Milad Jamal Livani, Mohsen Ghorbani, Hassan Mehdipour

(1. M.Sc Student of Chemical Engineering, Babol Noshirvani University of Technology, Shariati Ave., Babol, 47148-71167, Iran;2. Department of Chemical Engineering, Babol Noshirvani University of Technology, Shariati Ave., Babol, 47148-71167, Iran)

Abstract: A low cost activated carbon was prepared from hazelnut shells by chemical activation with sodium hydroxide at 600 ℃ in a N2 atmosphere and then combined with magnetic NiFe2O4 nanoparticles by hydrothermal and co-precipitation methods. Samples were characterized by FESEM, TEM, XRD, FT-IR, nitrogen adsorption and magnetic measurements. Results indicated that the NiFe2O4 nanoparticles synthesized by the hydrothermal method had a higher saturation magnetization and smaller average particle size than those produced by the co-precipitation method. The specific surface area and total pore volume of the activated carbon decreased from 314 to 288 m2/g and 0.363 9 to 0.333 8 cm3/g, respectively by forming a hybrid with the magnetic NiFe2O4 nanoparticles synthesized by the hydrothermal method. NiFe2O4 nanoparticles were mainly distributed on the surface, although a few were inside the pores of the activated carbon. Their sizes were the same as those of the original ones. The saturation magnetization of the hybrids was lower than those of the original NiFe2O4 nanoparticles due to the existence of the activated carbon. They showed superparamagnetic behavior at room temperature and were easily separated from solutions by an external magnet.

Key words: Nanoparticle; Activated carbon; Co-precipitation; Hydrothermal; Nanocomposite; Magnetic properties

1 Introduction

There are many researches in recent years to obtain low-cost activated carbons from agricultural wastes, such as acorn shell[1], chestnut shell[2], hazelnut shell[3], olive stone[4], pistachio shell[5], tamarind wood[6], walnut shell[7], broom corn stalk[8], mango peels[9]and rice husks[10], owing to their abundant resources and low prices. Furthermore, converting the agricultural wastes into valuable activated carbons provides a new way for agricultural waste treatment[7]. The main reason for the application of agricultural wastes as precursor materials is their high carbon contents and low ash contents and their primary porous structures, which make it possible to prepare activated carbons with a well-developed porous structure[11].Activation of activated carbon can be realized by two methods, physical and chemical activation. Typically, physical activation include pyrolysis of the raw materials, followed by steam or carbon dioxide activation of the carbonized materials. Chemical activation is a single step process, in which the precursor is impregnated or mixed with an activating reagent such as NaOH, ZnCl2, K2CO3, KOH, FeCl3, H2SO4, and H3PO4[11-14], followed by heat treatment under inert atmosphere. Chemical activation normally takes place at lower temperatures than physical activation. Activated carbons (ACs) have attracted great interest in the field of environmental protection because of their excellent adsorption performance owing to their high surface area and appropriate pore volume[15].

In the last two decades, attention has been paid to the understanding of optical, electrical, chemical and magnetic properties of nanostructured materials such as nanowires, multi-layers, nanocrystals and nanocomposite materials[16]. Ferrites are a very important group of magnetic materials with a wide range of applications in low wavenumber to microwave range and from low to high permeability. Conventional techniques for preparation of nanoparticles (such as ferrites) include hydrothermal, co-precipitation, sol-gel, sonochemical approach, microemulsion technique, reverse micelles and combustion method[16-21]. In order to control the size of synthesized nanoparticles, hydrothermal and co-precipitation method have been used. The electronic, optical,magnetic and catalytic properties of these nanoparticles depends crucially on their size, structure, magnetic stability and purity, which make them possible to be used in electronic, recording and medical industries. The spinel ferrites are often denoted by the formula of MFe2O4(M=Co, Ni, Zn, Cu, Mn or other elements) where M2+and Fe3+ions occupy either tetrahedral (A) or octahedral (B) sites[22-25].

In the present study, nickel ferrite nanocrystals were prepared from an aqueous solution containing metal Fe and Ni through co-precipitation and hydrothermal routes. Nanoparticles synthesized with the hydrothermal method were pure, and possessed a high saturation magnetization. Thus the hydrothermal technique in an alkaline solution was chosen to prepare NPH/AC nanocomposite. The morphological, structural, and magnetic properties of as-prepared nanoparticles were characterized by VSM, FESEM, EDX, XRD, TEM, TGA and nitrogen adsorption.

2 Experimental

2.1 Materials

Iron (III) chloride hexahydrate (FeCl3·6H2O,99%), nickel chloride hexahydrate (NiCl2·6H2O, 98%), Poly vinyl pyrrolidone (PVP), oleic acid(C18H34O2, 99%), ammonia solution 25%, ethanol (96%), sodium hydroxide (NaOH) and hydrochloric acid 37% (HCl) were all obtained from Merck and used without further purification. All aqueous solutions were prepared with deionized water.

2.2 Preparation of AC

Hazelnut shells from Qom province were used as raw materials for synthesis of AC. Firstly, hazelnut shells were crushed, repeatedly washed with distilled water to remove dust particles and impurities, dried at 105 ℃ for 48 h and sieved to obtain particle size under 0.3 mm. Chemical activation of the shells was performed with NaOH solution in a mass ratio of 2∶1 (NaOH∶shell). The mixture maintained at room temperature for 24 h and evaporated at 110 ℃ for 48 h. Impregnation of samples was carried out in a stainless steel vertical tubular reactor followed by heating in a furnace under a stream of nitrogen with a flow rate of 150 cm3/min. The sample was heated to the final carbonization temperature (600 ℃), with a heating rate of 10 ℃/min, and held at this temperature for 1 h. Finally, the activated sample was cooled under nitrogen flow and washed with 0.05 mol/L solution of HCl and hot deionized water to remove any residual chemicals until the pH of the solution reached to approximately 6-7, the sample was then dried at 105 ℃.

2.3 Preparation of NiFe2O4 nanoparticles by a co-precipitation method

An aqueous solutions of 0.2 mol/L NiCl2·6H2O and 0.4 mol/L FeCl3·6H2O was prepared in deionized water, and 3 mol/L solution of sodium hydroxide as the precipitating agent was slowly added dropwise to the solution until the pH of the solution reached a constant value of 12. The solution was continuously stirred using a magnetic stirrer for 2 h at 80 ℃, followed by addition of 5 mL oleic acid as the surfactant. The liquid precipitate was later cooled to room temperature and the resulting product was thoroughly washed with distilled water and ethanol to remove impurities and excess surfactant. The sample was centrifuged for 30 min at 6 000 r/min and dried overnight at 80 ℃. The dried powder was calcinated at temperature of 800 ℃ for 2 h in order to crystallize the amorphous nanoparticles[16,26]. The NiFe2O4nanoparticles obtained by the co-precipitation method was denoted as NPC. The NiFe2O4formation reaction is described by the following equation:

NiCl2+2FeCl3+8NaOH → NiFe2O4+4H2O

+ 8NaCl

(1)

2.4 Preparation of NiFe2O4 nanoparticles by a hydrothermal method

The NiFe2O4powder was also synthesized by using the hydrothermal method in an alkaline solution. In brief, 1.188 g of NiCl2·6H2O and 2.6 g of FeCl3·6H2O were dissolved in 50mL distilled water to form a transparent solution. Ammonia aqueous solution (25%) was added dropwise to the above mixture, the suspension was stirred continuously until a pH level of 11-12 was achieved. A certain amount of PVP was added into 10 mL of deionized water under stirring. Finally, the liquid precipitate was transferred into a 100 mL Teflon-lined stainless steel autoclave, sealed and placed in an oven with the temperature rising slowly to 180 ℃ and maintained at this temperature for 12 h. Then the autoclave was allowed to cool to room temperature naturally. The resulting product was washed several times with absolute ethanol and distilled water to remove impurities. The brown precipitate was collected and dried at 80 ℃ overnight[27-29]. The NiFe2O4nanoparticles obtained by the hydrothermal method was denoted as NPH.

2.5 Preparation of NiFe2O4 /AC nanocomposites by a hydrothermal method

In order to prepare nanoparticles with the hydrothermal method, 2 g AC was added in the above precipitated solution before it was transferred into an autoclave. Then liquid precipitate was dispersed with the aid of an ultrasonic bath at 30 ℃ for 20 min. The preparation process is similar to the one described for nanoparticle through the hydrothermal method in section 2.4. The NiFe2O4/AC nanoparticles obtained by the hydrothermal method was denoted as NPH/AC.

2.6 Characterization

The magnetization hysteresis (M-H) measurements was performed using a MDK6 vibrating sample magnetometer (VSM), Meghnatis daghighe kavir company (Kashan, Iran), at room temperature in a magnetic field of kOe. The structural and phase identification of all the samples were revealed by X-ray diffraction patterns (X’PERT MPD, Philips) using Cu Kαradiation with a wavelength of 0.154 059 nm at 30 mA and 40 kV working voltage. Patterns were recorded at 2θrange of 10° to 80 ° with a step size of 0.02 (°)/s. Morphology and microstructure of as-synthesized products was observed by a Zeiss- EM10c microscope, Field Emission Scanning Electron Microscope (FESEM) at an accelerating voltage of 80 kV.

Energy-dispersive X-ray spectroscopy (EDX) was taken using an Oxford Instrument (England). Transmission electron microscopy (TEM) imaging was performed on an EM10C-zeiss (Germany) operating in a voltage range of 180-200 kV (sample were ultrasonicated with a Misonix- S3000 sonicator for uniform dispersion of nanoparticles for TEM analysis). FT-IR spectra were measured on a Burker tensor 22 spectrometer in the range of 4 000-400 cm-1. FTIR samples were prepared by using tablets of pressed KBr (ratio of sample to KBr is 1∶100) with a diameter of 1 cm. Surface area, pore volume and pore diameter of the obtained activated carbon were determined by N2adsorption-desorption at 77 K. The moisture and atmospheric gases in AC were outgassed at 180 ℃ in vacuum for at least 36 h. The sample temperature was reduced to that of liquid nitrogen and nitrogen adsorption isotherms were measured over a relative pressure (p/p0) range from approximately 0.005 to 0.99 and surface area was determined by means of the standard Brunauer-Emmett-Teller (BET) plot. Then, the total surface area was divided by sample weight in order to calculate the specific surface area. The total pore volume was calculated from the amount of adsorbed nitrogen at (p/p0=0.99) and the pore size distribution was derived from the desorption branch of the isotherm based on the Barrett-Joyner- Halenda (BJH) model[30]. Thermal decomposition of samples was studied in a nitrogen atmosphere by using thermo gravimetric analysis (TGA) through a Netzsch TG 209 F1 Iris, at a heating rate of 10 K/min over a temperature range of 25-800 ℃.

3 Results and discussion

3.1 Characterization of hazelnut shells and AC

Chemical analysis of the hazelnut shells and AC was conducted using proximate and elemental analysis which are summarized in Table 1. of Agricultural waste (as raw material) with relatively high carbon content and low ash content is considered as a good candidate for preparation of activated carbon. The carbon content of the activated samples substantially increased from 46.47 wt.% to 84.30 wt.% after pyrolysis and activation of the raw hazelnut shells. In addition, hydrogen, nitrogen, sulfur and oxygen contents followed an opposite trend. As a result of decomposition, volatile compounds (e.g. hydrogen, nitrogen, oxygen and sulfur) are released, accompanied by degradation of organic substances in carbonaceous product and an increase of the carbon content[31].

Table 1 Proximate and ultimate analysis of the precursor AC and NPH/AC nanocomposites.

a: Wet basis;b: Estimated by difference;c: Dry and ash-free basis.

3.2 XRD analysis

Fig. 1 shows XRD patterns of AC, NPC, NPH, and NPH/AC nanocomposite samples. Chemical activation with NaOH disordered the hazelnut shell structure, this was revealed in the XRD patterns of the AC that did not possess any peak and also had no reflections in the pattern of the NPH/AC nanocomposites sample[32,33]. The XRD patterns of nickel ferrite have seven characteristic peaks at 18.37°(111), 30.28°(220), 35.66°(311), 43.31°(400), 53.78°(422), 57.34°(511), 62.93°(440) with the corresponding crystallographic orientations. The diffraction peaks of NPC and NPH were in good agreement with the standard values for NiFe2O4(JCPDS card #75-0035). No peaks of any other phases were observed in XRD patterns of NPH particles, indicating the high purity of the products. However, the X-ray diffraction patterns for NPC displayed a number of additional small peaks (the three weak peaks between (111) and (220), (220) and (311), (400) and (422) ), which can be ascribed to the presence ofα-Fe2O3phase in the sample.

Fig. 1 XRD patterns of (a) AC， (b) NPC， (c) NPH and (d) NPH/AC.

It should be noted that, when NPH and NPH/AC were compared from structural point of view, the crystallite structure of NiFe2O4particles did not change in the presence of AC. Hence, it confirmed that NPH/AC nanocomposites were synthesized successfully with a high purity. In fact, activated carbon cannot affect the crystalline structure due to its intrinsic amorphous structure. The size of the crystallites deduced from Debye-Scherrer’s formula (eq. (2)) with the (311) peak were estimated as 26.38, 8.84 and 8.81 nm for NPC, NPH and NPH/AC, respectively. The average crystallite size of magnetic nanoparticles in the NPH/AC composite was almost identical to that of bare NiFe2O4nanoparticles.

(2)

whereλis the wavelength of X-ray (λ=0.154 18 nm),Kis the Scherrer constant (K= 0.89),βis full-width-at-half-maximum (FWHM) of the (311) plane andθis the diffraction angle.

3.3 Morphological analysis

In order to investigate the surface morphology of as-synthesized NPC, NPH, AC and NPH/AC samples, FESEM technique was used to obtain their morphology under the optimal preparation conditions (Fig. 2(a-d)). Fig. 2(a) and (b) illustrate the FESEM images of as-prepared NPC and NPH, respectively. As shown in the Fig. 2(b), the morphology of NPH nanoparticles was rather spherical, homogeneously distributed with very small sizes while NPC (Fig. 2(a)) was heterogeneous with agglomerated nanoparticles. Large and well-developed pores with different sizes were clearly found on the surface of the activated carbon and the NPH/AC nanocomposites. All of the pores observed by FESEM analysis were macropores (>50 nm), while the nitrogen adsorption data showed that the pores in AC and NPH/AC nanocomposites were mainly mesoporous. The surface of the NPH/AC nanocomposite was not smooth and magnetic nanoparticles were observed on the surface. Moreover, Fig. 2(d) shows that the NiFe2O4nanoparticles were uniformly distributed on activated carbon.

Fig. 3(a,b) shows the EDX results for the magnetic nanoparticles and nanocomposites. The atomic ratio of Ni to Fe was very close to the stoichiometric Ni/Fe ratio in NiFe2O4, which further confirmed the existence of NiFe2O4nanoparticles on AC. Intense peaks in the EDX spectra of magnetic NPH/AC revealed the existence of Ni and Fe elements. Moreover, a comparison of the relative content of Ni, Fe and O element in NPH and NPH/AC showed that nanoparticles of NiFe2O4were synthesized successfully via the simple method.

Fig. 2 FESEM images of (a) NPC， (b) NPH， (c) AC and (d) NPH/AC.

Fig. 3 EDX spectra of (a)NPH and (b)NPH/AC.

3.4 Magnetic properties

The magnetization properties and magnetization curves of NPH, NPH/AC nanocomposite and NPC were measured with a vibrating sample magnetometer at room temperature, the corresponding hysteresis loops are depicted in Fig. 4. The saturation magnetization (Ms), coercive field (Hc) and remanent magnetization (Mr) values of samples are summarized in Table 2. A comparison of the obtained results for NPH and NPC showed that the NPC nanoparticles exhibited ferromagnetic behavior at room temperature while the NPH nanoparticles possessed superparamagnetic behavior with Hcvalues of 137 and 1.64 Oe, respectively[15,25,34-36]. The magnetic analysis for NPH and NPH/AC indicated that the Msand Mrvalues of NPH/AC is 16.21 and 0.055 emu/g, respectively. These values are smaller than those reported for bare NPH, 36.51 and 0.137 emu/g. This reduction can be most likely assigned to the existence of AC and the much small particle sizes of NPH[37,38]. Hcvalue of NPH/AC was higher than pure nanoparticles, the increased coercivity might be due to the increase of the surface anisotropy after NiFe2O4incorporation in AC[39]. The facile separation of the NPH/AC magnetic sample from water was checked by placing a magnet near the glass bottle and the black nanocomposite was attracted to the magnet in a short period of time (inset in Fig.4).

Fig. 4 Magnetic hysteresis loops of (a) NPH, (b) NPC and (c) NPH/AC.

SamplesMs (emu/g)Mr (emu/g)Hc (Oe)NPC36.515.130137.00NPH28.170.1371.64AC-NPH16.210.0551.95

3.5 TEM analysis

Further observation on the structure of AC, NPH and NPH/AC was conducted by TEM analysis (Fig. 5). The NiFe2O4crystal sizes and inner pore distribution of AC were not easily observed from SEM analysis, therefore TEM analysis was accomplished for these samples. Fig. 5(a) shows a developed porous

Fig. 5 TEM micrographs of (a) AC, (b) NPH and (c) NPH/AC.

structure of activated carbon and Fig. 5(b) the NiFe2O4particles size with average diameter of 9.6 nm. Fig. 5(c) also indicates that NiFe2O4nanoparticles with a size of approximately 9.2 nm were homogeneously distributed on AC surface and pores inside, which agreed well with the results obtained by the analysis of XRD and Debye-Scherrer’s formula (black regions are NiFe2O4nanoparticles and light regions are AC)[30, 40].

3.6 Fourier transform infrared spectroscopy

Fig. 6 FTIR spectra of (a) AC, (b) NPH and (c) NPH/AC.

3.7 Nitrogen adsorption

Figs. 7 and 8 show the N2adsorption and desorption isotherms and pore size distribution curves of AC and NPH/AC nanocomposites. Detailed characteristics of the porosity of AC and NPH/AC (surface area, pore volume and average pore diameter) are summarized in Table 3. As shown in Table 3, the results indicated that the surface area and the pore volume of AC reduced from 314 to 288 m2g-1and 0.363 9 to 0.333 8 cm3g-1, respectively by loading NPH. This may be attributed to the formation of NiFe2O4nanoparticles in the structure of AC[40]. The results of BET analysis indicated that although some of the canals of activated carbon were blocked by the presence of NiFe2O4nanoparticles, the nanocomposites still retained high BET surface area and pore volume. The average pore diameters for AC, NPH/AC nanocomposite were 4.24 and 5.05 nm, respectively. Both AC and NPH/AC samples exhibited type IV isotherms according to the IUPAC classification of adsorption isotherms, which revealed a mesoporous structure in samples[49].

Fig. 7 Nitrogen adsorption isotherms of (a) AC and (b) NPH/AC nanocomposite at 77 K.

Fig. 8 Pore size distributions of (a) AC and (b) NPH/AC nanocomposite.

SamplesSBET /m2g-1Vtotal /cm3g-1daverage /nmAC3140.36394.24NPH/AC2880.33385.05

SBET: Brunauer-Emmett-Teller surface area,Vtotal: Adsorption total pore volume,daverage: BJH Desorption average pore diameter.

3.8 Thermal analysis

Thermogravimetric analysis of activated carbon and NPH/AC nanocomposite in the protection of nitrogen was evaluated and their results are shown in Fig. 9. The initial weight loss in temperatures below 200 ℃ corresponded to the loss of water present as moisture and light volatile compounds in the samples, which were 7% and 2% for activated carbon and NPH/AC, respectively. The activated carbon curve in Fig. 9(a) showed a 8.6% weight loss from 250 to 650 ℃, which can be accounted for by the release of volatile products. The second weight loss stage in NPH/AC (Fig. 9(b)) between 350 and 500 ℃ was rapid, which might be ascribed to the processes such as decomposition of residual PVP, release of volatile and gasification of carbons. The weight loss of samples at temperatures above 600 ℃ can be assigned to release of strongly held volatile components and probably decomposition of activated carbon[13].

Fig. 9 TGA curves of (a) AC and (b) NPH/AC.

4 Conclusions

Superparamagnetic NiFe2O4nanoparticles and NPH/AC nanocomposite were synthesized by the hydrothermal and co-precipitation method. Magnetic properties, crystallite structure, morphology, size of pores, chemical structure, surface area, pore volume and average pore diameter, thermal stability of AC, NiFe2O4nanoparticles and NPH/AC nanocomposite were investigated. Saturation magnetization and coercivity of NPH/AC nanocomposite were in the range of 39.15 emu/g and 1.64 Qe, respectively. The TEM and SEM investigation indicated that the surface and pores of the AC was coated with NiFe2O4nanoparticles with a size of approximately 9.2 nm. The diffraction peaks of NPH/AC and NPH were in good agreement with the standard values for NiFe2O4with crystallite sizes of approximately 8.81 nm, which agreed well with the TEM observations. All bands of FTIR in the NPH/AC nanocomposite were clearly conformed to the peaks of NPH and AC. As-synthesized nanocomposite had appropriate surface area and large average pore diameter (～5 nm), which can be used as an absorbent for high adsorption rate and capacity for organic pollutants and large dye molecules in wastewater. It is expected that the NPH/AC magnetic nanocomposite derived from agricultural residues with appropriate properties can be used as a potential sorbent for removal of various toxic pollutants and dye molecules from wastewater.

Acknowledgements

This study was supported by “The Chemical Engineering Department of Babol University of Technology in Iran”. The authors wish to thank the Director, Polymer Laboratory, for his kind permission to publish this paper.