Transformation of calcium oxalate hydrates

doi:10.1016/0022-0248(86)90131-4

Journal of Crystal Growth

Volume 74, Issue 2, February–March 1986, Pages 399-408

https://doi.org/10.1016/0022-0248(86)90131-4 Get rights and content

Abstract

The transformation kinetics of calcium oxalate trihydrate and calcium oxalate dihydrate to the thermodynamically stable monohydrate have been studied in batch precipitation experiments. A combination of size distribution measurements, optical microscopy, X-ray diffraction and thermogravimetric analysis were used to characterise the processes involved. The transformation appears to be a solution mediated process in which dissolution of the unstable phases is followed by growth of the monohydrate; corresponding dissolution and growth rates have been determined. The distribution of hydrates in the initial stages of the precipitation was very sensitive to the mixing process; for example the use of a magnetic stirrer produced a different distribution to that formed with a propeller agitator.

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Preparation and characterization of calcium oxalate dihydrate seeds suitable for crystal growth kinetic analyses
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The precipitation system was mixed again for 30 s, and then left unstirred for 20 min. The applied experimental procedure was used after considering the known literature data, [29,31–35] and the results of preliminary experiments. The systems were stirred at a constant rate by three different means: mechanical and magnetic stirring, and ultrasonic irradiation.
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).
The impact of oxalate ions on barium sulfate crystallization
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In this manuscript we investigate the impact of oxalate ions on the crystallization of barium sulfate. The organic anion, oxalate, was found to inhibit both nucleation and growth according to atomic force microscopy measurements and dynamic light scattering data. Raman confocal imaging was used to try and determine how the oxalate interacts with the barium sulfate surface. It was found that barium oxalate is only seen at very high oxalate concentrations. At lower concentrations no direct evidence for the presence of oxalate was observed, however, the most probable mode of action for oxalate is via adsorption onto the barium sulfate surface including into key kink or step sites, impacting growth.
Effectiveness of oxalic acid treatments for the protection of marble surfaces
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In the case of the calcitic marble, just whewellite (CaC2O4·H2O) (see Raman and XRD results, below) could precipitate due to the absence of Mg2 +, whereas for the dolomitic marble the precipitation of whewellite and magnesium oxalate phases, such as glushinskite (MgC2O4·2H2O), was thermodynamically possible under the experimental conditions for D60 samples. The distribution of precipitates shown in Fig. 4 suggests that during the interaction of the marbles with oxalic acid solutions, whewellite precipitated earlier than glushinskite due to its lower solubility (log Ksp − 8.69 [35] vs log Ksp − 5.18 [36], respectively). The perfect pseudomorphism observed in these partially replaced samples, resulting in the preservation of the textural features of the calcitic and dolomitic substrates, suggests that the replacement took place via an interface-coupled dissolution-precipitation mechanism [37] as previously indicated by Ruiz-Agudo et al. [30].
Naturally formed rims of calcium oxalates developed on calcareous stones have been recognized as effective protective coatings. Inspired in nature, it has been recently proposed the use of oxalate salts for the protection of stone surfaces via dissolution of the calcitic substrate and the subsequent precipitation of oxalate phases. In contrast, the application of an oxalic acid solution on carbonate stones has been generally avoided due to assumed hazards associated with enhanced substrate dissolution. Nonetheless, it has been reported that coherent oxalate layers and preservation of textural features only occurs at low pH, which could be beneficial from a conservation point of view. Here, the application of oxalic acid treatments on two calcitic and dolomitic Spanish marbles from Macael area has been studied as a means to develop effective oxalate protective coatings. Morphological and compositional analyses show that reacted marble surfaces develop μm-thick calcium or calcium and magnesium oxalate rims on calcitic and dolomitic marble, respectively. The presence of such oxalate layers strongly reduces chemical weathering due to acid dissolution and sulfation, without altering the color of the marble substrates. This protection methodology overcomes the limitations of previous oxalate treatments and may represent a highly efficient conservation methodology.
Solubility, structure, and morphology in the co-precipitation of cadmium and zinc with calcium-oxalate
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Calcium-oxalates (Ca-Ox), which are widely produced by microorganisms and plants, are ubiquitous and persistent biominerals in the biosphere. We investigated the potential trapping of two phytotoxic metals, cadmium (Cd) and zinc (Zn) by isomorphous substitution into the crystalline structure of Ca-Ox precipitated over a wide range of Cd²⁺/Ca²⁺ or Zn²⁺/Ca²⁺ ratio in solution. We employed atomic absorption spectroscopy, X-ray diffraction (XRD), and optical microscopy to evaluate our hypotheses that favorable solid-solution conditions and structural framework of crystal habits promote selective metal trapping within Ca-Ox precipitates. Chemical analysis demonstrated more effective Cd-Ox/Ca-Ox than Zn-Ox/Ca-Ox co-precipitate formation at the same trace metal mole fraction in solution. The XRD results revealed sequestration of Cd, but not Zn, within Ca-Ox monohydrate (whewellite). Comparative chemical analysis with Cd-Ox formation in the absence of Ca-Ox showed that the whewellite solid-solution formation lowered the solubility of Cd²⁺ below that of pure Cd-Ox. The XRD patterns indicated that Zn²⁺ precipitated as a separate pure Zn-Ox crystal that is largely excluded from the Ca-Ox structure. Furthermore, the presence of Zn²⁺ in solution favored the formation of the less stable Ca-Ox dihydrate (weddellite) over whewellite. In agreement with the XRD data, visualization of the co-precipitates by optical microscopy illustrated combined mineral phases of Cd-Ox with Ca-Ox whereas Zn-Ox and Ca-Ox exhibited two distinct mineral morphologies. Our findings shed light into the structural factors that are most critical in facilitating the trapping of toxic trace metals within Ca-Ox crystals.
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Eulerian and Lagrangian computational fluid dynamical simulations by Rielly and Marquis (2001) show that the flow field in crystallizers is composed of such regions (LSEs) of strong mean flow as well as regions of nearly quiescent flow. The LSEs are primarily determined by tank geometry, impeller position, and speed (Tavare, 1995; Brecevic et al., 1986). However, crystallization also depends on micromixing and molecular scale reactions (Garside and Tavare, 1985; Tavare, 1989; Söhnel and Garside, 1992; Leubner, 2002).
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Transformation of calcium oxalate hydrates

Abstract

J. Crystal Growth

J. Urol.

J. Crystal Growth

J. Crystal Growth

J. Pharm. Sci.

J. Crystal Growth