Effects of different parameters on the synthesis of silica aerogel microspheres in supercritical CO2 and their potential use as an adsorbent

ORIGINAL PAPE R: NANO- AND MACR OP OR OU S MA T ERI AL S ( A E RO G E L S , X E R O G E L S , CR Y O G E L S , E T C. )

Effects of different parameters on the synthesis of silica aerogel

microspheres in supercritical CO

2

and their potential use as an

adsorbent

Ersin Başaran1●Tuğba Alp Arıcı2●Adnan Özcan1,3●Özer Gök1,3●A. Safa Özcan1,3

Received: 27 August 2018 / Accepted: 15 November 2018 / Published online: 28 November 2018 © Springer Science+Business Media, LLC, part of Springer Nature 2018

Abstract

Synthesis of metal oxide microspheres in supercritical carbon dioxide (SCO2) is very attractive for a wide range of

applications such as catalysis, controlled release, and separation science due to their identical properties. The aim of this study is the synthesis of silica microspheres (SM) in SCO2by sol–gel method. The effects of temperature, pressure, flow

rate, time, co-solvent and its ratio on the synthesis of silica microspheres in SCO2 were investigated. The synthesized

microspheres were characterized by several techniques includingfield emission scanning electron microscopy (FE-SEM), Fourier transform infrared (FT-IR) spectroscopy, X-ray diffraction (XRD), dynamic light scattering (DLS), and surface area analysis. The results indicated that the optimum condition for the synthesis of SM was determined at 150 bar, 80 °C, and 2.5 mL min−1of SCO2flow rate. The usability of SM as an adsorbent for the removal of dyes and heavy metals from aqueous

solutions was also examined. Kinetic and isotherm studies for the adsorption of Pb(II) ions and Acid Blue 260 (AB260) onto SM were carried out.

Graphical Abstract

SCO2 + TEOS

Keywords Silica● _Microspheres● _{Supercritical CO}

2● Characterization● Adsorption

Highlights:

● _{Silica microspheres were synthesized in supercritical CO2.}

● _{The optimum condition for the preparation of microspheres was determined.} ● _{Adsorption behavior of silica microspheres was investigated.}

* A. Safa Özcan asozcan@anadolu.edu.tr

1 _{Department of Chemistry, Faculty of Science, Eskişehir Technical}

University, 26470 Eskişehir, Turkey

2 _{Department of Chemical Technology, Emet Vocational School,}

Kütahya Dumlupınar University, 43700 Kütahya, Turkey

3 _{Department of Chemistry, Faculty of Science, Anadolu University,}

26470 Eskişehir, Turkey

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0();,:

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1 Introduction

In recent years, the researches, which are focused on the synthesis of microsphere-based materials, have been increased with the advanced synthesis processes and the development of technology. Among these materials, metal oxide spheres which have micron and sub-micron size have drawn considerable attention since they have been used in many application areas such as sensors, catalysis, bio-chemistry, encapsulation, acoustic insulation, drug delivery, water treatment, cosmetics, and medical [1–9]. These microspheres have been favored in many areas because of their lower densities, tunable porosities and surface areas, etc. [10]. In addition, they are unique materials for phar-maceutical products as a carrier and controlled delivery agent due to their biocompatibility, nontoxicity, and che-mical stability [11,12].

Several techniques are available for the production of metal oxide-based microspheres. These methods are Stöber [13], surfactant template route [14], emulsion-gelation [15], spray drying [16], and nuclei controlling method [17]. Most of these methods have some disadvantages, i.e., using a large quantity of alcohols, toxic organic solvents, and hazard inorganic acids or bases [13]. Therefore, recent researches have sought to find alternative, innovative, convenient, environmentally friendly, and manageable methods for producing these materials. One of the great interesting methods for the synthesis of these materials is to apply sol–gel method in supercritical fluids (SFs) [18–21]. Supercritical carbon dioxide (SCO2) is used as a reaction

medium because it is nontoxic, nonflammable, easily recyclable, inert, and inexpensive [22]. Due to its unique properties such as low viscosity, high diffusivity, and very low surface tension, the solvent power of SCO2 may be

controlled by tuning these properties.

A number of studies are encountered for the synthesis of silica by using SCO2in the literature. For instance, Sui et al.

[21] enhanced a sol–gel route in SCO2 to obtain silica

aerogel particles. The influence of some parameters on the synthesis of citric acid catalyzed with silica aerogels was investigated by Wagh et al. [23]. Garcia-Gonzalez and Smirnova [24] had used SFs technology to prepare aerogel particles for delivery systems. Hollow silica microspheres (SM) were also prepared in SCO2 by Sun et al. [25].

Another study was about the preparation of the fabrication of hollow silica particles in SCO2[26].

The synthesis of SM in SCO2 was studied by several

researches but the operating conditions and process set up were not examined intensively. In this study, the prepared SM was also characterized by using various techniques. Then, the effects of co-solvent, co-solvent’s ratio, flow rate, pressure, temperature, and time were investigated in order to optimize the conditions for the synthesis of SM. Finally,

the usability of SM as an adsorbent for the removal of dyes and heavy metal ions from aqueous solutions was also examined.

2 Experimental

2.1 Synthesis of silica microspheres

The synthesis of SM was carried out by using tetraethyl orthosilicate (TEOS, 98%, Sigma-Aldrich) as the silica source under acidic condition which was provided with formic acid (98%, Sigma-Aldrich) in SCO2 (99.9%,

HABAS) as a reaction medium.

The SCO2 apparatus for the synthesis of SM is

sche-matically illustrated in Fig.1.

The apparatus mainly consisted of a CO2cylinder with a

syringe pump (Isco 260D Model), a cooler (Polyscience 9505 Model), a heating tape to supply the desired tem-perature, a SS precursor vessel, and a 450 mL pressure– temperature controlled reactor (Buchi Limbo Model).

For the synthesis, 3 mL of TEOS was placed in the precursor vessel which was filled with glass beads (<230 mesh), and the mixture of 3 mL of formic acid, a known amount of co-solvent, and 1 mL of deionized water was inserted in the reactor. First, the temperature was adjusted to the desired value and then the vessel and the reactor were simultaneously pressurized to the required pressure. After a certain time, the CO2 at the constant flow rate was

trans-ferred to the reactor by passing through the precursor vessel and the pressure of the system was thenfixed at the constant value. When the process was completed, the system was slowly depressurized and SM were collected from the reactor. Finally, SM were washed with methanol in order to remove excess precursor and dried in an oven at 120 °C for overnight.

2.2 Characterization

The surface morphology of SM was characterized by FE-SEM (Zeiss Ultra Plus) at 5 kV. For this purpose, the Fig. 1 Scheme of experimental apparatus. 1: CO2 tank; 2: syringe

pump; 3: cooler; 4: heating tape; 5: SS extraction cell; 6: SS reactor

samples were firstly coated with gold to enhance con-ductivity before to obtain surface analysis. Energy dis-persive X-ray diffraction analysis (EDX) attachment was also used for FE-SEM instrument in order to determine the chemical composition of SM. The crystalline structure of SM was determined by using powder X-ray diffractometer (XRD, Bruker D8 Advance) with Cu K_αradiation at a range of 10°–50° (2θ). BET specific surface area was determined via nitrogen adsorption isotherm at 77 K (Quantachrome Instrument Nova 2200e). The FT-IR spectrum of the sample was performed (Perkin-Elmer Spectrum 100 Model) by preparing KBr pellets to observe the functional groups of SM. Zeta potential measurement of SM in water was also carried out by using the DLS method (Malvern Zetasizer Nano ZS 90 Series).

2.3 Adsorption experiments

Adsorption experiments were also carried out in a batch system. For this purpose, 250 mg dm−3 of dye (Acid Blue 260 (AB260) and Reactive Blue 19 (RB19)) solutions and 50 mg dm−3 of heavy metal (Pb(II) and Cu(II)) ions solu-tions, and 0.5 g dm−3of SM were used in the experiments. The pH values of the solutions were adjusted to 1.5 for dyes and 5.5 for heavy metal ions by using HCl or NaOH solutions. Each experiment was carried out at 20 °C for 60 min. The solutions were then filtered and analyzed using UV–visible (Shimadzu 1201UV) or atomic absorption spectrophotometer (PerkinElmer Analyst 800).

The amount of adsorbed AB260 or Pb(II) ions onto SM is determined by using Eq. (1).

q¼ðCi C_m tÞV ð1Þ

whereC_iandC_t(mg dm−3) are the concentration of AB260 or Pb(II) ions (mg dm−3) at initial andt time, respectively. V is the volume of solution (dm3) andm is the mass of SM (g). The effects of pH, temperature, contact time, and con-centration were also examined for the adsorption of AB260 and Pb(II) ions from aqueous solutions due to their high adsorption selectivity towards SM. The adsorption kinetic and isotherm models were also applied to the experimental data.

3 Results and discussions

3.1 Effects of co-solvent and its ratio

Carbon dioxide is very attractive reaction media to dissolve relatively non-polar compounds. The addition of a small amount of co-solvent enhances the solvent power of the

SCO2and enables it to interact with more polar compounds.

Typical co-solvents such as ethanol, acetone, isopropanol, dichloromethane, and cyclohexane were used to observe how co-solvent effects the synthesis of SM (Fig.2).

It can be seen from Fig. 2 that the smallest sized microspheres were synthesized using acetone as a co-sol-vent, but these microspheres were more aggregated than others. When dichloromethane was used as a co-solvent in the process, the more homogenous distribution was obtained. The polarity indexes of the co-solvents play an important role in this medium. Polarity index decreases from ethanol (5.2) to cyclohexane (0.0). Dichloromethane was chosen as the most suitable co-solvent for the synthesis of SM in SCO2because of its moderate polarity index (3.1).

Therefore, the nuclei interactions can be prevented by adding a small amount of dichloromethane and the forma-tion of smaller size particles can be acquired.

The effect of co-solvent’s ratio was carried out at various amounts, e.g., 1–5% (v/v) and FE-SEM images are shown in Fig.3.

As it can be seen from thefigure, the optimum dichlor-omethane ratio can be determined as 2.5% (v/v). When the amount of dichloromethane is increased from 2.5 to 5% (v/ v), the formation of SM negatively affects as the nucleation rate depends on the polarity. Smaller particles can be gen-erated by using polar media. In contrast, small particles tend to agglomerate due to low colloidal stabilities [27].

3.2 Effect of flow rate

Theflow rate of the fluid is another important parameter for the synthesis of SM. FE-SEM images of SM at variousflow rates are depicted in Fig.4.

The results revealed that the synthesis of the particles was favored at moderate flow rates (see Fig. 4). At high flow rates, the shapes of the particles were not spherical due to rapid drift. In addition, the amorphous structured particles were formed at high flow rates. Therefore, the optimum flow rate of SCO2for the synthesis was chosen as 2.5 mL

min−1.

When a sol attains the gel point, it is generally supposed that the hydrolysis and condensation reactions of precursor (alkoxide) complete. During this phenomenon, initially, the primary particles are formed, then they aggregate into sec-ondary particles, andfinally link together [28]. The results revealed that the formed nuclei at low flow rates depart more easily from each other. On the other hand, at highflow rates, the nuclei are drifted on top of each other and the aggregation occurred. It can also be mentioned that the sphere formation was not observed due to the accumulation (Fig. 4e).

Fig. 2 The effect of co-solvent: ethanol a, acetone b, iso-propanol c, dichloromethane d, and cyclohexane e. [The synthesis of SM with TEOS: formic acid:co-solvent (3:3:11.5 mL),flow rate: 2.5 mL min−1at 80 °C, 5 h, and 200 bar]

Fig. 3 The effect of co-solvent ratio: 1% a, 2% b, 2.5% c, 4% d, and 5% (v/v) e. [The synthesis of SM with TEOS:formic acid (3:3 mL),flow rate: 2.5 mL min−1, at 80 °C, 5 h, and 200 bar]

Fig. 4 Effect offlow rate: 1 mL min−1a, 2.5 mL min−1b, 5 mL min−1c, 7.5 mL min−1d, and 10 mL min−1e. [The synthesis of SM with TEOS: formic acid (3:3 mL), at 80 °C, 5 h, and 200 bar]

3.3 Effect of pressure

The pressure influences on the density of the reaction medium. Therefore, the effects of SCO2 pressure were

studied at various pressures (125–250 bar). FE-SEM images are also exhibited in Fig.5.

The images (Fig. 5) showed that the formation of the microspheres was favored between 150 and 200 bar. The clustering of microspheres was increased at low pres-sure (125 bar). The irregular formations were observed at 250 bar, because the precursor was compressed and the formed nuclei get together partially. Therefore, the mixture of microspheres and shapeless particles occur at the same time.

3.4 Effect of temperature

The effect of temperature was investigated between 50 and 100 °C and at a constant pressure of 150 bar. When the temperature was increased from 50 to 100 °C, the micro-spheres were effectively handled in good-shape at 80 °C (Fig.6). The rate of sol–gel reactions at room temperature is slow and often requires several days to complete. The density of SCO2is decreased and the diffusion of the

pre-cursor increases with increasing temperature. Acceptable smooth-shaped microspheres were obtained at the tem-perature range of 60–80 °C due to the increase in Brownian motion at these temperatures. This way, the particle clusters are contacted and react with each other in the process, Fig. 5 The effect of pressure: 125 bar a, 150 bar b, 200 bar c, and 250 bar d. [The synthesis of SM with TEOS: formic acid:dichloromethane (3:3:11.5 mL),flow rate: 2.5 mL min−1, at 80 °C, and 5 h]

Fig. 6 The effect of temperature: 50 °C a, 60 °C b, 70 bar c, 80 °C d, 90 °C e, and 100 °C f. [The synthesis of SM with TEOS: formic acid: dichloromethane (3:3:11.5 mL),flow rate: 2.5 mL min−1, 150 bar, and 5 h]

Fig. 7 The effect of operating times: 1 h a, 2 h b, 3 h c, 4 h d, and 5 h e. [The synthesis of SM with TEOS:formic acid:dichloromethane (3:3:11.5), flow rate: 2.5 mL min−1_{, at 80 °C, and 150 bar]}

increasing the number of siloxane bridges [28]. In contrast, the solubility of the mixture can be decreased with the increasing temperature due to the low density of the med-ium. As a result, microspheres could not occur at 90 and 100 °C (Fig.6).

3.5 Effect of time

The aging time is another important parameter for the sol– gel process. The effect of operating time was also carried out at various times (Fig.7).

The optimum time was found to be as 3 h and there were no changes in the particle formation at high operating times (Fig. 7). The gel formation by the traditional method can proceed days or weeks, supercritical drying is then applied to gel to form aerogel [29]. It can be concluded that supercritical method is much quicker than conventional methods.

3.6 Characterization studies

The characterization of SM was carried out by using various chemical analysis techniques. The chemical composition of the microspheres was determined via FE-SEM-EDX ana-lysis (Fig.8). EDX analysis revealed that Si (53.94%) and O (42.56%) were the main elements in SM. Carbon (3.50%) was the impurity due to unreacted TEOS.

XRD is capable of giving knowledge about the mor-phology of silica. The XRD pattern of SM is shown in Fig.

9. According to thefigure, it was indicated that the broad peak which was around at 2θ = 25° corresponds to amor-phous silica.

BET adsorption–desorption isotherm graph for SM is shown in Fig. 10. The specific surface area of SMs was determined as 300.6 m2g−1 by using the BET isotherm equation. Pore size diameter of SM was evaluated from the

BJH method and found to be as 3.34 nm. It can be concluded that SM is a mesoporous silica according to the IUPAC classification. This result revealed that SM may be used as an adsorbent and porous carrier for many applications.

Zeta potential measurements give information about the application of the materials in the surface science. The effects of pH onto the zeta potential of SM were also examined in this study (Fig.11). As it can be seen from the figure, the surface charge of SM was positively charged at low pH values. Point of zero charge of SM (pzc) was determined as pH= 7.48. The microspheres were negatively charged above pH= 7.48. The zeta potentials in the studied pH’s (pH = 2‒11) were changed from + 40 to −40 mV.

The FT-IR spectrum of SM was performed in the 4000– 400 cm−1wavenumber range (Fig.12).

The absence of ethoxy groups in the spectrum indicates that the hydrolysis of the precursor to form silicon was totally completed [30]. The bands between 3637 and 3417 cm−1 assign–O–H stretching vibrations to conduct SiO–H stretching of surface silanol groups hydrogen-bonded water Fig. 8 EDX spectrum of the synthesized SM

2θ (o ) 5 10 15 20 25 30 35 40 45 50 Intensity (C ps ) 0 1000 2000 3000 4000 5000 6000

Fig. 9 XRD pattern of the synthesized SM

P/P₀ 0.0 0.2 0.4 0.6 0.8 1.0 Adsorbed v o lum e (cm 3 g − 1) 0 100 200 300 400 Adsorption Desorption

Fig. 10 BET isotherm plot of N2adsorption for the synthesized SM

molecules [31] and the bending vibration of water mole-cules is observed at 1634 cm−1 [32]. The Si–O in-plane stretching vibrations of the silanol Si–OH groups occur at 938 cm−1[33]. The intense Si–O vibrations occur between

1250 and 1000 cm−1and oxygen atoms of the silica in these bands set up to bridge between each two silicon sites. At this range, very intense and broad main bands at 1085– 1081 cm−1 corresponds to Si–O–Si asymmetric stretching vibrations [34]. The symmetric stretching vibration of Si– O–Si appears at 797 cm−1 [31] and its bending vibration appears at 463–467 cm−1[35]. The low wavenumber at 587 cm−1 is assigned to Si–O stretching of the silica network defects [31].

3.7 Adsorption studies

Adsorption ability of heavy metal ions and dyes onto SM was examined at 20 °C. The results are given in Table1.

According to the table, the high adsorption capacities were obtained for Pb(II) ions and AB260. Therefore, the further adsorption experiments were carried out with these pollutants. The effect of pH on the adsorption of Pb(II) ions and AB260 was examined and the results are illustrated in Fig. 13.

As it can be easily seen from Fig.13, the adsorption of dye or metal ions by SM is directly related to the pH of the solution. At low pH values (1–2) there is no Pb(II) ions adsorption, the surface of SM is positively charged due to the high concentration of hydronium ions (Fig. 13a). The adsorption capacity increases sharply from pH 2.0 to pH 5.5

Table 1 Adsorption results of heavy metal ions and dyes by SM Adsorbates Adsorbed amounts (mg g−1)

Pb(II) ions 89.59 Cu(II) ions 42.26 AB260 415.10 RB19 164.32 pH 0 2 4 6 8 10 12 Zeta potential ( m V) -40 -20 0 20 40 60 pHpzc=7.48

Fig. 11 The effect of pH on the zeta potential

4000 3500 3000 2500 2000 1500 1000 500 3637 3417 1634 1085 1081 938 797 587 467 463 Relative transmittance Wavenumber (cm−1)

Fig. 12 FT-IR spectrum for the synthesized SM at the optimum conditions pH q (m g g −1 ) 0 20 40 60 80 100 120 140 160 pH 0 1 2 3 4 5 6 7 0 2 4 6 8 10 q (m g g −1) 100 150 200 250 300 350 400 450 500

b

a

and then it is stable up to pH 6.0. Experiments were not conducted beyond pH 5.5 due to the fact that Pb(II) pre-cipitation appears at higher pH values and interferes with the accumulation. The maximum adsorption for Pb(II) ions is observed at pH= 5.5.

The maximum adsorption capacity for the adsorption of AB260 (Fig.13b) was acquired at pH= 1.5 because there are strong interactions between the negatively charged dye molecules and the positively charged SM surface. The adsorption capacity decreases sharply until pH 6.0 and then it becomes stable up to pH 9.0. This is because that the repulsion forces between the negatively charged dyestuff and the negatively charged adsorbent increases.

The effect of temperature on the adsorption was also studied at the range of 10–30 °C. The obtained results indicated that the amounts of adsorbed Pb(II) ions or AB260 did not change with the temperature too much. For this reason, the further experiments were performed at 20 °C.

The adsorption kinetics of Pb(II) ions or AB260 onto SM was investigated to explain the adsorption mechanism. The relation between the amounts of adsorbed Pb(II) ions or AB260 and contact time is shown in Fig.14.

The equilibrium time reached in 10 min for the adsorp-tion of Pb(II) ions, while this value was 30 min for the adsorption of AB260.

Adsorption kinetics models are very important to clarify for the adsorption mechanism. Lagergren first-order [36] and the pseudo-second-order models [37], and the intra-particle diffusion equation [38], which are given in Table2, were applied to the experimental data.

The calculated parameters for the kinetic models are given in Table3. t (min) 0 20 40 60 80 100 120 140 q (m g g − 1) 0 20 40 60 80 100 120 140 t (min) 0 20 40 60 80 100 120 140 q (m g g −1) 0 100 200 300 400 500 600 b a t (min) 0 20 40 60 80 100 120 140 t/qt (m in g m g ) 0.00 0.05 0.10 0.15 0.20 0.25 0.30 t (min) 0 20 40 60 80 100 120 140 t/qt (m in g m g ) 0.0 0.2 0.4 0.6 0.8 1.0 1.2

Fig. 14 The effect of contact time on the adsorption of Pb(II) ions a and AB260 b. (Inset: pseudo-second-order model)

Table 2 The description of adsorption kinetic models applied to the experimental data

Kinetic model Linear equation Symbols

Lagergrenfirst-order lnðq_e qtÞ ¼ ln qe k1t qe: maximum adsorbed amount at equilibrium (mg g−1)

qt: adsorbed amount att time (mg g−1)

k1: Lagergrenfirst-order rate constant (min−1)

Pseudo-second-order _qt t¼ 1 k2q22   þ 1 q2  

t k2: pseudo-second-order rate constant (g mg−1min−1)

q2: maximum adsorbed amount at equilibrium (mg g−1)

Intraparticle diffusion qt¼ kpt1=2þ C kp: Intraparticle diffusion rate constant (mg g−1min−1)

C: constant related to layer thickness (mg g−1₎

Table 3 Kinetic parameters for the adsorption of Pb(II) ions and AB260

Kinetic model Pb(II) AB260

Lagergrenfirst-order k1(min−1) 1.65 × 10−2 3.56 × 10−2 q1(mg g−1) 2.679 21.11 r12 0.388 0.642 Pseudo-second-order k2(g mg−1min−1) 3.66 × 10−2 4.46 × 10−3 q2(mg g−1) 123.8 472.0 r22 0.999 0.999 Intraparticle diffusion kp(mg g−1min−1/2) 3.161 5.610 C (mg g−1_{) 114.0 423.5} rp2 0.699 0.976

Among these models, the pseudo-second-order model exhibited the best correlation coefficient for the adsorption data. The maximum adsorption capacities obtained experi-mentally were found to be as 123.8 mg g−1 for Pb(II) ions and 472.0 mg g−1 for AB260. These values were also con-sistent with the calculated values from the pseudo-second-order model.

The evaluation of favorable adsorption plays an important role in the process. Isotherm studies were also carried out with different concentrations at 20 °C for the adsorption of Pb(II) ions and AB260. Two widely used isotherm models (Lang-muir and Freundlich [39,40]) were applied to the experimental data. The related equations are given in Table4.

Langmuir isotherm model describes the monolayer coverage on the homogeneous surface. Freundlich isotherm model suggests that the adsorption takes place on the het-erogeneous surface. Adsorption isotherms of Pb(II) ions and AB260 onto SM are plotted and shown in Fig.15.

Adsorption of Pb(II) ions and AB260 increases with an increase in the concentration of Pb(II) ions or AB260 due to a rise in the interactions between Pb(II) ions or AB260. The isotherm constants for each adsorption process are given in Table5.

Table 4 The description of adsorption isotherm models applied to the experimental data

Isotherm model Equation Symbols Langmuir q_e¼qmKLCe

1þKLCe qe: maximum adsorbed amount at

equilibrium (mg g−1)

KL: Langmuir isotherm constant

(dm3mg−1)

qm: monolayer adsorption capacity

(mg g−1)

RL¼1þK1LC0 RL: Langmuir dimensionless constant

RL> 1: unfavorable RL= 1: linear

RL= 0: irreversible

0 <R_L< 1: favorable

Freundlich q_e¼ K_FCn_e K_F: Freundlich capacity constant (dm3_g−1₎

n: Freundlich intensity constant n = between 2 and 10: good adsorption

n = between 1 and 2: moderately difficult n < 1: poor adsorption Ce (mg dm ) qe (m g g −1 ) 0 50 100 150 200 250

a

b

Fig. 15 Adsorption isotherm curves for the adsorption of Pb(II) ions a and AB260 b. (Inset: Langmuir isotherm model)

Table 5 Calculated isotherm constants for the adsorption of Pb(II) ions and AB260

Isotherm model Pb(II) AB260

Langmuir KL(dm3mg−1) 0.041 0.012 qm(mg g−1) 290.7 661.2 RL 0.149 0.238 rL2 0.990 0.993 Freundlich n 2.475 1.982 KF(dm3g−1) 39.51 34.08 rF2 0.938 0.974

Langmuir model was bestfitted (r2= 0.990 and 0.993) with the experimental data. The lower separation factor (R_L) for the adsorption of Pb(II) ions and AB260 was calculated as 0.149 and 0.238, respectively. This suggests that the adsorption of Pb (II) ions and AB260 on SM is favorable in this study.

4 Conclusion

In this study, SM were successfully synthesized from TEOS under moderate acidic condition by using environmentally friendly supercritical fluid technology. The supercritical apparatus was designed for this synthesis procedure. The effects of system variables (co-solvent and its ratio, flow rate, pressure, temperature, and operating time) on the synthesis of SM in SCO2 were investigated. The

experi-mental results show that the most suitable co-solvent was dichloromethane and its ratio was found to be as 2.5% (v/v). The optimumflow rate, pressure, temperature, and operat-ing time were chosen as 2.5 mL min−1, 150 bar, 80 °C, and 3 h, respectively. The characterization of the synthesized SM was performed by using FE-SEM, BET, XRD, FT-IR, and DLS techniques. The results indicated that the synthe-sized SM mainly consist of Si and O according to the SEM– EDX analysis. XRD analysis also revealed that the micro-spheres possess amorphous structure. The specific surface area was calculated as 300.6 m2g−1 from BET surface analysis. The point of zero charge for SM was found to be as pH= 7.48. The surface functional groups of SM were determined by using FT-IR analysis.

The contact time curves revealed that the removal of Pb (II) ions (10 min) and AB260 (30 min) was too fast at the initial times and then it was continuously leading to surface saturation. The free active surface sites are available for the adsorption during initial stage and then the surface of SM has monolayer coverage of Pb(II) ions or AB260, resulting in the diminish of free sites for the adsorption. The adsorption kinetic results showed that the pseudo-second-order rate equation was the most fitted model to the experimental data.

Due to the high correlation coefficient constant, Lang-muir isotherm is the most approved isotherm for the adsorption of Pb(II) ions and AB260 onto SM at 20 °C. It can be said that the monolayer adsorption occurred for Pb (II) ions and AB260 favorably. Adsorption experiments indicated that SM can be used as an effective adsorbent for the removal of many contaminants such as dyes, heavy metals, etc. from aqueous solutions.

Acknowledgements The authors gratefully acknowledge thefinancial support provided by Anadolu University for the Scientific Research Projects (Project Nos.: 1306F256 and 1401F011).

Compliance with ethical standards

Conflict of interest The authors declare that they have no conflict of interest.

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