The variable charge of dioctahedral smectites

One of the most important properties of smectites is the cation exchange capacity resulting both from the structural permanent charge and the pH depending variable charge. The variable charge, in turn, mainly results from the edge aluminol groups (in the case of dioctahedral smectites). The permanent charges can be calculated using the layer charge density; the variable charges, however, were rarely quantified accurately. The present study was conducted to (i) test and compare different methods for the measurement of the variable charge of dioctahedral smectites (mainly montmorillonites), (ii) characterize the range of differences of the variable charge of smectites from different deposits, and (iii) identify the reason for different variable charge values. Considering a 0.1×0.1 μm montmorillonite particle, a variable charge of 8 meq/100 g (about 10% of the CEC) was calculated. Two CEC (Cu(trien)) based methods provided slightly larger values. These values ranged from 2% to 14% between pH 4 and 6 and from approximately 10% to 30% between pH 4 and 9. With the potentiometric titration method, even larger values for the variable charge were determined (15-35%). Interestingly, both the CEC based methods and the titration method provided comparable trends. Accordingly, any of these methods is suitable to distinguish materials with larger or lower variable charge. One of the reasons for the differences of the variable charge of different bentonites is the variable chemical composition of the smectites, particularly the structural Mg content. Further reasons may be the roughness of the edge surface (not considered in the calculation) and the submicron particle size distribution, which are both difficult to determine.