Recyclable magnetic nanocomposites for efficient removal of cadmium ions from water: performance, mechanism and isotherm studies

ABSTRACT

Herein, succinic acid functionalized Fe₃O₄@AMPA nanocomposite was synthesized as an effective adsorbent and used for the efficient removal of Cd(II) ions from aqueous media. Zeta potential, BET, TEM, TGA, and FTIR spectra were applied to characterize the Fe₃O₄@AMPA@SA nanocomposite. According to the BET analysis, the surface area of Fe₃O₄@AMPA@SA was found to be 43 m²/g. The influence of pH value, initial Cd(II) concentration, adsorbent dosage, temperature, and contact time on the adsorption process was achieved via the batch experiment method. The result showed that the optimal parameters were 6, 250 mg/L, 0.03 g/50 mL, 150 min, and 25°C, respectively. The experimental data fitted well to the Langmuir isotherm and the pseudo-first-order models. The maximum monolayer adsorption capacity of Cd(II) ions was 265 mg g⁻¹. The adsorption mechanism of Cd(II) on magnetic nanocomposite is physical adsorption. The Fe₃O₄@AMPA@SA nanocomposite shows excellent reusability with a removal efficiency of 81.4% after three cycles.

GRAPHICAL ABSTRACT

KEYWORDS:

• Magnetic nanocomposite
• adsorption
• mechanism
• toxic Cd(II)
• isotherm
• succinic acid

1. Introduction

Cadmium (Cd(II)) is one of the most toxic heavy metals introduced into the environmental system via different industrial processes such as the chemical industry, battery manufacturing, mining and electroplating [1]. Cd(II) is non-biodegradable and carcinogenic and it can damage the human immune system, kidneys, digestive system and nervous system, leading to hypertension, anemia, human hepatitis, cancer and other diseases [2,3]. Owing to the dangerous effects of Cd(II) on humans, the WHO has set the permissible limit of Cd(II) in drinking water is 5 μg/L [4]. Therefore, the remediation of toxic Cd(II) ions from the environmental water before discharge into the environment is an important task. Therefore, it is essential to develop an effective and eco-friendly technology for remediation of toxic Cd(II) ions from the aqueous environment before discharging into the receiving water body.

There are many techniques available to eliminate heavy metal ions from wastewater, including chemical precipitation, ion exchange, electrochemical removal, solvent extraction, adsorption, etc. [5–12]. Among these, adsorption is a unique and promising technique for the elimination of Cd(II) and other pollutants from an aqueous environment owing to its simplicity of design, environmentally friendly, low-cost, ease of recovery, high efficiency, easy operation and does not produce secondary pollutants in water. Many adsorbents have been applied for the adsorptive elimination of Cd(II) ions from wastewater under different operating conditions like CaO-SA [13], NaOH-FA [14], MgO-ATP [15], MnS-fBC [16], zeolite [17], bentonite-fly ash [18], industrial chili seeds (CS) [19], biochars [20]. Exploring the excellent adsorbent with easy separation of the analyte metal from the adsorbent surface and easy regeneration with high recovery efficiency would improve the adsorption process. Adsorbent materials based on magnetic nanoparticles have received extensive attention due to their magnetic properties and exhibit high removal capacity of pollutants and low toxicity [21–23]. It can easily separate the pollutants from the aqueous solution via a magnet. Various adsorbent materials were coated magnetic nanoparticles to improve the adsorption process toward pollutant. Alqadami et al. prepared Fe₃O₄ @TSC nanocomposites for the removal of Cd(II) from an aqueous media [1]. Elham Sadati Behbahani et al. synthesized Fe₃O₄/FeMoS₄/MgAl-LDH adsorbent for removal of Cd(II), Pb(II) and Cu(II) ions [24]. They found that the adsorption capacities of the adsorbent were 140.50, 190.75 and 110.25 mg g⁻¹, respectively. Introducing various functional groups such as amino, carboxyl, hydroxyl, amide, and thiol will improve the adsorption process of pollutants. 2-Amino-3-mercaptopropionic acid (L-Cysteine) (AMPA) is a neutral organosulfur that consists of sulfhydryl, amino, and carboxyl groups on the side chain. These functional groups were used as chelate mercury (Hg (II)) [24–26]. Succinic acid (SA) has a dicarboxylic group that serves as an adsorption active site for heavy metal ions [27,28]. Karnitz Jr. et al. introduced succinic anhydride in sugarcane bagasse using succinylation reaction for Cu(II), Pb(II), and Cd(II). They found that a good adsorption capacity for Cu(II), Pb(II), and Cd(II) [27]. Therefore, the SA-modified surface of Fe₃O₄@AMPA could create a new magnetic nanocomposite for the removal of Cd(II) from aqueous media. To our knowledge, no work has been reported on the introduction of succinic acid to Fe₃O₄@AMPA.

This work aimed to synthesize a new magnetic nanocomposite (Fe₃O₄@AMPA@SA) by in situ co-precipitation and amidition methods. The Fe₃O₄@AMPA@SA adsorbent was applied for the elimination of toxic Cd(II) from an aqueous environment. The synthesized adsorbent was characterized using Zeta potential, BET surface area, TEM, FTIR and TGA analysis. The influence of efficient factors such as initial Cd(II) concentrations, adsorbent amount, solution pH, temperature, and contact time was investigated. The nonlinear isotherm and kinetic adsorption models were used to analyze experimental data. Thermodynamic parameters were also calculated. The mechanism and reusability of Cd(II) removal were studied.

2. Experiment

2.1. Chemicals and reagents

Succinic acid (SA) (C₄H₆O₄), Dimethylformamide (DMF) was purchased from Panreac, UK 2-Amino-3-mercapto-propionic acid (AMPA) or (L-Cysteine, C₃H₇NO₂S) (C₃H₇NO₂S), Ferrous chloride tetrahydrate (FeCl₂.4H₂O), 1-ethyl-3-(3 dimethylaminopropyl) carbodiimide (EDC), and Ferric chloride hexahydrate (FeCl₃.6H₂O) were purchased from Sigma-Aldrich. NaOH, 25% NH₄OH, and HCl were obtained from Merck, Germany. Cd(NO₃)₂ 4H₂O were purchased from BDH, England.

2.2. Synthesis of Fe₃O₄ @AMPA

Amino-3-mercaptopropionic (AMPA) modified Fe₃O₄ nanoparticles (Fe₃O₄@AMPA) were prepared by in situ co-precipitation method as follows: 10.81 g of FeCl₃.6H₂O and 3.98 g of FeCl₂.4H₂O were dissolved in beakers containing 100 ml of 0.01 M HCl under ultra-sonication and N₂ gas, separately. Next, 1 g of AMPA was added in beaker containing 50 ml of D.I water under ultra-sonication for 3 h. Then the salt solution was transferred to a three-neck flask (500 ml). After that, the mixture was heated at 75°C under mechanical stirring and N₂ gas for 10 min. Then, 35 mL of NH₄OH solution was added into the mixture at the seam time with 50 mL of AMPA under mechanical stirring and N₂ gas until it reached pH 10. Next, the mixture was kept under stirring N₂ gas at room temperature for 2 h. The black precipitate was isolated from the solution by a magnet and washed several times with deionized water and EtOH. The black precipitate was dried overnight at 50°C (Figure 1).

Figure 1. The synthesis of the Fe₃O₄@AMPA@SA nanocomposites.

2.3. Synthesis of Fe₃O₄ @AMPA@SA nanocomposite

Fe₃O₄@AMPA@SA nanocomposite was synthesized in the following procedure: 1.15 g of succinic acid (SA) was dissolved in 30 mL DMF under ultra-sonication for 10 min. Then, 0.9 g of EDC was added to the solution of SA to activation of a carboxylic group under mechanical stirring for 30 min. After that, 1.5 g of Fe₃O₄@AMPA was added into the solution of SA. The reaction mixture was stirred vigorously under N₂ gas at 25°C for 24 h. Finally, the Fe₃O₄@AMPA@SA was isolated via a magnet and then washed three times with DMF several times with D.I. water (Figure 1).

2.4. Adsorption and desorption studies

The adsorption process of Cd(II) using Fe₃O₄@AMPA@SA nanocomposite in an aqueous solution was conducted in batch method at room temperature. The effect of different factors on the adsorption process, such as adsorbent dosage, pH solution, initial Cd(II) concentration, equilibrium time, and the temperature was accomplished in the range of (0.01–0.07 mg/L), (2.3–9), (25–350 mg/L), (5–240 min), and (25–45°C), respectively. In each experiment, a given load of Fe₃O₄@AMPA@SA (0.03 g/50 mL) was added to 50 mL of 25 mg/L Cd(II) aqueous solution; then, the sample was adjusted to pH = 7.4 using 0.1 HNO₃ or NaOH. Next, the sample was shaken in a thermostatic water bath oscillator for 120 min at 25°C. after that, the sample was isolated via a magnet. Finally, the concentration of Cd(II) in the aqueous solution was quantified by AAS. By Equations (1)(1) $\mathrm{\%}\mathrm{a}\mathrm{d}\mathrm{s}\mathrm{o}\mathrm{r}\mathrm{p}\mathrm{t}\mathrm{i}\mathrm{o}\mathrm{n}=\frac{\mathrm{C}\mathrm{o}-\mathrm{C}\mathrm{e}}{\mathrm{C}\mathrm{o}}\hspace{0.17em}\times 100$(1) and (2(2) $\mathrm{q}\mathrm{e}=\left(\mathrm{C}\mathrm{o}-\mathrm{C}\mathrm{e}\right)\frac{\mathrm{V}}{\mathrm{m}}\hspace{0.17em}$(2) ) the adsorption capacity and percentage efficiency of Cd(II) ions were calculated, respectively.

(1) $\mathrm{\%}\mathrm{a}\mathrm{d}\mathrm{s}\mathrm{o}\mathrm{r}\mathrm{p}\mathrm{t}\mathrm{i}\mathrm{o}\mathrm{n}=\frac{\mathrm{C}\mathrm{o}-\mathrm{C}\mathrm{e}}{\mathrm{C}\mathrm{o}}\hspace{0.17em}\times 100$(1)

(2) $\mathrm{q}\mathrm{e}=\left(\mathrm{C}\mathrm{o}-\mathrm{C}\mathrm{e}\right)\frac{\mathrm{V}}{\mathrm{m}}\hspace{0.17em}$(2)

where C₀ and C_e (mg/L) are the concentration of Cd(II) at initial time, respectively; m (g) and V(L) are the mass of Fe₃O₄@AMPA@SA adsorbent and volume of Cd(II) solution, respectively.

For each adsorption-desorption cycle, 0.03 g Fe₃O₄@AMPA@SA was used for 50 mL of 25 mg/L Cd(II) solution and the sample was adjusted to pH = 7.4 using 0.1 HNO₃ or NaOH. Then, the sample was shaken for 150 min at 25°C. After that the sample was separated with a magnet to collect the Cd(II) loaded Fe₃O₄@AMPA@SA and the concentration of Cd(II) in the aqueous solution was quantified by AAS. The attained Cd(II) loaded Fe₃O₄@AMPA@SA was rinsed with deionized water to remove unabsorbed traces of Cd(II) and then its added to conical flask containing 50 ml of 0.01 M HNO₃ or 0.01 HCl solution, separately, shaking at 25°C for 150 min. The sample was separated with a magnet and the concentration of Cd(II) in the aqueous solution was quantified by AAS. The obtained Fe₃O₄@AMPA@SA nanocomposite was washed with deionized water and dried at 50°C for successive cycles. This cycle was repeated three times. The % desorption was calculated using Equation 3(3) $\mathrm{\%}\mathrm{d}\mathrm{e}\mathrm{s}\mathrm{o}\mathrm{r}\mathrm{p}\mathrm{t}\mathrm{i}\mathrm{o}\mathrm{n}=\frac{C_m}{C_e}\times 100$(3)

(3) $\mathrm{\%}\mathrm{d}\mathrm{e}\mathrm{s}\mathrm{o}\mathrm{r}\mathrm{p}\mathrm{t}\mathrm{i}\mathrm{o}\mathrm{n}=\frac{C_m}{C_e}\times 100$(3)

where C_m (mg/L) and C_e (mg/L) refer to the Cd(II) concentrations released in the solution and the initially adsorbed Cd(II) concentration, respectively.

3. Results and discussion

3.1. Characterization of adsorbents

The TGA measurements of Fe₃O₄ nanoparticles and nanocomposite were performed using TGA: Mettler Toledo TGA/SDTA851, USA. The Zeta potential was measured at a range of 3–9 using Nano Plus Series, USA. The morphologies of the Fe₃O₄@AMPA@SA were analyzed using a transmission electron microscope (TEM, PHILIPS CM120). Fourier transform infrared spectroscopy (FTIR, Nicolet 6700, Thermo Scientific, USA) was used to determine the functional groups of Fe₃O₄@AMPA@SA, Fe₃O₄, SA, and Fe₃O₄@AMPA@SA//Cd(II). The BET surface area of Fe₃O₄@AMPA@SA nanocomposite was determined using nitrogen adsorption-desorption measurements. TEM images of Fe₃O₄@AMPA@SA nanocomposite are shown in Figure 2(a). It can be seen that the image of Fe₃O₄@AMPA@SA nanocomposite reveals spherical particles in shape with an average diameter of 12.6 nm as presented in Figure 2(a) inset. A grey layer at the edges of the magnetite nanoparticles may be assigned to organic compounds (AMPA and SA). To confirm the existence of sulfhydryl, amide, carboxyl groups on the surface of Fe₃O₄@AMPA@SA, FT-IR of the structure of Fe₃O₄@AMPA@SA, Fe₃O₄, SA, and Fe₃O₄@AMPA@SA//Cd(II) were tested was examined and as displayed in Figure 2(b). For magnetite nanoparticles, the characteristic peak of magnetite nanoparticles appeared at 572 cm⁻¹ refer to the Fe-O bond. In the case of Fe₃O₄@AMPA@SA nanocomposite, the board peaks at 3439 cm⁻¹ refers to the stretching vibration of the O-H/N-H group, the peaks at 2861 cm⁻¹ and 2925 cm⁻¹ is due to ν(C-H) stretching in AMPA and SA. The characteristic peak for Fe-O bond appeared at 572 cm⁻¹, confirmed the formation of magnetic nanocomposite [23], in addition, the characteristic peak for vibration stretching of S–H appeared at 2548 cm⁻¹, indicates the presence of AMPA on the surface of magnetic nanocomposite [29–31]. The weak peak at 1686 cm⁻¹, a strong peak at 1635 cm⁻¹, a weak peak at 1557 cm⁻¹ may refer to the stretching of carbonyl of carboxylic, amide I, and amide II vibrations, respectively which are consistent with the literature [32–34], indicated the formation of Fe₃O₄@AMPA@SA nanocomposite successfully. The peaks at 1517 cm⁻¹, 1398 cm⁻¹, (1322–1233 cm⁻¹), and (1186–1090 cm⁻¹), attributed to the stretching of N-H, COO⁻, C-N, C-O groups, respectively [35,36]. For SA, the ν(-C = O) of the stretch of COOH revealed at 1710 cm⁻¹, ν(C–H), ν(O–H), and anti-symmetric (COO-) at around 2939 cm⁻¹, 2648 cm⁻¹, and 2529 cm⁻¹, respectively [37]. Remarkable alters are observed in the FTIR of spectrum Fe₃O₄@AMPA@SA nanocomposite after Cd(II) adsorption. A peak of sulfhydryl (S-H) at 2548 cm⁻¹ was disappeared, indicated the bending of Cd(II) ion with S-H bond by electrostatic interaction on the surface of Fe₃O₄@AMPA@SA nanocomposite [29]. In addition, the intensity of peak at1398 cm⁻¹ was increased and shifted to 1395 cm⁻¹ due to binding Cd(II) with COO- group on the surface of Fe₃O₄@AMPA@SA nanocomposite. The intensity of peaks in the range of 1322 − 1090 cm⁻¹ was reduced and shifted to 1304–1080 cm⁻¹ due to binding Cd(II) with C-N and C-O groups, respectively. The peaks of the O-H bond and NHCO− were shifted to 3418 and 1620 cm⁻¹, respectively [38]. Therefore, it is clearly that the Cd(II) ions are combined with functional groups onto Fe₃O₄@AMPA@SA nanocomposite

Figure 2. (a) TEM image of Fe₃O₄@AMPA@SA with size particles distribution (inset) and (b) FTIR spectrum of Fe₃O₄, SA, Fe₃O₄@AMPA@SA, and Fe₃O₄@AMPA@SA/Cd(II).

Figure 3(a) exhibited thermal stability of the Fe₃O₄ and Fe₃O₄@AMPA@SA adsorbents. It was noticed that the magnetite nanoparticles and Fe₃O₄@AMPA@SA were degraded in two and three stages, respectively. In the first stage, 4% and 9% of initial weight loss at temperature up to 150°C for Fe₃O₄ and Fe₃O₄@AMPA@SA, respectively, which can be related to the elimination of adsorbed water and impurities [39,40]. In the second stage, a 5% of weight loss was observed in the range of 150°C to 750°C for pure magnetite owing to the conversion of some hydroxide to oxide [41]. For the Fe₃O₄@AMPA@SA, 9% and 10% of weight loss from (150 to 250°C) and (250 to 800°C) were observed attributed to the decomposition of the organic structure of SA and AMPA [29]. 9% and 28% are the total weight loss for Fe₃O₄ and Fe₃O₄@AMPA@SA nanocomposite, respectively, confirmed synthesized Fe₃O₄@AMPA@SA successfully.

Figure 3. (a) TGA analysis of Fe₃O₄ and Fe₃O₄@AMPA@SA, (b) N₂ adsorption-desorption isotherms of Fe₃O₄@AMPA@SA nanocomposite, and (c) BJH pore-size distribution graph.

The mesoporous structure of the adsorbent material is critical for the fast and effective adsorption of heavy metals from aqueous solutions due to their large surface area, pore structure, and tunable pore sizes [42]. Therefore, the heavy metal ions are adsorbed not only on the external surface area but also within the adsorbents’ mesoporous. Figure 3(b) displays the N₂ adsorption-desorption analysis of Fe₃O₄@AMPA@SA nanocomposite which exhibited type IV isotherms. The BET surface area was calculated by using the Brunauer-Emmett-Teller (BET) method in the relative pressure (P/P₀) range from 0.05 to 0.15 at 77 K in liquid nitrogen. The BET surface area of Fe₃O₄@AMPA@SA was found to be 43 m²/g. The values of C constant and correlation coefficient (R²) were 92.018 and 0.999921, respectively. The pore size distribution of Fe₃O₄@AMPA@SA was determined using the Barrett-Joyner-Halenda (BJH) method and found to be 12.3 nm as depicted in Figure 3(c), which allows internal and external diffusion of Cd(II) by physical adsorption. The diameter of the Cd(II) ions is 0.15 nm [42], enabling the pore-filling sorption mechanism. Thus, Cd(II) ions could get into the pore network of Fe₃O₄@AMPA@SA nanocomposite and promote pore-dependent adsorption by physical adsorption. Compared to the BET surface area of pure magnetite, the BET surface area is less than that reported in the literature [43]. The decrease in surface area after functionalizing with AMPA and SA may be due to disruption in the aggregation after surface modification and also may be present a slight layer of SA and AMPA onto the magnetic surface. Therefore, the interaction between N₂ molecules and the particle surface of Fe₃O₄@AMPA@SA nanocomposite is decreased. A similar result has been reported by Ghosh et al [44]., Mohapatra and Pramanik [45].

3.2. Adsorption performance of Fe₃O₄@AMPA@SA

3.2.1. Effect of pH and Zeta potential

The initial pH is a factor parameter affecting the adsorption performance behavior of adsorbents which influences the adsorbate ionization state and the surface charge of adsorbents, Therefore, the influence of initial pH on the adsorption process was tested at different pH (pH = 2.3–9) with at parameters fixed [C_o: 25 mg/L, temperature: 25°C, dose: 0.03 g/50 mL, time: 24 h, and agitated: 100 rpm] as represented in Figure 4(b). It can be observed that the adsorption capacity of Fe₃O₄@AMPA@SA toward Cd(II) was increased from 4.6 to 40 mg g⁻¹ as the solution pH value rising from 2.3 to 7, respectively. The maximum removal efficiency and adsorption capacity were 96% and 40 mg g⁻¹, respectively at pH 7. Slightly decrease in the adsorbed Cd(II) was observed from (pH = 7–9). The removal efficiency was only 11.2% at strong acidic pH = 2.3 due to the competition between Cd(II) ions and hydronium ion (H₃O⁺) for adsorption sites onto Fe₃O₄@AMPA@SA resulting in reduced the adsorption capacity of adsorbent and removal efficiency of Cd(II). After the further increase in pH from 2 to 7, a good removal efficiency was observed due to the increase of charge negatively on the surface of Fe₃O₄@AMPA@SA nanocomposite, resulting in higher adsorption capacity. The results of the Zeta potential test reveal that the isoelectric point (IEP) of the Fe₃O₄@AMPA@SA nanocomposite was 5, which means the Fe₃O₄@AMPA@SA nanocomposite was protonated at pH_zpc<5 and the Cd(II) adsorption was reduced via the electrostatic repulsion (Figure 4(a)). After pH_zpc >5, the active sites of the Fe₃O₄@AMPA@SA surface became a negative charge, resulting in higher removal efficiency due to the electrostatic attraction between negative Fe₃O₄@AMPA@SA surface and positive Cd(II) ions. At a pH higher than 7, the Cd(II) ions suffer hydrolysis, reducing the adsorption process and forming hydroxide precipitates (Cd(OH)₂) [46]. Thus, the optimal pH = 7 was selected as optimal pH for all adsorption experiments.

Figure 4. (a) Zeta potential of Fe₃O₄@AMPA@SA, (b) Effects of pH, (c) adsorbent dosage, (d) contact time, and (e) initial Cd(II) concentration on adsorption of Cd(II) onto Fe₃O₄@AMPA@SA nanocomposite.

3.2.2. Effect of adsorbent dosage

The influence of adsorbent dose on the removal efficiency of Cd(II) was studied in the range between 0.01 to 0.07 g/50 mL (T: 298 K; C_o: 25 mg/L; pH: 7; agitation: 100 rpm) as presented in Figure 4(c). The elimination efficiency of Cd(II) was increased from nearly 48.64% to 95.54% as the adsorbent dose of the Fe₃O₄@AMPA@SA improving from 0.01 g to 0.07 g/50 mL, respectively owing to improve the active sites on the Fe₃O₄@AMPA@SA surface, which resulted in higher removal efficiency. Behind 0.03 g/50 mL, no change in removal efficiency was observed. On the other hand, the adsorption capacity of Fe₃O₄@AMPA@SA was reduced to 39.61 mg g⁻¹ with improving the adsorbent to 0.03 g/50 mL due to the initial Cd(II) concentration is still constant in equation (1)(1) $\mathrm{\%}\mathrm{a}\mathrm{d}\mathrm{s}\mathrm{o}\mathrm{r}\mathrm{p}\mathrm{t}\mathrm{i}\mathrm{o}\mathrm{n}=\frac{\mathrm{C}\mathrm{o}-\mathrm{C}\mathrm{e}}{\mathrm{C}\mathrm{o}}\hspace{0.17em}\times 100$(1) with increasing adsorbent dose, leading to the reduction of adsorption capacity. Also, the reason for the decrease in adsorption capacity may be due to the aggregation of adsorption sites [47]. A similar effect is achieved by Aldawsari et al [35]. Therefore, 0.03 g/50 mL of Fe₃O₄@AMPA@SA was chosen as a proper dose for all adsorption experiments.

3.2.3. Effect of contact time

The effect of equilibrium time on the Cd(II) adsorption onto Fe₃O₄@AMPA@SA surface was performed by varying the time between 5 and 240 min [T: 298 K; C_o: 25 mg/L; pH: 7; agitation: 100 rpm] as shown in Figure 4(d). The outcomes result was revealed that the adsorption capacity of Fe₃O₄@AMPA@SA and percentage adsorption of Cd(II) ion was increased gradually from (2.5 to 39.58 mg g⁻¹) and (6% to 96%), respectively with rising equilibrium time from 5 min to 150 min and then remained constant. Fast adsorption was observed in the first 15 and 30 min with 50% and 74% removal, respectively due to the high availability of functional groups on the surface of Fe₃O₄@AMPA@SA nanocomposite which binding of Cd(II) ions easily. Therefore, 150 min was the desirable choice for all adsorption experiments.

3.2.4. Effect of initial MG concentration and temperatures

The effect of the initial Cd(II) concentration (25–350 mg/L) on adsorption capability at various temperatures was performed [C_o: 25 mg/L; time: 150 min; pH: 7; agitation: 100 rpm] as shown in Figure 4(e). It’s clear that the adsorption capacities of Fe₃O₄@AMPA@SA were augmented from 37.5 to 250 mg g⁻¹ as the initial Cd(II) concentration improving from 25 mg/L to 350 mg/L, respectively, which might be elucidated that the increasing Cd(II) concentration gradient acts as a driving force for the mass transfer of Cd(II) to Fe₃O₄@AMPA@SA surface and enhance the effective collision between the Cd(II) ions and the active sites onto Fe₃O₄@AMPA@SA surface which leading to the improve in adsorption capacity. A similar result has been reported by Xiaohui Zhou et al. [48]. The adsorption capacity of Fe₃O₄@AMPA@SA adsorbent toward Cd(II) was reduced with a rise in the temperature from 25 to 45°C as displayed in Figure 4(e), which means that the adsorption of Cd(II) onto Fe₃O₄@AMPA@SA was exothermic.

3.3. Adsorption modeling

3.3.1. Isotherm studies

To evaluate the adsorption capacity of the Fe₃O₄@AMPA@SA toward Cd(II) adsorption, four nonlinear isotherm adsorption include Langmuir Equation (4)(4) $q_e=\frac{q_m{K}_L{C}_e}{1+K_L{C}_e}$(4) [49], Freundlich Equation (5)(5) $q_e=K_F{C_e}^{1/n}$(5) , Timken Equation (6)(6) $q_e=B_{T}ln\left(A_T{C}_e\right)$(6) and Dubinin–Radushkviech (D–R) Equations 7(7) $q_e=q_s{e}^{-K_{D-R}{\epsilon }^2}$(7) –9(9) $E=\frac{1}{\sqrt{2K_{D-R}}}$(9) [50] isotherm was applied.

(4) $q_e=\frac{q_m{K}_L{C}_e}{1+K_L{C}_e}$(4)

(5) $q_e=K_F{C_e}^{1/n}$(5)

(6) $q_e=B_{T}ln\left(A_T{C}_e\right)$(6)

(7) $q_e=q_s{e}^{-K_{D-R}{\epsilon }^2}$(7)

(8) $\epsilon =RTln\left(1+\frac{1}{C_e}\right)$(8)

(9) $E=\frac{1}{\sqrt{2K_{D-R}}}$(9)

where K_L and K_F, and K_D-R represent the Langmuir, Freundlich and D–R isotherm constants; n is the Freundlich intensity parameter; E (kJ/mol) is the energy of adsorption; A_T is the equilibrium binding constant and B_T is the heat of adsorption (J/mol). Table 1 summarizes the analysis of various parameters of isotherm models. The fitting curves are shown in Figure 5(a-c). Among the isotherm models, the Langmuir isotherm model can fit the data well and had a highest R²(0.98891) compared to other models Timken (0.9498), Freundlich (0.9104), and D–R (0.9000), indicating that the adsorption process of Cd(II) ions were monolayer adsorption with maximum q_e (265 mg g⁻¹) at 25°C. Compared to the recently reported magnetic nanocomposite for Cd(II) adsorption, the adsorption capacity of Fe₃O₄@AMPA@SA toward Cd(II) adsorption is the highest [46,48,51–57](Table 4). Therefore, these results confirmed that the Fe₃O₄@AMPA@SA is a promising adsorbent for the effective removal of Cd(II) ions from contaminated water. From the D-R isotherm (Table 1), the values of E were in the range (0.0842–0.0614 kJ /mol) at various temperatures from 25°C to 45°C, indicating that physical adsorption was dominant in the adsorption process [58,59].

Figure 5. Nonlinear isotherm adsorption of Cd(II) by Fe₃O₄@AMPA@SA nanocomposite at various temperature (a) 25°C, (b) 35°C, and (c) 45°C, (d) Nonlinear kinetic adsorption of Cd(II) by Fe₃O₄@AMPA@SA nanocomposite.

Table 1. Isotherm parameters for Cd(II) adsorption on Fe₃O₄@AMPA@SA nanocomposite.

Model	Temperature (K)
	298	308	318
Langmuir
qm, mg/g	265.0	240.1	188.7
KL (L/mg)	0.036	0.030	0.027
RL	0.526	0.571	0.597
R^2	0.9846	0.9885	0.9889
Freundlich
Kf, (mg/g) (L/mg)^1/n	40.68	29.76	23.24
n	2.74	2.68	2.73
R^2	0.9104	0.9117	0.8979
Dubinin-R
qs, mg/g	235.9	191.6	151.9
KD-R(mol^2 KJ^−2)	70.53	112.4	132.6
E (kJ mol^−1)	0.0842	0.0666	0.0614
R^2	0.9000	0.8735	0.8982
Temkin
bT	43.87	52.02	65.43
AT (L/g)	0.542	0.346	0.269
BT (J /mol)	56.50	49.24	40.42
R^2	0.9498	0.9690	0.9710

Table 4. Comparison of Cd(II) adsorption on various magnetic adsorbents.

Adsorbent	qm(mg/g)	Ref.
Magnetic graphene oxide	128.2	^[Citation45]
MBCI	197.96	^[Citation47]
rGO-PDTC/Fe3O4	116.28	^[Citation50]
M-Fe3O4-GO	125	^[Citation51]
NH2-MS	51.81	^[Citation52]
Mag@Fh-P	64.1	^[Citation53]
Magnetic ferrites	44.13	^[Citation54]
MGO	91.29	^[Citation55]
Fe3O4@CdTe-IIP	17.57	^[Citation56]
Fe3O4@AMPA@SA	265	This wprk

3.3.2. Kinetics studies

To investigate the controlling mechanisms of Cd(II) adsorption on Fe₃O₄@AMPA@SA, nonlinear pseudo-first-order (PFO) Equation (10)(10) ${\mathrm{q}}_{\mathrm{t}}={\mathrm{q}}_{\mathrm{e}}(1-{\mathrm{e}}^{-{\mathrm{k}}_1\mathrm{t}})$(10) , pseudo-second-order (PSO) Equation (11)(11) ${\mathrm{q}}_{\mathrm{t}}=\frac{{\mathrm{q}}_{\mathrm{e}}^2{\mathrm{k}}_2\mathrm{t}}{1+{\mathrm{q}}_{\mathrm{e}}{\mathrm{k}}_{2\hspace{0.17em}}\mathrm{t}}$(11) [60] and Elovich kinetic models (EKM) Equation (12)(12) $\hspace{0.17em}{\mathrm{q}}_{\mathrm{t}}=\frac{1}{\beta }ln\left(1+\alpha \beta \mathrm{t}\right)$(12) were applied [61].

(10) ${\mathrm{q}}_{\mathrm{t}}={\mathrm{q}}_{\mathrm{e}}(1-{\mathrm{e}}^{-{\mathrm{k}}_1\mathrm{t}})$(10)

(11) ${\mathrm{q}}_{\mathrm{t}}=\frac{{\mathrm{q}}_{\mathrm{e}}^2{\mathrm{k}}_2\mathrm{t}}{1+{\mathrm{q}}_{\mathrm{e}}{\mathrm{k}}_{2\hspace{0.17em}}\mathrm{t}}$(11)

(12) $\hspace{0.17em}{\mathrm{q}}_{\mathrm{t}}=\frac{1}{\beta }ln\left(1+\alpha \beta \mathrm{t}\right)$(12)

where k₁ and k₂ refer to the constants of adsorption rates PFO and PSO model, respectively; α is the constant (desorption) and represent surface coverage, and α is the initial adsorption rate. Table 2 summarizes the analysis of various parameters of kinetic models. The fitting curves are presented in Figure 5(d). As shown in Table 2, the R² of the PFO kinetic model (0.9625) were much better than those of PSO (0.9308) and EKM (0.875). The experimental (39.83 mg g⁻¹) values are very close to the calculated q_{e. cal} (39.35 mg g⁻¹) by PFO kinetic model, indicating that the adsorption process was dominated by physisorption. This result is consistent with the literature reported by Pawar et al. for Pb(II), Cd(II) and As(V) adsorption using magnetic composite beads [62].

Table 2. Kinetics parameters for Cd(II) adsorption on Fe₃O₄@AMPA@SA nanocomposite.

Co (mg/L)	qe,exp. (mg/g)	Pseudo-first-order			Pseudo-second-order			Elovich
		qe, cal. (mg/g)	K1 (1/min)	R^2	qe2, cal. (mg/g)	K2 (g/mg-min)	R^2	Α (mg/g min)	Β (mg/g)	R^2
25	39.83	39.35	0.0407	0.9625	45.146	0.0012	0.9308	3.471	0.1017	0.8755

3.3.3. Thermodynamic of Cd(II) adsorption

To determine the thermodynamic properties of the Cd(II) adsorption onto Fe₃O₄@AMPA@SA, energy changes such as Gibbs free energy change (ΔG°) Equation (13)(13) $\mathrm{\Delta }{G}^{\circ }=-RTlnKc$(13) , enthalpy (ΔH°) Equation (14)(14) $Ln{K}_c=-\frac{\mathrm{\Delta }{H}^o}{RT}+\frac{\mathrm{\Delta }{S}^o}{R}$(14) , and entropy change (ΔS°) was analyzed:

(13) $\mathrm{\Delta }{G}^{\circ }=-RTlnKc$(13)

(14) $Ln{K}_c=-\frac{\mathrm{\Delta }{H}^o}{RT}+\frac{\mathrm{\Delta }{S}^o}{R}$(14)

Where Kc is equal (q_e/C_e) which refers to an equilibrium constant at various temperatures [63]. The ΔH^o and ΔS^o were calculated from the plot of ln Kc versus 1/T. Table 3 shows the thermodynamics factors for the Cd(II) adsorption on Fe₃O₄@AMPA@SA nanocomposite. The outcomes showed that at all temperatures the ΔG^o values were negative, which depicted that the spontaneous and feasibility of the adsorption process. The values of ΔS^o and ΔH^o were negative at all temperatures, suggesting decreasing disorder of the interface during the adsorption process and the adsorption process was exothermic, respectively [35].

Table 3. Thermodynamics factors for the Cd(II) adsorption on Fe₃O₄@AMPA@SA nanocomposite.

Metal ions	(-) ΔH° (kJ/mol)	(-) ΔS° (J/mol.K)	(-) ΔG° (kJ/mol)
			298 K	308 K	318 K
25	34.11	138.4	9.56	7.97	6.81
50	24.36	55.12	7.91	7.45	6.53
75	27.97	68.40	7.67	6.72	6.31

3.4. Desorption study

Recycling performance is an important index for evaluating an adsorbent. To achieve the reuse performance of Fe₃O₄@AMPA@SA nanocomposite, 0.01 M HNO₃ and 0.01 M HCl solution were applied as eluents for recovery of Cd(II) from Fe₃O₄@AMPA@SA adsorbent. The outcomes revealed that the removal recovery of Cd(II) using 0.01 M HNO₃ (91%) was better than HCl (79%). The reusability of Fe₃O₄@AMPA@SA nanocomposite was achieved using determining the percentage elimination of 25 mg/L of Cd(II) with recovered adsorbent in different cycles. After three recycle, the removal efficiency was reduced to 81.21%, indicating the excellent potential of Fe₃O₄@AMPA@SA nanocomposite as reusable adsorbent (Figure (6)).

Figure 6. The recycle tests of Cd(II) removal onto Fe₃O₄@AMPA@SA nanocomposite.

3.5. Adsorption mechanism

Figure 7 exhibits the mechanism of Cd(II) adsorption onto Fe₃O₄@AMPA@SA nanocomposite. It can be observed that a large number of S-H, -COOH, and -CONH functional groups incorporating on the surface of Fe₃O₄@AMPA@SA nanocomposite. Isotherm, kinetic adsorption, thermodynamic parameters and FTIR spectrum were used to evaluate the mechanism of Cd(II) adsorption. As shown in Figure 2(b), after Cd(II) adsorption onto Fe₃O₄@AMPA@SA nanocomposite, the intensity of peaks in the range (1322–1090 cm⁻¹) was reduced and shifted to (1304 to 1080 cm⁻¹) due to binding Cd(II) with C-N and C-O groups. The peaks of the O-H bond and -CONH were moved to 3418 cm⁻¹ and 1620 cm⁻¹, respectively. In addition, a peak of sulfhydryl (S-H) at 2548 cm⁻¹ was disappeared, indicated the bending of Cd(II) ion with S-H bond by electrostatic interaction on the surface of Fe₃O₄@AMPA@SA nanocomposite [29]. All these changes indicate the adsorption of Cd(II) onto the surface of Fe₃O₄@AMPA@SA nanocomposite [38]. In addition, the isotherm, kinetic adsorption and thermodynamic parameters confirm the physical adsorption between the active site on magnetic adsorption and Cd(II) ions.

Figure 7. Mechanism of Cd(II) adsorption onto Fe₃O₄@AMPA@SA nanocomposite.

Conclusion

In this study, a new magnetic nanocomposite (Fe₃O₄@AMPA@SA) was synthesized for the elimination of toxic Cd(II) from aqueous media. The synthesized adsorbent was characterized using Zeta potential, FT-IR, TEM, TGA and BET surface area analysis. Based on the TEM image, the average particle size of Fe₃O₄@AMPA@SA nanocomposite was 12.6 nm. The adsorption of Cd(II) onto Fe₃O₄@AMPA@SA was performed using a batch method and the results reveal that the best optimized experimental conditions were dose: 0.03 g: contact time: 150 min; pH: 7; initial Cd(II) concentration: 250 mg/L; and temperature: 25°C. Kinetic and isotherm data exhibited a good correlation to pseudo-first-order and Langmuir with maximum monolayer adsorption capacity 265 mg g⁻¹ at 25°C. Based on the D-R model, the activation energy (E_a) was found in the range (0.0842–0.0614 kJ /mol) at various temperatures, confirming the mechanism adsorption of Cd(II) on magnetic nanocomposite is physical adsorption. Based on thermodynamic results, the adsorption process was exothermic and spontaneous. Fe₃O₄@AMPA@SA nanocomposite shows excellent reusability with a removal efficiency of 81.4% after three cycles. In conclusion, the characteristic advantages of use Fe₃O₄@AMPA@SA nanocomposite show great potential for use as an effective adsorbent for the removal of toxic Cd(II)) from aqueous media.

Acknowledgments

The authors express their appreciation for the search and innovation agency, the Ministry of Education in Saudi Arabia to finance this research work through the project number (UB-27-1442).

Disclosure statement

No potential conflict of interest was reported by the author(s).

Additional information

Funding

The authors express their appreciation for the search and innovation agency, the Ministry of Education in Saudi Arabia to finance this research work through the project number (UB-27-1442).