Synthesis of Novel Magnetic Microspheres with Bimetal Oxide Shell

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Cite This: ACS Sustainable Chem. Eng. 2017, 5, 10298-10306

Synthesis of Novel Magnetic Microspheres with Bimetal Oxide Shell for Excellent Adsorption of Oxytetracycline Lili Lian, Jinyi Lv, and Dawei Lou* Department of Analytical Chemistry, Jilin Institute of Chemical Technology, No. 45 Chengde Street, Jilin 132022, People’s Republic of China S Supporting Information *

ABSTRACT: Multifunctional magnetic microspheres (Mag@ ZnO−Co3O4) with bimetal oxide shell were synthesized via a facile method. The Mag@ZnO−Co 3 O 4 composite was characterized through a vibrating sample magnetometer, N2 adsorption analysis, Fourier transform infrared spectroscopy, powder X-ray diffraction, X-ray photoelectron spectroscopy, scanning electron microscopy, and transmission electron microscopy analyses. The structural analysis showed that the as-prepared microsphere possessed a mean diameter of 150 nm, a bimetal oxide shell thickness of ∼10.0 nm, and a magnetic core, which endowed it with good dispersibility, strong magnetism, and excellent adsorption property for oxytetracycline (OTC). Several experimental conditions, such as solution pH, adsorption time, OTC concentration, and ionic strength were systematically investigated. The Mag@ZnO−Co3O4 composite exhibited a fast adsorption rate, that is, the equilibrium was reached within 90 min, when the concentration of OTC was between 10 and 30 mg/L. The composite showed favorable performance for the adsorption of OTC in the pH range 5−9. According to the adsorption isotherms and kinetics, the removal of OTC was efficient and obeyed the pseudo-second-order rate equation and Langmuir adsorption model. KEYWORDS: Magnetic, Adsorption, Antibiotic, Oxytetracycline, Core−shell

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antibiotics from water. Specifically, Saitoh and Shibayama19 reported that tetracyclines in water can be rapidly removed using montmorillonite clay. Fu20 used powdered activated carbon to remove six frequently used quinolone (QN) antibiotics base on hydrophobic interaction, electrostatic interaction, and π−π dispersion force. Copper-based metal organic framework materials also exhibited an excellent adsorptive capacity for toxic sulfonamide antibiotics mainly through p−p stacking, hydrogen bonding, and electrostatic interactions.2 Although some of these mentioned adsorbents show a good adsorption capacity for antibiotics, the difficulty in collecting them from the solution impedes their further application. Magnetic nanocomposites are a new generation of separation media designed to face this challenge. Previous research indicated that incorporating magnetic nanoparticles into a multifunctional shell facilitates convenient recovery of the adsorbent and the absorbed contaminants from the treated solution by magnetic separation.21−24 In the present work, we demonstrate a facile, mild, and ecofriendly method for synthesis of magnetic microspheres with a magnetic core and a bimetal oxide shell (ZnO−Co3O4). The as-fabricated magnetic microspheres (Mag@ZnO−Co3O4) not

INTRODUCTION The discovery of antibiotics is one of the greatest scientific achievements in the 20th century.1,2 However, antibiotics are regarded as environmental contaminants due to their potential long-term adverse effects and complex vicious cycle of transformation and bioaccumulation in the environment.3 Given that sewage treatment plants (STPs) are geared toward traditional pollutants, such as metal and organic pollutants, antibiotics are not efficiently removed from the environment.4,5 Hence, treatment of antibiotics-polluted environmental water has been a great challenge to the health care sector in the 21st century.6 Scholars must develop a highly efficient method for eliminating antibiotics in the aquatic environment. Physical and chemical processes have been employed to remove antibiotics from environmental waters and wastewaters; these processes include photocatalytic, flocculation/coagulation, oxidation, fungal treatment, adsorption, and membrane techniques.7−12 Adsorption is the most commonly used technique because of its facile operation, high efficiency, cost effectiveness, and low energy requirement; moreover, this process does not produce secondary pollutants. Many potential adsorbents have been applied in pollutant adsorption and degradation.13−15 In recent years, various adsorbents, such as activated carbon,16 zeolites,17 and ion exchange resins18 have been used for adsorption of © 2017 American Chemical Society

Received: July 11, 2017 Revised: September 6, 2017 Published: September 13, 2017 10298

DOI: 10.1021/acssuschemeng.7b02320 ACS Sustainable Chem. Eng. 2017, 5, 10298−10306

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also prepared using the same technique with various Zn/Co molar ratios to determine the effect of the molar ratio on the adsorbent characteristics. Characterization. The surface area of Mag@ZnO−Co3O4 was measured using an ASAP 2010 Micrometrics instrument (USA) by the Brunauer−Emmett−Teller (BET) method. The magnetic property was assessed at 300 K on a superconducting quantum interference device magnetometer (MPMS XL-5, Quantum Design, USA). Fourier transform infrared (FTIR) spectroscopic analysis was conducted through a Vertex 80 V spectrometer (Bruker, Germany) within the range 4000−400 cm−1. Powder X-ray diffraction (XRD) analysis was conducted at room temperature by use of a Bruker D8 Advanced X-ray diffractometer (Bruker Optik GmbH, Germany) with Cu Kα radiation in the range 2θ = 10−90°. X-ray photoelectron spectra (XPS; Thermo Fisher Scientific, K-Alpha, USA) were recorded to ascertain the chemical composition and element chemical state in Mag@ZnO− Co3O4. The morphology of Mag@ZnO−Co3O4 was observed by a field-emission transmission electron microscope (TEM; JEOL, JEM2100F, Japan) and scanning electron microscope (SEM; JEOL, JSM7500F, Japan) equipped with an energy dispersive spectrometer (EDS; Oxford, X-Max, U.K.). The surface charge of the product was measured on a ζ-potential analyzer (Malvern, Zetasizer Nano ZS90, U.K.). Oxytetracycline Analytical Method. The OTC residue remaining in the supernatant was analyzed at 280 nm through ultrahighperformance liquid chromatography (UPLC; Waters, U.S.A.) with a TUV detector using a BEH C18 column (Waters, 2.1 mm × 50 mm, 1.7 μm particle size) at 30 °C. The mobile phase consisted of 10% methanol, 20% acetonitrile, and 70% oxalic acid buffer (0.02 mol/L). The flow rate was maintained at 0.2 mL/min, and the injection volume was set as 10 μL. Pollutant Adsorption Experiments. Scheme 1 shows the application of the nanoadsorbent in the adsorption process. First, magnetic composites prepared using different dosages of Co and Zn raw materials were used as adsorbent to uptake OTC from the solution to validate the optimal Zn/Co molar ratio. The effect of coexisting ions on the adsorption was studied by shaking 0.0025 g of the nanoadsorbent with 10 mL of OTC at the initial dye concentration of 10 mg/L. The equilibrium time was 6 h. The effect of pH (2−11) on the adsorption was also evaluated under the same testing conditions. The pH of the sample solution was adjusted by adding 0.1 mol/L HCl and NaOH solutions. Adsorption isotherm experiments were conducted with a constant dosage of 0.0025 g of the adsorbent and initial OTC concentrations between 20 and 200 mg/L (V = 10 mL) at 298 K under the same testing conditions. For the effect of concentration on the adsorption, 0.0025 g of Mag@ZnO− Co3O4 was suspended in 10 mL of the OTC solution (10−30 mg/L). In brief, 1 mL of a sample was taken at different intervals for each condition. The magnetic adsorbent was finally separated by a magnet, and the OTC residue remaining in the supernatant was determined by UPLC analysis. Adsorption capacity (qt, mg/g) at time t was calculated using the following equation:

only retain the functions of the magnetic nanoparticles but also provide additional unique physiological activities conferred by the bimetal oxide shell. To investigate the growth mechanism for the unique core/shell Mag@ZnO−Co3O4 microspheres, we performed a series of experiments by using different dosages of Zn and Co as raw materials. Under the optimal Zn/Co molar ratio, the as-synthesized microspheres exhibited a fast adsorption rate for the removal of oxytetracycline (OTC), one of the most widely used tetracycline antibiotics, from aqueous solution. The effects of OTC concentration, coexisting ionic species, and pH conditions were also evaluated to investigate and quantify the operational processes affecting the uptake capacity of the composite in the treated solution.

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EXPERIMENTAL SECTION

Materials and Reagents. OTC (≥95% by HPLC) was purchased from Aladdin Chemistry Co., Ltd. (Tianjin, China). A stock solution of OTC (1000 mg/L) was prepared by dissolving a required amount of OTC in methanol. The solution was diluted to the desired concentrations by adding water. Ferric chloride hexahydrate (FeCl3· 6H2O), cobalt nitrate hexahydrate (Co(NO)3·6H2O), and zinc nitrate hexahydrate (Zn(NO 3)2·6H2O) were purchased from Damao Chemical Reagent Factory (Tianjin, China). Pure water was obtained from a Milli-Q water purification system (Millipore Ltd., Bedford, MA, USA) and used for all dilutions. All other chemicals were of analytical grade and used as received. Preparation of Bimetal Oxide Modified Magnetic Microspheres. Scheme 1 illustrates the strategy for preparing the core−

Scheme 1. Schematic of the Synthesis and Application of Core−Shell Mag@ZnO−Co3O4 Microspheres

shell Mag@ZnO−Co3O4 nanoadsorbents. The Fe3O4 nanoparticles were synthesized through a facile solvothermal process according to our previous report.25 In a beaker, 1.35 g of Fe(III) salts, 3.60 g of anhydrous sodium acetate, and 1.00 g of polyethylene glycol (Mr = 2000) as a reductant were mixed in 40 mL of ethylene glycol. The mixture was continuously agitated by a magnetic stirrer for 30 min to form a homogeneous yellow solution. The resulting mixture was transferred into the hydrothermal synthesis reactor and heated at 190 °C for 8 h. The mixture was cooled to room temperature, and the black precipitate was magnetically separated and washed several times with ethanol and deionized water. The synthesized Fe3O4 nanoparticles were then dried at 60 °C under vacuum. ZnO−Co3O4 shell-coated microspheres were prepared via a facile hydrothermal process. Fe3O4 nanoparticles were used as seeds for subsequent ZnO−Co3O4 shell growth, by adding 0.1 g of Fe3O4 nanoparticles to a homogeneous solution consisting of 30 mL of deionized water, 1 mmol of Zn(NO3)2·6H2O, 1 mmol of Co(NO3)2· 6H2O, 4 mmol of ammonium fluoride, and 8 mmol of urea. The mixture was transferred to a 50 mL hydrothermal synthesis reactor and heated at 120 °C for 6 h. The resulting magnetic microspheres were collected by a magnet, washed thoroughly with deionized water to remove the residues, and dried under vacuum at 60 °C for 6 h. After annealing at 350 °C in air for 2 h, Mag@ZnO−Co3O4 magnetic microspheres were successfully prepared. Magnetic composites were

qt =

(C0 − Ct )V m

(1)

where C0 (mg/L) is the initial concentration; Ct (mg/L) and Ce (mg/ L) are the remaining concentrations of OTC in solution at time t and equilibrium, respectively; m is the mass of adsorbent (g); and V is the volume of the solution (mL).

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RESULTS AND DISCUSSION Characterization of Mag@ZnO−Co3O4. As shown in Figure 1, the magnetization curve indicates the ferromagnetic feature of the nanocomposite. The saturation magnetization (Ms) is 30.1 emu/g; this high value allows the pollutantsadsorbed microspheres to aggregate together and then separate from the treated samples by using a magnet within 1 min. The 10299

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shown in Figure 2, all peaks of Fe3O4 correspond to the standard diffraction patterns of the cubic inverse spinel lattice structure of Fe3O4 (JCPDS No. 19-629).26 The new diffraction peaks indicated by diamonds can match well with the diffraction from the (100), (002), (101), (103), (110), and (202) planes of wurtzite ZnO (JCPDS No. 36-1451).27 The other peaks indicated by stars correspond to the diffractions of the face-centered-cubic spinel Co3O4 (JCPDS No. 42-1467); this finding suggests that the multifunctional shell was composed of both ZnO and Co3O4.28,29 However, distinguishing Fe2O3 from Fe3O4 in the XRD patterns was difficult due to their very similar structures. Therefore, other characterization methods, namely, XPS and high-resolution TEM analyses, were conducted to ascertain the element chemical state and crystallite size of Mag@ZnO−Co3O4. The XPS survey spectrum in Figure 3a indicates the presence of Fe, Zn, Co, and O. Figure 3b shows the high-resolution spectrum of the Fe 2p region. The core-level binding energy peaks at 711 and 725 eV match well with Fe 2p3/2 and Fe 2p1/2. Moreover, the Fe 2p3/2 and 2p1/2 main peaks of the sample are accompanied by a shakeup satellite structure, which is the fingerprints of the electronic structures of Fe3+, at 719 eV (indicated by arrow), indicating the absence of Fe2+.30,31 Basing on these results, we conclude that Fe3O4 was transformed into γ-Fe2O3 form during the annealing process. As shown in Figure 3c, the characteristic peaks of Co 2p3/2 and Co 2p1/2 at 780 and 795 eV are in accordance with the presence of Co3O4, indicating that spinel Co3O4 successfully deposited on the magnetic nanoparticle surface.32 Additionally, the spin−orbit splitting between the two peaks is 15 eV, consistent with the reported data of spinel Co3O4.33 Notably, after the incorporation of Co3O4 with ZnO, the lattice parameters did not change, and the crystal structure was maintained (Figure 3). EDS analysis was performed to determine the chemical composition of the as-obtained Mag@ZnO−Co3O4. As shown in Figure 4a, the EDS spectrum peaks belonging to Zn and Co, with weight percentages of about 12 and 6%, respectively,

Figure 1. Magnetic hysteresis loop of Mag@ZnO−Co3O4.

N2 adsorption−desorption isotherm of Mag@ZnO−Co3O4 is presented in Figure S1. The BET surface area of the prepared magnetic microspheres is measured to be 25.40 m2/g. The XRD patterns of Fe3O4 and as-prepared Mag@ZnO− Co3O4 magnetic microspheres are presented in Figure 2. As

Figure 2. XRD patterns of Fe3O4 (a) and Mag@ZnO−Co3O4 nanoadsorbent (b).

Figure 3. XPS spectra of Mag@ZnO−Co3O4 microspheres: (a) full spectrum, (b) Fe 2p spectrum, (c) Co 2p spectrum, and (d) Zn 2p spectrum. 10300

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Figure 4. (a) EDX analysis. (b) SEM image; inset, high magnification SEM image. (c) TEM image. (d, e) High-resolution TEM images of Mag@ ZnO−Co3O4.

1. In the first step, Fe3O4 nanoparticles were directly obtained by reduction of Fe3+ at 190 °C in the presence of polyethylene glycol and ethylene glycol under alkaline media. In the second step, the metal complexes were first obtained by reacting Zn2+ and Co2+ with F−. When the solution temperature was increased to 120 °C, the contents of CO2 and OH− increased gradually because of urea decomposition (CO(NH2)2 + H2O → 2NH3 + CO2; H2O + NH3 → OH− + NH4+). With increasing OH−, Zn(OH)2 was generated and agglomerated on the Fe3O4 nanoparticles surface due to the low solubility product constant value (Ksp). The CO32− produced by CO2 hydrolysis could react with Co2+, instead of F−, to form metal basic carbonate because of strong affinity (CoF+ + xOH− + 0.5(2 − x)CO32− + nH2O→ Co(OH)x(CO3)0.5(2−x)·nH2O + F−).42 Finally, a thin layer of ZnO−Co3O4 shell was developed in the final annealing treatment (Zn(OH)2 → ZnO + H2O; Co(OH)x(CO3)0.5(2−x)·nH2O + O2 → Co3O4 + CO2 + H2O).43,44 During synthesis, the presence of F− is necessary for the development of ZnO−Co3O4 layer shell. A system consisting of irregular separate Fe3O4@ZnO nanosheets and individual Co3O4 nanorods was obtained in the absence of F−. Scholars reported different methods for the synthesis of pCo3O4/n-ZnO heterojunction. The well-dispersed core−shell nanoadsorbent evidently differs from the reported p-Co3O4/nZnO heterojunction45−47 and [email protected] The monodispersed magnetic Fe3O4 nanoparticles are believed to play a beneficial role in the functional behavior, and serve as substrates to facilitate subsequent metal oxide shell growth. Otherwise, hierarchical or clustered bimetal oxide is easily received, thereby

indicate the homogeneous distribution of these elements in the magnetic microspheres. Consistent with the XRD and XPS results, the EDS data confirm the successful modification of a thin layer of ZnO−Co3O4. The morphologies of Mag@ZnO− Co3O4 and its core−shell structure were characterized using SEM and TEM images. The SEM image shows that the assynthesized microspheres mainly present a spherical structure with an average diameter of 150 nm (Figure 4b). The highmagnification SEM image (inset of Figure 4b) reveals the rough surface feature of Mag@ZnO−Co3O4. The morphology of the received adsorbent (Figure 4c) has an obscure interface between the magnetic core and ZnO−Co3O4 shell regions. The morphology differs from the reported core−shell magnetic microspheres, such as SiO2, dopamine, and polyethylenimine,34−37 possibly due to the similar crystal structure between the magnetic core and the bimetal oxide shell. High-resolution TEM was used to reveal the detailed microstructures and crystallite size of the microspheres (Figure 4d,e). The bimetal oxide layer is as thin as ∼10 nm. The spaces 0.482 and 0.264 nm in the core regions correspond to the (111) and (310) planes of γ-Fe2O3, indicating that the Fe3O4 core changed into γ-Fe2O3 after calcination.38 The marked d-spacing of 0.467 nm in the shell region is in good agreement with the d-spacing of the (111) plane of Co3O4.39−41 Moreover, the interplanar spacing of 0.248 nm corresponds well to the spacings of the (101) lattice planes of ZnO.30 Synthesis Mechanisms for Mag@ZnO−Co3O4 Microspheres. The core−shell microspheres with a magnetite core and a ZnO−Co3O4 shell were synthesized according to Scheme 10301

DOI: 10.1021/acssuschemeng.7b02320 ACS Sustainable Chem. Eng. 2017, 5, 10298−10306

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Figure 5. (a) Adsorption of OTC by magnetic composites with different Zn and Co raw materials. Effect of (b) ion species, (c) pH, and (d) concentration on OTC adsorption.

Interestingly, the presence of Na+ and Cl− ions and the increase in the NaCl concentration have a negligible influence on the OTC adsorption. By contrast, the presence of KNO3 and CaCl2 decreased the adsorption capacity for OTC. These results indicate that the OTC uptake by Mag@ZnO−Co3O4 was not influenced by the increase in ionic strength, but was significantly influenced by the coexisting ionic species. The phenomenon can be associated with the surface properties of the nanoadsorbent and the molecular structure of OTC. OTC contains several electron donor groups such as diketone, dimethylammonium, and tricarbonylmethane, which could complex with high polarizing transition metals by forming stable metal−OTC complexes. Consequently, the Mag@ZnO−Co3O4 microspheres, with a bimetal oxide shell, can easily attract the OTC molecule, leading to the formation of Zn−OTC and Co−OTC complexes. Adsorption of OTC in water is affected by the coexisting metal ions, which can chelate. Consequently, binding of Ca2+ with the diketone, dimethylammonium, and β-keto−enol groups decreased the availability of N-electrons of OTC, resulting in inhibition of OTC adsorption.49 Not only the metal types but also some anions in the water environment play an important role in the adsorption of OTC. For instance, NO3− could compete with the electron-rich functional groups of OTC for the binding of metal. Hence, a decline in the OTC uptake is observed when the concentration of KNO3 increased from 0 to 0.1 mol/L. All these phenomena certified that OTC adsorption by the magnetic composite was realized by the formation of surface complexes at the active sites on the surface of the adsorbent. The complexation between OTC and Mag@ZnO−Co3O4 was further certified by the FTIR spectra. Figure S2 shows FTIR spectra of OTC and of the Mag@ZnO−Co 3 O 4 microspheres before and after adsorption of OTC. As expected, the peaks at 661, 590, and 453 cm−1 belonging to Co−O, Fe− O, and ZnO−O stretching vibrations of Mag@ZnO−Co3O4 indicate the successful coating of bimetal oxide shell (Figure S2a,b). By comparison with the OTC spectrum, it was found

diminishing the magnetic response and reducing the recycle efficiency. Determination of Optimal Zn/Co Molar Ratio. A series of magnetic composites with different dosages of Co and Zn raw materials were tested as adsorbents for OTC adsorption. The final phases and adsorption amount of OTC for the obtained magnetic nanosorbent can be seriously affected by the amount of Zn and Co raw materials. As shown in Figure 5a, the adsorption capacity increased with increasing amount of Zn and Co raw materials from 0.5 to 1 mmol. However, the adsorption amount decreased slightly with further increase in Zn and Co raw materials to 2 mmol; under this condition, the surplus Zn2+ and Co2+ generate other byproducts, thereby reducing the adsorption performance. In addition, the adsorption amount of OTC decreased as the dosage of the Zn2+ and Co2+ changed to 2:1 or 1:2. The differences in the adsorption capacity of the mentioned adsorbents can be attributed to their different structures. The SEM and TEM analyses show that the homogeneous core/shell microspheres were synthesized using 1 mmol of Zn2+ and Co2+ as raw material (Figure 4). The welldefined bimetal oxide shell and the small size of the microspheres are believed to play a beneficial role in improving the adsorption capacity. However, a portion of the separated ZnO nanosheet and Co3O4 nanorods would mix in the core− shell microspheres when the ratio of Zn2+ and Co2+ was changed to 2:1 and 1:2. As a consequence, the coexistence of ZnO and Co3O4 phases, with intimate contact between Mag@ ZnO−Co3O4 magnetic microspheres, led to reduction of the adsorption performances. Interferences and Complexation Mechanism. The coexisting ions could compete with OTC for adsorption by Mag@ZnO−Co3O4; as a result, these species can deteriorate the adsorption capacity for OTC. To study the influence of coexisting ionic species on the adsorption of OTC by the magnetic nanocomposite, we investigated several commonly encountered inorganic salts (NaCl, CaCl2, and KNO3) in tap water and environmental water samples (Figure 5b). 10302

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ACS Sustainable Chemistry & Engineering Table 1. Kinetic Models and Other Statistical Parameters of Mag@ZnO−Co3O4 Nanoadsorbent at 298 °C pseudo-first-order model C0 (mg/L) 20 30

qe (mg/g) 13.6 14.6

k1 (min−1) −2

1.47 × 10 1.43 × 10−2

pseudo-second-order model R2

qe (mg/g)

0.8543 0.6435

49.3 68.0

k2 (g min/mg) −3

7.35 × 10 4.58 × 10−3

R2

qe,exp (mg/g)

0.9977 0.9993

50.8 66.8

reached within 90 min. The fast uptake kinetics is derived from the small size of Mag@ZnO−Co3O4 microspheres and the particular bimetal oxide layer on the adsorbent surface. On one hand, submicrometer-sized particles could not only accelerate the accessibility of the metal active binding sites, but also increase the intraparticle diffusion rate of OTC onto the adsorption sites. Moreover, the thin coating of bimetal oxide shell on the magnetic core is beneficial for the full exposure of active metal sites, and enables the easy adsorption of OTC onto bimetal oxide surface by means of complexation between metal and OTC. Uptake Kinetics. To further understand the adsorption behaviors of Mag@ZnO−Co3O4, the batch experimental data (20, 30 mg/L) were analyzed in terms of kinetic models via pseudo-first-order and pseudo-second-order equations,54 which can be expressed as follows:

from the spectrum of OTC-adsorbed microspheres that the characteristic stretching absorption of phenolic OH at 3317 cm−1 partially shifted to 3379 cm−1, and a new peak appeared at 1394 cm−1, implying cation−π interactions and the formation of surface complexation between OTC and the surface active sites of Mag@ZnO−Co3O4.50,51 Moreover, in order to study the interferences in real water, the uptake of OTC by the Mag@ZnO−Co3O4 composite was demonstrated for the treatment of a tap water sample (Jilin, China). As illustrated in Figure S3, the OTC uptake capacity in tap water was found to be slightly lower compared with pure water. Although the presence of cation and anion in the tap water causes a slight decrease in the adsorption capacity, the favorable adsorption performance and magnetic recovery capability of the Mag@ZnO−Co3O4 composite are significant for its practical use in treating real water bodies. Effect of pH and Electrostatic Interaction. The pH of the solution is one of the significant factors for the uptake of pollutants, which is essential to understanding the effect of the electrostatic interactions on the adsorption process. The adsorption of OTC using Mag@ZnO−Co3O4 was investigated within the pH range 2−11 with an initial concentration of 10 mg/L at 298 K. As shown in Figure 5c, the adsorption of OTC is more favorable at neutral pH over acidic and basic conditions; from pH 2 to pH 5, the adsorption capacity linearly increases, while above pH 5 and up to pH 9 the adsorption capacity is relatively stable with the highest value at pH 7. Above pH 9, the adsorption tends to progressively decrease again. The results can be associated with the surface properties of the nanoadsorbent and the pH-dependent speciation of OTC. ζpotential measurements results indicate that the surface of Mag@ZnO−Co3O4 has negative charges over the pH range 5− 11 (Figure S4). Moreover, OTC is one of the typical tetracyclines that form a series of species at different pH values. The charge of the OTC molecule could be positive (pH 9.58).52,53 At pH 0.997). In addition, the calculated qe values obtained are also very close to the experimental values of qe,exp. These facts indicate that the adsorption process obeyed the pseudo-second-order model. Therefore, adsorption of OTC onto the Mag@ZnO−Co3O4 nanoparticles could be a chemisorption process involving electron sharing or electron transfer between adsorbent and adsorbate.55 Adsorption Isotherms. Langmuir and Freundlich isotherm models were employed to fit the adsorption equilibrium data. Figure 6 shows the uptake of OTC versus concentrations for the typical Mag@ZnO−Co3O4 microspheres. While the dosage of the magnetic microspheres remains constant (0.0025 g), the uptake amount of OTC shows a very rapid increase with the initial OTC concentration increasing from 20 to 80 mg/L for the synthesized Mag@ZnO−Co3O4 microspheres; subsequently, the adsorption rate decreases gradually and the uptake reaches saturation when the OTC concentration reaches 160 mg/L. As shown in Table S1, by comparing the correlation coefficients (R2), the Langmuir model could describe the experimental data better than the Freundlich model, indicating a monolayer adsorption onto the Mag@ZnO−Co3O4 surface.56,57 The n value calculated from the Freundlich isotherm model is larger than unity, indicating the strong interaction 10303

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microsphere structure and a thin well-defined bimetal oxide layer on the surface, resulting in an excellent adsorption performance for OTC. The super adsorption performance of Mag@ZnO−Co3O4 can be attributed to its unique structure, high isoelectric point, and strong surface complexation. The ratio of Zn and Co raw materials played a crucial factor in controlling the core−shell structure formation, and the presence of F− during the modification of Zn and Co should be responsible for the formation of the bimetal oxide shell. Moreover, this bimetal oxide modified magnetic adsorbent had preferential adsorption of OTC in aqueous mixtures at neutral and weak basic conditions. This work provides a novel method to prepare bimetal oxide modified magnetic microspheres with well-defined core−shell morphologies. Furthermore, the novel magnetic microspheres can be also expected to have potential applications in catalysis, photodegradation, and other fields due to the multifunctional transition metal oxide shell.

Figure 6. OTC adsorption isotherm curve on Mag@ZnO−Co3O4 microspheres.

force between OTC and Mag@ZnO−Co3O4 adsorbent. The maximum adsorption capacity (qmax) of Mag@ZnO−Co3O4 is found to be 128 mg/g for OTC, calculated by the Langmuir model, which indicates that the experimental values of qmax are in good agreement with the theoretical results. Additionally, we compared the uptake of OTC on various adsorbents in Table 2.

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S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acssuschemeng.7b02320. Isotherm and statistical parameters (Table S1), N2 adsorption−desorption isotherms (Figure S1), FTIR spectra (Figure S2), time-course uptake of OTC by Mag@ZnO−Co3O4 in pure water and tap water (Figure S3), ζ potentials (Figure S4), pseudo-second-order kinetics plots (Figure S5), potential regeneration of Mag@ZnO−Co3O4 magnetic microspheres (Figure S6) (PDF)

Table 2. Uptake Parameters of OTC on Various Adsorbents Recently Reported adsorbent

qmax (mg/g)

equipment time

ref

goethite/hematite activated sludge resin MN-100/150 graphene oxide MFe2O4 GO-MPS zerovalent iron Mag@ZnO−Co3O4

2 91 60/120 212 0.1 45 34 128

72 h 48 h 72 h 90 min 10 min 10 min 480 min 90 min

58 51 59 60 61 62 63 this work

ASSOCIATED CONTENT

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Fax: 86-432-63083163. Tel.: 86432-63083163.

Obviously, some carbon materials showed the highest adsorption capacity. However, they always needed so much time to obtain adsorption equilibrium or were too expensive to use in the adsorption. Obviously, the Mag@ZnO−Co3O4 magnetic composite possesses a relatively large adsorption capacity and a fast adsorption rate as compared to previously reported adsorbents. Moreover, the high saturation magnetization enables the pollutants-adsorbed microspheres conveniently recovered from the treated samples using a magnet compared with the other metal oxides, including Co3O4 and ZnO. Recycling the Nanoadsorbent. The regeneration performance of adsorbents is one of the vital factors in practical water treatment applications. Consequently, the UPLC mobile phase (consisting of 10% methanol, 20% acetonitrile, and 70% 0.02 mol/L oxalic acid buffer) was used as eluent for the desorption and reusability of Mag@ZnO−Co3O4. The results (Figure S6) show that the mobile phase could efficiently cause the adsorbed OTC to undergo desorption. The uptake capacity decreased slightly at the first five cycles (uptake loss