Synthesis and characterization of Fe₃O₄@TSC nanocomposite: highly efficient removal of toxic metal ions from aqueous medium

Abstract

In the present study, trisodium citrate (TSC) modified magnetite (Fe₃O₄) nanocomposite was synthesized and characterized using various analytical techniques. The transmission electron microscope images show that the Fe₃O₄@TSC nanoparticles are well dispersed due to the presence of the TSC coating on Fe₃O₄, and the particles sizes are in the range of 5–10 nm. The S_BET and the total pore volume of Fe₃O₄@TSC are 245.42 m² g⁻¹ and 0.368 cm³ g⁻¹, respectively. The saturation magnetization values of Fe₃O₄ and Fe₃O₄@TSC are 78.4 and 55.4 emu g⁻¹, respectively. The Fe₃O₄@TSC is a magnetic adsorbent and was used for the removal of Cr³⁺ and Co²⁺ metal ions from aqueous medium. The adsorption of both metal ions onto Fe₃O₄@TSC is rapid and efficient. The adsorption process is performed at diverse temperatures and the outcomes are investigated kinetically. The results show that adsorption was exothermic and followed the pseudo-second-order kinetic model. Isotherm modelling reveals that the Langmuir equation described the adsorption of both metal ions. Moreover, the loaded Fe₃O₄@TSC could be recovered easily from aqueous solution by magnetic separation and regenerated by simply washing with 0.1 M HCl solution. Consequently, Fe₃O₄@TSC nanocomposite could be utilized as an efficient and recyclable adsorbent for the removal of toxic metal ions from aqueous solution.

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1. Introduction

Magnetic nanoparticles (MNPs) have been widely studied due to their many potential applications in the chemical, biological and medical sciences such as pre-concentration and separation of ions,^1–3 bioseparation^4–6 protein separation,⁷ DNA recovery,⁸ cancer treatment,⁹ biomedicine and targeted drug delivery.^10,11 MNPs are effective adsorbents due to their magnetic properties and large specific surface area that enables effective separation in a short time using an external magnetic field.^12–14 Nano-sized iron oxide nanoparticles have gained significant consideration owing to their simple, low-cost synthesis in comparison to commercially accessible activated carbon.¹³ Nevertheless, nano-sized iron oxide nanoparticles suffer from some shortcomings such as agglomeration, and low chemical stability making industrial applications inconvenient.¹⁵ Hence, to attain stability, MNPs are usually functionalized with specific groups. Citric acid and trisodium citrate (TSC) have proven to be suitable and easily accessible materials to stabilize MNPs.^16–20 Due to several advantages over bulk magnetite –greater adsorption capacity, less equilibrium time and easy magnetic separation of solids after adsorption – MNPs have also been utilized as adsorbents for the removal of heavy metal ions and other pollutants from aqueous systems.^21–23 Heavy metal ions are pollutants with toxic to living organisms if present at a certain concentration.^24–26 The existence of toxic metals in the environment can be harmful to human beings and other living species even at low concentrations.^27,28 The most toxic heavy metals are chromium, mercury, lead, nickel, copper, cobalt and cadmium which can be distinguished from other pollutants, because they are non-degradable and accumulate in living organisms. The main sources of heavy metal pollution in water are agricultural and industrial wastewaters; agricultural sources arise from fertilizer and fungicidal spray and industrial sources include paint, plating, tanneries, chemical, mining and petroleum refining.²⁹ So far, a variety of techniques, such as ion-exchange, chemical coagulation, membrane separation, extraction, chemical precipitation, electro deposition and adsorption have been used for the removal of heavy metals from water.^30–34 Most of these techniques have limitations such as high cost, complicated treatment processes and high energy requirements, or they risk introducing secondary pollution. Adsorption is a very efficient, simple, and cost-effective method for the removal of contaminants from aqueous medium.^35–39

Herein, we have synthesized MNPs modified with TSC solution. The modified magnetite nanocomposite (Fe₃O₄@TSC) was characterized using various analytical techniques, and was utilized for the removal of Cr³⁺ and Co²⁺ metal ions from aqueous medium. Kinetic and thermodynamic studies were conducted to obtain optimum metal ion removal capacity and certain constants relating to the adsorption phenomena.

2. Experimental

2.1. Materials and methods

All chemicals and reagents used for this study were of analytical reagent grade. Metal nitrates, chlorides and TSC were purchased from Sigma-Aldrich, USA. All solutions were prepared in Milli-Q water.

Fe₃O₄ nanoparticles were synthesized in a three-necked round bottom flask equipped with mechanical stirrer, nitrogen gas inlet and dropping funnel by using a co-precipitation process. In the first step, iron(II)chloride and iron(III)chloride were mixed in 1 : 2 molar ratio. This ratio was achieved by dissolving 10.8 g FeCl₃·6H₂O in 100 mL 0.5 M HCl and 3.98 g FeCl₂·4H₂O in 100 mL 0.5 M HCl under N₂ gas for 30 min. Nitrogen gas was used to remove O₂ and to prevent the unwanted critical oxidation of Fe(II). Then, these two solutions were mixed with vigorous stirring and heated at 75 °C under N₂ for 1 h. An aqueous solution of NH₄OH (28%, 20 mL) was added drop-wise into solution at the same temperature (75 °C) for 5 h. Black colored precipitates were obtained and were isolated from the supernatant liquid by decantation. The precipitates were washed with demineralized water and separated using centrifugation at 5000 rpm for 10 min. Then, 0.05 M HCl solution was added to neutralize the anionic charge on the particle surface. The formed magnetite nanoparticles (Fe₃O₄) were re-dispersed in 150 mL of 0.5 M solution of TSC (to prevent Fe₃O₄ NPs from possible oxidation in air as well as from agglomeration) and heated at 80 °C under ultra-sonication vibration for 60 min. The as-formed reaction product was kept in excess of citric acid and so, the nanoparticle dispersion was centrifuged. The Fe₃O₄@TSC nanocomposites were washed with deionized water and acetone to remove the excess citrate and then dried under vacuum oven for 12 h at 50 °C.

Throughout the study, the pH measurements were performed using a single electrode pH meter (Orion 2 star, Thermo Scientific, USA). The concentrations of chromium and cobalt were determined using atomic absorption spectrophotometry (AAS, model AAnalyst 400, Perkin Elmer). The FTIR spectra were obtained between 500 to 4000 cm⁻¹ by KBr disc method using an FTIR spectrophotometer (Nicolet 6700, Thermo Scientific, USA). The surface area and other related parameters of the magnetite and Fe₃O₄@TSC were evaluated using an automated surface area and porosity analyzer, (Quadrasorb evo, Quantachrome, USA). The scanning electron micrographs were obtained using a high-performance scanning electron microscope with a high resolution of 3.0 nm (JSM-6380 LA, Tokyo, Japan). TEM (JEM-2100F, JEOL, Japan) and X-ray diffraction (XRD, Shimadzu model 6000) characterizations were also performed on Fe₃O₄ and Fe₃O₄@TSC nanocomposites.

2.2. Batch adsorption experiment

The adsorption of Cr³⁺ and Co²⁺ metal ions using Fe₃O₄@TSC was determined by performing adsorption tests in 100 mL Erlenmeyer flasks. 150 mL of 10 mg L⁻¹ metal ion solutions were shaken with 30 mg Fe₃O₄@TSC nanocomposite at 100 rpm for 24 h to reach equilibrium. After the adsorption process, the samples were separated using a magnetic field, filtered and analyzed by AAS. All experiments were done in triplicate and the average values were taken. The pH of the solution was adjusted using 0.1 M HNO₃/NaOH solutions. The amount of the metal ions adsorbed at equilibrium (q_e, mg g⁻¹) was evaluated by the following equation:⁴⁰where, C_o and C_e are the initial and equilibrium concentrations of metal ions in the solution phase, respectively, V is the initial volume of the metal ion solution (L), and m is the mass of the adsorbent (g).

Batch studies were also performed to obtain kinetic, thermodynamic and adsorption isotherm parameters for the adsorption of Cr³⁺ and Co²⁺ metal ions onto Fe₃O₄@TSC.

2.3. Desorption studies

The desorption efficiency was carried out by batch process using HCl and HNO₃ solutions of different concentrations as the eluents. 100 mL of 10 mg L⁻¹ metal ion solutions were treated with 100 mg of Fe₃O₄@TSC nanocomposites in an Erlenmeyer flask at 120 rpm for 180 min. After 180 min, Fe₃O₄@TSC nanocomposites were filtered off and washed several times with Milli-Q water to remove the excess metal ions. Then, Fe₃O₄@TSC nanocomposites were treated with 100 mL of the aforementioned eluents in an Erlenmeyer flask under the above mentioned conditions. After 180 min, the solution was filtered and the residual concentration of metal ions in the solution phase was determined by AAS as described above.

3. Results and discussion

3.1. Characterization

The procedure for the preparation of the Fe₃O₄@TSC nanocomposite is illustrated in Scheme 1. The FTIR spectra of Fe₃O₄, Fe₃O₄@TSC and Fe₃O₄@TSC after Cr³⁺ and Co²⁺ adsorption are illustrated in Fig. 1. In the FTIR spectrum of Fe₃O₄ nanoparticles, the broad band at 595–610 cm⁻¹ corresponds to Fe–O bonds. In the Fe₃O₄@TSC spectrum, the bands at 1410 and 1620 cm⁻¹ come from symmetric and asymmetric vibrations of carboxylate groups on ligands on the surface of Fe₃O₄ due to the presence of TSC. The other two bands at 2925 and 2854 cm⁻¹ were due to the presence of C–H stretching vibrations.⁴¹ Other bands at 3325 and 3380 cm⁻¹ support the presence of stretching vibrations of O–H and the broadening in this region shows hydrogen bonding. Comparing the original spectrum of Fe₃O₄@TSC before and after adsorption of Cr³⁺ and Co²⁺, the FTIR spectrum of Fe₃O₄@TSC@Cr/Co shows characteristic absorption bands at 540/567 cm⁻¹ corresponding to Cr–O and Co–O.

Scheme 1 Preparation of Fe₃O₄@TSC nanocomposite.

Fig. 1 FTIR spectra of (A) Fe₃O₄, (B) Fe₃O₄@TSC, (C) Cr³⁺ adsorbed Fe₃O₄@TSC, (D) Co²⁺ adsorbed Fe₃O₄@TSC.

The thermal stability of Fe₃O₄ and Fe₃O₄@TSC powder was characterized by thermogravimetric analysis (TGA) as shown in Fig. 2. The weight loss during the thermal treatment was observed for Fe₃O₄ and Fe₃O₄@TSC up to 750 °C.

Fig. 2 TGA and XRD patterns of Fe₃O₄ and Fe₃O₄@TSC.

In the case of Fe₃O₄ nanoparticles, the thermal degradation was divided into two stages, the first stage is up to 200 °C, where 4.3% weight loss was observed due to the loss of adsorbed water of humidity. The second phase was the key degradation stage in the case of Fe₃O₄, where 4.8% weight loss was observed up to 400 °C, possibly due to the removal of crystalline water and conversion of some hydroxide to oxide.

On the other hand, the thermal degradation of Fe₃O₄@TSC nanocomposites was divided into three stages. The first weight loss (up to 200 °C) is associated with the evaporation of physio absorbed water and the solvent. The second stage (between 200 and 300 °C) links to a similar removal of TSC, and further 18% weight loss occurs on increasing the temperature from 300 to 750 °C. During this stage, TSC was slowly removed, resulting in an uninterrupted weight loss owing to the elimination of various volatile compounds including H₂O, CO₂, CO, C₂H₆, H₂ and CH₄.

Fig. 2 shows the XRD studies of Fe₃O₄ and Fe₃O₄@TSC. The X-ray diffraction lines at 2θ; 18.28, 30.06, 35.48, 43.70, 53.81, 57.51, 62.80, 70.80 and 74.92 correspond to (111), (220), (311), (400), (422), (511), (440), (620) and (533) planes of the face-centered cubic (fcc) lattice of Fe₃O₄, respectively (JCPDS card no. 19-0629). TSC stabilized Fe₃O₄ shows low intense XRD peaks due to the low content of Fe₃O₄. Moreover, the peaks became broader due to the amorphous nature of TSC and more broadening occurred at a 2θ value of 20–30. The FTIR and XRD results support the formation of magnetic Fe₃O₄ nanoparticles and Fe₃O₄ nanoparticles being stabilized by TSC.

The magnetic properties of Fe₃O₄ nanoparticles and Fe₃O₄@TSC were subsequently studied by using a vibrating sample magnetometer at room temperature. As shown in Fig. 3, no obvious magnetic hysteresis loops were observed for Fe₃O₄ indicating that all nanoparticles exhibited superparamagnetic behavior. The magnetic Fe₃O₄ nanoparticles had a saturation magnetization (M) of 78.4 emu g⁻¹. The saturation magnetization value of the Fe₃O₄@TSC nanocomposite was 55.4 emu g⁻¹. The saturation magnetization value of Fe₃O₄@TSC nanocomposite was lower in comparison to the original magnetic Fe₃O₄ nanoparticles which might be due to the decrease of Fe₃O₄ content in the nanocomposite, but the magnetic attraction of Fe₃O₄@TSC was strong enough to be proficiently separated from a solution using an external magnetic field.

Fig. 3 Magnetization behavior of Fe₃O₄ and Fe₃O₄@TSC.

The morphology of Fe₃O₄ and the Fe₃O₄@TSC nanocomposite was characterized using scanning electron microscopy (SEM). The results show that the prepared nanoparticles were spherical, within the diameter range of 5–8 nm. Monodisperse Fe₃O₄@TSC magnetic microsphere composites showed a similar spherical form with an average diameter of 5–10 nm (Fig. 4A and B).

Fig. 4 (A) SEM of Fe₃O₄, (B) SEM of Fe₃O₄@TSC, (C and D) TEM of Fe₃O₄, (E and F) TEM of Fe₃O₄@TSC.

TEM and high resolution transmission electron microscopy (HRTEM) of the Fe₃O₄, Fe₃O₄@TSC are illustrated in Fig. 4C–F and 5, respectively. The TEM images of Fe₃O₄ nanoparticles were agglomerated due to the involvement of the hydroxyl groups and magnetic interaction. The average particle sizes were 5–8 nm for the Fe₃O₄ (Fig. 4C and D). On the other hand, the Fe₃O₄@TSC nanoparticles were well dispersed due to the presence of TSC coating on Fe₃O₄ and the particles size were 5–10 nm (Fig. 4E and F). The lattice fringes with an interfering distance of 0.25 ± 0.05 nm corresponding to the (311) plane of face centered (fcc) of Fe₃O₄, similarly, an interfering distance of 0.25 ± 0.05 nm corresponding to the (311) plane of Fe₃O₄@TSC as shown in Fig. 5A and B, respectively.

Fig. 5 HRTEM of (A) Fe₃O₄, (B) Fe₃O₄@TSC.

The composition of Fe₃O_4, Fe₃O₄@TSC, Fe₃O₄@TSC@Cr and Fe₃O₄@TSC@Co were measured by EDX. The elements detected were Fe and O in the case of Fe₃O₄, and Fe, O and C in the case of Fe₃O₄@TSC. The EDX and elemental mapping images illustrated in Fig. 6, supported the presence of Cr³⁺ and Co²⁺ metal ions after the adsorption onto Fe₃O₄@TSC.

Fig. 6 EDX spectra and elemental mapping of (A) Fe₃O₄, (B) Fe₃O₄@TSC, (C) Cr³⁺ adsorbed Fe₃O₄@TSC, (D) Co²⁺ adsorbed Fe₃O₄@TSC.

Nitrogen adsorption–desorption measurements were carried out to measure the porosity and surface nature of the Fe₃O₄ and Fe₃O₄@TSC as shown in Fig. 7. The adsorption isotherms were type IV in the case of both Fe₃O₄ and Fe₃O₄@TSC which supports the nanoporous nature. The corresponding Barrett–Joyner–Halenda (BJH) pore size distribution was calculated using the adsorption–desorption isotherm which revealed a uniform pore size of 5–10 nm for nanocomposites prepared after coating with TSC. The Brunauer–Emmett–Teller (BET) surface area and the total pore volume for Fe₃O₄@TSC was higher than that of the parent Fe₃O₄, found to be 245.42 m² g⁻¹ and 0.368 cm³ g⁻¹, respectively.

Fig. 7 Adsorption–desorption isotherms.

3.2. Effects of operating parameters

The time dependent adsorption behavior was measured by performing a series of adsorption experiments at different contact times (1–240 min). It was found that the removal of Cr³⁺ and Co²⁺ metal ions was very fast for both metal ions. However, the adsorption of Cr³⁺ was faster than Co²⁺. The equilibrium was attained within 120 min for Cr³⁺ metal ions where the adsorption was 100% (q_e, 50 mg g⁻¹). While, for the Co²⁺ metal ion the equilibrium was achieved in 240 min where the adsorption was 95% (q_e, 47.4 mg g⁻¹). Thus, 240 min was taken as the optimal contact time for the removal of both metal ions. The high removal efficiency of Fe₃O₄@TSC nanocomposite for these metal ions may be attributed to the presence of various acetate groups on the Fe₃O₄@TSC nanocomposite (Fig. 8A). The oxygen atoms of the acetate groups are electron rich (electronegative) and the metal ions (Cr³⁺ and Co²⁺) are electropositive. So there is an electrostatic attraction between the oxygen atoms and the metal ions. In addition, there are several other oxygen atoms on the carbonyl group and internal ester groups on the core of Fe₃O₄@TSC which were also responsible for the bonding of Fe₃O₄@TSC to the metal ions through electrostatic attraction. The fast adsorption at the initial stage of adsorption was attributed to the large number of active sites available. Thus, the progressive decrease of adsorption sites resulted in a slower adsorption reaction.

Fig. 8 Removal of Cr³⁺ and Co²⁺ metal ions using Fe₃O₄@TSC nanocomposite at different (A) time, (B) pH, (C and D) Fe₃O₄@TSC dose.

The pH of the solution plays an important role in the adsorption of any adsorbate. Fig. 8B shows the effect of pH on the adsorption of Cr³⁺ and Co²⁺ metal ions from aqueous solution onto Fe₃O₄@TSC nanocomposite by keeping all other parameters constant (contact time 240 min, Fe₃O₄@TSC dose 1 g L⁻¹, temperature 25 ± 2 °C). It was found that the adsorption capacity for Cr³⁺ metal ions was only 0.20 mg g⁻¹ at pH 2.5 and reached up to 24.8 mg g⁻¹ increasing the pH to 6.0. After that, it started to decrease. The same trend was observed for Co²⁺ metal ions. The adsorption capacity was 1.5 mg g⁻¹ at pH 2.5 and increased up to 22.4 mg g⁻¹ at pH 6.0, and then decreased with further pH increase. At pH > 6.0, the adsorption of both metal ions decreased due to the formation of metal hydroxides.⁴² As expected, the adsorptive removal of Cr³⁺ and Co²⁺ metal ions was low in the acidic medium; the concentration of H⁺ is high, so protonation of the active sites of Fe₃O₄@TSC dominates the adsorption process.

The effect of Fe₃O₄@TSC dose on the adsorption capacity of Cr³⁺ and Co²⁺ metal ions is shown in Fig. 8C and D. It was observed that adsorption of Cr³⁺ and Co²⁺ metal ions increases from 89 to 96% and 91 to 97%, respectively, as the dose of Fe₃O₄@TSC increased from 100 to 500 mg. While the adsorption capacity for Cr³⁺ and Co²⁺ sharply decreased from 22.4 mg g⁻¹ to 4.8 mg g⁻¹ and 22.7 mg g⁻¹ to 4.8 mg g⁻¹, respectively. This decrease in the adsorption capacity of both metal ions with the increase in the adsorbent dose might be due to the fact that some adsorption sites remained unsaturated during the adsorption process when the number of available adsorption sites increased.^43,44 Moreover, an increase in Fe₃O₄@TSC dose resulted in aggregation of adsorbent particles, leading to a decrease in total surface area of the Fe₃O₄@TSC and increased diffusional path length.⁴⁵ Thus, a dose of 100 mg/25 mL was chosen as the optimum dose of Fe₃O₄@TSC nanocomposite for the further studies as it brought down the Cr³⁺ and Co²⁺ metal ion concentration to below the WHO permissible limit.

The effects of initial metal ion concentrations at different temperatures (298–328 K) were also studied. The adsorption capacity of the Fe₃O₄@TSC nanocomposite for both metals increased with increasing metal ion concentration (Fig. 9). This could be ascribed to more targets of metal ions affording a higher driving force to enable the ion diffusion from the solution phase to Fe₃O₄@TSC, and more collisions between metal ions and active sites of Fe₃O₄@TSC.⁴⁶ However, in relation to the temperature, it was found that the adsorption capacity for Cr³⁺ and Co²⁺ metal ions decreased with increasing temperature, showing the exothermic nature of adsorption. The highest adsorption was achieved at 298 K for both metal ions.

Fig. 9 The effect of initial ion concentration on the adsorption of Cr³⁺ and Co²⁺ metal ions onto the Fe₃O₄@TSC nanocomposite at different temperatures.

3.3. Adsorption kinetics

The kinetics of Cr³⁺ and Co²⁺ metal ions adsorption onto the Fe₃O₄@TSC nanocomposite was examined using pseudo first-order and pseudo second-order kinetic models. The conformity between experimental data and the model predicted values was expressed by the correlation coefficients (R²). A relatively high R² value showed that the model effectively described the kinetics of Cr³⁺ and Co²⁺ metal ions adsorption.

3.3.1. The pseudo-first-order equation. It was given by Lagergren:⁴⁷where, q_t and q_e are the amounts of metal ions adsorbed at time t, and at equilibrium, respectively, and k₁ is the rate constant for pseudo-first-order adsorption (min⁻¹). The values of rate constant were determined from the slope of the plot log(q_e − q_t) versus t.

3.3.2. The pseudo-second-order equation. The pseudo-second-order kinetic rate equation is given by Ho and Mckay:⁴⁸where k₂ is the pseudo-second-order rate constant. The values of k₂ for all studied concentrations were determined from the intercepts of the plot t/q_t versus time.

The kinetic parameters are given in Table 1. The fitted linear regression plot shows data that fit the pseudo-second-order model well with a better value of correlation coefficient, compared to the pseudo-first-order (Fig. S1†). The low values of correlation coefficient (R²) for pseudo-first-order ascertained the unsuitability of this model for the adsorption of Cr³⁺ and Co²⁺ metal ions adsorption onto the Fe₃O₄@TSC nanocomposite. The experimental equilibrium adsorption capacity values (q_e,exp) for Cr³⁺ and Co²⁺ metal ions at various concentrations are in good agreement with the calculated equilibrium adsorption capacity values (q_e,cal) for the pseudo-second-order kinetics model.

Table 1 Isotherm constants for the adsorption of Cr³⁺ and Co²⁺ metal ions onto Fe₃O₄@TSC

Metal ions	Temperature (K)	qm,exp (mg g^−1)	Langmuir isotherm				Freundlich isotherm
			qm,cal (mg g^−1)	b (L mg^−1)	RL	R^2	Kf (mg g^−1)(L mg^−1)1/n	n	R^2
Cr(III)	298	549.125	1667	0.004	0.961	0.994	32.94	1.117	0.976
	308	525.325	1000	0.0091	0.911	0.994	15.16	0.941	0.967
	318	507.6	625	0.024	0.805	0.993	9.28	0.883	0.965
	328	487.2	495	0.118	0.456	0.997	6.88	0.871	0.973
Co(II)	298	452.5	588	0.007	0.939	0.997	22.42	1.33	0.984
	308	399.2	555	0.011	0.904	0.990	15.46	1.334	0.992
	318	382.5	380	0.036	0.735	0.993	6.01	1.08	0.998
	328	352.5	370	0.059	0.631	0.993	3.52	1.012	0.993

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3.4. Adsorption isotherms

Isotherm results were examined using Langmuir and Freundlich isotherms.^49,50

The linear form of the Langmuir isotherm model can be written as:

where, q_e is the amount of adsorbed phenol (mg g⁻¹), C_e is the equilibrium concentration of phenol (mg L⁻¹), Q_m and b are the Langmuir constants related to maximum monolayer adsorption capacity and energy of adsorption, respectively.

The values of Q_m and b were evaluated from the intercept and slope of the linear plots of 1/q_e vs. 1/C_e, respectively (Table 2). To see the efficiency of the adsorption process, the value of dimension less equilibrium parameter (R_L) was determined using the following equation:⁵¹

where, C_o (mg L⁻¹) is the lowest initial metal ion concentration and b is the Langmuir constant (L mg⁻¹). The values of R_L in the present study were <1 which shows a favorable adsorption of Cr³⁺ and Co²⁺ metal ions onto Fe₃O₄@TSC (Table 2).

Table 2 Kinetic parameters for the adsorption of Cr³⁺ and Co²⁺ metals onto Fe₃O₄@TSC

Metal ions	(Co, mg L^−1)	qe,exp (mg g^−1)	Pseudo-first-order			Pseudo-second-order
			qe,cal (mg g^−1)	K1 (min^−1)	R^2	qe,cal (mg g^−1)	K2 (g mg^−1 min^−1)	R^2
Cr(III)	10	50	4.50	0.012	0.363	38.91	0.032	0.975
Co(II)	10	46.58	13.31	0.024	0.926	46.73	0.0079	0.999

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The linear form of the Freundlich isotherm is given by the following equation:

The Freundlich isotherm constants (K_f and n) were determined from the intercept and slope of the linear plots of log q_e vs. log C_e, respectively (Table 2). K_f decreased with increasing temperature which shows the exothermic nature of the adsorption process. The value of n was found to be greater than one which suggested the favorable adsorption of Cr³⁺ and Co²⁺ metal ions onto Fe₃O₄@TSC nanocomposites.⁵² As seen in Table 2, the Langmuir isotherm fitted well with the experimental data (correlation coefficient R² > 0.99), whereas, the low correlation coefficients showed poor agreement with the Freundlich isotherm experimental data (Fig. S2†).

3.5. Adsorption thermodynamics

The values of enthalpy change (ΔH°) and entropy change (ΔS°) were calculated from the slopes and intercepts of the plots of ln K_c versus 1/T using the following equation:

The values of ΔG° (free energy change) were calculated from the following relation:

ΔG° = ΔH° − TΔS° (8)

The results for the thermodynamic parameters are presented in Table 3. The values of ΔG° were negative and decreased with the increase in temperature showing that the spontaneous adsorption process and spontaneity decreased with increasing temperature. The negative values of ΔS° suggest a decrease in randomness at the solid/solution interface. The adsorption of both metal ions onto Fe₃O₄@TSC nanocomposite was physical and exothermic in nature as the values of ΔH° were negative. The decrease in adsorption capacity with the increase in temperature may be ascribed to the weakening of adsorptive forces between the active sites present on the Fe₃O₄@TSC nanocomposite surface and the metal ions, and also between the adjacent molecules of the adsorbed phase.

Table 3 Thermodynamic parameters for the adsorption of Cr³⁺ and Co²⁺ metals onto Fe₃O₄@TSC

Metal ions	Co (mg L^−1)	−ΔH° (kJ mol^−1)	−ΔS° (J mol^−1 K^−1)	−ΔG° (kJ mol^−1)
				298 K	308 K	318 K	328 K
Cr(III)	10	41.84	119.67	6.385	4.691	3.715	2.753
	100	21.76	59.81	3.858	3.451	2.801	2.068
	250	19.12	47.93	4.903	4.256	3.870	3.443
Co(II)	10	47.56	141.63	5.256	4.208	2.252	1.185
	100	41.19	124.29	4.495	2.391	1.625	0.658
	250	18.26	53.73	2.389	1.459	1.204	0.702

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3.6. Desorption study

To recover the adsorbed Cr³⁺ and Co²⁺ metal ions, desorption studies were performed. It was noticed that better recovery of both metal ions was observed in HCl as compared to HNO₃ (Fig. 10). It might be due to the smaller size of Cl⁻ ions in comparison to the NO₃⁻ ion. It is also interesting to note that as the concentration of HCl increased, the recovery was decreased from 88.3% to 80.2% for Cr³⁺, and 90% to 70.3% for Co²⁺, which might be due to the deterioration of Fe₃O₄@TSC surface functional groups at higher concentrations of HCl solution.

Fig. 10 Adsorption/desorption studies.

3.7. Proposed adsorption/desorption mechanism

The suggested mechanism for the adsorption/desorption of Cr³⁺ and Co²⁺ metal ions onto Fe₃O₄@TSC is given in Scheme 2. It can be seen that Fe₃O₄@TSC had three types of active groups: –COONa, –C=O and –OH which were capable of bonding with the metal ions. However, –COONa was more active towards metal ions in comparison to –C=O and –OH groups. Actually, the oxygen atoms of acetate groups are electron rich (electronegative) and the metal ions (Cr³⁺ and Co²⁺) are electropositive. So, there is electrostatic attraction between the electron rich oxygen atoms and the electropositive metal ions. As can be seen in Section 3.6, the desorption of both metal ions was performed efficiently using 0.1 M HCl solution.

Scheme 2 Proposed mechanism for the adsorption/desorption of Cr³⁺ and Co²⁺ metal ions onto Fe₃O₄@TSC nanocomposite.

4. Conclusion

In the present study, TSC modified magnetite nanocomposite was synthesized and used as a magnetic adsorbent for the removal of toxic Cr³⁺ and Co²⁺ metal ions from aqueous medium. The Fe₃O₄@TSC was characterized using various analytical techniques and it was found that Fe₃O₄@TSC particles were in the range of 5–10 nm. The saturation magnetization value of Fe₃O₄@TSC microspheres was lower in comparison to the original magnetic Fe₃O₄ nanoparticles which is due to the decrease of Fe₃O₄ content in the nanocomposites, the magnetic receptiveness of Fe₃O₄@TSC is still sufficient for separation from a solution using external magnetic fields. The surface area and the total pore volume for Fe₃O₄@TSC was higher than that of parental Fe₃O₄ and found to be 245.42 m² g⁻¹ and 0.368 cm³ g⁻¹. The prepared magnetic adsorbent could be well dispersed in the water and easily separated magnetically from the medium after adsorption. Isotherm modelling revealed that the Langmuir equation could better describe the adsorption of both metal ions onto Fe₃O₄@TSC as compared to the Freundlich isotherm model. The adsorption process was kinetically studied by fitting the experimental data with two kinetic models, and the results designated that the adsorption followed the pseudo second order kinetic model well.

Acknowledgements

This project was supported by King Saud University, Deanship of Scientific Research, College of Science Research Center.

References

1. B. R. White, B. T. Stackhouse and J. A. Holcombe, J. Hazard. Mater., 2009, 161, 848.
2. L. Zhou, Y. Wang, Z. Liu and Q. Huang, J. Hazard. Mater., 2009, 161, 995.
3. T. Tuutijarvi, J. Lu, M. Sillanp and G. Chen, J. Hazard. Mater., 2009, 166, 1415.
4. S. Bucak, D. A. Jones, P. E. Laibinis and T. A. Hatton, Biotechnol. Prog., 2003, 19, 477.
5. X. Song, X. Luo, Q. Zhang, A. Zhu, L. Ji and C. Yan, J. Magn. Magn. Mater., 2015, 388, 116.
6. S. V. Salihov, Y. A. Ivanenkov, S. P. Krechetov, M. S. Veselov, N. V. Sviridenkova, A. G. Savchenko, N. L. Klyachko, Y. I. Golovin, N. V. Chufarova, E. K. Beloglazkina and A. G. Majouga, J. Magn. Magn. Mater., 2015, 394, 173.
7. H. Gu, K. Xu, C. Xu and B. Xu, Chem. Commun., 2006, 9, 941.
8. Y. Maeda, T. Toyoda, T. Mogi, T. Taguchi, T. Tanaami, T. Yoshino, T. Matsunaga and T. Tanaka, Colloids Surf., B, 2016, 139, 117.
9. I. Akira, T. Kouji, K. Kazuyoshi, S. Masashige, H. Hiroyuki, M. Kazuhiko, S. Toshiaki and K. Takeshi, Cancer Sci., 2003, 94, 308.
10. D. K. Kim, M. Mikhaylova, F. H. Wang, J. Kehr, B. Bjelke, Y. Zhang, T. Tsakalakos and M. Muhammed, Chem. Mater., 2003, 15, 4343.
11. A. K. Gupta and S. Wells, IEEE Transactions on NanoBioscience, 2004, 3, 66.
12. B. Zargar, H. Parham and A. Hatamie, Chemosphere, 2009, 76, 554.
13. A. Afkhami and R. Moosavi, J. Hazard. Mater., 2010, 174, 398.
14. C. H. Weng, Y. T. Lin, C. L. Yeh and Y. C. Sharma, Water Sci. Technol., 2010, 62, 844.
15. J. P. Jolivet, C. Chanéac and E. Tronc, Chem. Commun., 2004, 481.
16. L. C. A. Oliveira, D. I. Petkowicz, A. Smaniotto and S. B. C. Pergher, Water Res., 2004, 38, 3699.
17. L. C. A. Oliveira, R. V. R. A. Rios, J. D. Fabris, V. Garg, K. Sapag and R. Lago, Carbon, 2002, 40, 2177.
18. G. Crini, Dyes Pigm., 2008, 77, 415.
19. G. Crini and P. M. Badot, Prog. Polym. Sci., 2008, 33, 399.
20. P. R. Chang, J. Yu, X. Ma and D. P. Anderson, Carbohydr. Polym., 2011, 83, 640.
21. M. H. Liao and D. H. Chen, J. Mater. Chem., 2002, 12, 3654.
22. Y. F. Shen, J. Tang, Z. H. Nie, Y. D. Wang, Y. Ren and L. Zuo, Sep. Purif. Technol., 2009, 68, 312.
23. Y. F. Shen, J. Tang, Z. H. Nie, Y. D. Wang, Y. Ren and L. Zuo, Bioresour. Technol., 2009, 100, 4139.
24. M. Kumari, C. U. Pittman Jr and D. Mohan, J. Colloid Interface Sci., 2015, 442, 120.
25. S. A. Nabi and M. Naushad, Colloids Surf., A, 2007, 293, 175.
26. S. A. Nabi, M. Naushad and A. M. Khan, Colloids Surf., A, 2006, 280, 66.
27. A. A. ALqadami, M. A. Abdalla, Z. A. ALOthman and K. Omer, Int. J. Environ. Res. Public Health, 2013, 10, 361.
28. A. Shahat, M. R. Awual, M. A. Khaleque, M. Z. Alam, M. Naushad and A. M. Sarwaruddin Chowdhury, Chem. Eng. J, 2015, 273, 286.
29. S. Babel and T. A. Kurniawan, J. Hazard. Mater., 2003, 97, 219.
30. F. An, R. Du, X. Wang, M. Wan, X. Dai and J. Gao, J. Hazard. Mater., 2012, 201, 74.
31. O. B. piridon, E. Preda, A. Botez and L. Pitulice, Environ. Sci. Pollut. Res., 2013, 20, 1.
32. M. Caetano, C. Valderrama, A. Farran and J. L. Cortina, J. Colloid Interface Sci., 2009, 338, 402.
33. M. Ehtash, M. C. Fournier-Salaün, K. Dimitrov, P. Salaün and A. Saboni, Chem. Eng. J., 2014, 250, 42.
34. Y. Ng, N. Jayakumar and M. Hashim, J. Hazard. Mater., 2010, 184, 255.
35. Z. A. ALOthman, M. Naushad and R. Ali, Environ. Sci. Pollut. Res., 2013, 20, 3351.
36. M. Naushad, T. Ahamad, Z. A. Alothman, M. A. Shar, N. S. AlHokbany, S. M. Alshehri and J. Ind, J. Ind. Eng. Chem., 2015, 29, 78.
37. M. Rabiul Awual, M. M. Hasan, A. Shahat, M. Naushad, H. Shiwaku and T. Yaita, Chem. Eng. J., 2015, 265, 210.
38. M. Naushad, Z. A. ALOthman and G. Sharma, Ionics, 2015, 21, 1453.
39. M. R. Awual, M. M. Hasan, M. Naushad, H. Shiwaku and T. Yaita, Chem. Eng. J., 2015, 209, 790.
40. R. Bushra, M. Naushad, R. Adnan, Z. A. ALOthman and M. Rafatullah, J. Ind. Eng. Chem., 2015, 21, 1112.
41. Y. Meng, D. Chen, Y. Sun, D. Jiao, D. Zeng and Z. Liu, Appl. Surf. Sci., 2015, 324, 745.
42. M. R. Awual, G. E. Eldesoky, T. Yaita, M. Naushad, H. Shiwaku, Z. A. AlOthman and S. Suzuki, Chem. Eng. J., 2015, 279, 639.
43. S. E. Bailey, T. J. Olin, R. M. Bricka and D. D. Adrian, Water Res., 1999, 33, 2469.
44. M. M. Saeed, Nucl. Chem., 2003, 256, 73.
45. A. Ozer, G. Akkaya and M. Turabik, J. Hazard. Mater., 2005, 126, 119.
46. J. Gong, X. Wang, X. Shao, S. Yuan, C. Yang and X. Hub, Talanta, 2012, 101, 45.
47. S. Lagergren, Handlingar, Band, 1898, 24, p. 1.
48. Y. S. Ho and G. McKay, Chem. Eng. J., 1998, 70, 115.
49. I. Langmuir, J. Am. Chem. Soc., 1918, 40, 1361.
50. H. M. F. Freundlich, J. Phys. Chem., 1906, 57, 385.
51. M. Naushad, T. Ahamad, Z. A. Alothman and A. Aldalbahi, Chem. Eng. J., 2014, 254, 181.
52. C. Namasivayam, R. T. Yamura and D. J. S. E. Arasi, Environ. Geol., 2001, 41, 269.

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Footnote

† Electronic supplementary information (ESI) available. See DOI: 10.1039/c5ra27525c

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