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Fluoride removal from water and wastewater with a bach cylindrical electrode using electrocoagulation
Chemical Engineering Journal 223 (2013) 110–115
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Fluoride removal from water and wastewater with a bach cylindrical electrode using electrocoagulation Umran Tezcan Un a,⇑, A. Savas Koparal b, Ulker Bakir Ogutveren b a b
Department of Environmental Engineering, Anadolu University, 26555 Eskisehir, Turkey Applied Research Centre for Environmental Problems, Anadolu University, 26555 Eskisehir, Turkey
h i g h l i g h t s
g r a p h i c a l a b s t r a c t
An electrochemical reactor in a
unique design was used. 97.6% Fluoride removal was obtained
using Al cylindrical anode and rotating impeller cathode. The recommended fluoride concentration of 1.2 mg/L by WHO was obtained within 5 min. The effects of anions, cations and all ions together were determined.
a r t i c l e
i n f o
Article history: Received 4 December 2012 Received in revised form 26 February 2013 Accepted 28 February 2013 Available online 14 March 2013 Keywords: Defluoridation Electrocoagulation Aluminium Iron Treatment
a b s t r a c t In this study, an electrochemical reactor with a unique design was used for defluoridation. A rotating impeller aluminium cathode and a cylindrical aluminium anode, which until now have not been employed for fluoride removal in the literature, were used, and various operating parameters, such as the electrode material (aluminium and iron), the current density (in the range of 0.5–2 mA/cm2), the duration of electrolysis, the supporting electrolyte dosage (in the range of 0.01–0.03 M Na2SO4), the ini2 tial pH (in the range of 4–8) and the presence of other ions(Ca2+, Mg2+, PO3 4 , SO4 ), were examined to achieve optimal performance of the process. The experimental results revealed that the fluoride removal could be enhanced at pH 6, higher current density and higher electrocoagulation time using aluminium 3 electrode. The presence of Ca2+ and Mg2+ ions also enhanced the removal efficiency while SO2 4 and PO4 ions effected adversely. The fluoride concentration was reduced from the initial value of 5.0–0.12 mg/L, with a removal efficiency of 97.6% after 30 min treatment at the current density of 2 mA/cm2, pHi of 6 and presence of 0.01 M Na2SO4. The required electrocoagulation time to reach the WHO-recommended fluoride limit of 1.2 mg/L at 0.5 mA/cm2 was 5 min, with an energy consumption of 0.47 kW h/m3. The obtained results show that this specially designed electrochemical reactor is an efficient alternative for the defluoridation of the water and the wastewater. Ó 2013 Elsevier B.V. All rights reserved.
1. Introduction Fluorine is a natural element which occurs in geochemical deposits, minerals, and natural water systems and gets involved in food chains with drinking water, plants and cereals. Although ⇑ Corresponding author. Tel.: +90 222 321 35 50x6418; fax: +90 222 323 95 01. E-mail address: [email protected] (U. Tezcan Un). 1385-8947/$ - see front matter Ó 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.cej.2013.02.126
the fluoride present in drinking water is essential for human health, an excessive intake of fluoride causes severe dental or skeletal fluorosis [1]. Therefore, the 1984 World Health Organization (WHO) guidelines suggested an optimal fluoride concentration level in the range of 1–1.2 mg/L [2]. The literature reports that many countries have regions where the water contains more than 1.5 mg/L of fluoride. A study conducted by UNICEF showed that fluorosis was endemic in at least
U. Tezcan Un et al. / Chemical Engineering Journal 223 (2013) 110–115
27 countries across the globe [3]. In the several regions of the world especially in Africa, Asia and Turkey, groundwater contains high fluoride levels [2]. In some parts of Turkey surface and ground water contains high fluoride concentration in the range of 1.5 and 13.70 mg/L. For example, in the Isparta Province situated southwest of Turkey, the fluoride content is between 1.5 and 6.0 mg/L. In the Dogubeyazit area, the fluoride concentration is between 6.5 and 12.5 mg/L, and in the areas south and north of the Tendurek Volcano, the fluoride content ranges from 5.7 to 15.2 mg/L and from 10.3 to 12.5 mg/L, respectively [4]. For these reasons, the removal of excess fluoride from water and wastewater is important for protecting public health and the environment. Several methods have been attempted to remove fluorides from water, namely, adsorption [5], precipitation [6], and ion exchange [7]. Recent investigations have revealed that electrocoagulation (EC) is an effective alternative for fluoride removal, both in drinking water [3] and in industrial wastewater [8]. On the other hand, the deficiency of electrocoagulation studies is the lack of the variety in reactor design. A survey of the literature showed that most EC studies have been carried out with parallel plate monopolar or bipolar electrode configuration systems. For example, fluoride removal has been performed using bipolar connections with two aluminium electrodes [9,10], three aluminium electrodes [11], four aluminium electrodes [12], seven aluminium electrodes [13,14], and nine aluminium electrodes [15] and using two monopolar aluminium electrodes [16]. An external-loop airlift reactor was also used for fluoride removal [17,18]. In summary, all the works mentioned focused mainly on the design of the electrodes. In this study, a cylindrical aluminium reactor with a rotating impeller aluminium cathode designed in a unique manner from those reported in the literature was used. In our previous studies [3], design performance was evaluated using iron as the electrode material for fluoride removal. However, further investigations for fluoride removal were considered using aluminium electrodes because of their superior performance in a similarly designed reactor. Therefore, improving the performance of aluminium electrodes was the purpose of this study, although iron electrodes were used for comparison. The effects of the electrode material, the current density (i; mA/cm2), the duration of electrolysis, the supporting electrolyte dosage (CNa2 SO4 ), the initial pH (pHi), and the presence of other ions on the performance of the reactor for fluoride removal from synthetic solutions were investigated, and the electrical energy consumptions were determined. 2. Experimental 2.1. Electrochemical reactor In this study, a cylindrical aluminium reactor with a rotating impeller cathode was used for the defluoridation of drinking water. The anode was a cylindrical aluminium reactor with a diameter of 10 cm, and the effective wet area of the electrode was 238 cm2. The rotating impeller cathode had two aluminium blades with the width of 2 cm and the length of 6 cm, and it was located in the center of the reactor. It mechanically stirred the solution at 100 rpm to maintain homogeneity of the solution and prevent the particles in the reactor from settling during the electrocoagulation. The experimental system used in this study is shown in Fig. 1. The experiments were carried out batch-wise.
111
Fig. 1. Experimental set-up.
conductivity of the solutions was adjusted by adding Na2SO4 salt as an electrolyte. The conductivity measurements were carried out using an Inolab conductivity meter (level 1). The solution pH was measured by a pH meter (Orion 420 A).The initial pH values of the solutions were adjusted with diluted H2SO4 or NaOH solutions. The chemicals used in the experiments were of analytical degree. 2.3. Experimental protocol A sample solution of 400 mL was placed into the reactor for each test run. The rotating aluminium impellers (Heidolph RZR 2102) were placed in the reactor and stirred the mixture at 100 rpm while simultaneously operating as the cathode. The electrodes were connected to the power supply (Statron T-25), and a constant current was applied for 30 min for each run. The temperature and pH of the solution were not controlled but were monitored during the experiments. To follow the performance of the electrocoagulation process, samples were taken from the reactor at several time intervals during the course of electrocoagulation and were centrifuged. The supernatant liquid was analysed for fluoride content. The fluoride concentrations in the sample were determined by the SPANDS method from Standards Methods. All analyses were performed twice, and an further measurements were conducted when necessary. The removal efficiency (RE%) was calculated using the following equation:
RE% ¼
C0 C 100 C0
ð1Þ
where C0 and C are the concentrations of fluoride before and after the treatment, respectively, in mg/L. 3. Results and discussion
2.2. Materials and methods 3.1. Effect of the electrode material The synthetic solutions were prepared by mixing stoichiometric amounts of sodium fluoride with deionised water. The initial fluoride concentration in the synthetic solution was 5 mg/L. The
Aluminium and iron are the most widely used materials as sacrificial anodes in electrocoagulation studies. For comparative
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purposes, electrocoagulation was carried out with both materials under identical conditions. Both reactors had the same anode and cathode dimensions. For both electrode materials at the same current density, the efficiency was found to be higher for the aluminium electrodes, as seen in Fig. 2. Conversely, the removal was poorer when using iron electrodes. The effluent fluoride concentration with the aluminium electrodes was found to be lower than that with the iron electrodes. A removal efficiency of 94.2%, corresponding to a final fluoride concentration of 0.29 mg/L, was achieved with the Al electrodes, whereas a removal efficiency of 83.6%, corresponding to 0.82 mg/L for the final fluoride concentration, was obtained using the Fe electrodes at 1 mA/cm2, pHi 6 with 0.01 M Na2SO4. In the case of the iron electrodes, a satisfactory final fluoride concentration could be attained thus the treated water could be used for drinking purposes. In our previous study [3], in which iron reactor was used for fluoride removal, the maximum removal efficiency was 85.9% at 3 mA/cm2. An improved efficiency using Al electrodes was most likely achieved because of the reaction between aluminium hydroxide and fluoride to form aluminium fluoride hydroxide complexes [AlnFm(OH)3nm] [10]. It was proposed that fluoride removal is progressed by chemical adsorption process with F replacing the OH group from the Aln(OH)3n flocs [13,16,18–20], as shown below: Coprecipitation: 3þ
nAlðaqÞ þ 3n mOHðaqÞ þ mFðaqÞ ! Aln Fm ðOHÞ3nmðsÞ
ð2Þ
Adsorption on the Al(OH)3 particles:
Aln ðOHÞ3nðsÞ þ
mFðaqÞ
! Aln Fm ðOHÞð3nmÞðsÞ þ
m¼
itM nF
ð3Þ
3.2. Effect of the current density The current density on the fluoride removal efficiency was investigated using various current densities with an aluminium anode at an initial pH of 6 and 0.01 M Na2SO4. The effect of the current density on fluoride removal was studied at 0.5, 1, and 2 mA/ cm2. The variation of the fluoride removal efficiency and the energy consumption versus time at various current densities are shown in Fig. 3. The removal efficiency was increased as the current density increased, as expected. The removal efficiency depended on the quantity of aluminium generated. The amount of aluminium
95 90 85 80
Fe
75
Al
VIt
65 0
5
10
15 20 Time, min
25
30
35
Fig. 2. The effect of the electrode material on the effluent fluoride concentration. i:1 mA/cm2, CNa2 SO4 :0.01 M, pHi:6.
ð5Þ
m
where EEC is the electrical energy consumption (kW h/m3), V is the potential (V), I is the current (A), t is the time (h), and m is the volume of the solution treated (m3). In these experiments, the energy consumption increased from 2.28 kW h/m3 to 23.3 kW h/m3 with the increase of current density from 0.5 to 2 mA/cm2 after 30 min of electrocoagulation, as seen from Fig. 3b. The system energy consumption decreased as the current density decreased, and the removal efficiency also decreased. 3.3. Effect of the duration of electrocoagulation on removal efficiency, pH and energy consumption An increase in the removal efficiency was obtained by increasing the duration of electrocoagulation. As time progressed, the dissolved coagulants from the aluminium electrode increased according to Faraday’s Law (Eq. (4)). A sufficient amount of coagulant dissolved from the aluminium electrode trapped the fluoride ions, and higher removal efficiency at a longer duration was observed, as seen in Fig. 3a. The duration of electrocoagulation also affected the pH of the solution. It should be noted that the pH increased continuously during the electrocoagulation process, as seen in Fig. 4. Within the first 5 min, the pH of the solution increased up to pH 10 because of the hydrogen evolution at the cathode (Eq. (6)) [21], and after 5 min, a slight increase in the pH was observed. Because of the buffer capacity of aluminium hydroxide, the final pH and residual fluoride concentration did not change very much with time.
3H2 O þ 3e !
70
ð4Þ
where m is the mass of the aluminium dissolved (g Al/cm2), i is the current density (A/cm2), t is the time (s), M is the molecular weight of Al (M = 27), n is the number of electrons involved in the oxidation reaction (n = 3), and F is Faraday’s constant, 96,500 C/mol. The dissolved ions from the sacrificial anode increases with the increase in current density according to the Faraday’s Law. Because sufficient current was passed through the solution, the dissolved metal ions were hydrolysed and metallic hydroxide species were formed. Thus, the production of floc increased by resulting in an increase in the removal efficiency. As seen from Fig. 3a the initial fluoride ion concentration of 5 mg/L was reduced to 0.56, 0.29 and 0.12 mg/L, corresponding to removal efficiencies of 88.8%, 94.2% and 97.6% after 30 min of electrocoagulation at 0.5, 1 and 2 mA/cm2, respectively. The electrical energy consumption of electrocoagulation is a major operating cost and depends on the operation time and the applied voltage and current, as given in Eq. (5). The electrical energy consumption was determined as kW h per m3 of effluent treated using the following equation:
EEC ¼ mðOHÞðaqÞ
Defluoridation is achieved by forming AlnFm(OH)3nm, which can be separated effectively from water. The results of this experiment led to further experiments that were performed with the Al electrodes.
Removal Efficiency, %
dissolved in the electrocoagulation process was theoretically calculated according to Faraday’s Law (Eq. (4)), which can be written simply as the relation between the current density and the amount of substance dissolved.
3 H2ðgÞ þ 3OH 2
ð6Þ
The energy consumption depends on time as well as the current and potential that are applied (Eq. (5)). As the reaction time increased, the system energy consumption also increased. As seen in Fig. 3a, the WHO-recommended fluoride concentration level of 1.2 mg/L was reached within 5 min for all current densities. The required electrocoagulation time to reach this fluoride level was 5 min at 0.5 mA/cm2, and the energy consumption was
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113
Fig. 3. The effect of the current densities (a) on the effluent fluoride concentration obtained from the experiments; (b) on the electrical energy consumptions computed from Eq. (5). pHi:6 CNa2 SO4 :0.01 M.
11
pH
10.5 10
pH4
9.5
pH5
9
pH6 pH7
8.5
pH8
8 7.5 7 0
10
20 Time. min
30
40
Fig. 4. Variation of the pH with time for different initial pH values. i:1 mA/cm2, CNa2 SO4 :0.01 M.
0.47 kW h/m3. The energy consumptions at 5 min for 1 and 2 mA/ cm2 were 1.35 and 4.58 kW h/m3, respectively. According to the obtained results, the energy consumption at 5 min of electrocoagulation decreases the fluoride concentration to the value recommended by the WHO. Furthermore, the long reaction time was not preferred from an economical viewpoint, because as the time proceeds the final fluoride concentration did not change significantly beyond 5 min since the reaction was first order as seen from Fig. 3a. Furthermore the fluoride concentration in the potable water below 1.2 mg/L was not recommended because of the adverse effect on the teeth. Thus the entire electrocoagulation process should be optimised to obtain the desired removal efficiency. As compared to the parallel plate electrodes in the literature the reactor mentioned in this study was found to be more effective in the manner of removal efficiency and time. For instance in the one of these studies [12] the initial fluoride concentration of 4 mg/L was reduced to 1 mg/L after 35 min electrocoagulation. In the other study [16] the initial fluoride concentration of 5 mg/L was treated with the removal efficiency around 85% after 10 min electrocoagulation at pH 6. 3.4. Effect of the supporting electrolyte The conductivity of the electrolyte solution is a key factor in an electrochemical process. The conductivity determines the cell
resistance, whereas the properties of the solvent and the electrolyte determine their interaction with the electroactive species and thereby influence the electrode reactions [22]. The effect of the supporting electrolyte concentration on fluoride removal was studied at various Na2SO4 concentrations at 1 mA/cm2 and pHi:6. The results presented in Fig. 5a indicate that the efficiency of the electrocoagulation process was influenced by the salt concentration. A higher concentration reduced the performance of the process in terms of the fluoride removal efficiency. The highest removal efficiency was obtained at an electrolyte concentration of 0.01 M. When the electrolyte concentration was higher than 0.01 M, the removal efficiency of fluoride was reduced as the electrolyte concentration increased. Similar results were reported in previous research [23], in which the removal efficiency decreased as a result of the interaction of excess SO2 4 ions with hydroxyl ions at high concentrations of salt and of the inhibition of the localised corrosion of aluminium electrodes by excess SO2 4 ions, which decreased the removal efficiency. The most important factor in any electrochemical method is the energy consumption. The ohmic potential drop in the solution and the anode and cathode overpotentials cause higher electrical energy consumptions in electrochemical systems. The conductivity of solution is increased by addition of a supporting electrolyte and reduces the ohmic resistance between the anode and the cathode to minimal levels. Therefore, the energy consumption decreases because of the reduction of the applied potential. In this study, the energy consumptions were reduced as the Na2SO4 concentration increased, as seen in Fig. 5b. Energy densities of 5.88, 3.82 and 2.77 kW h/m3 were consumed for Na2SO4 concentrations of 0.01, 0.02 and 0.03 M, respectively. As seen in Fig. 5b, more electricity was consumed when the electrical conductivity of the solution was low. Although adding salt to wastewater can reduce electricity consumption significantly, such an addition does not help increase the pollutant removal efficiency, and it can be inconvenient due to environmental considerations in the treatment of drinking water. 3.5. Effect of the initial pH The pH of the solution and the amount of the aluminium dissolved effect the electrocoagulation process. Because of the amphoteric characteristics of aluminium hydroxide, the pH affects the formation of Al(OH)3 flocs. In the pH range of 4–9, the hydroxide species having positive charge such as Al(OH)2+, AlðOHÞþ 2,
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7
0.01M 0.02M
4
0.03M
3 2 1 0
Electrical Energy Cons., kWh/m3
Fluoride Conc., mg/L
5
6 5 4 3 2 1 0
0
10
20 Time, min
30
40
(a)
0
10
20 Time, min
30
(b)
Fig. 5. The effect of the Na2SO4 concentration (a) on the effluent fluoride concentration; (b) on the electrical energy consumptions. i:1 mA/cm2, pHi:6.
Fluoride Removal Efficiency, %
7þ Al2 ðOHÞ4þ 2 , Al(OH)3, and Al13 ðOHÞ32 have formed and they have big capacity for adsorption and net catching reaction. At pH > 10, AlðOHÞ 4 which has little adsorption capacity is dominant. The main species is Al3+ at low pH which has not coagulation effect [24]. Solid Al(OH)3 is the most prevalent between a pH of 6 and 7, and at higher or lower pH, the solubility of the hydro-fluoro-aluminium precipitate, which is produced by Al(OH)3 and fluoride, increases [21]. Therefore, to determine the effect of the pH of the electrolyte on the removal efficiency, experiments were conducted to determine the most favourable initial pH value for the removal of fluoride at 1 mA/cm2 and 0.01 M Na2SO4. The effect of the initial pH on the removal efficiency is shown in Fig. 6. When the initial pH increased from 4 to 6, the removal efficiency of fluoride increased. When the initial pH increased from 6 to 8, the removal efficiency of fluoride decreased. The highest removal rates were achieved when the initial pH was 6. As seen in Fig. 6, when the pH was 6, the removal efficiency was approximately 94.2% after a treatment period of 30 min. For the same time period, removal efficiencies of 92.6%, 91.6%, 90.8% and 87% were obtained at pH values of 5, 4, 7 and 8, respectively. It can be concluded that the aluminium can form different species depending on the pH of the solution. In the studies of Zhao et al. [25] the formation and decomposition of polymeric aluminium species at different pH were investigated by using ESI mass spectrometry. They found that the amorphous flocs of Al(OH)3 was the final product of the polymerization and decomposition at the pH of 6.4. Gong et al. [26] investigated the distribution of the fluoride species and fluoride removal in the pH range of 4–9.
They obtained best fluoride removal at pH 7. Similarly the optimal pH range for fluoride removal was determined as 6–7 by Drouiche et al. [27], as 5.5–6.5 by Zhu et al. [16] and as 7 by Vasudevan et.al. [28]. 3.6. Effect of co-existing ions Wastewater containing fluoride may also include some coexisting ions that can affect fluoride removal by the electrocoagulation process. In this study, the effects of anions accompanied by cations such as Ca2+ and Mg2+ were investigated differently from the reports in the literature [13] by considering national drinking water limits and the presence in the water in Turkey. The effects of the co-existing ions on defluoridation were quantified using 3 2+ 2 Ca2+, SO2 4 , Mg , and PO4 ions at 1 mA/cm and a pHi of 6. A number of experiments were performed with solutions containing each ion individually and all ions together. Fig. 7 shows the effects of these ions on the fluoride removal efficiency. The effect of the SO2 ion (added as Na2SO4) concentrations on the removal effi4 ciency is presented in Fig. 5, which shows that the removal efficiencies decreased as the concentration of SO2 ions increased. 4 Removal efficiencies of 94.2%, 92.6% and 91.4% were obtained in 2 the presence of 0.96 g SO2 4 =L (0.01 M Na2SO4), 1.92 g SO4 =L (0.02 M Na2SO4) and 2.98 g SO2 =L (0.03 M Na SO ) ions, respec2 4 4 tively. A similar adverse effect was observed with the addition of PO3 (added as Na3PO4). A fluoride removal efficiency of 71.8%, 4 which was the lowest value obtained in this study, was obtained in the presence of 5 mg of PO3 4 =L. Similar results for these anions were observed by Hu et al. [13] and Vasudevan et al. [29] for deflu-
95 90 85 pH 4 80
pH 5
75
pH 6
70
pH 7 pH 8
65 0
10
20
30
40
Time, min Fig. 6. The effect of the pH on the fluoride removal efficiency as a function of time. i:1 mA/cm2, CNa2 SO4 :0.01 M.
Fig. 7. The effects of the co-existing ions on the fluoride removal efficiency as a function of time. i:1 mA/cm2, pHi:6.
U. Tezcan Un et al. / Chemical Engineering Journal 223 (2013) 110–115
oridation. The effect of the cations on the defluoridation efficiency was also investigated by using Ca2+ (as CaCl22H2O) and Mg2+ (as MgCl26H2O) ions. As seen from Fig. 7, the presence of Ca2+ and Mg2+ ions enhanced the removal efficiency. The initial F concentration of 5 mg/L was reduced to 0.12 mg/L (97.6%) in the presence of 200 mg/L Ca2+ ions and to 0.21 mg/L (95.8%) in the presence of 50 mg/L Mg2+ ions after 30 min of electrocoagulation. These curative effects are the results of the reactions between F–Ca and F– Mg as shown below: Mg2þ ðaqÞ þ 2FðaqÞ ! MgF2ðsÞ
ð7Þ
Ca2þ ðaqÞ þ 2FðaqÞ ! CaF2ðsÞ
ð8Þ
In the experiment carried out with a solution including all ions 3 2+ (Ca2+, SO2 4 , Mg , PO4 ), the removal efficiency values were observed to be the lowest among all experiments (Fig. 7).
4. Conclusion The performance of a specially designed batch electrocoagulation reactor for the treatment of fluoride from water was investigated. The effects of different parameters, including the electrode material, the current density, the electrolysis time, the supporting electrolyte dosage, the initial pH and the presence of other ions, were evaluated. The experimental results revealed that the fluoride removal efficiency in the aluminium electrode was higher than that observed in iron electrodes because of the reaction between the aluminium hydroxide and fluoride to form aluminium fluoride hydroxide complexes. Fluoride removal could be enhanced by increasing either the current density or the electrocoagulation time. The fluoride removal efficiency was increased to 97.6% at 2 mA/cm2 after 30 min of electrocoagulation. To avoid excessive energy consumption, Na2SO4 was used as the supporting electrolyte in the experiments. However, a higher salt concentration negatively affected the performance of the process in terms of the fluoride removal efficiency, although the energy consumption was reduced. The pH of the solution effects the EC because the aluminium could form different species depending on the pH. The highest removal efficiencies were achievable at an initial pH of 6. Wastewater containing fluoride may also contain some co-existing ions, and the effects of the co-existing ions on defluoridation were 3 2+ investigated using Ca2+, SO2 ions. The removal 4 , Mg , and PO4 3 efficiencies decreased as the concentration of SO2 4 and PO4 ions 2+ 2+ increased, but the presence of Ca and Mg ions enhanced the removal efficiency. In conclusion, the removal of fluoride to achieve the WHO-recommended fluoride concentration level of 1.2 mg/L was reached within 5 min with relatively low energy consumption. The results showed that the electrochemical reactor designed for this purpose can effectively used for the defluoridation of potable waters and wastewaters.
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Mohanraj, G. Sozhan, Effects of alternating and direct current in electrocoagulation process on the removal of fluoride from water, J. Chem. Technol. Biotechnol. 86 (2011) 428–436. [29] S. Vasudevan, J. Lakshmi, G. Sozhan, Studies on a Mg–Al–Zn alloy as an anode for the removal of fluoride from drinking water in an electrocoagulation process, Clean 37 (2009) 372–378.
Contents lists available at SciVerse ScienceDirect
Chemical Engineering Journal journal homepage: www.elsevier.com/locate/cej
Fluoride removal from water and wastewater with a bach cylindrical electrode using electrocoagulation Umran Tezcan Un a,⇑, A. Savas Koparal b, Ulker Bakir Ogutveren b a b
Department of Environmental Engineering, Anadolu University, 26555 Eskisehir, Turkey Applied Research Centre for Environmental Problems, Anadolu University, 26555 Eskisehir, Turkey
h i g h l i g h t s
g r a p h i c a l a b s t r a c t
An electrochemical reactor in a
unique design was used. 97.6% Fluoride removal was obtained
using Al cylindrical anode and rotating impeller cathode. The recommended fluoride concentration of 1.2 mg/L by WHO was obtained within 5 min. The effects of anions, cations and all ions together were determined.
a r t i c l e
i n f o
Article history: Received 4 December 2012 Received in revised form 26 February 2013 Accepted 28 February 2013 Available online 14 March 2013 Keywords: Defluoridation Electrocoagulation Aluminium Iron Treatment
a b s t r a c t In this study, an electrochemical reactor with a unique design was used for defluoridation. A rotating impeller aluminium cathode and a cylindrical aluminium anode, which until now have not been employed for fluoride removal in the literature, were used, and various operating parameters, such as the electrode material (aluminium and iron), the current density (in the range of 0.5–2 mA/cm2), the duration of electrolysis, the supporting electrolyte dosage (in the range of 0.01–0.03 M Na2SO4), the ini2 tial pH (in the range of 4–8) and the presence of other ions(Ca2+, Mg2+, PO3 4 , SO4 ), were examined to achieve optimal performance of the process. The experimental results revealed that the fluoride removal could be enhanced at pH 6, higher current density and higher electrocoagulation time using aluminium 3 electrode. The presence of Ca2+ and Mg2+ ions also enhanced the removal efficiency while SO2 4 and PO4 ions effected adversely. The fluoride concentration was reduced from the initial value of 5.0–0.12 mg/L, with a removal efficiency of 97.6% after 30 min treatment at the current density of 2 mA/cm2, pHi of 6 and presence of 0.01 M Na2SO4. The required electrocoagulation time to reach the WHO-recommended fluoride limit of 1.2 mg/L at 0.5 mA/cm2 was 5 min, with an energy consumption of 0.47 kW h/m3. The obtained results show that this specially designed electrochemical reactor is an efficient alternative for the defluoridation of the water and the wastewater. Ó 2013 Elsevier B.V. All rights reserved.
1. Introduction Fluorine is a natural element which occurs in geochemical deposits, minerals, and natural water systems and gets involved in food chains with drinking water, plants and cereals. Although ⇑ Corresponding author. Tel.: +90 222 321 35 50x6418; fax: +90 222 323 95 01. E-mail address: [email protected] (U. Tezcan Un). 1385-8947/$ - see front matter Ó 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.cej.2013.02.126
the fluoride present in drinking water is essential for human health, an excessive intake of fluoride causes severe dental or skeletal fluorosis [1]. Therefore, the 1984 World Health Organization (WHO) guidelines suggested an optimal fluoride concentration level in the range of 1–1.2 mg/L [2]. The literature reports that many countries have regions where the water contains more than 1.5 mg/L of fluoride. A study conducted by UNICEF showed that fluorosis was endemic in at least
U. Tezcan Un et al. / Chemical Engineering Journal 223 (2013) 110–115
27 countries across the globe [3]. In the several regions of the world especially in Africa, Asia and Turkey, groundwater contains high fluoride levels [2]. In some parts of Turkey surface and ground water contains high fluoride concentration in the range of 1.5 and 13.70 mg/L. For example, in the Isparta Province situated southwest of Turkey, the fluoride content is between 1.5 and 6.0 mg/L. In the Dogubeyazit area, the fluoride concentration is between 6.5 and 12.5 mg/L, and in the areas south and north of the Tendurek Volcano, the fluoride content ranges from 5.7 to 15.2 mg/L and from 10.3 to 12.5 mg/L, respectively [4]. For these reasons, the removal of excess fluoride from water and wastewater is important for protecting public health and the environment. Several methods have been attempted to remove fluorides from water, namely, adsorption [5], precipitation [6], and ion exchange [7]. Recent investigations have revealed that electrocoagulation (EC) is an effective alternative for fluoride removal, both in drinking water [3] and in industrial wastewater [8]. On the other hand, the deficiency of electrocoagulation studies is the lack of the variety in reactor design. A survey of the literature showed that most EC studies have been carried out with parallel plate monopolar or bipolar electrode configuration systems. For example, fluoride removal has been performed using bipolar connections with two aluminium electrodes [9,10], three aluminium electrodes [11], four aluminium electrodes [12], seven aluminium electrodes [13,14], and nine aluminium electrodes [15] and using two monopolar aluminium electrodes [16]. An external-loop airlift reactor was also used for fluoride removal [17,18]. In summary, all the works mentioned focused mainly on the design of the electrodes. In this study, a cylindrical aluminium reactor with a rotating impeller aluminium cathode designed in a unique manner from those reported in the literature was used. In our previous studies [3], design performance was evaluated using iron as the electrode material for fluoride removal. However, further investigations for fluoride removal were considered using aluminium electrodes because of their superior performance in a similarly designed reactor. Therefore, improving the performance of aluminium electrodes was the purpose of this study, although iron electrodes were used for comparison. The effects of the electrode material, the current density (i; mA/cm2), the duration of electrolysis, the supporting electrolyte dosage (CNa2 SO4 ), the initial pH (pHi), and the presence of other ions on the performance of the reactor for fluoride removal from synthetic solutions were investigated, and the electrical energy consumptions were determined. 2. Experimental 2.1. Electrochemical reactor In this study, a cylindrical aluminium reactor with a rotating impeller cathode was used for the defluoridation of drinking water. The anode was a cylindrical aluminium reactor with a diameter of 10 cm, and the effective wet area of the electrode was 238 cm2. The rotating impeller cathode had two aluminium blades with the width of 2 cm and the length of 6 cm, and it was located in the center of the reactor. It mechanically stirred the solution at 100 rpm to maintain homogeneity of the solution and prevent the particles in the reactor from settling during the electrocoagulation. The experimental system used in this study is shown in Fig. 1. The experiments were carried out batch-wise.
111
Fig. 1. Experimental set-up.
conductivity of the solutions was adjusted by adding Na2SO4 salt as an electrolyte. The conductivity measurements were carried out using an Inolab conductivity meter (level 1). The solution pH was measured by a pH meter (Orion 420 A).The initial pH values of the solutions were adjusted with diluted H2SO4 or NaOH solutions. The chemicals used in the experiments were of analytical degree. 2.3. Experimental protocol A sample solution of 400 mL was placed into the reactor for each test run. The rotating aluminium impellers (Heidolph RZR 2102) were placed in the reactor and stirred the mixture at 100 rpm while simultaneously operating as the cathode. The electrodes were connected to the power supply (Statron T-25), and a constant current was applied for 30 min for each run. The temperature and pH of the solution were not controlled but were monitored during the experiments. To follow the performance of the electrocoagulation process, samples were taken from the reactor at several time intervals during the course of electrocoagulation and were centrifuged. The supernatant liquid was analysed for fluoride content. The fluoride concentrations in the sample were determined by the SPANDS method from Standards Methods. All analyses were performed twice, and an further measurements were conducted when necessary. The removal efficiency (RE%) was calculated using the following equation:
RE% ¼
C0 C 100 C0
ð1Þ
where C0 and C are the concentrations of fluoride before and after the treatment, respectively, in mg/L. 3. Results and discussion
2.2. Materials and methods 3.1. Effect of the electrode material The synthetic solutions were prepared by mixing stoichiometric amounts of sodium fluoride with deionised water. The initial fluoride concentration in the synthetic solution was 5 mg/L. The
Aluminium and iron are the most widely used materials as sacrificial anodes in electrocoagulation studies. For comparative
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purposes, electrocoagulation was carried out with both materials under identical conditions. Both reactors had the same anode and cathode dimensions. For both electrode materials at the same current density, the efficiency was found to be higher for the aluminium electrodes, as seen in Fig. 2. Conversely, the removal was poorer when using iron electrodes. The effluent fluoride concentration with the aluminium electrodes was found to be lower than that with the iron electrodes. A removal efficiency of 94.2%, corresponding to a final fluoride concentration of 0.29 mg/L, was achieved with the Al electrodes, whereas a removal efficiency of 83.6%, corresponding to 0.82 mg/L for the final fluoride concentration, was obtained using the Fe electrodes at 1 mA/cm2, pHi 6 with 0.01 M Na2SO4. In the case of the iron electrodes, a satisfactory final fluoride concentration could be attained thus the treated water could be used for drinking purposes. In our previous study [3], in which iron reactor was used for fluoride removal, the maximum removal efficiency was 85.9% at 3 mA/cm2. An improved efficiency using Al electrodes was most likely achieved because of the reaction between aluminium hydroxide and fluoride to form aluminium fluoride hydroxide complexes [AlnFm(OH)3nm] [10]. It was proposed that fluoride removal is progressed by chemical adsorption process with F replacing the OH group from the Aln(OH)3n flocs [13,16,18–20], as shown below: Coprecipitation: 3þ
nAlðaqÞ þ 3n mOHðaqÞ þ mFðaqÞ ! Aln Fm ðOHÞ3nmðsÞ
ð2Þ
Adsorption on the Al(OH)3 particles:
Aln ðOHÞ3nðsÞ þ
mFðaqÞ
! Aln Fm ðOHÞð3nmÞðsÞ þ
m¼
itM nF
ð3Þ
3.2. Effect of the current density The current density on the fluoride removal efficiency was investigated using various current densities with an aluminium anode at an initial pH of 6 and 0.01 M Na2SO4. The effect of the current density on fluoride removal was studied at 0.5, 1, and 2 mA/ cm2. The variation of the fluoride removal efficiency and the energy consumption versus time at various current densities are shown in Fig. 3. The removal efficiency was increased as the current density increased, as expected. The removal efficiency depended on the quantity of aluminium generated. The amount of aluminium
95 90 85 80
Fe
75
Al
VIt
65 0
5
10
15 20 Time, min
25
30
35
Fig. 2. The effect of the electrode material on the effluent fluoride concentration. i:1 mA/cm2, CNa2 SO4 :0.01 M, pHi:6.
ð5Þ
m
where EEC is the electrical energy consumption (kW h/m3), V is the potential (V), I is the current (A), t is the time (h), and m is the volume of the solution treated (m3). In these experiments, the energy consumption increased from 2.28 kW h/m3 to 23.3 kW h/m3 with the increase of current density from 0.5 to 2 mA/cm2 after 30 min of electrocoagulation, as seen from Fig. 3b. The system energy consumption decreased as the current density decreased, and the removal efficiency also decreased. 3.3. Effect of the duration of electrocoagulation on removal efficiency, pH and energy consumption An increase in the removal efficiency was obtained by increasing the duration of electrocoagulation. As time progressed, the dissolved coagulants from the aluminium electrode increased according to Faraday’s Law (Eq. (4)). A sufficient amount of coagulant dissolved from the aluminium electrode trapped the fluoride ions, and higher removal efficiency at a longer duration was observed, as seen in Fig. 3a. The duration of electrocoagulation also affected the pH of the solution. It should be noted that the pH increased continuously during the electrocoagulation process, as seen in Fig. 4. Within the first 5 min, the pH of the solution increased up to pH 10 because of the hydrogen evolution at the cathode (Eq. (6)) [21], and after 5 min, a slight increase in the pH was observed. Because of the buffer capacity of aluminium hydroxide, the final pH and residual fluoride concentration did not change very much with time.
3H2 O þ 3e !
70
ð4Þ
where m is the mass of the aluminium dissolved (g Al/cm2), i is the current density (A/cm2), t is the time (s), M is the molecular weight of Al (M = 27), n is the number of electrons involved in the oxidation reaction (n = 3), and F is Faraday’s constant, 96,500 C/mol. The dissolved ions from the sacrificial anode increases with the increase in current density according to the Faraday’s Law. Because sufficient current was passed through the solution, the dissolved metal ions were hydrolysed and metallic hydroxide species were formed. Thus, the production of floc increased by resulting in an increase in the removal efficiency. As seen from Fig. 3a the initial fluoride ion concentration of 5 mg/L was reduced to 0.56, 0.29 and 0.12 mg/L, corresponding to removal efficiencies of 88.8%, 94.2% and 97.6% after 30 min of electrocoagulation at 0.5, 1 and 2 mA/cm2, respectively. The electrical energy consumption of electrocoagulation is a major operating cost and depends on the operation time and the applied voltage and current, as given in Eq. (5). The electrical energy consumption was determined as kW h per m3 of effluent treated using the following equation:
EEC ¼ mðOHÞðaqÞ
Defluoridation is achieved by forming AlnFm(OH)3nm, which can be separated effectively from water. The results of this experiment led to further experiments that were performed with the Al electrodes.
Removal Efficiency, %
dissolved in the electrocoagulation process was theoretically calculated according to Faraday’s Law (Eq. (4)), which can be written simply as the relation between the current density and the amount of substance dissolved.
3 H2ðgÞ þ 3OH 2
ð6Þ
The energy consumption depends on time as well as the current and potential that are applied (Eq. (5)). As the reaction time increased, the system energy consumption also increased. As seen in Fig. 3a, the WHO-recommended fluoride concentration level of 1.2 mg/L was reached within 5 min for all current densities. The required electrocoagulation time to reach this fluoride level was 5 min at 0.5 mA/cm2, and the energy consumption was
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113
Fig. 3. The effect of the current densities (a) on the effluent fluoride concentration obtained from the experiments; (b) on the electrical energy consumptions computed from Eq. (5). pHi:6 CNa2 SO4 :0.01 M.
11
pH
10.5 10
pH4
9.5
pH5
9
pH6 pH7
8.5
pH8
8 7.5 7 0
10
20 Time. min
30
40
Fig. 4. Variation of the pH with time for different initial pH values. i:1 mA/cm2, CNa2 SO4 :0.01 M.
0.47 kW h/m3. The energy consumptions at 5 min for 1 and 2 mA/ cm2 were 1.35 and 4.58 kW h/m3, respectively. According to the obtained results, the energy consumption at 5 min of electrocoagulation decreases the fluoride concentration to the value recommended by the WHO. Furthermore, the long reaction time was not preferred from an economical viewpoint, because as the time proceeds the final fluoride concentration did not change significantly beyond 5 min since the reaction was first order as seen from Fig. 3a. Furthermore the fluoride concentration in the potable water below 1.2 mg/L was not recommended because of the adverse effect on the teeth. Thus the entire electrocoagulation process should be optimised to obtain the desired removal efficiency. As compared to the parallel plate electrodes in the literature the reactor mentioned in this study was found to be more effective in the manner of removal efficiency and time. For instance in the one of these studies [12] the initial fluoride concentration of 4 mg/L was reduced to 1 mg/L after 35 min electrocoagulation. In the other study [16] the initial fluoride concentration of 5 mg/L was treated with the removal efficiency around 85% after 10 min electrocoagulation at pH 6. 3.4. Effect of the supporting electrolyte The conductivity of the electrolyte solution is a key factor in an electrochemical process. The conductivity determines the cell
resistance, whereas the properties of the solvent and the electrolyte determine their interaction with the electroactive species and thereby influence the electrode reactions [22]. The effect of the supporting electrolyte concentration on fluoride removal was studied at various Na2SO4 concentrations at 1 mA/cm2 and pHi:6. The results presented in Fig. 5a indicate that the efficiency of the electrocoagulation process was influenced by the salt concentration. A higher concentration reduced the performance of the process in terms of the fluoride removal efficiency. The highest removal efficiency was obtained at an electrolyte concentration of 0.01 M. When the electrolyte concentration was higher than 0.01 M, the removal efficiency of fluoride was reduced as the electrolyte concentration increased. Similar results were reported in previous research [23], in which the removal efficiency decreased as a result of the interaction of excess SO2 4 ions with hydroxyl ions at high concentrations of salt and of the inhibition of the localised corrosion of aluminium electrodes by excess SO2 4 ions, which decreased the removal efficiency. The most important factor in any electrochemical method is the energy consumption. The ohmic potential drop in the solution and the anode and cathode overpotentials cause higher electrical energy consumptions in electrochemical systems. The conductivity of solution is increased by addition of a supporting electrolyte and reduces the ohmic resistance between the anode and the cathode to minimal levels. Therefore, the energy consumption decreases because of the reduction of the applied potential. In this study, the energy consumptions were reduced as the Na2SO4 concentration increased, as seen in Fig. 5b. Energy densities of 5.88, 3.82 and 2.77 kW h/m3 were consumed for Na2SO4 concentrations of 0.01, 0.02 and 0.03 M, respectively. As seen in Fig. 5b, more electricity was consumed when the electrical conductivity of the solution was low. Although adding salt to wastewater can reduce electricity consumption significantly, such an addition does not help increase the pollutant removal efficiency, and it can be inconvenient due to environmental considerations in the treatment of drinking water. 3.5. Effect of the initial pH The pH of the solution and the amount of the aluminium dissolved effect the electrocoagulation process. Because of the amphoteric characteristics of aluminium hydroxide, the pH affects the formation of Al(OH)3 flocs. In the pH range of 4–9, the hydroxide species having positive charge such as Al(OH)2+, AlðOHÞþ 2,
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7
0.01M 0.02M
4
0.03M
3 2 1 0
Electrical Energy Cons., kWh/m3
Fluoride Conc., mg/L
5
6 5 4 3 2 1 0
0
10
20 Time, min
30
40
(a)
0
10
20 Time, min
30
(b)
Fig. 5. The effect of the Na2SO4 concentration (a) on the effluent fluoride concentration; (b) on the electrical energy consumptions. i:1 mA/cm2, pHi:6.
Fluoride Removal Efficiency, %
7þ Al2 ðOHÞ4þ 2 , Al(OH)3, and Al13 ðOHÞ32 have formed and they have big capacity for adsorption and net catching reaction. At pH > 10, AlðOHÞ 4 which has little adsorption capacity is dominant. The main species is Al3+ at low pH which has not coagulation effect [24]. Solid Al(OH)3 is the most prevalent between a pH of 6 and 7, and at higher or lower pH, the solubility of the hydro-fluoro-aluminium precipitate, which is produced by Al(OH)3 and fluoride, increases [21]. Therefore, to determine the effect of the pH of the electrolyte on the removal efficiency, experiments were conducted to determine the most favourable initial pH value for the removal of fluoride at 1 mA/cm2 and 0.01 M Na2SO4. The effect of the initial pH on the removal efficiency is shown in Fig. 6. When the initial pH increased from 4 to 6, the removal efficiency of fluoride increased. When the initial pH increased from 6 to 8, the removal efficiency of fluoride decreased. The highest removal rates were achieved when the initial pH was 6. As seen in Fig. 6, when the pH was 6, the removal efficiency was approximately 94.2% after a treatment period of 30 min. For the same time period, removal efficiencies of 92.6%, 91.6%, 90.8% and 87% were obtained at pH values of 5, 4, 7 and 8, respectively. It can be concluded that the aluminium can form different species depending on the pH of the solution. In the studies of Zhao et al. [25] the formation and decomposition of polymeric aluminium species at different pH were investigated by using ESI mass spectrometry. They found that the amorphous flocs of Al(OH)3 was the final product of the polymerization and decomposition at the pH of 6.4. Gong et al. [26] investigated the distribution of the fluoride species and fluoride removal in the pH range of 4–9.
They obtained best fluoride removal at pH 7. Similarly the optimal pH range for fluoride removal was determined as 6–7 by Drouiche et al. [27], as 5.5–6.5 by Zhu et al. [16] and as 7 by Vasudevan et.al. [28]. 3.6. Effect of co-existing ions Wastewater containing fluoride may also include some coexisting ions that can affect fluoride removal by the electrocoagulation process. In this study, the effects of anions accompanied by cations such as Ca2+ and Mg2+ were investigated differently from the reports in the literature [13] by considering national drinking water limits and the presence in the water in Turkey. The effects of the co-existing ions on defluoridation were quantified using 3 2+ 2 Ca2+, SO2 4 , Mg , and PO4 ions at 1 mA/cm and a pHi of 6. A number of experiments were performed with solutions containing each ion individually and all ions together. Fig. 7 shows the effects of these ions on the fluoride removal efficiency. The effect of the SO2 ion (added as Na2SO4) concentrations on the removal effi4 ciency is presented in Fig. 5, which shows that the removal efficiencies decreased as the concentration of SO2 ions increased. 4 Removal efficiencies of 94.2%, 92.6% and 91.4% were obtained in 2 the presence of 0.96 g SO2 4 =L (0.01 M Na2SO4), 1.92 g SO4 =L (0.02 M Na2SO4) and 2.98 g SO2 =L (0.03 M Na SO ) ions, respec2 4 4 tively. A similar adverse effect was observed with the addition of PO3 (added as Na3PO4). A fluoride removal efficiency of 71.8%, 4 which was the lowest value obtained in this study, was obtained in the presence of 5 mg of PO3 4 =L. Similar results for these anions were observed by Hu et al. [13] and Vasudevan et al. [29] for deflu-
95 90 85 pH 4 80
pH 5
75
pH 6
70
pH 7 pH 8
65 0
10
20
30
40
Time, min Fig. 6. The effect of the pH on the fluoride removal efficiency as a function of time. i:1 mA/cm2, CNa2 SO4 :0.01 M.
Fig. 7. The effects of the co-existing ions on the fluoride removal efficiency as a function of time. i:1 mA/cm2, pHi:6.
U. Tezcan Un et al. / Chemical Engineering Journal 223 (2013) 110–115
oridation. The effect of the cations on the defluoridation efficiency was also investigated by using Ca2+ (as CaCl22H2O) and Mg2+ (as MgCl26H2O) ions. As seen from Fig. 7, the presence of Ca2+ and Mg2+ ions enhanced the removal efficiency. The initial F concentration of 5 mg/L was reduced to 0.12 mg/L (97.6%) in the presence of 200 mg/L Ca2+ ions and to 0.21 mg/L (95.8%) in the presence of 50 mg/L Mg2+ ions after 30 min of electrocoagulation. These curative effects are the results of the reactions between F–Ca and F– Mg as shown below: Mg2þ ðaqÞ þ 2FðaqÞ ! MgF2ðsÞ
ð7Þ
Ca2þ ðaqÞ þ 2FðaqÞ ! CaF2ðsÞ
ð8Þ
In the experiment carried out with a solution including all ions 3 2+ (Ca2+, SO2 4 , Mg , PO4 ), the removal efficiency values were observed to be the lowest among all experiments (Fig. 7).
4. Conclusion The performance of a specially designed batch electrocoagulation reactor for the treatment of fluoride from water was investigated. The effects of different parameters, including the electrode material, the current density, the electrolysis time, the supporting electrolyte dosage, the initial pH and the presence of other ions, were evaluated. The experimental results revealed that the fluoride removal efficiency in the aluminium electrode was higher than that observed in iron electrodes because of the reaction between the aluminium hydroxide and fluoride to form aluminium fluoride hydroxide complexes. Fluoride removal could be enhanced by increasing either the current density or the electrocoagulation time. The fluoride removal efficiency was increased to 97.6% at 2 mA/cm2 after 30 min of electrocoagulation. To avoid excessive energy consumption, Na2SO4 was used as the supporting electrolyte in the experiments. However, a higher salt concentration negatively affected the performance of the process in terms of the fluoride removal efficiency, although the energy consumption was reduced. The pH of the solution effects the EC because the aluminium could form different species depending on the pH. The highest removal efficiencies were achievable at an initial pH of 6. Wastewater containing fluoride may also contain some co-existing ions, and the effects of the co-existing ions on defluoridation were 3 2+ investigated using Ca2+, SO2 ions. The removal 4 , Mg , and PO4 3 efficiencies decreased as the concentration of SO2 4 and PO4 ions 2+ 2+ increased, but the presence of Ca and Mg ions enhanced the removal efficiency. In conclusion, the removal of fluoride to achieve the WHO-recommended fluoride concentration level of 1.2 mg/L was reached within 5 min with relatively low energy consumption. The results showed that the electrochemical reactor designed for this purpose can effectively used for the defluoridation of potable waters and wastewaters.
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