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. 2023 Feb 10;8(7):6337–6348. doi: 10.1021/acsomega.2c06555

Arabic Gum-Grafted-Hydrolyzed Polyacrylonitrile@ZnFe2O4 as a Magnetic Adsorbent for Remediation of Levofloxacin Antibiotic from Aqueous Solutions

Fereshte Hassanzadeh-Afruzi †, Farhad Esmailzadeh †, Golnaz Heidari †, Ali Maleki †,*, Ehsan Nazarzadeh Zare ‡,*
PMCID: PMC9947993  PMID: 36844579

Abstract

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The Arabic gum-grafted-hydrolyzed polyacrylonitrile/ZnFe2O4 (AG-g-HPAN@ZnFe2O4) as organic/inorganic adsorbent was obtained in three steps using grafted PAN onto Arabic gum in the presence of ZnFe2O4 magnetic nanoparticles and then hydrolysis by alkaline solution. Fourier transform infrared (FT-IR), energy-dispersive X-ray analysis (EDX), field emission scanning electron microscopy (FESEM), X-ray diffraction (XRD), thermogravimetric analysis (TGA), vibrating sample magnetometer (VSM), and the Brunauer–Emmett–Teller (BET) analysis analyses were used to characterize the chemical, morphological, thermal, magnetic, and textural properties of the hydrogel nanocomposite. The obtained result demonstrated that the AG-g-HPAN@ZnFe2O4 adsorbent showed acceptable thermal stability with 58% char yields and superparamagnetic property with magnetic saturation (Ms) of 24 emu g–1. The XRD pattern showed that the semicrystalline structure with the presence of ZnFe2O4 has distinct peaks which displayed that the addition of zinc ferrite nanospheres to amorphous AG-g-HPAN increased its crystallinity. The AG-g-HPAN@ZnFe2O4 surface morphology exhibits uniform dispersion of zinc ferrite nanospheres throughout the smooth surface of the hydrogel matrix, and its BET surface area was measured at 6.86 m2/g, which was higher than that of AG-g-HPAN as a result of zinc ferrite nanosphere incorporation. The adsorption effectiveness of AG-g-HPAN@ZnFe2O4 for eliminating a quinolone antibiotic (levofloxacin) from aqueous solutions was investigated. The effectiveness of adsorption was assessed under several experimental conditions, including solution pH (2–10), adsorbent dose (0.0015–0.02 g) contact duration (10–60 min), and initial concentration (50–500 mg/L). The maximum adsorption capacity (Qmax) of the produced adsorbent for levofloxacin was found to be 1428.57 mg/g (at 298 k), and the experimental adsorption data were well explained by the Freundlich isotherm model. The pseudo-second-order model satisfactorily described the adsorption kinetic data. The levofloxacin was mostly adsorbed onto the AG-g-HPAN@ZnFe2O4 adsorbent via electrostatic contact and hydrogen bonding. Adsorption–desorption studies demonstrated that the adsorbent could be efficiently recovered and reused after four consecutive runs with no significant loss in adsorption performance.

1. Introduction

Water scarcity is one of the major issues facing societies today. The quality of the water resources that are currently available is deteriorating as a result of the expansion of industrial activity, ineffective management of municipal waste, and disregard for water resource management.1−5 Antibiotics, for example, have been extensively developed and used to treat bacterial infections in both veterinary and human medicine. Although antibiotics are vital components of medicine, their release into environments and water sources is linked to serious adverse consequences, even at low doses.6−9 Ciprofloxacin, levofloxacin, and pefloxacin are only a few examples of quinolone antibiotics that have low metabolic stability and release a lot of basic chemicals into the ecosystem.10,11 The presence of these antibiotics in soil or water resources has detrimental health consequences, including allergies, nausea, vomiting, diarrhea, acute renal failure, anxiety, and tremors. Additionally, antibiotic residues have a considerable negative influence on both the rates of algal growth and the photosynthetic processes of aquatic species.12−15 Also, their discharge into the environment increases the resistance of bacterial infections to widely used medications.16−18 Levofloxacin, a common fluoroquinolone that has been widely used to treat serious bacterial infections, is recognized as a harmful contaminant with irreversible effects in freshwater systems. Therefore, it is vital to establish an efficient technique to eliminate them from the aquatic habitat.19,20

Quinolone antibiotics are removed from water via a variety of ways, including biological processes, oxidation, osmosis, ion exchange, and adsorption. Among these methods, adsorption is rapidly becoming the most often used method.21 It not only uses fewer chemicals and solvents but also has a wider range of concentrations, making it a lot more efficient and an environmentally friendly solution. Thus, a variety of micro/nanostructures were considered suitable adsorbents for the efficient removal of levofloxacin residuals from water, as either single phases or composites.22−27 Biopolymer-based composites have lately been employed often as an environmentally benign adsorbent to remove water pollutants due to their biodegradability, availability, fair cost, and low toxicity to biological systems.28,29 Tragacanth gum, ghatti gum, xanthan gum, karaya gum, and guar gum have all been employed in the fabrication of natural polymer hydrogels. These natural polymers could capture contaminants effectively in a three-dimensional (3D) network. Natural polymers enhance adsorption efficiency by increasing the contact between pollutants and the surface functional groups of the hydrogel.30

Arabic gum (AG), commonly known as acacia gum, is derived from specific Arabic tree species and is biocompatible, stable, inexpensive, and widely accessible.31 Depending on the source, it could have relatively variable physicochemical characteristics and a distinct composition proportion of its constituent parts. Typically, AG is composed of 97% polysaccharides and 3% protein units. Galactopyranose units connected by a (1,3)-glycosidic bond make up the main chain of AG. The linear chain and the branches are joined by 1,6 glycosidic connections, with the branches typically consisting of two to five 1,3-linked galactopyranosyl units. In addition to galactose, the major backbone and branch composition also include the monomers of methyl glucuronic acid, glucuronic acid, rhamnose, and arabinose.32,33 Meanwhile, reactive functional groups like hydroxyl and carboxylic acid on its polymeric chain offer distinctive chemical changes that could well be employed in a range of eco-friendly processes. The modification of natural polymer-based hydrogel adsorbents with other compounds, on the other hand, is one approach for enhancing their adsorption ability. A more active adsorption site for efficient interaction with the desired contaminant and enhanced specific surface area could be developed by designing the natural polymer-based hydrogels grafted by vinyl monomers and adding magnetic nanoparticles to their matrix.34,35

Here, a nanocomposite adsorbent was prepared based on Arabic gum-grafted-hydrolyzed polyacrylonitrile and ZnFe2O4 magnetic nanoparticles for effective removal of levofloxacin from aqueous solution. The existence of various functional groups, including hydroxyl, amide, and carboxylate, could effectively interact with levofloxacin and can eliminate it from water.

2. Experimental Section

2.1. General Information

Compounds: Arabic gum (Sigma-Aldrich, CAS no. G9752), zinc chloride (Sigma-Aldrich, powder, ≥99.995%), ammonium persulfate ((NH4)2S2O8, Sigma-Aldrich, ≥98.0%), acrylonitrile (AN, CH2=CHCN, Sigma-Aldrich, ≥99%), N,N′-methylenebis(acrylamide), bis-acrylamide (MBA, (H2C=CHCONH)2CH2), Sigma-Aldrich, 99%), sodium hydroxide (NaOH, Sigma-Aldrich, reagent grade, ≥98%, pellets (anhydrous)), ferric chloride hexahydrate (FeCl3·6H2O, ACS reagent, 97%), ammonium acetate (Merck, ACS reagent, ≥97%), acetone (CH3COCH3, Merck, ACS reagent, ≥99.5%), ethylene glycol (Merck, ReagentPlus, ≥99%), ethanol (Sigma-Aldrich, 96%). A Shimadzu 8400 S (Japan) spectrometer was used to record Fourier transform infrared (FT-IR) spectra utilizing a KBr pellet (a part of the powder samples was mixed with KBr and pressed into pellets, and the corresponding spectra were recorded at room temperature). Energy-dispersive X-ray analysis (EDX) was used for elemental detection of the adsorbent by a Numerix JEOL-JDX 8030 spectrometer (Japan), operated at 30 kV, 20 mA. X-ray diffraction (XRD) patterns were recorded on a Bruker D8 Advance (Billerica, USA) X-ray diffractometer with a Cu Kα anode (λ = 0.1542 nm) operating at 40 kV and 30 mA. Field emission scanning electron microscopy (FESEM) images were recorded on a KYKY-EM8000 (China) device. The magnetic behavior of nanostructured magnetic materials was measured by vibrating sample magnetometer (VSM) analysis (Meghnatis, Daneshpajooh Kashan Company, Iran). The thermal gravimetric analysis (TGA) measurement was performed by a TGA BAHR-STA 504 (USA) instrument. The N2 adsorption–desorption was conducted by BET analysis on a micrometric ASAP 2010 (USA) instrument. The concentration of levofloxacin in the solutions was determined using UV–visible spectroscopic techniques (Shimadzu, 2550, 220 V, Japan).

2.2. Synthesis of Zinc Ferrite Nanospheres

The zinc ferrite nanospheres were synthesized utilizing a solvothermal procedure.36 1.092 g of FeCl3·6H2O (4 mmol), 0.27 g of ZnCl2 (2 mmol), and 70 mL of ethylene glycol were combined to create a clear solution. After that, 2.312 g of ammonium acetate (30 mmol), as a protective agent, was added to the solution and stirred vigorously at 600 rpm for 30 min at room temperature by a magnetic stirrer. This resulted in a yellowish-dark mixture. The mixture was later put in a 100 mL Teflon-lined stainless-steel autoclave at 210 °C for 48 h. The black precipitate was collected by magnetic field decantation and washed many times with ethanol and distilled water. Lastly, the final component was dried at 60 °C for 12 h in a vacuum oven.

2.3. Preparation of Arabic gum-grafted-hydrolyzed PAN@ZnFe2O4 (AG-g-HPAN@ZnFe2O4) Nanocomposite

The nanocomposite was prepared according to our previous work with slight modifications.35 A solution of AG (1 g) in 100 mL of distilled water was heated to 50 °C and stirred until the colorless solution was visible. The solution was then degassed for 15 min using nitrogen gas. Within 30 min, the (NH4)2S2O8 solution (1 mmol/50 mL) was added dropwise to the above solution and the solution was kept at 55 °C for 45 min. After that, 6 mL of AN (91.59 mmol) was gradually added to the mixture and left to continue for another 40 min. Then, 1.0 g of premade zinc ferrite and 1.6 g of MBA as a cross-linking agent were added to the polymerization solution. Over 16 h, the reaction was held at 55 °C. The AG-g-PAN@ZnFe2O4 nanocomposite precipitate (in hydrogel form) was obtained by adding acetone (250 mL) as an antisolvent to the mixture. After 3 h, the magnet-collected hydrogel was rinsed three times with deionized water and acetone before being dried in a vacuum oven at room temperature. For the preparation of Arabic gum-grafted-hydrolyzed PAN@ZnFe2O4 (AG-g-HPAN@ZnFe2O4) nanocomposite hydrogel, in a round-bottom flask, 0.5 g of AG-g-PAN@ZnFe2O4 was mixed with 50 mL of 15% NaOH at 55 °C for 16 h. The AG-g-HPAN@ZnFe2O4 was then washed multiple times with deionized water and dried at room temperature. The preparation route of the AG-g-HPAN@ZnFe2O4 hydrogel was depicted in Figure 1.

Figure 1.

Figure 1

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Schematic pathway for the fabrication of the AG-g-HPAN@ZnFe2O4 hydrogel nanocomposite.

2.4. Adsorption Experiments

Some tests were conducted to evaluate the capacity of the AG-g-HPAN@ZnFe2O4 hydrogel nanocomposite to remove levofloxacin from aqueous solutions. The influence of key factors on adsorption capacity, such as solution pH, adsorbent quantity, contact duration, and starting levofloxacin concentration in an aqueous solution, was investigated. HCl (0.1 N) and NaOH (0.1 N) were used to adjust the pH from 2 to 8. Following that, different quantities of adsorbent (0.015–0.02 g), contact times (10–60 min), and initial levofloxacin concentrations (50–500 mg/L) were tested to determine the best adsorption conditions. Additionally, the adsorption isotherms were investigated by contrasting the results of the experimental data with those indicated by the Freundlich and Langmuir models. The pseudo-first-order and pseudo-second-order models were also used to evaluate the adsorption kinetics. The experimental tests were conducted three times, and the findings were given as an average. Levofloxacin concentration was measured using a UV–visible spectrometer at λmax = 288 nm (Figure S1 in the Supporting Information). The calibration curve of levofloxacin is shown in Figure S2 in the Supporting Information. Equations 1 and 2 were used to calculate the capacity and adsorption efficiency of levofloxacin onto AG-g-HPAN@ZnFe2O4 hydrogel nanocomposite, respectively.37,38

2.4. 1
2.4. 2

where Ci and Ce are the initial and the equilibrium concentration of levofloxacin in the aqueous solutions (mg/L), respectively. m is the weight of AG-g-HPAN@ZnFe2O4 adsorbent (g), and V is the solution volume (L).

2.5. Desorption and Reusability

The levofloxacin adsorbed onto AG-g-HPAN@ZnFe2O4 was submerged in ethanol and agitated at room temperature for 2 h to explore its desorption and recoverability. A magnet was then used to separate the adsorbent. Following that, the concentration of released levofloxacin in the elution medium was measured using a UV–visible spectrophotometer. The following equation was used to calculate the desorption percentage.39,40

2.5. 3

where A is the mg of the levofloxacin desorbed to the elution medium and B is the mg of the levofloxacin adsorbed on the AG-g-HPAN@ZnFe2O4 biosorbent.

3. Results and Discussion

3.1. Characterization

The functional groups of the prepared materials were confirmed using FT-IR spectroscopy. Figure 2 depicts the FT-IR spectra of (a) ZnFe2O4, (b) AG, (c) AG-g-PAN@ZnFe2O4, and (d) AG-g-HPAN@ZnFe2O4. The most significant absorption peaks of ZnFe2O4 were observed at 500 cm–1 (Fe–O bond vibrations), 1400 cm–1, and 3400 cm–1 (vibration of surface OH groups).41,42 FT-IR spectrum of AG revealed different absorption bands at 2500 to 3500 cm–1 (O–H and COOH stretching vibration), 2933 and 2898 cm–1 (C–H asymmetric and symmetric stretching vibrations), 1625 cm–1 (C=O bond of the carboxylic acid group stretching vibration), 1421 cm–1 (CH2 bending vibration), and 970–1140 cm–1 (glycosidic bridge (C–O–C) and C–O–H stretching vibration).43 The FT-IR spectrum of AG-g-PAN@ZnFe2O4 showed two extra sharp bands at 615 and 2243 cm–1 compared to AG. These bands were attributed to stretching vibrations of Fe–O in zinc ferrite particles and the CN groups of PAN grafted on the AG backbone. Because the CN groups of PAN in the AG-g-PAN@ZnFe2O4 were transformed to carboxylate the absorption band of CN groups in the FT-IR spectrum of AG-g-HPAN@ZnFe2O4 was erased.

Figure 2.

Figure 2

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FT-IR spectra of (a) ZnFe2O4, (b) AG, (c) AG-g-PAN@ZnFe2O4, and (d) AG-g-HPAN@ZnFe2O4.

The chemical composition and presence of elements in the prepared samples were examined using EDX analysis. Figure 3 depicts the spectra of (a) neat AG, (b) AG-g-PAN, (c) AG-g-HPAN, (d) ZnFe2O4, and (e) AG-g-HPAN@ZnFe2O4. The EDX spectrum of pure AG revealed C and O peaks. The EDX spectrum of AG-g-PAN reveals the N peak in addition to the C and O peaks which are related to acrylonitrile copolymerization residue on the AG chain. The same C, N, and O peaks with different percentages could be found in the EDX spectrum of AG-g-HPAN. The conversion of the nitrile functional group to carboxylate was validated by increasing the quantity of O while reducing the amount of N in AG-g-HPAN. Zn, Fe, and O were all present in the EDX spectrum of ZnFe2O4. In the EDX spectrum of AG-g-HPAN@ZnFe2O4, the C, N, O, Fe, and Zn peaks can all be detected. Furthermore, the EDX mapping images (Figure 3f) show the distribution of components in this nanocomposite hydrogel.

Figure 3.

Figure 3

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EDX analysis of (a) AG, (b) AG-g-PAN, (c) AG-g-HPAN, (d) ZnFe2O4, and (e) AG-g-HPAN@ZnFe2O4 and (f) EDX mapping of AG-g-HPAN@ZnFe2O4.

TGA analysis was done to investigate the thermal stability and breakdown process of (a) AG, (b) ZnFe2O4, and (c) AG-g-HPAN@ZnFe2O4 in the temperature range 50–800 °C with a heating rate of 10 °C/min under air atmosphere (Figure 4). In the temperature range 50–200 °C, water evaporation resulted in a slight weight loss (2%) in the TGA thermogram of ZnFe2O4. In addition, the 8% weight loss at 400 °C might be explained by dehydroxylation and decomposition of carbanions precursor for ferrite production.44 Furthermore, due to ferrite crystallization, weight loss remained exceedingly gradual up to 600 °C, resulting in ZnFe2O4’s extraordinary thermal stability with a char generation of 90% at 800 °C. According to TGA curves of AG (with 8% char yield) and AG-g-HPAN (Figure S3 in Supporting Information) with 41% char yield, the amount of the hydrolyzed PAN (HPAN) in the fabricated nanocomposite was estimated to be 33%. In addition, by comparing the TGA curves of AG-g-HPAN (with 41% char yield) and the AG-g-HPAN@ZnFe2O4 (with 58% residual weight), the amount of ZnFe2O4 can be estimated at 17%. Eventually, based on the estimated contents of ZnFe2O4 and HPAN, the amount of AG in the nanocomposite was calculated at 50%. The graft polymerization of HPAN and particularly the addition of zinc ferrite nanospheres to the hydrogel matrix boosted its heat resistance as compared to the AG thermogram. Thus, the AG-g-HPAN@ZnFe2O4 adsorbent is suitable for high-temperature reactions due to its acceptable thermal resistance and weight retention of 58%.35

Figure 4.

Figure 4

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TGA thermograms of ZnFe2O4, AG, and AG-g-HPAN@ZnFe2O4.

Field emission scanning electron microscopy was used to evaluate particle size distribution, surface morphology, and particle aggregation in prepared samples (FESEM). Figure 5 presents the FESEM of ZnFe2O4 nanospheres, AG-g-HPAN, and AG-g-HPAN@ZnFe2O4. The FESEM images of zinc ferrite (a and b) indicated a significant number of evenly formed nanosphere particles created during a hydrothermal procedure. These observed images were similar to reported FESEM images of zinc ferrite in the literature.45 The zinc ferrite nanospheres had an average diameter of 98 nm.46 In FESEM images of AG-g-HPAN, swollen spherical topologies with uniform distribution throughout the smooth surface were observed.47 They were ascribed to cross-linking, alkaline hydrolysis, and homogeneous graft copolymerization of acrylonitrile onto the AG chain. The surface morphology of the AG-g-HPAN@ZnFe2O4 adsorbent exhibits dispersion of zinc ferrite nanospheres inside the hydrogel matrix.

Figure 5.

Figure 5

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FESEM images of (a, b) ZnFe2O4, (c, d) AG-g-HPAN, and (e, f) the AG-g-HPAN@ZnFe2O4..

The magnetic properties of the AG-g-HPAN@ZnFe2O4 nanocomposite hydrogel were tested using a vibrating sample magnetometer (VSM) at 300 K and compared to ordinary zinc ferrite nanospheres (Figure 6). The magnetic-hysteresis (M-H) curves of both tested materials show roughly zero values for coercivity (Hc) or remanence (Mr), suggesting that they are superparamagnetic and could be readily separated using a magnet. As a result, the AG-g-HPAN@ZnFe2O4 saturation magnetization was approximately 24 emu g–1, whereas the zinc ferrite saturation magnetization was around 68 emu g–1. The nanocomposite is mainly composed of nonmagnetic organic components like AG-g-HPAN, hence supporting the decrease.

Figure 6.

Figure 6

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Magnetization curves of ZnFe2O4, and AG-g-HPAN@ZnFe2O4.

XRD was used to analyze the crystalline nature of the samples (Figure 7). The XRD pattern of ZnFe2O4 revealed diffraction peaks at 2θ = 30.5°, 35.31°, 43°, 57.11°, and 62.25°. The lattice structure of zinc ferrite is cubic, which is consistent with standard card JCPDS No. 22-101246 and also in agreement with reported values of the XRD pattern of zinc ferrite MNPs in literature.45 The amorphous form of AG was as described, with two broad typical peaks at 2θ ∼ 6–8° and 18–20°.48 The diffractograms of AG-g-HPAN, which resulted from the modification of AG with PAN copolymer grafting, indicate its semicrystalline structure. The existence of zinc ferrite in AG-g-HPAN@ZnFe2O4 was verified by multiple distinct peaks 2θ: 31.2°, 35.30°, 43°, 57.1°, 62.3°. The addition of zinc ferrite nanospheres to AG-g-HPAN increased its crystallinity. The typical crystallite size of ZnFe2O4 is roughly 32 nm, according to the Scherer equation.

Figure 7.

Figure 7

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XRD patterns of ZnFe2O4, AG-g-HPAN, and AG-g-HPAN@ZnFe2O4.

The N2 adsorption–desorption isotherm (Figure S4 in the Supporting Information) of AG-g-HPAN and AG-g-HPAN@ZnFe2O4 by applying the BET analysis were examined to study their superficial behavior and the effect of zinc ferrite nanosphere incorporation within the AG-g-HPAN matrix on the specific surface area. The BET surface area of AG-g-PAN@ZnFe2O4 was measured at 6.58 m2/g which was higher than that for AG-g-HPAN (0.080 m2/g). The reason for this observation is the presence of zinc ferrite nanosphere in the polymer matrix of AG-g-HPAN since the reported value of BET surface area for zinc ferrite nanosphere which was prepared by a similar procedure was 42 m2/g.44

The surface charge of the AG-g-HPAN@ZnFe2O4 adsorbent was evaluated employing zeta potential measurements. The zeta potential of the AG-g-HPAN@ZnFe2O4 hydrogel nanocomposite was investigated at pH 4, 6, 8, and 10 in this study. Table 1 demonstrates that the zeta potentials of nanocomposite grow highly negatively as the pH rises, which is mostly due to the deprotonation of hydroxyl and carboxylic acid functional groups in its structure. At pH 6, the zeta potential of AG-g-HPAN@ZnFe2O4 was −41.6 mV, which was greater than the zeta potential of AG-g-PAN@ZnFe2O4 (− 21.8). This observation can be related to the hydrolysis of hydrogel nanocomposite and the conversion of CN groups to carboxylate groups.

Table 1. Zeta Potential of the AG-g-PAN@ZnFe2O4 and AG-g-HPAN@ZnFe2O4 Hydrogel Nanocomposite at Room Temperature and Various pHs.

Sample pH Zeta Potential (mV)
AG-g-HPAN@ZnFe2O4 4 –38.8
AG-g-HPAN@ZnFe2O4 6 –41.6
AG-g-HPAN@ZnFe2O4 8 –61.3
AG-g-HPAN@ZnFe2O4 10 –80.1
AG-g-PAN/ZnFe2O4 6 –21.8
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3.2. Levofloxacin Adsorption Optimization by the Use of Effective Parameters

Solution pH, adsorbent dose, contact time, and levofloxacin initial concentration are all important factors in levofloxacin adsorption onto the adsorbent. By addressing these parameters, antibiotic removal efficiency can be considerably promoted.

3.2.1. Solution pH

The pH of the aqueous solutions was varied between 2 and 10 to evaluate the effect of solution pH on the adsorption effectiveness of levofloxacin onto the AG-g-HPAN@ZnFe2O4 hydrogel nanocomposite adsorbent, and the results of these studies are shown in Figure 8a. The adsorption capacity grew and reached its maximum value of around 336.06 mg/g as the pH rose from 2 to 6, but when the pH increased to 8 and 10, the adsorption capacity marginally decreased to 332.88 and 326.72 mg/g, correspondingly. The condition of the pollutant molecule, such as whether it was present as an ionic or molecular species, affected how pH affected the removal rate. In a neutral pH solution, levofloxacin antibiotic occurs as zwitterions; however, at lower pH values, it appears in cationic species, and at higher pH values, anionic species predominate.28,48 In other words, the molecule has two separate acid dissociation constant (pKa) values and includes both a basic and an acidic group. It was found that the elimination % of levofloxacin was efficient under neutral or weakly acidic conditions. The removal mechanisms of levofloxacin antibiotic onto the prepared nanobiosorbent49 may include different binding processes, i.e., hydrogen bonding, and electrostatic attraction. The adsorbent with a cross-linked structure and high specific surface area leads to the rapid diffusion of antibiotic molecules into the network and interaction with its functional groups for efficient levofloxacin.

Figure 8.

Figure 8

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(a) Influence of solution pH (2, 4, 6, 8, 10) (adsorbent dose = 0.005 g, initial concentration = 200 mg/L, time = 30 min and temperature = 298 K, (b) adsorbent dosage (0.0015, 0.0025, 0.05, 0.01, 0.015, 0.02 g) (pH 6, initial concentration = 200 mg/L, time = 30 min and temperature = 298 K), (c) time (10, 15, 20, 25, 30, 35, 45, 50, 55, 60 min) (pH 6, adsorbent dosage = 0.0025 g, initial concentration = 200 mg/L), V = 10 mL), temperature = 298 K), (d) initial concentration (50,100, 150, 200, 250,300, 350, 400, 425,450, 475, 500 mg/L) (pH 6, adsorbent dosage = 0.0025 g, V = 10 mL, time = 45 min, temperature = 298 K).

3.2.2. Adsorbent Amount

Adsorption studies were done in the presence of varying quantities of the adsorbent at optimum pH to evaluate the connection between the amount of the AG-g-HPAN@ZnFe2O4 adsorbent and its adsorption ability for levofloxacin. The adsorption capacity reduced from 427.4 to 68.58 mg/g as the adsorbent quantity was increased from 0.0015 to 0.02 g, according to the results (Figure 8b). The high amount of antibiotic available for adsorption by the AG-g-HPAN@ZnFe2O4 adsorbent is connected to the high amount of adsorption capacity at a lower adsorbent dose. Adsorption efficiency, on the other hand, increased with increasing adsorbent quantity since it solely depended on the starting and equilibrium antibiotic concentrations. The ideal volume of adsorbent for ensuing tests was found to be 0.0025 g.

3.2.3. Contact Time

The effect of contact time on the adsorption capacity of the AG-g-HPAN@ZnFe2O4 adsorbent for levofloxacin removal was examined. As shown in Figure 8c, the adsorption capacity increased up to 588.86 mg/g by increasing the contact time from 10 to 45 min at the optimal pH and adsorbent dose. However, the adsorption capacity did not increase and instead decreased slightly to 582.3 mg/g when the reaction time reached 60 min. Hence, 45 min was determined to be the ideal contact time for further trials. Many unoccupied biosorbent sites are accessible to interact with antibiotic molecules in an aqueous solution at the start of the adsorption process. The interactions between the functional groups of the adsorbent and the antibiotic get stronger when the contact duration is increased to 45 min, reaching their maximum equilibrium adsorption capacity.48 However, beyond this time, there was no further improvement in adsorption capacity, which may be attributed to the AG-g-HPAN@ZnFe2O4 biosorbent active sites being occupied and the approach to the equilibrium state.

3.2.4. The initial Concentration

By adjusting the levofloxacin concentration from 50 to 500 mg/L at the optimum pH (6.0), adsorbent dose (0.0025 g), and contact time (45 min), the relationship between the initial levofloxacin concentration and the adsorption capacity of the AG-g-HPAN@ZnFe2O4 adsorbent was evaluated. According to the data shown in Figure 8d, the initial concentration of levofloxacin has an impact on the adsorbent’s adsorption capacity. As the initial concentration of the antibiotic was increased from 50 to 400 mg/L, the adsorption capacity intensity rose to 1097.36 mg/g with a steep slope while, by increasing the concentration from 400 to 500 mg/L, the slope of this increase decreases sharply. It can be concluded that the adsorption capacity increases when the antibiotic initial concentration is increased while the amount of adsorbent remains constant, and this process continues until the antibiotic initial concentration reaches an equilibrium level.

3.3. Adsorption Isotherm

The adsorption isotherm study was used to examine the interaction between levofloxacin and the AG-g-HPAN@ZnFe2O4 adsorbent. In most recent investigations, the maximum adsorption capacity and equilibrium adsorption isotherms are explained using Langmuir and Freundlich’s models.43,50 The single-layer adsorption of chemicals onto the adsorbent surface is referred to as the Langmuir isotherm. It is assumed that all sites on the adsorbent surface would interact with pollutants with an equal amount of energy and affinities, resulting in homogeneous adsorption.51−53 In particular, the adsorption capacity reaches its highest levels when contaminants form a full monolayer on the adsorbent surface. In contrast, the Freundlich model relies on the multilayer adsorption of pollutants on the heterogeneous surface of the adsorbent.

The Langmuir (4) and Freundlich (5) models are both expressed mathematically in the following equations.54

3.3. 4
3.3. 5

where Ce is the equilibrium concentration of levofloxacin (mg/L); Qe and Qmax are the equilibrium and maximum adsorption capacity (mg/g), respectively; KL (L/mg) and KF (L/mg) are the Langmuir and Freundlich constants calculated from the plot between Ce/Qe and Ce and between ln Qe and ln Ce, respectively. n is a factor to determine the favorability of the adsorption process; when n > 1, the uptake of levofloxacin onto adsorbent is desirable at high concentrations.55

Figure 9 (a and b) shows the plots for the Langmuir and Freundlich isotherm models. Table 2 provides all the specific data from these two isotherms. The Freundlich isotherm was found to be more consistent with the experimental data than the Langmuir isotherm, according to the corresponding correlation coefficient (R2) of isotherm models. This demonstrates antibiotic multilayer adsorption over the heterogeneous surface of the AG-g-HPAN@ZnFe2O4 adsorbent. When compared to other adsorbents published in recent years, the produced biosorbent has a maximum adsorption capacity for levofloxacin of 1428.571 mg/g (Table 3). The AG-g-HPAN@ZnFe2O4 adsorbent has a greater Qmax than other reported adsorbents. This is clarified by the existence of ZnFe2O4 nanoparticles in the three-dimensional hydrogel nanocomposite, which has many adsorption reactive sites including hydroxyl, amide, and carboxylate groups and can effectively interact with levofloxacin (via electrostatic interaction and hydrogen bonding) and remove the antibiotic.

Figure 9.

Figure 9

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(a) Langmuir and (b) Freundlich isotherms (condition: initial concentration (50–500 mg/L), pH 6, adsorbent dosage = 0.0025 g, contact time = 45 min, T = 298 K).

Table 2. Isotherm Constants, Correlation Coefficients, and Statistical Parameters, for Adsorption of Levofloxacin on the AG-g-HPAN@ZnFe2O4.

Isotherm Parameter  
Freundlich KF, mg g–1 154.3156
  n 2.3337
  R2 0.9901
Langmuir Qm 1428.571
  KL 0.04142
  R2 0.978
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Table 3. Evaluation of the Maximum Adsorption Capacity of AG-g-HPAN@ZnFe2O4 Hydrogel Adsorbent Compared to the Previously Reported Studies.

Adsorbent Qmax (mg/g) Ref
Biochar/MgFe2O4 115 (55)
NiFe2O4/biochar 172 (56)
Granular silica pillared clay 72.5 (57)
Magnetic NiFe-LDH/N-MWCNTs nanocomposite 454.5 (58)
Fe2O3/GO core–shell nanocomposite 129.9 (59)
Co-modified MCM-41 120 (60)
AG-g-HPAN@ZnFe2O4 1428.6 Present work
ZnFe2O4 51.2 Present work
AG 50.3 Present work
AG-g-PAN 1098.8 Present work
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3.4. Adsorption Kinetics

Adsorption kinetics is an effective tool for exploring and assessing potential adsorption processes, equilibrium time, and rate-limiting phase of the adsorption process. The pseudo-first-order and pseudo-second-order models were performed in this work to estimate the levofloxacin adsorption rates onto the AG-g-HPAN@ZnFe2O4 adsorbent. They are stated mathematically in eqs 6 and 7, respectively.61,62

3.4. 6
3.4. 7

where Qt (mg/g) and Qe (mg/g) are the adsorption capacity (or amount of levofloxacin adsorbed onto biosorbent) at time t and equilibrium, respectively. k1 (1/min) and k2 (g/mg·min) are the rate constants of the pseudo-first-order and pseudo-second-order, respectively. Figure 10 (a and b) and Table 4 both display the kinetic linear plots and their calculated details. The pseudo-second-order model is more suited to understanding the levofloxacin adsorption kinetics on the AG-g-HPAN@ZnFe2O4 by considering the correlation coefficients and difference between the estimated Qe and observed Qe values of the two examined kinetic models.

Figure 10.

Figure 10

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(a) Pseudo-first-order and (b) pseudo-second-order models (conditions: contact time (10–60 min), pH 6, adsorbent dosage = 0.0025 g, initial concentration = 200 mg/L, T = 298 K)).

Table 4. Kinetic Constants, Correlation Coefficients, and Statistical Parameters, for Adsorption of Levofloxacin on the AG-g-HPAN@ZnFe2O4.

Models Parameter  
Pseudo-first-order k1 0.0765
  Qe calculated 440.0991
  Qe experimental 588.86
  R2 0.9524
Pseudo-second-order k2 0.000449
  Qe 625
  Qe experimental 588.86
  R2 0.9992
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3.5. Regeneration Experiment

Adsorbent recoverability and reusability are critical in terms of environmental protection and cost, time, and energy savings. So, the adsorbents with these qualities significantly lessen the challenges associated with old adsorbent disposal and the new adsorbent manufacturing process. Four successive cycles of adsorption and desorption experiments were carried out to determine the recoverability and regeneration of the AG-g-HPAN@ZnFe2O4 adsorbent. For this propose, the levofloxacin adsorption test on AG-g-HPAN@ZnFe2O4 was performed (reaction conditions: pH 6, adsorbent dosage = 0.0025 g, contact time = 45 min, initial concentration = 200 mg/L, T = 298 K). Next, levofloxacin-loaded AG-g-HPAN@ZnFe2O4 was submerged into 10 mL of ethanol and stirred at ambient temperature for two hours to desorb levofloxacin. After release, the adsorbed levofloxacin was separated using a magnet, rinsed many times with distilled water, and then dried in preparation for the subsequent adsorption/desorption studies. Subsequently, using a UV–visible spectrophotometer, the amount of released levofloxacin in the elution medium was measured. Following the four-cycle, the adsorption percentage dropped from 88.33% to 84.32% and the desorption percentage from 84.39% to 81.8%, as shown in Figure 11. These findings demonstrate that the adsorbent could continue to remove levofloxacin after four consecutive cycles of adsorption and desorption without noticeably losing adsorption capacity.

Figure 11.

Figure 11

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Reusability of the AG-g-HPAN@ZnFe2O4 adsorbent for adsorption of levofloxacin (adsorption condition (pH 6, adsorbent dosage = 0.0025 g, contact time = 45 min, initial concentration = 200 mg/L, T = 298 K).

3.6. Proposed Adsorption Mechanisms

The textural property, chemical structure, and functional groups of AG-g-HPAN@ZnFe2O4 adsorbent are critical in the antibiotic adsorption process. The hydrogel nanocomposite contains a large number of reactive functional groups or adsorption reactive sites, such as carboxyl groups, amide, and hydroxyl, as well as a three-dimensional network. Levofloxacin was effectively eliminated from an aqueous solution by interacting with the hydrogel nanocomposite through electrostatic contact and hydrogen bonding, as shown in Figure 12.

Figure 12.

Figure 12

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Possible adsorption mechanisms for the adsorption of levofloxacin by the AG-g-HPAN@ZnFe2O4..

4. Conclusions

The AG-g-HPAN@ZnFe2O4 nanocomposite was obtained in three steps and employed as an adsorbent for levofloxacin elimination. The hydrogel nanocomposite was characterized using FT-IR, EDX, TGA, SEM, VSM, BET, and XRD techniques. The pH value of 6, the adsorbent dose of 0.0025 g, the contact period of 45 min, and the levofloxacin starting concentration of 500 mg/L were the most efficient parameters for the adsorption of levofloxacin onto AG-g-HPAN@ZnFe2O4. The characterizing analyses revealed that the AG-g-HPAN@ZnFe2O4 adsorbent showed acceptable thermal stability with 58% char yields and superparamagnetic properties. The XRD pattern of nanocomposite showed a semicrystalline nature with the addition of zinc ferrite nanospheres to amorphous AG-g-HPAN. The FESEM images of AG-g-HPAN@ZnFe2O4 showed uniform distribution of zinc ferrite nanospheres throughout the smooth surface of the AG-g-HPAN hydrogel matrix, and its BET surface area was increased in comparison with that of AG-g-HPAN as a result of zinc ferrite nanosphere incorporation. The experimental adsorption kinetics were satisfactorily matched to pseudo-second-order (R2 = 0.9992) and adsorption isotherm data, supplemented by a Freundlich isotherm model (R2= 0.9901) with Qmax of 1428.571 mg/g at 298 K and pH 6. The formation of hydrogen bonds and electrostatic interactions between adsorption reactive sites in the biosorbent and Levofloxacin can clarify the operation of antibiotic adsorption on the hydrogel network. Using exceptional thermal stability and superparamagnetic behavior at 800 °C, the AG-g-HPAN@ZnFe2O4 nanocomposite could be readily recovered from the reaction mixture with an external magnet and reused up to four times without losing substantial activity.

Acknowledgments

The authors are grateful to the Iran University of Science & Technology (IUST) for their partial support.

Supporting Information Available

The Supporting Information is available free of charge at https://pubs.acs.org/doi/10.1021/acsomega.2c06555.

  • UV–vis spectrum and a calibration curve of levofloxacin; TGA thermogram of AG-g-HPAN and N2 adsorption–desorption isotherm of adsorbent (PDF)

The authors declare no competing financial interest.

Supplementary Material

ao2c06555_si_001.pdf (262.1KB, pdf)

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