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

Authors:
Murray B McBride
Murray B McBride
Cornell University
United States
Meredith Frenchmeyer
Meredith Frenchmeyer
School of Integrative Plant Science
Ithaca | United States
Sabrina E Kelch
Sabrina E Kelch
School of Integrative Plant Science
Dr. Ludmilla Aristilde, PhD
Dr. Ludmilla Aristilde, PhD
Cornell University
Associate Professor
Environmental Chemistry; Environmental Biochemistry; Environmental Engineering.
Ithaca, NY | United States

J Colloid Interface Sci 2017 Jan 30;486:309-315. Epub 2016 Sep 30.

Soil and Crop Sciences Section, School of Integrative Plant Science, College of Agriculture and Life Sciences, Cornell University, Ithaca, NY, USA; Biological and Environmental Engineering, College of Agriculture and Life Sciences, Cornell University, Ithaca, NY, USA. Electronic address:

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.

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http://dx.doi.org/10.1016/j.jcis.2016.09.079DOI Listing
January 2017
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