Change of surface properties and performance due to chlorination of crosslinked polyamide membranes

The objective of this work was to systematically investigate the effect of hypochlorite oxidant on surface properties and performance of crosslinked aromatic polyamide membranes, and to propose a mechanism of the performance change of the membranes. The effects of chlorine on the membranes were evaluated using several analytical tools (SEM, AFM, XPS, FTIR, Contact Angle Analyzer, Streaming Potential Analyzer, and Filtration Test System).; Experimental results from chlorination showed that atomic percent of chlorine bound to the LFC1 membrane increased with increasing chlorine exposure and decreasing pH of the soaking bath. The introduction of electronegative chlorines created unbalanced dipole moments on the membrane surface, increasing hydrophilicity and made the zeta potential of the membrane more negative. The replacement of hydrogen with chlorine on the amide group of the membrane caused the loss of hydrogen bonding sites, confirmed by the systematic shift of the amide I band (C=O stretching vibration) to the higher wavenumbers and the disappearance of the amide II band (N-H bending vibration) by FTIR spectrum analysis.; Performance changes caused by the chlorination of crosslinked polyamide membranes were determined to be dependent on three factors: pH, concentration of chlorine, and filtration time. Chlorination at low pH and high chlorine concentration broke most hydrogen bonds and caused compaction of the polymer chains, resulting in blocking water passage through the membrane. After the initial flux drop, rearrangement of the flexible polymer chains induced a systematic flux increase of highly chlorinated polyamide membranes. Chlorination at high pH and low chlorine concentration broke small amounts of hydrogen bonds with an intact polymer structure and released the restriction of water passage through the membrane. Continuous high pressure caused distortion of polymer chains and monotonic flux decrease of slightly chlorinated membranes.; Systematic investigation of chlorine exposure to the polyamide membranes on the change of performance (water flux and salt rejection) demonstrated that the change of hydrogen bonding behavior due to the chlorination caused the change of rotational freedom or the flexibility of polymer chains followed by the change from initial performance and subsequent flux changes with filtration time.