Fenton-like degradation of 2,4-dichlorophenol using calcium peroxide particles: performance and mechanisms

Article abstract of DOI:10.1039/C6RA26754H

The degradation process of 2,4-dichlorophenol (2,4-DCP) in aqueous solution by a Fenton-like system using calcium peroxide (CaO₂) particles as a source of solid H₂O₂ was investigated. EDTA was added to the system to chelate iron and promote the Fenton-like reaction. The results show that 2,4-DCP can be readily degraded in a CaO₂/EDTA-chelated Fe(ii) system. Compared with an H₂O₂/EDTA-chelated Fe(ii) system, 2,4-DCP degradation showed a moderate reaction rate owing to the slow dissolution process of the CaO₂ particle. Complicated degradation kinetics were observed and the possible reason was revealed. The initial amount of EDTA chelated iron lost during the Fenton-like reaction was in the range 20-30%. The effects of different reaction parameters, such as initial pH, CaO₂ dosage and amount of EDTA-chelated iron, on 2,4-DCP degradation were studied. According to the identification of dominant reactive species and degradation intermediates, a possible theoretical degradation pathway of 2,4-DCP was proposed.

Full text of DOI:10.1039/C6RA26754H

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Fenton-like degradation of 2,4-dichlorophenol
using calcium peroxide particles: performance and
mechanisms
Cite this: RSC Adv., 2017, 7, 4563
ab
ab
c
ab
a
Hefei Wang, Yongsheng Zhao, Yan Su, Tianyi Li, Meng Yao
ab
and Chuanyu Qin*
The degradation process of 2,4-dichlorophenol (2,4-DCP) in aqueous solution by a Fenton-like system
using calcium peroxide (CaO ) particles as a source of solid H was investigated. EDTA was added to
the system to chelate iron and promote the Fenton-like reaction. The results show that 2,4-DCP can be
readily degraded in a CaO /EDTA-chelated Fe(II) system. Compared with an H /EDTA-chelated Fe(II)
system, 2,4-DCP degradation showed a moderate reaction rate owing to the slow dissolution process of
the CaO particle. Complicated degradation kinetics were observed and the possible reason was
revealed. The initial amount of EDTA chelated iron lost during the Fenton-like reaction was in the range
0–30%. The effects of different reaction parameters, such as initial pH, CaO dosage and amount of
2
2 2
O
2
2 2
O
2
Received 14th November 2016
Accepted 22nd December 2016
2
2
EDTA–chelated iron, on 2,4-DCP degradation were studied. According to the identification of dominant
reactive species and degradation intermediates, a possible theoretical degradation pathway of 2,4-DCP
was proposed.
DOI: 10.1039/c6ra26754h
www.rsc.org/advances
c
2
ꢀ
1
Introduction
FeðIIÞ þ HO /HOO þ FeðIII
Þ
(6)
The Fenton-like process, which was developed from traditional
Fenton technology, provides a promising alternative for the
oxidative treatment of contaminated soil and groundwater.
Conventional Fenton's reagent involves the catalyzed decom-
position of hydrogen peroxide (H O ) by Fe(II) to form a suite of
HOc + HOc / H O
(7)
(8)
2
2
HOc + organics / products + CO
2
2
+ H O
2
2
However, the conventional Fenton process, which is used
primarily for treating waters and wastewaters, is not feasible for
in situ soil or groundwater remediation. This is because the
conventional Fenton's reaction is usually conducted by gradu-
ally adding dilute H O to a solution of excess Fe(II) in condi-
highly active species, including the hydroxyl radical (HOc),
c
ꢀc
perhydroxyl radicals (HO ), superoxide radical anions (O2 ),
2
ꢀ
1–6
and hydroperoxide anions (HO ) (reaction (1)–(8)). Among
2
them, HOc generated via reaction (1) is the main species. It has
a high oxidation potential (2.76 V) and can react indiscrimin-
ately with most organic contaminants at near diffusion-limited
2
2
tions where the optimal pH is below 3. Fenton's reagent is
usually modied for in situ chemical oxidation (ISCO) using
a one-time addition of higher concentration of H
1
rates.
2 2
O and
ꢀ
varying the type of catalyst (i.e. Fe(II), Fe(III), iron chelates, or
iron minerals). A high concentration of liquid H O is typi-
2 2
H
2
O
2
+ Fe(II) / HOc + OH + Fe(III)
(1)
(2) cally injected in the subsurface for the supply of H O in the
7,8
þ
c
2
FeðIIIÞ þ H
2
O
2
/FeðIIÞ þ H þ HO
2 2
modied Fenton system (MF, or Fenton-like system). However,
(3) the H O utilization efficiency is very low because of the insta-
c
2
þ
FeðIIIÞ þ HO /FeðIIÞ þ H þ O
2
2 2
9
bility of H
2 2
O in the subsurface. It has been reported that liquid
ꢀ
H O can only persist from a few minutes to several hours aer
Fe(II) + HOc / Fe(III) + OH
(4)
(5)
2 2
injected into the subsurface. Disproportionation (reaction (9))
c
HOc þ H
2
O
2
/H
2
O þ HO
10,11
2
constitutes the major loss of H O at neutral pH.
It
2
2
consumes H
2
O
2
without producing HOc and releases O
2
gas,
a
which clogs pores around the injection wells and promotes
College of Environment and Resources, Jilin University, Changchun 130021, China.
12,13
E-mail: qincyu@jlu.edu.cn; Tel: +86 43188502608
b
contaminant volatilization.
Therefore, means for slow and
Key Laboratory of Groundwater Resources and Environment, Ministry of Education, controlled release of the H O into the subsurface are highly
2
2
Jilin University, Changchun 130021, China
desirable.
c
Shenyang Academy of Environmental Sciences, Shenyang 110167, China
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2
H O / H O + O
(9)
2
2
2
2
2
Materials and methods
2
.1 Materials
By far, one of the most useful solid forms of peroxide for
1
4,15
environmental applications is calcium peroxide (CaO
Previously, CaO was mostly used as an oxygen release
compound (ORC), which could slowly decompose to release
2
).
Analytical grade calcium peroxide (75% CaO
and 2,4-dichlorophenol (2,4-DCP 99.0%) were purchased from
Aladdin Reagent Co. Ltd. (Shanghai, China). Chromatographic
2
2
, 25% Ca(OH) )
2
1
6–18
oxygen when in contact with water (reaction (10)).
Recent
3
grade methyl alcohol (CH OH, 99.9%) was purchased from
studies showed that CaO can be a more effective source of
2
TEDIA High Purity Solvent Co. Ltd. (Faireld, America). Aceto-
1
9,20
H O than liquid H O in MF-ISCO.
In fact, CaO2 can
2
2
2
2
nitrile (C H N, 99.9%) was purchased from Thermo Fisher
2
3
dissolve in water to form H
range, liberating a maximum of 0.47 g H
According to the available literature, the yield of H
2 2
O via eqn (11) at a large pH
Scientic Co. Ltd. (Shanghai, China). Carbon tetrachloride (CT,
9.5%) was purchased from Tianjin Aoran Chemical Research
Institute (Tianjin, China). Monosodium phosphate dihydrate
NaH PO $2H O, 99.0%) and disodium phosphate dodecahy-
drate (Na HPO $12H O, 99%), anhydrous disodium ethylene-
diaminetetraacetate (C H O N Na , EDTA, 99%), sodium
2
1
2
O
2
per g CaO
2
.
9
2
O
2
from
CaO
2
decreased, while production of O
2
9,22
was elevated with
And about 70–80%
(
2
4
2
1
the increase of pH and temperature.
2
4
2
of CaO2 was transformed to H O at moderate pH and
2
2
1
0
14
8
2
2
temperature. Moreover, H O released from CaO2 is auto-
2
2
formate (NaCHO , 99.5%), sodium acetate (NaC H O , 99.0%),
2 2 3 2
regulated by the rate of CaO dissolution, reducing dispro-
2
acetic acid (C H O , 99.5%), sodium oxalate (Na C O , 99.8%),
2 4 3 2 2 4
portionation of H
2
O
2
to O
2
since not all the H
2
O
2
is available
ammonium acetate (C H O N, 99%), tert-butyl alcohol
2
7 2
at once as it is with liquid H
2
O
2
.
(
(CH
ferrous sulfate heptahydrate (FeSO
purchased from the Sinopharm Chemical Reagent Co. Ltd.
Beijing, China). Ammonium molybdate (((NH Mo
, 99.0%), nitrobenzene (NB,
9.0%), sodium hydroxide (NaOH, 96%) and methanol
3
)
3
OH, TBA, 98.0%), hydrogen peroxide (H
2 2
O , 30%) and
4
$7H O, 99.0%) were
2
CaO
2
+ H
2
O / Ca(OH)
2
Y + 1/2O
2
[
(10)
(11)
(
9
9
4
)
6
7
O24),
CaO + 2H O / Ca(OH) Y + H O
2
2
2
2
2
9.0%), trichloromethane (CHCl
3
The excellent properties of CaO
2 2 2
to release H O at
(CH OH, 99.9%) were purchased from Beijing Chemical Works
3
a controlled rate attracted more and more researchers to apply
23,24
25
(Beijing, China). Ultrapure water from a Milli-Q water process
Classic DI, ELGA, Marlow, UK) was used for preparing aqueous
solutions.
CaO
2
in MF-ISCO.
Ndjou'ou and Cassidy
applied
(
a commercially available CaO -based oxidant to treat soils
contaminated with total petroleum hydrocarbons (TPH) and
2
20
found that CaO removed 96% of TPH. Bogan et al. reported
2
2
.2 Preparation
2 2 2
that CaO performed better than liquid H O for removing
polycyclic aromatic hydrocarbons (PAHs) from soil. Xiang The buffer solutions used in this study were prepared with
26
Zhang et al. used CaO
2
activated with ferrous ions to treat NaH PO $2H O and Na HPO $12H O in de-ionized water.
2 4 2 2 4 2
trichloroethylene (TCE) with 100% removal efficiency. Northup Buffer solutions with three different pH values (6.0, 7.0 and 8.0)
19
and Cassidy investigated CaO
2 2 2
dissolution to yield H O at were chosen to evaluate the effect of pH on the degradation
various pH conditions and the degradation performance of performance. The buffer strength of the solutions was 0.05 mol
ꢀ
1
perchlorethylene (PCE) in the modied Fenton system.
L . Preliminary testing veried that each buffer solution was
Although the preliminary experimental results look able to maintain the desired pH with the dose of CaO and other
2
promising, the relationship between 2,4-DCP removal effi- reagents used. A stock solution of 2,4-DCP was prepared by
ciency and the main inuencing factors in a CaO
system, particularly CaO dosage, chelate-Fe(II) content, and brate with buffer solutions of desired pH value under gentle
solution pH, need to be further studied. In addition, the stirring and then diluted to the desired concentration. In the
mechanisms of CaO -based MF oxidation and the degrada- studies, each reactor also received EDTA chelated Fe(II) to
tion route of chlorinated aromatic hydrocarbons are still not catalyze the MF reaction. The initial concentration of 2,4-DCP
2
-based MF allowing the pure non-aqueous phase liquid 2,4-DCP to equili-
2
2
ꢀ
1
thoroughly elucidated. In this research, 2,4-dichlorophenol was kept at 100 mg L (0.61 mM). In all experiments except for
2,4-DCP) was used as the representative target contaminant the effect of CaO dosage and the amount of EDTA–Fe(II) on the
(
2
of chlorinated aromatic hydrocarbons. The aim of this degradation performance, the initial molar ratio of CaO /EDTA–
2
experiment is rst to explore a practical system, which can Fe(II)/2,4-DCP was set as 16/4/1.
degrade 2,4-DCP with high removal efficiency, and nd out
the effect of main experimental parameters on 2,4-DCP
2.3 Procedure
removal. Second, molecular probe tests and scavenger tests
were conducted to identify the reactive oxygen species
responsible for 2,4-DCP degradation. Third, the mineraliza-
tion of 2,4-DCP was monitored, the main intermediate
products were analyzed and the 2,4-DCP degradation
pathway was proposed.
All experiments were conducted in a 250 mL jacketed cylindrical
ꢁ
ꢁ
glass reactor with a constant temperature of 22 C ꢂ 0.5
C
using a thermostat circulating water bath. A magnetic stirrer
was used to ensure the uniformity of contaminants. Aer the
chemicals participating in the reaction, except for CaO2 (for
instance, 2,4-DCP, EDTA chelated ferrite, etc.), were dissolved in
the reactor and the reaction was started by adding the desired
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2
CaO dosage. Samples were withdrawn at desired time intervals
3
Experimental results and discussion
and immediately quenched with methyl alcohol before the
analysis of 2,4-DCP. In order to identify the reactive oxygen
species generated during the reaction, probe tests were con-
3.1 The performance of 2,4-DCP degradation in a Fenton-
like system
ducted in accordance with the 2,4-DCP degradation procedure The degradation of 2,4-DCP along time under different experi-
2
7,28
ꢀc
and 2,4-DCP was replaced by NB (HOc probe),
probe).
or CT (O2
mental conditions was evaluated as shown in Fig. 1.
29,30
Furthermore, to evaluate the contribution of the
The experiments were carried out with a xed 2,4-DCP
reactive oxygen species to 2,4-DCP degradation, TBA (HOc concentration at different CaO (or H O )/EDTA–Fe(II)/2,4-DCP
2
2 2
28
ꢀc
29
scavenger) and CHCl
3
(O2 scavenger), as radical scavengers, molar ratios. The pH value was maintained at 7.0 with phos-
ꢁ
had been applied in the experiments to observe 2,4-DCP phate buffer and the temperature was kept at 22 C. It was
degradation changes. Phosphate buffer solutions at three pH observed that within 450 min of reaction time, less than 5%
values (6.0, 7.0 and 8.0) were chosen to observe the effect of pH removal of 2,4-DCP was obtained with a molar ratio of 16/0/1. In
on the degradation performance and in other experiments, the the H O /EDTA–Fe(II)/2,4-DCP system, 2,4-DCP was removed
2
2
pH of solution was maintained at 7.0. All experiments were completely within only 20 min and as the degradation reaction
conducted in duplicate and the mean values were reported. The progressed, a large amount of micro-bubbles were generated in
standard deviations in all experiments were in the range of the solution. It means that a large amount of H O decomposed
2
2
0.012–0.050.
to O during this violent degradation reaction and this problem
2
would be magnied when H O is applied to actual site reme-
2
2
10,11
diation according to previous studies.
Remarkable degradation of 2,4-DCP was also achieved in the
2.4 Analyses
The concentrations of 2,4-DCP and NB were measured by CaO /EDTA–Fe(II) system. However, the degradation rate was
2
means of an Agilent 1100 high performance liquid chromato- relatively moderate compared with that of the conventional
graph (HPLC) with a C18 reversed-phase column (5 mm, 4.6 mm H O /EDTA–Fe(II) system. In the CaO /EDTA–Fe(II) system, the
2
2
2
ꢃ 150 mm) and a diode array detector (DAD). To determinate removal efficiency of 2,4-DCP reached 83.0% within 30 minutes,
the concentration of 2,4-DCP, the mobile phase was methanol/ and then the degradation rate (slope of the degradation curves)
ꢀ1
water (70 : 30, v/v) with a ow rate of 1.0 mL min . The UV slowed down gradually. It indicates that remarkable degrada-
detector was set at 225 nm. To measure aqueous samples tion of 2,4-DCP was also achieved in the CaO /EDTA–Fe(II)
2
containing NB, the mobile phase was acetonitrile/acetate system with a relatively moderate degradation rate compared
ꢀ1
buffer solution (65 : 35, v/v) with a ow rate of 1.0 mL min
The UV detector was set at 262 nm. The degradation interme- According to the literature and our previous studies,
diates were identied using a Waters ultra performance liquid can slowly release H O aer being dissolved in water. It can
.
2 2
with that of the conventional H O /EDTA–Fe(II) system.
19,22
CaO2
2
2
chromatography (UPLC) with an Acquity UPLC C18 column (1.7 thus be reasonably deduced that EDTA–Fe(II) reacted quickly
mm, 2.1 ꢃ 100 mm) and Quattro Premier™ XE mass spec- with H O released by CaO2 and the reaction facilitated the
2
2
trometer. The mobile phase was methanol/water (70 : 30, v/v) CaO dissolution simultaneously, generating a large amount of
2
ꢀ
1
with a ow rate of 0.25 mL min . The desolvation gas was reactive oxygen species. Along with the generation of H O ,
2
2
ꢁ
N
2
and the gas temperature was 400 C. The ion source Ca(OH) can also be formed through reaction (11), but phos-
2
ꢁ
temperature was set at 110 C. CT in the solution was rst phate buffer solution was able to maintain the desired pH. In
extracted with hexane (1 mL) for 3 min using a vortex stirrer addition, only a small amount of bubbles, which were consid-
and kept standing for 5 min for separation. Extracts containing ered as O , were generated in the CaO /EDTA–Fe(II) system. It is
2
2
CT were then measured by means of an Agilent 6890A gas
chromatograph with an autosampler (Agilent 7693), a DB-VRX
column (1.4 mm, 0.25 mm ꢃ 60 m) and an electron capture
detector (ECD). The temperatures of the injector, column and
ꢁ
ꢁ
ꢁ
ECD detector were set at 240 C, 80 C and 260 C, respectively.
Total organic carbon (TOC) was measured by a total organic
carbon analyzer (TOC-L, Shimadzu, Japan). The H O
2
2
concentration in aqueous phase was determined using
a spectrophotometer (Evolution, Thermo, United States) at
a wavelength of 340 nm aer the color was developed by
ammonium molybdate. The solution pH was analyzed using
a yellow spring instrument (YSI-100, America) pH-meter. To
ꢀ
determine the concentration of chloride ions (Cl ), carboxylic
acids and EDTA, an ion chromatograph (ICS-2100, Dionex,
Germany) equipped with a Dionex RFICTM IonPac® AS 19
analytical column (4 mm ꢃ 250 mm) was used throughout
the experiment. The eluent was 3.5 mM KOH with a ow rate of
Fig. 1 Degradation of 2,4-DCP by CaO
2
2 2
or H O in a modified Fenton
ꢀ
1
1
.0 mL min
.
system ([2,4-DCP]
0
¼ 0.61 mM).
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concluded that CaO
2
, compared with H
2
O
2
, can dramatically power and can degrade both 2,4-DCP and EDTA indiscrimin-
prolong the contaminant removal reaction time by releasing ately to form carboxylic acids of small molecules or CO2.
H O at a controlled rate and enhance the utilization efficiency Previous studies also showed the degradation of EDTA in the
2
2
31,32
of peroxide by reducing the O yield, which is not responsible MF system.
In addition to this, the identication results of
2
for the MF reaction. The abovementioned results demonstrated deposits in the bottom of the reactor showed the presence of
that it was much more feasible to use CaO
source of H to promote MF-ISCO.
2
as a long-term EDTA, which indicates that a small amount of EDTA initially
dissolved in the solution precipitated out during the reaction
2 2
O
To investigate the activity of EDTA–Fe(II) in the degradation time.
of 2,4-DCP, a control experiment was conducted with the molar
It can also be observed that the variation of iron ion
ratio of CaO /Fe(II)/2,4-DCP, consistent with that of other concentration was in accordance with EDTA, which is because
2
systems. It can be observed that only 13% of 2,4-DCP was the dissociated iron ions with the decay of the chelating agent
ꢀ
removed without the addition of EDTA, whereas 95% of 2,4-DCP will soon react with OH to form sediment and precipitate out
2
was removed in the system of CaO /EDTA–Fe(II)/2,4-DCP. Total from the solution. Although concentrations of both EDTA and
iron testing (not shown) showed that only little soluble iron iron ions were reduced by 20–30% at the end of the experi-
remained in the system at the end of experiments. By contrast, ments, the concentration le in the MF system was still enough
about 70% of initial iron was detected in the CaO
2
2
concentration of soluble iron. Two reasons can account for the
diminution of Fe(II) catalytic activity. On one hand, phosphate
used to buffer pH can combine most of the Fe(II) in solution to
form sediment as soon as Fe(II) is added into the solution. On
the other hand, the other part of Fe(II) involved in Fenton
degradation was oxidized to Fe(III) immediately by HOc and then
2
/EDTA–Fe(II)/ for catalytic reaction during the following experiment.
,4-DCP system. The low degrading efficiency in the CaO /Fe(II)/
,4-DCP system is probably ascribed to the sharp decrease of the
2
3.3 Effects of reaction parameters on 2,4-DCP removal
performance
3
.3.1 Effect of CaO
2
dosage. The oxidation of 2,4-DCP at pH
ꢁ
7
.0 and a temperature of 22 C with different CaO
2
dosages was
investigated and shown in Fig. 3. It can be seen from the gure
that the removal efficiency of 2,4-DCP within 240 min was
rapidly increased from 47.0% to 95.7% as the molar ratio of
converted to a precipitate of Fe(OH)
3
completely within the rst
CaO
molar ratio further increasing to 32/1, the degradation removal
was decreased to 92.4%. This is because an excess of CaO can
release an overly high concentration of H , which was re-
2
to 2,4-DCP increased from 4/1 to 16/1. However, with the
5
min. Once iron was converted to a precipitate, it could no
longer participate in the radical propagation cycle. It can be
concluded that chelation of EDTA can effectively maintain the
catalytic activity of Fe(II) in Fenton chemistry based on CaO
2
2 2
O
2
at
ported to scavenge HOc as expressed by reaction (5).
moderate pH.
Therefore, the initial molar ratio of CaO /2,4-DCP was
2
chosen as 16/1 ([CaO2]0 ¼ 10 mM) for the investigation dis-
3.2 Decay of EDTA–Fe(II) in a Fenton-like system
cussed below.
To better understand the role of EDTA chelated Fe(II) in the 2,4-
3.3.2 Effect of EDTA–Fe(II) concentration. Fig. 4 illustrates
DCP degradation process, the decay mechanisms of EDTA and the effect of EDTA–Fe(II) concentration on 2,4-DCP removal
ꢁ
iron ions were also investigated. As observed in Fig. 2, similar to efficiency at pH 7.0 and a temperature of 22 C with an initial
the degradation process of 2,4-DCP, the concentration of EDTA CaO
shows a prominent decrease during the reaction time. This is removal efficiencies of 2,4-DCP at the molar ratios of CaO
because HOc generated in the MF system have a great oxidizing EDTA–Fe(II)/2,4-DCP varying from 16/1/1 to 16/8/1 were similar
2
/2,4-DCP molar ratio of 16/1. It can be seen that the
2
/
Fig. 2 Decay of EDTA–Fe(II) in the degradation process of 2,4-DCP. Fig. 3 Effect of CaO
2
dosage on 2,4-DCP degradation ([2,4-DCP]
0
¼
(
The molar ratio of CaO /EDTA–Fe(II)/2,4-DCP was kept at 16/4/1.) 0.61 mM and the initial concentration of EDTA–Fe(II) was 2.45 mM).
2
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Fig. 4 Effect of EDTA–Fe(II) addition on 2,4-DCP degradation ([2,4- Fig. 5 Effect of pH on 2,4-DCP degradation performance (molar ratio
DCP]
¼ 0.61 mM).
of CaO /EDTA–Fe(II)/2,4-DCP in the system was kept at 16/4/1, [2,4-
DCP]
¼ 0.61 mM).
0
2
0
and were all over 95.0% within 400 min of reaction time.
Nevertheless, the curve slopes representing the degradation rate solution pH rises quickly to more than 11.0, at which point
varied widely with the increase of EDTA–Fe(II) concentration. As CaO is difficult to dissolve to form H . A pH of 7 was chosen
observed in the gure, the removal efficiency within 60 min of for the following study.
reaction time was 57.4% with the molar ratio kept at 16/1/1,
3.3.4 2,4-DCP degradation kinetics. It is known that
whereas it increased to 88.6% with the molar ratio of EDTA– a modied Fenton reaction is a pseudo-rst-order reaction with
O
2 2
2
35–38
Fe(II)/2,4-DCP enhanced to 16/8/1. It suggests that the degra- respect to the concentration of the contaminant.
However,
dation rate of 2,4-DCP was elevated with the molar ratio the 2,4-DCP degradation in the MF system based on CaO
2
,
increased from 16/1/1 to 16/8/1, though the degradation particularly with the addition of EDTA, showed a very complex
performances for 2,4-DCP were similar. This can be explained behavior; the 2,4-DCP concentration rst decreased rapidly,
by iron consumption and a regeneration cycle composed of followed by a much slower degradation process (shown in
reactions (1) and (2). The rate constant for reaction (1) was Fig. 1). This led to a situation where the 2,4-DCP degradation
ꢀ
1
ꢀ1
tested by former researchers as 63 M
s
, whereas that for could be tted by neither zero-, rst-, nor second-order kinetics.
This means that Fe(II) ions In fact, the degradation process was controlled by the release of
were consumed faster than they were produced. Accordingly,
from CaO decomposition to a large extent and in our
increasing the Fe(II) concentration accelerated the degradation previous study we had concluded that the H releasing
process by promoting the decomposition of H
to produce process from CaO decomposition followed a pseudo-zero-order
more HOc at a time and reducing the regeneration cycle kinetics pattern.
numbers. However, when the molar ratio was further increased
Then, we conducted a series of experiments with the addi-
from 16/8/1 to 16/16/1, both the degradation performance and tion of an equivalent amount of CaO and with no addition of
the degradation rate decreased dramatically. This is ascribed to 2,4-DCP to water to nd out the relationship between the 2,4-
the scavenging of HOc or other radicals by present iron species DCP degradation process and release of H from CaO
releasing process from
dissolution (see the red line) and the 2,4-DCP degradation
was investigated in the pH range from 5.0 to 8.0 maintained by performance (see the black line) by an equivalent amount of
phosphate buffer solution. It should be noted that solution pH CaO . In Fig. 6, C /CA0 on the le vertical axis represents the
variation before and aer reaction was less than 0.2. As seen relative concentration of 2,4-DCP, whereas C /CBmax on the
from Fig. 5, within 300 min of reaction time, the removal effi- right is the H productivity. The results show that without the
ciency decreased from 99.9% to 77.2% with the increase of pH addition of 2,4-DCP, H can be generated gradually from
ꢀ
1
ꢀ1 29
reaction (2) was only 0.01 M
s .
H
2
O
2
2
2 2
O
2
O
2
2
2
2
2
2
O
2
2
33,34
through undesirable reactions (3), (4) and (6).
.3.3 Effect of pH. The effect of pH on 2,4-DCP degradation CaO
2 2
decomposition. Fig. 6 shows the H O
3
2
2
A
B
O
2 2
2 2
O
from 5.0 to 8.0. The decreased oxidation efficiency at higher pH CaO
2
dissolution at a certain rate, regardless of the decreasing
content in solution (see the solid red line). It needs to be
values was attributed to the lower oxidation potential of CaO
2
hydroxyl radicals and the undesirable decomposition of CaO
form O
at higher pH (eqn (10)). In the degradation process taneously once CaO
without addition of buffer solution, the concentration of experiment. Therefore, the H
2
to noted that a signicant amount of H
was added to the solution to initiate the
releasing process from CaO
2
2 2
O was produced instan-
2
2
O
2
2
contaminant diminished quickly within 3 min and then tended dissolution can be tted approximately by a line disjointed to
to keep a constant value with the nal removal efficiency of the zero point. Consequently, in the initial stage of the degra-
5
1.2%. The reaction ceased probably because CaO can react dation reaction, the H
2
O
2
concentration was relatively high to
(eqn (11)), which caused a dramatic react with EDTA–Fe(II) and form HOc, which can degrade 2,4-
increase in pH. Without the addition of buffer reagents, the DCP at a higher rate. With the reaction going on, H , which
2
with water to form Ca(OH)
2
2 2
O
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Fig. 6 Releasing curve of H
2
O
2
from CaO
2
decomposition and the
(the
/EDTA–Fe(II)/2,4-DCP in the system was kept at
¼ 0.61 mM).
Fig. 7 The degradation of probe compounds in the CaO
system ([NB] ¼ 0.05 mM, [CaO ¼ 10 mM, [Fe(II)]
¼ 1 mM, [CT]
0 mM, T ¼ 22 C ꢂ 0.5 C).
2
/EDTA–Fe(II)
degradation curve of 2,4-DCP by a comparable amount of CaO
molar ratio of CaO
6/4/1, [2,4-DCP]
2
0
0
2
]
0
0
¼
2
ꢁ
ꢁ
2
1
0
Literature indicates that TBA reacts with HOc at high rates (kHOc
was released from CaO
insufficient” for the following degradation process. Accord-
ingly, the 2,4-DCP degradation in the Ca O /EDTA–Fe(II) system
2
at a certain slow rate, was relatively
8
ꢀ1 ꢀ1
¼
5.2 ꢃ 10 M
s ), and chloroform has minimal reactivity
“
6
ꢀ1 ꢀ1
ꢀc
with HOc (kHOc ¼ 7 ꢃ 10 M
s
) and high reactivity with O2
2
2
10
ꢀ1 ꢀ1
29,31
(
kO2ꢀc ¼ 3 ꢃ 10
M
s
).
Based on these properties, TBA
showed a moderate reaction rate because of the dissolution
process of CaO particles.
was used to scavenge HOc, whereas chloroform was selected as
2
ꢀc
the O2 scavenger. As shown in Fig. 8(a), the 2,4-DCP removal
efficiency in 60 min of reaction time reached 83.8% in the
absence of the scavenger. As TBA was added to the system from
3
.4 Identication of predominant reactive oxygen species
0
mM to 30 mM and 100 mM, the 2,4-DCP removal efficiencies
3.4.1 Free radical probe compound tests. To verify the
within 60 min decreased to 47.0% and 18.5%, respectively. The
removal efficiency decreased dramatically with scavenging of
HOc. This demonstrates that HOc is the dominant reactive
oxygen species responsible for 2,4-DCP degradation in the
system.
generation of reactive oxygen species, NB and CT were used as
the probe compounds on the basis of their reactivity with each
type of species potentially presented in the system. NB was
selected as the oxidant probe to demonstrate the presence of
9
ꢀ1
HOc since it has high reactivity with HOc (k
¼ 3.9 ꢃ 10 M
ꢀc
HOc
The result of the O2 scavenging test with chloroform is
ꢀ
1
27
s
). Apart from the oxidative species, the system may also
shown in Fig. 8(b). As seen from it, the removal efficiency of 2,4-
DCP within 200 min of reaction time was decreased from 90.2%
to 84.5% and 82.3%, respectively, in the presence of chloroform
from 0 to 3 mM, 10 mM, and 30 mM. It means that the degra-
dation performance of 2,4-DCP shows a minor decrease with the
ꢀc
have potential in generating reductive species such as O2 . CT,
which is a highly oxidized compound, was used for identifying
ꢀc
O2 because of its high reactivity with reductants (k ¼ 1.6 ꢃ
1
0
ꢀ1 ꢀ1
10
M
s ), but extremely low reactivity with hydroxyl radi-
6
ꢀ1 ꢀ1 29
cals (kHOc < 2 ꢃ 10 M
s
).
ꢀc
ꢀc
scavenging of some O2 . It can be deduced that O2 was
generated in the system, but its role of promoting the degra-
The generation of HOc in a CaO
2
-based Fenton system,
measured by NB loss, is shown in Fig. 7. The NB removal effi-
ciency was above 95% in 240 min of reaction time, proving that
HOc was generated in the system. The presence of HOc
ꢀc
dation of 2,4-DCP is limited. It is potentially because the O2
was generated in the reaction steps, which were not essential to
31
the degradation. The result agrees well with former studies.
2 2 2
demonstrated the ability of CaO to replace H O to participate
in the modied Fenton system. The removal efficiency of CT can
reach 24% within 240 min of reaction time in the modied
3
.5 Degradation intermediates and reaction mechanism
ꢀc
Fenton system, indicating the presence of O2 in the system. As chloride substituents on the benzene ring are responsible for
28
ꢀc
Previous studies have shown that O2 is likely a transforming the toxicity of aromatic compounds, the fate of chloride
species in the modied Fenton's process and is probably substituents must be given much more concern. As observed
formed in the processes represented by eqn (5) and (12):
from Fig. 9(a), the initial concentration of 2,4-DCP was 77.4 mg
ꢀ
1
L
and removal efficiency was 95.7%. The calculated dechlo-
c
ꢀc
þ Hþ
HO 4O
2
(12)
2
rination degree of 2,4-DCP reached 90%, showing that most of
the chlorine on the aromatic ring was released and formed
3.4.2 Free radical scavenger tests. In order to identify the chloride ions. The mineralization degree of 2,4-DCP in this MF
major reactive oxygen species for 2,4-DCP removal, free radical system was determined to investigate the oxidation efficiency of
scavenger tests with two types of scavengers were conducted. 2,4-DCP (as depicted in Fig. 9(a)). The initial concentration of
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Fig. 8 Effect of radical scavengers on the degradation of 2,4-DCP. The radical scavengers in (a) and (b) were TBA and chloroform, respectively.
The molar ratio of CaO /EDTA–Fe(II)/2,4-DCP was 16/4/1, [2,4-DCP]
¼ 0.61 mM.)
(
2
0
Fig. 9 (a) 2,4-DCP degradation efficiency, TOC removal and dechlorination degree of 2,4-DCP; (b) chromatogram of degradation intermediates
detected by UPLC-MS; (c) the concentration of degradation intermediates detected by IC. (The molar ratio of CaO
2
/EDTA–Fe(II)/2,4-DCP was
kept at 16/4/1.)
ꢀ
1
EDTA was 711.2 mg L and the initial concentration of TOC 30 min and then were generated slowly during the remaining
(
3
L
including 2,4-DCP and EDTA) in the system was measured as reaction time. It should be noted that the total concentration of
ꢀ
1
74.9 mg L . The nal TOC at 300 min was tested as 327.7 mg carboxylic acid was relatively high compared to that of the direct
ꢀ
1
. Therefore, the calculated mineralization of 2,4-DCP in the products from 2,4-DCP, which is ascribed to the degradation of
oxidant system is 12.6%. This suggests that some of the inter- a small amount of EDTA (shown in Fig. 2). Previous researchers
mediates derived from 2,4-DCP decomposition, such as have reported the degradation intermediates of mono-
carboxylic acids, remained in the solution.
chlorophenol and phenol, but they were not detected in our
39,40
Through UPLC-MS/MS and IC analyses, the main interme- system.
diates were 2-chlorohydroquinone, 4,6-dichlororesorcinol, ace-
tic acid, formic acid and oxalic acid (shown in Fig. 9(b) and (c)). literature,
According to the results and the information reported in the
41–44
the degradation pathway was proposed and shown
The concentration of 2,4-DCP decreased quickly in the solution in Fig. 10. The chlorine atom located in the para-position on the
during the rst 30 min and then decreased slowly till complete aromatic ring of 2,4-DCP was apparently the preferred location
removal within 300 min. 2-Chlorohydroquinone and 6-dichlor- for radical attack because of steric effects. Hence, 2,4-DCP was
oresorcinol were generated quickly aer the degradation reac- substituted by HOc and degraded to 2-chlorohydroquinone. This
tion was initiated and began to disappear within 30 min. The 2-chlorohydroquinone was subsequently dehydrogenated to give
acetic acid and formic acid were formed quickly during the rst 2-chloro-1,4-benzoquinone with further attack of HOc. Moreover,
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Fig. 10 The suggested pathway for the degradation of 2,4-DCP in the MF system.
the mechanism also allows an electrophilic HOc group to be for the support provided by the Key Project of National Natural
added onto the aromatic ring of the 2,4-DCP, forming hydroxyl- Science Foundation of China (41530636).
ation intermediates such as 4,6-dichlororesorcinol. Further
oxidation of chlorobenzoquinone and 4,6-dichlororesorcinol
leads to dechlorination, followed by ring cleavage and formation
of maleic and fumaric acids. These two acids were subsequently
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