Applied Clay Science 67–68 (2012) 106–116

Contents lists available at SciVerse ScienceDirect

Applied Clay Science

j ourna l homepage: www.e lsev ie r .com/ locate /c lay

Research paper

Adsorptive removal of thorium (IV) using calcined and flux calcined diatomite fromTurkey: Evaluation of equilibrium, kinetic and thermodynamic data

Sabriye Yusan a,⁎, Cem Gok b, Sema Erenturk a,c, Sule Aytas a

a Ege University, Institute of Nuclear Sciences, 35100, Bornova, Izmir, Turkeyb Pamukkale University, Faculty of Science and Letters, Physics Department, 20070 Kınıklı, Campus Denizli, Turkeyc Istanbul Technical University, Energy Institute, Ayazaga Campus, Maslak, Istanbul, Turkey

⁎ Corresponding author. Tel./fax: +90 232 3886466.E-mail address: sabriye.doyurum@ege.edu.tr (S. Yus

0169-1317/$ – see front matter © 2012 Elsevier B.V. Alldoi:10.1016/j.clay.2012.05.012

a b s t r a c t

a r t i c l e i n f o

Article history:Received 9 May 2011Received in revised form 8 May 2012Accepted 26 May 2012Available online 5 October 2012

Keywords:DiatomiteThoriumAdsorptionIsothermKineticThermodynamic

Adsorption experiments were performed under batch process, using Th(IV) initial concentration, pH of solu-tion, contact time and temperature as variables. Th(IV) uptake by calcined (C-D) and flux calcined diatomite(FC-D) was pH dependent (pH range, 3.0–6.0) and maximum sorption was observed at pH 4.0. Sorption ca-pacities for Th(IV) were found to be 0.52 (121.22 mg Th/g) and 0.50 mmol/g (116.69 mg Th/g) for calcineddiatomite (C-D) and flux calcined diatomite (FC-D), respectively. Adsorption process is well described byFreundlich and Dubinin–Radushkevich (D-R) isotherms in comparison with Langmuir, Temkin and Flory–Huggins isotherms. Thermodynamic data (ΔH°, ΔS°, ΔG°) were calculated from the temperature-dependent sorption isotherms. Results suggested that the adsorption of Th(IV) on calcined and flux calcineddiatomite was a spontaneous and endothermic process. A comparison of kinetic models applied to the ad-sorption of Th(IV) onto C-D and FC-D was evaluated for the pseudo-first-order, pseudo-second-order,intraparticle diffusion and film diffusion kinetic models. The experimental data fitted very well the pseudo-second-order kinetic model and also followed by intra-particle diffusion model, whereas intraparticle diffu-sion and film diffusion are both the rate limiting steps for Th(IV) onto calcined and flux calcined diatomite.

© 2012 Elsevier B.V. All rights reserved.

1. Introduction

Long-lived radionuclides in radioactive waste are considered to bedangerous pollutants and their migration by groundwater is stronglyaffected by their adsorption on the geologic materials. Presence of ra-dionuclides and toxic metals in wastes is a major environmental con-cern. Such wastes arise from technologies producing nuclear fuels,and from laboratories working with radioactive materials (Humeinicuet al., 2004).

Thorium which is only stable at its (IV) valence in solution is a nat-urally occurring radioactive elementwidely distributed over the earth'scrust. It is also an important model element for other tetravalent acti-nides such as Np(IV), U(IV), and Pu(IV) (Zuo et al., 2011). The presenceof thorium in the environment not only originates from the nuclear in-dustry, but also from other human activities such as lignite burning inpower plants, ore processing and the use of fertilizers. The amount ofTh(IV) in the environment is controlled by sorption, desorption, migra-tion and diffusion of Th(IV) in clay minerals and oxides. Several meth-odologies including evaporation, chemical precipitation, ion exchangeand adsorption have been developed to remove thorium from aqueoussolutions (Hu et al., 2010). For adsorption of Th(VI) from aqueous solu-tions, many researchers have used various adsorbents such as,

an).

rights reserved.

diatomite (Sheng et al., 2008), XAD-4 resin (Dev et al., 1999), hematite(Murphy et al., 1999), TiO2 (Jakobsson, 1999; Tan et al., 2007), gibbsite(Hongxia et al., 2006), silica (Chen and Wang, 2007a) and PAN/zeolitecomposite (Kaygun and Akyil, 2007). Desorption studies of Th(IV)ions were also realized from the adsorbents using various desorbing re-agents (Bhainsa and D'Souza, 2009; Humelnicua et al., 2010; Wu et al.,2007).

Adsorption is considered to be a particularly competitive and ef-fective process for the removal of trace quantities of heavy metalsfrom aqueous solutions (Huang and Blankenship, 1984). In principle,any solid material with a microporous structure can be used as an ad-sorbent, e.g. bone and coal char, clays, iron oxides, synthetic and nat-ural zeolites, molecular sieves and activated carbon. The mostimportant property of any adsorbent is its surface area and structure.Furthermore, the chemical nature and polarity of the adsorbent sur-face can influence the attractive forces between the adsorbent andadsorbate (Khraisheh et al., 2004).

Diatomite is a siliceous sedimentary rockhaving an amorphous formof silica (SiO2·nH2O) containing a small amount ofmicrocrystallinema-terial and available in various locations around the world. In addition, ithas received attention for its unique combination of physical and chem-ical properties (such as high permeability, high porosity, small particlesize, large surface area, low thermal conductivity and chemical stability,inert) and as low costmaterial for the removal of pollutants fromwaste-water. The silica surface contains silanol groups that spread over the

107S. Yusan et al. / Applied Clay Science 67–68 (2012) 106–116

matrix of the silica (Al-Ghouti et al., 2007; Khraisheh and Al-Ghouti,2005). Diatomite is used in a number of industrial applications, e.g. asa filtration media for various beverages, inorganic and organicchemicals and as an adsorbent for pet litter and oil spills. Although diat-omite has a unique combination of physical and chemical properties, itsusage as an adsorbent inwastewater treatment has not been investigat-ed in detail (Aytas et al., 1999; Lemonas, 1997).

The aim of this study is to investigate and compare the adsorptionproperties of thorium onmineral adsorbents, namely: calcined (C-D)and flux-calcined diatomite (FC-D). The natural adsorbents werecharacterized in respect to their physicochemical properties andTh(IV) adsorption at different pH and initial metal concentrations.The applicability of theoretical models for the equilibrium datafitting was tested. The results obtained from the present study canbe used to evaluate and compare the selected adsorbent materialsfor Th(IV) removal from aqueous solutions and gain insight intothe phenomena that occur at diatomite/Th(IV) interface in naturalsystems.

2. Materials and methods

2.1. Materials

Calcined diatomite (C-D) and flux calcined diatomites (FC-D)were supplied from Turkish Sugar Factories Inc., Kieselguhr Factory,Etimesgut — Ankara, Turkey. Thermal processing at 870–1100 °C,depending on the properties of the raw material and the methodof production is called calcination. This process partially fuses mi-cropores and increases particle size owing to the formation of struc-tured clusters with larger pores and reduced surface area — anenhanced benefit for filter applications. In white (flux) calcination,3–8% soda ash (Na2CO3) or salt (NaCl or KCl) was added to diatomitebefore the sintering or calcination stage. The flux allows iron oxides toenter a glassy phase, where it is colorless and produces greater agglom-eration of the diatomite fragments (Ediz et al., 2010).

In order to improve the mechanical properties, diatomite samplewas calcinated with Na2CO3 at 1600 °C by Kieselguhr Factory, whichis called flux-calcined diatomite (FC-D).

The chemical composition and the physical properties of calcinedand flux-calcined diatomite determined by the Kieselguhr Factorywere given in Table 1 (Analysis Reports of Diatomites, 1986).

2.2. Characterization of adsorbents

The sample characterization analysis was carried out with scan-ning electron microscope (SEM) using a Jeol Jsm 6060 Model, X-ray

Table 1Chemical composition and physical properties of diatomites.Analysis Reports of Diatomites, 1986.

Chemical composition,%

Calcined diatomite(C-D)

Flux calcined diatomite(FC-D)

SiO2 92.68 89.81Fe2O3 1.83 1.72Al2O3 2.60 2.40CaO 0.66 1.02MgO 0.44 0.30Na2O 0.89 3.49K2O 0.34 0.26Ignition loss, % 0.50 0.50

Physical propertiesColor Pink WhiteLoose wt, g/L 95.00 100.00pH (20 °C) 7.00 10.00Average diameter, μm 15.00 20.00

diffraction (XRD) using Phillips X'Pert Pro and Fourier Transform In-frared Spectroscopy (FTIR) using a IRPRESTIGE-21 Model Shimadzuspectrometer. The specific surface area, pore volume and pore sizewere measured by BET N2 adsorption method; surface area and po-rosity analyzer (Micromeritics model ASAP 2020).

In order to investigate the surface characteristics of C-D and FC-D,FTIR analyses were carried out in the range from 400to 4400 cm−1.The sample pellets were prepared for FT-IR measurements by mixingand grinding a small quantity of C-D and FC-D powders (0.005 g)with spectroscopic grade dry KBr powder (0.1 g) and then com-pressing the mixtures to form thin pellets. The FT-IR spectra of C-Dand FC-D were taken before and after adsorption in order to obtaininformation on the nature of probable interactions between the func-tional groups on the surface of diatomite and thorium ions. Identifica-tion of diatomite samples were determined by XRD with Cu-Kα

radiation, Ni filter, λ=1.54 Ǻ.

2.3. Batch adsorption experiments

In this study, sorption potential of thorium ions was investigatedusing calcined and flux calcined diatomite as adsorbents in aqueoussolutions by batch technique in 10 mL polyethylene tubes. Thebatch technique was carried out in a thermostated shaker bath GFL-1083 model. The shaking rate was same for all experiments. The pHof each test solution was adjusted to the required value with dilutedacetic acid (CH3COOH) and sodium acetate (CH3COONa) solutions atthe start of the experiment. Reagent blank was run for every samplesolution. The ability of C-D and FC-D to adsorb thorium(IV) fromaqueous solution was studied at different experimental conditionsof pH of solution, initial concentration of Th(IV), contact time andtemperature on thorium (IV) sorption by C-D and FC-D by changinga parameter and keeping the others constant. The solution was sepa-rated from the solids by filtration. Then the residual thorium (IV) ionsin aqueous solution was determined by using Arsenazo III {3,6-bis[(2-arsonophenyl)-azo]-4,5-dihydroxy-2,7-nphthtalenedisulphonic acid}method at 665 nm against reagent blank using Shimadzu UV–VIS1601 Spectrophotometer (Onishi, 1989). The experiments were carriedout in duplicate. Each experimental result was obtained by averagingthe data from two parallel experiments. Adsorption yields (%) werecalculated by:

Adsorption yield ð%Þ ¼ Ci−Ceð ÞCi

� 100 ð1Þ

where Ci is the concentration of the initial solution (mg/L), Ce is the con-centration of the solution in equilibrium (mg/L).

2.4. Study of kinetics

Kinetic studies were carried out in a thermostated shaker withpolyethylene tubes at 25 °C and 55 °C. 0.1 g of diatomite sampleswere added to 10 mL of thorium solution (50, 100, 150 mg Th/L).The pH of the solution was maintained constant throughout the ex-periment. Kinetic studies were carried out at two different tempera-tures between 10 min and 180 min. Samples were filtered by usingWhatman filter paper no. 44 and then the filtrate was analyzed to de-termine the remaining thorium.

3. Results and discussion

3.1. Surface characteristics of adsorbents

Scanning electronmicroscopy (SEM) is a primary tool for character-izing the fundamental physical properties of the adsorbent. It is usefulfor determining the particle shape and appropriate size distribution ofthe adsorbent. Scanning electron micrographs of calcined diatomite

108 S. Yusan et al. / Applied Clay Science 67–68 (2012) 106–116

and flux calcined diatomite are shown in Fig. 1. When the SEM imagesin the literature for natural diatomite are compared with calcined diat-omite sample (Fig. 1a,b), it can be seen that the solid structure of diatomfor calcined diatomite becomesmore noticeable after calcinations rang-ing from 870to 1100 °C (Bağci, 2011; Chaisena and Rangsriwatananon,2004; Chen et al., 2012; Mohamedbakr and Burkitbaev, 2009). Calcineddiatomite has notable pores as disks or cylindrical shapes. Thus, there isa good possibility for thorium ions to be adsorbed into these pores.However, solid structure of diatom changes after flux calcination pro-cess. From the SEM images of flux calcined diatomite, it can be seenthat the particles became aggregated to coarser size. As shown inFig. 1c,d, diatom structure shrank and got agglomerated due to the re-moval of organic matter and bulk humidity after calcinating at1600 °C. Therefore, the particle sizes of the calcined diatomite werealso increased (Ediz et al., 2010).

The results of FTIR spectrums are shown in Fig. 2a,b. The broad en-velope around 3500 cm−1 is due to O―H stretching of the frameworkSi–OH group and physically adsorbed water molecules for C-D andFC-D (Zuo et al., 2011). The spectral bands at 1647 cm−1 reflect thebending vibration of water molecules retained in the silica matrix ofdiatomite (Zhao et al., 2008). The peaks at 472, 474, 1093 and1087 cm−1 may be attributed to the asymmetric stretching modesof Si―O―Si bonds, the peak at 791 cm−1 may correspond to thesymmetric stretching vibration of Si―O―Si for C-D and FC-D, respec-tively (Sheng et al., 2008; Zhang et al., 2009a,b). As seen from Fig. 2a,the peak at 2361 cm−1 can be assigned to organic impurities. In addi-tion the acuti-peak at 1512 cm−1 can be assigned to unsaturateddouble bond C=C (Fig. 2b).

After the adsorption of Th(IV) on the C-D and FC-D, mentionedbands have preserved but small bands of C-D and FC-D at about2400 cm−1 have disappeared. Hence the FTIR pattern show no con-siderable difference between C-D and FC-D before and after interac-tion with Th(IV), which has been suggested a physical adsorptionphenomenon occurs as electrostatic repulsion between the negativecharges (Si―O―) onto diatomite and thorium ions with positivecharge under examination (Kandil et al., 2012; Sari et al., 2010).

XRD patterns of the C-D and FC-D are shown in Fig. 3a,b. for the C-D sample, amorphous structure of diatomite was essentially seen in

Fig. 1. SEM images of calcined diatomite (

sections between 18° and 30° of 2-theta. Fig. 3a reveals that themain peaks in the calcinated diatomite sample correspond tocristobalite, tridymite and small amount of quartz. The peaks ob-served around d=4.36, 4.28, 4.08, 3.78 and 3.55 Å are assigned tocristobalite (PDF no: 11-695). The C-D sample also contains tridymiteand quartz, characterized by reflections at the vicinity of 4.36, 4.28,4.08 Å and 4.28, 3.35 Å, respectively (PDF no: 14-260, 5-490). Similarresults were also reported by several authors (Du et al., 2011; Wanget al., 1991; Zhu et al., 2011).

It is clear that the X-ray pattern of the C-D is very similar to thepattern of the FC-D. However, the intensity of lines for FC-D areslightly higher than those of the lines for C-D, which might be dueto the calcination process in the presence of Na2CO3 and eliminationof some impurities above 1100 °C. As seen from Fig. 3b, the amor-phous phases of the sample change to crystalline phases. It alsonoted that the amount of cristobalite was remarkably increased,while tridymite was decreased. On the other hand the peaks of quartzminerals were completely disappeared. Similar behaviors of diato-mites were noted by Khraisheh and Al‐Ghouti (2005) andSprynskyy et al.(2010).

The BET test was conducted to determine the textural parameters,such as BET surface area, pore volume and pore size of C-D and FC-D.The BET surface area of the calcined diatomite was found as21.93 m2/g, which is greater than that of the flux calcined diatomite(1.25 m2/g). The pore size of C-D is estimated to be 117 Ǻ, while thatof FC-D is 123 Ǻ. Although the pore volume had the same trend withthe surface area, calcination with Na2CO3 at 1600 °C in diatomitecaused the pore volume of diatomite to be decreased from 0.055 to0.0015 cm3/g. Results showed that the pore size of the diatomitesare suitable for thorium adsorption. Since ionic radius of thoriumion is 1.08 Ǻ, it can be concluded that these pore sizes are higherthan thorium ions, and therefore thorium ions can enter in to poreseasily.

3.2. Effect of pH

The pH value of the solution is an important controlling parameterfor the sorption of radionuclides on the sorbents, and influences the

a,b) and flux calcined diatomite (c,d).

Fig. 2. FT-IR spectra of (a) calcined diatomite and (b) flux-calcined diatomite before and after adsorption.

109S. Yusan et al. / Applied Clay Science 67–68 (2012) 106–116

metal speciation and surface metal binding sites. It is well known thatsurface charge of a sorbent can be modified by changing the pH of thesolution and the chemical species in the solution depends on the pH.The effect of pH on the adsorption of Th(IV) at a fixed initial Th(IV)concentration (50 mg/L) at 25 °C is shown in Fig. 4. The resultsshow that adsorption of Th(IV) on calcined and flux-calcined diato-mite is obviously affected by pH values and mainly occurs at pH 4.0.Below pH 3.0, the predominant thorium ion would be the positivelycharged Th4+. When the pH of solution is around 4.0, the 1:1 and1:2 positively charged thorium acetate complexes [ThCH3COO]3+

and [Th(CH3COO)2]2+ appear as characteristic ions while the com-plexes [Th(CH3COO)2]2+ and [Th(CH3COO)3]+ are dominant whenpH is around 5.0 (Chen and Wang, 2007a; Kaygun and Akyil, 2007;Liao et al., 2004). Thorium sorption yield on the adsorbents reacheda maximum of 99% at pH 4.0. Above pH 4.0, the sorption yield de-creases with increasing pH because the hydrolysis products and theprecipitation begin to play an important role in the sorption ofTh(IV) (Chen and Wang, 2007b; Chen et al., 2007; Xu et al., 2007).

3.3. Effect of initial thorium concentration

The relative thorium uptake on the adsorbents as a function ofthorium concentration was studied from 25 to 150 mg/L. Fig. 5 illus-trates the effect of initial Th(IV) concentration on adsorption. Th(IV)adsorption increased in the initial concentration range from 25 to50 mg/L and slightly decreased with increasing initial thorium con-centration for calcined and flux-calcined diatomite. By increasing tho-rium concentration, a series of polynuclear complexes of thoriumcan probably be formed with the anions except perchloride anionsin the solution. Although the hydrolyzed thorium ions can increaseand these ions cannot reach the bounding sites of the adsorbentwhen the thorium concentration is increased (Aslani et al., 2001).

Therefore, in further experiments 50 mg/L thorium concentrationwas used for the adsorbents.

3.4. Effect of contact time

The effect of contact time on the adsorption of Th(IV) onto C-Dand FC-D was studied in the range of 10–90 min using 50 mg/L initialTh(IV) concentration at pH 4.0 for both adsorbents. Fig. 6 shows thatthe sorption yield slightly increases with increasing shaking time andattains equilibrium at 98% and 89% sorption within 20 min for C-Dand FC-D, respectively. The result of this study showed that the up-take of thorium by calcined and flux calcined diatomite was rapid.Then the adsorption remains constant level with increasing time.The quick adsorption of Th(IV) suggests that Th(IV) adsorption ontoC-D and FC-D are physical adsorption rather than chemical adsorp-tion (Guerra et al., 2009). Based on these results, a contact time of20 min was selected for the following experiments to assure the ad-sorption equilibrium.

3.5. Effect of temperature and thermodynamic study

Fig. 7 shows the effect of temperature on the adsorption of thori-um on C-D and FC-D. Uptake of thorium increases with increasingtemperature, which indicates the endothermic nature of the process.

Thermodynamic parameters such as enthalpy change (ΔH°), en-tropy change (ΔS°) and free energy change (ΔG°) were estimatedusing the following equations.

lnKd ¼ ΔS°R

−ΔH°RT

ð2Þ

ΔG° ¼ ΔH°−TΔS° ð3Þ

Fig. 3. XRD pattern of C-D (a) and FC-D (b) (T: Tridymite, C: Cristobalite, Q: Quartz).

110 S. Yusan et al. / Applied Clay Science 67–68 (2012) 106–116

The values of ΔH° and ΔS° were determined from the slopes andintercepts of the plots of lnKd vs. 1/T. The plot of lnKd against 1/T forthorium adsorption is shown in Fig. 8 for both adsorbents. Gibbsfree energy (ΔG°) was calculated by using Eq. (3). The values of thethermodynamic parameters for the adsorption of Th(IV) on C-D andFC-D are given in Table 2.

The positive values of the enthalpy change suggest the endother-mic nature of the adsorption process. When the value of ΔH° islower than 40 kJ/mol, the type of adsorption can be accepted as phys-ical process. ΔH° value is obtained as 35.99 kJ/mol and 23.62 kJ/mol

Fig. 4. Effect of pH on the adsorption of Th(IV) to C-D (●) and FC-D (◊). (Ci[Th(IV)]:50 mg/L,m/V:0.01 g/mL, T:293 K).

for C-D and FC-D, respectively. It indicates that the adsorption isphysical by nature and involves weak forces of attraction (Ho andMcKay, 1999), which is consistent with the results found from iso-therm models. The positive value of entropy change indicates the in-creased randomness at the solid-solution interface during theadsorption of thorium on adsorbents. The positive entropy favorscomplexation and stability of adsorption.

The negative values for the Gibbs free energy change show thatthe adsorption process is thermodynamically feasible and the degreeof spontaneity of the reaction increases with increasing temperature.

Fig. 5. Effect of initial Th(IV) concentration on the adsorption of Th(IV) onto C-D (●)and FC-D (◊) (pH: 4.0, m/V: 0.01 g/mL, T: 293 K).

Fig.6. Effect of contact time on the adsorption of Th(IV) onto C-D (●) and FC-D (◊) (Ci[Th(IV)]:50 mg/L, pH: 4.0, m/V: 0.01 g/mL, T: 293 K).

Fig. 8. Plots of lnKd versus 1/T for thorium adsorption on C-D and FC-D.

111S. Yusan et al. / Applied Clay Science 67–68 (2012) 106–116

The increase in adsorption with temperature may be attributed to ei-ther increase in the number of active surface sites available for ad-sorption on the adsorbents or the desolvation of the sorbing species.The decrease in the thickness of the boundary layer is surroundingthe adsorbent with temperature, so that the mass transfer resistanceof adsorbate in the boundary layer decreases (Aytas et al., 2009;Singha and Das, 2011). Generally, the absolute magnitude of thechange in Gibbs free energy for physisorption is between −20 and0 kJ/mol, and chemisorption has a range from −80 to −400 kJ/mol(Jaycock and Parfitt, 1981). The results found in this study arebetween −53.94 and −61.43 for C-D and −39.26 and −44.72 forFC-D. These values are in the middle between physisorption andchemisorption. It can be interpreted that physical adsorption wasenhanced by a chemical effect. In addition to this, since ΔG° valuesare between 20 and 80 kJ/mol, adsorption type can be explained asion exchange. Presumably the ion-exchange has a range from −20 to−80 kJ/mol (Gereli et al., 2006; Yu et al., 2001). It can be inferred thatadsorption process was administrated by combined control of severalmechanisms.

3.6. Adsorption isotherms of thorium

The adsorption data was subjected to five different adsorption iso-therms, namely Langmuir, Freundlich, Dubinin–Radushkevich, Temkinand Flory–Huggins to explain the adsorption equilibrium data, variousparameters of which are depicted in Table 3 and discussed below.

3.6.1. Langmuir isothermLangmuir adsorption isotherm assumes that finite numbers of

binding sites are distributed homogeneously over the surface of the

Fig.7. Effect of temperature on the adsorption of Th(IV) onto C-D (●) and FC-D (◊) (Ci[Th(IV)]:50 mg/L, pH: 4.0, m/V: 0.01 g/mL, t: 20 min).

adsorbent (Freundlich, 1906). Linearized equation of this model canbe expressed as follows:

Ce

qe¼ 1

Q0bLþ Ce

Q0ð4Þ

where qe is the amount of metal ions sorbed onto adsorbent, Ce is theequilibrium concentration of thorium in solution, and Q0 and bL areLangmuir constants related to adsorption capacity and adsorption en-ergy, calculated from the slope and intercept of linear plots of Ce/qeversus Ce, respectively.

3.6.2. Freundlich isothermThe Freundlich isotherm assumes heterogeneous surface of the

adsorbent, and linearized form of the model is as follows(Helfferich, 1962):

logq ¼ logKF þ1nF

logCe ð5Þ

where KF represents the adsorption capacity of adsorbent (mg/g), nFis a dimensionless constant related to adsorption intensity given inTable 3. Linear plot of log qe versus logCe shows the applicability ofthis isotherm for thorium adsorption onto C-D and FC-D. (Fig. 9)Among the isotherm models, high correlation coefficients were ob-served in the case of Freundlich model, which is regarded as hetero-geneous multilayer adsorption.

3.6.3. Dubinin–Radushkevich isothermDubinin–Radushkevich (D-R) isotherm describes the adsorption

on a single type of uniform pores that the characteristic sorptioncurve is related to the porous structure of the sorbent. Linearizedform of the D-R equation is given as:

lnCads ¼ lnXm−βε2 ð6Þ

where Cads (mmol/g) is the amount of solute adsorbed per unitweight of solid, Xm (mmol/g or mg/g) is the adsorption capacity,

Table 2Thermodynamic parameters of thorium adsorption on C-D and FC-D.

ΔH° ΔS° ΔG°(kJ/mol)

(J/mol) (J/mol K) 288 K 298 K 308 K 318 K 328 K

C-D 35.99 187.31 −53.94 −55.81 −57.68 −59.56 −61.43FC-D 23.62 136.34 −39.26 −40.63 −41.99 −43.35 −44.72

Table 3Adsorption isotherm constants for the adsorption of thorium on C-D and FC-D.

Isotherm models Parameters C-D values FC-D values

Langmuir Qe (mg/g) 12.89 11.49bL (L/mg) 0.09 0.08R2 0.8518 0.9020

Freundlich KF (L/mg) 0.97 0.74nF 0.70 0.68R2 0.9702 0.9702

Dubinin–Radushkevich Xm (mg/g) 121.22 116.69E (kJ/mol) 4.63 4.49R2 0.9607 0.9586

Temkin aTE (L/g) 0.64 0.54bTE (kJ/mol) 0.30 0.30R2 0.8245 0.8149

Flory–Huggins nFH 2.75 2.65ΔG° (kJ/mol) −10.01 −8.16R2 0.8739 0.8890

Fig. 10. D-R plots for the adsorption of thorium on C-D and FC-D.

112 S. Yusan et al. / Applied Clay Science 67–68 (2012) 106–116

β (mol/K)2 is a constant related to energy, and ε is the Polanyi poten-tial calculated using the following equation:

ε ¼ RT ln1

1þ Ce

� �ð7Þ

where R is the gas constant in kJ/mol, and T is the temperature inKelvin. If lnCads is plotted against ε2, β and Xm will be obtained fromthe slope and intercept, respectively (Fig. 10). The straight linesobtained were useful in calculating the D‐R isotherm constants,which are given in Table 3. The mean energy of adsorption (E) iscalculated by the following equation:

E ¼ 1ffiffiffiffiffiffiffiffiffiffiffi−2β

p ð8Þ

The adsorption energy (E) is useful for estimating the type of theadsorption process. If Eb8 kJ/mol, the adsorption process is physicalin nature, and in the range from 8 to 16 kJ/mol, it has an ion-exchange mechanism (Helfferich, 1962). The E value was calculatedas 4.63 kJ/mol and 4.49 kJ/mol, respectively. Since the values arelower than 8 kJ/mol, it is very likely that thorium adsorption on C-Dand FC-D is physical in nature.

3.6.4. Temkin isothermThe Temkin isotherm is based on the assumption that fall in the

heat of sorption is linear rather than logarithmic, as given inFreundlich equation (Aharoni and Ungarish, 1977). The heat of sorp-tion of all molecules in the layer decrease linearly with coverage dueto the sorbate/sorbent interactions (Hosseini et al., 2003).

Fig. 9. Freundlich plots for the adsorption of thorium on C-D and FC-D.

A linear form of the Temkin equation is formulated as:

qe ¼RTbTe

lnaTe þRTbTe

lnCe ð9Þ

where bTe is the Temkin constant related to heat of adsorption(J/mol), aTe the Temkin isotherm constant (L/g), R the gas constantand T the absolute temperature (K). According to this equation theplot of the qe versus lnCe gives a straight line, and bTe and aTeconstants can be calculated from the intercept and slope of thisstraight line, respectively. Lower values of bTe shows a weak ionicinteraction between thorium and adsorbents. Therefore, the thoriumremoval seems to involve a physical adsorption, which also supportsthe predictions from Dubinin–Radushkevich isotherm.

3.6.5. Flory–Huggins isothermThis model was applied to find out the degree of surface coverage

of the sorbate on the sorbent (Horsfall and Spiff, 2005). The linearizedequation of the Flory–Huggins isotherm can be expressed as:

logΘCi

¼ lnKFH þ nFH log 1−Θð Þ ð10Þ

where θ=(1−Ce/Ci) is the degree of surface coverage, KFH is equilib-rium constant and nFH the Flory–Huggins model exponent.

The equilibrium constant (KFH) was used for the calculation of theGibbs free energy of spontaneity (ΔG°) by the following equation:

ΔG° ¼ −RT lnKFH ð11Þ

Table 4The comparison of adsorption capacity of diatomite for thorium with those of varioussorbents reported in the literature.

Sorbent Sorption capacity (mmol/g) Reference

Activated carbon 0.087 Kutahyali and Eral, 2010PAN/zeolite 0.04 Kaygun and Akyil, 2007Resin(MCM) 0.984 Raju and Subramanian, 2007Amberlite XAD-4 0.25 Dev et al., 1999SiO2 0.001 Chen and Wang, 2007aMX-80 0.275 Zhao et al., 2008Amberlite XAD 0.113 Seyhan et al., 2008Perlite 0.025 Talip et al., 2009Modified clay MTTZ 0.116 (average) Guerra et al., 2009Attapulgite 0.067(average) Chen and Gao, 2009Raw diatomite 0.03 Sheng et al., 2008Calcined diatomite 0.06 This workFlux Calcineddiatomite

0.05 This work

113S. Yusan et al. / Applied Clay Science 67–68 (2012) 106–116

The feasibility of the process and the spontaneous nature of thori-um adsorption on C-D and FC-D were confirmed by the negativevalues of ΔG°.

In addition, Table 4 presents the comparison of adsorption capac-ity of diatomite for thorium with that of various sorbents reported inthe literature. In the present study, Th(IV) adsorption capacity by C-Dand FC-D was found as 0.52 mmol/g and 0.50 mmol/g, respectively.As seen from the table, the obtained Th(IV) adsorption capacity of di-atomite (present study) is higher than PAN/zeolite, SiO2, perlite andraw diatomite adsorbents. In particular, high capacity of Th(IV) byC-D and FC-D may be due to the calcination process. The similarstudy was performed by Sheng et al. (2008) with raw diatomite. Sowe can conclude that calcination process improves adsorption capac-ity for Th(IV) ions. On the other hand, adsorption capacities of activecarbon, resin, amberlite XAD-4, MX-80, amberlite XAD and modifiedclay MTTZ for Th(IV) are higher than the results obtained by thisstudy. However the result of the study by Chen and Gao (2009) isquite similar to with C-D and FC-D capacities for thorium (Table 4).As mentioned above, the adsorption isotherms for both adsorbentfit Freundich and D-R isotherms with high correlation coefficients.Thus, we considered the Th(IV) capacity of diatomites according toD-R isotherm model. Since diatomites have pore structure, data foradsorption equilibrium represent by D-R isotherm model which isthe characteristic sorption curve, is related to the porous structureof the sorbent.

3.7. Adsorption kinetics of thorium

The adsorption kinetic shows the evolution of the adsorption ca-pacity with time. To examine the potential rate-controlling step(e.g. chemical reaction, diffusion control and mass transfer), severalkinetic models were used to test the experimental data (Pérez-Marìn et al., 2007).

3.7.1. Pseudo-first-order modelThe employed Lagergren's (pseudo-first order) rate equation is

given by:

log qe−qtð Þ ¼ logqe−k1t

2:303ð12Þ

where qe is the amount of metal ion adsorbed onto adsorbent at equi-librium (mg/g), qt is the amount of metal ion adsorbed at varioustimes, t is the time of adsorption duration and k1 is the rate constantof the equation (min−1) (Hameed et al., 2009).

Table 5Kinetic constants at different concentrations of Th(IV) at 25 and 55 °C.

Concentration (mg/L)

25 °C

C-D FC-D

50 100 150 50 100 150

Pseudo-first order equationqc0.927

0.027 0.018 0.008 0.074 0.130

k1 0.006 0.005 0.003 0.007 0.001 0.001

Pseudo-second order equationqc14.25

4.93 9.85 14.94 4.48 9.21

k2 1.80 1.69 1.60 0.15 0.25 0.22

Intraparticle diffusionK0 0.005 0.003 0.001 0.015 0.003 0.029

Slopes and intercepts of plots of log (qe−qt) vs. time wereobtained for C-D and FC-D to determine the first-order rate constantk1 at different Th(IV) concentrations and temperatures (plots notshown) but the correlation coefficients for the first-order kineticmodel at different concentrations were relatively low, and indicatingthat the adsorption of Th(IV) ions onto C-D and FC-D was not of first-order reaction. Results of the model are shown in Table 5.

3.7.2. Pseudo-second-order modeThe pseudo-second-order model can be represented in the follow-

ing form by:

tqt

¼ 1k2qe

2 þtqe

ð13Þ

where k2 (g/mol min) is the second-order rate constant. The linearplot of t/qt as a function of t provided not only the rate constant k2,but also an independent evaluation of qe. The pseudo-second-orderrate constant k2 was determined from the slopes and intercepts ofplots of t/q vs. t (Ijagbemi et al., 2009). The values of model parame-ters (k2, R2) for different concentrations at 25 °C and 55 °C were cal-culated from Eq. (13) (Table 5). The correlation coefficients for thepseudo-second-order kinetic model at different concentrations wereabove 0.997. The data gives perfect fit for this model for both calcinedand flux calcined diatomite. Therefore, the rate-limiting step may bechemical sorption or chemisorption through sharing or exchange ofelectrons between sorbent and adsorbate (Namasivayam andSureshkumar, 2008; Ofomaja and Ho, 2007; Wang et al., 2006). Be-side this, the rate constant of pseudo-second order (k2) was foundto decrease with increasing initial metal ion concentrations for C-Dand FC-D at studied temperatures. That is, time required for the equi-librium adsorption increased with minute differences as initial metalions concentration increased (Fig. 11). However, the values of k2 wereincreased with increasing initial metal concentrations for FC-D at25 °C. It means the values of the second-order rate constant foundfrom the slopes of the plots for FC-D (k2: 0.254 g/mg min for100 mg/L), FC-D (k2: 0.153 g/mg min for 50 mg/L) and FC-D (k2:0.215 g/mg min for 150 mg/L) indicate that rate of Th(IV) removal isfaster for 100 mg/L by FC-D than for 50 and 150 mg/L by FC-D.

3.7.3. Adsorption mechanismIn general, sorption process can be interpreted by a series of

steps, mass transfer from liquid to the particle surface across theboundary layer, diffusion within the pores and deposition on thesurface of the particles. In order to gain insight into the mechanismsand rate controlling steps affecting the kinetics of adsorption, the

55 °C

C-D FC-D

50 100 150 50 100 150

1.238 0.012 0.043 0.142 0.169 0.345

0.0035 0.010 0.003 0.015 0.018 0.020

13.47 6.76 9.96 14.10 4.50 9.38

1.25 0.96 0.58 0.14 0.16 0.04

0.002 0.006 0.014 0.047 0.026 0.060

Fig. 11. Pseudo-second-order plot for the adsorption of Th(IV) by C-D (a) and FC-D (b)for 25 °C and 55 °C. Fig. 13. Reichenberg plot for the adsorption of Th(IV) by C-D (a) and FC-D (b) for 25 °C

and 55 °C.

114 S. Yusan et al. / Applied Clay Science 67–68 (2012) 106–116

kinetic experimental results were fitted to the diffusion models(Zhang et al., 2009a,b). Two diffusion models can be written as:

Film diffusion model :Bt ¼ −0:4977 ln 1−Xð Þ ð14Þ

Intraparticle dif f usion model : qt ¼ kp � t0:5 ð15Þ

Fig. 12. Weber and Moris plot for the adsorption of Th(IV) by C-D (a) and FC-D (b) for25 °C and 55 °C.

where X=qt/qe and Bt is amathematical function of X, which can be cal-culated for each value of X and kp (mmol/g min1/2), is the intraparticlediffusion rate constant, which can be evaluated from the slope of the lin-ear plot of qt versus t1/2. Values of kint (mg/g h1/2) were calculated fromthe slope of the linear plots of qt versus t1/2 (Fig. 12a,b). A plot of Bt ver-sus t is also shown in Fig. 13a,b, which is a straight line. From these fig-ures, it can also be observed that none of the straight lines passesthrough the origin, which indicates that intraparticle diffusion andfilm diffusion are both the rate limiting steps for Th(IV) onto C-D andFC-D (Ahmad, 2005). This is the indicative of some degree of boundarylayer control, and further show that the intra-particle diffusion is notthe only rate-limiting step, but also other kinetic models may controlthe rate of adsorption, all of which may be operating simultaneously(Mezenner and Bensmaili, 2009).

4. Conclusion

The adsorption phenomena of Th(IV) ions onto calcined diatomiteand flux-calcined diatomite from Turkey was investigated. It wasfound that the adsorption of Th(IV) on diatomite depends on pH, ini-tial concentration and temperature. The adsorptions (%) for thoriumions under the optimized experimental conditions were found as99±0.1 and 91±0.4 for C-D and FC-D, respectively. The resultsshow that Th(IV) adsorption on calcined and flux calcined diatomiteis very fast and effective. The FT-IR spectrum of C-D and FC-D beforeand after adsorption of Th(IV) indicated that small shifts in thewave number of specific bands of Si–OH groups on the surface of di-atomite mainly was involved in the adsorption of Th(IV). Therefore,diatomite can be used as a highly effective low-cost adsorbent forthe removal of Th(IV).

The adsorption datawaswell fitted by the Freundlichmodel for bothadsorbents. The kinetic data conformed better to the pseudo-second-order equation, which suggests that the rate-limiting step is a chemicaladsorption involving valency forces through sharing or exchangeof electrons between thorium ions and adsorbents. Thermodynamic

115S. Yusan et al. / Applied Clay Science 67–68 (2012) 106–116

parameters ΔH°, ΔS° and ΔG° were calculated and these parameters in-dicate that adsorption is endothermic and spontaneous.

The all experimental results of this study propose that natural,cheap and effective adsorbent calcined and flux calcined diatomitecan be successfully used for thorium removal from geological, miningor radioactive wastes.

Acknowledgment

The authors are particularly grateful to Dr. Hatice Yilmaz from theDepartment of Mining Engineering of Dokuz Eylul University forinterpreting the results of XRD analysis and to Prof. Dr. Turgay Karali,Assoc. Prof. Dr. Nesrin Ozalp and Mr. Denizcan Olmez for English lan-guage editing.

References

Aharoni, C., Ungarish, M., 1977. Kinetics of activated chemisorption. Part 2. Theoreticalmodels. Journal of the Chemical Society. Faraday Transactions 73, 456–464.

Ahmad, R., 2005. Sawdust: cost effective scavenger for the removal of chromium (III)ions from aqueous solutions. Water, Air, and Soil Pollution 163, 169–183.

Al-Ghouti, M.A., Khraisheh, M.A.M., Ahmad,M.N., Allen, S.J., 2007. Microcolumn studies ofdye adsorption onto manganese oxides modified diatomite. Journal of HazardousMaterials 146, 316–327.

Analysis Reports of Diatomite. Turkish Sugar Fact. Inc., Kieselguhr Factory, Etimesgut-Ankara/Turkey.

Aslani, M.A.A., Akyil, S., Eral, M., 2001. Thorium (IV) sorption on ignited Sarcotragusmuscarum, its kinetic and thermodynamic parameters. Journal of Radioanalyticaland Nuclear Chemistry 250, 153–157.

Aytas, S., Akyil, S., Aslani, M.A.A., Aytekin, U., 1999. Removal of uranium from aqueous so-lutions by diatomite (Kieselguhr). Journal of Radioanalytical and Nuclear Chemistry240, 973–976.

Aytas, S., Yurtlu, M., Donat, R., 2009. Adsorption characteristic of U(VI) ion onto ther-mally activated bentonite. Journal of Hazardous Materials 172, 667–674.

Bağci, C., 2011. Microstructural characterisation of β-SiC powders synthesised bycarbothermally reduction of Turkish diatomite. Scientific Research and Essays 6,542–551.

Bhainsa, K.C., D'Souza, S.F., 2009. Thorium biosorption by Aspergillus fumigatus, a fila-mentous fungal biomass. Journal of Hazardous Materials 165, 670–676.

Chaisena, A., Rangsriwatananon, K., 2004. Effects of thermal and acid treatments onsome physico-chemical properties of lampang diatomites. Suranaree Journal of Sci-ence and Technology 11, 289–299.

Chen, L., Gao, X., 2009. Thermodynamic study of Th(IV) sorption on attapulgite. Ap-plied Radiation and Isotopes 67, 1–6.

Chen, C., Wang, X., 2007a. Sorption of Th (IV) to silica as a function of pH, humic/fulvicacid, ionic strength, electrolyte type. Applied Radiation and Isotopes 65, 155–163.

Chen, C.L., Wang, X.K., 2007b. Influence of pH, soil humic/fulvic acid, ionic strength andforeign ions on sorption of thorium(IV) onto γ-Al2O3. Applied Geochemistry 22,436–445.

Chen, C., Li, X., Zhao, D., Tan, X., Wang, X., 2007. Adsorption kinetic, thermodynam-ic and desorption studies of Th(IV) on oxidized multi-wall carbon nanotubes.Colloids and Surfaces A: Physicochemical and Engineering Aspects 302,449–454.

Chen, Y., Xiao, M., Wang, S., Han, D., Lu, Y., Meng, Y., 2012. Porous diatomite-immobilized Cu–Ni bimetallic nanocatalysts for direct synthesis of dimethyl car-bonate. Journal of Nanomaterials 1–8 (Article ID 610410).

Dev, K., Pathak, R., Rao, G.N., 1999. Sorption behaviour of lanthanum(III), neodymium(III),terbium(III), thorium(IV) and uranium(VI) on amberlite XAD-4 resin functionalizedwith bicine ligands. Talanta 48, 579–584.

Du, Y.C., Shia, S.L., Bu, C.Y., Dai, H.X., Guo, Z.G., Tang, G.Y., 2011. Effect of particle sizedistribution of calcined diatomites on the extinction performance. Particulate Sci-ence and Technology 29, 368–377.

Ediz, N., Bentli, I., Tatar, I., 2010. Improvement in filtration characteristics of diatomiteby calcination. International Journal of Mineral Processing 94, 129–134.

Freundlich, H.M.F., 1906. Uber die adsorption in losungen. Zeitschrift für PhysikalischeChemie (Leipzig, Germany) 57A, 385–470.

Gereli, G., Seki, Y., Kusoglu, I.M., Yurdakoc, K., 2006. Equilibrium and kinetics for thesorption of promethazine hydrochloride onto K10 montmorillonite. Journal of Col-loid and Interface Science 299, 155–162.

Guerra, D.L., Viana, R.R., Airoldi, C., 2009. Adsorption of thorium cation on modifiedclays MTTZ derivative. Journal of Hazardous Materials 168, 1504–1511.

Hameed, B.H., Salman, J.M., Ahmad, A.L., 2009. Adsorption isotherm and kinetic model-ing of 2,4-D pesticide on activated carbon derived from date stones. Journal ofHazardous Materials 163, 121–126.

Helfferich, F., 1962. Ion Exchange. McGraw-Hill, New York, USA.Ho, Y.S., McKay, G., 1999. Pseudo-second-order model for sorption processes. Process

Biochemistry 34, 451–465.Hongxia, Z., Zheng, D., Zuyi, T., 2006. Sorption of thorium(IV) ions on gibbsite: effects of

contact time, pH, ionic strength, concentration, phosphate and fulvic acid. Colloidsand Surfaces A: Physicochemical and Engineering Aspects 278, 46–52.

Horsfall Jr., M., Spiff, I., 2005. Equilibrium sorption study of Al3+, Co2+ and Ag+ inaqueous solutions by fluted pumpkin (Telfairia occidentalis HOOK f) waste biomass.Acta Chimica Slovenica 52, 174–181.

Hosseini, M., Mertens, S.F.L., Ghorbani, M., Arshadi, M.R., 2003. Asymmetrical Schiffbases as inhibitors of mild steel corrosion in sulphuric acid media. Materials Chem-istry and Physics 78, 800–807.

Hu, B., Cheng, W., Zhang, H., Sheng, G., 2010. Sorption of radionickel to goethite: effectof water quality parameters and temperature. Journal of Radioanalytical and Nu-clear Chemistry 285, 389–398.

Huang, C.P., Blankenship, D.W., 1984. The removal of mercury (II) from dilute aqueous so-lution by activated carbon. Water Research 18, 37–43.

Humeinicu, D., Drochioiu, G., Popa, K., 2004. Bioaccumulation of thorium and uranyl on Sac-charomyces cerevisiae. Journal of Radioanalytical and Nuclear Chemistry 260, 291–293.

Humelnicua, D., Bulgariu, L., Macoveanu, M., 2010. On the retention of uranyl and tho-rium ions from radioactive solution on peat moss. Journal of Hazardous Materials174, 782–787.

Ijagbemi, O.C., Baek, M., Kim, D., 2009. Montmorillonite surface properties and sorptioncharacteristics for heavy metal removal from aqueous solutions. Journal of Hazard-ous Materials 166, 538–546.

Jakobsson, A.-M., 1999. Measurement andmodeling of Th sorption onto TiO2. Journal ofColloid and Interface Science 220, 367–373.

Jaycock, M.J., Parfitt, G.D., 1981. Chemistry of interfaces. Ellis Horwood, Onichester,pp. 12–13.

Kandil, A.E.H.T., Saad, E.A., Aziz, A.A.A., Aboelhasan, A.E., 2012. Study on adsorption be-havior and separation efficiency of naturally occurring clay for some elements bybatch experiments. European Journal of Chemistry 3, 99–105.

Kaygun, K.A., Akyil, S., 2007. Study of the behaviour of thorium adsorption on PAN/zeolite composite adsorbent. Journal of Hazardous Materials 147, 357–362.

Khraisheh, M.A.M., Al-Ghouti, M.S., 2005. Enhanced dye adsorption by microemulsion-modified calcined diatomite (μE-CD). Adsorption 11, 547–559.

Khraisheh, M.A.M., Al-Degs, Y.S., Mcminn, W.A.M., 2004. Remediation of wastewatercontaining heavy metals using raw and modified diatomite. Chemical EngineeringJournal 99, 177–184.

Kutahyali, C., Eral, M., 2010. Sorption studies of uranium and thorium on activated car-bon prepared from olive stones: kinetic and thermodynamic aspects. Journal ofNuclear Materials 396, 251–256.

Lemonas, J.F., 1997. Diatomite. American Ceramic Society Bulletin 76, 92–95.Liao, X., Li, L., Shi, B., 2004. Adsorption recovery of thorium(IV) by Myrica rubra tannin

and larch tannin immobilized onto collagen fibres. Journal of Radioanalytical andNuclear Chemistry 260, 619–625.

Mezenner, Y.N., Bensmaili, A., 2009. Kinetics and thermodynamic study of phosphate ad-sorption on iron hydroxide-eggshell waste. Chemical Engineering Journal 147, 87–96.

Mohamedbakr, H., Burkitbaev, M., 2009. Elaboration and characterization of natural di-atomite in Aktyubinsk/Kazakhstan. The Open Mineralogy Journal 3, 12–16.

Murphy, R.J., Lenhart, J.J., Honeyman, B.D., 1999. The sorption of thorium (IV) and ura-nium (VI) to hematite in the presence of natural organic matter. Colloids and Sur-faces A: Physicochemical and Engineering Aspects 157, 47–62.

Namasivayam, C., Sureshkumar, M.V., 2008. Removal of chromium(VI) from water andwastewater using surfactant modified coconut coirpith as biosorbent. BioresourceTechnology 99, 2218–2225.

Ofomaja, A.E., Ho, Y., 2007. Effect of pH on cadmium biosorption by coconut coprameal. Journal of Hazardous Materials 139, 356–362.

Onishi, H., 1989. Photometric Determination of Traces of Metals. Part IIB John Wiley &Sons Inc., pp. 449–492.

Pérez-Marìn, A.B., Zapata, M.V., Ortuño, J.F., Aguilar, M., Sáez, J., Lloréns, M., 2007. Re-moval of cadmium from aqueous solutions by adsorption onto orange waste. Jour-nal of Hazardous Materials 139, 122–131.

Raju, Ch.S.K., Subramanian, M.S., 2007. Sequential separation of lanthanides, thoriumand uranium using novel solid phase extraction method from high acidic nuclearwastes. Journal of Hazardous Materials 145, 315–322.

Sari, A., Citak, D., Tuzen, M., 2010. Equilibrium, thermodynamic and kinetic studies onadsorption of Sb(III) from aqueous solution using low-cost natural diatomite.Chemical Engineering Journal 162, 521–527.

Seyhan, S., Merdivan, M., Demirel, N., 2008. Use of o-phenylene dioxydiacetic acid im-pregnated in amberlite XAD resin for separation and preconcentration ofuranium(VI) and thorium(IV). Journal of Hazardous Materials 152, 79–84.

Sheng, G., Hu, J., Wang, X., 2008. Sorption properties of Th(IV) on the raw diatomite—effects of contact time, pH, ionic strength and temperature. Applied Radiation andIsotopes 66, 1313–1320.

Singha, B., Das, S.K., 2011. Biosorption of Cr(VI) ions from aqueous solutions: kinetics,equilibrium, thermodynamics and desorption studies. Colloids and Surfaces. B, Bio-interfaces 84, 221–232.

Sprynskyy, M., Kovalchuk, I., Buszewski, B., 2010. The separation of uranium ions bynatural and modified diatomite from aqueous solution. Journal of Hazardous Mate-rials 181, 700–707.

Talip, Z., Eral, M., Hicsonmez, U., 2009. Adsorption of thorium from aqueous solutionsby perlite. Journal of Environmental Radioactivity 100, 139–143.

Tan, X., Wang, X., Chen, C., Sun, A., 2007. Effect of soil humic and fulvic acids, pH andionic strength on Th(IV) sorption to TiO2 nanoparticles. Applied Radiation and Iso-topes 65, 375–381.

Wang, D.Z., Xu, J.F., Fan, C.G., Lu, J., Yan, X.M., 1991. Electron microscopic studies onstructure of diatomite calcined. Journal of Inorganic Materials 3, 354–356 (inChinese).

Wang, X., Xia, S., Chen, L., Zhao, J., Chovelon, J., Nicole, J., 2006. Biosorption of cadmium(I1) and lead(I1) ions from aqueous solutions onto dried activated sludge. Journalof Environmental Sciences 18, 840–844.

116 S. Yusan et al. / Applied Clay Science 67–68 (2012) 106–116

Wu, W., Fana, Q., Xu, J., Niu, Z., Lu, S., 2007. Sorption–desorption of Th(IV) onattapulgite: effects of pH, ionic strength and temperature. Applied Radiation andIsotopes 65, 1108–1114.

Xu, D., Chen, C., Tan, X., Hu, J., Wang, X., 2007. Sorption of Th(IV) on Na-rectorite: effectof HA, ionic strength, foreign ions and temperature. Applied Geochemistry 22,2892–2906.

Yu, Y., Zhuang, Y.Y., Wang, Z.H., 2001. Adsorption of water-soluble dye onto function-alized resin. Journal of Colloid and Interface Science 242, 288–293.

Zhang, W., Hong, C., Pan, B., Zhang, Q., Jiang, P., Jia, K., 2009a. Sorption enhancement of1-naphthol onto a hydrophilic hyper-cross-linked polymer resin. Journal of Haz-ardous Materials 163, 53–57.

Zhang, J.B., Yang, Y.X., Wang, Z.L., Huang, Z., Chen, Y.R., 2009b. Adsorption properties ofZhejiang diatomite modified by supramolecule. Journal of Dispersion Science andTechnology 30, 83–91.

Zhao, D.L., Feng, S.J., Chen, C.L., Chen, S.H., Xu, D., Wang, X.K., 2008. Adsorption ofthorium(IV) on MX-80 bentonite: effect of pH, ionic strength and temperature. Ap-plied Clay Science 41, 17–23.

Zhu, Q., Zhanga, Y., Zhou, F., Lv, F., Ye, Z., Fan, F., Chu, P.K., 2011. Preparation and char-acterization of Cu2O–ZnO immobilized on diatomite for photocatalytic treatmentof red water produced from manufacturing of TNT. Chemical Engineering Journal171, 61–68.

Zuo, L., Yu, S., Zhou, H., Tian, X., Jiang, J., 2011. Th(IV) adsorption on mesoporous mo-lecular sieves: effects of contact time, solid content, pH, ionic strength, foreignions and temperature. Journal of Radioanalytical and Nuclear Chemistry 288,379–387.