Properties of Hydroxy-Al and -Cr Interlayers in Montmorillonite

Abstract

Abstract-In environments contaminated 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 AIC13 and CrC13 to give five A1/(AI + Cr) molar ratios (1.0, 0.67, 0.5, 0.33, 0) with a total trivalent cation (M 3+ ) 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 exchangeability 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 spectroscopy 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 exchangeable interlayers, therefore, may be found in environments containing the most Cr.

Molecular Simulations of Montmorillonite Intercalated with Aluminum Complex Cations. Part II: Intercalation with Al(OH)3-Fragment Polymers

“…The basal spacings obtained for the single ring, d(001) = 14.58 ,~, and double ring, d(001) = 14.18 ,~, in the interlayer led to the conclusion that in the intercalated smectites with basal spacing --19 ,~, the Al(OH)3-fragment polymers are arranged in 2 layers in the interlayer. This model was already suggested by Hsu (1992) and Dubbin et al (1994).…”

“…Many recent structure investigations of clay minerals intercalated with A1-OH polymers focused on the nature of the intercalating agent (Plee et al 1987;Hsu 1992;Schoonheydt et al 1993Schoonheydt et al , 1994Dubbin et al 1994). The 2VA1 nuclear magnetic resonance (NMR) spectra of the intercalating solutions normally show the presence of 2 species: octahedral A1 and tetrahedral AI, included in All3 oligomer-Keggin cation (Figueras et al 1990;Hsu 1992;Schoonheydt et al 1994).…”

Electron Microscope Observations of Chromium Hydroxide-Induced Montmorillonite Microclusters

“…Therefore, most of the hydroxy-Cr polymers were sorbed to the montmorillonite surface at the end of the aging period. Previous analyses with X-ray photoelectron spectroscopy found all the Cr in these sorbed hydroxy polymers to be Cr(III) (Dubbin et al 1994).…”

“…Aluminum chloride and Crfl 3 were added to separate clay suspensions to give each suspension a total trivalent cation (M 3 § ) concentration of 400 cmol(M 3+ )/kg clay. The two clay-cation suspensions were stirred vigorously and titrated at 1 mL/min with Copyright 9 1995, The Clay Minerals Society 0.1 N NaOH until a final NaOH/M 3 § molar ratio of 2.5 was reached (Dubbin et al 1994). A third clay suspension containing 5 g of the < 2 gm fraction, which did not have any A1, Cr, or NaOH added, served as the control.…”

“…These clays are then heated to produce fixed oxide pillars which give the clay catalytic properties. Other workers have intercalated smectite simply to elucidate the structure of the unheated hydroxy-Cr polymers (Carr 1985, Drljaca et al 1992, Dubbin et al 1994. Despite advances in understanding the nature of these hydroxy-Cr polymers, little is known of their effect on the arrangement of individual smectite tactoids.…”