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Removal of sodium dodecylbenzenesulfonate using surface-functionalized mesoporous silica nanoparticles
Microporous and Mesoporous Materials 275 (2019) 270–277
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Microporous and Mesoporous Materials journal homepage: www.elsevier.com/locate/micromeso
Removal of sodium dodecylbenzenesulfonate using surface-functionalized mesoporous silica nanoparticles
T
Daewon Kima, Jongho Kima, Kune-Woo Leeb, Taek Seung Leea,∗ a b
Organic and Optoelectronic Materials Laboratory, Department of Organic Materials Engineering, Chungnam National University, Daejeon, 34134, South Korea Decontamination and Decommissioning Research Division, Korea Atomic Energy Research Institute, Daejeon, 34057, South Korea
A R T I C LE I N FO
A B S T R A C T
Keywords: Sodium dodecylbenzenesulfonate Mesoporous silica Cs ions Nuclear decontamination waste
Sodium dodecylbenzenesulfonate (SDBS) is widely used as an efficient detergent in various fields. It is also used to remove radioactive atoms including Cs ions, and as a result, Cs-bound SDBS are generated during a nuclear decontamination process. A silica-based adsorbent with mesopores was prepared and surface-functionalized with amine groups to have a positively-charged pocket for SDBS with electrostatic attraction. The removal of SDBS using such an adsorbent was investigated under various conditions. The removal of high SDBS concentration (more than 2000 ppm) was successfully carried out using both a mesoporous structure and electrostatic attraction. The effects of the initial SDBS concentration, the concentration of adsorbent, the SDBS adsorption time, and the pH in the adsorption of SDBS with the mesoporous silica-based adsorbent were investigated in detail. In addition, the adsorption of Cs ion-bound SDBS was investigated for a practical decontamination process, implying that the adsorbent did not remove Cs ions together with SDBS.
1. Introduction Radioactive liquid waste (RLW) is mainly generated during the operation of nuclear power plants and the decontamination after their lifetime use. Among the radioactive wastes, laundry wastewater, which is a low-level radioactive waste, is generated when washing contaminated clothes and taking a shower after work. A high level of radioactive waste was produced involuntarily during the decontamination of a shut-down nuclear power plant [1,2]. The liquid radioactive waste consists of radioactive isotopes, surfactants, electrolytes, and suspended and dissolved solid contaminants [3–5]. One useful method for treating such liquid radioactive wastes is the membrane separation technique. Membrane filtration presents an advantage in the treatment of liquid radioactive waste because it provides both water purification and impurity concentration, enabling volume reduction. However, membrane fouling frequently occurs because of the adsorption of organic species, such as surfactants, on the membrane surface, which dramatically reduces the effectiveness of liquid radioactive waste treatment [6–8]. Another way to treat radioactive liquid waste is the evaporation process, which can significantly reduce the volume of radioactive waste. However, organic substances contained in liquid radioactive waste may cause difficulties in the evaporation process because of the generation of foams during the decontamination by the evaporation method. Therefore, in order to increase the efficiency of ∗
membrane purification in the membrane separation method or to treat the decontamination wastewater without foaming in the evaporation method, it is necessary to remove organic materials from various types of liquid radioactive wastes. The International Atomic Energy Agency (IAEA) recommends using a combined process of membrane filtration and adsorption to enhance the efficiency of treatment of radioactive waste [1]. Various techniques including membrane filtration [9], photodegradation [10], chemical oxidation [11], and adsorption [12] have been investigated and applied in the removal of surfactant contained in radioactive aqueous wastes. Although the removal of surfactants at low concentrations, such as in household wastewater, has been handled by using a variety of sorbents such as zeolites [13], silica gel [14], ionexchange resins [15], and activated carbon [16,17], the adsorption of highly concentrated surfactants in liquid radioactive waste has been rarely reported, presumably because of low removal efficiency. SDBS, an anionic surfactant, is widely used in various fields such as electroplating, laundry, cosmetics, car washing and food processing [18]. However, SDBS does not degrade for a long time and generates foams in rivers and surface waters, reducing oxygen penetration in water and causing environmental problems [19]. SDBS can also be used as a decontaminating agent to remove radioactive nuclides, including Cs ions. During the decontamination process, such as washing clothing contaminated with radioactive nuclides, low-level liquid wastes are
Corresponding author. E-mail address: [email protected] (T.S. Lee).
https://doi.org/10.1016/j.micromeso.2018.09.007 Received 21 July 2018; Received in revised form 2 September 2018; Accepted 9 September 2018 Available online 11 September 2018 1387-1811/ © 2018 Elsevier Inc. All rights reserved.
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obtained using a Tecnai G2 F30 instrument. X-ray diffraction (XRD) patterns were obtained using a Bruker AXS D8 diffractometer with Cu-K radiation. The inductively coupled plasma mass spectrometer (ICP-MS) used in this work was an ELAN DRC II (PerkinElmer SCIEX). X-ray photoelectron spectroscopy (XPS) was measured by a VG Multilab 2000 spectrometer (Thermo Electron Corporation). Nitrogen adsorption−desorption isotherms were obtained on a BELSORP-max instrument. The Brunauer−Emmett−Teller (BET) and Barrett−Joyner−Halenda (BJH) methods were used to determine the surface area, pore size distribution, and pore volume.
generated. The washing water used in this process contains complexing agents and surfactants as decontamination agents. Among the various decontamination agents, SDBS is frequently used because it can combine easily with positively charged radioactive nuclides and it is inexpensive and easily obtainable anywhere. For these reasons, SDBS is widely used as a decontamination agent. As a result, numerous types of RLW containing Cs-bound SDBS are generated during a decontamination process [5]. The Cs-bound SDBS complexes generated in the RLW can reduce the treatment efficiency of the decontamination wastewater in membrane filtration methods. Mesoporous silica nanoparticles (MSNs), which consist of porous channels (mesopores), are a type of important nanomaterial that has attracted interest in the field of material science. Their unique properties-such as large surface area, tunable particle sizes and shapes, controllable pore sizes, high pore volume, uniform pore size distribution, and easy functionalization-offer potential applications in smallmolecule loading and controlled delivery [20–22]. Moreover, the surface modification of MSNs has been investigated, as they can provide versatile applications in the biomedical field and in industrial chemistry [23–28]. Accordingly, such MSNs have been explored as efficient sorbents for heavy metal ions [29], phenolic compounds [30], radioactive ions [31], and organic dyes [32]. In this work, surface-functionalized MSNs were prepared to obtain combined effects of electrostatic attraction and physical adsorption within mesopores for the removal of high-concentration SDBSs. SDBS adsorption by the effects of surface functionality and mesopores was investigated by the elucidation of surfactant-sorbent interaction. The removal efficiency of SDBSs was compared using MSNs and amine- and phenyl group-functionalized MSNs. In addition, changes in SDBS removal efficiency were investigated using regular silica NPs (SNPs) to determine the role of the mesopores. Among many purification processes including distillation, membrane separation, and evaporation, adsorption process has been reported to be poor removal efficiency at high concentrations of SDBS. The removal at a high SDBS concentration (more than 2000 ppm) was attempted using MSNs. Finally, an adsorption experiment was conducted to determine whether the SDBS and the Cs ion could be simultaneously removed by MSNs using a mixed solution of SDBS and Cs ions that mimicked the liquid waste generated after decontamination.
2.2. Synthesis of MSNs MSNs were synthesized according to a modified literature method [33]. CTAB (1.032 g) and 2 M sodium hydroxide aqueous solution (4.2 mL) were added to water (600 mL). After stirring for 15 min at room temperature, TEOS (6 mL) was added to the solution. The reaction mixture was stirred vigorously at 80 °C for 2 h. The precipitate was isolated by centrifugation, washed with methanol repeatedly and, then dried under vacuum for 20 h. For the complete removal of the template that might remain in the mesopores, the solid was dispersed in a mixture of ethanol (160 mL) and hydrochloric acid (1.6 mL). The mixture was stirred at 80 °C for 16 h and cooled to room temperature. The MSNs were washed with ethanol and water repeatedly, and finally dried under vacuum for 24 h. 2.3. Synthesis of amine-functionalized MSNs (MSN-NH2) The MSNs (0.65 g) were added to toluene (195 mL) and uniformly dispersed. Next, APTES (1.82 mL) was injected into the mixture and refluxed under vigorous stirring for 24 h. After cooling to room temperature, the MSN-NH2 obtained was washed with methanol at least three times and dried under vacuum. 2.4. Synthesis of phenyl-functionalized MSNs (MSN-Ph) MSNs (0.5 g) were dispersed in toluene (150 mL). PhTES (0.45 mL) was then injected into the mixture and refluxed for 24 h. The subsequent procedure was similar to that used for the synthesis of MSNNH2.
2. Experimental 2.1. Materials and characterization
2.5. Synthesis of amine-functionalized SNPs (SNP-NH2)
Triethoxyphenylsilane (PhTES, 98%), SDBS (CH3(CH2)11C6H4SO3Na, Scheme 1), N-cetyltrimethylammonium bromide (CTAB), and cesium chloride were purchased from Sigma-Aldrich (US). Tetraethyl orthosilicate (TEOS, 95%), toluene, ethanol, methanol, and ammonia solution (28 wt%) were purchased from Samchun Chemicals (Korea). (3-Aminopropyl)triethoxysilane (APTES, 98%) was obtained from TCI (Japan). All reagents were used without further purification. Fourier transform infrared (FT-IR) spectra were obtained with a Bruker Tensor 27 spectrometer. Ultraviolet visible (UV-vis) absorption spectra were recorded on a PerkinElmer Lambda 365 spectrometer. Zeta-potentials were determined using dynamic light scattering (DLS, Zetasizer Nano ZS, Malvern). Scanning electron microscopy (SEM) images were obtained using a Hitachi S4800 instrument. Transmission electron microscopy (TEM) images were
Regular SNPs were synthesized according to a literature method [34]. TEOS (16.75 mL) was mixed with ethanol (133.25 mL). At the same time, a mixture of ammonia (3.81 mL), water (40.78 mL), and ethanol (105.46 mL) was prepared. Then, both solutions were mixed and stirred at room temperature. After stirring for 210 min, the mixture was washed three times with methanol. The precipitated solid was dried in a vacuum oven for 24 h. The resulting SNPs were functionalized with APTES. SNPs (1 g) were dispersed in toluene (300 mL). APTES (1.4 mL) was added to the mixture and refluxed for 24 h. The mixture was washed with methanol at least three times and then dried under vacuum to obtain SNP-NH2. 2.6. Batch adsorption of SDBS An adsorbent was dispersed in the aqueous solution of SDBS and the mixture was stirred for 24 h. The adsorbent was then removed from the mixture using a syringe filter (0.20 μm). The absorbance of the solution at 224 nm was measured with a UV-vis spectrometer to determine the SDBS concentration in the solution, using a predetermined calibration curve [35,36]. The equilibrium adsorption capacity of the adsorbent and the SDBS removal efficiency were determined by the following equations:
Scheme 1. Chemical structure of SDBS. 271
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Fig. 1. TEM images of (a) MSNs, (b) MSN-NH2, (c) MSN-NH2@SDBS, and (d) MSN-Ph.
60 to 90 nm (Fig. S1 and Table S1). MSNs, MSN-NH2, MSN-Ph, and SNP-NH2 showed spherical particles, irrespective of the different surface functionalization and degree of mesoporosity. The chemical structure of silica-based particles was confirmed by FT-IR, to exhibit characteristic bands of silica such as the Si—O—Si stretching vibration at 1095 cm−1, and Si—OH at 954 cm−1 (Fig. S1e). Alkyl chains were found in SNP-NH2 and MSN-NH2 at 2900 to 3000 cm−1, and the characteristic band of the amine groups overlapped that of the hydroxyl groups above 3300 cm−1. The characteristic bands of the aromatic ring of MSN-Ph were observed at 3078 and 3059 cm−1, in addition to the C—H stretching vibration of the phenyl group at 1431 cm−1 and to the –CH2– asymmetric bending vibration of the aromatic ring at 738 and 698 cm−1 [37]. The spectroscopic results indicated that the silica adsorbents were successfully prepared as expected. The mesoporous structure of the MSNs was confirmed by TEM and was maintained after the surface functionalization for the preparation of MSN-NH2 and MSNPh, and even after the adsorption of SDBS by MSN-NH2 (designated as MSN-NH2@SDBS) (Fig. 1). The XRD patterns of MSNs and MSN-NH2 showed a broad peak at 2θ = 15–30°, indicative of the amorphous structure of silica-based adsorbents (Fig. S2a). To investigate the mesoporous structure of the silica-based adsorbents, the nitrogen adsorption/desorption isotherms (using the BET method) were measured (Fig. S2b). In the nitrogen adsorption–desorption isotherm, MSNs and MSN-NH2 were classified as type IV isotherms with an H3-type hysteresis loop at a relative pressure
Table 1 Mesoporous structure of MSN, MSN-NH2, and MSN-NH2@SDBS.
MSN MSN-NH2 MSN-NH2@SDBS
qe = (Co − Ce )
Surface area (m2 g−1)
Pore size (nm)
Pore volume (cm3 g−1)
1035.8 264.4 62.771
3.33 2.44 N.A.
1.1308 0.336 0.2177
V m
Removal efficiency (%) =
(1)
(Co − Ce ) Co
(2)
where qe (mg/g) is the equilibrium adsorption capacity, and C0 and Ce (mg/L) are the initial and equilibrium concentrations of SDBS in the solution, respectively. V (L) is the sample volume and m (g) is the weight of the adsorbent used. 3. Results and discussion 3.1. Characterization TEOS-based silica particles were prepared by a modified sol-gel method, and the shapes were confirmed by SEM with sizes ranging from 272
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Fig. 2. SDBS adsorption efficiency of MSN-NH2, SNP-NH2, and MSN-Ph. [adsorbents] = 1 mg/mL; [SDBS] = 0.5 mg/mL.
Fig. 4. Changes in (a) equilibrium adsorption capacity and (b) SDBS adsorption efficiency of MSN-NH2 according to the concentrations of MSN-NH2.
(P/Po) between 0.2 and 1.0, indicative of the typical characteristics of mesoporous materials [38]. The zeta potentials of the adsorbents were determined using DLS and are summarized in Table S1. MSN-NH2 and SNP-NH2 showed positive zeta potentials (+3.02 and + 5.3 mV, respectively) because of the amine functionalization, whereas MSN-Ph exhibited a negative zeta potential (−9.5 mV). To determine the total surface area of the silica-based materials, the pore-size distribution curves were investigated using the BJH method (Fig. S2c). The surface area and pore volume of MSNs were determined as 1035.8 m2 g−1 and 1.1308 cm3 g−1, respectively. After the surface modification, the surface area and pore volume of the MSN-NH2 decreased to 264.4 m2 g−1 and to 0.336 cm3 g−1, respectively. Considerable reductions in the surface area (62.771 m2 g−1) and pore volume (0.2177 cm3 g−1) were observed after adsorption of SDBS (MSN-NH2@ SDBS) (see Table 1). It was not possible to obtain feasible data for the pore size of MSN-NH2@SDBS, because the mesopores of MSN-NH2 were filled with SDBS. In the XPS spectra of MSN-NH2 and MSN-NH2@SDBS (Fig. S3a), the
Fig. 3. Changes in (a) equilibrium adsorption capacity and (b) SDBS adsorption efficiency of MSN-NH2 by various initial concentrations of SDBS.
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Accordingly, the removal efficiency of SDBS by MSN-NH2 was high because of the electrostatic attraction between MSN-NH2 and SDBS. As the mesopores of MSN-NH2 were filled with SDBS according to the BJH data, it is thought that the adsorption of SDBS by MSN-NH2 occurred in the positively charged mesopores of the adsorbent. The removal of SDBS using SNP-NH2 that was positively charged and not mesoporous, was investigated to determine whether the mesoporous structure affected the removal of SDBS (Fig. 2). Efficient adsorption of SDBS occurred at 1 mg/mL concentration of adsorbent and 0.5 mg/mL concentration of SDBS. It was confirmed that the removal efficiency and equilibrium adsorption (qe) of MSN-NH2 were much higher than those of SNP-NH2, indicating that the mesoporous structure was essential for the effective removal of SDBS. We attempted to investigate the possible effect of the π−π interaction or the hydrophobic interaction between benzene rings in SDBS and MSN-Ph. The removal efficiency of SDBS was about 30% for MSN-Ph; both the removal efficiency and qe were considerably lower than in MSN-NH2, suggesting that the adsorption of SDBS by the electrostatic interaction of MSN-NH2 was more effective than the hydrophobic or π−π interaction of MSNPh. In addition, the qe of MSN-NH2 increased dramatically with an increase in the initial concentration of SDBS (C0) (Fig. S6). The qe value reached a maximum when C0 was 1000 μg/mL and became saturated above that concentration. The critical micelle concentration (CMC) of SDBS has been reported to range from 553 to 1400 μg/mL and varies in the literature [43–46]. In our case, it seems that the qe value was not affected by the CMC of SDBS, although the mechanism of adsorption of surfactants above CMC has not been clearly understood [47]. In contrast, MSN-Ph showed a steady increment in qe with increase in Ce regardless of CMC, presumably because of the poor adsorption.
Fig. 5. Effects of pH on SDBS adsorption efficiency of MSN-NH2. [SDBS] = 0.5 mg/mL; [SDBS/Cs] = 0.5 mg/mL; [MSN-NH2] = 1 mg/mL.
N 1s peak was observed, indicating the presence of amine groups on the silica surface. In the N 1s spectra (Fig. S3b and c), a free -NH2 peak at 399 ± 0.2 eV and the peak by hydrogen bonded and protonated -NH2 at 401.5 ± 0.1 eV were identified [39–42]. The -NH2 peak from hydrogen bonded and protonated N was predominant in MSN-NH2, while the free -NH2 peak was dominated in MSN-NH2@SDBS. Because the adsorption of SDBS on MSN-NH2 might hinder the hydrogen bonding between the amine groups in MSN-NH2, it was assumed that the amount of -NH2 peak from hydrogen bonded and protonated N decreased and N from the free-NH2 increased. 3.2. Adsorption
3.2.2. Adsorption behavior of SDBS by MSN-NH2 To elucidate further the change in qe and the removal by MSN-NH2, the effect of various initial concentrations of SDBS was investigated. As C0 increased, the value of qe increased tremendously (Fig. 3a). The most efficient removal of SDBS was observed when C0 was 200 μg/mL, except when 4 mg/mL of MSN-NH2 was used (Fig. 3b). Above the C0 value of 200 μg/mL, the removal efficiency decreased. Interestingly, adsorption by MSN-NH2 showed highly efficient adsorption of SDBS at the higher concentration of 2000 μg/mL (2000 ppm), which is a much higher concentration than the CMC of SDBS (553–1400 ppm). As the concentration of MSN-NH2 increased, the qe value decreased above 1 mg/mL of MSN-NH2 (Fig. 4a). A noticeable change in removal values above an MSN-NH2 concentration of 4 mg/mL was observed (Fig. 4b). The pH value is an important parameter affecting adsorption efficiency in aqueous solutions. The effect of pH on the removal efficiency of SDBS by MSN-NH2 was investigated at various pH values (Fig. 5). For
The removal efficiency of SDBS was investigated using various adsorbents, including MSNs, MSN-NH2, and MSN-Ph to confirm the effects of various factors (adsorbent concentration, initial SDBS concentration and pH) on adsorption. 3.2.1. Effect of surface-functionalization of adsorbents on the removal of SDBS Effect of surface charge of the adsorbent was investigated in terms of the removal efficiency of SDBS. The negatively charged MSNs showed a remarkably lower removal efficiency than the positively charged MSN-NH2 (Fig. S4). The adsorption of SDBS by MSNs was negligible, whereas MSN-NH2 showed about 80% removal efficiency at the SDBS concentration of 0.4 mg/mL (400 ppm). It is thought that such a low adsorption by the MSNs resulted from the electrostatic repulsive force between the negative MSNs and the anionic sulfonate ion in SDBS.
Scheme 2. Schematic illustration of adsorption of SDBS by MSN-NH2. 274
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Fig. 6. (a) Effects of contact time on adsorption capacity of MSN-NH2 towards SDBS, Fitting plots of (b) pseudo first order and (c) pseudo second order kinetic models.
potential at low pH, which strengthened the electrostatic interaction between MSN-NH2 and SDBS. SDBS was not adsorbed on MSN-NH2 at basic conditions such as pH 11 or 12. This was because the silica structure of MSN-NH2 was destroyed in the alkaline solution. To understand whether Cs ions were removed by MSN-NH2 together with SDBS or whether only SDBS was removed in a mixed solution of SDBS and Cs ions, the Cs ion concentrations was measured before and after adsorption by MSN-NH2 in the mixed solution. Interestingly, it was found that the MSN-NH2 could adsorb SDBS only, because the concentrations of Cs ions before and after adsorption of SDBS/Cs were similar. This indicates that Cs ions were separated during the adsorption process of SDBS by the MSN-NH2 (Scheme 2). To confirm the effect of main ion species on adsorption of SDBS using MSN-NH2, SDBS removal efficiency of MSN-NH2 was measured in the presence of NaCl or KCl (Fig. S7). SDBS removal efficiency of MSNNH2 in the presence of NaCl and KCl was similar to that of salt-free conditions, indicating that the presence of NaCl and KCl salts did not significantly affect SDBS adsorption.
Table 2 Adsorption kinetic parameters of the adsorption of SDBS onto the MSN- NH2. Pseudo-first-order qe,exp (mg/g) qe,cal (mg/g) K1 * 10−3 (min−1) R2
Pseudo-second-order
105.3 14.1 0.9958
161.4 qe,cal(mg/g) K2 * 10−3 (g mg−1 min−1) R2
167.2 0.317 0.9907
The initial SDBS concentration is 200 mg/L.
the practical use of MSN-NH2, the same adsorption experiment was also performed using an aqueous solution containing SDBS pretreated with Cs ions to form Cs-bound SDBS (SDBS/Cs). The SDBS/Cs compound was prepared by simple mixing of SDBS and CsCl, and the effect of Cs ions on the removal of SDBS was checked using the MSN-NH2. The removal of SDBS attained about 80% at pH values ranging from 4 to 9, and the removal was more than 90% at the low pH of 2–3, regardless of the binding of the Cs ion to SDBS. The MSN-NH2 showed a positive zeta
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kinetic models. Pseudo first order and pseudo second order models are given as follows: Pseudo first order
ln(qe − qt ) = ln qe − k1 t
(3)
Pseudo second order
t 1 t = + qt qe k2 qe2
(4)
where qe and qt (mg/g) are the amounts of SDBS adsorbed on MSN-NH2 at equilibrium and at time t (min), and k1 (min−1) and k2 (g mg−1 min−1) are the rate constant for the pseudo first order and pseudo second order equation, respectively. The linear fittings of the pseudo first order and pseudo second order kinetic models are shown in Fig. 6b and c. The kinetic parameters were calculated based on the slope and intercept for the fitting curve. As can be seen in Table 2, the correlation coefficient R2 values for the pseudo first order and pseudo second order models are 0.9958 and 0.9907, respectively. Although the R2 value of the pseudo first order is slightly higher than that of the pseudo second order, the qe value (qe,exp) obtained from the experiment is very similar to the qe value (qe,cal) calculated from the pseudo second order. Therefore, it is confirmed that the pseudo second order model is a more suitable model than the pseudo first order model. Adsorption isotherms are widely used to evaluate the adsorption characteristics of adsorbents and help understand the adsorption process. Langmuir and Freundlich isotherm models were used to understand the adsorption behavior of SDBS onto MSN-NH2 at room temperature. The linear equation of Langmuir model is given as follows:
Ce C 1 = e + qe qm KL qm
(5)
Ce (mg/mL) and qe (mg/g) are the equilibrium concentration of SDBS and the adsorption capacity at equilibrium, respectively. qm (mg/ g) is the maximum adsorption capacity of SDBS adsorbed on MSN-NH2, and KL (L/mg) is the Langmuir constant. The linear equation of the Freundlich model can be expressed as:
lnqe = lnKF +
Table 3 Adsorption isotherm parameters for SDBS by MSN-NH2. Parameter
Parameter value
Langmuir
qm (mg/g) KL (L/mg) R2 1/n KF R2
854.70 0.0053 0.9197 0.7027 11.030 0.9818
Freundlich
(6)
KF and 1/n were the Freundlich model constants, indicating capacity and intensity of adsorption, respectively. Fig. 7 shows the Langmuir and Freundlich fitting curves for SDBS adsorption on MSN-NH2 and the values obtained from fitting curves are shown in Table 3. Among the two isotherm models, R2 value of the Freundlich model is closer to 1 than that of the Langmuir model, indicating that the adsorption is more suitable for the Freundlich model. Therefore, it was confirmed that adsorption behavior of SDBS on MSNNH2 was not limited to monolayer adsorption. The R2 value of the Langmuir isotherm model is relatively low, but the calculated qm value is 854.70 mg/g, which is much higher than the qm value of previously reported SDBS adsorbents such as zeolite modified with CTAB (30.7 mg/g) [18], polyaniline doped with CuCl2 (32.3 mg/g) [19], activated carbon (29.4 mg/g, 468.8 mg/g) [48,49], and activated carbon modified with quaternary ammonium (77.8 mg/g) (Table S2) [50].
Fig. 7. Fitting plots of (a) Langmuir and (b) Freundlich isotherm models for the adsorption of SDBS onto MSN-NH2.
Model
1 Ce n
3.3. Adsorption isotherms and kinetics
4. Conclusions
The adsorption kinetics can be used to determine the adsorption rate and help understand the adsorption mechanism. Fig. 6a shows the effect of contact time on the adsorption capacity of MSN-NH2 towards SDBS. The adsorption capacity of MSN-NH2 for SDBS increased rapidly until the first 100 min and reached to equilibrium within 240 min. The adsorption was applied to pseudo first order and pseudo second order
In this study, the removal efficiency of SDBS using silica-based adsorbents was investigated under various conditions. MSN-NH2, had a mesoporous structure and strong electrostatic interaction with SDBS, and was found to be effective in the removal of SDBS, in terms of initial concentration of SDBS, concentration of adsorbent, and pH value. In the adsorption of SDBS or SDBS/Cs, very similar adsorption performance 276
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was observed for MSN-NH2 in both cases. The concentrations of Cs ions were almost identical before and after adsorption by MSN-NH2, indicating that Cs ions were not removed together with SDBS. In addition, the maximum adsorption capacity of MSN-NH2 for SDBS was much higher than previously reported adsorbents. Therefore, the MSN-NH2 material was considered to be a suitable adsorbent for the removal of anionic surfactants that might be contained at high concentration in the decontamination waste solution, such as SDBS, because of its simple preparation, low cost, and excellent adsorption efficiency for SDBS even above CMC. In particular, removal of SDBS using MSN-NH2 prior to treatment such as evaporation or membrane separation may increase the efficiency of the evaporation or membrane separation process. MSN-NH2 can also be used in decontamination processes to remove SDBS bound to cesium in addition to SDBS in the radioactive waste treatment process.
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Microporous and Mesoporous Materials journal homepage: www.elsevier.com/locate/micromeso
Removal of sodium dodecylbenzenesulfonate using surface-functionalized mesoporous silica nanoparticles
T
Daewon Kima, Jongho Kima, Kune-Woo Leeb, Taek Seung Leea,∗ a b
Organic and Optoelectronic Materials Laboratory, Department of Organic Materials Engineering, Chungnam National University, Daejeon, 34134, South Korea Decontamination and Decommissioning Research Division, Korea Atomic Energy Research Institute, Daejeon, 34057, South Korea
A R T I C LE I N FO
A B S T R A C T
Keywords: Sodium dodecylbenzenesulfonate Mesoporous silica Cs ions Nuclear decontamination waste
Sodium dodecylbenzenesulfonate (SDBS) is widely used as an efficient detergent in various fields. It is also used to remove radioactive atoms including Cs ions, and as a result, Cs-bound SDBS are generated during a nuclear decontamination process. A silica-based adsorbent with mesopores was prepared and surface-functionalized with amine groups to have a positively-charged pocket for SDBS with electrostatic attraction. The removal of SDBS using such an adsorbent was investigated under various conditions. The removal of high SDBS concentration (more than 2000 ppm) was successfully carried out using both a mesoporous structure and electrostatic attraction. The effects of the initial SDBS concentration, the concentration of adsorbent, the SDBS adsorption time, and the pH in the adsorption of SDBS with the mesoporous silica-based adsorbent were investigated in detail. In addition, the adsorption of Cs ion-bound SDBS was investigated for a practical decontamination process, implying that the adsorbent did not remove Cs ions together with SDBS.
1. Introduction Radioactive liquid waste (RLW) is mainly generated during the operation of nuclear power plants and the decontamination after their lifetime use. Among the radioactive wastes, laundry wastewater, which is a low-level radioactive waste, is generated when washing contaminated clothes and taking a shower after work. A high level of radioactive waste was produced involuntarily during the decontamination of a shut-down nuclear power plant [1,2]. The liquid radioactive waste consists of radioactive isotopes, surfactants, electrolytes, and suspended and dissolved solid contaminants [3–5]. One useful method for treating such liquid radioactive wastes is the membrane separation technique. Membrane filtration presents an advantage in the treatment of liquid radioactive waste because it provides both water purification and impurity concentration, enabling volume reduction. However, membrane fouling frequently occurs because of the adsorption of organic species, such as surfactants, on the membrane surface, which dramatically reduces the effectiveness of liquid radioactive waste treatment [6–8]. Another way to treat radioactive liquid waste is the evaporation process, which can significantly reduce the volume of radioactive waste. However, organic substances contained in liquid radioactive waste may cause difficulties in the evaporation process because of the generation of foams during the decontamination by the evaporation method. Therefore, in order to increase the efficiency of ∗
membrane purification in the membrane separation method or to treat the decontamination wastewater without foaming in the evaporation method, it is necessary to remove organic materials from various types of liquid radioactive wastes. The International Atomic Energy Agency (IAEA) recommends using a combined process of membrane filtration and adsorption to enhance the efficiency of treatment of radioactive waste [1]. Various techniques including membrane filtration [9], photodegradation [10], chemical oxidation [11], and adsorption [12] have been investigated and applied in the removal of surfactant contained in radioactive aqueous wastes. Although the removal of surfactants at low concentrations, such as in household wastewater, has been handled by using a variety of sorbents such as zeolites [13], silica gel [14], ionexchange resins [15], and activated carbon [16,17], the adsorption of highly concentrated surfactants in liquid radioactive waste has been rarely reported, presumably because of low removal efficiency. SDBS, an anionic surfactant, is widely used in various fields such as electroplating, laundry, cosmetics, car washing and food processing [18]. However, SDBS does not degrade for a long time and generates foams in rivers and surface waters, reducing oxygen penetration in water and causing environmental problems [19]. SDBS can also be used as a decontaminating agent to remove radioactive nuclides, including Cs ions. During the decontamination process, such as washing clothing contaminated with radioactive nuclides, low-level liquid wastes are
Corresponding author. E-mail address: [email protected] (T.S. Lee).
https://doi.org/10.1016/j.micromeso.2018.09.007 Received 21 July 2018; Received in revised form 2 September 2018; Accepted 9 September 2018 Available online 11 September 2018 1387-1811/ © 2018 Elsevier Inc. All rights reserved.
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obtained using a Tecnai G2 F30 instrument. X-ray diffraction (XRD) patterns were obtained using a Bruker AXS D8 diffractometer with Cu-K radiation. The inductively coupled plasma mass spectrometer (ICP-MS) used in this work was an ELAN DRC II (PerkinElmer SCIEX). X-ray photoelectron spectroscopy (XPS) was measured by a VG Multilab 2000 spectrometer (Thermo Electron Corporation). Nitrogen adsorption−desorption isotherms were obtained on a BELSORP-max instrument. The Brunauer−Emmett−Teller (BET) and Barrett−Joyner−Halenda (BJH) methods were used to determine the surface area, pore size distribution, and pore volume.
generated. The washing water used in this process contains complexing agents and surfactants as decontamination agents. Among the various decontamination agents, SDBS is frequently used because it can combine easily with positively charged radioactive nuclides and it is inexpensive and easily obtainable anywhere. For these reasons, SDBS is widely used as a decontamination agent. As a result, numerous types of RLW containing Cs-bound SDBS are generated during a decontamination process [5]. The Cs-bound SDBS complexes generated in the RLW can reduce the treatment efficiency of the decontamination wastewater in membrane filtration methods. Mesoporous silica nanoparticles (MSNs), which consist of porous channels (mesopores), are a type of important nanomaterial that has attracted interest in the field of material science. Their unique properties-such as large surface area, tunable particle sizes and shapes, controllable pore sizes, high pore volume, uniform pore size distribution, and easy functionalization-offer potential applications in smallmolecule loading and controlled delivery [20–22]. Moreover, the surface modification of MSNs has been investigated, as they can provide versatile applications in the biomedical field and in industrial chemistry [23–28]. Accordingly, such MSNs have been explored as efficient sorbents for heavy metal ions [29], phenolic compounds [30], radioactive ions [31], and organic dyes [32]. In this work, surface-functionalized MSNs were prepared to obtain combined effects of electrostatic attraction and physical adsorption within mesopores for the removal of high-concentration SDBSs. SDBS adsorption by the effects of surface functionality and mesopores was investigated by the elucidation of surfactant-sorbent interaction. The removal efficiency of SDBSs was compared using MSNs and amine- and phenyl group-functionalized MSNs. In addition, changes in SDBS removal efficiency were investigated using regular silica NPs (SNPs) to determine the role of the mesopores. Among many purification processes including distillation, membrane separation, and evaporation, adsorption process has been reported to be poor removal efficiency at high concentrations of SDBS. The removal at a high SDBS concentration (more than 2000 ppm) was attempted using MSNs. Finally, an adsorption experiment was conducted to determine whether the SDBS and the Cs ion could be simultaneously removed by MSNs using a mixed solution of SDBS and Cs ions that mimicked the liquid waste generated after decontamination.
2.2. Synthesis of MSNs MSNs were synthesized according to a modified literature method [33]. CTAB (1.032 g) and 2 M sodium hydroxide aqueous solution (4.2 mL) were added to water (600 mL). After stirring for 15 min at room temperature, TEOS (6 mL) was added to the solution. The reaction mixture was stirred vigorously at 80 °C for 2 h. The precipitate was isolated by centrifugation, washed with methanol repeatedly and, then dried under vacuum for 20 h. For the complete removal of the template that might remain in the mesopores, the solid was dispersed in a mixture of ethanol (160 mL) and hydrochloric acid (1.6 mL). The mixture was stirred at 80 °C for 16 h and cooled to room temperature. The MSNs were washed with ethanol and water repeatedly, and finally dried under vacuum for 24 h. 2.3. Synthesis of amine-functionalized MSNs (MSN-NH2) The MSNs (0.65 g) were added to toluene (195 mL) and uniformly dispersed. Next, APTES (1.82 mL) was injected into the mixture and refluxed under vigorous stirring for 24 h. After cooling to room temperature, the MSN-NH2 obtained was washed with methanol at least three times and dried under vacuum. 2.4. Synthesis of phenyl-functionalized MSNs (MSN-Ph) MSNs (0.5 g) were dispersed in toluene (150 mL). PhTES (0.45 mL) was then injected into the mixture and refluxed for 24 h. The subsequent procedure was similar to that used for the synthesis of MSNNH2.
2. Experimental 2.1. Materials and characterization
2.5. Synthesis of amine-functionalized SNPs (SNP-NH2)
Triethoxyphenylsilane (PhTES, 98%), SDBS (CH3(CH2)11C6H4SO3Na, Scheme 1), N-cetyltrimethylammonium bromide (CTAB), and cesium chloride were purchased from Sigma-Aldrich (US). Tetraethyl orthosilicate (TEOS, 95%), toluene, ethanol, methanol, and ammonia solution (28 wt%) were purchased from Samchun Chemicals (Korea). (3-Aminopropyl)triethoxysilane (APTES, 98%) was obtained from TCI (Japan). All reagents were used without further purification. Fourier transform infrared (FT-IR) spectra were obtained with a Bruker Tensor 27 spectrometer. Ultraviolet visible (UV-vis) absorption spectra were recorded on a PerkinElmer Lambda 365 spectrometer. Zeta-potentials were determined using dynamic light scattering (DLS, Zetasizer Nano ZS, Malvern). Scanning electron microscopy (SEM) images were obtained using a Hitachi S4800 instrument. Transmission electron microscopy (TEM) images were
Regular SNPs were synthesized according to a literature method [34]. TEOS (16.75 mL) was mixed with ethanol (133.25 mL). At the same time, a mixture of ammonia (3.81 mL), water (40.78 mL), and ethanol (105.46 mL) was prepared. Then, both solutions were mixed and stirred at room temperature. After stirring for 210 min, the mixture was washed three times with methanol. The precipitated solid was dried in a vacuum oven for 24 h. The resulting SNPs were functionalized with APTES. SNPs (1 g) were dispersed in toluene (300 mL). APTES (1.4 mL) was added to the mixture and refluxed for 24 h. The mixture was washed with methanol at least three times and then dried under vacuum to obtain SNP-NH2. 2.6. Batch adsorption of SDBS An adsorbent was dispersed in the aqueous solution of SDBS and the mixture was stirred for 24 h. The adsorbent was then removed from the mixture using a syringe filter (0.20 μm). The absorbance of the solution at 224 nm was measured with a UV-vis spectrometer to determine the SDBS concentration in the solution, using a predetermined calibration curve [35,36]. The equilibrium adsorption capacity of the adsorbent and the SDBS removal efficiency were determined by the following equations:
Scheme 1. Chemical structure of SDBS. 271
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Fig. 1. TEM images of (a) MSNs, (b) MSN-NH2, (c) MSN-NH2@SDBS, and (d) MSN-Ph.
60 to 90 nm (Fig. S1 and Table S1). MSNs, MSN-NH2, MSN-Ph, and SNP-NH2 showed spherical particles, irrespective of the different surface functionalization and degree of mesoporosity. The chemical structure of silica-based particles was confirmed by FT-IR, to exhibit characteristic bands of silica such as the Si—O—Si stretching vibration at 1095 cm−1, and Si—OH at 954 cm−1 (Fig. S1e). Alkyl chains were found in SNP-NH2 and MSN-NH2 at 2900 to 3000 cm−1, and the characteristic band of the amine groups overlapped that of the hydroxyl groups above 3300 cm−1. The characteristic bands of the aromatic ring of MSN-Ph were observed at 3078 and 3059 cm−1, in addition to the C—H stretching vibration of the phenyl group at 1431 cm−1 and to the –CH2– asymmetric bending vibration of the aromatic ring at 738 and 698 cm−1 [37]. The spectroscopic results indicated that the silica adsorbents were successfully prepared as expected. The mesoporous structure of the MSNs was confirmed by TEM and was maintained after the surface functionalization for the preparation of MSN-NH2 and MSNPh, and even after the adsorption of SDBS by MSN-NH2 (designated as MSN-NH2@SDBS) (Fig. 1). The XRD patterns of MSNs and MSN-NH2 showed a broad peak at 2θ = 15–30°, indicative of the amorphous structure of silica-based adsorbents (Fig. S2a). To investigate the mesoporous structure of the silica-based adsorbents, the nitrogen adsorption/desorption isotherms (using the BET method) were measured (Fig. S2b). In the nitrogen adsorption–desorption isotherm, MSNs and MSN-NH2 were classified as type IV isotherms with an H3-type hysteresis loop at a relative pressure
Table 1 Mesoporous structure of MSN, MSN-NH2, and MSN-NH2@SDBS.
MSN MSN-NH2 MSN-NH2@SDBS
qe = (Co − Ce )
Surface area (m2 g−1)
Pore size (nm)
Pore volume (cm3 g−1)
1035.8 264.4 62.771
3.33 2.44 N.A.
1.1308 0.336 0.2177
V m
Removal efficiency (%) =
(1)
(Co − Ce ) Co
(2)
where qe (mg/g) is the equilibrium adsorption capacity, and C0 and Ce (mg/L) are the initial and equilibrium concentrations of SDBS in the solution, respectively. V (L) is the sample volume and m (g) is the weight of the adsorbent used. 3. Results and discussion 3.1. Characterization TEOS-based silica particles were prepared by a modified sol-gel method, and the shapes were confirmed by SEM with sizes ranging from 272
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Fig. 2. SDBS adsorption efficiency of MSN-NH2, SNP-NH2, and MSN-Ph. [adsorbents] = 1 mg/mL; [SDBS] = 0.5 mg/mL.
Fig. 4. Changes in (a) equilibrium adsorption capacity and (b) SDBS adsorption efficiency of MSN-NH2 according to the concentrations of MSN-NH2.
(P/Po) between 0.2 and 1.0, indicative of the typical characteristics of mesoporous materials [38]. The zeta potentials of the adsorbents were determined using DLS and are summarized in Table S1. MSN-NH2 and SNP-NH2 showed positive zeta potentials (+3.02 and + 5.3 mV, respectively) because of the amine functionalization, whereas MSN-Ph exhibited a negative zeta potential (−9.5 mV). To determine the total surface area of the silica-based materials, the pore-size distribution curves were investigated using the BJH method (Fig. S2c). The surface area and pore volume of MSNs were determined as 1035.8 m2 g−1 and 1.1308 cm3 g−1, respectively. After the surface modification, the surface area and pore volume of the MSN-NH2 decreased to 264.4 m2 g−1 and to 0.336 cm3 g−1, respectively. Considerable reductions in the surface area (62.771 m2 g−1) and pore volume (0.2177 cm3 g−1) were observed after adsorption of SDBS (MSN-NH2@ SDBS) (see Table 1). It was not possible to obtain feasible data for the pore size of MSN-NH2@SDBS, because the mesopores of MSN-NH2 were filled with SDBS. In the XPS spectra of MSN-NH2 and MSN-NH2@SDBS (Fig. S3a), the
Fig. 3. Changes in (a) equilibrium adsorption capacity and (b) SDBS adsorption efficiency of MSN-NH2 by various initial concentrations of SDBS.
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Accordingly, the removal efficiency of SDBS by MSN-NH2 was high because of the electrostatic attraction between MSN-NH2 and SDBS. As the mesopores of MSN-NH2 were filled with SDBS according to the BJH data, it is thought that the adsorption of SDBS by MSN-NH2 occurred in the positively charged mesopores of the adsorbent. The removal of SDBS using SNP-NH2 that was positively charged and not mesoporous, was investigated to determine whether the mesoporous structure affected the removal of SDBS (Fig. 2). Efficient adsorption of SDBS occurred at 1 mg/mL concentration of adsorbent and 0.5 mg/mL concentration of SDBS. It was confirmed that the removal efficiency and equilibrium adsorption (qe) of MSN-NH2 were much higher than those of SNP-NH2, indicating that the mesoporous structure was essential for the effective removal of SDBS. We attempted to investigate the possible effect of the π−π interaction or the hydrophobic interaction between benzene rings in SDBS and MSN-Ph. The removal efficiency of SDBS was about 30% for MSN-Ph; both the removal efficiency and qe were considerably lower than in MSN-NH2, suggesting that the adsorption of SDBS by the electrostatic interaction of MSN-NH2 was more effective than the hydrophobic or π−π interaction of MSNPh. In addition, the qe of MSN-NH2 increased dramatically with an increase in the initial concentration of SDBS (C0) (Fig. S6). The qe value reached a maximum when C0 was 1000 μg/mL and became saturated above that concentration. The critical micelle concentration (CMC) of SDBS has been reported to range from 553 to 1400 μg/mL and varies in the literature [43–46]. In our case, it seems that the qe value was not affected by the CMC of SDBS, although the mechanism of adsorption of surfactants above CMC has not been clearly understood [47]. In contrast, MSN-Ph showed a steady increment in qe with increase in Ce regardless of CMC, presumably because of the poor adsorption.
Fig. 5. Effects of pH on SDBS adsorption efficiency of MSN-NH2. [SDBS] = 0.5 mg/mL; [SDBS/Cs] = 0.5 mg/mL; [MSN-NH2] = 1 mg/mL.
N 1s peak was observed, indicating the presence of amine groups on the silica surface. In the N 1s spectra (Fig. S3b and c), a free -NH2 peak at 399 ± 0.2 eV and the peak by hydrogen bonded and protonated -NH2 at 401.5 ± 0.1 eV were identified [39–42]. The -NH2 peak from hydrogen bonded and protonated N was predominant in MSN-NH2, while the free -NH2 peak was dominated in MSN-NH2@SDBS. Because the adsorption of SDBS on MSN-NH2 might hinder the hydrogen bonding between the amine groups in MSN-NH2, it was assumed that the amount of -NH2 peak from hydrogen bonded and protonated N decreased and N from the free-NH2 increased. 3.2. Adsorption
3.2.2. Adsorption behavior of SDBS by MSN-NH2 To elucidate further the change in qe and the removal by MSN-NH2, the effect of various initial concentrations of SDBS was investigated. As C0 increased, the value of qe increased tremendously (Fig. 3a). The most efficient removal of SDBS was observed when C0 was 200 μg/mL, except when 4 mg/mL of MSN-NH2 was used (Fig. 3b). Above the C0 value of 200 μg/mL, the removal efficiency decreased. Interestingly, adsorption by MSN-NH2 showed highly efficient adsorption of SDBS at the higher concentration of 2000 μg/mL (2000 ppm), which is a much higher concentration than the CMC of SDBS (553–1400 ppm). As the concentration of MSN-NH2 increased, the qe value decreased above 1 mg/mL of MSN-NH2 (Fig. 4a). A noticeable change in removal values above an MSN-NH2 concentration of 4 mg/mL was observed (Fig. 4b). The pH value is an important parameter affecting adsorption efficiency in aqueous solutions. The effect of pH on the removal efficiency of SDBS by MSN-NH2 was investigated at various pH values (Fig. 5). For
The removal efficiency of SDBS was investigated using various adsorbents, including MSNs, MSN-NH2, and MSN-Ph to confirm the effects of various factors (adsorbent concentration, initial SDBS concentration and pH) on adsorption. 3.2.1. Effect of surface-functionalization of adsorbents on the removal of SDBS Effect of surface charge of the adsorbent was investigated in terms of the removal efficiency of SDBS. The negatively charged MSNs showed a remarkably lower removal efficiency than the positively charged MSN-NH2 (Fig. S4). The adsorption of SDBS by MSNs was negligible, whereas MSN-NH2 showed about 80% removal efficiency at the SDBS concentration of 0.4 mg/mL (400 ppm). It is thought that such a low adsorption by the MSNs resulted from the electrostatic repulsive force between the negative MSNs and the anionic sulfonate ion in SDBS.
Scheme 2. Schematic illustration of adsorption of SDBS by MSN-NH2. 274
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Fig. 6. (a) Effects of contact time on adsorption capacity of MSN-NH2 towards SDBS, Fitting plots of (b) pseudo first order and (c) pseudo second order kinetic models.
potential at low pH, which strengthened the electrostatic interaction between MSN-NH2 and SDBS. SDBS was not adsorbed on MSN-NH2 at basic conditions such as pH 11 or 12. This was because the silica structure of MSN-NH2 was destroyed in the alkaline solution. To understand whether Cs ions were removed by MSN-NH2 together with SDBS or whether only SDBS was removed in a mixed solution of SDBS and Cs ions, the Cs ion concentrations was measured before and after adsorption by MSN-NH2 in the mixed solution. Interestingly, it was found that the MSN-NH2 could adsorb SDBS only, because the concentrations of Cs ions before and after adsorption of SDBS/Cs were similar. This indicates that Cs ions were separated during the adsorption process of SDBS by the MSN-NH2 (Scheme 2). To confirm the effect of main ion species on adsorption of SDBS using MSN-NH2, SDBS removal efficiency of MSN-NH2 was measured in the presence of NaCl or KCl (Fig. S7). SDBS removal efficiency of MSNNH2 in the presence of NaCl and KCl was similar to that of salt-free conditions, indicating that the presence of NaCl and KCl salts did not significantly affect SDBS adsorption.
Table 2 Adsorption kinetic parameters of the adsorption of SDBS onto the MSN- NH2. Pseudo-first-order qe,exp (mg/g) qe,cal (mg/g) K1 * 10−3 (min−1) R2
Pseudo-second-order
105.3 14.1 0.9958
161.4 qe,cal(mg/g) K2 * 10−3 (g mg−1 min−1) R2
167.2 0.317 0.9907
The initial SDBS concentration is 200 mg/L.
the practical use of MSN-NH2, the same adsorption experiment was also performed using an aqueous solution containing SDBS pretreated with Cs ions to form Cs-bound SDBS (SDBS/Cs). The SDBS/Cs compound was prepared by simple mixing of SDBS and CsCl, and the effect of Cs ions on the removal of SDBS was checked using the MSN-NH2. The removal of SDBS attained about 80% at pH values ranging from 4 to 9, and the removal was more than 90% at the low pH of 2–3, regardless of the binding of the Cs ion to SDBS. The MSN-NH2 showed a positive zeta
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kinetic models. Pseudo first order and pseudo second order models are given as follows: Pseudo first order
ln(qe − qt ) = ln qe − k1 t
(3)
Pseudo second order
t 1 t = + qt qe k2 qe2
(4)
where qe and qt (mg/g) are the amounts of SDBS adsorbed on MSN-NH2 at equilibrium and at time t (min), and k1 (min−1) and k2 (g mg−1 min−1) are the rate constant for the pseudo first order and pseudo second order equation, respectively. The linear fittings of the pseudo first order and pseudo second order kinetic models are shown in Fig. 6b and c. The kinetic parameters were calculated based on the slope and intercept for the fitting curve. As can be seen in Table 2, the correlation coefficient R2 values for the pseudo first order and pseudo second order models are 0.9958 and 0.9907, respectively. Although the R2 value of the pseudo first order is slightly higher than that of the pseudo second order, the qe value (qe,exp) obtained from the experiment is very similar to the qe value (qe,cal) calculated from the pseudo second order. Therefore, it is confirmed that the pseudo second order model is a more suitable model than the pseudo first order model. Adsorption isotherms are widely used to evaluate the adsorption characteristics of adsorbents and help understand the adsorption process. Langmuir and Freundlich isotherm models were used to understand the adsorption behavior of SDBS onto MSN-NH2 at room temperature. The linear equation of Langmuir model is given as follows:
Ce C 1 = e + qe qm KL qm
(5)
Ce (mg/mL) and qe (mg/g) are the equilibrium concentration of SDBS and the adsorption capacity at equilibrium, respectively. qm (mg/ g) is the maximum adsorption capacity of SDBS adsorbed on MSN-NH2, and KL (L/mg) is the Langmuir constant. The linear equation of the Freundlich model can be expressed as:
lnqe = lnKF +
Table 3 Adsorption isotherm parameters for SDBS by MSN-NH2. Parameter
Parameter value
Langmuir
qm (mg/g) KL (L/mg) R2 1/n KF R2
854.70 0.0053 0.9197 0.7027 11.030 0.9818
Freundlich
(6)
KF and 1/n were the Freundlich model constants, indicating capacity and intensity of adsorption, respectively. Fig. 7 shows the Langmuir and Freundlich fitting curves for SDBS adsorption on MSN-NH2 and the values obtained from fitting curves are shown in Table 3. Among the two isotherm models, R2 value of the Freundlich model is closer to 1 than that of the Langmuir model, indicating that the adsorption is more suitable for the Freundlich model. Therefore, it was confirmed that adsorption behavior of SDBS on MSNNH2 was not limited to monolayer adsorption. The R2 value of the Langmuir isotherm model is relatively low, but the calculated qm value is 854.70 mg/g, which is much higher than the qm value of previously reported SDBS adsorbents such as zeolite modified with CTAB (30.7 mg/g) [18], polyaniline doped with CuCl2 (32.3 mg/g) [19], activated carbon (29.4 mg/g, 468.8 mg/g) [48,49], and activated carbon modified with quaternary ammonium (77.8 mg/g) (Table S2) [50].
Fig. 7. Fitting plots of (a) Langmuir and (b) Freundlich isotherm models for the adsorption of SDBS onto MSN-NH2.
Model
1 Ce n
3.3. Adsorption isotherms and kinetics
4. Conclusions
The adsorption kinetics can be used to determine the adsorption rate and help understand the adsorption mechanism. Fig. 6a shows the effect of contact time on the adsorption capacity of MSN-NH2 towards SDBS. The adsorption capacity of MSN-NH2 for SDBS increased rapidly until the first 100 min and reached to equilibrium within 240 min. The adsorption was applied to pseudo first order and pseudo second order
In this study, the removal efficiency of SDBS using silica-based adsorbents was investigated under various conditions. MSN-NH2, had a mesoporous structure and strong electrostatic interaction with SDBS, and was found to be effective in the removal of SDBS, in terms of initial concentration of SDBS, concentration of adsorbent, and pH value. In the adsorption of SDBS or SDBS/Cs, very similar adsorption performance 276
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was observed for MSN-NH2 in both cases. The concentrations of Cs ions were almost identical before and after adsorption by MSN-NH2, indicating that Cs ions were not removed together with SDBS. In addition, the maximum adsorption capacity of MSN-NH2 for SDBS was much higher than previously reported adsorbents. Therefore, the MSN-NH2 material was considered to be a suitable adsorbent for the removal of anionic surfactants that might be contained at high concentration in the decontamination waste solution, such as SDBS, because of its simple preparation, low cost, and excellent adsorption efficiency for SDBS even above CMC. In particular, removal of SDBS using MSN-NH2 prior to treatment such as evaporation or membrane separation may increase the efficiency of the evaporation or membrane separation process. MSN-NH2 can also be used in decontamination processes to remove SDBS bound to cesium in addition to SDBS in the radioactive waste treatment process.
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