INTRODUCTION

Water is among the most basic and essential life enablers, and its scarcity affects food production and could put the survival of many life forms in jeopardy[1]. Between continuous population growth, climate change, industrial development, and ecosystem contamination, provisions for clean and ample water have become one of the most significant challenges of the century. One culprit of water contamination is heavy metals whose excessive levels could cause various diseases[2]. The industrialization of communities and the rise in environmental pollution, along with the depletion of potable water resources, begs for the development of new water and wastewater purification techniques.

Nanotechnology is one of such techniques that is currently on the rise and has wide usage in the cleaning of natural resources such as surface, underground, industrial and potable water basins and has far better effectiveness compared with existing wastewater and contaminated water purifications techniques[3]. Nanomaterials have gained the attention of researchers as a low-cost, effective technology for water purification[4]. Zero-capacity ferrite nanoparticles[5], zeolites[6], Nano-filtration[7], Nano-photo-catalysts[8], magnetic nanoparticles[9], and Nano-sensors[10] are among the applications in which nanotechnology could be used in order to develop water and sewage purification and pollutant detection.

Iron ferrite, Fe3O4, nanoparticles could be used to process a wide range of environmental pollutants such as chlorinated methane, chlorinated benzene, organic paint, tri-halo-methane, arsenic, nitrates, and heavy metals such as mercury, copper, cadmium, lead, etc.[11]. The following characteristics render Fe3O4 nanoparticles as suitable additives for water applications: effectiveness in the reformation of a wide range of environmental pollutants, low cost, and non-toxic[12]. Fe3O4 nanoparticles are especially interesting due to their low inherent toxicity and excellent magnetic properties. Among the more interesting features of the magnetic nanoparticles is their ability to refine surfaces with various organic and nonorganic coatings to alleviate a wide range of waterborne heavy metals. Magnetic nanoparticles have high flocculation tendencies due to direct inter-particle effects such as the van der Waals force or magnetic interactions. These effects form micron-sized or larger aggregates, which reduce the surface/volume ratio and consequently their reactivity. Therefore, the magnetic nanoparticles are usually coated with organic compound ions and used to remove metallic ions from aqueous solutions [13, 14]. Previous studies indicate that Tri-ethylene glycol (TEG) coating reduces the surface spin disorder in ferrite nanoparticles and consequently increases the magnetization of nanoparticles[15]. Also, this coating reduces the interparticle magnetic dipole-dipole and exchange interactions and causes less aggregation of nanoparticles[15, 16]. The aggregation is expected to be even less common in aqueous solutions of TEG-coated ferrite nanoparticles. Therefore, the surface to volume increases, which is an advantage for heavy metal, nitrates, dye, and other pollutions removal from wastewater. To the best of our knowledge, there is no report in literature about using TEG coated nanoparticles in wastewater treatment and heavy metal removal. Using TEG coating and the formation of the hydroxyl groups at the surface of magnetic nanoparticles, the adsorption capacity could increase through electrostatic attraction between heavy metal cations and surface negatively charged groups.

In light of the discussions above, and water-related issues, bare and TEG coated Fe3O4 nanoparticles powders and aqueous solution (ferrofluid) were synthesized and the morphological and magnetic characteristics were analyzed. Finally, the effect of TEG coating on the removal of lead was investigated.

EXPERIMENTAL AND DATA TREATMENT

Chemicals

The raw chemicals compounds with high purity for the synthesis include tetra aqua iron chloride (II) FeCl2.4H2O, (Carlo Erba), Hexa aqua iron chloride (III) FeCl3.6H2O, (Merck), Sodium hydroxide, NaOH (Merck), Tri-ethylene glycol (TEG) C6H14O4, in liquid form(Merck) and Lead(II) nitrate, Pb(NO3)2, (Merck).

Samples Synthesis

The nanoparticle samples were synthesized by co-precipitation, from saline solutions of FeCl2.4H2O, and FeCl3.6H2O with 1:2 molar ratios under atmospheric pressure on a heater and stirred using a mechanical stirrer to reach 80 °C. Subsequently, NaOH solution was added to the reaction mixture solution as the sedimentation agent. Black sediments formed as soon as the base was added. The stirring then continued for another 30 minutes. After cooling down, the beaker was placed on a magnet to separate the nanoparticles. For structural and magnetic characterizations, the prepared powders were washed with distilled water and dried under an air atmosphere. To synthesize the TEG coated sample, a similar method was used and in the last step, 100 mg of the prepared Fe3O4 nanoparticles in an aqueous solution, were added to a beaker containing 10 ml of Tri-ethylene glycol at 140 °C and stirred for 30 minutes. After that, the powders were washed with distilled water several times and then dried to obtain a TEG-coated nanocomposite. The sample was named TEG-Fe3O4.

Adsorption experiments

For adsorption experiments, a specific amount of ferrofluid of nanoparticles was synthesized using the method mentioned above. After washing the samples with deionized water, the nanoparticles in aqueous form were used for Pb (II) removal experiments. Batch adsorption experiments were carried out to investigate the effects of contact time (0-30 min), solution pH (3-9), and adsorbent dose (100-500 mg.L-1) under solution temperature of 25 °C and Pb (II) concentration of 50 mg.L-1. Moreover, HCl and NaOH were used to adjust the pH of the solution.

Equipment

To study the structural characteristics of nanoparticles, which were prepared in powder form, a PW1730 Phillips XRD, with an X-ray wavelength of 1.54 Å, was used. The average crystallites size, áDñXRD was obtained using the Debye-Scherrer formula:

(1)

where the K is Scherrer constant (~ 0.9), l the X-ray wavelength of incident X-ray, b the full-width at half-maximum (FWHM) of the Bragg peaks, and q is Bragg angle.

The FTIR analyses were performed by a TENSOR27 Brucker device under ambient temperature and in the 400-4000 cm-1 range. In order to study and analyze the shape, size estimation of the particles, and surface morphology, a MIRA3-TESCAN field emission scanning electron microscope (FESEM), and a Philips/FEI model CM120 transmission electron microscope (TEM) were used. The particles size dispersion was fitted by a log-normal function
f(D)=exp[−ln2(D/D0)/2σ2]. Using fit results, the average size and standard deviation σD were obtained by =D0 exp(σ2/2) and σD=[exp(σ2)−1]1/2. A thermal analysis device TGA/SDTA 851 Mettler Toledo under N2 atmosphere and a constant rate of 10 °C per minute was used from 30 to 550 °C. For magnetic characterization, a vibrating sample magnetometer (Meghnatis Daghigh Kavir) was used. The surface area was pore size and was determined by Brunauer-Emmett-Teller (BET) method through nitrogen adsorption-desorption using a BELSORP-mini II instrument. To measure the Pb (II) content in the water, a NovaAA 400 atomic absorption spectrophotometer (Analytik Jena) was used.

RESULTS AND DISCUSSION

Structural Properties

X-ray diffraction (XRD) and FTIR spectroscopy of the samples were performed to confirm the crystalline structure and the absence of impurities, as well as the quality of the Tri-ethylene glycol polymeric coating. Fig. 1 displays the XRD for Fe3O4 and TEG-Fe3O4 samples refined by the Full-Prof software and Rietveld method. Irregular peaks, which represent impurities, are not present in the XRD patterns. This result shows the pure spinel structure of the samples. Using the X’pert Highscore Plus software, the position of the peaks and their relative intensity match well with the diffraction data of iron ferrite (JCPDS card no. 01-088-0866). For example, the main peaks of samples have been assigned to Bragg’s reflection planes of spinel structure and the data were collected in Table 1.

From Fig. 1, the experimental are well fitted by calculated patterns using the Rietveld refinement method. Also, the obtained a and Vu.c values using the Rietveld refinement method (see Table 1) are near to that for bulk Fe3O4 (a=8.384 Å). Also by regarding the calculations error in brackets, the estimated crystallite sizes using the Scherrer equation are almost identical, about 10 nm for both the samples (see Table 2).

Fig. 2a shows the FTIR spectrum of the bare Fe3O4 sample. The peaks at 3434, 3739, and 3850 cm-1 correspond with the bending O-H groups and residual water [17]. The twin peaks at 2922 and 2852 cm-1 correspond to the symmetric and asymmetric C-H tensile bonds [18, 19]. Absorptions at 1533 and 1632 cm-1 correspond with the stretching O-H groups[15]. In addition to these bonds, the spinel structures are characterized by two significant peaks observed below 1000 cm-1. The observed peak at 574 cm-1 corresponds to the Fe-O bonds in the tetrahedral sites of the spinel structure, and the peak close to 445 cm-1 represents Fe-O bonds in the octahedral sites [20].

Fig. 2b shows the FTIR spectrum of the TEG-Fe3O4 sample. Besides the peaks observed in Fig. 2a, there is a distinct peak centered at 1394 cm-1 which corresponds to CH2 bonds as a typical signal of TEG [15]. Fig. 2c shows the FT-IR spectrum for two samples with and without Tri-ethylene glycol polymeric coating. It can be seen from the Fe3O4 spectrum that the Fe-O adsorption bond at 574 cm-1 accounts for 16% of the total absorption spectra. Compared with the 1.5% IR spectrum absorbed by the TEG coated sample, it is inferred that the coating of Fe3O4 nanoparticles with TEG prevents any further absorption in the spectrum due to this specific bond. The effect of TEG coating on the surface of nanoparticles as well as the amount of trapped water in the powders is the possible reason for the observed different intensities of FTIR peaks of the samples. It seems that the TEG coating weakened the vibration of other bonds including spinel tetrahedral and octahedral bonds through adsorbing the infrared wave’s energy. Also, the amount of trapped water in the bare sample is probably higher than that of the TEG coated sample which affects the intensity of peaks. And the observed peaks in the bare sample show higher intensity when compared with the coated sample.

Fig. 3a shows the recorded TGA result for Fe3O4and TEG-Fe3O4nanoparticles under ambient temperature to 550 °C in an Ar atmosphere. The first weight loss at around 100-200 °C corresponds to the evaporation of trapped water molecules from the powders. As it can be seen from Fig. 3a, the amount of trapped water in the Fe3O4 is higher than that of the TEG-Fe3O4 sample which is attributed to the difference in synthesis condition of the samples. This result is well supported by the FTIR data in which the intensity of O-H peaks in the uncoated sample is higher than the TEG-coated one. There is a second weight loss in the TGA curve of both samples. For the Fe3O4 sample, the second loss may be the gradual crystallization and reduction of oxygen deficiencies on the surface of the Fe3O4 nanoparticles. On the other hand, the second weight loss in the TEG-Fe3O4 sample is attributed to the presence of the polyol coating [15]. The boiling point of TEG is about 280 °C from which it is decomposed and eliminated from the sample. Fig. 3b displays the differential thermal analysis (DTG) curve, derived from the TGA curve. In Fig. 3b, the coated sample has a peak around 261°C, which confirms the presence of Tri-ethylene glycol in the TEG- Fe3O4 sample [15].

To study the surface morphology of the powders, FE-SEM images were analyzed at different magnifications. Figs. 4a-b show the FE-SEM micrographs in magnification of 100 kx. The observed aggregation is due to the surface spin canting and magnetic interaction between the nanoparticles [16]. As can be seen from Fig. 4b, the TEG coating has significantly changed the aggregation, causing separated spherical particles, a characteristic that contributes to colloidal stability. It is evident from data in Table 2 that the áDñSEM of the coated sample is larger than bare Fe3O4, which can be a result of polyol molecules bonding at the surface of aggregates [15].

Figs. 4c-d show FESEM images at the high magnification of 330 kx. These images show that each aggregate is formed by particles with a size of about 10 nm. This value is very close to those obtained by the Debye-Scherer formula (Eq. 1), which predicts the formation of single-crystallite particles in both samples.

The TEM image of the Fe3O4 sample was recorded (Fig. 5a) to determine the size of the nanoparticles. Using the log-normal function the average particle size was obtained at 9.9 nm (Fig 5b), which is in good agreement with crystallite size and confirms single-crystallite particles. However, there are some larger particles in the TEM image, which are multi-crystallite particles. The single-crystalline nanoparticles can show superparamagnetic behavior that is an essential property for efficient wastewater treatment. Therefore, the magnetic characterization of samples was analyzed in the next section.

Magnetic Properties To compare the magnetic properties of uncoated and coated samples, and study the effect of TEG coating, the M-H curves were recorded at 300 K (Figs.6a-b). The near-zero coercive field, Hc (see Table 3)suggests superparamagnetic behavior of both the samples, at 300 K.For an ensemble of superparamagnetic nanoparticles, a modified Langevin function is applied to estimate the particles core saturation magnetization, Ms, the moment (super spin) value mp of particles and surface susceptibility,c which shows the surface spins contribution to the total magnetization of nanoparticles [21, 22]:

(2)

where, kB the Boltzmann constant, and L(x) implies the Langevin function [21-23]. The solid lines in Fig. 6a-b display the fit results of data using the modified Langevin function(Eq.2). The inset in Fig. 6a-b shows the high field linear behavior of M-H curves. The obtained fit parameters, Ms, mp, and c are presented in Table 3.

We found that the TEG coated sample has higher mp and Ms when compared with the uncoated Fe3O4 sample. However, the c of the coated sample has a smaller value compared with the Fe3O4 sample. This result shows a reduction in nanoparticles’ surface spin canting and magnetic disorder by TEG coating. The bonding between TEG and atoms at the surface of nanoparticles reduced the broken metal-oxygen bonds and surface deficiencies and consequently results in lower surface magnetic disorder. Due to broken bonds and lattice deficiency at the surface of Fe3O4 nanoparticles, there are plenty of metal ions (Fe2+ and Fe3+) that are ready to interact with oxygen atoms of the TEG molecule to reduce the surface energy and reach a stabilized surface as reported in refs[24, 25].

In summary of the magnetometry results, we can conclude that the TEG coating increases the saturation magnetization by reducing the surface spin canting, and this can influence the application of samples in different areas such as heavy metal removal efficiency from wastewater. In the next section, we studied the effect of TEG coating on Pb(II) removal.

Adsorption Results

One of the numerous applications of magnetic nanoparticles is the use of these materials in heavy metals removal from wastewater. In previous sections, we have shown that TEG coating affects the structural and magnetic properties of samples. Here, we reported an experimental study on Pb (II) removal from a laboratory-prepared solution. To calculate the Pb (II) elimination ratio and the adsorption capacity of the samples, the following equations are used [26]:

(3)

(4)

where R is adsorption efficiency, C0 and C is the initial and final Pb (II) concentration (mg.L-1), q is adsorption capacity (mg.g-1), m is mass of adsorbent (g), and V is the volume of solution (L). Different contact times between the nanoparticles and metal ions were investigated from 0 to 30 min (Fig. 7). It is evident from Fig. 7 that most of the metals adsorbed within 15 min contact time. Therefore, this period was considered for further calculations and kinetics.

For further analysis, kinetic models were investigated to describe the adsorption kinetics. The rate of reaction and reaction pathways are described by adsorption kinetics which depends upon the physical and chemical characteristics of the adsorbent. In order to understand the kinetics of the adsorption of Pb (II) ions and analyze the adsorption process, pseudo-first-order (Eq. 5) and pseudo-second-order (Eq. 6) kinetic models were applied [27]:

ln(qe-qt)=lnqe-(K1/2.303)t (5)

t/qt=1/K2.qe2+t/qe (6)

Here, K1 is the rate constant of the first-order kinetic model, K2 is the rate constant of the second-order kinetic model, and qt and qe represent the removal capacity at time t and equilibrium, respectively. The fitting results are shown in Fig. 8 and 9, while the related parameters are listed in Table 4. Based on the correlation coefficients of these fits (R2), the experimental data were better fitted to the pseudo-second-order kinetic model. This indicates that the rate-limiting step in the adsorption is the chemisorption, revealing that the adsorption process is based on chemical interactions rather than physical forces.

The pH of the aqueous solution is an important parameter in the adsorption process and affects the interaction between the adsorbent and adsorbate. The effect of the initial solution pH on the Pb (II) adsorption was studied at 25 °C; 10 mg of nanoparticles was contacted to the 20 ml of Pb (II) solution with the initial concentration of 50 mg.L-1 with different pH in the range 3–9. It was found that the maximum adsorption onto nanoparticles occurs at the pH range of 5–7 (Fig. 10). The results revealed that maximum adsorption occurs at pH=7, at which the R=92.55 % obtained. Using this result, in the next experiments, the pH of the solution was adjusted at 7.

In the next step, the effect of nanoparticle dose on Pb (II) adsorption capacity was studied. For this purpose, various amounts of the nanocomposite were contacted with 20 ml of Pb(II) solution with an initial concentration of 50 mg.L-1 and after 15 min adsorption Capacity was calculated. From Fig. 11, it can be seen that the adsorption capacity decreased with increasing the number of nanoparticles. The primary reason is that adsorption sites remain unsaturated during the adsorption process, whereas the number of sites available for adsorption increases by increasing the adsorbent amount. According to Fig. 12, it can be seen that the removal efficiency increased with increasing the number of nanoparticles. Maximum value of q=363.4 mg.g-1 with R=90.8% was obtained for m=100 mg.L-1, between adsorbent dose range of 100-500 mg.L-1.

The N2 adsorption-desorption isotherms (Fig. 13) were employed to investigate the specific surface area and pore structure of the magnetic ferrite. The BET surface area, pore-volume, and mean pore size was calculated to be 117.51 m2.g-1, 0.3614 cm3.g-1, and 12.3 nm, respectively, suggesting that the pore type was mainly inter-particles space.

Fig. 14 shows the schematic mechanism of Pb (II) adsorption by TEG-coated Fe3O4 nanoparticles. The presence of –OH groups at the surface of nanoparticles increases electrostatic interaction between TEG molecules and Pb (II) and consequently the adsorption capacity and efficiency increase when compared with bare Fe3O4 sample. Table 5 compares the Pb (II) adsorption results of the present study with some similar adsorbents reported in the literature. As can be seen from this table, the obtained results of this study predict the potential application of the prepared nanocomposite for heavy metals removal from wastewater.

CONCLUSION

In this work, bare and TEG-coated Fe3O4 nanoparticles were prepared using a modified co-precipitation route. The TEG coating reduces the spin canting at the surface of nanoparticles. The adsorption results showed the efficiency of TEG coating on Pb (II) removal from wastewater. By TEG coating, the efficiency reached a high value of 97 %, and almost all the heavy metal ions were eliminated. Based on the obtained results, we concluded Tri-ethylene glycol to be a suitable coating for adsorption capacity and elimination ratio improvement purposes.

CONFLICTS OF INTEREST

The authors declare that there are no conflicts of interest.