Study of structural properties of ion treated and mechanically deformed commercial bentonite

Abstract

This study deals with the investigation of the structural changes of Ca-montmorillonite present in a commercial bentonite produced by mechanical deformation via high-energy ball milling or by ionised argon interaction, both in controlled thermodynamic environment. Scanning electron microscopy (SEM), transmission electron microscopy (TEM), differential scanning calorimetry (DSC), Fourier-transform infra-red (FTIR), X-ray powder diffraction (XRPD), particle size laser diffraction (PSLD) and swelling index (SI) were used for the characterization of the samples. (i) Ball milling. We have observed that high-energy ball milling produced a progressive reduction of the d (001) lattice spacing of the montmorillonite from 1.50 to 1.30 nm as a function of time (at a constant cycle frequency). Simultaneously, a severe decrease and broadening of the XRD spectrum peaks were also detected, indicating a reduction of crystallinity and an increase of microstrain. Analysis by laser-diffraction indicates a bimodal distribution of the particle size, and a progressive increase of the number of particles with size in the range 0.5–1 μm and greater than 20 μm, for the material milled up to 20 h. However, by using increased spatial resolution, sub-micrometric rounded particles were observed after 20 h of milling by SEM analysis, and the presence of nanometric particles by TEM. FTIR and DSC clearly showed a structural destabilization both in the interlayer and in the intralayer and a progressive loss of interlayer water. The swelling index drastically reduced as a function of milling time. Finally, as expected, the more pronounced transformations were observed for milling in medium vacuum of 0.13 Pa (10 −3 Torr), than for in inert gas or air atmospheres because of the more dehydrating environment. (ii) Argon-ion treatment. X-ray diffraction analysis showed that the mean d (001) montmorillonite lattice spacing shifted from 1.50 to 1.35 nm, whilst the peak intensity decreased and broadened, with a decrease of the crystallite size in the 001 direction. SEM showed an increased presence of sub-micrometric rounded particles at the surface of the treated samples than for the untreated ones. DSC indicated a decrease in water content after the gas treatment. Laser diffraction showed that the ion treatment increased the number of submicron-sized particles with a concomitant reduction of the number of particles with bigger sizes, leading to an increase of the volumetric surface. These very preliminary results suggest that ionised argon treatment performed in a controlled environment can be exploited to reduce the amount of interlayer water molecules in montmorillonite, to modify its surface characteristics and particle size distribution in a way to increase the specific surface without particle agglomeration or compaction. Applications to other clay minerals (for example, talc and kaolinite, etc.) are envisaged and in progress. Hence, both mechanical deformation and interactions of particular ionised gases can induce significative changes of the physical and chemical properties of bentonite and other clay minerals and further systematic studies are in progress in order to evaluate the potentiality for low-cost industrial applications.