Removal of indigo carmine and green bezanyl-F2B from water using calcined and uncalcined Zn/Al + Fe layered double hydroxide

Synthetic dyes have long been used in various industries, such as textiles, cosmetics, paper, leather, pharmaceuticals and foods (Yuan et al. 2009). 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. 2001). 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 1996).

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. 2005; El Gaini et al. 2009). 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. 2001). 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. 2005; El Gaini et al. 2009). 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. 2012; González et al. 2015). LDHs are scarce in nature, but can be prepared easily and in large quantities in the laboratory using inexpensive precursors (Seftel et al. 2008). Their general formula is [ (OH)₂] (A⁠)_x/n mH₂O, where M²⁺ and M³⁺ are di- and trivalent metal cations, Aⁿ is an exchangeable organic or inorganic anion with negative charge n, m is the number of interlayer water molecules and x = M³⁺/ (M²⁺ + M³⁺) is the layer charge density of the LDH (Kameda et al. 2015). 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.

Calcination transforms LDHs into mixed metal oxides with high specific areas and homogeneous dispersion of metal cations (Ni et al. 2007). 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. 2006).

In the present study, ZAF-HT LDH was synthesized by co-precipitation and calcined at 500 °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°), enthalpy (ΔH°) and entropy (ΔS°) were calculated. The reusability study of CZAF for both dyes for four thermal cycles was also studied.

IC (purity 99%) was produced by Ciba Society (Zurich, Switzerland) and F2B, was provided by the company Soitex, Tlemcen, Algeria. The dyes were used as received. Dye solutions were prepared by dissolving an accurately known amount of dye (1 g L⁻¹) in distilled water and subsequently diluting to required concentrations. Aluminium chloride hexahydrate (purity 99%) and urea (purity 98%) were obtained from Sigma-Aldrich, Germany. Zinc chloride (purity 98%, Paureac, Spain) and ferric chloride (purity 99%, Chemopharma, Austria) were used as received.

Zn/(Al + Fe) DLH with a molar ratio of 3/(0.85 + 0.15) was synthesized by co-precipitation according to the literature with some modifications (Mantilla et al. 2010). A mixture of 0.06 moles of ZnCl₂, 0.017 moles of AlCl₃.6H₂O and 0.003 moles of FeCl₃ was dissolved in 200 mL of deionized water. The latter solution was added dropwise to a glass reactor vessel containing 100 ml of urea solution (20% w/v) as precipitant agent, with vigorous stirring. The pH of the suspension was adjusted to 10.5 by using 2 M NaOH. The resulting precipitate was vigorously stirred for 4 h at room temperature and then heated at 90 °C under reflux for 36 h. The material obtained was separated by centrifugation, washed thoroughly with deionized water several times until the precipitate was free from Cl⁻ (AgNO₃ test) and air-dried at 100 °C overnight. This material, designated as ZAF-HT, was ground and sieved to obtain a particle size of <0.250 mm. A portion of ZAF-HT was calcined in air at 500°C for 4 h and designated as CZAF.

The samples were characterized by several physical and chemical techniques. Powder X-ray diffraction (XRD) patterns were recorded using a Phillips X'Pert MPD diffractometer with monochromatic CuKα = 1.5418 Å (40 kV, 30 mA). Fourier transform infra-red (FTIR) spectra were obtained in a transmission mode on a Nicolet Avatar 330 FTIR spectrometer by using KBr disks containing 1wt% samples. The spectra were recorded with 2 cm⁻¹ resolution in the range 4,000–400 cm⁻¹. Thermogravimetric analysis (TGA) of solid samples was conducted using a NETZSCH STA 409 PC/PG simultaneous thermal analyser, heated from 20 to 900 °C at a heating rate of 10°C/min under a N₂ flow rate of 50 mL/min. The Brunauer–Emmett–Teller (BET) specific surface areas and pore sizes of the samples were determined by N₂ adsorption–desorption isotherms using a Micromeritics ASAP (2010). The particle morphologies of ZAF-HT and CZAF were observed by scanning electron microscopy (SEM) using a Hitachi S-2600N variable pressure scanning electron microscope.

The temperature effect was studied on suspensions of CZAF in IC and F2B solutions with initial dye concentration of 260 and 800 mg.L⁻¹, respectively (solid/solution ratio = 0.5 g L⁻¹). The suspensions were stirred during the equilibrium time at three constant temperatures (25, 35 and 45 °C) and then centrifuged, and the residual concentrations were determined as above.

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°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.

The retention of IC by CZAF was demonstrated by the appearance of new peaks at 681 cm⁻¹, attributed to the C–C single vibration; at 1,614 cm⁻¹, assigned to the double bonding of the ethylenic band; and the bands at 1,473–1,399 cm⁻¹, which are probably due to the double bonding of the aromatic ring system (El Gaini et al. 2009). The C = O vibration band was also present at 1,640 cm⁻¹ and the S–O vibration bands appeared at 1,153 and 1,199 cm⁻¹. The band assigned to ν(S = O) at 1,104 cm⁻¹ was shifted to 1,110 cm⁻¹ (El Gaini et al. 2009). The intensity of vibration bands decreased after the fixation of the IC dye. The FTIR spectra of F2B and ZAF-F2B showed the presence of many common peaks indicating the uptake of F2B dye by CZAF. The FTIR results were consistent with the XRD patterns.

The kinetic parameters of the models were calculated and are reported in Table 1. Based on the values of determination coefficient R², 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-second-order model (Ni et al. 2007; Bouraada et al. 2009; El Gaini et al. 2009; Ahmed & Gasser 2012).

In this study, ZAF-HT and CZAF were synthetized and characterized by several experimental techniques (XRD, BET, FTIR spectroscopy and SEM). The material was used to take up IC and F2B from water in a batch mode. The adsorption capacities of CZAF were found to be 488.94 and 1,487.91 mg g⁻¹ for IC and F2B, respectively. The sorption of IC and F2B does not depend on the pH of the solution. The kinetic data of both dyes fitted the pseudo-second-order kinetic model well, and the isotherm sorption data were described by the Langmuir model. The thermodynamic parameters were calculated and showed that the sorption process was spontaneous in nature. The reusability study of CZAF for the removal of IC and F2B for four cycles showed that the efficiency of the calcined material decreased slightly.

The authors would like to thank Professor Jersy Zajac, Director of Laboratory AIME-UMR, University of Montpellier, France, for the acceptance in his laboratory allowing us to achieve our characterizations of materials and for inspiring discussions and helpful comments. We would like also to thank all the co-workers cited in the references below for their valuable contributions to the work described here. This work was financially supported by the CNEPRU-Algeria, Project E02220120028.