Two-Dimensional FTIR Spectroscopic Characterization of Functional Groups of NaOCl-Exposed Alginate: Insights into Membrane Refouling after Online Chemical Cleaning

Abstract

In-depth understandings of the interactions between NaOCl and foulants (e.g., alginate) at a molecular level can provide a deeper insight into the optimization of chemical cleaning. In this study, two-dimensional Fourier transform infrared correlation spectroscopic (2D-FTIR-COS) analysis was used to probe the hypochlorite oxidation of alginate at normal wastewater treatment operating pH (i.e., 6, 7, and 8). The 2D-FTIR-COS characterization indicated that the groups in alginate were degraded by NaOCl in a similar sequence at all investigated pHs: carboxylate groups (-COOH at pH 6 and 7) > C-O-C bonds, C-O bonds of uronic acids > C-O-C bonds. The results also showed that the presence of hemiacetals, which are formed by uronic acids, can protect alginate chains from oxidation during their initial exposure to NaOCl (30 min) and thus maintain alginate at a constant molecular size. Additionally, the rupture of C-O-C bonds resulting from initial oxidation (30 min) increased the viscosity of alginate solution because of the decreased stiffness of alginate chains and the rise of stronger intermolecular junctions. Long-term exposure to NaOCl (480 min) largely degraded uronic acids and the C-O-C bonds of alginate molecules, leading to decreases in molecular weight and viscosity at pH 6 and 7. However, because of the low degree of oxidation at pH 8, molecular weight and viscosity of alginates were slightly changed after long-term exposure (480 min) to NaOCl. Our results imply that the partial oxidation of alginate during hypochlorite scavenging enhanced its fouling propensity, which could have a potential implication for the implementation of chemical cleaning.