Microporous and Mesoporous Materials 297 (2020) 110051
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Microporous and Mesoporous Materials journal homepage: http://www.elsevier.com/locate/micromeso
Enhanced surface hydroxyl groups by using hydrogen peroxide on hollow tubular alumina for removing fluoride Lei Huang a, b, Zhihui Yang a, b, Zhuanxia Zhang a, Linfeng Jin a, Weichun Yang a, b, Yingjie He a, Lili Ren a, Haiying Wang a, b, * a b
School of Metallurgy and Environment, Central South University, Changsha, 410083, PR China Chinese National Engineering Research Center for Control and Treatment of Heavy Metal Pollution, Changsha, 410083, PR China
A R T I C L E I N F O
A B S T R A C T
Keywords: Adsorption Fluoride Hollow alumina Tubular structure Hydrogen peroxide
A novel modified method of hollow tubular alumina was investigated for improving the ability to remove fluoride. In this paper, we designed two parts to improve the adsorption capacity of fluoride: (1) increased the exposed surface area by compounding hollow tubular structure; (2) hydroxylated with different concentrations of H2O2 from 0 to 10 wt%. The modified alumina had uniform hollow tubular and crystal form of γ-Al2O3. Modified hollow tubular alumina exhibited a good performance of 74.25% with 1% H2O2. The kinetic and thermodynamic were thoroughly explained to fit different models. It demonstrated that the pseudo-second-order model and Freundlich model, indicating that chemical and heterogeneous adsorption. The adsorptive velocities of 1% H2O2/Al2O3 were 8.07 � 10 8 and 1.39 � 10 6 m/s when the initial concentrations were 10 ppm and 100 ppm, respectively. The adsorption mechanism was discussed and the adsorbent was stable after adsorbing fluoride. The addition of hydrogen peroxide promoted the more hydroxyl groups, and the exchange hydroxyl groups with fluoride were the main mechanism of adsorption. The ratio of hydroxyl groups with the form of Al–O–OH and Al–OH descended from 80%, 29.3%–30.4%, 20.1%, respectively. These results show hollow structure and modified with H2O2 are the economic and environmental methods for enhancing to remove fluoride.
1. Introduction Fluoride pollution is still a worldwide problem, especially in devel oping countries, such as China, India, South Africa, Bangladesh, etc. Long-term ingestion of fluoride contaminated wastewater could lead to dental and skeletal fluorosis, cancer, and low hemoglobin level, etc [1–3]. Fluoride comes from ingredients or etching liquids in these fac tories [4]. It is estimated that 300 million people worldwide who are drinking water with concentrations of fluoride more than 1.5 mg/L (limited by the World Health Organization). The drinkable water could not exceed the Chinese guidelines of 1.0 mg/L, while the plant effluent must not more than 10.0 mg/L in China [5–7]. However, the concentration of fluoride in wastewater could reach 2000 mg/L or higher. There are many available methods of removing fluoride include chemical precipitation, ion exchange, electro coagulation, membrane separation, Donnan dialysis, adsorption tech nology [8–16]. Among these technologies, adsorption technology is an
economical, widespread and efficient method for dealing with fluoride-containing wastewater [17,18]. Sorbents are the key compo nent of adsorption technology. Various adsorptive materials had been successfully investigated for removing fluoride, such as activated alumina, carbon nanotube, polypyrrole, magnesia, zirconium oxide, MOFs [19–27]. Activated alumina was recommended as the best adsorbent for removing fluoride by the United States Environmental Protection Agency (US EPA) [28]. However, it has some disadvantages, such as the long equilibrium time, lower capacity. Different kind methods of modified activated alumina were investi gated to improve the adsorptive performance. It can divide into two parts: the first part could change the properties of alumina [29–31]. Just as, Amit Bhatnagar et al. used nano-alumina to remove fluoride from wastewater treatment. The adsorption capacity of fluoride was investi gated to be strongly pH-dependent on removal of fluoride (14.0 mg g-1) at pH 6.15 by using nano-alumina. Mesoporous hierarchical alumina was synthesized to remove fluoride very fast with high adsorption
* Corresponding author. School of Metallurgy and Environment, Central South University, Changsha, 410083, PR China. E-mail address:
[email protected] (H. Wang). https://doi.org/10.1016/j.micromeso.2020.110051 Received 26 September 2019; Received in revised form 24 January 2020; Accepted 27 January 2020 Available online 29 January 2020 1387-1811/© 2020 Elsevier Inc. All rights reserved.
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capacity (26 mmol g 1) that reported by Andrea Melchior et al. Porous alumina was also used to investigate for removing fluoride by Wei-Guo Song and Yong Jia. Another part was the composite with alumina. There were different kinds of composite materials, such as carbon nano tubes/amorphous alumina, Copper oxide/mesoporous alumina, man ganese dioxide/activated alumina, etc [32–35]. Our group also reported that used Fungus hyphae/alumina and Fe3O4/Al2O3 to enhance the removal of fluoride. However, it still lacks an effective and simple way to quickly improve the adsorptive property. In this present study, hollow tubular alumina had been designed for removing fluoride from wastewater. Different proportions of H2O2 were used to modify hollow tubular alumina to affirm the effectiveness of removing fluoride. Effect of pH, Solid-to-liquid ratio, and co-existing ions had been thoroughly considered. Batch of adsorption kinetics and adsorption thermodynamics were conducted to understand the process of adsorption. The adsorption mechanism of fluoride was characterized by TEM-EDS, X-Ray Diffraction (XRD) and X-ray photoelectron spec troscopy (XPS) Brunauer–Emmett–Teller (BET), etc. These results had been detailed discussed to better comprehend the adsorption mecha nism of fluoride using H2O2 modified hollow tubular.
2.3. Batch fluoride extraction experiments The solution containing fluoride was prepared using sodium fluoride and ultrapure water. The concentration of the stock solution was 1000 mg/L. This solution was stored in a plastic bottle. Firstly, it was diluted with 1000 mg/L fluoride ion solution to prepare 100 mg/L fluoride ion solution, and 50 mL of fluoride-containing solution was separately placed in 16 polyethylene white bottles (pH ¼ 3, 4, 5, 6, 7, 8, 9, 10, two bottles each experiment), which were adjusted with hydrochloric acid and sodium hydroxide solution, respectively. Accurately weighed 16 parts of 0.050 g of adsorbent into the above small and white bottle, and then placed in a constant temperature water bath shaker for 2 h at 30 � C. The adsorbed solution was filtered, and the concentration of fluoride ions in the filtrate was measured. First, 40 mL of the test solution and 10 mL of TISAB were shaken to mix in white bottle for the test. TISAB was prepared as the following scheme to avoid the effect of other ions. 10 g of Na3C6H5O7⋅2H2O and 58 g of NaCl were added and dissolved into a 1 L beaker with 800 mL of deionized water, 57 mL of acetic acid. The solution was adjusted the pH of the solution to 5.0–5.5, then transferred the solution to a volume of 1 L. The volumetric flask of TISAB was kept for using. Fluoride ion-selective electrode was used to measure the concentration of fluoride in the solution and waited for the measure ment system to stabilize, and then recorded a stable reading. The extent of fluoride adsorption was acquired by the following equation:
2. Materials and methods 2.1. Synthesize of Al2O3-based materials The hollow tubular Al2O3 was prepared by the hydrothermal-calcine method [36]. 4 g Al(NO3)3⋅9H2O, 8 g polyethylene glycol (PEG), 10 g carbamide were mixed and stirred into 35 ml ultrapure water. The dissolved mixture was transferred into a Teflon liner, and the outer steel sleeve was screwed and placed in an oven at 120 � C for 12 h. After the hydrothermal reaction was completed, the liner was cooled to room temperature and opened. The obtained product was washed with C2H5OH and deionized water alternately until the supernatant was colorless, and the precipitate at the bottom of the centrifuge tube was dried in an oven at 60 � C for 6 h, and then placed at a temperature of 500 � C/h. The hollow alumina fiber material (Al2O3) could be obtained by calcining at a high temperature (1000 � C) for 2 h in a furnace. The modified Al2O3 was acquired in this way. Different concentra tions of H2O2 solutions (0.5%, 1%, 2%, 4%, 8%, and 10%) were sepa rately measured and prepared. The 100 mg of Al2O3 were added into 100 mL of 0.5%, 1%, 2%, 4%, 8%, and 10% H2O2 solution, placing on a magnetic stirrer with the rotation speed of 500 rpm for 12 h. After standing to settle, the sample solution was filtered using multi-purpose vacuum pump. The solid sample on the filter was dried in an oven at 60 � C for 6 h to obtain H2O2 modified hollow Al2O3 fibers. These sample were named as X% H2O2/Al2O3 (X ¼ 0.5%, 1%, 2%, 4%, 8%, and 10%).
qt ¼
ðC0 –Ct ÞV m
(1)
where Co was the original concentration of fluoride, Ct was the imme diate concentration of fluoride at a certain time. The qt was adsorption capacity of fluoride at certain time. First, 100 mg/L fluoride ion solution by diluting with a 1000 mg/L fluoride ion solution was prepared and adjusted the most suitable pH with hydrochloric acid. These bottles were placed at 30 � C with different concentrations of 10 ppm, 50 ppm, and 100 ppm. The adsorption time was controlled to be 5, 10, 20, 30, 40, 60, 120, 480, 600, 720, 1440 min, and the adsorbed solution was filtered, and the concentration of fluoride ions in the filtrate was measured. The obtained kinetic data were fitted in the different adsorption kinetics models. These kinetic models were shown as follow: Pseudo
first
Pseudo
second
� � order model: Log qe qt ¼ log qe
order model:
k1 t 2:303
t 1 t ¼ þ qt k2 q2e qe
(3) (4)
Elovich Equation: qt ¼ A þ 2:303Blogt
2.2. Materials characterization
1
Intraparticle Diffusion Kinetic Equation: qt ¼ Kid ⋅t2 þ C
The X-ray powder diffraction (XRD) patterns were collected using D/ MAX 2500 VB þ XX diffractometer with a Cu-Kɑ radiation (λ ¼ 0.15406 nm) at 40 kV and 35 mA. Infrared spectra were obtained from Nicolet IS10 (Thermo Fisher Scientific, USA). Micromorphology was investi gated using Scanning electron microscope (JSM-IT300LA, Japan) and Transmission electron microscope (JEM-2100F, Japan) coupled with Energy dispersive X-ray analysis and Energy Dispersive Spectrometer (EDS). A phase diagram was also detected with an electron microscope. The Zeta potential was carried out on Zano-zs (Malvern Instruments, England) to understand the electrical property of these materials. The heat stability of these materials was obtained from Thermogravimetric analyzer (Netzsch 449F3, Germany). Specific surface area and N2 adsorption-desorption isotherm were performed using fully automatic specific surface and pore analyzer (ASAP2020, America). The X-ray photoelectron spectroscopy was tested on X-ray photoelectron spec trometer (ESCALAB250Xi, the United States) to know about adsorption mechanism.
External Diffusion Kinetic Equation: In
(2)
Ct ¼ C0
Mass Transfer Model: Cm ¼ rexp(-kfxt) þ z
KP t
(5) (6) (7)
The qe was the biggest adsorption capacity during the research time. K1 (min 1), K2 (g⋅mg 1⋅min 1), Kf (m/s), Kid (mg⋅g 1⋅min 0.5), and Kp (min 1) were kinetics constants of different models, respectively. In the Mass Transfer model, Cm was the concentration of F ions in a solution, the Kf was the mass transfer coefficient (m/s), x was the useful area for mass transfer per unit volume of the adsorbent (3.06 � 105 m2/m3). The others were constants without any special meaning. Eight concentration gradients with initial fluoride concentrations of 25 mg/L, 50 mg/L, 75 mg/L, 100 mg/L, 125 mg/L, 150 mg/L, 175 mg/L and 200 mg/L fluoride ion solution were employed to investigated the adsorption thermodynamics. The solution contained fluoride ion was adjusted to the most suitable pH with hydrochloric acid. The adsorption 2
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Fig. 1. (a) The X-ray diffraction of modified alumina by using hydrogen peroxide with different concentrations; (b) The X-Ray diffraction of Al2O3 and 1% H2O2/Al2O3.
thermodynamics were conducted at 30 � C, 40 � C, 50 � C for the optimum time. The achieved thermodynamic data were fitted to the adsorption thermodynamic models. These models were demonstrated as these formulas: qmax bCe 1 þ bCe
(8)
Freundlich model: qe ¼ KF C1=n e
(9)
Langmuir model: qe ¼
Temkin model: qe ¼
RT InðKT TCe Þ bT
D-R model: Inqe ¼ Inqm – βε2
(10) (11)
The qe was the same as qt as aforesaid, and Ce was equivalent to Co. The qmax and b were relevant parameters with respect to the maximal adsorption capacity and the Langmuir isotherm constants. KF and n were Freundlich constants that correspond to adsorption capacity and adsorption strength. The bT and KT were Temkin isotherm constant and equilibrium affinity constant of Temkin isotherm. The ε and β were the Polanyi potential and activity coefficient as regards sorption average free energy (mol2/J2). The 20 groups of solutions with fluoride ion concentration of 100 mg/L were prepared with competitive anions (PO34 , SO24 , NO3 , Br ) of 0.1, 0.5, 1, 2, 5 mmol/L. The solution was adjusted to the best condition to know the effect of anions. Chloride ion hardly influenced the removal of fluoride, owing to a conditioning agent (HCl solution) for pH.
Fig. 2. Different qualities of Al2O3 and 1% H2O2/Al2O3 for removing fluoride.
H2O2/Al2O3 were shown in Fig. S1. The characteristic peaks remained the same location (3409, 2977, 1647, 1448, 1270, 1092, 1052, 822, and 567 cm 1). However, the color of the peak which was 3409 cm 1 that corresponded to –OH became green to blue, indicating that H2O2 modification increased the surface hydroxyl on the material. The H2O2 had rich of hydroxyl sites. However, too much hydrogen peroxide might cause excessive accumulation on Al2O3 which brought about covering the owner sites. The 1% H2O2/Al2O3 exhibited the blue color which meant that it could acquire the best hydroxylation function among X% H2O2/Al2O3. The adsorption experiments were investigated to further know the modified influence on removing fluoride. The adsorption capacity of X% H2O2/Al2O3 and Al2O3 with different liquid-solid ratio was shown in Fig. S2. It could be inferred from Fig. S2 that the adsorption capacity of the X% H2O2/Al2O3 increased first and decreased afterward in contrast to Al2O3 with gradual increasing the concentration of H2O2. The crimson was displayed in Fig. S2, hinting that 1% H2O2/Al2O3 was the best capability to adsorb fluoride. At low H2O2 concentration, the adsorption capacity of fluoride slowly increased with the promotion of H2O2 con centration. When the concentration of H2O2 increased to 1%, the adsorption capacity of fluoride increased to maximum values of about 71 mg/g, 72 mg/g, 72.8 mg/g. While the H2O2 concentration was increased from 1% to 10%, the adsorption capacity was gradually low ered. It revealed that an appropriate amount of H2O2 improved the
3. Results and discussion 3.1. The influence of hydrogen peroxide concentration on alumina The XRD results of the Al2O3 and X% H2O2/Al2O3 were shown in Fig. 1a and Fig. 1b. There were diffraction patterns corresponding to γ-Al2O3 at 2θ ¼ 32.4� , 34.6� , 37.0� , 39.1� , 42.8� , 45.1� , 46.3� , 60.9� and 67.3� (JCPDS card 47–1770), indicating the presence of γ-Al2O3 was confirmed in Fig. 1b [36,37]. The structure of X% H2O2/Al2O3 main tained steady after the modification of the comparative material indi cated that the H2O2 modification did not destroy the original γ-Al2O3 crystal structure in Fig. 1a. The X-Ray Diffractions of 1% H2O2/Al2O3 and Al2O3 were compared in Fig. 1b. It demonstrated that these mate rials had the same lattice planes (108), (217), (305), (317), (2213), and (442). However, the crystal faces of 1% H2O2/Al2O3 had shifted left slightly, suggesting H2O2 inoculated on the Al2O3. Furthermore, the γ-Al2O3 had the cubic system. The infrared spectra of Al2O3 and X% 3
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Fig. 3. Characterizations of 1% H2O2/Al2O3: (a) Scanning electron microscope (SEM) pictures; (b) Transmission electron microscope (TEM) pictures; (c) High Resolution Transmission electron microscope (HRTEM); (d) Energy dispersive spectrum (EDS) after adsorption.
hydroxyl content of Al2O3, while the excess H2O2 molecule occupied the binding site of the hydroxyl group and the aluminum. What was more, the adsorption capacity also went up with the adsorbents range 20 mg to 40 mg. Moreover, 1% H2O2/Al2O3 and Al2O3 were used to contrastive analysis in Fig. 2. The ability to remove fluoride enhanced 8%–15% with the mass varying from 20 mg to 50 mg. Considering the amount of dose and the modification effect, 1% H2O2 modified solution and 50 mg adsorbent were selected as conditions of adsorptive experiment.
1% H2O2/Al2O3 was 20–50 nm. It could discover that d ¼ 0.21 nm (3 1 7), d ¼ 0.15 nm (4 4 2), d ¼ 0.26 nm (2 1 7) from High Resolution Transmission electron microscope in Fig. 3c. The fluoride could be uniformly adsorbed on the material which was demonstrated by using surface scanning EDS in Fig. 3d. This most component of fluoride was more than 7% which could be calculated average adsorption sites under the normal condition (the initial concentration of fluoride: 100 ppm/50 mg 1% H2O2/Al2O3). Some adsorbing sites were almost 50% (mass fraction), suggesting that it had a good potential of removing fluoride in Fig. S3. The thermogravimetry of these materials was the critical data for calculating the content of hydroxide radical in Fig. S4. The density of hydroxy group was calculated as the equation in SI.1. The density of hydroxy group of Al2O3, 1% H2O2/Al2O3, and 1% H2O2/Al2O3–F were 2.553 nm 2, 3.837 nm 2, 1.448 nm 2, respectively. On the one hand,
3.2. Characterization of 1% H2O2/Al2O3 The TEM-EDS diagram of 1% H2O2 modified Al2O3 was demon strated in Fig. 3. From the SEM and TEM images, the surface of material was rough and appeared as a hollow fiber structure. The mean width of
Fig. 4. (a) The relationship of pH and Zeta potential; (b) the relationship of pH and fluoride removal efficiency. 4
Microporous and Mesoporous Materials 297 (2020) 110051
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Fig. 5. (a) Adsorption kinetics at different concentrations; (b) Pseudo-second-order model.
this phenomenon discovered that this method was able to make H2O2 incorporate with Al2O3 validly. Furthermore, the hydroxide radical had contributed to the exchange of fluoride when the adsorption happened. The N2 adsorption-desorption isotherm of 1% H2O2/Al2O3 had emerged in Fig. S5 and Fig. S6. The test of BET indicated that the material had a large specific surface area (306.14 m2/g), which was beneficial to the removal of F . The adsorption isotherm of nitrogen on 1% H2O2/Al2O3
was Type II isotherms namely S isotherms, indicating the adsorption capability of mesoporous material which had certain adsorptive ad vantages due to its own characteristics in Fig. S5. The material pore size was hierarchical porous structure, which was higher than the molecular dimension of hydrogen peroxide, offering a good interface for combining H2O2 in these pores [38]. On another hand, the average pore diameter was 9.358 nm, while the dominating distributions were 3.794 nm and 14.945 nm in Fig. S6. The ion transport of fluoride would benefit from the pore distribution. What was more, the contrastive parameters of 1% H2O2/Al2O3 and activated alumina were shown in Table S2. Not only the zero-charge point of 1% H2O2/Al2O3 but also pore volume and specific surface area of 1% H2O2/Al2O3 were higher than the same pa rameters of activated alumina. Hence, the mesoporous structure offered a better adsorptive interface.
Table 1 The kinetic models of 1% H2O2/Al2O3. Pseudo-first-order model Co(mg/ L)
Function
10 50 100
� Log qe
qt Þ ¼
k1 (min 1)
R2
0.025701 0.017917 0.014325
0.69354 0.87032 0.79449
k1 t 2:303 Pseudo-second-order model Co(mg/ Function k2 (g⋅mg 1⋅min 1) L) t 1 t 10 0.02605 ¼ þ qt k2 q2e qe 50 0.00384 100 0.00140 Elovich model Co(mg/ Function A L) 10 qt ¼ Aþ2.303Blogt 45.32194 50 32.95339 100 7.96364 Intraparticle Diffusion Kinetic model Co(mg/ Function kid (mg⋅g 1⋅min L) 1 0.02755 10 50 0.28769 qt ¼ kid⋅t2 þ C 100 0.80998 External Diffusion Kinetic model Co(mg/ Function kp (min 1) L) Ct 0.03714 10 ¼ KP t In C0 50 0.02895 100 0.01792 Mass Transfer model Co(mg/ Function r L) 10 Cm ¼ rexp (-kfxt) þ z 21.52179 log qe
50
27.51528
100
34.4638
qe (cal) (mg/ g) 9.406 47.281 84.317
0.5
)
3.3. Effect of pH The zeta potential of 1% H2O2/Al2O3 in different pH solutions was as shown in Fig. 4a. It could be seen that the surfaces of the material were positively charged in the range of solution pH ¼ 3–10, while the material surfaces were negatively charged within the scope of solution pH ¼ 10–12, indicating that 1% H2O2/Al2O3 had a higher zero charge point (pHpzc ¼ 9.8). In general, when the pH of the solution was lower than 9.8 (pHpzc), the surface of 1% H2O2/Al2O3 was positively charged, and the electrostatic attraction was favorable for the F when the fluoride close to the surface of adsorbent, promoting the process of adsorption and defluorination. While the pH of the solution was higher than 9.8 (pHpzc), the surface of 1% H2O2/Al2O3 was negatively charged, and electrostatic repulsion was not conducive to the fluoride ion, hindering the adsorp tive process. Therefore, the migration of F to the adsorbent was enhanced by electrostatic attraction, thereby improving the effect of adsorbing fluoride. The effect of initial solution pH on the adsorption of F by 1% H2O2/ Al2O3 was shown in Fig. 4b. It would be found that pH had a great in fluence on the adsorption of fluoride by 1% H2O2/Al2O3. When the pH was increased from 3 to 4, the adsorption capacity of fluoride decreased sharply from 74 mg/g to 37 mg/g. The fluoride removal capacity decreased slowly from 37 mg/g to 27 mg/g with the pH continue improving to 7. The adsorption capacity tended to be gentle after pH > 7. The acidic pH was favorable for the adsorption and removal of fluo ride by 1% H2O2/Al2O3. The effect of removing fluoride was mainly through the influence of competitive adsorption and electrostatic attraction. The higher adsorption at acidic pH was mainly attributed to the consumption of OH in high acidic conditions, which reduced the
R2 0.99995 0.99965 0.99989
B
R2
0.255 2.200 5.895
0.32340 0.71954 0.89656
C
R2
8.72833 38.84993 60.59021
0.64854 0.45638 0.09061 R2 0.05626 0.53032 0.55245
kf (m/s)
z
1.39 � 10 6 1.52 � 10 7 8.07 � 10 8
0.63182 3.25685 17.58388
5
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Fig. 6. (a) Adsorption thermodynamics at different temperatures; (b) fitting of the thermodynamic model at 30 � C; (c) fitting of the thermodynamic model at 45 � C; (d) fitting of the thermodynamic model at 60 � C.
competitive adsorptive effect of OH , thereby promoting the exchange between F and M-OH. The lower adsorption capacity at alkaline pH was mainly attributed to the competitive adsorption between F and OH . The higher pH with larger concentration of OH , and the more intense effect of competitive adsorption. The electrostatic attraction mainly depended on the pHpzc of adsorbent and the pH of the adsorptive circumstances. Hence, pH ¼ 3 was chosen as the more suitable acid-base degree.
that adsorption kinetics was better fitted to the pseudo-secondary order model in Table 1 (R2 ¼ 0.999) and Fig. 5b. Due to the high correlation coefficient, it demonstrated that the external adsorption was the domi nant and control stage. What was more, its speed control step was chemical adsorption, indicating that the exchange between OH and F on the surface of the adsorbent [39]. The much smaller value of K2, the much more adsorption sites existed. The Mass Transfer model was applied to estimate the mass transfer velocity during the process of adsorption with different original concentration solution (10 ppm, 50 ppm, 100 ppm) in Fig. S8. The Kf was the mass transfer velocity which could be simulated by fitting the experimental data in Table 1. The slower diffusion rate was reflected with the higher concentration in the solution. Therefore, 8 h should be a well-advised adsorption time for these materials in these experiments.
3.4. Kinetic characteristics of adsorption The adsorption kinetics of 1% H2O2/Al2O3 was conducted to realize the effect of adsorptive reaction time for removing fluoride. Different concentrations of fluoride (10 ppm, 50 ppm, 100 ppm) were investi gated to estimate the adsorption time in Fig. 5a. From the adsorption kinetics of 1% H2O2/Al2O3, it showed that the adsorption process was very rapid within 2 h during the beginning of adsorption. The adsorptive efficiency increased sharply, reaching 99.41%, 97.11%, 91.99%, respectively. The adsorptive velocity slowed down from 2 h to 4 h, and then the adsorption rate gradually became flat and reached the adsorption equilibrium at 8 h. The adsorption capacities that corre sponded to the initial concentration of 10 ppm, 50 ppm, 100 ppm were 9.53 mg/g, 47.5 mg/g, and 84.25 mg/g, respectively. It was also found that the higher original concentration, the longer time needs. However, it was able to reach the best adsorption efficiency fast. The kinetic data were fitted with the pseudo-first-order model, pseudo-second-order model, Elovich model, Intraparticle Diffusion Kinetic model, External Diffusion Kinetic model, respectively in Fig. S7 and Fig. 5b. It indicated
3.5. Thermodynamic characteristics of adsorption The adsorption thermodynamics experiment was carried out using 1% H2O2/Al2O3 at different temperatures. The experimental data were fitted with the Langmuir models, Freundlich models, and Temkin model, respectively. The fitting results were shown in Fig. 6. The adsorption capacity went up with the improvement of temperature in Fig. 6a. It could reach the adsorption capacities of 147 mg/g, 152 mg/g, 155 mg/g at 30 � C, 45 � C, 60 � C, respectively. It could be primarily inferred that the adsorptive fluoride using 1% H2O2/Al2O3 was an endothermic process. The experimental data were best matched well with the Freundlich model, and the relevant thermodynamic parameters were displayed in Table 2. The higher regression coefficient values (R2 ¼ 6
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Table 2 The parameters of 1% H2O2/Al2O3 using different thermodynamic models at different temperatures. Temperature(K)
Langmuir model qe ¼ R2
303 318 333
0.995 0.995 0.996
qmax bCe 1 þ bCe qm (mg⋅g 687.68 753.25 836.44
Freundlich model qe ¼ 1
)
b (L⋅mg
1
)
0.00067 0.00044 0.00047
Temkin model
KF C1=n e
qe ¼
R2
KF (mg1
0.997 0.997 0.998
1.05 1.09 1.18
(1/n)
L1/ng 1)
RT ln (KT TCe ) bT
n
R2
KT
bT
1.10 1.07 1.06
0.913 0.946 0.946
0.00015 0.00012 0.00011
43.67 40.31 41.03
Fig. 8. The X-Ray Diffractions of 1%H2O2/Al2O3 after and before adsorption. Fig. 7. Different concentrations of anion competition.
agent. The results were shown in Fig. 7. It could be discovered that the other two anions had little effect on the adsorption of F except for PO34 and SO24 by using 1% H2O2/Al2O3. With the increasing concentration of SO24 , the adsorption capacities of F went down to 97%, 94%, 89%, and 81%, respectively. However, the adsorption capacities of F decreased to 94%, 84%, 71% and 53% with the concentration of PO34 went up. The effect of trivalent and divalent coexisting anions on removing fluoride removal was significantly greater than the monovalent coex isting anions. The electrostatic attraction between the positively charged material and high negative charge density was stronger so that the higher anion was adsorbed preferentially than the monovalent anion. To the surface of the material, the active sites for adsorbing fluoride ions were reduced and the adsorption capacity was lowered. The result indicated that the affinity of F with 1% H2O2/Al2O3 was stronger. The adsorption of fluoride was more difficult instead of other anions using 1% H2O2/Al2O3, but it was easily interfered by other high-charge coexisting anions. However, it could not influence by lower concentra tions and the practical water contained little PO34 and SO24 .
0.997–0.998) indicated that the experimental results were consistent with the Freundlich adsorption isotherms, which might be attributed to the adsorptive sites with different adsorptive energies on the surface of the material, including the original hydroxyl sites of Al2O3 and H2O2 [40]. The difference in adsorptive energy of the modified hydroxyl sites was also different, and the adsorptive sites were diverse in adsorptive energy due to different positions, such as inner and outer surfaces of hollow fibers. In addition, the n values obtained by the fitting models were all greater than 1, indicating that 1% H2O2/Al2O3 had a strong affinity with fluoride ion. The adsorption process was not simply phys ically adsorption but formed stable chemical adsorption with chemical bonds. As the experimental temperature increased, the values of KF went up, indicating that the temperature rose could promote the adsorption process. This further proved that the adsorption process in the kinetics was an endothermic reaction. Furthermore, the Dubinin-Radushkevich model also was investigated in Fig. S9 and Table S1. However, it did not accommodate at different temperatures. Hence, the adsorption process was chemistry and endothermic reaction.
3.7. Adsorption mechanism of fluoride
3.6. Effect of co-existing ions
The adsorption mechanism of 1% H2O2/Al2O3 for removing fluoride was discussed, the materials before and after adsorption were charac terized by X-Ray diffraction (XRD) and X-ray photoelectron spectros copy (XPS) in Figs. 8–9. The X-Ray diffraction pattern would not disappear when the removal of fluoride happened. Only this intensity of peaks brought down due to the combine fluoride with 1% H2O2/Al2O3. This crystal structure kept the same crystal faces suggesting structural stability after adsorbing fluoride. On the other hand, the modification of Al2O3 using H2O2 had less influence on the morphology in Fig. S10. In addition, the morphology was also very stable after adsorbing fluoride in Fig. 3. The full spectrum and binding energy peaks corresponding to O 1s and Al 2p were demonstrated in Fig. 9. A peak of distinct F 1s appeared
In the natural water and wastewater containing fluoride, most anions such as PO34 , SO24 , NO3 , Br , and Cl were usually present. The coexisting anion may compete with fluoride ion for adsorptive sites during the adsorptive process, inhibiting the removing efficiency of fluoride. The competitive adsorption strength of coexisting anions depended on the relative ion concentration and its affinity with the adsorbing material, and the affinity was intrinsically related to the ion radius and amount of charge. The experiment mainly studied the in fluence with different concentrations (1, 2, 5, 10 mmol/L) of anions (PO34 , SO24 , NO3 , Br ) on removing fluoride. In addition, the Cl ions suffered from little effect due to hydrochloric acid as pH conditioning 7
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Fig. 9. The X-ray photoelectron spectroscopy before and after adsorption (a) typical wide survey; (b) F 1s; (c) Al 2p before fluoride adsorption; (d) Al 2p after fluoride adsorption; (e) O 1s before fluoride adsorption; (f) O 1s after fluoride adsorption.
Fig. 10. The dominating adsorption mechanism of 1% H2O2/Al2O3.
on the full spectrum after adsorption. The binding energy of fluoride was 685.01 eV indicating that adsorption of fluoride happened on the modified Al2O3 in Fig. 9b. The XPS of Al 2p spectrum before and after adsorption by using modified Al2O3 was displayed in Fig. 9c–d. The peak of Al 2p shifted from 74.28 eV to 74.37 eV, confirming the strong interaction between fluoride and aluminum atoms. Furthermore, the proportions of Al–O–OH went down from 80% to 30.4%, while the percentages of Al2O3 rose from 20% to 69.6%. It reflected that the ex change hydroxide radical and fluoride. In order to prove the change of Al–OH about 1% H2O2/Al2O3, the O 1s XPS spectrum of the 1% H2O2/ Al2O3 was studied before and after adsorption. The results had revealed that the O 1s spectrum was shown in Fig. 9e–f. It could be divided into three peaks, which corresponded to Al–O, Al–OH, and H2O, respectively [41,42]. The peak area ratio of Al–O decreased slightly from 50.9% to 49.2% before and after adsorption, while the peak area ratio of Al–OH declined significantly from 29.3% to 20.1%. This result indicated that
Al–OH was the main adsorption site of 1% H2O2/Al2O3. The process of removing fluoride was principally ion exchange between Al–OH and fluoride. The adsorption process gave rise to the loss of hydroxide radicals. In summary, the mechanism of adsorption was divided into two parts as follows [43–45]: (1) Electrostatic interaction, 1% H2O2/Al2O3 adsorbed F to the surface by electrostatic action; (2) Ion exchange, the complex of Al-(OH)2 released OH to form stable Al–F compound. On the other hand, the main mechanism of removing fluoride using 1% H2O2/Al2O3 was ion exchange. The H2O2 could incorporate with hollow aluminum oxide. The fluoride exchanged with OH from 1% H2O2/Al2O3. The primary adsorption mechanism was revealed in Fig. 10. These adsorption equations were demonstrated as follows: Electrostatic interaction: Al2O3Hþ þ F → Al2O3HF 8
(12)
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Al2O3
… …
O2H2 þ F → Al2O3
……
O2H2F
(13)
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Ion exchange: (14)
Al2O3Hþ þ F → Al2O2F þ OH Al2O3
… …
O2H2 þ F → Al2O3
……
OHF þ OH
(15)
4. Conclusions The paper proved the effective modified method using H2O2 to improve the ability to adsorb materials to adsorb fluoride. It exhibited good adsorption capacity (84.25 mg/g) using this method. The adsorption kinetics and thermodynamics were well described by using the pseudo second-order model and Freundlich model, indicating that chemical sorption and heterogeneous adsorption sites. The 1% H2O2/ Al2O3 had good stability after removing fluoride in adsorptive solution. The mechanism of 1% H2O2/Al2O3 for removing fluoride was electro static interaction and ion exchange. Ion exchange played a significant role in removing fluoride. Hence, hollow structure was main structural design that exposed the adsorptive sites. In future, the modified mate rials improved the density of hydroxide radical using H2O2. Moreover, it was very economical to apply to factories. Declaration of competing interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper. CRediT authorship contribution statement Lei Huang: Writing - original draft. Zhihui Yang: Conceptualiza tion, Data curation. Zhuanxia Zhang: Conceptualization, Data curation. Linfeng Jin: Formal analysis. Weichun Yang: Formal analysis. Yingjie He: Software. Lili Ren: Software. Haiying Wang: Writing - review & editing. Acknowledgements This research is financially supported by the Key Project of Chinese National Research Programs (2016YFC0403003) and the National Natural Science Foundation of China (General Program) (51374237). Appendix A. Supplementary data Supplementary data to this article can be found online at https://doi. org/10.1016/j.micromeso.2020.110051. References [1] X. Li, H. Zhang, P. Wang, J. Hou, J. Lu, C.D. Easton, X. Zhang, M.R. Hill, A. W. Thornton, J.Z. Liu, B.D. Freeman, A.J. Hill, L. Jiang, H. Wang, Fast and selective fluoride ion conduction in sub-1-nanometer metal-organic framework channels, Nat. Commun. 10 (2019) 1–12. [2] A. Bansiwal, P. Pillewan, R.B. Biniwale, S.S. Rayalu, Copper oxide incorporated mesoporous alumina for defluoridation of drinking water, Microporous Mesoporous Mater. 129 (2010) 54–61. [3] A. Dhillon, Sapna, B.L. Choudhary, D. Kumar, S. Prasad, Excellent disinfection and fluoride removal using bifunctional nanocomposite, Chem. Eng. J. 337 (2018) 193–200. [4] R. Mudzielwana, M.W. Gitari, S.A. Akinyemi, T.A.M. Msagati, Synthesis and physicochemical characterization of MnO2 coated Na-bentonite for groundwater defluoridation: adsorption modelling and mechanistic aspect, Appl. Surf. Sci. 422 (2017) 745–753. [5] E. Tchomgui-Kamga, V. Alonzo, C.P. Nanseu-Njiki, N. Audebrand, E. Ngameni, A. Darchen, Preparation and characterization of charcoals that contain dispersed aluminum oxide as adsorbents for removal of fluoride from drinking water, Carbon 48 (2010) 333–343.
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