Formation Of Aragonite Research Articles

Twinning and homoepitaxy cooperation in the already rich growth morphology of CaCO3 polymorphs. I. Aragonite

Calcite and aragonite are the two most abundant among the CaCO3 polymorphs and are also well known for their rich morphology and habit, to which twinning relevantly contributes. Moreover, the calcite → aragonite transformation has been debated for a long time, even though the homo-epitaxies occurring within each polymorph have been overlooked, to date, from both experimental and theoretical points of view. Twinning is common and unfortunately can be deceptive as it can be mistaken for homo-epitaxy, thus leading to confusion regarding the growth mechanisms of many crystal aggregates. Here, experimental and theoretical studies were carried out to investigate the twinning and homo-epitaxy that operate in aragonite: (i) the connection between twinning and homo-epitaxy for contact and penetration aggregates made by the lateral {010} and {110} aragonite forms; (ii) the homo-epitaxial relationships among the {001} pinacoid and both the {010} pinacoid and the prismatic {110} aragonite forms. This work attempts to provide a new approach to monitoring the growth mechanisms of aragonite in cases where it is obtained not as a single crystal but as an aggregate. An analogous problem will be explored in our forthcoming work on calcite.

Aragonite formation induced by triethylene glycol and its enhancement to flexural strength in carbonated rankinite cement

In this study, triethylene glycol (TEG) was used as a modifier for rankinite (C3S2) cement carbonation to induce the formation of aragonite whiskers and to prepare rankinite cement with high flexural strength. The effects of different admixture amounts of TEG on the in situ aragonite formation and the enhancement in the mechanical strength of C3S2 mortar were mainly explored. The results showed that when the TEG/C3S2 molar ratio, reaction pressure, and temperature were 0.1, 0.1 MPa, and 70 °C, respectively, the aragonite whiskers generated in the hardened cement paste reached 35 % of the solid content. Under the above reaction conditions, the cement mortar specimens that were carbonated for 1 and 7 d had flexural strengths of 3.0 MPa and 9.6 MPa, respectively, which were 50 % and 95.9 % higher than those of the reference sample; furthermore, they had compressive strengths of 9.2 MPa and 60.8 MPa, respectively, which were 31.4 % and 60 % higher than those of the reference sample. In addition, under the same reaction conditions, the CO2 uptake of cement after carbonation for 72 h was 28.4 %. The analysis results showed that the increase in flexural strength could be attributed to the interlaced 3D spatial structure formed by the growth of aragonite whiskers in the cement paste. The results of this study could provide a new route for the preparation of high-flexural-strength cements while utilizing CO2.

Unravelling calcite-to-aragonite evolution from a subsurface fluid - Formation pathway, interfacial reactions and nucleation effects

Calcite-aragonite alternations are documented in sedimentary deposits worldwide, but their formation is still poorly understood and individual CaCO3 precipitation pathways are rarely confirmed experimentally. Therefore, (sub)recent CaCO3 sinter formation in a historic subsurface adit at Erzberg (Austria) was used as a natural laboratory to monitor and assess the calcite-to-aragonite evolution pathway and intergrowth mechanism in terms of solid-liquid-atmosphere dynamics and their relevance for solid-liquid interface reactions and nucleation effects. Our results indicate an initial homogeneous nucleation of low-Mg calcite (LMC: ∼3 ± 1 mol% MgCO3), induced by CO2 degassing from the percolating geogenic fluid, which is originated from seepage of local meteoric water and incongruent dissolution of Mg-Ca-Fe-bearing minerals from the host rock. Progressive LMC growth leads to an increase in the aqueous molar ratio of Mg/Ca, causing a Mg/Ca zonation pattern with transitions to high-Mg calcite (HMC: up to 7 mol% MgCO3). At a critical Mg concentration, the available Mg calcite crystal surfaces are acting as a nucleation site for heterogenous aragonite formation. In this way, fast growing acicular aragonite crystals are initiated, which impede further calcite growth. The calcite-to-aragonite transition is thus controlled by the reaction kinetics and mechanisms of Mg-calcite formation and the chemical evolution of the precipitating solution at the nano- to micro-spatial scale, creating Mg-enriched HMC surface sites for aragonite to be nucleated and preferentially grown. In the present case, the dynamics of the formation of calcite-aragonite sequences are triggered by distinct local environmental changes, in particular seasonal variations in seepage fluid flow behavior and progress in CO2 degassing. These considerations are relevant for a better understanding of proxy signal development and preservation in calcareous sedimentary sequences forming under highly dynamic environmental conditions.

Calcium isotopic fractionation during aragonite and high-Mg calcite precipitation at methane seeps

The formation of authigenic carbonate in marine environments represents a process that buffers ocean chemistry and atmospheric carbon dioxide levels. The isotopic composition of calcium (δ44/40Ca) in authigenic carbonate can be used to investigate the calcium (Ca) cycle, seawater chemistry, and diagenesis of carbonate rocks, archiving key information on the evolution of the Earth's surface environments. Laboratory experiments have shown that Ca isotopic fractionation during carbonate precipitation – the basis of the δ44/40Ca proxy – is mainly determined by both mineral phase and precipitation rate. However, the precipitation rate of submarine authigenic carbonate is much lower than rates in laboratory experiments. This study investigated the Ca isotopic composition of pore water and carbonate nodules collected from two gravity piston cores and one push core from the Haima seeps in the northern South China Sea. Methane fluxes calculated for the three cores range from 12 to 134 µmol cm−2 yr−1. The core with intermediate methane flux showed evidence of aragonite precipitation, while high-Mg calcite was the dominant mineral phase in the other two cores as indicated by Mg/Ca and Sr/Ca ratios of pore water. Numerical modeling of pore water geochemistry revealed that: (1) precipitation rates of carbonate per unit reactive surface area range from 0.05 to 21 µmol m−2 h−1, which is one to three orders of magnitude lower than rates in precipitation experiments; (2) Ca isotopic fractionation for aragonite precipitation is –1.8 ± 0.3 ‰, close to the reported equilibrium isotopic fractionation in precipitation experiments; and (3) Ca isotopic fractionation for high-Mg calcite precipitation is similar for the two cores (−0.9 ± 0.3 ‰ and −0.8 ± 0.3 ‰), although the modeled precipitation rates differed by two orders of magnitude. These results indicate that mineralogy rather than precipitation rate is the primary control on Ca isotopic fractionation during carbonate precipitation at seeps, resulting in lower δ44/40Ca values of aragonite (0.91±0.06 ‰, N = 4) than of high-Mg calcite (1.41±0.04 ‰, N = 9) reported relative to NIST SRM915a. This study documents distinguishable Ca isotopic fractionation during the formation of methane-derived high-Mg calcite and aragonite, providing fundamental parameters for further exploration of the calcium isotopic composition of marine authigenic carbonates in the geological record.

Effect of conditions on wet carbonation products of recycled cement paste powder

Wet carbonation is deemed a successful approach to recycling waste concrete powder. Controlling the growth and properties of products in the recycled cement paste powder (CPP) is highly dependent on the wet carbonation conditions. Therefore, this study aims to examine the effect of wet carbonation parameters on wet carbonation products in CPP. The wet carbonation of CPP resulted in mixed distribution of silica gel and CaCO3 without significant layering on the CaCO3. In contrast, silica gel covered the outside of the CaCO3 in the case of dry carbonation. Increasing the temperature from 25 °C to 80 °C promoted and accelerated the crystallization and growth of amorphous CaCO3, increased the crystallite size of CaCO3 by 1.1–5.1 nm, and favored the formation of aragonite and vaterite; however, it also led to a significant crystal defect and/or distortion within CaCO3. Higher temperatures also promoted silica gel formation and increased its polymerization degree by 68.2%. On the other hand, increasing the CO2 flow rate from 3 L/min to 5 L/min facilitated the crystallization and growth of CaCO3 and contributed to the increase in the crystallite size of CaCO3 by 3.0–6.6 nm. Higher CO2 flow led to the incorporation of aluminum into silica gel and resulted in the transformation of its gel pores into capillary pores. The findings of this study contribute to a better understanding of the best experimental parameters to achieve a more efficient and effective concrete waste recycling process.

Development of 3D Printable Calcium Phosphate Cement Scaffolds with Cockle Shell Powders.

Three-dimensional (3D) printed calcium phosphate cement (CPC) scaffolds are increasingly being used for bone tissue repair. Traditional materials used for CPC scaffolds, such as bovine and porcine bone, generally contain low amounts of calcium phosphate compounds, resulting in reduced production rates of CPC scaffolds. On the other hand, cockle shells contain more than 99% CaCO3 in the form of amorphous aragonite with excellent biocompatibility, which is expected to increase the CPC production rate. In this study, 3D-printed cockle shell powder-based CPC (CSP-CPC) scaffolds were developed by the material extrusion method. Lactic acid and hyaluronic acid were used to promote the printability. The characterization of CSP-CPC scaffolds was performed using Fourier transform infrared spectra, X-ray diffraction patterns, and scanning electron microscopy. The biocompatibility of CSP-CPC scaffolds was evaluated using cell viability, Live/Dead, and alkaline phosphatase assays. In addition, CSP-CPC scaffolds were implanted into the mouse calvarial defect model to confirm bone regeneration. This study provides an opportunity to create high value added in fishing villages by recycling natural products from marine waste.

Investigation of the acicular aragonite growth behavior in AOD stainless steel slag during slurry-phase carbonation

This study presents a novel method for producing acicular aragonite using argon oxygen decarburization (AOD) slag while controlling the reaction temperature, reaction time, stirring speed, and the magnesium-to‑calcium stoichiometric ratio. This approach provides steel plants with an opportunity to decrease their CO2 emissions and promote efficient resource utilization and CO2 storage through the production of high-quality value-added products. The experimental results showed that reaction temperature was the most significant factor affecting the carbonation efficiency of AOD slag, followed by reaction time, stirring speed, CO2 partial pressure, and the liquid-to-solid ratio (L/S). The study also found that elevated temperature and prolonged reaction duration favored the preferential precipitation of aragonite. Additionally, raising the temperature and the magnesium-to‑calcium stoichiometric ratio was shown to enhance the formation of aragonite, affecting its crystal growth orientation and dimensions. The optimal combination of reaction parameters for the preparation of acicular aragonite was found to be the reaction time of 8 h, the magnesium-to‑calcium stoichiometric ratio of 0.8, the reaction temperature of 120 °C, and the stirring speed of 200 r·min−1. Under these conditions, the resulting acicular aragonite exhibited excellent overall uniformity, a large aspect ratio, and a smooth crystal surface, with a content of 91.49 %, a single crystal length ranging from 9.86 to 32.6 μm, and a diameter ranging from 0.63 to 2.15 μm. This study provides valuable insights into the efficient production of acicular aragonite from steel slag while reducing CO2 emissions and promoting the sustainable use of resources.

Preparation of reactive urchin-like recycled concrete aggregate by wet carbonation: Towards improving the bonding capability of the interfacial transition zone in recycled aggregate concrete

To enhance the bonding capability between recycled concrete aggregate (RCA) and paste, an innovative carbonation method was developed to modify the surface characteristic of RCA by promoting the formation of needle-like aragonite using accelerated carbonation in Mg(NO3)2 solution under an elevated temperature (75 °C). The evolution of surface microstructure, phases and reaction kinetics was investigated using multiple testing methods including scanning electron microscopy, nanoindentation, X-ray diffraction, etc. The results revealed that reactive urchin-like RCA could be prepared within less than an hour after exposing to CO2. The urchin-like wrapping with a thickness of about 100 μm was seen rapidly grown on the surface of RCA, consisting of an outermost layer of aragonite coating, a thin layer of brucite and a silica-rich layer. The mineral wrapping induced by carbonation significantly modified the roughness, topography and geochemistry of RCA's surface entirely, contributing to enhanced bonding strength between RCA and new mortar (33.54%).

Nano-Hydroxyapatite Isolation and Characterisation of Echinometra mathaei from the Persian Gulf.

The study focuses on the preparation and characterisation (physicochemical and mechanical) of hydroxyapatite [Ca10(PO4)6(OH)2] (HA) from sea urchin, Echinometra mathaei. Therefore, nano-sized HA prepared from sea urchin shells were collected from beaches of the Persian Gulf in Iran. Sea urchin shells were found a source of calcium carbonate in the form of aragonite (calcite) that crystallised in an organic matrix. HA is one of the polymers used in coating the nanoparticles extracted from various sources. The calcined aragonite converted to nanosized hydroxyapatite powder by chemical reaction with orthophosphoric acid while maintaining stoichiometry, Ca/P = 1.667 at 80°C. To determine the purity of the nano-hydroxyapatite (nHA) numerous analytical procedures were used. Fourier transforms infrared spectroscopy (FTIR) confirmed the presence of the peak of 961 cm-1 is related to the symmetric tensile band of the P-O bond, and the peak of 1038 cm-1 and 1091 cm-1 is related to the tensile solid absorption of the PO4 as functional groups of nHA. The nanocrystalline HA can be observed from the SEM images. Thermogravimetric analysis (TGA-DTA) demonstrates the thermal stability of nHA powder. The results show successful isolation and characterisation study of this crucial nano-material shows it is valuable in biomedical applications, particularly in bone tissue engineering. Indeed, its fabrication is easy and economical.

Preparation and carbonation hardening of low calcium CO2 sequestration materials from waste concrete powder and calcium carbide slag

A series of new low calcium CO2 sequestration cementitious materials (LCC) with different Ca/Si ratios were prepared by sintering waste concrete fine powder (WCP) and calcium carbide slag (CCS) at different ratios. Their carbonation reaction activity, carbonation hardening properties, phase assemblages, and microstructure were systematically characterized. The results showed that when the WCP: CCS ratio was 70:30, the compressive strength of LCC reached 38 MPa after carbonation for 24h. With the decrease in the WCP: CCS ratio, the carbonation activity of LCC increased significantly. When the WCP: CCS ratio was 45:55, the compressive strength was increased to 109 MPa. Microstructure tests showed that the minerals of LCC gradually changed from CS and C2AS to C3S2 and C2S. After carbonation, unreacted clinker particles, crystalline CaCO3, and highly polymerized silica formed a dense microstructure, and the impurities of Mg element in the LCC triggered the formation of aragonite.

Increasing flexural strength of CO2 cured cement paste by CaCO3 polymorph control

This study investigates the impact of Mg(OH)2 on the in-situ formation of fibre-like aragonite and the mechanical properties of the cement mortar by accelerated carbonation. The results showed that the addition of Mg(OH)2 showed a weak effect on the compressive strength of carbonated samples, but it significantly improved the flexural strength of the carbonated samples. The flexural strength of carbonated sample increased with the addition of Mg(OH)2 content and the carbonated sample with 10% Mg(OH)2 showed approximately twice times than that of the hydrated sample at 28 d. The FTIR and XRD analysis also confirmed that aragonite became the dominate CaCO3 polymorph instead of calcite in carbonated sample with beyond 5% Mg(OH)2 content. The in-situ formed fiber/needle-like aragonite increased the total porosity of carbonated sample when adding Mg(OH)2 compared to the formed calcite in control sample due to the lower carbonation degree, while it refined the pore sizes of carbonated samples. Rhombohedral/cubic calcite and small amounts of long rod-like aragonite were present in the control sample. However, the dominant CaCO3 polymorph of aragonite with a length of 10–20 μm and an ammeter of 1–2 μm were generated in the samples incorporation of Mg(OH)2 and the length/diameter ratio of aragonite increased with the increased Mg(OH)2 dosages. This paper confirmed that reinforced cement composites can be prepared by controlled in-situ formation of fiber-like aragonite under carbonation curing and that an appreciable amount of CO2 can be sequestered.

Biomimetic mineralization of calcium carbonate under synergistic regulation of Sr2+ and egg-white proteins

The content of Sr in nacre is stable and second only to Mg, but its ability to induce aragonite is rarely reported. Therefore, the paper studied the effects of Sr2+ concentration, reaction rate and temperature on the biological mineralization of calcium carbonate in presence of egg-white proteins. The results show that Sr2+ could induce the formation of aragonite when coexisting with egg-white proteins as Mg2+. However, the large ion radius of Sr2+ makes it difficult to enter the calcite lattice, so its induction efficiency is much lower than Mg2+. Reducing the mineralization rate and increasing the mineralization temperature are beneficial to the formation of aragonite.

Quantification of the effect of gas–water–equilibria on carbonate precipitation

The expanding geothermal energy sector still faces performance issues due to scalings in pipes and surface level installations, which require elevated operation pressure levels and costly maintenance. For facilities in the North Alpine Foreland Basin, the precipitation of {hbox {CaCO}}_{3} is the main problem which is a consequence of the disruption of the lime-carbonic acid equilibrium during production. The formation of gas bubbles plays a key role in the scaling process. This work presents experiments in a bubble column to quantify the effects of gas stripping on carbonate precipitation and an extension of PhreeqC to include kinetic exchange between a gas phase and water for the simulation of the experimental results. With the same hybrid model not only precipitation of {hbox {CaCO}}_{3} but also the dissolution of scalings by the injection of {hbox {CO}}_{2} could be quantified. The bubble column was filled with tap water and brine. By varying the ionic strength of the solution, a wider range of geothermal waters was covered. Air and {hbox {CO}}_{2} were introduced at the bottom. The precipitates built on the column wall were analyzed with Raman spectroscopy: injecting air into tap water at low ionic strength led to the formation of aragonite with 59.8% of the precipitates remaining at the column wall and the rest as particles in dispersion. At moderate ionic strength the dominant polymorph was calcite and 81.5% of the crystals were attached to the wall. At high ionic strength precipitation was inhibited. The presence of crystallization nuclei reduced the time for precipitation, but not the amount of scalings formed. Injecting {hbox {CO}}_{2} into the solution completely removed the scalings from the column wall. The model and its experimental backup lay the foundation for a process-based prediction of the scales (not only) in geothermal systems.

Thermodynamic modelling of CaCO3 polymorphs during CO2 sequestration by cement slurry with the addition of MgCl2

As the largest consumed man-made materials, the production of cement releases a huge amount of CO2 into the atmosphere and dramatically accelerates the global warming. However, the main mineral component of cement can solidify CO2 and convert it into stable calcium carbonate, thus producing low-carbon building materials. In this paper, a thermodynamic model was proposed to simulate the formation of different calcium carbonate polymorphs during the aqueous carbonation of Portland cement with the addition of the most common ionic crystal stabilizer MgCl2. Based on the variations of the calculated Gibbs free energy due to the substitution of magnesium ions for calcium ions, the model can be applied to quantify the proportion of different calcium carbonate polymorphs under different CO2 input amount, MgCl2 concentrations, and temperatures. By comparing the experimental results with the modelling results, the developed model was proved to be able to predict the proportion of calcium carbonate polymorphs with an average relative error of 5.51%. Through experimental and simulation studies, it has been found that 80–90 °C is the most favorable temperature range for the formation of aragonite in carbonation systems. When the concentration of MgCl2 exceeds 0.18 mol/L, calcium carbonate can exist as pure aragonite at near room temperature (25 °C). Considering both the maximum CO2 sequestration rate and the proportion of aragonite, the appropriate CO2 input range is M(CO2)/M(PC) = 0.45–0.5.

Biocrystal assembly patterns, biopolymer distribution and material property relationships in mytilus galloprovincialis, Bivalvia, and haliotis glabra, Gastropoda, shells

Biologically secreted structural materials serve many purposes: protection, defense, locomotion, support, reinforcement. Their specific functions are achieved through the intimate interaction of biopolymer and mineral tissue characteristics: composition, microstructure/fabric, biomaterial properties. Here we highlight and discuss for bivalve and gastropod shells the interrelation between mineral microstructure, texture, organic component fabric and distribution. We investigate these on the micrometer to submicrometer scale. We choose two model organisms living in distinct marine environments constructing their shells with distinct mineral microstructures and biopolymer assemblies. We compare shell structural characteristics for the mollusk H. glabra that forms its purely aragonitic shell of a prismatic and a columnar tablet portion. In contrast, M. galloprovincialis constructs its shell of calcite and aragonite. Calcite fibers comprise outer and sheeted arrangements of aragonite tablets inner shell portions, respectively. Glutaraldehyde fixed/MOPS buffer soluble organic material exists between aragonite prisms, while insoluble organic membranes encase calcite fibers and aragonite tablets. Electron backscatter diffraction demonstrates for the shell layers the marked difference in microstructure and texture. Quasi-static nanoindentation documents that, of the four investigated microstructures, prismatic aragonite forms the hardest and stiffest material. Nacreous aragonite, columnar or sheeted, is less hard and stiff, while the assembly of calcite fibers yields the softest and the most compliant biomaterial. Is toughness considered, the trend is reverted. We show that the toughness of calcite fibers is capable to outperform the tablet arrangements, despite the fact that fibrous calcite gives the weakest microstructure, relative to aragonitic microstructures.

Halimeda tuna (Bryopsidales, Ulvophyceae) calcification on the depth transect in the northern Adriatic Sea; carbonate production on the microscale of individual segments.

Halimeda tuna (J. Ellis & Solander) J.V. Lamouroux is the only Halimeda species found in the Mediterranean Sea, and it is an important habitat former. In the northern Adriatic, H. tuna is among the ten most abundant seaweeds in the upper-infralittoral belt in spring and autumn. The modular thalli consist of serially arranged calcified segments. Calcification is closely related to photosynthesis, which causes alkalinization of the inter-utricular space and triggers aragonite formation. Understanding of the complex patterns of segment shape plasticity in relation to CaCO3content at different depth levels is still incomplete. Geometric morphometrics was used to investigate H. tuna segment shape variation on the depth transect at Cape Madona Nature Monument in the northern Adriatic Sea. The position on the thallus and the CaCO3 content of each studied segment were recorded, allowing slight changes in mineral content to be detected at the microscale of the segments. Our results showed that shape, size, or asymmetry of H. tuna segments were not significantly affected by depth. On the other hand, plants that grew deeper were generally more calcified. The apical and subapical segments contributed to the increase in CaCO3 content at the deeper sites, whereas the basal segments did not. This indicates that reniform or oval segments positioned apically or subapically play a key role in calcification of H. tuna in Mediterranean ecosystems.

Effects of amino acids on the multiscale properties of carbonated wollastonite composites

This article presents an insight into the effects of amino acids on the moisture susceptibility, nanomechanical properties, and fracture properties of carbonated wollastonite composites. Paste samples containing various concentrations of amino acids were subjected to a CO2-rich environment. The addition of amino acids resulted in the formation of amorphous calcium carbonate (ACC), vaterite, and aragonite, instead of calcite, in the carbonated composites. Thermogravimetric analysis (TGA), Scanning electron microscopy (SEM), Fourier-transform infrared spectroscopy (FTIR), and nanoindentation were performed for microstructural analysis. Inductively coupled plasma-optical emission spectrometry (ICP-OES) was performed to determine the leaching of Ca2+ and Si2+ ions from carbonated wollastonite composites. It was observed that the chain lengths of the amino acids have an influence on the formation of crystal phases. For determining fracture properties, the Jenq-Shah model was performed using a notched beam. It was observed that amino acids reduced carbonation reaction and improved fracture properties by 156 %. The formation of metastable CaCO3 enhanced the nanomechanical properties of this carbonated system.

Stability of alkalinity in ocean alkalinity enhancement (OAE) approaches – consequences for durability of CO2 storage

Abstract. According to modelling studies, ocean alkalinity enhancement (OAE) is one of the proposed carbon dioxide removal (CDR) approaches with large potential, with the beneficial side effect of counteracting ocean acidification. The real-world application of OAE, however, remains unclear as most basic assumptions are untested. Before large-scale deployment can be considered, safe and sustainable procedures for the addition of alkalinity to seawater must be identified and governance established. One of the concerns is the stability of alkalinity when added to seawater. The surface ocean is already supersaturated with respect to calcite and aragonite, and an increase in total alkalinity (TA) together with a corresponding shift in carbonate chemistry towards higher carbonate ion concentrations would result in a further increase in supersaturation, and potentially to solid carbonate precipitation. Precipitation of carbonate minerals consumes alkalinity and increases dissolved CO2 in seawater, thereby reducing the efficiency of OAE for CO2 removal. In order to address the application of alkaline solution as well as fine particulate alkaline solids, a set of six experiments was performed using natural seawater with alkalinity of around 2400 µmol kgsw−1. The application of CO2-equilibrated alkaline solution bears the lowest risk of losing alkalinity due to carbonate phase formation if added total alkalinity (ΔTA) is less than 2400 µmol kgsw−1. The addition of reactive alkaline solids can cause a net loss of alkalinity if added ΔTA > 600 µmol kgsw−1 (e.g. for Mg(OH)2). Commercially available (ultrafine) Ca(OH)2 causes, in general, a net loss in TA for the tested amounts of TA addition, which has consequences for suggested use of slurries with alkaline solids supplied from ships. The rapid application of excessive amounts of Ca(OH)2, exceeding a threshold for alkalinity loss, resulted in a massive increase in TA (> 20 000 µmol kgsw−1) at the cost of lower efficiency and resultant high pH values > 9.5. Analysis of precipitates indicates formation of aragonite. However, unstable carbonate phases formed can partially redissolve, indicating that net loss of a fraction of alkalinity may not be permanent, which has important implications for real-world OAE application. Our results indicate that using an alkaline solution instead of reactive alkaline particles can avoid carbonate formation, unless alkalinity addition via solutions shifts the system beyond critical supersaturation levels. To avoid the loss of alkalinity and dissolved inorganic carbon (DIC) from seawater, the application of reactor techniques can be considered. These techniques produce an equilibrated solution from alkaline solids and CO2 prior to application. Differing behaviours of tested materials suggest that standardized engineered materials for OAE need to be developed to achieve safe and sustainable OAE with solids, if reactors technologies should be avoided.

Experimental study on solidification and remediation of lead–zinc tailings based on microbially induced calcium carbonate precipitation (MICP)

With the development of industry, the human demand for lead and zinc is increasing gradually, and the heavy metal pollution caused by the mining process of lead–zinc tailings is becoming more and more serious. To alleviate the pollution problem, microbially induced calcium carbonate precipitation (MICP) technology has been used to solidify and remedy lead–zinc tailings containing heavy metals based on the multi-shell curing method. Based on unconfined compression strength (UCS) and the parameters of heavy metal ions leaching concentration, exploring the addition of lead–zinc tailings on the impact of microbially induced calcium carbonate precipitation on solidifying and repairing lead–zinc tailings, using SEM, XRD, FTIR, EDS and XPS tests, and revealing the mechanism of MICP to solidify lead–zinc tailings. The results of the experiment showed that MICP had a good solidifying and repairing effect on heavy metals in lead–zinc tailings. When the thickness of lead–zinc tailings was 2 cm, the unconfined compression strength of the specimen was the maximum up to 0.93 MPa and the calcium carbonate content was up to 7.41 %. At the same time, Fe3+, Zn2+, Pb2+, Cu2+ and Cd2+ in the leaching solution could be completely removed, and the removal of Mn2+ and Cr3+was up to 98.24 % and 95.56 % respectively, which was the best condition for solidification. The samples of lead–zinc tailings being solidified and repaired based on MICP showed that calcium carbonate was primarily precipitated as the commonly crystal forms of aragonite, calcite, and vaterite, with strong cohesiveness, which made lead–zinc tailings particles connected together. At the same time, the CO32– and alkaline substances produced by microbial metabolism made the heavy metal ions released by the lead–zinc tailings mainly precipitate in the form of metal carbonate, which reduced the pollution to the environment. This technology provides a new method for solidifying and repairing lead–zinc tailings.

Transformation of amorphous calcium carbonate in the presence of magnesium, phosphate, and mineral surfaces

Amorphous calcium carbonate (ACC) is known as a precursor to crystalline calcium carbonate polymorphs (CaCO3) in various natural and synthetic systems. While ACC has been shown to form by heterogeneous nucleation in biological systems, it is less obvious whether it plays any role in inorganic settings where carbonate minerals grow on pre-existing surfaces. Tight assemblies of calcium carbonate (CaCO3) phases and clay minerals occur in rock-forming quantities in marlstones, are abundant in Earth’s critical zone, in the sediments of aqueous environments, and in the atmosphere as aerosol particles. Previous studies suggested that the co-occurrence of clay and carbonate minerals (especially smectite and calcite) results from the heterogeneous nucleation of CaCO3 on the surface of smectite. In order to understand the role of ACC in the heterogeneous nucleation of carbonate minerals and the effects of common silicate mineral surfaces on the stability of ACC, we synthesized ACC both in the presence and absence of smectite, kaolinite, and quartz, monitored the pH of the solution during the experiments, and studied the precipitated phases with various transmission electron microscopy techniques. Since Mg2+ and (PO43−) ions are known to prolong the lifetime of ACC and are important components in natural aqueous environments, we tested their effects on ACC transformation in both homogeneous and heterogeneous systems.In all studied systems ACC was the first condensed carbonate phase. The presence of smectite in the solution reduced the lifetime of ACC and led to the rapid formation of calcite. We suggest that the main factor driving the transition was the enhanced growth of ACC particles on the surfaces of smectite flakes. On the other hand, the presence of kaolinite and quartz slightly increased ACC stability; ACC typically rimmed the silicate particles and transformed slower than separated, individual ACC particles. When Mg2+ was present in the solution, ACC stability significantly increased and led to the formation of aragonite instead of calcite in both homogeneous and heterogeneous systems. The presence of (PO4)3− ions caused only a minor delay in ACC transformation and the crystalline product was calcite, irrespective of the presence of silicate minerals. These observations suggest that smectite minerals play important roles in the formation of carbonate minerals by providing a suitable surface for the rapid growth and coalescence of ACC particles, leading to larger particle sizes which, in turn, result in the conversion of ACC into calcite.