J. Cent. South Univ. (2020) 27: 1176-1185

DOI: https://doi.org/10.1007/s11771-020-4358-2

Preparation of magnetic Fe₃O₄@Cu/Ce microspheres for efficient catalytic oxidation co-adsorption of arsenic(III)

JIN Lin-feng(金林锋)¹, CHAI Li-yuan(柴立元)^{1, 2}, SONG Ting-ting(宋婷婷)³,YANG Wei-chun(杨卫春)^{1, 2}, WANG Hai-ying(王海鹰)^{1, 2}

1. School of Metallurgy and Environment, Central South University, Changsha 410083, China;

2. Chinese National Engineering Research Center for Control and Treatment of Heavy Metal Pollution, Changsha 410083, China;

3. Beijing Urban Construction Design and Development Group Co., Ltd., Beijing 100037, China

Central South University Press and Springer-Verlag GmbH Germany, part of Springer Nature 2020

Abstract: Magnetic Fe₃O₄@Cu/Ce microspheres were successfully prepared by one-step solvothermal approach and further utilized to remediate toxic arsenic (As(III)) pollution. The effects of Cu/Ce elements co-doping on the crystal structure, catalytic oxidation and adsorption behaviors of magnetic microspheres were researched systematically. The results showed that with the aid of Cu/Ce elements, the grain size reduced, lattice defects increased, and the oxygen vacancies and surface hydroxyl groups were improved. Therefore, Cu/Ce elements endowed magnetic Fe₃O₄@Cu/Ce microspheres with excellent As(III) removal performance, whose maximum adsorption capacity reached 139.19 mg/g. The adsorption mechanism mainly involved catalytic oxidant co-adsorption. This research developed a feasible strategy for the preparation of high efficiency magnetic adsorbent to enhance the removal of As(III).

Key words: Cu/Ce; magnetic composites; As(III); catalytic oxidation; adsorption

Cite this article as: JIN Lin-feng, CHAI Li-yuan, SONG Ting-ting, YANG Wei-chun, WANG Hai-ying. Preparation of magnetic Fe₃O₄@Cu/Ce microspheres for efficient catalytic oxidation co-adsorption of arsenic(III) [J]. Journal of Central South University, 2020, 27(4): 1176-1185. DOI: https://doi.org/10.1007/s11771-020-4358-2.

1 Introduction

Arsenic (As) pollution is seriously hazardous to human beings and ecological systems, due to its toxic and carcinogenic nature [1-4]. The World Health Organization stipulates that the maximum concentration of arsenic contamination shall not exceed 10 μg/L [5]. Arsenic mainly exists in the forms of arsenate (As(V)) and arsenite (As(III)), and the latter is more mobile and toxic than the former [6, 7]. Therefore, the remediation of As(III) pollution is extremely urgent. At present, various technologies have been reported, such as membrane separation [8], ion exchange [9] and precipitation [10]. Among them, adsorption is an alternative method because of high efficiency, low cost and easy regeneration. Therefore, the development of high-performance adsorbents for As(III) removal is of significant importance.

Magnetic composites, especially magnetite (Fe₃O₄) composites, have aroused increasing attention owing to its outstanding physicochemical properties, easy recycle and high efficiency in the environmental fields [11-16]. However, magnetic composites can’t effectively remove As(III) due to the uncharged property of As(III) in acid or neutral solution. In contrast, As(V) is negatively charged in a wide range of pH. Therefore, the conversion from As(III) to As(V) is a critical factor in the removal process. Accordingly, the strategy of catalytic oxidation co-adsorption is proposed. However, the performance of traditional magnetic composites converting As(III) to As(V) is unfavorable.

Generally, copper (Cu) possesses outstanding catalytic properties in the forms of isolated Cu²⁺ ions and highly dispersed clusters [17, 18]. In addition, Ceria (Ce) has also attracted increasing attention due to its good redox and catalytic properties [19, 20]. Also, the synergistic effects of dispersed Cu and Ce would enhance the catalytic oxidant performances of composites [21].

In this work, the strategy of enhancing catalytic oxidation co-adsorption of magnetic composites was proposed by co-doping Cu/Ce elements. The as-prepared magnetic Fe₃O₄@Cu/Ce microspheres were characterized in detail and applied to remove As(III). Characterizations and adsorption experiments were performed to understand the property of microspheres and As(III) adsorption behavior. Finally, the As(III) removal mechanism was proposed.

2 Material and methods

2.1 Materials

Cerium chloride heptahydrate (CeCl₃·7H₂O, 99.9 %), cupric chloride dihydrate (CuCl₂·2H₂O, AR), ferric chloride hexahydrate (FeCl₃·6H₂O, AR), anhydrous sodium acetate (CH₃COONa, AR) and ethylene glycol (EG, AR) were purchased from Shanghai Aladdin Biochemical Technology Co., Ltd. (China). As(III) solution was prepared by dissolving NaAsO₂ in deionized (DI) water. All other chemicals are analytical grade and do not require further purification.

2.2 Synthesis of magnetic Fe₃O₄@Cu/Ce microspheres

Magnetic Fe₃O₄@Cu/Ce microspheres with different mass ratios of Cu to Ce were synthesized through a facile solvothermal approach. Firstly, FeCl₃·6H₂O, CuCl₂·2H₂O and CeCl₃·7H₂O were dissolved in 36 mL EG solution, and then 3.6 g CH₃COONa was added into above solution with vigorously stirring for 40 min. Secondly, the mixed solution was put into a Teflon-lined autoclave at 200 °C for 6 h. Finally, the precipitate was filtrated, washed with plenty of DI water and ethanol. The obtained composites were dried at 65 °C for 6 h. For comparison, pure Cu doped magnetic microsphere was prepared under the same condition without adding Ce. The as-prepared magnetic microspheres doped with Cu and Ce elements with different molar ratios (i.e., 1:0, 10:1, 5:1, 2.5:1, 1:1, 0.5:1) were synthesize in turn, and labeled as CCFX (X=0, 1, 2, 3, 4, 5).

2.3 Characterization

The morphology and structure of the magnetic CCFX microspheres were characterized by scanning electron microscopy (SEM-EDS, JSM-6360), X-ray diffraction (XRD, Rigaku D/Max-RB diffractometer), Raman spectroscopy (Renishow Invia, 532 nm), X-ray photoelectron spectroscopy (XPS, Thermo Fisher Scientific KAlpha 1063, Al K_α), Fourier transformed infrared spectroscopy (FT-IR, Nicolet IS10), respectively. Hydrogen temperature-programmed reduction (H₂-TPR) was performed in PCA-1200 instrument.

2.4 Batch experiments

Adsorption experiments were conducted at 25 °C, 180 r/min, 0.5 g/L dose. Different pH values were regulated by HCl or NaOH solution. Within the specified time, adsorbent was separated by external magnetic field. The initial and residual arsenic concentrations were detected through inductively coupled plasma-optical emission spectroscopy (ICP-OES, ICAP 7000). The regeneration experiments were conducted using 1 mol/L NaOH solution. The absorbent was shaken in NaOH solution for 12 h, and then separated by magnetic and rinsed with water. Finally, the adsorbent was applied to the next adsorption experiments.

3 Results and discussion

3.1 Materials characterization

The morphologies of CCFX microspheres are seen from Figure 1. CCFX microspheres exhibited spherical grains and the diameter of CCF0 was ~150 nm (Figure 1(a)). In contrast, CCF1-5 have the diameters range from ~120 to ~50 nm (Figure 1(b)-(f)). The decrease in the diameter of CCF1-5 was largely due to the introduction of Cu/Ce elements, which acted as an inhibitor. SEM-EDS mappings of CCF0 and CCF4 are shown in Figures 1(g) and (h), and Fe, O, Cu and Ce elements of CCF4 microsphere were more homogeneous than that of CCF0. Therefore, the existence of Cu/Ce elements enhanced the elements dispersion, which was conducive to the improvement of catalytic activity and adsorption performance [22].

Figure 1 SEM images of CCF0-5 (a-f) and SEM-EDS mappings of CCF0 (g) and CCF4 (h)

XRD, FT-IR, Raman and XPS technologies were applied to characterize the structure of CCFX microspheres, respectively. As shown in Figure 2(a), CCFX microspheres showed similar peaks with the typical peaks of Fe₃O₄ and Cu without the peaks of Ce or Ce oxide. The high-magnified XRD patterns of Cu(111) are shown in Figure 2(b). The Cu(111) diffraction peaks shifted obviously, which probably originated from the strong interaction between Cu and Ce elements [23]. In addition, XRD patterns also revealed that the characteristic peaks of CCF1-5 microspheres are broad and strong than that of CCF0. The phenomenon was attributed to the decrease of grain size and strain, as well as the generation of oxygen vacancies in the lattice. Meanwhile, the reverse trend was observed when further increasing the molar ratio of Ce to Cu (CCF4 and CCF5) because of the tight aggregation of microspheres [24].

As shown in Figure 2(c), the peaks of O—H bond (3000-3600 cm^-1), H—O—H bond (1637 cm^-1) and hydroxyl groups on metal oxides (M—OH) (1060 cm^-1) showed increased intensity, which originated from the enhanced oxygen vacancies [25]. Raman spectra are seen from Figure 2(d). The peaks of CCF0 at 218 and 282 cm^-1 were attributed to Cu₂O and CuO, respectively [26]. The peak at 662 cm^-1 was attributed to the magnetite band [27]. After co-doping Cu/Ce elements, the peaks of CCF1-5 at 218 and 282 cm^-1 missed, indicating that the introduction of Ce element led to microscopic strain effects and high disorder. Meanwhile, the peaks at 677 cm^-1 were strengthened. Furthermore, these peaks shifted slightly to a high frequency, indicating the formation of lattice defects and oxygen vacancies [28].

Figure 2 XRD patterns (a), high-magnified XRD patterns (b), FT-IR spectra (c), Raman spectra (d), XPS spectra of CFX: (e) Cu 2p; (f) O 1s

The chemical states of magnetic CCFX microspheres were further analyzed, and Cu 2p XPS spectra were shown in Figure 2(e). The Cu 2p_3/2 peaks of CCF0 and CCF4 were observed at 932.5 and 933.9 eV respectively, and the two satellite peaks at 940.6 and 943.7 eV implied the existence of CuO, Cu₂O and Cu. The peak at 952.6 eV was assigned to Cu 2p_1/2, and the peak at 961.8 eV belonged to the satellite peaks of Cu 2p_1/2 [29]. Hence, the introduction of Ce element induced an imbalance charge and unsaturated chemical bonds on the CCF4 surface, which would enhance oxygen vacancies and bring large amounts of hydroxyl groups [29, 30]. The O 1s spectra (Figure 2(f)) were also divided into three peaks (530.0, 531.1 and 533.1 eV), which were originated to lattice oxygen, surface adsorbed oxygen as well as chemisorbed water, respectively [31]. The shift of lattice oxygen peak to low binding energy would facilitate to catalytic oxidation of As(III) [24].

3.2 Adsorption experiments

3.2.1 Adsorption isotherms

Adsorption isotherms of CCFX microspheres with different initial As(III) concentrations were investigated. As shown in Figure 3(a), CCF0 had the lowest As(III) adsorption capacity. However, CCF1-4 microspheres exhibited the enhanced adsorption performance for As(III), implying that the co-doping Cu/Ce elements facilitated the removal of As(III). It was noted that the adsorption performance of CCF5 microspheres was decreased, probably due to the serious aggregation of microspheres, as shown in Figure 1(f). The adsorption data were matched by Langmuir and Freundlich isotherm models, which were depicted in Eqs. (1) and (2), respectively [32].

                        (1)

                        (2)

The calculated isotherm parameters were shown in Table 1. The data possessed favorable correlation to Freundlich model, which described the multilayer adsorption process [33]. In addition, the maximum As(III) removal performance of CCF4 microsphere was 139.19 mg/g, which was compared with that of other typically magnetic adsorbents, as seen from Table 2, indicating the bright prospects of CCF4 microspheres in the field of As(III) remediation.

Figure 3 Adsorption isotherms of CCFX microspheres (a), adsorption kinetics of CCF0 and CCF4 microspheres (b), pseudo-first-order (c) and pseudo-second order models (d) of CCF0 and CCF4 microspheres

3.2.2 Adsorption kinetics

The effect of time on the removal performance of CCF0 and CCF4 microspheres was explored, as shown in Figure 2(b). The uptake amounts of As(III) quickly increased at the initial stage, and then slowly increased until the adsorption equilibrium. In addition, CCF4 microspheres showed a favorable As(III) removal performance compared with CCF0. Pseudo-first-order and pseudo-second-order kinetic models were utilized to simulate the removal process (Figures 2(c), (d)). The mathematical expressions of pseudo-first-order and pseudo-second-order adsorption models are depicted in Eqs. (3) and (4), respectively [39, 40].

ln(q_e-q_t)=lnq_e-k₁t                         (3)

                          (4)

The corresponding kinetic parameters of CCF0 and CCF4 microspheres were presented in Table 3. According to the coefficient values, pseudo-second- order model was more suitable to describe the kinetic process, suggesting that As(III) adsorption process was a chemical adsorption process [41].

Table 1 Langmuir and Freundlich coefficients of magnetic CCFX microspheres

Table 2 Comparison of removal performance between magnetic CCF4 microspheres and other typically magnetic adsorbents

Table 3 Coefficients of pseudo-first-order and pseudo- second-order kinetic models of CCF0 and CCF4 microspheres

3.2.3 Regeneration

Recyclability is a crucial factor in the practical application of adsorbents. Regeneration experiments were performed to examine the recyclability of CCF4 microspheres. As seen from Figure 4, there is a slight decrease in the adsorption efficiency of CCF4 microsphere when increasing recycles. The decreased adsorption performance may be attributed to the reduction of binding sites, which were occupied by adsorbed As(III). Nevertheless, the adsorption performance of CCF4 microspheres remained about 75 % after five cycles, revealing a promising future.

3.3 As(III) adsorption mechanism

H₂-TPR and XPS technologies were performed to investigate the As(III) adsorption mechanism, as shown in Figure 5(a). The H₂-TPR of CCF0 microsphere presented three reduction peaks at ~220, ~600 and ~710 °C, which were ascribed to the reduction of copper ions (Cu²⁺ → Cu⁺ → Cu⁰), iron ions (Fe₃O₄ → FeO → Fe⁰) and bulk oxygen, respectively [42]. Interestingly, the synergistic interactions of Cu/Ce elements obviously decreased hydrogen consumption and reduction temperatures of CCF4 microsphere. These changes not only decreased the consumption of active sites by lower the temperature, but also promoted the migration of oxygen in the crystal structures. Furthermore, the oxygen vacancies would improve the reducibility and catalytic ability of CCF4 microspheres.

Figure 4 Regeneration of CCF4 microsphere (insets: photograph of As(III) solution before and after adsorption)

XPS spectra of CCF0 and CCF4 microspheres treated with As(III) were analyzed and the chemical state of As(III) on the surface of the microspheres were investigated, as presented in Figure 5(b). The As 3d peak was divided into two peaks at ~44.1 and ~45.3 eV [43, 44], indicating the existence of As(III) and As(V) species after treatment. In addition, As(V) contributed up to 80% of adsorbed As species on the surface of CCF4 microsphere, which was higher than CCF0 microsphere (63.7%). This effect indicated that Ce/Cu elements facilitated the catalytic oxidation of As(III) to As(V).

XPS spectra of CCF4 microsphere before and after As(III) adsorption were recorded to further investigate the adsorption mechanism, as seen from Figures 5(c) and (d). The Fe 2p peaks of CCF4 were similar to CCF4-As(III), suggesting no phase transformation of Fe₃O₄. The hydroxyl groups decreased from 37.2 % to 28.0 % after As(III) adsorption, revealing the significant interactions in the adsorption process. Moreover, the binding energy of lattice oxygen shifted to a low energy region, and the proportion of M-O increased from 51.0 % to 54.0 %. The improvement may be ascribed to the formation of M-O, As-O and As-O-M species on the CCF4 surface after adsorption.

Figure 5 H₂-TPR spectra (a), As 3d spectra after treating As(III) (b), Fe 2p spectra (c) and O 1s spectra (d) before and after As(III) adsorption

4 Conclusions

This study illustrated the catalytic oxidation co-adsorption of magnetic Fe₃O₄@Cu/Ce microspheres for efficient As(III) removal from aqueous solutions. The co-doping Cu/Ce elements obviously improved the concentration of surface oxygen vacancies on the surface of magnetic Fe₃O₄@Cu/Ce microspheres. Furthermore, magnetic Fe₃O₄@Cu/Ce microspheres possessed excellent As(III) adsorption capacity (139.19 mg/g), far higher than other typical magnetic adsorbents. Profoundly, the effective catalytic activity to convert As(III) to As(V) was successfully realized by dissolved oxygen and lattice oxygen with the aid of Cu/Ce elements. In summary, co-doping Cu/Ce elements provided a feasible strategy for efficient removal and detoxification of As(III) from aqueous solution.

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(Edited by HE Yun-bin)

中文导读

磁性Fe₃O₄@Cu/Ce微球催化氧化协同吸附重金属三价砷

摘要：本文通过一步溶剂法成功制备了磁性Fe₃O₄@Cu/Ce微球，并将其应用于毒性重金属三价砷的处理。系统考察了Cu/Ce元素共掺杂对复合材料结构晶型、催化氧化性能以及对三价砷的去除性能的影响。揭示了Cu/Ce元素的共掺杂效应可以显著降低磁性微球的晶粒尺寸，增加晶格缺陷，提高氧空缺位和丰富活性官能团。研究表明磁性Fe₃O₄@Cu/Ce微球对重金属三价砷具有较强的去除性能，最大吸附量可达139.19 mg/g。三价砷的去除机理主要涉及到催化氧化协同吸附，将三价砷氧化为五价砷的同时将其吸附在微球上。本文提出了一种可行的高性能磁性复合材料的制备方法，为三价砷的处理提供了技术思路。

关键词：铜/铈；磁性材料；三价砷；催化氧化；吸附

Foundation item: Project(2018YFC1802204) supported by the National Key R&D Program of China; Project(51634010) supported by the Key Project of National Natural Science Foundation of China; Project(2018SK2026) supported by the Key R&D Program of Hunan Province, China

Received date: 2019-11-14; Accepted date: 2020-03-18

Corresponding author: WANG Hai-ying, PhD, Professor; Tel: +86-731-88830875; E-mail: haiyw25@yahoo.com; ORCID: 0000-0002- 5446-3217