Calcined ZnAl-LDH trapping performance in alginate beads for adsorption of Congo Red dye

ABSTRACT In this study, we report the adsorption process of Congo Red (CR) dye by calcined Layered Double Hydroxide (ZAC) at 550°C. The urea hydrolysis method was applied to produce highly crystalline Zn2Al-LDH. Samples characterisation was carried out by X-ray diffraction (XRD), Thermogravimetric/differential thermal analysis (TG/DTA), and Fourier transform infrared spectroscopy (FTIR). The results show that the adsorption of CR on calcined ZnAl-LDH was accompanied by the reconstruction of the layered structure. The ZAC showed higher adsorption efficiency versus other adsorbent under investigation and optimum conditions of contact time, adsorbent dose, pH and temperature were found to be 50 min, 0.02 g, 7 and 318 K, respectively. Isotherm studies show that the Langmuir isotherm model was appropriate to describe the CR adsorption data compared with Freundlich and Temkin isotherm models, with a higher correlation coefficient (R2 > 0.99) and maximum adsorption capacity of 406.5 mg/g. The thermodynamic parameters revealed that the adsorption process was spontaneous, endothermic, and governed by chemisorption. To further facilitate the recovery process in CR adsorption, we test the feasibility of applying ZnAl-LDH and its calcined product at 550°C entrapped in alginate beads. Experimental results indicate that alginate-coated ZAC beads, which exhibited remarkable adsorption performance, were more stable and require less effort to pull and separate than the alginate/ZnAl-LDH composite beads.


Introduction
Dyes are the synthetic aromatic compounds that are extremely employed for the dyeing of cotton, jute, paper, silk, wool, and leather products in many parts of the world.Although, excessive ingestion of them can cause accumulative poisoning, cancer, nervous system damage and so on [1].Congo Red (CR) is an azo dye, with chromophore of aromatic ring being joined to the azo group.However, under anaerobic conditions, the azo linkage can be reduced to form aromatic amines which are colourless but which can be toxic and carcinogenic.Hence, proper treatment of effluent containing CR dye is extremely necessary for the protection of environment and other living organisms.
Various treatments including physical-chemical and biological methods have been applied to reduce the harmful impacts of residual dye in textile effluents, as membrane filtration [2], coagulation-flocculation [3], ozonation [4], oxidation [5], precipitation [6], and biological degradation [7].Although these traditional methods are high-energy consumption and required expensive chemicals, thus adsorption remains one of the most widely used techniques due to its low cost, high efficiency, and easy to implement [8].Due to its high adsorption capacity for organic materials, activated carbon has been the oldest and widely used adsorbent for dyeing wastewater treatment.However, it is generally quite expensive and difficult to regenerate [9].Therefore, many researchers have used hydrotalcite-like materials to treat the dye wastewater due to its low cost, easy to prepare, and regenerate.
Layered Double Hydroxides (LDH) -also known as hydrotalcite-like or anionic clay-, which acquires high surface area, high porosity, and exchangeable anions between layers, are widely used in wastewater treatment [10], medicine [11], catalysis [12], flame retardant additives [13], and as precursors to magnetic materials [14].LDH structure can be described using this general formula: [M II  1−x M III x (OH) 2 ] (A n− ) x/n .mH 2 O where 'M II (Mg 2+ , Zn 2+ , Ni 2+ , Mn 2+ , Fe 2+ . ..)' and 'M III (Al 3+ , Cr 3+ , Fe 3+ , Co 3+ , Mn 3+ )' represent the divalent and trivalent metal ions, respectively, that composite the brucite-like sheet.The molar ratio M II /M III for pure LDH phases is ranging from 2 to 4, above or below this value additional impurity phases will be formed [15,16].The substitution of divalent by trivalent ions, leads to an excess of positive charge in the layers.The presence of exchangeable anions (A n-) as well as some water molecules (nH 2 O) in the interlayer space ensures the electroneutrality of the structure [17].Moreover, the mixed oxides derived from the LDH calcination process exhibit higher surface areas and porosity, and therefore a beneficial for the adsorption of dyes from wastewater [18,19].Besides, the LDH derived mixed metal oxides have the ability to be reconstructed by 'memory effect' to form the original layered structure of LDHs in the presence of water or aqueous solution containing anion [20,21].
Lei et al. [22], for example, demonstrated that the adsorption capacity of CR on calcined NiFe-LDH adsorbent was improved two times (330 mg/g) compared with the original NiFe-LDH precursor (205 mg/g), showing very good adsorption of dyes onto calcined LDHs.Lei et al. [23], also reported that the calcined NiMgAl-LDHs has high adsorption performance to remove CR ions in water solution other than the non-calcined NiMgAl-LDHs.Further, Zhang et al. [24] confirmed that the maximum adsorption capacity of Ca-Al layered double oxides for CR dye reached to 1536.1 mg/g larger than that of non-calcined Ca-Al-LDH (285.7 mg/g).
Based on above stated facts, this work was sketched out to evaluate the adsorption performance of calcined LDH at 550°C (ZAC) towards CR dye.To further facilitate the recovery process in CR adsorption, we test the feasibility of applying ZnAl-LDH and its calcined product at 550°C entrapped in alginate beads.For efficient removal of CR dye, various process variables were optimised.

Synthesis of ZnAl-LDH
Carbonate-intercalated Layered Double Hydroxide (ZnAl-CO 3 -LDH), with Zn/Al molar ratio of 2 was synthesised by thermal hydrolysis of urea.The layered material preparation procedure is described in detail in our previous work [10].The calcined product was obtained by thermal decomposition of the as-prepared Zn 2 Al-CO 3 -LDH at 550°C for 2 hours under air flow.Finally, the obtained powder (ZAC) was air-cooled to room temperature before use.The detailed experimental procedure has been provided in supporting information (S1).

XRD analysis
XRD patterns of the investigated samples are shown in Figure 1.Several clearly and sharp reflections located at 11.6, 23.3, 34.5, 39.2, 46.7 and 60.2, corresponding to the (003), (006), (009), (015), (018), and (110), respectively, similar to reflections of layered double hydroxides [25] appear in the XRD pattern of the as-prepared LDH (Figure 1(a)).According to XRD analysis, no signal corresponding to metal hydroxides M(OH) (Zn(OH) 2 or Al(OH) 3 ) phase were detected, proving the successful preparation of highly crystalline typical LDH material.The as-prepared LDH display a basal d-spacing of 7.57 Å, corresponding to CO 3 2anions occupied the interlayer space of ZnAl hydrotalcite-like compounds [25].These results are related to the strong affinity of LDH for the carbonate anions compared to other anions [26].The XRD pattern of ZAC (Figure 1(b)) presents peaks correspond to the (100) and (101) plans of zinc oxide (ZnO) (JCPDS No 01-079-2205).This result indicates that thermal treatment at temperature 550°C leads to decomposition of the LDH structure and formation of ZnO oxide phase.XRD pattern of ZAC after CR adsorption (Figure 1(c)) reveals a layered structure similar to the XRD patterns of as-prepared ZnAl-LDH, showing that the lamellar structure was regenerated after adsorption through 'memory effect'.After CR adsorption on ZAC, the interlayer spacing (d 003 ) value is 7.68 Å, and greater than that of as-prepared ZnAl-LDH.Thus, because the size of dye molecules is larger than that of CO 3 2− and OH − molecules, the interlayer spacing of the reconstructed hydrotalcite was enlarged, which indicates that some dye molecules rush into the interlayer space during the adsorption process [27].

FTIR spectroscopy analysis
FTIR spectra of all studied samples are illustrated in (Supplemental file, Figure S2).The IR spectrum of LDH sample (Supplemental file, Figure S2) exhibit a wideband at 3400-3500 cm −1 corresponding to the hydroxyl groups stretching vibrations ν(O-H) of the brucitelike layer and interlayer water [25,28].The weak band at 1638 cm −1 due to the stretching vibrations of the water molecule.The bands observed at 1364 and 771 cm −1 were assigned to the symmetric stretching of interlayer carbonate anions ν (C-O) [25,29].These results are consistent with XRD analysis, confirming the presence of carbonate anions in the gallery space of LDH.While, the peaks in the range of 700 to 400 cm −1 assigned to metal-oxygen (Zn-O, Al-O) vibrations in the brucite-like layers [25].After heat treatment at 550°C, significant changes in the intensity of ν(O-H) and ν(C-O) bands indicate the decomposition of layered structure resulting in the formation of ZnO oxide as shown in (Supplemental file, Figure S2).The absorption band of carbonate species decreased and shifted to lower wavenumbers 1364→1351 cm −1 after the adsorption of CR species onto ZAC (Supplemental file, Figure S2), suggesting that the removal process of CR dye involves anion exchange of CO 3 2-anions [30].Furthermore, the band around 1638 cm -1 is related to -OH bending vibration, whereas the sharp peak at 1601 cm -1 is due to the C = C stretching vibration of the aromatic ring in the CR molecule [31].It is noticed that after adsorbing CR, the adsorption band at around 1638 cm −1 disappeared, which is most likely overlapped by the stretching mode of C = C [31].The peak at 1051 cm −1 is ascribed to the S = O stretching vibration [23,31].Meanwhile, the metal-oxygen (M-O) bands were shifted, indicating that the metal ions (Zn 2+ and Al 3+ ) of LDH were complexed with the sulphonate group (SO 3 − ) [24].These results are consistent with the XRD analysis cited above, implying that the CR adsorption mechanism may involve surface complexation and ion exchange.

TGA
The TGA curves of all the analysed samples are shown in (Supplemental file, Figure S3).According to the TGA curve (Supplemental file, Figure S3), thermal decomposition of LDH sample occurs in three main steps.The first weight loss stage happens up to 200°C, is associated with the removal of the physisorbed and the interlayer water molecules.The second loss in the 200-370°C temperature range is due to the dehydroxylation of the brucite-like sheet and the decomposition of the interlayer CO 3 2− anions.Finally, the last weight loss step observed for the as-prepared LDH appeared between 370 and 500°C and corresponded to continuous elimination of strongly bounded anions [32].There is no weight loss observed above 550°C.It implies that there is no phase change after 550°C, and hence the calcination temperature is fixed at 550°C for the calcined ZnAl-LDH adsorbent.The ZAC exhibit different decomposition curves (Supplemental file, Figure S3) from those of LDH.The overall weight loss up to the final temperature of 550°C was about 10.73%, mainly due to moisture, carbonate remained after calcination or adsorbed during storage [33,34].After adsorbing CR dye, the TGA curve was similar in terms of thermal decomposition to as reported previously for hydrotalcite-like materials (Supplemental file, Figure S3).The loss of interlayer water up to about 220°C in ZnAl-LDH (~16%) was larger than that of ZAC after CR adsorption (~ 13%).This indicated that less water molecules are accommodated in the interlayer region of ZAC after CR adsorption.The second loss occurs from 220°C, assigned to the decomposition of carbonate anions and organic species intercalated in gallery space.The decomposition of CR ions took place in the range of 250-570°C [23] and, therefore, a greater loss was observed below 220°C [24].Although, the total weight loss of ZnAl-LDH (33.20%) and ZAC after CR adsorption (31.97%), were relatively similar because CR concentration was less than the amount of ZAC.

Effect of pH
The solution pH is an important parameter influencing the adsorption properties of the adsorbent towards dye molecules.Thus, pH affects not only the adsorbent surface charge but also the ionisation degree of the adsorbate molecule [35].Multi-flasks containing 20 mg of ZAC suspended in 25 mL of 100 mg/L CR solution were employed to investigate the effect of solution pH on the adsorption capacity of the calcined LDH.The pH value of the solution was adjusted in the pH range 2-11 by using 0.1 M HCl or 0.1 M NaOH.The results are shown in Figure 2. The pH had no significant impact on CR species adsorption on ZAC in the pH range 2 to 11. Although, it can be observed that the highest amount of CR uptake was noted between pH values 6 and 8, near-neutral pH (CR dye (pH ≈ 7) [36]).At pH values higher than 9, a notable decrease in the adsorption capacity is observed, probably due to competition between OH − ions and dye species on occupying the adsorption sites.Thus, all batch experiments were conducted without adjusting the solution pH.Whereas, according to our previous work [10], the highest adsorption capacity of non-calcined ZnAl-LDH for CR was recorded at acid medium (pH = 3), which is possibly caused by the dissolution of LDH material [37].These results are consistent with those reported by Guo et al. [38] and Ni et al. [39].
The point of zero charge (pHpzc) determined for ZAC (Sposito.G et al. [40]) was found to be 10.82, similar to values of pHpzc for the calcined LDH reported by other authors [41].At pH below pHpzc, the ZAC surface is positively charged, which enhances the adsorption amount of the negatively charged CR dye through electrostatic attraction.As the pH become higher than pHpzc, the adsorption capacity of ZAC decreased due to an electrostatic repulsion between the adsorbent surface negatively charged and anionic dye molecule.

Effect of adsorbent dosage
The influence of adsorbent dose on CR adsorption on ZAC sample was studied by varying the adsorbent amount (0.01-0.07 g) in the initial CR concentration (100 mg/L) and at room temperature.Based on the experimental results shown in Figure 3, the percentage CR dye removal increases significantly from 64% to 98%, while the adsorption capacity of ZAC decreased from 161.65 to 34.93 mg/g when the adsorbent dosage increased from 0.01 to 0.07 g.These results revealed that the available adsorption sites increase with more adsorbent present, while the initial concentration of CR was fixed at 100 mg/L, resulting in a surface equilibrium state and a reduction in the adsorption capacity per unit mass of adsorbent [42,43].The removal rate of CR reached the saturated value at adsorbent mass of 0.02 g then it remains almost constant, therefore, this value has been selected for further studies.

Effect of contact time and initial concentration of dye
The effect of the contact time (0-210 min) on ZAC adsorption capacity at different initial dye concentrations (20-200 mg/L) is illustrated in (Supplemental file, Figure S4).The adsorption capacity of ZAC for CR increases with contact time and then reached a maximum value in 50 min.No remarkable change in ZAC adsorption capacity after 50 min up to 210 min, while the first 50 min for the adsorption process was a fast adsorption.This may be due to the numerous accessible-free active sites on the external surface during the initial stage of adsorption, and after 50 min of contact time the adsorption active sites get occupied by CR dye molecules that lead to create a repulsive force between the adsorbate on the adsorbent surface and in bulk phase [35,44,45].Furthermore, the percentage removal increased from 90.88% to 96.93% for CR dye, as the initial dye concentration increased from 20 mg/L to 200 mg/L.The improvement of interaction between dye and adsorbent surface as the initial concentration increase, which leads to an increase in the driving force for mass transfer [45][46][47].Hence, the higher adsorption rate of ZAC and utilisation of all active sites for the adsorption at higher concentration, and could therefore be a potential adsorbent for wastewater treatment [35,44,46,47].

Adsorption kinetics
The adsorption kinetics studies are used to describe the solute uptake rate and time needed for the process to reach equilibrium [42,48].To identify the kinetic mechanism of adsorption process, the obtained results were modelled using the pseudo-first-order [49], pseudo-second-order [50], and intra-particle diffusion [51] kinetic models, and the correspondent parameters related to these models are incorporated in Table 1.
The kinetic equations of these models are given as follows: Where Q e (mg/g) and Q t (mg/g) are the amounts of adsorbed CR at equilibrium and time, respectively.K 1 (min −1 ), K 2 (g/mg/min) and K ip (mg/g/min 0.5 ) are the rate constants of the pseudo-first-order, pseudo-second-order, and intra-particle diffusion kinetic models, respectively.
Figure 4 shows the fitted plots of the three adsorption kinetics models for CR adsorption at 298 K.The results indicated that the pseudo-second-order kinetic model offers the best fit to the data over the other kinetic models mentioned above, with a correlation coefficient R 2 close to 1 and a calculated equilibrium adsorption amount Q e (cal) value  close to that of Q e (exp).This finding suggested that the adsorption kinetic of CR onto ZAC was through the chemisorption process.Furthermore, the possibility of dye molecules diffusion from the liquid phase to the adsorbent surface was described using the intraparticle diffusion kinetic model.Figure 4(c) clearly shows that the plot did not pass through the origin, indicating that the intra-particle diffusion was not the only ratecontrolling step.Although the value of constant C increases from 19.57 to 201.47 mg/g as the CR concentration increases from 20 to 200 mg/L, suggesting that the thickness of the boundary layers may involve in the adsorption process [25].

Adsorption isotherm
The adsorption isotherm describes the interaction of adsorbate molecules and adsorbent surface at a constant temperature [52,53].The adsorption isotherms of CR on ZAC were modelled using Langmuir, Freundlich, and Temkin isotherms models.
The Langmuir model assumes that the adsorption occurs on a homogeneous and energetically uniform surface [54].The linear form of Langmuir model is expressed as follows: Where Q m (mg/g) is the maximum adsorption capacity of adsorbent (mg/g) and K L (L/mg) is the Langmuir constant (g/L).
Moreover, the essential characteristic parameter of the Langmuir isotherm can be expressed by a dimensionless constant called separation factor R L that is defined by the following equation: The value of R L indicates the nature of the adsorption process, whether it is favourable (R L <1), linear (R L = 1), or unfavourable (R L >1).
Although, Freundlich model is typically used to described the reversible adsorption onto heterogeneous surface [52,54].This model has the following form: Where K F (mg/g)(L/mg) 1/n is the Freundlich constant and n is the degree of sorption related to adsorption intensity.
Similar to the Freundlich isotherm model, the Temkin model considers the interactions between the adsorbent and adsorbate and assumes that the adsorption heat of all molecules decreases linearly when the layer is covered [44,55].The Temkin isotherm model is calculated using the following equation: where b T (J/mol) and K T (L/g) are Temkin's constants, R is the universal constant of gases (8.314 J/mol K), and T is the absolute temperature (K).
The experimental adsorption data fitted with Langmuir, Freundlich, and Temkin models are plotted in Figure 5, and the corresponding parameters related to these models are incorporated in Table 2.As seen from Figure 5 and Table 2, the Langmuir model much better fits the isotherms data than the Freundlich model with high R 2 values.The results imply that the sorption activity of the ZAC surface is energetically identical, and the CR adsorbed form a monolayer coverage on the adsorbent surface.Moreover, the adsorption capacity calculated from the Langmuir model increased from 166.1 to 406.5 mg/g with increasing the temperature from 298 to 318 K, indicating that higher temperature improves the adsorption process [54,56].The values of R L and n listed in Table 2 for different temperatures (298-318 K) are less than 1, suggesting that the CR adsorption on ZAC is favourable.However, the Temkin isotherm model did not fit the experimental data as well as the Langmuir model, suggesting that the adsorption process is simply governed by surface adsorption.The adsorption capacities (mg/g) of CR dye over various adsorbents are list in (Supplemental file, Table S5), whereas the ZAC material shows a significant adsorption capacity at neutral pH.

Thermodynamic study
Thermodynamic parameters, such as Gibb's free energy (ΔG°), standard enthalpy (ΔH°), and standard entropy (ΔS°), were used to characterise the adsorption process as a function of temperature.The linear Van's Hoff equation was used to find out the values of various thermodynamics parameters as follows: Where K d is the distribution coefficient, Q e and C e are the equilibrium concentrations of dye ions on adsorbent (mg/g) and in solution (mg/L), respectively.R (8.314 J/mol K) is the gas constant and T (K) is the absolute temperature.
Plot of ln K d versus 1/T for CR adsorption on ZAC is shown in (Supplemental file, Figure S6) and calculated G, H and S values are given in (Supplemental file, Table S7).The ΔG° showed negative values at different temperatures and decreased with increasing temperature, suggesting that the adsorption of CR onto ZAC becomes favourable and spontaneous at higher temperatures [57,58].Moreover, the value of ΔH° is greater than 40 KJ/mol, confirming the chemical and endothermic nature of adsorption, while the positive ΔS° (304.75J/mol/K) indicates that randomness increased at the solid/solution interface during the adsorption of CR onto the ZAC.

Proposed adsorption mechanism
The adsorption mechanism of CR was disclosed through XRD, and FT-IR analyses.The XRD analysis indicated that lamellar structure was regenerated after adsorption through 'memory effect'.However, the d 003 spacing value of LDH after CR adsorption was enlarged from 7.57 to 7.68 Å, suggesting the existence of ion exchange between CR molecules and interlayer anions.The FTIR spectra of ZAC before and after CR adsorption were also performed and depict in (Supplemental file, Figure S2).It is evident that the hydroxyl groups participating in the adsorption process through electrostatic interaction between the positively charged adsorbent surface and negatively charged SO 3 − groups of CR.
Moreover, the new peaks that appeared after CR adsorption confirmed the intercalation of SO 3 − groups in the gallery space.Based on the above discussion, the adsorption mechanisms of CR on ZAC may involve physical and chemical interactions involving surface adsorption, electrostatic, and ion exchange, respectively.The adsorption mechanisms for uptake of CR by ZAC is illustrated in (Supplemental file, Figure S8).

ZnAl-LDH, and ZAC entrapped in alginate beads
Adsorption of dyes using low-cost, easy to handle, and non-toxic materials has become efficient in comparison to other traditional and conventional methods.Sodium alginate (SA) is a neutral, linear polysaccharide found in algae, and described as a copolymer composed of β-D-mannuronic acid (M-blocks) and α-L-guluronic acid (G-blocks) linked via 1-4 glycosidic bonds [59,60].Due to its low cost, biocompatibility, biodegradability, nontoxicity, and hydrophilicity, the hydrogels of alginate can be used to remove dye from wastewater [61][62][63].The combination of LDHs with biopolymer matrices can significantly improve the morphology, mechanical, and thermal stability of the obtained composite, leading to an increase in their environmental applications [59,64].To further facilitate the recovery process in CR adsorption, we test the feasibility of applying ZnAl-LDH, and its calcined product at 550°C entrapped in alginate beads.To achieve strong adsorption ability of these porous adsorbents, alginate coated ZnAl-LDH beads were performed in solution with pH 3, while alginate coated ZAC beads, were typically conducted in neutral pH media.These conditions were chosen in accordance with our previous work [10].The adsorption performance of the beads was evaluated by contacting 0.25 g of composite beads with 25 mL of 100 mg/L of dye solutions at 25°C for 24 h to reach equilibrium.After adsorption, all the beads settled in the bottom of the bottle and they were easily separated leaving a clear supernatant (Figure 6).Furthermore, our results revealed that under the optimum pH, the uncalcined and calcined ZnAl-LDH adsorbents are effective in removing CR dye from aqueous solution, leaving a clear supernatant solution (Figure 6).On the other hand, Figure 7 indicates that the physical properties of alginates depend strongly on the pH of the solution.Independently alginate beads shrink in acidic pH (pH = 3) and swell when placed in a basic or neutral pH solution (pH = 7).This is confirmed by a significant decrease in the mean values of alginate coated who showed that this behaviour can be correlated with the negative charge on the alginate molecules that decrease with decreasing pH due to reducing the number of anionic groups on the alginate molecules.It is also worth mentioning that the adsorption of CR by both the adsorbents was favourable at acidic pH and neutral pH.This may be ascribed to the surface charges of composite beads and the distributed species of CR in solution.The pHpzc was determined experimentally for alginate coated ZnAl-LDH and alginate coated ZAC beads as 7.81, and 8.4, respectively.These results revealed that the surfaces are strongly positively charged at pH < pHpzc [68][69][70].Therefore, the adsorbents can expect to exhibit high adsorption performances for anionic dyes.However, at neutral pH, the alginate coated ZAC beads, which exhibit excellent adsorption power for CR, were the more stable, and requiring less effort to pull and separate.Although, this work showed that the inclusion of LDHs in alginate beads for dyes removal applications have great promise for water purification.

Conclusion
This paper is a continuation of our previous work and aims to understand the mechanisms involved in the adsorption of CR onto calcined LDH material.The effects of dye concentration, adsorbent dosage, contact time, pH, and solution temperature on the adsorption process were investigated in detail.Adsorption isotherm data for CR adsorption on ZAC at neutral pH and 318 K temperature fitted well to Langmuir isotherm model with a maximum adsorption capacity of 406.5 mg/g.The adsorption process follows ideally the pseudo-second-order kinetic model.XRD, FTIR and TGA analyses confirmed that the removal of CR onto ZAC was carried out by the rehydration process with the incorporation of CR ions, leading to the reconstruction of the layered structure of LDH via the memory effect.The fast adsorption rate and high adsorption capacity of uncalcined and calcined ZnAl-LDH, indicates their applicability in wastewater treatment.Moreover, the porous alginate coated ZAC beads exhibited remarkable adsorption performance and showed good potential for use as a practical adsorbent for removal of dyes from effluents.In addition, ZAC entrapped in alginate beads could separate easily from the solution and showed a good stability at neutral pH in comparison to alginate/ZnAl-LDH composite beads in the acidic medium (pH = 3).

Figure 4 .
Figure 4. Linear fitting plots of (a) pseudo-first-order, (b) pseudo-second-order, and (c) Intra-particle diffusion kinetic model for CR adsorption on ZAC.

Figure 7 .
Figure 7. Removal of CR dye from aqueous solution by adsorption conducted at pH 3 for Zn/Al LDH and ZnAl-LDH/Alginate beads, and at neutral pH for ZAC and ZAC/Alginate beads.

Table 1 .
Parameters of the pseudo-first and pseudo-second order, and intra-particle diffusion kinetic models.