Adsorption of fluoride from aqueous solution on magnesia-loaded fly ash cenospheres

Adsorption of ?uoride from aqueous solution on magnesia-loaded ?y 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 b s t r a c t

a r t i c l e i n f o Article history:

Received 13October 2010

Received in revised form 8January 2011Accepted 11January 2011

Keywords:Adsorption

Magnesia-loaded ?y ash cenospheres Fly ash cenospheres Fluoride

A novel adsorbent,magnesia-loaded ?y ash cenospheres (MLC),was prepared by wet impregnation of ?y 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 ?uores-cence spectrometry.Adsorption experiments were conducted to test the effects of pH,adsorbent dosage,contact time,reaction temperature and coexisting anions on ?uoride removal.The coexisting ions had a large impact on ?uoride removal by MLC in order comprehensive N dihydric phosphate N nitrate N sulfate.The adsorption process ?tted the Langmuir isotherm and the adsorption kinetics followed the pseudo-second-order rate equation.The values of ΔG 0(318K),ΔH 0and ΔS 0were ?0.409kJ mol ?1,20.04kJ mol ?1and 63.80Jmol ?1K ?1,respectively.The maximum adsorption capacity of MLC was about 6.0mg g ?1in the solution with 100mg L ?1of ?uoride ions at pH 3.0and 318K.MLC is low cost and more effective for ?uoride adsorption so as to be used widely in wastewater treatment.

?2011Elsevier B.V.All rights reserved.

1.Introduction

Fluoride is attached to positively charged calcium in bones and teeth because of its strong electronegativity.Moderate ?uoride (0.5–1.5mg L ?1)in drinking water is an essential micronutrient for the calci ?cation of the dental enamel and bone formation [1].However,the long-term intake of inappropriate ?uoride not only causes dental and skeletal ?uorosis,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 ?uoride in drinking water and its limiting value is between 0.5and 1.0mg L ?1[3].US EPA established the ef ?uent standard of 4mg L ?1for ?uoride from the wastewater treatment plant [4].The Environmental Protection Agency of China recommended a discharge standard I of 10mg L ?1for ?uoride from the industrial wastewater.

With the increase in the industrial activities,including pharmacy,?uorspar mining,semiconductor process,aluminum electrolysis,electroplating,generating electricity,rubber and fertilizer production,the excessive ?uoride has been drained into water bodies.Water resources substitution is impossible,therefore,the removal of ?uoride from aquatic environment is necessary.Various techniques of de ?uoridation,for instance,coagulation and precipitation [5],reverse osmosis [6],nano ?ltration [7,8],electrodialysis and electrolysis [9–14],membrane processes [15,16],ion-exchange [17,18],Donnan

dialysis [19]and adsorption,have been used to reduce the excess ?uoride.The precipitation process has been widely used because it is economical and simple,but it often results in additional dif ?culties in eliminating excessive chemicals and the ?nal ?uoride concentration in the water depends greatly on the solubility of the precipitated ?uoride and precipitation reagents.Furthermore,some high concen-tration residues in the treated water may create a danger to human health.Reverse osmosis,nano ?ltration,electro dialysis,etc.,have disadvantages in terms of maintenance cost and economic viability,thus they have not been broadly used for ?uoride removal from water solution.By contrast,adsorption is arguably regarded as one of the most suitable techniques for the de ?uorination because it is economical,robust,environmentally benign and ef ?cient.Many adsorbents have been reported to be effective for ?uoride adsorption,such as magnesia-amended silicon dioxide [20],KMnO 4-modi ?ed activated carbon [21],zirconium(IV)-impregnated collagen ?ber [22],granular activated carbons coated with manganese oxides [23],alum-impregnated activated alumina [24],copper oxide incorporated mesoporous alumina [25],magnesia/chitosan composite [26],mag-nesia-amended activated alumina granules [27,28],hydrous manga-nese oxide-coated alumina [29],and more.

Coal ?y 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 ?y ash is,therefore,attracting the broad concern of the researchers.Fly ash cenospheres,obtained from coal ?y ashes,as raw material for the preparation of an adsorbent,have lots of advantages.For example,the ?y ash cenospheres are low cost;due to its small density,?y ash cenospheres can ?oat on top of water and are easily

Desalination 272(2011)233–239

?Corresponding author.Tel./fax:+862583592903.E-mail address:jpzhai@https://www.360docs.net/doc/806819333.html, (J.

Zhai).

0011-9164/$–see front matter ?2011Elsevier B.V.All rights reserved.doi:

10.1016/j.desal.2011.01.028

Contents lists available at ScienceDirect

Desalination

j o u r n a l h o me p a g e :w w w.e l sev i e r.c om /l oc a te /de sa l

recovered.In the present study,the magnesia-loaded?y ash ceno-spheres(MLC)were synthesized by impregnating magnesium chloride (MgCl2)solution on the?y ash cenospheres,and its adsorption performance in the?uoride removal from aqueous solution was investigated.

2.Materials and methods

2.1.Materials

The cenospheres used in this study were obtained by screening coal?y ashes from the combustion of a Chinese soft coal.The coal?y ashes were collected from the third of four rows of electrostatic precipitator ash hoppers of No.6CFBC 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 arti?cial?uoride stock solution was prepared by dissolving2.21g sodium?uoride solid granules in1L of deionized water and subsequently diluted to the required concentrations for the adsorp-tion experiments.The adjustments for pH were done using HCl or NaOH.

2.2.Preparation of the adsorbent

The preparation of MLC was completed as follows:an excess of 100–150μm?y ash cenospheres was washed in deionized water and the nitric acid solution to remove impurities.The cenospheres were dried at378K,and30g of the dried cenospheres was added to100mL of0.6mol L?1MgCl2solution.The mixture was then settled in an incubator at298K for48h.After incubation,the excess solution was removed by?ltration.The solid material was dried at378K,and subsequently calcined at773K for4h.The sample was then cooled to room temperature and transferred to airtight bottles for storage.

2.3.Characterization of the adsorbent

MLC and the cenospheres were characterized using several techniques.Surface morphology was observed with a S-3400NII (Hitachi,Japan)scanning electron spectroscope(SEM).Fourier transform infrared(FT-IR)spectroscopy was performed on a Nexus 870infrared spectrometer(Nicolet,USA)with2cm?1resolution.The crystalline phase composition was obtained using an X-ray diffrac-tometer(XRD)(XRD-6000,Shimadzu,Japan)with Cu Kαradiation.A 9800XP+X-ray?uorescence spectrometer(XRF)(ARL,USA)was employed for analyzing the chemical composition.Table1shows that the percentage content of the magnesium oxide(MgO)in the cenospheres was low.After modi?cation,the percentage content of MgO increased signi?cantly in MLC.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?uoride solution were shaken in polyethene bottles in an oscillation incubator.At predetermined time intervals,the solutions were collected utilizing a0.45μm millipore?lter and the?uoride concentration after the adsorption was determined at once using an Ion Chromatography ICS-1000(Diana,USA).The amount of adsorp-tion(q e)and removal ef?ciency(E)were calculated using the following equations:

q e=

c0?c e

eTV

W

e1TE=

c0?c e

c0

×100:e2T

3.Results and discussion

3.1.Properties of MLC and the cenospheres

Fig.1illustrates 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 with100–150μm diameters.In contrast,Fig.1(d)and(e)demon-strated 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 modi?cation process.This is completely consistent with the chemical composition of the cenospheres (Table1).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.3shows the FTIR spectra of the cenospheres and MLC.In Fig.3 (a),the broad,strong band around3449.7cm?1was due to the stretching vibration of H2O and structural OH groups of the ceno-spheres,indicating the presence of surface hydroxyl groups and physically absorbed water.The bands at1418.2and1636.4cm?1 indicated the production of N–O in the washing process of the cenospheres because of ion exchange.A sharp band at1084.1cm?1 was related to the stretching vibrations of the Si–O groups,and the band at458.5cm?1is due to Si–O–Si bending vibrations.This is similar to the previous report by Eren[30].The bands centered at 795.7cm?1and around553.4cm?1represented Al–O bending vibrations and Fe–O stretching vibrations in the tetrahedral sheet, respectively.In Fig.3(b),the sharp,strong band around3411.1cm?1 could be attributed to Al–OH bond stretching,while the bands around 3246.2cm?1and3556.2cm?1were due to the Mg–OH stretching vibrations.In Fig.3(c),the band at3446.8cm?1indicated that the H2O-stretching vibration appeared again after adsorption,while the emergence of the sharp peak centered3695.8cm?1was due to the removal of the?uoride ions by replacing the hydroxyl ions.

Table1

Chemical composition of?y ash cenospheres and MLC.

Element as oxide Fly ash cenospheres MLC

MgO0.8985611.22036

S i O257.6896748.08131

Al2O329.5436225.84257

Fe2O3 3.75557 3.32275

K2O 2.87349 2.88439

CaO 2.48648 1.85839

TiO20.934770.84307

Na2O0.274460.14325

P2O50.074310.05127

SO30.031510.02558

MnO0.023500.02003

ZnO0.006770.01179

CuO0.009120.01170

LOl? 1.23400–

?LOl,loss on ignition at1233K.

234X.Xu et al./Desalination272(2011)233–239

3.2.Effect of initial pH

Generally,pH is a key factor affecting ?uoride adsorption at the water adsorbent interface [31].Fig.4presents the ?uoride 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 ?uoride solution.The maximum ?uoride 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 ?uoride.In other words,the maximum ?uoride 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 ?uoride ions.In the acidic medium,the reasons for the decrease of the ?uoride adsorption are the change in the surface charge of MLC and the decrease of the H +ions and ?uoride ions in solution.The reduction of the ?uoride removal in alkaline medium is due to the increasing electrostatic repulsion between the ?uoride ions and the negatively charged surface sites of MLC.Another cause for the decrease in the alkaline

medium is the competition between the OH ?ions and the ?uoride ions for the adsorption sites,because ?uoride is similar to OH ?in the charge and ionic radii.3.3.Amount of adsorbent

The effect of the adsorbent dosage on the ?uoride adsorption on MLC is shown in Fig.5.The ?uoride removal capacity of MLC increased from 0.9to 2.4mg g ?1with the increase of the adsorbent dosage ranging from 1.0to 2.5g L ?1,while it decreased from 2.4to 1.8mg g ?1with the increase of the adsorbent dosage ranging from 2.5to 5.0g L ?1.Therefore,2.5g L ?1MLC was used in all the experiments as the optimum adsorbent dose.The increase in the sorbent dosage at a changeless ?uoride concentration leads to the decrease of the ?uoride adsorption,because there are too many adsorbent doses for the limited amount of ?uoride.The removal ef ?ciency increased with the dosage because of more active sites from an increase in the amount of the

adsorbent.

Fig.1.SEM images of (a)?y ash cenospheres,(b)the whole cenospheres,(c)the broken cenosphheres,(d)MLC under high magni ?cation (e)MLC under low magni ?cation;EDAX spectrum of (f)?y ash cenospheres and (g)

MLC.

Fig.2.XRD pattern of (a)?y ash cenospheres and (b)

MLC.

Fig.3.FT-IR spectra of (a)?y ash cenospheres;(b)MLC before adsorption and (c)MLC after adsorption.

235

X.Xu et al./Desalination 272(2011)233–239

3.4.Effect of time and the ?uoride concentration

Fig.6illustrates the amount of ?uoride ions adsorbed by MLC as a function of the contact time in the range of 5–480min.The initial ?uoride concentrations in solution are 5,10and 20mg L ?1,respectively.It was seen that at all concentrations the adsorption capacity increased with time up to about 60min and then the curves became quite level,indicating the attainment of the adsorption equilibrium.The equilibrium time is usually independent of the ?uoride concentration,therefore,60min is ?xed as minimum contact time for the maximum ?uoride 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,?uoride removal by MLC,was governed by complicated adsorption.The adsorption capacity increased with the initial ?uoride concen-tration in solution from 5to 20mg L ?1.3.5.Effect of coexisting anions

The effect of the coexisting anions on ?uoride 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 ?uoride concentration in solution was ?xed at

10mg L ?1while the initial concentrations of the coexisting anions were 5,10and 20mg L ?1.As a whole,the decrease of the ?uoride adsorption was in order comprehensive N dihydric phosphate N nitrate N sulfate.This decreased de ?uorination may be due to the lower af ?nity of MLC for ?uoride adsorption,or due to a competition between the ?uoride 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.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-?rst-order and pseudo-second-order models,were studied by using the data obtained from the ?uoride adsorption on MLC at different time intervals.The equations are expressed as [33]:log q e ?q t eT=log q e ?k 1t e3T

t q t =1k 2q 2e

+t q e :e4

T

Fig.4.Effect of initial pH on the adsorption of ?uoride (T =298K;W =2.5g L ?1;c 0=10mg L ?1;t =1440

min).

Fig.5.Adsorption of ?uoride on MLC as a function of the adsorbent dosage (T =298K;

c 0=10mg L ?1;pH=3.0;t =1440

min).Fig.6.Adsorption of ?uoride on MLC as a function of contact time and the initial ?uoride concentration (pH =3.0;T =298K;W =2.5g L ?1

).

Fig.7.Effect of coexisting anions on the ?uoride adsorption capacity (pH =3.0;T =298K;W =2.5gL ?1;c 0=10mg L ?1;t =1440min).

236X.Xu et al./Desalination 272(2011)233–239

The values of k 1can be determined from the slope of the linear plot of log (q e ?q t )versus t ,and k 2can be calculated from the slope and intercept of the linear plot t /q t versus t .

The linear plots of two kinetic models are presented in Fig.8and Fig.9,respectively.The values of k 1,k 2,q e and the correlation coef ?cient (r 2)from the linear plots are shown in Table 2.The pseudo-second-order linear plots resulted in higher r 2values than the pseudo-?rst-order.The values of q e (cal)from the pseudo-second-order were more close to q e (exp)than that from the pseudo-?rst-order.These indicated the better applicability of the pseudo-second-order model.As reported previously,the system of the ?uoride 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 ?uoride from water and express the speci ?c relation between the ?uoride 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 ?uoride,but also indicate the maximum amount of ?uoride 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]:q e =

q m k a c e

a e

:

e5T

It can also be replaced by the linearized equation:c e q e =1q m c e +1k a q m

:e6T

The values of q m and k a are calculated from the slope and the intercept of the linear plots c e /q e versus c e .

The most important multisided adsorption isotherm for the heterogeneous surfaces is the Freundlich model,which is described by Ref.[35]:q e =k F c 1=n

e

e7T

and is represented by the following linearized expression:ln q e =ln k F +

1

n

ln c e :e8T

The values of k F and n were obtained from the intercept and slope of the linear plot of ln q e versus ln c e .

The linearization of the Langmuir isotherm for the ?uoride adsorption on MLC at different temperatures are presented in Fig.10.The correlation coef ?cient (r 2)obtained from different isotherm models are given in Table 3.The Langmuir model showed signi ?cantly higher correlations (r 2N 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 de ?ned in order to be able to evaluate the ?t of the isotherm to the experimental equilibrium data.The χ2test 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,χ2will be a small number,while if they are different,χ2will be a large number.The equivalent mathematical statement is χ2=∑q e ?q e ;m

q e ;m

0@

1A :e9T

The result of the χ2-analysis is presented in Table 3.The lower χ2-values of the Langmuir isotherm indicated the better applicability for the ?uoride sorption on MLC.

In order to further ?nd out the feasibility of the isotherm,the essential characteristics of the Langmuir isotherm can be described by an equilibrium parameter,R L [36],de ?ned as:R L =

1

a 0

:

e10

T

Fig.8.Pseudo-?rst-order plots for ?uoride adsorption by MLC (pH =3.0;T =298K;W =2.5gL ?1

).

Fig.9.Pseudo-second-order plots for ?uoride adsorption by MLC (pH =3.0;T =298K;W =2.5g L ?1).

Table 2

Parameters and correlation coef ?cients of two kinetics models.c 0

q e (exp a )

Pseudo-?rst-order Pseudo-second-order

k 1

q e (cal a

)

r

2

k 2

q e (cal a )r 2

5 1.56480.00820.670.96430.0263 1.90550.9902Y=?0.0082x ?0.1764Y=0.5248x +10.452810 1.79200.00940.710.98830.0393 1.93680.9928Y=?0.0094x ?0.1502Y=0.5163x +6.775920

1.9754

0.01070.640.92460.0376 2.24170.9961Y=?0.0107x ?0.1903Y=0.4461x +5.2851

a

Exp,experimental data;cal,calculation data.

237

X.Xu et al./Desalination 272(2011)233–239

The value of R L indicates the shape of the isotherms to be either unfavorable (R L N 1),linear (R L =1),favorable (0b R L b 1)or irrevers-ible (R L =0)[37].R L values (Table 4)calculated from the present system are within 0and 1,which indicate a favorable adsorption process.The ?uoride adsorption on MLC was more favorable at higher concentrations.

3.8.Thermodynamic treatment of sorption process

Temperature has a major in ?uence on the sorption process.The ?uoride sorption on MLC was monitored at three different tempera-tures 298,308,and 318K under the optimized condition.Thermody-namic parameters,viz.,ΔG 0,ΔH 0and ΔS 0(Table 5)were calculated according to the thermodynamic equations,which are described by Ref.[38]:

ln K 0=

ΔS 0R ?ΔH 0

RT

e11TΔG 0

=?RT ln K 0:

e12T

ΔH 0

and ΔS 0

are determined from the slope and intercept of the

plot of ln K 0versus 1/T .The thermodynamic equilibrium constant (K 0)is determined by the proposed Stephen and Sulochana method [39],namely,K 0equals to q m ×k a of the Langmuir isotherm.

The negative value of ΔG 0at 318K con ?rmed the spontaneous nature of the ?uoride sorption on MLC,while the positive values of ΔG 0at 298K and 308K indicated the passive reaction.The value of ΔH 0was positive,indicating that the sorption reaction was endo-thermic.The positive value of ΔS 0,indicating the increased disorder in

the system with the changes in the hydration of the adsorbing ?uoride ions,showed the sorption process was irreversible and stable.4.Conclusions

The ?uoride adsorption from the aqueous solutions by MLC depended on contact time,pH of the solution,reaction temperature,adsorbent dosage and the initial ?uoride concentration.The optimum reaction conditions were contact time (60min),pH 3.0and adsorbent dosage (2.5mg L ?1).The coexisting ions had a large impact on the ?uoride sorption on MLC and the decrease of the ?uoride adsorption was in order comprehensive N dihydric phosphate N nitrate N sulfate.Experimental data was well ?tted to the pseudo-second order kinetic model and followed the Langmuir isotherm.The Gibb's free energy changes (ΔG 0)showed that the ?uoride removal by MLC was the spontaneous nature process at 318K while non-spontaneous at 298K and 308K.The maximum adsorption capacity was about 6.0mg g ?1in the solution with 100mg L ?1of the ?uoride ions at pH 3.0and 318K.MLC,which was prepared by using coal ?y ashes as raw material,is low cost and has considerable ?uoride adsorption capacity,thus it will be a potential candidate for the ?uoride removal from wastewater.Effective regeneration of MLC and cost –bene ?t analysis of its application will be studied in future experiments.Symbols c 0the initial ?uoride concentration,mg L ?1

c e the equilibrium ?uoride concentration,mg L ?1V the volume of solution,L W the mass of dry adsorbent,g

q e the amount of adsorption at equilibrium,mg g ?1E the removal ef ?ciency,%

q t the amount of adsorption at time t,mg g ?1

q m the theoretical maximum adsorption capacity,mg g ?1

q e,m the equilibrium adsorption capacity obtained by calculating from the model,mg g ?1

k a the Langmuir isotherm constant,L mg ?1

k 1the rate constant of pseudo-?rst-order model,min ?1k 2the rate constant of pseudo-second-order model,g mg ?1min ?1

k F the Freundlich empirical constant n the Freundlich empirical constant t time,min

ΔH 0the standard enthalpy change,kJ mol ?1ΔS 0the standard entropy change,J mol ?1K ?1ΔG 0th standard free energy change,kJ mol ?1T temperature in Kelvin,K

R the universal gas constant,8.314Jmol ?1K ?1K 0

the thermodynamic equilibrium constant,L g ?

1

Fig.10.Linearization of the Langmuir isotherm for ?uoride removal (pH=3.0;W =2.5gL ?1;t =1440min).

Table 3

Comparison of correlation coef ?cients and χ2-values of the Langmuir and Freundlich isotherms.T

Langmuir Freundlich r 2

Equation

χ2

r 2

Equation

χ2

2980.9868Y=0.1959x+1.51650.12520.9728Y=0.2636x+0.39240.75433080.9900Y=0.1887x+1.16650.16480.9683Y=0.2257x+0.60200.72023180.9912Y=0.1658x+0.85650.12020.9287Y=0.1860x+0.88430.7848

Table 4

Equilibrium parameter R L values of the Langmuir isotherm at initial ?uoride concentrations 5–100mg L ?1.c 0510203050100R L

298K 0.60760.43630.27900.20510.13410.0718308K 0.55280.38200.23670.17080.11000.0582318K

0.5082

0.3406

0.2053

0.1469

0.0936

0.0491

Table 5

Thermodynamic parameters of ?uoride adsorption onto MLC.Anion

K 0ΔH 0

ΔS 0

ΔG 0298K

308K 318K 298K 308K 318K F ?

0.6594

0.8573

1.1675

20.04

63.80

1.032

0.394

?0.409

238X.Xu et al./Desalination 272(2011)233–239

Acknowledgements

Financial support from the Foundation of State Key Laboratory of Pollution Control and Resource Reuse of China is greatly appreciated by the authors.The authors would also like to acknowledge Sinopec Jinling Petrochemical Power Plant for donating CFBC?y ashes. References

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