Aragonite Surface Research Articles

Structural and Functional Analyses of the Acidic and Basic Amino Acid Repeat Sequence (DDRK) in Pif 80 from Pinctada fucata on the Aragonite Crystal Surface Using NMR

Shellfish and crustaceans use calcium carbonate (CaCO3) as a protective shell and exoskeleton. CaCO3 is a major biomineral component in invertebrates. The Pinctada fucata shell contains two crystal polymorphs, with outer prismatic and inner nacreous layers. Diverse shell organic materials play significant roles in microstructure formation with organic–inorganic interactions. Pif 80 has a repeated acidic and basic amino acid low-complexity region that specifically binds to aragonite crystals. In this study, we aimed to elucidate the microstructure formation mechanism by analyzing the interaction between the low-complexity region and CaCO3. We used solution NMR to evaluate the structure and activity of the Pif 80 acidic region. We designed synthesis methods for aragonite nanoparticles that can be dispersed and rotated in solution. The results revealed that the “GDDRKDDRKG” peptide has the appropriate structure for aragonite surface binding, wherein the four carboxy groups of the Asp side chain are in the same plane, supported by Arg and Lys residues. The saturation transfer difference also supported this structure. These results suggest that the Pif 80 acidic region binds the aragonite crystal surface via Asp side chains with basic residues, providing new insights into the function of acidic and basic amino acids in biomineral formation.

Effect of acidic microbial polysaccharides on calcium carbonate polymorph formation in hot water at 40-80 °C

Microbial organic molecules are widely involved in the formation of calcium carbonate (CaCO3) minerals in various environments on the earth’s surface including hot springs. To evaluate the effect of acidic microbial polysaccharides on the CaCO3 polymorph formation in hot water environments, batch experiments on the CaCO3 formation were performed in the systems containing alginic acid (A system) or gellan gum (G system) at 40, 60, and 80 oC for 24 h. Each system contained 5.0 mmol/L Ca2+ and 20.0 mmol/L HCO3- ions with 0.0, 1.0, 5.0, 10, 50, and 100 mg/L alginic acid or gellan gum. Results showed that needle- or rod-like shaped aragonite crystals were formed as the predominant polymorph in the absence of polysaccharides at 40 - 80 oC. In the presence of polysaccharides, alginic acid greatly inhibited the aragonite formation and favored the rhombohedral calcite formation as the predominant polymorph with increasing alginic acid concentration at 40 oC. The effect of alginic acid on the polymorphs slightly decreased as temperature increased but was maintained up to 80 oC. Gellan gum also exhibited a similar effect to a lesser extent than alginic acid at the same concentrations and temperatures. Thus, acidic microbial polysaccharides exhibited significant inhibitory effect on the aragonite formation and favored the calcite formation depending on their molecular structures in hot water environments. This effect of polysaccharides on the CaCO3 polymorphs is attributed to the high adsorption affinity of the polymer molecules for the aragonite surface.

The mineralization and early diagenesis of deep-sea coral Madrepora oculata

The skeletons of aragonitic corals carry essential information about oceanic environmental changes in the past. However, coral-based geochemical proxies are sensitive to vital effect and post-depositional diagenetic processes, which have not been well understood for deep-sea corals. In this study, we have investigated the mineralization and early diagenetic process of the dendroid stem of a dead deep-sea scleractinian coral sample (Madrepora oculata) collected by Remote Operated Vehicle (ROV) from a seamount in the Caroline Ridge, western Pacific. Despite being only ~270 years old, as dated by the U-series disequilibrium method, this sample already has extensive diagenesis including a visible chalky layer. The outermost chalky layer and the adjacent pristine skeleton of the sample were characterized by multiple mineralogical (SEM, XRD, EBSD, Raman Spectroscopy) and geochemical (ICP-OES, LA-ICP-MS, IR-MS, MC-ICP-MS) techniques. Skeletal stable C and O isotopes display strong linearity, with a trend pointing toward the theoretical isotopic composition of inorganically-precipitated aragonite from local seawater. This observation suggests that the epithecal mineralization of M. oculata is far from equilibrium with seawater and can be quantitatively explained by bio-mineralization models developed for solitary scleractinian corals. We further show that chalking of the outer layer, a common feature in sub-fossilized deep-sea corals, is caused by microbial-driven boring and μm-scale dissolution voids at an average rate of 3.5 μmol/cm2/y (scaled to the skeleton surface) without notable secondary infillings or mineralogical alternations. The higher average δ13C and δ18O of the chalky part than the pristine part might be due to the preferential dissolution of carbonate relatively depleted in 13C and 18O during alteration, and/or reduced isotope fractionation from seawater during coral mineralization of the outer layer. The chalky layer shows significantly higher Mn/Ca and Th/Ca ratios than the pristine part, characterized by Th/Mn ratios similar to that of the dissolved component of local seawater. Our study suggests that the incorporation of external Th into the coral skeleton is most likely associated with organic binding rather than detrital contamination, secondary carbonate mineral formation, adsorption onto surfaces of chalky aragonite or ferromanganese oxides, with important implications for coral U-series geochronology.

Density Functional Theory (DFT) as a Nondestructive Probe in the Field of Art Conservation: Small-Molecule Adsorption on Aragonite Surfaces.

Humans have incorporated minerals in objects of cultural heritage importance for millennia. The surfaces of these objects, which often long outlast the humans that create them, are undeniably exposed to a diverse mixture of chemicals throughout their lifetimes. As of yet, the art conservation community lacks a nondestructive, accurate, and inexpensive flexible computational screening method to evaluate the potential impact of chemicals with art, as a complement to experimental studies. In this work, we propose periodic density functional theory (DFT) studies as a way to address this challenge, specifically for the aragonite phase of calcium carbonate, a mineral that has been used in pigments, marble statues, and limestone architecture since ancient times. Computational models allow art conservation scientists to better understand the atomistic impact of small-molecule adsorbates on common mineral surfaces across a wide variety of environmental conditions. To gain insight into the surface adsorption reactivity of aragonite, we use DFT to investigate the atomistic interactions present in small-molecule-surface interfaces. Our adsorbate set includes common solvents, atmospheric pollutants, and emerging contaminants. Chemicals that significantly disrupt the surface structure such as carboxylic acids and sulfur-containing molecules are highlighted. We also focus on comparing adsorption energies and changes in surface bonds, which allows for the identification of key features in the electronic structure presented in a projected-density-of-state analysis. The trends outlined here will guide future experiments and allow art conservators to gain a better understanding of how a wide range of molecules interact with an aragonite surface under variable conditions and in different environments.

Site-specific interactions enhanced dissolution of natural aragonite (110) surfaces in succinic acid (SUC) solutions: Implications for the oceanic aragonite dissolution fluxes

Aragonite, one of the most common biological calcium carbonate minerals, is widespread in marine plankton and neritic sediments (e.g., accounting for 89% of pelagic calcification of CaCO3 in the surface ocean). Its dissolution directly affects the export fluxes of CaCO3 in seawater. The aragonite dissolution in the ocean correlates with the increase of partial pressure of carbon dioxide and is pertinent to acidic biomolecules. However, aragonite dissolution in seawater with acidic biomolecules has been overlooked, and the interaction mechanism is unclear. In-situ atomic force microscopy (AFM) was employed to observe the dissolution features of aragonite (110) growth surfaces in succinic acid (SUC, HOOC-CH2-CH2-COOH) solutions. The results demonstrate that (1) both the morphologies and spreading rates of the etch pits formed on aragonite (110) surfaces are altered by the interactions between SUC molecules and the surfaces; (2) the [11‾1] and [1‾11] steps of the etch pits on aragonite (110) surfaces in SUC solutions are kinetically controlled; (3) dissolution rates of aragonite (110) surfaces are proportional to SUC concentrations, which is attributed to the strong complexation between SUC molecules and surface-bounded Ca atoms (≡Ca+); (4) etch pits morphologies of aragonite (110) and calcite (10.4) surfaces are different in SUC solutions, and the spreading rates of the etch pits of the former are one to two orders of magnitude lower than those of the latter. Furthermore, aragonite is found to be more sensitive to SUC molecules than calcite, suggesting that dissolving aragonitic materials could be more easily affected by circumambient biomolecules than dissolving calcitic materials. In seawater that contains organic molecules, the dissolution fluxes of aragonite could be much slower than those of calcite. These findings not only reveal the site-specific interactions between SUC molecules and the ≡Ca+ on aragonite (110) surfaces but also emphasize its implication for the oceanic aragonite dissolution fluxes.

Low concentrated phosphorus sorption in aqueous medium on aragonite synthesized by carbonation of seashells: Optimization, kinetics, and mechanism study

Phosphorus (P) concentration beyond threshold limit can trigger eutrophication in stagnant water bodies nevertheless it is an indispensable macronutrient for aquatic life. Even in low P concentration (≤1 mg L−1), P can be detrimental for ecosystem's health, but this aspect has not been thoroughly investigated. The elimination of low P content is rather expensive or complex. Therefore, a unique and sustainable approach has been proposed in which valorized bivalve seashells can be used for the removal of low P content. Initially, acicular shaped aragonite particles (~21 μm) with an aspect ratio of around 21 have been synthesized through the wet carbonation process and used to treat aqueous solutions containing P in low concentration (P ≤ 1 mg L−1). Response surface methodology based Box-Behnken design has been employed for optimization study which revealed that with aragonite dosage (140 mg), equilibrium pH (~10.15), and temperature (45 °C), a phosphorus removal efficiency of ~97% can be obtained in 10 h. The kinetics and isotherm studies have also been carried out (within the range P ≤ 1 mg L−1) to investigate a probable removal mechanism. Also, aragonite demonstrates higher selectivity (>70%) towards phosphate with coexisting anions such as nitrate, chloride, sulfate, and carbonate. Through experimental data, elemental mapping, and molecular dynamic simulation, it has been observed that the removal mechanism involved a combination of electrostatic adsorption of Ca2+ ions on aragonite surface and chemical interaction between the calcium and phosphate ions. The present work demonstrates a sustainable and propitious potential of seashell derived aragonite for the removal of low P content in aqueous solution along with its unconventional mechanistic approach.

Heterogeneous Nucleation and Growth of CaCO3 on Calcite (104) and Aragonite (110) Surfaces: Implications for the Formation of Abiogenic Carbonate Cements in the Ocean

Although near-surface seawater is supersaturated with CaCO3, only a minor part of it is abiogenic (e.g., carbonate cements). The possible reason for such a phenomenon has attracted much attention in the past decades. Substrate effects on the heterogeneous nucleation and growth of CaCO3 at various Mg2+/Ca2+ ratios may contribute to the understanding of the origin of abiogenic CaCO3 cements. Here, we used in situ atomic force microscopy (AFM), scanning electron microscopy (SEM), X-ray diffraction (XRD) and Raman spectroscopy to study the heterogeneous nucleation and growth of CaCO3 on both calcite (104) and aragonite (110) surfaces. The results show that (1) calcite spiral growth occurs on calcite (104) surfaces by monomer-by-monomer addition; (2) the aggregative growth of aragonite appears on aragonite (110) surfaces through a substrate-controlled oriented attachment (OA) along the [001] direction, followed by the formation of elongated columnar aragonite; and (3) Mg2+ inhibits the crystallization of both calcite and aragonite without impacting on crystallization pathways. These findings disclose that calcite and aragonite substrates determine the crystallization pathways, while the Mg2+/Ca2+ ratios control the growth rate of CaCO3, indicating that both types of CaCO3 substrate in shallow sediments and aqueous Mg2+/Ca2+ ratios constrain the deposition of abiogenic CaCO3 cements in the ocean.

Impact of selenite, arsenate, and silicate oxyanions on the polymorphism and precipitation rate of calcium carbonate minerals in solutions with Mg2+ ions

Abstract Selenic acid (Se(IV)), arsenic acid (As(V)), and silicic acid are widespread oxyanions in some Earth’s surface environments. To evaluate the effect of these oxyanions on the polymorphism and precipitation rate of calcium carbonate (CaCO3) minerals, precipitation experiments were performed in the systems containing these oxyanions by the batch method using 100 mL solution at 25 °C. Each solution contained 5.0 mmol/L Ca2+ and Mg2+, and 20.0 mmol/L HCO3− ions with 0.00, 0.01, 0.05, 0.1, 0.5, and 1.0 mmol/L of selenic, arsenic, or silicic acids. Additionally, adsorption experiments with the oxyanions on the surface of calcite and aragonite were conducted to evaluate the adsorption affinity for the surface of CaCO3 minerals. Results indicated that the oxyanions exhibited inhibition effect on the formation of aragonite resulting in the growth of calcite as a predominant polymorph with the decrease in the precipitation rate of CaCO3 minerals in the following order: selenic acid > arsenic acid > silicic acid. The adsorption experiments confirmed that the oxyanions showed higher adsorption affinity for the aragonite surface than the calcite surface in the following order: selenic acid > arsenic acid > silicic acid. This order of adsorption affinity is consistent with the effectiveness of oxyanions on the formation of CaCO3 minerals. Thus, the adsorption of oxyanions on the CaCO3 surface contributed to the inhibition of aragonite formation and to the decrease in overall precipitation rate of CaCO3 minerals depending on the degree of adsorption affinity of the oxyanions.

Use of Powdered Cockle Shell as a Bio‐Sorbent Material for Phosphate Removal from Water

In this study, powdered cockle shell was used as a bio‐sorbent material for the removal of phosphates in water. The SEM–EDX, XRD, and FT‐IR analyses showed that calcium carbonate (CaCO3) exists in aragonite state in powdered cockle shell. The influences of pH, initial phosphate concentrations and contact time were conducted in a series of batch sorption experiments. Among the various factors studied, the pH was turned out to be a key variable for phosphate removal. Based on the experimental results for pH, it was found that the phosphate is chemically sorbed onto aragonite surface. At lower phosphate concentration, roughly 88% of sorption efficiency was accomplished by powdered cockle shell. The Pseudo‐second‐order kinetic and the Langmuir isotherm model were fitted well to the sorption experimental data of phosphate because of their higher correlation coefficients (R2).

Atomic-Scale Quantification of Interfacial Binding between Peptides and Inorganic Crystals: The Case of Calcium Carbonate Binding Peptide on Aragonite

Using a specific explicitly solvated interface model between a calcium carbonate binding peptide and crystalline aragonite, we investigate the electronic structure, atomic bonding, solvation effect, and the role of hydrogen bonding on the cohesion, stability, and functionality of this complex hybrid system using density functional calculation. The large interface model is strategically constructed using a stepwise procedure followed by ab initio molecular dynamics to obtain the optimal conformation. The calculated data on the electronic structure and bonding are analyzed in terms of three structural parts: aragonite, peptide, and water. Next, we focus on the binding between aragonite (001) surface and the peptide mediated by water. Finally, specific interatomic bonding between the amino acids in peptide and the (001) surface of aragonite is quantified. A single quantum mechanical metric, the total bond order density (TBOD), infers the dynamic interplay of different competing interactions. Four amino acids...

Study of Binding Interaction between Pif80 Protein Fragment and Aragonite.

Pif is a crucial protein for the formation of the nacreous layer in Pinctada fucata. Three non-acidic peptide fragments of the aragonite-binding domain (Pif80) are selected, which contain multiple copies of the repeat sequence DDRK, to study the interaction between non-acidic peptides and aragonite. The polypeptides DDRKDDRKGGK (Pif80-11) and DDRKDDRKGGKDDRKDDRKGGK (Pif80-22) have similar binding affinity to aragonite. Solid-state NMR data indicate that the backbones of Pif80-11 and Pif80-22 peptides bound on aragonite adopt a random-coil conformation. Pif80-11 is a lot more effective than Pif80-22 in promoting the nucleation of aragonite on the substrate of β-chitin. Our results suggest that the structural arrangement at a protein-mineral interface depends on the surface structure of the mineral substrate and the protein sequence. The side chains of the basic residues, which function as anchors to the aragonite surface, have uniform structures. The role of basic residues as anchors in protein-mineral interaction may play an important role in biomineralization.

Atlanta ariejansseni, a new species of shelled heteropod from the Southern Subtropical Convergence Zone (Gastropoda, Pterotracheoidea).

The Atlantidae (shelled heteropods) is a family of microscopic aragonite shelled holoplanktonic gastropods with a wide biogeographical distribution in tropical, sub-tropical and temperate waters. The aragonite shell and surface ocean habitat of the atlantids makes them particularly susceptible to ocean acidification and ocean warming, and atlantids are likely to be useful indicators of these changes. However, we still lack fundamental information on their taxonomy and biogeography, which is essential for monitoring the effects of a changing ocean. Integrated morphological and molecular approaches to taxonomy have been employed to improve the assessment of species boundaries, which give a more accurate picture of species distributions. Here a new species of atlantid heteropod is described based on shell morphology, DNA barcoding of the Cytochrome Oxidase I gene, and biogeography. All specimens of Atlanta ariejansseni sp. n. were collected from the Southern Subtropical Convergence Zone of the Atlantic and Indo-Pacific oceans suggesting that this species has a very narrow latitudinal distribution (37–48°S). Atlanta ariejansseni sp. n. was found to be relatively abundant (up to 2.3 specimens per 1000 m3 water) within this narrow latitudinal range, implying that this species has adapted to the specific conditions of the Southern Subtropical Convergence Zone and has a high tolerance to the varying ocean parameters in this region.

Incorporation of Mg2+ in surface Ca2+ sites of aragonite: an ab initio study

First-principles calculations of Mg2+-containing aragonite surfaces are important because Mg2+ can affect the growth of calcium carbonate polymorphs. New calculations that incorporate Mg2+ substitution for Ca2+ in the aragonite {001} and {110} surfaces clarify the stability of Mg2+ near the aragonite surface and the structure of the Mg2+-containing aragonite surface. The results suggest that the Mg2+ substitution energy for Ca2+ at surface sites is lower than that in the bulk structure and that Mg2+ can be easily incorporated into the surface sites; however, when Mg2+ is substituted for Ca2+ in sites deeper than the second Ca2+ layer, the substitution energy approaches the value of the bulk structure. Furthermore, Mg2+ at the aragonite surface has a significant effect on the surface structure. In particular, CO3 groups rotate to achieve six-coordinate geometry when Mg2+ is substituted for Ca2+ in the top layer of the {001} surface or even in the deeper layers of the {110} surface. The rotation may relax the atomic structure around Mg2+ and reduces the substitution energy. The structural rearrangements observed in this study of the aragonite surface induced by Mg2+ likely change the stability of aragonite and affect the polymorph selection of CaCO3.

<sup>13</sup>C-<sup>18</sup>O Bonds in Precipitated Calcite and Aragonite: An <i>ab Initio</i> Study

The 13C-18O bonds in carbonates are potential single-phase geo-thermometers. However, their theoretical distributions (noted as Δ47s) in CO2 degassed from calcite and aragonite with phosphoric acid are unclear. Thus, the isotope reactions of 13C-18O bonds on the growing surfaces of calcite (0001) and aragonite (001) planes were investigated using ab initio techniques. It was found that these reactions determined 13C-18O clumped isotope signatures in bulk calcite and aragonite minerals with novel Δ47 polynomials: and for temperatures ranging from 260 to 1500 K. These theoretical results were in good agreement with the experimental data. In addition, the influence of phosphoric acid on these polynomials was at the level of 0.01‰.

Oxygen isotope fractionation between aragonite and seawater: Developing a novel kinetic oxygen isotope fractionation model

Oxygen isotope fractionation factors between aragonite and seawater are studied at T=25–55°C and pH=7.4–8.1 in a set of ‘free-drift’ precipitation experiments with various CO2-degassing rates (0–75cc/min). The measured fractionation factors correlate weakly with degassing rate, but strongly with temperature in the following form (R2=0.998):1000lnαaragonite–seawater=22.5(±0.5)(103/T)-46.1(±1.6)where α is the fractionation factor and T is the temperature in Kelvin. Along with results from previous studies, these experiments help calibrate two extreme cases of a new kinetic model – extremely fast (e.g., during spontaneous precipitation) and slow aragonite precipitation processes (e.g., during slow ‘free-drift’ precipitation experiments) – at pH >7.5, T=0–55°C and salinity=∼5–39.6‰. The model assumes little isotopic fractionation between DIC species and aragonite during their attachment and detachment on aragonite surfaces at low temperatures and the δ18O value of aragonite equals the average δ18O value of all contributing DIC species. During the fast precipitation of aragonite, the contribution of each DIC species to the δ18O value of aragonite is proportional to its concentration, whereas in a slow precipitation process when the system reaches a steady state, the contribution is determined by both its concentration and a modifying factor. Although the physical meaning of this modifying factor depends on the method of derivation and other assumptions made, three different derivations based on precipitation/dissolution kinetics, isotope exchange reactions, and isotope disequilibrium among DIC species all lead to the same formulation. At pH >7.5, the natural logarithm of the modifying factor (kHCO3-) for adjusting the contribution of the bicarbonate ion relative to the carbonate ion is calibrated, using experimental data covering a range of temperature, salinity, pH, and precipitation rate, as follows:lnkHCO3-=(2.22±0.08)×(pH-pH0)+(846±78)·S-(40.3±2.1)×103RT-(0.30±0.03)·S-(4.7±1.2)where S is the salinity, R is the gas constant, pH0 is the pH difference between free scale and the scale used to calculate the concentration of DIC species. Quantitative evaluation of these two extreme cases using our model suggests that oxygen isotope fractionation between aragonite and water varies significantly with a pH change between 7.5 and 10 (e.g., ∼4‰ at 25°C), but little with a salinity change between 5‰ and 35‰ (<0.1‰ at 25°C) during extremely fast precipitation, whereas during slow precipitation little variation occurs with pH varying between 7.5 and 10 at a given salinity (<0.3‰ in δ18O at T=0–55°C). This model connects aragonite precipitation kinetics with oxygen isotope fractionation between aragonite and water, and provides a framework to reconcile the discrepancy among experimental calibrations of aragonite–water fractionation in different labs and empirical calibrations using biogenic aragonites by variations in precipitation conditions (including pH, salinity and precipitation rate).

Hydrothermal replacement of Aragonite by Calcite: interplay between replacement, fracturing and growth

The hydrothermal transformation of single aragonite crystals into polycrystalline calcite has been studied under hydrothermal conditions. The transformation involves a fluid-mediated replacement reaction, associated with fracturing of the initial aragonite crystal and growth of calcite throughout various parts of the reacted aragonite. The observed overall preservation of the crystal morphology is typical of an interface-coupled dissolution-reprecipitation mechanism. Analysis of the crystallographic orientation of the product calcite using electron backscatter diffraction (EBSD) showed little to no link between the two phases under the studied conditions, with calcite crystallites exhibiting dominantly different crystallographic orientations compared to those of the aragonite and of neighbouring calcite domains. The complexity of the observed textures is mainly a result of the combination of fracturing of the crystal and initiation of nucleation and growth at different points of the exposed aragonite surface. Experiments performed with solutions enriched in 18O as a tracer for oxygen exchange and monitored by Raman spectroscopy, showed that carbonate ions in the starting solution are mixed with carbonate from the dissolving aragonite, resulting in an 18O concentration in the product calcite which depended on the local fluid transport through the fractures. As replacement processes among the CaCO3 phases are relevant to a wide range of applications, understanding the mechanisms is essential for the interpretation of observations of natural and/or experimental samples. This study describes the interplay between the replacement process, the fracturing of the crystal and growth of the new phase, calcite, and provides new insights into the mechanism of the aragonite to calcite transition. The combination of the two methods, EBSD and Raman spectroscopy, demonstrates the importance of local fluid composition and transport pathways in determining the isotope and element exchange in mineral replacement reactions.

Atomic-Resolution Imaging of Aragonite (001) Surface in Water by Frequency Modulation Atomic Force Microscopy

Aragonite is a high-pressure phase of calcium carbonate crystals. However, aragonite is formed under normal pressure with the help of biomineralization. Although it is important to observe growth features to understand the growth mechanism of the aragonite surface, only a few research groups have succeeded in observing the growth steps. In this study, we performed observation of the (001) face of a natural aragonite crystal surface at the atomic scale by the newly developed frequency modulation atomic force microscopy (FM-AFM). On the (001) face of aragonite, several growth islands with flat terraces were observed. We have succeeded in obtaining atomic-resolution images of the pseudo-hexagonal arrangement of calcium ions on the terraces. We also obtained atomic-resolution images on the calcite (101̄4) face by FM-AFM, which highlights the difference in the surface atomic arrangements of the two polymorphs was clearly visualized.

Atomic-Resolution Imaging of Aragonite (001) Surface in Water by Frequency Modulation Atomic Force Microscopy

Atomic-Resolution Imaging of Aragonite (001) Surface in Water by Frequency Modulation Atomic Force Microscopy

Crystallization of CaCO3 in the presence of sulfate and additives: Experimental and molecular dynamics simulation studies

The effects of sulfate and BHTPMP (Bis (hexamethylene) triaminepentakis (methylene phosphonic acid)) on the crystallization rate, phase composition and morphology of calcium carbonate have been studied. It was observed that sulfate reduces the nucleation rate and favors the formation of aragonite form in the calcium carbonate precipitate. Moreover, in the presence of sulfate the rhombohedral morphology of the calcite crystals is modified, and during the formation of calcite, the development of {104} faces are more significantly prohibited than {110} faces. In the presence of sulfate together with BHTPMP, the crystallization process is inhibited and the modified morphology and the dominant calcite form are observed in the solid. The results from molecular dynamics simulations show the more strong combination of sulfate with calcite surface, in particular the {104} face, in comparison with the aragonite surface. The strong interaction of BHTPMP with sulfate and the aragonite surface favors the formation of the dominant calcite phase in the precipitate.

ワラストナイト系晶析型脱リン材におけるリン除去メカニズムに関する研究

Wollastonite consolidated by carbonation is one of promising materials as a crystallization-type phosphorous adsorbent. The detailed crystallization mechanism of phosphorous, however, has not been clarified yet. In this study, the crystallization of calcium phosphate on the surface of the consolidated body of wollastonite was examined in detail by SEM, EDX and XRD in order to clarify the detailed mechanism of phosphorous removal by crystallization.It was found that the crystallization of calcium phosphate occurred to a depth of 100μm from the surface of the wollastonite consolidated body, which was the region fully carbonated and mainly composed SiO2 gel-like compound and CaCO3 (calcite and aragonite). Further investigation of crystallization of calcium phosphate using SiO2 gel and CaCO3 powders consolidated by gypsum revealed that a crystallization site of calcium phosphate was probably the surface of CaCO3 crystals generated by the carbonation of wollastonite. Moreover, it was found from the investigation using calcite and aragonite single crystals that the crystallization of calcium phosphate occurred on the surfaces of both crystals and that the surface of aragonite was slightly advantageous for crystallization. XRD analysis showed that Ca-deficient hydroxylapatite (Ca9(HPO4)(PO4)5OH, d-HAP) and dicalcium phosphate dehydrate (CaHPO4·2H2O, DCPD) were mainly crystallized on the surfaces of aragonite and calcite, respectively.