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Removal of Anionic Dye from Aqueous Media by Adsorption onto SBA-15/Polyamidoamine Dendrimer Hybrid: Adsorption Equilibrium and Kinetics Maryam Mirzaie,† Abosaeed Rashidi,† Habib-Allah Tayebi,*,‡ and Mohammad Esmail Yazdanshenas§ †

Department of Textile Engineering, Science and Research Branch, Islamic Azad University, Tehran, Iran Department of Textile Engineering, Qaemshahr Branch, Islamic Azad University, Qaemshahr, Iran § Department of Textile Engineering, Yazd Branch, Islamic Azad University, Yazd, Iran ‡

ABSTRACT: Extremely ordered mesoporous SBA-15 was prosperously synthesized through a plain sol−gel technique and then was functionalized with polyamidoamine (PAMAM) dendrimer. The structure and chemical properties of the produced materials were identified by Fourier transform infrared spectroscopy, X-ray diffraction, nitrogen adsorption− desorption isotherms, and thermogravimetric analyzer. The surface morphology was recognized by transmission electron microscopy and field emission scanning electron microscopy. The adsorption behavior was determined by adsorption of acid dye on SBA-15/PAMAM. It was considered that SBA-15/ PAMAM indicated a very good adsorptive ability of Acid Blue 62 molecules. The effect of different experimental parameters such as pH, adsorbent dosage, contact time, initial dye concentration, and temperature on the adsorption was studied. The isotherm, kinetic, and thermodynamic characteristics were analyzed to define the adsorption manner of produced Nano adsorbent. The Langmuir, Freundlich, and Temkin adsorption isotherm models were applied for analyzing the experimental data, and the results showed a desirable simulation with the Langmuir adsorption isotherm model with a great adsorption valence of 1428.57 mg g−1, and the adsorption kinetic model was well-fitted by a pseudo-second-order kinetic equation. The adsorption data supported its spontaneous and exothermic property.

1. INTRODUCTION

In the past two decades, the wide use of mesoporous silica (MPS) as SBA-3, MCM-41, SBA-15, and MCM-48 has proven it to be a promising adsorbent, because of its high level and uniform pore structure, pore volume, thermal and mechanical stability, and versatility.11,12 The especially porous structure and texture of mesoporous materials allow the active sites of the molecule to be more easily targeted. Therefore, they are used for a variety of applications that demonstrate their good performance.13 Dendrimers are a class of molecules with a highly branched structure order, a great number of end groups, and attractive guest molecules that are easily identified. Dendrimers have low toxicity and are safe.14 Modified dendrimers provide the possibility of removing dyes from liquid−liquid or liquid−solid systems (dyeing polypropylene fibers).15 Polyamidoamine (PAMAM) dendrimers are used to remove acid dyes, and parameters such as pH and concentration show the effects of dendrimer generation on dye removal percentage.16

A lack of sufficient water (physical deficiency) is a growing problem. Wastewater treatment and the reuse of it as input to the system will significantly reduce the water consumption problem.1 According to reports, there are more than 100 000 different dyes. About 1−20% of the total world productions of dyes are destroyed during the dying process and so many dyes enter into the sewage systems.5,6 The dyes of fabric wastewater is one of the most dangerous substances that can contaminate water and cause problems for human and animal health. The low biodegradability of dangerous substances is due to the high molecular weight and complex structures of the materials.10 Acid dyes are used in several industries such as textile, printing, leather, plastics, paper, food processing, cosmetics, pharmaceutical, and paint. Water pollution by industrial wastewater is a common problem for several countries.2−4 The elimination of dyes from water is extremely significant because the dyes reduce water quality and even the attendance of small amounts of dye (less than 1 mg) in water is visible and unpleasant. In addition, the majority of these dyes contribute to health diseases such as cancer, allergic dermatitis, and skin irritation.7−9 © XXXX American Chemical Society

Received: October 31, 2016 Accepted: March 10, 2017

A

DOI: 10.1021/acs.jced.6b00917 J. Chem. Eng. Data XXXX, XXX, XXX−XXX

Journal of Chemical & Engineering Data

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Kovvali and Sirkar17 first declared the utilization of PAMAM dendrimers for CO2 removal. Pure PAMAM G0 dendrimer was immobilized in the pores of a hydrophilic microporous polymeric velum.17 Other studies of the same dendrimer were represented by Kouketsu et al.,18 in which the PAMAM dendrimer (G = 0) was collected in a porous ultrafiltration velum. Melamine-based dendrimer amines (MDAs) are therefore ideal dendrimer ligands owing to strongly binding amine sites, as well to their raised hydrophilic silica surface contrasted to similar adsorbents. Simanek and Zhang in 200019 had preliminarily characterized MDAs for adsorption of heavy metal cations. Researchers20−23 have utilized ligands including polyamine groups. Jiang et al.24 synthesized SBA-15/PAMAM and SBA-15-PAMAM-EDTA organic−inorganic hybrid materials to augment removal efficiency of heavy metal cations into the NPS pores, but they were not utilized for acid dye removal. Yijun and Qiuming prepared nanoparticle catalysts stabilized by SBA-15/PAMAM (G = n) organic−inorganic hybrid composites for removal of heterogeneous Pd(0).24 Several chemical, physical, and biological techniques such as ultrafiltration, ion exchange, reverse osmosis, and adsorption on different adsorbents have been expanded for the removal of dye from aqueous media.25 Among these techniques, adsorption is beneficial to industries due to wide performance, easy operation, cost-effectiveness, and water recyclability. Various kinds of adsorbent are applied for the removal of synthetic dyes as graphene oxide, zeolite, activated carbons, fly ash, and nanoporous silica. 26−32 For acquiring high-performance adsorbent, it is important to choose the more effective and cheaper adsorbents by better adsorption capability.33 Nanoporous silica have enticed enhancing regards because of vast surface area, pore volume, tunable pore diameter, and their easy synthesis and great thermal and hydrothermal stability.34 Adsorption is a suitable method to remove pollutants from wastewater. It permits flexibility in terms of both operation and design and produces pollutant-free effluents which are appropriate for reuse. Adsorption is mostly determined by the adsorbent as compared to the other materials. The process of adsorption/desorption of solid phases is time dependent; therefore, understanding the dynamic interactions of pollutants in the solid phase will allow a prediction of the outcome with the passage of time. The adsorption thermodynamic consideration is based on how to estimate the phase equilibrium between a liquid and a solid adsorbent.35 Because of the unique attributes of dendrimers, SBA-15 and SBA-15-Cl were synthesized and modified by the PAMAM dendrimer (G = 0) and were analyzed by Fourier transform infrared (FTIR) spectroscopy, transmission electron microscopy (TEM), X-ray diffraction (XRD), Brunauer−Emmett−Teller (BET), field emission scanning electron microscopy (FE-SEM), and thermogravimetric analysis (TGA). It is concluded that SBA15-Den nanoabsorbent has already been used for the removal of Acid Blue 62 (AB62) solutions. In addition AB62 adsorption isotherms, kinetics, and thermodynamic models were studied.

Figure 1. Chemical structure of AB62.

from Merck. The solutions’ pH were set by using H2SO4 (10% V/V), CH 3 COOH (10%V/V), and NaOH (10%V/V) solutions. To measure the absorption, the T80+ UV−vis spectrometer (Korea) was used. PAMAM dendrimer (Generation Zero) with four terminal end groups, the molecular weight of 517 g/mol and a diameter of 1.5 nm from the (Sigma-Aldrich, USA) was prepared (Figure 2).

Figure 2. Chemical structure of PAMAM dendrimer (chemical formula C22H48N10O4, MW = 517 g/mol, G0).

2.2. Characterization. TEM observation was carried out with a Hitachi, HF2000, Hitachi High-Technologies Europe GmbH and Krefeld. FE-SEM observation was carried out with MIRAll TESCAN (Czech Republic). FT-IR spectra were registered with PerkinElmer spectrum 400 in the range of 400−4000 cm −1 using KBr pellets. XRD (Bruker D8 ADVANCE with Ni-filtered Cu Kα radiation at 1.5406 Å) patterns were recorded with a speed of 2° min−1 and a step of 0.05°. Nitrogen adsorption−desorption isotherms were carried out at 77.35 K with a PHS-1020(PHS China) apparatus. The distributions of specific surface area and pore size were acquired, using the Brunauer−Emmett−Teller (BET) analysis and Barrett−Joyner−Halenda (BJH) method from the isotherms’ desorption branch. TGA was done with a Shimadzu TGA50H appliance. The specimens were heated in nitrogen with a heating rate of 10 °C min−1 from 25 to 600 °C, and as a function of temperature the weight losses were registered. 2.3. Synthesis of SBA-15. SBA-15 was synthesized by the procedure qualified by Stucky and co-workers.36 Accordingly 4 g of block copolymer surfactant Pluronic-123 (MW = 15800 g/ mol, Sigma-Aldrich) was mixed with 125 cm3 of aqueous solution of 1.9 M HCL.The homogenized mixture at a temperature of 40 °C (homogeneous and uniform) was stirred by stirring continuously and consistently. An 8.58 g aliquot of TEOS was then added, and the mixture was stirred for 20 h. The prepared mix was placed for 24 h at 100 °C in a polyethylene container, and the resulting solution was dried, filtered, and then was calcined at 550 °C for 10 h. 2.4. Synthesis of SBA-15-Cl. SBA-15-Cl was prepared by a modification method used by Adam et al.37 A mixture containing 10.0 g of silica and 9.42 g of 3-(chloropropyl) triemethoxysilane was prepared in 15 mL of toluene, stirred for

2. EXPERIMENTAL SECTION 2.1. Materials. AB62 (422.4 g/mol) is manufactured by Dystar Company. The chemical structure of AB62 is represented in Figure 1. Other materials are analytical grade and have been supplied from the Merck Company. The P123 Surfactant (EO20PO70EO20, Sigma-Aldrich, USA), hydrochloric acid (HCl, 37.8%, Merck), tetra-ethylorthosilicate (TEOS, 99%, Merck), and 3-(chloropropyl)trimethoxysilane were supplied B



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Figure 3. Chemical structure factor of SBA-15-Cl.

Figure 4. Scheme of the SBA-15/PAMAM preparation.

estimate the dye concentration. The maximum absorption wavelength (λmax) was applied to specify the residual’s concentration of AB62 in the solution using the UV−vis spectrophotometer set at 620 nm. For this purpose, the T80+ series of UV−visible spectrophotometers was used. The quantity of dye adsorbed on SBA-15/PAMAM and dye removal percentage (removal efficiency %) were calculated as eqs 1, 2, and 3:39

15 min at room temperature magnetically, and then refluxed for 24 h. The reaction mixture was cooled, filtered, and repeatedly washed with toluene, methanol, and acetone, respectively, and then under a vacuum oven-dried at 70 °C for 4 h to provide SBA-15-Cl (Figure 3). 2.5. Synthesis of SBA-15/PAMAM. PAMAM dendrimer was grafted on mesoporous silica modified by 3-(chloropropyl) trimethoxysilane. For this purpose PAMAM (5.16 g) was added to chloromesoporous silica (1 g) and dispersed in 50 mL of dry toluene. Then, the mix was refluxed at 70−80 ◦C under stirring for 24 h; the produced material was filtered, washed with toluene, ethanol, and diethyl ether, and dried at 70 °C for 8 h. The results were signified as SBA-15/PAMAM (Figure 4).38 As seen, the schematic drawing and structures of SBA-15 grafted with CPTMS and PAMAM dendrimer are illustrated in Figures 3 and 4. The modifications of nanoporous silica surface are applied by grafting in the postsynthesis method. By applying CPTMS as the silylation reagent, the chloropropyl functionalization of SBA-15 was performed. The amine end groups of PAMAM dendrimer were then grafted on chlorofunctionalized mesoporous silica. 2.6. Adsorption Studies. Dye absorption measurements using 0.01−0.1 g of SBA-15/PAMAM for AB62 in containers containing 100 mL of 40−600 ppm solution at various pH values (2−12) for different times (15−120 min) at various temperatures (25−45 °C) were carried out. The solution’s pH was set by H2SO4, CH3COOH, and NaOH. Finally, specimens were centrifuged for 40 min at a speed of 4000 rpm in order to

qe =

C0 −Ce V W

(1)

qt =

C0 −Ct V W

(2)

where qe and qt are the amount of dye adsorbed on SBA-15/ PAMAM (mg g−1) at equilibrium and at any time, respectively. C0 and Ce are the initial and final dye concentrations (mg L−1) and Ct is the concentration of dye (mg L−1) following adsorption time t, respectively. V is the dye bath’s volume, and W is the adsorbent’s weight (g).

R% =

A 0 −A1 100 A0

(3)

where R% is the removal percentage, A0 and A1 are the initial adsorption of dye before and after the adsorption procedure, respectively. C



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Figure 5. FESEM images of (a)SBA-15 (b)SBA-15-Cl (c) SBA-15/PAMAM and (d)TEM image of SBA-15.

3. RESULTS AND DISCUSSION

substance’s common constructional ordering might be straightly seen in the TEM. Figure 5d indicates TEM images in the pore axis orientation and in the perpendicular orientation to the SBA-15 pore axis. The TEM images of SBA-15 display the maintenance of identical channels’ hexagonal order with a distinctive honeycomb outward within the compound’s grafting.40 The physical adsorption of gas molecules on a solid surface might be described by the nitrogen adsorption−desorption isotherms (BET) and suffice as a significant analysis method for the evaluation of a substance’s specific surface area. BET isotherms of SBA-15, SBA-15-Cl, and SBA-15-Den and their results are shown in Figure 6 and Table 1. SBA-15 has a uniform pore diameter of 8.2 nm, a specific surface area of 743.45 m2 g−1, and a pore volume of 0.98 cm3 g−1, and the BET surface area of SBA-15-Cl was organized to be 348.78 m2 g−1 while SBA-15-Den has a uniform pore diameter of 4.1 nm, a specific surface area of 165.94 m2 g−1, and a pore volume of 0.36 cm3 g−1. Consecutive CPTMS and PAMAM dendrimer functionalization upon mesoporous silicas results in reduced specific surface area, pore volume, and pore diameter. Such considerable

3.1. Characterization. The morphology of the SBA-15, SBA-15-Cl, and SBA-15-Den were studied by FE-SEM to confirm the high mesoscale order of the produced samples. A regular and ordered structure with comparatively identical size that is in eligible accordance with SBA-15 can be observed in the FE-SEM image (Figure 5a−c). This can be seen in the BET report (Table 1). Higher magnifications permit observation of channel-like constructions extending parallel to the lengthy direction. These channels satisfactorily correlate to a 2D typical hexagonal structure of SBA-15, as confirmed by TEM. The mesoporous Table 1. BJH Results Obtained for SBA-15, SBA-15-Cl, and SBA-15-Den

sample

specific surface area (BET) (m2 g−1)

average pore diameter (BJH) (nm)

pore volume (BJH) (cm3 g−1)

SBA-15 SBA-15-Cl SBA-15-Den

743.45 348.78 165.94

8.2 6.3 4.1

0.98 0.64 0.36 D



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15 sample is ascribed to the adsorption of water molecules and stretching of the −OH vibrations of silanol groups. These peaks are allocated to the framework of SBA-15 and also might be seen in the SBA-15/PAMAM spectrum. In SBA-15-Cl the bands at 646 and 650 cm−1 denote the existence of absorption bands relevance to CH2−Si stretching, and a vast band at 698 cm−1 might be allocated to the C−Cl bond. In SBA-15-Den, the potent extensive band at 3411 cm−1 might be attributed to the O−H stretching vibrations of water molecules adsorbed and silanol groups. The band at 1647 cm−1 is ascribed to water adsorption. Evanescence of these negligible peaks at nearly 3450 cm−1 ascertains the reaction among imperfect Si−OH group and amine groups in SBA-15/PAMAM. The IR bands at 3411 and 3297 cm−1 in the SBA-15/PAMAM are because of the asymmetric and symmetric −NH2 stretching group, and its intentions are perceived to be slim.44,45 In SBA-15/PAMAM the enhancement in the number of amine groups causes the asymmetric and symmetric −NH2 stretching band, and the C− N stretching band might be perceived at 1462−1556 cm−1. The specific bending vibration of ethyl chain and the −CH stretching band in PAMAM dendrimer molecule are perceived at 2923−2958 and 688−798 cm−1, respectively. Low angle XRD template of SBA-15, SBA-15-Cl, and SBA15-Den displayed the (100), (110), and (200) peaks (Figure 8 panels a−c) which are generic of an arranged mesoscopic formation silica that displays the nanochannels’ two-dimensional hexagonal symmetry order. SBA-15-Cl and SBA-15-Den were specified by the same pattern (Figure 8b,c) showing that neither the chloropropyl functionalization nor the PAMAM dendrimer’s grafting hurt the constructional impeccability of SBA-15. The severity of the prevailing peak at 2θ = 0.91 that is ascribed to the (100) diffraction peak46 was reduced pending the surface modification steps due to the silica wall’s decreasing scattering power since modifications.47 The rate of weight changes in TGA in a material is subordinate to temperature (or time) following an adjusted atmosphere. This reflects the material’s thermal stability, polymers’ filler value, moisture, and solvent amount, and the percentage of combination’s ingredients. TGA is carried out by

Figure 6. N2 adsorption/desorption isotherm of SBA-15, SBA-15-Cl, and SBA-15/PAMAM.

reduction of the porous materials’ textural properties on grafting was previously reported.41,42 Having the PAMAM dendrimer attached on the SBA-15 surface causes an attended surface area reduction and reduction in mesoporous volume. The hysteresis loop was perceived in the 0.5 < P/P0 < 1.0 range that is affiliated with capillary condensation occurring through the mesopores. This demonstrates the attendance of mesoporous perforations in the silica material. Pursuant to IUPAC classification, the represented isotherm is type IV and shows an H1 hysteresis loop. This is specific of a highly arranged mesoporous substance.43 Figure 7 represented the FT-IR spectra of SBA-15, SBA-15Cl, and SBA-15/PAMAM. In SBA-15, the bands 801 and 1100 cm−1 are contingent on the symmetric and antisymmetric oscillations of the Si−O−Si bond. The peaks at 468, 1382, and 3437 cm−1 are respectively an effect of the deviation of the Si− O−Si bond’s vibration and the Si−OH group’s vibration. An extensive absorption band 3450 cm−1 for the unmodified SBA-

Figure 7. FT-IR spectra of (a) SBA-15, (b) SBA-15-Cl, and (c) SBA-15-Den. E



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Figure 8. Low angel XRD patterns of (a) SBA-15 (b) SBA-Cl, (c) SBA-Den.

Figure 10. Effect of pH on AB62 removal: 100 mL of 40 mg/L AB62, time = 2 h, T = 25 °C, and adsorbent dosage = 10 mg.

gently increasing the specimen’s temperature in a furnace since its weight is evaluated utilizing an analytical scale which subsists outside the furnace. In TGA, mass loss is considered if a thermal incident includes the loss of a temporary part. Moreover, TGA was applied to consider the amount of organic loading on the silica framework.48 The TGA results were represented in Figure 9. In any curve, weight loss less than 120 °C was due to the water’s loss and remaining solvents. In Figure 9a) the gentle weight-loss from 120 to 600 °C affirmed that the SBA-15 substance was constant up to 600 °C. As seen in Figure 9b, the SBA-15-Cl specimen weight-loss corresponds with the 13% of chloropropyl that is covalently anchored on the silica network. From Figure 9c loading of about 23% PAMAM dendrimer groups on the silica surface were received.49 This means that the chloropropyl groups represent a little additional thermal stability compared with the amine groups. 3.2. Adsorption Studies. 3.2.1. Effect of pH. pH is one of the significant parameters in an adsorption process.50 The tests were performed at a broad pH range of 2−12, 0.01 g of different adsorbent (SBA-15, PAMAM and SBA-15/PAMAM), 100 mL of AB62 solution (40 mg L−1), time contact of 60 min, and temperature of 25 °C. The effect of pH on AB62 adsorption by (SBA-15, PAMAM, and SBA-15/PAMAM), is represented in Figure 10. The utmost dye adsorption takes place at pH 2. AB62 is a polar molecule (R−SO3−). SBA-15 is evaluated as an acceptable adsorbent owing to its wide surface area, vast pore volume, and regular construction to surface functionalization, and SBA-15/ PAMAM has primitive amine end groups at any branch that

might be agitated by the solution’s pH. Thus, the electrostatic attraction, moreover the organic attributes and dye molecules’ structure and PAMAM dendrimer on the silica surface, might play a very significant role in dye adsorption on SBA-15/ PAMAM. At pH 2, an electrostatic attraction is found among the anionic dye and positively charged surface of the PAMAM dendrimer on the silica surface. As the pH of the system rises, the number of positively charged sites reduces as does the dye anions’ adsorption; therefore, a pH of 2 was applied in subsequent studies.51 3.2.2. Effect of Adsorbent Dosage. The adsorbent dosage’s effect was propagated ranging from 0.01 to 0.1 g of different adsorbent (SBA-15, PAMAM, and SBA-15/PAMAM) at a pH of 2, 100 mL of AB62 solution (40 mg/L), and a time contact of 60 min at 25 °C. The acquired results are shown in Figure 11. By increasing the adsorbent dose to 0.03 g, removal efficiency might increase as anticipated. At 0.03 g quantity, the utmost efficiency is near (94.071%). A greater increment of adsorbent dosage does not affect elimination efficiency of AB62 likely because of the accumulation of the adsorbent specks.52,53 Therefore, 0.03 g of adsorbent is represented as an optimal adsorbent amount for AB62 removal in this study. 3.2.3. Effect of Contact Time and Temperature. In this section, AB62 adsorption by the different adsorbents (SBA-15, PAMAM, and SBA-15/PAMAM) was investigated as a function of contact time. The obtained results were represented in Figure 12. It may be seen that, due to accessibility to active sites, the elimination of AB62 has a faster rate at the earlier

Figure 9. Thermogravimetric curves of (a) SBA-15, (b) SBA-15-Cl, (c) SBA-15-Den. F



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absorption processes to describe the adsorption of AB62 on SBA-15/PAMAM. The underlying assumption of the Langmuir theory is that during adsorbence, absorption occurs at particular sites into the adsorbent.55,56,63 The Langmuir equation may be written as follows: qe =

Q 0KLCe 1 + KLCe

(4)

As qe, ce, kL, and q0 are the equilibrium of AB62 adsorbed rate over SBA-15/PAMAM (mg/g), the concentration of the AB62 solution at equilibrium (mg/L), Langmuir constant (L/g), and the adsorption’s maximum capacity (mg/g), respectively. The linear form of the Langmuir equation is shown by eq 5: Ce C 1 = + e qe KLQ 0 Q0

(5)

The values of qe and kL are calculated from the intercept and slope of the plot of the ce/qe versus ce (Figure 13). The kL, qe, and R12 values are listed in Table 2.

Figure 11. Effect of adsorbent dosage on dye adsorption: 100 mL of AB62 solution with 40 mg/L concentration at 25 °C, contact time = 60 min and pH 2.

Figure 12. Effect of dye adsorption vs contact time at various temperatures: 25, 35, and 45 °C (100 mL of AB62 solution with 40 mg L−1 concentration, 30 mg of SBA-15/PAMAM at pH 2).

Figure 13. Langmuir isotherm plot for AB62 adsorption on SBA-15/ PAMAM.

The isotherm datum was examined by the Freundlich isotherm and expressed by eq 6:57−59,63

stages and then eventually a slower rate until equilibrium is achieved at 60 min when the sites of adsorption are filled.54 As represented in Figure 12, the quantity of adsorbate per adsorbent’s unit mass at time t (mg/g), qt, increases with decreasing temperature from 45 to 25 °C representing the exothermic nature of the adsorption processes. 3.3. Adsorption Isotherm. Adsorption is a mass transfer’s reposition procedure which may usually be described as material at the interface among two phases. Equilibrium relationships between adsorbate and adsorbent are modified by isotherms of adsorption, in general the ratio between the amount of adsorbed and residual in the solution at equilibrium at a steady temperature. Isotherm data ought to exactly appropriate into various isotherm models to detect an appropriate model which might be applied for the design procedure. So, to study the effect of concentration on the adsorption isotherm, the experiments were performed at various concentrations of AB62, and then Ce and qe values were determined. Various isotherms as the Langmuir, Freundlich, and Temkin isotherms were considered. The Langmuir isotherm prosperously has been used for several

qe = KFCe1/ n

(6)

Here KF is absorption valence in concentration units and 1/n is the absorption strength. Values of 1/n show that the isotherms are invariable (1/n = 0), desirable (0 < 1/n < 1), or undesirable (1/n > 1). In eq 4 we can see the linear form of eq 7: 1 ln qe = ln KF + ln Ce (7) n The KF and n and amounts calculated from the intercept and slope of the plot of the ln qe versus ln ce (Figure 14). The KF, n, and R22 amounts are listed in Table 2. Isotherm of Temkin is shown by eq 8: qe =

RT b ln(KTCe)

(8)

This can be linearized as shown by eq 9: qe = B1 ln KT + B1 ln Ce G

(9) DOI: 10.1021/acs.jced.6b00917 J. Chem. Eng. Data XXXX, XXX, XXX−XXX


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Table 2. Linearized Isotherm Coefficient for AB62 and SBA-15/PAMAM Adsorbents in the Ideal Rates of the Process Parameters Langmuir isotherm −1

Freundlich isotherm

adsorbent

q0 (mg g )

KL (L/mg)

R12

SBA-15-Den

1428.57

0.032

0.9933

KF (mg/g

(L/mg1/n)

87.41

and eq 10 is as follows: RT b

n

R2

1.7492

0.9387

KT (L/mol)

B1

R32

0.4049

297.91

0.9541

A model fit to the experimental data is generally considered in terms of linear regression analysis where the value of R2 is applied as a sign of the supremacy of the model’s fit (Table 2); the AB62 adsorption on the SBA-15/PAMAM may be appraised as a method that mostly conforms to the Langmuir model. The data of Table 2 suggested that the Langmuir model better fit the AB62 adsorption (R2 = 0.9933) which meant that the adsorbed dye had monolayer coverage on the adsorbent’s surface and indicated that all adsorption sites were equal with identical adsorption energies without any interaction among the adsorbed molecules.55,56 3.4. Adsorption Mechanism. Generally, the adsorption capacity relies on the physical and chemical processes on the surface of adsorbent. The hydroxyl groups on the silica surface break down and intensely interact with AB62, so a pure silica surface does not process strong adsorption sites which interact forcefully with AB62. In the adsorption of AB62 onto the surface of SBA-15/ PAMAM, various mechanisms may be present such as ionic attraction between the anionic sulfonate groups of dissolved AB62 and the dendrimer’s cationic amino end groups which are protonated. The conceivable mechanisms of the SBA-15/ PAMAM and AB62 adsorption process are considered: In aqueous media, first the AB62 is dissolved and then the AB62 sulfonate groups (D−SO3Na) are intercepted and transformed to anionic dye ions.

Figure 14. Freundlich isotherm Plot for AB62 adsorption on SBA-15/ PAMAM.

B1 =

Temkin isotherm 2

(10)

It is assumed that in the equation of the Temkin isotherm, the heat absorption of all molecules, according to adsorbate− adsorbent capacities, linearly decreases and the absorption is recognized by the distribution of identical bonding energies up to utmost binding energy.60,61,63 A plot of qe versus ln ce is used to determine the constants KT and B1 from the intercept and slope, respectively (Figure 15).

H 2O

dye−SO3Na ⎯⎯⎯→ dye−SO−3 + Na +

As well, in the attendance of H+, the amino end groups of PAMAM dendrimer on the silica surface (−NH2) are protonated. SBA‐15−PAMAM−NH 2 + H+ → SBA‐15−PAMAM−NH+3

The adsorption procedure then advances due to the electrostatic attraction among these two counterions: SBA‐15−PAMAM−NH+3 + Dye−SO−3 → SBA‐15−PAMAM−NH+3 −3OS−Dye

The mechanism of AB62 adsorption on SBA-15/PAMAM is illustrated in Figure 16. Furthermore, a vast contradistinction of AB62 is the number of the hydrophilic functional groups that have an intense inclination to make hydrogen bonds with SBA15/PAMAM. 3.5. Adsorption Kinetic. Various models may be applied to describe the absorption’s mechanism of absorbent material on the solute. To consider the mechanism of absorption, sorption constants are specified by applying the pseudo-first-order Lagrange equation62 and the pseudo-second-order equation.59,63 The pseudo-first-order equation in linear form (11) is as follows:

Figure 15. Temkin isotherm plot for AB62 adsorption on SBA-15/ PAMAM.

The binding constant equilibrium KT (L/mol) is related to the utmost binding energy, and constant B1 is related to the heat of adsorption. Also R, T, and b are decisive temperature (K), the universal gas constant (8.314 J/(mol K)) and a constant B1 connected to heat of adsorption, respectively. The KT, B1, and R32 correlation factors for the Temkin adsorption isotherm are shown in Table 2. H



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Figure 16. Illustration of adsorption mechanism of AB62 on SBA-15/PAMAM.

ln(qe − qt ) = ln(qe) − k1t

(t) assessed the pseudo-second-order kinetics’ correlation coefficients (R2) which represent the AB62 removal kinetics by SBA-15/PAMAM that could be approached as pseudosecond-order kinetics. The qe (mg g min−1) and k2 can be calculated from the intercept and slope of the plot of t/qt against t (Figure 18). The amounts of qe, k2, and R2 are listed in Table 3.

(11)

where k1, q1, and qe are dye adsorbed values at equilibrium (mg/g), the dye absorbed value by the time t (mg/g), and the pseudo-first-order (1/min) kinetic equilibrium rate constant, respectively. The qe and k1 values are estimated from the intercept and slope plot of ln (qe−qt) against t (Figure 17). The k1, qe, and R2 amounts are listed in Table 3.

Figure 17. Pseudo-first-order kinetics for AB62 adsorption on SBA15/PAMAM. Figure 18. Pseudo-second-order kinetics for AB62 adsorption on SBA15/PAMAM.

Table 3. Kinetic Constants for Pseudo-First-Order and Pseudo-Second-Order Model of Process Parameters pseudo-first-order (qe)cal. (mg/g)

K1 (1/min)

65.1179

0.0525

pseudo-second-order R12

(qe)cal. (mg/g)

K2 (mg/(g min))

R22

0.9932

131.5789

0.0017

0.9996

3.6. Adsorption Thermodynamics. Thermodynamic analysis is exerted to assess the mechanism and nature of the adsorption procedure. ΔH, ΔS, and ΔG quantities are evaluated from eqs 13 and 14 to define the thermodynamic manner of the adsorption process: ΔG = −RT ln Kd

Linear correlation between ln (qe−qt) and time (t) at pH0 2 can be represented by kinetic H0 and Mackay’s pseudo-secondorder (eq 12): ⎛1⎞ 1 t = 2 + ⎜⎜ ⎟⎟t qt qe k 2 ⎝ qe ⎠

(13)

ΔS ΔH − (14) R RT where ΔS (J/(mol K)) and ΔH (kJ/(mol K)) are prognosticated from the intercept and slope of ln k versus 1/ T data plotting, respectively (Figure 19). In Table 4, the thermodynamic parameters are depicted. The negative values of ΔG emphasis the spontaneous temperament of AB62 adsorption. The physisorption process ln Kd =

(12)

where qe is the dye adsorbed at equilibrium (mg/g) and K2 is the pseudo-second-order (g/mg·min) equation’s equilibrium rate constant. The linear fit between the t/qt and contact time I



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Figure 19. Thermodynamic plot for AB62 adsorption on SBA-15/ PAMAM.

Table 4. Thermodynamic Parameters of AB62 Adsorbed by SBA-15/PAMAM ΔG (kJ/mol) ΔH (kJ/mol)

ΔS (J/(mol K))

298 K

308 K

318 K

328 K

R2

−5.05

5.94

−6.82

−6.88

−6.95

−6.99

0.9991

of AB62 adsorption is identified from the negative values of ΔG which is in the range of −20 to 0 kJ/mol.63,64 Besides, negative values of ΔH for AB62 indicate its exothermic temperament. The positive amount of ΔS indicates that the adsorption of AB62 by SBA-15/PAMAM is reversible.

4. CONCLUSIONS In recent study, SBA-15/PAMAM nanoadsorbent was synthesized, characterized, and used for AB62 adsorption from aqueous media. Appropriate parameters for removing AB62 were a pH of 2, adsorbent dosage of 0.03 g/L, and contact time of 60 min at 25 °C. The experimental results suitably fit the Langmuir model, and the supreme adsorption valence was appraised to be 1428.57 mg/g. AB62 adsorption accompanied the pseudo-second-order kinetic model. Consequences confirmed that SBA-15/ PAMAM nanoadsorbent is a suitable adsorbent for removing AB62 from aqueous media. Adsorption thermodynamics indicated that the AB62 elimination mechanism by SBA-15/ PAMAM nanoadsorbent is physisorptive and exothermic.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Tel.: +989112160238. ORCID

Habib-Allah Tayebi: 0000-0001-9891-3489 Notes

The authors declare no competing financial interest.

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