New data on the microporosity of bentonites

Abstract

This comparative study on a very wide group of samples shows that the crystal size and crystalline defects of smectites influence microporosity of bentonite considerably more than previously considered. The smectite crystals and surface properties, including microporosity and micropore volume, were studied using high resolution transmission electron microscopy and N2 adsorption, respectively. The specific surface area obtained varied between 25 and 278 m2g−1. The micropore area ranged between 6 and 76 m2g−1, and the external area ranged from 18 to 208 m2g−1. The external surface area was related to the size of the crystals in [001] direction because of the smaller particles with few stacked 2:1 layers have more basal surfaces accessible to the N2 molecules. However, the microporosity can be related to 1) the size of the crystals, owing to the partial ability of N2 to penetrate into the interlayer space, 2) the abundance of crystalline defects affecting the stacking of the 2:1 layers, and 3) the arrangement of the crystals forming particles, in which sub-parallel aggregates generate micro and mesopores. The study shows that these bentonites have pores in the full range, from smaller micropores (related to the crystalline structure in the interior of the interlayer) to micrometric macropores. The amounts of micro, meso, and macropores varied between samples but were similar and characteristic for samples from the same geological area.