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Clays and Clay Minerals; December 2002; v. 50; no. 6; p. 784-790
© 2002 Clay Minerals Society
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INFLUENCE OF GIBBSITE SURFACE AREA AND CITRATE ON Ni SORPTION MECHANISMS AT pH 7.5

Noriko U. Yamaguchi1,*, Andreas C. Scheinost2 and Donald L. Sparks3

1 Department of Quantum Engineering and Systems Science, Graduate School of Engineering, The University of Tokyo, 7-3-1 Hongo Bunkyo-ku Tokyo, 113–8656, Japan
2 Institute of Terrestrial Ecology, ETHZ, 8952 Schlieren, Switzerland
3 Department of Plant and Soil Sciences, University of Delaware, Newark, DE 19717–1303, USA

* E-mail address of corresponding author: n-yamaguchi{at}q.t.u-tokyo.ac.jp

We investigated the sorption of Ni to gibbsite of two different surface areas at pH 7.5, in the presence and absence of citrate, over a time period of 180 days. Extended X-ray absorption fine-structure spectroscopy was employed to elucidate the sorption mechanisms at the molecular level. In agreement with former results, Ni-Al layered double hydroxide (LDH) formed in the presence of gibbsite of low surface area. However, gibbsite of high surface area suppressed the formation of the surface precipitate. Instead, two Al atoms neighboring Ni at distances of 2.95–2.98 Å indicated formation of an inner-sphere sorption complex, where each NiO6-octahedron shares edges with two AlO6-octahedra. Focused multiple scattering arising from Al atoms at a distance of 6 Å suggest that sorbed Ni(OH)2(OH2)4 monomers epitaxially extend the hexagonal arrangement of Al atoms in gibbsite. Only after 30 days or more was a small amount of LDH formed. The presence of citrate prevented the formation of LDH, while maintaining the formation of inner-sphere sorption complexes.

Key Words: Citrate • EXAFS • Gibbsite • Layered Double Hydroxide • Ni • Sorption • Surface Area • Surface Precipitate






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