Dredged-Sediment-Promoted Synthesis of Boron-Nitride-Based

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Dredged Sediment-Promoted Synthesis of Boron Nitride-based Floating Photocatalyst with Photodegradation of Neutral Red under UV-light irradiation Yong Guo, Ruxia Wang, Peifang Wang, Lei Rao, and Chao Wang ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.7b15638 • Publication Date (Web): 05 Jan 2018 Downloaded from http://pubs.acs.org on January 7, 2018

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ACS Applied Materials & Interfaces

Dredged Sediment-Promoted Synthesis of Boron Nitride-based

Floating

Photocatalyst

with

Photodegradation of Neutral Red under UV-light irradiation Yong Guoa*, Ruxia Wanga, Peifang Wanga*, Lei Rao b, Chao Wanga a. Key Laboratory of Integrated Regulation and Resource Development on Shallow Lakes, Ministry of Education, College of Environment, Hohai University, Nanjing, 210093, P.R. China. E-mail: [email protected]; E-mail: [email protected] b. College of Mechanics and Materials, Hohai University, Nanjing, 210093, P.R. China.

KEYWORDS:

Boron

Nitride;

Oxygen-limit

method;

Dredged

Sediment;

Floating

Photocatalyst; Neutral Red

ACS Paragon Plus Environment

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ABSTRACT ：A novel floating photocatalyst (BN-DS-7) has been successfully synthesized by calcinating the mixture of boron nitride (BN) and dredged sediment (DS) with a specific mass ratio (3:7) at 1100 oC for half hour. BN is first time synthesized through oxygen-limited method, which has a shape of nanoplate with size around 30 nm and a bandgap at 3.94 eV. The assynthesized BN can degrade NR under UV light irradiation. For BN-DS-7, X-ray photoelectron spectroscopy analysis suggests that BN mainly interacts with DS through the strong coordination between these N atoms in BN and these Si, Al atoms in DS. This leads to that BN-DS-7 has a good compression strength (around 9 Mpa). Thermogravimetric analysis for BN shows that a few BN (around 13%) synthesized with oxygen-limited method will pyrolyze at 1100 oC and the released gas can be sealed in the inside of DS at 1100 oC, which results in that BN-DS-7 can float on water surface. Photodegradation results show that BN-DS-7 can degrade 84% of NR (20 mg/L) under UV light irradiation for five hours as well as the active species are •OH and photoinduced hole. Total organic carbon analysis for NR solution before and after photodegradation show that about 70% of NR has been mineralized into inorganic carbons. This work is helpful to develop new type of BN-based floating material and enlarge the application field of DS.


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1 Introduction Hexagonal boron nitride (BN) has a similar structure to that of graphite, which has lots of attractive performances, such as low dielectric constant, large thermal conductivity, high mechanical strength and chemical inertness.1-3 So, BN has been widely used as protective coating material, 4 and reinforcing material.5 Due to the polarity of B-N bond, BN also has good adsorption performance for hydrogen 6 and organic pollutants (such as oil, dyes and benzene).7-9 For example, the adsorption quantity of the chemical-treated BN for methylene blue is around 400 mg/g10, while the adsorption quantity of nano BN sphere for benzene is around 700 mg/g.11 Thus, BN has great application potential in the purification of waste water. Recent research has found that carbon doped BN material can split water to produce hydrogen or oxygen and reduce carbon dioxide under visible irradiation.12 But, the photocatalytic performance of BN has not been investigated so far. This is due to that the band gap of the pure BN is around 5.5 eV 2, 12 and even the ultraviolet light (UV-light) in solar light is also very difficult to excite its electron from valence band to conduction band. Up to now, the synthesis of BN all proceeds in inert atmosphere, such as nitrogen gas.1, 9, 12-13 It has reported that some materials synthesized in inert atmosphere can also be produced with oxygen-limited method. For example, biochar has been produced with oxygen-limited method.14-15 The core principle of oxygen-limited method is to creat a anoxic condition during the theraml reaction of precursors under high temperature treatment. This can be realized by putting precursors into a closed space (such as a crucible covered by a lid) since the heating treatment and the released gas from the thermal reaction of precursors under high temperature will drive out oxygen from the inside of crucible. The advantage of oxygen-limited method related to anaerobic method is that it decreases the cost


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since inert gas does not use in whole process and the requirement for gas tightness of muffle furnace is not too high. So far, BN has not be synthesized with oxygen-limited method. Furthermore, BN exists in the form of powder and it is not easily recycled after using it to adsorb (or degrade) the organic pollutants in water. For solving this problem, the floating BN aerogel has been produced by dispersing aminated BN in water with the subsequent freezedrying7 and using carbon aerogel as template.16 However, new strategy is still necessary to be developed for synthesizing novel BN-based floating materials for purifying waste water. Every year, a large number of sediment have been dredged out from rivers, lakes and reservoirs.17 How to deal with the dredged sediment (DS) has become a challenging topic since DS may contain organic and metallic pollutants, and can not be directly discarded or recycled. Some methods have been developed to fix or remove the pollutants in DS before they are recycled. Bioremediation method has been used to degrade the organic contaminants (three tetracyclines) in river sediment.18 Sodium alginate modified zero-valent iron has been applied to immobilize Cd2+ in river sediment.19 Cement has been adopted to combine sediment and ash together to form low-strength material.20 The combination of bioleaching and Fenton-like reaction can effectively remove trace metals (including Cd, Zn, Cu, and Pb) in DS.21 Bioremediation has also coupled with electro-kinetic method to remove phthalate esters in DS.22 Many useful building materials have been synthesized by calcinating DS at high temperature so far. The advantage of recycling DS at high temperature is that the organic pollutants in DS can be burned into carbon dioxide and the metallic pollutants in DS can be converted into metal oxide when DS is transformed into useful material at high temperature.23-25 Glass foam are produced at temperatures of 750–1050 oC with sediment and Na2CO3 as precursors.23 Lightweight aggregate is made from DS and sewage sludge at the temperature of


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1100 oC.24 DS has also been used to synthesize ultra-lightweight ceramsite with the treatment temperature of 1600 oC.25 All of the building materials mentioned above are solid rather than powder, which are from that sodium ion, aluminum silicate in sediment can crosslink together at high temperature.23,25 Furthermore, the floating material (such as ceramsite) can be synthesized by mixing sediment with these materials that can release gas at high temperature since the released gas can be sealed in the inside of sediment by the glass phase formed from the calcination of sediment at 1100 oC.23, 25 The quantities of silicon and aluminum elements in DS are very rich.23-25 It has reported that the pure SiO2 and Al2O3 can be used as supporters in catalytic field.26-27 Thus, DS treated at high temperature should also has the potential of being used in catalytic field. So far, the application of DS in catalytic field has not been reported and this will enlarge the application field of DS. Most of non-metallic adsorbent or photocatalytic materials (such as resin, activated carbon, etc.) will decompose at high temperature and are unsuitable for mixing with DS at high temperature to form new type of material. BN is synthesized at high temperature (usually above 1000 oC) and has a good thermostability.1,

4, 7-9

It has reported that silicon nitride can be

synthesized by calcinating silicon powder above 1300

o

C under nitrogen atmosphere.28

Aluminum nitride can also prepared by evaporating metallic aluminum at 1200 oC under nitrogen/ammonia atmosphere.29 In DS, the quantities of silicon and aluminum elements are very rich. Thus, BN may link DS together by forming the interactions between the Si, Al elements in DS and N elements in BN through calcinating the mixture of BN and DS at high temperature. This promotes us to further presume that a BN-DS floating material is also possible able to be synthesized by calcinating the DS-BN mixture at high temperature. To date, the report using DS as linker to bind BN together to produce DS-BN floating material has not been found.


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Herein, BN is first synthesized with oxygen-limited method and the as-synthesized BN can photodegrade neutral red under UV light irradiation since its bandgap is 3.94 eV. Then, BN-DS composites with different mass ratio of BN:DS have been synthesized and one of them can float on water surface for a long time and can also photodegrade neutral red under UV-light irradiation. The objectives of this work are included as the following: (1) synthesizing BN with oxygen-limited method and investigating its photodegradation mechanism for neutral red under UV-light irradiation; (2) clarifying the formation mechanism of BN-DS floating material and investigating the photodegradation performance of BN-DS floating material for neutral red under UV-light irradiation. 2 Experimental section 2.1 Regents DS used in this work was taken from the Hejiabin River, which was located in Zhoutie town, Yixin city, Jiangsu Province, China. DS was dried in oven at 100 oC for 24 hours. After that, the sample was grinded and sieved with No. 60 mesh. The obtained powder sample was named as DS (Fig.S1) and was put into a valve bag for the use in the following experiments. Urea (analytical grade), diboron trioxide (B2O3, analytical grade), hydrochloric acid (HCl, analytical grade) and neutral red (analytical grade) were purchased from the Sinopharm Chemical Reagent limited corporation, P. R. China. 2.2 Preparation of BN BN was synthesized with oxygen-limited method in the first time. The detailed procedure was as the following: 12 g B2O3 and 24 g urea were first grinded in mortar for 30 minutes to make sure that they mixed together in uniform way; then, these mixtures were put into alumina crucible and covered with crucible lid; After that, the crucible was put in the muffle furnace and


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treated under 1100 oC for 4 hours (please see detailed scheme for oxygen-limited method in Fig. S2). The heating rate was 10 oC/min. The obtained white sample was first washed with HCl (1mol/L) to remove the unreacted B2O3 and other impurities, and then washed with deionized water until the pH of the supernatant was neutral. The washed sample (Fig.S3) was dried at 100 o

C for over night with the subsequent putting into a valve bag for the use in the following

experiments. This sample was named as BN. 2. 3 Preparation of DS ball (DSB) A certain amount DS and deionized water were mixed together to form mud, and followed by pelletizing the mud into mud balls (Fig. S4(a)). Then, these balls were put on the surface of alumina wafer and treated at 100 oC in muffle furnace for four hours, followed by calcination at 1100 oC for 0.5 hour. The heating rate was 10 oC/min. the obtained samples still kept the shape of the ball (Fig. S4(b)) and these balls would sink to the bottom of beaker if they were put into the beaker (Fig. S4(c)). This sample was named as DSB. 2. 4 Preparation of BN-DS composites BN powder and DS powder were dispersed in deionized water and pelletized together into balls with the mass ratio (1:9). Then, these balls were put on the surface of alumina wafer, followed by treatment in muffle furnace at 100 oC for four hours and then at 1100 oC for half hour, respectively. The heating rate was 10 oC/min. The obtained sample was named as BN-DS9. Then, similar procedures had also been adopted to treat the mixtures of BN-DS with the different mass ratios (2:8, 3:7, 4:6 and 5:5). The obtained samples were all balls, and named as BN-DS-8, BN-DS-7, BN-DS-6 and BN-DS-5, respectively (Fig.S5). 2. 5 Characterization of the samples


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The components in DS was determined by X ray fluorescence spectrometer (ARL-9800, Switzerland). The morphology of DS, DSB, BN and BN-DS-7 were investigated with JEM-2100 electron microscope (TEM). The X-ray diffraction (XRD) patterns of DS, DSB, BN and BN-DS7 were collected with the X-ray Diffractomer radiation (Utilma III Tokyo, Japan) with Cu Kα ( λ=1.540562 Ǻ ), in which the X-ray tube was operated at 40 kV and 40 mA. XPS characterization of BN and BN-DS-7 were performed with PHI5000 VersaProbe X-ray photoelectron spectroscopy. Surface areas of BN, DSB and BN-DS-7 were determined with HD88, ASAP2020 micropore analyzer (USA). The Fourier transform infrared spectrum (FTIR) of BN was acquired using Nexus 870 FT-IR instrument. UV-vis diffuse reflection spectra of BN, DSB and BN-DS-7 were measured using a UV-vis spectrophotometer (Varian CARY 100, USA). 2. 6 The adsorption and photocatalytic performance of BN The UV-vis spectrum of neutral red (NR) was shown in Fig. S6. There were two adsorption peaks in this UV-vis spectrum: one was at 267 nm, being attributed to the adsorption of aromatic rings in NR, while another was at 524 nm, being assigned to the adsorption of chromophoric group in NR (the conjugated three rings which make the solution of NR has red color). This adsorption peak at 524 nm was usually used as an indicator to evaluate the photodegradation performance of photocatalysts for NR under light irradiation. 30 In addition, the photostability of methylene blue (MB), NR and rhodamine B (RhB) were also investigated and the obtained results shown that NR was the most stable among the three dyes (Fig. S7). Thus, NR was chosen as proxy of dyes pollutants in this work. According Fig.S8, the UV peak of NR at 524 nm had a good linear correlation with the concentration of NR since R2 was 0.999. Thus, the concentration


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change of NR during adsorption and photodegradation could be determined based on the change of its adsorption peak at 524 nm. The adsorption and photocatalytic performance of BN was assessed by treating NR without and with UV light irradiation. Specifically, 0.15 g BN was put into 100 ml NR solution (50 mg/L). The solution was first stirred in darkness for 1 hour. During the stirring process, the solution was taken every 15 minutes and the supernatants from centrifugation for these samples were tested by UV-visible spectrometer at the wavelength of 524 nm.30 According to the results of adsorption experiment, the adsorption quantity of NR by BN was 24.01 mg/g in one hour and 24.45 mg/g at ten hours for NR (Fig. S9), suggesting that the adsorption equilibrium nearly achieved within 1 hour. Thus, the dark reaction time to evaluate the photocatalytic performance of BN was determined as one hour. In addition, the influences of temperature, pH and ionic strength on the adsorption of NR by BN were also investigated at the same conditions mentioned above. The temperature ranged from 15 oC to 35oC. The pH was controlled from 2.15 to 5.15. Usually, CaCl2 was adopted to control the ionic strength of solution for evaluating the influence of ionic strength on adsorption.14 So, different amount of CaCl2 were added into NR solution to make that the concentration of CaCl2 ranged from 0.01 M to 0.1 M. All of this results were provided in Fig.S10. After the adsorption equilibrium achieved, the NR solution containing BN was irradiated by UV light for five hours. The light source was a 125 mercury lamp (GGZ123, shanghai Jiguang Lighting Electrical Appliance Factory). Once the light was on, the solution of NR was taken every one hour. And the supernatants from centrifugation for these samples were tested by UVvisible spectrometer at the wavelength of 524 nm.30


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For better understanding the photodegradation performance of BN for NR under UV light irradiation, the photostability of NR under UV-light irradiation was investigated as well, in which the initial concentration of NR was same as that of NR solution after adsorption equilibrium by BN. By this way, a reasonable assessment for the photodegradation performance of BN for NR could be made by subtracting the self-degradation of NR under UV light irradiation. For

deep

understanding

the

photodegradation

mechanism

of

NR

by

BN.

Ethylenediaminetetraacetic acid disodium salt, p-benzoquinone and tert-butanol had been adopted to capture the active species in the photodegradation. The detailed experimental procedures were same as mentioned above. 2.7 Photocatalytic performance of DSB, BN and BN-DS-7 The photocatalytic activities of DSB, BN and BN-DS-7 were also assessed by degrading NR under UV light irradiation. From the preliminary experimental results, the adsorption performances of DSB and BN-DS-7 for NR were very weak. Thus, the concentration of NR herein was 20 mg/L. Specifically, 0.5 g DSB was put into 100 ml NR solution (20 mg/L). The solution was first stirred in darkness for 1 hour to achieve adsorption equilibrium, and then followed by UV light irradiation for five hours. The light source was a 125 mercury lamp (GGZ123, shanghai Jiguang Lighting Electrical Appliance Factory). Once the light was on, the solution of NR was taken every one hour. And the supernatants from centrifuging these samples were tested by UV-visible spectrometer at the wavelength of 530 nm. Similar procedure had also been performed for BN and BN-DS-7 samples to assess their photocatalytic performances. For deep understanding the photodegradation mechanism of NR by BN-DS-7. Ethylenediaminetetraacetic acid disodium salt, p-benzoquinone and tert-butanol had


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been adopted to capture the active species in the photodegradation. The detailed experimental procedures were same as mentioned above. 2. 8 Recycle usage test of BN-DS-7 0.5 g BN-DS-7 was put into 100 ml NR solution (20 mg/L). The solution was first stirred in darkness for 1 hour to achieve adsorption equilibrium, and then followed by UV light irradiation for five hours. Then, the sample was collected, washed and dried at 80℃ for overnight. After that, the collected sample was reused to photodegrade fresh NR solution (20 mg/L) in the same condition mentioned above. The concentration of NR after five hours was tested by UV-visible spectrometer at the wavelength of 524 nm. The same procedure repeated four times. 2. 9 Computational section For deep understanding the structure and band of BN synthesized with oxygen-limited method, the structures of the pure BN and the hydroxylated BN had been investigated with m062x/6-31g(d,p) method 31 in Gaussian 09 program.32 For BN, a model containing 36 boron atoms and 36 nitrogen atoms were designed and the unsaturated atoms at the edge were saturated with hydrogen atoms (Fig. S11(a)). According to FTIR result of the as-synthesized BN that was given in the following section, hydroxyl groups existed in the as-synthesized BN. So, a model containing 36 boron atoms, 36 nitrogen atoms and 2 hydroxyl groups were designed for simulating the structure of the as-synthesized BN, which was named as BNOH (Fig. S11(b)). All molecular structures were constructed with Gview 5.0 program based on these optimized results. 3 Results and discussion BN is synthesized with oxygen-limited method (please see the detailed procedure in Fig. S2) by calcinating the mixture of diboron trioxide and urea at 1100 oC. This reason choosing the calcination temperature as 1100 oC is that it has reported that BN can be produced at 1100 oC.9


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(a)

(b)

(c)

(d)

Fig.1 (a) N1s peak of BN in XPS spectrum; (b) FT-IR spectrum of BN; (c) XRD spectrum of BN; (d) TEM image of BN.

XPS (Fig.1a) result supports that the synthesized material is BN since the peak at 397.4 eV is assigned to the B-bounded N elements in BN.33-34 Furthermore, FTIR (Fig.1b) results also support this conclusion since the FTIR peaks at 1377 cm-1 and 779 cm-1 are attributed to the


12

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stretching vibration of the B–N bond and the bending vibration of B–N–B bond.33-35 In addition, FTIR results also suggests that the as-synthesized BN is hydroxylated since the FTIR peaks at 3195 cm-1 and 1192 cm-1 are most likely from the vibration of the B-bound OH33-34 and B-O bond

33-37

, respectively. It has reported that the bulk BN can been peeled into nanosheet by

simultaneous thermal exfoliation and hydroxylated functionalization at 1000 oC in air.33-34 XRD analysis also supports that the synthesized material is BN since the characteristic diffraction peaks (26.6 oC and 41.8 oC) of hexagonal BN are found in its XRD spectra (Fig.1c).7, 33-34 In addition, the two XRD peaks are broadened (Fig.1c), indicating that the hydroxylated BN exists in nanoscale.7 This is further confirmed by the TEM images of the hydroxylated BN. From Fig.1d and Fig.S12, the hydroxylated BN is nanoplate with size around 30 nm. This TEM results are different from that of BN synthesized in inert atmosphere with also diboron trioxide and urea as precursors, which are big sheets with size around several hundred nanometers.38 This is most likely from that these sheets are peeled into nanoplates under oxygen-limited condition.33-34 According to the TG result of BN (Fig.S13), about 13% BN is decomposed when the temperature increases to 1100 oC. This is due to that the synthesized BN in this work is hydroxylated nanoplate with size around 30 nm. It has reported that the thermal stability of BN is related to its size39 and thermal exfoliation of bulk BN into nano BN will decrease its thermal stability.33 It is well accepted that the pure BN has a wide indirect bandgap around 5.5 eV, implying that even UV light is also very difficult to excite the electron from valence band to conduction band.12, 12

However, the BN synthesized in this work has obvious adsorption peaks at 255 nm and 333

nm (Fig 2a). The band gap of the synthesized BN has been further analyzed based on Tauc method [(Ahν)1/n versus hν] 40 and the obtained result shows that BN has an indirect bandgap of


13


(a)

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(b)

Fig.2 (a) UV-vis spectra of BN; (b) the bandgap of BN.

3.94 eV (Fig 2b). So, the BN synthesized with oxygen-limited method can most likely degrade pollutant under UV light irradiation. It has reported that the electronic state (or band gap) of BN is largely dependent on the structural changes in morphology and size.41 For example, the bandgap of BN whiskers is 3.8 eV 42 and that of BN hollow nanoribbon is 5.3 eV.43 So, the nano structure of BN makes the contribution for its adsorption at 255 and 333 nm as well as the bandgap of 3.94 eV. In addition, the previous study 44 shows that oxygen-containing groups also have significant influence for the electronic state and photocatalytic performance of carbon quantum dot. Herein, molecular simulation has adopted to investigate the influence of hydroxyl group on the electronic state of BN. According to Fig.3(a) and Fig.3(b), HOMO orbital of BN is composed by the 2p orbitals of N atoms, while the LUMO orbital of BN is composed by the 2p


14

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(a)

(b)

(c)

(d)

Fig.3 (a) HOMO orbital of BN model; (b) LUMO orbital of BN model; (c) HOMO orbital of BNOH model, (d) LUMO orbital of BNOH model.

orbitals of B atoms. This is similar to the reported result,12 supporting that the adopted calculation method can provide a reasonable description for the electronic state of the investigate BN system. After the hydroxylation at high temperature, hydroxyl groups appear at the edge of BN.33-34 From Fig.3(c) and Fig.3(d), hydroxyl group mainly influences the HOMO orbital of BN since the HOMO orbitals of BN and BNOH are different, while the LUMO orbitals of BN and BNOH are nearly identical. Furthermore, the bandgap of BNOH is 0.2 eV smaller than that of


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BN based on the calculation results, suggesting that hydroxylation can also narrow the bandgap of BN. Thus, OH groups in BN sample also make the contribution for its adsorption at 255 and 333 nm as well as the bandgap of 3.94 eV. In summary, both nanostructure and hydroxylation of the BN sample are responsible for its adsorption at 255 and 333 nm as well as the bandgap of 3.94 eV. So far, the photocatalytic performance of BN has not been investigated. NR is often used to synthesize neutral red paper and as basic indicator, which widely exists in the printing and dyeing wastewater. So, many researchers have use NR as proxy of dyes pollutants to test the performance of photocatalysts that treat dyeing wastewater.30 In addition, it is well known that dye pollutants will self-degrade under UV light irradiation with mercury lamp as light source.30 The photostabilities of several commonly used dyes [including methylene blue (MB), NR and rhodamine B (RhB)] are compared. From Fig.S7, 48% of NR is degraded after five hours UV light irradiation, 52.9% of MB is degraded after five hours UV light irradiation, and 71.9% of RhB is degraded after five hours UV light irradiation. This means that NR is the most stable dye under UV light irradiation among MB, RhB and NR. Thus, NR is chosen as a proxy of environmental organic pollutants in this work. Then, the photocatalytic performance of the synthesized BN is investigated by using it to degrade NR under the light irradiation of 125 mercury lamp. Two periods often are included in the photodegradation of pollutant by photocatalyst: (1) the first one is adsorption, which undergoes in dark and is used to evaluate the adsorption performance of photocatalyst for pollutant, (2) the second one is photodegradation, which undergoes under light irradiation and is used to evaluate the photodegradation performance of photocatalyst for pollutant. According to the previous reports, the mesoporous9 and chemical-treated BN

10

all have good adsorptions for dyes. Thus, a preliminary experiment


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has been done to determine the dark reaction time. From Fig. S9, the adsorption quantity of NR by BN was 24.01 mg/g in one hour and 24.45 mg/g at ten hours for NR, suggesting that the adsorption equilibrium nearly achieved within 1 hour. Thus, the dark reaction time in the photodegradation of NR by BN is chosen as one hour. In addition, the influences of temperature, pH and ionic strength on the adsorption of NR by BN have investigated as well and the results are given in Fig. S10. From Fig. S10(a), one can see that 25 oC is the most appropriate temperature for NR adsorption by BN since the order of NR adsorption quantities is q25oC > q35oC > q15oC. This is due to that: (1) high temperature is favorable for the NR adsorption by BN (q25oC > q15oC); (2) but when the temperature is too high, the adsorbed NR will desorb from the surface of BN, which results in that q25oC is also larger than q35oC. From Fig. S10(b), the adsorption quantities of NR by BN decrease with the increased ionic strength, which is controlled with the added quantities of CaCl2. This is most likely from that these increased ions (Ca2+ and Cl-) can occupy the adsorption sites on the surface of BN, which leads to the decrease of NR adsorption quantity by BN. The influence of pH on the NR adsorption by BN is shown in Fig. S10(c). One can see that the adsorption quantities of NR by BN increase when the pH of NR solution increases to 5.15 from 2.15. This is most likely from that the protons (H+) in the solution may also occupy the adsorption sites on the surface of BN and inhibit the adsorption ability of BN for NR. Herein, we just consider the pH influence on the adsorption performance of BN for NR in acidic condition (pH range from 2.15 to 5.15, please see Fig. S10(c)). This is because that we find: NR will flocculate from solution when pH of NR solution is above 6.15 (please see Fig. S14). The starting states of NR solution at different pH values ranged from 2.15 to 10.15 are shown in Fig. S14(a). After five hours, the color of NR solution at 7.13 become light and obvious flocculates can be found at the bottom of tube containing NR solution with pH ranged


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from 7.75 to 10.15 (please see Fig. S14(b)). A further analysis has been performed to understand why NR will flocculate out from water when the pH of NR solution is above 6.15 (please see Fig. S14(b)). It is well known that NR is cationic dye and has an N-bound proton (please see the red circle part in Fig. S15, the proton may also take by other N atoms in NR). When the pH of NR solution increases from acidity to basicity, the proton will dissociate from NR, which will lead to that NR exists in water as a neutral molecule rather than a cation. From the Fig. S15, one can see that NR has conjugated aromatic rings. The solubleness of the neutral NR molecule in water will be low, which results in the flocculate of NR molecule from water. Obviously, the pH around 7.13 may be the transition region, in which cationic NR and neutral NR coexist. The pH change for the NR adsorption by BN is also investigated. The initial pH of NR solution (concentration: 50 ppm) is 4.14. After the addition of BN into the NR solution with one hour adsorption, the pH of NR solution is 5.36. Based on the research mentioned above, NR does not flocculate from solution in this pH range. Thus, we can sure that BN synthesized in this work can adsorb NR. The pH of NR solution changes with the change of concentration: it is 4.14 when the concentration of NR is 50 ppm, while it is 5.13 when the concentration of NR is 20 ppm. The pH test experiment shows that the synthesized BN does not change the pH of deionized water. Thus, the pH change (from the initial 4.14 to the final 5.36) of NR adsorption by BN is attributed to the decrease of NR concentration after adsorption. Then, the photocatalytic performance of BN is investigated. According to Fig. S16, nearly 75% of NR has been adsorbed by BN in 1 hour, in which the red line represents concentration of NR after adsorption equilibrium and the black line is corresponding to the initial concentration of NR. Then, Fig.S16 is re-treated into Fig.4(a) by using the concentration represented by the red line (the NR concentration after one hour adsorption by BN) as the initial concentration. This


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(a)

(b)

Fig. 4 (a) the photodegradation of NR by BN under UV light irradiation with the concentration after adsorption as initial concentration; (b) the photostability test of NR under UV light irradiation, in which BN is absent. The initial concentration of NR in the photostability test is same as the NR concentration after adsorption by BN

concentration is also adopted as the initial concentration for the photostability test of NR (Fig. 4(b)). By this way, it is convenient to consider the influence of NR self-degradation on the NR photodegradation by BN under UV light irradiation. According to Fig.4(a) and Fig.4(b), nearly 82% of NR is degraded under UV light irradiation for five hours with BN as photocatalyst, while, nearly 48% of NR degrades under UV light irradiation for five hours without the presence of BN. The degradation amount of NR under UV light irradiation with BN as photocatalyst is obvious higher than that with the absence of BN (82% vs 48%), suggesting that BN is UV-responsive


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photocatalyst. As we know, this is the first time to report that BN can photodegrade organic pollutant (NR) under UV light irradiation. From Fig. 4(a) and Fig.4(b), the characteristic peaks of NR decrease and no new peaks appear. This suggests that the aromatic structures in NR have been decomposed completely since the previous work shows that a new adsorption peak will appear in ultraviolet region if the conjugated aromatic structure is partially destroyed by photocatalyst.45 TOC analysis results for NR solution before and after photodegradation support this deduction since about 80% NR has been mineralized into inorganic carbon species. It has reported that ethylenediaminetetraacetic acid disodium salt (EDTA-2Na), pbenzoquinone (BQ) and tert-butanol (TB) can capture hole, •O2- radical and •OH radical that appear during the process of photodegradation.46 For deep understanding the photocatalytic performance of BN, a series of experiments have been done by adding these molecules mentioned above into the photodegradation systems. And the obtained results are shown in Fig. 5(a) and 5(b). Since the text descriptions in this two figures are too much and the adsorption peak used to characterize the NR photodegradation is at 524 nm, the wavelength ranged from 350 nm to 1050 nm is used in this two figures. Firstly, the influences of these three molecules on the NR self-degradation under UV-light irradiation are investigated. According to Fig.5(a), the NR self-degradation is great inhibited by TB, implying that •OH radical plays a very important role in the self-degradation of NR under UV light irradiation. In addition, •O2- radical and hole also take part in the self-degradation of NR since the self-degradations of NR are also inhibited by BQ and EDTA-2Na. All of these active species maybe from the photodecomposition of NR under UV light irradiation, which results in the formation of •OH radical, •O2- radical and hole. Then, the influences of TB, BQ and EDTA-2Na on the NR degradation by BN under UV-light irradiation are investigated. From Fig.5(b), one can see that •OH radical makes the most


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(a)

(b)

Fig. 5 (a) the influences of TB, BQ and EDTA-2Na on the self-degradation of NR under UV light irradiation; (b) the influences of TB, BQ and EDTA-2Na on the photo-degradation of NR by BN under UV light irradiation

important contribution for the photodegradation performance of BN for NR since TB has a obvious inhibition effect for the photodegradation of NR by BN under UV light irradiation. In addition, hole also makes a little contribution for the photodegradation performance of BN for NR since the photodegradation of NR is just a little inhibited by EDTA-2Na. •O2- radical does not make contribution for the photodegradation performance of BN for NR since the degradation results with and without BQ are nearly identical. In summary, the photodegradation performance of BN for NR is mainly from the contribution of •OH radical, then followed by hole, which are


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formed by the excitation of electron from valence band of the as-synthesized BN to its conduction band under UV light irradiation. At the present time, the as-synthesized BN exists in

Table 1 Chemical compositions of the dredged sediment by XRF analysis.

powder and it is not easy to recover it after using it to treat the waste water. Thus, it is necessary to develop the BN-based floating material. DS has shown the potential of working as crosslinker to produce floating material since glass phase can be formed when DS is treated under high temperature, which can seal gas in the inside of DS. Herein, the compositions of DS is first analyzed via X ray fluorescence (XRF) spectrometer. According to Table 1, the mass percent of SiO2 in DS is 66.3%, then followed by Al2O3 (15.8%), Fe2O3 (4.55%), K2O (2.02%), SO3 (1.33%), CaO (1.11%), MgO (1.0%), TiO2 (0.80%) and Na2O (0.69%). Obviously, Si, and Al are the main components in DS. The loss on ignition for DS is about 6%, implying that a certain amount of organic components exist in DS since the DS has been dried before this characterization. The mineral phases for these components in DS are further analyzed based on the XRD analysis result. From Fig.6(a), the peaks at 20.8 oC, 26.6 oC and 59.9 oC are attributed to the


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characteristic ones of quartz20,23 ； the peaks at 35.1 oC and 50.2 oC are designed to the characteristic ones of kaolin20, and the peaks at 36.5 oC, 39.5 oC and 42.2 oC are corresponding to

(a)

(b)

Fig.6 (a) XRD spectrum of DS sample; (b) TEM image of DS sample.

the characteristic ones of vermiculite.20 From the TEM images (Fig.6(b) and Fig. S17), DS has a loose structure since obvious space between components can be observed. Some of these components have flake-like structure (please see the yellow circle in Fig.6(b) and Fig.S17(a) and (b)), and the others look like aggregate (please see the red circle in Fig.6(b)). In addition, there still have some components, which are solid and have smooth surface (please see the blue circle in Fig.S17 (c) and (d)). All of these suggest that DS is composed of multiple components and this is consistent with the XRF and XRD results.


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For keeping the synthesis conditions of DSB and BN-DS samples are same as that of BN sample, the calcination temperatures of DSB and BN-DS samples are all at 1100 oC. If the synthesis conditions of DSB and BN-DS samples are different from that of BN sample, the analysis and comparison between these characterization datas of DSB, BN-DS and BN sample will become difficult and complicated. In addition, the previous report shows that the glass phase in DS will form and has good viscosity when DS is calcinated above 1050 oC.23 After calcination of DS at 1100 oC for 0.5 hour, these mud balls (Fig.S4(a)) have converted into solid and insoluble balls (DSB) (Fig.S4(b)). This maybe from that these components in DS have been dehydrated and crossed link together at the calcination temperature of 1100 oC. From Table 1, DS contains sodium, silicon, aluminum ions and iron oxide. It has reported that the sodium ions can react with aluminum silicates to form a glassy phase when the calcination temperature is above 1050 oC and iron oxides can help the formation of glassy phase during calcination process.23 TEM analysis for DSB further confirms this point since just huge solid particle with smooth surface can be found in the TEM image (Fig.S18), suggesting that these components in DS have crossed link together at the calcination temperature of 1100 oC. The main XRD peaks of DS and DSB are nearly identical except some small peaks in DS disappear after calcination at 1100 oC (Fig S19). Similar experimental phenomenon has been reported, which is most likely from the dehydration, and crosslinking of these components in sediment sample at high calcination temperature.47 Then, BN-DS composites are synthesized by calcinating the mixtures of BN and DS with different BN: DS ratios at 1100 oC for half hour. According to Fig. S5(a), the size of BN-DS-9 is similar to that before calcination and no obvious bloating phenomenon is observed. With the increase of BN amount in the BN-DS mixture, BN-DS-8, BN- DS-7, BN-DS-6 and BN-DS-5


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samples all show obvious bloating behavior (Fig S5(b)-(e)). From Fig S4, the size of DSB before and after calcination has not obvious change, suggesting that the bloating behaviors of BN-DS-8, BN-DS-7, BN-DS-6 and BN-DS-5 samples are from the contribution of the added BN. According to Fig. 13, about 13% of BN is decomposed when the temperature increases to 1100 o

C from 30 oC. It is well known that DS contains sodium, silicon, aluminum ions, and the sodium

ions can react with aluminum silicates to form a glassy phase when the calcination temperature is above 750 oC.23 Thus, the released gas from the pyrolysis of the small amount BN is sealed in the inside of BN-DS samples by the glassy phase formed through the reaction between the sodium ions and aluminum silicates in DS. And this leads to the bloating behavior of BN-DS-8, BN-DS7, BN-DS-6 and BN-DS-5 samples. The floating tests show: (1) BN-DS-8 can not float on water surface; (2) BN-DS-6 can just float on water surface about 22 days and followed by the disintegration into powder, while BN-DS-5 samples can just float on water surface about 14 days and followed by the disintegration into powder; (3) just BN-DS-7 can float on water surface without the disintegration (Fig. S20(a)). From Fig. S20(b), one can see that there are lots of pores with varying size in the inside of BN-DS-7, being an another evidence to support that the released gas from the pyrolysis of the small amount BN is sealed in the inside of BN-DS samples. The floating behaviours of BN-DS-8, BN-DS-7, BN-DS-6 and BN-DS-5 samples suggest that there is an optimal ratio between BN and DS in BN-DS composites. For BN-DS-9 and BN-DS-8 samples, the percentage of BN in composite are not too high and the gas released from BN pyrolysis at high temperature is not too much, which leads to that the gas sealed in the inside of BN-DS-9 and BN-DS-8 are insufficient to make the two samples floating on water surface. For BN-DS-6 and BN-DS-5 samples, they can float on water surface, but will disintegrate into powder after floating water surface for two-three weeks. This maybe from that


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the percentage of BN in composite are too high and the existence of BN will impede the crosslink of DS at high temperature. This conclusion is supported by the TEM results of BN-DS-7 and DSB. From the TEM Figs in Fig. S21 and Fig. S18, the surface of BN-DS-7 is less smooth than that of DSB and has become rugged. This is most likely from that the existence of BN impedes the cross-link of

(a)

(b)

Fig.7 (a)XRD comparison of BN-DS-7, BN and DSB samples; (b) N1s XPS comparison of BNDS-7 and BN samples.

DS during calcination process at high temperature. The compression strength of BN-DS-7 is around 9 Mpa, which is similar to the strength of ceramsite,48 suggesting that the structure of BN-DS-7 is strong and stable. As we know, this is first time to report that BN-DS composite with specific ratio can float on water surface. Floating is a very attractive performance for the materials treating waste water since this kind of materials are easily recycled.49-50 Thus, BN-DS-


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7 is chosen in this work to investigate the interaction between BN and DS in it as well as its adsorption and photodegradation performance for NR. For investigating the interaction between BN and DS in BN-DS-7, BN-DS-7 is first shattered into pieces, and subsequently grinded into powder with the following XRD, UV and XPS characterizations. According to Fig. 7(a), the characteristic peak of BN at 26.6 oC is overlapped with the quartz peak (26.6 oC) of DSB, and can not be used to determine if BN still exists in BNDS-7. However, BN still has another characteristic peak at 41.8 oC, and DSB has not peak at this position (Fig. 7(a)). This characteristic peak of BN at 41.8 oC can also be found in the spectrum of BN-DS-7 (Fig. 7(a)), suggesting that BN still exists in BN-DS-7. This intensity of the peak at 41.8 oC is not strong, being due to that the percentage of BN in BN-DS-7 is just 30%. It is easily to understand why BN still exists in BN-DS-7 since BN itself is also synthesized at 1100oC. From Fig. S22, the UV spectrum of BN-DS-7 is different from that of BN and DSB, suggesting that the interaction between BN and DSB is so strong that the UV spectrum of the composite is different from that the two precursors. The band gap value of BN-DS-7 sample is also acquired based on the Tauc method.40 From Fig. S23, one can see the band gap of BN-DS-7 is 3.13 eV, which is smaller than that of the as-synthesized BN. This will be helpful for BN-DS-7 to harvest more solar energy than BN. XPS is a common way to investigate the interaction between elements in materials or composite. From Fig.7(b), the N1s spectrum of N element in BN-DS-7 is different from that of N element in BN. For the N1s spectrum of N element in BN-DS-7, the N1s peak at 399.1 eV is attributed to the N atom coordinated with Si atom in Si3N4, 51 while the N1s peak at 396.0 eV is assigned to the N atom coordinated to aluminum,

52

respectively. This

suggests that the N elements in BN of BN-DS-7 has a strong interaction with the Si, Al elements in DS of BN-DS-7. The N1s peak at 397.4 eV

38

of BN is not found in the spectra of BN-DS-7


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(Fig.7(b)). It has reported that the N1s peak of BN is also found at 398.2 eV. 8, 53 So, the peak at 398.0 eV in the N1s spectra of BN-DS-7 is attributed to the B-bound N atoms in BN. By comparing the N1s spectrum of BN and BN-DS-7 (Fig.7(b)), one can see that the N1s peak of the B-bound N atoms blue-shifts to the 398.0 eV in BN-DS-7 sample from the 397.4 eV in BN sample. This is most likely due to that the strong interaction between BN and DS makes the valence electron excitation of B-bound N atoms in BN-DS-7 become more difficult than that in BN. From the XRF analysis result (Table 1), the quantities of silicon and aluminum elements in DS are very rich. It has reported that silicon nitride can be synthesized by calcinating silicon powder above 1300 oC under nitrogen atmosphere.28 Aluminum nitride can also prepared by evaporating metallic aluminum at 1200 oC under nitrogen/ammonia atmosphere.29 BN-DS-7 is synthesized at 1100 oC. So, it is very possible that the N elements in BN can built a strong interaction with the silicon and aluminum elements in DS during the treatment process at high temperature. Then, the adsorption and photodegradation performance of DSB and BN-DS-7 are investigated by using it to treat the NR solution, in which the concentration of NR is 20 mg/L since the adsorption ability of DSB and BN-DS-7 are very weak based on the preliminary experimental results. According to Fig.8(a), the adsorption ability (0.45 mg/g) of BN-DS-7 for NR is larger than that (0.21 mg/g) of DSB, but is much less than that (24 mg/g) of BN since NR in solution has been completely adsorbed by the as-synthesized BN in this experimental condition. This is consistent with the surface area order of BN (16.1 m2/g), BN-DS-7 (4.6 m2/g) and DSB (0.9 m2/g). The adsorption performance of BN-DS-7 for NR is obvious smaller than that of BN, being from the following two reasons: (1) the surface area of BN (16.1 m2/g) is larger than that of BN-DS-7 (4.6 m2/g). Furthermore, BN-DS-7 is a solid sample and its surface area is


28

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acquired by crushing it into powder with the subsequent characterization with ASAP2020 micropore analyzer. Thus, the real surface area of BN-DS-7 should less than 4.6 m2/g. The reason that the surface area of BN-DS-7 is much less than that of BN is most likely attributed to that the BN and DS parts in BN-DS-7 sample have been tightly fused together after the calcination at 1100 oC; (2) XPS analysis for BN and BN-DS-7 samples suggests that there are

(a)

(b)

Fig.8 (a) the photodegradation results of NR by DSB, BN and BN-DS-7 under UV light irradiation. The used quantities of DSB, BN and BN-DS-7 are 0.5 g, and the concentration of NR is 20 mg/L; (b) the influences of TB, BQ and EDTA-2Na on the photo-degradation of NR by BN-DS-7 under UV light irradiation

strong interactions between BN and DS, which means that the adsorption active sites on BN surface are occupied by the silicon and aluminum elements in DS. This makes that the adsorption


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active sites on BN surface are not available for NR molecule during the adsorption of NR by BN-DS-7 sample. After five hours irradiation, 51% NR self-degrades under UV-light irradiation. With the presence of DSB, the degradation rate of NR increases to 59%, being from that the interfacial interaction between DSB and NR is favorable for the self-degradation of NR. When BN-DS-7 is used, the degradation rate of NR increases to 84%, suggesting that BN-DS-7 is a UV-responsive floating photocatalyst since the degradation ratio (84%) is higher than selfdegradation ratio (51%) of NR. The photodegradation performance of BN-DS-7 is most likely from the contribution of BN in BN-DS-7 since BN can photodegrade NR under UV light irradiation (Fig. 4). The influences of EDTA-2Na, BQ and TB on the photodegradation of NR by BN under UV light irradiation are investigated as well. Since the text descriptions in Fig.8(b) are too much and the adsorption peak used to characterize the NR photodegradation is at 524 nm, the wavelength ranged from 350 nm to 1050 nm is used in Fig.8(b). According to Fig.8(b), the NR photo-degradation by BN-DS-7 is great inhibited by TB, implying that •OH radical plays a very important role in this process. In addition, hole also makes a little contribution for the photodegradation of NR by BN-DS-7 since the photodegradation of NR is just a little inhibited by EDTA-2Na (Fig. 8(b)). •O2- radical does not make contribution for the photodegradation of BN for NR since the degradation results with and without BQ are nearly identical. This result is similar to that of NR photodegradation by BN under UV light irradiation, further supporting that the photodegradation performance of BN-DS-7 is mainly from the contribution of BN. From Fig. 8(a), the characteristic peaks of NR decrease and no new peaks appear, suggesting that the aromatic structures in NR have been destroyed completely. TOC analysis results for NR solution before and after photodegradation support this point since about 70% NR has been mineralized into inorganic carbon after photodegradation. Recently, many researchers have pay their effort to


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develop the floating photocatalyst, such as polyetherimide/graphitic carbon nitride floating photocatalyst,54 F-Ce-TiO2/expanded perlite floating photocatalyst,55 TiO2/expanded graphite photocatalyst56 and amphiphilic black TiO2 foam.57 As we know, this is first time to report that a floating photocatalyst can be synthesized by calcinating BN and DS in a specific mass ratio. The pH changes of NR solution before and after photodegradation of NR by DSB and BNDS-7 are also investigated. The pH value at the starting time of the NR photodegradation by BNDS-7 is 5.14, and it increases to 6.51 after the photodegradation for five hours. For the selfdegradation of NR under light irradiation, the initial pH value is 5.14, and the final pH value after five hours irradiation is 5.55. For the NR photodegradation by DSB, the initial pH value is 5.14, and the final pH value after five hours irradiation is 5.62. From the Fig. 8(a), one can see that the pH values of the NR solutions after photodegradation have a direct relationship with their concentration after photodegradation. BN-DS-7 has the best photodegradation effect for NR within five hours, which leads to that the residual concentration of NR is the least. So, it has highest final pH values (6.51). It is true for NR photodegradation by DSB and NR selfphotodegradation, which have the final pH values: 5.62 and 5.55, respectively. This is consistent with our finding: the pH value of NR solution is linearly dependent with its concentration. In addition, the pH test experiment show that DSB and BN-DS-7 do not change the pH of deionized water. This further supports that the pH change of NR solution before and after photodegradation is from the concentration change of NR solution before and after photodegradation. Brief mechanisms have been further proposed to understand the synthesis and photodegradation performance of BN-DS-7 sample (Scheme.1). A BN-DS mixture with the mass ratio (1:7) of BN: DS is first prepared, then followed by the calcination at 1100 oC. The gases released from the pyrolysis of a few BN are sealed in the inside of DS through the glass phase


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formed from the calcination of DS at 1100 oC, which leads to the floating BN-DS-7 sample. The BN on the outside and inner surface of the floating BN-DS-7 sample can be excited under UV light irradiation since it makes main contribution for the photocatalytic performance of BN-DS-7 sample. The photo-induced hole can react with water to produce •OH radical, which makes a main contribution for the photodegradation of NR by the floating BN-DS-7 sample. In addition, a few of photo-induced holes can also directly degrade NR.

Scheme. 1 the proposed synthesis mechanism and photodegradation mechanism of BN-DS-7 sample.

The recyclability of BN-DS-7 is also investigated. 0.5g BN-DS-7 is put into 100 ml NR solution (20 mg/L), followed by UV-light irradiation for 5 hours. Then, the sample is collected, washed and dried at 80 oC for overnight before the second use to photodegrade NR under the same conditions. This procedure is repeated four times. According to Fig.9, the photodegradation efficiencies of NR by BN-DS-7 are 82% in the first time, 78% in the second time, 77% in the


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third time and 77% in the fourth time, respectively. One can see that the photodegradation performance of BN-DS-7 for NR becomes stable from the second time to the fourth time, suggesting that BN-DS-7 is good floating photocatalyst for removal of NR from dyeing wastewater.

Fig.9 The recycle usage test of BN-DS-7 photocatalyst under UV light irradiation, in which the quantity of BN-DS-7 is 0.5 g, and the concentration of NR is 20 mg/L.

4 Conclusions In summary, BN is synthesized with oxygen-limited method for the first time, which is of a band gap 3.94 eV and is a UV-light responsive photocatalyst. Then, a series of BN-DS composites are synthesized by calcinating the mixture of BN and DS with different ratio at 1100


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o

C for half hour, in which the composite (BN-DS-7) can float on water surface and photodegrade

NR under UV light irradiation. The gas released from the pyrolysis of BN is sealed in the inside of DS, which leads to that BN-DS-7 can float on water surface. Based on XPS analysis for BNDS-7, BN mainly interacts with DS through the strong coordination between these N atoms in BN and these Si, Al atoms in DS, resulting in that BN-DS-7 has a good compression strength, being around 9 Mpa. Photodegradation results show that BN-DS-7 can effectively photo-degrade NR under UV light irradiation and the photodegradation performance of BN-DS-7 is mainly from the contribution of BN. The adsorption ability of BN-DS-7 is obvious weaker than that of BN, being due to that BN has a strong interaction with DS. This work is helpful to develop new type of BN-based floating materials and enlarge the application field.

Corresponding Author * E-mail: [email protected]; * E-mail: [email protected] Supporting Information: the figures of DS, BN, DSB and BN-DS composites, the photostability tests of MB, NR and RhB, the influence of temperature, pH and ionic strength on the adsorption of NR by BN, the results of molecular simulation about BN and BNOH models, the TEM images of BN, DSB and BN-DS-7 sample etc, are provided in supporting information. Notes The authors declare no competing financial interest ACKNOWLEDGMENT We are grateful for the grants for Project supported by the Key Program of National Natural Science Foundation of China (No. 91647206), National Science Funds for Creative Research


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Groups of China (No.51421006), the National Major Projects of Water Pollution Control and Management Technology (No. 2017ZX07204003)，the National Key Plan for Research and Development of China (2016YFC0502203)，Natural Science Foundation of Jiangsu Province (BK20151494), the Fundamental Research Funds for the Central Universities (2014B18614 2015B25314), A Project Funded by the Priority Academic Program Development of Jiangsu Higher Education Institutions. REFERENCES 1

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Graphical Abstract


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Dredged-Sediment-Promoted Synthesis of Boron-Nitride-Based

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