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Calcium carbonate (CaCO3) is an important inorganic mineral in biological and geological systems. Traditionally, it is widely used in plastics, papermaking, ink, building materials, textiles, cosmetics, and food. Over the last decade, there has been rapid development in the controlled synthesis and surface modification of CaCO3, the stabilization of amorphous CaCO3 (ACC), and CaCO3-based nanostructured materials. In this review, the controlled synthesis of CaCO3 is first examined, including Ca2+–CO3 2_ systems, solid– liquid–gas carbonation, water-in-oil reverse emulsions, and biomineralization. Advancing insights into the nucleation and crystallization of CaCO3 have led to the development of efficient routes towards the controlled synthesis of CaCO3 with specific sizes, morphologies, and polymorphs. Recently developed surface modification methods of CaCO3 include organic and inorganic modifications, as well as intensified surface reactions. The resultant CaCO3 can then be further engineered via template induced biomineralization and layer-by- layer assembly into porous, hollow, or core–shell organic–inorganic nanocomposites. The introduction of CaCO3 into nanostructured materials has led to a significant improvement in the mechanical, optical, magnetic, and catalytic properties of such materials, with the resultant CaCO3-based nanostructured materials showing great potential for use in biomaterials and biomedicine, environmental remediation, and energy production and storage. The influences that the preparation conditions and additives have on ACC preparation and stabilization are also discussed. Studies indicate that ACC can be used to construct environmentally-friendly hybrid films, supramolecular hydrogels, and drug vehicles. Finally, the existing challenges and future directions of the controlled synthesis and functionalization of CaCO3 and its expanding applications are highlighted.

Boc-L-DOPA(OBn)2-OH is a simple synthetic molecule that promotes hydrogelation through electrostatic and π-πstacking interactions. Hydrogelation can occur in alkaline conditions by the use of triggers. Four hydrogels were prepared varying the base, NaOH or Na2CO3, and the trigger, GdL or CaCl2. When the hydrogel formed in the presence of Na2CO3 and CaCl2, the concomitant production of CaCO3 crystals occurred, generating an organic/inorganic composite material. It was observed that the hydrogel once self-assembled preserved its status even if the trigger, the calcium ions, was removed. The viscoelastic behavior of the hydrogels was analyzed through rheological experiments, which showed a solid-like behavior of the hydrogels. The corresponding xerogels were analyzed mainly by scanning electron microscopy (SEM) and synchrotron X-ray diffraction analysis (XRD). They showed differences in structure, morphology, and fiber organization according to their source. This research presents a hydrogel system that can be applied as a soft biomaterial for tissue engineering, cosmetics, food, and environmental science. Moreover, it represents a model for biomineralization studies in which the hydrogel structure can act as an analogue of the insoluble matrix that confines the calcification site, provides Ca2+, and preserves its structure.

Understanding the effect that specific amino acids (AA) exert on calcium phosphate (CaPs) formation is proposed as a way of providing deeper insight into CaPs’ biomineralization and enabling the design of tailored-made additives for the synthesis of functional materials. Despite a number of investigations, the role of specific AA is still unclear, mostly because markedly different experimental conditions have been employed in different studies. The aim of this paper was to compare the influence of different classes of amino acids, charged (aspartic acid, Asp and lysine, Lys), polar (asparagine, Asn and serine, Ser) and non-polar (phenylalanine, Phe) on CaPs formation and transformation in conditions similar to physiological conditions. The precipitation process was followed potentiometrically, while Fourier transform infrared spectroscopy, powder X-ray diffraction, electron paramagnetic spectroscopy (EPR), scanning and transmission electron microscopy were used for the characterization of precipitates. Except for Phe, all investigated AAs inhibited amorphous calcium phosphate (ACP) transformation, with Ser being the most efficient inhibitor. In all systems, ACP transformed in calcium-deficient hydroxyapatite (CaDHA). However, the size of crystalline domains was affected, as well as CaDHA morphology. In EPR spectra, the contribution of different radical species with different proportions in diverse surroundings, depending on the type of AA present, was observed. The obtained results are of interest for the preparation of functionalized CaPs’, as well as for the understanding of their formation in vivo.

The biomineralization of corals occurs under conditions of high and low supersaturation with respect to aragonite, which corresponds to day- or night-time periods of their growth, respectively. Here, in vitro precipitation of aragonite in artificial seawater was investigated at a high supersaturation, allowing spontaneous nucleation and growth, as well as at low supersaturation conditions, which allowed only the crystal growth on the deliberately introduced aragonite seeds. In either chemical systems, soluble organic matrix (SOM) extracted from Balanophyllia europaea (light sensitive) or Leptopsammia pruvoti (light insensitive) was added. The analyses of the kinetic and thermodynamic data of aragonite precipitation and microscopic observations showed that, at high supersaturation, the SOMs increased the induction time, did not affect the growth rate and were incorporated within aggregates of nanoparticles. At low supersaturation, the SOMs affected the aggregation of overgrowing crystalline units and did not substantially change the growth rate. On the basis of the obtained results we can infer that at high supersaturation conditions the formation of nanoparticles, which is typically observed in the skeleton's early mineralization zone may occur, whereas at low supersaturation the overgrowth on prismatic seeds observed in the skeleton fiber zone is a predominant process. In conclusion, this research brings insight on coral skeletogenesis bridging physicochemical (supersaturation) and biological (role of SOM) models of coral biomineralization and provides a source of inspiration for the precipitation of composite materials under different conditions of supersaturation.

Plate-like calcite crystals with expressed unstable {001} planes are interesting research model for investigations of interfacial interactions of different additive molecules, but also the crystal growth mechanisms. The aim of this study is to reproducibly prepare a significant amount of well-defined plate-like calcite crystals and to investigate the critical experimental parameters. Thus, in precipitation system c(NaHCO3) = c(CaCl2) = 0.1 mol dm−3, the influence of hydrodynamic parameters (mode of mixing of the reaction components) and a presence of lithium ions Li+ within a wide range of concentrations, 0.0 mol dm−3 &lt, c(Li+) &lt, 1.0 mol dm−3, have been studied. In addition, the kinetics of the solution mediated transformation of the initially formed metastable polymorph, vaterite, were followed in order to reproducibly describe the formation of stable calcite with expressed unstable morphology. The results indicate that the plate-like calcite is formed predominantly when the ultrasound irradiation is applied at c(Li+) ≥ 0.3 mol dm−3. On the other hand, when the magnetic and mechanical stirring are applied at higher Li+ concentrations, truncated rhombohedral crystals in a mixture with plate-like crystals are obtained. It was also found that the Li+ addition significantly prolonged the transformation, mainly by inhibiting the crystal growth of calcite.

The constantly growing need for advanced bone regeneration materials has motivated the development of calcium phosphates (CaPs) composites with a different metal or metal-oxide nanomaterials and their economical and environmentally friendly production. Here, two procedures for the synthesis of CaPs composites with TiO2 nanoplates (TiNPl) and nanowires (TiNWs) were tested, with the immersion of TiO2 nanomaterials (TiNMs) in corrected simulated body fluid (c-SBF) and precipitation of CaP in the presence of TiNMs. The materials obtained were analyzed by powder X-ray diffraction, spectroscopic and microscopic techniques, Brunauer–Emmett–Teller surface area analysis, thermogravimetric analysis, dynamic and electrophoretic light scattering, and their hemocompatibility and ability to induce reactive oxygen species were evaluated. After 28 days of immersion in c-SBF, no significant CaP coating was formed on TiNMs. However, the composites with calcium-deficient apatite (CaDHA) were obtained after one hour in the spontaneous precipitation system. In the absence of TiNMs, CaDHA was also formed, indicating that control of the CaP phase formed can be accomplished by fine-tuning conditions in the precipitation system. Although the morphology and size of crystalline domains of CaDHA obtained on the different nanomaterials differed, no significant difference was detected in their local structure. Composites showed low reactive oxygen species (ROS) production and did not induce hemolysis. The results obtained indicate that precipitation is a suitable and fast method for the preparation of CaPs/TiNMs nanocomposites which shows great potential for biomedical applications.

CaCO3 precipitation is physical-chemical basis of biomineral formation of hard tissue (shells, skeletons) in marine calcifying organisms (=biomineralization). Processes controlling biomineralization are still not fully clarified, so the study of influence of pH on basic processes of CaCO3 precipitation should contribute to better understanding of biomineralization under climate change. This paper reports on the effect of initial pH (pH0) and type of stirring (mechanical and magnetical) on spontaneous precipitation and phase composition, size and morphology of spontaneously precipitated CaCO3 formed at the identical initial supersaturation, ionic strength and a(Ca2+)/a(CO32−) ratio. The initial pH varied in a range 8.50 ≤ pH0 ≤ 10.50 and included values relevant for mimicking the conditions related to biomineralization in marine organisms. In all systems two CaCO3 polymorphs were found: calcite and/or vaterite. The increase of pH0 favoured the formation of rhombohedral calcite no matter the type of stirring. This was exclusively influenced by the systems’ pH0 (other relevant initial parameters were identical). Furthermore, increase of pH0 caused change of vaterite morphology from cauliflower-like spheroids to regular spherulites. The mechanically stirred systems produced larger calcite and vaterite particles and higher content of calcite.

To assess the application potential of novel biomaterials, their behaviour in model media and upon sterilization should be investigated, as well as the stability related to their storage condi-tions. Such data are lacking for Mg-substituted HAP (Mg-HAP). Therefore, the changes in the local structure of non-substituted and Mg-HAP after irradiation and immersion in corrected simulated fluid and saline solution for 28 days were followed by electron paramagnetic resonance (EPR) spectroscopy for the first time. To better understand the stability of radical species induced by steriliza-tion, EPR spectra of samples kept for 2 h at temperatures up to 373 K were recorded to provide an insight into the stability of the sample storage conditions by the accelerated aging method. Samples were characterized by PXRD, FTIR, SEM, EDS, AAS and TGA. Results confirmed that irradiation does not induce changes in the composition or the structure of any of the investigated materials. Fading or the complete disappearance of radical signals in the EPR spectra after immersion in both media was accompanied by the disappearance of other phases formed as a minor byproduct in the synthesis of substituted HAP, as confirmed by PXRD and FTIR analysis. Obtained results confirm the great potential of Mg-HAPs for biomedical applications, although closer attention should be given to the processes related to sample storage stability at different temperatures. © 2022 by the authors. Licensee MDPI, Basel, Switzerland.

Liposomes (lipid vesicles) are often considered to be a versatile tool for the synthesis of advanced materials, as they allow various control mechanisms to tune the materials’ properties. Among diverse materials, the synthesis of calcium phosphates (CaPs) and calcium carbonates (CaCO3) using liposomes has attracted particular attention in the development of novel (bio)materials and biomineralization research. However, the preparation of materials using liposomes has not yet been fully exploited. Most of the liposomes used have been anionic and/or zwitterionic, while data on the influence of cationic liposomes are limited. Therefore, the aim of this study was to investigate and compare the influence of differently charged liposomes on CaPs and CaCO3 formation. Zwitterionic 1,2-dimyristoyl-sn-glycero-3-phosphocholine (DMPC), negatively charged 1,2-dimyristoyl-sn-glycero-3-phospho-L-serine (DMPS), and positively charged 1,2-dioleoyl-sn-glycero-3-ethylphosphocholine (EPC) lipids were used to prepare the respective liposomes. The presence of liposomes during the spontaneous precipitation of CaPs and CaCO3 affected both the precipitation and transformation kinetics, as well as the morphology of the precipitates formed. The most prominent effect was noted for both materials in the presence of DMPS liposomes, as (nano) shell structures were formed in both cases. The obtained results indicate possible strategies to fine-tune the precipitation process of CaPs and CaCO3, which may be of interest for the production of novel materials.

Amino acids (AAs) attract attention for elucidating the role of proteins in biomineralization and the preparation of functionalized biomaterials. The influence that AAs exert on calcium phosphate (CaP) mineralization is still not completely understood, as contradictory results have been reported. In this paper, the influence of the addition of different classes of AAs, charged (L-aspartic acid, Asp, L-lysine, Lys), polar (L-asparagine, Asn, L-serine, Ser, L-tyrosine, Tyr), and non-polar (L-phenylalanine, Phe), on CaP growth in the presence of octacalcium phosphate (OCP) and calcium hydrogenphosphate dihydrate (DCPD) seeds was investigated. In control systems (without AAs), a calcium-deficient apatite (CaDHA) layer was formed on the surface of OCP, while a mixture of CaDHA and OCP in the form of spherical aggregates was formed on the surface of DCPD crystals. Charged and non-polar promoted, while polar AAs inhibited CaDHA formation on the OCP seeds. In the case of DCPD, Lys, Asp, and Phe promoted CaP formation, while the influence of other AAs was negligible. The most efficient promotor of precipitation in both cases was non-polar Phe. No significant influence of AAs on the composition and morphology of precipitates was observed. The obtained results are of interest for understanding biomineralization processes and additive controlled material synthesis.

In many living organisms, biomolecules interact favorably with various surfaces of calcium carbonate. In this work, we considered the interactions of aspartate (Asp) derivatives, as models of complex biomolecules, with calcite. Using kinetic growth experiments, we investigated the inhibition of calcite growth by Asp, Asp2, and Asp3.This entailed the determination of a step-pinning growth regime as well as the evaluation of the adsorption constants and binding free energies for the three species to calcite crystals. These latter values are compared to free energy profiles obtained from fully atomistic molecular dynamics simulations. When a flat (104) calcite surface is used in the models, the measured trend of binding energies is poorly reproduced. However, a more realistic model comprised of a surface with an island containing edges and corners yields binding energies that compare very well with experiments. Surprisingly, we find that most binding modes involve the positively charged ammonium group. Moreover, while attachment of the negatively charged carboxylate groups is also frequently observed, it is always balanced by the aqueous solvation of an equal or greater number of carboxylates. These effects are observed on all calcite features including edges and corners, the latter being associated with dominant affinities to Asp derivatives. As these features are also precisely the active sites for crystal growth, the experimental and theoretical results point strongly to a growth inhibition mechanism, whereby these sites become blocked, preventing further attachment of dissolved ions and halting further growth.

This work is focused on a practical procedure by which pure and well-formed COD crystals were obtained. The effect of different type of stirring on the precipitation process and properties of COD crystals is studied. Mechanical and magnetic stirring are two common stirring types for precipitation of calcium oxalate and method using ultrasonic irradiation is newly described. The precipitation processes are systematically investigated varying solution stoichiometry (defined as a ratio of concentrations of the constituent ions in solution, (Ca+/(C2O42−) and concentration of additive (citrate). The differences of the structure, morphology and crystals properties between precipitates were investigated by SEM, FT-IR, TG and PXRD. In the ultrasonically stirred systems, COD was the only precipitated phase regardless of solution stoichiometry. Pure and well formed COD crystals with well-formed planes were obtained in the system with c(Ca2+)/c(C2O42−) = 3.86 and c(citrate) = 0.009 mol dm−3. The results demonstrate that ultrasonically assisted precipitation could be a new way in synthesis of COD crystals having different potential applications (in the fields of medicine and biotechnology).

Alginate hydrogels are natural materials which can be used for biomedical purposes in order to trigger natural remineralization of damaged hard tissues. Mineralization of alginate hydrogels by using diverse protocols is extensively studied, with a goal of obtaining biocomposite materials having improved mechanical properties. The majority of the studies described in the literature are focused on the use of alginate in the form of beads, while alginate in bulk form has been more rarely studied. In this work, the influence of parameters, like phosphate concentration, type of media (water, HEPES and TRIS buffer) and pH (5.0, 7.4, 9.0), on gelation and mineralization of bulk alginate hydrogels (approx. 2 × 2 cm) was studied. Hydrogels were produced using semi- permeable membranes. In all experiments, calcium chloride solution was added as a mineralizing solution to the alginate dissolved in medium containing disodium phosphate, which simultaneously triggered gelation and mineralization of hydrogels. Hydrogels were mineralized for 5 days with daily exchange of mineralizing solution. Such hydrogels were characterized using a combination of infrared spectroscopy, powder X-ray diffraction and electron microscopy. The obtained results demonstrated that the media and pH have a profound influence on gelation and the stability of the hydrogels. Octacalcium phosphate was formed in all hydrogels except in TRIS buffer at pH = 9.0 where amorphous calcium phosphate (ACP) was formed. ACP was stabilized during the experiments, as well as during the extraction from the hydrogel. The obtained results point to a protocol suitable for fine-tuning the properties of the mineralized hydrogel, by a simple change of experimental conditions.

Spontaneous precipitation of CaCO3 polymorphs in the presence of selected amino acids (AA) has been investigated. The l -aspartic acid (Asp), l -lysine (Lys), l -asparagine (Asn), l -tyrosine (Tyr), l -phenylalanine (Phe), l -serine (Ser) and l -alanine (Ala) were selected because of different charge and polarity of their side chains at applied experimental conditions. The investigated AA are building units of soluble macromolecules, putatively responsible for biomineralization of molluscs’ exoskeletons. It was assumed that not only the acidic, but also the polar (hydrogen bonding) AA might contribute to macromolecules’ interactions with the mineral surfaces. The mineralogical composition, structure and morphology of precipitates formed in the presence of wide range of concentrations of AA have been determined by XRD, FT-IR and EPR spectroscopy, HPLC-MS and SEM. In the reference system, without AA addition, a mixture of typical calcite rhombohedral crystals and vaterite spherulites has been observed, while AA with negatively charged or polar side chains (Asp, Tyr, Lys, Asn, Ser) significantly changed the morphology, phase composition and crystal structure of the precipitates. The effects of nonpolar AA (Phe, Ala) on the structural and morphological properties of precipitates are less pronounced. The stronger impact observed for polar AA and particularly negatively charged Asp, may be correlated with the additional electrostatic interactions of side-chain groups with mineral surfaces.

The application of solar-light activated semiconductors in advanced water treatment, among which BiVO4 is considered a prospective candidate which suffers from rapid recombination of photogenerated charges, poor electron transfer kinetics, and low oxidative power of photogenerated holes. To address some of these limitations of BiVO4, we have tailored an isotype homojunction BiVO4. Herein, a novel, simple, and energy efficient protocol based on co-precipitation synthesis of BiVO4 using ethylenediaminotetracetic acid (EDTA) in water is presented. The obtained isotype BiVO4 has superior opto-electronic and photocatalytic properties in comparison to hydrothermally synthesized BiVO4. The formation of an isotype homojunction resulted in an increase in photocurrent density and improved charge separation. The isotype BiVO4 has achieved 2.5 times higher DCF removal than its hydrothermal counterpart. The degradation mechanism was investigated using scavengers for reactive oxygen species/sites; superoxide radical, photogenerated holes and hydroxyl radicals, indicating the latter as the primary reactive species responsible for DCF degradation.

This paper reports on the investigation of experimental conditions relevant for spontaneous precipitation of significant amount of pure calcium oxalate dihydrate (COD). For this purpose, the hydrodynamic and thermodynamic parameters, such as mode of agitation, temperature, supersaturation and concentration of additives (citrate ions), have been studied. The results show that in the model systems, without the citrate addition and applied mechanical stirring, calcium oxalate monohydrate (COM) was observed as dominant modification after 20 min of aging, while the magnetic stirring resulted in a formation of a mixture of COM and calcium oxalate trihydrate (COT), regardless of the temperature applied. In the mechanically stirred systems, the addition of citrate ions in the range of concentrations, 0.001 mol dm−3

The aim of this study is to contribute to understanding the mechanisms underlying the formation of biologically relevant minerals by comparing the properties of solid phases formed in calcium phosphate (CaP) or calcium carbonate (CaCO3) precipitation systems, at defined initial experimental conditions: supersaturation, constituent ions ratio, ionic strength, and/or presence of relevant inorganic ions. Thus, three systems of different chemical complexities were investigated: (a) system containing constituent ions, (b) system containing additional co-ions, and (c) system with higher ionic strength and addition of Mg2+. The respective precipitation diagrams were constructed, and supersaturation domains of different CaP and CaCO3 solid phases formation were identified. The obtained results may have implications not only for biomineralization and geochemistry, but also for materials science in general.

This work focuses on systematic research of spontaneous precipitation of calcium oxalate in model systems mimicking the chemical conditions typical for hyperoxaluria. For this purpose, model systems of three different complexities (simple, NaCl and artificial urine system) and two initial pH (pHi = 5.0 and pHi = 9.0) at 37 ºC are studied. In all investigated systems, dominant precipitation of calcium oxalate monohydrate (COM) is observed, except in artificial urine system at pHi = 9.0, ci(C2O42-) ≥ 6.0 mmol dm-3 and ci(Ca2+) ≥ 7.5 mmol dm-3 where precipitation of a mixture of calcium oxalate monohydrate and calcium oxalate dihydrate (COD) occurrs. The significant difference is mainly observed in COM crystal morphology. In a simple system, COM precipitates in a dendritic form, in NaCl system dendritic form and irregular shapes are found, while in the artificial urine, prismatic hexagonal and aggregated COM crystals precipitate.

The increased utilization of one-dimensional (1D) TiO2 and titanate nanowires (TNWs) in various applications was the motivation behind studying their stability in this work, given that stability greatly influences both the success of the application and the environmental impact. Due to their high abundance in aqueous environments and their rich technological applicability, surfactants are among the most interesting compounds used for tailoring the stability.The aim of this paper is to determine the influence of surfactant molecular structure on TNW stability/aggregation behavior in water and aqueous NaBr solution by dynamic and electrophoretic light scattering. To accomplish this, two structurally different quaternary ammonium surfactants (monomeric DTAB and the corresponding dimeric 12-2-12) at monomer and micellar concentrations were used to investigate TNW stability in water and NaBr. It was shown that TNWs are relatively stable in Milli-Q water. However, the addition of NaBr induces aggregation, especially as the TNW mass concentration increases. DTAB and 12-2-12 adsorb on TNW surfaces as a result of the superposition of favorable electrostatic and hydrophobic interactions. As expected, the interaction of TNWs with 12-2-12 was stronger than with DTAB, due to the presence of two positively charged head groups and two hydrophobic tails. As a consequence of the higher adsorption of 12-2-12, TNWs remained stable in both media, while DTAB showed an opposite behavior.In order to gain more insight into changes in the surface properties after surfactant adsorption on the TNW surface, a surface complexation model was employed. With this first attempt to quantify the contribution of the surfactant structure on the adsorption equilibrium according to the observed differences in the intrinsic log K values, it was shown that 12-2-12 interacts more strongly with TNWs than DTAB. The modelling results enable a better understanding of the interaction between TNWs and surfactants as well as the prediction of the conditions that can promote stabilization or aggregation.

One of the most common crystal habits of the thermodynamically stable polymorph of calcium carbonate, calcite, is the rhombohedral one, which exposes {10.4} faces. When calcite is precipitated in the presence of Li+ ions, dominantly {00.1} faces appear together with the {10.4}, thus generating truncated rhombohedrons. This well-known phenomenon is explored in this work, with the aim of obtaining calcite crystals with smooth {00.1} faces. In order to achieve this objective, the formation of calcite was examined in precipitation systems with different c(Ca2+)/c(Li+) ratios and by performing an initial high-power sonication. At the optimal conditions, a precipitate consisting of thin, tabular {001} calcite crystals and very low content of incorporated Li+ has been obtained. The adsorption properties of the tabular crystals, in which the energetically unstable {00.1} faces represent almost all of the exposed surface, were tested with model dye molecules, calcein and crystal violet, and compared to predominantly rhombohedral crystals. It was found that the {00.1} crystals showed a lower adsorption capability when compared to the {10.4} crystals for calcein, while the adsorption of crystal violet was similar for both crystal morphologies. The obtained results open new routes for the usage of calcite as adsorbing substrates and are relevant for the understanding of biomineralization processes in which the {00.1} faces often interact with organic macromolecules.

Calcium sulfate (CS) is a reasonable alternative to autogenous dental bone graft and is used as a bone filler or for the treatment of bone defects. Medical grade CS must be sterilized, usually by gamma-irradiation. In this study the structures of CS polymorphs, calcium sulfate dihydrate and calcium sulfate hemihydrate, have been investigated with electron paramagnetic resonance (EPR) spectroscopy, X-ray diffraction, thermogravimetric analyses and infrared spectroscopy. The influences of saline and artificial saliva on gamma-induced radical concentrations have been monitored.

Calcium phosphates (CaPs) composites with different TiO2 nanomaterials (TiNMs) have recently attracted attention due to their potential application as implant materials for hard tissue regeneration. Among different synthetic routes for such composites, biomimetic synthesis attracts special attention as a green synthesis route. To rationalize biomimetic synthesis, the influence of surface structure and surface charge density of different TiNMs on CaPs precipitation should be determined. In this paper, for the first time, the influence of TiO2 nanoparticles (TiNPs) and titanate nanotubes (TiNTs) on CaPs spontaneous precipitation was investigated. In addition, the ability of TiNMs to induce CaPs formation in corrected simulated body fluid (c-SBF) was tested. The advancement of the CaPs precipitation was followed by potentiometric measurements, while the obtained materials were characterized by Fourier transform infrared spectroscopy (FTIR), Raman spectroscopy (RS), powder X-ray diffraction (PXRD), scanning (SEM) and transmission (TEM) electron microscopy, electron paramagnetic resonance spectroscopy (EPR), Brunauer-Emmett-Teller surface area analysis (BET), dynamic (DLS) and electrophoretic light scattering (ELS), atomic force microscopy (AFM) and thermogravimetric (TGA) analysis. Both TiNMs showed poor ability to promote CaPs formation in c-SBF which can be attributed to their low absolute zeta-potential value. However, during spontaneous precipitation, they exhibited uncommon dual influence on precipitation kinetics, i.e. inhibition of initially formed amorphous calcium phosphate (ACP) transformation at lower and promotion at higher concentrations. After one hour of reaction time, in the presence of both TiNMs, calcium deficient hydroxyapatite was formed (CaDHA). Morphology of the CaDHA depended on the type and concentration of TiNMs. However, PXRD, EPR and TGA characterization confirmed that there is no significant difference between CaDHA formed in the presence of different TiNMs, which indicates that CaDHA composites with different TiNMs could be prepared at similar conditions. In addition, they can be prepared in a much shorter time compared to the usual procedure with SBF. Obtained results confirm the importance of surface structure and charge in CaPs – TiNMs interactions. They as well contribute to a deeper insight into precipitation processes on the nanosurfaces, which enables a rational approach not only in the biomimetic synthesis of novel biocomposites for hard tissue regeneration but also of composites for other applications.

Amorphous calcium carbonate (ACC) and calcium phosphate (ACP) increasingly attract attention as initial solid phases in vertebrate and invertebrate hard tissue formation, as well as in materials science as a possible new synthetic route for advanced materials preparation. Although much is known about these two amorphous phases and similarities in the mechanisms of their formation are recognized, no attempt has been made to investigate their formation under defined and comparable initial experimental conditions viz supersaturation, constituent ions ratio, ionic strength and presence of relevant inorganic additives. In this paper, the formation of ACC and ACP in three model precipitation systems of increased chemical complexity were investigated: (a) systems containing constituent ions, (b) systems containing additional co-ions, and (c) systems with higher ionic strength and addition of Mg2+. The results have shown that ACP is more stable and was formed at lower relative supersaturations in comparison to ACC. The precipitation domain of both phases expanded with increasing complexity of precipitation systems, with the ACP precipitation domains always being larger than that of ACC. In addition to stability, the presence of inorganic ions, especially Mg2+, influences the composition of both amorphous phases. The obtained results indicate that general similarity between ACC and ACP exists, but it could also be concluded that similar chemical environment in which they form not necessary lead to similar structural properties.

Calcium carbonate polymorphs were crystallized in alginate and xanthan hydrogels in which a degree of entanglement was altered by the polysaccharide concentration. Both hydrogels contain functional groups (COOH and OH) attached at diverse proportions on saccharide units. In all systems, the precipitation process was initiated simultaneously with gelation, by the fast mixing of the calcium and carbonate solutions, which contain the polysaccharide molecules at respective concentrations. The initial supersaturation was adjusted to be relatively high in order to ensure the conditions suitable for nucleation of all CaCO3 polymorphs and amorphous phase(s). In the model systems (no polysaccharide), a mixture of calcite, vaterite and amorphous calcium carbonate initially precipitated, but after short time only calcite remained. In the presence of xanthan hydrogels, precipitation of either, calcite single crystals, porous polyhedral aggregates, or calcite/vaterite mixtures were observed after five days of ageing, because of different degrees of gel entanglement. At the highest xanthan concentrations applied, the vaterite content was significantly higher. In the alginate hydrogels, calcite microcrystalline aggregates, rosette-like and/or stuck-like monocrystals and vaterite/calcite mixtures precipitated as well. Time resolved crystallization experiments performed in alginate hydrogels indicated the initial formation of a mixture of calcite, vaterite and amorphous calcium carbonate, which transformed to calcite after 24 h of ageing.

Negatively ionisable poly-L-glutamic acid (pGlu) and poly-L-aspartic acid (pAsp), considered as analogues of the naturally occurring acidic macromolecules involved in biomineralization processes, were used as additives in the calcium carbonate precipitation systems in order to investigate their interactions with the vaterite crystallites. Poly-L-lysine (pLys), a positively ionisable poly(amino acid), was also used in order to elucidate the impact of the side chain charge. The growth kinetics of vaterite was found parabolic, indicating that the integration of growth units into the spiral step at the vaterite crystal surfaces is the rate-determining mechanism. The presence of small amounts of pGlu and pAsp inhibited the crystal growth. At the highest concentrations of both acidic macromolecules the exponential rate law was observed, which indicates the surface nucleation as the rate controlling mechanism. The addition of pLys in the range of applied concentrations did not significantly influence the crystal growth of the vaterite. Thus, the kinetic results, corroborated by morphological observations, pointed out to the significance of the negative charge of the side chains of selected polypeptides in the interfacial interactions with mineral planes. This work is licensed under a Creative Commons Attribution 4.0 International License.

In this study, the influence of Mg2+ on the aggregation and morphology of precipitated aragonite crystals is investigated at different temperatures. Different from some investigations described in the literature, aragonite precipitates in chemical systems in which Mg2+ acts specifically as crystal habit modifiers and not as polymorphic selectors. The results show that at increased Mg2+ concentration and temperature the aragonite crystals are less aggregated and that the amount of crystals sharing {; ; 110}; ; faces and having larger extension of {; ; 001}; ; faces increases. These outcomes may be relevant in biological, geochemical and technological contexts.

This paper reports on the precipitation of CaCO3 polymorphs, having various crystal morphologies under different conditions. In particular, systems that were subject to ultrasonic irradiation were compared to the corresponding reference systems in the absence of such a treatment. The application of ultrasonic irradiation predominantly resulted in a change of particle size distribution and polymorphic composition of the precipitate, in comparison to the reference systems. Thus, it was found that the supersaturation and temperature influenced the size distribution, in both the reference and sonicated systems. A mixture of calcite, vaterite and aragonite was obtained in all reference systems, at 25 °C. At this temperature, the sonication caused the vaterite content to increase, while aragonite was not detected. In reference and sonicated systems at 80 °C, only aragonite precipitated. The results also indicate that the principle parameter responsible for the morphology of vaterite was the initial supersaturation: at higher supersaturation spherical vaterite particles precipitated, while at lower supersaturation hexagonal platelets were obtained. The morphological investigations also indicated different mechanisms of vaterite formation in the systems in which precipitation was initiated at higher supersaturation: spherulitic growth of vaterite was observed in sonicated systems, while the aggregation of primary particles was predominant in the reference systems. At lower supersaturation, the effect of c(Ca2+)/c(CO32-) on the morphology of hexagonal platelets of vaterite was observed as well. By varying the c(Ca2+)/c(CO32-), significant changes of the polymorphic composition were observed only in the sonicated systems, at 25 ˚C.

Recently, the results of experimental and theoretical investigations have revealed that, in vaterite, two or even more crystalline structures coexist. In this communication we report evidence of diverse vaterite structures in biogenic samples of different origin. In addition, it is shown that the synthetic vaterite precipitated in the presence of poly-L-aspartate has structures similar to those of biogenic samples.

Purpose This study used multi-element signatures of stream sediments to assess both natural and human-induced impact on fluvial system in the River Neretva delta receiving environment over time. The river basin actually comprises several sub-catchments, and the geochemical features of major elements, trace metals radiometric and mineralogical characterisation of river bed sediments were used to assist the interpretation of the environment of deposition and its subsequent modifications caused by various anthropogenic pressures within the river basin. Materials and methods Five sites were chosen for sediment sampling at key locations within the study area with assumed undisturbed, continuous sedimentation process. At each of the sites, three representative cores were taken by scuba divers. Sample sites were selected in order to reflect the influence of different sub-catchments they belong to and the land-use pattern of the surrounding area. Samples were analysed for pH, redox-potential, granulometry, mineralogy, thermogravimetry, major and trace element concentrations and radionuclide activities. The univariate and multivariate statistics were applied. The geochemical normalisation of data was done using Al, the procedure based on calculation of the regression line of the metal on the normaliser followed by testing the ratios metal/normaliser on all data-points. Results and discussion All studied sediments are classified as silt, ranging from clayey silt to silt and sandy silt. Mineralogically, the sediments were dominated by carbonates and quartz. The chemical contaminant data are generally of good quality, mostly below guideline levels. Sedimentation rates were estimated using vertical distribution of 137Cs activities. Normalisation of TMs done by using Al shows strong R2 adj values for the regressions of Al and V, Al and Cr, and Al and Ni. However, Al cannot fairly explain the fluctuation of the concentrations of Cd, Cu, Pb and Zn in sediment cores from all of the sampling sites. Generally, inorganic scavengers such as clay minerals followed by Fe and Mn oxides and S (pyrite and gypsum) seem to be dominant factors controlling TMs in studied sediments. Conclusions Although the River Neretva delta occupies a rather small area, the geochemical features of major and trace elements and 137Cs activities show complex sediment provenances. Each of the sampling sites reflect exactly different effects of anthropogenic intervention that particularly refer to the changes in river morphology and ecology, along with the altered flow regimes within the catchment on sediment loads and quality.

Mineralized tissues grow through biologically controlled processes in which specific macromolecules are involved. Some of these molecules, which are present in very low concentrations and are difficult to localize and characterize, become entrapped inside the mineralized tissue. Herein, a protein fragment, GP, which was obtained by the alkaline digestion of the green sheet of the abalone shell, is used as a probe to study the changes in molecular structure that occur during the precipitation of calcium carbonate. This important goal was achieved by exploiting a fluorescent tag in GP. The experimental results that were obtained by using spectroscopic-, chromatographic-, and microscopic techniques indicate that GP controls the precipitation kinetics and morphology of calcium carbonate crystals, and that it only undergoes structural reorganization when entrapped inside calcium carbonate crystals. To the best of our knowledge, this report represents one of the first studies on the conformational changes of a protein fragment that is involved in biomineralization processes on moving from the solution phase into the mineral phase.

Investigation of basic interactions between the active pharmaceutical compounds and calcium carbonates is of great importance because of the possibility to use the carbonates as a mineral carrier in drug delivery systems. In this study the mode and extent of interactions of salicylic acid and its amino acid derivates, chosen as pharmaceutically relevant model compounds, with calcite crystals are described. Therefore, the crystal growth kinetics of well defined rhombohedral calcite seed crystals in the systems containing salicylic acid (SA), 5-amino salicylic acid (5-ASA), N-salicyloil-L-aspartic acid (N-Sal-Asp) or N-salicyloil-L-glutamic acid (N-Sal-Glu), were investigated. The precipitation systems were of relatively low initial supersaturation and of apparently neutral pH. The data on the crystal growth rate reductions in the presence of the applied salicylate molecules were analyzed by means of Cabrera & Vermileya's, and Kubota & Mullin's models of interactions of the dissolved additives and crystal surfaces. The crystal growth kinetic experiments were additionally supported with the appropriate electrokinetic, spectroscopic and adsorption measurements. The Langmuir adsorption constants were determined and they were found to be in a good correlation with values obtained from crystal growth kinetic analyses. The results indicated that salicylate molecules preferentially adsorb along the steps on the growing calcite surfaces. The values of average spacing between the adjacent salicylate adsorption active sites and the average distance between the neighbouring adsorbed salicylate molecules were also estimated.

A novel method was proposed for the preparation of aluminosilicate-based ceramic materials by temperature induced transformation of amorphous aluminosilicates (zeolite precursors) as an alternative to conventional methods based on the thermal treatment of kaolin, feldspar and other silicate, aluminosilicate and/or oxide mixtures, or by sol–gel method. Amorphous aluminosilicate gels that are usually used as precursors for the crystallization of zeolites after exchanging their host's cations (sodium) with ammonium cations from solution were thermally treated (at 1263 K for 3 h) and yielded a mixture of mullite and amorphous silica. Further chemical treatment of the mixture with sodium hydroxide solution yielded a pure mullite phase having particle size below 100 nm.

Poly-L-glutamic (pGlu) and poly-L-aspartic (pAsp) acids, as analogues of naturally occurring soluble acidic proteins, were used to investigate specific interactions between the acidic polypeptides and calcite as a mineral surface. A basic polypeptide (poly-L-lysine, pLys) served as a negative control for testing the role of conformity between the substrate and the adsorbed polypeptides in biomineralization processes. The kinetics of calcite crystal growth was determined in a simplified precipitation model system by inoculating well defined calcite seed crystals into a moderately supersaturated solution containing one of the polypeptides. From the kinetic data, the mode and extent of the calcite/polypeptide interactions were determined. The parabolic rate law was found to be valid for the calcite crystal growth, the integration of ions into the spiral steps at the calcite crystal surface being the rate-determining mechanism. Small amounts of pGlu or pAsp caused an inhibition of calcite crystal growth, the effect being pAsp > pGlu, and the exponential dependence of the growth rate on supersaturation confirmed that surface nucleation was the growth controlling mechanism in the presence of the two acidic polypeptides. The pLys non-selective, weak, electrostatic adsorption at the crystal surface was probably responsible for increasing the calcite growth rate at low concentrations and for inhibiting it at higher concentrations. The strongest interactions between the crystal surfaces and the polypeptides were observed for the calcite/pAsp systems. They could be accounted for by coordinative interactions between the side chain carboxylic groups of the predominantly planar arrangement of the pAsp structure (β -pleated sheet) and Ca2+ ions from the calcite surface.

The production of precipitated calcium carbonate, PCC, by a semicontinuous process of slaked lime carbonation was performed in a bench-scale chemical reactor, fully controlled by means of custom built electronics and software for the personal computer. Calcite crystals, with different characteristic morphologies (rhombohedral, truncated prismatic, scalenohedral, spheroidal or chain-like agglomerates) were produced by varying a range of process parameters, like temperature, supersaturation, gas mixture flow rate, stirring rate and mass concentration of Ca(OH)2 suspension. In order to identify the effects of the chosen process parameters on the PCC morphology and on the related specific surface area, as well as on the extent of CO2 conversion, an empirical approach based on the experimental design techniques was employed. A multiple correlation analysis of the obtained data suggests that temperature and conductivity significantly influence the PCC morphology, while CO2 conversion is principally influenced by stirring rate, conductivity and gas mixture flow rate.

Many living organisms form mineral phases through biologically controlled processes, known as biomineralization. Thus created materials are composites of both, mineral and organic components. The shell of the gastropod mollusc red abalone (Haliotis rufescens) consists of calcite and aragonite layers, each of them containing characteristic biopolymers responsible for biomineralization. In this work, the effect of interstitial green sheet polypeptide, GP, extracted from the green layer of the mollusc red abalone shell, on the process of spontaneous precipitation of calcium carbonate polymorphs, was investigated. Three precipitation systems, in which the initial mineralogical composition of the precipitate was different, have been studied. Thus, in system (1) calcite appeared, in system (2) a mixture of calcite and vaterite was found, while in system (3), ASW, only aragonite precipitated. However, the precipitation kinetic measurements, X-ray diffraction, FT-IR spectroscopy, and light and scanning electron microscopy indicated that the addition of GP in the model systems caused the inhibition of precipitation and change of morphology of crystals as a consequence of GP adsorption on the crystal surfaces and its entrapment into the mineral structure. (doi: 10.5562/cca2197)

The influence of the initial reactant concentrations on the composition of the solid phases formed in the precipitation system MgCl(2)-NH(4)H(2)PO(4)-NaOH-H(2)O was investigated. The precipitation diagram constructed shows the approximate concentration regions within which struvite, newberyite, and their mixtures exist at 25 degrees C and an aging time of 60 min. It was found that immediately after mixing the reactant solutions, struvite (MgNH(4)PO(4).6H(2)O) precipitated in nearly the whole concentration area, while newberyite (MgHPO(4).3H(2)O) appeared mostly within the region of the excess of magnesium concentration. It was also found that after aging time of 60 min the precipitation domain of struvite alone is much broader than that of newberyite or the domain of their coexistence, and shows that struvite is more abundant in the systems in which the initial concentration of ammonium phosphate is higher than that of magnesium. The kinetics of struvite to newberyite transformation (conversion) was systematically studied under the conditions of different initial reactant concentrations and different initial pH in the systems in which a mixture of both phases precipitated spontaneously. The struvite to newberyite conversion period was found to be strongly related to the ratio of initial supersaturations, S(N)/S(S), rather than to the any particular physical quantity that can describe and predict the behavior of the precipitation system. Experimental data suggest a solution-mediated process as a most possible transformation mechanism. Along with a continuous monitoring of the changes in the liquid phase, the content of struvite in the solid phase was estimated by means of a Fourier transform infrared (FT-IR) method, developed for this particular precipitation system.

The kinetics of transformation of anhydrous calcium sulphate (AH) to calcium sulphate dihydrate (DH) in aqueous solutions was investigated using well-defined mixtures of AH and DH seed crystals to initiate the process. The progress of transformation was followed by recording the changes both in the liquid and in the solid phases. The transformation process was found to be solution-mediated, which was supported by a mathematical model proposed in this work. The model takes into account the experimentally determined initial solution calcium concentration and mass concentration of the solid phases as well as the rate constants and mechanisms of the processes involved in the transformation: dissolution of AH, dissolution of DH and crystal growth of DH. The kinetics of these processes were studied separately in the temperature range 10–40°C and the rate laws were found to be parabolic, linear and parabolic, respectively. The activation energies obtained for the temperature-dependent rate constants, Ea(AH)D=61.0 kJ mol−1, Ea(DH)D=24.0 kJ mol−1 and Ea(DH)G=64.0 kJ mol−1, support the assumption that the AH dissolution and the DH growth were controlled by processes at the crystal surface and that the DH dissolution was most probably a diffusion controlled process.

The kinetics of scalenohedral calcite crystal growth in the presence of N-salicyloil-Laspartic (N-Sal-Asp) and N-salicyloil-L-glutamic acid (N- Sal-Glu), as selected models of antiinflammatory drug-like substances, were analyzed in order to study the molecular interactions of the selected salicylates with scalenohedral {; ; ; 2 1 4}; ; ; surfaces. The results revealed that the growth is controlled by surface reactions, proceeding at the spiral step. The presence of the additives caused the inhibition of the growth, wherein the N-Sal-Asp interacted more strongly than N-Sal- Glu. The obtained data were supported with the electrokinetic, spectroscopic and adsorption measurements. The interactions between the selected additives and the scalenohedral calcite crystals were compared with the rhombohedral modification. The stronger interactions of salicylates with scalenohedral surfaces are explained in terms of their higher reactivity and, most likely, better structural matching of their surface calcium ions with the salicylate carboxylic groups. In addition, the spontaneous precipitation of scalenohedral or rhombohedral calcite crystals in the presence of selected salicylates indicated that, under the applied conditions, the salicylate incorporation into the scalenohedral modification was significantly higher and could be related to the enhanced adsorption efficiency and the kinetics of crystal growth. 2 The obtained results can contribute to understanding the basic interactions between the model drug derivatives containing carboxylic functional groups with different mineral surfaces, as well as to the development of biocompatible calcium carbonate drug carriers.

The kinetics of transformation of vaterite to calcite in aqueous solution was studied at temperatures between 25 °C and 45 °C and ionic strengths between 15 and 415 mmol dm^-3. Both vaterite and calcite precipitated spontaneously, with vaterite being initially the only solid phase in the system. The progress of the transformation was followed by recording pH as a function of time. It was found that this transformation was solution-mediated, and that the growth of calcite was the rate-determining process. This conclusion was additionally supported by a mathematical model made to predict changes of both the solid phase and the solution during the overall process. The model is based on the rate constants and the mechanism of the processes involved in the transformation, as well as on the experimentally determined number density of the crystals formed. The seeded growth kinetics of calcite was determined in separate experiments, at temperatures between 10 °C and 55 °C, and the rate law was found to be parabolic. The high activation energy obtained for the temperature dependent rate constants on temperature, E_a=55.29 kJ mol^-1, supports the assumption that the calcite growth was controlled by processes at the crystal surface.

A method for preparation of significant amount of hollow rhombohedral calcite nanoparticles, based on carbonation of calcium hydroxide suspension, is described. The mineralogical and morphological analyses of the precipitate confirmed the existence of exclusively stable polymorphic modification, calcite, with the mean particle size of about 100 nm and the diameter of the holes observed at the surfaces that are about 50 nm. The analysis of carbonation kinetics pointed out to a complex mechanism of hollow particles formation at high initial supersaturation, that assumed nucleation of amorphous precursor calcium carbonate phase and its solution mediated transformation into nanosized crystalline calcite. The holes obtained at the calcite surfaces are most probably the imprints remained after the dissolution of amorphous calcium carbonate particles.

The incorporation of divalent cations (Mg^2+, Mn^2+, Cu^2+, Sr^2+, Cd^2+, Ba^2+ and Pb^2+) in vaterite and calcite was studied by atomic absorption spectroscopy (AAS), optical and scanning electron microscopy (SEM), X-ray diffraction (XRD) and electron paramagnetic resonance (EPR) spectroscopy. The samples of vaterite and calcite were prepared by spontaneous precipitation from aqueous solutions under defined conditions. The particles of vaterite obtained were spheres, being aggregates of crystallites of 25-35 nm and the crystals of calcite were well defined rhombohedra. All ions examined, except Cu^2+, were found to be incorporated in vaterite probably forming solid solutions. Ba^2+ caused a distortion of the vaterite crystal lattice and Mg^2+ changed the shape of the particles. Mn^2+ ions were incorporated into vaterite in at least two different environments, segregated into clusters or substituted for Ca^2+, as was detected by EPR. The environment of those Mn^2+ ions substituted for Ca^2+ in vaterite was of lower symmetry than that in calcite. Characteristic EPR spectra of free radicals formed by gama-irradiation of vaterite and calcite are presented. These spectra were used to detect formation of calcite in the vaterite system, in the presence of Cd^2+ and Pb^2+ ions confirming the results obtained by EPR of Mn^2+ as a paramagnetic substitute for Ca^2+. Free radicals characteristic of Cd^2+ ions, were detected in both vaterite and calcite gama-irradiated under similar conditions.

Calcium carbonate monohydrate (CaCO3xH2O) has been synthesized and the crystals were characterized by means of optical microscopy, FT-IR spectroscopy, X-ray diffractometry and thermogravimetry. The solubility was found by recording pH during the dissolution of the monohydrate in water, preventing contact with carbon dioxide from the air. The solubility product was calculated frm the final, equilibrium, pH value taking all the relevant calcium and carbonate species into account. In the temperature range 15-50 degrees C the solubility product Kso could be expressed by -log Kso=7.050+0.000159 theta2 where temperature is expressed in degrees C. The kinetics of dissolution of calcium carbonate monohydrate crystals in aqueous solution were studied at temperatures between 15 and 45 degrees C. The progress of dissolution was followed by recording pH as a function of time. It was found that the dissolution kinetics follow a second order rate equation with the rate constant k equal to 2.205x10-4, 6.190x10-4, 1.218x10-3 and 4.369x10-3 l mol-1 s-1/mg l-1 at 15, 25, 35, 45 degrees C respectively, and that the activation energy for dissolution was 73.3 kJ mol-1, suggesting a surface-controlled process.

Precipitated calcium carbonate (PCC) was prepared by means of semicontinuous carbonation of Ca(OH)(2) suspension, at 35 and 45 degrees C and in the presence of non-ionic dextran and dextran sulfate. The carbonation process was regulated at different predetermined values of electrical conductivity, that corresponded to different concentrations of dissolved Ca(OH)(2): 0.5 mS cm(-1) < kappa(25) < 5.0 mS cm(-1). The results of physical-chemical characterization of the product showed that calcite was the only polymorphic modification obtained in the whole range of experimental conditions investigated. In addition, it was found that morphology, crystal size distribution and specific surface area of PCC strongly depended on the electrical conductivity / concentration of dissolved Ca(OH)(2), at which the process was performed: predominantly scalenohedral crystals of higher surface area (about 5 m(2) g(-1)) were produced at higher electrical conductivity, while at lower electrical conductivity predominantly rhomohedral calcite crystals of relatively low specific surface area were obtained. In the system of the highest electrical conductivity, kappa(25) = 5.0 mS cm(-1), and at 35 degrees C, the addition of non-ionic dextran significantly influenced the process by preventing the regulation. The crystals that appeared were in the form of irregular aggregates of high specific surface area, S approximate to 29 m(2) g(-1). FT-IR and TG analyses indicated that the non-ionic dextran was adsorbed onto the calcite surface, most probably by relatively strong and specific interactions between oxygen from the hydroxyl groups of dextran molecules and calcium ions from the crystal surface. On the contrary, the anionic dextran (dextran sulfate) exerted minor effects in the course of semicontinuous carbonation process and in the properties of the final product, PCC. However, the analysis of the precipitate indicated that dextran sulfate was adsorbed at the surfaces, most probably by the weak and non-specific electrostatic interactions.

Biomineralization processes are the subject of numerous investigations. This article gives a review of the study on interactions between the charged polypeptides and the mineral surfaces involved in biomineralization, with an additional kinetic approach. The influence of polypeptides on two types of precipitation processes is discussed: the spontaneous precipitation from supersaturated solution and the growth kinetics of calcite seed crystals. In the first case the phenomenon of the formation and stabilization of metastable phases was found while in the second case the influence of the applied polypeptides on the kinetics and mechanisms of calcite crystal growth was investigated. Calcium carbonate polymorphs, calcite and vaterite, were used as biomineral substrates and acidic polypeptides, poly-L-aspartic (pAsp) acid and poly-L-glutamic (pGlu) acid, as simplified models of naturally occurring soluble acidic proteins. A basic polypeptide, poly-L-lysine (pLys), was also used in experiments in order to find out whether conformity between the crystal surface and the adsorbed polypeptide, or just the electrostatic interactions, have a decisive role in these processes. The addition of a particular polypeptide into the precipitation system caused a significant inhibition of nucleation and growth of vaterite, the extent of inhibition being in the order InhpAsp > InhpGlu >> InhpLys. In addition to the inhibition of precipitation, the change of the polymorphic composition and the crystal morphology of the precipitate were also achieved. The explanation of such acidic polypeptide behaviour is a consequence of kinetic constraints through the diverse efficiency of inhibition of both calcite nucleation and vaterite growth caused by adsorption of acidic polypeptides. The acidic polypeptides also caused the inhibition of calcite crystal growth, the effect being pAsp > pGlu, and changed the observed mechanism of growth controlled by the integration of ions into the spiral steps, as found for the model systems, to the surface nucleation rate-determining mechanism. Nonselective, weak and electrostatic adsorption of pLys at the crystal surface was probably responsible for increasing the calcite crystal growth rate when pLys was present at low concentrations and for inhibiting it at pLys higher concentrations. The strongest interactions between the crystal surfaces and the polypeptides, observed for the calcite/pAsp system, can account for coordinative interactions between the side chain carboxylic groups of the predominantly planar arrangement of the pAsp structure (β-pleated sheet) and Ca2+ ions from the crystal surface. (doi: 10.5562/cca1809)

Influence of the batch molar ratio y = [SiO2/Al2O3]b on the size and shape of zeolite A crystals at three different molar ratios [SiO2/H2O]b and [Na2O/H2O]b was investigated. Four different morphological entities of zeolite A, namely regular cubic crystals with sharp edges and apexes, cubic crystals with truncated edges, cubic crystals with “rounded” edges and apexes and pseudo-spherical, face-less particles, were observed in the crystalline end products obtained by hydrothermal treatment (heating at 80 oC) of the hydrogels having [SiO2/Al2O3]b in the range 1.0 – 2.2, [SiO2/H2O]b in the range 0.006 – 0.0186 and [Na2O/H2O]b in the range 0.0136 - 0.0424. The chemical conditions with respect to the values of [SiO2/Al2O3]b, [SiO2/H2O]b and [Na2O/H2O]b for obtaining the particular morphological entities of zeolite A are defined, and the possible mechanisms of their formation are discussed. The influence of the mentioned factors on the crystal size and the surface properties of crystallized zeolites is also considered.

Precipitation of calcium carbonate in different carrageenan gelling environments was initiated by fast mixing of calcium chloride and sodium carbonate solutions. Carrageenan concentration and the gelling mechanism, controlled by the addition of different cross-linking cations, calcium or sodium, were varied in order to elucidate the mechanism and dynamic of solid phase formation in highly supersaturated precipitation systems in which initial formation of three calcium carbonate polymorphs and amorphous calcium carbonate is possible. The analysis of the samples isolated 5 days after initiating the precipitation showed that calcite was the only solid phase obtained in the whole range of carrageenan concentrations investigated. At lower carrageenan concentrations, the calcite aggregates were observed, while at higher concentrations rhombohedral crystals of uniform size distribution were found. Spherical imprints on the surfaces of calcite particles precipitated at lower gel concentrations were observed, thus pointing out the initial and simultaneous precipitation of stable calcite and metastable calcium carbonate phases, vaterite and possibly amorphous calcium carbonate. Time resolving precipitation experiments within gelling media of low concentration confirmed such an assumption and implied the solution mediated mechanism of transformation of metastable vaterite, which was in close contact with calcite crystals. Reference precipitation systems, performed at apparently similar initial supersaturation and hydrodynamic conditions but without the presence of carrageenan, showed a comparable pattern of stepwise solution mediated transformation of metastable amorphous calcium carbonate and vaterite into calcite

Spontaneous precipitation of calcium carbonate was investigated in two precipitation systems: (1) with initial supersaturation lower than that corresponding to the solubility of amorphous calcium carbonate (ACC), at which vaterite precipitated, and (2) with initial supersaturation higher than that of ACC solubility, at which a mixture of calcite and vaterite was formed. After the addition of an acidic polypeptide, poly-L-glutamic acid (pGlu) or poly-L-aspartic acid (pAsp), into (1) a significant inhibition of nucleation, expressed as an increase in induction time, and growth of vaterite, perceived as a dead zone, was observed. Extent of inhibition decreased in the order: Inh(pAps)>Inh(pGlu)>>Inh(pLys). The addition of a polypeptide into (2) caused the inhibition of precipitation and changed the morphology and polymorphic composition of the precipitate; only vaterite appeared at approximately c(pAsp)=3 ppm, c(pGlu)=6 ppm, or c(pLys)=7 ppm. This finding is explained as a consequence of kinetic constraints through the inhibition of calcite nucleation and stronger binding of acidic polypeptide by the calcite surfaces than by the vaterite surfaces. Laboratory precipitation studies using conditions that resemble those in living organism should be run at an initial supersaturation corresponding to the solubility of ACC as a limiting condition.

The influence of sodium stearate (NaSt) on the precipitation of calcium carbonate during the semicontinuous process of slaked lime carbonation was studied in the systems in which process parameters, like concentration of total dissolved calcium, temperature, CO(2) flow rate and initial addition rate of slaked lime, were controlled. It was found that calcite was the only calcium carbonate polymorph that appeared under the investigated experimental conditions, while FT-IR spectroscopy and thermogravimetric analysis of samples confirmed the presence of stearate on the surface of precipitated calcium carbonate (PCC). Specific surface area of PCC increased with increasing stearate content: the highest value, s = 52.8 m(2) g(-1), was obtained at t = 20 degrees C, c(tot) = 17.0 mmol dm(-3) and the stearate content of m(NaSt)/m(CaO) = 0.03. It was also found that hydrophobic calcite crystals in the form of rhombohedral and scalenohedral morphology can be produced at m(NaSt)/m(CaO)0.01. The exception is the case of nanosized PCC production, when much higher concentration of NaSt is needed, m(NaSt)/m(CaO) = 0.22. Minimal amount of stearate necessary to build up the monolayer and corresponding cross sectional area of one stearate molecule were estimated for the obtained calcite morphologies.

Quantitative analysis of mixtures of the crystalline polymorphs of calcium carbonate—vaterite, aragonite, and calcite—is of general interest. For example, in connection with kinetic studies of binary systems of these crystalline modifications there is need for rather fast and accurate methods for the determination of the composition of mixtures of the polymorphous forms mentioned above. Rao and Murthy some years ago published a paper where they describe a method for quantitative analysis of calcite-aragonite mixtures making use of infrared spectrophotometry and the alkali halide pellet technique. In connection with kinetic studies of the transformation of vaterite to calcite, we have developed a rather fast and precise procedure for quantitative analysis of vaterite-calcite mixtures. Like the method of Rao and Murthy, it is based on infrared spectrophotometry, employing the baseline method for obtaining the absorbances of the bands, but differs in some details from their procedure. After the conclusion of the present work, Xyla and Koutsoukos published a paper dealing with quantitative analysis by means of infrared spectroscopy of mixtures of calcium carbonate polymorphs including vaterite-calcite mixtures. As in the present work, the KBr pellet technique has been employed, but band areas have been used as a measure of the content of the polymorphs in the mixtures. For calcite-vaterite mixtures the areas under the bands at 746 cm−1 and 877 cm−1 were taken as a measure of the content of vaterite and calcite, respectively, although the latter band coincides with the vaterite band at 877 cm−1. I t is not obvious what justifies this procedure, nor is it explained how the band areas were obtained and overlapping bands separated.