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

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• 作者：Murray B. McBride^a ; ^{mbm7@cornell.edu" class="auth_mail" title="E-mail the corresponding author} ; Meredith Frenchmeyer^a ; Sabrina E. Kelch^a ; Ludmilla Aristilde^a ; ^b ; ^{ludmilla@cornell.edu" class="auth_mail" title="E-mail the corresponding author}
• 关键词：Oxalate ; Biomineral ; Metal sequestration ; Zinc ; Cadmium
• 刊名：Journal of Colloid and Interface Science
• 出版年：2017
• 出版时间：15 January 2017
• 年：2017
• 卷：486
• 期：Complete
• 页码：309-315
• 全文大小：1628 K

文摘

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.