Adsorption performance of an amine-functionalized MCM–41 mesoporous silica nanoparticle system for ciprooxacin removal in a batch system

: Antibiotic pollutants discharged from pharmaceutical industries are often present in the aquatic environment due to ineffective treatment of pharmaceutical wastewater and are hazardous to human and aquatic life. Therefore, effective treatment of antibiotic-containing wastewater is of utmost importance in the field of environmental protection. This study aims to evaluate the adsorption performance of an amine-functionalized MCM–41 mesoporous silica nanoparticles system (MCM–41–NH2) as an adsorbent for the removal of ciprofloxacin (CIP) antibiotic from aqueous solution. Surface and structural characteristics of MCM–41–NH2 were examined using scanning electron microscopy, X-ray diffraction, Brunauer–Emmett–Teller analysis, Fourier transform infrared spectroscopy, and point of zero charge analysis. In addition, thermal stability was investigated by thermogravimetric analysis. Via the proposed treatment, 99.25% CIP removal was achieved under the following conditions: pH = 7; MCM–41–NH2 dose = 0.8 g/L; CIP concentration = 10 mg/L; adsorption time = 120 min; and shaking speed = 200 rpm. Isotherm study showed that the experimental data fitted well with the Langmuir equation. Moreover, the maximum adsorption capacity of MCM–41–NH2 for CIP was 164.3 mg/g. Thermodynamic parameters showed that the adsorption process of CIP on MCM–41–NH2 effects of micropollutants Monitoring and Management performance of an amine -functionalized MCM – 41 mesoporous silica nanoparticle system for ciprofloxacin removal. In this study, an amine-functionalized MCM – 41 mesoporous silica nanoparticle system (MCM – 41 – NH 2 ) was prepared by modifying MCM – 41 mesoporous silica nanoparticle via amination and applied as an adsorbent for the removal of ciprofloxacin from aqueous solutions. The behavior and ciprofloxacin removal efficiency of MCM – 41 – NH 2 in the suggested treatment were investigated. Results show that MCM – 41 – NH 2 is an active and recyclable adsorbent for the removal of ciprofloxacin and may be a promising treatment agent for practical applications such as the treatment of antibiotic-containing wastewater. study new field Abstract Antibiotic pollutants discharged from pharmaceutical industries are often present in the aquatic environment due to ineffective treatment of pharmaceutical wastewater and are hazardous to human and aquatic life. Therefore, effective treatment of antibiotic-containing wastewater is of utmost importance in the field of environmental protection. This study aims to evaluate the adsorption performance of an amine-functionalized MCM – 41 mesoporous silica nanoparticles system (MCM – 41 – NH 2 ) as an adsorbent for the removal of ciprofloxacin (CIP) antibiotic from aqueous solution. Surface and structural characteristics of MCM – 41 – NH 2 were examined using scanning electron microscopy, X-ray diffraction, Brunauer – Emmett – Teller analysis, Fourier transform infrared spectroscopy, and point of zero charge analysis. In addition, thermal stability was investigated by thermogravimetric analysis. Via the proposed treatment, 99.25% CIP removal was achieved under the following conditions: pH = 7; MCM – 41 – NH 2 dose = 0.8 g/L; CIP concentration = 10 mg/L; adsorption time = 120 min; and shaking speed = 200 rpm. Isotherm study showed that the experimental data fitted well with the Langmuir equation. Moreover, the maximum adsorption capacity of MCM – 41 – NH 2 for CIP was 164.3 mg/g. Thermodynamic parameters showed that the adsorption process of CIP on MCM – 41 – NH 2 was endothermic and spontaneous. Additionally, the increase in solution temperature had a positive impact on the removal of CIP. The kinetic data obtained at different CIP concentrations (10, 25, 50, and 100 mg/L) were consistent with the pseudo-second-order model. MCM – 41 – NH 2 could be recycled eight times in the proposed adsorption process, with a slight loss in its adsorption capacity. Compared with other adsorbents, MCM – 41 – NH 2 was more effective for CIP removal.


Introduction
Studies on wastewater treatment and reuse are particularly important as they provide viable solutions for the prevention of environmental pollution Azarpira, 2016, Liu et al., 2011). Currently, one of the most dangerous and most widespread pollutants in the environment is antibiotics, which are common pollutants in pharmaceutical wastewater (Zhao et al., 2014).
Several studies have reported that in the last decade, the production rate of antibiotics in pharmaceutical factories has increased from 100 thousand tons/year to 200 thousand tons/year (Serwecińska, 2020, Miranda et al., 2018. For example, ciprofloxacin (CIP) is widely administered to humans and animals for the treatment of bacterial infections, specifically urinary tract, respiratory, and gastrointestinal infections (Peng et al., 2016, Amini et al., 2010. In fact, pollution of ecosystems by antibiotics or their by-products can pose serious risks to human health because of the high toxicity and carcinogenic properties of these compounds (Mohammed et al., 2020b, Nasseh et al., 2020. Therefore, it is critical to find an appropriate treatment for the removal of these compounds from pharmaceutical wastewater before reusing or discharging this wastewater into water bodies. Various treatments, including ozonation, nanofiltration, oxidation, photocatalytic degradation, biological methods, and adsorption, have been used alone or as complementary processes for the treatment of pharmaceutical wastewater (Gao and Pedersen, 2005, Peterson et al., 2012, Chang et al., 2012, Nasseh et al., 2020, Nasseh et al., 2021. Among the abovementioned techniques, the adsorption method has received significant attention than other treatment techniques because of its many advantages such as low initial cost, simple and flexible design, possibility of effluent reuse, easy operation, and insensitivity to toxic pollutants. In addition, intermediate by-products are not generated during this treatment (Balarak and Mostafapour, 2019, Balarak and Azarpira, 4 2016). Major mechanisms responsible for the removal of pollutants by the adsorption process include electrostatic processes, ion exchange, bonding processes, and chemical reactions (Mahvi et al., 2018). Several adsorbents, such as activated carbon, have an excellent ability to adsorb organic pollutants from air, water, and soil Al-Musawi, 2020, Brouers and. The adsorption ability of activated carbon is attributed to its superior characteristics including a mesoporous structure and large surface area (Rahardjo et al., 2011).
Because of the high cost and the loss of part of activated carbon during treatment, numerous researchers are working on developing new alternatives that do not have these shortcomings (Moussavi et al., 2013, Kerkez-Kuyumcu et al., 2016. From these perspectives, the use of nanotechnology in the adsorption process has increased due to the small size, and thus high adsorption ability of nanoparticles (Al-Jubouri et al., 2018, Samarghandi et al., 2015. Furthermore, several nanoadsorbents have high regeneration rates, which reduce the treatment cost (Kim et al., 2010).
Mesoporous silica has been used in many practical applications such as catalysis, drug transfer, and adsorption (Li et al., 2017, Yokoi et al., 2012. Accordingly, mesoporous silica nanoparticles have been considered as effective adsorbents for the removal of different pollutants. Main adsorption properties of mesoporous silica are high surface area, strong functional groups, structural stability, surface modification, and a regular channel structure (Lam et al., 2008, Bui andChoi, 2009). Moreover, this type of adsorbents has a negatively charged surface owing to the presence of silanol (Si-OH) groups; thus, these adsorbents are more useful for the adsorption of cations (Heidari et al., 2009). Santa Barbara Amorphous 15 (SBA-15) and Mobil Composition of Matter No. 41 (MCM-41) mesoporous silica nanoparticles are potential adsorbents because of their favorable surface and structural properties (Ebrahimi Getkesh et al., 2014). To enhance the adsorption capacity ( ) and kinetics of MCM-41, modification or functionalization of the pore walls of MCM-41 is considered a simple and effective way (Çıtak et al., 2012). In this direction, the amination of MCM-41 (MCM-41-NH2) has received substantial attention because amine group has a positive impact on the performance of the adsorption systems of heavy metals, dyes, and other recalcitrant organic compounds (Heidari et al., 2009, Ebrahimi Getkesh et al., 2014. As very few studies have been performed on the elimination of antibiotics using silicate nanoparticles, the purpose of this study was to investigate the and behavior of MCM-41-NH2 toward CIP. Surface and structural properties of MCM-41-NH2 with respect to adsorption were examined by advanced analyses. The effects of pH, MCM-41-NH2 dose ( / (g/L)), CIP concentration, adsorption time (t, min), shaking speed, temperature on the CIP removal efficiency (RE (%)) of MCM-41-NH2 were also analyzed. In addition, isotherm, kinetic, and thermodynamic studies were comprehensively conducted. Finally, the regeneration of MCM-41-NH2 was evaluated in several CIP adsorption-desorption cycles.

Preparation of MCM-41
MCM-41 was prepared by the method reported in the literature (Lam et al., 2008). First, 0.64 g sodium hydroxide was dissolved in 27 mL deionized water. Then, 1.8 g SiO2 nanoparticles were vigorously mixed with the as-prepared sodium hydroxide solution at 80 °C for 3 h to ensure complete dissolution. After cooling the resulting SiO2 solution to ambient temperature, CTMAB (5.46 g) was added to it followed by vigorous stirring for 1 h until a highly viscous homogeneous solution was acquired. Next, 0.3 mL HCl was added to the resulting mixture, and the reaction was allowed to proceed for 10 min. Thereafter, 24 mL deionized water was added, and at this stage, a white gel was obtained, which was stirred for 2 h at 300 rpm. The resulting mixture was then transferred to a polypropylene container and placed in an oven at 100 °C for 3 days. Finally, after being cooled to ambient temperature, the resulting solution was filtered using a typical filter, and the supernatant (MCM-41) was collected in a container. Subsequently, MCM-41 was

Preparation of MCM-41-NH2
Amination of MCM-41 was performed according to a previously reported method (Ho et al., 2003). Initially, 2.5 g calcined MCM-41 was soaked in 50 mL hexane for approximately 1 h.
Next, 2.5 g 3-APTMS was mixed with the MCM-41 solution. The resulting mixture was refluxed for 6 h followed by cooling to ambient temperature. Subsequently, the solution was filtered, washed twice with 20 mL hexane, dried, and then placed in a desiccator for further use.

Characterization
Surface and structural properties with respect to the adsorption behavior of MCM-41-NH2 were investigated using precise devices and powerful techniques. Pore size specifications and surface area of MCM-41-NH2 were determined by Brunauer-Emmett-Teller (BET) analysis using Belsorp mini II (Bel Japan Co.). Functional groups were analyzed by Fourier transform infrared (FTIR) spectroscopy (PerkinElmer, Spectrum GX). X-ray diffraction (XRD) (X'Pert Pro MPD) was conducted to examine the crystallographic characteristics of the adsorbent, and XRD patterns were achieved in the 2θ range from 20° to 80° at a wavelength of 1.5 Å. High-resolution scanning electron microscopy (SEM) was performed using a TESCAN microscope (MIRA3-XMU) for characterizing the morphology of MCM-41-NH2. In addition, the thermal stabilities of MCM-41 and MCM-41-NH2 were measured by evaluating the weight loss of these materials with an increase in temperature from 50 °C to 800 °C using a thermal analyzer (Model: Q600, TA Co.).

Determination of point of zero charge (pHpzc)
8 pHpzc is also an important and fundamental indicator of the characteristics of a material and is calculated to determine the net charge of an adsorbent as a function of solution pH. Moreover, the determination of pHpzc is an essential step in investigating the variation in the adsorption mechanism of the adsorbent with a change in the pH of the pollutant solution. In the present study, the pHpzc of MCM-41-NH2 was evaluated using a previously reported method (Al-Musawi et al., 2020).

Experiments
Experiments on the adsorption of CIP on MCM-41-NH2 were conducted in a batch adsorption system using several 200-mL Erlenmeyer flasks filled with 100 mL CIP solution. Adsorption reactions were performed by shaking these flasks in a shaker at 150 rpm for 120 min. Thereafter, the adsorption performance of MCM-41-NH2 toward CIP removal was tested at varying physicochemical parameters: pH: 2-11; / : 0.1-1.4 g/L; initial CIP concentration ( 0 (mg/L)): 25-100 mg/L; t: 0-120 mg/L; shaking speed: 0-200 rpm; and temperature: 20-60 °C. For the isotherm study, an experiment was conducted to determine the relationship between the equilibrium adsorption capacity ( (mg/g)) of MCM-41-NH2 and CIP concentration at different temperatures (283-328 K). Furthermore, for kinetic and thermodynamic studies, the experimental data of the analysis of the effects of 0 and temperature on the of MCM-41-NH2 were used. At a specific , a 5 mL sample was taken from each flask and centrifuged at 1509 x g for 10 min. The concentration of CIP in the supernatant was measured using highperformance liquid chromatography (HPLC) under the following conditions: an ODS C18 column; a UV detector; three mobile phases: 0.025 M orthophosphoric acid, methanol, and acetonitrile at a 13:75:12 ratio; wavelength: 280 nm; retention time: 5.5 min; and inflow: 1 mL min -1 . The collected samples were measured three times using HPLC, and the average values were adopted in the calculations. and RE of MCM-41-NH2 for CIP were determined using Equations (1) and (2), respectively (Li et al., 2017, Wang et al., 2016: where / denotes the MCM-41-NH2 dose (g/L), and 0 and are the CIP concentration (mg/L) in solution before and after specific time of adsorption process ( , min), respectively. The concentration and adsorption capacity at equilibrium time is denoted by (mg/L) and (mg/g), respectively.  (Heidari et al., 2009). Thus, it can be concluded that MCM-41-NH2 can serve as an appropriate adsorbent for the adsorption of CIP molecules from contaminated solution. Figure 1B presents the results of the BET analysis. The specific surface area, mean pore diameter, and volume of MCM-41-NH2 are 524 m 2 /g, 3.1 nm, and 0.87 cm 3 /g, respectively. A relatively narrow size distribution of MCM-41 with a noticeable peak was observed, which was related to the pores with 2-4 nm diameter. The isotherm shown in Fig. 1B is a typical type-IV isotherm with a well-defined hysteresis loop, which reveals the presence of mesopores (Yokoi et al., 2012).

Characterization of MCM-41-NH2
XRD patterns of MCM-41 and MCM-41-NH2 are shown in Fig. 1C. The XRD pattern of MCM-41-NH2 exhibited a strong high-intensity peak at 2θ = 1.8° (100). Furthermore, two lowintensity peaks were observed at 2θ = 3.2° (110) and 3.9° (200). These three peaks were not noticed in the XRD pattern of MCM-41. In fact, the presence of these peaks in the XRD pattern of MCM-41-NH2 indicates the formation of cavities with a regular hexagonal structure during amino-functionalization (Kirik et al., 2014). Numerous studies have reported that the formation of these cavities in an adsorbent may play an important role in the adsorption process as these cavities lead to a considerable number of active sites for the adsorption of pollutant molecules (Khodadadi et al., 2019, Mohammed et al., 2020b, Alwared et al., 2021. In the first step, the weight loss was approximately 3.0%, which was attributed to the desorption of physisorbed water retained in the pores. In the second step, the weight loss was approximately 5.0%, which was primarily owing to the oxidative destruction of the organic 11 functional groups. The weight loss in the final step, which was approximately 2.0%, was ascribed to the loss of water formed by the condensation of the Si-OH groups. FTIR spectra of MCM-41 and MCM-41-NH2 in the wavenumber range of 400-4000 cm −1 are shown in Fig. 1E. The broad band detected at 3540 cm -1 in the FTIR spectrum of MCM-41 is related to the reaction of the Si-OH groups with the absorbed water molecules and destruction of the active sites (Lam et al., 2006). overlapped with the broad band of the Si-OH group (Martin et al., 2001). By comparing the FTIR spectra acquired before and after the functionalization of MCM-41, it can be deduced that these two spectra are harmonic with no substantial differences. This implies that the amination of the pore walls of MCM-41 did not significantly change or damage the structure of MCM-41.
Instead, the structure of MCM-41 was improved after amination, with the emergence of new active N-H groups.
pHpzc of MCM-41-NH2 was found to be 7.3 (Fig. 1F). The analysis of pHpzc is very important to study the effects of solution pH on the adsorption process. Based on the results of this analysis, at pH 7.3, the net charge on the MCM-41-NH2 surface was zero. In addition, the surface of MCM-41-NH2 had a negative charge at pH greater than 7.3 and vice-versa.

Effects of physicochemical parameters
Many studies have demonstrated that the pollutant RE and of an adsorbent are significantly affected by various physicochemical parameters. Generally, solution pH and temperature have a considerable impact on the pollutant RE(%) of adsorbents as the type of surface charge of the adsorbent and the dissolution rate of the pollutant molecules mainly depend on these two parameters (Nasseh et al., 2021, Khodadadi et al., 2019. From an economic perspective and for designing large-scale treatment processes, determination of optimum adsorbent dose for the removal of pollutants is important. In addition, the most important analyses in the isotherm study depend on the relationship between the of the adsorbent for pollutant at a constant temperature. Investigation of the effects of pollutant concentration on t is essential for the kinetic study (Al-Musawi et al., 2018). Furthermore, numerous studies have reported that the effect of shaking speed on the adsorption of pollutants should be considered (Kuśmierek andŚwiątkowski, 2015, Abdelkareem et al., 2019). In the thermodynamic study, analyzing the effect of temperature on the adsorption process is crucial (Rostamian andBehnejad, 2016, Mahvi et al., 2018). In the present study, the effects of the abovementioned physicochemical parameters on the CIP adsorption process were studied (Fig. 2).

Effects of pH
Results of the effect of pH on the CIP RE of MCM-41-NH2 are shown in Fig. 2a (conditions of this experiment were as follows: pH = 2-11; / = 0.8 g/L; CIP concentration = 10 mg/L; t = 60 min; shaking speed = 200 rpm; and temperature = 25 ± 2 °C). The CIP RE increased with an increase in pH from 3 to 7, and the maximum CIP RE achieved herein was 97.48%. In addition, the CIP RE of MCM-41-NH2 decreased when pH was greater than 7. To explain these results, both the pHpzc of the adsorbent and the type of charges of the pollutant molecule in the studied pH range should be considered. Herein, the pHpzc of MCM-41-NH2 is 7.3 ( Figure 1F), which suggests that at a solution pH ≤ 7.3, the surface of MCM-41-NH2 will have a positive charge and vice-versa. On other hand, at pH ≤ 6.1 (pKa1 value of the CIP molecule), the surface of CIP will be positively charged due to the protonation of the amine functional groups on MCM-41 by surplus H + formed in the acidic pH medium (Amini et al., 2010). Thus, the repulsive forces occurring at pH ≤ 6.1 will hamper the adsorption of CIP molecules on MCM-41-NH2. The gradual increase in the repulsion rate with a decrease in the pH of the solution because of the increase in the amount of H + explains the decrease in the CIP RE with a decrease in pH, particularly to below 6 (Al- Musawi et al., 2018, Nasseh et al., 2020. In addition, the CIP RE decreased at pH above 7 because at a pH greater than 8.7 (pKa2 value of the CIP molecule), the CIP molecules exist as anionic species owing to the loss of protons from the carboxyl group (-COOH) of CIP (Table 1). Therefore, repulsion occurs between the negatively charged surfaces of MCM-41-NH2 and CIP molecules in the aqueous solution, specifically at pH above 8.
In the pH range from 6.1 to 8.7, deprotonation of -COOH results in the production of negatively charged CIP molecules (Peng et al., 2016). Furthermore, the amine group remains protonated

Effects of /
Results of the effect of / on the CIP RE of MCM-41-NH2 are shown in Fig. 2b (conditions of this experiment were as follows: pH = 7; / = 0.1-1.4 g/L; CIP concentration = 10 mg/L; t = 60 min; shaking speed = 200 rpm; and temperature = 25 ± 2 °C). When / was increased from 0.1 g/L to 0.8 g/L, the CIP RE increased from 49.60% to 97.09%. This is because with an increase in / at a fixed CIP concentration, the number of active sites also increases with respect to the concentration of CIP in the aqueous solution , Zha et al., 2013.
However, when / was further increased above 0.8 g/L, the CIP RE did not significantly vary.
This is because at low pollutant concentrations, the adsorption of pollutant molecules is more difficult (Li et al., 2017, Davis et al., 2003. Another reason that has been highlighted in the literature is that the agglomeration of some adsorbent particles at high doses reduces the availability of active sites for the pollutant molecules (Khodadadi et al., 2019). Based on these facts, 0.8 g was considered as the optimum / for the treatment of 1 L CIP solution with a CIP concentration of 10 mg/L.

Effects of
Effects of 0 on the CIP RE of MCM-41-NH2 were examined as a function of t (Figure 2c) as this information is important to accomplish the kinetic study. This experiment was perfomed under the following conditions: pH = 7; / = 0.8 g/L; CIP concentration = 10-100 mg/L; shaking speed = 200 rpm; t = 0-10 min; and temperature = 25 ± 2 °C. Clearly, the CIP RE increased from 82.25% to 99.25% when 0 was decreased from 100 mg/L to 10 mg/L. In addition, the plotted curves of the CIP RE of MCM-41-NH2 presented similar profiles at all analyzed 0 values. This is because in this experiment, / in the aqueous solution was fixed, and therefore, there were a limited number of active sites. With an increase in 0 , competition between CIP molecules to occupy the available active sites increased (Danalıoğlu et al., 2017, Kakavandi et al., 2014. Figure 1C also shows that the rate of adsorption of CIP on MCM-41-NH2 was high as the removal of more than 60% CIP was achieved in the first 30 min of adsorption at all CIP concentrations investigated herein. Furthermore, equilibrium adsorption occurred after 60 min of adsorption. The ascending limb of the plotted curves was very steep, and after 40 min of adsorption, the slopes of these curves gradually became mild ( Figure 1C).
This was because of the availability of a large number of active sites at the beginning of adsorption. In addition, this indicated rapid transport of the CIP molecules from the aqueous solution to MCM-41-NH2 via adsorption (Zha et al., 2013). These results showed that it is possible to achieve a CIP RE of up to 99% if adsorption is proceeded for 120 min. Accordingly, 120 min can be concluded as the optimum t for the adsorption of CIP on MCM-41-NH2 to ensure the achievement of full equilibrium status.

Effect of shaking speed
Shaking speed has a direct effect on the mixing degree and thus the rate of contact of pollutant molecules with adsorbent particles; therefore, the adsorption of pollutant molecules is highly dependent on this parameter. Figure

Adsorption kinetics
Kinetic analysis is essential to evaluate the practical applicability of an adsorbent as it is useful for understanding the mechanism and rate of adsorption Al-Musawi, 2020, Al-Musawi et al., 2018). In addition, determination of a mathematical formula that can describe the kinetic reactions of an adsorbent-adsorbate system is necessary for the precise design of largescale treatment processes (Balarak et al., 2020). Therefore, the experimental data shown in Fig.   2c were first treated using Equation 1 and then analyzed in terms of pseudo-first-order (PFO) and pseudo-second-order (PSO) kinetic models. Equations 3 and 4 are the linear forms of the PFO and PSO models, respectively (Azizian, 2004, Ho, 2006. Furthermore, the data presented in Fig.   2c were modeled using the intraparticle diffusion model (Equation 5) . In fact, the application of the intraparticle diffusion model is important as this model provides information about the role of intraparticle diffusion rate in controlling adsorption.
Moreover, (mg/g·min 0.5 ) is the rate constant of the intraparticle diffusion model, and I (mg/g) is the intraparticle constant that provides information about the thickness of the boundary layer.
Results of the CIP kinetic analysis are listed in Table 2. Regarding the PFO and PSO models, the relevant parameters were determined based on the equations of the trend lines of the two plots shown in Fig. 3a and b, respectively. 1 represents the slope of the trend line and is calculated from the plot of ( − ) vs. points. Furthermore, 2 is determined from the linear plot of vs. data. The fitting of each kinetic model with the kinetic data was estimated in accordance with the coefficient of determination (R 2 ) values and the convergence between the calculated uptake ( ( )) and experimental uptake ( ( )) ( Table 3) (Table 2). Therefore, it can be concluded that the intraparticle diffusion process is not the only rate-controlling step, and during adsorption, the rate-controlling mechanism may change (Yu et al., 2016).
To examine the fraction of the adsorbent surface occupied by the CIP molecules, surface coverage analysis was conducted. In fact, an increase in surface coverage implies an increase in the pollutant RE of the adsorbent. Surface coverage can be calculated using the following equation (Çalışkan and Göktürk, 2010): In the present study, the surface coverage of MCM-41-NH2 due to the adsorption of CIP molecules was calculated at different t, and the results are shown in Fig. 3d (Brouers and Al-Musawi, 2020). In this study, four isotherm models were applied, and their compatibility with the isotherm data was analyzed: Langmuir, Freundlich, Temkin, and Dubinin-Radushkevich isotherm (D-R) models. The Langmuir model (Equation 7) assumes that in adsorption systems, each pollutant molecule interacts with one active site located on the adsorbent surface (monolayer adsorption), and all the active sites located on the adsorbent are homogeneous and have equal binding energies. Thus, in the Langmuir model, adsorption is characterized as monolayer and homogeneous (Brouers and Al-Musawi, 2020, Khodadadi et al., 2019, Alhassani et al., 2020. where (mg/g) is a very important parameter in the adsorption studies, denoting the maximum of the adsorbent for the target pollutant, and (L/mg) is an equilibrium constant reflecting the affinity level of the active sites of the adsorbent.
In addition, the characteristics of adsorption using the Langmuir model can be defined based on the dimensionless separation factor ( ) (Equation 8). Based on the value, the favorable adsorption case can only be detected at 0 < < 1.
Moreover, it supposes that the total adsorption energy exponentially decreases during adsorption.
where KF is the Freundlich constant, indicative of the binding energy (mg/g)(L/mg) 1/n , and 1 is the heterogeneity parameter.
Temkin equation (Equation 10) is a complex isotherm model used to describe the indirect adsorption process of pollutant molecules on active sites (Zhao et al., 2014). Furthermore, this model hypothesizes that adsorption energy linearly decreases during the occupation of the adsorption center of an adsorbent (Gao et al., 2012). In general, this model is highly applicable to describe the chemical adsorption type (Nasseh et al., 2021).
where and B = are constants that provide information about the heat of sorption (J/mol), is the Temkin isotherm constant (L/g), R is the ideal gas constant (8.314 J/(mol·K)), and T is the thermodynamic absolute temperature (K).
For systems where the adsorption curve depends on the porous surface of the adsorbent, the D-R model (Equation 11) is appropriate. Using the D-R model, the nature (physical or chemical) of the adsorption mechanism can be determined Holmes, 2020, Guler andSarioglu, 2014).
where (mol 2 /kJ 2 ) is the activity coefficient constant associated with the mean free sorption energy, which is denoted by = increased with an increase in temperature, indicating that adsorption is superior at high temperatures and the adsorption process is endothermic. According to the value of the D-R model, which is less than 8 kJ/mol, the adsorption of CIP on MCM-41-NH2 is physical , Nguyen and Do, 2001, Zhao et al., 2014.  (283, 298, 313, and 328 K). These three parameters were evaluated using Equations 12-15) .
= , where (L/mg) is the equilibrium constant, R is the ideal gas constant (8.314 J/(mol·K)), and T is the thermodynamic absolute temperature (K). Note that ∆ and ∆ can be directly

Regeneration and recycling analysis
In the field of water and wastewater treatment using adsorption systems, one of the most important economic parameters is the reusability of the employed adsorbent (Huang et al., 2014).
Therefore, in the present study, the recyclability of MCM-41-NH2 was examined for eight consecutive CIP adsorption-desorption cycles under the following optimal conditions: pH = 7; / = 0.8 g/L; CIP concentration = 10 mg/L; t = 120 min; shaking speed = 200 rpm; and temperature = 25 ± 2 °C. At the end of each CIP adsorption cycle, MCM-41-NH2 was separated from the aqueous solution via centrifugation. Then, it was rinsed with ethanol and deionized water to desorb the adsorbed CIP molecules, dried overnight in a vacuum oven at 75 °C, and then reused in the next adsorption cycle. The results of this experiment are presented in Fig. 4 (Khoshnamvand et al., 2017)

Conclusion
In this study, MCM-41-NH2 was prepared and used as an adsorbent to eliminate CIP from contaminated solutions. Characterization analyses reveal that MCM-41-NH2 possesses unique properties and a high potential as an efficient adsorbent. In addition, these analyses showed that several characterization parameters, such as surface morphology, functional groups, and thermal stability, of the original MCM-41 material were improved after its modification via amination.
According to the results of the Langmuir model, which fitted best to the experimental data, the of MCM-41-NH2 for CIP was 164.3 mg/g at 328 K. Furthermore, the kinetic study demonstrated that the experimental kinetic data at different CIP concentrations followed the PSO model, and intraparticle diffusion was not the only process controlling the adsorption of CIP on MCM-41-NH2. Moreover, the optimum environmental conditions for achieving the maximum CIP RE (99.25%) were as follows: pH = 7; / = 0.8 g/L; CIP concentration = 10 mg/L; t = 120 min; and shaking speed = 200 rpm. Thus, MCM-41-NH2 prepared herein is a highly efficient adsorbent for the adsorption of CIP. In addition, MCM-41-NH2 can be reused eight times with only a 9% reduction in its adsorption capacity. Figure 1 Patterns of mass loss of two tree species of CWD (fresh wood) at each altitude (215m and 1400m) and forest-edge distance (edge and 60m from the forest edge) during 24 months of decomposition in Lushan Mountain of subtropical China.

Figure 2
Temperature and moisture content of CWD of the two tree species at each altitude (215m and 1400m) and forest-edge distance (edge and 60m from the forest edge) during the 24-month incubation.  Phospholipid fatty acid (PLFA) (mean ± SE; ng g-1 dry wood material) signatures of the CWD of the two tree species at each forest-edge distance (edge and 60m from the forest edge) and altitude (215m and 1400m) in Lushan Mountain of subtropical China. Total, total PLFA concentrations; B, bacterial PLFAs; F, fungal PLFAs; F/B, the fungal to bacterial ratio; G+, Gram-positive bacteria; G-, Gram-negative bacteria; G+/G-, ratio of Gram-positive to Gram-negative bacteria; AMF, arbuscular mycorrhizal fungi.