ArticlePDF Available

Both Degradation and AOX Accumulation Are Significantly Enhanced in UV/Peroxymonosulfate/4-Chlorophenol/Cl- System: Two Sides of the Same Coin?

Authors:

Abstract and Figures

The presence of external chloride can lead to a 47-fold increment in degradation rates of 4-chlorophenol than those in the absence of chloride in UV/peroxymonosulfate process. The other side of the same coin is an undesirable accumulation and increase in absorbable organic halogen (AOX) was observed in the presence of chloride, with formation of some more toxic tetrachlorinated byproducts.
Content may be subject to copyright.
Both degradation and AOX accumulation are
signicantly enhanced in UV/peroxymonosulfate/
4-chlorophenol/Cl
system: two sides of the same
coin?
Zhaohui Wang,*
ab
Min Feng,
a
Changling Fang,
a
Ying Huang,
a
Luoyan Ai,
a
Fei Yang,
a
Ying Xue,
a
Wenqian Liu
a
and Jianshe Liu
a
The presence of external chloride can lead to a 47-fold increment in degradation rates of 4-chlorophenol
than those in the absence of chloride in UV/peroxymonosulfate process. The other side of the same coin is
an undesirable accumulation and increase in absorbable organic halogen (AOX) was observed in the
presence of chloride, with formation of some more toxic tetrachlorinated byproducts.
Introduction
Recently sulfate radical-based Advanced Oxidation Processes
(AOPs) are attracting much attention in degrading recalcitrant
organic pollutants due to its strong oxidizing capacity (E¼2.5
3.1 V versus NHE) and longer half-time (3040 ps) of sulfate
radical (SO
4
c
).
1
Peroxymonosulfate (PMS) is known as an e-
cient precursor of SO
4
c
production, with assistance of transi-
tion metals,
25
heat,
6,7
base,
8,9
ultrasound,
10
carbon catalysts,
11,12
microwave
13
and ultraviolet (UV).
1416
Upon the cleavage of the
peroxo bond, PMS photolyzed by UV was suggested to produce
SO
4
c
and cOH in a benign and economic way.
17
UV/PMS
process can decompose organic pollutants either by direct
photolysis or indirectly by derived SO
4
c
and cOH. For example,
Sharma et al.
15
reported about 96.7% of 0.22 mmol L
1
bisphenol (BPA) was eciently degraded while 0.66 mmol L
1
PMS was applied in a UVC/PMS process. Our recent investiga-
tion also indicates that complete decomposition of 4-chloro-2-
nitrophenol (4C2NP) with UVA/PMS was feasible within
120 min.
16
Although PMS-based processes are known as promising
AOPs for the degradation of various organics, their reaction
eciency would be greatly constrained by the presence of
chloride, a common inorganic anion in industrial waste-
water.
1822
Chloride, from external (saline wastewater
20
)or
internal (released from dechlorination of chlorinated
compounds
18,21
) sources, may scavenge SO
4
c
or cOH to form
less reactive chlorine radical (Clc/Cl
2
c
) or react with PMS to
generate Cl
2
/HClO. In the past, eects of external or internal
chloride were oen separately investigated. A dual eect
(inhibitory and then accelerating) of external chloride was
observed in dye degradation with Co/PMS process.
5,23
Our more
recent work demonstrates that even the released chloride from
trichlorophenol also can be activated to generate poly-
chlorinated byproducts in PMS-based system.
18
In fact,
dechlorination and re-chlorination should simultaneously
occur when degrading chlorinated compounds in the presence
of chloride. Currently few studies concern the combined eects
of external and internal chloride in degradation of chlor-
ophenols in UV/PMS process where direct photodechlorination
may happen.
24
To assess the overall outcome of these
competing processes, absorbable organic halogen (AOX) is
applied as a measure of extent of chlorination.
21
For a simple
dechlorination process, AOX value should decline gradually.
Increase in AOX value implies accumulation of more chlori-
nated compounds.
Thus, the aim of this work is to examine eects of external
chloride on chlorophenol degradation and byproducts genera-
tion, and evaluate the overall balance between chlorination and
dechlorination reected by AOX. 4-Chlorophenol (4-CP) was
selected because both its dechlorination and chlorination are
comparatively easy and its degradation mechanism has been
well documented.
24,25
The results may provide useful reference
for UV/PMS application in the presence of chloride.
Experimental section
Chemicals
4-CP (>98.0%) were purchased from TCI (Shanghai, China).
Oxone® ([2KHSO
5
$KHSO
4
$K
2
SO
4
] salt, 95%) and methanol
(HPLC grade) were obtained from Sigma-Aldrich. NaCl, NaNO
2
,
a
State Environmental Protection Engineering Center for Pollution Treatment and
Control in Textile Industry, College of Environmental Science and Engineering,
Donghua University, Shanghai, 201620, China. E-mail: zhaohuiwang@dhu.edu.cn
b
International Centre for Balanced Land Use (ICBLU), The University of Newcastle,
Callaghan, NSW 2308, Australia
Electronic supplementary information (ESI) available: GC-MS spectra
(Fig. S1S7). See DOI: 10.1039/c7ra01294b
Cite this: RSC Adv.,2017,7,12318
Received 31st January 2017
Accepted 14th February 2017
DOI: 10.1039/c7ra01294b
rsc.li/rsc-advances
12318 |RSC Adv.,2017,7,1231812321 This journal is © The Royal Society of Chemistry 2017
RSC Advances
PAPER
NaNO
3
and Na
2
SO
3
were at least analytical grade and used
without further purication. Barnstead UltraPure water (18.2
MUcm) was used for all experiments. Stock solutions of all
chemicals were always freshly prepared. Prior to each experi-
ment, certain aliquots were added to the reactor vessel to obtain
the specic concentrations.
Procedure and analysis
The photodegradation of 4-CP was performed in an XPA-7 type
photochemical reactor (Xujiang Electromechanical Plant,
Nanjing, China). A 100 W medium-pressure mercury vapor
lamp (l
max
¼365 nm) was used as the light source in a water
cooled borosilicate glass immersion well. All experiments were
performed at room temperature in 50 mL continuously stirred
quartz tubes. Samples were withdrawn from the solution for
analysis at specic time intervals and quenched with methanol
(for HPLC analysis), NaNO
2
(for GC-MS analysis) immediately.
All experiments were performed in duplicate, with error bars in
gures representing one standard deviation.
The concentration of 4-CP was measured by an Agilent 1100
high-performance liquid chromatography (HPLC) instrument
(UV-vis detector) with GL Inertsil ODS-SP column (4.6 mm
250 mm, 5 mm) maintained at 30 C. The mobile phase was
methanol/water (65/35 (v/v)), with a ow rate of 1.0 mL min
1
.
The detector wavelength was set at 272 nm. Gas
chromatography-mass spectrometry (GC-MS, Agilent 7890A-
5975C, USA) analysis was conducted to identify the interme-
diate products formed during 4-CP degradation process. The
detailed procedure has been described in our previous work.
18
AOX determination was performed by instrumental analysis
(multi X 2500, Jena, Germany) aer enrichment on activated
carbon (European Standard EN 1485 H14, 1996). Samples were
pretreated by APU2 (Automatic Preparation Unit), an automatic
adsorption system for sample preparation. The detection range
of AOX analysis was from 1 mg to 100 mg (expressed as the
absolute content of chloride).
Results and discussion
Degradation kinetics
In agreement with literature,
24
photolyzed 4-CP is unstable and
about 80 mmol L
1
of 4-CP was removed under UVA irradiation
(data not shown). However, as shown in Fig. 1, its degradation
was largely enhanced in UV/PMS system. Nearly 90% of 4-CP
disappeared within 120 min under the given condition. Fig. 1
illustrates 4-CP degradation with chloride concentration
increasing from 1 to 300 mmol L
1
. Similar to our recent reports
in other PMS-based systems,
5,23
a dual eect of external chloride
was observed. At low level of chloride (<5 mmol L
1
), degrada-
tion rates of 4-CP declined very slightly. A further increase of
chloride content to 10 mmol L
1
resulted in a slight increase in
degradation rates. As chloride concentration continued to
increase from 30 to 300 mmol L
1
, degradation eciency of 4-
CP was enhanced signicantly. By tting curves with pseudo-
rst-order kinetics, reaction rate constants (k, min
1
) were ob-
tained and plotted as a function of chloride level. There was an
exponential increase in kwith an increase in external chloride
dosage, with a 47-fold increment in pseudo-rst-order rate
constant at [Cl
]¼300 mmol L
1
as compared to the UV/PMS
system.
AOX accumulation
Apparently, UV/PMS/Cl
process is supposed to be a favourable
technology in eciently degrading 4-CP as compared to original
UV/PMS system. However, complete degradation of target
pollutant does not necessarily mean all organochlorines have
been removed. To quantify the total chlorinated byproducts,
AOX values were measured with reaction time (Fig. 2). The
initial AOX value at t¼0 min represented the contribution of
4-CP alone. In the absence of chloride, AOX content gradually
decreased due to the (photo)dechlorination. However, as
Fig. 1 Pseudo-rst-order rate constants (k,min
1
)versus external
chloride concentration in the UV/PMS system. Inset: degradation
kinetics of 4-CP in the UV/PMS system at dierent level of chloride.
Conditions: [4-CP]
0
¼0.2 mmol L
1
; [PMS]
0
¼10 mmol L
1
;pH¼3.0.
Fig. 2 AOX levels as a function of reaction time in UV/PMS, PMS/Cl
,
and UV/PMS/Cl
processes. Conditions: [4-CP]
0
¼0.2 mmol L
1
;
[PMS]
0
¼10 mmol L
1
; [Cl
]¼100 mmol L
1
;pH¼3.0.
This journal is © The Royal Society of Chemistry 2017 RSC Adv.,2017,7, 1231812321 | 12319
Paper RSC Advances
100 mmol L
1
of chloride was added, AOX values did not drop,
but unexpectedly increased. In the investigated time scale (180
min), AOX values in UV/PMS/Cl
solution increased 2.5 times
from the initial AOX level originated from 4-CP itself. Since the
measured AOX reected the level of total chlorinated
compounds including residual 4-CP and newly generated
organochlorines, the AOX increase implies that external chlo-
ride has been transformed to organochlorines and re-
chlorination was an overwhelming process against dechlorina-
tion in UV/PMS/Cl
system.
Chlorinated byproducts identication
The main chlorinated intermediates during 4-CP degradation
were identied by GC-MS (Table 1, see GC-MS spectra in Fig. S1
S7). Only 2,3,6-trichlorophenol was identied in UV/PMS system
at the early stage of reaction (15 min). In contrast, in PMS/Cl
system, 5 chlorinated compounds were detected, including 2,3,6-
trichlorophenol, 2,6-dichloro-[1,4]benzoquinone, 2,3-dichlor-
ophenol, 2,4,6-trichlorophenol, 3,4,6-trichloro-benzene-1,2-diol.
Another two tetrachlorophenols, 2,3,4,6-tetrachlorophenol and
2,3,5,6-tetrachloro-benzene-1,4-diol, were identied in UV/PMS/
Cl
system. It should be emphasized that chlorine atom numbers
in all identied chlorinated compounds were higher than the
original 4-CP. Again, it indicates de novo formation of organo-
chlorines dominated even 4-CP has been completely removed
within 20 min (Fig. 1). Although acute toxicities of generated 2,3-
dichlorophenol and 2,4,6-trichlorophenol were lower than 4-CP,
two tetrachlorinated compounds show much greater toxicity in
UV/PMS/Cl
system. Quantication of all chlorinated
compounds is not reported in this communication, but will be
conducted while all chemicals are purchased or synthesized in
the laboratory.
Mechanism discussion
There are two major reaction pathways in UV/PMS/Cl
system.
5,23
The scission of the peroxo bond in PMS structure
under UV irradiation leads to generation of sulfate and hydroxyl
radicals (eqn (1)). Both chloride and 4-CP can be oxidized by
these strongly oxidizing radicals, leading to formation of chlo-
rine radicals (eqn (2)(9)) and photodechlorinaton of 4-CP.
Rates of 4-CP degradation may be decelerated in the presence of
chloride (<5 mmol L
1
), because the resulting chlorine radicals
are less oxidizing (E
0
(Clc/Cl
)¼2.41 V vs. NHE; E
0
(Cl
2
c
/2Cl
)¼
2.0 V vs. NHE).
26,27
Meanwhile, chloride can be directly oxidized
by PMS to produce reactive chlorine species (RCS, Cl
2
/HClO,
eqn (10) and (11)).
28
RCS is able to further decompose 4-CP (eqn
(12)) and therefore, 4-CP degradation was signicantly
enhanced while excess of external chloride was added (Fig. 1).
Dechlorination and re-chlorination may occur simulta-
neously in UV/PMS/4-CP/Cl
solution. It is known that CCl
bond cleavage is the rst step of 4-CP photoreaction.
24
Hydro-
quinone and other hydroxylated compounds aer dechlorina-
tion are the main products, however, which are expected to be
readily re-chlorinated. The released chlorine atom or chloride
ion, as an internal chloride, can be added to benzene ring aer
reaction with PMS or sulfate radical. This can explain the
occurrence of 2,3,6-trichlorophenol in UV/PMS system. Chlo-
rine radicals and RCS are excellent chlorination agents and lead
to formation of polychlorinated compounds, such as 2,3,4,6-
tetrachloro-phenol and 2,3,5,6-tetrachloro-benzene-1,4-diol.
Transformation of external chloride to organochlorine
accounts for the increased AOX values observed in PMS/Cl
and
UV/PMS/Cl
systems in Fig. 2.
HSO
5
+hn/SO
4
c
+cOH (1)
cOH + Cl
4ClOHc
(2)
ClOHc
+H
+
/Clc+H
2
O(3)
SO
4
c
+Cl
4Clc+SO
42
(4)
Clc+Cl
4Cl
2
c
(5)
Clc+Clc/Cl
2
(6)
Cl
2
c
+Cl
2
c
/Cl
2
+ 2Cl
(7)
Cl
2
c
+H
2
O4ClOHc
+H
+
+Cl
(8)
Table 1 Main identied chlorinated byproducts with GC-MS and their
reported acute toxicity (in parenthesis) in dierent systems
System UV/PMS PMS/Cl
UV/PMS/Cl
Identied product structure
a
n.a. ¼not available.
b
Rat LD
50
oral (mg kg
1
), data from MSDS for
each chemical. LD
50
oral for 4-CP is 670 mg kg
1
.
12320 |RSC Adv.,2017,7, 1231812321 This journal is © The Royal Society of Chemistry 2017
RSC Advances Paper
ClOHc
+H
+
4Clc+H
2
O(9)
HSO
5
+Cl
/SO
42
+ HOCl (10)
HSO
5
+ 2Cl
+H
+
/SO
42
+H
2
O+Cl
2
(11)
R–H + HOCl /R–Cl + H
2
O(12)
Conclusions
Eects of external chloride ion on degradation of 4-CP by UV/
PMS process were investigated in this study. A dual eect of
chloride (i.e. inhibitory and accelerating eect) on chlorophenol
degradation kinetics was observed. Chloride ions with high
concentrations (>30 mmol L
1
) can greatly increase the degra-
dation ecacy of 4-CP. AOX values declined gradually with the
degradation of 4-CP in UV/PMS system, whereas AOX accumu-
lation with the concentration of added chloride ion was
observed in the presence of external chloride. Based on GC/MS
data, several polychlorinated products, namely 2,3,6-tri-
chlorophenol, 2,6-dichloro-[1,4]benzoquinone, 2,3-dichlor-
ophenol, 2,4,6-trichlorophenol, 3,4,6-trichloro-benzene-1,2-
diol, 2,3,4,6-tetrachloro-phenol and 2,3,5,6-tetrachloro-
benzene-1,4-diol. Therefore, UV/PMS AOPs is not recom-
mended in highly saline wastewater treatment, as even this kind
of AOPs is able to rapidly degrade target organic pollutants. A
novel nanocarbon/PMS process seems promising because it can
achieve a fast organic mineralization in the presence of chloride
ions due to a moderate oxidative potential of the nonradical
pathway (compared with hydroxyl and sulfate radicals).
29
Acknowledgements
This work was supported by the National Science Foundation of
China (NSFC) (No. 21377023 and 21677031), National Key
Research and Development Program of China
(2016YFC0400501/2016YFC0400509), Shanghai Pujiang
Program and DHU Distinguished Young Professor Program.
The authors also appreciate the nancial support from the
Fundamental Research funds for Central Universities Central
(15D311312).
Notes and references
1 F. Ghanbari and M. Moradi, Chem. Eng. J., 2017, 310, 41.
2 G. P. Anipsitakis and D. D. Dionysiou, Appl. Catal., B, 2004,
54, 155.
3 P. Nfodzo and H. Choi, Chem. Eng. J., 2011, 174, 629.
4 Y. Huang, Z. Wang, C. Fang, W. Liu, X. Lou and J. Liu, RSC
Adv., 2016, 6, 70271.
5 R. Yuan, S. N. Ramjaun, Z. Wang and J. Liu, J. Hazard. Mater.,
2011, 196, 173.
6 M. G. Antoniou, A. A. de la Cruz and D. D. Dionysiou, Appl.
Catal., B, 2010, 96, 290.
7 S. Yang, P. Wang, X. Yang, L. Shan, W. Zhang, X. Shao and
R. Niu, J. Hazard. Mater., 2010, 179, 552.
8 C. Qi, X. Liu, J. Ma, C. Lin, X. Li and H. Zhang, Chemosphere,
2016, 151, 280.
9 X. Lou, C. Fang, Z. Geng, Y. Jin, D. Xiao, Z. Wang, J. Liu and
Y. Guo, Chemosphere, 2017, 173, 529.
10 J. Liu, J. Zhou, Z. Ding, Z. Zhao, X. Xu and Z. Fang, Ultrason.
Sonochem., 2017, 34, 953.
11 X. Duan, H. Sun, Z. Ao, L. Zhou, G. Wang and S. Wang,
Carbon, 2016, 107, 371.
12 S. Yang, T. Xiao, J. Zhang, Y. Chen and L. Li, Sep. Purif.
Technol., 2015, 143, 19.
13 C. Qi, X. Liu, C. Lin, H. Zhang, X. Li and J. Ma, Chem. Eng. J.,
2017, 315, 201.
14 T. Olmez-Hanci, C. Imren, I. Kabdaslı,O.T
¨
unay and
I. Arslan-Alaton, Photochem. Photobiol. Sci., 2011, 10, 408.
15 J. Sharma, I. M. Mishra, D. D. Dionysiou and V. Kumar,
Chem. Eng. J., 2015, 276, 193.
16 J. Zhou, J. H. Xiao, C. L. Fang, D. X. Xiao, Y. G. Guo, X. Y. Lou,
Z. H. Wang and J. S. Liu, China Environ. Sci., 2016, 36, 66.
17 Y. Guan, J. Ma, X. C. Li, J. Fang and L. Chen, Environ. Sci.
Technol., 2011, 45, 9308.
18 L. Xu, R. X. Yuan, Y. G. Guo, D. X. Xiao, Y. Cao, Z. H. Wang
and J. S. Liu, Chem. Eng. J., 2013, 217, 169.
19 J. Zhou, J. H. Xiao, D. X. Xiao, Y. G. Guo, C. L. Fang, X. Y. Lou,
Z. H. Wang and J. S. Liu, Chemosphere, 2015, 134, 446.
20 X. Y. Lou, Y. G. Guo, D. X. Xiao, Z. H. Wang and J. S. Liu,
Environ. Sci. Pollut. Res., 2013, 20, 6317.
21 C. L. Fang, D. X. Xiao, W. Q. Liu, X. Y. Lou, J. Zhou,
Z. H. Wang and J. S. Liu, Chemosphere, 2016, 144, 2415.
22 Y. Yang, J. J. Pignatello, J. Ma and W. A. Mitch, Environ. Sci.
Technol., 2014, 48, 2344.
23 Z. Wang, R. Yuan, Y. Guo, L. Xu and J. Liu, J. Hazard. Mater.,
2011, 190, 1083.
24 M. Czaplicka, J. Hazard. Mater., 2006, 134, 45.
25 F. J. Benitez, J. Beltran-Heredia, J. L. Acero and F. J. Rubio,
Chemosphere, 2000, 41, 1271.
26 R. E. Huie, C. L. Clion and P. Neta, Int. J. Radiat. Appl.
Instrum., Part C, 1991, 38, 477.
27 S. Steenken, in Free Radicals: Chemistry, Pathology and
Medicine, ed. C. Rice-Evans and T. Dormandy, Richelieu
Press, London, 1988, p. 51.
28 G. Lente, J. Kalm´
ar, Z. Baranyai, A. Kun, I. K´
ek, D. Bajusz,
M. Tak´
acs, L. Veres and I. F´
abi´
an, Inorg. Chem., 2009, 48,
17631773.
29 X. Duan, Z. Ao, L. Zhou, H. Sun, G. Wang and S. Wang, Appl.
Catal., B, 2016, 188, 98.
This journal is © The Royal Society of Chemistry 2017 RSC Adv.,2017,7,1231812321 | 12321
Paper RSC Advances
... Such an effect is much stronger than that of Cl − but weaker than that of Br − (Grebel et al., 2010). In PMS-based systems, halides have reportedly accelerated the contaminant removal because the reactions of PMS with halides form non-radical reactive halogen species (RHSs) (e.g., HOX, X 2 , X = Cl or Br) Luo et al., 2019;Wang et al., 2017). These non-radical RHSs can sometimes be very abundant compared to the original ROSs and result in significantly faster contaminant oxidation, although they may also lead to inhibited mineralization and formation of toxic halogenated byproducts Luo et al., 2019;Wang et al., 2017). ...
... In PMS-based systems, halides have reportedly accelerated the contaminant removal because the reactions of PMS with halides form non-radical reactive halogen species (RHSs) (e.g., HOX, X 2 , X = Cl or Br) Luo et al., 2019;Wang et al., 2017). These non-radical RHSs can sometimes be very abundant compared to the original ROSs and result in significantly faster contaminant oxidation, although they may also lead to inhibited mineralization and formation of toxic halogenated byproducts Luo et al., 2019;Wang et al., 2017). Given the ubiquitous presence of these anions in the aquatic environment, it is important to systematically evaluate their impacts on the performance of different AOPs under various conditions. ...
Article
Full-text available
As one of the most powerful approaches to mechanistically understanding complex chemical reactions and performing simulations or predictions, kinetic modeling has been widely used to investigate advanced oxidation processes (AOPs). However, most of the available models are built based on limited systems or reaction mechanisms so they cannot be readily extended to other systems or reaction conditions. To overcome such limitations, this study developed a comprehensive model on phenol oxidation with over 540 reactions, covering the most common reaction mechanisms in nine AOPs—four peroxymonosulfate (PMS), four peroxydisulfate (PDS), and one H2O2 systems—and considering product formation and the effects of co-existing anions (chloride, bromide, and carbonate). Existing models in the literature were first gathered and revised by correcting inaccurately used reactions and adding other necessary reactions. Extensive model tuning and validation were then conducted by fitting the model against experimental data from both this study and the literature. The effects of anions were found to follow PDS/CuO > H2O2/UV > other PDS or PMS systems. Halogenated organic byproducts were mainly observed in the PMS systems in the presence of halides. Most of the 543 reactions were found to be important based on the sensitivity analysis, with some involving anions being among the most important, which explained why these anions substantially altered some of the reaction systems. With this comprehensive model, a deep understanding and reliable prediction can be made for the oxidation of phenol (and likely other phenolic compounds) in systems containing one or more of the above AOPs.
... High chloride concentrations (> 5 mM) were found to significantly enhance dye decolorization and to some extent greatly inhibit dye mineralization when the azo dye AO7 was treated by Co/PMS system(Z. Wang et al., 2017;Yuan et al., 2011). Lai shows chlorine ion radicals promote the oxidative degradation of phenanthrene without forming chlorination byproducts in the Fenton/Cl − system, and the reactive species in the Fenton/Cl − system (Cl − ≤2000 mg/L) include •OH, •Cl, Cl 2 • -, and HO 2 •/O 2 • - (Lai et al., 2021(Lai et al., , 2023. ...
Article
Magnetic ion exchange resin (MIEX) pretreatment has been widely used to enhance the performance of waterworks, but the high-salinity organic desorbent produced during the resin regeneration is difficult to be safely disposed of. The present study aimed to investigate the removal of DOM and retention of salts from MIEX desorbent via zero-valent iron (ZVI)-assisted Fenton reaction. ZVI/H2O2 mediated Fenton reaction was further validated by various analytical approaches such as 3D-EEM, Molecular weight distribution, Zeta potential, size, dissolution of iron ions, etc. In batch experiments, 73.26% reduction of DOC and 97.66% of UV254 were achieved in 40 min at optimal H2O2 concentration (20 mM), 2 g/L dose of ZVI, and pH of 3, respectively. The removal of organics was ascribed to the direct oxidation of ZVI/H2O2 and flocculation of the iron released during the oxidation. It should be noted that the high concentration of Cl⁻ in system favored the organic removal and its enhanced performance lied in the production and oxidation of HOCl/OCl⁻. The treated liquid can be reused to regenerate the saturated resin with only a small amount of salt supplementation to obtain a similar recovery performance to that of a freshly configured 10% NaCl solution. Moreover, the retrieval and utilization of ZVI particles and other properties of MIEX desorbent after ZVI/H2O2 treatment were all promising and up to the mark: after four reuses, the mineralization of organic matter differs by 8.81% from the first time, which could be compensated by supplying new ZVI. This research will facilitate the recycling of resin desorbate and expand the market for MIEX applications.
Article
Although peroxymonosulfate (PMS) can be efficiently activated by ultrasound-driven piezocatalyst, there are still uncertainties on the mechanisms. Herein, molybdenum disulfide piezocatalysts with different sulfur vacancy (VS) types were prepared by hydrothermal (MoS2–H) and solvothermal methods (MoS2–S). The configuration of Vs in MoS2 was characterized by X-ray photoemission spectroscopy (XPS), electron paramagnetic resonance spectroscopy (EPR), and O2 diffuse reflectance infrared Fourier transform spectroscopy (O2-DRIFTS). Surface in-plane VS and edge VS dominated on MoS2–H and MoS2–S. MoS2–S exhibits higher PMS activation performance with 85.0% degradation rate of 4-chlorophenol within 2 hours. Although in-plane VS feature a stronger PMS adsorption energy compared with edge VS (−1.64 vs. −0.94 eV), PMS activation is not favorable at the in-plane VS sites of MoS2 (MoS2–H). In piezo-polarized MoS2, the separated charge carriers accumulate at the edges, where edge VS (MoS2–S), as the active site, strengthens the coupling between electrons and PMS. As a result, the O–O bond length in PMS is increased from 1.467 to 1.470 Å, enabling the facile activation of PMS with the generation of more abundant ·OH and ·SO4⁻ radicals. This work clearly explains the PMS activation mechanism from both experimental and theoretical aspects and provides directions to tailor the structure and atom vacancies of piezocatalysts.
Preprint
Full-text available
16 17 Highlights: 18  The role of Cl⁻ was deeply investigated in heat-activated peroxydisulfate (PS) and 19 peroxymonosulfate (PMS) processes for benzotriazole (BTA) degradation. 20  Cl⁻ exhibited positive and negative effects on BTA degradation in PMS and PS processes, 21 respectively. 22  Both organic (AOX) and inorganic (ClO x) chlorinated transformation products (TPs) were tracked. 23  Forty-two TPs were identified, and degradation pathways in the absence and presence of Cl⁻ were 24 proposed. 25  PMS/Cl⁻ system had more chlorinated TPs, and toxicity. 26 Abstract 27 The impact of chloride (Cl⁻) in heat-activated peroxydisulfate (PS) and peroxymonosulfate (PMS) 28 processes for benzotriazole (BTA) degradation was investigated. The PMS-mediated BTA degradation rate 29 showed an optimum at a Cl⁻ concentration of 1000 mg/L, while Cl⁻ inhibited PS-mediated BTA degradation. 30 BTA degradation was mainly driven by hydroxyl and sulfate radicals in both PS and PMS systems without 31 Cl⁻. However, reactive chlorine species (RCS) significantly boosted the PMS system for BTA degradation in 32 presence of Cl⁻. LC-MS/MS analysis identified forty-two transformation products (TPs). Based on the TPs, 33 polymerization, hydroxylation, benzene ring-opening, and carboxylic acid formation were hypothesized to 34 be the main degradation mechanisms in the absence of Cl⁻, whereas chlorination, triazole ring-opening, and 35 nitration were the additional degradation steps in presence of Cl⁻. Moreover, more chlorinated TPs were 36 found in PMS/Cl⁻ system than in PS/Cl⁻ combination, which was also reflected in AOX and end-product 37 toxicity analyses. The PMS/Cl⁻ process produced other undesirable by-products such as chlorates, while 38 these were not detected in the PS/Cl⁻ process. This shows that PS and PMS-based advanced oxidation Electronic copy available at: https://ssrn.com/abstract=4146213 1 processes can act notably different from the aspect of toxic by-product formation, and need to be critically 2 evaluated before applying for organic pollutant degradation under saline conditions 3 Statement of Environmental Implcation 4 As an emerging contaminant of concern frequently found in water resources, benzotriazole (BTA) 5 is a carcinogenic and mutagenic compound which is widely used in industries. In this research, 6 BTA degradation was investigated in SO 4 ·−-based advanced oxidation processes (SR-AOPs) using 7 peroxydisulfate (PDS) and peroxymonosulfate (PMS) activated by heat. With special focus on the 8 process application and feasibility, BTA removal was also studied in presence of chloride, as an 9 abundant anion of water and also industrial effluents. The results showed that PDS and PMS-based 10 AOPs can act notably different from the aspect of degradation efficiency and toxic chlorinated by-11 product formation. 12 13
Article
The transformation of halogenated organics in advanced oxidation processes (AOPs) has been extensively investigated. However, we currently know little about the fate of halogenated pollutants in the presence of exogenic halides (Cl⁻ or Br⁻). Herein, the degradability, mineralization rate, and accumulation capacity of adsorbable organic halogen (AOX) for chlorophenols (2-chlorophenol (2-CP), 3-chlorophenol (3-CP), 4-chlorophenol (4-CP), and 2,4,6-trichlorophenol (TCP)) were compared in the Fe²⁺/persulfate (PS) process with the addition of exogenic halides. Results indicate that exogenic X⁻ can lead to a decrease in chlorophenols degradation and mineralization rate, undesirable accumulation of AOX, and generation of halogenated by-products which are more toxic than precursor chlorophenols. Results of kinetics modeling show that Cl2•- played more important role than SO4•- with an addition of Cl⁻, while SO4•-, Br2•-, and Br2 were responsible for the effect of Br⁻. As well, the effect of endogenic chlorine atoms on chlorophenols reveals that the degradability and AOX formation potential of 3-CP are highest while that of TCP are the lowest. This study demonstrates the significant influence of endogenic chlorine atoms and exogenic X⁻ on the fate of typical organic halogen compounds. Consequently, the X⁻ level and position/number of halogen atoms should be considered simultaneously when treating organohalogen compounds.
Article
Full-text available
Recently, the degradation of non-chlorinated organic pollutants in saline pharmaceutical wastewater by SO4˙--based advanced oxidation processes (AOPs) has received widespread attention. However, little is known about the oxidation of chlorinated compounds in SO4˙--based AOPs. This study chose clofibric acid (CA) as a chlorinated pollutant model; the oxidation kinetics and mechanistic pathway were explored in the Co2+/peroxymonosulfate (PMS) system. Notably, a high removal efficiency (81.0%) but low mineralization rate (9.15%) of CA within 120 min were observed at pH 3.0 during Co2+/PMS treatment. Exogenic Cl- had a dual effect (inhibitory then promoting) on CA degradation. Several undesirable chlorinated by-products were formed in the Co2+/PMS system. This demonstrated endogenic chlorine and exogenic Cl- both reacted with SO4˙- to generate chlorine radicals, which participated in the dechlorination and rechlorination of CA and its by-products. Furthermore, SO4˙- was the dominant species responsible for CA degradation at low Cl- concentrations (≤1 mM), whereas Cl2˙- was the predominant radical at [Cl-]0 > 1 mM. A possible degradation pathway of CA was proposed. Our findings suggested that chlorinated compounds in highly saline pharmaceutical wastewater will be more resistant and deserve more attention.
Article
Degradation efficiency of 4-bromo-2-chlorophenol (BCP) containing both chlorine and bromine substituents in Co/PMS process was investigated in the light of a wide range of substrate, oxidant, catalyst concentrations and pH value. The effects of chloride ion (Cl⁻) on degradation kinetics, total organic carbon (TOC) removal and intermediates formation during BCP depletion in Co/PMS system were studied. The kinetics results demonstrated that the dual effect of Cl⁻ on BCP depletion in Co/PMS system due to different mechanisms involved. High concentrations of Cl⁻ (>5 mM) can significantly promote the degradation of BCP, but did inhibit BCP mineralisation to a certain extent which was closely related to Cl⁻ content. High degradation rates but lower mineralisation rates were found in the laboratory experiments, owing to the fact that BCP was mainly transformed to new halogenated intermediates instead of complete mineralisation. Gas chromatograph-mass spectrometer (GC-MS) data verified that a series of chlorinated by-products were formed during BCP decomposition process involving of the participation of Cl⁻. The proposed degradation pathways of BCP and its derivatives in presence of Cl⁻ were discussed on the basis of intermediate products including the undesirable halogenated by-products recognised by GC-MS. These results might offer some new perspectives on the transformation fates of BCP by utilising Co/PMS regent.
Article
Full-text available
Owing to the high concentration of pharmaceuticals in urine, the degradation of these organic pollutants before their environmental release is highly desired. Peroxymonosulfate (PMS) is a desirable oxidant that can be applied to environmental remediation; however, the performance and mechanism of PMS for the degradation of pharmaceuticals in the urine matrix have not been investigated. Herein, PMS was first discovered to efficiently degrade typical pharmaceuticals in hydrolyzed urine (HU) by selecting acetaminophen (ACE) as a target compound. Quenching experiments revealed that singlet oxygen (¹O2) and hydroxyl radicals (HO˙) were observed in the HU/PMS system, but the principal reactive species (RS) responsible for ACE removal was¹O2. The major constituents of HU, including SO4²⁻and organics (creatine, creatinine and hippuric acid), hardly affected the elimination of ACE, whereas Cl⁻, H2PO4⁻and NH4⁺would accelerate ACE degradation. Besides, HCO3⁻slightly inhibited this process. The ACE degradation efficiency was enhanced using photo-irradiation, including sunlight and visible light, although increasing the reaction temperature could, interestingly, hardly accelerate the degradation rate of ACE. Three-dimensional excitation-emission matrices (3D-EEMs) have indicated that other intermediates that have a higher fluorescence intensity might be generated in the HU/PMS system. Finally, nine intermediate products were determined and the degradation pathways of ACE were proposed. Overall, the results of this study illustrated that PMS is an efficient oxidant for the degradation of ACE in HU.
Article
Recently, studies on the degradation of emerging pollutants by activated persulfate (PS) have been widely reported. As a classic process, heat-activated peroxydisulfate (PDS) has been extensively used in practical engineering. However, the establishment of the quantitative relationship between the reduction of pollutants and PDS and the kinetic model of the heat-activated PDS process are rarely reported. In this study, triazine herbicide simazine (SMZ), which is frequently found in the environment, was selected as the target pollutant, and a series of coherent studies including kinetics calculation, radicals identification, and product analysis provided the basis for the establishment of the model. The steady-state model and unsteady-state model initially described the process of heat-activated PDS and degradation effect on SMZ. It was found that in the process of heat/PDS, the degradation of SMZ was significantly related to the decomposition of PDS. The decomposition of each unit of PDS at the beginning of the reaction could lead to the degradation of 1.616 units of SMZ. In addition, during the reaction, sulfate radical (SO4•−) was the dominant radical and could be partly transformed to hydroxyl radical (•OH) to participate in the reaction (with the ratio of 25:1). Finally, it was noted that the reaction intermediates could not be ignored in the establishment of the unsteady-state model. This study provides a set of coherent research methods, which is conducive to further simulation of the heat-activated PDS process.
Article
Recently, the degradation of organic compounds in saline dye wastewater by sulfate radicals (SO4•‒)-based advanced oxidation processes (AOPs) have attracted much attention. However, previous studies on these systems have selected non-chlorinated dyes as model compounds, and little is known about the transformation of chlorinated dyes in such systems. In this study, acid yellow 17 (AY-17) was selected as a model of chlorinated contaminants, and the degradation kinetics and evolution of oxidation byproducts were investigated in the UV/PDS system. AY-17 can be efficiently degraded (over 98% decolorization) under 90 min irradiation at pH 2.0-3.0, and the reaction follows pseudo-first order kinetics. Cl‒ accelerated the degradation of AY-17, but simultaneously led to an undesirable increase of absorbable organic halogen (AOX). Several chlorinated byproducts were identified by liquid chromatography-mass spectrometry (LC-MS/MS) in the UV/PDS system. It indicates that endogenic chlorine and exogenic Cl– reacted with SO4•‒ to form chloride radicals, which are involved in the dechlorination and rechlorination of AY-17 and intermediates. The possible degradation mechanisms of AY-17 photooxidative degradation are proposed. This work provides valuable information for further studies on the role of exogenic chloride in the degradation of chlorinated azo dyes and the kinetic parameters in the PDS-based oxidation process.
Article
In this study, peroxymonosulfate (PMS) activation was successfully achieved by microwave (MW) irradiation directly and subsequently applied for the degradation of bisphenol A (BPA, an endocrine disrupting chemical frequently detected in the environment), especially at temperatures above 60°C. The experiment results showed that a higher reaction temperature, MW power level, initial PMS dose, and initial solution pH had positive effects on the degradation efficiency of BPA. The degradation efficiency of BPA was slightly enhanced in real water compared to that in ultrapure water. The result of radical scavenger experiments indicated that both sulfate radical and hydroxyl radical were the dominant reactive oxygen species. Based on the results of high performance liquid chromatography and gas chromatography-mass spectrometry, several transformation pathways, including β-scission, hydroxylation, dehydration, oxidative skeletal rearrangement, and ring opening, were proposed. The complete degradation of several typical organic contaminants was also achieved using the MW/PMS process. This work would broaden the selection of PMS activation methods and provide an option for wastewater treatment.
Article
Magnetic Fe3O4 nanoparticles were synthesized and used as peroxymonosulfate (PMS) activator in the presence of ultrasound (US) irritation for azo dye degradation in this study. At pH 7.0 and temperature 298 K, Fe3O4 nanoparticles were able to activate PMS under ultrasound irritation to produce sulfate radicals for acid orange 7 (AO7) removal in 30 min. Higher catalysts dosage, neutral pH as well as optimum values of PMS dosage and ultrasound power favored the AO7 degradation. Fe3O4 displayed excellent stability and reusability in the US/PMS system. Both of the sulfate radicals and hydroxyl radicals were produced in the reaction and sulfate radicals were the dominant according to the scavenging tests and electron paramagnetic resonance (ESR) tests. The mechanisms of ultrasound irritation enhanced PMS activation in the presence of Fe3O4 were proposed based on the results and literatures.
Article
Addition order of reagents in the Acid Orange 7 (AO7) degradation process was investigated by varying the concentration of Fe(ii), the Fe(ii)/peroxymonosulfate (PMS) molar ratio and stepwise addition of Fe(ii) and PMS. The importance of addition order of reagents was confirmed and an order of Fe(ii)-PMS to improve the oxidation efficiency was recommended.
Article
Graphene-based materials have emerged as novel and green alternatives to metals/oxides for environmental catalysis. This study integrates deliberate material fabrication with density functional theory (DFT) calculations to probe intrinsic active sites, e.g. the defects and oxygen functionalities on graphene for activating OO bond in peroxymonosulfate (PMS) toward catalytic oxidation. The reaction rate constants of degradation efficiency were discovered to be closely related with the ID/IG values of thermally annealed reduced graphene oxides (rGOs). Three rGOs (rGO-CM, rGO-HH, and rGO-HT) with a similar oxygen level by different reduction methods were utilized to investigate the effect of different oxygen groups. The results indicate that rGO-HT with the highest contents of ketonic group (CO) presented the best activity. The theoretical calculations were applied to simulate the PMS chemisorption with all the possible active sites on rGO. The DFT results suggest that vacancies and defective edges are more reactive than the graphene basal plane with prolonged OO bond in PMS molecules, greater adsorption energy, and more electron transfer. Besides, the electron-rich ketonic groups may be the major active species among the oxygen functionalities. The findings will contribute to new insights in reaction mechanism and material design in heterogeneous carbocatalysis.
Article
Chloride ion is known to affect on degradation kinetics in different ways during HO and [Formula: see text] -based advanced oxidation processes (AOPs). However, its effect on absorbable organic halogen (AOX) evolution and acute toxicity of treated water remains unknown, despite the importance of the two parameters in evaluating the applicability of AOPs. In the present study, Co/peroxymonosulfate (Co/PMS) and UV/hydrogen peroxide (UV/H2O2) treatment of 2,4,6-trichlorophenol was compared in terms of AOX formation, chlorinated byproducts and acute toxicity. Both Co/PMS and UV/H2O2 systems were more reactive under acidic conditions, resulting in elevated AOX levels when compared with those at neutral pH. The presence of high levels of chloride led to an accumulation and increase of AOX in the Co/PMS system. The toxicity of chlorinated byproducts was evaluated using Photobacterium phosphoreum, and the results revealed a sharp increase in acute toxicity of Co/PMS reaction solutions on addition of chloride ion. However, addition of Cl(-) had no apparent impact on AOX and toxicity of UV/H2O2 reaction solutions. These findings may have significant technical implications for selecting feasible technologies to treat high salinity wastewater.