Glutamic acid containing supermacroporous poly(hydroxyethyl methacrylate) cryogel disks for UO22+ removal

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

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Glutamic acid containing supermacroporous poly(hydroxyethyl methacrylate) cryogel disks for UO22+ removal Nilay Bereli, Deniz Türkmen, Kazım Köse, Adil Denizli ⁎ Department of Chemistry, Hacettepe University, Ankara, Turkey

a r t i c l e

i n f o

Article history: Received 21 December 2011 Received in revised form 7 May 2012 Accepted 22 May 2012 Available online 28 May 2012 Keywords: Glutamic acid Metal-chelating cryogel Aminoacid-ligands Uranium removal

a b s t r a c t Supermacroporous cryogel with an average pore size of 10–200 μm in diameter was prepared by cryopolymerization 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, elemental analysis and SEM. The poly(HEMA–MAGA) cryogel had a specific surface area of 23.2 m2/g. The equilibrium 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 in the removal of UO22+ from aqueous solutions. Adsorption equilibrium of UO22+ was obtained in about 30 min. The adsorption of UO22+ ions onto the PHEMA cryogel disks was negligible (0.78 mg/g). The MAGA incorporation significantly 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. Consecutive 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 heavy metal originates from nuclear-related activities and cause significant economic and public health problems by its presence in terrestrial and aquatic ecosystems [1–3]. Uranium concentration in waste streams is too low for a conventional treatment and too high to allow its discharge into the environment [4]. It has been suggested that polymer based adsorbents and microorganisms could be used to remediate these waste waters and to concentrate heavy metals. Polymer based adsorbents having chelating groups would be of great importance in uranium removal from aqueous systems due to their high selectivity and efficiency, easy handling and cost effectiveness [5–14]. Several issues are important in the preparation of chelating polymers with higher stability for the selective removal of metal ions: specific and quick complex formation of the metal ions as well as reuse of the adsorbents [15]. In recent years, amino acids and/or polyamino acids attachment onto a polymer matrix has been attracted increasing interests [16–18]. The purpose of using amino acids by the researchers stems from the fact that these substances are highly reactive with metal ions. The higher flexibility and durability of the amino acid based ligands as well as significantly lower manufacturing and material costs are also very promising for environmental applications.

⁎ Corresponding author. E-mail address: [email protected] (A. Denizli). 0928-4931/$ – see front matter © 2012 Elsevier B.V. All rights reserved. doi:10.1016/j.msec.2012.05.015

The aim of this study was to prepare a novel glutamic acid containing metal-complexing cryogel disks for UO22+ removal. Conventional adsorption columns based are time consuming. Monolithic cryogels are efficient adsorbents for heavy metal removal considering many advantages [19–25]. Monolithic cryogel columns require higher flow-rates with a much lower pressure drop than a packed bed column. Such cryogels also posses several advantages over conventional adsorbents, e.g., macropores, short diffusion path and very short residence time for both adsorption and elution stage. In this technique, the toxic substance to be removed can be directly transported by convection to MAGA on the pore surface of the cryogel, higher throughput and faster processing times onto the cryogel column can be achieved. The macroporous adsorbents possesses higher adsorption capacity and mass transfer rate than microporous and non-porous adsorbents. Cryogels are also cheap materials. In the first part of this study, the metal-complexing ligand, N-methacryloyl-(L)-glutamic acid (MAGA), was prepared using methacryloyl chloride and glutamic acid hydrochloride. Then, the poly(2hydroxyethyl methacrylate-N-methacryloyl-(L)-glutamic acid) [poly (HEMA–MAGA)] cryogel disks were prepared by cryopolymerization of MAGA and HEMA. The poly(HEMA–MAGA) cryogel disks were characterized by swelling tests, FTIR, elemental analysis and SEM. Then, UO22+ adsorption on the poly(HEMA–MAGA) cryogel disks from aqueous solutions containing different amounts of UO22+, at different pHs, was also performed. Elution of UO22+ and reusability of the poly(HEMA–MAGA) cryogel disks was also evaluated. In our knowledge, supermacroporous cryogels have not been employed for the removal of UO22+ from aqueous solutions.

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

2. Experimental 2.1. Materials Glutamic acid hydrochloride and methacryloyl chloride were supplied by Sigma (St Louis, USA). Hydroxyethyl methacrylate (HEMA) was obtained from Fluka A.G. (Buchs, Switzerland), distilled under reduced pressure in the presence of hydroquinone inhibitor and stored at 4 °C until use. N,N′-Methylene-bis(acrylamide) (MBAAm) and ammonium persulfate (APS) were purchased from Sigma (St Louis, USA). All the other reagents were of analytical reagent grade and procured from Merck AG (Darmstadt, Germany). Water was purified using a Barnstead (Dubuque, IA) ROpure LP® reverse osmosis unit with a high flow cellulose acetate membrane (Barnstead D2731) followed by a Barnstead D3804 NANOpure® organic/colloid removal and ion exchange system. Buffer and all sample solutions were prefiltered through a 0.2 μm membrane (Sartorius, Göttingen, Germany). All glassware was washed with 1.0 M HNO3 and rinsed thoroughly with deionized water. 2.2. Synthesis of N-methacryloyl-(L)-glutamic acid The MAGA was selected as the metal chelating ligand. Details of the preparation and characterization of the N-methacryloyl-(L)glutamic acid (MAGA) were reported elsewhere [26]. The following experimental procedure was performed for the synthesis of MAGA: 5.0 g of L-glutamic acid and 0.2 g of hydroquinone were dissolved in 100 mL of dichloromethane solution. This solution was cooled down to 0 °C. Then, triethylamine (13.0 g) was added to the solution and methacryloyl chloride (4.0 mL) was poured slowly into this solution under N2 atmosphere. This solution was stirred magnetically (250 rpm) at room temperature for 2 h. The unreacted methacryloyl chloride was extracted with 10% NaOH. The aqueous phase was evaporated in a rotary evaporator and residue (i.e., MAGA) was dissolved in ethanol. 2.3. Preparation of poly(HEMA–MAGA) cryogel disks Water and MBAAm were added in the polymerization recipe as the pore-former and cross-linker, respectively. Preparation procedure is as follows: HEMA (1.3 mL) and MAGA (200 mg) were dissolved in deionized water (5.0 mL). MBAAm (0.283 g) was dissolved in deionized water (10 mL). Second solution was mixed with previous one. The cryogel was then prepared by free radical polymerization initiated by TEMED (25 μL) and APS (20 mg). After adding APS (1% (w/v) of the total monomers) the solution was cooled in an ice bath for 2–3 min. TEMED (1% (w/v) of the total monomers) was added and the reaction mixture was stirred for 1 min. Then, the reaction mixture was poured between two glass sheets. The polymerization solution was frozen between two glass sheets at −16 °C for 24 h and then thawed at room temperature. After washing with 200 ml of water, the cryogel was cut into circular disks (1.0 cm in diameter) and stored in 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 were obtained by using a FTIR spectrophotometer (FTIR 8000 Series, Shimadzu, Japan). A piece of dry cryogel disk (about 0.1 g) was thoroughly mixed with potassium bromide (KBr, 0.1 g, IR Grade, Merck, Germany), and pressed into a pellet form and FTIR spectrum was then recorded. 1 H NMR spectrum of MAGA monomer was taken in CDCl3 on a JEOL GX-400 300 MHz instrument. The residual non-deuterated solvent (CHCl3) served as an internal reference. Chemical shifts are reported in ppm (δ) downfield relative to CHCl3.

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The swelling degree of the cryogel disk (S) was determined as follows: cryogel disk was washed on porous filter until washing was clear. Then it was sucked dry and then transferred to pre-weighed vial and weighed (mwet gel). After drying to constant mass in the oven 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 investigated by scanning electron microscope (SEM, JEOL, JSM 5600, Tokyo, Japan). The sample was fixed in glutaraldehyde (2.5%) in 0.15 M sodium cacodylate buffer overnight, post-fixed in osmium tetroxide (1%) for 1 h. Then, the sample was dehydrated stepwise in ethanol and transferred to a critical point drier temperated to 10 °C where the ethanol was changed for liquid carbon dioxide as transitional fluid. The temperature was then increased to 40 °C and the pressure to ca. 100 bar. Liquid CO2 was transformed directly to gas uniformly throughout the whole sample without heat of vaporization or surface tension forces causing damage. Release of the pressure at a constant temperature of 40 °C resulted in dried cryogel sample. Finally, it was coated with gold–palladium (40:60) and examined by SEM. The pore volume and the average pore diameter greater than 20 Å were determined by mercury porosimeter up to 2000 kg/cm 2 using a Carlo Erba model 200 (Milano, Italy). The surface area of the cryogel sample was measured with a surface area apparatus (BET method). To evaluate the amount of MAGA into the cryogel structure, it was subjected 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 mL aliquots of aqueous solutions containing different amounts of UO22+ (in the range of 10–1000 mg/L) were treated with the cryogel disks at different pH (in the range of 3.0–8.0) (adjusted with NaOH–HCl). The cryogel disks were stirred with a uranium nitrate salt solution at room temperature for 2 h. The concentration of the UO22+ in the aqueous phase was determined by the Arsenazo III method. The adsorption capacity of the cryogel disks was calculated using the mass balance on uranyl ions. The selective adsorption experiments of Th4+, Fe3+ and Mn2+ with respect to UO22+ were carried out using PHEMA and poly(HEMA– MAGA) cryogels at batch system. The polymeric disks (100 mg) were added to 25 mL of aqueous solution of containing 100 mL Th4+/UO22+, Fe 3+/UO22+ and Mn2+/UO22+ and placed in a beaker. A solution (25 mL) containing 100 mg/L from each metal ion was incubated with the cryogel disks at a pH of 6.0 at room temperature, in the flasks stirred magnetically at 250 rpm. The concentration of Fe3+ and Mn2+ ions in the remaining solution was measured by flame atomic absorption spectroscopy (FAAS). Th4+ ions were determined spectrophotometric method. In the spectrophotometric determination of Th4+ ions with Lawsone (2-hydroxy-1,4-naphtoquinone, LAS) was studied spectrophotometrically in 40% (v/v) ethyl alcohol/water at 25 °C and ionic strength of 0.1 M (NaClO4). After the reaction of LAS with Th4+ ions, the absorbance of the complex solution was measured at 440 nm, against a blank reagent. Distribution coefficients of Th4+, Fe 3+ and Mn2+ with respect to UO22+ were calculated by using concentration 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 at room temperature. Elution volume was 50 mL. The final concentration

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

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

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 metal complexing ligand. In the first step, MAGA was synthesized from L-glutamic acid hydrochloride and methacryloyl chloride. Then, the poly(2-hydroxyethyl methacrylate-N-methacryloyl-(L)-glutamic acid) [poly(HEMA–MAGA)] cryogel disks were obtained by cryopolymerization of MAGA and HEMA. The chemical structures of the PHEMA and the poly(HEMA–MAGA) cryogels were illustrated in Fig. 1 in order to distinguish their differences. 1H NMR was used to determine the synthesis of MAGA structure. Fig. 2 shows the 1H NMR spectrum of MAGA. 1H NMR spectrum is shown to indicate the characteristic peaks from the groups in MAGA monomer. These characteristic peaks are as 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 vibration carboxyl–carbonyl and amide–carbonyl absorption bands at 1739 cm − 1 and 1640 cm − 1 respectively as shown in Fig. 3. The N–H bending peak appears at 1522 cm − 1 is associated with the amide vibration of MAGA. FTIR spectrum was recorded to determine the structure of the poly(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) and 1699 cm − 1 (carboxyl–carbonyl stretching vibration). The carbonyl peak 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 surface morphology and bulk structure of the poly(HEMA–MAGA) cryogel disks are shown in Fig. 5. The poly(HEMA– MAGA) cryogel produced in such a way has porous and thin polymer walls, large continuous interconnected pores (10–200 μm in diameter) that provide channels for the mobile phase to flow through the monolithic column. Pore size of the matrix is much larger than the size of the UO22+ ions, allowing them to transport easily through the pores. As a result of the convective transport, the mass transfer resistance is practically negligible. Polymeric cryogel is opaque, white, sponge like and elastic (Fig. 6). This cryogel can be easily compressed by hand to remove water accumulated inside the bulk structure. When the compressed piece of cryogel was submerged in water, it soaked in water and within 1–2 s restored its original size and shape due to its shape memory. Physicochemical properties of the PHEMA cryogel are shown in 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 isotherms by non-linear regression method for the adsorption of UO22+ ions onto the poly(HEMA–MAGA) cryogel disks. Adsorption of UO22+ ions onto the PHEMA cryogel disks as also performed. It should be noted that the elution of UO22+ onto the PHEMA cryogel disks was very low, about 0.78 mg/g. Because, PHEMA cryogel disks do not contain any reactive functional groups for complexation of UO22+ ions. This low adsorption value of UO22+ ions may be due to diffusion of UO22+ ions into the macropores of the cryogel disks and weak interactions between UO22+ ions and hydroxyl groups on the surface of the PHEMA cryogel disks. However, MAGA including into the polymer structure significantly increased the adsorption capacity to 92.5 mg/g. The adsorption values increased with increasing initial concentration of UO22+, and a saturation value is achieved at ion concentration of 800 mg/L, which represents saturation of the active binding sites on the poly(HEMA–MAGA) cryogel disks. Note that the MAGA content of the poly(HEMA–MAGA) cryogel disks used in this group of experiments was 878 μmol/g. The maximum UO22+ adsorption capacity obtained in the studied range is 342 μmol per gram of the cryogel disks. This seems to give a stoichiometry of around four MAGA groups per one uranium ion. MAGA as thought to be incorporated to the backbone through copolymerization and the pendant carboxyl groups in the MAGA are postulated to be responsible for uranium binding. Strong complex formation between carboxylic acid functional groups and uranyl ions has been thoroughly discussed in a recent review article [27], and the importance of carboxyl groups in metal binding by adsorbents has been addressed [28].

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

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Fig. 3. FTIR spectrum of MAGA monomer.

Adsorption isotherms are used to describe adsorption nature of adsorbents. Amount of adsorbed UO22+ (qe) against concentration of final UO22+ (Ce) is plotted. Langmuir and Freundlich–Peterson isotherm models are used to figure out the equilibrium data for UO22+ adsorption. According to Langmuir adsorption model, molecules are adsorbed with equivalent well-defined binding sites far from each other and in between there is no interaction — adsorbing only one molecule. Therefore, energies and enthalpies are equal. Langmuir model expression can be written by Eq. (2). qe ¼ qm Ka Ce =ð1 þ Ka Ce Þ:

ð2Þ

qe ¼ Kf Ce

1=n

:

ð4Þ

Linear form of this equation is:

This equation can be converted to linear form as: ðCe =qe Þ ¼ ðCe =qm Þ þ ð1=qm Ka Þ

Freundlich isotherm is another form of Langmuir isotherm for adsorption on heterogeneous surface with a non-uniform distribution of heat of adsorption over surface. Summation of UO22+ concentration adsorbed by all binding sites is equivalent UO22+ concentration adsorbed totally. Freundlich adsorption model denotes reversible adsorption and is not limited with monolayer adsorption whereas Langmuir model is. Empirical Freundlich equation is given by Eq. (4):

ð3Þ

where, Ka is the adsorption equilibrium constant, Ce and qe are unadsorbed UO22+ concentrations in solution and adsorbed UO22+ ions on the adsorbent of equilibrium, respectively. qm is the maximum amount of UO22+ ions per unit weight.

ln qe ¼ ln Kf þ ð1=mÞ ln Ce

ð5Þ

where, qe is the equilibrium UO22+ adsorbed amount; Ce is the residual UO22+ concentration at equilibrium; and Kf and n are the Freundlich constant related to the adsorption capacity and adsorption intensity of the adsorbent respectively.

Fig. 4. FTIR spectrum of poly(HEMA–MAGA) cryogel disks.

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

Redlich–Peterson isotherm is arranged to exhibit applicability of Langmuir and Freundlich adsorption models and so combination of Langmuir and Freundlich equations. Redlich–Peterson model is feasible for homogeneous and heterogeneous adsorption systems. The other models mentioned above described with two parameters but Redlich–Peterson did three. Redlich–Peterson isotherm is given by Eq. (6):   β qe ¼ ðKR Ce Þ= 1 þ aR Ce R

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 regression. At low concentrations the Redlich–Peterson isotherm approximates to Henry's law as βR goes to 1. The calculated isotherm parameters were shown in Table 2. The Langmuir adsorption capacity, qm, obtained in this study was found to be 131.6 mg/g. The adsorption capacity value (KF) for UO22+ obtained from Freundlich model was 1.77, this value indicated that the poly(HEMA–MAGA) had a good affinity for UO22+ ions. The Redlich– Peterson isotherm shows the value of βR to be 1.0 indicating that the isotherm tends towards the Langmuir form. The order of fitness

of these isotherm models to the experimental data was found to be Langmuir > Redlich–Peterson > Freundlich. Different types of natural and synthetic adsorbents/biosorbents have been used for the uranium removal from aqueous solutions and seawater. A comparison of the adsorption capacity of MIP particles with those of some other affinity adsorbents reported in literature is given in Table 3. The adsorption capacity of poly(HEMA– MAGA) cryogel was considered to be good when compared with other polymer based adsorbents. Differences of uranium adsorption capacity are due to the properties of each adsorbent such as structure, functional groups, ligand loading and accessible surface area. 3.1.2. Effect of pH pH is the most critical parameter for metal adsorption as it influences both the polymer surface chemistry as well as the solution chemistry of soluble metal ions [28, 31, 42–44]. In order to establish the effect of pH on the adsorption of UO22+ onto the both PHEMA and poly(HEMA–MAGA) cryogel disks, we repeated the batch adsorption equilibrium studies at different pHs in the range of 3.0–8.0. In this group of experiments, the initial concentration of UO22+ and the adsorption equilibrium time were 500 mg/L and 2 h, respectively. The pH dependence of adsorption values of UO22+ is shown in Fig. 8. In the case of PHEMA cryogel disks, adsorption is pH independent.

Fig. 6. Optical photograph of poly(HEMA–MAGA) cryogel disks.

N. Bereli et al. / Materials Science and Engineering C 32 (2012) 2052–2059 Table 1 Physicochemical properties of the PHEMA and poly(HEMA–MAGA) cryogel.

Specific surface area Pore size diameter Porosity MAGA loading Swelling degree Flow resistance

Table 2 Adsorption isotherm constants for UO22+ adsorption onto poly(HEMA–MAGA).

PHEMA

Poly(HEMA–MAGA)

Langmuir

20.2 m2/g 10–200 μm 71.6% – 8.56 g H2O/g cryogel 845 cm/h

23.2 (m2/g) 10–200 μm 84.3% 878 μmol/g 9.68 g H2O/g cryogel 624 cm/h

qm KL R2

But, it is indicated that the adsorption of UO22+ onto the poly(HEMA– MAGA) cryogel disks was pH dependent. The results show that uranium adsorption by the poly(HEMA–MAGA) cryogel disks was low at pH 3.0, but increased rapidly with increasing pH and then reached the maximum at pH 6.0. UO22+ adsorption around pH 3.0–4.0 was low. This can be explained by the fact that, at this pH, most of pendant carboxyl groups are protonated. It is well known in adsorption mechanisms, that a decrease in solubility favors an improvement in adsorption performance. The increase of pH was followed by a decrease in the uptake of uranyl ions. Because there is a decrease in dissolved uranyl ion concentration at higher pH due to the formation of solid schoepite (4UO2·9H2O) reducing uranium adsorption. In mildly acidic and neutral pHs 4.0–6.0, poly(HEMA–MAGA) cryogel disks are effective for removal of UO22+ from aqueous solutions.

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Freundlich 131.6 0.0025 0.9855

KF n R2

1.77 0.59 0.9459

Redlich–Peterson 0.35 0.002 1.0 0.9515

KR aR βR R2

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

Adsorption capacity

[R]

Furano uronate based resin Crosslinked chitosan PHEMA grafted tamarind fruit shell PHEMA grafted ligno-cellulosics Chitosan-tripolyphosphate beads Poly(acrylonitrile)-amidoxime Poly(acrylamidoxime) hydrogel Poly(glycidyl methacrylate)-amidoxime beads Tetramethylmalonamide chelating resins Poly(HEMA–MAGA) beads Hematite Marine algal biomass Nanoporous silica-PEI Microbial biomass Microorganisms Coir pith Poly(AAm–AAc) Poly(MAGA–EGDMA) imprinted beads Manganese oxide coated zeolite Poly(HEMA–MAGA) cryogel

26.3 mg/g 72.5 mg/g 100.1 mg/g 109.6 mg/g 240.0 mg/g 44.0 mg/g 39.5 mg/g 3.95 μmol 5–135 mg/g 204.8 mg/g 3.5 mg/g 300 mg/g 12.2 mg/g 180.0 mg/g 100–310 mg/g 255.3 mg/g 236.6 mg/g 181.0 mg/g 15.1 mg/g 92.5 mg/g

[3] [7] [8] [9] [10] [11] [12] [13] [14] [26] [29] [30] [32] [33] [34–37] [38] [39] [40] [41] In this study

3.1.3. Effect of contact time Fig. 9 shows the time dependence of the adsorption values of UO22+ on poly(HEMA–MAGA) cryogel disks. There is a direct relationship 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 adsorption rate was also relatively fast, the time required to reach equilibrium conditions was about 60 min. The initial concentration provides an important driving force to overcome all mass transfer resistance of UO22+ ions between the aqueous and solid cryogel phases. Hence, a higher initial concentration of UO22+ ions will increase the adsorption rate. High complexation affinity between UO22+ and MAGA groups in the cryogel disks structure may also contribute to this fast adsorption equilibrium. It is worth to point that UO22+ has a high affinity for MAGA. Experimental data on the adsorption kinetics of uranium by various adsorbents have shown a wide range of adsorption rates. For example, Wang et al. studied the adsorption of uranium on epichlorohydrin cross-linked chitosan and reported 3 h equilibrium

adsorption time [7]. Anirudhan and Radhakrishnan documented that the amine modified poly(hydroxyethyl methacrylate) grafted biomaterial (tamarind fruit shell) reached steady state at about 2 h [8]. Anirudhan et al. investigated recovery of uranium with carboxylate functionalized poly(hydroxyethyl methacrylate) grafted lignocellulosics and they reported that equilibrium was achieved in about 3 h [9]. Sureshkumar et al. studied adsorption of uranium using chitosan–tripolyphosphate beads and they reported that adsorption

Fig. 7. Comparison of the experimental and different isotherm models for the adsorption of UO22+ onto the poly(HEMA–MAGA) cryogel disks; pH: 6.0; T: 25 °C.

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.

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N. Bereli et al. / Materials Science and Engineering C 32 (2012) 2052–2059 Table 5 Kd values of Th4+, Fe3+ and Mn2+ with respect to UO22+.

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.

was rather slow and took 4 days for the beads to reached equilibrium [10]. Zhang et al. studied uranium adsorption on macroporous fibrous poly(acrylo nitrile) and they reported 6 h equilibrium adsorption time [11]. Shuibo et al. used hematite for the removal of UO22+ from aqueous solution and reported that equilibrium of uranium adsorption was not reached until after 6 h [29]. Khani reported that the uranium adsorption rate by algae biomass is high at the beginning but plateau values are reached for a wide uranium concentration range about 100 min long [30]. Olguin et al. achieved 30 min as a short equilibrium time in their uranium adsorption kinetic studies, in which they used natural Mexican erionite and Y zeolite as sorbent [45]. Several criteria are important in the design of chelating polymers with substantial stability for the selective adsorption of metal ions: specific and fast binding of the metal ions as well as the recyclability of the chelating polymeric adsorbents. All these experimental studies reported here have been carried out at different conditions. Therefore, it can be concluded that it is too difficult to compare the adsorption rates reported. However, the adsorption rates obtained with the affinity cryogel disks produced by us seem to be very satisfactory. The kinetic mechanism controlling and defining the efficiency of adsorption process was investigated using pseudo-first- and secondorder equations to test experimental data.     Pseudo  first order log qeq −qt ¼ log qeq −ðk1 tÞ=2:303

ð7Þ

    2 Pseudo  second order ðt=qt Þ ¼ 1=k2 qeq þ 1=qeq t

ð8Þ

Cryogels

UO22+ (mg/L)

Th4+ (mg/L)

PHEMA Poly(HEMA–MAGA)

100 100

100 100

Cryogels

UO22+ (mg/L)

Fe3+ (mg/L)

PHEMA Poly(HEMA–MAGA)

100 100

100 100

Cryogels

UO22+ (mg/L)

Mn2+ (mg/L)

Kd (UO22+)

Kd (Mn2+)

PHEMA Poly(HEMA–MAGA)

100 100

100 100

21.8 102.6

6.92 5.75

Kd (UO22+) 14.7 98.9 Kd (UO22+) 11.9 95.8

Kd (Th4+) 3.68 3.26 Kd (Fe3+) 4.26 3.88

adsorption of UO22+ ions on supermacroporous poly(HEMA–MAGA) cryogel disks can be described by pseudo-second order kinetic model. 3.1.4. Competitive adsorption Competitive adsorption of Th4+/UO22+, Fe3+/UO22+ and Mn2+/UO22+ from their binary solutions was also investigated in a batch system. Th4+/UO22+ was chosen as a competitive ions because uranium and thorium often coexists in the minerals, products and in even in wastestreams. Fe3+/UO22+ and Mn2+/UO22+ couples were chosen because these elements are in the same hard acid group and interfere each other. Table 5 shows Kd values for the poly(HEMA–MAGA) cryogel disks. A comparison of the Kd values for the poly(HEMA–MAGA) cryogel disks with the control PHEMA cryogel disks shows and increase in Kd for UO22+ while Kd decreases for Th4+, Fe 3+ and Mn2+ ions. 3.1.5. Behavior of the elution Elution of the UO22+ from the cryogel disks was performed in a batch experimental setup for improving cryogel's cost effectiveness by recycling the adsorbent for reuse in multiple cycles. Various factors are probably involved in determining rates of UO22+ elution, such as the extent of hydration of the metal ions and polymer microstructure. However, an important factor appears to be binding strength. In this study, the elution time was found to be 10 min. Elution ratios are very high (up to 98%). In order to obtain the reusability of the poly(HEMA– MAGA) cryogel disks, adsorption–elution cycles were repeated 10 times by using the same cryogel disks. The adsorption capacity of the recycled poly(HEMA–MAGA) cryogel disks can still be maintained at 97% level at the 10th cycle (Fig. 10). Thus, the undamaged cryogel disks could be reused after each repetition.

where, qe and qt are the amounts of UO22+ ions adsorbed (mg/g) at equilibrium time “t”. k1 and k2 are the pseudo-first order and pseudo-second order rate constants of adsorption respectively. Kinetic model parameters were determined at different initial concentrations of UO22+ ions. The results of kinetic analysis are summarized in Table 4. As a result of calculations, pseudo-second order kinetic model is well-fitted with experimental data meaning that adsorption is chemically controlled. The Table 4 The first and second order kinetic constants for poly(HEMA–MAGA) cryogel. Initial conc.

Experimental

First-order kinetic k1 (1/min)

qe (mg/g)

R2

k2 (1/min)

qe (mg/g)

R2

(mg/ml)

qe (mg/g)

Second-order kinetic

100 250 500 1000

24.2 52.58 78.6 92.5

0.0216 0.0308 0.0363 0.0377

4.99 32.59 75.80 87.09

0.24 0.68 0.75 0.74

0.00151 0.00030 0.00017 0.00017

29.1 73.5 113.5 128.2

0.9905 0.9667 0.9508 0.9394

Fig. 10. Adsorption–elution cycles for UO22+; Adsorption conditions; UO22+ initial concentration: 500 mg/L; pH: 6.0; T: 25 °C.

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

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