Journal of Sol-Gel Science and Technology https://doi.org/10.1007/s10971-018-4611-4
ORIGINAL PAPER: SOL-GEL AND HYBRID MATERIALS FOR ENERGY, ENVIRONMENT AND BUILDING APPLICATIONS
Preparation of SiO2–ZrO2 xerogel and its application for the removal of organic dye Guoliang Huang1 Wenxu Li1 Ying Song1 ●
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Received: 30 November 2017 / Accepted: 17 February 2018 © Springer Science+Business Media, LLC, part of Springer Nature 2018
Abstract SiO2–ZrO2 xerogel was prepared via a sol–gel method followed by ambient pressure drying. The xerogel was characterized by X-ray diffraction, thermal analysis, fourier transform infrared spectroscopy, scanning electron microscopy, and nitrogen adsorption/desorption analysis. The results showed that the SiO2–ZrO2 xerogel was amorphous and possessed a threedimensional network structure with a narrow distribution of pore size. Its specific surface area reached up to 525.6 m2/g after 600 °C heat treatment, with a pore volume of 1.16 cm3/g and an average pore size of 8.5 nm. In order to explore the potential application of the SiO2–ZrO2 xerogel for the removal of organic dyes, its adsorption capacity was studied by removal of Rhodamine B (RhB) from aqueous solution through batch experiments. The results showed that the adsorption process of RhB onto SiO2–ZrO2 xerogel was slightly promoted under acidic conditions and significantly inhibited under strong alkaline conditions. And adsorption equilibrium can be achieved in 30 min. The kinetic data of the adsorption were analyzed using pseudo-first-order and pseudo-second-order models. The results indicated that the pseudo-second-order model described the adsorption mechanism better. The sorption behavior was evaluated by Langmuir and Freundlich isotherm models. The results suggested that the Langmuir model could accurately describe the experimental data and the adsorption capacity qmax was 177.7 mg/g. Thermodynamic analysis revealed that the adsorption of RhB onto the SiO2–ZrO2 xerogel was both spontaneous and exothermic in nature. Thus, the as-prepared SiO2–ZrO2 xerogel might be used as an adsorbent for wastewater treatment, especially for the removal of dyes.
Graphical Abstract
* Wenxu Li
[email protected]
1
School of Chemistry and Chemical Engineering, Harbin Institute of Technology, 150001 Harbin, China
Journal of Sol-Gel Science and Technology
Keywords
SiO2–ZrO2 xerogel Sol–gel method Ambient pressure drying Dye adsorption Removal of Rhodamine B ●
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Highlights: Prepare SiO2–ZrO2 xerogel by ambient pressure drying. ● The porous performance of SiO2–ZrO2 xerogel was improved after calcination. ● The absorption mechanisms of RhB onto SiO2–ZrO2 xerogel have been discussed. ● The adsorption capacity of RhB onto SiO2–ZrO2 xerogel is high. ●
1 Introduction Removal of organic dyes, which are widely used in various industries, has attracted increasing attention due to their potential toxicity and bio-accumulative properties [1]. Dyes in wastewater are not only harmful to the environment but also hazardous to aquatic life as well as humans [2]. Therefore, the effective removal of dyes from wastewater is of great significance. Rhodamine B (RhB) is an important representative xanthene dye with high water solubility and good stability [3], which is widely used as a fluorescent dye in paint, leather, textile, and paper industries [4]. However, RhB not only causes severe environmental problems but also harms the health of animals and human beings, such as irritation to the skin, eyes, gastrointestinal, and respiratory tract [5]. Thus, RhB is often chosen as the adsorbate for the adsorption research. In recent years, various physical, chemical, and biological methods have been developed for the removal of toxic dyes from aqueous solutions, including photodegradation [6, 7], ion-exchange [8], advanced oxidation [3, 9], membrane separation [10], adsorption [11], and so on. For example, Abay et al. [12] prepared noble metal-free copper nickel oxysulfide nanoparticles for the catalytic reduction of methyl blue and RhB. Ahmedchekkat et al. [13] developed a sonophotocatalytic process to degrade RhB using a reactor geometry. Nidheesh et al. [9] used magnetite as a heterogeneous catalyst for the removal of RhB by a Fenton process. In addition, titania porous materials have been proved to be effective for the degradation of organic dyes [6, 7] and their adsorption/photolysis properties can be significantly improved by the suitable modification and functionalization [14]. Among these methods, adsorption is generally considered as one of the most effective and competitive methods for the removal of dyes from wastewater, due to simplicity of design, ease of operation, and low cost. Therefore, various adsorbents have been developed for the adsorption processes, such as reduced graphene oxide composites [15], activated carbons [16], hydrogels [17], xerogels [18], metal-organic frameworks [19], polymers [20], and so on. For instance, Bai et al. [11] used reduced graphene oxide supported ferrite hybrids to remove RhB from aqueous solution. Lacerda et al. [21] used carnauba
palm leaves, macauba endocarp, and pine nut shell as precursor lignocellulosic materials to prepare activated carbons for RhB removal from aqueous solution. However, most of these adsorbents still can not be widely used in the treatment of wastewater due to their limited adsorption capacity, low adsorption rate, or regeneration difficulty. Thus, it is necessary to develop some new adsorbents with large adsorption capacity and fast adsorption rate. As a mesoporous material with high specific surface area and large interior spaces, silica-based porous materials have obtained the widespread application in many fields, such as thermal insulation, photocatalytic activity, absorption for organic liquids or gases, etc. [22]. In recent years, SiO2–ZrO2 porous materials have been widely studied because of their higher thermal and chemical stability [23]. Some studies have been focused on their preparation and potential application in the field of thermal insulation. For example, Jing Ren et al. [24] prepared high surface area core-shell SiO2–ZrO2 aerogel by a sol–gel process followed by ambient pressure drying. It was reported that the coreshell SiO2–ZrO2 aerogel possessed a Brunauer–Emmett– Teller (BET) surface area of 619 m2/g and a pore size of 8.2 nm after heat treatment at 1000 °C. Jian He et al. [25] utilized hexamethyl-disilazane to modify the SiO2–ZrO2 gels during the aging process. The result showed that the specific surface area of ZrO2–SiO2 aerogels reached 537.2 m2/g. Furthermore, it is reported that the mechanical strength of SiO2–ZrO2 composites is higher than that of pure SiO2 samples [26, 27] due to the interface and solid solution of SiO2 and ZrO2 obtained via the solid-state reaction. Thus, SiO2–ZrO2 composites are better than single metal oxides both in structure and in the catalytic performance [28]. In addition, SiO2–ZrO2 composites as carriers or catalysts also have shown high catalytic properties in esterification, hydrogenation, and dehydrogenation [29], alcohol dehydration, CO methanation, and so on [30]. For example, Kuzminska et al. [31] immobilize heteropolyacids on a zirconia/silica support for the catalytic reactions of oleochemistry transesterification and esterification. Zhang et al. [32] fabricated the ZrO2–SiO2 supported Ni and NiCu catalysts for the guaiacol hydrodeoxygenation. Zhuang et al. [33] prepared ZrO2–SiO2 mixed oxides as catalysts for alcohol dehydration. Although SiO2–ZrO2 composites have been deeply studied for their application in many fields, few
Journal of Sol-Gel Science and Technology
work was reported on using SiO2–ZrO2 porous material as an adsorbent to remove organic dyes from wastewater. In this paper, SiO2–ZrO2 xerogel were prepared via a sol–gel method followed by ambient pressure drying. In order to explore the potential application of the SiO2–ZrO2 xerogel in the field of wastewater treatment, its adsorption capacity was studied by removal of RhB from aqueous solution through batch experiments.
the sol to gel. Then, the excessive reagent and water in the pores of gels were exchanged with ethanol three times in 8 h. Fifth, 15 vol.% TEOS ethanol solution was introduced for aging the SiO2–ZrO2 gel in a 60 °C oven for 48 h. Then, the TEOS and ethanol in the pores of gels were exchanged with N-hexane three times in 8 h. Finally, the gels were dried in ambient atmosphere followed by thermal treatment at 300 and 600 °C, respectively.
2.3 Characterization
2 Experimental 2.1 Materials Ethanol (EtOH), ZrO(NO)3·2H2O, formamid (CH3NO), 1,2-epoxypropane (C3H6O), and polyethylene glycol 400 (PEG-400) were purchased from Tianjin TIANLI Chemical Reagents Ltd. Tetraethoxysilane (TEOS), N-hexane (CH3 (CH2)4CH3), and RhB (C28H13ClN2O3) were purchased from Tianjin Kemiou Chemical Reagent Co., Ltd. All the chemical reagents were used as received.
2.2 Synthesis of SiO2–ZrO2 xerogel SiO2–ZrO2 xerogel was synthesized via a sol–gel method followed by ambient pressure drying. Figure 1 illustrated the schematic of the formation of SiO2–ZrO2 xerogels. First, 1.07 g ZrO(NO)3·2H2O was dissolved in 40 ml alcohol–water solution to obtain ZrO2 sol, in which the volume ratio of ethanol:water was 3:1. Then, 2.7 ml TEOS was dissolved in the ZrO2 sol to obtain mixed solution. Second, 0.64 ml formamide (FA, dry control chemical agent) and 0.6 ml PEG-400 (dispersant) were dissolved in the mixed solution, respectively. Third, 6.7 ml 1,2-epoxypropane (Po, gelation promoter) was added drop by drop to the mixed solution and stirred for 30 min. Fourth, the mixed sol was placed in a 40 °C oven to accelerate the transformation from
Fig. 1 Schematic of the formation of SiO2–ZrO2 xerogels
The morphologies and microstructures of these xerogels were characterized by using a scanning electron microscope (Helios Nanolab 600i, USA). Fourier-transform infrared spectroscopy was employed by using a Thermo Nicolet NEXUS 670 FTIR (USA) to investigate the chemical bonds of the mixed oxides and functional groups present in these xerogel samples. The samples were ground into refined powders, mixed with KBr and then pressed to form sample pellets. The IR spectra were recorded in the spectral range of 4000–400 cm−1. The crystalline phase of these xerogels were monitored with powder X-ray diffraction using a Philips Analytical X-ray Diffractometer (Holland) operated at 50 mA and 40 keV with Cu Kα radiation. Thermal gravimetric analysis (TGA) and differential scanning calorimetry analysis (DSC) of the resulting xerogels were performed by SDT Q600 Thermal Analyzer (USA) with a heating rate of 10 °C/min to 800 °C in the air. The nitrogen sorption isotherms of these xerogels were measured by a Micromeritic TriStarII 3020 Automatic Specific Surface and Porosity Analyze (USA). The specific surface areas and pore size distributions were calculated by BET analysis and Barrett–Joyner–Halenda (BJH) method, respectively. The pore volume was calculated by the amount of N2 when the relative pressure (P/P0) is 0.99. Prior to analysis, samples were heated to 150 °C under vacuum for at least 8 h to remove adsorbed species.
Journal of Sol-Gel Science and Technology
2.4 Adsorption experiments 2.4.1 Effect of solution pH value and adsorbent dose Adsorption behavior of the SiO2–ZrO2 xerogel was studied through batch experiments at constant temperature, before which the xerogel was ground to homogeneous powder. And all the adsorption experiments were performed in dark. 50 ml RhB aqueous solution (20 mg/L) was mixed with desired mass of SiO2–ZrO2 xerogel powder in 100 ml iodine flask and magnetic stirred (200 rpm) at room temperature (20 °C) for 24 h. Then the supernatant of the equilibrium solution was centrifuged at 10,000 rpm for 5 min. The concentration of RhB was determined through absorbance measurements at the maximum absorption wavelength of 554 nm using a visible spectrophotometer (Jinghua-721, China). Then, the concentration of RhB was calculated according to the standard correction curve. To determine the suitable pH of solution, adsorption experiments were performed with 0.1 g SiO2–ZrO2 xerogel powder and pH of RhB solutions (20 mg/L) was adjusted to 2.0, 3.0, 4.0, 5.0, 6.0, 7.0, 8.0, 9.0, 10.0, and 11.0 using HCl (0.1 mol/L) or NaOH (0.1 mol/L) solution. The concentration of RhB in solutions at different pH values was calculated according to the standard correction curve at the same pH value. To determine the suitable dose of adsorbent, adsorption experiments were performed with selected pH and certain amounts of SiO2–ZrO2 xerogel powder were added in 50 ml RhB solutions (20 mg/L), respectively. The removal ratio (%) of RhB was calculated by using the following equation: Ce Removalð%Þ ¼ 1 100%; ð1Þ C0 where C0 (mg/L) and Ce (mg/L) are the initial and equilibrium concentrations of RhB solution, respectively.
where C0 (mg/L) is the initial RhB concentration before removal, Ct (mg/L) is the residual RhB concentrations at time t, V (L) is the volume of the RhB aqueous solution, and m is the weight of the SiO2–ZrO2 xerogel powder. 2.4.3 Equilibrium experiments To confirm the adsorption isotherm model, 0.08 g SiO2–ZrO2 xerogel powder was placed in RhB solutions of various concentrations (4–240 mg/L). All batch equilibrium experiments were performed in iodine flasks and magnetic stirred (200 rpm) for certain time to establish the equilibration between adsorbent and adsorbed. The supernatant was drew from the solutions, centrifuged and analyzed using a visible spectrophotometer to determine the residual RhB concentrations at equilibrium (Ce, mg/L). The amount of RhB adsorbed onto the xerogel powder at equilibrium (qe, mg/g) can be calculated by using the following equation: qe ¼
ðC0 Ce ÞV ; m
ð3Þ
where Ce (mg/L) is the equilibrium concentrations of RhB solution, qe is the adsorption capacity at equilibrium of RhB per unit mass of adsorbent.
3 Results and discussion 3.1 Characterization of the SiO2–ZrO2 xerogels The XRD patterns of the SiO2–ZrO2 xerogels before and after 600 °C heat treatment are shown in Fig. 2, compared with the pure ZrO2 xerogel calcined at 600 °C. Typically, the diffraction peak nearby 10° (2θ) indicates that these
2.4.2 Kinetic experiments For the adsorption kinetics experiments, 0.32 g SiO2–ZrO2 xerogel powder was added to 200 ml RhB aqueous solution (20 mg/L). The suspensions were placed in iodine flasks and magnetic stirred (200 rpm) at 20, 30, and 40 °C, respectively. At appropriate time intervals, 5 ml supernatant was drew from the solutions, centrifuged and analyzed using a visible spectrophotometer to determine the residual RhB concentrations in the solutions. The amount of RhB adsorbed onto the xerogel powder at the given time (qt, mg/g) can be calculated by using the following equation: qt ¼
ðC0 Ct ÞV ; m
ð2Þ
Fig. 2 The XRD patterns of xerogels: a untreated SiO2–ZrO2 xerogel; b SiO2–ZrO2 xerogel calcined at 600 °C; c pure ZrO2 xerogel calcined at 600 °C
Journal of Sol-Gel Science and Technology
xerogels are typical mesoporous materials. The single diffuse peak at 20–30° in Fig. 2a and b indicates that SiO2–ZrO2 xerogels maintain an amorphous structure even after 600 °C calcination [34], which is a typical characteristic of porous adsorbents. In addition, the peak of SiO2–ZrO2 xerogel calcined at 600 °C is weaker than that of untreated SiO2–ZrO2 xerogel, which shows that the regularity of the SiO2–ZrO2 xerogel is weakened after calcination. The diffuse peak in XRD patterns of SiO2–ZrO2 xerogels is caused by high degree mixing of SiO2 and ZrO2, and the formation of amorphous xerogels is due to the incorporation of zirconium atoms in the framework of amorphous SiO2 [23]. In contrast, the pure ZrO2 xerogel calcined at 600 °C shows small but sharp crystallizing peaks at 30.1˚, 50.2˚, and 59.7˚, which are attributed to the tetragonal crystalline phase of ZrO2. The crystallization of ZrO2 in the SiO2–ZrO2 xerogel was prevented by SiO2 during the calcining process because the presence of Si–O–Zr bonds in SiO2–ZrO2 xerogels retard the crystal growth and phase transition [35]. The microstructure of untreated SiO2–ZrO2 xerogels and samples calcined at 300 and 600 °C are shown in Fig. 3. It can be observed that the untreated xerogel has a stack structure of flake particles. The microstructure of the xerogel changes after 300 °C heat treatment, which may attributed to the carbonization of residual organic matters. It is obviously that the SiO2–ZrO2 xerogel has a threedimensional reticulated porous structure after calcined at 600 °C, with a growth of xerogel skeleton particles and the increasing number of the pores. This may be attributed to the further oxidation and burning of the carbonized organic compounds, therefore more mesoporous were generated. The nitrogen sorption isotherms of these xerogels were measured to further investigate their microstructure, which are conducive to understanding the difference of SEM morphology. The N2 absorption–desorption isotherms of SiO2–ZrO2 xerogels are shown in Fig. 4. In the Fig. 4, all the samples exhibit typical IUPAC type IV adsorption– desorption isotherms [24]. It indicates that they are typical mesoporous materials with three-dimensional network structure [36]. Obvious hysteresis loops are observed in the three isotherms and are identified as type H1. This type of hysteresis loop is characteristic for a mesoporous structure with cylindrical pores [37] and reveals that the mesoporous material is aggregated by homogeneous spherical nanoparticles with a relatively narrow pore size distribution. The pore size distributions of SiO2–ZrO2 xerogels are shown in Fig. 5. It can be seen that the pore size distributions are relatively narrow and are centered around 10–25 nm within the mesopores region. The maximum of distribution takes a more and more big proportion with the increase of heat treatment temperature. It means that more mesoporous were generated in the range of 10–25 nm after calcination.
Fig. 3 The microstructure of SiO2–ZrO2 xerogels: a untreated; b 300 °C heat treatment; c 600 °C heat treatment
Journal of Sol-Gel Science and Technology
The specific surface area, pore volume, and average pore size of the composite xerogels under different heat treatment conditions are shown in Table 1. It can be seen that the BET-specific surface area of xerogel increases significantly after calcined at 600 °C, with an increase by 38.34% from 336.8 to 525.6 m2/g. Meanwhile, the pore volume increases from 0.74 to 1.16 cm3/g. When the heat treatment temperature is 300 °C, the BET-specific surface area and the pore volume increase slightly to 380.0 m2/g and 0.91 cm3/g, respectively. These observations show that the heat
Fig. 4 N2 adsorption–desorption isotherms of SiO2–ZrO2 xerogels
Fig. 5 Pore size distribution of SiO2–ZrO2 xerogels from BJH adsorption
Table 1 Properties of untreated SiO2–ZrO2 xerogel and xerogels calcined at 300 and 600 °C
treatment at 600 °C is favorable for improving the porous performance of SiO2–ZrO2 xerogel. Figure 6 shows the TGA and DSC curves of the untreated SiO2–ZrO2 xerogel. It can be found that the weight loss of the xerogel (TGA in Fig. 6) shows three stages (29.3 wt.%). The first weight loss of 3.8% below 80 ° C is attributed to the evaporation of residual solvent and the second weight loss of 7.7% occurred from 80 to 200 °C is attributed to the evaporation of water molecules. And a broad endothermic peak was observed in these two stages (DSC in Fig. 6), indicating the phenomenon of endothermic and evaporation in these two stages. The third weight loss of 17.8% occurred from 200 to 580 °C is attributed to the carbonization and oxidation of the organic compounds with a broad exothermic peak at approximately 300 °C. As can be seen from the TGA figure, the weight of the xerogel remains unchanged when the temperature is higher than 600 °C. It means that the SiO2–ZrO2 xerogel is free of organic impurities after heat treatment at 600 °C. The chemical bonds of the mixed oxides and functional groups presented in these xerogels were analyzed by FTIR as shown in Fig. 7. In the FTIR spectra of all the three xerogel samples, the absorption peaks at 3415 and 1625 cm−1 are attributed to –OH and H–O–H bonds, respectively [38]. They are caused by the stretching and bending vibration of the –OH group from the Zr–OH or Si–OH in the surface of the pores, as well as the absorbed water and ethanol [25]. Meanwhile, it is obvious that the absorption peaks at 3415 and 1625 cm−1 decrease with the increase of heat treatment temperature. It can be attributed to the evaporation of the
Fig. 6 TGA/DSC curves of the untreated xerogel
Samples
Heat treatment
Specific surface area (m2/g)
Pore volume (cm3/g)
Average pore size (nm)
1
Untreated
336.8
0.74
8.7
2
300 °C calcined
380.0
0.91
9.3
3
600 °C calcined
525.6
1.16
8.5
Journal of Sol-Gel Science and Technology
Fig. 7 The FTIR spectra of SiO2–ZrO2 xerogels
absorbed water and ethanol, which is in accord with the result of TGA and DSC (Fig. 6). These absorption peaks remain relatively large after heat treatment at 600 °C. It means Zr–OH and Si–OH bonds are stable and not easy to damage during the heat treatment process up to 600 °C. And during the adsorption process, the cationic dyes are easy to attach to these negative functionalities because of electrostatic interaction, which is helpful to improve the adsorption capacity. In the FTIR spectrum of the untreated xerogel, the 1625 cm−1 band shifted to 1680 cm−1. It is attributed to the stretching vibration of C=O bond from the residual FA. The big and sharp absorption peaks at 1075 cm−1 are attributed to the three-dimensional asymmetric stretching vibrations of Si–O–Si perturbed by the presence of Zr or the asymmetric stretching of Si–O–Zr band [39]. It shows that the main groups of these xerogels are Si–O–Si and Si–O–Zr. Meanwhile, the absorption peaks at 790 and 450 cm−1 in all three FTIR curves are attributed to bending vibration of Si–O [39]. The peaks at 2915 and 1384 cm−1 of the untreated xerogel are assigned to C–H bond. It indicates that there are some residual carbonaceous organic groups, which can be eliminated by calcination.
3.2 Adsorption properties In general, the adsorption properties of adsorbent mainly depends on its surface properties, especially the surface area [40]. From the previous discussion, it is known that the specific surface area of SiO2–ZrO2 xerogel increased significantly (525.6 m2/g) after calcined at 600 °C, which is more conducive to be applied in the field of adsorption. So, the SiO2–ZrO2 xerogel calcined at 600 °C was chosen as the adsorbent for subsequent adsorption experiments. pH value of aqueous solution plays an important role in controlling the adsorption process. It affects the chemistry properties of dye molecules and adsorbents, such as the
Fig. 8 Effect of pH in solution on the adsorption of RhB (conditions: 0.1 g adsorbent, 50 ml of 20 mg/L of RhB solution, magnetic stirring time of 24 h)
surface charge of adsorbents, the degree of ionization, and the structure of dye [41, 42]. The effect of solution pH value on the adsorption ratio of RhB was studied in the range of pH 2.0–11.0 at room temperature (20 °C). The experiments were performed with 0.1 g SiO2–ZrO2 xerogel powder and 50 mL RhB solutions (20 mg/L), and the results are shown in Fig. 8. It can be seen clearly from the Fig. 8 that the removal ratio reached over 95% and increased with the decrease of pH, when the pH was in the range of 2–4. When the pH was in the range of 5–8, the removal ratio was in the range of 92–93%. When the pH ≥ 9, the removal ratio was lower than 90% and decreased obviously with the increase of pH, especially when pH = 11 the removal ratio was only 51%. It is reported that the ionic forms of RhB are different in aqueous solutions with different pH values [18]. At pH < 4, the RhB ions exhibit cationic and monomeric molecular form. Thus, they can enter into the pore structure easily and the adsorption of the RhB is more likely to occur at lower pH. At pH > 4, the zwitterionic form of RhB exists as larger molecular units (dimers) in water because of the electrostatic interactions between the carboxyl and xanthene groups of the monomers [16]. Thus, they are not easy to enter into the pores. At pH > 8, the formation of hydrated ions of RhB hinders the entry of the RhB into the pores [21]. In short, the adsorption process is slightly promoted under acidic conditions and significantly inhibited under strong alkaline conditions. Since this work is designed to operate in wastewater, in which extreme pH is less required, pH was adjusted slightly to 4.0 in the subsequent experiments. The effect of xerogel dose on the removal ratio of RhB was studied in the range of 0.06–0.12 g at pH = 4.0. The experiments were performed with 50 mL RhB solutions (20 mg/L) at room temperature (20 °C), and the results are shown in Fig. 9. As expected, the removal ratio increases
Journal of Sol-Gel Science and Technology
Fig. 9 Effect of xerogel dose on the adsorption of RhB (conditions: 50 ml of 20 mg/L of RhB solution, pH = 4.0, magnetic stirring time of 24 h)
with the increase of xerogel dose, which can be attributed to the increase of the available surface area and availability of more adsorption sites [43]. It should be noted that excessive use of adsorbent will increase the cost of the wastewater treatment process. A suitable dose of xerogel must be determined for effective removal of RhB with relatively lower amount of xerogel. When dose = 0.08 g (1.6 g/L), the removal ratio reached to 92%, which is efficient enough. Therefore, the suitable dose of xerogel to adsorb RhB in this study is 0.08 g. Subsequent experiments were performed at a xerogel dose of 1.6 g/L. In the adsorption process, it is necessary to investigate the adsorption kinetics, which will help to understand the underlying absorption mechanisms [15]. Adsorption kinetics experiments were performed at 20, 30, and 40 °C, and the effect of contact time on the adsorption of RhB was presented in Fig. 10. It could be seen that the removal of RhB was rapid in the first 15 min and then adsorption rate gradually decreased till equilibrium was reached in 30 min. The removal ratio reached up to 89% when the contact time was over 30 min and no significant changes were observed with further increases in contact time. It was reported that the adsorption equilibrium time of RhB onto adsorbents in other literatures was 45 [44], 120 [40], 300 [16], 360 [43], 400 [11], 480 min [15], respectively. As can be seen from this comparison, the adsorption rate of RhB onto SiO2–ZrO2 xerogel is satisfactory for the practical applications in adsorption fields. In addition, it is obvious that the adsorption capacity decreases with the increase of temperature (11.57, 11.27, and 11.02 mg/g at 20, 30, and 40 °C, respectively), which indicates that adsorption is an exothermic process. To investigate the adsorption mechanisms, the kinetic data of the adsorption are analyzed using pseudo-first-order
Fig. 10 Effect of contact time on the adsorption of RhB at different temperatures (conditions: 0.32 g adsorbent, 200 ml of 20 mg/L of RhB solution, pH = 4.0)
and pseudo-second-order kinetics, which are generally expressed by the following linear equations [11, 44]: lnðqe qt Þ ¼ lnqe k1 t
ð4Þ
t 1 t ¼ þ ; 2 qt k 2 qe qe
ð5Þ
where qe (mg/g) is the sorption capacity at equilibrium and qt (mg/g) is the amount of adsorbed RhB at time t (min). The parameters k1 (min−1) and k2 (g/(mg.min)) are the pseudo-first-order and pseudo-second-order adsorption rate constant, respectively. Values of k1 can be calculated from the slope of a plot of ln(qe−qt) vs. t and k2 can be calculated from the intercept of a plot of t/qt vs. t. Additionally, the initial adsorption rate V0 (mg/(g.min)) can be calculated by the following equation: V0 ¼ k2 q2e :
ð6Þ
The plots of ln(qe−qt) vs. t and t/qt vs. t are given in Fig. 11a and b, respectively. The corresponding kinetic parameters and correlation coefficients (R2) are shown in Table 2. It can be seen from the Fig. 11a that the pseudo-firstorder model shows good fitting with high correlation coefficient values (R2) of 0.967, 0.981 at 20 and 30 °C, respectively. But it shows poor fitting with low R2 of 0.543 at 40 °C. In addition, the calculated equilibrium adsorption capacity (qe,cal.) are different from the experimental ones (qe, exp.). This finding indicates that the adsorption process can not be described by the pseudo-first-order kinetic model. In contrast, good linear relationships between t/qt and t are observed in Fig. 11b with correlation coefficient values (R2) of 0.999, 0.999, and 0.999, as well as the calculated qe,cal. are close to the qe,exp., which indicates the pseudo-secondorder model is more accuracy to describe the adsorption process of RhB onto SiO2–ZrO2 xerogel in this study. It
Journal of Sol-Gel Science and Technology Table 2 Kinetic parameters for the adsorption of RhB onto SiO2–ZrO2 xerogel at different temperatures Kinetic model
Kinetic parameters
20 °C
30 °C
40 °C
Pseudo-firstorder
k1 (min−1)
0.041
0.072
0.173
qe,cal. (mg/g)
1.913
1.584
2.921
qe,exp. (mg/g)
11.57
11.27
11.02
R2
0.967
0.981
0.543
Pseudok2 (g/(mg. second-order min))
0.107
0.124
0.113
V0 (mg/(g. min))
13.53
15.86
14.48
qe,cal. (mg/g)
11.25
11.31
11.32
qe,exp. (mg/g)
11.57
11.27
11.02
R2
0.999
0.999
0.999
α (mg/(g. min))
2.521 × 106 2.171 × 108 1.645 × 106
Elovich
β (g/mg)
1.701
2.112
1.640
R2
0.981
0.963
0.732
3.32 mg/(g.min) when the RhB initial concentration was 10 mg/L [11], 6.479 mg/(g.min) when the RhB initial concentration was 100 mg/L [16], and 4.57 mg/(g.min) when the RhB initial concentration was 50 mg/L [45], respectively. By comparison, it can be seen that the initial adsorption rate of RhB onto SiO2–ZrO2 xerogel is high. In reactions involving chemical adsorption, if adsorbates adsorb on the solid surfaces without desorption, the adsorption rates may decrease with time because of the increase of the surface coverage. Elvoich equation is one of the most widely used equations to investigate such chemical adsorption process, which can be expressed as follows [41]:
Fig. 11 Kinetic models for the adsorption of RhB at different temperatures: a pseudo-first-order, b pseudo-second-order, c Elvoich
suggests that the adsorption rate of RhB onto SiO2–ZrO2 xerogel depends mainly on the availability of adsorption sites rather than the RhB concentration in the solution [21]. In addition, the high values of V0 (13.53, 15.86, and 14.48 mg/ (g.min) at 20, 30, and 40 °C, respectively) also verify the fast adsorption rate for RhB within the first 15 min. It was reported that the values of V0 in other literatures were
1 1 qt ¼ lnðαβÞ þ ln t; ð7Þ β β where α is the initial sorption rate (mg/(g.min)) and β is the desorption rate constant (g/mg). Values of α and β can be calculated from the slope and intercept of the straight line plot of qt vs. lnt. The plots of qt vs. lnt are given in Fig. 11c and the corresponding parameters and correlation coefficients (R2) are shown in Table 2. As can be seen from the Fig. 11c, the Elvoich equation shows poor fitting for the experimental data and correlation coefficient values (R2) vary from 0.732 to 0.981, which indicates that the adsorption of RhB onto SiO2–ZrO2 xerogel in this study can be mainly attributed to the physical adsorption rather than the chemisorption. Adsorption isotherm shows the actual distribution of adsorbate between the bulk solution and solid adsorbent in equilibrium [46], which is useful for understanding the adsorption mechanism, surface properties, and affinity
Journal of Sol-Gel Science and Technology
between adsorbent and adsorbate [15]. To establish the adsorption isotherm model, 0.08 g SiO2–ZrO2 xerogel powder was placed in RhB solutions of various concentrations (4–240 mg/L) and batch experiments were carried out by 4 h to ensure adsorption equilibrium. The effect of initial dye concentration on the adsorption of RhB is shown in Fig. 12 and the adsorption isotherms of RhB adsorbed onto SiO2–ZrO2 xerogel in aqueous solution is given in Fig. 13. As shown in Fig. 12, the adsorption capacity in equilibrium (qe (mg/g)) increases with increasing initial concentration of RhB. This increase of the adsorption capacity may be due to the increase of the driving force of the concentration gradient, which decreases the mass transfer resistance of ions between the adsorbent and the aqueous medium [21]. The sorption behavior are evaluated by two common isotherm models including
Langmuir and Freundlich [47] (Fig. 13) and all the parameters and the correlation coefficients (R2) are listed in Table 3. The Langmuir model is based on the assumption that the uptake of dye occurs on a homogenous surface by monolayer adsorption and there is no interaction between adsorbate molecules, which can be applicable to both physical and chemical adsorption. It can be expressed as follows: qe ¼
qmax bCe 1 þ bCe
ð8Þ
or the linear form: Ce 1 1 ¼ þ Ce ; qmax qe bqmax
ð9Þ
where Ce is the equilibrium concentration of adsorbate in solution (mg/L), qe is the amount of adsorbate sorbed at equilibrium (mg/g), qmax is the maximum monolayer adsorption capacity (mg/g), and b is the Langmuir constant (L/mg). Values of qmax and b can be calculated from the slope and intercept of the plot Ce/qe vs. Ce. The essential characteristics of Langmuir model can be described by a dimensionless constant, namely, separation factor RL (also known as equilibrium parameter), which is defined by following expression: RL ¼
Fig. 12 Effect of initial dye concentration on the adsorption of RhB (conditions: 0.08 g adsorbent, 50 ml RhB solution, pH = 4.0, magnetic stirring time of 4 h)
1 ; ð1 þ bC0 Þ
ð10Þ
where C0 is initial concentration of adsorbate in solution (mg/L) and b is the Langmuir constant (L/mg). The value of RL indicates the shape of isotherms and predicts the feasibility of adsorption. Adsorption is irreversible if RL = 0, favorable if 0 < RL < 1, linear if RL = 1, and unfavorable if RL > 1. The Freundlich isotherm model is an empirical equation which is commonly used in describing the adsorption characteristics for the heterogeneous surface, considering interactions between adsorbed molecules. It can be Table 3 Langmuir and Freundlich isotherm parameters for RhB adsorbed onto SiO2–ZrO2 xerogel Isotherm model Langmuir
Freundlich Fig. 13 Adsorption isotherms of RhB adsorbed onto SiO2–ZrO2 xerogel in aqueous solution
Kinetic parameters
Value
qmax (mg/g)
177.7
b (L/mg)
0.0445
R2
0.998
KL
0.843–0.086
1/n
0.600
KF (mg/g)
12.8
R2
0.985
Journal of Sol-Gel Science and Technology Table 4 Comparison of dyes adsorption capacity of different adsorbents qmax (mg/g)
Adsorbent
References
Graphene oxide-ferrite hybrids
23.0
[11]
Activated carbons
36.4
[21] [41]
MIL-125(Ti)
59.9
Graphene oxide
94.1
[15]
Iron-pillared bentonite
98.6
[42]
Cupressus sempervirens cones
114.9
[43]
V2O5·nH2O/tetra-n-butyl titanate hybrid xerogels
179
[18]
Hydrophilic silica aerogel
189.6
[40]
SiO2–ZrO2 xerogel
177.7
This study
Table 5 Thermodynamic parameters for the adsorption of RhB onto SiO2–ZrO2 xerogel Temperature (K) ΔG (kJ mol−1) ΔH (kJ mol−1) ΔS (J mol−1 k−1) 293
−6.812
−23.572
303
−6.086
–
−57.376 –
313
−5.664
–
–
expressed as follows: qe ¼ KF Ce1=n
ð11Þ
or the linear form: 1 ð12Þ lnqe ¼ ln Ce þ ln KF ; n where KF (mg/g) and 1/n are Freundlich constant and exponent, respectively. Values of KF and 1/n can be calculated from the intercept and slope of the plot lnqe vs. lnCe. KF is a measure of the adsorptive capacity of an adsorbent for an adsorbate. And the value of 1/n is an indicator of the intensity of the adsorption, which indicates applicability of adsorbent and feasibility of adsorption. As can be seen from Fig. 13, Langmuir model shows a better fit to the experimental data than Freundlich model and the extremely high R2 value (0.998) indicates that Langmuir model can accurately describe the experimental data. The data fitting Langmuir model reveals the homogenous of the surface of the SiO2–ZrO2 xerogel and indicates there is no strong competition for occupying adsorption sites between the adsorbate and solvent [16]. The maximum adsorption capacity is 177.7 mg/g and the values of RL are between 0.086 and 0.843 (0 < RL < 1), which indicates favorable adsorption of RhB onto SiO2–ZrO2 xerogel. Table 4 shows the comparison of the maximum adsorption capacity of the as-prepared SiO2–ZrO2 xerogel in this study and other adsorbents in literatures. It can be
seen from Table 4 that most listed adsorbents have an adsorption capacity of less than 100 mg/g. It means that the maximum adsorption capacity of SiO2–ZrO2 xerogel is above the average values of the most listed adsorbents. However, it should be noted that it is infeasible to make an accurate comparison for these adsorbents, given the difference in operational conditions. The adsorption capacity decreases with the increase of temperature confirming the exothermic nature of the adsorption process. Thermodynamic parameters for the adsorption process, including the change in the Gibbs free energy (ΔG), enthalpy (ΔH), and entropy (ΔS), can be calculated from the following equations [48]: KC ¼
CA CS
ΔG ¼ RT ln KC
ð13Þ ð14Þ
ΔH ΔS ð15Þ þ ; RT R where KC is the equilibrium constant (or distribution coefficient), CA is the amount of dye ions adsorbed on adsorbent in each liter of solution at equilibrium (mg/L), CS is the equilibrium concentration of dye ions in solution (mg/ L), R is the ideal gas constant (8.314 J mol−1 K−1), and T is the absolute temperature (K). Thermodynamic parameters obtained are listed in Table 5. The negative ΔG values indicate that the adsorption process is feasible and spontaneous in nature. It is obvious that the negative of ΔG decreases with the increase of temperature, which implies that the process is more spontaneous at lower temperatures. The negative value of ΔH suggests the exothermic nature of the adsorption process. The negative value of ΔS indicates decreased randomness at the SiO2–ZrO2 xerogel/solution interface during the adsorption of RhB. The absolute value of ΔG is between 0 and 20 kJ/ mol and ΔH is below 40 kJ/mol, both of which are generally conceded to be the characteristics of physical adsorption [21]. So the high adsorption capacity may be due to the large surface area of the SiO2–ZrO2 xerogel as well as the electrostatic interaction between negatively charged xerogel surfaces and positively charged RhB molecules. ln KC ¼
4 Conclusions SiO2–ZrO2 xerogels were successfully prepared by a sol–gel method followed by ambient pressure drying and used for the removal of RhB from aqueous solution. The SiO2–ZrO2 xerogels are amorphous and possess a threedimensional network structure with a narrow distribution of
Journal of Sol-Gel Science and Technology
pore size, and their specific surface area can reach up to 525.6 m2/g after 600 °C heat treatment, with a pore volume of 1.16 cm3/g and an average pore size of 8.5 nm. The adsorption process is slightly promoted under acidic conditions and significantly inhibited under strong alkaline conditions. The equilibrium time is 30 min and the adsorption kinetics can be best described by the pseudosecond-order model. The adsorption isotherm data can be accurately described by Langmuir model with R2 of 0.998 and qmax of 177.7 mg/g. The values of ΔG and ΔH indicate that the adsorption of RhB onto SiO2–ZrO2 xerogel is both spontaneous and exothermic in nature. The high adsorption capacity of SiO2–ZrO2 xerogel for RhB can be attributed to the physical adsorption, which is promising for the removel of dyes from wastewater. Acknowledgements This work was financially supported by the Key Scientific and Technological Projects of Heilongjiang Province (grant no. GC13A102) and the Projects of 2013 Science and Technological Innovation Platform in the Field of Manufacturing, China.
Compliance with ethical standards Conflict of interest The authors declare that they have no conflict of interest.
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