Fenton degradation of 4-chlorophenol using H2O2: In situ generated by Zn-CNTs/O2 system

Article abstract of DOI:10.1039/c7ra08634b

In this study, Zn-CNT composites were prepared by the infiltration fusion method and characterized by TEM, XPS and N₂ adsorption/desorption experiments. The reaction of Zn-CNTs and O₂ in aqueous solution was performed for the in situ generation of H₂O₂, which was employed to react with Fe²⁺ for the Fenton degradation of 4-chlorophenol (4-CP). The effect of various parameters, including the initial pH, dosage of Zn-CNTs and Fe²⁺ concentration on 4-CP degradation was examined. The removal efficiencies of 4-CP and TOC (total organic carbon) were 98.8% and 87.4%, respectively, when the Fe²⁺ concentration was 20 mg L^-1, initial pH was 2.0, Zn-CNT dosage was 2 g L^-1, reaction time was 20 min and O₂ flow rate was 400 mL min^-1. When 4-CP was spiked in the secondary effluent of a municipal wastewater treatment plant, the removal efficiency of 4-CP and TOC was 47.0% and 45.6%, respectively, under the above-mentioned conditions. The intermediate products were detected by LC-MS and IC, and the possible degradation pathway of 4-CP and the reaction mechanism of the Zn-CNTs/O₂/Fe²⁺ system were tentatively proposed.

Full text of DOI:10.1039/c7ra08634b

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Fenton degradation of 4-chlorophenol using H2O2
in situ generated by Zn-CNTs/O2 system
Cite this: RSC Adv., 2017, 7, 49985
a
bc
Yong Liu,ab Yanlan Liu,
Zhao Yanga and Jianlong Wang
*
In this study, Zn-CNT composites were prepared by the infiltration fusion method and characterized by
TEM, XPS and N2 adsorption/desorption experiments. The reaction of Zn-CNTs and O2 in aqueous
solution was performed for the in situ generation of H2O2, which was employed to react with Fe2+ for
the Fenton degradation of 4-chlorophenol (4-CP). The effect of various parameters, including the initial
pH, dosage of Zn-CNTs and Fe2+ concentration on 4-CP degradation was examined. The removal
efficiencies of 4-CP and TOC (total organic carbon) were 98.8% and 87.4%, respectively, when the Fe2+
concentration was 20 mg Lꢀ1, initial pH was 2.0, Zn-CNT dosage was 2 g Lꢀ1, reaction time was 20 min
and O2 flow rate was 400 mL minꢀ1. When 4-CP was spiked in the secondary effluent of a municipal
wastewater treatment plant, the removal efficiency of 4-CP and TOC was 47.0% and 45.6%, respectively,
under the above-mentioned conditions. The intermediate products were detected by LC-MS and IC, and
the possible degradation pathway of 4-CP and the reaction mechanism of the Zn-CNTs/O2/Fe2+ system
were tentatively proposed.
Received 4th August 2017
Accepted 16th October 2017
DOI: 10.1039/c7ra08634b
rsc.li/rsc-advances
concentration of electrolyte or electrical energy or luminous
energy or nutrient substances, which limits their application in
1. Introduction
wastewater treatment.
The advanced oxidation processes (AOPs) based on the gener-
ation of hydroxyl radicals (cOH) are excellent alternatives for the
efficient removal of persistent organic pollutants.1,2 Among all
the AOPs, the Fenton process has been widely implemented due
to its simplicity and the use of environmentally iron-based
catalysts.3 During the conventional Fenton process, H2O2 is
usually provided by bulk feeding, which does not yield a high
efficiency and has safety hazards associated with the chemical
performance of H2O2. Recently, the Fenton process with the in
situ generation of H2O2 has been extensively studied for the
destruction of organic pollutants. One of the advantages of the
in situ generation of H2O2 is that the H2O2 can be consumed at
an appreciable controlled rate prior to decomposition into H2O
and O2, which leads to an improved performance of H2O2 in the
favored reaction direction.4 Moreover, the application of in situ
generated H2O2 has been found to be potentially safer and more
economical than that of bulk commercial H2O2.5 Various
methods have been used for the in situ generation of H2O2 for
the Fenton oxidation of organic pollutants, such as electro-
Fenton,6–8 photo-Fenton,9,10 and bio-Fenton processes11,12 etc.
The in situ generation of H2O2 can be obtained by a micro-
electrolysis (ME) process based on the electrochemical reac-
tion between O2 and the ME materials, which was conrmed by
the reduction of O2 on the surface of iron–carbon13,14 or bi-
metals.15,16 This may be a promising way for the in situ genera-
tion of H2O2. However, ME processes with a low capacity of
H2O2 generation due to the usage of unsuitable ME materials
have limited the application. It has been reported that H2O2
could be generated by the reaction of Zn with O2,17–19 and the
carbon nanotubes (CNTs) could improve the two-electron
reduction of O2 during an electrochemical reaction because of
their good electrical conductivity, high surface activity and
mechanical strength.20,21 Therefore, it can be assumed that Zn-
CNTs/O2 micro-electrolysis may have a high capacity of H2O2
generation. Moreover, the Zn-CNTs/O2 process has following
advantages for Fenton degradation: (1) the active hydrogen [H]
generated during the micro-electrolysis process has a high
chemical activity;13,22 (2) the regeneration of Fe3+ can be
improved by Zn0 and enables the maximal amount of cOH to be
generated via the decompose of H2O2 by Fe2+; (3) the freshly
formed Zn(OH)2 colloid through the reaction between Zn2+ and
OHꢀ during the micro-electrolysis process has a good adsorp-
tion capacity for the pollutants. In this study, 4-chlorophenol
was selected as the model pollutant to assess the efficiency of
the proposed Zn-CNTs/O2/Fe2+ system due to its high toxicity
and extensive application.23,24
However, these processes require the use of
a high
aCollege of Chemistry and Materials Science, Sichuan Normal University, Chengdu
610066, PR China
bCollaborative Innovation Center for Advanced Nuclear Energy Technology, INET,
Tsinghua University, Energy Science Building, Beijing 100084, PR China. E-mail:
wangjl@tsinghua.edu.cn; Fax: +86-10-62771150; Tel: +86-10-62784843
cBeijing Key Laboratory of Radioactive Wastes Treatment, Tsinghua University, Beijing
100084, PR China
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The objective of this study was to investigate the in situ (0.1 M). The experiments were carried out in duplicate and the
generation of H2O2 from the micro-electrolysis process and its average results were used.
use for the degradation of 4-chlorophenol. A Zn-CNT composite
To evaluate the effect of wastewater composition on the
was prepared, characterized and used for the reduction of dis- degradation of 4-CP, the secondary effluent of a municipal
solved O2 to generate H2O2. The generated H2O2 subsequently wastewater treatment plant spiked with 4-CP (25 mg Lꢀ1 or
reacted with Fe2+, forming Fenton's reagent that oxidizes 50 mg Lꢀ1) was used. The secondary effluent of wastewater was
contaminants in the wastewater. The degradation of 4-CP in the collected at a local wastewater treatment plant (WWTP) and
secondary effluent of a municipal wastewater treatment plant used as received within the next 2 days. The initial 4-CP and
was studied to examine the effect of wastewater composition on TOC of the secondary effluent were 0 mg Lꢀ1 and 9.17 ꢂ
4-CP degradation. The intermediate products were detected and 1 mg Lꢀ1, respectively.
the possible degradation pathway of 4-CP and the reaction
mechanism of the Zn-CNTs/O2/Fe2+ process were tentatively
proposed.
2.4 Chemical analyses
4-CP was analyzed by HPLC, equipped with an XDB-C18 (4.6 ꢃ
150 mm) reversed-phase column and a diode array detector
(DAD). The mobile phase was prepared with water and aceto-
2. Materials and methods
2.1 Materials and chemicals
nitrile with a ow rate of 1 mL minꢀ1. The ratio (v/v) was 40/60.
H2O2 was determined by a UV-Vis Spectrophotometry (Perki-
4-Chlorophenol (purity 99%) was reagent grade and purchased
nElmer Lambda 25) at a wavelength of 385 nm. Total organic
from Sigma-Aldrich. The solvents (acetonitrile) used to prepare
carbon (TOC) was measured using a TOC/TN analyzer (2100,
the mobile phase for the chromatographic analyses were HPLC
Analytik Jena, Germany).
grade. Hydroxyl-containing multi-walled CNTs (d < 8 nm, l ¼
The intermediate products were identied by HPLC-MS
10–30 mm) was purchased from the Beijing Deke Daojing Nano-
equipped with the above-mentioned column coupled to a Shi-
Company, China. Polyethylene glycol 4000 (PEG) was purchased
madzu 2010EV mass spectrometer with an ESI ion source (LC-
from the National Medicines Corporation Ltd., China. Zinc
MS 2010, Columbia, USA). It was equipped with a photo diode
powder was obtained from the Shandong Xiya Corporation Ltd.,
array (PDA) detector, and operated in a negative mode. The
China. All the other chemicals used in this study were analytical
analysis of the intermediate products was performed under the
grade from the National Medicines Corporation Ltd., China. All
aforementioned solvent conditions. The injected volume was
reagents were used without further purication. De-ionized
30 mL. The organic acids were determined by ion-exclusion
water was used in all experiments.
chromatography.
2.2 Preparation and characterization of Zn-CNTs
3. Results and discussion
The Zn-CNT particles were prepared as follows: zinc powder,
3.1 Characterization of Zn-CNT composites
multi-walled CNTs and 40% w polyethylene glycol 4000 (the
A typical TEM images of the newly prepared Zn-CNTs and the
used Zn-CNTs are presented in Fig. 1, which reveals that the Zn-
CNTs possessed a skeletal network of CNTs with some particles
adhered to the surface of CNTs. Fig. 1(b) shows that the surface
of the used Zn-CNT composite was covered by some large
aggregates, indicating the formation of precipitates during the
degradation of 4-CP by the Zn-CNTs/O2/Fe2+ system.
Fig. 2 illustrates the N2 adsorption–desorption isotherms
and BJH pore size distribution plots, which were determined
from the adsorption branch of the N2 isotherms for the newly
prepared and the used Zn-CNTs. The isotherms were identied
as type IV and the hysteresis loops as type H3, which are the
characteristics of mesoporous materials and have a slit pore of
plate structure.
mass ratio of Zn, CNTs and PEG was 3 : 1 : 2) were mixed, and
then dried at room temperature to form a slurry. The obtained
ꢁ
slurry was heated for 1 h at 550 C in a pipe furnace with a N2
ow rate of 60 mL minꢀ1, followed by slow cooling to room
temperature.
The morphology of Zn-CNTs was observed by using a trans-
mission electron microscope (TEM) (HRTEM, JEM2100 and
JEOL). The specic surface areas (BET) and the Barrett–Joyner–
Halenda (BJH) pore size distribution of Zn-CNTs were deter-
mined by nitrogen adsorption–desorption isotherm measure-
ments. X-ray photoelectron spectroscopy (XPS) analysis was
performed by using an Al Ka X-ray (1486.6 eV) source for the
excitation (NOVA 3200e).
The pore-size distribution obtained from the isotherm
indicated that most of the pores were in the range 2 to 10 nm,
2.3 4-Chlorophenol degradation experiments
The degradation experiments were performed in a 0.5 L glass and the average pore size was calculated to be 17 nm and 14 nm
bottle containing 0.25 L of solution. Pure O2 was fed with ow for the newly prepared and the used Zn-CNT composites,
rate of 400 mL minꢀ1 through a diffuser placed at the bottom of respectively. The BET specic surface areas were calculated
the reactor. To promote convection, a stirrer was placed in the from N2 isotherms, and were about 27 m2 gꢀ1 and 63 m2 gꢀ1 for
reactor at a constant rate of 300 rpm. The reaction temperature the newly prepared and the used Zn-CNTs, respectively. The
was kept at 25 ꢂ 1 ꢁC. Prior to the experiment, the desired initial total pore volume was 0.12 cm3 gꢀ1 for the newly prepared Zn-
pH of the solution was adjusted using H2SO4 (0.1 M) and NaOH CNTs and 0.23 cm3 gꢀ1 for the used Zn-CNTs. The extremely
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Fig. 1 TEM images of the newly prepared Zn-CNTs (a) and the used Zn-CNTs (b).
Fig. 2 N2 adsorption/desorption isotherms and pore size distributions of the newly prepared Zn-CNTs (a) and the used Zn-CNTs (b).
high BET surface area and large total pore volume strongly data, the Auger KE of Zn0 was about 992.1 eV, while ZnO was at
supported the fact that Zn-CNT composite had a porous struc- 987.7–988.5 eV. Thus, the peak at 991.7 eV could be assigned to
ture, which was consistent with the result of the TEM images. Zn0,25 and the peak at 988.5 eV could be assigned to ZnO.26
Compared with the newly prepared Zn-CNTs, the increase of the Compared with the newly prepared Zn-CNTs, the peak at
BET specic surface area and total volume of the pores for the 986.4 eV was assigned to Zn(OH)2,27 which was observed for the
used Zn-CNTs may be due to the formation of small pores used Zn-CNTs, demonstrating the interaction between Zn2+ and
caused by the corrosion of zinc and the deposition of zinc oxide OHꢀ species. The formation of Zn(OH)2 was consistent with the
on the surface of Zn-CNTs during the reaction between Zn-CNTs result of the TEM analysis. The Fe state in the used Zn-CNTs is
and oxygen. The similar average pore size of the newly prepared shown in Fig. 3(d). The coexistence of Fe(0), Fe(I) and Fe(II) was
Zn-CNTs and the used Zn-CNTs indicated that the pores of the evidenced by four shoulders. The main peak at 717.9 eV was
Zn-CNTs were not seriously blocked by the formed precipitates assigned to the Fe(0) species.28 The higher binding energy Fe
during the reaction.
2p3/2 peak at 710.7 eV and its shake-up satellites at 724.3 eV
Surface analysis of the newly prepared Zn-CNTs and the used (2p1/2) were reported for Fe3O4.29 The peak at 712.5 eV corre-
Zn-CNTs was carried out using XPS (Fig. 3). The peaks for O 1s sponds to a satellite transition that is the characteristic feature
and O KLL were observed at 529.6 and 995.0 eV, respectively. of Fe2+,30 indicating that the interaction occurred between Zn0,
Additional peaks (525.9 and 535.4 eV) for Zn 2p indicated Fe2+ and the oxidant.
different chemical environments. The peak for C 1s at approx-
imately 284.8 eV was observed. Compared with the newly
prepared Zn-CNTs, Fe 2p peaks were observed, indicating that
3.2 In situ generation of H2O2 in the Zn-CNTs/O2 system
a lot of iron was deposited on the surface of the Zn-CNTs aer
The variation of H2O2 concentration versus the reaction time at
different initial pH values is shown in Fig. 4.
the reaction of Zn-CNTs with Fe2+
.
Fig. 3(b) and (c) exhibit the Auger KE of Zn in the newly
prepared and the used Zn-CNT composites. From the standard
It was noted that at the initial pHs of 2.0 and pH 6.0, the
reaction of Zn-CNTs with O2 led to the formation of H2O2 and
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Fig. 3 XPS full survey spectra (a), Zn Auger KE spectra of the newly prepared Zn-CNTs (b), Zn Auger KE spectra of the used Zn-CNTs (c), Fe 2p
spectra of the used Zn-CNTs (d).
contacted wastewater. If dissolved O2 existed in the solution,
H2O2 was generated due to the reduction of O2 in the cathode of
the Zn0-CNT corrosion cells and the reactions are listed as
follows (eqn (1)–(3)):
Anode:
Zn ꢀ 2eꢀ / Zn2+
(1)
Cathode:
O2 + 2H+ + 2eꢀ / H2O2
2H+ + 2eꢀ / H2
(2)
(3)
In the acid solution, the competition of the reduction
between H+ and O2 (shown in eqn (3)) led to a lower concen-
tration of accumulated H2O2 at an initial pH of 2.0 than at an
initial pH of 6.0.
It was reported that a certain concentration of H2O2 was
found in the Zn0/O2 or Al0/O2 systems. However, the
concentration of the generated H2O2 in the Zn0/O2 system
was below 18 mg Lꢀ1 with a stoichiometry of the same dosage
as that of Zn in a neutral solution.17–19 In the Al0/O2 process,
the concentration of H2O2 was only 180 mM at an Al0 dosage
of 5.0 g Lꢀ1 and reaction time of 250 min.15 Therefore, the in
situ generation of H2O2 can be improved by the electrode
reaction in the corrosion cell with Zn0 as the anode and the
Fig. 4 Variation of H2O2 concentration with reaction time at different
initial pH values. Experimental conditions: O2 flow rate
400 mL minꢀ1, [Zn-CNTs] ¼ 2.0 g Lꢀ1, T ¼ 25 ꢁC.
¼
the concentration of H2O2 increased with reaction time. The
concentration of H2O2 within 20 min was 19.5 and 103 mg Lꢀ1
at the initial pHs of 2.0 and 6.0, respectively. The formation of
H2O2 was due to the micro-electrolysis process. In Zn-CNTs, Zn0
adhered to the surface of the CNTs and the numerous corrosion
cells were formed between Zn0 particles and CNTs when they
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CNTs as the cathode, which favored the two-electron reduc-
tion of O2.
It was reported that the rate of reaction (5) (k ¼ 63–
76 Mꢀ1 sꢀ1) was more rapid than that of reaction (6) (k ¼ (0.1–
1.0) ꢃ 10ꢀ2 Mꢀ1 sꢀ1), and the oxidative capability of cOH was
higher than that of HO2c.31,32 So, the regeneration of Fe2+ from
Fe3+ played a key role in increasing the degradation rate of 4-CP
by the Fenton oxidation process.33,34 In the Zn-CNTs/O2/Fe2+
process, the regeneration of the catalytic species (Fe2+ ions) was
expected to occur via reactions (6) and (7). The regeneration of
Fe2+ could be obtained from the intensication reduction of
Fe3+ by Zn (eqn (6)), which produced enough Fe2+ to catalyze the
H2O2 dissociation, which increased the mineralization rate of
the pollutants. This was one of the reasons for the higher
degradation efficiency of 4-CP in the Zn-CNTs/O2/Fe2+ system
than in the Fe2+/H2O2 system. Moreover, in the Zn-CNTs/O2/Fe2+
system, the dissolved oxygen could improve the production of
the ortho–para chlorophenolperoxyl radical (ClPPc) and increase
the extent of benzene ring cleavage. Therefore, 4-CP degrada-
tion in the Fenton/O2 system was better than that in the Fenton/
N2 system.35,36
The high concentration of H2O2, the rapid regeneration of
Fe2+ from the reduction of Fe3+ by Zn and the good adsorption
capability of Zn-CNTs for 4-CP all contributed to the higher
oxidative capability of the Zn-CNTs/O2/Fe2+ system. It was re-
ported that 50 mg Lꢀ1 of 4-CP was completely degraded aer 2 h
in the bimetallic Al–Fe/O2 process, while aer 20 min in the Zn-
CNTs/O2/Fe2+ process, many active intermediate species of
H2O2 could be generated.16 This result proved the effectiveness
of the Zn-CNTs/O2/Fe2+ process, which might be a promising
alternative for the degradation of recalcitrant organic pollutants
in wastewaters.
3.3 Degradation and mineralization of 4-CP in different
systems
To investigate the catalytic performance of the Zn-CNTs/O2
system for the degradation of 4-CP by Fenton oxidation, 4-CP
degradation experiments in aqueous solution were carried out
in different systems, including Zn-CNTs/O2, Fe2+/H2O2 and Zn-
CNTs/O2/Fe2+. The removal efficiencies for 4-CP and TOC with
reaction time are shown in Fig. 5. As depicted in Fig. 5(a), the
removal efficiencies of 4-CP in Zn-CNTs/O2, Fe2+/H2O2 and Zn-
CNTs/O2/Fe2+ systems were 15.57%, 45.50% and 98.8% aer
20 min of reaction at initial pH 2.0, respectively.
Fig. 5(b) shows that neither the Zn-CNTs/O2 nor the Fe2+/
H2O2 system was efficient for TOC removal, with only 14.0% and
9.94% of TOC removed, respectively, aer 20 min of reaction.
Compared with the Zn-CNTs/O2 and Fe2+/H2O2 systems, the Zn-
CNTs/O2/Fe2+ system led to an 87.4% TOC removal within
20 min, which clearly indicated that the combination of Zn-
CNTs/O2 and Fe2+ had a synergetic effect. The 4-CP removal
efficiency was quite similar to the TOC removal efficiency in the
Zn-CNTs/O2 system, suggesting that a low degradation of 4-CP
occurred in the Zn-CNTs/O2 system and the removal of 4-CP in
the Zn-CNTs/O2 process was due to the adsorption of 4-CP by
Zn-CNTs, which exhibited an excellent adsorption capacity for
4-CP and enhanced its degradation.
Therefore, the Zn-CNTs/O2/Fe2+ system is one of the most
efficient electrochemical AOPs, consistent with the continuous
production of cOH using Fenton oxidation with micro-
electrolysis generated H2O2. The oxidation mechanisms of the
Fenton oxidation process are shown in the following equations: 3.4 Involved active species
tert-Butanol (TBA) is known as an cOH scavenger, which are
widely used to examine the role of cOH. As shown in Fig. 6, the 4-
CP removal efficiency decreased obviously when TBA was added
to Zn-CNTs/O2/Fe2+ system, indicating that 4-CP was mainly
decomposed by the attack of cOH (including surface-bound cOH
and free cOH). The iodide ion is used to scavenge the surface-
O2 +2H+ + 2eꢀ / H2O2
H2O2 + Fe2+ / Fe3+ + OHꢀ + cOH
Fe3+ + H2O2 / Fe2+ + HO2c + H+
2Fe3+ + Zn / 2Fe2+ + Zn2+
(4)
(5)
(6)
(7) bounded cOH.21,37,38 It can be seen that the removal efficiency
Fig. 5 Comparison of 4-CP degradation in different reaction systems. Experimental conditions: [4-CP]0 ¼ 50 mg Lꢀ1, O2 flow rate ¼
400 mL minꢀ1, [Zn-CNTs] ¼ 2.0 g Lꢀ1, [Fe2+] ¼ 20 mg Lꢀ1, [H2O2] ¼ 20 mg Lꢀ1, T ¼ 25 ꢁC, pH ¼ 2.0.
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scavengers was similar, indicating that the degradation of 4-CP
was primarily attributed to the surface bound cOH.
3.5 Inuencing factors for 4-CP degradation
3.5.1 Effect of the initial pH. pH is an important parameter
for the generation of H2O2 by the reduction of O2 on the surface
of Zn-CNTs (Fig. 4), and pH also affects the dominant iron
species in aqueous solution and the generation of cOH through
the Fenton processes. The inuence of the initial pH on the
degradation of 4-CP by the Zn-CNTs/O2/Fe2+ process was inves-
tigated (Fig. 7). With the decrease of initial pH from 4.0 to 3.0, the
removal efficiency of 4-CP increased from 50.2% to 86.6% within
20 min. The removal efficiency of 4-CP was 91.4% at an initial pH
of 1.0, which was slightly lower than at an initial pH of 2.0 aer
20 min of reaction. The best degradation performance was ach-
ieved at an initial pH of 2.0, with 98.8% degradation within
20 min of reaction. The increased degradation efficiency at
a lower pH was attributed to the higher oxidation potential of
cOH and the higher stability of H2O2 in an acidic solution, which
could not immediately decompose to H2O and O2.8,33
Fig. 6 Effect of radical scavengers on the degradation of 4-CP.
Experimental conditions: [4-CP]0 ¼ 50 mg Lꢀ1, O2 flow rate ¼
400 mL minꢀ1, [Zn-CNTs] ¼ 2.0 g Lꢀ1, [Fe2+] ¼ 20 mg Lꢀ1, T ¼ 25 ꢁC,
pH ¼ 2.0.
of 4-CP was only 21.2% in 20 min when excess KI (20 mmol Lꢀ1
)
In order to explain the effect of the initial pH on the removal
was added into the system, which was lower than that of the efficiency of 4-CP, the variation of pH during the Zn-CNTs/O2/
system without scavengers (98.8% removal efficiency of 4-CP). Fe2+ process was determined and is shown in Fig. 7(b). It was
This suggested that surface-bound cOH played a signicant role
noted that the pH value increased gradually to be stable with
in the degradation of 4-CP. The inhibited degree of the two
a prolonged reaction time. The nal pH was 2.1, 5.0, 5.3 and 6.6,
Fig. 7 Influencing factors for 4-CP degradation in the Zn-CNTs/O2/Fe2+ system: (a) initial pH; (b) the variation of the solution pH; (c) Zn-CNTs
dosage; (d) Fe2+ concentration. Except for the investigated parameters, other parameters were fixed: [4-CP]0 ¼ 50 mg Lꢀ1, O2 flow rate ¼
400 mL minꢀ1, [Zn-CNTs] ¼ 2.0 g Lꢀ1, [Fe2+] ¼ 20 mg Lꢀ1, T ¼ 25 ꢁC, pH ¼ 2.0.
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respectively, when the initial pH was 1.0, 2.0, 3.0 and 4.0, respec- 50 mg Lꢀ1 4-CP were 47.0% and 45.6%, respectively. The
tively. Several processes performed simultaneously in the Zn- removal efficiencies of 4-CP and TOC in the secondary effluent
CNTs/O2/Fe2+ system, including the hydrolysis of the Zn(II) species were lower than that of the previous experiments in aqueous
or the Fe(III) species, the production of small-molecule organic solution at the same conditions, in which 98.8% and 87.7% of 4-
acids, and the generation of H2O2 or H2 by the reduction of O2 or CP and TOC were removed (Fig. 5). The reason may be due to
H+. H+ was released into the solution in the rst two processes and the presence of typical constituents, such as Clꢀ, SO42ꢀ, HPO4
2ꢀ
consumed in the third process. At the beginning, the generation and HCO3ꢀ, anions in the secondary effluent, which can act as
rate of H2O2 or H2 was faster than the consumption rate of H+, cOH scavengers and retard the Fenton reaction.42 Nevertheless,
which led to the increase of pH. When the generation rate of H+ the removal efficiency of TOC was over 45% in the secondary
was equal to its consumption rate, the solution pH was kept stable effluent with two different concentrations of 4-CP, suggesting
in a relative narrow range. This variation trend of the solution pH that the Zn-CNTs/O2/Fe2+ process was very effective for the
during the Zn-CNTs/O2/Fe2+ process could lead to a low pH envi- advanced treatment to remove persistent organic pollutants
ronment for pollutant degradation and high pH conditions for such as 4-CP in municipal wastewater.
neutralization without chemical addition, which is an important
advantage for its practical application.
3.5.2 Effect of Zn-CNTs dosage. The Zn-CNTs dosage will
affect the concentration of H2O2 in solution, which controlled
the productivity of cOH. The effect of Zn-CNTs dosage on 4-CP
3.7 The intermediate products and pathway of 4-CP
degradation
The variation of the concentration of formic, acetic and oxalic
acids during the degradation of 4-CP is illustrated in Fig. 9. As
can be seen, the concentration of formic and oxalic acids began
to increase within 5 min, reached a peak at about 6 min, and
then decreased. Additionally, the concentration of acetic acid
was very low, below 1 mg Lꢀ1 within 20 min.
The intermediate products generated during the 4-CP
degradation process by the Zn-CNTs/O2/Fe2+ system were
determined by LC-MS analysis. Intermediates at m/z 143, m/z
191 and m/z 195 were observed in the aqueous phase, which
indicated that the substitution reaction of cOH and the addition
reaction of cOH were involved in the Zn-CNTs/O2/Fe2+ process.
The appearance of intermediates at m/z 199 and m/z 91 sug-
gested the occurrence of the opening ring of aromatics and
dehalogenation in the degradation process. The intermediate at
m/z 189 might be the oxidized product of the intermediate at m/
z 199 by O2. Benzoquinone (BQ) was not detected, which indi-
cated that the direct dechlorination of 4-CP by cOH attack could
degradation was investigated. It can be observed that degrada-
tion of 4-CP increased with the increase of Zn-CNTs dosage.
However, when the dosage of Zn-CNTs was higher than 2.0 g Lꢀ1
,
the degradation of 4-CP was not signicantly increased, possibly
because higher Zn-CNTs dosage generated more H2O2 in solu-
tion, and excess H2O2 might also be converted to HO2c instead of
cOH because of scavenging effect of cOH by H2O2.39
cOH + H2O2 / HO2c/O2cꢀ + H2O
(8)
As shown in reaction (8), if H2O2 was excess, a less oxidative
radicals (HO2c and O2cꢀ) might be produced and a highly
oxidative radical (cOH) was consumed.40,41 The optimal Zn-CNTs
dosage was 2.0 g Lꢀ1 in this experiment.
3.5.3 Effect of Fe2+ concentration. The effect of Fe2+
concentration on the degradation of 4-CP is illustrated in Fig. 7(d).
It can be observed that the 4-CP removal efficiency was low (only
60.9%) at an Fe2+ concentration of 5 mg Lꢀ1. When the Fe2+
concentration increased to 10 mg Lꢀ1 and 20 mg Lꢀ1, the 4-CP
removal efficiency was 75.6% and 98.8%, respectively. This can be
explained by the Fenton reaction (9), in which Fe2+ ions promoted
H2O2 decomposition and increased the production of cOH.
Fig. 7(d) also shows that when the Fe2+ concentration was
increased from 20 mg Lꢀ1 to 50 mg Lꢀ1, the 4-CP removal efficiency
decreased from 98.3% to 73.5%, suggesting that an excess of iron
was detrimental to the degradation of 4-CP because excess iron
would consume cOH (eqn (10)) and reduce the amount of available
Fe2+ ion to catalyze H2O2 dissociation.7 Hence, for the effective
degradation of 4-CP, the Fe2+ concentration was 20 mg Lꢀ1
.
Fe2+ + H2O2 / Fe3+ + cOH
Fe2+ + cOH / Fe3+ + OHꢀ
(9)
(10)
3.6 Degradation of 4-CP in actual wastewater
Fig. 8 Effect of wastewater composition on the degradation of 4-CP
by the Zn-CNTs/O2/Fe2+ system. Experimental conditions: O2 flow
rate ¼ 400 mL minꢀ1, [Zn-CNTs] ¼ 2.0 g Lꢀ1, [Fe2+] ¼ 20 mg Lꢀ1, T ¼
The degradation of 4-CP in the secondary effluent spiked with 4-
CP is given in Fig. 8. It can be seen that the removal efficiencies
of 4-CP and TOC in the secondary effluent spiked with 25 ꢁC, pH ¼ 2.0.
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be neglected and 4-CP degradation was mainly by the 4-chlor-
ocatechol (4CC) pathway in the Zn-CNTs/O2/Fe2+ system.23,35
As a result, the main degradation pathway is proposed in
Fig. 10. According to previous studies, 4-chlorocatechol was
rapidly formed when the reaction was initiated by the attack of
cOH at the ortho-position of the hydroxyl group. 4-Chlor-
ocatechol was attacked by the addition or substitution reaction
of cOH to form chlorinated intermediates D2 and D3. D2 can be
easily transformed into quinine D5. Then, the C]C bond in the
aromatic rings was cleaved by the attack of cOH to yield D4 and
D6, which were oxidized to aliphatic carboxylic acid interme-
diates (e.g., formic acid, acetic acid, oxalic acid, etc.) that would
be further mineralize into CO2 and H2O.
Fig. 9 Evolution of the concentration of small organic acids formed
3.8 Reaction mechanism in the Zn-CNTs/O2/Fe2+ system
during the degradation of 4-CP. Experimental conditions: [4-CP]0
¼
50 mg Lꢀ1, O2 flow rate ¼ 400 mL minꢀ1, [Zn-CNTs] ¼ 2.0 g Lꢀ1, [Fe2+
¼ 20 mg Lꢀ1, T ¼ 25 ꢁC, pH ¼ 2.0.
]
On the basis of the aforementioned analyses and the Fenton
oxidation mechanism,8,20,31,43,44 the reaction mechanism of the
Zn-CNTs/O2/Fe2+ process is tentatively proposed (Fig. 11). In the
Fig. 10 Proposed degradation pathway of 4-CP by the Zn-CNTs/O2/Fe2+ system.
Fig. 11 The reaction mechanism of the Zn-CNTs/O2/Fe2+ system.
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Zn-CNT composite, Zn0 adheres to the surface of the CNTs and
numerous corrosion cells were formed between the particles of
Zn0 and CNTs, which was followed by reaction with dissolved
O2, leading to H2O2 formation. Then, surface chemisorbed
H2O2 diffused onto the surface of Zn-CNTs or in the solution,
and were decomposed into the cOH radical by Fe2+. In the last
stage, the organic molecules approached the cOH radical and
were oxidized.
Acknowledgements
This research was supported by the National Natural Science
Foundation of China (51338005) and the Program for Chang-
jiang Scholars and Innovative Research Team in the University
(IRT-13026).
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There are no conicts to declare.
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