Ángeles Fernández-González

Nucleation, growth, and zoning phenomena in crystallizing (Ba,Sr)CO3, Ba(SO4,CrO4), (Ba,Sr)SO4, and (Cd,Ca)CO3 solid solutions from aqueous solutions

Crystals of (Ba,Sr)CO3, Ba(SO4,CrO4), (Ba,Sr)SO4, and (Cd,Ca)CO3 solid solutions were obtained by counterdiffusion of reactants through a column of porous silica hydrogel. For each system, a set of experiments, starting with mother solutions of different concentrations, was carried out. The composition of the solids was analyzed by electron microprobe. The method allows the establishment of the influence of the supersaturation on the distribution of material between solid and aqueous phases. With this aim, equilibrium calculations and effective composition of the nucleating crystals are compared. Two extreme nucleation behaviours have been observed. The solid solutions (Ba,Sr)SO4 and (Cd,Ca) CO3, with endmember solubility products differing by several orders of magnitude, tend to nucleate from aqueous solutions in a bimodal way. In these systems there is a strong preferential partitioning and only a very narrow range of aqueous-phase compositions can coexist in equilibrium with intermediate solid solutions. At high supersaturations, the range of aqueous solutions from which intermediate solid solutions can nucleate expands, but the bimodal effect remains. At the other extreme, the solid solutions with close endmember solubility products, like (Ba,Sr)CO3 and Ba(SO4,CrO4), tend to nucleate in an evenly distributed way. For these solid solutions, a wide range of fluid compositions is in equilibrium with intermediate solid solutions. When nucleation occurs at high supersaturation, the substituting ions incorporate into the nuclei nearly in the same stoichiometric proportion as in the aqueous phase, and the partition coefficients reach values approaching unity. Finally, the growth process has been studied by monitoring the compositional evolution of the crystals. There is a close relationship between the bimodal trend of a solid solution and the appearance of high gradient compositional zoning. The solid solutions (Ba,Sr)SO4 and (Cd,Ca)CO3 develop sharp compositional zoning when the fluid composition passes through a certain range during the growth process. On the contrary, when the solid solutions (Sr,Ba) CO3 and Ba (SO4, CrO4) grow at high supersaturations, the solids have nearly the same stoichiomerry as the aqueous phase, and the crystals tend to be homogeneous or, at most, to undergo a very gradual compositional evolution.

Uptake of dissolved Cd by biogenic and abiogenic aragonite: a comparison with sorption onto calcite

Abstract The uptake of Cd2+ by aragonite and calcite is investigated by combining macroscopic measurements with some qualitative sorption experiments performed in a hydrogel medium. Both biogenic and abiogenic aragonites were studied in order to evaluate the process on materials with different textures. Assuming that sorption occurs by surface precipitation of metal-bearing solids, the gel produces a drastic decrease in the nucleation density, which allows for the precipitation of crystallites that are large enough to be analysed by scanning electron microscopy and characterized by glancing-incidence X-ray techniques. The macroscopic study reveals that aragonite is a powerful sorbent for cadmium in aqueous environments. Microscopic observations indicate that cadmium is sorbed onto aragonite by surface precipitation of (Cd, Ca)CO3 solid solutions with a calcite-type structure. The precipitating individuals grow randomly oriented on the surface to reach sizes in the micrometre range. As a consequence, the concentration of cadmium in the aqueous solution decreases dramatically to values controlled by the low solubility of the cadmium-rich end member. This mechanism involves simultaneous dissolution-crystallization and is the same for both abiogenic and biogenic aragonites, the only difference being a result of the higher specific surface area of the biogenic starting material. Long-term uptake of cadmium by calcite occurs through a similar dissolution-crystallization mechanism, the final outcome being virtually the same, that is, surface precipitation of (Cd,Ca)CO3 solid solutions. In this case, however, substrate and precipitate are isostructural and the process occurs by oriented overgrowth of thin lamellar crystallites, which spread to quickly cover the surface by a layer a few nanometers thick. This epitaxial layer armors the substrate from further dissolution, so that the process stops when only a small amount of cadmium has been removed from the fluid. As a result, the “sorption capacity” of calcite is considerably lower than that of aragonite. The study illustrates reaction pathways and “partial” equilibrium endpoints in surface-precipitation processes involving solid solutions.

Computing Lippmann Diagrams from Direct Calculation of Mixing Properties of Solid Solutions: Application to the Barite-Celestite System

The Lippmann diagram for the system(Ba, Sr)SO4-H2O was computed at 25 °Cby determining the solid-phase activity coefficientsfrom first principles calculations. Directcalculations of the mixing properties of thebarite-celestite series indicate this solid solutionbehaves as non-ideal and non-regular. At 25 °C,the enthalpy of mixing shows a minimum around 50 mole% SrSO4 due to an ordering tendency. Thefree energy of mixing shows two minima that delimit awide and symmetric miscibility gap (from 2.1 to 97.9 mole% SrSO4) at this temperature. The excessfree energy of mixing requires a Guggenheim expansionseries of 5 terms to be described, where the termswith odd exponents are null as a consequence of thesymmetric distribution of the mixing properties withcomposition. The Lippmann diagram shows a peritecticpoint that corresponds to the composition of an aqueoussolution which is simultaneously at equilibrium withthe two extremes of the miscibility gap. The largedifference between the solubility products of theendmembers involves a strong preferential partitioningof the less soluble endmember towards the solid phase,which explains the extremely Ba-poor composition ofthe aqueous solution (aqueous activity fraction forBa2+ = 0.000446 ) at the peritectic point.

Sorption of chromate ions diffusing through barite-hydrogel composites: implications for the fate and transport of chromium in the environment

The interaction of Cr(VI) with barite is studied by quantifying the effect of this mineral on the net flux of chromate ions diffusing through an artificial porous medium consisting of barite grains embedded in a matrix of silica hydrogel. The gel suppresses convection and advection, only allowing diffusion of the aqueous ions, which eventually can be sorbed on the surface of the embedded grains. We find that long-term Cr(VI) uptake by barite occurs by epitaxial overgrowth of a Ba(CrO4,SO4) solid solution with the barite structure. In these particular experiments, the epitaxial crystallites have compositions around BaCr0.89S0.11O4. Sorption on barite reduces the net flux of chromate ions in relation to the flow through an equivalent (with the same porosity and tortuosity) but unreactive quartz-gel composite. A linear sorption model with a factor Kd = 0.291 was used to account for the experimental results. This factor is a complex measure that depends on the bulk medium characteristics and on the tendency of CrO42− to partition into barite under the precipitation conditions. Here, we assess the operating precipitation conditions in terms of possible limiting scenarios of supersaturation and discuss their influence on the partitioning of CrO42− ions into barite. The results demonstrate that precipitation of Ba(SO4,CrO4) solid solutions may be an option to control the concentration of Cr(VI) in natural waters. Neglecting to consider such solid solution formation will lead to overestimates of the availability and mobility of Cr(VI) in the environment.

Crystallisation of Ba(SO4, CrO4) solid solutions from aqueous solutions

Barite (BaSO4) and barium chromate form a complete solid solution series, where SO42− ions substitute for CrO42−. Both endmembers are isomorphous and crystallise in the space group Pnma. In this work we present an experimental study of the crystallisation behaviour of this solid solution from aqueous solutions. Crystals of Ba(SO4,CrO4) were grown by counter-diffusion of BaCl2 and Na2SO4+Na2CrO4 through a column of silica hydrogel. A set of experiments starting with parent solutions of different SO42−/CrO42− ratios was carried out. The composition of the obtained crystals was studied by electron microprobe. A linear influence of the composition on the structural parameters was determined by X-ray diffraction. The equilibrium partitioning of sulphate between the aqueous and the solid phase can be described by means of a function XBaSO4(XSO4,aq), which relates the solid mole fraction and the aqueous activity fraction of sulphate. According to this function there is a moderate preferential partitioning of sulphate towards the solid phase. However, at high supersaturations SO42− and CrO42− ions tend to lie down in a ratio which approximates to the unity. As a consequence, the substituting ions incorporate into the solid in a stoichiometric proportion approaching that of the aqueous phase, and the obtained crystals are relatively enriched in the more soluble endmember BaCrO4.

Crystallization of the (Cd,Ca)CO3 solid solution in double diffusion systems: the partitioning behaviour of Cd2+ in calcite at different supersaturation rates

Abstract Considering an ideal solid solution, the partitioning of cadmium in calcite was studied at different levels of supersaturation (δ) using counter-diffusion experiments. The effective (non-equilibrium) distribution coefficients of Cd2+ in calcite (DCd), varied between 20 and 100 depending on the initial concentration of reactants and the evolution of supersaturation. A function that relates the DCd with the supersaturation at nucleation was derived: logDCd = −0.40logδ + 2.42. The relationship obtained suggests that in naturalwaters, Cd2+ can be incorporated to a greater degree in calcite when the solid solution-aqueous solutions system is near to equilibrium, which can occur in scenarios where the supersaturation rate and hence the supersaturation level are relatively small.

Growth of calcium carbonate in the presence of Se(VI) in silica hydrogel

Abstract The element selenium has become a considerable environmental concern due its accumulation in aquifers on the one hand and the high radiotoxicity of its long-lived isotope 79Se on the other hand. Se(VI) is the most mobile of the various Se cations. This study deals with the interaction of Se(VI) with rock-forming minerals (carbonates) to better understand how to mitigate the potential environmental hazards of Se. The effect of Se(VI) on the cystallization of CaCO3 at room temperature was studied using the silica hydrogel method. The CaCO3 crystals obtained were characterized by X‑ray powder diffraction, scanning electron microscopy and electron microprobe. The presence of Se(VI) in the growth medium has a clear effect on the polymorph selection of CaCO3, promoting the formation of vaterite and inhibiting that of aragonite. Se(VI) also affects the characteristics of calcite crystals, which show progressively more elongated habits and smaller sizes with increasing Se(VI) concentration in the growth medium. The effect of Se(VI) on both the polymorphic crystallization of CaCO3 and the characteristics of calcite crystals shows features strikingly similar to those of other tetrahedral anionic groups like S(VI) and Cr(VI). This similarity extends to the amount of Se incorporated into the structure of the different CaCO3 polymorphs, with calcite having Se contents up to 1200 ppm, vaterite up to 500 ppm, and aragonite growing virtually Se-free. The role of Se(VI) on the crystallization of CaCO3 is discussed taking into consideration the physicochemical conditions in the growth medium at nucleation, which were modeled using the PHREEQ code for low-temperature aqueous geochemistry. The possible effect of the incorporation of the Se(VI) on the relative stability and, by extension, on the solubility of CaCO3 polymorphs is also discussed.

Crystallization behaviour of the (Mn,Ca)CO3 solid solution in silica gel: nucleation, growth and zoning phenomena

Abstract The nucleation and growth behaviour of the (Mn,Ca)CO3 solid solution from aqueous solutions in silica hydrogel were studied at 25°C. In order to obtain different levels of supersaturation at nucleation, experiments were carried out in diffusion columns of different lengths. Zoning phenomena, compositional heterogeneity and morphological modifications of the crystals obtained were examined by electron microprobe and scanning electron microscopy and microanalyses. The large difference between the solubility of calcite (CaCO3) and rhodochrosite (MnCO3) implies a significant preferential partitioning of Mn towards the solid. The distribution of Mn2+ and Ca2+ ions between the aqueous and solid phase demonstrates that at elevated supersaturation, this preferential partitioning is softened. The morphologies of experimentally grown individuals show a clear dependence on the composition of the parent solution. Crystals obtained from aqueous solutions with large Mn contents exhibit a relatively sharp concentric zoning along the equatorial sections. On these bases, the roles of multiple kinetic and thermodynamic factors during nucleation and growth are also discussed. Finally, an X-ray diffraction analysis of the precipitates provided no evidence of the formation of the dolomite-type ordered kutnahorite [CaMn(CO3)2] in any of the experimental sets.

Crystallization of nanostructured cobalt hydroxide carbonate at ambient conditions: a key precursor of Co3O4

Abstract Crystals of Co2CO3(OH)2 have been synthesized under ambient conditions, in contrast to hydrothennal methods reported previously. We have developed a simple but efficient methodology to obtain an initial amorphous phase that evolves to a crystalline cobalt hydroxide carbonate after one week of maturation. X-ray diffraction analysis indicates that this phase crystallizes in the space group P21/a (a = 12.886(6), b = 9.346(3), c = 3. 156(1) Å, β = 110.358(6)°). The platelet morphology of Co2CO3(OH)2 agrees with its lamellar crystal structure. High-resolution transmission electron microscopy (HRTEM) reveals that each individual platelet is comprised of nanodomains disoriented with respect to their neighbours. The kinetics and the activation energy (Ea = 6.26 kJ mol-1) of the transformation process have been estimated using the rate constant method. The precipitation of solids leads to a decrease in the cobalt concentration in the solution (~88%) reaching values of ~150 ppm, which can be considered a successful reduction from the perspective of water quality. The calcination in air of the synthetized platelets produced exclusively Co3O4. The thermo-X-ray difraction results confirm that Co2CO3(OH)2 is transformed over a small range of temperatures (225 -235°C) into pure Co3O4. HRTEM images show that the lamellar nanomorphology is preserved in the Co3O4 phase. Therefore, understanding the crystallization behaviour of Co2CO3(OH)2 can help to minimize environmental problems caused by cobalt pollution and may facilitate the management of methods to obtain phases with specific nanomorphologies used widely in material sciences.

Crystal structure of cobalt hydroxide carbonate Co2CO3(OH)(2): density functional theory and X-ray diffraction investigation

The crystal structure of cobalt carbonate hydroxide Co2CO3(OH)2, a solid important in materials and environmental science, is investigated using density functional theory (DFT) simulations and powder X-ray diffraction (PXRD) measurements.