Role of visible light photocatalysis in alleviation and mechanism transformation of ultrafiltration membrane fouling caused by natural organic matter

In this study, a novel composite based on bismuth oxide, reduced graphene oxide and titanium dioxide (labeled as BRGT) was utilized for visible light photocatalysis to mitigate ultrafiltration membrane fouling caused by natural organic matter (NOM). The results show that the coexistence of NOM and the BRGT composite (without photocatalysis) did not aggravate membrane fouling, and the membrane preferentially retained a high molecular weight, hydrophobic component of NOM that formed a dense cake layer with BRGT composite. As the photocatalysis time increased, the extent of membrane fouling was alleviated. The maximum specific flux recovered from 0.21 to 0.73 at 60 min, and the hydraulic reversible and irreversible resistances ere 1.15 × 1010 and 4.23 × 1010 m−1, respectively. The reduction of NOM in the feed water, especially the removal of UV254, played a significant role in the alleviation of membrane fouling. Additionally, the changes in the properties of NOM, including the reduction of fluorescent humic and protein/polyphenol components corresponding to reversible and irreversible fouling, respectively, the photo-degradation of high molecular weight NOM into low molecular weight NOM (such as low molecular weight-acids), and the conversion of hydrophobic fractions into hydrophilic fractions, contributed to the mitigation of membrane fouling. We also found a strong negative correlation between the roughness of the cake layer and irreversible fouling. Overall, the changes in the properties of NOM in feed water can hinder or even inhibit the formation of the cake layer, and the use of visible light photocatalysis of the BRGT composite has great potential to alleviate ultrafiltration membrane fouling caused by natural organic matter.