# Photo-Transformation of Effluent Organic Matter by ZnO-Based Sunlight Irradiation
## 3 Results and Discussion
### EEM-PARAFAC Components
#### 3.2.2 Changes in PARAFAC Components during Photocatalysis  
Table 3 and Figure 5 provide the removal of the three PARAFAC components based on Fmax measurements (%). The apparent photodegradation rates (\(k_{app}\)) of the three PARAFAC components fit a pseudo-first-order model (R\({}^{2}\) = 0.91-1.00), suggesting that their photocatalytic degradation and kinetic rates could be directly compared. After 120 min of irradiation, a significant proportion of the three  
Figure 4: Percentage contribution of PARAFAC components in upstream, wastewater effluent, and downstream samples. components had been removed, ranging from 82.8% to 98.7%, which indicated that all of the identified fluorophore groups were subject to direct photochemical degradation. Consequently, the degradation of the fluorescing components was higher than the removal of DOC and UV\({}_{254}\). Apparently, a lower degradation of DOC than UV\({}_{254}\) and PAFAFAAC components may be due to the transformation of light-sensitive moieties in organics to non-light absorbing ones, and yet it could not be judged which PARAFAC components would be mostly transformed to the DOC-causing with non-light response (Figure 6a). However, assuming from the fact that C1 and C2 (the highest in both the upstream and effluent samples) exhibited a significant reduction in F\({}_{\text{max}}\) (Figure 6b,c), these two components are presumable contributors to residual DOCs. This also tells us that DOC measurement should be accompanied for addressing the mineralization of organic matter, because there are certain portions of the aliphatic or polysaccharides that are not responsive to the fluorescence spectroscopy. Thus, for a further understanding on fate and transport of EfOM by photocatalysis, it would be necessary to analyze the DOM using additional techniques, such as size exclusion chromatography with organic carbon and fluorescence detection simultaneously. The faster degradation of the fluorescence components as compared to UV-absorbing moieties was possibly due to the fluorescence that arises from the \(\pi\)-\(\pi^{\bullet}\) transitions in organic matter molecules and its rapid extinction under sunlight irradiation [8; 11; 12; 13]. The F\({}_{\text{max}}\) removal and \(k_{app}\) of the three PARAFAC components followed the order C3 \(>\) C1 \(>\) C2, indicating that the humic-like components were preferentially removed under sunlight irradiation. Our results were consistent with prior photobleaching studies using aquatic DOM samples [11; 12; 13], in which protein-like fluorescence peaks were more resistant to photodegradation than other peaks (e.g., peaks A, M, and C) and the humic-like fluorescence peaks (i.e., C3 in the present study), with the excitation wavelength falling within the  
Figure 5: Changes in three PARAFAC components during photocatalysis with 0.2 g/L of ZnO at pH 7 for upstream, wastewater effluent, and downstream samples: (**a**) C1 degradation curves, (**b**) C2 degradation curves, (**c**) C3 degradation curves, and (**d**) degradation rates. UVA band (i.e., 315-400 nm). Thus, the humic-like and the tyrosine-like components can be associated with photolabile organic matter and residual products, respectively. The removal and degradation rate of C3 was higher than that of C1, which can be explained by the preferable adsorption of organic molecules that are larger, that are more hydrophobic, or that have a higher degree of aromatic condensation compared to minerals and/or nanoparticles [11; 12; 13]. With peaks at longer wavelengths, C1 might be related to the structural condensation and polymerization of organic matter [13]. Indeed, more pronounced fluorescence at longer emission wavelengths in the EEMs of larger and/or more hydrophobic organic matter fractions has been previously reported [5; 13; 36].