Cation exchange capacity of natural and synthetic hectorite

Abstract

Cation exchange capacity (CEC) of smectites is a function of both layer charge and lateral layer dimension, the later ones determining their edge surface area. Two trioctahedral smectites similar in layer charge, the Na-saturated hectorite SHCa-1<0.2μm and the synthetic Laponite® RD, were studied differing by morphology and particle size with a proportion due to the edge specific surface area of about 2.0% and 6.9%, respectively, considering single layers. The CEC of the materials was evaluated using the Cu-trien method over the pH range of 6 to 13. For this purpose, the Cu-trien method was evaluated for a use at pH above 10, out of the usual pH range of application. In addition, stability of clays was followed by analysing aqueous concentrations issued from the dissolution of the minerals. The CEC were measured at 83±1cmol(+) kg−1 at pH7.9 for HtCa-0.2Na and 79±1cmol(+) kg−1 at pH8.9 for Laponite® RD following the usual Cu-trien procedure without pH adjustment. The CEC ranged from 50 to 150 cmol(+) kg−1 over the pH range of 6 to 13. The strong CEC increase at pH>10 was correlated to the deprotonation of the silanol edge sites. The larger CEC development observed for Laponite® RD was related to its morphology and larger edge specific surface area. Estimation of the exchangeable cation population following the usual Cu-trien procedure led to an overestimation of Mg2+ and Li+ due to partial clay dissolution at pH<10.