Abstract
Clays consist of fine particles (clay minerals) which under certain conditions can be dispersed in water, forming a fairly stable dispersion. The stability of the clay mineral dispersions depends on the pH, solid/liquid ratio, and most of all on the type and concentration of dissolved ions. The difference between the dispersibility of different clays is not yet understood. In the present study allophane and smectite (main constituent of bentonite) are compared. Allophane surfaces are dominated by aluminol groups. Therefore, they can be used to investigate the properties of the edges of platy dioctahedral clay minerals. Dispersion and coagulation of allophane was understood based on the charge of the surface functional groups. Dispersion of allophane was only possible at pH < 5 where the aluminol groups became positive and at low ionic strength (critical coagulation concentrations (CCC) < 10 mmol/L NaCl). Dispersion at high pH may also be possible but this was not tested in the present study. In contrast to allophane, bentonite dispersions coagulated when the edges of the smectites became positive. This is probably caused by heterocoagulation leading to edge (+)/face (−) aggregation. CCC values of different bentonite dispersions ranged from 10 to 750 mmol/L. Most of the Wyoming bentonite dispersions were more stable and even above the CCC, sedimented much slower compared to other bentonites. The reason for this phenomenon could not be identified and needs further research.
Highlights
IntroductionDispersion of a clay mineral in a liquid depends on the sample pretreatment (aggregate size, mechanical activation), primary particle size distribution, layer charge density, solution/solid ratio, pH, and ionic strength
Dispersion of a clay mineral in a liquid depends on the sample pretreatment, primary particle size distribution, layer charge density, solution/solid ratio, pH, and ionic strength
Most of the Wyoming bentonite dispersions were more stable, and even above the CCC, they coagulated much more slowly compared to other bentonites
Summary
Dispersion of a clay mineral in a liquid depends on the sample pretreatment (aggregate size, mechanical activation), primary particle size distribution, layer charge density, solution/solid ratio, pH, and ionic strength. Dispersions can be destabilized if dispersed particles are forced to coagulate, e.g., by increasing the ionic strength. This process called flocculation or coagulation, can be best explained by the DLVO (Derjaguin, Landau, Verwey, Overbeek) theory [2,3,4]. The resulting particle interaction causes coagulation and the dispersion is flocculated. According to Missana & Adell [6] the DLVO-theory fails to explain the dependency of the CCC on the pH value in the case of clays
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