Kinetics and mechanistic investigation into the degradation of naproxen by a UV/chlorine process

Article abstract of DOI:10.1039/c7ra04540a

In this study, UV irradiation combined with chlorine (UV/chlorine) was used to degrade naproxen (NPX), a typical non-steroidal anti-inflammatory drug (NSAID) widely used for the treatment of symptoms associated with inflammation, in water. Compared with UV irradiation alone and direct chlorination, the UV/chlorine process shows a synergistic effect on NPX degradation. The effects of different factors, including the chlorine dose, solution pH, and the presence of Cl^-, HCO₃^- or humic acid (HA), on NPX degradation in the UV/chlorine process were investigated. The results indicated that the degradation of NPX followed pseudo-first-order kinetics in all cases, and the rate constant increased as the chlorine dose increased and decreased as the pH increased. The effects of the water matrix on UV/chlorine treatment were species-dependent. The NPX degradation rate was inhibited by the presence of HCO₃^- and HA but significantly improved by Cl^-. LC/MS/MS analysis indicated that NPX decomposition in the UV/chlorine process was associated with decarboxylation, demethylation and hydroxylation. These results indicate that the UV/chlorine process is a promising technology for the treatment of water polluted by emerging contaminants, such as NPX. However, UV/chlorine can notably enhance the formation of disinfection by-products compared to direct chlorination, which should be carefully considered when integrating this process into drinking water treatment schemes.

Full text of DOI:10.1039/c7ra04540a

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PAPER
Kinetics and mechanistic investigation into the
degradation of naproxen by a UV/chlorine process†
Cite this: RSC Adv., 2017, 7, 33627
a
b
b
a
b
Yu-qiong Gao, * Nai-yun Gao, Wen-hai Chu, Qin-lin Yang and Da-qiang Yin
In this study, UV irradiation combined with chlorine (UV/chlorine) was used to degrade naproxen (NPX),
a typical non-steroidal anti-inflammatory drug (NSAID) widely used for the treatment of symptoms
associated with inflammation, in water. Compared with UV irradiation alone and direct chlorination, the
UV/chlorine process shows a synergistic effect on NPX degradation. The effects of different factors,
ꢀ
3ꢀ
including the chlorine dose, solution pH, and the presence of Cl , HCO or humic acid (HA), on NPX
degradation in the UV/chlorine process were investigated. The results indicated that the degradation of
NPX followed pseudo-first-order kinetics in all cases, and the rate constant increased as the chlorine
dose increased and decreased as the pH increased. The effects of the water matrix on UV/chlorine
3ꢀ
treatment were species-dependent. The NPX degradation rate was inhibited by the presence of HCO
ꢀ
and HA but significantly improved by Cl . LC/MS/MS analysis indicated that NPX decomposition in the
UV/chlorine process was associated with decarboxylation, demethylation and hydroxylation. These
results indicate that the UV/chlorine process is a promising technology for the treatment of water
polluted by emerging contaminants, such as NPX. However, UV/chlorine can notably enhance the
formation of disinfection by-products compared to direct chlorination, which should be carefully
considered when integrating this process into drinking water treatment schemes.
Received 23rd April 2017
Accepted 28th June 2017
DOI: 10.1039/c7ra04540a
rsc.li/rsc-advances
metabolized or simply pass through human body. NPX is
frequently detected in WWTFs, surface water, or even ground-
1
. Introduction
6–8
In recent years, pharmaceutical and personal care products water. It has been reported that naproxen can impair the lipid
PPCPs) have been the subject of increasing concern around the peroxidation system of bivalves and change microbial commu-
(
9
world as emerging contaminants. PPCPs are widely used as nity structures to yield antibiotic resistance. Like most other
veterinary therapeutic drugs to prevent and treat human and PPCPs, naproxen seems to be resistant to conventional waste-
1
animal diseases and improve the quality of life of humans.
water treatment. According to Carballa et al., the overall
Many PPCPs can be easily transported and distributed in water removal efficiency of NPX in a sewage treatment plant was
2
10
bodies due to their high polarity and low volatility. Because of approximately 40–55%. Therefore, it is important to make use
their extensive applications and poor removal in conventional of technologies that focus on removing such drugs from points
wastewater treatment plants, PPCPs are ubiquitous in aquatic of discharge.
3
environments. PPCPs are commonly detected in waters at
Advanced oxidation processes (AOPs) have oen gained
ꢀ1
ꢀ1
concentrations ranging from ng L to low mg L . Even these recognition as a promising approach towards removing recal-
trace levels are enough to cause hazardous effects on human citrant organic pollutants in water. The hydroxyl radicals (cOH)
health and ecosystems. As a typical PPCP, naproxen belongs formed during AOPs, with a high reduction potential of 2.9 V in
4,5
to the class of non-steroidal anti-inammatory drugs (NSAIDs) AOPs, are capable of destroying or mineralizing most organic
11
and is widely used in the treatment of arthritis, ankylosing compounds with high reaction rate constants. The combina-
spondylitis, acute muscle pain, and other symptoms associated tion of UV irradiation and chlorine (UV/chlorine) has recently
with inammation. Most of these drugs are usually partly been examined as an effective AOP for PPCPs removal from
12
water. Several reactive species, including cOH and reactive
ꢀ
chlorine species (RSC) such as cCl and Cl c , can be formed
2
a
School of Environment and Architecture, University of Shanghai for Science and
Technology, Shanghai 200093, China. E-mail: gaoyq@usst.edu.cn; Tel: +86 21
through reactions occurring in the UV/chlorine system (eqn (1)–
13
ꢀ
2
c have reduction potentials of 2.4 V and 2.0 V,
(
5)). cCl and Cl
5
5275979
b
respectively, which are comparable to that of cOH, and can
Yangtze River Water Environment, Ministry of Education, College of Environmental degrade the target contaminants.
State Key Laboratory of Pollution Control and Resource Reuse, Key Laboratory of
12
Science and Engineering, Tongji University, Shanghai 200092, China
+
ꢀ
†
Electronic supplementary information (ESI) available. See DOI:
0.1039/c7ra04540a
HOCl 4 H + OCl
(1)
1
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HOCl + hv / cOH + cCl
(2) 2.2. Experimental procedures
All experiments were conducted in a collimated beam apparatus
ꢀ
ꢀ
OCl + hv / cCl + cO
(3)
(4)
(5)
19
(Fig. S1†). The equipment consists of a low-pressure UV Hg
ꢀ
ꢀ
lamp (UV-C, 75 W, Philips, Netherlands), a lampshade used to
parallel the light, two reaction dishes, two magnetic stirrers,
and two rest pier. The average irradiation strength on the
cO + H
2
O 4 cOH + OH
ꢀ
ꢀ
cCl + Cl 4 Cl2c
ꢀ
2
solution was calculated to be approximately 261 mW cm
,
20
In general, the UV/chlorine process combines the advan- using atrazine chemical actinometry. Before each experiment,
tages of UV/H
and chlorination processes, which is an the lamp was turned on for at least 30 min to stabilize the lamp
2
O
2
economical, efficient, and easy to operate AOP. The UV/ output. The degradation of NPX by UV/chlorine process was
chlorine process is more efficient than UV photolysis or conducted by spiking specic amounts of chlorine into a 100
chlorination in the degradation of atrazine (ATZ), and the mL testing solution containing 25 mM NPX and 10 mM phos-
degradation efficiency of ATZ is sensitive to the solution pH, phate buffer to give an initial chlorine dosage of 125–375 mM
1
4
as well as common anions in real waters. There is a clear and pH of 5–9, and oxidation was initiated once the Petri dishes
synergistic effect between UV and chlorine on the degrada- were moved under the collimated beam apparatus. UV photol-
tion of carbamazepine during the UV/chlorine process, and ysis alone and direct chlorination of NPX were conducted
the initial oxidation products of CBZ resulted from hydrox- without spiking with chlorine solution and UV irradiation,
ylation, carboxylation and hydrogen atom abstraction by cOH respectively. At each determined time, 0.8 mL of sample was
1
5
2 2 3
and cCl. Wu et al. compared the change in the acute toxicity collected into an HPLC vial containing excess Na S O to
of a trimethoprim solution at different reaction times in the quench the residual chlorine. And the samples were immedi-
chlorination and UV/chlorine processes, and found that the ately analyzed by HPLC for NPX concentration and LC/MS/MS
acute toxicity resulting from UV/chlorine was lower than that for degradation products. In addition, 10 mL or 40 mL of
from direct chlorination, with the same removal rate of reaction solution was transferred into a glass volumetric bottle
1
6
trimethoprim. Additionally, UV irradiation can effectively for TOC or DBPs analysis, respectively.
inactive chlorine-tolerant pathogenic microorganisms (e.g.,
Cryptosporidium parvum and Giardia lamblia) and is oen
2
.3. Analytical methods
used in conjunction with chlorine in a complementary
The concentration of NPX was measured by using a high
performance liquid chromatography (HPLC) (Waters e2695,
USA) equipped with a symmetry C18 column (4.6 mm ꢁ 250
mm, 5 mm, Waters, USA) by an UV detector wavelength of
1
7
manner. However, as a chlorine-derivate treatment, this
process runs the risk of forming chlorinated products during
degradation. It has been reported that more disinfection by-
products (DBPs) precursors were formed during the UV/
chlorine process than during UV irradiation or direct chlo-
230 nm. The mobile phase for naproxen consisted of 70%/30%
(v/v) acetonitrile and 0.1% formic acid in Milli-Q water at a ow
1
8
rination, which is of concern in water treatment processes.
ꢀ1
rate of 0.8 mL min . The sample injection volume was 10 mL.
The evolution of the reaction mixtures was also monitored
using a TU-19 UV-vis spectrophotometer (Puxi, Beijing, China).
A stock solution of free chlorine was prepared from 5%
liquid sodium hypochlorite and standardized by the DPD (N,N-
diethyl-p-phenylenediamine, Sigma, >99%) colorimetric
method.
The intermediate products of NPX formed during UV/
chlorine process were analyzed by LC/MS/MS, which consisted
of an HPLC system (Waters 2010, USA) and a TSQ Quantum
mass spectrometer (ESI source, Thermo Scientic Inc., USA)
with a Hypersil Gold C18 column (2.1 ꢁ 150 mm, 5 mm). The
sample injection volume was 10 mL. The A and B mobile phases
were 0.1% formic acid in Milli-Q water and acetonitrile,
Therefore, the aim of this study is to evaluate the removal
efficiency of NPX by the UV/chlorine process, along with the
ꢀ
inuence of factors such as chlorine doses, solution pH, Cl ,
ꢀ
HCO3 and humic acid (HA) on NPX removal in water. More-
over, the degradation pathways of NPX during UV/chlorine were
proposed according to LC/MS/MS analysis. Finally, we compare
the disinfection by-products (DBPs) formation during the
treatment of NPX by direct chlorination and UV/chlorine
processes.
2. Materials and methods
2.1. Materials and reagents
ꢀ1
respectively, at a ow rate of 0.25 mL min . The elution started
Naproxen was purchased from Sigma-Aldrich (USA). Sodium at 5% B, then linearly increased to 95% B over 15 min and was
hypochlorite (NaOCl) containing 5% free chlorine was obtained kept at 95% B for 5 min. Aer that, the elution was decreased to
from Sinopharm Chemical Reagent Co., Ltd. (Shanghai, China). 95% B in the following 1 min and kept at 95% B for 4 min. The
HPLC-grade acetonitrile used as the mobile phase was obtained ESI conditions were as follows: spray voltage ¼ 4500 V, sheath
from J. T. Baker (USA). DBPs standard solutions were also gas pressure ¼ 20 au, aux gas pressure ¼ 5 au, capillary
ꢂ
purchased from Sigma-Aldrich. All other chemicals were at least temperature ¼ 320 C. MS chromatograms were obtained in
of analytical grade and were used without further purication. both total ion current (TIC) mode, using full scans (m/z 50–400)
All solutions were prepared with ultrapure water produced from for mass spectra acquisition, and selected ion monitoring (SIM)
a Milli-Q water purication system (Millipore, USA).
mode.
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The degree of NPX mineralization was quantied by a stronger oxidation ability than free chlorine. An additional
measurement of the reduction in the total organic carbon (TOC) experiment was conducted to conrm the role of radicals in
in water. TOC analysis was measured using a TOC analyzer NPX degradation. As shown in Fig. S2,† various concentrations
(Shimadzu TOC 5050A) equipped with an ASI5000 auto- of tert-butanol (TBA) inhibited NPX degradation to different
sampler. extent during the UV/chlorine process. As seen, the rate
ꢀ
1
ꢀ1
DBPs were quantied using a purge and trap sample constant decreased from 0.135 min to 0.058 min , respec-
concentrator (Eclipse 4660, OI, USA) and gas chromatography/ tively, with the addition of TBA increased from 0 mM to 25 mM
mass spectrometry (GC/MS) (QP2010, Shimadzu, Japan). ([TBA]
0
/[chlorine]
0
¼ 0–100). And the rate constant almost kept
21
Details on the analysis of DBPs are available elsewhere.
constant with further increased in TBA concentration. The re-
ported reaction rate constants for TBA with cOH and cCl are 6 ꢁ
8
ꢀ1 ꢀ1
8
ꢀ1 ꢀ1
1
0 M
s
and 3 ꢁ 10 M
s , respectively, and thus, TBA
3. Results and discussion
may compete for cOH and cCl, resulting in a decrease in the NPX
degradation rate.
5 mM TBA ([TBA]
NPX degradation by cOH and cCl. The above results indicated
that both direct chlorination and radical oxidation contributed
to the degradation of NPX by UV/chlorine oxidation.
24,25
3.1. Preliminary studies
Moreover, it seems that the addition of
/[chlorine]
2
0
0
¼ 100) can enough inhibited the
The results of NPX degradation by UV alone, direct chlorination
and the UV/chlorine process are shown in Fig. 1.
Generally, the degradation of NPX by UV irradiation alone
was negligible, resulting in the removal of 6.5% in 30 min. The
corresponding pseudo-rst-order rate constant was 0.00223
ꢀ
1
min . In contrast, it seems that NPX is vulnerable to both
chlorination and the UV/chlorine processes, as 77.8% and
3.2. Effect of chlorine dose
9
8.5% of NPX was removed aer 30 min by direct chlorination Semi-logarithmic graphs of [NPX]
and UV/chlorine, respectively. The degradation of NPX by direct rine doses as a function of time are shown in Fig. 2.
chlorination and UV/chlorine also followed pseudo-rst-order
NPX degradation by both direct chlorination and UV/
kinetics, and the corresponding rate constants were 0.053 chlorine under different doses of chlorine is well t by
t 0
/[NPX] with different chlo-
ꢀ
1
ꢀ1
min and 0.135 min , respectively.
Although the molar absorptivity of NPX at 254 nm can reach rate of NPX increased from 0.020 to 0.104 min as the ratio of
pseudo-rst-order kinetics. With chlorine only, the degradation
ꢀ
1
ꢀ1
ꢀ1
4
0 0
900 M cm , the low efficiency photolysis of NPX attributes [chlorine] /[NPX] increased from 5 to 15. Since direct chlori-
to its low quantum yield of 0.0093 mol per einstein. It has been nation plays an important role in the degradation of NPX, an
demonstrated that chlorine oxidation can readily transform increased concentration of chlorine will accelerate NPX degra-
NPX into intermediates. In addition, the synergistic effect was dation. In addition, the degradation rate of NPX by UV/chlorine
clear in the UV/chlorine process, as the degradation rate of NPX also increased from 0.057 to 0.215 min upon increasing the
increased by 1.54 times in the UV/chlorine process, compared to ratio of [chlorine] /[NPX] from 5 to 15. The reactive radicals,
22
23
ꢀ
1
0
0
that in direct chlorination. The combination of UV and chlorine including cOH and cCl, increased as the free chlorine increased,
results in a stronger oxidation than direct chlorination due to which strengthened the oxidation in the UV/chlorine process.
the formation of highly reactive radical species, such as Moreover, the degradation rate constant of NPX in the UV/
hydroxyl radicals (cOH) and chlorine atoms (cCl), which have chlorine process exhibited a linear tread as a function of
Fig. 1 Degradation of NPX by UV irradiation alone, direct chlorination Fig. 2 Effect of [chlorine]
and UV/chlorine process. Conditions: [NPX] chlorination and UV/chlorine process. Conditions: [NPX]
¼ 25 mM, [chlorine]
50 mM, pH ¼ 7. [chlorine] /[NPX] ¼ 5 : 1, 10 : 1, and 15 : 1, pH ¼ 7.
0
/[NPX]
0
on the degradation of NPX by direct
0
0
¼
0
¼ 25 mM,
2
0
0
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2
chlorine dose (k ¼ 0.631[chlorine] ꢀ0.0221, R ¼ 0.99). Previous respectively, the formation of cOH and cCl was reduced at higher
0
12
studies reported that an increase in the initial chlorine dose did pH as determined by the decreased quantum yield. Of note,
ꢀ
not continuously ensure an increase in the degradation rate of both HOCl and OCl are also cOH and cCl scavengers, and the
NPX due to the scavenging of the reactive species by excess free rate constants of HOCl with cOH and cCl are lower than that of
12,14
ꢀ
12,29
chlorine.
However, the inhibitory effect of excess chlorine OCl (eqn (6)–(9)).
was not observed, probably because the chlorine dose did not
reach the critical level at which NPX degradation begins to slow.
Considering that the effect of chlorine dose on the UV photol-
ysis of NPX can be neglected, we determined that the contri-
bution of the radical oxidation pathway slightly decreased from
4
ꢀ1 ꢀ1
s
cOH + HOCl / H
2
O + cClO, k ¼ 8.46 ꢁ 10 M
(6)
(7)
(8)
(9)
ꢀ
9
ꢀ1 ꢀ1
s
cOH + OCl / OH + cClO, k ¼ 9.0 ꢁ 10 M
+
ꢀ
9
ꢀ1 ꢀ1
s
cCl + HOCl / H + Cl + cClO, k ¼ 3.0 ꢁ 10 M
6
1% to 50% as the ratio of [chlorine] /[NPX] increased from 5
0 0
to 15.
ꢀ
ꢀ
9
ꢀ1 ꢀ1
s
cCl + OCl / Cl + cClO, k ¼ 8.2 ꢁ 10 M
As a result, UV/chlorine achieves a higher removal efficiency
at lower pH. In addition, the contribution of the radical oxida-
3.3. Effect of pH
Semi-logarithmic graphs of [NPX] /[NPX] at varies solution pH tion pathway increased from 17.0% to 81.3% as the solution pH
t
0
as a function of time are shown in Fig. 3.
increased from 5 to 9.
As seen, the degradation rate of NPX in both direct chlori-
nation and the UV/chlorine process is highly pH-dependent. As
the solution pH increased from 5 to 9, the pseudo-rst-order
rate constant of direct chlorination decreased from 0.392 to
3
.4. Effect of aqueous matrix species
ꢀ
3ꢀ
32ꢀ
ꢀ
1
The Cl anions, inorganic carbon (HCO /CO ) and humic
acid (HA) are common constituents in natural water, which
have been demonstrated to have different impacts on the
degradation of targeted pollutants by radical-based AOPs, it is
0
.003 min , while that of the UV/chlorine process decreased
ꢀ1
from 0.475 to 0.028 min , respectively.
Because the ratio of chlorine dissociation products (HOCl
ꢀ
and OCl ) is susceptible to the solution pH (pK
¼ 7.5),
a,HOCl
15
necessary to evaluate the effect of these constituents. Semi-
logarithmic graphs of [NPX] /[NPX] under different matrix
and HA) as a function of time are shown in
chlorine exists mainly as HOCl at lower pH, and HOCl gradually
transformed into OCl as the solution pH increased; therefore,
less HOCl is available for NPX oxidation. It has been reported
that the oxidation capacity of OCl towards drugs is weak
ꢀ
t
0
ꢀ
ꢀ
26
species (Cl , HCO
3
ꢀ
Fig. 4. As seen, their effects on NPX decomposition were
different. In the presence of 5 mM Cl , the NPX degradation
rate increased by 169.8% and 49.6% in the direct chlorination
and UV/chlorine processes, respectively. The result is due to the
ꢀ
27
compared with that of HOCl. Moreover, naproxen is a weak
acid with a pK of 4.15, and thus, the deprotonation of NPX
a
28
increased as the pH increased. Therefore, the reaction
between the two anionic forms at higher pH is less efficient than
that at lower pH. During the UV/chlorine process, despite the
ꢀ
fact that Cl
2
can be formed from HOCl and Cl , according to
ꢀ
eqn (10), and the presence of Cl shis the reaction to the
right. Since Cl
accelerates NPX degradation.
30
ꢀ
2
is more electrophilic than HOCl, its presence
fact that both HOCl and OCl photolysis can produce cOH
radicals, showing quantum yields at 254 nm of 1.4 and 0.97,
Fig. 3 Effect of solution pH on the degradation of NPX by direct Fig. 4 Effect of matrix species on the degradation of NPX by direct
chlorination and UV/chlorine process. Conditions: [NPX]
chlorine]
¼ 250 mM, pH ¼ 5, 6, 7, 8 and 9.
0
¼ 25 mM, chlorination and UV/chlorine process. Conditions: [NPX]
0
¼ 25 mM,
ꢀ
ꢀ
ꢀ1
[
0
[chlorine]
0
¼ 250 mM, pH ¼ 7, [Cl ] ¼ [HCO
3
] ¼ 5 mM, HA ¼ 5 mg L
.
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ꢀ
HOCl + H + Cl 4 Cl + H O
(10) 3.5. Degradation of NPX in natural water
2
2
The potential of the UV/chlorine process and direct chlorination
in removing NPX was examined using natural water samples.
The samples were collected from one waterworks in Yixin City,
Jiangsu Province. As shown in Table 1, the performance of NPX
degradation by direct chlorination and the UV/chlorine process
was signicantly inhibited when using natural water due to the
strong scavenging effects of anions and/or NOM, as discussed
earlier. In addition, the UV/chlorine process seems to be more
efficient and tolerant than direct chlorination for removing NPX
from natural water. UV/chlorine was also more suitable for the
On the other hand, cCl and cOH produced during the UV/
ꢀ
ꢀ
chlorine process reacts rapidly with Cl to form cCl
2
and
ꢀ
ClOHc separately, which are more selective and may affect the
12
ꢀ
degradation efficiency of NPX (eqn (11)–(15)). Altogether, Cl
has a positive effect on NPX degradation in this study.
ꢀ
ꢀ
9
ꢀ1 ꢀ1
cCl + Cl / Cl
2
c , k ¼ 6.5 ꢁ 10 M
s
(11)
(12)
ꢀ
ꢀ
5
ꢀ1
Cl
2
c
/ cCl + Cl , k ¼ 1.1 ꢁ 10 s
ꢀ3
removal of NPX in natural water. EEO (kW h m per order) was
(13) adopted to calculate the energy consumption by the UV/
ꢀ
ꢀ
9
ꢀ1 ꢀ1
cOH + Cl / ClOHc , k ¼ 4.3 ꢁ 10 M
s
33
chlorine process (eqn (19)).
ꢀ
ꢀ
9
ꢀ1
ClOHc / cOH + Cl , k ¼ 6.1 ꢁ 10 s
(14)
(15)
P ꢁ t ꢁ 1000
V ꢁ 60 ꢁ logðc =c Þ
E
EO
¼
(19)
ꢀ
ꢀ
ꢀ1
0
t
ClOHc / cCl + OH , k ¼ 23 s
where P is the total electrical power, kW; t is the irradiation
time, min; and V is the volume of the treated solution, L. The
calculated EEO, which denotes the electricity cost, in ultrapure
3ꢀ
As for HCO , the degradation rate of NPX decreased by
2
9.6% during the UV/chlorine process and remained almost
ꢀ
constant during direct chlorination. Once HCO
3
was added to
water, raw water and nished water was 213.20, 1370.57 and
the solution, it would proceed to react, and the carbonate
ꢀ3
6
54.14 kW h m , respectively.
system would eventually reach equilibration. As the pK values
a
of H CO are 6.35 and 10.33, at neutral pH, the primary
2
3
3
.6. Intermediate product analysis and degradation pathway
ꢀ
compositions at equilibrium were 4.083 mM HCO /0.002 mM
3
2
ꢀ
ꢀ
ꢀ
The UV-vis absorption spectra of the reaction mixtures as
a function of time are shown in Fig. 5. NPX has a main
absorption band centred at 230 nm, a second band centred
around 270 nm, and a third band of lower intensity around
CO
and CO
rate constants with cOH and cCl, which reduces the degradation
3
with a 5 mM initial concentration of HCO
3
. Both HCO
3
2
ꢀ
3
compete with NPX for cOH and cCl due to their high
12,31
rate of NPX (eqn (16)–(18)).
320 nm. It is obviously that the main absorption band of NPX
ꢀ
ꢀ
3
6
ꢀ1 ꢀ1
cOH + HCO3 / H O + CO c , k ¼ 8.5 ꢁ 10 M
s
(16) decreased in intensity as the reaction proceeds, indicating the
NPX disappeared quickly during UV/chlorine process, and the
(17) change of the absorbance in the 230–270 nm region cannot be
2
2
ꢀ
ꢀ
ꢀ
8
ꢀ1 ꢀ1
cOH + CO
3
/ CO
3
c
+ OH , k ¼ 3.9 ꢁ 10 M
s
described with a simple exponential curve, suggesting the
formation of the intermediates.
3ꢀ
2
3
ꢀ
8
ꢀ1 ꢀ1
s
cCl + HCO / H
O + CO
c , k ¼ 2.2 ꢁ 10 M
(18)
Besides NPX, eight intermediate products at detectable levels
ꢀ
1
In the presence of 5 mg L HA, the degradation rate of NPX
decreased by 50.9% and 61.5% in the direct chlorination and
UV/chlorine processes, respectively. Initially, HA competes with
NPX for oxidants and cOH and cCl. The reported rate constants
were identied by LC/MS/MS analysis. Their tentative struc-
tures, chromatograms and mass spectra are provided in Table
S1, Fig. S3 and S4.† The intermediate products of NPX were
denoted as P246, P186, P184, P200, P204, P264, P299 and P134.
P246 corresponds to the hydroxylated products of NPX,
which was attributable to cOH attack. In addition, P246 has
structural isomers with different retention times. However, the
MS data information cannot distinguish the exact position of
hydroxylation. According to Dulova et al., the methyl and
4
ꢀ1 ꢀ1
between NOM and cOH and cCl were 2.5 ꢁ 10 (mg L ) s and
4
ꢀ1 ꢀ1
12,32
1
.3 ꢁ 10 (mg L ) s , respectively.
Furthermore, NOM can
act as a UV inner lter, which reduces the rate at which chlorine
12
photolysis produces cOH and cCl. Finally, HA can also
consume chlorine directly.
a,b
Table 1 The water quality parameters and degradation rate constants of different samples
ꢀ1
k (min
)
ꢀ
ꢀ1
3ꢀ
ꢀ1
ꢀ1
pH
Cl mg L
HCO mg L
DOC mg L
UV/chlorine
Chlorination
Pure water
Raw water
Finished water
7
7.53
7.66
0
6
10
0
143
137
#0.02
6.189
4.311
0.135
0.021
0.044
0.053
0.007
0.012
a
b
[
NPX]
0
¼ 25 mM, [chlorine]
0
¼ 250 mM, reaction time ¼ 30 min. All the water samples are ltered through 0.45 mm hydrophilic polyethersulfone
membrane.
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abstraction to yield P204, and P205 may also be derived directly
from chlorine substitution in P184. In addition, P135 is attrib-
uted to a carboxylic acid, as the ultimate step in NPX mineral-
36
ization. To conclude, these products in the UV/chlorine system
reveal four main mechanisms, that is, hydroxylation, chlorine
substitution of naphthalene, decarboxylation and demethyla-
tion. Based on the above discussion, the postulated degradation
pathways of NPX during the UV/chlorine process are summa-
rized in Fig. 6.
3.7. Comparison of known DBPs formation during UV/
chlorine and chlorination treatment
Fig. 5 The UV-vis spectra of the NPX degradation in UV/chlorine
process. Conditions: [NPX]
0
¼ 25 mM, [chlorine]
0
¼ 250 mM, pH ¼ 7.
Fig. 7 displayed the formation of chlorinated DBPs during the
degradation of NPX by UV/chlorine and direct chlorination
processes. Four kinds of chlorinated DBPs, namely trichloro-
methoxyl groups on the naphthalene ring are two sites of the methane (TCM), chloral hydrate (CH), dichloropropanone
attack by cOH, and these two hydroxylated species can be (DCP) and trichloropropanone (TCP), were detected. As seen,
further oxidized or demethylated to form P184 (2-methoxy-6- the concentration of TCM and CH increased or kept stable
vinylnaphthalene)
and 2-(6-hydroxy-2-naphthyl)propanoic (DCP and TCP) as treatment was extended. Obviously, the
34
acid. However, we did not observe 2-(6-hydroxy-2-naphthyl) formation of TCM, CH, and TCP was enhanced in the UV/
propanoic acid probably due to its fast transformation rate in chlorine process compared to direct chlorination. Only 8.3%
our system. P200 is a typical photoinduced product of NPX mineralization was achieved for the UV/chlorine process
under UV irradiation. P200 is derived from the hydrogen under the tested conditions (date not shown), which indicates
abstraction of 1-(6-methoxy-2-naphthyl)ethanol, another that NPX may transform into intermediates that more easily
35
3
9,40
photoinduced product that was not detected in our study; form DBPs aer UV/chlorine treatment.
It has been re-
however, its oxidized precursor P187 was found. It has been ported that the addition of a hydroxy group on the aromatic
demonstrated that decarboxylation is a main step in the structure can increase the reactivity of compounds with chlo-
4
1
degradation of NPX, and P187 corresponds to the decarboxy- rine, which results in the formation of DBPs precursors.
36
lated product of NPX. Of note, P184 can also be formed by Similar results were also reported by Wu et al., the formation
eliminating molecule from 1-(6-methoxy-2-naphthyl) of four tested DBPs in their study, including TCM, CH,
ethanol. cCl is responsible for mono- and di-chloro NPX, cor- dichloroacetonitrile (DCAN) and trichloroacetonitrile (TCNM),
2
H O
37
responding to P264 and P299, respectively. In addition, two Cl- were enhanced in the UV/chlorine process compared to chlo-
1
6
containing products can also be formed aer electrophilic rination process. Therefore, this potential effect should be
38
attack by chlorine on the naphthalene ring of NPX. P264 can carefully considered in the practical use of the UV/chlorine
then undergo decarboxylation, demethylation and hydrogen process.
Fig. 6 Possible degradation pathways of NPX in UV/chlorine system.
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Fig. 7 The DBPs formation during NPX degradation by (a) UV/chlorine and (b) direct chlorination processes. Conditions: [NPX]
chlorine]
¼ 500 mM, pH ¼ 7.
0
¼ 50 mM,
[
0
6
J. T. Yu, E. J. Bouwer and M. Coelhan, Agr. Water Manag.,
006, 86, 72–80.
4
. Conclusion
2
This study investigated the degradation of NPX under different
experimental conditions by a UV/chlorine AOP. The UV/chlorine
process showed synergistic effects on NPX degradation
compared to UV alone or direct chlorination. Both chlorination
and radical oxidation are responsible for the degradation of
NPX in the UV/chlorine process. In a typical UV/chlorine
7 T. A. Ternes, Water Res., 1998, 32, 3245–3260.
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solution pH increased. The presence of Cl improves NPX
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ꢀ
degradation, while an inhibitory effect was observed in the 11 I. Sir ´e s, E. Brillas, M. A. Qturan, M. A. Rodrigo and
ꢀ
presence of HCO
3
and NOM. Analysis of the intermediate
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2878.
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This study demonstrates that the UV/chlorine process is
Water Res., 2016, 90, 15–23.
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water.
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This work was supported by the Foundation of Key Laboratory
of Yangtze River Water Environment, Ministry of Education,
Tongji University, China (YRWEF201601) and the National
Major Project of Science & Technology Ministry of China (No.
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