Shuangxi Wen,
Yili Wang* and
Shuoxun Dong
Beijing Forestry University, Beijing 100083, People's Republic of China. E-mail: wangyilimail@126.com; Fax: +86-10-62336596; Tel: +86-10-62336673
First published on 15th October 2015
Excessive fluoride in water can cause a series of health problems. In this study, a three-element adsorbent (γ-Fe2O3–graphite–La, MGLNP) was successfully developed for fluoride adsorption. Magnetic graphite nanoparticles (MGNP) were synthesized through the chemical deposition of Fe3O4 nanoparticles onto nanographite powder (NG) under alkaline conditions. The MGLNP was synthesized by immersing the MGNP in a saturated solution of La(NO3)3·6H2O and calcining at 300 °C for 3 h in a muffle furnace. These nanoparticles featured a quick and easy separation process with the saturation magnetization of 26.66 emu g−1. The surface charge of the MGLNP was highly pH-dependent, the corresponding pHpzc was approximately 7.9. The fluoride adsorption isotherm of the MGLNP could be well described by the Langmuir equation, and the maximum adsorption capacity was about 77.12 mg g−1 at 25 °C and pH = 7 ± 0.1. The kinetics of the fluoride adsorption was described by a pseudo first-order rate law. As the pH decreased, the fluoride adsorption capacity of the MGLNP continuously increased. The effects of the co-existing anions indicated that the anions had minimal effect on the fluoride adsorption. After six cycles of reuse, the MGLNP could still maintain 77.54% adsorption capacity. On the basis of the zeta potential analyses, FTIR spectroscopy, and XPS measurements, ion exchange, combination of La3+ and F−, as well as electrostatic attraction could be observed in the fluoride adsorption process.
Various technologies, such as ion exchange, precipitation, nanofiltration, electrocoagulation, and adsorption, have been employed to remove fluoride from water.5 Precipitation and adsorption are the most widely used techniques for defluorination. Precipitation is convenient and cheap, but its dosage requirement is high; moreover, the addition of alkali, calcium, and magnesium increases the alkalinity of the treated water, and the final concentration of fluoride usually exceeds the permissible limit.6 Adsorption is regarded as one of the most effective, economical, and environmentally friendly defluoridation techniques because of its high selectivity, easy handling, and low operating cost.7
For these reasons, several natural, modified, and synthetic adsorbents, such as activated alumina, bone char,8 Fe–Al–Ce trimetal oxide adsorbent,9 zeolite,10 biochar,11 hydrous zirconium oxide,12 metal ion-loaded fibrous protein,13 and Fe–Mg–La,1 have been employed to remove fluoride ions from water. However, these materials have certain disadvantages, including low adsorption capacity, separation difficulty, narrow available pH range, and poor mechanical strength. Thus, developing an effective adsorbent that features high fluoride removal efficiency and easy separation is necessary.
In recent years, a number of studies have proved that the rare earth element lanthanum (La) loaded on a fluoride adsorbent can increase fluoride adsorption capacity.14 Cheng et al.15 reported that a La3+-modified activated alumina adsorbent for the removal of fluoride from water had a significantly higher maximum adsorption capacity (qmax = 6.70 mg g−1) than an activated alumina (qmax = 2.74 mg g−1). Yu et al.16 proposed an innovative La-modified carbon (LMC) adsorbent rooted in Sargassum sp. for fluoride removal; the maximum adsorption capacity of the LMC adsorbent could reach 94.34 mg g−1 at a neutral pH and is thus substantially higher than that of many commercial adsorbent.
In addition, magnetic adsorbent materials have become a famous research focus in recent years. Magnetic carbon materials, such as magnetic-activated carbon nanoparticles17 and magnetic carbon nanotubes,18 have been widely developed as adsorbents for separating contaminants from aqueous solutions. Unlike traditional adsorbents, magnetic adsorbents feature a quick and easy separation process that does not require additional centrifugation or filtration procedures; these adsorbents also avoid time-consuming column passing operations encountered in the traditional solid phase extraction process.19
The aim of the present study is to develop and characterize a nanographite (NG)-loaded magnetic γ-Fe2O3 nanoparticle and La(III) adsorbent (MGLNP) for the removal of fluoride from drinking water. The characteristics of the adsorbent were analyzed using zeta potential analysis, field emission scanning electron microscopy, vibrating sample magnetometer (VSM), Fourier transform infrared spectroscopy (FTIR), and X-ray photoelectron spectroscopy (XPS). Batch experiments that considered adsorption kinetics, equilibrium, and the effects of initial pH, fluoride concentration, and coexisting anions on fluoride removal were also conducted to evaluate the adsorption performance of the MGLNP adsorbent.
The dried magnetic graphite was immersed in 200 mL saturated La(NO3)3·6H2O solution for 24 h under 160 rpm stirring in an air bath thermostat oscillator. The product was magnetically separated and calcined at 300 °C for 3 h in a muffle furnace. Upon cooling down to room temperature, the product was washed with distilled water thrice. The adsorbent was dried at 60 °C for further use.
The kinetic study was conducted under 25 °C, 35 °C, and 45 °C and pH 7.0 ± 0.1 at different time intervals with an initial fluoride concentration of 9.88 mg L−1. Samples were taken at predetermined times and immediately separated under a magnetic field for the residual F− measurement.
To further investigate the characteristics of the adsorption kinetics of the MGLNP, the pseudo first-order model, pseudo second-order model and the Elovich equation were used to fit the adsorption kinetics data.13
Pseudo first-order model:
log(qe − qt) = log![]() | (1) |
Pseudo second-order model:
t/qt = 1/(K2qe2) + t/qe | (2) |
Elovich equation:
qt = ln(αβ)/β + (ln![]() | (3) |
The apparent adsorption activation energy was calculated with the Arrhenius equation. The Arrhenius equation is given as follows:
![]() | (4) |
Taking the logarithm on the both sides forms the following equation:
![]() | (5) |
Isotherm experiments were carried out with initial F− concentrations from 1.97 mg L−1 to 244.6 mg L−1 at 25 °C, 35 °C, and 45 °C under pH 7.0 ± 0.1. The F− absorbed MGLNP was magnetically separated from the solution after the polyethylene vessels were shaken for 24 h, and the residual F− was analyzed. Moreover, La in the solution was analyzed with the method reported by Dong et al.20
To quantify the fluoride adsorption capacity of the adsorbent and to describe the adsorption mechanism, Langmuir, Temkin and Freundlich equations were adopted in defining the fluoride adsorption isotherm of the MGLNP.
The Langmuir equation is given as follows:
Qe = QmKLCe/(1 + KLCe) | (6) |
Temkin equation:
Qe = A + B![]() ![]() | (7) |
The Freundlich equation, which is indicative of the surface heterogeneity of the adsorbent, is given as follows:
Qe = KfCe1/n | (8) |
To evaluate thermodynamic feasibility and further analyze the nature of the adsorption process, the distribution constant K0 obtained at the mentioned temperatures was utilized in the computation of the three basic thermodynamic parameters using the following equations:21
![]() | (9) |
ΔG° = −RT![]() ![]() | (10) |
![]() | (11) |
The effect of equilibrium pH on F− adsorption was investigated by adjusting the solution pH to 4.0–10.0 at 25 °C under an initial F− concentration of 9.88 mg L−1 according to the procedure of the adsorption isotherm experiment.
The effects of co-existing anions (phosphate, arsenate, nitrate, chloride, and sulfate) on fluoride adsorption were investigated by performing fluoride adsorption under a fixed adsorbent dose of 200 mg L−1, initial fluoride concentration of 9.88 mg L−1 at 25 °C, solution pH of 7.0 ± 0.1, and a series of target anion concentrations. The reproducibility of the results was confirmed by performing all experiments in triplicate.
The defluorinated MGLNP was collected for the experiment by repeating the aforementioned adsorption and desorption procedures. The adsorbents were reused for six cycles in this study.
![]() | ||
Fig. 1 Physical properties of MGNP or MGLNP: (a) magnetization curves; (b) zeta potential vs. pH of MGLNP at 25 °C. |
As shown in Fig. 1b, the value of pHpzc of the MGLNP adsorbent was approximately 7.9, which indicated that the surface charge of the adsorbent was highly pH-dependent. When pH > 7.9, the zeta potentials were negative and further decreased as pH increased. This condition produced a strong electrostatic repulsion between the adsorbent and the fluoride anions. By contrast, the surface charge became positive when the solution pH was below pHpzc, which is conducive to the adsorption of fluoride. Compared with other fluoride adsorbents, such as the La-modified carbon (pHpzc = 6.12)16 and Fe–Al–Ce adsorbent (pHpzc = 7.5),9 MGLNP has a higher pHpzc, which equates to a wide pH range applicable in fluoride adsorption.
In ESI,† the SEM characterization results showed the layered structure of the NG (Fig. S1a†). After γ-Fe2O3 and La were loaded on the surface of the NG, the adsorbent particles became an amorphous structure comprising small, aggregated particles (Fig. S1c†). The size of the MGLNP is shown in Fig. S1c.† As revealed by the EDX analysis (Fig. S2†), La was successfully immobilized on the surface of the adsorbent, and the atomic ratio of La, Fe, and C in the adsorbent was La:
Fe
:
C = 1
:
1.3
:
3.3. The XRD spectra (Fig. S3†) also show that γ-Fe2O3 and La were successfully loaded on the NG.
![]() | ||
Fig. 2 Adsorption kinetics of fluoride on MGLNP (initial fluoride concentration = 9.88 mg L−1, adsorbent dosage = 200 mg L−1, natural pH, 25 °C). |
Three models were used to fit the adsorption kinetic curves in Fig. 2. The kinetic data under the three different temperatures were best fitted by the pseudo first-order adsorption rate model. The corresponding model parameters are provided in Table 1. K1 increased with temperature. A large K1 value usually indicates a fast adsorption rate.16 The theoretical and experimental qe values agreed well, implying that the fluoride adsorbed on the MGLNP underwent chemical adsorption, with the adsorption mechanism being the rate-controlling step.22 The apparent adsorption activation energy Ea = 19.73 kJ mol−1. This result indicated that the adsorption process occurred because of chemisorptions.23
Temperature (°C) | Pseudo-first-order | Pseudo-second-order | Elovich | |||||||
---|---|---|---|---|---|---|---|---|---|---|
qe,exp (mg g−1) | qe,cal (mg g−1) | K1 (min−1) | R2 | qe,cal (mg g−1) | K2 (L mg−1 min−1) | R2 | α (mg g−1 min−1) | β (g mg−1) | R2 | |
25 | 17.63 | 16.83 | 0.32 | 0.99 | 16.59 | 0.035 | 0.94 | 14.58 | 0.33 | 0.77 |
35 | 20.50 | 20.34 | 0.41 | 0.99 | 20.08 | 0.050 | 0.95 | 24.07 | 0.29 | 0.71 |
45 | 24.07 | 22.29 | 0.53 | 1.00 | 22.84 | 0.083 | 0.97 | 24![]() |
0.59 | 0.70 |
![]() | ||
Fig. 3 Fluoride adsorption isotherm at different temperatures (adsorbent dosage = 200 mg L−1, pH = 7 ± 0.1, equilibrium time: 24 h). |
The Freundlich, Langmuir and Temkin models were fit to describe the experimental adsorption results. The Langmuir parameters Qm and KL were calculated from the slope and intercept of the linear plots of 1/Qe versus 1/Ce. The values of the Freundlich parameters Kf and 1/n were obtained from the slope and intercept of the linear Freundlich plot of logQe versus log
Ce. The corresponding determination coefficients (R2) are presented in Table 2.
Temperature (°C) | Langmuir | Freundlich | Temkin | |||||||
---|---|---|---|---|---|---|---|---|---|---|
Qe,exp (mg g−1) | Qm (mg g−1) | KL (L mg−1) | R2 | 1/n | Kf (L mg−1) | R2 | A | B | R2 | |
25 | 72.33 | 77.12 | 0.038 | 0.99 | 0.49 | 6.29 | 0.95 | −0.38 | 29.72 | 0.97 |
35 | 79.00 | 87.27 | 0.041 | 0.99 | 0.47 | 8.04 | 0.96 | 2.50 | 32.51 | 0.96 |
45 | 94.67 | 100.90 | 0.045 | 0.99 | 0.46 | 10.00 | 0.96 | 5.76 | 36.84 | 0.97 |
In terms of the determination coefficients (R2), the Langmuir equation is more suitable to describe adsorption behavior than the Freundlich model. The maximal adsorption capacities for fluoride calculated with the Langmuir model were 77.12, 87.27, and 100.90 mg g−1 at 25 °C, 35 °C, and 45 °C, respectively. The concentration of La in the solution after fluoride adsorption was 0.12 mg L−1, which was much lower than 0.79 mg L−1 reported by Li et al.24
According to the KL values at different temperatures, the thermodynamic parameters were determined with eqn (9)–(11). The results are shown in Table 3.
Temperature (°C) | ΔG° (kJ mol−1) | ΔH° (kJ mol−1) | ΔS° (J mol−1 K−1) |
---|---|---|---|
25 | −2.53 | 21.13 | 79.34 |
35 | −3.24 | ||
45 | −4.12 |
A negative ΔG° confirmed the feasibility and spontaneous nature of the fluoride adsorption.13 The ΔG° value became increasingly negative as the temperature rose, thus indicating that the extent of spontaneity is proportional to temperature and that a high temperature facilitates the adsorption process.21 The temperature-dependent ΔH° value was 21.13 kJ mol−1. The positive ΔH° indicated the endothermic nature of the adsorption process25 and confirmed that the intensity of the adsorption process was enhanced at high temperatures.21 The positive ΔS° indicated the affinity of the adsorbent to fluoride ions and suggested an increased randomness at the solid/solution interface during the adsorption of fluoride ions onto the MGLNP. These findings may be related to the release of water of hydration during the adsorption process, which increased the randomness of the system.25
![]() | ||
Fig. 4 Effect of aqueous phase pH on the fluoride adsorption of MGLNP (initial fluoride concentration = 9.88 mg L−1, adsorbent dosage = 200 mg L−1, 25 °C). |
The drop in fluoride removal efficiency vs. pH may be explained as follows. The hydroxyl ions in a solution gradually increase with solution pH value,26 and hydroxyl ions have a similar ion radii with fluoride ions. Furthermore, it was found that the final pH of the solution was significantly higher than the initial pH, thus indicating that a number of hydroxide ions were released into the solution.14 Hence, the inhibition of fluoride adsorption mainly resulted from the competition for surface adsorption sites because of increasing hydroxyl ions. The changes in the pH-dependent electrostatic force existing between the adsorbent surface and fluoride also affected fluorine adsorption efficiency. A low pH favors the protonation of adsorbent surfaces. Enhanced protonation generates a high number of positively charged sites per unit surface area (Fig. 1b). This condition increases the electrostatic attraction force between a positively charged surface and negative fluoride ions, thus increasing the amount of adsorption at low pH values.12 In an acidic pH range (pH < 5), weak hydrofluoric acid is present in the solution; as pH increases, the fluoride with F− as the main existing form may affect defluoridation.27
![]() | ||
Fig. 5 Effect of coexisting anions on fluoride adsorption of MGLNP (initial fluoride concentration = 9.88 mg L−1, adsorbent dosage = 200 mg L−1, natural pH, 25 °C). |
As presented in Fig. 5, sulfate and chloride did not significantly interfere with the fluoride removal process even at a concentration of 100 mg L−1, whereas nitrate and carbonate showed adverse effects when the concentration reached 100 mg L−1. When the concentration of these anions was at 100 mg L−1, nitrate significantly reduced the fluoride adsorption among the anions on the MGLNP. Fluoride adsorption decreased from 18.27 mg g−1 to 11.33 mg g−1 as nitrate concentration increased from 0 mg L−1 to 100 mg L−1. The effects followed the decreasing order NO3− > CO32− > SO42− > Cl−.
As presented in Fig. 6a, a small amount of fluoride was released from the loaded adsorbent at a pH lower than 11.0. However, desorption efficiencies were 95.85% and 97.42% when the pH was increased to 12.0 and 13.0 (corresponding to NaOH concentrations of 0.4 and 4.0 g L−1, respectively), respectively. Considering the dosage of NaOH, pH = 12.5 was selected for the desorption of fluoride in the reuse experiment. The MGLNP adsorbent performed well after reuse (Fig. 6b). It maintained an adsorption capacity of 77.54% at the sixth cycle of fluoride adsorption.
In ESI,† the cost analysis of MGLNP adsorbent and comparison with other adsorbents were presented in Tables S1 and S2,† respectively. The results indicated that MGLNP adsorbent had a potential for fluoride removal from drinking water.
The stability experiments of MGLNP were carried out according to the procedure in Section 4 of the ESI.† Fig. S4 and Table S3† show that MGLNP had a stable performance in terms of magnetic separation and fluoride adsorption capacity when it was treated for 8 days with different pH values or temperatures.
After fluoride removal, the peaks at 1384 cm−1 almost disappeared; the peaks at 1434 and 1489 cm−1 became very weak and shifted to 1436 and 1473 cm−1, respectively, thus indicating that an interaction occurred between the adsorbents and fluoride during the adsorption process.31 Hence the –COOH and carbonate groups were partly lost after fluoride adsorption.26 In addition, the peak area at 3422 cm−1 (assigned to –OH) decreased after adsorption, thus indicating that the hydroxyl on the adsorbent participated in the fluoride adsorption process. Therefore, the surface ion exchange process based on –COOH, carbonate ions (CO32−), and surface hydroxyl ions (OH−) exchanged with F− may occur in the fluoride removal process.
A high-resolution scan of the O 1s spectrum of the fresh adsorbent is shown in Fig. 9a. The scan can be divided into three component peaks at 530.50, 532.04, and 533.44 eV, which can be assigned to metal oxide (M − O), hydroxyl group bonded to metal (M − OH), and carbonate group (C–O) in the adsorbent,16 respectively. As shown in Table 4, after fluoride adsorption, the relative area ratio for the peak could be attributed to the increase of M − O from 27.25% to 38.41%. By contrast, the relative areas for the peaks attributed to M − OH and C–O decreased from 41.62% to 33.37% and from 31.13% to 28.22%, respectively. The decreases suggested the participation of the O–H group in the fluoride adsorption process through the ion exchange process. The same result was achieved in the FTIR analysis.
Sample | Peak | B.E.a (eV) | FWHMb (eV) | G![]() ![]() |
Percentd (%) |
---|---|---|---|---|---|
a Binding energy (B.E.).b Full width at half maximum (FWHM).c Gaussian![]() ![]() |
|||||
Before adsorption | M–O | 530.50 | 1.42 | 30![]() ![]() |
27.25 |
O–H | 532.04 | 1.73 | 30![]() ![]() |
41.62 | |
C–O | 533.44 | 2.03 | 30![]() ![]() |
31.13 | |
After adsorption | M–O | 530.43 | 1.53 | 30![]() ![]() |
38.41 |
O–H | 532.12 | 1.38 | 30![]() ![]() |
33.37 | |
C–O | 533.32 | 2.35 | 30![]() ![]() |
28.22 |
On the basis of the comprehensive analysis of the FTIR and XPS results, the possible mechanism for the fluoride removal of the MGLNP could be conjectured as the surface ion exchange process based on –COOH, carbonate ions (CO32−), and surface hydroxyl ions (OH−) exchanged with F− and the combination of La3+ and F−, as described in Fig. 10.
Moreover, the fluoride removal data were estimated through quantitatively calculating the [OH−] increase and the loaded La3+ on the surface of MGLNP. The corresponding results were shown in Table S4,† which implied that surface La3+ complexation mechanism played an important role in the fluoride removal by MGLNP.
Footnote |
† Electronic supplementary information (ESI) available. See DOI: 10.1039/c5ra15215a |
This journal is © The Royal Society of Chemistry 2015 |