Microsoft Word - 43poletti.docx CHEMICAL ENGINEERING TRANSACTIONS VOL. 47, 2016 A publication of The Italian Association of Chemical Engineering Online at www.aidic.it/cet Guest Editors: Angelo Chianese, Luca Di Palma, Elisabetta Petrucci, Marco Stoller Copyright © 2016, AIDIC Servizi S.r.l., ISBN 978-88-95608-38-9; ISSN 2283-9216 Adsorption and Regeneration of Fluoride Ion on a High Alumina Content Bauxite Nicolò Maria Ippolito, Gianluca Maffei, Franco Medici, Luigi Piga Dipartimento di Ingegneria Chimica, Materiali e Ambiente, “Sapienza” Università di Roma, Via Eudossiana 18, 00184 Roma (Italy) franco.medici@uniroma1.it Adsorption of fluoride ion from aqueous solutions by using different silico-aluminous natural products was studied. Preliminary batch tests indicated how bauxite shows a high removal efficiency. Such optimum efficiency is due to the elevated presence of aluminium hydroxide (gibbsite) and aluminium oxide hydroxide (boehmite). Both batch and continuous experiments were performed: Freundlich equation well described batch equilibrium data. In continuous-flow column experiments, the effects of inlet fluoride concentration (5–50 mg L–1) and flow rate (up to 2.5 mL min–1) on breakthrough time and adsorption capacity were studied. Column studies showed that the dynamic adsorption capacity depends on the inlet fluoride concentration and the flow rate. After adsorption of fluorides, saturated bauxite, was regenerated by using two different solutions containing NaOH and H2SO4. Column tests indicated a different behaviour of original respect to regenerated bauxite. Key words: Fluoride, Bauxite, Aluminium hydroxide, Adsorption, Regeneration, Kinetics. 1. Introduction Fluoride is an essential constituent for humans, although accumulation of fluoride ions in the organic tissues is harmful to the human health. The maximum contaminant level of fluoride in drinking water depends on standards fixed by different countries, while the guideline value, set by the World Health Organization (2011), is 1.5 mg L-1. An excess of fluoride (>1.5 mg L-1) is harmful to the human health. In fact in children, consistent exposure to fluoride at high levels can discolour and disfigure emerging permanent teeth (dental fluorosis), in adults, the same high fluoride level, appears to increase the risk of bone fracture. What is more, a U.S. National Research Council panel (NCR, 2006) noted that fluorides may also trigger more serious problems (bone cancer and damage to the brain and thyroid gland). In Europe and also in Italy the maximum level for fluorides in drinking water is at the moment 1.5 mg L-1, but higher level of fluoride in groundwater are detected in various country from Africa, Asia, Europe and USA, so that this can be considered a world-wide problem. Excess of fluoride in drinking water is also evident in Italy, where more than 100 districts of two regions (Lazio and Toscana) exceed the suggested European limit. Fluoride removal techniques include chemical precipitation, membrane process (reverse osmosis), ion exchange and adsorption (Gogoi S. and Dutta R.K., 2015; and Loganathan et al., 2013; Habuda-Stanic et al. 2014), the latter process being widely accepted and used as general pollution removal technique, because of its ease of operation and low cost. Fluorides adsorption can be carried out utilizing various adsorbents such as fly ash (Chaturvedi et. al., 1990), gypsum (Thole et al. 2012), clays (Çengeloglu et al., 2002 ), activated carbon (Mohan et al., 2008), activated alumina (Ghorai and Pant, 2004; Ku and Chiou, 2002; Tripathy and Raichur 2008), metallurgical alumina (Pietrelli, 2005), zeolite (Gómez-Hortigüela et al. 2013), rare earth oxides (Raichur and Basu, 2001) and many natural products like tree bark, groundnut husk, sawdust, rice husk, chitin, etc. (Kamble et al., 2007; Chidambaram et al. 2003). The fluoride adsorption onto an industrial product like activated alumina appears to DOI: 10.3303/CET1647037 Please cite this article as: Ippolito N.M., Maffei G., Medici F., Piga L., 2016, Adsorption and regeneration of fluoride ion on a high alumina content bauxite, Chemical Engineering Transactions, 47, 217-222 DOI: 10.3303/CET1647037 217 be the most interesting operation due to the high surface area and crystalline form of this thermally activated material, in fact activated alumina processes may reduce fluorides content below 1 mg L-1. Between the products that could be employed as sorbent materials the most economic are the natural mineral products which do not need any expensive modification or activation (Malay and Salim, 2001). For this reason in this paper the performance of different silico-aluminous natural materials are experimented and compared; firstly batch tests were carried out and the results show that a natural bauxite, containing high percentage of aluminium hydroxide, can be considered a good natural adsorptive product even in absence of activation processes. 2. Experimental 2.1 Materials Sodium fluoride (CAS No. 7681-49-4) with purity >99% was purchased from Merck (Darmstadt, Germany). All other chemicals were analytical grade and used without further purification. Synthetic fluoride solutions were prepared by adding appropriate amounts of sodium fluoride to distilled water. Different materials based on ferric oxide (hematite and ocher), silico-aluminous products (lava, pozzolana) and aluminum hydroxide (bauxite) were used during the preliminary tests. Bauxite was obtained from a sedimentary deposit in Texas (USA), hematite and ocher were from Elba (Livorno, Italy), lava from Stromboli (Messina, Italy) and black pozzolana from Lunghezza (Roma, Italy). 2.2 Methods The sorption materials were preliminary ground and sieved to obtain powders below 0.5 mm of diameter. After careful washing with deionized water, the materials were dried at 110 °C in an electric oven for 3 hours. Then they were cooled in air to room temperature and characterized by standard analytical methods and by X-ray powder diffraction (XRPD). To assess the ability of each adsorbent to remove fluoride from water, preliminary batch screening tests were conducted. One gram of powdered sample was contacted with 100 mL of 10 mg L–1 fluoride solution for 1 h under gentle agitation. After this time, a sample of liquid was taken and the residual fluoride concentration was measured. A similar procedure was used to perform equilibrium experiments. In these experiments the liquid-to-solid ratio was set at 100 mL g–1 and the initial fluoride concentration was varied between 10 and 100 mg L–1 (Lavecchia et al., 2012). Then exhausted bauxite was dipped in 0.1 N NaOH solution and left for 12 h, then powders were washed repeatedly with water and then activated with 0.4 N H2SO4 solution. Afterwards water washing was carried to rise pH=7 followed by drying in oven for 5 h, this makes activated product ready for the next defluoridation cycle (Ghorai and Pant, 2004). Column studies were carried out in a glass column with an inner diameter of 1 cm and a length of 20 cm. A glass-wool plug was placed in the bottom of the column to support the adsorbent bed and prevent the outflow of particles. The amount of adsorbent was fixed at 4 g (corresponding to a bed height of 5.5 cm), the inlet fluoride concentration was varied from 5 to 50 mg L–1 and the flow rate from 0.5 to 2.5 cm3 min–1. Samples of the outlet solution were collected at definite time intervals and analyzed to determine the fluoride content. Batch and column experiments were performed at pH 7.0±0.1 and room temperature (20±2 °C). The concentration of fluoride in the aqueous solution was determined according to ASTM D 1179 Standard Method utilizing a potentiometer equipped with an ion-selective electrode (DC219-F, Mettler–Toledo, Novate Milanese, Italy). A series of standard fluoride solutions with concentrations ranging from 0.1 to 10 mg L–1 was used to construct the calibration curve. X-ray powder diffraction (XRPD) analysis was carried out with a diffractometer Bruker model Advance D8, DTA/TGA analyses were performed by using a Stanton Redcroft model 1500 thermobalance. 3. Results and discussion 3.1 Preliminary screening tests Batch screening tests gave the results presented in Table 1, from which it can be seen that percent removal of fluoride ranged from 3% (ocher) to 63.1% (bauxite). Because of its significantly higher efficiency, bauxite was used in subsequent experiments. Chemical analysis by standard gravimetric methods showed that Al2O3 (81.5 %), Fe2O3 (9.3%) and SiO2 (8.9%) were the main bauxite constituents. No heavy metals (Pb, Cd, As, Cr) were detected. XRPD analysis revealed the presence of hematite [Fe2O3], gibbsite [Al(OH)3)], boehmite [AlO(OH)] and anatase [TiO2] as the main phases. 218 Table 1: Results of preliminary screening tests (1 hour contact). FR is the percent fluoride removal Material Origin FR (%) Bauxite Texas (USA) 63.1 Hematite Elba (Italy) 7.3 Lava Stromboli (Italy) 10.0 Black pozzolana Lunghezza (Italy) 15.0 Ocher Elba (Italy) 3.0 3.2 Equilibrium studies The time needed to reach equilibrium was of the order of 3 h for the original bauxite and independent of the initial fluoride concentration, this results is in accordance with literature (Sujana and Anad, 2011). The equilibrium concentrations in the liquid (c*) were determined after this time, while the concentrations in the adsorbed phase (q*) were calculated as: *)(* cc w V q 0 L −= (1) where VL is the volume of the aqueous solution, w is the mass of adsorbent and c0 is the initial fluoride concentration. The data so obtained were well described by the two-parameter Freundlich equation: n1 F cKq /*)(* = (2) where KF is the Freundlich constant and 1/n is the heterogeneity factor. KF represents the amount of solute adsorbed at an equilibrium concentration of unity and thus is a measure of the adsorption capacity of the material, while n reflects the adsorption intensity. It is generally stated that values of n in the range 2–10, 1–2 and <1 indicate, respectively, good, moderate and poor adsorption characteristics. The Freundlich parameters were obtained from the slope and intercept of the log–log plot yielded KF = 0.957 mg1–1/n g–1 L1/n and n = 3.18 (Lavecchia et al., 2012). Saturated bauxite instead showed a different behavior: the time needed to reach equilibrium was of the order of 7 h. Equilibrium data were also well described by Freundlich equation (KF = 0.11 mg1–1/n g–1 L1/n and n = 1.81). 3.3 Column studies At first column experiments were carried out at different inlet fluoride concentrations and flow rates for the original bauxite. The results, expressed as the ratio between outlet and inlet fluoride concentration (c/c0) versus time, gave typical S-shaped breakthrough curves (Figure 1). As the flow rate (F) increased, the curves became steeper and the breakpoint time decreased. The area above the breakthrough curve (up to c/c0 = 1) provides a measure of the amount of adsorbed fluorides per unit weight of adsorbent, the so-called dynamic adsorption capacity. Figure 1: Breakthrough curves for fluoride removal (C0 = 25 g L –1) at different flow rates: a = 2.5 cm3 min–1; b = 1 cm3 min–1; c = 0.5 cm3 min–1 0.0 0.2 0.4 0.6 0.8 1.0 0 2 4 6 8 10 12 t, h c/ c 0 a b c 219 Then regenerated bauxite was subjected to column tests by using three different inlet fluoride concentrations (Co= 5, 25, 50 mg L-1) and the maximum flow rate (2.5 cm3 min-1) utilized in previous tests. Results of breakthrough curves for the original and regenerated bauxite are reported in Figure 2. Figure 2: Breakthrough curves for fluoride removal, symbols: o original bauxite, ∆ saturated bauxite Breakthrough curves were analyzed by determining the following parameters: the breakthrough time (tB), i.e., the time corresponding to c/c0 = 0.5; the adsorbent capacity at the breakthrough point (BPC), namely, the specific amount of fluoride adsorbed at t0.5 and the adsorbent capacity at the exhaustion point (EPC), namely, the specific amount of fluoride adsorbed at exhaustion. BPC and EPC were calculated as: w Vc EPC w Vc BPC E0B0 == ; (3) where VB and VE are the volumes of solution treated at, respectively, the breakthrough point and the exhaustion point. Results of evaluations are presented in Table 2. Table 2: Characteristic column adsorption parameters calculated from the breakthrough curves c0 (mg L–1) F (cm3 min–1) tB (h) BPC (mg g–1) EPC (mg g–1) 5 2.5 4.22 0.775 1.250 25 2.5 0.96 1.063 3.125 50 2.5 0.49 0.938 1.375 5 2.5 14.85 1.181 4.668 25 2.5 6.59 4.217 6.125 50 2.5 3.09 4.875 6.175 Examination of the data reveals that the breakthrough time decreased with increasing inlet fluoride concentration both for original or for regenerated bauxite. Regenerated bauxite shows a different behaviour with respect to the original one, namely a lower initial efficiency, but a capacity of adsorption prolonged in time and also a higher adsorbent capacity at the exhaustion point (EPC). 220 To explain this different behaviour samples of original and regenerated bauxite were subjected to X-ray diffraction tests (XRPD) and thermal analysis (DTA/TGA). The X-ray diffraction spectra reported in Figure 3 show that the spectra are identical and the same crystallographic phases are present. Thermal analysis highlights that the first weight loss up to 100 °C is due to water evaporation. The weight loss between 300 and 350 °C depends on the gibbsite, while the boehmite loses weight at temperatures between 450 and 570 °C. The contents of boehmite and gibbsite in the sample of the original bauxite were found to be respectively 56.6% and 23.1% while that of bauxite regenerated are 60% and 20.2%: the relationship between the quantities shows no changes inside the material. Figure 3: XRPD analyses The results can be summarized as follows: the original product and the regenerated bauxite present the characteristic peaks of gibbsite and boehmite, which do not disappear after regeneration. This indicates that a portion of “eliminable” fluoride ions are captured by gibbsite and/or boehmite with a prevalent chemical mechanism (namely. replacement of OH- with F-). After the regeneration treatment with sodium hydroxide the crystalline structure of gibbsite and boehmite are not destroyed and sulphuric acid activates the adsorption sites present in the matrix: for this reason bauxite longer works. To confirm this hypothesis final batch tests were carried out, three samples were considered (sample a: original bauxite, sample b: bauxite saturated with fluorides and regenerated, sample c: original bauxite regenerated). One gram of powdered these samples (a,b,c) was contacted with 100 mL of 10 mg L–1 fluoride solution for 24 h under gentle agitation. All tests were performed in duplicate, and the following results, regarding the removal of the fluoride ion, were obtained: (sample a: 81%, sample b: 94%, sample c: 96%), this means that, at long contact times (24 h), regenerated bauxite offers a greater reduction of fluorides. This result, together with the fact that the bauxite regenerated reaches the equilibrium conditions more slowly (7 h), may explain the different behaviour observed in the column tests. 4. Conclusions Bauxite constitutes a suitable adsorbent for the removal of fluoride from aqueous solutions. 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