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

Abstract

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 Cd/Ca or Zn/Ca 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 Cd below that of pure Cd-Ox. The XRD patterns indicated that Zn precipitated as a separate pure Zn-Ox crystal that is largely excluded from the Ca-Ox structure. Furthermore, the presence of Zn 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.

The role of zinc ions in calcium oxalate monohydrate crystallization

“…17 The ion has been revealed to have a weak tendency for incorporation within calcium oxalate minerals with preferred formation of a zinc oxalate species. 18 One of the most important organic molecules studied in urolithiasis is citric acid 19,20 , due to the natural occurrence within urinary medium. Similar small compounds that are also present within the human body have also been studied, these being mainly amino acids, with varied results.…”

Synthesis and Characterization of (Ca,Sr)[C2O4]∙nH2O Solid Solutions: Variations of Phase Composition, Crystal Morphologies and in Ionic Substitutions

“…The complex multicomponent composition of the crystallization medium (of urea and of other human physiological liquids), however, allows for the probability of such substitutions [22]. The probability of Cd-Ca substitutions in synthetic whewellite is proposed by McBride et al, 2017 [23].…”

Lead Solubility and Mineral Structures of Coprecipitated Lead/Calcium Oxalates

“…A previous study has also observed a similar effect of Zn 2+ ions in favoring the weddellite structure over whewellite. 10 The CaOx monohydrate and CaOx dihydrate have fundamentally different structures: whewellite has a sheet structure formed by polyhedral layers of Ca polyhedra parallel to the (001) plane; the weddellite structure has channels formed from Ca polyhedral chains that contain zeolitic water. 25,26 Therefore, we propose that trace metal cations such as Pb 2+ can be accommodated more readily into the channel structure of weddellite relative to the layered whewellite.…”

“…8 Trace metals such as zinc (Zn), cadmium (Cd), and Pb can be incorporated into Ca-oxalate (CaOx) during coprecipitation, defined as the simultaneous crystallization of two or more components from aqueous solution to form a single phase (solid solution) or multiple discrete phases. 9,10 The coprecipitation process in plant cells can selectively sequester trace metals into oxalate crystals to protect plants from metal toxicity. 11−13 For instance, it was found that Cd but not Zn was sequestered into CaOx crystals of the hyperaccumulator species, Gomphrena claussenii, 14 a result consistent with the greater tendency of Cd relative to Zn to substitute into structural positions of the CaOx mineral whewellite.…”

“…11−13 For instance, it was found that Cd but not Zn was sequestered into CaOx crystals of the hyperaccumulator species, Gomphrena claussenii, 14 a result consistent with the greater tendency of Cd relative to Zn to substitute into structural positions of the CaOx mineral whewellite. 10 Trace metal ions isomorphously substituted into the CaOx lattice can lower the solubility of these metals below that of the pure metal oxalate by the formation of solid solutions. In soils, fungi can promote the formation of Pb oxalate (PbOx) by exuding oxalic acid, causing the dissolution of quite insoluble Pb minerals such as pyromorphite.…”