Multi-layer structure toward simultaneous enhancement of forward osmosis membrane separation performance and anti-biofouling property

Biofouling is a critical issue in membrane-based water-treatment processes because the feed solution retains microorganisms despite rigorous pretreatment. The forward osmosis (FO) process has a drawback of severe biofouling tendency when the active layer faces the draw solution. Here, we fabricated a thin-film nanocomposite membrane with a multilayer structure consisting of an MXene/carbon nanotubes (MXene/CNT) interlayer and a CNT back layer (TFNi-CNT). The interlayer structure significantly enhanced the membrane separation performance whereas the CNT back layer did not significantly hamper the performance. The biofilm formed on the CNT back layer surface was reduced by approximately 90% compared to that on the pristine substrate, indicating that the CNT back layer has superior antibiofilm properties. The water flux of the TFNi-CNT membrane was well-maintained (approximately 46%) and reversibly recovered through facile physical flushing, even after four dynamic biofouling cycles, whereas that of the pristine membrane was reduced to approximately 10%. These results indicated that the TFNi-CNT membrane possesses excellent resistance to biofouling. The CNT layer acts as a barrier that effectively prevents bacteria from entering the inner porous substrate, thus alleviating the detrimental biofilm-enhanced internal concentration polarization. This study provides new insights into the rational design and fabrication of FO membranes to mitigate biofouling.