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Biological and Environmental Phenomena at the Interface

Enhanced photocatalytic simultaneous removals of Cr(VI) and bisphenol A over Co(II)-modified TiO2 Guixia Zhao, Yubing Sun, Yukun Zhao, Tao Wen, Xiangxue Wang, Zhongshan Chen, Guodong Sheng, Chuncheng Chen, and Xiangke Wang Langmuir, Just Accepted Manuscript • DOI: 10.1021/acs.langmuir.8b03214 • Publication Date (Web): 14 Dec 2018 Downloaded from http://pubs.acs.org on December 15, 2018

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Enhanced photocatalytic simultaneous removals of Cr(VI) and bisphenol A over Co(II)-modified TiO2 Guixia Zhao,† Yubing Sun,† Yukun Zhao,‡ Tao Wen,† Xiangxue Wang,† Zhongshan Chen,† Guodong Sheng,† Chuncheng Chen,‡ Xiangke Wang† * †College

of Environmental Science and Engineering, North China Electric Power University,

Beijing, 102206, P.R. China ‡Key

Laboratory of Photochemistry, Beijing National Laboratory for Molecular Sciences, Institute

of Chemistry, Chinese Academy of Sciences, Beijing 100190, P.R. China

ABSTRACT. To enhance the electron-hole separation and boost the practical performance of commercial titania (Degussa P25) under natural solar light, in this work, P25 was modified with Co(II) species (CoP25) through post-treatment with decomposition of CoEDTA

precursors

in

a

wet

chemical

anchoring

process.

With

appropriate Co(II) loading amount as molecular cocatalyst, the resulted

CoP25-4

showed

significantly

improved

photocatalytic

performance for Cr(VI) reduction and bisphenol A (BPA) oxidation under UV-light irradiation. The co-existence of Cr(VI) and BPA promoted

mutually

the

degradation

of

both

pollutants.

Under

simulated solar light (AM 1.5G) illumination, the Cr(VI) reduction rate over CoP25-4 was 8.5 times enhanced compared with that over

ACS Paragon Plus Environment

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P25, while the simultaneous degradation rate of BPA over CoP25-4 was 8 times higher than that over P25. Further investigations indicated

that

the

covalent

atomic

Co(II)

anchoring

on

P25

significantly promoted the photogenerated electron-hole separation and

facilitated

Cr(VI)

reduction

via

the

formation

of

Co(I)

intermediate and simultaneously boosted BPA oxidation. Our results demonstrated a facile strategy to modify P25 with remarkably improved

performance

for

the

practical

application

in

environmental pollution management under natural light excitation.

Introduction Hexavalent

chromium

Cr(VI)

is

a

well-known

mutagenic

and

carcinogenic pollutant widely existed in wastewater during the industrial processes like

leather, electroplating and pigment

production.1-3 Even when the concentration is above 0.05 ppm, Cr(VI) is of high toxicity to living organisms in biological systems.4 Therefore, Cr(VI)-containing effluent needs to be disposed before its discharge.2,5-7 Reduction of Cr(VI) into Cr(III) is one of the preferred method to reduce the contamination since Cr(III) is much less toxic in human nutrition, which can be easily removed through precipitation. In the past decades, photocatalytic reduction of Cr(VI)

has

been

considered

a

cheap method compared

with the

conventional chemical reduction methods using massive reductants like NH2NH2, FeSO4, NaHSO3, etc.6-8 On the other side, the organic


2

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pollutants like organochlorine pesticides, bisphenol A or organic dyes

also

cause

serious

environmental

problems.8,9

The

heterogeneous photo-degradation is also proved to be an economical approach

to

decompose

these

organic

pollutants

into

nontoxic

products like H2O, CO2, etc.10-12 Heavy metal ions and organic compounds are often discharged together in many real industrial processes like metal finishing, leather tanning and finishing, wood preserving, petroleum refining, which means it is common phenomenon for the simultaneous existence of organic pollutants and heavy metal ions in the wastewater. Therefore, the simultaneous degradation/elimination of organic chemicals and metal ions in wastewater is of particular importance for the practical pollutant remediation. Although the photocatalysis has been widely developed in the past decades,13-15 the most stable and commercialized semiconductor is still limited to TiO2.16-18 Prairie et al. first described the synergistic relationship between the reduction of metal ions and the oxidation of organic pollutant with TiO2 photocatalyst. The key issue in this synergistic effect is the enhanced separation of photo-generated electron-hole pairs due to the separately utilized redox half-reactions.8,19 Because of the original wide band gap of TiO2, most of TiO2-based photocatalysis is driven by UV light, which

triggered

much

research

work

on

the

visible-light

photocatalysis by TiO2 modification through doping20,21 and hetero-


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structure designing22-24. While it is often ignored that enhancing the quantum efficiency of TiO2 under UV light to maximally utilize the UV irradiation in the natural solar light via the cocatalyst engineering is also promising for the practical application. Co(II) has been reported as an effective cocatalyst for interfacial reduction reactions in recently reports,25-30 thus in this work, other than using precious metal nanoparticles as cocatalyst to boost the photocatalytic activity, we first introduced Co(II) species

modified

P25

(CoP25)

for

efficient

photocatalytic

degradation of Cr(VI) and BPA simultaneously. Detailed mechanism investigations

by

Electron

Spin

Resonance

(ESR)

and

X-ray

absorption spectroscopy (XAS) analysis disclose that the molecular cocatalyst anchored on P25 efficiently trap the photo-generated electron and the formed Co(I) intermediate facilitates Cr(VI) reduction, as well as the coupled BPA oxidation.

Experimental section Catalyst preparation and characterization P25 (purchased from Degussa) was used as purchased. For CoP25 preparation, Ethylenediamine-N,N,N',N'-tetraacetic Acid Cobalt(II) Disodium Salt Tetrahydrate (Co(II)-EDTA) was mixed with P25 in different mole ratios (2%, 4%, 6%) in methanol solution by stirring at

60

oC

for

solvent

calcinated at 350

oC

evaporation.

The

resulted

mixture

was

under argon for 1 hour. The resulted samples

were denoted as CoP25-2, CoP25-4, and CoP25-6, respectively.


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Powder XRD was conducted on an X-ray diffractometer (Rint 2000, Altima III, Rigaku Co. Japan) with a Cu Kα source. Transmission electron microscopy (TEM) images, were recorded with a field emission transmission electron microscope (JEM2100F, JEOL Co., Japan) operating at 200 kV, combined with energy dispersive X-ray spectroscopy (EDS) mapping. XPS measurement was carried out with electron spectrometer (PHI Quantera SXM, ULVAC-PHI Inc., Japan). FT-IR spectra were conducted over Nicolet 4700 spectrometer with reflection mode. The UV-Vis absorption was measured with a UVvisible spectrophotometer (Shimadzu, UV-2600) using BaSO4 as the reflectance standard reference. PL spectra were measured by a JASCO FP-6500 spectrofluorometer excited at 250 nm. ESR measurements were carried out at room temperature on a JEOL JES-FA-200. Co Kedge EXAFS spectra of CoP25 samples were recorded at 14 W1 beam line of Shanghai Synchrotron Radiation Facility (SSRF, China) in the fluorescence mode. The electron beam energy and mean stored current were 3.5 GeV and 300 mA, respectively. The X-ray energy was tuned with a fixed-exit double-crystal Si (111) monochromator. The

intensities

of

the

incident

and

fluorescence

X-ray

were

monitored by using standard N2-filled ion chamber and Ar-filled Lytle-type detector, respectively. Analysis of the obtained EXAFS data was completed using Athena and Artemis interfaces to the IFFEFIT software.31,32 The paths of Co-O and Co-Co shells were fitted from the crystal structures of CoO.33


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Photocatalytic activity evaluation and analysis The photocatalytic activity evaluations were carried out in a homemade glass reactor with 400 mL capacity. The light source includes a 300 W Xe lamp and solar simulator, i.e., Air Mass 1.5 Global (AM 1.5G). In each experiment, the reaction was performed in an open system at 25 oC and the distance between the Xe lamp and the reactor was set at 12 cm. Prior to photoreaction, the suspension containing photocatalyst, Cr(VI) and BPA (photocatalyst: 1g/L, Cr(VI): 10-30 mg/L, BPA: 20-40 mg/L, suspension volume: 200 mL) was magnetically stirred in dark for 30 min to ensure the adsorption-desorption balance. The suspension pH was adjusted with diluted NaOH or HCl. During the photoreaction, about 4 mL of suspension was taken out at a scheduled interval and then centrifuged at 10000 rpm for 5 min for analysis. The concentration of Cr(VI) was measured by spectrophotometry with 1,5-diphenylcarbazide method conducted on UV-Vis

spectrophotometer

(Shimadzu

UV-2500)

via

the

optical

adsorption intensity at 540 nm. The residual BPA concentration was detected using HPLC (Agilent 1200, USA) with a UV detector and a C18 reversed phase column (250*4.5*5mm, Agilent). A mobile phase is composed of acetonitrile and water (50/50, v/v) with a flow rate of 1.0 mL/min. Total organic carbon (TOC) was analyzed by an Apollo 9000 TOC analyzer (USA). To investigate the mechanism, different trapping agents such as p-benzoquinone and coumarin were added

into

the

suspension

to

understand

their

effect

to the


6

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degradation of BPA and Cr(VI). Further detection of the active ·O2radical was

conducted by

the

degradation

of

NBT (nitro blue

tetrazolium) as probe, which was monitored by the absorbance change at the wavelength of 259 nm.34,35 While •OH radicals were trapped by the terephthalic acid to form 2-hydroxyterephthalic acid, which can be measured by the fluorescence at 430 nm with excitation at 315 nm.36

Photoelectrical measurement The photoelectrical experiments were carried out using Autolab electrochemical

workstation in a typical three-electrode cell

including P25 or CoP25 electrode as working electrode, Ag/AgCl electrode as reference electrode and platinum as counter electrode. The photoresponse was evaluated by amperometry technique with chopped UV light irradiation at a potential of 0.2 V vs. Ag/AgCl, in an aqueous BPA solution (3 mg/L). Electrochemical impedance spectroscopy (EIS) was performed under UV-light irradiation at a potential of 0.2 V vs. Ag/AgCl, in the frequency range of 0.1 Hz100 kHz with potential amplitude of 10 mV in a 0.5 M Na2SO4 solution.

Results and discussion Structural characterization The TEM images of CoP25-4 indicate the maintained nanostructure after the anchoring process, except a thin amorphous layer on the surface

(Fig.

1).

EDS

mapping

of

Ti,

O

and

Co

implies

the

homogeneous distribution of Co on P25. Similar morphology can be


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found for CoP25-2, while some small particle emerged in the surface of CoP25-6 (Fig. S1).

Figure 1. (a-b) TEM and HRTEM images of CoP25-4 (Scale bar in a: 20 nm, Scale bar in b: 2 nm, inset in b is the magnification of the lattice in the square area); (c-f) STEM and EDS elemental mapping images of CoP25-4. The XRD patterns of the resulted CoP25 displayed the same characteristic diffraction peaks as P25 (Fig. 2a), suggesting that P25 kept in the same phase and no other new phase formed after the


8

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calcination with Co-EDTA. In the FTIR spectra of CoP25-2, CoP25-4 and CoP25-6 (Fig. 2b), the signals at 1148, 880 and 1452 cm-1 proved the existence of C-N, Co-N and -CH2-, suggesting the partially decomposition of EDTA-Co during the anchoring process. XPS spectra of CoP25-2, CoP25-4 and CoP25-6 confirmed the presence of C, N, O and Co elements (Fig. S2). The Co 2p3/2 and Co 2p1/2 peaks at around 780.9 and 796.7 eV in Co 2p spectra, accompanying with two shake-up satellite peaks at around 786.8 and 802.8 eV in Fig. 2c, indicated the dominating presence of Co(II) in the three samples.3739

As estimated from the decomposition of EDTA-Co under inert

atmosphere, the resulted Co(II) species may be composed of Co(II)organic clusters, where Co(II) is coordinated N and O.14

The UV-

Vis diffuse reflectance spectra of P25 and CoP25 were measured and converted into absorption spectra (Fig. 2d) using the Kubelka-Munk function.40 It shows that CoP25 samples have similar absorption edges to P25 except for the increased absorption in visible light area due to the deep black color of attached Co(II) species. Further microstructure investigation was carried out by Co K-edge X-ray absorption near edge structure (XANES) and extended X-ray absorption fine structure (EXAFS) spectroscopy. Fig. 2e shows the XANES spectra of references (Co2O3, Co(NO3)2) and samples (i.e., CoP25-2, CoP25-4, CoP25-6). It is observed that the type and absorption energy of near edges for CoP25-2, CoP25-4, CoP25-6 are very close to Co(NO3)2, indicating that the Co species on CoP25-2,


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CoP25-4, CoP25-6 are in divalent state.41,42 Fig. 2f shows the Co K-edge Fourier-transform (FT) EXAFS spectra of CoP25-2, CoP25-4, CoP25-6. The corresponding fitted parameters (coordination numberCN, interatomic distance-R, Debye-Waller factor and R-factor) were summarized

in

Table

S1.

Considering

the

similar

absorption

energies of O and N, the first shell was described as Co-O(N). As shown in Fig. 2f, the first FT features of CoP25-2 and CoP25-4 can be satisfactorily fitted by Co-O(N) with CN = 3.0 at R = 2.0 Å, whereas the value of CN for CoP25-6 (~ 2.0) is lower than those of CoP25-2 and CoP25-4 (Table S1). In addition, we tried to fit the second

FT

features

for

CoP25-2

and

CoP25-4

by

Co-Co

shell,

unfortunately, the unsatisfactory result (Debye-Waller factor > 0.03 Å) was obtained. However, the FT features of CoP25-6 at ~ 2.3 Å can be satisfactorily fitted by 9.4 Co (Co-Co shell) at 3.1 Å (Table S2). The results of EXAFS spectra indicated that the Co(II) species are mainly distributed and complexed molecularly on the surface of P25 with low Co loading, while the Co(II) species are stacked into clusters with high Co loading, as observed in HRTEM image (Fig. S1).


10

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Figure 2. a) XRD patterns; b) FT-IR spectra; c) XPS Co 2p spectra and d) UV-Vis diffuse reflectance spectra of P25 and CoP25; e) Co-K edge XANES spectra of CoP25 and the reference samples; f) Fourier transform of the K-edge EXAFS spectra of CoP25.

Photocatalytic activity evaluation

The effect of solution pH on the reduction of Cr(VI) was investigated over CoP25-4 (Fig. 3a). At pH 3, Cr(VI) underwent approximate 90% reduction over CoP25-4 within 60-min Full arc Xe lamp irradiation, while at pH 5, only


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47% Cr(VI) was reduced after 60-min photo-reaction. The efficiency further decreased when pH increased to 7 and 9. Since Cr(VI) reduction

process

consuming

protons

by

the

photogenerated

electrons can be expressed as Cr2O72- + 14 H+ + 6e- → 2Cr(III) + 7H2O, it is then inferred that the increase of H+ concentration would enhance the accepting-electron ability of Cr(VI). Thus, the following experiments were conducted at pH values (pH 3). Fig. 3b depicted the comparison of the photocatalytic activities over P25 and

CoP25

samples

with

different

anchoring

ratios

of

Co(II)

species. It shows that in the presence of P25, only 42% of Cr(VI) can be reduced after 60-min irradiation, and all CoP25 samples show enhanced activity compared with P25, among which, the CoP25-4 exhibits

the

highest

efficiency.

The

specific

efficiency

was

determined by the photocatalytic reaction rate constant (k), which was obtained by the fitting of time course curve using pseudofirst-order model (-ln(C/C0) = kt), and the fitted k values were listed in Table 1. It is inferred that the amount of loaded Co(II) species is critical towards the efficiency. The CoP25-6 showed decreased photocatalytic activity compared with CoP25-4, which was due to the inferior activity of the aggregated Co(II) species and the

superior

features

of

the

well

dispersed

Co(II)

species.

Concerning the fitting results of EXAFS spectra (Table S1), the low-coordinated

Co(II)

species

on

the

surface

of

CoP25-4

is

favorable to the interaction between catalyst and the reactants in


12

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solution, while the overwhelmed loading of Co(II) resulted in Co(II) stacking as inferred from the apparent Co-Co shell in the EXAFS spectra, which seriously affected the performance.

Figure 3. (a) Effect of pH on photocatalytic reduction of Cr(VI) over CoP25-4 under full arc Xe lamp irradiation with initial concentration of 20 mg/L; (b) Comparison of Cr(VI) removal over


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P25, CoP25-2, CoP25-4 and CoP25-6 at pH 3; (c) Effect of Cr(VI) concentration on the photocatalytic oxidation of BPA (30 mg/L) over CoP25-4 at pH 3; (d) Effect of Cr(VI) concentration on the photocatalytic oxidation of BPA (30 mg/L) over P25 at pH 3; (e) Effect of Cr(VI) concentration on the TOC removal during BPA oxidation over CoP25-4 at pH 3; (f) Effect of BPA concentration on the Cr(VI) reduction over CoP25-4 at pH 3. Table 1. Kinetic reaction rate constants for the degradation of Cr(VI) (with initial concentration of 20 mg/L) under different conditions at pH 3 Photocatalyst P25 CoP25-2 CoP25-4 CoP25-6 CoP25-4 CoP25-4 CoP25-4 P25 CoP25-4

Light source Full arc Xe lamp irradiation Full arc Xe lamp irradiation Full arc Xe lamp irradiation Full arc Xe lamp irradiation Full arc Xe lamp irradiation Full arc Xe lamp irradiation Full arc Xe lamp irradiation AM 1.5 G AM 1.5 G

Coexisted BPA concentration (mg/L)

reaction rate constants (k) min-1

0

0.0202

0

0.0738

0

0.1417

0

0.1030

15

0.1820

30

0.2545

40

0.2957

30 30

0.0004 0.0034

We further investigated the synergistic effect between Cr(VI) reduction and BPA oxidation by conducting the removal of 30 mg/L BPA at pH 3 with different initial concentration of Cr(VI) from 0


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to 30 mg/L. As depicted in Fig. 3c, the degradation efficiency of BPA increased with the increase of Cr(VI) concentration over CoP254. In the case of 20 mg/L Cr(VI), around 72% of BPA was removed in 10-min irradiation, which was 4 times as high as that without Cr(VI). In the presence of P25, the simultaneous BPA oxidation and Cr(VI) reduction were much less efficient although synergistic effect between photocatalytic Cr(VI) reduction and BPA oxidation still remained. As shown in Fig. 3d, only about 60% of BPA can be removed in the presence of initial 20 mg/L Cr(VI) after 60 minirradiation.

The

total

organic

carbon

(TOC)

measurement

also

suggested that the BPA mineralization was improved due to the addition of Cr(VI) (Fig. 3e). Specifically, without Cr(VI), only 21% of TOC removal yield was found after 180-min irradiation, while in the presence of 20 mg/L Cr(VI), TOC removal field approached to 50%

after

180-min

irradiation

over

CoP25-4.

Similarly,

the

synergistic effect between photocatalytic Cr(VI) reduction and BPA oxidation

was

studied

by

adjusting

different

initial

BPA

concentrations from 0-40 mg/L in the presence of 20 mg/L Cr (VI) at pH 3 over CoP25-4. It was shown in Fig. 3f that the reduction of

Cr(VI)

was

gradually

accelerated

with

the

increased

concentration of BPA, which was considered due to the enhanced electro-hole separation with the aid of hole-scavenger in the other half reaction.


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Even under the irradiation of AM 1.5G, about 19% Cr(VI) and 14% BPA can be removed after 60-min irradiation over CoP25-4 (Fig. 4), while only 4% Cr(VI) can be removed under the same condition over P25. The calculated photocatalytic reaction rate constant (k) for BPA degradation over CoP25-4 is about 0.0024 min-1, which is 8fold as high as that over P25 (0.0003 min-1).

Figure 4. Time course of BPA degradation and the simultaneous Cr(VI) reduction over P25 and CoP25-4 at pH 3 under AM 1.5 G irradiation.

Photoelectrochemical characterization To further investigate the effect of Co(II) species modification, photoelectrochemical response,

characterizations

electro-chemical

impedance

including spectroscopy

photocurrent (EIS)

were


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investigated

by

using

P25/CoP25

samples

as

photoanode

and

photocathode. Fig. 5a shows the amperometric response of P25 and CoP25 samples in the presence of BPA at the potential of 0.2 V vs. Ag/AgCl. The photocurrent response of CoP25 samples with different Co(II) loading amounts was higher than that of unmodified P25, and CoP25-4 exhibited the highest photocurrent density. The trend of the photocurrent on CoP25 samples is corresponding with the removal efficiency of BPA in the photocatalysis as discussed above, further confirming

that

the

CoP25-4

has

the

optimal

photocatalytic

activity. Photo-reduction of Cr(VI) was also investigated by the amperometric response in the presence of 5.2 mg/L Cr(VI) at the potential of 0 V vs. Ag/AgCl as shown in Fig. 5b. Compared with raw TiO2, the quick and larger initial current-response is supposed due to the efficient electron-trapping by Co(II). As the relative kinetically

sluggish

Cr(VI)

reduction,

the

photocurrent

is

decreased with time, but the overall photocurrent is still enhanced after Co(II) loading. EIS is an effective technique to evaluate the

charge

transfer

process

between

the

electrolyte

and

electrode.43 The Nyquist plots for P25 and CoP25 photoelectrodes show a semicircle that can be attributed to the electron transferlimited process.44 It can be seen clearly in Fig. S3 that under UVlight irradiation, the curve radius of CoP25 samples was smaller than that of P25, indicating that the modified Co(II) species could improve the separation of electron-hole pairs and thus decrease


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Page 18 of 26

the charge transfer resistance. The smallest semicircle for CoP254 photoelectrode also revealed the highest photoelectrocatalytic activity owning to the most facilitated charge transfer.

Figure 5. (a) The photocurrent responses of P25 and CoP25 photoelectrode in the presence of BPA (3 mg/L) at the potential of 0.2 V vs. Ag/AgCl; (b) The photocurrent responses of P25 and CoP25 photoelectrode in the presence of Cr(VI) (5.2 mg/L) at the potential of 0 V vs. Ag/AgCl.

Mechanism It is well-known that in the TiO2 catalysis process, the hydroxyl radical, superoxide radical and hole are considered as active species.

To

further

identify

the

mechanism

for

the

enhanced

performance, comparison experiments by addition of trapping agents were conducted in the photocatalytic reaction system. As shown in Fig. 6a, for P25, the addition of pBQ (short for p-benzoquinone, •O2– radical scavenger) and Cou (short for coumarin, •OH radicals scavenger) obviously blocked the photocatalytic degradation of BPA while both had no effect on Cr(VI) reduction. But for CoP25-4 (Fig.


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6b),

the

case

is

changed.

pBQ

addition

can’t

prevent

the

degradation of BPA, and only Cou addition decreased BPA degradation, while neither has obvious effect on Cr(VI) reduction. It is implied that for P25, the •O2– and •OH radicals are the main active species, while in the presence of CoP25-4, •O2– radical is negligible and only

•OH

radicals

contributes

to

BPA

oxidation.

It

is

then

suspected that the photogenerated electron is trapped rapidly on Co(II) sites, instead of reaction with O2 to form •O2–. To confirm this

speculation,

in-situ

ESR

measurements

over

CoP25-4

were

conducted. As shown in Fig. S4, CoP25 showed typical Co(II) broad resonance at g = 2.3072 in the dark condition, while the signal was

obviously

weakened

under

irradiation.

Based

on

the

diamagnetism of Co(I) species, it is deduced that the decreased signals were attributed to the boosted electron trapping by Co(II) on the surface of CoP25-4.45,46 Also as indicated in the quenched photoluminescence (Fig. S5), electron-hole recombination in CoP25 is largely decreased compared with that in P25. To further confirm the above speculation, the photo-generated •O2– and •OH over P25 were detected by NBT method34,35 and the terephthalic acid method 36,

respectively. As shown in Fig. S6, the photo-generated •O2– is

much less in the 40 min-irradiation of CoP25-4 than that of P25, while the photo-generation rate of •OH over CoP25-4 (Fig. S7) is enhanced compared with that over P25. Therefore, it is concluded that

after

modification

with

Co(II)

species,

under

UV-light


19

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irradiation,

the

photo-generated

Page 20 of 26

electron

can

be

efficiently

trapped by Co(II) to form Co(I), largely suppressing •O2– formation and

enhancing

promoting

the

charge-separation,

while

simultaneously

•OH generation. The active Co(I) is supposed to take

part in the Cr(VI) reduction and the increased •OH intermediates accelerate the decomposition of BPA.

Figure 6. Time course of the degradation of BPA (initial concentration of 30 mg/L) and Cr(VI) (initial concentration of 20 mg/L) in the presence of pBQ or Cou over P25 (a) and CoP25-4 (b) under full arc Xe lamp irradiation.

Conclusion In brief, it is concluded that through surface modification by a wet chemical anchoring process with EDTA-Co(II) as precursor, the resulted CoP25

showed

a

significant

improvement

in

the

photocatalytic

reduction of Cr(VI) and the simultaneous BPA degradation. In the case of CoP25-4, the Co(II) is covalently bonded with P25 with low-coordination

to

oxygen/nitrogen.

It

is

demonstrated

the

coexistence of Cr(VI) and BPA promoted the degradation of each


20

Page 21 of 26 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Langmuir

other. Specifically, the photocatalytic reaction rate constants (k) for Cr(VI) reduction rate over CoP25-4 is 8.5 times enhanced compared with that over P25, while the simultaneous BPA degradation over CoP25-4 is 8 times as efficient as that over P25 under AM 1.5G illumination. Detailed mechanism analysis implied that the covalent

anchored

photogenerated

Co(II)

electron-hole

electron-trapping

on

Co(II)

species separation sites.

in

CoP25-4

through

These

the

findings

promoted efficient provide

a

meaningful strategy for the facile modification of P25 with regards to the practical application in environmental remediation and purification process. ASSOCIATED CONTENT Supporting Information. HRTEM images of CoP25-2 and CoP25-6, XPS of CoP25 samples, Semi in situ ESR spectra, photoluminescence spectra, the degradation comparison of NBT (nitro blue tetrazolium) radical over P25 and CoP25, and the fluorescence of 2-hydroxyterephthalic acid, Fitting results of Co K-Edge EXAFS Spectra of CoP25, were included in the supporting information. AUTHOR INFORMATION Corresponding Author * [email protected] (X. Wang), Author Contributions


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Page 22 of 26

The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript. Notes The authors declare no competing financial interest. ACKNOWLEDGMENT Financial support from the National Key Research and Development Program of China (2017YFA0207000) and the National Natural Science Foundation of China (21577032) is acknowledged. X. K. Wang and C. C. Chen acknowledge the CAS Interdisciplinary Innovation Team of the Chinese Academy of Sciences. REFERENCES (1) Kieber, R. J.; Willey, J. D.; Zvalaren, S. D. Chromium Speciation in Rainwater: Temporal Variability and Atmospheric Deposition. Environ. Sci. Technol. 2002, 36, 5321-5327. (2) Zhang, S.; Zeng, M.; Xu, W.; Li, J.; Li, J.; Xu, J.; Wang, X. Polyaniline Nanorods Dotted on Graphene Oxide Nanosheets as a Novel Super Adsorbent for Cr(VI). Dalton Trans. 2013, 42, 7854-7858. (3) Kim, G.; Choi, W. Charge-Transfer Surface Complex of EDTA-TiO2 and Its Effect on Photocatalysis under Visible Light. Appl. Catal. B-Environ. 2010, 100, 77-83. (4) Wen, T.; Fan, Q.; Tan, X.; Chen, Y.; Chen, C.; Xu, A.; Wang, X. A Core–Shell Structure of Polyaniline Coated Protonic Titanate Nanobelt Composites for Both Cr(VI) and Humic Acid Removal. Poly. Chem. 2016, 7, 785-794. (5) Zhang, Y. C.; Li, J.; Zhang, M.; Dionysiou, D. D. Size-Tunable Hydrothermal Synthesis of Sns2 Nanocrystals with High Performance in Visible Light-Driven Photocatalytic Reduction of Aqueous Cr(VI). Environ. Sci. Technol. 2011, 45, 93249331. (6) Testa, J. J.; Grela, M. A.; Litter, M. I. Heterogeneous Photocatalytic Reduction of Chromium (VI) over TiO2 Particles in the Presence of Oxalate: Involvement of Cr (V) Species. Environ. Sci. Technol. 2004, 38, 1589-1594.


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