Design, synthesis, and characterization of a novel Zn(II)-2-phenyl benzimidazole framework for the removal of organic dyes

A novel Zn (II) organic framework comprising 2-phenyl benzimidazole (ZPBIF-1) was synthesized by using a solvothermal method. The characterization of the synthesized MOF was performed utilizing XRD, SEM, FT-IR, 1H-NMR, 13C-NMR, MS, XPS, TG/DTA, and N2 sorption analysis. ZPBIF-1 was successfully utilized to remove Acid red 88, Basic Violet 14, Basic Blue 54, and Congo red dyes in aqueous solutions. In this study, some parameters, including adsorbent dosage, initial dye concentration, contact time, temperature, and pH, were examined. To evaluate the experimental data, Freundlich, Langmuir, Temkin, and Dubinin-Radushkevich isotherm models were used. In this case, Langmuir is the most suitable model. Several kinetic models, including First-order, pseudo-first-order, second-order, and Pseudo-second-order kinetic models, Elovich's, and Weber's intraparticle diffusion models, were utilized to comprehend the detailed adsorption process. According to the pseudo-second-order kinetic model, dye sorption kinetics is best described. In addition, thermodynamic parameters like enthalpy (ΔH°), Gibbs free energy (ΔG°), and entropy (ΔS°) were also achieved and analyzed. The experimental studies thus suggest that Zn (II) metal–organic framework based on 2-phenyl benzimidazole could be a promising candidate for eliminating dyes from aqueous solution. Hence, the experimental studies suggest that a Zn (II) metal–organic framework based on 2-phenylbenzimidazole could be a promising candidate for eliminating dyes from aqueous solution. The maximum adsorption capacity of ZPBIF-1 was 1666.66, 1250, 1000, and 1250 mg/g for Acid red 88, Basic violet 14, Basic blue 54, and Congo red dyes, respectively. Furthermore, this method was used to remove contaminant dyes from textile wastewater, and an acceptable result was obtained.


1
H NMR and 13 C NMR spectra recorded on a on Bruker 250 MHz spectrometer with DMSO-d 6 as a solvent. The crystalline nature of ZPBIF-1 was analyzed by X-ray diffraction at room temperature with Co Kα radiation (model X'Pert PRO MPD, Panalytical, Made in the Netherland). The X-ray photoelectron spectroscopy (XPS) was conducted on the Bes Tec (USA) using Mg radiation. A Netzsch Thermoanalyze (Mettler Toledo, Swiss) was used for thermogravimetric analysis (TGA) with a heating rate of 50 ml/min in air. A combination of electrospray ionization mass spectrometry (ESI-MS) (5975C, Agilent Technologies, USA) was applied. The morphologies and elemental analysis were assessed by a scanning electron microscope SEM (Mira III, TeScan, Czech Republic) supplied with energy dispersive X-ray analysis (EDXA). The concentration level of remaining COD and BOD in wastewater sample was determined by the Standard Methods, using a USA-HACH spectrophotometer and Lutron DO meter. The UV-Vis spectrophotometer (Cary 60, Agilent Technologies USA) was utilized to determine the concentrations of the organic dyes at wavelengths of 497, 583, 497, and 559 nm, respectively. The specific surface area of samples was determined with Brunauer-Emmett-Teller (BET) method using the adsorption-desorption isotherms at 77 K (BEL, Belsorp mini II, Made in Japan).
Preparation of ZPBIF-1. Typically, A mixture of zinc nitrate hexahydrate (Zn(NO 3 ) 2 ·6H 2 O) (3.02 mmol, 0.9 g) and 2-phenyl benzimidazole (1.54 mmol, 0.3 g) was dissolved in methanol (35 mL), then autoclaved. Teflon-lined stainless steel (autoclave) was heated for 24 h at 90 °C in a programmable oven and cooled at room temperature at 0.4 °C/min. After the addition of chloroform to yellow liquid, the colorless polyhedral crystals of ZPBIF-1 were formed that were washed with methanol, then dried under vacuum for 6 h at 50 °C to give the desired product in 95% yield.
Adsorption experiments. The optimal conditions and influencing parameters for dye removal by ZPBIF-1 were investigated in the adsorption procedure. The Optimum adsorbent quantity was determined by combining different amounts of ZPBIF-1 (0.001-0.01 g) with dye solution (10 mL) for 20 min at 25 °C, with an initial dye concentration of 20 mg/L, and pH = 2-8 under continuous stirring at 400 rpm. A pH adjustment was achieved by adding sodium hydroxide (0.1 N) or hydrochloric acid (0.1 N) to the dye solution before combining it with the adsorbent. The effects of initial dye concentration (10-800 mg/L) were investigated using 0.008 g adsorbent at pH = 2 and 25-65 °C for 20 min. UV-Vis spectrophotometers were used to measure dye solution concentrations and adsorption. The dye solution concentration followed by adsorption was analyzed employing a UV-Vis spectrophotometer. Afterward, the amounts of dye adsorbed at different times (qt, mg/g) and dyes' removal efficiency were determined as follows: which, C e , C 0 , and C t (mg/L) were the equilibrium concentration, initial concentration, and concentration at different times, respectively. V was the volume of dye solution, and m (g) was the mass of ZPBIF-1. As part of the kinetic studies and optimization of adsorption capacity, the adsorbent dosage (0.01, 0.02, 0.06, 0.08, and 1 g/L) and dyes concentration (10-800 mg/L) were selected. The same procedure was repeated at 308, 318, and 338 K to assess the effects of temperature on dye adsorption.
Recyclability and stability study. The Efficiency of ZPBIF-1 as a selective dye adsorbent from waste water can be leveraged by its superior recyclability. Recycling was performed on ZPBIF-1 using optimized conditions such as dose of 0.08 g/L, dye concentration of 20 mg/L, pH 2, contact time of 20 min, and agitation rate of 400 rpm. The major recycling procedure can be described as follows: to organic dye-loaded ZPBIF-1 (0.008 g) from the first adsorption experiment, NaOH 0.1 M (10 mL) and ethanol %50 (10 mL) were added and mixed for 20 min. Following filtration and washing with H 2 O/EtOH, samples were dried and reactivated at 50 °C under vacuum for 60 min and used for reusability studies. UV-Vis spectroscopy was used to check for residual dye traces.

Results and discussion
Characterization of ZPBIF-1. A new zinc metal-organic framework, Tetrakis (2-phenylbenzoimidazol-1-yl) zinc (ZPBIF-1), was synthesized by autoclaving 2-phenyl benzimidazole and Zn (NO 3 ) 2 in methanol. The structure of ZPBIF-1 was characterized by FT-IR, 1 H-NMR, 13 C-NMR, and mass spectral data. The FT-IR spectrum of ZPBIF-1 exhibited the characteristic peaks of Zn moieties at 600/cm and aromatic moieties at 620-826/ Due to the symmetry of this compound, 1 H-NMR and 13 C-NMR only revealed peaks for one of the four ligands. Therefore, the 1 H NMR spectrum of ZPBIF in DMSO showed two doublet-doublet signals (AX system) in δ = 7.51 and 7.80 ppm for CHs-aromatic of benzimidazole ring along with characteristic signals for phenyl group at 2-position of imidazole moiety. The proton-decoupled 13 C-NMR spectrum of ZPBIF-1 in DMSO showed 8 distinct signals related to 28 CH and 24 C in agreement with the proposed structure. It showed one distinct resonance for the N-C=N group. Partial assignments of aromatic and imidazole resonances were given in the experimental report. C 52 13  The mass spectra of compound represented molecular ion peak at proper m/z value and the base peak appeared at m/z 194. EI-MS: m/z (%) = 836 (M+, 5), 194 (100), 193 (30), 116 (10), 104 (7), 91 (8), 90 (13), 77 (15). The IR, 1 HNMR, 13 CNMR, and Mass spectra for Zn (II)-2-phenyl benzimidazole framework (ZPBIF-1) are provided in the Supplementry file 5.
According to the diffraction pattern and comparison with similar composition ZIF-11standard card, it concludes ZPBIF-1 crystals with ideal inner structures had been successfully constructed. The simulated PXRD pattern of ZIF-11 and XRD pattern of Zn(II)-2-phenyl benzimidazole were shown in Fig. 2a,b, respectively. In the Fig. 2b pattern, the crystalline nature of the MOF structure crystallized at cubic crystal system was obtained at 2θ = 9. 14, 9.51, 10.86, 11.99, 13.43, 13.69, 15.88, 16.24, 16.76, 17.27, 18.80, 19.10, 20.24, 21.62, 24.99, 25.41, 26.11, 26.75, 32.01, 32. Metal-organic frameworks (MOFs) are hybrid materials comprising organic linkers and metal ions with high porosity and crystallinity. To study the morphologies, SEM images of the ZPBIF-1, investigated at the high magnifications, show regular rods with the hexagonal face (Fig. 3). Furthermore, it suggests the formation of ZPBIF-1 with homogeneous distribution throughout the MOF. The average size of 40.08 ± 4.62 µm, Energydispersive X-ray spectroscopy (EDAX) analyses (Fig. 4)  A XPS spectrum of the new MOF was prepared in order to determine its oxidation states and chemical bonds. According to Fig. 5a, the presence of constituted elements (N 1 s, C 1 s, and Zn 2p) was proved. As seen in Fig. 5b, high-resolution XPS of the C 1 s spectrum was explained by three well-fitted curves, as well as their respective binding energies. C-C/C-H (saturated carbons), C=N (unsaturated ring), and C-N bonds cause prominent peaks at 284.60, 285.30, and 286.40 eV. Figure 5c confirms the existence of coordination Zn-N bonds at 401.30 eV. N=C and N-H may be responsible for other curves at 400.50 and 399.29 eV, respectively 32,33 . Ultimately, the oxidation states of zinc elements: Zn 2+ 2p 1/2 and Zn 2+ 2p 3/2 could be proved at notable peaks at 1045.66 and 1023 eV, respectively (Fig. 5d).
To investigate the thermal stability of the prepared catalyst, derivative TG (DTG) and thermogravimetric analysis (TGA) were carried out. Thermal degradation assessments of solid materials are essential since several www.nature.com/scientificreports/ applications are based on their thermal stability. The result of the most significant property in the TGA for the ZPBIF-1 was that slight weight loss was found between the temperature range of 100-230 °C, revealing that the ZPBIF-1 was stable up to 400 °C (Fig. 6). It was indicated that a sharp weight-loss step of over 33.1602% was found by incrementing the temperature from 302.50 to 800 °C. This reveals the thermal decomposition of the ZPBIF-1 in that temperature range. This result of TGA confirmed the applicability of the ZPBIF-1 across various temperatures. The TG analysis of ZPBIF-1 revealed that the MOF was almost constant up to 400 °C with a bit of mass loss (12%). Then a fast and vast weight loss happened from 302.50 to 800 °C, probably due to the exothermic decomposition of ligand (2-phenyl benzimidazole) in these temperatures. N 2 adsorption-desorption analysis was employed for obtaining total pore volume (V m ) and specific surface area of sample. Langmuir surface area and V m of the ZPBIF-1 were 370.73 m 2 /g and 85.176 cm 3 /g, respectively (The related diagram has been shown in Supplementary file 6).
Effect of pH on dye adsorption. In dye adsorption, pH is crucial since it determines the degree of ionization of the adsorptive molecules and the surface charge of the adsorbent. The impact of initial solution pH According to Fig. 7, the highest adsorption capacity occurred at pH = 2 for all the dyes containing Acid red 88, Basic violet 14, Basic blue 54, and Congo red. As the solution pH increases from 2 to 8, the adsorption capacity q t (mg/g) decreases for Acid red 88 (an anionic dye). For the other dyes, the pH increase does not significantly reduce the adsorption capacity. This may be due to a large number of functional groups in dyes and the different interactions such as hydrogen bonding, dipole-dipole forces between dye and adsorbent 34,35 .
Effects of contact time. The effect of contact time on ZPBIF-1 adsorption capacity and dye removal percentage was evaluated under optimal conditions (adsorbent dosage 0.08 g/L, pH = 2, initial dye concentration 20 mg/L, and 25 °C). In this study, the removal efficiency of all dyes was determined as a function of contact time ranging from 10 to 60 min. Based on the results the percent removal and adsorption capacity of the ZPBIF-1 first increase significantly, then increment at a relatively slow speed with the contact time until equilibrium is reached. The most significant adsorption efficiency was obtained after 20 min of dye contact with ZPBIF-1. It is ascribed to the fact that many vacant surfaces are accessible for adsorption at the beginning. After a while, it becomes harder and harder to find the remaining vacant active sites. Hence, in this study, 20 min was selected as the equilibrium contact time.
Effect of ZPBIF dosage. Adsorbent concentration is an essential parameter because it indicates the most adsorbent capacity for an initial dye concentration. The optimal amount of ZPBIF-1 to adsorb dyes was determined by adsorption tests performed using 0.001-1 g/L adsorbent at pH = 2, 25 °C, and initial dye concentration (20 mg/L) for 20 min. Figure 8 shows that 100, 91, 86.8, and 85.87% of Acid red 88, Basic violet 14, Basic blue 54, and Congo red were removed from their aqueous solutions when 0.001 g ZPBIF-1 was used as an initial dosage. The best dosage of ZPBIF-1 found to remove Congo red was 0.008 g, while the dosages for Acid red 88, Basic violet 14, and Basic blue 54 were 0.001, 0.006, and 0.006 g, respectively. By increasing the dosage of ZPBIF-1 by 0.008 g, the removal percentage of all dyes increased and ultimately reached 100% for each dye. Further increase of the adsorbent dosage did not influence the removal of dyes, so the dosage of 0.008 g was selected. Additionally, Fig. 8 discusses the removal rate (R%), which increases with incrementing adsorbent amount until achieving equilibrium. This can be explained by an increase in the adsorbent's specific surface area and the availability of more active sites to adsorb dye. In this study, the removal rate of dyes was 100% at 0.008 g of ZPBIF-1, which was considered the optimal adsorbent dose.
Effect of temperature. The temperature is the main adsorption parameter that could play an essential role in the adsorption process. The temperature severely affects the solution viscosity and thus the adsorption capacity. Therefore, it is necessary to determine the optimum temperature for the adsorption procedure. At the initial concentration of dye 20 mg/L, the effects of temperature on the adsorption of ZPBIF-1 were investigated under optimal conditions (pH = 2, initial dyes concentrations 20 mg/L, stirring time 20 min). Figure 9 showed removal efficiencies for Acid red 88, Basic violet 14, Basic blue 54, and Congo red as a function of temperature ranging between 298 and 323 K. Also, the effect of temperature (298, 273,313, and 323 K) on the adsorption capacity of dye was represented. It is evident that the increase in temperature negatively impacted the removal of dyes. The optimal temperature for the removal of dyes by the ZPBIF-1 was 25 °C. www.nature.com/scientificreports/ Effects of initial dye concentration. The initial dye concentrations play a critical role in overcoming dye mass transfer resistance between the aqueous and solid phases 36 . Therefore, increasing initial dye concentrations would increase dye's adsorption capacity. On the other hand, a dye molecule can travel from bulk liquids to the outside surface of adsorbent, indicating that the adsorption of dye onto adsorbent is dependent on the concentration of dye molecules at the time of adsorption. Therefore, at low initial concentrations the percentage removal of dye is greater, but at high initial concentrations it is smaller. In this study, the effects of dye concentra-  Adsorption isotherms. During the adsorption procedure, equilibrium isotherms were used to determine the relationship between the dyes adsorbed (Q e , mg/g) on ZPBIF-1 and the equilibrium concentration of the solution (C e , mg/L). The optimal conditions for dye adsorption were an adsorbent dosage of 0.08 g/L, pH = 2, and an equilibrium time of 20 min. Generally, adsorption isotherms present valuable information about surface properties, the tendency of adsorbents, and the adsorption mechanism 37 . In this study, Langmuir and Freundlich, Temkin, and Dubinin-Radushkevi isotherms were employed to describe the equilibrium of dyes adsorbed  www.nature.com/scientificreports/ by ZPBIF-1. Among them, the Langmuir adsorption isotherm was the most suitable model. According to this model, adsorption occurs within homogeneous sites over the absorbent without any adsorbent transmission through the surface. The Langmuir isotherm equilibrium looks as below 38 : In which, C e (mg/L) and q e (mg/g) represent the dye concentration in solution and the quantity of dye adsorbed per unit mass of ZPBIF-1 at equilibrium, respectively. Q max (mg/g) represents the maximum adsorption capacity, as well as b (K L ) (L/mg) describes the Langmuir constant for the adsorbent and adsorbate requirements. In the Langmuir model, the isotherm constants (K L , Q max , and R 2 ) were found through linear plotting of 1/q e against 1/C e (Fig. 11).  www.nature.com/scientificreports/ It is indicated that the Langmuir model can acceptably provide the adsorption isotherm data. The Langmuir isotherm's fundamental features are communicated based on equilibrium parameter (R L ) or dimensionless constant separation factor as: In which C 0 (mg/L) is the most effective initial concentration of the adsorbent. The worth of R L demonstrates the kind of the isotherm as linear (R L = 1), unfavorable (R L > 1), irreversible (R L = 0), or favorable (0 < R L < 1). Based on Table 1, all the R L values were between 0 and 1, representing the favorable adsorption of dyes by ZPBIF-1. The utmost monolayer adsorption capacity Q max was 1666.66, 1250, 1000, and 1250 mg /g for dyes, respectively.  www.nature.com/scientificreports/ Freundlich isotherm relies on reversible, monolayer, and non-ideal adsorption, which might be accustomed to multiple-layer adsorption on heterogeneous surfaces with disparate affinities and adsorption energy. This model could even be represented as: 1/n and K F are Freundlich constants associated with the adsorption intensity and adsorption capacity per unit concentration, respectively. The n and K F are determined from the slope and axis intercept of the plot ( Table 1). The value of n denotes the adsorption favorability. The character of adsorption is favorable when n ˃ 1. Equation (6) can be reset in linear form as: The linear plot of log q e against log C e was used to investigate the Freundlich isotherm of dye removal. The value of n in all cases was more significant than 1, indicating calm adsorption. The Temkin isotherm model assumes that the adsorption heat linearly diminished with expanding the coverage of the absorbent surface. One of the properties of this model is a uniform distribution of the binding energy. This isotherm can be written in linear form as 37 : in which b T is that the Temkin constant represents the sorption point, (kJ/mol), and K T is the equilibrium binding constant associated with the foremost energy (L/g). T is the temperature (K), and R is the gas constant (8.314 J/mol K). The Temkin isotherm compatibly with dye adsorption data through ZPBIF-1 was investigated by the linear plot of q e against C e . Table 1 shows the isotherm constants (K T , R 2 , and B T ). These values are but the Langmuir, and Freundlich values reveal that the Temkin isotherm features worse experimental data compared to the other isotherms.
The Dubinin-Radushkevich (DRK) isotherm is used to formulate the adsorption mechanism over a heterogeneous surface with a Gaussian energy distribution 39 . The empirical form is given by the Eq. (8). The DRK isotherm constant ε is obtained using Eq. (9). The model is mainly used to differentiate between physical and chemical adsorption of metal ions with its mean free energy E per adsorbent molecule as: ln q e = ln(q s ) − K ad ε 2 ε = RT ln 1 + 1 C e www.nature.com/scientificreports/ here, q e is the amount of adsorbate (mg/g); q s shows the theoretical saturation capacity (mg/g); K ad and B DR are the DRK isotherm constants (mol 2 /kJ 2 ), and ε is the DRK isotherm constant. The gas constant (8.314 kcal/ mol K), adsorbate equilibrium concentration (mg/L), and absolute temperature (K) are represented by R, Ce, and T, respectively. The parameters of the models' isotherms are shown in Table 1. Based on the correlation coefficients (R 2 ) provided by the isotherm models, the Langmuir isotherm possesses the maximum correlation indicating dye adsorption on a homogeneous surface.
In Table 2, the adsorption capacity of ZPBIF-1 was 1666.66, 1250, 1000, and 1250 mg/g, substantially more signicant than the values reported for most other adsorbents. The superior adsorption properties of ZPBIF-1 are attributed to its unique structure, as described below.
1. ZPBIF-1 nanoparticles are distributed uniformly. It not only has adsorption sites by Zn, but also provides organic framework sites for adsorption. 2. It was found that ZPBIF-1 has a much higher surface area than simply agglomerated ZIF-8 nanoparticles 3. Due to the aromatic rings of the ligands of 2-Phenyl benzimidazole, ZPBIF-1 creates an π-π interaction with dyes.
The higher adsorption capacity of this adsorbent is related additionally to the porous structure, their great specific extent, and more active adsorption sites. It's concluded that the ZPBIF-1 possesses a relatively higher adsorption capacity representing its feasibility, and applicability as a low-cost, convenient, and effective adsorbent in the adsorption method to urge obviate dyes.
Adsorption kinetics study. The prediction of sorption rate is a key factor in designing batch adsorption procedures, and it is affected by adsorption kinetics. Chemical kinetic models are orientated by the adsorbent's chemical and physical properties, and their assessment is critical to providing insightful information about the factors influencing the reaction speed 53 . The experimental conditions were conducted at the enhanced points as solution pH = 2, the adsorbent dosage of 0.08 g/L, and initial dye concentrations of 50 and 250 mg/L. Furthermore, six adsorption models were introduced in this study.  Using the best-fit line, the coefficient value (R 2 ) was determined ( Table 3). The pseudo-first-order kinetic model can be integrated in terms of the solid capacity for sorption analysis as: Equation (12) represents a linear plot of ln (q e − q t ) against time (t), denoting a pseudo-first-order. qt (mg/g) shows the absorption quantity at time t, while ks 1 denotes the constant of the equilibrium rate pseudo-firstorder kinetic (/min). In the plot of ln (q e − q t ) against t, the straight line represents this kinetic model sufficiency presenting an association between the intercept and slope. The model parameters (ks 1 and estimated q e ) can be determined, respectively. The kinetic constants (ks 1 , R 2 , and (q e ) Cal .) are represented in Table 3.
Second-order reaction model. The integrated second-order rate equation is extensively utilized for the adsorption process, which can be represented mathematically as Eq. (13): It shows that 1/C t against time (t) is a linear plot, which indicates that the reaction rate is second order.
(11) lnC t = lnC 0 − k 1 t (12) ln q e − q t = lnq e − ks 1 t www.nature.com/scientificreports/ The pseudo-second-order model adsorption kinetic depends upon the solid adsorption capacity. Moreover, contrary to other models, it can estimate the absorption performance throughout the absorption range. The linear form of the model is: which, q eq and q t represent the numbers of dye adsorbed (mg/g) at equilibrium and time t, respectively; K 2 shows the pseudo-second-order kinetic model's equilibrium rate constant (g/mg/min). Linear plotting t/q t against t was used to investigate the compatibility of pseudo-second-order kinetics on dye removal at various adsorbent dosages from single systems. The value of q eq is obtained from the slope of the t/q t against t in the plot (Fig. 12), and K 2 is determined from the initial sorption rate value. Results showed that the adsorption process conformed better to the pseudo-second-order model. There was a linear plot with the highest correlation coefficients in this model (Table 3). Considering the coefficient of the determinant of the value, the pseudo-second-order reaction kinetic model was followed by the adsorption processes well compared to the other three kinetic models.
Adsorption is often a multi-stage process, including transferring aqueous solute molecules to the solid particles' surface, where they penetrate the pores (slower step). The diffusion mechanism is not determined by first and second-order kinetic models. Hence, it is preferred to use the intraparticle diffusion model to evaluate the results of kinetics as follows 56 : www.nature.com/scientificreports/ where Kp (mg/g min 1/2 ) denotes the intraparticle diffusion rate constant, and I is the intercept. The intraparticle diffusion kinetics constants (R 2 , I, and K p ) attained by linear plotting of q t against t 1/2 are presented in Table 3.
Another kinetic model equation in terms of the adsorption capacity is the Elovich equation by Eq. (16) 57 : α and β respectively represent the Elovich constants denoting the initial adsorption rate (g/mg min) and the desorption constant (mg/g min). The Elovich constants are attained from the graphs of q t versus ln t (Table 3). According to the results from Table 3, the Pseudo-second order model is the best fitting model representing the higher correlation coefficient Acid red88, Basic violet14, Basic blue54, and Congo red, R 2 = 50 mg/L (0.9798, 0.9926, 0.9979, 1.0), and 250 mg/L (0.9994, 1.0, 0.9999, 1.0) for the adsorption of dyes by ZPBIF-1.
Thermodynamics study and effects of temperature. Due to the importance of temperature in the adsorption process, its impacts on the adsorption of dyes on ZPBIF-1 were examined at 24, 45, and 65 °C, and pH = 2 and 100 mg/L of initial dye concentration. The thermodynamic parameters such as entropy (ΔS°), Gibbs free energy (ΔG°), and enthalpy (ΔH°) were determined as: where T represents the temperature in Kelvin, and R is the gas constant (8.314 J/mol K). K d shows the distribution coefficient determined by Eq. (17). C 0 denotes the initial dye concentration in solution (mg/l), m represents the adsorbent mass (g), V is the solution volume, C e is the equilibrium concentration (mg/l), and Q e is the quantity adsorbed per unit adsorbent mass at equilibrium (mg/g). Entropy (ΔS°) and enthalpy (ΔH°) were computed from the intercept and slope of the Van't Hoff plot of ln q e /C e versus 1/T. To calculate the value of Gibbs free energy (ΔG°), Eq. (18) was used ( Table 4).
The negative value of ΔH° represents the exothermic adsorption of Basic Violet14, Basic Blue 54, and Congo red onto ZPBIF-1. Moreover, the negative value of ΔS° suggests the reduction of randomness at the solid-solution interface during the adsorption procedure 55 . During the procedure, the mobility of dye molecules was increased by increasing the temperature, which caused them to escape from the solid to the liquid phase. Acid red 88 exhibited endothermic properties during its adsorption process, as indicated by its positive value of ΔH°. However, randomness increased at the solid-solution interface, as shown by its positive value of ΔS°. The negative values of Gibb's free energy ΔG° represent the spontaneous adsorption process for all dyes.
Adsorption mechanism. Different interactions are responsible for the adsorption mechanism of organic dyes on the ZPBIF-1 surface. The electrostatic attractions between the positively charged Zn groups occurred on the ZPBIF-1 surface with negatively charged sulfonate (SO 3 -) groups of Acid red 88 and Congo red dyes. Molecules were readily adsorbed on the positively charged ZPBIF-1, and the open sites within the ZPBIF-1 were www.nature.com/scientificreports/ simply complexed with NH 2 . This was probably due to a soft-soft interaction of the metal ions of ZPBIF-1 with the amino groups of the dyes 58 . Furthermore, there may be an π-π attraction between the dye molecules and phenyl benzimidazole ligands 59 . In total, three main types of interactions were considered in the adsorption mechanism, including π-π attraction, electrostatic interactions, and metal coordination. The proposed adsorption mechanism was presented as a schematic diagram (Fig. 13).
To demonstrate the interactions between dyes and ZPBIF-1, the FTIR spectra of adsorbent before and after adsorption of Congo red were compared. For example, the peak at 1388.78/cm in ZPBIF-1 was indicative of C-N vibration, which changed enormously after adsorption and became a dual peak in 1400-1460/cm. The peaks at 697.63/cm and 750.70/cm in ZPBIF-1 represented Z-N and C=C vibrations were reduced to 650.50/cm and 744.20/cm after adsorption of Congo red. Moreover, the peaks at 1061.54, 1178.78, and 1224.99/cm for S=O in Congo red were changed after adsorption on the ZPBIF-1 surface.
SEM images of ZPBIF-1 after adsorption of Acid red 88 are shown in Fig. 14, which can compare to the unused ZPBIF-1 (Fig. 5). The morphology of the unused ZPBIF-1 showed regular rods with hexagonal faces. However, its surface after adsorption of Acid red 88 dye was morphologically irregular, which is not surprising since the dye molecules adsorbed on the ZPBIF-1 surface caused morphological changes. These results showed that ZPBIF-1 could adsorb Acid red 88 molecules well.
XRD patterns of ZPBIF-1 composites before and after dye adsorption showed similar peak shapes, revealing the failure of adsorption to alter the crystalline form of ZPBIF-1.
Regeneration and recycling of ZPBIF-1 nanocomposites. In large-scale functional applications, recyclability and stability of adsorbents play an important role in reducing the material cost of water treatment. The practical potential for adsorbents can be evaluated substantially based on their recyclability. Here, the ZPBIF-1 adsorbent was recovered and used in 3 cycles for dye removal. The results of the reusability of ZPBIF-1 are demonstrated in Fig. 15. It should be noted that even after 3 cycles, the percent removal was above 80%.
Removal of dyes from wastewater sample. The applicability of the developed adsorbent was evaluated using a textile wastewater sample. The wastewater sample was filtered using a 0.45-mm pore size membrane www.nature.com/scientificreports/ filter to remove suspended particulate matter. One solution containing 50 mg/L of acid red 88 in wastewater was prepared as an actual sample. Then, 0.016 g of ZPBIF-1 was added to 100 mL wastewater at pH = 2. The mixed solution was shacked for 20 min. Because of the matrix effect, the tests were performed through the standard addition method, and the initial and final concentrations of the target species (before and after removal with the recommended procedure) were determined. For this aim, four 25 mL volumetric flasks were each filled with 20 mL of wastewater samples. Then, the different amounts of the standard acid red 88 solutions (0, 0.25, 0.5, and 0.75 mL of 1000 mg/L) were added, and the solutions in the flasks were diluted to the mark and mixed well. The amounts of dye in the solution were determined by the UV-Vis spectrophotometry method, and the absorption versus standard concentration was plotted. A simple Linear Least Squares analysis was conducted using the slope and intercept functions of Microsoft Excel. To find the initial concentration of the unknown, the value of X at y = 0 from y = mX + b was calculated. As shown in Table 5, the proposed method could be applied successfully for   www.nature.com/scientificreports/ the removal of acid red 88 from an actual sample of textile wastewater with acceptable efficiency. ZPBIF-1 had a removal efficiency of 92.9% for the real sample. Biodegradability, as well as biological oxygen demand (COD and BOD), were examined on an actual wastewater sample. The COD level decreased from the initial concentration of 1910-63 mg/L. Similarly, the BOD level gradually reduced from the initial concentration of 810-27 mg/L. Based on the acceptable amounts for irrigation and agricultural uses, 200 and 100 mg/L, respectively, the performance of this adsorbent has been evaluated very well, and the treated effluent can be reused. As a result, during the adsorption process, the organic portion of the dye molecules may be destroyed, decomposed, and even adsorbed.

Conclusion
A new MOF, Tetrakis (2-phenylbenzoimidazol-1-yl) zinc (ZPBIF-1), was successfully synthesized by the solvothermal method. According to the results of response surface methodology-based optimization, the optimal conditions to eliminate Acid red88, Basic violet14, Basic blue54, and Congo red were Co = 20 mg/L, contact time = 20 min , adsorbent dosage = 0.08 g/L, and pH = 2. In comparison with other adsorbents, ZPBIF-1 showed superior adsorption capacity at 1666.66, 1250, 1000, and 1250 mg/g for the above dyes. The adsorption kinetics followed the pseudo-second-order models for all organic dyes. In the meantime, according to the adsorption isotherms, all colors follow the Langmuir model. The exceptional adsorption properties of ZPBIF-1 can be attributed largely to the unique structure and interaction of active functional groups, such as electrostatic interaction, hydrogen bonding, π-π conjugation, and coordination effect of zinc. The thermodynamic parameters showed that adsorption on ZPBIF-1 was spontaneous and thermodynamically favorable. The dye adsorption for Basic violet 14, Basic blue 54, and Congo red by ZPBIF-1 was exothermic, while the dye adsorption for Acid red 88 was endothermic.
ZPBIF-1's unique properties, such as its high recycling capacity, the number of aromatic rings, high removal efficiency, and maximum adsorption capacities (1666.66-1250 mg/g) make it an effective dye adsorbent that can be used for the removal of dye from wastewater.

Data availability
All data generated or analyzed during this study are included in this published article and its supplementary information files (Supplementary files 1-6). Requests for material and additional data should be made to the corresponding authors.