Ni/Zn Layered Double Hydroxide (LDH) Micro/Nanosystems and Their Azorubine Adsorption Performance

: A membranous shaped Ni/Zn layered double hydroxide based nanohybrid was obtained using a low-cost template-free hydrothermal process at optimized growth conditions of 180 ◦ C for 6 h. The synthesized nanohybrid was structurally, texturally and morphologically characterized using different techniques such as X-ray diffraction, FTIR, XPS spectroscopy, BET analysis and FESEM microscopy. The adsorption performance of our product was estimated through the Azorubine dye removal from synthetic wastewater. We therefore studied the synergic effects of Ni/Zn adsorbent dosage, contact time, pH, adsorbate concentration, stirring speed and temperature on the Azorubine adsorption efﬁciency. In this investigation, we obtained bi-structure based nanoadsorbent with 54% crystallinity order composed of nickel hydrate and zinc carbonate hydroxides in irregular nanoﬂake-like mesoporous nanohybrid morphology. Interestingly, it was also revealed to have high speciﬁc surface area (SSA) of around 110 m 2 g − 1 with important textural properties of 18 nm and 0.68 cm 3 g − 1 average pore size and volume, respectively. Moreover, the adsorption results revealed that this novel Ni/Zn layered double hydroxide (Ni/Zn LDH) was an efﬁcient adsorbent for Az molecule and possesses an adsorptive ability exhibiting a short equilibrium time (60 min) and a high Az adsorption capability (223 mg g − 1 ). This fast removal efﬁciency was attributed to high contact surface area via mesoporous active sites accompanied with the presence of functional groups (OH − and CO 32 − ). In addition, the Langmuir and Freundlich isotherms were studied, and the results ﬁtted better to the Langmuir isotherm.


Introduction
Nowadays, one of the major challenges for researchers in chemistry is the elimination of dyestuff from liquid effluents before they are discharged to the external environment in view of their harmful effects on aquatic and terrestrial life [1][2][3][4]. This kind of contaminant has characteristics of excellent chemical stability, good solubility in water, high colority and a complex aromatic structure, which decomposition under natural conditions is difficult. The presence of these colorful contaminants in water can restrict light and oxygen getting into water, leading to serious threats to the survival of aquatic organisms. Moreover, most organic dyestuffs, such as azo and anthraquinone dyes, are toxic or carcinogenic to the human body [5][6][7][8].
Amongst these dyes, Carmoisine (E 122), also named Azorubine (Az), is used mainly in jams and preserves and was found by the US Certified Color Manufacturers Associ-

Adsorbent Ni/Zn Carbonate and/or Hydroxide Based Heterostructure Nanohybrid Synthesis
Nickel (II) chloride hexahydrate (NiCl 2 6H 2 O) (0.1 M) and zinc (II) chloride (ZnCl 2 ) (0.1 M) were dissolved in deionized water at room temperature. Afterwards, 0.5 M urea aqueous solution was added into the above solution and ultrasonicated until a dark green transparent solution was obtained, which was then transferred into a Teflon-lined hydrothermal autoclave system. The latter was sealed, maintained at growth conditions of Appl. Sci. 2021, 11, 8899 3 of 13 180 • C/6 h and then allowed to naturally cool down to room temperature. The obtained green-colored products were filtered and washed with distilled water and ethanol several times before drying in an oven at 80 • C overnight.

Adsorbent Characterization Techniques
The synthesized bi-transition metal hydroxide based nanohybrid was characterized using several techniques in order to identify the hydrothermal process product properties, such as structural, textural, morphology and composition, which influence its adsorption performance for environment applications.
Powder X-ray diffraction patterns of our solid adsorbent were collected on an XRD D8 ADVANCE-BRUKER AXS diffractometer equipped with a copper anticathode tube (λ CuKα = 1.5406 Å) and a graphite monochromator rear blade, operating at 40 kV-40 mA and employing a scanning rate of 0.2 • s −1 in the range 10 • -90 • .
Specific contact surface area (SSA) and porosity measurements were carried out using N 2 adsorption/desorption mechanism at nitrogen liquid temperature (77 K) on a Tristar an ASap2420 sorptometer (Micromeritics) surface area analyzer using Brunauer-Emmett-Teller (BET) and Barrett-Joyner-Halenda (BJH) methods, respectively.
Raman scattering measurement was carried out at room temperature using a Horiba JobinYvon Lab-RAM Aramis confocal Raman spectrometer equipped with a cooled CCD camera and an automated XYZ table at a laser excitation of 532 nm. Using a D2 filter, the estimated laser power that reached the samples was 0.33 mW.
The morphological aspect of this nanohybrid was identified via a field emission scanning electron microscopy FESEM (JEOL 6700-FEG microscope) operated at 3 kV.
Fourier-transform infrared (FTIR) spectra were recorded using a Bruker Vertex 77v spectrometer with a 4 cm −1 resolution in [400-4000 cm −1 ] range controlled with an Opus software analysis.

Analytical Method
A calibration curve was prepared using the standard solutions of Az with different concentrations from 0.5 to 50 mg L −1 . The efficiency of the proposed process was evaluated by monitoring Az removal after measuring the absorbance at λ max = 519 nm.
The azo dye adsorption capacity q t (mg g −1 )and the removal ratio R (%) were obtained from the following relations: where C 0 and C t are the initial and the equilibrium concentration of Az in the solution (mg L −1 ), where W the adsorbent dry weight (g) and V is the suspension volume (l).

Batch Adsorption
Batch adsorption tests were carried out at a constant temperature using a water bath. The initial concentration of Azorubine solution was 50 mg L −1 for all experiments, except for those carried out to determine the effect of the initial dye concentration. The adsorbent was put in selected flask of 100 mL dye solution and vigorously stirred at 25 • C, except experiments of the temperature effect. The experiments were conducted at natural pH of mixtures without further adjustment, except also for the step of solution pH effect where it was adjusted using (0.1 N) HCl or NaOH in separate experiments. Batch studies were performed as function of the adsorbent rate ranging from (0.01 to 0.05 g), pH solution from 2 to 12 and adsorbate concentrations from 20 to 200 mg L −1 .

Isotherm Models
The adsorption isotherm was used to represent the adsorption capacity at different solution concentrations that helps to have idea on the adsorbent and adsorbate interaction. The experimental results were fitted using Langmuir [33] and Freundlich [34] models. The Langmuir isotherm (Equation (3)) assumes a monolayer adsorption of the adsorbate on the adsorbent surface using a given relation: where q m is the maximum adsorption capacity (mg g −1 ) and K L is the Langmuir constant (L mg −1 ). Nevertheless, the Freundlich adsorption isotherm (Equation (4)) as an empirical equation often used to describe the chemisorption on heterogeneous surface is given by the following relation: where K F is the Freundlich constant and 1/nis the heterogeneity factor. In this Freundlich equation, 1/n values between 0 and 1 indicate that the adsorption is favorable.

Kinetic Studies
The pseudo-first-order and pseudo-second-order kinetic models are described as following [35]: Ln q e − q t = Ln q e − K 1 t (5) (6) where q e (mg g −1 ) is the equilibrium adsorption capacity, q t (mg g −1 ) is the adsorption capacity at time t; k 1 (min −1 ) is the rate constant of the first-order adsorption and k 2 (g·mg −1 min −1 ) is the rate constant of the second-order adsorption.

Characterization of Adsorbent
The description of the XRD pattern and FTIR spectrum of the as-synthesized Ni/Zn based nanohybrid which will be used as adsorbent in this investigation are presented in Figure 1. Figure 1a shows the formation of bi-nanostructures based on nickel hydrate hydroxide (α-Ni(OH) 2 0.75 H 2 O) and zinc carbonate hydroxide (Zn 5 (CO 3 ) 2 (OH) 6     The FTIR spectrum of this nanoadsorbent as shown in Figure 1b is typical of a hydrotalcite-like material showing a broad and intense band at 3336 cm -1 thatcan be attributed to hydroxyl (OH − ) stretching vibrations from structural hydroxyl groups and interlayer water molecules [36]. In addition, bands at 1362 cm −1 (υ 3 ), 1043 cm −1 (υ 1 ) and 745 cm −1 (υ 2 ) [37] show the presence of carbonate anion (CO 3 2-) in the interlayer [38]. However, the bands at 953 and 836 cm −1 are associated withmetal-oxygen (M-O) and metal-hydroxyl (M-OH) groups in the lattice of LDHs. More precisely, below 642 cm −1 , the recorded bands are attributed to δ Ni/Zn-OH or ν Ni/Zn-O as reported in many studies [39][40][41]. Moreover, the sharp band at 836 cm −1 is associated with in-plane quadrant bending [39][40][41]. These results clearly prove the XRD identification reported above.
The morphological and textural properties of this Ni/Zn based LDH nanohybrid illustrate that it possesses precisely special membranous morphology with non-homogeneous mesopores in an agglomerated way as shown in Figure 2a. The specific surface area (SSA-SBET) of this bi-phase nanohybrid was estimated from the adsorbed-desorbed nitrogen quantity in relation to its pressure at liquid boiling point and under normal atmospheric pressure. Figure 2b shows the obtained information of adsorption and desorption isotherms of nitrogen at 77 K and their pores size and volume distribution according to the BET and BJH model, respectively. The isotherm shows an hysteresis IV-type with SBET of about 110.38 m² g −1 . However, the pores distribution illustrated inset this figure indicates the collection of multiporous nanohybrid with average pore size essentially centered at 2, 3, 24 and 32 nm, which demonstratetheir mesoporous texture nanohybrid (pores between 2 and 50 nm) [42]. This distribution confirms, again, the dishomogeneous porous morphology observed in almost micro/nanospheric membranous architecture via FESEM microscopy as shown above.
To better understand the composition of the used nanohybrid based adsorbent as well as the existing functional groups that act on the adsorption mechanism, XPS analysis was performed. Figure 3 shows the deconvolution of main asymmetric peaks (Ni2p, Zn2p, C1s, and O1s) of high-resolution core-level XPS spectra, using L/G fitting method consistent The specific surface area (SSA-S BET ) of this bi-phase nanohybrid was estimated from the adsorbed-desorbed nitrogen quantity in relation to its pressure at liquid boiling point and under normal atmospheric pressure. Figure 2b shows the obtained information of adsorption and desorption isotherms of nitrogen at 77 K and their pores size and volume distribution according to the BET and BJH model, respectively. The isotherm shows an hysteresis IV-type with S BET of about 110.38 m 2 g −1 . However, the pores distribution illustrated inset this figure indicates the collection of multiporous nanohybrid with average pore size essentially centered at 2, 3, 24 and 32 nm, which demonstratetheir mesoporous texture nanohybrid (pores between 2 and 50 nm) [42]. This distribution confirms, again, the dishomogeneous porous morphology observed in almost micro/nanospheric membranous architecture via FESEM microscopy as shown above.
To better understand the composition of the used nanohybrid based adsorbent as well as the existing functional groups that act on the adsorption mechanism, XPS analysis was performed. Figure 3 shows the deconvolution of main asymmetric peaks (Ni 2p , Zn 2p , C 1s , and O 1s ) of high-resolution core-level XPS spectra, using L/G fitting method consistent with the literature data [36,43]. Ni 2p and Zn 2p show that the oxidation state of these elements is mainly based on ions, namely Ni 2+ (or Zn 2+ ) as in previously reported data, which will be very beneficial in our adsorption process [36,44]. distribution according to the BET and BJH model, respectively. The isotherm show hysteresis IV-type with SBET of about 110.38 m² g −1 . However, the pores distribution i trated inset this figure indicates the collection of multiporous nanohybrid with ave pore size essentially centered at 2, 3, 24 and 32 nm, which demonstratetheir mesopo texture nanohybrid (pores between 2 and 50 nm) [42]. This distribution confirms, ag the dishomogeneous porous morphology observed in almost micro/nanospheric m branous architecture via FESEM microscopy as shown above.
To better understand the composition of the used nanohybrid based adsorben well as the existing functional groups that act on the adsorption mechanism, XPS ana was performed. Figure 3 shows the deconvolution of main asymmetric peaks (Ni2p, Z C1s, and O1s) of high-resolution core-level XPS spectra, using L/G fitting method consis with the literature data [36,43]. Ni2p and Zn2p show that the oxidation state of these ments is mainly based on ions, namely Ni 2+ (or Zn 2+ ) as in previously reported data, w will be very beneficial in our adsorption process [36,44].

Adsorbent Dosage Effect on the Azorubine Removal
One of the parameters that significantly affect the adsorption capacity is the ad bent dosage [45]; Figure 4 illustrates theNi/Zn LDH dosage effect on Az adsorption. removal ratio of azo dye was increased when increasing the amount of Ni/Zn LDH ad bent that was used. High Az removal is observed, with the maximum reachinga valu 97% with 30 mg L -1 adsorbent. Thiscan be attributed to the larger pore diameter (aro 24 nm), which favors Az removal at lower adsorbent dosages [45]. In addition, the hi sorbent removal with Ni/Zn LDHs at low adsorbent dosage demonstrates the high aff between this adsorbent and the Az. Therefore, we fixed the adsorbent amount to 30 L −1 forall subsequent experiments. The adsorption as a function of contact time was conducted at room temperatur

Adsorbent Dosage Effect on the Azorubine Removal
One of the parameters that significantly affect the adsorption capacity is the adsorbent dosage [45]; Figure 4 illustrates theNi/Zn LDH dosage effect on Az adsorption. The removal ratio of azo dye was increased when increasing the amount of Ni/Zn LDH adsorbent that was used. High Az removal is observed, with the maximum reachinga value of 97% with 30 mg L -1 adsorbent. Thiscan be attributed to the larger pore diameter (around 24 nm), which favors Az removal at lower adsorbent dosages [45]. In addition, the higher sorbent removal with Ni/Zn LDHs at low adsorbent dosage demonstrates the high affinity between this adsorbent and the Az. Therefore, we fixed the adsorbent amount to 30 mg L −1 forall subsequent experiments.

Adsorbent Dosage Effect on the Azorubine Removal
One of the parameters that significantly affect the adsorption capacity is the adsorbent dosage [45]; Figure 4 illustrates theNi/Zn LDH dosage effect on Az adsorption. The removal ratio of azo dye was increased when increasing the amount of Ni/Zn LDH adsorbent that was used. High Az removal is observed, with the maximum reachinga value of 97% with 30 mg L -1 adsorbent. Thiscan be attributed to the larger pore diameter (around 24 nm), which favors Az removal at lower adsorbent dosages [45]. In addition, the higher sorbent removal with Ni/Zn LDHs at low adsorbent dosage demonstrates the high affinity between this adsorbent and the Az. Therefore, we fixed the adsorbent amount to 30 mg L −1 forall subsequent experiments. The adsorption as a function of contact time was conducted at room temperature (25 °C) using 30 mg L -1 of Ni/Zn LDH and is reported in Figure 5a. The azo dye uptake was rather fast initially (first 25 min), then increased slowly up until 60 min, when the equilibrium was reached with a removal ratio of 94.3%. At the beginning, there are phenomena The adsorption as a function of contact time was conducted at room temperature (25 • C) using 30 mg L -1 of Ni/Zn LDH and is reported in Figure 5a. The azo dye uptake was rather fast initially (first 25 min), then increased slowly up until 60 min, when the equilibrium was reached with a removal ratio of 94.3%. At the beginning, there are phenomena attributed to the existence of a large number of vacant active sites on the adsorbent surface. However, the remaining vacant surface sites are less available for adsorption with time probably due to the repulsive forces that occur between the Az molecules and the Ni/Zn LDH surfaces [46].
Appl. Sci. 2021, 11, x FOR PEER REVIEW 8 of 1 basic sites in the LDH structure and, consequently, it takes more time to reach the adsorp tion equilibrium [47]. The pH of the dye solution is also an important parameter which controls the rate o synthetic azo dye solution degradation. In order to study this pH effect, the degradation experiments were carried out using 30 mg of Ni/Zn hydroxide in a solution of 50 ppm A (Figure 5b) by varying the pH in the range 2-12 with addition of (0.1 M) either HCl o NaOH. The adsorption process affected by the milieu pH is possibly due to the interac tions between the dye anions in the solution and the adsorbent surface charge content [48,49]. Therefore, we noted firstly that the removal efficiency of Az increases from 82.52% to 96.02% when the pH is varied from 2 to 7, and the maximum was achieved at neutra pH. Then, the removal rate decreases in basic environment from 33.96% to 27.54% in th pH range from 10 to 12. This phenomenon can be attributed to the electrostatic attraction between the positively charged adsorbent surface layer and the anionic dye at pH les than the point of zero chare (PZC), which is equal to 7.53 for our Ni/Zn LDH adsorbent Therefore, at lower pH (acidic region), the surface of Ni/Zn LDH adsorbent with mor positively charged (pH < pHPZC) leads to increase the percentage of dye removal due to strengthening the attractive forces among the positive charge of the surface of the LDH and negative charge of the dye [50]. On the other hand, the surface of LDHs become mor Otherwise, the decrease in the Az adsorption kinetics is due to the presence of carbonates from LDH structure; the dye species are in competition with carbonate anions which originate from strong interactions between the CO 2 molecules and the strongest basic sites in the LDH structure and, consequently, it takes more time to reach the adsorption equilibrium [47].
The pH of the dye solution is also an important parameter which controls the rate of synthetic azo dye solution degradation. In order to study this pH effect, the degradation experiments were carried out using 30 mg of Ni/Zn hydroxide in a solution of 50 ppm Az (Figure 5b) by varying the pH in the range 2-12 with addition of (0.1 M) either HCl or NaOH. The adsorption process affected by the milieu pH is possibly due to the interactions between the dye anions in the solution and the adsorbent surface charge contents [48,49]. Therefore, we noted firstly that the removal efficiency of Az increases from 82.52% to 96.02% when the pH is varied from 2 to 7, and the maximum was achieved at neutral pH.
Then, the removal rate decreases in basic environment from 33.96% to 27.54% in the pH range from 10 to 12. This phenomenon can be attributed to the electrostatic attraction between the positively charged adsorbent surface layer and the anionic dye at pH less than the point of zero chare (PZC), which is equal to 7.53 for our Ni/Zn LDH adsorbent. Therefore, at lower pH (acidic region), the surface of Ni/Zn LDH adsorbent with more positively charged (pH < pH PZC ) leads to increase the percentage of dye removal due to strengthening the attractive forces among the positive charge of the surface of the LDHs and negative charge of the dye [50]. On the other hand, the surface of LDHs become more negatively charged along with the increase in solution pH (pH > pH PZC ), leading to a decline inthe nanohybrid adsorption ability. This decrease phenomenon is related to strong repulsion forces between the dye and negative surface of the LDHs containing inorganic anions. Consequently, for pH values under the pH PZC , there can be two probable mechanisms to propose for Az removed using this kind of LDH; the first mechanism could concern the anionic exchange of carbonate anions in the interlayer with dye anions Az-SO 3 − , while the second is the fixation by adsorption with the positively charged surface H 2 O + of Ni/Zn LDH and the Az anions [23]. Therefore, these solutions were used directly without any pH adjustment in the rest of the work.
According to the curves in Figure 5c, it can be seen that the capacity for retention of the dye increases slightly with increasing stirring speed, which acts favorably on the contact probability between substrate and support. Figure 5d presents Az removal as a function of time at three different temperatures (15, 25 and 45 • C) on Ni/Zn LDH adsorbent. A similar profile of the kinetic curves is observed for the three temperatures, and the sorption kinetic increases slightly with increasing temperature.
This indicates that the sorption process is endothermic. The phenomenon could be ascribed to the improvement of the mobility and the penetration of Az molecules within the Ni/Zn LDH based mesoporous bi-nanostructure by allowing the activation energy barrier to be easily overcome [51]. Figure 6 exhibitsthe adsorption capacity of the Ni/Zn LDH nanohybrid at various initial concentrations of Az. It can be seen that the removal percentage decreases from 98% to 66.8% and the Az adsorption amount increases from 123.74 to 710 mg g −1 when the Az initial concentration is increased from 25 to 200 mg L −1 . Therefore, the decrease in the dye removal efficiency with an increase in the dye concentration may be related to the initial amount of dye molecules (see pictures inset), which is lower than the available surface active sites, making the adsorption ratio independent of the initial dye concentration. Nevertheless, at higher concentrations, there would be fewer available sites than initial dye molecules in the solution which affect the Az removal percentage [46,52].

Adsorbent Concentration Effect on the Adsorption Process
Appl. Sci. 2021, 11, x FOR PEER REVIEW 9 of 15 negatively charged along with the increase in solution pH (pH > pHPZC), leading to a decline inthe nanohybrid adsorption ability. This decrease phenomenon is related to strong repulsion forces between the dye and negative surface of the LDHs containing inorganic anions. Consequently, for pH values under the pHPZC, there can be two probable mechanisms to propose for Az removed using this kind of LDH; the first mechanism could concern the anionic exchange of carbonate anions in the interlayer with dye anions Az-SO3 − , while the second is the fixation by adsorption with the positively charged surface H2O + of Ni/Zn LDH and the Az anions [23]. Therefore, these solutions were used directly without any pH adjustment in the rest of the work. According to the curves in Figure 5c, it can be seen that the capacity for retention of the dye increases slightly with increasing stirring speed, which acts favorably on the contact probability between substrate and support. Figure 5d presents Az removal as a function of time at three different temperatures (15, 25 and 45 °C) on Ni/Zn LDH adsorbent. A similar profile of the kinetic curves is observed for the three temperatures, and the sorption kinetic increases slightly with increasing temperature.
This indicates that the sorption process is endothermic. The phenomenon could be ascribed to the improvement of the mobility and the penetration of Az molecules within the Ni/Zn LDH based mesoporous bi-nanostructure by allowing the activation energy barrier to be easily overcome [51]. Figure 6 exhibitsthe adsorption capacity of the Ni/Zn LDH nanohybrid at various initial concentrations of Az. It can be seen that the removal percentage decreases from 98% to 66.8% and the Az adsorption amount increases from 123.74 to 710 mg g −1 when the Az initial concentration is increased from 25 to 200 mg L −1 . Therefore, the decrease in the dye removal efficiency with an increase in the dye concentration may be related to the initial amount of dye molecules (see pictures inset), which is lower than the available surface active sites, making the adsorption ratio independent of the initial dye concentration. Nevertheless, at higher concentrations, there would be fewer available sites than initial dye molecules in the solution which affect the Az removal percentage [46,52]. Moreover, our nanoadsorbent has a high elimination rate just in the first few minutes, and the adsorption equilibrium time was 60 min. It shows that the adsorption on Ni/Zn LDH nanohybrid asa micro/nanoporous system has a short equilibrium time, and the adsorption Moreover, our nanoadsorbent has a high elimination rate just in the first few minutes, and the adsorption equilibrium time was 60 min. It shows that the adsorption on Ni/Zn LDH nanohybrid asa micro/nanoporous system has a short equilibrium time, and the adsorption rate of this nanohybrid is much faster than other general bulk layered compound. This can be attributed to the mesoporous micro/nanosystem morphology, which is helpful for molecular transport by virtue of its porous texture architectural feature [53][54][55][56].

Fitting Kinetic Models
The kinetic curves of Az adsorption onto Ni/Zn LDH nanohybrid are shown in Figure 7a,b, and the parameters of these kinetics models were obtained by regression analysis and listed in Table 2. We observed that the R 2 coefficients determinedfor the pseudo-first-order model were higher than the pseudo-second-order model values for this nanohybrid adsorbent.
Appl. Sci. 2021, 11, x FOR PEER REVIEW 10 of 15 rate of this nanohybrid is much faster than other general bulk layered compound. This can be attributed to the mesoporous micro/nanosystem morphology, which is helpful for molecular transport by virtue of its porous texture architectural feature [53][54][55][56].

Fitting Kinetic Models
The kinetic curves of Az adsorption onto Ni/Zn LDH nanohybrid are shown in Figure 7a,b, and the parameters of these kinetics models were obtained by regression analysis and listed in Table 2. We observed that the R 2 coefficients determinedfor the pseudo-firstorder model were higher than the pseudo-second-order model values for this nanohybrid adsorbent.  In addition, the computed equilibrium uptakes of the pseudo-first-order (PFO) expression are closer to the experimental values, thus indicating the applicability of this model, which can describe very well the Az sorption process onto Ni/Zn LDH nanoadsorbent via a physical sorption [57].

Adsorption Isotherm Studies
In general, adsorption isotherms play a fundamental role in the optimization of adsorbent consumption and explains the reaction between the pollutants and the adsorbent material [58]. Hence, the choice of best isotherm fit was conductedby linear regression analyses and a comparison of their correlation coefficients (R 2 ).  In addition, the computed equilibrium uptakes of the pseudo-first-order (PFO) expression are closer to the experimental values, thus indicating the applicability of this model, which can describe very well the Az sorption process onto Ni/Zn LDH nanoadsorbent via a physical sorption [57].

Adsorption Isotherm Studies
In general, adsorption isotherms play a fundamental role in the optimization of adsorbent consumption and explains the reaction between the pollutants and the adsorbent material [58]. Hence, the choice of best isotherm fit was conductedby linear regression analyses and a comparison of their correlation coefficients (R 2 ).
Equilibrium studies which express the dye adsorbent capacity are explained through the adsorption isotherms. The kinetic curves of Az adsorption onto Ni/Zn LDH nanohybrid are shown in Figure 8, and their parameters were obtained by regression analysis and listed in Table 3.
Appl. Sci. 2021, 11, x FOR PEER REVIEW 11 of 15 Equilibrium studies which express the dye adsorbent capacity are explained through the adsorption isotherms. The kinetic curves of Az adsorption onto Ni/Zn LDH nanohybrid are shown in Figure 8, and their parameters were obtained by regression analysis and listed in Table 3. .123 and 2.097) at 288, 298 and 318 K temperatures, respectively, indicate that this Ni/Zn LDH nanohybrid has a higher adsorption capacity and affinity for the azo dye molecules, revealing the favorability of this adsorption [59]. Moreover, the theoretical maximum adsorption capacity of Ni/Zn LDH mesoporous nanoadsorbent calculated depending on the Langmuir model is 227.27 mg g −1 , which is four times more superior than that of other analogical or low-cost adsorbents such as the 3D magnetic hollow porous Ni-ferrites (3D-MHP-NiFe2O4) with qm = 53.13 mg g −1 [60] and copper oxide with qm = 1.59 mg g −1 [59].  The values of the correlation coefficient R 2 = 0.9952, 0.9934 and 0.9871 at 288, 298 and 318 K, respectively, of the Az adsorption process on our nanohybrid, demonstrate good linearity with the Langmuir expression compared to the Freundlich model. This result reveals the monolayer coverage of Az molecules on the NiZn/LDH nanohybrid accompanied byan homogeneous adsorption process. Moreover, the equilibrium parameter R L ( 1 1+bC 0 ) is in the range of [0-1], indicating that implying the Az adsorption process is favorable. Moreover, the higher values of K F (71.58, 72.35 and 43.12 mg g −1 ) and N (3.128, 3.123 and 2.097) at 288, 298 and 318 K temperatures, respectively, indicate that this Ni/Zn LDH nanohybrid has a higher adsorption capacity and affinity for the azo dye molecules, revealing the favorability of this adsorption [59].
Moreover, the theoretical maximum adsorption capacity of Ni/Zn LDH mesoporous nanoadsorbent calculated depending on the Langmuir model is 227.27 mg g −1 , which is four times more superior than that of other analogical or low-cost adsorbents such as the 3D magnetic hollow porous Ni-ferrites (3D-MHP-NiFe 2 O 4 ) with qm = 53.13 mg g −1 [60] and copper oxide with qm = 1.59 mg g −1 [59].

Conclusions
In summary, Ni/Zn layered double hydroxide based nanohybrid was obtained using low-cost template-free hydrothermal process at 180 • C growth temperature during 6 h via the formation of two phases based of nickel hydrate hydroxide (α*−Ni(OH) 2 0.75 H 2 O) and zinc carbonate (Zn 5 (CO 3 ) 2 (OH) 6 ), where the FTIR results also indicate the presence of functional groups that favor their electrostatic attraction with the anionic dye.
This multiscale porous hydroxide based nanohybrid with irregular-like nanoflakes in 3D spongeous shape hierarchical micro/nanostructures which possess very interesting specific surface area around 110 m 2 g −1 with reasonable pore volume and pore size distribution between [0.16-0.85 cm 3 g −1 ] and [12-32 nm], respectively, is accompanied by mixed oxidation states with two kinds of anions Ni 2+ (or Zn 2+ ) and Ni 3+ (or Zn 3+ ), allowing it to be a very good nanoadsorbent.
The Az adsorption ability of this new kind of nanoadsorbent was systematically investigated using kinetic and equilibrium studies as well as by examining the effect of dosage adsorbents, initial adsorbate concentration, initial pH, temperature, etc. Thus, the capacity sorption of Az increases with increasing Az concentration, with a very fast sorption process whereby an equilibrium time of 60 min was obtained. The adsorption behavior of dye onto this Ni/Zn LDH nanohybrid was obviously influenced by the pH with the best removal efficiency at pH range from 6 to 7. We also, noted that our new nanoadsorbent presents an important adsorption capacity (~223 mgg −1 ) compared to other materials. Moreover, the sorption kinetic data were more suitable with the pseudo-first-order (PFO) kinetic model. Langmuir adsorption isotherm model showed the best compatibility with the experimental data in comparison with other isotherm models.