Introduction

Water pollution has drastically increased in the last few decades due to the rapid growth of urbanisation and industrialization [1, 2]. Chemical industries such as textile, printing, paper, pharmaceutical, food, leather, photography, paints etc. are dumping tons and tons of untreated water to nearby lakes, rivers and sea and thereby accelerates the manmade water pollution [2,3,4,5]. Textile industries are one of the major contributors of water pollution and dispose large amount of toxic dyes to water bodies [6, 7]. Dyes are organic compounds which impart colour to the materials such as fabrics, paper, leather, food stuff etc. [8]. Based on their nuclear structures, dyes are classified into anionic, cationic and non-ionic dyes. Among them, cationic dye is known to be more toxic [8,9,10].

Methylene blue (MB) is a sapphire-coloured cationic dye which belongs to phenothiazine family (Fig. 1). It has been used as colorant, biological stain and redox indicator. However, due to its carcinogenic and mutagenic nature MB is considered as a harmful water pollutant [11, 12]. Hence researchers have implemented various physical, chemical and biological treatment methods for removing MB from water [13, 14]. Adsorption is one of the frequently used methods for removal of MB from aqueous solution. The interest in this technique mainly comes from its salient features such as low-cost, simplicity and easy handling [15,16,17]. There are several reports of using different adsorbent for the removal of MB from water [18,19,20,21]. For instance, Mouni et al. [22] reported that Algerian kaolin is an effective adsorbent for the removal of MB. The superior adsorption of kaolin is attributed to its high surface area. Huang et al. studied the removal efficiency of MB using graphene oxide modified zeolite [23] and kaolin [24]. The authors reported that the introduction of graphene oxide (GO) into zeolite/kaolin increases the active site on the composites and consequently improves its adsorption performance. Rida et al. [25] studies revealed that kaolin is a better adsorbent than zeolite for removing MB from aqueous solution. The high adsorption capacity (96%) of clay is attributed to its high surface area and pore diameter. Cao et al. [26] reported high MB removal efficiency and good regenerability of porous chitin. Aysan et al. [27] and co-workers employed chabazite to remove MB from aqueous environment and propose it as an excellent adsorbent for MB. Mayab et al. [28] successfully employed a low-cost bio adsorbent (walnut shell powder) to remove MB from aqueous solution.

Fig. 1
figure 1

Structural formula of methylene blue dye

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Both natural and synthetic adsorbents have been successfully employed for the removal of dye from the aqueous effluents. Nevertheless, most of the adsorbents are expensive, toxic, non-biodegradable, poor in regeneration capability and selectivity. Therefore, finding a cost effective and biodegradable adsorbent is a relevant area of research [29,30,31,32]. Zeolites are naturally occurring crystalline aluminosilicate mineral with porous structure. Owing to its high surface area, pore volume and pore diameter, zeolites have been increasingly studied for the liquid adsorption of dissolved pollutants in water [33,34,35,36]. The characteristic properties of zeolites such as pore size, pore volume and surface area could be modified by different templates starch, cellulose, chitosan, agricultural waste etc. [37,38,39,40]. Previous studies indicate that uniform pores and well-defined surface area in modified zeolite is responsible for enhancing its adsorption efficiency [41,42,43] reported high dye removal efficiency of mesoporous ZSM-5 zeolite synthesized through templating method. Meanwhile, [44, 45] examined the dye removal ability of natural and modified zeolites. The adsorption studies suggest that modified zeolite is superior to unmodified zeolites.

In this context, it is worthwhile to develop a modified zeolite for the adsorption of MB dye. We have chosen poly (diallyl dimethyl ammonium chloride) (PDADMAC), bio polymer as template to generate mesoporosity in the zeolite system. The samples were characterized by Fourier-transform infrared spectroscopy, scanning electron microscopy, transmission electron microscopy, X-ray diffraction, N2 adsorption analysis, thermogravimetric and 27Al Nuclear Magnetic Resonance. The influence of different parameters such initial dye concentration, zeolite dosage, contact time, pH and temperature was investigated. The adsorption isotherm and kinetics of adsorption is also discussed.

Experimental Section

Materials

Poly (diallyl dimethyl ammonium chloride) solution [(PDADMAC); average Mw 200,000–350,000)], tetrapropyl ammonium hydroxide [(CH3CH2CH2)4NOH; TPAOH], tetraethyl orthosilicate (C8H20O4Si; TEOS) and aluminium isopropoxide (C9H21AlO3; AIP) were purchased from Sigma Aldrich Co. Ltd (India). Methylene blue ((MB, C16H18ClN3S) used were procured from Merck, and were used without further purification.

Synthesis of Hierarchical Zeolite

The modified ZSM-5 zeolite was synthesized by employing hydrothermal crystallisation route. In a typical procedure, TEOS (3.46 g) and AIP (0.03 g) were mixed with stirring (300 rpm) to obtain a clear solution. To this, TPAOH (2.11 g) and PDADMAC (0.2 g) in aqueous solution was added and stirred for a whole day with controlled stirring (300 rpm), followed by rotavaporisation at 80 °C for 20 min. The transparent sticky solution thus obtained is transferred into autoclave and kept at 180 °C for 24 h. The synthesized zeolite was filtered, washed with water and later dried at 100 °C. Finally, the product was calcined in a muffle furnace at 550 °C for 5 h to remove the meso and micro templates from the framework. The PDADMAC modified ZSM-5 zeolite so obtained has been designated as PZSM-5. For comparison studies, we synthesized conventional ZSM-5 without adding meso template, and are denoted as ZSM-5 [40, 46,47,48,49]. Schematic representation of micro/mesopores formation in modified ZSM-5 zeolite is shown in Scheme 1.

Scheme 1
scheme 1

Formation of micro/mesopores in modified ZSM-5 zeolite

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Characterization

X-ray diffraction (XRD) patterns were obtained with a Rigaku Miniflex 2200 diffractometer using CuKα radiation. Scanning electron microscopic images were obtained by using a Hitachi SU6600 Variable Pressure Field Emission Scanning Electron Microscope (SEM). FT-IR spectrum was recorded in the range of 400–4000 cm−1 at room temperature using an FT-IR spectrometer (Jasco 4700). Thermogravimetric (TG) analysis of the uncalcined zeolite samples was done using a TGA (Instrument Q50) at a heating rate of 10 °C/min in nitrogen atmosphere. BET surface area and pore size distributions were measured using a Micromeritics Gemini V-2380 surface area analyser. Prior to the experiment, the samples were degassed at 200 °C under vacuum.27Al MAS NMR spectra were recorded on a Bruker Avance AV 300 spectrometer. Transmission electron microscopic images (TEM) were obtained with a JEOL JEM-2100 transmission electron microscope operated at an accelerating voltage of 200 kV.

Adsorption Experiment

In the present work, we have compared the adsorption property of modified ZSM-5 zeolite with conventional zeolite. Adsorption studies were conducted by using 0.1 g of the modified/conventional ZSM-5 zeolite in 50 ml of MB solution. After regular time intervals, the MB solution was withdrawn and the absorbance of the solution was measured using UV–Vis spectrophotometer (Shimduzu 2500) at λmax of 665 nm. The amount of MB adsorbed on the zeolite is calculated by the following equations.

$$\frac{\mathrm{q}=({\mathrm{C}}_{\mathrm{o}}-{\mathrm{C}}_{\mathrm{e}})\mathrm{V}}{\mathrm{W}}$$
(1)
$$\mathrm{R}(\mathrm{\%})=\frac{{\mathrm{C}}_{0}-{\mathrm{C}}_{\mathrm{e}}}{{\mathrm{C}}_{0}}\times 100$$
(2)

where, C0 is the initial MB concentration in liquid (mg L−1), Ce is the equilibrium MB concentration in liquid (mg L−1), V is the volume of MB (L) and W is weight (g) of the zeolite.

The effect of adsorption parameters such as initial MB concentration, zeolite dosage, contact time, pH and temperature, were also investigated and is discussed below.

Regeneration of Adsorbents

An adsorbent with good regeneration capacity is always preferred for practical applications. Here we checked the regeneration ability of the modified zeolite for six recycle runs. For recycling, the dye adsorbed zeolite was soaked in HCl: ethanol mixture for few hours, washed several times with distilled water and dried at high temperature in hot air oven. The dried adsorbent was further used for next adsorption–desorption cycle [23].

Result and Discussion

Fourier-transform infrared spectra of conventional ZSM-5 and PDADMAC modified ZSM-5 zeolite are illustrated in Fig. 2 (I). A broad absorption peak appeared at 3800–3400 cm−1 is due to Si–OH and Al–OH groups in zeolite. In addition to this, we can also observe distinct peaks at 1225 cm−1 (external asymmetric stretch), 1100-1050 cm−1 (internal asymmetric stretching), 795 cm−1 (external symmetric stretching), 549 cm−1 (asymmetric stretching of double five-membered ring of MFI-type zeolites) and 450 cm−1 (internal tetrahedral bending) [50]. It is evident from Fig. 2 (1) that the spectra of conventional and PDADMAC modified ZSM-5 zeolite is identical and indicates the successful formation of MFI framework. The FTIR result also indicate that the added template (PDADMAC) does not distort the characteristic MFI structure of zeolite [51]. The XRD diffraction pattern of conventional and PDADMAC modified ZSM-5 are displayed in Fig. 2 (II). It can be clearly seen that both samples displayed characteristic peak at 2θ = 7.98°, 8.82°, 14.82°, 23.14°, 23.96° and 24.44° attributed to the reflection from.[011], [020], [031], [051], [303] and [313] plane of zeolites [52]. Absence of new diffraction peaks in PDADMAC modified ZSM-5 samples confirm the successful development of MFI structure in the sample. However, the crystallinity of PDADMAC modified zeolites is found to diminish slightly probably due to the disruption of the order structure of zeolite by PDADMAC or due to the presence of mesopores in the system (Noor et al. [53]). This result where in accordance with previous studies where the authors report a decline in intensity with the addition of templates [54]. Low crystallinity in the zeolite structure is advantageous for large molecular reactions as it improves the pore connectivity and facilitates the diffusion of large molecules.

Fig. 2
figure 2

I FTIR spectra and II XRD pattern of a CZSM-5 b PZSM-5

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The micro/meso structure of zeolite were analysed by SEM and TEM analysis. Figure 3 show the SEM images of conventional and PDADMAC modified ZSM-5. Conventional ZSM-5 displayed uniform surface with micropores while a rough surface with non-uniform distribution of micro/mesoporosity is clearly visible in the SEM micrograph of PDADMAC modified zeolite. TEM images is complimentary to SEM results and supports the formation of mesopores in PDADMAC modified zeolite. A well-defined lattice fringes seen in the SAED pattern of PDADMAC modified samples implies good crystallinity (Fig. 4) [55]. The thermogravimetric curves of conventional and PDADMAC modified ZSM-5 are shown in Fig. 5. Conventional ZSM-5 zeolite shows two thermal degradation at 100 and 360 °C, corresponding to the loss of water and structure directing agent (TPAOH) respectively. However, in the case of PDADMAC modified zeolite an additional weight loss at 290 °C is also observed. This is attributed to the loss of mesotemplate (PDADMAC) from the system. The high weight loss of PDADMAC modified ZSM-5 (42%) hint towards the presence of large pores in the modified sample [56]. Figure 6 illustrated the N2 adsorption–desorption isotherm of conventional and PDADMAC modified ZSM-5 zeolite and their corresponding pore size distribution (inset of Fig. 6 (II)). As can be seen from Fig. 6, the conventional ZSM-5 exhibits type-I isotherm (Langmuir isotherm) without any hysteresis loop implying the microporous structure. On contrary, we can observe type-IV isotherm with a broad hysteresis loop at a relative pressure (P/Po) of 0.5–0.9 for PDADMAC modified ZSM-5 zeolite. This is attributed to capillary condensation of nitrogen and is characteristic of mesoporous systems. BJH pore size distribution of PDADMAC modified ZSM-5 revealed a prominent curve in the range of 10–30 nm, confirming the development of micro and mesopores in the system. The additional mesopores in the PDADMAC modified ZSM-5 is due to the removal of PDADMAC from the zeolite framework. The surface area and mesopore volume of conventional and hierarchical zeolite is displayed in Table 1. It can be seen that the PDADMAC templated sample possess high surface area and mesopore volume than conventional zeolite (440 m2/g and 0.44 cm3/g), respectively, which were much larger than those of other ZSM-5 catalysts (Table 1). Therefore, PDADMAC modified zeolite has high pore volume than conventional ZSM-5 thus suggesting a superior adsorption power for the system.27Al NMR spectra for PDADMAC modified ZSM-5 zeolites are shown in Fig. 7. We can observe two distinct signals: one at chemical shifts of ~ 54 ppm and the second at ~ 0 ppm. The peak observed at ~ 54 ppm and ~ 0 ppm corresponds to the tetrahedral and octahedral aluminium in the framework of zeolite. The 27Al NMR analysis thus confirms that most of the Al species are tetrahedrally and octahedrally coordinated in the framework [57].

Fig. 3
figure 3

SEM images of a CZSM-5 b and c PZSM-5

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Fig. 4
figure 4

TEM images of PZSM-5

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Fig. 5
figure 5

Thermogravimetric curve of: a CZSM-5 b PZSM-5 (DTG curves are given at the inset)

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Fig. 6
figure 6

a Nitrogen adsorption–desorption isotherms of (i) PZSM-5 (ii) (inset) CZSM-5; b Pore size distribution curve of: (i) PZSM-5 (ii) (inset) CZSM-5

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Table 1 Textural characteristics of conventional and PDADMAC modified ZSM-5 zeolite
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Fig. 7
figure 7

Solid state 27Al NMR spectra of PZSM-5

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Adsorption Studies

An idea of optimum adsorbent dosage is essential for large scale industrial applications. So, we have varied the zeolite dosage from 0.025 to 0.1 g by keeping other parameters constant. The effect of zeolite dosage on the adsorption of MB is graphically depicted in Fig. 8a. From the figure, one can see that on increasing the zeolite amount from 0.025 to 0.1 g, the adsorption capacity increases from 2.9 to 4.31 mg/g. This could be due to enhancement in the total surface area of the system which promotes the adherence of MB on the surface of zeolite. Since PDADMAC modified zeolite contain mesopores, an enhancement in its dosage could have increase the number of mesopores in the system and thus facilitates the penetration of large MB molecules [58]. The effect of contact time for the uptake of MB by modified zeolite was examined and the result is illustrated in Fig. 8b. It is evident that the adsorption take place in two stage. In the first stage, the adsorption capacity increases rapidly with contact time, probably due to the availability of large number of active sites. Meanwhile, in the second stage, only a slow increase in the adsorption capacity with contact time was observed and after 4 h the adsorption attains equilibrium. This may be allocated to the fact that the number of available adsorption site on the surface of zeolite decreases with contact time and eventually all adsorption sites will be occupied by MB molecule. Therefore, all experiments were conducted at a contact time of 4 h. In order to evaluate the effect of initial concentration of MB on modified ZSM-5 we have varied the initial concentration from 10 to 50 ppm. It is revealed that adsorption capacity increases with initial dye concentration but the removal percentage decreases. On increasing MB concentration, the mass driving force for the transfer of MB from solid to liquid phase increases and thus leads to enhancement in the adsorption capacity. The reduction in the removal percentage could be due to the decline in the ratio of number of available adsorption site to the number of MB molecules (Fig. 8c). This trend is in agreement with previous reports [59]. pH is an important factor that influence the adsorption process, so in this present work, adsorption of MB dye on zeolite have been studied at different pH (2–10) and is displayed in Fig. 8d. At low pH, due to the possible protonation, the surface of adsorbent becomes positive charged and thus repel the incoming MB molecule. However, at high pH, the surface becomes negative and thus provide a favourable condition (electrostatic attraction) for the attachment of MB molecule on surface of zeolite. From the above studies, it can be concluded that low pH disfavours the adsorption of MB on zeolite while high pH favours the adsorption process. Finally, we monitored the adsorption capacity as a function of temperature and is displayed shown in Fig. 8e. It is clear from Fig. 8e that upon increasing the temperature the adsorption capacity slightly increases, implying an endothermic nature of the adsorption process. Another plausible reason is that thermal energy provides a favourable driving force for the diffusion of large MB molecule from liquid solution into the solid adsorbent.

Fig. 8
figure 8

Effect of different parameters a zeolite dosage; b contact time; c initial dye concentration; d pH; e temperature on adsorption of MB on PDADMAC modified ZSM-5

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Comparison between PDADMAC Modified and Conventional ZSM-5

Figure 9 illustrates the comparison of adsorption performance of PDADMAC modified ZSM-5 zeolite with conventional ZSM-5 zeolite. It can be seen that PDADMAC modified zeolite exhibits high adsorption capacity than conventional zeolite, probably due to the difference in the porosity, which is evident from morphological and BET analysis. In the case of conventional zeolite, only surface adsorption is possible, as the small micropores in it will restrict the penetration of large MB molecules. However, mesopores in PDADMAC modified zeolite facilitates the penetration of large MB molecules and thus offer a higher adsorption capacity than conventional zeolite. This is schematically represented in Scheme 2.

Fig. 9
figure 9

Effect of different type of zeolite and reusability of MB dye on PDADMAC modified ZSM-5

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Scheme 2
scheme 2

Mechanism for MB adsorption on conventional and hierarchical ZSM-5 zeolite

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Reusability

An adsorbent with good regeneration ability is always recommended for the treatment of water treatment. In the present work, the regeneration capacity of PDADMAC modified zeolite was checked for six adsorption–desorption cycle. From Fig. 9 it is clear that the adsorption capacity of zeolite slightly reduced with number of cycles. Nevertheless, even after 6 recycle it possess good removal efficiency thus suggesting it as a potential candidate for dye removal application.

Adsorption Kinetics

Kinetic models were employed to understand the rate and mechanism of adsorption. In the present study, the kinetics of MB adsorption onto PDADMAC modified zeolite was evaluated by applying pseudo-first-order (PFO) and pseudo-second-order (PSO) model. The differential form of pseudo-first-order model is represented as

$$\frac{{\mathrm{dq}}_{\mathrm{t}}}{\mathrm{dt}}={\mathrm{K}}_{1}({\mathrm{q}}_{\mathrm{e}}-{\mathrm{q}}_{\mathrm{t}})$$
(3)

where qt (mg/g) is the amount of MB adsorbate on the surface of PDADMAC modified zeolite at time t, qe is the equilibrium adsorption capacity (mg/g). K1 is pseudo-first-order rate constant (L min−1). The linearized form of Eq. 3 is represented as

$$\mathrm{log}\left({\mathrm{q}}_{\mathrm{e}}-{\mathrm{q}}_{\mathrm{t}}\right)={\mathrm{logq}}_{\mathrm{e}}-\frac{{\mathrm{K}}_{1}\mathrm{t}}{2.303}$$
(4)

The kinetic parameter K1 and qe were obtained from the linear fit of log (qe − qt) against time.

The PSO model is represented as

$$\frac{{\mathrm{dq}}_{\mathrm{t}}}{\mathrm{dt}}={\mathrm{K}}_{1}{\left({\mathrm{q}}_{\mathrm{e}}-{\mathrm{q}}_{\mathrm{t}}\right)}^{2}$$
(5)

The Eq. (5) is integrated by applying boundary condition and on rearranging gives the linearized form which is represented as follows

$$\frac{\mathrm{t}}{{\mathrm{q}}_{\mathrm{e}}}=\frac{1}{{\mathrm{K}}_{2}{\mathrm{q}}_{\mathrm{e}}^{2}}+\frac{\mathrm{t}}{{\mathrm{q}}_{\mathrm{e}}}$$
(6)

where K2 is the rate constant of PSO (g/mg min). The value of qe and K2 can be calculated from the linear plot of t/qt vs time.

The goodness of fit (correlation coefficient R2) is used to identify the best kinetic model for adsorption process. Kinetic model with high correlation coefficient (R2) value is usually selected as the best model for a particular adsorption reaction [60, 61]. In order to identify the kinetic model, we have fitted the experimental data into PFO and PSO. The linear fitted graph is shown in Fig. 10 and the kinetic parameters are summarized in Table 2. On comparing the linear plot and R2, one can observe that PSO model has best agreement with the experimental adsorption data and is most suitable to describe MB adsorption on zeolite.

Fig. 10
figure 10

Linear kinetic plots of a pseudo-first-order b pseudo-second-order models for the adsorption of MB dye for the PDADMAC modified ZSM-5 zeolite

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Table 2 Adsorption kinetic parameters and correlation coefficient for the adsorption of MB dye on modified ZSM-5 zeolite with PFO and PSO
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Adsorption Isotherm

Adsorption isotherm describes the interaction between adsorbent and adsorbate and elucidates the adsorption mechanism. The experiment data of MB on PDADMAC modified ZSM-5 zeolite was analysed using two well-known models; Langmuir and Freundlich isotherms.

The Langmuir isotherm assumes an equivalent adsorption site on the surface of adsorbent and neglects any interaction between the adsorbed molecules. Thus, it suggests monolayer formation on the surface of the adsorbent. The linear form of Langmuir equation can be described as

$$\frac{{\mathrm{C}}_{\mathrm{e}}}{{\mathrm{q}}_{\mathrm{e}}}=(\frac{{\mathrm{C}}_{\mathrm{e}}}{{\mathrm{q}}_{\mathrm{max}}})+(\frac{1}{{\mathrm{K}}_{\mathrm{L}}*{\mathrm{q}}_{\mathrm{max}}})$$
(7)

where KL is Langmuir constant, Ce is equilibrium dye concentration (mg/L), qe is the amount of dye adsorbed onto the ZSM-5 (mg/g) and qmax is maximum adsorption (mg/g). The linear plot of Ce/qe against Ce gives a straight with slope 1/qm and intercept 1/qmKL. RL (equilibrium parameter) is another characteristic parameter obtained from Langmuir isotherm model and is used to predict the favorability of the adsorption process. The value of RL between 0 and 1 is considered as favorable for the adsorption process.

The Freundlich model assumes multilayer adsorption on a heterogenous surface. The linear form of Freundlich equation can be expressed as follows;

$$\mathrm{ln}\left({\mathrm{q}}_{\mathrm{e}}\right)={\mathrm{InK}}_{\mathrm{F}}+\frac{1}{\mathrm{n}}*\mathrm{In}({\mathrm{C}}_{\mathrm{e}})$$
(8)

where KF is Freundlich constant (mg/g) (L/mg)1/n) and n is the heterogeneity factor. KF is related to the adsorption capacity and ‘n’ indicates the feasibility of adsorption. Adsorption process is considered to be favorable if the value of 1/n is between 0 and 1 [62,63,64,65].

Comparison with Other Adsorbent

From the Table 3, it is clear that the hierarchical ZSM-5 possess fairly good adsorption capacity and its comparable with other adsorbents. Hence, PDADMAC modified ZSM-5 zeolite could be a promising candidate for the removal of MB from aqueous solutions

Table 3 Comparison study of MB on different adsorbent
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The plot of ln qe versus ln Ce and Ce/qe versus Ce for MB adsorption on PDADMAC modified ZSM-5 zeolite is shown in Fig. 11 and the corresponding adsorption parameter are displayed in Table 4. As can be seen Fig. 11 the experimental data fitted well in the Freundlich model with a high regression coefficient value (0.99). The value of 1/n is found to be (0.714) implying a favorable adsorption. Thus, MB molecules is distributed in multilayer fashion on the surface of PDADMAC modified zeolite.

Fig. 11
figure 11

Linear isotherm plots of a Langmuir b Freundlich model for the adsorption of MB dye for the PDADMAC modified ZSM-5 zeolite

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Table 4 Adsorption isotherm parameters and correlation coefficient for the adsorption of MB dye on modified ZSM-5 zeolite with Langmuir and Freundlich
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Conclusion

A modified ZSM-5 zeolite was successfully developed by using PDADMAC as the mesotemplate. The crystallinity and chemical structure of synthesised zeolite is confirmed by XRD and FTIR analysis. N2 adsorption and morphological analysis confirms the generation of mesopores in the PDADMAC modified samples. Adsorption studies indicate that PDADMAC modified ZSM-5 zeolite is as good adsorbent for the removal of cationic dye MB, from aqueous solution. The result further shows that adsorption process could be tuned by varying the operation parameters such as zeolite dosage, initial dye concentration, contact time, temperature and pH. Optimized conditions for removal efficiency are as follows (modified ZSM-5 zeolite:0.1 g, initial dye concentration:10 mg/L, pH: 10 and contact time: 300 min). The adsorption isotherm and kinetic studies indicate that Freundlich model and pseudo-second-order model is the best model to explain MB adsorption onto PDADMAC modified zeolite. The recyclability results suggest that PDADMAC modified ZSM-5 zeolite possess good regeneration capacity. Thus, we propose PDADMAC modified ZSM-5 zeolite as a potential adsorbent for removing methylene blue from water.