3 solutions (70–100 °C) led to formation of the biphasic aragonite–calcite druses of needle crystals (druses size ≈ 10–30 μm). Increasing the aging time for sediments in the mother liquor up to 1 h did not affect the shape of CaCO3 particles.

Structural analysis of CaCO3 nanoparticle/pulp fiber composites by tube flow fractionation

Two slurries that consisted of precipitated calcium carbonate (CaCO3) nanoparticles and unbeaten hardwood bleached kraft pulp (HBKP) were prepared via ultrafine bubble and mixing methods. In the ultrafine bubble method, a CO2 gas flow was bubbled into a HBKP slurry that contained Ca(OH)2 to prepare a precipitated CaCO3/HBKP composite slurry. The HBKP/water and precipitated CaCO3/water slurries were prepared separately and mixed to prepare a precipitated CaCO3/HBKP mixture slurry. Each of the two CaCO3/HBKP slurries was separated into five fractions using a tube flow fractionation (TFF) system. The first and second fractions consisted of long HBKP fibers and fiber aggregates. The third fraction had the highest mass ratio of the components in the five fractions, and it had approximately 1-mm-long HBKP fibers. The fourth fraction contained primarily HBKP short fibers and CaCO3 aggregates. Thus, the CaCO3/HBKP components in the slurry were separated adequately by TFF, depending on their hydrodynamic sizes. The average width of each fraction in the CaCO3/HBKP composite slurry was always larger than that of the corresponding fraction from the CaCO3/HBKP mixture slurry, which indicated that precipitated CaCO3 nanoparticles and their aggregates attached stably to long and short pulp fiber surfaces in the composite slurry.

Characteristics of CaCO3 nucleated around cyanobacteria: Implications for calcification process

To evaluate the influences of cyanobacterial extracellular polymeric substances (EPS) on CaCO3 precipitation, calcification experiments were conducted for 48 h (initial values of calcite saturation state and equilibrium CO2 partial pressure were ∼5-fold and ∼600 μatm, respectively) using cultures of four filamentous strains (Leptolyngbya sp. (NIES-2104), Scytonema sp. (NIES-2130), Phormidium ambiguum (NIES-2119), and Spirulina subsalsa (NIES-598)). All cultures induced CaCO3 precipitation mainly by photosynthesis to form mono-/poly crystalline calcite and amorphous calcium carbonate (ACC) around them. These features suggested a CaCO3 nucleation process via an ACC precursor phase on negatively charged substances generated by cyanobacteria (i.e., cell walls, EPS, and oxygen bubbles). The characteristics of the precipitated CaCO3 were strongly influenced by the surface properties of the cyanobacteria. For Leptolyngbya, which lacks EPS, a small number of CaCO3 was nucleated onto the smooth surface of negatively charged cell walls and entire cells were incorporated into a large single crystal of calcite. For Scytonema and Phormidium, which possess EPS sheaths, a relatively large number of CaCO3 was nucleated onto the rough surface of negatively charged sheaths, and their sheaths were encrusted and partially impregnated by calcite crystals. For Spirulina, which possesses a loose EPS matrix, CaCO3 nucleation was scarce and restricted to the surface of negatively charged oxygen bubbles stabilized by the EPS matrix to form hollow calcite crystals. These results suggest that chemical properties (i.e., abundance of dissociated acidic groups) primarily controlled the presence/absence of CaCO3 nucleation around cyanobacteria, while a physical property (i.e., submicron-scale structure of negatively charged substances) primarily controlled the numbers of crystal nuclei provided and stabilized ACC precursors as well as the calcification styles (sheath encrustation or impregnation).

Precipitation and Transformation of Vaterite Calcium Carbonate in the Presence of Some Organic Solvents.

In this paper, the production of CaCO3 particles via the carbonation route in the reaction of CaCl2 and CO2, using NH3 as a promoter of CO2 absorption, was studied. The solvents used as the reaction media for CaCO3 precipitation were aqueous solutions of methanol, isopropanol and dimethyl sulfoxide (DMSO), in a concentration range of 0–20% (v/v). It was found that the presence of an organic additive influenced the precipitation rate, the content of vaterite in the obtained product, the morphology and the size of the precipitated CaCO3 particles, as well as the rate of its transformation into calcite. The presence of all added organic solvents reduced the vaterite concentration in the produced CaCO3 both at the end of the reaction and after incubation in the reaction medium for 1 h. However, the transformation of vaterite particles into calcite in the tested solutions was slower when the 4 h and 24 h procedures were compared. The interactions of solvents with calcite and vaterite were compared using HPLC tests. DMSO molecules interacted with vaterite particles the most strongly, while the interaction of isopropanol with this polymorph was the weakest. The opposite effect was observed for interactions with calcite particles, and the affinity decreased in the series: isopropanol, methanol, DMSO.

Formation of calcium carbonates from Ca(OH)2-H2O-supercritical CO2 using a rapid spraying method

Particle formation techniques using supercritical fluid are simple processes that can control particle size and morphology, although high-pressure is required. The purpose of this study was to investigate how the experimental conditions affect the extent and rate of CaCO3 conversion and the size and morphology of the precipitated CaCO3 from the carbonation tests with rapid spraying of reactants causing rapid depressurization of supercritical fluid. The relatively low temperature and pressure conditions (35 °C and 7.5MPa) resulted in low CaCO3 conversion efficiency (41.4–51.9%), high vaterite content (70–78%) of CaCO3, and smaller-sized particles. The relatively high temperature and pressure conditions (80 °C and 12.0MPa) resulted in high CaCO3 conversion efficiency (66.8–73.2%), high calcite content (50–80%) of CaCO3, and larger-sized particles. The particle size of solid products ranged between 20 and 180nm with approximately a peak of 100 nm in the particle size distribution (PSD) curve, irrespective of the test conditions; however, shorter reaction times led to smaller particles. The optimal conditions under which the extent of CaCO3 conversion and calcite content were maximum were 50 °C, 9.0MPa, and 1 h of reaction time (CaCO3 conversion: 92.9%; calcite content of CaCO3: 87%).

Synthesis and Characterization of White Mineral Trioxide Aggregate Using Precipitated Calcium Carbonate Extracted from Limestone

White Mineral Trioxide Aggregate (WMTA) using precipitated CaCO3 (PCC) from limestone has been synthesized. PCC in calcite structure was extracted from limestone by calcination at 900 °C for 3 h, dissolved in 0.8 M nitric acid solution and followed with carbonation for 60 minutes. PCC was used for the synthesis of WMTA by mixing with tetraethoxyorthosilicate, bismuth oxide, aluminum oxide, catalyst of with HNO3 and NH3 solution and thermally treated at 1100 °C for 3 h. The products were characterized with Thermal Gravimetric Analysis-Differential Thermal Analysis (TGA-DTG), X-ray Diffraction (XRD), Frontier-Transform Infrared (FTIR) spectroscopy, Scanning Electron Microscopy (SEM) and X-ray Fluorescence (XRF). The results showed that the PCC dominated calcite structure was obtained with 75.25% in yield and 99.42% in purity. The WMTA has been successfully synthesized by low thermal treatment at 1100 °C using catalysts of HNO3 and NH3 solution, proven by the presence of tricalcium silicate (C3S), dicalcium silicate (C2S), tricalcium aluminate (C3A), and Bi2O3 in WMTA.

Precipitation Process of CaCO3 from Natural Limestone for Functional Materials.

This article aims to report on the production of precipitated CaCO3, with various morphologies and particle sizes through several methods, namely carbonation methods with CO2 gas bubbling, rapid mixing of the solutions, and carbonation with modified bubbling techniques. The simple bubbling method was performed by flowing CO2 gas into Ca(OH)2 solution until precipitation occurs. This method can produce calcite particles up to 100% at the intermediate temperatures. In the modified bubbling technique, the gas flow was interrupted by a breaker and porous material with variation of stirring speeds. Through this technique, it produces 99% aragonite particles at higher temperatures. Additionally, 95% spherically vaterite particles with porous structures can also be produced by the rapid mixing solution method at the low temperatures. Depending on the reaction temperature, pH and concentration of the solution, and CO2 gas flow rate, these methods provide alternative routes for producing precipitated CaCO3 particles from natural limestone extraction with certain morphology and particle size for additional applications.

Microbially induced calcium carbonate precipitation to design a new type of bio self-healing dental composite.

Crack propagation is one of the issues associating with dental composites which can significantly affect their performance. Current solutions for preventing and stopping the cracks include maximizing the filler to matrix ratio as well as fiber reinforcing of composites which are not always reliable. The precipitation of calcium carbonate (CaCO3) minerals by the generally recognized as safe (GRAS) bacteria can be seen as a novel approach to address this shortcoming. In the present study, the effect of microbially induced calcium carbonate precipitation (MICP) on filling dental composites' cracks and cavities was studied. In this first step, the capability of different GRAS bacteria to induce CaCO3 precipitation was investigated. In the next step, the capability of potent bacteria to initiate MCIP in solid matrix was evaluated. For this purpose, the CaCO3-bacteria along with necessary nutrients were introduced into different dental composites in two ways, namely, powder and paste form. The light-cured composites were analyzed using optical microscopy, scanning electron microscopy (SEM), and energy-dispersive X-ray (EDS) to identify and characterize the precipitated CaCO3 crystals. It was shown that the incorporation of powder healing compound in two composites resulted in precipitation of CaCO3, while no crystals were formed when a paste form of healing compound was mixed with composites. The results evidently show that MICP can be a feasible alternative to current inefficient approaches to address microcracking issues in dental composites.

Sandy Soil Improvement through Microbially Induced Calcite Precipitation (MICP) by Immersion.

The goal of this article is to develop an immersion method to improve the microbially induced calcite precipitation (MICP) treated samples. A batch reactor was assembled to immerse soil samples into cementation media. The cementation media can freely diffuse into the soil samples in the batch reactor instead of cementation media being injected. A full contact flexible mold, a rigid full contact mold, and a cored brick mold were used to prepare different soil sample holders. Synthetic fibers and natural fibers were selected to reinforce the MICP-treated soil samples. The precipitated CaCO3 in different areas of the MICP-treated samples was measured. The CaCO3 distribution results demonstrated that the precipitated CaCO3 was distributed uniformly in the soil sample by the immersion method.

Effect of Addition of Ca2+ and CO32- Ions with Temperature Control on Self-Healing of Hardened Cement Paste.

Concrete has a remarkably low ratio of tensile strength to compressive strength, and is widely used in construction. However, the occurrence of cracks in a concrete structure is inevitable. Nevertheless, in the presence of adequate moisture, small cracks in the concrete structure exhibit a propensity to self-heal by getting filled due to the rehydration of cement particles and the subsequent precipitation of calcium carbonate (CaCO3). According to previous studies, the self-healing performance can be maximized by optimizing the temperature and pH to control the crystal formation of CaCO3. This study focused on the crystal form of CaCO3 generated in the self-healing of a cement-based composite material. To evaluate the self-healing performance depending on the type of aqueous solution and the temperature, the weight change, the weight change rate, and the porosity reduction in each case were evaluated. Moreover, to increase the generation of CaCO3 (which is a self-healing precipitate), nanosized ultrafine CO2 bubbles using CO2 gas were used, along with an adequate supply of Ca2+ by adjusting the aqueous solution (Ca(OH)2, CaO + ethanol). For greater pore-filling effects by controlling the CaCO3 crystal forms in the cement matrix, the change in the crystal form of the precipitated CaCO3 in the hardened cement paste with changing temperature was analyzed by scanning electron microscopy and X-ray diffraction. As a result, the possibility of the effective generation and control of vaterite with a dense pore structure together with calcite was confirmed by adjusting the temperature to approximately 40 °C at a pH of 12.

Lab‐scale atmospheric CO2 absorption for calcium carbonate precipitation in sand

AbstractThe microbial‐induced calcite precipitation (MICP) process for ground improvement uses microorganisms to hydrolyze urea, producing carbonate ions to induce in situ calcium carbonate (CaCO3) precipitation in soil to improve its strength. This paper proposes using the hydroxide‐based absorption of CO2 instead to provide the carbonate ion source. This study utilizes direct air capture (DAC) to absorb atmospheric CO2 using potassium hydroxide (KOH) in a semi‐batch bubble absorption column. Potassium carbonate (K2CO3) was then injected into sand with calcium hydroxide (Ca(OH)2 as a calcium source to precipitate CaCO3 and regenerate KOH. Batch and continuous flow precipitation methods produced a poor distribution of CaCO3, with more CaCO3 precipitated on top, resulting in unconfined compressive strength (UCS) of 6.9 to 19.6 kPa. Sand pre‐mixed with Ca(OH)2 gave well distributed CaCO3, precipitated throughout the sample with 7.56 wt% and 6.87 wt% CaCO3 content and UCS of 39.2 and 35.3 kPa before failing for batch and continuous flow precipitation respectively. This differs from MICP strength improvement of 1000 kPa with 5.3 wt% CaCO3 due to poor binding of sand with the precipitated CaCO3 crystals. However, this application provides a stable sequestration source for atmospheric CO2 in soil. © 2019 Society of Chemical Industry and John Wiley & Sons, Ltd.

Reactivity of calcium carbonate prepared from flue gas desulfurization gypsum

Abstract Reactivity of various calcium carbonate samples for flue gas desulfurization was tested. Two groups of CaCO3 samples were considered; natural limestone containing calcite phase dominantly and samples prepared by the conversion of gypsum with ammonium and carbon dioxide (precipitated CaCO3) containing different amounts of calcite, aragonite and vaterite. Reactivity of precipitated calcium carbonate depends primarily on the particle size, similarly as in case of industrial samples. The initial reaction rate was comparable with the industrial limestones for samples with the average particle size lower than 15 μm. However, the conversion of laboratory samples was significantly higher after 5 min of the reaction. Phase composition of the precipitated calcium carbonate has a minor but noticeable impact on the reactivity. The presence of vaterite slightly increased the reactivity, which is in accordance with its lower compact structure in comparison with calcite and aragonite. Unexpected effect of the increased content of aragonite, which is the most compact phase in comparison with calcite and vaterite, was observed. If calcium carbonate contains up to approximately 30 % of aragonite the reactivity increases, which can be explained by the SEM pictures showing agglomerate composition with relatively high specific surface. At higher contents of aragonite, the reactivity decreases. All the obtained results proved the suitability of precipitated CaCO3 prepared from flue gas desulfurization gypsum to be recycled in the flue gas desulfurization process.

Effect of alternating electromagnetic field and ultrasonic on CaCO3 scale inhibitive performance of EDTMPS

Abstract In this study, the influence of Ethylene Diamine Tetra Methylene Phosphonic Acid Sodium (EDTMPS) on the precipitation of CaCO3 under alternating electromagnetic field (AEMF) and ultrasonic (US) treatment were investigated. The variations of pH value and the Ca2+ concentration of hard water solution were monitored, and the crystalline phase and morphology of precipitated CaCO3 crystals were characterized by XRD and SEM. The experimental results show that the presence of EDTMPS can inhibit the precipitation of CaCO3. In the 30 mmol/L Ca(HCO3)2 solution, when the EDTMPS concentration were 2.5 mg/L, 5 mg/L, 7.5 mg/L and 10 mg/L, the antiscale efficiencies were 34.08%, 63.67%, 70.79% and 71.16%, respectively. Thus, the antiscale efficiency correlates positively with EDTMPS concentration. The presence of EDTMPS can also inhibit the CaCO3 conversion from vaterite to calcite. The AEMF has the effect of hindering the precipitation of CaCO3 and inducing CaCO3 precipitates as vaterite crystals. Furthermore, it is significant that the AEMF can effectively improve the scale inhibition effect of EDTMPS, which means the EDTMPS used in combination with AEMF could be a good method to improve the scale inhibition. On the other hand, the US promotes the CaCO3 precipitation as calcite crystals with smaller size. Under the action of US, the EDTMPS loses its inhibition effect after a certain period of time, possibly due to the US destroys the connection between Ca and EDTMP and free the Ca2+ in the solution.

Experimental study on the extraction of calcium, magnesium, vanadium and silicon from steelmaking slags for improved mineral carbonation of CO2

In this paper X2PCC (Calcium containing material X to Precipitated Calcium Carbonate) process wherein a steelmaking slag is used as a source of Ca for mineral carbonation of CO2 was studied. Mineral carbonation refers to binding of CO2 into mineral carbonates, here CaCO3. In the process, we first dissolve Ca selectively from the slag, then separate the solid phase and finally, bubble CO2 into the solution to make precipitated calcium carbonate CaCO3. Based on our current understanding, the extraction step limits the process feasibility. In the experiments, ammonium chloride (NH4Cl) water solution was used as an extracting solvent with varying concentrations of 0, 0.5, 1 and 2 mol/L using slag to solvent ratio of 100 g/L. The main objective of this study was to determine the optimal concentrations of solvent and particle size for leaching out calcium from the slag. Tests were performed at room temperature with varying particle size and solvent concentration. The extraction behaviors of different size fractions of the slag, 0–50 μm, 50–74 μm and 74–125 μm were studied. Dissolution results showed that with high solvent concentrations calcium extraction efficiency increases but at the same time, other elements are leached, such as vanadium, silicon and magnesium that act as impurities in the subsequent carbonation stage. It was found that with the largest particles, Ca extraction becomes limited by mass transfer and the availability of Ca from the particle. Our hypothesis is that the surface layer of the particles gets blocked by reaction products and stops the reaction. With the smallest particle size, before the surface gets blocked, more calcium could be extracted and the maximum extraction efficiency was not reached.

Simulating speleothem growth in the laboratory: Determination of the stable isotope fractionation (δ13C and δ18O) between H2O, DIC and CaCO3

Here we present novel cave-analogue experiments directly investigating stable carbon and oxygen isotope fractionation between the major involved species of the carbonate system (HCO3−, CO2, CaCO3 and H2O). In these experiments, which were performed under controlled conditions inside a climate box, a thin film of solution flew down an inclined marble or glass plate. After different distances of flow and, thus, residence times on the plate, pH, electrical conductivity, supersaturation with respect to calcite, precipitation rate as well as the δ18O and δ13C values of the dissolved inorganic carbon (DIC) and the precipitated CaCO3 were obtained.Progressive precipitation of CaCO3 along the plate is accompanied by degassing of CO2 and stable isotope fractionation, and the system is driven out of isotope equilibrium. We observe a strong enrichment of the δ13C values with increasing residence time and a smaller enrichment in δ18O. The temporal evolution of the δ18O and δ13C values of both the DIC and the precipitated CaCO3 can be explained by a Rayleigh fractionation model, but the observed enrichment in δ13C values is much larger than expected based on isotope equilibrium fractionation factors.Our setup enables to determine the fractionation between CaCO3 and HCO3−, i.e., εCaCO3/HCO3−. Carbon isotope fractionation, 13εCaCO3/HCO3−, is strongly negative for all experiments and much lower than equilibrium isotope fractionation (0–1‰). In addition, 13εCaCO3/HCO3− decreases with increasing residence time on the plate, and thus decreasing supersaturation with respect to calcite. Thus, isotope fractionation depends on precipitation rate and consequently occurs under kinetic conditions. This is in contrast to previous studies, which found no rate-dependence and no or even a positive carbon isotope fractionation between CaCO3 and HCO3−. Oxygen isotope fractionation, 18εCaCO3/HCO3−, is also negative and dependent on precipitation rate. Since no literature values for 18εCaCO3/HCO3− are available, we calculated 18εCaCO3/HCO3− using equilibrium oxygen isotope fractionation factors between water and calcite and water and HCO3−, respectively. At the beginning of the plate, the fractionation is in agreement with the fractionation calculated using fractionation factors determined in cave systems.The observed fractionation between CaCO3 and water, 1000ln18α, is also in good agreement with the values determined in cave systems and shows a very similar temperature dependence 1000ln18α=16.516±1.267∗103T−26.141±4.356. However, with progressive precipitation of CaCO3 along the plate, the system is forced out of isotope equilibrium with the water, and 1000ln18α increases.The large, negative, rate-dependent isotope fractionations observed in this study suggest that precipitation of speleothem calcite is strongly kinetically controlled and may, thus, have a large effect on speleothem δ18O and δ13C values. Since these values may erroneously be interpreted as reflecting changes in past temperature, precipitation and/or vegetation density, these results have important implications for paleoclimate reconstructions from speleothems.

Controlled Crystallization of Calcium Carbonate via Cooperation of Polyaspartic Acid and Polylysine Under Double-Diffusion Conditions in Agar Hydrogels

We demonstrated the crystallization of calcium carbonate in the presence of polyaspartic acid (PAsp) and polylysine (PLys) under double-diffusion conditions in agar hydrogels. The crystallization of precipitated CaCO3 was discussed based on the equivalency rule, and it was suggested that the nucleation of CaCO3 in the presence of PLys and PAsp occurred heterogeneously on the polypeptide assemblies that act as templates for nucleation. With PLys alone, the precipitates had calcite as the major component and vaterite as the minor component, where calcite and vaterite adopted dendritic and spherulitic morphologies, respectively. In the presence of PAsp alone and a mixture of PLys and PAsp, the precipitates comprised calcite spherulites only. The spherulites had a hierarchal structure with an inner part (the core) having a loose structure and an outer part composed of calcite crystals that grew radically from the core. The cores completely dissolved during crystallization, resulting in the formation of spheru...

Polymer-modified microbially induced carbonate precipitation for one-shot targeted and localized soil improvement

The viscous nature of certain hydrophilic polymer solutions and the tendency of these solutions to retain moisture may make them suitable carriers for the process of microbially induced carbonate precipitation (MICP) for certain engineering proposes. Specifically, tuning the viscosity of the cementation solution enables the control of its infiltration process and thus control of the treatment region in highly permeable soils; the enhanced water retention also helps to maintain the moist conditions required for MICP in certain treatment cases. Therefore, it seems promising to utilize a hydrophilic polymer to develop a one-shot polymer-modified MICP approach for potential applications that require localized and targeted soil improvement. Presented in this paper are experiments conducted to investigate a new polyvinyl alcohol (PVA)-modified MICP approach. Standard Ottawa graded sand was treated using both polymer-modified and conventional water-based MICP solutions with different concentrations of cementation media. For the obtained specimens, the chemical components were identified through thermogravimetric analysis, the CaCO3 content was measured, and unconfined compression tests were conducted to evaluate the shear strength. The morphology, distribution, and crystal polymorphs of the precipitated CaCO3 were also characterized. The experimental results demonstrated the feasibility of the proposed polymer-modified MICP approach as a one-shot MICP treatment scheme. Moreover, two bench-scale soil improvement cases were conducted to highlight the advantages of the proposed polymer-modified MICP approach over the water-based approach for certain potential soil improvement applications.