Amine-Terminated Modified Succinic Acid-Magnetite Nanoparticles for Effective Removal of Malachite Green Dye from Aqueous Environment

Abstract

In this study, amine-terminated succinic acid-modified magnetic nanoparticles (MSA@TEPA) have been successfully synthesized using a facile two-step procedure as a new effective adsorbent for the removal of malachite green from aqueous solutions. The MSA@TEPA was characterized by scanning electron microscopy (SEM), Fourier-transform infrared spectroscopy (FTIR), zeta potential, thermal gravimetric analysis (TGA), and X-ray diffraction (XRD) analysis. The parameters influencing the adsorption capacity of MSA@TEPA, such as pH (3–8), contact time (t: 5–480 min), initial concentrations of MG dye (C_o: 20–200 mg/L), and adsorbent mass (0.05–0.5 g), were evaluated. It was observed that, under specified experimental conditions (C_o: 25 mg/L, pH: 7.1, T: 298 K, agitation rate: 100 rpm, and t: 420 min), the MSA@TEPA nanocomposite exhibits excellent adsorption efficiency (97.74%) for MG dye. The adsorption kinetics follow the PSO model, and the equilibrium data were fitted to the Langmuir isotherm with a maximum adsorption capacity of up to 282.65 mg/g. The thermodynamic parameters indicated that the adsorption process of MG dye was an exothermic process. After five consecutive cycles, MSA@TEPA nanocomposite still show good adsorption efficiency for MG dye. It is assumed that, because of the presence of amine group, adsorption mainly occurred through electrostatic interaction and H-bonding. In conclusion, the study shows a new and effective adsorbent with high adsorptive capacity, easy magnetic separation using an external magnetic field, and reusability for MG dye elimination from aqueous solutions.

1. Introduction

Synthetic organic dyes are the most harmful water pollutants to the environment and human health because of their carcinogenic, mutagenic, and teratogenic behavior [1]. Different dyes are extensively used as colors in the paper, textile, leather, cosmetic, and plastic industries [2,3,4]. It is estimated that around 7 × 10⁵ tons of dyestuff are manufactured each year and about 15% of the organic dyes are discharged into the aqueous environment, causing potential health hazards [3,5,6]. Malachite Green (MG, C₂₃H₂₅N₂Cl) is a cationic dye generally applied for coloring cotton, silk, leather, and paper products. It is also employed in aquaculture as a fungicide. By nature, MG is hazardous and toxic to human beings and other living organisms and its presence in a water environment causes damage to the liver, kidney, and respiratory system. Furthermore, it causes carcinogenic, mutagenic, and teratogenic effects on human cells [7,8,9]. Due to its noxious effects on living beings, it is therefore necessary to remove MG dye from wastewater.

Various physicochemical and biological techniques such as precipitation, membrane processes, coagulation, photocatalytic degradation, adsorption, chemical oxidation, ion exchange, and so on, have been applied for the elimination of dyes from contaminated water [10,11,12]. Amongst these, the adsorption technique has received significant attention from researchers because it is promising, efficient, low-cost, eco-friendly, and economically viable for water treatment [11,13,14,15]. Various adsorbent materials, including polymers [16], zeolites [17], activated carbon [18], metal-organic frameworks (MOF) [19], mesoporous silica [20], high-availability sustainable biomass (rock melon skin (RMS) [21] and Artocarpus odoratissimus leaves [22]), and cellulosic biochar [23] have been used for the elimination of dyes from wastewater. But, owing to their small size, some of these adsorbents suffer from the difficulty of separating them from aqueous solutions. To solve this problem, separation using a magnetic nanoparticle adsorption technique has attracted a lot of attention due to its remarkable properties of being a rapid and effective method, its biocompatibility, high reusability, as well as its high efficiency after regeneration cycles [24]. For example, iron oxide (magnetite) nanoparticles, with a large specific surface area and high magnetic nature, can be used as an easy-to-isolate adsorbent from aqueous solutions via a magnet and are of good chemical stability [25]. However, magnetic nanoparticles have disadvantages, such as the fact that they are unstable under acidic conditions, oxidize in air, and aggregate due to the strong dipole–dipole magnetic attraction between nanoparticles. To overcome these problems, large numbers of compounds with magnetic natures and different functional groups have been developed. Various magnetic nanocomposites, such as cobalt ferrite silica magnetic nanocomposite [26], Fe₃O₄@chitosan@ZIF-8 [27], magnetic biochar [28], poly(methyl methacrylate)/graphene oxide-Fe₃O₄ [28], MgO/Fe₃O₄ nanoparticles [29], and sodium alginate-coated Fe₃O₄ nanoparticles [30], were used to remove MG from aqueous solutions.

Succinic acid (C₄H₆O₄, SA) contains a dicarboxylic group that can serve as an active site for heavy metal ions adsorption [31,32]. The modification of magnetite nanoparticles using succinic acid (SA) leads to an improvement in the stability of magnetite nanoparticles and increases the pollutant’s adsorption. Various amine-based sorbents, such as amino functionalized graphenes [33], polyaniline (PANI)–Nickel ferrite (NiFe₂O₄) [34], and 3-MPA@PMNPs [35], have been developed for the removal of MG dye from waste water. The presence of amine groups in the structure of magnetic nanocomposites provide the desired environment for the adsorption of metal ions [36], anionic metal species [37], and dyes [38]. Patil et al. prepared polyaniline–nickel ferrite magnetic nanocomposite as an adsorbent for the removal of MG dye from an aqueous solution. They found that the adsorption capacity was 4.09 mg/g at an optimum condition of 30 mg/L [34]. Hu et al. studied the abilities of Fe₃O₄@SiO₂ and Fe₃O₄@SiO₂-NH₂ for the removal of MG dye and found that Fe₃O₄@SiO₂-NH2 showed a good removal efficiency of MG dye (90%) compared with Fe₃O₄@SiO₂ (60%) [39]. Tetraethylenepentamine (TEPA) is an organic polyamine molecule that contains five amino groups (two primary and three secondary functional groups) on its structure, which can serve as a binding site for metal ions [40], dyes [41], and CO₂ [42]. Ghasemi et al. used TEPFRCA and obtained the maximum adsorption of MG dye from the solvent phase. They reported a maximum adsorption capacity of 333.3 mg/g through the fast adsorption of MG dye from the solvent phase [43].

To the best of our knowledge, this was the first report of the design and synthesis of amine-terminated succinic acid-modified magnetic nanoparticles (MSA@TEPA) as an effective adsorbent using a facile two-step procedure. Firstly, magnetite nanoparticles functionalized with succinic acid were prepared using an in situ co-precipitation method and, next, were subsequently modified with tetraethylenepentamine (TEPA) via an amidation reaction. Then, the obtained MSA@TEPA product was characterized through SEM, FTIR, TGA, Zeta potential, and XRD analysis. The MSA@TEPA was used as an attractive adsorbent material for MG dye removal. The different parameters that influence adsorption processes, including, adsorbent dosage, initial MG concentration, solution pH, contact time, and temperature, were investigated. In addition, the adsorption nature was investigated through kinetic, isothermal, and thermodynamic parameters studies. Finally, this study provides a new effective magnetic nanocomposite as an adsorbent with high adsorptive capacity, easy magnetic separation using an external magnetic field, and reusability for MG dye removal from aqueous solutions

2. Materials and Methods

2.1. Materials

Tetraethylenepentamine (C₈H₂₃N₅, ≥97%), malachite green hydrochloride, ferric chloride (FeCl₃.6H₂O), ferrous chloride (FeCl₂.4H₂O), and 1-ethyl-3-(3 dimethylaminopropyl) carbodiimide (EDC) were purchased from Sigma–Aldrich, USA, India. Succinic anhydride (C₄H₄O₃, ≥99%) was purchased from Panreac, UK. Sodium hydroxide (NaOH, 97.5%), Nitric acid (HNO₃, 69–71), Hydrochloric acid (HCl, 37%), and ammonium hydroxide (NH₄OH, 25%) were purchased from Merck, Germany.

2.2. Synthesis of Magnetic Nanocomposite (MSA@TEPA)

Magnetic nanocomposites (MSA@TEPA) were prepared via a facile two-step procedure involving preparing magnetic succinic acid (MSA) using an in situ co-precipitation method and its subsequent modification with high nitrogen content tetraethylenepentamine (TEPA). Firstly, the modified succinic acid magnetite nanoparticles were prepared as follows: 2.03 g of ferric salt and 0.746 g of ferrous salt were dissolved separately in deionized water (75 mL, 0.1 HCl) with a molar ratio of 2:1 under N₂ gas and magnetic stirring for 15 min, then the mixture was moved into a round bottom flask. After that, 0.5 g of succinic acid was added to the mixture of iron salts under nitrogen gas and magnetic stirring for 30 min, then the mixture was heated at 75 °C followed by the dropwise addition of NH₄OH (30 mL) under mechanical stirring for 2 h until the pH reached 10. After mechanical stirring for another 2 h, the black precipitate was separated using a magnet. The modified succinic acid magnetite nanoparticles formed were washed with deionized water (Scheme 1). Secondly, the obtained MSA nanocomposites were dispersed into a beaker containing 50 mL of ethanol, followed by 0.931 g of EDC addition to this solution under ultra-sonication for 40 min. After 40 min of stirring, 10 mL of TEPA was added dropwise into the MSA dispersion under magnetic stirring and nitrogen gas for 24 h. After the reaction completion, the obtained magnetic nanocomposites (MSA@TEPA) were collected through magnetic separation. Finally, the product was washed with de-ionized water and ethanol, and then dried at 60 °C for 24 h (Scheme 1).

2.3. Adsorption Experiments

The removal of MG dye from aqueous media by MSA@TEPA was studied through the batch elimination process. The different parameters that influence the efficiency of MG removal, including solution pH (3–8), contact time (t: 5–480 min), adsorbent mass (0.05–0.5 g), and initial concentrations of MG dye (C_o: 20–200 mg/L), were studied. Generally, 20 mg of MSA@TEPA was added into an Erlenmeyer containing 20 mL of MG dye solution (25 mg/L). Then, the sample pH was adjusted up to ~7.1 by NaOH/HCl solution. Next, the sample was shaken at 100 rpm for the prescribed time (420 min) and then isolated using a magnet. The residual concentration of MG dye in the supernatant was determined using a UV–Vis spectrophotometer (Thermo Scientific, Evolution 600, Waltham, MA, USA) at the wavelength of 617 nm. The equilibrium removal efficiency of MG dye and the adsorption capacity (q_e mg/g) were calculated using Equations (1) and (2), respectively.

$${\mathrm{R}}_{\mathrm{e}} \%=\frac{{\mathrm{C}}_{\mathrm{o}}-{\mathrm{C}}_{\mathrm{e}}}{{\mathrm{C}}_{\mathrm{o}}}\times 100$$

(1)

$${\mathrm{q}}_{\mathrm{e}}=\left({\mathrm{C}}_{\mathrm{o}}-{\mathrm{C}}_{\mathrm{e}}\right)\times \frac{\mathrm{V}}{\mathrm{m}}$$

(2)

where C_o and C_e (mg/L) represented the initial and final concentrations of MG dye, respectively, q_e (mg/g) is the adsorption capacity of MSA@TEPA toward MG dye, m (g) is the mass of MSA@TEPA, and V (L) is the MG solution volume.

3. Results

3.1. Characterization of MSA@TPEA

The FT-IR spectra of magnetite nanoparticles, TEPA, MSA, MSA@TPEA, and MG-loaded-MSA@TPEA, are displayed in Figure 1. The FT-IR spectra of MSA display broad bands at 3442, 2947, 2872, (1648, 1382 cm⁻¹) 1428, 1069, and 570 cm⁻¹ that were assigned to hydroxyl (-OH), asymmetrical and symmetrical of C-H, carboxyl (COOH), C-O, and Fe-O bonds, respectively, indicating that the successful modification of succinic acid onto magnetite nanoparticles [44]. For TEPA, the characteristics bands at (3272 and 3349 cm⁻¹), 1451, and 1602 cm⁻¹ were described as amino (NH₂) and C-N-C and N-H bonds onto the TEPA surface [45,46,47]. Compared with MSA, the band at 1632 cm⁻¹ was described as the C=O stretching vibration of -CONH- formed in the MSA@TPEA product. In addition, the bands at 2855 and 2925 cm⁻¹ (C-H stretching) became stronger in intensity, indicating the large numbers of -CH₂- introduced which proves that TEPA had been successfully bonded on the surface of MSA through the reaction between the carboxyl group of MSA and the amino group of TEPA [48,49]. The bands at 3435, 1483, 1260, 1057, and 570 cm⁻¹ were assigned to the NH/OH, C-N-C bond, C-N, C-O, and Fe-O bonds. Based on these results, it is concluded that TEPA was grafted onto the MSA nanocomposite. After MG adsorption, the bands of functional groups decreased in their respective intensities. The bands at 3435, 1631, and 1265 cm⁻¹ for ν(-OH/NH), -CONH-, and C-N were decreased in intensity and shifted to 3442, 1627, and 1251 cm⁻¹, respectively. Based on these results, it was concluded that MG dye was successfully adsorbed onto the MSA@TPEA surface.

The thermal stability curve of the prepared magnetite nanoparticles and MSA@TEPA nanocomposite is shown in Figure 2a. During the first step, the weight change in Fe₃O₄ and MSA@TEPA before 180 °C corresponds to the loss of moisture. The next two steps of weight loss up (15%) at 370 °C and (21.5%) at 750 °C were related to the decomposition of the organic molecules (SA and TEPA) from the magnetite surface. The total weight loss of MSA@TEPA and pure magnetite nanoparticles was 41.5% and 9%, respectively. The overall TGA results indicate that the modified ratio of TEPA and SA onto magnetite nanoparticles is about 32.5%, which confirmed the formation of MSA@TEPA synthesis.

Figure 2b shows the XRD pattern of the Fe₃O₄ and MSA@TEPA nanocomposite. The diffraction patterns of both particles show eight peaks at 2θ = 30.35°, 35.71°, 43.53°, 53.82°, 57.26°, 62.86°, and 74.54, corresponding to (220), (311) (400), (422), (511), (440), and (533), planes of cubic spinel Fe₃O₄. These results are in agreement with the magnetite’s reflections (JCPDS file No. 019-0629) [50]. When compared with Fe₃O₄, the peaks of magnetite nanoparticles still exist in the XRD pattern of MSA@TEPA as well as the intensities of peaks in MSA@TEPA being almost the same as the pure magnetite nanoparticles, suggesting that the modification of magnetite with SA and TEPA did not change the phase of Fe₃O₄ nanoparticles [51]. The mean crystallite sizes (D) of Fe₃O₄ nanoparticles and MSA@TEPA nanocomposites were calculated using the Scherer equation:

$$\mathrm{D}=\frac{\mathit{k}\mathit{\lambda }}{\mathit{\beta } \mathit{c}\mathit{o}\mathit{s}\mathit{\theta }}$$

(3)

where K is a constant (0.94), λ (λ = 0.154 nm) is the X-ray wavelength, and β and θ are the line broadening at half-maximum intensity (FWHM) and the Bragg angle, respectively. The mean crystallite size for Fe₃O₄ nanoparticles and MSA@TEPA nanocomposite were 11.8 and 13.07 nm, respectively. The morphology of MSA@TEPA before and after MG dye adsorption is shown in Figure 3. It can be seen that, before adsorption, the MSA@TEPA has an irregular rough surface (Figure 3a,b) while, after MG dye adsorption, the surface of MSA@TEPA seems to be smooth, indicating the successful adsorption of MG dye on the MSA@TEPA surface (Figure 3c).

3.2. Optimization of Adsorption Process Conditions

3.2.1. Effect of pH

It is known that the molecular structure of MG depends on the pH solution. In strongly basic conditions, the structure of malachite green converts to a carbinol base and loses its color owing to the reaction of OH with MG dye. Thus, the adsorption of MG by MSA@TEPA was investigated in the pH range from 3 to 8.1, as shown in Figure 4a. It can be seen that the efficiency of MG removal and adsorption capacity of MSA@TEPA was gradually increased from 28.82%, (9.01 mg/g) to 88.320%, (27.60 mg/g) with increasing solution pH from 3 to 6.1, respectively, and it reached a maximum value of 93.31%, (29.16 mg/g) at pH 7.1 [52]. The low MG removal at lower pH was assigned to the existence of protons (H⁺) which might be because of the electrostatic repulsion between cationic MG ions and the protonated MSA@TEPA surface. On the other hand, with the pH increase, the positive charge of amine groups on the adsorbent surface decreases, thus, the electrostatic interactions with cationic MG⁺ increase, which leads to an improvement in the efficiency of MG dye removal. The zeta potential results further support the pH results, as depicted in Figure 4a. It can be seen that the pH_PZC of MSA@TEPA was 6.7, indicating that the surfaces of MSA@TEPA are positively charged at pH < pH_PZC and negatively charged at pH > pH_PZC. The different researchers also reported that low pH favors high adsorption, e.g., the MG adsorption on TiO₂ nanoparticles/anthracite [53] and reduced graphene oxide [54] reached a maximum value at pH = 8 and 8, respectively.

3.2.2. Effect of Adsorbent Dosage

The influence of prepared MSA@TEPA mass on the efficiency of MG elimination was tested by varying the mass of MSA@TEPA between 0.005 and 0.05 g against a solution volume of 25 mL of 25 mg/L MG dye, as shown in Figure 4b. The efficiency of MG removal improved from 59.04% to 96.74% with an increase in the amount of MSA@TEPA nanocomposite from 0.005 to 0.02 g. Meanwhile, the adsorption capacity of MSA@TEPA toward MG reduces from 73.80 to 33.23 mg/g with the increasing amount of MSA@TEPA nanocomposite from 0.005 to 0.02 g. The reduction in the adsorption capacity of MSA@TEPA may be because of the aggregation of adsorption sites resulting in a decrease in the total adsorption surface area available for the MG dye [55]. Therefore, 0.02 g mass of adsorbent was selected as an optimum value for the rest of the experiments.

3.2.3. Effect of Contact Time

The influence of equilibrium time on the adsorption of MG onto MSA@TEPA was investigated at various shaking times in the range of 5 to 480 min. The time-dependence efficiency of MSA@TEPA for MG removal is given in Figure 4c. By increasing the time from 5 to 360 min, the MG removal and adsorption capacity efficiency gradually increases from 25.47% to 94.32%, respectively, then slowly decreases until it reaches an equilibrium at 360 min with maximum removal (97.74%) and adsorption capacity (61.09 mg/g). This might be due to the sufficient time interaction between the MSA@TEPA adsorbent and the MG dye molecules [56]. At 5 min, the MG dye removal efficiency was 25.47% while, at the equilibrium time of 30 min, this reached a maximum value of 46.19%. As already reported, the MG adsorption on the AC@ATPA composite [57] and Fe₃O₄@ATPA@AMPA [58] reached maximum adsorption capacity values of 14 mg/g and 58.9 mg/g at equilibration times of 240 and 120 min, respectively. Thus, the optimum contact time was selected to be 420 min.

3.2.4. Effect of Initial Concentration and Temperature

The effect of MG concentration on the adsorption process was investigated at different initial concentrations between 20 and 200 mg/L under specific experimental conditions (m: 0.02 g; pH; 7.1; time: 420 min; T: 298 K; and agitation rate: 100 rpm), as displayed in Figure 4d. It can be seen that, with an increase in the initial concentration of MG dye from 25 to 350 mg/L, the MG removal efficiency decreases from 97.11% to 59.90%, while the adsorption capacity of MSA@TEPA toward MG dye increases from 29.72 to 262.06 mg/g. The increase in adsorption capacity with the increase in the initial MG concentration is because the higher the initial concentration of MG dye, the more collisions with active adsorption sites on the adsorbent surface, as well as the increase in the driving force of mass transfer leading to an increase in the adsorption capacity. At 25 mg/L, the MG dye removal efficiency was 97.11% while, at 100 mg/L, this reached the maximum value of 90.02%. The maximum adsorption capacity on the adsorbent was 262.06 mg/g at 25 °C. Figure 4d also reveals that, at higher temperatures, the sorption capacity is low. This means that the adsorption of MG dye on the adsorbent is an exothermic process.

3.3. Adsorption Modeling

3.3.1. Adsorption Isotherm

To better understand the maximum adsorption capacity of MSA@TEPA toward MG dye and also to study the mechanism of binding between MG dye and MSA@TEPA, different isotherm models, like the Langmuir [59], Freundlich [60], and Dubinin–Radushkevich (D-R) [61] isotherm models, were used. The nonlinear forms of these models are provided in Supplementary Information, Text S1.

Table 1. The values of parameters for each isotherm model.

Model	MG Dye
	298 K	308 K	318 K
Langmuir
qm, mg/g	282.65	267.6503	240.50
KL (L/mg)	0.071	0.042	0.029
RL	0.360	0.487	0.579
R2	0.99314	0.98715	0.99509
Freundlich
Kf, (mg/g) (L/mg)1/n	51.51	36.39	26.11
n	2.91	2.62	2.46
R2	0.96181	0.96088	0.96888
Dubinin–R
qs, mg/g	232.9	205.0	179.24
KD-R (mol2 KJ−2)	27.28	49.78	80.55
E (kJ mol−1)	0.135	0.100	0.053
R2	0.8876	0.82899	0.87887

gives a comparison between these isotherm models, and the obtained graph is shown in Figure 5a–c. The R² of Langmuir model (R² = 0.99314) is higher than that of the Freundlich (R² = 0.96181) and Dubinin–Radushkevich (R² = 0.8876) models, indicating that the MG adsorption onto MSA@TEPA follows the Langmuir isotherm model with qe (282.65 mg/g). This indicates a monolayer homogenous adsorption of the MG molecules on the MSA@TEPA surface. All these results indicate that our prepared MSA@TEPA adsorbent showed good adsorption capacity for MG when compared with other various magnetic adsorbents like magnetic hydroxyapatite nanoparticles (208.06 mg/g) [62], MGO@Cellulose@Lipase (51.87 mg/g) [63], GO/Fe₃O₄ (96.9 mg/g) [64], and magnetic cobalt oxide nanoparticles (238.10 mg/g) [65].

3.3.2. Adsorption Kinetic

To understand the MG adsorption mechanism on the MSA@TEPA surface, the experimental data were assessed using nonlinear kinetic models like the pseudo-first-order (PFO) [66], pseudo-second-order (PSO), and Elovich models [67]. These kinetic parameter models are given in Supplementary Information, Text S2. The adsorption kinetic parameter values of MG dye onto the MSA@TEPA surface are illustrated in

Table 2. The values of parameters for each kinetic model.

Co (mg/L)	qe,exp. (mg/g)	Pseudo-First-Order			Pseudo-Second-Order			Elovich
		qe,cal. (mg/g)	K1 (1/min)	R2	qe2,cal. (mg/g)	K2 (g/mg-min)	R2	A (mg/g min)	B (mg/g)	R2
25	61.23	57.21	0.023	0.9448	63.36	0.00053	0.9871	6.42	0.089	0.9731

, and the obtained graph is shown in Figure 5d. The correlation coefficients for the PSO model (R² = 0.9871) are greater than that of the Elovich (R² = 0.9731) and PFO (R² = 0.9448) models, indicating that the MG adsorption follows the PSO kinetic model. Furthermore, the calculated q_e,exp. (mg/g) value is close to the PSO model q_e,cal. (mg/g) value, suggesting that the adsorption process seems to be a chemisorption process, which might result from the electrostatic attraction between the MG dye and the functional groups of the MSA@TEPA nanocomposite [68,69].

3.3.3. Adsorption Thermodynamics

The data obtained from the adsorption isotherm process were analyzed to assess the thermodynamic parameters, namely the standard entropy change (ΔS°), standard enthalpy change (ΔH°), and Gibbs energy (ΔG°). The following equations were used to calculate these parameters (4)–(6) [70]:

$$∆\mathit{G}°=-\mathit{R}\mathit{T} \mathit{l}\mathit{n}\mathit{K}\mathit{c}$$

(4)

$${\mathit{K}}_{\mathit{c}}=\mathit{M}.\mathit{w}\mathit{t}\times 1000\times {\mathit{K}}_{\mathit{L}}$$

(5)

$$\mathit{l}\mathit{n}\mathit{K}\mathit{c}=\frac{∆\mathit{S}°}{\mathit{R}}-\frac{∆\mathit{H}°}{\mathit{R}\mathit{T}}$$

(6)

where K_L (L/g) is the Langmuir model constant, K_C referred to the adsorption equilibrium constant as calculated from Equation (4) [70], and M.wt is the molecular weight of MG dye. The corresponding thermodynamic parameter values are listed in

Table 3. Thermodynamic adsorption parameters for the adsorption of MG dye on MSA@TEPA.

ΔH° (kJ/mol)	ΔS° (J/mol.K)	ΔG° (kJ/mol)
		298 K	308 K	318 K
−34.76	−32.29	−25.19	−24.71	−24.56

. The negative ΔG° values obtained for the adsorption of MG dye at all studied temperatures (298–318 K) indicate the spontaneity of the adsorption process. The negative ΔH° value (−34.76 (kJ/mol) suggests the exothermic nature of the adsorption process. Furthermore, the value of enthalpy changes also shed light on the type of adsorption mechanism, as for physisorption (ΔH° < 20 kJ/mol), for electrostatic interaction (20 < ΔH° < 40 kJ/mol), and for chemisorption (80 < ΔH° < 450 kJ/mol) [71]. As in our case, the value of ΔH° was −34.76 (kJ/mol), suggesting the electrostatic interaction of MG on the MSA@TEPA surface [71,72]. The negative values of ΔS° indicate a degree of disorder of the adsorbed MG at the solid/liquid interface, which is a normal and general phenomenon.

3.4. Proposed Adsorption Mechanism

After careful mathematical and experimental calculations, an adsorption mechanism of MG on MSA@TEPA was proposed, as shown in Figure 6. As MSA@TEPA has five amino groups on its surface, it might have an affinity to adsorb MG dye by electrostatic interaction and H-bonding. The FT-IR analysis has affirmed that MSA@TEPA adsorbent contains groups like amino, hydroxyl, and amide groups and, after MG adsorption on MSA@TEPA, the intensities of most of the FT-IR bands were reduced and shifted to lower wave numbers. For example, the bands at 3435, 1631, and 1265 cm⁻¹ for ν(-OH/NH), -CONH-, and C-N were decreased in intensity and shifted to 3442, 1627, and 1251 cm⁻¹, respectively, indicating that some sort of interaction (the H- bonding and electrostatic interaction) was involved during MG adsorption onto MSA@TPEA. This also confirms the successful MG adsorption onto MSA@TPEA surface. According to the pH results, as pH increases, the positive charge of amine groups on the adsorbent surface decreases. Thus, the chances of electrostatic interactions with cationic MG⁺ increase, which lead to the MG dye removal efficiency. In conclusion, the interaction mechanism of MG onto the MSA@TEPA surface can be summarized as follows: (i) H- bonding between nitrogen of N(CH₃)₂ and -NH groups on the MSA@TEPA surface and (ii) electrostatic attraction between positive charges of ⁺N(CH₃)₂ on the MG dye surface and negative charges of amino groups on the MSA@TEPA surface.

3.5. Reusability of MSA@TEPA

To assess the reusability of MSA@TEPA nanocomposite, the removal of MG dye by MSA@TEPA was also tested over five consecutive cycles and with different eluents such as 0.01 M HCl, ethanol, and HCl/ethanol mixture. The results exhibited that the desorption efficiency was 32%, 73%, and 82% for HCl, ethanol, and HCl/ethanol mixture, respectively. Therefore, the eluent HCl/ethanol mixture was selected for subsequent experimental procedures. Figure 7 shows the reusability results of the MSA@TEPA nanocomposite. It was observed that the adsorption efficiency of MG was reduced to 61% after five consecutive adsorption–desorption cycles, which might be because of the depletion of adsorption active sites for MG and incomplete desorption of MG dye from MSA@TEPA.

4. Conclusions

In this study, amine-terminated modified succinic acid magnetite nanoparticles (MSA@TEPA) were prepared effectively using a simple, environmentally friendly co-precipitation method and were used as a new and efficient adsorbent for the elimination of malachite green from an aqueous medium. The removal of MG dye from aqueous media by MSA@TEPA was studied through the batch elimination process. MSA@TEPA were characterized using FTIR, XRD, and SEM techniques and the results showed successful modification of TEPA onto the MSA composite. MSA@TEPA exhibited good adsorption efficiency (97.74%) for MG under the specified experimental conditions (pH: 7.1, C_o: 25 mg/L, T: 298 K, and t: 360 min). Such a good adsorption efficiency was attributed to the presence of poly amino groups in the structure of MSA@TEPA. The Langmuir isotherm and PSO kinetic models were applied and seemed to be the best-fitted models for the adsorption process. Furthermore, the thermodynamic parameters showed that the adsorption of MG ions onto MSA@TEPA was an exothermic process and occurred through electrostatic interaction and H-bonding. After five consecutive cycles, the material still shows good adsorption efficiency (61%) making it a highly reusable absorbent for MG dye. Finally, based on the above results and discussion, it can be concluded that MSA@TEPA can function as highly efficient adsorbents for MG removal, making the process cost-effective, environmentally friendly, and sustainable.