Amalia Jiménez

Interaction of gypsum with As(V)-bearing aqueous solutions: Surface precipitation of guerinite, sainfeldite, and Ca2NaH(AsO4)2·6H2O, a synthetic arsenate

Abstract The interaction of arsenate-bearing aqueous solutions with gypsum at a starting pH of 9 and 25 °C results in surface precipitation of guerinite, Ca5(HAsO4)2(AsO4)2·9H2O, sainfeldite, Ca5(HAsO4)2(AsO4)2·4H2O, and occasionally Ca2Na(HAsO4)(AsO4)·6H2O, a new arsenate. These three solid phases are characterized by the simultaneous presence of HAsO42- and AsO43- groups in their structure, which is explainable since crystallization occurs within a pH range in which both HAsO42- and AsO43- are available in the aqueous solution. The interaction leads to a decrease in the As(V) concentration in the aqueous phase to reach values controlled by the solubility of these solid phases. The study combines several macroscopic experiments, in which changes in the solution chemistry are monitored as a function of time, with the characterization of solid phases by SEM-EDS and XRD. The crystal morphologies of the precipitating phases are interpreted on the basis of their respective structures. The thermodynamic solubility products of both guerinite and the new arsenate have been determined, being 10-31.17±0.05 and 10-13.83±0.03, respectively. The reaction paths followed by the system and the equilibrium endpoints have been modeled using the geochemical code PHREEQC.

Interaction of phosphate-bearing solutions with gypsum: Epitaxy and induced twinning of brushite (CaHPO4·2H2O) on the gypsum cleavage surface

Abstract Under slightly acidic conditions and 25 °C, the interaction between phosphate-rich aqueous solutions and gypsum cleavage fragments results in the surface precipitation of brushite (CaHPO4·2H2O) crystals, which grow epitaxially on the (010) surface of gypsum. Using an A-centered unit-cell setting for both brushite (Aa) and gypsum (A2/a), the epitaxial relationship implies matching of the planes (010) of both structures and correspondence between equivalent crystallographic directions within these planes. The crystal habit of the overgrowing brushite is thin tabular to laminar on {010} with {111} and {111} as side faces and a clear elongation on [101]. There are two orientations of the brushite plates on the gypsum surface related to each other by a twofold axis on [010]. Thus, the overgrowth is an aggregate of parallel brushite crystals that may be twin-related, with the twofold axis as the twin law. During the interaction, gypsum dissolution is coupled with brushite growth until saturation with respect to both minerals is reached. A model of this thermodynamically driven dissolution-crystallization process is presented using the geochemical code PHREEQC. The epitaxial relationships are explained by comparing the bond system and the crystallographic properties of both minerals.

AFM study of the epitaxial growth of brushite (CaHPO4·2H2O) on gypsum cleavage surfaces

Abstract The epitaxial overgrowth of brushite (CaHPO4·2H2O) by the interaction of phosphate-bearing, slightly acidic, aqueous solutions with gypsum (CaSO4·2H2O) was investigated in situ using atomic force microscopy (AFM). Brushite growth nuclei were not observed to form on the {010} gypsum cleavage surface, but instead formed in areas of high dissolution, laterally attached to gypsum [101] step edges. During the brushite overgrowth the structural relationships between brushite (Aa) and gypsum (A2/a) result in several phenomena, including the development of induced twofold twining, habit polarity, and topographic effects due to coalescence of like-oriented crystals. The observed brushite growth is markedly anisotropic, with the growth rate along the main periodic bond chains (PBCs) in the brushite structure increasing in the order [101] > [101] > [010], leading to tabular forms elongated on [101]. Such a growth habit may result from the stabilization of the polar [101] direction of brushite due to changes in hydration of calcium ions induced by the presence of sulfate in solution, which is consistent with the stabilization of the gypsum [101] steps during dissolution in the presence of HPO2-4 ions. The coupling between growth and dissolution was found to result in growth rate fluctuations controlled by the changes in the solution composition.

Structure and crystallization behavior of the (Ba,Sr)HAsO4·H2O solid-solution in aqueous environments

Abstract Crystals of different members of the (Ba,Sr)HAsO4·H2O solid solution have been synthesized, and the first structural studies indicate that they crystallize in the same space group, Pbca, with Z = 8. The unit-cell parameters are a = 7.436(2), b = 8.481(1), c = 14.348(6) Å, and a = 7.752(1), b = 8.759(1), c = 14.668(3) Å for the strontium and barium end-members, respectively. Both end-members have a layered structure with slices parallel to (001) linked by hydrogen bonds from the water molecules. These features are consistent with both the perfect cleavage on {001} and the morphological importance of this form in the crystals obtained. However, the two end-members are not isostructural and show differences in both the anionic hydrogen positions and number of hydrogen bonds. Complementary powder-diffraction measurements indicate that the cell parameters increase in a non-linear way with the barium content indicating that the solid solution is complete but could be non-ideal. Preliminary data suggest that barium partitions preferentially into the solid phase when crystallizing this solid solution from aqueous solutions.

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.

Dehydration behaviour of the Ca(SO4,HPO4).2H2O solid solution

Abstract Gypsum (CaSO4·2H2O) and brushite (CaHPO4·2H2O) are minerals with similar structures, despite not being isostructural. They both belong to the monoclinic system, but crystallize in different space groups. It is known that, to some extent, phosphate and sulphate groups can substitute for each other within the structure of these minerals. In the present work, the dehydration behaviour of experimentally produced Ca(SO4,HPO4)·2H2O solid solutions is assessed from thermogravimetrical (TG and DTG) and differential scanning calorimetry (DSC) analyses. For the applied experimental conditions it was not possible to obtain homogenous precipitates with compositions within 0.27 < XBr < 0.53, where XBr is the brushite mole fraction. The results reveal that the dehydration behaviour of solid solutions with compositions within the 0 < XBr < 0.27 interval tend to approach the dehydration process of pure gypsum. On the other hand, solid solutions with compositions within 0.53 < XBr< 1 involve higher dehydration temperatures than the brushite end-member, in a two-step process.

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.