Al–NO3 layered double hydroxides with varying layer charge density

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Applied Clay Science 40 (2008) 193 – 200 www.elsevier.com/locate/clay

Adsorption of 2,4-D on Mg/Al–NO3 layered double hydroxides with varying layer charge density Yia-Feng Chao, Pin-Chieh Chen, Shan-Li Wang ⁎ Department of Soil and Environmental Sciences, National Chung Hsing University, Taichung, Taiwan 40227 Received 27 June 2007; received in revised form 11 September 2007; accepted 11 September 2007 Available online 26 September 2007

Abstract Layered double hydroxides (LDHs) have high surface area and high anion exchange capacity, so they have been proposed to be an effective scavenger for contaminants. In this study, the adsorption of 2,4-dichlorophenoxyacetate (2,4-D) on Mg/Al–NO3 LDHs with varying layer charge density was investigated with particular attention on the effect of the orientation of the interlayer nitrate. Three Mg/Al LDHs were synthesized with Al3+/(Al3+ + Mg2+) molar ratios of 3.3 (LDH3), 2.6 (LDH4) and 2.1 (LDH5). The results of adsorption experiments showed that LDH5 exhibited an S-type isotherm with a low 2,4-D adsorption capacity due to the low accessibility of 2,4-D to the interlayer space. The accessibility was restricted by the small basal spacing of LDH5 as a result of the parallel orientation of the interlayer nitrate with respect to the hydroxide sheet. Thus, the 2,4-D adsorption occurred mainly on the external surface of the material. On the contrary, LDH3, which has the highest layer charge density among the samples, contains nitrate with an orientation perpendicular to the hydroxide sheet of LDH3. The interlayer nitrate was readily exchanged by 2,4-D. Thus, in addition to the adsorption on the external surface, the replacement of the interlayer nitrate by 2,4-D contributed to a higher adsorbed amount of 2,4-D; the 2,4-D adsorption of LDH3 exhibited an L-type isotherm. For LDH4 that contained interlayer nitrate with both parallel and perpendicular orientations, the adsorption characteristics were between those of LDH3 and LDH5. This work has demonstrated the dependence of 2,4-D adsorption characteristics on the nitrate orientation in LDHs, as a consequence of changing layer charge density. © 2007 Elsevier B.V. All rights reserved. Keywords: 2,4-D; Adsorption; Anion exchange; Guest orientation; Layered double hydroxide; Selective removal

1. Introduction 2,4-dichlorophenoxyacetic acid (2,4-D) is one of the phenoxyalkanoic acid herbicides, which are widely used for post-emergence control of broadleaf weeds in grain croplands, forests, domestic lawns, commercial turfs, and aquacultures (Kidd and James, 1991). The applica⁎ Corresponding author. Tel.: +886 4 22840373x3406; fax: +886 4 22862043. E-mail address: [email protected] (S.-L. Wang). 0169-1317/$ - see front matter © 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.clay.2007.09.003

tion of 2,4-D to soils and waters directly results in the presence of 2,4-D in the environment with subsequent adverse impacts on ecosystems. 2,4-D is water soluble and exists in its neutral and anionic forms in aqueous solutions. Due to its low pKa (i.e., 2.73) (Nelson and Faust, 1969), 2,4-D appears predominantly in its anionic form in the pH range that exists in the natural environment. This anionic contaminant is highly mobile and poorly biodegradable (Carter, 2000). The half-life of 2,4-D in water ranges from one to several weeks under aerobic conditions, and it can exceed 120 days under

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anaerobic conditions (WHO, 2003). Because of its high mobility and toxicity, 2,4-D poses a high threat to surface and ground water supplies and is among the priority contaminants of major environmental concern (Hamilton et al., 2003; WHO, 2003). Therefore, for water reclamation, there is a need for the development of an efficient and cost-effective adsorbent for removing 2,4-D from water. Layered double hydroxides (LDHs) and their calcined products possess high anion exchange capacity and have been considered to be promising adsorbents for anionic contaminants (Cavani et al., 1991; Li and Duan, 2006). LDHs are a group of materials that comprise positively charged hydroxide layers consisting of at least two different metal cations that are octahedrally coordinated by hydroxyl groups (Cavani et al., 1991; Evans and Slade, 2006). The general chemical formula of natural or synthetic LDHs is ½M2þ1−x M3þ x ðOHÞ2 xþ Ay− x=y ∙nH2 O

ð1Þ

where M2þ ¼ Ca2þ ; Mg2þ ; Zn2þ ; Co2þ ; Ni2þ ; Cu2þ ; Fe2þ or Mn2þ M3þ ¼ Al3þ ; Cr3þ ; Fe3þ ; Co3þ ; Ni3þ ; Mn3þ or Ga3þ A ¼ Cl− ; NO−3 ; CO2− 3 etc: The positive charges in the hydroxide sheets result from substitution of trivalent cations for divalent cations, and the M3+/(M3+ + M2+) ratio (i.e., x in Eq. (1)) in LDHs, representing the layer charge density, ranges from 1/5 to 1/3 (Brindley and Kikkawa, 1979; Cavani et al., 1991; Miyata, 1975). The interlayer spaces between hydroxide sheets are filled by water and anions (Ay−) that counterbalance the positive charges in the hydroxide sheets. Because the interlayer anions and water are often labile, LDHs exhibit anion exchange capacity (≤ ∼4 mmol g− 1) and have the unique property of being positively charged, regardless of pH. Such structural properties make LDHs useful as barriers for retarding the migration of anionic contaminants in groundwater or for scavenging hazardous anions in drinking water and industrial wastewater. Previous studies on the adsorption of some inorganic and organic anionic contaminants by LDHs have demonstrated their reactivity toward these anions (Cardoso and Valim, 2006; Chibwe and Jones, 1989; Kovanda et al., 1999;

Pavlovic et al., 2005; Tzou et al., 2006; Wang and Gao, 2006; Yang et al., 2005; You et al., 2001; You et al., 2002). For 2,4-D and similar compounds (e.g., 4-chloro2-methylphenoxyacetic acid), the adsorption capacities of various LDHs have been previously reported to be 0.1–5.2 mmol g− 1, depending on the types of cations and anions present in the structure of the LDHs and experimental conditions, such as pH and the concentration and type of background electrolytes (Cardoso and Valim, 2006; Inacio et al., 2001; Legrouri et al., 2005; Pavlovic et al., 2005). For example, Legrouri et al. (2005) reported that the 2,4-D adsorption capacities of Zn/Al–Cl LDHs were proportional to their Al3+/(Al3+ + Zn2+) molar ratios (x in Eq. (1)). Inacio et al. (2001) reported the 2,4-D adsorption capacities of Mg/Al LDHs were dependent on the anions present in the interlayer of the materials, following the order of CO32− b Cl− b NO3−. The selective binding of anions by LDHs is affected to a considerable extent by the properties of the anions. Generally, LDHs have affinities for anions following the order of the Hofmeister series (Collins and Washabaugh, 1985). That is, the affinity increases with increasing charge and decreasing ionic radius. For example, the affinity for monovalent inorganic anions decreases in the order of OH− N F− N Cl− N Br− N NO3− N I− (Bontchev et al., 2003; Miyata, 1983; Newman and Jones, 1999). Accordingly, LDHs have less affinity for nitrate and the interlayer nitrate is exchanged more readily than with other inorganic anions, such as Cl−, OH−, and CO32−. Thus, nitrate-containing LDHs have been considered as an important precursor for intercalation of other anions into LDHs (He et al., 2006). In our previous study, the orientation of the interlayer nitrate in Mg/Al LDHs was shown to be dependent on the layer charge density of the materials (Wang and Wang, 2007). The interlayer nitrate of Mg/Al LDHs has an orientation that is parallel to the hydroxide sheets with a low charge density (x = 0.20) and an orientation that is perpendicular to the hydroxide sheets with a high charge density (x = 0.33). In the Mg/ Al LDH whose charge density is the average of the above two end-member situations, nitrates with both parallel and perpendicular orientations co-exist in the same structure. The aim of this work was to further investigate the 2,4-D adsorption capacities and mechanisms of Mg/Al–NO3 LDHs with varying layer charge density and nitrate orientation in these materials. The results demonstrated how those structural properties of LDHs determined the kinetics and selectivity of 2,4-D adsorption. This information is essential for designing an optimum scavenger for selectively binding 2,4-D in waters that can be extended to other important anionic contaminants in the environment.

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2. Materials and methods

2.3. Adsorption kinetics

2.1. Sample preparation

The adsorption kinetic study was conducted using a waterjacketed reaction vessel, connected to a water bath to maintain a constant temperature at 25 °C during the experiments. Five hundred mL of a 100 mg L− 1 2,4-D solution was first placed in the reaction vessel after the pH value of the solution had been adjusted to 7.0. After attaining the desired temperature, 0.2 g of LDH was subsequently added to the 2,4-D solution under vigorous stirring and N2 purging. At given time intervals, 10 mL of the suspension was withdrawn from the reaction vessel and passed through a 0.45-μm membrane filter to collect the filtrate. The 2,4-D concentrations in the filtrates were determined using a Varian Cary 50 UV/VIS spectrometer at 284.0 nm. The kinetic data were fitted with the Elovich model (Low, 1960; Taylor and Thon, 1952) that has been applied to describe chemisorption kinetics with satisfactory results. The equation is expressed as follows:

The synthesis of Mg/Al–NO3 LDHs was conducted using a constant-pH co-precipitation method (Cavani et al., 1991; He et al., 2006). The solutions containing Al(NO3)3·9H2O and Mg(NO3)2·6H2O were prepared with a total cation concentration of 1.0 M and Al3+/(Mg2+ + Al3+) molar ratios of 1/3 (LDH3), 1/4 (LDH4) and 1/5 (LDH5) (Table 1). One hundred mL of each nitrate salt solution was added dropwise into 200 mL of deionized water in a reaction vessel during vigorous stirring. The pH of the mixture was maintained at a value of 10 by the simultaneous addition of a 2 M NaOH solution. After the addition of the nitrate salt solution was complete, the resulting suspension was aged at 25 °C for 1 h, while the solution's pH was maintained at 10. Afterward, the suspension was aged at 65 °C for 18 h. The suspension was filtered with a 0.45-μm membrane filter, and the sample headspace was flushed with N2 gas. The collected precipitate was subsequently washed twice with 500 mL of deionized water to eliminate any dissolved salts. The collected solid was then freeze-dried and stored in glass vials prior to use. 2.2. Sample characterization Samples for chemical composition analysis were dissolved in 1 N HCl. The Mg and Al contents were then determined using a Hitachi Z2000 Zeeman polarized atomic absorption spectrophotometer. The resultant Al3+/(Al3+ + Mg2+) molar ratios of the products were determined to be 0.33, 0.26, and 0.21, which were in good agreement with the initial values of the batch starting solutions (Table 1). Using the Al and Mg contents, the layer charges were calculated to be 3.9, 3.1, and 2.6 mmolc g− 1 for LDH3, LDH4, and LDH5, respectively. X-ray diffraction patterns were obtained on a Rigaku Miniflex diffractometer using Cu–Kα radiation (λ= 0.15418 nm). Data were collected with a scan rate of 2 °2θ min− 1. FTIR spectra were acquired for the samples randomly distributed in a KBr pellet, using a ThermoNicolet Nexus FTIR spectrometer equipped with a liquid nitrogen cooled MCT detector and a KBr beamsplitter. Spectra were obtained by co-addition of 64 individual scans with an optical resolution of 4 cm− 1. Table 1 Chemical compositions of Mg/Al–NO3 LDHs Sample

LDH3 LDH4 LDH5 †

Al3+/(Mg2+ + Al3+) molar ratio Initial

Actual

0.33 0.25 0.20

0.33 0.26 0.21

Anion exchange capacity † (mmolc g− 1)

3.9 3.1 2.6

Anion exchange capacity was calculated based on the corresponding Al3+ and Mg2+ contents.

dqt =dt ¼ αexpðβqt Þ

ð2Þ

or in the integrated form as qt ¼ ð1=βÞlnðαβÞ þ ð1=βÞlnðtÞ

ð3Þ

where qt is the amount of adsorbed 2,4-D (mmol g− 1) at time, t; α and β are the parameters of the model corresponding to the initial reaction rate (mmol g− 1 min− 1) and adsorption constant (g mmol− 1). The data before reaching equilibrium were selected to plot qt vs. ln(t) such that a straight line was obtained. The parameter β can be obtained from the slope of that line, and then the initial reaction rate α can be calculated using the value of β and the intercept of that line. 2.4. Adsorption isotherms The 2,4-D adsorption isotherms of various LDHs were obtained using the batch method. Twenty-five mL of 2,4-D solution was added to 0.01 g LDH in each of a series of 50-mL amber glass centrifuge tubes with Teflon-lined caps. The initial concentrations of the 2,4-D solutions ranged from 0 to 400 mg L− 1, and the tubes for each 2,4-D concentration were prepared in triplicate. The samples were shaken at 150 rpm and 25 °C for 2 h and the samples were filtered using a 0.45μm membrane filter. The final 2,4-D concentrations in the filtrates were determined using a Varian Cary 50 UV/VIS spectrometer at 284.0 nm. The adsorbed amount of 2,4-D was calculated as the difference between the initial and final concentrations of 2,4-D in the filtrate. Adsorption data of 2,4-D on LDHs were analyzed by use of the Langmuir model. The Langmuir equation can be rearranged in the following linear form: Ce =ðx=mÞ ¼ Ce =Sm þ 1=KL Sm

ð4Þ

where x / m and Ce are the equilibrium concentrations of 2,4-D in the adsorbed and liquid phases in mmol g− 1 and mmol L− 1,

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respectively; KL and Sm are the Langmuir constants related to the adsorption energy and adsorption capacity, respectively, and can be calculated from the slope and intercept of the linear plot of Ce / (x / m) vs. Ce.

3. Results and discussion 3.1. Adsorption kinetics Adsorption kinetics was investigated to determine the equilibrium time for 2,4-D adsorption on various LDHs at pH 7 and 25 °C (Fig. 1). This experiment was conducted with an initial 2,4-D concentration of 100 mg L− 1 and a solids concentration of 0.4 g L− 1. As seen from the curves in Fig. 1, the 2,4-D concentrations decreased rapidly after mixing LDHs with the 2,4-D solution. With LDH5, the removal rate of 2,4-D reached 19% at the first sampling point at 30 s and showed no discernible change afterward. This indicated that the 2,4-D adsorption on LDH5 occurred rapidly and reached equilibrium in seconds. With LDH4 and LDH3, the 2,4-D concentrations initially decreased rapidly for 4 min; after that, the reaction rates gradually slowed down and leveled off in 15 min. Thus, the reaction with LDH4 and LDH3 reached equilibrium in 15 min or less, a much longer time than was required to reach equilibrium when LDH5 was used. At the selected conditions, the maximum 2,4-D removal rates of LDH3, LDH4, and LDH5 were 93%, 62%, and 19%, respectively. With the Al3+ /(Mg2+ + Al3+ ) ratios of 0.33 (LDH3), 0.26 (LDH4), and 0.21 (LDH5), the corresponding anion exchange capacities were determined to be 3.9, 3.1, and 2.6 mmolc g− 1 (Table 1). The removal rate decreased as the anion exchange capacity of LDHs decreased.

Fig. 2. Adsorption kinetics of 2,4-D on Mg/Al–NO3 LDHs by a plot of the adsorbed 2,4-D vs. ln(t).

The Elovich equation was used to fit the kinetic data collected for LDHs (Fig. 2). The results of fitting showed a good linear plot covering the initial period of the kinetic study for LDH3 and LDH4 (R2 ≥ 0.998). After the reaction reached equilibrium, the curves became horizontal (data not shown in Fig. 2), and the corresponding value of the Elovich parameters α and β become zero and infinity, respectively. For the same reason, the kinetic data of LDH5 were not fitted using the Elovich equation, since the 2,4-D adsorption on LDH5 reached equilibrium in seconds and exhibited a horizontal curve throughout the experiment (Fig. 1). Therefore, the Elovich parameters α and β were determined only for LDH3 and LDH4 (Table 2). The values of α for LDH3 and LDH4 were 6.1 and 6.0 mmol g− 1 min− 1, respectively. The insignificant difference in the α values of these two materials indicates that the initial 2,4-D adsorption rates of these two materials were similar. The values of β for LDH3 and LDH4 were determined to be 5.5 and 7.4 g mmol− 1, respectively, and they were inversely correlated to the corresponding anion exchange capacities (Table 1). Although the fitting results with the Elovich equation did not provide any particular indication about the reaction mechanisms, these could be interpreted assuming that under the selected conditions the 2,4-D adsorption on LDH3 and LDH4 occurred by the same reaction mechanisms with different availabilities of surface sites for 2,4-D adsorption. On the other hand, the kinetic curve of

Table 2 The parameters of the Elovich model for 2,4-D adsorption on Mg/Al– NO3 LDHs Fig. 1. Adsorption kinetics of 2,4-D on Mg/Al–NO3 LDHs with Al3+/ (Al3+ + Mg2+) molar ratios of 0.33 (LDH3), 0.26 (LDH4), and 0.21 (LDH5). The initial 2,4-D concentration was 100 mg L− 1 and the LDH concentration was 0.4 g L− 1.

Sample

α (mmol g− 1 min− 1)

β (g mmol− 1)

R2

LDH3 LDH4 LDH5

6.1 6.0 n.d.

5.5 7.4 n.d.

0.998 0.998 n.d.

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Fig. 3. Adsorption isotherms of 2,4-D on Mg/Al–NO3 LDHs with Al3+/(Al3+ + Mg2+) molar ratios of 0.33 (LDH3), 0.26 (LDH4), and 0.21 (LDH5).

LDH5 demonstrated a fast adsorption that cannot be described by the Elovich equation. Fast adsorption may also occur in the cases of LDH3 and LDH4, but this cannot be distinguished from the kinetic curves of the overall reactions. Such hypotheses will have to be confirmed by additional experimental results.

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and after interacting with 400 mg L− 1 2,4-D were collected and investigated (Figs. 4 and 5). In the XRD patterns of the original LDHs, the two distinct reflections are (003) and (006). The values of the (003) reflections correspond to the basal spacings of two consecutive brucite-like hydroxide layers in various LDHs. With the Al3+/(Al3+ + Mg2+) ratios of 0.33 (LDH3), 0.26 (LDH4), and 0.21 (LDH5), the observed basal spacings of these materials were 0.899, 0.854, and 0.804 nm, respectively (Fig. 4). After interacting with 2,4-D of 400 mg L− 1, the amounts of 2,4-D adsorbed by LDH3, LDH4, and LDH5 were 2.73, 1.42, and 0.37 mmol g− 1, respectively (Fig. 1), giving rise to the basal spacings of 1.90, 1.91, and 0.804 nm (Fig. 4). The expansion of the basal spacings of LDH3 and LDH4 to 1.9 nm resulted from the replacement of interlayer nitrate ions by 2,4-D ions with a larger molecular size (Cardoso and Valim, 2006; Lakraimi et al., 2000; Legrouri et al., 2005). Meanwhile, the corresponding FTIR results also showed the decreasing

3.2. Adsorption characteristics Fig. 3 shows the adsorption isotherms of 2,4-D on the three LDH samples in 0.001 M KNO3 solution at 25 °C. With different Al 3+/(Mg2+ + Al3+) ratios, the 2,4-D adsorption rates of these materials were different. For LDH3 and LDH4, the adsorbed amounts of 2,4-D gradually increased upon increasing 2,4-D concentration until a plateau was reached. According to the classification of Giles and Smith (1974), the adsorption isotherms of LDH3 and LDH4 were both of L-type, indicating a minimum competition from water and background electrolyte (i.e., NO3−) for the adsorption sites on the LDH surfaces. Through fitting the adsorption data with the Langmuir equation, the maximum adsorptions for LDH3 and LDH4 were determined to be 2.86 and 2.02 mmol g− 1, respectively, corresponding to 73% and 65% of their anion exchange capacities (Table 1). Comparatively, the characteristics of 2,4-D adsorption on LDH5 were significantly different from those of LDH3 and LDH4. The 2,4-D adsorption on LDH5 exhibited the S-type isotherm (Giles and Smith, 1974), showing no significant increase in 2,4-D adsorption with increasing 2,4-D concentration up to 400 mg L− 1. These results indicated that the lower the Al3+/(Mg2+ + Al3+) molar ratio of the adsorbent, the weaker its affinity for the adsorbate. To clarify the structural implications to the differences in the 2,4-D adsorption isotherms of various LDHs, the XRD patterns and IR spectra of the LDH samples before

Fig. 4. XRD patterns of Mg/Al–NO3 LDHs interacting with a 400 mg L− 1 2,4-D solution in 0.001 M KNO3 and at pH 7.

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Fig. 5. FTIR spectra of Mg/Al–NO3 LDHs interacting with a 400 mg L− 1 2,4-D solution in 0.001 M KNO3 and at pH 7.

intensities of the nitrate peaks in the region of 1500– 1200 cm− 1 with the increasing intensities of the 2,4-D peaks after the 2,4-D adsorption (Fig. 5). On the contrary, the basal spacing of LDH5 showed no observable change after interacting with 2,4-D (Fig. 4); the FTIR spectrum of LDH5 collected after 2,4-D adsorption also showed weak 2,4-D peaks with no significant decrease in the intensities of the nitrate peaks (Fig. 5). These results indicated that the nitrate ions in the interlayers of LDH5 were not replaced by 2,4-D ions even after the material interacted with the high-concentration (400 mg L− 1) 2,4-D solution. Thus, the adsorption of 2,4-D may occur on the external surfaces of LDH5 so that the adsorbed amount of 2,4-D was much less than the amounts for LDH3 and LDH4, and no expansion of the basal spacing was observed after the adsorption. 3.3. Adsorption mechanisms The pKa of 2,4-D is 2.73 (Nelson and Faust, 1969), so it exists in its anionic form in the pH value of 7 used in these adsorption experiments. On the other hand, the structures of LDHs consist of positively charged hydroxide sheets. When 2,4-D is removed from aqueous solution by LDHs, the adsorption may occur primarily in response to Coulomb attractions between the anionic adsorbent and the positively charged external and interlayer surfaces of LDHs. This is confirmed by the FTIR spectra of LDHs containing 2,4-D (Fig. 5). The spectrum of pure 2,4-D contains peaks corresponding to the protonated –COOH group, particularly the one at 1733 cm− 1. This peak was replaced by the peaks at 1616 and 1338 cm− 1 (marked ⁎) in the IR spectra of LDHs containing 2,4-D, attributed to the –COO− group of 2,4-D.

Thus, 2,4-D adsorption on LDHs occurred essentially through the carboxylate anion of 2,4-D (Lakraimi et al., 2000; Legrouri et al., 2005). The differences in the intensities of these 2,4-D peaks in different LDHs reflected the corresponding amount of adsorbed 2,4-D in the materials. Among the three LDHs, LDH5 with the lowest layer charge density has the smallest basal spacing (i.e., 0.803 nm), in which the orientation of the interlayer nitrate is parallel to the hydroxide sheets (Wang and Wang, 2007). After interacting with the 2,4-D solution at a concentration of 400 mg L− 1, LDH5 exhibited low 2,4-D adsorption and its basal spacing showed no change after the adsorption. This indicated that 2,4-D has a low accessibility to the small interlayer space of LDH5. Thus, 2,4-D may be adsorbed only on the external surface of the LDH5 particles, and this adsorption is fast and can reach equilibrium in seconds as seen in the kinetic studies (Fig. 1). LDH3, on the other hand, has the largest basal spacing (i.e., 0.899 nm), resulting from the perpendicular orientation of the interlayer nitrate with respect to the hydroxide layers in the material (Wang and Wang, 2007). After interacting with 2,4-D, the basal spacing increased to 1.90 nm, indicating a high accessibility of the interlayer surface of LDH3 for 2,4-D. As a result of the perpendicular orientation of the interlayer nitrate, the large basal spacing facilitates the diffusion of nitrate and 2,4-D molecules in the interlayer space of LDH3. In conjunction with the adsorption on the external surface, the interlayer NO3− ions were replaced by 2,4-D for the positively charged binding sites in the hydroxide sheets, resulting in an increase of the basal spacing from 0.899 to 1.90 nm and a higher 2,4-D adsorption capacity (Fig. 4). The 2,4-D molecules may form an organic bilayer in the interlayer with their carboxylate groups toward the positively charged centers in the hydroxide layers on each side of the interlayer of LDHs (Lakraimi et al., 2000). The original basal spacing of LDH4 is 0.854 nm, which is the average of the above two end-member situations. It was suggested that LDH4 is an intermediate, randomly-interstratified phase, containing nitrates with both parallel and perpendicular orientations (Wang and Wang, 2007). After interacting with 2,4-D, the expansion of the basal spacing to 1.91 nm was also observed for LDH4, but its intensity was weaker than the counterpart of LDH3 (Fig. 4). Meanwhile, a broad shoulder appeared at 0.810 nm, which was not seen in the XRD pattern of LDH3 containing 2,4-D and was close to the value of the (003) reflection of LDH5. This broad reflection may be attributed to the layer structures containing interlayer nitrate with a parallel orientation. Compared with the

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counterparts in the XRD patterns of LDH3 containing 2,4-D, the broader reflections of LDH4 containing 2,4-D also revealed its less-ordered structure. Since the 2,4-D adsorption of LDH4 was also between those of LDH3 and LDH5, we therefore hypothesized that the interlayer nitrate molecules with a perpendicular orientation contributed to the adsorption of 2,4-D, while those with a parallel orientation were not replaced and remained in the structure. Therefore, the 2,4-D adsorption on LDH3 and LDH4 occurred through the same reaction mechanisms with different surface availabilities for 2,4-D, as suggested in the section on kinetics. 4. Conclusions Mg/Al–NO3 LDHs with different layer charge densities showed significant differences in their 2,4-D adsorption characteristics due to the different nitrate orientations in their interlayers. The 2,4-D adsorption proceeds primarily in response to the Coulomb attraction between the anionic adsorbate and the positively charged centers in the hydroxide sheets of LDHs. The interaction occurs on the external surface and in the interlayer region of the materials. The reaction rate of the former is much faster than that of the latter. When the interlayer nitrate was parallel to the hydroxide sheets, the resulting small interlayer spacing restricted the accessibility of 2,4-D, so the replacement of the interlayer nitrate by 2,4-D did not occur to an observable extent at the experimental conditions of this work. On the contrary, the interlayer nitrate with an orientation perpendicular to the hydroxide sheets was readily exchanged by 2,4-D for the positively charged sites in the hydroxide sheets, subsequently resulting in an higher amount of 2,4-D adsorption as well as the expansion of the basal spacing of LDHs. The extent and mechanism of 2,4-D adsorption are dependent on the orientation of interlayer nitrate, which is controlled by the layer charge density of LDHs. Although the molecular size of 2,4-D is much larger than that of nitrate, the interlayer nitrate can be readily exchanged by 2,4-D when the nitrate orientation is perpendicular to the hydroxide sheets. Therefore, for practical application as a contaminant scavenger, the selective retention of a specific contaminant against competing anions such as NO3−, CO32− and Cl− may be maximized by controlling the orientation of interlayer nitrate in LDHs, which is currently under investigation. Acknowledgement The authors are grateful for funding received from the National Science Council, Taiwan (NSC 94-2313-B-

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