(V

cienof expetharkrocps

e ofole

1. Introduction

rtant aingmedia, etc. [1]. Among theseuch incatalyst pot(VI),e authprodu

d magnl in theul in in

the naturally occurring form of -FeO(OH) [8]. It has since been

Uranium is a natural element and can be found in soils and rocks.In contact with water, it can be mobilised and, therefore, is found in

uranium has ve

previous work (-FeOOH) was synthesized in the laboratory by

Desalination 263 (2010) 233239

Contents lists available at ScienceDirect

Desalin

j ourna l homepage: www.e lrecognized as a major Fe-oxide component in soils and geothermalbrines. Akaganeite, -FeO(OH), is often observed as a corrosionproduct of some steels in chloride-containing environments, i.e.marine atmospheres. It occurs commonly in corroded archeologicaliron artifacts and is known to form continuously even in thecontrolled atmosphere of museums [9]. The crystal structure is welldetermined since the work of Mackay [10,11] who showed that it is of

precipitation from aqueous solutions of Fe(III) chloride and twodifferent precipitating agent and adsorbents obtained were called asAkaganeite-1 (AK-1) and Akaganeite-2 (AK-2). In that paper, therelative importance of test parameters like solution pH, contact time,temperature and concentration of adsorbate on adsorption perfor-mance of akaganeite for U(VI) ion were studied. And according toresults, AK-1 has a higher adsorption capacity than AK-2. Thethe hollandite type [12]. It is often assumed thparallel to the c axis of the tetragonal strucchloride ions and water molecules.

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

0011-9164/$ see front matter 2010 Elsevier B.V. Aldoi:10.1016/j.desal.2010.06.064hases [7].an was rst described as

In the present study, akaganeite (-FeOOH) which is synthesizedand as mentioned previous work was used as a sorbent. In theapplications associated to other iron oxide pAkaganeite from the Akagane mine in Japcatalysts, gas sensors, magnetic recordiron oxides, akaganeite has attractedmunique sorption, ion exchange, andinstance, akaganeite used as a low-coremoval of hexavalent chromium, Uaqueous solutions was studied by somakaganeite is used as a precursor in thephases such as hematite, goethite anparticle morphologies that are unusuathis way, akaganeite is indirectly usefvestigation because of itstic properties [2]. Forential adsorbent for theAs(V) and As(III) fromors [36]. Furthermore,ction of other iron oxideetite in order to obtainse iron oxide phases. Industrial and biomedical

health mostly results from its properties as a heavy metal [13]. Thereis increasing interest in searching for different adsorbents, such asactivated carbon [14,15], metal oxides, aluminosilicates [1618],goethite [19] for the uranium elimination from the aqueousenvironment. Uranium occurs as a mobile, aqueous uranyl UO22+ ionin nuclear wastes as well as in mill tailings, wastes material ofuranium mining, where soil and mine-water contamination plumeshave very high dissolved solutions and solid uranium concentrations[20].at the tunnels which areture can be occupied by

maximum U(VI)0.1 at initial conequal to 0.01 g.Langmuir, Freunisotherms and re1 and AK-2 ttthermodynamicsous and endothean).

l rights reserved.ry low levels of radioactivity. Its danger for human

Iron oxides, as a group, have impo pplications as pigments, many groundwaters as a trace component. Naturally occurringAdsorption equilibrium and kinetics of U

Sabriye Yusan , Sema Akyil ErenturkEge University Institute of Nuclear Sciences 35100 Bornova-Izmir, Turkey

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

Article history:Received 15 March 2010Received in revised form 24 June 2010Accepted 25 June 2010Available online 24 July 2010

Keywords:-FeOOHU(VI)AdsorptionIsothermsKinetic

The use of cheap, high efaqueous solutions. A seriesvariables, i.e. initial pH, temparameters were given inRadushkevich (DR) and Htemperatures. Adsorption pkinetics was found to followintraparticle diffusion as onchemical sorption plays a rI) on beta type of akaganeite

cy and low-risk adsorbent has been studied for the removal of U(VI) fromperiments was conducted in a batch system to assess the effect of the systemrature, initial uranium concentration and contact time. The results of thesee previous work. In this paper Langmuir, Freundlich, Temkin, DubinininsJura isotherms were used to analyze the equilibrium data at differentess tted to Langmuir and Temkin isotherm models. Also the adsorptioneudo-second-order rate kinetic model, with a good correlation (R2N0.99) andthe rate determining steps. The ndings of this investigation suggest that thein controlling the sorption rate.

2010 Elsevier B.V. All rights reserved.

ation

sev ie r.com/ locate /desa lremoval in AK-1 was obtained as N99% at pH 4.0centration of 50 mg L1 and amount of akaganeiteThe experimental results have been analyzed bydlich and DubininRadushkevich (DR) adsorptionsults showed that adsorption equilibrium data of AK-ed to Freundlich and Langmuir. The results of theof U(VI) ion/akaganeite system indicate spontane-rmic nature of the process. Also AK-1 and AK-2 were

were collected at room temperature in the range of 2 between 0 and70. The diffractograms of AK-1 and AK-2 are shown in Fig. 1(a) and (b).All diffraction patterns contain only the diffraction lines characteristic of-FeOOH (JCPDS card 34-1266). The corresponding Miller indices aremarked above the diffraction lines [23].

All XRD patterns exhibit the characteristic anatase diffractionpeaks at 2 values of AK-1 and AK-2: 35.3, 26.9, 11.9, 56.2 and16.9; 35.2, 26.8, 11.9, 56.1 and 16.8, respectively. This adsorbenthas low crystallinity with d values. Similar result has been obtained inthe literature [2326].

DebyeScherrer diffraction patterns are often used to characterizesamples, as well as to probe the structure of nanoparticles [27]. Wealso estimated the particle sizes of the synthesized AK-1 and AK-2from the line width as 8.64 and 17.35 nm, respectively using theDebyeScherrer equation:

L = k= B cos 2

where L is the length of the crystal in the direction of the d spacing, k isa shape factor of the particle (1 if spherical, typically 0.9 is used), B isthe line width at half maximum, and are the wavelength andincident angle of the X-rays, respectively [28].

Nitrogen multipoint BET analysis yielded a specic surface of AK-1and AK-2, 109.06 and 75.40 m2 g1, respectively. The average poreradius (nm) was found for AK-1 and AK-2 as 3.97 and 8.53.Incremental pore volume (cm3 g1) was 0.14 and 0.24, respectively.Khasanova et al. and Barrero et al. measured the BET surface area of41.7 and 25 m2 g1, respectively [29,30]. Cornell and Schwertmann,Solozhenkin et al., Parida et al., Watkins et al. andMartnez-Llad et al.obtained similar surface area of our work [5,3134].

FT-IR spectroscopywas performed on an IRPRESTIGE-21 ShimadzuFT-IR spectrometer. The sample was prepared with KBr and pressed

234 S. Yusan, S. Akyil Erenturk / Desalination 263 (2010) 233239characterized by powder X-ray diffraction for crystalline phaseidentication and scanning electron microscope (SEM).

In the current, the applications of the isotherm models have beenstudied to explain the adsorption characteristics of the akaganeite. Forthis aim, Langmuir, Freundlich, Temkin, DubininRadushkevich (DR)and HarkinsJura isotherms were used to analyze the equilibriumdata at different temperatures. And furthermore, kinetics as well asthe diffusion parameters for the adsorption of U(VI) onto theakaganeite is evaluated which are not studied in the previous workXRD, BET and FT-IR analysis are also discussed for characterizationstudies given in the previous paper.

2. Materials and methods

2.1. Materials

Synthesis of akaganeites (AK-1 and AK-2) was rst reported inour previous study [4]. All chemicals and reagents used for experi-ments and analyses were of analytical grades. A stock solution of1000 mg L1 U(VI) was prepared by dissolving an appropriateamount of UO2(NO3)26H2O in deionized water. The initial pH ofthe working solutions was adjusted by addition of HNO3 or NaHCO3.Dibenzoyl methane-tri-n-octyl phosphine oxide (DBM-TOPO), sal-icylic acid was obtained from Merck Co. The buffer solutions (pH 4, 7and 9) to calibrate the pH-meterModel 8521 fromHanna Instrumentswere also purchased from Merck.

2.2. Batch adsorption experiments

Batch adsorption experiments were carried in a thermostatedshaker bath, GFL-1083 model. AK-1 and AK-2 (0.01 g), which have75 m particle sizes, were added to 10 ml solution containing variousuranium concentrations at different temperatures for various contacttime. The pH was adjusted by adding HNO3 and Na2CO3 to thesolutions at the each experiment. The suspension was ltered byusing Whatman lter paper No: 44. A simple and sensitivespectrophotometric method was used in the experiments to deter-mine uranium in solution. The uranium remained in solution wasanalyzed with the DBM-TOPO as complexing agent at 405 nm againstreagent blank employing spectrophotometric method on ShimadzuUV-1601 UVVIS spectrophotometer [21,22]. The amount of adsorbeduranium was estimated from the difference of the uranium concen-trations in the aqueous phase before and after the adsorption. Eachexperiment was repeated at least three times and the results givenare the average values. The percentage adsorption of uranium fromaqueous solution was computed as follows:

Adsorption% = CintCfin =Cint 100 1

where Cint and Cn are the initial and nal uranium concentration,respectively.

Kinetics of adsorption was determined by analyzing adsorptiveuptake of the U(VI) from aqueous solution at different time intervals.For adsorption isotherms, U(VI) solutions of different concentrations(50200 mg L1) and at different temperatures (2050 C) wereagitatedwith known amounts of adsorbents until the equilibriumwasachieved.

2.3. Material characterization methodology and intermediate results

Scanning electron microscope (SEM) images were taken on JeolJsm 6060 scanning electron microscope. The SEM images showed thatcrystallites in both products had similar cigar-like shapes and similarcrystal sizes [4].

Powder X-ray diffraction (XRD) data were collected on a SHIMADZU

XDD-6000 diffractometer with Cu-X radiation (=1.5405 ). The data Fig. 1. X-ray diffractogram of the akaganeite (a) AK-1. (b) AK-2.

into pellet. As shown in Fig. 2, the spectrum of Akaganeite was col-lected in themid-IR range from 400 to 4000 cm1 with a resolution of1 cm1.

Infrared spectrawere used, for instance, to study the strength of thehydrogenbonds of iron(III) oxide hydroxides [35]. The infrared spectraof akaganeite (AK-1 and AK-2) were obtained with the KBr pressed-disk technique and are presented in Figs. 3 and 4. The spectral regionsof interest are the HOH banding region (~1630 cm1) and the OHstretching region of H2O (~3400 cm1). Sorbed water contributes tothe HOH banding region (~1630 cm1) and the OH stretchingregion of H2O (~3400 cm1). In akaganeite OH.Cl bonds are formedinstead of OHO bonds, with HCl ions being much weaker hydrogenbond acceptors than OH and O2 ions. So there are two sets oflibrations due to hydrogen bonds, those of OH librational ROH 847 and820 cm1 and those of 697 and 644 cm1, due to the two OH.Clhydrogenbondspresent. In the nanocrystalline sample prepared in thelaboratory, the bands at 847 and 820 cm1 were diminished andmostly disappeared due to the washing of the chlorine ions. Thecooperativity of hydrogen bonds such as OHOHOH mayresult in the band at 1064.5 cm1. The energy translational mode ofakaganeite due to the FeO stretches was observed at ~482 cm1

[31,36].

3. Results and discussion

3.1. Results of adsorbent characterization

The XRD analysis conrmed that iron(III) oxyhydroxides preparedin this study were crystalline and from the BET results they have highsurface area and it improves the sorption capacity.

Fig. 3. Lagmuir isotherm plots for the sorption of U(VI) on AK-1.

235S. Yusan, S. Akyil Erenturk / Desalination 263 (2010) 233239Fig. 2. FT-IR spectrum of Akaganeite nanoparticle, (a) AK-1. (b) AK-2.

for some temperatures especially for AK-2 adsorbent due to datascatter. The values of Qm and KL was showed variationwith increase intemperature. Adsorption capacity and intensity of adsorption aremore stable at lower temperatures for the removal of U(VI) ions fromaqueous efuent. Further, it conrmed the fact that an endothermicprocess and exothermic process took place in this adsorption systemfor AK-1 and AK-2, respectively [37]. Also difference of the processmay be due to major complexes which are seen in the range of acidicto near neutral pH values.

3.2.2. Temkin isotherm equationThis isotherm was rst developed by Temkin and Pyzhev [38], and

236 S. Yusan, S. Akyil Erenturk / Desalination 263 (2010) 233239From the FT-IR spectra it is speculated that uranium ions reactedrather directly with Fe substituted surface OH2 functional groups ofakaganeite. And the surface complexes, SOUO2+ and SOUO2OH,can be used to interpret the sorption of U(VI) on akaganeite.

This suggests that there is a specic adsorption between theuranium anions and the adsorbent, thus the substitution of SOHgroups by U(VI) ions plays an important role in the adsorptionmechanism. The fact that uranium ions are possibly coordinated todiscrete surface iron cations is supported by FT-IR results.

3.2. Adsorption equilibrium study

Equilibrium data, commonly known as adsorption isotherms, arebasic requirements for the design of adsorption systems. The sorptiondata have been subjected to different sorption isotherms, namelyLangmuir, Freundlich, Temkin, DR, HarkinsJura, Halsey sorptionand Handerson isotherms. For each isotherm, the temperature ofsolution was varied to 2050 C. According to results, AK-1 and AK-2sorption data tted Langmuir and Tempkin isotherm models.

3.2.1. Langmuir isotherm equationThe basic assumption of Langmuir adsorption isotherm is based on

monolayer coverage of the adsorbate on the surface of adsorbent. Thesaturated monolayer can be represented by the following equation:

Ce = qe = 1 =QmKL + Ce =Qm 3

where Ce is the equilibrium concentration of the adsorbate, qe is theamount of the adsorbate adsorbed at equilibrium, Qm and KL areLangmuir's constants related to the capacity and energy of theadsorption, respectively. The linear nature of the curve was found byplotting Ce/qe versus Ce at different temperatures (Figs. 3 and 4)

Fig. 4. Lagmuir isotherm plots for the sorption of U(VI) on AK-2.suggested the applicability of Langmuir isotherm for the presentsystem. The values of Qm and KL at different temperatures weredetermined from the slopes and intercepts of the respective plot andpresented in Table 1. As you see from Table 1, R2 values are very low

Table 1Values of Langmuir and Temkin isotherm constants.

Isotherm parameters Temperature (K)

AK-1

Langmuir 293 303 313KL (g L1) 80 16.67 1.825Qm (mmol g1) 2500 1000 137R2 0.976 0.995 0.607

TemkinB1 3406 2564 3329KT (g L1) 0.22 1.39 2.711

R2 0.971 0.969 0.889it is based on the assumption that the heat of adsorption woulddecrease linearlywith the increase of coverage of adsorbent [39]. Theyconsidered the effects of some indirect adsorbate/adsorbate interac-tions on adsorption isotherms. They suggested that, because of theseinteractions and ignoring very low and very large values ofconcentration, the heat of adsorption of all molecules in the layerwould decrease linearly with coverage [40]:

qe = RT = btln KtCe : 4

Eq. (4) can be linearized as

qe = B1 lnKt + B1 lnCe 5

where B1=RT/b, in which R is the gas constant, T the absolutetemperature in Kelvin, bt the constant related to the heat ofadsorption and Kt is the equilibrium binding constant (mg L1). TheTemkin isotherm equation has been applied to describe adsorption onheterogeneous surface [41,42].

The constants obtained for Temkin isotherm are shown in Table 1.The Temkin constant, B1, shows that the heat of adsorption increaseswith the increase in temperature for AK-1, however, the heat ofadsorption decreases with the increase in temperature, indicatingendothermic and exothermic adsorption for AK-1 and AK-2, respec-tively (Figs. 5 and 6).

3.3. Adsorption kinetics

To evaluate sorption as a unit operation, it requires considerationof two important physico-chemical aspects of the process: the kineticsand the equilibria of sorption. Kinetics of sorption describing thesolute uptake rate, which in turn governs the contact time, is one ofthe important characteristics dening efciency of sorption. The studyof the equilibrium established in any liquidsolid system is importantin determining distribution of the solute between the solid and liquidphases and determining feasibility and capacity of the sorbent forsorption [43].

In order to determine kinetic parameters and explain to themechanism of the adsorption processes, lots of researchers have usedrst and pseudo-second-order rate expressions [44]. Three kineticmodels were applied to adsorption kinetic data in order to investigate

AK-2

323 293 303 313 32350 100 10 1.5 16.672000 2500 500 333 10,0000.973 0.765 0.725 0.838 0.922

12,027 50,300 38,808 20,724 23,6902.86 0.17 0.04 1.22 0.230.633 0.944 0.860 0.283 0.137

outer surface of adsorbent, there is also possibility of transport of

Fig. 5. Tempkin isotherm plots for the sorption of U(VI) on AK-2.

Fig. 7. Lagergren rst-order plot for the adsorption of U(VI) by AK-1 and AK-2.

237S. Yusan, S. Akyil Erenturk / Desalination 263 (2010) 233239the behavior of adsorption process of U(VI) onto Akaganeite. Thesemodels are the pseudo-rst-order [45], the pseudo-second-order [28]and the intraparticle diffusion models [46].

The adsorption of U(VI) from a liquid phase to solid phase can beconsidered as a reversible process with equilibrium being establishedbetween the solution and solid phase. Adsorption phenomenon can bedescribed as thediffusion control process, assuming a non-dissociationmolecular adsorption of U(VI) on akaganeite nanoparticles [46]. TheLagergren rst-order rate expression based on solid capacity isgenerally expressed as follows:

logqeqt = log qek1t

2:3036

where qt and qe (mg g1) are the amounts of the metal ions sorbed atequilibrium (mg g1) and t (min), respectively, and k1 is the rateconstant of the equation (min1). The adsorption rate constants (k1)can be determined experimentally by plotting of ln(qeqt) versus t[45].

A plot of Eq. (6) is shown in Figs. 7 and 8 for AK-1 and AK-2. Table 2shows the rate constants and R2 values of the kinetic models. Theexperimental data gives not good t for both AK-1 (R2=0.8075) andAK-2 (R2=0.2375) indicating that the Lagergren model is notapplicable for these adsorbents.

The pseudo-second kinetic model developed by Ho and McKay, isbased on experimental information of solid phase sorption, generallyit has been applied to heterogeneous systems, where the sorptionmechanism is attributed to chemical sorption and the sorptioncapacity is proportional to the number of active sites on the sorbent.

The model can be represented by the following equation [46]:

tqt

=1

k2q2max

+tqmax

7

where k2 (g mol1 min1) is the second-order rate constant. Thelinear plot of t/qt as a function of t provided not only the rate constantk2, but also an independent evaluation of qe. The results are given inTable 2.Fig. 6. Tempkin isotherm plots for the sorption of U(VI) on AK-1.The plot for the above equation is shown in Figs. 9 and 10. The datagives perfect t for this model for both AK-1 (R2=0.999) and AK-2(R2=0.989). The values of the second-order rate constant found fromthe slopes of the plots for AK-1 (k2=0.17 gmg1 min1) and AK-2(k2=0.10 gmg1 min1) indicate that U(VI) removal rate is faster byAK-1 than AK-2.

Based on the obtained correlation coefcients (r), the experimen-tal data conformed better to the pseudo-second-order equation,evidencing chemical sorption as rate-limiting step of adsorptionmechanism [47]. The pseudo-second-order model is based on theassumption that the rate-limiting step may be chemisorptioninvolving valence forces through the sharing or exchange of electronsbetween adsorbent and adsorbate [48].

Another tested simplied kinetic model to the sorption processwas the intraparticle transport [49]. The kinetics of sorption of U(VI)on the AK-1 and AK-2 was also evaluated by the MorrisWeberequation [50]:

qt = kp t0:5 8

where qt is the concentration of the sorbed ion (mg g1) at time t, andkp is the rate constant for the intraparticle transport (mg g1h0.5).According to this model, a graphic plot for qt versus t0.5 could predictthe sorption mechanism. If a straight line (passing through the pointof origin) is obtained, therefore, sorption of the ions onto theimpregnated sorbent followed a diffusion mechanism [51].

When pore diffusion limits the adsorption process, the relation-ship between the initial solute concentration and the rate ofadsorption will not be linear. Besides for the adsorption on theFig. 8. Pseudo-second-order plot for the adsorption of U(VI) by AK-1 and AK-2.

adsorbent ions from the solution to the pores of the adsorbent due tostirring on batch process. This possibility was tested in terms ofintraparticle diffusion model. The linear portion of the plot for a widerange of contact time between the adsorbate and adsorbent does notpass through the origin. This deviation from the origin or nearsaturation may be perhaps due to difference in the rate of mass

[15] A. Mellah, S. Chegrouche, M. Barkat, The removal of uranium (VI) from aqueoussolutions onto activated carbon: kinetic and thermodynamic investigations, J.

effect, J. Environ. Radioactiv. 93 (2007) 127143.[17] A. Klncarslan, S. Akyil, Uranium adsorption characteristic and thermodynamic

behaviour of clinoptilolite zeolite, J. Radioanal. Nucl. Chem. 264 (2005) 541548.[18] S. Aytas, S. Akyil, M. Eral, Adsorption and thermodynamic behavior of uranium on

natural zeolite, J. Radioanal. Nucl. Chem. 260 (2004) 119125.[19] T. Missana, M. Garcia-Gutierrez, C. Mafotte, Experimental andmodelling study of

the uranium (VI) sorption on goethite, J. Colloid Interface Sci. 260 (2003) 291301.[20] M. Majdan, S. Pikus, A. Gajowiak, A. Gadysz-Paska, H. Krzyzanowska, J. Zuk, M.

Bujacka, Characterization of uranium(VI) sorption by organobentonite, Appl. Surf.Sci. 256 (2010) 54165421.

Table 2Values Lagergren rst-order model, pseudo-second-order model and Weber and Morismodel constants.

Adsorbent k1 (dk1) R1 k2 (g/mg dk) R2 kp (mg/g dk0.5)

AK-1 0.0050.02 0.81 0.170.01 0.999 10101.29AK-2 0.0040.04 0.24 0.100.05 0.989 7432.22

238 S. Yusan, S. Akyil Erenturk / Desalination 263 (2010) 233239transfer in the initial and nal stages of adsorption. Further suchdeviation from the origin indicated that the pore diffusion is not therate-limiting step [47]. From Fig. 9, it can also be observed that thestraight lines does not pass through the origin, which indicates thatintraparticle diffusion and lm diffusion are both the rate-limitingsteps for U(VI) diffusion onto AK-1 and AK-2 [52].

The Reichenberg equation [53] was applied to check that sorptionproceeds via lm diffusion or intraparticle diffusion mechanism.Reichenberg equation was tested in the following way [53]:

X = 1 62

eB t9

where X=qt/qe and Bt is a mathematical function of X which can becalculated for each value of X as.

Bt = 0:4977 ln1X 10

A plot of Bt versus t is also shown in Fig. 10, which is a straight line.It is clear from it that intraparticle diffusion is the rate controlling stepwith a small friction of the sorption that occurs through lm diffusionbecause the plot does not pass through origin [54].

4. Conclusion

The present study focuses on adsorption of U(VI) from aqueoussolution using the Akaganeite as a low cost and effective nanocrystalsorbent. The adsorption equilibrium and kinetics have beenexamined.

The experimental data were evaluated by Langmuir and Temkinisotherms. According to isotherm data, adsorption process isendothermic for AK-1 and exothermic for AK-2.

Kinetic evaluation of the equilibrium data showed that theadsorption of U(VI) on Akaganeite followed well the pseudo-second-order kinetic model and the lm diffusion and intraparticle diffusionare both the rate-limiting steps. Based on all the results, it can be alsoconcluded that the -FeOOH, Akaganeite is an effective and alternativeFig. 9. Weber and Moris plot for the adsorption of U(VI) by AK-1 and AK-2.Colloid Interface Sci. 296 (2006) 434441.[16] R. Han, W. Zon, Y. Wang, L. Zhu, Removal of uranium(VI) from aqueous solutions

by manganese oxide coated zeolite: discussion of adsorption isotherms and pHsorbent for removing U(VI) from aqueous solution because of its highadsorption capacity, low-risk feature.

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Adsorption equilibrium and kinetics of U(VI) on beta type of akaganeiteIntroductionMaterials and methodsMaterialsBatch adsorption experimentsMaterial characterization methodology and intermediate results

Results and discussionResults of adsorbent characterizationAdsorption equilibrium studyLangmuir isotherm equationTemkin isotherm equation

Adsorption kinetics

ConclusionReferences