Membrane fouling mechanisms in the presence of microplastics and organic matter: The unexpected mitigating role of Ca2+

Ultrafiltration (UF) is demonstrated to be highly effective in the removal of microplastics (MPs), but the presence of coexisting foulants introduces significant uncertainties into the associated membrane fouling behaviors. In this study, membrane fouling mechanisms were investigated when MPs, represented by polystyrene (PS), coexisted with typical organic foulants (sodium alginate, SA) and inorganic ions (Ca²⁺). Fouling tests revealed that the order of Ca²⁺ addition significantly impacted the fouling behavior of the SA-PS combined foulants. Specifically, the specific filtration resistance (SFR) was reduced by 40.82 % in the SA-PS-Ca²⁺ foulants and by 90.92 % in the SA-Ca²⁺-PS foulants, compared to the SA-PS foulants. X-ray photoelectron spectroscopy and density functional theory calculations indicated that sufficient cross-linking of Ca²⁺ with SA molecular chains in the SA-Ca²⁺-PS foulants, forming a large-scale 3D network that encapsulated more PS particles and resulted in larger flocs than those found in the SA-PS-Ca²⁺ foulants. According to extended Flory-Huggins theory, the improved filtration performance of the SA-PS combined foulants was due to substantial changes in chemical potential during their transition from gel to flocs upon Ca²⁺ addition. Furthermore, interfacial thermodynamic analyses suggested that increased repulsion between SA-Ca²⁺-PS foulants and between them and the membrane led to a looser fouling layer, significantly mitigating membrane fouling. This study elucidates the fouling mechanisms in the presence of MPs and other foulants from the perspectives of energy changes and molecular structures, providing novel insights for developing strategies to mitigate membrane fouling.