Chemical Engineering Journal 364 (2019) 349–360

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Chemical Engineering Journal

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Treatment of organic wastewater containing nitrogen and chlorine by T

combinatorial electrochemical system: Taking biologically treated landfill
leachate treatment as an example
⁎ ⁎
Yang Denga, Nan Chena, , Chuanping Fenga, , Fangxin Chena, Haishuang Wanga, Peijing Kuanga,
Zhengyuan Fenga, Tong Liua, Yu Gaob, Weiwu Huc
a
School of Water Resources and Environment, MOE Key Laboratory of Groundwater Circulation and Environmental Evolution, China University of Geosciences (Beijing),
Beijing 100083, China
b
College of Chemical and Environmental Engineering, Shandong University of Science and Technology, Qingdao 266590, China
c
Journal Center, China University of Geosciences (Beijing), Beijing 100083, China

H I GH L IG H T S G R A P H I C A L A B S T R A C T

• CES can simultaneously remove COD,

TN and available chlorine.
The mechanism diagram of CES.

• The formation and removal of avail-

able chlorine in CES is discussed.
• The changes of organics and cytotoxi-
city during electrolysis are demon-
strated.
• CES can reduce the volume of elec-
trocoagulation precipitates and in-
crease their sedimentation capacity.

A R T I C LE I N FO A B S T R A C T

Keywords: This study presents a combinatorial electrochemical system (CES) for the treatment of refractory organic was-
Combinatorial electrochemical system (CES) tewater containing nitrogen and chloride ion. The CES was composed of iron anode reactor (IAR), Ti/RuO2
Electrochemical oxidation anode reactor (TAR) and available chlorine removal reactor (CRR). Biologically treated landfill leachate (BTLL)
Electrocoagulation was selected as the treatment object to evaluate the performance of CES. The results showed that CES could
Chlorine removal
simultaneously remove chemical oxygen demand (COD) and total nitrogen (TN) by 94.6% and 98.3%, respec-
Landfill leachate
Ti/RuO2 anode
tively. Reduction of nitrite-N by cathode in IAR and oxidation of ammonium-N by available chlorine in TAR were
the major pathways for TN removal. Fluorescence spectroscopy and parallel factor analysis (PARAFAC) showed
that the main organic components in BTLL were humic-like substances and soluble microbial degradation
products. These substances were removed by CES and the remaining organics were some hydrocarbons and
carboxylic acids. The available chlorine was rapidly reduced into chloride ion by IAR precipitates, thus de-
creasing the cytotoxicity. In addition, the formation of stable Fe3+ crystals was promoted by the oxidation of
available chlorine in CRR, which increased the density of the precipitates and reduced their volume. Therefore,
the CES is a promising solution for the treatment of refractory organic wastewater containing nitrogen and
chlorine ion.

⁎
Corresponding authors.
E-mail addresses: chennan@cugb.edu.cn (N. Chen), fengcp@cugb.edu.cn (C. Feng).

https://doi.org/10.1016/j.cej.2019.01.176
Received 10 August 2018; Received in revised form 2 January 2019; Accepted 29 January 2019
Available online 30 January 2019
1385-8947/ © 2019 Elsevier B.V. All rights reserved.
Y. Deng et al. Chemical Engineering Journal 364 (2019) 349–360

1. Introduction Table 1
The characteristics of BTLL.
Refractory organic wastewater containing nitrogen and chlorine ion Property BTLL
(referred to as NCOW in this paper) is one of the main types of real
wastewaters, which is generally produced from landfill treatment pro- pH 6.5 ± 0.1
ORP (mV) 118 ± 9
cess [1], food industry [2], pharmaceutical industry [3] and printing
COD (mg/L) 3487.5 ± 12.5
and dyeing industry [4]. The NCOW contains a large amount of TOC (mg/L) 1001.5 ± 32.2
chloride ions, organic compounds and inorganic nitrogen, which are Cl− (mg/L) 2989.1 ± 2.5
difficult to deal with and can endanger the ecological environment as TN (mg/L) 589.8 ± 1.4
well as human health [5–8]. Biological treatment may be an economical Ammonium-N (mg/L) 233.6 ± 2.6
Nitrite-N (mg/L) 283.6 ± 0.8
method to degrade organics. However, its performance is greatly con-
Nitrate-N (mg/L) 24.4 ± 0.4
trolled by the C/N ratio, which means that the biological treatment
method is unstable [9]. Other methods, for example, advanced oxida-
tion process and adsorption require the use of additional chemicals In this study, we developed a combinatorial electrochemical system
harsh operating conditions [10–12]. (CES) for the simultaneous removal of TN, COD and available chlorine
In recent years, electrochemical processes such as electrochemical from NCOW. The biologically treated landfill leachate (BTLL) was se-
oxidation/reduction, electro-coagulation, and electro-flotation have lected as a typical NCOW to evaluate the performance of CES because
emerged as promising techniques for complex wastewater treatment BTLL has almost all the water quality characteristics of NCOW [25,26].
because of their environmental compatibility, easy operation, and high The ability of CES to remove TN and COD was demonstrated, and op-
pollutant removal efficiency [11,13]. Electrochemical oxidation process erational factors were systematically examined. In addition, the de-
can achieve the removal of low valence ions (e.g. ammonium and sul- gradation of residual available chlorine by using electro-coagulation
fide) and mineralization of refractory organics (e.g. humic acids and precipitates was investigated for the first time. The changes in products
fulvic acids) [14]. The electrochemical oxidation of pollutants can be (e.g. organic matters and precipitates) during the reaction process were
carried out by direct oxidation (which occurs on the surface of anode also analyzed. The findings from this study will assist the development
when pollutants exchange electrons with anode plate) and indirect of viable solutions for treatment of NCOW.
oxidation (which is carried out by the oxidants (e.g. available chlorine
and hydroxyl radical) produced during electrolysis). Apart from the
2. Materials and methods
electrochemical oxidation process, some high valence ions (e.g. nitrate
and nitrite) can be removed by electrochemical reduction process [15].
2.1. Samples and materials
Electro-coagulation process was shown to be effective for treating color,
suspended solids, heavy metals and macro-molecular organic matters
BTLL, collected from Asuwei Refuse Sanitary Landfill (Beijing,
[13]. Electro-flotation process can also occur in electrochemical sys-
China), was stored in a polypropylene bucket and kept at 4 °C before
tems since hydrogen bubbles are produced at the cathode and some
utilization. The characteristics of BTLL are summarized in Table 1. All
hydrophobic organic matters (e.g. bitumen and some antibiotics
reagents were of analytical grade or higher, and were purchased from
[16,17]) could be purge-trapped to the surface of solution [18].
Sinopharm Chemical Reagent Co., Ltd (Shanghai, China). Solutions
However, there are two main problems in the application of elec-
were prepared using ultrapure water. Iron anode, Ti/RuO2 anode and
trochemical process to NCOW treatment. Firstly, although the available
copper zinc alloy cathode (denoted as Cu/Zn) (Cu: 60 wt%; Zn: 40 wt
chlorine (e.g. chlorine and hypochlorite) generated from oxidation of
%), all with the surface area of 51.6 cm2 (12 cm × 4.3 cm), were pur-
chloride ion during electrochemical oxidation process is beneficial to
chased from a local factory in Beijing, China. The selection of Cu/Zn
remove chemical oxygen demand (COD) and ammonium-N from
cathode was mainly due to its high electrochemical reduction ability for
NCOW, it would inhibit the cathode reduction of nitrate-N [19].
the removal of nitrate [19].
Therefore, it is difficult to simultaneously remove TN and COD from
NCOW using a single electrolytic cell, making it necessary to develop an
integrated electrochemical process using several electrolyzers. In the 2.2. Reactor setup of CES
past, the combination of electro-coagulation and anodic oxidation was
used to remove COD and some inorganic ions (e.g., chromium, zinc and The CES consisted of an iron anode reactor (IAR), Ti/RuO2 anode
ammonium) from landfill leachate [20]. Electro-oxidation and electro- reactor (TAR), available chlorine removal reactor (CRR), DC power
coagulation processes were integrated to increase the removal effi- supply (KXN-645D, Zhaoxin Company, China) and peristaltic pump
ciency of total organic carbon (TOC) and turbidity from papermaking (LongerPump, China). The schematic diagram of CES is shown in Fig. 1.
wastewater [21]. However, few studies have reported the simultaneous The electrolysis cells (IAR and TAR) with volume of 300 mL were made
removal of TN and COD using electrochemical process. of acrylic material for the electrolysis reaction. The CRR with 300 mL
Secondly, excessive amounts of available chlorine are generated in volume was also made of acrylic material for available chlorine re-
the electrochemical oxidation process. The available chlorine has high moval. Iron and Ti/RuO2 anodes were used in IAR and TAR, respec-
cytotoxicity, which can cause secondary pollution to drinking water tively. In the treatment process, BTLL (250 mL) was first placed in IAR
and threaten public drinking water safety [8,22]. However, it is difficult for 40 min, and subsequently sent to the sedimentation tank for 60 min.
to achieve the removal of available chlorine by electrochemical oxi- Then, the effluent from the sedimentation tank was transferred to the
dation system alone. At present, the main methods for available TAR for 120 min of treatment. Finally, the treated BTLL and the IAR
chlorine removal are chemical reduction (such as adding zero valent precipitates were all transferred into the CRR for 120 min and dis-
iron, sodium thiosulfate, sodium sulfite and ascorbic acid, etc. [23]) charged.
and membrane technology [24]. Unfortunately, the use of extra che-
micals and the resulting membrane fouling would inevitably lead to 2.3. Experimental procedure
maintenance difficulties and high operating costs. Due to the above two
key problems, the practical engineering application of electrochemical The experimental design of IAR was as follows: (i) the influence of
technology in the treatment of NCOW is limited. Therefore, it is ne- current density influence was studied at a fixed initial pH value
cessary to develop a new electrochemical system for the simultaneous (pH = 6.5) and different current densities (30, 50, 70, 100 mA/cm2);
removal of TN, COD and available chlorine. (ii) the influence of initial pH value was investigated at a fixed current

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Fig. 1. Schematic diagram of combinatorial electrochemical system (CES).

density (70 mA/cm2) and different initial pH values (3.0, 6.5, 9.0, density, decreasing pH had relatively little effect on improving the ni-
11.0). Samples were taken out from the reactor at fixed time intervals trite-N removal. Therefore, it might be not necessary to adjust the pH
(0, 5, 15, 30, 40 min). The influent of TAR was the effluent of IAR value in IAR.
obtained under the optimal operating conditions (initial pH of 6.5 and
NO−2 + HClO = NO−3 + H2 O+ H+ + Cl− (1)
current density of 70 mA/cm2). The experimental design of TAR was as
follows: (i) the influence of current density was studied at a fixed initial NO−2 + 5H2 O+ 6e− = NH3 + 7OH− (2)
pH value (pH = 9.0) and different current densities (30, 50, 70,
100 mA/cm2); (ii) the influence of initial pH value was investigated at a NO−2 + 4H2 O+ 4e− = NH2 OH + 5OH− (3)
fixed current density (70 mA/cm2) and different initial pH values (3.0, In TAR, the tendency for electrochemical removal of TN was similar
7.0, 9.0, 13.0). Samples were taken out from the reactor after fixed time to that of ammonium-N. These results were in line with expectations
intervals (0, 5, 15, 30, 60,120 min). The influent of CRR was the ef- because ammonium-N was the main form of nitrogen after the treat-
fluent of TAR that was obtained under the optimal operating conditions ment by electro-coagulation process [20,27]. The nitrate-N, produced
(initial pH of 9.0 and current density of 70 mA/cm2). Then, the IAR by the oxidation of nitrite-N and ammonium-N, was mainly reduced
precipitate was added into the CRR for degradation of available into nitrogen gas or ammonium-N on the surface of cathode. Compared
chlorine. All experiments were conducted at room temperature to the initial pH value, the current density had a greater effect on
(22 ± 2 °C) in duplicate, and the mean values of experimental data ammonium-N removal. The rate constant (k) values of ammonium-N
were reported. removal at current densities of 30, 50, 70 and 100 mA/cm2 were 0.0110
(R2 = 0.9226), 0.0342 (R2 = 0.9331), 0.0706 (R2 = 0.9345) and
2.4. Analytical methods 0.0764 (R2 = 0.9866), respectively, within 40–100 min. The increase of
current density promoted the formation of available chlorine that was
The analytical methods used in this study were detailed in conducive to the removal of ammonium-N. However, with the increase
Supporting Information. in current density, the removal rate of ammonium-N did not increase
linearly because more side reactions (e.g. oxygen evolution reaction
3. Results and discussion and heat generation) would be occurred at high current density.
Therefore, the optimum current density of TAR for ammonium-N re-
3.1. Total nitrogen removal moval in the present experiment was 70 mA/cm2. When the initial pH
values were 7.0 and 9.0, the ammonium-N removal was the highest,
Fig. 2 shows the variation of TN, nitrate-N, nitrite-N and ammo- mainly due to the accumulation of available chlorine under neutral or
nium-N during electrolysis under different conditions. In IAR, nitrite-N weak alkaline conditions [29]. Moreover, the ammonium-N was easily
was converted into ammonium-N, while the concentration of TN hardly blown out of the liquid phase by air flotation under high pH conditions.
decreased. This was mainly because nitrite-N was rapidly reduced into Therefore, pH of 11.0 also resulted in a relatively high removal rate for
ammonium-N under the highly reducing environment obtained with ammonium-N [30]. In this study, considering the removal effect of TN
iron anode. The removal of nitrite-N under iron anode mainly depended and COD (discussed in Section 3.2), the pH value of TAR was de-
on the cathode reduction [27]. Therefore, the increasing current density termined to be 9.0.
could enhance the rate of cathode reduction and promote the removal The effluent obtained from the optimum conditions of IAR and TAR
of nitrite-N. It is worth noting that the residual nitrite-N from IAR could (IAR: initial pH of 6.5 and current density of 70 mA/cm2; TAR: initial
be rapidly oxidized into nitrate-N in TAR by available chlorine (Eq. pH of 9.0 and current density of 70 mA/cm2) and the IAR precipitate
(1)). Generally, the cathode reduction of nitrate-N occurs sequentially were transferred into CRR. In this process, the concentration of TN
from NO3− to NO2−, which then converts into N2 or NH4+ [15]. showed almost no change. The remaining concentration of TN (mainly
However, available chlorine would oxidize NO2− into NO3− and de- nitrate-N) was 7.84 mg/L, which was below the maximum contaminant
crease the reduction rate of nitrate-N in TAR, which meant that the level of nitrate-N recommended by the World Health Organization
removal of nitrate-N in TAR was relatively difficult. Therefore, the (11.3 mg/L) [31].
operating parameters of IAR should be set considering the nitrite-N
removal as much as possible. In this study, the reasonable current 3.2. Chemical oxygen demand removal
density for IAR was found to be 70 mA/cm2. Due to the promoting ef-
fect of acidic environment for the reactions of Eq. (2) and Eq. (3), the The stable form of iron compounds is controlled by oxidation-re-
nitrite-N had a higher removal rate at pH = 3.0 than at pH = 6.5, 9.0 duction potential (ORP) and pH of solution. In IAR, according to the
and 11.0 [28]. In this study, compared with the effect of current ORP-pH trajectories (Fig. 3), iron mainly existed as were Fe (II)-

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Fig. 2. The variation of TN, nitrate-N, nitrite-N and ammonium-N during electrolysis under different conditions in CES (the effects of initial pH and current density
for TAR were studied using the effluent of IAR that obtained under the optimal operating conditions (initial pH of 6.5 and current density of 70 mA/cm2)).

Fig. 3. The ORP-pH diagram in IAR (current density: 70 mA/cm2, initial pH: Fig. 4. The variation of COD during electrolysis under different conditions in
6.5). CES (the effects of initial pH and current density for TAR were studied using the
effluent of IAR that obtained under the optimal operating conditions (initial pH
compounds (e.g. FeOH+ and Fe(OH)2). It has been confirmed that iron of 6.5 and current density of 70 mA/cm2)).
hydroxides (e.g. Fe(OH)2 and Fe(OH)3) have strong adsorption capacity
for colloids and dispersed particles [32]. Due to the promoting effect of
high current density on electrode reaction (e.g. the formation of iron

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hydroxide floc and cathode reduction of nitrite-N) [33], the removal of performed to evaluate the oxidation characteristics of anode
COD was enhanced with the increase in current density (Fig. 4). In (Supporting Information Fig. S2). In the cyclic voltammogram, the
addition, some Fe(OH)2 flocs were converted to soluble anions (Fe oxidation peak of chlorine evolution (1.20–1.65 V) was observed [40],
(OH)4−) under alkaline conditions. Thus, an increase in the initial pH but no other obvious oxidation peaks were seen, indicating that the
value was detrimental to the COD removal [1]. direct oxidation of the anode plate was weak. This result was consistent
In TAR, the removal rate of COD during electrolysis increased with with a previous study, which reported that direct oxidation in the
the increase in current density over the range of 30–100 mA/cm2 treatment of organic wastewater (e.g. humic acid and landfill leachate)
(Fig. 4). Normally, the current density is a key factor in the electro- was weak, and the COD removal mainly depended on the indirect
chemical oxidation process because it exerts a significant influence on oxidation by available chlorine [1,25]. Therefore, when the chloride
the reaction kinetics and energy consumption [1]. When chloride ion is ions were present in solution, the indirect oxidation via available
present in the solution, chlorine gas would be generated at the anode chlorine played the main role in the removal of COD and ammonium-N.
and immediately react with water to form available chlorine [19]. In
addition, Ti/RuO2 is a high O2 overvoltage anode that facilitates the ClO− + HO·→ClO·+OH− (10)
production of hydroxyl radicals on the surface of anode (Eq. (4)) [34].
The reactions of oxidizers (available chlorine and hydroxyl radical) ClO−2 + HO·→ClO2 ·+OH− (11)
with COD are described in Eq. (5) and Eq. (6). Therefore, the increase in
current density increased the production rate of available chlorine and
Cl−2· + HO·→HClO + Cl− (12)
hydroxyl radicals, and thus increased the removal rate of COD.
MOx (DSA anode such as Ti/RuO2) + H2 O→ MOx − OH + H+ + e− However, the removal rate of COD did not increase linearly with the
(4) increase in current density. When the operating current density ex-
ceeded the limiting current density of electrochemical reaction, it was
HClO + COD → CO2 + H2 O+ Cl− + COP (oxidizing products) (5)
difficult to increase the reaction rate because the reaction was limited
Organic pollutants + HO·→CO2 + H2 O+ inorganic ions (6) by the mass transfer process [41]. Generally, the limiting current den-
sity for COD removal is about 73.8–115.6 mA/cm2 [25]. Therefore, the
The hydroxyl radical was detected in the first 15 min (Fig. 5) in IAR. current density of 70 mA/cm2 could achieve a relatively high COD re-
The reasons for the formation of hydroxyl radicals by iron anode were: moval efficiency and ACE (Supporting Information Table S1), and was
(i) dissolved oxygen transformed into hydroxyl radicals on the cathode determined to be the optimum current density for COD removal in TAR.
surface by directly obtaining electrons (Eq. (7)) [35]; (ii) hydroxyl ra- The pH value mainly affected the oxidant formation during elec-
dicals were formed by electro-Fenton process (Eqs. (8)–(9)) [36]. In trochemical oxidation process. Under strong acidic conditions, chlorine
addition, the hydroxyl radical was also detected in CRR because the ions are readily converted into chlorine gas which easily escapes from
reaction between Fe2+ and available chlorine could promote the for- acidic solution, thus reducing the removal rate of COD [42]. Further-
mation of hydroxyl radical [37]. Dissolved oxygen (DO) concentration more, under strong alkaline conditions, some available chlorine (e.g.
was an important factor for controlling hydroxyl radicals formation ClO− and Cl2) would be converted into ClO3- or ClO4- ions, which have
during electrolysis process [38], especially in IAR. During 15–40 min of poor oxidation ability for COD removal [8].
electrolysis, the concentration of DO was low (< 0.3 mg/L) (Supporting The electrochemical oxidation process performed well when the
Information Fig. S1). Thus, a low concentration of hydroxyl radicals initial pH value was 7.0–9.0 in the present experiments. This result was
was detected during 15–40 min. in agreement with the previous studies, which reported that the elec-
O2 + 2H+ + 3e− → OH·+OH− (7) trochemical oxidation efficiency was higher in the pH range of 6.0–9.0
[1]. It could be inferred that ClO- accumulation mainly occurred under
O2 + 2H+ + 2e− → H2 O2 (8) neutral or weak alkaline conditions [29,42]. Therefore, the optimum
Fe2 + + H2 O2 → Fe3 + + HO− + HO· (9) pH value of TAR for COD removal was selected as 9.0 in the present
study.
The concentration of DO in TAR was slightly higher than that on The COD concentration presented a slight increase (from
IAR. However, the concentration of hydroxyl radicals in the electrolyte 187.5 ± 4.7 to 237.5 ± 5.9 mg/L) after the IAR precipitate was added
was lower, because hydroxyl radicals could also be consumed by into CRR, which could be attributed to the release of COD from the
available chlorine and form chlorides (e.g. ClO% and HClO) (Eqs. precipitates. This process is discussed in the following Section 3.4.
(10)–(12)) [39]. In addition, cyclic voltammetry (CV) of Ti/RuO2 was

Fig. 5. Hydroxyl radical detection in electrolysis process.

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ClO4− under strong alkaline conditions [8], resulting in lower accu-

mulation rates of available chlorine and [chloride ion + available
chlorine].
The main process occurring in CRR was the interaction between
Fe2+ and ClOx− (e.g. ClO− and ClO3− (Eq. (19) and Eq. (20))) [37],
which led to the increase in chloride ion concentration.
ClO− + Fe2 + + H2 O→ Fe3 + + HO·+HO− + Cl− (19)

ClO−3 + 6Fe2 + + 3H2 O→ 6Fe3 + + 6HO− + Cl− (20)

In order to obtain the relationship between the precipitate dosage

and the contribution of available chlorine removal, different amounts of
precipitates were added into six pre-cleaned beakers with a capacity of
25 mL (TAR effluent). The mass of precipitates was set as 0.0, 1.6, 3.2,
4.9, 6.6 and 8.2 mg, respectively. A linear regression (Eq. (21))
(R2 = 0.9310) was generated between the six different precipitate do-
sages and the residual available chlorine concentration during
0–15 min.
Fig. 6. The variation of chloride ion and available chlorine during electrolysis [Available chlorine] = −1.181[precipitate] + 40.272 (21)
in CES.
In this study, the available chlorine concentration in effluent from
TAR was 40.5 mg, and the dissolved mass of Fe2+ was 493.6 mg
3.3. Generation and degradation of available chlorine (Supporting Information Eq. S1). The theoretical removal amount of
available chlorine with 493.6 mg of Fe2+ was 417.9 mg, which meant
Although the dissolution of iron anode was the primary reaction the IAR precipitate could achieve complete removal of available
occurring in IAR, the formation of free chlorine has been reported as a chlorine in TAR.
side reaction [43]. Moreover, in our previous studies, available chlorine
could be produced under iron anode, especially at high current density
3.4. Change in organics and cytotoxicity
[44]. In fact, the iron anode presented a high tendency to dissolve Fe2+
and a relatively low tendency to release free chlorine [1,45]. Therefore,
Based on the PARAFAC modeling, three fluorescent components
the concentration of available chlorine was extremely low in IAR
were identified in BTLL (Fig. 7). Two fluorescence peaks of the first
(0–40 min) (Fig. 6). In TAR, available chlorine was generated by the
component (Compound 1) and the second component (Compound 2)
reaction between free chlorine (Cl2) and water (Eq. (13)), and oxidation
were observed at Ex/Em intensity values of 315/405 and 345/445 nm,
of chloride ion (Eq. (14)) and electrolysis of water (Eq. (15)) were the
respectively, which represented the humic-like substances [49]. The
main reactions on the surface of anode. In this process, the ΔG° values
third component (Compound 3) showed a fluorescence peak at Ex/Em
of chloride ion oxidation and electrolysis of water were 2.72 and
of 305/375 nm, resembling soluble microbial byproduct-like materials
4.92 eV, respectively, which meant that the energy demand of chloride
[50,51].
ion oxidation was less than that of electrolysis of water [46]. Therefore,
Fig. 8 shows the results of EEMs for the removal of organics, and the
the Ti/RuO2 anode was selective in the reaction of chlorine evolution
main organic components in BTLL were humic-like substances (HSs).
and promoted the generation of free chlorine [47]. Moreover, the in-
The fluorescence intensity of HSs was significantly decreased over
direct process via a Volmer–Heyrovsky mechanism (Eq. (16) and Eq.
0–40 min in IAR, mainly because the iron compounds (e.g. Fe(OH)2 and
(17)) was also an important route to achieve the transformation of
Fe(OH)3) had strong adsorption ability for humic acids and other col-
chloride ions into free chlorine [48].
loids [32]. Moreover, the oxidation process of low-molecular-weight
Cl2 + H2 O→ 2H+ + ClO− + Cl− (13) organics also occurred in IAR [52]. The HSs were almost completely
eliminated within 40–160 min in TAR, and the remaining organic
2Cl− → Cl2 + 2e− (14)
compounds were mainly protein-like substances, with Ex/Em peak at
2H2 O→ O2 + 4H+ + 4e− (15) 220/347 nm [53]. The fluorescence intensity of protein-like substances
was increased after the removal of available chlorine in CRR, which was
M (anode) − O+ 2Cl− → M− OCl + e− + Cl− (16) mainly because some HSs adsorbed on the surface of precipitates were
resolved into low-molecular-weight organics (e.g. tyrosine and trypto-
M− OCl + e− + Cl− → M (anode) − O+ Cl2 + 2e− (17)
phan) by oxidants (e.g. hydroxyl radical and chlorine radical) produced
The available chlorine did not accumulate in TAR during during the removal of available chlorine [54].
40–70 min, because it was consumed by ammonium-N and COD In order to determine the main organic components released from
[25,37]. The oxidation process followed second-order kinetics for the the precipitates into the solution, GC–MS analysis was performed on the
reactions of available chlorine with ammonium-N and COD (Eq. (18)) effluents from TAR and CRR and the results are shown in Supporting
[42]: Information Fig. S3. The main organics in the effluent discharged from
the two reactors were hydrocarbons, carboxylic acids and aromatic
− d [B]/ dt = kapp [available chlorine][B] (18)
acids. Several peaks were observed around 4–8 min in the GC–MS
where kapp is the second-order rate constant, and [B] is the total con- chromatogram during the removal process of available chlorine. These
centration of B in solution (B is ammonium-N or COD). In this study, the peaks corresponded to small-molecule lipid substances (e.g. butanoic
kapp of ammonium-N oxidation was 0.0707 (R2 = 0.9346) and the kapp acid, pentanoic acid and oxalic acid) and aromatic acids (e.g. benzoic
of COD oxidation was 0.0165 (R2 = 0.9518), indicating that ammo- acid and alizaric acid). This result was consistent with a previous study
nium-N was the main constraint for the accumulation of available which reported that organic pollutants were attacked by hydroxyl ra-
chlorine. Therefore, the beginning of available chlorine accumulation dicals and converted into smaller and less complex substances [55]. The
meant that the removal of ammonium-N was almost complete. After the formation of these small-molecule acids could be explained by the
reaction for 130 min, some ClO− ions were converted into ClO3− or oxidation of humic-like substances by hydroxyl radicals generated from

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Fig. 7. Three fluorescent components of BTLL identified by EEM-PARAFAC.

the reaction between Fe2+ and ClO-/HClO (Eq. (19) and Eq. (22)) [37]. that when microorganisms were added to TAR effluent, they turned
This result consistent with previous study which reported that the white (normally the microorganisms were black). Bacterial counts
oxidation of humic-like substances by hydroxyl radicals could produce showed that microbial communities were almost impossible to detect
some carboxylic acids (e.g. fumaric acid, oxalic acid and acetic acid after the culture process in TAR effluent, even without dilution.
[56]). The production of carboxylic acids may lead to a slight increase Therefore, available chlorine was the main cytotoxic substance. The
in COD. Fortunately, they can be easily removed by microorganisms cytotoxicity of CRR effluent was decreased after removal of available
during further treatment at the sewage treatment plants [51]. In our chlorine by CRR.
previous study [57], the formation of these small-molecule organic
compounds (e.g. alkanes and carboxylic acids) was found to be bene-
3.5. Precipitate analysis
ficial for enhancing the biodegradability of BTLL. In addition, there was
no desorption of humic-like substances from the surface of precipitates,
Fig. 9 shows the XRD patterns of the precipitates obtained from IAR
since the adsorption ability of Fe (III)-compounds was stronger than
(P1) and CRR (P2). Generally, amorphous compounds lack long-range
that of Fe (II)-compounds.
crystallographic order and produce broad humps with low intensities in
HClO + Fe2 + → Fe3 + + HO·+ Cl− (22) XRD patterns (for example, the wide range region I and region II in
Fig. 9a) [58]. In this study, the P1 precipitates contained a large amount
In order to evaluate the change in toxicity during the treatment of amorphous iron compounds with valence states between 2 and 3 (e.g.
process, the untreated BTLL and effluents from TAR and CRR were used Fe4.67(SO4)6(OH)2H2O and Fe0.98O). The formation of amorphous iron
to cultivate microbial colonies. The bacterial colony counting during oxides could be due to the Fe2+–Fe3+ incomplete electron exchange in
the culture process was determined (Supporting Information Fig. S4). the presence of organic matters [59]. The oxidation of available
The results showed that the microbial community in CRR effluent was chlorine promoted the formation of Fe3+ stabilized crystals. The in-
significantly increased while the microbial community in BTLL was tegral areas of region I and region II decreased with the removal of
decreased, which indicated that CRR effluent was more conducive to amorphous components, and the XRD peaks of stable crystals were
bacterial propagation than BTLL. Moreover, the changes in total or- found to be sharper.
ganic carbon (TOC) and inorganic carbon (IC) in BTLL and CRR effluent The crystal width and crystallinity of amorphous iron oxides are
cultures during the culture process were examined (Supporting weak, while their specific surface area is large [60,61]. Therefore, the
Information Fig. S5). In BTLL, the values of TOC and IC showed almost macro structure of amorphous iron oxides is usually fluffy [62]. It was
no change. In contrast, obvious decrease in TOC and increase in IC was observed from the experiments that the sedimentation ability of P2 was
observed in CRR effluent. This result indicated that cellular respiration significantly higher than that of P1. In order to evaluate the sedi-
actively occurred in CRR effluent and led to the mineralization of or- mentation properties of precipitates, the sludge volume index (SVI) and
ganic matter. In addition, TAR effluent could almost entirely kill the settling velocity (SV) of P1 and P2 were determined. The results showed
microorganisms in a short time. It was observed during the experiment that the SVI of P1 was 48.13 mL/g while the SVI of P2 was 35.32 mL/g,

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Fig. 8. The EEM of DOM during the reaction process (in IAR at 0–40 min (current density: 70 mA/cm2, initial pH: 6.5); in TAR at 40–160 min (current density:
70 mA/cm2, initial pH: 9.0); in CRR at 160–280 min).

Fig. 9. The XRD analysis of the precipitates obtained from the IAR and CRR ((a) the precipitates obtained from the IAR; (b) the precipitates obtained from the CRR).

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Fig. 10. The EDS analysis of the precipitates obtained from the IAR and CRR (in bracket of this figure, the number in the front and back were spectral intensity and
elemental mass percentage, respectively).

which indicated that the volume of P2 was smaller than that of P1. In decomposed and mineralized by the reaction of available chlorine and
addition, the SV of P1 (48%) was lower than that of P2 (62%), indicating hydroxyl radicals (e.g. oxidation, addition reaction, base-catalyzed and
that the sedimentation ability of P2 was higher than that of P1. The electrophilic substitution reactions [42]), and the degradation products
masses of P1 and P2 were 2701 mg and 2690 mg, respectively, which of BTLL were some hydrocarbons and carboxylic acids.
are quite similar. Therefore, the formation of Fe3+ stabilized crystals Overall, in IAR, due to the low chlorine evolution tendency of Fe
increased the density of precipitates, which was conductive to the solid- anode, there was weak inhibition of cathode reduction by the low
liquid separation. In order to better depict the changes of precipitates in concentration of available chlorine. The cathode reduction rapidly
CRR, the EDS energy spectrum was obtained, and the results are shown converted nitrate-N and nitrite-N into ammonium-N. Furthermore,
in Fig. 10. The energy spectrum intensities of C, O and Fe in P2 were all coagulation of iron ions rapidly removed the macro-molecular organic
higher than the corresponding values in P1, indicating that the pre- matter. In TAR, ammonium-N and the remaining COD were effectively
cipitates were further compressed during the removal process of removed by the oxidation of available chlorine. Therefore, in this study,
available chlorine. Moreover, the mass percentage of Fe was increased the developed electrochemical reactor integrating both IAR and TAR
while the mass percentages of C and O were decreased, which indicated was able to simultaneously remove TN and COD.
that some organic compounds were removed during the process of After the reaction of TAR, the residual available chlorine was de-
available chlorine removal. The SEM analysis showed that the particle graded into chloride ion by the IAR precipitate (Eq. (19) and Eq. (22))
size of P2 was significantly smaller than that of P1 (Fig. 11). Moreover, in CRR. The cytotoxicity of effluent was decreased with the removal of
P1 presented a non-compact structure, while P2 was compressed. available chlorine. In addition, the compaction of precipitate flocs was
Therefore, CRR can not only effectively remove the available chlorine, promoted by the oxidation of available chlorine. Thus, the removal
but also reduce the sludge volume as well as increase the sedimentation process of available chlorine also reduced the precipitates volume and
ability of sludge. enhanced the sedimentation ability of precipitates.

3.6. Reaction mechanisms in CES for BTLL treatment 3.7. Economic analysis

The reaction mechanisms involved in the BTLL treatment by CES are Considering the energy and electrode consumption, the cost of BTLL
illustrated in Fig. 12. The transformation and degradation of TN could treatment by CES was calculated. The electricity cost and electrode cost
be explained as follows: (i) in IAR, the nitrite-N and nitrate-N were for treating 1 L of BTLL by CES under the optimum conditions were
mainly reduced to ammonium-N by cathode [19,27,63]; (ii) in TAR, the $0.015 and $0.0006, respectively. Therefore, the treatment cost of
ammonium-N was mainly oxidized to nitrogen gas and nitrate-N by BTLL by CES was $0.0156 per L. In the past, other technologies for
available chlorine and hydroxyl radicals [15,64]. Moreover, a minute treatment of refractory organic wastewater have been reported, such as
amount of nitrite-N was produced as an intermediate product during coagulation + granular activated carbon, O3/H2O2 and solar photo-
nitrate-N reduction process in TAR, which was probably rapidly re- Fenton with the corresponding treatment costs of $0.0166 [66],
duced or oxidized into nitrogen gas or nitrate-N, respectively [65]. $0.0125 [67] and $0.0147 [68] per L, respectively. In this study, it was
The removal of COD in BTLL involved the following steps: (i) in IAR, clear that the main cost of CES was power consumption. In fact, the cost
the macromolecular organic matters (e.g. humic-like and fulvic-like of power supply can be reduced by the usage of alternate forms of
substances) were removed by electro-coagulation of iron hydroxides energy (e.g. solar energy [69,70], wind energy [71] and biomass energy
(e.g. Fe(OH)+, Fe(OH)2 and Fe(OH)3). Furthermore, COD was also ra- [1]). These renewable energy sources can be easily obtained from
pidly lowered by the removal of nitrite-N; (ii) in TAR, COD was natural environment and would allow further reduction of CES

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Fig. 11. The SEM analysis of the precipitates obtained from the IAR and CRR ((a), (c) and (e) was the precipitates obtained from the IAR; (b), (d) and (f) was the
precipitates obtained from the CRR).

operation costs. simultaneous removal of TN, COD and available chlorine from NCOW.
In this study, BTLL was selected as a typical NCOW to evaluate the
performance of CES. The effects of operating factors (e.g. current den-
4. Conclusions
sity and initial pH value) and removal process of available chlorine
were investigated. The following conclusions were drawn from the
The constructed CES was demonstrated to be feasible for the

Fig. 12. The mechanism of CES for the removal of TN and COD and degradation of residual available chlorine.

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