Roles of hydroxyl and carbonate radicals in bisphenol a degradation: via a nanoscale zero-valent iron/percarbonate system: influencing factors and mechanisms

Article abstract of DOI:10.1039/d0ra08395j

In this work, nanoscale-zero-valent iron (nZVI) was applied to activate sodium percarbonate (SPC) to eliminate bisphenol A (BPA), which poses a risk to ecological and human health as a typical endocrine disruptor. The influence of nZVI loading, SPC dosing, initial pH, and the presence of inorganic anions (including Cl-, HPO42-, NO3- and NO2-) and humic acid on BPA removal by the nZVI/SPC system were investigated. Based on the scavenger test results, OH and CO3- participated in the degradation of BPA, and OH was illustrated to be the dominant radical. The X-ray diffraction (XRD) and X-ray photoelectron spectroscopy (XPS) analysis suggested that surface iron oxide generation, electron transfer and Fe2+ release were the main processes of the SPC activation by nZVI. Moreover, BPA transformation products were detected by LC-MS allowing the proposal of a possible degradation pathway of BPA. Along with the degradation of the parent compound BPA, the total organic carbon (TOC) gradually decreased, while the bio-toxicity increased at the initial stage of the reaction (0-3 min) and then decreased to a lower level rapidly at 20 min. Overall, this study evidenced the feasibility of the nZVI/SPC system to efficiently degrade BPA, broadening the applications of nZVI in wastewater treatment.

Full text of DOI:10.1039/d0ra08395j

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PAPER
Roles of hydroxyl and carbonate radicals in
bisphenol a degradation via a nanoscale zero-
valent iron/percarbonate system: influencing
factors and mechanisms†
Cite this: RSC Adv., 2021, 11, 3636
Yulun Xiao, Xiang Liu, Ying Huang, Wei Kang, Zhen Wang and Han Zheng *b
ab
c
d
e
bc
In this work, nanoscale-zero-valent iron (nZVI) was applied to activate sodium percarbonate (SPC) to
eliminate bisphenol A (BPA), which poses a risk to ecological and human health as a typical endocrine
disruptor. The influence of nZVI loading, SPC dosing, initial pH, and the presence of inorganic anions
ꢀ
42ꢀ
3ꢀ
2ꢀ
(
including Cl , HPO , NO and NO ) and humic acid on BPA removal by the nZVI/SPC system were
ꢀ
investigated. Based on the scavenger test results, cOH and CO
3
c
participated in the degradation of BPA,
and cOH was illustrated to be the dominant radical. The X-ray diffraction (XRD) and X-ray photoelectron
2+
spectroscopy (XPS) analysis suggested that surface iron oxide generation, electron transfer and Fe
release were the main processes of the SPC activation by nZVI. Moreover, BPA transformation products
were detected by LC-MS allowing the proposal of a possible degradation pathway of BPA. Along with
the degradation of the parent compound BPA, the total organic carbon (TOC) gradually decreased, while
the bio-toxicity increased at the initial stage of the reaction (0–3 min) and then decreased to a lower
level rapidly at 20 min. Overall, this study evidenced the feasibility of the nZVI/SPC system to efficiently
degrade BPA, broadening the applications of nZVI in wastewater treatment.
Received 1st October 2020
Accepted 19th December 2020
DOI: 10.1039/d0ra08395j
rsc.li/rsc-advances
strategies including coagulation, ltration, adsorption and
biodegradation. Therefore, it is urgent to develop an efficient
1
. Introduction
7,8
In recent decades, bisphenol A (BPA), as a precursor in the method to abate BPA from water and to simultaneously
1
manufacture of polycarbonate, epoxy resin and polystyrene,
decrease the bio-toxicity of BPA.
has resulted in the ubiquitous presence of BPA in mineral water
Currently, hydroxyl radical (cOH)-based advanced oxidation
bottles, medical equipment and food packaging. Due to the processes (AOPs) have been considered as a powerful tech-
9
–11
improper disposal and clearance of this BPA-containing waste, nology for BPA remediation, which is based on the activation
2
,3
BPA is directly or indirectly released into various water bodies.
It was found that the concentration of BPA in drinking water, system, H
2 2
of liquid H O to generate cOH. In the traditional Fenton
2
+
2 2
O could be catalyzed by Fe to form cOH for organic
ꢀ1
12
surface water and waste landll leachate could reach 0.1 mg L
,
2 2
contaminants elimination. But, H O was unstable and easily
ꢀ
1
ꢀ1
2,4
dozens of mg L and 17.2 mg L , respectively. As a typical decomposed, making it difficult to transportation and
endocrine disruptor, BPA poses a risk to aquatic wildlife and storage.
human beings even at low concentrations. However, BPA could carrier of H
not be effectively degraded from wastewater by the conventional the low cost, high stability, and effective activation.
5
13,14
Sodium percarbonate (SPC), as the solid phase
6
O
2
, is a promising alternative to liquid H due to
O
2 2
2
13,15
In
addition, the by-products of SPC decomposition contain
Na
(CO
2
CO
3
, which can react with cOH to form carbonate radical
ꢀ
a
b
Faculty of Science, Monash University, Clayton, VIC, 3800, Australia
3
) and accelerate the degradation of contaminants with
16,17
Hubei Key Laboratory of Mine Environmental Pollution Control and Remediation, electron-donating moieties (eqn (1)–(3)).
Based on these
Hubei Polytechnic University, Huangshi, 435003, China. E-mail: zhenghan@hbpu.
advantages, increasing attentions have been paid to the
homogeneous activation of SPC by Fe /Fe , which has great
power in the remediation of organic contaminants in water.
Miao et al. demonstrated that perchloroethylene was effi-
ciently oxidized by the Fe /SPC system in groundwater, and
edu.cn; Fax: +86-0714-6348286; Tel: +86-0714-6348671
2+
3+
c
School of Environmental Studies, China University of Geosciences, Wuhan 430074,
China
d
18
College of Chemical and Biological Engineering, Key Laboratory of Biomass Chemical
2
+
Engineering of Ministry of Education, Zhejiang University, Hangzhou 310027, China
e
School of Environmental Science and Engineering, Hubei Polytechnic University, cOH was the main active species, showing better performance
Huangshi, 435003, China
than the traditional Fenton process at the similar reaction
conditions (eqn (4)–(6)). Nevertheless, there are unavoidable
†
Electronic supplementary information (ESI) available. See DOI:
0.1039/d0ra08395j
1
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drawbacks such as narrow reaction pH rang, non-recycle of 2.3 Catalytic degradation experiments
catalyst and precipitation of ferric hydroxide (Fe(OH) ), which
3
In this study, unless otherwise noted, all batch experiments
19
results in large amount of iron sludge. In order to overcome
these limitations, nanoscale zero-valent iron (nZVI), as an non-
toxic, and easy recycled catalyst with relatively large specic
surface area, shows excellent potential in activating peroxide
oxidants, such as persulfate, peroxymonosulfate, bisulte and
ꢂ
were performed at room temperature (25 ꢁ 2 C) in 250 mL
beaker. The initial concentration of BPA was 0.1 mM and the
total volume was 100 mL. Following the addition of a desired
amount of SPC, the initial pH of mixture was adjusted with
2 4
0.1 M H SO as quickly as possible. BPA degradation was
20–24
hydrogen peroxide, for removing organic contaminants.
In
initiated by adding certain dosages of nZVI under continuously
mechanical agitation. 1.0 mL sample was collected and imme-
diately mixed with 1.0 mL of methanol to quench the residual
reactive species at predetermined time intervals. Aerwards,
the samples were ltered through 0.22 mm PVDF membranes.
To evaluate the reusability of catalyst, the used nZVI particles
were collected by magnetic separation and washed with ultrapure
water. Then, the recycled catalysts were utilized for the next
reaction under the identied conditions. All the experiments
were conducted in triplicate to ensure the accuracy of data.
the process of nZVI/hydroperoxide, organic pollutants are
0
mainly destroyed by the generated cOH and puried by the Fe
25
corrosion products (iron oxides). However, there have not
been any comprehensive mechanistic studies on the activation
of SPC by nZVI for the removal of contaminants of emerging
concern.
Na
2
CO
3
$1.5 H
2
O
2
/ Na
2
CO
3
+ 1.5 H
2
O
2
(1)
(2)
(3)
(4)
2
3
ꢀ
ꢀ
ꢀ
cOH + CO
/ CO
3
c
+ OH
3ꢀ
3
ꢀ
2
c + H O
cOH + HCO / CO
2.4 Analytical and characterization method
0
+
2+
The concentration of BPA was measured using high perfor-
mance liquid chromatography (HPLC, RIGOL L-3000, China),
Fe + O + 2H / Fe + H O
2
2
2
0
Fe + H
+
2+
2
O
2
+ 2H / Fe + 2H
2
O
(5) equipped with a Compass C18 column (5 mm, 4.6 ꢃ 250 mm)
and UV detector (wavelength at 280 nm for BPA). The mobile
2
Fe + H
+
3+
/ cOH + Fe + OH
ꢀ
2
O
2
(6)
phase was consist of acetonitrile and ultrapure water (40 : 60, v/
ꢀ1
v) with a constant ow rate of 1.0 mL min . The injection
Herein, in the present study, it is aimed to investigate the
insight of the mechanisms of SPC activation by nZVI for BPA
degradation in wastewater. The operation parameters of the
nZVI/SPC system were optimized, such as nZVI dosage, SPC
concentration, and initial pH. The inuences of the nature
water constituents on the BPA degradation were carefully eval-
ꢂ
volume was 20 mL and column temperature was set at 35 C.
2+
The concentrations of Fe
ions were colorimetrically
27
measured using the standard method. Briey, o-phenanthro-
line was added into the ltered reaction solution and the
resulting solution was measured at 510 nm wavelength on a UV-
vis light spectrophotometer. The contained Fe ions were
reduced to Fe ions by oxammonium hydrochloride to measure
3+
ꢀ
2ꢀ
ꢀ
ꢀ
uated, including the Cl , HPO , NO2 , NO3 anions, and
2+
4
humic acid (HA). The changes of the chemical compounds on
the surface of the pristine and used nZVI were characterized to
propose the possible mechanisms for SPC activation. Finally,
the transformation products (TPs) were detected to assist the
bio-toxicity analysis during the BPA degradation, providing the
fundamental knowledge and theoretic support for the future
applications of nZVI/SPC method in the wastewater treatment.
the concentrations of total iron ions in the reaction solutions.
Total Organic Carbon (TOC) was determined with a TOC/TN
analyzer (multi N/C 2100, Jena, German) to assess the miner-
alization rate of BPA. The transformation products of BPA were
determined by an ultrahigh performance Liquid Chromatog-
raphy (UPLC, UltiMate 3000, Dionex) coupled with a mass
spectrometer (MS, Q-Orbitrap, Thermo Scientic). The UPLC-
ꢀ
MS was performed with negative electrospray ionization (ESI )
ꢂ
in a full-scan range of 50–350 m/z at 40 C. The injection volume
2
. Materials and methods
was 1 mL. The biological toxicity during the BPA degradation
process was evaluated by the activated sludge inhibition test
2
.1 Chemicals
Bisphenol A (BPA, >99.8%), isopropyl alcohol (IPA, ACS, $
9.5%) and phenol (PhOH, ACS) were supplied by Aladdin
Chemistry. Sodium percarbonate (Na CO $1.5 H O , SPC, 98%)
28
(ASIT) as previously reported.
The structure and crystal phase of nZVI before and aer use
9
were analyzed by X-ray diffraction (XRD, D8 Advance, Bruker,
Germany) using Cu Ka radiation. The chemical states of Fe and
O were analyzed by X-ray photoelectron spectroscopy using
monochromatic 100 eV Al-Ka radiation (XPS, ESCALAB 250Xi,
Thermo Fisher).
2
3
2 2
was purchased from Sigma-Aldrich. Other analytical grade
chemicals provided by Sinopharm Chemical Reagent. No
further purication was employed for all reagents. The ultra-
pure water (18.2 U cm) was conducted to prepare reagents.
2
.5 Kinetics analysis
Based on the BPA removal pattern, the BPA degradation kinetics
The nZVI particle was synthesized by the liquid phase reduction in nZVI/SPC system should follow a pseudo-rst-order reaction,
2.2 Preparation of the nZVI catalyst
26
method. Detailed process was described in ESI.†
as shown in eqn (7).
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ln(C /C ) ¼ ꢀkobst
(7)
t
0
Where C
t
is the concentration of BPA (mM) at reaction time t
(
k
min) and C is the initial BPA concentration (mM), respectively.
obs represents the observed rate constant (min ).
0
ꢀ
1
3. Results and discussion
3.1 Degradation kinetics of BPA by nZVI/SPC
In this work, various reaction processes were performed to
assess the BPA degradation efficiency, including nZVI, SPC,
2
+
2 2
nZVI/SPC, nZVI/H O and Fe /SPC systems. As shown in Fig. 1,
BPA was slightly removed in the presence of nZVI or SPC alone
within 20 min. The results indicated that nZVI was ineffective to
2
9
adsorb molecular BPA (pk
a
¼ 9.6–10.2). Moreover, SPC has
limited oxidation ability to remove BPA without activation. Fig. 2 Reusability of nZVI: [BPA]
0
¼ 0.1 mM, [nZVI]
0
¼ 0.1 g Lꢀ , [SPC]
1
0
¼
3 mM, pH
0
¼ 4.0.
When nZVI combined with SPC, BPA was completely removed
within 20 min, suggesting that nZVI has an excellent perfor-
mance on the SPC activation. However, BPA degradation was
2
+
In order to assess the contribution of leached Fe from nZVI
to the SPC activation in the SPC system, the concentration of
deceased in the reaction system of nZVI/H
2
ꢀ
O
2
. Because
through eqn
2) and (3), which favor to oxidize the electron-rich organic
carbonates from SPC could transform to be CO c
3
2
+
2+
dissolved Fe was measured and the performance of Fe /SPC
was compared with nZVI/SPC. As shown in the Fig. 3, the
concentration of total Fe (including Fe and Fe ions) gradu-
ally increased to 2.3 mg L and then remained relatively stable,
which mainly consist of Fe . Fe could be rapidly reduced to
Fe by nZVI through eqn (8). Since the surface-associated Fe(II)
2 2
atoms and dissolved Fe ions could also activate H O to
2.3 mg L of Fe
was added with the SPC to evaluate the performance for BPA
removal. As shown in Fig. 1, only about 40% of BPA was elim-
inated in the Fe /SPC system, while more than 99% of BPA was
removed in the nZVI/SPC system. Unlike homogeneous catal-
ysis, surface active sites of nZVI could promote catalytic oxida-
tion reaction in heterogeneous processes. These results
suggested that both heterogeneous and homogeneous catalysis
contributed to BPA degradation by nZVI/SPC, but the hetero-
geneous reactions played a primary role.
(
13,16,30
pollutants,
resulting in the improvement of BPA degrada-
3
+
2+
tion efficiency in the nZVI/SPC system.
ꢀ1
The chemical stability and reusability of prepared nZVI were
evaluated by for its future applications in the wastewater
treatment. As shown in the Fig. 2, more than 99% of the BPA
was degraded in the rst two runs, while about 78% of the BPA
was removed in the third run. The catalytic activity of nZVI
decreased in the continuous operations, which might be
ascribed to two main reasons. The rst reason might be the
surface corrosion and iron ions dissolution, which reduced the
active sites on nZVI. The second one might be the iron ions
precipitation products that would deposit on the nZVI surface,
2
+
3+
2
+
2
+
32,33
ꢀ1
2+
decompose the organic contaminants,
2
+
33
31
hindering the activation of SPC. Overall, these results
conrmed that nZVI has a prominent catalytic ability with
limited reusability in the nZVI/SPC system.
Fig. 1 BPA degradation in various systems. General reaction condi-
ꢀ
1
2+
ꢀ1
,
tions: [BPA]
0
¼ 0.1 mM, [nZVI]
0
¼ 0.1 g L , [Fe
¼ 4.0.
]
0
¼ 2.3 mg L
Fig. 3 Fe leaching in nZVI/SPC system: [BPA]
0
¼ 0.1 mM, [nZVI] ¼
0
ꢀ1
[
H
2 2
O ]
0
¼ 4.5 mM, [SPC]
0
¼ 3 mM, pH
0
0.1 g L , [SPC]
0
¼ 3 mM, and pH ¼ 4.0.
0
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0
3+
2+
Fe + 2Fe / 3Fe
(8)
3
.2.2 Effect of initial pH on BPA degradation. According to
12
the previous work, solution pH was a crucial factor in organic
pollutants remediation in Fenton or Fenton-like systems. A
series of tests were conducted to investigate the effect of initial
3.2 Effects of operating parameters on BPA degradation
3
.2.1 Effects of nZVI loading and SPC dosing on BPA
pH (pH
that BPA oxidation efficiency in nZVI/SPC system was closely
related to pH , implying that BPA degradation was a pH
0
at 3.0, 4.0, 5.0 and 6.0) on BPA removal. Fig. 5a showed
degradation. To explore the effect of catalyst performance, the
degradation efficiency of BPA was investigated at different nZVI
dosages from 0.05 g L to 0.5 g L . As presented in Fig. 4a, BPA
removal rate was signicantly enhanced with the increase of nZVI
dosage from 0.05 g L
observed rate constant (kobs) was enhanced from 0.294 min to
.694 min (inset in Fig. 4a). More nZVI dosage could provide
more active sites for activating SPC, which was conducive to the
formation of more free radicals. However, with the further
increase of nZVI loading (0.2–0.5 g L ), BPA removal efficiency
was signicantly inhibited, as the kobs was reduced (inset in
Fig. 4a). It was deduced that excess nZVI would lead to agglom-
eration, which would reduce the effective collision between nZVI
0
ꢀ1
ꢀ1
dependent process. It was observed that BPA could be
completely removed during 10 min reaction at pH ¼ 3.0 and
0
ꢀ
1
ꢀ1
to 0.2 g L . Correspondingly, the
4.0, especially showing a higher removal rate (85.3%) at the
ꢀ
1
initial reaction (0–1 min). The results were in accordance with
the conclusion that acidic pH condition was benecial to
ꢀ
1
0
2+
37
release Fe (Fig. 5b), promoting the generation of cOH. In
contrast, the degradation efficiency of BPA gradually declined
34
ꢀ1
with the rise of pH
from 46.0% to 24.1% when the pH
0
. For instance, BPA removal rate decreased
increased from 5.0 to 6.0.
0
This phenomenon could be explained in three aspects: rst,
when the solution pH increased from 3.0 to 4.0, 5.0 and 6.0, the
0
34,35
2+
and H O ;
Besides, the superabundant Fe leached from the
2+
ꢀ1
2
2
concentration of Fe gradually dropped from 17.80 mg L to
.31 mg L , 0.14 mg L , 0.05 mg L aer reaction, respec-
tively. Less Fe was detected under higher pH, leading to the
lower production of reactive species. Second, the elevation of
24,25,34
excess nZVI might scavenge cOH through eqn (9).
ꢀ1
ꢀ1
ꢀ1
2
2
+
2
+
3+
ꢀ
Fe + cOH / Fe + OH
(9)
38
pH
0 0 3
(pH > 4) would benet the precipitation of Fe(OH) ,
inhibiting the availability of catalytic sites on the surface of
The inuence of SPC concentration (0.5 mM, 1 mM, 3 mM and
mM) in nZVI/SPC system was studied to evaluate the contribu-
tion of oxidant on BPA removal. As illustrated in Fig. 4b, BPA
degradation efficiency was enhanced when SPC concentration
increased from 0.5 mM to 3 mM. However, BPA degradation was
slightly restrained with further increase of SPC dose from 3 mM to
39
nZVI. Third, cOH has a higher oxidation potential at a lower
6
40
pH, which was 2.65–2.80 V at pH 3.0 and 1.90 V at pH 7.0.
Therefore, acid condition was preferred to BPA degradation in
the nZVI/SPC system.
6
mM. Moreover, kobs in nZVI/SPC system exhibited a consistent
3
.3 Effect of water matrix on BPA degradation
trend (inset in Fig. 4b). These results revealed that SPC with the
moderate concentration was benecial to form sufficient active
ꢀ
42ꢀ
3ꢀ
2ꢀ
Inorganic anions such as Cl , HPO , NO and NO inher-
ently exist in surface and ground water, which might react with
free radicals and play an inhibited impact on contaminant
degradation in AOPs.
different content of Cl , HPO
system was discussed separately. As depicted in Fig. 6a, Cl at
the level of 1–10 mM showed inhibitory effect on BPA removal.
It was probably because Cl could compete with BPA to
19,35
species, promoting the elimination of BPA.
, disaggregated from SPC in the bulk solution, could play as
a scavenger of cOH or other radicals as shown in eqn (10) and
However, excessive
2 2
H O
1
3,41,42
In this study, BPA removal by adding
ꢀ
2ꢀ
ꢀ
ꢀ
19,36
4
, NO
3
and NO
2
into nZVI/SPC
(11),
causing the decreasing of BPA degradation efficiency.
ꢀ
ꢀ
2
+
H O + cOH / H O + O c + H
(10)
2
2
2
ꢀ
ꢀ
ꢀ
O
2
c
+ cOH / OH + O
2
(11) consume cOH to form Cl
2
and HOCl with weak oxidation
¼ 0.1 g Lꢀ , [SPC]
1
¼
Fig. 4 Influence of nZVI loading (a) and SPC dosing (b) on BPA degradation in nZVI/SPC system: [BPA]
0
¼ 0.1 mM, [nZVI]
0
0
3
mM, pH
0
¼ 4.0.
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Fig. 5 (a) Effect of pH
0
on BPA degradation; (b) Fe leaching in the nZVI/SPC system. General reaction conditions: [BPA]
0
¼ 0.1 mM, [nZVI] ¼ 0.1 g
0
ꢀ1
L
, [SPC]
0
¼ 3 mM.
ꢀ
42ꢀ
3ꢀ
2ꢀ
Fig. 6 Effect of anions on BPA degradation in nZVI/SPC system. (a) Cl ; (b) HPO ; (c) NO ; (d) NO . React conditions: [BPA]
0
¼ 0.1 mM,
ꢀ1
[
nZVI]
0
¼ 0.1 g L , [SPC]
0
¼ 3 mM, pH ¼ 4.0.
0
43
42ꢀ
2ꢀ
ꢀ
ꢀ
capacity. Similarly, Fig. 6b showed that HPO
exhibited
HPO4 + cOH / HPO4c + OH
(12)
signicant negative inuence on BPA degradation. With the
2
ꢀ
increase of HPO
4
concentration from 1.0 to 5.0 and 10.0 mM,
3ꢀ
BPA removal decreased from 68.5% to 33.2% and 23.0%,
In case of NO , BPA degradation was slightly inhibited by
2
ꢀ
2+
ꢀ
ꢀ
respectively. HPO4 would complex with Fe to block the the addition of 1, 5, and 10 mM NO
3
(Fig. 6c), because NO
3
2
+
2+
3+
active sites on the nZVI surface and suppress the released Fe , was unable to complex with Fe or Fe , and also non-reactive
6
13
which might inhibit cOH generation. Additionally, the pres- to cOH. Conversely, a strong inhibitory effect on BPA removal
2
4
ꢀ
ꢀ
2
ence of HPO
, as one of cOH scavengers, could consume cOH was observed in the presence of NO
(Fig. 6d). This phenom-
6
by the following reaction (eqn (12)).
enon could be attributed to the scavenging reaction between
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Fig. 7 Effect of HA on BPA degradation in the nZVI/SPC system: [BPA]
0
Fig. 8 The effects of IPA and PhOH on BPA degradation in nZVI/SPC
ꢀ1
¼
0.1 mM, [nZVI]
0
¼ 0.1 g L , [SPC]
0
¼ 3 mM, pH ¼ 4.0.
0
system. General reaction conditions: [BPA]
0
¼ 0.1 mM, [nZVI] ¼ 0.1 g
0
ꢀ1
L
, [SPC]
0
¼ 3 mM, pH ¼ 4.0.
0
ꢀ
10
ꢀ1 ꢀ1
NO2 and cOH (k
ꢀ
,
¼ 1.2 ꢃ 10
M
s ), leading to the
NO2 cOH
ꢄ
$
ꢀ
2
44,45
generation of NO NO
2
with low oxidation potential.
system. But the contribution of O c was negligible. This is
2
ꢀ
ꢀ
ꢀ
In addition, natural organic matters (NOMs), the common due to the fact that HCO3 can scavenge O2c to be CO c ,
3
substances in water bodies, should be considered to investigate which selectively react with electron-rich compounds, like
5
0,52
the inuence on BPA removal in the nZVI/SPC process. There- phenols.
fore, humic acid (HA), as a kind of representative NOMs, was
selected to explore the effect of NOMs on BPA degradation. As
ꢀ1
shown in Fig. 7, when HA concentration increased to 5 mg L
3.5 Mechanistic insight of the nZVI/SPC system
ꢀ1
and 10 mg L , respectively, BPA removal rate was slightly
inhibited. With the further increase of HA concentration to
As presented in Fig. 9, the XRD patterns of nZVI before use were
corresponded to the standard peaks of Fe (JCPDS no. 36-
696). Aer 20 min reaction, the new peaks at 26.6 , 35.6 and
0.6 are observed in used nZVI, which are relevant to the
0
ꢀ
1
2
2
0 mg L , BPA removal efficiency still reached to 83% at
0 min. HA had the ability to consume a certain amount of cOH,
53
ꢂ
ꢂ
0
6
ꢂ
and compete with target compounds, leading to the lower effi-
ciency in AOPs.
3 4
crystalline structures of Fe O (JCPDS no. 89-6466), FeO (JCPDS
36,46
These results indicated that the nZVI/SPC
no. 89-0690) and FeOOH (JCPDS no. 70-0410), respectively.
These results suggested that iron oxides and oxide-hydroxides
were formed on the surface of nZVI during the activation of SPC.
To better understand the roles of Fe and O species on the
nZVI surface in SPC activation, the XPS spectra of nZVI before
system has potential to resist appropriate amount of NOM
interference on BPA degradation.
3.4 Identication of reactive species
ꢀ
ꢀ
3
As mentioned in previous literatures, cOH, O c and CO c were
expected in the SPC system.
2
16,17,36
In this work, isopropanol (IPA)
was used to study the existence of cOH, because it has a high
9
ꢀ1 ꢀ1
reaction rate with cOH (kIPA, cOH ¼ 3.9 ꢃ 10 M
s
) and a low
ꢀ
6
ꢀ1 ꢀ1
ꢀ
reaction rate with O
2
c
(kIPA, O2c
ꢀ
¼ 1 ꢃ 10 M
3
s ) and CO c
4
ꢀ1 ꢀ1 47–49
(
k
IPA, CO3c
ꢀ
¼ 4 ꢃ 10 M
s
).
Meanwhile, Phenol (PhOH)
ꢀ
was employed as cOH and CO
3
8
c scavenger, which reacts rapidly
ꢀ1 ꢀ1
ꢀ
with cOH (k
¼ 6 ꢃ 10 M
s
) and CO c (kPhOH, CO3c
ꢀ
PhOH, cOH
ꢀ1 ꢀ1 16,50
3
9
ꢀ
¼
1.2 ꢃ 10 M
s
),
and slowly with O c (kPhOH, O2c
ꢀ
¼ 5.8
2
2
ꢀ1 ꢀ1 51
ꢃ 10 M
s ). BPA degradation performance in the absence
and presence of quenching agents was presented in Fig. 8. Both
IPA and PhOH could prevent BPA removal in the nZVI/SPC
system, and the inhibitory effect of PhOH was more signi-
cant. Specically, in the absence of IPA or PhOH, BPA could be
completely degraded in 10 min, whereas BPA removal rate
decreased to 18.8% and 0.9% with the addition of 30 mM IPA
and PhOH, respectively. This result implied that both cOH and
ꢀ
CO
3
c
participated in the degradation of BPA, and cOH was the
main reactive oxygen radical for BPA removal in nZVI/SPC Fig. 9 XRD of nZVI before and after use.
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Fig. 10 XPS spectra on Fe 2p (a) and O 1s (b) of nZVI catalyst before and after use.
53
and aer reactions were recorded. As illustrated in Fig. 10a, the oxides, deposited on the surface of nZVI. On the other hand,
2
+
3+ 34,48
peaks of Fe 2p3/2 at 706.8 eV, 710.3 eV, and 712.7 eV indicated Fe could be regenerated from the reduction of Fe ,
further
0
2ꢀ
the existence of Fe , Fe O and FeOOH on the nZVI surface promoting the activation of SPC. In addition, CO3
Based on the Fe 2p3/2 spec- HCO3 , as by-products of SPC decomposition, could transform
trum, the Fe , Fe(II) and Fe(III) content of the nZVI before use to be CO
and
3
4
21,53–55
ꢀ
before reaction, respectively.
0
ꢀ
3
c
through the reaction with cOH as shown in eqn (2)
ꢀ
were 10.6%, 57.1% and 23.3%, but account for 6.5%, 52.3% and and (3). Finally, BPA was degraded to TPs by cOH and CO
3
c via
4
1.2% for the nZVI aer use, respectively. Aer the reactions, direct radical attachment and electron transfer, respectively.
0
the decrease of Fe and Fe(II) indicated that electron transfer
occurred on the nZVI surface and Fe was released from the
catalyst, resulting in the increase of Fe(III) fraction and cOH
2
+
3.6 BPA degradation pathways
20
production, according to the eqn (4).
In this study, the TPs of BPA in the nZVI/SPC system were
In addition, the O 1s spectrum is shown in Fig. 10b. The two identied by LC-MS. Four compounds were detected with
main peaks at 529.7 eV and 531.0 eV were assigned to iron oxide identied m/z values, namely B1 (monohydroxylated BPA, m/z ¼
and surface hydroxyl species before reaction, correspond- 243.101), B2 (dihydroxylated BPA, m/z ¼ 259.096), B3 (p-iso-
5
3,55
ingly.
iron oxide, an iron hydroxide layer was formed, making the ethan-1-one, m/z
Aer reactions, with the decrease of the fraction of propylphenol, m/z ¼ 151.075) and B4 (1-(2,4-dihydroxyphenyl)
¼
151.038). Detailed information was
composition of surface hydroxyl species increased. The results supplied in ESI.† The possible degradation pathway of BPA
further proved that the formation of iron oxides was the (Fig. 12) was proposed based on the monitored TPs and the
important step in activating SPC by nZVI.
Overall, based on the above analysis, the chemical reaction
mechanism between nZVI and SPC was proposed as shown in
Fig. 11. First, nZVI was corroded to generate Fe(II)-contained
2
+
oxides on the surface and Fe ions that leached into the reac-
2
+
tion solution (eqn (4) and (5)). Then, Fe ions could activate
3
+
2 2
H O directly, generated from SPC, to engender cOH and Fe .
3
+
The formed Fe would be transformed to Fe(III)-contained
Fig. 12 Possible degradation pathway of BPA in the nZVI/SPC system.
ꢀ1
Fig. 11 Proposed mechanism of BPA degradation in the nZVI/SPC Reaction conditions: [BPA]
0
¼ 0.1 mM, [nZVI]
0
¼ 0.1 g L , [SPC] ¼
0
system.
3 mM, pH
0
¼ 4.0.
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treatment. BPA degradation efficiency was enhanced with
increased nZVI loading and SPC dosing in the adequate range at
ꢀ
2ꢀ
ꢀ
low initial pH. Furthermore, the Cl , HPO , NO2 anions and
4
HA in aqueous solution inhibited BPA removal in the selected
range. In the nZVI/SPC system, cOH was the dominant active
ꢀ
species, while CO
3
c
slightly contributed to the degradation of
2
+
BPA. In addition, surface corrosion, Fe releasing, and iron
species conversion play signicant roles in the activation of SPC
by nZVI. The possible degradation pathways of BPA were
proposed based on the TPs detected by LC-MS. This work
supports that the nZVI/SPC system can be used as an efficient,
cost effective, and environmental-friendly technology for the
remediation of BPA in wastewater treatment, owing to its high
catalytic activity, good reusability, and treatment safety.
Fig. 13 Changes of TOC and bio-toxicity during BPA treatment in the Conflicts of interest
nZVI/SPC system. Reaction conditions: [BPA]
0
¼ 0.1 mM, [nZVI] ¼
0
ꢀ1
0
.1 g L , [SPC]
0
¼ 3 mM, pH
0
¼ 4.0.
The authors declare no conict of interest.
5
6,57
previous studies.
First, cOH might attack the aromatic ring Acknowledgements
of BPA through hydroxylation to produce B1, and further
hydroxylation to be B2. On the other hand, BPA could be con-
verted into phenolic-BPA radical cation under the attack of
CO3c by capturing electrons. Aer further interacted with
water, hydroxybenzene TPs (B1) was generated. Then, B1 was
hydroxylation into B2 following the same mechanism. Second,
cOH could attack the electron-rich alkyl carbon of BPA to
remove a benzene ring, resulting in the formation of alkyl
This work was nancially supported by the National Key
Research and Development Program of China
ꢀ
16
(2017YFC0212602), the Open Research Fund of Hubei Key
Laboratory of Mine Environmental Pollution Control & Reme-
diation, Hubei Polytechnic University (2020105), the China
Postdoctoral Science Foundation (2019M662064).
16
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