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Adsorption of fluoride from aqueous solution on magnesia-loaded fly ash cenospheres
Desalination 272 (2011) 233–239
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Desalination j o u r n a l h o m e p a g e : w w w. e l s ev i e r. c o m / l o c a t e / d e s a l
Adsorption of fluoride from aqueous solution on magnesia-loaded fly ash cenospheres Xiaotian Xu, Qin Li, Hao Cui, Jianfeng Pang, Li. Sun, Hao An, Jianping Zhai ⁎ State Key Laboratory of Pollution Control and Resource Reuse, School of the Environment, Nanjing University, Nanjing 210046, China
a r t i c l e
i n f o
Article history: Received 13 October 2010 Received in revised form 8 January 2011 Accepted 11 January 2011 Keywords: Adsorption Magnesia-loaded fly ash cenospheres Fly ash cenospheres Fluoride
a b s t r a c t A novel adsorbent, magnesia-loaded fly ash cenospheres (MLC), was prepared by wet impregnation of fly ash cenospheres with magnesium chloride solution. Its physicochemical properties were characterized by X-ray diffractometry, Fourier transform infrared spectrometry, scanning electron microscopy and X-ray fluorescence spectrometry. Adsorption experiments were conducted to test the effects of pH, adsorbent dosage, contact time, reaction temperature and coexisting anions on fluoride removal. The coexisting ions had a large impact on fluoride removal by MLC in order comprehensive N dihydric phosphate N nitrate N sulfate. The adsorption process fitted the Langmuir isotherm and the adsorption kinetics followed the pseudo-secondorder rate equation. The values of ΔG0 (318 K), ΔH0 and ΔS0 were − 0.409 kJ mol− 1, 20.04 kJ mol− 1 and 63.80 J mol− 1 K− 1, respectively. The maximum adsorption capacity of MLC was about 6.0 mg g− 1 in the solution with 100 mg L− 1 of fluoride ions at pH 3.0 and 318 K. MLC is low cost and more effective for fluoride adsorption so as to be used widely in wastewater treatment. © 2011 Elsevier B.V. All rights reserved.
1. Introduction Fluoride is attached to positively charged calcium in bones and teeth because of its strong electronegativity. Moderate fluoride (0.5– 1.5 mg L− 1) in drinking water is an essential micronutrient for the calcification of the dental enamel and bone formation [1]. However, the long-term intake of inappropriate fluoride not only causes dental and skeletal fluorosis, but may also lead to mutations in the user's deoxyribonucleic acid [2]. According to human health demand, The World Health Organization had set a limit to fluoride in drinking water and its limiting value is between 0.5 and 1.0 mg L− 1 [3]. US EPA established the effluent standard of 4 mg L− 1 for fluoride from the wastewater treatment plant [4]. The Environmental Protection Agency of China recommended a discharge standard I of 10 mg L− 1 for fluoride from the industrial wastewater. With the increase in the industrial activities, including pharmacy, fluorspar mining, semiconductor process, aluminum electrolysis, electroplating, generating electricity, rubber and fertilizer production, the excessive fluoride has been drained into water bodies. Water resources substitution is impossible, therefore, the removal of fluoride from aquatic environment is necessary. Various techniques of defluoridation, for instance, coagulation and precipitation [5], reverse osmosis [6], nanofiltration [7,8], electrodialysis and electrolysis [9– 14], membrane processes [15,16], ion-exchange [17,18], Donnan
⁎ Corresponding author. Tel./fax: +86 25 8359 2903. E-mail address: [email protected] (J. Zhai). 0011-9164/$ – see front matter © 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.desal.2011.01.028
dialysis [19] and adsorption, have been used to reduce the excess fluoride. The precipitation process has been widely used because it is economical and simple, but it often results in additional difficulties in eliminating excessive chemicals and the final fluoride concentration in the water depends greatly on the solubility of the precipitated fluoride and precipitation reagents. Furthermore, some high concentration residues in the treated water may create a danger to human health. Reverse osmosis, nanofiltration, electro dialysis, etc., have disadvantages in terms of maintenance cost and economic viability, thus they have not been broadly used for fluoride removal from water solution. By contrast, adsorption is arguably regarded as one of the most suitable techniques for the defluorination because it is economical, robust, environmentally benign and efficient. Many adsorbents have been reported to be effective for fluoride adsorption, such as magnesia-amended silicon dioxide [20], KMnO4-modified activated carbon [21], zirconium(IV)-impregnated collagen fiber [22], granular activated carbons coated with manganese oxides [23], alumimpregnated activated alumina [24], copper oxide incorporated mesoporous alumina [25], magnesia/chitosan composite [26], magnesia-amended activated alumina granules [27,28], hydrous manganese oxide-coated alumina [29], and more. Coal fly ash is one of the major industrial solid wastes and its amount is increasing year after year all over the world. The recycling of coal fly ash is, therefore, attracting the broad concern of the researchers. Fly ash cenospheres, obtained from coal fly ashes, as raw material for the preparation of an adsorbent, have lots of advantages. For example, the fly ash cenospheres are low cost; due to its small density, fly ash cenospheres can float on top of water and are easily
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recovered. In the present study, the magnesia-loaded fly ash cenospheres (MLC) were synthesized by impregnating magnesium chloride (MgCl2) solution on the fly ash cenospheres, and its adsorption performance in the fluoride removal from aqueous solution was investigated. 2. Materials and methods 2.1. Materials The cenospheres used in this study were obtained by screening coal fly ashes from the combustion of a Chinese soft coal. The coal fly ashes were collected from the third of four rows of electrostatic precipitator ash hoppers of No. 6 CFBC boiler unit at Sinopec Jinling Petrochemical Power Plant (Nanjing, China). All chemical reagents were of analytical grade. Deionized water was used in all experiments. An artificial fluoride stock solution was prepared by dissolving 2.21 g sodium fluoride solid granules in 1 L of deionized water and subsequently diluted to the required concentrations for the adsorption experiments. The adjustments for pH were done using HCl or NaOH.
2.4. Adsorption experiments Batch adsorption experiments were carried out to examine the effects of contact time, temperature, adsorption dose, coexisting anions and solution pH on the adsorption performance of MLC and to obtain equilibrium and kinetic data. Known quantities of adsorbent and fluoride solution were shaken in polyethene bottles in an oscillation incubator. At predetermined time intervals, the solutions were collected utilizing a 0.45 μm millipore filter and the fluoride concentration after the adsorption was determined at once using an Ion Chromatography ICS-1000 (Diana, USA). The amount of adsorption (qe) and removal efficiency (E) were calculated using the following equations:
qe =
E=
ðc0 −ce ÞV W c0 −ce × 100: c0
2.2. Preparation of the adsorbent
3. Results and discussion
The preparation of MLC was completed as follows: an excess of 100–150 μm fly ash cenospheres was washed in deionized water and the nitric acid solution to remove impurities. The cenospheres were dried at 378 K, and 30 g of the dried cenospheres was added to 100 mL of 0.6 mol L− 1 MgCl2 solution. The mixture was then settled in an incubator at 298 K for 48 h. After incubation, the excess solution was removed by filtration. The solid material was dried at 378 K, and subsequently calcined at 773 K for 4 h. The sample was then cooled to room temperature and transferred to airtight bottles for storage.
3.1. Properties of MLC and the cenospheres
2.3. Characterization of the adsorbent MLC and the cenospheres were characterized using several techniques. Surface morphology was observed with a S-3400 NII (Hitachi, Japan) scanning electron spectroscope (SEM). Fourier transform infrared (FT-IR) spectroscopy was performed on a Nexus 870 infrared spectrometer (Nicolet, USA) with 2 cm− 1 resolution. The crystalline phase composition was obtained using an X-ray diffractometer (XRD) (XRD-6000, Shimadzu, Japan) with Cu Kα radiation. A 9800XP + X-ray fluorescence spectrometer (XRF) (ARL, USA) was employed for analyzing the chemical composition. Table 1 shows that the percentage content of the magnesium oxide (MgO) in the cenospheres was low. After modification, the percentage content of MgO increased significantly in MLC.
Table 1 Chemical composition of fly ash cenospheres and MLC. Element as oxide
Fly ash cenospheres
MLC
MgO SiO2 Al2O3 Fe2O3 K2O CaO TiO2 Na2O P2O5 SO3 MnO ZnO CuO LOl⁎
0.89856 57.68967 29.54362 3.75557 2.87349 2.48648 0.93477 0.27446 0.07431 0.03151 0.02350 0.00677 0.00912 1.23400
11.22036 48.08131 25.84257 3.32275 2.88439 1.85839 0.84307 0.14325 0.05127 0.02558 0.02003 0.01179 0.01170 –
⁎ LOl, loss on ignition at 1233 K.
ð1Þ
ð2Þ
Fig. 1 illustrates SEM micrographs and EDAX spectrum of the cenospheres and MLC, respectively. The cenospheres (Fig. 1(a),(b), (c)) had a relatively uniform smooth surface and were hollow spheres with 100–150 μm diameters. In contrast, Fig. 1(d) and (e) demonstrated that MLC was velvet-like with a continuous and even distribution of Mg loaded on the surface of the cenospheres. Fig. 1 (f) and (g) further revealed that Mg was successfully loaded on the cenospheres during the modification process. This is completely consistent with the chemical composition of the cenospheres (Table 1). Velvet-like material was obviously fusion substances of Mg and the cenospheres. The XRD patterns of the cenospheres and MLC are shown in Fig. 2. In comparison to the JCPDS standard cards, the microstructures of the cenospheres (Fig. 2(a)) were mainly composed of the crystalline phases of mullite (Al6Si2O13) and sillimanite (Al2SiO5) in addition to the main amorphous glassy phase and quartz crystal (SiO2). Most diffraction peaks' intensity of Fig. 2(b) decreased to varying degrees. No peaks corresponded to Mg or any forms of Mg completely because there was some fusion between Mg and the cenospheres. The Fusion products were mostly magnesium–aluminum–silicate (MgAl2Si4O12) and pyrope (Mg3Al2(SiO4)3). Fig. 3 shows the FTIR spectra of the cenospheres and MLC. In Fig. 3 (a), the broad, strong band around 3449.7 cm− 1 was due to the stretching vibration of H2O and structural OH groups of the cenospheres, indicating the presence of surface hydroxyl groups and physically absorbed water. The bands at 1418.2 and 1636.4 cm− 1 indicated the production of N–O in the washing process of the cenospheres because of ion exchange. A sharp band at 1084.1 cm− 1 was related to the stretching vibrations of the Si–O groups, and the band at 458.5 cm− 1 is due to Si–O–Si bending vibrations. This is similar to the previous report by Eren [30]. The bands centered at 795.7 cm− 1 and around 553.4 cm− 1 represented Al–O bending vibrations and Fe–O stretching vibrations in the tetrahedral sheet, respectively. In Fig. 3(b), the sharp, strong band around 3411.1 cm− 1 could be attributed to Al–OH bond stretching, while the bands around 3246.2 cm− 1 and 3556.2 cm− 1 were due to the Mg–OH stretching vibrations. In Fig. 3(c), the band at 3446.8 cm− 1 indicated that the H2O-stretching vibration appeared again after adsorption, while the emergence of the sharp peak centered 3695.8 cm− 1 was due to the removal of the fluoride ions by replacing the hydroxyl ions.
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Fig. 1. SEM images of (a) fly ash cenospheres, (b) the whole cenospheres, (c) the broken cenosphheres, (d) MLC under high magnification (e) MLC under low magnification; EDAX spectrum of (f) fly ash cenospheres and (g) MLC.
3.2. Effect of initial pH Generally, pH is a key factor affecting fluoride adsorption at the water adsorbent interface [31]. Fig. 4 presents the fluoride adsorption on MLC over the pH range of 3–11. The adsorption capacity fell gradually along with the increase of the initial pH in the fluoride solution. The maximum fluoride adsorption was observed at pH 3.0, because in the surface of MLC there were more H+ ions, which led to the greater adsorption of the fluoride. In other words, the maximum fluoride removal was attributed to highly protonated MLC surface and the gradual increase in the attractive forces between the positively charged surface and the negatively charged fluoride ions. In the acidic medium, the reasons for the decrease of the fluoride adsorption are the change in the surface charge of MLC and the decrease of the H+ ions and fluoride ions in solution. The reduction of the fluoride removal in alkaline medium is due to the increasing electrostatic repulsion between the fluoride ions and the negatively charged surface sites of MLC. Another cause for the decrease in the alkaline
Fig. 2. XRD pattern of (a) fly ash cenospheres and (b) MLC.
medium is the competition between the OH− ions and the fluoride ions for the adsorption sites, because fluoride is similar to OH− in the charge and ionic radii. 3.3. Amount of adsorbent The effect of the adsorbent dosage on the fluoride adsorption on MLC is shown in Fig. 5. The fluoride removal capacity of MLC increased from 0.9 to 2.4 mg g− 1 with the increase of the adsorbent dosage ranging from 1.0 to 2.5 g L− 1, while it decreased from 2.4 to 1.8 mg g− 1 with the increase of the adsorbent dosage ranging from 2.5 to 5.0 g L− 1. Therefore, 2.5 g L− 1 MLC was used in all the experiments as the optimum adsorbent dose. The increase in the sorbent dosage at a changeless fluoride concentration leads to the decrease of the fluoride adsorption, because there are too many adsorbent doses for the limited amount of fluoride. The removal efficiency increased with the dosage because of more active sites from an increase in the amount of the adsorbent.
Fig. 3. FT-IR spectra of (a) fly ash cenospheres; (b) MLC before adsorption and (c) MLC after adsorption.
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Fig. 4. Effect of initial pH on the adsorption of fluoride (T = 298 K; W = 2.5 g L− 1; c0 = 10 mg L− 1; t = 1440 min).
Fig. 6. Adsorption of fluoride on MLC as a function of contact time and the initial fluoride concentration (pH = 3.0; T = 298 K; W = 2.5 g L− 1).
3.4. Effect of time and the fluoride concentration
10 mg L− 1 while the initial concentrations of the coexisting anions were 5, 10 and 20 mg L− 1. As a whole, the decrease of the fluoride adsorption was in order comprehensive N dihydric phosphate N nitrate N sulfate. This decreased defluorination may be due to the lower affinity of MLC for fluoride adsorption, or due to a competition between the fluoride ions and the coexisting anions for the active sites on the sorbent surfaces. Their competition capacity is usually decided by the change of the pH in solution or the concentration, charge and size of the anions, or the combination of both of them.
Fig. 6 illustrates the amount of fluoride ions adsorbed by MLC as a function of the contact time in the range of 5–480 min. The initial fluoride concentrations in solution are 5, 10 and 20 mg L− 1, respectively. It was seen that at all concentrations the adsorption capacity increased with time up to about 60 min and then the curves became quite level, indicating the attainment of the adsorption equilibrium. The equilibrium time is usually independent of the fluoride concentration, therefore, 60 min is fixed as minimum contact time for the maximum fluoride adsorption on MLC. If the adsorption process was only controlled by an ion exchange mechanism, equilibrium time would be short [32]. It suggested that the process, fluoride removal by MLC, was governed by complicated adsorption. The adsorption capacity increased with the initial fluoride concentration in solution from 5 to 20 mg L− 1. 3.5. Effect of coexisting anions
3.6. Adsorption kinetics The dynamics of sorption describes the rate of solute uptake, which in turn governs the duration time of the adsorption reaction. Two adsorption kinetic models, pseudo-first-order and pseudosecond-order models, were studied by using the data obtained from the fluoride adsorption on MLC at different time intervals. The equations are expressed as [33]:
The effect of the coexisting anions on fluoride removal by MLC is presented in Fig. 7. The coexisting anions studied here include sulfate, nitrate, dihydric phosphate and comprehensive (the mixture of sulfate, nitrate and dihydric phosphate; the ratio of concentration is 1:2:2). The initial fluoride concentration in solution was fixed at
log ðqe −qt Þ = log qe −k1 t
ð3Þ
t 1 t = + : qt qe k2 q2e
ð4Þ
Fig. 5. Adsorption of fluoride on MLC as a function of the adsorbent dosage (T = 298 K; c0 = 10 mg L− 1; pH = 3.0; t = 1440 min).
Fig. 7. Effect of coexisting anions on the fluoride adsorption capacity (pH = 3.0; T = 298 K; W = 2.5 g L− 1; c0 = 10 mg L− 1; t = 1440 min).
X. Xu et al. / Desalination 272 (2011) 233–239
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The values of k1 can be determined from the slope of the linear plot of log (qe − qt) versus t, and k2 can be calculated from the slope and intercept of the linear plot t/qt versus t. The linear plots of two kinetic models are presented in Fig. 8 and Fig. 9, respectively. The values of k1, k2, qe and the correlation coefficient (r2) from the linear plots are shown in Table 2. The pseudosecond-order linear plots resulted in higher r2 values than the pseudofirst-order. The values of qe (cal) from the pseudo-second-order were more close to qe (exp) than that from the pseudo-first-order. These indicated the better applicability of the pseudo-second-order model. As reported previously, the system of the fluoride adsorption on MLC is a chemisorption process, involving chemical bonding between adsorbent active sites and adsorbate valance forces [34]. 3.7. Adsorption isotherms The adsorption isotherms help in deciding the feasibility of the sorbents for removing the fluoride from water and express the specific relation between the fluoride concentration and its degree of accumulation onto the adsorbent surface at constant temperature. The most classic and commonly applied isotherms in solid/liquid systems are Langmuir and Freundlich. They not only provide the general idea of the adsorbent effectiveness in removing the fluoride, but also indicate the maximum amount of fluoride ions that will be adsorbed by the adsorbents. The Langmuir isotherm is applicable to the homogeneous sorption where the sorption of each sorbate molecule onto the surface has an equal sorption activation energy. The Langmuir equation is [35]: qe =
q m k a ce : 1 + k a ce
ð5Þ
It can also be replaced by the linearized equation: ce 1 1 = c + : qe qm e ka qm
ð6Þ
The values of qm and ka are calculated from the slope and the intercept of the linear plots ce/qe versus ce. The most important multisided adsorption isotherm for the heterogeneous surfaces is the Freundlich model, which is described by Ref. [35]:
Fig. 9. Pseudo-second-order plots for fluoride adsorption by MLC (pH = 3.0; T = 298 K; W = 2.5 g L− 1).
and is represented by the following linearized expression: ln qe = ln kF +
1 ln ce : n
ð8Þ
The values of kF and n were obtained from the intercept and slope of the linear plot of ln qe versus ln ce. The linearization of the Langmuir isotherm for the fluoride adsorption on MLC at different temperatures are presented in Fig. 10. The correlation coefficient (r2) obtained from different isotherm models are given in Table 3. The Langmuir model showed significantly higher correlations (r2 N 0.98) than the Freundlich model, and was selected as the most appropriate model. In the isotherm studies, the optimization procedure requires an error function to be defined in order to be able to evaluate the fit of the isotherm to the experimental equilibrium data. The χ2 test statistic is basically the sum of the squares of the differences between the experimental data and the calculation data. If the calculation data is similar to the experimental data, χ2 will be a small number, while if they are different, χ2 will be a large number. The equivalent mathematical statement is 1 0 qe −qe;m @ A: χ =∑ qe;m 2
1=n
qe = kF ce
ð7Þ
ð9Þ
The result of the χ2-analysis is presented in Table 3. The lower χ2values of the Langmuir isotherm indicated the better applicability for the fluoride sorption on MLC. In order to further find out the feasibility of the isotherm, the essential characteristics of the Langmuir isotherm can be described by an equilibrium parameter, RL [36], defined as: RL =
1 : 1 + k a c0
ð10Þ
Table 2 Parameters and correlation coefficients of two kinetics models. qe (expa)
c0
Pseudo-first-order k1
Fig. 8. Pseudo-first-order plots for fluoride adsorption by MLC (pH = 3.0; T = 298 K; W = 2.5 g L− 1).
5
1.5648
10
1.7920
20
1.9754
a
qe (cala)
Pseudo-second-order r2
0.0082 0.67 0.9643 Y = − 0.0082x − 0.1764 0.0094 0.71 0.9883 Y = − 0.0094x − 0.1502 0.0107 0.64 0.9246 Y = − 0.0107x − 0.1903
Exp, experimental data; cal, calculation data.
k2
qe (cala)
r2
0.0263 1.9055 0.9902 Y = 0.5248x + 10.4528 0.0393 1.9368 0.9928 Y = 0.5163x + 6.7759 0.0376 2.2417 0.9961 Y = 0.4461x + 5.2851
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X. Xu et al. / Desalination 272 (2011) 233–239 Table 4 Equilibrium parameter RL values of the Langmuir isotherm at initial fluoride concentrations 5–100 mg L− 1. c0 RL
298 K 308 K 318 K
5
10
20
30
50
100
0.6076 0.5528 0.5082
0.4363 0.3820 0.3406
0.2790 0.2367 0.2053
0.2051 0.1708 0.1469
0.1341 0.1100 0.0936
0.0718 0.0582 0.0491
the system with the changes in the hydration of the adsorbing fluoride ions, showed the sorption process was irreversible and stable. 4. Conclusions
Fig. 10. Linearization of the Langmuir isotherm for fluoride removal (pH = 3.0; W = 2.5 g L− 1; t = 1440 min).
The value of RL indicates the shape of the isotherms to be either unfavorable (RL N 1), linear (RL = 1), favorable (0 b RL b 1) or irreversible (RL = 0) [37]. RL values (Table 4) calculated from the present system are within 0 and 1, which indicate a favorable adsorption process. The fluoride adsorption on MLC was more favorable at higher concentrations. 3.8. Thermodynamic treatment of sorption process Temperature has a major influence on the sorption process. The fluoride sorption on MLC was monitored at three different temperatures 298, 308, and 318 K under the optimized condition. Thermodynamic parameters, viz., ΔG0, ΔH0 and ΔS0 (Table 5) were calculated according to the thermodynamic equations, which are described by Ref. [38]:
ln K0 =
ΔS0 ΔH 0 − R RT
ð11Þ
0
ΔG = −RT ln K0 :
ð12Þ
ΔH0 and ΔS0 are determined from the slope and intercept of the plot of ln K0 versus 1/T. The thermodynamic equilibrium constant (K0) is determined by the proposed Stephen and Sulochana method [39], namely, K0 equals to qm × ka of the Langmuir isotherm. The negative value of ΔG0 at 318 K confirmed the spontaneous nature of the fluoride sorption on MLC, while the positive values of ΔG0 at 298 K and 308 K indicated the passive reaction. The value of ΔH0 was positive, indicating that the sorption reaction was endothermic. The positive value of ΔS0, indicating the increased disorder in
The fluoride adsorption from the aqueous solutions by MLC depended on contact time, pH of the solution, reaction temperature, adsorbent dosage and the initial fluoride concentration. The optimum reaction conditions were contact time (60 min), pH 3.0 and adsorbent dosage (2.5 mg L− 1). The coexisting ions had a large impact on the fluoride sorption on MLC and the decrease of the fluoride adsorption was in order comprehensive N dihydric phosphate N nitrate N sulfate. Experimental data was well fitted to the pseudo-second order kinetic model and followed the Langmuir isotherm. The Gibb's free energy changes (ΔG0) showed that the fluoride removal by MLC was the spontaneous nature process at 318 K while non-spontaneous at 298 K and 308 K. The maximum adsorption capacity was about 6.0 mg g− 1 in the solution with 100 mg L− 1 of the fluoride ions at pH 3.0 and 318 K. MLC, which was prepared by using coal fly ashes as raw material, is low cost and has considerable fluoride adsorption capacity, thus it will be a potential candidate for the fluoride removal from wastewater. Effective regeneration of MLC and cost–benefit analysis of its application will be studied in future experiments. Symbols c0 ce V W qe E qt qm qe,m ka k1 k2 kF n t ΔH0 ΔS0 ΔG0 T R K0
the initial fluoride concentration, mg L− 1 the equilibrium fluoride concentration, mg L− 1 the volume of solution, L the mass of dry adsorbent, g the amount of adsorption at equilibrium, mg g− 1 the removal efficiency, % the amount of adsorption at time t, mg g− 1 the theoretical maximum adsorption capacity, mg g− 1 the equilibrium adsorption capacity obtained by calculating from the model, mg g− 1 the Langmuir isotherm constant, L mg− 1 the rate constant of pseudo-first-order model, min− 1 the rate constant of pseudo-second-order model, g mg− 1 min− 1 the Freundlich empirical constant the Freundlich empirical constant time, min the standard enthalpy change, kJ mol− 1 the standard entropy change, J mol− 1 K− 1 th standard free energy change, kJ mol− 1 temperature in Kelvin, K the universal gas constant, 8.314 J mol− 1 K− 1 the thermodynamic equilibrium constant, L g− 1
Table 3 Comparison of correlation coefficients and χ2-values of the Langmuir and Freundlich isotherms. T
Langmuir r2
Equation
Table 5 Thermodynamic parameters of fluoride adsorption onto MLC.
Freundlich χ2
r2
Equation
χ2
298 0.9868 Y=0.1959x+1.5165 0.1252 0.9728 Y=0.2636x+0.3924 0.7543 308 0.9900 Y=0.1887x+1.1665 0.1648 0.9683 Y=0.2257x+0.6020 0.7202 318 0.9912 Y=0.1658x+0.8565 0.1202 0.9287 Y=0.1860x+0.8843 0.7848
Anion
F−
K0 298 K
308 K
318 K
0.6594
0.8573
1.1675
ΔH0
ΔS0
ΔG0 298 K
308 K
318 K
20.04
63.80
1.032
0.394
− 0.409
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Desalination j o u r n a l h o m e p a g e : w w w. e l s ev i e r. c o m / l o c a t e / d e s a l
Adsorption of fluoride from aqueous solution on magnesia-loaded fly ash cenospheres Xiaotian Xu, Qin Li, Hao Cui, Jianfeng Pang, Li. Sun, Hao An, Jianping Zhai ⁎ State Key Laboratory of Pollution Control and Resource Reuse, School of the Environment, Nanjing University, Nanjing 210046, China
a r t i c l e
i n f o
Article history: Received 13 October 2010 Received in revised form 8 January 2011 Accepted 11 January 2011 Keywords: Adsorption Magnesia-loaded fly ash cenospheres Fly ash cenospheres Fluoride
a b s t r a c t A novel adsorbent, magnesia-loaded fly ash cenospheres (MLC), was prepared by wet impregnation of fly ash cenospheres with magnesium chloride solution. Its physicochemical properties were characterized by X-ray diffractometry, Fourier transform infrared spectrometry, scanning electron microscopy and X-ray fluorescence spectrometry. Adsorption experiments were conducted to test the effects of pH, adsorbent dosage, contact time, reaction temperature and coexisting anions on fluoride removal. The coexisting ions had a large impact on fluoride removal by MLC in order comprehensive N dihydric phosphate N nitrate N sulfate. The adsorption process fitted the Langmuir isotherm and the adsorption kinetics followed the pseudo-secondorder rate equation. The values of ΔG0 (318 K), ΔH0 and ΔS0 were − 0.409 kJ mol− 1, 20.04 kJ mol− 1 and 63.80 J mol− 1 K− 1, respectively. The maximum adsorption capacity of MLC was about 6.0 mg g− 1 in the solution with 100 mg L− 1 of fluoride ions at pH 3.0 and 318 K. MLC is low cost and more effective for fluoride adsorption so as to be used widely in wastewater treatment. © 2011 Elsevier B.V. All rights reserved.
1. Introduction Fluoride is attached to positively charged calcium in bones and teeth because of its strong electronegativity. Moderate fluoride (0.5– 1.5 mg L− 1) in drinking water is an essential micronutrient for the calcification of the dental enamel and bone formation [1]. However, the long-term intake of inappropriate fluoride not only causes dental and skeletal fluorosis, but may also lead to mutations in the user's deoxyribonucleic acid [2]. According to human health demand, The World Health Organization had set a limit to fluoride in drinking water and its limiting value is between 0.5 and 1.0 mg L− 1 [3]. US EPA established the effluent standard of 4 mg L− 1 for fluoride from the wastewater treatment plant [4]. The Environmental Protection Agency of China recommended a discharge standard I of 10 mg L− 1 for fluoride from the industrial wastewater. With the increase in the industrial activities, including pharmacy, fluorspar mining, semiconductor process, aluminum electrolysis, electroplating, generating electricity, rubber and fertilizer production, the excessive fluoride has been drained into water bodies. Water resources substitution is impossible, therefore, the removal of fluoride from aquatic environment is necessary. Various techniques of defluoridation, for instance, coagulation and precipitation [5], reverse osmosis [6], nanofiltration [7,8], electrodialysis and electrolysis [9– 14], membrane processes [15,16], ion-exchange [17,18], Donnan
⁎ Corresponding author. Tel./fax: +86 25 8359 2903. E-mail address: [email protected] (J. Zhai). 0011-9164/$ – see front matter © 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.desal.2011.01.028
dialysis [19] and adsorption, have been used to reduce the excess fluoride. The precipitation process has been widely used because it is economical and simple, but it often results in additional difficulties in eliminating excessive chemicals and the final fluoride concentration in the water depends greatly on the solubility of the precipitated fluoride and precipitation reagents. Furthermore, some high concentration residues in the treated water may create a danger to human health. Reverse osmosis, nanofiltration, electro dialysis, etc., have disadvantages in terms of maintenance cost and economic viability, thus they have not been broadly used for fluoride removal from water solution. By contrast, adsorption is arguably regarded as one of the most suitable techniques for the defluorination because it is economical, robust, environmentally benign and efficient. Many adsorbents have been reported to be effective for fluoride adsorption, such as magnesia-amended silicon dioxide [20], KMnO4-modified activated carbon [21], zirconium(IV)-impregnated collagen fiber [22], granular activated carbons coated with manganese oxides [23], alumimpregnated activated alumina [24], copper oxide incorporated mesoporous alumina [25], magnesia/chitosan composite [26], magnesia-amended activated alumina granules [27,28], hydrous manganese oxide-coated alumina [29], and more. Coal fly ash is one of the major industrial solid wastes and its amount is increasing year after year all over the world. The recycling of coal fly ash is, therefore, attracting the broad concern of the researchers. Fly ash cenospheres, obtained from coal fly ashes, as raw material for the preparation of an adsorbent, have lots of advantages. For example, the fly ash cenospheres are low cost; due to its small density, fly ash cenospheres can float on top of water and are easily
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recovered. In the present study, the magnesia-loaded fly ash cenospheres (MLC) were synthesized by impregnating magnesium chloride (MgCl2) solution on the fly ash cenospheres, and its adsorption performance in the fluoride removal from aqueous solution was investigated. 2. Materials and methods 2.1. Materials The cenospheres used in this study were obtained by screening coal fly ashes from the combustion of a Chinese soft coal. The coal fly ashes were collected from the third of four rows of electrostatic precipitator ash hoppers of No. 6 CFBC boiler unit at Sinopec Jinling Petrochemical Power Plant (Nanjing, China). All chemical reagents were of analytical grade. Deionized water was used in all experiments. An artificial fluoride stock solution was prepared by dissolving 2.21 g sodium fluoride solid granules in 1 L of deionized water and subsequently diluted to the required concentrations for the adsorption experiments. The adjustments for pH were done using HCl or NaOH.
2.4. Adsorption experiments Batch adsorption experiments were carried out to examine the effects of contact time, temperature, adsorption dose, coexisting anions and solution pH on the adsorption performance of MLC and to obtain equilibrium and kinetic data. Known quantities of adsorbent and fluoride solution were shaken in polyethene bottles in an oscillation incubator. At predetermined time intervals, the solutions were collected utilizing a 0.45 μm millipore filter and the fluoride concentration after the adsorption was determined at once using an Ion Chromatography ICS-1000 (Diana, USA). The amount of adsorption (qe) and removal efficiency (E) were calculated using the following equations:
qe =
E=
ðc0 −ce ÞV W c0 −ce × 100: c0
2.2. Preparation of the adsorbent
3. Results and discussion
The preparation of MLC was completed as follows: an excess of 100–150 μm fly ash cenospheres was washed in deionized water and the nitric acid solution to remove impurities. The cenospheres were dried at 378 K, and 30 g of the dried cenospheres was added to 100 mL of 0.6 mol L− 1 MgCl2 solution. The mixture was then settled in an incubator at 298 K for 48 h. After incubation, the excess solution was removed by filtration. The solid material was dried at 378 K, and subsequently calcined at 773 K for 4 h. The sample was then cooled to room temperature and transferred to airtight bottles for storage.
3.1. Properties of MLC and the cenospheres
2.3. Characterization of the adsorbent MLC and the cenospheres were characterized using several techniques. Surface morphology was observed with a S-3400 NII (Hitachi, Japan) scanning electron spectroscope (SEM). Fourier transform infrared (FT-IR) spectroscopy was performed on a Nexus 870 infrared spectrometer (Nicolet, USA) with 2 cm− 1 resolution. The crystalline phase composition was obtained using an X-ray diffractometer (XRD) (XRD-6000, Shimadzu, Japan) with Cu Kα radiation. A 9800XP + X-ray fluorescence spectrometer (XRF) (ARL, USA) was employed for analyzing the chemical composition. Table 1 shows that the percentage content of the magnesium oxide (MgO) in the cenospheres was low. After modification, the percentage content of MgO increased significantly in MLC.
Table 1 Chemical composition of fly ash cenospheres and MLC. Element as oxide
Fly ash cenospheres
MLC
MgO SiO2 Al2O3 Fe2O3 K2O CaO TiO2 Na2O P2O5 SO3 MnO ZnO CuO LOl⁎
0.89856 57.68967 29.54362 3.75557 2.87349 2.48648 0.93477 0.27446 0.07431 0.03151 0.02350 0.00677 0.00912 1.23400
11.22036 48.08131 25.84257 3.32275 2.88439 1.85839 0.84307 0.14325 0.05127 0.02558 0.02003 0.01179 0.01170 –
⁎ LOl, loss on ignition at 1233 K.
ð1Þ
ð2Þ
Fig. 1 illustrates SEM micrographs and EDAX spectrum of the cenospheres and MLC, respectively. The cenospheres (Fig. 1(a),(b), (c)) had a relatively uniform smooth surface and were hollow spheres with 100–150 μm diameters. In contrast, Fig. 1(d) and (e) demonstrated that MLC was velvet-like with a continuous and even distribution of Mg loaded on the surface of the cenospheres. Fig. 1 (f) and (g) further revealed that Mg was successfully loaded on the cenospheres during the modification process. This is completely consistent with the chemical composition of the cenospheres (Table 1). Velvet-like material was obviously fusion substances of Mg and the cenospheres. The XRD patterns of the cenospheres and MLC are shown in Fig. 2. In comparison to the JCPDS standard cards, the microstructures of the cenospheres (Fig. 2(a)) were mainly composed of the crystalline phases of mullite (Al6Si2O13) and sillimanite (Al2SiO5) in addition to the main amorphous glassy phase and quartz crystal (SiO2). Most diffraction peaks' intensity of Fig. 2(b) decreased to varying degrees. No peaks corresponded to Mg or any forms of Mg completely because there was some fusion between Mg and the cenospheres. The Fusion products were mostly magnesium–aluminum–silicate (MgAl2Si4O12) and pyrope (Mg3Al2(SiO4)3). Fig. 3 shows the FTIR spectra of the cenospheres and MLC. In Fig. 3 (a), the broad, strong band around 3449.7 cm− 1 was due to the stretching vibration of H2O and structural OH groups of the cenospheres, indicating the presence of surface hydroxyl groups and physically absorbed water. The bands at 1418.2 and 1636.4 cm− 1 indicated the production of N–O in the washing process of the cenospheres because of ion exchange. A sharp band at 1084.1 cm− 1 was related to the stretching vibrations of the Si–O groups, and the band at 458.5 cm− 1 is due to Si–O–Si bending vibrations. This is similar to the previous report by Eren [30]. The bands centered at 795.7 cm− 1 and around 553.4 cm− 1 represented Al–O bending vibrations and Fe–O stretching vibrations in the tetrahedral sheet, respectively. In Fig. 3(b), the sharp, strong band around 3411.1 cm− 1 could be attributed to Al–OH bond stretching, while the bands around 3246.2 cm− 1 and 3556.2 cm− 1 were due to the Mg–OH stretching vibrations. In Fig. 3(c), the band at 3446.8 cm− 1 indicated that the H2O-stretching vibration appeared again after adsorption, while the emergence of the sharp peak centered 3695.8 cm− 1 was due to the removal of the fluoride ions by replacing the hydroxyl ions.
X. Xu et al. / Desalination 272 (2011) 233–239
235
Fig. 1. SEM images of (a) fly ash cenospheres, (b) the whole cenospheres, (c) the broken cenosphheres, (d) MLC under high magnification (e) MLC under low magnification; EDAX spectrum of (f) fly ash cenospheres and (g) MLC.
3.2. Effect of initial pH Generally, pH is a key factor affecting fluoride adsorption at the water adsorbent interface [31]. Fig. 4 presents the fluoride adsorption on MLC over the pH range of 3–11. The adsorption capacity fell gradually along with the increase of the initial pH in the fluoride solution. The maximum fluoride adsorption was observed at pH 3.0, because in the surface of MLC there were more H+ ions, which led to the greater adsorption of the fluoride. In other words, the maximum fluoride removal was attributed to highly protonated MLC surface and the gradual increase in the attractive forces between the positively charged surface and the negatively charged fluoride ions. In the acidic medium, the reasons for the decrease of the fluoride adsorption are the change in the surface charge of MLC and the decrease of the H+ ions and fluoride ions in solution. The reduction of the fluoride removal in alkaline medium is due to the increasing electrostatic repulsion between the fluoride ions and the negatively charged surface sites of MLC. Another cause for the decrease in the alkaline
Fig. 2. XRD pattern of (a) fly ash cenospheres and (b) MLC.
medium is the competition between the OH− ions and the fluoride ions for the adsorption sites, because fluoride is similar to OH− in the charge and ionic radii. 3.3. Amount of adsorbent The effect of the adsorbent dosage on the fluoride adsorption on MLC is shown in Fig. 5. The fluoride removal capacity of MLC increased from 0.9 to 2.4 mg g− 1 with the increase of the adsorbent dosage ranging from 1.0 to 2.5 g L− 1, while it decreased from 2.4 to 1.8 mg g− 1 with the increase of the adsorbent dosage ranging from 2.5 to 5.0 g L− 1. Therefore, 2.5 g L− 1 MLC was used in all the experiments as the optimum adsorbent dose. The increase in the sorbent dosage at a changeless fluoride concentration leads to the decrease of the fluoride adsorption, because there are too many adsorbent doses for the limited amount of fluoride. The removal efficiency increased with the dosage because of more active sites from an increase in the amount of the adsorbent.
Fig. 3. FT-IR spectra of (a) fly ash cenospheres; (b) MLC before adsorption and (c) MLC after adsorption.
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X. Xu et al. / Desalination 272 (2011) 233–239
Fig. 4. Effect of initial pH on the adsorption of fluoride (T = 298 K; W = 2.5 g L− 1; c0 = 10 mg L− 1; t = 1440 min).
Fig. 6. Adsorption of fluoride on MLC as a function of contact time and the initial fluoride concentration (pH = 3.0; T = 298 K; W = 2.5 g L− 1).
3.4. Effect of time and the fluoride concentration
10 mg L− 1 while the initial concentrations of the coexisting anions were 5, 10 and 20 mg L− 1. As a whole, the decrease of the fluoride adsorption was in order comprehensive N dihydric phosphate N nitrate N sulfate. This decreased defluorination may be due to the lower affinity of MLC for fluoride adsorption, or due to a competition between the fluoride ions and the coexisting anions for the active sites on the sorbent surfaces. Their competition capacity is usually decided by the change of the pH in solution or the concentration, charge and size of the anions, or the combination of both of them.
Fig. 6 illustrates the amount of fluoride ions adsorbed by MLC as a function of the contact time in the range of 5–480 min. The initial fluoride concentrations in solution are 5, 10 and 20 mg L− 1, respectively. It was seen that at all concentrations the adsorption capacity increased with time up to about 60 min and then the curves became quite level, indicating the attainment of the adsorption equilibrium. The equilibrium time is usually independent of the fluoride concentration, therefore, 60 min is fixed as minimum contact time for the maximum fluoride adsorption on MLC. If the adsorption process was only controlled by an ion exchange mechanism, equilibrium time would be short [32]. It suggested that the process, fluoride removal by MLC, was governed by complicated adsorption. The adsorption capacity increased with the initial fluoride concentration in solution from 5 to 20 mg L− 1. 3.5. Effect of coexisting anions
3.6. Adsorption kinetics The dynamics of sorption describes the rate of solute uptake, which in turn governs the duration time of the adsorption reaction. Two adsorption kinetic models, pseudo-first-order and pseudosecond-order models, were studied by using the data obtained from the fluoride adsorption on MLC at different time intervals. The equations are expressed as [33]:
The effect of the coexisting anions on fluoride removal by MLC is presented in Fig. 7. The coexisting anions studied here include sulfate, nitrate, dihydric phosphate and comprehensive (the mixture of sulfate, nitrate and dihydric phosphate; the ratio of concentration is 1:2:2). The initial fluoride concentration in solution was fixed at
log ðqe −qt Þ = log qe −k1 t
ð3Þ
t 1 t = + : qt qe k2 q2e
ð4Þ
Fig. 5. Adsorption of fluoride on MLC as a function of the adsorbent dosage (T = 298 K; c0 = 10 mg L− 1; pH = 3.0; t = 1440 min).
Fig. 7. Effect of coexisting anions on the fluoride adsorption capacity (pH = 3.0; T = 298 K; W = 2.5 g L− 1; c0 = 10 mg L− 1; t = 1440 min).
X. Xu et al. / Desalination 272 (2011) 233–239
237
The values of k1 can be determined from the slope of the linear plot of log (qe − qt) versus t, and k2 can be calculated from the slope and intercept of the linear plot t/qt versus t. The linear plots of two kinetic models are presented in Fig. 8 and Fig. 9, respectively. The values of k1, k2, qe and the correlation coefficient (r2) from the linear plots are shown in Table 2. The pseudosecond-order linear plots resulted in higher r2 values than the pseudofirst-order. The values of qe (cal) from the pseudo-second-order were more close to qe (exp) than that from the pseudo-first-order. These indicated the better applicability of the pseudo-second-order model. As reported previously, the system of the fluoride adsorption on MLC is a chemisorption process, involving chemical bonding between adsorbent active sites and adsorbate valance forces [34]. 3.7. Adsorption isotherms The adsorption isotherms help in deciding the feasibility of the sorbents for removing the fluoride from water and express the specific relation between the fluoride concentration and its degree of accumulation onto the adsorbent surface at constant temperature. The most classic and commonly applied isotherms in solid/liquid systems are Langmuir and Freundlich. They not only provide the general idea of the adsorbent effectiveness in removing the fluoride, but also indicate the maximum amount of fluoride ions that will be adsorbed by the adsorbents. The Langmuir isotherm is applicable to the homogeneous sorption where the sorption of each sorbate molecule onto the surface has an equal sorption activation energy. The Langmuir equation is [35]: qe =
q m k a ce : 1 + k a ce
ð5Þ
It can also be replaced by the linearized equation: ce 1 1 = c + : qe qm e ka qm
ð6Þ
The values of qm and ka are calculated from the slope and the intercept of the linear plots ce/qe versus ce. The most important multisided adsorption isotherm for the heterogeneous surfaces is the Freundlich model, which is described by Ref. [35]:
Fig. 9. Pseudo-second-order plots for fluoride adsorption by MLC (pH = 3.0; T = 298 K; W = 2.5 g L− 1).
and is represented by the following linearized expression: ln qe = ln kF +
1 ln ce : n
ð8Þ
The values of kF and n were obtained from the intercept and slope of the linear plot of ln qe versus ln ce. The linearization of the Langmuir isotherm for the fluoride adsorption on MLC at different temperatures are presented in Fig. 10. The correlation coefficient (r2) obtained from different isotherm models are given in Table 3. The Langmuir model showed significantly higher correlations (r2 N 0.98) than the Freundlich model, and was selected as the most appropriate model. In the isotherm studies, the optimization procedure requires an error function to be defined in order to be able to evaluate the fit of the isotherm to the experimental equilibrium data. The χ2 test statistic is basically the sum of the squares of the differences between the experimental data and the calculation data. If the calculation data is similar to the experimental data, χ2 will be a small number, while if they are different, χ2 will be a large number. The equivalent mathematical statement is 1 0 qe −qe;m @ A: χ =∑ qe;m 2
1=n
qe = kF ce
ð7Þ
ð9Þ
The result of the χ2-analysis is presented in Table 3. The lower χ2values of the Langmuir isotherm indicated the better applicability for the fluoride sorption on MLC. In order to further find out the feasibility of the isotherm, the essential characteristics of the Langmuir isotherm can be described by an equilibrium parameter, RL [36], defined as: RL =
1 : 1 + k a c0
ð10Þ
Table 2 Parameters and correlation coefficients of two kinetics models. qe (expa)
c0
Pseudo-first-order k1
Fig. 8. Pseudo-first-order plots for fluoride adsorption by MLC (pH = 3.0; T = 298 K; W = 2.5 g L− 1).
5
1.5648
10
1.7920
20
1.9754
a
qe (cala)
Pseudo-second-order r2
0.0082 0.67 0.9643 Y = − 0.0082x − 0.1764 0.0094 0.71 0.9883 Y = − 0.0094x − 0.1502 0.0107 0.64 0.9246 Y = − 0.0107x − 0.1903
Exp, experimental data; cal, calculation data.
k2
qe (cala)
r2
0.0263 1.9055 0.9902 Y = 0.5248x + 10.4528 0.0393 1.9368 0.9928 Y = 0.5163x + 6.7759 0.0376 2.2417 0.9961 Y = 0.4461x + 5.2851
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X. Xu et al. / Desalination 272 (2011) 233–239 Table 4 Equilibrium parameter RL values of the Langmuir isotherm at initial fluoride concentrations 5–100 mg L− 1. c0 RL
298 K 308 K 318 K
5
10
20
30
50
100
0.6076 0.5528 0.5082
0.4363 0.3820 0.3406
0.2790 0.2367 0.2053
0.2051 0.1708 0.1469
0.1341 0.1100 0.0936
0.0718 0.0582 0.0491
the system with the changes in the hydration of the adsorbing fluoride ions, showed the sorption process was irreversible and stable. 4. Conclusions
Fig. 10. Linearization of the Langmuir isotherm for fluoride removal (pH = 3.0; W = 2.5 g L− 1; t = 1440 min).
The value of RL indicates the shape of the isotherms to be either unfavorable (RL N 1), linear (RL = 1), favorable (0 b RL b 1) or irreversible (RL = 0) [37]. RL values (Table 4) calculated from the present system are within 0 and 1, which indicate a favorable adsorption process. The fluoride adsorption on MLC was more favorable at higher concentrations. 3.8. Thermodynamic treatment of sorption process Temperature has a major influence on the sorption process. The fluoride sorption on MLC was monitored at three different temperatures 298, 308, and 318 K under the optimized condition. Thermodynamic parameters, viz., ΔG0, ΔH0 and ΔS0 (Table 5) were calculated according to the thermodynamic equations, which are described by Ref. [38]:
ln K0 =
ΔS0 ΔH 0 − R RT
ð11Þ
0
ΔG = −RT ln K0 :
ð12Þ
ΔH0 and ΔS0 are determined from the slope and intercept of the plot of ln K0 versus 1/T. The thermodynamic equilibrium constant (K0) is determined by the proposed Stephen and Sulochana method [39], namely, K0 equals to qm × ka of the Langmuir isotherm. The negative value of ΔG0 at 318 K confirmed the spontaneous nature of the fluoride sorption on MLC, while the positive values of ΔG0 at 298 K and 308 K indicated the passive reaction. The value of ΔH0 was positive, indicating that the sorption reaction was endothermic. The positive value of ΔS0, indicating the increased disorder in
The fluoride adsorption from the aqueous solutions by MLC depended on contact time, pH of the solution, reaction temperature, adsorbent dosage and the initial fluoride concentration. The optimum reaction conditions were contact time (60 min), pH 3.0 and adsorbent dosage (2.5 mg L− 1). The coexisting ions had a large impact on the fluoride sorption on MLC and the decrease of the fluoride adsorption was in order comprehensive N dihydric phosphate N nitrate N sulfate. Experimental data was well fitted to the pseudo-second order kinetic model and followed the Langmuir isotherm. The Gibb's free energy changes (ΔG0) showed that the fluoride removal by MLC was the spontaneous nature process at 318 K while non-spontaneous at 298 K and 308 K. The maximum adsorption capacity was about 6.0 mg g− 1 in the solution with 100 mg L− 1 of the fluoride ions at pH 3.0 and 318 K. MLC, which was prepared by using coal fly ashes as raw material, is low cost and has considerable fluoride adsorption capacity, thus it will be a potential candidate for the fluoride removal from wastewater. Effective regeneration of MLC and cost–benefit analysis of its application will be studied in future experiments. Symbols c0 ce V W qe E qt qm qe,m ka k1 k2 kF n t ΔH0 ΔS0 ΔG0 T R K0
the initial fluoride concentration, mg L− 1 the equilibrium fluoride concentration, mg L− 1 the volume of solution, L the mass of dry adsorbent, g the amount of adsorption at equilibrium, mg g− 1 the removal efficiency, % the amount of adsorption at time t, mg g− 1 the theoretical maximum adsorption capacity, mg g− 1 the equilibrium adsorption capacity obtained by calculating from the model, mg g− 1 the Langmuir isotherm constant, L mg− 1 the rate constant of pseudo-first-order model, min− 1 the rate constant of pseudo-second-order model, g mg− 1 min− 1 the Freundlich empirical constant the Freundlich empirical constant time, min the standard enthalpy change, kJ mol− 1 the standard entropy change, J mol− 1 K− 1 th standard free energy change, kJ mol− 1 temperature in Kelvin, K the universal gas constant, 8.314 J mol− 1 K− 1 the thermodynamic equilibrium constant, L g− 1
Table 3 Comparison of correlation coefficients and χ2-values of the Langmuir and Freundlich isotherms. T
Langmuir r2
Equation
Table 5 Thermodynamic parameters of fluoride adsorption onto MLC.
Freundlich χ2
r2
Equation
χ2
298 0.9868 Y=0.1959x+1.5165 0.1252 0.9728 Y=0.2636x+0.3924 0.7543 308 0.9900 Y=0.1887x+1.1665 0.1648 0.9683 Y=0.2257x+0.6020 0.7202 318 0.9912 Y=0.1658x+0.8565 0.1202 0.9287 Y=0.1860x+0.8843 0.7848
Anion
F−
K0 298 K
308 K
318 K
0.6594
0.8573
1.1675
ΔH0
ΔS0
ΔG0 298 K
308 K
318 K
20.04
63.80
1.032
0.394
− 0.409
X. Xu et al. / Desalination 272 (2011) 233–239
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