Ionic liquid based nanoporous organosilica supported propylamine as highly efficient adsorbent for removal of congo red from aqueous solution

2.1 Instruments and reagents

The following chemicals were commercially available: 3-aminopropyl-trimethoxysilane (97%, Aldrich), congo red (CR), NaOH, HCl, EtOH and toluene (Merck). The morphology of the ILNO-NH₂ was taken by digital scanning electron microscope KYKY-EM 3200. The transmission electron microscopy (TEM) images were obtained using an instrument model of FEI Tecnai TF20. X-ray diffraction (XRD) pattern of the sorbent was obtained by XRD diffractometer (X’pert, Philips). The diffuse reflectance infrared Fourier transform (DRIFT) spectra were determined using a Bruker-Vector 22. The concentration evaluation of the CR was carried out using Jusco UV–visible spectrophotometer model V-530 (Jasco, Japan) at a wavelength of 590 nm, while the pH/ion meter model-686 thermometer Metrohm was used for measurement of pH adjustment (Metrohm, Switzerland, Swiss). The stock CR solution was prepared by dissolving appropriate amounts of solid dye in double distilled water and the desired concentrations of test solutions were prepared by diluting the stock solution.

2.2 Preparation of ILNO-NH₂ nanostructure

At first the ionic liquid based ordered nanoporous organosilica (ILNO) material was prepared according to our previously reported procedure (Elhamifar et al., 2014a, 2014b; Elhamifar and Shábani, 2014; Karimi et al., 2011, 2012a, 2012b, 2014). Then, 0.5 mmol of 3-aminopropyl-trimethoxysilane was added into a flask containing a well-dispersed toluene solution of ILNO (1 g). The mixture was stirred and refluxed under argon atmosphere for 24 h (Elhamifar et al., 2014a, 2014b). After that, the obtained precipitate was filtered and completely washed with ethanol to remove unreacted 3-aminopropyl-trimethoxysilane. The final material was dried at 70 °C for 12 h and gave a white powder denoted as ILNO-NH₂ (Scheme 1).

Close

Scheme 1

Preparation of ILNO-NH₂ nanomaterial

Export to PPT

2.3 Measurements of dye uptake

To determine the influence of important parameters such as the pH, contact time and initial dye concentration, different experiments at different conditions, were conducted. For each experimental run, a 50 mL solution of known concentration of CR at known pH was prepared. This mixture was stirred at room temperature and controlled speed at different time intervals (2–36 min), filtrated and then analyzed to calculate remaining dye concentration. Effect of pH on dye uptake was studied over the range of 2–9 and further studies were carried out at optimum pH of 4.

The percentage of removing CR was calculated as follows:

Kinetics of adsorption was studied by analyzing removing of the CR from aqueous solution at various time intervals. To study the adsorption isotherms, different concentrations of CR were prepared and agitated with 0.025 g/50 mL of ILNO-NH₂ in batch system until the equilibrium was achieved, and the equilibrium sorption capacity was calculated as follows:

3.1 Characterization of ILNO-NH₂ nanostructure

The DRIFT spectroscopy was used to determine the functional groups of ILNO-NH₂ (Fig. 1). The broad and strong band around 3409 cm⁻¹ is assigned to the O—H stretching vibration of Si—OH groups (Elhamifar et al., 2014a, 2014b; Elhamifar and Shábani, 2014; Karimi et al., 2011, 2012a, 2012b, 2014). The strong bands cleared at 1078 and 952 cm⁻¹ are attributed to symmetric and asymmetric stretching vibrations of Si—O—Si bonds. Moreover, the absorption peaks of organic functional groups are observed at 2938 cm⁻¹ (aliphatic C—H stretching), 1363 cm⁻¹ (bending vibration of aliphatic groups), 1685 cm⁻¹ (C⚌N stretching of imidazolium ring), 1658 cm⁻¹ (C⚌C stretching of imidazolium ring) (Elhamifar et al., 2014a, 2014b; Elhamifar and Shábani, 2014; Karimi et al., 2011, 2012a, 2012b, 2014), 1446 cm⁻¹ (C—H deformation vibrations), 794 cm⁻¹ (for C—Si stretching vibrations), 3400 cm⁻¹ (N—H stretching vibration), and 458 cm⁻¹ (bending vibration of Si—O—Si), respectively. It is important to note the DRIFT spectroscopy of ILNO-NH₂ is very similar to ILNO spectroscopy (Elhamifar and Nazari, 2015). Importantly, for the later one (ILNO-NH₂) only the intensity of aliphatic C—H stretching peaks (about 3000 cm⁻¹) is increased. These observations successfully confirm victorious incorporation and inclusion of propylamine and alkyl imidazolium ionic liquid groups in the material framework. According to these data as well as the structure and properties of the adsorbate and adsorbent, the possible adsorption mechanism of CR onto/into ILNO-NH₂ is shown in Scheme 2. Accordingly, we conclude that CR molecules are adsorbed via hydrogen bonding interactions between amine groups of the sorbent and sulfonate moieties of CR molecules. The H-bonding between Si—OH of adsorbent and nitrogen atoms of CR also is also an effective way for this adsorption process. Moreover, another way for this successful adsorption may be attributed to π–π interactions between imidazolium ring of sorbent and aromatic rings of CR molecules (Scheme 2). The low-angle powder X-ray diffraction (LPXRD) and transmission electron microscopy (TEM) of the ILNO-NH₂ were used to investigate the type of mesochannels. The PXRD analysis of the material demonstrated a peak with high intensity at 2θ of 0.87 (d-spacing: 9.7 nm) and a peak with lower intensity at 2θ of 1.42 (d-spacing: 5.9 nm) which indexed as d100 and d110 reflections, respectively (Fig. 2). This is characteristic of materials with highly ordered 2-dimensional hexagonal mesostructure confirming that the material possesses a highly ordered nanostructure. The TEM image of the ILNO-NH₂ was in good agreement with PXRD analysis and showed highly ordered hexagonal arrays of mesopores with uniform pore size (Fig. 3). These uniformity and regularity strongly confirm high stability of the material during synthesis process as well as prove its possible adsorption capacity for the removal of dye molecules. The scanning electron microscopy (SEM) analysis was carried out to characterize both morphology and particle size of the materials. The SEM image of the ILNO-NH₂ material (Fig. 4a and b) revealed the presence of regular rope-like particles with uniform morphology and a quite uniform size distribution of 50 μm. The SEM image of the ILNO-NH₂ after adsorption of CR (CR@ILNO-NH₂) was also investigated (Fig. 4c). This image demonstrated a different morphology in comparison with its ILNO-NH₂ parent. This observation may be attributed to adsorption of the CR molecules onto/into the ILNO-NH₂ material. After characterization, the adsorption efficiency of the ILNO-NH₂ was investigated in the removal of CR under different conditions of pH, contact time, adsorbent dose, initial dye concentration and temperature.

Close

Figure 1

DRIFT spectrum of ILNO-NH₂

Export to PPT

Close

Scheme 2

The proposed mechanism for the adsorption of CR on ILNO-NH₂

Export to PPT

Close

Figure 2

X-ray diffraction (XRD) pattern of ILNO-NH₂ nanostructures

Export to PPT

Close

Figure 3

Transmission electron microscopy (TEM) images of ILNO-NH₂

Export to PPT

Close

Figure 4

Scanning electron microscopy (SEM) images of ILNO-NH₂ before the adsorption of CR (a: magnification = 2.50 KX; b: magnification = 500 X) and (c) magnification = 5.00 KX after the adsorption of CR

Export to PPT

3.2 Effect of pH

Fig. 5a shows the effect of pH value in the elimination of CR dye using ILNO-NH₂ sorbent. Some of dyes are anionic and/or cationic in nature; therefore, the sorption of these charged dyes onto the sorbent surface is affected by the surface charge of the sorbent which in turn is affected by the pH value of the solution. The ILNO-NH₂ framework is positively charged due to the presence of polar ionic liquid functional groups which are responsible for the excellent sorption capacity for anionic dyes. The other important functions in the surface of sorbent are OH and amine groups. The sorption of CR was investigated over a pH range of 2–9 with CR concentration of 40 mg L⁻¹, the adsorbent dose of 0.025 g/50 mL and the contact time of 30. The maximum removal for this anionic dye can be expected at lower pH values. Although ILNO-NH₂ exhibited notably high removals for CR at all pH values, the maximal of the dye sorption was observed at a pH of 4. Several reasons may be attributed to CR adsorption behavior of the adsorbent relative to solution pH. The surface of ILNO-NH₂ may contain a large number of active sites and CR uptake can be related to the active sites and also to the chemistry of the solute in the solution. Two possible mechanisms of CR adsorption on adsorbents may be considered: (a) electrostatic interaction between the protonated groups of carbon and acidic dye and (b) the chemical reaction between the adsorbate and the adsorbent. This might be due to the increase of positive charges, as a result of protonation of the amino functional groups, on the ILNO-NH₂ at lower pH values and there would be strong attraction forces between the anionic dye molecules and ILNO-NH₂ surface (Wang et al., 2008).

Close

Figure 5

Effect of (a) pH; (b) contact time; (c) adsorbent dosage and (d) initial dye concentration on the removal of CR by ILNO-NH₂

Export to PPT

3.3 Effect of contact time

To optimize the adsorption time for the removal of CR from the solution by the ILNO-NH₂ and SiO₂, adsorption studies were carried out at initial CR concentration of 40 mg/L at the adsorbent dose of 0.025 g/50 mL and various times from 2 to 36 min at the pH of 4.0. As can be seen, due to the quite high accessibility of the empty active sites of BPMO-IL-NH₂, the initial sorption rate is fast and the system is reached to equilibrium after about 30 min, while at higher times a slowdown in the sorption speed is observed and the remaining vacant surface sites are difficult to be occupied due to the diffusion of CR molecules from bulk solution onto the accessible sites of the sorbent and repulsive forces between the solute molecules on the solid and bulk phases. This study showed that up to 97% of CR removal occurs at 30 min (Fig. 5b). After that, the equilibrium time of 30 min under optimal conditions, was also tested for SiO₂ and ILNO sorbents. Interestingly, using SiO₂ and ILNO sorbents, respectively, 26% and 82% of CR were removed under the same conditions as before. This observation strongly confirms high adsorption capacity of ILNO-NH₂ sample which may be attributed to its high surface area as well as the key role of amine and ionic liquid functional groups of this sorbent in the attractive interaction with dye molecules.

3.4 Effect of adsorbent dosage

Measuring the uptake of CR as a function of time at various doses illustrated that sorption of CR increased with increasing dose of ILNO-NH₂. The studies were carried out at initial CR concentration of 40 mg/L at various adsorbent doses from 0.01 to 0.035 g/50 mL and the stirring time of 30 min, at the pH of 4.0. Increment of the uptake with increasing the adsorption dosage can be attributed to increment of the surface area of micropores as well as the accessibility of more vacant sites and active functional groups for sorption (Liu et al., 2011). The data are shown in Fig. 5c, and as can be seen from this Figure, the best result is obtained using 0.025 g/50 mL of ILNO-NH₂.

3.5 Effect of initial dye concentration

The efficacy of initial CR concentration over the range of 10–40 mg L⁻¹ on removing of CR dye was studied and the data are shown in Fig. 5d, and as can be seen from this, there is an opposite correlation between the initial amount of dye and the uptake percentage. At lower CR concentrations, the ratio of the amount of the sorbent to the dye molecules is quite high, causing an increment in dye uptake. It can be proposed that an increase in the initial dye concentration leads to saturation of sorbent and causes transfer of dye molecules from bulk solution to the solid surface.

3.6 Adsorption equilibrium study

Sorption equilibrium is established when the quantity of solute being adsorbed onto the sorbent is alike the quantity being desorbed. At this point, the concentration of the solution remains fixed. By plotting sorbent dosage against liquid phase concentration graphically it is feasible to demonstrate the equilibrium sorption isotherm. There are lots of theories related to sorption equilibrium.

3.6.1

3.6.1 Langmuir isotherm

The Langmuir isotherm (Langmuir, 1918) based on monolayer accumulation of target compound to the surface of the adsorbent is represented as follows Eq. (3):

(3)

$$q_e=Q_m\frac{K_a{C}_e}{1+K_a{C}_e}$$

To describe the constants K_a and Q_m, the linear form of the expression is as follows (Eq. (4)):

(4)

$$\frac{C_e}{q_e}=\frac{1}{K_a{Q}_m}+\frac{C_e}{Q_m}$$ where q_e represents the amount of dye adsorbed on the absorbent at equilibrium (mg g⁻¹); C_e describes the equilibrium dye concentration (mg L⁻¹); Q_m denotes the maximum adsorption capacity corresponding to complete monolayer coverage and K_a is the Langmuir adsorption equilibrium constant (L mg⁻¹). Thus, a plot of C_e/q_e vs. C_e (Fig. 6a) results in a straight line of slope (1/Q_m) and an intercept of 1/K_aQ_m. From the slopes and intercepts, the values of Q_m and K_a were calculated and are given in Table 1.

Close

Figure 6

(a) The Langmuir plot for the adsorption of CR on ILNO-NH₂ (adsorbent dose: 0.025 g/50 mL, pH: 4.0, 30 min agitation time at speed of 400 rpm). (b) Pseudo-second order kinetics plot for the adsorption of CR on ILNO-NH₂ (Dye concentration: 40 mg/L, adsorbent dose: 0.025 g/50 mL, pH: 4.0, stirrer speed: 400 rpm)

Export to PPT

Table 1 Isotherm constant parameters and correlation coefficients calculated for the adsorption of CR onto ILNO-NH₂ and SiO₂.

Isotherm	Parameters	Adsorbent
		ILNO-NH2	SiO2
Langmuir	Qm (mg/g)	43.10	1.48
	Ka (L/mg)	1.28	8.34
	R2	0.991	0.961
Freundlich	1/n	0.25	0.40
	KF (L/mg)	26.80	2.52
	R2	0.985	0.848
Temkin	BT	8.44	2.96
	KT (L/mg)	28.05	0.91
	R2	0.953	0.869

3.7 Kinetics evaluation

The kinetic analysis of the sorption information is based on reaction kinetics of pseudo first and second order mechanisms and Elovich equation. The linear forms of pseudo-first-order and pseudo-second-order equations are expressed as

Plot of q_t against ln(t) should yield a linear relationship with a slope and an intercept of (1/β) and ln(αβ), respectively. The correlation coefficients for the Elovich kinetic model acquired from all the data concentrations were high. This indicates that this sorption system is well enough for the system (Weber and Morris, 1963; Vimonses et al., 2010). Another alternative method for kinetic evaluation of an adsorption process is intra-particle-diffusion model that is based on the phenomena that dye adsorption on adsorbent material takes place through four steps such as bulk diffusion, film diffusion, intraparticle-diffusion and ion-exchange. The values of K_diff (mg g⁻¹ min^−0.5) were calculated from the slopes of q_t versus t^0.5 while C was obtained from its intercept (related to the thickness of the boundary layer). The R² values (0.916) are lower than pseudo-second-order which indicates no applicability of this model and makes possible final outcome that the rate-limiting step is not the intraparticle diffusion process. The kinetic parameters obtained from this study are listed in Fig. 6b and Table 2.

3.8 Thermodynamic studies

Temperature has important effect on adsorption process. Adsorption of CR at different temperatures in the range of 25–50 °C using 0.025 g/50 mL of ILNO-NH₂ was studied. The results clearly indicate that dye uptake increases with temperature. This may be explained on the basis of the fact that increase in temperature enhances the rate of diffusion of the adsorbate molecules across the external boundary layer and in the internal pores of the adsorbent particles as a result of the reduced viscosity of the solution. In addition, the mobility of sorbate molecules also increases with temperature, thereby facilitating the formation of surface monolayers. Changing the temperature will change the equilibrium capacity of the adsorbent for particular adsorbate. Thermodynamic parameters including change in Gibb’s free energy (ΔG⁰), entropy (ΔS⁰) and enthalpy (ΔH⁰) for the adsorption of CR over prepared adsorbent have been determined by using the Eqs. (13) and (14):

K_c is the distribution coefficient for adsorption and determined as

The plots of ln K_C against 1/T, and the values of ΔH⁰ and ΔS⁰ can be estimated from the slope and intercept. The values of ΔG⁰ were negative indicating that the adsorption of CR on the ILNO-NH₂ is feasible and spontaneous. The value of ΔH⁰ was observed to be positive (45.30 kJ/mol) for the adsorption of CR corresponding to an endothermic process. The positive value of ΔS⁰ suggests that the adsorbed CR molecules remain more randomly over the adsorbent surface (Gupta et al., 2011b). The values of these parameters have been given in Table 3.

3.9 Recovery of ILNO-NH₂

After the successful adsorption of the CR dye using ILNO-NH₂, this was washed completely with hot ethanol and then was recovered to study its stability and structure properties under applied conditions. Nitrogen adsorption–desorption isotherms showed that the BET specific surface area and pore volume of the fresh ILNO-NH₂ material decreased, respectively, from 586 m² g⁻¹and 1.1 cm³ g⁻¹ to 438 m² g⁻¹and 0.79 cm³ g⁻¹ for the recovered one (Fig. 7a, Table 4). Moreover the BJH calculation illustrated that the mean pore diameter of this material decreased from 9.9 to 9.2 nm for the recovered one. As shown in Fig. 7b, the BJH isotherm of such material significantly illustrates single sharp peak proving the uniform pore size distribution of recovered ILNO-NH₂ nanostructure. These data strongly confirm high stability and durability of the nanostructured ILNO-NH₂ material under adsorption process.

Close

Figure 7

(a) Nitrogen adsorption–desorption and (b) BJH pore size distribution isotherms of the recovered ILNO-NH₂ nanostructure

Export to PPT

3.10 Various adsorbents for CR removal

In the final study the efficiency of ILNO-NH₂ nanomaterial in the removal of CR was compared with some of recently reported sorbents in this regard (Table 5). Interestingly, the result showed that the present absorbent is much more efficient than other adsorbents and can remove CR dye from wastewater in nearly short time and quite low dosage of sorbent (0.025 g/50 mL of sorbent) (Ghaedi et al., 2012; Namasivayam and Kavitha, 2002; Namasivayam and Arasi, 1997; Namasivayam and Kanchana, 1993; Mall et al., 2005; Deo and Ali, 1993). The high adsorption capacity of ILNO-NH₂ may be attributed to its excellent porous structure as well as the presence of ionic liquid and amine functional groups in the material framework.