Magnetic NiFe2O4 nanoparticles have been synthesized and used as adsorbents for copper removal from aqueous solution. The NiFe2O4 nanoparticles were characterized by scanning electron microscope, transmission electron microscope, X-ray diffraction, and Fourier transform infrared spectroscopy. The batch removal of Cu2+ ions from aqueous solutions using NiFe2O4 magnetic nanoparticles under different experimental conditions was investigated. The effects of initial concentration, adsorbent dose, contact time, and pH were investigated. The adsorption process was pH dependent, and the maximum adsorption was observed at a pH of 6.0. Equilibrium was achieved for copper ion after 25 min. Experimental results showed that NiFe2O4 magnetic nanoparticles are effective for the removal of copper ions from aqueous solutions. The pseudo-second-order kinetic model gave a better fit of the experimental data as compared to the pseudo-first-order kinetic model. Experimental data showed a good fit with the Langmuir isotherm model.

INTRODUCTION

Today, because of industrial development and due to industrial wastewater entering into the environment, ecosystems around factories and also surface and ground waters are in danger of pollution (Nies 1999). The presence of metal ions in wastewater and surface water is one of the main concerns to human health and the environment. The excessive use of metals and chemical materials in industrial processes causes wastewater with high percentages of metal ions (Nasiruddin Khan & Farooq Wahab 2007). Copper is an essential element for plants and animals, but an excess is toxic. Human activities, such as mining, melting, usage of industrial and domestic sludge sewage on agricultural land, and also the use of copper in fungicides and insecticides, cause contamination of water and soil (Xu et al. 2006). Excessive concentration of copper in the human body can lead to gastrointestinal diseases (Nasiruddin Khan & Farooq Wahab 2007). The maximum acceptable concentration of copper in drinking water is 1.5 ppm as recommended by the World Health Organization (Karabulut et al. 2000). Common methods for removing metals from wastewater include chemical precipitation, ion exchange, adsorption by activated carbon, and electrolytic processes (Leyva-Ramos et al. 2005). Among several chemical and physical methods, the adsorption process is an effective technique that has been successfully employed for heavy metal removal from wastewater (Mohammed et al. 2011). Various adsorbents have been reported for the removal of Cu(II) from aqueous solutions, including zeolite (Zhang et al. 2011), diatomite (Safa et al. 2012), kaolinite (Guerra et al. 2008), chitosan (Juang et al. 1999), alumina (Rajurkar et al. 2011), functionalized polymers (Zhou et al. 2009), and zero-valent iron (Karabelli et al. 2008). However, most of these adsorbents are not the ideal choices due to their low adsorption capacity, inadequate adsorption efficiencies, complication separations from the solutions, or high costs.

In recent years, magnetic nanoparticles have attracted much attention because of their widespread application in different fields, such as mineral separation magnetic storage devices, catalysis, magnetic refrigeration systems, heat transfer application in drug delivery system, magnetic resonance imaging, cancer therapy, and magnetic cell separation (Liu et al. 2004; Sharma & Srivastava 2009; Hasany et al. 2013; Lee et al. 2013; Mody et al. 2014; Sobhani et al. 2014). The application of magnetic nanoparticles in wastewater treatment is becoming an interesting area of research (Hu et al. 2006; Banerjee & Chen 2007; Liu et al. 2008; Wang et al. 2009; White et al. 2009). Nanoparticles exhibit good adsorption efficiency, especially due to the higher surface area and more active sites for interaction with metallic species and dyes, and they can easily be synthesized (Mak & Chen 2005). Among the various magnetic nanoparticles, Ni ferrites with the general formula (AB2O4) are one of the most versatile materials as they have high saturation magnetization, high Curie temperature, and chemical stability (Goldman 2006). NiFe2O4 magnetic nanoparticles as a low cost, nontoxic adsorbent, containing active functional hydroxyl groups, are selected for their better application to and management of wastewater remediation. The objectives of this study are: (1) the synthesis of NiFe2O4 magnetic nanoparticles by the co-precipitation method (Zins et al. 1999) and their characterization with respect to scanning electron microscopy (SEM), transmission electron microscopy (TEM), X-ray diffraction (XRD), and Fourier transform infrared spectroscopy (FT-IR); (2) a comparative batch adsorption study of the adsorbent for Cu2+ with respect to various environmental parameters (initial concentration, adsorbent dose, contact time, and pH); and (3) corresponding isotherm and kinetic studies.

MATERIALS AND METHODS

Analytical-grade Cu(NO3)2.5H2O was obtained from Merck. A 1,000 mg/L stock solution of the salt was prepared in deionized water. All working solutions were prepared by diluting the stock solution with deionized water. Deionized water was prepared using a Millipore Milli-Q (Bedford, MA, USA) water purification system. All reagents (FeCl3, NiCl2, ammonium hydroxide (29.6%), NaOH, NaCl, and HNO3) used in the study were of analytical grade and purchased from Aldrich. Before each experiment, all glassware was cleaned with dilute nitric acid and repeatedly washed with deionized water.

XRD analysis was carried out using a PAN analytical X'Pert Pro X-ray diffractometer. Surface morphology and particle size were studied using a Hitachi S-4800 SEM instrument. TEM was performed using a Hitachi H-7650 microscope at 80 kV. FT-IR spectra were determined as KBr pellets on a Bruker model 470 spectrophotometer. All the metal ion concentrations were measured with a Varian AA240FS atomic absorption spectrophotometer.

Preparation of the NiFe2O4

The solution of metallic salts FeCl3 (160 mL, 1 M) and NiCl2 (40 mL, 1 M) was poured as quickly as possible into a boiling alkaline solution [NaOH (1,000 mL, 1 M)] under vigorous stirring. Then, the solution was cooled and continuously stirred for 90 min. The resulting precipitate was then purified by a four-times-repeated centrifugation (4,000–6,000 rpm, 20 min) and decantation (Zins et al. 1999).

Adsorption experiments

Batch adsorption of copper ions onto the adsorbent (NiFe2O4) was investigated in aqueous solutions under various operating conditions: pH 2–6, at a temperature of 298 K, for an initial Cu2+ ion concentration of 4 mg/L. About 0.05 g adsorbent was added to 20 mL of copper nitrate solution (4 mg/L). Then, the mixture was agitated on a shaker at 250 rpm. The initial pH values of the copper solutions were adjusted from 2 to 6 with 0.1 mol L−1 HNO3 or 0.1 mol L−1 NaOH solutions using a pH meter. After equilibrium, the samples were centrifuged and the adsorbent (NiFe2O4) removed magnetically from the solution. The Cu(II) concentration in the supernatant was measured by flame atomic absorption spectrometer. The effects of several parameters, such as contact time, initial concentration, pH, and adsorbent dose, on the extent of adsorption of Cu(II) were investigated.

The Cu(II) removal percentage was calculated as Equation (1): 
formula
1
where C0 and Ct (mg/L) are the concentration of Cu(II) in the solution at initial and equilibrium time, respectively.
The amount of Cu(II) adsorbed (Qe) was calculated using Equation (2): 
formula
2
where C0 and Ct are the initial and equilibrium concentrations of Cu(II) (mg/L), m is the mass of NiFe2O4 nanoparticles (g), and V is the volume of solution (L).

Adsorption isotherms

Adsorption isotherms were obtained by using 0.05 g of adsorbent and 20 mL of copper nitrate solution with different concentrations (4–60 mg/L) at 298 K. These solutions were buffered at an optimum pH (pH = 6) for adsorption and agitated on a shaker at 250 rpm until they reached adsorption equilibrium (25 min). The quantity of Cu(II) adsorbed was derived from the concentration change.

RESULTS AND DISCUSSION

Characterization of NiFe2O4 magnetic nanoparticles

NiFe2O4 nanocrystallites were prepared according to the reported procedure (Zins et al. 1999). NiFe2O4 nanocrystallites were characterized by FT-IR (Figure 1), XRD (Figure 2), TEM (Figure 3), and SEM (Figure 4). The FT-IR spectrum of NiFe2O4 (Figure 1) exhibits strong bands in the low-frequency region (1,000–500 cm−1) due to the iron oxide skeleton, which is consistent with the magnetite spectrum (Pol et al. 2009). The peak at 1,633 cm–1 showed the existence of Fe–O, and the peak at 3,446 cm−1 implied the existence of residual hydroxyl groups (Pol et al. 2009). To confirm the Ni ferrite formation in the synthesized magnetic nanoparticles, the XRD pattern of the sample was studied. The XRD pattern in Figure 2 shows that these nanoparticles have spinel structure, with all major peaks matching the standard pattern of bulk NiFe2O4 (JCPDS 10-325). The TEM and SEM photographs of the sample are illustrated in Figures 3 and 4, respectively. Both the SEM and TEM images demonstrate that the prepared magnetic nanoparticles are spherical, with average size of <50 nm in the diameter.

Figure 1

The FT-IR spectrum of NiFe2O4 nanoparticles.

Figure 1

The FT-IR spectrum of NiFe2O4 nanoparticles.

Figure 2

XRD pattern of NiFe2O4 nanoparticles.

Figure 2

XRD pattern of NiFe2O4 nanoparticles.

Figure 3

TEM image of NiFe2O4 nanoparticles.

Figure 3

TEM image of NiFe2O4 nanoparticles.

Figure 4

SEM image of NiFe2O4 nanoparticles.

Figure 4

SEM image of NiFe2O4 nanoparticles.

Adsorption and removal of copper from aqueous solution

Effect of pH

The acidity of the aqueous solution exerts a significant influence on the adsorption process, because it can affect the solution chemistry of contaminants and the state of functional groups on the surface of adsorbents (Sheng et al. 2010). The effect of solution pH on Cu(II) adsorption was investigated at pH 2–6 at 298 K. As shown in Figure 5, only a small amount of Cu(II) is adsorbed onto the adsorbent at pH = 2. The adsorption amount of Cu(II) increases with increasing pH values from 2 to 6. Cu(II) in aqueous solution can be present in several forms, such as , Cu(OH)+, Cu(OH)2, , and ; is the predominant species at pH < 6.0 (Sheng et al. 2010). The adsorption experiments at pH > 6 were not studied because of the precipitation of Cu(OH)2 in the solution. Thus, the optimum pH for Cu(II) ion sorption is found in the pH range of 5–6, which is in accordance with previous reports (Wang et al. 2011; Xin et al. 2012). The subsequent experiments were carried out at pH 6.0.

Figure 5

Percentage of copper removal at different pH values (initial Cu(II) concentration = 4 mg/L, adsorbent dose = 0.05 g, and T = 298 K).

Figure 5

Percentage of copper removal at different pH values (initial Cu(II) concentration = 4 mg/L, adsorbent dose = 0.05 g, and T = 298 K).

Effect of contact time

The effect of contact time on the amount of copper adsorbed was investigated at 4 mg/L initial concentration of copper. It could be observed from Figure 6 that with the increase of contact time, the percentage adsorptions also increased. Minimum adsorption was 91.75% for the time 5 min to maximum adsorption value 98.30% for the time 25 min. The adsorption characteristic showed a rapid uptake of the copper. The adsorption rate, however, decreased to a constant value with an increase in contact time, because all available sites were covered, and no active site was present for binding.

Figure 6

Effect of contact time (initial Cu(II) concentration = 4 mg/L, adsorbent dose = 0.05 g, pH = 6, and T = 298 K).

Figure 6

Effect of contact time (initial Cu(II) concentration = 4 mg/L, adsorbent dose = 0.05 g, pH = 6, and T = 298 K).

Effect of the adsorbent dosage

The effect of the adsorbent amount on the copper adsorption was studied, with the adsorbent amount ranging from 0.01 to 0.2 g. The results obtained are shown in Figure 7. From Figure 7, it is observed that the most suitable adsorbent dosage is 0.05 g. As the adsorbent dose increases, the percentage removal also increases, until it reaches a saturation point, where the increase in adsorbent does not change the percentage removal significantly.

Figure 7

The effect of adsorbent dosage on the removal percentage of Cu(II) ions (initial Cu(II) concentration = 4 mg/L, pH = 6, contact time = 25 min, and T = 298 K).

Figure 7

The effect of adsorbent dosage on the removal percentage of Cu(II) ions (initial Cu(II) concentration = 4 mg/L, pH = 6, contact time = 25 min, and T = 298 K).

Effect of initial Cu(II) concentration

The effects of initial Cu(II) concentrations at the range values from 4 to 60 mg/L on the adsorption of copper were investigated. As shown in Figure 8, the percentage of removal decreased with the increase in Cu(II) concentration. At higher Cu(II) concentrations, the ratio of available adsorbent surface to the initial Cu(II) concentration is low and the percentage of removal then depends upon the initial concentration.

Figure 8

Effect of initial Cu(II) concentration on the removal of Cu(II) (adsorbent dose = 0.05 g, pH = 6, contact time = 25 min, and T = 298 K).

Figure 8

Effect of initial Cu(II) concentration on the removal of Cu(II) (adsorbent dose = 0.05 g, pH = 6, contact time = 25 min, and T = 298 K).

Adsorption isotherms

The experimental data were correlated by Langmuir and Freundlich models. The related linear equations are shown in Equations (5) and (6), respectively.

Langmuir equation: 
formula
3
where qe is the amount of Cu2+ adsorbed per unit mass at equilibrium (mg/g); qm is the maximum amount of adsorbent that can be adsorbed per unit mass adsorbent (mg/g); Ce is concentration of adsorbent (in the solution at equilibrium (mg/L)); and ka is adsorption equilibrium constant.

A plot of versus Ce gives a straight line, with a slope of and intercept .

Freundlich equation: 
formula
4
where Ce (mg/L) and qe (mg/g) are the equilibrium concentration of adsorbent in the solution and the amount of adsorbent adsorbed at equilibrium, respectively; KF (mg1−(1/n) L1/n g−1) and n are the Freundlich constants, which show the adsorption capacity for the adsorbent and adsorption intensity, respectively.

A plot of log qe versus log Ce gives a straight line of slope 1/n and intercept log KF.

If , then the adsorption is favorable. If , the adsorption bond becomes weak and unfavorable adsorption occurs.

At first, we correlated the adsorption data at different initial concentrations of Cu(II) ion in terms of the Langmuir isotherm (Equation (3)). Furthermore, we examined the data according to the Freundlich isotherm (Equation (4)). The calculated parameters of the Langmuir and Freundlich models are given in Table 1 and Figure 9. The comparison of correlation coefficients (R2) of the linearized form of both equations indicates that the Langmuir model yields a better fit for the experimental equilibrium adsorption data than the Freundlich model. This suggests the monolayer coverage of the surface of NiFe2O4 nanoparticles by copper ions.

Table 1

Isotherm and kinetic model parameters for the Cu(II) adsorption on NiFe2O4 magnetic nanoparticles

Isotherm models 
Langmuir Freundlich  
R2 qm (mg/g) ka (L/mg) R2 KF (mg1−(1/n) L1/n g−11/n   
0.9985 200 2.5 0.9032 9.22 0.8684   
Kinetic models
 
Pseudo first-order Pseudo second-order 
R2 qe,cal. (mg/g) k1 (min−1R2 qe,exp. (mg/g) qe,cal. (mg/g) k2 (g/(mg.min)) 
0.646 1.590 0.089 0.999 9.829 9.823 0.531 
Isotherm models 
Langmuir Freundlich  
R2 qm (mg/g) ka (L/mg) R2 KF (mg1−(1/n) L1/n g−11/n   
0.9985 200 2.5 0.9032 9.22 0.8684   
Kinetic models
 
Pseudo first-order Pseudo second-order 
R2 qe,cal. (mg/g) k1 (min−1R2 qe,exp. (mg/g) qe,cal. (mg/g) k2 (g/(mg.min)) 
0.646 1.590 0.089 0.999 9.829 9.823 0.531 
Figure 9

The Langmuir adsorption isotherm of NiFe2O4 magnetic nanoparticles (0.05 g) for Cu(II) ions.

Figure 9

The Langmuir adsorption isotherm of NiFe2O4 magnetic nanoparticles (0.05 g) for Cu(II) ions.

The maximum adsorption capacity is compared in Table 2 with the data reported by other authors for copper adsorption. As can be seen, the maximum copper adsorption value of NiFe2O4 is higher than those reported in the literature. This comparison shows the great potential of NiFe2O4 magnetic nanoparticles for the removal of copper from wastewater.

Table 2

Maximum adsorption capacities of copper from aqueous media using various adsorbents

Adsorbents qm (mg/g) References 
NiFe2O4 nanoparticles 200 This study 
Hydroxyapatite nanoparticles 36.90 Wang et al. (2009)  
Maghemite nanoparticle 27.70 Hu et al. (2006)  
Magnetic gamma-Fe2O3 nanoparticles coated with poly-l-cysteine 42.90 White et al. (2009)  
Fe3O4 magnetic nanoparticles coated with humic acid 46.30 Liu et al. (2008)  
Magnetic nano-adsorbent modified by gum arabic 38.50 Banerjee & Chen (2007)  
Adsorbents qm (mg/g) References 
NiFe2O4 nanoparticles 200 This study 
Hydroxyapatite nanoparticles 36.90 Wang et al. (2009)  
Maghemite nanoparticle 27.70 Hu et al. (2006)  
Magnetic gamma-Fe2O3 nanoparticles coated with poly-l-cysteine 42.90 White et al. (2009)  
Fe3O4 magnetic nanoparticles coated with humic acid 46.30 Liu et al. (2008)  
Magnetic nano-adsorbent modified by gum arabic 38.50 Banerjee & Chen (2007)  

Adsorption kinetics

In this study, pseudo-first-order and pseudo-second-order kinetics models were applied to examine the controlling mechanism of copper ions' adsorption from aqueous solutions. Adsorption equilibrium was reached in 25 min (Figure 6). The linear form of the pseudo-first-order model and pseudo-second-order kinetics model can be described as shown in Equations (5) and (6), respectively: 
formula
5
 
formula
6
where qe and qt are the adsorption capacities at equilibrium and at time t (min), respectively. k1 (min−1) and k2 (g/(mg.min)) are the pseudo-first-order and pseudo-second-order rate constants, respectively (Ho 2006). The equilibrium experimental results were not well fitted with the pseudo-first-order model (Table 1). The values of qe and k2 can be calculated from the slope and intercept of the plot of t/qt versus t (figure not shown). The results listed in Table 1 show that the correlation coefficient is very high (R2 = 0.999). Furthermore, the calculated equilibrium adsorption capacity was consistent with the experimental result. This result suggested that the kinetics data were better described with a pseudo-second-order kinetics model, which supported the assumption that the rate-limiting step may be chemical sorption (Wu et al. 2001).

CONCLUSIONS

NiFe2O4 magnetic nanoparticles with average size less than 50 nm in diameter have been synthesized for efficient removal of copper from water. The prepared magnetic nanoparticles can be easily separated from the solution using an external magnet after adsorption. The following results were obtained in this study: (1) the equilibrium time for the adsorption of the copper ions onto adsorbent was 25 min (initial Cu(II) concentration = 4 mg/L, adsorbent dose = 0.05 g, pH = 6, and T = 298 K), and adsorption kinetic data were well described by a pseudo-second-order model with a high correlation coefficient (R2 = 0.999); (2) adsorption isotherm data of NiFe2O4 were fitted well with the Langmuir model (qm = 200 mg/g, ka = 2.5 L/mg, and R2 = 0.9985); and (3) removal of Cu(II) increases with increasing pH, and a maximum value was found at pH 6.0. At this pH (initial Cu(II) concentration = 4 mg/L, adsorbent dose = 0.05 g, and T = 298 K), the removal rate of copper ions was found to be 98.4%. Thus, the process of purifying water pollution presented here is efficient using magnetic nanoparticles.

ACKNOWLEDGEMENT

The authors are grateful to Islamic Azad University, Bandar Abbas Branch for financial support.

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