Sodium hypochlorite assisted membrane cleaning: Alterations in the characteristics of organic foulants and membrane permeability

Chemical cleaning is an important approach for alleviating severe fouling in membrane separation processes. In this study, lysozyme (LYS) was exposed to sodium hypochlorite (NaClO) with varied concentrations (0-2000 ppm) to understand the changes in the physicochemical properties and functional groups as well as the variations in membrane permeabilities. The results showed that membrane filterability exhibited an obvious 'U-shaped' trend, and the valley existed when the ratio of Cl/C (the ratio of NaClO and TOC concentrations in feed water) is among 1.35-3.09. Upon exposure to low dose NaClO, three-dimensional fluorescence excitation-emission matrix (3D-EEM) spectra showed that tryptophan protein substances were transformed to more hydrophobic humic-like substances. Fourier transform infrared (FTIR) analysis further confirmed that exposure to low dose NaClO promoted the breakage of aromatic substituents, leading to the formation of hydrophobic condensed aromatic substances. On the contrary, at high NaClO loads, protein structures were destroyed completely and almost no obvious fluorescent intensities could be detected, which promoted the recovery of membrane filterabilities. Notably, the chemical cleaning mechanisms of fouled membranes with NaClO were understood in depth in this study. These results provide new information about the oxidation products of LYS and the cleaning efficiency upon exposure to NaClO.