Rapid and long-lasting acceleration of zero-valent iron nanoparticles@Ti3C2-based MXene/peroxymonosulfate oxidation with bi-active centers toward ranitidine removal

Article abstract of DOI:10.1039/d1ta02046c

Advanced oxidation processes (AOPs) can effectively degrade ranitidine, a pharmaceutical that is a typical precursor of nitrosamine dimethylamine (NDMA), an extremely potent human carcinogen. Herein, novel magnetic Ti₃C₂-based MXene nanosheets decorated with nanoscale zero-valent iron particles (nZVIPs@Ti₃C₂nanosheets) were synthesized to boost catalytic peroxymonosulfate (PMS) activation, exhibiting remarkable reactivity and stability for the rapid removal of ranitidine under mild conditions. The response surface methodology (RSM) confirmed that the initial solution pH and PMS dosage were the main factors affecting the heterogeneous oxidation process. The long-lasting catalytic activity was mainly attributed to the rapid charge transfer between dual electron-rich active centers (≡Fe and ≡Ti), with SO₄˙^?and HO˙ derived from PMS activation being responsible for ranitidine degradation, among which SO₄˙^?was the major contributor. Density functional theory (DFT) calculations also confirmed that the as-synthesized nZVIPs@Ti₃C₂nanosheets provided bi-active centers for PMS activation, and this process triggered a series of thermodynamically favorable reactions. This study demonstrates a recyclable oxidation method for the rapid removal of ranitidine, which may be applicable to the degradation of other challenging pollutants.

Full text of DOI:10.1039/d1ta02046c

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Materials Chemistry A
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Rapid and long-lasting acceleration of zero-valent
iron nanoparticles@Ti C -based MXene/
3
2
Cite this: DOI: 10.1039/d1ta02046c
peroxymonosulfate oxidation with bi-active
centers toward ranitidine removal†
abc
d
e
abc
abf
Yiyang Ma,g Dongbin Xiong, Xiaofan Lv, Xuesong Zhao,
Chenchen Meng,
abc
Haijiao Xie and Zhenghua Zhang
*
Advanced oxidation processes (AOPs) can effectively degrade ranitidine, a pharmaceutical that is a typical
precursor of nitrosamine dimethylamine (NDMA), an extremely potent human carcinogen. Herein, novel
magnetic Ti -based MXene nanosheets decorated with nanoscale zero-valent iron particles
3
C
2
(
nZVIPs@Ti C nanosheets) were synthesized to boost catalytic peroxymonosulfate (PMS) activation,
3 2
exhibiting remarkable reactivity and stability for the rapid removal of ranitidine under mild conditions.
The response surface methodology (RSM) confirmed that the initial solution pH and PMS dosage were
the main factors affecting the heterogeneous oxidation process. The long-lasting catalytic activity was
mainly attributed to the rapid charge transfer between dual electron-rich active centers (hFe and hTi),
ꢀ
with SO
which SO
the as-synthesized nZVIPs@Ti
process triggered series of thermodynamically favorable reactions. This study demonstrates
4
c
and HOc derived from PMS activation being responsible for ranitidine degradation, among
ꢀ
4
c
was the major contributor. Density functional theory (DFT) calculations also confirmed that
nanosheets provided bi-active centers for PMS activation, and this
Received 10th March 2021
Accepted 20th April 2021
3 2
C
a
DOI: 10.1039/d1ta02046c
a recyclable oxidation method for the rapid removal of ranitidine, which may be applicable to the
degradation of other challenging pollutants.
rsc.li/materials-a
ꢀ
6
nitrosodimethylamine (NDMA), which presents a 10 -fold
increase in lifetime cancer risk at a concentration of only
1
. Introduction
ꢀ1
4
Pharmaceuticals and personal care products (PPCPs) are 0.7 ng L . Ranitidine is a histamine H -receptor antagonist
2
a newly emerging type of organic pollutants, which have been commonly used for the treatment of gastroesophageal reux
detected at various concentrations in surface water, ground- and ulcers. However, this compound is not metabolized
water and soils worldwide, mainly originating from anthropo- completely by the human body and more than 70% of
5
1–3
genic pollution sources. These PPCPs, especially some of untransformed ranitidine may be excreted in the urine.
them containing dimethylamine groups in their molecular Furthermore, recent studies have demonstrated that ranitidine
structure, have been veried as potential precursors of N- is a potentially potent NDMA precursor, exhibiting a high
NDMA molar conversion rate of 89.9–94.2% during the
6,7
decomposition process. Therefore, since ranitidine is an
emerging contaminant that has been widely introduced into the
natural environment, an effective treatment technique used to
remove the compound from water is highly desired.
a
Institute of Environmental Engineering & Nano-Technology, Tsinghua Shenzhen
International Graduate School, Tsinghua University, Shenzhen 518055, Guangdong,
China. E-mail: zhenghua.zhang@sz.tsinghua.edu.cn
3,8
b
Guangdong Provincial Engineering Research Center for Urban Water Recycling and
Environmental Safety, Tsinghua Shenzhen International Graduate School, Tsinghua
University, Shenzhen 518055, Guangdong, China
Heterogeneous Fenton-like oxidation is an advanced oxida-
tion process (AOP) which has developed rapidly in the past
decade, overcoming the limitations (e.g. the narrow pH range,
high yield of chemical sludge and poor stability) of the tradi-
c
School of Environment, Tsinghua University, Beijing 100084, China
d
Pillar of Engineering Product Development, Singapore University of Technology and
Design, Singapore 487372, Singapore
9–12
tional homogeneous Fenton process.
Compared with the
e
Beijing Key Laboratory of Water Resources & Environmental Engineering, China
short-lived hydrogen peroxide (H
2
O
2
), peroxymonosulfate (PMS,
University of Geosciences (Beijing), Beijing 100083, China
ꢀ
f
HSO
) belongs to a chemically stable and solid oxidant with an
College of Chemistry and Environmental Engineering, Shenzhen University, Shenzhen
5
5
18060, China
asymmetric structure, and serves as a precursor for the gener-
g
ꢀ
13,14
Hangzhou Yanqu Information Technology Co., Ltd, Hangzhou 310003, China
ation of sulfate radicals (SO
c ) via activation.
The standard
4
ꢀ
2ꢀ
†
Electronic supplementary information (ESI) available. See DOI:
0.1039/d1ta02046c
redox potential of E
0
(SO
4
c /SO
4
) (2.5–3.1 V) is higher than
1
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ꢀ
ꢀ
that of E
0
(HOc/OH ) (2.7 V), and therefore SO
4
c
has been used
In this study, a novel magnetic compound material
as a more aggressive oxidizing agent for aqueous contaminant (nZVIPs@Ti C nanosheets) was synthesized to rapidly remove
3
2
1
5,16
degradation.
Various transition metal elements such as Co, ranitidine from aqueous solution by activating PMS to yield
ꢀ
Ce, Fe and Ni have been widely applied for PMS activation (eqn highly reactive oxidizing species (ROS), such as SO4c and HOc.
(
1)), due to the low energy consumption requirements of these The main objectives of this study were to (1) evaluate the effects
17,18
activation systems.
of interactive relationships between experimental parameters
on the catalytic activity of the nZVIPs@Ti nanosheets/PMS
3 2
C
n+
M
5ꢀ
(n+1)+
ꢀ
42ꢀ
+ SO + 2HOc
+ 2HSO / M
+ SO
4
c
(1)
reactive system; (2) comprehensively identify the main domi-
nant ROS responsible for ranitidine degradation; (3) conduct
In recent years, heterogeneous Fenton-like oxidization
methods utilizing nanoscale zero-valent iron particles (nZVIPs)
have attracted considerable attention due to their high surface
reactivity, large specic surface area and low cost, and this
approach is established by replacing dissolved ferrous ions in
theoretical analysis of the catalytic roles of nZVIPs@Ti C2
nanosheets; (4) clarify the mechanism of PMS activation and the
Fe(II)/Fe(III) valence cycle.
3
19–21
2. Materials and methods
the Fenton reagent.
However, some disadvantages of nZVIPs
still greatly limited their application in water pollution treat- 2.1 Chemicals and reagents
ment. When their particle size decreases to the nanometer
All chemicals and reagents used in the preparation of catalytic
scale, agglomeration of nZVIPs in aqueous solution was inevi-
table due to magnetic and van der Waals forces, which would
directly result in a signicant decrease in the specic surface
materials were supplied by commercial sources without further
processing, with both pure nZVIPs and Ti C -based MXene
3
2
synthesized in the laboratory. Details of the above-mentioned
reagents and the preparation methods for pure nZVIPs and
22,23
area.
It is generally accepted that these nZVIPs exhibit
a distinct core–shell structure as a result of their high surface
energy, which triggers a series of reactions with environmental
media, causing the formation of a passivation layer on the
Ti
3 2
C -based MXene are described in Text S1 and Fig. S1, S2
(ESI†).
2
4,25
particle surface.
Fe(III) was blocked during the heterogeneous Fenton-like
oxidation reaction, which would also be the main reason for To ensure the uniformity of the lamellar structure of the
More importantly, the circulation of Fe(II)/
2
.2 Synthesis of magnetic nZVIPs@Ti C nanosheets
3 2
26
the passivation layer formation on the nZVIP surface. Partic- synthesized nZVIPs@Ti C nanosheets, Ti C -based MXene was
3 2 3 2
ularly for pure nZVIP samples, their catalytic activities were pre-treated by differential screening using high-speed centri-
driven by the oxidation of nZVIPs in the inner core. Therefore, fugation (3000 rpm for 15 min and then 8000 rpm for 15 min
the thickness and coverage area of the passivation layer on the followed by 5000 rpm for 10 min). Firstly, 0.35 g of ferrous
nZVIP surface are key factors that determine the charge and sulfate (FeSO
4
) and 0.35 g Ti
3 2
C -based MXene were placed in
22
degree of mass transport.
a 250 mL volumetric ask, with 200 mL deionized water added
To address these technical issues, the valence state cycle of to dissolve the mixture under ultrasonication for 30 min.
Fe(II)/Fe(III) in the reaction system can be accelerated by adding Secondly, the obtained suspension was transferred to a 500 mL
chelators or reductants to heterogeneous Fenton-like reactions three-neck ask, continually agitated at 20 rpm and then
induced by iron-based catalysts, resulting in the release of combined with 1.14 g of sodium borohydride (NaHB
a large amount of soluble ferrous ions into the bulk solution.
) dissolved
in 200 mL deionized water in a dropwise manner under
Reported literature indicates that the use of supporting mate- a continual ow of nitrogen. When the full dosage of NaHB was
4
27,28
4
rials with large specic surface areas is a highly effective added to the mixed solution, stirring was continued for 2 h until
method to prevent nZVIP aggregation and improve the chemical the ferrous ions were completely reduced to zero-valent iron and
stability of metal nanoparticles in heterogeneous oxidation the solution was black in color. The synthesis process is illus-
2
1
reactions.
carbides/carbonitrides (i.e., MXenes) have been applied in many eqn (2) as follows:
elds since their emergence, providing low-dimensional
Recently, two-dimensional transition metal trated in Fig. 1a and the whole reaction sequence is shown in

2
+
4ꢀ
0
Fe(H
2
O)
6
(aq) + 2BH (aq) / Fe (s) + 2B(OH)3(aq) + 7H2(g)[(2)
connement quantum effects and a unique nanosheet struc-
ture with a large specic area, hydrophilicity and electrical
conductivity, along with other benecial physical and chemical
29,30
characteristics.
Among them, Ti C -based MXene has been
3 2
the most widely used as the titanium element (valence states of 2.3 Batch experiment procedure
+
2, +3 and +4) in its molecular structure is metastable and could
All degradation experiments were conducted in 200 mL glass
further provide reactive sites for heterogeneous oxidation
vials sealed with a Teon-lined cap containing 150 mL of rani-
31,32
reactions.
systematic investigations have been reported on the catalytic
roles of nZVIPs@Ti for PMS activation, especially in-depth
However, to the best of our knowledge, no
ꢀ1
tidine (10 mg L ) with constant agitation at 250 rpm, with the
heterogeneous reaction initiated by the addition of 15.0 mg
PMS and 11.25 mg nZVIPs@Ti C nanosheets. The initial
3 2
C
3
2
analysis on the reaction mechanisms.
solution pH value was adjusted using 30 mM HCl and NaOH
without buffering. At selected time intervals, 5 mL of the
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Fig. 1 (a) Schematic illustration of the synthesis process for nZVIPs@Ti
3
C
2
nanosheets; (b) the SEM image of Ti
nanosheets; (e) the HRTEM image of nZVIPs@Ti
nanosheets including (g) carbon, (h) titanium, (i) iron and (j) oxygen elements; (k) the
nanosheets; (l) atomic structures of the Fe nanoparticle active-site and Ti nanosheet active-site. Blue, brown,
gold, yellow, red, and white balls represent Ti, C, Fe, S, O, and H atoms, respectively.
3 2
C -based MXene powder; (c)
Ti
nanosheets; (f) EDS-mapping images of nZVIPs@Ti
HRTEM image of nZVIPs@Ti
3
C
2
-based MXene in aqueous solution; (d) low-magnification TEM image of nZVIPs@Ti
3
C
2
3 2
C
3 2
C
3
C
2
3 2
C
aqueous sample was extracted and ltered through a 0.22 mm a pH meter (Thermo star-A211) and variation was controlled
membrane lter, followed by the addition of a quenching agent within a range of ꢁ0.1. Electron paramagnetic resonance (EPR,
(
50 mL of sodium thiosulfate, NaS O $5H O) to remove residual Bruker EMX 10/12, Bremen, Germany) spectrometry was
2 3 2
radicals before High Performance Liquid Chromatography employed to identify the free radicals, with EtOH and TBA
(
HPLC) detection. In order to reduce the impact of experimental utilized as radical scavengers and DMPO applied as the spin-
ꢀ
error, each group of experiments was carried out in parallel trapping agent for SO
4
c
and HOc. The concentrations of total
triplicates, with the standard deviation (SD) used to control iron and ferrous ions in solution during the catalytic process
experimental data, which was controlled within the range of were measured by water quality-determination of iron-
ꢁ
5%. Furthermore, in ranitidine cyclic degradation tests, in situ phenanthroline spectrophotometry (HJ/T 345-2007) with the
consecutive experiments were conducted to avoid further procedures shown in Fig. S3 (ESI†). Density functional theory
oxidation of nZVIPs before the next experimental cycle.
(DFT) calculations were conducted using the Vienna Ab Initio
Package (VASP) (Text S2, ESI†). Detailed information on electro-
chemical measurements is provided in Text S3 (ESI†).
2.4 Analytical methods
Ranitidine concentrations were analyzed using HPLC (Agilent
technologies, series 1100, America) with an Eclipse XDS-C18
2.5 Materials characterization
column (5 mm, 4.5 ꢂ 150 mm Agilent) with a mobile phase X-ray diffraction (XRD) spectra were measured using spec-
consisting of ultra-pure water and MeOH (1 : 1) at a ow rate of trometry (D8 Advance, Bruker, Germany) in the reection mode
ꢀ1
33
ꢃ ꢃ
The solution pH values were measured using with Cu/Ka radiation ranging from 5 to 70 at a scanning rate
1
mL min
.
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ꢃ
ꢀ1
5
min . In situ X-ray photo-electron spectroscopy (XPS, nanosheets. Additionally, the HRTEM image demonstrated that
PHI5000 VersaProbe II, Japan) was employed to record changes a large number of nZVIPs with diameters of 10–30 nm were
in surface elemental valence states. A combination of micro- deposited on the surface of Ti C -based MXene (Fig. 1f–j) with
3
2
laser confocal Raman spectrometry (Raman, MDTC-EQ-M15- the corresponding atomic and weight ratios of each element in
1, France) with the wavelength of 532.05 nm of the laser nZVIPs@Ti nanosheets presented in Table S1.† Also, the
0
3 2
C
source, and Fourier-transform infrared spectrometry (FTIR, surface oxidation of nZVIPs inevitably occurred during the
Nicolet iS50, America) were used to analyze the surface chem- synthesis process due to reactions of nZVIPs with the aqueous
istry components of the prepared samples. The Brunauer– solution or air, resulting in the formation of a core–shell
Emmett–Teller (BET, ASAP2020M+C, America) system was used morphology as illustrated in Fig. S7 (ESI†). Furthermore, the
to evaluate the specic surface area and pore size distribution of two lattice fringe spacings of nZVIPs@Ti C nanosheets were
3
2
materials. Magnetic data were recorded using a vibrating about 0.20 nm and 0.31 nm, which were assigned to the (110)
ꢃ
sample magnetometer (VSM, Quantum Design PPMS-9, Amer- crystal plane of zero-valent iron with a diffraction peak at 44.6
ica). The zeta potential of the prepared samples was recorded and (110) reection of Ti
under different pH conditions using a zeta potential analyzer peak at 60.7 , respectively (Fig. 1k).
3
C
2
-based MXene with a diffraction
ꢃ
34,35
Meanwhile, the opti-
(Nano ZS, England). The morphology of the prepared samples mized atomic structures of bi-active centers provided by
was characterized by eld-emission scanning electron micros- nZVIPs@Ti -based MXene were theoretically modeled as
3 2
C
copy (SEM, Hitachi SU8010, Japan), high-resolution trans- described in Fig. 1l, on which the adsorption of the PMS
mission electron microscopy (HTEM, FEI Tecnai G2 F30, molecule had the lowest energy (Fig. S8, ESI†).
America) and atomic force microscopy (AFM, Bruker Dimension
ICON, Germany).
Moreover, wide angle X-ray diffraction analysis was conducted
to determine whether the Ti AlC particle powder was successfully
3
2
3 2
converted into Ti C -based MXene as shown in Fig. 2a, S9 and S10
(
ESI†). XRD results clearly showed a shi of the (002) and (004)
3
. Results and discussion
peaks to lower angles and the weakening of the diffraction peak
from the (104) plane at 39.0 (2q), indicating that the interaction
3.1 Characterization of nZVIPs@Ti
3
C
2
nanosheets
ꢃ
36,37
Nanoscale zero-valent iron particles (nZVIPs) were well- between layers was weakened by HCl and LiF etching.
FT-IR and
established catalysts that are useful for water contamination Raman results (Fig. 2b and c) revealed that the etching process
remediation. However, their inherent magnetic force and high retained the functional end groups on the surface with abundant
surface energy cause an inevitable signicant increase in the C]O bonds and –OH groups on the surface of nZVIPs@Ti
average particle size. In order to overcome this disadvantage, nanosheets. Besides, the nitrogen adsorption–desorption
3 2
Ti C -based MXene was used as a support material for nZVIPs to isotherms of nZVIPs, Ti -based MXene and nZVIPs@Ti C
C
3 2
3
2
3
C
2
obtain novel physicochemical properties, including stable nanosheets are shown in Fig. 2d. The observed typical type IV
reactivity and nanoparticle dispersivity. Herein, the morpho- isotherms with type H3 hysteresis loops show that the synthesized
logical and structural features of nZVIPs@Ti
were established using SEM, TEM and HRTEM.
As shown in Fig. 1b, the nano-scale structure characteristics lish the corresponding pore size/volume distribution curve (Fig. 2d
of the exfoliated Ti AlC MAX phase (i.e., Ti
-based MXene) inset), further demonstrating that the synthetic nZVIPs@Ti
were investigated by SEM observation, and the black powder nanosheets were mainly mesoporous (range in 2–50 nm). The
was dissolved in aqueous solution (Fig. 1c). Compared with the specic surface area of pure Ti -based MXene was up to 107.82
. Importantly, the BET surface area of nZVIPs@Ti
3 2
C nanosheets catalytic materials exhibited mesoporous structure characteris-
38
tics. The BJH desorption technology was also employed to estab-
3
2
3
C
2
C
3 2
39
3 2
C
2
ꢀ1
irregular lumpy structure of the Ti AlC MAX phase (Fig. S4a,
m
g
3 2
C
3
2
2
ꢀ1
3 2
ESI†), the synthesized Ti C -based MXene samples exhibited nanosheets (28.25 m g ) increased by nearly 2-fold compared
2
ꢀ1
ultrathin sheet structures. In addition, a necklace-like structure with that of pure nZVIPs (14.56 m g ) (Table S2†).
of the pure nZVIP sample with partial agglomeration can be
observed in Fig. S4b,† and the corresponding particle sizes
range from 100 to 250 nm. The SEM image of nZVIPs@Ti C
3 2
nanosheets at low-magnication as shown in Fig. S5 (ESI†), and
3
.2 Interactive relationship between experimental
parameters
the corresponding elemental distribution of nZVIPs@Ti C
The response surface method (RSM) is an effective experimental
3
2
nanosheets, including C, Ti and Fe, further established the method that allows analysis of the interaction between various
relationship between the nZVIP distribution and Ti -based factors (e.g. pH, and dosage), including screening of the main
3 2
C
MXene combinations. It could be observed from the TEM image impact factors, polynomial modeling, and determination of the
that nZVIPs were uniformly immobilized on the surface of optimal combination of reactive conditions. Meanwhile, the
Ti
nZVIPs@Ti
microscopy image as shown in Fig. S6 (ESI†) suggested that ranitidine removal.
synthesized magnetic nanosheets with the thickness of around 3.2.1 Polynomial modeling and variance analysis. The
0–30 nm were successfully obtained. These experimental process conditions were optimized according to a Box–Behnken
results also further conrmed that nZVIPs were uniformly design (BBD) methodology using four factors at three levels, as
3
C
2
-based MXene (Fig. 1d). The actual geometric sizes of proposed method was also used to investigate the signicance
3 2
C
nanosheets stemming from the atomic force of one parameter and the interaction of these impact factors on
1
immobilized on the surface of the Ti
3
C
2
-based MXene listed in Table 1. The entire experimental design was
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Fig. 2 (a) XRD patterns of Ti
3
C
2
-based MXene and nZVIPs@Ti
-based MXene and nZVIPs@Ti
-based MXene and nZVIPs@Ti nanosheets (inset: pore size distribution curve for nZVIPs@Ti
3
C
2
nanosheets; (b) Raman spectra of Ti
3 2
AlC pre- and post-etching treatment, and
nZVIPs@Ti
3
C
2
nanosheets; (c) FTIR spectra of Ti
3
C
2
3 2
C
nanosheets; (d) nitrogen adsorption–desorption isotherms
for nZVI particles, Ti
3
C
2
3
C
2
3
C
2
nanosheets).
Table 1 Test factor levels and coding system
Levels and coding
Variate
Test factors
ꢀ1
0
1
X
1
X
2
X
3
X
4
Initial solution pH values
Dosage of PMS/(mg per 100 mL)
Dosage of nZVIPs@Ti nanosheets/(mg per 100 mL)
The mass ratio of nZVI : Ti
3.5
5
5
4.5
10
7.5
1:1
6.0
20
10
3
C
2
C
3 2
1:2
2:1
established using Design-Expert 8.0.6 soware as illustrated in
Table S3,† in which twenty-seven sets of experimental runs were
measured and three replicate centers were established to esti- between their corresponding terms and the surface response.
These coefficients effectively determined the relationship
41
mate the pure error. Taking the ranitidine removal efficiency as In addition, ANOVA analysis was performed to reliably verify the
42
the response value (Y) and tting the experimental data with relevance of the quadratic polynomial model, with the corre-
40
polynomial regression analysis, a quadratic polynomial model sponding outcomes for the proposed model shown in Table
for the response value with an independent variable (X ) was S4.† Based on the experimental results, the F-value of 181.45
i
obtained, as shown in eqn (3) as follows:
was much higher than 1 and the p-value was below the signi-
cance level of 0.05, indicating that this proposed model was
suitable for data tting. The corresponding F-values for initial
Y ¼ ꢀ275.53941 + 113.85124X
1
+ 6.13678X
+ 0.25721X
ꢀ 0.29444X
2
+ 14.28876X
3
43
+
+
24.64529X
0.065196X
4
ꢀ 0.37436X
1
X
2
1
X
3
+ 0.49375X X
1 4
solution pH (X
1
), the PMS dosage (X
2
), the nZVIPs@Ti
3 2
C
2
X
3
ꢀ 0.021541X
2
X
4
3 4
X
nanosheet dosage (X
), and the mass ratio of nZVI : Ti
C
4
(X )
3
3
2
2
2
2
2
ꢀ
12.53077X1 ꢀ 0.14397X ꢀ 1.01691X ꢀ 8.96577X4 (3)
2
3
were 451.74, 489.67, 15.86, and 14.46, respectively, conrming
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Fig. 3 Three-dimensional and contour maps of the interaction between different factors for ranitidine degradation efficiency, (a) and (b) initial
solution pH value and PMS dosage; (c) and (d) initial solution pH value and dosage of nZVIPs@Ti nanosheets; (e) and (f) PMS dosage and the
3 2
C
3 2 3 2
dosage of nZVIPs@Ti C nanosheets; (g) and (h) PMS dosage and the mass ratio of nZVI : Ti C .
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46,47
that the initial solution pH and PMS dosage were important removal.
Two distinct shapes could be observed in the 2D
factors affecting the ranitidine removal efficiency.
contour plots, circle and ellipse, with elliptical contour plots in
2
In general, the determination coefficient R and adjusted particular implying that the interactions between variables were
2
48
determination coefficient Radj are indispensable parameters signicant. The solution pH is an important parameter in
that are universally used to evaluate the reliability and accuracy iron-based oxidation systems, as it signicantly affects the
44,45
2
of polynomial models.
R
As shown in Table S4,† the R and series of reactions, such as the decomposition of oxidants and
adj values were 0.9953 and 0.9898, respectively, indicating that the generation of radicals. Furthermore, the solution pH also
the regression equation shown in eqn (3) could simulate the affects the speciation of activated radicals and the distribution
2
49,50
actual response surface well. Furthermore, the standardized of oxidants.
As shown in Fig. 3a and b, the ranitidine
residual indicated that the data points of standard deviation removal efficiency was enhanced with increasing PMS dosages
deviated from both the response value and the measured value at xed solution pH values, due to the generation of more active
ꢀ
were normally distributed. As shown in Fig. S11 (ESI†), these radicals (such as HOc and SO c ) in the solution. Although PMS
4
experimental data points were well tted with the modelled itself has a certain oxidative capacity, it still requires activation
ꢀ
line, indicating that the predicted values were close to the actual to generate SO
4
c
and HOc, which are the main oxidants
values.
required to achieve degradation of target contaminants, with
ꢀ
3.2.2 Effect of interaction between various factors on the SO
4
c
being prominent under acidic conditions while HOc is
51
removal of ranitidine. The three-dimensional (3D) response more dominant under basic conditions. However, HOc has
surface plots and two-dimensional (2D) contour plots were used a lower redox potential (1.8 V) under neutral and basic condi-
to reect the interactions between various factors on ranitidine tions than under acid conditions (2.7 V), and its life span was
Fig. 4 (a) Effects of common inorganic anions on ranitidine removal in the nZVIPs@Ti
different types of negative effects on ranitidine degradation by common inorganic anions, including (c) chlorine anions (Cl ), (d) bicarbonate
3
C
2
nanosheets/PMS system after 6 min of reaction; (b) the
ꢀ
ꢀ
ꢀ
2ꢀ
anions (HCO
nanosheets and nZVIPs@Ti
field); (h) reuse of nZVIPs@Ti
3
), (e) nitrate anions (NO
3
), and (f) sulfate anions (SO
4
), (g) magnetic hysteresis loops of freshly prepared nZVIPs@Ti
3 2
C
3
C
2
nanosheets after five cycles of reuse (inset: response of fresh nZVIPs@Ti C
nanosheets for five consecutive experimental cycles under different solution pH conditions; (i) zeta (z)-potential
3 2
nanosheets to an external magnetic
3
C
2
ꢀ1
ꢀ1
ꢀ1
,
as a function of solution pH (dosage of synthetic materials ¼ 0.5 g L ). Experimental conditions: [ranitidine]
0
¼ 10 mg L , [PMS]
0
¼ 0.1 g L
ꢀ1
[
Cata]
0
¼ 75 mg L , nZVI/Ti
3
C
2
mass ratio ¼ 1 : 1, the initial solution pH ¼ 4.5, at room temperature.
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ꢀ
52
shorter than that of SO
solution pH, the recombination rate of SO c was much higher dosage of Cl was increased to 5 and 10 mM, respectively. This
4
c . Furthermore, with an increase in gradually decreasing to 87.27% and 83.21% in 6 min as the
ꢀ
ꢀ
4
53–55
ꢀ
than the rate of conversion to HOc.
It has been demon- is possibly because Cl penetrates the passivating oxide layer by
strated that excessive PMS can also serve as a scavenger for diffusion, forming complexes with the zero-valent iron inner
ꢀ
18
60
SO
4
c as demonstrated by eqn (4). Since the maximum dosage core and greatly accelerating the corrosion of nZVIPs (Fig. 4c).
ꢀ
of PMS was below that limit in the present study, the ranitidine Additionally, excessive Cl can also consume activated radicals
removal efficiency exhibited a trend of continuous increase.
ꢀ
and HSO
5
, generating less reactive chloride radical anions (Clc,
and ClOc) and hypochlorous species (eqn (5)–(9)). The
ꢀ
Cl
2
c
HSO5ꢀ + SO4cꢀ / SO42ꢀ + SO5c
ꢀ
+
+ H
ꢀ
(4)
redox potentials of Clc and Cl2c are 2.4 V and 2.1 V, respec-
ꢀ
61–63
tively, both of which are lower than that of SO c (2.5–3.1 V).
4
ꢀ
When HCO3 was added to the bulk solution, signicant inhi-
bition was observed that the ranitidine removal efficiency
decreased from 92.62% to 90.06%, 87.83% and 81.31% in the
In addition, the ranitidine removal efficiency was enhanced
with increased concentrations of nZVIPs@Ti nanosheets
when the dosage was below 0.75 g L as depicted in Fig. 3c and
d. However, a further increase in the dosage of nZVIPs@Ti
3 2
C
ꢀ1
ꢀ
presence of 1, 5 and 10 mM HCO
3
, respectively. The dissolved
3
C
2
ꢀ
ꢀ
HCO
3
could serve as a scavenger, consuming both SO
4
c
and
nanosheets resulted in a negative effect on catalytic perfor-
mance and subsequently a decrease in ranitidine removal. The
effect of initial solution pH on ranitidine removal was investi-
gated over a pH range of 3.5–10.0 without buffering (Fig. S12,
S13 and Table S5†). Acidic conditions were more favorable for
the activation of PMS in this heterogeneous catalytic system, as
nZVIPs served as a controllable and slow-releasing source of
ferrous ions which could be corroded rapidly, especially under
HOc to yield less active radical species (eqn (10) and (11))
55
ꢀ
(
Fig. 4d). Furthermore, HCO anions are basic agents capable
3
of producing a buffering effect on the solution, resulting in an
increase in the solution pH that was unfavorable for ranitidine
64
removal.
ꢀ
ꢀ
42ꢀ
/ Clc + SO
Cl + SO
4
c
(5)
(6)
56
ꢀ
ꢀ
acidic conditions. The core–shell structure not only protects
the inner core of nZVIPs from rapid oxidation, but also assists
the adsorption of target contaminant molecules due to elec-
Cl + Clc / Cl2c
ꢀ
5ꢀ
+
42ꢀ
2
Cl + HSO + H / SO + Cl
2
2
+ H O
(7)
57
trostatic interactions and surface complexation. However,
when the initial solution pH values were excessively high,
various corrosion products (e.g. magnetite, and goethite) accu-
mulate and attach to the surface active sites, interrupting the
direct contact between the ZVIP inner core and ranitidine
molecules. Fig. 3e–h show that the ranitidine removal efficiency
Cl
2
+ H
2
O / HOCl
(8)
ꢀ
ꢀ
2ꢀ
Cl + HSO5 / SO4 + HOCl
(9)
ꢀ
3ꢀ
42ꢀ
+
+ H + CO
ꢀ
SO
4
c
+ HCO / SO
3
c
(10)
(11)
(12)
(13)
(14)
ꢀ
ꢀ
changed signicantly under the interaction of nZVIPs@Ti C
3
2
HOc + HCO3 / H O + CO c
2
3
nanosheets and PMS. PMS was found to have a more signicant
effect on the removal efficiency of ranitidine, and
nZVIPs@Ti C nanosheets, serving as an effective PMS acti-
3 2
vator, enhanced the generation of active radicals for ranitidine
removal. Moreover, the interactions between other various
factors on ranitidine removal were discussed in detail as shown
in Fig. S14.†
3ꢀ
ꢀ
3
HOc + NO / OH + NO c
ꢀ
3ꢀ
42ꢀ
SO
4
c
+ NO / SO + NO
3
c
ꢀ
aq
ꢀ
ꢀ
H O + e
2
+ NO3 / 2OH + NO2c
3ꢀ
42ꢀ
exhibited highly negative
In addition, both NO and SO
effects on ranitidine degradation, with SO
the most signicant inhibition effect. The signicant inhibitory
effect of NO on ranitidine removal was due to the reactions of
NO3 anions with dissolved electrons and activated radicals
SO4c and HOc) with the conversion to less active species
Fig. 4e), such as NO c and NO c (eqn (12)–(14)). When SO
3 2 4
was present in the bulk solution with the concentrations of 1, 5
and 10 mM, the ranitidine removal efficiency in 6 min
decreased to 84.26%, 73.55% and 67.45%, respectively. This
could be attributed to the formation of inner-sphere complexes
2
ꢀ
4
anions exerting
3.3 Effects of dissolved anions on ranitidine degradation in
nZVIPs@Ti C nanosheets/PMS system
3
2
ꢀ
3
ꢀ
ꢀ
Inorganic anions are commonly present in water bodies, which
may affect the performance of pollutant removal by catalysts,
including buffering the solution pH, capturing activated radi-
cals and mitigating the effect of electrostatic bonds between
(
(
55
2
ꢀ
5
8,59
ꢀ
reactants.
As such, common inorganic anions such as Cl ,
ꢀ
42ꢀ
ꢀ
HCO
3
, SO
and NO
3
were added to the bulk solution in the
form of sodium salts to investigate their interference to raniti-
3 2
dine degradation by the nZVIPs@Ti C nanosheets/PMS system
as shown in Fig. 4a–f.
on the nZVIP surface, blocking the electron transport pathway
65
(Fig. 4f).
Fig. 4a depicts the effects of the various inorganic anions
with the concentrations of 1, 5 and 10 mM on ranitidine
removal efficiency in the nZVIPs@Ti C nanosheets/PMS
3.4 Reusability and stability of nZVIPs@Ti
3
C
2
nanosheets
3
2
system. The ranitidine removal efficiency slightly increased The magnetic properties of nZVIPs@Ti
from 92.62% to 93.72% aer the addition of 1 mM Cl , while and aer the ranitidine degradation reaction were measured
3
C
2
nanosheets before
ꢀ
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with the corresponding magnetization hysteresis loops shown experimental cycle, nitrogen protection was conducted with the
in Fig. 4g. The saturation magnetization value of nZVIPs@Ti C detailed method of nZVIPs@Ti C recovery shown in Fig. S15.†
3
2
3 2
ꢀ
1
nanosheets before the reaction was 13.2 emu g , while that Additionally, the isoelectric point (pHpzc) of nZVIPs@Ti C2
increased to 15.3 emu g aer ve consecutive cycles of use. nanosheets was about 6.37 as shown in Fig. 4i, suggesting that
3
ꢀ
1
These results indicated that strong magnetic iron oxides were the surface of the composite exhibited a positive charge when
formed on the nZVIP surface. Additionally, nZVIPs@Ti
3
C
2
the solution pH was below 6.37. Therefore, when the initial
nanosheets could be conveniently separated from the reaction solution pH was adjusted to 3.5–6.0, the electrical interaction
ꢀ
mixture for reuse by applying an external magnetic eld, which between HSO
5
3 2
and nZVIPs@Ti C nanosheets favored their
is an important physical property for nanocomposite catalytic contact, accelerating the activation of PMS. When the initial
materials. solution pH was 4.5, the ranitidine removal efficiency reached
Considering that acidic conditions are conducive to etching 92.31%, 90.46%, 87.68%, 84.45%, and 80.29% in ve consecu-
of the passivated layer on the surface of nZVIPs, it is important tive cycles, respectively. As the initial solution pH further
to conduct cyclic experiments of nZVIPs@Ti
under different solution pH conditions. As shown in Fig. 4h, nZVIPs@Ti
although the removal efficiency slightly decreased with each the ranitidine removal efficiency was not signicantly reduced,
experimental cycle, nZVIPs@Ti nanosheets retained high indicating that nZVIPs@Ti nanosheets exhibited good
3
C
2
nanosheets increased to neutral and alkaline levels, the catalytic activity of
3 2
C
nanosheets was limited to varying degrees, while
3
C
2
3 2
C
catalytic activity especially under acidic conditions. To avoid stability in aqueous solution. Furthermore, the leaching
further oxidation of nZVIPs in composition before the next behavior of the Fe element at the acidic pH of 4.5 during the
Fig. 5 (a) EPR spectra of different reaction systems using DMPO as the trapping agent at 3 min ([DMPO]
b) the removal of ranitidine in different reaction systems; (c) comparison of ranitidine removal efficiency over different technologies; (d) AC
impedance plots for nZVIPs and nZVIPs/Ti nanosheets (inset: the equivalent circuit); (e) EPR spectra under various conditions at 3 min
[EtOH] ¼ 20 mM); (f) the effects of different quenching agent concentrations on ranitidine removal; XPS spectra of
¼ 20 mM, [TBA]
nZVIPs@Ti
0
¼ 0.1 M, VDMPO ¼ 50 mL, Vsample ¼ 2 mL);
(
3 2
C
(
0
0
3
C
2
nanosheets before and after reaction; narrow region scans of (g) Fe2p and (h) Ti2p; (i) O1s. Experimental conditions: [ranitidine]
0
¼
ꢀ
1
ꢀ1
ꢀ1
1
0 mg L , [PMS]
0
¼ 0.1 g L , [Cata]
0
¼ 75 mg L , nZVI/Ti
3
C
2
mass ratio ¼ 1 : 1, the initial solution pH ¼ 4.5, at room temperature.
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3 2
catalytic process in Fig. S16† shows that the maximum semicircle diameter of the plot for the nZVIPs@Ti C nanosheet
concentrations of total iron and ferrous ions in solution are electrode was signicantly smaller than that for the nZVIPs elec-
ꢀ
1
ꢀ1
2
.65 mg L and 0.41 mg L , respectively.
trode, mainly due to the enhanced electroconductibility of
nZVIPs@Ti3C2 nanosheets. The smaller Rct of nZVIPs@Ti3C2
nanosheets (187.3 U) demonstrated that this composite exhibited
a greater charge transfer capability compared to nZVIPs (227.0 U).
The results of these experiments indicated that PMS activation
3
.5 Identication of radical species generated in the
nZVIPs@Ti C nanosheets/PMS system
3 2
3 2
To clarify the mechanism of PMS activation, EPR experiments occurred mainly at the surface active sites of nZVIPs@Ti C
ꢀ
4
employing the spin-trapping agent DMPO were performed to nanosheets, with SO c and HOc being the main active oxidation
identify the free radicals involved in the nZVIPs@Ti
3
C
2
species for the degradation of ranitidine, but these results did not
ꢀ
nanosheets/PMS reaction system. Three groups of EPR signals determine whether SO
c
or HOc was the most dominant species.
4
with different response intensities were observed when nZVIPs, In generally, ethanol (EtOH) and tert-butyl alcohol (TBA) are
ꢀ
ꢀ1
Ti
3
C
2
-based MXene and nZVIPs@Ti
3
C
2
nanosheets were commonly used as scavengers for quenching free radicals (k(SO
4
c ,
7
ꢀ1 ꢀ1
9
present in the bulk solution with the addition of PMS, as shown EtOH) ¼ 1.6–7.7 ꢂ 10 M
s
, k(HOc, EtOH) ¼ 1.2–2.8 ꢂ 10 M
ꢀ
1
8
ꢀ1 ꢀ1
in Fig. 5a. These signal peaks were assigned to the hyperne
s
and k(HOc, TBA) ¼ 3.8–7.6 ꢂ 10
M
s
), with EtOH
and HOc while TBA scav-
When 20 mM TBA was added into the aqueous
ꢀ
splitting of DMPOX adducts, which may be attributed to highly considered as a scavenger of both SO
4
c
ꢀ
70–72
reactive oxidizing species (SO
PMS activation.
DMPO–SO
nZVIPs@Ti
4
c
and HOc) generated during enges HOc only.
66,67
ꢀ
Furthermore, the relative intensities of both solution, the DMPO–SO
c
4
signal detected in the nZVIPs@Ti
C
3
2
ꢀ
4
c
and DMPO–cOH adducts were higher in the nanosheets/PMS system was consistent with the selective scav-
nanosheets/PMS system compared to those of enging of HOc in the bulk solution by TBA, as illustrated in Fig. 5e.
3
C
2
both the nZVI/PMS system and the Ti
3 2
C
-based MXene/PMS However, the weak EPR signal of DMPO–HOc could still be
system, indicating that nZVIPs@Ti C nanosheets activated observed with the addition of 20 mM of EtOH, which might be due
3
2
ꢀ
PMS and enhanced the production of SO4c and HOc. Inter- to insufficient use of the scavenger, resulting in a small fraction of
estingly, both nZVIPs and Ti -based MXene in nZVIPs@Ti free radicals not being quenched completely. Fig. 5f shows that the
3
C
2
3 2
C
nanosheets exhibited catalytic activity for PMS activation, removal of ranitidine was inhibited signicantly when different
further conrming that this composite provided bi-active concentrations of EtOH and TBA were added into the solution. In
ꢀ
surface sites for the formation of SO
4
c
and HOc. In order to particular, the corresponding ranitidine removal efficiency was
verify these ndings, the degradation of ranitidine using reduced from 34.6% to 18.6% when the concentration of EtOH
different catalysts was compared, as shown in Fig. 5b. A increased from 20 mM to 40 mM. Comparison of the inhibition
ꢀ
4
signicantly higher removal efficiency was achieved in 6 min effects of EtOH and TBA on ranitidine removal shows that SO c
when any solid catalyst was added into the bulk solution and HOc derived from PMS activation are generated in the
ꢀ
compared to the case with ferrous ions, indicating that surface nZVIPs@Ti
3
C
2
nanosheets/PMS system, with SO
4
c
serving as the
active sites provided by solid catalysts played a major role in the dominant oxidative species for ranitidine degradation.
rapid decomposition of ranitidine. Both nZVIPs and Ti
3
C
2
-
ꢀ
based MXene could activate PMS to generate SO
4
c and HOc for
the degradation of ranitidine molecules, and the corresponding
removal efficiency by nZVIPs was higher than that by Ti
based MXene. However, it was difficult to achieve a high rani-
tidine removal efficiency when merely utilizing a mixture of The chemical states of Fe (2p), Ti (2p) and O (1s) elements on
nZVIPs, Ti -based MXene and PMS. This result indicates that the surface of nZVIPs@Ti nanosheets were analyzed by XPS
the synergistic interaction between nZVIPs and Ti -based measurements before and aer the ranitidine degradation
3
.6 Verication of surface element components of bi-active
3 2
C -
centers provided by nZVI@Ti nanosheets
3 2
C
3
C
2
3 2
C
3 2
C
MXene was more conducive to the catalytic degradation of reaction, as shown in Fig. 5g–i. The satellite peaks referring to
ranitidine in the PMS-induced heterogeneous oxidation system. Fe (2p1/2) and Fe (2p3/2) could be observed by spin–orbit
It is worth mentioning that the kinetic rate constant for the doublets with binding energies of 724.6 and 711.4 eV, respec-
3 2
degradation of ranitidine by the nZVIPs@Ti C nanosheets/ tively (Fig. 5g). Particularly for the Fe (2p3/2) peak, two compo-
PMS system was much larger (1.3–136.9-fold) than the re- nents centering at binding energies of 710.7 and 712.5 eV in
ported values as shown in Fig. 5c and Table S6,† indicating the both fresh and aged nZVIPs@Ti C nanosheets were indicative
3
2
3 2
remarkable efficiency of this nZVIPs@Ti C nanosheets/PMS of the existence of iron-based species derived from Fe(II) and
34
heterogeneous Fenton-like catalysis.
Fe(III). The relative ratios of Fe(II) and Fe(III) peak areas in the
Furthermore, electrochemical impedance spectroscopy (EIS) fresh sample were 58.64% and 41.36%, respectively, while the
measurements were carried out to investigate the conductivity of equivalent ratios in the aged sample were 66.19% and 33.81%.
nZVIPs and nZVIPs@Ti
Nyquist plots of the synthesized nZVIPs and nZVIPs@Ti
3
C
2
nanosheets. As shown in Fig. 5d, the It is of note that both Fe(II) and Fe(III) are involved in PMS
nano- activation, with an electron transfer cycle formed between Fe(II)
3 2
C
sheet electrodes consisted of semicircles in the high/medium and Fe(III). Moreover, the peak intensity of nZVI in aged
frequency ranges and straight lines in low frequency ranges, cor- nZVIPs@Ti C nanosheets was signicantly decreased, due to
3
2
responding to the charge transfer resistance (Rct) and Warburg nZVI particles serving as an electron donor and the increasing
68,69
impedance associated with Li-ion diffusion in the electrode.
The thickness of the oxide layer on the particle surface.
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0
Fe ( + hTi(IV) 4 hFe(II) + hTi(II)
(15) transition metals with the formation of surface hFe(II) and the
s)
release of ferrous ions into aqueous solution to stimulate the
The XPS spectra for Ti (2p) clearly show that there were heterogeneous and homogeneous Fenton-like reactions.
mainly three valence forms of Ti in Ti -based MXene Although these reactions were conducive to the rapid removal of
3 2
C
components (Fig. 5h). An increase in the proportion of Ti(II) ranitidine, catalysts were inevitably consumed. Aer the reac-
from 38.56% (fresh sample) to 54.56% (aged sample) suggested tion with PMS, distinct Ti–O and Fe O peaks were detected in
x
y
that Ti C -based MXene continuously accepted electrons from the aged sample, as shown in Fig. 5i, which could result from
3
2
nZVIPs (eqn (15)), further indicating that nZVIPs and Ti C - the consumption of the bi-active sites on the surface of
3
2
based MXene interacted through charge transfer between nZVIPs@Ti C nanosheets. The O (1s) spectrum results are in
3
2
Fig. 6 DFT calculations of PMS activation by nZVIPs@Ti
nZVIPs@Ti nanosheets; (b) calculated lO–O and lO–H bond lengths, and free energy evolution of PMS molecules on the surface of Ti
MXene and nZVIPs@Ti nanosheets, respectively; (c) reaction pathway of the activation of PMS on the surface of nZVIPs@Ti nanosheets,
3
C
2
nanosheets and Ti
3
C
2
-based MXene, (a) atomic structures of Ti
3
C
2
-based MXene and
3
C
2
3 2
C
-based
3
C
2
3 2
C
where IS, TS and FS represent the initial structure, transition structure and final structure, respectively; blue, brown, gold, yellow, red, and white
balls represent Ti, C, Fe, S, O, and H atoms, respectively.
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line with the evidence that the oxidation in the PMS-induced degradation, with nZVIPs@Ti
3
C
2
nanosheets providing a large
heterogeneous Fenton-like reaction favored the formation of number of active sites that can participate in PMS activation.
iron-based oxides.
These active sites replace the dissolved transition metal ions as
the catalytic oxidation reaction center, activating PMS to
generate highly active free radicals to continuously attack
3.7 DFT calculations of PMS activation by nZVIPs@Ti C
3
2
ranitidine molecules until mineralization. EPR results indi-
nanosheets
ꢀ
cated that SO
4
c
and HOc were the main products aer PMS
Density functional theory (DFT) calculations were performed to
investigate the adsorption and activation behavior of PMS
molecules on the surface of nZVIPs@Ti C nanosheets and to
understand the roles of bi-active surface centers in promoting
PMS activation. As shown in Fig. 6a, the optimized structures of
activation, with radical quenching experiments further con-
ꢀ

rming that SO
4
c
was the most important active oxidation
3
2
species for ranitidine removal.
.8.1 Mechanism of PMS activation by nZVIPs@Ti C2
3
3
nanosheets. Serving as a charge donor and electron-rich center,
nZVIPs exhibit excellent reactivity in aqueous solution, gener-
ating Fenton reagents involved in ranitidine degradation
through rapid charge transfer processes. However, due to the
magnetic force between these nanoscale particles, they contact
and easily agglomerate into a cluster of particles with a smaller
specic surface area. Based on these considerations, the selec-
tion of Ti C -based MXene as the supporting carrier overcame
the problem of nZVIP agglomeration. Therefore, compared with
pristine nZVIPs, the specic surface area of nZVIPs@Ti C
3 2
nanosheets was increased by nearly two-fold (Table S1†), which
is an important factor in the enhancement of catalytic perfor-
mance. Moreover, as discussed in Section 3.7, nZVIPs@Ti C
3 2
Ti
retically modeled. When PMS molecules were adsorbed on the
surface of Ti -based MXene and nZVIPs@Ti nanosheets,
3 2 3 2
C -based MXene and nZVIPs@Ti C nanosheets were theo-
3
C
2
3 2
C
the possible adsorption congurations with the lowest energy
are illustrated in Fig. 6b, and the corresponding adsorption
energies and bond length (lO–O and lO–H in the PMS molecule
structure) are also listed in Table S7.† The theoretical lO–O bond
73
3 2
˚
length of the free PMS molecule was 1.326 A, while the lO–O
bond was remarkably stretched when PMS molecules were
˚
adsorbed on the Fe nanoparticle (1.474 A) and Ti
3
C
2
nanosheet
˚
(
1.472 A) surface active-sites, indicating that these adsorbed
PMS molecules tend to decompose. In addition, PMS molecules
can be easily adsorbed on the Ti C nanosheet active-site (ꢀ5.43
3
2
nanosheets provide bi-active surface centers for PMS molecule
activation, allowing Ti C -based MXene in components to not
eV) and Fe nanoparticle active-site (ꢀ4.84 eV). The more favor-
3
2
able adsorption of PMS molecules on the Ti
active-site was due to the surface functional groups (–OH) of
Ti -based MXene. As such, nZVIPs@Ti nanosheets
3 2
C nanosheet
only be used as a substrate but also to contribute to the highly
catalytic effect. This conclusion is consistent with the ndings
3
C
2
3 2
C
3
1
of previous literature. PMS exhibits two different pK
a
values,
provided bi-active surface centers that could stimulate PMS
activation. The reaction energies required for the Fe nano-
53,75
ꢀ
pKa1 < 0 and pKa2 ¼ 9.4,
and as shown in Fig. S18,† HSO
5
is
the dominant species of PMS in the pH range from 3.5 to 9.0,
particle active-site and Ti C nanosheet active-site to stimulate
3
2
2ꢀ
while PMS mainly exists as SO
5
when solution pH increases to
PMS activation were calculated as shown in Fig. 6c. The reaction
energy barrier (E ) corresponding to the Ti C nanosheet active-
above 9.4. Furthermore, due to the activation of nZVIPs@Ti
nanosheets, the peroxide bond (lO–O) in the HSO5 molecule
3
C
2
b
3
2
ꢀ
site was calculated to be 0.08 eV, much lower than the critical
value (0.9 eV) that determined whether a chemical reaction
structure was broken to generate highly oxidizing sulfate radi-
ꢀ
cals (SO c ). On the basis of the above analysis, several possible
74
4
could proceed at room temperature. Besides, the strong
PMS activation mechanisms have been proposed according to
the location of the reactive sites in this composite and the
different types of radicals generated by activation, as illustrated
in Fig. 7a. Surface active sites composed of low-valence transi-
tion metal elements were the most important factors for PMS
activation. For nZVIPs@Ti C nanosheets, there were two types
interaction between positively charged Fe nanoparticle active-
ꢀ
sites and negatively charged PMS molecules (HSO
5
) resulted
in the spontaneous dissociation of adsorbed PMS molecules
ꢀ
without an energy barrier followed by the formation of SO4c
and HOc. Additionally, the PMS activation process that occurred
on the bi-active surface sites of nZVIPs@Ti C nanosheets and
3 2
the subsequent PMS dissociation are demonstrated in Fig. S17,†
which were thermodynamically favorable. These results further
3
2
of surface-bound active sites that could stimulate PMS activa-
ꢀ
tion to yield sulfate radicals (SO c ), hFe(II) generated by the
4
3 2
rapid charge transfer of nZVI and hTi(III) in the Ti C -based
conrm that both the Fe nanoparticle active-site and Ti
3 2
C
MXene lattice (eqn (16) and (17)). Meanwhile, hFe(II) can be
nanosheet surface active-site of nZVIPs@Ti nanosheets
3 2
C
ꢀ
converted to hFe(III), which subsequently reacts with HSO
5
to
enabled the activation of PMS molecules with the yield of highly
active radicals for pollutant removal.
ꢀ
76
produce SO
5
c
with the regeneration of hFe(II) (eqn (18)).
These rapid reaction processes lead to the enrichment of a large
ꢀ
number of SO4c
on the solid catalyst surface, some of which
ads
3
.8 Mechanism of ranitidine degradation in nZVIPs@Ti C
3 2
were directly involved in the decomposition of ranitidine
nanosheets/PMS system
ꢀ
molecules, while the remaining SO4c
are further converted
O/OH oxidation (eqn
ads
ꢀ
According to the experimental results discussed, the combina- to hydroxyl radicals (HOcads) through H
2
tion of nZVIPs@Ti C nanosheets and PMS contributed to rapid (19) and (20)).
3
2
and efficient removal of ranitidine from aqueous solution. In
this heterogeneous Fenton-like oxidation system, PMS was the
main source of active oxidation species for ranitidine
5ꢀ
4
ꢀads
ꢀ
+ OH
hFe(II) + HSO / hFe(III) + SO
c
(16)
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Fig. 7 (a) Schematic illustration of the ranitidine degradation mechanism by nZVIPs@Ti
3
C
2
nanosheets with the activation of PMS (transition
3 2
metal elements with “h” indicating adsorption on the solid catalyst surface); (b) possible charge transfer pathways in the nZVIPs@Ti C
nanosheets/PMS system.
5ꢀ
4
ꢀads
ꢀ
+ OH
hTi(III) + HSO / hTi(IV) + SO
c
(17) reagents, with this performance defect directly affecting the
26
service life of catalytic materials. The Ti element in the
ꢀ
ꢀ
+
+ H
hFe(III) + HSO5 / hFe(II) + SO5c
(18) molecular structure of Ti C -based MXene (valence states of +2,
ads
3 2
+
3 and +4) belonged to a metastable state and due to the stan-
+
SO
4
c
ꢀads
+ H
2
O / SO
42ꢀ
+ HOcads + H
(19)
(20)
(21)
(22)
3+
2+
4+
dard redox potential of Fe /Fe being 0.77 V, while that of Ti /
3
+
Ti was 0.19 V, the transfer of electrons from hTi(III) to hFe(III)
was thermodynamically favored (eqn (27)). Compared with
trivalent Ti, divalent Ti exhibits a stronger reduction capacity,
achieving Fe(II)/Fe(III) cycling through charge transfer (eqn (28)).
The above reaction processes provided the main route for
providing hFe(II) and hTi(III) to activate PMS, as well as the
4
c
ꢀads
+ OH / SO
ꢀ
42ꢀ
+ HOcads
SO
ꢀ
ꢀ
ꢀ
+ OH
Fe(II) + HSO5 / Fe(III) + SO4c
free
5ꢀ
ꢀfree
+
+ H
Fe(III) + HSO / Fe(II) + SO
5
c
ꢀ
2ꢀ
+
SO4c
+ H O / SO
+ HOcfree + H
(23) core reactions that can achieve recyclable catalysis oxidation.
free
2
4
ꢀ
42ꢀ
SO
4
c
ꢀfree
+ OH / SO
+ HOcfree
(24)
(25)
(26)
hFe(III) + hTi(III) 4 hFe(II) + hTi(IV)
hFe(III) + hTi(II) 4 hFe(II) + hTi(III)
(27)
(28)
(29)
HOcads/HOcfree + ranitidine / degraded products
ꢀ
ꢀ
0
SO4c ads/SO4c
+ ranitidine / degraded products
Fe (s) + 2hFe(III) 4 3hFe(II)
free
As discussed, the zero-valent iron inner core with a standard
Under acidic conditions, excess hydrogen ions in aqueous redox potential of ꢀ0.44 V can be used as an electron donor to
solution directly attack the core–shell structure of nZVIPs. This reduce hFe(III) to hFe(II). However, due to the short migration
process begins with etching and dissolving the passivated layer distance of electrons in aquatic media, only a portion of hFe(III)
20
on the surface of nZVIPs, with these hydrogen ions reacting with that surrounds the inner core of nZVIPs can receive electrons. The
the inner core to release ferrous ions through redox reactions. results of electrochemical experiments (Fig. 5d) showed that Ti C -
3 2
And then, ferrous ions resulting from the dissolution of nZVIPs based MXene with the excellent electrical conductivity provides
diffuse into the bulk solution and induce a series of homoge- stable electron transport channels, which greatly improves the
neous catalytic reactions, activating a small portion of free PMS charge transport efficiency between nZVIPs (eqn (29)). It can clearly
ꢀ
molecules and generating active radicals including SO
4
c
free be seen from Fig. 7b that the synergistic action of multi-valence
and HOcfree through a chain reaction (eqn (21)–(24)). Finally, state cycles was the key to achieving stable catalytic oxidation of
under the combined action of homogeneous and heteroge- ranitidine in the nZVIPs@Ti nanosheets/PMS heterogeneous
neous catalysis, ranitidine molecules were degraded mainly by reaction system, which was also consistent with the ranitidine
3 2
C
ꢀ
ꢀ
SO4c ads/SO4c
and HOcads/HOcfree
a surface-bound form on nZVIPs@Ti
the bulk solution (eqn (25) and (26)).
,
existing either in removal efficiency dynamics observed in cyclic experiments.
nanosheets, or free in
free
3
C
2
3
.8.2 Insight into the mode of multiple valence cycles 4. Conclusions
between Fe and Ti elements. In general, the Fe(II)/Fe(III) cycle of
most iron-based solid catalysts in heterogeneous Fenton reac-
tions is inferior to that of traditional homogeneous Fenton
In this study, novel magnetic Ti C -based MXene nanosheets
with nZVIPs uniformly immobilized on the surface in the form
of a single-layer covering, served as a solid catalyst for the rapid
3
2
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removal of ranitidine from aqueous solution. The response
surface method (RSM) was used to determine the optimal
combination of reaction conditions, among which the initial
solution pH and the PMS dosage were the main factors affecting
4 M. Selbes, D. Kim, N. Ates and T. Karanl, The roles of
tertiary amine structure, background organic matter and
chloramine species on NDMA formation, Water Res., 2013,
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the whole heterogeneous oxidation process. This nZVIPs@Ti
3
C
2
5 J. Rivas, O. Gimeno, A. Encinas and F. Beltr ´a n, Ozonation of
the pharmaceutical compound ranitidine: reactivity and
kinetic aspects, Chemosphere, 2009, 76, 651–656, DOI:
10.1016/j.chemosphere.2009.04.028.
nanosheets/PMS heterogeneous reaction system exhibited
a strong resistance to inorganic anion interference and retained
excellent reusability and a high level of catalytic activity aer

ve cycles of reuse. This was mainly ascribed to the presence of
6 R. Shen and S. A. Andrews, Demonstration of 20
pharmaceuticals and personal care products (PPCPs) as
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two types of surface-bound active centers capable of stimulating
PMS activation, with hFe(II) generated by the rapid charge
transfer of nZVI and hTi(III) existing in the Ti C -based MXene
3
2
ꢀ
lattice. EPR results indicated that SO c and HOc were the main
4
products formed aer PMS activation, with radical quenching
7 J. Lv, L. Wang and Y. M. Li, Characterization of N-
nitrosodimethylamine formation from the ozonation of
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8 Y. Valc ´a rcel, S. Gonz ´a lez-Alonso, J. L. Rodr ´ı guez-Gil, A. Gil
and M. Catalz ´a , Detection of pharmaceutically active
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j.chemosphere.2011.05.014.
ꢀ
experiments further conrming that SO
4
c
was the most
important active oxidation species for ranitidine removal. DFT
calculations demonstrated that the PMS activation process that
occurred on the bi-active surface centers of nZVIPs@Ti C
3
2
nanosheets and the subsequent PMS dissociation were ther-
modynamically favorable. Overall, this study established
a recyclable Fenton-like oxidation process for the rapid removal
of ranitidine and provides novel insights into PMS molecule
activation by surface bi-active centers instead of dissolved
transition metal ions as the catalytic oxidation reaction centers,
indicating its potential as a promising and recyclable Fenton-
like oxidation process for the degradation of other chal-
lenging pollutants.
9 F. Duarte, F. J. Maldonado-Hodar and L. M. Madeira,
Inuence of the particles size of activated carbons on their
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Conflicts of interest
There are no conicts to declare.
using Fe
response surface methodology, Front. Mater. Sci., 2016, 10,
5–55, DOI: 10.1007/s11706-016-0324-z.
3 4
O /MWCNTs as catalyst: process optimization by
4
Acknowledgements
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The research was supported by the National Natural Science
Foundation of China (52000113), Tsinghua SIGS Start-up
Funding (QD2020002N), and Committee of Science and Tech-
nology Innovation of Shenzhen (KQJSCX20180320171226768,
and JCYJ20190813163401660).
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