Removal of fluoride from wastewater solution using Ce-AlOOH with oxalic acid as modification

Removal of fluoride from wastewater solution using Ce-AlOOH with oxalic acid as modification

Journal of Hazardous Materials xxx (xxxx) xxxx Contents lists available at ScienceDirect Journal of Hazardous Materials journal homepage: www.elsevi...

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Journal of Hazardous Materials xxx (xxxx) xxxx

Contents lists available at ScienceDirect

Journal of Hazardous Materials journal homepage: www.elsevier.com/locate/jhazmat

Removal of fluoride from wastewater solution using Ce-AlOOH with oxalic acid as modification ⁎

Wen Taoa, Hong Zhonga, Xiangbo Panc, Peng Wangb, Haiying Wangb, , Lei Huangb,



a College of Chemistry and Chemical Engineering, Hunan Provincial Key Laboratory of Efficient and Clean Utilization of Manganese Resources, Central South University, Changsha 410083, Hunan, China b School of Metallurgy and Environment, Central South University, Changsha 410083, China c Changsha neptunus pharmaceutical co, ltd, China

G R A P H I C A L A B S T R A C T

A R T I C LE I N FO

A B S T R A C T

Editor: Deyi Hou

In this paper, Ce-AlOOH were investigated to develop as an adsorbent for removing fluoride. Oxalic acid was selected as an effectively modified reagent to improve the performance of adsorption. Cerium existed in the form of CeO2 and kept good stability during the adsorption process through XRD, TEM, BET, Raman, and Infrared spectra. The adsorption capacity could be improved with the addition of cerium (62.8 mg/g). Specially, the oxalic acid modification significantly promoted the adsorption capacity to 90 mg/g. There adsorption isotherm and kinetics were estimated independently. These adsorption behaviors were in accordance with the Freundlich model and pseudo-second-order model, indicating that chemisorption was the rate-determining step. the obtained adsorbents all exhibited good recycling performance using oxalic acid as the regeneration reagent. The species of tetravalent cerium was the important adsorption sites. The mechanism was carefully explored by XPS analysis. The fluoride adsorption process can be ascribed to the combined effect of the electrostatic action, surface coordination, and ion exchange between M−OH and F−. Furthermore, modification of oxalic acid exhibited a new easier way to quickly increase M−OH content, which contributed to the dominated adsorption sites.

Keywords: Fluoride removal Cerium oxide Oxalic acid AlOOH Adsorption



Corresponding authors. E-mail addresses: [email protected] (H. Wang), [email protected] (L. Huang).

https://doi.org/10.1016/j.jhazmat.2019.121373 Received 11 August 2019; Received in revised form 28 September 2019; Accepted 30 September 2019 0304-3894/ © 2019 Elsevier B.V. All rights reserved.

Please cite this article as: Wen Tao, et al., Journal of Hazardous Materials, https://doi.org/10.1016/j.jhazmat.2019.121373

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method for improving adsorptive property. It is significant to find a simple reagent for enhancing the ability to removal of fluoride. In this paper, we chose oxalic acid as a modifier. AlOOH was prepared using the cerium element under hydrothermal method. Different molar ratios of Ce/Al were investigated to determine the best ratio for removing fluoride. Different concentrations of oxalic acid were also used to improve the adsorption capacity. The characteristics of related materials were studied using scanning electron microscope (SEM), EDS, X-ray diffraction spectra (XRD), and Fourier Transform Infrared Spectroscopy (FTIR). The batch of adsorptive experiments had been carried on, such as pH, adsorption isotherm, adsorption kinetics, etc. The mechanism of adsorption was also elucidated.

1. Introduction Fluoride is a significant element for flora and fauna. However, too many fluorides might restrain metabolism, respiration, and photosynthesis in plants. Too many fluorides also give rise to dental fluorosis and harm for the human body (Kumari et al., 2019; Camacho et al., 2010; Millar et al., 2017). Hence, the concentration of fluoride ion in drinking water should be lower than 1.5 mg/L according to the emission standard of World Health Organization (WHO, 2011), while the effluent standard of China is 1.0 mg/L in drinking water (GB 57492006) (Chen et al., 2017; Tripathy and Raichur, 2007). There are over 60 million people in more than 20 provinces who drink excessive fluoride ion in China, especially in the countryside (Li et al., 2001; Gong et al., 2012). To achieve the main pollutant discharge target, many technologies have been reported to remove fluoride ion from wastewater, including chemical precipitation, ion exchange, reverse osmosis, nanofiltration, electric flocculation, electro-adsorption, and adsorption (Dehghani et al., 2016; Zhang et al., 2014; Wang et al., 2020; Bansiwal et al., 2010; Kumar et al., 2011; Jagtap et al., 2011; Karthikeyan et al., 2009a; Lanas et al., 2016; Li et al., 2019a). Among them, adsorption technology is the most commonly used technology with its advantages: easy operation, bargain price, and well-adaptability (He et al., 2019; Ghorai and Pant, 2005). The adsorbing materials are the critical portion in the adsorption technology. There were many papers that reported about different adsorbing materials. Carbon-based material, metal oxide, metal hydroxide, polymer, Fungus hyphae, chitosan, zeolite, natural materials, and metal-organic frameworks had been devoted to developing as adsorbents during these years (Dhillon et al., 2018; Kumari et al., 2020a; Li et al., 2019b; Qin et al., 2016a; Mena et al., 2019; Ayoob et al., 2008; Wu et al., 2016; Wang et al., 2017; Oladoja et al., 2017; Yang et al., 2017). It had been proved that Lanthanum, Cerium, Zirconium could improve the performance for removing fluoride due to hard alkaline (Huang et al., 2019; Maliyekkal et al., 2006; Mondal and Purkait, 2019; Qin et al., 2016b). Dinesh Kumar et al, Ce–Zn BMO adsorbent exhibited a good adsorption capacity of 194 mg/g at a neutral solution and could treat 94 bed volume using nature water (Dhillon et al., 2017). Kriveshini Pillay and Arjun Maity et al reported that hydrous [email protected] polyaniline fibers were synthesized to remove fluoride and also easy to recycle with good adsorption capacity (117.64 mg/g) (Chigondo et al., 2018). The bone char modified with Ce4+ had better adsorption ability of defluorination was reported by A. Bonilla-Petriciolet, et al (ZúñigaMuro et al., 2017). Dilru R. Ratnaweera et al used the tri-metal composites of Fe-La-Ce which had obvious good adsorption capacity of wastewater that contained fluoride ions (Thathsara et al., 2018). Cerium was a kind of cheap metal element. There are many kinds of literature about cerium-based adsorbents, including Mn–Ce oxide, Al–Ce hybrid adsorbent, Fe-Al-Ce trimetal hydrous oxide, cerium/ chitosan composite, La, Ce/modified alumina (Liu et al., 2010; Tripathy et al., 2006; Zhu et al., 2017; Deng et al., 2011; Wu et al., 2013). Hence, cerium ions could be used as a very good choice for modified materials. However, there was also no paper that designed Al-Ce mixed metal ions hydroxyl compound and discussed the function of different valence states. Moreover, it is meaningful to investigate the species of cerium for removing fluoride. Generally speaking, two kinds of decorated methods were used to improve the properties of removing fluoride. On the one hand, changing the characters of adsorbing materials should be an effective way, just like the size, pore diameter, morphology, crystal form. nano-alumina, alumina cement granule, mesoporous alumina, activated alumina had been investigated to have better adsorption capacity of fluoride (Mohapatra et al., 2009; Liao et al., 2019; Yu et al., 2019; Yang et al., 2018; Bouhadjar et al., 2019; Chai et al., 2013). On the other hand, different material collaborated with each other. The combined action of two or three materials can play an important role in stimulating synergistic effects. (Li et al., 2011). However, there lacks a valid and easy

2. Materials and methods 2.1. Materials Cerium nitrate hexahydrate, aluminum nitrate nonahydrate, and sodium citrate were purchased from Aladdin company. Hydrochloric acid and sodium hydroxide were applied to adjusting acid-base property and obtained from Chinese National Medicines. NaF, NaCl, NaBr, NaNO3, Na2SO4, Na3PO4 were also acquired from Chinese National Medicines to prepare for the concentration of fluoride solution. The solution was dissolved with ultrapure water produced by an ultra-pure water system in our school. 2.2. Preparation of Ce-AlOOH The Ce-AlOOH was synthesized by the following solvothermal method: 2 mmol nitrate (Al(NO3)3·9H2O and Ce(NO3)3·6H2O) and 0.5 mmol Na3C6H5O7·2H2O were dissolved into a mixed solvent (20 ml ultra-pure water and 20 ml absolute ethyl alcohol). Ce(NO3)3·6H2O was 0.12 mmol, 0.22 mmol, 0.40 mmol, 0.67 mmol, and 1 mmol, respectively. Ultrasonic shaking was used to mixed homogeneously. Then the mixed solution was added into polyethylene liner and reacted at 220 °C for 24 h in the oven. These materials were dried at 60 °C for 6 h. These materials were prepared for removing fluoride ions and modification. These materials were named as Cex-AlOOH, X = the dosage of Ce atoms. 2.3. Modified of Ce-AlOOH Cex-AlOOH was prepared as mentioned above. Cex-AlOOH were immersed using the concentration of oxalic acid (0.5%, 1%, 2%, 3%, 4%, 5%) for 12 h. Y% OA/Cex-AlOOH were acquired to remove fluoride, Y = the concentration of oxalic acid (OA). 2.4. Characterization of materials The transmission electron microscope and EDS spectra were carried out using TECNAI G2 (FEI company, USA). The X-Ray Diffraction (XRD) of materials was studied through a D/MAX 2500 VB + XX diffractometer with a Cu-Kɑ radiation (λ =0.15406 nm) at 40 kV and 35 mA. The Fourier transformed infrared (FTIR) were collected on KAlpha 1063 (Thermo Fisher Scientific, USA). The Raman spectrum was taken on a Renishaw Raman instrument (InVia Raman Microscope) equipped with a Leica microscope. The thermogravimetry was recorded with an STA 449F3 instrument at a heating rate of 10 °C/min form 25 °C – 1200 °C under argon atmosphere. The specific surface areas of materials were performed by using Autosorb 1 (Quanta chrome Corporation). To confirm the charge of materials, Zeta potentials were calculated using kinetic light scatterer (Malvern zeta sizer instrument Nano-ZS, UK). The X-ray photoelectron spectroscopy (XPS) was acquired using XSAM800 equipment (KRATOS company, UK) to investigate the mechanism. 2

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Cerium oxide could be obtained when Ce(NO3)3 was added into reaction kettle under hydrothermal reaction, and too much CeO2 might deposit and aggregate on the surface and pores of the materials, resulting in a decrease in their specific surface area. These also were shown in Table 1. The content of O atoms was enhanced after modifying due to the doping of oxalic acid. Some structural parameters of Ce-AlOOH and Ce-AlOOHoa were listed in Table 1. The BET test indicated that the specific surface area (155.86 m2/g) was reduced due to doping relative to γ-AlOOH (172.92 m2/g), but Ce-AlOOH had a large specific surface area, which was beneficial to the removal of F−. However, the specific surface of Ce-AlOOHoa was improved to 161.58 m2/g. The material pore size exhibited mesoporous characteristics of 2–50 nm, which essentially had a certain adsorption advantage. The pore diameter and pore volume increased a little due to the incorporation of oxalic acid. It was interesting to find that the best adsorption capacity (62.8 mg/g) of Cex-AlOOH reached when the dose of Ce(NO3)3 was 0.67 mmol. Therefore, Ce0.67-AlOOH was used in the following experiments, which was abbreviated to Ce-AlOOH in this paper. Further, based on surface strengthening by oxalic acid, the adsorption capacity of modified Ce-AlOOH was shown in Fig. 1b. It could be seen that the oxalic acid-modified Ce-AlOOH had different degrees of improvement in fluoride adsorption capacity, compared with the unmodified material. The adsorption capacity of removal increased with the increase of oxalic acid concentration. When the concentration of oxalic acid increased to 2%, the adsorption capacity of fluoride increased to the maximum value. The adsorption capacity did not change significantly when the concentration of oxalic acid increased to more than 2%. It indicated that Ce-AlOOH had been fully modified when the mass fraction of oxalic acid was 2% with the adsorption capacity about 88 mg/g. Considering the amount of oxalic acid and its modification effect, the oxalic acid solution with 2% mass fraction can be considered as the chemical impregnation modifier to decorate Ce-AlOOH, which was abbreviated to Ce-AlOOHoa in this whole paper (just as naming method in 2.2, 2.3).

2.5. Batch adsorption experiments The NaF was dissolved into ultra-pure water to prepare containing F (1000 ppm) as a stock solution. Then the main concentration of solutions for testing to remove fluoride was 100 ppm. These conditions of main adsorption experiments were conducted using 50 mg adsorbents at pH 3.0 for 2 h. The concentration of fluoride was detected using Orion CHN090 with the fluoride-selective electrode. The removal efficiency of fluoride was calculated using the following equation:

C − Ct ⎞ qt = ⎛ o V ⎝ m ⎠

(1)

Where Co was the original fluoride concentration (mg/L), Ct was the concentration of fluoride at time. Different values of pH were tested using hydrochloric acid and sodium hydroxide as modifiers from pH = 1-13. To better understand the adsorption process, different initial concentrations ranging from 10 ppm to 1000 ppm were investigated as adsorption isotherms at different temperatures (30 °C, 40 °C, 50 °C). The obtained data attempted to fit the Langmuir model and Freundlich model. These models were shown as follows:

Langmuir model: q e =

qmax bCe 1 + bCe

(2)

Freundlich model:q e = KF C1/n e

(3)

Where Ce was the equilibrium concentration of fluoride in solution, qmax was the maximum adsorption capacity. In these models, qe was the amount of equilibrium per gram, b (L/mg) was the Langmuir constant related to the affinity of fluoride with the binding sites. KF was the Freundlich isotherm constants related to adsorption capacity, and n was associated with the heterogeneity of the adsorption site energies and intensity. The different reaction time changed from 5 min to 240 min was studied as adsorption kinetics at different temperatures (30 °C, 40 °C, 50 °C), too. Pseudo-first order kinetic model and Pseudo-second order kinetic model were used to know the adsorption kinetics. These models were exhibited in these equations.

Pseudo− first order kinetic model:

dqt = k1(q e − qt) dt

Pseudo− second order kinetic model:

dqt = k2 (q e − qt)2 dt

3.2. Characterization of materials To realize the change in the effect of oxalic acid, Fig. 2a–d showed the TEM-EDS diagram of Ce-AlOOH and Ce-AlOOHoa. The surface of the material was rough and appeared as a spherical cluster structure, and there was no significant difference between the materials before and after the modification in Fig. 2a, c. EDS analysis showed that the main constituent elements of Ce-AlOOH were O, Al, and Ce. Furthermore, the molar ratio of Al to Ce was about 7:1, indicating that most of the metal sites in the material were still as the initial solution in Fig. 2b, d. The XRD results of Ce-AlOOH and Ce-AlOOHoa were shown in Fig. 3. There were diffraction peaks corresponding to γ-AlOOH at 2θ = 14.3°, 28.6°, 38.4° and 49.3° (JCPDS card 74–1895), indicating that there was γAlOOH in the material. At 28.7°, 33.0°, 47.8°, and 56.4°, there were four distinct diffraction peaks different from γ-AlOOH, corresponding to CeO2, which strongly indicated that the morphology of the cerium contained in the material was crystalline CeO2. Therefore, it indicated that the boehmite material was obtained, but the crystallinity of CeO2 was better than that of γ-AlOOH. The Raman spectrum of Ce-AlOOH was shown in SI Fig. 1. There was a significant peak between 400500 cm−1, indicating that the material contained cerium oxide (Cho et al., 2019), further confirming the presence of CeO2 in the above XRD results. Ce-AlOOHoa still retained the original crystal configuration after oxalic acid impregnation. However, at 25.0° and 43.9°, two weak new diffraction peaks appeared, which might be hydroxyl oxalate generated on the surface of modified materials. The infrared spectrum of these materials was shown in Fig. 4. The Ce-AlOOH had characteristic peaks at 3307, 3088, 2077, 1637, 1161, 1068, 740, 611, and 484 cm−1 before adsorption, corresponding to the peak positions of γ-AlOOH at 3296, 3094, 2091, 1640, 1164, 1071, 751,

(4)

(5)

In these functions, qt was the amount of the adsorption capacity at the time. K1 and K2 were the constants in different kinetic models, respectively. Different concentrations of negative ions were added into fluoride solutions (1, 2, 5, 10 mmol/L). The negative ions included PO43−, SO42, NO3-, Br-, Cl- demonstrated the effect of coexisting anions. Different concentrations of oxalic acid were used to estimate recycle the adsorbing materials (0.2%–5%). The materials were investigated in five cycles about regeneration performance. Cex-AlOOH and Y% OA/CexAlOOH were both applied in a batch of adsorption experiments to judge this performance of modification. 3. Results and discussion 3.1. Effect of cerium and oxalic acid To prove the influence on the dose of Ce(NO3)3 by Ce-AlOOH about the adsorption of fluoride, the adsorption effect of Cex-AlOOH was shown in Fig. 1a. The results showed that the adsorption capacity of Cex-AlOOH increased rapidly with the increase of Ce(NO3)3 dose. The proper amount of Ce in Cex-AlOOH significantly improved the ability to remove fluoride of γ-AlOOH. However, the adsorption capacity of CexAlOOH decreased when AlOOH took the serious Ce(NO3)3 overdose. 3

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Fig. 1. (a) The concentration of Ce(NO3)3 of modified AlOOH, (b) fluoride removal effects of modified Ce-AlOOH with different concentrations of oxalic acid. Table 1 Specific surface area of Ce-AlOOH and Ce-AlOOHoa. Sorbent

BET surface area

Pore diameter

Pore volume

Ce-AlOOH Ce-AlOOHoa

155.86 m2/g 161.58 m2/g

17.62 nm 18.23 nm

0.58 cm3/g 0.63 cm3/g

630, and 484 cm-1 (Karthikeyan et al., 2009b). The modified CeAlOOHoa materials had absorption peaks at 3308, 3088, 2096, 1619, 1160, 1074, 739, 616, and 484 cm−1. After modification by oxalic acid impregnation, the material shifted at the original absorption peak position of Ce-AlOOH, confirming the formation of modified Ce-AlOOH. The infrared spectrum generated red shift due to more active groups. Due to the doping of Ce, the material shifted at the original absorption peak position of γ-AlOOH, confirming the formation of γ-AlOOH. The absorption peak at 1068 cm−1 belonged to the MeOH superposition peak of Al−OH and Ce−OH, indicating that cerium doping caused hydroxylation of the surface of the material, and Ce-AlOOH had abundant metal hydroxyl sites. The Ce-AlOOHoa also had a sharp absorption peak at 1074 cm−1, which belonged to the MeOH superposition peak of Al−OH and Ce−OH, indicating that the modified CeAlOOH had rich metal hydroxyl sites. The Ce-AlOOHoa had a sharp new absorption peak at 1315 cm−1, which belonged to the absorption peak

Fig. 3. The XRD spectra of Ce-AlOOH and Ce-AlOOHoa.

of C]O, indicating that hydroxy oxalate might exist on the surface of Ce-AlOOHoa. The thermogravimetric analysis of Ce-AlOOH and Ce-AlOOHoa was shown in SI Fig. 2. It could be seen that Ce-AlOOH and Ce-AlOOHoa had

Fig. 2. (a) TEM images of Ce-AlOOH, (b) EDS spectra of Ce-AlOOH, (c) TEM images of Ce-AlOOHoa, (d) EDS spectra of Ce-AlOOHoa. 4

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Fig. 5. Effect of pH on fluoride removal by Ce-AlOOH and Ce-AlOOHoa.

Fig. 4. The FTIR spectra of AlOOH, Ce-AlOOH, Ce-AlOOHoa.

competitive adsorption between F− and OH− with the increase of pH. The higher the concentration of −OH was, the more remarkable competitive adsorption effect was. The electrostatic attraction might be improved and competitive adsorption would weaken with adding oxalic acid, which corresponded to the zeta potential of Ce-AlOOH and CeAlOOHoa.

good thermal stability. The weight loss of Ce-AlOOH was about 2% before 200 °C, implying that the material after drying treatment contained less adsorbed water. While the weight loss of about 5% about CeAlOOHoa before 200 °C demonstrated that the material after drying treatment contained not only a small amount of adsorbed water but also oxalic acid. There was a significant weight loss step at 200–800 °C, which was mainly attributed to the high-temperature conversion of γAlOOH to form Al2O3. The weight of the material remained unchanged after 800 °C, and the main components were Al2O3 and CeO2. The zeta potential of Ce-AlOOH and Ce-AlOOHoa demonstrated in SI Fig. 3 at different pH solutions. It could be discovered that the surface of CeAlOOH was positive in the range from pH 1 to 10.3, while the surface of the material was negative in the range from pH 10.3 to 13, indicating that Ce-AlOOH had a high zero charge point (pHpzc = 10.3). In contrast with Ce-AlOOH, the Ce-AlOOHoa had a higher zero charge point (pHpzc = 10.6), suggesting better performance for removing fluoride. Generally, when the adsorbent was positively charged, the opposite phase would tend to adsorb the anionic fluoride. While the surface of the adsorbent was negatively charged, the same-charge repelling greatly hindered the adsorption of fluoride (Mohammadi et al., 2017). Therefore, when the pH of the adsorption solution was lower than 10.3 or 10.6, the surfaces of Ce-AlOOH and Ce-AlOOHoa were positively charged, and the migration of fluoride to the adsorbent was enhanced by electrostatic attraction, thereby this phenomenon said modification improved the effect of adsorption and removal of fluoride.

3.4. Adsorption thermodynamics To further investigate the effect of initial fluoride ion concentration on removing fluoride, the adsorption thermodynamics experiment was carried out on Ce-AlOOH and Ce-AlOOHoa. The experimental data were fitted with the Langmuir and Freundlich models respectively. It obviously discovered that Freundlich model was more suitable than Langmuir models in Fig. 6. The relevant thermodynamic parameters of the Freundlich model were shown in Table 2. The higher regression coefficient values of Ce-AlOOH and Ce-AlOOHoa (R2 = 0.9988∼0.9997) indicated that the experimental results were consistent with the Freundlich adsorption isotherms. That might be ascribed to the adsorption sites with different adsorption energies on the surface of the material, proving that the oxalic acid was relative to γ-AlOOH with doping cerium. The surface of Ce-AlOOH and CeAlOOHoa had different adsorption energy of main hydroxyl sites, such as Al−OH and Ce−OH, and MeOH. In addition, the n values obtained by the fitting curve were all greater than 1, meaning that there was a strong affinity between the adsorbents and the adsorbate (F−) (Zhang et al., 2019). The adsorption process was not simply physically adsorbed but formed stable chemical adsorption of chemical bonds (MF). The n values of Ce-AlOOHoa was larger than that of Ce-AlOOH, which meant that the concentration of fluoride was a greater influence on CeAlOOHoa (Aghaei et al., 2015). As the experimental temperature increased, the values of KF also grew, indicating that the temperature could promote the adsorption process, which further proved that the adsorption process was an endothermic reaction. The much bigger value of KF, the more combining capacity was reflected (Rahmani et al., 2009). Therefore, the combination of fluoride and Ce-AlOOHoa was much easier than that of Ce-AlOOH.

3.3. Effect of pH The effect of initial solution pH on the adsorption of F− by CeAlOOH and Ce-AlOOHoa was shown in Fig. 5. It would be found that pH had a great influence on the adsorption of fluoride by Ce-AlOOH and Ce-AlOOHoa. The adsorption capacity of Ce-AlOOH was about 62 mg/g, while the adsorption amount of Ce-AlOOHoa was about 88 mg/g when the pH was 1-3. The fluoride removal capacity of Ce-AlOOH decreased slowly from 62 mg/g to 53 mg/g with increasing from pH 3 to 10, and the adsorption amount of Ce-AlOOHoa descended to 69 mg/g. The adsorption capacity of these materials dropped sharply at pH = 10-13. These materials had good fluoride removal performance in a wide pH range, acidic pH was favorable for the adsorption of fluoride. Then other adsorption experiments were conducted at pH = 3. Compared with Ce-AlOOH, the adsorption capacity of modified Ce-AlOOHoa increased by 25%∼40% in pH = 1-10. The effect of pH on fluoride removal mainly included the influence of competitive adsorption and electrostatic attraction. The higher adsorption at acidic pH was mainly attributed to less competitive adsorption of OH− and electrostatic attraction of H+. Therefore, it promoted the exchange of groups between F- and MeOH. The lower adsorption was mainly attributed to the

3.5. Adsorption kinetics The effect of adsorption reaction time on fluoride removal of CeAlOOH was carried out to study the adsorption kinetics. The results were shown in Fig. 7. Fig. 7a, d showed that the adsorption rate of F− was fast in the material. In the first 30 min, the adsorption process was very rapid at the beginning of adsorption. The adsorption capacity of Ce-AlOOH increased sharply, reaching 50.86, 55.21, 61.43 mg/g, respectively. The adsorption capacity of Ce-AlOOHoa reached 68.81, 5

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Fig. 6. Adsorption isotherm models of fluoride on Ce-AlOOH (a) the Langmuir model; (b) the Freundlich model; adsorption isotherm model of fluoride on CeAlOOHoa (c) the Langmuir model; (d) the Freundlich model.

73.25, 78.52 mg/g in the first 30 min. The adsorption rate slowed down within 30 min to 1 h, but still maintained a high adsorption rate. The adsorption rate gradually became flat within 1 h–2 h and reached the adsorption equilibrium in about 2 h. The equilibrium adsorption amounts of Ce-AlOOH were 61.43, 65.36, and 71.33 mg/g, respectively at different temperatures. And the equilibrium adsorption amounts of Ce-AlOOHoa were also 88.62, 91.12, and 94.68 mg/g, respectively. It could also be seen from Fig. 7 that led to accelerating the adsorption rate and increasing the equilibrium adsorption amount with the temperature rose. It can be also concluded that F− adsorption of Ce-AlOOH and Ce-AlOOHoa were an endothermic process. Compared with the equilibrium adsorption amount of Ce-AlOOH, the amount of fluoride removal increased by 30%∼40% using Ce-AlOOHoa. The kinetic data were fitted with the pseudo-first-order model and pseudo-secondary

Table 2 Isotherm parameters for the Freundlich model at different temperatures about Ce-AlOOH and Ce-AlOOHoa. Temperature (℃) Ce-AlOOH 30 40 50 Ce-AlOOHoa 30 40 50

KF

n

R2

2.642 2.835 3.183

1.168 1.138 1.105

0.9794 0.9890 0.9961

13.32 15.32 17.23

1.608 1.571 1.415

0.9985 0.9983 0.9991

Fig. 7. (a) Adsorption kinetics of fluoride on Ce-AlOOH; (b) the pseudo-first-order model on Ce-AlOOH; (c) the pseudo-second-order model on Ce-AlOOH; (d) adsorption kinetics of fluoride on Ce-AlOOHoa; (e) the pseudo-first-order model on Ce-AlOOHoa; (f) the pseudo-second-order model on Ce-AlOOHoa. 6

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amount of F- and PO43− were present in the solution, a co-adsorption treatment method might be tried. However, Ce-AlOOH and Ce-AlOOHoa had higher selectivity to F- than PO43− in water, and the adsorption process was not easily affected by other interference with coexisting anions.

Table 3 Kinetic parameters for the pseudo-second-order models at different temperatures about Ce-AlOOH and Ce-AlOOHoa. Temperature (℃) Ce-AlOOH 30 40 50 Ce-AlOOHoa 30 40 50

qe(mg/g)

k2× 103(g/mg ∙ min)

R2

62.77 66.67 72.73

3.58 3.78 4.07

0.9991 0.9995 0.9997

90.91 93.20 96.53

1.96 2.16 2.43

0.9988 0.9992 0.9994

3.7. Regeneration The recycling performance of Ce-AlOOH and Ce-AlOOHoa were conducted and shown in Figs. 9 and 10. The F− adsorbed on materials could be desorbed by substances such as acid, alkali, and salt. In order to ensure the adsorption performance of the adsorbent after desorption, the choice of desorbent depended largely on the content of −OH. In this experiment, an oxalic acid solution with rich carboxyl groups and weak acidity was selected as the desorbent. The desorption effect of F− in each concentration of the oxalic acid solution was shown in Fig. 9. With the increase of the concentration of the oxalic acid solution, the desorption rate of F− gradually increased and tended to be gentle. The mass concentration of the oxalic acid solution was selected as 2% and 3% for Ce-AlOOH and Ce-AlOOHoa, respectively. The results of the adsorption-desorption 5-cycle experiment were shown in Fig. 10. The amount of adsorbed F- on Ce-AlOOH and Ce-AlOOHoa slowly decreased with the increase of the number of cycles, and the adsorption capacity of the fifth cycle still maintained good adsorption capacity. They both kept 76% and 70% of the initial adsorption capacity. In summary, the adsorption of fluoride by Ce-AlOOH and Ce-AlOOHoa had good recycling performance.

model respectively. The fitting results were shown in Figs. 7.b, c, e, f. The data was well fitted to the pseudo-secondary model and its related dynamics. The parameters of the pseudo-secondary model were shown in Table 3. Due to the higher correlation coefficient (R2 = 0.9988–0.9997), the kinetics of Ce-AlOOH and Ce-AlOOHoa for removing fluoride would be well described using the pseudo-secondary model, which indicated that the surface adsorption reaction was the dominant and control stage, and the rate-control step was chemisorption. The smaller value of K2, the much more adsorption site was obtained (Kong et al., 2019). Hence, oxalic acid improved the ability of exchanged −OH with F- on the surface of the adsorbent.

3.6. Effect of coexisting anions In the fluoride-containing natural water and wastewater, a plurality of anions such as PO43−, SO42−, NO3−, Br−, and Cl− were usually present. The coexisting anion might compete with the adsorbate ions for adsorption sites during the adsorption process, reducing the removal efficiency of the adsorbent. The adsorption competition of coexisting anions depended on the relative ion concentration and its affinity with the adsorbent, and the affinity was intrinsically related to the ion radius and charge amount. The experiment mainly studied the effects of different concentrations (1, 2, 5, 10 mmol/L) of anions (PO43−, SO42−, NO3−, Br−, and Cl−) on F− adsorption behavior. The results were shown in Fig. 8. It can be seen from the figure that except for PO43−, the other four anions had little effect on the adsorption of F− of CeAlOOH and Ce-AlOOHoa. The removal rate of F- decreased to 97% and 91%, 80% and 65% about Ce-AlOOH when the concentration of PO43− increased, while the removal rate of F- decreased to 94%, 86%, 75%, and 61% about Ce-AlOOHoa. The competitive adsorption of PO43− might be ascribed to a high negative charge density, leading to easily adsorption onto the surface of a positively charged material. The charge radius ratios of PO43−(3/3.40) and F-(1/1.33) were similar (George et al., 2010). The PO43− in the solution could gradually occupy the active adsorption sites on the surface of the adsorbent, making it impossible for the adsorbent accepting more F−. Therefore, when a large

3.8. Mechanism of adsorption The adsorption mechanism of Ce-AlOOH and Ce-AlOOHoa was investigated by X-ray photoelectron spectroscopy (XPS), as shown in Fig. 11. There was a full spectrum with binding energy peaks corresponding to O 1s, Al 2p, and Ce 3d. The apparent F 1s peak had binding energy of 685.09 eV, indicating that F− was adsorbed on Ce-AlOOH in Fig. 11a (Pillai et al., 2019). To investigate the morphology distribution of fluoride adsorbed on Ce-AlOOH, the F 1 s XPS spectrum after adsorption was applied (Fig. 11b, g). Obviously, the F 1 s spectrum was divided into two peaks corresponding to Al-F and Ce-F, respectively. The ratio of peak area changed from 3:1 to 1.6:1 with modification using oxalic acid. On the one hand, it meant that the adsorbed F formed a chemical bond with Al and Ce in Ce-AlOOH. The improved contribution of Ce on removing fluoride might be the combination of oxalic acid with Al atom. Particularly, the lower ratio of Ce4+ suggested that Ce4+ played a more important role in removing fluoride. To further understand the change of MeOH during the removal of fluoride, the O 1s XPS spectrum of Ce-AlOOH before and after adsorption was studied. The O 1s spectrum could be divided into three peaks as shown in Fig. 11e, f, h, i. The peaks corresponded to MO, MeOH, and H2O, respectively, and the peak areas before and after adsorption varied in

Fig. 8. Effect of coexisting anions on the adsorption of fluoride about Ce-AlOOH and Ce-AlOOHoa. 7

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Fig. 9. Effect of oxalic acid concentration on the desorption of fluoride about Ce-AlOOH and Ce-AlOOHoa.

Fig. 10. Effect of cyclic adsorption-regeneration runs on the adsorption of fluoride about Ce-AlOOH and Ce-AlOOHoa.

Fig. 11. XPS spectra of Ce-AlOOH: (a) typical wide survey; (b) F 1s; (c) Ce of before fluoride adsorption; (d) Ce after fluoride adsorption; (e) O 1s before fluoride adsorption; (f) O 1s after fluoride adsorption; XPS spectra of Ce-AlOOHoa (g) F 1s; (h) O 1s before fluoride adsorption; (i) O 1s after fluoride adsorption. 8

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Fig. 12. This mechanism of Ce-AlOOHoa.

HOOCCOOH + F− → HOOCCOF + OH

different degrees (Teng et al., 2009). The peak area ratio of MO before and after adsorption decreased slightly from 55.4% to 51.3%, while the peak area ratio of MeOH decreased significantly from 25.5% to 20.4% (Fig. 11e, f). Simultaneously, the peak area ratio of MO decreased slightly from 49.2% to 46.2%, while the peak area ratio of MeOH decreased significantly from 33.7% to 23.5% (Fig. 11h, i). It strongly implied that MeOH was the main adsorption site. By comparing the MeOH peak area ratio of Ce-AlOOH and Ce-AlOOHoa before adsorption (25.5% and 33.7%), oxalic acid modification definitely increased the surface hydroxyl content of Ce-AlOOH. Considering the defluorination capacity of Ce-AlOOH and Ce-AlOOHoa (61.43 mg/g and 88.62 mg/g) and the decrease of MeOH peak area ratio before and after adsorption (10.2% and 5.1%), the oxalic acid-modification obviously enhanced the fluoride removal by increasing surface MeOH contents. The process originated from ion exchange between MeOH and fluoride. In summary, the adsorption of fluoride by Ce-AlOOH could be attributed to the ion exchange of F− and Al−OH, Ce−OH. Oxalic acid modification and the doping of cerium greatly increased the MeOH content in the adsorbents. Thereby more adsorption sites generated and thus increased the amount of fluoride removal. The fluoride removal mechanism of Ce-AlOOH and Ce-AlOOHoa was shown in Fig. 12 (O’Connor et al., 2018; Kumari et al., 2020b). (1) electrostatic action: Ce-AlOOH and Ce-AlOOHoa adsorbed F− to the surface by electrostatic attraction; (2) surface coordination: F-complex reacted with Al or Ce to form a complex of Al-F or Ce-F on the surface of Ce-AlOOH and Ce-AlOOHoa. The species of tetravalent cerium were the important adsorption sites; (3) ion exchange: the generation of M-F complex released OH− to form a stable AlO-F, CeO-F, and CO-F compound. Therefore, the mechanism of adsorption and removal of fluoride by Ce-AlOOH and Ce-AlOOHoa has included the combined effect of electrostatic interaction, surface complexation, and ion exchange. The chemical reactions involved were as follows: Electrostatic action: AlOOHH+ + F− → AlOOHHF

(6)

CeOOH+ + F− → CeOOHF

(7)

HOOCCOOH + F− → HOOCCOOHF

(8)

This mechanism was clearly shown in Fig. 12. 4. Conclusion In this work, Ce-AlOOH and Ce-AlOOHoa were compounded as the adsorbing materials. This conclusion could be divided into two parts: (1) Ce-AlOOH would have a good performance when the molar ratio of nAl: nCe was 2: 1. The adsorption property could be stable in a wide range of pH. Chemisorption was a rate-determining step because it conformed to the Freundlich model and pseudo-second-order model. Adsorption equilibrium reached fast in 120 min (62.8 mg/g). It had good cycle performance, maintaining 74% of the initial adsorption capacity after 5 cycles. (2) Ce-AlOOHoa modified by 2% oxalic acid exhibited similar adsorption stability in a wide range of pH, adsorption equilibrium rate and cycling performance, by comparing with CeAlOOH. However, their adsorption capacity was significantly improved by 25%∼40%. The tetravalent cerium was the crucial adsorption species. Ion exchange between M−OH and fluoride was demonstrated to be main process during fluoride adsorption. Specially, oxalic acid modification and the doping of cerium greatly increased the M−OH content in the adsorbents. Oxalic acid modification could be used as an effective strategy for structural design and synthesis of adsorbent for fluoride removal. Acknowledgment This research is financially supported by the Hunan province Natural Science Foundation of China (General Program) (2018SK2026) and the Key Project of Chinese National Research Programs (2016YFC0403003). Appendix A. Supplementary data Supplementary material related to this article can be found, in the online version, at doi:https://doi.org/10.1016/j.jhazmat.2019.121373. References

Surface coordination: AlOOH + F− → F-AlOOH CeOO + F− → F-CeOO

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Ion exchange: AlOOH + F− → AlO-F + OH

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CeOOH+ + F− → CeO-F + OH

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