A simple Fe3+/bisulfite system for rapid degradation of sulfamethoxazole

Article abstract of DOI:10.1039/d0ra05962e

Sulfate radical (SO4-) based oxidation technologies have been widely used in the remediation of antibiotic-containing wastewater. Activated persulfates are efficient reagents for achieving SO4-, but the storage and transportation of concentrated persulfates present associated safety issues. In this study, bisulfite (BS) was used as an alternative precursor for replacing persulfates, and a simple advanced oxidation system (Fe3+/BS) for generating SO4- and hydroxyl radical (HO) was formulated and evaluated for removing sulfamethoxazole (SMX) from contaminated water. The initial pH, dosages of Fe3+ and BS, as well as the water matrix were investigated to improve the SMX degradation. The results indicated that 1 μmol L-1 SMX was completely removed within 5 min at optimum initial pH of 4.0, Fe3+ dosage of 10 μmol L-1, BS dosage of 100 μmol L-1 and temperature of 25 °C. The presence of HCO3- and natural organic matter (NOM) showed obviously negative effects on SMX degradation, while Cu2+ could slightly promote the degradation of SMX if its concentration was in an appropriate range (～1 μmol L-1). Scavenger quenching experiments confirmed the presence of SO4- and HO, which resulted in efficient SMX degradation in the Fe3+/BS system. During the radical chain reactions, Fe2+ and Fe3+ could be converted into each other to form self-circulation in this system. The degradation pathway of SMX by Fe3+/BS was proposed including hydroxylation and bond cleavage.

Full text of DOI:10.1039/d0ra05962e

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A simple Fe3+/bisulfite system for rapid degradation
of sulfamethoxazole
Cite this: RSC Adv., 2020, 10, 30162
*
Shixiang Wang, Guangsheng Wang, Yongsheng Fu, Hongbin Wang and Yiqing Liu
Sulfate radical (SO4cꢀ) based oxidation technologies have been widely used in the remediation of
antibiotic-containing wastewater. Activated persulfates are efficient reagents for achieving SO4cꢀ,
but the storage and transportation of concentrated persulfates present associated safety issues. In
this study, bisulfite (BS) was used as an alternative precursor for replacing persulfates, and a simple
advanced oxidation system (Fe3+/BS) for generating SO4cꢀ and hydroxyl radical (HOc) was
formulated and evaluated for removing sulfamethoxazole (SMX) from contaminated water. The
initial pH, dosages of Fe3+ and BS, as well as the water matrix were investigated to improve the SMX
degradation. The results indicated that 1 mmol Lꢀ1 SMX was completely removed within 5 min at
optimum initial pH of 4.0, Fe3+ dosage of 10 mmol Lꢀ1, BS dosage of 100 mmol Lꢀ1 and temperature
ꢀ
of 25 ꢁC. The presence of HCO3 and natural organic matter (NOM) showed obviously negative
effects on SMX degradation, while Cu2+ could slightly promote the degradation of SMX if its
concentration was in an appropriate range (ꢂ1 mmol Lꢀ1). Scavenger quenching experiments
Received 8th July 2020
Accepted 10th August 2020
confirmed the presence of SO4cꢀ and HOc, which resulted in efficient SMX degradation in the Fe3+
/
BS system. During the radical chain reactions, Fe2+ and Fe3+ could be converted into each other to
form self-circulation in this system. The degradation pathway of SMX by Fe3+/BS was proposed
including hydroxylation and bond cleavage.
DOI: 10.1039/d0ra05962e
rsc.li/rsc-advances
may be extensively used because it is more environmentally
1 Introduction
friendly than Co2+ or Cr6+ and more chemically stable than Fe2+
.
Fe3+ activated BS may have a lot of similarity to Fe2+/persulfate
system. Once SO4cꢀ is generated, it can propagate a series of
reactions involving the formation of other reactive species,
particularly the hydroxyl radical (HOc). The possible chain
reactions in the presence of oxygen during above process are
shown in eqn (1)–(15) and (Table 1).7–14 However, few works
focused on the application of such system in the oxidation of
organic or inorganic contaminants.
Advanced oxidation processes (AOPs), a method mainly based
on the reactive species generated from the decomposition of
precursors such as hydrogen peroxide, peracetic acid and per-
2ꢀ
sulfate (S2O8 ),1 have been widely used in wastewater remedi-
ation due to their applicability to a wide range of contaminants,
high efficiency and cost effectiveness.2 Among all commonly
used AOPs, sulfate radical (SO4cꢀ) based oxidation technologies
have been paid more attention due to its high redox potential
(E0 ¼ 2.6–3.1 V) and strong selectivity.3 Even though SO4cꢀ can
be effectively generated through the heat, UV and alkaline pH
activation of persulfate, Fe(II) or Fe(III) based activation is
a viable method for practical application due to its high radical
yield and additional coagulation function for contaminant
coprecipitation.4
Sulfamethoxazole (SMX), a synthetic antimicrobial that is
extensively used in both human and veterinary medicine, has
been frequently detected in many water bodies and effluents
from wastewater treatment plants due to its incomplete
removal during conventional wastewater treatment,15 which
leads to an increase of antibiotic resistance in humans. Fe3+
/
BS based advanced oxidation process may be a suitable
method for the removal of SMX from wastewater due to the
generation of oxysulfur radicals. However, little is known
about the mechanism of SMX oxidation by Fe3+/BS and the
primary reactive species involved, although this system with
or without light irradiation was widely used in the treatment
of wastewater containing dyes or heavy metal ions.13,16,17
Furthermore, few works focused on the Fe(II)/Fe(III) self-
circulation during the degradation process of contaminants
in Fe3+/BS system.
Nowadays, some novel systems using bisulte (BS) as an
alternative precursor for replacing persulfate to generate SO4cꢀ
have been developed to overcome the safety risks from trans-
portation and storage processes of persulfate due to its chem-
ical instability.5 Furthermore, several metal ions, including
Fe2+, Fe3+, Co2+ and Cr6+, haveꢀbeen proved to be effective for
catalyzing BS to generate SO4c .6 Among these catalysts, Fe3+
Faculty of Geosciences and Environmental Engineering, Southwest Jiaotong University,
Chengdu 611756, China. E-mail: liuyq@swjtu.edu.cn
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Table 1 Possible reactions involved in the Fe3+/BS system
Reactions
k (Mꢀ1 sꢀ1
)
(1) Fe(III) + nHSO3 / FeIII(HSO3
)
ꢀ
ꢀ
n
(n ¼ 1, 2, 3)
(2) FeIII(HSO3
)
n / Fe(II)
ꢀ
+ (n ꢀ 1)HSO3ꢀ + H+ + SO3cꢀ
(3) Fe(III) + SO3cꢀ + H2O / Fe(II)
+ HSO4ꢀ + H+
(4) SO3cꢀ + O2 / SO5cꢀ
k ¼ (1.1–2.5) ꢃ 109
k ¼ 1.2 ꢃ 104
(5) SO5cꢀ + HSO3ꢀ/SO4cꢀ + SO42ꢀ + H+
(6) SO4cꢀ + HSO3ꢀ / SO3cꢀ + SO4 + H+
k ¼ 1.3 ꢃ 108–2.5 ꢃ 109
k ¼ 104–108
2ꢀ
(7) 2SO5cꢀ / 2SO4cꢀ + O2
(8) 2SO5cꢀ / S2O82ꢀ + O2
k ¼ 107–108
(9) Fe(II) + SO4cꢀ / Fe(III) + SO4
2ꢀ
k ¼ 4.6 ꢃ 109
(10) Fe(II) + SO5cꢀ + H+ / Fe(III) + HSO5
k ¼ 2.0 ꢃ 108
ꢀ
(11) Fe(II) + HSO5ꢀ / Fe(III) + SO4cꢀ + OHꢀ
k ¼ 3.0 ꢃ 104
Fig. 1 Effect of radical scavenger on SMX removal. Experimental
conditions: [BS]0 ¼ 100 mmol Lꢀ1, [Fe3+
]
0
¼ 10 mmol Lꢀ1, [SMX]0 ¼ 1
(12) SO4cꢀ + OHꢀ / SO42ꢀ + HOc
k ¼ (1.4–6.5) ꢃ 107
k ¼ 1.1 ꢃ 101
mmol Lꢀ1, [IPA]0 ¼ [TBA]0 ¼ 1000 mmol Lꢀ1, pH0 ¼ 4.0, T ¼ 25 ꢁC.
(13) SO4cꢀ + H2O / SO4 + HOc + H+
2ꢀ
(14) HOc + HSO3ꢀ / SO3cꢀ + H2O
(15) Fe(II) + HOc / Fe(III) + OHꢀ
k ¼ 4.5 ꢃ 109
k ¼ 4.3 ꢃ 108
Na2S2O3. In the radical scavenging experiment, TBA and IPA
were added into the pure SMX solution before BS addition.
Thus, the aim of this work was to propose the in situ simulta-
neous activation of BS by Fe3+ and degradation of SMX by radicals
generated from Fe3+/BS. The effects of operational factors, such as
the initial pH, Fe3+ and BS dosages, as well as the common water
quality constituents including natural organic matter (NOM),
inorganic anions and cations on the degradation of SMX were
discussed. Besides, the dominant reactive species and Fe(II)/Fe(III)
cycle in this system were investigated. Finally, the possible degra-
dation pathways of SMX were tentatively proposed based on the
detected reaction products.
2.3 Analysis
The SMX concentration was measured using Thermo' HPLC
apparatus (Ultimate 3000) at 270 nm. The mobile phase was
a mixture of methanol and 0.1% acetic acid (v/v ¼ 60/40) at
a ow rate of 0.8 mL minꢀ1. The column temperature was set at
30 ꢁC and the injection volume was 50 mL. The products during
SMX degradation were analyzed by UPLC-Q-TOF-MS (ACQUITY
UPLC, Quattro Premier XE, Waters, America) with a C18
reversed-phase column (100 ꢃ 2.1 mm, 1.7 mm). The mobile
phase contained 0.1% formic acid water solution (A) and
acetonitrile (B) at a ow rate of 0.5 mL minꢀ1. The gradient was
90% A for 0.2 min, decreasing to 5% for 4.8 min, maintaining
for 0.5 min, and then increasing back to 90% for 2 min. The
solution pH was monitored by a pH meter (PHS-3C+, Leici).
Phenanthroline colorimetric method was used for Fe2+/Fe3+
concentration measurement.
2 Materials and methods
2.1 Materials
SMX, tert-butyl alcohol (TBA) and fulvic acid were produced by
Aladdin Reagent Company, China. Ferric sulfate and sodium
bisulte were obtained from Sinopharm Chemical Reagent
Corporation Ltd., China. Methanol and isopropanol (IPA) were
the products of Fisher corporation. H2SO4, NaOH, NaCl, NaNO3,
NaHCO3, Na2SO4, Na2S2O3, CuSO4, Mg(NO3)2, Ca(NO3)2 were all
of analytical grade and purchased from Chengdu Kelong
Chemical Reagent Co., Ltd, China. Deionized water was used in
all experiments.
2.2 Experimental procedure
The SMX degradation experiments were carried out in a 250 mL
glass beaker containing 100 mL SMX solution (1 mmol Lꢀ1) at
298 K. In each experiment, BS was added and the mixture was
rotated for 10 min at 400 rpm. During this process, NaOH (0.5
M) and H2SO4 (0.5 M) were used to adjust the initial pH of
solution. When the Fe3+ with specic amount was put into the
SMX solution, the reaction between Fe3+ and BS resulted in the
degradation of SMX. Samples (1.0 mL) were extracted at pre-
determined time intervals and ltered by 0.2 mm lter
Fig. 2 Variation in concentrations of Fe2+ and Fe3+ during SMX
removal. Experimental conditions: [BS]0 ¼ 100 mmol Lꢀ1, [Fe3+
]
¼ 10
0
membrane, which were then rapidly quenched with 0.1 mol Lꢀ1 mmol Lꢀ1, [SMX]0 ¼ 1 mmol Lꢀ1, pH0 ¼ 4.0, T ¼ 25 ꢁC.
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Table 2 Degradation products of SMX in Fe3+/BS system
proper conditions, and its degradation could be described by
pseudo rst-order kinetic model with the rate constant of
0.85 minꢀ1. This result demonstrated that Fe3+/BS was an effi-
cient system for SMX degradation and the radicals generated
from Fe3+/BS might play an important role during this process.
It is widely accepted that HOc and SO4cꢀ can be generated via
the reaction of Fe(II) with peroxymonosulfate or persulfate and
the transformation of reactive oxygen species,18 which might
also occur in a Fe(III)/S(IV) system. Thus, the radical scavenging
experiments were carried out to explore the dominant active
Mass to
Serial
charge
number
ratio (m/z)
Formular
Proposed structure
SMX
254
270
C10H11N3O3S
radicals which were responsible for the removal of SMX by Fe3+
/
1
C10H11N3O4S
BS. TBA was selected as a strong HOc scavenger with the second-
order rate constant of 3.8–7.6 ꢃ 108 Mꢀ1 sꢀ1
,
while IPA was
17
used as a nonselective scavenger for both HOc and SO4cꢀ, with
the second-order rate constants of 1.9 ꢃ 109 Mꢀ1 sꢀ1 and 8.2 ꢃ
107 Mꢀ1 sꢀ1, respectively.19 As shown in Fig. 1, the SMX degra-
dation rate constant in the system without TBA/IPA addition
was much higher than those in Fe3+/BS/TBA (0.39 minꢀ1) and
Fe3+/BS/IPA systems (0.051 minꢀ1). This result proved the
participation of both HOc and SO4cꢀ in the SMX degradation
process. Meanwhile, the SMX degradation rate in Fe3+/BS/TBA
system was 7.6-fold faster than that in Fe3+/BS/IPA system,
demonstrating that SO4cꢀ was the dominant radical in Fe3+/BS
system under current conditions.
2
3
129
113
C4H4N2O3
C4H4N2O2
4
99
C4H6N2O
3.1.2 Variation in concentrations of Fe2+ and Fe3+ during
SMX degradation. To clarify if Fe2+ and Fe3+ could be converted
into each other in this system (as shown in eqn (1)–(3) and (9)–
(11)), the concentration variation of Fe2+ and Fe3+ during SMX
degradation was investigated. As shown in Fig. 2, the concen-
tration of total Fe was maintained at about 10 mmol Lꢀ1 during
the whole reaction. A decrease of Fe3+ concentration from 10 to
3 Results and discussion
3.1 Degradation mechanism of SMX by Fe3+/BS
3.1.1 Identication of reactive species in Fe3+/BS system.
Fig. 1 displayed the complete removal of SMX by Fe3+/BS under
Fig. 3 Possible removal pathways of SMX by Fe3+/BS: (1) hydroxylation and (2) bond cleavage.
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0.06 mmol Lꢀ1 was obviously observed in the initial 3 min, while
Fe2+ concentration increased from 0 to 9.7 mmol Lꢀ1 corre-
spondingly. However, 65.1% of Fe2+ was inversely transformed
to Fe3+ aer 20 min reaction, indicating that the oxidation of
Fe2+ was occurred based on the eqn (9)–(11) and (15). This result
proved the occurrence of Fe(II)/Fe(III) self-circulation in the Fe3+
/
BS system. For this reason, the radicals could be continuously
generated during chain reactions between precursors and
regenerated Fe(II)/Fe(III) species.
3.1.3 Degradation pathways of SMX by Fe3+/BS. The main
degradation products of SMX in Fe3+/BS system were detected
and identied by UPLC-Q-TOF-MS. Except SMX itself, there
were four substances identied in the reaction, and their
formula and structures are shown in Table 2. Based on these
products, the possible reaction pathway of SMX by Fe3+/BS was
proposed in Fig. 3, including hydroxylation and bond cleavage.
(1) Hydroxylation (pathway (1)) was considered as one of the
classical degradation pathways of SMX, which resulted in the
electrophilic replacement on the aromatic chain of SMX and the
generation of m/z 270 due to the electron-rich nature of aniline
group in SMX structure.20
(2) Bond cleavage (pathway (2)) was resulted from the radical
attack at S–N bond in the acidic medium,21 which led to the
formation of 3-amino-5-methylisoxazole (m/z 99). The amino group
of m/z 99 could be further oxidized, forming m/z 113 and 129. It
was worth noting that the 4-amino benzene sulfonic acid (m/z 174)
might also be generated in this pathway. We could not exclude its
formation, although this intermediate was not detected.
Fig. 5 Effect of BS dosage on SMX removal. Experimental conditions:
[SMX]0 ¼ 1 mmol Lꢀ1, [Fe3+
]
¼ 10 mmol Lꢀ1, pH0 ¼ 4.0, T ¼ 25 ꢁC.
0
efficiently removed in the rst 3 min. When the pH decreased to
3.0 or increased to 8.5, the SMX removal aer 20 min reaction
decreased to 37.0% and 2.9%, respectively. Speciation under the
aforementioned conditions might primarily account for the
increase in SMX degradation at pH 4.0. As reported by Wang
et al.,6 the predominant S(IV) species at pH 4.0 is BS, and thus
facilitating the formation of FeIII(HSO3ꢀ)n and the consequent
reactions shown in eqn (1)–(15). Increasing pH might accelerate
2ꢀ
2ꢀ
both the ionization of BS to form SO3 or S2O6 and the
hydrolyzation of Fe(III), which inhibited the speciation of the
system. pH 3.0 (or a lower pH value) had a negative effect on
both radical generation and SMX removal, because part of S(IV)
was volatilized at pH < 3.0. Consequently, 4.0 was deemed as the
optimal pH and was used in subsequent experiments.
3.2 Factors inuencing SMX degradation by Fe3+/BS
3.2.1 Effect of initial pH. Free iron ions can activate BS to
generate a series of oxysulfur radicals (e.g., SO3cꢀ, SO4cꢀ and
SO5cꢀ), and the pH plays an important role in this process. To
3.2.2 Effect of BS dosage. Since BS was the only precursor
of active radicals in a homogeneous Fe3+/BS system under acidic
conditions, the dose of BS had a signicant effect on the
formation of reactive species and SMX degradation. As shown in
Fig. 5, the SMX removal efficiency signicantly increased with
the increasing BS dosage from 10 to 100 mmol Lꢀ1, owing to the
determine the optimum initial pH for SMX degradation in Fe3+
/
BS system, experiments at initial pH values between 3.0 and 8.5
were conducted. As shown in Fig. 4, the fastest and complete
SMX removal was obtained at pH 4.0 and most of SMX was
Fig. 4 Effect of initial pH on SMX removal. Experimental conditions:
[Fe3+
]
¼ 10 mmol Lꢀ1, [BS]0 ¼ 100 mmol Lꢀ1, [SMX]0 ¼ 1 mmol Lꢀ1, T ¼ Fig. 6 Effect of Fe3+ dosage on SMX removal. Experimental condi-
0
25 ꢁC.
tions: [BS]0 ¼ 100 mmol Lꢀ1, [SMX]0 ¼ 1 mmol Lꢀ1, pH0 ¼ 4.0, T ¼ 25 ꢁC.
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according to eqn (5), (6) and (14). Hence, the balance between
the reducing nature of BS and its activity to form radicals is
critical. Considering the factors above and economic efficiency,
100 mmol Lꢀ1 was used as the optimum BS concentration for
this system.
3.2.3 Effect of Fe3+ dosage. Besides the solution pH and BS
concentration, speciation of Fe(III) under the aforementioned
conditions might primarily account for the increase in chain
reaction rate, which was crucially inuenced by initial Fe3+
dosage in the system. As shown in Fig. 6, a conspicuous
enhancement in the SMX removal was observed when the Fe3+
dosage increased from 1 to 10 mmol Lꢀ1. Nevertheless, further
increasing Fe3+ concentration only resulted in the acceleration
of SMX degradation in the rst 1 min, which indicated that 10
mmol Lꢀ1 Fe3+ was enough for activating 100 mmol Lꢀ1 BS.
Therefore, 10 mmol Lꢀ1 of Fe3+ was chosen as the optimum
condition in this study.
Fig. 7 Effect of NOM on SMX removal. Experimental conditions: [BS]0
¼ 100 mmol Lꢀ1, [Fe3+]0 ¼ 10 mmol Lꢀ1, [SMX]0 ¼ 1 mmol Lꢀ1, pH0 ¼ 4.0,
T ¼ 25 ꢁC.
3.2.4 Effect of NOM. Natural organic matter can be both
a source and a sink of radicals, the inuence mechanism of
which depends on the concentration and characteristics of
dissolved organic compound.22 Carboxylate ligands are known
increasing generation rate and accumulation of FeIII(HSO3
)
n
ꢀ
and the subsequent radicals with the increasing BS concentra-
tion. However, excessively high BS concentration (such as >500
mmol Lꢀ1) could retard SMX degradation in the system because
of the competitive reactions of excess BS with the active radicals
to accelerate the advanced oxidation process by forming
ꢄ
complex with Fe(III) which reacts faster with H2O2=HOꢄ =O2
ꢀ
2
than hydrate in the Fenton process, although they are always
Fig. 8 Effect of SO42ꢀ (a), NO3ꢀ (b), HCO3ꢀ (c) and Clꢀ (d) on SMX removal. Experimental conditions: [BS]0 ¼ 100 mmol Lꢀ1, [Fe3+]0 ¼ 10 mmol Lꢀ1
,
[SMX]0 ¼ 1 mmol Lꢀ1, pH0 ¼ 4.0, T ¼ 25 ꢁC.
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assumed as the scavengers for radicals, as shown in eqn (16) on SMX degradation. Aer 20 min reactiꢀon, the removal of SMX
and (17).23 In order to investigate the effect of NOM on the in the presence of 0.5 mmol Lꢀ1 HCO3 fell to 7.5%. Possible
Fe(III)/S(IV) system, fulvic acid (FA), a naturally existing macro- explanations for this result include: (1) HCO3ꢀ could react with
molecular carboxyl-containing substance, was added in the HOc and SO4cꢀ with the second-order rate constants of 3.9 ꢃ 108
system. As shown in Fig. 7, the SMX removal in 20 min Mꢀ1 sꢀ1 and 9.1 ꢃ 106 Mꢀ1 sꢀ1, respectively (as shown in eqn
decreased from 100% to 4.5% with the FA concentration (18) and (19)), which was inefficient for SMX oxidation;23 and (2)
increasing from 0 to 10 mg Lꢀ1, which indicated that the the addition of HCO3ꢀ resulted in the increase of solution pH to
degradation of SMX was inhibited signicantly in the presence alkaline range, which was unfavourable for the radical genera-
of NOM and the radical scavenging effect of NOM surpassed its tion due to the formation of Fe(III) precipitate. For Clꢀ, as re-
promoting effect on radical generation.
ported by Lutze et al.,24 SO4cꢀ can react with Clꢀ and the
oxidation product Clc is favoured at weakly acidic pHs. No
obvious difference on SMX removal was observed between the
Fe3+/BS system and Fe3+/BS/Clꢀ system, because the oxidation
of SMX by Clc and its derivate HOc (eqn (20)) might occur
synchronously.
HOc + NOM / products k ¼ 2.23 ꢃ 108 L (mol C)ꢀ1 sꢀ1 (16)
SO4cꢀ + NOM / products k > 6 ꢃ 106 L (mol C)ꢀ1 sꢀ1 (17)
3.2.5 Effect of inorganic anions. SMX-contaminated waters
usually contain not only SMX but also substantial concꢀentra-
tions of other inorganic ions (e.g., SO42ꢀ, NO3ꢀ, HCO3 and
Clꢀ), which may affect the overall efficiency of Fe3+/BS system.
In this study, these four typical background anions were
selected for examination. As presented in Fig. 8, the presence of
SO4 and NO3 both had little effect on SMX degradation,
because they could ꢀhardly participate in the radical chain
reactions. The HCO3 ion exerted an obvious inhibition effect
ꢀ
HOc + HCO3 / CO3cꢀ + H2O
(18)
(19)
(20)
ꢀ
2ꢀ
SO4cꢀ + HCO3 / CO3cꢀ + SO4 + H+
Clc + H2O / HOc + Clꢀ + H+
2ꢀ
ꢀ
3.2.6 Effect of metal cations. The cations in the solution
might have different impacts on SMX removal by binding with it
Fig. 9 Effect of Cu2+ (a), Ca2+ (b), and Mg2+ (c) on SMX removal. Experimental conditions: [BS]0 ¼ 100 mmol Lꢀ1, [Fe3+]0 ¼ 10 mmol Lꢀ1, [SMX]0 ¼ 1
mmol Lꢀ1, pH0 ¼ 4.0, T ¼ 25 ꢁC.
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to change its chemical properties or reacting with precursor of Fundamental Research Funds for the Central Universities
radicals directly.25 In this study, the inuence of three common (2682018CX32).
cations (i.e., Cu2+, Ca2+ and Mg2+) on SMX degradation was
evaluated. As shown in Fig. 9, the degradation of SMX was
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Cu2+ concentration was 1 mmol Lꢀ1. A reasonable explanation
for this result was that SO4cꢀ could be formed extra from
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(21)
(22)
2ꢀ
2ꢀ
2ꢀ
Cu2+ + SO4cꢀ / SO4 + Cu3+
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4 Conclusion
Fe3+/BS, a simple homogeneous advanced oxidation system that
efficiently generates multiple radicals was developed. The
inuence of operational parameters on SMX oxidation by Fe3+
/
BS, the dominating reactive species contributing to SMX
degradation and the reaction pathways were investigated.
Under the conditions of pH of 4.0, Fe3+ dosage of 10 mmol Lꢀ1
,
BS dosage of 100 mmol Lꢀ1, initial SMX concentration of 1 mmol
Lꢀ1 and temperature of 25 C, the removal of SMX was about
ꢁ
100% in 5 min reaction. The presence of SO42ꢀ, NO3ꢀ, Clꢀ, Ca2+
and Mg2+ showed little inuence on SMX removal, while obvi-
ꢀ
ously negative effects of HCO3 and NOM were observed. Cu2+
could slightly promote the destruction of SMX if its concentra-
tion was in an appropriate range (ꢂ1 mmol Lꢀ1). It has to be
mentioned that both HOc and SO4cꢀ were the contributing
reactive species for SMX degradation. Fe2+ and Fe3+ could be
converted into each other to form a self-circulation in this
system. According to the identied products, two degradation
pathways of SMX including hydroxylation and bond cleavage
were proposed.
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Conflicts of interest
There are no conicts to declare.
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Acknowledgements
This research was nancially supported by Sichuan Science and
Technology Programs (2017SZ0175 and 2018SZDZX0026). Yiq- 26 X. Xu, Q. Ye, T. Tang and D. Wang, J. Hazard. Mater., 2008,
ing Liu also gratefully appreciates nancial support from the
158, 410–416.
30168 | RSC Adv., 2020, 10, 30162–30168
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