On the balance of hydrophilicity and electrostatic interactions during particle filtration and backwash at microfiltration pores

Abstract

Colloidal fouling is a notoriously limiting factor in many membrane processes. Although intensely studied in mostly macroscopic filtration systems, the underlying phenomena leading to generally decreased fouling remain unknown. This study systematically investigates fouling and backwashing of anionic polystyrene particles in microfluidic structures with engineered surface charge and hydrophilicity. We interpret the observed phenomena with the extended Derjaguin–Landau–Verwey–Overbeek (xDLVO) theory for interaction potentials. Polyelectrolyte coatings of the membrane-mimicking structure allow us to investigate particle–membrane interactions in (1) hydrophobic structures with negative zeta potential, (2) hydrophilic structures with positive zeta potential, and (3) hydrophilic structures with negative zeta potential. We assess the qualitative changes in membrane–particle interaction potentials regarding their hydrophilic and electrostatic double-layer interactions on the basis of the extensive study of particle deposition and resuspension during filtration and backwash. The results generally confirm the known trend of decreased fouling with increased hydrophilicity. However, we also show that hydrophilicity alone is an insufficient measure to estimate a membrane’s particle deposition and removal behavior. More important to achieve a low-fouling membrane that can be cleaned with pure flow reversal during regular backwashing is the design of a hydrophilic membrane with an engineered surface charge.