Synthesis of a Novel Ce-bpdc for the Effective Removal of Fluoride from Aqueous Solution

Ce-1,1-biphenyl-4,4-dicarboxylic acid (Ce-bpdc), a novel type of metal organic framework, was synthesized and applied to remove excessive fluoride from water. The structure and morphology of Ce-bpdc were measured by X-ray diffraction, scanning electronmicroscopy, Fourier transform infrared spectroscopy, and X-ray photoelectron spectroscopy.The effects, such as saturated adsorption capacity, HCO3 , and pH, were investigated. The optimal pH value for fluoride adsorption was the range from 5 to 6. The coexisting bicarbonate anions have a little influence on fluoride removal. The fluoride adsorption over the Ce-bpdc adsorbent could reach its equilibrium in about 20min. The Ce-bpdc coordination complex exhibited high binding capacity for fluoride ions. The maximum adsorption capacity calculated from Langmuir model was high up to 45.5mg/g at 298K (pH = 7.0) and the removal efficiency was greater than 80%. In order to investigate the mechanism of fluoride removal, various adsorption isotherms such as Langmuir and Freundlich were fitted. The experimental data revealed that the Langmuir isotherm gave a more satisfactory fit for fluoride removal. Finally, the tested results of ground water samples from three places, Yuefang, Jiangji, and Sanyi which exhibited high removal efficiency, also demonstrate the potential utility of the Ce-bpdc as an effective adsorbent.


Introduction
Fluoride ions are ubiquitous in surface and underground water and are one of the critical components for dental and bone health of mammals which are found to be beneficial for the mineralization of hard tissues in the mammal body in a narrow concentration with range of 0.5-1.5 mg/L [1][2][3][4], and fluorine and its compounds are valuable and extensively used in industry of fertilizers, high purity graphite, semiconductors, electrolysis of alumina, and so forth [5].Fluorine ions are found in trace amounts in various tissues of the human body, particularly teeth and bones, where 80%-90% of the fluorine ions in the human body are present.Fluoride ions are also important constituents of bone and teeth; a healthy body requires approximately 1.0 mg of fluorine ions every day.On the other hand, severe side effects, such as bone cancer and fluorosis, can occur when people are exposed to fluoride concentrations in excess of 1.5 mg/L for long periods [6,7].
Though the presence of fluoride in drinking water is essential for human health, excessive intake of fluoride will result in diseases such as dental, skeletal, or crippling fluorosis in children and fetal cerebral function, neurotransmitters and brain damage, Alzheimer's syndrome, thyroid disorder, and cancer in adults [8][9][10][11].The World Health Organization (WHO) has recommended a guidance value of 1.5 mg/L for fluoride in drinking water, and the permissible concentration of fluoride ions is less than 1.0 mg/L in China [12,13].This concern greatly promotes the water community for searching an effective way of defluoridation.
Excess fluoride ions from groundwater for meeting the requirement of drinking water can be removed by various techniques, including ion-exchange [14][15][16], reverse osmosis [17,18], dialysis and electrodialysis [19,20], precipitation [21,22], membrane separation [23,24], coagulation sedimentation [25], electrocoagulation [26,27], and adsorption and biosorption [28][29][30].In the previous period, the traditional 2 Advances in Condensed Matter Physics method for fluoride removal is to form insoluble salts by adding calcium salt, aluminum salt, or iron salt as precipitant.On the other hand, these classical methods need to add a large amount of precipitate or coprecipitating agent; therefore, these methods are limited because they cannot meet the drinking water standards by the World Health Organization (WHO) [31].Up to now, several methods are being developed to remove fluoride ions, such as adsorption through the use of metal oxide, metal-loaded synthetic polymer, biopolymer, and some anion exchange resin as efficient defluoridation adsorbents [32,33].Although they have already been modernized or studied by some researchers, these methods are still tedious and time-consuming and have no practical applications [34].Adsorption is a more acceptable defluoridation method and plays an important role in the purification of water with trace levels of fluoride because of the simplicity of design, environmental considerations, effectiveness, and convenience as compared to other methods [31][32][33][34].Adsorbents with favorable structures, high adsorption capacities, and ease of separation are highly desirable.Among the adsorbents used for defluoridation, such as activated carbon [35], metal oxides [36][37][38], and aluminum based adsorbents [39,40], aluminum hydroxides receive more attention because of their strong affinity for fluoride and low cost for large-scale production [41].In recent years, a variety of adsorbents, like Ce(III)-incorporated cross-linked chitosan, Zr(IV) immobilized cross-linked chitosan, and so forth, have been identified for the purpose of defluoridation [9,42].
Metal organic frameworks (MOFs) constructed by metalcontaining nodes connected by organic bridges, as potential adsorbents, attract a great deal of interest because of their large surface areas, adjustable pore sizes, and controllable properties, as well as acceptable thermal stability.MOFs can exhibit ultrahigh porosity and high thermal and chemical stability and have been altered for use in various adsorptionrelated areas [43].Recently, a number of researchers started to investigate the capability of MOFs to remove compounds including removal of metal ions [44][45][46] and toxic dyes [47,48] in aqueous environment.These studies unveiled the fact that several types of MOFs can remain intact in aqueous solutions and show exceptional capacities to remove pollutants from water compared to traditional adsorbents.Lately, MOFs has also been evaluated for defluoridation in water [23,49,50].
More researchers have focused on rare earth metals, and rare earth element hydrous oxides, for example, cerium and lanthanum and its salts, have strong affinity to fluoride, arsenate, and other anions [51].Cerium is investigated using first-principles calculations based on density functional theory to stabilize the structural and electronic properties [52].In our present study, Ce-1,1  -biphenyl-4,4  -dicarboxylic acid (Ce-bpdc), a novel type of metal organic framework and novelly defined adsorption for rapid removal of fluoride from aqueous solution, was prepared for the first time.Ce-MOFs adsorbents were prepared and used for fluoride removal, respectively.Ce-MOFs adsorbents were investigated by X-ray diffraction, scanning electron microscopy, Fourier transform infrared spectroscopy, and X-ray photoelectron spectroscopy.The as-prepared adsorbent was also used for treatment of underground water with high fluoride concentration in Yuefang, Jiangji, and Sanyi in Anhui province of China.Thus, it is desirable to develop low cost adsorbents which can efficiently work in a real sample.The maximum adsorption capacity and removal efficiency were studied and had most crucial part in adsorption.This study indicated that Ce-MOFs membrane could be developed into a very viable technology for highly effective removal of fluoride from drinking water.

Preparation of Disodium 4,4 󸀠 -Biphenylenedicarboxylate.
All chemicals used in this study were of pure analytical grade and used without further purification.5.0 g of 1,1  -biphenyl-4,4  -dicarboxylic acid was dissolved in 80 mL of water in a 250 mL beaker with vigorous stirring, and a uniform cloudy solution was obtained.Subsequently, a 10 M NaOH solution was added dropwise to the above mixed solution with vigorous stirring, until the precipitation disappeared.The mixture solution was then heated and crystallized.The obtained crystals were washed several times with Milli-Q water (18 MΩ) to yield the final product, disodium 4,4 biphenylenedicarboxylate.

Preparation of Ce-bpdc Coordination
Complex.Briefly, 3.0 g of disodium 4,4  -biphenylenedicarboxylate was placed into a 100 mL beaker, followed by the addition of an appropriate amount of water for complete dissolution under ultrasonic irradiation.Subsequently, the solution was stirred vigorously with a magnetic agitator.Ce(NO 3 ) 3 ⋅6H 2 O was dissolved in water and added slowly to the above solution, resulting in a turbid solution, which was stirred for 1 h.The resulting precipitate was centrifuged and washed several times with Milli-Q water and vacuum dried at 50 ∘ C for 24 h.

Characterization.
The products were characterized by X-ray diffraction (XRD, Shimadzu XRD-6000, with high-intensity Cu K radiation with a wavelength of 1.54178 Å), scanning electron microscopy (SEM, Hitachi S-4800, operated at 5 kV), and Fourier transform infrared (FT-IR, IRPrestige-21) spectroscopy.X-ray photoelectron spectroscopy (XPS, Thermo ESCALAB 250) was carried out using an Al-K 150 W nonmonochromatized X-ray source and a hemispherical energy analyzer at pass energy of 30 eV.

Detection of Fluoride Ions.
The experiments were performed using the fluoride ion-selective electrode method to detect F − .Because the electromotive force of the primary cell consisted of a fluoride ion-selective electrode and F − solution and the logarithm of F − activity had a linear relationship, the electromotive force was measured first, followed by the concentration of the F − solution.The standard curve method was applied to an analysis of the linear relation: a set of different concentrations of standard F − solutions were prepared before the concentration of the testing F − solution was examined.The linear formula could be obtained by checking the electromotive force of the primary cell consisting of an electrode and the standard F − solution.The concentration of the testing F − solution can then be obtained using the linear formula and the electromotive force of the primary cell consisted of electrode and the testing F − solution.The main steps of this method were to immerse the selective electrode into F − and constitute primary cell and measure the electromotive force of the primary cell using pH meter (model: PHS-3C).

Results and Discussion
3.1.Structure and Morphology.The crystal phase of the asprepared products was characterized by XRD.As shown in Figure 1(a), the XRD peaks of the Ce-bpdc were sharp and intense, indicating the high crystallinity of the Ce-bpdc, and there was a sharp characteristic peak of M-O at approximately 6 ∘ .However, it is hard for us to obtain the Ce-bpdc single crystals and the single crystal structure of the new metal organic compound Ce-bpdc cannot be determined by X-ray single crystal structure analysis.Figure 1(b) presents a SEM image of the sample.The as-prepared Ce-bpdc sample had micrometer-sized rod-like and sphere-like shapes.
FT-IR spectroscopy was used to confirm all the functional groups of Ce-bpdc.The FT-IR spectra of the dried Ce-bpdc and diphenyldicarboxylic acid were investigated, as shown in Figure 2. A comparison of the two spectra revealed a red shift in the spectrum of Ce-bpdc, and the characteristic peaks observed at 1584 cm −1 and 1528 cm −1 were assigned to the COO-Ce.The peak at 3400 cm −1 was ascribed to the -OH stretching mode in the coordination complex or H 2 O in the sample.The peak at 600-1000 cm −1 was assigned to the atomic vibration of benzene ring.
The surface compositions and chemical states of the asprepared Ce-bpdc were evaluated by XPS.The XPS survey spectrum in Figure 3 indicates the presence of Ce.The peaks at 882.0 eV, 884.0 eV, and 902.0 eV were assigned to Ce 4+ , Ce3d 5/2 , and Ce3d 3/2 , respectively, which indicates that the valence state of the majority of cerium is +3 and also contains a small amount of Ce 4+ .In the O1s spectrum, the peaks at 531.9 eV, 532.5 eV, and 532.9 eV were assigned to crystal lattice oxygen, and the peak at 533.9 eV was attributed to adsorbing oxygen in the sample.In the C1s spectrum, the peaks at 285.4 eV, 287.0 eV, and 289.0 eV were ascribed to C-OR, C-X, and MCO 3 , respectively, indicating the presence of CeCO 3 in Ce-bpdc.

Adsorption of pH Dependence.
Because OH − and F − are isoelectronic species but OH − is more active than F − , it can compete with F − during the adsorption process [53].As a result, most of the adsorbent material of F − needs to be used under acidic conditions, which is unavailable for drinking water.Therefore, it is essential to study materials that can Advances in Condensed Matter Physics remove F − under neutral solution conditions.In these experiments, the optimal pH for the removal of F − by Ce-bpdc was examined; the adsorption experiments were performed at pH = 3.0, 4.0, 5.0, 6.0, 7.0, and 8.0.As shown in Figure 4, when the pH was increased from 3.0 to 5.0, the adsorption capacity increased with increasing pH.On the other hand, the adsorption capacity decreased rapidly when the pH was higher than 6.0.From the above data, Ce-bpdc has the optimal adsorption capacity when the pH is ca.6.0.Therefore, all the following experiments were conducted at pH = 6.0.

Saturated Adsorption Capacity.
The curve of the adsorption capacity of F − with Ce-bpdc changed with various initial concentrations of solution F − .As shown in Figure 5, the adsorption capacity increased with increasing F − concentration.The experimental data was simulated using the Langmuir and Freundlich models, as shown in Figure 5 and Table 1.In Figure 5, the solid line is Langmuir model simulation, and the dashed line is the Freundlich model simulation.From Table 1, the Langmuir model can better represent the data ( 2 = 0.97515) than the Freundlich model ( 2 = 0.81688), indicating that the adsorption isotherms fit the Langmuir model.Therefore, the adsorption of F − is due mainly to chemical adsorption with physical adsorption as a complement.The maximum saturation capacity of Ce-bpdc towards F − is 45.5 mg/g.

Removal Efficiency.
If an adsorbent has high adsorption capacity, it does not mean that it has high removal efficiency.Therefore, for further analysis of the maximum adsorption capacity, more calculations are needed when the experiments are finished, which can determine if Ce-bpdc can remove fluoride ions from water efficiently.Figure 6 shows that the removal efficiency is greater than 80% when the F − concentration is no more than 35 mg/L.The concentration is below 1 ppm after treatment when the initial concentration of F − is 6 mg/L.This shows that the Ce-bpdc not only has high adsorption capacity but also has high removal efficiency.

Adsorption Kinetics.
A kinetic study was performed at pH = 6 utilizing a 12 mg/L F − solution and 1.0 g/L adsorbent in a water bath.8 mL of the mixed solution was taken out at a certain time.The adsorbent was separated by centrifuge.The concentration of fluoride ions was measured using a fluoride ion-selective electrode.Finally, the adsorption capacities were calculated at different times.Figure 7 shows the adsorption capacities at 2, 5, 10, 15, 20, 30, 60, and 120 min.From Figure 7, the reaction proceeded rapidly in the first 20 min, after which, the reaction reached equilibrium.A pseudo-second-order rate equation was used to simulate the kinetic adsorption as follows: where  2 (g/mg⋅h) is the pseudo-second-order rate constant of adsorption,   (mg/g) is the amount of F − adsorbed by the adsorbent at time  (min), and   (mg/g) is the equilibrium adsorption capacity. Figure 7 shows a linear plot of /  versus .The  2 value was calculated from the slope and the intercept was 0.067 g/mg⋅min.The correlation coefficient ( 2 > 0.999) for the linear plots is very close to 1, indicating that kinetic adsorption can be described very well using pseudo-second-order kinetics equation.

3.2.5.
Effect of HCO 3 − .In nature, there is a complex chemical environment in water, in which a variety of anions and cations can be found.Among them, the competition among similar type of ions, such as OH − , HCO chemical methods.For the removal of fluoride ions, OH − and HCO 3 − have great influence because OH − is more active than F − [54].Therefore, OH − has the greatest impact on the removal of F − .The influence of HCO 3 − has two main aspects: one is the competition with itself and the other is the hydrolysis of HCO 3 − , which can produce OH − and affect the removal efficiency of F − .To avoid the effects of OH − , the pH of the solution was adjusted to 6.0.In this study, the effects of HCO 3 − on the adsorption efficiency were also investigated.By adding 1.0 g/L of adsorbent to 3.0 mg/L of initial concentration of F − solution, the pH of the solution was adjusted to 6.0, and the solution was shaken for 24 h.The final results are shown in Figure 8.The removal efficiency was 84% when there was no HCO 3 − in the solution.On the hand, the removal efficiency decreased sharply to 40% when there was 50 mg/L HCO 3 − in solution.The removal efficiency decreased with increasing HCO 3 − concentration.

Study of Real Water Samples.
The real water samples from Yuefang, Jiangji, and Sanyi were used to check the application of the product.First, the pH of the real water samples was adjusted to 6.0, and 1 g/L of Ce-bpdc was then added.Table 2 lists all the experimental data; the initial concentrations of F − in the real water samples from Yuefang, Jiangji, and Sanyi were 3.69, 3.69, and 3.83 mg/L, respectively.After the reaction with Ce-bpdc for 24 h, the concentration of F − in the real water samples from Yuefang, Jiangji, and Sanyi decreased to 0.86, 1.29, and 1.88 mg/L, respectively, giving corresponding removal efficiencies of 78%, 64%, and 50%.Therefore, Ce-bpdc has high practical value in the removal of fluoride ions from real water samples.

Conclusions
Ce-bpdc, a novel type of metal organic framework, was synthesized by a facile simple chemical method.This study  investigated these factors affecting fluoride adsorption onto Ce-bpdc, including coexisting bicarbonate anions, pH value of fluoride solution, and contact time.The Langmuir isotherm mode fitted the equilibrium data well, and the maximum adsorption capacity was up to 45.5 mg/g for fluoride at 298 K and natural pH (6.0), and the removal efficiency was greater than 80%.The adsorption of F − was mainly due to chemical adsorption with physical adsorption as a complement.The tested results of ground water samples from three places, Yuefang, Jiangji, and Sanyi which exhibited high removal efficiency, indicated that the Ce-bpdc could be effectively employed as a promising defluoridating agent.

Table 1 :
Parameters for the Langmuir and Freundlich models.

Table 2 :
Treatment of real water samples by Ce-bpdc.