Treating PFOA emulsified oil in membrane distillation: Relationship between membrane fouling and surface properties

Perfluorooctanoic acid (PFOA), an amphiphilic compound with ecotoxicity, contains fluorinated alkyl chains terminated with weak acidic groups, making it challenging to degrade or eliminate from water sources. Herein, two hydrophilic-hydrophobic composite membranes (PDA/SiNPs-PTFE and PDDA/SiNPs-PTFE) with different surface charges were developed to concentrate and remove PFOA from model contaminated water via direct contact membrane distillation (DCMD). The composite membranes were fabricated through nanocomposite coating and characterized by SEM, AFM, FTIR, XPS, Zeta potential and contact angle. During 24-h DCMD tests using an oily solution without PFOA, both PDA/SiNPs-PTFE and PDDA/SiNPs-PTFE membranes exhibited relatively stable average fluxes (13.5 and 13.2 kg m −2 h −1 , respectively) and excellent anti-fouling performance due to their oleophobic property, while pristine PTFE membrane showed predictably poor anti-fouling performance. When treating PFOA emulsified oily solution, PDA/SiNPs-PTFE membrane exhibited exceptional anti-fouling performance, attributed to synergistic effect of hydration layer and electrostatic repulsion. Theoretical analysis was conducted to further elucidate the anti-fouling mechanism. Extended Derjaguin-Landau-Verwey-Overbeek (XDLVO) analysis revealed Lifshitz-van der Waals and acid-base interactions play dominant roles in determining the total energy between foulant and membrane surface. Molecular dynamic simulation also illustrated higher foulant-membrane interaction energy corresponds to a greater probability of polymer chain detachment from the contaminant molecules. This study provides valuable guidance to concentrate PFOA emulsified solution by MD and offers new insight into interfacial interactions in such system. • Janus membranes with varied charges were prepared by coating PTFE with PDA/SiNPs or PDDA/SiNPs. • PDA/SiNPs-PTFE showed high flux (13.2 kg m −2 h −1 ) and rejection (96.8 %) for PFOA-emulsified solution. • Synergistic hydration and electrostatic repulsion confer PDA/SiNPs-PTFE superior anti-fouling properties. • RDF showed fewer PFOA around PDA/SiNPs-PTFE than other two membranes.