Effects of Ca2+ and Mg2+ on Defluoridation in the Electrocoagulation

Because aqueous ions can influence the defluoridation of the electrocoagulation (EC) process, the effects of Ca2+ and Mg2+ were investigated. The beha...
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Environ. Sci. Technol. 2010, 44, 9112–9116

Effects of Ca2+ and Mg2+ on Defluoridation in the Electrocoagulation Process H U A Z H A N G Z H A O , * ,†,‡ B I N Z H A O , †,‡ W E I Y A N G , †,‡ A N D T I A N H O N G L I †,‡ Department of Environmental Engineering, Peking University, Beijing 100871, People’s Republic of China, and The Key Laboratory of Water and Sediment Sciences, Ministry of Education, Beijing 100871, People’s Republic of China

Received July 26, 2010. Revised manuscript received October 3, 2010. Accepted October 5, 2010.

Because aqueous ions can influence the defluoridation of the electrocoagulation (EC) process, the effects of Ca2+ and Mg2+ were investigated. The behaviors and mechanisms of EC defluoridation in Ca2+-containing systems were different from those in Mg2+-containing systems. An increase in Ca2+ concentration improved the defluoridation efficiency (εF), but it could not change the optimal molar ratio of OH- and F- to Al3+ (rOH+F). The highest εF can usually be obtained at rOH+F ) 3 for defluoridation. Only a small portion of Ca2+ entered into the flocs, and Ca2+ could not influence the mechanism of EC defluoridation. For the Mg2+-containing system, the optimal rOH+F increased with increasing Mg2+ concentration. The optimal rOH+F was maintained at 3 after the Mg2+ concentration was corrected using the obtained correction coefficient of 0.3435. About 50% to 70% of the total Mg2+ entered into the flocs. From the XRD analysis, it was found that some Mg-Al-F layered double hydroxides (LDHs) were formed by Mg2+, F-, and Al3+ during electrolysis. It is proposed for the first time that the formation of Mg-Al-F LDH is one of the mechanisms for EC defluoridation in systems containing both F- and Mg2+.

Introduction Whether fluoride in the drinking water is beneficial or harmful to human health depends on the fluoride concentration. Proper amounts of fluoride can prevent dental caries and osteoporosis, but excess fluoride can cause fluorosis (1). Various defluoridation methods, including adsorption (2-4), coagulation and precipitation (5-7), membrane separation (8-10), and electrocoagulation (EC) (11, 12), have been developed to remove excess fluoride from water and to improve drinking water quality. The EC defluoridation method has attracted much attention. The defluoridation mechanism of the EC system includes adsorption (13), coprecipitation (14), electrocondensation (15), and other processes. Zhu et al. (15) divided the fluoride in the EC defluoridation process into three parts: the part that remained in the solution, the part that was removed by the electrodes, and the part that was adsorbed by the flocs. The adsorption of fluoride by flocs contributes largely to the defluoridation efficiency (εF). It is generally * Corresponding author phone: +86-10-62754292-815; fax: +8610-62756526; e-mail: [email protected]. † Peking University. ‡ Ministry of Education. 9112

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thought that the coprecipitation of Al3+, F-, and OH-, as well as the adsorption by the Al(OH)3 precipitate, contributes to the defluoridation by the flocs (16). The εF of EC is influenced by factors including pH, alkalinity, and coexisting ions (17). The highest εF of EC can be obtained when the pH of the final system is about 6 (13). Because the pH of F--containing water is sensitive to the addition of acid or base, it is difficult to control the pH during the actual operation. Hu et al. (14) proposed that a pH adjustment can be replaced by controlling the molar ratio of OH- and F- to Al3+ (rOH+F) and that the highest εF can be obtained at rOH+F ) 3. The rOH+F is defined by eq 1: rOH+F )

[OH-]0 + [OH-]EC + [F-]0 - [H+]0 [Al3+]T

(1)

where [OH-]0 is the initial OH- concentration, [OH-]EC is the concentration of OH- generated in the electrolysis process, [Al3+]T is the total concentration of Al3+, and [H+]0 and [F-]0 are the initial concentrations of H+ and F-, respectively. The principle of optimal rOH+F (rOH+F(opt)) ) 3 would be affected if there were other ions in the system and can only be used after the ion concentrations are corrected. Zhao et al. (18) corrected the concentration of bicarbonate that is ubiquitous in water. Using the obtained correction coefficient, the highest εF would be still achieved at rOH+F ) 3. Until now, no reports have been made on whether this principle is influenced by other ions (Ca2+, Mg2+, NO3-, etc.) or what correction should be applied to these ions. Many adsorption materials, such as zeolite, bone char, and active carbon, have been used in defluoridation processes (19, 20). Some new adsorption materials have been developed recently, and the study of layered double hydroxides (LDHs), which have similar structure to the natural hydrotalcite, is an active area of research (21, 22). The chemical composition of LDHs is often described by the general formula, [M2+1-xM3+x(OH)2]x+(An-x/n) · mH2O, where M2+ and M3+ are divalent and trivalent cations, respectively (such as Mg2+ and Al3+), and An- is the exchangeable anion (such as F-, Cl-, and CO32-) (23). Many methods, such as coprecipitation, ion exchange, calcination and reconstruction, and template synthesis, can be used to synthesize LDHs (24). LDHs also have relatively large surface areas (20-120 m2 g-1) and high anion-exchange capacities (3.0-4.8 meq g-1) (25). Because of these properties, LDHs have been used in water purification and treatment processes, especially in the removal of the anions, including F- and AsO43- (26, 27). Mandal et al. (28) used Zn-Al LDHs to remove fluoride from water and found that the LDH with a Zn/Al ratio of 0.97 had the highest capacity for fluoride adsorption (1.14-4.16 mg g-1). Wang et al. (29) used synthetic Mg-Al hydrotalcite-like compounds and their calcined products (HTCs) to treat fluoridecontaining water, and the results demonstrated that the HTCs were rather effective in F- adsorption. Lv et al. (25) developed a Mg-Al-CO3 LDH to treat highly concentrated fluoride solutions and concluded that the maximum adsorbed fluoride by the LDH was 319.8 mg g-1 by fitting the equilibrium data with the Langmuir-Freundlich model. In addition to using the preprepared LDHs, LDHs can also be prepared in situ to remove contaminants from water. Chen et al. (30) employed Mg-Al LDHs synthesized in situ to treat wastewater containing Cr(VI). Their results indicated that the factors influencing the efficiency of Cr(VI) removal included the concentrations of Mg2+ and Al3+, the Mg/Al molar ratio, the system pH, and the original Cr(VI) concentration. Zhu 10.1021/es102540t

 2010 American Chemical Society

Published on Web 10/26/2010

et al. (31) synthesized Mg-Al LDHs in situ to remove Congo Red anion-containing dyestuff from wastewater, with a removal efficiency of 100% at pH of 9.0, Mg/Al molar ratio of 2:1, and reaction duration of 2 h. By dropwise addition of a NaOH solution into a rapidly stirred solution system containing AlCl3, MgCl2, and NaF, Peng et al. (32) synthesized Mg-Al LDH in situ, and the εF reached 92.77%. The aforementioned process of synthesizing LDHs in situ is similar to the coagulation process caused by aluminum salts, but it is unknown whether LHDs are formed in an aluminum-based EC process. Ca2+ and Mg2+ are the common ions in water, and they have been shown to affect the EC defluoridation performance (13, 33). These results demonstrate that the εF rises with the increasing Ca2+ concentration (13, 33); however, in the case of Mg2+, there is an optimal Mg2+ concentration to obtain the highest εF (13). Excessive Mg2+ would result in the precipitation of Mg(OH)2 onto the surface of Al(OH)3 flocs. This phenomenon leads to the decrease in the εF because the affinity of fluoride to Mg(OH)2 is much smaller than that to Al(OH)3 (13, 34). In addition, the presence of Ca2+ and Mg2+ would also affect the structure and composition of the flocs produced during the EC process. As a result, the EC defluoridation mechanism would change due to these common cations. However, this viewpoint has been neither put forward nor comprehensively discussed before. In this work, the effects of Ca2+ and Mg2+ on the optimal defluoridation rOH+F in the EC process were investigated. The distributions of the Ca2+ and Mg2+ in the EC system and the composition of the flocs were explored to illuminate the specific defluoridation mechanism in the Ca2+- and Mg2+containing systems. Moreover, a correction method was proposed to extend the EC defluoridation principle of optimal rOH+F ) 3 for the Mg2+-containing system.

Experimental Section Materials. Synthetic raw water, which was prepared by adding NaF (final concentration of 10 mg L-1) and NaCl (final concentration of 2 mM) to deionized water, was used in the experiments. The Mg2+- or Ca2+-containing fluorinated water was prepared by adding the desired amount of MgCl2 or CaCl2 to the raw water. EC Defluoridation. An aluminum anode with a purity of 99.9999% and a copper cathode were used in the EC defluoridation process. The effective dimensions of the two electrodes were both 90 × 60 mm, and the distance between the electrodes was 10 mm. For each experiment, 300 mL of raw water was electrolyzed for 10 min with a 0.15 A current, and the rOH+F value of the raw water was adjusted by adding either a diluted HCl or NaOH solution. A magnetic stirrer was used to ensure the good diffusion of Al3+ during the electrolysis process. After each batch of electrolysis, the electrodes were taken out, and the aluminum anode was washed thoroughly with water to remove any solid residues on the surfaces and then dried and reweighted. Its mass loss due to electrolysis was expressed as the total Al3+ released from the anode, and the Al dose ([Al3+]T) was around 1.2 mM for each run. After the electrolysis, the solution was slowly stirred for 10 min. Samples were then taken out of the reactor and filtered through a 0.22-µm membrane. The fluoride concentration in the filtration was then measured. The flocs were investigated by X-ray diffraction (XRD) analysis after centrifugation, washing, and drying. To study the effect of aging on the structure and the composition of the flocs generated in the EC process, the test solutions were transferred into sealed Erlenmeyer flasks after electrolysis and aged at 50 °C for 24 h. The solutions were then filtered through a 0.22-µm membrane, and the flocs were detected by XRD after washing and drying.

FIGURE 1. Variation of εF with rOH+F at different Ca2+ (a) and Mg2+ (b) concentrations. Distribution of Ca2+ and Mg2+ in the EC System. A method similar to that proposed by Zhu et al. (15) was used, in which the Mg2+ in the EC system was divided into three parts: the part that remained in the solution ([Mg2+]solution), the part that was retained on the electrodes ([Mg2+]electrodes), and the part that was transformed into flocs ([Mg2+]flocs). The initial Mg2+ concentration was denoted as [Mg2+]initial. After electrolysis, a water sample was removed. Part of this sample was filtered through a 0.22-µm membrane, and the Mg2+ concentration in the filtration was denoted as [Mg2+]solution. The other part was acidified with a HCl solution to pH < 2 to completely dissolve the flocs and release the Mg2+ into solution; therefore, the measured Mg2+ concentration in this acidified solution was denoted as [Mg2+]flocs+solution. According to the above definition, [Mg2+]flocs and [Mg2+]electrodes can be obtained by equations [Mg2+]flocs ) [Mg2+]flocs+solution [Mg2+]solution and [Mg2+]electrodes ) [Mg2+]initial - [Mg2+]flocs+solution. Similarly, to investigate the distribution of Ca2+ in the EC system, the Ca2+ in the system was also divided into [Ca2+]solution, [Ca2+]flocs, and [Ca2+]electrodes. Synthesis of the Mg-Al-F LDH. The Mg-Al-F LDH was synthesized by the coprecipitation method. A mixed salt solution was prepared by adding MgCl2 · 6H2O and AlCl3 · 6H2O to the boiled deionized water with a Mg/Al molar ratio of 2. With rapid stirring, a 1 mol L-1 NaOH solution and an excessive NaF solution were added dropwise into the mixed salt solution until the pH reached 10. The solution was then aged at 50 °C for 24 h. The flocs were obtained by filtration with a 0.22-µm membrane and were detected by XRD after washing and drying. Analysis Methods. The concentrations of F-, Mg2+, and Ca2+ in the solution were measured by ion chromatography (Dionex, ICS2500). The pH was measured by a pH meter (Hanna, pH-201), which was calibrated before each test. The structures of the flocs and the synthesized Mg-Al-F LDH were characterized by X-ray diffraction (Rigaku, DMAX-2400).

Results and Discussion Effects of Ca2+ and Mg2+ on the Optimal rOH+F. The effects of Ca2+ and Mg2+ on the optimal rOH+F were investigated in the electrolysis process with a fixed Al dose of 1.2 mM. As shown in Figure 1a, the εF varied with changing rOH+F and it VOL. 44, NO. 23, 2010 / ENVIRONMENTAL SCIENCE & TECHNOLOGY

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FIGURE 3. X-ray diffraction patterns of the flocs in the different EC defluoridation systems. Fluoride-containing water with 10 mg L-1 NaF and 2 mM NaCl (a), 10 mg L-1 NaF, 2 mM NaCl, and 1 mM CaCl2 (b), 10 mg L-1 NaF, 2 mM NaCl, and 1 mM MgCl2 (c).

FIGURE 2. Distributions of Ca2+ (a) and Mg2+ (b) in the EC system and the corresponding efficiency of defluoridation at different rOH+F. rose slightly with the increasing Ca2+ concentration at the fixed rOH+F. However, the Ca2+ concentration did not affect the optimal rOH+F. The optimal rOH+F values were close to 3 at different Ca2+ concentrations, and the maximum εF values were higher than 90%. Figure 1b shows that the Mg2+ concentration had a remarkable influence on the optimal rOH+F and that the optimal rOH+F values rose with the increasing Mg2+ concentration in the study range (Mg2+ e 1.872 mM). When the Mg2+ concentrations were 0, 0.468, 0.936, 1.404, and 1.872 mM, the corresponding optimal rOH+F values were 3.0, 3.3, 3.6, 4.3, and 4.8, respectively. By forming CaF2, soluble calcium salts can be used to remove fluoride from solution (13, 33, 35). However, because the F- and Ca2+ concentrations were low in the present experiment, the amount of CaF2 formed was small and the value of εF increased slightly with the increasing Ca2+ concentration. In addition, forming CaF2 was not supposed to be strongly related to or change the system pH; therefore, the presence of Ca2+ would not change the optimal rOH+F. The effect of Mg2+ on the fluoride removal by EC was quite different from that of Ca2+. Precipitates, which will be discussed later, are probably formed during the EC process in the presence of Mg2+. These reactions involved the participation of OH- and the change of the system pH. Moreover, F- was also able to be removed by adsorption on these precipitates. As a consequence, the optimal rOH+F in the EC defluoridation changed with the Mg2+ concentrations. In the next experiment, the distributions of Ca2+ and Mg2+ in the solution, in the flocs, and on the electrodes were measured to explain the different influences of Ca2+ and Mg2+ in the EC defluoridation. Distributions of Ca2+ and Mg2+ in the EC Process. For the test solutions with the initial Ca2+ or Mg2+ concentration of 1 mM, the concentrations of Ca2+ and Mg2+ in the solution, in the flocs, and on the electrodes were measured after the EC defluoridation. Figure 2 shows the distributions of Ca2+ and Mg2+ and the corresponding εF at different rOH+F values. The maximum εF of 94.23% was obtained at rOH+F ) 3.0 for Ca2+, while the maximum εF of 92.47% was obtained at rOH+F ) 3.8 for Mg2+. The amount of Ca2+ in the flocs accounted for only 10% of the total Ca2+. Overall, 90% of the Ca2+ 9114

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remained in solution, which means that the majority of Ca2+ was not involved in the EC defluoridation process. However, in the case of Mg2+, 50% to 70% of the total Mg2+ was transformed into the flocs. For both systems, the amounts of Ca2+ and Mg2+ on the electrodes were small enough to be ignored. As a consequence, the different influence on the EC defluoridation between Ca2+ and Mg2+ would mainly depend on the floc composition. For the Ca2+-containing system, both the formation of CaF2 and the adsorption on Al(OH)3 would lead to the existence of Ca2+ in the flocs. Since the rOH+F was initially adjusted to 2.7-3.3 (the system pH before electrolysis was 3.1 to 3.8, and after electrolysis was 5.4 to 8.1), Ca(OH)2 was not able to be deposited according to its solubility product (KSP ) 5.5 × 10-6, 25 °C) (36). Coprecipitation of AlnFm(OH)3n-m was the prevailing mechanism for the fluoride removal in this case (16). In addition to the AlnFm(OH)3n-m, Mg(OH)2 and Mg-Al-F LDH might be formed in the Mg2+-containing system, resulting in a different mechanism for EC defluoridation compared to the Ca2+-containing system. Because the formation of Mg(OH)2 or Mg-Al-F LDH can change the solution pH, as a consequence, the highest εF was obtained at initial pH value between 9.87 and 10.9 for the Mg2+containing system (the system pH decreased to 5.5-7.8 after electrolysis). This finding is quite different from that in the Ca2+-containing system. The solubility product of Mg(OH)2 is 1.8 × 10-11 at 25 °C (36) suggesting that Mg(OH)2 would not be formed in this case. The composition of the flocs will be further investigated by XRD techniques. XRD Analysis for the Flocs. Figure 3 shows the XRD results for the flocs produced in the different EC defluoridation systems. The XRD patterns of the flocs generated in the EC with Ca2+ (b) were almost the same as those without Ca2+ or Mg2+ (a). No obvious diffraction peaks were observed, suggesting that no crystals were generated in either condition. Four obvious diffraction peaks were observed for the flocs generated in the Mg2+-containing EC system (c), although these peaks were wide and had poor symmetry. This difference indicated that some crystalline material was generated in this process and that the wide diffraction peaks might result from the doping of the noncrystalline in the crystal materials. The XRD patterns of the flocs generated in the Mg2+containing system in this study were very similar to those of the Mg-Al LDH intercalated with L-ascorbic acid (ASA) in the study from Aisawa et al. (37). Therefore, we inferred that the Mg-Al LDH was generated in the Mg2+-containing EC process. However, due to lack of necessary aging conditions especially the optimal aging temperature and aging time (26, 27), the crystallinity of the produced Mg-Al LDH in the present Mg2+containing EC process was not well developed.

rOH+F )

FIGURE 4. Comparison of XRD patterns of the artificially synthesized Mg-Al-F LDH and the Mg-containing flocs generated in the EC process. Synthesized Mg-Al-F LDH: Mg/ Al molar ratio ) 2, aging time ) 24 h (a); flocs generated in EC defluoridation: Mg/Al molar ratio ) 2, aging time ) 24 h (b); Mg/Al molar ratio ) 2, aging time ) 0 h (c); Mg/Al molar ratio ) 1, aging time ) 24 h (d); Mg/Al molar ratio ) 1, aging time ) 0 h (e). XRD Pattern Comparison of the Mg2+-Containing Flocs Generated in the EC Process and the Artificially Synthesized Mg-Al-F LDH. The XRD patterns of the artificially synthesized Mg-Al-F LDH were compared with those of the LDH generated in the EC process as shown in Figure 4. The characteristic diffraction peaks ((003), (006), (009), (015), (018), (110), and (113)) of the synthesized Mg-Al-F LDH (38) were observed in Figure 4a. After an aging treatment for 24 h, the flocs generated in the EC defluoridation process with different Mg/Al molar ratios exhibited characteristic diffraction peaks similar to those of the synthesized Mg-Al-F LDH. The sample with a Mg/Al molar ratio of 2 was more crystalline (Figure 4b). The flocs without aging showed wide peaks at the corresponding diffraction angles, which meant that the crystalline structures were not well developed and the crystalline and the noncrystalline materials were doped with each other (Figure 4c and e). The above results indicated that the Mg-Al-F LDH was generated in the EC defluoridation process but that its crystallinity was poor. This finding demonstrated that the Mg2+-containing EC defluoridation system had the basic conditions for the LDH synthesis but that the conditions were not optimal for the crystalline growth of LDHs. If we adjusted the conditions of EC defluoridation, such as optimizing the Mg/Al molar ratio, the temperature, and the aging time, then more and better crystalline Mg-Al-F LDH would be generated in the Mg2+-containing system. Because Mg2+ is common in water, especially in the groundwater, the conclusion can be drawn that the formation of a Mg-Al-F LDH is one of the important defluoridation mechanisms for the practical use of the EC technology. Correction of Mg2+. Because the Mg-Al LDH is formed in the Mg2+-containing EC defluoridation system, the Mg2+ concentration affects the optimal rOH+F. The optimal rOH+F was found to rise with the increasing Mg2+ concentration (Figure 1b). It would be more convenient for practical applications if the optimal rOH+F value was adjusted to 3.0 by correcting the effect of Mg2+. In the Mg2+-containing system, as mentioned above, Fions can be removed by the formation of AlnFm(OH)3n-m and Mg-Al-F LDH. Since both Mg2+ and Al3+ contributed to the defluoridation, the Mg2+ concentration should be considered together with the Al3+ concentration in the defined equation of rOH+F. Therefore, the initial Mg2+ concentration ([Mg2+]0) is multiplied by a coefficient and then added to [Al3+]T in the denominator of eq 1. By assuming that the correction coefficient of Mg2+ was 1, eq 2 was obtained:

[OH-]0 + [OH-]EC + [F-]0 - [H+]0

(2)

[Al3+]T + [Mg2+]0

The values of rOH+F were recalculated according to eq 2. When the [Mg2+]0 were 0, 0.468, 0.936, 1.404, and 1.872 mM, the optimal rOH+F values were 3, 2.37, 2.02, 1.98, and 1.88, respectively. It can be found that the principle of the optimal rOH+F ) 3 was not available for the Mg2+-containing systems if the correction coefficient was set as 1. To obtain the effective correction coefficient for Mg2+, some assumptions were made: (1) The rOH+F(opt) is equal to 3.0 at varied concentrations of Mg2+. (2) The correction coefficient of Mg2+, denoted as R, is constant. (3) The corrected rOH+F (rOH+F(cor)) is calculated using eq 3: rOH+F(cor) )

[OH-]0 + [OH-]EC + [F-]0 - [H+]0

(3)

[Al3+]T + R[Mg2+]0

According to the first assumption, the highest εF can be obtained when rOH+F(cor) is 3. For the different Mg2+ concentrations, the optimal rOH+F must comply simultaneously with eqs 2 and 3. Equation 4 will be obtained if eq 2 is divided by eq 3: (1 - R)[Mg2+]0 3 - rOH+F ) 3 [Al3+]T + [Mg2+]0

(4)

In the present work, the value of [Al3+]T was 1.2 mM for each run. Then, eq 4 can be rewritten as follows: 3 - rOH+F (1 - R)[Mg2+]0 ) 3 1.2 + [Mg2+]0

(5)

We defined x and y as follows: y)

x)

3 - rOH+F 3

(6)

[Mg2+]0

(7)

1.2 + [Mg2+]0

Then, eq 8 can be obtained: y ) (1 - R)x

(8)

By using the values of the optimal rOH+F from eq 2 and the corresponding Mg2+ concentration, the linear relationship between the values of y and x was proved with a correlation coefficient (R2) of 0.9691. The slope of the fitted line (1-R)

FIGURE 5. Variation of εF with rOH+F(cor) at different Mg2+ concentrations. VOL. 44, NO. 23, 2010 / ENVIRONMENTAL SCIENCE & TECHNOLOGY

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can be used to calculate the correction coefficient (R) of Mg2+. The R value for the present Mg2+-containing system is 0.3435. The rOH+F(cor) can be obtained according to eq 3 using the calculated correction coefficient. The variation of εF with rOH+F(cor) at different initial Mg2+ concentrations is shown in Figure 5. After the initial Mg2+ concentrations in the range of 0-1.872 mM were corrected, the highest εF values were able to be achieved at rOH+F(cor) close to 3.0. This finding demonstrates that the obtained correction coefficient for Mg2+ can be used successfully for the extension of EC defluoridation principle of optimal rOH+F ) 3 in the Mg2+containing system.

Acknowledgments The authors are grateful for the financial support from the National Natural Science Fund (Grant No. 21077001) and the National Eleventh Five-Year Technology Support Programme (Grant No. 2006BAD01B02) of China.

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