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


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][6][7][8][9][10][11][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][22][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 Fe 3 O 4 @TSC nanocomposites for the removal of Cd(II) from an aqueous media [1].Elham Sadati Behbahani et al. synthesized Fe 3 O 4 /FeMoS 4 /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][25][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 SAmodified surface of Fe 3 O 4 @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 Fe 3 O 4 @AMPA.
This work aimed to synthesize a new magnetic nanocomposite (Fe 3 O 4 @AMPA@SA) by in situ coprecipitation and amidition methods.The Fe 3 O 4 @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.

Synthesis of Fe 3 O 4 @AMPA
Amino-3-mercaptopropionic (AMPA) modified Fe 3 O 4 nanoparticles (Fe 3 O 4 @AMPA) were prepared by in situ co-precipitation method as follows: 10.81 g of FeCl 3 .6H 2 O and 3.98 g of FeCl 2 .4H 2 O were dissolved in beakers containing 100 ml of 0.01 M HCl under ultrasonication and N 2 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 N 2 gas for 10 min.Then, 35 mL of NH 4 OH solution was added into the mixture at the seam time with 50 mL of AMPA under mechanical stirring and N 2 gas until it reached pH 10. Next, the mixture was kept under stirring N 2 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).

Synthesis of Fe 3 O 4 @AMPA@SA nanocomposite
Fe 3 O 4 @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 Fe 3 O 4 @AMPA was added into the solution of SA.The reaction mixture was stirred vigorously under N 2 gas at 25°C for 24 h.Finally, the Fe 3 O 4 @AMPA@SA was isolated via a magnet and then washed three times with DMF several times with D.I. water (Figure 1).

Adsorption and desorption studies
The adsorption process of Cd(II) using Fe 3 O 4 @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 Fe 3 O 4 @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 HNO 3 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) and (2) the adsorption capacity and percentage efficiency of Cd(II) ions were calculated, respectively.

% adsorption
where C 0 and C e (mg/L) are the concentration of Cd(II) at initial time, respectively; m (g) and V(L) are the mass of Fe 3 O 4 @AMPA@SA adsorbent and volume of Cd(II) solution, respectively.For each adsorption-desorption cycle, 0.03 g Fe 3 O 4 @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 HNO 3 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 Fe 3 O 4 @AMPA@SA and the concentration of Cd(II) in the aqueous solution was quantified by AAS.The attained Cd(II) loaded Fe 3 O 4 @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 HNO 3 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 Fe 3 O 4 @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 3Figure 1.The synthesis of the Fe 3 O 4 @AMPA@SA nanocomposites.

% desorption
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.

Characterization of adsorbents
The TGA measurements of Fe 3 O 4 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 Fe 3 O 4 @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 Fe 3 O 4 @AMPA@SA, Fe 3 O 4 , SA, and Fe 3 O 4 @AMPA@SA//Cd(II).
The BET surface area of Fe 3 O 4 @AMPA@SA nanocomposite was determined using nitrogen adsorptiondesorption measurements.TEM images of Fe 3 O 4 @AMPA@SA nanocomposite are shown in Figure 2(a).
It can be seen that the image of Fe 3 O 4 @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 Fe 3 O 4 @AMPA@SA, FT-IR of the structure of Fe 3 O 4 @AMPA@SA, Fe 3 O 4 , SA, and Fe 3 O 4 @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 Fe 3 O 4 @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][30][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][33][34], indicated the formation of Fe 3 O 4 @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 Fe 3 O 4 @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 Fe 3 O 4 @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 COOgroup on the surface of Fe 3 O 4 @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 Fe 3 O 4 @AMPA@SA nanocomposite Figure 3(a) exhibited thermal stability of the Fe 3 O 4 and Fe 3 O 4 @AMPA@SA adsorbents.It was noticed that the magnetite nanoparticles and Fe 3 O 4 @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 Fe 3 O 4 and Fe 3 O 4 @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 Fe 3 O 4 @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 Fe 3 O 4 and Fe 3 O 4 @AMPA@SA nanocomposite, respectively, confirmed synthesized Fe 3 O 4 @AMPA@SA successfully.
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 N 2 adsorption-desorption analysis of Fe 3 O 4 @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/P 0 ) range from 0.05 to 0.15 at 77 K in liquid nitrogen.The BET surface area of Fe 3 O 4 @AMPA@SA was found to be 43 m 2 /g.The values of C constant and correlation coefficient (R 2 ) were 92.018 and 0.999921, respectively.The pore size distribution of Fe 3 O 4 @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 Fe 3 O 4 @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 N 2 molecules and the particle surface of Fe 3 O 4 @AMPA@SA nanocomposite is decreased.A similar result has been reported by Ghosh et al [44]., Mohapatra and Pramanik [45].

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 Fe 3 O 4 @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 (H 3 O + ) for adsorption sites onto Fe 3 O 4 @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 Fe 3 O 4 @AMPA@SA nanocomposite, resulting in higher adsorption capacity.The results of the Zeta potential test reveal that the isoelectric point (IEP) of the Fe 3 O 4 @AMPA@SA nanocomposite was 5, which means the Fe 3 O 4 @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 Fe 3 O 4 @AMPA@SA surface became a negative charge, resulting in higher removal efficiency due to the electrostatic attraction between negative Fe 3 O 4 @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.

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 Fe 3 O 4 @AMPA@SA improving from 0.01 g to 0.07 g/50 mL, respectively owing to improve the active sites on the Fe 3 O 4 @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 Fe 3 O 4 @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) 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 Fe 3 O 4 @AMPA@SA was chosen as a proper dose for all adsorption experiments.

Effect of contact time
The effect of equilibrium time on the Cd(II) adsorption onto Fe 3 O 4 @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 Fe 3 O 4 @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 Fe 3 O 4 @AMPA@SA nanocomposite which binding of Cd(II) ions easily.Therefore, 150 min was the desirable choice for all adsorption experiments.

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 Fe 3 O 4 @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 Fe 3 O 4 @AMPA@SA surface and enhance the effective collision between the Cd(II) ions and the active sites onto Fe 3 O 4 @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 Fe 3 O 4 @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 Fe 3 O 4 @AMPA@SA was exothermic.

Isotherm studies
To evaluate the adsorption capacity of the Fe 3 O 4 @AMPA@SA toward Cd(II) adsorption, four nonlinear isotherm adsorption include Langmuir Equation (4) [49], Freundlich Equation (5), Timken Equation ( 6) and Dubinin-Radushkviech (D-R) Equations 7-9 [50] isotherm was applied. ) q e ¼ q s e À K DÀ R ε 2 (7) 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 R 2 (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 Fe 3 O 4 @AMPA@SA toward Cd(II) adsorption is the highest [46,48,[51][52][53][54][55][56][57](Table 4).Therefore, these results confirmed that the Fe 3 O 4 @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].
where k 1 and k 2 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.As(V) adsorption using magnetic composite beads [62].

Thermodynamic of Cd(II) adsorption
To determine the thermodynamic properties of the Cd(II) adsorption onto Fe 3 O 4 @AMPA@SA, energy changes such as Gibbs free energy change (ΔG°) Equation ( 13), enthalpy (ΔH°) Equation ( 14), and entropy change (ΔS°) was analyzed: 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 Fe 3 O 4 @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].

Desorption study
Recycling performance is an important index for evaluating an adsorbent.To achieve the reuse performance of Fe 3 O 4 @AMPA@SA nanocomposite, 0.01 M HNO 3 and 0.01 M HCl solution were applied as eluents for recovery of Cd(II) from Fe 3 O 4 @AMPA@SA adsorbent.The outcomes revealed that the removal recovery of Cd(II) using 0.01 M HNO 3 (91%) was better than HCl (79%).The reusability of Fe 3 O 4 @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 Fe 3 O 4 @AMPA@SA nanocomposite as reusable adsorbent (Figure ( 6)).MBCI 197.96 [47] rGO-PDTC/Fe 3 O 4 116.28 [50] M-Fe 3 O 4 -GO 125 [51] NH 2 -MS 51.81 [52] Mag@Fh-P 64.1 [53] Magnetic ferrites 44.13 [54] MGO 91.29 [55] Fe 3 O 4 @CdTe-IIP 17.57  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 Fe 3 O 4 @AMPA@SA nanocomposite [29].All these changes indicate the adsorption of Cd(II) onto the surface of Fe 3 O 4 @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.

Conclusion
In this study, a new magnetic nanocomposite (Fe 3 O 4 @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 Fe 3 O 4 @AMPA@SA nanocomposite was 12.6 nm.The adsorption of Cd(II) onto Fe 3 O 4 @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.Fe 3 O 4 @AMPA@SA nanocomposite shows excellent reusability with a removal efficiency of 81.4% after three cycles.In conclusion, the characteristic advantages of use Fe 3 O 4 @AMPA@SA nanocomposite show great potential for use as an effective adsorbent for the removal of toxic Cd(II)) from aqueous media.

Figure 7
Figure7exhibits the mechanism of Cd(II) adsorption onto Fe 3 O 4 @AMPA@SA nanocomposite.It can be observed that a large number of S-H, -COOH, and -CONH functional groups incorporating on the surface of Fe 3 O 4 @AMPA@SA nanocomposite.Isotherm, kinetic adsorption, thermodynamic parameters and FTIR spectrum were used to evaluate the mechanism of Cd(II) adsorption.As shown in Figure2(b), after Cd(II) adsorption onto Fe 3 O 4 @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 Fe 3 O 4 @AMPA@SA nanocomposite[29].All these changes indicate the adsorption of Cd(II) onto the surface of Fe 3 O 4 @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.
Table2summarizes the analysis of various parameters of kinetic models.The fitting curves are presented in Figure5(d).As shown in Table2, the R 2 of the PFO kinetic model (0.9625) were much better than those of PSO (0.9308) and EKM (0.875).

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