Structural analysis of metal-doped calcium oxalate

Abstract

Calcium oxalate crystals are the most common biominerals found in plants. They also make their presence known as painful kidney stones in humans and animals. Their function in plants is extraordinarily versatile and encompasses calcium storage and defense mechanisms against herbivores and detoxification processes. Since plants containing calcium oxalate crystals are often exposed to metallic environments, we studied the interaction of such environmental metals with calcium oxalate in vitro. We showed that selected metals are indeed incorporated into synthetic calcium oxalate, and in a manner that depends on their ionic radius when precipitated in solution. One such mechanism of incorporation is based on the replacement of calcium ions by other metal cations within the host unit cell. The unit cell of calcium oxalate expands when incorporating elements with larger atomic radii and shrinks when doped with elements possessing ionic radii smaller than that of calcium. In this systematic study, metal-doped calcium oxalate crystals were characterized by means of high-resolution synchrotron X-ray powder diffraction, energy-dispersive X-ray spectroscopy, inductively coupled plasma atomic emission spectroscopy, and transmission electron microscopy. Better understanding of metal incorporation into mineral hosts might lead to ways of developing new and more efficient sorbent materials for detoxification processes.

Solubility, structure, and morphology in the co-precipitation of cadmium and zinc with calcium-oxalate

“…The measurements of Cd removal are consistent with a process of constant partitioning of Cd into solid solutions possessing the Ca-Ox structure, in agreement with previous data by Weber et al [11]. A separate experiment in which Ca was not added to solutions containing Cd and oxalate showed that no Cd was precipitated from solutions unless the dissolved Cd concentrations exceeded about 1.3 x 10 -4 M (Fig.…”

“…Appreciable precipitation of Zn oxalate was measured at Zn mole fractions greater than 0.25 (Fig 1C). In agreement with these data, Weber et al [11] reported that Zn is not as readily incorporated into the crystalline Ca-Ox structure as Cd. Up to Zn mole fractions of 0.25, our data were consistent with a tendency for Zn to be largely excluded from the precipitated oxalate (Fig.…”

“…This may be explained by the slightly lower ionic radius of Cd 2+ (0.95 Å) compared to Ca 2+ (1.00 Å). This phenomenon of unit cell contraction has been observed by XRD to result from Cd 2+ substitution into both Ca-Ox [11] and vaterite (CaCO 3 ) exuded by tobacco leaf trichomes [25].…”

“…The solubility product constants of these trace metal oxalates can be above or below that of Ca-Ox, with values of 1.42 x 10 -8 for Cd-oxalate (Cd-Ox) trihydrate [9] and 1.38 x 10 -9 [9] for Zn-oxalate (Zn-Ox) dihydrate. These trace metals and others have been shown to substitute readily into Ca-Ox crystallites [10][11], raising the possibility that Ca-Ox biominerals could both limit their leaching in soils and promote their detoxification processes in plants or other cellular environments. However, the mechanisms underlying the dependence of this phenomenon on the solubility and physical properties of the specific trace metal are not well understood.…”

The synthesis of polyaspartic acid derivative PASP-Im and investigation of its scale inhibition performance and mechanism in industrial circulating water

“…All the O and N atoms of PASP-Im have a high degree of a negative charge, which means that PASP-Im can provide Ca 2+ with a large amount of binding sites. [35][36][37][38][39] As a result, the N and O atoms in the molecular structure of PASP-Im can be well bound to Ca 2+ ion, thereby inhibiting the growth of calcium carbonate crystals as well as the formation of calcium carbonate scales.…”