Imidacloprid photo-degradation on Ag/AgBr modified TiO2: critical impacts and quantitative study on mechanism

Article abstract of DOI:10.1007/s11164-019-03940-y

To improve the visible-light catalytic performance of TiO₂ for practical application, Ag/AgBr?modified TiO₂ was fabricated in ionic liquid–water medium via a facile hydrothermal reduction method. Imidacloprid (IMI) was employed as the target pollutant to evaluate the critical impacts on the catalytic activity of as-prepared Ag/AgBr/TiO₂, where the factors relating to the catalyst (composition, dosage, recycle and reuse) and solution chemistry (pH and concentration, simulated wastewater samples and floodwater of rice paddy field) were studied. The results suggested that benefitting from visible-light sensitive Ag/AgBr and high specific surface area TiO₂, Ag/AgBr/TiO₂ exhibits superior visible-light photocatalytic activity and stability to Ag/AgBr and TiO₂. The quenching tests with various scavengers indicate that ^·O₂ ^? and h⁺, and ^·O₂ ^? and ^·OH may be the key roles in IMI photo-degradation on Ag/AgBr and on Ag/AgBr/TiO₂, respectively. Thus, the photocatalytic performance of Ag/AgBr/TiO₂ differs from that of Ag/AgBr. Furthermore, the contribution of such active species is clarified for quantitative mechanisms study. The photo-degradation conducted in the simulated wastewater and the floodwater of rice paddy field proves favorable anti-interference capacity of Ag/AgBr/TiO₂. The tests conducted on real wastewater sample verified that Ag/AgBr/TiO₂ can mineralize IMI promptly and almost completely.

Full text of DOI:10.1007/s11164-019-03940-y

Research on Chemical Intermediates
https://doi.org/10.1007/s11164-019-03940-y
Imidacloprid photo‑degradation on Ag/AgBr modifed
TiO2: critical impacts and quantitative study on mechanism
Jialin Sun1 · Jingtao Dai1 · Rui Liu2 · Xiaowei Yang1 · Jian Chen1
Received: 21 April 2019 / Accepted: 24 July 2019
© Springer Nature B.V. 2019
Abstract
To improve the visible-light catalytic performance of TiO2 for practical application,
Ag/AgBr modifed TiO2 was fabricated in ionic liquid–water medium via a facile
hydrothermal reduction method. Imidacloprid (IMI) was employed as the target pol-
lutant to evaluate the critical impacts on the catalytic activity of as-prepared Ag/
AgBr/TiO2, where the factors relating to the catalyst (composition, dosage, recy-
cle and reuse) and solution chemistry (pH and concentration, simulated wastewater
samples and foodwater of rice paddy feld) were studied. The results suggested that
beneftting from visible-light sensitive Ag/AgBr and high specifc surface area TiO2,
Ag/AgBr/TiO2 exhibits superior visible-light photocatalytic activity and stability to
·
−
Ag/AgBr and TiO2. The quenching tests with various scavengers indicate that O2
·
−
·
and h+, and O2 and OH may be the key roles in IMI photo-degradation on Ag/
AgBr and on Ag/AgBr/TiO2, respectively. Thus, the photocatalytic performance of
Ag/AgBr/TiO2 difers from that of Ag/AgBr. Furthermore, the contribution of such
active species is clarifed for quantitative mechanisms study. The photo-degradation
conducted in the simulated wastewater and the foodwater of rice paddy feld proves
favorable anti-interference capacity of Ag/AgBr/TiO2. The tests conducted on real
wastewater sample verifed that Ag/AgBr/TiO2 can mineralize IMI promptly and
almost completely.
Keywords Ag/AgBr/TiO2 · Imidacloprid · Visible-light photocatalysis · Wastewater
Electronic supplementary material The online version of this article (https://doi.org/10.1007/s1116
4-019-03940-y) contains supplementary material, which is available to authorized users.
*
Jian Chen
xchenjian@163.com
1
Institute of New Energy on Chemical Storage and Power Sources, Institute of Environmental
Toxicology and Environmental Ecology, College of Chemistry and Environmental
Engineering, Yancheng Teachers University, 2 Xiwang South Road, Yancheng 224007, Jiangsu,
People’s Republic of China
2
Key Laboratory of Pollution Ecology and Environmental Engineering, Institute of Applied
Ecology, Chinese Academy of Sciences, 72 Wenhua Road, Shenyang 110016, Liaoning,
People’s Republic of China
Vol.:(0123456789)
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J. Sun et al.
Introduction
To enhance agricultural production, a wide variety and large quantity of pesti-
cides have been registered and marketed for pest control around the world [1].
However, less than 1% of the applied pesticide reaches its biological target under
typical feld applications; the remaining part is released into the soil, the air and
eventually the water [2]. Thus, the pesticides constitute one of the most abundant
organic pollutants in aqueous environment [3]. Among these pesticides, imidaclo-
prid frequently detected in surface water and groundwater has been recognized as
a serious environmental issue [4].
Imidacloprid [IMI, the chemical structure is in Fig. S1 of Electronic Supple-
mentary Material (ESM)], as a class of broad-spectrum insecticide, works as a
block on the postsynaptic nicotinic acetylcholine receptors, causes overstimula-
tion on motor neurones, and ultimately kills the target insects [5]. Besides the
toxicity to invertebrates, IMI can also block the acetylcholine receptors in verte-
brates’ motor neurons [4]. Accordingly, IMI might be toxic to both invertebrates
and vertebrates. The inference has been verifed that IMI and its degradation
products impose great risk on ecosystems and human health [1, 3, 4, 6–12]. The
situation is more aggravated in the developing countries like India, Pakistan and
China [8], where the IMI production and application are increasing vigorously.
Thus, it is an urgent need to develop efcient and cost-efective methods to
remove IMI and its degradation products from the aqueous environment. How-
ever, early researches proved that the conventional strategies are severely
restricted by the high mobility and persistency of IMI [4, 5]. More recently, the
advanced oxidation processes have attracted explosive attention [1, 5, 8]. Herein,
photocatalysis is a promising strategy in organic pollutant remediation [5, 13].
Encouraging studies have been conducted in IMI photocatalytic degradation,
where various photo-catalysts were employed in these studies [2, 5, 7, 8, 14–19].
Among these photo-catalysts, TiO2 is a robust candidate [20], and pilot-scale
tests have been launched to evaluate photocatalytic performance of TiO2 [18, 19].
However, previous eforts revealed that TiO2 is subjected to weak visible-light
photocatalytic activity and the recombination of photo-generated electron–hole
pairs [21]. Therefore, it is of great signifcance to improve the visible-light photo-
catalytic performance of TiO2 [22].
For this purpose, various strategies have been explored in TiO2 modifcation [2,
7, 17, 23]. Among these strategies, indirect band-gap in AgBr (direct band-gap:
4.29 eV and indirect band-gap: 2.69 eV) may retard the electron–hole recombination
and provide more powerful photocatalytic activity [24–27]. Furthermore, the plas-
monic Ag located over AgBr can efectively conduct and separate photo-induced
carriers [24, 28–33]. These works mainly focus on photo-catalyst preparation, char-
acterization, photocatalytic performances and qualitative research in mechanisms
[24, 25, 30, 31, 34–36]. However, besides these promising improvements achieved,
few studies were conducted to investigate insecticide photo-degradation on Ag/AgBr
(plasmonic Ag over AgBr) decorated TiO2, let alone the critical impacts on pesticide
photo-degradation and quantitative study on photocatalytic mechanisms.
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Imidacloprid photo‑degradation on Ag/AgBr modifed TiO2:…
Motivated by these remarkable works above and the gap between laboratory
study and practical necessity, we fabricated Ag/AgBr decorated TiO2 employing
a facile one-pot hydrothermal reduction method. The photocatalytic performance
of as-fabricated catalysts toward representative insecticide, IMI, was studied in
the medium of aqueous solution, simulated wastewater samples and foodwater of
rice paddy feld. The major objectives of the current study are to: (1) evaluate the
critical impacts relating to the catalyst (composition, dosage, recycle and reuse)
and solution chemistry (pH and concentration of IMI aqueous solution, simulated
wastewater samples and foodwater of rice paddy feld) on the catalytic activity of
as-prepared Ag/AgBr/TiO2; (2) explore the mechanism underlying the photocata-
lytic performance and quantify the contribution of the active species driving IMI
photo-degradation.
Experimental
Materials
All analytical chemicals were commercially available and used without any further
purifcation: AgNO3 (purity≥99%), anhydrous pyridine (purity 99.8%), absolute
ethanol (purity≥99.8%), IMI (purity 98.5%), brominated n-butane (purity≥99.7%)
and p-Benzoquinone (purity 99%) were purchased from Aladdin chemistry Co., Ltd.
(Shanghai, China); the other reagents were purchased from Sinopharm Chemical
Reagent Co., Ltd. (Shanghai, China). All ultrapure water was freshly obtained from
a Milli-Q system (18.2 MΩ/cm, Millipore Corp., USA).
Synthesis of ionic liquid
The [Bpy]Br ionic liquid (IL) was synthesized by the method described elsewhere
[37]. In brief, accurately weighed pyridine (0.1 mol, 7.91 g) and n-butyl bromide
(0.1 mol, 13.8 g) were stirred and refuxed at 80 °C for 2 h. After the reaction system
cooled to room temperature, the white solid, [Bpy]Br IL, was obtained [38] and sub-
jected to the analysis of nuclear magnetic resonance (NMR) and Fourier translation
infrared spectroscopy (FT-IR). The diagrammatic drawing of the reaction and char-
acterization data of [Bpy]Br IL is provided in Scheme. S1 (EMS).
Synthesis of photo‑catalyst
Typically, 10 mL 0.1 M of AgNO3 solution was well dispersed into the IL-H2O
system (IL-H2O, 20 g-20 g), and the mixture was referred to as dispersion A
(AgBr precursor). Tetrabutyl titanate (0.01 mol) was well dispersed into another
IL-H2O system, and the mixture was referred to as dispersion B (TiO2 precur-
sor). The dispersion A and B were mixed with given volume ratio in a fask and
subjected to a vigorous stirring for 20 min [38–40]. After that, the mixture was
heated at 120 °C for 12 h and then air-cooled to room temperature [41]. The
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J. Sun et al.
product was washed with ultrapure water and ethanol for 2-3 times and dried
at 60 °C for 2 h [30]. The powder obtained was dispersed into methyl orange
solution (10 mg/L), irradiated under a 300 W Xe arc lamp with a UV-cut flter
(λ > 420 nm) (PLS-SXE 300, Beijing Perfectlight Co., Ltd.) for 5 min, then col-
lected by centrifugation, and washed with ultrapure water, then dried at 60 °C
under vacuum for 8 h. The as-prepared catalyst is the fnal product, Ag/AgBr/
TiO2. In addition, Ag/AgBr or pure TiO2 was prepared through a similar way,
using only the dispersion A or B, separately.
Catalyst characterization
The crystallographic structures of the samples were determined using X-ray dif-
fraction spectroscopy (XRD, Ultima-IV, Rigaku) equipped with Cu Kα radiation
source, at a scan rate of 8°/min from 10° to 70° (2θ). The morphology and micro-
structure of the as-prepared catalysts were characterized using S-4800 feld emis-
sion scanning electron microscope (SEM, S-4800II FESEM, Hitachi, Japan), with
an accelerating voltage of 20 kV. X-ray photoelectron spectroscopy (XPS) was
obtained on a PHI5300 (PHI-5300, PE, USA) to identify the surface elements
and chemical valences. The textural properties of the catalysts were investigated
by N2 adsorption–desorption method executed on a Micromeritics ASAP 2400
(Micromeritics Ltd., USA). Ultraviolet–visible spectroscopy difuse refectance
spectra (UV–Vis DRS) of the catalysts were obtained on a PerkinElmer UV–Vis
spectrophotometer (PerkinElmer Corporation, America). Inductively coupled
plasma optical emission spectroscopy (ICP-OES; PerkinElmer, Optima 8000DV,
America) and ion chromatograph (IC; ICS-5000+, Thermo Fisher, America) were
adopted to assist Ag and Br elemental detection in catalysts or solutions.
Degradation procedure
The photocatalytic experiment was carried out on a PCX50A Discover multichan-
nel photocatalytic reaction system (Beijing Perfectlight Science and Technology
Co., Ltd., Beijing, China). A light-emitting diode was used to simulate solar light
irradiation (5 W, its emission spectrum is presented in Fig. S2, EMS). In a typical
experiment, 40 mL of 10 mg/L IMI solution and 1.0 g/L of catalyst were mixed
well in the reactor in the dark for 30 min to establish the adsorption–desorption
equilibrium between the solution and the catalyst before the photocatalytic test.
The pre-equilibrated mixture was stirred magnetically and sampled at defnite
time intervals, during the photocatalytic test. After the catalyst was separated
from the sample by centrifugation, IMI concentration in the residue was analyzed
with a high-performance liquid chromatography (HPLC) (Agilent 1200 HPLC,
Agilent Ltd., Santa Clara, CA, USA) at the detection wavelength of 269 nm. The
photocatalytic degradation rate (DR, %) was calculated based on the concentra-
tion variation.
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Imidacloprid photo‑degradation on Ag/AgBr modifed TiO2:…
Tests conducted on real wastewater samples
The real wastewater samples were the foodwater of a rice paddy feld (N33.29°,
E120.41°). Before test, the samples were fltrated through quantitative flter paper
and 0.45 μm flter membrane in sequence to remove the impurities, and then 4 L
such fltrate was mixed with 1 L 50 mg/L IMI solution to obtain a wastewater sam-
ple. The chemical oxygen demand (COD) measurements were executed by the
dichromate titration method on the aqueous residue of wastewater samples [42]. The
total organic carbon (TOC) was measured on a vario TOC cube (Elementar, Ger-
many) [42].
Results and discussion
Morphology and crystal structure
The SEM images of Ag/AgBr, TiO2 and Ag/AgBr/TiO2 are shown in Fig. 1. The
Ag/AgBr briquettes consist of spherical Ag/AgBr particles with a diameter about
50 nm (Fig. 1a). The diameter of TiO2 particles is in the range of 1–2 μm (Fig. 1b).
Compared with pure TiO2, Ag/AgBr/TiO2 microsphere particles are rougher, with
tiny Ag/AgBr particles over the surface of TiO2 (Fig. 1c).
The powder XRD patterns are recorded in Fig. 2 to study the crystal phases of as-
prepared catalysts. For Ag/AgBr, the distinct difraction peaks at 2θ of 26.8°, 31.0°,
44.4°, 55.1°, 64.5° and 73.3° correspond to (111), (200), (220), (222), (400) and
(420) crystal faces of AgBr [21, 26, 32, 43]; the weak difraction peaks centered at
38.2°, 64.5° and 77.9° can be indexed to (111), (220) and (311) crystal faces of Ag
[36, 44]. Thus, it is reasonable to deduce that AgBr has been partially converted to
metallic Ag (Ag0) by treating with methyl orange solution during the preparation
process. As for TiO2, XRD patterns are well matched with the standard cards of
Anatase TiO2 (PDF#06-0438). For Ag/AgBr/TiO2, the blurred baseline indicates a
lower crystallinity of the composite. The difraction peaks at (004), (105), (211) and
(204) coincide well with the standard anatase [21, 45, 46]. In addition, typical dif-
fraction peaks of AgBr were detected in the Ag/AgBr/TiO2 composite.
The as-prepared Ag/AgBr/TiO2 sample was further analyzed by XPS to inves-
tigate the surface compositions and bonding states. The binding energies were
Fig. 1 SEM images of Ag/AgBr (a), TiO2 (b) and Ag/AgBr/TiO2 (c)
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J. Sun et al.
Fig. 2 XRD patterns of as-prepared Ag/AgBr, TiO2 and Ag/AgBr/TiO2
calibrated using C1s photoelectron peak (284.6 eV). As presented in Fig. 3a, besides
the C1s signal, peaks of Ag 3d, Br 3d, Ti 2p, and O 1s were observed in the full
survey of XPS spectra. In Fig. 3b, two peaks at 367.6 and 373.6 eV correspond to
the binding energies of Ag 3d5/2 and Ag 3d3/2, respectively [24, 28, 31, 47]. The two
bands can be deconvoluted into group 1 at: 367.61 and 373.62 eV, and group 2 at:
368.3, 374.2 eV. The former is assigned to the Ag(I) of Ag/AgBr, and the latter is
attributed to Ag0. Based on the integrals of the corresponding peak areas from the
deconvolution, the molar ratio of Ag0 to Ag(I) in the sample is about 1:12; further-
more, initial molar ratio of AgNO3 and tetrabutyl titanate is about 1:10 during Ag/
AgBr/TiO2 preparation process. Such a low content may be the major reason for the
undetected Ag0 difraction peak in XRD analysis. In Fig. 3c, two individual peaks
at 458.8 eV and 464.5 eV are attributed to the binding energies of Ti 2p3/2 and Ti
2p1/2, respectively [28, 31]. In Fig. 3d, two overlapped peaks at 68.8 eV and 67.9 eV
are assigned to Br 3d5/2 and Br 3d3/2, respectively [28, 47]. The results from XRD
and XPS analysis indicate that Ag/AgBr has been successfully introduced over the
surface of TiO2.
The specifc surface area, pore radius and pore specifc volumes of as-prepared
catalysts were determined by N2 adsorption–desorption experiments. The corre-
sponding data are provided in Fig. 4 and Table 1, where the specifc surface area
of TiO2 is 204.78 m2/g, which is about twice of the value obtained from the sur-
factant–ethanol system in solvothermal method [31]. While the specifc surface area
of Ag/AgBr is only 20.29 m2/g, for it is compacted in briquettes [48], as depicted in
Fig. 1a. The specifc surface area of Ag/AgBr/TiO2 is about 61.45 m2/g and lower
than the value reported by Velmurugan [35], probably because of the larger grain
size (micrometer-scale) of the Ag/AgBr/TiO2 prepared in this study. However, the
increase in the specifc surface area from 20.29 m2/g (Ag/AgBr) to 61.45 m2/g (Ag/
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Imidacloprid photo‑degradation on Ag/AgBr modifed TiO2:…
Fig. 3 XPS spectra of as-prepared Ag/AgBr/TiO2
Fig. 4 Textural properties of Ag/AgBr (a), TiO2 (b) and Ag/AgBr/TiO2 (c)
Table 1 Data from N2
Catalyst
Specifc
surface area
(m2/g)
Pore radius (Å) Pore specifc
adsorption–desorption
experiments for as-prepared
catalysts
volume
(cm3/g)
TiO2
Ag/AgBr
Ag/AgBr/TiO2
204.78
20.29
61.45
25.308
25.290
45.109
0.262
0.046
0.091
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J. Sun et al.
AgBr/TiO2) indicates that TiO2 acts as a proper supporter and efectively promotes
the dispersion of Ag/AgBr.
To investigate the optical response of the as-prepared catalysts, the UV–Vis DRS
was recorded over wavelength range of 200–800 nm. As shown in Fig. 5a, TiO2
exhibits strong absorption band throughout the UV region, but negligible optical
response in the visible-light region. As an indirect band-gap semiconductor rein-
forced by Ag0, Ag/AgBr exhibits responses over both UV and visible-light regions.
Comparing with TiO2, the absorption band edge of Ag/AgBr/TiO2 is red-shifted,
distinctly. In addition, the band-gap value is calculated from (αhv)1/2−(hv) and pro-
vided in Fig. 5b. The result shows that the band-gap energies of as-prepared Ag/
AgBr, Ag/AgBr/TiO2 and TiO2 are 2.42 eV, 2.73 and 3.20 eV, respectively. The
band-gap of TiO2 matches well with the values reported elsewhere [49], while that
of Ag/AgBr is remarkably lower than the literature value, due to the efect of local-
ized surface plasmon resonance (LSPR) from Ag0 [32, 36]. Further, it is reasonable
to conclude that visible-light response is readily induced for Ag/AgBr/TiO2 by com-
bining Ag/AgBr with TiO2.
Efect from molar ratio of AgBr/TiO2
A series of composite catalysts were achieved with diferent AgBr (or Ag/AgBr)
and TiO2 molar ratio by varying the volume ratio of dispersion A and B during the
preparation. The photocatalytic activity of these composite catalysts was evaluated
through the batch experiments settled under the conditions as: IMI concentration,
15 mg/L; catalyst dosage, 1.0 g/L and pH of IMI solutions, 7.0. As shown in Fig. 6a,
DR increases with raised AgBr (or Ag/AgBr) and TiO2 molar ratio beneath 10%,
and then decreases gradually with the rising molar ratio. The results indicated that
enhancing the molar ratio reinforces the photocatalytic activity of the composite
catalysts, efectively. But excessive AgBr (or Ag/AgBr) dispersed over TiO2 surface
might motivate the recombination of the photo-generated electron–hole pairs and
further depress the photocatalytic activity. Thus, the AgBr (or Ag/AgBr) and TiO2
molar ratio were settled at 10% in the following tests.
Fig. 5 UV–Vis DRS (a) and the Tauc plots of as-prepared catalysts (b)
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Imidacloprid photo‑degradation on Ag/AgBr modifed TiO2:…
Fig. 6 Efect from molar ratio of AgBr/TiO2 (a), catalyst dosage (b), solution pH (c) and initial concen-
tration (d) on IMI photo-degradation
Efect from catalyst dosage
To evaluate the efect of catalyst dosage, the experiments were carried out with the
IMI concentration fxed at 15 mg/L and pH of IMI solutions settled at 7.0. As shown
in Fig. 6b, DR increases with the raised photo-catalyst dosage from 0.5 to 1.5 g/L.
The maximum DR is achieved at 1.5 g/L, higher dosage decreases the DR. This
trend was confrmed on TiO2, Ag/AgBr and Ag/AgBr/TiO2. Reasonable explanation
for this trend is that excessive catalyst may block the light transmission and intensify
light scattering in IMI solution [35]. In addition, the comparison suggested distinctly
that Ag/AgBr/TiO2 exhibits superior photo-degradation activity to Ag/AgBr and
TiO2.
Efect of pH on IMI photo‑degradation
The pH value is a signifcant and intricate role in IMI photo-degradation [4, 50]. So
the solutions with varying initial pH value, with the catalyst dosage fxed at 1.5 g/L
and IMI concentration fxed at 15 mg/L were chosen to inquire the efect of pH on
IMI photo-degradation under visible-light irradiation. It is obvious in Fig. 6b, c that
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J. Sun et al.
the DRs obtained on TiO2 are far beneath that on Ag/AgBr and Ag/AgBr/TiO2.
Thus, TiO2 was eliminated from the following tests. In Fig. 6c, the DR acquired
from Ag/AgBr/TiO2 is higher than that from Ag/AgBr through the pH series, espe-
cially in alkaline pH range. And the DR obtained from Ag/AgBr decreases over the
elevated pH value, which is coincident with studies reported elsewhere [7, 18, 51],
while the DR acquired from Ag/AgBr/TiO2 increases with raised pH value. The
distinction in the DR trend over the pH series suggests the diference among IMI
photo-degradation mechanisms over Ag/AgBr and Ag/AgBr/TiO2.
Efect from initial IMI concentration
To investigate the efect of initial IMI concentration, the experiments were con-
ducted with a fxed catalyst dosage at 1.5 g/L and settled solutions pH at 7.0. As
revealed in Fig. 6d, the DR on Ag/AgBr/TiO2 increases with initial IMI concen-
tration beneath 15 mg/L. This signifcative phenomenon suggested an autocatalytic
process during IMI photo-degradation over Ag/AgBr/TiO2 [3], while the trend is
reversed by raised initial IMI concentration above 15 mg/L. Possible reason is that
besides the increasing mass of IMI to be photo-degraded, more IMI molecules or
intermediates block the active sites of catalysts and depress the generation of active
species. For the same reason, the DR obtained from Ag/AgBr decreases with the
raised IMI concentration from 5 to 30 mg/L. Furthermore, the photocatalytic rate
constants obtained from the kinetic ftting were also presented as inset.
Recycle and reuse tests of catalysts
As an important parameter, the stability is essential to a photo-catalyst in its practi-
cal application. The stability of as-prepared Ag/AgBr and Ag/AgBr/TiO2 was inves-
tigated by cycle degradation experiments settled in: catalyst dosage, 1.5 g/L; pH
of IMI solutions, 7.0; IMI concentration, 15 mg/L. As shown in Fig. 7, after fve
cycles, the DR obtained on Ag/AgBr declines from 93.4 to 85.0% and that on Ag/
AgBr/TiO2 declines from 94.0 to 89.5%. Calculated relative maintenance of DR on
Ag/AgBr and Ag/AgBr/TiO2 is about 91.0% and 95.2%, respectively. Based on the
maintenance of DR, comparison with similar catalysts elsewhere indicated that the
particle size regulated into micrometer-scale in this study might favor the recycle
and reuse performance of Ag/AgBr/TiO2 [8, 23, 24].
The recycled Ag/AgBr and Ag/AgBr/TiO2 after degradation experiments were
collected for ICP-OES, IC and XRD analysis. The molar ratios (nAg: nBr, average
of three replicates) obtained from ICP-OES and IC analysis conducted on raw and
recycled catalysts are listed in Table S1 (EMS). The XRD patterns for recycled cata-
lysts are also recorded in Fig. S3 (EMS). There is no signifcant diference in molar
ratios and XRD pattern recorded on raw and recycled catalysts. The results demon-
strate that both Ag/AgBr and Ag/AgBr/TiO2 are fairly stable and reusable in poten-
tial application.
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Imidacloprid photo‑degradation on Ag/AgBr modifed TiO2:…
Fig. 7 Efect from cycle number on IMI photo-degradation
Impacts from coexisting ions in simulated wastewater
To characterize the catalytic activity and anti-interference of Ag/AgBr/TiO2
beyond high-concentration inorganic ions, IMI photo-degradation was executed
in the simulated wastewater prepared with high salinity: catalyst dosage, 1.5 g/L;
pH of IMI solutions, 7.0; IMI concentration, 15 mg/L and inorganic salt con-
centration, 5 mM, which is signifcantly higher than the mean value of common
concentration in surface and ground water. Besides, batch experiments were con-
ducted under paralleled conditions, but with lower inorganic salts concentration
of 0.5 mM, and the results are presented in Fig. S4 (EMS). In Fig. 8a, b, distinct
restraint is observed from 5 mM inorganic salts against IMI photo-degradation,
comparing with 0.5 mM ones. Among the cations, Ca2+ and Mg2+ moderately
depress the DR from 84.6 to about 75%, while K+ and Na+ exhibit a negligi-
2−
ble inhabitation. Similar comparison was executed for anions, where the SO4
and HCO3 distinctly inhibit IMI photo-degradation, especially for SO42−, due
−
−
to their competitive photo-absorption against IMI [52]. However, NO3 acts as
a photosensitizer reinforcing IMI photo-degradation [53]. Thus, before potential
application, the impacts from coexisting ions on IMI photo-degradation need to
be evaluated, especially in high-salinity cases, since the coexisting ions might
impact IMI photo-degradation depending on their species and nature.
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J. Sun et al.
Fig. 8 Efect from cations (a) and anions (b) on IMI photo-degradation
Tests conducted on real wastewater samples
The variations of COD, TOC and IMI concentration were measured during the
photocatalytic degradation experiments (catalyst dosage, 1.5 g/L), which were
conducted on foodwater of a rice paddy feld. As it is presented in Fig. 9a, b,
the increase trends of COD, TOC and IMI removal rates acquired on Ag/AgBr
resemble that on Ag/AgBr/TiO2, but the values obtained on Ag/AgBr are beneath
the corresponding values obtained on Ag/AgBr/TiO2.
Furthermore, comparing with the results presented in Fig. 6c (IMI concentra-
tion, 10 mg/L). The relative maintenance of DR is 80.0% and 86.7% for Ag/AgBr
and Ag/AgBr/TiO2, respectively, which suggested that Ag/AgBr and Ag/AgBr/
TiO2 both exhibit favorable anti-interference capacity beyond the foodwater from
rice paddy feld, and Ag/AgBr/TiO2 performs better than Ag/AgBr in the dimen-
sion of anti-interference.
Fig. 9 COD, TOC and IMI removal in real wastewater by Ag/AgBr (a) and Ag/AgBr/TiO2 (b)
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Imidacloprid photo‑degradation on Ag/AgBr modifed TiO2:…
The inconspicuous diference among COD, TOC and IMI removal rates indi-
cates that IMI is almost completely mineralized and the coexisting substances in the
foodwater of rice paddy feld exert only weak interference on IMI mineralization.
In IMI photocatalytic degradation, the prompt mineralizing capacity of as-prepared
catalysts is crucial to eliminate the potential environmental and ecological risks,
because some degradation products have almost the same (even higher) toxicity as
(than) IMI [5, 17].
Mechanism analysis
To identify the main reactive species driving the photo-degradation of IMI on Ag/
AgBr and Ag/AgBr/TiO2, a series of control experiments with scavengers were car-
ried out under settled conditions: IMI concentration, 15 mg/L; pH of IMI solutions,
7.0; catalyst dosage, 1.5 g/L. Various scavengers were employed to quench the pos-
sible active species: dimethyl sulfoxide (DMSO; 5 mM) for e−, ethylenediamine
tetraacetic acid disodium (EDTA; 5 mM) for h+, p-benzoquinone (BQ; 5 mM) for
−
·
·O2 and tert-butyl alcohol (TBA; 5 mM) for OH, respectively [21, 24, 31, 34]. As
illustrated in Fig. 10a and Table S2 (EMS), for Ag/AgBr, DRs were dramatically
reduced by EDTA and BQ and moderately reduced by TBA, while the reduction by
·
−
DMOS was almost negligible. The results indicate that for Ag/AgBr, O2 , h+ and
·OH may be key roles in IMI photo-degradation, especially for ·O2− and h+, e− may
be not the main reactive species [21]. For Ag/AgBr/TiO2, DRs were dramatically
decreased by BQ and TBA, while the decrease from EDTA and DMOS was almost
negligible. Thus, for Ag/AgBr/TiO2, both ·O2− and ·OH may be the key roles in IMI
photo-degradation, e− and h+ may be not the main reactive species, which is par-
tially consistent with the studies elsewhere [24, 31, 34–36, 54].
To quantify the contribution of such active species, a hypothesis was proposed
that the IMI photo-degradation was mostly driven by e−, h+, ·O2− and ·OH. The alge-
·
−
·
braic sum of the contribution from e−, h+, O2 and OH was denoted as the SUM
in Table S2, and the percentage of the DR under given scavenger versus that under
Fig. 10 Efect from scavengers on IMI photo-degradation
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J. Sun et al.
the blank control was denoted as the relative photo-degradation rate (RD-R). Thus,
RD-R under specifc scavenger equals the value of SUM subtracting the contribu-
tion from corresponding active species (label as “Equivalence” in Table S2). There-
fore, the RD-Rs from the quenching experiments provide the four linear equations
·
−
·
relating to four unknowns (contribution from e−, h+, O2 and OH). The results of
such equations are listed in Table S2 and designated as “Contribution.” To facilitate
the comparison between the results obtained from Ag/AgBr and Ag/AgBr/TiO2, all
the data in column “Contribution” were normalized into column “Adjusted contri-
bution” of Table 2, where the SUM was scaled up into 100%. The quantifed and
the adjusted contribution from specifc active species are presented in Table 2 and
Fig. 10b. On the basis of the results mentioned above and from previous studies
[24, 30–32, 36], the proposed reaction mechanism is depicted in Table 2 in column
marked as “Contribution method.” Furthermore, such distinctions in the contribu-
tion method of active species cause diferent photocatalytic performance between
Ag/AgBr and Ag/AgBr/TiO2 in this study.
Conclusions
As-prepared Ag/AgBr/TiO2 exhibits superior photocatalytic activity and stabil-
ity to Ag/AgBr and TiO2. The optimized conditions for Ag/AgBr/TiO2 are: Ag/
AgBr and TiO2 molar ratio, 10%; catalyst dosage, 1.5 g/L; initial IMI concentration,
15 mg/L. Furthermore, the photocatalytic performance of Ag/AgBr/TiO2 difers
from that of Ag/AgBr. The rising solution pH decreases the DR obtained from Ag/
AgBr, while elevates the DR acquired from Ag/AgBr/TiO2. Throughout fve cycles,
the maintenance of DR on Ag/AgBr and Ag/AgBr/TiO2 is about 91.0% and 95.2%,
Table 2 Quantitive results of quenching experiments and the proposed mechanisms
Catalysts
Active species Adjusted
Contribution method
contribution
(%)
−
Ag/AgBr
e−
−63.5
77.7
e− +O2 →·O2
−
·O2
·O2−+IMI→intermediates and/or mineralization
products
·O2− +H+→HO2· →O2 +H2O2;
H2O2 +·O2−→·OH+OH−+O2
h+
55.2
30.5
h++IMI→intermediates and/or mineralization products
·OH
·OH+IMI→intermediates and/or mineralization
products
−
Ag/AgBr/TiO2 e−
−26. 1
83.5
e− +O2 →·O2
−
·O2
·O2−+IMI→intermediates and/or mineralization
products
h+
−20.6
63.2
h++OH−→·OH
·OH
·OH+IMI→intermediates and/or mineralization
products
1 3

Imidacloprid photo‑degradation on Ag/AgBr modifed TiO2:…
respectively. The impacts from coexisting ions in the simulated wastewater depend
−
on their species and nature: the SO42− dramatically depresses the DR; HCO3 , Ca2+
and Mg2+ moderately inhibit IMI photo-degradation; Cl−, K+ and Na+ exhibit a
−
negligible inhabitation; while NO3 reinforces IMI photo-degradation. Ag/AgBr
and Ag/AgBr/TiO2 drive IMI photo-degradation by diferent mechanisms, and cor-
responding contribution of active species is quantifed. As-prepared Ag/AgBr/TiO2
exhibits favorable anti-interference capacity beyond the foodwater of rice paddy
feld, where IMI is almost completely mineralized and the coexisting substances in
the foodwater of rice paddy feld exert only weak interference on IMI mineraliza-
tion. Thus, the results from critical impacts and mechanism study demonstrate the
robust photocatalytic activity, anti-interference capacity, stability and reusability of
Ag/AgBr/TiO2 in potential application.
Acknowledgements This work is supported by the National Natural Science Foundation of China (Grant
Nos. 21307103, 41877118, 51702279), Qing-Lan Project of Jiangsu Province, and the Natural Science
Foundation of the Jiangsu Higher Education Institutions of China (Grant No. 17KJB610013).
Compliance with ethical standards
Confict of interest The authors declare that they have no competing interests.
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