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Optimization of a Fe–Al–Ce nano-adsorbent granulation process that used spraycoating in a fluidized bed for fluoride removal from drinking water

Lin Chen a, Ting-Jie Wang a,⁎, Hai-Xia Wu a, Yong Jin a, Yu Zhang b, Xiao-Min Dou c

a Department of Chemical Engineering, Tsinghua University, Beijing 100084, Chinab Research Center for Eco-Environmental Sciences, Chinese Academy of Sciences, Beijing 100085, Chinac School of Environmental Science and Engineering, Beijing Forestry University, Beijing 100083, China

a b s t r a c ta r t i c l e i n f o

Article history:Received 9 November 2009Received in revised form 22 September 2010Accepted 25 September 2010Available online 1 October 2010

Keywords:CoatingGranulationLatexFluoride removalOptimization

A coating granulation technology comprising the spraying of a Fe–Al–Ce nano-adsorbent suspension ontoglass beads in a fluidized bed was developed. An acrylic-styrene copolymer latex was used as a binder. Thegranulated adsorbent was used in a packed bed for fluoride removal from drinking water. The effects ofcoating temperature, latex/Fe–Al–Ce ratio, and coating amount on granule compressive strength andadsorption capacity were investigated. With increased coating temperature, cross linking in the polymer inthe coated layer increased, which resulted in increased granule strength but decreased adsorption capacity.With increased latex/Fe–Al–Ce ratio, more active sites were covered by the polymer, which also resulted inincreased granule strength but decreased adsorption capacity. The optimal parameters for making highperformance adsorbent granules were for the granules to be coated at 65 °C using a latex/Fe–Al–Ce ratio of0.5:1 and a coating amount of 27.8%. These granules had a fluoride adsorption capacity of 2.77 mg/g (coatedgranules) for water with an initial fluoride concentration of 0.001 M that was treated at pH 7.

© 2010 Elsevier B.V. All rights reserved.

1. Introduction

Although the fluoride present in drinking water is essential forhealth, an excessive intake of fluoride causes severe dental or skeletalfluorosis [1]. Therefore, it is necessary to treat fluoride-contaminatedwater and control the fluoride concentration to a permissible limit,which is 1.5 mg/L in theWHO guidelines [2]. Adsorption is consideredone of the most efficient technologies for fluoride removal fromdrinking water, and is more efficient than many other technologiesincluding reverse osmosis, nanofiltration, electrodialysis and Donnandialysis [3]. Activated alumina and bone char that are 1–2 mm indiameter are the most widely used adsorbent because they are lowcost and can be packed into columns with acceptable hydraulicconductivity. However, they have low compressive strength, whichresults in a lack of resistance to hydraulic shock and leads to thefracturing of the adsorbent in a packed bed [4].

Many adsorbents are synthesized as fine powders or as hydroxidefloc [5] with sizes from nanometers to micrometers. Recently, a newlysynthesized Fe–Al–Ce trimetal hydroxide adsorbent (Fe–Al–Ce) witha high adsorption capacity was reported [3]. However, the use of thisadsorbent was limited by its low hydraulic conductivity in a packedbed [6]. This work focuses on producing granules of the above

material that have a sufficiently high compressive strength, so thatthey can be used for adsorption in a packed bed.

Recently, an approach was proposed that used the immobilizing ofa powder adsorbent on an inert support to overcome the largepressure drop when small powder particles are used in a watertreatment process. Iron oxide-coated sand was prepared by theimpregnation of sand with a mixed solution of salts, titration of theprecipitator, and subsequent drying [7,8]. However, the thickness ofthe coated layer was only several micrometers and the coated layershedded easily, which resulted in a low adsorption capacity andsecondary pollution in the drinking water.

To overcome the above problems, Ake et al. [9,10] developed a newgranulation method in which clay minerals were coated on silicasurfaces using mucilage and carboxymethylcellulose as binders. Thesegranules were used as a wastewater filter to remove contaminant ionssuch as Ni2+. In the reported procedure, the polymers were firstmixed with the solid support (either silica sand or glass beads), andthen the clay minerals were added into the mixture. After calcining ata high temperature and rinsingwithwater, a composite adsorbent of aclay-silica material was obtained. However, the calcining resulted in alow adsorption capacity for the granules because most of the activesites on the adsorbent were destroyed by the high temperature [3].

Other polymers, such as polyacrylamide (PAM) and polyvinylalcohol (PVA), were also promising binders for the binding of the clayby hydrogen bonding and ion exchanger [11]. Wu et al. [12] reportedthe use of an acrylic-styrene copolymer latex as binder for coating anano-adsorbent on an inert carrier that did not use calcining at a high

Powder Technology 206 (2011) 291–296

⁎ Corresponding author. Tel.: +86 10 62788993; fax: +86 10 62772051.E-mail address: wangtj@tsinghua.edu.cn (T.-J. Wang).

0032-5910/$ – see front matter © 2010 Elsevier B.V. All rights reserved.doi:10.1016/j.powtec.2010.09.033

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temperature. The latex can crosslink and be cured at roomtemperature. The use of the latex increased the compressive strengthof the coated layer but the latex covered some active sites on theadsorbent surface. Therefore, the latex/Fe–Al–Ce adsorbent ratio hasto be optimized for the optimum compressive strength andadsorption capacity of the granules [13].

In this work, a Fe–Al–Ce nano-adsorbent was coated onto glassbeads using an acrylic-styrene copolymer latex as a binder in afluidized bed. To optimize the coating granulation, the effects of thecoating temperature, latex/Fe–Al–Ce ratio, and coating amount ongranule adsorption capacity and compressive strength were investi-gated. The aim was to prepare a high performance adsorbent granulethat can be used in a packed bed for fluoride removal from drinkingwater.

2. Experimental

2.1. Materials

FeSO4·7H2O, Al2(SO4)3·12H2O and Ce(SO4)2·4H2O used were ofanalytical grade (Chemical Engineering Company of Beijing, China).The other chemicals used were of analytical reagent (AR) grade.

FeSO4·7H2O, Al2(SO4)3·12H2O and Ce(SO4)2·4H2Owere dissolvedin deionized water to form a mixed solution with concentrations of0.1 M, 0.2 M and 0.1 M, respectively. A 6 M NaOH solution was slowlytitrated into the mixed solution until the pH reached 9.5. The solutionwas stirred at 200 rpm during the whole process [3]. The precipitateobtainedwere centrifuged andwashedwith deionized water until thepH of the filtrate was 6.5±0.2. The product was Fe–Al–Ce trimetalhydroxide adsorbent (Fe–Al–Ce), which has a high specific surfacearea of 90 m2/g and diameter of 40 nm. This was kept in deionizedwater.

The glass beads (Technology Development Center of China PaintAssociation, Beijing, China) were sieved to give a 2–3 mm fraction.These were soaked in the HCl solution (pH=1) for 4 h, rinsed withdeionized water until the pH reached 6±0.2, and dried at 105 °C for24 h. The glass beads obtained were kept in a capped bottle.

An acrylic-styrene copolymer latex that can crosslink and cure atroom temperature was supplied by the Institute of Polymer Scienceand Technology (Dept. of Chem. Eng., Tsinghua University, China). Thelatex had a solid fraction of 40% and dynamic viscosity of 20 cP at20 °C. The glass transformation temperature of the latex was 22.8 °C.

2.2. Coating granulation procedure

A fluidized bed in a Perspex column with an inner diameter of55 mm and height of 500 mm was used for the coating granulation ofthe Fe–Al–Ce nano-adsorbent. The glass beads were fluidized bycompressed air. A suspension of Fe–Al–Ce mixed with the acrylic-styrene copolymer latex at a set concentrationwas used as the coatingreagent. The coating reagent was atomized and sprayed onto the glassbeads. The particles in the fluidized bed were dried by controlling thetemperature of the fluidizing gas.

The procedure for the coating granulation followed that reportedby Chen et al. [13]. The experimental apparatus is shown in Fig. 1. Thecoating reagent was agitated in a vessel to prevent the sedimentationof the Fe–Al–Ce adsorbent. The feed rate of the reagent was controlledby a peristaltic pump. After the spray coating of a fixed amount of thecoating reagent, the granules were sampled for analysis.

2.3. Characterization

A high resolution scanning electron microscope (HRSEM, JSM7401, JEOL Co., Japan), was used to examine the morphology of thegranules and microstructure of the coated layer. A particle strengthmeter (0–200 N, KQ-2, Jiangyan, China) was used to measure the

compressive strength of the granules. The pressure at which a crackfirst appeared in the coated layer was recorded. For each sample, anaverage value from the measurements of 10 particles was used.

The coating amount was characterized by the mass ratio of Fe–Al–Ce trimetal hydroxide to glass bead in the granules. A TG analysisshowed that the acrylic-styrene copolymer latex decomposed at390 °C and was completely burnt off at 450 °C. Thus, the coatingamountwas determined by the following procedure. A knownmass ofgranules was heated in a muffle furnace at 550 °C until the acrylic-styrene copolymer was burnt off to leave the glass beads (m1). Thenthe glass beads were soaked in a 1 M HCl solution for 3 h withagitation at 160 rpm until the Fe–Al–Ce trimetal oxide on the glassbead surface was completely dissolved in the acid solution. Theremaining granules were washed, filtered, dried at 105 °C for 3 h, andthen weighed (m2).

From the TG analysis of the Fe–Al–Ce nano-adsorbent powder, theweight of the remaining solid became constant at 72% of the initialweight for temperatures higher than 550 °C. Therefore, the coatingamount R can be calculated by

R =m1−m2ð Þm2 × 72%

× 100%: ð1Þ

2.4. Fluoride adsorption capacity determination [3]

A 1000 mg/L fluoride solution was prepared by dissolving 1.1050 gNaF in 500 mL deionized water. Fluoride containing solutions wereprepared by diluting the above solution to specified concentrationswith deionized water. Known volumes of these fluoride solutionswere added separately into conical flasks. A 0.5 g adsorbent was dosedinto 100 mL of fluoride containing solution. The pH of the test solutionwas kept at 6.5–7.5 by titration with either 0.05 M HClO4 or 0.05 MNaOH solution. The test solution was shaken at 180 rpm and kept at25 °C for 36 h during the adsorption. After adsorption, the granuleswere filtered and the filtrate was analyzed. KNO3 solution was addedto the filtrate as the background electrolyte at a concentration of0.2 M. The fluoride ion concentration remaining in the filtrate wasmeasured with a fluoride selective electrode connected to an ionmeter (PXS-450, Shanghai Kang-Yi Instruments Co., LTD, China).

The adsorption capacity, qe (mg/g), was calculated using

qe =C0−Cf

m× V ð2Þ

where C0 (mg/L) is the initial fluoride concentration and Cf (mg/L) isthe final fluoride concentration after adsorption, V is the volume of thesolution containing fluoride ions, i.e. 100 mL in text, and m is theadsorbent granules dose of 0.5 g.

Fig. 1. Coating granulation apparatus. 1. Coating reagent vessel; 2. Peristaltic pump;3. Atomized gas; 4. Flowmeter; 5. Compressed gas; 6. Flowmeter; 7. Fluidized bed; and8. Nozzle.

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3. Results and discussion

3.1. Core particles and coated granules

The compressive strengths of alumina (~1 mm), sand (1–2 mm)and glass bead (2–3 mm) are shown in Table 1. The compressivestrength of the alumina granules is 12.4 N, which means these wouldeasily fracture in a packed bed. Sand with a compressive strength of49.6 N is promising as a carrier because of its high strength and lowcost, which is less than 1% of the cost of the Fe–Al–Ce adsorbent [12].However, due to the irregular shape of the sand, the SEM images of themicrostructure showed a large variation with different locations onthe sand particles, which made it difficult to compare samples. Tocompare the samples, glass beads with a spherical shape were used asthe carrier particles. The glass beads, shown in Fig. 2 (a), have a muchhigher compressive strength of over 200 N.

In our previous work, the experimental parameters of a latex/Fe–Al–Ce mass ratio of 1:1 and coating temperature (fluidization gas) of35 °C were found to give high adsorption capacity and good stability[13]. In order to use those results as a reference for the present work,the glass beads were coated under the above conditions by spraycoating with the reagent in a fluidized bed with the coating amountfixed at 18.0%. The coated granules obtained have a uniform size andspherical shape as shown in Fig. 2(b).

3.2. Effect of the coating temperature

The acrylic-styrene copolymer latex used can crosslink and cure atroom temperature, so the coating temperature in the previous workwas set at 35 °C. After the latex was sprayed onto the substrate, itcondensed and coated on the substrate by water evaporation duringwhich the latex deformed, cross-linked and cured to form a film layer[12]. The experiments showed that as the temperature increased, thecross-linking degree of the latex increased. A higher cross-linkingdegree produced a higher strength in the polymerized copolymer.

The acrylic-styrene copolymer latex played the role of binder inthe layer coating process. The strength of the coated layer, which wascomposed of the copolymer and Fe–Al–Ce particles, was investigatedas a function of increasing coating temperature. Fig. 3 shows themicrostructure of the granules. Fig. 3(a), (b), and (c) show thesamples obtained at temperatures of 35 °C, 50 °C, and 65 °C,respectively, and Fig. 3(d), (e), and (f) are the corresponding imagesat a higher magnification. It can be seen that the surface of thegranules became smoother and more uniform as the temperatureincreased. Fig. 3 (d), (e), and (f) show the pore structure, whichbecame denser with smaller pores as the temperature increased.However, the highest temperature that could be used was limitedbecause too high a temperature makes the granules adhere to eachother in the fluidized bed and this adhesion of particles would tear offthe coated layer.

Table 2 shows that the compressive strength of the granulesincreased from 160 N to 200 N as the coating temperature increasedfrom 35 °C to 65 °C. As mentioned above, with increased temperature,the cross-linking degree increased, whichmade the network structureof the layer denser. However, some of the active sites on the adsorbentthen get covered by the polymer. This caused a decrease in theadsorption capacity, which is shown in Table 2. Therefore, increasingthe coating temperature and decreasing the latex/Fe–Al–Ce ratiowould lead to a condition for the optimum compressive strength and

adsorption capacity. The coating temperature was set at 65 °C tooptimize the other coating parameters.

3.3. Effect of the latex/Fe–Al–Ce ratio

The active site for fluoride adsorption is the hydroxyl group on theFe–Al–Ce surface [4]. When a chemical bond is formed between thelatex and Fe–Al–Ce adsorbent, this leads to the loss of active sites forfluoride adsorption and a decrease in the adsorption capacity [13].However, a low latex/Fe–Al–Ce ratio leads to a low compressivestrength of the granules. Therefore, the latex/Fe–Al–Ce ratio requiresoptimization for the best compressive strength and adsorption capacity.

In our previous work, the ratio of latex/Fe–Al–Ce of 1:1 wasthought to be suitable, which was based on the measurement of theadhesion of the coated layer on a glass plate, and had been used byWuet al. [12]. The shortcoming of this method was that it did not exactlysimulate the layer on the spray-coated granules. Therefore, in thiswork, a particle strength meter was used to measure the compressivestrength of the granules for determining a more suitable latex/Fe–Al–Ce ratio for the coating layer.

It can be seen that a good structure of the coating layer would beone in which the latex amount is just enough to bind the adsorbentparticles and also forms a network structure with many pores thathave plentiful active sites for the adsorption of fluoride ions. Since thedensity of latex to Fe–Al–Ce is about 1:4, when the mass ratio of latex/Fe–Al–Ce is 1:1, the volume ratio of latex/Fe–Al–Ce is about 4:1. It isinferred that when the mass ratio of latex/Fe–Al–Ce is 1:1, thepolymer would form a continuous phase in the coating layer, whichwould lead to the adsorbent getting easily covered. However, if the

Table 1Compressive strength of the core particles.

Particle Alumina Sand Glass bead

Size, mm ~1 1–2 2–3Compressive strength, N 12.4 49.6 N200

10mm

10mm

a

b

Fig. 2. Images of glass beads (a) and coated granules (b) (latex/Fe–Al–Ce=1:1, coatingtemperature at 35 °C, coating amount=18.0%).

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amount of latex is not enough to bind the adsorbent particles, thecoated layer on the granule surface would be easily peeled off byhydraulic shock in the packed bed. Therefore, there is an optimal ratioof latex/Fe–Al–Ce, where the polymer is present in the best amount toplay the role of binder.

Fig. 4 shows the surface structure of the granules at different ratiosof latex/Fe–Al–Ce, from 1:1 to 0:1, with the coating amount kept at

a b c

d e f

Fig. 3. Surface morphology of granules coated at different temperatures (latex/Fe–Al–Ce=1:1, coating amount=18.0%). Temperature: (a), (d): 35 °C; (b), (e): 50 °C; (c), (f): 65 °C.

Table 2Compressive strength and adsorption capacity vs coating temperature.

Temperature, °C 35 50 65Compressive strength, N 160 180 N200Adsorption capacity, mg/g 2.61 2.45 2.40

(Conditions: latex/Fe–Al–Ce=1:1, coating amount=18.0%, granule dose=5 g/L,initial fluoride concentration=0.001 M, and adsorption time=36 h).

Cracks

100µm

Uncoated Surface

a b c

d e f

Fig. 4. Surface morphology of granules coated at different ratios of latex/Fe–Al–Ce (the inset is the corresponding SEM image at higher magnification, coating temperature at 65 °C,coating amount=18.0%). Latex/Fe–Al–Ce ratio: (a) 1:1; (b) 0.75:1; (c) 0.5:1; (d) 0.375:1; (e) 0.25:1; and (f) 0:1.

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18.0%. The higher magnification SEM image of the surface is shown inthe inset. Changes in the adsorption capacity and compressivestrength vs decrease in the latex/Fe–Al–Ce ratio are shown in Fig. 5.

When the latex/Fe–Al–Ce ratios were 1:1 and 0.75:1, there werefew pores on the surface and a continuous phase was formed, asshown in Fig. 4(a) and (b). It can be inferred that some active sites ofthe adsorbent were covered by the continuous polymer phase in thelayer. From the compressive strength value shown in Fig. 5, theamount of latex was sufficient for binding the adsorbent particles andfor efficiently eliminating stress in the coated layer. When the latex/Fe–Al–Ce ratios were 0.5:1 and 0.375:1, the external surfaces werecoarse and comprised many small particles that adhered together, asshown in Fig. 4(c) and (d). From the compressive strength shown inFig. 5, the amount of latex (with the latex/Fe–Al–Ce ratio at 0.5:1) wasinsufficient for forming a continuous layer. In this case, the latex,however, could still bridge the adsorbent particles. The coating layerhas many pores and most of the active sites on the adsorbent surfacewere not covered. When the latex/Fe–Al–Ce ratios were 0.25:1 and0:1, the coating layer had many cracks and defects. This is shown inFig. 4(e). There were some uncoated surface patches on the granules,which are shown in Fig. 4(f). The compressive strength in Fig. 5 wasalso very low.

Based on the above discussion, the latex/Fe–Al–Ce ratio of 0.5:1was used as the optimum value for the subsequent experiments.

3.4. Effect of coating amount

Themorphology of the granules as the coating amount increased isshown in Fig. 6. The adsorption capacity increased as the coatingamount increased. This is shown in Table 3. When the coating amount

increased, stress accumulated and more cracks appeared on the layersurface [13]. The cracks made the coated layer fragile, and it sheddedeasily, which led to a decrease in the compressive strength of thegranules from 90 N to 42.1 N, as shown in Table 3. Considering theutilization efficiency of the Fe–Al–Ce nano-adsorbent [13], the coatingamount of 27.8% with a compressive strength of 60 N is suggested tobe optimal.

3.5. The optimal adsorbent granules

The optimal parameters of the coating granulation for highperformance adsorbent granules with an acceptable compressivestrength and high adsorption capacity were obtained. This conditionwas for the granules to be coated at 65 °C, a mass ratio of latex/Fe–Al–Ce of 0.5:1, and a coating amount of 27.8%. These granules had afluoride adsorption capacity of 2.77 mg/g (coated granules) for waterwith an initial fluoride concentration of 0.001 M that was treated atpH 7. As compared to the granules prepared under our previouscondition [13], the adsorption capacity of the granules prepared underthis optimal condition was increased by 25%.

The durability of these optimal granules was measured by the lossin weight of the granules in the adsorption process. 0.5 g granuleswere added to 100 mL of water containing fluoride and then shaken at160 rpm. After 36 h of adsorption, the granules were filtered and driedat 100 °C for 12 h, and weighed. This latter weight of the granules was0.496 g. In addition, there was no visible peeling from the particlesduring the soaking and shaking process. The filtrate was also clear.The results showed that the durability of the optimal granules wasvery promising.

The adsorption equilibrium isotherm of the Fe–Al–Ce adsorbentgranules was measured and fitted using the Langmuir isotherm, asshown in Fig. 7. The fitted parameters are listed in Table 4.

According to the Langmuir isotherm model,

1qe

=1

qmaxb×

1Ce

+1

qmaxð3Þ

where qe is the adsorption capacity, qmax is the maximum adsorptioncapacity (mg/g), b is the Langmuir constant, which is indirectly relatedto the energy of adsorption (L/mg), and Ce is the concentration offluoride at equilibrium. The values of qmax and bwere calculated using anonlinear estimation method to fit the Langmuir isotherm to theexperimental data, and were 5.9 mg/g and 0.066 L/mg, respectively.

1.0 0.8 0.6 0.4 0.2 0.0

2.4

2.6

2.8

3.0

3.2

0

50

100

150

200

250

Temperature, 65ΟCCoating amount, 18.0%

(a)(b)

Ads

orpt

ion

capa

city

, mg/

g

Com

pres

sive

Str

engt

h, N

Latex/Fe-Al-Ce

Fig. 5. Fluoride adsorption capacity (a) and compressive strength (b) of the granules vslatex/Fe–Al–Ce ratios (granule dose=5 g/L, initial fluoride concentration=0.001 M,adsorption time=36 h).

a b c

Fig. 6. Surface morphology of granules with different coating amounts (coating temperature at 65 °C, latex/Fe–Al–Ce=0.5:1). Coating amounts: (a) 18.0%; (b) 27.8%; and (c) 41.7%.

Table 3Compressive strength and adsorption capacity vs coating amount.

Coating amounts,% 18.0 27.8 41.7Compressive strength, N 90 60 42.1Adsorption capacity, mg/g 2.63 2.77 2.86

(Conditions: coating temperature at 65 °C, latex/Fe–Al–Ce=0.5:1, granule dose=5 g/L,initial fluoride concentration=0.001 M, and adsorption time=36 h).

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The adsorption capacity of the Fe–Al–Ce adsorbent granules werecompared to that of other commonly used adsorbents at theequilibrium fluoride concentration of 1 mg/L, which is a reasonableconcentration of fluoride in safe drinking water. For the activatedalumina and activated carbon, the adsorption capacities at anequilibrium fluoride concentration of 1 mg/L were 0.96 mg/g and0.075 mg/g, respectively [14]. From the fitted Langmuir isothermmodel, the adsorption capacity of the Fe–Al–Ce adsorbent granuleswas calculated to be 0.37 mg/g, which was higher than the adsorptioncapacity of activated carbon but lower than that of activated alumina.

However, adsorbents are evaluated not only by the adsorptioncapacity but also by compressive strength, cost, feasibility of massproduction, etc. The compressive strength of the optimized granuleswas much higher than that of the activated alumina shown in Table 1.Although the cost of the Fe–Al–Ce nano-adsorbent is high, the maincomponent of the granule, sand, is low cost. The estimated cost of thegranules is close to the activated alumina. The granulator and thecoating granulation process can be easily scaled up for massproduction, e.g., by using a fluidized bed or a drum-fluidized bedgranulator.

A conventional method for the regeneration of the Fe–Al–Ceadsorbent was by rinsing with a NaOH solution [3]. However, thisregenerationmethod is costly since it requires much NaOH andwater,and it produces new waste water as well. Hence, a new method foradsorbent granule regeneration needs to be developed in the future.

4. Conclusions

The parameters for the granulation of a Fe–Al–Ce nano-adsorbentby spray coating onto glass beads in a fluidized bed were optimized.The optimal conditions resulted in a better structure of the coatinglayer, where the adsorbent particles were just bonded by the latex,which formed a network structure with many pores that has plentifuladsorbent particles for the adsorption of fluoride ions.

The cross-linking degree of the polymer in the coated layerincreased as the coating temperature increased. This led to increasedgranule strength and decreased adsorption capacity. As the mass ratioof latex/Fe–Al–Ce increased, the granule strength increased but the

adsorption capacity decreased because some active sites on theadsorbent particles were covered by the polymer.

The optimal parameters for producing high performance adsor-bent granules were a coating temperature of 65 °C, a latex/Fe–Al–Ceratio of 0.5:1, and a coating amount of 27.8%. These granules had anadsorption capacity for fluoride of 2.77 mg/g (coated granules) forwater with an initial fluoride concentration of 0.001 M that weretreated at pH 7. These granules have a suitable compressive strengthof 60 N and can be used as a filter in a packed bed for fluoride removalfrom drinking water.

Acknowledgements

The authors wish to express their appreciation of financial supportof this study by the National High Technology Research andDevelopment Program of China (863 Program, No. 2007AA06Z319),and the National Natural Science Foundation of China (NSFC No.20906055).

References

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[14] N.A. Medellin-Castillo, R. Leyva-Ramos, R. Ocampo-Perez, R.F. Garcia de la Cruz, A.Aragon-Piña, J.M. Martinez-Rosales, R.M. Guerrero-Coronado, L. Fuentes-Rubio,Adsorption of fluoride from water solution on bone char, Ind. Eng. Chem. Res. 46(26) (2007) 9205–9212.

0 20 40 60 80 100 120 140

1

2

3

4

5

6

fitted with the Langmuir isotherm model

Experiment data

q e(mg/

g)

Ce(mg/L)

Fig. 7. Adsorption isotherm of the adsorbent granules.

Table 4Fitted parameters of the Langmuir adsorption isotherm.

Parameters Maximum adsorptioncapacity

Langmuir constant Correlation coefficient

qmax, (mg/g) b, (L/mg) R2, (−)Values 5.9 0.066 0.972

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