Sea-Buckthorn-Like MnO2 Decorated Titanate Nanotubes with

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Sea-Buckthorn-Like MnO Decorated Titanate Nanotubes with Oxidation Property and Photocatalytic Activity for Enhanced Degradation of 17#-Estradiol under Solar Light Penghui Du, Junjun Chang, He Zhao, Wen Liu, Chenyuan Dang, Meiping Tong, Jinren Ni, and Baogang Zhang ACS Appl. Energy Mater., Just Accepted Manuscript • DOI: 10.1021/acsaem.8b00197 • Publication Date (Web): 04 May 2018 Downloaded from http://pubs.acs.org on May 5, 2018

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ACS Applied Energy Materials

Sea-Buckthorn-Like MnO2 Decorated Titanate Nanotubes with Oxidation Property and Photocatalytic Activity for Enhanced Degradation of 17β-Estradiol under Solar Light Penghui Du,1,2 Junjun Chang,2 He Zhao,2 Wen Liu,*,1,3 Chenyuan Dang,1 Meiping Tong,1 Jinren Ni,1 Baogang Zhang*,4 1

The Key Laboratory of Water and Sediment Sciences, Ministry of Education, College

of Environmental Sciences and Engineering, Peking University, Beijing, 100871, China 2

Beijing Engineering Research Center of Process Pollution Control, Division of

Environment Technology and Engineering, Institute of Process Engineering, Chinese Academy of Sciences, Beijing 100190, China 3

School of Civil and Environmental Engineering, Georgia Institute of Technology,

Atlanta, GA 30332, USA 4

Key Laboratory of Groundwater Circulation and Evolution, School of Water

Resources and Environment, China University of Geosciences Beijing, Beijing 100083, China

*Corresponding authors Tel: +1-334-444-7129, E-mail: [email protected] (W. Liu) Tel: +86-138-1026-6109, E-mail: [email protected] (B. Zhang) 1 ACS Paragon Plus Environment

ACS Applied Energy Materials 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

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ABSTRACT A new class of sea-buckthorn-like composite material, MnO2/TNTs, was developed and used for 17β-estradiol (E2) degradation in this study. The new material was characterized as amorphous MnO2 nanoparticles decorated on titanate nanotubes, with chemical formula of 0.4MnO2•Na1.1H0.9Ti3O7. The MnO2 fraction can pre-activate E2 through one-electron oxidation, while the TNTs skeleton is the primary photocatalysis center. Unlike the traditional weak oxidation and photocatalytic degradation, synergetic effect of these two processes lead to efficient E2 removal by MnO2/TNTs under simulated solar light. The apparent first-order rate constant (k1) was determined to be 0.198 min-1, which was ~28 times of that for MnO2/TNTs direct oxidation without light and ~15 times of that for calcined TNTs photocatalysis with light. Moreover, higher TOC elimination rate (82.6% at 1 h) was also obtained compared to that in pure MnO2 system. Dual-enhanced mechanisms are proposed to interpret the high E2 degradation efficiency: 1) heterojunction structure of MnO2 and titanate results in inhibited electron-hole recombination and promoted visible-light-driven photocatalytic activity, and 2) synergy of pre-oxidation and photocatalysis leads to high reactivity on activated E2 radical and •OH coupling. Products identification and density functional theory (DFT) calculation further confirm the reaction pathway of the radical coupling, which is a key linkage between pre-oxidation and photocatalysis. The developed MnO2/TNTs materials appear promising for degradation of emerging phenolic pollutants under solar light.

KEYWORDS:

synergy,

titanate,

MnO2,

pre-oxidation,

17β-estradiol

2 ACS Paragon Plus Environment

photocatalysis,

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1. INTRODUCTION Endocrine disruptor chemicals (EDCs) have drawn increasing attentions because of their world-widely occurrence in surface waters and wastewaters. The dramatic interruption effect of EDCs on hormonally mediated processes can cause developmental disorders even at low doses.1, 2 17β-Estradiol (E2), one of the most detected EDCs, is a chemical with particular concern due to its important role in the regulation and maintenance of female reproductive system.3, 4 The removal of trace EDCs challenges traditional treatment technologies such as advance oxidation processes (AOPs), because these compounds in aquatic environment are generally at levels of µg/L or lower,5-7 and the ubiquitous humic substances usually consume most produced reactive oxygen species (ROS) from AOPs systems.8,

9

Interestingly,

manganese dioxide (MnO2), which is non-toxic and earth abundant, is found to be a selective and efficient oxidant for phenolic contaminants due to the formation of phenoxyl radical through one-electron transfer.10, 11 Considering the advantages of AOP and MnO2 oxidation, a combination of these two technologies can be an effective approach for trace EDCs degradation. Heterogeneous photocatalysis using TiO2-based materials is one of the most concerned AOPs for organic pollutants removal due to the low-cost raw materials and high energy utilization efficiency.12-15 In recent years, the one-dimensional (1D) titanate nanotubes (TNTs) have attracted great research interests considering their specific characteristics, including large specific area, optical and electrical quantum effects, easy separation performance, high stability, and good ion exchange property.16-20 Therefore, TNTs are widely used as adsorbents and supporting skeletons for composite materials synthesis.21-24 However, the photocatalytic activity of pure titanate is blamed because of the fast electron-hole recombination rate.25-27 Hence,



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numerous methods have been applied to improve the photocatalytic activity of TNTs, especially the metal/non-metal doping, CdS doping and heterojunction architecture.23, 25, 28-30

great

Besides oxidation property, MnO2 also manifests semiconductor properties and optical

absorption

in

the

visible

light

region,31-33

thus

enhanced

photo-degradation efficiency of organic pollutants by various MnO2/TiO2 composites under solar light have been reported.34-37 TNTs show different properties compared with TiO2 in view of structure and crystalline phase, however, few studies focus on MnO2 modified TNTs heterojunction photocatalysts. Based on the selective oxidation property and potential optical quantum effect of MnO2, we propose that MnO2 decorated TNTs (MnO2/TNTs) could be promising photocatalysts for E2 removal, while the degradation mechanisms should be deeply investigated. Therefore, the specific goals of this work were to: (1) develop a method to synthesize novel MnO2 nanoparticles decorated TNTs composite materials (MnO2/TNTs); (2) test the effectiveness of MnO2/TNTs for E2 removal under solar irradiation; (3) propose the underlying degradation mechanisms on pre-oxidation and photocatalytic degradation, and interpret the specific role of pre-oxidation; (4) elucidate the E2 degradation pathway and further explain the reactions by means of density functional theory (DFT) calculation, and (5) examine the reusability of the materials. The key objective of this study is to clearly illustrate the synergetic mechanism on pre-oxidation and photocatalysis using MnO2/TNTs for phenolic organics degradation.

2. EXPERIMENTAL SECTION 2.1. Chemicals TiO2 (P25, ~80% of anatase and ~20% of rutile) nanoparticles were obtained from Degussa (Evonik) of Germany. NaOH, HClO4, MnCl2·4H4O, KMnO4, ethylene


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diamine tetraacetic acid (EDTA), p-benzoquinone (BQ) and terephthalic acid were purchased from Acros Organics (Morris Plains, NJ, USA). Alcohol, methanol, acetonitrile and acetic acid glacial were of HPLC grade (>99% purity) and purchased from EMD Millipore (Billerica, MA, USA). 17β-estradiol (properties shown in Table S1) was got from Sigma-Aldrich (St. Louis, MO, USA), and a stock solution of 5 mM was prepared using menthol as solvent. All other solutions were prepared using deionized (DI) water (Milli-Q DI water system, 18.2 MΩ·cm). 2.2. Synthesis and Characterizations of MnO2/TNTs TNTs were firstly synthesized through an alkaline hydrothermal method based on our previous studies.38, 39 Specifically, 1.2 g TiO2 (P25) was dispersed into 10 M NaOH solution (66.7 mL). After magnetically stirring for 12 h, the suspension was transferred into a Teflon reactor and heated at 130 °C for 72 h. Then the white precipitates were washed with DI water to neutral and final products were obtained after dried at 105 °C for 4 h. Decoration of MnO2 on TNTs was modified according to the classical Murrayʼs method.40, 41 0.24 mmol MnCl2·4H4O was dispersed into 100 mL DI water, and N2 gas (>99.7%) was injected to remove all the dissolved oxygen in solution. Afterwards, 0.2 g TNTs was added into the solution and then the suspension was shaken at 200 rpm and 25 °C for 4 h to reach adsorption equilibrium of Mn2+. 3.2 mmol of KMnO4 and 6.4 mmoL of NaOH were mixed in 100 mL DI water to form a homogeneous solution, which was then slowly poured into the TNTs-Mn2+ suspension under stirring. Brown precipitates were formed and the mixture was shaken for another 1 h. After that, the obtained material was allowed to stay still for 24 h for the purpose of MnO2 growth at room temperature. Final MnO2/TNTs composite was obtained after washing with DI water for three times, dried at 105 °C for 4 h and calcined at 400 °C for 2 h in a



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muffle oven. The Mn content in MnO2/TNTs was determined based on U.S. Environmental Protection Agency (USEPA) Method 3050B.42 Pure MnO2 nanoparticles were also synthetized according to the Murrayʼs method. Transmission electron microscopy (TEM) images of the materials were recorded on a Tecnai30 FEG on a microscopy (FEI, USA) operated at 300 kV, and energy dispersive spectra (EDS) was also obtained at the same time. X-ray diffractometer (XRD) analysis was conducted using a D/max-2400 diffractometer (Rigaku, Japan) at 100 kV and 40 mA with the Cu Kα radiation (λ = 1.542 Å), and a scanning rate of 4°/min was set. X-ray photoelectron spectroscopy (XPS) analysis was carried out on an AXIS-Ultra XPS apparatus (Kratos, England) operated at 15 kV and 15 mA with the Al Kα X-ray as the irradiation source. All the peaks were calibrated by the reference of standard C 1s peak at binding energy (Eb) of 284.8 eV. The Brunauer-Emmett-Teller (BET) surface area of the sample was obtained on an ASAP 2010 surface area analyzer (Micromeritics, USA) in the relative pressure (P/P0) range of

0.06–0.20,

while

the

pore

size

distribution

was

got

following

the

Barrett-Joyner-Halenda (BJH) method. The nitrogen adsorption at the relative pressure of 0.99 was used to determine the pore volumes and the average pore diameters. Zeta potentials of the materials at different pH were measured using a Nano-ZS90 Zetasizer (Malvern Instruments, UK). Diffuse reflectance UV-visible absorption spectra (UV-DRS) were obtained on a UV-2400 spectrophotometer (Shimadzu, Japan), where BaSO4 powder was used as the reference at all energies (100% reflectance) and the reflectance measurements were converted to absorption spectra through the Kubelka-Munk function.43 2.3. Batch Oxidation and Photocatalysis Experiments Batch experiments on removal of E2 by different materials were carried out in


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dark using brown glass bottles (40 mL) with Teflon cap sealed. Reaction was started with initial E2 concentration of 4 µM and material dosage of 0.1 g/L at pH 5. The bottles were shaken at 200 rpm and room temperature (25 °C) for 60 min. At selected time intervals, 1 mL of sample was collected and mixed with 1 mL methanol, and then the mixture was immediately filtered by a polytetrafluoroethylene (PTFE) membrane (0.22 µm). The remaining E2 concentration in the filtrate was determined. The photocatalysis experiments were conducted in a cylindrical glass reactor (total volume of 250 mL) with a quartz cover. A 300 W Xenon arc lamp (Ushio model UXL-500 W) coupled with an AM1.5 solar spectrum filter was used as the simulated solar light source, which were placed above the reactor (10 cm) away. The light intensity in the center of the reactor was determined to be 85 ± 2 mW/cm2 using a broadband radiant power meter (Newport Corporation). After 100 mL of E2 solution (C0 = 4 µM) was mixed with 0.1 g/L of material at pH 5, and light was turned on to start the reaction. Circulating water was injected surround the reactor to maintain the temperature of reaction system of 25.0 ± 0.2 °C. To test the effects of pH, 4 µM of E2 was mixed with 0.1 g/L MnO2/TNTs, and solution pH was adjusted to 4−9 using diluted HClO4 and NaOH. Photolysis of E2 without any material addition indicated almost no E2 removal (neat TNTs, which is in good agreement with the E2 21 ACS Paragon Plus Environment


photocatalytic degradation results (Figure 4). Therefore, decoration of MnO2 can inhibit electron-hole pairs recombination attributed to photo-excited electron migration, thus promoting photocatalytic activity. More production of ROS (especially •OH) leaded to higher photocatalytic degradation efficiency of E2 by MnO2/TNTs under solar light. To evaluate the roles of h+ and ROS (i.e. •OH and •O2-) in E2 degradation, a widely-used scavenger-quenching method was employed.74, 75 Figure 8 shows that the photocatalytic reaction was greatly inhibited with the addition of EDTA and isopropanol, as the E2 removal efficiency decreased from 99.8% to 35.3% and 41.7%, respectively. After h+ and •OH quenching, removal of E2 was attributed to MnO2 direct oxidation. Inhibition on h+ formation by EDTA will affect the organic direct oxidation process by h+ and diminish •OH generation.74, 76 Great effect of isopropanol indicated •OH was the primary ROS for E2 degradation, and it is widely known that •OH plays the dominant role in photocatalysis systems using Ti-based materials.71 In comparison, •O2- played a negligible role in E2 photocatalytic degradation as BQ almost did not inhibit the reaction.


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Figure 8. Photocatalytic degradation of E2 by MnO2/TNTs in the presence of various radical scavengers (Initial E2 = 4 µM, scavenger concentration = 1 mM, material dosage = 0.1 g/L, solution pH = 5.0 ± 0.2, simulated solar light source).

In view of the hetero-architecture of MnO2/TNTs, we got both high E2 and TOC removal efficiency in the photocatalysis system. Compared to MnO2/TiO2 composites reported previously,34-37 MnO2/TNTs with TNTs as the skeletal material have the following advantages: 1) good ion exchange property of TNTs leads to incorporation of Mn2+ into the lattice interlayer of titanate and subsequent uniform growth of MnO2 nanoparticles on TNTs during synthesis, 2) large surface area of TNTs skeleton facilitates interaction with target organic pollutant, and 3) efficient electron transfer due to photoelectric quantum response of titanate and hetero-structure of the composite. 3.4. E2 Degradation Pathway and Synergetic Effect Degradation products of E2 by MnO2/TNTs under solar irradiation were identified by HPLC-MS, and samples at different reaction times were taken so as to collect the intermediates/products in different degradation stages. Computational parameters including Fukui index and spin density population were further applied to predict the regioselectivity for ROS attacks and interpret the synergetic effect. Based on the radical identification, products determination, and theoretical calculation results, E2 degradation pathway was proposed (Figure 9).



Figure 9. Proposed E2 degradation pathway by MnO2/TNTs under solar light. Table S6 lists the main identified intermediates and products. In the early stage of reaction, estrone (m/z 269) and E2 dimer (m/z 541) were observed, which were the products of direct oxidation by MnO2 fraction in MnO2/TNTs (Equations 5 and 6). Estrone (E1) is another well-known EDC,77 so only MnO2 oxidation (E2→A) is not a complete and environmental-friendly process for E2 removal. Meanwhile, one-electron oxidation of E2 (E2→B) was also found, and phenoxyl radical (B) formed when E2 lost one electron. E2 dimer (D) was produced through the following radical resonance (B→C) and coupling reaction (C→D).78 The exact structure of E2 dimer was further confirmed by means of quantum chemistry. Spin and charge population calculations quantify the amount of charge and unpaired spin at specific positions and thus explain the regioselectivity in radical coupling reactions (Figure 10). Figure 10b shows that the sites (O1, C3, C5, and C7) on E2 radical with higher spin populations are likely to be more reactive. However, all of these sites are negatively charged, the O1 site has the highest negative charge (-0.492), leading to unfavorable coupling reaction when approaching to each other due to electrostatic repulsion. Moreover, C5 is fully substituted and can not be connected. Therefore, for 24 ACS Paragon Plus Environment

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the coupling reaction of E2 dimer formation, the O1 and C5 sites are excluded, and coupling between C3–C3’, C3–C7’, and C7–C7’ are the most likely reaction pathway. E2 dimer with the linking bond at C3–C3’ (Product D) is proposed to be the primary dimer product, as its single-point energy is calculated to be 5.73 and 8.74 kcal/mol lower than that formed through C3–C7’ and C7–C7’ coupling, respectively.

Figure 10. NBO analysis for E2 radical and molecule at B3LYP/6-31+G(d,p) level. (a) E2 radical structure, (b) Natural population analysis (NPA) spin and charge populations, (c) Condensed Fukui index distribution for radical attacks (f 0), and (d) Condensed Fukui index distribution for electrophilic attacks (f -).

In the later stage, E2 degradation was mainly due to photocatalysis under solar light. Compounds E (m/z 287) and F (m/z 335) were two important photocatalysis products, which were also detected in the photocatalysis system using calcined TNTs. Therefore, •OH radical attack (E2→E→F) is the typical pathway for E2 photocatalytic degradation, which is in accordance with the previous reports.79-81 Specifically, compound with m/z 287 is verified to be a •OH addition product. To 25 ACS Paragon Plus Environment


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elucidate the accurate structure of this compound, Fukui function based on DFT calculation is used to investigate the regioselectivity of •OH attack (Text S1 in Supporting Information). Figure S8 clearly presents the calculated Fukui index distribution for radical (f 0) and electrophilic attacks (f -) on E2 molecule. It is found that the attacks mainly occur on the benzene ring, while the Fukui index values of C8–C14 sites are relatedly low. Highest occupied molecular orbital (HOMO) and lowest unoccupied molecular orbital (LUMO) distributions also confirm the reactive sites are located on the benzene ring of E2 (Figure S9). Figure 10c and 10d further display the Fukui index distribution on the benzene ring of E2 for radical attacks (f 0) and electrophilic attacks (f -), respectively. However, little difference is found on radical attacks for different sites (Figure 10a), so electrophilic attacks (f -) are considered in this study. •OH is reported to be a strong electrophilic radical, so it is more likely to attack the sites which can easily lose electrons, as marked with high f values in Figure 10d.82 The C5 site with the highest f

–

value (0.180) is fully

substituted, so we propose that the C3 site with the second-highest f – value (0.068) should be favorable for •OH electrophilic radical addition, resulting in formation of Product E shown in Figure 9. Product F (m/z 335) is the following ring-opened carboxylic acid of Product E, which can be further oxidized to small organic carboxylic acids, and finally mineralized to CO2 and H2O.79, 80 Besides the traditional MnO2 direct oxidation and photocatalytic degradation, it is very interesting that a new reaction pathway for E2 degradation by MnO2/TNTs in this system was observed, and the key step is transformation of E2 radical (C) to Product G. Based on HPLC-MS analysis, Product G (m/z 287) with a retention time (RT) of 19.5 min was identified a constitutional isomer of Product E (RT = 18.7 min). In the reaction system with existence of MnO2/TNTs, a large amount of E2 phenoxy


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radicals (B and C) form through one-electron oxidation by MnO2. In the meanwhile, the produced •OH through photocatalysis will then attack the E2 radicals, forming Product G (C→G). E2 radicals (B and C, i.e. activated *E2 in Equation 12) are more active species compared to E2 molecule, and therefore, radical reaction between •OH and *E2 (C→G) is much more preferable, which can greatly promote E2 degradation. Moreover, the preference of radical coupling also can be interpreted by the following two reasons: 1) C5 is found with relatively high spin population and strong nucleophilicity (Figure 10d), so it is the most reactive site for •OH addition, and formation of E2 radical C can release the C5 site for further •OH electrophilic attack; 2) E2 radical prefer to coupling with •OH instead of self-coupling because of the steric effect and charge repulsion. The MnO2 induced pre-activation can promote the transformation of E2 to its hydroxylated form, which subsequently undergoes further oxidation and mineralization. Based on the material characterizations and E2 degradation pathway, it can be concluded that the high E2 and TOC removal efficiency in the MnO2/TNTs system is highly related to two mechanisms: 1) heterojunction effect of decorated MnO2 and titanate, resulting in enhanced photocatalytic activity, and 2) synergetic effect of pre-oxidation and photocatalysis, leading to radical reaction between activated E2 and •OH. Compound G is found to be a characteristic product in this synergetic system, which is a solid evidence on the linkage of pre-oxidation and photocatalysis. Considering the specific structure and composition of MnO2/TNTs, we propose that the phenolic (e.g., E2, phenol, chlorophenols, etc.) and aniline-like (e.g., aniline and sulfonamides) compounds can be efficiently degraded by this material due to the synergetic effect. 3.5. Reusability and Stability of MnO2/TNTs



Figure 11 presents the E2 degradation by MnO2/TNTs over 5 operating cycles. Although k1 value decreased from 0.198 min-1 for the 1st to 0.103 min-1 for the 5th cycle (Table S7), the E2 removal efficiency also could reach up to 94.4% at 1 h in the 5th cycle, suggesting good reusability of MnO2/TNTs. Decrease on reactivity is mainly because the pre-oxidation reaction is a self-sacrifice process for MnO2. Therefore, regeneration of the material through further KMnO4 hydrothermal treatment is necessary and can be an available way for restoration of MnO2 sites. Little Mn ions dissolution in solution (

Sea-Buckthorn-Like MnO2 Decorated Titanate Nanotubes with

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