Preparation of Novel Magnetic Microspheres with the La and Ce

Article Cite This: J. Chem. Eng. Data 2019, 64, 3641−3651

pubs.acs.org/jced

Preparation of Novel Magnetic Microspheres with the La and CeBimetal Oxide Shell for Excellent Adsorption of Fluoride and Phosphate from Solution Minyuan Han,† Jinghua Zhang,*,‡ Yanyan Hu,† and Runping Han*,† †

College of Chemistry and Molecular Engineering, Zhengzhou University, No. 100 of Kexue Road, Zhengzhou 450001, PR China Department of Chemistry and Chemical Engineering, Huanghuai University, No. 599 of Wenhua Road, Zhumadian 463000, PR China

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‡

ABSTRACT: Magnetic core−shell Fe3O4@La−Ce composites were prepared through a rapid coprecipitation method for adsorption of phosphate and fluoride anions from aqueous solution in a single system. Fe3O4@La−Ce was characterized by scanning electron microscopy and X-ray photoelectron spectroscopy, and it could be easily separated from aqueous solution by an external magnet. The effects of solution pH and co-existing anions on phosphate and fluorine uptake by Fe3O4@La−Ce were evaluated. The adsorption process was highly pH-dependent, and the optimal condition was pH 3 for phosphate and pH 4 for fluorine ions. The mechanism may be ligand exchange, electrostatic interaction, and Lewis acid−base interaction. The results showed that the adsorption capacity at 303 K was 51.6 and 56.8 mg/g for phosphate (calculation according to P element) and fluorine, respectively. The kinetic process was better predicted by the Elovich equation and double constant equation. Phosphate adsorption isotherms were well described by the Koble Corrigan model, while fluorine adsorption isotherms were fitted better by the Freundlich model and Redlich−Peterson model. There was coordination between La or Ce from the adsorbent and O or F from the adsorbate. Thermodynamic results demonstrated that the adsorption processes of phosphate and fluorine were spontaneous and endothermic. Fe3O4@La−Ce was highly selective and efficient to remove phosphate and fluoride from solution. magnetite (Fe3O4 or γ-Fe2O3) is potentially attractive to prepare magnetic adsorbents.11,12 The magnetic carrier is usually composed of the magnetic core to ensure a stronger magnetic response and a metal-oxide shell to provide favorable functional groups and features for various applications. Rare metal-modified adsorbents have been presented for removal of phosphate and fluorine from solution. Trivalent metal lanthanum with hard Lewis acid property has strong affinity toward phosphate.1 It is noteworthy that fluoride ions have high electronegativity and small ionic sizes, and La ions show high chemical attraction for fluoride ions.3,11 The advantage of cerium (Ce) is that it promotes particle dispersion and helps obtain greater surface area, pore size, and pore volume, as well as more active functional sites, which are beneficial to the adsorbent.12 There are several adsorbents modified by lanthanum and cerium, such as Fe−Si−La and Al−Ce binary oxide Fe3O4@alginate−La. However, there are few reports on magnetic core−shell Fe3O4@La−Ce for removal of phosphate and fluorine from solution.1,3,13 However, magnetic core−shell Fe3O4@La−Ce for removal of phosphate and fluorine from solution has not been reported.

1. INTRODUCTION Phosphate and fluorine can be harmful or beneficial to human beings and aquatic animals depending on the concentration. Excessive phosphorus-containing wastewater discharge into lakes can easily lead to eutrophication of lakes. Phosphorus in the eutrophication threshold ranges from 0.02 to 0.1 mg/L.1,2 The fluoride level in the range of 0.5−1.0 mg/L is critical to humans, and excess fluoride ions in drinking water may cause osteoporosis, nervous disorders, and so forth, which are not conducive to human health.3 Therefore, wastewater containing phosphate and fluorine must be treated before being discharged. Phosphate and fluorine can be separated from wastewater by ion exchange and chemical precipitation processes.4,5 However, the adsorption process has been considered for its efficiency, selectivity, and accessibility.5,6 Adsorbent materials for phosphate and fluorine have been widely reported, such as the La(III)-modified zeolite adsorbent, La−SBA-15, La−Al-loaded scoria, and manganese oxide-modified activated carbon.7−10 Solid can be separated from liquid by conventional means such as filtration and sedimentation. In recent years, the magnetic adsorbent has received extensive attention. Magnetic-based solid/liquid separation offers several advantages, for example, higher separation efficiency, lower energy consumption, and shorter time. Because of the magnetic response and larger surface area, © 2019 American Chemical Society

Received: May 17, 2019 Accepted: July 3, 2019 Published: July 16, 2019 3641

DOI: 10.1021/acs.jced.9b00434 J. Chem. Eng. Data 2019, 64, 3641−3651

Journal of Chemical & Engineering Data

Article

coprecipitated onto the surface of Fe3O4. The driving force between Fe3O4 and La−Ce may be heterogeneous precipitation, hydrogen bonding, or physical interactions. 2.3. Characterization of Fe3O4 and Fe3O4@La−Ce. The magnetic hysteresis loop of Fe3O4 and Fe3O4@La−Ce was with an in-plane external field (H), as determined using a VSM (Quantum Squid-VSM, America). The surface area and pore volume were measured based on the Brunauer−Emmett− Teller surface area (BET surface area, Micromeritics ASAP2020, America). The element contents on Fe3O4 and Fe3O4@La−Ce were determined using XRF (Bruker S4 PIONEER, Germany). XRD (PANalytical X’Pert PRO, Holland) was applied to identify the mineral phases. The microstructure and morphology of Fe3O4@La−Ce particles were imaged by SEM (Hitachi Su8020, Japan). The pH at point zero charge (pHpzc) of Fe3O4@La−Ce was evaluated by the solid addition method. The valances of specific elements of magnetic core−shell Fe3O4@La−Ce before and after adsorption phosphate (Fe3O4@La−Ce−P) and fluorine (Fe3O4@ La−Ce−F) were analyzed by XPS (Thermo Fisher ESCALAB 250Xi, England). 2.4. Adsorption Tests. Adsorption was performed in an air bath (SHZ-82, Guohua enterprise, China) at 303 K and 120 rpm in the batch mode. Phosphate or fluorine solution was introduced in a flask at the magnetic particle concentration of 0.5 g/L. After that, Fe3O4@La−Ce could be easily separated from the aqueous solution by the external magnetic field. The concentration of phosphate and fluorine was measured by using the spectrophotometry (752, Shanghai Sunny Hengping Science Instrument Co., Ltd., China) method. Mo-Sb Antispectrophotometric and Fluorine reagents spectrophotometry were used to determine the concentration for phosphate and fluorine at the wavelength of 700 and 620 nm (maximum absorbance), respectively. The adsorption quantities of phosphate and fluorine onto the unit weight of Fe3O4@La−Ce (qt or qe, mg/g) were expressed as following

In this paper, Fe3O4@La−Ce composites were obtained via a rapid coprecipitation method. The characteristics were presented by the following techniques: VSM (vibrating sample magnetometer), XRF (X-ray fluorescence), XRD (X-ray diffraction), SEM (scanning electron microscopy), isoelectric point analysis, and XPS (X-ray photoelectron spectroscopy). Several factors, such of solution pH, co-existing anions, and temperature were presented to study the effects on the removal of phosphate and fluorine. The mechanism between adsorbents and adsorbates as well as adsorption kinetics and isotherms were presented.

2. MATERIALS AND METHODS 2.1. Materials. Table 1 shows the CAS number, source, and purity (determined by supplier) of the chemicals used in this work. All chemicals are of analytical grade. Table 1. CAS Registry Number, Sources, and Mass Fraction Purity of the Chemicals chemical name

CAS reg. no.

supplies

FeCl3·6H2O

7705-08-0

FeSO4·7H2O

7782-63-0

Ce(SO4)2·4H2O

10294-42-5

LaCl3·7H2O

10025-84-0

NaOH

1310-73-2

HCl

7647-01-0

KH2PO4

7778-77-0

NaF

7681-49-4

Tianjin Fengchuan Chemical Reagent Technologies Co., Ltd. Sinopharm Chemical Reagent Co., Ltd. Sinopharm Chemical Reagent Co., Ltd. Tianjin Kemiou Chemical Reagent Co., Ltd. Tianjin Kemiou Chemical Reagent Co., Ltd. Luoyang Chemical Reagent Co., Ltd. Luoyang Chemical Reagent Co., Ltd. Luoyang Chemical Reagent Co., Ltd.

mass fraction analytical grade analytical grade analytical grade analytical grade analytical grade analytical grade analytical grade analytical grade

Stock solutions of 500 mg/L phosphate (concentration related to phosphorus element) or fluorine (F) were prepared from KH2PO4 and NaF in distilled water. The working solutions were obtained by stock solutions through dilution with distilled water. 2.2. Preparation of the Adsorbent. Preparation of magnetite was through the coprecipitation method using Fe3+ and Fe2+ salts at a molar ratio of 2:1. The procedure is as follows: 0.04 mol of FeC13·6H2O solution and 0.02 mol of FeSO4·7H2O solution were dissolved in 200 mL of 0.5 mol/L HCl solution and placed in a beaker. Under magnetic stirring, 300 mL of NaOH solution (1.25 mol/L) was added dropwise to the mixed solution, and then, stirring was continued for 30 min at room temperature. Next, the mixed solution was adjusted to neutral with 25% HCl solution. Finally, magnetite, named as Fe3O4, was washed several times with distilled water, dried at 333 K, and stored in a glass bottle. Fe3O4 (2.5 g) and 500 mL of solution (0.01 mol/L LaCl3· 7H2O and 0.005 mol/L Ce(SO4)2·4H2O) were added to one 1000 mL beaker. The end pH of mixtures was adjusted to about 10.5, and the reaction was done with mechanical stirring at 323 K for 4 h. Modified Fe3O4 (Fe3O4@La−Ce) was separated by the magnetic method and washed several times by using distilled water. Finally, Fe3O4@La−Ce was obtained, dried at 333 K, and stored in a glass bottle. La−Ce was

q=

V (C 0 − C ) m

(1)

where C0 is the initial phosphate or fluorine concentration (mg/L, according to P element), C is the phosphate or fluorine concentration at any time t or equilibrium (mg/L), V is the phosphate or fluorine solution volume (L), and m is the weight of Fe3O4@La−Ce in grams. Adsorption experiments were repeated three times and averages were recorded. The error is less than 5%.

3. RESULTS AND DISCUSSION 3.1. Characterization of Materials. The magnetic hysteresis loop of Fe3O4 and Fe3O4@La−Ce is shown in Figure 1. The magnetic hysteresis loops obtained using VSM (measured at 300 K) showed that Fe3O4 and Fe3O4@La−Ce were superparamagnetic, so the particles were easily separated from solution using an external magnetic field. VSM analysis results also showed that the saturation magnetization values of Fe3O4 and Fe3O4@La−Ce were 69.5 and 41.5 emu/g, respectively. The BET surface area and pore volume of Fe3O4 were 65.3 m2/g and 0.326 cm3/g, respectively, while those of Fe3O4@ La−Ce was 39.7 m2/g and 0.202 cm3/g, respectively. Because of La and Ce loading on Fe3O4, the BET surface area and pore 3642



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SEM photographs can be presented to observe the property of the surface, and SEM micrographs of Fe3O4 and Fe3O4@ La−Ce are shown in Figure 3a,b, respectively. They exhibited a

Figure 3. SEM micrographs of Fe3O4 (a) and Fe3O4@La−Ce (b).

rough and heterogeneous surface structure with a diameter of 100 nm. It was also found from Figures 2 and 3 that Fe3O4@ La−Ce presented more roughness and protrusion, which might have resulted from La, Ce hydroxyls onto the shell of the particles. The values of pHpzc (pH at point zero charge) were evaluated, and the results were pH 5.6 for Fe3O4 and pH 6 for Fe3O4@La−Ce, which were slightly higher after the treatment (figures are not shown). 3.2. Adsorption Results. 3.2.1. Comparative Studies of Adsorption Capacity of Phosphate and Fluorine on Fe3O4@ La−Ce. The effectiveness of phosphate and fluorine removal is determined by using Fe3O4 and Fe3O4@La−Ce as adsorbents and is shown in Figure 4. It was observed from Figure 4 that the adsorption quantity for phosphate was found to be 6.23 mg/g of Fe3O4 and 47.77 mg/g of Fe3O4@La−Ce at C0 = 30 mg/L. Adsorption quantities for fluorine were 0.73 mg/g of Fe3O4 and 15.26 mg/g of Fe3O4@La−Ce at C0 = 10 mg/L. For phosphate and fluoride adsorption, the adsorption capacity of Fe3O4@La−Ce greatly increased compared to that of Fe3O4. It means that Fe3O4@La−Ce has high affinity for phosphate and fluorine because the hydroxyl group on the surface of Fe3O4@La−Ce provides abundant active adsorptive sites.14,15 3.2.2. Effect of pH on Adsorption Quantity. The pH of solution is one of the important factors that affect the adsorption quantity. The effects of pH on phosphate and fluorine adsorption by Fe3O4@La−Ce are shown in Figure 5.

Figure 1. Magnetic hysteresis loop of Fe3O4 and Fe3O4@La−Ce.

volume decreased, which provided indirect evidence of successful loading of La and Ce species on the particles of Fe3O4. The XRF data of materials were analyzed. The concentrations of La, Ce, and Fe in Fe3O4@La−Ce were 16.4, 9.03, and 41.8%, respectively, while the concentration of Fe in Fe3O4 was 59.2%, and La and Ce were not detected. As expected, lanthanum and cerium were loaded on Fe3O4. The XRD patterns of Fe3O4 and Fe3O4@La−Ce are illustrated in Figure 2. It was markedly observed that the XRD of Fe3O4@La−Ce was combined by the spectrum of Fe3O4 and La hydroxyl. The characteristic peaks at 35.4°, 56.9°, 62.5° were assigned to magnetite FeO·Fe2O3 and 24.3°, 30.1°, 43.7° were assigned to lanthanum hydroxide. After loading with La and Ce, the diffraction pattern of Fe3O4@La− Ce showed the superimposition of reflections from the FeO· Fe2O3 and La(OH)3 phase. No characteristic peak of Ce emerges, indicating that the hydrated Ce oxide exists on the surface of the magnetite as an amorphous phase. The crystal structures of Fe3O4 and Fe3O4@La−Ce were well defined by the diffraction peaks.

Figure 2. XRD spectra of Fe3O4 and Fe3O4@La−Ce. 3643



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complexes with the protonated and positively charged magnetic adsorbent. With the increase of pH (>6), the functional groups on the surface of Fe3O4@La−Ce are gradually deprotonated, so it is difficult to form metal ligand complexes. Meanwhile, another reason for the decrease of the adsorption capacity at higher pH is the competitive adsorption of OH− and phosphate. The mechanism of phosphate removal on Fe3O4@La−Ce composites at different initial solution pH is plotted in Scheme 1. Scheme 1. Proposed Mechanisms for the Removal of Phosphate and Fluoride on Magnetic Core−Shell Fe3O4@ La−Ce at Different Initial Solution pH

Figure 4. Comparison of the adsorption capacity of Fe3O4 and Fe3O4@La−Ce for the removal of phosphate (C0 = 30 mg/L, background electrolyte = 0.04 mol/L Na2SO4, pH = initial pH, T = 303 K) and fluorine (C0 = 10 mg/L, pH = 4, T = 303 K).

Figure 5. Effect of solution pH on phosphate (C0 = 30 mg/L, contact time = 12 h, T = 303 K) and fluorine (C0 = 10 mg/L, contact time = 12 h, T = 303 K) removal by Fe3O4@La−Ce particles.

It was evidently seen that the values of qe about phosphate and fluorine significantly declined with the increase of pH. A similar trend with other phosphate or fluorine adsorption were also reported, that is, the CeO2-covered nanofiber,16 Fe3O4@ LDHs composites,14 cerium-modified chitosan,17 and Fe− Mg−La composites.18 For phosphate, the adsorption quantity was 46.6 mg/g at pH 3 and 40.3 mg/g at pH 5. For fluorine, the adsorption quantity showed a decrease from 19.0 to 3.73 mg/g within the pH from 4 to 12. At solution pH 3−7, H2PO4− is an important form of the phosphate solution and HPO42− is the dominant species of pH 8−12. Regarding the adsorption free energy (H2PO4− < HPO42−), the results show that H2PO4− is more easily adsorbed on the surface of Fe3O4@La−Ce.14 When pH was less than 3, H3PO4 mainly existed (pKa = 2.12) and binding action between H3PO4 and Fe3O4@La−Ce became weak. At pH 2−6, the anion H2PO4− readily form metal ligand

As pKa of HF is 3.18 and there are more species of HF at pH less than 4, the experimental design is performed at solution pH 4. It shows that higher pH induces lower adsorption capacity and the maximum adsorption is at pH 4. The reason may be the competition of fluoride and hydroxyl ions for active sites on the surface of the adsorbent. It was obtained from Figure 5 that there was positive about surface of Fe3O4@La− Ce particles when the solution pH was in the range of 4−6. Therefore, there was electrostatic attraction between Fe3O4@ La−Ce and the fluoride negative ion during the adsorption process. As pH increased from 6 to 12, the electrostatic repulsion force that existed between fluorine and Fe3O4@La− Ce was not in favor of adsorption. These results showed that 3644



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competitive adsorption between fluoride and phosphate onto active sites of the Fe3O4@La−Ce surface. It was clearly shown from Figure 6b that the effects of Cl− and SO42− on F− adsorption were negligible. On the other hand, F− adsorption exhibited a sharp decrease from 18.1 to 0.567 mg/g when the concentration of HCO3− ions ranged from 0.01 to 0.018 mol/L. This showed that more hydroxide anions were produced if more HCO3− coexisted in the system, and competitive adsorption induced lower adsorption quantity. 3.2.4. Adsorption Kinetic Study. The values of qt toward phosphate and fluoride onto Fe3O4@La−Ce change with time, and the results are shown in Figure 7 at various initial

there was both ligand-exchange action and the Lewis acid− base action between Fe3O4@La−Ce and fluoride.11 Possible mechanisms for the removal of F− on magnetic core−shell Fe3O4@La−Ce are also shown in Scheme 1. 3.2.3. Effect of Co-Existing Anions. Usually, wastewater contained common salts (anions). Therefore, the effects of typical co-existing anions such as Cl−, SO42−, HCO3−, and F− on the phosphate and fluorine adsorption by Fe3O4@La−Ce are investigated, and the results are shown in Figure 6.

Figure 6. (a) Effect of co-existing anions on phosphate (C0 = 30 mg/ L, pH = initial pH, contact time = 12 h, T = 303 K) adsorption. (b) Effect of co-existing anions on fluorine (C0 = 10 mg/L, pH = 4, contact time = 1 h, T = 303 K) adsorption. Figure 7. (a) Effect of contact time on phosphate (background electrolyte = 0.04 mol/L Na2SO4, pH = initial pH, T = 303 K) adsorption onto Fe3O4@La−Ce. (b) Effect of contact time on fluorine (pH = 4, T = 303 K) adsorption onto Fe3O4@La−Ce.

It was found from Figure 6a that values of qe for phosphate increased with the increase of concentration (Cl−, SO42−). Therefore, phosphates were bound onto the surface of Fe3O4@ La−Ce, mainly in the form of an internal ligand.5 The influence of SO42− was greater than that of Cl− because there was higher charge with SO42− and the impact on the electrical double layer was greater. However, when F− and HCO3− coexisted in solution, values of qe toward phosphate decreased to 17.1 and 55.7%, respectively. It was found that phosphate adsorption, very sensitive to F−, existed in the solution, which showed an adverse effect of F− on the phosphate adsorption process. HCO3− leads to hydroxide anions and thus to a lower adsorption quantity. The phosphate adsorption quantity reduced to 31.9 mg/g and almost remained unchanged with increasing concentration of HCO3−; it could be a buffer solution. There was an adverse effect of F− because of

adsorbate concentrations. The earlier equilibrium time was found at lower concentrations of phosphate and fluorine. For phosphate, adsorption could achieve near-equilibrium at 3 h (20 mg/L), slower than that for fluorine, which is at 1 h (20 mg/L). The difference in the kinetic process of phosphate and fluorine was probably due to ion sizes. Fluoride was close to the hydroxy radius (lower radius), so the complexation degree with active sites onto the surface of Fe3O4@La−Ce was greater.19 In order to investigate the adsorption kinetics of phosphate and fluorine on Fe3O4@La−Ce, the pseudo-second-order model, Elovich model, and double constant model were applied.20,21 3645



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Table 2. Parameters of Kinetic Models for the Adsorption of Phosphate and Fluorine Onto Fe3O4@La−Ce C0 (mg/L)

pseudo second-order kinetic equation PO4 −P 3−

F−

Elovich equation PO4 −P 3−

F−

double constant PO43−−P

F−

qe(exp) (mg/g)

10 20 30 20 30 50 C0 (mg/L) 10 20 30 20 30 50 C0 (mg/L) 10 20 30 20 30 50

19.8 37.6 46.1 23.6 36.8 40.5 A

19.6 37.8 44.8 22.0 36.0 40.1 B

10.5 7.29 15.8 16.7 9.71 7.86 A′

(2)

The Elovich equation is given by the following equation qt = A + B ln t

(3)

The double constant equation is generally expresses as follows qt = A′t KS

k2 1.30 × 2.26 × 3.33 × 0.100 3.13 × 2.15 ×

−2

10 10−3 10−3 10−3 10−3 R2

R2

SSE

0.893 0.874 0.866 0.648 0.871 0.871

6.28 59.8 62.1 18.9 47.1 68.6 SSE

0.732 0.963 0.952 0.941 0.962 0.956 R2

15.6 17.4 22.2 3.14 13.9 23.3 SSE

0.684 0.947 0.926 0.937 0.940 0.941

18.5 25.3 34.4 3.39 21.9 31.6

Figure 8 illustrates the adsorption isotherms of phosphate and fluorine on Fe3O4@La−Ce in single systems at three temperatures. It was observed from Figure 8 that the equilibrium adsorption qe increased with the increase of adsorbate concentration. It was further observed that values of qe increased at higher temperature, which was in agreement with the endothermic process. It was found that adsorption quantity at 313 K was 53.2 mg/g for phosphate and 60.3 mg/g for fluoride. Four isotherm models, Langmuir model, Freundlich model, Koble Corrigan model, and Redlich−Peterson model, were applied to fit the adsorption equilibrium process. The nonlinear expression of the Langmuir isotherm equation is as follows15

k 2qe 2t 1 + k 2qet

1.68 5.39 5.30 1.16 4.73 5.70 Ks 9.26 × 10−2 0.187 0.141 5.69 × 10−2 0.164 0.186

11.8 13.3 20.7 16.9 14.5 14.1

The pseudo-second-order equation is expressed as qt =

qe(theo) (mg/g)

(4)

where qt is adsorption quantity (mg/g) at time t; qe is adsorption quantity at equilibrium (mg/g); k2 is pseudosecond kinetic rate constant (mg/g min); A and B are the constants relating to fraction of the surface covered and chemisorption activation energy, respectively; A′ is the constant; and Ks is the adsorption rate coefficient. Relative parameters of models, determined coefficients (R2), and values of errors (SSE) are obtained according to minimum residual sum of squares (SSE) by nonlinear regressive analysis. Table 2 lists fitting results from three kinetic models and values of qe from experiments about phosphate and fluorine adsorption onto Fe3O4@La−Ce. The fitted curves are also shown in Figure 7. It was observed from Table 2 that values of qe obtained from the pseudo-second-order kinetic equation agreed better with the values of qe from experiments at the same conditions. Therefore, this kinetic model can be used to predict the equilibrium adsorption quantity. The pseudo-second-order kinetic equation was used to describe the chemical process, showing the adsorption system contains the chemical process. However, the Elovich and double constant kinetic equations can obtain higher values of R2 and lower values of SSE, indicating ion exchange and heterogeneous diffusion in the adsorption process. 3.2.5. Adsorption Isotherm Study. Adsorption isotherms can often present the property of the adsorption process.

qe =

qmKLce 1 + KLce

(5)

where qm is qe for a complete monolayer (mg/g), a constant related to adsorption capacity; and KL is a constant related to the affinity of the binding sites and energy of adsorption (L/ mg). Ce is equilibrium concentration (mg/L). The Freundlich model can be expressed by the following equation3 qe = KFce1/ n

(6)

where KF is the constant of the Freundlich isotherm and 1/n is the constant which is related to the adsorption capacity and the adsorption intensity. The Koble-Corrigan isotherm is a three-parameter equation, which incorporates both Langmuir and Freundlich isotherm models for representing the equilibrium adsorption data22 qe =

ACe n 1 + BCe n

(7)

where A, B, and n are the Koble Corrigan isotherm constants. The three-parameter Redlich−Peterson model has linear dependence on concentration in the numerator, and an exponential function in the denominator has been proposed 3646



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where A, B, and g are the Redlich−Peterson parameters, and g lies between 0 and 1. The results of nonlinear regression analysis are presented in Figure 8 and Table 3. It was clearly seen from Table 3 that values of qm obtained from the Langmuir model were close to values of qe from experiments (for phosphate), while the Koble-Corrigan model was better described phosphate adsorption according to values of R2 and SSE. For fluorine, it was found that values of KF indicated that adsorption was easy, and the value increased at higher temperature. It means that fluorine adsorption was an endothermic process. The values of 1/n were between 0.1 and 0.5, and this confirmed the process was favorable. The values of R2 from the Redlich−Peterson isotherm were more than 0.912, and both the Freundlich model and Redlich− Peterson model could better predict the data. Moreover, the Langmuir model and Koble Corrigan model were not considered better to predict the equilibrium process of fluoride. As the Freundlich model and Redlich−Peterson model were not suitable to fit the equilibrium results of phosphate while the Langmuir model and Koble Corrigan model were not suitable to fit the equilibrium results of fluoride, the fitted results are not presented in Figure 8 and Table 3. The values qm of phosphate or fluoride onto Fe3O4@La−Ce are compared with other adsorbents and the results are listed in Table 4. It was obviously observed that Fe3O4@La−Ce had relatively higher adsorption capacity. In addition, concerning the magnetic effect of composites, there are some advantages and challenges to remove phosphate and fluoride from solution. 3.2.6. Thermodynamic Analysis. The effect of temperature on phosphate and fluorine adsorption can be used to obtain thermodynamic parameters, such as enthalpy change (ΔH0), Gibbs free energy change (ΔG0), and entropy change (ΔS0), according to equilibrium results by the following equations15

Figure 8. (a) Adsorption isotherms of phosphate (C0 = 5−60 mg/L, background electrolyte = 0.04 mol/L Na2SO4, pH = initial pH) on Fe3O4@La−Ce. (b) Adsorption isotherms of fluorine (C0 = 10−80 mg/L, pH = 4) on Fe3O4@La−Ce.

Kc = Cad,c/Ce

to improve the fit by the Langmuir or Freundlich equation and is given as follows23 ACe qe = 1 + BCe g

(8)

(9)

ΔG 0 = −RT ln Kc

(10)

ΔG 0 = ΔH 0 − T ΔS 0

(11)

Table 3. Parameters of Adsorption Isotherm Models for Phosphate and Fluorine Adsorption Langmuir

TK

KL (L/g)

qe(exp) (mg/g)

qe(theo) (mg/g)

R2

SSE

PO4 −P

293 303 313

0.928 1.10 1.14

47.1 51.6 53.2

0.867 0.813 0.964 R2

158 264 53.1 SSE

3−

Koble Corrigan PO4 −P 3−

Freundlich −

F

Reddish−Peterson F−

T/K 293 303 313 T/K 293 303 313 T/K 293 303 313

A

B

49.3 51.7 52.5 n

63.6 162 62.5

1.38 3.33 1.20

2.39 3.43 1.06

0.928 0.880 0.955

KF

1/n

R2

16.7 17.3 18.7

0.268 0.302 0.297

0.934 0.952 0.970

68.8 136 52.6 SSE

A

B

g

R2

3.57 × 109 8.14 × 109 1.29 × 109

2.13 × 108 4.71 × 108 6.90 × 108

0.732 0.698 0.703

0.912 0.936 0.960

3647

38.1 40.9 28.9 SSE 38.1 40.9 28.9



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and fluorine were successfully adsorbed onto the surface of Fe3O4@La−Ce. The scan spectra of O 1s high resolution before and after phosphate adsorption are shown in Figure 9b and the fitting parameters are listed in Table 6. The O 1s peak from Fe3O4@ La−Ce was fitted into two peaks at 530.05 and 531.50 eV, which were assigned to the oxide oxygen (O2−) and hydroxyl group (−OH) peaks, respectively.39 After adsorption, the O2− and −OH peaks shifted to higher binding energies (0.73 and 0.33 eV). This was due to the formation of a new lanthanum and cerium species. The −OH percentage decreased from 77.8 to 46.0%, while the O2− percentage increased from 22.2 to 54.0%. The −OH on the surface of Fe3O4@La−Ce before and after adsorption of the phosphate-containing solution was studied. Then, the reduction in the −OH percentage was analyzed to contribute to the phosphate adsorption, which suggested that the −OH which existed on the Fe3O4@La−Ce surface surely participates in phosphate sorption. Similar results were studied at phosphate adsorption onto hydrous zirconium oxide, lanthanum-incorporated zeolite, and Fe− Mg−La composite.40,41 Additionally, La 3d high-resolution scan spectra of Fe3O4@La−Ce and Fe3O4@La−Ce−P are displayed in Figure 9c. Clearly, at binding energy investigated, the peaks of La 3d changed and shifted from 835.95−839.34 eV. Then, the present results about phosphate adsorption showed that both effects, that is, the hydroxyl-substituted phosphate anion and the formation of new La species (La−O− P), were involved before and after the adsorption process. The peaks of Ce 3d before and after phosphate adsorption are shown in Figure 9d. The complex XPS of Ce 3d showed the chemical state of the cerium element on Fe3O4@La−Ce and Fe3O4@La−Ce−P according to the published method.42 Six fitting peaks labeled as the royal color were determined as the property of Ce4+, while other four peaks with orange color were assigned to the presence of Ce3+. The Fe3O4@La−Ce−P complex XPS of Ce 3d was collected for Ce3+ oxide powder (CePO4).42 The change of the oxidation state (from Ce4+ to Ce3+) confirmed that phosphate was adsorbed on Fe3O4@La− Ce. Moreover, the P 2p spectrum of Fe3O4@La−Ce−P agreed with the peak at 133.74 eV in Figure 9e, implying that the binding energy is more than P 2p in NaH2PO4·2H2O (at 132.9 eV) and the peak corresponds to the pentavalent P of LaPO4.43,44 Concerning the binding energy of P 2p at 133.74 eV, the present results showed that phosphate ions existed on the surface of Fe 3 O 4 @La−Ce through adsorption or coordination reactions which occurred between phosphate and lanthanum. La 3d high-resolution scan spectra of Fe3O4@La−Ce and Fe3O4@La−Ce−F are also displayed in Figure 9c. Then, the peaks of La 3d was observed and shifted from 835.46 and 838.85 to 836.59 and 839.70 eV. It was seen that fluoride ions interacted with lanthanum ions. The high-resolution Ce 3d spectra of Fe3O4@La−Ce and Fe3O4@La−Ce−F composites are reported in Figure 9d. However, the Ce 3d spectrum of Fe3O4@La−Ce−P similarly remained unchanged after fluoride

Table 4. Comparison of Various Adsorbents for Phosphate and Fluorine Removal adsorbate PO43−−P

F−

qmax/(mg/g)

adsorbent Fe−Si−La Fe3O4@Zn−Al−LDH Fe3O4@Mg−Al−LDH iron-doped activated carbon ferric sludge La(III)-modified bentonite La(III)−Al-modified pillared clay Fe(III)-modified bentonite shaddock peel loaded Zr and La Zr modified carbon nanotube functioned magnetic polymer adsorbents Fe3O4@La−Ce Al−Ce binary oxide Fe3O4@n-HAp Alg composite La−AA magnetic Fe3O4@Fe−Ti Al(OH)3/SiO2/Fe3O4 ferric hydroxide Ce−Ti@Fe3O4 powder Ce−Ti oxide powder Fe3O4@La−Ce

27.8 36.9 31.2 14.1 25.5 14.0 10.0 11.2 45.2 10.9 102 53.2 91.4 4.05 6.70 41.8 38.0 7.00 91.0 44.4 60.3

refs 1 14 14 24 25 26 27 28 29 30 31 this study 13 32 33 34 35 36 37 37 this study

where Kc is the distribution coefficient for adsorption, Cad,e is the concentration of phosphate and fluorine on the adsorbent at equilibrium (mg/L), R [8.314 J/(mol·K)] is the universal gas constant, and T is the absolute temperature (K). Values of ΔG0, ΔH, and ΔH0 are calculated and listed in Table 5. Then, concerning the negative ΔG0 values, the adsorption process was spontaneous in the phosphate or fluorine systems. The values of ΔG0 decreased from −4.72 to −7.16 kJ/mol (phosphate) and −1.61 to −2.28 kJ/mol (fluorine) in the 293−313 K temperature range, which indicated that higher temperature increased the spontaneity of the adsorption reaction. Positive values of ΔH0 agree with the endothermic nature of the reaction. Concerning that the enthalpy change of phosphate (31.2 kJ/mol) was similar to the enthalpy change of chemisorption (40−120 kJ/mol), it was inferred that there is the strength of binding between phosphate and the Fe3O4@La−Ce surface.38 The present results agreed with the degree of disorder which increased with the increase of the phosphate and fluorine number at the surface of Fe3O4@La−Ce composites because the entropy change ΔS0 was also positive.14 The adsorption process of phosphate with a high value of ΔS0 also indicated remarkable change of entropy.15 3.2.7. XPS Analysis. To further understand the adsorption mechanism of phosphate and fluorine onto Fe3O4@La−Ce, XPS of Fe3O4@La−Ce before and after adsorbate adsorption is displayed in Figure 9a. The appearance of P 2p and F 1s peaks in the XPS wide scan spectrum indicated that phosphate

Table 5. Thermodynamic Parameters of Phosphate and Fluorine Adsorption onto Fe3O4@La−Ce ΔG0/(kJ·mol−1) −1

−1

−1

adsorbate

ΔH /(kJ·mol )

ΔS /(J·mol ·K )

293 K

303 K

313 K

PO43−−P F−

31.2 8.18

122 33.4

−4.72 −1.61

−5.18 −1.88

−7.16 −2.28

0

0

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Figure 9. (a) Wide-scan XPS spectrum of Fe3O4@La−Ce, Fe3O4@La−Ce−P, and Fe3O4@La−Ce−F; XPS spectra for (b) O 1s of the Fe3O4@ La−Ce adsorbent before and after phosphate; XPS spectra for (c) La 3d (d) Ce 3d, of the Fe3O4@La−Ce adsorbent before and after phosphate, fluorine adsorption; (e) P 2p XPS spectra of Fe3O4@La−Ce−P; (f) F 1s XPS spectra of Fe3O4@La−Ce−F.

Considering the results of XPS analysis and the effect of common anions that existed in solution, it was referred that the main mechanism between Fe3O4@La−Ce and phosphate or fluoride anions be coordination.

Table 6. Fitting Parameters of the O 1s peak of Fe3O4@La− Ce and Fe3O4@La−Ce−P sample Fe3O4@La−Ce Fe3O4@La−Ce−P

peak

binding energy (eV)

area

fwhm (eV)

percent (%)

O2− OH− O2− OH−

530.05 531.50 530.78 531.83

3317 11 595 7490 6382

2.85 2.85 2.72 2.72

22.2 77.8 54.0 46.0

4. CONCLUSIONS The aim of this paper was to investigate the magnetic core shell Fe3O4@La−Ce composites for adsorption of phosphate and fluorine from solution. As expected, the core shell Fe3O4@La− Ce composites was successfully synthesized and the adsorption capacity was enhanced after La and Ce modification. Phosphate and fluorine adsorption were highly dependent on solution pH. The existence of Cl− and SO42− in solution had a positive effect on phosphate adsorption, while the existence of F− and HCO3− had a negative impact on phosphate removal. There was no significant effect with co-existing anions Cl− and SO42− on fluoride adsorption. The adsorption kinetics

adsorption. The satellite peak at 916.9 eV was a fingerprint of Ce4+ in Ce compounds.39 Besides, Figure 9f implied that the binding energy of F 1s on Fe3O4@La−Ce−F was 684.91 eV, showing that the binding energy was slightly higher than NaF (684.5 eV)45 and was assigned to the La−F formation,3 suggesting the good removal ability for fluoride observed in this work. 3649



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processes of the phosphate and fluorine were better fitted by the Elovich equation and double constant model. The equilibrium data of phosphate adsorption were better predicted by the Koble Corrigan model, while the adsorption isotherms of fluorine adsorbed on Fe3O4@La−Ce were described by the Freundlich model and Redlich−Peterson model. The phosphate and fluorine adsorption processes were endothermic and spontaneous in nature. Fe3O4@La−Ce was effective to remove phosphate and fluorine anions from solution and the main mechanism was in coordination.

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AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected] (J.Z.). *E-mail: [email protected]. Phone +86 371 67781757. Fax +86 371 67781556 (R.H.). ORCID

Runping Han: 0000-0002-1585-4522 Notes

The authors declare no competing financial interest.

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Preparation of Novel Magnetic Microspheres with the La and Ce

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