Organo/Zn-Al LDH Nanocomposites for Cationic Dye Removal from Aqueous Media
© Starukh et al. 2016
Received: 30 November 2015
Accepted: 4 April 2016
Published: 27 April 2016
Cationic dye sorption by Zn-Al-layered double hydroxides (LDHs) modified with anionic surfactants was examined using methylene blue (MB) dye as a compound model in aqueous solutions. The modification of Zn-Al LDHs was performed by reconstruction method using dodecyl sulfate anion (DS) solutions. DS contained Zn-Al LDHs were characterized by XRD, FTIR, thermogravimetric, and SEM analysis. The reconstructed organo/Zn-Al LDHs comprise the crystalline phases (DS-intercalated LDHs, hydrotalcite), and the amorphous phase. The intercalation of DS ions into the interlayer galleries and DS adsorption on the surface of the LDHs occurred causing the MB adsorption on the external and its sorption in the internal surfaces of modified LDHs. The presence of DS greatly increased the affinity of organo/Zn-Al LDHs for MB due to hydrophobic interactions between the surfactants and the dye molecules. The optical properties of sorbed MB were studied.
KeywordsLayered double hydroxides Organo/LDH nanocomposites Intercalated surfactants Cationic dye sorption Wastewater treatment
Facing the ever-growing demand for materials combining several properties, research is now extensively devoted to the preparation of multifunctional assembly in which the different components may act in synergism. In such approach, the layered compounds are of great interest , and some interesting multifunctionalities are reported such as drug delivery systems using lamellar nanocomposites with incorporated drug , the flexible solar cell using layered compound-metal particle composites , and flat-panel displays using graphene oxide liquid crystals .
A special attention is now paid to the layered double hydroxides (LDHs) because of their tunable charge density and large chemical versatility [5, 6]. LDHs or hydrotalcite-like compounds are a class of ionic lamellar compounds made up of positively charged brucite-like layers with an interlayer region containing charge compensating anions and solvation molecules. The metal cations occupy the centers of edge-sharing octahedra, whose vertexes contain hydroxide ions and that are connected to form infinite 2D sheets. The chemical composition of LDHs is expressed by the general formula [M2+ 1−xM3+ x(OH)2][An−]x/n·zH2O, where M2+ may be common; Mg2+, Zn2+, or Ni2+, and M3+ may be common; Al3+, Ga3+, Fe3+, or Mn3+. An− is nonframework charge-compensating inorganic or organic anion, e.g., CO3 2−, Cl−, SO4 2−, and RCO2−, and x is normally between 0.2 and 0.4. LDHs layers gain a positive charge by isomorphous substitution of M3+ for M2+, which is compensated by interlayer anions .
Some attempts have been made to prepare the intercalated compounds with a large variety of anionic species for various purposes and applications [7–9]. Recently, LDHs began to be used in the removal of inorganic and organic contaminants. Grover et al.  used synthetic Mg-Al LDHs to the uptake of arsenite ions from aqueous solution by an ion-exchanged process. Alves and coworkers  reported that Mg-Al LDHs removed the colored oxidation debris from wastes originated from carbon nanotubes chemical processing. Calcined LDHs were effectively used to the uptake of anionic dyes from wastewater by sorption in the interlayers, and/or adsorption on the external surfaces of LDHs [12, 13]. As the inorganic LDHs host lamella improve the thermal and photochemical stability of the dyes , the dye sludge can be reutilized as colorant filler for polymer materials exhibiting the resistance to bleeding and fire. The LDH-based pigments can be applied for the production of coatings, paints, and composite materials .
Due to their anionic exchange capacity, LDHs are suitable for intercalation and sorption of negatively charged species but are not applicable for positively charged ones. Recently, some researchers used anionic surfactants to modify LDH’s surface properties, and then to adsorb many types of organic molecules [15–17]. The organic phase in the interlayer which was formed with the intercalated organic ions, acted as an adsolubilization medium increasing the affinity of LDHs for organic compounds. Sodium dodecyl sulfate-modified LDHs demonstrated the greatly enhanced sorption of ionizable and nonionic organic contaminants such as pesticides, herbicides, and aromatic hydrocarbons [16, 18, 19]. Surfactant-intercalated LDHs allowed obtaining an organic–inorganic hybrid pigments based on cationic dyes . Mg-Al LDHs intercalated with DS were effectively used for the removal of cationic dyes as safranin  and methylene blue .
There are several ways of LDHs modification reported in literature, for example, co-precipitation in the presence of organic species, ion exchange method, reconstruction method, etc. [5, 6]. Among these, the reconstruction method has been found to be more facile and could be applied to carry out the modification of large amounts of LDHs at a time. This method is based on a unique property of the layered materials. Most of the LDHs clays can regenerate an original structure from their oxide form, when the latter is dispersed in an aqueous solution containing the anion present in the original material. This phenomenon was also confirmed for obtaining of DS-modified Mg-Al LDHs .
Sodium dodecyl sulfate is a widespread contaminant of aquatic environmental as it is intensively used in many industrial processes, such as colloids stabilization, metal treatments, mineral flotation, and production of daily-used detergents and pesticides [23, 24]. Adsorption technology can offer a potential low-cost treatment of this compound, by using soil, activated carbon, and clays, but after DS adsorbing, their products were difficult to recycle [23–25]. It follows that the LDHs are reasonable to use for DS removal and to exploit the obtained product as valuable resources to dispose organic matter from wastewater.
The investigation of Zn-Al interlayers modified with DS using reconstruction method is scarcely reported in the literature. The published results were devoted to DS-intercalated Zn-Al LDHs synthesized by direct co-precipitation and ion-exchange methods. Therefore, the purpose of this study was to synthesize the surfactant-modified LDHs by reconstruction of calcined Zn-Al LDHs in solutions with different DS concentration and to evaluate the sorption performance of the prepared materials as for the capacity of cationic dye (methylene blue) removal from water, comparing with the sorption capacity of unmodified Zn-Al LDHs.
Preparation of Zn-Al LDHs
All chemicals were analytical grade and used without further purification. Zn-Al LDHs with carbonate as the interlayer anion, with [Zn]/[Al] = 1/2 were synthesized by co-precipitation at a constant pH 10, following the method described in . A mixed solution of 0.1 mol of Zn(NO3)2 and 0.05 mol of Al(NO3)3 in 200 ml of distilled water was added dropwise under vigorous stirring to 200 ml of an aqueous solution containing 0.3 mol of NaOH and 0.1 mol of Na2CO3. The pH 10 was maintained constant by the addition of NaOH. Once addition was completed, the temperature was raised up to 85 °C and the slurry was being kept for 6 h at this temperature under continuous stirring. After that, the slurry was cooled down to the room temperature within several hours. The product was isolated by filtration and washed several times with the deionized water until pH 7. Afterwards, the solid was dried at 100 °C. The sample was labeled as ZnAl LDH.
The Zn-Al LDHs were modified with sodium dodecyl sulfate (CH3(CH2)11SO4Na) by reconstruction method. Zn-Al LDHs have been calcined at 450 °C over 2 h to destroy the layered structure. The suspensions contained 1 g of calcined LDHs and 50 ml of CO2 —free aqueous solutions of DS were stirred for 24 h at room temperature. The concentration of DS ranged from 0.012 mol L−1 to 0.205 mol L−1. This sample was labeled as ZnAl LDH/DS.
XRD patterns of the samples were recorded with the DRON-4-07 diffractometer (CuKα radiation). Calculation of apparent crystallite size for LDHs has been performed by Debye-Scherrer formula: β(2θ) = 0.94λ/(Dcos θ°), using (003) and (110) reflection, employing the FWHM procedure. The thermogravimetric analysis (TGA) and differential thermal analysis (DTA) were carried out using Derivatograph Q-1500 D MOM (Hungary) equipment operated under a flow of an air at the heating rate of 10° min−1. Infrared spectra were obtained in the range of 4000–400 cm−1 on a Thermo Nicolet NEXUS FT-IR spectrophotometer (Nicollet, USA). The morphology and microstructure of Zn-Al LDHs were examined by the scanning electron microscope (SEM; JSM-6490-LV, JEOL, Japan). Diffuse reflectance spectra were obtained with a Lambda 35 UV–Vis (Perkin Elmer) spectrometer equipped integrated with Labsphere RSA-PR-20 in the range of wavelength 200–1000 nm. The UV–visible spectra of the solutions were recorded on a Lambda 35 UV–Vis (Perkin Elmer) spectrometer using a quartz cell (1-cm path length), with distilled water as a blank.
MB adsorption isotherms were determined using a batch adsorption approach. Typically, 0.02 g of freshly as-synthesized and calcined Zn-Al LDHs were introduced into 100-mL glass tubes containing 40 mL of freshly prepared aqueous MB solutions with concentrations ranging from 8 × 10−7 mol L–1 to 5 × 10−5 mol L−1. The adsorption process was allowed to last 4 h under continuous stirring. The particles were removed by centrifugation at 6000 rpm, and the residual concentration of MB in the solution was determined using UV–Vis spectrometry at a detecting wavelength of 663 nm. The equilibrium adsorption amount of MB in the sample was calculated according to equation: q е = (С 0 − С е )V/m, where q e is the amount of MB adsorbed at equilibrium, C 0 is the initial MB concentration, C e is the equilibrium concentration in solution, V is the total volume of solution, and m is the sorbent mass.
Results and Discussions
Structural parameters of ZnAl LDHs
a = 2d110, Å
c = 3d003, Å
Basal spacing d003, Å
Interlayer distancea, Å
Crystal size, Å
ZnAl LDH reconstructed
ZnAl LDH was calcined in air at 450 °C for 2 h to obtain a mixture of zinc and aluminum oxides. The reflections in XRD pattern of calcined ZnAl LDH are characteristic of the ZnO (Fig. 1). It is known that at certain temperature, the original hydrotalcite is converted into a mixture of oxides which have “memory” of the original structure . In fact, in the presence of aqueous solutions, the oxides regenerate the double hydroxides in the form of brucite-like sheets and the positive charges are balanced by anions presented in solutions. The hydration of calcined ZnAl LDH in aqueous suspension caused to the reconstruction of the hydrotalcite phase (Fig. 1). The source of anions in this case is atmospheric CO2 from which CO3 2− ions are formed.
Al (ZnAl LDH):DS molar ratio, DS concentration, d-spacing of LDH/DS, and hydrotalcite phase content of ZnAl LDH modified with DS
Al(ZnAl LDH):DS molar ratio
CDS, mol L−1
Basal spacing d003, Å
Integral intensities I (3.4°)/I (11.7°) ratio
Hydrotalcite phase content, %
7.6; 26.4; 38.4
The thermal decomposition stages of Zn-Al LDHs
Temperature intervals, °C
Total mass loss, %
ZnAl LDH reconstructed
ZnAl LDH/DS (Al:DS = 1:0.33)
LDHs containing CO3 2− anions have the characteristic bands for various modes of infrared sensitive vibrations of the anion. In most LDHs, these bands are observed in the range of 850–880 cm−1 (bending non-planar mode), 1350–1380 cm−1 (the asymmetric stretching mode), and 670–690 cm−1 (the bending angular mode) . In the case of the Zn-Al LDHs, the sharp band at 1360 cm−1 without any distinct shoulder or degeneration indicating the absence of splitting of the asymmetric stretching vibrations (Fig. 3). It has been explained  by the fact that carbonate ions occupy a highly symmetric site between the hydroxide sheets. A small shoulder around 690 cm−1 in ZnAl LDH spectra indicates the bending angular mode vibrations and the band around 870 cm−1 indicates that the bending non-planar vibrations are overlapped with stretching M–OH vibrations (Fig. 3). A broad band at 3200–3700 cm−1 is observed for all the compounds. It is attributed to OH stretching vibrations of the octahedral layer and intercalated water molecules . The shoulder at 3050 cm−1 of IR spectra of ZnAl LDH is attributed to hydrogen bonding of H2O molecules to CO3 2− ions in the interlayer space . The bands at 1510 and 1385 cm−1 are signed to polydentate carbonates while the sharp band at 2340 cm−1 corresponds to the vibrations of gaseous CO2 present in the experimental conditions. The vibrations below 700 cm−1 are characteristic to metal-oxygen bond stretching. The various lattice vibrations associated with metal hydroxide sheets caused to the appearance of the sharp bands around 780, 550, and 430–450 cm−1 .
In the case of calcined LDHs, the absorption at 3800–3500 cm−l is pointed to the stretching vibrations of surface-free hydroxyl groups. The bands at 1510 and 1385 cm−1 are due to residual bulk polydentate carbonates while the sharp bands at 2340 cm−1 belong to gaseous CO2 present in the experimental conditions. Thus, the crystal-layered structure is destroyed after calcination of LDHs at 450 °C (as was confirmed with the XRD pattern in Fig. 1), but the calcined products still contain the interlayer carbonate anions and bounded water.
IR spectrum of the ZnAl LDH/DS contains the vibration of carbonate ions located at 1360 cm−1 that indicates the presence of carbonate form of ZnAl LDH. All the typical bands of DS were observed at corresponding wavenumbers, such as C–H stretching and bending bands (2840–2955 cm−1), –OSO3 − anions stretching and bending bands (1211, 1060, and 720 cm−1) . The stretching vibrations of lattice water and –OH groups were appeared at 3480 and 3636 cm−1 as the strong overlapping bands. The peak at 1620 cm−1 was attributed to the H–O–H bending vibration of the interlayer water molecules in organo/Zn-Al LDH.
The smaller total mass loss of the reconstructed ZnAl LDH is explained by the fact that some part of calcined LDHs was not restored by hydration. So, the reconstructed LDH contained fewer hydroxides and carbonate ions in comparison with the as-synthesized ZnAl LDH. As shown by XRD analysis, the traces of ZnO are remained in the pattern of the reconstructed ZnAl LDH (Fig. 3).
The thermal decomposition of DS-modified LDH is almost identical to ZnAl LDH (Fig. 4c). The temperature of interlayer water loss seems to be lower than that of the as-synthesized and the reconstructed ZnAl LDH indicating the changes of LDH structure. It can be caused by the fact that DS ions intercalated into interlayer of LDHs reduce the interactions of water with the environment in ZnAl LDH. In other words, DS intercalation into ZnAl LDH increases the hydrophobic nature of LDH interlayer surface . The second step of dehydroxylation of the brucite-like sheets is accompanied by DS ions destruction. The decomposition of DS ions takes place in the range 210–250 °C , and therefore, a greater loss is observed above 200 °C. The mass loss of DS-modified LDHs at 300–500 °C occurs due to the total dehydroxylation and the collapse of the layered structure. The thermogravimetric analysis coupled with mass spectroscopy revealed that the pyrolysis of the organic anion takes place under thermal treatment of LDHs . The mass loss between 800 and 900 °C can be ascribed to SO3 evolution as a result of the decomposition of (Zn, Al) sulfate formed by decomposition of DS anions during the second mass loss stage . The total mass loss of ZnAl LDH/DS is calculated as 51 % that is on 24 % more than the total mass loss for ZnAl LDH reconstructed in aqueous suspensions. This increase in the mass loss is resulted from the loading of DS, whose decomposition is reflected by the mass loss in 400–900 °C and the exothermal peak at 642 °C.
Study of MB Dye Removal
Comparison of the MB sorption over ZnA LDH modified with DS solutions of various concentrations
Al (ZnAl LDH):DS molar ratio
CDS, mol L−1
q (MB), mmol g−1
q (MB), mg g−1
% of MB removal
calcined ZnAl LDH
ZnAl LDH with adsorbed DS
The results indicate that the organic phase (DS) greatly increased the affinity of organo/Zn-Al LDHs for MB due to hydrophobic and/or electrostatic interactions between the surfactants and the dye molecules. The pH value of the solution is an important parameter for control of the sorption process since it can affect sorption of the dye by changing the surface charge of the sorbents and ionization behavior of the sorbents and dye. The effect of pH on the removal of MB was studied using the dye concentration of 2 × 10−4 mol L−1 and at pH values in the range of 3–9. From the results presented in Fig. 6b, it was clear that the sorption capacity was not particularly dependent on pH. This might be due to the buffering properties of the DS intercalated sample.
The electrostatic attraction between the dye anions and the positively charged hydroxide surface in LDHs was the rate-limiting step in most of the surface adsorption studies. But in our case, the organic modification of ZnAl LDH produced high hydrophobicity due to the linear arrangement arising from the assemblage of DS anions. Such aggregation has been reported to enhance the adsorption capacity over a broad spectrum of dye molecules irrespective of their ionic charges and structure . Under such circumstances, the hydrophobic/hydrophobic interactions between the alkyl chains of the surfactant and MB molecules are predominate over the electrostatic attractions. There was confirmed the prevailing influence of the hydrophobicity on the high adsorption of the cationic dyes by various sorbents modified with surfactants [18, 22].
MB adsorbed on interlamellar clay sites can be distinguished spectroscopically from MB adsorbed on external surface sites . The diffuse reflectance spectra of ZnAl LDH/DS with sorbed MB (q = 32 mg g−1) were analyzed (Fig. 7, #2). According to , the absorption band at 652 nm indicates the adsorption of MB monomers on the internal surface sites of organo/LDHs (Fig. 7). The diffuse reflectance spectra of ZnAl LDH/DS do not contain the absorbance band of an unsymmetrical MB dimer at 718 nm that was observed for MB adsorbed on the interlayer surface of hectorite . The existence of the MB dimers on the external surface is confirmed by the adsorption band at 615 nm. The absorption at 673 nm that is typical for the MB monomers adsorbed on the external surface  was absent. Thus, the interlayer space of LDHs included monomers of MB and the external LDHs surface contained MB dimmer forms. Evidently, the organo/Zn-Al LDHs consist of DS in two forms: intercalated into the interlayer galleries of the LDHs and adsorbed on the surface.
The diffuse reflectance spectra of ZnAl LDH/DS with higher loading of MB (110 mg g−1) show the broadening of absorption to 450 nm (Fig. 7, #3). The absorption of MB trimer state is observed at 570 nm [42, 43]. It is possible that aggregates with larger than three units are formed on the surface of ZnAl LDH/DS with higher MB loading.
Adsorption capacity of (q max calculated from Langmuir model) of adsorbed MB by several adsorbents
The modification of ZnAl LDHs in DS solutions by reconstruction method is the facile way of preparation of organo/LDH nanohybrids for the sorption of cationic dye MB from the aqueous medium. The reconstructed organo/Zn-Al LDHs contained as the crystalline (DS-intercalated LDH and hydrotalcite structure), so amorphous phases. The internal and external surfaces of LDHs are modified with DS that causes MB molecules embedding in the interlayers and its adsorption on the surfaces of LDHs. The organo/LDH sorbents are stable to leaching of DS anions from the interlayer spacing. The study of pH effect on MB sorption by ZnAl LDH/DS leads to the conclusion that the hydrophobic/hydrophobic interactions between the alkyl chains of the surfactant and MB molecules are predominated over the electrostatic attractions. Despite the fact that the organo/ZnAl LDHs contained the DS-intercalated phase and hydrotalcite phase, they showed sorption capacity to MB, comparable to LDHs composites, which exclusively consisted of DS-intercalated phase. The colored organo/Zn-Al LDHs are easy to separate from the solution, and the resulting sludge could be reutilized in other applications like as pigments, colorant additives for polymers, and light-activated antimicrobial surfaces.
The current research work provides a further insight into the applications of LDHs-based composites as surfactants and dyes sorbents for wastewater treatment.
Authors acknowledge Dr. Mykola Borysenko for thermogravimetric and differential thermal analyses.
Dr. hab. S.B. Shekhunova for SEM measurements in the Center for shared use of scientific equipment of Institute of Geologic Sciences of NASU.
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