Simultaneous adsorption of cationic and anionic dyes using a novel multifunctional mesoporous silica

Received 17 Nov 2020 Revised form 22 Jan 2021 Accepted 15 Feb 2021 Available online 29 Mar 2021 *Corresponding Author: Amir Vahid Email: avahid753@gmail.com & vahida@ripi.ir https://doi.org/10.24200/amecj.v4.i01.127 -----------------------


Introduction
Dyes as a important raw material were used in different industries including cloth, plastics, tanning, cosmetics and food [1]. Additionally, the dyes are one of the most problematic groups as a results of their environmental impact. Furthermore, the industrial effluents containing dyes may be have a carcinogenic effect in humans when, discharge to waters without any proper treatment.
Methylene Blue (MB) and Direct red (Dr) as the cationic and anionic dyes have harmful effects on human life and environment. Acute exposure to MB causes many health effect such as increased heart rate and cyanosis jaundice quadriplegia in mammals [2,3]. Up to now, the dye adsorption studies have focused on solutions containing single dye and mixture of dyes [4][5][6]. The most industrial effluents include a mixture of several dyes, so it is necessary to study the simultaneous adsorption of two or more dyes from aqueous solutions [7,8]. However, the dyes are resistance to biodegradation due to their aromatic structure [9]. Many industrial technologies, including chemical oxidation [10], adsorption [11], the coagulation/flocculation [12], the membrane separation and ion exchange [13] was used for the adsorption of dyes from industrial effluents. Among them, adsorption has been known as one of the practical physical processes for the treatment and cleaning of wastewaters, because it is cost effective and very effective. The main problem of this technology is the synthesis of low cost adsorbent possesses with high adsorption capacity [14]. In multi-dye adsorption, the interference of one dye on the other dyes is very important [15]. The liquid chromatography [16], the capillary electrophoresis [17], the spectrometry [18] and the electrochemistry methods [19] are used for the analysis of dyes. Due to advantages of UV-Vis spectrophotometry such as accuracy of results, sensitivity, low cost, easy operation and reproducibility, it was used for analysis of colored samples [20]. The main drawback of this technique is the overlap and interference of broad absorption peaks. So, the simultaneous analysis of mixture dyes could be carried out using derivative spectrophotometry [21]. It was determined based on the derivative values of interest compound while other components have zero value [22]. Also, many applications were used by UVM-7 in different sciences [23][24][25][26]. In this work, the synthesis of a multifunctional mesoporous silica contains both acidic and basic functional groups were reported. At the second step, the adsorption of MB / Dr from aqueous solution was carried out simultaneously and the adsorption phenomenon was studied.

Synthesis of UVM-7
UVM-7 was synthesized via well-known atrane route in which triethanol amine (TEAH3), has effect on the rate of hydrolysis and condensation of teraethyl orthosilicate (TEOS). In a typical synthesis, mixture of TEOS and TEAH3 heated up to 120 °C and then CTAB (cetyltrimethylammonium bromide) was added when the temperature of the solution reached to 70°C. After addition of water a suspension was formed and aged for 4 hours at ambient temperature. The final molar composition of the synthesis mixture was 1.0 TEOS : 3.5 TEAH3 : 0.25 CTAB : 90 H 2 O. The product was filtered, washed with water and acetone and dried in an oven at 80°C overnight and calcined at 550°C for 6 hours.

Synthesis of NH 2 -/UVM-7/SO 3 H
First, 1.0 g of aminopropyle triethoxysilane (APTES) was added to the aqueous solution of CTAB and after 5 minutes, mixture of TEOS and TEAOH, was added to the micellar solution. The reactants molar ratio was 1.0 TEOS: 3.5 TEAH3 : 0.25 CTAB : 90 H 2 O: 0.20 APTES. After aging, the white suspension was filtered and washed thoroughly with water and acetone. The white precipitate dried in oven over night at 80 °C. At the second step, 1.0 g of the as-synthesized NH 2 /UVM-7 and 1.0 g of triethoxysilane propanethiol (TPTES) was refluxed in 50 mL of toluene for 12 hours and then, the mixture filtered and dried in oven again. Then, thiol groups grafted on the external surface of the sample was oxidized in 15% H 2 O 2 and 1 molar solution of H 2 SO 4 . Then, CTAB of the assynthesized NH 2 /UVM-7/SO 3 H was removed by reflux in 1 molar ethanolic solution of HCL for 24 hours to get access to the internal pores and amine groups. The final product was dried in oven at 80 °C in vacuum oven overnight.

Batch adsorption procedure
In each test, 0.1 g of adsorbent was added to the 10 mL of a solution containing a known concentration of each dyes while agitating at 250 rpm at 25 °C. After 120 minutes the concentration of residual dyes in supernatant was determined by UV-Vis spectrophotometer at 502 and 664 nm for Dr and MB, respectively. In binary solution, the best wavelength for each dye was find out utilizing firstorder derivative spectrometry. The adsorption percentage (R%) and adsorption capacity of each dye q e , (mg dye/g adsorbent) was calculated according to the following equations (Eq.1 and Eq.2): Where C 0 and C e (mg L -1 ) are the initial and equilibrium concentration of dye, respectively. V (L) is the volume of the solution and M (g) is the mass of adsorbent used.

Characterization
X-ray diffraction (XRD) patterns were recorded using a Philips 1840 diffractometer using nickelfiltered Cu Kα (1.5418˚ A) X-ray source, operating at 35.4 kV and 28 mA. Textural properties of the synthesized samples were measured by nitrogen adsorption at 77 K using a BELSORP-max apparatus after being out-gassed in vacuum at 120 °C. Elemental analysis was carried out using CHNS-Elementar. FT-IR spectra was recorded using a FT-IR 70 VERTX Bruker spectrophotometer using KBr powder. UV-Vis spectrophotometer was used to measure the concentration of the Methylene Blue and Direct red23 in aqueous solution. FE-SEM images were captured using Mira TESCAN 3-XMU. Samples were sputtered with gold prior to imaging. TEM images were recorded at 150 kV operating voltage using a Zeiss microscope.
A strong and broad diffraction peak can be observed which is characterized in bimodal meso/macroporous material. This peak can be attributed to diffraction of incident X ray from the d 100 plane. According to the hexagonal symmetry, two weak and broad peaks at higher diffraction angles can be assigned to d 110 and overlap of d 200 and d 210 planes, respectively.

Nitrogen physisorption
The isotherms of UVM-7 and NH 2 /UVM-7/SO 3 H is shown in Figure 3. The isotherms showed the four type of isotherm according to the IUPAC classification. In both isotherms, at low relative pressure, the inflection is not sharp. By approaching to the saturated relative pressure, increasing of gas uptake led to the sharp increasing of isotherm. This can be attributed to the inter particles of UVM type material which is also present in NH 2 /UVM-7/ SO 3 H after internal and external grafting of organic moieties. The high BET surface area and pore volume means that the pore structure/size almost preserved after grafting. However, after grafting of organic moieties on UVM-7, the sharpness of these region slightly decreases. The two inflections in NH 2 /UVM-7/SO 3 H can be attributed to the presence of two types of pore system. Adsorption of dyes by multifunctional mesoporous silica Amir Vahid et al

TEM and SEM
Electron microscopy is a powerful technique provides a direct view of porous structure, particle morphology and many other information of porous materials at nanoscale. The SEM of NH 2 / UVM-7/SO 3 H reveals the particle size, shape and morphology of nanomaterials. Figure 4 illustrates the SEM image of NH 2 /UVM-7/SO 3 H. The sample composed of nanosize particles without presence of large agglomerates. The shape of the nanoparticles is almost irregular. The TEM image of the NH 2 /UVM-7/SO 3 H is taken perpendicular to d 100 direction of pores and is displayed in Figure 5. Pore openings are clearly visible. Particle size in sub-ten nanometers and is in agreement with those seen in SEM image.
FT-IR spectra of SO 3 H/UVM-7/NH 2 is shown in Figure 6. FT-IR spectroscopy was utilized to indicate the existence of NH 2  These results proof presence of amine and sulfonic acid functional groups on NH 2 /UVM-7/SO 3 H.

Determination of MB and Dr in single and binary solution
The calibration curve obtained from absorbance spectra of MB and Dr versus concentration at λ max of the corresponding dye and used for the determination of dyes in single solution. The concentration of dyes in binary solution of MB and Dr is obtained by first order derivative spectrophotometry by plotting the change rate of absorbance against wavelength. The obtained spectra for raw and derivative spectra are displayed in Figure 7a and 7b. The calibration curves for determination of each dye in the presence of the other one were obtained by measuring of first-order derivative at the zero crossing points of Dr (502 nm) and MB (664nm) [27,28]. Several standard binary mixtures of MB and Dr were used for the validation of the method.
The regression equations and coefficients of determinations, are given in Table 1.

Equilibrium modeling 3.5.1.Single-component adsorption isotherm
Two well-known adsorption model, Langmuir and Freundlich, were applied to the adsorption data of single-component dye solution (Equation 3 and 4) [29,30]. In single solution, the R 2 of Langmuir isotherm was higher than that of Freundlich, which implies that suggestions of Langmuir model. The homogenous adsorption sites, equal activation energy and monolayer formation, is predominantly determine the mechanism of adsorption. The isotherm constants are presented in Table 2.

Multi-component adsorption isotherms
In this case, competition for occupation of adsorption sites by adsorbate molecules occur and  led to a complex adsorption model. Furthermore, for the design of treatment/refining systems, understanding the mechanism of multi-component adsorption is very important. Despite the difficulty of obtaining of a model for adsorption of binary mixtures, extended-Langmuir model was developed to explain equilibrium adsorption model in such system is as Equation 5:

(Eq. 5)
Using non-linear regression technique, parameters of Equation 5 were obtained given in Table 2. High regression coefficients (R 2 : 96.98-99.86) indicate that the extended Langmuir model has the ability to explain the adsorption equilibrium of binary solution of MB and Dr.

Adsorption kinetic
The kinetic data of adsorption was obtained with the initial concentrations of 50 mg L -1 and 100 mg L -1 for MB and Dr, respectively. Solutions prepare in 25 mL flask with the stirring rate of 300 rpm and the contact time was changed in the range of 10 to 120 minutes. to evaluate if there are changes/ interferences in the adsorption process in the presence of other dyes in the solution.

Pseudo-first order model
The pseudo-first order kinetic model has been widely used to explain kinetic of adsorption. Where q e and q t are the adsorption capacities (mg g -1 ) at equilibrium and at time t, respectively. K 1 is the rate constant of pseudo-first order adsorption (L min -1 ).  Where q e is the equilibrium adsorption capacity, and K 2 is the pseudo-second order constants (g mg -1 .min -1 ). These terms can be determined experimentally from the slope and intercept of plot t/q t versus t, shown in Figure 8. The coefficients of both models are shown in Table 3. In all cases including single and binary solutions, the pseudosecond order model has better coefficient of determination, R 2 , which means that kinetic of adsorption od MB and Dr on NH 2 /UVM-7/SO 3 H can be explained by The difference between two kinetic model can be attributed to the number and accessibility of adsorption sites, specific surface area, pore size, nature of the functional groups of adsorbent and chemical structure of dyes.
Adsorption of dyes by multifunctional mesoporous silica Amir Vahid et al  Figure 8 displays that NH 2 /UVM-7/SO 3 H can selectively adsorb the anionic Dr from the Dr/MB mixture and pretreated at pH=5. While, the NH 2 / UVM-7/SO 3 H can selectively adsorb the cationic MB dye from the Dr/MB mixture and pretreated at pH=10. This test verifies with the easy, the special treatment and selective adsorption of cationic or anionic dyes by NH 2 -SO 3 H/UVM-7.

Thermodynamic studies
The effect of temperature on the adsorption of MB and Dr onto NH 2 /UVM-7/SO 3 H was investigated. The obtained results indicate that the amount of equilibrium adsorption (qe) of MB and Dr on the NH 2 /UVM-7/SO 3 H slightly increases with increasing temperature which reveals the endothermic nature of adsorption. Main thermodynamic parameters including standard free energy (ΔG), enthalpy (ΔH) and entropy (ΔS) were obtained using Equation 8.

(Eq. 8)
Where R is gas constant (8.314 Jmol -1 K -1 ) and T is temperature in Kelvin. The apparent equilibrium constant (K c ) of the adsorption is defined as  Where C a and C b is the equilibrium concentration (mg L -1 ) a of corresponding dye on the adsorbent and in the solution, respectively. The K c value is used in the Equation 10 to determine the Gibbs free energy of adsorption (ΔG).

(Eq. 10)
Furthermore, ΔH and ΔS were calculated from the slope and intercept of the linear plot of lnKc versus 1/T, respectively. The obtained thermodynamic parameters are summarized in Table 4. All ΔG values are negative which imply that the spontaneous nature of adsorption. Furthermore, the value of ΔG becomes more negative with increasing of temperature, indicating that the adsorption process is more favorable at higher temperature. As can be seen in Table 4, the positive value of adsorption enthalpy, ΔS, confirms the endothermic nature the adsorption process.

Conclusions
In This work, a novel synthesis method was developed for the preparation of an adsorbent contains both sulfonic acid and basic amine groups in the exterior and interior pores of UVM-7, respectively. This multifunctional adsorbent, NH 2 /UVM-7/SO 3 H, can be used for simultaneous removal of multiple dyes with different electrostatic charge. In real aqueous sample and wastewater there are several dyes which differ in their nature. Furthermore, one of the most effective parameters in the removal of dye is its electrostatic charge. On the other hand, large pore size and surface area of UVM-7 can improve the diffusion of the dye toward the adsorption sites. This improved diffusion rate can be accompanied by small size of the adsorbent particles, UVM-7, to improve adsorption yield. However, it would be very useful in terms of industrial point of view, to have a single adsorbent to remove a variety of pollutants. The proposed method can be applied to use a variety of adsorbents and open us a way for treatment of wastewaters which normally contain many types of pollutants.