# Photo-Transformation of Effluent Organic Matter by ZnO-Based Sunlight Irradiation
## 3 Results and Discussion
### EEM-PARAFAC Components  
#### 3.2.1 Spectral Characteristics of PARAFAC Components  
Using PAFAFAC modeling based on 197 EEM samples, three components (C1, C2, and C3) were identified (Figure 3). The spectral characteristics of these three components were similar to those of organic matter that was previously identified in other aquatic environments and the OpenFluor database (Table S2). C1 (Ex/Em maxima at \(\leq\)230(325)/406 nm) has been categorized as microbial humic-like [25], humic-like (peaks A + M) [26], an intermediate that forms as a result of the photochemical degradation of terrestrial DOM [34], and a phytoplankton-degradation humic component that arises from microbial activity [35]. The C1 detected in the river samples may be either terrestrially derived or autochthonous produced in aqueous environments from terrestrial organic substrates [8; 25; 26]. C2 (Ex/Em maxima at \(\leq\)230(276)/304 nm) was assigned to autochthonous tyrosine-like fluorescence peak B [8; 25; 26; 36]. The C2 in the wastewater effluent samples could be formed during the biological wastewater treatment process [8; 26], while the C2 in the upstream and downstream samples may be anthropogenically derived organics in wastewater-impacted waters, plankton-derived organics in fresh water, or algal organic matter in the river water [26; 36]. C3 (Ex/Em maxima at 261/490 nm) resembled a combination of traditionally defined peak A and peak C, and it has  
Figure 2: DOC and UV\({}_{254}\) removal from upstream, wastewater effluent, and downstream samples using photocatalysis with 0.2 g/L of ZnO at pH 7: (**a**) DOC removal and (**b**) UV\({}_{254}\) removal. been ascribed to terrestrial-like humic substances [13; 14; 24; 25]. Prior studies have demonstrated that C3 may be either terrestrially derived or possibly produced autochthonous in aquatic environments that were largely affected by terrestrial input [13; 24; 25]. C1 and C3 were both associated with the presence of humic-like substances; however, C1 had a peak at a shorter emission wavelength than C3, suggesting that C3 appeared to have a larger molecular size and was more hydrophobic than the C1 [11; 12; 13]. Thus, the three components were identified as a humic-like component (C1), a tyrosine-like component (C2), and a terrestrial-like humic component (C3). Figure 3: Contour plots and excitation/emission loadings for three PARAFAC components from upstream, wastewater effluent, and downstream samples. Figure 4 shows the relative distributions of the three PARAFAC components (i.e., %C1, %C2, and %C3) that were calculated using Fmax. C1 and C2 accounted for over 84% of the total PARAFAC components, which indicated that the majority of the fluorophores in the organic matter in the present study could be described by a combination of the humic-like (C1) and the tyrosine-like (C2) components. Furthermore, the contributions of the PARAFAC components from different sources of organic matter in the receiving water differed. Specifically, the contributions of C1 and C3 followed the order of wastewater effluent \(>\) downstream \(>\) upstream, whereas the contribution of C2 followed the order of upstream \(>\) downstream \(>\) wastewater effluent. This reveals an increase in both humic-like components (C1 and C3) from the upstream to the downstream samples with a simultaneous decrease in the protein-like component (C2), reflecting the impact of EfOM on the PARAFAC components of organic matter in the receiving water. To understand the impact of EfOM on the receiving water from the perspective of organic matter characteristics, changes in the contribution of the three PARAFAC components in the DOM/EfOM mixtures were investigated. As the EfOM ratio increased, %C1 and %C3 increased linearly (R\({}^{2}\) = 0.91-0.99), while %C2 decreased linearly (R\({}^{2}\) = 0.95) (Figure 4 and Figure S5). The ratio of the humic-like/protein-like components, i.e., C1/C2, C3/C2, and (C1 + C3)/C2, also increased linearly with a greater EfOM ratio (R\({}^{2}\) = 0.97-1.00).