The colloidal stability of variable-charge mineral suspensions

Abstract

AbstractA method of analysing the colloidal stability of variable-charge mineral suspensions as a function of pH is described, which combines light scattering with soluble element and surface charge measurements. The absorbance of the suspensions closely follows the A = k−m equation, where m = −dlgA/dlgλ is an aggregate light-scattering index which is inversely related to the limiting sedimentation speed v = dA600nm/dt. Usually, suspensions of variable-charge minerals scatter less near their point of zero charge where the aggregation process is a maximum, in contrast to environments where they are charged and well-dispersed. Although the morphology of goethite is that of elongated needles, kaolinite plates, and imogolite tubes, the coalesced sphere approach of the Mie theory allows rapid correlation of the absorbance at 600 nm with aggregate-size distribution, indirectly measured by the light- scattering aggregate-size index m confirmed by electron microscopy experiments. Kaolinite and goethite suspensions obey the diffraction scattering law with a variation in size of 2–20 µm from the dispersed to the aggregated particles. Imogolite and alumina gel suspensions possibly obey the anomalous diffraction scattering law and are present as 0·05–1 µm aggregates of their 50–100 Å structural units. Silica gel and the more dispersed imogolite suspensions possibly obey the anomalous Rayleigh scattering law with a variation of aggregate size from 700–3000 Å.