Mechanism of humic acid fouling in a photocatalytic membrane system

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Photocatalytic membrane filtration has emerged as a promising technology for water purification because it integrates both physical rejection and chemical destruction of contaminants in a single unit, and also largely mitigates membrane fouling by natural organic matter (NOM). In this study, we evaluated the performance of a photocatalytic membrane system for mitigating fouling by a humic acid, which is representative NOM, and identified critical properties of the humic acid that determined membrane fouling. We prepared a partially oxidized humic acid (OHA) through the photocatalysis of a purified humic acid (PHA), and the OHA showed reduced fouling for polyvinylidene fluoride (PVDF) ultrafiltration membranes compared to PHA. Molecular-level characterizations indicated that OHA had a reduced molecular size, an increased oxygen content, and increased hydrophilicity. OHA also formed smaller aggregates on the fouled membrane surfaces than PHA. The introduction of oxygen-containing, hydrophilic functional groups, e.g., -OH and -COOH, to the humic acid and the depolymerization or mineralization of the humic acid in photocatalysis could result in the reduction of the foulant-membrane and foulant-foulant interactions, as characterized by atomic force microscopy (AFM), thereby mitigating membrane fouling. Foulant-membrane adhesion forces were always larger than foulant-foulant adhesion forces in our study, irrespective of the humic acid before or after photocatalytic oxidation, which may suggest that the reduction of foulant-membrane interactions is critical for membrane fouling control. In summary, this study sheds light into humic acid fouling in a photocatalytic membrane system through a systematic and comprehensive research approach, and provides insights for the design of novel membrane materials and processes with improved performance for water purification.