Alejandro Burgos-Cara

A non-classical view on calcium oxalate precipitation and the role of citrate

Although calcium oxalates are relevant biominerals, their formation mechanisms remain largely unresolved. Here, we investigate the early stages of calcium oxalate formation in pure and citrate-bearing solutions. Citrate is used as a well-known oxalate precipitation inhibitor; moreover, it resembles the functional domains of the biomolecules that modulate biomineralization. Our data suggest that calcium oxalate forms after Ca2+ and C2O42− association into polynuclear stable complexes that aggregate into larger assemblies, from which amorphous calcium oxalate nucleates. Previous work has explained citrate inhibitory effects according to classical theories. Here we show that citrate interacts with all early stage CaC2O4 species (polynuclear stable complexes and amorphous precursors), inhibiting calcium oxalate nucleation by colloidal stabilization of polynuclear stable complexes and amorphous calcium oxalate. The control that citrate exerts on calcium oxalate biomineralization may thus begin earlier than previously thought. These insights provide information regarding the mechanisms governing biomineralization, including pathological processes (e.g., kidney stone formation).The formation mechanism of abundant calcium oxalate biomaterials is unresolved. Here the authors show the early stages of calcium oxalate formation in pure and citrate-bearing solutions by using a titration set-up in conjunction with solution quenching, transmission electron microscopy and analytical ultracentrifugation.

Exploring the effect of poly(acrylic acid) on pre- and post-nucleation BaSO4 species: new insights into the mechanisms of crystallization control by polyelectrolytes

Barium sulphate (BaSO4) precipitation has been suggested to occur by non-classical pathways that include the formation of a dense liquid precursor phase, nucleation of primary nanoparticles and two levels of oriented aggregation resulting in micron-sized barite single crystals. In this study we build from this previous knowledge and explore how Ba2+ and SO42− ions associate in solution prior to nucleation of a solid phase and the effects of poly(acrylic acid) (PAA) in these pre-nucleation and post-nucleation stages. With this aim, in situ potentiometric experiments and transmission electron microscopy (TEM) observations of time-resolved quenched samples were carried out. Additional bulk precipitation experiments in which supersaturation was achieved by rapid mixing of Ba- and SO4-bearing solutions were performed. The resultant precipitates were characterized by scanning electron microscopy (FESEM and ESEM), transmission electron microscopy (TEM), X-ray diffraction (XRD) and thermogravimetric (TG) analyses. Our study provides evidence that barium sulphate precipitation occurs via the formation of ion associates in solution (ion pairs and/or clusters), that are significantly destabilized in the presence of PAA. This is associated with a noticeable delay in nucleation in the presence of PAA. Thus, our results provide indirect evidence that suggests that prenucleation ion associates must form prior to solid BaSO4 nucleation. Alternatively, BaSO4 mineralization in the presence of PAA seems to occur by a different route that consists in the formation of Ba-PAA globules in solution followed by an amorphous hydrated BaSO4 phase that transform into crystalline barite nanoparticles. PAA seems to stabilize this amorphous phase, which nevertheless also forms in the absence of PAA. Finally, single micron-sized crystals are formed by the oriented attachment of distinguishable smaller subunits, thus forming mesocrystals in the presence and absence of PAA.

Crystallization and Colloidal Stabilization of Ca(OH)2 in the Presence of Nopal Juice (Opuntia ficus indica): 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.

Influence of pH and citrate on the formation of oxalate layers on calcite revealed by in situ nanoscale imaging

The influence of pH and citrate concentration on the replacement of calcite by calcium oxalate has been investigated by in situ nanoscale observations in flow-through experiments performed using atomic force microscopy (AFM). Significant changes in the morphology of the precipitated whewellite (CaC2O4·H2O) crystals were observed, as well as in the degree of interface coupling of the dissolution–precipitation reactions, depending on the pH of the solution. Citrate also influences whewellite morphology; different citrate species seem to preferentially adsorb on different whewellite faces according to our experimental observations. The results of this study may help in the design of effective treatments for the protection of calcareous stone, based on the natural process of patina formation that occurs in monuments, as well as to better understand the process of unwanted oxalate precipitate formation and its control by small organic molecules such as citrate.

Porosity generated during the fluid-mediated replacement of calcite by fluorite

Mineral replacement reactions are common phenomena in natural and laboratory environments where solids have re-equilibrated with aqueous solutions and are characterized by the generation or destruction of porosity in the product phase(s). Here, the evolution of porosity during the replacement of calcite by fluorite is used as a model system to characterize the kinetics of volume variations. Non-porous single crystals of calcite were reacted with sodium fluoride solutions for different reaction times. The crystals were pseudomorphically replaced by highly porous fluorite. Complementary use of porosimetry techniques, high resolution imaging, and mass-balance calculations revealed the total, open, and closed porosity in the samples. The infiltration of aqueous fluids in the Earth depends on the evolution of porosity and hence porosity variations are important in various geological processes such as, rock weathering and soil formation, fluid-controlled metamorphism, mineral ore emplacement, or oil and gas reservoir compaction. Such mechanisms can also be used to develop geo-inspired materials designed for industrial and medical applications.

Effect of ferrous iron on the nucleation and growth of CaCO3 in slightly basic aqueous solutions

The precipitation of calcium carbonate, CaCO3 and the growth of calcite has been studied in aqueous solutions containing ferrous iron (Fe2+). Two different types of bulk experiments have been carried out: nucleation experiments at constant pH, following the procedure developed by Gebauer et al. (2008), revealed the stabilisation of aragonite induced by the presence of the foreign ions at the expense of calcite and vaterite. Growth experiments, using the constant composition method (Tomson and Nancollas, 1978), clearly showed that calcite growth is inhibited by the presence of Fe2+ and the characterisation of the re-grown calcite crystals by HR-TEM confirmed the predictions obtained with molecular dynamics simulations. Additionally, in situ atomic force microscopy (AFM) flow-through growth experiments directly showed the distortion of the normal calcite growth spirals that lead to macroscopic inhibition of calcite growth. Finally, thermodynamic considerations for the solid solution – aqueous solution system Ca–Fe–CO2–H2O are discussed that allow the modelling of geochemical processes involved in this system, such as geological carbon storage in basaltic rocks.