Abstract-A structure model for amorphous hydrated or dehydrated silico-aluminas with composition varying between 0 < AI : Al + Si < 1 is presented. A central core made from a tetrahedral network in which silicon is partially substituted by aluminium carries a net negative electrical charge. This charge is balanced by more or less polymerized hydroxyaluminium cations forming a coating around the core.As AI: AI + Si increases, the number of substitutions in the core increases as well as the complexity of the hydroxyaluminium cations in the coating.For AI : AI + Si >~ 0.8, a demixing is observed, leading to the formation of a crystalline pseudoboehmite and bayerite.Upon heating, the coating as well as the demixed phases are transformed into a spinel structure containing tetrahedral aluminium, while the core structure remains unaffected.This model could explain the solubility features, the phosphate reaction and the catalytic properties of amorphous silico-aluminas.

A B S T R A C T:Calcium-cadmium and calcium-zinc exchange equilibria were studied at 20~ and constant ionic strength (0.015) on four clay minerals, viz. montmorillonite, bentonite, illite and vermiculite. Vermiculite and, to a lesser extent, illite and bentonite showed the strongest affinity for Cd 2+ or Zn 2+ over Ca 2 § ions, whereas on the Camp Berteau montmorillonite nearly nonpreferential exchange isotherms were observed. With this one exception, selectivity for the heavy metals was greatly enhanced in the trace Cd or Zn regions, suggesting the presence of specific adsorption sites in these clays. Generally, the adsorption increased with the polarizing power of the exchangeable cation. Thermodynamic equilibrium constants and standard free energy changes for the complete exchange of Ca-clay to Cd-or Zn-clay were calculated.

The adsorption of aniline (ANL) and p-chloroaniline (PCA) from 0.02 M methanol and chloroform solution on air-dried montmorillonite is determined by the conjunction of several factors: (i) basicity of the solute, (ii) complexing ability, Br6nsted and Lewis acidity of the exchangeable cation, (iii) polarity and Lewis basicity of the solvent. The principal adsorption mechanisms, inferred from infrared data, are direct or indirect coordination and protonation of the adsorbate. They take place simultaneously and to a relative extent depending mainly on the exchange cation.On Fe-, A1-and H(A1)-montmorillonite ANL and PCA give rise to type II complexes, characterised by an intense charge-transfer band in the infrared region and by the formation of radical cations, evidenced by ESR spectroscopy. This oxidation process occurs on heating in air, but with Fe 3+ in vacuo also and even at ambient temperature in methanol solution. On Cu-montmorillonite ANL, contrary to PCA, does not give a type II complex; however, radical species are produced in air and in vacuo and their production is enhanced by heating.

Abstract--The form under which ethylenediamine (EDA) is adsorbed from aqueous solution by hydrogen-and ethylenediammonium-montmorillonite was studied as a function of the amount of amine present in the system. EDA added to the acid clay in quantities lower than or equal to the cation exchange capacity 9 " 2+ (C.E.C.) was exclusively adsorbed as ethylenedlammonmm (EDAH2) ion. On further addition of diamine the pH of the suspension rose to alkaline values and the monoprotonated species (EDAH § was the main charge balancing cation. Evaporating at room temperature "EDA-H-montmorillonite-H20" or "EDA-EDAH2-montmorillonite-H20" systems containing 300 me EDA/100 g clay did not cause loss of nitrogen, but degassing under high vacuum (10 -~ rnm Hg) did. Nevertheless, excess EDA molecules with respect to the C.E.C. were retained on the clay surface, at the expense of water molecules, through strong asymmetrical hydrogen bonds between their NH2 groups and the NH3 + groups of EDAH + ions. On heating up to 160~ under vacuum the nitrogen content decreased further, but still remained at a level significantly higher than the C.E.C. value, all NH3 + groups remaining involved in strong hydrogen bonding. It is suggested that a "condensation" process takes place, implying evolution of EDA molecules and giving rise to "polymeric" associations between protonated and unprotonated diamine.Washing the clay suspensions with distilled water did not completely remove excess EDA either, as a consequence of the equilibrium existing between the ionic species in solution (EDAH2 ~+ and EDAH § and on the clay surface. It seems that these species were preferentially adsorbed as "trimeric" associations in which two out of four NH3 § groups are hydrogen bonded to NH2 groups.After heating at 200~ nitrogen retained on the clay surface was mainly in the form of NH4+ ions. Ammonium formation was enhanced by the presence of excess EDA and was considerably faster than in montmorillonite systems containing EDA coordinated to Cu ~+ ions.

Aniline, adsorbed from aqueous solution in 1% (w/v) clay suspensions, forms coloured complexes and polymerizes on Fe(III)- and Cu(II)-montmorillonite. As evidenced by the adsorption isotherms, and the coloration and spectroscopic (IR, ESR) characteristics of the organo-clay associations formed, the conditions under which these reactions take place and the mechanisms involved differ according to the exchange cation. Fe(III) interacts with the π electrons of the aromatic ring to give rise to radical cations or so-called type II complexes, provided the aniline concentration does not exceed 500 p.p.m. With Cu(II). an aniline concentration above 500 p.p.m. is required and the reaction occurs at the amine group, proceeding through coordination followed by free-radical formation.