Novel insights into variation of dissolved organic matter during textile wastewater treatment by fluorescence excitation emission matrix

Accepted Manuscript Novel insights into variation of dissolved organic matter during textile wastewater treatment by fluorescence excitation emission matrix Cheng Cheng, Jing Wu, Luodan You, Jiukai Tang, Yidi Chai, Bo Liu, Muhammad Farooq Saleem Khan PII: DOI: Reference:

S1385-8947(17)31773-4 https://doi.org/10.1016/j.cej.2017.10.059 CEJ 17843

To appear in:

Chemical Engineering Journal

Received Date: Revised Date: Accepted Date:

19 July 2017 9 September 2017 13 October 2017

Please cite this article as: C. Cheng, J. Wu, L. You, J. Tang, Y. Chai, B. Liu, M.F.S. Khan, Novel insights into variation of dissolved organic matter during textile wastewater treatment by fluorescence excitation emission matrix, Chemical Engineering Journal (2017), doi: https://doi.org/10.1016/j.cej.2017.10.059

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Novel insights into variation of dissolved organic matter during textile wastewater treatment by fluorescence excitation emission matrix Cheng Chenga, Jing Wu a*, Luodan Youa, Jiukai Tanga, Yidi Chaia, Bo Liu a, Muhammad Farooq Saleem Khana a State Key Joint Laboratory of Environment Simulation and Pollution Control, School of Environment, Tsinghua University, Beijing 100084, China

*Corresponding author footnote E-mail: [email protected] Fax: +86 10 62785687 Phone: +86 10 62789121

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Abstract In this work, the textile wastewater samples along two full-scale treatment trains were characterized by fluorescence excitation emission matrix (EEM). Four fluorescent components (C1 – C4) were identified by parallel factor (PARAFAC) analysis. The polarity and apparent molecular weights (MW) of C1 – C4 were investigated by high performance liquid chromatography (HPLC). By comparison of EEM spectra and HPLC chromatograms, the tyrosine-like component (C1) and the tryptophan-like components (C2 and C3) were related to Dispersant MF. Component C4 with two emission peaks suggested the formation of an intramolecular exciplex. Despite the remarkable difference of untreated textile wastewater, the polarity and apparent MWs of C1 – C4 as well as their variations along the treatment trains (‘biological + oxidative’ processes) were highly similar between different wastewater treatment plants. The weights distribution of Dispersant MF among C1 – C4 indicates its dominance in the protein-like fluorescence of textile wastewater. Component C1 – C4 had different fate in various treatment stages. C1 – C4 were poorly removed (< 25%) during the anaerobic or anoxic process. A better removal efficiency of C3 (65% – 80%) than C1, C2 and C4 (15% – 50%) was achieved in the aerobic process. Both Fenton and chlorination process could significantly decrease the concentrations of C1 – C4 (70% – 100%). The concentration of C2 was strongly correlated with dissolved organic carbon (DOC), chemical oxygen demand (COD) and UV absorbance at 254 nm (UVA254). The fluorescence of C2 could be proposed as an indicator of COD and a supplement to UVA254 to evaluate treatment efficiency of textile wastewater. The results provide a better understanding of DOM variation of textile wastewater during treatment.

Keywords: Textile wastewater; Dissolved organic matter; Fluorescence excitation emission matrix; Parallel factor analysis; High performance liquid chromatography

1. Introduction Nomenclature and acronym in alphabetical order COD

Chemical oxygen demand

NH 3-N

Ammonia nitrogen

DAD

Diode array detector

PARAFAC

Parallel factor

DOC

Dissolved organic carbon

SEC

Size exclusion chromatography

DOM

Dissolved organic matter

SMPs

Soluble microbial products

EEM

Excitation emission matrix

SNFC

Sulfonated naphthalene

FLD

Fluorescence detector

FRI

Fluorescence regional integration

formaldehyde condensates SUVA254

Specific UV absorbance at 254 nm

HPLC

High performance liquid chromatography

TN

Total nitrogen

HRT

Hydraulic retention time

TP

Total phosphorus

IFE

Inner filter effect

UVA254

Ultraviolet absorbance at 254 nm

MW

Molecular weight

WWTP

Wastewater treatment plant

Due to rapid measurement, high sensitivity and good selectivity, fluorescence excitation emission matrix (EEM) is considered as a helpful tool to characterize dissolved organic matter (DOM) in natural and engineering water systems [1-3]. It can be used to distinguish different types of DOM (e.g. humic 2

substances, protein and various anthropogenic chemicals) according to their differences in light excitation/emission of fluorophores. EEM is very informative, and several methods including peak-picking, fluorescence regional integration (FRI) and parallel factor (PARAFAC) analysis have been developed to interpret EEM during the last two decades [4-6]. By multivariate analysis of PARAFAC, the complex fluorescence spectra could be mathematically decomposed into individual fluorescent components for both qualitative and quantitative analysis [7]. Besides, high performance liquid chromatography (HPLC) could relate polarity and molecular weight (MW) of fluorescent DOM to their EEM spectra, which could provide a more comprehensive understanding of fluorophores [8, 9]. EEM exhibited immense potential of real-time monitoring in drinking water and municipal wastewater treatment [3, 10]. Recently, increasing attention has been paid to the application of EEM to characterize DOM in industrial wastewater, including detection of wastewater pollution and control of wastewater treatment processes [11-13]. As one of the least eco-friendly industry sectors, the textile industry in developing countries, especially like China and India, generates an enormous amount of wastewater with high chroma and organic loads [14, 15]. The discharge of textile wastewater into the natural water without proper treatment will cause visual pollution and impose serious harm to aquatic organisms [16, 17]. The composition of real textile wastewater is very complex including various dyestuffs and auxiliaries, and the removal of DOM is the top concern during treatment [18]. Up to now, biological process is still the most extensively used method in practice [18, 19]. The DOM in bio-treated textile effluent includes soluble microbial products (SMPs) such as polysaccharide, protein and humic substances [20] besides various dyes, auxiliaries and their incomplete degradation products [21]. With increasingly stringent discharge standards in recent years, advanced treatment processes like Fenton oxidation and ozonation have been applied for bio-treated textile effluent to achieve further decoloration and COD removal [22, 23]. EEM was used to assess DOM removal performance of the advanced treatment process by illustrating the variation of protein-like and humic-like fluorescence. The origin of the fluorescent DOM in textile wastewater and its variation along the whole treatment train including biological and chemical process remain attractive to be explored, which is very meaningful to optimization and development of treatment process. So far, few studies have applied EEM to track DOM of textile wastewater along the full-scale treatment process. Hence, this study focuses on the following objectives: (1) to characterize DOM in textile wastewater from various treatment stages by EEM coupled with PARAFAC and HPLC analysis; (2) to investigate treatment efficiency and selectivity of DOM in different treatment stages; (3) to explore possible relations between water quality parameters and concentrations of fluorescent components. The results could identify the origin of some fluorescent components and provide novel insights into the behavior of DOM in textile wastewater during treatment.

2. Materials and methods 2.1 Chemicals and reagents Methanol (HPLC grade), acetonitrile (HPLC grade) and ammonium acetate (HPLC grade) were purchased from J&K, Scientific. Ultrapure water was produced by a Milli-Q Reference system (Merck, Germany). Several model compounds were analyzed to explore possible origins of fluorescence in textile wastewater (Table S1).

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2.2 Sampling campaigns Textile wastewater samples were taken from two full-scale wastewater treatment plants (WWTPs) located in Wuxi City, Jiangsu Province, China between April 2016 and March 2017. The detailed information on each WWTP was listed in Table 1. For each sampling, 24h flow-weighted composite samples were collected from each treatment stage (Fig. S1), which were named as CD-Reservoir, CD-Anaerobic, CD-Aerobic, CD-Fenton and JY-Reservoir, JY-Anaerobic, JY-Anoxic, JY-Aerobic, JY-Chlorination, respectively. The biological treatment refers to the anaerobic-aerobic process in CD WWTP and the anaerobic-anoxic-aerobic process in JY WWTP, respectively. Basic water characteristics of these samples including pH, chemical oxygen demand (COD), ammonia nitrogen (NH3-N), total nitrogen (TN), total phosphorus (TP) measured with standard methods [24] are shown in Table S2. All the samples were immediately filtered through pre-washed 0.45 µm membrane and stored at 4 °C until analysis. Table 1 Treatment capacity, wastewater source, treatment train and the number of sampling events (n) for each WWTP. WWTP

Treatment capacity

CD

16000 m3/d

JY

20000 m3/d

Wastewater sources

Treatment train

n

Anaerobic – Aerobic - Fenton process*

4

Anaerobic – Anoxic – Aerobic - Chlorination**

4

Textile wastewater from 25 textile factories Textile wastewater from 2 large textile factories

* During Fenton oxidation (HRT, 0.8 h), the mass concentration ratio of COD, H2 O2 and FeSO4 •7H2 O was about 1:5:5 with pH 3.0-3.5. The Fenton oxidation effluent was coagulated (HRT, 0.5 h) at pH 6.5–8.0 with Fe(Ⅲ) produced by the Fenton reaction. The ‘Fenton process’ here was defined as the combination of Fenton oxidation, subsequent coagulation and sedimentation. ** During chlorination (HTR, 1 h), NaClO was used to for further decoloration. The mass concentration ratio of NaClO to COD was about 10:1. The ‘Chlorination process’ here was defined as the combination of chlorination, subsequent coagulation and sedimentation as above.

2.3 DOC and UVA254 measurement The DOC concentrations of the filtered samples were measured by a TOC-VCPH analyzer (Shimadzu, Japan). UVA254, an extensively used indicator of DOM concentration [25, 26], was measured using a U-3900 spectrophotometer (Shimadzu, Japan). For every sample, the specific UV absorbance at 254 nm (SUVA254), an indicator of DOM aromaticity, was calculated by dividing the UVA254 by the corresponding DOC concentration [27].

2.4 EEM spectra and PARAFAC analysis EEM spectra were obtained using a fluorescence spectrophotometer (Hitachi F-7000, Japan) equipped with a 150 W Xenon lamp. The measurement was carried out in a 1-cm quartz cell at a scan rate of 2400 nm/min with a PMT voltage of 700 V. EEMs were collected with an excitation range of 220-450 nm with an interval of 5 nm and an emission range of 250-550 nm with an interval of 2 nm, respectively. All the samples were diluted until UVA254 < 0.05 cm-1 to minimize inner filter effect (IFE) [28]. Thus according to UVA254 values in Table 2, 200-fold and 100-fold dilution with ultrapure water were necessary for CD-Reservoir and JY-Reservoir, respectively. The other samples from CD and JY 4

WWTPs were diluted by the same times as CD-Reservoir and JY-Reservoir, respectively. All fluorescence data were presented in Raman units (R.U.) after Raman correction [29].

Table 2 DOC, UVA254 and SUVA254 of samples along the treatment trains in CD and JY WWTPs Samples

DOC* (mg/L)

UVA254* (cm-1)

SUVA254* (L/(mg· m))

CD-Reservoir

184.2 ± 26.2

5.96 ± 0.73

3.24 ± 0.22

CD-Anaerobic

111.5 ± 20.7

4.70 ± 0.62

4.22 ± 0.19

CD-Aerobic

55.20 ± 10.4

3.06 ± 0.51

5.54 ± 0.12

CD-Fenton

14.74 ± 0.69

0.376 ± 0.023

2.55 ± 0.13

JY-Reservoir

256.7 ± 29.7

4.21 ± 0.55

1.64 ± 0.12

JY-Anaerobic

150.4 ± 19.8

2.62 ± 0.36

1.74 ± 0.15

JY-Anoxic

123.7 ± 18.7

2.46 ± 0.22

1.99 ± 0.18

JY-Aerobic

22.52 ± 1.06

0.930 ± 0.050

4.13 ± 0.09

JY-Chlorination

12.35 ± 1.16

0.253 ± 0.010

2.05 ± 0.05

* Mean value ± standard deviation, n=4 for CD and JY treatment trains, respectively.

The PARAFAC analysis was carried out with the DOMFluor toolbox [7] for Matlab R2012a and non-negative constraints were applied for excitation and emission loadings. In order to examine the soundness of the PARAFAC modeling, a series of criteria was applied to a dataset of 37 EEMs (36 textile wastewater samples + 1 Dispersant MF sample) [7]: (1) the examination of the core consistency, (2) the evaluation of the shape of the spectral loading, (3) the leverage analysis regarding the influence of a specific sample or certain excitation and emission wavelengths, (4) the residuals analysis and (5) the split half analysis. With an initial exploratory analysis, no outlier was found.

2.5 HPLC analysis The Agilent 1260 LC Infinity system with DAD and FLD was used in this study. The Waters Atlantis T3-C18 column (150 × 4.6 mm, 2.7 µm) was applied using a gradient elution program (Table S3) with the sample injection volume of 5 µL. The UV wavelength 254 nm was selected while the excitation and emission wavelength pairs were set up according to the peak locations of fluorescent components identified by PARAFAC analysis. With the C18 column, the more hydrophilic components will be eluted with a shorter retention time. The DOM fractions with retention time 1.0 – 2.5 min, 2.5 – 5.0 min and 5.0 – 9.0 min were designated as hydrophilic, relatively hydrophilic and hydrophobic clusters, respectively. As for size exclusion chromatography (SEC) analysis, the PL aquagel-OH 30 (300 mm×7.5 mm, 8 µm) and guard column (50 mm×7.5 mm, 8 µm) were installed inside the HPLC system. The sample injection volume was 20 µL and the phosphate buffer solution (1.0336 g/L KH2PO4 + 2.8272 g/L K2HPO4·3H2O, pH=7.0) was used as the mobile phase at a flow rate of 1 mL/min. The DOM species with higher MW are eluted with shorter retention time. A polyethylene glycol kit (Agilent, EasiVial) were used as SEC calibration standards to determine apparent MW of DOM (Fig. S2).

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3. Results and discussion 3.1 DOC, UVA254 and SUVA254 As shown in Table 2, both DOC and UVA254 decrease along CD or JY treatment train. However, the average SUVA254 values of CD-Aerobic and JY-Aerobic reached 5.54 and 4.13 L/(mg•m), which were maximum along the CD and JY treatment train, respectively. This indicates that the DOM aromaticity of textile wastewater increases during biological treatment and decreases after the Fenton or chlorination process.

3.2 EEM and PARAFAC analysis The EEMs of the textile wastewater samples along the treatment trains from CD and JY WWTPs were displayed in Fig. 1. With peak-picking method [30], three major peaks were found at Ex/Em 230/338 nm, Ex/Em 275/320 nm and Ex/Em 250/454 nm. By spectral comparison (Fig. 1 vs. Fig. 2), the two protein-like peaks (Ex/Em 230/338 nm and 275/320 nm) could be related to Dispersant MF (Fig. S3), which was extensively used as dye dispersants, surfactants, tanning agents or plasticizers of concrete [31]. The mass ratio of Dispersant MF to disperse or vat dyes needs to be more than 60% to acquire stable dye dispersion and most of Dispersant MF is commonly discharged with raw textile wastewater [32]. Hence, the EEMs of untreated textile wastewater (CD-Reservoir and JY-Reservoir) consist of the two characteristic peaks (Ex/Em 230/338 nm and 275/320 nm). The main components of Dispersant MF are sulfonated naphthalene formaldehyde condensates (SNFC), and the critical fluorescent structure is naphthalene (Fig. S4). SNFC are highly water-soluble and can’t be eliminated during the biological treatment [33]. Thus SNFC should not be negligible even when interpreting protein-like DOM in final textile effluent and natural water [34].

Fig. 1. EEMs of textile wastewater from various treatment stages in CD WWTP and JY WWTP

6

Fig. 2. EEM of Dispersant MF

In previous research [21, 35, 36], the fluorescence peaks at Ex/Em 230/338 nm and 275/320 nm were prevalent in treated textile wastewater after biological or advanced treatment. Besides Dispersant MF, the two fluorescence peaks were associated with protein-like SMPs [37] and aromatic amines like aniline [9]. The protein-like SMPs were released by microbial metabolism in biological treatment [20] while aromatic amines were produced by the degradation of azo dyes [19]. By PARAFAC analysis, four fluorescent components denoted as C1 – C4 were identified (Fig. 3). Component C1 displayed two excitation maxima (at 225, 275 nm) corresponding to the same emission maximum (at 316 nm). C2 and C3 were composed of two excitation peaks at Ex/Em 230, 280/338 nm and Ex/Em 235, 290/350 nm, respectively. Component C1 – C3 were found in various types of DOM in previous reports (Table 3). In summary, C1, C2 and C3 were ascribed to protein-like fluorescence in municipal wastewater, and C1 could also be attributed to phenol-like fluorescence in coke wastewater. However, C1 – C3 could be attributed to various monomers and oligomers of Dispersant MF in this study. Component C4 consisted of shoulder emission peaks, and the similar phenomena were reported in previous research [38, 39]. The dual excitation peaks on the left shoulder (Ex/Em 250, 305/362 nm) were similar to phenanthrene-like components [40] or protein-like components [41]. The dual excitation peaks on the right shoulder in parallel with the left shoulder (Ex/Em 250, 305/450 nm) were similar to humic-like components [3, 42] or 1-amino-2-naphthol moieties [21]. However, emission spectra should exhibit only one maximum [7]. One possible explanation for the two maxima of C4 emission spectra is the intramolecular formation of charge-transfer complexes in the excited state [43].

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Fig. 3. Contour maps, excitation and emission loadings of the four fluorescent components (C1 – C4) identified by PARAFAC analysis

Table 3 PARAFAC-derived fluorescent components C1 – C3 by previous reports. Component

Description

Reference

Tyrosine-like fluorescence in municipal wastewater

[39, 41]

Phenol-like fluorescence in coke wastewater

[13]

C2 (230, 280/338 nm)

Tryptophan-like fluorescence in municipal wastewater

[42, 44]

C3 (235, 290/350 nm)

Tryptophan-like fluorescence in municipal wastewater

[3, 38]

C1 (225, 275/316 nm)

3.3 Polarity and MW of fluorescent DOM Due to the similar peak locations of C2 and C3, Ex/Em 275/316 nm, Ex/Em 230/338 nm and Ex/Em 250/450 nm were selected as excitation/emission wavelength pairs to explore the polarity and MW distribution of C1 – C4 along the treatment trains. Despite the remarkable differences of untreated textile wastewater (CD-Reservoir vs. JY-Reservoir) between CD WWTP and JY WWTP (Fig. S5), the polarity and apparent MWs of the corresponding fluorescent components are highly similar (Fig. S6, Fig. S7). C1 of CD-Reservoir and JY-Reservoir mainly consists of hydrophilic and hydrophobic clusters while C2 and C3 include hydrophilic, relatively hydrophilic and hydrophobic clusters. C4 is composed of hydrophilic substances. As for the apparent MW distributions, C1 of CD-Reservoir and JY-Reservoir consists of a major cluster with MW of 270 Da – 955 Da and a minor cluster with larger MW of 955 Da – 2.9 kDa. C2 and C3 both range from 234 Da to 18.2 kDa. The major clusters of C4 are 9.0 – 18.2 kDa. Compared with the C18/SEC-fluorescence chromatograms of Dispersant MF (Fig. S8), the same retention time of different clusters further demonstrates that C1 – C3 are related to Dispersant MF. Besides, the variations of C1 – C4 along the treatment train in CD WWTP were also highly similar with the variations in JY WWTP (Fig. 4 vs. Fig. 5). In the anaerobic or anoxic process, no obvious changes occur on the polarity of C1 – C4. The hydrophobic clusters of C1 – C3 seem to be more removed than the hydrophilic clusters during the aerobic process. The apparent MW distributions of C1 – C4 were almost unaffected by various stages in the biological process. The polarity and apparent MWs of CD-Aerobic and JY-Aerobic are in agreement with prior reports on bio-treated textile wastewater [9, 21], which further demonstrates that the components and physicochemical properties of fluorescent DOM in bio-treated textile wastewater are relatively invariable. However, the clusters of C1 – C4 with different polarity and apparent MWs were indiscriminately removed by Fenton and chlorination process, respectively.

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Fig. 4. C18-fluorescence chromatograms with specific Ex/Em wavelength pairs: (a) CD WWTP and (b) JY WWTP

Fig. 5. SEC-fluorescence chromatograms with specific Ex/Em wavelength pairs: (a) CD WWTP and (b) JY WWTP

3.4 Variation of fluorescent components along the treatment train The concentration scores of C1 – C4 were obtained by PARAFAC analysis to provide a quantitive analysis of the fluorescent components. For a certain treatment stage in CD or JY WWTP, the concentration scores of C1 – C4 were averaged over the entire sampling campaign. As shown in Fig. 6, 9

the weights distribution of Dispersant MF among C1 – C4 was 41%, 37%, 21% and 1%, respectively. In both CD and JY WWTPs, the weights distribution among C1 – C4 stayed relatively invariable in the anaerobic or anoxic process. Unlike little change of C1 and C4, C2 had a dramatic decrease (30%→13% for CD WWTP, 30%→15% for JY WWTP) while C3 had a moderate increase (27%→41% for CD WWTP, 32%→41% for JY WWTP) in the aerobic process. Moreover, the weights distribution among C1 – C4 varied substantially in the Fenton process (CD-Fenton: C1–25%, C2–18%, C3–25% and C4–32%) or chlorination process (JY-Chlorination: C1–62%, C2–31%, C3–5% and C4–2%). Except for CD-Fenton, the sums of the weights of C1 – C3 for the textile wastewater samples and Dispersant MF were all > 90%, indicating the dominance of Dispersant MF in the protein-like fluorescence of textile wastewater.

Fig. 6. The component weights distribution among C1 – C4 in terms of the concentration scores

To describe the fluorescent components variation along the treatment train, the averaged score of a given component was normalized by the corresponding value of the first sample. The removal efficiencies of various treatment stages were calculated using the averaged scores. For CD WWTP (Fig. 7a), the concentrations of C1 – C3 were decreased by 12.9%, 13.1% and 20.2% while the concentration of C4 was almost unchanged (-2.2%) during the anaerobic process. In the following aerobic process, the removal efficiencies of C1 – C4 were successively 46.3%, 22.2%, 77.3% and 36.1%. During Fenton process, the removal efficiencies of C1 – C4 were all > 95%. As for JY WWTP (Fig. 7b), the removal efficiencies of C1 – C4 in the anaerobic process were 22.1%, 24.4%, 7.9% and 3.4%, respectively. In the following anoxic process, little change occurred on the concentrations of C1 – C4 with the removal efficiencies of -0.1%, -7.8%, 1.3% and -8.1%. The aerobic process decreased the concentrations of C1 – C4 by 30.3%, 16.4%, 66.7% and 20.3%. Finally, the removal efficiencies of the chlorination process reached 77.4%, 98.4%, 74.1% and 97.1% for C1 – C4 in succession. During the whole biological treatment of both WWTPs, C1 and C2 were moderately removed (30% – 55%) while C3 was effectively removed (70% – 85%). However, protein-like C1 – C3 in municipal wastewater were all effectively removed (> 75%) by biological treatment [38, 44]. The possible reasons are as follows: (1) Dispersant MF consists of readily biodegradable monomers and less biodegradable oligomers [45]; (2) the cleavage of ‘N=N’ bond of azo dyes by anaerobic and anoxic treatment would result in the production of aromatic amines like aniline and other dye intermediates, which could decrease the removal efficiency of C1 – C3 and even increase the concentration of C1 – C3 [18, 46]; (3) the removal efficiencies of various aromatic amines and other dye intermediates are different during aerobic treatment. As for C4, its unsuccessful removal during the biological treatment (34.7% for CD WWTP, 16.8% for JY WWTP) was consistent with humic-like components in municipal wastewater [44, 47]. 10

Fig. 7. Removal dynamics and removal efficiencies of fluorescent components along the treatment trains in (a) CD WWTP and (b) JY WWTP

Although C1 – C4 in textile wastewater had different fate in the biological process, their concentrations could be all significantly decreased by Fenton or chlorination process. Fluorescent DOM in bio-treated textile wastewater mostly consists of the aromatic structure with amino, hydroxy or sulfonic groups, which was rather refractory [48]. Hydroxyl radicals (·OH) generated by Fenton process could nonselectively break down organic pollutants, resulting in the decrease of MW and biodegradability improvement [49, 50]. Sodium hypochlorite (NaClO) could attack the amino group, thus initiating and accelerating azo bond cleavage of dyes for decoloration [51]. However, the residual chlorine and chlorinated organic compounds would increase biotoxicity of textile effluent [52]. Ozonation was also reported as an effective method to eliminate fluorescence of bio-treated textile wastewater [36]. In summary, the process with strong oxidation was highly effective for advanced treatment of textile wastewater.

3.5 Relations between water quality parameters and concentrations of fluorescent components As fluorescence is generally related to organic matters, its correlation with other water quality parameters such as NH3-N, TN and TP are not taken into consideration here. The correlation coefficients (r) between the concentration scores of C1 – C4 and surrogate parameters for organic matters (UVA254, DOC and COD) were calculated (Table 4). In both WWTPs, the concentrations of C2 were well correlated with COD (r > 0.89). The phenomenon of strong correlation is consistent with reported correlations between COD and 11

concentrations of the tryptophan-like component in municipal wastewater by PARAFAC analysis [38]. Thus, the concentration of C2 could be proposed for predicting COD of textile wastewater during treatment. The correlations of C1 – C4 scores with UVA254 were stronger than those with the corresponding DOC or COD, which is probably because fluorescent DOM has a more similar removal pattern with nonfluorescent aromatics than aliphatics during treatment. Considering its strong correlation with UVA254 (> 0.90) and association with dye auxiliaries and degradation products of dyestuffs, the fluorescence of C2 could also be a supplement to UVA254 to evaluate treatment efficiency of textile wastewater. Table 4 Correlation coefficients (r) between the concentration scores of C1 – C4 and surrogate parameters for organic matters in the investigated WWTPs; m is the number of samples included in the correlation. CD WWTP, m=16

JY WWTP, m=20

UVA254

DOC

COD

UVA254

DOC

COD

C1

0.9763

0.9517

0.8783

0.8427

0.7334

0.8119

C2

0.9565

0.9560

0.9084

0.9193

0.7357

0.8909

C3

0.9564

0.9257

0.8195

0.7844

0.6629

0.7278

C4

0.9182

0.9213

0.8042

0.7695

0.5171

0.6561

4. Conclusion Real textile wastewater samples in the two different WWTPs along the treatment trains were characterized by EEM coupled with PARAFAC and HPLC analysis. The main conclusions could be drawn as follows: •

By PARAFAC analysis, four fluorescent components (C1 – C4) were identified. By comparison of EEM spectra and HPLC chromatograms, the tyrosine-like component (C1) and the tryptophan-like components (C2 and C3) could be related to Dispersant MF. Component C4 with dual emission peaks suggested the formation of some intramolecular exciplex.

•

Despite the remarkable difference of untreated textile wastewater, the polarity and apparent MWs of C1 – C4 as well as their variations along the treatment trains (‘biological + oxidative’ processes) were highly similar between different WWTPs.

•

The weights distribution of Dispersant MF among C1 – C4 indicates its dominance in the protein-like fluorescence of textile wastewater. Component C1 – C4 had different fate in various treatment stages. C1 – C4 were poorly removed (< 25%) during the anaerobic or anoxic process. In the aerobic process, a better removal efficiency of C3 (65% – 80%) than C1, C2 and C4 (15% – 50%) was achieved. Both Fenton and chlorination process could significantly decrease the concentrations of C1 – C4 (70% – 100%).

•

The concentration of C2 was strongly correlated with DOC, COD and UVA254. The fluorescence of C2 could be proposed as an indicator of COD and a supplement to UVA254 to evaluate treatment efficiency of textile wastewater.

Acknowledgements We sincerely thank financial support provided by Scientific Instrument and Equipment Development Major Project (2017YFF0108500) and the special fund of State Key Joint Laboratory of Environment Simulation and Pollution Control (17Y01ESPCT). We also thank Mr. Chao Chen, Ms. 12

Jing Jiang, Ms. Qiuxia Zhou and Mr. Li Shao from Jiangyin Environmental Monitoring Station (Wuxi City, Jiangsu Province, China) for their generous help in the sampling campaign and measurement of COD, NH3-N, TN and TP.

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Figure legends Fig. 1. EEMs of textile wastewater from various treatment stages in CD WWTP and JY WWTP Fig. 2. EEM of Dispersant MF Fig. 3. Contour maps, excitation and emission loadings of the four fluorescent components (C1 – C4) identified by PARAFAC analysis Fig. 4. C18-fluorescence chromatograms with specific Ex/Em wavelength pairs: (a) CD WWTP and (b) JY WWTP Fig. 5. SEC-fluorescence chromatograms with specific Ex/Em wavelength pairs: (a) CD WWTP and (b) JY WWTP Fig. 6. The component weights distribution among C1 – C4 in terms of the concentration scores Fig. 7. Removal dynamics and removal efficiencies of fluorescent components along the treatment trains in (a) CD WWTP and (b) JY WWTP Table legends Table 1 Treatment capacity, wastewater source, treatment train and the number of sampling events (n) for each WWTP. Table 2 DOC, UVA254 and SUVA254 of samples along the treatment trains in CD and JY WWTPs Table 3 PARAFAC-derived fluorescent components C1 – C3 by previous reports. Table 4 Correlation coefficients (r) between the concentration scores of C1 – C4 and surrogate parameters for organic matters in the investigated WWTPs; m is the number of samples included in the correlation.

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DOM of textile wastewater during treatment was characterized by EEM with PARAFAC. Protein-like fluorescent components were related to Dispersant MF. Polarity and MWs of fluorescent DOM along the treatment trains were investigated. Fenton oxidation or chlorination process could significantly remove fluorescence.

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