Tracking fluorescent dissolved organic matter in hybrid ultrafiltration systems with TiO2/UV oxidation via EEM-PARAFAC

Abstract

In this study, different fluorescent constituents of dissolved organic matter (DOM) with terrestrial sources and their membrane fouling potentials were tracked in the hybrid ultrafiltration (UF) processes adopting UV photooxidation with TiO2 nanoparticles (NPs) as a form of pretreatment via fluorescence excitation emission matrix coupled with parallel factor analysis (EEM-PARAFAC). The photocatalytic oxidation processes consistently resulted in enhanced removal rates of DOM and membrane fouling mitigation regardless of DOM sources. EEM-PARAFAC decomposed bulk DOM into four different fluorescent DOM (FDOM) components including three humic-like components (C1, C2, and C3) and one protein/polyphenol-like component (C4). The results showed that direct hole oxidation was primarily involved in attacking large sized humic-like C1 on TiO2 surfaces, while indirect oxidation with hydroxyl radicals was responsible for the removal of the small sized humic-like C3 and C4. Under a short irradiation period of 30min, the intermediate-sized humic-like C2 was released into solutions from adsorbed C1. Among the FDOM components, C4 was the most associated with reversible fouling of the hybrid UF system, while C1 contributed most of the fouling of the UF system without the photocatalysis. Meanwhile, C3 was the most important FDOM component responsible for irreversible fouling, and the contribution was more pronounced at longer irradiation. This study revealed the great applicability of EEM-PARAFAC in probing the extent of membrane fouling in TiO2/UV-UF hybrid systems. Considering the unique characteristics of the individual FDOM components, EEM-PARAFAC can provide valuable information providing further insight into system optimization as well as predictions of the treated water quality.