Silk sericin-coated polyamide thin-film composite membranes to simultaneously improve anti-scaling properties, long-term storage, and waste-to-resource recovery

A polyamide thin-film composite (PA-TFC) membrane, which is one of the widely used for desalination and water softening processes, features a lot of carboxyl groups inevitably formed by hydrolysis of unreacted acyl chlorides after interfacial polymerization. However, since carboxyl groups interact with multivalent cations and thereby cause membrane scaling, it is desirable to reduce the density of carboxyl groups on the membrane surface. In this regard, silk sericin (SS) can be an excellent alternative to modifying the surface characteristics to mitigate scaling because it has various hydrophilic amino acids with a much lower density of carboxyl groups per unit mass. Given the properties, we prepared anti-scaling TFC membranes by coating SS on the PA active layer. According to the result, the SS-PA-TFC (16.4% decrease) significantly reduced the flux decline to half of the PA-TFC (36.7% decrease) during the 48-h scaling test under the condition for heterogeneous gypsum crystal nucleation. More importantly, the comparison of operation duration until cleaning obviously revealed the anti-scaling properties of the SS-PA-TFC, which was evidenced by about 7.5 times longer duration than the PA-TFC. Furthermore, the cyclic scaling test in 19 mM CaSO4 showed that the SS-PA-TFC (3.6%) could decrease the irreversible fouling to about one-fifth of the PA-TFC (15.9%). This outstanding anti-scaling performance of the SS-PA-TFC stemmed from the less negatively charged (-40.8 and -28.1 mV for the PA-TFC and SS-PA-TFC, respectively) and much more hydrophilic surface (52.2 degrees and 18.2 degrees for the contact angles of the PA-TFC and SS-PA-TFC, respectively). Suprisingly, the above-mentioned anti-scaling properties of the SS-PA-TFC membrane were achieved without the trade-off between water flux and salt rejection thanks to a very thin SS coating layer (ca. 16 nm). Apart from the anti-scaling properties, SS coating also improved long-term storage and waste-to-resource recovery.