Materials Science and Engineering C 32 (2012) 2052–2059

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Materials Science and Engineering C

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Glutamic acid containing supermacroporous poly(hydroxyethyl methacrylate)cryogel disks for UO2

2+ removal

Nilay Bereli, Deniz Türkmen, Kazım Köse, Adil Denizli ⁎Department of Chemistry, Hacettepe University, Ankara, Turkey

⁎ Corresponding author.E-mail address: denizli@hacettepe.edu.tr (A. Denizli

0928-4931/$ – see front matter © 2012 Elsevier B.V. Alldoi:10.1016/j.msec.2012.05.015

a b s t r a c t

a r t i c l e i n f o

Article history:Received 21 December 2011Received in revised form 7 May 2012Accepted 22 May 2012Available online 28 May 2012

Keywords:Glutamic acidMetal-chelating cryogelAminoacid-ligandsUranium removal

Supermacroporous cryogel with an average pore size of 10–200 μm in diameter was prepared bycryopolymerization of N-methacryloyl-(L)-glutamic acid (MAGA) and 2-hydroxyethyl methacrylate (HEMA).The poly(HEMA–MAGA) cryogel was characterized by surface area measurements, FTIR, swelling studies, ele-mental analysis and SEM. The poly(HEMA–MAGA) cryogel had a specific surface area of 23.2 m2/g. The equilib-rium swelling ratio of the cryogel is 9.68 g H2O/g for poly(HEMA–MAGA) and 8.56 g H2O/g cryogel for PHEMA.The poly(HEMA–MAGA) cryogel disks with a pore volume of 71.6% containing 878 μmol MAGA/g were used inthe removal of UO2

2+ fromaqueous solutions. Adsorption equilibriumofUO22+was obtained in about 30 min. The

adsorption of UO22+ ions onto the PHEMA cryogel diskswas negligible (0.78 mg/g). TheMAGA incorporation sig-

nificantly increased the UO22+ adsorption capacity (92.5 mg/g). The adsorption process is found to be a function

of pH of the UO22+ solution, with the optimum value being pH 6.0. Adsorption capacity of MAGA contained

PHEMA based cryogel disks increased significantly with pH and then reached the maximum at pH 6.0. Consecu-tive adsorption and elution cycles showed the feasibility of repeated use for poly(HEMA–MAGA) cryogel disks.

© 2012 Elsevier B.V. All rights reserved.

1. Introduction

Uranium, a hazardous and radioactive heavymetal originates fromnuclear-related activities and cause significant economic and publichealth problems by its presence in terrestrial and aquatic ecosystems[1–3]. Uranium concentration in waste streams is too low for a con-ventional treatment and too high to allow its discharge into the envi-ronment [4]. It has been suggested that polymer based adsorbentsand microorganisms could be used to remediate these waste watersand to concentrate heavy metals. Polymer based adsorbents havingchelating groups would be of great importance in uranium removalfrom aqueous systems due to their high selectivity and efficiency,easy handling and cost effectiveness [5–14]. Several issues are impor-tant in the preparation of chelating polymers with higher stability forthe selective removal of metal ions: specific and quick complex for-mation of the metal ions as well as reuse of the adsorbents [15]. In re-cent years, amino acids and/or polyamino acids attachment onto apolymer matrix has been attracted increasing interests [16–18]. Thepurpose of using amino acids by the researchers stems from the factthat these substances are highly reactive with metal ions. The higherflexibility and durability of the amino acid based ligands as well assignificantly lower manufacturing and material costs are also verypromising for environmental applications.

).

rights reserved.

The aim of this study was to prepare a novel glutamic acid con-taining metal-complexing cryogel disks for UO2

2+ removal. Conven-tional adsorption columns based are time consuming. Monolithiccryogels are efficient adsorbents for heavy metal removal consideringmany advantages [19–25]. Monolithic cryogel columns require higherflow-rates with a much lower pressure drop than a packed bed col-umn. Such cryogels also posses several advantages over conventionaladsorbents, e.g., macropores, short diffusion path and very short res-idence time for both adsorption and elution stage. In this technique,the toxic substance to be removed can be directly transported by con-vection to MAGA on the pore surface of the cryogel, higher through-put and faster processing times onto the cryogel column can beachieved. The macroporous adsorbents possesses higher adsorptioncapacity andmass transfer rate than microporous and non-porous ad-sorbents. Cryogels are also cheap materials.

In the first part of this study, the metal-complexing ligand,N-methacryloyl-(L)-glutamic acid (MAGA), was prepared usingmetha-cryloyl chloride and glutamic acid hydrochloride. Then, the poly(2-hydroxyethyl methacrylate-N-methacryloyl-(L)-glutamic acid) [poly(HEMA–MAGA)] cryogel disks were prepared by cryopolymerizationof MAGA and HEMA. The poly(HEMA–MAGA) cryogel disks werecharacterized by swelling tests, FTIR, elemental analysis and SEM.Then, UO2

2+ adsorption on the poly(HEMA–MAGA) cryogel disksfrom aqueous solutions containing different amounts of UO2

2+, at differ-ent pHs, was also performed. Elution of UO2

2+ and reusability of thepoly(HEMA–MAGA) cryogel disks was also evaluated. In our knowl-edge, supermacroporous cryogels have not been employed for theremoval of UO2

2+ from aqueous solutions.

2053N. Bereli et al. / Materials Science and Engineering C 32 (2012) 2052–2059

2. Experimental

2.1. Materials

Glutamic acid hydrochloride and methacryloyl chloride were sup-plied by Sigma (St Louis, USA). Hydroxyethyl methacrylate (HEMA)was obtained from Fluka A.G. (Buchs, Switzerland), distilled under re-duced pressure in the presence of hydroquinone inhibitor and storedat 4 °C until use. N,N′-Methylene-bis(acrylamide) (MBAAm) and am-monium persulfate (APS) were purchased from Sigma (St Louis, USA).All the other reagents were of analytical reagent grade and procuredfrom Merck AG (Darmstadt, Germany). Water was purified using aBarnstead (Dubuque, IA) ROpure LP® reverse osmosis unit with ahigh flow cellulose acetate membrane (Barnstead D2731) followedby a Barnstead D3804 NANOpure® organic/colloid removal and ionexchange system. Buffer and all sample solutions were prefilteredthrough a 0.2 μm membrane (Sartorius, Göttingen, Germany). Allglassware was washed with 1.0 M HNO3 and rinsed thoroughly withdeionized water.

2.2. Synthesis of N-methacryloyl-(L)-glutamic acid

The MAGA was selected as the metal chelating ligand. Details ofthe preparation and characterization of the N-methacryloyl-(L)-glutamic acid (MAGA) were reported elsewhere [26]. The followingexperimental procedure was performed for the synthesis of MAGA:5.0 g of L-glutamic acid and 0.2 g of hydroquinone were dissolved in100 mL of dichloromethane solution. This solution was cooled downto 0 °C. Then, triethylamine (13.0 g) was added to the solution andmethacryloyl chloride (4.0 mL) was poured slowly into this solutionunder N2 atmosphere. This solution was stirred magnetically(250 rpm) at room temperature for 2 h. The unreacted methacryloylchloride was extracted with 10% NaOH. The aqueous phase was evap-orated in a rotary evaporator and residue (i.e., MAGA) was dissolvedin ethanol.

2.3. Preparation of poly(HEMA–MAGA) cryogel disks

Water and MBAAm were added in the polymerization recipe asthe pore-former and cross-linker, respectively. Preparation procedureis as follows: HEMA (1.3 mL) and MAGA (200 mg) were dissolved indeionized water (5.0 mL). MBAAm (0.283 g) was dissolved in deion-ized water (10 mL). Second solution was mixed with previous one.The cryogel was then prepared by free radical polymerization initiat-ed by TEMED (25 μL) and APS (20 mg). After adding APS (1% (w/v) ofthe total monomers) the solution was cooled in an ice bath for2–3 min. TEMED (1% (w/v) of the total monomers) was added andthe reaction mixture was stirred for 1 min. Then, the reaction mixturewas poured between two glass sheets. The polymerization solutionwas frozen between two glass sheets at −16 °C for 24 h and thenthawed at room temperature. After washing with 200 ml of water,the cryogel was cut into circular disks (1.0 cm in diameter) and storedin a buffer containing 0.02% sodium azide at 4 °C until use.

2.4. Characterization of poly(HEMA–MAGA) cryogel disks

FTIR spectra of MAGA and poly(HEMA–MAGA) cryogel disks wereobtained by using a FTIR spectrophotometer (FTIR 8000 Series,Shimadzu, Japan). A piece of dry cryogel disk (about 0.1 g) was thor-oughly mixed with potassium bromide (KBr, 0.1 g, IR Grade, Merck,Germany), and pressed into a pellet form and FTIR spectrum wasthen recorded.

1H NMR spectrum of MAGA monomer was taken in CDCl3 on aJEOL GX-400 300 MHz instrument. The residual non-deuteratedsolvent (CHCl3) served as an internal reference. Chemical shifts arereported in ppm (δ) downfield relative to CHCl3.

The swelling degree of the cryogel disk (S) was determined as fol-lows: cryogel disk was washed on porous filter until washing wasclear. Then it was sucked dry and then transferred to pre-weighedvial and weighed (mwet gel). After drying to constant mass in theoven at 60 °C, the mass of dried cryogel disk was determined(mdry gel). The swelling degree was calculated as:

S ¼ mwet gel−mdry gel

� �=mdry gel: ð1Þ

The surface morphology and bulk structure of the cryogel were in-vestigated by scanning electron microscope (SEM, JEOL, JSM 5600,Tokyo, Japan). The sample was fixed in glutaraldehyde (2.5%) in0.15 M sodium cacodylate buffer overnight, post-fixed in osmiumtetroxide (1%) for 1 h. Then, the sample was dehydrated stepwise inethanol and transferred to a critical point drier temperated to 10 °Cwhere the ethanol was changed for liquid carbon dioxide as transi-tional fluid. The temperature was then increased to 40 °C and thepressure to ca. 100 bar. Liquid CO2 was transformed directly to gasuniformly throughout the whole sample without heat of vaporizationor surface tension forces causing damage. Release of the pressure at aconstant temperature of 40 °C resulted in dried cryogel sample. Final-ly, it was coated with gold–palladium (40:60) and examined by SEM.

The pore volume and the average pore diameter greater than 20 Åwere determined by mercury porosimeter up to 2000 kg/cm2 using aCarlo Erba model 200 (Milano, Italy). The surface area of the cryogelsample was measured with a surface area apparatus (BET method).

To evaluate the amount of MAGA into the cryogel structure, it wassubjected to elemental analysis using a Leco Elemental Analyzer(Model CHNS-932, USA).

2.5. UO22+ adsorption/elution studies

Adsorption of UO22+ from aqueous solutions was investigated in

batch experiments. Effects of UO22+ concentration and pH of the

medium on the adsorption rate and capacity were studied. 100 mLaliquots of aqueous solutions containing different amounts of UO2

2+

(in the range of 10–1000 mg/L) were treated with the cryogel disksat different pH (in the range of 3.0–8.0) (adjusted with NaOH–HCl).The cryogel disks were stirred with a uranium nitrate salt solutionat room temperature for 2 h. The concentration of the UO2

2+ in theaqueous phase was determined by the Arsenazo III method. Theadsorption capacity of the cryogel disks was calculated using themass balance on uranyl ions.

The selective adsorption experiments of Th4+, Fe3+ and Mn2+withrespect to UO2

2+ were carried out using PHEMA and poly(HEMA–MAGA) cryogels at batch system. The polymeric disks (100 mg) wereadded to 25 mL of aqueous solution of containing 100 mL Th4+/UO2

2+,Fe3+/UO2

2+ and Mn2+/UO22+ and placed in a beaker. A solution

(25 mL) containing 100 mg/L from each metal ion was incubated withthe cryogel disks at a pH of 6.0 at room temperature, in the flasksstirred magnetically at 250 rpm. The concentration of Fe3+ and Mn2+

ions in the remaining solution was measured by flame atomic absorp-tion spectroscopy (FAAS). Th4+ ions were determined spectrophoto-metric method. In the spectrophotometric determination of Th4+ ionswith Lawsone (2-hydroxy-1,4-naphtoquinone, LAS) was studiedspectrophotometrically in 40% (v/v) ethyl alcohol/water at 25 °C andionic strength of 0.1 M (NaClO4). After the reaction of LAS with Th4+

ions, the absorbance of the complex solution was measured at440 nm, against a blank reagent. Distribution coefficients of Th4+,Fe3+ andMn2+with respect to UO2

2+were calculated by using concen-tration difference in initial and final solutions.

Elution of UO22+ was studied in 0.1 M NaHCO3 solution. The poly

(HEMA–MAGA) cryogel disks adsorbed with UO22+ were placed in this

elution medium and stirred (at a stirring rate of 100 rpm) for 1 h atroom temperature. Elution volume was 50 mL. The final concentration

Fig. 1. Chemical structures of the (A) PHEMA and (B) poly(HEMA–MAGA) cryogels.

2054 N. Bereli et al. / Materials Science and Engineering C 32 (2012) 2052–2059

of UO22+ in the aqueous phase was determined by the Arsenazo III

method. The elution ratio was calculated from the amount of UO22+

adsorbed on the cryogel disks and the final concentration of UO22+ in

the elution medium, by using the mass balance.In order to show the reusability, adsorption–elution cycles were

repeated 10 times by using the same group of the poly(HEMA–MAGA) cryogel disks.

3. Results and discussion

N-Methacryoyl-L-glutamic acid (MAGA) was selected as the metalcomplexing ligand. In the first step, MAGA was synthesized fromL-glutamic acid hydrochloride and methacryloyl chloride. Then, thepoly(2-hydroxyethyl methacrylate-N-methacryloyl-(L)-glutamic acid)[poly(HEMA–MAGA)] cryogel disks were obtained by cryopoly-merization of MAGA and HEMA. The chemical structures of thePHEMA and the poly(HEMA–MAGA) cryogels were illustrated in Fig. 1in order to distinguish their differences. 1H NMRwas used to determinethe synthesis of MAGA structure. Fig. 2 shows the 1H NMR spectrum ofMAGA. 1H NMR spectrum is shown to indicate the characteristic peaksfrom the groups in MAGA monomer. These characteristic peaks areas follows: 1H NMR (DMSO): δ 1.19–1.23 (m; 2H, CH2), 1.50–1.55(m; 2H, CH2), 1.90 (s; 3H, CH3) 4.49–4.59 (m; 1H, CNH), 5.33 (s; 1H,H2C_C), 5.73 (s; 1H, H2C_C); 6.87 (δ; 1H, NH), 9.58 (δ; 6s, 2H, OH).

FTIR spectrum of MAGA has the characteristic stretching vibra-tion carboxyl–carbonyl and amide–carbonyl absorption bands at1739 cm−1 and 1640 cm−1 respectively as shown in Fig. 3. TheN–H bending peak appears at 1522 cm−1 is associated with theamide vibration of MAGA.

FTIR spectrum was recorded to determine the structure of thepoly(HEMA–MAGA) cryogel disks (Fig. 4). The FTIR spectra of poly(HEMA–MAGA) with characteristic peaks appear at 3310 cm−1

(characteristic hydroxyl, OH stretching vibration), 2928 cm−1 (CH3

stretching vibration), 2889 cm−1 (CH2 stretching vibration) and1699 cm−1 (carboxyl–carbonyl stretching vibration). The carbonylpeak appears at 1652 cm−1 is associated with the amide and carbonyl

Fig. 2. 1H NMR spectrum of MAGA monomer.

vibration of MAGA. These data proved that the poly(HEMA–MAGA)cryogel disks were formed with the functional groups MAGA.

The SEM images of the surfacemorphology and bulk structure of thepoly(HEMA–MAGA) cryogel disks are shown in Fig. 5. The poly(HEMA–MAGA) cryogel produced in such a way has porous and thin polymerwalls, large continuous interconnected pores (10–200 μm in diameter)that provide channels for the mobile phase to flow through the mono-lithic column. Pore size of the matrix is much larger than the size ofthe UO2

2+ ions, allowing them to transport easily through the pores.As a result of the convective transport, the mass transfer resistance ispractically negligible. Polymeric cryogel is opaque, white, sponge likeand elastic (Fig. 6). This cryogel can be easily compressed by hand to re-move water accumulated inside the bulk structure. When the com-pressed piece of cryogel was submerged in water, it soaked in waterand within 1–2 s restored its original size and shape due to its shapememory. Physicochemical properties of the PHEMA cryogel are shownin Table 1.

3.1. UO22+ ion adsorption on poly(HEMA–MAGA) cryogel disks

3.1.1. Effect of UO22+ concentration

Fig. 7 shows the experimental and calculated adsorption isothermsby non-linear regressionmethod for the adsorption of UO2

2+ ions ontothe poly(HEMA–MAGA) cryogel disks. Adsorption of UO2

2+ ions ontothe PHEMA cryogel disks as also performed. It should be noted thatthe elution of UO2

2+ onto the PHEMA cryogel disks was very low,about 0.78 mg/g. Because, PHEMA cryogel disks do not contain anyreactive functional groups for complexation of UO2

2+ ions. This lowadsorption value of UO2

2+ ions may be due to diffusion of UO22+ ions

into the macropores of the cryogel disks and weak interactions be-tween UO2

2+ ions and hydroxyl groups on the surface of the PHEMAcryogel disks. However, MAGA including into the polymer structuresignificantly increased the adsorption capacity to 92.5 mg/g. Theadsorption values increased with increasing initial concentration ofUO2

2+, and a saturation value is achieved at ion concentration of800 mg/L, which represents saturation of the active binding sites onthe poly(HEMA–MAGA) cryogel disks.

Note that the MAGA content of the poly(HEMA–MAGA) cryogeldisks used in this group of experiments was 878 μmol/g. The maxi-mum UO2

2+ adsorption capacity obtained in the studied range is342 μmol per gram of the cryogel disks. This seems to give a stoichi-ometry of around four MAGA groups per one uranium ion. MAGA asthought to be incorporated to the backbone through copolymeriza-tion and the pendant carboxyl groups in the MAGA are postulatedto be responsible for uranium binding. Strong complex formation be-tween carboxylic acid functional groups and uranyl ions has beenthoroughly discussed in a recent review article [27], and the impor-tance of carboxyl groups in metal binding by adsorbents has beenaddressed [28].

Fig. 3. FTIR spectrum of MAGA monomer.

2055N. Bereli et al. / Materials Science and Engineering C 32 (2012) 2052–2059

Adsorption isotherms are used to describe adsorption nature ofadsorbents. Amount of adsorbed UO2

2+ (qe) against concentration offinal UO2

2+ (Ce) is plotted. Langmuir and Freundlich–Peterson iso-therm models are used to figure out the equilibrium data for UO2

2+

adsorption. According to Langmuir adsorption model, molecules areadsorbed with equivalent well-defined binding sites far from eachother and in between there is no interaction — adsorbing only onemolecule. Therefore, energies and enthalpies are equal. Langmuirmodel expression can be written by Eq. (2).

qe ¼ qmKaCe= 1þ KaCeð Þ: ð2Þ

This equation can be converted to linear form as:

Ce=qeð Þ ¼ Ce=qmð Þ þ 1=qmKað Þ ð3Þ

where, Ka is the adsorption equilibrium constant, Ce and qe areunadsorbed UO2

2+ concentrations in solution and adsorbed UO22+

ions on the adsorbent of equilibrium, respectively. qm is the maxi-mum amount of UO2

2+ ions per unit weight.

Fig. 4. FTIR spectrum of poly(HE

Freundlich isotherm is another form of Langmuir isotherm foradsorption on heterogeneous surface with a non-uniform distributionof heat of adsorption over surface. Summation of UO2

2+ concentrationadsorbed by all binding sites is equivalent UO2

2+ concentrationadsorbed totally. Freundlich adsorption model denotes reversibleadsorption and is not limited with monolayer adsorption whereasLangmuir model is.

Empirical Freundlich equation is given by Eq. (4):

qe ¼ KfCe1=n

: ð4Þ

Linear form of this equation is:

ln qe ¼ ln Kf þ 1=mð Þ ln Ce ð5Þ

where, qe is the equilibrium UO22+ adsorbed amount; Ce is the re-

sidual UO22+ concentration at equilibrium; and Kf and n are the

Freundlich constant related to the adsorption capacity and adsorptionintensity of the adsorbent respectively.

MA–MAGA) cryogel disks.

Fig. 5. SEM images of the (A) PHEMA and (B) poly(HEMA–MAGA) cryogel disks.

2056 N. Bereli et al. / Materials Science and Engineering C 32 (2012) 2052–2059

Redlich–Peterson isotherm is arranged to exhibit applicability ofLangmuir and Freundlich adsorption models and so combination ofLangmuir and Freundlich equations. Redlich–Peterson model isfeasible for homogeneous and heterogeneous adsorption systems.The other models mentioned above described with two parametersbut Redlich–Peterson did three.

Redlich–Peterson isotherm is given by Eq. (6):

qe ¼ KRCeð Þ= 1þ aRCeβR

� �ð6Þ

where, qe and Ce are the UO22+ concentration adsorbed at equilibrium

and UO22+ concentration unadsorbed in solution, respectively. KR, aR

and βR are the Redlich–Peterson constant calculated by non-linear re-gression. At low concentrations the Redlich–Peterson isotherm ap-proximates to Henry's law as βR goes to 1.

The calculated isotherm parameters were shown in Table 2. TheLangmuir adsorption capacity, qm, obtained in this study was foundto be 131.6 mg/g. The adsorption capacity value (KF) for UO2

2+ ob-tained from Freundlich model was 1.77, this value indicated that thepoly(HEMA–MAGA) had a good affinity for UO2

2+ ions. The Redlich–Peterson isotherm shows the value of βR to be 1.0 indicating thatthe isotherm tends towards the Langmuir form. The order of fitness

Fig. 6. Optical photograph of poly(

of these isotherm models to the experimental data was found to beLangmuir>Redlich–Peterson>Freundlich.

Different types of natural and synthetic adsorbents/biosorbentshave been used for the uranium removal from aqueous solutionsand seawater. A comparison of the adsorption capacity of MIP parti-cles with those of some other affinity adsorbents reported in litera-ture is given in Table 3. The adsorption capacity of poly(HEMA–MAGA) cryogel was considered to be good when compared withother polymer based adsorbents. Differences of uranium adsorptioncapacity are due to the properties of each adsorbent such as structure,functional groups, ligand loading and accessible surface area.

3.1.2. Effect of pHpH is the most critical parameter for metal adsorption as it influ-

ences both the polymer surface chemistry as well as the solutionchemistry of soluble metal ions [28, 31, 42–44]. In order to establishthe effect of pH on the adsorption of UO2

2+ onto the both PHEMAand poly(HEMA–MAGA) cryogel disks, we repeated the batch adsorp-tion equilibrium studies at different pHs in the range of 3.0–8.0. Inthis group of experiments, the initial concentration of UO2

2+ and theadsorption equilibrium time were 500 mg/L and 2 h, respectively.The pH dependence of adsorption values of UO2

2+ is shown in Fig. 8.In the case of PHEMA cryogel disks, adsorption is pH independent.

HEMA–MAGA) cryogel disks.

Table 1Physicochemical properties of the PHEMA and poly(HEMA–MAGA) cryogel.

PHEMA Poly(HEMA–MAGA)

Specific surface area 20.2 m2/g 23.2 (m2/g)Pore size diameter 10–200 μm 10–200 μmPorosity 71.6% 84.3%MAGA loading – 878 μmol/gSwelling degree 8.56 g H2O/g cryogel 9.68 g H2O/g cryogelFlow resistance 845 cm/h 624 cm/h

Table 2Adsorption isotherm constants for UO2

2+ adsorption onto poly(HEMA–MAGA).

Langmuir Freundlich

qm 131.6 KF 1.77KL 0.0025 n 0.59R2 0.9855 R2 0.9459

Redlich–Peterson

KR 0.35aR 0.002βR 1.0R2 0.9515

Table 3Comparison of the adsorption capacities for uranium of various adsorbents.

Material Adsorptioncapacity

[R]

Furano uronate based resin 26.3 mg/g [3]Crosslinked chitosan 72.5 mg/g [7]PHEMA grafted tamarind fruit shell 100.1 mg/g [8]PHEMA grafted ligno-cellulosics 109.6 mg/g [9]Chitosan-tripolyphosphate beads 240.0 mg/g [10]Poly(acrylonitrile)-amidoxime 44.0 mg/g [11]Poly(acrylamidoxime) hydrogel 39.5 mg/g [12]Poly(glycidyl methacrylate)-amidoxime beads 3.95 μmol [13]Tetramethylmalonamide chelating resins 5–135 mg/g [14]Poly(HEMA–MAGA) beads 204.8 mg/g [26]Hematite 3.5 mg/g [29]Marine algal biomass 300 mg/g [30]Nanoporous silica-PEI 12.2 mg/g [32]Microbial biomass 180.0 mg/g [33]Microorganisms 100–310 mg/g [34–37]Coir pith 255.3 mg/g [38]Poly(AAm–AAc) 236.6 mg/g [39]Poly(MAGA–EGDMA) imprinted beads 181.0 mg/g [40]Manganese oxide coated zeolite 15.1 mg/g [41]Poly(HEMA–MAGA) cryogel 92.5 mg/g In this study

2057N. Bereli et al. / Materials Science and Engineering C 32 (2012) 2052–2059

But, it is indicated that the adsorption of UO22+ onto the poly(HEMA–

MAGA) cryogel disks was pH dependent. The results show that urani-um adsorption by the poly(HEMA–MAGA) cryogel disks was low atpH 3.0, but increased rapidly with increasing pH and then reachedthe maximum at pH 6.0. UO2

2+ adsorption around pH 3.0–4.0 waslow. This can be explained by the fact that, at this pH, most of pendantcarboxyl groups are protonated. It is well known in adsorptionmechanisms, that a decrease in solubility favors an improvement inadsorption performance. The increase of pH was followed by adecrease in the uptake of uranyl ions. Because there is a decrease indissolved uranyl ion concentration at higher pH due to the formationof solid schoepite (4UO2·9H2O) reducing uranium adsorption. Inmildly acidic and neutral pHs 4.0–6.0, poly(HEMA–MAGA) cryogeldisks are effective for removal of UO2

2+ from aqueous solutions.

3.1.3. Effect of contact timeFig. 9 shows the time dependence of the adsorption values of

UO22+ on poly(HEMA–MAGA) cryogel disks. There is a direct relation-

ship between the initial UO22+ concentration and the adsorption rate.

When the initial UO22+ concentration is increased from 10 to

1000 mg/L, the UO22+ adsorption amount of the poly(HEMA–MAGA)

cryogel disks increased from 5.1 to 92.5 mg/g at pH 6.0. The adsorp-tion rate was also relatively fast, the time required to reach equilibri-um conditions was about 60 min. The initial concentration providesan important driving force to overcome all mass transfer resistanceof UO2

2+ ions between the aqueous and solid cryogel phases. Hence,a higher initial concentration of UO2

2+ ions will increase the adsorp-tion rate. High complexation affinity between UO2

2+ and MAGAgroups in the cryogel disks structure may also contribute to this fastadsorption equilibrium. It is worth to point that UO2

2+ has a highaffinity for MAGA.

Experimental data on the adsorption kinetics of uranium byvarious adsorbents have shown a wide range of adsorption rates.For example, Wang et al. studied the adsorption of uranium onepichlorohydrin cross-linked chitosan and reported 3 h equilibrium

Fig. 7. Comparison of the experimental and different isotherm models for the adsorp-tion of UO2

2+ onto the poly(HEMA–MAGA) cryogel disks; pH: 6.0; T: 25 °C.

adsorption time [7]. Anirudhan and Radhakrishnan documented thatthe amine modified poly(hydroxyethyl methacrylate) grafted bio-material (tamarind fruit shell) reached steady state at about 2 h[8]. Anirudhan et al. investigated recovery of uranium with carboxyl-ate functionalized poly(hydroxyethyl methacrylate) grafted ligno-cellulosics and they reported that equilibrium was achieved in about3 h [9]. Sureshkumar et al. studied adsorption of uranium usingchitosan–tripolyphosphate beads and they reported that adsorption

Fig. 8. Effect of pH on adsorption of UO22+ on the PHEMA and poly(HEMA–MAGA)

cryogel disks: Initial concentration of UO22+: 500 mg/L; T: 25 °C.

Fig. 9. Time dependent adsorption of UO22+ on the poly(HEMA–MAGA) cryogel disks;

A: 1000 mg/L, B: 500 mg/L, C: 250 mg/L, D: 100 mg/L, pH: 6.0 and T: 25 °C.

Table 5Kd values of Th4+, Fe3+ and Mn2+ with respect to UO2

2+.

Cryogels UO22+ Th4+ Kd Kd

(mg/L) (mg/L) (UO22+) (Th4+)

PHEMA 100 100 14.7 3.68Poly(HEMA–MAGA) 100 100 98.9 3.26

Cryogels UO22+ Fe3+ Kd Kd

(mg/L) (mg/L) (UO22+) (Fe3+)

PHEMA 100 100 11.9 4.26Poly(HEMA–MAGA) 100 100 95.8 3.88

Cryogels UO22+ Mn2+ Kd Kd

(mg/L) (mg/L) (UO22+) (Mn2+)

PHEMA 100 100 21.8 6.92Poly(HEMA–MAGA) 100 100 102.6 5.75

2058 N. Bereli et al. / Materials Science and Engineering C 32 (2012) 2052–2059

was rather slow and took 4 days for the beads to reached equilibrium[10]. Zhang et al. studied uranium adsorption on macroporous fibrouspoly(acrylo nitrile) and they reported 6 h equilibrium adsorption time[11]. Shuibo et al. used hematite for the removal of UO2

2+ from aque-ous solution and reported that equilibrium of uranium adsorptionwas not reached until after 6 h [29]. Khani reported that the uraniumadsorption rate by algae biomass is high at the beginning but plateauvalues are reached for a wide uranium concentration range about100 min long [30]. Olguin et al. achieved 30 min as a short equilibriumtime in their uranium adsorption kinetic studies, in which they usednatural Mexican erionite and Y zeolite as sorbent [45]. Several criteriaare important in the design of chelating polymers with substantialstability for the selective adsorption of metal ions: specific and fastbinding of the metal ions as well as the recyclability of the chelatingpolymeric adsorbents. All these experimental studies reported herehave been carried out at different conditions. Therefore, it can be con-cluded that it is too difficult to compare the adsorption rates reported.However, the adsorption rates obtained with the affinity cryogel disksproduced by us seem to be very satisfactory.

The kinetic mechanism controlling and defining the efficiency ofadsorption process was investigated using pseudo-first- and second-order equations to test experimental data.

Pseudo� first order log qeq−qt

� �¼ log qeq

� �− k1tð Þ=2:303 ð7Þ

Pseudo� second order t=qtð Þ ¼ 1=k2q2eq

� �þ 1=qeq

� �t ð8Þ

where, qe and qt are the amounts of UO22+ ions adsorbed (mg/g) at equi-

librium time “t”. k1 and k2 are the pseudo-first order and pseudo-secondorder rate constants of adsorption respectively. Kinetic model parame-ters were determined at different initial concentrations of UO2

2+ ions.The results of kinetic analysis are summarized in Table 4. As a result ofcalculations, pseudo-second order kinetic model is well-fitted with ex-perimental data meaning that adsorption is chemically controlled. The

Table 4The first and second order kinetic constants for poly(HEMA–MAGA) cryogel.

Initialconc.

Experimental First-order kinetic Second-order kinetic

qe k1 qe R2 k2 qe R2

(mg/ml) (mg/g) (1/min) (mg/g) (1/min) (mg/g)

100 24.2 0.0216 4.99 0.24 0.00151 29.1 0.9905250 52.58 0.0308 32.59 0.68 0.00030 73.5 0.9667500 78.6 0.0363 75.80 0.75 0.00017 113.5 0.95081000 92.5 0.0377 87.09 0.74 0.00017 128.2 0.9394

adsorption of UO22+ ions on supermacroporous poly(HEMA–MAGA)

cryogel disks can be described by pseudo-second order kinetic model.

3.1.4. Competitive adsorptionCompetitive adsorption of Th4+/UO2

2+, Fe3+/UO22+ andMn2+/UO2

2+

from their binary solutions was also investigated in a batch system.Th4+/UO2

2+ was chosen as a competitive ions because uranium andthorium often coexists in the minerals, products and in even in waste-streams. Fe3+/UO2

2+ and Mn2+/UO22+ couples were chosen because

these elements are in the same hard acid group and interfere eachother. Table 5 shows Kd values for the poly(HEMA–MAGA) cryogeldisks. A comparison of the Kd values for the poly(HEMA–MAGA)cryogel disks with the control PHEMA cryogel disks shows and increasein Kd for UO2

2+ while Kd decreases for Th4+, Fe3+ and Mn2+ ions.

3.1.5. Behavior of the elutionElution of the UO2

2+ from the cryogel diskswas performed in a batchexperimental setup for improving cryogel's cost effectiveness byrecycling the adsorbent for reuse in multiple cycles. Various factorsare probably involved in determining rates of UO2

2+ elution, such asthe extent of hydration of the metal ions and polymer microstructure.However, an important factor appears to be binding strength. In thisstudy, the elution time was found to be 10min. Elution ratios are veryhigh (up to 98%). In order to obtain the reusability of the poly(HEMA–MAGA) cryogel disks, adsorption–elution cycles were repeated 10times by using the same cryogel disks. The adsorption capacity of therecycled poly(HEMA–MAGA) cryogel disks can still be maintained at97% level at the 10th cycle (Fig. 10). Thus, the undamaged cryogeldisks could be reused after each repetition.

Fig. 10. Adsorption–elution cycles for UO22+; Adsorption conditions; UO2

2+ initialconcentration: 500 mg/L; pH: 6.0; T: 25 °C.

2059N. Bereli et al. / Materials Science and Engineering C 32 (2012) 2052–2059

4. Conclusion

The poly(HEMA–MAGA) cryogel disks was prepared for the selec-tive removal of UO2

2+ from aqueous solutions. This novel approach forthe preparation of adsorbent has many advantages over conventionaltechniques, those need the activation of the matrix for metal-complexing ligand attachment. In this procedure, MAGA acts as themetal-complexing ligand, and there is no need to activate the matrixfor the metal-complexing ligand attachments. MAGA is polymerizedwith HEMA and no leakage of the metal-complexing ligand is ob-served. We have shown that N-methacryloyl-(L)-glutamic acid con-taining cryogel disks have high adsorption capacity for UO2

2+

(92.5 mg/g). Repeated adsorption and elution cycles showed the fea-sibility of the novel poly(HEMA–MAGA) cryogel disks for UO2

2+

adsorption.

References

[1] B. Benedict, T.H. Pigford, H.W. Lewi, Nuclear Chemical Engineering, McGraw-Hill,New York, 1981.

[2] M. Fomina, J.M. Charnock, S. Hillier, R. Alvarez, G.M. Gadd, Environ. Microbiol. 9(2007) 1696.

[3] E. Guibal, C. Roulp, P.L. Cloirec, Water Res. 8 (1992) 1139.[4] A.A. Atia, Hydrometallurgy 80 (2005) 13.[5] B. Garipcan, N. Bereli, S. Patır, Y. Arıca, A. Denizli, Macromol. Biosci. 1 (2001) 332.[6] N. Kabay, Sep. Sci. Technol. 29 (1994) 375.[7] G. Wang, J. Liu, X. Wang, Z. Xie, N. Deng, J. Hazard. Mater. 168 (2009) 1053.[8] T.S. Anirudhan, P.G. Radhakrishnan, J. Environ. Radioact. 100 (2009) 250.[9] T.S. Anirudhan, L. Divya, P.S. Suchitra, J. Environ. Manage. 90 (2009) 549.

[10] M.K. Sureshkumar, D. Das, M.B. Mallia, P.C. Gupta, J. Hazard. Mater. 184 (2010)65.

[11] A. Zhang, G. Uchiyama, T. Asakura, React. Funct. Polym. 63 (2005) 143.[12] O. Hazer, S. Kartal, Talanta 82 (200) (1974).[13] T. Çaykara, S.S. Alaslan, R. Inam, J. Appl. Polym. Sci. 104 (2007) 4168.[14] I.M. Ismail, M. Nogami, K. Suzuki, Sep. Purif. Technol. 31 (2003) 231.[15] A. Denizli, R. Say, B. Garipcan, S. Emir, A. Karabakan, S. Patır, Sep. Purif. Technol. 30

(2003) 3.[16] A. Denizli, N. Şanlı, B. Garipcan, S. Patır, G. Alsancak, Ind. Eng. Chem. Res. 43

(2004) 6095.

[17] E. Tamahkar, N. Bereli, R. Say, A. Denizli, J. Sep. Sci. 34 (2011) 3433.[18] L. Uzun, D. Türkmen, E. Yılmaz, S. Bektaş, A. Denizli, Colloids Surf. A 330 (2008)

161.[19] Ç. Arpa, N. Bereli, E. Özdil, S. Bektaş, A. Denizli, J. Appl. Polym. Sci. 118 (2010)

2208.[20] P. Arvidsson, F.M. Plieva, V.I. Lozinsky, I.Y. Galaev, B. Mattiasson, J. Chromatogr. A

986 (2003) 275.[21] I. Koç, G. Baydemir, E. Bayram, H. Yavuz, A. Denizli, J. Hazard. Mater. 192 (2011)

1819.[22] N. Bereli, M. Andaç, G. Baydemir, R. Say, I.Y. Galaev, A. Denizli, J. Chromatogr. A

1190 (2008) 18.[23] V.I. Lozinsky, I.Y. Galaev, F.M. Plieva, I.N. Savina, H. Jungvid, B. Mattiasson, Trends

Biotechnol. 21 (2003) 445.[24] A. Derazshamshir, G. Baydemir, M. Andaç, R. Say, I.Y. Galaev, A. Denizli, Macromol.

Chem. Phys. 211 (2010) 657.[25] N. Bereli, Y. Saylan, L. Uzun, R. Say, A. Denizli, Sep. Purif. Technol. 82 (2011) 28.[26] A. Denizli, R. Say, B. Garipcan, S. Patır, React. Funct. Polym. 58 (2004) 123–130.[27] J. Leciewicz, N.W. Alcock, T.J. Kemp, Struct. Bond. 82 (1995) 43.[28] G. Geesey, L. Jang, in: H.L. Ehrlich, C.L. Brierley (Eds.), Microbial Mineral Recovery,

McGraw-Hill, New York, 1990, p. 223.[29] X. Shuibo, Z. Chun, Z. Xinghuo, Y. Jing, Z. Xiaojian, W. Jingsong, J. Environ.

Radioact. 100 (2009) 162.[30] M.H. Khani, Environ. Sci. Pollut. Res. 18 (2011) 790.[31] S. Yusan, N. Yenil, S. Kuzu, M.A.A. Aslani, J. Chem. Eng. Data 56 (2011) 2013.[32] Y. Jung, S. Kim, S.J. Park, J.M. Kim, Colloids Surf. A 33 (2008) 162.[33] M. Tsezos, B. Volesky, Biotechnol. Bioeng. 23 (1981) 583.[34] Ö. Genç, Y. Yalçınkaya, E. Büyüktuncel, A. Denizli, M.Y. Arica, S. Bektaş, Int. J.

Miner. Process. 68 (2003) 93.[35] P. Sar, S.F. D'Souza, J. Chem. Technol. Biotechnol. 76 (2001) 1286.[36] M. Tsezos, Z. Georgousis, E. Remoudaki, Biotechnol. Bioeng. 55 (1997) 16.[37] M.Z.C. Hu, J.M. Norman, B.D. Faison, Biotechnol. Bioeng. 51 (1996) 237.[38] H. Parab, S. Joshi, N. Shenoy, R. Verma, A. Lali, M. Sudersanan, Bioresour. Technol.

96 (2005) 241.[39] L. Raj, G.S. Chauhan, Polym. Bull. 64 (2009) 363.[40] R. Say, A. Ersöz, A. Denizli, Sep. Sci. Technol. 38 (2003) 3447.[41] R. Han, W. Zou, Y. Wang, L. Zhu, J. Environ. Radioact. 93 (2007) 127.[42] A. Denizli, R. Say, S. Patır, Y. Arıca, React. Funct. Polym. 46 (2000) 157.[43] A. Denizli, R. Say, S. Patır, Y. Arıca, Sep. Sci. Technol. 36 (2001) 2213.[44] W. Li, H. Zhao, P.R. Teasdale, R. John, Polymer 43 (2002) 4803.[45] M.T. Olguin, M. Solache, M. Asomoza, D. Acosta, P. Bosch, S. Bulbulian, Sep. Sci.

Technol. 29 (1994) 2161.