Fabrication of the magnetic mesoporous silica Fe-MCM-41-A as efficient adsorbent: performance, kinetics and mechanism

Antibiotics are emerging pollutants and increasingly present in aquaculture and industrial wastewater. Due to their impact on the environment and health, their removal has recently become a significant concern. In this investigation, we synthesized nano zero-valent iron-loaded magnetic mesoporous silica (Fe-MCM-41-A) via precipitation and applied the adsorption of oxytetracycline (OTC) from an aqueous solution. The effects of competing ions such as Na+, Ca2+ and Cu2+ on the adsorption process under different pH conditions were studied in depth to providing a theoretical basis for the application of nanomaterials. The characterization of the obtained material through transmission electron microscopy demonstrates that the adsorbent possesses hexagonal channels, which facilitate mass transfer during adsorption. The loaded zero-valent iron made the magnetic, and was thus separated under an applied magnetic field. The adsorption of OTC onto Fe-MCM-41-A is rapid and obeys the pseudo-second-order kinetic model, and the maximum adsorption capacity of OTC is 625.90 mg g−1. The reaction between OTC and Fe-MCM-41-A was inner complexation and was less affected by the Na+. The effect of Ca2+ on the adsorption was small under acidic and neutral conditions. However, the promotion effect of Ca2+ increased by the increase of pH. Cu2+ decreased the removal efficiencies continuously and the inhibitory effects decrease varied with the increase of pH. We propose that surface complexing, ion-exchange, cationic π-bonding, hydrogen bonding, and hydrophobicity are responsible for the adsorption of OTC onto Fe-MCM-41-A.

Water is our most necessary resource. It is also becoming a scarce resource in many parts of the world. The pollutants in our water are also becoming more difficult to remove. Levels and kinds of antibiotics in our drinking water are all on the rise 1 . Oxytetracycline (OTC), as a broad-spectrum antibiotic, is extensively used in human therapy, livestock breeding and aquaculture 2,3 . Recent studies indicate that the OTC is roughly 100-340 ng L −1 in surface water 4,5 , for example, the maximum detected and mean concentrations of oxytetracycline in Honghu lake, China are up to 2796.6 and 161.9 ng L −1 6 . The continuous presence of OTC in environment can harm the growth of plants and animals and cause resistance genes 7 .
Oxytetracycline (OTC) is difficult to degrade owing to its toxicity and resistance to microorganism 8 . The elimination of OTC in few of the biological treatments has been distinguished by quick sorption and slow biodegradation 9 . Traditional water treatment technologies have a limited removal efficiency of 20-30% for antibiotics 10 . Besides, antibiotic pollution is characterized by pH dependence and heavy metal contamination 11 . OTC is a complex organic compound and show positive, zwitterion or negative charges vary with pH. The active groups of OTC will combine with metal ions to form complex pollutants, which will affect each other's characteristics and enhance toxic effects 12 . Therefore, the demand for effective removal of OTC is urgent.
Treatment techniques based on physical and chemical methods, such as adsorption, are the forefront of water remediation 13 . In the past few decades, the emergence of nanomaterials has greatly promoted the development of water treatment technology 14,15 . MCM-41 is a silica-based material which can be adopted as an adsorbent owing www.nature.com/scientificreports/ to its larger surface area, flexible pore structure, larger pore volume, and nontoxicity 16 . The hexagonal phase channel expedites the adsorption of oxytetracycline, a polycyclic macromolecule. The nanoscale pore channels inside the material can provide a large number of adsorption reaction micro-interfaces, which can change the material and energy transfer process on the interface, promote concentration accumulation of pollutants, reaction activation, and advance the reaction 17 . The MCM-41 has been assembled with zeolites (e.g., zeolite A, zeolite X), iron oxide nanoparticles and et al. to enhance its performance [18][19][20] . Nanoscale zero-valent iron (NZVI) has recently drawn rising attention for water pollution remediation due to its high activity and separability 21 . However, to the best of the author's knowledge, little information has been reported on the performance and mechanism of removal of OTC from water using NZVI-loaded MCM-41-zeolite composites, which is the objective of this investigation.
Herein, an efficient method was developed for the remediation of OTC. The specific objectives were (1) to synthesize and characterize nano zero-valent iron (NZVI)-loaded magnetic mesoporous silica (Fe-MCM-41-A); (2) to examine OTC removal performance under different conditions, i.e., initial solution pH initial concentrations, adsorbent dosages and competition ions; (3) to carry out the adsorption kinetics and thermodynamics analysis; and (4) to elucidate the possible adsorption mechanisms for the removal of OTC.

Preparation of Fe-MCM-41-A.
We synthesized MCM-41-A through impregnation-assisted one-step crystallization method 22 . The specific preparation process was described in the Supplementary information. Batch experiments. Adsorption efficiency experiments. The adsorption efficiency experiment was carried out by single factor experiment 8 . Adsorption studies were conducted at different contact times, pH, and initial concentrations to assess the potential removal of OTC. Moreover, the adsorption mechanisms have been presented based on the achieved results. The specific experimental steps were described in the Supplementary information.

Results and discussion
Characterization of Fe-MCM-41-A. The SEM image and EDX of Fe-MCM-41-A showed in Fig. 1a,b demonstrates the linear nano-structure of the material and that the sample has loose texture and uniform structure. According to the HRTEM and TEM images (Fig. 1d,e), it can be seen that the sample exhibited a hexagonal pore structure and Fe 0 is uniformly loaded in the sample. These results demonstrate that the mesopore structure was not undermined by Fe loading. Figure 1d shows that the sample has an ordered arrangement of hexagonal channels with uniform pore size of 3.4 nm. This indicates that the material keeps the pore structure well after the loaded of zero-valent iron. This mesoporous structure of the material is conducive to the mass transfer process of the adsorption reaction and the increase of the adsorption capacity.
As indicated in Fig www.nature.com/scientificreports/ that Fe in the material is mainly zero-valent 24 . The small peak at 37° represents iron oxidation which may be due to the oxidation of Fe0 during preservation. It can be seen from Fig. 2b that the characteristic peak of Al in Fe-MCM-41-A decreases significantly, while the characteristic peak of Fe in Fe-MCM-41-A increases accordingly. This demonstrated that the content of Al in MCM-41-A decreases and the content of Fe element increases after the loading modification of NZVI. It can be seen from Fig. 2d that photoelectron peaks appear in the Fe 2p3/2 spectra. The peak of 707.1 eV corresponds to the fe2p3/2 electron orbital peak of Fe 025 , indicated that the sample in freshly synthesized contains zero valent iron. This analysis is consistent with the XRD analysis. So, we speculated that a decrease in the Al content of Fe-MCM-41-A during modification may be due to the replacement of Fe in the skeleton of MCM-41-A 26 .
To further investigate the nature and binding form of Fe in Fe-MCM-41-A, DRS UV-Vis spectroscopy experiments were conducted and the results are shown in Fig. 3. Silicon based multihole materials have no bands in 200-270 nm region 27 and the absorption bands at 287 nm may be contributed by the pore structure of Fe-MCM-41-A 28 . However, the sample shows absorption bands centered at 210-250 nm, this indicates that Fe has been dispersed within the material. The bands at 211 nm and 248 nm may be assigned to the d¼-p¼ charge-transfer transition between the iron and oxygen atoms in the framework of Fe-O-Si in Fe-MCM-41-A 29 . So, we assume that the schematic diagram of displacement reaction in the modification process as shown in Fig. 4. Figure 5 shows the Mössbauer spectrum of Fe-MCM-41-A, and this spectrum is a superposition of one major paramagnetic doublet and one minor broad magnetic sextet. The six-line pattern of the spectrum is characteristic of the body-centered cubic face of metallic α-Fe confirmed by the hyperfine interaction parameter values shown in Table 1. The major paramagnetic doublet indicating that the paramagnetic state of the sample 30 , therefore, the agglomeration of Fe-MCM-41-A caused by excessive residual magnetism can be avoided. The small contribution www.nature.com/scientificreports/ of the sextet may be due to refinements of the structure to the nanometer level of Fe. As shown in Table 1     Since the sample was subjected to Mössbauer analysis more than 2 months after its preparation, the sample may be partially oxidized which is consistent with the XPS analysis. Uncertainty of the values is given in parentheses for the last significant number. IS isomer shift relative to α-iron, QS quadrupole splitting for the doublet and quadrupole shift for the sextet, B hf hyperfine magnetic field, Γ half width at half maximum of spectral lines.
The zero-valent iron loaded material is magnetic and can be separated quickly under applied magnetic field. The magnetization properties of Fe-MCM-41-A were analyzed by hysteresis loop test. Figure 6 shows that Fe-MCM-41-A is strongly magnetic and can be separated within 1 min under applied magnetic field. Removal kinetics. The adsorption of OTC onto Fe-MCM-41-A is rapid especially in the first 20 min (Fig. 7a). This high adsorption was mainly due to the large surface area, hexagonal mesoporous structure and appropriate pore size of Fe-MCM-41-A which promote internal mass transfer during the adsorption 35 . In this work, the adsorption capacity of OTC was 196.8 mg g −1 , when 100 mg L −1 OTC solution was remediated with Fe-MCM-41-A (0.5 g L −1 ) at pH 5.0. Correlation coefficient (R 2 = 0.9999) indicates that the adsorption obeys the pseudosecond-order kinetic model (Table 2). This indicates that the removal of OTC was rate-controlling limiting step. Figure 7b reveals that from pH 2.0 to 5.0, the removal efficiency increased from 92.2 to 98.4%, and then from pH 5.0 to 11.0, the efficiency decreased from 98.4 to 76.7%. This proves that a weak acidic condition could favor the adsorption process. The adsorption of OTC onto Fe-MCM-41-A reached equilibrium in 40 min at pH 4.0 and 5.0; however, the adsorption lasted for more than 150 min at pH 2.0 and 11.0. This could be due to the digestion of Fe and the desilication of Fe-MCM-41-A under strong acidic and alkaline conditions. The highest adsorption efficiency was achieved at pH 5.0. When the pH is higher than 7.4 (pK a2 ), HL − and L 2− become predominant species and Kow of OTC also decreases, thereby Fe-MCM-41-A surfaces become negative simultaneously. By this, we could infer that a decrease in the removal efficiency at pH 11.0 might be largely due to the weakening of hydrophobic function 35 .

Effect of pH.
Effect of adsorbent dosage. Figure 7c exhibits that the removal efficiency of OTC increased with the amount of adsorbent. When the dosage reached 0.5 g L −1 , the removal efficiency no longer increased with the amount of adsorbent owing to agglomeration and particle precipitation of Fe-MCM-41-A at high solid to liquid ratio.
Effect of initial OTC concentration. It has been noticed that the initial concentration of OTC can significantly influence the removal efficiency. Figure 7d shows that the adsorption capacity of OTC increased and the removal efficiency increased at first and then decreased with an increase in the initial concentration. The highest removal efficiency was 99.1% at an initial concentration of 50 mg L −1 , and the maximum equilibrium adsorption capacity was 609.0 mg g −1 at an initial concentration of 500 mg L −1 . The main reason for the decrease of adsorption efficiency was the lack of adsorption sites. The enhancement of the adsorption capacity was due to the high driving  , hence, the longer reaction equilibrium time. Figure 8 shows that the removal efficiency of OTC reduced less than 7% while the concentration of Na + in background solution increased from 0.0 M to 0.1 M in each pH condition. The adsorption was less affected by the Na + , this was owing to the inner complexation between OTC and Fe-MCM-41-A 37 , as inner layer complexation can be hardly affected by the ionic strength in the background solution 38,39 . Figure 8 shows that the effect of Ca 2+ on the adsorption was small under acidic and neutral conditions. However, the removal efficiency increased with the increasing of the concentration of Ca 2+ under alkaline condition, and the promotion effect increased by the increase of pH. This was mainly benefit from the bridging effect of Ca 2+ , OTC combined with Ca 2+ to form TCs-Ca 2+ -Fe-MCM-41-A 40 .

Kinetic model Equation Parameters Value
Pseudo-first-order log(q e − q t ) = logq e − k 1 /2.303t k 1 (min −1 ) 0.1011 q e (mg g −1 ) 30.49 Pseudo-second-order (t/q t ) = 1/(k 2 q e ) + 1/q e (t) www.nature.com/scientificreports/ Another reason for the removal efficiency of TCs increased with the increasing of Ca 2+ concentration under alkaline condition was the effect of surface chemical precipitation. TCs molecules may complex with Ca 2+ and deposited on the surface of Fe-MCM-41-A 41 . Besides Ca 2+ could reduce the hydrophilicity of OTC and helped to prompt TCs adsorbed onto Fe-MCM-41-A under hydrophobic forces 42 .
The removal efficiencies of TCs onto Fe-MCM-41-A decreased continuously with the increase of Cu 2+ (Fig. 8), meanwhile, the inhibitory effects of Cu 2+ decrease varied with the increase of pH. The inhibiting effect of Cu 2+ was mainly due to the complexation between Cu 2+ and TCs. The surface charge of OTC increased after complexation with Cu 2+ and the electrostatic repulsion between OTC and Fe-MCM-41-A also increased, which may inhibit the removal of OTC.
Adsorption isotherm. According to the adsorption isotherms analysis on the removal of OTC, we found that the equilibrium concentration increased along with the amount of OTC adsorbed onto Fe-MCM-41-A. The adsorption of OTC onto Fe-MCM-41-A obeys the Langmuir model ( Table 3). The constant q max at 25 °C obtained from Langmuir model was 625.90 mg g −1 . This indicates that the adsorption of OTC onto Fe-MCM-41-A follows monolayer adsorption which occurs on a relatively uniform surface.
Adsorption mechanism. The functional groups of Fe-MCM-41-A, OTC-Fe-MCM-41-A and OTC were measured by FT-IR. As shown in Fig. 9, the characteristic peaks of OTC are principally placed at 1200-1700 cm −1 . Peaks at 1615 cm −1 , 1587 cm −1 ,1458 cm −1 and 1366 cm −1 can be attributed to attributed to a stretching of the C=O groups, C-H bonds, amide-NH groups and C-N bonds, respectively 43,44 . The typical bands of C-N and amide-NH shifted to higher frequencies after adsorption, indicating the reaction between amide groups in OTC with Fe-MCM-41-A. The peak located in 3200-3500 cm −1 can be assigned to -OH band. The broadening and peak of -OH band strength weakening after the adsorption, indicated that the -OH band in OTC changed from free state to association state 45 . This may be due to the hydrogen bonding between neutral -OH band and nonionized -Si-OH band in TCs 46 .
XPS were further employed to study the adsorption mechanism (Fig. 2), the results of analysis prove that the intensities of N 1s increased, whereas the intensities of Na 1s decreased after adsorption. This could be due to that parts of OTC adsorb onto Fe-MCM-41-A through ion-exchange. In Fig. 2c the peaks at 399.9 eV and 401.9 eV www.nature.com/scientificreports/ could be correspond to the amino group and its disturbation caused by Fe after adsorption, respectively, indicated that Fe-MCM-41-A could be combined with the amino bands in OTC by cationic-bonding. The Fe2p 3/2 and Fe2p 1/2 spectra emerged at the binding energies of 725.2 eV and 711.4 eV, respectively, the chemical shift in the spin-orbit coupling was 13.8 eV (Fig. 2d). This suggests that the cationic-bond between Fe-MCM-41-A and -NH on OTC could be formed through polarization 47 . Besides, according to the law of removal efficiency and octanol-water distribution coefficient (Kow) of OTC, it can be inferred that an increase in the hydrophobicity of OTC is conducive to the adsorption of OTC on Fe-MCM-41-A.
Comparison of various adsorbent. Performances of different materials for the adsorption of OTC were compared in Table 4. The q max of OTC onto Fe-MCM-41-A was higher than other similar materials. This honeycomb-like magnetic mesoporous silica Fe-MCM-41-A synthesized in this study was superior to other materials in both adsorption rate and efficiency.

Regeneration of Fe-MCM-41-A.
In the regeneration experiment, the removal rate remained above 78% ( Fig. 10) after five rounds of sorption-desorption cycles, indicating that Fe-MCM-41-A has a good regenerability. From Fig. 1c and f it can be seen that after the adsorption of OTC the adsorbent retains the original morphology and pore structure well, which also proves that the structural stable material is renewable. Leaching of iron was investigated after each cycle as an indication of the stability of Fe-MCM-41-A. As shown in Fig. 10, the amount of leached iron ions was the highest in the first cycle (0.87 mg L −1 ). After third cycle, leached iron reduced to < 0.09 mg L −1 and reasonably stable for the subsequent cycles reducing to 0.05 mg L −1 at the fifth cycle. The change law of iron loss is consistent with that of adsorbent regeneration property, so we assumed that this trivial iron loss is an important factor for reuse of the adsorbent. The decrease in removal efficiency occurred mainly in the second cycle, this might have been caused by an irreversible occupation of partial-adsorption sites and the loss of nano-absorbent, as has been observed in previous studies 8 .

Conclusions
A kind of magnetic hexagonal mesoporous silica Fe-MCM-41-A was synthesized and demonstrated to be an efficient adsorbent for the removal of OTC from aqueous solution. The ordered hexagonal pore structure of Fe-MCM-41-A which can facilitate mass transfer was observed by TEM and XRD analysis. The adsorption  We propose that surface complexing, ion-exchange, cationic π-bonding, hydrogen bonding, and hydrophobicity are responsible for the adsorption of OTC onto Fe-MCM-41-A.