Efficient detoxification of triclosan by a S-Ag/TiO2@g-C3N4 hybrid photocatalyst: Process optimization and bio-toxicity assessment

Article abstract of DOI:10.1039/c9ra03279g

Owing to their persistency and toxicity, development of an effective strategy to eliminate antibiotic residues from the aquatic system has become a major environmental concern. Doping TiO₂ with hetero atoms and forming a hybrid structure with g-C₃N₄ could serve as an efficient visible light active photocatalytic candidate. In this study, a novel S-Ag/TiO₂@g-C₃N₄ hybrid catalyst was prepared for visible light degradation and detoxification of triclosan (TS) antibiotic. The effect of various operational parameters towards the photocatalytic degradation was systematically evaluated through response surface methodology (RSM) based on central composite design (CCD). The highest TS degradation (92.3%) was observed under optimal conditions (TS concentration = 10 mg L^-1, pH = 7.8, and catalyst weight = 0.20 g L^-1) after 60 min. Efficient charge separation resulted from the doped nanoparticles (silver and sulphur), the existing integrated electric field of the heterojunction and the overlying light response of hybridized TiO₂ and g-C₃N₄, thus the S-Ag/TiO₂@g-C₃N₄ composite showed impressively higher activity. The main degradation products of TS were identified by LC/ESI-MS analysis. In addition, the toxicity of the degradation products was investigated through an Escherichia coli (E. coli) colony forming unit assay and the results revealed that under optimal conditions a significant reduction in biotoxicity was noticed.

Full text of DOI:10.1039/c9ra03279g

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Efficient detoxification of triclosan by a S–Ag/
TiO2@g-C3N4 hybrid photocatalyst: process
optimization and bio-toxicity assessment †
ac
Cite this: RSC Adv., 2019, 9, 20439
Xiangfeng Xie,ab Chen Chen,a Xiaoxiang Wang,ac Jie Li
*
cd
*
and Saraschandra Naraginti
Owing to their persistency and toxicity, development of an effective strategy to eliminate antibiotic residues
from the aquatic system has become a major environmental concern. Doping TiO2 with hetero atoms and
forming a hybrid structure with g-C3N4 could serve as an efficient visible light active photocatalytic
candidate. In this study, a novel S–Ag/TiO2@g-C3N4 hybrid catalyst was prepared for visible light
degradation and detoxification of triclosan (TS) antibiotic. The effect of various operational parameters
towards the photocatalytic degradation was systematically evaluated through response surface
methodology (RSM) based on central composite design (CCD). The highest TS degradation (92.3%) was
observed under optimal conditions (TS concentration ¼ 10 mg Lꢀ1, pH ¼ 7.8, and catalyst weight ¼ 0.20 g
L
ꢀ1) after 60 min. Efficient charge separation resulted from the doped nanoparticles (silver and sulphur), the
existing integrated electric field of the heterojunction and the overlying light response of hybridized TiO2
and g-C3N4, thus the S–Ag/TiO2@g-C3N4 composite showed impressively higher activity. The main
degradation products of TS were identified by LC/ESI-MS analysis. In addition, the toxicity of the
degradation products was investigated through an Escherichia coli (E. coli) colony forming unit assay and
the results revealed that under optimal conditions a significant reduction in biotoxicity was noticed.
Received 2nd May 2019
Accepted 24th June 2019
DOI: 10.1039/c9ra03279g
rsc.li/rsc-advances
difficult to eliminate by conventional wastewater treatment
methods. Moreover, physicochemical and biological technologies
do not exhibit an excellent outcome to deal with triclosan.7 This
1. Introduction
The continuous and long-term usage of antibiotics has raised
important environmental concerns since these antibiotics are
being detected in lakes, rivers and wastewater effluents and is
thought to be the reason for the increment in antibiotic resistant
bacteria and genes.1,2 For instance, during 2013 China consumed
about 92 700 tons of antibiotics in which 53 800 tons were released
into the environment.3 Triclosan is a bactericidal agent which has
been added to many PCPs (personal care products) like soap,
toothpaste, shampoo etc.4 However, triclosan has been detected in
wastewater and conrmed to be dangerous to many organisms
and humans.5 This necessitates the removal or decomposing of
triclosan from the environment to safe guard human beings6 and
other organisms. However, as a recalcitrant pollutant triclosan is
necessitates the development of efficient materials and tertiary
oxidative treatments for the enhanced degradation of triclosan.
Advanced Oxidation Processes (AOPs) are conceived as one
of the most attracting solutions because of their rapidity and
non-selective degradation power towards a wide range of
organic compounds.8 TiO2 is one of the best photocatalytic
materials due its stability, non-toxicity, low cost and high
performance,9 but its band gap (ꢁ3.2 eV) and higher recombi-
nation rate restricting its usage in practical applications.10
Doping TiO2 matrix with metal and non-metal dopants has
attracted signicant attention since they can exhibit higher
photo-catalytic activity and unique characteristics.11–14 Even so,
the dopants should be in small quantities to suppress the
recombination of photo-generated electrons and holes.15
Hence, the low concentration doping could be an effective
solution to enhance the visible light absorption efficiency.
Recently, graphitic carbon nitride (g-C3N4) has pulled ample
scientic interests because of its suitable band gap, stability and
efficient transferability to further enhance the light harvesting
capacity of TiO2 based photocatalysts.16,17 Though g-C3N4 is visible
light active material, its UV response is relatively lower, when
compared to TiO2.18 Coupling lower bandgap g-C3N4 with higher
bandgap TiO2 doped with metal and non-metal atoms as visible
aJiangsu Academy of Environmental Industry and Technology Corp., Nanjing 210036,
China. E-mail: enveng@126.com
bSchool of Energy and Environment, Key Laboratory of Environmental Medicine
Engineering of the Ministry of Education, Southeast University, Nanjing 210096, China
cMinistry of Education Key Laboratory of Integrated Regulation and Resource
Development on Shallow Lakes, College of Environment, Hohai University, Nanjing
210098, China
dBiofuels Institute, School of the Environment, Jiangsu University, 301 Xuefu Road,
Zhenjiang 212013, China. E-mail: saras@ujs.edu.cn
† Electronic supplementary information (ESI) available. See DOI:
10.1039/c9ra03279g
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light sensitizer to form a heterojunction can superimpose the light
response activity of both the semiconductors, owing to the special
electronic band structure.19,20 Furthermore, heterojunction
formation is a promising scheme to improve charge separation, by
the build-in electric eld of heterojunction which can drive the
photogenerated electrons and holes to transfer to opposite direc-
tions, therefore inhibiting their recombination.21 Thus, it is suit-
able to harvest more sunlight through the combination of g-C3N4
and TiO2 forming the g-C3N4/TiO2 heterojunction.
The optimization of operational parameters is an important
strategy to improve photocatalytic efficiency to obtain satisfactory
degradation of organic pollutants. Response surface method-
ology (RSM) is a statistical and experimental strategy to get the
optimum conditions for a multivariable system and it has been
successfully applied to a different photocatalytic processes for
achieving its optimization using experimental designs.22–24
This study aims to fabricate Ag and S co-doped TiO2@g-C3N4
hybrid photocatalyst for efficient degradation of triclosan anti-
biotic. The plausible mechanism of degradation for the efficient
catalytic activity of the S–Ag/TiO2@g-C3N4 hybrid composite was
proposed. Furthermore, response surface methodology was
successfully applied for optimizing and maximizing the degra-
dation. In addition, water reuse demands the toxicity assessment
of triclosan and its degradation products. Hence the biotoxicity of
triclosan and its degradation products was also investigated on E.
coli to dene the environmental impact of the treated water.
2.3 Characterization techniques
BRUKER D8 Advance X-ray diffractometer was used for XRD
analysis with Cu Ka source (l ¼ 1.5406 A). For imaging a high
ꢀ
resolution transmission electron microscope was used (TEM, FEI
TecnaiG2 F20 S-TWIX, at 200 KV) at different magnications.
JASCO V-670 UV-Vis spectrophotometer was used for diffused
reectance spectra analysis. Raman analysis was carried out by
using Thermo Scientic DXR2 at room temperature. XPS data
was acquired using Kratos Axis Ultra 165 spectrometer with
a monochromated Al Ka X-ray source (ha ¼ 1486.6 eV).
2.4 Photocatalytic degradation of triclosan
A 250 W Xe lamp was used as the visible light source with a lter (l
# 420 nm). The samples at regular intervals were collected and the
concentration of leover TS was analyzed using Agilent 1260 series
HPLC equipped with Eclipse XDBC18 (5 mm) reverse phase column
(4.6 ꢃ 150 mm). Water and acetonitrile at 30 : 70 v/v ratio was used
as the mobile phase at an injection volume of 1 mL minꢀ1 for 10
minutes. Further, the degradation products identication was
carried out by liquid chromatography tandem mass spectrometry
(Agilent 1290 Innity Binary LC system, Agilent 6460 Triple
Quadrupole LCMS/MS system employing the Zorbax eclipse plus
C18 column; rapid resolution, 2.1 ꢃ 50 mm, 1.8 mm). The mobile
phase of water and acetonitrile at 30 : 70 (v/v) was used for about
60 min. To electro spray ionization (ESI) was used to obtain the
mass spectra under the helium gas ow at approximately 1
mL minꢀ1 and 16 V of fragment voltage.
2. Experimental
2.1 Materials
Titanium(IV) isopropoxide (95%), silver nitrate ($99.8%), thio-
urea ($99.5%), 2-propanol ($99.5%), Tween 20 ($99%),
hydrazine hydrate ($80%), melamine (99%) and triclosan
(97%) were obtained with analytical grade from Alibaba chem-
icals and used without further purication.
2.5 Experimental design
In the present study to optimize the degradation efficiency of TS,
statistical modeling soware Design-Expert®11 was utilized.
Four independent variables i.e., TS concentration, pH, photo-
catalyst weight and time were selected on the percentage degra-
dation of TS as the response (dependent variable). The ranges
and the levels of the independent variables are shown in Table 1.
A Central Composite Design (CCD) based on RSM was employed
for the evaluation of four operational factors effects on the pho-
tocatalytic degradation process. Table 2 represents the ranges of
the factors along with the twenty four tests (24 runs) designed.
Accordingly, the test results were tted to a quadratic polynomial
model intended by the soware to correlate the measured
responses with the independent factors (Table 2).
2.2 Preparation of the photocatalyst
First, the g-C3N4 was synthesized according to previous report.25
In brief, 5 g of melamine was heated at 600 ꢂC for 2 h at a heating
rate of 2 ꢂC minꢀ1 in a muffle furnace; the obtained faint yellow
colored g-C3N4 was naturally cooled to room temperature. Later,
a calculate amount of g-C3N4 (0.1 g) and 5 mL of titanium iso-
propoxide (TTIP) was dispersed in 50 mL of isopropanol which
was further added to 200 mL of distilled water at pH ꢁ 1.5. To the
above solution required amounts of AgNO3 and thiourea aqueous
solutions were added followed by 5 mL of Tween 20 while stir-
ring. The resultant sol was then sonicated at 80 MHz for 90 min.
The ꢂobtained gel was further dried followed by calcination at
2.6 Biotoxicity assessment on Escherichia coli
400 C for 5 h and denoted as S/Ag–TiO2@g-C3N4. Ag–TiO2@g- The bio-toxicity assessment was determined by evaluating the
C3N4 nanoparticles were also prepared as described in the above growth of E. coli K-12 (CGMCC1.365) while treating with pure
method without addition of thiourea. For control experiment, TS, degradation products and control (without TS). The solu-
TiO2 nanoparticles were prepared as follows; a mixture of 5 mL of tion's biotoxicity is reciprocal to the number of E. coli colonies
titanium(IV) isopropoxide in 50 mL isopropanol was added grown on LB agar plates. In other words, the detoxication
dropwise to 200 mL of distilled water maintained at pH 1.5 while efficiency of the sample was inversely proportional to the
the solution was continuously stirred. This TiO2 sol was dried at growth inhibition of E. coli with respect to the control. The
100 ꢂC for 24 h, followed by calcination at 400 ꢂC for 5 h.
detoxication efficiency was calculated as follows:
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Table
variables
1
Experimental range and levels of the independent test
photocatalysts. XRD pattern of pure g-C3N4 showed two distinct
peaks at 13.3ꢂ and 27.3ꢂ which could be ascribed to (100) and
(002) planes respectively (JCPDS: 87-1526).26 The XRD pattern of
Ag/TiO2@g-C3N4 and S–Ag/TiO2@g-C3N4 showed characteristic
peaks for anatase phase with 2q values at 25.3ꢂ, 37.7ꢂ, 47.7ꢂ,
55.0ꢂ and 62.5ꢂ which could be assigned to (101), (103), (004),
(200), (105), (211), and (204) planes respectively (JCPDS: 21-
1272). Due to lower dopant concentration, signals corresponded
to Ag were not observed, this could be suggested that the doped
metal atoms did not alter phase crystallinity of TiO2.27 The
absence of g-C3N4 peaks in Ag/TiO2@g-C3N4 and S/Ag–TiO2@g-
C3N4 was attributed to the combination of smaller and highly
dispersed g-C3N4 particles with the other nanoparticles, or the
hybridization had no signicant effect on the crystal phase of
TiO2, in which the TiO2 peaks were highly strong so as to hinder
the g-C3N4 diffraction peaks.19 Furthermore, Raman analysis
was carried out to conrm the crystalline formation of the
prepared TiO2/g-C3N4 composites. As shown in Fig. 1b the pure
TiO2 showed peaks at 145, 395, 514 and 639 cmꢀ1 attributed to
anatase TiO2.28 While all other composites showed additional
two peaks at 1306 and 1589 cmꢀ1 which are attribute to A1g
phonon pattern and E2g symmetric phonon vibration of the k
point in the graphite-like structure and disordered sp2 micro-
domains enclosed by linking with N atoms, showing the
successful coupling of TiO2 on the g-C3N4 sheet and the results
are in complete agreement with the literature.29,30 The inset of
Fig. 1b shows the Raman spectrum of pure g-C3N4 with D and G
bands located at 1306 and 1589 cmꢀ1 respectively.
The optical properties of the photocatalyst play a crucial role
in deciding its catalytic activity, hence the UV-Vis absorption
spectra of pure TiO2, g-C3N4, Ag/TiO2@g-C3N4 and S/Ag–
TiO2@g-C3N4 were investigated (Fig. S1a†). Pure TiO2 did not
show any absorption in the visible region because of its intrinsic
wide band gap. However, the doped Ag could strongly absorb
visible light due to its SPR effect.31,32 TiO2 nanoparticles doped
with single atom (silver) showed lower shi in comparison to
two atoms (sulphur and silver) (Fig. S1a†). g-C3N4 can absorb
the light in UV and visible region up to 450 nm. However, g-C3N4
light adsorption potential in UV region is lower than TiO2, in
particular at lower light wavelengths (<300 nm). By coupling Ag/
TiO2 and S/Ag–TiO2 with g-C3N4 it showed much higher
absorption intensity than pure g-C3N4 and TiO2 both in the UV
and visible regions. Furthermore, the absorption edge of S/Ag–
TiO2@g-C3N4 had an evident higher red-shi when compare to
g-C3N4 and Ag–TiO2@g-C3N4 (Fig. S1b†). It is suggested that
TiO2 modied with sulphur and silver followed by coupling with
g-C3N4 showed decrease in the band energy, revealed that the
combination of g-C3N4 and S/Ag–TiO2 not only led to an effi-
cient light adsorption performance but also a narrowed band
gap.
Coded levels
Symbol
Variables
ꢀ1
10
5
0.1
0
+1
X1
X2
X3
X4
TS concentration (mg Lꢀ1
pH
)
20
8
0.2
60
30
11
0.3
90
Catalyst weight (g Lꢀ1
Time
)
30
Table 2 Response surface central composite design (CCD) and their
experimental results
Actual values
Coded values
X4 X1 X2 X3
Response
Run X1 X2 X3
X4 Actual Predicted
1
2
3
4
5
6
7
8
20
8
0.2 30
0
0
0
ꢀ1 85.60
87.93
63.90
69.46
86.90
67.12
64.70
61.86
60.64
66.44
60.72
61.92
92.26
61.93
67.25
64.72
68.48
66.93
86.00
71.78
63.64
61.21
88.47
90.74
62.20
10 11 0.1 30 ꢀ1 +1 ꢀ1 ꢀ1 66.30
10
20
5
8
0.3 90 ꢀ1 ꢀ1
+1 +1 67.90
+1
0
0.3 60
0
0
0
+1
0
0
88.70
65.30
20 11 0.2 60
30
30 11 0.1 30 +1
30 11 0.3 30 +1
5
0.1 90 +1 ꢀ1 ꢀ1 +1 66.20
+1 ꢀ1 ꢀ1 63.40
+1
+1 ꢀ1 60.30
9
10
5
0.1 90 ꢀ1 ꢀ1 ꢀ1 +1 66.80
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
30 11 0.1 90 +1
+1 ꢀ1 +1 59.80
10 11 0.3 30 ꢀ1
+1
0
+1
+1 ꢀ1 60.40
92.80
10
8
0.2 60 ꢀ1
0
0
10 11 0.3 90 ꢀ1
+1 +1 62.90
+1 ꢀ1 68.20
+1 ꢀ1 65.90
+1 +1 66.10
10
30
30
10
20
20
30
5
5
5
5
8
5
5
0.3 30 ꢀ1 ꢀ1
0.3 30 +1 ꢀ1
0.3 90 +1 ꢀ1
0.1 30 ꢀ1 ꢀ1 ꢀ1 ꢀ1 66.00
0.1 60
0.2 60
0
0
0
ꢀ1
ꢀ1
0
0
84.20
73.60
0
0.1 30 +1 ꢀ1 ꢀ1 ꢀ1 62.70
10 11 0.1 90 ꢀ1
+1 ꢀ1
+1 60.00
+1 90.80
20
30
8
8
0.2 90
0.2 60 +1
0
0
0
0
0
0
90.20
30 11 0.3 90 +1
+1 +1
+1 63.10
Inhibition rate of E. coli in the presence of TS (X)
¼ (no. of E. coli colonies in control)
ꢀ (no. of E. coli colonies in pure TS)
Inhibition rate of E. coli in the presence of degradation
products (Y) ¼ (no. of E. coli colonies in control)
ꢀ (no. of E. coli colonies in degradation products)
ꢀ
ꢁ
X ꢀ Y
Detoxification efficiency ð%Þ ¼
ꢃ 100
X
The TEM image shown in Fig. 2a indicated that the S/Ag
doped TiO2 nanoparticles were in agglomerated form with
small irregular shape. The presence of Ag, TiO2 and g-C3N4 was
also observed in the TEM images. The high resolution TEM
(HRTEM) image (Fig. 2b) evidently showed the close interface
3. Results and discussion
3.1 Characterization of nanocomposite
Fig. 1a represented the XRD patterns of the prepared pure TiO2, between the 2D akes of g-C3N4 and S/Ag doped TiO2 nano-
g-C3N4,
Ag/TiO2@g-C3N4,
and
S–Ag/TiO2@g-C3N4 particles. The observed lattice spacing value of 0.33 nm was
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Fig. 1 (a) XRD patterns and (b) Raman spectra of the prepared composites; inset of (b) Raman spectra of pure g-C3N4.
ascribed to the (101) plane of anatase phase TiO2 (JPCDS no. 78- and Ti 2p1/2 binding energies (Fig. 3d) were observed at 459.3
2486), and the value of 0.21 nm was assigned to the (111) plane and 464.7 eV respectively.38,39 Fig. 3e represents the C 1s spectra,
of cubic structure silver (JPCDS no. 89-3722). The close inter- in which the peak at 284.7 eV was attributed to the surface
facial contact between doped TiO2 and g-C3N4 is of extreme unessential carbon or sp2 C–C bonds. The peaks at 286.4 eV and
implication for alleviating the photo generated electrons to 288.1 eV attributed to C–O and C–N–C groups of g-C3N4,
transfer from g-C3N4 to S/Ag doped TiO2.33
respectively.33,40 Fig. 3f showed the high resolution XPS spec-
XPS analysis was carried out for S/Ag–TiO2@g-C3N4 in order trum of N, in which peaks at 398.7, 399.6, and 401.2 eV were
to investigate the elemental composition of the prepared cata- corresponded to sp2 hybridized aromatic N bonded to carbon
lyst. Fig. 3a showed the survey spectrum which indicated the atoms (C]N–C), the tertiary N bonded to carbon atoms in the
presence of C, N, Ti, O, Ag, and S elements in the prepared form of N–(C)3, and N–H side groups, respectively.40 The week
catalyst. The high-resolution XPS spectrum of Ag at 3d core peak at 404.6 eV was ascribed to the p-excitations.33,41 From the
levels (Fig. 3b) revealed that the Ag 3d5/2 and Ag 3d3/2 binding XPS results the atomic percentages of Ag, S, Ti, C, O and N
energies were observed at 368.3 and 374.2 eV respectively elements in S/Ag–TiO2@g-C3N4 composite were found to be
indicates the presence of Ag0.34,35 The S 2p spectra (Fig. 3c) 5.75, 2.16, 16.04, 29.29, 34.91 and 11.85 respectively.
showed two peaks at 169.8 eV and 170.2 eV, which could be
ascribed to S 2p3/2 and S 2p1/2, respectively. This could be
3.2 Modeling, optimization and kinetics
emphasized that the S cations in the sample are present in S6+
state which is similar to the previous studies.36,37 The Ti 2p3/2
The photocatalytic degradation of triclosan by S/Ag–TiO2@g-
C3N4 nanocomposite was modeled through RSM framework by
Fig. 2 (a) TEM image and (b) HRTEM image of S/Ag–TiO2@g-C3N4.
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Fig. 3 XPS survey spectrum (a), high-resolution Ag 3d (b), S 2p (c), Ti 2p (d), N 1s (e), and C 1s (f) spectra of S/Ag–TiO2@g-C3N4.
a Central Composite Design. The RSM modeling led to the polynomial expression shown in eqn (1) consisting of 15 coef-
development of a quadratic polynomial model (Table 2) to cients was attained from the analysis of variance (ANOVA) at
ascertain the degradation percentage of TS. A second-order 95% condence level (p < 0.05).
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Table 3 ANOVA results for quadratic modela
Source
Sum of squares
Degree of freedom
Mean square
F-value
p-value
Model
X1
X2
X3
X4
2803.40
3.64
97.53
10.28
1.28
14
1
1
1
1
1
1
1
1
1
1
1
1
1
1
9
23
200.24
3.64
97.53
10.28
1.28
5.29
0.5625
7.29
1.56
4.84
2.40
41.27
1118.53
2.57
13.27
5.97
33.54
0.6105
16.33
1.72
0.2144
0.8860
0.0942
1.22
0.2617
0.8106
0.4024
6.91
187.33
0.4302
2.22
<0.0001
0.4547
0.0029
0.2220
0.6544
0.3711
0.7659
0.2978
0.6213
0.3914
0.5417
0.0274
<0.0001
0.5283
0.1702
X1X2
X1X3
X1X4
X2X3
X2X4
X3X4
5.29
0.5625
7.29
1.56
4.84
2.40
2
X1
X2
X3
X4
41.27
1118.53
2.57
13.27
53.74
2857.13
2
2
2
Residual
Cor total
a
R2 ¼ 0.9812; adjusted R2 ¼ 0.9519; predicted R2 ¼ 0.8683.
the adjusted-R2 value (0.9519) which conrmed the close rela-
tionship between the actual and predicted values of TS degra-
dation percentage suggested by the model (Fig. 4a).
Furthermore, the normal probability plot of the residuals
showed that there is almost no violation of the assumptions:
errors are normally distributed and independent, as the error
variance is homogeneous (Fig. 4b). Moreover, the present model
has an adequate precision due its signal to noise ratio value of
16.36, which is >4.42 Accordingly, the model developed in the
present study could be dependably used to navigate the design
space. The F-values of variables suggested the order of inde-
pendent parameters that inuenced the TS degradation effi-
ciency as follows: pH (X2) > catalyst weight (X3) > TS
concentration (X1) > reaction time (X4).
Y (%) ¼ + 90.49 + 0.4500X1 ꢀ 2.33X2 ꢀ 0.7556X3 + 0.2667X4
ꢀ 0.5750X1X2 + 0.1875X1X3 + 0.6750X1X4 + 0.3125X2X3
2
2
ꢀ 0.5500X2X4 + 0.3875X3X4 ꢀ 4.04X1 ꢀ 21.04X2
2
2
+ 1.01X3 ꢀ 2.29X4
(1)
where Y represents the degradation efficiency (%) and X1, X2, X3
and X4 indicated the uncoded values of TS concentration, pH,
catalyst weight and time respectively.
The analysis of variance (ANOVA) results incurred for the
present model (Table 3) suggested the quadratic model which is
signicant because of its high F-value (33.54) and very low p-
value (<0.0001). Furthermore, it is also indicated that the lack-of
t was not signicant relative to the pure error. The correlation
coefficient-R2 value (0.9812) was in reasonable agreement with
Fig. 4 (a) The relationship of predicted and actual values of the RSM model; (b) the internally studentized residuals versus normal % probability
distribution.
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Fig. 5 Contours and 3D surface plots of TS degradation efficiency (%) for (a and b) catalyst weight vs. pH, (c and d) TS concentration vs. pH and (e
and f) time vs. pH, respectively.
points among the utmost contour levels in each of the
showed gures indicate the corresponding parameters
optimum value. Fig. 5(a and b) indicated that the TS degra-
dation efficiency was gradually increased with the increase in
pH from 5–8 regardless TS concentration at low or high level.
The information about the surface properties and inuence
of solution pH on photocatalytic degradation by S/Ag–
TiO2@g-C3N4 nanocomposite has not been available in the
literature; it is hard to understand the pH role on TS degra-
dation by S/Ag–TiO2@g-C3N4. The triclosan can be trans-
formed to triclosan anion in the basic pH (pH > 8) since the
pKa value of triclosan is between 7.6 and 8.1. Hence, the tri-
closan adsorption was less in basic and acidic pH due to the
repulsive force works between triclosan and TiO2 catalyst in
pH higher than 8. Thus at pH 7.5 the maximum degradation
was achieved. The reactant/substrate quantity adsorbed on
the catalyst surface is an important parameter since only this
Fig. 6 TS degradation efficiency using the different prepared catalysts
amount imparts to photocatalytic process apart from the
whole solution. This adsorption quantity depends on the
initial concentration. The TS degradation efficiency was
under the optimized conditions (catalyst weight ¼ 0.20 g Lꢀ1, pH ¼ 7.8,
and TS concentration ¼ 10 mg Lꢀ1).
decreased while increasing in concentration further to its
optimum value (Fig. 5c and d). This could be justied that
when the concentration of TS increases, more number of TS
molecules are adsorbed on catalyst surface, while less
number of photons are accessible to reach the catalyst
surface which further leads to lower amounts of active
species generation, thus leading to lower degradation effi-
ciency. Meanwhile, the TS degradation efficiency increased
The response surfaces could be visualized as contours or/
and 3D plots to constitute the variations in the response with
respect to two variables, by keeping the other variable xed.43
Fig. 5 shows the contours and 3D surface response plots for
TS degradation efficiency (%) as a function of (a and b)
catalyst weight vs. pH, (c and d) TS concentration vs. pH and
(e and f) time vs. pH, respectively. The central coordinate
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Fig. 7 Plausible degradation pathway of triclosan by S/Ag–TiO2@g-C3N4 under the optimized conditions.
Table
4 Detoxification efficiency of the TS and its degradation
optimal conditions were practical. Thus, it could be
concluded that the system to optimize the degradation
conditions using RSM for the photocatalytic degradation of
TS in this study was successful.
Furthermore, LC-ESI/MS analysis was performed in order
to investigate the plausible degradation pathway of TS
during visible light irradiation by S/Ag–TiO2@g-C3N4 under
the optimized conditions. LC-MS analysis of the TS
degraded samples evidenced the presence of compounds
with molecular weights 319.3, 287.9, 255.2, 164.1, 147.0,
144.6 and 110.2 which could be interpreted as (M+), (M+2),
(M+), (M+), (M+), (M+), and (M+) peaks of structure A, B, C,
D, E, F and G respectively (Fig. 7 and S3†).
products by using the prepared ternary composite, inoculum (E. coli) ¼
50 mL; TS concentration ¼ 20 mg Lꢀ1
Irradiation
time
Detoxication
efficiency (%)
System
CFU
Control
Untreated TS
Degradation products
—
—
1326
0
—
0
60 min
852 ꢄ 53
64.3 ꢄ 3.9
with the increase in catalyst weight up to an optimum value
(0.20 g Lꢀ1) followed by decrease in the efficiency. The
increase in catalyst weight higher to the optimum value could
decrease the light penetration, by shielding effect and
increases the agglomeration and sedimentation of nano-
particles which lead to decrease in the availability of active
sites to the organic molecules.44
A verifying experiment was performed in order to check
the optimum combination of the key operating parameters
for degradation efficiency of TS under optimized conditions
3.3 Biotoxicity assessment
The toxicity tests could be based on animal, plant and bacterial
bioassays. In general the bacterial bioassay methods includes
bioluminescence and metabolic (i.e. population growth, respi-
ration) inhibition assay. The various species includes Vibrio
suggested by RSM model. Compared to the other prepared scheri/Photobacterium phosphoreum, Vibrio harveyi and Pseu-
composites, the highest TS degradation efficiency (92.3%) domonas uorescens were used for bioluminescence inhibition
was achieved under optimised conditions by S/Ag–TiO2@g-
C3N4 nanocomposite (Fig. 6 and S2†), indicating that the
assay, while E. coli and Pseudomonas putida were used for
metabolic inhibition assay. Many researchers chosen E. coli
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Fig. 8 ESR signals of $OH (a) and O2cꢀ (b) at ambient temperature.
Our results indicated that during visible light irradiation the
toxicity and antibacterial activity of the pure TS has been
dramatically reduced to provide safety release of the solution
into the water bodies in order to safe guard the several
organisms in the ecosystem.
3.4 Photocatalytic degradation mechanism
The ESR technique was utilized to investigate the photocatalytic
reaction mechanism and to analyze the active species partici-
pated in photodegradation of TS and over S/Ag–TiO2@g-C3N4
under visible light. The signals corresponded to DMPO–$OH
and DMPO–O2cꢀ species could be substantiated from the
standard spectra of $OH (AN ¼ 14.1 G and AH ¼ 14.3 G; g ¼
ꢀ
2.0132) and $O2 (AN ¼ 13.3 G and AH ¼ 11.3 G; g ¼ 2.0132)
which was simulated by Bruker WinEPR SimFonia programmꢀe
(Fig. 8a and b).47–49 From the results it conrmed that $OH, O2c
and photogenerated holes were the major species involved in
photocatalytic process.
Fig. 9 Photocatalytic degradation mechanism of S/Ag–TiO2@g-C3N4
nanocomposite under visible light.
Based on the ESR results, the enhanced photocatalytic
degradation mechanism of S/Ag–TiO2@g-C3N4 under visible
light was explained by Fig. 9. The CB and VB potentials of
TiO2 are around ꢀ0.50 and 2.71 eV,50,51 while potentials of g-
C3N4 are around ꢀ1.11 and 1.65 eV respectively.52,53 The CB
potential of g-C3N4 (ꢀ1.12 eV) is more negative than TiO2
(ꢀ0.5 eV), hence the photogenerated electrons initially
transferred from VB of g-C3N4 to CB, later could easily
transfer from CB of g-C3N4 to CB of TiO2. Meanwhile, TiO2
absorb the UV light and transfer the electrons to CB from its
VB. The Ag Fermi levels are lower than the CB of TiO2.54 Thus,
the electrons on the TiO2 CB could further transfer to the
surface of silver nanoparticles. Furthermore, the doped
sulfur leads to the formation of mid-gap energy levels such as
S 2p between the valence band of O 2p and conduction band
of Ti 3d.55 This further reduces the TiO2 band gap. The
enhanced visible light activity is attributed to the formation
strains because of their ability to easily respond with high
sensitivity to different toxic compounds.45,46
In our study, according to rate of E. coli growth the pure TS
showed 100% toxicity towards the bacteria due to its strong
antibacterial activity. Aer 60 min of visible light irradiation,
the mineralization products of TS showed 64.3 ꢄ 3.9%
(average data) of detoxication efficiency (Table 4 and S4†).
Our study was according to the rate of E. coli growth (meta-
bolic inhibition) in which the colony forming unit (CFU)
assay measures the ability of E. coli to multiply. Thus, it
represents and evaluates the reproductive power or condition
of bacterial cells under normal physiological conditions or
being exposed to other chemical compounds. The toxic
products interfere with bacterial metabolism that involved in
DNA replication, cell division and growth. Thus, the study
gives a reliable measure of biotoxicity to the test chemicals.
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of hetero-junction electric eld among the g-C3N4/TiO2. The
transfers of electrons from g-C3N4 CB to TiO2 CB and further
from CB of TiO2 to silver was greatly suppressed the recom-
bination of electron–hole pairs. These photogenerated elec-
trons further react with adsorbed O2 on the catalyst surface to
form O2cꢀ radicals which further combine with H2O to
transforms to $OH radicals. In the meantime, the holes in g-
C3N4 VB was less positive than $OH/H2O redox potential
(2.68 eV vs. SHE),56 indicating that the holes on g-C3N4 may
not able to oxidize H2O into $OH radicals, while they directly
participate in the degradation. Meanwhile, holes on TiO2 VB
are more positive than $OH/H2O redox potential which could
react with H2O to generate highly reactive $OH radicals.
These O2cꢀ, $OH, h+ and eꢀ species are highly reactive
towards TS degradation to generate the degradation prod-
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Conflicts of interest
There are no conicts to declare.
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Acknowledgements
This study was nancially supported by the Major Science and
Technology Program for Water Pollution Control and Treat-
ment (2018ZX07208010-01), Six Talent Peaks Project in Jiangsu
Province, and 333 Talent Project Foundation of Jiangsu
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