Removal efficiency on magnetite (Fe 3 O 4 ) of some multicomponent systems present in synthetic aqueous solutions Andra Predescu, Ecaterina Matei, Andrei Predescu, Andrei Berbecaru Faculty of Materials Science and Engineering Politehnica University of Bucharest Splaiul Independentei Street, nr. 313, Bucharest ROMANIA andra.predescu@ecomet.pub.ro http://www.ecomet.pub.ro Abstract: - Environmental pollution remains an important issue attracting great global attention despite many efforts being put in place worldwide. Heavy metals represent one of the most harmful pollutants in surface and ground waters. The paper refers to researches regarding the removal efficiency on magnetite of some multicomponent systems (Cd, Ni, Cu, Cr) present in synthetic aqueous solutions. 10 nm nano-sized crystals of magnetite iron oxides were synthesized and characterized with several spectroscopic techniques as well as transmission electron microscopic measurements or x-ray diffraction analysis. The removal efficiency on magnetite was established through adsorption tests. Key-Words: - nanomaterials, adsorption, magnetite, heavy metal ions, waste waters, aqueous solutions 1 Introduction Over the past decades, extensive synthetic efforts from chemists and process engineers have led to discovery of many novel metal and metal oxide nanoparticles [1]. Iron nanoparticles are made usually in the size range of 1-100 nm, and are highly reactive because of their large specific surface areas. Iron-based nanoparticles are inexpensive, easily scalable and highly reactive towards a wide array of organic and inorganic pollutants. The industrial waste water effluents contains mainly the various heavy metals, which are a real threat to the environment and public health, because of their toxicity and persistence in the environment [2]. The removal of toxic metals such as chromium, cadmium, copper, lead, nickel, mercury and zinc from waste waters became a necessity due to their toxicity and carcinogenicity. There are many removal techniques such as adsorption, chemical precipitation, ion exchange, filtration, membrane separation, and reverse osmosis in order to diminish the pollution impact of these metals [3]. Also, due the development in nanotechnology, nanoscale iron oxides have became a raw material for treating the waste waters and soils, accelerating the coagulation of sewage, removing radionuclides, adsorbing organic dyes and cleaning up the contaminated soils [4]. There are many types of nanomaterials with these characteristics, but magnetic iron oxide nanoparticles are efficient adsorbents, which couple magnetic separation with ionic exchange capacity for removal of heavy metals pollutants [5]. 2 Experimental 2.1 Preparation and characterization of magnetite nanoparticles There are various methods for preparation of Fe 3 O 4, using different reagents for synthesis. In this paper the nanoparticles were prepared by coprecipitation of ferric chloride (FeCl 2.4H 2 O) and ferrous chloride (FeCl 3.6H 2 O) with NaOH solution. D-sorbitol was used to prevent the agglomeration between the nanoparticles [6]. The iron solutions were strongly stirred in water, after adding NaOH solution. The precipitates were separated by magnetic decantation or slow filtration after which it was washed several times with distilled water and ethanol. The magnetite nanoparticles were dried into oven at 60 o C. For ph adjustment at 13, a small quantity of NaOH was added. The magnetite formation took place once with the solution blackening. The obtained suspension was kept in agitation for 2 hours. After 2 hours of stirring at high temperatures, the stirring was continued at room temperature for 30 minutes. The Fe 3 O 4 nanoparticles were then washed with distilled water and alcohol and dried at 55 o C. The structure and morphology of the Fe 3 O 4 nanoparticles was characterized by X-ray powder diffraction, which was carried out in a SHIMADZU ISBN: 978-1-61804-069-5 63
diffractometer with high-intensity Cu Kα radiation (λ = 1.54065 Å) with the 2θ range from 10 o to 90 o. The particle size and distribution were detected by transmission electron microscopy (TEM).The energy dispersive X-ray (EDX) and selected area electron diffraction (SAED) spectra were recorded with a TECNAI F30 G 2 high resolution transmison electron micrsoscope with 1Å line resolution and with an EDX detector with 133 ev resolution. Fig. 1: XRD patterns for Fe 3 O 4 nanoparticles The obtained samples were analyzed by X-ray diffraction (XRD) using a diffractometer with highintensity Cu Kα radiation (λ = 1.54065 Å). The diffraction pattern for maghemite is shown into Figure 1. The diffraction peaks reveal a cubic spinel structure with no other phases existing in the sample. The morphology and structure of the maghemite samples were investigated by transmission electron microscopy through bright field (TEMBF) as seen in Figure 2(a). Spherical particles with a reduced size and a good dispersion can be observed in this figure. In order to identify the planes associated with the maximum values for the nanoparticles a selected area electron diffraction (SAED) image was recorded (Figure 2(b)). The SAED diffraction peaks corresponding to the (533), (800), (440), (511), (422), (222), (311), (220) and (111) planes for Fe 3 O 4 match with the XRD pattern. The SAED image confirmed the formation of the Fe 3 O 4 nanoparticles. These nanoparticles have been used as adsorption material for pollutants from wastewaters, in accordance with their small particle size and high specific surface area. Fig. 2: (a) Transmission electronic microscopy through bright field (TEMBF) for Fe 3 O 4 ; (b) Selected area electron diffraction (SAED) image associated to the sample from figure (a) ISBN: 978-1-61804-069-5 64
2.2. Reactives and materials The metal solutions were prepared from Merck stock standard solutions, consisting of dissolved metal into nitric acid, in case of copper, nickel and cadmium and from potassium chromate (K 2 CrO 4 ) salt dissolved into distilled water in case of chromium. Each initial metal standard solution concentration was. The initial standard solutions were used in order to evaluate the maximum quantity adsorbed at ph 2.5. The metal solutions with the same concentrations were mixed together for obtaining a multi-component system (Ni, Cd, Cu, Cr). The evaluation took place by analyis from 10 to 10 minutes, during 2 hours. The time for recovery was 24 hours and all magnetite nanoparticles and nanocomposite were recycled by the magnetic separation. All samples containing hexavalent chromium were analyzed by diphenylcarbazide method with a molecular absorption spectrometer (Cintra 202 GBC) with spectral domain between 190 and 1000 nm. Total chromium quantity and the others metals were measured with an atomic absorption spectrometer (GBC 932 AB Plus) with spectral domain between 185 and 900 nm. Cd soluńion Ni soluńion Cu soluńion K2CrO4 dissolved salt in ultrapure water Ultra pure water dilution for each metal 10 mg/l Cd (II) 10 mg/l Ni (II) 10 mg/l Cu (II) 10 mg/l Cr (VI) Mixture form every solution with the same concentration, final concentrations: 10 mg each metal /L, 20 mg each metal/l, 50 mg each metal/l Fig. 3. Procedure scheme for obtaining the quaternary system of metal ions (Cd, Ni, Cr) in synthetic aqueous solutions to test the adsorption capacity of magnetite nanoparticles 2.3 Adsorption tests The adsorption studies were conducted by addition of 0.1 g of Fe 3 O 4 nanoparticles in the quaternary system shown in Figure 3. The concentrations were 10, 20 respectively 50 mg / L of each metal. The magnetite was dispersed by stirring at a speed of 600 rpm with a magnetic stirrer. For studies on the kinetics of adsorption process, the quaternary solutions were kept in contact for different periods of time (between 10 and 100 minutes). The work temperature was 22 degrees C and the ph values were chosen according to specific conditions of industrial waste water, in our case, 2.5. Here are the results for adsorption in acid at ph 2.5. The nanoparticles separation was performed using a magnet and 5 ml of supernatant from the quaternary solution was taken every 10 minutes for analysis of the metal ion concentration in the remaining solution. The analysis method were those indicated in the previous paragraphs. The removal efficiency of Cr, Cd, Ni, Cu at ph 2.5 is shown in the table 1 the results beeing shown for the three types of concentrations and the contact time between 10 and 100 minutes. As can be seen from the table, for the 10 mg/l, the retain tendency on the adsorbents increases in the order: Cr <Cu <Ni <Cd. For 20 mg/l concentrations, removal efficiency is: Cu <Ni <Cd <Cr, and for 50 mg/l the results are: Cu<Cd <Ni <Cr. It appears that the values become stable after cca. 10 minutes, which can lead to the conclusion that the adsorption can be instantaneous. The metals retention varies, but the efficiency removal is over 90%. ISBN: 978-1-61804-069-5 65
Removal efficiency (η, %) of heavy metals from quaternary solutions with different concentrations: Time (minutes) 10 mg/l 20 mg/l 50 mg/l Cr Cd Ni Cu Cr Cd Ni Cu Cr Cd Ni Cu 10 95.40 97.70 95.10 97.46 96.05 96.00 95.53 95.17 98.16 97.05 98.19 98.03 20 97.50 97.85 97.51 97.49 96.00 98.23 99.41 95.12 98.18 97.98 98.77 98.02 30 98.30 97.83 93.08 97.43 97.45 98.83 99.57 95.28 98.22 98.43 99.17 98.16 40 100.00 98.77 99.07 97.22 100.00 99.11 99.89 95.17 99.06 98.88 99.40 98.15 50 100.00 98.30 97.11 97.02 100.00 99.39 100.00 95.10 100.00 99.19 99.66 96.25 60 100.00 98.82 100.00 97.01 100.00 99.60 100.00 95.11 100.00 99.39 99.78 96.66 70 100.00 99.07 100.00 97.03 100.00 99.55 100.00 95.22 100.00 99.47 99.86 96.88 80 100.00 99.85 100.00 97.12 100.00 99.81 100.00 95.08 100.00 99.60 100.00 96.33 90 100.00 99.64 100.00 97.21 100.00 99.78 100.00 95.28 100.00 99.59 100.00 96.33 100 100.00 99.50 100.00 97.10 100.00 99.69 100.00 95.11 100.00 99.50 99.98 96.35 Table 1: Removal efficiency of heavy metals from quaternary solutions with different concentrations After the analysis for the determination of Cr (VI) by the two analysis methods, it was found that the values are about the same. This phenomenon is explained by the amount of magnetite phpzc which is between 6.3 to 6.8 [7]. The evolution of competitive ions in the quaternary system is shown in Figure no. 4. The highest amount adsorbed at balance was reached after the first 10 minutes and corresponds to the 50 mg / L concentration. The results decrease as shown bellow: Cr (48.5 mg/g)> Ni (47.29 mg/g)> Cu (46.72 mg/g)> Cd (45.39 mg/g). Increasing the contact time up to 100 minutes does not increase the amount of metal retained on magnetite nanoparticles. Fig. 4. The amount of metal adsorbed at equilibrium at ph 2.5 as function of contact time ISBN: 978-1-61804-069-5 66
3. Conclusions The magnetite nanoparticles with an average diameter of 10 nm were synthesized using a coprecipitation method. These nanoparticles were successfully tested for the removal of some toxic metals from synthetic aqueous solutions, such as hexavalent chromium, copper, cadmium and nickel. The adsorption process was conducted into acidic environment, at ph 2.5. The obtained adsorption data indicated a good adsorption capacity for metal ions removal and a higher adsorption tendency for hexavalent chromium in comparison with the other metal ions. The regeneration of the adsorbents was investigated with good results, in presence of sodium hydroxide solution. The adsorption study showed that the electrostatic attraction was responsible for the metal removal in case of magnetite nanoparticles. The obtained data represents only the preliminary result obtained for achieving a systematic study regarding the removal of heavy metals from wastewaters using as adsorbents the nanoparticles with high capacity of adsorption due theirs high surface area. [4] Sun Y. k., Ma M., Zhang Y., Gu N., Colloids and Surfaces A: Physicochem. Eng. Aspects, 245, 15 (2004) [5] Predescu A., Researches regarding the dimension and distribution of magnetic iron oxide nanopowders and theirs characterization, (In Romanian), Bulletin of the Polytechnic Institute of Jassy, LVI (LX), (2010), 95 102. [6] Hu J., Chen G., Lo M.C.I., Removal and recovery of Cr (VI) from wastewater by maghemite nanoparticles, Water Research, 39, (2005), 4528 4536. [7] Ris 1. Ecaterina Matei, A. M. Predescu, E. Vasile, C. Predescu Investigations on nano-iron oxides properties used for different industrial applications, în Optoelectronics and Advanced Materials Rapid Communications, vol. 5, No. 3, March 2011, p. 296 301. References: [1] Hu J., Lo M.C.I., Chen G., Comparative study of various magnetic nanoparticles for Cr (VI) removal, Separation and Purification Technology, 56, (2007), p. 249 256. [2] Lo M.C.I., Hu J., Chen G., Iron-Based Magnetic Nanoparticles for Removal of Heavy Metals from Electroplating and Metal-Finishing Wastewater, In: Nanotechnologies for Water Environment Applications, Zhang C.T., Surampali Y.R., Lai K.C.K, Hu Z., Tyagi R.D., Lo M.C.I. (Eds.), American Society of Civil Engineers, Virginia, (2009), 213-264. [3] Wang X., Zhao C., Zhao P., Dou P., Ding Y., Xu P., Gellan gel beads containing magnetic nanoparticles: An effective biosorbent for the removal of heavy metals from aqueous system, Bioresource Technology, 100, (2009), 2301-2304. ISBN: 978-1-61804-069-5 67