  We present a comprehensive study of cross-flow ultrafiltration (UF) of
charge-stabilized suspensions, under low-salinity conditions of
electrostatically strongly repelling colloidal particles. The axially varying
permeate flux, near-membrane concentration-polarization (CP) layer and osmotic
pressure profiles are calculated using a macroscopic diffusion-advection
boundary layer method, and are compared with filtration experiments on aqueous
suspensions of charge-stabilized silica particles. The theoretical description
based on the one-component macroion fluid model (OCM) accounts for the strong
influence of surface-released counterions on the renormalized colloid charge
and suspension osmotic compressibility, and for the influence of the colloidal
hydrodynamic interactions and electric double layer repulsion on the
concentration-dependent suspension viscosity $\eta$, and collective diffusion
coefficient $D_c$. A strong electro-hydrodynamic enhancement of $D_c$ and
$\eta$, and likewise of the osmotic pressure is predicted theoretically, as
compared with their values for a hard-sphere suspension. We also point to the
failure of generalized Stokes-Einstein relations describing reciprocal
relations between $D_c$ and $\eta$. According to our filtration model, $D_c$ is
of dominant influence, giving rise to an only weakly developed CP layer having
practically no effect on the permeate flux. This prediction is quantitatively
confirmed by our UF measurements of the permeate flux using an aqueous
suspension of charged silica spheres as the feed system. The experimentally
detected fouling for the largest considered transmembrane pressure values is
shown not to be due to filter cake formation by crystallization or
vitrification.
