Visible-light-driven photocatalytic degradation of naproxen by Bi-modified titanate nanobulks: Synthesis, degradation pathway and mechanism

Journal Pre-proof Visible-light-driven photocatalytic degradation of naproxen by Bi-modified titanate nanobulks: Synthesis, degradation pathway and mechanism Gongduan Fan, Rongsheng Ning, Jing Luo, Jin Zhang, Pei Hua, Yang Guo, Zhongsheng Li

PII:

S1010-6030(19)31081-0

DOI:

https://doi.org/10.1016/j.jphotochem.2019.112108

Reference:

JPC 112108

To appear in:

Journal of Photochemistry & Photobiology, A: Chemistry

Received Date:

25 June 2019

Revised Date:

21 September 2019

Accepted Date:

25 September 2019

Please cite this article as: Fan G, Ning R, Luo J, Zhang J, Hua P, Guo Y, Li Z, Visible-light-driven photocatalytic degradation of naproxen by Bi-modified titanate nanobulks: Synthesis, degradation pathway and mechanism, Journal of Photochemistry and amp; Photobiology, A: Chemistry (2019), doi: https://doi.org/10.1016/j.jphotochem.2019.112108

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Visible-light-driven photocatalytic degradation of naproxen by Bi-modified titanate nanobulks: Synthesis, degradation pathway and mechanism

Gongduan Fana, b, *, Rongsheng Ninga, Jing Luoa, Jin Zhangc, Pei

College of Civil Engineering, Fuzhou University, 350116 Fujian, China b

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Huad, Yang Guoa, Zhongsheng Lia

State Key Laboratory of Photocatalysis on Energy and Environment,

Institute of Groundwater and Earth Sciences, Jinan University, 510632 Guangdong, China

SCNU Environmental Research Institute, Guangdong Provincial Key

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Fuzhou University, 350002 Fujian, China

Laboratory of Chemical Pollution and Environmental Safety & MOE Key Laboratory of Environmental Theoretical Chemistry, South China Normal University, 510006 Guangzhou, China *Corresponding authors: Page 1 of 53

Gongduan Fan, E-mail: [email protected]

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GRAPHICAL ABSTRACT

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HIGHLIGHTS

Visible-light-driven Bi-TNB was synthesized for photodegradation of NPX.

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Different factors on the degradation efficiency of NPX were investigated.

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Kinetics mechanism in the photodegradation process of NPX were studied.

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The coexistence of ACT and NPX hindered the photocatalytic degradation of NPX.

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Bi-TNB has the potential to degrade the PPCPs in practical applications.

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ABSTRACT Naproxen (NPX) as one of the typical pharmaceuticals and personal care products (PPCPs) has been constantly detected in aquatic environment recently which has potentially hazard to the human health and ecosystem. However, the practical applicability of photocatalysts in degradation of NPX is still restricted by challenges that most nanomaterials need to be stimulated by ultraviolet light and their limited photocatalytic activity under visible light. Therefore, we synthesized bismuth titanate nanobulks (Bi-TNB) through the two-step method of hydrothermal-calcining. The

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crystal structure, morphology, UV-vis diffuse reflectance spectra and surface adsorption performance of the as-prepared were investigated by XRD, SEM, UV-vis and BET. Several parameters which might influence the degradation efficiency were studied

including initial NPX concentration, catalyst dosage, solution pH and concentration of

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anions, cations and humus. The results indicated that more than 99.9% of NPX (0.25 

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mg/L) was removed by Bi-TNB (0.5 g/L) at pH  =  7. Reactive species scavenging experiments indicated that h+ and O2·- were the dominant active species involved the

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degradation of NPX. In photodegradation process, NPX was firstly decarboxylated and then further photocatalytic oxidized to form carboxylic acids of lower molecular weights and would be finally transformed into CO2 and H2O. PPCPs coexistence

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experiment showed that acetaminophen would hinder the removal of NPX by Bi-TNB. Both the reaction in different water matrices and degradation under the condition of

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sunlight indicated that Bi-TNB could be applied in degradation of NPX in environmental water systems. This study provides a new strategy for enhancing

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material photocatalytic performance to be used in practical applications.

KEYWORDS Bi-modified; Titanate nanomaterials; Naproxen; Photocatalysis; Visible light; Photodegradation

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1. Introduction Pharmaceuticals and personal care products (PPCPs), as one of the micropollutants[1], have raised concerns about caused new environmental pollution problems in aquatic environment. The pharmaceuticals include anti-inflammatory drugs, antibiotics, antihypertensive drugs and antiepileptic drugs, etc., which main sources are medical wastewater, animal feeding wastewater, sewage leakage and domestic sewage discharge[2]. Due to the effects of water diffusion, soil adsorption and bioaccumulation[3-5], the existence of PPCPs in water environment has potential

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negative influence on human health and ecosystem. Naproxen (NPX), as one of the most commonly used non-steroidal anti-inflammatory drugs (NSAIDs)[6], is mainly used to treat arthritis, dysmenorrhea and acute gout which is widely used around the world[7,8]. The traditional wastewater treatment processes cannot completely degrade

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or mineralize NPX because of its high resistance to biodegradation, resulting that it can

be detected in a variety of environmental water bodies and even in drinking water[9].

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Drinking water containing NPX for a long time can harm human health and increase the risk of cardiovascular disease and stroke[10]. The growth of fish populations will

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also be somewhat inhibited due to long-term contact to NPX in water[11]. Hence, it is urgent to find an economical and efficient method to remove NPX from water.

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In recent years, many scholars have studied the removal of NPX, and the main removal methods include biological method, physical method and chemical method. For example, Ding et al. used Scenedesmus quadricauda as culture medium to degrade

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58.8% in 30 days[12]; Im et al. utilized ultrasonic method to remove 74.8% of NPX in 10 min at 1000 kHz[13]; Luo et al. oxidized and degraded about 15% of NPX in

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synthetic urine within 1 min by Ferrate (VI)[14]. Alternatively, photocatalytic degradation is an efficient and green technology for removal of organic pollutants[15]. Photocatalysis not only can be driven by solar light, but also can effectively oxidize and decompose organic contaminants[16]. Among them, TiO2 catalyst series is the most extensively studied photocatalyst, which has strong catalytic oxidation ability, stable chemical properties, no poisonous harm and other advantages[17]. The photocatalytic activity of TiO2 nanocrystals mainly depends on specific surface area and Page 4 of 53

crystallinity[18]. The titanate nanomaterials formed by the reaction of NaOH and TiO2, on the basis of the original characteristics of TiO2, have the unique intercalated structure of titanate, excellent ion-exchange property, large specific surface area and other merits[19], which have been successfully applied in the adsorption and removal of organic dyes (such as basic magenta)[20]. However, the degradation effect of titanate materials on methylene blue and rhodamine B could be improved by twice under ultraviolet light compared with visible light[21]. Thus, photoelectron hole with high recombination rate and low visible light utilization rate hinder titanate materials further

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application in water treatment by photocatalysis[22]. To solve this problem, metal oxides with narrow band gap can be produced on the carrier surface by metal doping.

It can not only expand the absorption range of titanate nanomaterials from ultraviolet

light to visible light, but also solve the problem of low photoelectron hole pair

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separation rate, and has a good prospect in visible light driven catalysis[23]. Recently,

it has been found that Bi metal has nice photocatalytic activity[24], meanwhile bismuth-

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based semiconductors have been proven to be an efficient way to improve charge separation and accelerate the separation of photogenic charge carriers[25,26]. It is

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commonly believed that the surface plasmon resonance (SPR) of Ag and other precious metal nanoparticles can be used to expand the light capture range, and Dong et al. found

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that bismuth metal also has a similar SPR effect[27]. Bismuth metal is cheaper than precious metal materials, therefore bismuth has the possibility to replace precious metal ions. Moreover, He et al. used BiOI quantum dot and g-C3N4 compound to form Z-

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scheme structure[28], which effectively promoted the separation of photoelectron and hole and further improve the photocatalytic activity. Similarly, the photocatalytic

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activity of titanate nanomaterials modified by bismuth series compounds is also hopeful to be significantly enhanced. In addition, nanomaterials with different morphologies (such as sheet, ribbon and

bulk) have different physicochemical properties and various photocatalytic effects on PPCPs[29]. Among them, the crystallinity of bulk nanomaterials is better. When nanoparticles tend to be deposited forming random multilayers over bulk nanomaterials rather than the layer-by-layer deposition, which can reduce the loss of specific surface Page 5 of 53

area after loading to a certain extent[30]. Therefore, bismuth series compound modified bulk titanate nanomaterials can further enhance the photocatalytic activity and improve the degradation effect of NPX. In this study, the visible-light-driven titanate nanobulks (TNB) photocatalysts by Bi modified was prepared by hydrothermal-calcining two-step method to degrade NPX and its physicochemical properties have been characterized. The effects of various factors on photocatalytic degradation were studied, including initial concentration of NPX, Bi-TNB dosage and initial pH value. Furthermore, the intermediate products

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produced during the degradation of NPX were analyzed by high performance liquid chromatography-ion trap mass spectrometry (HPLC-IT-MS), and the transformation pathway of NPX was speculated. Subsequently, the photocatalytic mechanism of

photocatalytic degradation of NPX by Bi-TNB was proposed by active material capture

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experiments. Finally, the effects of Bi-TNB photocatalytic degradation of NPX under

the co-existence of two PPCPs (ACT and NPX), in the actual water matrix and under

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sunlight condition were investigated respectively, so as to prove the feasibility of BiTNB photocatalyst in natural water body. In conclusion, this work provides an idea for

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the morphology and metal modification of titanate nanomaterials and applies it to remove PPCPs in natural water matrices under visible light, founding a basis for

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popularizing the future practice application of titanate photocatalysis. 2. Experimental

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2.1 Reagents

NPX, Bi(BNO3)3·5H2O, titanium butoxide (Ti(OC3H7)4), Bi2O3, p-benzoquinone

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(C6H4O2, BQ), sodium oxalate (Na2C2O4), phosphoric acid (H3PO4), sodium sulfate (Na2SO4), copper sulfate (CuSO4), magnesium sulfate (MgSO4), ferric chloride (FeCl3) and humic acid (HA) were purchased from Aladdin Reagent Co. Ltd. TiO2 (P25, 80% anatase, and 20% rutile), sodium hydroxide (NaOH), calcium sulfate (CaSO4·2H2O), sodium bicarbonate (NaHCO3), sodium nitrate (NaNO3), sodium chloride (NaCl), isopropyl alcohol (C3H8O), fulvic acid (FA) and methanol were obtained from Sinopharm Chemical Reagent Co. Ltd., China. All chemicals for the synthesis of Page 6 of 53

nanomaterials were used as received without any further purification, except for the methanol which was of chromatographic purity. 2.2 Synthesis of nanomaterials 2.2.1 Synthesis of unmodified TNB and Bi20TiO32 TNB with high specific surface area have been synthesized through the traditional hydrothermal method[31]. Typically, 1.2 g of TiO2 (P25) and 29 g of NaOH were mixed and dispersed into 66 mL of deionized water. After the mixture was heated in a Teflon reactor at 403.15 K for 3 days, the products were washed to neutral with deionized

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water and dried at 353.15 K for 12 hours. Bi20TiO32 nanoparticles were synthesized

through solvent thermal method[32]. The detailed methods are shown in the Supporting information.

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2.2.2 Synthesis of Bi-modified titanate nanobulks (Bi-TNB)

The Bi-TNB were fabricated via a modified calcination method based on previous

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studies[33]. In a typical procedure, 2.0425 g of Bi(NO3)3·5H2O was added to 100 mL of deionized water before being mixed thoroughly with supersonic stiring for 30 min

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until it was completely dissolved. Then 0.3775 g of TNB were added to the solution of Bi(NO3)3·5H2O and mixed with supersonic stirring for 20 min. Afterwards, the powder was washed with deionized water to neutrality and dried at 343.15 K overnight. The

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dried powder was put into a muffle furnace for sintering (773.15 K) and incubated for 2 hours to obtain Bi-TNB.

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2.3 Characterization of titanate nanomaterials The powder X-ray diffraction (XRD) (PANalytical, X’Pert Pro, Netherlands)

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patterns were used to investigate the crystal structure and phases of samples by using X-ray diffractometry equipped with Co adiation of λ = 1.5418 Å operated at 35 kV and 35 mA over the interval ranging from 5 to 85° at scan rate of 20° second-1. The morphology and particle size of the as-prepared samples were observed by scanning electron microscope (SEM) (Hitachi, S-4800, Japan). The Brunauer-Emmett-Teller (BET) specific surface area of the samples were analyzed by a by nitrogen adsorption isotherm measurements on a surface area and porosity analyzer (Micromeritics, Page 7 of 53

ASAP2020 HD88, USA) at liquid nitrogen temperature (77 K). Prior to the experiment the samples were degassed for 6 h at 423 K under vacuum. UV–vis diffuse reflection spectra (UV-vis) was recorded by using a UV-vis spectrophotometer (PerkinElmer, Lambda950, USA) at scanning wavelength range of 300~800 nm with BaSO4 as a reflectance standard. 2.4 Photocatalytic degradation of NPX The photocatalytic degradation experiments were performed in a photochemical reactor (Fig. S1), which equipped with a magnetic stirrer at a speed of 300 rad/min to

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ensure adequate reaction. The light source was provided by a 500 W metal halogen lamp which combined with a UV and IR cut-off filter to remove wavelengths less than 400nm and larger than 800 nm, respectively. The control experiments were performed

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at the same condition but without photocatalyst. All experimental groups and control

groups were repeated 3 times. Prior to visible light catalysis, the solution was placed in

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dark (60 min with constant stirring) for obtaining the adsorption-desorption equilibrium. During irradiation, approximately 1.0 mL of aliquot was collected by using a 0.45 μm

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PTFE syringe filter at each time interval and centrifuged (4500 rpm/10 min). The residual concentration of NPX was determined by high performance liquid chromatography system (HPLC) (Shimadzu, LC-2030, Japan). The reusability of Bi-

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TNB was evaluated by the repeated use of the photocatalyst powder for three runs under the identical conditions. After each cycle of the photocatalytic reaction, Bi-TNB were

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separated and recycled from the solution by centrifugal collection, then washed thoroughly with deionized water to remove residual pollutants and dried in an oven at

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a temperature of 333.15 K, and the recycled catalyst were used in the succeeding. For the indirect analysis of reaction mechanism, quenching experiments were

conducted by adding ROSs scavengers during the photocatalytic process. The quenchers used were benzoquinone (BQ, 1.0 mM) for superoxide free radical (O2·−), sodium oxalate (Na2C2O4, 1.0 mM) for the holes (h+), and isopropanol (IPA, 1.0 mM) for hydroxy radicals(·OH), respectively. 3. Result and discussion Page 8 of 53

3.1 Characteristics of the photocatalysts 3.1.1 XRD analysis of photocatalysts The XRD patterns of TNB and Bi-TNB were recorded to investigate phase composition and crystallinity. It can be seen from Fig. 1 that TNB shows diffraction peaks at 2θ values of 9.87°, 24.65°, 28.38° and 47.81°. Compared with the JCPDS Card No. 31-1329, they matched the Na2Ti3O7 (100), (201), (111), (020) crystal plane, respectively. The diffraction peaks intensities of Bi-TNB sample were very strong, indicating that catalyst had good crystallinity[34]. They were relative to the diffraction

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peaks of Bi20TiO32 (JCPDS file No. 42-0202), Bi2O3 (JCPDS file No. 50-1088), and

TiO2 (JCPDS file No. 46-1237). New peaks appeared at 2θ of 24.60°, 32.78°, 48.70°,

53.61° and 73.72°, which were corresponding to the (200), (130), (116), (333), and (029)

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planes of Bi2O3, indicating that Bi2O3 nanocrystals have been successfully deposited on TNB by the calcination process. The reason why Bi4Ti3O12 appeared in Bi-TNB was

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that a small fraction of Bi20TiO32 and TiO2 reacted in the synthetic material to generate Bi4Ti3O12[33]. The non-obvious diffraction peak of titanium dioxide in Fig. S2(a) may

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be due to the coating of titanium dioxide by surface Bi20TiO32[35]. Additionally, from the Fig. S2(b) we can see that the Bi20TiO32 nanoparticles have been successfully

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synthesized.

3.1.2 SEM imaging of photocatalysts The morphology of photocatalyst before and after Bi modification was studied by

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scanning electron microscope. Fig. 2(a) presents the SEM image of TNB produced by mixing reaction of NaOH and TiO2. Obviously, the TNB aggregated to form spheres,

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which may make them difficult to disperse in aqueous solutions. Besides, part of the stacking structure may prevent visible light from reaching the catalyst surface in a certain degree, affecting the photocatalytic degradation effect. It is observed in Fig. 2(b) that after Bi modification of TNB, nanobulks were effectively separated, and the shape of nanobulks was more obvious. The separated Bi-TNB was more easily dispersed into the solution while more active sites and light absorption positions were exposed on the surface. The dispersed particles on the surface of Bi-TNB may be Bi2O3 Page 9 of 53

nanoparticles[36]. Furthermore, the surface of nanobulks is anomalistic owing to the random doping induced in the sonication process[37]. 3.1.3 Diffuse reflectance UV-vis spectra of photocatalysts Fig. 3 displays the UV-vis diffuse reflectance spectra of TNB and Bi-TNB. Based on the data, The band gap energy (Eg) can be calculated by the equation (1)[38]：

ah = K(h - Eg)n/ 2

(1)

Where α, h, ν, K, Eg are the absorption coefficient, Planck constant, light frequency,

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proportionality constant, and band gap, respectively. The value of n depends on the type of optical transition in the semiconductor (n = 1 for a direct absorption and n = 4 for an indirect absorption), and n = 4 for the titanate materials in this study.

The TNB has strong absorption peak intensities in ultraviolet region. Compared to

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the unmodified titanate, the absorption edge of Bi-TNB was about 555nm, which

slightly redshifted to visible region compared with TNB (405 nm), Bi20TiO32 (550 nm)

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and Bi2O3 (451 nm), indicating the photo-effect under visible light was enhanced for Bi-TNB. The band gap energy (Eg) of the Bi-TNB was determined to be 2.19 eV as

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shown in the illustration, which was narrower than that of the TNB (3.21 eV). Therefore, a new e-donor level was formed in Bi-TNB, which promoted the transfer of photo-

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generated electrons and inhibited the electron-hole pair recombination. Moreover, the narrower bandgap can increase the utilization of the solar spectrum and thus generate additional electrons and holes to improve the photocatalytic performance.

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3.1.4 BET analysis of photocatalysts

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The results of N2 adsorption–desorption isotherms, surface area and porosity of as-prepared photocatalysts are shown in Fig. 4, BET size of different samples followed the following order: TNB > Bi-TNB. The TNB sample showed type H2 (b) hysteresis loop according to IUPAC classification, and H2 (b) was an ink-bottle mesoporous material with relatively wide "neck". The nitrogen adsorption and desorption isotherms curves of the Bi-TNB displayed isotherm of type IV according to BDDT category. This confirmed the porous characteristics of the Bi-TNB composite, indicating the presence Page 10 of 53

of mesoporous structures and slit holes. After being doped with Bi element, the material hysteresis loop was belonging to type H3. In general, the type H3 ring is associated with slit holes, which is consistent with the layered structure of the TNB. Compared with unmodified TNB (SBET = 375.6 m2/g), Bi-TNB samples had a smaller surface area (SBET). The catalyst specific surface area decreased from 177.9576 m2/g to 39.1436 m2/g after doping with Bi. This may be due to the large number of particles sintered during the calcination process[39]. This suggested that the photocatalytic activity of BiTNB was not closely related to BET surface area. Meanwhile, these results also showed

TNB. 3.2 Degradation characterization of NPX using Bi-TNB

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that the introduction of Bi ion did not significantly change the structural properties of

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3.2.1 Photodegradation of NPX by Bi20TiO32, Bi2O3, TNB and Bi-TNB

In order to compare the modified and unmodified titanate photocatalyst photocatalytic

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activity, the photocatalytic degradation of NPX by photocatalyst was studied with the conditions of 0.25 mg/L NPX initial concentration, 1.0 g/L catalyst dosage and 7.0 pH

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under the visible light irradiation. The photodegradation efficiency of NPX for TNB and Bi-TNB photocatalysts can be seen in Fig. 5(a). The removal effect of Bi-TNB on

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NPX was 99.9%, significantly better than that of TNB, Bi20TiO32 and Bi2O3. Fig. 5(b) depicts the effect of titanate nanomaterials before and after modification on the kinetic rate constant of NPX. After being loaded with Bi, kinetic rate constant increased

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obviously from 0.00229 to 0.04168 min-1, indicating that the photocatalytic activity of titanate nanomaterials was significantly enhanced and Bi played a key role. This may

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because that there are some spaces existing in Ti atoms and Bi20TiO32 crystals have inherent intrinsic point defects. The empty Ti atoms can be replaced by Bi, and positive holes are generated from adjacent oxygen atoms in the tetrahedron[40]. These holes enhance the absorption of Bi-TNB in the visible region, and thus produce a higher photocatalytic activity. 3.2.2 Effect of the initial concentration of NPX

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The photocatalytic performance on the degradation of NPX was studied at different initial concentrations ranging from 0 to 2.8 mg/L with 1 g/L catalyst dosage, controlled at 298.15 K and the pH of the solution was 7. From Fig. 5(c) shows the effect of variable initial concentrations of NPX on the photocatalytic degradation efficiency of Bi-TNB. The results show that the removal rate of NPX decreased from 99.99% to 35.43% as the initial NPX concentration increased from 0.08 to 0.25 mg/L. There were two reasons for this: firstly, the optical transmittance decreased as the initial NPX concentration increased, and these might reduce the number of photons received by the

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catalyst; secondly, the intermediate products and parent molecular would compete for activity site, resulting in a low removal of NPX by the photocatalyst[41]. Fig. 5(d) shows the fitting kinetics results of Bi-TNB degradation at different initial

concentrations. Obviously, –ln (Ct/C0) and time t have a good linear relationship. At

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different initial concentrations of NPX, the photocatalytic reaction rate constants k of catalysts for NPX were 0.00211, 0.01914, 0.02486, 0.0356, and 0.05064 min−1,

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respectively. When the initial NPX concentration was less than 0.25 mg/L, the degradation rate increased as an increasing initial concentration of NPX, which matched

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the experimental results of the concentration. The results show that the photocatalytic degradation reaction of NPX by Bi-TNB with different initial concentrations followed

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the pseudo first-order kinetic equation. 3.2.3 Effect of catalyst dosage

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The photocatalytic degradation reaction occurs on active sites of photocatalyst by governing the adsorption of the pollutant and generating oxidants with high activity.

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For this reason, the amount of catalyst is a key parameter during the photodegradation reactions. The amount of catalyst was studied in the range of 0 g/L up to 1.2 g/L. As presented in Fig. 5(e), the photocatalytic efficiency of NPX raised with increasing BiTNB dosage from 0.2 g/L to 0.5 g/L. This led to an increasing of the catalyst active sites, which could accelerate the reactions to produce ·OH to promoting the NPX degradation efficiency. However, NPX degradation decreased with the amount of photocatalyst further increases by more than 0.5 g/L. It can be explained by the fact that Page 12 of 53

the excess photocatalyst and NPX molecular would gather into cluster, thus reducing the number of active sites on the surface of the Bi-TNB composite, hindering the light penetration in the reactor and reducing the generation of ·OH[42]. Meanwhile, when the catalyst dosage was 0.5 g/L, the kinetic rate constant reached the highest which was.05562 min-1 (Fig. 5(f)). Overall, 0.5 g/L was chosen as the optimal dose for BiTNB degradation of NPX in this experiment. 3.2.4 Effect of the pH value of the NPX solution. As shown in Fig. 5(g), the rate constant of NPX decreased by almost 58 times

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(from 0.07 to 0.0012 min-1) as the pH was varied from 3 to 11, during the degradation

using the Bi-TNB. The results indicated that NPX photolysis was beneficial under

acidic conditions, while the degradation rate decreased under conditions close to neutral

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or alkaline. In Fig. 5(h), the surface charge of the nanocomposite reduced with the

increasing of pH. The reason was that surface functional groups could be ionized by

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reacting with OH- ion in high alkaline medium. However, the surface charge becomes more positive due to the adsorption of h+ on the catalyst surface in acidic medium[43].

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In general, adsorption of organic compounds on the surface of photocatalysts is the first step of photocatalysis. The isoelectric point of NPX is about pH = 3, while the zero potential of Bi-TNB is around pH = 5.2. Thus, the electrostatic attraction between

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positively charged catalyst and negatively charged NPX causes more pollutants to adsorp on the material surface at pH < 5.2, thereby increasing the photocatalytic

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degradation rate. However, both materials and contaminants were negatively charged at alkaline pH. Less contaminants could be adsorbed on the material surface because of

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charge repulsion, resulting in the reduction of both photocatalytic reaction and degradation rate. This was similar with the study by Zarrin et al., which the MO of negative charge could be absorbed by the positive charge on the surface of the photocatalyst under the acidic condition[44]. This facilitated the decolorization reaction and thus led to high photocatalytic activity. By contrast, the coulomb repulsion between the negative photocatalyst surface and the dye anions under alkaline conditions reduce the absorption rate of MO, thus reducing photodegradation efficiency. Page 13 of 53

3.2.5 Effect of anions concentration in solution It has been reported that Cl- has no effect on photocatalytic reactions under neutral or alkaline conditions[45]. Results of this study showed Cl- did not show significant influence on the photocatalytic performance of naproxen degradation by Bi-TNB(Fig. S3(a)), which may be due to the neutral state of Bi-TNB photocatalytic degradation of NPX. Cl- may not be able to effectively remove h+ or ·OH under such neutral condition. Another possibility is that OH reacted with Cl- to form chlorine radicals, which using OH (ψOH = 2.3 eV) to form weak oxidation Cl (ψCl = 2.1 eV)[46]. However, the main

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active substance in the degradation process of NPX in this study was h+ instead of ·OH, so almost no ·OH and Cl participated in the reaction. Therefore, Cl- had little effect on the degradation of NPX which the degradation reaction rate was slightly inhibited. Moreover, the degradation rate did not change significantly after 180 min.

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Fig. S3(b) improved that after adding a certain concentration of CO32-, the

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degradation rate of photocatalytic NPX degradation by Bi-TNB increased as reaction time progresses. Nevertheless, it can be observed that carbonate still had a slight

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inhibitory effect, which became stronger with the increase of CO32-. This may be because the carbonate can react with ·OH radical which the reaction rate constant was 108 M-1s-1, thus forming CO3·- as shown in equation (2)[47]. It indicated that the

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scavenging effect of CO32- resulted in the decrease of degradation rate, which was due to the fact that the reactivity of CO3·-with NPX was lower than that of ·OH. During the

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photocatalytic reactions, ·OH is an important active substance, and CO3·-is a weaker oxidant than ·OH. With the progress of the reaction, ·OH was captured, thus affecting

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the photocatalytic degradation efficiency of NPX. Furthermore, once CO32- was added to the solution, the hydrolysis of CO32- in water will eventually reach the equilibrium of CO32-/ HCO3- system (equation (3)). This is because HCO3- is one of the most important anion in water, and the natural balance of water pH value is maintained by the balance of CO32-/ HCO3- ion pairs[48]. Besides, in the presence of CO32-, the pH value of the solution will increase. According to the experimental results of the effect of pH mentioned above, the alkaline environment was not conducive to the

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photodegradation of NPX, so the photocatalytic degradation rate of NPX was inhibited.

CO32 OH  OH   CO3  CO32  H 2O  HCO3-· ·OH

(2) (3)

Fig. S3(c) shows that SO42- inhibited the photocatalytic degradation rate of NPX by Bi-TNB, but had no significant effect on the removal rate of NPX. This may be because SO42- can react with h+ and ·OH to form SO42- (equation (4) and (5))[49]. The oxidation capacity of SO42- was lower than that of ·OH and h+, so the photocatalytic

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degradation rate of NPX was reduced. At the same time, there may be competitive adsorption between SO42- and NPX on the surface of Bi-TNB, which may also lead to the decrease of photocatalytic degradation rate[50].

SO42  h  SO4

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SO42 OH  SO4  OH 

(4) (5)

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It is observed in Fig. S3(d) that when the concentration of nitrate ion was 5 mg/L, NO3- had little influence on the photodegradation process of NPX, mainly because NO3-

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can react with ·OH to form NO3·with oxidative activity (equation (6))[51]. However, its reaction rate constant with ·OH is very small, and the concentration of NO3- is low, thus it has little influence on the degradation reaction. When the concentration of NO3-

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was 10 mg/L, the photocatalytic degradation of NPX by Bi-TNB was suppressed. The higher concentration of NO3- might compete with NPX for the active substance in the

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solution, causing a large consumption of the active substance. However, there were many active substances produced by the Bi-TNB visible light catalytic system in the

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solution, and a part of the consumed active substances did not induce a significant drop in NPX. Therefore, the removal rate was only decreased compared to the NO3concentration of 0 mg/L.

NO3 OH  NO3 OH 

(6)

3.2.6 Effect of cations concentration in solution According to Fig. S4(a), Cu2+ concentration had no significant effect on Bi-TNB Page 15 of 53

photocatalytic degradation of NPX, possibly because the addition of Cu2+ did not change the pH value of the solution. According to above experiment results, if the pH of the solution changed, the final result of the experiment would be affected. As can be seen from Fig. S4(b), the presence of Ca2+ slightly inhibited the degradation of NPX, and this inhibition increased with the increase of Ca2+. When the concentration of Ca2+ was 0, 5 and 10 mg/L, the degradation rates of NPX were 99.9%, 99.7% and 97.8%, respectively. This may be because the reduction potential of Ca2+ is -2.84 eV which is higher than Bi-TNB[49], so Ca2+ can not capture photoelectrons and does not affect the

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photocatalytic reaction of NPX. It can be seen from Fig. S4(c) that the presence of Mg2+ slightly inhibited the visible light catalytic degradation of NPX by Bi-TNB. When the Mg2+ concentration was 5 mg/L and 10 mg/L, the degradation rate of NPX was about

97.4% and 91.4%, respectively. In this process, Mg2+ might occupy the adsorption and

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reactivity sites of NPX, and then inhibit the visible light catalytic degradation of

NPX[52]. The presence of Fe3+ slightly inhibited the visible light catalytic degradation

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of NPX by Bi-TNB (Fig. S4(d)). This might be due to the dark color of the aqueous

the degradation rate[53].

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solution caused by Fe3+ hydrolysis, which weakened the light intensity and thus reduces

3.2.7 Effect of humus concentration in solution

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It can be seen from Fig. S5(a) that when FA concentration was increased from 5.0 to 10.0 mg/L, the reaction rate constant reduced from 0.0252 min-1 to 0.0102 min-1 (Fig.

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S5(b)), which the reaction was still according with the first-order reaction kinetics. It is because that FA is the dissolved organic matter (DOM) with photochemical activity in

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natural water. Additionally, FA can be used as a photosensitizer to promote the photodegradation of pollutants, or as a photomasking agent or free radical scavenger to inhibit the photocatalytic degradation of pollutants, which is closely related to its composition[54]. HA is a major component of humus and a ubiquitous substance in the water environment and has been found to have the unique potential to accelerate photodegradation of pollutants[55]. Previous studies has reported that HA as a Page 16 of 53

photosensitizer could significantly enhance the photodegradation of steroidal estrogen, which was different in water due to its special composition and aquatic properties[56]. , Consequently, it has an important meaning to study the effect of HA concentration on the photocatalytic degradation of NPX by Bi-TNB and the results are shown in Fig. S5(c). In the presence of 5 mg/L HA, HA could effectively induce the photodegradation of NPX, and its photodegradation rate was twice as fast as that of deionized water, which was consistent with the results reported by Wang et al.[57]. The accelerated

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function of HA was mainly attributed to reactive oxygen species production. Under photochemical action, it would form hydrated electrons and react with dissolved oxygen in water to form O2-·, which in turn combined with oxygen ions in water to

produce H2O2. A series of free radical reactions were initiated to generate new active

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species such as ·OH, 1O2, e-, etc. to promote the photodegradation reaction[58]. Due to the dark color of HA, the chromaticity of the reaction solution is relatively increased

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under conditions of high concentration of HA, which hinders photons from passing through the solution, thereby reducing the rate of photodegradation. In the meantime,

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HA is also a scavenger for the active species. When concentration of HA increased, HA was more likely to capture the active substance ·OH, which had a competitive effect

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with pollutants, thus reducing the photodegradation rate[59]. Accordingly, low concentration of HA can promote the photodegradation of NPX by Bi-TNB. However, when HA concentration exceeds the critical concentration, increasing HA concentration

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has an inhibitory effect on photodegradation.

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3.3 Photocatalytic mechanisms of Bi-TNB 3.3.1 Roles of reactive species The photochemical degradation of organics based on a series of reactive species,

for example, superoxide radicals(O2·-), photoexcited holes (h+), hydroxy radicals(·OH) and electron(e-)[60,61]. In order to provide useful information for the photocatalytic mechanism and ractive species involved in the degradation of NPX, trapping experiments using different scavengers such as IPA (hydroxyl scavenger), BQ Page 17 of 53

(superoxide scavenger) and sodium oxalate (hole scavenger) have been investigated. The contribution of different reactive species toward photocatalytic degradation of NPX is shown in Fig. 6(a). As illustrated in Fig. 6(a), the degradation efficiency of NPX was remarkably prohibited when the scavenger BQ for trapping holes was added. The photocatalytic degradation rate of NPX was decreased by 6 times without benzoquinone (Fig. 6(b)), suggesting the role of superoxide radicals during reactions. The photodegradation performance was quenched drastically while applying sodium oxalate as a scavenger.

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Its photocatalytic degradation rate was reduced by 16 times which was more obvious than that of benzoquinone, revealing that h+ played a major role in the process of degradation. Photocatalytic degradation of NPX by Bi-TNB was hardly affected by the addition of isopropanol, indicating that ·OH was not an important reactive species in

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explaining the NPX degradation behavior. This might be due to the fact that ·OH was an important reactive species in photocatalytic reaction, and its higher redox potential

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(+1.99 V) was unable to directly originated from the path of hole oxidation[62]. Moreover, the redox potential of Bi V/Bi III (+1.59 V) is more negative than the

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conduction band edge potential of ·OH/H2O (+2.77 V) and ·OH/OH- (+1.99 V), therefore Bi-based photocatalysts are difficult to generate ·OH[63]. Thus, h+ and O2·-

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were the main oxidative species responsible for the NPX degradation, and h+ played a relatively important role.

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3.3.2 Mechanisms of enhanced photocatalytic performance According to the model of charge transfer in the conventional type II

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heterojunction, it is difficult to explain the formation of active species on the basis of the experimental results above[64]. Under the experimental conditions, the results showed that h+ and O2·- were the major active species in the Bi-TNB photocatalytic system which had high photocatalytic activity, a simplified mechanism diagram was offered in Scheme 1 to represent the proposed reaction mechanism. Based on the previous research, we have calculated the VB and CB of Bi20TiO32 and Bi2O3 which the results shown in Table S1. (see Supporting information for detail calculation Page 18 of 53

process). In the Bi-TNB photocatalytic system, the CB position of the Bi2O3 (0.41 V) and Bi20TiO32 (0.36 V) were more negative than the standard redox potential of O2/H2O2 (0.7 V), therefore could capture O2 to product H2O2[65-67]. Furthermore, it can be known that the Bi-based photocatalytic system is not conducive to the formation of ·OH from the above description of quenching experiment, thus were hard to react with OHto form ·OH. According to the above, all h+ in the VB of the Bi2O3 directly oxidize NPX. Furthermore, the photoelectrons in the CB of Bi2O3 and the photoholes in the VB of Bi20TiO32 recombined at the interface of Bi20TiO32 and Bi2O3 to form a visible light-

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driven Z-scheme[68]. This scheme could further improve photocatalytic performance by eliminating the relatively useless electrons (from CB of Bi2O3) and holes (from VB

of Bi20TiO32), but holding the useful electrons (from CB of Bi20TiO32) and holes (from

VB of Bi2O3). As a result, the two main reactive species (h+ and O2·-) actively

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participated in the photocatalytic reaction, thereby NPX was degraded.

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3.3.3 Possible degradation pathway of NPX in Bi-TNB

Degradation of intermediates and by-products was as important as degradation of

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maternal drug contaminants. The intermediates produced during the NPX degradation were analyzed via the high-performance liquid chromatography–ion trap mass spectrometry (HPLC-IT-MS) (Agilent Technologies, USA) and some intermediates that

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have been reported in the literature have been detected. The structural elucidation of the degradation products were based on mass spectrum analysis (Table 1). The

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determination of the by-products indicated that decarboxylation was the main initial process of NPX degradation[69]. Among them, intermediates with m/z of 158, 186, 200,

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202, and 218 were assigned as the decarboxylation products of NPX. The spectral analysis showed that the major fragments of m/z 218.25, m/z 202.0991, m/z 158 and m/z 186.1042 respectively correspond to 1-(6-methoxy-2-naphthylethyl) hydrogen peroxide, 1-(6-methoxy-2-naphthyl methyl) ethanol, 2-naphthyl methyl ether and 2ethyl-6-methoxynaphthalene. The first pathway to form intermediate was the decarboxylation of the carboxyl groups, and then reacted with superoxide radical. The presence of the product with m/z 200.23 corresponded to 6-methoxy- 2Page 19 of 53

acetylnaphthalene, which could be formed by dehydration of 1-(6-methoxy-2naphthylmethyl) ethanol. The previous literatures have reported that these compounds were by-products of NPX photoconversion[70,71]. Some small molecular acids have also been detected from the MS mass spectrum which proved to be ring-opening products of NPX, such as malic acid, succinic acid, acetic acid, propionic acid, etc. The results showed that NPX and its degradation intermediates might be further decomposed by Bi-TNB, ring-opening products appeared and were finally mineralized to CO2 and H2O.

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On the basis of intermediate identification, the NPX degradation pathways were proposed, as shown in Scheme 2. Similar to the study by Wang et al[72], photocatalytic

degradation of NPX by Bi-TNB was mainly through two primary pathways, including hydroxylation and decarboxylation reactions, were involved in the photocatalytic

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degradation of NPX. In photocatalytic path I , O2·- attacked on the most positive point charge C (22) atom to cleave the carboxyl, while generating carbon center radicals T1.

and 218, which

are

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Afterwards, T1 was further oxidized by O2·- or 1O2 to form the product of m/z 186, 200 respectively 2-ethyl-6-methoxynaphthalene,

2-acetyl-6-

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methoxynaphthalene, 1-(6-methoxy-2-naphthylethyl) hydrogen peroxide[73]. T1 could also react with ·OH to form 1-(6-methoxy-2-naphthyl methyl) ethanol, which was

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further oxidized to be product of m/z 200[74]. Another decarboxylation reaction occurred by h+ attacked on C (1), resulting in the formation of 2-naphthylmethyl ether which was product of m/z 158. Finally, some of the small molecule carboxylic acids,

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which were ring-opening products, were eventually mineralized to CO2 and H2O.

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3.4 Practical application of Bi-TNB 3.4.1 Effect of PPCPs coexistence on photocatalytic degradation As is known, PPCPs seldom appear in the environment alone. In order to

investigate how the maternal and its degradation products interact in the environment, this paper selected the mixture of two PPCPs to study, ACT and NPX. These two kinds of PPCPs have a high usage rate and are frequently detected in aquatic environment, such as surface water and effluent of the wastewater plant. Degradation of ACT and Page 20 of 53

NPX as separate PPCPs under visible light irradiation has been reported[75]. However, reports on the photocatalytic degradation of PPCPs coexistence are still lacking. Therefore, this study aimed to determine the efficiency of visible photocatalytic degradation of ACT and NPX mixture by Bi-TNB. Since the concentration discrepancies of PPCPs in the environment may be greatly different, the influence of PPCPs mixture with different concentrations on photocatalytic degradation need to be considered. We considered equal (1:1) and nonequal (ACT:NPX = 1:2 or ACT:NPX = 2:1) concentration conditions to investigate the

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effect of photodegradation by Bi-TNB under the visible light. Experimental results (Fig. 7(a), 7(b) and 7(c)) sugguseted that NPX degraded faster than ACT in the mixture of

PPCPs. Moreover, the degradation of NPX seemed to be affected by the increase of ACT or NPX concentration.

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The degradation observed in the PPCPs mixture can be adequately described by the pseudo-first-order kinetics model in Table 2. The effect of higher concentrations of

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ACT and NPX was obvious that the degradation rate of NPX decreased from 0.3674 min-1 to 0.0251 and 0.0141 min-1, which requiring almost 180 min of irradiation to

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achieve complete degradation. Nevertheless, it could be completely degraded in a 2:1 mixture for only 90 minutes. When the concentration of NPX was twice that of ACT,

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although the degradation rate of NPX increased slightly from 0.0141 min-1 to 0.0251 min-1, there was no substantial difference in the total degradation time of NPX.

When

the mixture was not equimolar, the degradation rate constant of NPX decreased, while

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that of ACT increased slightly.

The presence of chemicals or pollutants in the water systems may interact

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with ·OH and/or compete for ·OH radicals produced during the process of Bi-TNB photocatalytic. Thess results were consistent with that of Pereira et al., who found that when contaminants existed in a mixture, the initial reaction rate of oxolinic acid and oxytetracycline decreased due to competition of holes or ·OH radicals[76]. Besides, the properties of PPCPs molecules and intermediates formed during the photocatalytic treatment of Bi-TNB perhaps affect the degradation rates of ACT and NPX[77]. Comparison of degradation rates of individual PPCPs and their mixtures is plotted Page 21 of 53

in Fig. 7(d). The pseudo-first-order rate constant of ACT (0.00385 min-1) in an iequimolar mixture was higher than that in the presence of ACT alone (0.00336 min-1). However, there was no difference in the rate of ACT in the mixture of non-equal concentrations. For NPX, the pseudo-first-order rate constants for the individual and equimolar mixtures were similar, while the rate in the non-equal concentration mixtures was reduced. In addition, NPX was more easily degraded by Bi-TNB under the visible light alone. These results revealed that PPCPs behaved differently not only under various conditions but also under the same conditions, which further highlighted the

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complexity of processing a variety of PPCPs pollutants in aqueous. Although the behavior of PPCPs was different in the mixture, Bi-TNB photocatalyst might be an effective way to degrade them when they exist simultaneously.

Although ACT and NPX are both antiinflammatory analgesics, they have different

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adsorption capacities on the surface of Bi-TNB. The photodegradation of pollutants depends not only on the formation of hydroxyl radicals but also on the adsorption

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capacity of pollutants of photocatalysts surface. Moreover, the two pollutants would compete on the surface of photocatalyst adsorption because of their different physical

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and chemical properties[78]. Therefore, the reason why the photocatalytic degradation effect of Bi-TNB on ACT and NPX was different might be that Bi-TNB had selective

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adsorption effect on different PPCPs and the adsorption efficiency of PPCPs was contrary to the order of molecular size. These quantitative results showed that Bi-TNB exhibited a highly selective photocatalytic effect on the charge and size adsorption of

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PPCPs on the surface of Bi-TNB[79]. Consequently, the selective photocatalytic rate constant changed with the order of adsorption capacity: NPX > ACT.

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Fig. 7(d) also shows the effect of the coexistence of ACT and NPX on the

photocatalytic degradation of naproxen and ACT by Bi-TNB under visible light. Both ACT and NPX concentrations were 0.5 mg/L. The coexistence of ACT and NPX enhanced the degradation of ACT by Bi-TNB. This may be due to the rapid consumption of h+ during the photocatalytic degradation of NPX, resulting in recombination of e- and h+ to be inhibited. Therefore, ACT can be oxidized by more h+. On the other hand, because the molecular structure of the ACT and NPX was different, Page 22 of 53

there might be competition relations between the h+ of ACT and the e- of NPX. In the presence of NPX, the photocatalytic degradation of ACT slowly consumes h+ to make the recombination of e- and h+ easilier to occur. Thus, the coexistence of ACT and NPX hindered the photocatalytic degradation of NPX by Bi-TNB under the visible light, which was similar to the study by Xu et al. that the coexistence of MO and RhB hindered the photocatalytic reduction of Cr (VI)[80]. 3.4.2 Photocatalytic degradation in different water matrices The presence of various substances in the natural water sample has effect on the

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removal efficiency of the target contaminant. In order to assess whether the Bi-TNB photocatalytic system might be applied to the decomposition of NPX under ambient

conditions, the degradation of NPX was performed in different natural water samples.

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The water quality of the four real samples was exhibited in Table S2.

As shown in Fig. 8(a), the degradation rate of NPX in deionized water (DW) and

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reservoir water (RW) was basically identical. The concentration of NPX in reservoir water was reduced about 96.9% in 360 min under solar-simulated irradiations. The NPX

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degradation rates were reduced to 69.8%, 51.2% and 14.6%, respectively, in the effluent of second sinking pool (SSP), the disinfected water (DFW) and the sewage plant raw water (SPRW). As illustrated in Fig. 8(b), the apparent velocity constants of NPX

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degradation in DW, RW, SSP, DWF and SPRW was respectively 0.02532 min-1, 0.00784 min-1, 0.0034 min-1, 0.00186 min-1 and 0.0004 min-1, indicating that the NPX

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degradation efficiency in natural water samples was much lower than that in deionized water, which was attributed to the consumption of ROS by DOM[81]. The properties

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of DOM include electron donating ability, molecular weight fraction, hydrophobicity/ hydrophilicity, and chromophore/fluorophore, which would affect reactivity with free radical species[82]. Furthermore, the natural water sample contains various inorganic, organic and dissolved substances (such as HS and salts), different types and intensity of ions in the water, and the hindrance to light conduction, which ultimately resulted in different degradation effects. It was reported that the clearance rate constant of ·OH by DOM was 2.5×104 (mg·L-1)-1s-1[83]. DOM as a precursor to TOC may be the major Page 23 of 53

competitor of NPX for reactive species, based on water quality index data, including TOC, Cl- (chloride) and NO3- (for N). In addition, the DOM could also cover and firmly adhere to the surface of the photocatalyst and prevent the penetration of light, which together deactivated the photocatalyst and inhibited the production of active substances degraded by NPX[84]. Nevertheless, NPX could be completely remove by Bi-TNB after the real reservoir water was extended to 6 h of irradiation time, indicating that the photocatalyst had good water purification practicality. The correlation analysis between photocatalytic degradation efficiency of NPX

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and other influencing factors are shown in the Table S3. Except for the fact that the positive correlation between the TP and degradation efficiency of NPX was weak, degradation rate of NPX had a negative correlation with other quality metrics. Among them, the correlativity between NPX degradation efficiency and TN was the largest,

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suggesting that TN was the primary factor of photocatalytic degradation of NPX and

showed a negative correlation. This might be due to the fact that nitrate ions in TN

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reduced the rate of photocatalytic degradation of NPX, and inhibited the photodegradation of NPX at higher concentrations.

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Nitrate (NO3-) is one of the primary reactive nitrogen sources playing a critical role in the natural nitrogen cycling in aquatic ecosystems[85]. The inhibition of

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photocatalytic degradation of NPX by NO3- was mainly attributed to the competition of active substances or adsorption, between NO3- and NPX, thus occupying the

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photocatalytic active site of Bi-TNB and affecting its catalytic activity. 3.4.3 Photocatalytic degradation of NPX under the condition of the sunlight

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In order to examine the photodegradation of PPCPs exposed to sunlight under natural conditions, the NPX solution was exposed to sunlight in an open environment, which is more realistic than simulated light. As illustrated in Fig. 8(c), the photocatalytic degradation rate of NPX by Bi-TNB under solar sunlight is greatly accelerated under simulated visible light conditions. This might be due to the fact that sunlight contains ultraviolet light, which made it more energetic to accelerate photocatalytic degradation of NPX. Generally, natural water was more or less turbid Page 24 of 53

owing to the presence of suspended matters. Moreover, light scattering and quenching of PPCPs might slow down the degradation of PPCPs, or accelerate the process and then sensitize 3O2 to 1O2[86]. These results indicated that the degradation of NPX by Bi-TNB under visible light has practical value in the natural water. This technology is expected to be applied to solve the problem of drug contamination in water in the future, which found a basis for the popularization of visible photocatalytic degradation of PPCPs by titanate nanomaterials. 3.4.4 Reusability of recycled Bi-TNB photocatalyst

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The reusability and stability in photocatalytic reaction processes were evaluated

for the Bi-TNB sample by cycle experiments. As can be seen from Fig. 8(d), the

insignificant decrease in the degradation efficiency of Bi-TNB towards the NPX

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degradation after three consecutive test confirmed photocatalyst could still effectively maintain its excellent photocatalytic activity. In addition, we could see no obvious

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change in X-ray diffraction patterns of Bi-TNB after the third cycle of the photocatalytic degradation in Fig S6. It was likely due to the formation of heterostructures, resulting

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in a faster transfer of photoelectron-hole pairs. Therefore, we can say that TNB composited with Bi results in an efficient and reclaimable photocatalyst. Additionally, the Bi-TNB composition Bi2O3 and titanate are both atoxic which shows the potential

4. Conclusion

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of Bi-TNB nanophotocatalysts for using in the environment.

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A visible-light-driven recoverable Bi-TNB nanophotocatalyst was developed through hydrothermal-calcining method to degrade NPX. Under the optimum operating

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condition (initial concentration of NPX was 0.25 mg/L, catalyst dosage was 0.5 g/L, and pH value was 7.0), the degradation rate of NPX was 99.9% within 180 minutes and the degradation process of NPX followed pseudo first-order kinetics. When anions, cations and humus existed in aqueous solution, NPX could also be effectively degraded. The reactions in the presence of radical quenchers revealed that h+ and O2·- were the dominant active species for the oxidization of NPX, in which hole h+ played a relatively important role. The identification of transformation intermediates indicated that eleven Page 25 of 53

intermediates and by-products were formed during the degradation of NPX. The photocatalytic degradation of NPX by Bi-TNB was mainly through two main pathways, including hydroxylation and decarboxylation, and it was speculated that most NPX and intermediate products would be mineralized into final products CO2 and H2O with the increase of irradiation time. In practical applications, the coexistence of ACT and NPX hindered the photocatalytic degradation of NPX by Bi-TNB in visible light. Reactions in different water matrices indicated that the Bi-TNB can be effectively used for the degradation of NPX under ambient water conditions. In addition, the photodegradation

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rate of NPX using Bi-TNB under the actual sunlight condition was much faster than that under the simulated visible light condition, which laid a foundation for the actual

Conflict of interest statement

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None

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application of titanate photocatalyst.

Acknowledgements

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The authors would like to gratefully acknowledge the financially support from the National Natural Science Foundation of China (No. 51778146), the Outstanding Youth

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Fund of Fujian Province in China (No. 2018J06013), and the Open Project Program of

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National Engineering Research Center for Environmental Photocatalysis (No. 201901).

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Page 36 of 53

Figure Caption Fig. 1. X-ray diffraction patterns of TNB and Bi-TNB. Fig. 2. SEM images of TNB (a) and Bi-TNB (b). Fig. 3. UV-Vis spectra of Bi20TiO32, Bi2O3, TNB and Bi-TNB (Inset is the Tauc plot). Fig. 4. N2 adsorption-desorption isotherms of TNB and Bi-TNB. Fig. 5. Photodegradation of NPX under visible light (a) effect of Bi20TiO32, Bi2O3, TNB and Bi-TNB, (b) reaction kinetics fit by different photocatalysts (c) effect of initial NPX concentration,

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(d) reaction kinetics fit at different initial concentrations, (e) effect of Bi-TNB catalyst dosages, (f) reaction kinetics fit under different dosages, (g) effect of various pH values, (e) zeta potential of NPX and Bi-TNB in deionized water at different pH values.

Fig. 6. Photodegradation of NPX by Bi-TNB under visible light (a) effect of different scavengers, (b)

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pseudo-first-order kinetic constants with different scavengers, (c) cycling experiments with recycled Bi-TNB.

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Fig. 7. Photocatalytic degradation of NPX and ACT coexisting in deionized water (a) ACT: NPX (concentration ratio) = 1:1, (b) ACT: NPX (concentration ratio) = 1:2, (c) ACT: NPX

their mixtures.

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(concentration ratio) = 2:1, (d) Comparison of degradation rates of individual PPCPs and

Fig. 8. (a) Effect of different water matrix conditions on photocatalytic degradation of NPX, (b)

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rate constant of photocatalytic reaction mixtures, (c) photocatalytic degradation of NPX by Bi-TNB under actual sunlight. (DW refers to deionized water; RW refers to reservoir

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water; SSP refers to second sinking pool; DFW refers to disinfected water; SPRW refers to sewage plant raw water)

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Scheme 1. Possible photocatalytic reaction mechanism of NPX over Bi-TNB under visible light irradiation.

Scheme 2. Proposed pathway for photocatalytic degradation of NPX in Bi-TNB solution.

Page 37 of 53

 Na2Ti3O7 pdf: 31-1329

TiO2 pdf: 46-1237

 Bi20TiO32 pdf: 42-0202

Bi2O3 pdf: 50-1088

Intensity (a.u)



 



TNB



20







Bi-TNB

30 40 50 60 2Theta (degrees)

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na

lP

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Fig. 1. X-ray diffraction patterns of TNB and Bi-TNB.

Page 38 of 53





-p

10



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



70



80

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Fig. 2. SEM images of TNB (a) and Bi-TNB (b).

Page 39 of 53

1.4

2.5 Bi20TiO32

1/2

1.0

(ahv)

2.0

Bi2O3

1.5

TNB Bi-TNB

1.0

0.8

2.10 eV

0.5

0.6

0.0

2.19 eV

3.07 eV

2.63 eV

1.6 1.8 2.0 2.2 2.4 2.6 2.8 3.0 3.2 3.4 3.6 3.8

hv(eV)

0.4

Bi20TiO32

0.2

Bi2O3

0.0 300

400

500

600

TNB Bi-TNB

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Absorbance (a.u.)

1.2

700

800

-p

Wavelength (nm)

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Fig. 3. UV-Vis spectra of Bi20TiO32, Bi2O3, TNB and Bi-TNB (Inset is the Tauc plot).

Page 40 of 53

TNB-desorption TNB-adsorption Bi-TNB-desorption Bi-TNB-adsorption

140 120 100 80 60 40 20 0 0.0

0.2

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3

Quantity Adsorbed (cm /g STP)

160

0.4 0.6 0.8 Relative Pressure (P/P0)

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re

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Fig. 4. N2 adsorption-desorption isotherms of TNB and Bi-TNB.

Page 41 of 53

1.0

(a)

1.0

Bi-TNB TNB Bi20TiO32

0.8

Bi2O3

t

C /C

0

0.6 0.4 0.2 0.0 30

60

90

120

Time (min)

(b) 8

5

Bi2O3

180

-1

k=0.04168 min -1 k=0.00428 min -1 k=0.00306 min -1 k=0.00229 min

4 3

re

-ln (Ct/C0)

-p

6

Bi-TNB TNB Bi20TiO32

7

150

ro of

0

2

0 0

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1 20

40

60

80

100 120 140 160 180

Time (min)

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1.0

(c)

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0.8

Ct/C0

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0.6

0.08 mg/L 0.25 mg/L 0.5 mg/L 1.5 mg/L 2.8 mg/L

0.4 0.2 0.0

0

30

60

90 120 Time (min)

Page 42 of 53

150

180

10

-1

(d) 8 6

-ln (Ct/C0)

k=0.05064 min -1 k=0.03560 min -1 k=0.02486 min -1 k=0.01914 min -1 k=0.00211 min

0.08 mg/L 0.25 mg/L 0.5 mg/L 1.5 mg/L 2.8 mg/L

4 2 0 30

60

90 120 Time (min)

(e) 1.0 0.8

180

0 g/L 0.2 g/L 0.5 g/L 1.0 g/L 1.2 g/L

-p

0.4

re

Ct/C0

0.6

150

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0

0.0 0

30

60

90 120 Time (min)

150

180

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10

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0.2

(f)

0.2 g/L 0.5 g/L 1.0 g/L 1.2 g/L

9 8

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7

-ln (Ct/C0)

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6 5

-1

k=0.05562 min -1 k=0.04711 min -1 k=0.04570 min -1 k=0.00834 min

4 3 2 1 0

0

20

40

60

80 100 120 140 160 180 Time (min)

Page 43 of 53

(g)

1.0 0.8

Ct/C0

0.6

pH=3 pH=5 pH=7 pH=9 pH=11

0.4 0.2 0.0 60

2

4

90 120 Time (min)

10 0 0

6

8

-20

-50

10

12

re

-30 -40

180

-p

-10

150

NPX Bi-TNB

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Zeta Potential (mV)

(h)

30

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0

pH

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Fig. 5. Photodegradation of NPX under visible light (a) effect of Bi20TiO32, Bi2O3, TNB and Bi-TNB, (b) reaction kinetics fit by different photocatalysts (c) effect of initial NPX concentration, (d)

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reaction kinetics fit at different initial concentrations, (e) effect of Bi-TNB catalyst dosages, (f) reaction kinetics fit under different dosages, (g) effect of various pH values, (e) zeta

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potential of NPX and Bi-TNB in deionized water at different pH values.

Page 44 of 53

(a) 1.0

Control IPA BQ SS

0.8

Ct/C0

0.6 0.4 0.2 0.0 30

60

90

120

Time (min)

(b)

0.05

0.04711 0.04208

0.02

0.00821

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0.01 0.00

180

-p

0.03

re

-1

k (min )

0.04

150

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Control

IPA

BQ

0.00293 SS

Species

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Fig. 6. Photodegradation of NPX by Bi-TNB under visible light (a) effect of different scavengers, (b)

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pseudo-first-order kinetic constants with different scavengers.

Page 45 of 53

(a) 1.0

ACT NPX

0.9 0.8 0.7

Ct/C0

0.6 0.5 0.4 Photodegradation

0.3 0.2

Dark adsorption

0.1 -30

0

30

60 90 Time (min)

120

1.0

(b)

180

Photodegradation 0.8

-p

0.6 Dark adsorption

0.2

re

0.4

ACT NPX

0.0 -60

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Ct/C0

150

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0.0 -60

-30

0

30

60

90

120

150

180

Time (min)

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Photodegradation

(c)

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Ct/C0

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0.6 0.4

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0.2

Dark adsorption 0.0 -60

-30

0

30

60

90

Time (min)

Page 46 of 53

120

150

180

0.05486

(d) 0.05

-1

k (min )

0.04

0.03024

0.03 0.02 0.01

0.00336

PX ACT CT T single A ACT in NPX+ single N X in NPX+AC NP

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0.00

0.00385

Fig. 7. Photocatalytic degradation of NPX and ACT coexisting in deionized water (a) ACT: NPX (concentration ratio) = 1:1, (b) ACT: NPX (concentration ratio) = 1:2, (c) ACT: NPX (concentration

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ratio) = 2:1, (d) Comparison of degradation rates of individual PPCPs and their mixtures.

Page 47 of 53

1.0

(a)

SPRW

0.8 0.6

Ct/C0

DFW

0.4

SSP

0.2 RW DW

0

(b) 0.025

60

120

180 240 Time (min)

0.02532

0.010

0.00784

0.005

0.00186

DW

RW

SSP DFW Water sample

0.00040

SPRW

na

(c) 1.0

0.00340

lP

0.000

360

-p

0.015

re

-1

k (min )

0.020

300

ro of

0.0

0.8

Jo

Ct/C0

ur

0.6 0.4

Control Sunlight Simulated visible light

0.2 0.0 0

30

60

90 120 Time (min)

Page 48 of 53

150

180

(d) 1.0 st

nd

1 run

2

rd

3

run

run

Ct/C0

0.8 0.6 0.4 0.2 0.0 100

200

300

400

Time (min)

500

600

ro of

0

Fig. 8. (a) Effect of different water matrix conditions on photocatalytic degradation of NPX, (b)

rate constant of photocatalytic reaction mixtures, (c) photocatalytic degradation of NPX by Bi-TNB

-p

under actual sunlight. (d) cycling experiments with recycled Bi-TNB. (DW refers to deionized

water; RW refers to reservoir water; SSP refers to second sinking pool; DFW refers to disinfected

Jo

ur

na

lP

re

water; SPRW refers to sewage plant raw water)

Page 49 of 53

ro of -p

Jo

ur

na

lP

re

Scheme 1. Possible photocatalytic reaction mechanism of NPX over Bi-TNB under visible light irradiation.

Page 50 of 53

ro of

Jo

ur

na

lP

re

-p

Scheme 2. Proposed pathway for photocatalytic degradation of NPX in Bi-TNB solution.

Page 51 of 53

Table 1. Intermediates and reaction by-products identified by HPLC-MS analysis. m/z

Proposed structure

C13H14O

186.1042

21388-17-0

C13H14O2

202.0991

77301-42-9

C13H12O2

200.23

3900-45-6

C11H10O

158.197

93-04-9

C13H14O3

218.0865

C4H6O5

134.0217

C2H4O2

60.0216

lP

re

-p

-

132.0064

6915-15-7

64-19-7

328-42-7

na

C4H4O5

CAS no

ro of

Intermediates

118.03

100-15-6

74.0213

79-09-4

C3H6O3

90.0313

50-21-5

C3H4O2

72.0207

79-10-7

ur

C4H6O4

Jo

C3H6O2

Page 52 of 53

Table 2. Photocatalytic degradation kinetics of different initial concentration ratios of NPX:ACT. NPX:ACT=1:1

NPX:ACT=1:2

NPX:ACT=2:1

NPX

0.03674

0.0141

0.0251

ACT

0.00329

0.0038

0.0033

Jo

ur

na

lP

re

-p

ro of

kapp (min-1)

Page 53 of 53