Layered Double Hydroxides (Ldhs) with Hydrotalcite-Like Structure

Clay Science9, 187-197 (1995)

UPTAKE BEHAVIOR OF CHELATING AGENTS BY MAGNESIUM- ALUMINIUM OXIDE PRECURSOR WITH RECONSTRUCTION OF HYDROTALCITE-LIKE LAYER STRUCTURE

EIICHI NARITA1, TOSHIHIDE YAMAGISHI2, KAZUHARU TAZAWA2, OSAMU ICHIJO1 and YOSHIO UMETSU1 1 Department of Applied Chemistry and Molecular Science, Faculty of Engineering, Iwate University, Ueda, Morioka 020, Japan 2 Department of Chemical and Biological Engineering , Hachinohe National College of Technology, Tamonoki, Hachinohe 039-11, Japan (Accepted February 22, 1995)

ABSTRACT

The uptake behavior of anionic chelating agents into the magnesium-aluminium

oxide precursor, Mg0.58Al0.28• 0 .14O (• : vacancy), prepared by the thermal decomposition of a synthetic Mg-Al carbonated layered double hydroxide, Mg0.67Al0.33 (OH)2(CO3)0.16.0.46 H2O, was investigated at 25•Ž. The oxide solid solution was found to include highly the chelating agents such as ethylenediaminetetraacetate, its metal complexes, nitrilotriacetate and other similar anions as guest anion instead of CO32- ion from aqueous solutions with reconstructing the original hydrotalcite-like layer structure. The effect of initial solute concentration on the uptake was studied and isotherm parameters were evaluated. The Freundlich adsorption isotherm was found to be more suitable for all systems tested here and the values of the

parameters, k and n, were quite higher. On the basis of the data, the relationship between the isotherm parameters and the number of negative charge, structure and arrangement of these anions in the interlayer of the resulting hydrotalcite-like compound was also discussed.

Key words: Hydrotalcite, Layered Double Hydroxide, Uptake, Intercalation, Adsorption, EDTA, NTA, Chelating Agent.

INTRODUCTION

Layered double hydroxides (LDHs) with hydrotalcite-like structure represent an important group of ionic lamellar solids with positively charged layers and have, therefore, a mirror image of the much studied family of cationic clay minerals. They are represented by the following general formula (Frondel, 1941; Allmann, 1970):

where M2+=Mg2+, Ni2+ Zn2+, etc.; M3+=Al3+, Cr3+, Fe3+, etc.; An-= NO3-, Cl-,

CO32-, SO42-, etc.; and x=0.2•`0.33. The interlayer anion, An-, is exchangeable.

Hydrotalcite with typical composition of Mg0 .67Al0.33(OH)2(CO3)0.16•E0.5 H2O is one of the naturally occurring LDHs (anionic clay minerals) and is well-known that it is easy and 188 E. Narita et al.

relatively inexpensive to be prepared in laboratory. The crystal structure consisted of positively charged brucite-like octahedral hydroxide layers, which are stacked on top of each other. Carbonate ions are located between the layers as interlayer anions, and water molecules occupy the remaining interlayer space (Allmann, 1968; Taylor, 1973). An extensive literature exists on the structure, composition (Feitknecht et al., 1942; Gastuche et al., 1967; Reichle et al., 1986), and physicochemical properties (Miyata, 1975, 1980, 1983; Sato et al., 1986b, 1988) of the Mg2+-Al3+-CO32- LDH. In these studies, it has been observed to be decomposed to the NaCl-type magnesium-aluminium oxide solid solution in the temperature range of 500•`800•Ž (Miyata, 1980; Sato et al., 1986a).

(1)

The most interesting feature of the resulting oxide is the intrinsic rehydration reac- tivity. When the oxide precursor is added into an aqueous solution, it readily rehydrates and incorporates anions to reconstruct the original LDH structure as expressed by eq 2.

(2)

It suggests that it is possible to incorporate many kinds of anions from solutions as the guest anions in LDH, if the magnesium-aluminium oxide precursor is used as starting material. According to the literature (Sato et al., 1986a), the uptake of various inorganic ions such as Cl-, MnO, HPO42-, CrO42-, HVO42-, SO24-, S2O62-, etc. by the calcined Mg-Al oxide precursor has been carried out from the viewpoint of the removal of these anions. As regards the uptake of organic anions, sebacate ion (Chibwe et al., 1989), various mono- and dicarboxylate ions (Narita et al., 1990), anionic dyes (Narita et al., 1992a) and aromatic sulfonate ions (Narita et al., 1992b), benzenecarboxylate ions (Sato et al., 1992), naphthalenecarboxylate ions (Tagaya et al., 1993) have been incorporated in the gallery of LDHs by using the reaction of eq 2. Organic chelating agents such as ethylenediaminetetraacetate (EDTA) and nitrilotriacetate (NTA) are used frequently in chemical industries. However, the waste-water treatment containing these toxic agents is known to be comparatively difficult because of their higher stability in solutions. In addition, the intercalation of metal complexes of EDTA in the gallery of LDH recently became an important technique to synthesize LDH-metal compound intercalates such as LDH-CdS and LDH-CdS/ZnS (Sato et al., 1990). In the present study, therefore, the uptake behavior of various anionic chelating agents by the calcined Mg-Al oxide precursor was investigated quantitatively.

EXPERIMENTAL

Materials All reagents used in this investigation were of analytical grade. The parent LDH,

Mg0 .67A10.33(OH)2(CO3)o.16•E0.46H2O, was synthesized according to the earlier literature (Miyata, 1980) using MgC12 6H2O, AlC13.6H2O, NaOH and Na2CO3. The X-ray powder diffraction pattern revealed that the product has the typical hydrotalcite-like Reconstruction of Hydrotalcite-like Layer Structure 189

FIG. 1. X-ray powder diffraction patterns for the parent LDH, Mg0 .67 Al0 ,33(OH)2(CO3)016.0.46H2O (A) and for the calcined precursor, Mg0,58A10_28• 0.14O (B). Symbol -•›; LDH, a; Periclase (MgO).

structure as shown in Fig. 1-A. Before using this in the adsorption+ experiment, it was

heated at 500•Ž for 2 h to prepare the calcined precursor, Mg0 ,58Al0_28• 0.14O. It is found to be changed to the NaCl-type oxide with low crystallinity as also shown in Fig. 1-B . As an adsorbate, disodium salt of EDTA, various metal complex salts of EDTA

(Fe(III)-edta-, Ni(II)-edta2-, Cu(II)-edta2-, Zn(II)-edta2-), trisodium salt of NTA, sodium potassium tartrate (Rochelle salt), or trisodium citrate were tested.

Adsorption procedure For the investigation of the adsorption isotherms, 0.2 g of the calcined precursor and 50 cm3 of a chelating agent solution of a known initial concentration (Co: mg dm-3) were mechanically shaken in a 100 cm3 Erlenmeyer flask at 25•Ž for 20 h. The adsorption equilibrium was found to reach within about 12 h in all tests. The amount of adsorbed chelating agent (X: mg dm-3) was determined from the equilibrium concentration (C: mg dm-3) after centrifugation. The analysis was done with the aid of a Shimadzu TOC-500 total organic carbon analyzer. The solid product obtained after the adsorption experiment was dried at 60•Ž for 24 h and examined by using a Rigaku Geigerflex RAD-IIC X-ray diffractometer with Ni filtered Cu Ka radiation. TG and DTA were carried out using a Sinku-Riko TGD-7000RH thermal analyzer in the temperature range of room temperature 800•Ž. Ignited ƒ¿-Al2O3 was used as a reference material for DTA. The heating•` rate of the furnace was 10 deg. /min.

+ Though the uptake phenomenon observed here is not a simple adsorption , it has been represented as an adsorption for convenience. 190 E. Narita et al.

RESULT AND DISCUSSION

The adsorbed amount (X/M) of edta4- and metal-edta complex anions by the calcined precursor was determined at various initial concentrations of the chelating agents, and the results are shown in Fig. 2. The adsorbed amount was found to increase slightly with the increase of the equilibrium concentration in bulk solutions. In all cases, the Freundlich isotherm was well applicable: (3) where k and n are constants and M is the amount of the calcined precursor added

(g dm-3). Table 1 gives the values of Freundlich's parameters, k and n, which were obtained from the intercept and slope of the isotherms shown in Fig. 2. Generally, the values were quite higher than expected at the wide concentration range tested here. This extraordinarily high adsorption capacity was thought to be due to the intercalation in the gallery, not to a simple adsorption mechanism. Figures 3 and 4 show the X-ray diffraction patterns of the solid products containing edta4- and Ni(II)-edta2- ions, respectively. All of the patterns of the 003 and 006 reflection region showed district two peaks at 2ƒÆ, 11.5•‹ and 23.0•‹, which are quite similar to those of the parent LDH. It means that the calcined precursor reconstructed the original hydrotalcite-like structure with incorporating the chelating agents and water molecules by the reaction of eq 2. Usually adsorbates are attracted only on the surface or pore of an adsorbent, while in this case chelating agents were incorporated in the gallery of the resulting layered compound.

FIG. 2. Adsorption isotherms of EDTA and its metal complex anions by the calcined Mg-Al oxide precursor at 25•Ž. Symbol-•›; edta4-,•œ; Fe(III)-edta-, •œ; Ni(II)-edta2-, •œ; Cu(II)-edta2-, •›; Zn(II)-edta2-. Reconstruction of Hydrotalcite-like Layer Structure 191

TABLE 1. Adsorption data of various chelating agents by the calcined Mg-Al oxide precursor at 25•Ž

FIG. 3. X-ray powder diffraction patterns in 003 and 006 reflection region

for the solid products containing edta4-. Adsorbed amount (mg g-1)-(A) 165, (B) 225, (C) 330. Symbol-• ; Gibbsite (ƒÁ-Al(OH)3), •¡; Bayerite (ƒÀ-Al2O3.3H2O) 192 E. Narita et al.

FIG. 4. X-ray powder diffraction patterns in 003 and 006 reflection region for the solid products containing Ni(II)-edta2-. Adsorbed amount (mg g-1)-(A) 90, (B) 382, (C) 738.

The adsorbed amount of edta4- ion was the lowest in the data of the EDTA anions tested here because the negatively charged number is the largest. As can be understood by eq 2, the adsorbed amount is naturally decreased with the increase in the charge number of the guest anion. Figure 3 shows that the (003) basal spacing of the solid

products was almost constant as 7.68 A regardless of the adsorbed amount. However, the height of the diffraction peaks was decreased and broadened with the increase in the adsorbed amount of the guest anions. This was thought to be due to the partial overlap of the guest anions each other or of the guest anion and hydroxide ion in gallery space. It might cause the heterogeneous increase of the interlayer distance, i.e., the ununiformity in gallery height. Similar tendency has also been observed in our previous researches about the uptake of various carboxylate ions (Narita et al., 1990), aromatic sulfonate ions

(Narita et al., 1992b) and oxometalate ions (Yamagishi et al., 1993) by the reaction of eq. 2. Three peaks at 2 0, 18.4•‹, 20.4•‹, and 27.8•‹ in Fig. 3-C were assigned to the recrystallized

gibbsite (ƒÁ-Al(OH)3) and bayerite (ƒÀ-Al203 3H20). With increasing the edta4- concentration, the dissolution of the calcined precursor is enhanced, and then gibbsite, bayerite and amorphous magnesium hydroxide tend to precipitate with the elapse of the reaction time by the hydrolysis of Mg-edta2- and Al-edta- complex anions.

In this case, the concentration of Mg2+ and Al3+ ions in bulk solution was analyzed and Reconstruction of Hydrotalcite-like Layer Structure 193 the actual amount of the calcined precursor, M, was corrected. At the equilibrium edta4- concentration of more than 2,000 mg dm-3, both of the dissolution of the calcined precursor and the precipitation of brucite, gibbsite and bayerite were remarkably observed. On the other hand, the adsorbed amounts of Ni(II)-edta2- and Zn(II)-edta2- complex anions were similar each other and were much higher in the wide range of the equilibrium concentration. The charge number for these complex anions is -2 and the steric configu- lation is a conpact octahedral structure for Ni(II)-edta2- ion (it is known that one water molecular is coordinated at the solid state) and a conpact tetrahedral structure for Zn(II)- edta2- ion as shown in Fig. 5. In Fig. 4, the (003) basal spacing for the solid product containing a small amount of Ni(II)-edta2- ion was similar to that of the parent LDH and another diffraction peaks based on 003 and 006 reflection at the range of lower scattering angles was observed in the patterns of the solid products having more than 600 mmol g-1 of the guest anion. The (003) basal spacing of the sample C in Fig. 4 was found to be 14.72 A, in which the adsorbed amount of the guest anion was about 85% of the theoretical adsorption capacity. As the thickness of the LDH host layer is 4.77 A, the interlayer distance of the sample C is 9.95 A. This agrees with value of 10 A expected for Ni(II)-edta2- ion with octahedral structure as shown in Fig. 5. The same tendency was also observed in the X-ray diffraction patterns of the solid products containing a great deal of Fe(III)-edta- ion. The reason of the comparatively lower adsorbed amount of Cu(II)-edta2- ion was unclear, but it may be depend on the large hydrated steric structure, viz., planar square as shown in Fig. 5 accompanying two H2O molecules at top and bottom vertical positions. In the case of Fe(III)-edta- ion, the adsorbed amount was quite high at high equilibrium concentrations because of the low charge number of -1. However, the n value for Fe(III)-edta- ion was lower than those of the other EDTA complex anions having the charge number of -2. The lower charge number of the guest anion was already found to cause the lower n value in the cases of the uptake of carboxylate ions (Narita et al., 1990) and oxometalate ions (Yamagishi et al., 1993). At lower equilibrium concentration of guest anion with charge number of -1, hydroxide ion, OH-, is predominantly incorporated rather than the guest anion because of the comparatively high solution pH of 10-11. Figure 6 shows the adsorption isotherms of NTA, tartrate and citrate ions. The

planer square octahedral tetrahedral

FIG. 5. Molecular model of metal-EDTA complex anions. 194 E. Narita et al.

FIG. 6. Adsorption isotherms of NTA, tartrate, and citrate anions by the calcined Mg-Al oxide precursor at 25•Ž. Symbol -•›; citrate, •œ; tartrate, •œ; nta3-.

FIG. 7. X-ray powder diffraction patterns in 003 and 006 reflection region for the solid products containing nta3- (A), tartrate (B), or citrate (C). Adsorbed amount (mgg-1)-(A) 160, (B) 245, (C) 275. Reconstruction of Hydrotalcite-like Layer Structure 195

Freundlich isotherm was also well applicable and the values of Freundlich's parameters , k and n, are given in Table 1. The adsorbability for carboxylate anions such as tartrate and citrate were quite high as same as for metal-EDTA complex anions. In addition, citrate ion with longer carbon chain and tree carboxyl groups were incorporated more than tartrate ion. However, the adsorbability for nta3- ion was considerably lower just like as the case of edta4- ion. Definite reason of the extraordinarily high adsorbability for citrate ion is as yet unclear. It can be generally said that the adsorbability of the calcined Mg-Al oxide precursor indicated in Table 1 is quite higher than that of the much used activated carbon (Shirakashi et al., 1983). The X-ray diffraction patterns of the typical solid products were shown in Fig. 7. All these patterns were the same each other and are quite similar to that of the parent LDH in Fig. 1-A. The (003) basal spacing of the solid products was also 7.69 A. The increase in the interlayer distance observed in the cases of mono- and dicarboxylate ions having linear carbon chain (Narita et al., 1990, 1994) was

FIG. 8. TG and DTA diagrams of the solid product containing edta4- (A), Ni(II)-edta2- (B), or nta3- (C). Samples were obtained from the solution of the equilibrium concentration of 15 mmol dm-3 . 196 E. Narita et al.

not observed any more in these cases because of the difficulty in regular arrangement of the guest ions in gallery. The TG-DTA diagrams of the typical solid products are shown in Fig. 8. The endothermic peak around 200-250•Ž due to the elimination of interlayered water was observed for all specimens. The exothermic peak was also observed at about

390•Ž, which was attributed to the combustion of chelating agent. The second weight loss was almost agreed with the amount of the incorporated chelating agent in gallery. The same pattern was observed in the diagrams of the other solid products. It was also found that the calcined Mg-Al oxide precursor reprepared by heating the resulting LDH intercalates indicated the approximately same adsorption capacity. It means the probability of the repeated use in the uptake of chelating agents. In conclusion, it was found that various chelating agents can be incorporated highly in the gallery of LDH by the unique reconstitution reaction, and the isotherms of the uptake were obtained by the quantitative experiments. The results obtained here could be useful not only for the removal of chelating agents from waste solutions but also for the preparation of the LDH derivatives with ultra fine particles of metal sulfide, oxide, etc. as catalyst.

ACKNOWLEDGEMENTS

We wish to express our thank to Mr. T. Oosaki of Hachinohe National College of Technology for his helpful assistance in experiments.

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