Clays and Clay Minerals, Voi. 42, No. 3, 331-336, 1994. PROPERTIES OF HYDROXY-A1 AND -Cr INTERLAYERS IN MONTMORILLONITE W. E. DUBBIN, * TEE BOON GOH, 1 D. W. OSCARSON, 2 AND F. C. HAWTHORNE 3 Department of Soil Science, University of Manitoba, Winnipeg, Manitoba, Canada 2 AECL Research, Whiteshell Laboratories, Pinawa, Manitoba, Canada 3 Department of Geological Sciences, University of Manitoba, Winnipeg, Manitoba, Canada Abstract- In environmentscontaminated with Cr, the interlayers of expandable layer silicates may serve as sinks for this potentially toxic element. As a means of determining the potential for smectites to serve as sinks for Cr, the precipitation products of A1 and Cr in the interlayers of a montmorillonite were examined. Five montmorillonite (SWy-1) day suspensions were treated with preweighed amounts of AIC13and CrC13to give five A1/(AI + Cr) molar ratios (1.0, 0.67, 0.5, 0.33, 0) with a total trivalent cation (M3+) concentration of 600 cmol(+)/kg clay. The clay-cation suspensions were titrated with 0.1 N NaOH to give a NaOH/M 3+ molar ratio of 2.5. Analysis of the solid-phase reaction products showed that the cation exchange capacity and specific surface of all clays were reduced. Chromium reduced the exchange- ability of the interlayers while A1 increased the thermal stability. X-ray diffraction analysis revealed that all Al-containing interlayer materials formed similar gibbsite-like polymers. Data from infrared spec- troscopy indicated that both AI and Cr were present within the same polymer. Differential thermal analysis and thermogravimetric tracings showed that the rapid collapse of the interlayer in the Cr end-member upon heating was due to a low-temperature loss of hydroxyls. It was not possible to identify all interlayer structures in the Cr end-member. Data from X-ray photoelectron spectroscopy showed all Cr to be Cr(III). Displacement of the interlayer material became more difficult as Cr content increased. The least ex- changeable interlayers, therefore, may be found in environments containing the most Cr. Key Words--Aluminum, Chromium, Interlayering, Montmorillonite, XPS. INTRODUCTION Intercalation ofhydroxy-Al polymers in expandable layer silicates is a well-known natural process. Labo- ratory 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 (Yamanaka and Brindley, 1979), Ni (Yamanaka and Brindley, 1978), and Cr (Brindley and Yamanaka, 1979; Carr, 1985). The hydroxy-Cr interlayered montmorillonite pre- pared by Brindley and Yamanaka (1979) displayed a basal spacing near 16.8/~, considerably larger than the 14.4/~ spacing of chlorite. As the first observed dif- fraction peak was broad and higher orders were weak or absent, the authors suggested that the basal spacings were probably variable. 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 inter- layer region catalyzed the formation of larger hydroxy- Cr polymers. The first-order reflections of the inter- layered clays varied from 10 ~k to 15 ~ and tended to be broad, indicating variable basal spacings. As the exact composition of the hydroxy-Cr polymers was not known, the structures of the interlayered species were not determined. Copyright 9 1994,The Clay MineralsSociety Hydroxy-aluminochromium interlayers have been synthesized by those interested in the petrochemical applications of clays (Carrado et al., 1986). Upon cal- cination, the hydroxy polymers convert to fixed oxide pillars, which give the clay catalytic properties. Al- though the properties of the oxide pillars have received much attention, the nature of unheated hydroxy-alu- minochromium species remains unclear. In environ- ments contaminated with Cr, the interlayers of ex- pandable layer silicates may serve as sinks for Cr, which can be toxic when in solution (Bartlett and Kimble, 1976). The structure and stability of the hydroxy-alu- minochromium polymers, therefore, merit attention. In this study, hydroxy interlayers were formed with five different A1/(A1 + Cr) molar ratios in a standard montmorillonite. The structure and stability of the in- terlayer material were examined to determine the pos- sible interactions between AI and Cr. MATERIALS AND METHODS The montmorinonite (SWy-1, Crook County, Wy- oming) was obtained from the Source Clays Repository of The Clay Minerals Society. Prior to fractionation, the clay was Na-saturated by washing five times with 1 N NaC1 then washed free of CI- as determined by the AgNO3 test. Five grams of the < 2 gm fraction were 331