## 3 Results and Discussion
### Optical Indices of SHS  
Precise UV/Vis and fluorescent analysis can obtain valuable DOM information [24, 36, 37, 40, 46, 47]. In this study, the specific ultraviolet absorbance at wavelength 254 nm (SUVA\({}_{254}\)) and the humification index (HIX) was used to analyze the chemical properties of the bulk and size-fractionated SHS samples. The SUVA\({}_{254}\) and HIX of the bulk and size-fractionated SHS for the AD and FD samples are shown in Figure 1a,b. The SUVA\({}_{254}\) value is a surrogate for the abundance of aromaticity for SHS [12, 24, 37]. The HIX is an indicator of material age and recalcitrance. HIX has been widely applied to investigate the extent of humification in SHS [11, 48]. The SUVA\({}_{254}\) and HIX values were 1.98 \(\pm\) 0.51 L/mg-C/m and 2.10 \(\pm\) 0.30, respectively, of the total SHS solutions, which were lower than the average values of 4.2 \(\pm\) 1.9 L/mg-C/m (SUVA\({}_{254}\)) and 5.45 \(\pm\) 3.80 (HIX) of the alkaline-extracted SHS samples taken from 43 stream sediments [5]. The SUVA\({}_{254}\) and HIX values were comparable to the alkaline-extracted SHS samples measured in sediments taken from a lake, industrial water, and a stream [6, 10, 20].  
The SUVA\({}_{254}\) and HIX values of the AD bulk SHS were significantly higher than the FD bulk SHS (\(p\) = 0.047 and 0.018, respectively). In individual size-fractionated SHS samples, the SUVA\({}_{254}\) and HIX values of the AD samples were higher than the values of the FD samples, though insignificantly different (except the SUVA\({}_{254}\) of HHS). The high SUVA\({}_{254}\) and HIX values implied that the AD treatment had developed more aromatic functional groups and recalcitrant humic-like substances than the FD treatment due to biological activity. In the HHS samples, the average AD SUVA\({}_{254}\) value was significantly higher than the FD samples (\(p\) = 0.02), consistent with the fluorescence regional integration (FRI) results.  
We compared the indicator difference between the size-fractionated SHS samples; for each sized SHS sample, the AD and FD samples were combined as a pooled sample. The average SUVA\({}_{254}\) values were 2.29 \(\pm\) 0.11 and 2.12 \(\pm\) 0.31 L/mg-C/m for HHS and MHS, respectively, which were significantly higher than the average LHS value of  
Figure 1: **(a,b)** Optical indicators varied with size-fractionated AEOM for AD and FD samples.  
1.18 \(\pm\) 0.08 L/mg-C/m (\(p\) < 0.001) (Figure 1a). However, the HIX values were 1.86 \(\pm\) 0.18, 2.15 \(\pm\) 0.22, and 2.33 \(\pm\) 0.40 for HHS, MHS, and LHS, respectively. The LHS HIX values were significantly higher than the HHS HIX values (\(p\) = 0.036) (Figure 1b). Most studies have shown high molecular weight SHS with a high HIX [9; 19]; however, sediment low molecular weight water extracted organic matters (WEOM) had a higher HIX value than observed with the high molecular weight [3]. The SHS composition may cause these reverse conditions. The SUVA\({}_{254}\) values of all samples were <3.0, which suggested that the SHS composition was predominantly a hydrophilic substance with poor aromaticity [40]. The HIX values of all samples were <4, which suggested the SHS was of biological or aquatic origin with a low degree of humification [23; 49]. The SHS composition characteristics were further examined with the FRI method as shown in Section 3.3.