Capture of cationic metal ions from aqueous solutions by layered double hydroxides intercalated with organic acid anions

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1 Capture of cationic metal ions from aqueous solutions by layered double hydroxides intercalated with organic acid anions TOMOHITO KAMEDA,* TETSU SHINMYOU, TOSHIAKI YOSHIOKA Graduate School of Environmental Studies Tohoku University Aoba, Aramaki, Aoba-ku, Sendai JAPAN Abstract: - Layered double hydroxides (LDHs) have anion-exchange capabilities and have been used for the preservation of aqueous environments. While typical LDHs cannot take up cationic metals from aqueous solutions, LDHs intercalated with organic acid anions can take up cationic metal ions from aqueous solutions. In this paper, we have demonstrated the uptake of metal ions such as Nd 3+ and Sr 2+ in the cationic form from aqueous solutions using Cu-Al LDHs intercalated with triethylenetetramine-hexaacetic acid. Key-Words: - layered double hydroxides; intercalation; chelating agent; complex; triethylenetetraminehexaacetic acid; capture technology; cationic metal ions 1 Introduction Layered double hydroxides (LDHs) are capable of anion exchange and have therefore, attracted attention as promising functional materials for a number of applications, including water purification [1]. LDHs are typically represented by the generic formula [M 2+ 1 xm 3+ x(oh) 2 ](A n- ) x/n mh 2 O, where M 2+ and M 3+ are divalent and trivalent metal ions, respectively, x is the M 3+ /(M 2+ + M 3+ ) molar ratio (0.20 x 0.33), and A n- is an anion, such as CO 3 2- or Cl - [2,3]. An LDH consists of a stack of brucitelike octahedral layers, in which some M 2+ moieties are replaced with M 3+. The positive charge of the layer arising from this substitution is neutralized by interlayer anions [4], and the interlayer space is occupied by water molecules belonging to the hydration shell of these anions. While the typical LDHs can take up various types of anionic metals from aqueous solutions, owing to their anionexchange capabilities, they cannot remove cationic metals from aqueous solutions. Therefore, cationic clays and chelate resins are usually used for cationic metal uptake. However, LDHs intercalated with organic acid anions can take up cationic metals from aqueous solutions [5]. Ethylenediamine tetraacetate (EDTA) anions can be intercalated in the LDH interlayers [6]. Similarly, stable anionic chelates formed between metal ions and aminocarboxylates such as EDTA can also be intercalated in the LDH interlayers. For example, Ni(EDTA) 2, Co(EDTA) 2, Cu(EDTA) 2, Zn(EDTA) 2, and Cd(EDTA) 2 intercalations have been examined [6-11]. Previously, we developed for the first time a scavenger by intercalating an EDTA anion in the interlayers of LDH and utilizing the scavenger for decreasing the concentrations of heavy metal ions in aqueous solutions [12]. In order for the scavenger to take up heavy metal ions from aqueous solutions, the EDTA anion should function as a chelating agent in the interlayers of LDH. We have also found that the Mg Al LDHs intercalated with citrate, malate, and tartrate are able to take up heavy metal ions such as Cu 2+ and Cd 2+ ions from aqueous solutions [13-15]. The uptake was shown to occur by chelation between the heavy metal ions and organic acid anions. The rate equation for the heavy metal ion uptake was found to be dependent on the type of chelate formed. The reaction proceeded at a certain rate, which was different for each combination of heavy metal ion and organic acid anion. The Ni Al LDH intercalated with citrate was also able to take up Cu 2+ from aqueous solutions [16]. In addition, we also clarified the uptake of Sc 3+, Y 3+, and La 3+ from aqueous solutions using EDTA-intercalated Cu Al LDHs reconstructed from Cu Al oxide [17,18]. The EDTA Cu Al LDH was found to selectively take up rare earth ions from mixed solutions. The degree of uptake was high, in the order of Sc 3+ > Y 3+ > La 3+ for all the time durations considered, which was attributed to the differences in the stabilities of Sc(EDTA), Y(EDTA), and La(EDTA). In addition, we developed a method for the preparation of Cu Al LDH intercalated with EDTA by co- ISBN:

2 precipitation, and investigated its uptake of rare earth ions from aqueous solutions [19]. In addition, we also realized the desorption of rare earth ions from EDTA-intercalated Cu-Al LDH using Fe 3+ [20]. In other words, we demonstrated that the organic-modified LDHs that have previously taken up metal ions could be recovered in solution. Further, we conducted kinetics and equilibrium studies on the uptake of rare earth ions from aqueous solutions using a Cu Al LDH intercalated with EDTA [21] and found that the uptake is more adequately described by the mass-transfercontrolled shrinking core model than by the surfacereaction-controlled model. This reaction can also be expressed by Langmuir-type adsorption, suggesting the formation of a chelate complex between rare earth and EDTA ions in the EDTA Cu-Al LDH interlayer. Recently, we developed a method for the preparation of Zn Al LDH intercalated with triethylenetetramine-hexaacetic acid (TTHA) by coprecipitation, and investigated its uptake of rareearth metal ions such as Nd 3+ and Sr 2+ from aqueous solutions [22]. TTHA Zn Al LDH was found to be superior to EDTA Zn Al LDH for the uptake of Nd 3+, which can be attributed to the higher stability of the Nd-TTHA complex compared to the Nd- EDTA complex. Continuing along the above lines, in this paper, TTHA Cu Al LDH has been prepared by coprecipitation, and its uptake of Nd 3+ and Sr 2+ from aqueous solutions has been investigated. The aim of this study is to examine the influence of the M 2+ ion in the host layer in the LDH on the uptake of metal ions from aqueous solutions. In particular, we aim to understand whether Zn 2+ or Cu 2+ is more suited for the uptake of Nd 3+ and Sr 2+ from aqueous solutions. 2 Experimental All the reagents were of chemical reagent grade and used as-received without further purification. 2.1 Preparation TTHA Cu Al LDH was prepared by the drop-wise addition of a Cu Al nitrate solution to a TTHA (C 18 H 30 N 4 O 12 ) solution at a constant ph of 8.0. At this ph, the stable anionic species of TTHA was C 18 H 26 N 4 O 4 12 [23]. Therefore, the theoretical formula for TTHA Cu Al LDH may be written as Cu 0.67 Al 0.33 (OH) 2 (C 18 H 26 N 4 O 12 ) The co-precipitation reactions are expressed by Eq. (1), in which the stoichiometric coefficient of the C 18 H 26 N 4 O 4 12 ion was calculated using the neutralization of the excess positive charge in the Al-bearing brucite-like octahedral layers by the replacement of Cu 2+ ions with Al 3+ ions at a Zn/Al molar ratio of 2.0 as follows: 0.67 Cu Al C 18 H 26 N 4 O OH Cu 0.67 Al 0.33 (OH) 2 (C 18 H 26 N 4 O 12 ) (1) Cu Al solutions ([Cu 2+ ] + [Al 3+ ] = 0.5 mol/l) with Cu/Al molar ratios of 2.0 were prepared by dissolving the required amount of Cu(NO 3 ) 2 3H 2 O and Al(NO 3 ) 3 9H 2 O in 250 ml of deionized water. The TTHA solution was prepared by dissolving twice the stoichiometric amounts of C 18 H 30 N 4 O 12, as defined by Eq. (1) in 250 ml of deionized water. The Cu Al solution was added drop-wise to the TTHA solution at a rate of 10 ml/min at 30 C with mild agitation. The resulting solution was adjusted to ph 8.0 by adding 1.25 mol/l NaOH solution until the desired ph was attained. The resulting suspensions were maintained at 30 C for 1 h at a constant ph of 8.0. TTHA Cu Al LDH particles were obtained by filtering the resulting suspension, washing repeatedly with deionized water until neutral ph was achieved, and drying under reduced pressure (133 Pa) for 40 h. N 2 gas was bubbled into the solution throughout the operation to minimize the effect of dissolved CO 2. NO 3 Cu Al LDH was also prepared by co-precipitation in a similar manner. The co-precipitation reactions are expressed by Eq. (2): 0.67 Cu Al NO OH Cu 0.67 Al 0.33 (OH) 2 (NO 3 ) (2) 2.2 Uptake of Nd 3+ and Sr 2+ from aqueous solutions The TTHA Cu Al LDH prepared using the above procedure was added to 500 ml of 1.0 mmol/l Nd(NO 3 ) 3 or Sr(NO 3 ) 2 solution, and the resulting suspension was maintained at 30 C for 120 min under 300 rpm stirring. N 2 was bubbled into the solution throughout the operation. Samples of the suspension were extracted at different time intervals and immediately filtered through a 0.45 μm membrane filter. The filtrates were then analyzed for the target metal ions. The molar ratios of TTHA in TTHA Cu Al LDH to the Nd 3+ and Sr 2+ ions in the nitrate solutions were set at 1 and 5. To demonstrate the effect of the interlayer anion, NO 3 Cu Al LDH was also used as a reference material. 2.3 Characterization methods The TTHA Cu Al LDH and NO 3 Cu Al LDH samples were analyzed by X-ray ISBN:

3 diffraction (XRD) using Cu Kα radiation. Furthermore, the materials were dissolved in 1 mol/l HNO 3 and analyzed for Cu 2+ and Al 3+ ions by inductively coupled plasma-atomic emission spectrometry (ICP-AES). In addition, the materials were also dissolved in 1 mol/l HNO 3 and analyzed for TTHA based on the total organic carbon (TOC) content. For the adsorption experiments, the residual concentrations of Nd 3+ and Sr 2+ ions in the filtrates were determined by ICP-AES. 3 Results and discussion 3.1 Preparation Fig. 1(a) and (b) show the XRD patterns for NO 3 Cu Al LDH and TTHA Cu Al LDH, respectively. The XRD peaks for NO 3 Cu Al LDH and TTHA Cu Al LDH can be attributed to copper aluminum carbonate hydroxide hydrate (JCPDS card No ), which has the formula Cu 6 Al 2 (OH) 16 CO 3 4H 2 O with an LDH structure, although the peaks observed by us were broader. This result indicates that NO 3 Cu Al LDH and TTHA Cu Al LDH have an LDH structure. For NO 3 Cu Al LDH, the observed basal spacing, d 003, was 8.7 Å, with an LDH host layer thickness of 4.8 Å and interlayer spacing of 3.9 Å. On the other hand, Fig. 1b shows that the TTHA Cu Al LDH has a basal spacing of 16.6 Å with interlayer spacing of 11.8 Å. In the case of TTHA Cu Al LDH, the intercalation of the TTHA ions, which are larger than the NO 3 ions, in the interlayer of Cu Al LDH, probably increased the basal spacing from 8.7 to 16.6 Å. Further, in our previous paper [22], the length of TTHA was calculated to be 12.1 Å, which is similar to the interlayer spacing derived from the XRD measurements. Therefore, the TTHA ion was most likely oriented vertically with respect to the brucite-like Cu Al LDH layers. Table 1 shows the chemical compositions of Relative intensity 003 d 003 =8.7 Å d 003 =16.6 Å Cu 6 Al 2 (OH) 16 CO 3 4H 2 O 1010 (a) (b) Table 1 Chemical compositions of NO 3 Cu Al LDH and TTHA Cu Al LDH. (a) NO 3 Cu Al LDH (b) TTHA Cu Al LDH wt% Cu Al TTHA Cu/Al molar ratio TTHA/Al NO 3 Cu Al LDH and TTHA Cu Al LDH. In the case of NO 3 Cu Al LDH, the Cu/Al molar ratio was close to the value calculated using Eq. (2), indicating that Cu 2+ and Al 3+ ions in the solutions were precipitated as Cu Al LDH. On the other hand, in the case of TTHA Cu Al LDH, the Cu/Al molar ratio was lower than the value calculated using Eq. (2), suggesting the formation of a complex between some Cu 2+ and TTHA ions. Owing to the strong stability of the complex, some Cu 2+ could not be precipitated as LDH, and remained in the solution. The TTHA/Al molar ratio was found to be 0.19, whereas the theoretical TTHA/Al molar ratio based on the charge balance with the Zn Al LDH was 0.25, as shown in Eq. (1). The lower TTHA/Al molar ratio compared to the theoretical value indicated that the C 18 H 26 N 4 O 4 12 ion was intercalated in the Cu Al LDH interlayer. Of course, [Cu-TTHA] complexes such as [Cu-C 18 H 25 N 4 O 12 ] 3 formed between some Cu 2+ and TTHA ions may also be intercalated in the Cu Al LDH interlayer. In summary, the co-precipitation method afforded Cu Al LDHs intercalated with TTHA ions. 3.2 Uptake of Nd 3+ and Sr 2+ ions from aqueous solutions Fig. 2 shows the change in Nd 3+ uptake as a function of time during the suspension of TTHA Cu Al and NO 3 Cu Al LDHs in the Nd(NO 3 ) 3 solution. The TTHA/Nd 3+ molar ratio is calculated as the molar ratio of TTHA in the added TTHA Cu Al LDH to the Nd 3+ ions in the Nd(NO 3 ) 3 solution. The amount of NO 3 Cu Al LDH was equal to that of TTHA Cu Al LDH at TTHA/Nd 3+ = 5. The Nd 3+ ion uptake increased with time for TTHA Cu Al LDHs. The Nd 3+ ion uptake increased with increasing TTHA/Nd 3+ molar ratios for all the durations considered, reaching 20.3% at TTHA/Nd 3+ = 5 at 120 min. In contrast, the Nd 3+ ion uptake was 5.2% for NO 3 Cu Al LDH after 120 min. This difference in the Nd 3+ ion uptake can be 4 attributed to the presence of the C 18 H 26 N 4 O 12 ion and [Cu-TTHA] complexes in the interlayer of the TTHA Cu Al LDH. [Nd-C 18 H 25 N 4 O 12 ] θ/deg.(CuKα) Fig.1 XRD patterns for (a) NO 3 Cu Al LDH and (b) TTHA Cu Al LDH. ISBN:

4 complex likely formed in the interlayer of TTHA Cu Al LDH, according to Eq. (3). 30 TTHA/Nd 3+ = 1 TTHA/Nd 3+ = 5 NO 3 Cu Al LDH* LDH was equal to that of TTHA Cu Al LDH at TTHA/Sr 2+ = 5. For all the Cu Al LDHs, the Sr 2+ ion 30 TTHA/Sr 2+ = 1 TTHA/Sr 2+ = 5 NO 3 Cu Al LDH* Nd 3+ uptake / % Sr 2+ uptake / % Time / min Fig.2 Variations in Nd 3+ uptake with time during the suspension of TTHA Cu Al LDH and NO 3 Cu Al LDH in the Nd(NO 3 ) 3 solution. The amounts of NO 3 Cu Al LDH and TTHA Cu Al LDH are equal at TTHA/Nd 3+ = 5. Nd 3+ + C 18 H 26 N 4 O 4 12 [Nd-C 18 H 25 N 4 O 12 ] 2 + H + (3) For a [Cu-TTHA] complex such as the [Cu- C 18 H 25 N 4 O 12 ] 3 complex, the [Nd- C 18 H 25 N 4 O 12 ] 2 complex was most likely formed in the interlayer of TTHA Cu Al LDH according to Eq. (4). Nd 3+ + [Cu-C 18 H 25 N 4 O 12 ] 3 [Nd-C 18 H 25 N 4 O 12 ] 2 + Cu 2+ (4) Therefore, the difference in the Nd 3+ uptake behavior may be caused by the difference in the stabilities of the [Nd-C 18 H 25 N 4 O 12 ] 2 and [Cu- C 18 H 25 N 4 O 12 ] 3 complexes, as evidenced by the metal-chelate formation constants for the [Nd- C 18 H 25 N 4 O 12 ] 2 and [Cu-C 18 H 25 N 4 O 12 ] 3 complexes, which were 22.8 and 19.2, respectively [24,25]. The values of the formation constants suggest that the [Nd- C 18 H 25 N 4 O 12 ] 2 complex was more stable than the [Cu-C 18 H 25 N 4 O 12 ] 3 complex. Fig. 2 also shows that the Nd 3+ ion uptake for NO 3 Cu Al LDH was 5.2% after 120 min, probably due to the precipitation of Nd 3+ ions as Nd(OH) 3 caused by the increase in ph as a result of the addition of the LDH. However, this value is low. Although a small amount of Nd 3+ may be precipitated as Nd(OH) 3 when TTHA Cu Al LDH is used, TTHA Cu Al LDH is capable of taking up almost all of the Nd 3+ ions from aqueous solutions in the cationic form. Fig. 3 shows the changes in the Sr 2+ uptake as a function of time during the suspension of TTHA Cu Al and NO 3 Cu Al LDH in the Sr(NO 3 ) 2 solution. The TTHA/Sr 2+ ratio is calculated as the molar ratio of TTHA in the added TTHA Cu Al LDH to the Sr 2+ ions in the Sr(NO 3 ) 2 solution. The amount of NO 3 Cu Al Time / min Fig.3 Variations in Sr 2+ uptake with time during the suspension of TTHA Cu Al LDH and NO 3 Cu Al LDH in the Sr(NO 3 ) 2 solution. The amounts of NO 3 Cu Al LDH and TTHA Cu Al LDH are equal at TTHA/Sr 2+ = 5. uptake increased slowly with time, reaching <10% after 120 min. The Cu Al LDHs were barely able to take up the Sr 2+ ions from the solutions, and the uptake behaviors of TTHA Cu Al LDH and NO 3 Cu Al LDH were 4 similar. This indicates that the C 18 H 26 N 4 O 12 ion and the [Cu-TTHA] complex such as the [Cu-C 18 H 25 N 4 O 12 ] 3 complex in the interlayer of the TTHA Cu Al LDH did not function as chelating agents. If they reacted with the Sr 2+ ions, the [Sr-C 18 H 25 N 4 O 12 ] 3 complex would be formed in the interlayer of the TTHA Cu Al LDH. Further, the metal chelate formation constant for [Sr-C 18 H 25 N 4 O 12 ] 3 was 9.3 [24,25], indicating that the [Sr-C 18 H 25 N 4 O 12 ] 3 complex was less stable than the [Cu-C 18 H 25 N 4 O 12 ] 3 complex. Owing to the low stability of the [Sr- C 18 H 25 N 4 O 12 ] 3 complex, TTHA Zn Al LDH was unable to form a metal-chelate complex in the interlayer. In particular, Sr 2+ ions were unable to exchange with the Cu 2+ ions in the [Cu-C 18 H 25 N 4 O 12 ] 3 complex (formation constant value of 19.2, as stated previously). As mentioned in our recent paper [22], the Sr 2+ ion uptake by TTHA Cu Al LDHs and NO 3 Zn Al LDH can be attributed to the co-precipitation of the Sr 2+ ions and Al hydrolysate derived from LDH. Finally, we compare TTHA Zn Al LDH and TTHA Cu Al LDH in terms of the uptake of Nd 3+ ions. In the case of TTHA Zn Al LDH, the Nd 3+ ion uptake was 91.5% at TTHA/Nd 3+ = 5 at 120 min [22]. During the uptake of Nd 3+ by TTHA Zn Al LDH, the following reaction (Eq.(5)) would occur in addition to the reaction described in Eq. (3) [22]. Nd 3+ + [Zn-C 18 H 25 N 4 O 12 ] 3 [Nd-C 18 H 25 N 4 O 12 ] 2 + Zn 2+ (5) ISBN:

5 On the other hand, in the case of TTHA Cu Al LDH, the Nd 3+ ion uptake was 20.3% at TTHA/Nd 3+ = 5 at 120 min, as mentioned previously. In other words, the uptake of Nd 3+ ions by TTHA Cu Al LDH was lower than that by TTHA Zn Al LDH, which may be attributed to the difference in the metal-chelate formation constants of the [Zn-C 18 H 25 N 4 O 12 ] 3 and [Cu- C 18 H 25 N 4 O 12 ] 3 complexes (16.7 and 19.2, respectively). In other words, the [Cu- C 18 H 25 N 4 O 12 ] 3 complex is more stable than the [Zn-C 18 H 25 N 4 O 12 ] 3 complex. Therefore, the reaction expressed in Eq.(4) may be less feasible than the reaction described in Eq.(5). Based on the data presented above, it can be concluded that TTHA Zn Al LDH is superior to TTHA Cu Al LDH in its ability to take up Nd 3+ ions. Stated differently, Zn 2+ is superior to Cu 2+ as the M 2+ ion in the host layer in the LDH. 4 Conclusions TTHA Cu Al LDH was prepared by the drop-wise addition of Cu Al nitrate solution to a TTHA solution at a constant ph of 8.0. The assynthesized TTHA Cu Al LDH contained C 18 H 26 N 4 O 12 4 and [Cu-TTHA] complexes such as [Cu-C 18 H 25 N 4 O 12 ] 3 in its interlayer. TTHA Cu Al LDH was found to take up Nd 3+ ions from aqueous solutions, which can be attributed to the metalchelating functions of C 18 H 26 N 4 O 12 and [Cu- 4 C 18 H 25 N 4 O 12 ] 3 in the interlayer of the TTHA Cu Al LDH. In other words, the [Nd- C 18 H 25 N 4 O 12 ] 2 complex could be formed in the interlayer. However, the TTHA Cu Al LDH was barely able to take up Sr 2+ ions from aqueous solutions, suggesting that the C 18 H 26 N 4 O 4 12 and [Cu-C 18 H 25 N 4 O 12 ] 3 species in the interlayer of the TTHA Cu Al LDH did not function as chelating agents for Sr 2+ ions. This can be attributed to the low stability of the [Sr-C 18 H 25 N 4 O 12 ] 3 complex. TTHA Cu Al LDH was found to take up Nd 3+ ions more preferentially than the Sr 2+ ions from aqueous solutions, which can be explained by the following order of stabilities of the metal-chelate complexes: [Nd-C 18 H 25 N 4 O 12 ] 2 > [Cu-C 18 H 25 N 4 O 12 ] 3 > [Sr- C 18 H 25 N 4 O 12 ] 3. Furthermore, TTHA Zn Al LDH was superior to TTHA Cu Al LDH in its ability to take up Nd 3+ ions. Stated differently, Zn 2+ was superior to Cu 2+ as the M 2+ ion in the host layer in LDH. Thus, the degree of metal ion uptake can be controlled by selecting a suitable M 2+ in the host layer in the LDH. References: [1] F. Cavani, F. Trifirò, A. Vaccari, Hydrotalcitetype anionic clays: Preparation, properties and applications, Catalysis Today, 11, 1991, [2] L. Ingram, H.F.W. Taylor, The crystal structures of sjögrenite and pyroaurite, Mineralogical Magazine, 36, 1967, [3] R. Allmann, The crystal structure of pyroaurite, Acta Crystallographica, B24, 1968, [4] S.J. Mills, A.G. Christy, J.-M.R. Génin, T. Kameda, F. Colombo, Nomenclature of the hydrotalcite supergroup: natural layered double hydroxides, Mineralogical Magazine, 76, 2012, [5] J. Cuppoletti (Ed.), Metal, ceramic and polymeric composites for various uses, InTech, Croatia, [6] E. Narita, T. Yamagishi, K. Tazawa, O. Ichijo, Y. Umetsu, Uptake behavior of chelating agents by magnesium-aluminum oxide precursor with reconstruction of hydrotalcitelike layer structure, Clay Science, 9, 1995, [7] T. Sato, H. Okuyama, T. Endo, M. Shimada, Preparation and photochemical properties of cadmium sulphide-zinc sulphide incorporated into the interlayer of hydrotalcite, Reactivity of. Solids, 8, 1990, [8] A.I. Tsyganok, K. Suzuki, S. Hamakawa, K. Takehira, T. Hayakawa, Alternative approach to incorporation of nickel into layered structure of Mg Al double hydroxides: intercalation with [Ni(edta)] 2 species, Chemistry Letters, 30, 2001, [9] A.I. Tsyganok, K. Suzuki, S. Hamakawa, K. Takehira, T. Hayakawa, Mg Al layered double hydroxide intercalated with [Ni(edta)] 2 chelate as a precursor for an efficient catalyst of methane reforming with carbon dioxide, Catalysis Letters, 77, 2001, [10] A. V. Lukashin, A. A. Vertegel, A. A. Eliseev, M. P. Nikiforov, P. Gornert, Y. D. Tretyakov, Chemical design of magnetic nanocomposites based on layered double hydroxides, Journal of Nanoparticle Research, 5, 2003, [11] A. Tsyganok, A. Sayari, Incorporation of transition metals into Mg Al layered double hydroxides: Coprecipitation of cations vs. their pre-complexation with an anionic chelator, Journal of Solid State Chemistry, 179, 2006, [12] T. Kameda, S. Saito, Y. Umetsu, Mg Al layered double hydroxide intercalated with ethylenediaminetetraacetate anion: Synthesis ISBN:

6 and application to the uptake of heavy metal ions from an aqueous solution, Separation and Purification Technology, 47, 2005, [13] T. Kameda, H. Takeuchi, T. Yoshioka, Uptake of heavy metal ions from aqueous solution using Mg Al layered double hydroxides intercalated with citrate, malate, and tartrate, Separation and Purification Technology, 62, 2008, [14] T. Kameda, H. Takeuchi, T. Yoshioka, Hybrid inorganic/organic composites of Mg Al layered double hydroxides intercalated with citrate, malate, and tartrate prepared by coprecipitation, Materials Research Bulletin, 44, 2009, [15] T. Kameda, H. Takeuchi, T. Yoshioka, Kinetics of uptake of Cu 2+ and Cd 2+ by Mg Al layered double hydroxides intercalated with citrate, malate, and tartrate, Colloids and Surfaces A: Physicochemical and Engineering Aspects, 355, 2010, [16] T. Kameda, H. Takeuchi, T. Yoshioka, Ni Al layered double hydroxides modified with citrate, malate, and tartrate: Preparation by coprecipitation and uptake of Cu 2+ from aqueous solution, Journal of Physics and Chemistry of Solids, 72, 2011, [17] T. Kameda, K. Hoshi, T. Yoshioka, Uptake of Sc 3+ and La 3+ from aqueous solution using ethylenediaminetetraacetate-intercalated Cu Al layered double hydroxide reconstructed from Cu Al oxide, Solid State Sciences, 13, 2011, [18] T. Kameda, K. Hoshi, T. Yoshioka, Thermal decomposition behavior of Cu Al layered double hydroxide, and ethylenediaminetetraacetate-intercalated Cu Al layered double hydroxide reconstructed from Cu Al oxide for uptake of Y 3+ from aqueous solution, Materials Research Bulletin, 47, 2012, [19] T. Kameda, K. Hoshi, T. Yoshioka, Preparation of Cu-Al layered double hydroxide intercalated with ethylenediaminetetraacetate by coprecipitation and its uptake of rare earth ions from aqueous solution, Solid State Sciences, 17, 2013, [20] T. Kameda, K. Hoshi, T. Yoshioka, Desorption of rare earth ions from ethylenediaminetetraacetate-intercalated Cu-Al layered double hydroxide using Fe 3+, Research and Reviews in Materials Science and Chemistry, 2, 2013, [21] T. Kameda, K. Hoshi, T. Yoshioka, Kinetics and equilibrium studies on the uptake of rare earth ions from aqueous solution using a Cu Al layered double hydroxide intercalated with ethylenediaminetetraacetate, Fresenius Environmental Bulletin, 23, 2014, [22] T. Kameda, T. Shimmyo, T. Yoshioka, Preparation of Zn Al layered double hydroxide intercalated with triethylenetetraminehexaacetic acid by coprecipitation: uptake of rare-earth metal ions from aqueous solutions, RSC Advances, 4, 2014, [23] P. Letkeman, A. E. Martell, Nuclear magnetic resonance and potentiometric protonation study of polyaminopolyacetic acids containing from two to six nitrogen atoms, Inorganic Chemistry, 18, 1979, [24] L. Harju, A. Ringbom, Compleximetric titrations with triethylenetetramine-hexaacetic acid, Analytica Chimica Acta, 49, 1970, [25] L. Harju, The stability constants of some metal chelates of triethylenetetraminehexaacetic acid (TTHA), Analytica Chimica Acta, 50, 1970, ISBN: