Effect of mechanochemical treatment on structure and electrical properties of montmorillonite

Abstract

This study investigates the effect of grinding on the structure of montmorillonite (MMT). Both XRD and IR analyses indicate a gradual breakdown of the mineral layers and a decrease of the crystallinity with grinding. The prolonged grinding is accompanied with an amelioration of oxidant capacity of MMT which could be attributed to the conversion of structural Fe2+ to Fe3+ cations caused by adsorbed atmospheric oxygen and/or to the exposition of more Fe3+ cation to the surface of the mineral. The evolution of acidity of MMT during grinding has been carried out by determining the point of zero charge (pHpzc) of MMT and the pH of the aqueous suspension of MMT. It was shown that pHpzc increases with grinding from 6.2 to 8.1 respectively for untreated MMT and that ground for 30 min. On the other side the pH of MMT suspension increases from 4.75 to 8.18 respectively for untreated MMT and that ground for 75 min. These two results indicate that acidity of MMT has been lost during grinding. TGA analysis shows that the thermal decomposition of the ground MMT for 90 min takes place in a single step. This behavior has been explained by the formation of less tightly bound hydroxyl groups. Electrical and dielectric properties have been studied using impedance spectroscopy. It has been established that grinding is accompanied by an increase of dc conductivity and dielectric constant in the range of MHz. Moreover, it has been observed a shift of bound water relaxation frequency to lower frequency. These changes could be attributed to the enhancement of mobility of interlayer cations and to the perturbation of the state of interlayer water which could be connected to the defects in the octahedral sheet rather than being entirely localized at the interlayer space of MMT.