Factors affecting formation of deethyl and deisopropyl products from atrazine degradation in UV/H2O2 and UV/PDS

Article abstract of DOI:10.1039/c7ra03660d

In this study, the formation of deethyl products (DEPs) (i.e., atrazine amide (Atra-imine) and deethylatrazine (DEA)) and deisopropyl product (i.e., deisopropylatrazine (DIA)) from parent atrazine (ATZ) degraded in UV/H₂O₂ and UV/PDS processes under various conditions was monitored. It was found that SO₄^- displayed a more distinctive preference to the ethyl function group of ATZ than HO, leading to the higher ratio of DEPs/DIA in UV/PDS system than that in UV/H₂O₂ system in pure water. The effects of water matrices (i.e., natural organic matter (NOM), carbonate/bicarbonate (HCO₃^-/CO₃^2-), and chloride ions (Cl^-)) on ATZ degradation as well as formation of DEPs and DIA were evaluated in detail. The degradation of ATZ by UV/PDS was significantly inhibited in the presence of NOM, HCO₃^-/CO₃^2- or Cl^-, because these components could competitively react with SO₄^- and/or HO to generate lower reactive secondary radicals (i.e., organic radicals, carbonate radicals (CO₃^-) or reactive chlorine radicals (RCs)). The yields of these DEPs and DIA products from ATZ degradation were not impacted by NOM or HCO₃^-/CO₃^2-, possibly due to the low reactivity of organic radicals and CO₃^- toward the side groups of ATZ. Howbeit, the increase of DIA yield companied with the decrease of DEPs yield was interestingly observed in the presence of Cl^-, which was attributed to the promotion of Cl^- at moderate concentration (mM range) for the conversion of SO₄^- into HO. Comparatively, in the UV/H₂O₂ process, NOM and HCO₃^-/CO₃^2- exhibited a similar inhibitory effect on ATZ degradation, while the influence of Cl^- was negligible. Differing from UV/PDS system, all these factors did not change DEPs and DIA yields in UV/H₂O₂ process. Moreover, it was confirmed that RCs had a greater selectivity but a lower reactivity on attacking the ethyl function group than that of SO₄^-. These findings were also confirmed by monitoring the degradation of ATZ as well as the formation of DEPs and DIA in three natural waters.

Full text of DOI:10.1039/c7ra03660d

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PAPER
Factors affecting formation of deethyl and
deisopropyl products from atrazine degradation in
UV/H2O2 and UV/PDS
Cite this: RSC Adv., 2017, 7, 29255
a
a
a
Congwei Luo,a Jin Jiang,
Chaoting Guan, Jun Ma, Suyan Pang,b Yang Song,a
*
*
Yi Yang,a Jianqiao Zhang,c Daoji Wud and Yinghong Guane
In this study, the formation of deethyl products (DEPs) (i.e., atrazine amide (Atra-imine) and deethylatrazine
(DEA)) and deisopropyl product (i.e., deisopropylatrazine (DIA)) from parent atrazine (ATZ) degraded in UV/
H2O2 and UV/PDS processes under various conditions was monitored. It was found that SO4cꢀ displayed
a more distinctive preference to the ethyl function group of ATZ than HOc, leading to the higher ratio of
DEPs/DIA in UV/PDS system than that in UV/H2O2 system in pure water. The effects of water matrices
(i.e., natural organic matter (NOM), carbonate/bicarbonate (HCO3ꢀ/CO32ꢀ), and chloride ions (Clꢀ)) on
ATZ degradation as well as formation of DEPs and DIA were evaluated in detail. The degradation of ATZ
by UV/PDS was significantly inhibited in the presence of NOM, HCO3ꢀ/CO3 or Clꢀ, because these
2ꢀ
components could competitively react with SO4cꢀ and/or HOc to generate lower reactive secondary
radicals (i.e., organic radicals, carbonate radicals (CO3cꢀ) or reactive chlorine radicals (RCs)). The yields of
2ꢀ
these DEPs and DIA products from ATZ degradation were not impacted by NOM or HCO3ꢀ/CO3
,
possibly due to the low reactivity of organic radicals and CO3cꢀ toward the side groups of ATZ. Howbeit,
the increase of DIA yield companied with the decrease of DEPs yield was interestingly observed in the
presence of Clꢀ, which was attributed to the promotion of Clꢀ at moderate concentration (mM range)
2ꢀ
for the conversion of SO4cꢀ into HOc. Comparatively, in the UV/H2O2 process, NOM and HCO3ꢀ/CO3
exhibited a similar inhibitory effect on ATZ degradation, while the influence of Clꢀ was negligible.
Differing from UV/PDS system, all these factors did not change DEPs and DIA yields in UV/H2O2 process.
Moreover, it was confirmed that RCs had a greater selectivity but a lower reactivity on attacking the ethyl
function group than that of SO4cꢀ. These findings were also confirmed by monitoring the degradation of
ATZ as well as the formation of DEPs and DIA in three natural waters.
Received 30th March 2017
Accepted 7th May 2017
DOI: 10.1039/c7ra03660d
rsc.li/rsc-advances
attracted great concerns in the protection of aquatic ecosys-
tems.5 Numerous technologies have been developed to remove
Introduction
Atrazine (6-chloro-N-ethyl-N0-(1-methylethyl)-1,3,5-triazine-2,4-
diamine; ATZ), which has been most widely used as
a synthetic herbicide to kill broadleaf weeds over the last half
century,1,2 has been frequently detected in ground and surface
water.2–4 Since many studies reported the high toxicity and
endocrine disruption potentials of ATZ even at trace levels, the
effective removal of ATZ in drinking water treatment has
ATZ, including biological treatment, physical absorption, and
chemical oxidation.6–10 Among these, chemical degradation has
been generally recognized as a promising and decisive step.
Recently, advanced oxidation processes (AOPs) based on
hydroxyl radicals (HOc, E0 ¼ 1.89–2.72 V vs. NHE) and sulfate
radicals (SO4cꢀ, E0 ¼ 2.5–3.1 V vs. NHE) have been of increasing
interest as an option of ATZ degradation.1,11,12 The reaction rates
of these two radicals with ATZ have been reported to be similar
and very fast (i.e., k ¼ 3.0 ꢁ 109 Mꢀ1 sꢀ1).13,14 The effects of water
matrices, including carbonate/bicarbonate (CO32ꢀ/HCO3ꢀ),
chloride (Clꢀ), and natural organic matters (NOM) on the
degradation kinetics of ATZ in these AOPs have been evaluated
as well. It has been suggested that these ubiquitous species can
react with HOc and/or SO4cꢀ to different degrees, thus inu-
encing the degradation rate of ATZ. For instance, it was reported
aState Key Laboratory of Urban Water Resource and Environment, Harbin Institute of
Technology, Harbin, 150090, China. E-mail: jiangjinhit@126.com; majun@hit.edu.
cn; Fax: +86-451-86283010; Tel: +86-451-86283010
bKey Laboratory of Green Chemical Engineering and Technology of College of
Heilongjiang Province, College of Chemical and Environmental Engineering, Harbin
University of Science and Technology, Harbin 150040, China
cLuohu District Environment Protection and Water Affairs Bureau, China
dSchool of Municipal and Environmental Engineering, Shandong Jianzhu University,
Jinan, 250010, China
ꢀ
that the presence of CO32ꢀ/HCO3 (>4 mM) inhibited ATZ
oxidation by HOc and SO4cꢀ generated from magnetic porous
eSchool of Water Conservancy and Civil Engineering, Northeast Agricultural University,
copper ferrite catalyzed peroxymonosulfate.15 In our previous
Harbin 150040, China
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study, it was found that Clꢀ over the concentration range of 0.5 reactive intermediates such as singlet oxygen (1O2), peroxy
3
to 10 mM had a slight scavenging effect on ATZ removal in UV/ radical (ROOc), and DOM* in direct UV photolysis, and these
hydrogen peroxide (UV/H2O2) process but signicantly sup- species are considerably reactive toward organic contaminants
pressed the degradation of ATZ in UV/persulfate (UV/PDS) such as atenolol, carbamazepine, and glimepiride.37 In our
2ꢀ
process.16 Additionally, ATZ was degraded more efficiently by previous work, the inuence of water constituents (i.e., HCO3
/
SO4cꢀ than by HOc in the presence of natural organic matter CO32ꢀ, Clꢀ, and NOM) on ATZ degradation kinetics have been
(NOM), which was attributed to the lower SO4cꢀ reaction rates investigated, and these factors on the relative contributions of
ꢀc,ATZ
with NOM than HOc (kSO
¼ 6.8 ꢁ 103 mg Cꢀ1 sꢀ1 vs. HOc and SO4cꢀ in UV/PDS and UV/H2O2 processes have been
4
kHOc,ATZ ¼ 1.4 ꢁ 104 mg Cꢀ1 sꢀ1).12
evaluated.16 However, the effects of various water constituents
Moreover, the mechanisms of ATZ transformation in HOc on the formation of primary products (i.e., DEPs and DIA) from
and/or SO4cꢀ based oxidation processes have been extensively ATZ are still unknown.
investigated,6,17–19 and dealkylation is suggested as a preferred
The main objective of this study was to investigate the
reaction pathway.20–23 HOc and/or SO4cꢀ abstract H-atom from formation of primary dealkylation products (i.e., DEPs and DIA)
the carbon adjacent to nitrogen to produce a carbon-centered along with the degradation of ATZ in UV/PDS and UV/H2O2
radical intermediate, and the addition of O2 to this interme- processes in the absence vs. presence of various water constit-
diate results in generation of a peroxide radical. Then, followed uents, including HCO32ꢀ/CO32ꢀ, Clꢀ, and NOM. Then the
by the loss of perhydroxyl radical, atrazine amide (Atra-imine) characteristics of secondary reactive radicals were evaluated by
can be produced, and its subsequent hydrolysis leads to the monitoring the changes of the molar ratio of DEPs to DIA.
formation of deethylatrazine (DEA).1 A similar mechanism is Furthermore, the formation of DEPs and DIA was also examined
expected for the formation of simazine amide and its hydrolysis in three natural waters.
product deisopropylatrazine (DIA). The hydrolysis of simazine
amide is much faster than Atra-imine.24 In addition, another two
pathways including alkyl chain oxidation and dechlorination-
hydroxylation can also occur during ATZ oxidation.25
Materials and methods
Reagents
ATZ, DEA, DIA, potassium peroxodisulfate, nitrobenzene (NB),
sodium carbonate, hypochlorite (HClO), and sodium chloride of
analytical grade were purchased from Sigma-Aldrich Chemical
Co. Ltd. (USA). Suwannee River Natural Organic Matter (SRNOM)
was purchased from the International Humic Substances
Society. H2O2 solution (35% w/w), sodium hypochlorite, phos-
phate monobasic monohydrate, sodium phosphate dibasic,
sodium thiosulfate were of analytical-reagent grade and
purchased from Sinopharm Chemical Reagent Co., Ltd., China.
Other chemicals were of analytical grade or better and used
without further purication. Three real water samples were
collected from Songhua River (SW) (China), drinking water
plants (EW), and groundwater (GW), respectively. All of the three
samples were ltered through 0.45 mm glass ber membranes
(Whatman) prior to using. The water quality parameters of these
samples were shown in Table 1. ATZ was spiked to the real water
samples to prepare ATZ-containing water.
Further, the percentages of deethyl products (DEPs) and
desisopropyl product (i.e., DIA) were calculated, and their total
yields accounted for about 70% degradation of ATZ.1,26 The total
yield of DEPs was determined to be higher than the yield of DIA.
For instance, the molar ratio of DEPs to DIA was calculated to be
3 in the case of HOc, while for SO4cꢀ it was 10.12,27 The higher
yield of DEPs than DIA is generally explained by the facts that:
(i) the N–H bond at the N-ethyl group is more acidic than that at
the N-isopropyl group, and the deprotonation would occur more
preferably at the H–N-ethyl site,12 (ii) both DEPs and DIA are
unstable toward the further attack of reactive radicals, and the
rate constant for the reaction of SO4cꢀ and/or HOc with DIA was
slightly higher than that with DEPs (i.e., a slightly faster
consumption of DIA),12,27 and (iii) steric hindrance effect of
isopropyl group is much more signicant than that of ethyl
group, thus limiting the formation of DIA.28
Generally, secondary reactive organic or inorganic radicals
(e.g., carbonate radical CO3cꢀ), reactive chlorine radicals (RCs)
can be generated by HOc and/or SO4cꢀ reacting with these water
matrices.29–31 CO3cꢀ as a highly selective one-electron oxidant
Analytical methods
ATZ, DEA, and DIA were analyzed using high performance
liquid chromatography (HPLC) with Hitachi 5110 pump and
5420 UV detector (Hitachi Chromaster), and the separation was
carried out with a symmetry C18 column (4.6 ꢁ 150 mm, 5 mm
particle size, Waters). For the mobile phase, various isocratic
(E0
¼
1.78 V vs. NHE) can oxidize some electron-rich
compounds (e.g., S-containing organics, N-containing
organics, and phenols) through electron transfer or hydrogen
abstraction.32–34 Chlorine atoms (Clc, E0 ¼ 2.4 V vs. NHE) and
dichloride radical (Cl2cꢀ, E0 ¼ 2.0 V vs. NHE) can also react with
various organic compounds, and the pathways mainly involve
one-electron oxidation, H-abstraction, and addition to unsatu-
rated C–C bond.35,36 The steady-state concentrations of CO3cꢀ
and RCs have been estimated to be higher than those of HOc
and SO4cꢀ for both UV/ꢀH2O2 and UV/PDS processes in the
presence of CO32ꢀ/HCO3 and Clꢀ (mM range).33 In addition,
previous studies have been reported that NOM as a photosen-
sitizer can lead to the photochemical formation of various
Table 1 Water quality of the actual water
pH
TOC (mg Lꢀ1
)
Alkalinity (mM)
Clꢀ (mM)
SW
GW
EW
7.6 ꢂ 0.02
7.9 ꢂ 0.02
6.8 ꢂ 0.02
3.12
1.62
1.53
0.4
2.3
0.3
0.15
3.50
0.22
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at room temperature (20 ꢃC). Phosphate buffer (5 mM) was used
to adjust pH value at 7.0. The samples were withdrawn at pre-
determined time intervals and immediately quenched using
excess sodium sulte. Then, they were kept at room tempera-
ture for 72 h in order to achieve complete conversion of Atra-
imine and simazine amide to DEA and DIA, respectively.
Thus, the concentrations of initially formed deethyl products
(DEPs) and deisopropyl products (DIPs) could be represented by
the concentrations of DEA and DIA, respectively.1 All kinetic
experiments were replicated independently at least three times,
and the error bars represented the standard deviations.
Results and discussion
Degradation of ATZ and formation of DIA and DEPs
Fig.
1 The experimental device used for the UV photolysis
experiments.
Fig. 2a showed the time courses of DEPs and DIA formation
during ATZ oxidation in direct UV, UV/H2O2 and UV/PDS
processes. As could be seen, the degradation rates of ATZ in
mixtures and gradients of 1& acetic acid and methanol were UV/PDS and UV/H2O2 processes were much higher than that in
used. The ow rate was set at 1.0 mL minꢀ1, and the sample
direct UV process. DEPs and DIA were generated along with ATZ
degradation in UV/PDS and UV/H2O2 processes, and then were
completely depleted within the investigated time scales in the
UV/PDS process. Comparatively, there were still residuals of the
products aer 60 min of reaction in the UV/H2O2 process. For
injection volume was 100 mL. The wavelengths of UV detection
were set at 230 nm (ATZ), 260 nm (DEA), and 237 nm (DIA),
respectively.23 PDS and H2O2 concentrations were measured by
the iodometric method.38 The stock solutions of residual chlo-
rine and active chlorine were standardized by using the triio- direct UV process, neither DEPs nor DIA was detected, which
dide spectrophotometric method.39 NOM (mg C Lꢀ1) was was consistent with the nding that hydroxylated atrazine
measured with a Multi 3100 N/C TOC analyzer (Shimadzu).
rather than dealkylation products was the primary product of
direct ATZ photolysis in previous study.41
The second-order rate constants of parent ATZ and its
primary oxidation products (e.g. Atra-imine, DEA, DIA, terbu-
Experimental procedures
The bench-scale UV irradiator equipped with four Low Pressure thylazine, and propazine) with HOc and SO4cꢀ were compa-
Hg UV lamps (254 nm, Heraeus, GPH212T5 L/4). 100 mL of rable.12 Nevertheless, the higher molar extinction coefficient ꢀa1t
solution with water depth of 4.0 cm was placed directly beneath 254 nm and the larger quantum efficiency of PDS (21.1 M
the collimated tube for UV irradiation. The surface of the cmꢀ1 and 1.4 mol einsteinꢀ1) than those of H2O2 (18 Mꢀ1 cmꢀ1
solution was approximately 30 cm from the UV lamp (Fig. 1). and 1.0 mol einsteinꢀ1) would result in higher steady-state
The irradiance of the UV irradiator was measured by a UV concentrations of SO4cꢀ than HOc16. For instance, Lutze et al.
detector UVC radiometer (Photoelectric Instrument Factory of reported that the concentration of SO4cꢀ in UV/PDS process was
Beijing Normal University, Beijing, China) and calibrated 1.5–1.7 fold higher than that of HOc in UV/H2O2 process.42
following the suggestion of Bolton and Linden,40 which was These factors lead to the faster degradation of ATZ as well as
estimate to be 0.16 mW cmꢀ2. All experiments were conducted DEPs and DIA in the UV/PDS process.
Fig. 2 (a) Degradation of ATZ and formation of DEPs and DIA as a function of time in direct UV, UV/H2O2, and UV/PDS processes. (b) Yields of
DEPs and DIA as a function of time in UV/H2O2 and UV/PDS processes. The insert showed the DEPs to DIA ratio. Experiment conditions: [ATZ] ¼
10 mM, [H2O2] ¼ [PDS] ¼ 1.5 mM, and pH ¼ 7.
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In initial stage of ATZ reaction with SO4cꢀ (i.e., UV/PDS), the comparable to ethanol and 2-propanol.12 The second order rates
formation of DIA and DEPs accounted for approximate 7% and of CO3cꢀ plus 2-propanol and ethanol were <4.0 ꢁ 104 Mꢀ1 sꢀ1
75% of ATZ consumption, respectively, as shown in Fig. 2b. As and 2.2 ꢁ 104 Mꢀ1 sꢀ1, respectively.44,45 This suggested the low
for ATZ degradation by HOc, the initial yields of DIA and DEPs reactive CO3cꢀ toward the side groups of ATZ was unable to
were 20% and 51%, respectively. Accordingly, the DEPs to DIA impact the yields of DEPs and DIA.
ratio was calculated to be >10 in UV/PDS process, while it was
z2.5 in UV/H2O2 process. This nding was consistent with
Effect of NOM
previous reports,12,27,43 which could be attributed to that SO4cꢀ
As shown in Fig. 4a, the degradation of ATZ by UV/PDS or UV/
displayed a more distinctive preference to the ethyl function
group than HOc.
H2O2 was inhibited in the presence of NOM (1 mg C Lꢀ1), and
the former was still more efficient. Meanwhile, the degradation
of generated DEPs and DIA was also signicantly suppressed
ꢀ
Effect of CO32ꢀ/HCO3
(Fig. 4b and c). Both yields of these two products as well as the
The effect of CO32ꢀ/HCO3 on ATZ degradation as well as the DEPs to DIA ratio were not affected by NOM (Fig. 4b and c
ꢀ
formation of predominant products (i.e., DEPsꢀand DIA) was inserts and Fig. 4d), similar to the case of CO32ꢀ/HCO3ꢀ. The
investigated. It was observed that CO32ꢀ/HCO3 (5 mM) had signicant inhibition of NOM on ATZ and these two products
a signicant inhibitory effect on ATZ removal in both processes degradation could be explained as following: (i) NOM usually
of UV/PDS and UV/H2O2, and the efficiency of ATZ degradation acted as radical scavenger via competing for HOc and SO4cꢀ
in UV/PDS was still much higher than that in UV/H2O2 (Fig. 3a). (kHOc,NOM ¼ 1.4 ꢁ 104 L mg Cꢀ1, kSO c ,NOM ¼ 6.8 ꢁ 103 L mg
ꢀ
4
The negative effect of CO32ꢀ/HCO3ꢀ on these two systems might Cꢀ1);12 (ii) NOM could exert an inner lter effect for the
result from the fact that signicant amounts of HOc and SO4cꢀ photolysis of oxidants.46 Previous studies reported that the
could react with CO32ꢀ/HCO3 to form the inorganic radical reaction of HOc or SO4cꢀ with organic matters (e.g., aromatic,
ꢀ
CO3cꢀ.30 Previous studies have reported that CO3cꢀ presumably aliphatic, ester, and ketone etc.) could yield organic radicals.
attacked on the site of the nitrogen atom of ATZ by electron Howbeit, the redox potentials of varied organic radicals (i.e.,
transfer or hydrogen transfer, but it was much less reactive than ꢀ1.09 V to +0.40 V) have been determined in previous
SO4cꢀ or HOc, with a relatively low second order rate constant of studies,47,48 which are much lower than those of HOc and SO4cꢀ.
4 ꢁ 106 Mꢀ1 sꢀ1 (vs. kSO c ,ATZ ¼ 3 ꢁ 109 Mꢀ1 sꢀ1).34
Thus, the yield of oxidation products could not be impacted by
these organic radicals similar to CO3cꢀ.
ꢀ
4
Further, results exhibited that the impact of CO32ꢀ/HCO3
on the formation of products (i.e., the concentrations of DEPs
ꢀ
In addition, singlet oxygen (1O2), peroxy radical (ROOc), and
and DIA as well as the DEPs to DIA ratio) was relatively slight 3DOM* could be produced by direct photolysis of NOM as above
(Fig. 3b–d). Lutze et al. suggested that the reactivity of the side mentioned. Minero et al. reported that the rate of ATZ photol-
groups of ATZ (i.e., ethylamine and isopropylamine group) was ysis was increased and the formation of dealkylation products
Fig. 3 Effect of CO32ꢀ/HCO3ꢀ on the degradation of ATZ (a), the formation of DIA (b), the formation of DEPs (c), and the DEPs to DIA ratio (d) as
a function of time in UV/H2O2 and UV/PDS processes. The inserts in (b) and (c) showed the effect of CO32ꢀ/HCO3ꢀ on the yield of DEPs and DIA,
respectively. Experiment conditions: [ATZ] ¼ 10 mM, [H2O2] ¼ [PDS] ¼ 1.5 mM, and pH ¼ 7.
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Fig. 4 Effect of NOM on the degradation of ATZ (a), the formation of DIA (b), the formation of DEPs (c), and the DEPs to DIA ratio (d) as a function
of time in UV/H2O2 and UV/PDS processes. The inserts in (b) and (c) showed the effect of NOM on the yield of DEPs and DIA, respectively.
Experiment conditions: [ATZ] ¼ 10 mM, [H2O2] ¼ [PDS] ¼ 1.5 mM, NOM ¼ 1 mg C Lꢀ1, and pH ¼ 7.
was enhanced in the presence of humic material, suggesting formation of DEPs and DIA. As shown in Fig. 6, Clꢀ had
that NOM promoted the formation of various reactive inter- a negligible effect on the transformation of ATZ in UV/H2O2
mediates in direct UV process.49 However, the result in this work process, including the kinetics and products formation.
indicated that the degradation of ATZ by direct photolysis was However, the presence of Clꢀ inhibited ATZ degradation and
slightly inhibited in the presence of NOM, and no DEPs or DIA signicantly inuenced the formation trend of DEPs and DIA in
was detected (Fig. 5), consistent with Torrents' study.41 This UV/PDS process.
discrepancy could be explained by the fact that different func-
It was observed that the initial yield of DIA increased from
tional groups present in NOM could inuence the photo- approximate 6.8–18.9% when 5 mM Clꢀ was present, while the
chemical reactions.37
initial yield of DEPs decreased from approximate 75–46.4%
(Fig. 6b and c insets). Correspondingly, the initial ratio of DEPs
to DIA decreased from 13.6 to 3.3. In UV/H2O2 process, Clꢀ
scavenging for HOc was apparently weak considering the fast
backward reaction process (i.e., 6.1 ꢁ 109 sꢀ1) in the equilib-
rium reaction of HOc with Clꢀ to form ClOHꢀc,50,51 which
explained no impact of Clꢀ on ATZ degradation. In contrast, the
signicant inhibitory effect of Clꢀ on ATZ oxidation by UV/PDS
could be attributed to the much higher second-order rate
constant of Clꢀ reacting with SO4cꢀ (i.e., 3.0 ꢁ 108 Mꢀ1 sꢀ1) than
that of HOc (i.e., 1.8 ꢁ 105 Mꢀ1 sꢀ1) at neutral pH.52
Effect of Clꢀ
Experiments were carried out in the presence of Clꢀ (5 mM) to
investigate its effect on ATZ degradation as well as the
It was well-known that Clꢀ could be oxidized by SO4ꢀc to
form reactive RCs such as Clc, ClOc and Cl2cꢀ.53 Recent studies
reported that RCs were responsible for the degradation of
certain
organic
pollutants
(e.g.,
N,N-diethyl-3-
methylbenzamide, caffeine, trimethoprim, and acid orange
7).50,54 In order to clarify the pattern of DEPs and DIA formation
during the reaction of RCs with ATZ, UV/HClO, a typical process
of generating RCs, was employed for oxidation of ATZ. NB was
used herein as an effective quencher for HOc (kNB, HOc ¼ 6.0 ꢁ
108 Mꢀ1 sꢀ1), but not for RCs (<106 Mꢀ1 sꢀ1) (e.g., Clc, ClOc and
Cl2cꢀ).53,55–57 As shown in Fig. 7, ATZ was effectively oxidized in
UV/HClO process, and the initial yields of DIA and DEPs were
Fig. 5 Effect of NOM on the degradation of ATZ in UV direct process.
Experiment conditions: [ATZ] ¼ 10 mM, pH ¼ 7.
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Fig. 6 Effect of Clꢀ on the degradation of ATZ (a), the formation of DIA (b), the formation of DEPs (c), and the DEPs to DIA ratio (d) as a function of
time in UV/H2O2 and UV/PDS processes. The inserts in (b) and (c) showed the effect of Clꢀ on the yield of DEPs and DIA, respectively. Experiment
conditions: [ATZ] ¼ 10 mM, [H2O2] ¼ [PDS] ¼ 1.5 mM, Clꢀ ¼ 5 mM, and pH ¼ 7.
Fig. 7 (a) Degradation of ATZ and formation of DEPs and DIA as a function of time in UV/HClO process. (b) Yields of DEPs and DIA as a function of
time in UV/HClO process. HClO ¼ 1.5 mM, ATZ ¼ 10 mM, NB ¼ 100 mM, and pH ¼ 7.
approximately 12% and 45%, respectively. Hence, the DEPs to acid (pNBA) vs. 4-chlorobenzonic acid (pCBA) could be used as
DIA ratio could be calculated to be z4 (Fig. 7b). The co- an indicator for the transformation of SO4cꢀ to HOc in the UV/
existence of NB signicantly inhibited the degradation of ATZ, PDS with Clꢀ.42 It was found that the slope (ln C/C0 (pNBA) vs.
as expected. It was noteworthy that the yield of DEPs was about ln C/C0 (pCBA)) for UV/PDS in presence of Clꢀ was equal to the
45% but no DIA was detected in the presence of NB, suggesting UV/H2O2 without Clꢀ present, indicating HOc were the major
that RCs had a great selectivity on attacking the ethylamine reactive species in both processes. In this study, the initial ratio
group of ATZ to form DEPs. Accordingly, it seemed likely that of DEPs to DIA in the presence of Clꢀ in UV/PDS process (i.e., 3)
the promoted yield of DIA in the UV/PDS process in the pres- was very close to that in UV/H2O2 process (i.e., 2.5), which
ence of Clꢀ could not be attributed to the generation of RCs.
provided further evidence for the facilitated effect of Clꢀ on the
Actually, in the literature one could nd an important transformation of SO4cꢀ to HOc.
conclusion on the inuence of Clꢀ on SO4cꢀ based oxidation,
Clꢀ could turn SO4cꢀ into HOc at pH $ 7. Zhang et al. investi-
Degradation of atrazine under natural water background
gated the effect of Clꢀ on pharmaceuticals degradation in UV/
PDS process, and obtained that the steady-state concentration
of HOc increased from 1.75 ꢁ 10ꢀ13 M to 2.78 ꢁ 10ꢀ11 M due to
the addition of 0.1 M Clꢀ by kinetic modeling.58 Lutze et al.
suggested that the ratio of degradation rates of 4-nitrobenzoic
The degradation of ATZ as well as the formation of DEPs and
DIA in three natural waters were investigated. As could be seen
in Fig. 8a, UV/PDS system was more effective than UV/H2O2
system under the identical conditions. This was ascribed to that
the quantum efficiency for photolysis and the molar extinction
29260 | RSC Adv., 2017, 7, 29255–29262
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Fig. 8 The degradation of ATZ (a), the formation of DIA (b), the formation of DEPs (c), and the DEPs to DIA ratio (d) as a function of time in UV/
H2O2 and UV/PDS processes in actual waters. Experiment conditions: [ATZ] ¼ 10 mM, [H2O2] ¼ [PDS] ¼ 1.5 mM, and pH ¼ 7.
2ꢀ
coefficient for S2O8
were higher than H2O2, as described
(iv) Clꢀ suppressed the degradation of ATZ in UV/PDS, but
above. The yields of DIA and DEPs in these three natural waters had a slight effect on that in UV/H2O2. The decrease of DEPs
were fairly close in UV/H2O2 system. Distinguishingly, the yield concentration and the increase of DIA concentration in UV/PDS
of DIA in UV/PDS system in the effluent from GW was much process were observed when a certain concentration of Clꢀ was
higher than those in effluents from SW and EW, and meanwhile present, attributed to that Clꢀ enhanced the transformation of
the yield of DEPs showed the reverse change trend. As shown in SO4cꢀ into HOc. In addition, RCs (i.e., Clc, ClOc and Cl2cꢀ) were
Fig. 8b and c, the initial yields of DIA and DEPs were about 15% found to be more selective on attacking the ethylamine group of
and 50% in GW during UV/PDS process, respectively, while the ATZ.
initial yields of DIA and DEPs were approximately 5% and 65%
in SW or EW, respectively. This could be attributed to the higher
Acknowledgements
concentration of Clꢀ in GW than that in SW and EW (Table 1),
and this further conrmed that Clꢀ played an important role in
the transformation of reactive oxidative species as well as the
yields of ATZ oxidation products.
This work was nancially supported by the National Natural
Science Foundation of China (51578203 and 51408107), the
Funds of the State Key Laboratory of Urban Water Resource and
Environment (HIT, 2016DX13), the Foundation for the Author
of National Excellent Doctoral Dissertation of China (201346),
the National Key Research and Development Program
(2016YFC0401107), and the Heilongjiang Province Postdoctoral
Science Foundation (LBH-Q15057).
Conclusion
The formation of DEPs and DIA was investigated in UV/H2O2
and UV/PDS processes under various conditions. For all studied
processes the following conclusions were obtained:
(i) UV/PDS system was more effective than UV/H2O2 system
for the degradation of ATZ and its oxidation products under the
same conditions.
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