Effect of ionic strength and pH on hydraulic properties and structure of accumulating solid assemblages during...

The structure and hydraulic behaviour of colloidal montmorillonite assemblages formed during constant-pressure microfiltration of feed suspensions under various pH and ionic strengths have been investigated with flux versus time data analysed using both conventional cake filtration theory and a more rigorous sorptivity-diffusivity approach. Size distribution and fractal dimension analyses revealed a shift in assemblage structure from porous to compact as a result of a step-increase in electrolyte concentrations. The hydraulic conductivity of the filter cakes was dramatically affected by suspension ionic strength with significantly higher hydraulic conductivity observed at the higher ionic strengths compared to that observed at lower ionic strengths. Results obtained using the sorptivity-diffusivity model were consistent with conventional cake filtration theory and provided useful insights into the bulk properties of the filter cakes. Cake moisture ratio profiles of the montmorillonite system showed that high suspension ionic strength resulted in denser or less voluminous filter cakes that retained less water than was the case at the low ionic strength. These results suggest that, under low ionic strength conditions, the clay particles associate in suspension in assemblages of high aspect ratio which subsequently form highly "cross-linked" voluminous honeycomb type structures of low permeability once deposited upon the membrane. However, under sufficiently high ionic strength conditions, the high aspect ratio montmorillonite assemblages form nematic structures on deposition on the membrane that are denser yet more permeable than the structures formed at lower salt concentration. The distinct change in properties of the deposited clay on increase in salt concentration may well be indicative of transition from a gel to a nematically ordered phase.