Citrate Stabilizes Hydroxylapatite Precursors: Implications for Bone Mineralization

Mineralization of hydroxylapatite (HAp), the main inorganic phase in bone, follows nonclassical crystallization routes involving metastable precursors and is strongly influenced by organic macromolecules. However, the effect of small organic molecules such as citrate on the formation of HAp is not well constrained. Using potentiometric titration experiments and titration calorimetry, in combination with a multianalytical approach, we show that citrate stabilizes prenucleation species as well as a liquid-like calcium phosphate precursor formed before any solid phase nucleates in the system. The stabilization of a liquid-like precursor phase could facilitate infiltration into the cavities of the collagen fibrils during bone mineralization, explaining the enhancement of collagen-mediated mineralization by citrate reported in previous studies. Hence, citrate can influence bone mineralization way before any solid phase (amorphous or crystalline) is formed. We also show that HAp formation after amorphous calcium phosphate (ACP) in the absence and presence of citrate results in nanoplates of about 5-12 nm thick, elongated along the c axis. Such nanoplates are made up of HAp nanocrystallites with a preferred c axis orientation and with interspersed ACP. The nanoplatelet morphology, size, and preferred crystallographic orientation, remarkably similar to those of bone HAp nanocrystals, appear to be an intrinsic feature of HAp formed from an amorphous precursor. Our results challenge current models for HAp mineralization in bone and the role of citrate, offering new clues to help answer the long-standing question as to why natural evolution favored HAp as the mineral phase in bone

Citrate Stabilizes Hydroxylapatite Precursors: Implications for Bone Mineralization

This research was funded by the Spanish Government (grant nos. RTI2018.099565.B.I00 and CGL2015-64683-P), the European Commission (ERDF funds), the University of Granada ("Unidad Cientifica de Excelencia" UCE-PP201605), and the Junta de Andalucia (no. P11-RNM-7550 and research group RNM-179). The authors thank M. Abad and Haidour Benamin from CIC-UGR for their assistance during microscopy and NMR studies. C.R.A. thanks project A7 from SFB1214 (DFG-Deutsche Forschungsgemeinschaf) and Zukunftstkolleg (University of Konstanz).Mineralization of hydroxylapatite (HAp), the main inorganic phase in bone, follows nonclassical crystallization routes involving metastable precursors and is strongly influenced by organic macromolecules. However, the effect of small organic molecules such as citrate on the formation of HAp is not well constrained. Using potentiometric titration experiments and titration calorimetry, in combination with a multianalytical approach, we show that citrate stabilizes prenucleation species as well as a liquid-like calcium phosphate precursor formed before any solid phase nucleates in the system. The stabilization of a liquid-like precursor phase could facilitate infiltration into the cavities of the collagen fibrils during bone mineralization, explaining the enhancement of collagen-mediated mineralization by citrate reported in previous studies. Hence, citrate can influence bone mineralization way before any solid phase (amorphous or crystalline) is formed. We also show that HAp formation after amorphous calcium phosphate (ACP) in the absence and presence of citrate results in nanoplates of about 5−12 nm thick, elongated along the c axis. Such nanoplates are made up of HAp nanocrystallites with a preferred c axis orientation and with interspersed ACP. The nanoplatelet morphology, size, and preferred crystallographic orientation, remarkably similar to those of bone HAp nanocrystals, appear to be an intrinsic feature of HAp formed from an amorphous precursor. Our results challenge current models for HAp mineralization in bone and the role of citrate, offering new clues to help answer the long-standing question as to why natural evolution favored HAp as the mineral phase in bone.Spanish Government European Commission RTI2018.099565.B.I00 CGL2015-64683-PEuropean Commission European Commission Joint Research CentreUniversity of Granada ("Unidad Cientifica de Excelencia") UCE-PP2016-05Junta de Andalucia P11-RNM-7550 RNM-179DFG-Deutsche Forschungsgemeinschaf SFB1214Zukunftstkolleg (University of Konstanz

The Carbonation of Wollastonite: A Model Reaction to Test Natural and Biomimetic Catalysts for Enhanced CO2 Sequestration

One of the most promising strategies for the safe and permanent disposal of anthropogenic CO2 is its conversion into carbonate minerals via the carbonation of calcium and magnesium silicates. However, the mechanism of such a reaction is not well constrained, and its slow kinetics is a handicap for the implementation of silicate mineral carbonation as an effective method for CO2 capture and storage (CCS). Here, we studied the different steps of wollastonite (CaSiO3) carbonation (silicate dissolution -> carbonate precipitation) as a model CCS system for the screening of natural and biomimetic catalysts for this reaction. Tested catalysts included carbonic anhydrase (CA), a natural enzyme that catalyzes the reversible hydration of CO2(aq), and biomimetic metal-organic frameworks (MOFs). Our results show that dissolution is the rate-limiting step for wollastonite carbonation. The overall reaction progresses anisotropically along different [hkl] directions via a pseudomorphic interface-coupled dissolution–precipitation mechanism, leading to partial passivation via secondary surface precipitation of amorphous silica and calcite, which in both cases is anisotropic (i.e., (hkl)-specific). CA accelerates the final carbonate precipitation step but hinders the overall carbonation of wollastonite. Remarkably, one of the tested Zr-based MOFs accelerates the dissolution of the silicate. The use of MOFs for enhanced silicate dissolution alone or in combination with other natural or biomimetic catalysts for accelerated carbonation could represent a potentially effective strategy for enhanced mineral CCS.This research was funded by the Spanish Government (grants CGL2015-70642-R, CGL2015-73103-EXP, CTQ2017-84692-R), EU FEDER funding, the University of Granada (“Unidad Científica de Excelencia” UCE-PP2016-05) and the Junta de Andalucía (grant P11-RNM-7550 and Research Group RNM-179). We thank the personnel of the Centro de Instrumentación Científica (CIC) of the University of Granada for their help during TG-DSC, FESEM, -XRD, and ICP-OES analyses

Reaction of pseudowollastonite with carbonate-bearing fluids: Implications for CO2 mineral sequestration

The kinetics of silicate carbonation in aqueous solutions are typically sluggish, especially at neutral to alkaline conditions. This hampers the complete understanding of the mechanisms and parameters that control mineral carbonation during carbon capture and storage (CCS). Here we study the hydrothermal dissolution and carbonation of pseudowollastonite (psw; alpha-CaSiO3), one of the most reactive silicates known, under a range of geochemical conditions ranging from acidic to strongly alkaline pH, presence/absence of different background alkali metal ions and carbonate sources (K2CO3 and Na2CO3, pH similar to 13, or NaHCO3 and KHCO3 , pH similar to 9). We show that in addition to amorphous silica precipitation, the formation of secondary Na + Ca- or K + Ca-silicates in the presence of Na+ and K+ background ions, respectively, fosters the progress of psw carbonation. However, the formation of Ca-containing secondary crystalline silicates and Ca-containing amorphous silica is shown to be a strong handicap for a fully effective carbonation. In all cases a higher conversion into CaCO3 (up to similar to 70 mol%) is achieved when using bicarbonate salts (i.e., lower initial pH). By using a reactor with a pressurized CO2-solution, with and without Na+ or K+ background ions, rapid and nearly complete conversion of psw with a CaCO3 yield similar to 92 mol% is achieved because, in addition to the initial low pH (similar to 3.7) that favored alpha-CaSiO3 dissolution, abundant Ca-free non-passivating amorphous silica formed along with calcite. These results imply that the presence (e.g., use of sea water during CO2 injection or mixing with saline formation solutions) or the release of different alkali metal ions (e.g., after feldspar and/or basaltic glass dissolution) in combination with a reaction-induced pH increase during in situ CCS scenarios may strongly limit carbonation due to the capture of alkaline-earth metals in secondary silicates and a reduction in reaction rates. In turn, our results show that the high conversion achieved in pure CO2 -aqueous systems, while relevant for ex situ CCS, may not reflect the actual conversion in multicomponent natural systems following reactive transport during in situ CCS. Moreover, the precipitation of secondary silicate and calcium carbonate phases have a direct cementing effect, which could be detrimental for in situ CCS, as it would likely reduce host rock permeability, but would be relevant and beneficial for the setting of novel CaSiO3 -based non-hydraulic cements with reduced CO2 footprint. Keyword

The multiple roles of carbonic anhydrase in calcium carbonate mineralization

status: Published onlin

Nonclassical Crystallization of Calcium Hydroxide via Amorphous Precursors and the Role of Additives

In many systems, prenucleation clusters (PNC), dense liquids, and solid amorphous phases precede the formation of crystalline phases, which can grow via a nanoparticle aggregation mechanism. Despite intensive efforts, the current understanding of the mechanisms of such nonclassical crystallization processes is far from complete. Here by means of calcium potentiometric titration tests complemented by X-ray diffraction, dynamic light scattering, and electron microscopy analyses, we show that in the case of calcium hydroxide (CH), one of the main components of set Portland cement, PNCs and dense liquid precursors (prenucleation stage), and amorphous CH (ACH) and a metastable nanocrystalline CH (postnucleation stage) precede the formation of stable CH crystals. Such a phase sequence is also observed in the presence of additives commonly used as cement set-retarders and plasticizers (polysaccharides, lignosulfonate, and polyacrylate). We show that the main action of additives occurs during the prenucleation stage via destabilization/stabilization of PNCs, and the promotion/stabilization of dense liquid precursors leading to a significant delay in the onset of ACH nucleation at high supersaturations. Additives also stabilize amorphous and metastable crystalline CH phases and modify the number, size, and morphology of stable CH crystals. In contrast to classical crystallization theory, an inverse relationship between supersaturation at the onset of nucleation and the final number and size of CH crystals is observed. This unexpected result is explained by the fact that CH crystals nucleate after dissolution of ACH, whose solubility marks the maximum supersaturation in the system with respect to primary CH nanoparticles, which subsequently undergo oriented attachment to form large CH particles that further grow via aggregation of ACH nanoparticles. These results help to understand how CH forms, show that nonclassical crystallization can take place in cement systems, and shed light on how cement admixtures work

New polymer-based treatments for the prevention of damage by salt crystallization in stone

Salt crystallization can produce severe damage in porous stones, with a dramatic impact on cultural heritage conservation. Such damage is related to the fact that repulsive forces arise between the salt crystals and the pore wall, generating a disjoining pressure that frequently exceeds the tensile strength of stone. In this paper, new treatments are proposed, aimed at preventing salt damage by depositing a thin layer of polymeric coatings over the stone\u2019s pore surfaces. These coating are expected to change the surface chemistry, eliminating the repulsion between the growing crystals and the pore wall and hence the development of the disjoining pressure. Several biopolymers were tested on these substrates: silica glass, calcite, and calcite subjected to a pre-treatment with diammonium hydrogen phosphate (DAP), aimed at preventing calcite dissolution and acting as an anchoring substrate for the polymer coating. Selected polymer treatments were applied to porous Globigerina limestone samples, which were subjected to crystallization tests with sodium sulfate, obtaining promising results (i.e., significant reduction in stone damage), especially when the polymers were applied after the DAP treatment

Sequestration of Selenium on Calcite Surfaces Revealed by Nanoscale Imaging

Calcite, a widespread natural mineral at the Earth’s surface, is well-known for its capacity to sequester various elements within its structure. Among these elements, selenium is important because of its high toxicity in natural systems and for human health. In the form of selenite (Se^(IV)), selenium can be incorporated into calcite during growth. Our in situ atomic force microscopy observations of calcite surfaces during contact with selenium-bearing solutions demonstrate that another process of selenium trapping can occur under conditions in which calcite dissolves. Upon the injection of solutions containing selenium in two states of oxidation (either Se^(IV) or Se^(VI)), precipitates were observed forming while calcite was still dissolving. In the presence of selenate (Se^(VI)), the precipitates formed remained small during the observation period. When injecting selenite (Se^(IV)), the precipitates grew significantly and were identified as CaSeO₃·H₂O, based on SEM observations, Raman spectroscopy, and thermodynamic calculations. An interpretation is proposed where the dissolution of calcite increases the calcium concentration in a thin boundary layer in contact with the surface, allowing the precipitation of a selenium phase. This process of dissolution–precipitation provides a new mechanism for selenium sequestration and extends the range of thermodynamic conditions under which such a process is efficient

Crystallization and Colloidal Stabilization of Ca(OH)₂ in the Presence of Nopal Juice (<i>Opuntia ficus indica</i>): Implications in Architectural Heritage Conservation

Hydrated lime (Ca­(OH)₂) is a vernacular art and building material produced following slaking of CaO in water. If excess water is used, a slurry, called lime putty, forms, which has been the preferred craftsman selection for formulating lime mortars since Roman times. A variety of natural additives were traditionally added to the lime putty to improve its quality. The mucilaginous juice extracted from nopal cladodes has been and still is used as additive incorporated in the slaking water for formulation of lime mortars and plasters, both in ancient Mesoamerica and in the USA Southwest. Little is known on the ultimate effects of this additive on the crystallization and microstructure of hydrated lime. Here, we show that significant changes in habit and size of portlandite crystals occur following slaking in the presence of nopal juice as well as compositionally similar citrus pectin. Both additives contain polysaccharides made up of galacturonic acid and neutral sugar residues. The carboxyl (and hydroxyl) functional groups present in these residues and in their alkaline degradation byproducts, which are deprotonated at the high pH (12.4) produced during lime slaking, strongly interact with newly formed Ca­(OH)₂ crystals acting in two ways: (a) as nucleation inhibitors, promoting the formation of nanosized crystals, and (b) as habit modifiers, favoring the development of planar habit following their adsorption onto positively charged (0001)_Ca(OH)₂ faces. Adsorption of polysaccharides on Ca­(OH)₂ crystals prevents the development of large particles, resulting in a very reactive, nanosized portlandite slurry. It also promotes steric stabilization, which limits aggregation, thus enhancing the colloidal nature of the lime putty. Overall, these effects are very favorable for the preparation of highly plastic lime mortars with enhanced properties