Desalination 275 (2011) 102–106
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Desalination j o u r n a l h o m e p a g e : w w w. e l s ev i e r. c o m / l o c a t e / d e s a l
Analysis and the understanding of ﬂuoride removal mechanisms by an electrocoagulation/ﬂotation (ECF) process Mohammad M. Emamjomeh a,⁎, Muttucumaru Sivakumar b, Ali Safari Varyani c a
Research Center for Community Development and Health promotion, Qazvin university of Medical Sciences, Qazvin, Iran Sustainable Water and Energy Research Group, School of Civil, Mining and Environmental Engineering, Faculty of Engineering, University of Wollongong, Wollongong, NSW 2522, Australia c Occupational Health Group, Faculty of Health, Qazvin university of Medical Sciences, Qazvin, Iran b
a r t i c l e
i n f o
Article history: Received 6 December 2010 Received in revised form 12 February 2011 Accepted 14 February 2011 Available online 8 March 2011 Keywords: Electrocoagulation/ﬂotation (ECF) process Fluoride removal mechanism Analysis Sludge characteristics
a b s t r a c t Electrocoagulation is a method of applying direct current to sacriﬁcial electrodes that are submerged in an aqueous solution. Dissolving aluminum (Al3+) is predominant in the acidic condition and aluminum hydroxide has tendency soluble. The deﬂuoridation process was found to be efﬁcient for a pH ranging from 6 to 8. The ﬂuoride removal mechanisms are investigated based on the solution speciation (Al and Al–F complexes) and dried sludge characteristics in the electrocoagulator. The XRD analysis of the composition of the dried sludge shows the formation of Al(OH)3 − xFx and provides conﬁrmation for the main mechanism for ﬂuoride removal. The mechanism of the ﬂuoride removal was conﬁrmed to be not only the competitive adsorption between OH– and F− but also the formation of solid cryolite in pH range of 5–8. © 2011 Elsevier B.V. All rights reserved.
1. Introduction Inorganic constituents, which may be presented in natural waters or in contaminated source waters, are found to become a major public health problem in drinking water. The presence of ﬂuoride, as an inorganic ion, is serious more than limits in drinking water and it is a public health problem. The ﬂuoride removal in drinking water and in wastewater has been the subject of many publications and studies that have progressively developed the aspects of toxicity on man and on the environment. When the ﬂuoride concentration is increased to more than 4 mg L−1 in drinking water, ﬂuorosis deformity at hips, knees and other joints are seen in skeletal ﬂuorosis . However, long-term use of low ﬂuoride concentration (less than 0.5 mg L−1) is the cause of dental caries . Because of the public health signiﬁcance of high ﬂuorides consumption in drinking water, deﬂuoridation is important. The maximum acceptable concentration of ﬂuoride in water is 1.5 mg L−1 . Fluoride pollution occurs through two different sources including natural and anthrogenic sources. In groundwater sources, the natural concentration of ﬂuoride depends on the geological, chemical and physical characteristics of the aquifer, as concentrations of up to 38.5 mg L−1 have been reported in India. Some of the water supplies can be polluted by discharging of industrial wastewater without any ⁎ Corresponding author at: Environmental Health Engineering Dep, Faculty of Health, Qazvin university of Medical Sciences, Bahonar Street, Qazvin, Iran. Tel.: +98 281 3344504; fax: +98 281 3368778. E-mail address: [email protected]
(M.M. Emamjomeh). 0011-9164/$ – see front matter © 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.desal.2011.02.032
ﬂuoride treatment, including glass manufacturing industries  and semiconductor industries  in the natural environment. Several methods were globally studied for deﬂuoridation of water, such as: adsorption [6–8], chemical precipitation [9–15], electrodialysis , and electrochemical methods [16,17]. These methods can be divided in two categories as precipitation and sorption methods . Lime addition is the most common technology of the ﬁrst group and is used for high ﬂuoride concentration. Lime is used to form CaF2 precipitate and reduce the ﬂuoride concentration. In laboratory experiments, a two-column limestone reactor has been designed to reduce ﬂuoride concentrations from 109 mg L−1 to 4 mg L−1 . The second groups (sorption methods) need regular column regeneration and not costeffective to treat wastewaters with ﬂuoride concentration greater than 10 mg L−1 . Lime is the cheapest chemical used for the deﬂuoridation of wastewater, however, it is impossible to reduce the ﬂuoride concentration to 1 mg L−1 using only lime  .Toyoda and Taira  proposed a new method for treating high ﬂuoride concentration (100 mg L−1) to reduce sludge and running costs at two stages, e.g., the formation of CaF2 by addition of Ca salt such as Ca (OH)2 and the adsorption of the residual ﬂuorine by Al(OH)3 by addition of an Al salt. In the 1st step of the conventional treatment system, ﬂuoride concentration was decreased from 100 to 20 mg L−1, since it can be decreased to 2.5 mg L−1 by addition of Al salt and neutralizing in the 2nd step. Using chemical coagulants for precipitation is one of the most essential methods in conventional water and wastewater treatment. However, the generation of large volumes of sludge, the hazardous waste categorization of metal hydroxides, and high costs associated with chemical treatments have made chemical
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Co d I i M pHi pHf Eci t T
Initial ﬂuoride concentration (mg L−1) Distance between electrodes (m) Current (A) Current density (A m−2) Molar mass (g mol−1) Initial pH Final pH Initial conductivity (mS m−1) Electrolysis time (min) Temperature (°C)
coagulation less acceptable compared to other methods. A new process, electrocoagulation/ﬂotation (ECF) process, which produces less waste sludge, could replace the conventional chemical coagulation. ECF process, which is an electrochemical technique, is the combination of oxidation, ﬂocculation and ﬂotation. In its simplest form, an ECF reactor may be made up of an electrolytic cell with one anode and one cathode as current is passed through the electrodes, electrolyzing the water and producing bubbles of hydrogen and oxygen gas. These bubbles ﬂoat to the top of the tank, colliding with particles suspended in the water on the way up, adhering to them and ﬂoating them to the surface of the water. Some researchers [14–20] have demonstrated that electrocoagulation (EC) using aluminum anodes are effective in deﬂuoridation. Their main objectives were to investigate the electrochemical removal of ﬂuoride and elucidate the inﬂuence of some factors including current, contact time, solution pH, ﬂuoride concentration and electrolyte concentration on the ﬂuoride removal efﬁciency, as mechanisms of ﬂuoride removal have been missed in more these researches. However, Ming, Yi S.R. et al. (1983; Cheng 1985; Mameri, Lounici et al. 2001; Shen, Chen et al. 2003) reported that the mechanism of the ﬂuoride removal was conﬁrmed to be the competitive adsorption between OH− and F− when were used an electrocoagulation (EC) process followed by an electroﬂotation (EF) operation, but more investigation needs to be done . To elucidate this mechanism for a combined system (EC and EF processes), a monopolar ECF process has been conducted for this research, as the effective design factors have not been considered in this research. These design factors were reported before . The main aim of the research is to elucidate the mechanism of ﬂuoride removal in a batch monopolar ECF. The understanding of the mechanisms of speciation and its impact on ﬂuoride removal are the main interest of this research and mechanisms of ﬂuoride removal will hence be explained in more detail. To explore the mechanism of F− removal, the characteristics of the sludge and initial pH inﬂuence were studied in this research. 2. Theory 2.1. Aluminum speciation in water The aqua aluminum ion is the hardest of the trivalent metal ions, with an effective ionic radius of 0.5 Å, which is smaller than other commonly encountered trivalent metal ions. Aluminum has a strong tendency to hydrolyze in aqueous solution . The speciation of aluminum can exist in several different forms when depending on the several factors such as: pH, temperature of the water during treatment, the type of organic and inorganic elements in the raw water, and treatment conditions . In the basic and acidic solution, + Al ion will be found in anionic (Al(OH)− 4 ) and cationic (Al(OH)2 ) forms, respectively. Without signiﬁcant concentrations of other anions, the aqueous Al will form to various hydroxyl complexes
when the relative amounts of which will depend on both pH and initial concentration of the Al in solution . However, the hydrated ion Al(H2O)3x + exists in the solution at pH b 4 but the hydroxide complexes are formed at pH N 4 . For example, the aqua complex Al (H2O)36 +predominates at pH b 4 . Concerning the aluminium ionic species in an aqueous solution, the relationships of aluminium solubility are considered by using the thermodynamic properties. The equilibrium relationships can be expressed as: ð1Þ
aA + bB↔cC + dD
Where A and B are reactants, C and D are products, and a, b, c, and d are the number of moles of each involved in the reaction. The thermodynamic equilibrium constant (K0) at standard temperature and pressure (STP) is deﬁned , as: 0
ðC Þc ðDÞd ð AÞa ðBÞb
The solubility of aluminum in equilibrium with solid phase Al(OH)3 depends on the surrounding pH. At pH between 5 and 6, the predominant hydrolysis products are found to be Al(OH)2+ and Al (OH)+ 2 . The solid Al(OH)3 is most prevalent in pH range of 5–8. The soluble species Al(OH)− 4 is the predominant species at pH values more than 9. MINEQL+ software  was used to show Fig. 1 how different pH would inﬂuence the solubility of Gibbsite (Al(OH)3). The equilibrium concentrations of the soluble complexes are calculated from the data in MINQEL+ software . At this concentration of Al, there are no polymeric species. Solid Al(OH)3 precipitates in pH of 5 when it predominates over soluble complexes in the pH range of 5–9. At higher pH (pH N 10), the soluble aluminates,Al(OH)− 4 , predominate as it is the only species present. 2.2. Aluminum-ﬂuoride complexation Thermodynamic calculations show the aluminum ﬂuoride complexes are generally the dominant inorganic aluminum species [29,30]. The presence of ﬂuoride can produce changes in the progress of Al in water. Since ﬂuoride ion forms strong complexes with aluminum, it can considerably increase the solubility of Al . In the presence of 1 × 10−5 M ﬂuoride, the ﬂuoride complexes AlF2+, AlF+ 2 , AlF3, and AlF− 4 predominate in acid solution until Al(OH)3 precipitates. These complexes of Al are not precipitated until the solution pH is reached to 6. In the alkaline solution, the complex of Al(OH)− 4 is formed. The excess solid Al(OH)3 maintains a constant concentration of Al(OH)3 complex. 10
Log concentration (Al species)
-5 Al(OH) + 2 -10
pH Fig. 1. Solubility of aluminium hydroxide at various pH values by using MINEQL+ software.
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2.3. Mechanisms of electrocoagulation/ﬂotation (ECF) Aluminum is naturally present in drinking water predominantly using aluminum sulfate (Alum) in water treatment process. Although, there has been the move away from Alum in the water industry due to its possible link to Alzheimer's disease , it remains in use as a coagulant agent in some countries. As noted, Al electrodes are used instead of Al salts such as Alum in the electrocoagulation/ﬂotation (ECF) process. The electrolytic dissolution of Al anodes by oxidation produces aqueous Al3+ species  and the electrode reactions are outlined below: −
At the aluminum cathodes reduction takes place which results in hydrogen bubbles being produced by the following reaction: −
Cathodes : 2H2 O + 2e →H2ðgÞ + 2OH
The H2 bubbles ﬂoat in ECF reactor. The Al3+ ions further react as shown in Eq. (3) to a solid Al(OH)3 precipitate (see Fig. 1). Those precipitates form ﬂocs that combine water contaminants as well as a range of coagulant species and metal hydroxides formed by hydrolysis as shown below: Al
+ 3H2 O↔AlðOHÞ3ðSÞ + 3H
These coagulants destabilize suspended particles or precipitate and adsorb dissolved contaminants. For example, the Al(OH)3 ﬂoc is believed to adsorb F− strongly  as shown by Eq. (6): −
AlðOHÞ3 + xF ↔AlðOH Þ3−x Fx + xOH
Freshly formed amorphous Al(OH)3 precipitates that are required for “sweep coagulation” have large surface areas, which are beneﬁcial for rapid adsorption of soluble compounds and trapping of colloidal particles. These ﬂocs polymerize usually at high Al concentrations as: nAlðOH Þ3 →Aln ðOHÞ3n
Co-precipitation (Eq. (6)) or adsorption (Eq. (7)) reaction may occur when aluminum salt is used for ﬂuoride removal . −
nAlðaqÞ + 3n−mOHðaqÞ + mFðaqÞ →Aln Fm ðOHÞ3n−mðsÞ −
Aln ðOHÞ3nðsÞ + mFðaqÞ →Aln Fm ðOH Þ3n−mðsÞ + mOHðaqÞ
3. Experimental study 3.1. Experimental setup A laboratory batch monopolar electrocoagulation reactor was designed and constructed to the dimensions shown in Fig. 2. In the electrochemical cell, ﬁve aluminum (purity of Al 95–97%, Uldrich Aluminium Company Ltd, Sydney) plate anodes and cathodes (dimension 250 × 100 × 3 mm) were used as electrodes. The electrodes were connected using monopolar conﬁguration in the electrocoagulation reactor. In the case of monopolar electrodes, individual electrochemical cells can be combined in assemblies by parallel coupling. These were dipped 200 mm into an aqueous solution (3.6 L) in a Perspex box (dimension 300 × 132 × 120 mm). In the reactor, stirring was achieved using a magnetic bar placed between the bottom of the electrodes and the box. A gelatinous deposition layer is present on the surface of the anode after EC process. This is because the electro-condense effect induces the accumulation of ﬂuoride ions near the anode and leads to surface co-precipitation on the anode. This layer was cleaned for each run by hydrochloric acid solutions. The gap between the two neighboring electrode plates was 5 mm. Direct current from a DC power supply (0–30 V, 0–2.5 A, ISO-TECH, IPS-1820D) was passed through the solution via ﬁve electrodes. The ECF reactor operated in a galvanostatic mode which means that the current was held constant while the cell potential varied to maintain the required current. Current was varied over the range of 0.5–2.5 A, however, it was held constant for
Also, ﬂuoride may be segregated as Na3AlF6 (Cryolite) or other compounds that contain the complex ionAlF36 −, as the process is highly dependent on the pH of the water or wastewater to be treated. The following reactions may occur by complexation of F and Al3+, AlF2+, AlF3 and subsequent precipitation of Cryolite (Na3AlF6). Al
+ 3Na →Na3 AlF6ðsÞ
The ECF process is highly dependent on the pH of the water or wastewater [33,34]. Coagulation by aluminum salts occurs at a wide range of pH due to different mechanisms, the amorphous aluminum hydroxide is least soluble at a pH close to 8 . MINEQL+ equilibrium speciation software was utilized to show how different pH would inﬂuence the solubility of Gibbsite [(Al(OH)3)] (Fig. 1). In ECF process, the solution pH increased when electrolysis time was increased. At pH value of less than 4, aluminum remains in the form of Al3+, which caused no precipitation to occur, so the ﬂuoride concentration cannot
6 Water level
Anodes : AlðsÞ →Al
be reduced. The hydrogen ion concentration remains relatively high in the pH range of 4–5.5. Consequently, a high zeta potential and a strong repulsive force among aluminum hydroxide colloidal ﬂocs are maintained . At a pH range of 5.5–6.5, the concentration of OH− is lower than when pH is increased to 8. In alkali solution, OH− ions are increased. In the anodic adsorption layer, the concentration of OH− ions is higher than the concentration of F−, so the production of the complex AlF36 − ion becomes difﬁcult. The results show that the deﬂuoridation process is more efﬁcient for a pH ranging from 5.5 to 7.5. It is agreement with results of Mameri et al. . For better understanding of the mechanisms of speciation and its impact on ﬂuoride removal, the initial pH inﬂuence and the characteristics of the dried sludge were further studied in this research by designing an ECF monopolar reactor.
1. Electrolytic box 2. Aluminum electrodes 3. Magnetic bar-stirrer 4. Drain tube 5. Sampling valve 6. Electrode support
120 mm 132 mm Fig. 2. Schematic diagram of the electrocoagulation reactor (EC).
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each run. Electrocoagulation experiments were performed for each run and samples were taken from the drain tube section in the electrocoagulator for pH and ﬂuoride measurement. 3.2. Solution chemistry
4.2. Characteristics of sludge The composition of the sludge produced in the bottom of EC reactor was analyzed using X-ray diffraction (XRD) spectrum and then soft analysis by relevant software at the ﬁnal pH range of 6–8. As seen in Fig. 4, the strongest peaks appeared at degrees 18 and 20, which were identiﬁed to be aluminum ﬂuoride hydroxide and aluminum hydroxide, respectively. In the ﬁnal pH range of 6–8, the sludge characterization results showed that the residual ﬂuoride may occur in solids particles such as aluminum ﬂuoride hydroxide complexes. When the ﬁnal pH is more than 8, the concentration of OH− ions is higher than the concentration of F− in the anodic adsorption layer, so production of the complex AlF36 − ion becomes difﬁcult. Other peaks were identiﬁed to be Al(OH)2, AlFO, Al(OH)F, AlFO2H, and AlF2 when the composition of the sludge was analyzed using X-ray photoelectron spectroscopy (XPS) and time-of-ﬂight secondary ion mass spectroscopy (TOF-SIMS).
# Aluminum Fluoride Hydroxide * Aluminum Fluoride Hydroxide Hydrate + Aluminum Hydroxide
* * #
* # + + + *
500 450 400 350 300 250 200 150 100 50 0 10
The rate of aluminium-ﬂuoride complex formation is generally dependent on the solution pH values. The presence of ﬂuoride can produce comprehensible changes in the progress of Al in water. Since aluminum hydroxide is an amphoteric hydroxide, pH is a sensitive factor to the formation of Al(OH) 3 ﬂocs. Although coagulation with Al salts occurs at a wide range of pH due to different mechanisms . The amorphous aluminum hydroxide is least soluble at a pH close 8. Effect of initial pH was experienced by using the synthetic solution (distilled water + NaF salt). The pH of the feed and product water was measured for each experiment as the ﬁnal pH was kept to be constant among 5–8 during experiments by adding sodium hydroxide and hydrochloric acid solutions. The results, obtained from Fig. 3, show that deﬂuoridation process is efﬁcient at a ﬁnal pH range of 6–8 when the effect of initial pH was investigated in a big range from 2 to 10. No formation of Al(OH)3 ﬂoc was formed when the inﬂuent pH was found to be 2. It is the main reason for decreasing of the ﬂuoride removal efﬁciency. As seen in Fig. 3, by increasing the initial pH from 3 to 7, the ﬁnal pH was increased from 6 to 8.7. The ﬂuoride removal efﬁciency reached to 92% when the ﬁnal pH was more than 6. When the initial pH increased from 4 to 8, the ﬁnal pH was increased from 8.5 to 8.9 in the EC reactor. Where it remains almost constant because of the
− At higher pH, pH ≥ 9, due to formation of Al(OH)− 4 and AlO2 , which is soluble and useless for deﬂuoridation, ﬂuoride removal efﬁciency is decreased. No Al(OH)3 ﬂoc was observed visually when pH is beyond 10. The results, obtained from Fig. 3, show that the highest treatment efﬁciency was obtained for the ﬁnal pH ranging between 6 and 8. It may be explained due to low solubility of Gibbsite at a pH range of 6–8 (see Fig. 1). The strong presence of the hydroxyaluminum, which maximizes the ﬂuorohydroxide aluminum complex formation, is the main reason for deﬂuoridation by electrocoagulation. More details will be explained in the next section when the composition of the sludge was analysed.
4.1. Inﬂuence of initial pH
4. Results and discussions
AlðOHÞ3 + OH ↔AlðOH Þ4
Fluoride concentration was determined using the ionometric standard method  with a ﬂuoride selective electrode (Metrohm ion analysis, Fluoride ISE 6.0502.150, Switzerland). To prevent interference from the Al3+ion, TISAB buffer (58 g of NaCl, 57 mL of glacial acetic acid, 4 g 1,2 cyclohexylenediaminetetraacetic (CDTA), and 125 mL 6 N NaOH were dissolved in 1000 mL distilled water with stirring until pH 5.3–5.5 was reached) was added to the samples. Direct current from a DC power supply was passed through the solution via the ﬁve electrodes. Cell voltage and current were readily monitored using a digital power display. Conductivity and pH were measured using a calibrated pH meter and conductivity meter, respectively. The composition of sludge was analyzed by X-ray diffraction (XRD). The XRD measurements were carried out by the Philips no RN 1730 with CUKα source. The analyzers were ﬁtted by ICCD standard patterns and Trace 5 software. Pass voltage of 40 kV and current of 20 mA were used for its spectra.
buffering capacity of the system Al(OH)3/Al(OH)− 4 .
3.3. Analytical techniques
Fig. 3. Effect of initial pH on the deﬂuoridation removal by ECF process (i = 18.75 A/m2, d = 5 mm, Eci = 10 ms/m, Co = 10 mg L−1, t = 60 min).
All experiments were conducted at 25± 1 °C with an initial F concentration of 10 mg L−1 and 25 mg L−1 where synthetic water was added to make up to the same concentration.The inﬂuence of the experimental design factors on the deﬂuoridation process was investigated with “synthetic” water (distilled water + NaF salt + NaCl + NaHCO3) in a batch reactor. Sodium chloride (0.1 M) was added to the aqueous solution to promote conductivity to 1000 mS/m in the electrocoagulator. 1 M Sodium hydroxide and 1:5 hydrochloric acid solutions were added for pH adjustment (values 5 to 8.5). Sodium bicarbonate (5× 10−4 M) was only added in synthetic samples to maintain alkalinity. The alkalinity acts to buffer the water in a pH range where the coagulant can be effective.
Degrees 2 -Theta Fig. 4. The composition of dried settled sludge analyzed by XRD on deﬂuoridation by ECF process (pHf = 6, Eci = 10 ms/cm, Co = 10 mg L−1, t = 60 min, i = 18.75 A/m2, T = 25 ± 1 °C).
* Aluminum Hydroxide # Aluminum Fluoride Hydroxide
500 450 400 350 300 250 200 150 100 50 0 15
M.M. Emamjomeh et al. / Desalination 275 (2011) 102–106
Degrees 2- Theta Fig. 5. The composition of dried sludge collected in the surfaces of electrodes analyzed by XRD on deﬂuoridation by ECF process (pHf = 7, A = 0.08 m2, Eci = 10 ms/cm, Co = 10 mg L−1, t = 60 min, i = 18.75A/m2, T = 25 ± 1 °C).
To conﬁrm and understand the ﬂuoride removal mechanism, the composition of dried sludge, which was collected in the surfaces of electrodes, was again analyzed by XRD spectrum. As seen in Fig. 5, there were two strong peaks at degrees 18 and 20 which could be due to the formation of aluminum hydroxide ﬂoc and the structure of aluminum ﬂuoride hydroxide complexes [Al(OH,F)3]. As seen in Fig. 5, the strongest peaks are due to the formation of aluminum hydroxide and aluminum ﬂuoride hydroxide, which are similar to results obtained from Fig. 4. It is clear that there is no difference among ﬂoc characteristics that were collected from electrodes surface and bottom of an electrocoagulator. In summary, in the ﬁnal pH range of 5–8, the residual ﬂuoride may occur in different dissolved forms (F−, AlF2+, AlF4−) or ﬁnely formed to solid (cryolite, Al(OH)3 − xFx). The mechanism of the ﬂuoride removal was conﬁrmed to be not only the competitive adsorption between OH− and F− but also formations of solid cryolite in the pH range of 5–8. The solid cryolite appeared when the pH range is found to be between 5 and 6. In the ﬁrst stage of the treatment process the electrolysis time is short and solid cryolite may be formed, deﬂuoridation efﬁciency is low. By increasing the pH from 6 to 8, the ﬂuorohydroxide aluminum complex formation is maximized that it is the main reason for deﬂuoridation by electrocoagulation. 5. Conclusion A monopolar batch ECF reactor was experienced for understanding of the mechanism of ﬂuoride removal. The experimental results elucidated that the electrocoagulation/ﬂotation process is highly dependent on the pH of the solution. At a pH range of 5–8, the solid Al (OH)3 is most prevalent. The soluble species Al(OH)− 4 is the predominant species when the ﬁnal pH is increased to 10. The ﬂuoride − complexes such as AlF2+,AlF+ 2 , AlF3, and AlF4 predominate in acid solution until Al(OH)3 precipitates. The removal efﬁciency decreased when the ﬁnal pH was increased from 8 to10. The residual ﬂuoride may occur in different dissolved forms (F−, AlF2+, AlF4−) or ﬁnely formed to solid (cryolite, Al(OH)3 − xFx). The mechanism of the ﬂuoride removal was conﬁrmed to be not only the competitive adsorption between OH− and F− but also the formation of solid cryolite in the ﬁnal pH range of 5–8. It could be resulted that the deﬂuoridation process is more efﬁcient for the ﬁnal pH ranging between 6 and 8. Concerning the XRD results, the ﬂuorohydroxide aluminum complex formation is maximized which is the main reason for deﬂuoridation by electrocoagulation. References  National Research Council (U. S.), Health inﬂuence of ingested ﬂuoride / Subcommittee on Health Inﬂuence of Ingested Fluoride, Committee on Toxicology, Board on Environmental Studies and Toxicology, Commission on Life Sciences, National Research Council, National Academy Press, Washington, D.C, 1993.
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