International Journal of PhotoenergyVolume 2012 (2012), Article ID 608298, 5 pagesdoi:10.1155/2012/608298

Research Article

Synthesis and Characterization of Iron Oxide Nanoparticles and Applications in the Removal of Heavy Metals from Industrial Wastewater

Zuolian Cheng,¹ Annie Lai Kuan Tan,¹ Yong Tao,² Dan Shan,² Kok Eng Ting,¹ and Xi Jiang Yin²

¹School of Chemical & Life Sciences, Singapore Polytechnic, 500 Dover Road, Singapore, 139651²Advanced Material Technology Centre, Singapore Polytechnic, 500 Dover Road, Singapore, 139651

Received 16 January 2012; Accepted 17 February 2012

Academic Editor: Stéphane Jobic

Copyright © 2012 Zuolian Cheng et al. This is an open access article distributed under the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

Abstract.

This study investigated the applicability of maghemite (γ-Fe₂O₃) nanoparticles for the selective removal of toxic heavy metals from electroplating wastewater. The maghemite nanoparticles of 60 nm were synthesized using a coprecipitation method and characterized by X-ray diffraction (XRD) and scanning electron microscopy (SEM) equipped with energy dispersive X-ray spectroscopy (EDX). Batch experiments were carried out for the removal of Pb²⁺ ions from aqueous solutions by maghemite nanoparticles. The effects of contact time, initial concentration of Pb²⁺ ions, solution pH, and salinity on the amount of Pb²⁺ removed were investigated. The adsorption process was found to be highly pH dependent, which made the nanoparticles selectively adsorb this metal from wastewater. The adsorption of Pb²⁺ reached equilibrium rapidly within 15 min and the adsorption data were well fitted with the Langmuir isotherm.

1. Introduction

With heavy metal pollution becoming one of the most serious environmental problems, various methods for heavy metal removal from wastewater have been extensively studied during the past decades, such as chemical precipitation, electrochemical techniques, membrane filtration, ion exchange, and adsorption [1]. To date, considerable research attention has been paid to the removal of heavy metals from contaminated water via adsorption process. In theory, the adsorption process can offer flexibility in design and operation and in many cases will produce high-quality treated effluent. In addition, as the adsorption is sometimes reversible and adsorbent can be regenerated by suitable desorption process, various types of adsorbents have found application in the removal of heavy metals, including activated carbon [2, 3], carbon nanotubes [4–6], polymeric adsorbents [7], metal oxides [8], and bioadsorbents [9–12]. Among these adsorbents, iron-based magnetic nanomaterials have distinguished themselves by their unique properties, such as larger surface area-volume ratio, diminished consumption of chemicals, and no secondary pollutant. However, with another special property of this kind magnetic materials are realized and utilized in the context of environmental remediation. More and more magnetic separation has been combined with adsorption for the removal of heavy metals from contaminated water at laboratory scales [13–15]. Especially in industries, magnetic separation is desirable because it can overcome many drawbacks occurring in the membrane filtration, centrifugation, or gravitational separation and is easy to achieve a given level of separation just via external magnetic field.

Iron oxides exit in many forms in nature, with magnetite (Fe₃O₄), hematite (α-Fe₂O₃) and maghemite (γ-Fe₂O₃), being most probably common and important technologically [16]. It has been reported that surface effects have a strong influence on the magnetic properties of iron oxide nanoparticles [17]. As the surface area of iron-oxide-based magnetic materials decreased, their responses to external magnetic field decreased, making it difficult to recover the adsorbents after treatment has been completed [14]. On the other hand, it has also been noted that the adsorption capacities of adsorbents rely largely on the available surface areas, and the increase of the surface area is normally obtained by the decrease of the particle size of adsorbents. As a result, there is a need to synthesize such absorbents with proper particles sizes for the removal of heavy metals from industrial wastewater.

Up to now, there are several methods that can be used to synthesize iron-oxide-based nanomaterials. These methods include hydrothermal synthesis [18, 19], thermal decomposition [20, 21], co-precipitation [22, 23], sol-gel method [15], and colloidal chemistry method [24]. Among these synthesis methods, coprecipitation has proven to be the most promising method for the production of nanomaterials as the procedure is relatively simple and the particles can be obtained with controlled particle size.

The specific objective of the present study was (1) to synthesize γ-Fe₂O₃ nanoparticles using a modified method, which involved urea as a uniformity precipitation reagent, (2) to characterize γ-Fe₂O₃ nanoparticles synthesized using different kinds of analytical instruments, and (3) to evaluate synthesized γ-Fe₂O3 nanoparticles as adsorbents to remove heavy metals such as Pb²⁺ from industrial wastewater.

2. Experimental

2.1. Preparation and Characterization of γ-Fe₂O₃ Nanoparticles

The synthesis of γ-Fe₂O₃ nanoparticles involves the following steps: (1) a designated molar ratio of iron chloride and urea was dissolved in deionized water; (2) the mixture was continuously stirred for 45 min at 90°C in a water bath before it was cooled to room temperature; (3) the produced precipitation was centrifuged and washed by deionized water and followed by ethanol; (4) after being dried at 75°C for 4 hours, the collected powder was slowly calcined to 650°C in air and dwelt for 2 hours. The resultant product of γ-Fe₂O₃ nanoparticles was obtained for subsequent characterization.

A scanning electron microscope (SEM, JEOL JEM2010, Japan) was used to characterize the structure properties of the synthesized materials. The element composition of the synthesized materials was identified by an energy dispersive X-ray spectroscopy system (EDX) coupled to the SEM. The crystallization phase analysis was executed by a powder X-ray diffraction (XRD) (Philips PW-1830, Netherlands). Magnetization measurement was carried out with a vibration sample magnetometer (VSM) at room temperature.

2.2. Removal of Heavy Metals from Wastewater

A stock solution containing Pb²⁺ was prepared by dissolving a known quantity of lead nitrate (Pb(NO₃)₂) in deionized water. Batch adsorption studies were performed by mixing 0.5 g of γ-Fe₂O₃ with 50 mL of solutions of varying Pb²⁺ concentrations (50, 100, and 150 mg/L) in 100 mL glass vials. The adsorption on γ-Fe₂O₃ was first studied at pH values of 2.5 to 6.5 to investigate the effects of pH values on the Pb²⁺ adsorption. 0.1 M HCl and 0.1 M NaOH solutions were used to adjust the pH values of water samples. The pH values of water samples were stable over the the experiment period. All the adsorption experiments were carried out at a room temperature of and were performed in triplicate. The total aqueous concentrations of Pb²⁺ were measured using an inductively coupled plasma-optical emission spectrometer (ICP-OES, Thermo, icap 6000). Sample dilution was conducted before the ICP-OES measurement, where necessary.

3. Results and Discussion

3.1. Synthesis and Characterization of γ-Fe₂O₃

The crystalline grain size is mainly determined by both the formation energy of growth unit and the lattice energy, besides different synthesis conditions. In the present study, Fe₂O₃ nanoparticles were synthesized by varying pH values, ageing time, the mass ratio of FeCl₃ and urea, and so forth. Figure 1(a) shows an SEM image of the synthesized γ-Fe₂O₃, confirming that the particles obtained were indeed in the nanometer range. Upon ageing for different durations, it was observed that the grain size increased with increasing ageing time. The smallest grain size ( nm) was obtained after ageing for 45 minutes. In addition, strong peaks for Fe and O can be observed in the spectrum illustrated in Figure 1(b) for particles after ageing time of 45 min. The insert of Figure 1(b) reveals that the O/Fe atomic ratio of the γ-Fe₂O₃ analyzed was 1.56, which was relatively consistent with the theoretical O/Fe atomic ratio of 1.50.

(a)

(b)

Figure 1: SEM image and EDX spectrum for γ-Fe₂O₃ nanoparticles.

The magnetization with respect to applied field was recorded at room temperature. The hysteresis loop, shown in Figure 2, suggested a weak magnetic nature of the samples with little hysteresis. The weak magnetism might be caused by the presence of α-Fe₂O₃ as detected by XRD. From Figure 2, the MS was calculated to be 0.025 emu/g and the HC to be 1250 Oe.

Figure 2: VSM measurements for γ-Fe₂O₃ nanoparticles.

Figure 3 shows the results of XRD analysis for the synthesized nanoparticles that were obtained with different initial concentrations of FeCl₃. It indicates that the particles consist of γ-Fe₂O₃ (peaks denoted by *) and α-Fe₂O₃ (peaks denoted by #). It also indicates that a concentration of 0.02 M of FeCl₃ resulted in the particles with the smallest Z-average diameter. Therefore, 0.02 M was the optimized concentration for FeCl₃.

Figure 3: XRD patterns of synthesized γ-Fe₂O₃ nanoparticles with different concentrations of FeCl₃.

3.2. Effect of Adsorption Time and Initial Concentration

The adsorption of Pb²⁺ onto γ-Fe₂O₃ nanoparticles was monitored for 120 min. The initial Pb²⁺ concentrations were 50, 100, and 150 mg/L, respectively. The initial pH value of water samples was 5.5, and the solution temperature was . As seen in Figure 4, Pb²⁺ ions were adsorbed onto γ-Fe₂O₃ nanoparticles rapidly, and equilibrium was established within 30 minutes. This could be due to the small size of γ-Fe₂O₃ nanoparticles, which was favorable for the diffusion of Pb²⁺ ions from bulk solution onto the active sites of the solid surface. External adsorption dominated and no pore diffusion was observed to slow down the adsorption rate. Despite the short equilibrium time, a 24-h contact time was adopted for the subsequent experiment to ensure that adsorption equilibrium was achieved. The short equilibrium time was in agreement with that reported by other researchers for the adsorption of other metal ions onto iron oxide nanoparticles [25–27]. This is in contrast to other conventional porous adsorbents in which adsorption occurs through pore diffusion steps, which in turn slow down the adsorption rate. The γ-Fe₂O₃ nanoparticles are nonporous adsorbents, as confirmed by the surface area and porosity measurements, where only external adsorption occurred. This type of adsorption mass transfer requires less time to reach the equilibrium [28]. This result is promising as equilibrium time plays a major role in economic viability for wastewater treatment plant. Furthermore, as shown in Figure 4, the amount adsorbed of Pb²⁺ increased with the increase in the initial concentration of Pb²⁺ in water samples. This can be attributed to the increase in the ion occupancy number, which favors the adsorption process.

Figure 4: Effect of contact time on the removal of Pb²⁺ at different initial concentrations. Adsorbent dose: 10 g/L, shaking rate: 200 rpm, pH: 5.5, T: .

3.3. Effect of pH

It is well known that pH is one of the most important factors that affect the adsorption process of heavy metals in water samples. The experiments were carried out to find the optimum pH on the adsorption of Pb²⁺ ions onto γ-Fe₂O₃ nanoparticles using different initial pH values of 2.5 to 6.5. The experimental pH values of up to 6.5 were chosen as precipitation of lead hydroxide would occur at pH values equal to or higher than 7.0, even though other variables such as the amount of nanoparticles were fixed [29]. Therefore, it was not feasible to carry out the adsorption experiments for Pb²⁺ at pH > 6.5 without introducing some uncertainties to the results. Figure 5 shows the effects of pH on the adsorption of Pb²⁺. As observed in the graph, the removal efficiency of Pb2+ ions from watersamples by the γ-Fe₂O₃ nanoparticles was clearly pH dependent and the highest adsorption efficiency was obtained at pH ≥5.5. Lee et al. also observed a similar pH effect for the adsorption of Pb²⁺ onto bulk iron oxides in water samples [30]. It indicated that a water sample with a higher pH value was favorable for the deprotonation of sorbent surface [31, 32]. Increased deprotonation could result in the increase of negatively charged sites, which enhanced the attractive forces between the sorbent surface and the Pb²⁺ ions. Therefore, it will result in the increase in the adsorption capacity. On the other hand, in a water sample with lower pH, the positively charged sites dominate and this could enhance the repulsion forces existing between the sorbent surface and the Pb²⁺ ions and therefore decrease the adsorption of Pb²⁺ ions.

Figure 5: Effect of pH on Pb²⁺ adsorption onto γ-Fe₂O₃ nanoadsorbents. Initial Pb²⁺ concentration: 50 mg/L. Adsorbent dose: 10 g/L, shaking rate: 200 rpm, T: .

3.4. Effects of Salinity

Increasing NaCl salinity from 0% to 3.5% (the salinity of seawater) had no effects on the removal of Pb²⁺ by γ-Fe₂O₃ nanoadsorbents. This suggested that no interaction occurred among NaCl, Pb²⁺ and γ-Fe₂O₃ nanoadsorbents, and the complexation of Pb²⁺, and Cl⁻ was much weaker than the coordination between Pb²⁺ and the adsorptive sites on the surfaces of γ-Fe₂O₃ nanoadsorbents.

4. Conclusions

This study showed that the prepared γ-Fe₂O₃ nanoparticles could be used as an alternate to the conventional adsorbents for the removal of metal ions from wastewater with high removal efficiency within a very short time. The removal of Pb²⁺, as a typical metal ion commonly present in wastewater, by adsorption onto γ-Fe₂O₃ nanoparticles was successfully accomplished. Adsorption was very rapid and equilibrium was achieved within 15 min. It also showed that adsorption was highly dependent on the initial concentration of Pb²⁺ and initial pH value. Maximum removal efficiency was achieved at pH 5.5 at room temperature. Increasing NaCl from 0% to 3.5% (the salinity of seawater) had no effects on the adsorption of Pb²⁺ on γ-Fe₂O₃ nanoparticles. The adsorption isotherms were also determined and were appropriately described by both Langmuir and Freundlich models, with a better fitting to the Langmuir model than the Freundlich model. Therefore, γ-Fe₂O₃ nanoparticles were recommended as fast, effective, and inexpensive nanoadsorbents for rapid removal and recovery of metal ions from industrial wastewater.

Acknowledgment

This work was supported by the Ministry of Education Innovation Fund of Singapore (MOE2008-IF-1-010).

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