*Article* **Enhanced Removal of Sb (III) by Hydroxy-Iron/Acid–Base-Modified Sepiolite: Surface Structure and Adsorption Mechanism**

**Yu Zou 1,2, Bozhi Ren 1,2,\*, Zhendong He 1,2 and Xinping Deng <sup>3</sup>**


**Abstract:** To improve the removal of antimony (Sb) from contaminated water, sepiolite (Sep) was chosen as the feedstock, modified with an acid–base and a ferric ion to yield a hydroxy-iron/acid– base-modified sepiolite composite (HI/ABsep). The surface structure of the HI/ABsep and the removal effect of the HI/ABsep on Sb (III) were investigated using potassium tartrate of antimony as the source of antimony and HI/ABsep as the adsorbent. The structural features of the HI/ABsep were analyzed by SEM, FTIR, PXRD, BET, and XPS methods. Static adsorption experiments were performed to investigate the effects of adsorption time, temperature, adsorbent dosage, and pH on the Sb (III) adsorbed by HI/ABsep. This demonstrates that sepiolite has a well-developed pore structure and is an excellent scaffold for the formation of hydroxy-iron. HI/ABsep adsorption of Sb (III) showed the best fit to the pseudo-second-order model and the Freundlich model. The maximum saturated adsorption capacity of the HI/ABsep regarding Sb (III) from Langmuir's model is 25.67 mg/g at 298 K. Based on the research results, the HI/ABsep has the advantages of easy synthesis and good adsorption performance and has the potential to become a remediation for wastewater contaminated with the heavy metal Sb (III).

**Keywords:** antimony; hydroxy-iron; sepiolite; adsorption

#### **1. Introduction**

The heavy metal pollution of water supplies and sewers has become a common problem in the production of food safety and human health [1]. Antimony and its compounds, as a crucial strategic nonrenewable metal, have been extensively used in flame retardants, alloys, ceramics, pigments, and synthetic fibers [2,3]. However, antimony is a widely distributed toxic element. Antimony (Sb) is classified as one of the main pollutants by the US Environmental Protection Agency and the European Environmental Protection Agency due to its proven and potential carcinogenicity, immunotoxicity, genotoxicity, and toxicity to the reproductive system [4–6]. Antimony in water exists primarily in two forms, Sb (V) and Sb (III), among which the toxicity of Sb (III) is approximately ten times higher than that of Sb (V) [7]. China also has the largest reserve of antimony and is the world's largest coal miner. Extensive antimony mining makes antimony pollution more serious in the southwest [8]. Therefore, it is crucial to find a rapid and efficient way to remove antimony.

In the past few decades, standard methods for removing Sb mainly include adsorption [9], ion exchange [10], coagulation [11], and microbial treatment [12]. The adsorption method is still one of the most widely used of these methods due to its advantages: simple operation, low cost, and greenness of the materials. The adsorbents include modified nonmetallic minerals, natural or synthetic metal oxides, and carbon materials, etc. The current research results show that iron oxyhydroxide has a good removal effect on antimony in water [13,14]. However, ferric hydroxide in water is often in a loose, amorphous flocculation

**Citation:** Zou, Y.; Ren, B.; He, Z.; Deng, X. Enhanced Removal of Sb (III) by Hydroxy-Iron/ Acid–Base-Modified Sepiolite: Surface Structure and Adsorption Mechanism. *Water* **2022**, *14*, 3806. https://doi.org/10.3390/w14233806

Academic Editor: Hai Nguyen Tran

Received: 20 October 2022 Accepted: 15 November 2022 Published: 23 November 2022

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**Copyright:** © 2022 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (https:// creativecommons.org/licenses/by/ 4.0/).

state, with the disadvantage of not being easy to disband nor easy to separate from water. These shortcomings increase the cost of removing antimony from iron oxides and limit their practical application. The key approach to solving this problem is to fix iron oxide in coarse, porous granular carriers with a large specific surface area. At present, iron modification is widely applied in clay minerals. Bahabadi et al. [15] compared the adsorption effects of several naturally occurring clay minerals and iron-modified clay minerals on heavy metal adsorption and found that the modification of Fe3+ ions can be localized in the channel or that oxygen- or hydrogen–oxygen complexes are formed at the mineral surface to enhance the active sites of the mineral. Xu et al. [16] prepared a composite material of iron oxide and quartz sand from iron oxide-filled quartz sand and used it to adsorb Sb (III) into the water.

The mineral sepiolite (Sep) is a naturally occurring magnesium-rich silicate clay mineral with a standard crystalline chemical formula of Mg8Si12O30(OH)4(OH2)4·8H2O. China's sepiolite resources are mainly distributed in Hunan Province, with proven reserves exceeding 21.4 million tons. Therefore, it is an excellent choice to study iron modification with sepiolite from local materials. In the unique channel of the sepiolite nanostructure, the presence of "zeolite water" and some Mg2+ and Ca2+ exchangeable ions cause it to have strong adsorption properties and ion exchange capacities, so its potential application in the heavy metal adsorbing field has been widely investigated [17,18]. Sepiolite is cheap, readily available, and has a large surface area. It has great potential in the treatment of heavy metal wastewater. Natural sepiolite, however, has shortcomings, such as low surface acidity, a small channel, and low metal binding constant, so it needs proper modification processing [19].

Hence, this study is mainly conducted from the following three aspects: (I) the modified two-step sepiolite synthesis of antimony adsorption material, and SEM-EDS, FTIR, PXRD, BET, and XPS technology enhanced for analyzing the adsorbing process. (ii) The surface structure and adsorption conditions of HI/ABsep are investigated, including contact time, adsorbent dosage, pH, and initial concentration of Sb (III). (iii) The adsorption behavior and adsorption type of HI/Absep-adsorbed Sb (III) were determined by quasi-primary and quasi-secondary kinetic models, and Langmuir and Freundlich isothermal models.

#### **2. Materials and Methods**

#### *2.1. Preparation of HI/ABsep*

Sepiolite purity >80%, procured from Xiangtan Sepiolite Technology Co., Ltd. (Xiangtan, China). All reagents are of analytical grade and procured from Sinopharm Chemical Reagent Co., Ltd. (Shanghai, China).

Sepiolite (10 g) is dispersed in 2.5 mol/L HCl solution (250 mL) at room temperature, acid activation is performed at 250 r/min agitation speed for 24 h, then ammonia water is added, and added dropwise under magnetic stirring in the solution until the pH of the solution reaches 10.0. The supernatant was stirred for 2 h and then removed by centrifugation, followed by several items of deionized water and absolute ethanol. The precipitate is finally collected, dried in a drying oven at a constant temperature of 80 ◦C, crushed, and sieved to obtain a combined acid–base-modified sepiolite (abbreviated as ABsep). Disperse ABsep (10 g) in 1 mol/L FeCl3 solution (10 mL) and sonicate for 10 min. Then, 1 mol/L NaOH (10 mL) was added, stirred with a magnetic stirrer at 250 r/min for 2 h, washed several times with deionized water, centrifuged, dried at 80 ◦C, then ground, continued to be cleaned until nearly neutral, and then after drying, passed through a 200-mesh sieve, and, finally, HI/ABsep is prepared.

#### *2.2. Characterization Methods*

Before the option of Sb (III) in simulated wastewater, sepiolite and HI/ABsep derived from sepiolite were characterized. Fourier-transform infrared spectroscopy (FTIR) (ALPHA, Bruker) was used to analyze the functional groups in the samples and their changes during the reaction. The surface morphology and structural characteristics of the sample were observed by scanning electron microscope (SEM) (JEOL JSM-6700F) with energy spectrum; X-ray powder diffraction (PXRD) was obtained by X-ray diffractometer (D/Max 2500, Rigaku). The material pore structure was measured by the specific surface area and porosity analyzer (ASAP2020M). The composition and distribution information of chemical elements was obtained by X-ray photoelectron spectroscopy (XPS) (ESCALAB 250Xi, Thermo fisher, Waltham, MA, USA).

#### *2.3. Batch Adsorption Experiments*

The powdered potassium tartrate of antimony dissolved in diluted hydrochloric acid was prepared for 500 mg/L, mother liquor pH 6, and diluted by mother liquor to obtain simulated wastewater of different antimony concentrations. The separated or modified sepiolite powders were added to the simulated antimony wastewater and the vibrating adsorption at 1500 r/min under different experimental conditions and requirements. This was filtered through a 0.45 μm syringe diaphragm filter. An atomic absorption spectrometer (TAS-990) was used to determine the equilibrium concentration of Sb (III) in the supernatant. The equilibrium adsorption capacity (*qe*) and adsorption efficiency *η* (%) of Sb (III) were calculated by the following formula:

$$q\_{\varepsilon} = \frac{(\mathbb{C}\_0 - \mathbb{C}\_{\varepsilon}) \times V}{\mathbb{C}\_0} \tag{1}$$

$$\eta = \frac{\mathcal{C}\_0 - \mathcal{C}\_\varepsilon}{\mathcal{C}\_0} \times 100\% \tag{2}$$

where *C*<sup>0</sup> and *Ce* are the initial and equilibrium (mg/L) concentrations of Sb (III), respectively, *V* is the volume of solution (L), and *m* is the mass of the adsorbent (g).

#### 2.3.1. Adsorption Kinetics Experiment

Sep (1.0 g) and HI/ABsep (1.0 g) were added to the 20 mg/L Sb (III) solution (500 mL). At this time, the initial pH of the solution was not adjusted (in this case, pH = 6). The beaker with the mixture was transferred to a magnetic stirrer and continuously stirred at 150 r/min speed at room temperature. At each sampling time (3–240 min), samples were collected and immediately filtered through a 0.45 μm syringe membrane filter to determine the equilibrium concentration of Sb (III) in the supernatant.

The pseudo-first-order Equation (3) and pseudo-second-order Equation (4) are used for nonlinear fitting. The formula are as follows:

$$q\_t = q\_\varepsilon \left(1 - e^{-K\_1 t}\right) \tag{3}$$

$$q\_l = \frac{K\_2 q\_e^2 t}{1 + K\_2 q\_e t} \tag{4}$$

where *K*<sup>1</sup> is the pseudo-first-order kinetic equation rate constant (min−1); *K*<sup>2</sup> is the pseudo-second-order kinetic equation rate constant (g·mg−1·min<sup>−</sup>1); *<sup>t</sup>* is the adsorption time (min); *qt* and we are the Sb (III) adsorption amount (mg·g<sup>−</sup>1) at time *<sup>t</sup>* and adsorption equilibrium, respectively.

#### 2.3.2. Effect of Adsorbent Dosage and pH Value

Experimental conditions stipulate a temperature of 35 ◦C, a volume of solution 50 mL, an initial concentration of Sb (III) 10 mg/L, an initial pH of the system 6, an adsorption time of constant temperature oscillation 240 min, a dosage of HI/ABsep (0.5–3 g/L), respectively, for the effect on adsorbing heavy metal Sb (III).

In the same way, HCl (0.1–1 mol/L) and NaOH (0.1–1 mol/L) were used to adjust the initial pH value of the adsorption system (2–12), the dosage of the adsorbent is 2 g/L, and the other conditions are the same as mentioned above. Multiple adsorption tests were carried out on heavy metal Sb (III), and the effect of pH on the adsorption of Sb (III) at HI/ABsep was obtained.

#### 2.3.3. Adsorption Thermodynamics

Sep and HI/ABsep (0.1 g) were added to a series of 50 mL solutions of Sb (III) (10, 20, 50, 80, 100, and 150 mg/L), respectively. The solution was placed in a box with constanttemperature gas bath agitation and stirred at 150 r/min for 240 min at a temperature of 298 K, 308 K, and 318 K, respectively. Sb (III) was measured by filtering the supernatant through a 0.45 μm membrane syringe filter.

The experimental data were analyzed using two adsorption models. Langmuir Equation (5) and Freundlich Equation (6) perform non-linear fits of the adsorption isotherms at different temperatures (289 K, 308 K, and 318 K), respectively.

$$q\_{\mathcal{E}} = \frac{K\_L Q\_m \mathcal{C}\_{\mathcal{E}}}{1 + K\_L \mathcal{C}\_{\mathcal{E}}} \tag{5}$$

$$q\_{\varepsilon} = K\_F \mathbb{C}\_{\varepsilon}^{1/n} \tag{6}$$

where *qe* is the equilibrium adsorption amount (mg/g); *Ce* is the equilibrium concentration (mg/L); *Qm* is the saturation adsorption amount (mg/g); *KL* (L·mg−1) and *KF* [mg·g−1· (L·mg−1) 1/*n*] are Langmuir adsorption constant and Freundlich adsorption constant, respectively; 1/*n* is a constant for adsorption strength, which varies with the heterogeneity of the material.

#### 2.3.4. Thermodynamic Parameters

In order to further explain the spontaneous characteristics and energy changes in the preparation of HI/ABsep when adsorbing Sb (III) in the solid–liquid system, the thermodynamic parameters under three different temperatures (standard free energy Δ*G*◦ standard enthalpy Δ*H*◦ , standard entropy Δ*S* ◦ ) were calculated by using Equations (7) [20–22], (8), and (9).

$$K\_{Eq}^{\circ} = \frac{K\_T \times \mathcal{C}\_{AdSorbit}^{\circ}}{r\_{AdSorbit}} \tag{7}$$

$$
\Delta G^{\circ} = -RT\ln K\_{Eq}^{\circ} \tag{8}
$$

$$
\ln K\_{Eq}^{\circ} = \frac{\Delta S^{\circ}}{R} - \frac{\Delta H^{\circ}}{RT} \tag{9}
$$

where *K*◦ *Eq* (dimensionless) is the standard thermodynamic equilibrium constant of adsorption based on molar concentrations; *KT* is the distribution coefficient obtained from the Langmuir isotherm (L/mol); *C*◦ *Adsorbate* (mol/L) is the standard concentration of adsorbate; and *rAdsorbate* (dimensionless) is the activity coefficient of the adsorbate.

Using *lnK*◦ *Eq* and 1/T as the plot, the intercept and slope obtained by linear regression analysis can be used to calculate Δ*S* ◦ and Δ*H*◦ , respectively, and Δ*G*◦ can be directly calculated by Equation (8).

#### **3. Results and Discussion**

#### *3.1. Characterization of Sep and HI/ABsep*

#### 3.1.1. N2 Adsorption–Desorption Isotherm and Pore Size Distribution

Before compounding hydroxy iron into sepiolite, the sepiolite needs to be modified with acid and alkali. The N2 adsorption–desorption isotherm at 77 K and pore size distribution of sepiolite, ABsep, and HI/ABsep are shown in Figure 1a,b. Both of these two materials are of type IV adsorption. Their pores are concentrated in size distribution at 2–50 nm, indicating that there are a large number of mesopores in the sample, and antimony can penetrate the internal sepiolite surface, which is beneficial for adsorption [23]. The mean pore diameters of the ABsep composite and Sep materials calculated by BJH are 1.4 nm and 3.7 nm, respectively. The BET formula gives a significantly higher average specific surface area for the ABsep composite material (260.5 m2/g) than Sep (160.1 m2/g); it can be seen that the surface performance after the acid–base modification has been greatly

improved compared with that before modification, which provides a good foundation for the composite of hydroxyl iron. HI/ABsep surface area and average pore diameter were 152.4 m2/g and 1.7 nm, respectively, both smaller than the surface area and average pore diameter of ABsep. The reason for this may be that, during the introduction of iron and supported iron oxide, some channels are blocked, forming a thick membrane on the surface that prevents N2 from adsorption and desorption [24].

**Figure 1.** N2 adsorption–desorption isotherm at 77 K (**a**) and BJH pore size distribution (**b**) of Sep, ABsep, and HI/ABsep.

#### 3.1.2. SEM Analysis

As shown in Figure 2a, the original structure of the sepiolite material is a thin straight rod-like fiber structure. In Figure 2b, the SEM image of the ABsep material obtained by treatment of sepiolite with hydrochloric acid and ammonia is shown. As can be seen, the surface of the composite is non-uniform, and the nanofibers are fragmented in comparison to the SEM image of the original sepiolite structure. With the massive structure, the dispersibility is reduced, the agglomeration appears, and the pores increase. The result is consistent with Figure 1. When compared to the surface without iron modification, the iron-modified HI/ABsep showed irregular ripples and a highly roughened surface (Figure 2c), which could be affected by hydroxyl iron.

**Figure 2.** SEM image of Sep (**a**) and ABsep (**b**) and SEM-EDS image of HI/ABsep (**c**).

#### 3.1.3. PXRD and FTIR Analysis

The PXRD spectrum of the sample is shown in Figure 3b. The Sep spectrum showed apparent diffraction of 7.2, 11.8, 19.7, 23.7, 26.7, 33.5, and 40.2 degrees. The normalized card number ((JCPDS: 14-0001) indicates that (110), (130), (300), (340), (270), (510), and (490) are known to correspond to the sepiolite orders. After modification, the sepiolite peak disappears, and the characteristic diffraction peaks 2θ of HI/ABsep are 20.9◦, 26.6◦, and 50.1◦, which are following SiO2 (JCPDS:46-1045) [25], indicating that the main component of HI/ABsep used in this study is SiO2 [26,27]. In addition, the characteristic peaks of HI/ABsep, (130), (021), (221), and (151), correspond to the angles 2θ = 33.2◦, 34.7◦, 53.2◦, and 59.0◦, which are consistent with the standard card number (JCPDS: 81-0464). It shows that the main form of iron supported by modified sepiolite is iron oxyhydroxide FeOOH.

**Figure 3.** PXRD (**a**) and FTIR spectrum (**b**) of Sep and HI/ABsep. The red line in Figure 3a represents the XRD image of HI/ABsep, the black line is the XRD image of Sep, and the light blue refers to the data image corresponding to JCPDS:81-0464; Green is the data image corresponding to JCPDS:46-1045; The dark blue is the data image corresponding to JCPDS:14-0001. The red line in Figure 3b is the image produced by the infrared spectral data of HI/ABsep, and the black line is the infrared spectrum of Sep.

Figure 3a shows the FTIR spectra obtained by Sep and HI/ABsep. In the wavenumber range of 1300–900 cm−1, it is a broad and strong stretching vibration band produced by Si-O-Si [28]; the strong absorption band near 3462 cm−<sup>1</sup> belongs to the structure of the sepiolite ore fine octahedral coordination. The stretching vibration of hydroxyl water (Mg-OH) and coordination water (-OH2), and the absorption peak near 687 cm−<sup>1</sup> are lattice vibrations of MgO, Al2O3, and Ca2O3 oxides; the band of 479 cm−<sup>1</sup> corresponds to vibrations on the Si-O-Si plane [29]; the 1658 cm−<sup>1</sup> belongs to the peak of C=O stretching vibration; moreover, there are some differences in the peaks of the HI/ABsep spectrum. The hydroxyl peak (-OH) at 3429 cm−<sup>1</sup> is enhanced. The -OH could be the hydroxyl peak of water. The absorption band characteristic at 1421 cm−<sup>1</sup> can be caused by Cl-vibration. Especially at 900–450 cm<sup>−</sup>1, which is the characteristic absorption peak of metal oxide, the change before and after modification is related to the appearance of FeOOH oxyhydroxide [30,31].

#### *3.2. Adsorption of Sep and HI/ABsep on Sb (III)*

#### 3.2.1. Adsorption Kinetics

However, as shown in Figure 4, the amount of Sb (III) adsorbed by Sep and HI/ABsep increased rapidly and then stabilized. At 30 min contact time, the rate of Sb (III) removal by HI/ABsep had reached 67.28%. Within 30–120 min, the absorption rate of the material gradually decreases and tends to equilibrium, reaching an adsorption equilibrium at about 180 min. Sb (III) has a maximum adsorption capacity of 6.14 mg/g and a removal rate of 92%%, which is significantly higher than the original sepiolite (2.65 mg/g); the reason is that hydroxy-iron increases the number of active adsorption sites on the sepiolite surface and the adsorptive capacity is enhanced. The initial rapid adsorption may be due to the external diffusion of Sb (III), which is attached to the external surface of the material. Its main functions are ion exchange and physical adsorption. However, as time increases, on the one hand, the surface of the adsorbent can be used for the adsorption of Sb (III). However, as the concentration gradient of Sb (III) on the solution surface and adsorbent is reduced, the driving force for the concentration gradient decreases so that the resistance of the Sb (III) to adsorbate in solution increases, and the rate of adsorption slowly decreases [32,33].

**Figure 4.** Adsorption kinetics of Sep and HI/ABsep.

In Table 1, the R<sup>2</sup> value of the quasi-second-order kinetics for the adsorption of Sb (III) by HI/ABsep is 0.9865, which is greater than that of the fitting quasi-first-order kinetics, suggesting that the quasi-second-order kinetics can more accurately describe the adsorption process of Sb (III) by HI/ABeep. Based on the assumption of the quasi-second-order kinetic model, combined with the related literature, the HI/ABsep adsorption process of Sb (III) can be known to be primarily chemical adsorption. The adsorbent and Sb (III) may form chemical bonds by sharing or exchanging electrons. Adsorption occurs through complexation, coordination, or ion exchange [6,9,13,34–37].


**Table 1.** Kinetic parameters of adsorption on Sb (III).

3.2.2. Effect of HI/ABsep Dosage and pH on the Removal of Sb (III)

It has obvious practical significance for the industrial application of sorbent to study the effect of the amount of sorbent and pH on sorbent adsorption performance and the maximum adsorption effect of the sorbent on different heavy metal ions. From Figure 5a, it can be seen that, as the dosage of HI/ABsep is increased, the rate of elimination gradually increases. When the dosage is 2 g/L, the removal rate reaches 95.46%. Further analysis shows that, when the dosage is low, the number of adsorption sites that can be provided is correspondingly more negligible. The adsorption site of the adsorbent material has a positive correlation with the adsorption efficiency. As a result, the rate of Sb (III) elimination by HI/ABsep at low doses is lower. When the dosage of the adsorbent gradually increases, on the one hand, the adsorption site of the solution also increases rapidly. Conversely, the probability that the reaction between the adsorbed substance and the adsorbate collide with each other also increases as a result; thus, the removal rate of Sb (III) is rapidly increased. Taking into account the economic benefits, we use a dose of 2 g/L in other single-factor experiments.

**Figure 5.** Effect of different dosages of adsorbent and pH change on Sb (III) adsorption. (**a**). Effect of different doses of adsorbents on the adsorption of Sb (III); (**b**). Effect of pH on the adsorption of Sb (III) by adsorbents.

From Figure 5b, it can be seen that the pH of the solution does not significantly affect the adsorption efficiency of Sb (III) by HI/ABsep. When pH is 2–6 and pH = 8–12, the removal rate of Sb (III) increased slowly. When pH is 6–8, the removal rate of Sb (III) decreased slowly. On the whole, the removal rate of Sb (III) remained at a high level (92–95.5%). When the pH of the solution is 2~10, the main form of Sb (III) in the solution is the neutral complex Sb (OH)3. Under the strong acid condition, Sb (III) mainly exists as SbO+; all of them may be adsorbed on the surface of goethite with a positive charge as inner sphere complexes [38]. Under the strong alkali condition, its preferential form is SbO2 [35,39–41]. Sb (OH)3 easily reacts with Fe ions to form X ≡ Fe-Sb (OH)2 precipitate [41], and the reaction process is little affected by pH, so the pH has little effect on the adsorption and removal of Sb (III) by HI/ABsep.

#### 3.2.3. Adsorption Isotherm

As can be seen from the nonlinear fitting results in Figure 6 and related parameters in Table 2, the Freundlich model has a higher temperature than the Langmuir model (R2 = 0.9595, 0.9062 and 0.9545) at 298 K, 308 K, and 318 K temperatures. The values of the correlation coefficient are 0.9671, 0.9854, and 0.9660. This result demonstrates that the Freundlich model can describe the process of Sb (III) adsorption onto HI/ABsep well. Furthermore, at different temperatures, the Langmuir and Freundlich isotherm models can fit well. This shows that the intermolecular forces play a major role in the magnitude of the intermolecular forces: The higher the temperature, (1) the stronger the adsorption and (2) The greater the concentration of the solution. Langmuir isotherms are appropriate for single-layer adsorption where the adsorption sites are uniformly distributed over the adsorbent surface and there is no interaction between adjacent sites and the particles [42]. The Freundlich adsorption isotherm model is an empirical formula for multilayer adsorption [43]. It can be deduced that Sb (III) is dominated by the uniform adsorption of multiple molecular layers under a variety of temperature conditions. Furthermore, the range of values of 1/n of the adsorption models of the Langmuir isotherm and Freundlich isotherm is from 0 to 1, indicating that the prepared HI/ABsep has a high affinity to Sb (III), and adsorption may spontaneously develop in the direction of adsorption [44]. According to the Langmuir isotherm (HI/ABsep) adsorption model, the maximum adsorption capacity of Sb (III) is found to be 25.73 mg/ at 298 K.

**Figure 6.** Isothermal adsorption line of HI/ABsep to Sb (III). (**a**). the Langermuir isotherm model for HI/ABsep adsorbed Sb (III), 298K, 308K, 318K; (**b**). the Freundlich isotherm model for HI/ABsep adsorbed Sb (III), 298K, 308K, 318K.

**Table 2.** Two adsorption isotherm-related parameters.


The actual maximum adsorption capacity of the adsorbent HI/ABsep prepared in this study under optimal experimental conditions is 25.73 mg/g, which has adsorption potential compared with other Sb (III) adsorption materials, as shown in Table 3 below. At the same time, considering that the adsorption material in this study has the advantages of simple preparation process and easy physical separation from the treatment system, it has great application potential for the treatment of Sb (III)-contaminated wastewater

**Table 3.** Comparison of Sb (III) adsorption by various adsorbents.


#### 3.2.4. Thermodynamic Parameters

As can be seen from Table 4, as the temperature is increased from 298 K to 318 K, the adsorption of Sb (III) on HI/ABsep has Δ*G*◦ < 0 and Δ*H*◦ > 0, indicating that the adsorption is a spontaneous endothermic reaction. If the absolute value of ΔH is between 0 and 20 kJ/mol, the adsorption reaction is related to physical adsorption, and if the absolute value of Δ*H* is between 40 and 80 kJ/mol, then the adsorption reaction is related to chemical adsorption [47]. The value of Δ*H*◦ used in this study is 160.18 kJ/mol at 298–318 K, which indicates that the adsorption of Sb (III) by HI/ABsep is chemical adsorption [48]. The mechanism of adsorption may be an electrostatic force and the filling of pores. This binding increases with an increasing temperature, indicating that increasing temperature is beneficial for the progress of adsorption.

**Table 4.** Thermodynamic parameters for Sb (III) adsorption by HI/ABsep.


3.2.5. The Mechanism of HI/ABsep Adsorption on Sb (III)

In order to gain insight into the HI/ABsep adsorption mechanism of Sb (III), XPS is used to analyze the existence of elements on HI/ABsep and their interaction during the process of adsorption. The high-resolution spectra of HI/ABsep O and Sb before and after adsorption are shown in Figure 7a. The characteristic peaks of 531.84 eV binding energy and 532.53 eV binding energy in the pre-adsorption O1s high-resolution spectrum correspond mainly to Fe-O-OH and Si=O [49] (SiO2), respectively. After adsorption, the two characteristic peaks at the binding energy of 530.65 eV and the binding energy of 540.28 eV correspond to the positions of the standard binding energy peaks of the two orbitals of Sb 3d5/2 and Sb 3d3/2, respectively. Among the valence states are Sb (III) and Sb (V), which indicates that the redox reaction took place during the adsorption process [50]. The binding energies for Fe 2p3/2 and Fe 2p1/2 of HI/ABsep are about 710.96 eV and 724.26 eV (Figure 7b). The photoelectron peak at 710.96 eV, which is characteristic of iron oxide species in HI/ABsep, shifts to the position of 710.72 eV in HI/ABsep-Sb, which indicates the interaction between adsorbed Sb (III) and iron oxide in HI/ABsep [51]. The removal mechanism of HI/ABsep-strengthened removal of Sb (III) is shown in Figure 8.

**Figure 7.** XPS spectra before and after HI/ABsep adsorption on Sb (III). (**a**) is the fine spectrum of O1s before and after the reaction, and (**b**) is the fine spectrum of Fe2p before and after the reaction). The red line in Figure 7a indicates the peak fitted to the data, indicating that it is composed of the peaks in the green, cyan, and purple lines; the green and cyan lines represent the two characteristic peaks of O1S corresponding to FeOOH and Si=O; and the purple line represents the characteristic peak of Sb (III); The XPS spectra before and after the adsorption of Sb (III) by HI/ABsep are shown in Figure 7b, respectively. The red line indicates the peak made by fitting the data, and the fuchsia line, cyan line and blue line are made by splitting the peak from the red line. The fuchsia line and the blue line indicate the two satellite peaks of the Fe2p3/2 orbit. The cyan line indicates the characteristic peak of Fe2p1/2 orbit.

**Figure 8.** Mechanism of Sb (III) Removal by HI/ABsep.

Briefly, the adsorption of HI/ABsep Sb (III) is mainly chemical and physical adsorption. HI/ABsep adsorption onto Sb (III) is a very complex process, which can be combined through a variety of adsorption mechanisms such as redox, coordination complexation, and physical adsorption. A schematic diagram of hydroxyl iron formation and the mechanism of enhanced Sb (III) removal by HI/ABsep can be seen in Figure 8. At pH = 2–10, the main forms of antimony in water are Sb (OH)3 and Sb (OH)6 −. It is speculated that the surface complexation between antimony ions in the simulated water and the hydroxyl iron on the surface of HI/ABsep is shown in the following formula:

$$t \equiv MOH + Sb(OH)\_3 = MOSbO\_2 + 2H\_2O \tag{10}$$

$$2 \equiv MOH + Sb(OH)\_3 = MOSb(OH)OM + 2H\_2O \tag{11}$$

$$\rm{Sb(OH)\_3 + 3H\_2O - 2e^- = Sb(OH)\_6^- + 3H^+} \tag{12}$$

$$\overline{\mathbf{a}} \equiv \mathbf{M}\mathbf{O}\mathbf{H} + \mathbf{S}\mathbf{b}(\mathbf{O}\mathbf{H})\_6^- = \mathbf{M}\mathbf{O}\mathbf{S}\mathbf{b}(\mathbf{O}\mathbf{H})\_5^- + \mathbf{H}\_2\mathbf{O} \tag{13}$$

$$2 \equiv MOH + Sb(OH)\_6^- = MOSb(OH)\_4^-OM + 2H\_2O \tag{14}$$

where *M* stands for iron, silicon, magnesium, etc.

#### **4. Conclusions**

HI/ABsep modified based on sepiolite can be used as a promising material for adsorbing heavy metal Sb (III). XRD and XPS show that the Fe species supported on sepiolite are mainly FeOOH. Furthermore, the dispersion of FeOOH on sepiolite led to the exposure of a few new adsorption locations. The kinetics of adsorption follow a pseudo-second-order kinetic model. The Langmuir and Freundlich isotherm models were used to fit the composite adsorbent kinetics successfully, and the maximum adsorption capacity is 25.73 mg/g at 298 K, 26.89 mg/g at 308 K, and 32.45 mg/g at 318 K.

HI/ABsep adsorption of Sb (III) is barely affected by pH but is proportional to the content of the adsorbent. Future research will focus on the practical implementation of the live demonstration.

**Author Contributions:** Conceptualization, B.R. and Y.Z.; Methodology, Y.Z. and Z.H.; Software, Z.H.; validation, Y.Z., Z.H. and X.D.; formal analysis, Y.Z.; investigation, X.D.; Resources, X.D.; Data curation, Y.Z. and Z.H.; Writing—original draft, Y.Z. and Z.H.; Writing—review & editing, B.R. and Y.Z. All authors have read and agreed to the published version of the manuscript.

**Funding:** This work was financially supported by the National Natural Science Foundation of China (Nos. 41973078) and the Hunan Provincial Natural Science Foundation of China (2022SK2073).

**Institutional Review Board Statement:** The study did not deal with ethics.

**Informed Consent Statement:** The study did not involve humans.

**Data Availability Statement:** The study did not report any data.

**Acknowledgments:** We sincerely thank the National Natural Science Foundation of China and the Hunan Provincial Natural Science Foundation of China for their support in experiments and data analysis.

**Conflicts of Interest:** The authors declare no conflict of interest.

#### **References**


## *Article* **Efficient Uptake of Angiotensin-Converting Enzyme II Inhibitor Employing Graphene Oxide-Based Magnetic Nanoadsorbents**

**Miguel Pereira de Oliveira 1, Carlos Schnorr 2, Theodoro da Rosa Salles <sup>1</sup> , Franciele da Silva Bruckmann <sup>3</sup> , Luiza Baumann 3, Edson Irineu Muller 3, Wagner Jesus da Silva Garcia <sup>4</sup> , Artur Harres de Oliveira <sup>5</sup> , Luis F. O. Silva <sup>2</sup> and Cristiano Rodrigo Bohn Rhoden 2,6,\***


**Abstract:** This paper reports a high efficiency uptake of captopril (CPT), employing magnetic graphene oxide (MGO) as the adsorbent. The graphene oxide (GO) was produced through an oxidation and exfoliation method, and the magnetization technique by the co-precipitation method. The nanomaterials were characterized by FTIR, XRD, SEM, Raman, and VSM analysis. The optimal condition was reached by employing GO·Fe3O4 at pH 3.0 (50 mg of adsorbent and 50 mg L−<sup>1</sup> of CPT), presenting values of removal percentage and maximum adsorption capacity of 99.43% and 100.41 mg g−1, respectively. The CPT adsorption was dependent on adsorbent dosage, initial concentration of adsorbate, pH, and ionic strength. Sips and Elovich models showed the best adjustment for experimental data, suggesting that adsorption occurs in a heterogeneous surface. Thermodynamic parameters reveal a favorable, exothermic, involving a chemisorption process. The magnetic carbon nanomaterial exhibited a high efficiency after five adsorption/desorption cycles. Finally, the GO·Fe3O4 showed an excellent performance in CPT removal, allowing future application in waste management.

**Keywords:** adsorption; carbon nanomaterials; magnetite; captopril

#### **1. Introduction**

Water contamination by emerging pollutants (EPs) has become an important problem around the world [1]. Amongst these, drugs and personal care products are widely found in water bodies, due to their intrinsic characteristics and incomplete removal by current methods [2]. Additionally, due to their unique properties and xenobiotic-like character, these compounds present a high tendency to bioaccumulation, which leads to the development of diverse damages to human health, animals, and the environment [3]. In addition, high consumption, inadequate disposal, and incomplete metabolism are associated with the high and constant generation of waste. Due to low concentration and traces in water, the conventional systems are not able to remove these compounds, resulting in a secondary source of exposition and contamination [4].

Captopril (CPT) (1-(3-mercapto-2-D-methyl-1-oxopropyl)-L proline is a hydrophilic drug used to inhibit angiotensin-converting enzyme II (ACE II) and is one of the most commonly prescribed medicines for the treatment of high blood pressure and heart failure [5,6].

**Citation:** de Oliveira, M.P.; Schnorr, C.; da Rosa Salles, T.; da Silva Bruckmann, F.; Baumann, L.; Muller, E.I.; da Silva Garcia, W.J.; de Oliveira, A.H.; Silva, L.F.O.; Rhoden, C.R.B. Efficient Uptake of Angiotensin-Converting Enzyme II Inhibitor Employing Graphene Oxide-Based Magnetic Nanoadsorbents. *Water* **2023**, *15*, 293. https://doi.org/ 10.3390/w15020293

Academic Editor: Hai Nguyen Tran

Received: 13 December 2022 Revised: 5 January 2023 Accepted: 6 January 2023 Published: 10 January 2023

**Copyright:** © 2023 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (https:// creativecommons.org/licenses/by/ 4.0/).

Although this medicine is effective for control of diseases, around 50% of CPT is excreted by kidneys in unchanged form, contributing to water contamination [7]. Additionally, this pharmaceutical compound may cause diverse side effects and toxicity to organisms [8,9].

In this scenario, diverse methods have been explored for EPs removal from aqueous medium, such as Fenton [10], photocatalyst [11], ozonation [12], and adsorption [13,14]. The adsorption technique is characterized by the capture of contaminant (adsorbate) using a solid surface (adsorbent). This method presents advantages compared with other processes, such as low energy requirements and easy operation, without byproducts and generation of sludge [15]. Regarding the uptake of captopril, recent studies reported its removal using alternative materials from the biomass and lignocellulosic waste [9,16]. Considering the high consumption of captopril and the potential damage to health, and the limited studies that report its removal from the aquatic environment, it is necessary to use and design alternative and novel adsorbents. Additionally, nanotechnology is an excellent tool to enhance the process, using adsorbents at nanoscale, improving the properties and characteristics of conventional adsorbents, as well as the adsorption efficiency [17,18].

Among these, carbon nanomaterials, such as graphene oxide (GO) can be used in the adsorption process, due to the high specific surface area, reactivity, and presence of oxygenated groups [19]. Nonetheless, the unique properties of GO allow combination with other nanoparticles, as magnetic nanoparticles (MNPs), conferring new properties as well as avoiding the filtration and centrifugation steps. Recently, Li et al. [20] synthesized a magnetic nanocomposite with GO and hydroxyapatite for Pb (II) removal from water. The results demonstrate that nanomaterials exhibited a high adsorption capacity and maintained efficiency after several cycles of adsorption/desorption.

This work reports captopril adsorption using magnetic graphene oxide (GO·Fe3O4), synthesized through a straightforward method, using only Fe2+ as the iron source [21]. The adsorption behavior was evaluated using different experimental variables. The isotherm and kinetic models were used to understand the adsorption equilibrium and kinetic profile. Additionally, the adsorbent regeneration was investigated to determine the lifetime and efficiency after diverse adsorption/desorption processes. The mechanistical hypotheses for captopril adsorption onto graphene oxide-based magnetic adsorbent are also proposed, based on chemical species and magnetite incorporation on the GO surface.

#### **2. Materials and Methods**

#### *2.1. Synthesis of Graphene Oxide and Graphene Oxide-Based Magnetic Nanoadsorbent*

Graphene oxide was synthesized following Salles et al. [22]. 1 g of graphite in flakes (Sigma-Aldrich®, St. Louis, MO, USA) was added to a flask with 60 mL of H2SO4 98% (Synth®) under magnetic stirring for 10 min at room temperature. Sequentially, 6 g of KMnO4 99% (Synth®) was slowly added over 20 min. The solution was heated at 40 ◦C and stirred continuously for 5 h. Afterward, 180 mL of distilled H2O was slowly added and stirring for 12 h at room temperature. Sequentially, the reaction was heated at 40 ◦C for 2 h. Finally, 300 mL of distilled H2O and 10 mL of H2O2 29% (Synth®) was added into the solution. The yellow solution was washed until pH 5.0 and dried in an oven (DeLeo) at 50 ◦C for 24 h.

The magnetic graphene oxide was produced by the co-precipitation method [21]. In a 250 mL round-bottom flask containing 120 mL of ultrapure water, 100 mg of GO were added to 1000 mg of FeCl2 ≥ 99% (Sigma-Aldrich®). Afterwards, ammonium hydroxide (Synth®) was added to reach pH 9.0. Sequentially, the mixture was submitted to ultrasonic radiation (Elma, power 150 W) for 90 min at room temperature. Finally, the product was purified with distilled water and acetone (Synth®), and dried in an oven (DeLeo) at 50 ◦C for total solvents evaporation.

#### *2.2. Adsorbent Characterization*

Graphene oxide and magnetic graphene oxide were characterized by several different techniques. The functional groups were determined through the Fourier Transform spectroscopy (Perkin-Elmer, model Spectro One). The crystallinity was determined by X-ray diffraction (Bruker diffractometer, model D2 Phaser), and Raman Spectroscopy (Renishaw inVia spectrometer system). The morphological characteristics were evaluated using a scanning electron microscopic (Zeiss Sigma 300 VP). The magnetic properties were evaluated with a vibrating sample magnetometer (Stanford Research Systems Model SR830 DSP lock-in amplifier, coupled to a Stanford Research Systems low-noise pre-amplifier model SR560, including a Kepco bipolar operation power supply).

The zero point of charge (pHZPC) of MGO was evaluated by the following procedure. 10 mg of adsorbent was added to a 100 mL flask containing 50 mL of ultrapure water (Milli-Q®, Darmstadt, Germany) and pH adjusted in the range of (2.0–12.0) using NaOH (0.01 mol L−1) and HCl (0.01 mol L−1). Sequentially, the samples were maintained under stirring (120 rpm) for 24 h at room temperature. Subsequentially, the final pH was measured with a potentiometer (Digimed).

#### *2.3. Adsorption Procedure and Mathematical Modeling*

The captopril adsorption was performed in a batch system. The influence of magnetite was initially evaluated employing GO and GO·Fe3O4 to determine the effect of iron nanoparticles in GO surface on the adsorption efficiency. The adsorption equilibrium study was evaluated using 50 mg of GO·Fe3O4 and 50 mg L−<sup>1</sup> of initial concentration of captopril, pH 3.0, and different temperatures (20 ◦C, 30 ◦C, and 40 ◦C). The adsorption kinetic was performed employing different concentrations of CPT ≥ 98% (10–200 mg L<sup>−</sup>1), 50 mg of adsorbent, at room temperature, and pH 3.0. Thermodynamic parameters were evaluated under the same conditions of the equilibrium study. Besides, this the influence of pH was verified in a pH range between 2.0 and 10.0, using HCl and NaOH solutions (0.1 mol L−1) for adjustment of the pH of the solution. The effect of ionic strength was performed employing a gradual NaCl concentration (0.01–1 mol L<sup>−</sup>1). Following this, the effect of initial concentration and adsorbent dosage were performed using 10–200 mg L−<sup>1</sup> of CPT and 12.5–50 mg of GO·Fe3O4, respectively. The adsorbent regeneration was carried out employing NaOH (0.25 mol L−1) as the desorbing agent at room temperature for 1 h and 150 rpm. The residual concentration of CPT was measured using a UV-vis spectrophotometer (Shimadzu-1650PC) at λ = 217 nm. The removal percentage and adsorption capacity at equilibrium are shown in Table 1 [23].

Isotherms models were employed to understand the relationship between the adsorbate and adsorbent molecules. The Langmuir, Freundlich, and Sips models were used to fit the experimental results, and the equations are shown in Table 1 [24].

The kinetic study is useful to explain the mass transfer phenomenon and the effect of contact time on the adsorption rate. The models used in this work were Pseudofirst-order (PFO), Pseudo-second-order (PSO), and Elovich. Thermodynamic parameters were calculated by Van 't Hoff equation (Equation (10)) to determine the adsorption behavior [25,26]. The equilibrium and kinetic data were estimated through non-linear regression, using the Statistical 10 software (StatSoft, Tulsa, OK, USA). The coefficient of determination (R2), adjusted coefficient of determination (R2 adj), sum of squared errors (SSE), and average relative error (ARE) were used to verify the validity of models, and mean square error (MSE) are shown in Table 1 [27].

The mathematical modeling used to calculate the adjustment parameters are shown in Table 1.


**Table 1.** Equations of removal percentage, adsorption capacity, adsorption isotherms, kinetics, thermodynamics models, and mathematical modelling.

#### **3. Results and Discussion**

*3.1. Fourier Transform Infrared Spectroscopy (FTIR)*

The presence of functional groups was evaluated by FTIR analysis (Figure 1). According to the results, it was possible to observe an intense signal around 3405 cm−1, corresponding to the stretching vibration of OH groups in GO composition [28]. The bands at 1734, 1651, 1342, and 1048 cm−<sup>1</sup> are related to the C=O stretch vibration, C=C bonds, C-OH vibration, and C-O bonds, respectively [29]. Besides this, for the GO·Fe3O4, it was

possible to verify the appearance of band around 617 cm−<sup>1</sup> characteristics of Fe-O, which suggests the presence of magnetite into GO surface [30,31].

**Figure 1.** FTIR of GO and GO·Fe3O4.

#### *3.2. X-ray Diffraction (XRD)*

The XRD results of graphene oxide and magnetic graphene oxide are shown in Figure 2. The GO diffraction shows the characteristic signal around 2θ ≈ 10.5◦, corresponding to the (001) plane, and the absence of a peak at 2θ ≈ 26◦ indicate the complete oxidation of graphite (precursor material) [32]. The peak at 2θ ≈ 44.2◦ (100) suggests a small distance between GO layers. For the GO with iron oxide nanoparticles, it was possible to verify the appearance of peaks around 2θ ≈ 30.3◦, 35.2◦, 43.5◦, 57.02◦, and 63.3◦, corresponding to the indices (220), (311), (400), (511) and (440), typical of magnetite [33]. The position and intensity of all peaks are compatible with Fe3O4 (ICDD card no:88-0315). The absence of GO signal in the GO·Fe3O4 diffractogram is attributed to the surface coated by iron nanoparticles, which is possible to observe in SEM results (Figure 3) [21].

**Figure 2.** XRD of (**a**) GO and (**b**) GO·Fe3O4.

**Figure 3.** SEM of (**a**) GO and (**b**) GO·Fe3O4.

#### *3.3. Scanning Electron Microscopy (SEM)*

The morphology of GO and GO·Fe3O4 was determined by scanning electron microscopy (SEM) and is shown in Figure 3. For the GO (Figure 3a), it was possible to verify a wrinkles sheet, which is characteristic of a nanomaterial with few layers [34]. In Figure 3b, for the magnetic graphene oxide, the GO surface was covered by magnetite. The high amount of FeCl2 employed in the magnetization process results in a magnetic nanocomposite with high magnetic nanoparticle content [21].

#### *3.4. Raman Spectroscopy*

The Raman spectroscopy results are shown in Figure 4. The three bands can be observed for GO andgraphene oxide-based magnetic nanoadsorbent (GO·Fe3O4). For both materials, it was possibile to observe the bands at 1366, 1580, and 2698 cm−<sup>1</sup> that represent the bands D (defects and disorder of graphite structure), G (sp<sup>2</sup> vibration by carbon atoms), and 2D (second order of the D band), which is characteristic of graphene materials [21]. Additionally, the ID/IG ratio increased after the magnetization procedure, showing values of 0.96 and 1.22 for GO and GO·Fe3O4. The increases are related to the involvement of carbon atoms in partial Fe3+ reduction [35].

**Figure 4.** Raman spectroscopy of GO and GO·Fe3O4.

Nonetheless, the co-precipitation method is widely used to synthesize magnetic nanoparticles, due to the low energy requirements, low temperature, and high-quality products. In this work, the magnetization process increased the ID/IG ratio; however, the low values showed that the MGO was not significantly affected by the procedure. Alongside this, using ammonium hydroxide as a precipitant agent, the graphene structure was preserved, resulting in excellent magnetic graphene oxide surface and low values for ID/IG ratio [21]. Zhang et al. [36] synthesized a magnetic nanocomposite, employing a microwave absorbing method with a mixture of Fe2+ and Fe3+ (1:2 molar ratio). The results of ID/IG ratio showed a high disorder in the nanocomposite structure. Nevertheless, Hatel et al. [37] developed GO/Fe3O4 nanorods using ultrasonic radiation and NaOH as the precipitating agent. Raman spectroscopy results demonstrated a nanocomposite with a low crystallinity degree when compared to the GO sample.

#### *3.5. Vibrating Sample Magnetometer (VSM)*

Figure 5 shows the magnetization profile of magnetic graphene oxide at room temperature. According to the graph, it was possible to verify that GO·Fe3O4 presented a ferromagnetic behavior; magnetization saturation (MS), remanent magnetization (MR), and MR/MS ratio values were 45 emu g−1, 3.44 emu g−1, and 0.076, respectively. Similarly, Ghosh et al. [38] developed a magnetic GO functionalized with nitrogen groups, and the materials showed a ferromagnetic behavior.

**Figure 5.** VSM of GO·Fe3O4.

#### *3.6. Captopril Adsorption*

3.6.1. Effect of Magnetite Incorporation onto Graphene Oxide Surface

The synthesis of nanocomposites is an important tool for the design of adsorbent materials with different characteristics and properties. In this study, the effect of the magnetite incorporation on the GO surface on the captopril adsorption behavior was evaluated (Figure 6).

According to Figure 6, it was possible to verify that both adsorbents exhibited excellent performance when removing the emerging pollutant. However, the graphene oxide-based magnetic adsorbent presented the highest adsorption capacity and removal percentage values compared to the pristine carbon nanomaterial. The GO and GO·Fe3O4 display removal values of 91.42 and 99.43% and adsorption capacities of 91.84 and 99.41 mg g−1, respectively. The development of composite materials not only improve the physicochemical stability, but also can enhance their efficiency as an adsorbent. Recently, diverse studies have reported the effects of magnetite on the GO surface in terms of the adsorption performance [21,39,40].

#### 3.6.2. Effect of Initial Concentration of CPT and Adsorbent Dosage

The initial adsorbate concentration and the adsorbent dosage play an important role in the adsorption efficiency, due to the higher ability to overcome the mass transfer resistance phenomenon and the more available surface area, respectively. Here, the effect of the initial concentration of CPT and GO·Fe3O4 was investigated in the range of 10–200 mg L−<sup>1</sup> and 0.125–1.0 g L<sup>−</sup>1, respectively.

The influence of CPT concentration is shown in Figure 7. Remarkably, the adsorption capacity values improved with the gradual increase in the initial concentration of CPT (e.g., the qe value raised from 14.68 to 390.31 mg g−<sup>1</sup> when the CPT concentration was changed from 10 to 200 mg L−1), thus implying that CPT adsorption is favored at higher concentrations. However, the removal percentage displayed a slight decrease with rising concentration. The removal value reduced by around 23% from the lowest to the highest concentration. The improvement in the adsorption performance can be attributed to the higher concentration gradient, impelling the motions of the adsorbate to the boundary layer, favoring the interaction between the liquid phase and the solid phase (surface of the adsorbent) and, therefore, the mass transfer phenomenon [41,42].

**Figure 7.** Effect of the initial concentration of CPT (GO·Fe3O4 dosage (0.5 g L−1), C0 = 10−200 mg L−1, pH 3.0, and 293.15 K).

Regarding the effect of adsorbent dosage (Figure 8), it was observed that an initial increase in the GO·Fe3O4 mass lead to an improvement in the removal percentage values. However, at the highest dosages (0.75 and 1.0 g L−1), a slight reduction was verified (the removal value decreased from 99.43 to 88.71% when the adsorbent dosage changed from 0.5 to 1.0 g L−1). On the other hand, the adsorption capacity decreased inversely with the gradual rise in the adsorbent quantity. This is related to the imbalance between available adsorption sites and absorbate molecules, which is caused bt higher adsorbent dosages [43,44].

**Figure 8.** Effect of the adsorbent dosage on CPT adsorption (GO·Fe3O4 dosage (0.125–1.0 g L−1), initial concentration of CPT (50 mg L<sup>−</sup>1), pH 3.0, and 293.15 K).

#### 3.6.3. Effect of pH and Adsorption Mechanisms

The pH of the solution is an important experimental parameter, due to its influence on chemical speciation and adsorbent surface charge. The effect of pH on CPT adsorption was evaluated in the range of 2–9, keeping other parameters constant. At the same time, the zero potential of charge (pHZPC) was estimated by the typical 11-point experiment.

From the results presented in Figure 9, it was possible to observe that the pH had a significant effect on the adsorption capacity and removal percentage values. The adsorption was reached at pH 3.0 (qe = 99.41 mg g−<sup>1</sup> and R% = 99.43). Captopril is a drug used for arterial hypertension that contains an ionizable group (-COOH, pKa = 3.7), i.e., it has an anionic character in this pH condition [45]. Under acidic conditions, the GO·Fe3O4 presents a positive surface charge (pHZPC = 7.41), which allows the occurrence of electrostatic interactions between the adsorbent and adsorbate (pH 3.0). In contrast, in the range between 4–7, the adsorbent and captopril have a cationic characteristic, decreasing the affinity of adsorbate by binding sites. At the same time, in acidic media, the H<sup>+</sup> ions can compete for the available active sites, justifying the considerable reduction in the adsorption performance. In addition, under alkaline conditions, the adsorbent material exhibits an anionic surface, also affecting the interaction between the systems [21].

**Figure 9.** (**a**) Effect of pH on CPT adsorption (C0 = 50 mg L−1, pH = 2–9, adsorbent dosage = 0.5 g L−1, V = 100 mL, and 293.15 K) and (**b**) Zero point of charge of GO·Fe3O4.

Adsorption mechanisms are closely related to the chemical structure of the adsorbate, such as functional groups, aromaticity, hydrophilicity, and pH sensitivity (ionizable groups). Additionally, the interaction depends on the charge and specific surface area of the adsorbent material. Thus, some hypotheses regarding the CPT adsorption onto GO·Fe3O4 were proposed (Figure 10). Considering the influence of pH solution on the chemical speciation of adsorbate and the adsorbent surface charge, this suggests that the captopril adsorption phenomenon is mainly governed by electrostatic interactions (optimal condition was reached at pH 3.0) [45,46]. Although attraction forces can act on the CPT removal, the abundance of oxygenated functional groups in nanoadsorbent structure and carboxylic acids on the captopril molecule also allows the occurrence of other interactions, such as hydrogen bonds (Yoshida H-bonding and dipole-dipole hydrogen bonding) [44,47].

**Figure 10.** Hypotheses of the captopril adsorption mechanism.

#### 3.6.4. Effect of Ionic Strength

The evaluation of the effect from diverse experimental conditions is an important requirement considering the potential application in wastewater treatment systems. In this study, the influence of ionic strength on CPT adsorption was performed using different concentrations of sodium chloride (0.01–1.0 mol L−1), keeping other parameters constant. As shown in Figure 11, it was possible to observe that an increase in NaCl molarity caused a considerable reduction in the adsorption capacity value. For instance, the system without NaCl presents a qe of 99.41 mg g<sup>−</sup>1, while, at a concentration of 1.0 mol L−1, the adsorption capacity was 32.38 mg g−1. The Na+ and Cl¯ ions can hamper the binding between the adsorbate and adsorbent due to the competition by adsorption sites. Additionally, the presence of inorganic salts may decrease the specific surface area, as well as reduce the solubility of the adsorbate [44,48,49].

**Figure 11.** Effect of ionic strenght on CPT adsorption (C0 = 50 mg L<sup>−</sup>1, pH = 3.0, adsorbent dosage = 0.5 g L−1, NaCl concentration (0.01–1.0 mol L<sup>−</sup>1), V = 100 mL, and 293.15 K).

#### 3.6.5. Kinetic Modeling

Kinetic models are valuable mathematical expressions, widely used to understand the adsorption behavior in relation to the experimental time. In this work, three models were employed: pseudo-first-model, pseudo-second-order, and Elovich to construct the kinetic profile and calculate the adjustment parameters for captopril adsorption onto GO·Fe3O4. The kinetic study was conducted at room temperature using different concentrations of adsorbate. Table 2 shows the adjust of experimental data to nonlinear kinetic models.


**Table 2.** Kinetic parameters for captopril adsorption onto GO·Fe3O4.

<sup>a</sup> (Mean ± standard deviation).

According to the results presented in Table 2, it was possible to assume that the captopril adsorption onto graphene oxide-based magnetic nanoadsorbent was well-described by the Elovich model in all concentrations tested. This because the model exhibited high values for coefficient of determination (R2 ≥ 0.994) and low values for error functions (ARE, MSE and SSE). Additionally, the Elovich parameters related to sorption rate (α) and surface coverage (β), respectively, indicated that the adsorption occurs in the heterogeneous surface, i.e., the adsorbent exhibits adsorption sites with different energy levels [24,50].

The kinetic profile of captopril adsorption is shown in Figure 12. As demonstrated by kinetic curves, it was possible to verify that the adsorption presents a characteristic behavior of fast kinetic, with three different stages. In the first minutes, a high amount of adsorbate is removed, followed by a stationary phase (plateau) and, finally, equilibrium is achieved. In addition, the adsorption capacity at equilibrium is proportional to the initial concentration of the contaminant. The increase in adsorption efficiency can be attributed to greater ease when overcoming of the mass transfer phenomenon and effective occupation of binding sites [51].

**Figure 12.** Kinetic curves for CPT adsorption onto GO·Fe3O4.

3.6.6. Adsorption Equilibrium Isotherms and Thermodynamic Study

The isotherm models are useful mathematical expressions that help us to understand the interaction between adsorbate and adsorbent; they are also used to verify the influence of temperature in the adsorption equilibrium. In this present work, Langmuir, Freundlich, and Sips isotherm models were used to predict the best adjustment of equilibrium data at different temperatures. As shown in Table 3, it was possible to determine that the Sips isotherm was the best model to describe the CPT adsorption onto GO·Fe3O4. Concerning values for coefficients of determination (R2) and error functions (ARE and SSE), the descending order of fit of the models is as follows:

#### Sips < Freundlich < Langmuir

The Sips model is a combined equation of the Langmuir and Freundlich isotherms, used to predict the adsorption on the heterogenous surface [52]. Regarding the Sips constant associated to the heterogeneity (ns), it is remarkable that the heterogeneity of the system increased with the temperature (ns = 2.26 at 20 ◦C and ns = 7.88 at 40 ◦C). Additionally, the maximum adsorption capacity estimated by this model decreased with the rise in the temperature, indicating that adsorption is an exothermic process (in agreement with the thermodynamic study) [45]. On the other hand, in the Sips constant related to equilibrium (Ks), a considerable decrease was observed with the gradual increase in temperature, i.e., suggesting that the process is disadvantaged at high temperatures [53].

Figure 13 shows the Sips isotherms curves. According to the results, it is possible to observe a characteristic "L-shape" type for 20 and 30 ◦C, indicating an adsorbent surface with a high presence of binding sites [54]. However, for 40 ◦C, the isotherm showed an "S-shape" suggesting a solute–solute attraction and/or an adsorption site competition [55].


**Table 3.** Equilibrium parameters for CPT adsorption onto GO·Fe3O4.

<sup>a</sup> (Mean ± standard deviation).

**Figure 13.** Sips model for CPT adsorption onto GO·Fe3O4.

To understand adsorption behavior under the influence of temperature, this study was performed at different conditions (293.15, 313.15, and 333.15 K). The thermodynamic equilibrium constant (Ke) was calculated using the KSips (best adjustment for isotherm model), and the captopril molecular weight (217.29 g mol−1) [16,56–59] Thermodynamic parameters for CPT adsorption onto GO·Fe3O4 were calculated by the Van 't Hoff equation and are shown in Table 4. From the values for Gibbs free energy variation (ΔG0), it was possible to assume that the captopril adsorption was a thermodynamically favorable process [56]. Furthermore, the negative value for enthalpy change (ΔH0) indicated exothermic adsorption [47]. Additionally, the magnitude (−64.19 kJ mol−1) suggests that the adsorption phenomenon is mainly governed by chemical interactions [60]. Regarding the entropy change (ΔS0), the negative value is related to a decrease in the randomness of the system.

**Table 4.** Thermodynamic parameters for CPT adsorption onto GO·Fe3O4.


The adsorption performance was compared to previous studies employing different adsorbents and experimental conditions. It is possible to observe, in Table 5, that all the materials employed in CPT removal showed a removal percentage. Karperiski et al. [16] synthesized an activated carbon with ZnCl2 from Caesalpina ferrea (CFAC) and obtained a qmax of 535.5 mg g−1. The results showed that the best adsorbent performance was reached by CFCAC.1.5 and the process was spontaneous and favorable for removal of 97.67% of CPT from the aqueous solution. Cunha et al. [9] developed an activated carbon derivate from Butia catarinensis (ABc) with a high surface area (1267 m2 g−1) and with high efficiency removal (up to 99%). The maximum adsorption capacity was dependent on the temperature, rising with the temperature. In this work, magnetic graphene oxide removed up to 99.0% of the drug, with a high adsorption capacity (100.41 mg g−1) at pH 3.0. Therefore, the magnetite functionalization improved the adsorption process, allowing easy adsorbent regeneration and reusability and avoiding centrifugation/filtration steps.


**Table 5.** Removal percentage of different adsorbents of CPT.

#### 3.6.7. Regeneration and Reuse

The study of the regeneration of adsorbents is a crucial step in the adsorption processes, aiming at the development and application of materials with high performance and cost-effectiveness. In this work, the regeneration of GO·Fe3O4 was performed using a solution of sodium hydroxide (0.25 mol L<sup>−</sup>1), stirring at room temperature for 60 min. Thus, five consecutive adsorption-desorption cycles were conducted to estimate the efficiency for several experiments. According to Figure 14, it was possible to observe that the nanoadsorbent exhibits high efficiency, even after diverse cycles of regeneration and reuse. The percentage removal value displayed a slight decrease over time, which can be attributed to incomplete desorption and mass loss during the adsorbent recovery step [26,47].

**Figure 14.** GO·Fe3O4 performance after several adsorption cycles.

#### **4. Conclusions**

The magnetic graphene oxide was synthesized employing a straightforward method with low energy requirements and easy operation. The FTIR, XRD, Raman, SEM, and VSM techniques demonstrated that the nanocomposite was successfully obtained, exhibiting excellent properties (crystalline structure with low defects) and high value for saturation magnetization (Ms = 45 emu g−1). The graphene oxide-based magnetic nanoadsorbent exhibits higher performance when compared with the pristine carbon nanomaterial. The maximum adsorption capacity and removal percentage (99.43% and 100.41 mg g−1) was reached at pH 3.0, using 50 mg of adsorbent, 50 mg L−<sup>1</sup> of CPT, and 293.15 K. These results can be attributed to the synergic effect between graphene oxide and iron oxide nanoparticles. The pH dependence and influence of the adsorbent type suggested that the CPT adsorption onto GO·Fe3O4 is mainly driven by electrostatic interactions and hydrogen bonds. Sips and Elovich models were well-suited to describe the equilibrium and adsorption kinetics data, assuming a process on a heterogeneous surface. The thermodynamic study inferred that the adsorption was spontaneous, exothermic, and occurred predominantly by chemical mechanisms. Additionally, the GO·Fe3O4 demonstrated high efficiency after five adsorption/desorption cycles. From the experimental results, it can be concluded that the magnetic adsorbent is a promising material for wastewater management, especially regarding emerging pollutants like captopril.

**Author Contributions:** M.P.d.O., C.S.: Methodology, Investigation. T.d.R.S., F.d.S.B., L.B.: Investigation, Writing—Original Draft. E.I.M., W.J.d.S.G.: Visualization, Data Curation. A.H.d.O., L.F.O.S.: Visualization, Writing—Review & Editing. C.R.B.R.: Conceptualization, Writing—Review & Editing, Supervision (cristianorbr@gmail.com). All authors have read and agreed to the published version of the manuscript.

**Funding:** This research received no external funding.

**Data Availability Statement:** Not applicable.

**Acknowledgments:** The authors thank Laboratório de Materiais Magnéticos Nanoestruturados— LaMMaN, Laboratório de Magnetismo e Materiais Magnéticos—LMMM, UFSM, and CAPES, CNPq, FAPERGS (TO 22/2551-0000609-4) for their support.

**Conflicts of Interest:** The authors declare no conflict of interest.

#### **References**


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