Fluoride is an essential micronutrient for humans. Nonetheless, when the amount of fluoride ion is greater than required, it will cause skeletal fluorosis and dental fluorosis to threaten human health. In this paper, a series of sodium alginate (SA)-based foam materials are prepared by freeze-drying technique and anchored with the nano-activated alumina (nAl2O3) in the SA to obtain a novel adsorbent of SA-nAl2O3 foam used for fluoride ions removal. The SA-nAl2O3 foam morphology was further explored and confirmed that nAl2O3 existed stably in the SA. The adsorption results showed that the maximal fluoride ion adsorption capacity was 5.09 mg/g with 20 mg/L fluorine solutions at a pH of 3. The adsorption isotherm fitted adequately to the Langmuir isotherm model, which demonstrated that the adsorption process is closer to monolayer adsorption. The adsorption kinetics behavior of SA-nAl2O3 foam was described by a pseudo-second-order model, and the adsorption process occurred by chemisorption. Adsorption thermodynamics analysis emphasized that the adsorption process was spontaneous and endothermic. The main mechanism of the foam is ion exchange. The SA-nAl2O3 foam exhibited excellent regeneration performance and stability after three cycles.

  • A novel adsorption material of SA-nAl2O3 foam was obtained for fluoride ion removal.

  • SA-nAl2O3 foam exhibited steady fluoride ion adsorption capacity.

  • SA-nAl2O3 foam has excellent regenerability after three cycles

Graphical Abstract

Graphical Abstract
Graphical Abstract

Fluoride ions are widely found in the atmosphere, water and the Earth's crust, and they are generated from geological scouring and wastewater discharge of industries such as semiconductor manufacturing, metal smelting, electroplating and so on (Chigondo et al. 2018). Fluoride ions damage equipment, pollute the environment, and affect the health of animals and plants in a way that adversely affects human health. Proper intake of fluoride helps prevent dental caries and strengthen bones. In contrast, long-term and excessive intake of fluoride will affect the growth and development of the organism, leading to fluorosis, neurological disorders, and liver injury (Tao et al. 2020; Gai & Deng 2021). Therefore, finding an efficient and low-cost technology for fluoride ion removal is very important.

The traditional technology for fluoride removal has been confirmed effective, including coagulation precipitation, ion exchange, membrane separation, electrodialysis and adsorption, etc. However, the adsorption method is more attractive than other fluorine ions removal methods due to the low cost, simple design and operation (Belkada et al. 2018; Chang et al. 2019; Robshaw et al. 2019). Activated alumina is an important adsorption material that has been widely used and studied in fluoride ion removal (Cheng et al. 2014; Jin et al. 2015). Compared with conventional activated alumina, nano-activated alumina (nAl2O3) has the advantages of small particles, large specific surfaces and abundant active sites to facilitate the removal of fluoride ions. In the meantime, nAl2O3 is difficult to recycle in practical applications because of the small particles (Tangsir et al. 2016). Accordingly, the key to the problem is how to design an environmentally friendly, safe and stable carrier to immobilize nAl2O3. It has been noticed that sodium alginate (SA) can load with metal materials (Wang et al. 2018; Wang et al. 2019). In addition, SA has abundant hydroxyl groups, contributing to fluoride ion exchange in the solution (Fang et al. 2003).

In this work, the SA=based foam materials are prepared by freeze-drying technique and the nAl2O3 was introduced and anchored in the SA foam to synthesize a novel adsorbent SA-nAl2O3 foam. The foam microstructure and adsorption properties of fluoride ions were systematically analyzed. This preparation process can provide a new matrix for removing fluoride ions, which is avoided by applying the tiny particles of nAl2O3 independently to produce the second pollution.

Materials

The SA, nAl2O3 (γ Crystal, grain size of 20 nm) and anhydrous calcium chloride were purchased from Shanghai Macklin Biochemical Co. Ltd China. Sodium fluoride ion (NaF) was provided by Tianjin Jinke Fine Chemicals, China. Deionized (DI) water was produced by Millipore Milli-Q Advantage A10 (Billerica, MA, USA). All other reagents used were analytical grade in the experiment.

Preparation of SA foam

The SA-nAl2O3 foam were prepared with a mass ratio of nAl2O3/SA of 0.5–1.0. It was selecting different content of SA (2 wt%, 3 wt%, 4 wt%) and 2 wt% nAl2O3 to dissolve in DI water, stirring at 50 °C constant temperature for 3 h to obtain a homogeneous mixture solution, which was poured into a cylindrical mold (Φ 0.5 cm and height 2 cm) and transferred in a refrigerator freeze at −20 °C for 24 h. Subsequently, the mold and frozen mixtures were put into a frozen dryer to dehydrate the material and form a porous structure. Then, the porous materials were put into a calcium chloride solution with a concentration of 0.5 mol/L for curing and cross-linking to form the composite foam. In the end, wash foam with DI water several times to remove excess calcium chloride from the surface and put it in an oven at 60 °C for 24 h to obtain dry SA-nAl2O3 foam. The weight of each SA-nAl2O3 foam (pcs) was 0.065 ± 0.005 g. The prepared SA-nAl2O3 were respectively labeled as SA2-nAl2O3, SA3-nAl2O3 and SA4-nAl2O3 respectively with different SA weight percentages. The same procedure synthesized SA foam, which content of SA was 2 wt% without adding nAl2O3 (Hong et al. 2018). Figure 1 describes the synthetic procedure of SA-nAl2O3 foam.
Figure 1

Synthetic procedure of SA-nAl2O3 foams.

Figure 1

Synthetic procedure of SA-nAl2O3 foams.

Close modal

Characterization of SA-nAl2O3 foam

A scanning electron microscope (SEM, Tescan, MIRA4, Czech Republic) examined the surface morphology of foams, and the elements on the foam were determined by energy dispersive spectrometry (EDS, Oxford, Xplore 30, UK), with an accelerating voltage of 10 keV. Fourier transform infrared spectroscopy (FTIR, NicoletiS10, USA) was used to characterize the functional group of SA-nAl2O3 foam by a wavelength ranging from 500 cm‒1 to 4,000 cm‒1. The BJH pore size-distribution was calculated by the Kelvin equation. The foams were degassed at 100 °C under vacuum at less than 1 × 10−5 bar for 6 h before analysis. X-ray photoelectron spectroscopy (XPS) served to determine the surface elemental composition of the samples with a Quanta 200 spectrometer (FEI Co. Ltd, USA) with a monochromatic Al-Ka X-ray source (1,486.6 eV photons) at a pass energy of 93.9 eV. The fluoride ion concentration was measured by ion chromatography (Dionex Aquion, USA).

Fluoride ion adsorption experiments

The static adsorption testing assessed the adsorption capacity and optimum pH of the SA-nAl2O3 foam. The SA-nAl2O3 foam was at first dried and weighted before the fluoride ion adsorption experiment. Subsequently, it was placed in a plastic tube with an 80 mL predetermined NaF solution concentration. Then, the tube was shaken in the shaker for 24 hours under 25 °C. The fluoride ion uptake capacity of SA-nAl2O3 foam was obtained according to the following equation:
formula
(1)
where qe is the adsorption capacity (mg/g), C0 and Ce are respectively the initial and adsorption equilibrium of fluoride ion concentration in the solution (mg/L), V is the volume of solution (L) and m is the mass of the dry adsorbent of SA-nAl2O3 foam (g).
The Langmuir and Freundlich adsorption isotherm models further fitted and explained the experimental data for the adsorption isotherms. Both models have the form of:
formula
(2)
formula
(3)
where qe is the experimental uptake (mg/g) at equilibrium concentration, Ceq is the fluoride ion concentration at equilibrium (mg/L), qmax is the SA-nAl2O3 foam maximum adsorption capacity (mg/g). b is the Langmuir sorption coefficient (L/mg), KF and are the constant and heterogeneity coefficient of the Freundlich isotherm respectively (Shi et al. 2019).
There is a certain error in fitting the adsorption process with the fitted correlation coefficient R2. Therefore, in order to minimize error, we used the sum squared error (SSE) to fitting the experimental values (AbdulRazak & Rohani 2018).
formula
(4)
where Qc and Qe are the calculated and experimental values, respectively.
The adsorption kinetics test was investigated to depict the effect of adsorption time on the SA-nAl2O3 foam that removed the fluoride ions. The foam was placed into a plastic tube with 80 ml 20 mg/L fluoride ion concentration and shook at a speed of 200 rpm/min under 25 °C. The solution was withdrawn at predetermined time intervals of 10 min, 20 min, 30 min, 1 h, 2 h, 3 h, 4 h, 8, 12 and 24 h to measure the fluoride ion concentration. The adsorption kinetics data were evaluated by the kinetic models of pseudo-first-order, pseudo-second-order and intra-particle diffusion. The equations were written as follows:
formula
(5)
formula
(6)
formula
(7)
where qe and qt (mg/g) are the uptake of fluorine ion at the equilibrium and time of t (min), respectively, k1 (min−1), k2 (g/mg·min) and kp (mg/g·min0.5) respectively are the reaction rate constant of the pseudo-first-order, pseudo-second-order and intra-particle diffusion rate constant (Ramteke & Gogate 2016).
The adsorption thermodynamics analyzed the effect of absolute temperature on fluoride ions removal of SA-nAl2O3 foam from 283 to 313 K. The adsorption process of the free energy change (ΔG, kJ/mol), enthalpy change (ΔH, kJ/mol), and entropy change (ΔS, J/(mol·K)) were deduced by the equation form:
formula
(8)
formula
(9)
where Kc is the equilibrium constant (L/mol), T is the absolute temperature (K), R is the gas constant (8.314 J/mol·K) (Gheju et al. 2016).

Regeneration experiments

The fluoride ions loaded on SA-nAl2O3 foam (the saturated adsorption capacity was q1) were desorbed via an alkaline solution of 0.5 mol/L NaOH for 24 h at room temperature (Maliyekkal et al. 2006). Subsequently, SA-nAl2O3 foam was washed by DI water until neutral. Then, SA-nAl2O3 foam adsorbed the fluoride ions again, and the uptake q2 was measured. Repeating the process above three times and calculating the regeneration efficiency of SA-nAl2O3 foam as follows:
formula
(10)
where q1 and q2 are the initial adsorption capacity and the cycled use (mg/g).

All the experiments were carried out in triplicate, the results were averaged and the error was calculated.

Characterization of adsorbents

Figure 2 illustrated the SEM and EDS analysis images of SA foam and SA-nAl2O3 foams. Result showed that the original SA foam exhibited a smooth sheet structure (Figure 2(a)). When nAl2O3 was added, we can clearly see that the surface of foam becomes rugged, and the nAl2O3 particles were anchored on the SA foam surface (Figure 2(b)2(d)), which confirmed that the nAl2O3 particles existed in the SA foam. The EDS analysis certified that the Al element principally exists in the composited foam (Figure 2(e)2(h)). The foam surface appears to be compact with the increase of SA content. Meanwhile, the Ca element content was progressively expanded from 12.08 wt% (SA2-nAl2O3) to 26.02 wt% (SA4-nAl2O3). It was due to that the ―OH group increased with SA content, leading to an increased cross-linking degree with CaCl2 and arise the compact process of SA-nAl2O3 foam surface (Borgogna et al. 2014; Hecht & Srebnik 2016). The BET surface area and average pore diameter were displayed in Table 1. It implied that the specific surface area and average pore diameter of SA-nAl2O3 foam were diminished with the increase of SA content, which showed the same tendency to EDS results. It disclosed the dense structure caused the specific surface area to decrease, and will lead to the permeability of the foam simultaneously decreasing. SA2-nAl2O3 foam has the highest specific surface area of 26.22 m2/g. This value is larger than the specific surface area of SA foam (0.37 m2/g) previously reported (Hong et al. 2018), which is a significant improvement for foam adsorbent.
Table 1

BET surface area analysis of SA-based foams

BET surface areaAverage pore diameter
(m2/g)(nm)
SA foam 22.20 7.25 
SA2-nAl2O3 foam 26.22 7.74 
SA3-nAl2O3 foam 16.40 6.78 
SA4-nAl2O3 foam 11.97 6.10 
BET surface areaAverage pore diameter
(m2/g)(nm)
SA foam 22.20 7.25 
SA2-nAl2O3 foam 26.22 7.74 
SA3-nAl2O3 foam 16.40 6.78 
SA4-nAl2O3 foam 11.97 6.10 
Figure 2

SEM images and EDS analysis of (a,e) SA foam, (b,f) SA2-nAl2O3 foam, (c,g) SA3-nAl2O3 foam, (d,h) SA4-nAl2O3 foam.

Figure 2

SEM images and EDS analysis of (a,e) SA foam, (b,f) SA2-nAl2O3 foam, (c,g) SA3-nAl2O3 foam, (d,h) SA4-nAl2O3 foam.

Close modal
The FTIR spectrum of SA and SA-nAl2O3 foam was illustrated in Figure 3(a). The Al―O stretching vibration peak of SA-nAl2O3 foam appeared at 556 cm‒1 compared with SA foam. In addition, the broadband peak belongs to the ―OH groups stretching vibration at 3,420 cm‒1. This result may be induced by the nAl2O3 particles around attract abundantly ―OH group. The results further showed that the nAl2O3 was present in the SA foam. With increased SA content, the peak intensity of ―OH was gradually weakened and revealed that the functional groups decreased was against the fluoride ion adsorption process in the foam. Other peak intensities remained consistent. The peak position of 2,922 cm‒1, 1,628 cm‒1, 1,081 cm‒1, and 1,030 cm‒1 respectively belonged to saturated C―H stretching vibration, asymmetric carboxyl stretching, Si―O stretching, and deformation vibration peak of C―O (Sun et al. 2016; Feng et al. 2022). We tried to add more SA during the experiment to introduce the ―OH group, but the foam compactness increased and ―OH group content declined persistently. Accordingly, the amount of SA added in the SA-nAl2O3 was regulated within 2 wt% to optimize the content of active sites and the structure of SA-nAl2O3 foam.
Figure 3

ATR-FTIR spectra of SA and SA-nAl2O3 foams.

Figure 3

ATR-FTIR spectra of SA and SA-nAl2O3 foams.

Close modal

Adsorption isotherms and kinetics

As can be seen from Figure 4(a), the nonlinear curve was plotted according to the relationship between fluoride ion equilibrium concentration and the adsorption capacity, which was used to be analyzed by the Freundlich and Langmuir adsorption isotherm models. It showed that the adsorption capacity presented a rising tendency initially and then became stable with the increase in fluoride ion concentration. It was due to the increase of the active site in the SA2-nAl2O3 foam reacting with the fluoride ion at a high concentration, and demonstrated higher adsorption capacity toward the removal of fluoride ion in water. It was beneficial to generate the concentration gradient to increase the diffusion rate (Al-Ghouti & Da'ana 2020). The adsorption isotherm models' parameters and the linear fitted curve are respectively shown in Table 2 and Figure 4(b) and 4(c).
Table 2

The isotherm parameters for fluoride ions adsorption onto SA2-nAl2O3 foam

Langmuir isotherm model
Freundlich isotherm model
qmax (mg/g)b (L/mg)R2SSEKF (L/mg)nR2SSE
SA2-nAl2O3 foam 5.398 0.457 0.997 0.610 2.119 3.878 0.549 0.138 
Langmuir isotherm model
Freundlich isotherm model
qmax (mg/g)b (L/mg)R2SSEKF (L/mg)nR2SSE
SA2-nAl2O3 foam 5.398 0.457 0.997 0.610 2.119 3.878 0.549 0.138 
Figure 4

(a) Nonlinear fitting of the Freundlich and Langmuir isotherm models of fluoride ion on SA2-nAl2O3 foam; (b) Linear fitting of Langmuir model; (c) Linear fitting of Freundlich model.

Figure 4

(a) Nonlinear fitting of the Freundlich and Langmuir isotherm models of fluoride ion on SA2-nAl2O3 foam; (b) Linear fitting of Langmuir model; (c) Linear fitting of Freundlich model.

Close modal

It indicated that the Langmuir adsorption isotherm better described the relevant result by the high correlation coefficient value and low SSE value (AbdulRazak & Rohani 2018). The maximum adsorption capacity of SA2-nAl2O3 foam was 5.815 mg/g. The adsorption process was monolayer adsorption and gave rise to the chemical reaction between the active site of SA2-nAl2O3 foam and fluoride ion. Several representative research of adsorbents has been summarized in Table 3. Under the similar concentration range and temperature, the adsorption capacity of SA-nAl2O3 foam demonstrates distinct advantages in the listed.

Table 3

The fluoride ion adsorption capacity comparison of adsorbents

AbsorbentTemperatureqmaxConcentration rangeReferencess
(°C)(mg/g)(mg/L)
SA‒nAl2O3 foam 25 5.82 2.00‒100.00 This study 
Al2O3 modified expanded graphite 30 5.75 3.00‒100.00 Jin et al. (2015)  
Manganese‒oxide‒coated alumina 30 ± 2 2.85 5.13‒32.09 Maliyekkal et al. (2006)  
Activated alumina Room 1.41 2.50‒14.00 Ghorai & Pant (2005)  
PUF‒Al2O3 composite foam Room 2.60 5.84‒45.99 Wang et al. (2021)  
Porous granular ceramic containing dispersed aluminum and iron oxides 25 ± 1 1.79 10.00 Chen et al. (2011)  
Alkoxide origin alumina 30 ± 2 2.00 5.00‒25.00 Kamble et al. (2010)  
Aluminium titanate 30 3.01 2.00‒10.00 Karthikeyan & Elango (2009)  
Fe–Al–Ce nano‒adsorbent 25 2.22 5.50 Chen et al. (2009)  
Al(OH)3 modified magnetite 30 1.51 1.00‒30.00 García-Sánchez et al. (2016)  
Al compound modified siderite 25 4.42 2.00‒25.00 Shan & Guo (2013)  
Aluminum hydroxide modified diatomaceous earth 23 ± 2 1.67 5.00‒70.00 Akafu et al. (2019)  
Bauxite Room 5.16 4.00‒12.00 Nazari & Halladj (2014)  
Nanomagnesia/alumina adsorbents 25 5.60 5.00‒20.00 Sujana & Anand (2011)  
Fe–Zr loaded calcium alginate bead 25 ± 2 0.98 2.00‒50.00 Swain et al. (2013)  
Al2O3‒Fe3O4‒expanded graphite Nano‒sandwich adsorbent 30 3.38 5.00‒50.00 Xu et al. (2016)  
AbsorbentTemperatureqmaxConcentration rangeReferencess
(°C)(mg/g)(mg/L)
SA‒nAl2O3 foam 25 5.82 2.00‒100.00 This study 
Al2O3 modified expanded graphite 30 5.75 3.00‒100.00 Jin et al. (2015)  
Manganese‒oxide‒coated alumina 30 ± 2 2.85 5.13‒32.09 Maliyekkal et al. (2006)  
Activated alumina Room 1.41 2.50‒14.00 Ghorai & Pant (2005)  
PUF‒Al2O3 composite foam Room 2.60 5.84‒45.99 Wang et al. (2021)  
Porous granular ceramic containing dispersed aluminum and iron oxides 25 ± 1 1.79 10.00 Chen et al. (2011)  
Alkoxide origin alumina 30 ± 2 2.00 5.00‒25.00 Kamble et al. (2010)  
Aluminium titanate 30 3.01 2.00‒10.00 Karthikeyan & Elango (2009)  
Fe–Al–Ce nano‒adsorbent 25 2.22 5.50 Chen et al. (2009)  
Al(OH)3 modified magnetite 30 1.51 1.00‒30.00 García-Sánchez et al. (2016)  
Al compound modified siderite 25 4.42 2.00‒25.00 Shan & Guo (2013)  
Aluminum hydroxide modified diatomaceous earth 23 ± 2 1.67 5.00‒70.00 Akafu et al. (2019)  
Bauxite Room 5.16 4.00‒12.00 Nazari & Halladj (2014)  
Nanomagnesia/alumina adsorbents 25 5.60 5.00‒20.00 Sujana & Anand (2011)  
Fe–Zr loaded calcium alginate bead 25 ± 2 0.98 2.00‒50.00 Swain et al. (2013)  
Al2O3‒Fe3O4‒expanded graphite Nano‒sandwich adsorbent 30 3.38 5.00‒50.00 Xu et al. (2016)  

Further investigated the kinetic adsorption performance of SA2-nAl2O3 foam, the fitting curve was shown in Figure 5(a). It showed that the adsorption equilibrium of SA2-nAl2O3 foam emerged after 4 h. These results indicated the SA composite foam reached the adsorption equilibrium in a relatively short time, and which has commercial application value. The efficient adsorption process could be due to abundant active sites of activated nano-Al2O3 being dispersed and exposed on the SA surface, which is more likely to access fluoride ion to complete the adsorption process (Wang et al. 2021). The three typical kinetic models, including pseudo-first-order, pseudo-second-order and intra-particle diffusion were used to analyze the experimental data. The kinetics parameters were exhibited in Table 4. It indicated that the pseudo-second-order kinetic model agrees well with describing the kinetic behaviour of SA2-nAl2O3 foam. It is regarded that chemical adsorption is the critical factor to affect the rate-limiting step through exchanging electrons between adsorbent and adsorbate in this adsorption process. In addition, the fitting curve of the intra-particle diffusion model did not go through the origin, which further confirmed the chemical reaction decided the process of kinetic adsorption (Jiang et al. 2013; Kim et al. 2020).
Table 4

The kinetics parameters for fluoride ions adsorption onto SA2-nAl2O3 foam

Pseudo-first order model
Pseudo-second order model
Intra-particle diffusion model
SA2-nAl2O3 foamk1 (min−1)qe (mg/g)R2k2 (g/mg·min)qe (mg/g)R2kp (mg/g·min0.5)R2
 0.027 3.899 0.814 0.009 4.251 0.939 0.080 0.744 
Pseudo-first order model
Pseudo-second order model
Intra-particle diffusion model
SA2-nAl2O3 foamk1 (min−1)qe (mg/g)R2k2 (g/mg·min)qe (mg/g)R2kp (mg/g·min0.5)R2
 0.027 3.899 0.814 0.009 4.251 0.939 0.080 0.744 
Figure 5

(a) Adsorption kinetics of fluoride ion on SA2-nAl2O3 foam with three models; (b) Adsorption thermodynamics of fluoride ion on SA2-nAl2O3 foam at different temperature.

Figure 5

(a) Adsorption kinetics of fluoride ion on SA2-nAl2O3 foam with three models; (b) Adsorption thermodynamics of fluoride ion on SA2-nAl2O3 foam at different temperature.

Close modal

Adsorption thermodynamics

The thermodynamics adsorption effect on fluoride ions of SA2-nAl2O3 foam was investigated at different temperatures and shown in Figure 5(b). The result was revealed from tripartite aspects. First, ΔH > 0 revealed the endothermic nature of the fluorine ions adsorption process. Second, ΔS > 0 suggested that the disorder of SA2-nAl2O3 foam enhanced at the solid-liquid interface because of the movement of fluorine ion from the free motion in aqueous solution to the two-dimensional motion after binding to the binding site. Third, the free energy ΔG < 0 demonstrated the spontaneous nature of the adsorption process (Shi et al. 2013).

Effect of solution pH

The pH effect on the fluoride ion removal capacity with 20 mg/L fluorine solution was presented in Figure 6(a). The results indicated that the maximal adsorption capacity was 5.09 mg/g at a pH of 3, and the adsorption capacity decreased gradually with the increase of pH value. Due to many H+ dominant species existing at acid conditions, the SA2-nAl2O3 foam active site was protonated further to generate electrostatic attraction and hydrogen-bond interaction. At the alkaline state, the hydroxyl group competing with fluoride ions leads to deterioration in adsorbent adsorption capacity (Paudyal et al. 2012).
Figure 6

The effect of (a) pH, (b) dosage and (c) competitive anions on the adsorption of fluoride ion by SA2-nAl2O3 foam; (d) Regeneration of SA2-nAl2O3 foam.

Figure 6

The effect of (a) pH, (b) dosage and (c) competitive anions on the adsorption of fluoride ion by SA2-nAl2O3 foam; (d) Regeneration of SA2-nAl2O3 foam.

Close modal

Effect of adsorbent dosage

The additive dose effect of SA2-nAl2O3 foam on fluoride ions removal was studied in a 20 mg/L fluoride ion solution of pH 3 to optimize the amount of adsorbent. Control the dosage range of addition of SA2-nAl2O3 foam was 1‒10 pcs. As shown in Figure 6(b), the removal rate increased significantly with the dosage increase, the highest removal rate is 98.17%. It indicated that a higher adsorbent content implied more adsorption sites to remove the fluoride ions. However, the fluoride ions can be virtually exhausted and adsorption capacity decrease progressively when introduced a high dose of SA2-nAl2O3 foam adsorption process. Therefore, the dosage of SA2-nAl2O3 foam should be satisfied with the fluoride ion emission limits while making full use of the adsorption capacity of the foam in practical applications.

Effect of co-existent ions

In fluoride wastewater was commonly accompanied by external anions to compete with fluoride ions in the adsorption process. Evaluating their effects on fluoride ion absorption is vital for promoting the practical application of SA2-nAl2O3 foam. However, the degree of competition of various anions depends on their relative concentration and affinity for adsorbents. Therefore, the ions competitive experiments were executed by introducing Na2CO3, Na2SO4, NaNO3 and NaCl to obtain the 0.1 mol/L and 1.0 mol/L coexisting ionic solutions, and the results were illustrated in Figure 6(c). The fluoride ions adsorption capacity was affected by anions in order of CO32− > SO42− > Cl > NO3. This phenomenon may be explained by hydrated radius and ion valence, which indicated that a smaller hydrated radius of ions more easily produced the stronger influence.

Regeneration of SA2-nAl2O3 foam composite foam

The regeneration performance result was displayed in Figure 6(d) and used to estimate the economic perspective of SA2-nAl2O3 foam. It emerged that the SA2-nAl2O3 foam adsorption capacity moderately diminished to reach 4.62 mg/g, but the regeneration efficiency was still maintained above 90% after three cycles. In addition, the nAl2O3 steadily existing in the SA foam prevented falling off and avoided secondary pollution. Therefore, SA2-nAl2O3 foam has recycling performance and good application prospects.

Adsorption mechanism

The SA2-nAl2O3 foam adsorbed fluoride ions before and after were investigated by XPS, and the result was shown in Figure 7(a)7(c). It revealed that the wide scans spectra show a new peak of F 1s at 691 eV compared with SA2-nAl2O3 foam adsorb before and after (Figure 7(a)). The O 1s spectra showed the Al―O and Al―OH species appeared at 531.2 eV and 532.7 eV, respectively (Figure 7(b)) (Lindsay et al. 2016). The peak intensity of Al―OH decreased from 62.31% to 59.17%, and the percentage of Al―O species increased from 37.69% to 40.83% after adsorption. It indicated that the ―OH group played an important role in the process of fluoride ion removal. The new peaks of AlF2.7(OH)0.3 and AlF3 appeared in the Al 2p spectrum after adsorption at 75.74 eV and 76.55 eV, respectively (Figure 7(c)). The AlF2.7(OH)0.3 was the reaction intermediate in the adsorption process (Chen et al. 2021). In addition, the FTIR was further analyzed with the SA2-nAl2O3 foam adsorption before and after (Figure 3(a)). Meanwhile, the ―OH peak intensity was obviously weakened when SA2-nAl2O3 foam after adsorption because the fluoride ion was more prone to replace the ―OH group. The fluoride ion solution pH change was simultaneously measured in the kinetic experiment process under the fluoride ions solution initial pH of 3 conditions (Figure 7(d)). The pH value increased rapidly in the first four hours until it reached a plateau of 6.72. This indicates that fluoride ions have replaced the hydroxyl groups on SA2-nAl2O3 foam and released OH into the solution to increasing the pH value (Zhang & Jia 2018). It revealed that the SA2-nAl2O3 foam primary adsorption mechanism was ion exchange based on the above analysis and discussion (Zhang et al. 2021).
Figure 7

The XPS analysis of the wide scans (a), O 1s (b) and Al 2p (c) before and after the adsorption of fluoride ion; (d) change of pH value according to the contact time of adsorption kinetics; (e) the mechanism of fluoride ion adsorption on SA-nAl2O3 foam.

Figure 7

The XPS analysis of the wide scans (a), O 1s (b) and Al 2p (c) before and after the adsorption of fluoride ion; (d) change of pH value according to the contact time of adsorption kinetics; (e) the mechanism of fluoride ion adsorption on SA-nAl2O3 foam.

Close modal

In this study, a novel material of SA-nAl2O3 foam was successfully prepared by freeze-drying SA-based foam materials and mixing the nAl2O3 to remove fluoride ions. SA2-nAl2O3 foam introduced more active sites and presented an obvious advantage of a high adsorption capacity of 5.09 mg/g at 20 mg/L fluoride ion solution compared to other adsorbents. The adsorption process is closer to monolayer adsorption. The pseudo-second-order model better described the adsorption kinetics behavior and which is dominated by the chemical adsorption. Adsorption thermodynamics analysis illustrated that the adsorption process was spontaneous and endothermic. The SA2-nAl2O3 foam revealed excellent regeneration performance after three cycles and maintained the nAl2O3 stability that exists in the SA. The primary adsorption mechanism of SA2-nAl2O3 foam was ion exchange for fluoride ions removal because of the foam has a larger of ―OH group and the fluoride ion was more prone to replace the ―OH group. SA composite foam has excellent application prospects in environmental water samples. The feasibility of SA-nAl2O3 foam in theory of fluoride ion removal has been proved, and the application in the treatment of more complex and large-scale practical fluoride wastewater is the next work focus to be explored.

The work was supported by Tianjin Municipal Science and Technology Bureau of China (Project No. 18PTZWHZ00140, 20JCZDJC00380, 20JCYBJC00560) and TG Hilyte Environment Technology (Beijing) Co., LTD. (Project No. M-P-0-181001-001).

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

The authors declare there is no conflict.

AbdulRazak
A. A.
&
Rohani
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