Hydration States of Interlamellar Chromium Ions in Montmorillonite

Abstract

Abstraet--Hydroxychromium montmorillonites were prepared from solutions containing chromium nitrate and varying amounts of NaOH. The reactant Cr ions were hydrolyzed and, in some experiments, were dimerized to a significant extent. The extent of polymerization in the product formed from a solution containing a maximum amount of [Cr2(H20)s(OH)2] a+ was compared with that in products obtained from solutions containing monomer, [Cr(H20)5OH] 2 § and lesser amounts of the dimer. Despite the uncertainty about the nature and amounts of product interlamellar species, the catalytic effect of the montmorillonite interlayer on the hydrolysis and polymerization of Cr(III) appears to be independent of the nature of the reactant species. The validity of this conclusion depends to a large extent on the reliability of water determination in the clay mineral species, which is estimated to be no better than 5%. The assumption that H20+ is 'combined' and H20-is 'free' water is shown to be partially true, thereby causing some uncertainty in the interpretations of interlayer compositions.

Sorptive capacity of montmorillonite for hydroxy-Cr polymers and the mode of Cr complexation

“…Additional evidence for the inner-sphere complexation of Cr is found in the work of Carr (1985), which revealed the surface-catalysed polymerization of intercalated hydroxy-Cr oligomers. Hydrolytic polymerization of Cr(III) requires the replacement of H20 with OH-within the coordination sphere of Cr(III).…”

“…The adsorption of various heavy metals by smectites has received considerable attention in the past. The adsorption of Cd (Egozy, 1980;lnskeep & Baham, 1983;Van Bladel et al, 1993), Co (Egozy, 1980), Cr (Rengasamy & Oades, 1978;Brindley & Yamanaka, 1979;Carr, 1985;Dubbin et aI., 1994), Cu (Inskeep & Baham, 1983), Hg (Blatter, 1973), Pb (Griffin & Au, 1977), and Zn (Asher & Bar-Yosef, 1982;Van Bladel et al, 1993) are among those metats that have received the most attention. Most of the above-mentioned work, however, has focused only on equilibrium studies or adsorption mechanisms.…”

“…The inner-sphere complexation of Cr with the siloxane oxygen of montmorillonite (i.e. exchange of O ~ for H20) would provide this first ligand exchange and allow for the enhanced polymerization observed by Carr (1985).…”

Properties of Hydroxy-Al and -Cr Interlayers in Montmorillonite

“…Laboratory studies have also been successful in producing hydroxy interlayers of A1 (Rich, 1968;Goh and Huang, 1986;Barnhisel and Bertsch, 1989), Fe (Carstea et al, 1970;Martin-Luengo et al, 1989), Zr , Ni (Yamanaka and Brindley, 1978), and Cr Carr, 1985). The hydroxy-Cr interlayered montmorillonite prepared by Brindley and Yamanaka (1979) displayed a basal spacing near 16.8/~, considerably larger than the 14.4/~ spacing of chlorite.…”

“…These workers were unable to determine the structure of the hydroxy-Cr polymers. While studying hydration states of interlamellar Cr in montmorillonite, Carr (1985) observed that the interlayer region catalyzed the formation of larger hydroxyCr polymers. The first-order reflections of the interlayered clays varied from 10 ~k to 15 ~ and tended to be broad, indicating variable basal spacings.…”