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Aluminum and lanthanum effects in natural materials on the adsorption of fluoride ions
Journal of Fluorine Chemistry 148 (2013) 6–13
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Journal of Fluorine Chemistry journal homepage: www.elsevier.com/locate/fluor
Aluminum and lanthanum effects in natural materials on the adsorption of fluoride ions A. Teutli-Sequeira a,b, V. Martı´nez-Miranda b, M. Solache-Rı´os a,*, I. Linares-Herna´ndez b a
Instituto Nacional de Investigaciones Nucleares, Depto. de Quı´mica, Apdo. Postal 18-1027, 11801 Me´xico, D.F., Mexico Centro Interamericano de Recursos del Agua, Facultad de Ingenierı´a, Universidad Auto´noma del Estado de Me´xico, Km. 14.5, Carretera Toluca-Ixtlahuaca, Toluca, Estado de Me´xico, Mexico b
A R T I C L E I N F O
A B S T R A C T
Article history: Received 6 December 2012 Received in revised form 11 January 2013 Accepted 17 January 2013 Available online 4 February 2013
The removal of fluoride ions from aqueous solutions and drinking water with aluminum and lanthanum modified natural materials was studied. Drinking water containing naturally 5.87 mg of fluoride ions per liter was characterized. The hematite, zeolitic tuff and calcite were aluminum modified by an electrochemical method. Hematite and a zeolitic tuff were lanthanum modified by ion exchange. The results show that the electrochemical method is useful to modify these materials with aluminum. The presence of this element improves the sorption efficiencies for fluoride ions from drinking water and synthetic solutions. The fluoride adsorption capacities increase with increasing the concentration of the aluminum in the samples. The sorption capacities for hematite 3A-2h (containing 11.92% of Al) and hematite-La (containing 1.24% of La) with drinking water were 0.53 mg/g and 0.36 mg/g respectively and the sorption capacities for zeolite 3A-3h (containing 34.74% of Al) and zeolite-La (containing 7.15% of La) were 0.56 mg/g and 0.36 mg/g respectively. The aluminum modified hematite is more effective than aluminum modified zeolitic tuff, the presence of iron may be responsible for this behavior. The presence of lanthanum in hematite and zeolitic tuff improves their sorption efficiencies for fluoride ions, but they are lower than the efficiencies found for aluminum modified materials. ß 2013 Elsevier B.V. All rights reserved.
Keywords: Aluminum Lanthanum Fluoride Adsorption Natural materials
1. Introduction Excess of fluoride in groundwater has been recognized as a serious problem worldwide [1]. According to the World Health Organization guide lines, the permissible limit of fluoride is 1.5 mg/L. Epidemiological evidences show that concentrations above this value carry an increasing risk of dental fluorosis and that progressively higher concentrations lead to increasing risks of skeletal fluorosis. The last value is higher than that recommended for artificial fluoridation of water supplies, which is usually 0.5– 1.0 mg/L [2]. Fluoride is attracted by positively charged calcium in teeth and bones due to its strong electronegativity, which results in dental, skeletal, and non-skeletal forms of fluorosis [3,4]. Longterm drinking of water containing high fluoride content can result in softening of bones, ossification of tendons and ligaments, and abnormal bone growth in both humans and animals called as skeletal fluorosis [5]. The dental and skeletal fluorosis is irreversible and no treatment exists. The only remedy is prevention by keeping fluoride intake within the safe limits [6].
* Corresponding author. Tel.: +52 5553297200x2271; fax: +52 5553297301. E-mail address: [email protected] (M. Solache-Rı´os). 0022-1139/$ – see front matter ß 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.jfluchem.2013.01.015
Fluoride is a persistent and non-degradable poison that accumulates in soil, plants, wild life, and humans. Waters with high fluoride content are found mostly in calcium deficient ground waters in many basement aquifers, such as granite, in geothermal waters and in some sedimentary basins. Fluoride can be enriched in natural waters by geological processes [6]. Anthropogenic fluoride contamination of groundwater is associated with mineral processing industries including coal-fired power station, beryllium extraction plants, brick and iron works, and aluminum smelter [7]. Groundwater with high fluoride concentrations can be found in many parts of the world, particularly in parts of India, China, Central Africa and South America [2], and Mexico. For these reasons, the removal of the excess of fluoride from waters and wastewaters is important in terms of protection of public health and environment [8]. The treatment technologies that have been developed to remove fluoride ions from drinking water are adsorption [9], nanofiltration [10], ion exchange [11], reverse osmosis, electrodialysis [12], donnan dialysis [13,14], and electrochemical treatment [15]. Adsorption has been widely studied in recent years as a separation and purification method due to its high selectivity, easy handling and lower operating cost [8,16]. For these reasons, several natural, modified and synthetic adsorbents have been employed to remove fluoride ions from water.
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In this study, hematite, zeolitic material and calcite were used. The hematite is a mineral composed of iron oxide (Fe2O3). Iron is one of the most abundant elements and iron oxides are widely distributed in the environment [17]. Hematite deposits are mostly of sedimentary origin, also found in igneous and metamorphic rocks, they are found worldwide [18]. Zeolitic material is a highly porous material. The skeletal framework of zeolitic material is made of alumina and silica tetrahedral and has a high concentration of negative charges induced by oxygen atoms. The alumina tetrahedron is also negatively charged, but the charges are balanced by the loosely bound cations [19]. The large internal surface area of zeolitic material was utilized to create active sites for fluoride sorption by modifying the material with Al3+ or La3+ ions [19–22]. Calcite has been suggested as a fluoride removal medium by many authors and they have concluded that fluorine precipitation is the main mechanism of fluoride removal because the mass of fluoride lost from solution was independent of the weight of calcite [7]. Several adsorbents for fluoride ions are aluminum or lanthanum enriched materials [20–26]. Therefore, hematite, zeolitic material and calcite used in this work were treated with Al3+ or La3+ ions. Natural materials have been modified by different chemical methods. Electrochemical methods offer some advantages over traditional chemical treatment: less coagulant ion is required and less sludge is formed. Usually, aluminum or iron plates are used as anodes in the electrocoagulation process [27]. Electrocoagulation is a method of applying direct current to electrodes that are submerged in an aqueous solution. Dissolving aluminum is predominant under acidic condition [15]. The aim of this work was to evaluate the removal of fluoride ions from aqueous solutions and drinking water with electrochemically aluminum modified materials (hematite, zeolitic tuff and calcite) and ion-exchange lanthanum modified hematite and zeolitic tuff.
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Fig. 1. Schematic diagram of the electrochemical cell.
with the Faraday constant (F = 96,500 C mol1) and the charge on the cation (z = 3+).
2. Experimental 2.1. Materials Hematite (Hematita AP de Quı´micos, Reactivos y Minerales, S.A. de C.V.), grains with diameters between 30 and 140 meshes were selected for the experiments. The zeolitic material (natural zeolitic material obtained from Oaxaca, Mexico) and calcite (obtained from Zacatecas, Mexico) were ground and sieved to obtain particle sizes between 30 and 50 meshes. 2.2. Modification of the materials 2.2.1. Aluminum modification A batch electrochemical cell (Fig. 1) was employed for the modifications of materials (hematite, zeolitic material and calcite). The electrode system is monopolar. The reactor cell contains an array of Al electrodes. The electrode dimensions were 0.08 m long and 0.03 m wide. The anode electrode has a surface area of 0.0048 m2. A direct current (DC) power source supplied the system with 1–3 A at 13 V, corresponding to current densities of 208– 625 A m2. The modification of adsorbent material was performed placing 20 g of adsorbent inside the cell with deionized water acidified with HCl as supporting electrolyte at a pH of 2. The samples were labeled with the name of the material, the amperes used were 1, 2 and 3 A and the times of treatment were 30 min, 1, 2 and 3 h. Faraday’s law was used to calculate the maximum amount of aluminum produced (n) in the electrochemical process (Eq. (1)), it was calculated considering the experimental conditions of I = 1– 3 A of current intensity and t = 30–180 min of electrolysis along
n¼
It zF
(1)
The aluminum concentration in solution was calculated using Eq. (2): C¼
n V
(2)
where n is the moles of aluminum and V is the working volume (0.15 L) of the cell. The produced aluminum concentration was determined by atomic absorption spectroscopy. Also the amount of aluminum on the adsorbent material was determined by performing the electrochemical process in the presence of the adsorbent materials (hematite or zeolitic material) in the same experimental conditions, different aliquots were taken at different times and the concentrations of aluminum were determined. 2.2.2. Lanthanum modification Hematite and zeolitic materials were modified with lanthanum nitrate. The hematite was modified (hematite-La) by adding 100 mL of a 0.09 M NaOH and 50 mL of 0.06 M La(NO3)36H2O solutions to 20 g of hematite. After mixing for 24 h, the material was washed with distilled water. The zeolitic material was placed in reflux with 150 mL of a 0.03 M La(NO3)36H2O solution for 6 h, after this time, the mixture was left for 48 h, then the material was washed repeatedly with distilled water (zeolite-La).
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2.3. Characterization Powder diffractogram were obtained with a Siemens D5000 diffractometer coupled to a copper-anode X-ray tube. The conventional diffractograms were used to identify the compound and to verify its crystalline structure. The spectroscopic studies were performed by using a scanning electron microscopy (JEOL JSM-5900 LV SEM). 2.4. Sorption A batch system was used for the sorption experiments; 10 mL of a fluoride solution (4 and 10 mg F L1) or drinking water (5.8 mg F L1) and 100 mg of the each material were used (the mixtures were shaken for 72 h at 120 rpm at 30 8C). Each experiment was carried out in duplicate. 2.5. Fluoride ion measurements The concentration of fluoride ions in the solutions was determined with a selective electrode for fluoride ions (ISEC301F Radiometer analytical, combined fluoride electrode). Total ionic strength adjustment buffer solution was added to all fluoride standards and samples to control the pH and ionic strength. The calibration line was obtained by using fluoride standards solutions with fluoride concentrations from 0.5 to 10 mg/L. 2.6. Drinking water sampling The sampling was performed according with the Mexican Official Standard [28], which sets sanitary procedures for sampling water supply systems for human use and consumption. In Mexico [29], there are many locations where fluoride ions are present in excess of acceptable limits (>1.5 mg/L) in drinking water. A drinking water sample was collected from the State of Zacatecas, Mexico. 2.7. Drinking water characterization Alkalinity, total dissolved solids dissolved oxygen, chloride, bicarbonate, carbonate, fluoride; pH and electrical conductivity were determined by using the standard methods [30]. 3. Results and discussion The aluminum concentration produced and the one calculated by Faraday’s law are shown in Fig. 2. As it can be observed, the actually measured values were higher than those calculated by Faraday’s equation, after 180 minutes. The difference is a particular effect that has been recently noted, the so-called ‘‘superfaradaic efficiencies’’. This term describes the difference between the theoretical amount of aluminum in aqueous solution and the actual aluminum detected. The detected aluminum concentration in the aqueous solution was 27.89 g/L (previous to settling). The theoretical calculation corresponded to 20.83 g/L, which implies that there is an excess of 34%. A possible explanation of this difference is that a chemical process takes place at the cathode promoting aluminum dissolution, which is not accounted by the Faraday’s law. The electrochemical oxidation and reduction of water can modify the pH on the anode and cathode surfaces with respect to the bulk pH. This is especially important on the cathode, where the pH can become strongly alkaline and the aluminum may go to the solution as aluminum hydroxide. This can justify the important contribution of the aluminum chemical dissolution in the cathode to the total dissolution rate [27,31]. The difference between the
Fig. 2. Measured and calculated aluminum concentration at different times in the presence and absence of adsorbents in the electrochemical cell.
aluminum in solution with and without adsorbents could predict the amount of aluminum deposited on the adsorbent materials and it was verify semi quantitatively by elemental analysis (Fig. 3). As it can be observed, the adsorption was similar for both hematite and zeolitic materials; the adsorption increases as the time increases. Once the aluminum is dissolved chemically or electrochemically different aluminum species can be formed, depending on the pH of the solution and on the presence of other ions in the aqueous media. 3.1. Sorbents characterization 3.1.1. X-ray diffraction The hematite did not show any significant changes after modification (Fig. 4), the diffraction peaks intensities were higher for the modified than the unmodified hematite, this behavior could be attributed to the crystallinity of the samples. The diffractograms corresponded to the hematite structure according with the JCPDS (Joint Committee on Powder Diffraction Standards) 00-002-0915-hematite (Fig. 4). In the case of unmodified zeolitic material and zeolitic material 3A-3h (modified at 3 A for three hours), the peaks of the diffractograms correspond to the JCPDS 00-025-1349-Clinoptilolite, JCPDS 01-074-3676-Mordenite and JCPDS 01-074-4856 Muscovite (Fig. 5). The diffractograms are similar and it is not observed any important change in the structure of the material after treatments. The diffractogram
Fig. 3. Content of aluminum in the adsorbent materials at different times in the electrochemical cell.
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Fig. 4. X-ray diffractogram of the unmodified hematite and hematite 3A-2h.
Fig. 5. X-ray diffractogram of the unmodified zeolitic material and zeolitic material 3A-3h compared with the JCPDS 00-025-1349 ($ Clinoptilolite), JCPDS 01-074-3676 (D Modernite) and JCPDS 01-074-4856 (^ Muscovite).
of unmodified calcite corresponded to the JCPDS 01-089-1305Calcite, magnesium as shown in Fig. 6. 3.1.2. Scanning electron microscopy The scanning electron microscopy photographs of hematite, zeolitic material and calcite are shown in Figs. 7–9 respectively. Hematite show grains of different sizes, the zeolitic material shows
some morphology characteristics of the Clinoptilolite which occurred as euhedral plates and laths. The crystals display a characteristic monoclinic symmetry, and some are coffin-shaped and cubic-like crystals [32]. For the calcite, small particles are observed and they are adhered on the surface. In general, the treated materials did not show any important morphological difference with respect to the unmodified materials.
Fig. 6. X-ray diffractogram of the calcite compared with the JCPDS 01-089-1305 (calcite and magnesium).
3.1.3. Elemental analysis of the unmodified and modified materials Table 1 shows the elemental analysis of the unmodified and modified materials. As expected the main elements found in the hematite samples were iron and oxygen, as found elsewhere [33]. The quantities of aluminum in the samples increased as the amperes and treatment times increased. Also, Table 1 shows that lanthanum was detected in the sample treated with lanthanum nitrate. The main components of a zeolitic material are silicon, aluminum and oxygen, other elements were found in the zeolitic material samples such as carbon, calcium, iron, sodium and potassium, the last four may be cations that could be exchanged by aluminum in the zeolitic material. Aluminum increased as the amperes and treatment times increased. Lanthanum was found in the sample treated with lanthanum nitrate but in less quantity than in hematite.
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Fig. 7. Scanning electron micrographs of hematite.
Fig. 9. Scanning electron micrographs of calcite.
The main components of the calcite samples were calcium, oxygen and carbon. Aluminum increased and calcium decreased after treatments. 3.2. Drinking water characterization Table 2 shows the characterization of the drinking water used in the experiments, the concentration of fluoride ions is higher than the limit allowed by the World Health Organization (2011) guidelines. The pH of the drinking water was 7.46 which is acceptable, the recommended pH values are between 6.5 and 8.5 for drinking water [2,34]. Chloride occurs naturally in drinking water; the concentration of chloride was 26.93 mg/L, and should not affect the human beings. The total dissolved solids were 231 mg/L. Similar results for this drinking water have been reported elsewhere [35]. In general, besides fluoride concentration, all other parameters are within the values accepted by the regulations. Fig. 8. Scanning electron micrographs of zeolitic material.
Fig. 10. Fluoride adsorption capacities and equilibrium pH values for the unmodified and modified hematite, zeolitic material and calcite.
0.05 0.03 0.03 0.02 0.06 0.01 0.04 0.03 – – – – – – – – – – – – – – 0.28 0.05 0.10 0.03 0.11 0.04 0.13 0.04 0.29 0.07 0.23 0.04 – – – – 31.93 2.64 28.55 5.12 20.04 2.58 19.86 3.11 Calcite Unmodified 1A, 30 min 2A, 30 min 3A, 30 min
– – – – 51.04 1.52 52.17 2.51 54.41 1.49 53.27 0.92 16.70 1.27 18.79 2.61 16.50 1.20 18.49 1.97
0.16 0.03 0.32 0.12 8.42 1.22 8.00 0.83
– – – – – – – – 2.08 0.59 1.49 0.25 1.79 0.26 1.65 0.56 1.68 0.56 0.84 0.25 0.63 0.14 2.32 1.11 3.23 0.21 2.96 0.23 2.41 0.14 2.89 0.85 2.71 0.69 2.12 0.34 0.83 0.35 2.44 0.37 – – – – – – – 1.66 0.76 – – – – – 1.43 0.49 2.15 0.78 – 37.41 1.00 30.44 3.67 30.30 1.38 30.34 1.24 30.30 1.06 21.62 3.99 11.84 3.47 36.28 1.22 1.53 0.64 0.84 0.11 1.32 0.26 0.90 0.38 0.95 0.43 0.57 0.19 0.77 0.65 1.16 0.34 0.59 0.12 0.42 0.09 0.40 0.05 0.73 0.78 0.80 0.73 0.33 0.07 – 0.61 0.25 – – 0.21 0.01 0.16 0.03 – – – – 48.37 1.84 48.89 1.17 46.71 1.02 46.95 1.40 46.72 1.65 47.52 1.20 49.20 2.10 48.37 2.00 Zeolitic material Unmodified – 1A, 30 min 10.67 2.31 2A, 30 min 9.72 1.86 3A, 30 min 8.20 0.68 3A, 1 h 8.60 0.49 3A, 2 h 14.17 1.36 3A, 3 h – La –
6.80 0.12 6.43 0.54 7.26 0.46 8.28 1.46 8.25 1.20 11.41 2.20 34.74 4.94 7.15 0.69
– – – – – – 3.47 3.02 – – 0.22 0.12 – 0.35 0.07 0.48 0.13 – – 1.49 0.78 2.63 0.61 2.36 0.27 2.01 0.30 1.51 0.48 2.54 0.26 72.36 4.76 51,02 11.3 58.34 6.09 47.49 4.41 47.11 4.68 39.19 5.73 46.41 7.59 1.50 0.18 0.29 0.14 0.65 0.30 0.47 0.17 0.37 0.11 0.30 0.08 1.01 0.12 1.15 0.21 2.13 1.22 2.44 0.69 2.48 0.24 5.81 0.72 11.92 1.74 1.24 0.24 0.38 0.06 – – 0.50 0.05 0.40 0.15 – 0.61 0.18 17.70 1.39 20.20 6.54 28.22 4.90 27.92 3.23 30.50 1.49 33.39 2.49 32.83 2.45 6.92 3.98 16.33 19.1 7.50 1.24 7.62 0.97 13.44 3.34 13.14 2.45 11.89 4.43 Hematite Unmodified 1A, 30 min 2A, 30 min 3A, 30 min 3A, 1 h 3A, 2 h La
Mg O C
% Elemental composition Material
Table 1 Elemental analysis of the unmodified and modified materials.
Al
Ca
Fe
Si
Cl
La
Na
– – – – – – –
K
– – – – – – –
P
– – – – – – –
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3.3. Sorption capacities Figs. 10 and 11 show the fluoride sorption capacities and the removal percent respectively, determined for the different materials using fluoride solutions of 4 and 10 mg/L and drinking water containing 5.87 mg/L (naturally). The equilibrium pH values are also shown (Fig. 10). As it can be observed the adsorption capacities determined are proportional to the content of fluoride ions in the aqueous phase. Figs. 12 and 13 show the initial percent of aluminum in the samples vs. the adsorption capacities, for the hematite and zeolitic material samples, respectively. The adsorption behaviors were similar for both materials, with the adsorption capacities increasing up to a plateau. For hematite, the plateaus for fluoride solutions and drinking water were reached when the percentage of aluminum was a little higher than 2% and for the zeolitic material when it was about 10%. These results indicate that aluminum is more effective for fluoride adsorption in hematite than in the zeolitic material; iron may be responsible for this behavior. In general, hematite is more efficient than zeolitic materials for the removal of fluoride ions [22,24]. Most equilibrium pH values for drinking water were between 6.5 and 8 which are acceptable according to the regulations [2,34]. Onyango et al. [19] reported the removal of fluoride ions by zeolitic material F9; they reported that the presence of lanthanum improves the sorption efficiency of this material and found a sorption capacity of 59.12 mg/g. In this work the hematite and zeolitic materials were modified with lanthanum and it was found that the presence of this element improves the sorption efficiencies of these materials for fluoride ions as shown in Figs. 10 and 11. Similar results have been reported elsewhere with lanthanum incorporated chitosan beads, lanthanum impregnated chitosan flakes, lanthanummodified chitosan [36–38]. The aluminum modified materials show higher sorption capacities than the lanthanum modified materials (Figs. 10 and 11). Jain and Jayaram [25] found that aluminum modified calcite is an efficient material for the removal of fluoride ions, in this work a calcite sample was aluminum modified, according to Table 1 the unmodified material contains 0.16 0.03% of aluminum, after modification with 1 A and 30 min the content increased to 0.32 0.12% and with 2–3 A for 30 min, the aluminum was about 8%, however the adsorption capacities for fluoride removal from drinking water were less than 0.2 mg/g for the unmodified material and about 0.4 mg/g for the aluminum modified materials, as shown in Table 1 and Fig. 10, it seems that the adsorption capacities are not directly proportional to the quantities of aluminum in the calcite, the origin and the composition of the samples may be responsible for these different behaviors. In general, the presence of aluminum improves the sorption efficiencies for fluoride ions from drinking water and the most
Table 2 Characterization of drinking water. Parameter
Value
Alkalinity Total dissolved solids % Saturation oxygen Dissolved oxygen Chloride Bicarbonate Carbonate Fluoride pH Temperature Electrical conductivity
207.20 mg/L as CaCO3 231 mg/L 59.8 4.59 mg/L 26.93 mg/L 247.92 mg/L 0.006 mg/L 5.87 mg/L 7.46 12.7 8C 463 ms/cm
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Fig. 11. Fluoride adsorption percentages for the unmodified and modified hematite, zeolitic material and calcite.
efficient materials are those with the highest quantities of aluminum. 4. Conclusions
Fig. 12. Fluoride adsorption capacities vs. aluminum percent in hematite.
The natural materials were aluminum modified by an electrochemical method. The presence of aluminum improves the sorption efficiencies for fluoride ions from drinking water and the most efficient materials are those with the highest quantities of aluminum. Aluminum modified hematite is more effective than aluminum modified zeolitic tuff for fluoride adsorption and the presence of iron may be responsible for this behavior. The presence of lanthanum in hematite and zeolitic tuff improves their sorption efficiencies for fluoride ions, but the aluminum modified materials show higher sorption capacities than the lanthanum modified materials. Acknowledgments We acknowledge financial support from CONACYT, project 131174Q and scholarship Grant No. 231465 for ATS. References
Fig. 13. Fluoride adsorption capacities vs. aluminum percent in zeolitic material.
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Aluminum and lanthanum effects in natural materials on the adsorption of fluoride ions A. Teutli-Sequeira a,b, V. Martı´nez-Miranda b, M. Solache-Rı´os a,*, I. Linares-Herna´ndez b a
Instituto Nacional de Investigaciones Nucleares, Depto. de Quı´mica, Apdo. Postal 18-1027, 11801 Me´xico, D.F., Mexico Centro Interamericano de Recursos del Agua, Facultad de Ingenierı´a, Universidad Auto´noma del Estado de Me´xico, Km. 14.5, Carretera Toluca-Ixtlahuaca, Toluca, Estado de Me´xico, Mexico b
A R T I C L E I N F O
A B S T R A C T
Article history: Received 6 December 2012 Received in revised form 11 January 2013 Accepted 17 January 2013 Available online 4 February 2013
The removal of fluoride ions from aqueous solutions and drinking water with aluminum and lanthanum modified natural materials was studied. Drinking water containing naturally 5.87 mg of fluoride ions per liter was characterized. The hematite, zeolitic tuff and calcite were aluminum modified by an electrochemical method. Hematite and a zeolitic tuff were lanthanum modified by ion exchange. The results show that the electrochemical method is useful to modify these materials with aluminum. The presence of this element improves the sorption efficiencies for fluoride ions from drinking water and synthetic solutions. The fluoride adsorption capacities increase with increasing the concentration of the aluminum in the samples. The sorption capacities for hematite 3A-2h (containing 11.92% of Al) and hematite-La (containing 1.24% of La) with drinking water were 0.53 mg/g and 0.36 mg/g respectively and the sorption capacities for zeolite 3A-3h (containing 34.74% of Al) and zeolite-La (containing 7.15% of La) were 0.56 mg/g and 0.36 mg/g respectively. The aluminum modified hematite is more effective than aluminum modified zeolitic tuff, the presence of iron may be responsible for this behavior. The presence of lanthanum in hematite and zeolitic tuff improves their sorption efficiencies for fluoride ions, but they are lower than the efficiencies found for aluminum modified materials. ß 2013 Elsevier B.V. All rights reserved.
Keywords: Aluminum Lanthanum Fluoride Adsorption Natural materials
1. Introduction Excess of fluoride in groundwater has been recognized as a serious problem worldwide [1]. According to the World Health Organization guide lines, the permissible limit of fluoride is 1.5 mg/L. Epidemiological evidences show that concentrations above this value carry an increasing risk of dental fluorosis and that progressively higher concentrations lead to increasing risks of skeletal fluorosis. The last value is higher than that recommended for artificial fluoridation of water supplies, which is usually 0.5– 1.0 mg/L [2]. Fluoride is attracted by positively charged calcium in teeth and bones due to its strong electronegativity, which results in dental, skeletal, and non-skeletal forms of fluorosis [3,4]. Longterm drinking of water containing high fluoride content can result in softening of bones, ossification of tendons and ligaments, and abnormal bone growth in both humans and animals called as skeletal fluorosis [5]. The dental and skeletal fluorosis is irreversible and no treatment exists. The only remedy is prevention by keeping fluoride intake within the safe limits [6].
* Corresponding author. Tel.: +52 5553297200x2271; fax: +52 5553297301. E-mail address: [email protected] (M. Solache-Rı´os). 0022-1139/$ – see front matter ß 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.jfluchem.2013.01.015
Fluoride is a persistent and non-degradable poison that accumulates in soil, plants, wild life, and humans. Waters with high fluoride content are found mostly in calcium deficient ground waters in many basement aquifers, such as granite, in geothermal waters and in some sedimentary basins. Fluoride can be enriched in natural waters by geological processes [6]. Anthropogenic fluoride contamination of groundwater is associated with mineral processing industries including coal-fired power station, beryllium extraction plants, brick and iron works, and aluminum smelter [7]. Groundwater with high fluoride concentrations can be found in many parts of the world, particularly in parts of India, China, Central Africa and South America [2], and Mexico. For these reasons, the removal of the excess of fluoride from waters and wastewaters is important in terms of protection of public health and environment [8]. The treatment technologies that have been developed to remove fluoride ions from drinking water are adsorption [9], nanofiltration [10], ion exchange [11], reverse osmosis, electrodialysis [12], donnan dialysis [13,14], and electrochemical treatment [15]. Adsorption has been widely studied in recent years as a separation and purification method due to its high selectivity, easy handling and lower operating cost [8,16]. For these reasons, several natural, modified and synthetic adsorbents have been employed to remove fluoride ions from water.
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In this study, hematite, zeolitic material and calcite were used. The hematite is a mineral composed of iron oxide (Fe2O3). Iron is one of the most abundant elements and iron oxides are widely distributed in the environment [17]. Hematite deposits are mostly of sedimentary origin, also found in igneous and metamorphic rocks, they are found worldwide [18]. Zeolitic material is a highly porous material. The skeletal framework of zeolitic material is made of alumina and silica tetrahedral and has a high concentration of negative charges induced by oxygen atoms. The alumina tetrahedron is also negatively charged, but the charges are balanced by the loosely bound cations [19]. The large internal surface area of zeolitic material was utilized to create active sites for fluoride sorption by modifying the material with Al3+ or La3+ ions [19–22]. Calcite has been suggested as a fluoride removal medium by many authors and they have concluded that fluorine precipitation is the main mechanism of fluoride removal because the mass of fluoride lost from solution was independent of the weight of calcite [7]. Several adsorbents for fluoride ions are aluminum or lanthanum enriched materials [20–26]. Therefore, hematite, zeolitic material and calcite used in this work were treated with Al3+ or La3+ ions. Natural materials have been modified by different chemical methods. Electrochemical methods offer some advantages over traditional chemical treatment: less coagulant ion is required and less sludge is formed. Usually, aluminum or iron plates are used as anodes in the electrocoagulation process [27]. Electrocoagulation is a method of applying direct current to electrodes that are submerged in an aqueous solution. Dissolving aluminum is predominant under acidic condition [15]. The aim of this work was to evaluate the removal of fluoride ions from aqueous solutions and drinking water with electrochemically aluminum modified materials (hematite, zeolitic tuff and calcite) and ion-exchange lanthanum modified hematite and zeolitic tuff.
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Fig. 1. Schematic diagram of the electrochemical cell.
with the Faraday constant (F = 96,500 C mol1) and the charge on the cation (z = 3+).
2. Experimental 2.1. Materials Hematite (Hematita AP de Quı´micos, Reactivos y Minerales, S.A. de C.V.), grains with diameters between 30 and 140 meshes were selected for the experiments. The zeolitic material (natural zeolitic material obtained from Oaxaca, Mexico) and calcite (obtained from Zacatecas, Mexico) were ground and sieved to obtain particle sizes between 30 and 50 meshes. 2.2. Modification of the materials 2.2.1. Aluminum modification A batch electrochemical cell (Fig. 1) was employed for the modifications of materials (hematite, zeolitic material and calcite). The electrode system is monopolar. The reactor cell contains an array of Al electrodes. The electrode dimensions were 0.08 m long and 0.03 m wide. The anode electrode has a surface area of 0.0048 m2. A direct current (DC) power source supplied the system with 1–3 A at 13 V, corresponding to current densities of 208– 625 A m2. The modification of adsorbent material was performed placing 20 g of adsorbent inside the cell with deionized water acidified with HCl as supporting electrolyte at a pH of 2. The samples were labeled with the name of the material, the amperes used were 1, 2 and 3 A and the times of treatment were 30 min, 1, 2 and 3 h. Faraday’s law was used to calculate the maximum amount of aluminum produced (n) in the electrochemical process (Eq. (1)), it was calculated considering the experimental conditions of I = 1– 3 A of current intensity and t = 30–180 min of electrolysis along
n¼
It zF
(1)
The aluminum concentration in solution was calculated using Eq. (2): C¼
n V
(2)
where n is the moles of aluminum and V is the working volume (0.15 L) of the cell. The produced aluminum concentration was determined by atomic absorption spectroscopy. Also the amount of aluminum on the adsorbent material was determined by performing the electrochemical process in the presence of the adsorbent materials (hematite or zeolitic material) in the same experimental conditions, different aliquots were taken at different times and the concentrations of aluminum were determined. 2.2.2. Lanthanum modification Hematite and zeolitic materials were modified with lanthanum nitrate. The hematite was modified (hematite-La) by adding 100 mL of a 0.09 M NaOH and 50 mL of 0.06 M La(NO3)36H2O solutions to 20 g of hematite. After mixing for 24 h, the material was washed with distilled water. The zeolitic material was placed in reflux with 150 mL of a 0.03 M La(NO3)36H2O solution for 6 h, after this time, the mixture was left for 48 h, then the material was washed repeatedly with distilled water (zeolite-La).
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2.3. Characterization Powder diffractogram were obtained with a Siemens D5000 diffractometer coupled to a copper-anode X-ray tube. The conventional diffractograms were used to identify the compound and to verify its crystalline structure. The spectroscopic studies were performed by using a scanning electron microscopy (JEOL JSM-5900 LV SEM). 2.4. Sorption A batch system was used for the sorption experiments; 10 mL of a fluoride solution (4 and 10 mg F L1) or drinking water (5.8 mg F L1) and 100 mg of the each material were used (the mixtures were shaken for 72 h at 120 rpm at 30 8C). Each experiment was carried out in duplicate. 2.5. Fluoride ion measurements The concentration of fluoride ions in the solutions was determined with a selective electrode for fluoride ions (ISEC301F Radiometer analytical, combined fluoride electrode). Total ionic strength adjustment buffer solution was added to all fluoride standards and samples to control the pH and ionic strength. The calibration line was obtained by using fluoride standards solutions with fluoride concentrations from 0.5 to 10 mg/L. 2.6. Drinking water sampling The sampling was performed according with the Mexican Official Standard [28], which sets sanitary procedures for sampling water supply systems for human use and consumption. In Mexico [29], there are many locations where fluoride ions are present in excess of acceptable limits (>1.5 mg/L) in drinking water. A drinking water sample was collected from the State of Zacatecas, Mexico. 2.7. Drinking water characterization Alkalinity, total dissolved solids dissolved oxygen, chloride, bicarbonate, carbonate, fluoride; pH and electrical conductivity were determined by using the standard methods [30]. 3. Results and discussion The aluminum concentration produced and the one calculated by Faraday’s law are shown in Fig. 2. As it can be observed, the actually measured values were higher than those calculated by Faraday’s equation, after 180 minutes. The difference is a particular effect that has been recently noted, the so-called ‘‘superfaradaic efficiencies’’. This term describes the difference between the theoretical amount of aluminum in aqueous solution and the actual aluminum detected. The detected aluminum concentration in the aqueous solution was 27.89 g/L (previous to settling). The theoretical calculation corresponded to 20.83 g/L, which implies that there is an excess of 34%. A possible explanation of this difference is that a chemical process takes place at the cathode promoting aluminum dissolution, which is not accounted by the Faraday’s law. The electrochemical oxidation and reduction of water can modify the pH on the anode and cathode surfaces with respect to the bulk pH. This is especially important on the cathode, where the pH can become strongly alkaline and the aluminum may go to the solution as aluminum hydroxide. This can justify the important contribution of the aluminum chemical dissolution in the cathode to the total dissolution rate [27,31]. The difference between the
Fig. 2. Measured and calculated aluminum concentration at different times in the presence and absence of adsorbents in the electrochemical cell.
aluminum in solution with and without adsorbents could predict the amount of aluminum deposited on the adsorbent materials and it was verify semi quantitatively by elemental analysis (Fig. 3). As it can be observed, the adsorption was similar for both hematite and zeolitic materials; the adsorption increases as the time increases. Once the aluminum is dissolved chemically or electrochemically different aluminum species can be formed, depending on the pH of the solution and on the presence of other ions in the aqueous media. 3.1. Sorbents characterization 3.1.1. X-ray diffraction The hematite did not show any significant changes after modification (Fig. 4), the diffraction peaks intensities were higher for the modified than the unmodified hematite, this behavior could be attributed to the crystallinity of the samples. The diffractograms corresponded to the hematite structure according with the JCPDS (Joint Committee on Powder Diffraction Standards) 00-002-0915-hematite (Fig. 4). In the case of unmodified zeolitic material and zeolitic material 3A-3h (modified at 3 A for three hours), the peaks of the diffractograms correspond to the JCPDS 00-025-1349-Clinoptilolite, JCPDS 01-074-3676-Mordenite and JCPDS 01-074-4856 Muscovite (Fig. 5). The diffractograms are similar and it is not observed any important change in the structure of the material after treatments. The diffractogram
Fig. 3. Content of aluminum in the adsorbent materials at different times in the electrochemical cell.
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Fig. 4. X-ray diffractogram of the unmodified hematite and hematite 3A-2h.
Fig. 5. X-ray diffractogram of the unmodified zeolitic material and zeolitic material 3A-3h compared with the JCPDS 00-025-1349 ($ Clinoptilolite), JCPDS 01-074-3676 (D Modernite) and JCPDS 01-074-4856 (^ Muscovite).
of unmodified calcite corresponded to the JCPDS 01-089-1305Calcite, magnesium as shown in Fig. 6. 3.1.2. Scanning electron microscopy The scanning electron microscopy photographs of hematite, zeolitic material and calcite are shown in Figs. 7–9 respectively. Hematite show grains of different sizes, the zeolitic material shows
some morphology characteristics of the Clinoptilolite which occurred as euhedral plates and laths. The crystals display a characteristic monoclinic symmetry, and some are coffin-shaped and cubic-like crystals [32]. For the calcite, small particles are observed and they are adhered on the surface. In general, the treated materials did not show any important morphological difference with respect to the unmodified materials.
Fig. 6. X-ray diffractogram of the calcite compared with the JCPDS 01-089-1305 (calcite and magnesium).
3.1.3. Elemental analysis of the unmodified and modified materials Table 1 shows the elemental analysis of the unmodified and modified materials. As expected the main elements found in the hematite samples were iron and oxygen, as found elsewhere [33]. The quantities of aluminum in the samples increased as the amperes and treatment times increased. Also, Table 1 shows that lanthanum was detected in the sample treated with lanthanum nitrate. The main components of a zeolitic material are silicon, aluminum and oxygen, other elements were found in the zeolitic material samples such as carbon, calcium, iron, sodium and potassium, the last four may be cations that could be exchanged by aluminum in the zeolitic material. Aluminum increased as the amperes and treatment times increased. Lanthanum was found in the sample treated with lanthanum nitrate but in less quantity than in hematite.
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Fig. 7. Scanning electron micrographs of hematite.
Fig. 9. Scanning electron micrographs of calcite.
The main components of the calcite samples were calcium, oxygen and carbon. Aluminum increased and calcium decreased after treatments. 3.2. Drinking water characterization Table 2 shows the characterization of the drinking water used in the experiments, the concentration of fluoride ions is higher than the limit allowed by the World Health Organization (2011) guidelines. The pH of the drinking water was 7.46 which is acceptable, the recommended pH values are between 6.5 and 8.5 for drinking water [2,34]. Chloride occurs naturally in drinking water; the concentration of chloride was 26.93 mg/L, and should not affect the human beings. The total dissolved solids were 231 mg/L. Similar results for this drinking water have been reported elsewhere [35]. In general, besides fluoride concentration, all other parameters are within the values accepted by the regulations. Fig. 8. Scanning electron micrographs of zeolitic material.
Fig. 10. Fluoride adsorption capacities and equilibrium pH values for the unmodified and modified hematite, zeolitic material and calcite.
0.05 0.03 0.03 0.02 0.06 0.01 0.04 0.03 – – – – – – – – – – – – – – 0.28 0.05 0.10 0.03 0.11 0.04 0.13 0.04 0.29 0.07 0.23 0.04 – – – – 31.93 2.64 28.55 5.12 20.04 2.58 19.86 3.11 Calcite Unmodified 1A, 30 min 2A, 30 min 3A, 30 min
– – – – 51.04 1.52 52.17 2.51 54.41 1.49 53.27 0.92 16.70 1.27 18.79 2.61 16.50 1.20 18.49 1.97
0.16 0.03 0.32 0.12 8.42 1.22 8.00 0.83
– – – – – – – – 2.08 0.59 1.49 0.25 1.79 0.26 1.65 0.56 1.68 0.56 0.84 0.25 0.63 0.14 2.32 1.11 3.23 0.21 2.96 0.23 2.41 0.14 2.89 0.85 2.71 0.69 2.12 0.34 0.83 0.35 2.44 0.37 – – – – – – – 1.66 0.76 – – – – – 1.43 0.49 2.15 0.78 – 37.41 1.00 30.44 3.67 30.30 1.38 30.34 1.24 30.30 1.06 21.62 3.99 11.84 3.47 36.28 1.22 1.53 0.64 0.84 0.11 1.32 0.26 0.90 0.38 0.95 0.43 0.57 0.19 0.77 0.65 1.16 0.34 0.59 0.12 0.42 0.09 0.40 0.05 0.73 0.78 0.80 0.73 0.33 0.07 – 0.61 0.25 – – 0.21 0.01 0.16 0.03 – – – – 48.37 1.84 48.89 1.17 46.71 1.02 46.95 1.40 46.72 1.65 47.52 1.20 49.20 2.10 48.37 2.00 Zeolitic material Unmodified – 1A, 30 min 10.67 2.31 2A, 30 min 9.72 1.86 3A, 30 min 8.20 0.68 3A, 1 h 8.60 0.49 3A, 2 h 14.17 1.36 3A, 3 h – La –
6.80 0.12 6.43 0.54 7.26 0.46 8.28 1.46 8.25 1.20 11.41 2.20 34.74 4.94 7.15 0.69
– – – – – – 3.47 3.02 – – 0.22 0.12 – 0.35 0.07 0.48 0.13 – – 1.49 0.78 2.63 0.61 2.36 0.27 2.01 0.30 1.51 0.48 2.54 0.26 72.36 4.76 51,02 11.3 58.34 6.09 47.49 4.41 47.11 4.68 39.19 5.73 46.41 7.59 1.50 0.18 0.29 0.14 0.65 0.30 0.47 0.17 0.37 0.11 0.30 0.08 1.01 0.12 1.15 0.21 2.13 1.22 2.44 0.69 2.48 0.24 5.81 0.72 11.92 1.74 1.24 0.24 0.38 0.06 – – 0.50 0.05 0.40 0.15 – 0.61 0.18 17.70 1.39 20.20 6.54 28.22 4.90 27.92 3.23 30.50 1.49 33.39 2.49 32.83 2.45 6.92 3.98 16.33 19.1 7.50 1.24 7.62 0.97 13.44 3.34 13.14 2.45 11.89 4.43 Hematite Unmodified 1A, 30 min 2A, 30 min 3A, 30 min 3A, 1 h 3A, 2 h La
Mg O C
% Elemental composition Material
Table 1 Elemental analysis of the unmodified and modified materials.
Al
Ca
Fe
Si
Cl
La
Na
– – – – – – –
K
– – – – – – –
P
– – – – – – –
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3.3. Sorption capacities Figs. 10 and 11 show the fluoride sorption capacities and the removal percent respectively, determined for the different materials using fluoride solutions of 4 and 10 mg/L and drinking water containing 5.87 mg/L (naturally). The equilibrium pH values are also shown (Fig. 10). As it can be observed the adsorption capacities determined are proportional to the content of fluoride ions in the aqueous phase. Figs. 12 and 13 show the initial percent of aluminum in the samples vs. the adsorption capacities, for the hematite and zeolitic material samples, respectively. The adsorption behaviors were similar for both materials, with the adsorption capacities increasing up to a plateau. For hematite, the plateaus for fluoride solutions and drinking water were reached when the percentage of aluminum was a little higher than 2% and for the zeolitic material when it was about 10%. These results indicate that aluminum is more effective for fluoride adsorption in hematite than in the zeolitic material; iron may be responsible for this behavior. In general, hematite is more efficient than zeolitic materials for the removal of fluoride ions [22,24]. Most equilibrium pH values for drinking water were between 6.5 and 8 which are acceptable according to the regulations [2,34]. Onyango et al. [19] reported the removal of fluoride ions by zeolitic material F9; they reported that the presence of lanthanum improves the sorption efficiency of this material and found a sorption capacity of 59.12 mg/g. In this work the hematite and zeolitic materials were modified with lanthanum and it was found that the presence of this element improves the sorption efficiencies of these materials for fluoride ions as shown in Figs. 10 and 11. Similar results have been reported elsewhere with lanthanum incorporated chitosan beads, lanthanum impregnated chitosan flakes, lanthanummodified chitosan [36–38]. The aluminum modified materials show higher sorption capacities than the lanthanum modified materials (Figs. 10 and 11). Jain and Jayaram [25] found that aluminum modified calcite is an efficient material for the removal of fluoride ions, in this work a calcite sample was aluminum modified, according to Table 1 the unmodified material contains 0.16 0.03% of aluminum, after modification with 1 A and 30 min the content increased to 0.32 0.12% and with 2–3 A for 30 min, the aluminum was about 8%, however the adsorption capacities for fluoride removal from drinking water were less than 0.2 mg/g for the unmodified material and about 0.4 mg/g for the aluminum modified materials, as shown in Table 1 and Fig. 10, it seems that the adsorption capacities are not directly proportional to the quantities of aluminum in the calcite, the origin and the composition of the samples may be responsible for these different behaviors. In general, the presence of aluminum improves the sorption efficiencies for fluoride ions from drinking water and the most
Table 2 Characterization of drinking water. Parameter
Value
Alkalinity Total dissolved solids % Saturation oxygen Dissolved oxygen Chloride Bicarbonate Carbonate Fluoride pH Temperature Electrical conductivity
207.20 mg/L as CaCO3 231 mg/L 59.8 4.59 mg/L 26.93 mg/L 247.92 mg/L 0.006 mg/L 5.87 mg/L 7.46 12.7 8C 463 ms/cm
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Fig. 11. Fluoride adsorption percentages for the unmodified and modified hematite, zeolitic material and calcite.
efficient materials are those with the highest quantities of aluminum. 4. Conclusions
Fig. 12. Fluoride adsorption capacities vs. aluminum percent in hematite.
The natural materials were aluminum modified by an electrochemical method. The presence of aluminum improves the sorption efficiencies for fluoride ions from drinking water and the most efficient materials are those with the highest quantities of aluminum. Aluminum modified hematite is more effective than aluminum modified zeolitic tuff for fluoride adsorption and the presence of iron may be responsible for this behavior. The presence of lanthanum in hematite and zeolitic tuff improves their sorption efficiencies for fluoride ions, but the aluminum modified materials show higher sorption capacities than the lanthanum modified materials. Acknowledgments We acknowledge financial support from CONACYT, project 131174Q and scholarship Grant No. 231465 for ATS. References
Fig. 13. Fluoride adsorption capacities vs. aluminum percent in zeolitic material.
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