Influence of the pretreatment in the cation exchange capacity and surface properties of absorbent clays

Abstract

Sepiolite, palygorskite, and smectites are special clays bellonging to the group of adsorbent minerals, and their industrial applications in this field are related to the physical-chemical properties, mainly to the surface properties and the cation exchange capacity. These properties vary among samples from different deposits, similar to most industrial minerals, and their description and comparison is a common topic in studies of these special clays. For instance, the specific surface area of sepiolite may vary from 77 m2 g-1 to 399 m2 g-1, as shown in a comparative study of sepiolites from 22 localities around the world [1]. The presence of impurities, the variation in the crystal-chemistry, and the texture may influence greatly these properties; however, there are also differences for samples from a same deposit because the conditions of study, mainly the pre-treatment of the sample, may be very different. In this study we compare the results obtained from a) sample with its original texture, in grains of ~4 mm, b) ground sample in manual agate mortar, c) ground sample by mechanical grinding, and d) re-sedimented sample. The studied samples are i) two smectites, a trioctahedral smectite from the Tajo Basin (Spain) and a dioctahedral Na-smectite from Wyoming (U.S.A.), ii) two sepiolites, from Tajo Basin (Spain) and from Polattli area (Turkey), and iii) a palygorskite from Attapulgus (U.S.A.). The mineralogical composition from X-Ray diffraction, textural features from scanning electron microscopy, the surface properties from the adsorption of N2 isotherms and the cations exchange capacity (CEC) and exchangeable cations were studied. The X-Ray diffraction patterns of samples b and c show that mechanical grinding produces a loss of crystallinity with partial amorphization of the sample as can be deduced by the diminution off the diffraction peaks of the clay mineral and the small elevation of the background in the central zone of the pattern. The results obtained show a variation in all properties for each sample that may be very large; for instance Wyoming smectite displays a four-fold increase of the specific surface area (BET) after mechanical grinding. The variations are different for the different minerals, and the highest values of BET and total volume of pores was obtained either for natural, powdered or re-sedimented samples. However, there is a general increase of microporosity accessible to N2 in all samples when they are ground mechanically, with the exception of the sepiolite from Polattli in which the textural microporosity of natural sample is probably high. When only the manual and mechanical grinding were compared, all samples displayed higher microporous area and micropore volume after mechanical grinding. The cation exchange capacity is also higher for samples ground by the mechanical procedure with the exception, of the Wyoming smectite in which the CEC is higher for the sample ground in the agate mortar. The X-Ray patterns show that this sample has higher crystal order than its counterpart ground mechanically. The increase of of CEC and the amount exchangeable Mg2+ obtained after grinding might be related with partial dissolution of the octahedral Mg. The change of the natural texture by grinding or suspension and sedimentation affects to the surface properties in different ways depending on the meso and macroporosity. In general, more extensive grinding causes an increase of the microporosity and the CEC.