Creation of Nanomagnetite Aggregated Iron Oxide Hydroxide for Magnetically Removal of Fluoride and Phosphate from Wastewater

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1 , pp Creation of Nanomagnetite Aggregated Iron Oxide Hydroxide for Magnetically Removal of Fluoride and Phosphate from Wastewater Akbar ESKANDARPOUR, Kensuke SASSA, Yoshiyuki BANDO, Hiroshi IKUTA, Kazuhiko IWAI, Masazumi OKIDO and Shigeo ASAI Graduate School of Engineering, Nagoya University, Nagoya Japan. (Received on August 3, 2006; accepted on November 10, 2006) A new chemical adsorbent for fluoride and phosphate removals was produced by applying a novel nanomagnetite aggregation process through the formation procedure of an iron oxide hydroxide, i.e., schwertmannite. Although there was no evidence of magnetite related peaks in the XRD pattern of the new adsorbent, because of the very small amount of the used magnetite particles, the SEM picture reveals a surface alteration on the crystal structure of the new adsorbent comparing to the schwertmannite. The results of magnetic removal of fluoride and phosphate using the new adsorbent indicate that nanomagnetite aggregation process not only improves the magnetic property, but also provides a highly-promoted fluoride and phosphate adsorption capacities comparing to the schwertmannite. KEY WORDS: fluoride removal; phosphate removal; magnetic filter; wastewater treatment; schwertmannite; nanomagnetite; electromagnetic processing of materials. 1. Introduction Application of nanomagnetic particles has recently received a great deal of attention due to their high surface reactivity, high saturation magnetization, very large surface area and so on. A wastewater treatment process with the aid of magnetic field requires a relatively high magnetic susceptibility of particles to be separated through a magnetic filter. To overcome this obstacle, the technology using magnetic seeding particles has been developed to enable the separation of small particles which are not magnetic materials. By varying the surface characteristics, attaching the desired species onto the surface, and entrapping magnetic seeding particles, the selective recovery of colloidal and macromolecule species has been achieved. That is, nonmagnetic adsorbents can convert to apparent magnetic materials via magnetic seeding aggregation, which can be easily adopted in various chemical and biochemical processes. 1 4) However, it requires consuming relatively large amounts of magnetic particles which are hard to work and recycle as well. Excessive fluoride concentration in drinking water contaminated by discharging of wastewater of semiconductor, metal processing, fertilizers, and glass-manufacturing industries to the surface water, results in mottling of teeth, softening of bones, and ossification of tendons and ligaments. 5) According to the World Health Organization (WHO), the permitted limitation of fluoride for drinking water quality is 1.5 mg/l. The most common processes for fluoride removal are chemical precipitation, adsorption and electrochemical method, where among them; the most widely used method is adsorption of fluoride on adsorbents. 5 8) On the other hand, removal of phosphate from wastewater has become a great environmental concern due to its direct impact on the prevention of eutrophication in inland and coastal waters. Among the chemical precipitation, biological removal and crystallization which are the most well known methods for the phosphate removal, the adsorption and chemical precipitation have been widely used for phosphate removal. 9 12) The removal of phosphate from wastewater consists of the conversion of soluble phosphate to an insoluble solid phase. This solid phase can be separated from water by means of sedimentation or filtration. In wastewater treatment, the most common and successful methods to precipitate phosphate involve the application of dissolved cations such as Al 3, Ca 2, Fe 3 and to a lesser extent of Fe 2. 9) Schwertmannite which is an iron oxide hydroxide with the ideal formula of Fe 8 O 8 (OH) 6 SO 4 has been found as a new and strong fluoride and phosphate adsorbent. It can be easily applied in an industrial scale. 13) Its very fine particle size, however, makes it difficult to be applied to the classical filtration techniques for solid liquid separation. Thus, magnetic filtration can be considered as a suitable candidate for separation of fine schwertmannite particles from its suspended liquid. In the magnetic filtration, however, in order to increase the trapping efficiency of adsorbent particles, the conversion of the magnetic property of the adsorbent is crucial. 558

2 In this study, by using of nanomagnetite particles, a new iron oxide hydroxide chemical adsorbent has been created. The effect of the nanoaggregated magnetite particles on the physicochemical properties such as phosphate and fluoride adsorptions has been studied and the magnetic filtration test using the new adsorbent has been conducted. 2. Experimental The experimental work to produce the new adsorbent can be divided into two sequential steps: (1) the formation of nanomagnetite particles by means of a chemical precipitation and (2) the production of an iron oxide hydroxide adsorbent using a wet chemical process where the nanomagnetite particles are embedded on it via an aggregation process presented in this work. Magnetite nanoparticles were prepared by co-precipitation from a mixture of FeCl 2 and FeCl 3 (1 : 2 molar ratio) throughout the slow addition of diluted NH 3 solution. 1) Schwertmannite, which is an iron oxide hydroxide adsorbent, was generated by homogenous hydrolysis of Fe 2 (SO 4 ) 3 nh 2 O using urea as a neutral agent. 13) After dissolving about 25 g of Fe 2 (SO 4 ) 3 5H 2 O in 500 ml of distilled water, the solution was preheated at about 60 C and slowly stirred for 10 min and then a 10 ml of the slurry containing the fresh nanomagnetite particles was added to the solution. Then, 500 ml of 5 M urea solution was added drop-wise for about 2 4 h into the solution. When precipitating of crystals begun, the color of the solution was changed from red to brownish yellow and the reaction was continued until half of the initial solution was vaporized. After solid liquid separation, solid part was rinsed for removal of impurities. Figure 1 shows a flow chart of the specimen fabrication procedure. The yielded material which was dried below 40 C was a newly developed adsorbent in this work. That is, the adsorbent produced under without addition of magnetite was schwertmannite, and the adsorbent under with that of magnetite was the new adsorbent. The ratio of the magnetite to the yielded adsorbent was about 0.3 to 0.5%, which is quite small amount. The physicochemical, magnetic and adsorption properties of the new adsorbent were compared with the ordinary schwertmannite. In a batch fluoride and phosphate adsorptions tests, 50 ml of an artificial wastewater with desired concentrations of fluoride or phosphate and 0.05 mg of the adsorbent were mixed and shaken for about 3 h to reach the equilibrium. Then, the yielded solution was passed through a paper filter and finally, fluoride or phosphorous concentrations in a filtrate was analyzed using the standard methods. The magnetic filtration trial was also carried out in a super conducting magnet. As schematically shown in Fig. 2, 200 ml of the solution pretreated by adding 10 g/l of the adsorbent to the artificial wastewater was passed through a cylindrical glass tube with 10 cm length and 2.5 cm diameter, packed with ferromagnetic wires, the center of which was located at the place with the highest gradient of a magnetic field in the magnet. Then, the amount of the adsorbent passed through the filter was weighted to obtain the separation efficiency of the magnetic filter. The magnetic susceptibility of both the adsorbents, namely the new one created in this work and the ordinary schwertmannite was investigated using a vibrating sample magnetometer, VSM, under Fig. 2. Fig. 1. Flow chart of the adsorbent fabrication procedure. A schematic view of the experimental setup of the magnetic filtration. the maximal external field of 0.3 T. The surface charge of the adsorbents was characterized using an electrophoretic light scattering spectrophotometer. The crystallographic structure and morphology of the adsorbents were examined by a X-ray diffractometer using copper Ka operating at 40 kv/40 A and a scanning electron microscope, respectively. 3. Results and Discussion 3.1. Effects of Initial Fluoride and Phosphate Concentrations and Adsorption Isotherm In order to investigate the role of the aggregated magnetite nanoparticles on the adsorption capacity of the new adsorbent, the batch adsorption tests were performed by varying the initial concentrations of the phosphate and fluoride ions at optimum ph values and 1 g/l of the adsorbent. Then an isotherm model was applied to evaluate the fluoride and phosphate adsorption capacities of the adsorbents. Figure 3 shows the phosphate and fluoride adsorptions results with the new adsorbent and the ordinary schwertmannite. It was observed that the fluoride and phosphate ad- 559

3 Fig. 3. F (a) and P (b) adsorptions results for the schwertmannite and new adsorbent under various initial P and F concentrations. Table 1. Langmuir isotherm constants for the adsorption data. Fig. 4. Langmuir plots of fluoride and phosphate adsorptions for the new adsorbent and schwertmannite. sorption capacities of the new adsorbent were much higher than that of the adsorbent without magnetite, i.e., the ordinary schwertmannite. These adsorption data were analyzed in terms of Langmuir model as shown in Fig. 4. The correlation coefficients of more than 0.99 confirm that the data are well fit with this model. The Langmuir model assumes that adsorption occurs at specific homogeneous adsorption sites within the adsorbent and that intermolecular forces decrease rapidly with the distance from the adsorption surface. 14) It can be expressed by the following equations; QbCe qe...(1) 1 bce Ce 1 Ce...(2) qe bq Q where q e is the surface concentration which equals to the amount of the sorbate at equilibrium (mg (adsorbate)/g (adsorbent)), Q and b the Langmuir isotherm constant, C e the fluoride or phosphate concentration in the solution at equilibrium. That is, the amount of q e is obtained by the following equation; q e C 0 C e...(3) where C 0 is the initial P or F concentration in each batch of the experiment. The isotherm constants, Q and b were estimated by fitting a set of experimental data to the isotherm model given in Eqs. (1) and (2). The Langmuir plots of fluoride and phosphate adsorptions on both the adsorbents were depicted as shown in Fig. 4. For the maximum initial concentrations of 100 mg/l, the fluoride and phosphate adsorption capacities Q of both the adsorbents were determined from the slope of the Langmuir plots. Langmuir constants and correlation coefficients (R 2 ) are given in Table 1. The maximum adsorption capacities given in Table 1 for both the adsorbents indicate that the adsorption capacity in the new adsorbent was remarkably increased, which implies to the unique effect of the nanomagnetite particles aggregated in the new adsorbent Crystal Structure of New Adsorbent From the SEM observation, it was found that the particle size of the magnetite ranged from 10 to 30 nm. The main advantage of nanomagnetite, with such an ultra fine of particle size, is its very large external surface. 15) In order to find out the crystal structure of the new adsorbent in comparison with the schwertmannite, the XRD patterns of both the adsorbents and magnetite were shown in Fig. 5. The new adsorbent particles showed the same X-ray diffraction peaks as those of schwertmannite and the peaks relating with added magnetite were not observed on it. This is because of a quite small amount of the nanomagnetite particles aggregated into the new adsorbent structure where the ratio of the magnetite particles to the schwertmannite 560

4 ranged from %. This figure also implies that the nanomagnetite particles had no altering effect on the basic crystal structure of the schwertmannite. Since from the XRD pattern of both the adsorbents, any significant alteration in the crystallographic structure was not observed (Fig. 5), a scanning electron microscopy (SEM) was employed to characterize the morphology of the adsorbents. Figures 6(a) and 6(b) show the SEM pictures of both the adsorbents with and without magnetite, corresponding to the new adsorbent and the ordinary schwertmannite, respectively. The schwertmannite is known as a poorlycrystalline iron oxide hydroxide that consists of spherical to ellipsoidal particles with hedge-hog like aggregates and several mm in size. 16) The needle-like structures with average width and thickness of 2 4 nm and length of nm radiate from the particle surfaces to devote a pincushion morphology to this material. 17,18) It was clearly observed, that the needle-like surface structure of the schwertmannite seen in Fig. 6(a) has almost disappeared in Fig. 6(b). It may be denoted that the better achievement in fluoride and phosphate adsorptions of the new adsorbent as shown in Fig. 3 might attribute to the alteration in the surface morphology of the ordinary schwertmannite. The surface charge of the adsorbents was also measured by the zeta potential meter under 0.2 g/l of the adsorbent solution in the range of ph Figure 7 illustrates the zeta potential data for both the adsorbents, i.e., the ordinary schwertmannite and the new adsorbent of magnetite-aggregated schwertmannite. Although the zeta potential curves of both the adsorbents are almost the same beyond the ph value of around 6, but below this ph value, the two curves are deviated from each other and consequently the isoelectric point, which is the most important point on the surfacebehavior of a colloidal system, is different. The direct estimation of the contribution of the surface charge to the fluoride and phosphate adsorption capacities of the adsorbents is difficult, since the surface charge of both the adsorbents are nearly the same around the neutral ph value in which the adsorption was performed in the experiments. Fig. 5. XRD patterns of the adsorbent with and without magnetite Magnetic Properties and Magnetic Separation Figure 8 indicates that the adsorbent without magnetite, namely, the ordinary schwertmannite is paramagnetic. From the magnetization curve, the magnetic susceptibility of the schwertmannite was about By aggregating nanomagnetite particles in the schwertmannite, the new adsorbent has appeared as a composite of ferromagnetic and paramagnetic materials. That is, the new adsorbent can be effectively separated through a magnetic filter rather than schwertmannite. The magnetic separation trial was performed using the experimental setup shown in Fig. 2. Table 2 shows the Fig. 6. SEM pictures of the adsorbent (a) without magnetite, (b) with entrapped magnetite. Fig. 7. Zeta potential curves for the adsorbent with and without entrapped magnetite. Fig. 8. Magnetization curve belong to the adsorbent with and without magnetite. 561

5 Table 2. Magnetic separation efficiency for the adsorbents (%). magnetic separation results of the two adsorbents from their suspended liquids through a magnetic filter, in which the separation efficiency of adsorbent was defined as the ratio of the capturing particles to the entering ones. By applying a magnetic intensity of 0.5 T, the magnetic separation efficiency of the new adsorbent reached to more than 99% and nearly whole of the adsorbent particles was separated through magnetic filter under rather weak magnetic field. That is, it is noticed that the new adsorbent can be easily attracted by an ordinary permanent magnet. High temperature and strong stirring of the preheated Fe 2 (SO 4 ) 3 nh 2 O solution, as well as the addition of the fresh nanomagnetite particles in the solute state during the making procedure of the new adsorbent may be responsible for rather high magnetic susceptibility under such a trivial amount of magnetite nanoparticles. 4. Conclusions By introducing a magnetite aggregation process to the nucleation stage of schwertmannite, a new adsorbent has firstly been produced. Following results have been obtained in this study; (1) A new adsorbent which is a composite of ferromagnetic and paramagnetic materials has been produced. (2) The fluoride and phosphate adsorption capacities of the new adsorbent are much higher than those of the adsorbent without magnetite particles, i.e., the ordinary schwertmannite. (3) Surface alteration of the adsorbent due to the aggregated nanomagnetite particles is responsible for the high adsorption capacity of the new adsorbent. (4) Consuming a trivial amount of magnetite nanoparticles in this aggregation process results in the alteration in the surface morphology. Acknowledgements This work was partly supported by JSPS Asian CORE Program. The authors wish to thank Associate Prof. H. Asano and Mr. Y. Takeda (Dept. of materials, physics and energy engineering, Nagoya University) for their kind help and assistance on magnetization measurement. REFERENCES 1) C. F. Chang, P. H. Lin and W. Holl: Colloid Surf. A: Physicochem. Eng. Asp., 280 (2006), ) G. Moffat, R. A. Williams, C. Webb and R. Stirling: Miner. Eng., 7 (1994), ) N. Gokona, A. Shimada, H. Kanekoa, Y. Tamaura, K. Itob and T. Ohara: J. Magn. Mag. Matern., 238 (2002), 47. 4) A. Uheida, M. Iglesias, C. Fonta s, Y. Zhang and M. Muhammed: Sep. Sci. Technol., 41 (2006), ) F. Shen, X. Chen, P. Gao and G. Chen: Chem. Eng. Sci., 58, (2003), ) Y. Zhou., C. Yu and Y. Shan: Sep. Pur. Tech., 36 (2004), 89. 7) B. Grzmil and J. Wronkowski: Desalination, 189 (2006), ) R. Aldaco, A. Irabien and P. Luis: J. Chem. Eng., 107 (2005), ) M. Y. Can and E. Yildiz: J. Hazard. Mater., B135 (2006), ) S. Tillotson: Filt. Sep., 43 (2006), ) S. Irdemez, Y. S. Yildiz and V. Tosunoglu: Sep. Pur. Tech., 52 (2006), ) K. Karageorgiou, M. Paschalis and G. N. Anastassakis: J. Hazard. Mater., 139 (2007), ) A. Eskandarpour, K. Sassa, Y. Bando, M. Okido and S. Asai: Mater. Trans., 47 (2006), ) N. Unlu and M. Ersoz: J. Hazard. Mater., B136 (2006), ) J. T. Mayo, S. Yean, L. Cong, W. Yu, J. Falkner, A. Kan, M. Tomson and V. Colvin: Proc. of ISNEPP (Asia Pacific Nanotechnology Forum), http;// 160&cf 5. (aceessed July 2, 2006). 16) R. M. Cornel and U. Schwertmann: The Iron Oxides, 2nd ed., Wiley- Vch, Weinheim, Germany, (2003), ) J. M. Bigham, L. Carlson and E. Murad: Miner. Maga., 58 (1994), ) L. Carlson, J. M. Bigham, U. Schwertmann, A. Kyek and F. Wagner: Environ. Sci. Technol., 36 (2002),