Analysis of the stability behaviour of colloids obtained from different smectite clays

Abstract

The analysis of the conditions affecting clay colloid stability is needed to assess the relevance of bentonite erosion processes and of bentonite colloids transport in the frame of a high-level radioactive waste repository.

Clay colloids (< 1 μm particle size) were extracted from several bentonites or smectite-rich clays with different physicochemical properties (i.e., smectite content, main exchanging cation, layer charge, charge distribution,etc.), by dispersing them in deionized water and collecting the supernatant after centrifuging. First, colloids were characterized to evaluate their initial size and electrophoretic mobility (surface potential). Then,their aggregation behaviour upon the addition of Na+ or Ca2+ was studied, by analysing the evolution of both colloid size and electrophoretic mobility. The concentration of monovalent or divalent cation needed to start coagulation process (critical coagulation concentration, CCC) was estimated for each clay.

Despite the different initial clay colloids properties, their aggregation behaviour upon electrolyte additions was quite similar in all the cases. The aggregating power of Ca was observed to be much higher than that of Na, a

Ca concentration of 0.3mM being enough to aggregate any clay, compared to 1–20mM of Na. The presence of Ca in solution is a critical and prevailing factor for clay particles aggregation, independently of other smectite

bulk properties. The range of Na concentration needed to start the aggregation of different clay colloids is wider, but a relation between the measured CCC and other smectite bulk properties cannot be clearly established.

Nevertheless, the samples more resistant to aggregation were: the NANOCOR®, a Na-clay with the lowest percentage of tetrahedral charge, and the as-received and exchanged MX-80, which presents the lowest layer charge. The samples less resistant to aggregation were: the SAz-2, a Ca-clay with the highest layer charge and the lowest initial Na content, and the as-received and exchanged SBId-1, a beidellite, which presents a very high percentage of charge in tetrahedral position.

In stability tests, upon Na addition and consequent particle aggregation, an increase of the ζ-potential was observed, which is not in agreement with the double layer theory. Furthermore, the sample (Na)-SBId-1, which

developed the highest surface potential, was the less resistant to aggregation. These results are a clear indication that the ζ-potential, alone, is not a very significant parameter to indicate or predict smectite colloid stability.