Zn-Al layered double hydroxide: synthesis, characterization and application for orthophosphates ions adsorption in aqueous medium

This paper aims at studying the preparation of Zn-Al-LDH anionic clay and its adsorption proprieties in removing orthophosphate ions from aqueous medium. The LDH phase was synthesized by co-precipitation method and then calcined at 500 °C. Structural and textural properties of the prepared LDHs were determined by using x-ray diffraction, FTIR, BET, SEM and pHPZC analysis. Batch adsorption experiments were carried out under different parameters such as stirring time, pH and initial orthophosphate concentration. Results show that the prepared LDHs are efficient in removing orthophosphate and the maximum uptake was observed after 2 h. Experimental kinetic data are well described by the pseudo-second order model. It was also found that the adsorption process is significantly affected by the pH value. The sorption mechanisms include ion exchange, reconstitution of LDH phase and/or electrostatic attraction. Also, the isotherm study reveals that the Freundlich equation best fit the equilibrium data.


Introduction
Phosphorus (P) is an essential compound which is required for all living organisms [1]. This element becomes from natural origin such as the dissolution of soils and decomposition of organic matter [2]. However, the anthropogenic source of P is related to agricultural activities, industrial effluents, detergents, animal excrement and fertilizers [3]. Discharging of elevated amounts of P into aquatic environment generates eutrophication phenomenon and leads to the degradation of aquatic ecosystems [4].
At nowadays, there is a serious regulation for the control of P in wastewaters effluent. The recovery of P from polluted waters would reduce pollution and can present a new source of phosphorus. Therefore, several methods have applied in phosphorus elimination including biological treatments [5], chemical precipitation by calcium, aluminum or magnesium salts [1], phosphate crystallization [6], ion exchange [7], and adsorption [8]. The latter method is considered as a very promising technology for phosphate recovery from wastewater due to its efficiency and simplicity.
Recently, synthetic clays such as layered double hydroxides (LDHs) are demonstrated their high adsorption performance for the removal of organic and inorganic compounds from aqueous solutions. LDHs are anionic clays with a relatively high surface area and important exchange anionic capacity. The general formula of LDH is represented as follow [9]: The aim of the present work is to prepare Zn-Al-CO 3 LDH and its calcined phase (Zn-Al-Cal). The prepared materials are employed as adsorbent to remove orthophosphate ions from aqueous solutions. Various physicochemical techniques such as XRD, FTIR, BET and pH PZC were used to identify the different proprieties of the synthetized LDH clays. The kinetic and equilibrium studies were investigated to understand the adsorption mechanisms.

Preparation of Zn-Al-CO 3
Zn-Al-CO 3 LDH was synthesized by the conventional co-precipitation method [10]. A solution of NaOH (2M) and Na 2 CO 3 (1M) was added dropwise to acid solution containing chlorides of the metal cations (Zn 2+ and Al 3+ ). The solution was agitated at 80°C until crystallization. Then, the slurry was separated by centrifugation and washed with deionized water until chloride free. Finally, the obtained material was dried at 65°C then ground and stored. 2 g of Zn-Al-CO 3 was calcined at 500°C which is named Zn-Al-Cal for the rest of this study.

Characterization of samples
Characterizations of the synthetized LDHs have been studied by x-ray powder diffraction 'PANalytical X'pert HighScore Plus', Infrared spectra (SHIMADZU IR Affinity-1) and N 2 adsorption measurements (Micromeritics ASAP 2020 instrument). The pore size distribution was calculated from desorption isotherm using barrettjoyner-hallender (BJH) method. The surface morphology adsorbents were studied by scanning electron microscopy (SEM) analysis 'Tescan Vega 3'. Determination of point zero charge (pH PZC ) was performed according to the method described by Miyah et al [11]. In brief, solutions of 0.01 M NaCl (50 ml) were adjusted to pH range of 2-12 by adding drops of 0.1 M HCl or 0.1 M NaOH. Then, 50 mg of adsorbent was added to each solution. Mixtures were stirred for 24 h at constant speed, and the final pH of solutions was measured. Value of pH PZC was obtained from the plot of ΔpH (pH f -pH 0 ) versus pH 0 .

Experiments of orthophosphate adsorption
In this study, the orthophosphate content is expressed in mg/l -PO 4 3 (1 mg l −1 -PO 4 3 =0.326 mg l −1 P). Stock solution of phosphate was prepared at 1 g l −1 by dissolving an accurate amount of Na 2 HPO 4 . Adsorption experiments were carried out in batch manner at room temperature (18±1°C). A desired amount of adsorbent was added to phosphate solution (50 ml) at known concentration. The dispersions were stirred at speed of 200 rpm and a constant pH (pH=6.5). After adsorption, solutions were filtered through a 0.45 μm membrane filter, and the residual concentration of phosphate ions was detected by UV-vis spectrophotometer at a wavelength λ=800 nm [12].

Analysis of adsorption data
The adsorbed quantity (q ads , mg g −1 ) of orthophosphates was determined as follow: where C 0 and C e are the initial and equilibrium concentration (mg l −1 ), respectively; whereas V is the solution volume (L) and m is the adsorbent mass (g).
In order to investigate the kinetic of orthophosphates ions adsorption onto LDHs, the experimental data were analyzed using the pseudo-first-order [13] and the pseudo-second-order [14] kinetic models. The adsorption mechanism was studied by fitting the isotherm data to the Langmuir [15], Freundlich [16], and [17] models. Table 1 shows the equations and parameters of these models.
The nonlinear coefficient of determination R 2 [18] and the residual root mean square error RMSE [19] were used to identify the best-fit model: where q e,exp (mg g −1 ) and q e,cal (mg g −1 ) is the experimental and calculated quantity of adsorbed orthophosphates, respectively. q e,mean (mg g −1 ) is the mean of q e experimental values. n is the number of observations in the adsorption experiment.

Results and discussion
3.1. Characterization of samples X-ray powder diffraction patterns of LDHs phases (Zn-Al-CO 3 and Zn-Al-Cal) are depicted in figure 1. The pattern of Zn-Al-CO 3 corresponds to typical LDH phase and shows that the prepared powder consists of a single crystalline phase ( figure 1(a)). The d-spacing interlamellar value (d 003 ) of Zn-Al-CO 3 is 7.20 Å, which is close to other values cited in the existing literature [20,21]. Zn-Al-Cal phase shows a destruction of the crystal lattice by decarbonization and dehydroxylation, consequently leading to the formation of spinel form (ZnAl 2 O 4 ) as shown in figure 1(b).
FTIR spectrum of Zn-Al-CO 3 in figure 2(a) shows absorption bands of LDH containing CO 3 2− in the interlayer space [21]. The broad band in the range (3600 to 3100) cm −1 corresponds to the vibration mode of the O-H groups of layers and interlayer water molecules [22]. The weak peak at 1630 cm −1 is attributed to interlayer water vibrations [23]. The intercalated carbonates anions in the interlayer LDH were observed at 1358 cm −1 (ν 3 ),

Kinetic and Equation Parameters
Curve q e (mg g −1 ) and q t (mg g −1 ) are the adsorbed quantity at equilibrium and at time t, respectively.  [25]. After calcination, all peaks characterizing the Zn-Al-CO 3 phase were decreased considerably ( figure 2(b)) [26,27]. The N 2 adsorption-desorption plots of LDHs samples show type IV isotherm according to IUPAC classification ( figure 3). These plots have a hysteresis loops of H 3 indicating a mesoporous structure [28,29]. Results of BET measurements show that the specific surface area increases from 30 to 129.0 m 2 g −1 after calcination of Zn-Al-CO 3 . It can be observed also that the loss of -CO 3 2 ions leads to develop the total pore volume of mesoporous LDH. All textural characteristics of LDH phases are summarized in table 2.
The morphology of studied LDH phases is provided in figure 4. Results show that the samples consist of relatively uniform hexagonal platelet-like sheets [27]. The particles were welldefined and the platelet size is about 400-800 nm. In the Zn-Al-Cal LDH, an increase in the pore diameter was observed, indicating that the interlayer carbonates were disappeared after calcination of the sample. This finding was also confirmed by the increase of pore volume in the BET analysis.  The point of zero charge is defined as the pH value at which the total surface charge of adsorbent is zero [28]. It plays a critical role in studying the surface charge properties of the material and understanding the adsorption mechanisms. Result of experimental determination of pH PZC of Zn-Al-CO 3 sample is 7.96 ( figure 5). When pH of solution >7.96, Zn-Al-CO 3 reacts as negative surface and as a positive surface when solution pH<7.96.

Kinetic study
The effect of contact time on the adsorbed amount of orthophosphates was examined at initial concentration of 20 mg l −1 ( figure 6). Results show that the uptake of orthophosphates ions onto LDHs surface is a rapid phenomenon in the first minutes, and becomes slow after stirring time of 60 min. The quantity of    (table 3). This finding suggests that the PSO model is more suitable for the adsorption of orthophosphate ions onto Zn-Al-CO 3 and Zn-Al-Cal phases.

Adsorption mechanisms
Adsorption tests were performed at various initial pH values ranging from 2 to 12 to investigate the interaction mechanisms between orthophosphates and LDHs. Depending on the value of pH solution, orthophosphates can exist in several forms H 3 PO 4 , -H PO , 2 4 -HPO , 4 2 and -PO 4 3 [29]. Results of figure 7 show that the affinity of adsorbent to remove orthophosphates is low at pH3, this can be explained by the instability of LDHs structure [30]. At 4<pH<pH PZC , the dominant species of orthophosphates could be -H PO 2 4 and -HPO , 4 2 and the LDHs surface is positively charged. The adsorption is primarily result of ion exchange between orthophosphate anions and interlayer -CO 3 2 of Zn-Al-CO 3 phase [31], and reconstitution in the case of calcined Zn-Al-LDH (figure 8). At the same time, the electrostatic attraction (surface positive charge-anion) promotes the anionic orthophosphates uptake [32].
As the increase of pH (pH PZC to 12), the amount of orthophosphate anions adsorbed onto LDHs decreased. At this pH range, the LDHs surface becomes negatively charged and the anionic forms of orthophosphate are not favorably attracted by the adsorbent sites. Meanwhile, this decrease of adsorption capacity at higher pH values implies a competition between -PO 4 3 and -OH for the adsorption sites [33].

Equilibrium study
The isotherm of adsorption is represented by the plot of the equilibrium adsorbed amount of orthophosphate qe against the equilibrium concentration Ce. The fitting of equilibrium data using the nonlinear Langmuir, Freundlich and Dubinin-Radushkevich models are depicted in figure 9.
Results of isotherm parameters in table 4 show that the Freundlich equation gives the best fit for equilibrium data, which is confirmed by the high value of R 2 and minimum value of RMSE. This finding suggests the multilayer adsorption of orthophosphate ions onto heterogeneous surface of LDHs [34]. . Adsorption kinetic of orthophosphate using Zn-Al-CO 3 (a) and Zn-Al-Cal (b) and fitting curves of kinetic models (C 0 =20 mg l −1 , pH=6.5, adsorbent amount=50 mg, V=50 ml).

Conclusion
In the present study, Zn-Al-CO 3 LDH was prepared by co-precipitation method and calcined at 500°C. The synthetized adsorbents were used to remove orthophosphate from aqueous solutions.
The obtained results show that the prepared material corresponds to typical LDH phase with d-spacing of 7.20 Å. The adsorption tests indicate a high efficiency of the used LDHs and the adsorption reaches its maximum after 2 h. It was also found, that the maximum of removal was obtained at pH ranging from 4 to 9. The adsorption of orthophosphate anions can result of ion exchange, reconstitution and/or electrostatic attraction. The Freundlich equation adequately describes the uptake of orthophosphate anions onto LDHs surface.