Bioinspired synthesis of mesoporous ZrO₂ nanomaterials with elevated defluoridation performance in agarose gels

Abstract

Mesoporous ZrO₂ nanomaterials were synthesized in an agarose gel medium by a bioinspired approach, and their adsorption capacity for the fluoride anion was also evaluated. The obtained ZrO₂ samples after calcination at 600 °C for 2 h are primarily composed of tetragonal nanocrystals about 11.8 nm in size. Moreover, the samples possess bimodal mesopores about 3.5 nm and 5.9 nm in diameter, which are generated by removing the agarose gel fibers embedded in the ZrO₂ particles during their growth process. The pore volume and specific surface area of mesoporous ZrO₂ nanomaterials are 0.091 cm³ g⁻¹ and 40.14 m² g⁻¹, respectively, which increase with the increase of gel concentration, while decrease with the increase of initial ZrOCl₂ concentration. The ZrO₂ samples synthesized in agarose gel show a higher fluoride adsorption capacity than those synthesized in the aqueous solution by the same method. This study may provide a broader insight into the bioinspired synthesis of metal oxides in hydrogel medium under mild conditions.

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1. Introduction

Recently, much effort has been devoted to the bioinspired synthesis of inorganic nanomaterials in hydrogel, because such medium is regarded as a popular matrix for imitating the biomineralization processes.^1–3 The hydrogel creates a tunable and stable diffusion-dominated mass transport environment for the nucleation and growth of inorganic nanomaterials, in which the morphology, structure and physico-chemical properties of the resultant materials can be well controlled.^4–6 In particular, the gel fibers can be incorporated into inorganic single crystals by manipulating both gel mechanical strength and crystal growth kinetics, rendering a new approach for the synthesis of inorganic materials with mesopores.¹

Till now, a wide variety of inorganic crystals, such as CaCO₃,^7,8 hydroxyapatite,⁹ CaC₂O₄ ¹⁰ and calcium tartrate tetrahydrate,¹¹ have been prepared within various types of hydrogels. Moreover, the nanotubes¹² and porous monoliths^13–20 of metal oxides including TiO₂, SiO₂, ZrO₂, Nb₂O₅, and SnO₂, have also been synthesized by using different types of hydrogels as templates. In our previous study, hollow TiO₂ nanospheres with bimodal mesopores about 3.8 and 5.5 nm in size have been firstly prepared by using an agarose gel as the medium,²¹ which exhibits the different morphology and microstructure from their counterpart prepared in the aqueous solution under identical conditions. It is speculated that a spherical composite formed by protamine and agarose molecules may take a crucial role in their formation, and the mesopores may be formed through removing the agarose gel fibers embedded in the shell of hollow TiO₂ nanospheres during their nucleation and growth. However, this bioinspired method needs extend to other metal oxides, and the formation mechanism of metal oxides in the gel medium also needs to be further elucidated.

Zirconia nanomaterials have gained their popularity during the past few decades due to their distinctive properties containing thermodynamical stability, corrosion resistance, high refractive index, dielectric constant and mechanical strength, which have been utilized as catalyst and catalyst carrier, absorbent, photocatalyst, memory device and dense ceramics etc.^22–27 To date, various physical and chemical strategies have been adopted to prepare ZrO₂ nanomaterials, e.g., the sol–gel method,²⁸ forced hydrolysis,^29,30 hydrothermal method,³¹ modified emulsion precipitation,³² combustion synthesis³³ and electric explosion of wires.³⁴ However, these conventional physical and chemical methods often involve the extremely harsh conditions, such as high temperature, inflammable organic solvents and high energy heating. Therefore, it is still a challenge for preparing ZrO₂ nanomaterials by a green, efficient, facile, generic method.

In this study, porous ZrO₂ particles are prepared by using ZrOCl₂ and NH₄OH as the reactants in agarose gels. The agarose, a natural neutral polysaccharide, is chosen as the gel medium, because it is easily available and commonly acts as the model hydrogel in the synthesis of inorganic materials. ZrOCl₂ and NH₄OH are chosen as the reactants because of their low cost and easy scale-up for industrial application. Furthermore, the defluoridation performance of resultant ZrO₂ particles is evaluated. This study may be useful for an intensive understanding about bioinspired metal oxides synthesis in the hydrogel medium.

2. Experimental

2.1 Chemicals

Zirconium oxychloride (ZrOCl₂·8H₂O) and ammonium hydroxide (NH₄OH) were purchased from Tianjin Guangfu Company. Agarose was supplied by Gene Tech. Co. (Shanghai) with a gel strength of 750 g cm⁻² under 1 wt% gel concentration. A 100 mg L⁻¹ fluoride ion stock solution was prepared by dissolving 0.221 g NaF in 1000 mL of deionized water. All other chemicals were of analytical grade, and used without further purification. Deionized water was used in all experiments.

2.2 Synthesis of ZrO₂ samples in agarose gels

The detailed synthetic method is as follows: 1 g of agarose powder was dissolved in 100 mL water, and then heated to 85–90 °C with vigorous stirring until all the powder dissolved completely. The hot agarose solution was poured into a Petri dish at room temperature until the hydrogel formed completely. The cooled agarose gel was cut into cubic pieces about 1 cm³ in volume, which were soaked in a 100 mL ZrOCl₂ solution (0.05 mol L⁻¹) to incubate for 4 h at room temperature. Then, the gel pieces were washed, and immersed into a 2.5 wt% NH₄OH solution for 4 h to induce the ZrO₂ precipitation. Thereafter, the gel pieces containing ZrO₂ precipitates were rinsed with deionized water till the pH of the washing water is near 7, followed by freeze-drying to get agarose–ZrO₂ composites. After these composites were calcined at different temperatures in air for 2 h, ZrO₂ samples were obtained finally. They are denoted as “ZrO₂–AG-Y” (Y = 400, 600 and 800), where AG and Y represent the agarose gel and calcination temperature in Celsius degree, respectively. For comparison, ZrO₂ materials were also prepared by the same procedure in an aqueous solution, and designated as ZrO₂–W, where W represents the water.

2.3 Characterization

The X-ray powder diffraction (XRD) patterns were obtained by a Philips X-Pertpro diffractometer using Cu Kα radiation at a scan rate of 4° min⁻¹ with an accelerating voltage of 40 kV and a current of 40 mA, which is used to determine the crystallographic information of ZrO₂ samples. Transmission electron microscope (TEM) observation was measured on a JEM-100CX II instrument. High resolution transmission electron microscopy (HRTEM) was performed on a Tecnai G2 F-20 instrument. Thermogravimetric/derivative thermogravimetric analysis (TG-DTG) was conducted on a STA 449F3 (Netzsch) thermogravimetric analyzer at a heating rate of 10 °C min⁻¹ under N₂ flow of 30 mL min⁻¹ from room temperature to 800 °C. The BET specific surface area of ZrO₂ samples were evaluated by N₂ adsorption–desorption isotherm measurements performed at 77 K on a Tri-Star 3000 gas adsorption analyzer. Prior to measurements, the samples were pretreated at 100 °C for 2 h. The infrared (IR) spectra of ZrO₂ samples (1 wt% in KBr) were carried out in the 400–4000 cm⁻¹ region with a resolution of 4 cm⁻¹ for each spectrum on a Nicolet-560 Fourier transform infrared (FT-IR) spectrometer.

2.4 Defluoridation performance of ZrO₂ samples

2.4.1 Defluoridation. All the defluoridation measurements of ZrO₂ samples were carried out at room temperature. Fifty milligram ZrO₂ samples were added into a 50 mL NaF solution with the initial F⁻ concentration of 30 mg L⁻¹. The suspension was sealed, put into a constant temperature incubator shaker, and kept for 4 h. After filtered by a 0.45 μm filter membrane, the F⁻ concentration in the supernatant liquid was determined by a F-ion selective electrode method.³⁵ The adsorption capacity (q_e) was calculated according to the equation as follows:where c₀ and c_e represent the F⁻ concentration before and after adsorption; V represents the volume of NaF solution; m represents the quality of ZrO₂ samples.

2.4.2 Effect of adsorption conditions on ZrO₂ defluoridation. The effect of adsorption conditions including initial F⁻ concentration, pH value of NaF solution, and co-existing anions, on the q_e of obtained ZrO₂ samples for removal of F⁻ ions were investigated using ZrO₂–AG-600 as a representative sample. The effect of initial F⁻ concentration on the q_e of ZrO₂–AG-600 was determined in a series of 50 mL NaF solutions with different F⁻ concentrations (10–50 mg L⁻¹, pH 7.1–7.4) without changing other conditions. The optimum pH was determined by measuring the q_e of ZrO₂–AG-600 in a 30 mg L⁻¹ F⁻ solution with different pH values (3.0–11.0), and other conditions remained unchanged. The pH of NaF solution was adjusted by using dilute HCl or NaOH solution. The effect of co-existing anions on the q_e of ZrO₂–AG-600 was determined in a 30 mg L⁻¹ F⁻ solution containing 30 mg L⁻¹ of Cl⁻, NO₃⁻, PO₄³⁻, SO₄²⁻ and HCO₃⁻, without changing other conditions.

3. Results and discussion

3.1 Characterization of ZrO₂ samples

When the agarose gel pieces containing ZrOCl₂ were immersed into the NH₄OH solution, some white substance produces inside the gels within a few minutes. Furthermore, its amount increases with prolonging the immersion time gradually. However, the reaction rate is much lower than that in the aqueous solution, which can be attributed to the diffusion inhibition of agarose gel for the reactants.⁸ The produced agarose gel–ZrO₂ composites cannot be dissolved in hot water about 90 °C like agarose gel/TiO₂ composites,²¹ indicating that ZrO₂ binds strongly with the agarose gel. After the agarose gels were removed by calcination at high temperature, white ZrO₂ powders were obtained. From Fig. 1a, it can be clearly observed that ZrO₂–AG-600 comprises uniformly quasi-spherical nanoparticles about 10 nm in size. The corresponding HRTEM image directly shows the clear lattice fringes with lattice spacing of 0.3 nm, which is indexed to the (111) plane of tetragonal ZrO₂.^36,37 In comparison, ZrO₂–W-600 (Fig. 1b) exhibits a monolithic structure, which may be formed via the fusion of ZrO₂ particles at high temperature.

Fig. 1 Typical TEM images of ZrO₂–AG-600 (a) and ZrO₂–W-600 (b). The inset of (a) is a HRTEM image of ZrO₂–AG-600.

The XRD technique was employed to characterize the crystal structure of ZrO₂ products after calcination at high temperature. The diffraction patterns in Fig. 2 clearly display that both of ZrO₂–AG-600 and ZrO₂–W-600 are mixtures of monoclinic phase (JCPDS no. 88-2390) and tetragonal phase (JCPDS no. 80-0965). ZrO₂–AG-600 exhibits high diffraction peaks at 30°, 50° and 60°, which correspond to the diffractions of the (111), (112) and (211) plane of tetragonal phase, respectively. Comparatively, there are sharp diffraction peaks at 28.3° and 31.5° in the XRD spectrum of ZrO₂–W-600, which belong to the (−111) and (111) plane of monoclinic phase, respectively. The weight fraction of tetragonal phase can be calculated via the following equation:¹⁸

where F_t represents the weight fraction of tetragonal phase, (111)_t and (−111)_m are the intensities of the corresponding tetragonal and monoclinic diffraction peaks, respectively. Compared with ZrO₂–W-600, the weight fraction of tetragonal phase in ZrO₂–AG-600 increases from 49.5% to 73.1%, indicating that incorporation of agarose gel fibers inhibits the phase transformation of ZrO₂ from tetragonal phase to monoclinic phase at 600 °C. Zhao et al.³⁷ demonstrated that the ratio of tetragonal ZrO₂ enhanced with increasing the hydrocarbon chain length of the capping agents. It is inferred that the agarose gel fiber may play a similar role like the capping agent of ZrO₂. The apparent crystallite size (D_x) of ZrO₂ samples is evaluated according to Scherrer's equation:³⁸where β is the half-height width of diffraction peak at θ value of the (111) reflection of tetragonal phase, K = 0.89 is a coefficient and λ is the X-ray wavelength corresponding to the Cu K_α irradiation. The D_x of ZrO₂–AG-600 is about 11.8 nm, well in agreement with the TEM result.

Fig. 2 XRD patterns of ZrO₂–W-600 and ZrO₂–AG-600.

The thermal properties of ZrO₂–W and the agarose–ZrO₂ composite before calcination were investigated by TGA. The weight loss of ZrO₂–W, as shown in Fig. 3a, occurs mainly from 40 °C to 240 °C as corroborated by an exothermic peak at about 104 °C in the DTG curve, which is attributed to the bound-water loss of ZrO₂·xH₂O.³⁹ When the temperature increases to 800 °C, the hydrous zirconia loses 16.8% of total weight. It is speculated that the theoretical x value in ZrO₂·xH₂O is 1.4. Actually, the weight loss almost ceases at 480 °C in the TG curve, indicating this temperature seems to be sufficient for the transformation of zirconia from amorphous phase into crystal.⁴⁰ As depicted in Fig. 3b, the degradation of the agarose–ZrO₂ composite is divided into two stages: from room temperature to 240 °C, the weight loss is about 10%, which is attributed to the bound-water evaporation in ZrO₂·xH₂O. In the range of 240–500 °C, a significant weight loss of about 38% occurs, and there is a corresponding exothermic peak at 268 °C in the DTG curve, which is primarily ascribed to the decomposition of agarose molecules.¹³ It should be mentioned that the water loss of agarose–zirconia composites is less than that of ZrO₂ precipitates obtained in aqueous solution, which is consistent with the previous report.¹⁸ This phenomenon may be caused by the incorporation of agarose gel fibers, which occupy the binding sites around ZrO₂, thus decreasing the bound water.

Fig. 3 TG and DTG curves of ZrO₂–W (a) and the agarose–ZrO₂ composite (b) before calcination.

Fig. 4 shows the nitrogen adsorption–desorption isotherms of ZrO₂–AG-600 (Fig. 4a) and ZrO₂–W-600 (Fig. 4b), in which the insets are the corresponding pore-size distribution curves. According to IUPAC classification, the adsorption–desorption isotherm of ZrO₂–AG-600 can be classified as type IV with a hysteresis loop of type H3 at high relatively pressures from 0.4 to 0.95, which indicates the existence of connected mesopores. The pore-size distribution curve calculated by the BJH (Barrett–Joyner–Halenda) method from the desorption branch shows two mesopores about 3.5 nm and 5.9 nm in diameter, respectively. Comparatively, the adsorption–desorption isotherm of ZrO₂–W-600 is classified as type II, and does not have hysteresis loop, suggesting no mesopores. The Brunauer–Emmett–Teller (BET) surface area and pore volume of ZrO₂–AG-600 are determined to be 40.14 m² g⁻¹ and 0.091 cm³ g⁻¹, respectively, which are comparative with the reported result,^13,18 and much higher than those of ZrO₂–W-600 sample (2.63 m² g⁻¹ and 0.002 cm³ g⁻¹). It is known that the diameter of agarose gel fibers usually varies from 3 to 30 nm depending on the experimental conditions,³ and the average diameter of agarose gel fibers incorporated into gel-grown calcite crystals is about 13 ± 5 nm.¹ In the previous study, it was reported that hollow TiO₂ nanospheres synthesized in agarose gels possessed bimodal pores about 3.8 and 5.5 nm in diameter.²¹ Moreover, porous ZrO₂ nanoparticles synthesized by using the agarose gel as the template exhibited the mesopores around 4–8 nm in size.¹⁸ Therefore, these two mesopores should be formed by removing the agarose gel fibers embedded in the ZrO₂ particles. The difference between agarose fibers and mesopore diameters of ZrO₂ can be attributed to the freeze-drying process before the N₂ adsorption–desorption isotherm measurement.

Fig. 4 Nitrogen adsorption–desorption isotherms of ZrO₂–AG-600 (a) and ZrO₂–W-600 (b). The inset is the corresponding pore-size distribution curve by the BJH (Barrett–Joyner–Halenda) method.

3.2 Forming mechanism of ZrO₂ in agarose gels

In order to investigate the forming mechanism of ZrO₂ in agarose gels, the effect of agarose gel and ZrOCl₂ concentration on the ZrO₂ structure was committed. A series of ZrO₂–AG-600 samples were synthesized in agarose gels with different concentrations, in which other reaction conditions were fixed. As listed in Table 1, when the gel concentration increases from 0.5 wt% to 4 wt%, the specific surface area and pore volume increase from 37.05 m² g⁻¹ and 0.093 cm³ g⁻¹ to 63.44 m² g⁻¹ and 0.169 cm³ g⁻¹, respectively. Meanwhile, the pore size is almost unchanged, indicating that the size of agarose gel fibers changes little. It is speculated that the diffusion is dominant for inorganic material growth in agarose gels, since the interaction between inorganic reactants and agarose gel is little, and 99% of agarose gel is water.⁶ As the gel concentration increases, the gel network becomes denser. This change decreases the diffusion rate of inorganic reactants and increases the growth resistance of ZrO₂, thus promoting the formation of small-size particles.⁸ Furthermore, the increase of gel concentration also enhances the incorporation of gel fibers in ZrO₂, consequently resulting in the increase of mesopore amount. Smaller particles and more mesopores endow the ZrO₂ sample prepared in agarose gels with high concentration larger specific surface area and pore volume.

Table 1 BET data of ZrO₂–AG-600 prepared in agarose gels with different concentrations

Gel concentration (%)	Specific surface area (m^2 g^−1)	Pore volume (cm^3 g^−1)	Pore size (nm)
0.5	37.05	0.093	7.0
1	40.14	0.091	3.5, 5.9
2	43.34	0.090	3.6, 8.1
3	46.79	0.136	3.6, 6.2
4	63.44	0.169	3.6, 7.3

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The influence of initial ZrOCl₂ concentration (10–200 mmol L⁻¹) on the ZrO₂–AG-600 structure was also investigated. As the initial ZrOCl₂ concentration increases, the pore size changes little; while the specific surface area and pore volume decrease gradually. When the ZrOCl₂ concentration is as low as 10 mmol L⁻¹, the specific surface area and pore volume of ZrO₂ are 60.8 m² g⁻¹ and 0.201 cm³ g⁻¹, respectively; while the specific surface area and pore volume reduce to 30.1 m² g⁻¹ and 0.077 cm³ g⁻¹, respectively, as the ZrOCl₂ concentration reaches 200 mmol L⁻¹. It is inferred that the produced ZrO₂ particles are few and small at low ZrOCl₂ concentration, because they cannot acquire sufficient reactants; while ZrO₂ particles can grow larger at high ZrOCl₂ concentration due to the sufficient reactant supplying. It is accepted that the materials composed of small particles usually have higher specific surface area and more stacking pores. Therefore, ZrO₂ samples prepared at low ZrOCl₂ concentration possess the larger specific surface area and pore volume than those at high ZrOCl₂ concentration. These above results illustrate that the structure of ZrO₂ prepared by this bioinspired method can be controlled via simply changing the reaction conditions.

Based on the experimental results, a tentative mechanism of mesoporous ZrO₂ nanomaterials formed in agarose gels is proposed. When ZrOCl₂ is dissolved in water, a tetramer complex [Zr(OH)₂·4H₂O]₄⁸⁺ forms as the dominant species.^30,41 As depicted in Scheme 1, when the agarose gels are dipped into the ZrOCl₂ solution, [Zr(OH)₂·4H₂O]₄⁸⁺ diffuses uniformly into the agarose gels. Since [Zr(OH)₂·4H₂O]₄⁸⁺ forms the hydrogen-bonding and coordination interaction with the –OH groups of agarose molecules, it can be rich around the agarose gel fibers. After agarose gels containing [Zr(OH)₂·4H₂O]₄⁸⁺ are soaked subsequently in a NH₄OH solution, OH⁻ ions promote the hydrolysis and polycondensation of [Zr(OH)₂·4H₂O]₄⁸⁺ to form the hydrous ZrO(OH)₂ nanoparticles. Meanwhile, the agarose gel fibers are entrapped in the hydrous ZrO(OH)₂ particles during their growing process.^6,18 After these ZrO(OH)₂–agarose composites are freeze-dried in vacuum and calcined at high temperature, the agarose gel fibers are removed thoroughly, thus giving rise to the formation of mesoporous ZrO₂ particles finally.

Scheme 1 A tentative mechanism of mesoporous ZrO₂ particles formed in agarose gels.

In the previous study, hollow TiO₂ nanospheres with mesopores in the shell were prepared in agarose gels by the similar procedure, in which a spherical protamine/agarose composite may take a key role in the formation of hollow nanospheres.²¹ However, no organic reactant is involved in the synthetic process of ZrO₂, and the intermediate similar to the protamine/agarose composite cannot form. Therefore, the prepared ZrO₂ particles do not have the hollow-sphere structure like TiO₂. It is worth noting that there are also bimodal mesopores about 3.8 and 5.5 nm in size in the shell of hollow TiO₂ nanospheres prepared in 1 wt% agarose gel, which are almost same as those of ZrO₂ particles. This result confirms further that these mesopores come from the removal of agarose gel fibers embedded in the oxide materials. This may be a versatile approach for the preparation of inorganic oxide materials with bimodal mesopores of several nanometers to use the hydrogel fiber as the template.

Compared with other reaction media including water, organic solvent, and ionic liquid, the hydrogel medium has three advantages as used to synthesize metal oxide materials. Firstly, the synthesis in the hydrogel is a facile green process under ambient conditions, which does not need harsh conditions including high temperature, high pressure and complicated post-treatments. Secondly, the products prepared in the hydrogel can availably avoid the cluster aggregation in water environment, since the hydrogel provides a stable diffusion-dominated mass transport environment for the nucleation and growth of inorganic materials. Thirdly, the hydrogel fibers can be used as the template to form the mesopores in the produced materials, which can be regulated via simply changing the reaction conditions. In comparison with the method applied the freeze-dried agarose gel scaffold as the template, this bioinspired approach does not need the complicated preparation of template. Moreover, the acquired metal oxide materials have the higher mechanical strength than those prepared by the templating method, while keeping the comparative specific surface area and pore volume.

3.3 Defluoridation performance of ZrO₂–AG-Y particles

Water pollution by fluoride has been a global environmental concern for decades, since excessive fluoride intake is expected to cause skeletal fluorosis as well as disruption of normal calcium and phosphorus metabolism.⁴² The defluoridation performance of ZrO₂–AG-Y samples was evaluated by their adsorption capacity for F⁻ ions at room temperature. As listed in Table 2, ZrO₂–AG-600 has the highest fluoride adsorption capacity of 4.69 mg g⁻¹ in all of ZrO₂ samples, almost 5 times of ZrO₂–W-600. It is well known that the surface –OH group of ZrO₂ is essential to the fluoride adsorption. During the adsorption process, F⁻ ions replace the surface –OH groups of ZrO₂, and form Zr-oxyfluoride species, such as ZrO₂F₅ and ZrO₃F₄.⁴³ In the FTIR spectra of ZrO₂–AG-600 and ZrO₂–W-600 (Fig. 5), a broad band at 3200–3600 cm⁻¹ and a sharp peak at 1630 cm⁻¹ are assigned to the stretching and bending mode of –OH groups, respectively.^18,44 It can be observed that the –OH characterized peaks of ZrO₂–AG-600 are apparently stronger than those of ZrO₂–W-600, indicating more –OH groups on the surface of ZrO₂–AG-600. This result can be assigned to smaller particle size and higher specific surface area of ZrO₂–AG-600 than those of ZrO₂–W-600. Therefore, ZrO₂–AG-600 has a higher defluoridation performance than ZrO₂–W-600. Although ZrO₂–AG-400 exhibits much higher specific surface area than ZrO₂–AG-600, the adsorption capacity of ZrO₂–AG-400 is a little lower than that of ZrO₂–AG-600. As illustrated in Fig. 3, the agarose fiber embedded in the ZrO₂–AG sample cannot be removed completely at 400 °C. The residual agarose molecules may occupy some active sites on the ZrO₂ surface and prevent the fluoride adsorption, resulting in lower defluoridation performance of ZrO₂–AG-400. When the calcination temperature reaches 800 °C, both the specific surface area and surface –OH groups of ZrO₂ crystal particles reduce because of their larger size. Consequently, the defluoridation performance of ZrO₂–AG-Y samples exhibits such a trend that rises first, and then falls, with increasing the calcination temperature.

Table 2 BET data and fluoride adsorption capacity of ZrO₂–AG-Y samples

Samples	Specific surface area (m^2 g^−1)	Pore volume (cm^3 g^−1)	qe (mg g^−1)
ZrO2–W-600	2.63	0.002	0.97
ZrO2–AG-400	136.46	0.086	4.28
ZrO2–AG-600	40.14	0.091	4.69
ZrO2–AG-800	17.74	0.103	2.08

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Fig. 5 FTIR spectra of ZrO₂–AG-600 and ZrO₂–W-600.

The effect of adsorption conditions, such as initial F⁻ concentration, pH value of NaF solution, and co-existing anions, on the adsorption capacity (q_e) of ZrO₂–AG-600 for F⁻ ions were investigated. Fig. 6a illustrates the effect of initial F⁻ concentration on the q_e of ZrO₂–AG-600 for F⁻ ions without changing other conditions. With the increase of initial F⁻ concentration, the q_e of ZrO₂–AG-600 gradually increases at the beginning, and then reaches the saturation adsorption nearly as the F⁻ concentration is beyond 30 mg L⁻¹. The pH value of NaF solution would dramatically affect the adsorption behavior taking place at the solid–liquid interface. The effect of pH value of NaF solution on the q_e of ZrO₂–AG-600 is presented in Fig. 6b. It can be seen that ZrO₂–AG-600 demonstrates a superior defluoridation performance in the range of pH 3.0–7.0 and a maximum q_e of 5.5 mg g⁻¹ at pH 5.0. As the pH increases continuously, the q_e of ZrO₂–AG-600 drops monotonously. This result suggests that these kinds of ZrO₂ materials are suitable for the weakly acidic fluoride solution rather than the basic solution. The pH value of NaF solution can influence the surface electric property of ZrO₂ particles and the existing state of F⁻. At pH < 5, the surface of ZrO₂ particles carries positive charges, which can attract F⁻ ions by the electrostatic interaction. However, a part of F⁻ ions are hydrolyzed to HF molecules at strongly acidic condition, thus lowering the adsorption capacity. At pH > 5, the surface of ZrO₂ particles carries negative charges, which can repel F⁻ ions with the same electronic property. Furthermore, OH⁻ ions can be adsorbed competitively on the surface of ZrO₂ particles under basic condition, decreasing the adsorption capacity for F⁻ ions.

Fig. 6 Effect of initial NaF concentration (a), pH value of NaF solution (b) and co-existing ions (c) on the fluoride adsorption of ZrO₂–AG-600.

Besides the F⁻ ion in the environmental water, other ubiquitous co-existing anions, such as Cl⁻, NO₃⁻, PO₄³⁻, SO₄²⁻ and HCO₃⁻, would compete for active sites of ZrO₂ absorbents, and affect their working capacity for fluoride removal. In order to study the potential application of prepared ZrO₂ samples for the practical system, the q_e of ZrO₂–AG-600 in the NaF solution with different co-existing anions, such as Cl⁻, NO₃⁻, PO₄³⁻, SO₄²⁻ and HCO₃⁻, was determined. As shown in Fig. 6c, the order of co-existing anions affecting the q_e of ZrO₂–AG-600 is Cl⁻ < NO₃⁻ < PO₄³⁻ < SO₄²⁻ < HCO₃⁻. Since the HCO₃⁻ ion can form a stable penta cyclic complex with Zr⁴⁺ on the surface of ZrO₂, it occupies competitively the active adsorption sites of ZrO₂, resulting in the undesirable reduction of defluoridation performance. As for PO₄³⁻ and SO₄²⁻, they carry more charges than Cl⁻ and NO₃⁻, thus exhibiting the stronger adsorption capacity on the surface of ZrO₂ samples.

4. Conclusions

In this study, a facile bioinspired approach has been explored to synthesize mesoporous ZrO₂ particles in agarose gels. After calcined at 600 °C for 2 h, the ZrO₂ samples are primarily composed of tetragonal nanocrystals about 11.8 nm in size. Meanwhile, the ZrO₂ samples possess bimodal mesopores of about 3.5 nm and 5.9 nm in diameter, which are generated by the templating of entrapped agarose gel fibers during the formation of ZrO₂ particles. In addition, these ZrO₂ particles exhibit much higher fluoride adsorption capacity than their counterpart synthesized in aqueous solution medium, which can be attributed to their high specific surface area and mesoporous structure. Hopefully, this study may afford a versatile approach for the preparation of mesoporous metal oxide materials in the gel medium under mild conditions.

Acknowledgements

This research was supported by National Science Fund for Distinguished Young Scholars of China (no. 21125627), National Basic Research Program of China (2009CB724705), and the Program of Introducing Talents of Discipline to Universities (no. B06006).

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