Investigation of kinetics, isotherms, thermodynamics and photocatalytic regeneration of exfoliated graphitic carbon nitride/zeolite as dye adsorbent

A novel exfoliated graphitic carbon nitride and clinoptilolite nanocomposites (Ex.g-C3N4/CP and g-C3N4/CP with a various ratios of g-C3N4 to CP) were prepared by facile method. This study evaluates the adsorption of methylene blue (MB) on the surface of synthesized adsorbents. The as-prepared composites were characterized by XRD, FT-IR, FESEM, BET and DRS. Batch experiments were carried out under various conditions, such as the amount of adsorbent and solution pH. The optimum batch experimental conditions were found under the response surface methodology. The Ex.g-C3N4/CP presented maximum removal of MB as compared to others. The removal efficiency of the as-prepared nanocomposite was significantly elevated owing to the synergistic effects. The adsorption capacities of MB (10 ppm) on Ex.g-C3N4/CP was 54.3 mg/g. The adsorption process by both composites (g-C3N4/CP and Ex.g-C3N4/CP) showed well-fitting with the Elovich kinetic model, and Langmuir isotherm. The thermodynamic study suggested that the adsorption of MB was a spontaneous and endothermic process. The reusability of g-C3N4/CP1:2 and Ex. g-C3N4/CP in removing of MB (10 ppm, pH = 9) was studied by photocatalytic regeneration under visible irradiation for three consecutive cycles. The results obtained from the experimental analyses showed that the removal of MB was easy treatment, eco-friendly, and high yield.


Preparation of adsorbents.
In order to remove natural zeolite impurities, it was washed with distilled water for one hour and then dried in an oven at 70 °C for one hour and named as CP.Bulk graphitic carbon nitride was synthesized by thermal polymerization.For synthesis, 16 g of urea was poured into a 100 ml aluminum crucible with a lid, then it was heated for 4 h at 500 °C (rate of 2 °C/min), and finally a yellow powder product was obtained.It was named as bulk g-C 3 N 4 .Also, exfoliated graphitic carbon nitride was synthesized by thermal oxidation.First, 0.4 g of synthesized bulk g-C 3 N 4 was poured into a 100 ml alumina crucible, then it was heated for 2 h at 520 °C (5 °C/min) and finally a pale yellow powder was obtained.It was named as Ex.g-C 3 N 4 (Fig. 1).
Different amounts of synthesized bulk g-C 3 N 4 were dispersed in methanol (50 ml) and exposed to ultrasonic radiation for 30 min.Then, different amounts of CP were added and stirred for 24 h.At the end, it was dried in the oven at 70 °C for 24 h.The synthesis conditions of three samples are given in the Table 1.
For fabrication of the nanocomposite, 0.62 g of synthesized Ex.g-C 3 N 4 was dispersed in water (50 ml) and exposed to ultrasonic radiation for 30 min.Then, 5 g of CP was added and stirred for 24 h.Then the obtained sample was dried in an oven at 70 °C for 24 h.The synthesized sample was named as Ex.g-C 3 N 4 /CP.

Characterization of adsorbents.
Brunker model D8 X-ray diffractometer was used to check the crystal structure.Fourier transform infrared spectroscopy (FTIR) Magna-IR Nikolate 550 was used to identify organic compounds.Zeiss sigma 300-HV field emission scanning electron microscope was used to determine the average size of particles and morphology.Also, Philips X-ray energy diffraction spectrometer (EDS) model XL30 was used.The transmission electron microscope (TEM) image was obtained from a Phillips model EM208S device.Nitrogen adsorption/desorption was done using Belsorp mini x device.Shimadzu model UV3600Iplus Diffuse Reflectance/Transmission Spectrometer (DRS) was used to determine light absorption ability.

Adsorption experiments.
Adsorption experiments were performed in batch mode by adding a specific amount of adsorbent to the known volume of MB solution on magnetic stirrers at room temperature.HCl and NaOH (1 M) solutions were used to adjust the pH.Finally, the adsorbent was separated by centrifugation and the concentration of the filtrate was determined by UV-Vis spectrophotometer at 664 nm.Removal efficiency® and equilibrium adsorption capacity were calculated by Eqs. ( 1) and (2), respectively.Where, q e is the equilibrium adsorption capacity (mg/g), C i is the initial concentration of MB (mg/L), C e is the concentration of MB at the equilibrium (mg/L), V is the volume of MB solution (L) and M is the mass of the adsorbent (g) 21 .
In order to determine the isotherm models, different concentrations of MB (5, 10, 30, 50 mg/L) was prepared at pH equal to 9 (according to the optimal conditions) with an adsorbent concentration of g-C 3 N 4 /CP1:2 and EX.g-C 3 N 4 /CP equal to 0.33 and 0.2 g/L, respectively.The solutions were stirred on a magnetic stirrer at ambient temperature and after the desired time; the samples were separated and their adsorption rate was determined.The regression coefficient was used to show how well the regression equation fits the data.Adsorption kinetics were investigated using g-C 3 N 4 /CP 1:2 and Ex.g-C 3 N 4 /CP in optimal conditions specified according to RSM results.The heat of adsorption refers to the thermal effect during the adsorption process, and the magnitude of it can reflect the degree of adsorption.The Gibbs free energy change (∆G o ), enthalpy change (∆H o ) and entropy change (∆S o ) were investigated to determine whether the adsorption was spontaneous and analyze the driving force of (2) Steps for the synthesis of bulk g-C 3 N 4 and Ex.g-C 3 N 4 samples.

Results and discussion
Characterizations of the adsorbents.Crystal structure and phase purity of synthesized samples have been shown in Fig. 2a.The XRD pattern of natural zeolite show that it consists of significant amounts of Clinoptilolite, Hollandite, Biotite, Feldspar and Quartz.The characteristic XRD peaks of natural zeolite observed at 9.8, 11.24, 17.4, 20.8, 22.4, 26.7, 30 and 33°.This is well matched with the patterns of Clinoptilolite [22][23][24] .Besides, the 2θ values at 27.2 and 13° were corresponds to (002) and (100) planes of crystal faces of g-C 3 N 4 , that matched with the JCPDS card no (JCPDS No. 87-1526) 25 .These peaks have also appeared in the pattern of the Ex.g-C 3 N 4 .But after exfoliation, the intensity of the 27.2° was significantly reduced, and this indicated the successful synthesis 13 .Availability of both types of peaks in all composites confirms the successful incorporation of g-C 3 N 4 into natural zeolite.Though, the characteristic peaks of natural zeolite was not shifted much, the stacking of g-C 3 N 4 layers corresponds to the peak at 27.2°2 6 and in all composites has shifted to higher angles, which shows a reduction of the distance between graphitic carbon nitride layers 13 .The distance between layers in bulk g-C 3 N 4 , Ex.g-C 3 N 4 , g-C 3 N 4 /CP1:1, g-C 3 N 4 /CP1:2, g-C 3 N 4 /CP2:1 and Ex.g-C 3 N 4 /CP are equal to 0.324, 0.327, 0.316, 0.315, 0.317 and 0.323 nm.This can be explained that part of g-C 3 N 4 inserted into the layer of zeolite.This is clearly indicated that the zeolite was well decorated on the g-C 3 N 4 .Also, the intensity of the peak at 27.2° gradually increases with the increase of g-C 3 N 4 , which indicates the successful integration of g-C 3 N 4 with CP.
To further determine the composition information of g-C 3 N 4 , CP and all composites was further evidenced by FT-IR analysis (Fig. 2b).For bulk g-C 3 N 4 and Ex.g-C 3 N 4 , three main regions can be seen.A sharp peak was observed at 811 and 1891 cm −1 , which corresponds to 3-S triazine units.The characteristic peaks at 1231, 1327, 1417 and 1573 cm −1 were assigned to the N-C stretching vibrational state of aromatic rings, the peak at 1643 cm −1 was assigned to the N = C stretching vibrational state, and the peak at 2180 cm −1 was assigned to C≡N.Broad www.nature.com/scientificreports/absorption peaks in the range of 3100 to 3300 cm −1 are related to the stretching vibrations of primary (NH 2 groups) and secondary amines (NH = groups), and the peak near 3400 cm −1 is related to the stretching vibration of the interlayer H-OH bond 27,28 .In the structure of Ex.g-C 3 N 4 , a peak at 3500 cm −1 was observed, which was attributed to -OH groups which are caused by the oxidation of g-C 3 N 4 in air 29 .In the FT-IR spectra of Clinoptilolite, characteristic peaks in the range of 3750-2900 cm −1 are attributed to OH.Finally, the bending vibration of water is observed at 1627 cm −1 .The sharp peak at 1013 cm −1 corresponds to Al atoms, the peaks observed in the range of 420-500 cm −1 are related to T-O 24,30 .The FT-IR spectra of the composites can be clearly show that peaks from 900 to 1800 cm −1 are related to C-N H-C units.The broad peaks at 3500-3000 cm −1 are assigned to N-H and O-H stretching vibrations, which indicate the presence of NH or NH 2 groups in them 30 .Also, the intensity of the peaks is weakened, which indicates that g-C 3 N 4 with CP is not physically mixed together, but forms a lower energy structure 27 .
The surface morphology of the bulk g-C 3 N 4 , Ex.g-C 3 N 4 and CP was analyzed by SEM.Bulk g-C 3 N 4 (Fig. 3a) has an irregular layered morphology similar to a honeycomb 15,32 and is formed in micrometer dimensions 31 .Ex.g-C 3 N 4 (Fig. 3b)) has an irregular and layered morphology after thermal exfoliation 33 .It can be seen that CP (Fig. 3c)) has a flat and layered morphology.Also, flat particles are stacked together 34,35 .According to Fig. 4a, both of g-C 3 N 4 and CP are intertwined and block some mesoporous channels on the g-C 3 N 4 , which leads to a decrease in the surface area of the g-C 3 N 4 /CP1:2 composite compared to bulk g-C 3 N 4 .In other word, the surface and edges of the CP were coated uniformly and tightly with layer g-C 3 N 4 .The elemental composition of the assynthesized composites was also confirmed by EDS (Fig. 4b).It was indicated that the synthesized compound was in high purity form.
According to Fig. 5, the Ex.g-C 3 N 4 was appearing to be sheet like morphology.It is clearly confirmed that CP was randomly decorated on the g-C 3 N 4 nano sheets.
The optical property of g-C 3 N 4 /CP1:2 and Ex.g-C 3 N 4 /CP composites during photocatalytic regeneration were investigated by UV-vis DRS (Fig. 6).Both of them are effective and great light harvesting efficiency for visible light.Therefore, both samples can be regenerated during the photocatalytic process under visible irradiation.
The surface area and pore size of samples were studied by N 2 adsorption/desorption isotherm (Fig. 7, Table 2).
According to the IUPAC classification, their isotherms are type IV and have a type of H3 hysteresis loop and show mesoporous materials.Surface area of bulk g-C 3 N 4 is equal to 170.07 m 2 g −1 , which by peeling, it has increased to 336.39 m 2 g −122, 36, 37 .The g-C 3 N 4 was having lesser surface area than the composite, so the composite  Adsorption of MB by synthesized samples.Comparison of performance of synthesized samples in adsorption of MB was investigated (Fig. 8).Results indicated that all composites showed an improved performance compared to bulk g-C 3 N 4 .Also, the g-C 3 N 4 /CP 1:2 has the higher adsorption efficiency than g-C 3 N 4 /CP 1:1, g-C 3 N 4 /CP 2:1.And, the Ex.g-C 3 N 4 /CP nanocomposite has the highest adsorption efficiency.Therefore, by combining CP and g-C 3 N 4 , a synergistic effect was achieved in the composite in such a way that an adsorbent with high adsorption property along with the ability to regenerate under visible light radiation has been synthesized.In the continuation of the present study, the g-C 3 N 4 /CP1:2 and Ex.g-C 3 N 4 /CP samples were investigated and compared as the best synthesis samples and optimization of adsorption was done by using them.
With the aim of investigating the effective factors on the process, the interactions between the effective factors and their optimization, an experimental design based on response surface methodology (RSM) was carried out.Since the Box Behnken Design (BBD) set a mid-level between the original low and high level of the factors, avoiding the extreme axial points as in the central compound design (CCD), the latter was applied.Besides, BBD cannot estimate the full quadratic model for less than four factors 38,39 .Two important operational parameters of the adsorption process (adsorbent concentration and initial pH value of the solution) were optimized by Design Expert 11 software.The removal efficiency of MB was selected as the response.The results of the design are presented in Table 3.
The obtained data was in good agreement with the reduced quadratic model.The obtained models for predicting the removal percentage in the coded form for g-C 3 N 4 /CP1:2 and Ex.g-C 3 N 4 /CP composites are as Eqs.6 and 7, respectively.The adequacy of the obtained model was checked with analysis of variance (Table 4).The regression coefficient of the models for both composites showed the adequacy and importance of the models.The 3D surface response interaction diagram for both composites is presented in Fig. 9.It can be seen that for higher amounts of adsorbent and at alkaline pH, the amount of adsorption has been more efficient.An important parameter in adsorption is pH pzc .At this pH, the surface charge of the adsorbent is zero.At pH lower than pH pzc , the adsorbent surface has a positive charge and the adsorbent surface has a negative charge at pH higher than pH pzc , which tends to adsorb cations.The pH pzc value of g-C 3 N 4 /CP 1:2 and Ex.g-C 3 N 4 /CP adsorbents was  www.nature.com/scientificreports/determined to be 6.5 and 6.2, respectively.Based on the pH pzc values of the two adsorbents, at pH higher than pH pzc , their surface has a negative charge and the electrostatic attraction between the adsorbent and MB as a cationic pollutant with a positive charge increases the amount of adsorption.The increase in adsorption with higher amounts of adsorbent can be attributed to the increase in the adsorbent surface and the availability of more adsorption sites 40 .At pH lower than pH pzc , due to the increase of H + ions in the solution, electrostatic repulsion has occurred and less adsorption has occurred.www.nature.com/scientificreports/ The optimization of the investigated operating parameters was defined by choosing the minimum amount of adsorbent and the maximum achievable removal efficiency (Table 5).
In the following, the adsorption kinetics, isotherm, thermodynamic study and regeneration and the stability of the adsorbents in the process in the resulting optimal conditions have been investigated.To further explore the adsorption process, four kinetic models were used in the study (Table 6) 41 .Elovich's model showed excellent linearity with high fitting, which suggested that the adsorption process might be controlled by chemical adsorption, which is in agreement with the isotherm results.
To further analyze the adsorption isotherm quantitatively of MB onto the g-C 3 N 4 /CP1:2 and Ex.g-C 3 N 4 /CP, four typical isotherm models were used (Table 7).It could be noted that the equilibrium adsorption capacities increased when the MB concentration was increased.The Langmuir model presumes adsorption occurs as   www.nature.com/scientificreports/ a mono-layer on a homogenous surface.The Freundlich model describes the multiple-layer adsorption on a heterogeneous surface 42,43 .The value of R 2 is regarded as the good-to-fit of the experimental data on the Langmuir model.Obviously, the adsorption of MB on g-C 3 N 4 /CP1:2 and Ex.g-C 3 N 4 /CP was better simulated by the Langmuir adsorption isotherm than the others and the values of q m (34.1 and 54.3 mg/g) calculated by the Langmuir adsorption isotherm basically agreed with the experimental data, which speculated that the adsorption on the composites was monolayer adsorption.Also, such adsorption capacity demonstrated that the Ex.g-C 3 N 4 /CP was more effective adsorbents for MB adsorption than g-C 3 N 4 /CP1:2.For the purpose of evaluating the adsorption feasibility, the separation factor RL was obtained.As we can see from Table 7, the R L values were between 0 and 1, suggesting the MB adsorption was a favorable process 44 .
According to Table 8, the positive values of ∆H o and the negative ∆G o values for both composites indicated that the adsorption of MB on them is endothermic and spontaneous.As the temperature rose from 25 to 45 °C, the value of ∆G o became more negative, leading to the stronger adsorption, which demonstrated that it was in Table 6.Pseudo-first-order, pseudo-second-order, Elovich and intra particle adsorption kinetics for composites.www.nature.com/scientificreports/favor of the adsorption for MB at a high temperature.The positive value of ∆S o indicated an increase in randomness and a significant change in the internal structure of the adsorbent.Table 9 gave the comparison of the adsorption capacity of different sorbents for MB adsorption.It could be seen that the Ex.g-C 3 N 4 /CP possessed similar or higher adsorption capacity of MB compared to the other commonly used adsorbents.
Regeneration and stability of adsorbents.Recyclability of adsorbents was crucial for their practical applications.Thus, in the next step, the regeneration of the g-C 3 N 4 /CP1:2 and Ex.g-C 3 N 4 /CP was elucidated by photocatalytic process.Their reusability (g-C 3 N 4 /CP: 0.33 g/L and Ex.g-C 3 N 4 /CP: 0.2 g/L) in removing MB (10 ppm, pH = 9) was studied for 180 min.After each experiment, the adsorbent was separated, dispersing in water and exposed to visible light radiation (500 watts for 7 h).Then, the adsorbent was separated and used in the next cycle of adsorption.This was repeated three times.The results indicated that both composites have good chemical stability and simple reusability for three cyclic removal of MB (Table 10).Also, the used composite was analyzed by XRD and FT-IR analysis (Fig. 10), which indicates its good stability.

Conclusion
Herein, a novel exfoliated graphitic carbon nitride/Clinoptilolite (Ex.g-C 3 N 4 /CP) with porous structure was successfully prepared via a facile method using urea as the g-C 3 N 4 precursor.The as-prepared nanocomposite was characterized by XRD, FT-IR, SEM and BET.Batch experiments were carried out under various conditions, such as the amount of adsorbent and solution pH.In optimizing the operational parameters (Adsorbent concentration-pH of solution) by g-C 3 N 4 /CP1:2 and Ex.g-C 3 N 4 /CP composites, the optimal conditions were obtained as 0.33 g/L-pH = 9 and 0.2 g/L-pH = 9, respectively.Ex.g-C 3 N 4 /CP was demonstrated a high adsorption capacity of 54.3 mg.g −1 , which is much higher than that of single g-C 3 N 4 , or other synthesized composites and many reported adsorbents.The removal efficiency of the as-prepared composite was significantly elevated owing to the synergistic effects.The MB adsorption process on it can be well described using a Langmuir isotherm and Elovich kinetic model.The thermodynamic study suggested that the adsorption of MB was a spontaneous, chemisorption and endothermic process.After the dye adsorption, Ex.g-C 3 N 4 /CP was regenerated simply and retained a high adsorption efficiency for MB after three uses.Our findings were indicated that Ex.g-C 3 N 4 /CP is a low cost, green and promising adsorbent and low environmental impact which can potentially be applied for the removal of extensive pollutants from aqueous solution.

Figure 10 .
Figure 10.XRD pattern and FT-IR spectrum of Ex.g-C 3 N 4 /CP composite (A) before and (B) after MB adsorption.

Table 1 .
The synthesis conditions of the samples.

Table 2 .
The comparison of surface area, pore size and pore volume of synthesized samples.

Table 3 .
The design matrix of the central complex; Factors, levels, experimental Responses (MB: 10 ppm, time: 180 min).

Table 5 .
Optimum operating conditions for the adsorption of MB (10 ppm).

Table 9 .
Comparison of the adsorption capacities for MB on various adsorbents.

Table 10 .
Investigation of photocatalytic regeneration of g-C 3 N 4 /CP 1:2 and Ex.g-C 3 N 4 /CP adsorbents in three consecutive cycles.