One-Pot synthesis, characterization and adsorption studies of amine-functionalized magnetite nanoparticles for removal of Cr (VI) and Ni (II) ions from aqueous solution: kinetic, isotherm and thermodynamic studies

Background Discharge of heavy metals such as hexavalent chromium (Cr (VI)) and nickel (Ni (II)) into aquatic ecosystems is a matter of concern in wastewater treatment due to their harmful effects on humans. In this paper, removal of Cr (VI) and Ni (II) ions from aqueous solution was investigated using an amino-functionalized magnetic Nano-adsorbent (Fe3O4-NH2). Methods An amino-functionalized magnetic Nano-adsorbent (Fe3O4-NH2) was synthesized by compositing Fe3O4 with 1, 6-hexanediamine for removal of Cr (VI) and Ni (II) ions from aqueous solution. The adsorbent was characterized by Scanning Electron Microscope (SEM), Transmission Electron Microscopy (TEM), powder X-Ray Diffraction (XRD), and Vibrating Sample Magnetometry (VSM). Also, the effects of various operational parameters were studied. Results According to our finding, Fe3O4-NH2 could be simply separated from aqueous solution with an external magnetic field at 30 s. The experimental data for the adsorption of Cr (VI) and Ni (II) ions revealed that the process followed the Langmuir isotherm and the maximum adsorption capacity was 232.51 mg g−1 for Cr (VI) at pH = 3 and 222.12 mg g−1 and for Ni(II) at pH = 6 at 298 °K. Besides, the kinetic data indicated that the results fitted with the pseudo-second-order model (R2: 0.9871 and 0.9947 % for Cr (VI) and Ni (II), respectively. The results of thermodynamic study indicated that: standard free energy changes (ΔGɵ), standard enthalpy change (ΔHɵ), and standard entropy change (ΔSɵ) were respectively −3.28, 137.1, and 26.91 kJ mol−1 for Cr (VI) and −6.8433, 116.7, and 31.02 kJ mol−1 for Ni (II). The adsorption/desorption cycles of Fe3O4-NH2 indicated that it could be used for five times. Conclusions The selected metals’ sorption was achieved mainly via electrostatic attraction and coordination interactions. In fact, Fe3O4-NH2 could be removed more than 96 % for both Cr (VI) and Ni (II) ions from aqueous solution and actual wastewater.


Background
Discharge of heavy metals into aquatic ecosystems is a matter of concern in wastewater treatment due to their harmful effects on humans even at low concentrations [1,2]. Among heavy metals, Cr (VI) is among the toxic elements that may enter the environment due to effluent discharge by some industries, such as tanning, textile, wood preservations, paint, metal and mineral processing, pulp, and paper industries [3,4]. Evidence has shown that these elements can be carcinogenic and mutagenic to living organisms [5]. Nickel is also another heavy metal used in different industries, such as porcelain enameling, electroplating, storage batteries, dying, steel manufacturing, and pigment industries. The acceptance tolerance of nickel has been reported to be 0.01 mgL −1 and 2.0 mgL −1 in drinking water and industrial wastewater, respectively [6]. Due to the problems remarked above, some effective wastewater treatment approaches have to be employed for Cr (VI) and Ni (II) removal. Up to now, many methods have been used in this regard, including chemical precipitation, ion exchange, membrane technologies, coagulation, electrocoagulation, reduction, bio sorption, filtration, adsorption, reverse osmosis, foam flotation, granular ferric hydroxide, electrolysis, and surface adsorption [7][8][9][10][11]. Most of these methods have economic and technical disadvantages and could not achieve the discharge standards. Yet, adsorption is an effective and flexible method, generating high-quality treated effluent [12]. Until now, many adsorbents have been grown, including maple sawdust, walnut, hazelnut, almond shell [2], carbon nanotubes [13], aminofunctionalized polyacrylic acid (PAA) [14], and Lewatit FO36 Nano [15]. However, in many cases, these materials do not have the sufficient adsorption efficiency because of not having enough active surface sites. Furthermore, these materials have a lot of problems, including high cost, difficulty in separation, desorption, and regeneration of adsorbents, and secondary wastes. Therefore, new materials, such as various functional groups, including amide, amino groups, and carboxyl, are to develop new adsorbents that have high selectivity toward toxic metals [16][17][18]. In this respect, amino-groups have attracted more attention as chelation sites due to their large specific surface areas. Thus, amino-groups are capable of adsorbing a number of metal anions and cations from aqueous solution [19].
As described above, due to the high specific surface area created through grafting of appropriate organic amino-groups on inorganic magnetic Fe 3 O 4 particles, with strong magnetic properties, low toxicity, and easy separation, it could be used as a sorbent for removing heavy metals [18,20]. Another advantage is that it is useful for recovery or reuse of the magnetite nanoparticles modified with amino-groups [17,21].
In this study, we prepared a novel amino-functionalized magnetic Nano-adsorbent (Fe 3 O 4 -NH 2 ) developed by grafting amino-groups onto the surfaces of Fe 3 O 4 nanoparticles and used nanocompostie as the adsorbent for removal of Cr (VI) and Ni (II) from aqueous solution. The adsorbent was characterized by Transmission Electron Microscopy (TEM), powder X-Ray Diffraction (XRD), Vibrating Sample Magnetometry (VSM), Scanning Electron Microscope (SEM), and zeta-potential measurement. The effects of pH, initial concentrations of Cr (VI) and Ni (II), adsorption kinetics, thermodynamics, and adsorption isotherm were studied, as well.

Synthesis of amino-functionalized magnetic Nano-adsorbent (Fe 3 O 4 -NH 2 ) by one-pot synthesis
Amino-functionalized magnetic Nano-adsorbent (Fe 3 O 4 -NH 2 ) was prepared according to hydrothermal reduction method. In doing so, a solution of 1, 6-hexanediamine (13 g), anhydrous sodium acetate (4.0 g), and FeCl 3 · 6H 2 O as a single Fe ion source (2.0 g) was added to ethylene glycol (80 mL). The above mixture was stirred at 50°C under vigorous stirring for 30 min. Then, this solution was heated at 198°C in a Teflonlined autoclave for 6 h. Thereafter, the mixture was cooled down to room temperature. The magnetite nanoparticles were collected with a magnet and were then washed with water and ethanol (3 times) to effectively remove the solvent and unbound 1, 6-hexanediamine. Finally, the amino-functionalized magnetic Nano-adsorbent (Fe 3 O 4 -NH 2 ) was dried in a vacuum oven at 50°C before characterization and application [22]. The size and morphology of the Fe 3 O 4 -NH 2 were showed by SEM (Holland, company: Philips). Besides, the magnetic property (M-H loop) of the typical magnetic nanoparticles bound with 1, 6-hexadiamine at 25°C was characterized by VSM (MDKFD, Iran). The crystal structure and phase purity of Fe 3 O 4 -NH 2 were also examined by XRD (Philips, Holland) using Cu Kα radiation (λ = 0.1541 nm) at 2 θ , 30 kV, and 30 mA. Finally, the TEM image of Fe 3 O 4 -NH 2 was examined using TEM, Model EM10C-100KV (Zeiss, Germany).

Adsorption experiments
The absorption experiments were conducted in 1000 ml Erlenmeyer flasks containing 50 ml Ni (II) and Cr (VI) solutions at 5 to 100 mg L −1 concentrations and 0.05 g of Fe 3 O 4 -NH 2. The mixtures were stirred (200 rpm) at room temperature from 10 to 90 min. After adsorption, Fe 3 O 4 -NH 2 with adsorbed Cr (VI) and Ni (II) was separated from the solution under the external magnetic field. The concentrations of Cr (VI) and Ni (II) ions in the solutions were measured by an Inductive Coupled Plasma (ICP-OES, Spectro arcos, Germany (Company: SPECTRO)).
In order to determine the effects of various factors, the experiments were performed at different Fe 3 O 4 -NH 2 doses (0.1 to 0.3 g/L), initial concentrations of Cr (VI) and Ni (II) (5 to 100 mg/L), and temperatures (298.15 to 338.15°K). Besides, each experiment was carried out in duplicate. The removal of Cr (VI) and Ni (II) by Fe 3 O 4 -NH 2 and removal efficiency have been figured by equations in Table 1 [18].

Characterization of Fe 3 O 4 -NH 2
The SEM, VSM, XRD, and TEM of Fe 3 O 4 -NH 2 were recorded. The SEM of Fe 3 O 4 -NH 2 image has been shown in Fig. 1. Based on the results, the SEM image indicated that the size of Fe 3 O 4 -NH 2 was much smaller than that of naked particles, confirming the coating of 1, 6 hexanediamine [18].
The magnetic hysteresis loops measured at room temperature has been illustrated in Fig. 2 Fig. 3, the magnetic Fe 3 O 4 -NH 2 was dispersed in water. In addition, it could be collected by external magnetic field and be re-dispersed through slight shaking, making the solid and liquid phases separate easily.
The particle size was obtained via XRD analysis through Debye-Sherrer's formula [23] Where λ is the wavelength of the X-rays, θ is the diffraction angle, and β is the corrected full width. The result of size distribution demonstrated that the size of the prepared Fe 3 O 4 -NH 2 was under 90 nm. Additionally, the sharp and strong peaks of the products revealed its appropriate crystallinity. Moreover, the six characteristic peaks of Fe 3 O 4 showed that amino-groups did not cause any measureable alter in the phase property of Fe 3 O 4 cores. Therefore, the amino-groups were fixed on the surface of Fe 3 O 4 cores, making a core-shell structure. In other words, binding and amino-functionalization (NH 2 ) occurred only on the surface of Fe 3 O 4 cores to form a core-shell structure [22].
The TEM image of Fe 3 O 4 -NH 2 has been shown in Fig. 5. Accordingly, Fe 3 O 4 -NH 2 particles were multidispersed with an average diameter of around 25 nm. It has been reported that magnetic particles of less than 30 nm would show paramagnetism [24].

The effect of initial concentration and pH on the adsorption properties and zeta potential analyses
The effect of initial concentration on the adsorption properties was intensively studied for Fe 3 O 4 -NH 2 by varying C 0 of Cr (VI) and Ni (II) ions at 5, 25, 50, and 100 mg L −1 . The results have been presented in Figs. 6 and 7. Under corresponding pH values from 2.0 to 9.0, the adsorption efficiency of Cr (VI) and Ni (II) respectively decreased and increased with increase in the initial Cr (VI) and Ni (II) concentrations. Accordingly, the percentage of uptake of Cr (VI) and Ni (II) ions at the Fe 3 O 4 -NH 2 concentration of 5 mg L −1 decreased from 98.02 to 36.85 % for Cr (VI) and increased from 46.21 to 93.03 % for Ni (II) with increasing the pH from 2.0 to 9.0. This can be justified by the fact that for a fixed adsorbent dosage, the total available adsorption sites would be relatively settled. Thus, increasing the initial Cr (VI) and Ni (II) concentrations led to a decrease in the adsorption percentage of the adsorbate [25]. Table 1 The kinetic, isotherm, and thermodynamic equations used for adsorption of Cr (VI) and Ni (II) onto Fe 3 O 4 -NH 2

Kinetic models
Isotherm equations Thermodynamic equations Removal efficiency and equilibrium adsorption capacity Ref.
Langmuir adsorption model [27,31,[33][34][35][36][37]39] Separation factor (R L ) R L = 1/(1 + K L C o ) [18,25,26] qe (mg/g), K 1 (1/min), K 2 (g/mgmin), qm (mg/g), K F [(mg g −1 ) (mgL −1 ) n], Kp (mg/g min -0. 5  To assess the effect of pH, the study was conducted from pH 2 to 9 for both Cr (VI) and Ni (II) ions. The maximum sorption was perceived at pH = 6 for Ni (II), but at pH = 3 for Cr (VI). The adsorption of Cr (VI) at lower pH levels was also observed in other magnetic materials, such as the mesoporous magnetic ɤ-Fe 2 O 3 [26]. pH value affected the adsorption efficiency due to its influence on the amino-groups modified on the surface of Fe 3 O 4 -NH 2 . The plot of pH initial vs. pH final depicted that the pHzpc was 5.8 for Fe 3 O 4 -NH 2 . Hence, at pH >5.8, the surface charge of Fe 3 O 4 -NH 2 was negative and the electrostatic interactions between the metal ions and the adsorbent enhanced. Considering Ni (II), the interaction between the adsorbents and the Ni (II) ions might be defined by Equations 1-5 [14,27].
R À NH 2 OH À þ Ni 2þ ↔R À NH 2 OH À …Ni 2þ ð4Þ The protonation/deprotonation reactions of the Fe 3 O 4 -NH 2 amino-groups in the solution have been presented in Equation 1. Based on Equation 2, the ability of NH 2 to be protonated was weakened at higher pH levels, resulting in more -NH 2 on the surface of the adsorbent to coordinate with Ni (II). At higher pH levels, OH − in the solution is competitively adsorbed by amino-groups (−NH 2 ), and the electrostatic adsorption is prevailed gradually compared to coordination. Considering Cr (VI), a large number of H + exists under acidic conditions (pH levels: 2-3.5), causing amino-groups (−NH 2 ) to be protonated to NH 3+ more easily and electrostatic attraction to occur between these two oppositely charged ions (Equation (6)) [14,27].
Kinetic, equilibrium, and thermodynamic studies Adsorption isotherms of Fe 3 O 4 -NH 2 were gained at pH = 3 for Cr (VI) and pH = 6 for Ni (II) with the initial concentrations of 5 to 100 mg L −1 . The relevant equations for kinetic, equilibrium, and thermodynamic studies have been shown in Table 1 [18]. Besides, the Langmuir and  Freundlich isotherm [18]. The value of 1/n reported in Table 2 was less than 1; hence, adsorption by the Freundlich model was unfavorable. Kinetics of the adsorption process is essential for aqueous solution since it gives essential information on the rate of adsorbate uptake on the adsorbent and controls the equilibrium time. The results presented in Table 2 indicated that the adsorption capacity of Fe 3 O 4 -NH 2 for Cr (VI) and Ni (II) was high (q m for Ni (II) = 232.51 mg/g −1 at pH = 6 and q m for Cr (VI) = 222.12 mg/g −1 at pH = 3) compared to other adsorbents. Afkhami et al. also reported that the adsorption capacity of DNPH-γ-Al 2 O 3 for Ni (II) was 18.18 (mg g −1 ) at pH = 5 and that the process followed the Langmuir isotherm [26]. In another study, the experimental data for the adsorption of Ni (II) on Fe 3 O 4 -GS revealed that the process followed the Langmuir isotherm and that the maximum adsorption capacity was 158.5 mg g −1 at pH = 6 [28]. The parameters of the pseudo-first-order and pseudo-second-order sorption kinetic models have been presented in Table 3. In order to evaluate the applicability of these kinetic models to fit the experimental data, K 1 and K 2 constants were determined experimentally from the slope and intercept of straight-line plots.  [18]. In addition, the slope and intercept of the plot of lnK vs. 1/T indicated the ΔH θ and ΔS θ values [18]. The values of standard enthalpy change (ΔH θ ) and standard entropy change (ΔS θ ), which were related to distribution coefficient (K D ), were calculated and presented in  [25]. Hence, the results indicated that adsorption of Ni (II) on DETA-NMPs could be followed spontaneously, was endothermic, and was entropy favored in nature [25]. One other study also reported that ΔG, ΔH, and ΔS were −1.599, 8.438, and 83.1, respectively for Ni (II) adsorption on Nano-HAP [30]. Overall, simple preparation, fast separation, and high adsorption capacity of Fe 3 O 4 -NH 2 make it a potential  If: RL > 1, the adsorption is unfavorable. RL = 1, the adsorption is linear. 0 < RL < 1, the adsorption is favorable. RL = 0, the adsorption is irreversible. [17,44,45] If: 1/n < 1, the adsorption is unfavorable. If: 0.1 < 1/n < 1, the adsorption is favorable. [46] Tem.  If: RL > 1, the adsorption is unfavorable. RL = 1, the adsorption is linear. 0 < RL < 1, the adsorption is favorable. RL = 0, the adsorption is irreversible. [17,44,45] If: 1/n < 1, the adsorption is unfavorable. If: 0.1 < 1/n < 1, the adsorption is favorable. [46] applicant for Cr (VI) and Ni (II) removal. Considering the larger adsorption capacity of Fe 3 O 4 -NH 2 attributed to the amino-groups modified on the surface of Fe 3 O 4 -NH 2 , the amino-groups played a very important role in the adsorption process of Cr (VI) and Ni (II) in aqueous solution. This indicated that the increase of nitrogen percentage in Fe 3 O 4 -NH 2 could result in an increase in the value of q m . Similar results were also obtained by Shen et al. [27] and Zhao et al. [25].

Conclusion
In this study, Fe 3 O 4 -NH 2 was prepared using a simple, cost-effective, and environmentally friendly method for the removal of Cr (VI) and Ni (II) ions from aqueous solution and was characterized by SEM, TEM, XRD, and VSM. The effects of controlling parameters, such as contact time, temperature, pH, Fe 3 O 4 -NH 2 dose, and initial concentration of both heavy metals, were studied, as well. Based on the results, the Langmuir model fitted the isotherm data for both heavy metals and the maximum sorption capacity was 232.51 mg g −1 at pH = 3 for Cr (VI) and 222.12 mg g −1 at pH = 6 for Ni (II). Moreover, the adsorption kinetic data for Cr (VI) and Ni (II) were based on the assumption of a pseudo-second-order model and thermodynamic parameters showed that the adsorption process was endothermic, spontaneous, and entropy favored in nature. In addition, this nanoadsorbent was able to remove over 96 % of both heavy metals from tap water and industrial wastewater. The Fe 3 O 4 -NH 2 could be regenerated with acid after adsorption and the adsorption capabilities only decreased with 6-7 % for both metal ions after five cycles. Overall, this study indicated that an amino-functionalized magnetic nano-adsorbent was promising for removal of Cr (VI) and Ni (II) ions in field application.

Highlights
A sensitive method was developed for removal of Cr (VI) and Ni (II) from aqueous solution.
In-lab synthesized magnetic nanoadsorbent was developed by grafting amino-groups onto the surfaces of Fe 3 O 4 nanoparticles.
The effects of pH, initial concentrations of Ni (II) and Cr (VI), adsorption kinetics, thermodynamics, and adsorption isotherm were studied.