# Photo-Transformation of Effluent Organic Matter by ZnO-Based Sunlight Irradiation
## Materials and Methods
### Analytical Methods  
The DOC concentration was measured using a TOC-V\({}_{\text{CPH}}\) analyzer (Shimadzu, Kyoto, Japan). Potassium hydrogen phthalate with the concentration range of 1-20 mgC/L was used as a standard solution for DOC calibration. The samples for DOC analysis were filtered with a 0.45-um filter, acidified to pH \(\leq\) 2. UV/Vis absorption spectra were recorded at a wavelength range of 200-800 nm using a double-beam UV/Vis spectrophotometer (SPECORD 200 PLUS, Analytik Jena AG, Jena, Germany) with a 1-cm quartz cuvette. Several indicators extracted from the UV/Vis spectra were used to investigate the properties of the organic matter. UV absorbance at 254 nm (UV\({}_{254}\)) was used to assess the aromaticity and molecular weight of the organic matter [15], while the specific ultraviolet absorbance at 254 nm (SUVA\({}_{254}\)), which was calculated as the ratio of the normalized UV\({}_{254}\) to the DOC concentration (UV\({}_{254}\)\(\times\) 100/DOC), was used to determine its hydrophobicity and hydrophilicity [16]. The absorption ratio at 250 and 365 nm (E2/E3) was used to indicate the molecular size of the organic matter [17]. Spectra slopes, including S\({}_{275-295}\) and S\({}_{350-400}\), were obtained from the linear regression of log-transformed absorption coefficients (computed using MATLAB 2018b, Mathworks, USA) from wavelengths of 275-295 nm and 350-400 nm, respectively. The spectra slope ratio (S\({}_{\text{R}}\)) was calculated from the ratio of S\({}_{275-295}\) and S\({}_{350-400}\). The spectra slopes and S\({}_{\text{R}}\) were used to assess the molecular weight and aromaticity of the organic matter [18]. The molar absorptivity at 280 nm (\(\epsilon_{280}\)), which was defined as the absorption per mole of organic carbon at 280 nm, was used to indicate the aromaticity and apparent molecular weight of the organic matter [19]. Fluorescence EEMs were recorded using a Cary Eclipse fluorescence spectrophotometer (Agilent Technologies Inc., Santa Clara, CA, USA) with a 1-cm quartz cuvette. All of the EEMs were obtained by scanning the emission spectra in the range of 250-600 nm at 1-nm intervals and the excitation spectra in the range of 230-550 nm at 1-nm intervals. The bandwidths were adjusted to 10 nm for both excitation and emission. The scanning speed was set at 9600 nm/min. We used a 290-nm cutoff for all samples to limit the influence of second-order Rayleigh scattering. To eliminate the inner-filter effect, the samples were diluted to a final DOC close to \(\sim\)1 mgC/L before the measurement of the EEMs. To eliminate the effect of Raman scattering, the EEMs were blank-subtracted using the EEMs of a water blank (Milli-Q water) that was measured on the same day as the samples. The EEMs were calibrated to Raman units (RUs) by dividing the EEMs by the Raman peak area of 370-700 nm of Milli-Q water measured on the same day as the samples. The FI, humidification index (HIX), and biological/autochthonous index (BIX), calculated using the corrected fluorescence data, were utilized to define and classify the characteristics of the organic matter. The FI was used to determine the relative contribution of allochthonous and autochthonous organic matter in aquatic ecosystems. The FI was calculated from the ratio of the fluorescence intensity at an emission wavelength of 450 nm with that at an emission wavelength of 550 nm at an excitation wavelength of 370 nm [20]. The HIX was used to indicate the degree of humidification of the organic matter and it was calculated as the ratio of two spectral regions at emission wavelengths of 435-480 nm and 300-345 nm at an excitation wavelength of 254 nm [21]. The BIX, which is also known as the \(\beta\):\(\alpha\) ratio, was used to estimate the degree of biological degradation of the organic matter.