Research Progress in Lanthanum Extraction from Boehmite

Abstract

The purpose of this study was to examine the utilization of 0–45 μm aluminum hydrate fraction (gibbsite) manufactured by Alum SA Tulcea, as a precursor in the hydrothermal synthesis process of nanosize boehmite from gibbsite suspensions. Furthermore, the use of the produced material as adsorbent of lanthanum (III) ions was investigated using aqueous solutions. Thermogravimetric analysis (TG), Fourier Transform InfraRed Spectroscopy (FT-IR), Wide-Angle X-Ray Scattering (WAXS), and atomic force microscopy (AFM) were used to demonstrate the preparation of the required material. Additionally, the material point of zero charge (pHpzc) and material specific surface (using BET approach) were evaluated. By conducting adsorption studies, the specific parameters for the lanthanum adsorption process were evaluated. The effects of pH, the ratio of adsorbent material to La(III) quantity from aqueous solution, contact time, and La(III) initial concentration were also assessed. Based on the obtained experimental data, it was proved that the maximum adsorption capacity of 97.7 mg/g was obtained for a solid:liquid ratio = 0.1 g:25 mL, pH between 3 and 6, temperature of 298 K, and contact time of 90 min. The studied adsorption process is most effectively described by the Sips model, suggesting an intricate interaction among the adsorbent material and La (III) ions. The prepared adsorbent exhibited a good desorption capacity (higher than 93%) when 20% HCl was used for desorption.

1. Introduction

According to the well-known phase transition series (gibbsite–boehmite–γ–δ–θ–α), common boehmite represents a mineralogical phase that is produced after the thermal dehydration process of alumina hydrate [1,2]. One of the widely used precursors for the synthesis of different types of special aluminas is the boehmite γ-AlOOH, which is founded in a large variety of crystallographic forms with distinct structural properties. It can be used for multiple applications in different fields, such as: synthesis of biomaterials [3,4,5,6,7,8]; catalyst and catalyst support preparation [9,10,11,12,13,14]; composite material synthesis [1,15,16,17,18,19]; coating materials [6,20,21,22,23,24,25]; thermal insulation materials [26,27,28,29,30]; flame retardants [20,31,32,33,34,35]; luminescent materials [36,37,38,39,40]; and adsorbents [41,42,43,44,45]. Regarding the boehmite as a precursor for different syntheses, different technologies have been developed to satisfy the highest quality standards for of its applications. The most commonly used technologie for the conversion of aluminum trihydrate and of aluminum into boehmite are the Bayer modified precipitation technology [46,47,48,49,50] and the water oxidation of aluminum under hydrothermal conditions [51,52,53,54]. In fact, because pH is one of most important parameters in hydrothermal reactions, the boehmite as a precursor synthesis could be produced in acidic [55,56,57,58,59], alkaline [60,61,62,63,64], or neutral solutions [50,59,65,66,67], using the common adequate reactants solutions and water. Some particular ways used for boehmite synthesis are: hydrolysis of aluminum alkoxides; hydrolysis of aluminum double salts [50,68,69,70]; high technologies using treatments in inductively coupled non-thermal plasma [71,72]; ultrasonic assistance [73]; and amorphous alumina cluster mediated processes [74,75]. Alumina hydrate of good quality for boehmite synthesis is available in many alumina Bayer factories that produce calcined alumina. SC Alum SA Tulcea is the only producer of Bayer alumina in Romania. In this factory, the main product is calcined alumina, as well, as a secondary product, as the dried, milled, and classified aluminum trihydrate (dry gibbsite). Some additional data concerning these products are available in the papers published by authors [76,77,78,79,80,81,82,83,84,85,86,87].

In this study, we describe the usage of γ-AlOOH obtained from aluminum hydrate (gibbsite) manufactured by Alum SA Tulcea, as an adsorbent material for La(III) ion recovery. The adsorbent material was prepared through the hydrothermal synthesis process starting from gibbsite suspensions. The raw material (gibbsite) used was the fraction 0–45 μm of SC Alum SA Tulcea product, consisting of dried, milled, and classified aluminum trihydrate. Previous studies reported the usage of γ-AlOOH as an efficient adsorbent for silica removal [88], heavy metal ions removal [89], and organic pollutants removal [90]. The surface properties of the prepared γ-AlOOH were studied using advance analytical techniques (BET, AFM, WAXS). The impact of the adsorbent amount, pH, time, and temperature, adsorption thermodynamics, and kinetics were examined by carrying out batch experiments.

2. Material and Methods

2.1. Preparation

In order to investigate the hydrothermal recrystallization of boehmite in acidic, alkaline and neutral conditions, three separate experiments were conducted. The experiments were conducted under similar conditions, following the methods used by Kozerozhets [58,59].

2.2. Characterization

Thermal analysis (TGA) was performed by using a Thermogravimetric analyzer 5500 from TA Instruments, New Castle, DE, USA. The sample, with a mass of around 11 mg, was positioned into a platinum HT pan of 100 µL. The experiment was performed in a dynamic nitrogen atmosphere with a flow rate of 40 mL/min, in a range of 25–800 °C, at a rate of 10 °C/min.

The attenuated total reflection (ATR) spectrum was recorded using infrared Fourier transform spectroscopy (ATR/FT-IR) on a Nicolet™ iS50 FT-IR Spectrometer (Thermo Fisher Scientific, Waltham, MA, USA), equipped with a diamond crystal. These spectra were recorded in the spectral range of 4000–400 cm⁻¹ with 64 scans per spectrum at a resolution of 4 cm⁻¹.

Wide-Angle X-ray Scattering (WAXS) (Xenocs Xeuss 3.0, Xenocs SAS, Grenoble, France) was utilized to acquire data about the structure of the materials at the mesoscale.

The following stage involved analyzing the produced material by recording the AFM image using a Scanning Probe Microscopy Platform (MultiView-200 system, Nanonics Imaging Ltd., Jerusalem, Israel), in intermittent mode in normal conditions (25 °C, atmospheric pressure). This analysis was carried out by using a chromium-doped tip with a 20 nm radius and 30–40 kHz resonance. Atomic force microscopy (AFM), MultiView-200 system, Nanonics Imaging Ltd., Jerusalem, Israel, was used to quantify the material surface roughness.

Nitrogen desorption isotherms were recorded at 77 K using a Nova 1200e (QUANTACHROME) system; pore size distributions were determined with the Barrett–Joyner–Halenda (BJH) method; and specific surface areas were obtained using the Brunauer–Emmett–Teller (BET) method. In previous evaluations, the probes were degassed in vacuum for 5 h at room temperature. All the experiments were performed in duplicate to assess the standard error, which was less than 5%.

The point of zero charge—pHpZc, was determined by bringing the system to equilibrium. In this experiment, 0.1 g of material was mixed with 25 mL of 0.1 N KCl solution, stirred at 200 rpm, maintaining the entire system at 298 K using a Julabo SW23 water bath equipped with thermostatic control and stirring. The pH of the KCl solutions was adjusted in the range of 1–14 by adding NaOH solutions with concentrations between 0.05 N and 2 N, or HNO₃ solutions in the same concentration range. After filtering the samples, the pH of the resulting solution was measured using a METTLER TOLEDO SevenCompact S 210 pH meter. pH values were measured after a contact time of 60 min, which was chosen to allow the system to reach equilibrium. This time value was selected based on the preliminary experiments, which demonstrated that the system pH did not change after this time.

2.3. Application: Adsorption Experiments

2.3.1. Effect of Operating Conditions on the Performance of La(III) Adsorption

Influence of Solid:Liquid Ratio

To determine the optimum solid:liquid (S:L) ratio for obtaining maximum efficiency of the La(III) ions recovery process, the amount of γ-AlOOH adsorbent involved was varied (0.05, 0.1, 0.2, 0.3, 0.4, and 0.5 g), while maintaining a constant 25 mL volume of a 500 mg La(III) per liter solution (prepared from LaCl₃·7H₂O, Merck). Adsorption experiments were performed using a JULABO SW23 shaker, at 298 K, for a contact time of 60 min. Adsorption efficiency was determined using the following equation:

$$\mathrm{E}\mathrm{f}\mathrm{f}\mathrm{i}\mathrm{c}\mathrm{i}\mathrm{e}\mathrm{n}\mathrm{c}\mathrm{y}={\displaystyle \frac{\mathrm{C}\mathrm{i}-\mathrm{C}\mathrm{r}\mathrm{e}\mathrm{z}}{\mathrm{C}\mathrm{i}}}100,[\%]$$

(1)

where: C_i—La(III) initial concentration, mg/L.

C_rez—La(III) residual concentration, mg/L.

Influence of the Solution pH

The pH effect on La(III) adsorption was investigated, considering its influence on both the speciation of La(III) in solution and γ-AlOOH surface properties. The study focused on a pH range of 1 to 6. P. Patnaik’s “Handbook of Inorganic Chemical Compounds” states that La(III) precipitation begins at pH 8 and, because of that, the experiments were not carried out at pH values higher that 6 [91].

For these experiments, 0.1 g of γ-AlOOH adsorbent was mixed with 25 mL of a 500 mg/L La(III) solution. Adsorption experiments were carried out at 298 K for a contact time of 60 min. The solution pH was adjusted using HCl or NaOH solutions with concentrations ranging from 0.1 to 1 N.

Material adsorption capacity was evaluated using the following equation:

$$\mathrm{q}={\displaystyle \frac{\left(\mathrm{C}\mathrm{i}-\mathrm{C}\mathrm{r}\mathrm{e}\mathrm{z}\right)\mathrm{V}}{\mathrm{m}}},[\mathrm{m}\mathrm{g}/\mathrm{g}]$$

(2)

where: q—adsorption capacity, mg/g

Ci—La(III) initial concentration, mg/L.

Crez—La(III) residual concentration, mg/L.

V—solution volume, L.

m—material mass, g.

Contact Time and Temperature Effect

The effects of temperature and contact time on the adsorption capacity of La(III) ions were examined through a series of experiments. In all the experiments, 25 mL La(III) solution with an initial concentration of 500 mg of La(III) ions per liter were mixed with 0.1 g of adsorbent material. These mixtures were subsequently agitated at 200 rpm for different contact times (15, 30, 45, 60, 90, and 120 min) and temperatures (298, 308, 318, and 328 K).

Influence of La(III) Ions Initial Concentration

Solutions with initial concentrations of 10, 25, 50, 100, 250, 500, 600, 700, 800, 900, and 1000 mg/L were used to determine the impact of La(III) ion concentration on the maximum adsorption capacity. All the adsorptions were carried out into the pH 3–6, at 298 K, for an adsorption time of 90 min.

3. Results and Discussion

3.1. γ-AlOOH Synthesis and Characterization

All the samples involved in this study were obtained by hydrothermal recrystallization of boemithe. Preliminary experiments were conducted in smaller autoclaves (100 mL) to determine the optimal parameters for maximizing gibbsite—boehmite conversion. From these experiments, the parameters determined included: the autoclave degree of fullness; solid/liquid ratio; experimental times; and heating/cooling time duration (specific parameters are depicted in

Table 1. The parameters of the hydrothermal recrystallization process of boehmite from gibbsite suspensions.

Product	Acid Boehmite Recystallization (A)	Alkaline Boehmite Recrystallization (B)	Neutral Boehmite Recrystallization (C)
Autoclave volume, mL	2000	2000	2000
Degree of fullness, %	50	50	80
Solid/liquid ratio, g/mL	375/850	375/850	592/1360
Temperature, °C	200	200	200
Heating time till 200 °C	1	1	1
Experiment time, h	5	5	5
Cooling time, h	1	1	1
Agitation, rpm	150	150	150
Pressure, 200 °C, bar	19	19	20
pH initial	2.5	11.0	5.1 (corrected)
pH final	4.0	11.5	11.0
Boehmite purity, %	99.3	97.9	98.8

). Furthermore, to obtain the adsorbent material required for adsorption tests, all the experiments were conducted in a 2L autoclave. Boehmite purity was evaluated by comparing the recorded WAXS spectra with a specific CPDS PDF file.

For the test of hydrothermal boehmite as an adsorbent for La(III) ions from aqueous solutions, based on preliminary results obtained was choose the recrystallized boehmite obtained from pure water in the initial stage.

3.2. Thermal Analysis, TGA

The thermal evolution of γ-AlOOH was investigated through thermogravimetry analysis, and the recorded TG-DTG curves of the boemithe are depicted in Figure 1.

The optimal temperature required for synthesis can be determined by estimating the thermal stability of γ-ALOOH. The TG pattern recorded for the sample used reveals three decomposition steps specific to the material, which take place in the range of 25 to 800 °C. The first step occurs at around 224 °C, with a mass loss of 0.2%, which is attributed to the dehydration of the chemisorbed water molecules [92].

The second peak occurred at 384 °C with an associated mass loss of 1.7%; this is attributed to the loss of interstitial water trapped between the γ-AlOOH layers, concomitant with the partial formation of γ-Al₂O₃ [93,94].

The main process of mass loss of approximately 15% occurs in the last stage, as observed through an endothermic effect at 519 °C. In this process, dehydration–dehydroxylation of the material take place, transforming it from boehmite into γ-Al₂O₃ [93,94].

3.3. The Attenuated Total Reflection, ATR /FT-IR

The ATR/FT-IR spectrum of γ-AlOOH in the 4000–400 cm⁻¹ region is presented in Figure 2.

The infrared spectrum in the OH stretching region shows two bands, at 3083 cm⁻¹ and 3263 cm⁻¹, which are ascribed to the boehmite hydroxyl group stretching vibrations [92]. Peaks located at 1077 cm⁻¹ and at 1145 cm⁻¹ are associated with the symmetric bending vibrations of Al-O-H and with asymmetric bending vibration of the Al-O-H groups [94].

The three bands at 732, 579, and 478 cm⁻¹ attributed to the symmetric stretching and bending vibrations of the Al-O-Al bond are also observed in the spectrum [92,95,96].

3.4. Wide-Angle X-Ray Scattering, WAXS

The degree of material crystallinity was determined using the WAXS scattering method. The analysis was carried out with a Genix 3D (Cu) source at room temperature using Kapton in vacuum. The data analysis for the scattering pattern was conducted in the range of 10–55 2θ degree with a DECTRIS detector.

Figure 3 shows the 2D WAXS patterns and profile of the material obtained.

The obtained scattering patterns indicate that the sample presents well-defined crystallinity. Compared with the database and the literature, it is observed that the material presents a structure specific to γ-AlOOH (boehmite), with an orthorhombic crystallographic system with space group Amam 63, according to JCPDS PDF sheet No. 21-1307 [93,97].

The full width at half maximum (FWHM) of the peak (0 2 0) was used to compute the crystallite size based on the Scherrer equation, yielding a value of 215 nm.

Experiments carried out by Li et al. in 2019 [98] proved the interdependence between particle size and surface area. In this context, it was concluded that the larger surface area is associated with the smaller size of the constituent particles [98]. On this basis, it is clear that smaller value of the surface area recorded for the prepared material is due to the particle size being above 200 nm.

3.5. Atomic Force Microscopy, AFM

Given the importance of surface morphology in the adsorption process, AFM analysis for the γ-AlOOH was performed. AFM (2D and 3D) images, surface profile on the selected area, together with roughness data, were obtained; they are presented in Figure 4 and Figure 5 and

Table 2. Values obtained from AFM analysis.

Sample Name	Ironed Area (µm2)	Sa (nm)	Sq (nm)	Sp (nm)	Sv (nm)	Sy (nm)	Sku	Ssk
Boehmite, γ-AlOOH	443.877	73.948	91.982	209.977	−213.977	423.810	2.6737	−0.4507

.

From the AFM image presented, specific parameters were evaluated (Average roughness (Sa), Mean Square Root Roughness (Sq), Maximum peak height (Sp), Maximum valley depth (Sv), Maximum peak-to valley height (Sy), Surface kurtosis (Sku), and Surface skewness (Ssk)), as shown below in Table 2:

The 2D AFM image (Figure 4) reveals formations of similar morphology patterns, organized in compact clusters, as shown in the additional profile images (Figure 5). The surface roughness of the sample is determined by the Ra and Rq, whereas both have the same importance. However, their value differs due to distinct calculation formula [99]. Additional information linked with the material surface is described by the Sku and Ssk values. Symmetrical height distribution is determined by Ssk = 0, whereas an asymmetrical height distribution with a positive Ssk value implies the prevalence of peaks, a negative Ssk indicates the prevalence of valleys. The density sharpness of a profile is determined by Sku, indicating the preponderance of “sharper” (Sku > 3) and “flatter” (Sku < 3) surface aspects. Sp and Sv parameters indicate the height of the highest peak; the largest pit within the area can be correlated with the Sku and Ssk. Sy is defined as the sum of the largest peak height value (Sp) and the largest pit depth value (Sv) within the defined area [100]. The obtained results highlight the prevalence of valleys, as indicated by the Ssk value in addition to the more negative Sv value compared to the Sp. Additionally, the Sku is <3, indicating a “flatter” surface due to the presence of deep valleys. The obtained results are in concordance with the profile image on the selected areas, indicating the presence of wide peaks and valleys, with occasional smaller valleys on the top of the peaks. The negative Ssk, in combination with an Sku < 3, indicates a rough surface. However, given the profile image and nitrogen adsorption-desorption isotherms data, the obtained results indicate a limited surface porosity. In this context, the good adsorptive properties of prepared adsorbent material can be associated with the valleys observed in the images recorded in Figure 5. These valleys possibly act as traps in the adsorption process, leading to an increase of the adsorption capacity of the adsorbent material.

3.6. Nitrogen Adsorption-Desorption Isotherms: Brunauer–Emmett–Teller (BET) Method

The nitrogen adsorption-desorption isotherm and pore size distribution are depicted in Figure 6.

According to IUPAC, the γ-AlOOH material presents a type II nitrogen adsorption isotherm [101]. This reversible type II isotherm is specific for materials that have macropores or no pores at all. Evaluating the data obtained, the specific surface area can be modeled using the BET (Brunauer, Emmet, Teller) method, obtaining a value of 14 m²/g.

Using the BJH (Barrett–Joyner–Halenda Model) and DFT (Density Functional Theory) methods, the pore distribution data were evaluated, showing an average value of approximately 4.2 nm. The total pore volume derived from the final point of adsorption at P/Po = 0.99037 is 0.0114 cm³/g, with mesopores smaller than 201.5 nm.

The Fractal Dimension showed by the FHH method (The Frenkel–Halsey–Hill model) indicates a value of D = 2.0151, which is specific for flat surfaces (2D). Correlating with AFM measurements, we can observe that even if the surface is in 2D, the material still presents porosity but with macropores.

3.7. Point of Zero Charge, pHpzc

A fundamental concept in surface chemistry, point of zero charge (pHpzc), is used to characterize different adsorbent materials. This study reveals the nature of the functional groups presented on the surface of the adsorbent material surface. The pHpzc is the pH value at which there are as many positively charged groups as there are negatively charged ones on the adsorbent material surface. In order to obtain the optimum pH required for optimal adsorption based on the nature of the adsorbate, it is crucial to know the value of the adsorbent pHpzc. In Figure 7, the experimental data used for pHpzc determination are presented.

Taking into account the information depicted in Figure 7, it can be observed that the pHpzc of γ-AlOOH has a value, equal to 9.8. The evaluation of the adsorbent material surface charge indicates the charging behavior of the adsorbent material surface as a function of the pH of the solution in which it is immersed. Taking into account the value obtained during the experiments, we can conclude that if the solution pH is lower than 9.8, the adsorbent material surface will have a positive net charge, because the higher concentration of H⁺ ions from the solution tends to protonate the superficial groups from the material surface. At a pH higher than 9.8, the material surface will be negatively charged due to the higher concentration of HO⁻ ions present in the solution that tend to deprotonate the superficial groups. When the solution pH is equal to 9.8, the surface of the adsorbent material is electrically neutral. In this context, it is evident that the surface net charge plays an important role for all the adsorptive processes [102]. Thus, since our studies were performed at a pH lower than 9.8, we can state that an efficient recovery of the La(III) species is possible only if we admit the presence of Al–O groups located on the surface of adsorbent materials, as stipulated by as Li J. et al. [98].

3.8. Effect of Operating Conditions on the Performance of La(III) Adsorption

3.8.1. Influence of the Solid:Liquid Ratio

The effect of adsorbent type and its concentration is illustrated in Figure 8.

The analysis of the data presented in Figure 8 reveals that the increase in the quantity of the adsorbent material correlates with a rise in the percentage of the La(III) recovery. Based on these data, we can observe that the increase in the adsorbent efficiency is insignificant when the quantity of adsorbent material is higher than 0.1. Therefore, taking into account all the determining factors, we can conclude that the optimum solid:liquid ratio is 0.1 g of adsorbent material:25 mL of liquid, when the La(III) adsorption efficiency is around 62%, corresponding to approximatively 75 mg of La (III) ions per gram of adsorbent material.

3.8.2. Influence of the Solution pH

Figure 9 illustrate how the concentration of hydrogen ions affect the adsorption of La(III) ions on a pH range between 1 and 6.

It is well known that the characteristics of adsorption processes are dependent on the pH values. In this context, we assessed the optimal pH range for La(III) ion adsorption and concluded that La(III) adsorption is most effective when the solution pH is between 3 and 6, with a maximum adsorption capacity of around 76 mg of La(III) per g of adsorbent material. The pH-insensitive behavior of the lanthanum during adsorption is a notable observation [103]. In this condition, is important to take into account that at a pH below 6, lanthanum ions in an aqueous solution exist as La³⁺ ions [104]. This low sensitivity can be related to the weak electrostatic forces established between La(III) ions and the produced adsorbent material.

3.8.3. Influence of the Contact Time and Temperature

La(III) adsorption experiments were conducted with a contact time ranging from 15 to 120 min and a temperature ranging from 298 to 328 K in order to evaluate the impact of these two variables on the studied adsorptive process. The obtained results are presented in Figure 10.

From the data presented in Figure 10, it can be seen that for 90 min the adsorption takes place rapidly, after which the amount of adsorbed ions reaches a maximum that does not change significantly for any other increase in the adsorption time. In this context, it can be observed that the adsorption capacity increases up to 90 min, after which it remains constant (q_e~77 mg/g at 298 K) regardless of the time increase. Also, it can be observed that as the temperature increases, the maximum adsorption capacity also increases, but not significantly enough to justify usage at temperatures higher than 298 K during La(III) ions adsorption. Even though the temperature effect on the studied adsorption is negligible, such experiments are important to determine if the process must be conducted at a temperature different from room temperature.

3.8.4. Influence of La(III) Ions Initial Concentration

Under ideal adsorption circumstances (pH = 3–6, S:L ratio = 0.1 g:25 mL, contact time = 90 min and optimal temperature of 298 K), the impact of the initial concentration of the La(III) ions on the adsorptive process has been investigated. In Figure 11, the data showing the influence of the initial concentration of La(III) ions on the adsorption capacity are presented.

The information presented in Figure 11 proves that the maximum adsorption capacity increases with the increase in the La(III) ions initial concentration. The maximum adsorption capacity increases as the initial concentration of La(III) ions increases, until it reaches 800 mg/L, when a maximum adsorption capacity of 97.7 mg/g is attained. We may conclude that 97.7 mg/g represents the maximum adsorption capacity under the considered experimental conditions because any additional increase in the La(III) initial concentration results in no increase in the maximum adsorption capacity.

3.9. Adsorption Mechanism

3.9.1. Adsorption Kinetics

To understand the rate and mechanism of La(III) adsorption, kinetic modeling was performed. Specifically, pseudo-first-order (Lagergren) and pseudo-second-order (Ho and McKay) kinetic models were used to describe the adsorption kinetics.

Intraparticle diffusion, the movement of La(III) within the adsorbent’s pores after surface attachment, was analyzed using the Weber–Morris model. This step is crucial in the overall adsorption process, alongside film diffusion (transport from the bulk solution to the adsorbent surface) and surface adsorption (interaction between La(III) and the adsorbent).

Lagergren model is [105]:

$$\mathrm{l}\mathrm{n}\left({\mathrm{q}}_{\mathrm{e}}-{\mathrm{q}}_{\mathrm{t}}\right)={\mathrm{l}\mathrm{n}\mathrm{q}}_{\mathrm{e}}-{\mathrm{k}}_1\mathrm{t}$$

(3)

where: q_e—equilibrium adsorption capacity (mg/g)

q_t—adsorption capacity at a specific time—t (mg/g)

k₁—pseudo-first order speed constant (min⁻¹)

t—contact time (min)

The Ho and McKay model is [106,107,108]:

$${\displaystyle \frac{\mathrm{t}}{{\mathrm{q}}_{\mathrm{t}}}}={\displaystyle \frac{1}{{\mathrm{k}}_2{\mathrm{q}}_{\mathrm{e}}^2}}+{\displaystyle \frac{\mathrm{t}}{{\mathrm{q}}_{\mathrm{e}}}}$$

(4)

where: q_e—equilibrium absorption capacity (mg/g)

q_t—adsorption capacity at a specific time—t (mg/g)

k₂—pseudo-second-order speed constant (g/mg·min)

t—contact time (min)

A linear plot of ln(q_e − q_t) = f(t) gives the equilibrium adsorption capacity, q_e,calc, as the intercept, while the slope gives the k₁ rate constant for the pseudo-first-order kinetic model. For the pseudo-second-order kinetic model, the linear plot of t/qt function of time yields the slope as equilibrium adsorption capacity q_e,calc and the intercept as the k₂ rate constant.

The Weber and Morris model is [109,110] described by the following equation:

$${\mathrm{q}}_{\mathrm{t}}={\mathrm{k}}_{\mathrm{d}\mathrm{i}\mathrm{f}\mathrm{f}}·{\mathrm{t}}^{{\displaystyle \frac{1}{2}}}+\mathrm{C}$$

(5)

where: q_t—adsorption capacity at t time, mg/g

k_diff—speed constant for intraparticle diffusion, mg/g·min^1/2

C—constant correlated with the thickness of the liquid film surrounding the adsorbent particles.

The obtained experimental data have been modeled using pseudo-first-order, pseudo-second-order, and Weber and Morris kinetic models (the obtained data are presented in Figure 12). The data obtained from the modeling of the experimental data are presented in

Table 3. Kinetic parameters for the adsorption of La(III) onto γ-AlOOH.

Pseudo-first order
Temperature (K)	qe,exp (mg g−1)	k1 (min−1)	qe,calc (mg g−1)	R2
298	76.8	0.0198	51.7	0.9368
308	77.1	0.0197	45.1	0.9390
318	77.5	0.0203	40.9	0.9205
328	78.1	0.0249	42.1	0.9531
Pseudo-second-order
Temperature (K)	qe,exp (mg g−1)	k2 (g mg−1∙min−1)	qe,calc (mg g−1)	R2
298	76.8	37.9 · 103	90.0	0.9931
308	77.1	45.2 · 103	88.4	0.9936
318	77.5	51.2 · 103	87.7	0.9958
328	78.1	57.9 · 103	89.2	0.9970
Intraparticle diffusion model (IPD)
Temperature (K)	Kdiff (mg·g−1 min−1/2)	C		R2
298	15.6	0.543		0.9301
308	18.1	0.448		0.9209
318	19.8	0.402		0.8911
328	20.7	0.321		0.9003

.

As can be seen from the data presented in Table 3, the obtained experimental data fit the second-order kinetic model quite well, as indicated by the coefficient of determination (R²), which ranges from 0.993 to 0.997. Furthermore, can be observed that the calculated adsorption capacity has values close to the experimentally determined adsorption capacity, confirming that the pseudo-second-order kinetic model fits the experimental data.

It can also be observed that the La(III) adsorption mechanism takes place through several steps since the straight lines obtained by plotting the dependence q_t as a function of t^1/2 at different temperatures do not pass through the origin, which means that the adsorption kinetics are influenced by both intraparticle and film diffusion. Similarly, from the data presented in Table 3, it can be observed that as the temperature increases, so does the value of K_diff. It can also be observed that the constant specific for stage 1 is higher than the diffusion constant specific for stage 2, which signifies that stage 2 determines the adsorption process speed [111].

3.9.2. Adsorption Isotherms

Adsorption equilibrium was analyzed using the following isotherm models:

(i)

Langmuir Isotherm: This model assumes monolayer adsorption onto a homogeneous surface;

(ii)

Freundlich Isotherm: This model describes adsorption onto heterogeneous surfaces; and

(iii)

Sips Isotherm: This model combines aspects of both the Langmuir and Freundlich isotherms, offering a more versatile description.

Equilibrium isotherms were obtained by plotting the equilibrium adsorption capacity (q_e) against the equilibrium concentration (C_e). The parameters for each isotherm model were then determined. Specifically, the Langmuir and Freundlich constants were calculated using the linearized forms of the respective isotherm equations after fitting the experimental data.

The Langmuir isotherm is [112]:

$${\mathrm{q}}_{\mathrm{e}}={\displaystyle \frac{{\mathrm{q}}_{\mathrm{L}}·{\mathrm{K}}_{\mathrm{L}}·{\mathrm{C}}_{\mathrm{e}}}{1+{\mathrm{K}}_{\mathrm{L}}·{\mathrm{C}}_{\mathrm{e}}}}$$

(6)

where: q_e—equilibrium adsorbtion capacity (mg/g)

C_e—equilibrium concentration (mg/L)

q_L—Langmuir maximum adsorption capacity (mg/g)

K_L—Langmuir constant.

The Freundlich isotherm is [113] described by the following equation:

$${\mathrm{q}}_{\mathrm{e}}={\mathrm{K}}_{\mathrm{F}}{\mathrm{C}}_{\mathrm{e}}^{{\displaystyle \raisebox{1ex}{$1$}\!\left/ \!\raisebox{-1ex}{${\mathrm{n}}_{\mathrm{F}}$}\right.}}$$

(7)

where: q_e—maximum adsorption capacity (mg/g)

C_e—equilibrium concentration (mg/L)

K_F and n_f—characteristic constants that can be associated with the relative adsorption capacity of the adsorbent and the adsorption intensity.

Sips isotherm is [114] described by the following equation:

$${\mathrm{q}}_{\mathrm{e}}={\displaystyle \frac{{\mathrm{q}}_{\mathrm{S}}{\mathrm{K}}_{\mathrm{S}}{\mathrm{C}}_{\mathrm{e}}^{\raisebox{1ex}{$1$}\!\left/ \!\raisebox{-1ex}{${\mathrm{n}}_{\mathrm{S}}$}\right.}}{1+{\mathrm{K}}_{\mathrm{S}}{\mathrm{C}}_{\mathrm{e}}^{\raisebox{1ex}{$1$}\!\left/ \!\raisebox{-1ex}{${\mathrm{n}}_{\mathrm{S}}$}\right.}}}$$

(8)

where: q_S—Sips maximum adsorption capacity (mg/g)

K_s—constant related to adsorbent adsorption capacity

n_s—heterogeneity factor.

Experimental data were also modeled using Langmuir, Freundlich, and Sips isotherms. The obtained results are presented in Figure 13 and

Table 4. Parameters of isotherm model for adsorption of La(III) onto γ – AlOOH.

Langmuir isotherm
qm,exp(mg/g)	KL(L/mg)	qL(mg/g)	R2
97.7	0.01	119.2	0.9929
Freundlich isotherm
KF(mg/g)	1/nF		R2
6.8	0.43		0.9498
Sips isotherm
KS	qS(mg/g)	1/nS	R2
0.01	118.6	0.01	0.9920

.

From the data presented in Figure 13, we can conclude that the studied adsorption process is effectively explained by the Langmuir isotherm, as evidenced by a high coefficient of determination—0.9929 (from the data presented in

Table 5. Thermodynamic parameters for adsorption of of La(III) onto γ – AlOOH.

ΔH° (kJ/mol)	ΔS° (J/mol∙K)	ΔG° (kJ/mol)				R2
1.77	18.37	298 K	308 K	318 K	328 K	0.9303
		−5.4	−5.6	−5.8	−6.0

). The obtained results also suggest that temperature has a positive influence on the studied adsorptive process.

3.9.3. Adsorption Thermodynamics

Thermodynamic studies are useful for evaluating the viability of the adsorptive process, and provide general information on how temperature affects the studied process. To determine how La(III) ion adsorption takes place on the surface of the adsorbent material, Gibbs’ free energy (ΔG°) was calculated using the Gibbs–Helmholtz equation [115]:

$$∆{\mathrm{G}}^{\mathrm{o}}=∆{\mathrm{H}}^{\mathrm{o}}-\mathrm{T}·∆{\mathrm{S}}^{\mathrm{o}}$$

(9)

where:

ΔG^o—free Gibbs energy standard variation (J/mol)

ΔH^o—enthalpy standard variation (J/mol)

ΔS^o—entropy standard variation (J/mol·k)

T—absolute temperature (K)

Standard variations of enthalpy and entropy were evaluated from the linear dependence of ln K_d versus 1/T (linear form of van’t Hoff equation), where K_d is the equilibrium constant, calculated as the ratio between the equilibrium adsorption capacity (q_e) and equilibrium concentration (C_e).

$$\mathrm{l}\mathrm{n}\mathrm{K}\mathrm{d}={\displaystyle \frac{∆{\mathrm{S}}^{\mathrm{o}}}{\mathrm{R}}}-{\displaystyle \frac{∆{\mathrm{H}}^{\mathrm{o}}}{\mathrm{R}\mathrm{T}}}$$

(10)

where: Kd—constant of equilibrium;

ΔS°—standard variation of entropy (J/mol·K)

ΔH°—standard variation of enthalpy (kJ/mol)

T—absolute temperature (K);

R—ideal gas constant (8.314 J/mol·K).

The equilibrium constant is given by the ratio of the qe and Ce:

$$\mathrm{K}\mathrm{d}={\displaystyle \frac{\mathrm{q}\mathrm{e}}{\mathrm{C}\mathrm{e}}}$$

(11)

Figure 14a shows the dependence between ln(Kd) and 1/T, while Figure 14b presents the dependence between lnK₂ and 1/T.

From the information presented in Figure 14, the values of the thermodynamic parameters associated with the La(III) adsorption were evaluated (parameters presented in Table 5).

Analyzing the data presented in

Table 6. Activation energy and determination coefficient.

Material	Activation Energy, Ea kJ/mol	R2
γ-AlOOH	1.11	0.9965

, we can observe that La(III) ion adsorption is an endothermic process due to the positive value of the enthalpy. We can also conclude that the studied adsorption is a spontaneous process, as indicated by the negative values of the free Gibbs energy obtained for at all the studied temperatures.

The activation energy value can provide information about the nature of the adsorption process, whether it is physical or chemical. Thus, the activation energy E_a was calculated using the Arrhenius equation and the velocity constant from the pseudo-order kinetic model two k₂.

$${\mathrm{k}}_2=\mathrm{l}\mathrm{n}\mathrm{A}-{\displaystyle \frac{{\mathrm{E}}_{\mathrm{a}}}{\mathrm{R}\mathrm{T}}}$$

(12)

where: k₂—Speed constant, g/min·mg

A—Arrhenius constant, g·min/mg¹

E_a—Activation energy de activare, kJ/mol¹

T—Absolut temperature, K

R—The ideal gas constant, 8.314 J/mol·K.

The activation energy of adsorption La (III) on γ-AlOOH was calculated from the linear equation associated with the graphical representation ln k₂ = f (1/T). In Table 6, the calculated values for activation energy and for the determination coefficient are presented.

Based on the dependence depicted in Figure 14b, we evaluated the value of the activation energy, which has a value of 1.11 kJ/mol. This low value is in concordance with the thermodynamic parameters, suggesting that La(III) adsorption is spontaneous. Because the value of the activation energy is lower than 40 kJ/mol, we can stipulate that the adsorption process is a physical one [116].

To demonstrate that the produced adsorbent material is suitable for La(III) recovery using adsorption, the maximum adsorption capacities shown by different materials were compared (data presented in

Table 7. Values of “q” for La(III) obtained with other adsorbents.

Adsorbent	pH	Adsorption Capacities, mg/g	References
Activated carbon	4	10.14	[117]
Magnetic nanoparticles functionalized with diethylenetriamine- pentaacetic acid (DTPA)	3	0.091	[118]
Magnetite nanoparticles were functionalized with a phosphonic acid group (PA-MNPs)	4	18.4	[119]
Dowex 50W-X8,	5.5	10.80	[120]
MOF@BC	6	288.89	[121]
γ-AlOOH	3–6	97.7	This paper

). Although, based on these data, we can observe that γ-AlOOH does not have the highest adsorption capacity, the exhibited adsorption capacity can be considered to be good. Due to the lower production price in comparison with the one for the MOF adsorbent, we my deduce that γ-AlOOH represents a suitable adsorbent for La(III) ion recovery.

3.9.4. Absorbent Regeneration

To ascertain the potential for reusing γ-AlOOH as an adsorbent material, La(III) desorption tests were performed. For these tests, 2 g of adsorbent material was loaded with the maximum amount of La(III) by performing adsorption. Furthermore, exhausted adsorbent material was used in desorption tests, which were performed using a 10 and 20% HCl solution for 4 h. Upon analyzing the amount of desorbed La(III) ions, it was found that more than 93% of the adsorbed La(III) ions could be desorbed when 20% HCl was used. Based on this observation, we can conclude that the γ-AlOOH adsorbent material lends itself to repeated adsorption–desorption cycles (adsorbent material was able to regenerate five times).

4. Conclusions

One objective of this study was to evaluate the use of aluminum hydrate (gibsite) as a precursor in the hydrothermal synthesis process of boehmite from gibbsite suspensions. The parameters of the boehmite synthesis process are also presented, along with their effect on the main properties of hydrothermal recrystallized boehmite. The final goal was to test synthesized nanomaterial as an adsorbent for lanthanum (III) ion from aqueous solutions.

Thermogravimetric analysis was performed to estimate the optimum temperature required for the synthesis of boehmite (γ-AlOOH). Furthermore, in the recorded FT-IR spectra, a band located at 1077 cm⁻¹ and at 1145 cm⁻¹ can be observed; these are associated with the symmetric and asymmetric bending vibrations of Al-OH groups, suggesting that the desired material was obtained. The WAXS scattering method was used to determine the crystallinity of the produced boehmite, proving that the synthesized material presents well-defined crystallinity, with an orthorhombic crystallographic system. AFM analysis reveals the presence of different formations with similar morphological shapes organized in compact clusters. BET analysis demonstrates that γ-AlOOH has a specific surface of 14 m²/g, with a pore distribution of approximately 4.2 nm. The pHpzc has a value equal to 9.8, meaning that, in the specific study conditions (pH = 3–6), the adsorbent material surface is positively charged, and the adsorbed La species must therefore be negatively charged for an efficient adsorption process.

To determine the optimum adsorption conditions, we investigated the effects of the adsorbent dose, pH, contact time, temperature, and initial conditions. In terms of the adsorbent dose, experimental data reveal that the maximum adsorption capacity increases with the increase in the adsorbent dose, yielding a maximum adsorption capacity for 0.1 of g γ-AlOOH. From the experiments carried out, in order to evaluate the influence of the pH, it was found that La adsorption occurs with good efficiency in the pH range of 3 to 6; at a pH higher than 6, precipitation of the dissolved La(III) ions occurs.

Experiments carried out to evaluate the optimum contact time revealed that the maximum adsorption capacity increases as the contact time increases, up to a maximum adsorption capacity obtained for a contact time of 90 min contact time. Any further increase in the contact time proved insignificant for the increase in adsorption capacity, and it can therefore be concluded that the optimum contact time is 90 min. Furthermore, these experiments proved that La(III) adsorption occurs with good results at 298 K, with any temperature increase leading to an insignificant increase in adsorption capacity. From all the obtained experimental data, it was observed that the maximum adsorption capacity has a value of 97.7 mg/g, obtained for an initial concentration of 800 mg La(III) ions per L, in established optimum sorption conditions.

From the adsorption isotherms, it was established that the Langmuir isotherm is most appropriate for describing the studied adsorption. Based on thermodynamic studies can conclude that La(III) adsorption is an endothermal process reflected by the positive value of the enthalpy and spontaneously one as indicated by the negative value of Gibbs free energy. Performed desorption studies proved that when 20% HCl was used approximately 93% of the amount of La(III) loaded on the adsorbent materials could be desorbed, meaning that the adsorbent material can be reused for several adsorption/desorption cycles.