**1. Introduction**

Dissolved organic matter (DOM) is a heterogeneous compound of organic matters, mainly composed of proteins, humus and other aromatic and aliphatic organic compounds [1,2]. DOM is ubiquitous in surface water, pore water, sediments and other natural environments [3,4]. DOM is also considered as a tracer in biogeochemical cycling, which plays an important role in the course of material circulation and energy exchange [5]. DOM characteristics are associated with their source and environment conditions. In general, there are two main sources of DOM, including allochthonous inputs and autochthonous production [6]. Allochthonous DOM represents the terrigenous input materials, and autochthonous DOM predominantly originates from the microbial decomposition of organic

**Citation:** Zhang, H.; Liu, Z.; Xu, J.; Yang, J.; Zhang, X.; Tao, S. Characterization of Dissolved Organic Matter of Sediments in Urban Streams Using EEMs– PARAFAC and Absorption Spectroscopy: A Case Study in Wuhan, China. *Water* **2022**, *14*, 3181. https://doi.org/10.3390/w14193181

Academic Editors: Qiting Zuo, Fuqiang Wang, Jiaqi Zhai, Xiuyu Zhang, Dunxian She, Lei Zou, Rong Gan, Zengliang Luo and Roohollah Noori

Received: 24 August 2022 Accepted: 6 October 2022 Published: 10 October 2022

**Publisher's Note:** MDPI stays neutral with regard to jurisdictional claims in published maps and institutional affiliations.

**Copyright:** © 2022 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (https:// creativecommons.org/licenses/by/ 4.0/).

matter and macrophytes and algae production [7,8]. DOM has been discussed in studies of biogeochemical processes in estuaries, reservoirs and lakes, due to its importance in element cycling as well as ecosystem function [9–11]. Chromophoric dissolved organic matter (CDOM) refers to the fraction of DOM based on the absorption of ultraviolet and photosynthetically active radiation [12]. In general, light absorption by CDOM can promote not only the primary production of aquatic ecosystems, but also photochemically induced transformations in natural waters [13].

During recent years, rapid industrialization and increased urbanization have brought about a variety of urban water problems, including an impact on aquatic ecosystems and the deterioration of water quality in urban streams [14]. The increase of impervious surface in urban catchments changes the hydrological process of urban streams, and enhanced water disturbance promotes the interactions between sediment and overlying water, especially under heavy rain conditions [15]. Many pollutants accumulated in sediments are then released into the overlying waters, among which the speciation, transformation and characterization of DOM have become a hot topic in water environmental protection studies [16]. Urbanization also has an effect on the components and characteristics of DOM in the water environment [17]. Previous studies have explored the interactions between DOM and water pollutants, such as TN and TP, and revealed the relationship between DOM in natural waters and water quality indexes [18]. DOM content in water has been reported to be regulated by sediment release, which provides a perspective on the response of water quality to sediment characteristics [19]. Previous studies have suggested that DOM from surface sediments and overlying waters exchanges at the water–sediment interface, increasing the proportion of autochthonous DOM [2]. Therefore, systematic analysis of DOM in sediments is helpful to provide scientific basis and management strategy for the control of water environmental pollution.

Wuhan is a typical city suffering from urban water problems, such as eutrophication, decreased self-purification capacity and water ecological degradation, which is a threat to the balance of aquatic ecosystems [20]. Point (mainly domestic sewage) and non-point pollution sources caused by runoff washing the roads in urban areas lead to spatial distribution differences of water pollution in Wuhan. Previous studies in Wuhan have mostly investigated the common water quality parameters of urban lakes [21]. Our team has also studied geochemical and isotopic characteristics of urban streams during the late spring of 2019 and revealed the fragmentation of urban hydrological connectivity [22]. Some studies have explored the water environment carrying capacity of urban lakes [23], and others have examined the distribution of organic compounds in the Yangtze River [24]. However, few studies have focused specifically on DOM of sediments in urban streams, especially using fluorescence spectroscopy, which can reflect water quality and the characteristics and variations of fluorescent components simultaneously. Analyzing DOM is a rapid and effective way to monitor the current situation and source of water pollution, especially when non-point pollution is hard to detect. The main purposes of this research were to (1) characterize properties of sediments in urban streams, including the CDOM absorption properties, DOM components and their abundance, etc.; (2) reveal the effect of urbanization on CDOM properties and DOM composition; (3) explore the correlations between water quality, CDOM and DOM in Wuhan.

### **2. Materials and Methods**

#### *2.1. Site Description and Sample Collection*

Wuhan (113◦410–115◦050 E and 29◦580–31◦220 N), the capital of Hubei Province, is located in central China and the middle reaches of the Yangtze River (Figure 1). Precipitation is abundant and unevenly distributed within the year, with an annual average rainfall of 1140–1265 mm [25]. As a new first-tier city in China, the resident population of Wuhan is increasing year by year, reaching 12.45 million in 2020 [26]. The urban core consists of seven districts and accounts for approximately 57.89% of Wuhan's population. As an important national transportation junction, Wuhan is obviously a modern metropolis with rapid economic development and intense human activities. Wuhan has an inland area of 2217.6 km<sup>2</sup> , accompanied by two major rivers (the Yangtze and Han rivers) and plenty of lakes, reservoirs and urban streams (shallow waterbodies and canals) that are interconnected [25]. Due to the development of commerce, tourism and industrial activities, water environment problems, including water pollution, eutrophication and the reduction of biodiversity, have gradually become serious in the urban streams in Wuhan. water samples and 14 sediment samples were collected in urban streams of the central regions, including five districts. In addition, samples from Mulan Lake, far from the central districts, were also collected as the reference for non-urban areas, totaling five water samples and one sediment sample. In general, the urban streams in this study were shallow in depth, ranging from 0.2 m to 2.2 m, averaging 1.15 m. The samples were quickly collected and then transported to the laboratory in dark and cold conditions.

biodiversity, have gradually become serious in the urban streams in Wuhan.

seven districts and accounts for approximately 57.89% of Wuhan's population. As an important national transportation junction, Wuhan is obviously a modern metropolis with rapid economic development and intense human activities. Wuhan has an inland area of 2217.6 km2, accompanied by two major rivers (the Yangtze and Han rivers) and plenty of lakes, reservoirs and urban streams (shallow waterbodies and canals) that are interconnected [25]. Due to the development of commerce, tourism and industrial activities, water environment problems, including water pollution, eutrophication and the reduction of

An urban stream and sediment sampling program was conducted during a rainless period in mid-August, 2020. For each urban stream, three water samples were sampled along the stream flow. Owing to the weak fluidity and relative stability of sediments, only one sediment sample was taken by the grab sampler from each urban stream. In total, 42

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**Figure 1.** Location map of the study area, and the water and sediment sampling sites. **Figure 1.** Location map of the study area, and the water and sediment sampling sites.

*2.2. Sample Pretreatment and Biogeochemical Parameter Measurement*  Water temperature (Temp), pH, dissolved oxygen (DO), turbidity (Turb), salinity (Sal), chlorophyll a (Chl-a), oxidation-reduction potential (ORP), and electrical conductivity (EC) were measured in situ using a Hydrolab DS5 multi-probe water quality analyzer (Hach company, Loveland, CO, USA). Concentrations of heavy metals were determined using an inductively coupled plasma mass spectrometer (ICP-MS) (Nexion 350, PerkinElmer, Waltham, MA, USA) using the sample measurement method described in detail in our previous work [22]. An urban stream and sediment sampling program was conducted during a rainless period in mid-August, 2020. For each urban stream, three water samples were sampled along the stream flow. Owing to the weak fluidity and relative stability of sediments, only one sediment sample was taken by the grab sampler from each urban stream. In total, 42 water samples and 14 sediment samples were collected in urban streams of the central regions, including five districts. In addition, samples from Mulan Lake, far from the central districts, were also collected as the reference for non-urban areas, totaling five water samples and one sediment sample. In general, the urban streams in this study were shallow in depth, ranging from 0.2 m to 2.2 m, averaging 1.15 m. The samples were quickly collected and then transported to the laboratory in dark and cold conditions.

#### *2.2. Sample Pretreatment and Biogeochemical Parameter Measurement*

Water temperature (Temp), pH, dissolved oxygen (DO), turbidity (Turb), salinity (Sal), chlorophyll a (Chl-a), oxidation-reduction potential (ORP), and electrical conductivity (EC) were measured in situ using a Hydrolab DS5 multi-probe water quality analyzer (Hach company, Loveland, CO, USA). Concentrations of heavy metals were determined using an inductively coupled plasma mass spectrometer (ICP-MS) (Nexion 350, PerkinElmer, Waltham, MA, USA) using the sample measurement method described in detail in our previous work [22].

Sediment samples were screened through 100 mesh after natural air drying and grinding. The contents of carbon and nitrogen (C and N) in the sediments were then determined

by an elemental analyzer (Elementar Vario Macro cube, Hanau, Germany). Sediments DOM were extracted by preparing 200 g/L suspended solution, stirred continuously for 24 h at a velocity of 70 r/min and a constant temperature of 25 ◦C without light. After centrifuging at a speed of 7500 r/min for 5 min and standing for 30 min, the obtained solution was then filtered with 0.22 µm polyethersulfone filter membrane [19]. CDOM and DOM were measured using the filtered supernatant, and the analysis methods are described in detail in Sections 2.3 and 2.4.

#### *2.3. CDOM Absorption Analysis*

A Shimadzu UV-3600 UV-Vis spectrophotometer was used to determine the UV-Vis spectra of filtered supernatant. The CDOM absorption coefficient *a*(λ) was expressed as Equation (1) [13,27]:

$$a(\lambda) = 2.303D(\lambda)/L \tag{1}$$

where *D*(*λ*) is the absorbance after deducting the corresponding absorbance of Milli-Q water, and *L* is the cuvette length (m). The absorption coefficient of CDOM at 254 nm (*a*254) was used to quantify the content of CDOM, with higher *a*<sup>254</sup> indicating higher concentration of CDOM.

The spectral slope parameter *S* was computed by Equation (2) [27]:

$$a(\lambda) = a(\lambda\_0) \mathbf{e}^{\left(S(\lambda\_0 - \lambda)\right)} \tag{2}$$

where *λ*<sup>0</sup> is the reference wavelength (440 nm). *S*275–295 and *S*350–400 refer to the spectral slopes of 275–295 nm and 350–400 nm, respectively. The humification signal of CDOM is inversely proportional to *S*. The lower the *S*275–295 value, the stronger the terrestrial humic acid signal [28].

The calculation formula of the spectral slope ratio (*S*R) is shown in Equation (3) and is sensitive to the characteristics of CDOM [27]. A lower *S*<sup>R</sup> value indicates higher DOM molecular weight and greater aromaticity [27,29].

$$S\_R = \frac{S\_{275-295}}{S\_{350-400}}\tag{3}$$

#### *2.4. DOM Fluorescence Measurement*

The excitation–emission matrix spectra (EEMs) of filtered supernatant were measured using a FS5 fluorescence spectrometer (Edinburgh Instruments, Scotland) [30]. The EEMs of Milli-Q water were measured to remove the interferences of water Raman scattering peaks. Moreover, the fluorescence intensity of 3D fluorescence spectra was calibrated as a Raman unit (R.U.) by the ratio of fluorescence integral intensity of Milli-Q water at the wavelength of 350 nm (Ex) and 371–428 nm (Em).

Three fluorescent indexes, including the fluorescence index (FI), biological index (BIX) and humification index (HIX), were computed to further distinguish the source of DOM and quantify humification degree. The calculation formulae are shown in Equations (4)–(6) [31–33]:

$$\text{FI} = \frac{F\_{\text{E}\_{\text{x}}=370 nm, E\_{\text{m}}=470 nm}}{F\_{\text{E}\_{\text{x}}=370 nm, E\_{\text{m}}=520 nm}} \tag{4}$$

$$\text{BIX} = \frac{F\_{\text{E}\_{\text{x}}=310 nm, \text{E}\_{\text{m}}=380 nm}}{F\_{\text{E}\_{\text{x}}=310 nm, \text{E}\_{\text{m}}=450 nm}} \tag{5}$$

$$\text{HIX} = \frac{A\_{\text{Ex}=254nm,\text{Em}=435-480nm}}{A\_{\text{Ex}=254nm,\text{Em}=300-345nm}} \tag{6}$$

where *F* is the fluorescence intensity and *A* is the integrated intensity. FI was usually applied to analyze the source of DOM, with higher FI indicating more autochthonous DOM from microbial decomposition [34]. BIX was generally used to estimate the relative contribution of autochthonous DOM, and HIX was used to characterize the humification degree [35,36].

#### *2.5. Data Analysis*

Three fluorescent components of EEMs of sediments were identified by parallel factor analysis (PARAFAC) using the "DOMFluor" toolbox on the MATLAB platform, after Raman and Rayleigh scattering of spectral data were removed. For the accuracy and reliability of the results obtained, the optimal fluorescence fraction was extracted through half-split verification and residual examination. Redundancy analysis (RDA) and Spearman's correlation analysis were performed on the standardized PARAFAC components, calculated fluorescent indexes and biogeochemical parameters, using Canoco5 and R 4.2.0, respectively.

#### **3. Results and Discussion**

#### *3.1. Biogeochemical Characteristics*

The biogeochemical parameters of water and sediment samples in urban streams and the reference are presented in Table 1.

**Table 1.** Biogeochemical parameters of water and sediment samples in urban streams and the reference.


Note: ND refers to not detected.

The pH of urban streams ranged from weakly acidic to weakly alkaline. Clearly, physicochemical parameters of urban streams in Wuhan varied greatly, especially DO, Turb and EC. The mean concentrations of Chl-a (6.11 ± 10.72 µg/L) in urban streams were relatively low compared with the content of Chl-a of urban lakes previously reported in Wuhan, with a mean of 114.56 µg/L [37]. Some studies have also shown that Chl-a concentrations in urban rivers were significantly higher, compared to peri-urban rivers [38], which is in agreement with our research. Increased impervious surfaces lead to urban rainfall runoff being discharged into urban streams rather than seeping directly downward [39]. Enriching nutrients such as nitrogen and phosphorus in urban streams increase the Chl-a content and contribute to algae bloom. Less than half of urban streams (46.7%) had DO concentrations higher than 3 mg/L (Grade III surface water quality standards). The concentration of DO in the non-urban area was higher, with an average close to 7.5 mg/L (Grade I). As confirmed by the previous study, urban streams that received point source pollution, mainly through sewage effluent, had a lower value of DO [40]. The impact of domestic sewage, industrial wastewater and runoff pollution on urban streams may be serious, as water volume of these streams is not large [39]. The mean ORP in urban streams was 336.4 ± 64.1 mV, slightly less than the reference water (395.0 ± 31.9 mV), revealing a lower oxidability. Compared with the EC of the reference water (71.08 ± 1.13 µS/cm), urban streams were clearly less purified, with a mean EC of 383.9 ± 243.5 µS/cm. The C/N of sediments was within a wide range, demonstrating that sediments collected were considerably different in physical properties.

Heavy metal in natural surface water is generally at low values. However, with the development of agriculture and industries in urban areas, a mass of wastewater derived from anthropogenic activities has carried lots of heavy metals into surface water, resulting in severe heavy metal pollution in some urban streams [41]. Therefore, many studies pay attention to the types and contents of heavy metals when investigating the urban water environment. In this research, the content of heavy metals has also been examined. The results demonstrated that concentrations of heavy metals in urban streams were mostly higher than those in the reference, except for Cr (Figure 2). Overall, urban streams in Wuhan contain metals at relatively low levels, with the concentrations of Ni, Cu, Cr, Pb lower than 10 µg/L. were considerably different in physical properties. Heavy metal in natural surface water is generally at low values. However, with the development of agriculture and industries in urban areas, a mass of wastewater derived from anthropogenic activities has carried lots of heavy metals into surface water, resulting in severe heavy metal pollution in some urban streams [41]. Therefore, many studies pay attention to the types and contents of heavy metals when investigating the urban water environment. In this research, the content of heavy metals has also been examined. The results demonstrated that concentrations of heavy metals in urban streams were mostly higher than those in the reference, except for Cr (Figure 2). Overall, urban streams in Wuhan contain metals at relatively low levels, with the concentrations of Ni, Cu, Cr, Pb lower than 10 μg/L.

streams was 336.4 ± 64.1 mV, slightly less than the reference water (395.0 ± 31.9 mV), revealing a lower oxidability. Compared with the EC of the reference water (71.08 ± 1.13 μS/cm), urban streams were clearly less purified, with a mean EC of 383.9 ± 243.5 μS/cm. The C/N of sediments was within a wide range, demonstrating that sediments collected

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**Figure 2.** Variations in the concentrations of heavy metals (μg/L) in the urban streams. The blue dashed line refers to the reference level. **Figure 2.** Variations in the concentrations of heavy metals (µg/L) in the urban streams. The blue dashed line refers to the reference level.

#### *3.2. CDOM Absorption Characteristics 3.2. CDOM Absorption Characteristics*

As demonstrated in Figure 3, absorbance decreases exponentially with increased wavelength, except for the absorption peaks in 200–230 nm and 260–280 nm. There was no absorption peak in the CDOM absorption spectra of sediments in most urban streams, consistent with that of natural waterbodies [27]. The CDOM absorption coefficients of sediments in most urban streams were apparently higher than the reference, with high and flat curves for urban streams. It is well known that inorganic ions have significant UV absorption at wavelengths lower than 230 nm [42]. CDOM absorption spectra of sediments in some urban streams showed a similar strong adsorption peak. Low intensity absorption peaks in the 260–280 nm wavelength range may be induced by an unconjugated group with lone pair electrons, such as carboxyl [42]. As demonstrated in Figure 3, absorbance decreases exponentially with increased wavelength, except for the absorption peaks in 200–230 nm and 260–280 nm. There was no absorption peak in the CDOM absorption spectra of sediments in most urban streams, consistent with that of natural waterbodies [27]. The CDOM absorption coefficients of sediments in most urban streams were apparently higher than the reference, with high and flat curves for urban streams. It is well known that inorganic ions have significant UV absorption at wavelengths lower than 230 nm [42]. CDOM absorption spectra of sediments in some urban streams showed a similar strong adsorption peak. Low intensity absorption peaks in the 260–280 nm wavelength range may be induced by an unconjugated group with lone pair electrons, such as carboxyl [42].

Among 15 sediment samples, *a*<sup>254</sup> ranged from 25.7 m−<sup>1</sup> to 197.6 m−<sup>1</sup> , with an average of 116.32 <sup>±</sup> 60.5 m−<sup>1</sup> for urban streams and 51.52 m−<sup>1</sup> for the reference, demonstrating that CDOM contents of sediments in most urban streams were significantly higher than that in the reference. This may be attributed to the large magnitude of conjugated structure and high degree of humification of CDOM in sediments from urban streams. The variation of CDOM of sediments in urban streams normally corresponds to those of urban waters [18]. In a previous investigation, values of *a*<sup>254</sup> of natural waters from rivers were reported to be higher than those of lakes and coastal marine environments, with means of 50.5 m−<sup>1</sup> , 13.2 m−<sup>1</sup> and 6.4 m−<sup>1</sup> , although all at a relatively low level [43]. Moreover, *a*<sup>254</sup> of sediments in urban streams exhibited considerable variability, even in the same district.

A previous study has reported that higher amounts of CDOM were observed in urban waters, due to high population density, water pollution and an increase of surrounding artificial surfaces [5]. These were also thought to be the factors leading to the variation and higher concentration of CDOM of sediments in the study area. *Water* **2022**, *14*, x FOR PEER REVIEW 7 of 16

**Figure 3.** UV-Vis absorption spectra of CDOM in sediments from urban streams and the reference, with special curves in the upper left and lower right corner. **Figure 3.** UV-Vis absorption spectra of CDOM in sediments from urban streams and the reference, with special curves in the upper left and lower right corner.

Among 15 sediment samples, *a*254 ranged from 25.7 m−1 to 197.6 m−1, with an average of 116.32 ± 60.5 m−1 for urban streams and 51.52 m−1 for the reference, demonstrating that CDOM contents of sediments in most urban streams were significantly higher than that in the reference. This may be attributed to the large magnitude of conjugated structure and high degree of humification of CDOM in sediments from urban streams. The variation of CDOM of sediments in urban streams normally corresponds to those of urban wa-The *S*275–295 varied greatly, from 14.6 µm−<sup>1</sup> to 24.9 µm−<sup>1</sup> , and the mean was 17.88 <sup>±</sup> 2.66 <sup>µ</sup>m−<sup>1</sup> for urban streams and 18.8 µm−<sup>1</sup> for the reference. *S*275–295 of sediments in urban streams ranged widely, indicating differences in their content, molecular weight and sources of CDOM. Since *S*275–295 was also an indicator of DOM sources, sediments of urban streams were inferred to contain different amounts of allochthonous DOM from terrigenous substances (mainly humus-like matter with a large molecular weight) [44].

ters [18]. In a previous investigation, values of *a*254 of natural waters from rivers were reported to be higher than those of lakes and coastal marine environments, with means of 50.5 m−1, 13.2 m−1 and 6.4 m−1, although all at a relatively low level [43]. Moreover, *a*254 of sediments in urban streams exhibited considerable variability, even in the same district. A previous study has reported that higher amounts of CDOM were observed in urban waters, due to high population density, water pollution and an increase of surrounding artificial surfaces [5]. These were also thought to be the factors leading to the variation and higher concentration of CDOM of sediments in the study area. The *S*275–295 varied greatly, from 14.6 μm−1 to 24.9 μm−1, and the mean was 17.88 ± 2.66 μm−1 for urban streams and 18.8 μm−1 for the reference. *S*275–295 of sediments in urban The mean value for *S*<sup>R</sup> (0.93 ± 0.13) of sediments in urban streams was higher than in the reference (0.88), indicating a lower molecular weight of CDOM. *S*R, representing the structural changes of DOM, is inversely correlated with the molecular weight of DOM. A small *S*<sup>R</sup> indicates that DOM is mostly newly generated (photocatalytic oxidation or microbial activities) or mainly imported from external sources. On the contrary, a larger *S*<sup>R</sup> demonstrates that DOM is mainly endogenous (self-decomposition of microorganisms in water) or that photobleaching is strong [28]. The fact that *S*<sup>R</sup> of sediments in urban streams ranged extensively apparently indicates enormously different DOM sources, mainly autochthonous and allochthonous, respectively.

#### streams ranged widely, indicating differences in their content, molecular weight and *3.3. Variations and Characteristics of DOM*

sources of CDOM. Since *S*275–295 was also an indicator of DOM sources, sediments of urban streams were inferred to contain different amounts of allochthonous DOM from terrigenous substances (mainly humus-like matter with a large molecular weight) [44]. The mean value for *S*R (0.93 ± 0.13) of sediments in urban streams was higher than Based on the PARAFAC model, three-dimensional fluorescence spectra analysis and split half validation were carried out on the sediment samples, and three DOM components were eventually identified (Table 2 and Figure 4).



tochthonous and allochthonous, respectively.

*3.3. Variations and Characteristics of DOM* 

nents were eventually identified (Table 2 and Figure 4).

**Figure 4.** Identification of the three components extracted from the PARAFAC model. (**a**–**c**): contour plots of C1, C2 and C3. (**d**–**f**): excitation and emission loadings of C1, C2 and C3. **Figure 4.** Identification of the three components extracted from the PARAFAC model. (**a**–**c**): contour plots of C1, C2 and C3. (**d**–**f**): excitation and emission loadings of C1, C2 and C3.

Based on the PARAFAC model, three-dimensional fluorescence spectra analysis and split half validation were carried out on the sediment samples, and three DOM compo-

**Table 2.** Descriptions of the three fluorescence components identified by PARAFAC analysis.

C1 260, 365 475 Humic-like acid C2 325 402 Humic-like (fulvic) acid C3 275 334 Protein-like (tryptophan) component

**Component Ex (nm) Em (nm) Type** 

Component1 (C1) demonstrated two fluorescence peaks at an Ex/Em wavelength of 260 nm (365 nm)/475 nm, consistent with humic-like peaks, as previously reported [6,45]. Component2 (C2), with a primary fluorescence peak at an Ex/Em wavelength of 325 nm/402 nm, was similar to fulvic-like acid in the visible region, a humus-like substance with a relatively small molecular weight. The fulvic acid was considered to be derived from the degradation and transformation of macromolecular humus-like substances [46]. Component3 (C3) showed a primary fluorescence peak with Ex/Em wavelength of 275 nm/334 nm and was considered as a tryptophan-like matter, related to carboxyl functional groups and aromatic protein structures generated by microbial degradation [4,47]. C2/C1 is the ratio of humic-like acid and fulvic-like acid fluorescence peaks, which is often used to characterize the composition of humus and reflect the degree of humus aggregation to a certain extent. C2/C1 of sediments in urban streams were in a wide range and mostly lower than the reference, with means of (1.53 ± 0.36) and 1.88, respectively, accounting for the variation of the humus aggregation degree and worse quality of sediments in urban streams. Component1 (C1) demonstrated two fluorescence peaks at an Ex/E<sup>m</sup> wavelength of 260 nm (365 nm)/475 nm, consistent with humic-like peaks, as previously reported [6,45]. Component2 (C2), with a primary fluorescence peak at an Ex/E<sup>m</sup> wavelength of 325 nm/402 nm, was similar to fulvic-like acid in the visible region, a humus-like substance with a relatively small molecular weight. The fulvic acid was considered to be derived from the degradation and transformation of macromolecular humus-like substances [46]. Component3 (C3) showed a primary fluorescence peak with Ex/E<sup>m</sup> wavelength of 275 nm/334 nm and was considered as a tryptophan-like matter, related to carboxyl functional groups and aromatic protein structures generated by microbial degradation [4,47]. C2/C1 is the ratio of humic-like acid and fulvic-like acid fluorescence peaks, which is often used to characterize the composition of humus and reflect the degree of humus aggregation to a certain extent. C2/C1 of sediments in urban streams were in a wide range and mostly lower than the reference, with means of (1.53 ± 0.36) and 1.88, respectively, accounting for the variation of the humus aggregation degree and worse quality of sediments in urban streams.

The maximum fluorescence intensity (Fmax) of components extracted, based on the PARAFAC model, can be used to characterize their abundance (Figure 5a). The total fluorescence intensity (FT) and component fluorescence intensity of sediments in urban streams were significantly higher than those in the reference. It is worth noting that F<sup>T</sup> of sediments in urban streams was nearly four times that of the reference, with means of 2.51 ± 0.97 R.U. and 0.65 R.U., respectively, indicating a higher DOM concentration of sediments in urban streams. A large amount of nutrients and organic matter such as humus-like substances from runoff input were found to be carried into urban streams by point source or non-point source pollution, resulting in higher allochthonous organic matter in waterbodies [48]. Therefore, it is reasonable to believe that the external inputs influenced by the process of urbanization increase the DOM content of sediments in urban streams. In general, the concentration of C3 of sediments in most urban sampling sites was the highest and ranged widely, with a distinct difference in the spatial distribution. Although slightly lower than C3, the content of C2 was rather uniformly distributed. Moreover,

the concentration of C1 was clearly the lowest, demonstrating that the humus of DOM of sediments in most urban streams was mainly composed of micromolecules. slightly lower than C3, the content of C2 was rather uniformly distributed. Moreover, the concentration of C1 was clearly the lowest, demonstrating that the humus of DOM of sediments in most urban streams was mainly composed of micromolecules.

The maximum fluorescence intensity (Fmax) of components extracted, based on the PARAFAC model, can be used to characterize their abundance (Figure 5a). The total fluorescence intensity (FT) and component fluorescence intensity of sediments in urban streams were significantly higher than those in the reference. It is worth noting that FT of sediments in urban streams was nearly four times that of the reference, with means of 2.51 ± 0.97 R.U. and 0.65 R.U., respectively, indicating a higher DOM concentration of sediments in urban streams. A large amount of nutrients and organic matter such as humuslike substances from runoff input were found to be carried into urban streams by point source or non-point source pollution, resulting in higher allochthonous organic matter in waterbodies [48]. Therefore, it is reasonable to believe that the external inputs influenced by the process of urbanization increase the DOM content of sediments in urban streams. In general, the concentration of C3 of sediments in most urban sampling sites was the highest and ranged widely, with a distinct difference in the spatial distribution. Although

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**Figure 5.** Distribution of the (**a**) abundance and (**b**) relative abundance of three PARAFAC components of the sediment samples in Wuhan; 1–14 refer to the urban streams, 15 refers to the reference. **Figure 5.** Distribution of the (**a**) abundance and (**b**) relative abundance of three PARAFAC components of the sediment samples in Wuhan; 1–14 refer to the urban streams, 15 refers to the reference.

The relative abundance of three fluorescence components is displayed in Figure 5b. The average relative abundance of C2 of sediments in urban streams was highest, C3 was the next highest and C1 was the lowest. Humus-like substances (C1 + C2) were significantly higher than protein-like matter (C3), with means of 66.81% and 33.19%, respectively. Humus-like components are commonly considered to be mainly related to external inputs, including runoff input and farmland water regression [49]. Protein-like components are commonly supposed to be derived from domestic sewage and also probably originate from microbial degradation of algae or aquatic plant residues [50]. The relative abundance of humus-like and protein-like components were possibly associated with land use, population density and the economic system, as there were obvious individual differences in each sampling sediment. Research in Jilin Province has also found that urban waters have higher relative abundance of humic-like acid (74%), accompanied by lower relative abundance of protein-like substances (26%) [5]. In this study, more than 71% of sediments in urban streams were characterized by lower relative abundance of C3 and higher relative abundance of C1 + C2 compared to the reference, indicating that DOM of sediments is influenced by urbanization, with increasing humus content and decreasing tryptophan content. The results above indicate that DOM of sediments in urban streams was affected by both allochthonous and autochthonous origins, with the terres-The relative abundance of three fluorescence components is displayed in Figure 5b. The average relative abundance of C2 of sediments in urban streams was highest, C3 was the next highest and C1 was the lowest. Humus-like substances (C1 + C2) were significantly higher than protein-like matter (C3), with means of 66.81% and 33.19%, respectively. Humus-like components are commonly considered to be mainly related to external inputs, including runoff input and farmland water regression [49]. Protein-like components are commonly supposed to be derived from domestic sewage and also probably originate from microbial degradation of algae or aquatic plant residues [50]. The relative abundance of humus-like and protein-like components were possibly associated with land use, population density and the economic system, as there were obvious individual differences in each sampling sediment. Research in Jilin Province has also found that urban waters have higher relative abundance of humic-like acid (74%), accompanied by lower relative abundance of protein-like substances (26%) [5]. In this study, more than 71% of sediments in urban streams were characterized by lower relative abundance of C3 and higher relative abundance of C1 + C2 compared to the reference, indicating that DOM of sediments is influenced by urbanization, with increasing humus content and decreasing tryptophan content. The results above indicate that DOM of sediments in urban streams was affected by both allochthonous and autochthonous origins, with the terrestrial inputs being slightly higher.

trial inputs being slightly higher. FI varied from 1.53 to 1.76, distributed within the range of 1.4–1.9, indicating the simultaneous presence of terrigenous and biogenic DOM (Figure 6). Most BIX of sediments in urban streams was within 0.8–1.0, demonstrating the moderate autochthonous inputs. Nonetheless, BIX of some sediment samples was less than 0.8 or higher than 1.0, indicating distinct spatial distribution differences and multiple sources of DOM of sediments in urban streams. The range of HIX was from 1.04 to 5.30, providing evidence of various degrees of humification, since HIX represents DOM humification degree. Significantly higher HIX was observed compared to the reference, demonstrating that DOM of sediments in urban streams were more humified and aromatic. This discovery was consistent with a previous study, which reported that urban waterbodies had a wide range of fluorescence indexes, due to differences in land use types [51]. The influence of allochthonous and autogenous sources on DOM of surface sediments has also been found in a past case study [52]. The lower FI and lower BIX, along with higher HIX, than the reference indicates that DOM of sediments in urban streams in Wuhan were more affected by the terrestrial input and less generated by aquatic organisms with higher degree of humification.

FI varied from 1.53 to 1.76, distributed within the range of 1.4–1.9, indicating the simultaneous presence of terrigenous and biogenic DOM (Figure 6). Most BIX of sediments in urban streams was within 0.8–1.0, demonstrating the moderate autochthonous inputs. Nonetheless, BIX of some sediment samples was less than 0.8 or higher than 1.0, indicating distinct spatial distribution differences and multiple sources of DOM of sediments in urban streams. The range of HIX was from 1.04 to 5.30, providing evidence of various degrees of humification, since HIX represents DOM humification degree. Significantly higher HIX was observed compared to the reference, demonstrating that DOM of sediments in urban streams were more humified and aromatic. This discovery was consistent with a previous study, which reported that urban waterbodies had a wide range of fluorescence indexes, due to differences in land use types [51]. The influence of allochthonous and autogenous sources on DOM of surface sediments has also been found in a past case study [52]. The lower FI and lower BIX, along with higher HIX, than the reference indicates that DOM of sediments in urban streams in Wuhan were more affected by the terrestrial input and less generated by aquatic organisms with higher degree of humification.

**Figure 6.** Fluorescence indexes of (**a**) FI and HIX (**b**) FI and BIX of DOM of sediments. The blue points refer to urban sampling sites, and the red ones were the reference. **Figure 6.** Fluorescence indexes of (**a**) FI and HIX (**b**) FI and BIX of DOM of sediments. The blue points refer to urban sampling sites, and the red ones were the reference.

#### *3.4. Relationship between DOM Optical Parameters and Water Environmental Factors 3.4. Relationship between DOM Optical Parameters and Water Environmental Factors*

The degradation of water ecosystems has always been a hot issue in urban rivers and also a threat to the sustainable development of water resources, making it essential to monitor and control water quality [14]. In general, surface water varies rather greatly under different scenarios. As a cumulative environmental medium, sediment could reflect the situation in past periods and the influence on future water quality. The content and components of DOM were influenced by the sources and concentrations of some biogeochemical factors, so the relationship among them has been widely studied in some oceans, rivers and lakes[11,18,53]. The RDA of optical characteristics and biogeochemical factors is shown in Figure 7. C1 was significantly positively correlated with C2, *a*254, BIX, Chl-a, Sal and EC. C2 and *a*254 share the same correlation with C1. C3 was positively correlated with both BIX and FI. HIX was significantly positively correlated with Temp, C/N, Turb, DO, pH and ORP, and negatively correlated with FI and HIX. The degradation of water ecosystems has always been a hot issue in urban rivers and also a threat to the sustainable development of water resources, making it essential to monitor and control water quality [14]. In general, surface water varies rather greatly under different scenarios. As a cumulative environmental medium, sediment could reflect the situation in past periods and the influence on future water quality. The content and components of DOM were influenced by the sources and concentrations of some biogeochemical factors, so the relationship among them has been widely studied in some oceans, rivers and lakes [11,18,53]. The RDA of optical characteristics and biogeochemical factors is shown in Figure 7. C1 was significantly positively correlated with C2, *a*254, BIX, Chl-a, Sal and EC. C2 and *a*<sup>254</sup> share the same correlation with C1. C3 was positively correlated with both BIX and FI. HIX was significantly positively correlated with Temp, C/N, Turb, DO, pH and ORP, and negatively correlated with FI and HIX. *Water* **2022**, *14*, x FOR PEER REVIEW 11 of 16

**Figure 7.** RDA of spectral parameters and biogeochemical factors. which might be due to the different source of some industrial wastewater around the sam- **Figure 7.** RDA of spectral parameters and biogeochemical factors.

Since C1 and C2 were positively correlated and had similar correlations with other factors, it could be concluded that they might be obtained from common allochthonous

DOM. The non-fluorescent organic matter in phytoplankton can be transformed and degraded into humus-like fluorescent substances by bacteria, contributing to the positive correlation between Chl-a, C1 and C2. Some ions have been shown to be able to complexate or chelate with soil humus, which explains the positive correlation between EC and C1, as well as C2 [54]. Since Sal and EC are interchangeable, they showed the same correlation with C1 and C2. The positive correlation between C3, FI and BIX demonstrates that tryptophan-like substances might be closely associated with the proportion of autochthonous DOM and allochthonous DOM. Protein-like substances with small molecular weight have lower C/N ratios, which fully explains the negative correlation between C3 and C/N. The inverse correlations between HIX and FI, and between HIX and BIX, indicate the higher fluorescence intensity, the more allochthonous DOM and the lower the degree of humification. A previous study has reported that DO was regarded as an oxidant in accelerating the generation of organic matters in waterbodies [55]. Due to the positive correlation between HIX and other physicochemical parameters, we can conclude that the degree of humification of DOM of sediments was partly influenced by overlying water. Further research is still required to reveal the relationships between DOM of sediments

The relationship between DOM components of sediments and heavy metals was explored by Spearman's correlation analysis (Figure 8). The results showed positive correlations between most heavy metals, indicating that they might be derived from a similar origin of water pollution. However, Cr was negatively correlated with all other metals,

and water quality.

Since C1 and C2 were positively correlated and had similar correlations with other factors, it could be concluded that they might be obtained from common allochthonous sources. The strong positive correlation between C1, C2 and *a*254, and relatively weak correlation between C3 and *a*254, reiterated that humic-like acid was the major component of DOM. The non-fluorescent organic matter in phytoplankton can be transformed and degraded into humus-like fluorescent substances by bacteria, contributing to the positive correlation between Chl-a, C1 and C2. Some ions have been shown to be able to complexate or chelate with soil humus, which explains the positive correlation between EC and C1, as well as C2 [54]. Since Sal and EC are interchangeable, they showed the same correlation with C1 and C2. The positive correlation between C3, FI and BIX demonstrates that tryptophan-like substances might be closely associated with the proportion of autochthonous DOM and allochthonous DOM. Protein-like substances with small molecular weight have lower C/N ratios, which fully explains the negative correlation between C3 and C/N. The inverse correlations between HIX and FI, and between HIX and BIX, indicate the higher fluorescence intensity, the more allochthonous DOM and the lower the degree of humification. A previous study has reported that DO was regarded as an oxidant in accelerating the generation of organic matters in waterbodies [55]. Due to the positive correlation between HIX and other physicochemical parameters, we can conclude that the degree of humification of DOM of sediments was partly influenced by overlying water. Further research is still required to reveal the relationships between DOM of sediments and water quality.

The relationship between DOM components of sediments and heavy metals was explored by Spearman's correlation analysis (Figure 8). The results showed positive correlations between most heavy metals, indicating that they might be derived from a similar origin of water pollution. However, Cr was negatively correlated with all other metals, which might be due to the different source of some industrial wastewater around the sampling sites. Interestingly, all three components were positively correlated with most metals, especially Ni, Fe and Pb. A previous investigation has confirmed the toxic metal (Ni2+, Pb2+) binding affinity of DOM, and that the metal complexing capacity was influenced by DOM concentrations, structure and components [56]. It is considered that the existence of DOM in surface waters might reduce the precipitation of heavy metals by inhibiting the formation of carbonate and hydroxide precipitations, and then enhancing their migration ability [57,58]. However, when heavy metals and macro-molecular DOM form insoluble chelates, the migration ability of heavy metals may be decreased instead, which would lead to the precipitation of DOM as well as metals [59]. Some studies have also considered that the existence of metals may promote the aggregation of humic substances by decreasing the intermolecular repulsion [60]. The presence of such processes might be also influenced by salinity, as it was found in RDA analysis that EC and Sal in surface water had a significantly positive correlation with three components of sediments. Therefore, heavy metals in overlying water may enhance the accumulation of DOM in sediments. Heavy metals, especially Fe and Mn, with relatively higher concentration levels, were not significantly correlated with FI and BIX, because heavy metals in urban streams were mainly from allochthonous sources (point and non-point water pollution), while the sources of DOM included allochthonous and autochthonous inputs. In addition, the relatively weak negative correlation between Fe/Mn and HIX indicates that heavy metals have a slight negative influence on the humification process, i.e., the biological transformation process. Overall, the pollution by heavy metals was not serious and interactions between heavy metals and DOM were not obvious in urban streams in Wuhan.

metals and DOM were not obvious in urban streams in Wuhan.

pling sites. Interestingly, all three components were positively correlated with most metals, especially Ni, Fe and Pb. A previous investigation has confirmed the toxic metal (Ni2+, Pb2+) binding affinity of DOM, and that the metal complexing capacity was influenced by DOM concentrations, structure and components [56]. It is considered that the existence of DOM in surface waters might reduce the precipitation of heavy metals by inhibiting the formation of carbonate and hydroxide precipitations, and then enhancing their migration ability [57,58]. However, when heavy metals and macro-molecular DOM form insoluble chelates, the migration ability of heavy metals may be decreased instead, which would lead to the precipitation of DOM as well as metals [59]. Some studies have also considered that the existence of metals may promote the aggregation of humic substances by decreasing the intermolecular repulsion [60]. The presence of such processes might be also influenced by salinity, as it was found in RDA analysis that EC and Sal in surface water had a significantly positive correlation with three components of sediments. Therefore, heavy metals in overlying water may enhance the accumulation of DOM in sediments. Heavy metals, especially Fe and Mn, with relatively higher concentration levels, were not significantly correlated with FI and BIX, because heavy metals in urban streams were mainly from allochthonous sources (point and non-point water pollution), while the sources of DOM included allochthonous and autochthonous inputs. In addition, the relatively weak negative correlation between Fe/Mn and HIX indicates that heavy metals have a slight negative influence on the humification process, i.e., the biological transformation process. Overall, the pollution by heavy metals was not serious and interactions between heavy

**Figure 8.** Spearman's correlation analysis results between heavy metals and (C)DOM optical characteristics of sediments. Blue color means positive correlations and the red refers to negative correlations. **Figure 8.** Spearman's correlation analysis results between heavy metals and (C)DOM optical characteristics of sediments. Blue color means positive correlations and the red refers to negative correlations.

#### **4. Conclusions**

In this study, the biogeochemical characteristics of waterbodies and (C)DOM of sediments in urban streams in Wuhan have been investigated. The results showed that CDOM and DOM concentrations of sediments in urban streams ranged widely and were at relatively high levels, with *<sup>a</sup>*<sup>254</sup> (116.32 <sup>±</sup> 60.5 m−<sup>1</sup> ) and F<sup>T</sup> (2.51 ± 0.97 R.U.) nearly twice and four times higher than the reference (51.52 m−<sup>1</sup> , 0.65 R.U.), respectively. The fluorescence components of sediments in urban streams identified by the EEM–PARAFAC model included humic-like acid (C1), fulvic-like acid (C2) and protein-like substance (C3). The calculated results of FI, BIX and HIX demonstrated that DOM of sediments in urban streams had allochthonous inputs and autochthonous production simultaneously and a high degree of humification. The relative abundance of humus-like acid (66.81%) was much higher than that of protein-like matter (33.19%), indicating a higher allochthonous input. Comparison of the urban and reference samples revealed that urbanization played a remarkable role in the increase of CDOM and DOM contents and the variation of DOM components and sources. RDA and Spearman's correlation analysis showed that DOM optical characteristics of sediments in urban streams were associated with most heavy metals and other water quality parameters. This study could support the use of the EEM–PARAFAC method to identify diverse sources of DOM, and it promotes a better understanding of DOM characteristics of sediments in urban streams and their relationship with biogeochemical parameters.

**Author Contributions:** Conceptualization, X.Z.; methodology, J.X. and Z.L.; software, H.Z.; validation, H.Z. and S.T.; formal analysis, H.Z. and Z.L.; investigation, H.Z., Z.L. and S.T.; resources, J.X. and J.Y.; writing—original draft preparation, H.Z.; writing—review and editing, S.T. and X.Z.; visualization, H.Z. and S.T.; supervision, S.T. and X.Z.; project administration, J.X., J.Y. and X.Z.; funding acquisition, J.X., J.Y. and X.Z. All authors have read and agreed to the published version of the manuscript.

**Funding:** This research was funded by the Strategic Priority Research Program of the Chinese Academy of Sciences (No. XDA23040302), the National Natural Science Foundation of China (No. 41890823) and the Key Research and Development Project of Hubei Province (No. 2021BCA128).

**Institutional Review Board Statement:** Not applicable.

**Informed Consent Statement:** Not applicable.

**Data Availability Statement:** The data that support the findings of this study are available from the authors upon reasonable request.

**Acknowledgments:** Comments from the anonymous reviewers are appreciated.

**Conflicts of Interest:** The authors declare no conflict of interest.

#### **References**

