Journal of Environmental Management 156 (2015) 252e256
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Journal of Environmental Management journal homepage: www.elsevier.com/locate/jenvman
Kinetics and equilibrium studies on MgeAl oxide for removal of fluoride in aqueous solution and its use in recycling Tomohito Kameda*, Jumpei Oba, Toshiaki Yoshioka Graduate School of Environmental Studies, Tohoku University, 6-6-07 Aoba, Aramaki, Aoba-ku, Sendai 980-8579, Japan
a r t i c l e i n f o
a b s t r a c t
Article history: Received 26 February 2015 Received in revised form 24 March 2015 Accepted 25 March 2015 Available online
MgeAl oxide obtained by the thermal decomposition of MgeAl layered double hydroxide (LDH) intercalated with CO2 3 (CO3$MgeAl LDH) was found to take up fluoride from aqueous solution. Fluoride was removed by rehydration of MgeAl oxide accompanied by combination with F. Using five times the stoichiometric quantity of MgeAl oxide, the residual concentration of F was decreased from 100 to 6.3 mg/L in 480 min, which was below the effluent standard in Japan (8 mg/L). Removal of F can be represented by pseudo-second-order reaction kinetics. The apparent rate constants at 10 C, 30 C, and 60 C were 2.3 103, 2.2 102, and 2.5 101 g mmol1 min1, respectively. The apparent activation energy was 73.3 kJ mol1. The rate-determining step for F removal by MgeAl oxide was consistent with chemical adsorption involving intercalation of F into the reconstructed MgeAl LDH due to electrostatic attraction. The adsorption of F by MgeAl oxide follows a Langmuir-type adsorption. The values of the maximum adsorption and the equilibrium adsorption constant were 3.0 mmol g1 and 1.1 103, respectively, for MgeAl oxide. The F in the F$MgeAl LDH thus produced was found to be anionexchanged with CO2 3 in solution. The MgeAl oxide after regeneration treatment had excellent properties for removal of F in aqueous solution. In conclusion, the results of this study indicated that MgeAl oxide has potential for use in recycling to remove F in aqueous solution. © 2015 Elsevier Ltd. All rights reserved.
Keywords: MgeAl oxide Recycling Fluoride Removal Kinetics Equilibrium
1. Introduction MgeAl Layered double hydroxides (MgeAl LDHs) are typically 3þ n represented by the formula ½Mg2þ 1x Alx ðOHÞ2 (A )x/n$mH2O, where x is the Al3þ/(Mg2þ þ Al3þ) molar ratio (0.20 x 0.33), and An is anion, such as CO2 3 or Cl (Cavani et al., 1991; Ingram and Taylor, 1967; Allmann, 1968; Mills et al., 2012). MgeAl LDH intercalated with CO2 (CO3$MgeAl LDH) can be transformed into 3 MgeAl oxide by calcination at 450 Ce800 C, as expressed by Eq. (1): Mg1xAlx(OH)2(CO3)x/2 / Mg1xAlxO1þx/2 þ x/2CO2þH2O
(1)
MgeAl oxide can rehydrate and combine with anions to reconstruct the LDH structure: Mg1xAlxO1þx/2 þ x/n An þ (1 þ x/2)H2O / Mg1xAlx(OH)2Ax/n þ xOH
* Corresponding author. E-mail address: [email protected] (T. Kameda). http://dx.doi.org/10.1016/j.jenvman.2015.03.043 0301-4797/© 2015 Elsevier Ltd. All rights reserved.
(2)
MgeAl oxide has been examined for use in removing nitrate, Cr(VI), and orange II from aqueous solutions (Hosni and Srasra, 2008; Carriazo et al., 2007; Geraud et al., 2007). We found that MgeAl oxide is effective for the treatment of HCl, H2SO4, and HNO3 (Kameda et al., 2000, 2002, 2003, 2006, 2011, 2012). Fluoride-containing wastewater is produced by the electronics industry, glass industry, and etching processes. The effluent standard in Japan for F is 8 mg L1. In primary treatment, F is treated as slightly soluble CaF2 by adding calcium salts, such as Ca(OH)2 and CaCl2, to the wastewater. However, F remains present in the treated water, and therefore aluminum salts, such as polyaluminum chloride, are added as secondary treatment. Gelatinous aluminum hydroxide is produced and adsorbs F, which then co-precipitates and can be removed. However, this method requires two processes and is complicated. Furthermore, this method results in the generation of huge amounts of sludge caused by the addition of large amounts of aluminum salts and calcium salts. Therefore, a simple process that produces less sludge is required for treatment of fluoride-containing wastewater. Here, we propose a new treatment method for aqueous NaF using MgeAl oxide, as shown in Fig. 1. MgeAl oxide is used for the treatment of F-contaminated
T. Kameda et al. / Journal of Environmental Management 156 (2015) 252e256
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The removal of F from aqueous solution by MgeAl oxide results in the formation of MgeAl LDH with intercalation of F (F$MgeAl LDH). Desorption of F from the F$MgeAl LDH in Na2CO3 solution was examined. The theoretical equation is as follows: Mg0.67Al0.33(OH)2(F)0.33 þ 0.17CO2 3 / Mg0.67Al0.33(OH)2(CO3)0.17 þ 0.33F
Fig. 1. Proposal for treatment of aqueous NaF using MgeAl oxide.
wastewater to produce MgeAl LDH with intercalation of F (F$MgeAl LDH). The F$MgeAl LDH is treated with CO2 3 in aqueous solution. Due to anion exchange between F in MgeAl LDH and CO2 3 in aqueous solution, the CO3$MgeAl LDH is regenerated. The CO3$MgeAl LDH is calcined to obtain MgeAl oxide again, and the regenerated MgeAl oxide is reused for the treatment of aqueous NaF. MgeAl oxide has been shown to remove F from aqueous solution (Lv et al., 2006a,b; Yoshioka et al., 2007). However, these studies did not consider treatment after F removal. In this study, we systematically examined the removal of F by MgeAl oxide. The effects of the amount of MgeAl oxide and temperature were examined. In addition, kinetics and equilibrium studies were conducted, and the adsorption behavior was examined. Furthermore, 2 the effects of Cl, NO 3 , and SO4 on the removal of F were also examined. Desorption of F from the F$MgeAl LDH thus produced was investigated using Na2CO3 solution, and the reproduced CO3$MgeAl LDH was calcined to obtain MgeAl oxide. The ability of the regenerated MgeAl oxide to take up F in aqueous solution was examined. 2. Experimental All reagents were of chemical reagent grade and used without further purification. MgeAl oxide was obtained by the thermal decomposition of MgeAl LDH intercalated with CO2 3 (CO3$MgeAl LDH). The MgeAl oxide contained 13.6 wt% Mg and 7.8 wt% Al, and the Mg/Al molar ratio was 1.9. NaF solution was prepared by dissolving NaF in deionized water. MgeAl oxide was added to 100 mg/L NaF solution (500 mL), and the resultant suspensions were stirred at 10 Ce60 C for 840 min. The solutions were bubbled with N2 throughout the entire procedure. Samples of the suspension were withdrawn at different times and immediately filtered through 0.45-mm membrane filters. The filtrates were analyzed for residual F. The amount of MgeAl oxide used for removal of fluoride was 1e5 times the stoichiometric quantity indicated by Eq. (3), and is indicated using the notation Eqs. (1)e(5), respectively. Mg0.67Al0.33O1.17 þ 0.33F þ 1.17H2O / Mg0.67Al0.33(OH)2(F)0.33 þ 0.33OH
(4)
F$MgeAl LDH was prepared by suspension of MgeAl oxide in NaF solution at 30 C for 8 h at Eq. (1). F$MgeAl LDH contained 3.1 wt% of F; 0.1 g of F$MgeAl LDH and 20 mL of Na2CO3 solution (0.01e1 M) were placed in 50 mL screw-top tubes and shaken at 30 C for 120 min. The MgeAl oxide was regenerated from F$MgeAl LDH in 1 M Na2CO3 solution at 30 C for 120 min, followed by calcination at 500 C for 2 h. The regenerated MgeAl oxide was suspended in NaF solution at 30 C at Eq. (1). The materials were examined by X-ray diffraction (XRD) analysis with Cu Ka radiation. For the adsorption experiments, the residual concentrations of F and anions in the filtrates were determined using a Dionex DX-120 ion chromatograph and a Dionex model AS-12A column (eluent: 2.7 mM Na2CO3 and 0.3 mM NaHCO3; flow rate: 1.3 mL min1).
3. Results and discussion Fig. 2 shows the changes in F concentration over time in the MgeAl oxide suspension in NaF solution at various stoichiometric quantities (eqs) at 30 C. In all cases, the concentrations of F decreased with time and with increasing stoichiometric quantity. The lowest residual concentration of F was 6.3 mg/L at 480 min at Eq. (5), and was below the effluent standard in Japan (8 mg/L). Fig. S1 in Supplementary materials shows the changes in pH over time in MgeAl oxide suspension in NaF solution at various stoichiometric quantities (eqs) at 30 C. In all cases, the pH initially increased rapidly with time, increased gradually, and then remained constant at pH 10.5e11.5. These observations suggest that OH was released due to rehydration of MgeAl oxide. That is, the removal of F by MgeAl oxide was attributed to rehydration and combination with F according to Eq. (3). On the other hand, F could not be less removed by MgeAl oxide at Eq. (1), which was attributed to the intercalation of OH, as shown in Eq. (5).
(3)
To determine the adsorption isotherm of F adsorbed by MgeAl oxide, 20 mL of NaF solution (0.01e0.06 mol/L) and 0.1 g of MgeAl oxide were placed in 50-mL screw-top tubes and shaken at 30 C for 1 week. To examine the effects of coexisting anions on F removal, NaCl, NaNO3, or Na2SO4 was added to the NaF solution. The amounts of 2 Cl, NO were 0.5e10 times the molar equivalent 3 , and SO4 (mol.eq.) to that of F. In this case, a stoichiometric quantity (Eq. (1)) of MgeAl oxide was used for F in aqueous solution.
Fig. 2. Changes in the concentration of F over time in MgeAl oxide suspension in NaF solution at various stoichiometric quantities (eqs) at 30 C.
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Mg0.67Al0.33O1.67 þ 0.33OH þ 1.17H2O / Mg0.67Al0.33(OH)2(OH)0.33 þ 0.33OH
(5)
Fig. S2 in Supplementary materials shows the XRD patterns for the products. The XRD peaks for (b) product were assigned to hydrotalcite (JCPDS card 22e700), indicating that the LDH structure was formed after removal of F. This supports the reconstruction of MgeAl LDH from MgeAl oxide with the intercalation of F, as shown in Eq. (3). Fig. 3 shows the changes in amount of F removed over time by a suspension of MgeAl oxide in NaF solution at various temperatures. The removal of F increased with increasing temperature. Based on these results, we next examined the kinetics of F removal by MgeAl oxide. First, the data were arranged according to first-order kinetics, but none of the first-order plots for removal of F at various temperatures showed good linearity, indicating that F removal cannot be represented by first-order reaction kinetics. Pseudosecond-order kinetics were expressed as follows (Ho and McKay, 1999; Ho, 2006; Wu et al., 2009): dqt/dt ¼ k(qe qt)2
(6)
where qt (mmol g1) is the amount of F removed at reaction time t, qe (mmol g1) is the amount of F removed at equilibrium, and k (g mmol1 min1) is the rate constant for F removal. Integration of Eq. (6) gives:
. t=qt ¼ 1 kq2e þ t=qe:
Fig. 4. Pseudo-second-order plot of F removal by a suspension of MgeAl oxide in NaF solution at various temperatures. A stoichiometric quantity (Eq. (1)) of MgeAl oxide was used.
(7)
A pseudo-second-order reaction can predict adsorption behavior by assuming that the rate-determining step consists of chemical adsorption involving valence forces through the sharing or exchange of electrons between the adsorbent and adsorbate (Ho and McKay, 1999; Ho, 2006; Liang et al., 2005a,b; Kragovi c et al., 2013). Fig. 4 shows the pseudo-second-order plots for F removal by a suspension of MgeAl oxide in NaF solution at various temperatures. Plots at all temperatures showed good linearity, indicating that F removal can be represented by pseudo-second-order reaction kinetics. The apparent rate constants at 10 C, 30 C, and 60 C were 2.3 103, 2.2 102, and 2.5 101 g mmol1 min1, respectively. Thus, the apparent rate constant increased with increasing temperature. An Arrhenius plot of the rate constants (Fig. S3 in Supplementary materials) yielded an apparent activation
Fig. 3. Changes in the amount of F removed over time by a suspension of MgeAl oxide in NaF solution at various temperatures. A stoichiometric quantity (Eq. (1)) of MgeAl oxide was used.
energy of 73.3 kJ mol1 for F removal by MgeAl oxide. With chemical adsorption, the reaction rate is known to vary with temperature according to the finite activation energy (8.4e83.7 kJ mol1) in the Arrhenius equation (Kragovi c et al., 2013; Aksu, 2002; Zou et al., 2006). The apparent activation energy obtained here was within the range for chemical adsorption. Therefore, the removal of F was well described by a pseudo-secondorder reaction and the rate-determining step for F removal by MgeAl oxide was consistent with chemical adsorption involving intercalation of F into the reconstructed MgeAl LDH caused by electrostatic attraction. Fig. S4 in Supplementary materials shows the adsorption isotherms for adsorption of F by MgeAl oxide. The equilibrium adsorption amount increased with increasing equilibrium concentration. The adsorption isotherms showed Langmuir-type behavior, which was confirmed by arranging the experimental data according to the Langmuir equation, expressed as follows: qe ¼ CeqmKL/(1 þ CeKL)
(8)
Fig. 5. Ce/qe versus Ce plots for the adsorption isotherms for the adsorption of F by MgeAl oxide. MgeAl oxide quantity: 0.1 g; Initial F concentration: 0.01e0.06 M; temperature: 30 C; time: 1 week.
T. Kameda et al. / Journal of Environmental Management 156 (2015) 252e256
where qe (mmol g1) is the equilibrium adsorption, Ce (mM) is the equilibrium concentration, qm (mmol g1) is the maximum adsorption, and KL is the equilibrium adsorption constant. This equation can also be expressed as: Ce/qe ¼ 1/qmKL þ Ce/qm
(9)
Fig. 5 shows the Ce/qe versus Ce plots for the adsorption isotherms for adsorption of F by MgeAl oxide. Good linearity was obtained, indicating that this process followed Langmuir-type adsorption. The values of qm and KL, determined from the slope and intercept of the straight line in Fig. 5, were 3.0 mmol g1 and 1.1 103, respectively. Bhatnagar et al. examined the adsorption capacity of F (Bhatnagar et al., 2011). MgeAl oxide showed higher F removal capacity than both Schwertmannite and chitosan-coated silica (2.9 and 2.3 mmol g1, respectively). 2 Fig. 6 shows the effects of the presence of Cl, NO 3 , and SO4 on changes in F concentration over time in the MgeAl oxide suspension in NaF solution at 30 C. The concentrations of F for Cl 1 mol eq. and NO 3 1 mol.eq. were only higher maximum 10 mg/L than those without co-anion, and were around 65 mg/L after 480 min. Although the Cl and NO 3 concentrations increased from 1 to 10 mol eq., the F concentration decreased from 100 to around 70 mg/L after 480 min. These observations suggested that MgeAl oxide preferentially takes up F rather than Cl or NO 3 . This is probably due to the higher charge density of F than Cl or NO 3. The concentrations of F for SO2 4 0.5 and 5 mol.eq. were much higher than those without co-anion at all time points examined. Even for SO2 4 0.5 mol eq., the concentration of F did not decrease below 85 mg/L. The prevention of F removal by MgeAl oxide in the 2 presence of SO2 4 was attributed to the removal of SO4 . MgeAl 2 oxide showed preferential uptake of SO4 than F in solution. The charge density of SO2 4 is greater than that of F . Fig. S5 in Supplementary materials shows the changes in F desorption over time in F$MgeAl LDH in Na2CO3 solution at 30 C. The F desorption at all concentrations increased rapidly with time, and then remained constant (>90% at all concentrations after 120 min). The maximum F desorption of 96.1% was observed from F$MgeAl LDH in 1 M Na2CO3 solution for 120 min. The F in F$MgeAl LDH was found to be anion-exchanged with CO2 3 in solution according to Eq. (4). F was found to be effectively desorbed from F$MgeAl LDH using Na2CO3 solution. Fig. S6 in Supplementary materials shows the changes in
255
concentration of F over time in MgeAl oxide before and after the regeneration treatment in NaF solution at 30 C. Similar to the original MgeAl oxide, the concentration of F decreased rapidly with time, and then decreased gradually over time by regenerated MgeAl oxide. The concentrations of F were 57.7 and 65.1 mg/L before and after treatment at 480 min. Despite regeneration, the F removal performance of MgeAl oxide was almost completely retained. Fig. S2(c) in Supplementary materials shows the remaining LDH structure in CO3$MgeAl LDH regenerated after F desorption. In addition, the crystallinity of the regenerated MgeAl oxide (Fig. S2(d) in Supplementary materials) was almost the same as that of the original MgeAl oxide (Fig. S2(a) in Supplementary materials). Furthermore, the crystallinity of the regenerated F$MgeAl LDH (Fig. S2(e) in Supplementary materials) was almost as same as that of regenerated CO3$MgeAl LDH (Fig. S2(c) in Supplementary materials). The similar performance of MgeAl oxide for F removal after regeneration treatment was attributed to the maintenance of its crystallinity and the smaller amount of F remaining in the MgeAl oxide. The MgeAl oxide after regeneration treatment showed excellent removal of F in aqueous solution. 4. Conclusions MgeAl oxide was found to take up F from aqueous solution. F was removed by rehydration of MgeAl oxide accompanied with combination with F. The lowest residual concentration of F was 6.3 mg/L at 480 min at Eq. (5), and below the effluent standard in Japan (8 mg/L). F removal can be represented by pseudo-secondorder reaction kinetics. The apparent rate constants at 10 C, 30 C, and 60 C were 2.3 103, 2.2 102, and 2.5 101 g mmol1 min1, respectively. The apparent activation energy was 73.3 kJ mol1. The rate-determining step for F removal by MgeAl oxide was consistent with chemical adsorption involving intercalation of F into the regenerated MgeAl LDH caused by electrostatic attraction. The adsorption of F by MgeAl oxide follows Langmuir-type adsorption. The values of maximum adsorption and the equilibrium adsorption constant were 3.0 mmol g1 and 1.1 103, respectively, for MgeAl oxide. MgeAl oxide preferentially took up F rather than Cl or NO 3 . On the other hand, MgeAl oxide preferentially took up SO2 4 rather than F in solution. The F in the F$MgeAl LDH thus produced was anion-exchanged with CO2 3 in solution. The MgeAl oxide after regeneration treatment showed excellent removal of F in aqueous solution. In summary, this study clarified the possibility of use of MgeAl oxide in recycling for F removal as shown in Fig. 1. Acknowledgments This research was supported by the Environment Research and Technology Development Fund (5RFb-1201) of the Ministry of Environment, Japan. Appendix A. Supplementary data Supplementary data related to this article can be found at http:// dx.doi.org/10.1016/j.jenvman.2015.03.043. References
2 Fig. 6. Effects of the presence of Cl, NO 3 , and SO4 on changes in the concentration of F over time in MgeAl oxide suspension in NaF solution at 30 C. A stoichiometric quantity (Eq. (1)) of MgeAl oxide was used.
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Carriazo, D., Arco, M., Martin, C., Rives, V., 2007. A comparative study between chloride and calcined carbonate hydrotalcies as adsorbents for Cr(VI). Appl. Clay Sci. 37, 231e239. , F., Vaccari, A., 1991. Hydrotalcite-type anionic clays: preparation, Cavani, F., Trifiro properties and applications. Catal. Today 11, 173e301. Geraud, E., Bouhent, M., Derriche, Z., Leroux, F., Prevot, V., Forano, C., 2007. Texture effect of layered double hydroxides on chemisorption of orange II. J. Phys. Chem. Solids 68, 818e823. Ho, Y.S., McKay, G., 1999. Pseudo-second-order model for sorption processes. Process Biochem. 34, 451e465. Ho, Y.S., 2006. Review of second-order models for adsorption systems. J. Hazard. Mater. 136, 681e689. Hosni, K., Srasra, E., 2008. Nitrate adsorption from aqueous solution by MII-Al-CO3 layered double hydroxide. Inorg. Mater. 44, 742e749. €grenite and pyroaurite. Ingram, L., Taylor, H.F.W., 1967. The crystal structures of sjo Mineral. Mag. 36, 465e479. Kameda, T., Miyano, Y., Yoshioka, T., Uchida, M., Okuwaki, A., 2000. New treatment methods for waste water containing chloride ion using magnesiumaluminum oxide. Chem. Lett. 29, 1136e1137. Kameda, T., Yoshioka, T., Uchida, M., Miyano, Y., Okuwaki, A., 2002. New treatment method for dilute hydrochloric acid using magnesium-aluminum oxide. Bull. Chem. Soc. Jpn. 75, 595e599. Kameda, T., Yabuuchi, F., Yoshioka, T., Uchida, M., Okuwaki, A., 2003. New method of treating dilute mineral acids using magnesiumaluminum oxide. Water Res. 37, 1545e1550. Kameda, T., Yoshioka, T., Hoshi, T., Uchida, M., Okuwaki, A., 2006. Treatment of hydrochloric acid with magnesiumaluminum oxide at ambient temperatures. Sep. Purif. Technol. 51, 272e276. Kameda, T., Fubasami, Y., Yoshioka, T., 2011. Kinetics and equilibrium studies on the treatment of nitric acid with MgAl oxide obtained by thermal decomposition of NO-3-intercalated MgAl layered double hydroxide. J. Colloid Inter. Sci. 362,
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Contents lists available at ScienceDirect
Journal of Environmental Management journal homepage: www.elsevier.com/locate/jenvman
Kinetics and equilibrium studies on MgeAl oxide for removal of fluoride in aqueous solution and its use in recycling Tomohito Kameda*, Jumpei Oba, Toshiaki Yoshioka Graduate School of Environmental Studies, Tohoku University, 6-6-07 Aoba, Aramaki, Aoba-ku, Sendai 980-8579, Japan
a r t i c l e i n f o
a b s t r a c t
Article history: Received 26 February 2015 Received in revised form 24 March 2015 Accepted 25 March 2015 Available online
MgeAl oxide obtained by the thermal decomposition of MgeAl layered double hydroxide (LDH) intercalated with CO2 3 (CO3$MgeAl LDH) was found to take up fluoride from aqueous solution. Fluoride was removed by rehydration of MgeAl oxide accompanied by combination with F. Using five times the stoichiometric quantity of MgeAl oxide, the residual concentration of F was decreased from 100 to 6.3 mg/L in 480 min, which was below the effluent standard in Japan (8 mg/L). Removal of F can be represented by pseudo-second-order reaction kinetics. The apparent rate constants at 10 C, 30 C, and 60 C were 2.3 103, 2.2 102, and 2.5 101 g mmol1 min1, respectively. The apparent activation energy was 73.3 kJ mol1. The rate-determining step for F removal by MgeAl oxide was consistent with chemical adsorption involving intercalation of F into the reconstructed MgeAl LDH due to electrostatic attraction. The adsorption of F by MgeAl oxide follows a Langmuir-type adsorption. The values of the maximum adsorption and the equilibrium adsorption constant were 3.0 mmol g1 and 1.1 103, respectively, for MgeAl oxide. The F in the F$MgeAl LDH thus produced was found to be anionexchanged with CO2 3 in solution. The MgeAl oxide after regeneration treatment had excellent properties for removal of F in aqueous solution. In conclusion, the results of this study indicated that MgeAl oxide has potential for use in recycling to remove F in aqueous solution. © 2015 Elsevier Ltd. All rights reserved.
Keywords: MgeAl oxide Recycling Fluoride Removal Kinetics Equilibrium
1. Introduction MgeAl Layered double hydroxides (MgeAl LDHs) are typically 3þ n represented by the formula ½Mg2þ 1x Alx ðOHÞ2 (A )x/n$mH2O, where x is the Al3þ/(Mg2þ þ Al3þ) molar ratio (0.20 x 0.33), and An is anion, such as CO2 3 or Cl (Cavani et al., 1991; Ingram and Taylor, 1967; Allmann, 1968; Mills et al., 2012). MgeAl LDH intercalated with CO2 (CO3$MgeAl LDH) can be transformed into 3 MgeAl oxide by calcination at 450 Ce800 C, as expressed by Eq. (1): Mg1xAlx(OH)2(CO3)x/2 / Mg1xAlxO1þx/2 þ x/2CO2þH2O
(1)
MgeAl oxide can rehydrate and combine with anions to reconstruct the LDH structure: Mg1xAlxO1þx/2 þ x/n An þ (1 þ x/2)H2O / Mg1xAlx(OH)2Ax/n þ xOH
* Corresponding author. E-mail address: [email protected] (T. Kameda). http://dx.doi.org/10.1016/j.jenvman.2015.03.043 0301-4797/© 2015 Elsevier Ltd. All rights reserved.
(2)
MgeAl oxide has been examined for use in removing nitrate, Cr(VI), and orange II from aqueous solutions (Hosni and Srasra, 2008; Carriazo et al., 2007; Geraud et al., 2007). We found that MgeAl oxide is effective for the treatment of HCl, H2SO4, and HNO3 (Kameda et al., 2000, 2002, 2003, 2006, 2011, 2012). Fluoride-containing wastewater is produced by the electronics industry, glass industry, and etching processes. The effluent standard in Japan for F is 8 mg L1. In primary treatment, F is treated as slightly soluble CaF2 by adding calcium salts, such as Ca(OH)2 and CaCl2, to the wastewater. However, F remains present in the treated water, and therefore aluminum salts, such as polyaluminum chloride, are added as secondary treatment. Gelatinous aluminum hydroxide is produced and adsorbs F, which then co-precipitates and can be removed. However, this method requires two processes and is complicated. Furthermore, this method results in the generation of huge amounts of sludge caused by the addition of large amounts of aluminum salts and calcium salts. Therefore, a simple process that produces less sludge is required for treatment of fluoride-containing wastewater. Here, we propose a new treatment method for aqueous NaF using MgeAl oxide, as shown in Fig. 1. MgeAl oxide is used for the treatment of F-contaminated
T. Kameda et al. / Journal of Environmental Management 156 (2015) 252e256
253
The removal of F from aqueous solution by MgeAl oxide results in the formation of MgeAl LDH with intercalation of F (F$MgeAl LDH). Desorption of F from the F$MgeAl LDH in Na2CO3 solution was examined. The theoretical equation is as follows: Mg0.67Al0.33(OH)2(F)0.33 þ 0.17CO2 3 / Mg0.67Al0.33(OH)2(CO3)0.17 þ 0.33F
Fig. 1. Proposal for treatment of aqueous NaF using MgeAl oxide.
wastewater to produce MgeAl LDH with intercalation of F (F$MgeAl LDH). The F$MgeAl LDH is treated with CO2 3 in aqueous solution. Due to anion exchange between F in MgeAl LDH and CO2 3 in aqueous solution, the CO3$MgeAl LDH is regenerated. The CO3$MgeAl LDH is calcined to obtain MgeAl oxide again, and the regenerated MgeAl oxide is reused for the treatment of aqueous NaF. MgeAl oxide has been shown to remove F from aqueous solution (Lv et al., 2006a,b; Yoshioka et al., 2007). However, these studies did not consider treatment after F removal. In this study, we systematically examined the removal of F by MgeAl oxide. The effects of the amount of MgeAl oxide and temperature were examined. In addition, kinetics and equilibrium studies were conducted, and the adsorption behavior was examined. Furthermore, 2 the effects of Cl, NO 3 , and SO4 on the removal of F were also examined. Desorption of F from the F$MgeAl LDH thus produced was investigated using Na2CO3 solution, and the reproduced CO3$MgeAl LDH was calcined to obtain MgeAl oxide. The ability of the regenerated MgeAl oxide to take up F in aqueous solution was examined. 2. Experimental All reagents were of chemical reagent grade and used without further purification. MgeAl oxide was obtained by the thermal decomposition of MgeAl LDH intercalated with CO2 3 (CO3$MgeAl LDH). The MgeAl oxide contained 13.6 wt% Mg and 7.8 wt% Al, and the Mg/Al molar ratio was 1.9. NaF solution was prepared by dissolving NaF in deionized water. MgeAl oxide was added to 100 mg/L NaF solution (500 mL), and the resultant suspensions were stirred at 10 Ce60 C for 840 min. The solutions were bubbled with N2 throughout the entire procedure. Samples of the suspension were withdrawn at different times and immediately filtered through 0.45-mm membrane filters. The filtrates were analyzed for residual F. The amount of MgeAl oxide used for removal of fluoride was 1e5 times the stoichiometric quantity indicated by Eq. (3), and is indicated using the notation Eqs. (1)e(5), respectively. Mg0.67Al0.33O1.17 þ 0.33F þ 1.17H2O / Mg0.67Al0.33(OH)2(F)0.33 þ 0.33OH
(4)
F$MgeAl LDH was prepared by suspension of MgeAl oxide in NaF solution at 30 C for 8 h at Eq. (1). F$MgeAl LDH contained 3.1 wt% of F; 0.1 g of F$MgeAl LDH and 20 mL of Na2CO3 solution (0.01e1 M) were placed in 50 mL screw-top tubes and shaken at 30 C for 120 min. The MgeAl oxide was regenerated from F$MgeAl LDH in 1 M Na2CO3 solution at 30 C for 120 min, followed by calcination at 500 C for 2 h. The regenerated MgeAl oxide was suspended in NaF solution at 30 C at Eq. (1). The materials were examined by X-ray diffraction (XRD) analysis with Cu Ka radiation. For the adsorption experiments, the residual concentrations of F and anions in the filtrates were determined using a Dionex DX-120 ion chromatograph and a Dionex model AS-12A column (eluent: 2.7 mM Na2CO3 and 0.3 mM NaHCO3; flow rate: 1.3 mL min1).
3. Results and discussion Fig. 2 shows the changes in F concentration over time in the MgeAl oxide suspension in NaF solution at various stoichiometric quantities (eqs) at 30 C. In all cases, the concentrations of F decreased with time and with increasing stoichiometric quantity. The lowest residual concentration of F was 6.3 mg/L at 480 min at Eq. (5), and was below the effluent standard in Japan (8 mg/L). Fig. S1 in Supplementary materials shows the changes in pH over time in MgeAl oxide suspension in NaF solution at various stoichiometric quantities (eqs) at 30 C. In all cases, the pH initially increased rapidly with time, increased gradually, and then remained constant at pH 10.5e11.5. These observations suggest that OH was released due to rehydration of MgeAl oxide. That is, the removal of F by MgeAl oxide was attributed to rehydration and combination with F according to Eq. (3). On the other hand, F could not be less removed by MgeAl oxide at Eq. (1), which was attributed to the intercalation of OH, as shown in Eq. (5).
(3)
To determine the adsorption isotherm of F adsorbed by MgeAl oxide, 20 mL of NaF solution (0.01e0.06 mol/L) and 0.1 g of MgeAl oxide were placed in 50-mL screw-top tubes and shaken at 30 C for 1 week. To examine the effects of coexisting anions on F removal, NaCl, NaNO3, or Na2SO4 was added to the NaF solution. The amounts of 2 Cl, NO were 0.5e10 times the molar equivalent 3 , and SO4 (mol.eq.) to that of F. In this case, a stoichiometric quantity (Eq. (1)) of MgeAl oxide was used for F in aqueous solution.
Fig. 2. Changes in the concentration of F over time in MgeAl oxide suspension in NaF solution at various stoichiometric quantities (eqs) at 30 C.
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Mg0.67Al0.33O1.67 þ 0.33OH þ 1.17H2O / Mg0.67Al0.33(OH)2(OH)0.33 þ 0.33OH
(5)
Fig. S2 in Supplementary materials shows the XRD patterns for the products. The XRD peaks for (b) product were assigned to hydrotalcite (JCPDS card 22e700), indicating that the LDH structure was formed after removal of F. This supports the reconstruction of MgeAl LDH from MgeAl oxide with the intercalation of F, as shown in Eq. (3). Fig. 3 shows the changes in amount of F removed over time by a suspension of MgeAl oxide in NaF solution at various temperatures. The removal of F increased with increasing temperature. Based on these results, we next examined the kinetics of F removal by MgeAl oxide. First, the data were arranged according to first-order kinetics, but none of the first-order plots for removal of F at various temperatures showed good linearity, indicating that F removal cannot be represented by first-order reaction kinetics. Pseudosecond-order kinetics were expressed as follows (Ho and McKay, 1999; Ho, 2006; Wu et al., 2009): dqt/dt ¼ k(qe qt)2
(6)
where qt (mmol g1) is the amount of F removed at reaction time t, qe (mmol g1) is the amount of F removed at equilibrium, and k (g mmol1 min1) is the rate constant for F removal. Integration of Eq. (6) gives:
. t=qt ¼ 1 kq2e þ t=qe:
Fig. 4. Pseudo-second-order plot of F removal by a suspension of MgeAl oxide in NaF solution at various temperatures. A stoichiometric quantity (Eq. (1)) of MgeAl oxide was used.
(7)
A pseudo-second-order reaction can predict adsorption behavior by assuming that the rate-determining step consists of chemical adsorption involving valence forces through the sharing or exchange of electrons between the adsorbent and adsorbate (Ho and McKay, 1999; Ho, 2006; Liang et al., 2005a,b; Kragovi c et al., 2013). Fig. 4 shows the pseudo-second-order plots for F removal by a suspension of MgeAl oxide in NaF solution at various temperatures. Plots at all temperatures showed good linearity, indicating that F removal can be represented by pseudo-second-order reaction kinetics. The apparent rate constants at 10 C, 30 C, and 60 C were 2.3 103, 2.2 102, and 2.5 101 g mmol1 min1, respectively. Thus, the apparent rate constant increased with increasing temperature. An Arrhenius plot of the rate constants (Fig. S3 in Supplementary materials) yielded an apparent activation
Fig. 3. Changes in the amount of F removed over time by a suspension of MgeAl oxide in NaF solution at various temperatures. A stoichiometric quantity (Eq. (1)) of MgeAl oxide was used.
energy of 73.3 kJ mol1 for F removal by MgeAl oxide. With chemical adsorption, the reaction rate is known to vary with temperature according to the finite activation energy (8.4e83.7 kJ mol1) in the Arrhenius equation (Kragovi c et al., 2013; Aksu, 2002; Zou et al., 2006). The apparent activation energy obtained here was within the range for chemical adsorption. Therefore, the removal of F was well described by a pseudo-secondorder reaction and the rate-determining step for F removal by MgeAl oxide was consistent with chemical adsorption involving intercalation of F into the reconstructed MgeAl LDH caused by electrostatic attraction. Fig. S4 in Supplementary materials shows the adsorption isotherms for adsorption of F by MgeAl oxide. The equilibrium adsorption amount increased with increasing equilibrium concentration. The adsorption isotherms showed Langmuir-type behavior, which was confirmed by arranging the experimental data according to the Langmuir equation, expressed as follows: qe ¼ CeqmKL/(1 þ CeKL)
(8)
Fig. 5. Ce/qe versus Ce plots for the adsorption isotherms for the adsorption of F by MgeAl oxide. MgeAl oxide quantity: 0.1 g; Initial F concentration: 0.01e0.06 M; temperature: 30 C; time: 1 week.
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where qe (mmol g1) is the equilibrium adsorption, Ce (mM) is the equilibrium concentration, qm (mmol g1) is the maximum adsorption, and KL is the equilibrium adsorption constant. This equation can also be expressed as: Ce/qe ¼ 1/qmKL þ Ce/qm
(9)
Fig. 5 shows the Ce/qe versus Ce plots for the adsorption isotherms for adsorption of F by MgeAl oxide. Good linearity was obtained, indicating that this process followed Langmuir-type adsorption. The values of qm and KL, determined from the slope and intercept of the straight line in Fig. 5, were 3.0 mmol g1 and 1.1 103, respectively. Bhatnagar et al. examined the adsorption capacity of F (Bhatnagar et al., 2011). MgeAl oxide showed higher F removal capacity than both Schwertmannite and chitosan-coated silica (2.9 and 2.3 mmol g1, respectively). 2 Fig. 6 shows the effects of the presence of Cl, NO 3 , and SO4 on changes in F concentration over time in the MgeAl oxide suspension in NaF solution at 30 C. The concentrations of F for Cl 1 mol eq. and NO 3 1 mol.eq. were only higher maximum 10 mg/L than those without co-anion, and were around 65 mg/L after 480 min. Although the Cl and NO 3 concentrations increased from 1 to 10 mol eq., the F concentration decreased from 100 to around 70 mg/L after 480 min. These observations suggested that MgeAl oxide preferentially takes up F rather than Cl or NO 3 . This is probably due to the higher charge density of F than Cl or NO 3. The concentrations of F for SO2 4 0.5 and 5 mol.eq. were much higher than those without co-anion at all time points examined. Even for SO2 4 0.5 mol eq., the concentration of F did not decrease below 85 mg/L. The prevention of F removal by MgeAl oxide in the 2 presence of SO2 4 was attributed to the removal of SO4 . MgeAl 2 oxide showed preferential uptake of SO4 than F in solution. The charge density of SO2 4 is greater than that of F . Fig. S5 in Supplementary materials shows the changes in F desorption over time in F$MgeAl LDH in Na2CO3 solution at 30 C. The F desorption at all concentrations increased rapidly with time, and then remained constant (>90% at all concentrations after 120 min). The maximum F desorption of 96.1% was observed from F$MgeAl LDH in 1 M Na2CO3 solution for 120 min. The F in F$MgeAl LDH was found to be anion-exchanged with CO2 3 in solution according to Eq. (4). F was found to be effectively desorbed from F$MgeAl LDH using Na2CO3 solution. Fig. S6 in Supplementary materials shows the changes in
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concentration of F over time in MgeAl oxide before and after the regeneration treatment in NaF solution at 30 C. Similar to the original MgeAl oxide, the concentration of F decreased rapidly with time, and then decreased gradually over time by regenerated MgeAl oxide. The concentrations of F were 57.7 and 65.1 mg/L before and after treatment at 480 min. Despite regeneration, the F removal performance of MgeAl oxide was almost completely retained. Fig. S2(c) in Supplementary materials shows the remaining LDH structure in CO3$MgeAl LDH regenerated after F desorption. In addition, the crystallinity of the regenerated MgeAl oxide (Fig. S2(d) in Supplementary materials) was almost the same as that of the original MgeAl oxide (Fig. S2(a) in Supplementary materials). Furthermore, the crystallinity of the regenerated F$MgeAl LDH (Fig. S2(e) in Supplementary materials) was almost as same as that of regenerated CO3$MgeAl LDH (Fig. S2(c) in Supplementary materials). The similar performance of MgeAl oxide for F removal after regeneration treatment was attributed to the maintenance of its crystallinity and the smaller amount of F remaining in the MgeAl oxide. The MgeAl oxide after regeneration treatment showed excellent removal of F in aqueous solution. 4. Conclusions MgeAl oxide was found to take up F from aqueous solution. F was removed by rehydration of MgeAl oxide accompanied with combination with F. The lowest residual concentration of F was 6.3 mg/L at 480 min at Eq. (5), and below the effluent standard in Japan (8 mg/L). F removal can be represented by pseudo-secondorder reaction kinetics. The apparent rate constants at 10 C, 30 C, and 60 C were 2.3 103, 2.2 102, and 2.5 101 g mmol1 min1, respectively. The apparent activation energy was 73.3 kJ mol1. The rate-determining step for F removal by MgeAl oxide was consistent with chemical adsorption involving intercalation of F into the regenerated MgeAl LDH caused by electrostatic attraction. The adsorption of F by MgeAl oxide follows Langmuir-type adsorption. The values of maximum adsorption and the equilibrium adsorption constant were 3.0 mmol g1 and 1.1 103, respectively, for MgeAl oxide. MgeAl oxide preferentially took up F rather than Cl or NO 3 . On the other hand, MgeAl oxide preferentially took up SO2 4 rather than F in solution. The F in the F$MgeAl LDH thus produced was anion-exchanged with CO2 3 in solution. The MgeAl oxide after regeneration treatment showed excellent removal of F in aqueous solution. In summary, this study clarified the possibility of use of MgeAl oxide in recycling for F removal as shown in Fig. 1. Acknowledgments This research was supported by the Environment Research and Technology Development Fund (5RFb-1201) of the Ministry of Environment, Japan. Appendix A. Supplementary data Supplementary data related to this article can be found at http:// dx.doi.org/10.1016/j.jenvman.2015.03.043. References
2 Fig. 6. Effects of the presence of Cl, NO 3 , and SO4 on changes in the concentration of F over time in MgeAl oxide suspension in NaF solution at 30 C. A stoichiometric quantity (Eq. (1)) of MgeAl oxide was used.
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