Examining the adsorption efficiency of iron oxide-decorated magnetic halloysite nanotubes for tetracycline elimination from solutions

In our research, we investigated the potential of HNT-Fe3O4, a magnetic adsorbent composed of halloysite nanotubes(HNTs) and iron oxide nanoparticles, for removing tetracyclines(TCs) from water. We utilized a range of analytical techniques such as FT-IR, VSM, XRD, BET, TEM, TGA, and SEM to characterize the materials. Furthermore, we evaluated HNT-Fe3O4’s capability to remove TCs from water, particularly noting the influence of solution pH on its adsorption properties. Results indicated that the adsorption process adhered to the pseudo-second-order kinetics, with the Langmuir model best describing the adsorption isotherms. The reusability study revealed that after six repeated uses, HNT-Fe3O4 maintained most of its adsorption efficiency. An adsorption–desorption experiment further confirmed its potential as a valuable adsorbent for wastewater treatment. In summary, our study underscores the potential of HNT-Fe3O4 as a durable and effective medium for TCs removal in water treatment.


Introduction
In recent years, antibiotics have gained significant usage in combating microbial infections in both humans and animals.Notably, TCs have gained substantial popularity owing to their broad-spectrum antimicrobial effects, ranking as the second most commonly used antibiotics worldwide [1].Due to their natural tendency to mix with water, TCs are often expelled from the body through feces and urine, leading to a buildup of TCs residues in the environment.This buildup can potentially encourage the growth of antibiotic-resistant microbes, posing a threat to both human and animal health [2].As a result, there is an immediate need to find a cost-effective solution to remove TCs from our environment [3].
HNTs are aluminosilicate clays with a chemical formula of Al 2 Si 2 O 5 (OH) 4 •2H 2 O), which exhibit a unique tube-like structure formed by curving the halloysite sheets due to the lattice mismatch between the silicone tetrahedral and aluminum octahedral layers [17].The nano-tubular structure, combined with regular openending pores and abundant hydroxyl groups on their surfaces, makes HNTs highly promising as inorganic materials with versatile applications across various fields [18,19].Notably, they find extensive use in catalysis [20], drug delivery [21], nano-materials [22], and separation technologies [23].Moreover, HNTs offer a significant advantage over other nano-materials like MWNTs, as they are environmentally sustainable and highly abundant in nature as raw materials.
Magnetic separation technologies (MSTs) have been gaining traction lately [24], offering a promising method to extract magnetic substances from water, irrespective of their particle size.Among the various magnetic materials, Fe 3 O 4 , or magnetite, is particularly favored due to its superior magnetic characteristics, chemical robustness, and biocompatibility.
Integrating MSTs with the adsorption process has been extensively explored [25].However, there are relatively few reports on the synthesis of magnetic HNTs composites and their application as adsorbents for TCs removal.Addressing this research gap, we prepared HNT-Fe 3 O 4 composite by incorporating Fe 3 O 4 nanoparticles into HNTs.The composite was comprehensively characterized to assess its morphology, BET surface area, FT-IR spectra, XRD, TGA, and magnetic properties.Subsequently, we investigated the adsorption potential of HNT-Fe 3 O 4 for TCs, aiming to evaluate its efficacy as a magnetic adsorbent.

Materials
The HNTs were procured from Henan (PRC) and subjected to multiple sedimentation cycles to eliminate any quartz impurities.The purified HNTs were then dried at 80 °C for 12 h.The TCs were obtained from Shunbo Biological Engineering Co., Ltd (PRC), while FeCl 3 •6H 2 O, FeCl 2 •4H 2 O, and an aqueous solution of ammonia were sourced from Sinopharm Chemical Reagent Co., Ltd (PRC).All chemicals were of p.a. grade.Ultrapure deionized (DI) water was prepared using Purelab ultra (Organo, Japan).The stock solutions of TC (1 g L −1 ) were prepared by dissolving the compound in DI water, and the pH was adjusted by 1 M HCl or 1 M NaOH.

Synthesis of HNT-Fe 3 O 4
The HNT-Fe 3 O 4 composite was synthesized using a modified co-precipitation method [26].For the synthesis of HNT-Fe 3 O 4 , 1.0 g of HNT was suspended in a solution containing 1.18 g FeCl 3 •6H 2 O and 0.52 g FeCl 2 •4H 2 O with a final volume of 180 ml.The suspension was then heated to 60 °C under nitrogen atmosphere, and aqueous ammonia solution was added drop by drop to create Fe 3 O 4 .The pH of the mixture was kept between 9 and 11 throughout the process.Subsequently, the suspension was allowed to age at 70 °C for 4 h and afterwards washed with DI water (3×).The resulting products were finally oven-dried at 60 °C under reduced pressure.

Characterization
The FT-IR spectra were obtained using a Nicolet Nexus 470 FT-IR device (Thermo) at a resolution of 2 cm −1 between 400 and 4000 cm −1 , using KBr pellets as the sample medium.UV-vis spectra were recorded with a Specord 2450(Shimadzu, Japan).Magnetic properties were measured using a vibrating sample magnetometer (VSM, HH-15 from PRC) operating at magnetic fields up to 10 kOe.
For crystal structure characterization, XRD analysis was performed with a D/max-RA X-ray diffractometer (Rigaku, Japan) equipped with Ni-filtered Cu K α source (45 kV, 200 mA).SEM images were collected using an S-4800 SEM device (HITACHI, Japan), while TEM micrographs were captured with a JEOL-JEM-2010 (JEOL, Japan) operating at 200 kV.
Nitrogen adsorption-desorption isotherms were measured at 77 K using a gas adsorption analyzer (Autosorb-iQ, Quantachrome, USA).Before the measurements, the samples underwent degassing at 473 K for 4 h under reduced pressure.Based on these measurements, the BET specific surface area and the pore volume of the nanocomposite were derived from the corresponding mathematical models.

Adsorption experiments
The adsorption of TCs on HNT-Fe 3 O 4 composite was analyzed using in a thermo shaker with a rotating speed of 100 rpm and a temperature of 25 °C for 80 min.A predetermined mass of HNT-Fe 3 O 4 was equilibrated with 10 ml of TCs solution having known, previously determined concentrations and pH values.The final pH of the suspension was set by adding 1 M HCl or 1 M NaOH.Once the adsorption process was completed, the HNT-Fe 3 O 4 particles were removed from the solution using a magnet, and the TCs concentration in the supernatant was analyzed via UV-vis spectrophotometry at 275.6 nm.
The adsorption tests were performed under various time intervals (0-180 min), solution pH values (3.0-10.0),and initial TCs concentrations (200-800 mg l −1 ) in a final volume of 10 ml.The amount of TCs adsorbed on HNT-Fe 3 O 4 at equilibrium was calculated from the formula: In this equation, Q e and C stand for adsorption capacity (in mg l −1 ) and concentration of TCs (in mg l −1 ) at the beginning of the experiment (index 0) and at the equilibrium (index e).V represents the volume of the system, and m is the mass of HNT-Fe 3 O 4 composite.
To study the adsorption kinetics, TCs solution with a starting concentration of 200 mg L −1 was used, and aliquots were collected at regular time intervals to follow the process.At each interval t, the corresponding adsorption capacity was derived using the formula equivalent to equation (1):

Results and discussion
3.1.FT-IR Figure 1 displays the FT-IR spectra of both HNTs and HNT-Fe 3 O 4 .In the spectra, the two sharp maxima at 3692 and 3620 cm −1 are assigned to the stretching vibrations of HNTs -OH groups, while the peak observed at 1030 cm −1 is associated with Si-O stretching modes.Furthermore, the peaks at 468, 543, and 911 cm −1 are attributed to the deformation modes of Si-O-Si, Al-O-Si, and O-H vibrations, respectively [27].Notably, the vibration band of HNTs at 543 cm −1 is found to overlap with the high-intensity Fe 3 O 4 absorption band at ∼580 cm −1 in HNT-Fe 3 O 4 [28].This observation suggests the successful incorporation of Fe 3 O 4 nanoparticles into the HNTs structure, indicating the formation of the desired composite.

SEM and TEM analysis
The morphology of both HNTs and HNT-Fe 3 O 4 composite was thoroughly examined using SEM, and the data are presented in figures 2. In figure 2(a), the HNTs exhibit a tubular structure with irregular morphology, where their lengths vary between 1.0 and 3.0 μm, according to the particle size distribution in SEM images, 50% of the length of HNTs is 1.5 μm, 30% of the length of HNTs is 2 μm.

Magnetic characteristics of HNT-Fe 3 O 4
The magnetization properties of HNT-Fe 3 O 4 were investigated using VSM, and the results are presented in figure 6.The magnetization curves exhibited symmetrical behavior with no hysteresis, indicating typical superparamagnetic characteristics of the particles [32].The saturation magnetization values of HNT-Fe 3 O 4 measured at 298 K were found to be 16.7 emu g −1 , and were positively correlated with the magnetic field strength.
Furthermore, a separability test was conducted to demonstrate the fast separation capacity of HNT-Fe 3 O 4 , as depicted in figure 6.In the presence of an external magnetic field, the HNT-Fe 3 O 4 particles were rapidly attracted to the vial wall within approximately 90 s, resulting in a homogeneous dispersion without the need for an extrinsic magnetic field.This exceptional property indicates the easy separability of HNT-Fe 3 O 4 using an external magnet, underlining its fast separation ability after the adsorption of TCs.Therefore, it can be concluded that HNT-Fe 3 O 4 has practical applications as a highly effective adsorbent for TCs due to its fast separation ability with the assistance of an external magnet.The superparamagnetic behavior of HNT-Fe 3 O 4 ,  combined with its high saturation magnetization, enables efficient and swift removal of TCs from aqueous solutions, making it a promising candidate for practical applications in environmental remediation.

Thermogravimetric analysis
Thermal analysis is essential for identifying temperature ranges associated with significant weight loss.In figure 7, we present the TG curves of both HNTs and HNT-Fe 3 O 4 .Interestingly, the curves reveal a peak weight loss occurring at approximately 500 °C for both samples.The total weight loss for HNTs and HNT-Fe 3 O 4 was found to be 12.5% and 7.1%, respectively.This weight reduction can be attributed to the dehydroxylation of the aluminol groups within the HNTs structure [29].The result demonstrates that HNT-Fe 3 O 4 exhibits good thermal stability at high temperatures and is not susceptible to thermal decomposition or failure.The lower weight loss in HNT-Fe 3 O 4 compared to HNTs can be attributed to the protective effect of Fe 3 O 4 nanoparticles, which likely hinder the dehydroxylation reaction to some extent.

BET analysis
The BET isotherms of both HNTs and HNT-Fe 3 O 4 (figures 8 and 9) were assigned to type IV in the traditional BDDT classification, suggesting the presence of mesoporous features in these materials [33].However, the hysteresis loops observed in these isotherms were classified as type H3 according to IUPAC [34], suggesting the occurrence of slit-shaped pores.Furthermore, the specific surface area (S BET ) and total pore volume (V t ) of the HNTs were 33.422 m 2 g −1 and 0.289 cm 3 g −1 , respectively, while for HNT-Fe 3 O 4 , they increased significantly to 91.232 m 2 g −1 and 0.318 cm 3 g −1 , respectively.This notable increase in S BET and V t in HNT-Fe 3 O 4 can be attributed to the incorporation of Fe 3 O 4 nanoparticles into the HNT structure.This enhanced porosity and surface area hold significant promise for improved adsorption capacity and efficiency of HNT-Fe 3 O 4 as an adsorbent for TCs.

Adsorption kinetic studies
In this study, we investigated the adsorption behavior of TCs on HNT-Fe 3 O 4 and determined the optimal kinetic model to describe the process.Two kinetic models, the pseudo-first-order (PFO) and the pseudosecond-order (PSO) equations, were applied to elucidate the adsorption mechanism [35].The respective amounts of TCs adsorbed at equilibrium (Q e ) and time t (Q t ) are represented by equations (3) and (4): The rate constant k 1 (in min −1 ) for the PFO model is determined by plotting ln(Q e -Q t ) versus t, while the rate constant k 2 (in g mg −1 min −1 ) for the PSO model is derived from the plot of t/Q t versus t.  Figure 10 displays the fitting curves of HNT-Fe 3 O 4 using the PFO and PSO kinetic equations.The study reveals that the PSO kinetic model provides better correlation coefficients (R 2 ) and is more suitable for describing the adsorption of TCs on HNT-Fe 3 O 4 than the PFO kinetic model [29].table 1 summarizes the values for the starting adsorption rate (h, mg g −1 min −1 ) and the equilibrium half-time (t 1/2 , min) derived from PSO model represented by equations ( 5) and (6) [36].
The main result of this analysis is the confirmation that the adsorption of TCs on HNT-Fe 3 O 4 adheres to the PSO model between experimental and calculated values of Q e (with R 2 >0.99), indicating a physisorption process in the adsorption process for TCs.

Adsorption isotherm study
The equilibrium adsorption experiments were performed to investigate the binding properties of HNT-Fe 3 O 4 for TCs.To analyze the TCs adsorption data, two adsorption isotherms, namely the Langmuir and Freundlich models, were employed, represented by the equations [37] (figure11):  Langmuir model: Freundlich model: In these equations, Q e (mg g −1 ) represents the equilibrium amount of TCs adsorbed, C e (mg L −1 ) denotes the equilibrium concentration of TC in a solution, Q m (mg g −1 ) is a constant representing the maximum monolayer adsorption capacity of HNT-Fe 3 O 4 , and K L (L mg −1 ) is a constant indicating the adsorbent's affinity towards the adsorbate.Moreover, K F (mg g −1 ) and n represent Freundlich constants.Q e, c (mg Figure 11 shows the Langmuir and Freundlich plots of HNT-Fe 3 O 4 , with the parameters of both models listed in table 2. The Langmuir equation had higher R 2 values than the Freundlich model, indicating a better fit for the Langmuir model.The theoretical Q m for TCs agreed well with the experimentally determined value, confirming the homogeneity of the HNT-Fe 3 O 4 surface.After adsorption, the surface properties of the adsorbent are uniform, only adsorbing a single layer of tetracycline molecules, and there is no interaction between the adsorbed adsorbatesl [31].These results can be well explained by the Langmuir model [38].The Freundlich equation, which describes adsorption on heterogeneous surfaces with different energy levels of adsorption and nonidentical adsorption sites, is therefore not a suitable model for HNT-Fe 3 O 4 .

Effect of pH on TC binding
The pH value plays a crucial role in determining the ionization state and solubility of TCs in water.It can also influence the charge of the adsorbent surface and thus impact the mutual adsorbent/adsorbate interactions.In figure 8, the impact of pH on TCs adsorption is demonstrated.TCs are triprotic acids with pK a1 , pK a2 , and pK a3 of 3.3, 7.7, and 9.7, respectively.When the solution pH is below the pK a2 of TCs, the amount of TCs adsorbed ranges from 13 to 14.6 mg g −1 at initial concentrations of 200 mg L −1 , as illustrated in figure 12.However, when the pH is above pK a2 , TCs adsorption sharply decreases to 7 mg g −1 at pH 10.These results suggest the importance of electrostatic attractions, with TCs being adsorbed onto the HNT-Fe 3 O 4 surface through cation exchange.
The isoelectric point (pH iep ) of HNTs is 2.5 [39].Notably, all pH values, both before and after adsorption, significantly surpass this point, indicating a negatively charged surface on the HNTs throughout the entire adsorption process.At a lower pH, there may be some competition between H + ions and TCs, resulting in a decrease in TCs adsorption on HNT-Fe 3 O 4 surfaces.However, the constant amount of TCs adsorbed from pH 3 to pK a2 suggests that TCs in cationic and zwitterionic forms exhibit a higher affinity for HNT-Fe 3 O 4 surfaces and are less affected by H + ions.

Reusability studies
In considering the economic feasibility of utilizing HNT-Fe 3 O 4 for adsorption purposes, it is crucial to assess its ability to maintain adsorption capacity after repeated processes.To evaluate this aspect, we conducted six consecutive adsorption-desorption cycles using 0.01 M NaOH as the desorption agent.figure 13 illustrates the reusability and adsorption capacity of HNT-Fe 3 O 4 for TCs.Remarkably, we observed that the adsorption capacity decreased slightly from 97.4% to 91.2% after six cycles.This result indicates the superior reusability of HNT-Fe 3 O 4 , making it suitable for practical applications in removing TCs from wastewater.

Conclusions
We synthesized a magnetically retrievable adsorbent, HNT-Fe 3 O 4 , via the coprecipitation technique.Our study aimed to assess its efficacy in extracting TCs from aqueous solutions.The findings confirm that HNT-Fe 3 O 4 serves as a potent adsorbent for TCs.Importantly, this material demonstrates robust adsorption capabilities across a broad pH spectrum (up to 8) and retains its efficiency even in the presence of elevated TCs concentrations.
The adsorption process on HNT-Fe 3 O 4 is primarily governed by physical adsorption, driven by electrostatic attraction.We found that the pseudo-second-order equation provides a better fit for the adsorption kinetics, while the adsorption isotherms are better described by the Langmuir model.These findings shed light on the underlying mechanisms and the potential application of HNT-Fe 3 O 4 as an efficient adsorbent.
Another significant advantage of HNT-Fe 3 O 4 is its excellent reusability.After six repeated adsorptiondesorption cycles, the adsorbent displayed good stability, with the adsorption capacity only slightly decreasing from 97.4% to 91.2%.HNT-Fe 3 O 4 exhibited notable adsorption capacity and regeneration properties, and could be swiftly and effortlessly separated from the solution phase with the assistance of magnetic force.These characteristics potentially make them suitable for applications in wastewater treatment, biological molecule separation, and drug extraction.
Notably, figure 2(b) reveals the presence of Fe 3 O 4 clusters on the surface of HNTs, confirming the formation of HNT-Fe 3 O 4 composite [29].This observation is likely due to the plentiful hydroxyl groups, extensive surface area, and significant pore volume of HNTs.Additionally, figure 3 displays TEM images of both HNTs and HNT-Fe 3 O 4 .In figure 3(a), the HNTs are depicted as open-ended nanotubes with a hollow cavity measuring about 20 nm in diameter [30].
Figure 3(b) shows Fe 3 O 4 clusters situated on the surface of HNT-Fe 3 O 4 , with some Fe 3 O 4 nanoparticles distributed within the inner lumen of the HNTs.The TEM visuals indicate that HNTs have an expansive surface area, a suitable quantity and arrangement of hydroxyl groups, coupled with a notable pore volume, making them effective adsorbents for TCs.Notably, the TEM micrographs highlight the superior dispersion of HNTs and HNT-Fe 3 O 4 .The uniform and well-dispersed magnetic clusters on the HNTs surface are indicative of a successful synthesis process.This enhanced dispersion of magnetic nanoparticles on the HNTs can significantly contribute to the overall adsorption capacity and efficiency of the HNT-Fe 3 O 4 composite.

Figure 6 .
Figure 6.The VSM magnetization profiles of HNT-Fe 3 O 4 recorded at room temp.Inset: an image of HNT-Fe 3 O 4 aqueous dispersion with (left) and without (right) the presence of an external magnetic field.

Figure 10 .
Figure 10.The experimental data fitting for the kinetics of TCs adsorption on the surface HNT-Fe 3 O 4 using the PFO and PSO models.

Figure 11 .
Figure 11.The adsorption isotherms for the binding of TCs on HNT-Fe 3 O 4 surface.

Table 1 .
The statistics and kinetic constants obtained by fitting the experimental data for TCs adsorption on HNT-Fe 3 O 4 at 298 K into PFO and PSO models.PFO model PSO model C 0 (mg L −1 ) Q e , exp (mg g −1 )

Figure 12 .
Figure 12.The influence of pH values on the amount of TCs adsorbed on HNT-Fe 3 O 4 .