Synthesis of a novel porous organic polymer containing triazine and cyclohexanone rings as an efficient methyl red adsorbent from aqueous solutions

In this research, a new porous organic polymer based on triazine and cyclohexanone rings was synthesized via Schiff base condensation, and its performance as an adsorbent for the removal of Methyl Red dye from aqueous solution was investigated. The synthesized polymer was characterized by FT-IR, XRD, SEM, EDS, TEM, TGA, and BET analyses. Five important parameters of pH (4–10), contact time (10–120 min), adsorbent dose (5–10 mg), initial dye concentration (10–70 mg/L), and temperature (25–45 °C) were investigated to optimize the adsorption conditions. Solution pH of 4, contact time of 80 min, adsorbent dose of 8 mg, initial dye concentration of 50 mg/L, and temperature of 45 °C were obtained as the best conditions for the adsorption of methyl red dye. Two widely used Langmuir and Freundlich models were employed to investigate the adsorption isotherm, and the obtained data showed that the adsorption process follows the Langmuir isotherm with a correlation coefficient (R2 = 0.9784) which indicates monolayer adsorption. The achieved maximum adsorption capacity was 178.57 mg/g. Also, the results of kinetic studies indicate that the adsorption process follows the pseudo-second-order kinetic, which suggests that chemical interactions play an important role in dye removal. Furthermore, the results showed that the adsorption process of methyl red dye by polymer is endothermic.

Characterization techniques.Fourier transform infrared (FT-IR) spectra was registered on an ALPHA-Bruker spectrometer from 400 to 4000 cm −1 , using KBr pellet. 1 H-NMR (300 MHz) and 13 C-NMR (75 MHz) were carried out in CDCl 3 on a Bruker-Avance spectrometer.The thermogravimetry analysis (TGA) was performed on a Q600 TA instrument under an argon atmosphere at 25-800 °C.X-ray diffraction (XRD) data were examined by X'Pert-MPD PRO-PW3040/60 instrument.The surface morphology and atomic distribution were obtained by field emission scanning electron microscope (FE-SEM) brand TESCAN (MIRA 3 LMU).Zeiss EM900 microscope was used for Transmission electron microscopy (TEM) analysis.The surface area, pore volume, and pore size distribution were characterized on a BELSORP MINI II system at 77 K using N 2 as the adsorbate.The UV-Visible (UV-Vis) spectra of the methyl red dye solutions were recorded using a UV-2550 spectrophotometer.

Synthesis of 2,4,6-tris-(4-formylphenoxy)1,3,5-triazine (TFPT) (3). This triazine compound
(3) was preapred according to our earlier method 55 .Briefly, a solution of p-hydroxybenzaldehyde (1) (1.89 g, 15.5 mmol) and NaOH (0.62 g, 15.5 mmol) in acetone and water (40 mL, 1:1 v/v) was prepared over a magnetic stirrer at 0 °C.Then, a solution of cyanuric chloride (2) (0.92 g, 5.0 mmol) in acetone (20 mL) was added dropwise over 60 min.Afterward, the obtained solution was stirred for 2 h at room temperature and refluxed for 10 h.The reaction mixture was then poured into 50 mL water to precipitate the desired white product.The precipitate was filtered and washed with distilled water and ethanol three times and recrystallized from ethyl acetate to give pure product, which then dried at 70 °C under a vacuum for 12 h.

6-bis(4-aminobenzylidene)cyclohexanone (7).
A mixture of Na 2 S (0.80 g, 10.2 mmol) and NaHCO 3 (0.35 g, 4.16 mmol) was dissolved in 5 mL of water.Then, methanol (10 mL) was added, and the reaction mixture stirred for 30 min at room temperature.The white precipitate was filtered and the filtered solution was added to a mixture of 2,6-bis(4-nitrobenzylidene)cyclohexanone (6) (0.26 g, 0.714 mmol) and 15 mL methanol, and stirred for 3 h under reflux conditions.The mixture was concentrated using a rotary evaporator and the residue poured into water, the red product was filtered, washed well with water, and finally dried 57 .

Methyl red adsorption experiments.
A stock solution (1000 ppm) of methyl red in deionized water was prepared, and the desired solutions were also prepared by dilution of this stock solution.To investigate the removal of methyl red by POP, 8 mg of TC-POP was poured into 10 mL of 50 mg/L MR solutions and stirred at room temperature for 24 h by magnetic stirring The adsorbent was separated by centrifuge and the residual concentration of MR was determined by UV-vis spectrophotometer at λ max = 430 nm.Then, the effect pH (adjusted with 0.1 M of NaOH and 0.1 M HCl solutions), contact time, initial dye concentration, the amount of adsorbent, and temperature were examined in adsorption process.The adsorption follows Eqs.(1) and (2).
where C 0 , C e (mg/L), m (g), and V (L) refer to the initial and equilibrium concentration of methyl red, the mass of adsorbent and the volume of solution, respectively.
The thermal stability of TC-POP was investigated by thermogravimetric analysis (TGA) in the temperature limit of 25-800 °C and under an argon atmosphere.As can be seen in the TGA curve (Fig. 7), the synthetic polymer shows a good resistance to heat up to 400 °C due to strong covalent bonds between its constituent monomers.Below 250 °C, there is a mass reduction, which is related to the removal of the adsorbed moisture (water), unreacted monomers, and organic solvents trapped in TC-POP.
Scanning electron microscope (SEM) and transmission electron microscope (TEM) were used to investigate the morphology and structure porosity of the synthesized TC-POP (Fig. 8).According to the SEM image    Methyl red adsorption studies.After the synthesis of the polymer, the adsorption of methyl red dye was studied under optimal conditions to examine its performance as an adsorbent.In order to optimize the adsorption conditions, five important parameters of pH, contact time, adsorbent dose, initial dye concentration, and temperature were investigated.

Effect of pH.
The pH solution is one of the most sensitive factors in the adsorption process because it can affect the amount of ionization of the organic contaminants, the adsorbent level load, the structure of the pollutants, and the active sites on the adsorbent.Due to the presence of many nitrogen and oxygen atoms as active sites on the adsorbent surface, as well as the presence of functional groups N(CH 3 ) 2 , COOH and N=N of methyl red dye, pH plays an important role in the adsorption process.The adsorption of MR on TC-POP was studied at a pH range of 4-10 using 8 mg of TC-POP in 10 mL of 50 mg/L solutions of MR at 45 °C (Fig. 10a).The maximum adsorption of methyl red was observed at pH = 4. Considering the structure of TC-POP adsorbent and methyl red anionic dye, several interaction forces such as the π-π interactions between polymer and dye π orbitals, hydrogen bonding interactions, and electrostatic interactions between anionic dye and polarized sites due to change of pH on the adsorbent surface can be expected.At pH = 4, the active sites on the surface of the adsorbent are protonated, which creates positively charged sites on the adsorbent and strong electrostatic interactions with the anionic dye, as a result, the adsorption capacity increases at this pH.Also, the decrease in adsorption capacity at higher pH can be considered due to the decrease of positive charge sites on the adsorbent.However, the significant adsorption of anionic dye on the adsorbent at high pH is due to other interactions such as π-π and hydrogen bonding between MR and TC-POP.It is worth noting that the surface area of adsorbents has a significant impact on its ability to adsorb pollutants.Materials with a higher surface area have more available sites for pollutant molecules to bind to, resulting in higher adsorption capacity and efficiency.This is why many adsorbents, such as porous organic polymers (POPs), have high surface area and porosity to maximize their adsorption capabilities.Therefore, due to the relatively good Brunauer-Emmett-Teller surface area (S BET ) (108.27 m 2 g −1 ) of the synthesized TC-POP, there are many active sites on the polymer, which significantly contributes to the adsorption of methyl red dye.This property, in combination with the other favorable characteristics of the TC-POP, makes it as a promising adsorbent for the removal of methyl red dye from aqueous solutions.The adsorption process of methyl red dye was also investigated using FT-IR spectroscopy.A clear difference between the FT-IR spectra before (TC-POP) and after (TC-POP + MR) adsorption is observed, which supports the dye adsorption process by the TC-POP adsorbent (Fig. 11a-c).The comparison of the two FT-IR spectra and the presence of new absorption bands in the spectrum after the absorption process indicate the formation of a connection between TC-POP and MR.Also, some peaks are disappeared, and the intensity of some peaks reduced or shifted to lower wavelengths 58 .A possible mechanism of MR adsorption on TC-POP is briefly presented in Fig. 12.

Effect of contact time.
As shown in Fig. 10b, the process of methyl red dye adsorption on TC-POP was investigated at different times from 10 to 120 min.Initially, the adsorption process of MR is fast due to the high surface area of the adsorbent and subsequently many active sites of the adsorbent until it reaches equilibrium within 80 min.After that, with increasing contact time, no significant change in adsorption capacity was observed.

Effect of adsorbent dosage.
To investigate the effect of adsorbent amount on MR removal, different adsorbent amounts (5-10 mg) were used, while other parameters such as pH = 4, contact time 80 min, the concentration of MR solution 50 mg/L, and temperature of 45 °C were constant.By increasing the adsorbent dose, the dye removal efficiency also increases, which may be due to the increase in the adsorbent surface and the subsequent increase in the number of active sites for dye adsorption.The dye was rapidly adsorbed up to the adsorbent dose of 8 mg, and no noticeable change in adsorption efficiency was observed at higher concentra-  www.nature.com/scientificreports/tions.According to the obtained results, the amount of 8 mg of adsorbent was chosen as the optimal dosage to achieve the maximum dye removal efficiency (Fig. 10c).

Effect of methyl red dye concentration.
The effect of initial MR dye concentration on TC-POP adsorption capacity was examined in the range of 10-70 mg/L with an adsorbent dose of 8 mg at pH = 4 and temperature of 45 °C.As can be seen in Fig. 10d, As the initial dye concentration increases, the adsorption efficiency increases.This tendency continues up to the concentration of 50 mg/L, but at higher concentrations, the adsorption efficiency remains almost constant due to the constant amount of adsorbent and the number of active sites on the adsorbent.Therefore, with the increase of the MR concentration, the active sites of the adsorbent are gradually filled and the polymer is no longer able to adsorb the dye, and with the increase of the dye concentration, the adsorption capacity decreases.So, the optimal initial concentration of 50 mg/L was considered.

Effect of temperature.
Temperature plays an important role in the adsorption process in order to determine whether it is endothermic or exothermic.Therefore, to find the highest adsorption efficiency, the optimum temperature should be obtained.The effect of temperature the MR adsorption process by the TC-POP adsorbent was investigated at 25, 35, and 45 (pH: 4; dose: 8 mg; [MR]: 50 mg/L; time: 80 min).The results can be seen in Fig. 10e.As the increases, the dye adsorption efficiency also increases, which is a sign of the endothermic nature of the adsorption process.The optimal temperature was found to be 45 °C.Adsorption isotherms.The adsorption isotherm is one of the most important factors for designing the adsorption process system and also describing the reciprocal behavior between the adsorbent and the solute.In addition, the adsorption mechanism can be expressed through adsorption isotherms and is also used to calculate the adsorption capacity of the adsorbent.The Langmuir and Freundlich models are among the most important and widely used models for describing the experimental data of adsorption isotherms.Therefore, in this study, the data obtained from the dye adsorption process on the polymer were analyzed using Langmuir and Freundlich isotherms.In the Langmuir isotherm model, it is assumed that the adsorption of solutes on the adsorbent is carried out in a monolayer, and it also assumes that all the adsorption sites are the same in terms of energy and the adsorbent has a homogeneous structure.Linear Eq. ( 3) is used to express the Langmuir isotherm model 59 : where C e (mg/L), Q e (mg/g), Q max , and K indicate the concentration at equilibrium, the equilibrium adsorption capacity, the maximum adsorption capacity and the Langmuir constants, respectively.In the Freundlich isotherm, which is an experimental model, it is assumed that the solutes are adsorbed in several layers on the adsorbent surface, the adsorbent surface is not uniform and the surfaces have different adsorption power.The linear form of the Freundlich model is described by Eq. ( 4) 60 .
where n and K f (L/mg) are the Freundlich constants, which indicate the adsorption intensity and adsorption capacity, respectively.The results obtained from the calculation of Langmuir and Freundlich isotherm linear equations are given in Fig. 13 and Table 1.After analyzing the data and the values of the regression coefficient (R 2 ), it was found that the adsorption of methyl red by TC-POP follows the Langmuir isotherm model and it can be assumed that the adsorption is performed in a monolayer on the homogeneous adsorbent.
Adsorption kinetics.Adsorption kinetic models are used to investigate the contact time required to reach the adsorption equilibrium (speed of the adsorption process) and also to show the interaction mechanism between the adsorbent and the adsorbate.Therefore, in order to better understand the adsorption behavior and mechanism and to evaluate the kinetics of MR adsorption at the TC-POP level, two common kinetic models including pseudo-first-order and pseudo-second-order under optimal conditions at different times were used.Pseudo-first-order kinetic models 61 , Eq. ( 5), and pseudo-second-order kinetics models 62 , Eq. ( 6), show that adsorption occurs through physical adsorption and chemical adsorption processes, respectively.
In the above equations, Q e (mg g −1 ) is the adsorption capacity at equilibrium, Q t (mg g −1 ) is the adsorption amount at time t (min), K 1 (L min −1 ), and K 2 (g mg -1 min -1 ) are the pseudo-first-order and pseudo-second-order   www.nature.com/scientificreports/rate constants, respectively.The results of the adsorption kinetics of MR dye are shown in and Fig. 14 and Table 2.
According to the results and correlation coefficients (R 2 ), the adsorption of MR by TC-POP is consistent with the pseudo-second-order kinetic model.
Thermodynamic studies.The adsorption process can be endothermic or exothermic according to the nature of the adsorbent material and adsorbed molecules.In order to investigate whether MR adsorption on POP is exothermic or endothermic, thermodynamic parameters, Gibbs free energy ΔG° (kJ/mol), entropy ΔS° (J/mol/K), and enthalpy ΔH° (KJ/mol) at several temperatures (298, 308, and 318 K) were calculated by Eqs. ( 7) and ( 8) 63 .
where R is the gas constant (8.314J mol −1 K −1 ), K c (L mol −1 ) is the equilibrium constant, and T (K) is the adsorption temperature.The obtained thermodynamic parameters are presented in Fig. 15 and Table 3.The positive values of ΔH and ΔS indicate that the adsorption process of MR using TC-POP is endothermic and leads to an increase in the degree of randomness or a decrease in the degree of orderliness of the system.Furthermore, the negative values of ΔG for all temperatures indicate that the adsorption process is spontaneous and possible.
Desorption and reusability studies.One of the vital features of an ideal adsorbent is its ability to be regenerated and reused as well as its high adsorption capacity, which makes its use in practice and industry economical and cost-effective.Therefore, several adsorption-desorption cycles were performed to investigate this feature in the synthesized polymer.To test this feature, 10 mL of MR aqueous solution (50 mg/L) was mixed with 8 mg  Pseudo-first-order Pseudo-second-order q e,cal (mg g -1 ) K 1 (1 min −1 ) R 2 q e,cal (mg g −1 ) K 2 (g mg, min −1 ) R  www.nature.com/scientificreports/ of TC-POP adsorbent and stirred for 80 min.Then TC-POP was separated from the mixture by centrifugation and washed well with ethylene glycol and deionized water, respectively, dried and used for subsequent adsorption.This process was carried out for five cycles.The results show that the removal efficiency of MR by TC-POP does not change significantly in five cycles and has a slight decrease in each step (Fig. 16).Therefore, the results demonstrate the reusability of TC-POP for MR removal.
The comparison of the maximum adsorption capacity (Q max ) of the polymer synthesized in this study and other adsorbents for the adsorption of methyl red dye is reported in Table 4.The obtained results show the ideal and acceptable adsorption capacity for TC-POP.

Conclusion
To summarize, a novel triazine and cyclohexanone-based porous organic polymer, (TC-POP), was successfully prepared via Schiff base condensation using 2,4,6-tris-(4-formylphenoxy)1,3,5-triazine (TFPT) and 2,6bis (4-aminobenzylidene)cyclohexanone.The obtained polymer shows significant physiochemical and thermal resistance, which may be due to its rich π-conjugated structure.It was used as an adsorbent to remove the organic pollutant methyl red dye from an aqueous solution under optimized conditions with excellent adsorption efficiency and acceptable reusability.Adsorption isotherms and adsorption kinetics and thermodynamic parameters were also investigated to obtain the mechanism, speed, and endothermic or exothermic nature of the adsorption process.Analysis of the results showed that the adsorption of MR on TC-POP has pseudo-second-order kinetics and a Langmuir adsorption model.Also, the calculated thermodynamic parameters show that the adsorption process is endothermic and spontaneous.The existence of oxygen and nitrogen atoms as well as aromatic rings in the polymer structure causes strong π-π interactions and electrostatic, and hydrogen bonding between it and the dye and removal of the dye from the water.After 5 times of recycling in the adsorption process, the synthesized TC-POP showed a good performance and the adsorption efficiency was slightly changed.These results can indicate the capabilities of this polymer as a reliable and ideal adsorbent for the adsorption of methyl red dye and toxic pollutants from water and wastewater.

Figure 6 .
Figure 6.The XRD spectra of the triazine and cyclohexanone-based porous organic polymer.

Figure 12 .
Figure 12.The possible mechanism of MR adsorption on TC-POP.

Figure 14 .
Figure 14.The kinetic models for MR adsorption onto the TC-POP.

Figure 16 .
Figure 16.The reusability of the POP for methyl red removal in five consecutive cycles.

Table 4 .
The comparison of the maximum adsorption capacity of MR on various adsorbents.