Analytical s tudy on lead elimination by anionic clays: Characterization, adsorption kinetics, isotherm, thermodynamic, mechanism and adsorption

The co-precipitation method synthesized the synthetic anionicMg–Al and Ni-Al clays with three molar ratios (Mg/Al, Ni/Al). The samples were characterized by powder X-ray di ﬀ raction (XRD), Fourier transform infrared spectroscopy (FTIR), and scanning electron microscopy (SEM). No other crys talline phases were detected in the powder XRD patterns of the co-precipitated samples. The infrared spectra obtained all the functional groups that characterize these two types of anionic clays. SEM micrographs indicate the presence of particles and aggregates. The particles, or aggregates, are in the form of plates, supported by particles of acceptable sizes. The optimal pH for maximum lead adsorption is about 6.5 for both clays. The optimal adsorbent masses for the maximum percentages of lead removal are 0.2 g for Mg 3 AlCO 3 and 0.25 g for Ni 3 AlCO 3 . The Mg 3 AlCO 3 has a maximum adsorption capacity of lead, where q m =73.42 mg g -1 . The adsorbed amount increases with increasing temperature for both types of clays s tudied. The equilibrium time of Pb 2+ adsorption is reached after 5 min for both clays. The mos t appropriate models to describe the experimental data of adsorption kinetics and isotherms are pseudo-second-order and Langmuir. The detection limit (LOD) was 0.272 mg L -1 . The linearity range was 1 to 5 mg L -1 ; the correlation coe ﬃ cient in this range was 0.9997.

The s tructure of these clays consis ts of positively charged mixed metal hydroxide layers separated by charge-balancing anions and water molecules [4].The cationic sheets containing M(OH) 2 in octahedral surroundings are linked by three edges, as in the brucite s tructure, between which compensating layers of anions are found [5].These compounds have been the subject of much interes t and research in recent years thanks to their interes ting properties of anionic exchange [6,7], adsorption and porosity, which make it possible to envisage the intercalation of a large variety of anions (organic or inorganic) ------------------------and the immobilization of various species, giving these hybrid materials a particular reactivity [6,7].Therefore, LDH is considered a promising material [8].Heavy metals are one of the main categories of water pollution; these releases pose a real danger to humans and their environment due to their s tability and low biodegradability [9][10][11].The use of the adsorption technique to remove heavy metals in aqueous solutions on different solid supports, especially on new materials such as anionic clays, has been the subject of much work [3,5,12].Among the scientific results that use layered double hydroxides as adsorbents to remove lead are the s tudies of Yasin et al. (2014) [13].The types of LDHs used to remove lead (Pb) from the aqueous solution are MgAl-NO 3 and Tartrate-MgAl.In this s tudy, the maximum lead adsorption capacity calculated by the Langmuir model is 8.4 and 3.2 mg g -1 for Tartrate-MgAl and MgAlNO 3 , respectively [13].Yanming et al. (2017) [14] s tudied the removal of Pb 2+ ions from an aqueous solution by glutamate intercalated in layered double hydroxide.The maximum retention capacity of Pb 2+ is 68.49mg g -1 [14].The concentration of Pb and other metal ions can be determined by several techniques, which can be grouped under atomic spectrometry.Graphite furnace atomic absorption spectrophotometer (GF-AAS) is the bes t technique method.Several research s tudies use this technique to determine the lead concentration in the blood, like s tudies of Pacer et al. (2022) [15].In addition, GF-AAS is an excellent method currently applied for trace lead concentration with high accuracy and precision [15,16].Like GF-AAS, inductively coupled plasma mass spectrometry and ICP-MS is other assay technique with precise and accurate results.However, it is a simple and rapid method compared to ICP-MS [17].The comparative s tudy carried out by Trzcinka-Ochocka et al. for the determination of lead and cadmium shows that validation parameters for ICP-MS and GF-AAS were similar.However, ICP-MS for Pb determinations is better than GF-AAS.Also, the detection limits (LOD) of ICP-MS are better than GF-AAS for lead analysis [18].Inductively Coupled Plasma -Atomic Emission Spectroscopy (ICP-AES), sometimes named ICPoptical emission spectrometry (ICP-AES), is a device that results from the coupling between a highfrequency induced argon plasma and a spectrometer, which is used to calculate the concentration of metals in solid, liquid or gas samples.The LOD of the ICP-AES technique is lower than ICP-MS [19].In addition, there are other lead dosage techniques without spectroscopic techniques, such as cyclic voltammetry (CV).Riyanto et al. showed that the electroanalysis method for lead determination in was tewater is accurate, precise, reproducible and inexpensive, with acceptable correlation [20].Compared with spectroscopic techniques, the LOD of electroanalysis (0.929 mg L -1 ) is higher than that of spectroscopic methods.In this paper, lead removal from an aqueous solution was performed using two-layered double hydroxides, namely Mg 3 AlCO 3 and Ni 3 AlCO 3 .The co-precipitation method prepared these two adsorbents with a molar ratio of (Mg/Al) equal to 3. Ni 3 AlCO 3 can be considered among the LDHs not used to eliminate heavy metals from aqueous solution.The adsorption capacities of lead with two anionic clays at room temperature in optimized pH were compared and analyzed.We s tudied the effect of different parameters such as the Ni/ Al ratio, pH, contact time, dose of adsorbent and temperature in the adsorption of Pb 2+ ions from an aqueous solution.In addition, the mechanism and thermodynamic parameters were s tudied.

Reagents and Materials
The products of magnesium nitrate hexahydrate (Mg(NO 3 ) 2 .6H

Apparatus
The absorption measurements were made with AA-6200 Atomic Absorption Flame Emission Spectrophotometer SHIMADZU.Calibrating s tandard lead solutions were prepared by dilution from the s tock solution (1000 mg L -1 ).The linear working range was obtained between 1 to 5 mg L -1 .To es timate the sensitivity of the FAAS method, we calculated the limit of detection (LOD) and the limit of quantification (LOQ).The LOD and LOQ values were achieved at 0.272 mg L -1 and 0.825 mg L -1 , respectively.

Synthesis method
The LDHs s tudied are NiAlCO 3 (R=2, 3 et 4) and MgAlCO 3 (R=3).These clays were synthesized by the direct co-precipitation method, in which an aqueous solution containing appropriate amounts of nitrate elements (hydrated metal salts as sources) was added dropwise into an alkaline solution of Na 2 C0 3 and NaOH at room temperature under vigorous s tirring.During the synthesis, the pH was adjus ted to pH 10.The resulting suspension was s tirred for 18 hours at 65 °C.After cooling to room temperature, the precipitate was centrifuged and washed several times with bi-dis tilled water until there was no trace of nitrate (AgNO 3 tes t) and then dried overnight in an oven at 100 °C.

Adsorption method
The s tudy of the adsorption of lead was performed by the batch method.The lead s tock solution was prepared by dissolving Pb (NO 3 ) 2 in dis tilled water and diluting it to the desired concentration.Adsorption of Pb 2+ on the selected clay was carried out in a 50 ml conical flask by taking 50 ml of a solution of the desired Pb 2+ concentration to which 200 mg of the adsorbent was added.The adsorbate in the mixture was separated by centrifugation.An atomic absorption spectrophotometer determined the residual Pb2+ in the filtrate.All experiments except the pH variation s tudy were performed at the s tock solution pH.In the case of pH variation s tudies, a variable concentration of diluted NaOH and HCl solutions was used to adjus t the pH. Figure 1 presents the adsorption method.The adsorption of lead was calculated by Equation 1. (Eq.1) Where q e = lead adsorbed (mgg -1 ); V = solution volume (L); C i = initial concentration (mg Pb 2+ L -1 ); Ce (mgPb 2+ L -1 ) =equilibrium concentration and m adsorbent mass.% Removal of metal ions were calculated using Equation 2.

Characterization
A Bruker D8 diffractometer with CuKα radiation (λ= 0.15406 nm) was used to s tudy the s tructural properties of the clay.A scanning electronic microscopy ins trument (S-4800), a Hitachi model (Japan), was utilized to s tudy the surface properties.Fourier transform infrared spectroscopy (Perkin-Elmer model; USA) was applied to s tudy the functional group of the adsorbent.Moreover, this las t technique was used to examine the effect of the adsorption of Pb 2+ on the different bands of the functional groups after adsorption.The infrared spectra were carried out between 4000 cm -1 and 400 cm -1 .

3.1.1.Characterization by XRD
The X-ray diffraction patterns of the prepared phases (Fig. 2a and 2b) are characteris tic of layered double hydroxide materials (LDHs).Peaks 003 and 006 are sharp, narrow, and symmetric; the baseline is low and s table, which indicates a high degree of crys tallinity and a typical s tructure of anionic clays.These reflections correspond to the layer order along the c-axis [21].Interreticular dis tances are 7.70 °A and 7.75 °A for Ni/ Al (R=3) and Mg/Al (R=3), respectively.These values are in order of those reported in similar s tudies by Kris tina Klemkaite et al. [22]and Faour et al. [23].The cell parameters of Ni 3 AlCO 3 and Mg 3 AlCO 3, calculated as a = 2*d 110 [24], are 3.04 and 3.06 °A, respectively.The cons tant c calculated using the equation c=3*d 003 [25] shows that the corresponding values for Ni 3 AlCO 3 and Mg 3 AlCO 3 are (23.10°A) and (23.25 °A), respectively.These values always agree with those of Kris tina Klemkaite et al. [22] and Faour et al. [23].

3.1.2.Characterization by SEM
The images characterizing the surfaces of the different subs trates are presented in Figure3a and 3b.The images at different magnifications show surfaces with large porosities and different types and sizes.The large inter-particle pores are occupied by particles of smaller sizes for both clays, which indicate the presence of inter-particle attraction forces that form large aggregates.The particles, or aggregates, are in the form of plates, supported by particles of acceptable sizes.Mg 3 AlCO 3 particles are characterized by a rigid (compact) perimeter surrounding a highly porous surface.

Parameters of adsorption
In the following, we s tudied the effect of some parameters on lead adsorption, such as the solution's initial pH, the adsorbent's mass, contact time, molar ratio, and temperature.The concentration and volume of the aqueous lead solution were fixed at 50 mgL -1 and 50 mL, respectively, and the s tirring speed was set at 400 rpm.

3.2.1.Effect of pH
To find the optimal pH corresponding to the maximum adsorption of lead in the aqueous solution, we s tudied the effect of this factor on the retention of Pb 2+ at different pHs (from pH 3 to pH 9).The results obtained are shown in Figure 4.The amount retained as a function of the pH solution was determined from the concentration of Pb 2+ remaining in the solution after equilibrium by the atomic absorption technique.According to Figure 4, the curves can be divided as a function of pH into two regions: the firs t one represents the domain of pH lower than 6.5 in which the percentage removal of Pb 2+ retained on the selected anionic clays increases as the pH increases, reaching a maximum value at pH 6.5.At this optimum pH, the lead removal percentages are 95.4% and 81.34% for Mg 3 AlCO 3 and Ni 3 AlCO 3, respectively.The increase in Pb 2+ adsorption on both types of LDHs with increasing pH can be explained by the decrease in H + ion concentration with increasing pH.Where the clay surface at low pH became positively charged due to the protonation reaction on the surfaces (formation of SOH 2+ ) [25], which leads to repulsive forces between Pb 2+ ions and SOH 2+ groups on the adsorbent surface [26].According to Donglin Zhao, at pH values below 7, lead ions are present as Pb 2+ in the solution.The adsorption reactions are surface complexation reactions, including two surface reactions.The chemical bonding reaction occurs between the metal ions and the surface functional groups, forming surface complexes of the inner sphere.In the second region, an electros tatic binding reaction occurs between metal ions and oppositely charged surface functional groups, forming surface complexes of the outer sphere at some dis tance from the surface.The complex adsorption of lead on LDH samples can be described as follows [25,26].
Electros tatic binding adsorption At pH greater than 6.5, for Mg 3 AlCO 3 , there is a slight decrease and then s tability for lead removed until pH 9.These results almos t agree with those found by Donglin Zhao et al., who used anionic clay based on Mg 2 Al-LDH to remove lead [26].For the anionic clay based on Ni 3 AlCO 3 , the characteris tic curve shows a remarkable decrease in the amount of lead removed.At this pH range (pH ˃ 6.5), according to ZHAO, Donglin et al. (2011) [26], lead in the aqueous solution takes the forms of Pb (OH) and Pb (OH) 2 .Thus, the adsorption of Pb 2+ on both LDHs occurred by precipitation reaction, as explained by several authors in this pH range (pH ˃ 7) [26].According to our results, it can be noted that the lead precipitation reaction is better catalyzed on the Mg 3 AlCO 3 surface than that of Ni 3 AlCO 3 , where the amount adsorbed by Mg 3 Al remains cons tant from pH 7.5 and higher compared to Ni 3 Al-based clays, where the amounts of lead are decreased with the increase of pH (Figure 4).On the other hand, LIANG, Xuefeng et al. [25] conclude that the adsorption of Pb 2+ on a clay-type Mg 2 Al-Cl LDH results mainly from the precipitation induced by the surface.At optimum pH (pH = 6.5), the results show that the order of the quantity of lead retained for the clays used becomes Equation 3.
Whereas, at pH˂5.5 (an acidic medium), the percentage of Pb 2+ removal by Ni 3 AlCO 3 HDL is higher than that of Mg 3 AlCO 3 HDL, conversely for the pH ˃ 5.5 range.This can be explained by the s tart of the lead precipitation reaction occurring in parallel with the complexation reaction from pH 5.5 to the optimum pH of 6.5.

3.2.2.Effect of adsorbent quantity
Different amounts of the adsorbent (0.05-0.3 g) were added to other conical flasks containing 50 mL of the aqueous solution of Pb 2+ (pH 6.5). Figure 5 shows the variations of Pb 2+ adsorbed amounts as a function of adsorbent mass for the two anionic clays s tudied with a contact time of 2 hours.The initial adsorbate concentration used is 50 mg L -1 .Lead removal percentages increase as the adsorbent mass increases from 0.05 g to 0.2 g.Above 0.

3.2.3.Effect of temperature
The effect of temperature on lead adsorption was s tudied at 20 °C, 30 °C, 40 °C, 50°C, and as well as pH of the solution.Figure 6 shows the percentage of lead removal as a function of temperature for Mg 3 AlCO 3 and Ni 3 AlCO 3 .From the curves shown in Figure 6, the adsorbed amount increases with increasing temperature for both types of LDHs, where the lead removal percentages reach 94.16 % and 85.39 % for Mg 3 AlCO 3 and Ni 3 AlCO 3 , respectively.The percentage of lead removal by Mg 3 AlCO 3 is higher than that of Ni 3 AlCO 3 for all temperatures.This may indicate that the adsorption of lead onto the active sites of LDHs s tudied is endothermic [27].The increase in temperature can be enlarged and activate the adsorbent surface, which facilitates the mobility of lead ions from the bulk solution to the adsorbent surfaces and enhances the accessibility to the adsorbent active sites [27].

3.2.4.Effect of contact time and molar ratio
The s tudy of the contact effect was carried out using four LDHs of the types Mg Qads (mg g -1 ) Qads (mg g -1 ) Qads (mg g -1 )

Kinetic models of adsorptions
To determine the mos t appropriate kinetic model, we have chosen three models, the mos t used for modelling adsorption kinetic data, which are: the pseudo-firs t-order (Equation 5), the second-order (Equation 6), and the intraparticle diffusion model (Equation 7).The corresponding equations in linear forms are presented as follows [28,29]. (Eq.5) (Eq.6) (Eq.7) Where q e , q t , t, k 1, k 2 , k i , and C are respectively the quantity of Pb 2+ adsorbed at equilibrium (mg g -1 ), the quantity of Pb 2+ adsorbed at time t (mg g -1 ), the time (min), the rate cons tant of the pseudofirs t-order kinetic equation in g/mg min -1 , the rate cons tant of the pseudo-second-order kinetic equation in g/mg min -1 , the rate cons tant mg/ gmin 0.5 , and the boundary layer thickness.

3.3.2.Pseudo second-order model.
This model considers that the rate-limiting s tep in heavy metal adsorption is chemisorption and that chemisorptive bonds involving electron sharing or exchange between the absorbent and the adsorbent have been applied [5].According to the high values of the regression cons tant R 2 = 0.99 for all the s tudied clays, the evolution of t/qt vs. t is presented by pseudo-second-order kinetics (Fig. 9).The parameters of the two kinetic models are shown in Table 2. From these results and in contras t to the firs t-order model, the amount of Pb 2+ adsorbed at equilibrium determined experimentally is closer to that calculated using the second-order kinetic model (Table 2).
Study the elimination of lead by anionic clays Salah Bahah Fig. 8. Pseudo firs t-order model for anionic clays Table 2. Parameters of the pseudo-second-order model.

3.3.3.Weber -Morris internal diffusion model
The Weber-Morris intra-particle diffusion model is the mos t used technique to identify the mechanism involved in the adsorption process.Intra-particle diffusion plots (q t vs. t 0.5 ) (Fig. 10) were obtained from Equation 7.
All the parameters of this model are presented in Table 3. Figure 10 indicates that s traight lines do not pass through the point of origin before reaching the equilibrium s tate; therefore, the adsorption does not follow only the mechanism of intra-particle diffusion and that several processes affect the adsorption of Pb 2+ and that intra-particle diffusion is not the limiting s tep for the whole reaction.The values of the cons tant C, which presents the thickness of the boundary layer, are in the order The values of C determine the boundary layer effect; higher values indicate a more significant impact [29].

Lead retention equilibrium s tudies
To determine the adsorption characteris tics of Mg 3 AlCO 3 and NiAlCO 3 (R = 2, 3, and 4), a series of experiments are carried out in which solutions containing known concentrations of Pb 2+ are in contact with the adsorbent.This s tudy was carried out under the same conditions as the contact effect parameter, except for the concentrations of the Pb 2+ solutions.After 24 hours, the solution concentration is measured when equilibrium has been es tablished.

Characterization of adsorbents before and after lead adsorption and mechanism
The infrared (IR) spectra of the raw Mg 3 AlCO 3 and Ni 3 AlCO 3 samples before and after the retention of lead at different concentrations are shown in Figure 12.The spectra are subdivided into three regions: between (1000 cm -1 and 400 cm -1 ), (3000 cm -1 and 1000 cm -1 ), and (4500 cm -1 and 3000 cm -1 ).It can be noted that the IR spectra corresponding to the second region (1000 cm -1 to 3000 cm -1 ) indicate two essential bands between (1650 cm -1 and 1660 cm -1 ) and (1350 cm -1 and 1400 cm -1 ) for both adsorbents before and after Pb 2+ adsorption.These two bands are assigned to H2O and CO32-vibration modes, respectively.
In the low-frequency region between 1000 cm -1 and 400 cm -1 , the infrared spectra after lead adsorption show intense vibration bands and are shifted compared to those of Mg 3 AlCO 3 and Ni 3 AlCO 3 before adsorption, as shown in Figure 12(a, b, c).For Mg 3 AlCO 3 , the vibration bands observed at 446.49 cm -1 and 554.49cm -1 are attributed to the AlOH and MgOH bands, respectively [31].After adsorption of the lead at different concentrations, the intensity of these bands increases and shifts slightly to highfrequency regions (447.49cm -1 and 555.50 cm -1, respectively), which is explained by the fixation of Pb 2+ ions on the lamellar layers to form bands of Pb-Al-OH and Pb-Mg-OH or M-O-Pb [32].The band observed at 668.28 cm -1 for the Mg/Al-based clay before adsorption is assigned to the carbonate vibration mode [33].
After adsorption of Pb 2+ , the spectrum does not show a remarkable shift of this band.The same remark was observed at the carbonate vibration band located at about 1357.79 cm -1 (Figure 12b) (second region).For the high-frequency region between (4500-3000 cm -1 ), the vibration band of free hydroxyl groups located around 3446.55 cm -1 before adsorption is shifted to 3442 cm -1 for Mg 3 AlCO 3 solids after lead adsorption for different concentrations.This shift, accompanied by the decrease of free OH band area after adsorption, is explained by the reduction of free OH in the hydroxyl layer, which reacts with Pb 2+ ions according to Equation 8.
As Equation 8, M is a divalent or trivalent cation (Mg or Al).For the adsorbent based on Ni 3 AlCO 3 , several bands were observed in the low-frequency region of the spectrum (<600 cm −1 ) that characterize the lattice vibration modes [32].The spectra indicate a slight shift with increasing peak intensity (sharp peak) at about 418.56 cm -1 for all Ni/Al-based HDLs after Pb 2+ adsorption (Figure 12a).This band can be attributed to the formation of Al-O-Pb or Pb-Al-OH [32].The infrared spectra also show the shift of the vibration band peaks from about 560.28 cm -1 and 594.99 cm -1 before adsorption to 562.25 cm -1 and 592.15 cm -1, respectively, for all Ni 3 AlCO 3 clays after lead adsorption.The band's shift around 562 cm-1 has been assigned to hydroxyl groups associated with mainly Al [34], and bounds around 592 cm-1 have been given as hydroxyl groups associated with Al/ Ni.The shift of these two bonds is assigned to the formation of M-O-Pb according to the reaction proposed above (case of Mg 3 AlCO 3 ) (M = Ni 2+ or Al 3+ ) [32].Before the adsorption of Pb 2+ , the s trong and broad absorption band centred on 3483.20 cm −1 corresponds to the O-H s tretching vibration of the layer surface and interlayer water molecules, and the band in 1652.88 cm −1 is due to the O-H bending vibration of water molecules.After adsorption of the lead, we notice the reduction and slight shift of the bands, which are centred towards a low frequency at 3482.53 cm -1 , indicating that the hydroxyl groups electros tatically attracted Pb 2+ anions.

3.4.2.Model of adsorption isotherms
Modelling adsorption isotherm data is essential for predicting and comparing adsorption performance.Lead (Pb) adsorption was modelled using Langmuir, Freundlich, and Temkin models.The linear equations that correspond to the three models are presented in Equations 9, 10, and 11 [28,35].
(Eq. 9) (Eq. 10) (Eq. 11) Anal.Methods Environ.Chem.J. (a) region to 1000 cm -1 to 400 cm -1 , (b) region to 3000 cm -1 at 1000 cm -1 , and (c) region to 4500 cm -1 to 3000 cm -1  Where C e , q e , and q m (mg g -1 ) are the equilibrium concentration of lead (mg g -1 ), the quantity of Pb 2+ adsorbed at equilibrium (mg g -1 ), and the maximum monolayer adsorption capacity of adsorbent (mg g -1 ), respectively.n and K F are the Freundlich adsorption cons tants.K L is the Langmuir adsorption cons tant (Lmg -1 ).This las t parameter is used to calculate the dimensionless equilibrium parameter (R L ) that explains the favorability of the adsorption process; R L is calculated from Equation 12 [35].
(Eq. 12) B is a cons tant related to the heat of adsorption, which equals B = RT/b.R, T, and b are the gas cons tant (8.314 J.mol -1 K -1 ), the absolute temperature (K), and the Temkin cons tant (J mol -1 ).Typical adsorption isotherms for Pb 2+ on all selected anionic clays are shown in Figures 13, 14 and 15, respectively, for the Langmuir, Freundlich, and Temkin models.4, the Pb 2+ adsorption data on all the anionic clays followed the Langmuir and Temkin models.The values of the equilibrium parameter without dimension R L are between 0 and 1 for all the s tudied clays, showing that the adsorption of lead is favourable (Table 4).According to the correlation coefficients of the Freundlich model, the total experimental data on lead adsorption on anionic clays do not follow the Freundlich model (Table 4).The n value in the range of 1.62-3.37indicates a favourable adsorption process.The correlation coefficients for the Temkin model of all the clays s tudied show that this las t model adequately represents the experimental data on lead adsorption.The Temkin cons tant (BT) values presented in Table 4, related to the heat of sorption of Pb 2+ , increase with the increase of the molar ratio of Ni/Al.

Thermodynamic parameters
To describe the thermodynamic behaviour of the absorption of Pb 2+ ions in the aqueous solution, we use the following Equation13 and 14 [35,36]. (Eq.13) ΔG= -RT ln K D (Eq.14) where ΔH, ΔS, ΔG and T are the enthalpy, entropy, Gibbs free energy, and absolute temperature, respectively, and R is the gas cons tant (8.314 J k -1 .mol -1 ), K D = (q e /C e ), which depends on temperature.The thermodynamic parameters are determined s tarting from the lines of ln (k D ) vs. (1/T) in the linear domain of temperature, corresponding to the adsorption of lead, i.e., between 20°C and 50°C.
The thermodynamic parameters are presented in Table 5.The positive values of enthalpy sugges ted the endothermic nature of the adsorption.They reflected the affinity of the adsorbent for Pb 2+ ions [36].

Conclusion
As part of the s tudy of layered double hydroxides and the possibility of using them as adsorbents to remove lead from water, we have synthesized in our laboratory two types of anionic clays by the direct co-precipitation method, namely Mg The results showed that at pH below 6.5, the removal of Pb 2+ may be achieved by complexation reactions, and the lead precipitated at higher pH.The experimental data on lead adsorption kinetics show that the pseudo-second-order model bes t describes the adsorption kinetics.The results of the applied Pb 2+ adsorption isotherm models indicate that the Langmuir and Temkin models are the mos t adequate to represent the experimental data for both adsorbents.In addition, thermodynamic parameters show that the adsorption of lead by Mg 3 AlCO 3 and Ni 3 AlCO 3 is endothermic, spontaneous and random at the solute-solution interface.

Fig. 1 .
Fig. 1.Adsorption of lead by a batch method

Fig. 5 .
Fig. 5. Effect of mass on the adsorption of Pb 2+ onto Ni 3 Al-CO 3 and Mg 3 Al-CO 3

3
AlCO 3 , Ni 2 AlCO 3 , Ni 3 AlCO3, and Ni 4 AlCO 3 (to see the molar ratio effect, (R=2, 3, and 4).The mass of the adsorbent used is 0.2 g, the concentration of Pb 2+ in the solution is 50 mg Pb 2+ L -1 , the s tirring speed is 400 rpm, and the adsorption occurs at ambient temperature.The equilibration time is an important parameter that allows the determination of the rate of lead elimination, whether it is fas t or slow, as well as the evaluation of the effectiveness of the absorbent.The shape of the curves shown in Figure7is typical of saturation curves with a slight quantitative difference.The Pb 2+ retention kinetics consis ts of two dis tinct s teps: an initial fas t s tep with a contact time of up to 5 minutes for Mg 3 AlCO 3 and Ni 3 AlCO 3 and about 10 minutes for Ni 2 AlCO 3 and Ni 4 AlCO 3 , respectively, and a slower second s tep in which retention reaches a plateau, indicating the achievement of balance.The equilibrium times for Ni 2 AlCO 3 and Ni 4 AlCO 3 represent the double time required for equilibrium compared to Mg 3 AlCO 3 and Ni 3 AlCO 3 .This can be explained by the crys talline factor in Mg 3 AlCO 3 and Ni 3 AlCO 3 , which is good compared to Ni 2 AlCO 3 and Ni 4 AlCO 3 .The regular and repetitive dis tribution of atoms and functional groups bonded with these atoms, such as OH, facilitate the rapid attachment of Pb 2+ ions to these surface functional groups.It is known that the elimination of Pb 2+ for all adsorbents is done under the same conditions (temperature, s tirring speed).

Figure 11
Figure 11 shows the curves of adsorbed amounts versus equilibrium concentrations for Pb 2+ adsorption isotherms on Mg 3 AlCO 3 , Ni 2 AlCO 3 , Ni 3 AlCO 3 , and Ni 4 AlCO 3 clays.The adsorption capacities of these clays are proportional to the metal concentrations.According to Giles et al., the allure of the isotherms is of type L [30].Furthermore, the results showed that Mg 3 AlCO 3 clay has a higher adsorption capacity than NiAlCO 3 (R = 2, 3, and 4).

Fig. 12 .
Fig. 12. FT-IR spectra of Mg 3 AlCO3 and Ni 3 AlCO 3 before and after uptake of lead at different concentrations.

Fig. 13 .Fig. 14 .
Fig. 13.Langmuir isotherm model of the studied anionic clays The lead adsorption capacity values (Qm) found from the Langmuir model show that Mg 3 AlCO 3 has a large adsorption capacity (73.42 mg g -1 ).For Ni/ Al-based clays, the Pb 2+ ions adsorption capacity increases with the increase of the molar ratio, and Ni 4 AlCO 3 has a Pb 2+ adsorption capacity close to Mg 3 AlCO 3 .

Fig. 15 .
Fig. 15.Temkin isotherm model of the studied anionic clays The low H values for both adsorbents are 17.39 KJ mol -1 for Mg 3 AlCO 3 and 4.97 for Ni 3 AlCO 3 , which are less than 40 KJ mol -1 .It shows a physical adsorption between Pb 2+ ions and these clays[37].Positive entropy values showed that the randomness at the solute-solution interface increases with the adsorption of Pb 2+ in the adsorption process.Negative free energy values indicate a spontaneous process of the adsorption of Pb 2+ by Mg 3 AlCO 3 and Ni 3 AlCO 3 .These results obtained for Mg 3 AlCO 3 agree with those found by Ayawei et al.[38].

3
AlCO 3 and Ni 3 AlCO 3 .The analysis techniques used to characterize the LDHs show that the synthesized clays are materials from the family of layered double hydroxides.They have the same properties as the anionic clays of the Mg 3 AlCO 3 and Ni 3 AlCO 3 types.The pH, adsorbent mass, temperature, contact time, and molar ratio show that the adsorption capacity of Pb 2+ by Mg 3 AlCO 3 is higher than that of Ni 3 AlCO 3 .According to the Langmuir model,

Table 1 .
Pseudo-firs t-order model parameters

Table 4 .
Parameters of Pb 2+ adsorption isotherm models on selected anionic clays

Table 5 .
Thermodynamic parameters for the adsorption of Pb 2+ by Mg 3 AlCO 3 and Ni 3 AlCO 3 at various temperatures AlCO 3 and Mg 3 AlCO 3 clays have a high lead adsorption capacity, and the maximum adsorption capacity values are 72.51mg g-1 and 73.42 mg g -1 for Ni 4 AlCO 3 and Mg 3 AlCO 3 , respectively.At an optimal pH of 6.5, the removal percentages reach 95.4 % and 81.3 % for Mg 3 AlCO 3 and Ni 3 AlCO 3 , respectively.The adsorbed amount increases with increasing temperature for both types of LDHs, where the lead removal percentages reach 94.16 % and 85.39 % for Mg 3 AlCO 3 and Ni 3 AlCO 3 , respectively, and the adsorption capacities of Pb 2+ were obtained (Q Ni4AlCO3