Colloids and Surfaces A: Physicochem. Eng. Aspects 469 (2015) 307314

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Colloids and Surfaces A: Physicochemical andEngineering Aspects

journa l h om epage: www.elsev ier .com/ locate /co lsur fa

Uptake -COMg/Al ratio effect on its structure, electrical afnity and adsorptiveproper

Dongjin a State Key Labb School of Che

h i g h l

Higher Mginterlayer cal afnityon adsorpt

Pseudo-secmodel ttsatisfactor

From bothstudy, the and capacMg/Al = 4.

The adsorpneous and

The detrexisting Cl < SO42

a r t i c l

Article history:Received 9 SepReceived in reAccepted 11 JaAvailable onlin

Keywords:HydrotalciteFluorideMg/Al ratioAdsorptionInterlayer spacZeta-potential

CorresponE-mail add

http://dx.doi.o0927-7757/ ty

Wana,b, Yongde Liub, Shuhu Xiaoa,, Jing Chenb, Jian Zhangb

oratory of Environmental Criteria and Risk Assessment, Chinese Research Academy of Environmental Sciences, Beijing 100012, Chinamistry and Chemical Engineering, Henan University of Technology, Zhengzhou, Henan 450001, China

i g h t s

/Al ratio exhibited largerspacing and lower electri-

which had reverse effection.ond order and Langmuired kinetics and isothermily.

kinetics and equilibriummaximum adsorption rateity was achieved when

tion process was sponta-endothermic.imental effect of co-anion increased as:< PO43.

g r a p h i c a l a b s t r a c t

e i n f o

tember 2014vised form 9 January 2015nuary 2015e 29 January 2015

ing

a b s t r a c t

Previous studies have found that calcined Mg-Al-CO3 hydrotalcites exhibit high uoride removal capacity.The effect of Mg/Al ratio on its structure, electrical afnity and adsorptive property, however, remainedelusive, and their elucidation could enhance the functional optimization of the material. This studypresents signicant effect of Mg/Al ratio in calcined hydrotalcite (CHTx, x was the ratio: 2:1, 3:1, 4:1and 5:1) on uoride adsorption. The materials were characterized by X-ray diffraction (XRD), N2 adsorp-tion/desorption analysis using BET method, scanning electron microscopy (SEM), Zeta-potential andFourier transform infrared spectroscopy (FTIR) to conrm the adsorption mechanism. In this uniquestudy, we found that different Mg/Al ratios resulted in different interlayer spacing and electrical proper-ties, inuencing the substances adsorptive properties. As the Mg/Al ratio increased, the interlayer spacingincreased and Zeta potential decreased, which had a reverse effect on adsorption. The CHT4 exhibits thehighest adsorption rate and capacity (Qmax = 119.04 mg/g at 298 K). The adsorption kinetics data best t aPseudo-second-order kinetic model. The Langmuir isotherm model t experiment data better than Freund-lich model. The results of thermodynamic study highlighted the spontaneous and endothermic nature ofthe adsorption process. The detrimental effect of co-existing anion increased as: Cl < SO42 < PO43.

2015 Elsevier B.V. All rights reserved.

ding author. Tel.: +86 10 84928381; fax: +86 10 84917906.resses: dongjinwan@yeah.net (D. Wan), xiaoshuhu@126.net (S. Xiao).

rg/10.1016/j.colsurfa.2015.01.0452015 Elsevier B.V. All rights reserved. uoride from water by caclined Mg-Al 3 hydrotalcite:

308 D. Wan et al. / Colloids and Surfaces A: Physicochem. Eng. Aspects 469 (2015) 307314

1. Introduction

Fluoride contamination in groundwater is a signicant problemfor many countries [1]. Fluoride is a naturally occuring substance,and is releacesses thatproduced tproductionto uoride drinking waexcess ouosteoporosidamage [4,5set a strict nately, it is on drinking[7].

Primaryinclude chereverse osmthe adsorptis an effectcess that ymethod for studied as solutions, icalcium-bahydrotalciteproducts hacost, high a

The strumineral brucations (sucgenerates psated by aniwater molehave the gewhere M2+

N3+ is a trivthe interlayhydrotalcitehydroxyls aresulting manions fromture [1719

Previouscites exhibifacilitates athe uroideAl-CO3 hydand modiadsorption et al. compa(Mg/Al = 3:1exhibited hResearch alature and chydrotalcitestudies aboThe effect adsorption dation coul

In a prevenced interadsorptive studying th

solution is a natural next step. The goals of this study were to: (1)use a coprecipitation method to synthesize Mg-Al-CO3 hydrotal-cites of different Mg/Al ratio; (2) investigate the uoride adsorptionprocess of calcined hydrotalcite with different Mg/Al ratios; and (3)

terizpos

teria

sorb

-Al-Csized

a s6H2O

ratie als2COst soy drously aintaK foreioniecipir, antaine

netic

adsoand

our deio

wast dosThe Cws:

0 C

equag), V concerben

uilib

ilibr0.10 ns. Ts plaf 15

at 29ctionothor 24e anty of .dditiand 3sed into groundwater through natural geological pro- dissolve uorine-rich rocks. However, ouride is alsohrough industrial activities, such as glass and ceramic

and semiconductor manufacturing, also contributingpollution [2,3]. Although small amounts of ouride inter is considered benecial in reducing dental cavities,

ride intake can cause severe health problems, such ass, arthritis, brittle bones, cancer, infertility, and brain]. Therefore, the World Health Organization (WHO) hasguideline of 1.5 mg/L for drinking water [6]. Unfortu-estimated that over 200 million people worldwide rely

water with uoride concentration exceeding 1.5 mg/L

conventional methods to remove uoride from watermical-precipitation [8], membrane ltration such asosis [9], ion-exchange [10,11], electrodialysis [12], andion method [7,13]. Among these methods, adsorptionive and widely used physiochemical treatment pro-ields promising removal results and is an attractiveuoride removal [7,13]. A variety of materials have beenadsorbents to facilitate uride removal from aqueousncluding alumina and aluminum-based sorbents [14],sed sorbents [15], carbon based adsorbents [16], ands [17]. Among these, hydrotalcites and their calcinedve attracted increasing attention because of their lowdsorption capacity, and unique structure [18].cture of hydrotalcites can be derived from the layeredcite Mg(OH)2. Isomorphous substitution of divalenth as Mg2+, Zn2+) by trivalent ones (such as Al3+, Fe3+)ositive charges on the brucite layers which are compen-ons (e.g. CO32) located in the interlayer region. Besides,cules are also found in the interlayer space. Theyneral formula: [M2+(1 )N3+(OH)2] +[An]/nmH2O,is a divalent cation (Mg2+, Zn2+, Mn2+, Co2+, Ni2+, Cd2+),alent cation (Al3+, Fe3+, Cr3+, Ga3+), An is a anion iner and is dened as the N3+/(M2+ + N3+) ratio. Whens are calcined, the interlayer water and anions, and there removed, and the layered structure is destroyed. Theaterial is a mixture of metal oxides which can absorb

aqueous solution to recover its original layered struc-].

studies have found that calcined Mg-Al-CO3 hydrotal-t high uoride removal capacity, as the layered structuredsorption processes [18,20]. Lv et al. [18] investigated

adsorption kinetics and equilibrium by calcined Mg-rotalcite (Mg/Al = 2:1), the LangmuirFreundlich modeled multiplex model could be used to describe theequilibrium and kinetics process satisfactorily. Wangred the adsorptive property of Mg-Al-CO3 hydrotalcite) with their calcined products, the calcined products

igher adsorption capacity than uncalcined material [20].so demonstrates that parameters such as pH, temper-o-existing ions are factors in ouride adsorption by-like componds [18,2022]. However, there are fewut the effect of M2+/N3+ ratios on uoride adsorption.of Mg/Al ratio on its structure, electrical afnity andproperty, however, remained elusive, and their eluci-d enhance the functional optimization of the material.ious study, we found that the Mg/Al ratio greatly inu-layer spacing and electrical properties, further affectingfeatures during anion adsorption [23]. Based on this,e Mg/Al ratio effect on uoride adsorption from aqueous

characthe comties.

2. Ma

2.1. Ad

MgsynthecreatedMgCl2atomic5:1. Wand Nathe rdrop bvigorowas mat 338 with dThe prpowdewas ob

2.2. Ki

Thebents, part ofNaF intrationsorben298 K. as follo

qt = (C

In thist (mg/initial of adso

2.3. Eq

Equmass (solutioof askspeed otainedThe reaavoid aasks fmixturcapaci(mg/g)

In a308 K, e hydrotalcites with different Mg/Al ratios and exploreitional effect on their structure and adsorptive proper-

ls and methods

ents synthesis

O3 hydrotalcites (HTx, x was Mg/Al atomic ratio) was using a conventional coprecipitation method. We rstalt solution (200 mL) containing appropriate ratios of

(1.2 mol/L) and AlCl36H2O (1.2/x mol/L). The Mg/Alo (x) in the solution was selected as 2:1, 3:1, 4:1, ando created a second solution (200 mL) containing NaOH3, at sufcient concentrations to precipitate the salt inlution. The two solutions were simultaneously addedp into a 200 mL sample of deionized water, which we

stirred. The temperature was xed at 313 K, and the pHined at 1011. The resulting slurry was allowed to rest

12 h. The nal precipitate was centrifuged several timeszed water, until the superstratum water was free of Cl.tate was then dried at 378 K for 8 h to obtain the HTxd calcined hydrotalcites powder (CHTx, 100 meshes)d by calcining HTx in a mufe furnace at 773 K for 5 h.

s study

rption rate is an important factor when studying adsor-is best studied through kinetics experiments. For thisstudy, the uoride solution was prepared by dissolvingnized water and vigorously stirring. The initial concen-

xed at 200 mg/L; the solution volume was 500 mL; theage was 1 g/L; and the temperature was controlled atHTx adsorptive capacity toward uoride was calculated

t) Vm

(1)

tion, qt is the adsorptive capacity of adsorbent at timeis the volume of solution (L), C0 and Ct (mg/L) are thentration of uoride and that at time t, and m is the masst (g).

rium study

ium isotherms were obtained by subjecting a constantg) of CHTx to a range of different uoride concentrationhe CHTx and uoride solutions were agitated in a seriesced in a temperature controlled orbital shaker (stirring0 rpm). We used equal solution volumes (100 mL), main-8 K for a period of 24 h to achieve adsorb equilibrium.

n mixture pH was not controlled, because we wanted toer anion impacting uoride uptake. After shaking the

h, the sample solution was removed from the reactiond passed through a 0.20 m membrane. The adsorptionCHTx toward uoride at equilibrium was denoted as qe

on to these tests, we also tested the equilibrium at 288 K,18 K to investigate temperature effect.

D. Wan et al. / Colloids and Surfaces A: Physicochem. Eng. Aspects 469 (2015) 307314 309

2.4. Effect of co-existing anions

We also explored the inhibiting effect of co-existing anions, suchas Cl, SO42 and PO43 in this study. A series of CHT4 and syntheticsolutions were agitated by shaking them for 24 h in conical asksat 298 K to achieve adsorption equilibrium. The synthetic solutioncontained equal concentrations of F with other competitive anions(Cl, SO42 or PO43); the two anion concentrations ranged from0.001 mol/L to 0.05 mol/L. The solution volume was 100 mL, and thesorbent dosage was 1 g/L.

2.5. Analytical methods

Following the preparation discussed above, samples wereltered using a 0.20 m membrane before analysis. The u-oride concentration was analyzed using a uorine reagentspectrophoSpectrophosis was comfrom 5 to average poobtained ususing the Bimages wertransform idisks rangiFourier TraZeta-potent(0.001 mol/

3. Results

3.1. Charac

3.1.1. XRD aThe X-ra

ture. Fig. 1(after adsorcrystallizedreections (asymmetricreections pounds, indcell paramerespectivelying of two Therefore, ttracting thefrom the ba

The strupresented iratio compaincrease ining. This ph

0 10 20 30 40 50 60 70 80

0

500

100 0

1500

200 0

250 0

300 0

350 0

400 0

113110018015

012

006

HT5HT4

HT3

2 Theta (degre e)

Inte

nsity

HT2

003

20 30 40 50 60 70 80

0

500

100 0

150 0

200 0

250 0

300 0

350 0

400 0

CHT5CHT4

CHT3CHT2

Inte

nsity

2 Th eta (deg ree )

20 30 40 50 60 70 80

0

500

100 0

150 0

200 0

250 0

300 0

350 0

400 0

CHT5-ACHT4-A

CHT3-AIn

tens

ity

2 Theta (degree)

CHT2-A

(a)

(b)

(c)

ig. 1. The XRD patterns of HTx, CHTx and CHTx-A (after adsorption).

Table 1Structural data

Sample (nm) c (nm) Interlayer spacing (nm)

HT2 304 2.274 0.278HT3 300 2.318 0.293HT4 302 2.367 0.309HT5 302 2.415 0.326CHT2-A 299 2.281 0.280CHT3-A 299 2.316 0.292CHT4-A 300 2.387 0.316CHT5-A 302 2.424 0.328tometry method at 620 nm using a TU-1900 Perseetometer (Persee, China). The X-ray diffraction analy-pleted using X pert Pro MPD with Cu-Ka radiation

80 (2) (Panalytical, Netherlands). The surface area,re volume, and pore diameter of the samples wereing an ASAP 2020 apparatus (Micromeritics Co., USA),ET method. The scanning electron microscopy (SEM)e obtained using a Hitachi S-3000N microscope. Fouriernfrared spectra (FTIR) were recorded with KBr-pressedng from 400 to 4000 cm1 using an IR Prestige-21nsform Infrared Spectrometer (Shimadzu, Japan). Theial was measured using a Malvern Zetasizer 2000L NaCl, 298 K).

and discussion

terization

nalysisy diffraction analysis was used to explore sample struc-

shows the XRD patterns of HTx, CHTx, and CHTx-Aption). Before being calcined, the HTx showed a well

hydrotalcite-like structure, with sharp and symmetricplanes of 0 0 3, 0 0 6, 1 1 0, and 1 1 3), as well as wide and

reections (planes of 0 1 2, 0 1 5, and 0 1 8). All of thesewere characteristic planes of hydrotalcite-like com-icating that our preparation was successful [19]. Theters were calculated as a = 2 d1 1 0 and c = 3 d0 0 3,

[24]. The d0 0 3 values corresponded to the basal spac-consecutive brucite-like hydroxide layers in the HTx.he interlayer free spacing could be calculated by sub-

brucite sheet thickness (0.480 nm as reported [24])sal spacing d0 0 3.ctural data calculated on the bases of XRD analyses aren Table 1. The table shows that the increase in Mg/Alred to the parent hydrotalcite is accompanied by the

c parameter and, consequently, the interlayer spac-enomenon indicates weaker electrostatic interactions

0 10

0 10

F

calculated on the basis of XRD analyses.

2 of d 0 0 3 () 2 of d 1 1 0 () a

11.74 60.88 0.11.33 60.42 0.11.16 60.04 0.11.12 60.09 0.11.65 60.70 0.11.48 60.69 0.11.47 60.65 0.11.35 60.20 0.

310 D. Wan et al. / Colloids and Surfaces A: Physicochem. Eng. Aspects 469 (2015) 307314

-20

-10

0

10

20

30

40

Zeta

Pot

entia

l (m

V)

CHT2 CHT3 CHT4 CHT5

Fig. 2. The Ze298 K).

between laAl3+ in the bto 5:1 [25].

When thhydrotalcitemation of m

We subswhen placintion of uouoride in formed theculated on table showsin the inter

3.1.2. BET aThe adso

nicantly inare summaarea (152.7age pore diisotherm ofstarted at anature of th

Zeta potcess. Whenstructure dMg(Al)O. Zepositive chresults of Zconcluded Hence, all ta wide pH samples folnomenon c

Table 2Physical chara

Sample

CHT2 CHT3 CHT4 CHT5

400 0 0

0

0

0

0

0

T%

60

80

100

20

40

60

80

100

HT4 CHT4

. Th couldn solhe lo

FT-IR 3 repoad pH grles and hydroxyls groups attaching to Mg and Al in the-layers. The band at 1650 cm1 is attributed to the bendingon of the interlayer water. The adsorption bands observed3 cm1 are due to a carbonate group [26]. Finally, the bandsg from 400 to 800 cm1 can be attributed to the characteris-tching bands of magnesium and aluminum oxides. Whenas calcined, the band of OH (3480 cm1 and 1643 cm1)rbonate (1370 cm1) was weakened in the CHT4 spectra.

12 4 6 8 10 12 14pH

ta potential of CHTx as a function of solution pH (0.001 mol/L NaCl,

yer and interlayer anions, due to the lower amount ofrucite-type sheet when Mg/Al ratio increases from 2:1

e material was calcined at 773 K for 4 h, the layered-like structure disappeared (Fig. 1b), leading to the for-ixed oxide Mg(Al)O (attribute to peaks at 43 and 62).equently observed the layered structure reconstructiong the CHTx in uoride solution (Fig. 1c). After adsorp-ride, the layered structure of HTx was reformed; thethe water was adsorbed onto the positive layer and

new negative layer. The cell parameters of CHTx-A, cal-the basis of the XRD analyses, are listed in Table 1. This

that, compared with HTx, there is no signicant changelayer space after adsorption.

nd Zeta potential analysisrbents physical properties are very important and sig-uence adsorption capacity. The BET analysis results

rized in Table 2. All samples exhibited a similar surface5170.28 m2/g) and mesopore structure, with an aver-ameter of 13.8416.04 nm. The adsorptiondesorption

CHTx could be classied as type-IV. The hysteresis loop relatively high pressure, indicating the mesoporouse sample (Fig. S1).ential is an important parameter for adsorption pro-

the material was calcined, the layered hydrotalcite-likeisappeared, leading to the formation of mixed oxide

2

4

6

8

10

20

40

samplewhichoride iforce, t

3.1.3. Fig.

rst bring of Omolecubrucitevibratiat 139rangintic streHT4 wand cata-potential of CHTx was derived from the redundantarge on the mixed oxide surface. Fig. 2 presents theeta-potential as a function of solution pH. It could bethat the isoelectric points of CHTx were about pH 11.he CHTx had residual positive charge in suspension inrange (pH < 11). Besides, the Zeta-potentials of theselow the order: CHT5 < CHT4 < CHT3 < CHT2. This phe-ould be explained by the electric charge density of the

cterization of BET analysis.

BET (m2/g) Mesoporevolume (mL/g)

Average porediameter (nm)

157.07 0.58 14.29152.75 0.60 13.84169.63 0.55 15.83170.28 0.69 16.04

After adsor1641 cm1

frequency 1oxide mate

3.1.4. SEM Fig. S2 is

CHT4-A. Behydrotalcitis very thinture disappobserved (Cture becambe observedwas partly ysis, in whimuch lowe3600 320 0 2800 240 0 2000 160 0 1200 800 40 0

1388 cm-11641 cm-1

3541 cm-1

1370 cm-1

1650 cm-1

3480 cm-1

1393 cm-1

1643 cm-1

CHT4-A

CHT4

Wav e number (cm-1)

HT4

3532 cm-1

Fig. 3. FTIR spectra of HT4, CHT4 and CHT4-A.

at is, higher Mg/Al ratio results in lower Al3+ content, result in lower Zeta-potential of the sample. When u-

ution was adsorbed onto the positive layers by electricwer Zeta-potential was unfavorable for adsorption.

analysisresents the FTIR spectra of HT4, CHT4, and CHT4-A. Theeak, around 3532 cm1, can be classied as the stretch-oups. This stretching was caused by the interlayer waterption of uoride, the bands observed at 3541 cm andbecome stronger, the band at 1370 cm1 shifts to higher388 cm1, corresponding to the reconstruction of therial in the presence of uoride.

analysis a high vacuum SEM image of gold-coated HT4, CHT4,fore being calcined, the HT4 clearly shows a layerede sheet. The image demonstrates that the crystal sheet. When the material was calcined, the layered struc-eared, and the compact structure of material wasHT4). After adsorption of uoride, the compacted struc-e loose again. The layered structure of CHT4-A could not

as clearly as HT4, indicating that the layered structurerecovered. This result is consistent with the XRD anal-ch the characteristic reection intensity of CHTx-A wasr than HTx.

D. Wan et al. / Colloids and Surfaces A: Physicochem. Eng. Aspects 469 (2015) 307314 311

0 0

10

20

30

40

50

60

70

80

90

CHT2 CHT3 CHT4 CHT5 First order mode l Pseud o-sec ond order mode l Intrapa rticle di ffusion mode l

q t (m

g/g)

Fig. 4. Kineticdosage 1 g/L, V

3.2. Sorptio

Fig. 4 shcaclined M298 K. The adsorbent dthe initial to atten Both the aalong the CHT4 > CHTexperiencedfrom 2:1 tofrom 4:1 ttive ability.achieved 84over 240 micharacteristto t the e[18,23,26,2

ln(qe qt) t

qt= 1

k2 qqt = k3t0.5

In thesement (min)rate constan

Table 3 data againssecond-orde

0 200 400 600 80 0 1000

0

20

40

60

80

100

120

q e (m

g/g)

CH T2 CH T3 CH T4 CHT5 Langmuir mo del Freund lich model

sotherL, ad

0.10 a

orreles thate-l

rptio

Adsoilibre adothecrea

ver tty foxhibonceh eq

291.ed 3

comment

uir m

lich

these

Table 3Experimental

Adsorbent

CHT2 CHT3 CHT4 CHT5 50 100 15 0 20 0 250 300

Time (min)

s of uoride adsorbed by CHTx at 298 K. (C0 = 200 mg/L, adsorbent = 500 mL, initial pH 7.20 0.10).

n kinetics

ows the kinetics curves of uoride adsorption ontog/Al-CO3 hydrotalcites with different Mg/Al ratios atinitial uoride concentration was 200 mg/L and theosage was 1 g/L. Fluoride uptake increased rapidly in120 min for the four adsorbents; the curves startedafter that due to a much slower adsorption rate.dsorption rate and adsorbent capacity proceededfollowing sequence during the adsorption process:5 > CHT3 > CHT2. The results indicate that the CHTx

a greater adsorption rate as the Mg/Al ratio increased 4:1. Furthermore, when the Mg/Al ratio increased

o 5:1, the adsorbent did not exhibit higher adsorp- The CHT4 showed the highest adsorptive capacity (qt.31 mg/g) and achieved a removal efciency of 42.2%n of adsorption. To further our understanding of kineticics, we used three common adsorption kinetic modelsxperimental data. These models included (Eqs. (24))7]:

= ln(qe) k1t First-order kinetics model (2)

2e

+ tqe

Pseudo-second-order kinetics model (3)

Intraparticle diffusion model (4)

Fig. 5. IV = 100 m11.20

high cassumis the r

3.3. So

3.3.1. Equ

uoridrium isCHTx intion. OcapaciCHT4 erium cand higCe wasexceedat.

Twoexperi

Langm

Freund

In equations, t is the contact time of adsorption experi-; k1 (min1), k2 (g/mg min) and k3 (mg/g min0.5) is thet of each kinetics model.lists the results seen from tting the experimentalt the three models. Of the three models, the Pseudo-r model showed a better mathematical t, based on the

solution atthe adsorb(L/mg), anddependent

Table 4 sthat the Laneffectively t

data and parameters for the three kinetics models at 298 K.

qe expmg/g

First-order Pseudo-sec

qe cal (mg/g) k1 102min1

V0 102mg/g min

R2 qe cal (mg/g)

55.91 52.74 1.59 83.86 0.9916 67.57 64.16 56.59 1.76 99.60 0.9935 74.07 84.31 65.76 1.78 117.05 0.9856 90.09 75.61 60.95 1.68 102.40 0.9906 84.03 Ce (mg/L)

ms of uoride adsorbed by CHTx at 298 K. (Adsorbent dosage 1 g/L,sorption time was 24 h, initial pH 7.20 0.10, and the pH wast equilibrium without buffer and acid/alkaline solutions adjustment).

ation coefcient (R2). The Pseudo-second-order modelat the chemisorption of the adsorbate on the adsorbentimiting step [18].

n equilibrium

rption equilibrium of CHTxium studies were conducted to determine the maximumsorption capacity of CHTx. Fig. 5 shows the equilib-rm of CHTx. It indicates that the adsorption capacity ofsed steadily with the increase of equilibrium concentra-he range of tested concentrations, the CHTx adsorptionllowed this sequence: CHT2 < CHT3 < CHT5 < CHT4. Theits the highest adsorption capacity at both low equilib-ntrations (qe achieved 53.5 mg/g when Ce was 6.9 mg/L)uilibrium concentrations (qe achieved 108.7 mg/g when3 mg/L). When the equilibrium concentration of uoride00 mg/L, the CHTx adsorption isotherm curves became

mon equilibrium models were used to try to t theal data:

odel qe = Q0CeKL(1 + CeKL)(5)

model qe = KFCne (6) equations, Ce is the uoride concentration in the

equilibrium (mg/L), Q0 is the monolayer capacity of

ent (mg/g), KL is related to the energy of adsorption

KF (mg/g) (L/mg)n and n are the Freundlich temperatureconstants.hows the results from the tting exercise, and indicatesgmuir model describes the sorption equilibrium morehan the Freundlich model with higher coefcients (R2).

ond order Intra particlediffusion model

k2 104g/mg min

v0 102mg/g min

R2 k3 R2

2.80 127.84 0.9966 3.88 0.89383.57 195.86 0.9979 4.59 0.80065.34 433.40 0.9993 6.17 0.54994.03 284.56 0.9999 5.45 0.7106

312 D. Wan et al. / Colloids and Surfaces A: Physicochem. Eng. Aspects 469 (2015) 307314

Table 4Adsorption isotherm parameters.

Adsorbent Freundlich: qe = KFCneKF(mg/g) (L/mg)n

n R2

T = 298 KCHT2 10.46 0.341 0.8221CHT3 16.54 0.290 0.8106CHT4 28.20 0.240 0.8547CHT5 21.82 0.265 0.8344

T = 288 KCHT4 50.17 0.182 0.8873

T = 308 KCHT4 39.56 0.199 0.9375

T = 318 KCHT4 17.16 0.271 0.9198

The R2 for Lride adsorpof monolayfrom the socalcined mastructure wcarbonates,rial was a muoride froture. DurinCHTx positiadsorbed laattributed t

The maxculated usinand 109.89the materiaratios increincreased a4:1 to 5:1, Previous rereect the greater adssteadily whMg/Al ratiovalues was is related to

To avoiacid/alkalintests, whileThe initial was 11.20 of pH for thOH during[23].

Fig. S2 sand nal pthe NaF so10.87. The CpH varied fobserved a bly due to Cet al.s studing capacitiincreased wbuffering eftive capacit[29].

Effect of temperature on CHT4 adsorption equilibriumny researchers have reported that temperature greatly

CHTx anion adsorption, such as Cl [24] and Cr (VI) [30]. study, CHT4 adsorption isotherms under different temper-

(288 K, 298 K, 308 K, and 318 K) were explored, the resultswn in Fig. 6. We conclude from these results that the u-ptake was enhanced with the increase of the adsorption

rature. In addition, the adsorption data were tested using theuir model and Freundlich model. The tting results are listed in. It wl witained, 142tiveldue s. Oution

stanon:

RT

his etemp

of sta

0

0

0

0Langmuir: qe = Q0CeKL/1 + CeKL

Q0(mg N/g)

KL(L/mg)

R2

87.71 0.025 0.998697.09 0.041 0.9995

119.04 0.050 0.9990 109.89 0.047 0.9976

102.04 0.022 0.9993

142.86 0.055 0.9989

158.73 0.065 0.9992

angmuir model t was >0.99, indicating that the uo-tion toward CHTx was consistent with the hypothesiser adsorption. The mechanism used to remove anionslution could be reframed as the reconstruction of theterial [18]. That is, after being calcined, the HTx layeras destroyed; and the interlayer water, the interlayer

and the hydroxyls were removed. The resulting mate-ixture of metal oxides Mg (Al) O, which then picked upm the solution in an effort to recover its layered struc-g the adsorption process, uoride was adsorbed ontove surface, thereby forming the new negative layer. Theyer was a monolayer, hence, the adsorption isothermo the Langmuir model.imum monolayer adsorption capacity (Q0) of CHTx, cal-g the Langmuir model, achieved 87.71, 97.09, 119.04,

mg/g or CHT2, CHT3, CHT4, and CHT5, respectively. Allls exhibited excellent sorption abilities. When Mg/Alased from 2:1 to 4:1, the CHTx absorption capacityccordingly. When the Mg/Al ratio further increased fromthe CHTx adsorption capacity did not increase further.search indicated that the Langmuir constant KL couldadsorption ability; that is, a larger KL value indicatedorption afnity [28]. In this study, KL values increaseden Mg/Al ratios increased from 2:1 to 4:1, and whens further increased from 4:1 to 5:1, the increase in KLnot observed. The effect of the Mg/Al ratio on adsorption

its structure and will be further discussed in Section 3.4.d contaminating co-existing anions, the buffer ande solutions were not used to adjust pH in the adsorption

the initial and equilibrium pH values were monitored.pH of the NaF solution was 7.20 0.10, and the pH

0.10 at the studys equilibrium point. The increasee adsorption process is mainly due to the release of

the structural regeneration process of hydrotalcite

3.3.2. Ma

affectsIn thisaturesare shooride utempeLangmTable 4t welQ0 obt119.04respecature procesadsorp

Theequati

G0 =

In tKelvin values

10

12

14

16

/g)hows the effect of initial pH on adsorption capacityH. Solutions of 0.05 M HCl or NaOH were added tolution to adjust initial pH, which varied from 3.11 toHT4 exhibit high adsorption capacity when the initialrom 4.71 to 10.87. When the initial pH was 3.11, wesignicant decline in CHT4 adsorption capacity, possi-HT4 dissolution. This result is consistent with Ferreiray [29], which reported that given the high pH buffer-es of hydrotalcites, the solution pH at the equilibriumhen initial pH was low. In addition, because of thefect, hydrotalcites exhibited consistently high adsorp-y, irrespective of initial pH (which varied from 4 to 10)

020

40

60

80

q e (m

g

Fig. 6. Isotherdosage 1 g/L, V7.20, and the solutions adjuas discovered that the adsorption isotherm for uorideh Langmuir model. The maximum adsorption capacities

by the Langmuir model linear regression were 102.04,.86, and 158.73 mg/g at 288 K, 298 K, 308 K, and 318 K,y. Generally, adsorption decreases with higher temper-to the exothermic nature of the physical adsorptionr results for uoride adsorbed by CHTx suggest that theprocess involves an endothermic chemical interaction.dard free energy (G0) was calculated by the following

(7)

quation, R is the ideal gas constant (8.314 J/mol K), T iserature (K), and KL is the Langmuir constant (L/mol). Thendard enthalpy change (H0) and the standard entropy200 400 600 800 1000

288 K 298 K 308 K 318 K Langmuir model Freund lic h mod el

Ce (mg/ L)

ms of uoride adsorbed by CHT4 at different temperatures (adsorbent = 100 mL, adsorption time was 24 h, initial pH of the solution was

pH was 11.20 0.10 at equilibrium without buffer and acid/alkalinestment).

D. Wan et al. / Colloids and Surfaces A: Physicochem. Eng. Aspects 469 (2015) 307314 313

0 10 20 30 40 500

1

2

3

4

5

6

7

8

9

10

C0 (mmol/L)

q e (m

mol

/g)

Ce (mm ol/L)

Operation line: qe=(C0-Ce)V/m

Fig. 7. Isotherms of uoride adsorbed by CHT4 when competitive anion exists withequal substance concentrations at 298 K (the two anion initial concentration waspresented as C0 of corresponding operation line, adsorbent dosage 1 g/L, V = 100 mL,adsorption time was 24 h; initial pH of the solution was 7.20 0.10, and the pH was11.20 0.10 at equilibrium without buffer and acid/alkaline solutions adjustment).

change (S0

Vant Hoff p

ln KL =

R

Table 5 pas they adsride ions byare negativthat the adsin nature, adomness at

3.3.3. EffectA variet

contaminatcompete wiwe exploreside equal cor PO43). adsorption Freundlich mthis case, th

As Fig. 7the uoride

Table 5Thermodynam

Temperature

288 K 298 K 308 K 318 K

anions, especially PO43 are present. When equal concentrationsof Cl, SO42 and PO43 exist, the maximum adsorption capac-ity of uoride decreased from its level of 6.26 to 4.87, 4.16, and3.15 mmol/g, respectively. CHTxs ability to pick up uoride fromthe solution mainly depends on the electrical afnity of its positivesurface [19]. Multivalent anions are adsorbed more readily ontothe positive surface than monovalent anions. As such, the effectof co-existing anions decreased on uoride uptake followed thefollowing sequence: PO43 > SO42 > Cl.

3.4. Mechanism

The hydrotalcite structure could be derived from the layeredmineral brucite Mg(OH)2, where Mg2+ is located in the center of anedge-sharinwas isomorerated on thin this studystructure [1

In this sof HTx comWhen the M

HTx d, there rAl)Ored sed inilar

oride the

can ect oads sorp

Mg/A with

withed b

ed on/Al rrlayeed fs moed w

clus

Table 6Adsorption iso

Co-existing

Blank Cl

SO42

PO43) were calculated from the slope and intercept of thelot using regression:

H0

T+ S

0

R(8)

resents the thermodynamic parameter values of CHTxorb uoride. It can be seen that the adsorption of uo-

CHT4 is a spontaneous process, since the G0 valuese. Given that the value of H0 is positive, we concludeorption process of uoride ions by CHT4 is endothermicnd the positive values of S0 reveal the increasing ran-

the solid solution interface of the adsorption process.

of co-existing anions on CHT4 adsorption equilibriumy of other anions are commonly present in uoride-ed water, including Cl, SO42, and PO43. These mayth uoride for adsorption by CHTx. As such, in this study,d the isotherms of uoride adsorbed by CHT4 along-oncentrations of other competitive anions (Cl, SO42

The experimental results are presented in Fig. 7; thedata were also tted against the Langmuir model andodel and the tting results were listed in Table 6. In

e Langmuir model tted experimental data best. shows, when compared with the blank experiment,

adsorption capacity decreases signicantly when other

ing of calcinexyls wof Mg(its layeobservited simthe uformed

Weble effratio leride adhighersistentwater,conrm

Basthe Mgof inteincreasity waachiev

4. Conic parameters for uoride ions adsorbed by CHT4.

(K) ln KL G0 (kJ/mol) H0 (kJ/mol) S0 (J/mol K)

6.03 14.45 25.788 141.276.86 16.986.95 17.807.12 18.82

Higher Mgreater inteaffecting thratios increproperties Mg/Al atomtive impact

therm parameters for uoride ions adsorbed by CHT4 when competitive anion exists wi

anions Langmuir: qe = Q0CeKL/1 + CeKL

Q0(mmol/g)

KL(L/mmol)

R2

6.26 0.727 0.9979 4.87 0.485 0.9972 4.16 0.234 0.9936 3.15 0.138 0.9800 g octahedral surrounded with hydroxyls. When Mg2+

phously substituted by Al3+, positive charges were gen-e layers, which could be compensated by anions (CO32

) in the interlayer region. This formed a sandwich-like9], with water molecules found in the interlayer space.tudy, the XRD analysis indicated that the structure sizepounds is affected by Mg/Al ratios (Fig. 1a and Table 1).g/Al ratio increased from 2:1 to 5:1, the interlayer spac-increased from 0.278 nm to 0.326 nm. When HTx wase interlayer water, interlayer anions, and the hydro-emoved (Fig. 1b). The resulting material is a mixture, which takes up uoride from the solution to recovertructure. The layered structure reconstruction could be

Fig. 1c. After adsorption of uoride, the CHTx-A exhib- reection patterns compared with HTx, indicating that

in the water was adsorbed onto the positive layer and new negative layer.conclude, therefore, that the Mg/Al ratio has a dou-n the adsorption process. On one hand, higher Mg/Alto a larger interlayer spacing of HTx, benetting uo-tion. On the other hand, as reported elsewhere [31], thel ratio (which means lower Al3+ concentrations) is con-

weaker hydrogen bonding to the interlayer anions and decreasing charges on the brucite-type layer, which isy Zeta-potential analysis in this study (Fig. 2).

the adsorption kinetics and equilibrium results, whenatio increased from 2:1 to 4:1, the positive inuencer spacing was dominant. When Mg/Al ratio further

rom 4:1 to 5:1, the negative impact of electrical afn-re of a factor. The maximum adsorption capacity washen the Mg/Al ratio was 4:1 (Fig. 8).

ions

g/Al atomic ratios in Mg-Al-CO3 hydrotalcite led torlayer spacing and lower electrical afnities, adverselye adsorption of ouride ions. When Mg/Al atomicased from 2:1 to 4:1, the hydrotalcites adsorptionwere mainly affected by interlayer spacing. Whenic ratio further increased from 4:1 to 5:1, the nega-

of electrical afnity played more of a role. Based on

th equal substance concentration at 298 K.

Freundlich: qe = KFCneKF(mmol/g) (L/mmol)n

n R2

2.997 0.196 0.87012.258 0.209 0.70031.402 0.279 0.83100.833 0.324 0.8230

314 D. Wan et al. / Colloids and Surfaces A: Physicochem. Eng. Aspects 469 (2015) 307314

0.250

0.275

0.300

0.325

0.350

5

10

15

20

80

90

100

110

120

130

Inte

rlay

er sp

acin

g of

HTx

(nm

)Ze

ta p

oten

tial

of C

HTx

(mV

)

Qm

ax o

f CH

Tx

(mg/

g)

Fig. 8. Interlaymaterial.

both the kiadsorption CHT2 < CHTto understaprocess and

The pseucould be uisotherms. spontaneouMultivalentanions; as sfollowing o

Acknowled

This woChina (NSFCand Plan foTechnology

Appendix A

Supplemfound, in th2015.01.04

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Uptake fluoride from water by caclined Mg-Al-CO3 hydrotalcite: Mg/Al ratio effect on its structure, electrical affinity an...1 Introduction2 Materials and methods2.1 Adsorbents synthesis2.2 Kinetics study2.3 Equilibrium study2.4 Effect of co-existing anions2.5 Analytical methods

3 Results and discussion3.1 Characterization3.1.1 XRD analysis3.1.2 BET and Zeta potential analysis3.1.3 FT-IR analysis3.1.4 SEM analysis

3.2 Sorption kinetics3.3 Sorption equilibrium3.3.1 Adsorption equilibrium of CHTx3.3.2 Effect of temperature on CHT4 adsorption equilibrium3.3.3 Effect of co-existing anions on CHT4 adsorption equilibrium

3.4 Mechanism

4 ConclusionsAcknowledgmentsAppendix A Supplementary dataReferences