# Photo-Transformation of Effluent Organic Matter by ZnO-Based Sunlight Irradiation
## 3 Results and Discussion  
### DOC and Optical Parameters  
#### 3.1.1 Characterization of the DOM, EfOM, and EfOM-Impacted  
Table 1 summarizes the DOC and optical parameters (i.e., absorbance and fluorescence) of the organic matter from different sources. Initially, all of the samples exhibited a low SUVA\({}_{254}\) (1.124-1.234,  
less than 2), indicating that the samples mainly contained hydrophilic organic compounds and had a low molecular weight, a low charge density, and a higher non-humic than humic fraction [16; 26; 27; 28]. Organic matter with a higher non-humic fraction has a potentially negative impact on drinking water treatment processes because the removal of non-humic compounds is more difficult [28]. Fluorescence EEM measurements displayed two dominant peak areas (Figure S2a): a humic-like organic matter peak (at Ex/Em = 330-350 nm/420-480 nm and Ex/Em = 250-260 nm/380-480 nm) and a protein-like organic matter peak (Ex/Em = 250-280 nm/280-350 nm) [25; 29]. The protein-like peak in the wastewater effluent and wastewater-impacted samples may originate from soluble microbial products in biologically treated wastewater [3; 25; 26]. The protein-like organic matter peak detected in the drinking water DOM may originate from algal organic matter [2; 3]. The samples also exhibited a low HIX (HIX = 1.01-1.45, less than 4), a high BIX (BIX = 0.96-1.04), and a low FI (FI = 1.26-1.66), which suggested that the organic matter in samples was associated with autochthonous organic matter [3; 20; 21]. A comparison between wastewater EfOM and upstream DOM samples showed that the EfOM samples had a higher SUVA\({}_{254}\), \(\epsilon_{280}\), and \(\mathrm{S_{R}}\) than the DOM samples did, while E2/E3 and \(\mathrm{S_{350-400}}\) exhibited the opposite trend. These observations indicate that the aromatic content and average molecular weight of the EfOM samples were probably higher than those of the DOM samples [17]. The FI, HIX, and BIX for the EfOM samples were higher than those for the DOM samples, which indicated that the EfOM was derived from autochthonous and microbially derived sources [3; 6; 24]. In the comparison between the upstream and downstream samples, the changes in optical parameters (SUVA\({}_{254}\), \(\epsilon_{280}\), \(\mathrm{S_{R}}\), FI, HIX, and BIX) were similar to those observed in the comparison between the EfOM and DOM samples, which indicated that these changes were the result of the contribution of EfOM. The discharge of wastewater effluent may affect wildlife (such as fish, birds, etc. ), which uses the river water as a habitat or for drinking and it may contribute to algal blooms in the Han River due to the high nutrient levels from protein-like components. To further investigate the impact of EfOM on the quality of the receiving water, changes in DOC and optical parameters in DOM/EfOM samples were examined. Most of the measured parameters were close to the theoretical mixing lines (R\({}^{2}\) = 0.90-0.99), which suggested that the investigated indicators were primarily dependent on the physical mixing of the DOM and EfOM. The EfOM ratio had a positive linear relationship with DOC, UV\({}_{254}\), SUVA\({}_{254}\), FI, HIX, and BIX (Figure S2b), a positive nonlinear relationship with \(\epsilon_{280}\), and a negative nonlinear relationship with E2/E3. The positive linear changes in the FI with an increasing EfOM contribution were attributed to a higher proportion of microbially derived organic matter from the biological treatment at the WWTP [6; 7]. Unlike previous studies, the SUVA\({}_{254}\) exhibited a linear increase as the EfOM contribution increased, which indicated that the EfOM consisted primarily of more UV-sensitive fractions, providing a high relative index of DOC humic content [3; 6; 7; 30]. As the EfOM ratio increased, the EEM contours (Figure S2a) exhibited more humic-like and less protein-like organic matter peaks.