A new ion exchange adsorption mechanism between carbonate groups and fluoride ions of basic aluminum carbonate nanospheres

Abstract

Excessive fluoride in water has serious effect on people's health and removal of it through adsorption is very effective and important. Developing adsorbents with high adsorption capacities and elucidating the adsorption mechanisms are the two aspects needed to be enhanced. Herein, we reported that basic aluminum carbonate (denoted as Al(OH)CO₃) nanospheres with an abundance of carbonate groups exhibited excellent properties for removal of fluoride with maximum adsorption capacity of 59 mg g⁻¹ in neutral solution. In particular, other common anions in water, such as Cl⁻, NO₃⁻, PO₄³⁻, SiO₃²⁻ and SO₄²⁻, had no significant effects on the adsorption properties of Al(OH)CO₃ for fluoride. Based on the X-ray photoelectron spectroscopy (XPS) and Fourier transform infrared spectroscopy (FTIR) results, a new ion exchange mechanism involving carbonate groups and fluoride ions in solution was proposed.

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1 Introduction

Fluorides in drinking water can be beneficial or detrimental depending on their concentration and total amount ingested. The optimum fluoride level in drinking water set by the World Health Organization (WHO) is less than 1.5 mg L⁻¹.¹ An excess of fluoride in drinking water causes dental fluorosis and/or skeletal fluorosis. Therefore, it is very important to remove excessive fluoride in water. Compared to many other removal methods, such as chemical precipitation/coagulation,² ion exchange,³ membrane based process⁴ and electrodialysis,⁵ adsorption is considered as one of the most extensively explored methods because of its effectiveness, convenience and low cost.^6–8 Developing adsorbents with high adsorption capacities is the most important for this method.⁹

In the past few years, various materials have been developed to scavenge fluoride, such as active alumina,¹⁰ layered double hydroxides,¹¹ zeolites,¹² chitosan,¹³ red mud¹⁴ and MOFs.¹⁵ Among these adsorbents, aluminum-based materials have received more attention in recent years due to the abundance of aluminum on the earth and the selective affinity with fluoride based on the soft–hard acid base theory. For example, Li et al. reported the synthesis of highly ordered mesoporous alumina by a sol–gel route with block copolymers as soft templates which exhibited superb fluoride and arsenic removal capacities.¹⁶ Li et al. synthesized amorphous alumina supported on carbon nanotubes via heating CNTs and Al(NO₃)₃ mixture under N₂ atmosphere and the composite showed high fluoride adsorption efficiency (14.9 mg g⁻¹) from water in a broad range of pH values.¹⁷ Tripathy et al. investigated the adsorption capacity of manganese dioxide-coated activated alumina and the mechanism of fluoride uptake which was due to physical and intraparticle diffusion.¹⁸ Shi et al. impregnated commercially available granulated activated alumina with lanthanum oxide for effective fluoride removal with maximum capacity at 16.9 mg g⁻¹.¹⁹

Besides the adsorption property, the adsorption mechanisms for fluoride ions are also very important for rational design of adsorbents. In general, the mechanisms of adsorbents are mostly involved the exchange of fluoride ions with hydroxyl group and formation aluminum–fluoro complexes.^16,19–26 Li et al. found that the adsorption mechanism of mesoporous alumina was ligand exchange between hydroxyl groups and fluoride ions according to X-ray photoelectron spectroscopy (XPS) analysis results.¹⁶ Shi et al. studied lanthanum-impregnated activated alumina for fluoride removal and the interaction mechanism was the ion exchange between fluoride ions and surface hydroxyl groups especially La–OH.¹⁹ Karthikeyan et al. prepared the polyaniline/alumina and polypyrrole/alumina composites.²⁷ They proposed the mechanism for fluoride removal involved both the formation of aluminum–fluoro complexes on the alumina surface and fluoride ions replaced doped ionizable chloride in the polymer.²⁷ Chai et al. developed sulfate-doped Fe₃O₄/Al₂O₃ nanoparticles for fluoride removal and proposed that the anion exchange with sulfate by fluoride and formation of inner-sphere fluoride complex were the main mechanisms.²⁸ However, whether any other groups can also be exchanged with fluoride ions have not been determined.

Herein, we found that Al(OH)CO₃ nanospheres with abundance of carbonate groups exhibited excellent property for removal of fluoride with maximum adsorption capacity of 59 mg g⁻¹ at pH of 7. In particular, other common anions in water had no obvious effects on the adsorption property of Al(OH)CO₃ for fluoride. Based on the X-ray photoelectron spectroscopy (XPS) and Fourier transform infrared spectroscopy (FTIR) results, a new ion exchange mechanism between carbonate groups and fluoride was proposed.

2 Experimental sections

2.1 Synthesis of Al(OH)CO₃ nanospheres

All materials were purchased from Beijing Chemicals Co., Ltd. (Beijing, China) and used without further purification. Milli-Q purified water (18.2 MΩ) by Milli-Q Integral (Millipore, Bedford, MA) was used for all experiments. Al(OH)CO₃ nanospheres were synthesised through a microwave-assisted solvothermal process.²⁹ In brief, 3.75 g Al(NO₃)₃·9H₂O and 1.2 g of urea were dissolved in 100 mL anhydrous ethanol under sonication, and then 40 mL solution was poured into a Teflon autoclave. The autoclave was sealed and placed in a microwave oven (MDS-6, Shanghai Sineo Microwave Chemistry Technology Co., Ltd.). The oven was heated to 150 °C in 2 min and then kept at that temperature for another 5 min by microwave heating. After cooling down to room temperature, the precipitates were collected by centrifugation, washed with water and ethanol for three times and then dried at 80 °C for 6 h.

2.2 Adsorption experiments

Batch adsorption experiments were conducted at room temperature using 50 mL polyethylene (PE) vessels. Solutions with different concentrations of fluoride were prepared using analytical grade NaF. The pH value was adjusted to 7 by addition aqueous solution of 0.2 M NaOH or 0.2 M HCl. For the adsorption isothermal study experiment, 10 mg Al(OH)CO₃ was suspended in 20 mL sodium fluoride solutions with different concentrations under stirring for 12 h at room temperature. The solutions were then filtered through 0.22 μm syringe filters (Millipore Millex). The amount of remaining fluoride in the filtrate was measured by ion chromatography (ICS-900). The effects of co-existing anions (CO₃²⁻, SO₄²⁻, PO₄³⁻, SiO₃²⁻, Cl⁻ and NO₃⁻) on the fluoride adsorption were measured in 50 mg L⁻¹ fluoride solutions.

2.3 Characterizations

The morphology of the samples were characterized by field emission scanning electron microscopy (FE-SEM, JEOL-6701F) equipped with an energy-dispersive X-ray (EDX) analyser (Oxford INCA), transmission electron microscopy (TEM, JEOL-1011), and high resolution transmission electron microscopy (HRTEM, JEM 2100F, 200 kV). X-Ray photoelectron spectroscopy (XPS) was performed on the Thermo Scientific ESCALab 250Xi using 200 W monochromated Al K_α radiation. The concentration of fluoride was determined by ion chromatography (IC Dionex ICS-900) equipped with a Dionex AS 14A analytical column and a DS5 conductivity detector. The eluent is 3.5 mM Na₂CO₃ and 1 mM NaHCO₃ mixed solution and its flow rate is 1 mL min⁻¹. The injection volume of the samples is 25 μL. The zeta potential at pH of 7 was measured by Zetasizer Nano ZS ZEN3600.

3 Results and conclusions

3.1 Adsorption property

Basic aluminum carbonate (Al(OH)CO₃) nanospheres were first prepared according to our previous paper.²⁹ These basic aluminum carbonate nanospheres were existed as polymeric species with composition of Al(OH)CO₃. The exact structures are under further investigation. Fig. 1 shows the SEM and TEM images of Al(OH)CO₃ nanospheres with an average diameter of ca. 300 nm. Energy dispersive spectrum (EDS) suggested that the sample was composed of aluminum, carbon and oxygen (Fig. S1†). Elemental mapping analysis (Fig. 1c–f) showed that the elements were uniformly distributed on the particles. These Al(OH)CO₃ nanospheres had a high specific surface area of 484 m² g⁻¹ and abundant micropores at 1.0 nm (as shown in Fig. S2†), which were very beneficial for adsorption.²⁹ In addition, they had abundant surface carbonate groups. This is very different from many other common adsorbents. Therefore, we envision that these Al(OH)CO₃ particles may have particular adsorption property for fluoride.

Fig. 1 (a) SEM, (b and c) TEM, and elemental mapping images of (d) aluminum, (e) carbon and (f) oxygen of Al(OH)CO₃ nanospheres.

Fig. 2a shows the adsorption isotherm of Al(OH)CO₃ nanospheres for fluoride at pH 7. The adsorption data agree well with the Langmuir model. Langmuir model is the most frequently used for adsorption analysis. It can be described as follows:

q_e = q_mbC_e/(1 + bC_e)

where C_e is the equilibrium concentration of fluoride ions [mg L⁻¹], q_e is the amount of fluoride ions adsorbed per unit weight of the adsorbent at equilibrium [mg g⁻¹], q_m is the maximum adsorption capacity corresponding to complete monolayer coverage [mg L⁻¹] and b is the equilibrium constant related to the adsorption energy.

Fig. 2 (a) Adsorption isotherm of fluoride on Al(OH)CO₃ nanospheres at initial pH value of 7; (b) effects of co-existing anions on fluoride removal (the initial concentration of fluoride is 50 ppm, and other competing anions of Cl⁻, SO₄²⁻, CO₃²⁻, NO₃⁻, PO₄³⁻ and SiO₃²⁻ are 200 ppm, 200 ppm, 200 ppm, 10 ppm, 50 ppm and 50 ppm, respectively).

The maximum adsorption capacity for fluoride is 59 mg g⁻¹ according to the fitted results. This value is much higher than most of the reported adsorbents in the literatures, as shown in Table 1. More importantly, unlike many literatures conducted under acid condition, the values obtained in this study were at pH value of 7.

Table 1 Comparison of fluoride adsorption capacity of various adsorbents

Adsorbents	Adsorption capacity (mg g^−1)	pH	Ref.
Al(OH)CO3	59	7	This study
Activated alumina^10	2.41	7	10
Al2O3/CNT^17	14.9	6	17
LDH-n-MABS^26	32.4	5	26
La impregnated activated alumina^19	16.9	7	19
Basic aluminium sulfate@graphene^25	33.4	7.2	25

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In natural water, several common anions including PO₄³⁻, Cl⁻, SO₄²⁻, CO₃²⁻, NO₃⁻, and SiO₃²⁻ are simultaneously present with fluoride at different concentrations, which may compete with fluoride for the active adsorption sites. Hence, it is necessary to investigate the effects of these co-existing anions on the adsorption property of fluoride. In order to simulate the natural water, six solutions containing 200 ppm Cl⁻, 10 ppm NO₃⁻, 200 ppm SO₄²⁻, 50 ppm PO₄³⁻, 200 ppm CO₃²⁻ and 50 ppm SiO₃²⁻ were prepared with fixed initial fluoride concentration of 50 ppm.¹⁶ As shown in Fig. 2b, it can be seen that Cl⁻, NO₃⁻, PO₄³⁻, SiO₃²⁻ and SO₄²⁻ have no significant effects on the adsorption capacity for fluoride. However, CO₃²⁻ showed evident influence on the fluoride removal. This is related to the adsorption mechanism, which will be discussed in the following section. In addition, though the six co-existing anions were added to the solution at the same time, the adsorption efficiency was 68.1% compared to that solution containing only fluoride ions. The above results suggested that Al(OH)CO₃ particles possess specific affinity toward fluoride in natural water, making them suitable adsorbents for fluoride adsorption in water treatment. Basic aluminium carbonate nanospheres can be regenerated with Na₂CO₃ (0.1 M) for re-usability. Recycling test shows that the capacity for fluoride ions can be maintained as 24 mg g⁻¹ after 5th regeneration. Although Al(OH)CO₃ particles cannot be used as real sorbents currently for practical application due to the powder lost during water flow, they can be assembled to microspheres with sufficient mechanical strength for real applications.

3.2 Adsorption mechanism

In general, the mechanism for fluoride adsorption is electrostatic or/and ion exchange between hydroxyl groups and fluoride ions. The zeta potential value of Al(OH)CO₃ nanospheres in aqueous solution (pH = 7.0) was measured about 46.6 mV, suggesting the surface of as-prepared Al(OH)CO₃ was positively charged in neutral solution. Because fluoride ions are negatively charged, the electrostatic attraction was the initial driving force to bind the anions onto the surface of the adsorbent.

In order to investigate the interaction between fluoride ions and Al(OH)CO₃ particles, we prepared the fluoride-saturated sample first. As shown in Fig. 3a and b, the morphology of Al(OH)CO₃ nanospheres after adsorption was similar to that of the fresh sample, indicating that adsorption did not damage their structures. Information of fluoride appeared in the EDS spectrum of Al(OH)CO₃ after adsorption (Fig. S3†). Elemental mapping (Fig. 3c–f) results showed that fluoride ions were uniformly distributed on the surface of Al(OH)CO₃ particles, providing direct evidence of fluoride adsorbed onto the surface.

Fig. 3 (a) SEM, (b) TEM and elemental mapping images of (c) fluoride, (d) aluminum, (e) carbon and (f) oxygen of Al(OH)CO₃ nanospheres after fluoride adsorption.

Fig. 4a shows the full-range XPS spectra of Al(OH)CO₃ nanospheres before and after adsorption of fluoride. Two strong peaks located at 685 eV and 831 eV of fluoride were observed, which also confirmed fluoride ions were adsorbed on the surface of Al(OH)CO₃. High resolution XPS spectrum of F 1s suggested fluoride existed as F⁻ (Fig. 4b). The high resolution C 1s spectrum of the fresh Al(OH)CO₃ nanospheres showed a peak located at 288.4 eV, which can be assigned to CO₃²⁻. However, the intensity of CO₃²⁻ became significantly lower after fluoride adsorption, suggesting that carbonate groups have exchanged with fluoride ions. Comparing the FTIR spectra of Al(OH)CO₃ before and after fluoride adsorption, the intensity of carbonate groups was significantly decreased after adsorption, while that of hydroxyl groups was nearly the same (Fig. 5). O 1s spectra remained nearly the same before and after fluoride adsorption (Fig. S4†), also confirming that only carbonate groups were involved in the ion exchange process.

Fig. 4 (a) Full-range XPS spectra of Al(OH)CO₃ nanospheres before and after adsorption of fluoride. (b) F 1s spectrum of Al(OH)CO₃ after fluoride adsorption. C 1s spectrum of (c) Al(OH)CO₃, and (d) Al(OH)CO₃ after fluoride adsorption.

Fig. 5 FTIR spectra of Al(OH)CO₃ nanospheres before and after fluoride adsorption.

Based on the above results, we conclude that the electrostatic attraction is the initial driving force for fluoride binding to the surface of Al(OH)CO₃, and then ion exchange between carbonate groups and fluoride occurs. This is a new type of ion exchange mechanism involving carbonate groups in fluoride adsorption, and will inspire us to fabricate nanomaterials with higher adsorption capacities for fluoride. At the same time, it is reasonable to elucidate that CO₃²⁻ has obvious effect on adsorption of fluoride as mentioned above. Because under high concentration of CO₃²⁻, carbonate groups on Al(OH)CO₃ will get harder to exchange with fluoride in the solution.

4 Conclusions

In summary, we reported that Al(OH)CO₃ nanospheres with high surface area and abundant surface carbonate groups showed excellent adsorption ability for fluoride with maximum capacity of 59 mg g⁻¹ in neutral condition. In addition, other common anions in natural water, such as Cl⁻, SO₄²⁻, CO₃²⁻, NO₃⁻, PO₄³⁻ and SiO₃²⁻, have no significant influence on fluoride removal. A new adsorption mechanism involving ion exchange between carbonate groups and fluoride ions was proposed and confirmed. When in large scale applications, the adsorbents could be filled in columns in the process of waste water treatment. We believe that these Al(OH)CO₃ particles have highly potential applications in removing excessive fluoride ions in water, and such an ion exchange mechanism is beneficial for design and synthesis of other adsorbents with higher capacities.

Acknowledgements

We thank the financial supports from the National Basic Research Program of China (2011CB933700, 2012BAJ25B08), the National Natural Science Youth Foundation of China (NSFC 51402305) and the Chinese Academy of Sciences (XDA09030200, GJHZ1224, and KJCX2-YW-N41).

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Footnote

† Electronic supplementary information (ESI) available: EDS spectra, N₂ adsorption–desorption and pore size distribution isotherms of Al(OH)CO₃ nanospheres, O 1s spectra of Al(OH)CO₃ nanospheres before and after adsorption. See DOI: 10.1039/c4ra11018h

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