# Photo-Transformation of Effluent Organic Matter by ZnO-Based Sunlight Irradiation
## 3 Results and Discussion
### DOC and Optical Parameters
#### 3.1.2 Photocatalytic Degradation of DOM, EfOM, and EfOM-Impacted
During photocatalytic oxidation, a substantial decrease in SUVA\({}_{254}\) was observed (Figure S4a), which could be explained by the preferential removal of aromatic chromophores over aliphatic moieties, followed by the transition of transforming the DOM into non- or less-UV-absorbing substances upon irradiation [11; 12; 13]. Photocatalysis also resulted in decreases in \(\varepsilon_{280}\) (Figure S4b), S\({}_{275}\)-\({}_{295}\), and S\({}_{350}\)-\({}_{400}\), and an increase in S\({}_{\mathrm{R}}\) and E2/E3 (Figure S4c), indicating a reduction in the aromaticity, molecular size, and molecular weight of the organic matter [17; 31; 32]. The FI, HIX, and BIX of all the samples decreased after photocatalysis (Figure S4d,e,f), demonstrating the reduction of aromatic structures in the organic matter following solar irradiation [31; 32]. These observations are similar to the previously reported photocatalytic degradation of organic matter in aquatic environments [11; 31; 32]. Figure 2 and Table 3 illustrate changes in the DOC and UV\({}_{254}\) during photocatalysis. Solar irradiation led to a significant reduction in DOC and UV\({}_{254}\), suggesting that the organic matter underwent both photomineralization and photobleaching [8; 12; 31]. All of the observed degradation trends followed pseudo-first-order kinetic models (R\({}^{2}\) = 0.96-1.00), which has also been reported in previous studies on the photocatalytic degradation of organic matter [11; 12; 13; 14]. The DOC removal ranged from 28.8 \(\pm\) 0.8% to 43.4 \(\pm\) 0.3%, with empirical degradation rates ranging from 0.0029 \(\pm\) 0.0107 min\({}^{-1}\) to 0.0052 \(\pm\) 0.0054 min\({}^{-1}\). When compared to the DOC, UV\({}_{254}\) absorbance showed higher removal and degradation rates, ranging from 60.0 \(\pm\) 0.8% to 67.6 \(\pm\) 0.2% and from 0.0072 \(\pm\) 0.0206 min\({}^{-1}\) to 0.0102 \(\pm\) 0.0532 min\({}^{-1}\), respectively. This observation could be explained by the terminal functional groups (e.g., hydroxyl and carboxyl groups) of the aromatic compounds strengthened the adoption affinity of the catalyst surfaces and/or some of the aromatic chromophores were partially transformed into non- or less-UV-absorbing substances in the photochemical reaction [11; 12; 13]. The removal efficiency (DOC and UV\({}_{254}\)) of the upstream, wastewater effluent, and downstream samples followed the order of upstream \(>\) downstream \(>\) wastewater effluent. In particular, the DOC removal from the downstream sample was 4.4% lower than that from the upstream sample and it was 10.2% higher than that from the wastewater effluent sample. Similarly, the UV\({}_{254}\) removal from the downstream sample was 4.2% lower than that from the upstream sample and it was 3.4% higher than that from the wastewater effluent sample. These results reflect the impact of EOM on the degradation efficiency of the organic matter present in receiving waters. \begin{table}
\begin{tabular}{c c c c c c c c c c c} \hline \multirow{2}{*}{**Samples**} & \multicolumn{8}{c}{**Removal (\%)**} & \multicolumn{8}{c}{**Degradation Rate, k (min\({}^{-1}\))**} \\ \cline{2-11}  & **DOC** & **UV\({}_{254}\)** & **C1** & **C2** & **C3** & **DOC** & **UV\({}_{254}\)** & **C1** & **C2** & **C3** \\ \hline
100/0 & 43.4 & 67.6 & 98.4 & 82.8 & 98.7 & 0.0052 & 0.0102 & 0.0292 & 0.0142 & 0.0330 \\ (upstream) & 38.4 & 65.4 & 96.1 & 84.0 & 96.5 & 0.0041 & 0.0088 & 0.0256 & 0.0156 & 0.0278 \\
75/25 & 35.9 & 64.6 & 95.6 & 86.4 & 95.7 & 0.0036 & 0.0079 & 0.0242 & 0.0174 & 0.0255 \\
25/75 & 33.1 & 61.9 & 94.2 & 88.7 & 94.7 & 0.0035 & 0.0077 & 0.0223 & 0.0178 & 0.0238 \\
0/100 (WW effluent) & 28.8 & 60.0 & 93.3 & 90.8 & 93.5 & 0.0029 & 0.0072 & 0.0218 & 0.0202 & 0.0223 \\ Downstream & 39.0 & 63.4 & 94.6 & 86.5 & 94.8 & 0.0039 & 0.0070 & 0.0217 & 0.0152 & 0.0235 \\ \hline \end{tabular}
\end{table} 
Table 3: Changes in DOC, UV\({}_{254}\), and the three EEM-PARAFAC components for upstream, wastewater effluent, and downstream samples during photocatalysis with 0.2 g/L of ZnO at pH 7 for 120 min of irradiation (mean).