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Abstract

In this study, dimercaptosuccinic acid-functionalized magnetic chitosan (Fe₃O₄@CS@DMSA) was synthesized via in situ coprecipitation process and amidation reaction, aiming to eliminate cadmium (Cd(II)) ions from an aqueous environment. The structure, morphology, and particle size of the Fe₃O₄@CS@DMSA adsorbent were investigated using FTIR, TEM, EDX, TGA, zeta potential, and XRD techniques, and the obtained results approved the successful synthesis of the Fe₃O₄@CS@DMSA nanocomposite. The influence of external adsorption conditions such as pH solution, adsorbent mass, initial Cd(II) concentration, temperature, and contact time on the adsorption process was successfully achieved. Accordingly, pH: 7.6, contact time: 210 min, and adsorbent mass:10 mg were found to be the optimal conditions for best removal. The adsorption was analyzed using nonlinear isotherm and kinetic models. The outcomes revealed that the adsorption process obeyed the Langmuir and the pseudo-first-order models. The maximum adsorption capacity of Fe₃O₄@CS@DMSA toward Cd(II) ion was 314.12 mg/g. The adsorption mechanism of Cd(II) on Fe₃O₄@CS@DMSA nanocomposite is the electrostatic interaction. The reusability test of Fe₃O₄@CS@DMSA nanocomposite exhibited that the adsorption efficiency was 72% after the 5^th cycle. Finally, this research indicates that the Fe₃O₄@CS@DMSA exhibited excellent characteristics such as high adsorption capacity, effective adsorption-desorption results, and easy magnetic separation and thus could be an effective adsorbent for removing Cd(II) ions from aqueous solutions.

1. Introduction

Water contamination by a toxic cadmium (Cd(II)) metal is a widespread environmental issue owing to its long-term adverse effects on humans and ecosystems. Cd(II) is one of the most dangerous metal ion due to its nondegradable, strong bioaccumulate, and highly toxic even at low concentrations, which leads to a serious threat to human health [1, 2]. Cd(II) pollution can cause kidney, liver, and bone damage to humans with a long time exposure. Cd(II) has excellent solubility which can be easily released into the aqueous systems through industrial production processes such as alkaline batteries, electroplating, textile printing industries, and pigment [3]. Cd(II) is classified as a category one carcinogen by U.S. EPA, and the maximum concentration of Cd(II) in drinking water is 5 μg/L [4, 5]. Thus, the removal of extremely toxic cadmium from an aqueous environment is essential to avoid pollution to the environmental systems. Several techniques, namely, chemical precipitation [6], adsorption [7], membrane separation [8], ion exchange [9], and electrodeposition [10, 11], have been applied to treat the toxic metals from wastewater. Among them, the adsorption technique has been proven economical, simple, easy operation and ecofriendly, cost-effective, versatile in nature, and highly efficient for metal removal [12].

Many adsorbents have been applied to adsorption of Cd(II) from aqueous medium like sulfonated biochar [13], functionalized cellulose derived [14], amino-functionalized lignin [15], metal-organic framework (MOF) ZIF-8 [16], EDTA/mGO [17], and para-aminobenzoic acid-functionalized activated [18]. These adsorbents suffer from the difficulty of recovering metal after adsorption using traditional methods such as centrifugation and filtration, which may result in secondary pollution and loss of the amount of adsorbents [19]. Magnetic nanocomposite has received great attention as an efficient adsorbent owing to its many advantages such as easy magnetic separation, high surface area, low toxicity, biocompatibility, and the existence of a large number of surface hydroxyl groups that use them in surface modification. To improve the stability of Fe₃O₄ nanoparticles under acidic conditions and reduce the agglomeration of the nanoparticles, the surface of Fe₃O₄ nanoparticles can be modified with some materials like activated carbon [20], graphite oxide [21], and carboxylated MNP nanoparticles [22].

Naturally abundant polysaccharides such as chitosan are considered as one of the most promising surface stabilizing materials for magnetite nanoparticles due to their multifunctionality, nontoxicity, biocompatibility, and renewability [23]. Chitosan has a strong affinity with metal ions because of the existence of NH₂ and OH groups which can serve as the active adsorption sites for the removal of metals [24–26]. To improve the number of active adsorption sites for adsorption on magnetic chitosan, it needs to be surface modified to provide specific functional groups. Meso-2,3-dimercaptosuccinic acid (DMSA) is a suitable candidate for enhancing the adsorption process owing to DMSA having carboxyl and thiol groups, which can be used for the capture of heavy metals [27, 28]. In addition, DMSA acid is a nontoxic chelating agent and FDA approved drug which has been used to treat heavy metal poisoning in the human body [29–31]. To the best of our knowledge, the Fe₃O₄@CS@DMSA nanocomposite has not been used for the elimination of pollutants.

In this study, Fe₃O₄@CS@DMSA nanocomposite was synthesized by an in situ coprecipitation method followed by a covalent functionalization of Fe₃O₄@CS with DMSA acid via amidation reaction. The synthesized Fe₃O₄@CS@DMSA adsorbent was applied to eliminate Cd(II) ions from water. The synthesized Fe₃O₄@CS@DMSA was characterized using zeta potential, FTIR, XRD, TGA, TEM, and EDX techniques. The impact of external adsorption conditions such as pH solution, adsorbent mass, initial Cd(II) concentration, temperature, and contact time on the adsorption process was successfully achieved. To achieve the adsorption capacity and mechanisms of Cd(II) adsorption onto Fe₃O₄@CS@DMSA nanocomposite, the equilibrium kinetic and isotherm were studied. Thermodynamic parameters were also studied. The reusability test of Fe₃O₄@CS@DMSA nanocomposite was performed by carrying out five cycles of adsorption-desorption studies.

2. Experimental

2.1. Chemicals and Reagents

FeCl₂·4H₂O, ≥99%, FeCl₃·6H₂O, ≥99%, chitosan, medium MW, meso-2,3-dimercaptosuccinic acid, ~98%, and 1-ethyl-3-(3-dimethylaminopropyl)carbodiimide (EDC) were purchased from Sigma-Aldrich, USA. Dimethyl sulfoxide (DMSO) was obtained from Panreac, UK. HNO₃, 69%, NaOH, ≥98%, and Cd(NO₃)₂ were obtained from BDH Chemicals Ltd. (Poole, England). NH₄OH, 25%, and HCl, 36.5-38%, were procured from Merck, Germany. All chemicals and reagents utilized in these experiments were of AR grade.

2.2. Synthesis of Fe₃O₄@CS

Magnetic chitosan (Fe₃O₄@CS) was synthesized by an in situ coprecipitation method according to the literature [32] with some modifications. Typically, 1.8 g of chitosan (CS) was dissolved in acetic acid solution (100 mL, 3% $w / w$ ) under magnetic stirring and N₂ gas. Then, the chitosan solution was transferred into a three-necked-round bottom flask containing FeCl₂·4H₂O (2.98 g) and FeCl₃·6H₂O (8.11 g) with a molar ratio (1 : 2). The mixture was stirred mechanically at 75°C for 15 min, and then, 35 mL of ammonia (25 wt%) was added to the mixture dropwise until pH reaches 10. After mechanical stirring for another 120 min, the precipitate was cooled at room temperature and then was collected by a magnet. Finally, the Fe₃O₄@CS was washed with deionized water and then dried at 50°C for 24 h. A brief preparation procedure of Fe₃O₄@CS is demonstrated in Scheme 1.

Scheme 1

Schematic for preparation of Fe₃O₄@CS@DMSA nanocomposite.

2.3. Synthesis of Fe₃O₄@CS@DMSA

Fe₃O₄@CS@DMSA nanocomposite was synthesized by amidation reaction between the carboxylic group of DMSA and the amino group of magnetic chitosan (Fe₃O₄@CS). Typically, DMSA (0.91 g) was added into 50 mL of DMSO to be completely dissolved by ultrasonication; then, 0.776 g of EDC was added into the DMSA solution under ultrasonication and N₂ gas for 1.0 h. After that, 1.00 g of Fe₃O₄@CS was dispersed into the above solution by mechanical stirring under N₂ gas for 24 h. Then, the black product was isolated by a magnet and then washed with D.I. water. Finally, the Fe₃O₄@CS@DMSA was dried at 50°C for 24 h. A brief preparation procedure of Fe₃O₄@CS@DMSA is demonstrated in Scheme 1.

2.4. Characterization of Fe₃O₄@CS@DMSA Nanocomposite

The functional groups of Fe₃O₄ nanoparticles, DMSA, Fe₃O₄@CS@DMSA, and Cd(II)-loaded Fe₃O₄@CS@DMSA were recorded by Fourier transform infrared spectroscopy (FTIR) (Nicolet 6700, Thermo Scientific, USA). The morphology and particle size of the Fe₃O₄@CS@DMSA were performed using TEM (JEOL 2100, Japan). The surface composition of Fe₃O₄@CS@DMSA and Cd(II)-loaded Fe₃O₄@CS@DMSA was analyzed by energy-dispersive X-ray (EDX) using EDXS: AMETEK Nova 200. The crystallinity nature of Fe₃O₄ nanoparticles and Fe₃O₄@CS@DMSA nanocomposite was performed by XRD analysis using a Shimadzu model 6000. The zeta potential of Fe₃O₄@CS@DMSA nanocomposite was determined using a Nano Plus Series, USA. TGA curve was recorded Fe₃O₄@CS@DMSA nanocomposite using a thermogravimetric analyzer (Mettler Toledo GA/SDTA851) under an N₂ atmosphere.

2.5. Batch Adsorption Experiments

The removal efficiency of Cd(II) ions by Fe₃O₄@CS@DMSA from water was studied by batch method to achieve the impact of various process factors such as adsorbent mass, contact time, pH solution, temperature, and initial Cd(II) concentration on adsorption process. In this work, contact time, solution pH, and adsorbent mass were achieved in the range of 5-350 min, 1.8-9.1, and 5-30 mg whereas temperature and initial Cd(II) concentration were varied from 25 to 45°C and 25 to 300 mg/L. A known amount of Fe₃O₄@CS@DMSA was put into an Erlenmeyer containing 25 mL of known Cd(II) concentration, and the sample was then adjusted to the desired pH at 25°C. After that, the sample solution was shaken for 24 h. Then, the sample was isolated by a magnet, and the residual concentration of Cd(II) ions has been determined using AAS. The adsorbed amount ( $q_{e}$ (mg/g)) and percentage adsorption of Cd(II) were calculated using Equations (1) and (2), respectively. $\begin{matrix} (1) & q_{e} = (C_{o} - C_{e}) \frac{V}{m}, \\ (2) & % adsorption = \frac{C_{o} - C_{e}}{C_{o}} \times 100, \end{matrix}$ where $C_{o}$ and $C_{e}$ refer to initial and equilibrium Cd(II) concentration in the solution (mg/L), respectively; $V$ (L) refers to the volume of the Cd(II) solution; $m$ (g) is the weight of Fe₃O₄@CS@DMSA nanocomposite. The adsorption capacities for Fe₃O₄@CS and Fe₃O₄@CS@DMSA adsorbents toward Cd(II) ions were 52.5 mg/g and 58.8 mg/g, respectively, at condition parameters at constant adsorbent mass (0.01 g), initial Cd(II) concentration (25 mg/L), temperature (25°C), pH (7.6), stirring rate (100 rpm), and contact time (1440 min).

3. Results and Discussion

3.1. Characterization of Fe₃O₄@CS@DMSA Nanocomposite

Figure 1(a) shows the FTIR spectra of Fe₃O₄ nanoparticles, DMSA, Fe₃O₄@CS@DMSA, and Cd(II)-loaded Fe₃O₄@CS@DMSA. For Fe₃O₄ nanoparticles, the characteristic bands at 572 and 1612 cm⁻¹ are due to the Fe –O and –OH bonds, respectively [33]. In the spectrum of DMSA, the characteristic bands are shown at around 2551 and 1693 cm^-1 attributed to $ν$ (-SH) and $ν$ (COOH) groups, respectively [34, 35]. The bands 3520, 2983, 1413, 1296, and 1175 cm⁻¹ are due to the stretching vibration of $ν$ (-OH), $ν$ (-CH), $ν$ (-COO-), $ν$ (C-O), and $ν$ (O-C-O) bonds, respectively [34]. For Fe₃O₄@CS@DMSA, the bands at 3393 cm⁻¹ are attributed to the presence of the $ν$ (-OH) group overlapping with the -NH group. The characteristic band for (–SH) appeared at 2550 cm^-1 [34]. Besides, the bands at 1670 and 1554 cm^-1 are due to amide I and amide II or $δ$ (N–H) groups, respectively [36], which indicates the formation of amide bonds between Fe₃O₄@CS and DMSA [37]. The Fe-O band was decreased to 552 cm⁻¹, confirming the presence of magnetic nanoparticles. The bands at 1481, 1355, 1274, and 1044 cm^-1 are due to $ν$ (-COO-), $ν$ (-C-N), $ν$ (C-O), and $ν$ (O-C-O) stretching vibration, respectively [38, 39]. After adsorption of Cd(II), the spectra showed the bands were slightly shifted and decreased in intensity due to the binding of COOH, SH, and OH onto Fe₃O₄@CS@DMSA surface with Cd(II) ions through electrostatic attractions. In detail, the bands at 3393 and 2550 cm^-1 decrease in intensity owing to the interaction between Cd(II) and carboxyl (COOH), hydroxyl (OH), and SH groups, respectively, on the Fe₃O₄@CS@DMSA surface by electrostatic interaction. In addition, the band at 1274 cm^-1 for $ν$ (C-O) disappeared after Cd(II) adsorption onto the Fe₃O₄@CS@DMSA surface. The decreased intensity of the band at 1481 cm^-1 for $ν$ (COO^-) indicates the adsorption of Cd(II) onto the Fe₃O₄@CS@DMSA surface.

Figure 1

FTIR spectra of (a) Fe₃O₄, DMSA, Fe₃O₄@CS@DMSA, and Cd(II)-loaded Fe₃O₄@CS@DMSA and (b) XRD pattern of Fe₃O₄, and Fe₃O₄@CS@DMSA.

(b)

The XRD patterns of magnetite nanoparticles and Fe₃O₄@CS@DMSA are indicated in Figure 1(b). The major peaks at $2 θ = {18.3}^{°}$ , 30.2°, 35.7°, 43.13°, 54.4°, 57.6°, and 63.3° are associated with the (111), (220), (311), (400), (422), (511), and (400) crystal planes of cubic phase magnetite, which was consistent with a previous report [40]. Compared with magnetite, the XRD pattern of Fe₃O₄@CS@DMSA appeared a new broad reflection at a 2θ value of 22.5°, confirming the Fe₃O₄ nanoparticles covered by DMSA and chitosan [41, 42]. Using the Scherer equation (3), the average crystal size ( $D$ ) of Fe₃O₄@CS@DMSA nanocomposite was calculated: $\begin{matrix} (3) & D = \frac{k λ}{β \cos θ}, \end{matrix}$ where $λ = 0.154$ nm is the X-ray wavelength; $θ$ is Bragg’s peak; $β$ is the half-width of diffraction peak; $K$ is the Scherer constant. The mean crystal size of Fe₃O₄@CS@DMSA nanocomposite was 11.5 nm.

Figure 2(a) displays the TGA curves for Fe₃O₄ nanoparticles and Fe₃O₄@CS@DMSA nanocomposite. The total weight loss was ~9% and 17% observed for Fe₃O₄ nanoparticles and Fe₃O₄@CS@DMSA nanocomposite, respectively. The weight loss of mass for magnetite nanoparticles in a range of temperatures (30-700°C) due to evaporation of adsorbed H₂O and decomposition of oxygen-containing functional groups from the Fe₃O₄ nanoparticles [43, 44]. The thermal stability of Fe₃O₄@CS@DMSA exhibited a high loss in mass of approximately 17% with two stages. In the first one, the weight loss was ~3% in low temperature up to 200°C owing to elimination of adsorbed water and solvent absorbed onto the surface Fe₃O₄@CS@DMSA nanocomposite. In the second one, ~15% weight loss at around 200–700°C ascribes to the thermal decomposition of an organic part of CS and DMSA [45], confirming the successful synthesis of Fe₃O₄@CS@DMSA nanocomposites.

Figure 2

TGA analysis of (a) Fe₃O₄ and Fe₃O₄@CS@DMSA and (b) zeta potential plots as a function of Fe₃O₄@CS@DMSA nanocomposite.

(b)

To determine the point of zero charge (PZC) of Fe₃O₄@CS@DMSA, the surface charge of Fe₃O₄@CS@DMSA was measured under different pH values. The outcomes are displayed in Figure 2(b). It was seen that the zero of point charge value (pH_pzc) of Fe₃O₄@CS@DMSA nanocomposite was ~5.2. This value is lower than ~7.1 for Fe₃O₄ nanoparticles [46]. This behavior of the Fe₃O₄@CS@DMSA nanocomposite is mainly assigned to the existence of –OH, COOH, and –SH groups, which are being protonated at lower than ~5.2.

The size and morphology of the Fe₃O₄@CS@DMSA were studied by TEM, and the outcomes are displayed in Figure 3(a). It is clear that the nanoparticles were uniform spherical morphology with a bright of amorphous CS and DMSA over the dark spot crystalline core of magnetite nanoparticles [47]. The value particle size of Fe₃O₄@CS@DMSA was ~11.5 nm confirming the surface modification of magnetite nanoparticles with CS and DMSA (Figure 3(b)). The elemental surface of Fe₃O₄@CS@DMSA and Cd(II)-saturated Fe₃O₄@CS@DMSA nanocomposite is shown in Table 1. The EDX analysis of Fe₃O₄@CS@DMSA showed the presence of Fe, O, C, N, and S on the Fe₃O₄@CS@DMSA surface, confirming the successfully prepared Fe₃O₄@CS@DMSA nanocomposite. The Cd(II) ions were observed on the Fe₃O₄@CS@DMSA surface after adsorption by EDX analysis (Table 1), which indicated the good adsorption capability of the Fe₃O₄@CS@DMSA nanocomposite when applied to Cd(II) adsorption.

Figure 3

(a) TEM image and (b) particle size distribution plots of Fe₃O₄@CS@DMSA nanocomposite.

(b)

Table 1

EDX analysis.

Sample	Elemental content (weight %)
Sample	Fe	O	N	C	S	Cd
Fe₃O₄@CS@DMSA	59.82	21.38	1.17	16.61	1.02	0
Cd(II)-loaded Fe₃O₄@CS@DMSA	56.99	19.61	0.91	20.92	0.75	0.82

3.2. Adsorption Study

3.2.1. Effect of pH Solution

The functional groups on the surface of Fe₃O₄@CS@DMSA nanocomposite and the state of Cd(II) metal ions are greatly influenced by H⁺ concentration [48]. The effect of different initial pH values (1.8-9.1) on Cd(II) adsorption by Fe₃O₄@CS@DMSA nanocomposite was studied as shown in Figure 4(a). The other parameters were kept constant as initial Cd(II) concentration (25 mg/L), temperature (25°C), contact time (210 min), adsorbent mass (10 mg), and agitation speed (100 rpm). As implied in Figure 4(a), the adsorption capacity of Fe₃O₄@CS@DMSA toward Cd(II) was increased from 0.75 to 58.75 mg/g as the pH increased from 1.8 to 7.6, respectively. After that, it is slightly reduced and may be owing to the formation of Cd(II) hydroxide precipitate such as Cd(OH)⁺ and Cd(OH)₂, inhibiting the Cd(II) ion adsorption on Fe₃O₄@CS@DMSA nanocomposite [49–51]. The maximum value of Cd(II) elimination occurred at near pH (7.6). The zero-point charge of Fe₃O₄@CS@DMSA nanocomposite was determined as 5.2 as shown in Figure 2(b). Therefore, when the value of pH solution was lower than the pH_zpc (5.2), the surface of Fe₃O₄@CS@DMSA becomes positively charged and would form electrostatic repulsion between Fe₃O₄@CS@DMSA adsorbent and Cd(II) ions, which lead to reduced adsorption capacity. As the value of pH solution is greater than pH_pzc (5.2), the surface charge of Fe₃O₄@CS@DMSA nanocomposite becomes negatively charged, indicating the presence of electrostatic attraction between Cd(II) ions and Fe₃O₄@CS@DMSA nanocomposite which results in increased adsorption capacity. The $pH = 7.6$ was chosen as the optimum value for Cd(II) adsorption onto Fe₃O₄@CS@DMSA nanocomposite in this work.

Figure 4

Effect of (a) pH (adsorbent dose: 0.01 g; $C_{o}$ : 25 mg/L; agitation speed: 100 rpm; time: 1440 min), (b) adsorbent dose ( $T$ : 298 K; $C_{o}$ : 25 mg/L; $T$ : 298 K; time: 1440 min; agitation speed: 100 rpm), (c) contact time (adsorbent dose: 0.01 g; $C_{o}$ : 25 mg/L; $T$ : 298 K; agitation speed: 100 rpm), and (d) initial Cd(II) concentration on adsorption of Cd(II) by Fe₃O₄@CS@DMSA ( $T$ : 298-318 K; adsorbent dose: 0.01 g; agitation speed: 100 rpm).

(b)

(c)

(d)

3.2.2. Effect of Adsorbent Mass

Figure 4(b) demonstrates the Cd(II) adsorption onto Fe₃O₄@CS@DMSA nanocomposite under various adsorbent mass in the range between 5 mg and 30 mg at constant temperature (25°C), initial Cd(II) concentration (25 mg/L), speed agitation 100 rpm, and pH (7.6). It was observed that the percentage removal of Cd(II) ions was improved sharply from 52% to 93.6% with an increasing amount of adsorbent from 5 mg to 10 mg, respectively, due to increasing the availability of active adsorption sites on the Fe₃O₄@CS@DMSA surface. Conversely, the adsorption capacity of Fe₃O₄@CS@DMSA nanocomposite toward Cd(II) reduced from 65.0 to 58.5 mg/g with rising the amount of Fe₃O₄@CS@DMSA adsorbent to 10 mg. This is due to the adsorption capacity being inversely proportional to the adsorbent mass as per equation (Equation (1)) [38, 52]. After the adsorbent mass of 10 mg, no significant change in adsorption capacity was observed.

3.2.3. Effect of Contact Time

To find out the optimum contact time, experiments were conducted at various time intervals between 5 and 350 min at constant adsorbent mass (10 mg), initial Cd(II) concentration (25 mg/L), pH (7.6), stirring rate (100 rpm), and temperature (25°C) as presented in Figure 4(c). It was noticed that the amount of Cd(II) adsorbed onto the Fe₃O₄@CS@DMSA increased rapidly with increasing equilibrium time and the maximum adsorption capacity and removal efficiency reached up to 58.0 mg/g and 92.8%, respectively, at 210 min. In the initial stage of the adsorption process, the Cd(II) ions easily interacted with active sites of Fe₃O₄@CS@DMSA nanocomposite owing to the abundance of the active adsorption sites on the Fe₃O₄@CS@DMSA surface. After 210 min, no significant change in the adsorption capacity owing to the active sites of Fe₃O₄@CS@DMSA tended to saturate and could not easily adsorb the Cd(II) ions.

3.2.4. Effect of Initial Cd(II) Concentration and Temperatures

Figure 4(d) displays the influence of initial Cd(II) adsorption in the range of 25 to 300 mg/L with different temperatures (298-318 K) on the adsorption capacity of Fe₃O₄@CS@DMSA toward Cd(II) ions at constant of the other parameters ( $pH = 7.6$ , adsorbent $mass = 10$ mg, agitation $speed = 100$ rpm, and contact $time = 210$ min). The amount of Cd(II) adsorbed onto the Fe₃O₄@CS@DMSA increased from 57.5 to 295 mg/g with the rising initial Cd(II) ion concentration from 25 to 300 mg/L at 298 K. This phenomenon can be explained that a higher Cd(II) concentration rises the driving force and provides more collisions between Cd(II) ions and active sites of Fe₃O₄@CS@DMSA which could improve the adsorption rate. The influence of temperature on the adsorption process is presented in Figure 4(b). The adsorption capacity of Fe₃O₄@CS@DMSA toward Cd(II) was decreased from 295 to 195 mg/g when the temperature was improved from 298 K to 318 K at 300 mg/L, suggesting that the adsorption of Cd(II) on Fe₃O₄@CS@DMSA is exothermic. This could be explained by the weakening of the adsorptive forces between the active sites of Fe₃O₄@CS@DMSA nanocomposite and the Cd(II) ions [53], which is consistent with the previous report on Cd(II) adsorption by MGO-Trp [54] and CSAP [55].

3.3. Modeling

3.3.1. Isotherm Model

The adsorption of studied Cd(II) onto Fe₃O₄@CS@DMSA was studied at three various temperatures 298, 308, and 318 K by different nonlinear isotherm models, namely, Dubinin Radushkevich (Equations (4)–(6)) [56], Langmuir (Equation (7)) [57], Freundlich (Equation (8)) [58], and models. The fitting outcomes and parameters are indicated in Figure 5 and Table 2, respectively. $\begin{matrix} (4) & q_{e} = q_{s} e^{- K_{D ‐ R} ε^{2}}, \\ (5) & ε = RTln (1 + \frac{1}{C_{e}}), \\ (6) & E = \frac{1}{\sqrt{2 K_{D ‐ R}}}, \\ (7) & q_{e} = \frac{q_{m} K_{L} C_{e}}{1 + K_{L} C_{e}}, \\ (8) & q_{e} = K_{F} {C_{e}}^{1 / n}, \end{matrix}$

Figure 5

Adsorption isotherm fit for the Cd(II) adsorption onto Fe₃O₄@CS@DMSA at (a) 298 K, (b) 303 K, and (c) 318 (time: 210 min; $C_{o}$ : 25-300 mg/L; adsorbent dose: 0.01 g; agitation speed: 100 rpm); adsorption kinetic fit for the Cd(II) adsorption onto Fe₃O₄@CS@DMSA at (d) 298 K (T: 298 K; $C_{o}$ : 25 mg/L; adsorbent dose: 0.01 g; agitation speed: 100 rpm).

(b)

(c)

(d)

Table 2

Isotherm data for Cd(II) adsorption by Fe₃O₄@CS@DMSA nanocomposite (time: 210 min; $C_{o}$ : 25-300 mg/L; adsorbent dose: 0.01 g; agitation speed: 100 rpm).

Model	Cd(II)
Model	298 K	308 K	318 K
Langmuir
$q_{m}$ (mg/g)	314.12	302.73	258.73
$K_{L}$ (L/mg)	0.0698	0.0278	0.0151
$R_{L}$	0.3541	0.5897	0.7254
$R^{2}$	0.96915	0.97306	0.97069
Freundlich
$K_{f} (mg / g) {(L / mg)}^{1 / n}$	65.213	36.361	18.691
$n$	3.2730	2.6270	2.2381
$R^{2}$	0.96072	0.95405	0.96607
Dubinin-R
$q_{s}$ (mg/g)	261.86	231.17	182.33
$K_{D ‐ R}$ (mol² KJ^-2)	15.900	115.32	318.78
$E$ (kJ mol^-1)	0.1773	0.0658	0.0396
$R^{2}$	0.7983	0.82341	0.8159

where $C_{e}$ (mg/g) is the equilibrium aqueous-phase Cd(II) concentration. $q_{e}$ and $q_{m}$ refer to the equilibrium and maximum amount of Cd(II) adsorbed (mg/g), respectively; $K_{DR}$ (mol²/kJ²), $K_{F}$ , and $K_{L}$ are the constants of D-R, Freundlich, and Langmuir models, respectively; $E$ (kJ/mol) and $ɛ$ are the average free energy and the Polanyi potential, respectively; $n$ is the adsorption intensity.

Figure 5 displays the three nonlinear fitting parameter results of the adsorption isotherm models for the Cd(II) adsorption on Fe₃O₄@CS@DMSA nanocomposite. By comparison, it was observed that the $R^{2}$ values were 0.96215, 0.97072, and 0.7983 for Langmuir, Freundlich, and D-R isotherms, respectively. Thus, the experimental data was better described by the Freundlich ( $R^{2} = 0.97072$ ) model than those of the Langmuir and D-R models, which indicated the heterogeneous nature of Fe₃O₄@CS@DMSA and a contribution of electrostatic interaction to the Cd(II) adsorption on Fe₃O₄@CS@DMSA (physisorption nature) [59]. By applying the Freundlich equation, the values of $n$ (adsorption intensity) were in the range of 2.2381-3.2730, indicating that multilayer adsorption occurred onto the heterogeneous surface of Fe₃O₄@CS@DMSA. In addition, the Cd(II) adsorption is a favorable process. The higher value of $n = 3.2730$ at 298 K indicates that the Fe₃O₄@CS@DMSA nanocomposite has better adsorption performance [59, 60]. According to the Langmuir model, the maximum amount of Cd(II) adsorbed was 314.12 mg/g. This value is higher than other material adsorbents for adsorption of Cd(II) like Fe₃O₄/chitosan-glycine-PEGDE (171.06) [61], polyethyleneimine-modified magnetic porous cassava (143.6) [62], chitosan-modified kiwi branch biochar (126.58) [63], sulfhydryl-modified chitosan beads (183.1) [64], MNP-DMSA (25.44) [65], chitosan-pectin gel beads (177.6) [66], chitosan-iron oxide (CS-Fe₂O₃) (204.318) [67], and thiocarbohydrazide-chitosan gel (81.26) [68] (Table 3). According to the D-R isotherm, the values of means free energy ( $E$ ) were found to be in the range of 0.0396-0.1773 kJ/mol), which indicated that the Cd(II) adsorption onto Fe₃O₄@CS@DMSA nanocomposite classified as a physical adsorption process due to the $E$ value is less than 8 kJ/mol [69].

Table 3

Comparison of Cd(II) adsorption performance on Fe₃O₄@CS@DMSA nanocomposite with different adsorbents.

Adsorbent	Kinetic models	Isotherm models	q_m (mg/g)	Reference
Fe₃O₄/chitosan-glycine-PEGDE	Pseudo-second-order	Langmuir	171.06	[61]
Polyethyleneimine-modified magnetic porous cassava	Pseudo-second-order	Langmuir	143.6	[62]
Chitosan-modified kiwi branch biochar	Pseudo-second-order	Langmuir	126.58	[63]
Sulfhydryl-modified chitosan beads	Pseudo-second-order	Langmuir	183.1	[64]
MNP-DMSA	Pseudo-first-order	Langmuir	25.44	[65]
Chitosan-pectin gel beads	Pseudo-second-order	Langmuir	177.6	[66]
Chitosan-iron oxide (CS-Fe₂O₃)	Pseudo-second-order	Langmuir	204.318	[67]
Thiocarbohydrazide-chitosan gel	Pseudo-second-order	Langmuir	81.26	[68]
Fe₃O₄@CS@DMSA	Pseudo-first-order	Langmuir	314.12	This study

3.3.2. Kinetic Model

The kinetic studies were estimated at different contact times in the range of 5-350 min with fixed parameter conditions ( $pH = 7.6$ , initial Cd(II) concentration (25 mg/L), adsorbent $mass = 10$ mg, contact $time = 210$ min, and $temperature = 298$ K). Three nonlinear kinetic models, namely, Elovich (Equation (9)) [70], pseudo-first-order (PFO) (Equation (10)) [71], and pseudo-second-order (PSO) (Equation (11)) models, were applied to understand the mechanism of Cd(II) adsorption on Fe₃O₄@CS@DMSA nanocomposite. The fitting results and parameters are indicated in Figure 5(d) and Table 4, respectively. $\begin{matrix} (9) & q_{t} = \frac{1}{β} \ln (1 + α β t), \\ (10) & q_{q} = q_{e} (1 - e^{- k_{1} t}), \\ (11) & q_{t} = \frac{q_{e}^{2} k_{2} t}{1 + q_{e} k_{2} t}, \end{matrix}$

Table 4

Kinetic data for Cd(II) adsorption by Fe₃O₄@CS@DMSA nanocomposite ( $T$ : 298 K; $C_{o}$ : 25 mg/L; adsorbent dose: 0.01 g; agitation speed: 100 rpm).

Model	Parameters	Value
Model	$C_{o}$ : 25 mg/L, $q_{e, \exp .}$ : 58.75 mg/g	Value
Pseudo-first-order	$q_{e 1, cal .}$ (mg/g)	61.45
	$K_{1}$ (1/min)	0.0103
	$R^{2}$	0.97515
Pseudo-second-order	$q_{e 2, cal .}$ (mg/g)	79.82
	$K_{2}$ (g/mg-min)	0.00011
	$R^{2}$	0.9567
Elovich	$Α$ (mg/g min)	0.87664
	$Β$ (mg/g)	0.04278
	$R^{2}$	0.93427

where $q_{e}$ and $q_{t}$ (mg/g) refer to the amounts of Cd(II) adsorbed on Fe₃O₄@CS@DMSA at equilibrium and time $t$ , respectively; $k_{1}$ is the PFO rate constant; $k_{2}$ represents the PSO rate constant; $β$ (mg/g) is the Elovich kinetic parameter; $α$ refers to the desorption constant.

Based on $R^{2}$ values, PFO displays a better correlation coefficient ( $R^{2} = 0.97515$ ) than the PSO ( $R^{2} = 0.9567$ ) and Elovich ( $R^{2} = 0.93427$ ) models, suggesting the rate-limiting step for Cd(II) is physisorption involving electrostatic interaction between Cd(II) and Fe₃O₄@CS@DMSA nanocomposite. The value of $q_{e . cal}$ (61.45 mg/g) calculated is close to the experimental equilibrium adsorption capacities ( $q_{e, \exp} = 58.75$ mg/g). By applying the Elovich equation, the values of $α$ and $β$ were 0.877 mg/g min and 0.043 mg/g with $R^{2}$ (0.93427), respectively. The low value of $R^{2}$ indicates the absence of a chemisorption mechanism.

3.3.3. Adsorption Thermodynamics

The thermodynamic parameters, namely, enthalpy change ( $Δ H^{°}$ ) (Equation (12) and entropy change ( $Δ S^{°}$ ) (Equation (12)), can be obtained from the slope and intercept of the Van’t Hoff plot of $\ln K_{e}$ vs. $1 / T$ (Figure 6(a)) at different temperatures (298-318 K), and the free energy change ( $Δ G^{°}$ ) can be estimated from the equation (Equation (13)): $\begin{matrix} (12) & \ln K_{e}^{0} = - \frac{Δ H^{°}}{RT} + \frac{Δ S^{°}}{R}, \\ (13) & ∆ G^{°} = - RT \ln K_{e}^{0}, \\ (14) & K_{e}^{0} = \frac{(1000 \cdot K_{L} molecular weight of adsorbate) \cdot {[adsorbate]}^{°}}{γ}, \end{matrix}$

Figure 6

(a) Van’t Hoff plot for the adsorption of Cd(II), (b) comparison of eluents for desorption of Cd(II) from Fe₃O₄@CS@DMSA surface, and (c) reusability of Fe₃O₄@CS@DMSA.

(b)

(c)

where $K_{e}^{0}$ (Equation (14)) is the thermodynamic equilibrium constant at a certain temperature [59–61] and [adsorbate]°, $γ$ , and $K_{L}$ are the standard concentrations of the adsorbate (1.0 mol/L), activity coefficient, and Langmuir constant, respectively. The thermodynamic parameters for Cd(II) adsorption onto Fe₃O₄@CS@DMSA are summarized in Table 5. The negative values of free energy ( $Δ G^{°}$ ) was noticed, indicating that the Cd(II) adsorption onto Fe₃O₄@CS@DMSA is a spontaneous reaction and the values of $Δ G^{°}$ were increased from -22.36 to -19.66 kJ/mol with rising temperature from 298 to 318 K which demonstrates the favorability of the adsorption of Cd(II) at a lower temperature. Negative values of $Δ H^{°}$ and $Δ S^{°}$ indicated that the Cd(II) adsorption onto Fe₃O₄@CS@DMSA was exothermic and the decreased the reaction randomness.

Table 5

Thermodynamic data for the Cd(II) adsorption on Fe₃O₄@CS@DMSA.

Metal ions	$Δ H^{°}$ (kJ/mol)	$Δ S^{°}$ (J/mol·K)	$Δ G^{°}$ (kJ/mol)
Metal ions	$Δ H^{°}$ (kJ/mol)	$Δ S^{°}$ (J/mol·K)	298 K	308 K	318 K
Cd(II)	-60.344	-128.280	-22.220	-20.608	-19.668

3.4. Adsorption Mechanism

The proposed adsorption mechanism is shown in Figure 7. Based on the adsorption kinetic results, the adsorption process followed the pseudo-first-order model, suggesting a physical interaction through electrostatic attraction between the Cd(II) ions and the Fe₃O₄@CS@DMSA nanocomposite. According to the FTIR analysis (Figure 1(a)), the position peaks of functional groups declined in intensity and slightly shifted to a lower wavenumber. In detail, the bands at 3393 and 2550 cm^-1 decrease in intensity owing to the interaction between Cd(II) and carboxyl (COOH), hydroxyl (OH), and SH groups, respectively, on the Fe₃O₄@CS@DMSA surface by electrostatic interaction. In addition, the band at 1274 cm^-1 for $ν$ (C-O) disappeared after Cd(II) adsorption onto the Fe₃O₄@CS@DMSA surface. The decreased intensity of the band at 1481 cm^-1 for $ν$ (COO^-) indicates the adsorption of Cd(II) onto the Fe₃O₄@CS@DMSA surface. Besides, the Fe₃O₄@CS@DMSA exhibited a high adsorption capacity of Cd(II) owing to the presence of functional moieties (SH, COOH, OH, and NH₂) on the surface of Fe₃O₄@CS@DMSA, which was confirmed by the FTIR spectrum. The EDX analysis displays a new peak of Cd(II), which indicates Cd(II) ion adsorption onto the Fe₃O₄@CS@DMSA surface.

Figure 7

Mechanism adsorption of Cd(II) onto Fe₃O₄@CS@DMSA nanocomposite.

3.5. Reusability Study

To evaluate the reusability of the Fe₃O₄@CS@DMSA, adsorption and desorption tests were conducted using three eluents, namely, 0.01 M CH₃COOH, 0.01 M HNO₃, and 0.01 M HCL. The % desorption (Equation (15)) was estimated by the equation: $\begin{matrix} (15) & % desorption = \frac{C_{m}}{C_{e}} \times 100, \end{matrix}$ where $C_{m}$ and $C_{e}$ (mg/L) refer to the concentration of Cd(II) ion released in the solution and the initially adsorbed Cd(II) concentration, respectively. The results of the Cd(II) adsorption/desorption test on Fe₃O₄@CS@DMSA nanocomposite using eluents are indicated in Figure 6(b). It was observed that the percentage desorption was found to be CH₃COOH (47.48%), HNO₃ (87.26%), and HCl (91.3%), which indicate the best eluent for desorption of Cd(II) was 0.01 M HCl owing to the smaller ionic size of the Cl^- ion compared to CH₃COO^- and NO₃ ^-. For reusability of the Fe₃O₄@CS@DMSA study, the Cd(II)-loaded Fe₃O₄@CS@DMSA was isolated using a magnet and then the solid adsorbent was washed with deionized water, dried, and regenerated with 25 mL of 0.01 M HCl. After that, the sample was shaken at room temperature for 210 min. The solid/solution phase is separated via a magnet, and the supernatants are analyzed by the AAS method. After desorption, the Fe₃O₄@CS@DMSA nanocomposite was reused to Cd(II) adsorption and five adsorption/desorption cycles were applied. The results obtained are presented in Figure 6(c). It is seen that up to five cycles about 72% of Cd(II) were successfully removed. The reduction in removal efficiency of Cd(II) after five cycles may be owing to incomplete desorption of the Cd(II) ions on Fe₃O₄@CS@DMSA.

4. Conclusion

Fe₃O₄@CS@DMSA nanocomposite was synthesized via the in situ coprecipitation method followed by a covalent functionalization of Fe₃O₄@CS with DMSA acid by amidation reaction. The synthesized Fe₃O₄@CS@DMSA nanocomposite was characterized using zeta potential, FTIR, XRD, TEM, EDX, and TGA techniques. These techniques confirmed the formation of adsorbent successfully. After characterization, the Fe₃O₄@CS@DMSA was used to eliminate Cd(II) ions from aqueous systems. The Fe₃O₄@CS@DMSA adsorbent exhibited a high adsorption capacity (314.12 mg/g at the optimum condition pH: 7.6, contact time: 210 min, temperature: 298 K, adsorbent mass:10 mg, and stirring rate: 100 rpm). The FTIR and EDX results confirmed the existence of Cd(II) ions after adsorption on Fe₃O₄@CS@DMSA nanocomposite. The Freundlich isotherm data and pseudo-first-order kinetic data displayed more compatibility with the equilibrium data than that of other models. The mechanism of Cd(II) adsorption on Fe₃O₄@CS@DMSA nanocomposite is electrostatic attraction. The thermodynamic results confirmed the spontaneous and exothermic nature of adsorption. The reusability test of Fe₃O₄@CS@DMSA nanocomposite exhibited that the adsorption efficiency was 72% after five cycles. The results indicate that the Fe₃O₄@CS@DMSA has a good potential for the elimination of Cd(II) from an aqueous solution.

Footnotes

Data Availability

Anyone who wants to request research article data can contact me directly via the following email: aalkudami@ksu.edu.sa, aaaayoub17@gmail.com, Chemistry Department, College of Science, King Saud University.

Conflicts of Interest

There are no conflicts to declare.

Acknowledgments

The authors are thankful to the Deanship of Scientific Research at Najran University for funding this work under the General Research Funding program grant code (NU/-/SERC/10/509).

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Sulfhydryl Functionalized Magnetic Chitosan as an Efficient Adsorbent for High-Performance Removal of Cd(II) from Water: Adsorption Isotherms,Kinetic,and Reusability Studies

Abstract

1. Introduction

2. Experimental

2.1. Chemicals and Reagents

2.2. Synthesis of Fe3O4@CS

2.3. Synthesis of Fe3O4@CS@DMSA

2.4. Characterization of Fe3O4@CS@DMSA Nanocomposite

2.5. Batch Adsorption Experiments

3. Results and Discussion

3.1. Characterization of Fe3O4@CS@DMSA Nanocomposite

3.2. Adsorption Study

3.2.1. Effect of pH Solution

3.2.2. Effect of Adsorbent Mass

3.2.3. Effect of Contact Time

3.2.4. Effect of Initial Cd(II) Concentration and Temperatures

3.3. Modeling

3.3.1. Isotherm Model

3.3.2. Kinetic Model

3.3.3. Adsorption Thermodynamics

3.4. Adsorption Mechanism

3.5. Reusability Study

4. Conclusion

Footnotes

Data Availability

Conflicts of Interest

Acknowledgments

References

2.2. Synthesis of Fe₃O₄@CS

2.3. Synthesis of Fe₃O₄@CS@DMSA

2.4. Characterization of Fe₃O₄@CS@DMSA Nanocomposite

3.1. Characterization of Fe₃O₄@CS@DMSA Nanocomposite