Faten F. Nazzal , and Mark R. Wiesner
The permeate flux of clean a-alumina membranes is affected significantly by the pH of the feed stream. The 0.8 µm membrane used in this work operated at a significantly higher permeation rate at a pH well below the i.e.p. of the membrane. This conclusion is at some variance with a previous study of ceramic membranes in which membrane permeability was found to be at a maximum at the i.e.p. of the membrane. Both the permeation rate and its sensitivity to pH are reduced at higher ionic strengths.
These observations of membrane permeability cannot be accounted for in terms of electroviscosity alone. In the absence of other dissolved species and surface active materials, it appears that changes in the permeability of alumina membranes are largely due to changes in the composition of counter ions in membrane pores. At a pH above the i.e.p. of the membrane, the counter ions are cations with hydrated radii which are large and produce greater drag on permeate flow than the anions that predominate in the diffuse layer below the i.e.p.. It is important to note however, that the magnitute of the observed effects suggests that counterions influence permeate flux at distances from the pore wall substantially greater than a debye length.
The electrokinetic properties of silica deposits on the membranes played a negligible role in determining membrane permeability. Although the presence of silica reduced permeate flux, we attribute this primarily to the blockage of membrane pores by silica particles. Had permeate flux been reduced by the resistance of a deposited cake, an increase in the silica deposited on the membrane should have yielded a reduction in permeate flux.
The presence of humic materials in the feed water had a significant effect on membrane permeation. By adsorbing to membrane surfaces, humic materials both impede the flow of permeate and modify the surface chemistry of the membrane. In the presence of humic materials, the performance of ceramic membranes should be less sensitive to pH.