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Fluoride distribution in electrocoagulation defluoridation process
Separation and Purification Technology 56 (2007) 184–191
Fluoride distribution in electrocoagulation defluoridation process Jun Zhu, Huazhang Zhao, Jinren Ni ∗ Department of Environmental Engineering, Peking University, The Key Laboratory of Water and Sediment Sciences, Ministry of Education, Beijing 100871, PR China Received 20 September 2006; received in revised form 24 December 2006; accepted 31 January 2007
Abstract Electrocoagulation (EC) is an effective process to remove fluoride from water, but few of scientific literatures explore its inside mechanism. A new approach was used in this study to investigate fluoride distribution in the EC defluoridation process, which divided the fluoride into three parts: remained in water, removed by electrodes, and adsorbed on hydroxide aluminum flocs. The fluoride distribution was investigated in terms of several critical parameters such as pH, charge loading, current density and initial fluoride concentration. The experimental results showed that the removal by electrodes was primarily responsible for the high defluoridation efficiency, and the adsorption by hydroxide aluminum flocs gave a secondary effect. The parameters affected the efficiencies of defluoridation in a way of changing the fluoride distribution in the EC process. A chemical complex of Aln (OH)m Fk 3n−m−k was formulated to explain the mechanism inside the EC defluoridation process. The new approach provides a detailed insight of the electrocondensation effect, which helps to gain more scientific comprehension about the cooperation between electrochemical and chemical ways occurring inside the EC process. © 2007 Elsevier B.V. All rights reserved. Keywords: Fluoride; Electrocoagulation; Chemical coagulation; Electrocondensation
1. Introduction Fluoride is recognized as an essential constituent in the human diet. Low fluoride concentration (<1 mg/L) could prevent dental problem, but higher fluoride concentration (>1.5 mg/L) will cause dental and skeletal fluorosis [1]. Many countries, such as China, Egypt, India, Kenya, etc., have areas where fluorosis is endemic [1]. Fluoride pollution in environment occurs through two different channels: natural sources and anthropogenic sources. Fluoride is frequently encountered in minerals and in geochemical deposits. Because of the erosion and weathering of fluoride-bearing minerals, it becomes a surface species. The discharge of industrial wastewater, such as semiconductor industries, aluminum industries, and glass manufacturing industries, also contributes fluoride in water pollution, especially in groundwater. Many methods have been developed to remove excessive fluoride from drinking water. These methods can be categorized into four categories: adsorption [2,3], chemical precipitation [4,5], membrane separation [6,7], and electrocoagulation [8–10]. ∗
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1383-5866/$ – see front matter © 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.seppur.2007.01.030
The electrocoagulation (EC) is the process utilizing “sacrificed” anodes to form active coagulant which is used to remove pollutant by precipitation and flotation in situ. Compared with traditional chemical coagulation (CC), EC process reportedly requires less space and does not require chemical storage, dilution, and pH adjustment [11]. It is proven to be effective in water treatment such as drinking water supply for small or medium sized community [12]. EC process has been widely studied in water and wastewater treatment to remove heavy metals [13,14], organics [15,16], bacteria [17], turbidity [18] and inorganic anions [19–21]. Because of its proven ability to effectively remove wide range of pollutants, together with its inherent simplicity of design and operation, EC is being re-examined as potential treatment technology [22]. Some researchers [8–10,23,24] have demonstrated that EC using aluminum electrodes is effective in defluoridation. They focused on investigating the effects of critical operating parameters, such as inter-electrode distance, influent pH, charge loading, current density, electrode area/volume ratio, and initial F− concentration. The kinetics of EC defluoridation process was reported to follow the exponential function with time [9,25]. Emamjomeh [10] developed an empirical model using critical parameters to evaluate the rate constant (K) for fluoride removal by a monopolar EC-flotation process.
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The mechanism of fluoride removal process was a chemical adsorption process with F− replacing the –OH group from the Aln (OH)3n flocs [26]. Fluoride ions and hydroxide ions can clearly coprecipitate with Al3+ ions to form Aln Fm (OH)3n−m [27]: − − nAl3+ (aq) + 3n − mOH(aq) + mF(aq) → Aln Fm (OH)3n−m(s)
(1) However, the fluoride ions in the precipitate are very easily substituted for hydroxide ions: Aln Fm (OH)3n−m(s) + OH− (aq) → Aln Fm−1 (OH)3n−m+1(s) + F− (aq)
(2)
It was reported that EC system performed better than CC system in defluoridation efficiency [24,25,27]. However, only simple explanation was offered about the electrocondensation effect near the surface of electrode without directly experimental proof. EC is a complex electrochemical process, which comprises chemical and physical processes involving many surface and interfacial phenomena. The technology lies at the intersection of three more fundamental technologies—electrochemistry, coagulation and flotation [22]. When a potential is applied to form an external power source, the anode material undergoes oxidation, while the cathode will be subjected to reduction or reductive deposition of elemental metals. If aluminum electrodes are used, the generated Al3+ (aq) ions will immediately undergo further spontaneous reactions to produce corresponding hydroxides and/or polyhydroxides. These hydroxides compounds have strong affinity for dispersed particles as well as counter ions to cause coagulation. The gases evolved at the electrodes may impinge on and cause flotation of the coagulated materials. However, fundamental researches on physical, chemical and electrochemical interactions are still needed for successful engineering design of EC reactor [28]. A new approach to investigate the fluoride distribution could provide more detailed information on the defluoridation in EC process. The fluoride in the EC process was divided into three parts: (1) remain in water; (2) adsorbed by flocs generated and formed in situ; and (3) removed by the gelatinous layer attached on the electrodes which were immediately taken out of the reactor after the electrolysis reaction. The fluoride attaching on the electrodes resulted from the electrophoresis and/or electrocondensation effects in the electric field. The objective of this study is to explore the electrochemical and the chemical interaction in EC defluoridation process. The membrane filtration was used after the coagulation processes to eliminate the removal effects from precipitation or flotation. The effects of the influent pH, charge loading, current density and initial fluoride concentration on the fluoride distribution were further investigated. From the experimental results, a mechanism of fluoride removal by EC was proposed and described. The defluoridation performance of EC and CC was also compared.
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2. Experimental 2.1. Materials Deionized water with 3.0 mM NaHCO3 and 2.0 mM NaCl was used to simulate natural water with a certain degree of alkalinity and ionic strength. Desired concentrations of F− solution were made by adding proper amount of sodium fluoride into the water. The conductivities of the sample water in all the experiments are between 500 and 750 S/cm. 2.2. Analytical methods The pH was measured by a pH meter (Hanna, PH201), which was freshly calibrated by 2 points (6.86, 9.18) before each test. The fluoride concentration was determined by an ion chromatograph (Dionex, ICS 2500). 2.3. Electrocoagulation test A 300-mL of sample solution was placed into a batch reactor for each test run. Two monopolar aluminum electrodes, both having a purity of 99.999%, were used as the anode and cathode, respectively. The dimensions of the electrodes were 90 mm × 60 mm and the A/V is 18 m2 /m3 . Before each test, organic impurities on electrode surfaces were removed by washing with acetone and follow-up HCl solution (10% wt) prior to use. The two electrodes were placed at a distance of 10 mm. The current density was maintained constant by using a potentiostat in intensiostat mode. According to conventional mixing mode, the rapid mixing by a magnetic stirrer was used to enhance full contact between the solution and the coagulant produced in situ. After electrolysis, the electrodes were taken out and the solution was flocculated for 10 min by gentle mixing. Samples taken from the reactor were directly filtered by syringe filter (0.22 um, PSE membrane). The residual fluoride in the filtrate was then measured. At the end of each run, the electrodes were washed thoroughly with water to remove any solid residues on the surfaces, and then dried and re-weighted. The calculated current efficiency in this study was around 111%. 2.4. Coagulation test A proper amount coagulant of Al(NO3 )3 was added to the sample water containing fluoride, and then the water was mixed rapidly for 1 min. During the mixing, the pH of the water was adjusted to the desired value with dilute acid (HCl) or base (NaOH) solution. After mixing, the solution was flocculated for 10 min by gently mixing. The samples taken from coagulation test were also directly filtered by syringe filter. 2.5. Fluoride distribution experiment The ion chromatograph method has been used in several studies [24,27] to measure fluoride concentration, which is not limited by pH and coexisting cations. Thus, the fluoride adsorbed
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Fig. 1. Selected area energy dispersive X-ray spectra of the electrodes: (a) anode and (b) cathode.
on the flocs can be measured by adjusting the sample pH with 1 M NaOH to exceed 13, then completely dissolving aluminum hydroxide particles and releasing the F− ions adsorbed on the flocs. The eluents used in the ion chromatograph is NaOH and high purity water, so the high pH will not affect the result. Each test was run more than three times to eliminate the experimental errors. The experimental procedure was as follows: (1) Add 300 mL of test water with initial F− concentration ([F− ]initial ) into the EC reactor. (2) Take out the electrodes immediately when the electrolysis reaction is finished. (3) Flocculate the solution for 10 min by gentle mixing. (4) Take samples from the flocculated solution; filtrate the sample by 0.22 m filter for fluoride analysis and express the result as [F− ]remain (F− in water). (5) Mix the residual flocculated solution rapidly; take samples for pH adjustment (pH > 13) with 1 M NaOH. Take sample for fluoride analysis and express the result as [F− ]floc+remain (F− in water and flocs) until the flocs is completely dissolved.
3.2. Fluoride distribution in the EC process 3.2.1. Effect of influent pH The influent pH is one of the important factors affecting the performance of electrochemical process [15]. In the EC system, the fluoride removal efficiency is solely determined by the fluoride on electrodes and flocs. The fluoride distribution was shown in Fig. 2, from which the variation of overall removal efficiency of fluoride with the influent pH could be clearly identified. The optimal pH range was 5.5–6.5, at which higher fluoride removal efficiency could be reached. Similar results have also been reported by other investigators [26]. The adsorption on flocs is of primary importance to understanding of the fluoride distribution. Based on the chemical Eqs. (1) and (2), lower pH is favorable for fluoride removal. In the EC process, however, the pH could be increased not only because Table 1 The surface composition of the electrodes analyzed by XPS Atomic concentration (%)
C
O
F
Na
Al
Cl
Anode Cathode
28.44 24.26
48.60 46.69
1.93 0.56
0.58 4.03
19.19 23.89
1.25 0.56
Based on the aforementioned experiments, the distribution of fluoride concentration including [F− ]remain , [F− ]flocs and [F− ]electrodes could be obtained, of which [F− ]flocs (fluoride adsorbed on flocs) = [F− ]flocs+remain − [F− ]remain ; [F− ]electrodes (fluoride removal by electrodes) = [F− ]initial − [F− ]flocs+remain . Furthermore, the efficiency of fluoride removal can be defined as εF = 1 − [F− ]remain /[F− ]initial . 3. Results and discussion 3.1. Characteristics of Al electrodes The Al electrodes after EC defluoridation was analyzed by using EDX and XPS to prove the existence of fluoride ions in the surface layer of electrodes. The results were given in Fig. 1 and Table 1, respectively (operation: [F− ] initial = 1 mmol/L, pH 6.5, charge loading = 2.07 F/m3 ), which showed that the fluoride ions exist on both the anode and the cathode after EC defluoridation process.
Fig. 2. Fluoride distribution at various pH values; initial [F− ] = 5 mg/L, charge loading = 1.55 F/m3 , t = 10 min.
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of the hydrogen generation at the EC cathodes [29], but also the liberation of hydroxide ions from aluminum hydroxide due to the substitution of fluoride ions. Therefore, further increase of the influent pH would decrease the defluoridation capability. In Fig. 2, the adsorbed fluoride on flocs declined quickly to be neglected as the influent pH was getting over 7.5. Since aluminum hydroxide is an amphoteric hydroxide, high pH will lead to the formation of Al(OH)− 4 , which is soluble and useless for defluoridation. The defluoridation by electrodes is a major contributor to overall removal efficiency in an EC system. Higher removal efficiency of electrodes was corresponding to the influent pH range of 6.0–7.0, but the efficiency would decrease as the influent becoming acidic or basic. However, the removal efficiency of electrodes would become dominant even under basic conditions (pH ≥ 7.5). The variation of defluoridation with the influent pH implied that different interactions might exist around the electrodes, such as electrophoresis, adsorption and precipitation of hydrofluoroaluminum complexes. It is the complexes formed on electrodes that vary with the influent pH, which in turn lead to the change of fluoride removed by electrodes. 3.2.2. Effect of charge loading The quality of EC effluent depends on the amount of coagulant produced (mg) or applied charge loading [12]. In this study, charge loading was obtained at the same reaction time by changing current density. The fluoride distribution with different charge loading was illustrated in Fig. 3. Initial sharp increase of overall removal efficiency could be clearly seen. As charge loading was over 1.55 F/m3 , the overall removal efficiency approached a plateau at 91%. The same tendency was also observed by other investigators [15]. The optimal charge loading appeared at the end of the sharp increasing stage. Similar trends were found between variations of removal efficiency of electrodes and those of the overall removal efficiency, whereas removal efficiency of the hydro-aluminum flocs tended to decrease with increasing charge loading. The applied potential increased with the current densities rising, which enhanced the electrophoresis effect. The higher Al3+ concentration on the
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Fig. 4. γ e/f at various charge loadings.
surface of anode could also help to capture the fluoride ions by complexation, adsorption and coprecipitation. Consequently, fluoride ions were attracted to the electrodes and the fluoride concentration near the electrodes exceeded that in bulk solution. Lower fluoride concentration in bulk solution reduced the possibility of removal by flocs adsorption, although the amount of hydro-aluminum flocs was increased. This indicated that fluoride ions were apt to be removed by the electrodes, which proved the occurrence of the electrocondensation effect [24,25,27]. Therefore, greater defluoridation efficiency by electrodes could be expected than hydroxide aluminum flocs. In order to compare the removal effects of electrodes and flocs, we calculated the ratio (re/f ) of removal efficiency of electrodes (εelectrodes ) to that of flocs (εflocs ). As results, the rise of re/f with increasing charge loading as shown in Fig. 4 implied that electrochemical effect was enhanced significantly with chemical effect weakened relatively. 3.2.3. Effect of current density Many researchers studied the influence of current density based on varying charge loading [9,20]. In this study, the effect of current density was investigated based on a constant applied charge loading by varying reaction time. It was found that current density has little effect on defluoridation by flocs (Fig. 5). The reason is that the principal factors influencing adsorption of fluoride by flocs are coagulant dosage and solution pH. Moreover, the experimental results showed that (Table 2) the current efficiency changed little with varying current density for the same charge loading. Hence, change of the overall removal efficiency should be primarily attributed to the variation of defluoridation efficiency of the electrodes. The overall removal efficiency would decrease as current density exceeds 9.26 A/m2 as shown in Fig. 5. Current Table 2 Aluminum electrodes behavior during the EC test at various current densities
(A/m2 )
Fig. 3. Fluoride distribution at various charge loadings; initial [F− ] = 5 mg/L, pH 6.5, t = 10 min.
Current density [Al3+ ] (mg/L) (theoretical) [Al3+ ] (mg/L) Current efficiency (%)
1
2
3
4
5
4.63 9.33 9.77 104
9.26 9.33 9.77 104
18.52 9.33 9.93 106
37.04 9.33 10.50 112
92.59 9.33 10.03 107
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Fig. 7. γ e/f at various initial fluoride concentrations.
Fig. 5. Fluoride distribution at various current densities; initial [F− ] = 5 mg/L, charge loading = 1.04 F/m3 , pH 6.5.
density directly affected coagulant dosage and bubble generation rate, as well as the mixing intensity and mass transfer near electrodes [22], so increasing current density would accelerate the liberation rate of Al3+ and OH− ions. Furthermore, the run time was varied to keep the same charge loading, of which the biggest and smallest run times differed by a factor of about 20, e.g. the run time was only 1 min when the current density was applied at 92.59 A/m2 . Thus, both rapid liberation and short run time would lead to incomplete reaction around the electrodes due to insufficient time to reach dynamic equilibrium. In addition, the rapid liberation of Al3+ and OH− ions from the surface of electrode would make the solution around the anode and cathode extremely acidic or basic respectively, which would decrease the removal efficiency of electrodes as discussed in Section 3.2.1. Experiments on defluoridation under different applied current densities revealed that operating current was of less significance to removal efficiency of electrode as current density was below 9.26 A/m2 (Fig. 5). Furthermore, long reaction time was undesirable from the engineering viewpoint. For the current density of 9.26 A/m2 , 10-min reaction time was enough to equilibrate solution pH and form hydrofluoroaluminum complexes. Thus,
there should be an optimal current density at a certain applied charge loading for the EC system. 3.2.4. Effect of initial F− concentration Fig. 6 showed the general relation between the overall defluoridation efficiency and initial F− concentration, from which we could see that the former gradually decreased from 92 to 80% as the latter increased from 3 to 15 mg/L. Moreover, defluoridation efficiency and capacity were also shown in Fig. 6a and b, respectively. It was observed that the defluoridation efficiency of electrodes tended to decrease although defluoridation capacity increased relatively with the initial F− concentration rising. However, both removal efficiency and capacity of flocs increased when the initial F− concentration went up. The relationship between re/f and initial F− concentration as shown in Fig. 7 revealed that the increase of initial F− concentration would improve the chemical effect by increasing fluoride concentration in bulk solution. Adsorption isotherm of the EC process was drawn by the mass of fluoride adsorption and the equilibrium concentration of fluoride remained in the water. Both Freundlich and Langmuir equations were used to specify the data, and a better fit was found for the latter as shown in Fig. 8. Although the EC defluoridation process could be well described by the conventional Langmuir equation as also reported in other similar studies [30], the fluoride distribution provided more detailed information on
Fig. 6. Fluoride distribution at various initial F− concentrations (a) defluoridation efficiency (b) defluoridation capacity; charge loading = 2.07 F/m3 , pH 6.5, t = 10 min.
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Fig. 8. Equilibrium isothems.
the removal mechanisms inside EC process under different operating conditions. 3.3. Proposed mechanism of fluoride removal by EC From above discussions, it is found that the removal by electrodes was primarily responsible for the high defluoridation efficiency, and the adsorption by hydroxide aluminum flocs gave a secondary effect. In order to have a deeper understanding of the EC process, hydro-fluoro-aluminum complex Aln (OH)m Fk 3n−m−k was used to describe various chemical species containing fluoride, hydroxide and Al3+ , such as Al–F complexes, Al–OH complexes and the hydro-fluoro- aluminum colloid flocs. An schematic of Aln (OH)m Fk 3n−m−k complex based on ion structure was shown in Fig. 9. At 3n = m + k, Aln (OH)m Fk 3n−m−k complexes were likely to exist in the form of colloid species, which could be removed by the follow-up membrane filtration. At 3n < m + k, the negatively charged Aln (OH)m Fk 3n−m−k complexes would move towards the anode by electric migration. In the immediate region around the anode, there was a locally higher concentration of hydro-fluoro-aluminum complexes. The Aln (OH)m Fk 3n−m−k complexes could integrate with dense Al3+ ions on the surface of anode to form colloid flocs precipitated or adsorbed on the anode surface. At 3n > m + k, the positively charged Aln (OH)m Fk 3n−m−k complex would also be attracted to the adjacent area of the cathode by the electrophoretic effect. The liberated hydroxide ions near the cathode could neutralize
Fig. 9. Scheme of proposed defluoridation process in EC.
Fig. 10. Enhancement in fluoride removal from 2 to 40 times at higher Al3+ and F− concentrations. Final Al3+ = 10 mg/L, final F− = 5 mg/L, pH 6.5.
the charge of the Aln (OH)m Fk 3n−m−k complexes to form colloid flocs precipitated or adsorbed on the cathode surface. Both kinds of Aln (OH)m Fk 3n−m−k would be removed when electrodes were taken out of the reactor. It was electrical field that encouraged Aln (OH)m Fk 3n−m−k to be condensed near the electrodes, which made the fluoride adsorbed mostly on the electrodes so that the EC process had a higher defluoridation efficiency. According to the previous studies [31], the F− /Al3+ ratio of hydro-fluoro-aluminum precipitated at a high concentration of fluoride exceeded that at a low concentration of fluoride. A CC process with manually concentrating Al3+ and fluoride was designed to simulate the EC process for local condensation in the area near the electrodes [17]. Fig. 10 summarized the defluoridation efficiency for various Al3+ and F− concentration. From Fig. 10, it could be seen that locally high concentration of Al3+ and F− ions significantly improved F− removal. 3.4. Comparison of EC and CC Tests were conducted to compare EC and CC under various Al3+ dosages and pH values at an initial F− concentration of 5 mg/L. Samples obtained from CC and EC effluent were filtrated with micro-membrane (0.22 m) to eliminate precipitation or flotation. The experimental results are summarized in Fig. 11, in which the pH range of CC process was referred to other studies [32]. Fig. 11 showed that EC process significantly outperformed CC process for fluoridation removal for all the Al3+ concentrations tested in the pH range of 6.0–7.0, especially for low Al3+ dosages. For example, 80% reduction of fluoride was achieved by EC process at an Al3+ dosage of 10–20 mg/L, whereas the same reduction achieved by CC process required Al3+ dosage of 30–40 mg/L. It is well understood that it was electrocondensation that made EC outperform CC in the low coagulant dosage, which was essentially absent in the CC process. Although the present study indicated the contribution of electrodes to defluoridation in the EC process by investigating fluoride distribution, efforts are still needed to understand the fluoride removal mechanism with particular attention to the potential applications, such as removal by flotation and by pre-
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Fig. 11. Comparison of defluoridation efficiency between the EC process and CC process.
cipitation. The removal by electrodes is the dominant contributor to the defluoridation of EC process, but the complexes near the electrode would more or less passivate the electrodes. From the engineering viewpoint, more practical issues should be considered before the final purpose of designing a fluoride removal system would be achieved. 4. Conclusion The fluoride in the EC process could be distributed into three parts, i.e. remained in water, adsorbed by flocs generated and formed in situ, and removed by the gelatinous layer attached on the electrodes. In this study, the existence of the fluoride on the electrodes was proved experimentally with help of EDX and XPS analysis. The overall defluoridation efficiency varies with fluoride distribution that is altered by operating parameters such as pH, charge loading, current density and initial fluoride concentration. The removal efficiency of electrodes was compared with that of flocs, and the ratio between them (re/f ) was found to be increased with the rise of charge loading but decreased with increasing initial F− concentration. The existence of electric field would enhance the concentration of the Al3+ and F− around the surface of electrode, and thus significantly improve F− removal in EC process. As far as the fluoridation removal is concerned, the EC process has obvious advantages over the CC process at various Al3+ concentrations (especially low Al3+ dosages) with pH of 6.0–7.0. The approach proposed in this study could provide not only insight into electrocondensation around the surface of electrode but also more detailed information on electrochemical and chemical interactions in the EC process. Acknowledgements The authors are thankful to Professor Alistair Borthwick in University of Oxford and Professor C.F. Chiang in China Medical University for their valuable help in English editing during the preparation of the paper. Thanks are also to J.L Xie and W.W. Zhou, College of Chemistry and Molecular Engineering in Peking University, for their assistance in XPS and EDX analysis.
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Fluoride distribution in electrocoagulation defluoridation process Jun Zhu, Huazhang Zhao, Jinren Ni ∗ Department of Environmental Engineering, Peking University, The Key Laboratory of Water and Sediment Sciences, Ministry of Education, Beijing 100871, PR China Received 20 September 2006; received in revised form 24 December 2006; accepted 31 January 2007
Abstract Electrocoagulation (EC) is an effective process to remove fluoride from water, but few of scientific literatures explore its inside mechanism. A new approach was used in this study to investigate fluoride distribution in the EC defluoridation process, which divided the fluoride into three parts: remained in water, removed by electrodes, and adsorbed on hydroxide aluminum flocs. The fluoride distribution was investigated in terms of several critical parameters such as pH, charge loading, current density and initial fluoride concentration. The experimental results showed that the removal by electrodes was primarily responsible for the high defluoridation efficiency, and the adsorption by hydroxide aluminum flocs gave a secondary effect. The parameters affected the efficiencies of defluoridation in a way of changing the fluoride distribution in the EC process. A chemical complex of Aln (OH)m Fk 3n−m−k was formulated to explain the mechanism inside the EC defluoridation process. The new approach provides a detailed insight of the electrocondensation effect, which helps to gain more scientific comprehension about the cooperation between electrochemical and chemical ways occurring inside the EC process. © 2007 Elsevier B.V. All rights reserved. Keywords: Fluoride; Electrocoagulation; Chemical coagulation; Electrocondensation
1. Introduction Fluoride is recognized as an essential constituent in the human diet. Low fluoride concentration (<1 mg/L) could prevent dental problem, but higher fluoride concentration (>1.5 mg/L) will cause dental and skeletal fluorosis [1]. Many countries, such as China, Egypt, India, Kenya, etc., have areas where fluorosis is endemic [1]. Fluoride pollution in environment occurs through two different channels: natural sources and anthropogenic sources. Fluoride is frequently encountered in minerals and in geochemical deposits. Because of the erosion and weathering of fluoride-bearing minerals, it becomes a surface species. The discharge of industrial wastewater, such as semiconductor industries, aluminum industries, and glass manufacturing industries, also contributes fluoride in water pollution, especially in groundwater. Many methods have been developed to remove excessive fluoride from drinking water. These methods can be categorized into four categories: adsorption [2,3], chemical precipitation [4,5], membrane separation [6,7], and electrocoagulation [8–10]. ∗
Corresponding author. Tel.: +86 1 62751185; fax: +86 1 62756526. E-mail address: [email protected] (J.R. Ni).
1383-5866/$ – see front matter © 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.seppur.2007.01.030
The electrocoagulation (EC) is the process utilizing “sacrificed” anodes to form active coagulant which is used to remove pollutant by precipitation and flotation in situ. Compared with traditional chemical coagulation (CC), EC process reportedly requires less space and does not require chemical storage, dilution, and pH adjustment [11]. It is proven to be effective in water treatment such as drinking water supply for small or medium sized community [12]. EC process has been widely studied in water and wastewater treatment to remove heavy metals [13,14], organics [15,16], bacteria [17], turbidity [18] and inorganic anions [19–21]. Because of its proven ability to effectively remove wide range of pollutants, together with its inherent simplicity of design and operation, EC is being re-examined as potential treatment technology [22]. Some researchers [8–10,23,24] have demonstrated that EC using aluminum electrodes is effective in defluoridation. They focused on investigating the effects of critical operating parameters, such as inter-electrode distance, influent pH, charge loading, current density, electrode area/volume ratio, and initial F− concentration. The kinetics of EC defluoridation process was reported to follow the exponential function with time [9,25]. Emamjomeh [10] developed an empirical model using critical parameters to evaluate the rate constant (K) for fluoride removal by a monopolar EC-flotation process.
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The mechanism of fluoride removal process was a chemical adsorption process with F− replacing the –OH group from the Aln (OH)3n flocs [26]. Fluoride ions and hydroxide ions can clearly coprecipitate with Al3+ ions to form Aln Fm (OH)3n−m [27]: − − nAl3+ (aq) + 3n − mOH(aq) + mF(aq) → Aln Fm (OH)3n−m(s)
(1) However, the fluoride ions in the precipitate are very easily substituted for hydroxide ions: Aln Fm (OH)3n−m(s) + OH− (aq) → Aln Fm−1 (OH)3n−m+1(s) + F− (aq)
(2)
It was reported that EC system performed better than CC system in defluoridation efficiency [24,25,27]. However, only simple explanation was offered about the electrocondensation effect near the surface of electrode without directly experimental proof. EC is a complex electrochemical process, which comprises chemical and physical processes involving many surface and interfacial phenomena. The technology lies at the intersection of three more fundamental technologies—electrochemistry, coagulation and flotation [22]. When a potential is applied to form an external power source, the anode material undergoes oxidation, while the cathode will be subjected to reduction or reductive deposition of elemental metals. If aluminum electrodes are used, the generated Al3+ (aq) ions will immediately undergo further spontaneous reactions to produce corresponding hydroxides and/or polyhydroxides. These hydroxides compounds have strong affinity for dispersed particles as well as counter ions to cause coagulation. The gases evolved at the electrodes may impinge on and cause flotation of the coagulated materials. However, fundamental researches on physical, chemical and electrochemical interactions are still needed for successful engineering design of EC reactor [28]. A new approach to investigate the fluoride distribution could provide more detailed information on the defluoridation in EC process. The fluoride in the EC process was divided into three parts: (1) remain in water; (2) adsorbed by flocs generated and formed in situ; and (3) removed by the gelatinous layer attached on the electrodes which were immediately taken out of the reactor after the electrolysis reaction. The fluoride attaching on the electrodes resulted from the electrophoresis and/or electrocondensation effects in the electric field. The objective of this study is to explore the electrochemical and the chemical interaction in EC defluoridation process. The membrane filtration was used after the coagulation processes to eliminate the removal effects from precipitation or flotation. The effects of the influent pH, charge loading, current density and initial fluoride concentration on the fluoride distribution were further investigated. From the experimental results, a mechanism of fluoride removal by EC was proposed and described. The defluoridation performance of EC and CC was also compared.
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2. Experimental 2.1. Materials Deionized water with 3.0 mM NaHCO3 and 2.0 mM NaCl was used to simulate natural water with a certain degree of alkalinity and ionic strength. Desired concentrations of F− solution were made by adding proper amount of sodium fluoride into the water. The conductivities of the sample water in all the experiments are between 500 and 750 S/cm. 2.2. Analytical methods The pH was measured by a pH meter (Hanna, PH201), which was freshly calibrated by 2 points (6.86, 9.18) before each test. The fluoride concentration was determined by an ion chromatograph (Dionex, ICS 2500). 2.3. Electrocoagulation test A 300-mL of sample solution was placed into a batch reactor for each test run. Two monopolar aluminum electrodes, both having a purity of 99.999%, were used as the anode and cathode, respectively. The dimensions of the electrodes were 90 mm × 60 mm and the A/V is 18 m2 /m3 . Before each test, organic impurities on electrode surfaces were removed by washing with acetone and follow-up HCl solution (10% wt) prior to use. The two electrodes were placed at a distance of 10 mm. The current density was maintained constant by using a potentiostat in intensiostat mode. According to conventional mixing mode, the rapid mixing by a magnetic stirrer was used to enhance full contact between the solution and the coagulant produced in situ. After electrolysis, the electrodes were taken out and the solution was flocculated for 10 min by gentle mixing. Samples taken from the reactor were directly filtered by syringe filter (0.22 um, PSE membrane). The residual fluoride in the filtrate was then measured. At the end of each run, the electrodes were washed thoroughly with water to remove any solid residues on the surfaces, and then dried and re-weighted. The calculated current efficiency in this study was around 111%. 2.4. Coagulation test A proper amount coagulant of Al(NO3 )3 was added to the sample water containing fluoride, and then the water was mixed rapidly for 1 min. During the mixing, the pH of the water was adjusted to the desired value with dilute acid (HCl) or base (NaOH) solution. After mixing, the solution was flocculated for 10 min by gently mixing. The samples taken from coagulation test were also directly filtered by syringe filter. 2.5. Fluoride distribution experiment The ion chromatograph method has been used in several studies [24,27] to measure fluoride concentration, which is not limited by pH and coexisting cations. Thus, the fluoride adsorbed
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Fig. 1. Selected area energy dispersive X-ray spectra of the electrodes: (a) anode and (b) cathode.
on the flocs can be measured by adjusting the sample pH with 1 M NaOH to exceed 13, then completely dissolving aluminum hydroxide particles and releasing the F− ions adsorbed on the flocs. The eluents used in the ion chromatograph is NaOH and high purity water, so the high pH will not affect the result. Each test was run more than three times to eliminate the experimental errors. The experimental procedure was as follows: (1) Add 300 mL of test water with initial F− concentration ([F− ]initial ) into the EC reactor. (2) Take out the electrodes immediately when the electrolysis reaction is finished. (3) Flocculate the solution for 10 min by gentle mixing. (4) Take samples from the flocculated solution; filtrate the sample by 0.22 m filter for fluoride analysis and express the result as [F− ]remain (F− in water). (5) Mix the residual flocculated solution rapidly; take samples for pH adjustment (pH > 13) with 1 M NaOH. Take sample for fluoride analysis and express the result as [F− ]floc+remain (F− in water and flocs) until the flocs is completely dissolved.
3.2. Fluoride distribution in the EC process 3.2.1. Effect of influent pH The influent pH is one of the important factors affecting the performance of electrochemical process [15]. In the EC system, the fluoride removal efficiency is solely determined by the fluoride on electrodes and flocs. The fluoride distribution was shown in Fig. 2, from which the variation of overall removal efficiency of fluoride with the influent pH could be clearly identified. The optimal pH range was 5.5–6.5, at which higher fluoride removal efficiency could be reached. Similar results have also been reported by other investigators [26]. The adsorption on flocs is of primary importance to understanding of the fluoride distribution. Based on the chemical Eqs. (1) and (2), lower pH is favorable for fluoride removal. In the EC process, however, the pH could be increased not only because Table 1 The surface composition of the electrodes analyzed by XPS Atomic concentration (%)
C
O
F
Na
Al
Cl
Anode Cathode
28.44 24.26
48.60 46.69
1.93 0.56
0.58 4.03
19.19 23.89
1.25 0.56
Based on the aforementioned experiments, the distribution of fluoride concentration including [F− ]remain , [F− ]flocs and [F− ]electrodes could be obtained, of which [F− ]flocs (fluoride adsorbed on flocs) = [F− ]flocs+remain − [F− ]remain ; [F− ]electrodes (fluoride removal by electrodes) = [F− ]initial − [F− ]flocs+remain . Furthermore, the efficiency of fluoride removal can be defined as εF = 1 − [F− ]remain /[F− ]initial . 3. Results and discussion 3.1. Characteristics of Al electrodes The Al electrodes after EC defluoridation was analyzed by using EDX and XPS to prove the existence of fluoride ions in the surface layer of electrodes. The results were given in Fig. 1 and Table 1, respectively (operation: [F− ] initial = 1 mmol/L, pH 6.5, charge loading = 2.07 F/m3 ), which showed that the fluoride ions exist on both the anode and the cathode after EC defluoridation process.
Fig. 2. Fluoride distribution at various pH values; initial [F− ] = 5 mg/L, charge loading = 1.55 F/m3 , t = 10 min.
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of the hydrogen generation at the EC cathodes [29], but also the liberation of hydroxide ions from aluminum hydroxide due to the substitution of fluoride ions. Therefore, further increase of the influent pH would decrease the defluoridation capability. In Fig. 2, the adsorbed fluoride on flocs declined quickly to be neglected as the influent pH was getting over 7.5. Since aluminum hydroxide is an amphoteric hydroxide, high pH will lead to the formation of Al(OH)− 4 , which is soluble and useless for defluoridation. The defluoridation by electrodes is a major contributor to overall removal efficiency in an EC system. Higher removal efficiency of electrodes was corresponding to the influent pH range of 6.0–7.0, but the efficiency would decrease as the influent becoming acidic or basic. However, the removal efficiency of electrodes would become dominant even under basic conditions (pH ≥ 7.5). The variation of defluoridation with the influent pH implied that different interactions might exist around the electrodes, such as electrophoresis, adsorption and precipitation of hydrofluoroaluminum complexes. It is the complexes formed on electrodes that vary with the influent pH, which in turn lead to the change of fluoride removed by electrodes. 3.2.2. Effect of charge loading The quality of EC effluent depends on the amount of coagulant produced (mg) or applied charge loading [12]. In this study, charge loading was obtained at the same reaction time by changing current density. The fluoride distribution with different charge loading was illustrated in Fig. 3. Initial sharp increase of overall removal efficiency could be clearly seen. As charge loading was over 1.55 F/m3 , the overall removal efficiency approached a plateau at 91%. The same tendency was also observed by other investigators [15]. The optimal charge loading appeared at the end of the sharp increasing stage. Similar trends were found between variations of removal efficiency of electrodes and those of the overall removal efficiency, whereas removal efficiency of the hydro-aluminum flocs tended to decrease with increasing charge loading. The applied potential increased with the current densities rising, which enhanced the electrophoresis effect. The higher Al3+ concentration on the
187
Fig. 4. γ e/f at various charge loadings.
surface of anode could also help to capture the fluoride ions by complexation, adsorption and coprecipitation. Consequently, fluoride ions were attracted to the electrodes and the fluoride concentration near the electrodes exceeded that in bulk solution. Lower fluoride concentration in bulk solution reduced the possibility of removal by flocs adsorption, although the amount of hydro-aluminum flocs was increased. This indicated that fluoride ions were apt to be removed by the electrodes, which proved the occurrence of the electrocondensation effect [24,25,27]. Therefore, greater defluoridation efficiency by electrodes could be expected than hydroxide aluminum flocs. In order to compare the removal effects of electrodes and flocs, we calculated the ratio (re/f ) of removal efficiency of electrodes (εelectrodes ) to that of flocs (εflocs ). As results, the rise of re/f with increasing charge loading as shown in Fig. 4 implied that electrochemical effect was enhanced significantly with chemical effect weakened relatively. 3.2.3. Effect of current density Many researchers studied the influence of current density based on varying charge loading [9,20]. In this study, the effect of current density was investigated based on a constant applied charge loading by varying reaction time. It was found that current density has little effect on defluoridation by flocs (Fig. 5). The reason is that the principal factors influencing adsorption of fluoride by flocs are coagulant dosage and solution pH. Moreover, the experimental results showed that (Table 2) the current efficiency changed little with varying current density for the same charge loading. Hence, change of the overall removal efficiency should be primarily attributed to the variation of defluoridation efficiency of the electrodes. The overall removal efficiency would decrease as current density exceeds 9.26 A/m2 as shown in Fig. 5. Current Table 2 Aluminum electrodes behavior during the EC test at various current densities
(A/m2 )
Fig. 3. Fluoride distribution at various charge loadings; initial [F− ] = 5 mg/L, pH 6.5, t = 10 min.
Current density [Al3+ ] (mg/L) (theoretical) [Al3+ ] (mg/L) Current efficiency (%)
1
2
3
4
5
4.63 9.33 9.77 104
9.26 9.33 9.77 104
18.52 9.33 9.93 106
37.04 9.33 10.50 112
92.59 9.33 10.03 107
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Fig. 7. γ e/f at various initial fluoride concentrations.
Fig. 5. Fluoride distribution at various current densities; initial [F− ] = 5 mg/L, charge loading = 1.04 F/m3 , pH 6.5.
density directly affected coagulant dosage and bubble generation rate, as well as the mixing intensity and mass transfer near electrodes [22], so increasing current density would accelerate the liberation rate of Al3+ and OH− ions. Furthermore, the run time was varied to keep the same charge loading, of which the biggest and smallest run times differed by a factor of about 20, e.g. the run time was only 1 min when the current density was applied at 92.59 A/m2 . Thus, both rapid liberation and short run time would lead to incomplete reaction around the electrodes due to insufficient time to reach dynamic equilibrium. In addition, the rapid liberation of Al3+ and OH− ions from the surface of electrode would make the solution around the anode and cathode extremely acidic or basic respectively, which would decrease the removal efficiency of electrodes as discussed in Section 3.2.1. Experiments on defluoridation under different applied current densities revealed that operating current was of less significance to removal efficiency of electrode as current density was below 9.26 A/m2 (Fig. 5). Furthermore, long reaction time was undesirable from the engineering viewpoint. For the current density of 9.26 A/m2 , 10-min reaction time was enough to equilibrate solution pH and form hydrofluoroaluminum complexes. Thus,
there should be an optimal current density at a certain applied charge loading for the EC system. 3.2.4. Effect of initial F− concentration Fig. 6 showed the general relation between the overall defluoridation efficiency and initial F− concentration, from which we could see that the former gradually decreased from 92 to 80% as the latter increased from 3 to 15 mg/L. Moreover, defluoridation efficiency and capacity were also shown in Fig. 6a and b, respectively. It was observed that the defluoridation efficiency of electrodes tended to decrease although defluoridation capacity increased relatively with the initial F− concentration rising. However, both removal efficiency and capacity of flocs increased when the initial F− concentration went up. The relationship between re/f and initial F− concentration as shown in Fig. 7 revealed that the increase of initial F− concentration would improve the chemical effect by increasing fluoride concentration in bulk solution. Adsorption isotherm of the EC process was drawn by the mass of fluoride adsorption and the equilibrium concentration of fluoride remained in the water. Both Freundlich and Langmuir equations were used to specify the data, and a better fit was found for the latter as shown in Fig. 8. Although the EC defluoridation process could be well described by the conventional Langmuir equation as also reported in other similar studies [30], the fluoride distribution provided more detailed information on
Fig. 6. Fluoride distribution at various initial F− concentrations (a) defluoridation efficiency (b) defluoridation capacity; charge loading = 2.07 F/m3 , pH 6.5, t = 10 min.
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189
Fig. 8. Equilibrium isothems.
the removal mechanisms inside EC process under different operating conditions. 3.3. Proposed mechanism of fluoride removal by EC From above discussions, it is found that the removal by electrodes was primarily responsible for the high defluoridation efficiency, and the adsorption by hydroxide aluminum flocs gave a secondary effect. In order to have a deeper understanding of the EC process, hydro-fluoro-aluminum complex Aln (OH)m Fk 3n−m−k was used to describe various chemical species containing fluoride, hydroxide and Al3+ , such as Al–F complexes, Al–OH complexes and the hydro-fluoro- aluminum colloid flocs. An schematic of Aln (OH)m Fk 3n−m−k complex based on ion structure was shown in Fig. 9. At 3n = m + k, Aln (OH)m Fk 3n−m−k complexes were likely to exist in the form of colloid species, which could be removed by the follow-up membrane filtration. At 3n < m + k, the negatively charged Aln (OH)m Fk 3n−m−k complexes would move towards the anode by electric migration. In the immediate region around the anode, there was a locally higher concentration of hydro-fluoro-aluminum complexes. The Aln (OH)m Fk 3n−m−k complexes could integrate with dense Al3+ ions on the surface of anode to form colloid flocs precipitated or adsorbed on the anode surface. At 3n > m + k, the positively charged Aln (OH)m Fk 3n−m−k complex would also be attracted to the adjacent area of the cathode by the electrophoretic effect. The liberated hydroxide ions near the cathode could neutralize
Fig. 9. Scheme of proposed defluoridation process in EC.
Fig. 10. Enhancement in fluoride removal from 2 to 40 times at higher Al3+ and F− concentrations. Final Al3+ = 10 mg/L, final F− = 5 mg/L, pH 6.5.
the charge of the Aln (OH)m Fk 3n−m−k complexes to form colloid flocs precipitated or adsorbed on the cathode surface. Both kinds of Aln (OH)m Fk 3n−m−k would be removed when electrodes were taken out of the reactor. It was electrical field that encouraged Aln (OH)m Fk 3n−m−k to be condensed near the electrodes, which made the fluoride adsorbed mostly on the electrodes so that the EC process had a higher defluoridation efficiency. According to the previous studies [31], the F− /Al3+ ratio of hydro-fluoro-aluminum precipitated at a high concentration of fluoride exceeded that at a low concentration of fluoride. A CC process with manually concentrating Al3+ and fluoride was designed to simulate the EC process for local condensation in the area near the electrodes [17]. Fig. 10 summarized the defluoridation efficiency for various Al3+ and F− concentration. From Fig. 10, it could be seen that locally high concentration of Al3+ and F− ions significantly improved F− removal. 3.4. Comparison of EC and CC Tests were conducted to compare EC and CC under various Al3+ dosages and pH values at an initial F− concentration of 5 mg/L. Samples obtained from CC and EC effluent were filtrated with micro-membrane (0.22 m) to eliminate precipitation or flotation. The experimental results are summarized in Fig. 11, in which the pH range of CC process was referred to other studies [32]. Fig. 11 showed that EC process significantly outperformed CC process for fluoridation removal for all the Al3+ concentrations tested in the pH range of 6.0–7.0, especially for low Al3+ dosages. For example, 80% reduction of fluoride was achieved by EC process at an Al3+ dosage of 10–20 mg/L, whereas the same reduction achieved by CC process required Al3+ dosage of 30–40 mg/L. It is well understood that it was electrocondensation that made EC outperform CC in the low coagulant dosage, which was essentially absent in the CC process. Although the present study indicated the contribution of electrodes to defluoridation in the EC process by investigating fluoride distribution, efforts are still needed to understand the fluoride removal mechanism with particular attention to the potential applications, such as removal by flotation and by pre-
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Fig. 11. Comparison of defluoridation efficiency between the EC process and CC process.
cipitation. The removal by electrodes is the dominant contributor to the defluoridation of EC process, but the complexes near the electrode would more or less passivate the electrodes. From the engineering viewpoint, more practical issues should be considered before the final purpose of designing a fluoride removal system would be achieved. 4. Conclusion The fluoride in the EC process could be distributed into three parts, i.e. remained in water, adsorbed by flocs generated and formed in situ, and removed by the gelatinous layer attached on the electrodes. In this study, the existence of the fluoride on the electrodes was proved experimentally with help of EDX and XPS analysis. The overall defluoridation efficiency varies with fluoride distribution that is altered by operating parameters such as pH, charge loading, current density and initial fluoride concentration. The removal efficiency of electrodes was compared with that of flocs, and the ratio between them (re/f ) was found to be increased with the rise of charge loading but decreased with increasing initial F− concentration. The existence of electric field would enhance the concentration of the Al3+ and F− around the surface of electrode, and thus significantly improve F− removal in EC process. As far as the fluoridation removal is concerned, the EC process has obvious advantages over the CC process at various Al3+ concentrations (especially low Al3+ dosages) with pH of 6.0–7.0. The approach proposed in this study could provide not only insight into electrocondensation around the surface of electrode but also more detailed information on electrochemical and chemical interactions in the EC process. Acknowledgements The authors are thankful to Professor Alistair Borthwick in University of Oxford and Professor C.F. Chiang in China Medical University for their valuable help in English editing during the preparation of the paper. Thanks are also to J.L Xie and W.W. Zhou, College of Chemistry and Molecular Engineering in Peking University, for their assistance in XPS and EDX analysis.
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