Effect of Charge On Filtration Rejection Coefficients of Capsule-Shaped Particles in Porous Membranes

Bacteria such as Escherichia coli and Serratia marcescensthat are encountered during microfiltration and ultrafiltration processes are capsule shaped organisms. The shape of the pathogens becomes important for membrane selection, and for efficient design of membrane separation systems. In addition, surface charges on bacteria and membrane influence the filtration rejection coefficients. Previous models in the literature have not explored the combined effect of particle shape and surface charges on rejection by membranes.  

In the present work, a computational model was developed to predict the combined effect of steric, hydrodynamic and electrostatic interactions on the rejection coefficient for a capsule-shaped particle, representing a bacterium in a porous membrane. The non-linear Poisson-Boltzmann equation was used to determine the electrostatic particle-wall interaction energy, and a computational fluid dynamic model was developed to determine the hydrodynamic interactions. These were then incorporated in a rejection model.  To validate the model, a series of experiments were conducted to measure the rejection of gold nanorods carrying negative charge on the surface by polycarbonate track etch membranes.

Rejection coefficients are reported for particles with aspect ratio (length/diameter) = 2 and relative particle diameters (particle diameter/ pore diameter) ranging from 0.1 to 0.8. Theoretical results predicted for particle and pore having the same charge and fixed surface charge densities indicate that the rejection coefficients exceed that of an uncharged system, due to the stronger exclusion that results from the electrostatic repulsive interactions between the particle and the pore. Model results also revealed that 99.99% rejection is obtained for a membrane with pore size 1.66 times larger than the particle diameter. Experimental results will be compared to the theoretical predictions.