Tetracycline Adsorption from Aqueous Media by Magnetically Separable Fe3O4@Methylcellulose/APTMS: Isotherm, Kinetic and Thermodynamic Studies

This study aimed to synthesize Fe3O4@Methylcellulose/3-Aminopropyltrimethoxysilane (Fe3O4@MC/APTMS) as a new magnetic nano-biocomposite by a facile, fast, and new microwave-assisted method and to be utilized as an adsorbent for tetracycline (TC) removal from aqueous solutions. Fe3O4@MC/APTMS was characterized by Fourier transform-infrared (FTIR), Field emission scanning electron microscopy (FESEM), Energy dispersive spectroscopy (EDS), Mapping, X-ray diffraction (XRD), Thermal gravimetric analysis (TGA), Brunauer–Emmett–Teller (BET) and vibrating sample magnetometer (VSM). The point of zero charge (pHzpc) value of the nano-biocomposite was estimated to be 6.8 by the solid addition method. Optimum conditions were obtained in TC concentration: 10 mg L−1, adsorbent dosage: 80 mg L−1, contact time: 90 min, and solution pH 6 with the maximum TC removal of 90% and 65.41% in synthetic and actual samples, respectively. The kinetic and isotherm equations pointed to a pseudo-second order kinetic and Langmuir isotherm optimum fitting models. Based on the values of entropy changes (ΔS) (50.04 J/mol k), the enthalpy changes (ΔH) (9.26 kJ/mol), and the negative Gibbs free energy changes (ΔG), the adsorption process was endothermic, random, and spontaneous. The synthesized adsorbent exhibited outstanding properties, including proper removal efficiency of TC, excellent reusability, and simple separation from aqueous media by a magnet. Consequently, it is highly desirable that Fe3O4@MC/APTMS magnetic nano-biocomposite could be used as a promising adsorbent for TC adsorption from aqueous solutions.


Introduction
Over the last 30 years, pharmaceuticals have been regarded as one of the most important water pollutants because of their wide variety, consumption, and particular environmental resistance. These compounds are known as micropollutants, which can enter the environment from point sources such as sewage treatment plants and nonpoint sources such as herbicides, insecticides and excess fertilizers from agricultural lands [1][2][3]. Antibiotics have been categorized as emerging pollutants due to their negligent usage, continuous input, and persistence in various environmental matrices even at low concentrations [4][5][6]. Antibiotics consumption went up by 65% from 2000 to 2015. In 2017, United Nations (UN) experts warned that the improper disposal of antibiotics could produce the generation of "ferocious superbugs" [7]. Tetracyclines (TCs) are among the most consuming antibiotics and are produced by actinomycetes. These antibiotics can be divided into four groups, including oxytetracycline, tetracycline, chlortetracycline, doxycycline. They have been frequently used in human and veterinary medicines along with feed additives for several decades [8]. As a typical antibiotic, TC production and usage can be classified as the second rank worldwide. Approximately 70-90% of the primary TC is released into the environment over urine and feces due to their low biodegradability and stable chemical structure [9]. It was reported that TC concentration in the wastewater released from aquaculture farms up to 20 mg L −1 [10]. Multiple methods have been reported to remove antibiotics including biological treatment [11], chlorination [12], advanced oxidation processes (AOPs) [13][14][15], electrochemical treatment [16], adsorption [3,[17][18][19][20][21][22], membrane processes [23], and ultrasonic cavitation effect method [24,25]. AOPs may cause the formation of recalcitrant and ecotoxic oxidation by-products, which hinder their usage in wastewater treatment plants [26]. The use of membrane separation technologies has been limited due to fouling issues [27]. Conventional wastewater treatment plants (WWTPs) are not designed with the particular focus of removing pharmaceuticals. Stackelberg et al. [28] and Kulkarni et al. [29] reported that conventional WWTPs, mainly based on microorganisms, do not effectively destroy antibiotics due to their low concentrations in water and intricate molecular structure. The adsorption method has been regarded as the most successful technology to remove antibiotics from aqueous solutions owing to its low cost, high efficiency, and no risk of toxic by-products [8]. Several adsorbents, including activated carbons (ACs) [30], multi-walled carbon nanotubes (MWCNTs) [31], natural clay materials such as bentonite [32], ion exchange materials, and biochars (BCs), have been studied for antibiotic removal [33]. Methods based on nanotechnology are one of the newest methods to remove antibiotics. The application of nanomaterials is considered as fundamental for more efficient wastewater treatment [19,34]. Nanoadsorbents have been reported as a new generation of adsorbents with higher adsorption capacity, making them more efficient for the adsorption of antibiotics and other emerging pollutants [17,18,20,21,33]. Furthermore, there are numerous nanoadsorbents used to remove antibiotics, including graphene-based nanoadsorbents [35], gold nanoparticles [36], zero-valent iron (ZVI) nanoparticles [37] and titanium dioxide nanoparticles [38]. Filtration and centrifugation methods are used to separate the non-magnetic adsorbents from an aqueous solution [39]. These techniques are time-consuming and require extra cost [40]. Magnetic adsorbents are a new class of adsorbents that could be used to remove drugs, heavy metals, oil, toxic organic compounds, dyes, and several biomolecules [41]. The production of magnetic adsorbents can solve this problem by assimilating magnetic nanoparticles on the adsorbent surfaces and their separation by the efficient, fast, and economical magnetic separation method [42,43]. As the most abundant renewable biopolymer, Cellulose consists of three hydroxyl groups (-OH) per hydroglucose unit and is naturally produced about 10 11 -10 12 tons per year [44]. Cellulose-based nanomaterials are favorable adsorbents because of their high surface area-to-volume ratio, Affordability and intrinsic environmental inertness [45]. Cellulose-based adsorbents are fascinating materials, which could be readily adsorbed on the surfaces iron oxides such as Fe 2 O 3 and Fe 3 O 4 [46]. Cellulose etherification is considered as the most important route for cellulose derivatization, and methylcellulose is one of the main cellulose ethers as well as carboxymethylcellulose (CMC). Besides, MC derivatives such as hydroxypropyl methylcellulose (HPMC), hydroxyethylcellulose (HEC), hydroxypropyl cellulose (HPC), and ethylcellulose (EC) are conventionally used for coatings, films, membranes, drilling, pharmaceuticals, and foods [47,48]. MC has numerous outstanding properties, including good solubility, high chemical stability, and is toxicologically innocuous [48]. Crude cellulose has low solubility, weak interaction with contaminants, low surface area, weak interaction with contaminants, and reuse problems [46]. The conjugation of MC with magnetic nanoparticles leads to more efficient biomagnetic adsorbents for removing organic contaminants in water and wastewater. [21]. Some properties of the cellulose-based adsorbents could be changed via modification [49]. Also, the surface of cellulose nanomaterials (CNs) is readily functionalizable, which allows for the cohesion of chemical portions that can enhance the binding efficiency of pollutants to the cellulose nanomaterials finally [45]. Cellulose-based nanomaterials are used to remove various toxic pollutants such as heavy metals, dyes, oil, pharmaceuticals, pesticides, and radioactive metals from aqueous solutions [46]. Besides, the application of different cellulose-based magnetic adsorbents such as ZnFe 2 O 4 @MC [21], CoFe 2 O 4 @MC, ZnFe 2 O 4 @CMC [2] and CuFe 2 O 4 @MC [3] for antibiotic adsorption has been studied recently. In general, chemical modification of cellulose materials results in higher adsorption capacities than unmodified cellulose forms [50]. 3-Aminopropyltrimethoxysilane (APTMS) is an organofunctional silane compound that contains amino groups. Silanization with coupling agents such as APTMS and 3-Aminopropyltriethoxysilane (APTES) has attracted extensive interest for the functionalization of nanoparticles with free amine groups [51]. Some studies reported that the surface modification of the nanoparticles could improve the mechanical properties of the coating. Palimi et al. [52] and Kegl et al. [53] Reported that surface modification by APTMS causes an enhancement in both hydrophobicity and removal efficiency.
This study has been aimed to synthesize a magnetic and eco-friendly nano-biocomposite for the removal of TC from aqueous solutions. Process parameters, including solution pH, initial concentration, adsorbent dosage, and contact time, were optimized, and adsorption isotherms, kinetics, and thermodynamics were investigated. The nano-biocomposite reusability and stability were also determined. To the best of our knowledge, Fe 3 O 4 @ MC/ APTMS has neither been synthesized nor used for the adsorption and removal of TC from aqueous solutions.  ), methylcellulose, and sodium hydroxide (NaOH) were obtained from Merck (Germany), APTMS (C 6 H 17 NO 3 Si, 95%) was obtained from ACROS (USA). Stock standard solution of TC at a concentration of 20 mg L −1 was prepared with distilled water.

Preparation of Fe 3 O 4 @MC/APTMS
Fe 3 O 4 @MC/APTMS magnetic nano-biocomposite was prepared in two stages. In the first stage, stoichiometric amounts of metal chlorides and MC (10.8 g FeCl 3 , 4 g FeCl 2, and 1 g MC) were dissolved in 100 mL of distilled water. NaOH (12 g) was added within 1 h to alkalize the pH of the solution. Then, the suspension was placed in the microwave (SAMSUNG, 2450 MHz, 800 W) at 450 W for (3 × 5) min to obtain a dark brown precipitate. The obtained precipitate was separated using a magnet, washed several times with distilled water, and placed in an oven for 24 h at 70 °C. In the second stage of the synthesis, APTMS (4 mL) was dissolved in ethanol (50 mL). Next, the solution was added to the reaction container and stirred under reflux condition for 8 h at 80 °C to obtain Fe 3 O 4 @MC/APTMS as a product. The obtained residue was separated using a magnet and washed with distilled water to neutralize the final pH. Finally, the dark brown precipitate was dried for 24 h at 25 °C. The schematic of preparation of Fe 3 O 4 @MC/APTMS nanobiocomposite is shown in Fig. 1.

Batch Adsorption Experiments
Batch experiments were conducted to study the TC adsorption on synthesized Fe 3 O 4 @MC/APTMS in the synthetic solutions at room temperature in triplicates. A stock solution of TC was prepared by dissolving pure TC (0.02 g) in distilled water (100 mL). Different concentrations of TC used in batch experiments were obtained by diluting the individual Fig. 1 Schematic of preparation of Fe 3 O 4 @MC/APTMS magnetic nano-biocomposite stock solution. Specific amounts of adsorbent were added to a beaker containing 100 mL of the TC mixture. Suspensions were agitated by a magnetic stirrer at 300 rpm. Then, filtered using a 0.22 μm syringe filter (Jet Biofil), and the residual concentration of TC was measured at the maximum wavelength of 358 nm by UV/Vis spectrophotometer (Shimadzu 1800). We investigated the effective parameters on the adsorption process, including solution pH (3, 5, 6, 9, and 11) that adjusted with 0.1 M HCl or NaOH with a pH meter (HANNA 211), adsorbent dosage (60, 80, 100, 120, and 150 mg L −1 ), initial antibiotic concentration (5, 10, 15, 20, and 30 mg L −1 ), contact time (20,40,60,90, and 120 min), and temperature of the solution (298, 303, 308, 313, and 318 K). The removal efficiency of TC was calculated using Eq. (1) [54]. The adsorption capacity at equilibrium (q e ) and amount of soption at time t (q t ) were calculated using Eqs. (2) and (3), respectively [22].
where C 0 , C e, and C t are TC concentrations (mg L −1 ) at the initial time, equilibrium, and time t (min), respectively. V is the volume of the solution (L). M is the mass of the adsorbent (g).
Optimum conditions obtained from batch experiments on the synthetic solutions were investigated on the sanitary wastewater sample. High-performance liquid chromatography (HPLC) device (Shimadzu LC-10AD VP, Japan) was utilized to identify and measure TC concentration in the samples. Details of the HPLC analysis were included mobile phase: acetonitrile: deionized water 30:70 (v/v), column model: C18, 250 × 4.6 mm, 5 μm, detector: UV at the wavelength of 280 nm, injection volume: 20 μL at a flow rate of 1 mL min −1 [55].
The adsorbent used in the experiments was reused for the subsequent cycles. After each run, the nanoadsorbent was separated by a magnet, washed several times with ethanol and distilled water, dried at 40 °C, and reused in five runs.

Kinetics, Isotherms, and Thermodynamics Studies
Langmuir, Freundlich, Temkin, and Dubinin-Radushkevich (D-R) isotherm models were employed to study isotherms of TC adsorption by Fe 3 O 4 @MC/APTMS at initial antibiotic concentrations of 5, 10, 15, 20, and 30 mg L −1 at 25 °C. pseudo-first order, pseudo-second order, Intraparticle, and  (4) and (5) were used. In Eq. (4), ∆G is Gibbs free energy changes, R is the gas constant with a value of 8.314 J/mol K, T is the temperature of the reaction in Kelvin, and K d is the equilibrium constant. In Eq. (5), ∆S is the standard entropy changes (kJ/ mol), ∆H is the standard enthalpy changes (kJ/mol), and R is the ideal gas constant (J/mol. K). Both energy and entropy factors determine what processes would occur spontaneously [56,57].

Fe 3 O 4 @MC/APTMS Magnetic Nano-biocomposite Characterization
A new microwave-assisted co-precipitation method was employed to synthesize magnetic Fe 3 O 4 @MC/APTMS nano-biocomposite. Microwave assisted synthesis is a novel and promising technique in comparison to other methods existing for the synthesis of nanoparticles. Also, this technique has advantages including fast and uniform heating, no selective surface heating, energy savings, shorter preparation time and low preparation cost [58,59]. Several analytical techniques were employed to characterize the prepared magnetic nano-biocomposite. Figure 2 illustrates a comparison of the KBr pellet FTIR spectra of MC, APTMS and Fe 3 O 4 @MC/APTMS using an FTIR spectrophotometer between 500 and 4000 cm −1 . According to the FTIR spectra of MC, the broad peak observed at 3461 cm −1 was due to O-H stretching. In comparison, the peaks at 2931 cm −1 and 1644 cm −1 were related to asymmetric C-H stretching and adsorbed water stretching, respectively. Furthermore, the peaks at 1458.17 cm −1 1376.04 cm −1 confirmed the C-H bending of methylene and methyl groups, respectively. Also, the C-O stretching was observed at 1100-1150 cm −1 [60].
Moreover, in the FTIR spectra of APTMS, the broad peak at 3429 cm −1 could be due to O-H stretching, while the peak at 2931 cm −1 may be related to the asymmetric C-H stretching. The bands at ~ 1603 and ~ 1472 cm −1 correspond to the bending mode of the N-H group. The bands appearing at ~ 1030 and ~ 774 cm −1 could be assigned to vibrational modes in which bridging oxygen atoms participate in Si-O-Si and Si-O bonds. [61,62].
In the FTIR spectra of Fe 3 O 4 @MC/APTMS, the peak at 3433 cm −1 corresponded to the combined O-H and N-H stretching. The peaks at 2925 cm −1 and 1634 cm −1 corresponded to the -CH 2 and NH 2, respectively, which proves the existence of the functionalized NH 2 groups on the Fe 3 O 4 @ MC surface. Furthermore, the peak observed at 1384 cm −1 corresponded to N-O bending. In 586 and 635 cm −1 bands, two strong peaks can be attributed to the stretching vibration mode, which corresponds to the Fe-O absorption band in the Fe 3 O 4 crystal lattice [63]. Our findings are in good agreement with previous findings reported by Amirmahani et al. [62] and Palimi et al. [52]. Figure 3 depicts the morphology of the magnetic nano-biocomposite. FESEM images of Fe 3 O 4 @MC/ APTMS illustrated the establishment of spherical-shaped nanoparticles (average particle size: ~ 52 nm). The surface morphology of the spherical Fe 3 O 4 @ MC/APTMS particles is compact, smooth, uniform and without agglomeration.
The EDS spectrum of Fe 3 O 4 @MC/APTMS is presented in Fig. 4a. The EDS of Fe 3 O 4 @MC/APTMS can define the chemical combination and pureness of the prepared magnetic nano-biocomposite. Fe 3 O 4 @MC/APTMS was composed of (73.38% Fe, 18.79% O, 5.45% C, 1.99% N and 0.39% Si). In this matter, the iron signal arose from the existence of the Fe 3 O 4 , the carbon signal derived from the presence of MC, and the nitrogen and silicon signal were attributed to the presence of APTMS in the Fe 3 O 4 @ MC/APTMS nano-biocomposite. The results of EDS confirmed the presence of the expected chemical composition of Fe 3 O 4 @MC/APTMS [42,62].  Our results are in agreement with those obtained by Amirmahani et al. [62]. Also, the average particle size of Fe 3 O 4 @ MC/ATPMS was found to be around 0.3 nm from the Scherrer Eq. (6) [57].
where, D (nm), β, λ (nm) and θ are the particle diameter, the line broadening at half the maximum intensity X-ray wavelength and Bragg angle.
Concerning the importance of adsorbent thermal stability, the TGA of Fe 3 O 4 @MC/APTMS (Fig. 6) was carried out in a range from room temperature to 800 °C (20 °C/min). Thermal decomposition of Fe 3 O 4 @MC/APTMS took place in two stages. The first stage of weight loss of Fe 3 O 4 @MC/ The textural properties of the as-prepared magnetic nanobiocomposite were characterized by BET analysis. The BET, Barrett Joyner Halenda (BJH) adsorption-desorption isotherms of the as-prepared Fe 3 O 4 @MC/APTMS are shown in Fig. 7a-c. The specific surface area of the Fe 3 O 4 @MC/ APTMS magnetic nano-biocomposite was 67 m 2 /g. The mean pore diameter and total pore volume (p/p 0 = 0.990) were 15.2 nm and 0.26 cm 3 /g, respectively. The International Union of Pure and Applied Chemistry (IUPAC) reported that pores are divided into microporous, mesoporous, and macroporous materials. A mesoporous material is a nanoporous material consisting of pores with diameters between 2 and 50 nm. Furthermore, microporous and macroporous pore sizes are smaller than 2 nm and larger than 50 nm, respectively [18,63]. Based on evaluating the BET specific area of Fe 3 O 4 @MC/APTMS, The synthesized magnetic adsorbent could be classified as a mesoporous material [2,18].

Effect of TC Initial Concentration
Variations in the removal efficiency of TC and its q e by the magnetic Fe 3 O 4 @MC/ATPMS nano-biocomposite at different concentrations of TC and contact times, solution pH of 6 and adsorbent dosage of 80 mg L −1 are indicated in Fig. 9a.
As shown in Fig. 9a, with a growth in the initial TC concentration from 5 to 30 mg L −1 , the q e value increased from 5.7 to 24.04 mg g −1 . The initial concentration of TC provides a Significant driving force of concentration gradient to dominate the resistance force of mass transfer of TC among the solid and solution phase [64]. Conversely, an enhancement in the TC concentration reduced the removal efficiency from 91.36 to 64.11% (see Fig. 9b), owing to the decrease of available sites on the surface of adsorbent for TC adsorption [68].
Accordingly, the optimum TC concentration of 10 mg L −1 was regarded as the optimum TC concentration. Before saturation of the nano-biocomposite surface, with an extension of the contact time to 90 min, the removal efficiency also increased. At low TC concentrations, the ratio of the initial amount of TC molecules to the available active sites of the magnetic adsorbent is low, which contributes to the high removal efficiency. As a result, at higher concentrations of TC, access to the adsorption sites is considerably limited and leading to less antibiotic adsorption. Our results are in good agreement with other studies conducted by Malakootian et al. [17] and Babaei et al. [69].

Effect of Magnetic Nano-biocomposite Dosage
The optimal adsorbent dose is a crucial parameter that affects the amount of adsorbed adsorbate. As shown in Fig. 10, the removal efficiency was 78.02% for an adsorbent dosage of 60 mg L −1 . TC removal increased to 90% with an enhancement in the adsorbent dose to 80 mg L −1 . Further increase in the adsorbent dosage at its maximum level (150 mg L −1 ) lead to a maximum removal efficiency of 92.45%. However, with 80 mg L −1 of adsorbent dosage, the removal efficiency was almost constant, not remarkably different from the maximum level, and the adsorbent dosage of 80 mg L −1 was selected as the optimum amount for subsequent experiments.  The removal efficiency increases with increasing adsorbent dosage due to the increased number of accessible active sites of the magnetic adsorbent, as previously reported in literature works [70,71].

Effect of Solution pH
The Solution pH is considered to be a key factor in the adsorption processes used to remove contaminants. In this study, variable pH ranges (3)(4)(5)(6)(7)(8)(9)(10)(11) were evaluated to determine the optimum pH for TC removal by Fe 3 O 4 @MC/ APTMS. As Fig. 11a depicts, an enhancement in removal efficiency was observed with an increase in pH from 3 (73.84%) to 6 (89.81%), in opposition, more increase in pH from 6 to 11 (46.9%) lowered the removal efficiency significantly, owing to the electrostatic repulsion among TC and the negatively charged surface of the adsorbent which prevents adsorption.
The effect of solution pH can be explained by the pH zpc of the adsorbent. The pH zpc of Fe 3 O 4 @MC/APTMS was determined by the solid addition method and was found to be 6.8. As shown in Fig. 11b, TC has three acid dissociation constants (pKa), namely 3.3, 7.68, and 9.68. Consequently, depending on the pH value, it can form different ionic species by protonation or deprotonation. TC molecules are present in cationic (pH < 3.3), zwitterionic (3.3 < pH < 7.68), or anionic forms (pH > 7.68). It could be explained that at pH: 6, TC had both negative and positive charges. As mentioned above, the pH zpc value for Fe 3 O 4 @MC/APTMS was about 6.8, representing that it is positively charged at pH < 6.8, and electrostatic attraction occurs. In contrast, at pH > 6.8, the surface of Fe 3 O 4 @MC/APTMS is negatively charged, and electrostatic repulsion occurs, which hinders TC adsorption. Our results are in good agreement with previous studies [22,72].

Adsorption Isotherm Studies
Isotherm models express the interaction among adsorbate and adsorbent at constant temperature [73]. The results of the isotherm study are mentioned in Table 1. The Langmuir model presumes that the adsorption takes place at a number of specific and uniform sites in the adsorbent. The Freundlich model is acquired by presuming a homogeneous surface with a distribution of irregularities of the adsorbed heat on the surface. The Temkin isothermal model is based on the fact that the heat from the sorption process of all molecules in the layer would decrease linearly. The D-R isotherm model is used to take into account the effect of the porous structure of adsorbents. It gives an understanding of the adsorbent porosity and the adsorption energy [73,74].
According to Table 1, the linear regression coefficients of the Langmuir model were higher than other studied models (R 2 = 0.9918). The results were consistent with previous studies [17,42,72,75].
The maximum adsorption capacity (q max ) and K L obtained from the Langmuir model were equal to 23.77 mg g −1 and 0.71 L mg −1 , respectively. The dimensionless parameter of R L (0.21) was within the range of 0-1, which suggested that Fe 3 O 4 @MC/APTMS was a potential adsorbent for the removal of TC. The above results indicated that the TC adsorption was a monolayer sorption on a uniform adsorption surface between molecules with no interactions between molecules [75,76].  Langmuir q e = q m ⋅b⋅C e /1 + k L ⋅C e C e /q e = C e /q m + 1/q m ⋅k L C e /q e vs. C e q max (mg g −1 ) 23.77 k L (L/mg) 0.71 R 2 0.992 Freundlich q e = k f C e 1/n Log q e = log k f + 1/n log C e log q e vs. log C e K f (L/mg) 9.14 n 2.29 R 2 0.96 Temkin q e = RT/b ln (k f ⋅C e ) q e = B 1 ln k f + B 1 ln C e q e vs. ln C e K t (L/mg) 6

Kinetic Models
Kinetic models are employed to study the adsorption mechanisms. Adsorption is a Physico-chemical process in which the adsorbate is transferred from the solution phase to the surface of the adsorbent. Different kinetic models can be utilized to describe the transfer behavior of the adsorbate molecules [74]. Table 2 summarizes the properties of each model. As shown in Fig. 12a, the TC adsorption by Fe 3 O 4 @MC/ APTMS is well fitted on the pseudo-second-order kinetic model (R 2 = 0.98).
These results implied that the adsorption process of TC by Fe 3 O 4 @MC/APTMS was the rate-controlling limiting step. The value of q e estimated by the pseudo-second-order kinetic model was 11.45 mg g −1 and the calculated rate constant k 2 was found to be 0.011 g mg −1 min −1 . The obtained kinetic model fitting is consistent with the previous studies conducted for the adsorptive removal of various pollutants by different adsorbents [42,77].
Changes in the TC adsorption spectra due to the adsorption process are shown in Fig. 12b at various contact times. The TC adsorption peak is observed at 358 nm. The reduction in the intensity of adsorption peak by increasing the contact time indicated TC removal during the adsorption process.

Recovery of Fe 3 O 4 @MC/APTMS
The reusability is a significant factor in terms of practical applications of as-prepared magnetic nano-biocomposite.
The results of the recovery study are shown in Fig. 13. The results showed that the adsorptive activity of Fe 3 O 4 @MC/ APTMS reduced was to 81.35% after the second run. The adsorption percentage decrease can be driven by the adsorption of intermediates on the magnetic nano-biocomposite active sites, making them unavailable to remove a fresh TC solution [21]. However, after five runs, 74% of the TC could be successfully adsorbed.

TC Removal from Actual Wastewater
The optimal conditions determined through the synthetic samples were investigated on the sanitary wastewater collected from the Kerman University of Medical Sciences campus. The Wastewater sample was collected from the chlorination unit inlet. Under optimum conditions, the maximum TC and chemical oxygen demand (COD) removal efficiency was found to be 65% and 71% respectively. The decrease in removal efficiency compared to synthetic conditions could be described by the presence of wastewater anions and cations. Interfering factors such as cyclic and organic compounds along with turbidity in the wastewater led to a decrease in the TC removal efficiency in the actual samples, as previously reported [2,21]. Also, base on the COD removal efficiency, the amount of residual COD was about 31.1 mg L −1 which was lower than the effluent discharge standards [81,82]. The physicochemical characteristics of the employed actual wastewater sample included BOD (58 mg L −1 ), COD (107.4 mg L −1 ), TSS (16 mg L −1 ), TDS (1040 mg L −1 ), Nitrate (1 mg L −1 ), PO 4 (35.31 mg L −1 ), pH (7.57), and TC concentration (9.55 mg L −1 ).

Adsorption Mechanism
FTIR analysis of Fe 3 O 4 @ MC/APTMS before and after TC adsorption was compared in Fig. 14. It could be observed some differences in the FTIR spectra before and after TC adsorption due to TC binding on the surface of the adsorbent. After the adsorption process, the peaks at 2925 cm −1 and 1634 cm −1 , which corresponded to -OH and -NH 2 groups, shifted to a lower wavelength and intensity (2922 cm −1 for -OH group and 1630 cm −1 for -NH 2 group). Moreover, the peaks at 1060 cm −1 , 635 cm −1 and 459 cm −1 , which were related to Si-O, Fe-O and Si-O-Si functional groups, shifted to 997 cm −1 , 632 cm −1 and 443 cm −1 , respectively. As a result, it could be stated that the -CH, -NH 2 , Si-O,  Table 4. One of the common measures of an adsorbent efficiency is its maximum adsorption capacity (q max ), which is calculated by the adsorption isotherm models to determine the maximum amount of adsorbate that can be loaded by the adsorbent [83]. In the current study, Fe 3 O 4 @MC/APTMS magnetic nano-biocomposite showed a higher q max and is superior in terms of the removal efficiency, Reusability, contact time and adsorbent usage than compared studies.

Conclusion
Fe 3 O 4 @MC/APTMS was prepared by the microwaveassisted co-precipitation method in aqueous media. FESEM, EDS, Mapping, FT-IR, XRD, TGA, BET, and VSM techniques were carried out to identify the structure of as-prepared Fe 3 O 4 @MC/APTMS magnetic nano-biocomposite. S BET and the mean pore size of the adsorbent were 67 m 2 /g and 15.2 nm, respectively. Furthermore, the synthesized adsorbent represented excellent thermal stability as well as good magnetic properties with a saturation magnetization of 65.18 emu/g. q max of the synthesized Fe 3 O 4 @MC/APTMS for TC adsorption was obtained 23.77 mg g −1 . The adsorption data of TC were well fitted on the Langmuir isotherm model. Kinetic studies exhibited that the adsorption process followed the pseudo-second-order kinetic model. Fe 3 O 4 @MC/APTMS as a new nano-biocomposite can be utilized as an environment-friendly bioadsorbent to remove TC with high efficiency from aqueous solutions, as well as convenient and straightforward magnetic separation. Our findings may offer a new strategy to prepare high-performance magnetic biopolymer-based and amino-functionalized bioadsorbents to remove various water and wastewater pollutants such as dyes, heavy metals and other pharmaceutical compounds.

Data Availability
The datasets generated and analyzed during the current study are available from the corresponding author on reasonable request.