Spectrum evolution of dissolved aromatic organic matters (DAOMs) during electro-peroxi-coagulation pretreatment of coking wastewater

Separation and Purification Technology 235 (2020) 116125

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Spectrum evolution of dissolved aromatic organic matters (DAOMs) during electro-peroxi-coagulation pretreatment of coking wastewater

T

⁎

Xin Zhoua,b, , Zilong Houa,b, Jingjing Songa,b, Lin Lva a b

College of Environmental Science and Engineering, Taiyuan University of Technology, Taiyuan 030024, China Innovation Center for Postgraduate Education in Municipal Engineering of Shanxi Province, Taiyuan 030024, China

A R T I C LE I N FO

A B S T R A C T

Keywords: Dissolved aromatic organic matters (DAOMs) Electro-peroxi-coagulation (EPC) Coking wastewater GC/MS analysis Spectrum evolution

Enormous existence of dissolved aromatic organic matters (DAOMs) is considered to be a major cause that coking wastewater is hardly degraded. In this study, an advanced electro-chemical process i.e., electro-peroxicoagulation (EPC) was developed as a pretreatment for efficiently degrading DAOMs and improving the biodegradability of the raw coking wastewater. Gas chromatography-mass spectrometry (GC-MS) also confirmed that most of DAOMs (phenolics, heterocycles, polycyclic aromatic hydrocarbons, benzenes, organic nitriles and anilines) could be effectively decomposed by EPC pretreatment. Ultraviolet visible (UV–Vis), Fourier transform infrared spectroscopy (FTIR) and excitation-emission matrix (EEM) consistently found remarkable spectrum variations of DAOMs in the first 15 min, while spectrum changes became gentle until 2 h. The spectrum evolution of DAOMs suggests the strong attacks on the benzene rings and unsaturated bonds of DAOMs occurs initially and then low molecule intermediates are further degraded. Therefore, EPC should be a feasible option for coking wastewater pretreatment based on the combination of electro-oxidation and electro-precipitation.

Cathode: O2 + 2H+ + 2e− → H2O2

1. Introduction

−

Coking wastewater commonly generated during coal coking, gas purification and by-product recovery processes of a coke factory [1,2] is a typically industrial high-strength organic wastewater. Dissolved aromatic organic matters (DAOMs) are classified as hydrocarbons with single- or multi-benzene ring structure. The variety of DAOMs involving benzene series, phenol and their derivatives, polycyclic aromatic hydrocarbons and heterocyclic compounds are widely present in coking wastewater [3]. Due to complex structure, stable chemical property and high toxicity, refractory DAOMs are normally difficult to completely degrade by means of conventional either physico-chemical or biological treatment methods. As a result, the final effluent COD frequently fails to reach the stringent discharge standards of coking wastewater [4]. Currently, electro-peroxi-coagulation (EPC) process is recognized as a promising electro-chemical advanced oxidation process (EAOPs) [5,6], which is the use of iron as sacrificial anode in electro-Fenton. The principle of EPC process can be described as a series of reactions Eqs. (1)–(8). Anode: Fe − 2e− → Fe2+

(1)

2H2O − 4e− → O2 + 4H+

(2)

⁎

(3)

−

2H2O + 2e → H2 + 2OH

(4)

Solution: Fe2+ + H2O2 + H+ → Fe3+ + H2O + %OH

(5)

Fe2+ + 2OH− → 2Fe(OH)2

(6)

2Fe Fe

2+

3+

+ 5H2O + 1/2O2 → 2Fe(OH)3 + 4H

+

−

+ 3OH → 3Fe(OH)3 2+

(7) (8)

In the EPC process, Fe and H2O2 are respectively produced between the anode and cathode under the electricity field (Eqs. (1)–(4)). Hydroxyl radical (%OH) is then able to be electrogenerated by Fenton reaction (Eqs. (5)) in the bulk solution. %OH enables to non-selectively convert the hard-to-degrade organic pollutants into small micromolecule substances owing to strong attacks on the aromatic rings or long chains. In addition, EPC also allows the coagulation [7], precipitated by iron hydroxides formed as Fe(OH)n (n = 2, 3) (Eqs. (6)–(8)). Thus, refractory persistent organic pollutants such as DAOMs may be more easily transformed into biodegradable compounds and even removed based on combined effects of oxidation and coagulation in EPC process, which is quite suitable for the followed biological treatment. Until now, the EPC process has been effectively applied for

Corresponding author at: College of Environmental Science and Engineering, Taiyuan University of Technology, Taiyuan 030024, China. E-mail address: [email protected] (X. Zhou).

https://doi.org/10.1016/j.seppur.2019.116125 Received 17 July 2019; Received in revised form 21 August 2019; Accepted 22 September 2019 Available online 23 September 2019 1383-5866/ © 2019 Elsevier B.V. All rights reserved.

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the treatment of textile wastewater [8], petrochemical wastewater [9], composite wastewater [10] and land leachate [11]. As a pretreatment technology for raw coking wastewater, however, EPC process have paid few attentions until now. The spectrometry characterization provides a precise and rapid tool to identify and predict the structure of complex organic matters due to high sensitivity, simple operation and rapid detection without damaging the structure of the sample [12]. Especially, aromatic organics with π * π conjugated double bonds display stronger spectrum response. Spectral information on DAOMs have been explored using ultraviolet visible (UV–Vis) spectrum, fourier infrared (FTIR) spectrum and fluorescence excitation-emmission matrix (EEM) spectrum in previous studies [13,14]. However, the correlation between spectral evolution and removal behaviors of DAOMs during the electro-chemical treatment of wastewater were barely concerned yet, to our knowledge. In this study, a lab-scale EPC system was used for the pretreatment of raw wastewater from an actual coke plant (COD: 2500–3000 mg/L). Firsty, the COD removal and the biodegradability of coking wastewater were investigated under optimal process conditions and then the organic components of DAOMs were identified using gas chromatography-mass spectrometry (GC-MS) before and after EPC pretreatment. Based on above analysis, spectral evolution property (the composition, molecular structure and relative quantity) of DAOMs during the EPC pretreatment was clarified using UV–vis, FTIR and EEM techniques. This study offers more insights into the fate and degradation mechanism of DAOMs during EPC pretreatment of coking wastewater.

Table 1 Characteristics of coking wastewater. Parameter

Concentration range

Units

pH COD BOD BOD5/COD NH4+-N TKN

7.8–8.0 2500–3000 300–350 0.10–0.12 150–200 200–300

– mg/l mg/l – mg/l mg/l

adjustable by electric control cabinet.

2.2. The wastewater The wastewater in this study was collected from the ammoniastripping effluent in an actual coke-oven plant in Shanxi province of China. The main characteristics of the raw wastewater are listed in Table 1.

2.3. Experimental procedures Firstly, 4 L wastewater was collected in the EPC system at 25 °C. Na2SO4 as the electrolyte was added into the liquor to enhance the electroconductibility of wastewater and increase the hydroxyl radical production. The magnetic mixer provided continuous mixing of the reaction system. Continuous aeration with the rate of 500 mL/min was supplied by a compress air pump to increase the generation of H2O2 at the cathode. The initial pH value was controlled at 3.0 with 3.0 M H2SO4 and 6.0 M NaOH and the electricity density and reaction time were fixed at 2.8 mA/cm2 and 2 h, respectively. The above parameters have been optimized through our preliminary experiment. During 2 h reaction, samples at fixed time intervals were withdrawn from the reactor into the 100 mL beakers. The pH of the effluent samples were adjusted to 10.5 with NaOH and precipitated for 30 min. The supernatant after precipitation was put into water bath at 50 °C for 1 h to remove residual Fe2+ and H2O2 prior to COD determination [15].

2. Materials and methods 2.1. Experimental set-up A lab-scale self-made electro-peroxi-coagulation system mainly consisting of multi-stage electrodes with iron plate (150 mm × 100 mm × 2 mm) as anode and graphite rod (10 mm × 10 mm × 100 mm) as cathode, power control box and electrolytic tank is set up in Fig. 1. The electrolytic reactor with an effective volume of 7 L was a cylindrical plexiglass container with a diameter of 170 mm, a height of 300 mm. The current intensity was flexibly

Fig. 1. A lab-scale EPC experiment equipment. 2

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2.4. Analytical methods pH was measured by portable pH meter (FE20 Mettler Toledo, Germany). COD, BOD5, ammonia and total nitrogen was measured according to standard methods [16]. The wastewater biodegradability was indicated as the ratio of BOD5 and COD. All wastewater samples were filtered with a 0.45 μm filter membrane to obtain the DAOMs and immediately stored at 4 °C to prepare for measurement. 2.5. GC-MS analysis GC-MS technology was used for organic constitute analysis before and after EPC process. Prior to GC-MS analysis, 100 mL of water sample was extracted three times with 15 mL of CH2Cl2 (5 mL for each time). The remaining water sample was adjusted to pH ≥ 11 by adding NaOH and then extracted three times with 15 mL of CH2Cl2. In the extraction process, a certain amount of NaCl is added to reach the saturation state, which is conducive to improving the extraction efficiency. The extraction phases were combined and filtered with appropriate amount of Na2SO4 to remove excess moisture. Finally, the organic phase was concentrated to 5 mL by a rotary evaporator and purged to 1 mLwith high-purity nitrogen gas. 0.4 μL of concentrated liquor was injected into the GC/MS analyzer (Agilent 7890A/5975C, USA) equipped with a HP5MS chromatographic column (30 mm × 0.25 mm × 0.25 mm) with a column flow rate of 1 mL/min. The programmed heating was followed by retaining at 50 °C for 5 min and then increasing to 280 °C with 5 °C/ min. The gas was not separated with a carrier gas of high pure nitrogen. Analysis was undertaken with reference to the NIST 05 mass spectral library database. 2.6. Spectrum analysis The UV–Vis absorption spectrum was recorded using a UV–Vis spectrometer (Cary 300 Scan, Varian, USA) with the wave number range of 200–800 nm. The water samples to be tested were diluted 10 times and the pH value was adjusted to 7.0 using ultra-pure water as blank. FTIR spectrum was applied using a Infrared Spectrometer (Vertex 70, Brook, Germany) with wave number range from 40 to 4000 cm−1. 10 mL of water samples were firstly dried at 40 °C. Then solid sample was powdered and mixed with dry KBr (1:100 wt ratio) and then pressed into a pellet under 10 tons pressure for 1 min. EEM spectrum was recorded using 3D fluorescence spectrometer (CARY Eclipse, Varian, USA). with the scanning excitation wavelength from 200 to 450 nm and emission wavelengths from 250 to 550 nm. The excitation and emission slit widths were set to 10 nm, and the scan speed was maintained at 1200 nm/min with a scanning interval of 2 nm. All the wastewater samples was diluted 50 times, and then pH value was adjusted to 7.0.

flocculation played a leading role in eliminating the organics due to the formation of enough Fe(OH)n based on the Eqs. (6)–(8) at pH > 6.0. Owing to electro-Fenton combined with electro-flocculation, the final effluent COD dropped from 2858 mg/l to 880 mg/L with corresponding approximate 70% removal efficiency at the end of the reaction. The COD removal trend was also directly reflected by the color degree diminish of time-course treated wastewater samples, shown in Fig. 2b. Moreover, BOD5 stably went up as COD falling within the duration, leading to the significantly increase of B/C ratio from 0.11 to 0.43. It was acknowledged that the threshold of B/C ratio for suitable for biotreatment was 0.30 [17]. Therefore, it was obvious that the final treated EPC effluent with an improved B/C ratio and neutral pH became more feasible for subsequent biological treatment without pH adjustment.

3. Results and discussion

3.2. GC-MS analysis

3.1. Overall performance

To further evaluate the performance, GC-MS determination was undertaken to identify the main organic compounds of the raw and treated coking wastewater, summarized in Tables 2a and 2b. Compared the GC-MS spectrum in Fig. 3a and b, it was clearly that both the quantity and height of DAOMs spectrum peaks sharply decreased in the effluent, thus achieving high COD removal efficiency. According to GCMS results shown in Fig. 3a, over 100 different species of organic compounds were identified in the raw coking wastewater. Among them, more than 97% organics were DAOMs, which consisted of phenol and phenolic compounds (84%), heterocyclic compounds containing N and O (6.7%), polycyclic aromatic hydrocarbons (2.5%), benzenes (1.3%), benzene nitriles (1.1%) and anilines (0.9%). The presence of a variety of DAOMs was responsible for the high recalcitrance and low biodegrability of the crude coking wastewater. After the pretreatment,

Fig. 2. Overall performance during EPC reaction: (a) BOD5, COD, B/C ratio and pH; (b) visual inspection of different treated samples.

The concentrations of COD, BOD5, the ratio of B/C and pH during the EPC pretreatment under the optimum reaction conditions were presented in the Fig. 2a. As shown, the dramatic drop in COD were clearly observed in the initial 15 min with its corresponding removal rate of 45.49%, while COD concentration gradually tended to be flat as the reaction time. The tendency may suggest some readily degradable organic matters were quickly decomposed in a short time and then refractory organic matters were gradually transformed within a rather longer period. The observation was also closely with the basic theory of the EPC process. Initially, hydroxy oxidation predominated in COD removal at the very low pH (3.0–4.0). But later on, with the continuous consumption of H+, the effect of oxidation was reduced, while the 3

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Therefore, these UV–vis spectral discoveries well fitted with GC/MS results, suggesting phenols as dominating component of DAOMs in the influent. At 6 min, two weak absorption peaks at 201 and 204 nm were detected and the absorbance of the E2 band was slightly increased than the original sample. At the same time, the maximum absorption peak of phenolic substances at 269 nm were transferred to about 276 nm. Due to the introduction of substituent groups to benzene ring, a superconjugation effect might lead to obvious red-shift. During this time, only some readily degradable simple-structure compounds were directly removed, while almost DAOMs were hardly decomposed. But after 15 min, the characteristic absorption peaks of phenolic compounds at 276 nm swiftly disappeared. Due to low degree of symmetry and high compatibility with water [20], structures of high molecular weight such as PAHs- and HCs-containing DAOMs were quite unstable and prone to be primarily degraded into aromatic intermediates. Simultaneously, the main peak and shoulder peak at E2 band were red-shifted to 204 and 209 nm, respectively. These implied that PAHs were likely to be broken down attacked by the hydroxyl radicals. The longer conjugated system was in favor of migration of the absorption towards higher wavelength [21,22]. At 30 min, some weak peaks faded away and the main peak and shoulder peak at E2 band were blue-shifted to 201and 205 nm, respectively, but their intensity lowered. Some relatively refractory organics required higher energy to achieve electron transition of π → π*, thus leading to the blue shift of absorption peaks. On the other hand, the relative content of aromatic compounds had dropped to a low level. At 60 min, the absorption band was slightly changed in contrast with that at 30 min. With the continuous deletion of DAOMs, only one absorption peak was observed at this time, indicating part of intermediates got mineralized. During this time, pH was sharply increased from 3.85 to 6.35, and COD removal rate was quickly decreased. These results demonstrated that oxidation performance of EPC was gradually weakened due to the rapid loss of H+, which was consistent with the previous study [23]. Until 120 min, no apparent peaks was monitored and the falling rate of the absorption bands was faster. During this period, pH value persistently raised from 6.35 to 7.32. Due to pH elevation, less hydroxyl radicals obviously decreased the oxidizing rate of the DAOMs [24]. Instead, the coagulation and precipitation could remove organics due to Fe(OH)n formation at the pH-neutral condition [23]. In summary, these UV–vis results supported the efficient removal of DAOMs based on synergy effects of ·OH oxidation and Fe(OH)n flocculation during EPC pretreatment.

Table 2a Main organic compounds analysis before EPC pretreatment. Peak

Retention time (min)

Compounds

Relative content (%)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23

10.143 12.973 14.33 16.138 16.675 17.160 17.715 17.962 18.985 19.981 21.973 23.877 24.433 25.27 26.425 28.673 29.070 29.158 29.431 31.670 37.444 38.731 39.983

Phenol 4-methylphenol 2-nitrophenol 3,4-dimethyllphenol N-benzyl-2-phenethylamine 3-methyl-2-nitrophenol 4-methyl-2-nitrophenol 5-methyl-2-nitrophenol Indole 1H-indenol 1,4-naphthalenedione 3,3-thiobis-propanenitrile 2-naphthalenol 4-nitrophenol 3-methyl-4-nitrophenol 2-nitro-1-naphthol 4-isoquinolinol 5-isoquinolinol 2(1H)-QUINOLINONE Pyridine Benzoic acid Acridinone 2-methyl-acridinone

54.77 19.00 3.43 0.36 0.61 0.47 2.03 0.62 1.88 0.78 0.25 0.47 0.33 2.27 0.69 0.52 0.44 0.51 0.33 2.04 0.23 0.36 0.31

Table 2b Main organic compounds analysis after EPC pretreatment. Peak

Retention time (min)

Compounds

Relative content (%)

1 2 3 4 5 6 7 8 9 10

9.270 12.426 17.089 18.597 23.507 23.692 24.732 31.582 36.959 38.511

Phenol 4-methylphenol 2-ethyl-1-hexanol 2-amino-benzonitrile 1H-indazole 3,3-thiobis-propanenitrile 2-methyl-1h-benzimidazole Pyridine Benzenedicarboxylic acid Acridinone

58.47 4.56 1.57 1.05 1.73 5.25 3.15 7.12 4.85 2.20

only 28 substances remained in the effluent according to the GC-MS analysis in Fig. 3b. Lots of complex DAOMs such as most phenolic, polyring-aromatic and heterocyclic compounds were nearly disappeared. In particular, the relative abundance proportion of phenol and its derivatives remarkably decreased from 84% in the influent to 36% in the effluent. In the effluent, only a few mono-cyclic and long-chain compounds involving phenols, nitrogen heterocycles, nitriles, carboxylic acids and alcohols remained in the effluent. It was evident that a major fraction of various refractory DAOMs could be effectively degraded and even completely removed. Due to the transformation of the complex structures into small molecules, the biodegradability of the effluent was highly enhanced.

3.4. FITR analysis FTIR technique was applied to clearly identify the molecular characteristics, especially key functional groups of DAOMs during the pretreatment. Fig. 5(a)–(e) demonstrated the five FTIR adsorption spectrums along the time span (at 0, 15, 30, 60, 120 min) during EPC reaction. In the influent, a broad band between 3600 and 3200 cm−1 was attributed to the overlap of phenolic hydroxy stretching vibrations and NeH stretching vibrations of eNH and NH2. The peak at 1602 cm−1 was assigned to aromatic C]C and conjugated C]O strengthening [25,26]. The above indicates the main organics of the influent are the aromatic with benzene ring, N-containing heterocyclic compounds and anilines. The presence of the peak at 2923, 2830 and 1385 cm−1 derived from CeH stretching vibration of eCH2 and eCH3, and the 1353 cm−1 peak resulted from symmetrical contraction of aromatic eNO2, which might represent methylphenol and nitrophenol. The peaks at 2070 and 1385 cm−1 were respectively related to the stretching vibration of C^N triple bond and CeN bond, indicating the existence of nitriles and NHCs in the wastewater. The peak at 1146 cm−1 corresponding to CeO stretching and the peaks at 1043 and

3.3. UV–Vis analysis To determine and predict the molecular structure and content of DAOMs transformation during the EPC process, UV–vis analysis was used during the experiment. As depicted in Fig. 4, the UV–vis absorption bands of six wastewater samples (0, 6, 15, 30, 60, 120 min) were observed from 200 nm to 500 nm. At 0 min, high-intensity absorption peaks with precise structures occurred in the E2 absorption band in the region of 200–225 nm. This indicated conjugated molecule containing benzene rings were present in raw coking wastewater. Moreover, a wide peak at 269 nm was also observed in the B absorption band corresponding to phenolic aromatic compounds [18]. Meanwhile, the E2 absorption band of PAHs and HCs was potential contributor for also the appearance of the peak [19]. 4

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Fig. 3. GC/MS spectrum of influent (a) and effluent (b).

1002 cm−1 assigning to the asymmetric stretching vibration of CeOeC bonds were attributed to aromatic ethers. These FTIR observations closely accorded with the GC-MS results of the influent, which mainly consists of DAOMs including phenolics, N-heterocycles, aromatic aniline, aldehydes, ethers and organic nitriles. At 15 min, the peak intensities at 3440 cm−1 (OeH, NeH), 2070 cm−1(C^N), 1602 cm−1 (C]C or C]O), 1353 cm−1 (eNO2) were significantly dropped and some peaks at 1146 cm−1 (CeOH stretching), 1043 cm−1and 1002 cm−1 (CeOeC) nearly disappeared. Meanwhile, some peaks 1447 cm−1 (in-phase NeH bending and CeN stretching), 1130 cm−1 (CeO stretching), 875 cm−1 (aromatic CeH bending vibration), 837 cm−1(para substitution of phenol), 770 cm−1 (ortho substitution of phenol) and 619 cm−1 (NeH, OeN]O) newly emerged. Combined with the reason of red shift at 6 min in UV–vis spectrum, the peaks at 837 cm−1 and 770 cm−1 probably represent hydroquinone and catechol, respectively. Based on GC-MS of influent and UV spectrum, these observations proved that most of PAHs with eOH, eNO2, HCs containing N and O, organic nitriles, small amounts of phenolic compounds and some relatively degradable DAOMs like aromatic aldehydes, ketones, carboxylic acids tended to be primarily

degraded into aliphatic carboxylic acids, amides and aromatic amine and other simple intermediates in the short time. Presumably, high acid environment allows the strong formation of hydroxyl to powerfully open the loop structures of DAOMs via Fenton oxidation, significantly contributing to high COD removal rate, clearly illustrated in Fig. 2a. At 30 min, the peak at 2070 cm−1 on behalf of C^N bond and the peak at 837 cm−1 symbolizing aromatic CeH bond of hydroquinone nearly vanished, manifesting the organic nitriles containing C^N such as 3, 3-Thiobis-Propanenitrile and aromatic intermediate such as hydroquinone were resistant to be degraded due to their highly symmetrical structure. The deep degradation of DAOMs corresponds to the variation in absorption peaks at E2 band according to the result of UV spectra. As the reaction performing for 60 min, H2O2 was continuously consumed and oxidative effect of OH was lessened. The remarkable signal is that the remaining hydroxyl group can only remove a small amount of hydroquinone at 837 cm−1, but complete decomposition seemed not to proceed because of inadequate hydroxyl radicals at relatively high pH. Besides, there was no additional peaks formed or lost at this time, further demonstrating declining degradation rate. 5

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are summarized in Table 3. As shown in Fig. 6 and Table 3, five fluorescence peaks (B, T1, T2, M, C) were apparently observed with total fluorescence intensity of 738 a.u. in the EEM spectra of the influent, implying the high diversity and strength of DAOMs from the raw coking wastewater. Among these, the most strongest fluorescent centers were respectively located at Ex/Em = 350/430 nm of the peak C and Ex/Em = 324/380 nm of the peak M. This was linked with the identifications of the humic-acid (HA) [27], implying large molecular weight compounds with poly-rings like HCs and PAHs. The peak M is assigned to soluble microbial byproduct (microbial humic-like substances) [28], which are associated with phenolic substance, protein-like compounds and carbohydrates [29]. Besides, the peak B at Ex/Em = 278/300 nm labeled as aromatic protein I (tyrosines-like) in visible region [30] might indicate phenols [31] and PAHs [32] compounds. The peak T1 (Ex/Em = 240/370 nm) and T2 (Ex/Em = 292/360 nm) were categorized as aromatic protein II (tryptophan) of the ultraviolet region and visible region [33]. This probably meant the existence of bicyclic aromatic compounds, for instance, naphthalene and naphthalenol [34]. At 15 min, the peak C totally disappeared and the fluorescence intensities of peaks B, T2 and M declined by 45.5%, 57.7% and 40.2%, respectively in contrast with the original wastewater. The noticeable variation implied that HA with high molecular weight was more vulnerable to strike by hydroxyl groups. This was further confirmed by the vanishing of peak at 276 nm according to UV–Vis observation. Moreover, the aromatic protein I and II of visible region and microbial humic-like substances were also rapidly degraded or transformed. But the peak T1 had only a slight decline with a red-shift of 10 nm at the emission wavelength, implying the appearance of carbonyl, hydroxyl and alkoxy groups [35], which was in agreement with the red shift of phenolic substances in UV spectra and the appearance of catechol (770 cm−1) and hydroquinone (837 cm−1) of FITR result. Based on above, some aromatic intermediates might have been further decomposed into ketones or quinones, amides and aliphatic carboxylic acids, but a small number of aromatic intermediates still remain unbroken. At 30 min, peak T2 fully disappeared and the intensity of peaks B, and M also decreased a bit, while the intensity of T1 was almost unchanged. Probably, the disappearance of peak T2 was closely associated with the loss of 2070 cm−1(C^N), indicating low molecular weight organic amine or amide transformed through the cracking of C^N bond of nitriles. The slight decrease of fluorescence intensity also demonstrated the fact that the COD removal rate slowed down during this time. At 60 min, the fluorescence intensity of Peak B was gradually weakened, while a shoulder peak of aromatic protein I was newly observed in the region I, which may be concerned with the degradation of hydroquinone at 837 cm−1 into straight-chain hydrocarbons in FTIR. This indicated the structure of the benzene ring was destroyed but not fully mineralized owing to remarkably reduced oxidation ability at this period. At the end of the reaction, aromatic protein I was thoroughly removed, and the dropping speed of fluorescence intensities of aromatic protein II distinctly accelerated. These observations were quite fit with the disappearance of the peak at 201 nm in UV spectra and the peaks at 1447 cm−1 (NeH, CeN), 1130 cm−1 (CeO), 875 cm−1 (CeH) and 619 cm−1 (NeH or OeN]O) in FTIR spectra. Some small molecule aromatic acids including amino, carboxyl and C]C could be effectively deleted via coagulation at high pH. Meanwhile, the fluorescence centers of peak T1 and peak M were red-shifted by 10 nm and 20 nm toward the emission wavelength, respectively. This indicated electro-coagulation was conducive to the polymerization of polar molecules so as to production of fulvic acids and humic acids. The humic acids in visible region corresponding to high molecular weight HCs was in line with the 1, 5-Naphthyridine, 1H-Indazole and acridinone, while some low molecular weight MACs with eCOOH such as phenol, 4-methylphenol pyridine and benzenedicarboxylic acid might be related with the presence of the fulvic acids in ultraviolet region combined with the organic

Fig. 4. UV–Vis analysis of DAOMs during EPC pretreatment.

Fig. 5. FTIR analysis of DAOMs during EPC pretreatment. (a–e represent five FTIR adsorption spectrums at 0, 15, 30, 60, 120 min, respectively).

At 120 min, the peaks at 1447 cm−1 (CeN), 1130 cm−1 (CeO), 875 cm−1 (CeH) and 619 cm−1 (NeH or OeN]O) nearly vanished except the peaks at 3430 cm−1 (OeH or NeH), 2923 cm−1 (eCH2), 2070 cm−1 (C^N) and 1602 cm−1 (C]C and C]O). It could be concluded that small molecules of amides, alcohols, carboxylic acids and alkanes might be removed by Fe(OH)n precipitation at neutral pH, which was strongly confirmed by the molecular property according to UV spectra. As the increasingly weakened oxidative ability, small amount of DAOMs with eOH, eCH2, eNeH, eC^N, eC]C, eC]O and CeO groups mainly remained in the final effluent supported by the GC-MS results. 3.5. 3D-EEM analysis During the reaction, 3D-EEM were also employed to further reveal the composition and transformation characteristics of DAOMs illustrated in Fig. 6. The corresponding fluorescence intensities and peak positions and total fluorescence intensities at 0, 15, 30, 60 and 120 min 6

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Fig. 6. 3D-EEM analysis of DAOMs during EPC pretreatment. (a–e represent five 3D-EEM spectrums at 0, 15, 30, 60, 120 min, respectively). Table 3 Corresponding fluorescence intensities and peak positions and total fluorescence intensities during the EPC reaction. Time (min)

0 15 30 60 120

Peak B

Peak T1 (A)

Peak T2

Peak M

Peak C

Total intensitya.u.

Ex/Em nm

Intensity a.u.

Ex/Em nm

Intensity a.u.

Ex/Em nm

Intensity a.u.

Ex/Em nm

Intensity a.u.

Ex/Em nm

Intensity a.u.

278/300 278/300 278/300 278/300 /

123.55 67.347 57.371 54.744 /

240/370 240/380 240/380 240/380 240/400

70.35 66.238 65.656 65.128 32.428

292/360 292/360 / / /

164.31 69.56 / / /

324/380 324/380 324/380 324/380 324/410

190.18 113.667 90.757 84.664 46.538

350/430 / / / /

190.33 / / / /

component by GC/MS. As 89.3% less of total fluorescence intensities than the original wastewater, it could be strongly conclude that most of refractory DAOMs were effectively degraded and even removed by EPC pretreatment.

738.72 316.81 213.78 204.54 78.966

Research and Development Projects of Shanxi Province (No. 201803D31052) and State Key Laboratory of Pollution Control and Resource Reuse Foundation, Tongji University (No. PCRRF18011). References

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In this study, desirable COD removal performance (69.21%) with an improved B/C ratio (0.42) were achieved in a lab-scale EPC system for the coking wastewater pretreatment. GC-MS verified most of complex and non-degradable DAOMs were degraded or even completely removed, thus enhancing the wastewater biodegradability. UV–vis, FTIR and 3D-EEM spectroscopic characterizations also further revealed transformation characteristics and removal mechanisms of the DAOMs during the EPC process. Based on the spectrum observations, the aromaticity, conjugation, molecular weight and humification degree of the DAOMs significantly decreased during 2 h pre-treatment through the cleavage of rings and breaking of substituent groups. Therefore, the EPC process is an effective alternative to pretreat refractory coking wastewater prior to biological process. Acknowledgements The authors would like to acknowledge the financial support from National Natural Science Foundation of China (No. 21607111), Key 7

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