Kinetics and thermodynamics of methylene blue adsorption on the Fe-oxide nanoparticles embedded in the mesoporous SiO2

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Contents lists available at ScienceDirect

Advanced Powder Technology journal homepage: www.elsevier.com/locate/apt

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Original Research Paper

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Kinetics and thermodynamics of methylene blue adsorption on the Fe-oxide nanoparticles embedded in the mesoporous SiO2

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June Hyung Kim a,1, Byeong Jun Cha b,1, Young Dok Kim b, Hyun Ook Seo a,⇑

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a b

Department of Chemistry and Energy Engineering, Sangmyung University, Seoul 03016, Republic of Korea Department of Chemistry, Sungkyunkwan University, Suwon 16419, Republic of Korea

a r t i c l e

i n f o

Article history: Received 5 August 2019 Received in revised form 11 November 2019 Accepted 28 November 2019 Available online xxxx Keywords: Adsorbents Mesoporous SiO2 Fe-oxide nanoparticles Adsorption kinetics Adsorption thermodynamics

a b s t r a c t Fe-oxide nanoparticles with a diameter of
10 nm embedded into the porous structure of mesoporous SiO2 particles using temperature regulated chemical vapor deposition and a subsequent annealing process. The kinetics and thermodynamics of MB adsorption on bare SiO2 and Fe-oxide/SiO2 particles were investigated. MB adsorption kinetics of both SiO2 particles can be described by pseudo first order and intra particle diffusion plot. Thermodynamic parameters were calculated based on the modified Langmuir isotherm model. Equilibrium constant and spontaneity of MB adsorption was increased with the presence of Fe-oxide nanoparticles inside the SiO2 pores, which can be ascribed to the more negative values of DH of Fe-oxide/SiO2 particles compared to bare SiO2. A higher affinity of Fe-oxide nanoparticles for MB adsorption facilitated the intra particle diffusion of MB molecules at a longer time of MB adsorption and improve the maximum MB adsorption capacities of Fe-oxide/SiO2 compared to bare SiO2. The maximum MB adsorption capacities of bare SiO2 and Fe-oxide/SiO2 particles reduced as the initial pH of MB solution increased, however, Fe-oxide/SiO2 particles always exhibited the higher capacity of MB adsorption than bare SiO2. Ó 2019 The Society of Powder Technology Japan. Published by Elsevier B.V. and The Society of Powder Technology Japan. All rights reserved.

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1. Introduction

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A lot of concerns on water pollution have been raised in conjunction with the rapid industrialization. Various organic dye molecules with intense color and high stability were synthesized and extensively used in a variety field of industries to dye their products, such as leather, ceramics, textile, food, plastics [1–4]. Most of synthetic dyes are not biodegradable in water and discharge of small amount of dyes into water can cause serious environmental problems [5–8]. Due to their intense color, the color of water can be readily changed even with very small amount of the dyes discharged into the water disturbing the visible light penetration into water which is essential to aquatic ecosystem [3,7]. Various methods for the dye removal from wastewater have been investigated and they can be categorized into (1) chemical and (2) physical methods; (1) Organic dye molecules in wastewater can be degraded via chemical reactions, such as photocatalytic oxidation, electrochemical oxidation and Fenton reaction [9–13].

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⇑ Corresponding author at: Natural Science Building K214, Sangmyung University, 20, Hongjimun 2-gil, Jongno-gu, Seoul 03016, Republic of Korea. E-mail address: [email protected] (H.O. Seo). 1 These authors contributed equally to this work.

(2) On the other hands, organic dyes can be removed from the wastewater via adsorption [14–17]. Physical methods based on adsorption of organic molecules on adsorbent is much attractive solution than chemical methods owing to its simplicity, effectiveness and less possibility of the generation of secondary pollution [14–17]. A large number of investigations has been conducted on the removal of organic dyes from wastewater utilizing adsorbents and adsorption performances of a variety of materials, including carbon-based materials [18–21], SiO2 [22–24], zeolite [25–28] have been investigated. Recently, utilization of bio-materials, such as enzyme, as an adsorbent for organic dye removal has been also suggested [29,30]. Adsorption capacity of adsorbents are closely related not only to chemical composition of adsorbent surface but also the geometrical structure of adsorbent. Porous structured materials with high surface area have been often commonly chosen as adsorbents for wastewater purification [7,18–28,31,32]. Sometimes, the surfaces of porous materials were modified, and other metal or metaloxide materials were imbedded into the porous structure to improve adsorption capacity [31,33–38]. Recently, utilization of nano-materials as organic dye removal has been suggested and the combination of nano-materials with porous adsorbents has

https://doi.org/10.1016/j.apt.2019.11.036 0921-8831/Ó 2019 The Society of Powder Technology Japan. Published by Elsevier B.V. and The Society of Powder Technology Japan. All rights reserved.

Please cite this article as: J. H. Kim, B. J. Cha, Y. D. Kim et al., Kinetics and thermodynamics of methylene blue adsorption on the Fe-oxide nanoparticles embedded in the mesoporous SiO2, Advanced Powder Technology, https://doi.org/10.1016/j.apt.2019.11.036

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also reported [39–43]. When the size of materials reduced in the range of few tens of nanometer, they can exhibit unique properties (e.g., high surface to volume ratio, high surface energy, high adsorption ability, catalytic activity) differed from those seen in their bulk states. Recently, we combined the small Fe-oxide nanoparticles (
10 nm) with a commercially available mesoporous SiO2 and examined its adsorption performance for organic dye removal from aqueous phase especially in terms of their reusability [44]. Reusability of SiO2 as an adsorbent for organic dye removal was greatly improved by the incorporation of Fe-oxide nanoparticles. Adsorption capacity of Fe-oxide/SiO2 can be almost fully recovered by a thermal annealing (
100 °C) and Fe-oxide/SiO2 particles can be repeatedly used without losing its original adsorption capacity after the regeneration process (100 °C annealing). The improved reusability of Fe-oxide/SiO2 particles compared to bare SiO2 can be attributed to the thermal catalytic activity of Fe-oxide nanoparticles, based on our previous studies using various analysis technique including FT-IR, TGA, and XPS. However, the process of MB adsorption on the Fe-oxide/SiO2 particles was not examined in detail previously. Thus, here, we examined the detailed behaviors of methylene blue (MB) adsorption on Fe-oxide/SiO2 adsorbent and compared them with those of bare SiO2; Kinetics, thermodynamics, and pH dependency of MB adsorption on both samples were investigated.

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2. Materials and methods

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2.1. Sample preparation

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The Fe-oxides were deposited on mesoporous SiO2 particles by a temperature regulated chemical vapor deposition (TR-CVD) and the Fe-oxides were turned into small nanoparticles at inner pores of SiO2 by a subsequent annealing process (750 °C, 8 h, dry air flow of 30 sccm). The mesoporous SiO2 particles (a particle size of 250– 500 lm, average pore diameter of 15 nm) were used as a substrate, whereas the bis(cyclopentadienyl) iron (Fe(Cp)2, Aldrich) was used as a metal precursor. Both the SiO2 particles and Fe(Cp)2 were bought from Sigma Aldrich. The TR-CVD deposition of iron oxide on SiO2 particles was proceed via two steps; (1) vaporization and deposition of Fe(Cp)2 at 60 °C for 2 h, and (2) oxidation of Fe(Cp)2 forming Fe-oxide films on SiO2 at 200 °C for 12 h. After the deposition of Fe-oxide films, the samples were annealed at 750 °C for 8 h using a furnace at a constant dry air flow condition (30 sccm). The entire process for the preparation of Fe-oxide nanoparticles (TRCVD and post-annealing) was previously introduced and more details on the preparation processes can be found in elsewhere [44].

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2.2. Sample characterization

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The geometrical structure of Fe-oxide/SiO2 particles were analysed by means of high-resolution transmission electron microscopy (HR-TEM, JEOL, JEM ARM 200F), and X-ray diffraction pattern (XRD) of bare SiO2 and Fe-oxide/SiO2 samples were obtained using a X-ray diffractometer (Rigaku, Ultima UV) in order to check the crystallinity of the sample. XRD analysis was conducted using a Cu Ka radiation (40 kV, 30 mA, k = 1.54 A) at a scanning rate of 4° min1. The surface of Fe-oxide/SiO2 sample was analysed before and after MB adsorption by means of X-ray photoelectron spectroscopy (XPS). The XPS analysis was conducted in an ultra-high vacuum chamber with a base pressure of 5.0  109 torr at room temperature using an Mg-Ka-line and a concentric hemispherical analyzer (CHA, PHOIBOS-Has 2500, SPECS). N2 adsorption/desorption isotherms (3Flex, Micromerities) were measured with bare

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SiO2 and Fe-oxide/SiO2. The surface area and average pore diameter of both samples were determined by Brunauer-Emmett-Teller (BET) and Barret-Joyner-Halenda (BJH) methods, respectively. An inductively coupled plasma optical emission spectroscopy (ICPOES) was used to determine the loading amount of Fe on Feoxide/SiO2.

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2.3. MB adsorption test

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0.03 g of each SiO2 particles (bare SiO2 or Fe-oxide/SiO2) was placed on the bottom of a glass vial containing 30 ml of MB solution with various MB concentration (10, 20, 30 mg/l). pH of MB solution was measured before and after each MB adsorption experiment using a pH meter (Seven Compact S210, Mettler Toledo). Each sample was annealed at 400 °C for 3 h at a constant dry air flow condition (100 sccm) prior to MB adsorption experiments. MB solutions containing SiO2 particles were agitated at a constant shaking speed using a shaker (IST-4075R, JEIO TECH) during the adsorption tests and the shaking speed can be chosen in a range of 0–500 rpm. The upper part of vial was covered with a Teflon cap. A schematic description of experimental set-up and the procedure of methylene blue (MB) adsorption experiments can be found in supplementary data (Fig. S1). At a certain time-interval, the shaker was turned off and a 4 ml of aliquot of the solution was taken from the vial. The UV–vis absorbance of each aliquot was measured using a UV–vis spectrometer (Genesys 10S UV–vis spectrometer, Thermo Fischer) in a wavelength range of 400–900 nm. The aliquot was re-injected into the vial containing MB and SiO2, and the vial was covered with the cap. Then, the MB adsorption experiments were continued by agitating at a constant shaking speed using the shaker. Depends on the initial MB concentration, a quartz cell with different path length was chosen for each series of MB adsorption experiment. For the series of MB adsorption experiment with the MB solution with 10 ppm of the initial concentration, the cell with 10 mm of path length was used. Whereas quartz cells with 5 and 2 mm of path length were used for the series of MB adsorption tests starting with the solutions with MB concentrations of 20 and 30 mg/l, respectively. The linear relationship between the MB concentration and UV–vis absorbance was checked by obtaining calibration curve for each cell with different path length.

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3. Results and discussion

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3.1. Characterization of Fe-oxide/SiO2

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Fe-oxide/SiO2 samples prepared by a TR-CVD and a subsequent annealing (at 750 °C, for 8 h, dry air flow of 30 sccm) characterized by HR-TEM, XRD, N2-isotherm and ICP-OES analyses. In a HR-TEM image of the Fe-oxide/SiO2 sample, the lattice structure of small Fe-oxide nanoparticles (with a diameter of
10 nm) can be identified from the amorphous SiO2 particles (Fig. 1a). The lattice parameter of Fe-oxide nanoparticles was corresponding to (3 1 1) plane of Fe-oxide (a-Fe2O3 or Fe3O4) [45,46]. Fig. 1(b) are the XRD patterns of bare SiO2 and Fe-oxide/SiO2 samples. There was no noticeable feature in its XRD pattern of bare SiO2 due to its amorphous nature. On the other hands, two distinct peaks at 36 and 63° were found in the XRD pattern of Fe-oxide/SiO2 sample which can be attributed to (3 1 1) and (4 4 0) plane of Feoxide (a-Fe2O3 or Fe3O4) (JCPDS CARD No. 87-1166, 65-3107) [45,46]. The both analyses results of HR-TEM and XRD are in line with each other indicating existence of (3 1 1) plane of Fe-oxide. However, two different phases of Fe-oxides (a-Fe2O3 or Fe3O4) cannot be discriminated since the lattice constants of both phases are

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♦ ♦

θ

Fig. 1. Characterization results. (a) HR-TEM image of Fe-oxide/SiO2 (inset: an enlarged image). (b) XRD patterns of Fe-oxide/SiO2 and bare SiO2. (c) Fe 2p3/2 core-level XPS spectrum of Fe-oxide/SiO2. (d) BJH pore size distribution of Fe-oxide/SiO2 and bare SiO2.

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almost same [45,46], i.e., the Fe-oxide nanoparticles can be aFe2O3, Fe3O4 or their mixture. The surface of Fe-oxide/SiO2 sample was analyzed further by means of XPS and Fig. 1(c) show a Fe 2p core-level XPS spectrum of Fe-oxide/SiO2. The Fe 2p3/2 peak was centered at 710 eV and extended from 716 to 706 eV. It is important to note that Fe 2p core-level XPS spectrum exhibits inherent broadness arises from the co-existence of multiple elements due to the final state relaxation effects. Thus, although the center position of Fe 2p3/2 peak of Fe-oxide/SiO2 sample was close to Fe3O4 states (
710.4 eV) than a-Fe2O3 states (
710.9 eV), the oxidation states of Fe-oxide cannot be exactly determined. And the Fe-oxides were likely to be in a complex form of mixture of various oxidized-Fe. We would like to mention that Fe-oxide prepared on porous alumina via a similar TR-CVD and post-annealing process consisted of a complex mixture of several Fe-oxides including a-Fe2O3 and Fe3O4, which was confirmed by Mössbauer spectroscopy [47]. Fig. 1(d) shows pore size distribution curves of bare SiO2 and Feoxide/SiO2 derived by BJH method with N2-isotherms results. BET and BJH analyses results are summarized in Table 1. Only the pore volumes of which pore diameter is around
11 nm decreased upon the formation of Fe-oxide nanoparticles (via a TR-CVD and subsequent annealing at 750 °C). Whereas pore volumes with other sizes (pore diameter <10 nm and >11 nm) were almost identical between two samples (bare SiO2 and Fe-oxide/SiO2). It indicated

Table 1 Surface area (m2/g) and average pore diameter (nm) of bare SiO2 and Fe-oxide/SiO2 determined by BET and BJH methods, respectively, and the amount of Fe loading (wt %) of Fe-oxide/SiO2 sample determined by ICP-OES.

Bare SiO2 Fe-oxide/SiO2

Surface area (m2/g)

Average pore diameter (nm)

Fe loading (wt%)

302.6 224.3

13.1 14.8

– 8.3

that the Fe-oxide nanoparticles with a diameter of
10 nm were preferably formed in the pores of SiO2 with a specific pore diameter (10–11 nm). This resulted in increase of averaged value of pore diameter from 13.1 to 14.8 nm and decrease of specific BET surface area (from 302.6 to 224.3 m2/g) upon the formation Fe-oxide nanoparticles. The results of ICP-OES indicated that
8.3 wt% of Fe was loaded in Fe-oxide/SiO2 (Table 1).

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3.2. MB adsorption behaviors of bare SiO2 and Fe-oxide/SiO2

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0.030 g of each sample (bare SiO2 or Fe-oxide/SiO2) was placed in the vial containing 30 ml of MB solution with a concentration of 10 mg/l and each vial was sealed by a Teflon cap. The vials were kept at room temperature and agitated at a constant shaking speed

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of 200 rpm using the shaker. The concentration change of MB solution during the adsorption experiments were investigated by measuring the absorbance of MB solution at a regular time interval. For the first 5 h, the absorbance was measured every 60 min, and then it was measured again after 22 h of the adsorption experiments (Fig. 2(a) and (b)). MB molecules in the aqueous phase exhibited their characteristic absorption band in the region of visible light which consisted of two absorption peaks at
610 and 664 nm. Intensities of both peaks decreased with increasing adsorption time in the presence of each SiO2 particles (bare SiO2 or Fe-oxide/SiO2), while the positions and relative peak intensities of both peaks did not undergo any noticeable changes. This indicated that the changes of characteristic absorption band of MB molecules during the adsorption experiments were attributed to the MB adsorption on each SiO2 particles, and the degradation MB molecules was insignificant under our experimental conditions. In the earlier stage of adsorption (<240 min), decrease of MB adsorption band was much pronounced in the presence of bare SiO2 compared to the case of Fe-oxide/SiO2 particles. However, further decrease of MB adsorption band after 240 min of adsorption process was not noticeable when bare SiO2 particles were used as adsorbents. Whereas, the decrease of the adsorption band continued for the case of Fe-oxide/SiO2 under same conditions. After 22 h (1320 min) of adsorption process, MB adsorption band almost disappeared in the presence of Fe-oxide/SiO2 in the MB solution. In the contrast, for the case of bare SiO2, the adsorption band was still observable, and its intensity was very similar to that measured at 240 min. The maximum peak height of MB peak at 664 nm was obtained from each absorbance spectrum measured at each time interval, and then each peak height was converted to the respective concentration of MB solution. For peak height (664 nm) to concentration conversion, the experimentally determined linear relationship between peak height (664 nm) and MB concentration was used. Then the amount of adsorbed MB molecules (mg) per one gram of adsorbents at each adsorption time, denoted as Qt hereafter, was calculated for each case of SiO2 samples (bare SiO2 or Feoxide/SiO2) using the following equation.

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Qt ¼

243 244 245 246 247 248 249 250 251 252 253 254 255 256 257 258 259 260 261 262 263 264 265 266 267 268 269 270 271 272 273 274 275 276 277 278 279 280

282

ðC o  C t ÞV W

ð1Þ

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Co is the initial concentration of MB solution (mg/l) and Ct is the MB concentration (mg/l) at a given time of adsorption, and V and W are the volume of MB solution (l) and the weight of adsorbents (g), respectively. Fig. 2(c) shows the plot of calculated Qt values versus adsorption time, and one can notice the different behaviors of MB adsorption on both samples (bare SiO2 and Fe-oxide/SiO2). MB adsorption reached to its equilibrium state on bare SiO2 particles faster (within
200 min) than on Fe-oxide/SiO2 particles. However, the Qt value at the equilibrium, hereafter denoted as Qe, of Fe-oxide/ SiO2 particles was larger compared to the case of bare SiO2 particles.

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3.3. Kinetics of MB adsorption on bare SiO2 and Fe-oxide/SiO2

298

In order to compare the kinetics of MB adsorption on both SiO2 particles (bare SiO2 and Fe-oxide/SiO2), datum shown in Fig. 2(c) were fitted by pseudo-first-order (PFO) and pseudo-second-order (PSO) model (Fig. 3) [48]. Qt values of both cases (bare SiO2 and Fe-oxide/SiO2) below 90% of the respective Qe values were used for PFO and PSO fitting. Non-linear form of each model was used which can be expressed as below [48–50].

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Q t ¼ Q e ð1  ek1 t Þ

ð2Þ

Q 2e k2 t Qt ¼ 1 þ k2 Q e t

ð3Þ

Qt and Qe are the amount of adsorbate uptake (mg) per one gram of adsorbents at each adsorption time and equilibrium state, respectively. And k1 and k2 are the rate constant of the PFO (Eq. (2)) and PSO (Eq. (3)) equations, respectively. Experimental datum of both SiO2 particles (bare SiO2 and Feoxide/SiO2) fit better to PFO than PSO model. Although the PFO mode can describe kinetic data of MB adsorption on both SiO2 particles by the fitting, it did not reveal the adsorption mechanisms on both cases. It is likely that the diffusion rate into inner pores of both SiO2 particles contributed to the overall rate of MB adsorption on both particles. Following four stages can be involved in dye adsorption process on porous adsorbents in aqueous phase; (1) transport of dye from the bulk solution to external surface of adsorbents, (2) diffusion of dye across the boundary layer and dye adsorption on the external surface, (3) diffusion of dye into inner pores (intra particle diffusion), and (4) adsorption of dye on internal surface of pores. First two steps can be referred as a surface process while the last two steps can be referred as a pore process. In a well agitated system, which is likely the case of this study, surface process takes place very rapidly, therefore its contribution to the overall adsorption rate is very small. The adsorption process of which kinetic is influenced by the intraparticle diffusion can be interpreted based on the plot of Qt vs the square root of time, known as IPD plot (intra particle diffusion plot) (Fig. 4) [48]. The linearized form of the IPD plot can be expressed as below,

pffiffi Q t ¼ kp t þ C

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338

ð4Þ

340

where kp (mg/g  min1/2) is the rate constant of the IPD model and C (mg/g) is a constant related to the boundary layer thickness (higher the C values, greater influence of limiting boundary layer). The linear portion of each plot (bare SiO2 or Fe-oxide/SiO2) shown in Fig. 4 indicates that external surface process took place rapidly and the overall adsorption rate was dominantly controlled by intraparticle diffusion. MB molecules can diffuse into the inner pores either at aqueous states or at adsorbed states, and the major driving force of both cases is the concentration gradient of MB molecules (at aqueous or adsorbed states) from the exterior to interior of the pores. The concentration gradient of MB molecules reduced as the intraparticle diffusion took place during the MB adsorption experiments, which can explain the gradual decrease of slope of the first linear portion of each plot (Fig. 4). For the case of Fe-oxide/SiO2 particles, MB molecules reached to the interior of pores can adsorbed on the Fe-oxide nanoparticles in addition to the SiO2 surface. And if the surface of Fe-oxide nanoparticles has higher affinity for MB adsorption than bare SiO2 surface, it can provide additional driving force for intraparticle diffusion and slow down the decrease of concentration gradient from exterior to interior SiO2 surface. We would like to mention that Qe values of Feoxide/SiO2 particles was larger than the case of bare SiO2 particles as aforementioned, which imply that the surface of Fe-oxide nanoparticles exhibited higher affinity towards MB adsorption. The thermodynamic parameters such as equilibrium constant, Gibbs free energy, enthalpy, and entropy of MB adsorption of bare SiO2 and Fe-oxide/SiO2 particles were calculated based on the Langmuir isotherm model and it will be discussed in detail in the following section.

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3.4. Thermodynamics of MB adsorption

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Thermodynamic aspects of MB adsorption behaviors on bare SiO2 and Fe-oxide/SiO2 were investigated by analyzing MB adsorp-

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Fig. 2. Results of MB adsorption experiments. UV–vis absorption spectra of MB solution measured during the adsorption experiments conducted with (a) bare SiO2 and (b) Fe-oxide/SiO2. (c) Changes of Qt (mg/g) values as a function of adsorption time for the cases of bare SiO2 and Fe-oxide/SiO2 (black empty square: bare SiO2 and red empty circle: Fe-oxide/SiO2). (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)

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tion data obtained with both SiO2 particles at various temperatures (20, 25, 30, 35, 40, 50 °C) and initial MB concentrations (10, 20, 30 mg/l). Three different MB solutions with various concentrations (10, 20, 30 mg/l) were prepared and 30 ml of each MB solution was poured into the vial containing 0.030 g of SiO2 (bare SiO2 or Feoxide/SiO2, either). Then, each vial was sealed by a Teflon cap and agitated at a constant shaking speed of 300 rpm using a shaker. The concentration changes of MB solutions inside each vial was monitored by measuring the absorbance every 60 min for 3 h. This set of MB adsorption experiments was performed at six different temperatures (20, 25, 30, 35, 40, 50 °C). For all cases of study, MB adsorption was saturated within 150 min and Qt (t = 180 min) values of each case of experiment was taken as the respective Qe values. The obtained isotherm data were analyzed based on Langmuir isotherm model. The linear form of Langmuir model can be expressed as follow (Hanes-Woolf linearization of Langmuir model, Type 1) [48],

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Ce ¼ Qe

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391



 1 Ce þ o

Q Max

1 Q oMax K L

ð5Þ

where Ce and Qe are concentration of MB solution and Qt values at equilibrium, while Q oMax and KL are the maximum saturated monolayer adsorption capacity of an adsorbent and Langmuir adsorption constant. Ce/Qe values of each isotherm data were plotted as a function of the respective Ce values, and they were linearly fitted (supplementary material, Figs. S2 and S3). The linear fitting results are summarized in Tables 2 and 3. The correlation coefficients of linear fitting results were very close to 1 (R2 > 0.96) excepting for one case (bare SiO2 at 50 °C) (Table 1), which implies that thermodynamic parameters for the MB adsorption on bare SiO2 and Fe-oxide/SiO2 under our experimental conditions can be calculated based on Langmuir model. However, it is worth to mention that adsorption sites of bare SiO2 and Fe-oxide/SiO2 were heterogeneous rather than homogeneous, whereas Langmuir isotherm model assumes homogeneous adsorption sites. As mentioned in previous section, at least two and three different types of adsorption sites existed on bare SiO2 and Fe-oxide/SiO2 particles, respectively; exterior and interior surface of SiO2 and surface of Fe-oxide nanoparticles at interior of Fe-oxide/SiO2 particles. Therefore, one should note that the thermodynamic parameters (adsorption equilibrium constant, changes

Please cite this article as: J. H. Kim, B. J. Cha, Y. D. Kim et al., Kinetics and thermodynamics of methylene blue adsorption on the Fe-oxide nanoparticles embedded in the mesoporous SiO2, Advanced Powder Technology, https://doi.org/10.1016/j.apt.2019.11.036

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of Gibbs free energy, enthalpy, and entropy) calculated based on Langmuir model fitting are rather averaged values over the various types of adsorption sites on both SiO2 adsorbents (bare SiO2 and Fe-oxide/SiO2). Tables 2 and 3 summarize adsorption equilibrium constant for each isotherm data obtained at various temperatures (20–50 °C) which was calculated based on Langmuir model (denoted as KL in Tables 2 and 3). For the case of both SiO2 particles (bare SiO2 and Fe-oxide/SiO2 particles), KL values gradually decreased as the adsorption temperature increased, indicating the exothermic nature of MB adsorption on both adsorbents. Fe-oxide/SiO2 particles always shows higher KL values than bare SiO2 particles at same temperatures which implies that adsorption capacity of porous SiO2 for MB molecules can be enhanced by incorporation of Feoxide nanoparticles into the porous structure of SiO2. Further on, thermodynamic parameters (changes of Gibbs free energy DG, enthalpy DH, and entropy DS) of MB adsorption on both SiO2 particles (bare SiO2 and Fe-oxide/SiO2) were calculated. The changes of Gibbs free energy (DG) can be calculated using following equation,

DG ¼ RT ln K

ð6Þ

where R and T are the perfect gas constant and absolute temperature in Kelvin while K is the unitless adsorption equilibrium constant. The changes of enthalpy (DH) and entropy (DS) can be estimated using Van’t Hoff equation which can be expressed as follow [48],

ln K ¼ 

Fig. 3. PFO and PSO fitting results. Nonlinear PFO and PSO fitting results of Qt (mg/ g) vs adsorption time plots of (a) bare SiO2 and (b) Fe-oxide/SiO2. Solid lines: PFO fitting curves and dotted lines: PSO fitting curves. Qt values obtained from the experimental data were displayed as empty markers (black empty square: bare SiO2 and red empty circle: Fe-oxide/SiO2. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)

Fig. 4. IPD plot results. Intra-particle diffusion plots (IPD plot) for bare SiO2 and Feoxide/SiO2.

DH 1 DS þ R T R

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444

ð7Þ

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It is very important to mention that one should use dimensionless K values for the calculation of thermodynamic parameters (DG, DH, and DS) using above equations (Eqs. (6) and (7)). It has been pointed out that the calculated values of the parameters can be significantly changes if K values with various units are used [51]; the signs (positive or negative) as well as the magnitudes of calculated thermodynamic parameters can be varied depending on the units of K values. As shown in Tables 2 and 3, the adsorption equilibrium constant calculated using the Langmuir isotherm model (KL) has unit of L/mol. Recently, modified Langmuir isotherm model has been suggested which gives dimensionless equilibrium constant, hereafter denoted as KML, and we used the modified Langmuir isotherm model for the calculation of thermodynamic parameters (DG, DH, and DS). The modified model takes the dependence of solute desorption rates on solute concentration into consideration which is not considered in the original Langmuir model derived for gas phase adsorption process. The linear form of the modified Langmuir model can be expressed as follow,

447

  Ce K ML  1 Cs Ce þ ¼ K ML Q m Qe K ML Q m

466

ð8Þ

where Cs is the solubility of solute which can be either experimentally determined or founds in literatures (e.g., solubility handbooks), while Qm is the amount of adsorbed solute per one gram of adsorbent when the monolayer adsorption takes place. KML is the equilibrium constant for solute adsorption which has no dimension [51]. The change of Gibbs free energy (DG) for each isotherm data was calculated with the respective KML value obtained by the modified Langmuir model (Tables 2 and 3). For all cases, DG are negative values indicating MB adsorption on both SiO2 (bare SiO2 and Fe-oxide/SiO2) was thermodynamically favorable process (spontaneous process).

Please cite this article as: J. H. Kim, B. J. Cha, Y. D. Kim et al., Kinetics and thermodynamics of methylene blue adsorption on the Fe-oxide nanoparticles embedded in the mesoporous SiO2, Advanced Powder Technology, https://doi.org/10.1016/j.apt.2019.11.036

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Table 2 Linear fitting results (slope, intercept and R2 values) of Ce/Qe vs Ce plots, and the calculated adsorption equilibrium constants (KL, KML) of bare SiO2 based on Langmuir and the modified Langmuir model. Change of Gibbs free energy of MB adsorption for each case calculated with the respective KML value are also summarized. Bare SiO2

Slope Intercept R2 KL (L/mol) KML (unitless) DG (kJ/mol)

20 °C

25 °C

30 °C

35 °C

40 °C

50 °C

0.06376 0.05873 0.996 1.085 47,335 26.238

0.03958 0.0451 0.999 0.878 38,265 26.158

0.05665 0.10304 0.999 0.549 23,971 25.418

0.03645 0.075 0.966 0.486 21,190 25.522

0.068445 0.1782 0.994 0.384 16,748 25.323

0.06846 0.2584 0.86096 0.265 11,552 25.134

Table 3 Linear fitting results (slope, intercept and R2 values) of Ce/Qe vs Ce plots, and the calculated adsorption equilibrium constants (KL, KML) of Fe-oxide/SiO2 based on Langmuir and the modified Langmuir model. Change of Gibbs free energy of MB adsorption for each case calculated with the respective KML value are also summarized. Fe-oxide/SiO2

Slope Intercept R2 KL (L/mol) KML (unitless) DG (kJ/mol)

481 482 483 484 485 486 487 488

489 490 491 492 493 494

495 497 498

499 500 501 502 503 504 505 506 507 508 509 510 511 512 513 514 515 516 517 518 519 520 521

20 °C

25 °C

30 °C

35 °C

40 °C

50 °C

0.05959 0.00486 0.999 12.261 534,594 32.147

0.03891 0.00459 0.987 8.477 369,603 31.780

0.05416 0.00872 0.999 6.211 270,801 31.529

0.04376 0.00859 1 5.094 222,112 31.542

0.06803 0.01867 0.999 3.643 158,871 31.180

0.04541 0.01885 0.999 2.409 105,034 31.065

Other thermodynamic parameters (DH and DS) for MB adsorption on bare SiO2 and Fe-oxide/SiO2 were also obtained using the dimensionless KML values. The lnKML values were plotted as a function of 1/T for both SiO2 cases (bare SiO2 and Fe-oxide/SiO2) and then linearly fitted (Fig. 5). DH and DS values for both SiO2 cases were then calculated using Van’t Hoff equation (Eq. (7)) with the respective slope and y-intercept values of fitted straight line and obtained values are summarized in Fig. 5. The negative values of DH for both cases (bare SiO2 and Fe-oxide/SiO2) indicate the exothermic nature of MB adsorption on both SiO2 particles, while the negative sign of DS implies the decrease of randomness of solid/solution system during the MB adsorption on both particles. The DG values are related to the respective DH and DS by following equation,

DG ¼ DH  T DS

ð9Þ

The equation manifest that more negative values of DS make DG less negative (less spontaneous), whereas more negative DH values make DG more negative (more spontaneous). The magnitude of negative DH values increased upon the incorporation of Fe-oxide nanoparticles into mesoporous SiO2 resulted in more spontaneous MB adsorption on Fe-oxide/SiO2 (more negative DG) than bare SiO2 (Fig. 5). In addition, Fe-oxide/SiO2 particles exhibited slightly less negative DS values than bare SiO2. And it also makes DG values of Fe-oxide/SiO2 more negative than bare SiO2 although DH values seems much largely contributed to more favorable MB adsorption of Fe-oxide/SiO2 than bare SiO2 in the temperature range of 20–50 °C. After MB adsorption, Fe-oxide/SiO2 sample was dried at room temperature and further analyzed by means of XRD and XPS (Fig. 6). Any noticeable changes of XRD pattern of Fe-oxide/SiO2 upon the MB adsorption were not found indicated that the phase of Fe-oxide did not altered by the adsorption of MB molecules (Fig. 6a). On the other hands, XPS peak shape underwent notable changes upon the MB adsorption. New components at lower binding energy region appeared both in Fe 2p and C 1s core-level XPS spectra (Fig. 6b and c), which were attributed to the formation of Fe-carbide species after the MB adsorption process. It is not clear whether the formation of Fe-carbide took place during the MB adsorption process in aqueous phase or during the sample drying

process conducted prior to XPS analysis, considering that the TRCVD prepared Fe-oxide nanoparticles can exhibit catalytic activity at room temperature [52,53]. However, those results indicated the surface of Fe-oxide nanoparticles can interact with MB molecules strongly. And it is in line with our isotherm results indicating the increase of magnitude of negative DH values upon the incorporation of Fe-oxide nanoparticles. H. Mittal et al. studied MB adsorption on Fe-oxide nanoparticles incorporated Gum ghatti composites [54], and they reported somewhat different values of DG (15.2 kJ/mol at 25 °C), DH (40.49 kJ/mol) and DS (18.67 kJ/molK) values of the Fe-oxide/Gum ghatti composites towards MB adsorption compared to our case of study (Fe-oxide/SiO2 particles). They concluded that it was the positive DS values which made MB adsorption on the Fe-oxide nanoparticles-incorporated composite favorable (negative DG values), which is different from our case. It is important to note that there are large differences between their study and our present one, including the nature of substrate, sample preparation methods, and conditions of adsorption experiments, which all can influence the adsorption behaviors of MB on adsorbents. Since a direct comparison of adsorption behaviors of our sample to previously reported ones in other literatures is very difficult to be made due to differences in experimental conditions, we performed MB adsorption experiments with Fe-oxide nanoparticles under same experimental conditions used for the Fe-oxide/SiO2 particles. For this purpose, a commercially available Fe-oxide nanoparticles of which diameter was in the range of 50–100 nm was bought from Sigma-Aldrich and its MB adsorption capacity was examined (supplementary material, Fig. S4). Decrease of MB absorption peak was not detectable when the same MB adsorption experiments at 30 °C was performed with Fe-oxide nanoparticles. And it implied that the preparation methods of Fe-oxide nanoparticles are also important factors determining organic dye adsorption ability of Fe-oxide nanoparticles.

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3.5. Effect of pH

556

The effect of initial pH values of MB solution on adsorption capacity at equilibrium state of two SiO2 particles (bare SiO2 and Fe-oxide/SiO2) were investigated. Initial pH values of MB solutions

557

Please cite this article as: J. H. Kim, B. J. Cha, Y. D. Kim et al., Kinetics and thermodynamics of methylene blue adsorption on the Fe-oxide nanoparticles embedded in the mesoporous SiO2, Advanced Powder Technology, https://doi.org/10.1016/j.apt.2019.11.036

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Fig. 5. Results of the lnKML vs 1/T plot. The lnKML vs 1/T plots for the cases of (a) bare SiO2 (black empty square) and (b) Fe-oxide/SiO2 (red empty circle). (Solid lines: linear fitting results.) Changes of enthalpy and entropy for both cases (bare SiO2 and Fe-oxide/SiO2) calculated with the results of linear fitting of the respective lnKML vs 1/T plot are summarized. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)

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were varied in the pH range of 2–10, whereas initial MB concentrations were fixed at 20 mg/L. The same amount of each SiO2 particle (0.030 g) was placed in the vial containing MB solutions (30 ml) with various pH values (2, 4, 6, 8, 10). Then each vial was sealed with Teflon cap and agitated at a constant speed of 200 rpm using a shaker and the temperature was kept at 30 °C. The adsorption capacities at equilibrium state of two SiO2 samples for MB molecules (Qe) were plotted as a function of initial pH values of MB solution (Fig. 7). Both SiO2 particles showed similar trends of Qe values change upon the pH variation; Qe values increased as initial pH values of MB solution increased for both cases (bare SiO2 and Fe-oxide/SiO2). The point of zero charges (pHpzc) of both SiO2 particles were found to be
6.5 where the adsorbent surface was in an electronically neutral state. If pH of solution was lower than pHpzc values of adsorbent, the adsorbent surface became positively charged which can generate electrostatic repulsion force between adsorbent surface and cationic MB molecules. On the other hands, the negative charge appeared on adsorbent surface when solution pH was higher than the pHpzc values which can facilitate the cationic MB adsorption. One can notice that the trends of change of Qe values with increasing pH of MB solution varied over the pH range of 2–10. Qe values gradually increased in overall range of pH (2–10) as the pH of solution increased, however, in two pH ranges of 4–6 and 8–10, the increases of Qe values were less significant compared to other pH ranges (pH 2–4 and 6–8). It seems that, in those two pH ranges (4–6 and 8–10), MB adsorption on both SiO2 particles less subject to electrostatic interaction between MB and adsorbent surface. However, a further investigation utilizing other analysis techniques which can directly determine the surface charge state of adsorbent and adsorbate in aqueous phase still

needs to be done to get deeper insights in these phenomena. Nevertheless, it is worth to note that under our experimental conditions, Fe-oxide/SiO2 particles always show higher adsorption capacity than bare SiO2 in the pH range of 2–10.

591

4. Conclusion

595

Small Fe-oxide nanoparticles (
10 nm) were incorporated into the porous structure of mesoporous SiO2 particles using TR-CVD and a subsequent annealing. MB adsorption behaviors of bare SiO2 and Fe-oxide/SiO2 particles were studied. MB adsorption reached its equilibrium state faster on bare SiO2 than Fe-oxide/ SiO2 however, Fe-oxide/SiO2 has higher adsorption capacity of MB adsorption at equilibrium. The MB adsorption behaviors of two SiO2 particles (bare SiO2 and Fe-oxide/SiO2) were further investigated in terms of their kinetics, thermodynamics, pH dependency, and reusability. Kinetic behaviors of MB adsorption process on bare SiO2 and Feoxide/SiO2 can be described by pseudo first order kinetic model and intra particle model. The linear portion of IPD plot (Qt vs time0.5) of both SiO2 particles indicates the rapid MB adsorption on the external surface of both SiO2 particles and the dominant contribution of intraparticle diffusion on overall adsorption rate. Fe-oxide nanoparticles imbedded inside the pores of SiO2 provided additional adsorption sites with higher affinity of MB adsorption which facilitated the intra particle diffusion of MB molecules at longer time of MB adsorption. Thermodynamic parameters, such as K, DG, DH, and DS, of MB adsorption on both SiO2 particles were calculated using the modified Langmuir isotherm model. Fe-oxide/SiO2 particles exhibited higher K values than bare SiO2 in the temperature range of 20–

596

Please cite this article as: J. H. Kim, B. J. Cha, Y. D. Kim et al., Kinetics and thermodynamics of methylene blue adsorption on the Fe-oxide nanoparticles embedded in the mesoporous SiO2, Advanced Powder Technology, https://doi.org/10.1016/j.apt.2019.11.036

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θ

Fig. 6. Results of XRD and XPS analysis on Fe-oxide SiO2 before and after MB adsorption. (a) XRD patterns of Fe-oxide/SiO2 obtained before and after the MB adsorption. (b) Fe 2p3/2 and (c) C 1s core-level spectra of Fe-oxide/SiO2 obtained before and after the MB adsorption. MB adsorption experiments were performed at 25 °C for 3 h using 10 pppm of MB solution and samples were dried at room temperature for 2 days before the analysis.

Fig. 7. pH dependency of MB adsorption (Qe). The plot of Qe (mg/g) values (the adsorption capacity at equilibrium state) of bare SiO2 (black empty square) and Feoxide/SiO2 (red empty circle) as a function of pH values of MB solution. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)

40 °C, and the calculated K values decreased with increasing temperature for both SiO2 particles indicating the exothermic nature of MB adsorption on both particles. Both samples exhibited negative values of calculated DG, DH, and DS. Spontaneity of MB adsorption was increased by the incorporation of Fe-oxide nanoparticles into the porous structure of SiO2, which can be ascribed to the more negative values of DH of Fe-oxide/SiO2 particles compared to bare SiO2. Increase of initial pH of MB solution resulted in the reduced amount of MB adsorption on bare SiO2 and Fe-oxide/SiO2 at equilibrium. However, Fe-oxide/SiO2 always exhibited higher MB adsorption capacity than bare SiO2 in pH range of 2–10.

620

Acknowledgement

631

This research was supported by Basic Science Research Program through the National Research Foundation of Korea (NRF) funded by the Ministry of Education (2017R1D1A1B03034381).

632

Appendix A. Supplementary material

635

Supplementary data to this article can be found online at https://doi.org/10.1016/j.apt.2019.11.036.

636

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Please cite this article as: J. H. Kim, B. J. Cha, Y. D. Kim et al., Kinetics and thermodynamics of methylene blue adsorption on the Fe-oxide nanoparticles embedded in the mesoporous SiO2, Advanced Powder Technology, https://doi.org/10.1016/j.apt.2019.11.036

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