A B S T R A C T:The adsorption of cerium(IV) by both natural and Na-exchanged kaolinite from aqueous solutions of ceric ammonium nitrate was studied in equilibrium systems. Cerium may be adsorbed either as a monomeric species or as a polymeric hydroxy cation. The species adsorbed depends on the concentration of cerium in the stock solution, the age of this solution prior to the preparation of the equilibrium system and on the equilibration period. Adsorbed cations undergo polymerization on the surface of the kaolinite; the resulting polymeric species forms strong bonds with the clay surface--it is not desorbed by sulphuric acid--and may be considered fixed. Adsorption of cerium reaches a maximum in solutions containing 0.15 M sulphuric acid.Very little is known about the adsorption of lanthanides by clay minerals, although this process seems to be important in the geochemical cycle of these elements. Recently, the adsorption of a few lanthanides by montmorillonite was investigated by Bruque et al. (1980( ) and Miller et al. (1982( , 1983. The present communication deals with the mechanism of adsorption of cerium by kaolinite. EXPERIMENTAL MaterialsNatural kaolinite from Maktesh-Ramon, Israel, was manually ground using an agate pestle and mortar and the fraction passing an 80-mesh (0.177 mm) sieve was used. Na-kaolinite was prepared by dispersing 1 g of natural kaolinite:in 25 ml M NaC1 and, after a 24-h equilibration period, the excess NaCI was removed by repeated washing with distilled water, followed by centrifugation, until no chloride was detected. The exchangeable sodium content of the Na-kaolinite was 0.03 mEq Na/g clay. Equilibrium systemsThree stock solutions containing 0.1, 0.01 and 0.001 M ceric ammonium nitrate were used for the preparation of equilibrium systems. The cerium salt was dissolved either in distilled water or in aqueous solutions of sulphuric acid. Adsorption was carried out in equilibrium systems containing 1 g clay. Various amounts between 0.5 and 10.0 ml of each of the stock solutions of cerium(IV) were added. The suspension was diluted to 12.0 ml by adding distilled water. Some of the series of equilibrium systems were prepared from fresh and others from aged stock solutions. Equilibration periods ranged from 30 min to 1984 The Mineralogical Society

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A B S T R A C T:The thermal behaviour of sepiolite and palygorskite treated with pyridine (Py) was investigated by DTA, TG, DTG and EGA (by MS) under a flow of air and inert gases (N 2 or Ar). The thermal analysis showed that the organic base was adsorbed by the clays, penetrating into the clay pores and replacing zeolitic and bound water. Its presence in the pores shifted the dehydroxylation peak to temperatures higher than the corresponding peak in the untreated clays. Under N2 and Ar, thermal desorption of Py from sepiolite was observed at 260 and 650~ whereas for palygorskite only traces of Py were detected in the evolved gases. In addition to desorption, reactions of pyrolysis and condensation to coke were detected by the evolution of ammonia, methane and hydrogen, respectively. In air, oxidation of the organic matter led to the appearance of exothermic peaks. The profile of the DTA curve and the temperatures of the most intense exothermic peaks were determined by the rate of the carbon oxidation.In a previous publication (Shuali etal., 1990) we described the thermal behaviour, under air or N2 flow, of the porous clays sepiolite and palygorskite treated with n-butylamine (BA). Under N 2 flow, in addition to thermal desorption of the aliphatic base, reactions of pyrolysis and condensation to coke were detected. In air, oxidation of the sorbed organic matter led to the appearance of exothermic peaks. The temperatures of the most intense exothermic peaks are governed by the rate of carbon oxidation, but the shape of the DTA curves and the temperatures of exothermic shoulders are governed by the rate at which the hydrogen from the organic molecules combine with oxygen from the air.In the present investigation, the thermal behaviour of sepiolite and palygorskite treated with pyridine (Py), both in air and inert atmosphere (N2 or Ar) was studied. There are several publications on the sorption of Py by sepiolite and palygorskite. According to RuizHitzky et al. (1985), the sorption of Py on the external surfaces of these clays takes place in two well-defined steps. In the first step a monolayer is formed with the Py molecules lying with their planes parallel to the (001) plane of the clay crystal; in the second step a bilayer coverage is formed. Blanco et al. (1988) studied the surface acidity of palygorskite by infrared (IR) spectroscopy. After evacuating for 2 h at 150~ the thermally treated clay was saturated with Py. The IR spectra showed the formation of weak hydrogen bonds between Py and Bronsted acid sites which were developed as a result of the thermal treatment. Shuali et al. (1989) identified, by IR spectroscopy, several associations which are formed between palygorskite or sepiolite and Py. During the adsorption process, the base penetrates into the channels replacing part of the zeolitic water and forming hydrogen 9 1991 The Mineralogical Society