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Particuology 10 (2012) 317– 326

Contents lists available at SciVerse ScienceDirect

Particuology

jou rna l h omepage: www.elsev ier .com/ locate /par t ic

reparation and ion exchange properties of egg-shell glass beads with differenturface morphologies

hun Shen, Yujun Wang, Jianhong Xu, Yangcheng Lu, Guangsheng Luo ∗

tate Key Laboratory of Chemical Engineering, Department of Chemical Engineering, Tsinghua University, Beijing 100084, China

r t i c l e i n f o

rticle history:eceived 31 July 2011eceived in revised form 23 October 2011ccepted 3 November 2011

eywords:orous glassubcritical water treatmenteavy metal adsorptiongg-shell structure

a b s t r a c t

A subcritical water treatment method was developed for preparing porous-surfaced glass beads with anegg-shell structure in a batch reactor. Based on the “corrosion-ion-migration-recondensation” strategy,ordinary soda-lime glass beads with a diameter of about 100 �m were made first to react with subcriticalwater to effect controlled quantity of silicate dissolution of glass by adjusting treatment time and tem-perature. The dissolved silicate was then made to recondense on the glass core to form different porousshell morphologies: pores, flakes and fibers. Among these, glass beads coated with fibers with surfacearea of 154.5 m2/g, pore volume of 0.27 cm3/g and pore size of 7.1 nm were obtained at 573 K after 2 hof treatment. The prepared porous-surfaced glass beads were then used as adsorbent for heavy metalions, showing various ion exchange properties. Glass beads covered with fibers displayed fast kinetics

and high sorption capacity because of their egg-shell structure and high surface area. More than 90% ofcopper ions were adsorbed within 100 min from a solution with an initial concentration of 110 mg/L at313 K. Ion sorption capacities were 149.33, 81.33 and 42.96 mg/g respectively for Ag+, Cu2+ and Ni2+ at313 K. A green and low-cost method was thus developed to produce egg-shell-structured porous glasswith high sorption capacity.

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© 2011 Chinese So

. Introduction

Due to its high stability, easy preparation and low cost, porouslass poses as a potential material for water treatment (Petrellat al., 2010; Tokarev et al., 2010), heterogeneous catalysis (Hammel

Allersma, 1975; Schmöger et al., 2008, 2009; Yazawa, Machida,ubo, & Jin, 2009), biochemistry (Dartiguenave, Hamad, & Waldron,010; Karakus & Pekyardımci, 2009; Li et al., 2010), and some otherelds (Kuraoka, Chujo, & Yazawa, 2001). Petrella et al. (2010) usedecycled porous glass with a particle size in the range of 0.5–1 mmo adsorb lead ions from wastewater. Best column performanceas obtained with an initial concentration of 2 mg Pb2+/L and aow rate of 0.20 L/h, to realize a lead ions retention capacity of.10 mg/g. Mass diffusion was assumed to be the rate-determiningtep of the overall ion exchange reaction.

Glass beads used in industry are usually composed of 25.1 wt%

a2O, 9.8 wt% MgO, and 4.9 wt% CaO. Because of the high con-

ents of alkali and alkaline earth metals and their ion exchangeroperty with heavy metal ions, glass beads exhibit great potential

∗ Corresponding author. Tel.: +86 010 62788568; fax: +86 010 62788568.E-mail addresses: wangyujun@mail.tsinghua.edu.cn (Y. Wang),

sluo@mail.tsinghua.edu.cn (G. Luo).

GJYRRV&2

674-2001/$ – see front matter © 2011 Chinese Society of Particuology and Institute of Process Eoi:10.1016/j.partic.2011.11.002

of Particuology and Institute of Process Engineering, Chinese Academy of Sciences. Published by Elsevier B.V. All rights reserved.

n heavy metal adsorption. However, the specific surface area ofntreated glass beads is small, while most of the active ions insidehe glass beads are not available for exchange with heavy metalons. In order to better utilize the potential of glass, it is desirableo prepare porous glass beads with high surface areas. On the otherand, in as much as the overall rate of ion exchange process issually controlled by internal mass transfer resistance (Helfferich,962), a specific egg-shell structure with shorter diffusion length

s expected to significantly reduce the residence for mass transfer.hus, egg-shell-structured porous glass beads with high specificurface area seem to be an ideal answer.

Four main methods are available for preparing porous glass,amely, the phase separation method (Doremus, 1973; Nimjaroen,orimoto, & Tangsathitkulchai, 2009; Suzuki & Tanaka, 2008;eronesi, Leonelli, Pellacani, & Boccaccini, 2003), hydrother-al treatment under supercritical conditions (Sigoli, Feliciano,iotto, Davolos, & Jafelicci, 2003; Sigoli, Kawano, Davolos, &

afelicci, 2001), hydrothermal hot-pressing (Matamoros-Veloza,anagizawa, & Yamasaki, 1999; Matamoros-Veloza, Yanangisawa,endon-Angeles, & Oishi, 2004; Matamoros-Veloza, Yanangisawa,

endon-Angeles, Oishi, & Cisneros-Guerrero, 2004; Matamoros-eloza et al., 2008; Yanagizawa et al., 2006; Yoshikawa, Sato,

Tanaka, 2008), and corrosion by solutions (Suhiyama et al.,004; Tournie, Ricciardi, & Colomban, 2008). The phase separation

ngineering, Chinese Academy of Sciences. Published by Elsevier B.V. All rights reserved.

318 C. Shen et al. / Particuology 10 (2012) 317– 326

Nomenclature

d pore size (nm)C0 and Ce the initial and equilibrium concentrations (mg/L)F the function attainment of equilibriumk the rate constant in intraparticle diffusion model

(mg/(g min0.5) or the sorption rate constant in filmdiffusion model (min−1)

k2 the pseudo-second-order rate constant(mg/(g min))

KL the equilibrium constant (L/g)Mn+ the metal ion to be exchangedq the amount of adsorbed metal ions on unit adsor-

bent (mg/g)qe the amount of metal ions adsorbed on porous glass

beads after equilibrium (mg/g)qm the maximum sorption capacity (mg/g)qt and qet the amount of metal ions adsorbed on porous glass

at time t and after equilibrium, respectivelyV the volume of the solution (L)

modasmaslwt

cwsrw

C7omb0apThtEceaftos

F(

hmcdddtbflfltia

2

2

1c4frr

2

htp

W the dosage of the porous glass (g)t the contact time (min)

ethod calls for high temperature (773–873 K) and large amountsf acid. In the second method, the glass-transition temperature isecreased by supercritical water treatment followed by phase sep-ration, leading to a surface area of 11.5 m2/g of the treated porousilica matrix (Sigoli et al., 2003). The hydrothermal hot-pressingethod is much greener than the methods stated above because it

voids chemical reagents, though it is difficult to control the poretructure. In the method of corrosion by solution, alkali or alka-ine earth ions in glass exchange with hydrogen-bearing ions from

ater or hydroxyl ions in solution during hydrothermal treatment,hus leading to pores on the glass surface.

In our previous work (Sun, Wang, Lu, Wang, & Luo, 2008), a sub-ritical water treatment method for preparing porous glass beadsas developed. As far as we know, preparation of porous glass with

pecial egg-shell structure under subcritical water has not beeneported. Under the condition of 573 K and 10 MPa, water can reactith glass beads as follows:

Si O Na + H2O → Si O H + Na+ + OH−

Si O Si + OH− → Si O H + SiO−

SiO− + H2O → Si O H + OH−(1)

ompared to hydrothermal treatment with supercritical water at43 K, the treatment conditions were much milder. Because nother chemical reagents except subcritical water were used in thisethod, it is considered environmentally friendly. Porous glass

eads with a surface area of 34.05 m2/g and a pore volume of.158 cm3/g were made in this method, and showed ion exchangebility with the sorption capacities of 20.8 and 99.6 mg/g for cop-er and silver ions, respectively (Sun, Wang, Yang, Lu, & Luo, 2008).hat the adsorption capacities of porous glass beads were relativelyigh even though the glass beads had a small surface area, indicateshat the amount of exchangeable ions on unit surface area was high.vidently, if the surface area were further increased, the adsorptionapacity would be further improved. In their work, Sun, Wang, Lu,t al. (2008) used a tubular reactor to carry out the treatment in

continuous flow mode. The active ions, e.g., Na+, Ca2+, dissolved

rom the glass beads and most of them removed from the reactor byhe flowing water. So during the cooling stage only small amountsf ions were recrystallized, thus resulting in thin porous layer andmall specific surface area.

iptw

ig. 1. Experimental set up: (1) temperature controller; (2) stainless steel reactor;3) electric heating layer; (4) heat insulation layer.

To prepare porous glass beads with controllable structures andigh adsorptive capacity, a modified subcritical water treatmentethod was developed using a tank reactor instead to replace the

onventional tubular reactor in order to help the active ions recon-ense. This new method has several advantages: first, ions fromissolution need not be removed and they would simply recon-ense during the cooling stage; second, it is highly repeatable;hird, it is controllable, and easy to tailor the morphologies of glasseads by varying the treating conditions as is not possible in theowing mode. Three kinds of morphologies were obtained: pores,akes and fibers by varying the treatment conditions. The adsorp-ion of the prepared beads for copper, silver, palladium and nickelons was investigated, all resulting in higher ion sorption capacitiesnd faster adsorption kinetics.

. Experimental

.1. Materials and chemicals

Soda-lime glass microbeads with diameters ranging from 75 to50 �m were obtained from Hebei Chiye Corporation, with theomposition of 59.7 wt% SiO2, 25.1 wt% Na2O, 9.8 wt% MgO, and.9 wt% CaO. These beads were sieved before use, and those rangingrom 95 to 105 �m in size were taken as samples. Palladium chlo-ide, nickel nitrate, silver nitrate, and copper sulfate were analyticaleagents purchased from Beijing Chemical Plant.

.2. Preparation of porous glass beads

Fig. 1 shows the experimental set up, consisting of an electricallyeated reactor with a volume of 250 cm3 and controlled by a ±0.1 Kemperature controller. Being hermetically sealed, the reactionressure was temperature dependent. Experimental procedure

s as follows. First, 200 mL of water and 5.0 g of glass beads werelaced in the tank reactor, then the reactor was gradually heatedo the desired reaction temperature, ranging from 523 to 573 K,hile the pressure increased with temperature from ambient to

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C. Shen et al. / Particu

bout 4–8 MPa. The subcritical state was maintained for a fewinutes, after which the reactor was cooled to room temperature,

aturally. Porous glass beads were then separated from water byltration and washed several times with de-ionized water before

urther processing. The influences of treating temperature andreating time on the morphology of glass beads were studied.

.3. Adsorption performance

To investigate the isotherm of adsorption, 0.1 g of the prepareddsorbent was put into 50 mL of aqueous solution with variousnitial concentrations of metal ions: e.g., 30–180 mg/L Cu2+ and–13 mg/L Pd2+. The mixture was shaken for 24 h at 160 rpm in

temperature-controlled shaker at 313 K.To investigate the kinetics of adsorption, 0.1 g of porous glass

as added into 100 mL of copper solution with an initial concen-ration of 100 mg/L and other conditions identical to those in thesotherm study. The solution was regularly sampled to measure theopper ion concentration.

To investigate the adsorption capacity of Ni2+, Ag+, and Cu2+,.1 g of porous glass was added into 100 mL of solution with an ini-ial concentration of 100 mg/L. After equilibrium was reached, glasseads were separated from the solution by filtration and mixedith fresh solution again to get another equilibrium. This proce-ure was repeated until no more adsorption was detected. Thedsorptivity was the sum of the separate loadings. Other exper-mental conditions remained identical to those in the isothermtudies. Because adsorption capacity is independent of initial con-entration, and dependent only on the adsorbent materials, thedsorption capacities between different materials are comparable.

The concentration of metal ions was measured on an atomic-bsorption spectrophotometer (AAS, Z5000, Hitachi) with anir–acetylene flame. The amount of adsorbed metal was calculatedsing the following equation:

= V(C0 − Ce)W

, (2)

here q is the amount of adsorbed metal ion on unit adsorbentmg/g), C0 and Ce are the initial and equilibrium concentrationsmg/L), V is the volume of the solution, and W is the dosage of theorous glass (g).

.4. Characterization of prepared materials

The morphology of the porous glass beads was observedsing scanning electron microscopy (SEM) (JEOL JSM 7401F,

EOL Ltd., Japan). Surface composition was calculated throughhe elemental analysis using energy dispersive spectroscopyEDS) combined with the SEM. Nitrogen adsorption-desorptionsotherms were measured at 77 K on a Quantachrome Autosorb--C chemisorption–physisorption analyzer. Specific surface areaas calculated from the adsorption branches in the relativeressure range of 0.05–0.25. Pore diameter and pore size distri-ution were calculated from the desorption branches using thearrett–Joyner–Halenda (BJH) method, and the total pore volumeas evaluated at a relative pressure of about 0.99.

.5. Models used in adsorption performance

The sorption process includes three steps: (1) solute transferso the sorbent particle surface (film diffusion); (2) solute transfers

rom the surface to the active sites along channels in the sorbentintraparticle diffusion); (3) retention on the active sites via ionxchange. Depending on particle characteristics, the influence oflm diffusion resistance could be significant. In order to reduce

tb1Y

10 (2012) 317– 326 319

lm resistance, different conditions were tried and all experimentsere conducted under the best condition. As to the influence of

ntraparticle diffusion, it should have been weakened because ofhe egg-shell structure. In other words, the egg-shell structurehould be suitable for fast reactions limited by mass transfer, forxample, ion exchange. In order to validate which of the resistancess rate-limiting, three models were used to describe the adsorptioninetics, viz., pseudo-second-order model, intraparticle diffusionodel and film diffusion model.The pseudo-second-order model (Ho & Mckay, 1999) can be

xpressed as follows:

t

qt= 1

k2q2et

+ t

qet, (3)

here t is the contact time (min), qt and qet are the amount ofetal ions adsorbed on porous glass at time t and after equilib-

ium, respectively, k2 (mg/(g min)) is the pseudo-second-order rateonstant.

The intraparticle diffusion model (Allen, Mckay, & Khader, 1989)an be expressed as follows:

t = kt0.5, (4)

here qt and t are defined as before, k is the rate constantmg/(g min0.5)). The plot of qt versus t0.5 gives a straight line with

zero intercept when intraparticle diffusion process is the rate-etermining step.

The linear form of the film diffusion model is as follows:

n(1 − F) = −kt, F = qt

qe, (5)

here F is the function attainment of equilibrium, qt, qe and t areefined as before, k is the sorption rate constant (min−1). Accord-

ng to the film diffusion model, if the process is controlled by filmiffusion, the plot of ln(1 − F) versus t is linear with a zero intercept.

The Langmuir (1918) model was used to describe the sorptionrocess. It is based on the assumption of monolayer sorption on

homogeneous surface with constant sorption energy. Its linearorm is as follows:

Ce

qe= Ce

qm+ 1

KLqm, (6)

where Ce is the equilibrium concentration (mg/L); qe is themount of metal ions adsorbed on porous glass after equilibriummg/g); qm represents the maximum sorption capacity (mg/g); andL is the equilibrium constant (L/g). These constants, qm and KL, arevaluated from the slope and the intercept of the linear plot of Ce/qe

ersus Ce, respectively.

. Results and discussion

.1. Preparation of glass beads with different morphologies

Fig. 2(a) and (b) shows the surface morphology of the glass beadsntreated with subcritical water, both smooth and non-porous.ig. 2(c)–(h) shows the surface morphologies of the preparedorous glass beads under different treating conditions. Treatingemperature and treating time have significant effect on the sur-ace morphologies. Pores only were formed on the surface oflass beads treated at 523 K for 15 min as shown in Fig. 2(c)nd (d); for those treated with subcritical water at 573 K for 1 h,akes appeared besides pores as shown in Fig. 2(e)–(f); for those

reated with subcritical water at 573 K for 2 h, fibers appearedesides pores as shown in Fig. 2(g)–(h). In others’ work (Doremus,973; Matamoros-Veloza et al., 1999, 2008; Matamoros-Veloza,anangisawa, Rendon-Angeles, & Oishi, 2004; Matamoros-Veloza,

320 C. Shen et al. / Particuology 10 (2012) 317– 326

Fig. 2. (a and b) Surface morphology of untreated glass beads; (c and d) surface morphology of glass beads treated at 523 K for 15 min; (e and f) surface morphology of glassbeads treated at 573 K for 60 min; (g and h) surface morphology of glass beads treated at 573 K for 120 min.

C. Shen et al. / Particuology 10 (2012) 317– 326 321

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ctsatiw

Table 1BET results of the prepared porous glass beads under different conditions.

Treating condition Surface area(m2/g)

Pore volume(mL/g)

Pore size(nm)

Original 2.9 × 10−3 – –523 K, for 15 min 8.8 3.662 × 10−2 16.503

daa(aabw2tt(tstoc

Woddslsodnatiagsctimbptsccrystals on a macro level. In our previous work (Sun, Wang, Lu,et al., 2008), a continuous flow reactor was used, in which mostof the silicate dissolved from the glass beads was removed by the

Table 2Surface element composition of glass beads prepared in this work (A) and in Sun’swork (Sun, Wang, Lu, et al., 2008) (B).

Sample Surface elements content (mol%)

Fig. 3. Position relation of fiber layer and flake layer.

anangisawa, Rendon-Angeles, Oishi, & Cisneros-Guerrero, 2004;imjaroen et al., 2009; Sigoli et al., 2001, 2003; Suhiyama et al.,004; Suzuki & Tanaka, 2008; Tournie et al., 2008; Veronesi et al.,003; Yanagizawa et al., 2006, 2008), only pores were formed onhe surface of glass beads. Even in our previous work (Sun, Wang,u, et al., 2008), morphology involving fibers had not been dis-overed; and only flake morphology was reported. In addition, itas found further that the total surface of the glass bead was cov-

red with microstructures (pores, flakes or fibers), though in Sun’sork (Sun, Wang, Lu, et al., 2008), some parts of the surface of

he glass beads were not covered with flakes. Fig. 3 shows clearlyhat a flake layer underlies a top fiber layer. Because a tank reactoras used in this work, the silicate dissolved from the glass beadsnder the subcritical water environment remained in the reactornd re-crystallized when the temperature dropped, leading to neworphologies. Such new morphology is highly reproducible in the

resent tank reactor.Pore size distribution of glass beads untreated with subcriti-

al water is shown in Fig. 4(a), those of the porous glass beadsreated at 523 K for 15 min, at 573 K for 1 h and 2 h are respectivelyhown in Fig. 4(b)–(d). Their specific surface areas, pore volumes,nd mean pore sizes are listed in Table 1. It indicates no pores on

he surface of the untreated glass beads, however, after the subcrit-cal water treatment in the tank reactor, the specific surface area

as significantly enlarged. Different treating conditions resulted in

573 K, for 60 min 162.6 2.578 × 10−1 6.339573 K, for 120 min 154.5 2.743 × 10−1 7.099

ifferent pore structures. Fig. 4 shows that mesopores around 4 nmppeared on the surface of all treated glass beads. For those treatedt 523 K, however, larger mesopores around 30 nm also appearedas shown in Fig. 4(b)). After treatment at 573 K, some pores with

diameter of around 8 nm were obtained, as shown in Fig. 4(c)nd (d). In the following discussion, sample (A) denotes the glasseads covered with flakes (i.e., treated at 573 K for 1 h) in this work,hile sample (B) denotes those in Sun’s work (Sun, Wang, Lu, et al.,

008). Compared to sample (B) with a surface area of 34.05 m2/g,he surface area of sample (A) is greatly increased. Table 2 showshe comparison of surface element compositions between samplesA) and (B). As expected, the content of sodium increased from 1.8%o 5.1% by changing the reactor type, that is, both active ion den-ity and surface area increased by changing the tubular reactor to aank reactor. Therefore, more active ions remained on the surfacef the porous glass prepared in this work, presaging higher sorptionapacity.

The “corrosion-ions migration-recondensation” strategy (Sun,ang, Lu, et al., 2008) provides a framework for the mechanism

f formation. As temperature rises from room temperature to theesired constant temperature, the outer rind of glass is dissolved, asescribed in Eq. (1), and when temperature decreases, the dissolvedilicon sources recondense on the surface of the glass beads. Theonger the constant elevated temperature stage, the more siliconources and materials dissolve into the water. The basic structuref the silicate system is the Si O4 tetrahedron. According to itsifferent connection mode, there are five kinds of morphologies,amely, the layer, fiber, ring, island and shell structures. Some poresppeared on the surface of glass beads treated at 523 K because ofhe shorter treating time and lower reaction temperature, resultingn very few silicon sources to recondense. For the samples treatedt 573 K, as shown in Fig. 5, the formation of different morpholo-ies can be described as follows. At the beginning of the coolingtage, the temperature is high and the flake structure is a favorablerystalline texture. With decrease in temperature, the favored crys-alline texture gradually changes to a fiber structure, thus resultingn the appearance of a fibrous layer, provided there is still enough

aterial to recondense. The appearance of flakes and fibers coulde explained by the growth unit model of the anion coordinationolyhedron theory (Li, Shi, Chen, & Yin, 2001). At low temperatures,he favored growth unit is short fibers. At high temperatures, thesehort fibers are highly unstable and the five-united molecules withell structures are the favored growth units, which are laminated

Si Na Mg Ca O

(A) 28.9 5.1 1.2 6.2 58.6(B) 23.8 1.8 1.6 2.6 70.2

322 C. Shen et al. / Particuology 10 (2012) 317– 326

F 3 K, f

fld

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1isodnpttsafmfit

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ig. 4. Pore size distributions of porous glass beads for (a) original; (b) treated at 52

owing water, so little silicate recondensed when the temperatureropped, and no fibers appeared.

.2. Mechanism of ion exchange

According to Zachariasen’s random network theory (Holland,964), the initial configuration of glass is an extended network lack-

ng symmetry and periodicity. The glass forming cations (Si4+) areurrounded by a polyhedron of oxygen ions of two kinds, bridgingxygen ions and non-bridging oxygen ions in the form of polyhe-ra. Each bridging oxygen ion links two polyhedra, while eachon-bridging oxygen ion links only one polyhedron. Cations of lowositive charge (Na+, Ca2+, and Mg2+) may exist in holes betweenhe oxygen polyhedra, called network modifiers, to compensate forhe excess negative charge of the non-bridging oxygen ions. Afterubcritical water treatment, some new textures with large surfacerea appeared and part of the modifiers still remained on their sur-ace. Based on our previous work (Sun, Wang, Yang, et al., 2008),

etal ions with electronegativities higher than the network modi-

ers can exchange with the network modifiers and compensate forhe excess negative charge of the non-bridging oxygen ions instead.

Let Mn+ be the metal ion to be exchanged. In order to investi-ate the ion exchange mechanism, concentrations of the modifiers

pwcs

Fig. 5. Formation process of d

or 15 min; (c) treated at 573 K, for 60 min and (d) treated at 573 K, for 120 min.

nd Mn+ in solution were analyzed with atomic absorption spec-roscopy (AAS) before and after the adsorption process. As shownn Table 3, before the ion-exchange process, all concentrations ofa+, Ca2+ and Mg2+ were below detectable limits, while concentra-

ions of Na+, Ca2+ and Mg2+ all increased after the exchange process.onsidering the stoichiometric proportion, Mn+ can substitute na+ ions or n/2 Ca2+ ions, and the results apparently supported

he postulated mechanism. In addition, pH values of the solutionere monitored at intervals, showing a slight increase during thehole process, as shown in Fig. 6. For example, the pH value of cop-er solution increased from 4.52 to 4.90 during the process. Hence,he results indicate that metal ions exchanged with modifiers con-ained in the glass (Na, Ca, and Mg) stoichometrically in the formf cation, relying little on the pH value of the solution.

.3. Adsorption capacities

The prepared porous glass beads exhibit the ability for ionxchange with metal ions of higher electronegativity. Table 4 com-

ares the adsorption capacities of the prepared porous glass beadsith other adsorbents found in the literature. The ion sorption

apacities of the present porous glass beads with an egg-shelltructure achieved 149.33, 81.33 and 42.96 mg/g for Ag+, Cu2+ and

ifferent morphologies.

C. Shen et al. / Particuology 10 (2012) 317– 326 323

Table 3Element content in solution before and after adsorption (adsorbent 2 g/L; Cu2+/Ag+/Ni2+ 100 mg/L; Pd2+ 12 mg/L; 313 K; interaction time 24 h).

Experimentalnumber

Metal Concentration beforeadsorption (mg/L)

Concentration beforeadsorption (mmol/L)

Concentration afteradsorption (mg/L)

Concentration afteradsorption (mmol/L)

1

Cu2+ 99.300 1.602 8.880 0.143Na+ – – 53.800 2.445Ca2+ – – 5.211 0.137 0.096Mg2+ – – 2.120

2

Ag+ 91.424 0.854 – –Na+ – – 17.062 0.776 0.039Ca2+ – – 1.471 0.002Mg2+ – – 0.040

3

Ni2+ 71.844 1.283 8.628 0.154Na+ – – 40.024 1.819Ca2+ – – 4.847 0.128Mg2+ – – 1.700 0.077

Pd2+ 12.036 0.116 7.896 0.076+

Nraa1wsampiilotiuapsim

tta

sMasafrom 2.9 × 10−3 to 162.6 m2/g, thus multiplying the active part ofthe porous glass where active ions were exchanged with heavymetals. In addition, the density of active ions is increased by chang-ing the traditional continuous tubular reactor to a batch reactor.

Table 4Comparison of the adsorption capacity of the prepared porous glass beads withother adsorbents.

Adsorbate Adsorbent Adsorptioncapacity(mg/g)

Reference

Cu2+

Porous glass with pores 8.32 This workPorous glass with flakes 78.20 This workPorous glass with fibers 81.33 This workLin’an montmorillonite 19.26 Wu et al. (2011)Sporopollenin 13.44 Gubbuk (2011)Porous glass with flakes 20.80 Sun, Wang, Yang, et al.

(2008a)Wood sawdust-E 37.44 Ahmed (2011)

4Na – –

Ca2+ – –

Mg2+ – –

i2+ respectively at 313 K, much higher than those found in otheresearch. Wu, Zhao, and Yang (2011), used Lin’an montmorillonite,

natural layer-type silicate with a surface area of 39.5 m2/g, todsorb copper ions in aqueous solution, achieving a capacity of9.26 mg/g, as compared to 81.33 mg/g of our porous glass beadsith a surface area of 154.5 m2/g. If we assume that the den-

ity of active ions were almost the same because the compositionnd structure of glass beads are quite similar to those of Lin’anontmorillonite, it seems that adsorption capacity is more or less

roportional to specific surface area. Kolodynska (2011) used anon-exchange resin, Amberlite IRA 402, to remove copper ionsn wastewater, reporting a sorption capacity of 56.67 mg/g, alsoower than that of the porous glass beads in this work. Becausef the increase of both surface area and density of active ions,he sorption capacity of glass beads covered with fibers for silverons reached 149.33 mg/g. Priya, Basha, and Ramamurthi (2011)sed cation exchange resins as adsorbents for nickel in wastew-ter with a capacity of 45.98 mg/g, not much different from theorous glass beads covered with fibers in this work. Evidently, theorption capacities of porous glass are almost the same as those ofon-exchange resins while the preparation of these glass beads is

uch greener and cheaper.

These results indicate that glass beads covered with fibers have

he largest adsorption capacity while those covered with pores,he smallest. Both show an advantage in adsorption capacitiess compared to other adsorbents, primarily due to their large

Fig. 6. Changes of pH versus time during ion exchange.

0.780 0.03550.149 0.0040.032 0.001

urface area and high active ions density (25.1 wt% Na2O, 9.8 wt%gO and 4.9 wt% CaO in raw soda-lime glass beads). The ability of

lkali metal or alkali earth metals to exchange with heavy metalshows that glass beads have great potential in sorption capacity. Inddition, subcritical water treatment increased their surface area

Amberlite IRA 402 56.67 Kolodynska (2011)n-HAp/chitosancomposite

6.2 Gandhi, Kousalya, andMeenakshi (2011)

Ag+

Porous glass with pores 14.65 This workPorous glass with flakes 143.49 This workPorous glass with fibers 149.33 This workPorous glass with flakes 99.60 Sun, Wang, Yang, et al.

(2008)Macrofungus Pleurotusplatypus

46.70 Das, Das, and Mathew(2010)

Hollow chitosanmicrospheres

33.00 Wang and Yu (2010)

Dead cell ofcorynebacteriumglutamicum

52.50 Sneha, Sathishkumar,Mao, Kwak, and Yun(2010)

Ni2+

Porous glass with pores 2.25 This workPorous glass with flakes 36.01 This workPorous glass with fibers 42.96 This workActivated charcoal 9.45 Lalhruaitluanga, Prasad,

and Radha (2011)Cashew nut shell 18.87 Senthil Kumar et al.

(2011)Cation exchange resins 45.98 Priya et al. (2011)Manganese oxide coatedsand

3.33 Boujelben, Bouzid,Elouear, and Feki (2010)

324 C. Shen et al. / Particuology

Fa

Ti

3

wqi

aptt

t

rctatstaptietmm

ig. 7. Sorption of Cu2+ on the porous glass beads with three different morphologiess a function of time.

hese two reasons probably contributed to the high sorption capac-ty reported in this work.

.4. Adsorption kinetics

2+

Fig. 7 shows the adsorption of Cu on porous glass beadsith the three morphologies. The concentration of Cu2+ decreased

uickly in the first 100 min, during which more than 90% of Cu2+

ons were adsorbed on the porous glass beads, after which, the

3

i

Fig. 8. Results of m

10 (2012) 317– 326

dsorption rate slowed down until the equilibrium state. In com-arison, Lin’an montmorillonite (Wu et al., 2011), as an adsorbento remove copper ions through ion exchange, took about 360 mino absorb 90% of copper ions.

Results of model analysis (Section 2.5) are shown in Fig. 8, andhe related parameters are given in Table 5.

Correlation showed that the sorption data could be well rep-esented by the pseudo-second-order model, that is, the rateontrolling step of metal ions onto porous glass is the chemisorp-ion process. Neither intraparticle diffusion nor film diffusionlone is the rate-determining step, but both influence the sorp-ion process. Moreover, because of the egg-shell structure withhort diffusion length, the chemical exchange reaction becamehe rate-determining step instead of mass transfer in the over-ll ion exchange process. The major cause was that long diffusionath increased the resistance of intraparticle diffusion. Further, ifhe attack of subcritical water on the glass beads was radical, thenfluence of intraparticle diffusion should be much greater. Thegg-shell structure of the porous glass adopted in this work helpedo ameliorate mass transfer limitations. In other words, porous

aterials with an egg-shell structure are promising candidates forultiphase reactions suffering from intraparticle diffusion.

.5. Sorption isotherms

Using the prepared porous glass beads covered with flakes, thesothermal adsorption of Cu2+ and Pd2+ at 313 K was determined,

odel analysis.

C. Shen et al. / Particuology 10 (2012) 317– 326 325

Table 5Parameters and the statistical fits of the sorption data.

Treating condition Pseudo-second-order model Intraparticle diffusion model Film diffusion model

k (mg/(g min)) R2 k (mg/(g min0.5)) R2 k (min−1) R2

523 K, 15 min 0.0048 0.9690 0.14 0.8693 0.0050 0.9035573 K, 1 h 0.0013 0.9955 1.87 0.9938 0.0075 0.9641573 K, 2 h 0.0013 0.9959 1.88 0.9949 0.0071 0.9606

Fig. 9. Isotherm model of Cu2+ and Pd2+ ions

Table 6Parameters of Langmuir adsorption model for Cu2+ and Pd2+ on porous glass coveredwith flakes.

Metal qm KL R2

adado

4

sWmtebsh1afcaLta

A

SN

R

A

A

B

D

D

DG

G

H

HH

HK

K

K

L

L

L

Cu2+ 51.02 1.57 0.9973Pd2+ 0.98 1.04 0.9973

s shown in Fig. 9. Related model parameters of the adsorptionata to these equations are listed in Table 6. The adsorption of Cu2+

nd Pd2+ ions behaves in accordance with the Langmuir isotherm,emonstrating that the adsorption of metal ions onto the surfacef porous glass beads is monolayer.

. Conclusions

By varying treating conditions, glass beads with porous egg-hell surface of different morphologies were successfully prepared.

ith increasing treating temperature and treating time, the surfaceorphology changed from pores through flakes to fibers. This syn-

hesis route has the advantages of simple operation, low cost, andnvironmental friendliness. Ion sorption properties of the preparedeads were also investigated. The prepared porous glass beadshowed fast reaction kinetics and high adsorptive capacities foreavy metal ions. More than 90% of Cu2+ ions were adsorbed in00 min from a solution with an initial concentration of 110 mg/Lt 313 K. The egg-shell structure helped to promote mass trans-er. Glass beads covered with fibers showed the highest adsorptiveapacity: 81.33, 149.33, and 42.96 mg/g, respectively, for Cu2+, Ag+,nd Ni2+. The ion adsorption process could be described by theangmuir isotherm model. Compared to other inorganic sorbents,he porous glass beads prepared as above showed great potentials a heavy metal separation material and catalyst support.

cknowledgments

We gratefully acknowledge the support of the National Naturalcience Foundation of China (21036002 and 20976096) and theational Basic Research Program of China (2007CB714302).

L

M

on the porous glass beads with flakes.

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