Kinetic and mechanistic analysis of membrane fouling in microplastics removal from water by dead-end microfiltration

This study explores and analyses the kinetic and mechanistic aspects of microfiltration cellulose acetate membrane fouling by polyamide (PA) and polystyrene (PS) particles in dead-end configuration and the main interactions between the microplastics and the membrane during the filtration process. First, PA and PS particles were characterised to define the differences in shape (regular and irregular), particle size distribution (10–105 µm and 20–320 µm), and surface charge (neutral and negative). The results showed that the prevailing mechanisms during microplastic filtrations were complete pore blocking followed by cake layer formation in both cases. The mechanisms’ kinetics were positively correlated to MPs load through a power-law relationship which was stronger for PS than for PA particles because of higher steric hindrance effects. On the other hand, increasing the working transmembrane pressure led to an optimum working condition, between 0.3 and 0.5 bar for PA and 0.3 bar for PS filtration. Overall, higher fouling was induced by the PA particles due to the higher PA hydrophobicity and their smaller size, which caused a denser cake layer. Instead, PS particles with higher irregularities and repulsive electrostatic forces formed a more porous layer but induced a high degree of abrasion on the membrane surface. Finally, membrane fouling led to an increase in hydrophobicity and roughness, probably causing further fouling. To conclude, modelling membrane fouling can help predict the best working conditions and the membrane replacement cycles to increase the MPs removal efficiency and reduce secondary MP-based pollution.