Interaction between MIL-101(Cr) and natural organic matter in an integrated MOF-UF system

As an emerging metal organic framework (MOF) material, MIL-101(Cr) has a large specific surface area and good hydraulic stability, making it ideal in applications before ultrafiltration (UF). However, the unclear interaction between MIL-101(Cr) and natural organic matter (NOM) was identified as the main limitation to understand membrane fouling. To reveal the working mechanisms of NOM removal using MIL-101(Cr), an integrated system of MOF and UF was constructed and operated in separation mode (SM) and coating mode (CM). Based on the results, MIL-101(Cr) had a high NOM adsorption capacity of 68.454 mg/g for HA and 174.725 mg/g for BSA at saturation. Permeate quality improvement occurred in both modes over a 37.7% UV254 removal rate and a 15% fluorescence intensity decrease, and permeation performance of the hybrid system was simultaneously enhanced. We found that MIL-101(Cr) influenced membrane fouling behaviors and models because of the foulant transformation in SM and dispersed NOM in the MIL-101(Cr) layer in CM. Furthermore, MIL-101(Cr) played a dual role in the integrated system. It could remove NOM by adsorption, leading to membrane fouling reduction, but also caused additional filtration resistance. Notably, MIL-101(Cr) showed the overall positive effects due to its open hollow structure and good hydrophilicity. The π-π interactions and salt-bridges between MIL-101(Cr) and NOM played a dominant role in adsorption. Whilst SM was more efficient in membrane fouling mitigation due to reduced membrane fouling resistance and a normalized flux increase above 15%, CM could achieve the same permeate quality with less MIL-101(Cr) usage and reaction time. This study reveals the interaction between MIL-101(Cr) and NOM and their effects on UF, providing a scientific basis for the widespread application of MOFs.