Abstract

The present study deals with the preparation and structural and adsorbent characterization of the ternary layered double hydroxides (LDHs; ZFA-HT) with molar ratio Zn2+/Al3+/Fe3+ = 2/0.5/0.5 and its product calcined (ZFA-350) at 350 °C, which is examined for the removal of phosphate P(V) and chromate Cr(VI) from aqueous media. The as-obtained materials are characterized by X-ray diffraction (XRD), Fourier-transform infrared (FT-IR), thermogravimetric analysis-differential scanning calorimetry (TGA–DSC), scanning electron microscopy-X-ray energy dispersion (SEM-EDX) and Brunauer–Emmett–Teller (BET). Structural characterizations show that the LDHs is successfully synthesized and its calcined product is a mixed oxide. Batch sorption studies are conducted to investigate the effects of various experimental parameters such as contact time, solution pH, adsorbent amount, initial P(V) or Cr(VI) concentration and temperature. The isotherms, kinetics and thermodynamic parameters of adsorption of phosphate and chromium are studied. The adsorption processes are well described by the pseudo-second-order kinetic model than the other models examined. The adsorption isotherms data fit best to the Langmuir isotherm model instead of Freundlich and Dubinin–Radushkevich models. The maximum monolayer adsorption capacity of ZFA-350 was found to be 140.85 mg/g for P(V) and 52.63 mg/g for Cr(VI). The positive ΔH and ΔS and negative ΔG values reveal that the P(V) and Cr(VI) sorption onto ZFA-350 is endothermic, irreversible and spontaneous in nature.

HIGHLIGHTS

  • Synthesis and characterization of [Zn-Al-Fe-CO3] LDHs and its calcined product.

  • Adsorption of P(V) and Cr(VI) follow the Langmuir isotherm model and pseudo-second-order kinetics.

  • Thermodynamic analysis exhibits the spontaneous and endothermic nature of the sorption process.

  • Improvement of the adsorption capacities of P(V) and Cr(VI) compared to other adsorbents used.

INTRODUCTION

Water pollution has several origins: it can come from industrial, domestic and urban discharges, or agricultural. Chromium and phosphate are widely used in various industrial applications. The very large quantities of these products released into the environment can lead to changes in the structure of the ecosystem. Chromium is a toxic element; it is considered a dangerous pollutant and carcinogen (Fang et al. 2014). According to the US Environmental Protection Agency (EPA), the maximum allowable concentration of Cr(VI) in drinking water and that discharged in surface water are 0.05 and 0.1 mg/L, respectively (Kumar & Puri 2012). Phosphates are considered to be mainly responsible for the eutrophication process. This phenomenon occurs when the phosphate concentration in water bodies is greater than 0.02 mg/L (Xiong et al. 2011), hence the need to remove P(V) and Cr(VI) ions in wastewater. Various treatment methods such as ion exchange, reverse osmosis, precipitation, oxidation and reduction, electrocoagulation and adsorption show the capability to remove the toxic elements from wastewaters (Fu & Wang 2011; Ungureanu et al. 2015; Shao et al. 2018). However, adsorption, due mainly to its high performance, recoverability and reactive ability of adsorbent, can be considered as a suitable method in terms of economy (Rao et al. 2007). Considering the environmental and economic aspects, a wide variety of inexpensive adsorbents have been developed, to varying degrees of success, in particular natural and industrial mineral wastes, for their availability, efficiency, profitability and are safe for the environment. Among these adsorbents (Kostas et al. 2010; Qasemi et al. 2018), we cite: clays and soils, the different sludge, gravel and industrial-waste substrata, fly ash, iron or aluminum based compounds, natural zeolite, limestone, blast furnace slag and other materials. Comparisons of different adsorbents for removal of Cr(VI) (Pfeifer & Škerget 2020) and P(V) (Ruiting et al. 2018) have been reviewed. On the other hand, phosphorus-based products such as phosphatized dolomite have been studied to remove a various cations (Ivanets et al. 2016).

The layered double hydroxides (LDH) and their calcined forms are receiving considerable interest in the removal of various contaminant species. The general formula of LDH is (OH)2(An−)x/n].y(H2O), while M2+ is a divalent metal cation, M3+ is a trivalent metal cation, and An− is an interlayer anion. LDHs display attractive physical and chemical properties including effectual dispersion, large specific surface areas and high anion exchange capacities that make them ideal adsorbents for many cations and anions. In this present work, Zn/(Al + Fe) ternary double-layered hydroxide [ZFA-HT] is synthesized by co-precipitation and calcined at 350 °C [ZFA-350]. The obtained materials are analyzed by the diverse characterization methods. The calcined sample is used in the removal of phosphate and chromate ions from an aqueous solution. The effect of various sorption factors, such as contact time, solution pH, adsorbent amount and temperature, is studied. Kinetic models, equilibrium isotherms and thermodynamic parameters are used to analyze the batch experimental data and to describe the adsorption process.

MATERIALS AND METHODS

Preparation of the samples

All chemicals including ZnCl2, FeCl3.6H2O, AlCl3.6H2O, Na3PO4, and KCrO4 are supplied by Prolabo with analytical grade and are used without further purification.

Zn(II)/(Al, Fe)(III) LDH is prepared by the co-precipitation method at room temperature and constant pH. A 50 mL solution containing 0.1 moles of ZnCl2, 0.025 moles of AlCl3.6H2O and 0.025 moles of FeCl3.6H2O is added dropwise to 100 mL of aqueous solution containing Na2CO3 [2M] and NaOH [1M] with vigorous stirring. The pH is maintained at a constant value of 10 during the addition by the addition of the required drops of a 0.1 M NaOH solution. The as-obtained solution is kept in the oven at 85 °C for 24 h. After the ageing step, the precipitate is separated by centrifugation, washed extensively with distilled water until chloride free. The LDHs powder is dried in the oven overnight at a temperature of about 65 °C, then ground and this sample is designated ZFA-HT. Sample labeled [ZFA-350] is a derivative of LDHs, obtained by calcining ZFA-HT at 350 °C for 7 h in a muffle furnace.

Material characterization

Powder X-ray diffraction analyses are performed on a Phillips PW 1800 diffractometer with Cu-Kα radiation (λ = 1.5418 Å) at 40 KV and 20 mA between 5° and 80° (2 Theta).

The IR spectra are recorded in the 4,000–400 cm−1 range on a Bruker ATR-Diamant type device using KBr pellets. The TGA/DSC thermal analysis of the material is carried out using an apparatus of the NETZSCH STA 449F1 type. The linear temperature rise is carried out at a rate of 10 °C/min, from room temperature (20 °C) up to 800 °C under a continuous airflow. The specific surfaces of the samples [ZFA-HT] and [ZFA-350] are determined by the Brunauer–Emmett–Teller (BET) method using an apparatus of the TriStar 3000 Micrometrics type. The samples are degassed beforehand at 100 °C under vacuum for a period of 15 h. The particle morphology of the ZFA-HT and ZFA-350 samples is observed by scanning electron microscopy (SEM) brand JSM.6390LV. X-ray energy dispersion (EDX) analysis allowed us to confirm the elemental composition of the samples.

Adsorption experiments and methodology

It should be noted that the adsorption study for ZFA-HT is not included in this work due to the lower adsorption capacity of this material compared to that of [ZFA-350]. The two stock solutions of P(V) and Cr(VI) ions with a concentration of 1,000 mg/L are prepared by dissolving a known quantity of salt Na3PO4 or KCrO4 in distilled water, and the salt concentrations to be used are obtained by dilution. The adsorption experiments are done using a batch equilibration technique to study the effect of the main parameters that influence the adsorption capacity of the two anions on the ZFA-350 material such as contact time, adsorbent mass, pH, initial concentration and temperature.

All adsorption experiments are performed at room temperature using 25 mL of pollutant solution of 150 mg/L concentration. After completing each adsorption experiment, supernatant solution is separated from the adsorbent by centrifugation at 3,000 rpm for 5 min.

The phosphate concentration in the supernatant is evaluated spectrophotometrically by the molybdenum blue method, by measuring the absorbance at 880 nm on a UV-vis spectrophotometer. For the determination of the Cr(VI) concentrations, the 1,5-diphenylcarbazide is used as a complexing agent at the corresponding maximum absorption wavelength λ = 540 nm.

Effect of contact time

The determination of the adsorption kinetics of the two pollutants on the ZFA-350 material is carried out on a series of suspensions of 30 mg of material in 25 mL of P(V)or Cr(VI) solution with an initial concentration of 150 mg/L. The suspensions are stirred at room temperature for various time intervals from 10 to 180 min without adjusting the initial pH of the solutions (pH = 6 for P(V) and pH = 4 for Cr(VI)). The obtained data are fitted to pseudo-first-order, pseudo-second-order and intraparticle diffusion

Effect of adsorbent dosage

The study of the effect of the mass of ZFA-350 on the adsorption of P(V) and Cr(VI), is carried out by taking different masses of the adsorbent material from 20 to 120 mg, which are added to a series of 25 mL of phosphate (or chromate) solutions with an initial concentration of 150 mg/L at the corresponding pH, with a contact time of 120 min, and then the residual P(V) or Cr(VI) concentrations are analyzed.

Effect of initial pH

To study the influence of pH on the adsorption of P(V) and Cr(VI) by ZFA-350, 25 mL of solution of each pollutant with a concentration of 150 mg/L are brought into contact with 40 mg of adsorbent for 150 min at a pH varying between 2 and 12. The starting solution pH is adjusted to the designed value by adding an HCl solution (1M) or NaOH (1M) with stirring, by using pH meter. Determination of point of zero charge (pHPZC) is carried out by the so-called pH drift method to determine the adsorbent surface charge. For this purpose, 0.1 M NaCl solution is prepared and its initial pH is adjusted to successive initial values between 2 and 12 using either HCl (0.1 M) or NaOH (0.1 M). Then, 25 mL of 0.1 M NaCl is taken in flasks and 30 mg of ZFA-350 is added to each solution. These flasks are shaken for 48 h at room temperature then the final pH of the solutions is measured.

Adsorption isotherms

The adsorption isotherms are established using suspensions of 40 mg of ZFA-350 in 25 mL of salt solution at initial concentrations varying from 10 mg/L to 180 mg/L and at pH = 5 for P(V) and pH = 4 for Cr(VI). The suspensions are stirred for a period of 120 min and the equilibrium concentrations are determined as above. The experimental adsorption equilibrium data are studied by two-parameter isotherms, such as Langmuir, Freundlich and Dubinin-Radushkevich models. To test the adequacy of the isotherm equations to represent the equilibrium data, coefficient of determination (R2), root mean square error (RMSE) (Equation (1)) and Chi-Squares (X2) (Equation (2)) are chosen as criteria to determine the equation of best fit. When the RMSE and X2 values are small, that signifies curve fitting is better (Abdelwahab 2007).
formula
(1)
formula
(2)
where Qe, exp and Qe, cal (mg/g) are the experimental and calculated values pollutant uptake and n is the number of experimental data points.

Effect of temperature

The influence of temperature on the adsorption of chromate and phosphate ions has been realized in the temperature range of 20–60 °C, by taking 30 mg of adsorbent in 25 mL of each salt solution with Ci = 180 mg/L, pH = 5 for P(V) and Ci = 100 mg/L, pH = 4 for Cr(VI), according to the equilibrium time of 120 min.

The adsorption capacity qe (mg/L) is calculated using the following equation:
formula
(3)
where: C0 and Ce (mg/L) are the initial and residual solution concentrations at equilibrium, V (L) is the volume of solution and m(g) is the mass of the adsorbent. The thermodynamic parameters, such as the change in enthalpy (ΔH°), change in entropy (ΔS°), and change in free energy (ΔG°), are estimated from the considered temperature.

RESULTS AND DISCUSSION

Characterizations of adsorbents

X-ray diffraction analysis

The X-ray diffraction (XRD) patterns of the synthesized ZFA-HT LDHs samples and its product derived from its calcination [ZFA-350] are shown in Figure 1.

Figure 1

X-ray diffractograms of ZFA-HT sample and its calcination product ZFA-350.

Figure 1

X-ray diffractograms of ZFA-HT sample and its calcination product ZFA-350.

The diffractogram corresponding to ZFA-HT shows the presence of the entire strong and sharp peaks (003), (006), (012), (015), (00 12), (110) and (113) characteristic of hydrotalcite-type compounds and exhibits good crystallinity and the high purity of the prepared hydrotalcite nanosheets (Guo et al. 2012).

For this type of material, the interlayer space corresponding to d003 is evaluated at 7.513 Å; which can be assigned to the intercalation of carbonate ions and the extent of hydration between the inorganic lamellae. The values of the parameters of the hexagonal lattice calculated and refined correspond to the values obtained: a = 3.072 Å and c = 22.540 Å. They agree well with those reported for carbonated LDHs (Fatima et al. 2017). The a value, determined from the d110, is equivalent to the mean distance between adjacent cation centers and can be correlated with the average radii of metal cations in the layers.

The diffractogram relating to the calcined LDHs shows the disappearance of the diffraction peaks of the initial phase, which is transformed into an amorphous structure, due to the destruction of the interlamellar compounds by dehydration and decarbonation. On the other hand, we note the appearance of other new peaks characteristic of metal oxides Fe2O3 and ZnO, which are in the form of mixed oxides of Zn (Fe, Al) O, with a layered double oxides (LDOs) structure. Calcination at an excessive temperature leads to the appearance of spinel phases, which must be avoided in order to preserve the platelet morphology of the compound.

FTIR infrared spectroscopy analysis

The FTIR spectra of ZFA-HT and ZFA-350 samples are depicted in Figure 2.

Figure 2

IR spectra of the synthesized material (ZFA-HT) and that calcined (ZFA-350).

Figure 2

IR spectra of the synthesized material (ZFA-HT) and that calcined (ZFA-350).

It is easy to remark that the spectrum of the ZFA-HT sample resembles to that of other hydrotalcite-like phases with as the counter anions and exhibits their characteristic bands (Hassiba et al. 2017). The wide and intense band centered at 3,377 cm−1, is attributed to the stretching vibration of the O-H bond of the hydroxyl groups of layers and interlamellar water molecules. The broadness of the band indicates the existence of hydrogen bonds of varying strengths (Laura et al. 2010). The shoulder observed at about 3,050 cm−1 is due to the hydrogen bonding of H2O to ions in the interlayer space. The weak peak at 1,615 cm−1 is relative to the deformation vibration of interlayer water. A strong band observed at 1,356 cm−1 characteristic of the ν3 vibration linked to the shoulder revealed at 950 cm−1 are assigned to the interlayer carbonate anions. The shoulder at 917 cm−1 and the band at 769 cm−1 are due to the deformation and translation modes of M-OH respectively. The bands at 550 cm−1 and 602 cm−1 are assigned to the Al-OH and Zn-OH translation mode respectively (Silion et al. 2009).

The FT-IR spectrum of calcined product ZFA-350 shows a significant decrease in intensities of different vibration bands due to the disappearance of hydroxyl groups, water molecules and carbonates ions by dehydration and decarbonation during calcination. We still notice the existence of the weak adsorption bands located at 1,356 cm−1 and 1,615 cm−1, which corresponds to the carbonate and H2O species adsorbed when the LDH sample was in contact with air. The new absorption bands below 1,000 cm−1 are ascribed to the M-O and O-M-O vibration modes of the different oxides obtained after calcination.

Thermal analysis

Figure 3 shows the thermoanalytical measurements of ZFA-HT sample to evaluate the different transformations of the material during the heat treatment. The differential scanning calorimetry (DSC) and thermogravimetric analysis (TGA) curves allow four main stages of mass loss to be observed. Therefore, the DSC thermogram displays four prominent endothermic peaks at 120 , 230, 320 and 700 °C. The first mass loss (5%) occurs between ambient temperature and 120 °C; it corresponds to the evaporation of physically adsorbed water. In the second stage, extending from 120 °C to 280 °C, the 6% mass loss is related to the removal of the interlayer water. At this stage, the integrity of the double-layered structure is not affected, however, the material is dehydrated and a significant rearrangement of the octahedral brucite-like layer occurred.

Figure 3

TGA/DSC thermograms of ZFA-HT sample.

Figure 3

TGA/DSC thermograms of ZFA-HT sample.

The third mass loss step located in temperature range from 280 °C to 360 °C, rated at 14%, is attributed to the dehydroxylation of OH bonds with M3+ as well as the decomposition of the majority of interlayer carbonate anions (decarbonation), which occurred and which led to the collapse of the layered structures. During this treatment, LDH are converted into mixed, well-dispersed oxides with a high surface area that stems from the formation of significant porosity (Hutson et al. 2004). The last step, which extends from 360 °C to 700 °C, corresponds to the release of an insignificant quantity of remaining carbonates and which is accompanied by the diffusion of M3+ towards ZnO leading to the formation of spinel structures , justified by the endothermic peak at 700 °C (Hibino et al. 1995). The total mass loss of the pure sample in the temperature range up to 600 °C is about 28%, and is assigned to the removal of water and CO2.

Textural analysis (BET method)

The nitrogen adsorption-desorption isotherms of ZFA-HT and ZFA-350 materials are shown in Figure 4. The sorption isotherms of the two materials followed the type IV adsorption isotherms according to the IUPAC classification with a H3-type hystersis loop; they also exhibit an H3 type hysteresis loop at high relative pressure, and correspond to mesoporous solids with plate-like particles having slit-shaped irregular pores (Parida & Mohapatra 2012). These adsorption isotherms do not present a plateau at high p/p0 values, indicating that the physico-sorption of N2 took place between the platelet particle aggregates and exhibits the lamellar morphology of the two compounds. According to the determined values of the structural characteristics, it is noted that the specific surface of ZAF-HT increased from 55 to 99 m2/g by heating at 350 °C, while the volume and the average size of the pores decreased from 0.35 to 0,31 cm3/g and from 25.71 to 12.64 nm respectively. It is noticed that the area of ZFA-350 is 80% higher than ZAF-HT; this is due to the additional mesoporous region, which is created by the formation of channels and pores resulting from the removal of water and carbon dioxide during the calcination. These results are higher than those found by (Hassiba et al. 2017).

Figure 4

N2 adsorption-desorption isotherms for ZFA-HT and ZFA-350 samples.

Figure 4

N2 adsorption-desorption isotherms for ZFA-HT and ZFA-350 samples.

Morphological characterization (SEM-EDX)

The images obtained by scanning electron microscopy (SEM), for ZFA-HT and ZFA-350 are shown in Figure 5(a) and 5(b). Micrograph 5(a) indicates that ZFA-HT grains have a hexagonal platelet morphology with crystallite size varying from 50 to 180 nm. It can be seen that the shape of the grains is more regular and that their surface is smooth. This morphology is typical for LDH (Liang et al. 2010). During the calcination process, the structure of hydrotalcite crumbled as the plane shapes become brittle and the particles become smaller with an irregular shape and a high surface area, as shown in micrograph 5b, and the size of the crystallites ranges from 20 to 100 nm.

Figure 5

SEM micrographs of ZFA-HT (a) and ZFA-350 (b); EDX pattern of ZFA-HT (c).

Figure 5

SEM micrographs of ZFA-HT (a) and ZFA-350 (b); EDX pattern of ZFA-HT (c).

The result of the EDX analysis of ZFA-HT (Figure 5(c)) confirms that the material is composed of zinc, aluminum, iron, oxygen and carbon. These results attest the formation of the hydrotalcite type material and show also that the ratios between the different metals Zn, Al, and Fe are close to the ratios used during the synthesis.

Adsorption studies

Effect of contact time

In order to determine the adsorption equilibrium time, the adsorption of the P(V) and Cr(VI) ions onto ZFA-350 is studied as a function of contact time. Since the adsorbed species diffuse from volume to active sites on the surface, the contact time between the adsorbate and the adsorbent is of significant importance in the adsorption process. As displayed in Figure 6, the curve shows that ion adsorption rate is significantly more at the first 15 min, due to the availability of a large number of active sites on the surface of ZFA-350 at the start of the adsorption process. As the adsorption sites filled with adsorbate, and due to the slow diffusion of anions into the bulk of the adsorbent, the adsorption rate slowly increased and gradually reached equilibrium state after 120 min for both pollutants. This optimum process time is comparatively faster than those obtained by other adsorbents (Ramesh et al. 2005; Xiulan & Yuhong 2014). The adsorption capacity of the material at equilibrium is 115 mg/g for P(V) and 48.15 mg/g for Cr(VI). Thus, the equilibrium time closely depends on the structure and chemical composition of the adsorbent and adsorbate.

Figure 6

Effect of contact time on adsorption of P(V) and Cr(VI) by ZFA-350.

Figure 6

Effect of contact time on adsorption of P(V) and Cr(VI) by ZFA-350.

Adsorption kinetics

For the examination of the controlling mechanisms of adsorption process, such as chemical reaction, diffusion model control and mass transfer, three kinetic models are proposed to test the experimental data. The Lagergren pseudo-first-order (Equation (4)), pseudo-second-order (Equation (5)) and intraparticle diffusion (Equation (6)) models (Hassan et al. 2019), are applied to the experimental data from the contact time effect for the ZFA-350 sample. The kinetic data is analyzed based on the regression coefficient (R2), root mean square error (RMSE), Chi-Squares (X2) and the amount of salt adsorbed per unit mass of the adsorbent. These models can be expressed by the following equations:
formula
(4)
formula
(5)
where qe and qt are respectively the adsorption capacity of pollutant (mg·g−1) at equilibrium and at time t, k1 (min−1) and k2 (g·mg−1 min−1) is the adsorption rate constant of pseudo-first-order and second-order sorptions respectively. The obtained parameters values (k1, k2, qe) for the sorption of Cr(VI) and P(V) onto ZAF-350 systems, as well as their respective correlations coefficients (R2) are given in Table 1. From the results of this table, we notice that the correlation coefficient for the pseudo-second-order kinetic model is closer to unity and the values of RMSE and X2 are smaller than that of the pseudo-first-order model. It is concluded that the pseudo-second-order model better describes the adsorption process of the two pollutants, and the qe values calculated by this model are close to that determined experimentally. Several studies reported that the adsorption of Cr(VI) and P(V) by diverse adsorbent was described by this model (Xiang et al. 2009; Zhike et al. 2013). Figure 7 shows the fitting of the present experimental data to this model. Because ion adsorption follows pseudo-second-order kinetics, this suggested that the boundary layer resistance is not the rate-limiting step. The rate of Cr(VI) or P(V) adsorption may be controlled largely by a chemisorption process in conjunction with the chemical characteristics of the ZFA-350, Cr(VI) and P(V) species, and the adsorption capacity of ZFA-350 is proportional to the number of active sites on its surface.
Table 1

Kinetic parameters of the pseudo-first-order, pseudo-second-order, and intraparticle diffusion of P(V) and Cr(VI) onto ZFA-350

Pseudo-first-order
Pseudo-second-order
Intraparticle diffusion
qe (mg/g)K1 (min−1)qe (mg/g)K2 (g/mg/min)Kint (mg/g/min1/2)C (mg.g)
P(V) 105,84 0,034 142,85 0,044 5,03 52,94    
Cr(VI) 38,09 0,011 58,82 0,026 3,71 2,94    
R2RMSEχ2R2RMSEχ2R2RMSEχ2
P(V) 0.901 1,762 1,243 0.990 0.220 0,520 0.977 0,435 0,310 
Cr(VI) 0.920 1,036 0,843 0.988 0.609 0,430 0.985 0,641 0,458 
Pseudo-first-order
Pseudo-second-order
Intraparticle diffusion
qe (mg/g)K1 (min−1)qe (mg/g)K2 (g/mg/min)Kint (mg/g/min1/2)C (mg.g)
P(V) 105,84 0,034 142,85 0,044 5,03 52,94    
Cr(VI) 38,09 0,011 58,82 0,026 3,71 2,94    
R2RMSEχ2R2RMSEχ2R2RMSEχ2
P(V) 0.901 1,762 1,243 0.990 0.220 0,520 0.977 0,435 0,310 
Cr(VI) 0.920 1,036 0,843 0.988 0.609 0,430 0.985 0,641 0,458 
Figure 7

Pseudo-second-order kinetic model for adsorption of P(V) and Cr(VI) by ZFA-350.

Figure 7

Pseudo-second-order kinetic model for adsorption of P(V) and Cr(VI) by ZFA-350.

Intraparticle diffusion

Intraparticle diffusion is a transport process involving movement of species from the solution bulk to the solid phase. Intra­particle diffusion model is used to describe the adsorp­tion process occurring on a porous adsorbent; it is expressed by the following Weber and Morris equation.
formula
(6)
where qt, Kint, and C are the amount of pollutant adsorbent (mg/g) at time t (min), the diffusion rate constant in the pores (mg·g−1·min−1/2) and the intercept, respectively. Value of C gives an idea about the thickness of the boundary layer, because the greater its value, the greater the effect of the boundary layer. Kins and C are determined by plotting qt as a function of . The straight curves obtained of the intraparticle phosphate and chromate diffusion do not pass through the origin; this is indicative of some degree of boundary layer control, and suggests that intraparticle diffusion is not the only rate-limiting step (Mall et al. 2005), but also other kinetic models may control the rate of adsorption.

Dosage effect of the adsorbent

Adsorbent dose is an important parameter in the determination of adsorption capacity. Figure 8 shows the effect of the mass of ZFA-350 on the adsorption of P(V) and Cr(VI). It is observed that the adsorption capacity decreases with the increase in the ZFA-350 dose; this effect can be explained by the increase in the adsorbent specific surface area and active sites' unsaturated adsorption, while the adsorption efficiency increases to the same optimum value of the mass evaluated at 40 mg and then remains almost constant.

Figure 8

The adsorption capacity of P(V) and Cr(VI) as a function of the mass of ZFA-350.

Figure 8

The adsorption capacity of P(V) and Cr(VI) as a function of the mass of ZFA-350.

Effect of pH on sorption

The pH is an important factor in the adsorption process and can influence the surface charge of the adsorbent, the degree of ionization of the adsorbate and the degree of dissociation of functional groups from the active sites of the adsorbent (Xiaowen et al. 2017).

The obtained results of P and Cr(VI) adsorption onto ZFA-350, at different initial pH values ranging from 2.0 to 12.0 are illustrated in Figure 9(a).

Figure 9

Effect of initial pH on the adsorption (a), zero point charge determination (b).

Figure 9

Effect of initial pH on the adsorption (a), zero point charge determination (b).

The adsorbent surface charge is estimated by evaluating the point of zero charge (PZC) which is the pH where the net total particle charge is zero. The PZC can be used as a qualitative parameter for the adsorbent surface charge balance. As displayed in Figure 9(b), the curve, plotted between ΔpH (pHfinal – pHinial) and the initial pH, intercepts the axis at the point pHi = 7.6 (ΔpH = 0) which corresponds to the pHPZC. This reveals that at pH < pHPZC, the surface of the adsorbent is positively charged, which favors the adsorption of anionic species by electrostatic interaction, thus explaining the strong adsorption capacity of P(V) and Cr(IV) anions present in this pH range. However, at pH > pHPZC, the surface is negatively charged, thus causing a repulsive force between the anions and the ZFA-350 surface, thus justifying the rapid decrease in adsorption capacity (Figure 9(a)).

Figure 9(a) shows that the increase in pH leads to an increase in the adsorption capacity of P(V) and Cr(VI) by ZFA-350 until it reaches a maximum in the form of a plateau with a pH = 6–7 for P(V) and pH = 4–5 for Cr(VI). Then a decrease follows differently depending on the nature of the pollutant. Since the modification of pH can cause the formation of multivalent phosphate species,, and chromium species , , according to the following equations (Xiulan & Yuhong 2014).
formula
(7)
formula
(8)
formula
(9)
formula
(10)
formula
(11)

We conclude that adsorption strongly depends on the nature of the anionic form of the species existing in a given pH range. The low adsorption capacity at pH = 2 can be explained by the fact that a large amount of the species to be adsorbed is found in molecular form which is weakly attached to the ZFA-350 sites and mainly to the dissolution of an adsorbent quantity giving the cations of metals Zn2 +, Al3 + and Fe3 +, hence the ability of ZFA-350 to adsorb the anions of P(V) and Cr(IV) is considerably reduced. The maximum adsorption capacity corresponds to the domain of existence of the monovalent ion (or) which is the predominant form according to Equations (7) and (10) corresponding, as well as the most favorable adsorbed form. This may be explained by the electrostatic attraction between the positively charged material surface and the negatively charged or ion in solution. When the pH values increase in a basic medium, it is observed that the adsorption capacity decreases. This is probably due, on the one hand, to the double and/or triple anionic charge of the species, which requires more sites of the absorbent, and from their electrostatic repulsion from the surface of the negatively charged adsorbent and, on the other hand, to the increase in the competitive effect of adsorption of OH ions with the various existing anionic species. The more marked decrease for P(V) than for Cr(VI) can be explained by the presence of a large anionic charge (the trivalent anion). In summary, the surface property of the adsorbent and the nature of the anionic form of the adsorbate jointly affect the impact of pH on the adsorption of P(V) and Cr(VI) by ZFA-350. Dissimilar results have been reported (Ramesh et al. 2005; Lili et al. 2011; Xiulan & Yuhong 2014).

Modeling of the adsorption isotherm

Adsorption isotherms play an important role in designating the mechanism of interaction and the adsorption capacity of adsorbates on the adsorbent, and are of great interest in evaluating the efficiency of an adsorbent. The isotherms of the adsorption of P(V) and Cr(VI) by ZFA-350 are shown in Figure 10. They all display the typical ‘L’ shape according to Giles's classification. Isotherms with this profile are typical of systems where the functional adsorbate is strongly attracted by the adsorbent. It is noted that the adsorption capacity of P(V) on ZFA-350 is greater than that of Cr(VI).

Figure 10

Adsorption isotherms of phosphate and chromate ions onto ZFA-350.

Figure 10

Adsorption isotherms of phosphate and chromate ions onto ZFA-350.

Figure 11

Langmuir isotherm of the adsorption of P(V) and Cr(VI) by ZFA-350.

Figure 11

Langmuir isotherm of the adsorption of P(V) and Cr(VI) by ZFA-350.

In this study, the adsorption equilibrium is analyzed by applying the models of Langmuir (Equation (12)), Freundlich (Equation (14)) and Dubinin-Radushkevish (Equation (15)) (Abdelwahab 2007). The isotherm equation applicability is based on a comparison of the correlation coefficients (R2), root mean square error (RMSE) and Chi-Squares (X2). The best isothermal model is identified through the value of R2 closest to one and the smaller values of RMSE and X2. The calculated constants of the three isotherm equations, as well as the values of R2, RMSE and X2, are presented in Table 2.

Table 2

Adsorption isotherms parameters of P(V) and Cr(VI) onto ZFA-350

Langmuir
Freundlich
Dubinin-Radushkevich
Qmax (mg/g)KL (L/mg)RlKF (L/g)nqm (mg/g)β (mol2/k.J2)E (KJ/mol)
P(V) 140.85 0.61 0.009 5.0 3.4 111.58 7.45 0.26 
Cr(VI) 52.63 0.51 0.016 1.5 2.4 99.48 2.43 0.45 
R2RMSEχ2R2RMSEχ2R2RMSEχ2
P(V) 0.990 0.004 0,004 0.81 0.021 0,018 0.98 0.005 0,005 
Cr(VI) 0.997 0.011 0,054 0.941 0.214 0,211 0.890 0.410 0,398 
Langmuir
Freundlich
Dubinin-Radushkevich
Qmax (mg/g)KL (L/mg)RlKF (L/g)nqm (mg/g)β (mol2/k.J2)E (KJ/mol)
P(V) 140.85 0.61 0.009 5.0 3.4 111.58 7.45 0.26 
Cr(VI) 52.63 0.51 0.016 1.5 2.4 99.48 2.43 0.45 
R2RMSEχ2R2RMSEχ2R2RMSEχ2
P(V) 0.990 0.004 0,004 0.81 0.021 0,018 0.98 0.005 0,005 
Cr(VI) 0.997 0.011 0,054 0.941 0.214 0,211 0.890 0.410 0,398 

Langmuir isotherm

This isotherm model takes an assumption that the monolayer adsorption occurs at specific homogenous sites within the adsorbent. The equation of this model, in its linear form, is formulated as follows:
formula
(12)
where Ce (mg/L) is the equilibrium concentration, qe is the quantity adsorbed at equilibrium (mg/g), KL is the equilibrium constant relative to the Langmuir model and qmax is the quantity maximum adsorbed (mg/g). The plot of the curve 1/qe = f (1/Ce) (Figure 11) makes it possible to determine the values of qmax and KL (Table 2). The table indicates that the Langmuir equation gives the best satisfactory fitting to the adsorption isotherms of P(V) and Cr(VI) anions by the material ZFA-350 with higher correlation coefficient (0.999) and the smaller values of RMSE (0.004) and X2 (0.004) than those related to the other models, revealing the homogenous nature of the surface and the formation of a monolayer of P(V) or Cr(VI) on the adsorbent surface. It is estimated that the ions are chemically adsorbed at a fixed number of well-defined sites, that all sites are energetically equivalent, and that there is no interaction between ions adsorbed on neighboring sites (Do 1998). The maximum adsorption capacity obtained by the Langmuir model is 125 mg/g for P(V) and 47.62 mg/g for Cr(VI). These values are very close to those obtained experimentally, 116.82 mg/g and 46.9 mg/g, respectively. The important characteristic of the Langmuir isotherm is expressed by a dimensionless factor (RL) (Equation (13)), commonly known as separation factor. It is calculated according to the following equation:
formula
(13)
where C0 is the highest initial concentration (mg/L) and KL is Langmuir's constant (L/mg). Depending on the value of RL, the adsorption is said to be linear for RL = 1, irreversible for RL = 0, favorable when 0 < RL < 1 and unfavorable if RL > 1 (Fayoud et al. 2015). In our case, the values found for RL are between 0 and 1, which indicates a favorable adsorption for the two anions P(V) and Cr(VI).

Freundlich isotherm

The Freundlich isotherm is an empirical equation that describes single-component adsorption on heterogeneous surfaces, due to the variation in the presence of functional groups in the surface and admits an adsorption of a multilayer nature. The Freundlich isotherm is expressed by the following relation:
formula
(14)
where KF is the Freundlich constant (L/g). The constants KF and n indicate the adsorption capacity and the adsorption intensity respectively, they are determined from the linear form of the isotherm by plotting Ln (qe) = f (Ln Ce). For the two pollutants, the values of ‘n’ are greater than unity, which implies favorable and physical adsorption. The values of the correlation coefficients R2 are equal to 0.80 for P(V) and 0.94 for Cr(VI), and they are lower than those of the Langmuir model (R2 = 0.99).

Dubinin-Radushkevish isotherm

The Dubinin-Radushkevish (D-R) isotherm is chosen to study the nature of the adsorption process and to estimate the mean free energy of adsorption. The linear form of the isotherm of D-R is expressed as follows:
formula
(15)
where β is a constant related to the average free energy of adsorption per mole of adsorbent (mol2/J2), qm the theoretical saturation capacity and ε the Polanyi potential (Equation (16)) (Fatima et al. 2017). The plot of the curve, ln(qe) as a function of ɛ2, will give a straight line with slope β and the y-intercept will give the sorption capacity qm. Polanyi's potential is defined as follows:
formula
(16)
where R is the universal gas constant (J·mol−1·K−1), T: temperature (K).
The mean free sorption energy E (kJ/mol) (Equation (17)) is calculated using the following relationship:
formula
(17)

The value of E is useful for estimating the type of adsorption process. If E is between 8 and 16 kJ·mol−1, ion exchange is the dominant factor and if E < 8 kJ·mol−1, physisorption occurs. For E > 16 kJ·mol−1, the adsorption process is chemical. Using the D-R model, the mean adsorption energy E was found to be 0.45 (kJ/mol) for Cr(VI) and 0.26 (kJ/mol) for P(V), which are less than 8 KJ/mol (Table 2), confirming that the adsorption process is of a physical nature and the predominance of Van der Waals forces.

From the values of the coefficients R2, RMSE and X2, and for comparison, it can be inferred that the Dubinin-Radushkevich equations are a suitable description of the adsorption behavior only of P(V) on ZFA-350. It can be noted that this isotherm model is more general than the Langmuir model (Fatima et al. 2017).

Temperature effect and thermodynamic study

The effect of temperature of the P(V) and Cr(VI) adsorption on ZFA-350 is investigated under isothermal conditions in the temperature range 20–60 °C. We notice that an increase in temperature promotes an increase in the amount of P(V) or Cr(VI) adsorbed by the ZFA-350 material (Figure 12(a)), which suggests that the adsorption process of these ions is endothermic. This can be interpreted by the increase in the mobility of the ions of P(V) and Cr(VI) as a result of the increase in the thermal energy of the absorbent species when the temperature rises (Zhike et al. 2013). To examine the feasibility and spontaneity of the adsorption process, the study of thermodynamic parameters, such as changes in Gibbs free energy (ΔG°), enthalpy (ΔH°) and entropy (ΔS°), which depend on the temperature, is required. Thermodynamic parameters such as standard free enthalpy ΔG°, standard enthalpy ΔH° and standard entropy ΔS° are determined using the following equations (Equations (18)–(20)) (Seo-Young et al. 2014; Ioannis & George 2016).
formula
(18)
formula
(19)
formula
(20)
where kd is the distribution constant (dimensionless); a is the adsorbent dose (g/L); qe the adsorption capacity at equilibrium (mg/g); Ce the concentration at equilibrium of adsorbate in solution (mg/L); ΔG° (J/mol) the standard Gibbs energy change; R the ideal gas constant (8.314 J·(mol K)−1); and T the absolute temperature (K); ΔH° (J/mol): enthalpy change, and ΔS° (J.(mol K)−1): entropy change. The values of ΔH° and ΔS° are obtained from the slope and intercept found from the plot of lnK against 1/T, respectively (Figure 12(b)). The thermodynamic parameters of the adsorption are determined from the experimental results obtained at different temperatures and are summarized in Table 3. The obtained positive values of the enthalpy ΔH° indicate the endothermic nature of the process. Positive values of ΔS° indicate increased disorder at the adsorbate–adsorbent interface during adsorption of P(V) or Cr(VI) on the ZFA-350 surface. The negative values of ΔG° obtained indicate that the process of removing phosphate or chromium ions is spontaneous and feasible, which means a high affinity of ZFA-350 material toward anionic adsorbate. Further, it is noticed that there is a decrease in value for ΔG°, which is obtained with the rise in temperature, suggesting that adsorption is more favorable at high temperature.
Table 3

Thermodynamic parameters of the adsorption of P(V) and Cr(VI) ions on ZFA-350 material

T(K)ΔG°(KJ/mol)ΔH°(KJ/mol)ΔS°(J.mol−1.K−1)
P(V) 293 −3.076 43.91 160.37 
313 −6.284 
333 −9.491 
Cr(VI) 293 −1.094 13.00 48.11 
313 −2.056 
333 −3.018 
T(K)ΔG°(KJ/mol)ΔH°(KJ/mol)ΔS°(J.mol−1.K−1)
P(V) 293 −3.076 43.91 160.37 
313 −6.284 
333 −9.491 
Cr(VI) 293 −1.094 13.00 48.11 
313 −2.056 
333 −3.018 
Figure 12

Effect of temperature on the adsorption capacity of P(V) and Cr(VI) ions (a) and evolution of LnKd as function of 1000/T (b).

Figure 12

Effect of temperature on the adsorption capacity of P(V) and Cr(VI) ions (a) and evolution of LnKd as function of 1000/T (b).

Table 4 shows the comparison of the adsorption capacity of P(V) and Cr(VI) obtained using the ZFA-350 material and other calcined hydrotalcite-like materials reported in the literature. It can be seen that the adsorption capacity of each anion on ZFA-350 is high compared to other adsorbents. It can also be concluded that the removal of phosphate and chromate ions by ternary hydrotalcite materials is almost superior to that of binary materials. This can be interpreted by the electropositive character of iron which is greater than that of aluminum, therefore the substitution of Al3+ by Fe3+ generates an excess of positive charge on the sheets, which leads to better adsorption (Snehaprava & Suresh Kumar 2018).

Table 4

Comparison of the adsorption capacities of phosphate and chromium on the different calcined hydrotalcite adsorbents reported in literature

Adsorbants (calcined HDL)T(K) calcined HDLpH solutionT(K)solutionQmax (mg/g)References
 Mg2-Al 550 8.5 291 39.84 Abdalah & Mohamed (2006)  
 Mg2Al 500 7.5 308 70.75 Seon et al. (2019)  
 Zn2-Al 300 323 92.59 Xiang et al. (2009)  
P(V) Mg3-Mn 300 8.5 313 7.5 Ramesh et al. (2005)  
Mg2Al 350 303 97.09 Xiulan & Yuhong (2014
Mg3Fe0,4-Al0,6 450 6.5 298 117 Kostas et al. (2010)  
Mg3Fe0,8-Al0,2 450 6.5 298 134 Kostas et al. (2010)  
Zn2Fe0,5Al0,5 350 293 140.85 Current study 
Cr(VI) Mg–Zn)2–Al 450 298 33.82 Eshaq et al. (2015
Zn3-Al 250 293 37.5 Laura et al. (2010
Mg2-Al 150 Ambient 28.5 Ramos et al. (2009
Mg-3Al0,8-Fe0,2 500 295 52.5 Lili et al. (2011
Zn2Fe0,5Al0,5 350 293 52.63 Current study 
Adsorbants (calcined HDL)T(K) calcined HDLpH solutionT(K)solutionQmax (mg/g)References
 Mg2-Al 550 8.5 291 39.84 Abdalah & Mohamed (2006)  
 Mg2Al 500 7.5 308 70.75 Seon et al. (2019)  
 Zn2-Al 300 323 92.59 Xiang et al. (2009)  
P(V) Mg3-Mn 300 8.5 313 7.5 Ramesh et al. (2005)  
Mg2Al 350 303 97.09 Xiulan & Yuhong (2014
Mg3Fe0,4-Al0,6 450 6.5 298 117 Kostas et al. (2010)  
Mg3Fe0,8-Al0,2 450 6.5 298 134 Kostas et al. (2010)  
Zn2Fe0,5Al0,5 350 293 140.85 Current study 
Cr(VI) Mg–Zn)2–Al 450 298 33.82 Eshaq et al. (2015
Zn3-Al 250 293 37.5 Laura et al. (2010
Mg2-Al 150 Ambient 28.5 Ramos et al. (2009
Mg-3Al0,8-Fe0,2 500 295 52.5 Lili et al. (2011
Zn2Fe0,5Al0,5 350 293 52.63 Current study 

CONCLUSION

The obtained results show that ZFA-HT tertiary double lamellar hydroxide and its calcined product ZFA-350 have been successfully prepared by the co-precipitation method, and the adsorption of phosphates and chromates from the aqueous solution is proficiently carried out on ZFA-350. The determined specific surface area of the ZFA-350 material (98.7 m2/g) is greater than that of the original LDH form (ZFA-HT) (54.3 m2/g). The adsorption equilibrium time for both ions are reached after 120 min. The adsorption kinetics is better described by pseudo-second-order kinetic model. The adsorption efficiency is dependent on the initial pH of the solution and the adsorption is maximum at pH of 7 for P(V) and pH of 4 for Cr(VI). The experimental results show that the adsorption capacities follow the Langmuir isotherm model with a maximum monolayer adsorption of 140.85 mg/g for P(V) and 52.63 mg/g for Cr(VI). Thermodynamic analysis shows that sorption increases with increasing temperature from 20 to 60 °C, and exhibits the spontaneous and endothermic nature of the sorption process. The maximum adsorption capacities of phosphates and chromates obtained in this work are found to be greater than those obtained by other authors, using the same nature of adsorbents. The satisfactory adsorption results obtained show that ZFA-350 is a promising material for the removal of inorganic anionic pollutants in the treatment of industrial wastewater.

DATA AVAILABILITY STATEMENT

All relevant data are included in the paper or its Supplementary Information.

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