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Title: Heteroepitaxial growth of cadmium carbonate at dolomite and calcite surfaces: Mechanisms and rates

Here, the systematic variation of rates and the mechanism of cadmium uptake on the (104) surface of dolomite (CaMg(CO 3) 2) were investigated using in situ and ex situ atomic force microscopy (AFM), ex situ specular X-ray reflectivity (XR), and ex situ X-ray fluorescence (XRF). Selected experiments were performed on the calcite (CaCO 3) (104) surface for comparison. Aqueous solutions of CdCl 2, CaCl 2, and NaHCO 3, undersaturated with respect to calcite and supersaturated with respect to otavite (CdCO 3) and the (Cd xCa 1-x)CO 3 solid solution, were reacted with dolomite surfaces for minutes to days. Calcite substrates were reacted with solutions containing 1-50 μM CdCl 2, and with no added Ca or CO 3. Thin carbonate films following the Stranski-Krastanov growth mode were observed on both substrates. Specular XR and XRF revealed the formation of nm-thick Cd-rich carbonate films that were structurally ordered with respect to the dolomite (104) plane. Epitaxial films adopted the calcite crystal structure with a d 104- spacing (3.00 Å) larger than those of pure dolomite (2.88 Å) and otavite (2.95 Å) indicating either a solid solution with x approximate to 0.5, or a strained Cd-rich carbonate with a composition near that ofmore » otavite. The growth rate r of this phase increases with the initial supersaturation of the solution with respect to the solid solution, beta max, and follows the empirical relationship, as determined from XRF measurements, given by: r = 10 -4.88 ± 0.422.29 ± 0.24 max - 1), (in units of atoms of Cd/Å 2/h).The morphology of the overgrowth also varied with β max, as exemplified by AFM observations. Growth at step edges occurred over the entire β max range considered, and additional growth features including 3 Å high monolayer islands and ~ 25 Å high tall islands were observed when log β max > 1. On calcite, in situ XR indicated that this phase is similar to the Cd-rich overgrowth formed on dolomite and images obtained from X-ray reflection interface microscopy (XRIM) reveal the existence of laterally variable Cd-rich domains.« less
Authors:
 [1] ;  [2] ;  [3] ;  [4] ;  [5] ;  [6] ;  [2]
  1. Univ. of Illinois, Chicago, IL (United States); Univ. of California, Los Angeles, CA (United States)
  2. Argonne National Lab. (ANL), Argonne, IL (United States)
  3. Univ. of Chicago, Chicago, IL (United States)
  4. Oak Ridge National Lab. (ORNL), Oak Ridge, TN (United States)
  5. Univ. of Illinois, Chicago, IL (United States)
  6. Univ. of Delaware, Newark, DE (United States)
Publication Date:
Grant/Contract Number:
AC02-06CH11357
Type:
Accepted Manuscript
Journal Name:
Geochimica et Cosmochimica Acta
Additional Journal Information:
Journal Volume: 205; Journal Issue: C; Journal ID: ISSN 0016-7037
Publisher:
The Geochemical Society; The Meteoritical Society
Research Org:
Argonne National Lab. (ANL), Argonne, IL (United States)
Sponsoring Org:
National Science Foundation (NSF), Directorate for Geosciences Division of Earth Sciences (GEO/EAR); USDOE Office of Science (SC), Basic Energy Sciences (BES) (SC-22), Chemical Sciences, Geosciences, and Biosciences Division; USDOE
Country of Publication:
United States
Language:
English
Subject:
58 GEOSCIENCES; mineral-water reactivity, X-ray reflectivity, carbonate film growth; cadmium; X-ray reflectivity; X-ray fluorescence; atomic force microscopy
OSTI Identifier:
1372659
Alternate Identifier(s):
OSTI ID: 1398653