One-Step Synthesis of a Mn-Doped Fe2O3/GO Core-Shell Nanocomposite and Its Application for the Adsorption of Levofloxacin in Aqueous Solution.

This study describes for the first time the synthesis, characterization, and application of a MnFe2O3/GO core-shell nanocomposite as an adsorbent for the removal of levofloxacin (Lev) from real water samples. The formation of the proposed nanocomposite was confirmed using various characterization techniques. The structural techniques revealed a 20 nm average particle size of the MnFe2O3/GO core-shell nanocomposite, with a surface area of 70.7 m2 g-1, as shown by the BET results. The most influential parameters (adsorbent dosage, stirring rate, and Lev pH) that affected the adsorption process were optimized using the response surface methodology (RSM) based on a central composite design. The optimum conditions were 0.007 g, 2, and 7 for adsorbent dosage, stirring rate, and Lev pH, respectively. The adsorption behavior of Lev on the MnFe2O3/GO core-shell nanocomposite was examined using isotherm models, kinetics, and thermodynamics. The kinetic models demonstrated that the adsorption process was controlled by both intraparticle and outer diffusion. Furthermore, the results obtained revealed that the adsorption of Lev on MnFe2O3/GO was dominated by electrostatic interactions. Moreover, Dubinin-Radushkevich and Temkin isotherms confirmed that the sorption mechanism was dominated by electrostatic interactions, while Langmuir and Sips models confirmed a monolayer adsorption process. The maximum adsorption capacity of Lev onto the MnFe2O3/GO adsorbent was found to be 129.9 mg g-1. Furthermore, the thermodynamic data revealed that the adsorption system was spontaneous and exothermic. The synthesized MnFe2O3/GO core-shell nanocomposite showed significant recyclability and regenerability properties up to five adsorption-desorption cycles. As a proof of concept, the performance of the prepared adsorbent was evaluated for laboratory-scale purification of spiked real water samples. The prepared adsorbent significantly reduced the concentration of Lev in the real water samples and the removal efficiency ranged from 86 to 97%.

Introduction

The use of antibiotics to treat infectious diseases, such as chlamydia, malaria, urinary tract infections, etc., results in drug resistance in pathogenic bacteria in human beings and livestock.[1,2] However, antibiotics are regarded as persistent organic contaminants because of their incessant rapid increase in various water bodies.[3−5] Moreover, antibiotics are unmetabolized or partially metabolized, causing the release of metastable cyclic functional moieties such as benzene, piperazine, sulfonamides, morpholine, quinolone, and hexahydro-pyrimidines to the environment.[6] Therefore, contamination of drinking water by antibiotics has become a serious environmental concern as these contaminations can cause serious health risks such as headaches, nausea, diarrhea, nervousness, allergic reactions, etc.[7]

Levofloxacin (Lev) is a wide-spectrum fluoroquinolone antibiotic and has been widely used in both human and veterinary medical treatments.[8,9] Furthermore, Lev is effective for the treatment of various infections such as skin, genitourinary, gynecological, soft tissues, and lower and upper respiratory tracks.[5,10] In comparison with other antibiotics, a large amount of Lev is excreted unmetabolized and released into the environment through wastewater treatment plants.[5,10] Moreover, Lev has extensive antibacterial effects that could affect Gram-positive and Gram-negative bacteria and also other pathogens.[11,12] In addition, due to the low biodegradation rate of Lev, it is challenging to remove it from wastewater using traditional biological methods.[12] Thus, it is essential to develop effective methods for the complete removal of Lev from environmental matrices.

Numerous treatment methods such as adsorption,[11,13,14] catalytic degradation,[10,15,16] membrane filtration,[17] biological treatments,[18] disinfection,[19] electrochemical treatments,[20] and ozonation[15] have been investigated for the removal of antibiotics from aqueous solutions. Among these methods, the adsorption system demonstrated great potential for the removal of antibiotics from aqueous solutions and has advantages such as low cost, easy operation, high efficiency, reproducibility, and availability of several adsorbents.[12,21,22] Among the reported adsorbents, graphene oxide (GO) has received more attention in the removal of antibiotics in aqueous solutions.[23,24] This is because GO has remarkable features such as high adsorption efficiency, large specific surface area, high biocompatibility, modifiability, and various surface functional groups.[25] Despite the attractive properties, the major limitations of GO include low production yield during synthesis, high dispersibility in an aqueous medium, difficulty to recover from treated effluents, and GO layer agglomeration.[25,26] To overcome the abovementioned drawbacks, most researchers have recently reported that GO nanocomposites are emerging adsorbents for the adsorptive removal of antibiotics due to their large surface area, controllable surface properties, and fast and easy separation.[21,27] As a result, GO nanocomposites such as GPTMS/APTMS-MGO,[27] GO/CuMOF-Fe3O4,[28] reduced MGO/polyaniline composite,[26] amine-functionalized magnetic GO,[25] and Chitosan/Fe3O4/rGO[29] among others have been used for the removal of antibiotics.

Therefore, the main objective of the present study was to synthesize a Mn-doped Fe2O3/GO core–shell nanocomposite for the adsorptive removal of Lev from aqueous solution. In this study, Fe2O3 was doped with Mn to increase adsorption capacity as well as alter or enhance the surface properties of iron oxide. GO, on the other hand, is used as a support for the immobilization of nanoparticles. This was done to avoid agglomeration as well as leaching of Fe and Mn, thus causing secondary pollution during the adsorption process. Additionally, GO was used because of its remarkable surface features, leading to enhanced adsorption affinity toward Lev. The effects of pH, contact time, Lev concentration, and temperature on the adsorption of Lev were investigated. Lastly, the adsorption mechanism between Lev and the MnFe2O3/GO core–shell nanocomposite was investigated using kinetic, isotherm, and thermodynamic studies. According to the author’s knowledge, the MnFe2O3/GO core–shell nanocomposite has been reported for the first time to act as an adsorbent for the adsorptive removal of Lev from real water samples.

Experimental Details

Materials and Chemicals

Graphite powder (98%), iron chloride (≤99%), manganese nitrate (99%), potassium permanganate (98%), hydrogen peroxide (30%), levofloxacin (≥98%), sodium hydroxide (65%), hydrochloric acid (37%), absolute ethanol, nitric acid (65%), and sulfuric acid (98%) were all purchased from Sigma-Aldrich (Johannesburg, South Africa) and used without further purification.

Synthesis of Graphene Oxide (GO)

The synthesis of graphene oxide was carried out using the method adopted from the literature, where the improved Hummers’ method was used.[30] In a representative experiment, a 9:1 mixture of 360 mL of H2SO4 and 40 mL of H3PO4 was poured into a mixture of 3.0 g of graphite flakes and 18 g of KMnO4, generating a slightly exothermic reaction up to 40 °C. The reaction was then heated up to 50 °C under vigorous stirring overnight. The obtained sample was cooled down to room temperature and transferred onto an ice bath with 3 mL of H2O2. The mixture was filtered through a metal U.S. standard testing sieve and further sieved through a polyester fiber.[31] The filtrate was centrifuged at 4000 rpm for 4 h, and then the supernatant was discarded. The material obtained was then separately washed several times with deionized H2O, HCl, and ethanol. Finally, the black-brown precipitate obtained was vacuum-dried for 12 h at room temperature.

One-Step Synthesis of MnFe2O3 Core–Shell Nanoparticles

The MnFe2O3 core–shell nanoparticles were synthesized using a one-step hydrothermal method.[32] In a typical experiment, 2 g of FeCl3 is dissolved in 60 mL of deionized H2O and stirred for 10 min, followed by the addition of 0.1 g of Mn(NO)3·H2O and further stirred for another 10 min. The solution obtained is then transferred into a 100 mL Teflon autoclave container and placed on the muffle furnace at 180 °C for 10 h. The final product was further washed several times with deionized H2O and ethanol. Lastly, the precipitate was dried in an oven at 60 °C for 12 h.

One-Step Synthesis of the MnFe2O3/GO Core–Shell Nanocomposite

The MnFe2O3/GO core–shell nanocomposite was also prepared by the one-step hydrothermal method.[33] In a representative experiment, 1 g of synthesized GO was dissolved in 500 mL of deionized H2O with ultrasonication for 30 min. FeCl3 (2.0 g) was dissolved in 20 mL of deionized H2O and then transferred into the solution while stirring, followed by the addition of 100 mg of Mn(NO)3. The obtained solution was then transferred into a 100 mL Teflon autoclave container and positioned on the autoclave at 180 °C for 10 h. After that, the obtained material was cooled at room temperature, and then the solution was washed several times with deionized H2O and ethanol. Finally, the material obtained was dried in an oven at 60 °C for 12 h.

Characterization of the Synthesi zed Materials

The materials were characterized with powder X-ray diffraction (P-XRD, PANalytical, Almelo, the Netherlands), Micromeritics ASAP-2460 accelerated surface area and porosimetry system, transmission electron microscopy (TEM, JOEL Ltd., Tokyo, Japan), scanning electron microscopy/energy-dispersive spectroscopy (SEM/EDS, Tescan, Brno, Czech Republic), X-ray photoelectron spectroscopy (XPS, model ESCALAB 250 Xi, Thermo Fisher Scientific, Waltham), and Fourier transform infrared spectroscopy (FTIR, Bruker Tensor 27, Bruker Optics, GmbH, Germany). The detailed information and procedures for characterization techniques are presented in the Supporting Information (Sub-section 1.1).

Adsorption Experiments

All of the adsorption experiments were carried out in a round bottom flask sealed with an aluminum foil to prevent degradation by light exposure under vigorous stirring. Briefly, appropriate masses (10–30 mg) of the adsorbent were placed into a 250 mL round bottom flask, followed by the addition of a 100 mL model solution containing 10 mg L–1 Lev (pH ranging between 5 and 9). Then, the solutions were placed on a digital hot-plate stirrer and agitated at varying speeds of 2–4 at a fixed temperature (298 K) for 30 min. The optimization of dosage, stirring rate (SR), and pH were optimized using response surface methodology (RSM) based on a central composite design (CCD). The three parameters were investigated at five levels, and their values are presented in Table . The percentage removal (%RE) used as an analytical response was calculated using eq S1 (Supporting Information, Sub-section 1.2).

Table 1

Central Composite Optimization Parameters

parameters	–α	low level	central point	high level	+α
dosage (mg)	7.13	10	20	30	32.9
stirring rate (SR)	1.7	2	3	4	4.3
pH	4.4	5	7	9	9.6

During kinetic studies, 7 mg of MnFe2O3/GO core–shell nanocomposite was dispersed in 100 mL solution (10, 20, and 30 mg L–1) of Lev at pH 7. It must be noted that each experiment was conducted for 10–40 min. Moreover, the resultant mixtures were sampled and centrifuged for 10 min at 4500 rpm to separate the adsorbent from the supernatant solutions. The latter were then analyzed for Lev by UV–vis spectroscopy. The adsorption capacity at time t (q) was calculated using eq S2 (Supporting Information, Sub-section 1.2).

To investigate the isotherms, 100 mL of model solutions at various concentrations of Lev (5–35 mg L–1) was placed in a round bottom flask, followed by the addition of 7 mg of the MnFe2O3/GO core–shell nanocomposite. The mixtures were then stirred at a rate of 2 for 35 min at room temperature. Finally, the adsorbent and supernatant were separated by centrifugation (4500 rpm) at 10 min. After that, UV–vis spectroscopy (Shimadzu UV1800 spectrophotometer, RF-5301PC, Shimadzu, Kyoto, Japan, λmax: 200–800 nm) was used to determine the concentration of Lev at equilibrium. The adsorption capacity (qe) was calculated using eq S3 (Supporting Information, Sub-section 1.2).

Lastly, the adsorption thermodynamic studies were investigated by adding 7 mg of the adsorbent to 100 mL of Levofloxacin (30 mg L–1). The mixture was agitated at a stirring rate of 2 and the temperature was varied between 25 and 35 °C. Every 35 min, a 10 mL portion was sampled and centrifuged for 10 min at 4500 rpm, followed by Lev determination by UV–vis spectroscopy.

Application of the MnFe2O3/GO Core–Shell Nanocomposite to Real Water Samples

The adsorption of Lev onto the MnFe2O3/GO core–shell nanocomposite from real water samples (tap water, effluent wastewater, and influent wastewater) was investigated using the optimum conditions as described above. The samples were spiked with a 1.0 mg L–1 Lev standard solution. The water samples were collected from the Daspoort Wastewater Treatment plant (Pretoria, South Africa), and tap water was collected for the laboratory.

Reusability Studies

The reusability and the stability of the spent MnFe2O3/GO core–shell adsorbent were investigated. After the adsorption of Lev, the proposed adsorbent was treated with 25 mL of ethanol solution to desorb the adsorbate. The mixture was then centrifuged for 10 min at 4500 rpm speed to settle down the adsorbent. Then, the separation of the adsorbent was carried out using an external magnet and decantation. Furthermore, the adsorbent was dried in an oven at 60 °C for 2 h and the adoption/desorption procedure described above was repeated five times.

Results and Discussion

Brunauer Emmett Teller

Brunauer Emmett Teller (BET) measurements of the synthesized composites are presented in Table . The results demonstrated that the surface areas of GO, MnFe2O3, and the MnFe2O3/GO core–shell nanocomposite were 15.5, 7.7, and 70.7 m2 g–1, respectively. The nitrogen adsorption/desorption isotherm and size distribution of the MnFe2O3/GO core–shell nanocomposite are shown in Figure a,b. As seen in these figures, the nitrogen adsorption/desorption of MnFe2O3/GO core–shell nanocomposite exhibited a type IV isotherm, which is of the H3 hysteresis loop, as demonstrated in Figure a.[34] Furthermore, Figure b shows the average pore size distribution of the MnFe2O3/GO core–shell nanocomposite at around 30 nm, suggesting that the material was mesoporous in nature.

Table 2

BET Results of GO, MnFe2O3/GO, and the MnFe2O3/GO Core–Shell Nanocomposite

material	surface area (m2g–1)
GO	15.5
MnFe2O3	7.70
MnFe2O3/GO nanocomposite	70.7

Figure 1

(A) Nitrogen adsorption isotherm curve of the MnFe2O3/GO core–shell nanocomposite and (B) pore size distribution of the MnFe2O3/GO core–shell nanocomposite.

Fourier Transform Infrared Spectroscopy

Fourier transform infrared (FTIR) spectra of GO, MnFe2O3, and the MnFe2O3/GO core–shell nanocomposite are shown in Figure a–c. The GO peak was explored at 3293 cm–1 due to O–H stretching vibrations. The less sharp intensity peaks at 1727 and 1624 cm–1 were assigned to the stretching vibrations of the C=O and C=C groups, respectively, as shown in Figure a.[4,35] The peaks at 1036 and 665 cm–1 were attributed to the alkoxy C–O stretching and O–H bending vibrations.[36]Figure b shows the appearance of a low-intensity peak at 1620 cm–1, which is based on the asymmetric stretching vibration of C=O. The broad band at 3314 cm–1 was ascribed to the O–H group and a sharp peak at 660 cm–1 was attributed to the Fe–O bond in MnFe2O3. The MnFe2O3/GO core–shell nanocomposite showed low-intensity peaks due to the incorporation of MnFe2O3 to form a core–shell, as shown in Figure c.[36,37] These results confirm the successful synthesis of the nanocomposite.

Figure 2

FTIR spectra of (a) GO, (b) MnFe2O3, and (c) the MnFe2O3/GO core–shell nanocomposite.

Powder X-ray Diffraction Spectroscopy

The crystalline structures of GO, MnFe2O3, and the MnFe2O3/GO nanocomposite were, respectively, investigated by powder XRD, and representative diffraction patterns are shown in Figure a–c. The intense and sharp diffraction peaks in the 2θ of 12.7° (001) and 26.8° (002) with interlayer distances of 0.726 and 0.213 nm correspond to graphene oxide, respectively. These peaks were attributed to the oxygenated functional groups and the amorphous nature of GO (see Figure a).[23,38,39]Figure b shows the diffraction patterns of the MnFe2O3 nanoparticles, which show highly intense and sharp peaks corresponding to rhombohedral (MnFe2O3) [ICDD: 04-011-9587]. Moreover, MnFe2O3/GO core–shell nanocomposites showed less-intense and sharp diffraction peaks, and subsequently, the GO disappeared due to MnFe2O3 nanoparticles, which were successfully coated with GO, as shown in Figure c. The Fe (104) crystalline plane was used to calculate the crystallite average particle size of MnFe2O3 nanoparticles and MnFe2O3/GO core–shell nanocomposites according to the Debye–Scherrer equation as followswhere d is the average particle size in nm, λ is the wavelength of the X-ray used, β1/2 is the width of the diffraction peak at half height in radians, and θ is the angle peak maximum position in degrees. The obtained MnFe2O3 and MnFe2O3/GO core–shell nanocomposite sizes were 16.5 and 17.5 nm, respectively.

Figure 3

XRD patterns of (a) GO, (b) MnFe2O3, and (c) the MnFe2O3/GO core–shell nanocomposite.

Scanning Electron Microscopy

The morphology, elemental composition, and mapping of the synthesized materials (GO, MnFe2O3, and MnFe2O3/GO core–shell nanocomposite) were further investigated using a scanning electron microscope (SEM) coupled with an energy-dispersive spectrometer (EDS), as demonstrated in Figure a–c. The obtained results shown in Figure a indicate the formation of GO sheets. The formation of agglomerated spherical nanoparticles was observed, as shown in Figure b, confirming the interaction of Mn and Fe2O3. Furthermore, Figure c shows spherical MnFe2O3 nanoparticles supported with GO sheets, suggesting the formation of the core–shell-like structure of MnFe2O3/GO. These findings were compared with those reported in the literature.[32] In addition, the energy-dispersive spectroscopy (EDS) spectrum, as demonstrated in Figure d, confirms the successful synthesis of the MnFe2O3/GO core–shell nanocomposite due to the presence of Mn, Fe, O, and C signals. Lastly, elemental mapping (Figure e) was investigated to study the elemental distribution, as shown by different colors. As observed, the synthesized MnFe2O3 nanoparticles were uniformly distributed on the surface of GO sheets.

Figure 4

SEM images of (a) GO, (b) MnFe2O3, and (c) the MnFe2O3/GO core–shell nanocomposite. (d) EDX and (e) elemental mapping for the MnFe2O3/GO core–shell nanocomposite.

Transmission Electron Microscopy

The morphology and particle size distribution of the GO, MnFe2O3, and MnFe2O3/GO core–shell nanocomposite were investigated using a transmission electron microscope (TEM), as demonstrated in Figure a–c. The thin multilayer sheets of GO were observed in the TEM image shown in Figure a. The diameter of the aggregated MnFe2O3 nanoparticles ranges up to 389 nm, as shown in Figure b. The TEM image (Figure c) captured for the MnFe2O3/GO core–shell nanocomposite showed well-dispersed MnFe2O3 nanoparticles, supported by the score–shell GO sheets, which also act as a reducing agent. It is worth mentioning that Figure b,c confirms the formation of core–shell structures for MnFe2O3 and MnFe2O3/GO, respectively, with different shell thicknesses.[40,41] Additionally, the effect of reducing agent (GO) was also observed in BET results, where MnFe2O3 supported by GO demonstrated an increase in the surface area from 7.7 to 70.7 m2 g–1, causing a diameter decrease of the composite to approximately 20 nm average particle size (see Table ). It is worth mentioning that the average particle size of the composite is in line with the results obtained from P-XRD.

Figure 5

TEM images of (a) GO, (b) MnFe2O3, and (c) the MnFe2O3/GO core–shell nanocomposite.

X-ray Photoelectron Spectroscopy

X-ray photoelectron spectroscopy (XPS) was used to investigate the binding energies and oxidation states of elements in the MnFe2O3/GO core–shell nanocomposite. The survey spectrum showed the various elements (C, O, Fe, and Mn), as shown in Figure . The high-intensity peak that appeared at 286.09 eV corresponds to the binding energy of C 1s elements in GO.[10] The O 1s peak was observed at 529.89 eV, and it resulted from MnFe2O3 and GO.[42] The peaks located at 710.26 and 688.49 eV were attributed to Fe 2p and Fe 2s elements, respectively, in the Fe2O3 nanoparticles.[32] The very low intensity peaks at around 637.93 and 662.02 eV were attributed to Mn 2p3/2 and Mn 2p1/2, respectively, and were expanded, as shown in Figure .[32] Furthermore, the appearance of the small peaks at 59.08 eV for Mn 3p and 95.15 eV for Mn 3s suggested the successful doping of Fe2O3/GO by Mn.[43]

Figure 6

XPS survey spectrum of Mn/Fe2O3/GO core–shell nanocomposite.

Optimization Using a Response Surface Approach

The effect of influential parameters (adsorbent dosage, stirring rate (SR), and pH) on the adoptive removal of Lev from aqueous solutions were optimized using the response surface methodology (RSM) based on a central composite design (CCD). The Pareto charts generated from the analysis of variance (ANOVA) were used to view the significance of individual parameters, as shown in Figure S1. Furthermore, the red line of the Pareto chart signifies a 95% confidence limit, whereas the bars characterize the experimental parameters that were optimized. As shown in the Pareto charts, examined factors and their interaction were insignificant for the adsorption of Lev by the MnFe2O3/GO core–shell nanocomposite, as the bars did not cross the 95% confidence limit, except for the adsorbent dosage.[44] Moreover, the smaller the dosage of the adsorbent, the higher the adsorption capacity. It can be seen in Figure S1 that the increase in the experimental stirring rate increases the adsorption capacity of Lev.

A three-dimensional (3D) response surface approach (Figure ) has been used to study the effect of interaction between two variables to achieve maximum adsorption capacity. Figure a,c demonstrated that an increase in the dosage of the adsorbent decreases the adsorption capacity. Moreover, the pH between 6 and 8 increases the adsorption (Figure b,c). This might be influenced by the pKa values of Lev and the positively charged adsorbent (Figure S2). According to the literature, Lev has two acid dissociation constants, that is, pKa1 = 6.02 (tertiary amine) and pKa2 = 8.15 (carboxylic acid).[22] This suggests that between pKa1 and pKa2, Lev exists as a zwitterion species,[45] and the adsorption process increased due to electrostatic interactions between the positive surface and anionic species of Lev. At pH values below pKa1, the %RE was lower due to electrostatic repulsion between the positively charged adsorbent and protonated amine groups of Lev molecules.[45] It is worth mentioning that the experimental stirring rate was directly proportional to the maximum adsorption capacity.

Figure 7

3D plots for the interaction of optimal parameters ((a) dosage and stirring rate; (b) pH and stirring rate; and (c) pH and dosage) for adsorption of Lev onto the MnFe2O3/GO core–shell nanocomposite.

Furthermore, the desirability function was used for the simultaneous optimization of independent factors (dosage, pH, and starring rate). This desirability function predicted responses that changed into a dimensionless desirability value (D).[44,46] As reported in the literature, the desirability scale ranges from 0 (undesired response) to 1 (desired response).[47,48] In this study, the optimal values for the reported independent factors (experimental speed, dosage of the adsorbent, and Lev pH) were attained using D ≈ 1.0. As demonstrated in Figure S3, the corresponding optimal conditions for the adsorption of Lev were 4, 7 mg, and 7 for experimental stirring rate, adsorbent dosage, and pH, respectively. Therefore, the achieved optimal conditions were confirmed experimentally to investigate the validity of the RSM model and the maximum adsorption capacity of 46.1 mg g–1 was obtained. Lastly, the obtained results were in agreement with the predicted values at a 95% confidence level.

Adsorption Kinetics

The effect of contact time was investigated to confirm the complete equilibrium between Lev and the MnFe2O3/GO core–shell nanocomposite. Therefore, the kinetic studies were performed at three different concentration levels (10, 20, and 30 mg L–1) of Lev, and the results are presented in Figure S4. As shown in these graphs, the adsorption capacity at 10 mg L–1 drastically increased with time (from 10 to 35 min) and the Lev adsorption equilibrium was reached at 40 min due to the saturation of the available sites for adsorption.[49] The adsorption capacity of the adsorbent at 20 mg L–1 Lev significantly increased with time (10–20 min) and a plateau was reached from 25 to 35 min. Moreover, the adsorption capacity at 30 mg L–1 Lev increased rapidly with increasing time from 10 to 15 min. Further increase in time led to the attainment of adsorption equilibrium. Therefore, the kinetic studies showed that adsorption equilibrium was reached faster at higher adsorbate concentrations.

To investigate the adsorption rate and mechanism for Lev onto the MnFe2O3/GO core–shell nanocomposite, the kinetic data were fitted to the pseudo-first-order and pseudo-second-order parameters, as shown in Figures S5 and S6. The linear equations (eqs S4–S6, Supporting Information, Sub-section 1.2) were used to study these models, and the attained pseudo-first-order and pseudo-2nd-order kinetic parameters are demonstrated in Table . It can be seen that the obtained data evidently show a good fit for the pseudo-second-order model for all Lev tested concentrations, as confirmed by the higher regression coefficient values (R2 > 0.993) when compared to the pseudo-first-order model, suggesting an electrostatic interaction mechanism.[2,50] Additionally, the calculated adsorption capacity (qe) values of the pseudo-second-order kinetic model were closer to the experimental qe values. These findings suggest that the overall Lev adsorption was dominated by the electrostatic processes involving ion exchange forces, in accordance with the assumption of the pseudo-second-order kinetic model.[2]

Table 3

Parameters for the Kinetic Models of Lev Adsorbed on the MnFe2O3/GO Core–Shell Nanocomposite

concentration (ppm)	10	20	30
pseudo-first-order
qe (mg g–1)	72.4	38.3	22.3
k1 (min–1)	0.0012	0.0018	0.0944
R2	0.8805	0.8043	0.8993
pseudo-second-order
qe (mg g–1)	46.1	78.7	117.6
k2 (g mg–1 min)	0.024	0.040	0.0064
R2	1.0000	1.000	0.9994
intraparticle diffusion
Kid1 (mg g–1 min1/2)	1.16	1.08	8.07
C1 (mg g–1)	38.7	73.0	75.7
R12	0.9993	0.9975	0.9980
Kid2 (min–1)	0.35	0.95	1.53
C2 (mg g–1)	42.8	73.5	105
R22	0.9217	0.9848	0.9540

Furthermore, the intraparticle diffusion model was used to investigate the rate-limiting step in the adsorption process. According to previous studies, the intraparticle diffusion rate-determining step is a plot of Lev adsorbed against the square root of the contact time to produce a straight line.[51,52] The kinetic data were fitted to the intraparticle diffusion model (eq S7, Supporting Information, Sub-section 1.2), and the results are presented in Table and Figure S7. The latter revealed that the plot did not pass through the origin but instead exhibited multilinearity, suggesting that intraparticle diffusion was not the only rate-determining step.[51,52] The first steeper linear portion represents the rapid external surface adsorption, while the second portion represents the steady adsorption of Lev in which intraparticle diffusion is the rate-determining step. As seen in Table , the intraparticle diffusion rate constant, kid, for the adsorption of Lev onto the MnFe2O3/GO core–shell nanocomposite decreased while the intercept, C, increased.

Adsorption Isotherms of Lev onto the MnFe2O3/GO Core–Shell Nanocomposite

Adsorption isotherms were used to investigate the interaction of Lev with the MnFe2O3/GO core–shell nanocomposite. Adsorption methods can be validated by the adsorption isotherms, which are identified as the isothermal equilibrium that illustrate the amount of adsorbate adsorbed per unit mass of adsorbent.[53] Thus, the equilibrium adsorption isotherm of Lev onto the MnFe2O3/GO core–shell nanocomposite is shown in Figure . As shown in this figure, it can be seen that the amount of Lev adsorbed drastically increased with increasing equilibrium concentration (Ce), and the maximum adsorption capacity was 129.9 mg g–1.

Figure 8

Isotherm plots for the adsorption of Lev on the MnFe2O3/GO core–shell nanocomposite. Experimental conditions: sample pH (7), contact time (10–40 min), and dosage (0.007 g).

Six isotherm models, Langmuir, Freundlich, Redlich-Peterson, Sips, Dubinin-Radushkevich, and Temkin (eqs S8–S15, Supporting Information, Sub-section 1.2), were investigated, and the parameter values calculated from linear plots (Figure ) are presented in Table . The adsorption of Lev showed a higher correlation coefficient value for the Langmuir model when compared to the Freundlich model. The obtained results demonstrated that Lev was adsorbed on a monolayer surface, suggesting electrostatic interactions.[2,54]

Table 4

Fitting Parameters for the Isotherm Models of Lev Adsorbed on the MnFe2O3/GO Core–Shell Nanocomposite

isotherm model	parameter	value
Langmuir	KL	1.03
	qmax (mg g–1)	130
	R2	0.9964
Freundlich	KF (mg g–1)	64.4
	N	4.4
	R2	0.9765
Dubinin-Radushkevich	qmax (mg g–1)	122
	E (kJ mol–1)	8.45
	β	2.00 × 10–7
	R2	0.9321
Temkin	bT (kJ mol–1)	90.6
	AT (L g–1)	0.94
	R2	0.9840
Redlich-Peterson	KR	25.0
	αR	0.065
	βR	1.3
	R2	0.9901
Sips	KS (L mg–1)	0.93
	qs (mg g–1)	133
	R2	0.9961
	ns	1.1

The Dubinin-Radushkevich isotherm was studied to differentiate the physical and chemical adsorption of Lev with a Gaussian energy distribution (E) onto the MnFe2O3/GO core–shell nanocomposite surface.[55] It is known that adsorption can be considered as physical if the calculated value of E is below 8 kJ mol–1 and chemical if the value is in the range of 8–16 kJ mol–1.[56] Moreover, according to the calculated E value (see Table ), the adsorption can be confirmed by characterization of chemisorption and through electrostatic interactions.

Another model (Redlich-Peterson isotherm) is the improved version of Langmuir and Freundlich models and can be used either in homogeneous or heterogeneous systems due to its flexibility.[57] Therefore, Table shows that if the exponent βR tends to zero, it confirms the Freundlich isotherm model, whereas if the βR value is close to 1, it obeys the Langmuir isotherm.[49] Furthermore, based on the obtained βR value for the adsorption of Lev onto the MnFe2O3/GO nanocomposite, it can be confirmed that the adsorption favors the Langmuir isotherm model.

The Sips isotherm was used to investigate the monolayer adsorption of Lev; when ns = 1, the adsorption confirms the Langmuir model, whereas if ns > 1, it confirms the Freundlich model.[56] Moreover, according to the obtained data-fitting values demonstrated in Table , it can be noted that ns = 1 confirms the Langmuir model.

The Temkin isotherm was used to investigate the effects of indirect adsorbate/adsorbent interactions on the adsorption system. Additionally, it can be assumed that the heat of adsorption (ΔHads) of all particles in the layer decreases linearly with increasing surface coverage.[58] In this study, the Temkin isotherm was fitted to confirm that the adsorptions of Lev onto the MnFe2O3/GO core–shell nanocomposite follow a chemisorption process. Furthermore, based on the obtained bT value, as presented in Table , it can be confirmed that the adsorption was exothermic, suggesting that the sorption process was driven by the electrostatic interaction between the adsorbent and Lev. Finally, the fitting degree of these models was evaluated by comparing their correlation coefficiencies (R2), and the following sequence was observed: Langmuir = Sips > Redlich-Peterson > Temkin > Freundlich > Dubinin-Radushkevich.

Adsorption Thermodynamics of Lev onto the MnFe2O3/GO Core–Shell Nanocomposite

The adsorption isotherm of Lev onto the MnFe2O3/GO core–shell nanocomposite at different temperatures (25, 30, and 35 °C) is shown in Figure S8. The standard Gibbs free energy was calculated using eq S14, to describe the thermodynamic behavior of the Lev onto the MnFe2O3/GO core–shell nanocomposite. Based on the obtained results of ΔG (−8.78, −6.15, and −3.28 kJ mol–1), the adsorption was spontaneous. Furthermore, the negative sign on the ΔH values shows that the adsorption process was exothermic in nature. The ΔS and ΔH values were found to be 0.55 kJ mol–1 K–1 and 172.7 kJ mol–1, respectively, indicating that the adsorption process favors electrostatic interactions.[59]

Adsorption Mechanism of Lev onto the MnFe2O3/GO Core–Shell Nanocomposite

The FTIR spectra of the MnFe2O3/GO core–shell nanocomposite before and after adsorption of Lev are shown in Figure . In Figure b, the MnFe2O3/GO core–shell nanocomposite showed a broad peak at 3425 cm–1, which was ascribed to the O–H stretching vibration of O–H, while the peaks at 1620 and 660 cm–1 were assigned to C=C and Fe–O vibrations in the MnFe2O3/GO core–shell nanocomposite. By contrast, the O–H peak of the nanocomposite before adsorption was broader compared to MnFe2O3/GO-Lev (after adsorption, Figure c) and a slight shift was observed. This might be due to the reactions between the terminal hydroxyl groups and the Lev zwitterion molecule (hydrogen bonding formation). The O–H stretching vibrations for the carboxylic group at 3270 cm–1 for Lev (Figure a) disappeared because the Fe from the adsorbent combined with the O atom of the carboxyl group, thus forming a bridging bidentate complex. The peak around 1719 cm–1 in Figure a, which represents the C=O of the carboxylic group in the Lev molecule, disappeared after adsorption, illustrating that C=O was involved in the formation of the bridging bidentate complex between the Fe atom and carboxylate.[52] Furthermore, other peaks changed after adsorption by shifting upward or downward, while a reduction in the intensity of other bands was observed. These observations confirmed that the adsorption of Lev on MnFe2O3/GO was driven by electrostatic interaction processes such as a result of hydrogen bonding, complexation, and electrostatic interactions as well as the possibility of fluoride ion of Lev replacing one of the hydroxyl groups of the nanocomposite.[52]and

Figure 9

FTIR spectra of (A) Lev (B) the MnFe2O3/GO core–shell nanocomposite before and (C) after adsorption of Lev.

Comparison Studies

The maximum adsorption capacity of the synthesized MnFe2O3/GO core–shell nanocomposite was compared with other literature-reported adsorbents, as shown in Table . Most of the adsorbents listed in Table showed lower maximum adsorption capacity as compared to 133 mg g–1 of the proposed adsorbent.[34,60−63] However, there is only one study that reported a higher maximum adsorption capacity (181 mg g–1), but with poor correlation coefficiency (R2 = 0.981).[64] Therefore, the data tabulated in Table confirms that the MnFe2O3/GO core–shell nanocomposite is one of the excellent adsorbents for the adsorptive removal of Lev contaminants from the aqueous solution.

Table 5

Comparison of the Adsorption Capacities of Levofloxacin with the MnFe2O3/GO Core–Shell Nanocomposite with Other Nanocomposites

nanocomposite	adsorption capacity (mg g–1)	R2	references
Co-MCM-41	120	0.985	(60)
Fe/Mn-BC	181	0.981	(64)
zeolite	35.5	0.9928	(61)
CaO/MgO	107	0.98	(34)
MOFs, MIL-100 (Fe)	87.3	0.9109	(62)
Fe3O4@SiO2	6.85	0.99	(63)
MnFe2O3/GO core–shell	133	0.9964	this work

Application of the MnFe2O3/GO Core–Shell Nanocomposite in Real Water Samples

The feasibility of using the MnFe2O3/GO core–shell nanocomposite as an adsorbent for the adsorptive removal of Lev was explored in real water samples, including tap, river, effluent wastewater, and influent wastewater. These water samples were spiked with known amounts of Lev, and the adsorption efficiencies are shown in Table . From this table, it can be observed that the general uptake of Lev was between 96 and 97% for both river and tap water samples. However, there was a slight decrease (93%) in adsorption when effluent wastewater was tested, especially in influent wastewater (86%). The lower adsorption efficiency in wastewater samples may be due to the occurrence of other contaminants (organic and inorganic) since real water samples have different physicochemical properties as compared to model laboratory water. Moreover, a decrease in the adsorptive removal of Lev from wastewater samples can be caused by high concentration levels of the investigated analyte, when compared to river and tap water samples. Similar observations were also reported elsewhere.[65] Therefore, the attained results indicated that the MnFe2O3/GO core–shell nanocomposite can be applicable in the adsorptive removal of Lev in real water samples.

Table 6

Removal of Lev in Spiked Real Water Samples (Tap Water, Effluent Wastewater, and Influent Wastewater)a

samples	unspiked	spiked with 1.0 mg L–1	after adsorption	% recovery
concentration (mg L–1)
tap water	ND	0.991 ± 0.003	0.027 ± 0.005	97.3 ± 1.5
river water	ND	0.993 ± 0.005	0.036 ± 0.004	96.4 ± 1.4
WW effluent	0.010 ± 0.001	1.003 ± 0.004	0.068 ± 0.003	93.2 ± 1.6
WW influent	0.231 ± 0.009	1.224 ± 0.006	0.176 ± 0.005	85.6 ± 2.1

ND = not detected.

Reusability Study

Reusability studies or regeneration capacity of the MnFe2O3/GO core–shell nanocomposite was crucial to investigate the possibility of applying the proposed system on a large scale. The reusability studies of the MnFe2O3/GO core–shell adsorbent for the removal of Lev in aqueous media are shown in Figure . From this figure, it can be observed that the adsorption of the proposed adsorbent slightly decreased from 131 to 115 mg g–1 after five regeneration cycles. This means that the Lev removal efficiency by MnFe2O3/GO was nearly 88% in the fifth cycle. In overall conclusion, the proposed adsorbent showed remarkable regenerative properties for Lev removal as the removal efficiency was more than 80% for the fifth cycle.

Figure 10

Reusability of the MnFe2O3/GO core–shell nanocomposite for adsorption of Lev.

Safe Disposal of Spent Adsorbent

Disposal of spent adsorbents containing organic contaminants poses secondary environmental pollution, which could cause ecological and social risks.[66] Therefore, the safe disposal of spent Mn-doped Fe2O3/GO core–shell nanocomposite could only be done by discharging into the landfills or dumping sites after complete desorption of Lev using ethanol. The ethanol effluent can then be treated with oxidants such as peroxide and ozone to further reduce the concentration of Lev. Additionally, the presence of catalytic components such as Mn and Fe in the current adsorbent suggests that the overall adsorbent can be used as a catalyst for the degradation of Lev. Therefore, the regeneration of the spent adsorbent could be achieved by combining adsorption and photocatalytic processes before disposal in landfills. Other methods that could be explored include incineration of the spent adsorbent at high temperatures.[67]

Conclusions

In this study, a MnFe2O3/GO core–shell nanocomposite was successfully synthesized by a one-step hydrothermal method and further characterized with TEM/EDS, SEM, P-XRD, XPS, and FTIR. The adsorbent was used for the adsorption of Lev from aqueous solution. Under optimal conditions, the adsorption kinetics well fitted the pseudo-second-order parameter. The adsorption isotherm data best fitted the Langmuir model and was further supported by Dubinin-Radushkevich, Sips, and Temkin models. Moreover, the maximum adsorption capacity for Lev was found to be 129.9 mg g–1. The adsorbent was applied to real water samples and the percentage recoveries ranging from 85.6 to 97.3% were obtained. The synthesized MnFe2O3/GO core–shell nanocomposite could be reused for up to five cycles without losing its adsorption performance and easily separated. Lastly, the thermodynamics parameters of the MnFe2O3/GO core–shell nanocomposite showed that the adsorption system was spontaneous and exothermic, confirming the electrostatic interaction process.