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Journal of Hazardous Materials 246– 247 (2013) 199– 205

Contents lists available at SciVerse ScienceDirect

Journal of Hazardous Materials

jou rn al h om epage: www.elsev ier .com/ loc ate / jhazmat

emoval of radioactive iodine from water using Ag2O grafted titanateanolamina as efficient adsorbent

rixin Boa, Sarina Sarinaa, Zhanfeng Zhenga, Dongjiang Yangb, Hongwei Liua, Huaiyong Zhua,∗

School of Chemistry, Physics, Mechanical Engineering, Queensland University of Technology, GPO Box 2434, Brisbane, QLD 4001, AustraliaCollege of Chemistry, Chemical and Environmental Engineering, Laboratory of Fiber Materials and Modern Textile, the Growing Base for State Key Laboratory, Qingdao University,ingdao 266071, PR China

i g h l i g h t s

Ag2O nanocrystals were deposited on titanate nanolamina prepared from TiOSO4.The composite is efficient adsorbent for removal of radioactive Iodine from water.The adsorbent exhibited a high capacity of 3.4 mmol of iodine per gram of adsorbent in 1 h.Ag2O nanocrystals are firmly anchored on the surface of the titanate lamina by coherent interface.The adsorbent can be recovered easily for safe disposal and suitable for column adsorption-bed.

r t i c l e i n f o

rticle history:eceived 1 September 2012eceived in revised form0 November 2012ccepted 3 December 2012vailable online 10 December 2012

eywords:odine removalitanate nanolaminauclear waste

a b s t r a c t

Emergency treatment of radioactive material leakage and safety disposal of nuclear waste is a constantconcern all along with the development of radioactive materials applications. To provide a solution,titanate with large surface area (143 m2 g−1) and a lamina morphology (the thickness of the lamina isin range of tens of nanometers) was prepared from inorganic titanium compounds by hydrothermalreactions at 433 K. Ag2O nanocrystals (5–30 nm) were deposited onto the titanate lamina. The surfaceof the titanate lamina has crystallographic similarity to that of Ag2O nanocrystals. Hence, the depositedAg2O nanocrystals and titanate substrate join together at these surfaces, forming a well-matched phasecoherent interface between them. Such coherence between the two phases reduces the overall energyby minimizing surface energy and anchors the Ag2O nanocrystals firmly on the external surface of thetitanate structure. The composite thus obtained was applied as efficient adsorbent to remove radioactiveiodine from water (one gram adsorbent can capture up to 3.4 mmol of I− anions). The composite adsorbent

can be recovered easily for safe disposal. The structure changes of the titanate lamina and the compositeadsorbent were monitored by various techniques. The isotherm and kinetics of iodine adsorption, com-petitive adsorption and column adsorption using the adsorbent were studied to assess its iodine removalabilities. The adsorbent exhibited a capacity as high as 3.4 mmol of iodine per gram of adsorbent in 1 h.Therefore, Ag2O deposited titanate lamina is an effective adsorbent for removing radioactive iodine fromwater.

. Introduction

The environmental and health issues caused by nuclear wasteave drawn increasing attention around the world since the exten-ive application of nuclear energy started from early 20th century

1]. Substance in nuclear waste like radioactive iodine producedrom uranium fission can easily dissolve in water and thus, has neg-tive effects on lives living within the ecosystem. Yet, the concern

∗ Corresponding author. Tel.: +61 7 3138 1581; fax: +61 7 3138 1804.E-mail address: hy.zhu@qut.edu.au (H. Zhu).

304-3894/$ – see front matter © 2013 Elsevier B.V. All rights reserved.ttp://dx.doi.org/10.1016/j.jhazmat.2012.12.008

© 2013 Elsevier B.V. All rights reserved.

on nuclear waste leakage rose to a higher level globally after thenuclear accident at Fukushima, Japan in 2011. Dating back, a nuclearaccident known as the Chernobyl Disaster happened in 1986 atChernobyl, previous Ukrainian SSR, where radioactive iodine wasreleased as the major component of the radiation and led to 28 fatalcases of acute radiation sickness according to United Nations Scien-tific Committee of the Effects of Atomic Radiation (UNSCEAR) [2].Radioactive iodine is also widely applied to accurately diagnose and

treat various thyroid diseases based on thyroid’s iodine selectiveuptake ability [3]. Consequently, the disposal amount of radioactiveiodine from numerous medical research institutions is also con-siderable. Small amounts of radioactive iodine are produced from

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00 A. Bo et al. / Journal of Hazardous M

he operation of nuclear power plants, which can release minormounts to air and water. Once ingested by human being, mostodine decays in the body and the rest is exhaled or excreted inhe urine and decays naturally in the environment [4]. The radioac-ive isotopes of iodine are produced from nuclear fission of fissileuclides (especially uranium-235) and their half-life times differ

rom around 8 days (131I) to 1.6 × 107 years (129I). Since the iodinesotopes have the same electronic structure, their physical andhemical properties are indistinguishable in many circumstances5]. The iodine isotopes of short half-life times are also damag-ng to human body because thyroid collects and concentrates bothadioactive and non-radioactive iodide [6]. Therefore, effectivelyreventing the spread of the radioactive iodine wastes is quiteecessary and therefore, emergency treatment and safe storage ofadioactive iodine in nuclear dump and polluted water source is anmportant challenge around the world [7].

One approach reported for I− anion removal from water is tose compounds containing Bi3+, Hg2+, Ag+, Cu+, and Pb2+ cations toorm precipitates or sparingly soluble phases with I− anions [8–12].he precipitates or sparingly soluble phases formed on the sur-ace of a compound were then separated from the solutions andisposed of safely. However, these compounds displayed low sorp-ion capacities and slow uptake dynamics due to their small specificurface areas. To address this problem, Ag+ and Hg2+ cations wereoaded onto the surface of natural zeolites to capture I− anionsrom gaseous and aqueous wastes [13,14]. However, these zeolitedsorbents suffered from slow adsorption kinetic and thus, poorfficiency as most of the cations are located in the cages and chan-els of the zeolite, which are less than 1 nm wide. Diffusion of theations and anions through the narrow passages before their pre-ipitation is slow. Such a drawback is intrinsic to the microporoustructure of the substrates, impeding the practical application ofhe zeolite based sorbents. Moreover, concern is also raised on theossible release of radioactive species from the used sorbents if therecipitate particles did not bind solidly to the zeolite substrate.

I− anions can readily react with silver oxide (Ag2O) to form insol-ble silver iodide [15]. Therefore, Ag2O can potentially entrap I−

nions of low concentrations in water whereas using Ag2O directlyor the removal of I− anions is impractical as the capacity andynamics of the removal depend on the specific surface area ofhe Ag2O particles. If the specific surface area is small (large Ag2Oarticles), the removal ability is poor. When very fine Ag2O parti-les with a large specific surface area are used, the removal abilitys improved but the separation of the fine AgI particles from water

ill be extremely difficult and costly. A solution to this problem iso anchor Ag2O nanocrystals firmly onto a support with a large spe-ific surface area that allows them to disperse sufficiently withoutorming aggregates.

Recently, titanate nanomaterials have been found to be a poten-ial nuclear waste host for radioactive cations due to their highorption performance and stability to radiation, thermal, and chem-cals [16–18]. Additionally, we found that Ag2O crystals haveurfaces of crystallographic similarity to the surface of the titanateanostructures [19]. This property means that Ag2O crystals canrmly attach onto the external surface of titanate nanomaterialsy sharing oxygen atoms at the interfaces between Ag2O crys-als and the titanates. The attached Ag2O nanocrystals can capture− in aqueous solution at a considerable rate under high fluxecause they are on the external surface of the titanates and cane readily accessed by I− ions. In the present work, we synthesizedhe titanate lamina by a wet chemical reaction between concen-rated NaOH solution and TiOSO4 under hydrothermal conditions.

g2O nanocrystals were anchored on the surface of the titanateanolamina using facile chemical deposition method. The compos-

te adsorbents displayed high adsorption capacity, selectivity, andast kinetics for I− anions.

als 246– 247 (2013) 199– 205

2. Experimental

2.1. Preparation of titanate nanolamina

Titanate nanolamina (Na2Ti3O7) was synthesized by the reac-tion between concentrated NaOH solution and titanium (IV)oxysulfate hydrate (TiOSO4·XH2O) under hydrothermal conditions[16]. Specifically, 21.6 g of TiOSO4·XH2O (98%, from Fluka) was dis-solved in 160 mL of deionised water under shaking. The solutionwas shaken for 1.5 h and gradually became clear, and then mixedwith 80 ml 15 M NaOH solutions under stirring and continuouslystirred for 1 h. The white suspension was then equally divided intothree 125 ml autoclaves and kept at 423 K for 48 h to yield sodiumtitanate. The white solid in the autoclaved mixture was recoveredand washed with deionised water. Finally, the collected white pow-der was dried at 353 K for 24 h (labeled as T3NL).

2.2. Ag2O deposition

Prior to adsorption experiment, 3 g of T3NL was dispersed intoexcess amount of AgNO3 solution (0.1 M, 200 ml) by shaking atroom temperature for 48 h. Afterwards, the sample were collectedby centrifuge (4000 rpm, 10 min) and washed with deionised waterfor three times before drying in air at 353 K for 24 h. The color of thesample changed to black during the drying due to the formation ofAg2O and this sample was labeled as Ag2O–T3NL and used for I−

adsorption.

2.3. Adsorption experiments

The anion adsorption experiment was carried out with non-radioactive I− in aqueous solution considering the toxicity of itsradioactive isotope 131I. The I− anion adsorption isotherm wasdetermined by equilibrating 25 mg of Ag2O–T3NL with 25 ml NaIsolution possessing I− concentration in a range of 100–1100 ppmfor 48 h and measuring the concentration of the I− left in thesolution. A calibration curve was constructed (the R2 value of thestraight calibration line is 0.9998) and used to convert the UV–visdata to actual I− concentration in the adsorption tests.

2.4. Selective uptake of I− ions by Ag2O–titanate nanolaminaadsorbent

The selective adsorption test was conducted in the presence ofhigh concentration of Cl− anions. For example, 25 mg of adsorbentwas dispersed in 25 ml of aqueous solution containing 5 × 104 ppmNaCl and 5 × 102 ppm NaI (the molar ratio of Cl− ions to I− ionswas 100:1), the suspension was shaken for 48 h prior to UV–vismeasurement.

2.5. Desorption of the precipitated I− anions

Leaching or desorption was examined in various aqueous solu-tions. The adsorbents saturated with I− anion were collected viacentrifuge (4200 rpm, 10 min) and rinsed with 25 ml water threetimes to remove the remaining I− in the solution and the adsor-bents were redispersed into 25 ml of water, 0.01 M and 0.1 M NaClsolutions respectively. The suspension was shaken for 48 h at roomtemperature and the I− anion concentrations of the solutions weredetermined by UV–vis spectroscopy.

2.6. Competitive adsorption of I− ions on Ag2O–titanate

nanolamina in fixed-bed adsorption column

500 ppm NaI plus 5 × 104 ppm NaCl was used to test the adsorp-tion kinetics of Ag2O–T3NL in adsorption column. To build up the

aterials 246– 247 (2013) 199– 205 201

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Fig. 1. (A) The isotherm for I− adsorption was obtained by dispersing 25 mg ofAg2O–T3NL adsorbent in 25 ml NaI solution (100–1100 ppm) for 48 h. (B) Showsremoval of I− anions of different concentration using 25 mg of Ag2O–T3NL adsor-

−

than T3NT may due to the fact that lamina titanate are tend tooverlap on each other during the adsorption process where theywould obstruct, impede or block the reaction/interaction between

Table 1BET surface area of titanate samples and the I− removal capacity of Ag2O graftedtitanates.

Sample BET surfacearea (m2 g−1)

I− removal amount(mmol g−1)

T3NT 205.4 0.5Ag O–T3NT 156.5 4.5

A. Bo et al. / Journal of Hazardous M

olumn, a piece of filter paper was padded in the bottom of glassipette. 0.1 g Ag2O–T3NL adsorbent was then put into the glass col-mn in which it forms a 3 × 5 mm cylindrical adsorptive layer. 20 ml

−/Cl− mixture solution was passed through the column each timend measured at different flow rates. The flow rates were set at.2 ml/min, 0.8 ml/min and 0.4 ml/min which were controlled anddjusted by a barometer connected air pump. The I− concentrationn the collected solution was measured by UV–vis spectroscopy.

.7. Characterization

The titanate lamina and Ag2O–T3NL were characterized by pow-er X-ray diffraction (XRD) and Transmission electron microscopyTEM). The TEM study on the samples and the HRTEM investigationsere carried out on a FEI Tecnai F20 operating at 200 kV. The spec-

mens were deposited onto a copper microgrid coated with holeyarbon film. XRD patterns of the sample powder were recordedn a Philips PANalytical X’pert pro diffract meter equipped withraphite monochromator. Cu K radiation and a fixed power source40 kV and 40 mA) were used. The XRD data were collected over a� range between 3.5◦ and 75◦, at a scanning rate of 2.5◦/min. The

− concentration was measured and calculated by Carry 100 UV–vispectroscopy. Lastly, N2 adsorption–desorption isotherm and poreize distribution were obtained by Micromeritics TriStar II surfacerea and porosity analyser.

. Results and discussion

The equilibrium isotherm of Ag2O–T3NL (Fig. 1A) illustrated thathe maximum adsorption capacity of Ag2O–T3NL can reach up to.4 mmol g−1 (3.4 mmol of I− anions per 1 g of sorbent or 340 meq.er 100 g) while that of parent T3NL was substantially negligible.his uptake value is considerably higher than the values previouslyeported for conventional metallic compound sorbents in the liter-ture [19]. For the wastewater containing 5 ppm of the radioactiveodine 1 kg of the reported Ag2O–T3NL could fully clean up 86.2 tonf wastewater in terms of radioactive iodine removal. As shown inig. 1B, the Ag2O anchored titanate nanolamina (Ag2O–T3NL) isble to remove 80% of I− from solution with the I− concentration asigh as 500 ppm. More than 99% of I− ions can be removed whenhe concentration is 300 ppm or below (Fig. 1B). The reaction of I−

ons onto parent (without Ag2O particles) T3NLs was less than 1%,hich is in stark contrast with that of Ag2O–T3NL shown above.

he distribution coefficient Kd was derived from the systems con-aining iodide ions of various initial concentrations. Kd is the ratiof the amount of I− ions adsorbed by one gram of the sorbent to themount of I− ions remaining in solution (per milliliter) [20]. Thebtained Kd values are given in Fig. 1B.

It is notable that the adsorption ability of Ag2O anchoreditanates toward I− anions is strongly coupled with the morphol-gy of titanates. When Ag2O nanocrystals are loaded on the tubularitanate surface (Ag2O–T3NT), the maximum removal of I− can bes high as 4.5 mmol g−1 whilst on the fabric titanate (Ag2O–T3NF),t was decreased to around 3.0 mmol g−1 [19]. The morphologyetermines the specific surface area of the material which mayubsequently influence the amount of Ag2O loaded on the sur-ace. According to Table 1, the higher I− adsorption capacity isbtained when the surface area is larger. While not all the Ag+ ionsntroduced into titanates are in the form of Ag2O on the surface,art of them exists in the form of Ag+ ions in the interlayer region

fter ion exchange with Na+ ions in the layer of titanates. Theseg2O nanoparticles exposed on the surface of the fibers or tubesct as efficient capturers of I− anions that readily accessible to thenions even in a fast flux.

bent. For comparison, the I uptake of pristine titanate nanolamina (T3NL) is shown.The Kd values are calculated from the adsorption saturation data (at 500 ppm) asshown in Fig. 1B: Kd of Ag2O–T3NL is 4463.6 and that of parent T3NL is 38.8.

We obtained the practical Ag loading amount from EDS data andcalculated the efficiency of Ag+ utilized for capturing I− ions. Theaverage weight percentage of Ag in the Ag2O–T3NL adsorbent is40.3% which gives approximately 3.76 mmol Ag atoms per gram ofcombined titanate. Therefore, the theoretical adsorption capacityof I− goes to 3.76 mmol. Regarding to the adsorption amount of I−

obtained from the adsorption isotherm (Fig. 1A), the efficiency ofthe Ag2O–T3NL adsorbent is around 90.6%. The above iodine uptakeamount of T3NL is much higher than the I− adsorption capacitiesof other known adsorbent materials, such as clays, zeolites, syn-thetic mica, and niobates [21–26]. Efficiencies of T3NT can be ashigh as 99% and T3NF is only 78% respectively. The underliningreason is that the slightly lower utilizing efficiency of Ag+ on T3NL

2

T3NL 143.1 0.2Ag2O–T3NL 134.8 3.4T3NF 33.0 0.1Ag2O–T3NF 24.6 2.6

202 A. Bo et al. / Journal of Hazardous Materi

Fig. 2. Dynamic adsorption of I− in adsorption column. The mixture solution of I−

and Cl− (500 ppm and 5 × 104 ppm for I− and Cl− respectively) was passed throughthe column under different flow rates: round, 1.2 ml/min, square, 0.8 ml/min andtriangle for 0.4 ml/min. It shows the I− removal amount against the adsorption timewhen constant volume of I−/Cl− solution was passed through the adsorptive columnat each given flow rate. I− concentrations at the outlet during the column adsorptionw

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the external surface of T3NL and I− anion captured by the Ag O

ere measured at the above three flow rates and displayed in the inset.

g+ and I− ions, while tubular titanate does not have this drawbackecause of the well dispersed 1D structure. Fabric titanate showshe lowest efficiency of Ag+ utilization, this should be caused byhe phase transition of titanate fiber from the initial layered struc-ure to microporous tunnels, in which the Ag+ are blocked firmlynd permanently in a same way of Cs+ uptake by the T3NF [19].he Ag+ located in tunnels is not able to react with I−. In contrast,

morphology change took place in T3NT and T3NL but the lay-red structure was remained and this contributes to the higher Ag+

tilization efficiency.Looking into other Ag2O grafted titanates, it can be observed

hat titanate nanolamina has preferable Ag+ function efficiencynd higher I− removal ability comparing with T3NF. As for syn-hetic process, T3NL can be obtained from both TiO2 (includingutile minerals) and TiOSO4 as Ti source while T3NT can only beynthesized from TiO2. The characters made T3NL an effectivelynd economically advantageous I− capturer. Practically, T3NL func-ions better than T3NT in fixed-bed column adsorption due to lessmount of resistant pressure generated and they are precipitatedfficiently.

Column adsorption is widely used as a purification techniquen both research and industrial field [27]. To explore the poten-ial industrial application for I− removal, Ag2O–T3NL adsorbentas padded in the glass column and tested under different flow

ates to investigate the adsorption where the adsorbent was inhe form of sediment (optimal I− adsorption was usually obtainedy shaking the adsorption system). It can be seen from Fig. 2 thathe maximum adsorption was eventually reached at different flowates. In other words, Ag2O–T3NL in fixed-bed column has a simi-ar adsorption capacity to that we measured under shaking. Whenhe flow rate was faster, the saturation was reached in a shortereriod of time, this is because larger amount of I− solution willass through the adsorptive layer at a unit time that brings moredsorbate to Ag2O–T3NL which accelerate the speed of adsorptionaturation of Ag2O nanocrystals with I− that forms AgI on the T3NLurface. The I− ions from the outlet of the solution are almost unde-ectable at the beginning. At fast flow rate (1.2 ml/min), I− ionsave higher chance to escape from the fix-bed column causing

ncreasing concentration at the outlet at earlier stage. This escap-ng of I− ions is less significant at lower flow rates (0.8 ml/min and.4 ml/min). Therefore, the concentration at the outlet is dependent

als 246– 247 (2013) 199– 205

on the flow rate. After the pedded Ag2O–T3NL reaches adsorp-tion saturation, I− ion concentration at the outlet would increasebecause the fix-bed column would no longer detain I− ions. Overall,based on the ability of Ag2O–T3NL that can almost instantly cap-ture I− ions with considerable amount, Ag2O–T3NL can be appliedas effective adsorbent for radioactive iodine removal in the indus-trial scale and the radioactive iodine from nuclear accident could becaptured efficiently using Ag2O–T3NL materials pedded adsorptioncolumns.

Regardless of the shaking equilibrium or column adsorptionstudy, the key issue for I− anion adsorption ability of Ag2O anchoredT3NL sorbent (as well as the T3NT and T3NF) is that the Ag2Onanoparticles must firmly attach to the titanate nanostructures.If they detached from the support easily, it would be extremelydifficult and costly to recover these fine nanoparticles from asolution. Furthermore, the small Ag2O nanoparticles may aggre-gate together forming secondary particles with low surface areaand result in poor ability for capturing I− anions. Fortunately,Ag2O nanocrystals have similar surfaces with the surface of thetitanate nanostructures in crystallography, which means that theoxygen arrangement of Ag2O is similar with the oxygen arrange-ment of titanate surface [19]. When the Ag2O nanocrystals andtitanate substrate join together at these surfaces, the oxygen atomsat the interface between the two phases are shared and a fullcoordination of the atoms can be achieved forming well-matchedphase interfaces (coherent interface). Such coherence betweenthe two phases reduces the overall energy by minimizing surfaceenergy and thereby anchoring the Ag2O nanocrystals firmly to thetitanate structure surface. Additionally, the distribution of the Ag2Onanocrystals can be readily controlled by adjusting the concentra-tion of the Ag+ cations in the aqueous silver nitrate solution [28].

The TEM image indicated that (Fig. 3A) titanate nanolamina isoverlaid on top of each other and each of them has a plate-likesurface in the range of 20–100 nm. It can be clearly observed fromthe selected area that the titanate nanolamina has lattice fringewhich confirms the crystalline of T3NL composed of layer struc-ture with an interlayer distance of 0.74 nm. This length is close tothe interlayer distance of other reported titanates [29–31]. It canbe seen that Ag2O particles with a particle size range of 5–30 nmare firmly located on the surface of the titanate (Fig. 3B) by dis-persing T3NL into an aqueous silver nitrate solution in neutral orslightly basic suspension (pH = 7–8). The Ag2O nanocrystals canreadily react with I− ions to form AgI nanoparticles. Due to thecoherent interface between Ag2O and titanate surface, the Ag2Onanocrystals can still anchored steadily on the surface even after achemical reaction with I− anions (Fig. 3C).

Because of their laminar morphology, the sorbents are easilyseparated from liquid suspensions after adsorption. TEM image(Fig. 3C) confirms that the laminar morphology is maintained afterloading of Ag2O nanocrystals and uptake of I− ions. Nonetheless, theion exchange of a significant amount of Ag+ ions during the processof Ag2O loading can cause deformation of the titanate layers. Thestructural change was also verified by the XRD patterns (Fig. 3D).The T3NL shows a similar poor crystallinity as T3NT comparing withthe titanate nanofibre (T3NF). They present a common plane (1 0 0)[19] which indicates these titanate materials are formed by the TiO6octahedron slabs unit. The most obvious change is the disappear-ance of diffraction peak of (1 0 0) planes which appears at 9.84◦ afterprecipitation of Ag2O nanocrystals on the surface of titanates, this isdue to the stronger interaction of Ag+ than Na+ with the negativelycharged TiO6 layer.

Scheme 1 illustrates the formation of Ag2O nanoparticles on

2

nanoparticles. Firstly, in a basic environment, Ag+ ions diffuseonto the surface of T3NL forming silver hydrate Ag(OH)n(H2O)m

intermediates that deposit on the surface. The intermediates will

A. Bo et al. / Journal of Hazardous Materials 246– 247 (2013) 199– 205 203

Fig. 3. TEM image of (A) titanate lamina (T3NL), (B) titanate nanolamina anchored with Ag2O nanocrystals (Ag2O–T3NL). The inset panel is the EDS analysis of Ag2O–T3NL.T ed onn n in (CA

ddssdAedw(ta

ypical TEM image depicting the abundant Ag2O nanocrystals (5–30 nm) dispersanocrystals on titanate lamina converted to AgI nanocrystals (AgI–T3NL) is showg2O anchored T3NL before (Ag2O–T3NL) and after iodine adsorption (AgI–T3NL).

ehydrate with the surface Ti–OH and bond on the TiO6 octahe-ra by sharing the surface oxygen atoms of the TiO6 octahedronlabs in plane (1 0 0). The sharing of surface oxygen atoms causeserious deformation of the surface plane (1 0 0), resulting in theramatic loss of diffraction intensity of this plane. In addition, theg+ ions could exchange with the Na+ ions within the interlay-rs that also results in the shrinkage of the structural factor foriffraction. After the entrapment of iodine ions, Ag2O nanocrystals

ill convert to AgI nanocrystals on the surface of titanate lamina

Ag2O–T3NL). However, the phase interface between AgI nanocrys-als and titanate surface is composed of silver atoms and the oxygentoms of Ti–O frameworks [19]. Thus, the deformation was retained

Scheme 1. Formation of Ag2O nanoparticles on the hydroxyl group rich

the surface of titanate lamina and the used Ag2O–T3NL sorbent in which Ag2O) where its EDS graph is filled. (D) Shows XRD patterns of titanate lamina (T3NL),

after I− anion deposition. The iodine capturing mechanism is basedon the reaction below:

2I−(aq) + Ag2O(s) + H2O → 2AgI(s) + OH−(aq)

The kinetics adsorption depicts the adsorption curve approa-ching a plateau which indicates the saturate sorption capacity ofAg2O–T3NL obtained from the experiment as shown in Fig. 4.

At high concentration, the maximum adsorption capacity(3.4 mmol g−1) can be reached in 1 h. As comparison, at low I− con-centration (125 ppm, the I− amount in the solution is lower thanthe maximum adsorption capacity of Ag2O–T3NL) the I− can be

titanate nanolamina surface and subsequent removal of I− ions.

204 A. Bo et al. / Journal of Hazardous Materials 246– 247 (2013) 199– 205

Fig. 4. I− adsorption kinetics curve of Ag2O–T3NL. I− of a high and a low concen-tration were used in the competitive adsorption test: (A) 500 ppm and the insetgt1

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raph (B) 125 ppm. For competitive adsorption experiment, the concentration ofhe Cl− was set 100 times higher than that of I− in molar ratio (5 × 104 ppm and.25 × 104 ppm in (A) and (B) respectively).

ompletely removed within 15 min. This is because the adsorptionechanism is based on the preferable chemical reaction between

− and Ag2O rather than physical adsorption where adsorption isstablished by sufficient surface contact and distribution betweendsorbate and adsorbent which usually takes considerable amountf time to reach adsorption equilibrium [32]. The rapid adsorptionime was also observed on Ag2O grafted titanate nanotube andanofiber which further proves that surface deposited Ag2O canapture I− in a short period of time regardless of the surface area ofhe substrate. It can also be seen that the adsorption amount of I−

nions is barely affected by the existence of chlorine ions even theoncentration of Cl− ions was 100 times higher than that of I− ions.t is interesting that the time required to reach adsorption equilib-ium was shortened in the presence of Cl− ions. The result of I− andl− selective adsorption can be explained by the Gibbs energies ofhe reactions of Ag2O with NaI and with NaCl (as shown below):

g2O(s) + 2I− + H2O = 2AgI(s) + 2OH−

g2O(s) + 2Cl− + H2O = 2AgCl(s) + 2OH−

The Gibbs free energy is −32 kJ/mol for the reaction of Ag2O with− and +41 kJ/mol for that with Cl−. Evidently, the reaction with I−

ons is thermodynamically much more preferred than that with Cl−

ons [19].The high I− removal ability of Ag2O–T3NL is due to the large

mount of Ag2O uniformly loaded on large specific surface area of3NL substrate (143 m2 g−1). The BET surface area of T3NL was cal-ulated from the nitrogen isotherms at 77 K in Fig. 5 where it showsype IV isotherms and the voids indicate stacked pores which willisappear in aqueous solution. The large surface area and laminartructure allows the fast adsorption of I− from the solution with-ut diffusion problem. The Ag2O nanoparticles on the surface of theaminar adsorbent are the efficient capturers of I− anions as theyre readily accessible to the I− anions even in a fast flux.

Titanate with larger surface area has more space for the attach-ent of Ag2O and it provides more contact opportunities for the

adioactive anions and adsorbent [19]. Therefore, the surface area of

he substrate material is a crucial parameter for adsorbent in termsf its adsorption amount and efficiency. Here, titanate nanolam-na (T3NL) is compared with titanate nanotube (T3NT) and titanateanofibre (T3NF) in terms of surface area and their Ag2O grafted

Fig. 5. Nitrogen adsorption–desorption isotherms of T3NL parent sample (squareand round symbols correspond to the adsorption and desorption branches respec-tively).

samples’ I− removal capacity. As shown in Table 1, the surface areaof the titanates decreased after the Ag2O nanoparticles grew ontheir surfaces.

Although the surface area for titanate nanotube is comparablylarger, the inner area of the tubular titanate cannot fully play therole as Ag2O host since they are too narrow (4.0 nm in diameter) tosufficiently place Ag2O nanoparticles. For titanate nanolamina, asformed in the shape of plates, Ag2O are easier to stack up becausetheir larger contact area brings larger surface energy. Therefore,the contributing surface area is considered to be larger than themeasured amount from the BET. Generally, the I− removal abilityof different Ag2O attached titanate adsorbents are align with theirsurface area.

4. Conclusion

Having unique structural characteristic to firmly stabilize Ag2O,titanates with different morphologies have been demonstratedas cutting-edge radioactive capturers. Among these potentialadsorbents Ag2O–T3NL has shown superior I− removal ability,selectivity, stability and speed since it processes large surface areaand more than 90% of the Ag grafted on the T3NL surface canparticipate in the I− capturing. Titanate nanolamina can be easilyfabricated from various titanium sources at low cost, and have largesurface area and larger void between the particles, which guaran-tees a high capacity for the uptake of Ag2O particles and very fast I−

adsorption kinetics. The platelike morphology of the sorbents alsoallows facile recovery of the adsorbent from solutions for safe dis-posal. The potential to tailor these structural features to enhanceuptake and trapping of ions can be exploited for further develop-ment of new and selective adsorbents to remove other toxic cationsand anions that exist in various wastewater or liquid surroundings.

Acknowledgment

This research is supported by the Australian Research Coun-cil (ARC) and National Natural Science Foundation of China (No.21207073).

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