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Removal of synthetic azo dyetartrazine from wastewater by layered double hydroxide material Zn2-Al-Cl: kinetics, isotherms and thermodynamics study

Abstract
In this work, tartrazine dye is removed from wastewater by layereddouble hydroxide (LDH) Zn2-Al-Cl. LDH materials have proven to be highly effective in removing pollutants, with a low cost of synthesis and withoutbeing toxic or regenerating the sludge.
Several parameters were studied, the retention of dye by LDH is optimal for a pH between 5 and 8, the retentionequilibrium is obtained after 24 hours, and retention kinetics follows the pseudo-second order model. The isotherms are of L type and they follow the Langmuir model, retention capacity reaches 100% for optimalmass ratio adsorbate/adsorbent equal to0.5, and the maximum amount retainedofdye is 743.4 mg/g.
X-ray diffraction showed that the synthesized matrix is crystallized in lamellar structure, andtwo processes achieve retention of the dye: adsorptiononto surface of LDH and intercalation between the layers. Infrared analysis indicated the appearance of the valence bands corresponding to the dye in the spectrum of the phase obtainedafter retention. Scanning electron microscopydisplayed the lamellar character of the two phases obtained before and after retention. Thermodynamic study showed that the process is endothermic and the adsorption mechanism is governed by physisorption.

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Keywords:Lamellar double hydroxide, Tartrazine, Adsorption, Isotherm, Kinetic study, Thermodynamic study.?
Introduction
Today, dyestuffs are used extensively in virtually all fields: textiles, tanning, food processing, stationery, paints and cosmetics. However, industrial effluents containing dyes presents many problems for environment, such as eutrophication, under-oxygenation, color, turbidity, odor, persistence, bioaccumulation, mutagenicity and carcinogenicity 1.
The family of azo dyes is the most used, and is the most toxic, whose toxicity was already reported in 1895 by the increase of bladder cancer among workers in the textile industry 2. Indeed several studies have proved the carcinogenic effect of azo dyes for humans and animals 2-5, in fact the azo bond is the most labile portion of these molecules, then by enzymatic action the azo bond will be broken and therefore carcinogenic amine derivatives are regenerated 3,6,7.Therefore, it is necessary to pre-treat the wastewaters loaded with these dyes before rejecting them into environment.
Among the most toxic azo dyes istartrazine3, which is a yellow synthetic azoic dye, it’s used in many industries such as food (E102 – Yellow 5 in USA) and cosmetic (CI 19140). Several studies have highlighted the impact of tartrazine on health, causing hyperactivity, anxiety, depression, irritability, restlessness, sleep disturbanceand asthma 8,9. Other research has shown that tartrazine has genotoxic and cytotoxic effects 3. In this context, several research focus on the elimination of tartrazine dye from wastewater by different treatment methods. These methods include removalbyanion exchange resin 10, treatment by advanced oxidation processes such as photo–Fenton 11, or by photo-catalytic through TiO2 12 or treatment by electro-coagulation with electrochemical advanced oxidation processes 13. However, these treatment methods are expensive, require high technology and regenerate sludge andsometimesmore toxic and unknown products. The adsorption method remains the most used because of its simplicity, efficiency and it is relativelylow cost. Several materials are used for the removal of tartrazine, such as activated carbon 14-16, chitin, chitosan 17, etc.
Among these materials, layereddouble hydroxides (LDH) have proven effective to eliminate many pollutants, so they are inexpensive, recyclable, non-toxic, and do not regenerate sludge 18.
In this context, the aim of our work is to study the possibility of the removal of tartrazine dye from water byLDH material Zn2-Al-Cl.
LDH materials have been proved to have a high retention efficiency, thanks to their anionic exchange capacity, which is due to their lamellar structure, which can intercalate different anions in their inter-lamellar space 18.The general formula of LDH materials is expressed as MII1-xMIIIx(OH)2x+(Xm?x/m), nH2Ox-, where MII and MIII are divalent and trivalent metal cations that occupy octahedral sites in the hydroxide layers, Xm? is an exchangeable anion located in the interlayer space between two hydroxide layers 19 .

Fig. 1. Schematic representation of LDH phases 20.
Materials and methods
2.1. Synthesis and characterization of adsorbent
For the synthesis of the LDH precursor, we used the co-precipitation method at constant pH 21. The optimal conditions for the synthesis were pH = 9, molar ratio Zn/Al = 2 and the addition rate of the metal salts (Zn2+, Al3+) is 3 ml/h. The pH was maintained constant by the addition of NaOH(0.5M).
The synthesis carried out under a stream of N2 to avoid contamination by atmospheric CO2. The ripening time (72h), which is a decisive step in obtaining a relatively good product, the precipitate separated by filtration and washed by decarbonated water in order to remove the residual electrolytes (sodium chloride) formed during the synthesis. The product finally dried at room temperature and gave a powdery texture material.
Characterization of the powder obtained by X-ray diffraction (XRD) (Fig. 2) showed that the phase corresponds to a pure LDH 22. The obtainedmaterialwell crystallizedin single phase with large constituting crystallites. The XRDpatternshowed that our material crystallized in a rhombohedral structureand thatthe spatial symmetrygroup is R?3mwith the cell parameters: a = 0.307 nm;c = 2.322 nm andinterlamellar distance d = c/3 = 0.774 nm.

Fig. 2. XRD pattern of Zn2-Al-Cl precursor.

2.2. Retention procedure
Retention tests carried out at room temperature (25°C), at constant pH, maintained by adding NaOH orHCl dilutedsolutions, and under an inert atmosphere. The amounts of Zn2-Al-Cl were dispersed in 100 mL solutions of the dye prepared by dissolving the powder dye in the decarbonated water. The initial concentration of tartrazine were varied between 50 and 1200 mg/L. After filtration, the obtained solid were dried at room temperature before being analysed by XRD technique. The residual dye concentration was determined by UV-Vis spectroscopyat 410 nm.
The retained amount Q (mg/g) of the dye by Zn2-Al-Clwas calculated by equation (1):
(1)
With C0is the initial concentration,Cethe concentration at equilibrium (final) of the solution dye, m the mass of the adsorbent and V the volume of the solution.
2.3. Analytical techniques
XRD equipment used was a Siemens D 501 diffractometer. Samples of unoriented powder were exposed to copper K? radiation (?= 0.15415 nm). Measurement conditions were 2h, range 5-70°, step size: 0.08-2h, and step counting time: 4s. Data acquisition was effective on a DACO-MP microcomputer.
Absorbance IR spectra were recorded on a JASCO6300 PC spectrophotometer, at a resolution of 2 cm-1 and averaging over 100 scans, in the range 400–4000 cm-1. Samples were pressed into KBr discs.
The absorbance UV-Vis was measured by Jenway 6300 spectrophotometer.
Scanning electron microscopy(SEM) allows viewing the external morphology of materials, the principle of scanning is to scan the surface of the sample in successive rows and to transmit the sensor signal to a cathode ray tube, whose scan is exactly synchronized with that of the incident beam, the scanning microscope use a very fine beam which scans the surface point by point of the sample.
Results and discussion
Preliminary adsorption experiments conducted to determine the optimal conditions for the retention of tartrazine onto LDH regarding the pH value, contact time (tc), initial concentration (C0) of adsorbate and the mass ratio tartrazine/LDH.
3.1. Effect of pH
pH of the mixture (solution of dye and matrix) is a factor influencing the retention of tartrazine by the matrix. Initial concentration of tartrazine used wasC0=200 mg/L. The mixtures stirred for48 h and filtered on sintered glass; the filtrate assayed by UV-visible spectrophotometer. The result obtained is givenin Fig. 3.
Fig. 3. Retained amount of tartrazine by LDH Zn2-Al-Cl in function ofpH.
According to the graph, the retention is favoured in solution having a pH between 5 and 8. For more acidic solution there is a decrease in retention which can be explained by a partial dissolution of the host matrix, and as to the basic solution there is also a decrease in the retention that is related to competition with the carbonate ions which have a high affinity to LDH 22, 23.Therefore, the pH will be fixed at 7in the allfollowingparts of thisstudy.

3.2. Kinetic study
To follow kinetics of retention and to optimize the dye-matrix contact time, the adsorption kinetics were followed in a time interval up to 48 h, for initial dye concentrations of 50, 200, 400 and 800 mg/L,the used mass of the matrix is 50 mg.
Fig. 4. Adsorption kinetics of tartrazine by Zn2-Al-Cl.
From these results, it is observed that the process of retention of tartrazine by LDH Zn2-Al-Cl is rapid; the saturation was obtained in 1h for 50 mg/L, in 6h for 200 mg/L and in 24h for 400 and 800 mg/L.
Similarequilibrium time was obtained in other studies such as the retention of the herbicide 2,4-dichlorophenoxyacetic acid (2,4-D) by the same matrix Zn2-Al-Cl 18.
The adsorption sites provided by the hydroxyl functions are accessible for the low concentrations and the equilibrium is obtained for a shorter time, but for the high concentrations, there is competition between the anions of the dye for the adsorption sites, this competitiondelaysreaching the equilibrium state due toelectrostatic repulsions between anions. Another phenomenon that can delay the achievement of equilibrium is the exchange of chloride anions by dye anions in the interlamellar space, which will become an increasingly slow phenomenon depending on the concentration of dye 18.
The linear models according to pseudo-second order 24 (equation 2) and pseudo-first order25(equation 3) are givenon the following equations:
(2)
(3)
With Qeistheamount retained at equilibrium (mg/g), Qttheamount retainedat a given time (mg/g),k2 a kinetic constant of pseudo-second order (g/mg/h) and with k1 a kinetic constant of pseudo-first order (h-1).
Table 1
Parameters of pseudo-first and pseudo-second order models.
Table 1 above indicates that the retention kinetics of tartrazine dye is in good agreement with the pseudo-second order model and that the constant k2 decreases with the increase in the initial concentration of tartrazine.Values of the correlation coefficient are far from unity indicates that the pseudo-first order model is not suitable in this case.
3.3. Intra-particle diffusion
To have an idea of the different processes of diffusion of the adsorbate towards the adsorbent, the adsorption capacity is plotted as a function of the square root of the time according to the equation 4 26:
(4)
Fig. 5. Intra-particle diffusion kinetic for adsorption of tartrazine onto LDH.
For the high concentrations (400 and 800 mg/L), there are two diffusion processes, the first phase is due to external diffusion, while the second phase is linked to the diffusion in the interlayerspace by intercalation of dye between LDH sheets which is proved by XRD after retention of dye, this diffusion state corresponds to intra-particle diffusion. Whereas the third phase indicates the equilibrium state. For the low concentrations (50 and 200mg/L), it is noticed that the external diffusion is fast, and the equilibrium state reaches quickly. In this concentration range, the phenomenon of intra-particle diffusion is absent.
Adsorption isotherms
The tracing of the isotherms of retention gives us information on the maximum adsorption capacity and the adsorption mechanism. The study was realizedwith initial concentration range from 50 to 1200 mg/L with different LDH doses 30, 50, 80 and 100 mg.
Fig. 6. Adsorption isotherms of tartrazine onto LDH at different adsorbent doses.
It can be noticed that isotherms of retention are of L type 27.Initial parts of the isothermsare almost vertical; which indicates that the interaction between the adsorbed molecules and the surface of the solid are very strong, interaction adsorbate–adsorbent is much stronger than solvent–adsorbent at the adsorption sites. Isotherms with this profile are typical of systems where the functional groups of adsorbate is strongly attracted by the adsorbent, mostly by ion–ion interaction, which tends to reach a saturation value given by a plateau. These results suggest that tartrazine anions are preferentially removed 28.
The linearizationaccording toLangmuir 29 and Freundlich30models aresuccessively givenbyfollowing equations:
(5)
(6)

Table 2
Parameters of Langmuir and Freundlich models.
From these results, it can be seen that the maximum quantity retained increases with the mass of LDH and reaches 743.4 mg/g. The four isotherms are in good agreement with the Langmuir model, which suggests that the surface of LDH is homogeneous and the adsorption is done in monolayer.The values of the constant K are neighboursfor the four isotherms, which reflect that the nature of the interactions between adsorbate and adsorbent are similar 18.
3.5. Effect of the mass ratio (adsorbate/adsorbent)
To determine the influence of the mass ratio adsorbate/adsorbent on removal rate, the initial concentration of tartrazine was varied from 50 to 800 mg/L while using an LDH mass of 50 mg.
Fig. 7. Effect of mass ratio (adsorbate/adsorbent)
From this graph, it can be seen that the tartrazine has been eliminatedwithmass ratio between 0.1 and 0.5. To optimize the mass of the LDH used while maintaining the total removal, it is interesting to use the most importantratio with a smallest dose of LDH.That is, for any given tartrazine concentration, the optimal mass of LDH to remove totally the dye is determined for a mass ratio of 0.5.
3.6. Adsorption and intercalation of tartrazine
3.6.1. Study by X-ray diffraction
Fig. 8. XRD patterns of LDH andthe phases obtained after retention of tartrazine
attwo mass ratios R = mTAR/mLDH.
It can be clearly seen that for low mass ratio (adsorbate/adsorbent) (R=0.1), the lines of the matrix before and after retention of tartrazine are similar (Fig.8), this can be explained by adsorption of dye onto LDH surfacewithout affecting the interlamellar distance between the leaflets. Whereas from high mass ratio (R=3), there is a shift of the line (003) to the low values of 2? at 3.8o (this results is similar to that obtainedin another work with an LDH based on metals Mg and Al31).The interlamellar distance increasesfrom d = 0.777 nm to d = 2.25 nm due to the interchange of interlamellar Cl- ions with the larger tartrazine anions.
Accordingto these results, it can be concluded that two processes, adsorption for low mass ratio adsorbate/adsorbent, and adsorption and intercalation for high mass ratio do the retention of tartrazine dye. Knowing that the thickness of the brucitic sheets equal to 0.21 nm, the hydrogen bond has a distance of 0.27 nm and the length of the molecule of tartrazine is 1.8 nm 17, if we add these distances, we will have a value greater than 2.25 nm founded experimentally by XRD.Then we can explain this result by the fact that the anion representing tartrazine wasn’t intercalated vertically, but it is interposed a little inclined by 34°, other result was found with an interlayer distance of 2.30 nm close to our result equal to 2.25 nm 31. Therefore, an orientation of the tartrazine anions between LDHsheets enables us to propose astructural model for Zn2-Al-TAR in Fig. 9.

Fig. 9. Structural model proposed for Zn2-Al-TAR.
The cohesion within the material is ensured by hydrogen bonds between the sulfonate groups of dye and the hydroxyl groups of LDH sheets.
3.6.2. Study by infrared spectroscopy
This technique allows demonstrating the presence of the tartrazine anion and interactions, which may exist with the matrix.
Fig. 10. IR spectra of tartrazine dye, LDH-tartrazine and LDH matrix.
It is noted that the absorption bands of tartrazine appear in the spectrum of the phase obtained after retention, such as:
The broad band at 3400 cm-1that corresponds to the OH valence vibrations.
The characteristic bands of tartrazine about 1500 cm-1 which corresponds to the vibrations bands of C = C of benzene ring.
Vibrationstowards1000 and 1200 cm-1corresponds to the asymmetric and symmetric vibrations of the sulfonate groups S=O.
The bands between 700 and 800 cm-1corresponds to the deformationof (C-H) outside benzenering.
According to these results, it is confirmed that tartarzine dye was well retained by the LDH matrix.
3.6.3. Study by Scanning Electron Microscopy
SEM allows us to have information on the morphology of the matrix before and after retention of the tartarzinedye.SEM images are obtained with a magnification of 20,000 times(Fig. 11), it noted the appearance of the lamellar character of the LDH matrix, with the crystallites size go up to 1 µm. The lamellar character after retention decreases which is confirmed by XRD results.

Fig. 11. SEM images of phases before (a) and after retention of tartrazine dye by LDH (b).
3.7. Comparative study
Several studies have shown that adsorption, or elimination by different materials, is easily realizable. Many adsorbents are usedfor the removal of pollutants from wastewater such as activated carbon, food waste, cationic clays, chitosan, LDH, etc. The difference between these materials is due to their ability to fix pollutants and to their elimination rate.
In this work, we will compare our results with other works, based on the removal rate and maximum retention capacity of tartrazine by different materials (Table 3).
Table 3
Removal rate and amount retained at equilibrium of tartrazine by different materials.
From this comparative study, it is clear that the removal of tartrazine dye by LDH is total (100%), is much greater than that obtained with other materials, retention capacity reaches 743.4 mg/g, LDH Zn2-Al-Cl can be classified as having a relatively high retention capacity. This makes our material more efficient compared to other materials and promising for the adsorption and elimination of this type of pollutant.
In addition, LDHs do not regenerate sludge like food waste, the cost of synthesis is relatively less than the synthesisof activated carbon, and in addition, they are recyclable by anion exchange reactions.
3.8. Thermodynamic study
Temperature is an important thermodynamic factor, which influences the maximum amount retained at equilibrium. Then to determine the thermodynamic parameters for retention of tartrazine by LDH Zn2-Al-Cl, in a thermostat which adjusted three temperatures at 303, 313 and 333 K, for an initial concentration 400 mg/L of tartrazine dye and 50 mg of the matrix, we determined the concentration Ce at equilibrium and the maximum quantity retained Qeat equilibrium state.
Thermodynamic parameters such as standard free enthalpy ?G°, standard enthalpy ?H° and standard entropy ?S° were determined using the following equations 38:
(7)
(8)
(9)
With kdis the distribution constant; Qeadsorption capacity at equilibrium (mg/g); Ce equilibrium concentration of solute in solution (mg/L); R the ideal gas constant (J/mol/K) and T: absolute temperature (K).
Table 4
Thermodynamic parameters of retention
The results show that the retention capacity increases by the increase of temperature, so the retention mechanism is endothermic, whichis provedby the positive value of ?H°. The negative value of ?G ° confirms the spontaneous nature of the retention. The increase of ?G ° with the temperature indicates that the adsorption process is more favorable at high temperature.
The value of ?H°

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