Removal of indigo carmine and green bezanyl-F 2 B from water using calcined and uncalcined Zn / Al þ Fe layered double hydroxide

Layered double hydroxide Zn/(Alþ Fe) with a molar ratio of 3:(0.85þ 0.15), designated as ZAF-HT, was synthetized by co-precipitation. Its calcined product CZAF was obtained by heat treatment of ZAF-HT at 500 WC. The calcined and uncalcinedmaterialswere used to remove the acid dyes indigo carmine (IC) and green bezanyl-F2B (F2B) from water in batch mode. The synthetized materials were characterized by X-ray diffraction, scanning electron microscopy, Brunauer–Emmett–Teller analysis, Fourier transform infra-red spectroscopy and thermogravimetric/differential thermal analysis. The sorption kinetic data fitted a pseudo-second-order model. The adsorbed amounts of the calcined material were much larger than ZAF-HT. The maximum adsorption capacity of CZAF was found to be 617.3 mg g 1 for IC and 1,501.4 mg g 1 for F2B. The isotherms showed that the removal of IC and F2B by ZAF-HT and CZAF could be described by a Langmuir model. The thermodynamic parameters were also calculated. The negative values of standard free energy ΔGW indicate the spontaneity of sorption process. The reuse of CZAF was studied for both dyes and the calcined material showed a good stability for four thermal cycles. This is an Open Access article distributed under the terms of the Creative Commons Attribution Licence (CC BY 4.0), which permits copying, adaptation and redistribution, provided the original work is properly cited (http://creativecommons.org/licenses/by/4.0/). doi: 10.2166/wrd.2016.042 from https://iwaponline.com/jwrd/article-pdf/7/2/152/376792/jwrd0070152.pdf ber 2018 Hassiba Bessaha (corresponding author) Mohamed Bouraada Laboratoire de valorisation des matériaux, Faculté des Sciences exactes et de l’Informatiques, Université Abdelhamid Ibn Badis Mostaganem, B.P. 227, Mostaganem 27000, Algeria E-mail: hassiba2544@hotmail.fr; technologieverte@hotmail.fr Louis Charles Deménorval ICG–AIME–UMR 5253, Montpellier 2 University, Place Eugene Bataillon CC 1502, Montpellier Cedex 05 34095, France


INTRODUCTION
Synthetic dyes have long been used in various industries, such as textiles, cosmetics, paper, leather, pharmaceuticals and foods (Yuan et al. ).During and after the dyeing process, huge amounts of dyes are released as industrial discharges into the environment causing serious problems.
They are aesthetic pollutants, and the coloration of water by dyes reduces the penetration of light which affects photochemical activities (McMullan et al. ).As a result, the aquatic ecosystem is destroyed.Moreover, it has been reported that many dyes are toxic, carcinogenic and mutagenic for aquatic organisms, even at very low doses, and may cause severe damage to human beings, such as dysfunction of the kidneys, reproductive system, liver, brain and central nervous system (Mittal & Gupta ).
Chemical, electrochemical and photochemical methods, reverse osmosis, coagulation and aerobic and anaerobic biodegradation have been used to remove dye compounds from water and wastewater (Zhu et al. ; El Gaini et al. ).
However, photodegradation methods may not fully mineralize synthetic dyes, due to their resistance, and also may produce persistent degradation products which are toxic and carcinogenic for aquatic organisms (Lourenco et al. ).Furthermore, treatment of dyes in water by chemical or electrochemical methods, reverse osmosis and coagulation is not widely applicable because of economic considerations (Zhu et al. ; El Gaini et al. ).However, the adsorption process has widely been used to remove different types of dye from water using less expensive adsorbents.This process has several advantages over others, such as high efficiency, no harmful by-products and simplicity of operation.
Layered double hydroxides (LDHs), also known as hydrotalcite-like materials or anionic clay, can be good adsorbents for removal of organic and inorganic pollutants because of their high capacity for anion exchange and high layer charge densities (Extremera et al. ; González et al. ).LDHs are scarce in nature, but can be prepared easily and in large quantities in the laboratory using inexpensive precursors (Seftel et al. ).Their general formula is , where M 2þ and M 3þ are di-and trivalent metal cations, A n is an exchangeable organic or inorganic anion with negative charge n, m is the number of interlayer water molecules and x ) is the layer charge density of the LDH (Kameda et al. ).LDHs can be used to remove pollutants by anionic exchange with the original interlayer anions.
However, the latter process is not always feasible, especially when the initial interlayer anions have a strong affinity toward the LDH layers, e.g.carbonates ions.and entropy (ΔS W ) were calculated.The reusability study of CZAF for both dyes for four thermal cycles was also studied.

Adsorption equilibrium experiments
The removal of IC and F2B was studied in batch mode at (Equation ( 1)) and the removal percentage of the dyes (R %) (Equation ( 2)) were calculated using the following equations: where C i (mg L À1 ) and C e (mg L À1 ) are the initial and the equilibrium concentration of dyes, respectively, C t (mg L À1 ) is the concentration of the dye solution at time t, V (L) is the solution volume and m (g) is the adsorbent mass.

Temperature effect
The temperature effect was studied on suspensions of CZAF in IC and F2B solutions with initial dye concentration of 260 and 800 mg.L À1 , respectively (solid/solution ratio ¼ 0.5 g L À1 ).
The suspensions were stirred during the equilibrium time at three constant temperatures (25, 35 and 45 W C) and then centrifuged, and the residual concentrations were determined as above.

Reusability study
The regeneration of CZAF was based on a thermal recycling method.After the adsorption of dyes (IC or F2B), the material was recovered and calcined at 500 W C for 4 h, and then the calcined product was dispersed into a known concentration of dye solution.The residual concentration of dye was determined as above.This procedure was repeated three times.

Characterization of materials
The

Effect of contact time and kinetic modelling
The contact time between the pollutants and the adsorbent is an important parameter in treatment by adsorption.Figure 6 ln where Q t and Q e are the adsorbed amount at time t and at equilibrium state, respectively (mg g À1 ), k 1 : the pseudo-firstorder rate constant of adsorption (h À1 ), and k 2 : the pseudosecond-order rate constant of adsorption (g mg À1 h À1 ).
The kinetic parameters of the models were calculated and are reported in Table 1.Based on the values of determination coefficient R 2 , the sorption of the dyes was better expressed by a pseudo-second-order kinetic model (Figure 6(b)).Several studies reported that the adsorption of acid dyes by anionic clay was described by a pseudo-     Table 1 | Parameters of pseudo-first-and the pseudo-second-order models for sorption of IC and F2B by CZAF where C e (mg L À1 ) is the equilibrium concentration of dyes, Q e is the amount adsorbed at equilibrium, Q max is the maximum monolayer adsorption capacity (mg g À1 ), K L is the Langmuir constant refers to energy of adsorption and K F is the Freundlich isotherm constant (L g À1 ) related to the adsorption capacity.1/n is the heterogeneity factor that could be obtained from the slope of the plot lnQe versus ln C e .
The parameter values of Freundlich and Langmuir models are reported in Table 2.The sorption of both dyes by ZAF-HT and CZAF fitted very well to the Langmuir isotherm model (Figure 9) with determination coefficient values R 2 very close to 1.The maximum adsorption capacity of the calcined material was found to be 617.W ) and entropy (ΔS W ) were estimated using Equations ( 7) and ( 8):  where T is the absolute temperature (in K), R the gas constant (8.314J mol À1 K À1 ), and K d (cm 3 g À1 ) is distribution coefficient of adsorbates between liquid and solid phase.
K d was calculated using Equation ( 9): LDHs into mixed metal oxides with high specific areas and homogeneous dispersion of metal cations (Ni et al. ).The calcined-LDHs (CLDHs) are also used as adsorbents to remove anionic contaminants from water via a specific property called 'memory effect'.The CLDHs regain their original structure by the intercalation of anionic species such as acid dyes (Lv et al. ).In the present study, ZAF-HT LDH was synthesized by co-precipitation and calcined at 500 W C (CZAF).The adsorptive performances of the calcined and the uncalcined materials were tested on the uptake of two anionic dyes, indigo carmine (IC) and green bezanyl-F2B (F2B), from aqueous solution in batch mode.The effect of various sorption factors, such as kinetics, solution pH, isotherms and temperature, was investigated.The thermodynamic parameters of change in standard free energy (ΔG W ), enthalpy (ΔH W ) room temperature and at atmospheric pressure.The effects of contact time, solution pH, initial concentration of the dyes and temperature on the adsorption process were investigated.A suspension containing 25 mg of ZAF-HT or CZAF was added to 50 ml of IC or F2B solution with initial concentrations of 250 and 750 mg L À1 , respectively.The suspensions were stirred for various time intervals (0.5-24 h) without adjusting the initial pH of solution.To study the effect of pH, the suspension pH was adjusted in the range 5.0-9.5 by adding 1 N HCl or 1 N NaOH.Samples were stirred during equilibration and then centrifuged.The concentrations of dyes in the supernatant were determined by visible spectrophotometry on a HACH DR/4000 U spectrophotometer at 610 nm and at 646 nm for IC and F2B, respectively.The equilibrium sorption amount Q e (mg g À1 ) Figure 1 | XRD patterns: ZAF-HT (a), ZAF-IC (b), ZAF-F2B (c) and CZAF (d).

Figure 5
Figure 5 shows the thermoanalytical measurements of ZAF-HT to evaluate the different transformations of the sample during the heat treatment.At temperature range 80-220 W C, a small endothermic peak was observed and related to dehydration of water molecules from the internal gallery and the external non-gallery surfaces.In the second region from 220 to 400 W C, about 11% of weight was lost due to the dehydroxylation and the beginning of carbonate decomposition (Seftel et al. ).
(a) shows the time effect on the sorption of IC and F2B by CZAF: the adsorbed amount increased with contact time, and remained almost constant after 4 h and 20 h for IC and F2B, respectively, indicating an equilibrium state.The adsorption capacity of the material at the equilibrium was 488.9 mg g À1 for IC and 1,487.9mg g À1 for F2B.El Gaini et al. () studied the removal of IC by calcined Mg-Al-CO 3 where the equilibrium time was found to be 20 min.However, Bouraada et al. reported that the sorption of F2B by Mg-Al-SDS takes 2 h to reach the equilibrium state (Bouraada et al. ).Thus, the equilibrium time depends closely on the nature of the adsorbent (structure and chemical composition) and the dye structure.Adsorption kinetics models were used in order to explain the possible adsorption mechanism.Pseudo-firstorder (Equation (3)) and pseudo-second-order (Equation (4)) are the most used models (Mantilla et al. ), and they were applied to the experimental data of the uptake of IC and F2B by CZAF material.
second-order model (Ni et al. ; Bouraada et al. ; El Gaini et al. ; Ahmed & Gasser ).Effect of initial pHThe variation of the sorption capacity of CZAF at different initial pH solution is presented in Figure7.This figure showed that the sorption amount was almost constant in the studied pH range (5.0-9.5) for both dyes.These results are consistent with previous reports (El Gaini et al. ).Thus, it was decided for the rest of this study to carry out the sorption experiments without pH adjustment (pH i of IC equal 6.10 and pH i of F2B equal 5.01).Sorption isothermsSorption isotherms of IC and F2B by CZAF and ZAF-HT are shown in Figure8.The sorption capacities of CZAF were higher than those of ZAF-HT for both dyes.The adsorption amount of F2B by CZAF was about a hundred times greater than that of ZAF-HT.On the other hand, the mixed oxides metals CZAF regained their original structure by the intercalation of the dye molecules into interlayer space.Moreover, the sorption capacity of F2B by CZAF was three times greater than that of IC.This is probably related to the chemical structure of the dyes and their affinity towards the calcined material.These results suggest that calcined LDHs may be good adsorbents for removal of a wide range of anionic dyes from wastewater(You et al. ).The experimental isotherm data were investigated with the most frequently used isotherm models: Langmuir and Freundlich.The linearized formula of Langmuir model (Equation (5)) and Freundlich (Equation (6)) (Zhu et al. ; Bouraada et al. ) are expressed by the following equations:

Figure 6 |
Figure 6 | (a) Effect of contact time on removal of IC and F2B by CZAF; (b) pseudo- second-order plots for the sorption of IC and F2B by CZAF.

Figure 7 |
Figure 7 | Effect of pH on adsorption capacity of IC and F2B by CZAF.
g À1 for IC and F2B, respectively.These values were compared with other studies (Zhu et al. ; Ni et al. ; Bouraada et al. , ; Ahmed & Gasser ) and are shown in Table3; it can be seen that the negative charges number of the dye has no significant effect on the maximum adsorption capacity.This allows us to conclude that the nature of the adsorbent and the structure of the adsorbed have a significant influence on the adsorbed amount.Moreover, CZAF material showed a strong ability to remove a huge quantity of dyes.Effect of the temperatureThe sorption of IC and F2B by CZAF was studied at different temperatures(25, 35 and 45   W C) to evaluate the variation of the adsorbed amount.The thermodynamic parameters such as change in standard free energy (ΔG W ), enthalpy (ΔH
Figure 10 shows the plots of ln K d versus 1,000/T.The high values of the determination coefficient indicate the good linearity of the plots (R 2 > 0.995).The calculated values of ΔG W , ΔH W and ΔS W are shown in Table 4.The negative values of standard free energy indicate the spontaneity of the sorption process, which means a high affinity of CZAF material toward anionic dyes.It was seen also that the value of ΔH W was positive (endothermic) in the sorptionof IC, but it was negative (exothermic) for the removal of F2B.These results showed that for IC higher adsorption

Figure 10 |
Figure 10 | Van't Hoff plot of the sorption by CZAF of IC and F2B.

Figure 11 |
Figure11| Reuse of CZAF for the uptake of IC and F2B for four thermal cycles.

Table 2 |
Langmuir, Freundlich parameters and determination coefficient R 2 for the uptake of IC and F2B by CZAF and ZAF-HT

Table 3 |
Comparison of the maximum monolayer sorption capacities (Q max ) of some dyes on various calcined LDH

Table 4 |
Thermodynamic parameters for the sorption of IC and F2B by CZAF at several