Polyoxometalate supported on a magnetic Fe₃O₄/MIL-88A rod-like nanocomposite as an adsorbent for the removal of ciprofloxacin, tetracycline and cationic organic dyes from aqueous solutions

Abstract

In this work, a magnetic H₃PW₁₂O₄₀/Fe₃O₄/MIL-88A (Fe) rod-like nanocomposite as a stable and effective ternary adsorbent was fabricated by the hydrothermal method and utilized for the removal of ciprofloxacin (CIP), tetracycline (TC) and organic dyes from aqueous solution. Characterization of the magnetic nanocomposite was accomplished by FT-IR, XRD, Raman spectroscopy, SEM, EDX, TEM, VSM, BET specific surface area and zeta potential analyses. The influencing factors on the adsorption potency of the H₃PW₁₂O₄₀/Fe₃O₄/MIL-88A (Fe) rod-like nanocomposite including initial dye concentration, temperature and adsorbent dose were studied. The maximum adsorption capacities of H₃PW₁₂O₄₀/Fe₃O₄/MIL-88A (Fe) for TC and CIP were 370.37 mg g⁻¹ and 333.33 mg g⁻¹ at 25 °C, respectively. In addition, the H₃PW₁₂O₄₀/Fe₃O₄/MIL-88A (Fe) adsorbent had high regeneration and reusability capacity after four cycles. In addition, the adsorbent was recovered through magnetic decantation and reused for three consecutive cycles without a considerable reduction in its performance. The adsorption mechanism was mainly ascribed to electrostatic and π–π interactions. According to these results, H₃PW₁₂O₄₀/Fe₃O₄/MIL-88A (Fe) can act as a reusable effective adsorbent for the fast elimination of tetracycline (TC), ciprofloxacin (CIP) and cationic dyes from aqueous solutions.

---

1. Introduction

The rapid development of industrialization, has led to an increasing number of water pollutants such as dyes and antibiotics entering the environment. Organic dyes are a potential hazard to human health due to their various harmful effects.^1–4 The most important sources of industrial dye pollutants originate from different industries such as the textile, cosmetic, leather, food, pharmaceutical, paint and varnish, and pulp and paper industries.^5–8 Antibiotics play a major role in human and animal disease treatment, growth promotion, and prophylaxis.^9,10 Antibiotic pollution is becoming a serious problem. The main source of this contamination in the aquatic environment is wastewater from antibiotic manufacturers, large scale animal farming, and hospitals.¹¹ Tetracycline (TC), which belongs to polyketides has broad-spectrum antibacterial activities against various disease-causing pathogens.¹² Ciprofloxacin (CIP) is a type of antibiotic that is derived from the fluoroquinolones family, and used for healing the contagions induced by Gram-positive and Gram-negative bacteria.¹³ However, extensive use of antibiotics has resulted in their frequent detection in the effluents of wastewater treatment plants (WWTPs). The incomplete metabolism of humans or animals and the undesirable removal performance of traditional technologies caused the residual CIP and TC to be continuously discharged into the natural water. There are many methods to remove cationic dyes, CIP, and TC, such as electrochemical purification, membrane processes.^14,15 biodegradation, and adsorption process using new adsorbents such as MOFs and Mxenes to remove different types of antibiotics and dyes.^16–18 Adsorption is widely used as an excellent and promising technique due to the great advantages which are based on its lower expenses, higher performance, and convenience in use.^19–21

Polyoxometalates (POMs) are a big group of metal–oxygen clusters having acid–base properties, consisting of primary transition metals such as tungsten, vanadium, and molybdenum.^22,23 POMs are significant metal-oxide clusters with a highly negative charge and abundant topologies, which have been employed in many research fields such as optics, magnetism, catalysis, and biological medicine.^24,25 However, there are some disadvantages for the use of POMs as adsorbents: (1) their relatively small surface area seriously obstructs accessibility to the active sites and (2) their excellent solubility in aqueous solution determines that they cannot be reused and recycled in the process of wastewater treatment. To solve these limitations, substrates including mof have been utilized to anchor polyoxometalates to the formation of heterogeneous composites.²⁶

Metal–organic frameworks (MOFs) are hybrid crystalline porous materials with structures consisting of a regular array of positively charged metal ions or metallic clusters associated with organic linkers.²⁷ MOFs are porous materials with fascinating structures. The high surface area, crystallinity, controllable pore size, flexibility, and fictionalization of the porous surface are some of the main characteristics which determine the versatility of MOFs.^28,29 Among MOFs, one of the most attractive materials is MIL-88A (Fe). MIL-88A (Fe) was a 3D structured framework built up from trimers of Fe³⁺ octahedral linked to fumarate dianions. This structure exhibited a pore-channel system along the cages (5–7 Å).³⁰ Thus, these features make MIL-88A (Fe) to be a good candidate for environmental contaminant elimination.

Based on the above considerations, in this paper, a magnetic H₃PW₁₂O₄₀/Fe₃O₄/MIL-88A (Fe) rod-like nanocomposite was first fabricated by a hydrothermal method, and its chemical properties and physical structure were characterized. The adsorption of tetracycline (TC), ciprofloxacin (CIP), and organic dyes onto the H₃PW₁₂O₄₀/Fe₃O₄/MIL-88A (Fe) and the influences of the adsorbent dose, primary adsorbate concentration and temperature on the adsorption process were checked.

2. Experimental

2.1. Materials and procedures

Fumaric acid, iron(III) chloride (FeCl₃·6H₂O, 99%), H₃PW₁₂O₄₀ (98%), ammonium iron(III) sulfate ((NH₄)Fe(SO₄)₂·12H₂O, 98%), ammonium iron(II) sulfate ((NH₄)₂Fe(SO₄)₂·12H₂O, 98.5%), ammonia NH₃, methylene blue (C1₆H₁₈ClN₃S, MB, 99%), methyl orange (C₁₄H₁₄N₃NaO₃S, MO, 99%), and rhodamine B (C₂₈H₃₁ClN₂O₃,RhB, 99%) were obtained from Merck company. Tetracycline (C₂₂H₂₄N₂O_8, TC, 99.5%) and ciprofloxacin (C₁₇H₁₈FN₃O₃, CIP, 95.5%) drugs were obtained from Exir pharmaceutic company (Boroujerd, Iran). All of the chemicals were used without further purification.

2.2. Synthesis of iron-based MIL-88A (Fe) nanorods

The sample of MIL-88A (Fe) was prepared through a hydrothermal procedure. The accurate action to the synthesis of iron-based MOF is as follows: FeCl₃·6H₂O (1.2 g) and fumaric acid (0.49 g) were poured into 25 mL Water and the obtained combine was stirred for 15 min at ambient temperature. The mixture within a 30 mL Teflon-lined autoclave was transferred and warmed for 12 h at 65 °C. Then, it was slowly cooled at 25 °C and the obtained light brown solid was separated through centrifugation. The solid result was washed with water and also with ethanol solvent three times. Eventually, the product was dried in an oven for 1 h at 60 °C.

2.3. Synthesis of magnetic Fe₃O₄/MIL-88A (Fe) nanorods

The Fe₃O₄/MIL-88A (Fe) nanocomposite was produced as follows: FeCl₃·6H₂O (1.2 g), fumaric acid (0.49 g), Fe₃O₄ (0.1 g), in 25 mL of water solvent was poured and the combine was stirred for 15 min at 25 °C. The mixture within a 30 mL Teflon-lined autoclave was transferred and warmed at 65 °C for 12 h. Afterward slowly cooled at 25 °C and the light brown solid product was separated the solid product was detached from the solution by applying a magnet and then washed with Water solvent and dried in an oven at 70 °C for 1 h.

2.4. Synthesis of H₃P₂W₁₂O₄₀/Fe₃O₄/MIL-88A (Fe) rod-like composite

For the fabrication of H₃PW₁₂O₄₀/Fe₃O₄/MIL-88A (Fe), 1 g of the Fe₃O₄/MIL-88A (Fe) and 1 g of H₃PW₁₂O₄₀ were solved within 20 mL of deionized water. The combine was stirred at 25 °C for 20 h. The resulting solid was washed through deionized water and cooled gradually at 25 °C. Finally, the solid product was detached from the solution by applying a magnet.

2.5. Characterization methods

FTIR spectra were registered with a Shimadzu-8400S spectrometer (Japan) in the wavenumber range of 400–4000 cm⁻¹. Powder XRD patterns were recorded by an X-ray diffractometer under a current of 40 mA and voltage of 40 kV with Cu Kα radiation (k = 0.1542 nm). The distribution and morphology of pure MOF and magnetic nanocomposite were studied utilizing (SEM, MIRA3 TESCAN) scanning electron microscopy connected with (EDX) energy-dispersive X-ray analysis. Spectra of UV-vis were performed on a Carry 100 Conc Varian spectrophotometer. The surface area of the nanocomposites was performed by N₂ adsorption isotherm with the BET method (Micro metrics PHS-1020, Japan). The magnetic (VSM) measurement was checked by MDKFD vibrating magnetometer (Daneshpajoohan Co., Iran) via a high magnetic field of 10 kOe. The adsorption process of dyes was measured on a Varian Cary 100 spectrophotometer (USA). Raman spectra were obtained using a Raman microscope (Senterra 2009, Germany) with a 514 nm line.

2.6. Adsorption experiments of drug

The adsorption of TC and CIP was performed in a 50 ml glass beaker and the adsorption reaction temperature was maintained at 25 °C. The removal experiments were carried out in 50 ml reaction volume of TC and CIP 10 mg L⁻¹ and the parameters chosen for the optimization are nanoparticle concentration: 25–100 mg L⁻¹ and temperature: 25–65 °C. After the reaction was completed, the adsorbent was separated with an external magnet. All parallel experiments were performed in triplicate to ensure accuracy, and the average results were employed for further data analysis. The main absorbance peak of TC and CIP was observed at 375 and 326 nm and its removal efficiency were calculated according to the following equation: The degradation efficiency (R (%)) is defined as (C_o − C_t)/C_o × 100%, where C_o is the initial concentration of TC and CIP solution and C_t is the remaining concentration of CIP at reaction time t.

R (%) = (C_o − C_t) × 100/C_o

and equilibrium adsorption capacity q_e (mg g⁻¹) could be computed:

q_e = (C_o − C_t) × V/m

2.7. Adsorption experiments of organic dyes

The adsorption experiments were carried out using cationic dyes i.e., MB, rhodamine B (RhB), and anionic dye, i.e., methyl orange (MO) as model organic dye pollutants of the water at 25 °C. In a typical experiment, 20 mg of the as-prepared H₃PW₁₂O₄₀/Fe₃O₄/MIL-88A (Fe), the hybrid adsorbent was added into 30 ml of dye aqueous solution with the initial concentration of 30 mg L⁻¹ and stirred in the dark. The concentrations of MB, RhB, and MO were determined using UV-visible spectrophotometry at wavelengths of 664, 553, and 467 nm, respectively. The effects of important parameters on the adsorption performance, like amount of nanocomposite (10, 20, 30 and 40 mg) and initial dye concentration (25, 50, 75, 125, 150 and 200 mg), were studied under similar conditions as described above. To evaluate the selective adsorption ability of the hybrid nanomaterial, 20 mg of H₃PW₁₂O₄₀/Fe₃O₄/MIL-88A (Fe) was stirred with 30 ml of mixed dyes of MB + MO (C_o (MB) = C_o (MO) = 30 mg L⁻¹), MB + RhB (C_o (MB) = C_o (RhB) = 30 mg L⁻¹), MO + RhB (C_o (MB) = C_o (RhB) = 30 mg L⁻¹) and the ternary mixture of MB + MO + RhB (C_o (MB) = C_o (MO) = C_o (RhB) = 30 mg L⁻¹) and then the process was monitored using UV-visible spectroscopy at given time intervals. The elimination rate (R%) was computed based on the following equation:

(R%) = (C_o − C_t) × 100/C_o

3. Results and discussion

3.1. Characterization of the adsorbent

3.1.1. FTIR analysis. The FT-IR spectra of MIL-88A (Fe) (a), Fe₃O₄ (b), H₃PW₁₂O₄₀ (c), Fe₃O₄/MIL-88A (Fe) (d), and H₃PW₁₂O₄₀/Fe₃O₄/MIL-88A (Fe) rod-like composite (e) are displayed in Fig. 1. In Fig. 1(a), the broadband at about 3465 cm⁻¹ is related to the stretching vibration of water molecules. The two sharp peaks at 1400 and 1610 cm⁻¹ are assigned to symmetric and asymmetric vibrations of carboxyl groups, respectively, confirming the presence of the dicarboxylate linker within the sample.³¹ Also, the peak that appeared at 574 cm⁻¹ is specified to the Fe–O vibration. The FT-IR spectrum of Fe₃O₄ (Fig. 1(b)) demonstrated an adsorption peak at 567 cm⁻¹ which is related to the vibration of the Fe–O bond of Fe₃O₄.⁵ As revealed in Fig. 1(c), the characteristic peaks of 789 and 898 cm⁻¹ are related to the vibration of (W–O_b–W), and the bands appeared at 960, and 1083 cm⁻¹ are assigned to the vibrations of (W–O_d) and (P–O_a) of H₃PW₁₂O₄₀ polyanion, respectively.³² Comparing the spectra of H₃PW₁₂O₄₀/Fe₃O₄/MIL-88A (Fe) samples (Fig. 1(d)) shows that after forming this nanocomposite, the characteristic peaks related to iron-based MIL-88A, H₃PW₁₂O₄₀, and Fe₃O₄ particles are slightly shifted. These shifts emphasize that a strong interaction exists between the H₃PW₁₂O₄₀ anion and Fe₃O₄ nanoparticles with MIL-88A (Fe) MOF.

Fig. 1 The FTIR spectra of (a) MIL-88A (Fe), (b) Fe₃O₄, (c) H₃PW₁₂O₄₀, and (d) the H₃PW₁₂O₄₀/Fe₃O₄/MIL-88A Fe nanocomposite.

3.1.2. XRD diffraction patterns. The crystallinity and phase of the fabricated samples MIL-88A (Fe), Fe₃O₄, H₃PW₁₂O₄₀, Fe₃O₄/MIL-88A (Fe), and H₃PW₁₂O₄₀/Fe₃O₄/MIL-88A (Fe) rod-like composite were analyzed via XRD as elucidated in Fig. 2. As shown in Fig. 2(a), the XRD pattern of MIL-88A showed two diffraction peaks at 2θ = 10–12°, which was accordance with the reported information.³¹ As exhibited in Fig. 2(b), the typical peaks at 2θ = 75°, 63°, 53.7°, 43.3°, 36°, and 30.2° assigning to (440), (511), (422), (400), (311), and (220) the crystal planes of Fe₃O₄ particles (JCPDS card No. 19-0629).³³ In Fig. 2(c), the diffraction peaks of POM alone are observed. The XRD pattern of the nanocomposite sample in Fig. 2(d) exhibits the characteristic diffraction peaks corresponding to Fe₃O₄ and MIL-88A. Also, some diffraction peaks contributed to H₃PW₁₂O₄₀ phase are observed with low intensities because of the homogeneous distribution of the H₃PW₁₂O₄₀ molecules within the porous structure of MIL-88A (Fe). These findings confirm that the nanocomposite sample is composed of Fe₃O₄, MIL-88A and H₃PW₁₂O₄₀ components and it has been successfully prepared.³⁴

Fig. 2 The XRD patterns of (a) MIL-88A (Fe), (b) Fe₃O₄, (c) H₃PW₁₂O₄₀, and (d) the H₃PW₁₂O₄₀/Fe₃O₄/MIL-88A (Fe) nanocomposite.

3.1.3. Raman spectroscopy. Raman spectroscopy was used to confirm the structural phase of MIL-88A (Fe), H₃PW₁₂O₄₀, H₃PW₁₂O₄₀/MIL-88A (Fe), and H₃PW₁₂O₄₀/Fe₃O₄/MIL-88A (Fe) nanomaterials. In Fig. S1(a),† the bond of the fumaric acid molecule in MIL-88A was evidenced by the appearance of a band at 3059 cm⁻¹, corresponding to the stretching sp² C–H vibrations, and a strong peak at 1649 cm⁻¹ associated to the symmetric vibration modes C=C. The bands located at 1592 and 1435 cm⁻¹, in the case of MIL-88A, materials are attributed to the anti-symmetric and symmetric vibrations, respectively, of the carboxylate groups. The C–O bond was observed as an intense peak centered at 1280 cm⁻¹. Additionally, the bending vibrations outside the plane of the bond =C–H were placed at Raman shifts of 998, 896, and 763 cm⁻¹.³⁵ As exhibited in Fig. S1(b),† the Raman spectrum of H₃PW₁₂O₄₀ shows characteristic bands at 1010 cm⁻¹ (stretching vibration of P–O), 990 cm⁻¹ (stretching of W=O), 913 cm⁻¹ (bending of W–O_c–W) and 519 cm⁻¹ (bending of O–P–O). According to Fig. S1(c),† the appearance of some vibrational modes of the Fe₃O₄ and three bands of H₃PW₁₂O₄₀ at 292, 751, and 1010 cm⁻¹, beside the characteristic bands of MIL-88A (Fe), proved the existence of both Fe₃O₄ and H₃PW₁₂O₄₀ in the framework of MIL-88A (Fe).

3.1.4. SEM and TEM images. The morphology and microstructure of the iron-based MIL-88A (Fe), Fe₃O₄, Fe₃O₄/MIL-88A (Fe), and H₃PW₁₂O₄₀/Fe₃O₄/MIL-88A (Fe) samples were studied by utilizing SEM analyses as depicted in Fig. 3. According to Fig. 3(a) pure iron-based MIL-88a particles have a shape bar-like with two prismatic heads. In Fig. 3(b), the SEM image of Fe₃O₄ consists of small and near-spherical particles which is highly agglomerated. The SEM photographs of the binary Fe₃O₄/MIL-88A (Fe) (Fig. 3(c)) and ternary H₃PW₁₂O₄₀/Fe₃O₄/MIL-88A (Fe) nanocomposite (Fig. 3(d) and (e)) demonstrate that the morphology and shape of these samples are analogous to MIL-88A (Fe), which established that the framework of iron-based MIL-88A (Fe) remains unaltered after modification with Fe₃O₄ nanoparticles and H₃PW₁₂O₄₀. The morphology and microstructure of the as-prepared H₃PW₁₂O₄₀/Fe₃O₄/MIL-88A (Fe) nanocomposite were further investigated by TEM analysis. As evident from the image in Fig. 3(f), the morphology of the H₃PW₁₂O₄₀/Fe₃O₄/MIL-88A (Fe) rod-like composite sample from TEM images agreed with the SEM results.

Fig. 3 The SEM photographs of (a) iron-based MIL-88A, (b) Fe₃O₄, (c) Fe₃O₄/MIL-88A (Fe), (d and e) the H₃PW₁₂O₄₀/Fe₃O₄/MIL-88A Fe nanocomposite, and (f) TEM image of the H₃PW₁₂O₄₀/Fe₃O₄/MIL-88A (Fe) nanocomposite.

3.1.5. EDX analysis. The EDX analysis of the H₃PW₁₂O₄₀/Fe₃O₄/MIL-88A (Fe) rod-like composite is displayed in Fig. S2.† In Fig. S2(a),† the EDX elemental spectrum of the ternary nanohybrid sample exhibits elemental peaks corresponding to H₃PW₁₂O₄₀ (O, P and W), Fe₃O₄ (Fe, O), and MIL-88A (Fe) (C, Cl, and Fe), with no other impure peaks observed, showing that the composite sample was composed of H₃PW₁₂O₄₀, Fe₃O₄, and MIL-88A (Fe). Further, the EDX elemental mappings of the composite in Fig. S2(b)† show the homogeneous dispersion of elements within the nanocomposite.

3.1.6. BET analysis. The N₂ adsorption–desorption isotherms of the activated MIL-88A (Fe) and H₃PW₁₂O₄₀/Fe₃O₄/MIL-88A (Fe) are illustrated in Fig. 4(a) and (b). The Brunauer–Emmett–Teller (BET) surface area values for MIL-88A (Fe) and H₃PW₁₂O₄₀/Fe₃O₄/MIL-88A (Fe) were calculated to be 3.19 m² g⁻¹ and 4.16 m² g⁻¹, respectively, in agreement with literature.²⁴ Their average pore diameter of them was measured as 41.2 nm and 47.4 nm, respectively. The BJH pore-size dispersions in the inset of the figures affirm the mesoporous cages of MIL-88A (Fe) and showed that the overall volume changes through the incorporation. The results suggested that the H₃PW₁₂O₄₀ polyoxometalate was mostly encapsulated within the channels and on the surface of MIL-88A (Fe).

Fig. 4 N₂ adsorption–desorption isotherms and pore-size distribution curves (the insets) of (a) MIL-88A (Fe) and (b) the H₃PW₁₂O₄₀/Fe₃O₄/MIL-88A (Fe) nanocomposite.

3.1.7. VSM analysis. The magnetic properties of the pure Fe₃O₄, Fe₃O₄/MIL-88A (Fe), and H₃PW₁₂O₄₀/Fe₃O₄/MIL-88A (Fe) samples were investigated by VSM at room temperature, and the magnetic hysteresis loops are depicted in Fig. 5(a)–(c). The magnetic saturation values (M_s) of the Fe₃O₄ nanoparticles, Fe₃O₄/MIL-88A and H₃PW₁₂O₄₀/Fe₃O₄/MIL-88A (Fe) were about 72, 51 and 43 emu g⁻¹, respectively, which was sufficient for quick separation, via the aid of an outer magnet. This value was less than that of bare Fe₃O₄ (about 51 emu g⁻¹) because of the existence of nonmagnetic H₃PW₁₂O₄₀ and MIL-88A (Fe) components. The magnetic separability of the H₃PW₁₂O₄₀/Fe₃O₄/MIL-88A (Fe) nanocomposite was disclosed by approaching the glass container with a magnet.

Fig. 5 Magnetization curves of (a) Fe₃O₄, (b) Fe₃O₄/MIL-88A (Fe) and (c) the H₃PW₁₂O₄₀/Fe₃O₄/MIL-88A (Fe) nanocomposite.

3.1.8. Zeta potential analysis. The zeta potential investigation results got for analyzed MIL-88A (Fe) and H₃PW₁₂O₄₀/Fe₃O₄/MIL-88A (Fe) are presented in Fig. 6. As shown in Fig. 6, the positive zeta potential value of MIL-88A (Fe) (+4.59 mV) is changed to a negative charge for the H₃PW₁₂O₄₀/Fe₃O₄/MIL-88A (Fe) composite (−17.85 mV). This significant change in zeta potential value is mostly because of the introducing the negatively charged PW₁₂O₄₀³⁻ component into the MIL-88A (Fe) framework and makes it more suitable for selective adsorption of cationic species.

Fig. 6 Zeta potential distribution curve of the MIL-88A (Fe) and H₃PW₁₂O₄₀/Fe₃O₄/MIL-88A (Fe) samples in aqueous solutions.

3.2. TC and CIP drugs adsorption investigations

Different factors affect the adsorption process. The effect of H₃PW₁₂O₄₀/Fe₃O₄/MIL-88A (Fe) dosage, temperature and primary concentration of TC and CIP were investigated by a variable-controlling strategy. To determine the optimum adsorbent dosage, we explored the adsorbent effect on the TC and CIP removal efficiency. As shown in Fig. 7(a) and (b), the adsorption efficiency for TC is about 85% in 3 min and for CIP is approximately 100% in 12 min.

Fig. 7 UV-vis spectral changes of (a) TC and (b) CIP drugs in aqueous solutions over the H₃PW₁₂O₄₀/Fe₃O₄/MIL-88A (Fe) nanocomposite at different time intervals: conditions: C_o (drug) = 50 mg L⁻¹, V_drug = 50 mL, adsorbent dose: 10 mg at 25 °C.

3.2.1. Influence of adsorbent dose. Different amounts of H₃PW₁₂O₄₀/Fe₃O₄/MIL-88A (Fe) nanocomposite (5, 10, 15, 20 mg) were added to investigate the influence of adsorbent dose on the CIP and TC removal efficiency. The results in Fig. 8(a) and (b) show that the removal efficiency increased with the nanocomposite dosage from 5 to 20 mg.^17,32 As expected, the removal (%) of drugs sharply increased with increasing the H₃PW₁₂O₄₀/Fe₃O₄/MIL-88A (Fe) nanocomposite composite dosage; this mainly attributed to the presence of further active sites for adsorption of drugs molecules. According to the above results, the amount of 10 mg was selected used for the subsequent adsorption experiments.

Fig. 8 Effect of adsorbent doses on the removal of (a) TC and (b) CIP antibiotics.

3.2.2. Influence of temperature. The adsorption behavior of H₃PW₁₂O₄₀/Fe₃O₄/MIL-88A (Fe) demonstrated at different temperatures in Fig. 9. As shown in Fig. 9(a), the progressive increase of temperature from 25 to 65 °C is adversely affecting the removal of 10 mg L⁻¹ of TC by H₃PW₁₂O₄₀/Fe₃O₄/MIL-88A (Fe) nanocomposite, because the removal efficiency increase from 85% to 100%. This shows that the reaction between TC and H₃PW₁₂O₄₀/Fe₃O₄/MIL-88A (Fe) nanoparticles is the endothermic method. In Fig. 9(b), the influence of temperature on the elimination of CIP via H₃PW₁₂O₄₀/Fe₃O₄/MIL-88A (Fe) nanocomposite was checked at several temperatures from 25 to 65 °C under an adsorbent dosage of 10 mg. The results indicate that the adsorption process between ciprofloxacin and H₃PW₁₂O₄₀/Fe₃O₄/MIL-88A (Fe) nanoparticles is exothermic.^36,37

Fig. 9 Effects of temperature on the removal of (a) TC and (b) CIP antibiotics.

3.2.3. Influence of primary TC and CIP concentration. The influence of the various initial concentrations (25, 50, 75, and 100 mg L⁻¹) of TC and CIP drugs on the adsorption efficiency of H₃PW₁₂O₄₀/Fe₃O₄/MIL-88A (Fe) adsorbent under the constant conditions of 50 mg L⁻¹ absorbent, pH 6, and 25 °C are shown in Fig. 10. As can be seen in Fig. 10(a) and (b), the removal efficiency decreased with increasing the CIP and TC concentrations, probably due to the fast saturation of accessible active sites on the surface of adsorbent in higher initial concentration of drugs and the deficiency of active sites needed for high drug concentration.³²

Fig. 10 Effect of concentration on the removal of (a) TC and (b) CIP antibiotics.

3.2.4. Removal efficiency of the nanocomposite components. The removal efficiency of pure MIL88A (Fe) and Fe₃O₄/MIL101 (Fe) in the elimination of TC and CIP from aqueous solutions were studied under the optimum reaction conditions in Fig. 11. The removal efficiency of these components into TC and CIP was 59–85% and 35–100% after adsorption times of 3–12 minutes and 12–24 minutes, respectively. The elimination percentage of the H₃PW₁₂O₄₀/Fe₃O₄/MIL-88A (Fe) nanocomposite is higher than the individual components and Fe₃O₄/MIL101 (Fe) composite, which showed the magnetic nanocomposite to be a better choice for the removal of the antibiotic drugs.

Fig. 11 The efficiency of the nanocomposite components on the removal of (a) TC and (b) CIP antibiotics.

3.2.5. Comparison with some reported adsorbents. To indicate the advantage of the present adsorbent, we have compared the results in the removal of TC and CIP from aqueous solution by H₃PW₁₂O₄₀/Fe₃O₄/MIL-88A (Fe) nanocomposite with some reported similar adsorbents in the literature.^{16,17,38–53} From Table 1, concerning the adsorption capacities, the present method is more suitable and/or superior. We can see that the adsorption process in the presence of some reported adsorbents demonstrated lower adsorption capacities compared to the present nanocomposite. The higher adsorption capacity of the H₃PW₁₂O₄₀/Fe₃O₄/MIL-88A (Fe) is the because of the synergistic effect between MIL-88A (Fe) and H₃PW₁₂O₄₀.

Table 1 Comparison of the maximum adsorption capacities of some reported adsorbents for TC and CIP drugs

Adsorbent	Drug	Maximum adsorption capacity (mg g^−1)	Ref.
HAP/MIL101 (Fe)/Fe3O4	CIP	112.35	16
Fe3O4/cellulose	CIP	6.90	17
Fe3O4/cellulose	CIP	168.03	17
CS-g-PA/TSM	CIP	238.3	38
MIL101 (Cr)–HSO3	CIP	564.9	39
c-FD	CIP	328	40
mGOCP	CIP	283.44	41
Ga-cl-PAM/C3N4	CIP	169.49	42
Fe/Zn-SBC	CIP	74.2	43
Fe/Zn-SBC	TC	145	43
Fe3O4/MIL-53 (Al)–NH2	TC	84.8	44
MnO2/graphene	TC	195.31	45
Graphene oxide/ZnO	TC	159	46
Fe3O4@MIL-53 (Al)	TC	47.8	47
MnFe2O4/rGO	TC	41	48
Scallop shell coated Fe2O3 nanocomposite	TC	49.26	49
CTM/Fe3O4	TC	215.31	50
CDF/MF	TC	168.24	51
MWCNT/MIL-53 (Fe)	TC	180.68	52
CNT/MnFe2O4	TC	86.48	53
CNT/β-CD/MnFe2O4	TC	89.53	53
ZnAl-LDH/biochar	TC	41.98	54
H3PW12O40/Fe3O4/MIL-88A (Fe)	TC	370.37	This work
H3PW12O40/Fe3O4/MIL-88A (Fe)	CIP	333.33	This work

---

3.3. Dye adsorption investigations

To study and evaluate the absorption potential of H₃PW₁₂O₄₀/Fe₃O₄/MIL-88A (Fe) nanocomposite for the removal of dyes from contaminated waters, two organic dyes (MB and MO) with different charges were used for initial experiments. The adsorption was monitored using their special absorption bands, which are 664, and 463 nm for MB and MO, respectively. The time-dependent UV-visible absorption spectra of dyes in the presence of H₃PW₁₂O₄₀/Fe₃O₄/MIL-88A (Fe) are shown in Fig. 12. Fig. 12(a) and (b) shows that the characteristic absorption band of cationic MB dye at 664 nm almost completely disappeared within 6 min but the disappearance of band at about 463 nm for MO is almost negligible. This result shows that the nanocomposite is a poor absorbent for removing anionic MO dye from aqueous solution.

Fig. 12 The UV-vis spectra during adsorption of (a) cationic MB and (b) anionic MO dyes over the H₃PW₁₂O₄₀/Fe₃O₄/MIL-88A (Fe): C_o (dye) = 30 mg L⁻¹, V_dye = 30 mL, adsorbent dosage = 20 mg, temp = 25 °C.

The selective uptake of dyes was investigated using the binary mixtures of MB + MO, MB + RhB, MO + RhB, and MB + MO + RhB with H₃PW₁₂O₄₀/Fe₃O₄/MIL-88A (Fe) as absorbent. As shown in Fig. 13(a) and (b), the preferable uptake of cationic MB and RhB dyes from the MB + MO and RhB + MO mixtures can be attributed to the anionic nature of H₃PW₁₂O₄₀/Fe₃O₄/MIL-88A (Fe). For further comparison, the binary mixture of cationic dyes (MB + RhB) was selected and the results in Fig. 13(c) revealed that MB and RhB were also adsorbed completely in the initial 3 min. As exhibited in Fig. 13(d), the representative peaks of MB + MO + RhB all disappeared quickly for the mixed dyes, and only the characteristic absorption peaks of MO remained, suggesting that H₃PW₁₂O₄₀/Fe₃O₄/MIL-88A (Fe) could selectively capture cationic dyes when utilized in the corresponding ternary mixture. This finding confirms that H₃PW₁₂O₄₀/Fe₃O₄/MIL-88A (Fe) nanocomposite also possesses selective adsorption ability towards cationic dyes in wastewater. So, the H₃PW₁₂O₄₀/Fe₃O₄/MIL-88A (Fe) nanocomposite exceptional adsorption properties towards the cationic MB and RhB molecules were clearly demonstrated. Some diverse effects on removal of colorants can be linked to the structure of the dye molecules and the nanocomposite as well. This is due to the existence of vastly electronegative PW₁₂O₄₀³⁻, which mostly enhanced the adsorption ability of the porous material MIL-88A, which together with enormous numbers of negative charges may have a stronger force with the positive charges of the colorants. The fabricated material in this study is not an efficient adsorbent for the removal of MO, because of negative charges of both the adsorbent and dye molecules. As a consequence, there is neither electrostatic attraction nor adsorption property between the PW₁₂O₄₀³⁻ encapsulated into the nanocomposite and MO molecules.

Fig. 13 Selective adsorption capability of H₃PW₁₂O₄₀/Fe₃O₄/MIL-88A (Fe) into the combined solution of (a) MB + MO, (b) MB + RhB, (c) MO + RhB, and (d) MB + MO + RhB. Conditions: C_o (MB) = C_o (RhB) = C_o (MO) = 35 mg L⁻¹, and adsorbent dosage = 20 mg in 30 mL.

3.4. Possible adsorption mechanism

The behavior of TC and MB adsorption onto H₃PW₁₂O₄₀/Fe₃O₄/MIL-88A (Fe) were studied by the adsorbent properties, the TC and MB nature, electrostatic interaction, π–π interaction, hydrogen bonding, van der Waals, hydrophobic interaction sand surface complexation in this study. MB has a flat extended conjugated structure, which has stronger π–π stacking interactions compared to the TC. In addition, MB is a cationic dye, which exists as positively charged in an aqueous solution. The adsorption efficiency of the MIL-88A (Fe), sample, despite its higher surface area, was lower than that of the H₃PW₁₂O₄₀/Fe₃O₄/MIL-88A (Fe) composite. This confirms that other factors influence the adsorption efficiency besides to the specific surface area of the adsorbent. After introducing the PW₁₂O₄₀³⁻ polyanions into the cavities of MIL-88A (Fe), it was confirmed that the adsorption capacity of the nanocomposite changed, in comparison with that of MIL-88A (Fe). This is because of the rapid and effective adsorption of cationic MB dye by PW₁₂O₄₀³⁻ polyanions, whereas it is unfavorable for H₃PW₁₂O₄₀/Fe₃O₄/MIL-88A (Fe) to adsorb the anionic dye MO. These findings show that the PW₁₂O₄₀³⁻ polyoxoanion clusters encapsulated in the mesoporous cages of MIL-88A (Fe) act as the active sites for the adsorption of cationic dye molecules. The zeta potential of a material is another key factor influencing its adsorption capacity; therefore, this was tested to understand further why the H₃PW₁₂O₄₀/Fe₃O₄/MIL-88A (Fe) sample can remove the cationic dyes MB more effectively than the anionic MO dye. The zeta potentials of MIL-88A (Fe) and H₃PW₁₂O₄₀/Fe₃O₄/MIL-88A (Fe) samples were measured as +4.59 and −17.85 mV, respectively, in water at natural pH. After modification with H₃PW₁₂O₄₀, the H₃PW₁₂O₄₀/Fe₃O₄/MIL-88A (Fe) nanocomposite showed a more negative zeta potential than the pristine MIL-88A (Fe) sample, thus, it can adsorb more of the cationic dyes such as MB, whereas it is unfavorable to adsorb the anionic dyes such as MO.

Also in this work, the adsorption efficiency of H₃PW₁₂O₄₀/Fe₃O₄/MIL-88A (Fe) was found to be higher as compared to pure H₃PW₁₂O₄₀, MIL-88A (Fe), and Fe₃O₄/MIL-88A (Fe). First, there are many open metal sites on MIL-88A (Fe) that facilitate the adsorption of specific guest molecules with corresponding functional groups.^55,56 Since the adsorption could happen inside the MOFs, hydrogen bonding may also occur during the tetracycline adsorption process. TC molecule has a complex structure with several carbonyls, amino and hydroxyl functional groups, and O-containing groups of the H₃PW₁₂O₄₀/Fe₃O₄/MIL-88A (Fe) composite such as OH, NH₂ and carboxylate.⁵⁷ The aromatic structure of the drugs molecule adsorption onto MIL-88A (Fe), series mainly occurred by π–π interaction. The O-containing groups in MIL-88A (Fe) and PW₁₂O₄₀³⁻ play an important role in the adsorption process.⁵⁸ The reason for high adsorption of TC and MB on samples should be further investigated.

3.5. Stability and reusability of the magnetic nanocomposite

To check the reusability of the H₃PW₁₂O₄₀/Fe₃O₄/MIL-88A (Fe) as an adsorbent, cycle experiments were carried out by applying a combination of methanol and deionized water as desorption solvents. For this goal, 10 mg of nanocomposite was added in TC and CIP solutions with a concentration of 50 mg L⁻¹, and stirred for 30 minutes, in continuing the nanocomposite was thoroughly detached by utilizing a magnet. In the next, the magnetic nanocomposite was eluted with a methanol solution several times and finally dried for 12 h at 25 °C and reused for another adsorption process under the same condition. The results in Fig. 14(a) show the adsorbent is reusable for at minimum 3 runs without notable loss of activity. As indicated in Fig. 14 (b)–(d), SEM, FT-IR, and XRD analyses of the recycled H₃PW₁₂O₄₀/Fe₃O₄/MIL-88A (Fe) after 4 runs were identical to the fresh nanocomposite. These results established that the structure of the magnetic nanocomposite was stable and was not altered by the reactants.

Fig. 14 (a) Reusability of the H₃PW₁₂O₄₀/Fe₃O₄/MIL-88A (Fe) nanocomposite in the removal of TC and CIP antibiotics, (b) SEM image, (c) FT-IR spectrum and (d) XRD pattern of the fresh and recovered nanocomposite after three runs.

4. Conclusions

In summary, a magnetically separable H₃PW₁₂O₄₀/Fe₃O₄/MIL-88A (Fe) adsorbent was successfully prepared using a hydrothermal method and its adsorptive property of organic dyes and antibiotics entering the environment were investigated. The tests showed that the magnetic nanocomposite H₃PW₁₂O₄₀/Fe₃O₄/MIL-88A (Fe) offers high removal efficiency against cationic dyes, TC, and CIP antibiotics. The maximum adsorption capacities of TC and CIP drugs onto H₃PW₁₂O₄₀/Fe₃O₄/MIL-88A (Fe) composite were found to be 370.33 and 333.33 mg g⁻¹, respectively. Also, reusability tests showed that H₃PW₁₂O₄₀/Fe₃O₄/MIL-88A (Fe) composite can be reused for three cycles without a considerable reduction in its performance. As a result, magnetic nanocomposite could be used as an efficient to remove cationic dyes antibiotics from water.

Conflicts of interest

There are no conflicts to declare.

Acknowledgements

The authors give their sincere thanks to the Lorestan University and Iran Nanotechnology Initiative Council (INIC) for all provided supports.

References

1. R. Hajavazzadeh, M. Kargar Razi and A. R. Mahjoub, Int. J. Nano Dimens., 2021, 12, 67–75.
2. A. Golshan Tafti, A. Rashidi, H.-A. Tayebi and M. E. Yazdanshenas, Int. J. Nano Dimens., 2018, 9, 79–88.
3. S. Ahmadi, L. Mohammadi, A. Rahdar, S. Rahdar, R. Dehghani, C. Adaobi Igwegbe and G. Z. Kyzas, Nanomaterials, 2020, 10, 556.
4. S. Rahdar, A. Rahdar, M. N. Zafar, S. S. Shafqat and S. Ahmadi, J. Mater. Res. Technol., 2019, 8, 3800–3810.
5. A. Jarrah and S. Farhadi, J. Solid State Chem., 2020, 285, 121264.
6. L. Zhu, L. Zong, X. Wu, M. Li, H. Wang, J. You and C. Li, ACS Nano, 2018, 12, 4462–4468.
7. Z. Wang, L. Song, Y. Wang, X.-F. Zhang and J. Yao, J. Phys. Chem. Solids, 2021, 150, 109839.
8. M. Fayazi, D. Afzali, R. Ghanei-Motlagh and A. Iraji, Environ. Sci. Pollut., 2019, 26, 18893–18903.
9. X. Song, J. Mo, Y. Fang, S. Luo, J. Xu and X. Wang, Environ. Sci. Pollut., 2022, 29, 35204–35216.
10. G. Gopal, H. Sankar, C. Natarajan and A. Mukherjee, J. Environ. Manage., 2020, 254, 109812.
11. C. Liu, L. Tan, L. Zhang, W. Tian and L. Ma, Front. Environ. Sci., 2021, 9, 692298.
12. Y. Zhang, Z. Jiao, Y. Hu, S. Lv, H. Fan, Y. Zeng, J. Hu and M. Wang, Environ. Sci. Pollut., 2017, 24, 2987–2995.
13. N. A. Elessawy, M. Elnouby, M. H. Gouda, H. A. Hamad, N. A. Taha, M. Gouda and M. S. M. Eldin, Chemosphere, 2020, 239, 124728.
14. Z. Liu, M. Zhu, L. Zhao, C. Deng, J. Ma, Z. Wang, H. Liu and H. Wang, Chem. Eng. J., 2017, 314, 59–68.
15. X. Liu, D. Yang, Y. Zhou, J. Zhang, L. Luo, S. Meng, S. Chen, M. Tan, Z. Li and L. Tang, Chemosphere, 2017, 182, 306–315.
16. M. Beiranvand, S. Farhadi and A. Mohammadi-Gholami, RSC Adv., 2022, 12, 34438–34453.
17. A. Azizi, J. Iran. Chem. Soc., 2021, 18, 331–341.
18. L. Zhang, P. Ma, L. Dai, S. Li, W. Yu and J. Guan, Catal. Sci. Technol., 2021, 11, 3834–3844.
19. M. Martínez-Cabanas, M. Lopez-Garcia, J. L. Barriada, R. Herrero and M. E. S. de Vicente, Chem. Eng. J., 2016, 301, 83–91.
20. J. Yang, Q. Xiao, X. Jia, Y. Li, S. Wang and H. Song, J. Hazard. Mater., 2021, 403, 124016.
21. W. Wang, W. Zhao, H. Zhang, J. Xu, L. Zong, Y. Kang and A. Wang, Powder Technol., 2021, 390, 303–314.
22. N. I. Gumerova and A. Rompel, Nat. Rev. Chem., 2018, 2, 1–20.
23. O. R. Tambunan and R. Mohadi, J. Sci. Technol., 2017, 2, 1–8.
24. N. A. Ramsahye, T. K. Trung, L. Scott, F. Nouar, T. Devic, P. Horcajada, E. Magnier, O. David, C. Serre and P. Trens, Chem. Mater., 2013, 25, 479–488.
25. S. M. Wang, J. Hwang and E. Kim, J. Mater. Chem., 2019, 7, 7828–7850.
26. C. T. Buru, P. Li, B. L. Mehdi, A. Dohnalkova, A. E. Platero-Prats, N. D. Browning, K. W. Chapman, J. T. Hupp and O. K. Farha, Chem. Mater., 2017, 29, 5174–5181.
27. A. M. Andani, T. Tabatabaie, S. Farhadi and B. Ramavandi, RSC Adv., 2020, 10, 32845–32855.
28. F. Mahmoudi and M. M. Amini, J. Water Proc. Eng., 2020, 35, 101227.
29. R. Narayan, U. Y. Nayak, A. M. Raichur and S. Garg, Pharmaceutics, 2018, 10(1–49), 118.
30. A. Benítez, J. Amaro-Gahete, D. Esquivel, F. J. Romero-Salguero, J. Morales and Á. Caballero, Nanomaterials, 2020, 10, 424.
31. V. P. Viswanathan, K. Divya, D. P. Dubal, N. N. Adarsh and S. Mathew, Dalton Trans., 2021, 50, 2891–2902.
32. A. Jarrah and S. Farhadi, RSC Adv., 2018, 8, 37976–37992.
33. T. Wang, P. Zhao, N. Lu, H. Chen, C. Zhang and X. Hou, Chem. Eng. J., 2016, 295, 403–413.
34. J. Wu, S. Jin, X. Wei, F. Gu, Q. Han, Y. Lan, C. Qian, J. Li, X. Wang and R. Zhang, Chem. Eng., 2021, 412, 128712.
35. A. Benítez de la Torre, J. Amaro-Gahete, D. Esquivel, F. J. Romero-Salguero, J. Morales and Á. Caballero, Nanomaterials, 2020, 10, 424.
36. Y. Yue, Z. Peng, W. Wang, Y. Cai, F. Tan, X. Wang and X. Qiao, Powder Technol., 2019, 347, 1–9.
37. Y. X. Song, S. Chen, N. You, H. T. Fan and L. N. Sun, Chemosphere, 2020, 255, 126917.
38. Y. K. Manea, A. M. Khan, A. A. Wani, M. T. Qashqoosh, M. Shahadat and M. A. Salem, J. Mol. Liq., 2021, 335, 116144.
39. Z. Li, M. Ma, S. Zhang, Z. Zhang, L. Zhou, J. Yun and R. Liu, J. Porous Mater., 2020, 27, 189–204.
40. A. Jain, A. Sharma, A. Kapur, S. Wadhawan, M. Garg, S. K. Pandey, S. Singh and S. K. Mehta, J. Nanostruct. Chem., 2021, 11, 437–453.
41. Y. Zhou, S. Cao, C. Xi, X. Li, L. Zhang, G. Wang and Z. Chen, Bioresour. Technol., 2019, 292, 121951.
42. G. Sharma, B. Thakur, A. Kumar, S. Sharma, M. Naushad and F. J. Stadler, Macromol. Mater. Eng., 2020, 305, 2000274.
43. Y. Ma, M. Li, P. Li, L. Yang, L. Wu, F. Gao, X. Qi and Z. Zhang, Bioresour. Technol., 2021, 319, 124199.
44. G. Zhang, R. Wo, Z. Sun, L. Xiao, G. Liu, G. Hao, H. Guo and W. Jiang, Front. Chem., 2021, 9, 707559.
45. Z. Song, Y. L. Ma and C. E. Li, Sci. Total Environ., 2019, 651, 580–590.
46. D. Qiao, Z. Li, J. Duan and X. He, Chem. Eng. J., 2020, 400, 125952.
47. H. Molavi, H. Moghimi and R. A. Taheri, Appl. Organomet. Chem., 2020, 34, e5549.
48. J. Bao, Y. Zhu, S. Yuan, F. Wang, H. Tang, Z. Bao, H. Zhou and Y. Chen, Nanoscale Res. Lett., 2018, 13, 396.
49. D. Naghipour, K. Taghavi, J. Jaafari, I. Kabdaşlı, M. Makkiabadi, M. Javan Mahjoub Doust and F. Javan Mahjoub Doust, Environ. Technol., 2023, 44, 150–160.
50. T. Ahamad, M. Naushad, T. Al-Shahrani, N. Al-Hokbany and S. M. Alshehri, Int. J. Biol. Macromol., 2020, 147, 258–267.
51. T. Ahamad, A. A. Chaudhary, M. Naushad and S. M. Alshehri, Int. J. Biol. Macromol., 2019, 134, 180–188.
52. W. Xiong, G. Zeng, Z. Yang, Y. Zhou, C. Zhang, M. Cheng, Y. Liu, L. Hu, J. Wan, C. Zhou, R. Xu and X. Li, Sci. Total Environ., 2018, 627, 235–244.
53. R. Foroutan, S. J. Peighambardoust, P. Latifi, A. Ahmadi, M. Alizadeh and B. Ramavandi, J. Environ. Chem., 2021, 9, 106344.
54. L. P. Hoang, T. M. P. Nguyen, H. T. Van, M. Yılmaz, T. K. Hoang, Q. T. Nguyen, T. M. H. Vi and L. T. Q. Nga, Environ. Technol. Innovation, 2022, 28, 102914.
55. Y. Sun, M. Chen, H. Liu, Y. Zhu, D. Wang and M. Yan, Appl. Surf. Sci., 2020, 525, 146614.
56. J. Yu, W. Xiong, X. Li, Z. Yang, J. Cao, M. Jia, R. Xu and Y. Zhang, Microporous Mesoporous Mater., 2019, 290, 109642.
57. X. Zhang, X. Lin, Y. He and X. Luo, Int. J. Biol. Macromol., 2019, 136, 445–459.
58. N. Tian, Q. Jia, H. Su, Y. Zhi, A. Ma, J. Wu and S. Shan, J. Porous Mater., 2016, 23, 1269–1278.

---

Footnote

† Electronic supplementary information (ESI) available. See DOI: https://doi.org/10.1039/d2ra07898h

---