flotation (ECF) process

Desalination 275 (2011) 102–106

Contents lists available at ScienceDirect

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 [1]. However, long-term use of low fluoride concentration (less than 0.5 mg L−1) is the cause of dental caries [2]. 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 [3]. 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 [4] and semiconductor industries [5] in the natural environment. Several methods were globally studied for defluoridation of water, such as: adsorption [6–8], chemical precipitation [9–15], electrodialysis [13], and electrochemical methods [16,17]. These methods can be divided in two categories as precipitation and sorption methods [17]. 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 [12]. The second groups (sorption methods) need regular column regeneration and not costeffective to treat wastewaters with fluoride concentration greater than 10 mg L−1 [12]. 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 [6] .Toyoda and Taira [5] 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

M.M. Emamjomeh et al. / Desalination 275 (2011) 102–106

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 [17]. 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 [21]. 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 [22]. 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 [23]. 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 [24]. 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 [25]. For example, the aqua complex Al (H2O)36 +predominates at pH b 4 [26]. 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 [27], as: 0

K =

ðC Þc ðDÞd ð AÞa ðBÞb

ð2Þ

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 [28] 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 [28]. 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 [15]. 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)

Nomenclature

103

5 Al

3+

Al(OH)3(s) -

Al(OH)4

0 2+

Al(OH)

-5 Al(OH) + 2 -10

-15 1

3

5

7

9

11

13

pH Fig. 1. Solubility of aluminium hydroxide at various pH values by using MINEQL+ software.

M.M. Emamjomeh et al. / Desalination 275 (2011) 102–106

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 [31], 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 [27] and the electrode reactions are outlined below: −

ð3Þ

+ 3e

At the aluminum cathodes reduction takes place which results in hydrogen bubbles being produced by the following reaction: −

Cathodes : 2H2 O + 2e →H2ðgÞ + 2OH

−

ð4Þ

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

3 +

+ 3H2 O↔AlðOHÞ3ðSÞ + 3H

þ

ð5Þ

These coagulants destabilize suspended particles or precipitate and adsorb dissolved contaminants. For example, the Al(OH)3 floc is believed to adsorb F− strongly [17] as shown by Eq. (6): −

AlðOHÞ3 + xF ↔AlðOH Þ3−x Fx + xOH

−

ð6Þ

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

ð7Þ

Co-precipitation (Eq. (6)) or adsorption (Eq. (7)) reaction may occur when aluminum salt is used for fluoride removal [32]. −

3 +

−

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

ð8Þ

DC

+ -

ð9Þ

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

3−

AlF6

+ 6F

−

3− →AlF6

ð10Þ

þ

ð11Þ

+ 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 [27]. 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

1

300 mm

3+

+ -

2

250 mm

3 +

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 [14]. 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. [16]. 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.

5 4

1. Electrolytic box 2. Aluminum electrodes 3. Magnetic bar-stirrer 4. Drain tube 5. Sampling valve 6. Electrode support

50 mm

104

3

120 mm 132 mm Fig. 2. Schematic diagram of the electrocoagulation reactor (EC).

M.M. Emamjomeh et al. / Desalination 275 (2011) 102–106

105

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

* * #

*

* # + + + *

*

# 20

+

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 [34]. 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

ð12Þ

− 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 [35] 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).

Counts

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.

30

40

50

60

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

Counts

106

25

35

45

55

65

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 [1] 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.

[2] J.J. Murray, Fluorides in Caries Prevention, John Wright, Bristol, 1995, pp. 125–139. [3] NHMRC and ARMCANZ, Australian Drinking Water Guidelines, National Health and Medical Research Council and the Agriculture and Resource Management Council of Australia and New Zealand, (2004). [4] M.G. Sujana, R.S. Thakur, S.B. Rao, Removal of fluoride from aqueous solution by using alum sludge, J. Colloid Interf. Sci. 206 (1998) 94–101. [5] A. Toyoda, T. Taira, A new method for treating fluorine wastewater to reduce sludge and running costs, IEEE Trans. Semicond. Manuf. 13 (2000) 305–309. [6] Q. Zuo, X. Chen, W. Li, G. Chen, Combined electrocoagulation and electroflotation for removal of fluoride from drinking water, J. Hazard. Mater. 159 (2008) 452–457. [7] K.B. Jinadasa, S. Weerasooriya, C.B. Dissanayake, A rapid method for the defluoridation of fluoride-rich drinking waters at village level, Int. J. Environ. Stud. 31 (1988) 305–312. [8] M. Meenakshi, R.C. Maheshwari, Fluoride in drinking water and its removal, J. Hazard. Mater. 137 (2006) 456–463. [9] N. Drouiche, S. Aoudj, M. Hecini, N. Ghaffour, H. Lounici, N. Mameri, Study on the treatment of photovoltaic wastewater using electrocoagulation: fluoride removal with aluminium electrodes—characteristics of products, J. Hazard. Mater. 169 (2009) 65–69. [10] S. Saha, Treatment of aqueous effluent for fluoride removal, Water Res. 27 (1993) 1347–1350. [11] C.J. Huang, J.C. Liu, Precipitate flotation of fluoride-containing wastewater from a semiconductor manufacturer, Water Res. 33 (1999) 3403–3412. [12] E.J. Reardon, Y. Wang, A limestone reactor for fluoride removal from wastewaters, Environ. Sci. Technol. 34 (2000) 3247–3253. [13] Z. Amor, B. Bariou, N. Mameri, M. Taky, Fluoride removal from brackish water by electrodialysis, Desalination 133 (2001) 215–223. [14] V. Khatibikamal, A. Torabian, F. Janpoor, G. Hoshyaripour, Fluoride removal from industrial wastewater using electrocoagulation and its adsorption kinetics, J. Hazard. Mater. 179 (1–3) (2010 Jul 15) 276–280. [15] C.-Y. Hu, S.-L. Lo, W.-H. Kuan, Simulation the kinetics of fluoride removal by electrocoagulation (EC) process using aluminum electrodes, J. Hazard. Mater. 145 (2007) 180–185. [16] N. Mameri, H. Lounici, D. Belhocine, Y. Yahiat, Defluoridation of Sahara water by small plant electrocoagulation using bipolar aluminium electrodes, Sep. Purif. Technol. 24 (2001) 113–119. [17] F. Shen, X. Chen, P. Gao, G. Chen, Electrochemical removal of fluoride ions from industrial wastewater, Chem. Eng. Sci. 58 (2003) 987–993. [18] M.M. Emamjomeh, M. Sivakumar, An empirical model for defluoridation by batch monopolar electrocoagulation/flotation (ECF) process, J. Hazard. Mater. 131 (2006) 118–125. [19] M.M. Emamjomeh, M. Sivakumar, Review of pollutants removed by electrocoagulation and electrocogualtion/flotation process, J. Environ. Manage. 90 (2009) 1663–1679. [20] M.M. Emamjomeh, M. Sivakumar, Fluoride removal by a continuous flow electrocoagulation reactor, J. Environ. Manage. 90 (2009) 1204–1212. [21] M.M. Emamjomeh, M. Sivakumar, Fluoride removal by using a batch electrocoagulation reactor. 7th Annual Environmental Research Conference, Victoria (Australia), Environ. Eng. Res. Event (2003) )143–152. [22] N. Meunier, P. Drogui, C. Montané, R. Hausler, G. Mercier, J.-F. Blais, Comparison between electrocoagulation and chemical precipitation for metals removal from acidic soil leachate, J. Hazard. Mater. 137 (2006) 581–590. [23] P.T. Srinivasan, T. Viraraghavan, B. Kardash, J. Bergmann, Aluminum speciation during drinking water treatment, Water Qual. Res. J. Can. 33 (1998) 377–388. [24] R.W. Smith, Kinetic aspects of aqueous aluminum chemistry: environmental implications, Coordin. Chem. Rev. 149 (1996) 81–93. [25] Y. Avsar, U. Kurt, T. Gonullu, Comparison of classical chemical and electrochemical processes for treating rose processing wastewater, J. Hazard. Mater. 148 (2007) 340. [26] K. Scott, Electrochemical processes for clean technology, R. Soc. Chem. (2004) 85–126. [27] G. Sposito, The environmental chemistry of aluminum, Baca Raton, 2006, pp. 25–115. [28] Environmental Research Software, MINEQL+ computer program, The first choice in chemical equilibrium modeling, Environmental Research Software, Hallowell, USA, (2004). [29] P.T. Srinivasan, T. Viraraghavan, Characterisation and concentration profile of aluminum during drinking-water treatment, Water SA. 28 (2002) 99–106. [30] N. Radic, M. Bralic, Aluminum fluoride complexation and its ecological importance in the aquatic environment, Sci. Total Environ. 172 (1995) 237–243. [31] Z.S. Khachaturian, T.S. Radebaugh, Alzheimer's disease: cause(s), diagnosis, treatment, and care, CRC Press, Boca Raton, 1996. [32] C.Y. Hu, S.L. Lo, W.H. Kuan, Effects of co-existing anions on fluoride removal in electrocoagulation (EC) process using aluminum electrodes, Water Res. 37 (2003) 4513–4523. [33] N. Mameri, A.R. Yeddou, H. Lounici, Defluoridation of septentrional Sahara water of North Africa by electrocoagulation process using bipolar aluminium electrodes, Water Res. 32 (1998) 1604–1612. [34] M.Y.A. Mollah, R. Schennach, J.R. Parga, D.L. Cocke, Electrocoagulation (EC)— science and applications, J. Hazard. Mater. 84 (2001) 29–41. [35] AWWA, WEF, Standard Methods for the Examination of Water and Wastewater, American Water Works Association and Water Environment Federation, Washington, D.C., 2005