Improved photoremoval performance of boron carbon nitride–pyromellitic dianhydride composite toward tetracycline and Cr(VI) by itself to change the solution pH

Congcong Yan a, Yong Guo *a, Peifang Wang *a, Lei Rao b, Xin Ji a and Ying Guo a
aKey Laboratory of Integrated Regulation and Resource Development on Shallow Lakes, Ministry of Education, College of Environment, Hohai University, Nanjing, 210098, P. R. China. E-mail: guoyong@hhu.edu.cn; pfwang2005@hhu.edu.cn
bCollege of Mechanics and Materials, Hohai University, Nanjing, 210098, P. R. China

Received 20th April 2020 , Accepted 2nd June 2020

First published on 2nd June 2020


A series of boron carbon nitride–pyromellitic dianhydride (BCNPA) composites were successfully synthesized for the first time, where BCNPA3 exhibited the best adsorption and photodegradation performances for tetracycline (TC) under visible-light irradiation. 1H NMR characterization confirmed the fact that BCNPA3 was formed via the dehydration reaction between the OH groups in BCN and COOH groups in PA. Elemental N in BCNPA3 could capture protons and increase the pH of the TC solution from 3.73 to 6.80; consequently, TC changed from the positive state (pH 3.73) to the neutral state (pH 6.80). Subsequently, the adsorption capacity of BCNPA3 for TC increased from 66.24 mg g−1 (at pH 3.73) to 74.66 mg g−1 (at pH 6.80). This may be attributed to the fact that these captured protons can work as adsorption sites, too. Furthermore, the photodegradation efficiency of BCNPA3 for TC at pH 6.80 under visible-light irradiation was also higher than that at pH 3.73 (95% vs. 87%), because TC at pH 6.80 is less stable than that at pH 3.73. More interestingly, Cr(VI) could only be photoreduced into Cr(III) by BCNPA3 under visible-light irradiation when it coexisted with TC. This could be attributed to the fact that the photodegradation of TC increased the separation efficiency of the photoinduced electron and hole; consequently, the photoinduced electrons had sufficient time to reduce Cr(VI). Photoinduced ˙O2 and holes had the main contribution toward TC photodegradation by BCNPA3, while photoinduced electrons were responsible for Cr(VI) photoreduction. The adsorption and photodegradation performances of BCNPA3 for TC and Cr(VI) could be easily recovered under visible-light irradiation.


1. Introduction

Boron carbon nitride (BCN) is a type of ternary hybrid material containing B, C, and N elements, and its performance is considered to be between those of carbon and boron nitride (BN).1,2 In addition to good performance (such as high adsorption ability and chemical inertness) ascribable to carbon and BN,3,4 BCN also exhibits many fascinating performances (such as tunable bandgap) by controlling the ratio among elemental B, C, and N.5–7 Therefore, BCN materials have been widely used in many fields, such as metal-free electrocatalysts,8,9 photocatalysts,7,10,11 optical materials,6 capacitor materials,5 and adsorbing materials.12,13 Until now, many kinds of BCN materials have been developed, such as zero-dimensional BCN spheres,14 one-dimensional BCN rods,15 two-dimensional BCN nanosheets,3,11,13 three-dimensional BCN aerogels,10 and porous BCN materials.5,12 Till date, developing novel BCN-based materials has remained significant in the field of BCN materials.

BCN materials are usually synthesized in an inert atmosphere,1,2,8 which causes the formation of a few functional groups (such as hydroxyl groups) on their surface. Similarly, BN materials are also fabricated in an inert atmosphere and hydroxyl groups are present on their surfaces, too.3,4,11 BN materials can be peeled into nanosheets and simultaneously functionalized with hydroxylation when they are calcined in air at 1000 °C.16 Our earlier works have shown that BN nanoparticles and carbon-doped boron nitride (BCN) materials with hydroxyl groups can be directly generated by the one-step oxygen-limited method.17,18 Furthermore, a novel BN-based hybrid material can be formed from the reaction between the hydroxyl groups on the BN surface and the amino groups on the carbon nitride surface, which can be used to selectively remove protonated dyes from mixed dye wastewater.19 The reaction between the amino groups on the BN surface and the COOH groups on the graphene surface can also be used to synthesize new BN composites.20,21 In addition, new types of photocatalysts can be synthesized via the reaction between the amino groups on the surface of carbon nitride and the hydroxyl groups on the surface of carbon quantum dots22 and pyromellitic dianhydride (PA),23,24 too. Therefore, developing new hybrid materials based on the reactivity of the functional groups on the material surface is an efficient strategy. So far, synthesizing novel BCN-based materials based on this method has not been reported.

Photocatalytic technology can degrade organic pollutants into smaller molecules—even carbon dioxide and water—by using light.7,10,11,22 If the photocatalytic material itself has good adsorption properties, the photoactive species (such as photoinduced holes (h+), electrons (e), superoxide radicals (˙O2−), and hydroxyl radicals (˙OH)) can degrade more pollutants.25,26 For example, a photocatalyst (such as TiO2) can be coupled with adsorbents (such as carbon and zeolite) to regenerate their adsorption performance under light irradiation.27,28 However, the regenerated adsorption performance is usually not too high. This is due to the fact that TiO2 is loaded on the surface of the adsorbent, which makes it difficult for TiO2 to photodegrade the pollutants in the inner pores of the adsorbent.29 Furthermore, this material should have broad application prospects in wastewater treatment if it has both adsorption and photocatalytic properties. Some materials with these properties have been reported. For example, the adsorption performances of TiO2 nanotubes30 and boron nitride–pyromellitic dianhydride (BNPA) composites31 for dyes can be effectively recovered by light irradiation as well as the high adsorption and photocatalytic activities of porous BCN nanosheets for RhB can also be regenerated by light irradiation.32 Until now, it is still significant to develop a material with both adsorption and photocatalytic properties since the adsorption performance can be regenerated by using light, which is an environmentally friendly regeneration strategy.

Tetracycline (TC) is a kind of amphoteric molecule that has antibiotic properties and low biodegradability.33,34 Therefore, it is difficult to remove TC from water bodies by means of biological methods. Many methods (such as advanced oxidation,35 electrochemistry,36 adsorption,18,33,37 and photocatalysis38) have been used to remove it. Among these removal methods, photocatalytic technology has promising application prospects, since it is environmentally friendly, economical, and sustainable.26 Furthermore, the adsorption and photodegradation performances of photocatalysts are considerably affected by the pH of the solution.39,40 As shown in Fig. S1a (ESI), the charge of TC can be changed with a change in the pH of the solution, which is due to the fact that the functional groups in TC (including a dimethyl amino group, a carbonylamide group, and a phenolic deketone group) can undergo protonation–deprotonation reactions.41 From Fig. S1b (ESI), it is evident that TC exists as a cation species at pH < 3.3, zwitterion species within 3.3 < pH < 7.68, and anion species at pH > 7.68.33,41 Therefore, the pH value has a very significant influence on the adsorption and photodegradation performances of TC by photocatalysts since its existing state varies with a change in the pH of the solution. Several adsorbents (such as carbon nanotubes and biochar) act only as adsorbents and cannot change the pH of a solution.3,13,42 This is also true for photocatalysts. Our earlier study had shown that BN–carbon nitride composites have stronger affinity toward protons and can selectively remove protonated dyes (such as neutral red and malachite green) from wastewater.19 In addition, the adsorption and photodegradation behaviors of pollutants are different in the mixed or single states. For example, wheat-straw-derived biochar has higher adsorption capacity for atrazine (0.0556 mmol g−1) than that for chlorpyrifos (0.0537 mmol g−1) in separate adsorption processes, whereas its adsorption capacity (0.0470 mmol g−1) for chlorpyrifos is higher than that for atrazine (0.034 mmol g−1) in atrazine/chlorpyrifos mixed solutions.42 When a 2,4-DCP/Cr(VI) system is treated with phosphorus-doped porous ultrathin carbon nitride nanosheets, the photodegradation efficiency of 2,4-DCP increases from 90% (single 2,4-DCP solution) to 98%; further, the photoreduction efficiency of Cr(VI) increases from 18% (single Cr(VI) solution) to 85%.43 Moreover, when a TC/Cr(VI) mixed system is treated with a TiO2-/O-doped g-C3N4 composite, the removal efficiency of TC increases from 83% (single TC solution) to 89% and the photoreduction efficiency of Cr(VI) increases from 80% (single Cr(VI) solution) to 99%.38 Many intermediates of pollutants have been reported to be more biotoxic than the pollutants themselves.44,45 Furthermore, it is not easy for photocatalysts to completely mineralize pollutants in actual water bodies, and the impact of these photodegradation intermediates on the water environment can be significant.43,46 Therefore, it is necessary to study the toxicity of such intermediate products for evaluating the photodegradation properties of the photocatalysts.

In this study, a series of boron carbon nitride–pyromellitic dianhydride (BCNPA) composites have been successfully prepared for the first time by calcinating a mixture of BCN and PA at 350 °C for 4 h. Among all the BCNPA composites, BCNPA3 has the largest adsorption capacity as well as the best photodegradation efficiency for TC under visible-light irradiation. BCNPA3 is capable of changing the pH of the TC solution and increasing its adsorption and photodegradation performances for TC. More interestingly, Cr(VI) can only be photoreduced into Cr(III) by BCNPA3 under visible-light irradiation when it mixes with TC. This phenomenon is different from those discussed in earlier reports.38,43 The goals of this study are as follows: (1) investigating the reaction mechanism of BCNPA3 with BCN and PA as the precursors; (2) clarifying the adsorption and photodegradation mechanisms of TC by BCNPA3-induced pH change in the solution; (3) investigating the removal mechanism of the TC/Cr(VI) coexistence solution by BCNPA3 under visible-light irradiation; and (4) evaluating the toxicity of TC and its photodegradation intermediates.

2. Materials and methods

2.1. Reagents

Diboron trioxide (B2O3), melamine (C3H6N6), PA, potassium dichromate (K2Cr2O7), TC, sodium hydrate (NaOH), sodium sulfate (Na2SO4), 1,5-diphenylcarbazide (DPC), p-benzoquinone (BQ), disodium ethylenediamine tetraacetate (EDTA-2Na), tert-butanol (TB), potassium bromate (PB), isopropanol (IPA), sulfuric acid (H2SO4), and hydrochloric acid (HCl) were of the analytical grade. These reagents were purchased from Shanghai Yuanye Biotechnology Co., Ltd.

2.2. Preparation of BCNPA composites via oxygen-limited method

In our earlier work, BCN and PA were ground in a mortar at a mass ratio of 1[thin space (1/6-em)]:[thin space (1/6-em)]1 for 30 min to ensure that they were evenly mixed together. BCN was prepared by calcinating a mixture of B2O3 and melamine at 1100 °C for 3 h,18 which was a white-colored sample (Fig. S2a, ESI). The color of PA was also white (Fig. S2b, ESI). Then, the mixture was transferred into a closed graphite crucible with a lid. After that, this crucible was placed in a muffle furnace for calcination at 350 °C for 4 h at a heating rate of 5 °C min−1. The obtained powder was washed multiple times with ultrapure water to remove the unreacted PA and other impurities. The washed sample was dried in an oven at 60 °C for 12 h. This light-yellow sample was termed BCNPA1 (Fig. S3a, ESI). Based on the colors of BCN, PA, and BCNPA1 shown in Fig. S2a, b and S3a (ESI), respectively, BCNPA1 was clearly a new material with respect to BCN and PA.

For considering the effect of PA ratio on the properties of BCNPA composites, BCNPA2, BCNPA3, and BCNPA4 composites were also synthesized by using the same synthesis method mentioned above, except for the fact that the mass ratios of BCN and PA were changed as 1[thin space (1/6-em)]:[thin space (1/6-em)]2, 1[thin space (1/6-em)]:[thin space (1/6-em)]3, and 1[thin space (1/6-em)]:[thin space (1/6-em)]4, respectively. By comparing the illustrations of BCNPA1, BCNPA2, BCNPA3, and BCNPA4 composites (Fig. S3a–d, respectively, ESI), the color of the BCNPA samples changed from yellow to ochre as the proportion of PA increased.

Similarly, BCN (1 g) was calcined by the same synthesis method as that mentioned above. The obtained sample was still white in color. Furthermore, PA (1 g) was treated by the same synthesis method, too. However, nothing remained in the calcined graphite crucible. This was due to the combustion and evaporation of PA under the condition of oxygen limitation at 350 °C.

2.3. Sample characterizations

The X-ray diffraction (XRD) measurements of BCN, PA, and BCNPA3 were obtained using an XRD radiation instrument (Ultima III, Tokyo, Japan). The FTIR patterns of BCN, PA, and BCNPA3 were measured by a Fourier-transform infrared (FTIR) instrument (NEXUS 870, USA). The XPS characterizations of BCN, PA, and BCNPA3 were acquired using an X-ray photoelectron spectroscopy (XPS) instrument (PHI 5000 VersaProbe, Japan). The morphologies of BCN, PA, and BCNPA3 were obtained by using a transmission electron microscope (TEM; JEM-200CX, Japan). The hydrogen spectra (1H NMR) of BCN, PA, and BCNPA3 were obtained by means of a solid nuclear magnetic resonance spectrometer (Bruker AVANCE III 600 MHz, Switzerland). The BET specific surface area and pore size distribution of BCN and BCNPA composites were acquired using a microporous analyzer (ASAP 2020 HD88, USA). The UV-visible diffuse reflectance spectra (UV-DRS) of BCN, PA, and BCNPA3 were collected with a UV-visible spectrophotometer (Agilent CARY 100, USA). The pH of the solution was measured with a pH meter (PHS-3E, China). The zeta potential values of BCNPA3 at different pH values were measured by using a potential analyzer (Zetasizer Nano ZS, UK). The signal of the ˙O2 radical of BCNPA3 in solution during the photocatalytic process under visible light (λ > 420 nm) was determined using electron spin resonance (ESR; JES FA200). The radical signal tests were operated in the dark and under visible-light irradiation for 5 and 10 min. The photodegradation intermediates of TC were determined using high-performance liquid chromatography-mass spectrometry (HPLC-MS) (Agilent 6530, Japan). The mobile phase was acetonitrile/water (80/20, v/v) at a flow rate of 0.4 mL min−1 and an injection volume of 10 μL.

The transient photocurrent (TPC) response, electrochemical impedance spectra (EIS), and conduction band edge of the BCNPA composites were obtained using a CHI 660D workstation with a three-electrode model. Pt and Ag/AgCl electrodes were utilized as the counter and reference electrodes, respectively. In general, the working electrodes were prepared by drop-coating a colloid as follows. First, 5 mg photocatalyst was ultrasonically dispersed in a mixed solvent containing 0.6 mL deionized water, 0.2 mL IPA, and 0.01 mL Nafion solution to obtain a homogeneous colloid. Then, 100 μL of the prepared colloid was transferred to a 1 cm × 4 cm fluorine-doped tin oxide (FTO) slice, and its effective working area was 1 cm2. A 300 W Xe arc lamp was used as the light source, and 0.1 M Na2SO4 aqueous solution served as the working solution. In addition, the intensity and spectrum of the visible-light source were measured with a strong optical power meter (CEL-NP2000, China) and an electrochemical workstation (CHI 660, China), respectively.

The removal quantities of TC were obtained according to the UV characteristic peaks of the reaction solution, which was determined by using a UV-visible spectrometer (TU-1901, China). From Fig. S4 (ESI), it is evident that the UV characteristic peaks of the TC solution at different pH values had different adsorption wavelengths: the UV characteristic peaks of the TC solution at pH 3.73 and 6.80 were 357 and 365 nm, respectively. In this study, the pH of the TC solution changed from the initial value of 3.73 to 6.80 after the addition of BCNPA composites. Therefore, the standard curves of the TC solution at different pH values were plotted based on the UV characteristic peaks of the TC solution at the corresponding pH value. According to Fig. S5a and b (ESI), the concentrations of the TC solution at pH 3.73 and 6.80 showed good linear correlation with the corresponding UV characteristic peaks because their correlation coefficient (R2) were greater than 0.9996. Based on the pH value of the TC solution, the corresponding characteristic curve mentioned above was selected to calculate the removal efficiencies of the BCNPA composites for TC.

As evident from Fig. S5c (ESI), the Cr(VI) concentration was detected by the diphenylcarbazide spectrophotometric method43,47 according to its UV characteristic peak at 542 nm. From Fig. S5d (ESI), the concentration of the Cr(VI) solution showed good linear correlation with its UV characteristic peak (correlation coefficient, R2 = 0.9995).

2.4. Adsorption and photodegradation experiments of BCNPA composites for TC

For investigating the effect of PA dosage on the properties of BCNPA composites, the adsorption and photodegradation experiments involving BCNPA1, BNCPA2, BCNPA3, and BCNPA4 samples for TC were performed. The detailed steps were as follows. The BCNPA composite (25 mg) was added to 100 mL TC solution (concentration: 30 mg L−1). It was then stirred for 300 min at 200 rpm under dark conditions at room temperature to reach adsorption equilibrium. Thereafter, 3 mL TC solution was taken and filtered with a 0.22 μm cellulose membrane, which was placed in a centrifuge tube for characterizing its concentration with a UV-visible spectrometer. After that, a xenon lamp (300 W, CEL-PF300-T8, Beijing Zhongjiao Jinyuan Technology Co., Ltd) equipped with a 420 nm filter was turned on. Therefore, the radiated light was visible light at a wavelength greater than 420 nm. Then, 3 mL of the water sample was taken into a centrifuge tube by the same method mentioned above after every 30 min. As shown in Fig. S6 (ESI), the cellulose membrane did not affect the TC concentration since the UV spectrum of the TC solution had no change before and after filtration. The above experiments were repeated three times to ensure the accuracy of the experimental results.

2.5. Adsorption experiments of BCNPA3 for TC

The kinetic adsorption experiments of TC by BCNPA3 were performed according to the following detailed procedures. BCNPA3 (25 mg) was added to 100 mL TC solution (concentration: 30 mg L−1). It was then stirred at 200 rpm under dark condition at room temperature. For ensuring the reasonability of the obtained results, ten adsorption experiments of TC by BCNPA3 were simultaneously performed. These experiments were stopped at 5, 10, 20, 30, 50, 90, 150, 210, 270, and 330 minutes. Then, 3 mL of the TC solution was intermittently withdrawn and filtered with a 0.22 μm cellulose membrane for characterizing its concentration with a UV-visible spectrometer.

The isotherm adsorption experiments of BCNPA3 for TC were performed according to the above procedures, except for changing the TC concentration (ranging from 20 to 100 mg L−1). Then, 3 mL of the water samples were taken into a centrifuge tube with the above method after adsorption for 300 min to reach adsorption equilibrium.

The thermodynamic adsorption experiments of TC by BCNPA3 were also performed with the above procedures, except for changing the reaction temperature (ranging from 25 to 55 °C). Then, 3 mL of the water samples were taken into a centrifuge tube with the same method after adsorption for 300 min to reach adsorption equilibrium.

The adsorption performance of TC by BCNPA3 under acidic conditions was tested by adding 0.1 mmol L−1 HCl to adjust the pH value of the TC solution to 3.73. The adsorption quantity of TC by BCNPA3 at pH 3.73 was investigated using the same method mentioned above. The above adsorption experiments were also repeated three times.

2.6. Photodegradation experiments of BCNPA3 for TC

The photodegradation experiments of TC by BCNPA3 were performed in the following detailed procedures. BCNPA3 (25 mg) was added to 100 mL TC solution (concentration: 30 mg L−1). It was then stirred at 200 rpm for 300 min under dark condition at room temperature to achieve adsorption equilibrium. After that, the solution was irradiated with visible light by using a xenon lamp. Then, 3 mL of the water sample was taken every 30 min and filtered with a 0.22 μm cellulose membrane for characterizing its concentration with a UV-visible spectrometer. Furthermore, in order to identify the active species during the photodegradation of TC by BCNPA3, EDTA-2Na, TB, BQ, and PB were added in the photodegradation experiments of TC by BCNPA3 to capture the photoinduced h+, ˙OH, ˙O2, and e, respectively.

The photodegradation performance of TC by BCNPA3 under acidic conditions (pH 3.73) was tested by adding 0.1 mmol L−1 HCl to adjust the pH value of the TC solution to 3.73 after reaching adsorption equilibrium. The detailed photodegradation procedures of TC by BCNPA3 were performed according to those mentioned above. All the above photodegradation experiments were repeated three times, too.

2.7. Removal (including adsorption and photoreduction) experiments of Cr(VI) by BCNPA3

The removal experiments of BCNPA3 for Cr(VI) were performed according to the same procedure mentioned above, except for the fact that the contaminant solution was Cr(VI) solution (K2Cr2O7 solution; concentration: 10 mg L−1). Here, 3 mL of the water samples were taken out and filtered with a 0.22 μm cellulose membrane after adsorption for 300 min to reach adsorption equilibrium. These experiments were also repeated three times.

2.8. Adsorption, photodegradation, and photoreduction experiments of TC/Cr(VI) by BCNPA3

The removal experiments of TC/Cr(VI) by BCNPA3 were performed according to the same procedures mentioned above, except for the fact that the contaminant solution was the TC/Cr(VI) coexistence solution (TC concentration: 30 mg L−1; Cr(VI) concentration: 10 mg L−1). In order to determine the active species during the photoremoval process of TC/Cr(VI) by BCNPA3, EDTA-2Na, BQ, TB, and PB were added in the photoremoval experiments involving TC/Cr(VI) by BCNPA3 to capture the photoinduced h+, ˙O2, ˙OH, and e, respectively. All the above experiments were repeated three times, too.

2.9. Photodriven recycling of BCNPA3 for the degradation of TC/Cr(VI) coexistence solution

BCNPA3 (25 mg) was put into 100 mL TC/Cr(VI) coexistence solution (TC concentration: 30 mg L−1; Cr(VI) concentration: 10 mg L−1). Initially, the solution was stirred in the dark for 300 min to reach adsorption equilibrium. After that, it was irradiated with visible light for 180 min. Then, 3 mL of the water samples were correspondingly taken and placed into a centrifuge tube for characterizing their removal efficiencies. Then, BCNPA3 was collected by centrifugation and placed on the surface of a watch glass with ultrapure water and further exposed to visible light for 180 min. Finally, this sample was collected by centrifugation and dried in an oven at 60 °C for 12 h. The treated BCNPA3 was reused to remove the TC/Cr(VI) coexistence solution under consistent experimental conditions. In addition, for further testifying the stability and reusability of BCNPA3, the XRD measurements of BCNPA3 before and after five cycles were obtained by using an XRD radiation instrument (Ultima III, Tokyo, Japan). These experiments were also repeated three times.

2.10. Comparative experiments of BCN, BNPA, biochar, and activated carbon with BCNPA3

For evaluating the adsorption and photoremoval performances of BCNPA3, comparative experiments of TC involving adsorbents (BCN, biochar, and activated carbon) and photocatalyst (BNPA) were performed. The BNPA composite was synthesized by calcinating BN and PA at 350 °C for 4 h and biochar was generated by calcinating the straw powder at 750 °C as per our earlier works.31,48 Activated carbon was a commercial product purchased from Environmental Protection Materials Co., Ltd. The experimental procedures for these materials were the same as those mentioned above. The above experiments were also repeated three times.

2.11. Computational section

The molecular structures of BCN, PA, and BCNPA3 were determined by using the M06-2x/6-31G(d,p) method49 in the Gaussian 09 program.50 As shown in Fig. S7a (ESI), the constructed BCN model comprised 31 boron atoms, 8 carbon atoms, 33 nitrogen atoms, and 5 oxygen atoms, as well as the fact that its edges were saturated with 24 hydrogen atoms. As shown in Fig. S7b (ESI), the full-atom model was adopted for PA. As shown in Fig. S7c (ESI), the BCNPA3 model was built with 31 boron atoms, 18 carbon atoms, 33 nitrogen atoms, and 10 oxygen atoms, as well as the fact that its edges were saturated with hydrogen atoms. Furthermore, the interaction energy of BCNPA3 with the protons and the adsorption energy of BCNPA3 with TC were calculated. As shown in Fig. S7d (ESI), the molecular structure of neutral TC (TCH02) at pH 6.80 was also determined, where one proton in TC was lost from the O atom. According to these optimization results, the above molecular structures were built by using GaussView 5.0 program.

3. Results and discussion

3.1. Determination of the optimum mass ratio between BCN and PA in BCNPA composites

As shown in Fig. S2 and S3 (ESI), with an increase in the PA dosage, the colors of BCNPA1, BCNPA2, BCNPA3, and BCNPA4 changed from pale yellow to ochre. This is different from the colors of BCN (white) and PA (white). Therefore, the BCNPA composites are new materials with respect to BCN and PA. TC is a typical antibiotic and has been widely adopted to evaluate the adsorption and photocatalytic properties of synthetic materials.35,51,52 Therefore, the adsorption and photodegradation performances of BCNPA composites for TC were initially evaluated in this work to determine the optimum mass ratio between BCN and PA.

As evident from Fig. 1a, the adsorption capacity of TC by BCNPA3 is the highest (74.66 mg g−1), followed by BCNPA2 (69.58 mg g−1), BCNPA4 (69.07 mg g−1), and BCNPA1 (68.64 mg g−1). A xenon lamp with a 420 nm filter is turned on when the adsorption equilibrium is reached. From Fig. 1b, BCNPA1, BCNPA2, BCNPA3, and BCNPA4 can further photodegrade TC under visible-light irradiation, indicating that BCNPA composites are photocatalytic materials with visible-light response. BCNPA3 has the largest removal efficiency (95%) for TC, where 62% TC can be attributed to adsorption and 33% TC to photodegradation under visible-light irradiation. BCNPA2 can remove 91% TC (adsorption: 59%; photodegradation: 32%), BCNPA4 can remove 87% TC (adsorption: 57%; photodegradation: 30%), and BCNPA1 can remove 83% TC (adsorption: 57%; photodegradation: 26%). Furthermore, the photostability of TC under visible-light irradiation has also been studied. According to Fig. S8a (ESI), TC cannot be photodegraded under visible-light irradiation. In addition, the photodegradation kinetics of all the BCNPA composites for TC are fitted, which is consistent with the first-order kinetic model (Fig. S8b, ESI). The calculated TC photodegradation rate of BCNPA3 is 0.01513 min−1, which is 1.23, 1.71, and 2.14 times that of BCNPA2, BCNPA4, and BCNPA1, respectively. This also indicates that BCNPA3 has the best photodegradation performance for TC under visible-light irradiation.


image file: d0nj01987a-f1.tif
Fig. 1 (a) Adsorption quantities of TC by BCNPA composites (dosage: 25 mg; TC concentration: 30 mg L−1; volume: 100 mL). (b) Adsorption and photodegradation efficiencies of TC by BCNPA composites under visible-light irradiation (dosage: 25 mg; TC concentration: 30 mg L−1; volume: 100 mL). (c) Transient photocurrent response of BCN and BCNPA composites. (d) EIS Nyquist plots of BCN and BCNPA composites.

The measurement of the transient photocurrent is an effective method to study the photoresponse ability and photogenerated charge recombination efficiency of the photocatalysts.43 According to Fig. 1c, it is evident that all the electrodes of the BCNPA composites show stable and fast photocurrent responses, and this photoresponsive phenomenon is completely reversible under light irradiation. Among all the BCNPA composites, BCNPA3 exhibits the highest photocurrent density, implying that it has the highest light-harvesting and lowest photogenerated charge recombination efficiencies. EIS is also an efficient method that can be adopted to explain the efficiency of electron transfer on the electrode.53 The semicircular Nyquist plots for BCN and all the BCNPA composites are shown in Fig. 1d. BCN has the largest diameter, indicating its poor electrical conductivity. Therefore, electron transfer from BCN to the back-contact electrode is inhibited. In contrast, BCNPA3 has the smallest diameter, which implies that the charge transfer impedance is the lowest. Therefore, both TPC and EIS results effectively explain why BCNPA3 has the best photodegradation performance for TC, which is identical to the photodegradation results of TC by BCNPA composites (as shown in Fig. 1b). Therefore, BCNPA3 is chosen for further investigating the formation of BCNPA composites as well as its adsorption and photocatalytic mechanisms for TC and TC/Cr(VI) coexistence solutions.

3.2. Characterizations of BCNPA3

BCN was synthesized using B2O3 and melamine as the precursors, as per our earlier study.18 According to Fig. 2a, BCN has two typical characteristic diffraction peaks in its XRD pattern, namely, 26.7° and 42.5°. Meanwhile, the XRD pattern of BCNPA3 is almost identical to that of BCN, indicating that they have a similar skeletal structure.11,54 Therefore, BCNPA3 is a BCN-based material. The FTIR spectra of BCN, PA, and BCNPA3 are shown in Fig. 2b. In the FTIR spectrum of BCN, the peak at 786 cm−1 can be assigned to the bending vibration of the B–N–B bond, and the broad peak at about 1527 cm−1 belongs to the stretching of the B–N bond.16,18,55 The peak at 921 cm−1 can be ascribed to the N–B–O bond.56,57 Furthermore, the peak at 2369 cm−1 can be attributed to the C–N bending vibration.58 The broad peak at around 3400 cm−1 can be attributed to the stretching vibration of the O–H and N–H bonds.56 Therefore, BCN synthesized with the oxygen-limited method contributes toward the existence of the OH and NH groups on its surface. In the FTIR spectrum of PA, the peak at 1771 cm−1 can be attributed to the vibration of its COOR group.59 Furthermore, a peak at 3400 cm−1 can also be found, which can be ascribed to the vibration of the C-bound OH group.59 This can be attributed to the hydrolysis of PA in air. Moreover, in the FTIR spectrum of BCNPA3, the peak at 1771 cm−1 can also be found, which suggests that BCN and PA were successfully coupled since the residual PA in BCNPA3 was washed off. Further, the FTIR peak at 3400 cm−1 suggests that the OH and NH groups exist in BCNPA3, too. However, the intensity of the peak at 3400 cm−1 in the FTIR spectrum of BCNPA3 is lower than that of BCN, implying that the OH groups on the BCN surface may be involved in the coupling reaction between BCN and PA. The FTIR spectrum of BCNPA3 is similar to that of BCN, too (Fig. 2b), further indicating that BCNPA3 is a BCN-based material.
image file: d0nj01987a-f2.tif
Fig. 2 (a) XRD patterns of PA, BCN, and BCNPA3. (b) FTIR spectra of PA, BCN, and BCNPA3. (c) B1s spectra of BCNPA3. (d) 1H NMR spectra of PA, BCN, and BCNPA.

XPS can be adopted to determine the electronic structure of the elements for studying the interaction between elements. As shown in Fig. S9a (ESI), the B1s peak of BCN can be divided into three subpeaks, namely, 190.3, 190.9, and 192.1 eV, which correspond to B–C, B–N, and B–O bonds, respectively.2,18 According to Fig. S9b (ESI), the C1s signal of BCN can be divided into three subpeaks, which are centered at 284.4, 285.6, and 286.8 eV. These subpeaks are composed of sp2 carbons of C–C, C–B, and C–N bonds,11 respectively. From Fig. S9c (ESI), the N1s signal of BCN can be divided into two subpeaks, namely, 397.9 and 398.6 eV, which can be attributed to N–B and N–C bonds,2,56 respectively. According to Fig. S9d and e (ESI), the C1s and N1s peaks of BCNPA3 are almost the same as those of BCN, indicating that BCNPA3 is a BCN-based composite as well as the fact that elemental C and N do not participate in the coupling reaction of BCN with PA. From Fig. S9f (ESI), in the O1s spectra of BCN, PA, and BCNPA3, the O1s peak of BCNPA3 is also identical to that of BCN and is obviously not from PA. This can probably be attributed to the strong O1s peak intensity of BCN. From Fig. 2c, the B1s peak of BCNPA3 can also be divided into three subpeaks, namely, 190.3, 190.9, and 192.5 eV, which correspond to B–C, B–N, and B–O bonds, respectively. The peak of B–O bonds in BCNPA3 is different from that of BCN, suggesting that elemental B may be involved in the formation of BCNPA3. In the literature, it has been described that metal-free photocatalysts can be synthesized using carbon nitride and PA, which is based on the dehydration reaction between PA and residual NH2 groups in carbon nitride.23 Therefore, BCNPA3 is also most likely formed based on the dehydration reaction between the B-bound OH groups in BCN and C-bound COOH groups in PA.

1H NMR characterization is an effective way to study the participation of hydrogen in the reaction. As shown in Fig. 2d, the signals at 8.0 and 4.4 ppm can be ascribed to the OH and NH groups, respectively, in the 1H NMR spectrum of BCN.60 However, the signal at 8.0 ppm cannot be found in the 1H NMR spectrum of BCNPA3. This supports the fact that the OH groups on the BCN surface participate in the formation of BCNPA3. Furthermore, in the 1H NMR spectrum of PA, the two peaks at −1.3 and 2.1 ppm can be assigned to the proton combined with the oxygen atom in the carboxyl group and the proton combined with the carbon atom in aromatic ring, respectively.31 Moreover, in the 1H NMR spectrum of BCNPA3, the signal at 4.0 ppm can also be ascribed to the NH groups.61 Further, there are two peaks at −1.0 and 1.2 ppm, which are very close to those of PA (namely, −1.3 and 2.1 ppm, respectively). Therefore, these two peaks of BCNPA3 can also be attributed to the proton in the carboxyl group of the PA part and the proton combined with the carbon atom in the aromatic ring in BCNPA3, respectively. The 1H NMR peaks of PA and PA part in BCNPA3 are different since they are in different chemical environments. As shown in Fig. S7c (ESI), due to the spatial structure, neither of the two COOH groups on the hydrolyzed PA surface can easily react with the OH groups on the BCN surface. Therefore, the OH groups in the carboxyl group can still be found in the 1H NMR and FTIR spectra of BCNPA3. Therefore, the B1s peak of BCN at 192.1 eV and the B1s peak of BCNPA3 at 192.5 eV are most likely due to the B–O–H and B–O–C bonds, respectively.

As shown in Fig. 3a, the surface of BCN is a plate-like structure with circular holes.62,63 According to Fig. 3b, the TEM images of PA confirm the fact that PA exists in the form of aggregates with a particle size of around 5 nm. As shown in Fig. 3c, BCNPA3 exists in the form of certain crotch-like structures. This is probably due to the fact that PA is a linear molecule with functional groups on both its sides (Fig. S7d, ESI). One side of PA can react with BCN, and the other side can react with another BCN, too. Therefore, PA can act as a linker that links BCN together to form a chain-like structure. Crotch-like structures are formed (Fig. 3c) when more than one PA molecule is bound to BCN. According to Fig. 3a and c, the structure of BCNPA3 is looser than that of BCN. Therefore, the BET specific surface area of BCNPA3 should be greater than that of BCN. The BET area and pore size distribution data are shown in Table S1 and Fig. S10 (ESI); the BET area of BCNPA3 is indeed greater than that of BCN (28.611 m2 g−1vs. 18.708 m2 g−1). Meanwhile, the average adsorption particle size of BCNPA3 is 3.276 nm, which is lower than that of BCN (6.372 nm). In addition, the proportion of BCNPA3 with pore size less than 4 nm is significantly larger than that of BCN (Fig. S10, ESI), and the proportion of BCNPA3 with pore size less than 2 nm is 2.22 times that of BCN (Table S1, ESI). Therefore, the coupling reaction of BCN with PA is favorable to increase the BET specific surface area and change the pore size of BCN. Furthermore, their BET area and pore size distribution results are consistent with those of the TEM data.


image file: d0nj01987a-f3.tif
Fig. 3 (a) TEM image of BCN. (b) TEM images of PA; scales: 5 and 50 nm. (c) TEM image of BCNPA3.

UV-DRS is an effective method to determine the bandgap and optical properties of photocatalytic materials.43,64 The UV-DRS data for BCN, PA, and BCNPA3 are shown in Fig. 4a. In the UV-DRS data of BCN, it has an obvious peak at 252 nm and a small peak at 336 nm. Further, PA has an obvious peak and small peak in its UV-DRS data, namely, 292 and 222 nm, respectively. However, in addition to the two absorption peaks at 232 and 318 nm, BCNPA3 exhibits absorption in the wavelength range of the visible-light region (400 nm < λ < 800 nm) in its UV-DRS. This further indicates that BCNPA3 is a new type of material that is different from PA and BCN. Furthermore, as shown in Fig. 4b, based on the Tauc method [(αhν)1/nvs. hν],65 the bandgaps of BCN, PA, and BCNPA3 were calculated, namely, 3.81, 3.73, and 2.79 eV, respectively. Therefore, the bandgap of BCN is 1.37 times higher than that of BCNPA3, which supports the fact that the bandgap of BCN can be decreased by coupling with PA. Moreover, the Mott–Schottky (M–S) plots can be used for both BCN and BCNPA3 to further determine their exact band-edge locations. As shown in Fig. 4c and d (ESI), the CB edges of BCN and BCNPA3 are −0.57 and −0.45 V vs. NHE, respectively.


image file: d0nj01987a-f4.tif
Fig. 4 (a) UV-DRS of PA, BCN, and BCNPA3 composites. (b) Bandgap energies of PA, BCN, and BCNPA3. (c) CB edge of BCN with plant band potential method. (d) CB edge of BCNPA3 with plant band potential method.

The band structures of BCN and BCNPA3 were obtained based on the bandgap and plant band potential results, which are shown in Fig. 5a. The CB and VB values of BCN are −0.57 and 3.24 V vs. NHE, while those for BCNPA3 are −0.45 and 2.34 V vs. NHE, respectively. The band structures of BCN and BCNPA3 have been investigated via theoretical studies. As shown in Fig. S11a and b (ESI), the N2p orbital is responsible for the VB of BCN and the B2p orbital mainly contributes toward the CB of BCN. The above result is consistent with that in the literature,11,66 which supports the fact that the calculation method mentioned above can reasonably describe the investigated system. As shown in Fig. S12 (ESI), two BCNPA3 models have been designed: one is the BCNPA3 model, in which the interaction between the carboxyl oxygen in PA and B atom in BCN is considered, and a six-member heterocyclic ring is formed via the acid–base interactions between the B atom in CBN and O atom in PA. The other is the BCNPA3a model, which does not consider the acid–base interactions between B in CBN and O in PA. The calculated energy of BCNPA3 model is 20.05 kcal mol−1, which is lower than that of the BCNPA3a model, suggesting that BCNPA3 probably exists in the BCNPA3 model. This is consistent with the XPS results since the B1s peaks of BCNPA3 are different from those of BCN. Therefore, the band structure analysis is performed based on the BCNPA3 model. As shown in Fig. 5b, the VB (2.34 V vs. NHE) of BCNPA3 is mainly composed of the O2p and C2p–C2p orbital in the PA part of BCNPA3. As shown in Fig. 5c, the CB (−0.45 V vs. NHE) of BCNPA3 is mainly composed of B2p–N2p, C2p–C2p, and N2p–C2p orbitals in the BCN part of BCNPA3.


image file: d0nj01987a-f5.tif
Fig. 5 (a) Band structures of BCN and BCNPA3. (b) HOMO orbital of the BCNPA3 model. (c) LUMO orbital of the BCNPA3 model.

3.3. Adsorption kinetics, isotherms, and thermodynamics of BCNPA3 for TC

The adsorption mechanism of TC by BCNPA3 has been investigated. According to Fig. 6a, the adsorption capacity of BCNPA3 for TC reaches 71.41 mg g−1 at 60 min and then achieves adsorption equilibrium with the largest adsorption capacity (74.66 mg g−1) after 150 min. As shown in Fig. S13 and Table S2 (ESI), the R2 value is calculated to be 0.9763 for the pseudo-first-order kinetic model and 0.9994 for the pseudo-second-order kinetic model. Furthermore, the calculated value of qe from the pseudo-second-order kinetic model is 72.73 mg g−1, which is very close to the experimental value (74.66 mg g−1). Therefore, the adsorption kinetics of BCNPA3 for TC is more suitable to be described by the pseudo-second-order kinetic model.33
image file: d0nj01987a-f6.tif
Fig. 6 (a) Adsorption kinetics curve of BCNPA3 for TC (dosage: 25 mg; TC concentration: 30 mg L−1; volume: 100 mL). (b) Adsorption isotherm curve of BCNPA3 for TC (dosage: 25 mg; TC concentration ranging from 20 to 100 mg L−1; volume: 100 mL). (c) Adsorption capacity of BCNPA3 for TC at different temperatures (dosage: 25 mg; TC concentration: 30 mg L−1; volume: 100 mL; temperature ranging from 25 to 55 °C).

The influence of TC concentration on the adsorption performance of BCNPA3 has also been investigated. As shown in Fig. 6b, when the concentration of TC ranges from 20 to 100 mg L−1, the adsorption capacity of TC by BCNPA3 increases from 57.09 to 110.26 mg g−1. According to Fig. S14 and Table S3 (ESI), the R2 value of the Langmuir isotherm model (0.9996) is higher than that of the Freundlich isotherm model (0.9124). Therefore, the Langmuir isotherm model is suitable to describe the adsorption isotherm of TC by BCNPA3.51

The relationship between TC adsorption quantity by BCNPA3 and temperature has also been studied. As shown in Fig. 6c, when the temperature ranges from 25 to 55 °C, the adsorption quantity of TC by BCNPA3 increases from 76.74 to 83.21 mg g−1. Therefore, this adsorption process is endothermic. In addition, as shown in Fig. S15 and Table S4 (ESI), the adsorption thermodynamics parameters of BCNPA3 for TC are also calculated. All the calculated ΔG° values are negative at different temperatures, suggesting that the adsorption process of BCNPA3 for TC occurs spontaneously.67 As the temperature increases, the absolute values of image file: d0nj01987a-t1.tif increase as well as the values of image file: d0nj01987a-t2.tif (14.19 kJ mol−1) and image file: d0nj01987a-t3.tif (97.58 kJ mol−1) are both positive (Table S4, ESI). This suggests that a higher temperature is conducive toward the adsorption of TC by BCNPA3.51

3.4. Adsorption mechanism of BCNPA3 for TC

For determining whether the specific surface area is the driving force of TC by BCNPA3, the relationship between the adsorption capacity of all the BCNPA composites for TC and their BET specific surface areas were studied. The adsorption capacity of TC by BCNPA3 is 74.66 mg g−1, followed by BCNPA2 (69.58 mg g−1), BCNPA4 (69.07 mg g−1), and BCNPA1 (68.64 mg g−1) (Fig. 1a). From Table S5 (ESI), the BET surface area of BCNPA3 is 28.611 m2 g−1, followed by BCNPA2 (27.867 m2 g−1), BCNPA1 (27.805 m2 g−1), and BCNPA4 (27.706 m2 g−1). The order of the adsorption capacities of these BCNPA composites is not the same as their BET specific surface areas. Therefore, the BET specific surface area may not be the main driving force for BCNPA3 toward TC adsorption.

At the beginning of the experiment, the pH of the TC solution was 3.73. Therefore, TC existed in two forms (neutral ions (TCH02) and cations (TCH3+)) in solution (Fig. S1b, ESI). After adding BCNPA3 to the TC solution, its pH value increased to 6.80 within 2 min. Consequently, TC loses protons and mainly exists in the form of a neutral species (TCH02), but negative species (TCH1) also start to appear (Fig. S1b, ESI). Furthermore, the zeta potential of BCNPA3 at pH 3.73 is −21 mV. However, it is −30 mV at pH 6.80. If the main driving force for the adsorption of BCNPA3 for TC is electrostatic interactions, the adsorption capacity of BCNPA3 for TC at pH 3.73 should be higher than that at pH 6.80. However, the surface of BCNPA3 is negative and TC exists as a mixture of its neutral form ((TCH02)) and positive form (TCH3+) at pH 3.73. However, the adsorption quantity of TC by BCNPA3 at pH 3.73 is 66.24 mg g−1, which is lower than that at pH 6.80 (74.66 mg g−1). Therefore, electrostatic interactions are also not the main driving force for BCNPA3 toward TC adsorption.

For deeply understanding the different adsorption performances of BCNPA3 for TC at different pH values and the reason why BCNPA3 could increase the pH value of the TC solution, the interactions of BCNPA3 with protons and TC were investigated via detailed theoretical studies. The BCNPA3 model was adopted to simulate the molecular structure of BCNPA3 (Fig. S12a, ESI). According to Fig. 7, there are three ways for the interaction of protons with the BCNPA3 model: (1) two protons interact with the two edge N atoms in the BCN part of the BCNPA3 model (Fig. 7a), and the interaction energy between the BCNPA3 model and the protons is 286.66 kcal mol−1; (2) two protons interact with the N atoms near the C atom in the middle of the BCNPA3 model (Fig. 7b), and the interaction energy between the BCNPA3 model and the protons is 285.63 kcal mol−1; (3) one proton interacts with the N atom in the BCN edge part of the BCNPA3 model and the other proton interacts with the carbonyl group in the PA edge part of the BCNPA3 model (Fig. 7c), and the interaction energy between the BCNPA3 model and the protons is 287.09 kcal mol−1. Therefore, protons prefer to be combined with the N atoms in the BCN edge part and the carbonyl groups in the PA edge part of the BCNPA3 model, which is different from BCN since protons prefer to interact with the middle N atoms in the BCN model.18 Furthermore, the high interaction energy between the BCNPA3 model and protons (287.09 kcal mol−1) effectively illustrates that BCNPA3 can increase the pH value of the TC solution from 3.73 to 6.80 by capturing protons. Therefore, BCNPA3 is protonized in the TC solution. BCN-based composites can exist in the protonated form in solution, which has been reported for the first time. This finding effectively explains why the surface electrostatic potential of BCNPA3 is −21 mV at pH 3.73 and −30 mV at pH 6.80, which can be attributed to the fact that some protons captured by BCNPA3 get released when the pH value of the solution increases from 3.73 to 6.80.


image file: d0nj01987a-f7.tif
Fig. 7 (a) Two protons are combined with the two edge N atoms in the BCN part of the BCNPA3 model. (b) Two protons interact with the N atoms adjacent to the C atom in the middle of the BCNPA3 model. (c) One proton interacts with the N atom in the BCN edge part of the BCNPA3 model and another proton interacts with the O atom in the carbonyl group in the PA edge part of the BCNPA3 model. (d) Optimized structure of the BCNPA3-TC model.

According to the FTIR results of BCNPA3 (Fig. 2b), the functional groups (OH groups, NH groups, etc.) exist on the surface of BCNPA3. Furthermore, the functional groups (OH groups, NH2 groups, etc.) exist in TC, too (Fig. S1, ESI). Therefore, the interaction between these functional groups in BCNPA3 and TC can mainly contribute toward the adsorption of TC by BCNPA3, which has been investigated by theoretical studies. Based on the above experimental and theoretical studies, BCNPA3 can be protonized in solution (Fig. 7c). The neutral form (TCH02) is adopted for TC since the pH of the TC solution changes to 6.80 after BCNPA3 is added (Fig. S1b, ESI). From Fig. 7d, the calculated adsorption energy between the BCNPA3 model and TC is 104.3 kcal mol−1 based on the optimized structure of the BCNPA3-TC model. The distance between the O-bound proton in the PA part of BCNPA3 and the C-bound N atom in TC is 2.43 Å. The distance between the N-bound proton in the BCN part of BCNPA3 and the C-bound O atom in TC is 2.19 Å. The distance between the C-bound O atom in the PA part of BCNPA3 and the proton in TC is 1.09 Å. The distance between the O-bound proton in the PA part of BCNPA3 and the C-bound O atom in TC is 1.36 Å. Therefore, the high adsorption energy (104.3 kcal mol−1) of BCNPA3 for TC can not only be attributed to the interaction between the functional groups of BCNPA3 and TC, but also to the interaction between the captured protons in BCNPA3 and the functional groups in TC. This effectively explains why the adsorption capacity (74.66 mg g−1) of BCNPA3 for TC at pH 6.80 is higher than that at pH 3.73 (66.24 mg g−1), since these captured protons can function as the adsorption sites for BCNPA3 to adsorb TC. The above discussions indicate that the hydrogen bonding between the functional groups of BCNPA3 and TC is mainly responsible for the adsorption of TC by BCNPA3. Furthermore, the distance between the O-bound B atom in the BCN part of BCNPA3 and the C-bound O atom in TC is 1.46 Å, which can be attributed to the donor–acceptor interaction between them. The number of OH groups on the BCNPA composites surface is obtained via the Boehm titration method.68 BCNPA3 has the most OH groups (0.214 mmol g−1), followed by BCNPA2 (0.201 mmol g−1), BCNPA4 (0.196 mmol g−1), and BCNPA1 (0.191 mmol g−1). This is identical with the order of the TC adsorption capacity by the BCNPA composites (Fig. 1a): BCNPA3 has the largest adsorption capacity (74.66 mg g−1) for TC, followed by BCNPA2 (69.58 mg g−1), BCNPA4 (69.07 mg g−1), and BCNPA1 (68.64 mg g−1). Therefore, this indicates that the interaction between the OH groups in BCNPA3 and the functional groups in TC mainly contribute toward TC adsorption by the BCNPA composites. In summary, BCNPA3 can increase the pH value of the TC solution from 3.73 to 6.80 by capturing protons, and the existing state of TC in solution is also changed. These captured protons by BCNPA3 can further act as the active sites for TC adsorption. Consequently, the adsorption capacity of BCNPA3 for TC at pH 6.80 is higher than that at pH 3.80 (74.66 mg g−1vs. 66.24 mg g−1). The interactions between the functional groups of BCNPA3 and TC mainly contribute toward TC adsorption by BCNPA3.

3.5. Studying the photodegradation mechanism of BCNPA3 for TC

The photodegradation performances of BCN and BCNPA3 for TC have also been studied. According to Fig. S8a (ESI), the photostability tests suggest that TC cannot be photodegraded under visible-light irradiation. Further, the photodegradation property of PA for TC is not considered, too, since it is soluble in water and BCNPA3 was washed with ultrapure water to remove the residual PA. As shown in Fig. 8a, the removal experiment is divided into two periods: adsorption and photodegradation periods. The adsorption time is 300 min, which is sufficient for BCN and BCNPA3 to reach adsorption equilibrium (Fig. 6a). In the experiment on BCN removal, nearly 61% TC is removed by adsorption. However, the concentration of TC is almost the same before and after visible-light irradiation. Therefore, BCN cannot degrade TC under visible-light irradiation. This is probably due to the fact that the bandgap of BCN is 3.81 eV (Fig. 4b), because of which the pollutant cannot be degraded by BCN under visible-light irradiation. However, for BCNPA3, nearly 95% TC is removed, where 62% is removed by adsorption and another 33% is removed by photodegradation. The intensity and light spectrum of the visible-light source are also investigated (Fig. S16, ESI). The intensity of visible light is measured to be 164.4 mW cm2. More than 99% of the light with wavelength less than 420 nm is removed by the 420 nm filter, and light with wavelength more than 580 nm mainly contributes toward the intensity of visible light. Therefore, BCNPA3 is a kind of visible-light-responsive photocatalyst and has high adsorption and photodegradation efficiencies for TC.
image file: d0nj01987a-f8.tif
Fig. 8 (a) Removal efficiencies of TC by BCN and BCNPA3 (dosage: 25 mg; concentration: 30 mg L−1; volume: 100 mL). (b) Influences of BQ, TB, EDTA-2Na, and PB on the photodegradation of TC by BCNPA3. (c) DMPO spin-trapping ESR spectra of BCNPA3. (d) Removal of TC by BCNPA3 at different pH values of TC solution (dosage: 25 mg; concentration: 30 mg L−1; volume: 100 mL).

The photodegradation mechanism of TC by BCNPA3 was further investigated by identifying the photoactive species. In the literature, EDTA-2Na, TB, BQ, and PB have been shown to capture h+, ˙OH radical, ˙O2 radical, and e, respectively.7,17,22 Therefore, a series of photodegradation experiments have been conducted by adding these photoactive molecules. As shown in Fig. 8b, their addition has no obvious impact on TC adsorption by BCNPA3 since the removal efficiency of BCNPA3 for TC by adsorption is essentially the same as that in the absence of these molecules. However, the photodegradation performance of TC by BCNPA3 is obviously affected after these photoactive molecules are added. The photodegradation of TC is marginally inhibited by EDTA-2Na because the removal efficiency decreases from 95% to 89% in the absence of EDTA-2Na. This indicates that h+ is involved in the photodegradation of TC by BCNPA3. Furthermore, the photodegradation of TC by BCNPA3 can be accelerated with the addition of PB to capture e, which further proves that h+ is the active species in the photodegradation of TC. The inhibiting effect of TB for TC photodegradation by BCNPA3 is weaker than that of EDTA-2Na since the removal efficiency decreases from 95% to 92% in the presence of TB. Therefore, the ˙OH radical has a marginal contribution in TC photodegradation by BCNPA3. BQ has the highest inhibition on TC photodegradation by BCNPA3 because the removal efficiency decreases from 95% to 83% in the absence of BQ. This indicates that ˙O2 is mainly responsible for the photodegradation of BCNPA3 for TC under visible-light irradiation. To further reveal the formation of ˙O2 radical under visible-light irradiation, the ESR spin-trapping technique is used. As shown in Fig. 8c, the ESR signal of the ˙O2 radical is basically absent in the dark and is detected at 5 and 10 min of visible-light irradiation. Furthermore, the intensity of the ˙O2 radical signal is enhanced by prolonging the time of visible-light irradiation. Therefore, this indicates that BCNPA3 can produce ˙O2 radicals under visible-light irradiation.69

Since the pH value of the TC solution can be increased from 3.73 to 6.80 by BCNPA3, the influence of pH on the photodegradation performance of TC by BCNPA3 is investigated, too. The pH of the TC solution is adjusted to 3.73 by adding 0.01 mol L−1 HCl after reaching adsorption equilibrium. Then, the solution is irradiated under visible light. According to Fig. 8d, the photodegradation efficiency of BCNPA3 at pH 6.80 is higher than that at pH 3.73 (95% vs. 87%). This can be mostly attributed to the fact that TC mainly exists in the form of neutral species (TCH02) at pH 6.80, which may be more easily photodegraded by BCNPA3 under visible-light irradiation than that at pH 3.73. For confirming this inference, the stabilities of TC at pH 3.73 and pH 6.80 were tested under ultraviolet-light irradiation since TC itself cannot photodegrade under visible-light irradiation. From Fig. S17a and b (ESI), TC at pH 3.73 cannot be photodegraded under visible-light irradiation, whereas TC at pH 6.80 can decompose under ultraviolet-light irradiation. This implies that TC at pH 6.80 is less stable than that at pH 3.73. Therefore, BCNPA3 can enhance the photodegradation performance for TC by changing the pH of TC and converting TC from its stable existence into less stable existence.

To clarify the photodegradation products of TC by BCNPA3 in a single TC solution, HPLC-MS was used to analyze the reacted liquids.70,71 This HPLC-MS method has been widely used to determine the intermediates of TC photodegradation by photocatalysts under light irradiation.38,72,73 The HPLC-MS spectra reveal the formation of intermediate products with m/z values of 427, 412, 384, 362, 318, 274, 244, 228, 218, and 149 under visible-light irradiation (Fig. S18, ESI). Based on these values, a potential photocatalytic degradation pathway of TC during photocatalysis has been proposed, as shown in Fig. 9. The product with m/z = 455 corresponds to the molecular weight of TC. Product 1 (m/z = 427) is generated from the process of dehydration at C6. Subsequently, product 2 (m/z = 412) is formed by substituting the formaldehyde group at C2 in product 1 with acylamino. Product 3 (m/z = 384) is generated by the demethylation of the dimethylamino group at C4 in product 2. Then, product 3 loses the formaldehyde group and removes the double bonds at C2 and C12, forming product 4 (m/z = 362). Next, product 5 (m/z = 318) is formed by cleaving the carboatomic ring in product 4, which is consistent with the literature.72 After that, product 6 (m/z = 274) is generated as a result of the decarboxylation at C13 in product 5. Product 6 could be further photodegraded to form product 7 (m/z = 244). Product 8 (m/z = 228) results from the dehydroxylation at C12 in product 7 for breaking the right ring. Subsequently, product 9 (m/z = 218) and product 10 (m/z = 149) are further formed by cleaving the carboatomic ring in product 8. Finally, the above ring-opening products are photodegraded into inorganic intermediates such as CO2, H2O, NH4+, and carboxylic acids.


image file: d0nj01987a-f9.tif
Fig. 9 Possible degradation pathways of TC photodegraded by BCNPA3 in a single TC solution.

Since the properties of the intermediates have a considerable influence on water bodies, it is necessary to explore the toxicity of pollutants in the photodegradation reaction.43,46 The acute toxicity, bioaccumulation factors, developmental toxicity, and mutagenicity of TC and its degradation intermediates were determined to evaluate their toxicity. These parameters were tested via the quantitative structure–activity relationship (QSAR) method by using the Toxicity Estimation Software Tool (T.E.S.T.).38 From Fig. 10a, the LD50 value of TC for rats is 806.96 mg kg−1, which is regarded as “very toxic.” Except for P5, P6, P7, and P8, the LD50 values of the other TC intermediates are higher, suggesting that the acute toxicity of most intermediates is lower. Although P5, P6, P7, and P8 are highly toxic, P9 and P10 are the main final intermediates, which have lower acute toxicity than TC. However, as shown in Fig. 10b, the bioaccumulative factors of P6, P7, P8, P9, and P10 increase significantly. Furthermore, except for P3, P6, P7, P8, and P9, the developmental toxicity of the TC intermediates decreases (Fig. 10c). Although P3, P6, P7, P8, and P9 show higher developmental toxicity, the value for the final intermediate, P10, is much lower than that of TC. In addition, TC is “mutagenic positive” (Fig. 10d). The TC intermediates, except for P2, P3, and P6, decreased to “mutagenic negative.” The above toxicity predictions indicate that the toxicity of most TC intermediates is reduced. Therefore, the photodegradation of BCNPA3 for TC can decrease the toxicity. Therefore, prolonging the photocatalytic reaction time is favorable to obtain a better photodegradation effect when BCNPA3 is used to photodegrade TC in water.


image file: d0nj01987a-f10.tif
Fig. 10 (a) Acute toxicity LD50, (b) bioaccumulation factor, (c) developmental toxicity, and (d) mutagenicity of TC and its photodegradation intermediates by BCNPA3 in a single TC solution.

3.6. Studying the removal (including adsorption and photoremoval) mechanism of Cr(VI) and TC/Cr(VI) coexistence solution by BCNPA3

The photodegradation experiment of BCNPA3 for Cr(VI) was also performed. However, the UV-visible spectra of the Cr(VI) solution after adsorption and visible-light irradiation by BCNPA3 remain unchanged when compared with those of the initial single Cr(VI) solution (Fig. S19, ESI). Therefore, in the single Cr(VI) solution, Cr(VI) cannot be photoreduced by BCNPA3. The photostability of the Cr(VI) solution under visible-light irradiation is investigated, too. As shown in Fig. S20a (ESI), the UV-visible spectra of Cr(VI) remains unchanged under visible-light irradiation for 180 min, indicating that Cr(VI) is stable under the experimental condition. Pollutants coexist in the real situation and this kind of coexistence can influence their photodegradation behaviors. For example, when a TC/Cr(VI) mixed system is treated by TiO2-/O-doped g-C3N4 composite under visible-light irradiation, the removal efficiency of TC increases from 83% (single TC solution) to 89% and the removal efficiency of Cr(VI) increases from 80% (single Cr(VI) solution) to 99%.38 Hence, the photoremoval of the TC/Cr(VI) coexistence system by BCNPA3 under visible-light irradiation was investigated, too. First, the UV-visible spectra of TC and Cr(VI) solution without a photocatalyst also remain unchanged under visible-light irradiation for 180 min when TC and Cr(VI) coexist (Fig. S20b and c, ESI). This suggests that TC and Cr(VI) do not exhibit strong interactions. From Fig. 11a, after adding BCNPA3 into the TC/Cr(VI) system, the color of the Cr(VI) solution via the diphenylcarbazide spectrophotometric method varies from dark to colorless, and the experimental results also indicate that 97% Cr(VI) can be effectively photoreduced by BCNPA3 under visible-light irradiation when TC and Cr(VI) coexist.
image file: d0nj01987a-f11.tif
Fig. 11 (a) Comparison of the Cr(VI) photoremoval efficiencies in TC/Cr(VI) and single Cr(VI) systems by BCNPA3 (dosage: 25 mg; Cr(VI) concentration: 10 mg L−1; TC/Cr(VI) concentration: 30/10 mg L−1; volume: 100 mL). (b) High-resolution XPS spectra of the sample obtained by freeze-drying the TC/Cr(VI) solution. (c) Comparison of TC adsorption and photodegradation efficiencies in TC/Cr(VI) and single TC system by BCNPA3 (dosage: 25 mg; TC concentration: 30 mg L−1; TC/Cr(VI) concentration: 30/10 mg L−1; volume: 100 mL).

To understand the photoreduction product of Cr(VI) by BCNPA3 under TC/Cr(VI) coexistence solution. XPS is used to analyze the sample obtained by freeze-drying the TC/Cr(VI) solution. From Fig. S21a (ESI), a new peak appears in the XPS spectrum of BCNPA3 after photocatalysis. According to Fig. 11b, the XPS spectra of Cr2P can be fitted into two peaks, namely, 577.44 and 586.97 eV, which are identical to those of the standard Cr(III) samples (CrCl3·6H2O) (Fig. S21b, ESI) and different from those of the standard Cr(VI) samples (K2Cr2O7) (Fig. S21c, ESI). Therefore, these peaks can be attributed to Cr(III). These results obviously imply that Cr(VI) can be photoreduced to Cr(III) by BCNPA3. In the presence of Cr(VI), the removal efficiency of TC by adsorption is nearly the same as that in the single TC solution (38%), but the photodegradation efficiency of TC by BCNPA3 increases from 57% to 60% (Fig. 11c). It is very interesting that Cr(VI) can be considerably photoreduced and the photodegradation efficiency of TC can be promoted under visible-light irradiation when Cr(VI) and TC coexist.

In order to comprehensively understand the photodegradation mechanism of BCNPA3 for a TC and Cr(VI) coexistence solution, a series of experiments were performed involving the addition of EDTA-2Na, TB, BQ, and PB. As shown in Fig. 12a, the photoreduction efficiency of Cr(VI) increased during the Cr(VI) photoreduction process when EDTA-2Na is added, but it decreased after the addition of PB. This result supports the fact that e is the main contributor toward the reduction of Cr(VI), which is also consistent with another study.74 Furthermore, the inhibition of EDTA-2Na on the photodegradation of TC by BCNPA3 increases when Cr(VI) and TC coexist when compared with that in the single TC solution (Fig. 8b and 12b), indicating that the formation of h+ and e can be promoted when TC and Cr(VI) coexist. All the above indicate that the photodegradation efficiency of TC can be promoted and Cr(VI) can be effectively photoreduced to Cr(III) when TC and Cr(VI) coexist.


image file: d0nj01987a-f12.tif
Fig. 12 (a) Influence of BQ, TB, EDTA-2Na, and PB on Cr(VI) photoreduction by BCNPA3 in a TC/Cr(VI) solution. (b) Influence of BQ, TB, EDTA-2Na, and PB on TC photodegradation by BCNPA3 in a TC/Cr(VI) solution.

All the above discussions indicate that Cr(VI) can be photoreduced to Cr(III) by BCNPA3 under visible-light irradiation when TC and Cr(VI) coexist. This can be most likely attributed to the fact that the lifetime of a photoinduced electron is limited and is insufficient to reduce the Cr(VI) species during the experimental process. The photodegradation of TC increases the separation efficiency of the photoinduced e and h+ as well as enhances the lifetime of the photoinduced electron, ensuring that the photoinduced electrons have sufficient time to interact with Cr(VI) and reduce it to Cr(III). Cr(VI) is nonbiodegradable, mutagenic, toxic, and carcinogenic. However, the toxicity of Cr(III) when obtained from the photoreduction of Cr(VI) is only 0.01 times that of Cr(VI).38 From Fig. S22 (ESI), the HPLC-MS analysis of the photodegradation intermediates of TC by BCNPA3 in a TC/Cr(VI) solution is found to be the same as that in a single TC solution.

3.7. Recycling performances of BCNPA3 for TC and Cr(VI)

The adsorption performance of adsorbents can be regenerated with light by coupling with a photocatalyst, such as zeolite/TiO2 and activated carbon/TiO2 as reported in earlier studies.75,76 The advantages of recovering the adsorption performance of the adsorbent by using light are as follows: (a) the light energy is green, convenient, and low-cost; (b) no organic solvent is used throughout the process and there is no risk of secondary pollution; (c) organic pollutants can be directly photodegraded into smaller molecules (even carbon dioxide and water). Based on the above experiments, TC/Cr(VI) can be effectively removed by BCNPA3 under visible-light irradiation. Therefore, light can be utilized to recover the removal performance of BCNPA3 for TC and Cr(VI). The stability and reusability of BCNPA3 for TC and Cr(VI) can be investigated under the same conditions. As shown in Fig. 13a, BCNPA3 has excellent reaction stability toward TC after 5 cycles of experiments. The removal efficiency of TC by BCNPA3 is 95% (adsorption: 62%; photodegradation: 33%) in the first attempt; for the second, it is 95% (adsorption: 63%; photodegradation: 32%); for the third, it is 93% (adsorption: 62%; photodegradation: 31%); for the fourth, it is 92% (adsorption: 62%; photodegradation: 30%); and for the fifth, it is 91% (adsorption: 62%; photodegradation: 29%). Furthermore, the stability of BCNPA3 toward Cr(VI) during photocatalytic reduction is also investigated. According to Fig. 13b, BCNPA3 exhibits fantastic stability, too, since 90% Cr(VI) can be photoreduced by BCNPA3 in the fifth reuse cycle.
image file: d0nj01987a-f13.tif
Fig. 13 (a) Recycling experiment of TC adsorption and photodegradation by BCNPA3 under visible-light irradiation when TC and Cr(VI) coexist (dosage: 25 mg; TC/Cr(VI) concentration: 30/10 mg L−1; volume: 100 mL). (b) Recycling experiment of BCNPA3 for the photoreduction of Cr(VI) under visible-light irradiation when TC and Cr(VI) coexist (dosage: 25 mg; TC/Cr(VI) concentration: 30/10 mg L−1; volume: 100 mL). (c) XRD patterns of BCNPA3 before and after photocatalytic cycling for 5 times.

For further verifying the stability and reusability of BCNPA3, the XRD patterns of BCNPA3 before and after five cycles were determined, as shown in Fig. 13c. The XRD patterns of BCNPA3 do not significantly change after five cycles, indicating that the crystal structure of BCNPA3 does not change during the photoremoval reaction. Therefore, BCNPA3 is a fairly stable and reusable photocatalyst. The above results suggest that BCNPA3 is a good water-cleaning material with better practical application potential. The adsorption and photoremoval performances of BCNPA3 for TC/Cr(VI) can be effectively recovered under visible-light irradiation. This is probably due to the fact that BCNPA3 itself is a visible-light-responsive photocatalyst. Furthermore, the reason why BCNPA3 needs further irradiation treatment after the adsorption and photodegradation experiments was also investigated. As shown in Fig. S23 (ESI), BCNPA3 after photodegrading TC and photoreducing Cr(VI) with further visible-light irradiation can still remove 95% TC (62% TC is removed by adsorption and 33% is removed by photodegradation). However, BCNPA3 after photodegrading TC and photoreducing Cr(VI) without further visible-light irradiation can remove only 75% TC (51% is from adsorption and 24% is from photodegradation). This may be attributed to the fact that only BCNPA3 near the water surface is capable of adopting visible light to photodegrade the adsorbed TC molecule and photoreduce Cr(VI) during the stirring process. This means that there are still certain TC molecules on the surfaces of the BCNPA3 samples after the photodegradation experiment. Therefore, it is necessary to further photodegrade these residual TC molecules on the surface with visible-light irradiation for the BCNPA3 sample.

3.8. Comparison with other adsorbents and photocatalysts

For better evaluating the adsorption and photoremoval properties of BCNPA3, comparative experiments involving adsorbents (BCN, biochar, and activated carbon) and photocatalysts (BNPA) for TC were performed. As shown in Fig. 14a, the adsorption capacities of BNPA, biochar, BCN, BCNPA3, and activated carbon for TC were 43.52, 64.88, 71.48, 74.66, and 98.83 mg g−1, respectively. Therefore, the adsorption properties of BNCPA3 are obviously better than those of BNPA, biochar and BCN. However, the adsorption performance of BCNPA3 seemed to be weaker than that of activated carbon. Moreover, the specific surface area of BCNPA3 was only 28.611 m2 g−1, which is far less than that of activated carbon (607.719 m2 g−1). The adsorption capacity per square meter for TC was calculated to further compare the adsorption performances of BCNPA3 and activated carbon; for BCNPA3, it was 2.609 mg m−2 and for activated carbon, it was 0.163 mg m−2. This indicates that BCNPA3 has better adsorption performance for TC than activated carbon. Furthermore, the adsorption performance of activated carbon was recycled by washing with water or calcinations at higher temperatures. This is not environmentally friendly and needs to consume energy.
image file: d0nj01987a-f14.tif
Fig. 14 (a) Adsorption capacity of adsorbents and photocatalysts for TC. (b) Removal efficiencies of BNPA and BCNPA3 for TC.

For better evaluating the photoremoval performances between BCNPA3 and BNPA,31 as shown in Fig. 14b, nearly 95% TC (62% is from adsorption and 33% is from photodegradation) can be removed by BCNPA3 under visible-light irradiation, which is higher than that of BNPA since it can only remove 68% TC (36% is from adsorption and 32% is from photodegradation). Moreover, BCNPA3 can increase the adsorption and photodegradation performances by capturing protons in the solution and can also photoreduce Cr(VI) under visible-light irradiation in a TC/Cr(VI) coexistence solution. This suggests that BCNPA3 has better adsorption and photodegradation performances than BNPA. Therefore, BCNPA3 is a good water-cleaning material and has potential application prospects in wastewater purification.

4. Conclusion

A series of BCNPA composites have been successfully synthesized for the first time by calcinating a mixture of BCN and PA at 350 °C. Among them, BCNPA3 has the best adsorption quantity (74.66 mg g−1) as well as optimum photodegradation performance for TC. 1H NMR characterization confirmed that BCNPA3 was formed by the dehydration reaction between the OH groups in BCN and the COOH groups in PA. The adsorption of TC by BCNPA3 follows the pseudo-second-order kinetic model and Langmuir isotherm model, and it is an endothermic process. BCNPA3 can increase the pH value of the TC solution from 3.73 to 6.80 by capturing protons, and it exists in its protonated form in the solution. The adsorption capacity of BCNPA3 for TC at pH 6.80 is higher than that at pH 3.73 (74.66 mg g−1vs. 66.24 mg g−1), which can be attributed to the fact that these captured protons can work as adsorption sites. The bandgap of BCNPA3 is 2.79 eV. The photodegradation efficiency of BCNPA3 for TC under visible-light irradiation increases from 87% (at pH 3.73) to 95% (at pH 6.80), which can be attributed to the fact that TC at pH 6.80 is less stable than that at pH 3.73. The toxicity of these intermediates can be evaluated by the QSAR method. More interestingly, Cr(VI) can be photoreduced by BCNPA3 under visible-light irradiation when it coexists with TC. This can be attributed to the fact that the photodegradation of TC increases the lifetime of the photoinduced electron. The removal performance of BCNPA3 for TC and Cr(VI) can be easily recovered under visible-light irradiation. Therefore, BCNPA3 has promising application prospects in the purification of wastewater.

Conflicts of interest

The authors declare no competing financial interest.

Acknowledgements

This work was supported by National Science Funds for creative Research Groups of China (No. 51421006), the Key Program of National Natural Science Foundation of China (No. 91647206), Research project of Yangtze Institute for Conservation and Development (B200204004), Nantong Ocean and Coastal Engineering Research Institute, Hohai University (JC2019152), 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), A Project Funded by the Priority Academic Program, Development of Jiangsu Higher Education Institutions. The Joint International Research Laboratory of Global Change and Water Cycle (JLGCWC). We thank High performance Computing Center of Nanjing University for computational study.

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Footnote

Electronic supplementary information (ESI) available: The structure of tetracycline (TC), the dotted areas represent the structural moieties of TC associated with the three acidic dissociation constants (pKa); speciation distributions of TC in different pH values; the image of BCN, PA, BCNPA1, BCNPA2, BCNPA3 and BCNPA4; the UV spectra of TC solution at different pH values. See DOI: 10.1039/d0nj01987a

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