Preparation of chlorine resistant thin-film-composite reverse-osmosis polyamide membranes with tri-acyl chloride containing thioether units

A novel tri-acyl chloride monomer containing reductive thioether units (T-TDC) was prepared. It was conducted to partially replace trimesoyl chloride (TMC) and react with m-phenylenediamine (MPD) by interfacial polymerization to prepare a series of T-TDC/TMC/MPD copolymer thin-film-composite reverse-osmosis polyamide membranes. Through this routine, the reductive thioether units was introduced into the film molecular chain to protect amide bonds from being chlorinated by oxidizing during the membrane cleaning process. The enhanced chlorine resistance of these resultant membranes was revealed by FT-IR and XPS characterization: it could be found thioether was prior to amide oxidation, and its oxidation process can be divided into two steps (the thioether units was first converted to sulfoxide groups, and then further oxidized to sulfone groups). Through this routine, the active chlorine was consumed partially, then the amide bond was protected. The experiment result is consistent with the DFT theoretical calculation. The chlorine exposure concentration of the normalized flux and rejection decreased to 1 increased gradually with the addition of copolymerization ratio of T-TDC, especially for the sample 20%-TTDC-TFC, its chlorine exposure concentration was 3 and 4 times higher respectively than that of 0%-TTDC-TFC when its normalized flux and rejection at 1. In addition, it can be found that the polar sulfone and sulfoxide groups can form hydrogen bonds to prevent the membrane from degrading immediately. This method provides an effective way for improving the service life of RO film.

Automated summary

In this study, the chlorine resistance of polyamide membranes was improved by introducing reductive thioether units, which protected the amide bonds. Increasing the copolymerization ratio of T-TDC also reduced the chlorine exposure concentration. The sulfone and sulfoxide groups further prevented membrane degradation by forming hydrogen bonds.