Treatment of textile effluents by chloride-intercalated Zn-, Mg- and Ni-Al layered double hydroxides

This work involved the preparation, characterization and dyes removal ability of Zn-Al, Mg-Al and Ni-Al layered double hydroxide (LDH) minerals intercalated by chloride ions. The materials were synthetized by co-precipitation method. X-ray diffraction, Fourier transform infrared, thermogravimetric-differential thermal analysis and transmission electron microscopy characterization exhibited a typical hydrotalcite structure for all the samples. Adsorption experiments for methyl orange were performed in terms of solution pH, contact time and initial dye concentration. Experimental results indicate that the capacity of dye uptake augmented rapidly within the first 60 min and then rested practically the same regardless of the concentration. Maximum adsorption occurred with acidic pH medium. Kinetic data were studied using pseudo-first-order and pseudo-second-order kinetic models. Suitable correlation was acquired with the pseudo-second-order kinetic model. Equilibrium data were fitted to Langmuir and Freundlich isotherm models. The maximum Langmuir monolayer adsorption capacities were 2,758, 1,622 and 800 mg/g, respectively, for Zn-Al-Cl, Mg-Al-Cl and Ni-Al-Cl. The materials were later examined for the elimination of color and chemical oxygen demand (COD) from a real textile effluent wastewater. The results indicated that the suitable conditions for color and COD removal were acquired at pH of 5. The maximum COD removal efficiency from the effluent was noted as 92.84% for Zn-Al-Cl LDH.


INTRODUCTION
Industries such as textiles, leather, plastics, paper-making, Many industries currently use activated carbon to remove dyes from wastewater (Santhy & Selvapathy ). However, the high cost of activated carbon diminishes its comprehensive use. In recent years, search for the creation of new low-cost adsorbents, such as clay minerals (Elmoubarki et al. ), phosphate (Barka et al. ) and natural biosorbents (Tounsadi et al. ) has grown. However, these materials generally possess low adsorption capacities and hence, a large adsorbent dosage is needed to eliminate a feeble dye concentration of the discharges. In order to decrease the cost of wastewater treatment, tests have been invented to find other relatively lowercost adsorbents, which have higher adsorption ability.
Some innovative materials have attracted significant attention in effective and suitable purification techniques over the last few decades (Dawood & Sen ). The objective and target of this work is the preparation, characterization and usage of three LDHs with different divalent metal as precursors for the adsorption of a model of anionic dyes from aqueous solution and the use of them for color and chemical oxygen demand (COD) removal from effluents produced by a textile industry. The samples were characterized by X-ray diffraction (XRD), infrared (IR) spectroscopy, transmission electron microscopy (TEM) and simultaneous thermogravimetric-differential thermal analysis (TGA-DTA). The influence of solution pH, contact time and initial dye concentration were studied in batch mode.
Kinetic and equilibrium parameters were explored to interpret the adsorption mechanism. To verify the efficacy of the adsorbents for the purification of textile effluents, adsorption tests were carried out with effluent discharged from a dyeing mill. The effects of different parameters on the removal of color and COD by these precursors such as solution pH and adsorbent dose were studied in detail.

Materials
All the chemicals used in this study were of analytical grade.

Study of MO adsorption
Synthetic dye solution at 1 g/L was prepared by dissolving a The adsorbed quantity was determined using the following equation (Vanderborght & Van Grieken ): where q (mg/g) is the adsorbed amount, C 0 (mg/L) is the initial dye concentration, C (mg/L) is the dye concentration Association ). The COD reduction (%) was calculated using the following equation: where COD 0 is the initial COD concentration (mg/L), and COD is the COD concentration after treatment (mg/L).

XRD analysis
The XRD patterns of the three LDHs are displayed in for Zn-Al-Cl, Mg-Al-Cl and Ni-Al-Cl, respectively.

TEM observation
The surface morphology plays an important role in the adsorption properties of the LDH. Figure 4 shows the TEM images of synthesized LDHs. As can be seen, Zn-Al-Cl shows the platelet particles confirming the lamellar structure that was obtained by XRD analysis, and the particle size  protonated and was charged positively when the pH value was below the PZC and deprotonated at pH values above the PZC.
At pH values lower than the PZC: At pH values above the PZC: where Surface means the surface of the LDHs.
In the dissolution of the MO dye in the aqueous solution, the sulfonate groups of the dye (D -SO 3 Na) are dissociated and transformed to: The dissociation constant pKa for MO is 3.46, so MO molecules were predominantly present as monovalent anions above this equilibrium pH (Zollinger ).
At pH values between the PZC of LDHs and the pKa of The rapid adsorption at the first contact time is due to the existence of a number of vacant sites on the surface of LDHs that are available for adsorption throughout the first step. The later slow rate is probably due to the anionic exchange between the dye and interlayer Cl À anion.
In order to describe the kinetics involved in the process of adsorption, pseudo-first-order and pseudo-second-order rate equations were suggested and the kinetic data were examined founded on the regression coefficient (r 2 ) and the quantity of dye adsorbed per unit weight of the LDHs.
The first-order rate of Lagergren () is founded on solid capacity and is generally shown as follows: where q e and q (both in mg/g) are, respectively, the amounts of dye removed at equilibrium and at any time 't' and k 1 (1/min) is the rate constant of adsorption.
The pseudo-second-order model suggested by Ho & McKay () is based on the hypothesis that the uptake follows second order chemisorption. This model can be expressed as: where k 2 (g/mg . min) is the rate constant of pseudo-secondorder adsorption.
The kinetic data determined for each model and the correlation coefficient (r 2 ) are shown in Table 2. As can be shown from the results, the two models exhibit a reasonably good agreement with the experimental data, but comparison between the r 2 values, indicates that the pseudo-second-order model gives the perfect overall fit. Furthermore, the adsorbed amounts at equilibrium calculated (q e,cal ) by this model are also a good fit where q m (mg/g) is the maximum amount of MO fixed per unit mass of adsorbent and K L (L/mg) is the Langmuir constant related to adsorbent/adsorbate affinity. C e is the equilibrium concentration.
The Freundlich model isotherm represents an empirical equation that presumes that the adsorption surface is heterogeneous for the course of the adsorption process. The Freundlich isotherm is defined by the following equation: where K F represents a value for the system, associated to the bonding energy, it can be motioned as the adsorption or distribution coefficient and indicates the quantity of dye adsorbed or fixed onto adsorbent for unit equilibrium concentration. 1/n represents the adsorption intensity of the dye molecules onto the sorbent or surface heterogeneity, which becomes more heterogeneous when the 1/n value becomes proximate to zero. A value for 1/n below 1 represents a normal Langmuir isotherm, while 1/n above 1 indicates a cooperative adsorption. result was in agreement with XRD analysis, which demonstrated that Zn-Al-Cl presents a high interlayer distance.

Treatment of textile effluent
Tests were carried out on industrial wastes from a dyeing factory that used different dyes and chemical substances such as dispersants, detergents and salts. Their quantities in effluents vary in different production processes. Initial characterization of the effluent showed various physicochemical parameters (Table 4). The pH of the sample effluent was observed as basic (8.9) and the temperature value was 30 W C. It was also observed that the COD value (614.6 mg/L) was higher compared to the standard value.
For the verification of the capacity of LDHs to treat the effluent, the effect of the variation of adsorbents dose and pH on the percentage of decolorization and COD removal     from industrial textile effluent were investigated. Figure 8 shows the effect of pH on COD removal. The optimum pH was observed in the range of 4-7. The maximum COD removal was shown to be 92.95% at the pH value of 5. On the other hand, Figure 9 represents the evolution of the UV-Vis spectra of the TWW treated by Zn-Al-Cl at different solution pH values. The results show that the decolorization of TWW was obtained at pH of 5 and 6. So these results can be explained by pH PZC and stability of LDHs which is reported and observed in the effect of pH on MO adsorption.
The effect of adsorbent dosage on the decolorization of TWW on the three adsorbents is presented by the UV-Vis spectrum in Figure 10. This figure indicates that the decrease of the absorbance is associated with the increase in adsorbent dosage. These results are explained by a reduction of chromophore groups which is responsible for colorization of textile effluent. A rapid decrease in the absorbance of TWW was observed and 90% of the color was removed at 5 g/L for Zn-Al-Cl and there was a great capacity of decolorization at low adsorbent dose for Mg-Al-Cl and Ni-Al-Cl.
These results suggest that the precursors present a high capacity of decolorization of the textile effluent.
The effect of the LDH dose on COD removal for the three adsorbents was evaluated by the variation of the dose from 0.5 to 5 g/L. The results are given in Figure 11 and indicate that COD removal increases with increasing the quantity of adsorbent. The figure also shows that increasing the dose beyond 2 g/L had little effect on the COD reduction and hence this value was considered as the optimum. The maximum percentage of COD removed from the TWW is noted as 92.84% in the case of Zn-Al-Cl.

CONCLUSION
From the present study, it can be concluded that the M/Al-Cl LDHs (M ¼ Zn, Mg, Ni) were successfully prepared by the co-precipitation method. The XRD patterns display high crystallinity of Zn-Al-Cl compared to Ni-Al-Cl and Mg-Al-Cl, and the interlayer distance was dependent on  The maximum percentage of COD removed from the TWW is noted as 92.84% in the case of Zn-Al-Cl.