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1. New insights into the early stage nucleation of calcium carbonate gels by reactive molecular dynamics simulations

   https://doi.org/10.1063/5.0127240  

   Qin, Ling ; Mao, Xingtai ; Cui, Yifei ; Bao, Jiuwen ; Sant, Gaurav ; Chen, Tiefeng ; Zhang, Peng ; Gao, Xiaojian ; Bauchy, Mathieu ( December 2022 , The Journal of Chemical Physics)

   The precipitation of calcium carbonate (CaCO3) is a key mechanism in carbon capture applications relying on mineralization. In that regard, Ca-rich cementitious binders offer a unique opportunity to act as a large-scale carbon sink by immobilizing CO2 as calcium carbonate by mineralization. However, the atomistic mechanism of calcium carbonate formation is still not fully understood. Here, we study the atomic scale nucleation mechanism of an early stage amorphous CaCO3 gel based on reactive molecular dynamics (MD) simulations. We observe that reactive MD offers a notably improved description of this reaction as compared to classical MD, which allows us to reveal new insights into the structure of amorphous calcium carbonate gels and formation kinetics thereof.

    

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2. Isotopic fractionation accompanying CO2 hydroxylation and carbonate precipitation from high pH waters at The Cedars, California, USA

   https://doi.org/doi.org/10.1016/j.gca.2021.01.003  

   Christensen, J. ( January 2021 , Geochimica)

   Teagle, Damon A (Ed.)

   The Cedars ultramafic block hosts alkaline springs (pH > 11) in which calcium carbonate forms upon uptake of atmospheric CO2 and at times via mixing with surface water. These processes lead to distinct carbonate morphologies with ‘‘floes” forming at the atmosphere-water interface, ‘‘snow” of fine particles accumulating at the bottom of pools and terraced constructions of travertine. Floe material is mainly composed of aragonite needles despite CaCO3 precipitation occurring in waters with low Mg/Ca (<0.01). Precipitation of aragonite is likely promoted by the high pH (11.5–12.0) of pool waters, in agreement with published experiments illustrating the effect of pH on calcium carbonate polymorph selection. The calcium carbonates exhibit an extreme range and approximately 1:1 covariation in d13C (
   9 to
   28‰ VPDB) and d18O (0 to
   20‰ VPDB) that is characteristic of travertine formed in high pH waters. The large isotopic fractionations have previously been attributed to kinetic isotope effects accompanying CO2 hydroxylation but the controls on the d13C-d18O endmembers and slope have not been fully resolved, limiting the use of travertine as a paleoenvironmental archive. The limited areal extent of the springs (0.5 km2) and the limited range of water sources and temperatures, combined with our sampling strategy, allow us to place tight constraints on the processes involved in generating the systematic C and O isotope variations. We develop an isotopic reaction–diffusion model and an isotopic box model for a CO2-fed solution that tracks the isotopic composition of each dissolved inorganic carbon (DIC) species and CaCO3. The box model includes four sources or sinks of DIC (atmospheric CO2, high pH spring water, fresh creek water, and CaCO3 precipitation). Model parameters are informed by new floe D44Ca data (
   0.75 ± 0.07‰), direct mineral growth rate measurements (4.8 to 8  10
   7 mol/m2/s) and by previously published elemental and isotopic data of local water and DIC sources. Model results suggest two processes control the extremes of the array: (1) the isotopically light end member is controlled by the isotopic composition of atmospheric CO2 and the kinetic isotope fractionation factor (KFF (‰) = (a
   1)  1000) accompanying CO2 hydroxylation, estimated here to be
   17.1 ± 0.8‰ (vs. CO2(aq)) for carbon and
   7.1 ± 1.1‰ (vs. ‘CO2(aq)+H2O’) for oxygen at 17.4 ± 1.0
   C. Combining our results with revised CO2 hydroxylation KFF values based on previous work suggests consistent KFF values of
   17.0 ± 0.3‰ (vs. CO2(aq)) for carbon and
   6.8 ± 0.8‰ for oxygen (vs. ‘CO2(aq)+H2O’) over the 17–28
   C temperature range. (2) The isotopically heavy endmember of calcium carbonates at The Cedars reflects the composition of isotopically equilibrated DIC from creek or surface water (mostly HCO- 3, pH = 7.8–8.7) that occasionally mixes with the high-pH spring water. The bulk carbonate d13C and d18O values of modern and ancient travertines therefore reflect the proportion of calcium carbonate formed by processes (1) and (2), with process (2) dominating the carbonate precipitation budget at The Cedars. These results show that recent advances in understanding kinetic isotope effects allow us to model complicated but common natural processes, and suggest ancient travertine may be used to retrieve past meteoric water d18O and atmospheric d13C values. There is evidence that older travertine at The Cedars recorded atmospheric d13C that predates large-scale combustion of fossil fuels. 

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3. Coprecipitation of Crystalline Calcium Silicates and Carbonates from the Hydrothermal Reaction of Pseudowollastonite

   https://doi.org/10.1021/acssuschemeng.3c05110  

   Tolliver, Coleman ; Nguyen, Suzanne ; Dela Cerna, Kimmie ; McNamara, Rachel ; Opila, Elizabeth J. ; Clarens, Andres F. ( February 2024 , ACS Sustainable Chemistry & Engineering)

   Calcium silicates are abundant, but sparingly soluble, feedstocks of interest for making low-carbon alternative cements. Under hydrothermal and alkaline conditions, they can form crystalline calcium silicate hydrate (CCSH) products, which are abundant in Roman concrete, or they can form carbonates when CO2 is present. To understand when co-precipitation of CCSH and carbonate phases is possible, we studied the hydrothermal carbonation of a model calcium silicate, pseudowollastonite (-CaSiO3), at 150ºC and high pH as a function of CO2 source (CO2(g) or Na2CO3) and different concentrations of sodium, alumina, and silica. Our experiments produced a range of CCSH phases including tobermorite – 13Å, rhodesite, and pectolite, as early as one day after the start of our experiments. About 10.7% hydrated product was observed after 7 days of curing in 2 M NaOH solution. We also observed the formation of CaCO3 as both aragonite and calcite when carbon was introduced to our experimental system. The carbon source impacted the ratio of CaCO3 to CCSH phases in the reaction products. Availability of Na2CO3 produced a balance between CaCO3 and CCSH phases whereas CO2(g) produced more CaCO3 at about 36.4% by mass at the highest. Higher concentrations of Na+ increased precipitation of both CaCO3 and/or CCSH phases. The presence of excess silica, in the form of dissolved borosilicate glass from our reaction vessels under alkaline reaction conditions, also enhanced the formation of CCSH phases formed in some experiments. Supplemental Al2O3, a common constituent in many silicate feedstocks, also enhanced CCSH formation, likely by forming aluminum substituted phases under the conditions tested here. These chemical insights can be enabling in designing formulation and curing guidelines for novel cementitious materials. 

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4. Kinetics of calcite precipitation by ureolytic bacteria under aerobic and anaerobic conditions

   https://doi.org/10.5194/bg-16-2147-2019  

   Mitchell, Andrew C. ; Espinosa-Ortiz, Erika J. ; Parks, Stacy L. ; Phillips, Adrienne J. ; Cunningham, Alfred B. ; Gerlach, Robin ( January 2019 , Biogeosciences)

   Abstract. The kinetics of urea hydrolysis (ureolysis) and induced calcium carbonate(CaCO3) precipitation for engineering use in the subsurface wasinvestigated under aerobic conditions using Sporosarcina pasteurii(ATCC strain 11859) as well as Bacillus sphaericus strains 21776and 21787. All bacterial strains showed ureolytic activity inducingCaCO3 precipitation aerobically. Rate constants not normalized tobiomass demonstrated slightly higher-rate coefficients for both ureolysis(kurea) and CaCO3 precipitation (kprecip)for B. sphaericus 21776 (kurea=0.10±0.03 h−1, kprecip=0.60±0.34 h−1) compared toS. pasteurii (kurea=0.07±0.02 h−1,kprecip=0.25±0.02 h−1), though these differences werenot statistically significantly different. B. sphaericus 21787showed little ureolytic activity but was still capable of inducing someCaCO3 precipitation. Cell growth appeared to be inhibited duringthe period of CaCO3 precipitation. Transmission electron microscopy (TEM) images suggest this is dueto the encasement of cells and was reflected in lower kureavalues observed in the presence of dissolved Ca. However, biomass regrowthcould be observed after CaCO3 precipitation ceased, which suggeststhat ureolysis-induced CaCO3 precipitation is not necessarilylethal for the entire population. The kinetics of ureolysis andCaCO3 precipitation with S. pasteurii was furtheranalyzed under anaerobic conditions. Rate coefficients obtained in anaerobicenvironments were comparable to those under aerobic conditions; however, nocell growth was observed under anaerobic conditions with NO3-,SO42- or Fe3+ as potential terminal electronacceptors. These data suggest that the initial rates of ureolysis andureolysis-induced CaCO3 precipitation are not significantlyaffected by the absence of oxygen but that long-term ureolytic activity mightrequire the addition of suitable electron acceptors. Variations in theureolytic capabilities and associated rates of CaCO3 precipitationbetween strains must be fully considered in subsurface engineering strategiesthat utilize microbial amendments. 

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5. Thermal equation of state of post-aragonite CaCO3-Pmmn

   https://doi.org/10.2138/am-2020-7279  

   Lv, Mingda ; Liu, Jiachao ; Greenberg, Eran ; Prakapenka, Vitali B. ; Dorfman, Susannah M. ( September 2020 , American Mineralogist)

   null (Ed.)

   Abstract Calcium carbonate (CaCO3) is one of the most abundant carbonates on Earth's surface and transports carbon to Earth's interior via subduction. Although some petrological observations support the preservation of CaCO3 in cold slabs to lower mantle depths, the geophysical properties and stability of CaCO3 at these depths are not known, due in part to complicated polymorphic phase transitions and lack of constraints on thermodynamic properties. Here we measured thermal equation of state of CaCO3-Pmmn, the stable polymorph of CaCO3 through much of the lower mantle, using synchrotron X-ray diffraction in a laser-heated diamond-anvil cell up to 75 GPa and 2200 K. The room-temperature compression data for CaCO3-Pmmn are fit with third-order Birch-Murnaghan equation of state, yielding KT0 = 146.7 (±1.9) GPa and K′0 = 3.4(±0.1) with V0 fixed to the value determined by ab initio calculation, 97.76 Å3. High-temperature compression data are consistent with zero-pressure thermal expansion αT = a0 + a1T with a0 = 4.3(±0.3)×10-5 K-1, a1 = 0.8(±0.2)×10-8 K-2, temperature derivative of the bulk modulus (∂KT/∂T)P = –0.021(±0.001) GPa/K; the Grüneisen parameter γ0 = 1.94(±0.02), and the volume independent constant q = 1.9(±0.3) at a fixed Debye temperature θ0 = 631 K predicted via ab initio calculation. Using these newly determined thermodynamic parameters, the density and bulk sound velocity of CaCO3-Pmmn and (Ca,Mg)-carbonate-bearing eclogite are quantitatively modeled from 30 to 80 GPa along a cold slab geotherm. With the assumption that carbonates are homogeneously mixed into the slab, the results indicate the presence of carbonates in the subducted slab is unlikely to be detected by seismic observations, and the buoyancy provided by carbonates has a negligible effect on slab dynamics. 

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