Fouling mechanism of forward osmosis membrane in domestic wastewater concentration: Role of substrate structures

An elaborate structural and chemical depth profile of the thin film composite (TFC)-forward osmosis (FO) membrane is indispensable to the complete understanding of how the substrate structures influence the in-situ polyamide (PA) layer formation and ultimately the wastewater concentration performance of the resultant TFC-FO membrane. Herein, the X-ray photoelectron spectroscopy paired with Ar-2500(+) gas cluster ion beams sputtering is introduced for the first time to conduct the depth profiling analysis of the PA-based TFC-FO membrane. The fine chemical and structural properties of the TFC-FO membranes were successfully reconstructed from the depth profiles, which revealed that the sponge-like substrate with small surface pores favored the formation of a smooth and thin PA layer with a highly cross-linked inner layer, while the macroporous substrate with large surface pores facilitated the development of a rough and defective PA layer with deep formation inside substrate pores. Benefiting from the thin effective thickness and dense inner selective layer, the PA layer formed on sponge-like substrate exhibited superior water permeability and salt selectivity. Besides, even suffered from more serious internal concentration polarization within substrate, the smooth PA layer surface with fewer carboxylic groups endowed the sponge-like substrate supported TFC-FO membrane with much higher antifouling capacity and average water flux than its counterpart. However, the TFC-FO membrane with highly cross-linked PA layer still exhibited poor NH4+-N rejections (56.93 +/- 0.95%). This work opens up a frontier for depth profiling of the ultrathin PA layer and provides theoretical guidance for the preparation of high-performance and anti-fouling TFC-FO membrane.