Enhanced adsorption and photocatalytic activities of ultrathin graphitic carbon nitride nanosheets: Kinetics and mechanism

Chemical Engineering Journal 381 (2020) 122760

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Enhanced adsorption and photocatalytic activities of ultrathin graphitic carbon nitride nanosheets: Kinetics and mechanism Cheng Tiana, Hui Zhaoa,b, Hongli Sunb, Kemeng Xiaob, Po Keung Wongb,c,

T

⁎

a

Jiangsu Key Laboratory of Anaerobic Biotechnology, School of Environment and Civil Engineering, Jiangnan University, 1800 Lihu Avenue, Wuxi 214122, China School of Life Sciences, The Chinese University of Hong Kong, Shatin, NT, Hong Kong Special Administrative Region, China c Institute of Environmental Health and Pollution Control, School of Environmental Science & Engineering, Guangdong University of Technology, Guangzhou 510006, China b

H I GH L IG H T S

G R A P H I C A L A B S T R A C T

CNNS exhibited efficient adsorption• photocatalytic ability for TC removal. adsorption followed pseudo• TC second-order kinetics and type 1 Langmuir isotherm.

TC degradation was • Photocatalytic fitted for zero- and first-order kinetics. reactive species and degrada• Major tion products were determined.

A R T I C LE I N FO

A B S T R A C T

Keywords: g-C3N4 nanosheets Antibiotic removal Adsorption-photocatalytic activity Kinetics Mechanism

Different from most composite structures with a common configuration of adsorbent-catalyst, g-C3N4 thin nanosheets (CNNS) with the thickness of 8–11 nm had more efficient and stable adsorption and photocatalytic activities in the removal of tetracycline (TC) from aqueous solution. In comparison with common g-C3N4 counterparts, the adsorption performance of CNNS increased by at least 10.3 times and that for photocatalytic efficiency increased for more than 4.8 times. The large contact area and delocalized π bonds provided by CNNS resulted in the enhanced TC adsorption capacity. The efficient separation and prolonged-lifetimes of photogenerated charge carriers, and the strong adsorption brought about a reinforced photocatalytic activity for TC degradation. The adsorption of TC onto CNNS was well followed the pseudo-second-order adsorption kinetics, and the obtained adsorption isotherm was well fitted to the Langmuir isotherm model. Due to the efficient adsorption and photocatalysis of CNNS, the zero-order and first-order kinetics models were presented during the different reaction time for TC photodegradation, the h+, %O2− and %OH radicals were confirmed as the major reactive oxidative species, and the degradation products were determined. This study provided a new route for the development of catalysts with efficient adsorption and photocatalytic activity in the purification of organic wastewaters.

1. Introduction Recently, the presence of pharmaceutical residues in wastewater

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and their harmful effects on ecosystems have attracted tremendous attention worldwide. By means of the pharmaceutical industry, the effluent from hospital, excretion from humans and livestock, etc. [1],

Corresponding author. E-mail address: [email protected] (P. Keung Wong).

https://doi.org/10.1016/j.cej.2019.122760 Received 21 May 2019; Received in revised form 5 September 2019; Accepted 6 September 2019 Available online 07 September 2019 1385-8947/ © 2019 Elsevier B.V. All rights reserved.

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Graphitic carbon nitride (g-C3N4) is a layered semiconductor and several advantages for itself have been presented such as being environmentally friendly, easy accessibility, robust stability and earthabundant elemental composition [32,33]. Due to the strong covalent CN bonds in each layer and weak van der Waals force between layers, bulk g-C3N4 can be easily exfoliated to nanosheet g-C3N4 [34]. When gC3N4 is reported to have the nanosheet structure with thin thickness, it often has two apparent advantages. One is the high surface area for providing abundant adsorption sites. For example, Cai et al. found 2D gC3N4 nanosheets with high-surface-area presented effective adsorption for cadmium ions and methylene blue [35]. Another advantage is the short bulk diffusion length for reducing the recombination of photogenerated charges and thus enhancing photocatalytic activity. For example, Niu et al. found g-C3N4 nanosheets with a thickness of approximate several nanometers had an improved charge separation that significantly accelerated photocatalytic H2 evolution from water [36]. Xia et al. synthesized the thin nanosheet assemblies of g-C3N4 with efficient separation of electron-hole pairs which promoted the enhancement of photocatalytic CO2 reduction activity [37]. These reports make us fully believe that the g-C3N4 with structure of thin nanosheet would be a potential candidate which possesses synergistic adsorptionenrichment and efficient photocatalysis in the process of antibiotic wastewater treatment. There were few studies of g-C3N4 thin nanosheets that have been reported to have such synergistical performance for the removal of antibiotics in wastewaters. To date, the exfoliation methods of g-C3N4 can be divided into thermal exfoliation and liquid exfoliation where thermal exfoliation is considered as a low cost, large scale and environmentally friendly method for preparing g-C3N4 nanosheets [34]. Herein, in this study gC3N4 thin nanosheets had been synthesized through using thiourea as the precursor based on a common two-step thermal treatment method. Tetracycline (abbreviated as TC below) a typical antibiotic was used as a model pharmaceutical compound. As expected, g-C3N4 thin nanosheets exhibited efficient adsorption and photocatalytic activity for TC removal, and such performance was very stable in the repeated experiment. To explain the improved adsorption and photocatalytic activity, the possible reasons were revealed. In addition, the adsorption kinetics and isotherm models were detailed discussed, and the photocatalytic kinetics and mechanism were deeply investigated.

these pharmaceutical residues are released into the aquatic environment and gives rise to the pollution of environment. As known, the utilization of antibiotics has afforded the obvious improvement in infectious disease treatment and agricultural productivity [2], and thus antibiotics has a high consumption rate among pharmaceuticals. For this reason, antibiotic residues contribute a large proportion of pharmaceutical contaminations [3]. Antibiotic residues in aqueous systems, even with a low concentration, could potentially produce some environmental problems, such as antibiotic resistance to bacteria, perturbations in ecosystems, and possible risks to human health through drinking water and the food chain [4-7]. Therefore, elimination of antibiotics from wastewater has become an essential issue to be addressed for human health and environmental protection. During the past years, various strategies, such as biodegradation, hydrolysis, oxidation and reduction [8–10], have been developed for removal of antibiotics from water. However, the high cost, low stability, and poor recycle ability inhibit the application of these methods [2]. Notably, adsorption and photocatalysis have shown superiority to other techniques in antibiotic removal and thus widely utilized and extensively investigated. Adsorption technique has multiple advantages such as low cost, high efficiency, and easy operation [11–14]. Common adsorbent materials include carbon materials [15–17], silica [18,19], and minerals [20–22], etc. Nevertheless, in the adsorption process the pollutants are just transferred from aqueous phase to solid phase rather than mineralized to non-polluting substances. Under this circumstance, the secondary pollution is produced in wastewater treatment. Additionally, the adsorbent is generally difficult to be regenerated and should be treated to release pollutants once the adsorption saturation is achieved [23,24]. Photocatalysis is an economic, efficient, and green technology using sunlight under ambient reaction conditions and is one of the most promising method for degrading antibiotics [2]. However, the photocatalyst usually has the limitations of surface area, low adsorption capacity, and high recombination of photogenerated electron-hole paris [25,26], which largely hinders the industrial application of photocatalysis in antibiotic wastewater treatment. To overcome the limitations of both adsorption and photocatalysis, a combinational approach on the basis of both the adsorption and photocatalysis method has been developed [25,26]. The adsorbed organic contaminations can be effectively degraded and mineralized by photocatalysis while the presence of adsorption significantly enhances the contact between a pollutant and photocatalyst resulting in an enhanced photocatalytic activity. Undoubtedly, the development of an elegant structure coupling with adsorption enrichment and efficient photocatalysis is crucial to take advantage of this technology. To date, the structure of a photocatalyst supported on the surface or in the porous channel of an absorbent has been widely reported to have such performance [27–29]. Furthermore, to accelerate the separation and transfer of photoexcited charge carriers, the absorbent also usually has an excellent electron-transfer performance. For example, Yu et al. synthesized a composite structure of TiO2 photocatalyst loaded on carbon nanotube surface with enhanced adsorption and photocatalytic activity for pollutant degradation [30]. Yang et al. settled the 3D flower-like Bi2WO6 photocatalyst in the 3D porous graphene hydrogel that exhibited excellent synergistic effects of adsorption-enrichment and photocatalytic degradation [31]. However, there are two limitations that constrain the large-scale application of such composites. On the one hand, their preparation strategies are usually extremely complicated and the porous structure of absorbent probably suffers from destruction in the harsh loading process of photocatalyst. On the other hand, the loaded photocatalyst inevitably blocks the adsorption sites of an absorbent, resulting in a declined adsorption performance. To solve these two drawbacks, one of the feasible solutions is to develop a catalyst with efficient photocatalytic activity simultaneously exhibits strong adsorption for target molecules without introducing any absorbent component.

2. Experimental 2.1. Materials Thiourea (CH4N2S, CAS: 62-56-6), dicyanamide (C2H4N4, CAS: 46158-5), tetracycline (C22H24N2O8·xH2O, CAS: 60-54-8), ethylenediaminetetraacetic acid disodium salt (C10H14N2Na2O8·2H2O, CAS: 6381-926) are analytical reagents and purchased from Aladdin (China). Melamine (C3H6N6, CAS: 108-78-1) is chemically pure reagent and isopropanol (C3H8O, CAS: 67-63-0) is an analytical reagent. They are purchased from Sinopharm (China). 2.2. Catalyst preparation The preparation of g-C3N4 thin nanosheets is based on a two-step thermal treatment strategy. Firstly, thiourea with the weight of 10 g is put in a 50 mL ceramic crucible with a cover and heated to 550 °C in air at a ramp rate of 2 °C min−1 for 2 h. The resultant product is subsequently grinded to fine powder and uniformly tiled on the bottom of the ceramic crucible, which is then heated at 500 °C for 2 h in air without a cover. After that, a light-yellow g-C3N4 powder consisting of thin nanosheets is acquired and is labelled as CNNS. It is note that the utilization of crucible cover in the first thermal treatment process is to prevent the volatilization of intermediates in the thermal polymerization from thiourea to g-C3N4. To exfoliate g-C3N4 to nanosheet structure through thermal oxidation etching in air, the second heat treatment 2

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Fig. 1. (a) TEM images of CNNS; (b) TEM image of the sheet surface of CNNS; (c) High resolution TEM image of CNNS; (d) SEM image of CNNS; (e) High magnification SEM image of CNNS; (f) Trapping-mode AFM image of g-C3N4 nanosheet deposited on the mica wafer substrate and the height curve in the AFM image; (g) XRD patterns of CNNS; (h) FTIR spectrum of CNNS; (i) High-resolution XPS spectra of C1s in CNNS; (j) High-resolution XPS spectra of N1s in CNNS; (k) Highresolution XPS spectra of S2p in CNNS; (l) UV-vis absorption spectrum of CNNS and the inset is the Kubelka-Munk transformed reflectance spectrum and estimated optical absorption band gap of CNNS.

electron microscopic (SEM) image, atomic force microscope (AFM) image, X-ray diffraction (XRD) patterns, Fourier transform infrared (FTIR) spectra, X-ray photoelectron spectroscopy (XPS) spectra, UV-vis absorption spectra, and BET specific surface area are recorded to characterize the catalyst structure. The photoinduced current density with time (i-t curve) and the time resolved fluorescence decay spectra are recorded to characterize the charge separation and transfer of the as-prepared photocatalyst. The active oxygen species in the photocatalytic reaction are characterized and recognized based on the trapping experiment and electron spin resonance (ESR) technique. The products of TC degradation were determined by ultra-performance liquid chromatography-mass spectrometry (UPLC-MS). These

process should be exposed with air and does not require a cover. For comparison, several common g-C3N4 catalysts derived from different precursors (dicyanamide, thiourea, melamine) are synthesized according to our recently reported method with minor modification [38]. In short, 10 g of above-mentioned precursor is thermally treated at 550 °C for 2 h in air. The as-obtained product is denoted as D-CN, TCN and M-CN when dicyanamide, thiourea and melamine are employed, respectively.

2.3. Catalyst characterization The transmission electron microscopic (TEM) image, scanning 3

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FTIR spectrum of CNNS (Fig. 1h). The adsorption band in the range from 1700 to 1200 cm−1 is corresponding to the characteristic vibrations of CN heterocycles in g-C3N4, and the characteristic band at 809 cm−1 is due to the vibrational mode of triazine units in g-C3N4 [39–42]. The band appeared in the region of 3400–2850 cm−1 is ascribed to residual N–H and O–H component that is as a result of the uncondensed amino groups and adsorbed water molecules on the surface of g-C3N4 [39–42]. In addition, it is obviously observed that three deconvolution peaks could be fitted in the high-resolution C1s XPS spectrum (Fig. 1i). They are positioned at 284.8, 286.4 and 288.4 eV, respectively. These peaks are ascribed to graphite carbon atom, carbon atoms in C-NH2 species and the sp2-hybridized carbon in the aromatic ring of g-C3N4, respectively [39–43]. And the N1s spectrum (Fig. 1j) could be also separated into three deconvolution peaks appeared at 398.8, 399.6 and 401.0 eV, respectively. They are corresponding to the sp2-hybridized nitrogen atoms in the triazine units, bridging nitrogen in N-(C)3 and the nitrogen atoms in the heterocycles and cyano groups of g-C3N4, respectively [39–43]. In the high-resolution S2p XPS spectrum (Fig. 1k), the appeared peak centered at 169.4 eV is ascribe to sulfur oxide species (SO42−) [44–46]. The formation of SO42- is due to the presence of oxygen during the pyrolysis step which can play an important role in capturing sulfur decomposed from thiourea into SO42− [46]. No obvious peak is appeared in the range of 162–167 eV, indicating that no sulfur is doped into CNNS network [44–46]. Furthermore, CNNS has an absorption edge of ca. 430 nm and the obtained band gap value is ca. 2.88 eV (Fig. 1l). Evidently, CNNS has a larger band gap when compared with common g-C3N4 (ca. 2.7 eV), that is as a result of the quantum confinement effect in the structure of thin nanosheet [36].

corresponding characterization conditions and/or procedures are shown in the supplementary information. 2.4. Photocatalytic TC removal The TC removal experiment is performed under the irradiation of visible light (λ > 420 nm) by using a 300 W Xenon lamp with a 420 nm cutoff filter and the reaction temperature is controlled at 25 °C. Prior to the experiment, 20 mg of photocatalyst should be added into 100 mL of TC aqueous solution with concentration of 20 mg L−1 to obtain the reaction solution. During the photocatalytic process, a certain volume of suspension is taken out at intervals. The TC concentration is determined based on a colorimetric method at the wavelength of 357 nm. The degradation efficiency of TC is calculated using the following equation:

TC degradation(%) = (C0 − Ct )/ C0 × 100

(1)

where C0 (mg/L) and Ct (mg/L) represent the TC concentration at the initial and after reaction time t (min) in the reaction process, respectively. For comparison, the blank experiment is carried out in the absence of any catalyst to evaluate the photolysis of TC. 2.5. TC adsorption The TC adsorption experiment is carried out at 25 °C without light irradiation. Before the experiment, the reaction solution should be prepared in which 20 mg of photocatalyst is added into the 100 mL of TC aqueous solution with different concentrations (20–500 mg L−1). The pH value of this solution is controlled in the neutral range (6.5–7.5). The system is agitated for 60 min to reach equilibrium. At intervals, a certain volume of suspension is fetched out for determination. The concentration of surface adsorbed TC is calculated using:

TC adsorbed (%) = (C0 − Ct )/ C0 × 100

3.2. Efficient adsorption and photocatalytic activity As seen from Fig. 2a, the presence of CNNS significantly enhanced the removal of TC under visible light irradiation. During the reaction time of 60 min, ca. 75% of TC could be removed while only ca. 4% was determined in pure photolysis. When compared with several common g-C3N4 catalysts (T-CN, D-CN and M-CN), this efficiency is at least 18.8 times higher. In addition, it is found that the removal of more than half of TC occurred within the first minute of reaction time. This is as a result of strong adsorption of TC molecules on the surface of CNNS. Therefore, the improved removal of TC in the CNNS photocatalytic process resulted from the combination of efficient adsorption and photocatalytic activity. As expected, as high as ca. 41% of TC could be quickly adsorbed on CNNS surface (Fig. 2b). This value is as least ca. 10.3 times higher than that for comparative g-C3N4 catalysts. The photocatalytic performance of CNNS was also investigated (Fig. 2c). Before illumination, the system should reach the adsorption–desorption equilibrium so that the adsorption could be discounted. It can be seen that ca. 38% of TC could be photodegraded during the irradiation time of 30 min in the presence of CNNS while the highest TC degradation efficiency of g-C3N4 counterpart (T-CN) was only ca. 8% under the same conditions. Evidently, CNNS exhibited efficient photocatalytic activity. In accelerating the large-scale application of a catalyst, the stability is a pivotal factor to be taken into account. The time-circle adsorption and photocatalytic experiments on CNNS were carried out to evaluate the stability of CNNS. As shown in Fig. 2d, the adsorption and photocatalytic efficiency nearly maintained unchanged after three runs. Moreover, the morphology and crystal structure of recycle CNNS were found to almost keep the same as fresh sample according to the characterizations of TEM, XRD and FTIR (Fig. S1). In addition, the change of reaction temperature (5–40 °C) hardly affected the TC adsorption and photodegradation activities onto CNNS (Fig. S2). These results demonstrated that CNNS possessed an excellent stability of both adsorption and photocatalysis for TC removal. It has been reported the mechanisms of ion exchange, van der Waals forces and π-π electron donor–acceptor (EDA) interaction to explain the

(2)

where C0 (mg/L) and Ct (mg/L) denote the TC concentration at the initial and after reaction time t (min) of adsorption, respectively. The adsorption amount (qt) retained on per gram of photocatalyst at reaction time t is calculated using:

qt = (C0 − Ct )/ m × V

(3)

where m is the used weight of photocatalyst and V is the volume of reaction solution. 3. Results and discussion 3.1. Catalyst structure According to the TEM characterization (Fig. 1a), CNNS has a thinlayer nanosheet structure, and the sheet edges seem to be ragged that probably results from the minimization of the sheet surface energy [36]. As presented in the high-magnification TEM image (Fig. 1b), it is found that the thin-sheet surface of CNNS tends to be rough. This is as a result of the gradual oxidation decomposition of polymetic melon units in the layer of g-C3N4 at a high thermal treatment treatment [36]. No obvious lattice fringe is observed in the high resolution TEM image (Fig. 1c), indicating CNNS has a low crystallinity. From the SEM images (Fig. 1d and e), it is apparently found that CNNS has the hierarchical morphology over the whole surface which is consist of crumpled nanosheets with numerous wrinkles and folds. This result is in good accordance with TEM characterization. As shown in Fig. 1f, the representative AFM image of CNNS exhibits a single g-C3N4 nanosheet deposited on the mica wafer substrate. As the result from the TEM image, the nanosheet is not smooth with the thickness varying from 8 to11 nm. In the XRD patterns (Fig. 1g), CNNS exhibits two diffraction peaks at 13.0° and 27.9° that could be ascribed to the (1 1 0) and (0 0 2) planes of g-C3N4 with graphitic structure (JCPDS#87-1526). In the 4

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Fig. 2. (a) TC removal in the blank and photocatalytic systems; (b) TC adsorption activity over different catalyst; (c) TC photodegradation activity over different catalyst; (d) TC adsorption and photodegradation activity over CNNS in the recycle experiment.

donor ability of π-electron was increased in CNNS, and more TC molecules could be interacted on the surface based on the π-π EDA effect. As a result, it was concluded that the large contact area and delocalized π bonds provided by CNNS resulted in the enhanced TC molecular adsorption capacity. The studies on the light harvesting, separation and transfer of charge carriers, and surface catalytic reaction were performed to reveal the reasons for the improved photocatalytic activity of CNNS. As presented in the UV-vis adsorption spectra (Fig. S5), CNNS had a lower optical adsorption than T-CN. Hence, the highly efficient photocatalytic activity of CNNS could not be attributed to the optical adsorption. Since more TC molecules were adsorbed on CNNS surface, the surface reaction between active species with TC molecule would become more convenient and effective. To evaluate the separation and transfer of photoinduced charge carriers, the photoinduced i-t curves and timeresolved fluorescence decay spectra were recorded. As displayed in the Fig. 3a, it was clearly observed that CNNS exhibited higher photocurrent intensity than T-CN under visible light irradiation. This result manifested more efficient separation and transfer of photogenerated electron-hole pairs occurred in CNNS. Though the fluorescent intensities for CNNS and T-CN decay exponentially in the time-resolved fluorescence decay spectra (Fig. 3b), CNNS presented slow decay kinetics when in comparison with T-CN. And the radiative lifetimes of photogenerated charge carriers in CNNS (τ1 = 1.23 ns, τ2 = 4.27 ns, and τ3 = 17.25 ns) were all longer than that in T-CN (τ1 = 0.87 ns, τ2 = 3.18 ns, and τ3 = 14.31 ns). These results further illustrated more efficient separation and transfer of photoexcited charge carriers in CNNS. Based on the above results, we can make a conclusion that the efficient separation and prolonged radiative lifetimes of photogenerated electron-hole pairs in CNNS would allow more charge carriers to take part in the photocatalytic reaction, together with more TC molecules adsorbed on CNNS surface that made surface degradation reaction become more convenient and effective, finally brought about a reinforced photocatalytic activity for TC degradation.

Table 1 The surface structural characteristics of CNNS and T-CN. Sample

Zeta potential (mV)

Specific BET surface area (m2/g)

Surface atom percent of sp2hybridized C and N (%) a

CNNS T-CN

−14 −19

178 38

82 and 61 49 and 56

a

Surface atomic percent is calculated based on the corresponding XPS peak area (S), surface atom percent of sp2-bonded C = Ssp2-bonded C/SC1s × 100%, Surface atom percent of sp2-bonded N = Ssp2-bonded N/SN1s × 100%.

strong adsorptive interaction of TC with the absorbent [47]. Within the test pH of 6.5–7.5 in the present study, the zwitterion of TC was predominated. And through the determination of surface zeta potential value (Table 1), the surface of as-prepared g-C3N4 was negatively charged (-14 and −19 mV for CNNS and T-CN, respectively). Therefore, cation exchange reactions were expected to occur between the zwitterionic TC molecules and the respective ionic sites of g-C3N4. However, T-CN with more negatively charged surface exhibited lower TC adsorption activity than CNNS. Hence, the cation exchange mechanism was not the major factor responsible for the strong adsorption of TC on CNNS surface [47]. The intensities of van der Waals forces of an adsorbed molecule were proportional to its contact surface area [47]. As summarized in Table 1, the BET specific surface area of CNNS (178 m2/ g) was obviously higher than that of T-CN (38 m2/g). Different from the bulk morphology of T-CN (Fig. S3), CNNS had the thin-sheet structure that provided a beneficial adsorption platform for TC molecule. These results mean the large contact area was provided for TC molecule adsorption on CNNS. The TC molecule had a large planar ring structure; therefore, strong van der Waals forces were likely to occur between the TC molecule and CNNS. In the π-π EDA mechanism, the TC molecule adsorption largely depended on the π-electron-donor ability of the absorbent [48]. Through the XPS characterization (Fig. 1i and 1j, Fig. S4 and Table 1), the higher contents of sp2-hybridized C and N atoms were determined on the surface of CNNS (82 and 61%, respectively) when compared with T-CN (49 and 56%, respectively), indicating that the large delocalized π bonds were generated on CNNS [49]. Therefore, the 5

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Fig. 3. (a) Photoinduced i-t curves over different catalysts under visible light irradiation; (b) Time-resolved fluorescence decay spectra over different catalysts monitored at 457 nm acquired from their fluorescence emission spectra with the excitation wavelength of 325 nm. The catalysts were excited by the incident light of 375 nm from a picosecond pulsed light-emitting diode. The inset are the radiative lifetimes of photoexcited charge carriers in different catalysts.

Fig. 4. (a) The adsorption kinetic curve of TC (20 mg L−1) on CNNS; (b) Pseudo-first-order plot for adsorption of TC on CNNS; (c) Pseudo-second-order plot for adsorption of TC on CNNS; (d) Intra-particle diffusion model for adsorption of TC on CNNS.

3.3. Adsorption kinetics and isotherm

qt = kid t 1/2 + C

Fig. 4a displayed the adsorption kinetics curve of TC with the concentration of 20 mg L-1 on CNNS. Obviously, the adsorption equilibrium was quickly reached within the reaction time of lower than 5 min. And the equilibrium adsorption amount of TC reached to ca. 40 mg g−1 on GCN surface. In order to evaluate the kinetics mechanism that controlled the adsorption process, the pseudo-first-order equation, pseudo-second-order equation and intra-particle diffusion model were employed to interpret these experimental data. The pseudo-first-order equation can be expressed as:

where kid (mg g−1 min−1/2) is the intra-particle diffusion rate constant and C is the thickness of boundary layer [50]. The plot of qt versus t1/2 was given in Fig. 4d. In comparison with the pseudo-first-order plot (R2 = 0.93718) and intra-particle diffusion model (R2 = 0.69514), it was obviously observed that the adsorption data in the pseudo-secondorder plot correlated more better (R2 = 0.99999). Previous studies had invoked that good correlation could explain the adsorption mechanism of TC on the solid phase surface [51]. The calculated qe value (41 mg g−1) from the pseudo-second-order kinetics plot was nearly the same as the experimental value of 40 mg g−1 in Fig. 4a. Therefore, CNNS had the optimum pseudo-second-order kinetics model fitting for the adsorption of TC. This result further demonstrated that the chemical adsorption was the rating-limiting step of the adsorption process [51], which can be easily understood by the above conclusion that the π-π EDA interaction were one of the main driving forces for the adsorption between TC molecule and CNNS. The calculated rate constant k2 value was 0.26 g mg−1 min−1 in CNNS. In addition, the TC molecule adsorption on comparative T-CN also followed the pseudo-second-order kinetics (Fig. S6). When compared with CNNS, the lower qe (4 mg g−1) and k2 (0.15 g mg−1 min−1) values were determined in the T-CN adsorption process. And the initial adsorption rate (h) was also evaluated which could be calculated from qe and k2 values [52]. In CNNS the h

1/ qt = k1/(qe × t ) + 1/ qe

(4)

where k1 (min−1) is the adsorption rate constant of pseudo-first-order, qe (mg/g) is the amount adsorbed at equilibrium. The plot of 1/qt versus 1/t was presented in Fig. 4b. The pseudo-second-order equation is given by:

t / qt = t / qt 1/(k2 × qe2)

(5)

where k2 (g mg−1 min−1) is the pseudo-second-order adsorption rate constant. The plot of t/qt versus t was shown in Fig. 4c. And the intraparticle diffusion model could be explored by using the modified Weber and Morries equation that is represented by: 6

(6)

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in Fig. 5b. The Freundlich isotherm equation is given by:

Table 2 The R2 parameters in different kinetics models for the adsorption of TC with different concentrations on CNNS and the experimental and calculated qe values according to the pseudo-second-order kinetics model. Ce of TC (mg/ L)

R2 (pseudofirstorder)

R2 (pseudosecondorder)

R2 (interparticle diffusion)

Experimental qe (mg g−1)

Calculated qe (mg g−1)

20 80 160 220 280 340 400 500

0.93178 0.65111 0.70697 0.66449 0.80688 0.58371 0.56033 0.50874

0.99999 0.99938 0.99949 0.99948 0.99963 0.99921 0.99838 0.99919

0.69514 0.93198 0.94772 0.88431 0.84872 0.62981 0.73258 0.69885

40 59 73 78 80 81 79 82

41 60 74 79 81 83 80 84

ln (qe ) = ln (Ce )/ n + ln (KF )

where both KF and n are the Freundlich constants that are related to the adsorption capacity and intensity, respectively [53]. Fig. 5c provided the plot of ln(qe) versus ln(Ce). The linear form of the Tempkin isotherm has been used in the following form:

qe = RT / bT × ln (Ce ) + RT / bT × ln (AT )

(9)

where AT is the equilibrium binding constant corresponding to the maximum binding energy (L mg−1), bT is the constant of Tempkin isotherm, T is the temperature (K) and R is the ideal gas constant (8.315 J mol−1 K−1). The plot of qe versus ln(Ce) was depicted in Fig. 5d. It can be seen that the Langmuir isotherm model obviously correlated better (R2 = 0.99784) than Freundlich (R2 = 0.94881) and Tempkin (R2 = 0.96427) isotherm models in the CNNS adsorption process. And the calculated qm value (89 mg g−1) based on the Langmuir isotherm model was close to the experimental value (85 mg g−1) in Fig. 5a. Hence, the adsorption isotherm of TC on CNNS was well suitable for the Langmuir isotherm model, suggested that the monolayer adsorption of TC occurred on CNNS. The calculated KL value in CNNS was 3.3 × 10-2 L mg−1. For comparison, it was found that the TC adsorption isotherm in T-CN also followed the Langmuir isotherm model (Fig. S8), and the calculated qm (12 mg g−1) and KL (2.7 × 10-2 L mg−1) values were lower than that in CNNS.

value was 437 mg g−1 min−1 that was approximately as high as ca. 219 times than that in T-CN (2 mg g−1 min−1). In addition, with the TC concentration increasing to 500 mg L−1, through the determination of the adsorption capacity (Fig. S7), it was also found that these adsorption data well followed the pseudo-secondorder adsorption equation due to the good correlation and close result between experimental and calculated qe values (Table 2). Therefore, the variation of TC concentration (20–500 mg L−1) did not change the adsorption kinetics of CNNS. According to the calculated qe values under the condition of different TC concentrations, the adsorption isotherm of CNNS was established as shown in Fig. 5a. It can be seen that the adsorption capacity of TC on CNNS increased with the equilibrium concentration of TC in solution progressively saturating the adsorbent. To interpret the adsorption isotherm, the Langmuir, Freundlich, and Tempkin isotherm models were employed, respectively. The Langmuir isotherm equation is expressed as:

Ce / qe = (1/ qm ) Ce + [1/(qm × KL)]

(8)

3.4. Photocatalytic kinetics In the photocatalytic process, the adsorbed TC molecule can be degraded by the photogenerated active oxidation species (mainly h+, % OH and/or %O2− radicals) of the as-prepared g-C3N4. Consequently, the degradation reaction can be expressed as:

CN⋯TC + n AOS → products

(7)

−1

where qm (mg g ) is the maximum adsorption capacity and KL (L mg−1) is the Langmuir constant corresponding to the affinity of the sorbate for the binding sites [53]. The plot of Ce/qe versus Ce was shown

(10)

where CN⋯TC is the TC molecules adsorbed on the surface of gC3N4; AOS is the generated active oxygen species in g-C3N4 under the irradiation of visible light.

Fig. 5. (a) Adsorption isotherm of TC with different concentrations on CNNS; (b) Langmuir isotherm for TC adsorption on CNNS; (c) Freundlich isotherm for TC adsorption on CNNS; (d) Tempkin isotherm for TC adsorption on CNNS. 7

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degradation data well fitted for the zero-order kinetic rate equation, as shown in Fig. 6b. The determined rate constant of k0 approached to 0.37 mg L−1 min−1. With the gradual decrease of TC concentration, the measured data during the reaction time from 16 to 36 min transformed to correlate the first-order kinetic rate equation (Fig. 6c). The determined k1 rate constants was 0.014 min−1. When the reaction time was over 36 min, TC molecule was hardly degraded. This was mainly because the generated small-molecular intermediates from TC degradation effectively adsorbed on CNNS surface and reacted more easily with active species. As for T-CN, due to the low adsorption and photocatalytic activity, the TC degradation data in the whole reaction process just adapted to the zero-order kinetic rate equation with the k0 value as low as 0.05 mg L−1 min−1 (Fig. S9). Therefore, the efficient adsorption and photocatalytic activity was the major reason for the different degradation kinetics of TC molecule in the CNNS photocatalytic process. Furthermore, when the reaction rate (k0) was normalized to specific surface area (S) for CNNS and T-CN (Table S1). Evidently, CNNS exhibited the highest k0/S value which further demonstrated more efficient photocatalytic activity of CNNS.

The kinetic equation of Eq. (10) can be described as:

− d[CN⋯TC]/dt = −nd[AOS]/ dt = k [CN···TC][AOS]n

(11)

It has been widely reported that the [AOS] is largely dependent on the amount of separated electron-hole pairs on the surface of a photocatalyst [54,55]. Due to the structural stability of g-C3N4 in the photocatalytic reaction [32,33], the performance of charge separation and transfer can be proximately regarded to have no change and thus the amount of surface charge carriers is almost unchanged. Hence, [AOS] can be basically regarded as a constant. When the concentration of TC in reaction solution is sufficiently high for the surface of g-C3N4 to be saturated with adsorbed TC molecule, [CN⋯TC] would be a constant and equal to qm. Therefore, in this circumstance the degradation of TC would follow a zero-order kinetic rate equation and Eq. (11) can be transformed as: (12)

− dC /dt = k 0 and

k 0 = kqm [AOS ]n

(13)

So

3.5. Active oxidation species and degradation products

C = C0′ − −k 0 t

(14) It can be seen from Fig. 7a that CNNS had a declined photocatalytic activity for TC degradation when N2, EDTA-2Na and IPA were added. The TC degradation followed the zero-order kinetics equation in the whole reaction process which was largely due to the decrease of photocatalytic activity of CNNS in the presence of these scavengers. In detail, when N2 gas was added into the reaction system, the TC degradation efficiency dropped from 38 to 24% along with the degradation rate decreasing from 0.37 to 0.081 mg L−1 min−1. This result suggested the %O2− radicals participated for TC degradation. When IPA was added, the TC degradation efficiency and rate reduced to be 26% and 0.086 mg L−1 min−1, respectively, which indicated the %OH radicals were also the active oxidation species. When the EDTA-2Na was presented in the photocatalytic system, both the TC degradation efficiency and rate were inhibited as well (24% and 0.087 mg L−1 min−1, respectively), illustrating the h+ also played an important role in the photocatalytic process. Hence, the h+, %O2− and %OH radicals were confirmed as the major active species for TC degradation in the CNNS photocatalytic process. As displayed in Fig. 7b, the generated active species were further verified by the ESR technique. Evidently, the characteristic peaks for %OH and %O2− radicals were found when CNNS was irradiated by visible light, demonstrating that the %OH and %O2− radicals could be generated. These results were in good accordance with the above results of trapping experiment. In addition, as presented in Fig. S10, the determined conduction band minimum (−1.25 V vs. NHE) of CNNS was more negative than EO2/%O2– (-0.33 V vs. NHE), the %O2− radicals were thus generated from the reduction of dissolved oxygen by electrons. Because the determined valence band maximum (1.63 V vs. NHE) of CNNS was more negative than EOH−/%OH (1.99 V vs.

and

C / C0 = −(k 0/ C0 ) t + C0′/ C0 −1

(15)

−1

where k0 (mg L min ) is the zero-order kinetic rate constant; C (mg L−1) and C0′ (mg L−1) is the concentration of TC in the reaction solution at reaction time t and initial time of photocatalysis, respectively. When the TC concentration in the reaction solution is not enough for the surface of g-C3N4 to be saturated with adsorbed TC molecule, [CN⋯TC] would be less than qm and decreases gradually during the degradation. In this case, the fitting of experimental data would deviate from the straight and the TC degradation would follow a first-order kinetic rate equation. Therefore, Eq. (11) could be transformed as: (16)

− dC /dt = k1 C and

k1 = k [AOS]n

(17)

so

ln(C / C0′) = −k1 t

(18)

and

ln(C / C0) = −k1 t + ln(C0′/ C0)

(19)

−1

where k1 (min ) is the first-order kinetic rate constant. Fig. 6a recorded photocatalytic TC degradation activity on CNNS. Evidently, different TC degradation rates were observed within the different reaction time. In the reaction time of the first 16 min, the TC

Fig. 6. (a) Photocatalytic TC degradation on CNNS. Prior to visible light irradiation, the reaction system should achieve the adsorption equilibrium; (b) Zero-order kinetics curve of TC degradation on CNNS; (c) First-order kinetics curve of TC degradation on CNNS. 8

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Fig. 7. (a) Photocatalytic TC degradation activities and zero-order kinetics curves of TC degradation on CNNS in the presence of N2, EDTA-2Na and IPA; (b) DMPO spin-trapping ESR spectra of ·OH and %O2– radicals on CNNS in the absence and presence of visible light irradiation during the reaction time of 3 min.

NHE), the formation of %OH radicals cannot come directly from the transformation of the h+ with reaction with OH− ions, which probably resulted from further reduction of %O2− radicals, which was an indirect way to form %OH radicals [56,57]: %O2− + e− + 2H+ → H2O2 %O2

−

+ 4H2O → 2%OH + 2OH

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(20) −

+ O2

H2O2 → 2%OH

(21) (22)

Furthermore, the products after the CNNS photocatalytic process were determined by UPLC-MS. As shown in Fig. S11 and Table S2, the m/z of 445 was assigned to the residual TC. Products with m/z of 416 (P1) and 395 (P2) were attributed to the dealkylation products of TC. The m/z of 375 (P3), 362 (P4), 318 (P5), 246 (P6), 226 (P7) were ascribed to the further oxidation products. These results were consistent with previously reported studies [58–60].

4. Conclusions In summary, the as-prepared CNNS exhibited efficient and stable adsorption and photocatalytic activity for TC removal. Such performance was significantly higher when compared with the g-C3N4 counterparts. The enhancement of adsorption on CNNS mainly resulted from the large contact area and delocalized π bonds, and the TC adsorption followed the pseudo-second-order adsorption kinetics and Langmuir isotherm. The efficient separation and prolonged radiative lifetimes of photogenerated charge carriers and the strong adsorption were responsible for the improved photocatalytic activity of CNNS. Different kinetics models including zero-order and first-order had been determined for TC degradation, and the h+, %O2− and %OH radicals were confirmed as the major reactive species in the CNNS photocatalytic system. This study provided a new route for the development of catalysts with efficient adsorption and photocatalytic activity in the remediation of organic wastewaters.

Acknowledgements This study is supported by the National Natural Science Foundation of China (No. 21806059), Hong Kong Scholar Program (No. XJ2018031), Natural Science Foundation of Jiangsu Higher Education institution (No. 17KJB610006), China Postdoctoral Science Foundation (No. 2018M630524), Research Grant Council (No. GRF14100115) and Technology and Business Development Fund, The Chinese University of Hong Kong (No. TBF18SCI006).

Appendix A. Supplementary data Supplementary data to this article can be found online at https:// doi.org/10.1016/j.cej.2019.122760. 9

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