Cadmium Removal Performances of Different Dye Ligands Attached Cryogel Disks

Poly(HEMA) cryogel disks were synthesized by free radical polymerization of 2-hydroxyethylmethacrylate (HEMA), and then Cibacron Blue F3GA (CB), Reactive Green 19 (RG) and Congo Red (CR) were immobilized as dye ligands. Disks were characterized by Fourier transform infrared spectroscopy (FTIR), scanning electron microscope (SEM) and swelling degree, porosity calculations. Cd2+ adsorption experiments were performed for 60 min. Maximum adsorption capacities were determined as 25.5 mg/g; 48.0 mg/g and 28.5 mg/g at pH = 7.0 for poly(HEMA)-CB; poly(HEMA)-RG and poly(HEMA)-CR, respectively. Langmuir isotherm fitted best with the adsorption data and adsorption thermodynamics showed that Cd2+ adsorption is thermodynamically favorable and a physisorption process. A great majority of adsorbed Cd2+ desorbed with 1 M NaCl and cryogel disks can be re-used in adsorption experiments. Cd2+ removal efficiencies of disks from human plasma are approximately 45 %. Dyeattached cryogel disks synthesized in this study have potential in use for environmental and therapeutic applications.


INTRODUCTION
Although it is pointed out that metals play important roles in biological processes and some of them are classified as essential, toxic symptoms are exhibited above threshold levels of metal ions. 1 Heavy metals, known as teratogenic and carcinogenic, can be bioaccumulated by vegetables, hydrophytes and aquatic inhabitants and, finally enriched by human beings through food chains. 2,3Unlike organic toxicants, heavy metals are not appropriate to biological degradation. 4Cadmium, which is highly toxic to human, plants and animals even at low concentrations, has been classified by US Environmental Protection Agency as a probable human carcinogen. 2 It has an extremely long biological half-life and causes kidney damage, altered renal tubular function, impaired regulation of calcium and phosphorus, bone demineralization, osteomalacia, and pathological fractures. 5It also causes liver and lung damage. 6Men may become exposed to cadmium due to its use in electroplating, smelting, plastics, pigments, ceramics, battery, cadmium-rich phosphate fertilizers and mining. 3,6Cadmium concentration in blood of healthy person is 0.01 mg/L and it is excreted in the urine.Cadmium concentration above 0.05 mg/L is a strong sign of cadmium intoxication due to excessive exposures to external cadmium sources. 5,7o date there are no proven effective treatments for chronic cadmium intoxication.Besides supportive therapy and hemodialysis, metal poisoning is often treated with a chelating agent. 8Different chelating agents that are available commercially for the treatment of cadmium poisoning are British anti lewisite and calcium disodium EDTA.Recently, one of the most promising techniques for blood detoxification is extracorporeal affinity adsorption.][11][12][13] Polymeric materials are suitable candidates due to their reactive functional groups, good mechanical properties, ease of preparation method and applicability to introduce bio-friendly components for improving biocompatibility. 14Polymeric gels have applications in many different areas of biotechnology. 15One of the new types of polymer gels with considerable potential in biotechnology is 'cryogels'. 16Several advantages of cryogels are large pores, short diffusion path, low pressure drop, and very short residence time for both ad-sorption and elution.Cryogels are also cheap materials and they can be used as disposable avoiding cross contamination between batches.Furthermore, cryogels can be formed in any desirable shape, for example, blocks, cylinders, tubes, granules and disks. 16 number of textile dyes, known as reactive dyes, have been used in dye-ligand affinity systems.Dyes are commercially available, inexpensive and can be easily immobilized, especially on matrices bearing hydroxyl groups.Most of the reactive dyes consist of a chromophore, linked to a reactive group (often a mono-or dichlorotriazine ring).They also have sulfonic acid groups to provide the desired solubility of the molecule in aqueous media.Some dyes contain carboxyl, amino, chloride, or metal complexing groups; most contain nitrogen both in or outside on aromatic ring. 17he aim of this work was to evaluate the performances of cryogel disks with different dye ligands for cadmium removal from aqueous solutions and to determine their efficiencies for Cd 2+ removal from human plasma.

Materials
HEMA (≥ 99 %), N,N,N',N'-tetramethylethylenediamine (TEMED, more than 99 %) and amoniumpersulfate (APS, ≥ 98 % for electrophoresis) were supplied by Fluka (Fluka A.G. (Buchs, Switzerland).N,N'-methylene-bis(acrylamide, 99 %) (MBAAm), sodium chloride (99 %) and cadmium sulfate (≥ 99.0 %) were obtained from Sigma (St Louis, USA).All other chemicals were of the highest purity commercially available and were used without further purification.All water used in the adsorption experiments was purified using a Millipore S.A.S 67120 Molsheim-France facility whose quality management system is approved by an accredited registering body to the ISO 9001.Before use the laboratory glassware was rinsed with deionised water and dried in a dust-free environment.

Preparation of Poly(HEMA) Cryogel Disks
For the purpose of cadmium removal, poly(HEMA) cryogel disks were synthesized by free radical polymerization started with TEMED and APS.A typical preparation procedure was given as follows: 21.43 mmol HEMA was mixed with MBAAm solution (3.67 mmol in 20 mL).The cryogel was then produced by free radical polymerization initiated by TEMED and APS.After adding APS (0.175 mmol), the solution was cooled in an ice bath for 2-3 min.TEMED (0.335 mmol) was added and the reaction mixture was stirred for 1 min.Then, the reaction mixture was poured between two glass plates separated with 1.5 mm thick spacers.The polymerization mixture was frozen at -16 °C for 24 h and then thawed at room temperature.Poly(HEMA) cryogel disks were washed with 2 L water to remove unreacted monomers.The cryogel was cut into circular disks (0.8 cm in diameter) and mass of cryogel disks was determined over a range of 20-25 mg.Cryogel disks stored in buffer containing 0.02 % sodium azide at 4 °C until use.

Immobilization of Dye Ligands onto the Poly(HEMA) Cryogel Disks
Immobilization procedures of CB, RG and CR (Figure 1) were performed using the book of immobilized affinity ligand techniques. 18Poly(HEMA) disks were divided into three parts.Every part of disks was bottled up into dye solutions (100 mg dye dissolved in 30 mL deionized water).After shaking at 150 rpm for 30 min at 60 °C, 1.5 g NaCl was added to reaction mixture and shaked 1 h more.Then, temperature was increased to 70 °C and 0.15 g Na2CO3 added.After 2 h shaking, reaction mixture was cooled to room temperature and disks were washed with water until washing water was colorless.Dye immobilization processes were checked by UVabsorbance measurements of dyes with using a UVspectrophotometer (Schimadzu 1601, Japan).Finally, disks were washed with ethanol-water (50:50) solution until washing solutions have no UV-absorbance and CB, RG and CR-attached cryogel disks were stored at 4 °C until use.

Characterization of the Poly(HEMA) Cryogel Disks
To determine the swelling degree of the disks (S), disks were dried to constant mass at vacuum oven at 55 °C and 100 mbar and masses of dried pellets were determined (mdried disk).The dried disks were bottled up to 50 mL ionized water and masses of swollen disks were determined (mswollen disk).The swelling degree was calculated as: The water content of cryogel disks were determined according to the studies of Plieva et al., and Horak et al. 19,20,21 Dried disks were mounted into chamber saturated with water vapor.Water vapor adsorbed onto cryogel disk by time was determined and then weight of cryogel disks adsorbed water vapor was measured (mwater bound disk).Sponge like cryogel disks were squeezed by hand without damaging their physical forms and weight of squeezed disks were determined (msqueezed disk).The porosity (P, expressed in %) and porosity for macropores (Pmacropores, expressed in %) were also calculated as follows: The surface morphology of the poly(HEMA) cryogel disks was examined using SEM.The samples were initially dried in air at 25 °C for 7 days before being analyzed.A fragment of the dried disk was mounted on a SEM sample mount and was sputter coated for 2 min.The sample was then mounted in a SEM (Phillips, XL-30S FEG, Germany).The surface of the sample was then scanned at the desired magnification to study the morphology of the disks.
FTIR spectra of poly(HEMA) cryogel disk and CB, RG and CR-attached poly(HEMA) cryogel disks were obtained by using a FTIR spectrophotometer (Perkin Elmer spectrum 100 FT-IR spectrometer) with universal ATR sampling accessory.

Cadmium Adsorption Studies from Aqueous Solution
Cadmium adsorption studies were performed in a batch system at 25 °C with stirring continuously.Some variables such as time, pH, initial cadmium concentration and temperature were studied to optimize adsorption conditions.To determine effect of time, adsorption was completed at different time periods.Cadmium solutions were prepared in universal buffer between pH 3.0-8.0 to observe the effect of pH on the Cd 2+ adsorption.Different initial Cd 2+ concentrations (10-500 mg/ L) were used to determine the effect of initial Cd 2+ concentration and temperature was changed between 15 °C and 45 °C.The amount of adsorbed Cd 2+ per unit mass of the disk (q) was calculated by using the following expression: Where q is adsorption capacity (in mg/ g), c0 and ce are the concentration of Cd 2+ in the initial solution and in the aqueous phase after treatment for certain period of time, respectively (in mg/ L), V is the volume of Cd 2+ solution (in L) and mdisks is the mass of disks used (in g).Adsorption experiments were conducted for 60 min which was the equilibrium period.Initial and final cadmium concentrations were determined by Atomic Adsorption Spectrophotometer (AAS) (Perkin-Elmer AA 700).

Desorption and Reusability Studies
Desorption efficiencies of different agents at various concentrations were investigated.NaCl, 2-mercaptoethanol and thiourea were used as desorption agents and desorption ratios were calculated from the amount of Cd 2+ adsorbed and the final metal concentration in the desorption medium.Cd 2+ adsorbed disks were placed within the desorption medium containing 0.5-1.0M NaCl, 0.5-1.0M 2-mercaptoethanol and 0.5-1.0M thiourea at room temperature for 6 h.It must be pointed out that there was no CB, RG and CR release in this case which shows that dye-molecules are bonded strongly to poly(HEMA) cryogel disks.
After desorption process, disks were washed with water several times and re-conditioned with universal buffer of pH = 7.0.These disks were used in Cd 2+ adsorption-desorption cycle to test the reusability of dyeattached cryogel disks.

Cd 2+ Ions Removal from Human Plasma
Removal of Cd 2+ ions from human plasma was carried out in a batch system.Human blood is collected from thoroughly controlled voluntary blood donors.Human blood was collected into EDTA-containing vacutainers without adding preservatives and red blood cells were separated by centrifugation at 4000 rpm for 30 min at room temperature, then filtered (3 µm Sartorius filter) and frozen at -20 °C.Before use, plasma was thawed for 1 h at 37 °C and samples were prepared with different dilutions.Then, adsorption studies were performed with the overloaded plasma samples by using the same procedure mentioned above.

Characterization of the Dye-Attached Poly(HEMA) Cryogel Disks
The scanning electron micrograph of the internal structure of the poly(HEMA) cryogel disks is shown in Figure 2, from which it can be seen that dye attached p(HEMA) cryogel disks have a porous structure and thin polymer walls, with large continuous interconnected pores that provide channels through which the mobile phase can flow.
CB, RG and CR dye ligands were immobilized onto poly(HEMA) cryogel disks.The functional hydroxyl groups on the surface of the pores in the p(HEMA) cryogel disks produced by polymerization in the frozen state of the HEMA in the presence of APS/TEMED as an initiator/activator pair allowed their modification with dye ligands.The FTIR spectra of the poly (HEMA) and dye-attached poly(HEMA) cryogel disks are shown in Figure 3.
The FTIR bands of dye attached poly(HEMA) cryogel disks have vibration bands around 1055 cm -1 and 1075 cm -1 .These bands correspond to -S=O stretching and -SO3H stretching vibrations, respectively.The presence of aromatic C=C stretching vibration band as a shoulder in the spectra of dye attached poly(HEMA) cryogel disks indicated the immobilization of dyes onto poly(HEMA) cryogel disk.-OH stretching band of poly(HEMA) was located in 3312 cm -1 .Immobilization of dyes broadened the -OH stretching band because of the presence of -NH stretching vibrations.Additionally, visual observations (the colors of the cryogel disks) ensured the attachment of CB, RG and CR.
Dye-attached cryogel disks were sponge like and elastic.These can be easily compressed by hand to remove water accumulated inside the pores.When compressed cryogel disks were submerged in water, they soaked in water and within 1-2 s restored its original size and shape.Swelling degrees (S), P/ % and Pmacropores/ % were calculated and given in Table 1.

Effect of Contact Time
Adsorption studies were performed for different time periods until 90 min.As shown in Figure 4, Cd 2+ adsorption increased with time and reached a plateau of saturation at 60 min.Therefore, all adsorption studies were performed at 60 min in further experiments.

Adsorption Kinetics
The kinetics of adsorption describes the rate of Cd 2+ uptake and it controls the equilibrium time.The pseudofirst-order kinetic model has been widely used to predict adsorption kinetics.The model given by Langergren and Svenska 22 is defined as: Where qe and qt (in mg/g) are the amounts of Cd 2+ adsorbed at equilibrium and at any time, t (h), and k1 (h -1 ) is the adsorption rate constant.When a graph of ln(qeqt) vs. t is drawn, a straight line with slope of k1 and intercept of ln qe is obtained.
The pseudo-second-order equation 23 based on equilibrium adsorption is expressed as: Where k2 (in g/ mg h) is the rate constant.The linear plot of t/qt vs. t gives 1/qe as the slope and 1/k2q 2 e as the intercept.This procedure is more likely to predict the behavior over the whole range of adsorption.
The Elovich equation, analyzed for applicability of adsorption data, is expressed as: 24   Where α (in mg/ g h) is the initial adsorption rate and β (in g/mg) is related to the extent of surface coverage.The parameters (1/ β) and (1/ β) ln (αβ) can be obtained from the slope and intercept of the linear plot of qt versus ln t.The value of (1/ β) is indicative of the number of sites available for adsorption while the (1/β) ln (αβ) is the adsorption quantity when ln t is equal to zero.This value is helpful in understanding the adsorption behavior of the first step. 25The qe values calculated from Elovich equation agreed quite well with the experimental values.As the above kinetic models were not able to identify the diffusion mechanism, thus intraparticle diffusion model based on the theory proposed by Weber and Morris 26 was tested.It is an empirically found functional relationship, common to the most adsorption processes, where uptake varies almost proportionally with 1/ 2 t rather than with the contact time.According to this theory: Where kpi (in mg/ g 1/ 2 h ), the rate parameter of stage i, is obtained from the slope of the straight line of qt versus 1/ 2 t .ci, the intercept of stage i, gives an idea about the thickness of boundary layer, i.e., the larger the intercept, the greater the boundary layer effect.It can be concluded from the data obtained that adsorption process was not controlled by intraparticle diffusion by the reason of correlation coefficient values.
All the correlation coefficient, R 2 values and constants obtained from the adsorption kinetics are given in Table 2.The correlation coefficient values for both the pseudo-first-order and the pseudo-second-order kinetic model are high (R 2 = 0.9919 for poly(HEMA)-CB, R 2 = 0.9889 for poly(HEMA)-RG, R 2 = 0.9623 for poly-(HEMA)-CR and R 2 = 0.9931 for poly(HEMA)-CB, R 2 = R 2 = 0.9937 poly(HEMA)-RG, R 2 = 0.9962 for poly-(HEMA)-CR, respectively).Besides, calculated qe values in both pseudo-first and pseudo-second order kinetic models are notably close to the experimental qe values.Experimental data were rather accurately fitted onto both first-order and second-order kinetic models for cadmium adsorption. 27,28 Initial adsorption rates of poly(HEMA)-CB, poly(HEMA)-RG and poly(HEMA)-CR cryogel disks were calculated as 197.40; 176.79 and 211.44 mg/ g h, respectively, by using Elovich equation.

Effect of pH
The pH of solution is an important factor influencing complex formation and stability.Most chelating agents are unstable at low pH, whereas at high pH metals tend to form insoluble hydroxides which are less accessible to chelating agents. 29The solubility of cadmium is governed by hydroxide or carbonate concentration.It is well known that precipitation of cadmium ions becomes significant at pH = 8.5.In order to establish the effect of pH on Cd 2+ adsorption, studies were carried out at different pH in the range of 3.0-8.0.
As seen in Figure 5, Cd 2+ adsorption by dye attached p(HEMA) cryogel disks was low at pH = 3.0 due to protonation of the functional groups on the structure of dyes, but increased with increasing pH.The increasing pH favors complex formation between the dye molecules and Cd 2+ ions.Maximum adsorption was observed at pH = 7.0.In all solutions, hydronium ions (H3O + ) and Cd 2+ ions compete for adsorption.At low pH values, hydronium ions have high concentration and more tendencies to be adsorbed, so hydronium ions are adsorbed more than Cd 2+ ions. 30,311][32] .The reason of higher adsorption at higher pH values may also be that Cd 2+ interact with CB, RG and CR not only through the nitrogen and oxygen atoms by chelating, but also through -SO3H groups by cation-exchange, which is unprotonated at high pH.

Effect of Initial Cd 2+ Concentration
Figure 6 shows the effect of initial concentration.Maximum adsorption capacities were determined as 25.5 mg/ g; 48.0 mg/ g and 28.5 mg/ g for poly(HEMA)-CB; poly(HEMA)-RG and poly(HEMA)-CR cryogel disks, respectively.While the adsorption capacities reached to a plateau value at about 200 mg/ L for poly(HEMA)-CB and poly(HEMA)-CR; maximum adsorption capacity of poly(HEMA)-RG was determined at 400 mg/ L. These data demonstrates that Cd 2+ adsorption capacity of poly(HEMA)-RG disks were higher than the others under the same conditions.Furthermore, non-specific Cd 2+ adsorption on the poly(HEMA) cryogel disks was determined as negligibly small (0.18 mg/ g).It can be concluded that dye immobilization increased the adsorbed Cd 2+ amounts.

Adsorption Isotherms
The adsorption isotherm is the most important information indicating how the adsorbate molecules distribute between liquid and solid phase when the adsorption process reaches an equilibrium state.It is important to create the most appropriate correlation for the equilibrium curves to optimize the design of an adsorption system.Langmuir isotherm assumes monolayer adsorption onto a surface containing a finite number of adsorption sites of uniform strategies of adsorption with no transmigration of adsorbate in the plane of surface. 33The linear form of Langmuir isotherm equation is given as: Where, qe is the amount of Cd 2+ adsorbed per unit weight of disk at equilibrium (in mg/ g) and ce is the equilibrium concentration of Cd 2+ in solution (in mg/ L). qm is the monolayer adsorption capacity (in mg/ g) and L is related with the adsorption energy (in L/ mg).When ce/ qe is plotted against ce, a straight line with slope of 1/ qm and intercept of 1/ Lqm is obtained.The essential characteristics of Langmuir isotherm can be expressed by a dimensionless constant called equilibrium parameter, RL, defined by Weber and Chakkravorti 33 as: Where, L is the Langmuir constant and c0 is the highest initial Cd 2+ concentration (in mg/L).RL values bigger than 1 represent unfavorable, and between 0 and 1 represent favorable adsorption.Adsorption type is linear when RL = 1 and irreversible when RL = 0. Adsorption type can be commented as favorable at batch adsorption conditions due to RL values were calculated between 0 and 1.
Freundlich model is an empirical equation based on adsorption on a heterogeneous surfaces or surfaces supporting sites of varied affinities.It is assumed that the stronger binding sites are occupied first and the binding strength decreases with the increasing degree of site occupation. 34A linear form of the Freundlich expression can be presented as below: Where, KF is the Freundlich constants giving the adsorption capacity and nf is the Freundlich constants indicating favorability of the adsorption process.The plot of log qe versus log ce gave a straight line with slope of nf and intercept of log Kf.The slope of nf ranging between 0 and 1 is a measure of surface heterogeneity.The value of nf below one indicates a normal Langmuir isotherm while nf above one is indicative of cooperative adsorption. 35emkin isotherm 36 contains a factor that explicitly takes into account the adsorbent-adsorbate interactions.The adsorption is characterized by a uniform distribution of binding energies, up to some maximum binding energy.The Temkin isotherm is expressed as: Where, R T/ bT = B (in J/ mol), which is the Temkin constant related to heat of adsorption whereas A (in L/ g) is the equilibrium binding constant corresponding to the maximum binding energy.R (8.314 J/ mol K) is the universal gas constant and T (in K) is the absolute solution temperature.
In order to calculate the mean free energy value of adsorption, Dubinin-Radushkevich (DR) isotherm has also been applied.The DR equation can be defined by the following equation: 37  Where, β is the constant related to sorption energy (in mol 2 / J 2 ), Xm is the Dubinin-Radushkevich monolayer capacity (in mol / g), qe is the amount of adsorbed Cd 2+ (in mg/ g), ε is the Polanyi potential which can be obtained as follows: Where, ce is the equilibrium concentration of Cd 2+ (in mol / L), R gas constant (8.314J/mol K) and T the temperature (in K).By plotting ln qe versus ε 2 , β can be determined from the slope and Xm from the intercept.The mean free energy E (in kJ/ mol) of adsorption can be estimated by using β values in the following equation: All the correlation coefficients and the constants obtained from the four isotherm models applied for Cd 2+ adsorption are given in Table 3.In this work, the correlation obtained from the fitting of the Langmuir model was better than the fit using either the Freundlich or Temkin models.The result of the modeling therefore indicates monolayer sorption on a homogenous surface.Energy values ranging between 1 and 8 kJ/ mol indicate that the adsorption is due to physical interactions between adsorbent and adsorbate. 38The E values between 7.00-7.45kJ/ mol calculated from Equation 15indicate that physisorption due to weak Van der Waals forces plays a significant role in the adsorption process.

Effect of Temperature
The adsorption studies were performed over a range of temperatures from 25 to 45 °C.As seen in Figure 7, adsorption capacities of dye-attached cryogel disks decreased with increasing temperature.Except for the poly(HEMA)-CB cryogel disks, decrease at the adsorption capacity with increasing temperature is not statistically significant.This result can be explained by the exotermic nature of adsorption. 39,40Also, the decrease in adsorption capacity with increasing temperature could be derived from the increase in the average kinetic energy of Cd 2+ ions.This increased kinetic energy causes insufficient attractive forces between Cd 2+ ions and dyeattached p(HEMA) cryogel disks. 41ermodynamic Parameters Thermodynamic parameters such as Gibbs free energy change (∆G°), enthalpy change (∆H°), and entropy change (∆S°) were estimated for Cd 2+ adsorption.∆G° Where qe is the amount of Cd 2+ adsorbed at equilibrium (in mg/g), ce is the equilibrium Cd 2+ concentration (in mg/ L), Kd is the distribution coefficient.∆H° is obtained from Van't Hoff equation: Where T is the absolute temperature, R is gas constant (8.314J/mol K). ∆H° and ∆S° values were calculated from the slope and the intercept of the plot of ln Kd versus 1/ T. Calculated ∆H° and ∆S° values and also ∆G° values at different temperatures are presented in Table 4.
As seen in Table 4, adsorption presented unfavorable ∆G° values.On the other hand, all interactions presented exothermic enthalpy changes which suggest that Cd 2+ adsorption by dye attached cryogel disks is thermodynamically favorable.It is supported by the decreasing adsorption of Cd 2+ with the increase in temperature.The negative enthalpy values obtained with this study also shows that Cd 2+ adsorption is a physisorption process.However, the adsorption was accompanied by a decrease of entropy.The negative entropy indicates a decrease in the randomness in the system solid/solution interface during the adsorption process.Thus, the adsorption was not favorable at higher temperatures.

Desorption and Reusability Studies
To prepare and use low cost and reusable adsorbents has a growing interest because of economical limitations.Desorption of Cd 2+ from dye attached-p(HEMA) cryogel disks was performed in a batch system using different desorption agents with varying concentrations.Desorption ratio of NaCl, 2-mercaptoethanol and thiourea of different concentrations are given in Figure 8.
It can be concluded that 1.0 M NaCl is a suitable desorption agent.Desorption values for 1.0 M NaCl were over than 90% for poly(HEMA)-CB and poly(HEMA)-RG, but over than 95% for poly (HEMA)-CR.
In order to test reusability, adsorption-desorption cycle was repeated five times.Results showed that there was no remarkable decrease in the adsorption capacities of dye-attached cryogel disks and that these disks can be repeatedly used for Cd 2+ removal.

Cd 2+ Ions Removal from Human Plasma
Human plasma samples kept at -20°C thawed for 1 h at 37 °C and diluted with buffer solution of pH = 7.0 at various dilutions.Samples were overloaded with 100 mg/ L Cd 2+ solution and disks were used in removal experiments under the optimum conditions.As seen in Figure 9, the most convenient dilution ratio for Cd 2+ removal is 1:5.This result can be concluded that Cd 2+ removal increases with increasing dilution due to decrease of shielding effect of blood components.

CONCLUSION
Attractive features of polymeric cryogels, that have excessive applications, make them appropriate chromatographic materials for various areas of biotechnology.Synthesized supermacroporous cryogel disks were modified with different textile dyes which are commercially available, inexpensive and having several reactive groups and incorporation of these dye ligands increased the adsorption capacity of the cryogel.Maximum Cd 2+ adsorption of dye-attached cryogel disks was observed at pH = 7 and at ϑ = 25 °C and Cd 2+ adsorption capacity of poly(HEMA)-RG disks were higher than the others under these conditions.On the other hand, adsorption capacities of all of the dye-attached cryogel disks are quite well among the other adsorbents (Table 5).Langmuir model fitted better the experimental results, indicating monolayer adsorption on a homogenous surface.In addition, the adsorption energy, calculated by the Dubinin-Radushkevich equation, implied that the adsorption was dominated by the physical interactions between Cd 2+ and dye attached p(HEMA) cryogel disks.The negative ΔH value indicated that the adsorption of Cd 2+ onto the dye attached p(HEMA) cryogel disks was spontaneous.Cd 2+ adsorption capacity of dye attached p(HEMA) cryogel disks decreased only 14 % after five adsorption-desorption cycles.Moreover, dye-attached cryogel disks were used for Cd 2+ removal from overloaded human plasma samples.Cd 2+ removal efficiencies of poly-(HEMA)-CB; poly(HEMA)-RG and poly(HEMA)-CR cryogel disks from human plasma were determined as 47.18 %; 46.24 % and 43.41 %, respectively.These results suggest that dye-attached poly(HEMA) cryogel disks, that are non-hazardous, easy to use and prepare, can be used for environmental and therapeutic applications.

Figure 9 .
Figure 9. Cd 2+ removal ratio of dye-attached cryogel disks from human plasma at various dilutions.

Table 1 .
Swelling degree, S; porosity and porosity for macropores percentages for dye-attached cryogel disks

Table 3 .
Adsorption isotherm model constants and correlation coefficients for cadmium adsorption at 25°C d

Table 4 .
Thermodynamic parameters for cadmium adsorption at 25 °C