Adsorption Thermodynamics and Kinetics of Uridine 5

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Adsorption Thermodynamics and Kinetics of Uridine 50-Monophosphate on a Gel-Type Anion Exchange Resin Xiqun Zhou,† Jiansheng Fan,† Nan Li,† Wenbin Qian,† Xiaoqing Lin,† Jinglan Wu,† Jian Xiong,† Jianxin Bai,† and Hanjie Ying*,† †

State Key Laboratory of Materials-Oriented Chemical Engineering, College of Life Science and Pharmaceutical Engineering, Nanjing University of Technology, Nanjing 210009, Jiangsu Province, China ABSTRACT: The adsorption behavior of uridine 50 -monophosphate (UMP) on a gel-type anion-exchange resin SD3 at different temperatures was investigated by the batch method. The dissociation equilibrium of UMP in aqueous solution has been studied. Effects of solution pH, contact time, and initial concentrations of UMP on the adsorption have been discussed. Adsorption equilibrium data had been fitted to three different isotherms including Langmuir, Freundlich, and Sips isotherms, which have been widely used in biosorption processes, and the accuracy for all models has been evaluated by the residual-root-mean-square error. For adsorption kinetics, the adsorption rate of UMP on the resin was interpreted by the Fick model, first- and second-order kinetic models, and the adsorption process was found to be well represented by the Fick model. The solution diffusivities of UMP at different temperatures were estimated by the Wilke-Chang equation. The sorption process was found to be controlled by the intraparticle diffusion. The Fick model and Sips isotherm were chosen to simulate the concentration diffusion of UMP on SD3 resin during the adsorption process. An intraparticle two-dimensional profile of SD3 resin at 0, 1, 4, 15, and 25 min for 293.15, 303.15, and 313.15 K was shown with a satisfactory description of the adsorption process. The thermodynamic parameters such as Gibbs free energy and enthalpy and entropy changes were calculated, and the values indicated that the adsorption process of UMP on SD3 resin was spontaneous and endothermic.

1. INTRODUCTION Uridine 50 -monophosphate (UMP), one of the four nucleotides, is the integral part of genetic materials. With more and more investigations on the mechanisms of the nucleotides and their derivatives,1 nucleotides have been widely used in the food industry, medicine, and many other domains. As an important medical intermediate, uridine 50 -monophosphate -Na2, the main preservation form of UMP, can be used to synthesize various drugs, such as cytidine diphosphate choline (CDPC), uridine diphosphate glucose (UDPG), uridine triphosphate (UTP), uridine diphosphate (UDP), cytidine triphosphate (CTP), etc.2 UMP as a significant biochemical reagent is also a useful acridine for synthesizing pharmaceuticals. It participated in the biosynthesis of glucuronic acid anhydride which has the function of detoxification in the liver, and it was investigated for the treatment of hepatitis, coronary heart disease, rheumatoid arthritis, and leukopenia conscious symptoms.3 The nucleotides having the basic group and the phosphoric acid radical are the amphoteric compounds. In the solution with certain pH values, nucleotides exist in the forms of charged ions. Theoretically, the ion-exchange resin could be used to extract nucleotides.4 Since 1950, ion-exchange chromatography (IEC) as one of the most commonly used separation methods, has been widely applied in the separation and purification of ribonucleotides.57 For the separation of four nucleotides in the fermentation broth, cation-exchange resins are generally used to absorb three kinds of nucleotides including adenosine monophosphate (AMP), cytidine monophosphate (CMP), and guanosine monophosphate (GMP). Then, anion-exchange resins were used for the further separation of UMP.8 r 2011 American Chemical Society

For IEC, it is very important to design the adsorption processes effectively, which requires equilibrium adsorption data and kinetic data to establish the models for the prediction of the adsorption performance with a range of operating conditions.9 So far as concerned, there are a few reports about the adsorption of nucleotides but far from thoroughly studied. Ruan et al. had separated CTP from the mixture of CTP, CDP, and CMP and investigated the kinetics and thermodynamics of CTP on Duolite A-30 resin.10 Ying et al. had done some studies in the purification of CTP from CMP by an anion ion-exchange resin and established the optimal purification conditions.11 However, there is few literature which could offer a minute description about the thermodynamics and kinetics of adsorption of UMP on such a gel-type anion exchange resin. In order to provide theoretical basis for future engineering applications, the ion-exchange thermodynamics and kinetics of UMP on a geltype anion exchange resin (SD3 resin) have been investigated by batch experiments, and the concentration of UMP was measured by high performance liquid chromatography (HPLC). The dissociation equilibrium of UMP in aqueous solution has been studied. Effects of solution pH, contact time, and initial concentrations of UMP on the adsorption have also been discussed. Adsorption equilibrium data have been fitted to three different isotherms including Langmuir, Freundlich, and Sips isotherms, which have been widely used in biosorption processes and the accuracy for all Received: August 15, 2010 Accepted: June 16, 2011 Revised: June 1, 2011 Published: June 16, 2011 9270

dx.doi.org/10.1021/ie101721a | Ind. Eng. Chem. Res. 2011, 50, 9270–9279

Industrial & Engineering Chemistry Research

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models has been evaluated. For adsorption kinetics, the adsorption rate of UMP on the resin was investigated by the Fick model, first- and second-order kinetic models, and one of the models was chosen to predict the rate of adsorption. Further, the solution diffusivities of UMP at different temperatures were studied. The effects of the operating temperature and average particle radius on the adsorption rate were also investigated. The thermodynamic parameters such as Gibbs free energy and enthalpy and entropy changes were calculated.

2. MATERIALS AND METHODS

Figure 1. The SEM observation of SD3 resin.

2.1. Materials. The gel-type anion exchange resin (SD3 resin) used in this study was synthesized with the condensation polymerization method.12 The resin was sieved to the average particle radius of 3.35 and 5.57 104 m. The surface morphology of the SD3 resin was investigated by SEM (scanning electron microscope) (SEM, JSM-5900, JEOL, Tokyo, Japan) which is presented in Figure 1, and the particle diameter was measured by a particle size analyzer (Microtrac S3500, Microtrac Inc., USA). The structure and physical properties of the resin are listed in Table 1. Uridine 50 -monophosphate disodium (UMPNa2) with the purity >98% was obtained by recrystallization in our lab. Standard sample of UMPNa2 which was used for the establishment of calibration curve was purchased from Sigma Chemical Co. The mass of UMP (mUMP) in this study was calculated by the following equation mUMPNa2 MUMP ð1Þ mUMP ¼ MUMPNa2

(v/v) phosphoric acid (adjusted pH to 6.60 with triethylamine). The column temperature was 298.15 K, and flow rate was 1.0 mL/min. A calibration curve of the standard UMP was first measured with different known concentrations. A good linearity was obtained with a correlation coefficient R of 0.9995. The concentrations of UMP in this study were calculated based on the calibration curve, and the determination error was estimated to be less than 0.5%.

where MUMP and MUMPNa2 are the molecular weight of UMP and UMPNa2, respectively, and mUMPNa2 is the mass of UMPNa2. Other chemicals used in this study were of analytical grade. 2.2. Equilibrium Experiments. The ion-exchange equilibrium between UMP and SD3 resin at 293.15 K, 303.15, and 313.15 K had been studied in a constant temperature shaker (type HYG-II a, Shanghai Xinrui Automatic Instrument Co., Ltd.). Solutions of UMP (0.612 g/L) were prepared by dilution of the sample in deionized water. Each test was held in the thermostatic shaker at least 4 h to make sure that the adsorption equilibrium was achieved. The initial and final concentrations of UMP were analyzed by high performance liquid chromatography, and the adsorption capacity was determined by the eq 213 qe ¼ ðC0 Ce ÞV =m

ð2Þ

All experiments were repeated three times to get a mean value, and the relative error was less than 3%. 2.3. Batch Kinetic Experiments. The UMP solutions (100 mL) were put in the vessel and left in the water bath with continuous stirring at 200 rpm to reach thermal equilibrium in different systems, respectively. Then, the resin (2.0 g) was quickly poured into the vessel. The concentration of UMP was measured at different time by sampling of a 0.05 mL solution. The experiments were carried out at different temperatures in a thermostatic water bath (type DC-2030, Shanghai Sunny Hengping Scientific Instrument Co., Ltd.) which maintained the temperature at a certain value within (0.05 K by circulating water. Each test was repeated for 3 times, and the relative error was less than 3%. 2.4. Analysis of the Concentration of UMP. The concentrations of UMP were analyzed by high performance liquid chromatography (1200 Series, Agilent1100, USA) with the Aminex HPX-87H ion-exclusion column (7.8 300 mm, 9 μm, Bio-Rad Laboratories, Inc., USA) at 260 nm. The mobile phase was 6%

3. RESULTS AND DISCUSSION 3.1. Adsorption Equilibrium. 3.1.1. Effect of Solution pH. Since SD3 is a kind of weak basic resin which has a certain application range, the pH value of solution was set from 2 to 8, in order to dissociate as many counterions as possible and make sure that the resin exerts its maximal adsorption capacity. Within the range of pH values, the dissociation of phosphate radical of UMP takes place with its pKa1 and pKa2 of 1.0 and 6.4. The dissociation reaction of UMP could be represented as follows pKa1 ¼ 1:0

UMP rsf UMP + H+ pKa2 ¼ 6:4

UMP rsf UMP2 + H+

ð3Þ ð4Þ

The concentration of UMP in the aqueous solution is the sum of the concentration of three different forms C ¼ ½UMP + ½UMP + ½UMP2

ð5Þ

where [UMP], [UMP], and [UMP2-] are the concentrations of three forms, respectively. According to the above three equations, the distribution coefficients δ for three forms could be represented as follows ½UMP ½H + ½H + ¼ 2 + C ½H + KR1 ½H + + KR1 KR2 ½UMP KR1 ½H + ¼ δUMP ¼ C ½H + 2 + KR1 ½H + + KR1 KR2 2 ½UMP KR1 KR2 ¼ δUMP2 ¼ 2 + C ½H + KR1 ½H + + KR1 KR2

δUMP ¼

ð6Þ

The change of the distribution coefficients with pH for three forms, UMP, UMP, and UMP2, was calculated by eq 5 and shown in Figure 2, and the effect of the solution pH on the adsorption capacity for UMP on SD3 resin at 293.15 K was shown in Figure 3. It could be concluded that the dissociation of UMP has an apparent influence on the adsorption capacity. Overall, the adsorption capacity increases with the pH value. The effect of pH can be divided into two parts to discuss. The first part is when the pH value is from 2 to 4. In this part, the main component of the solute in the solution is UMP. The distribution coefficient of 9271

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Table 1. Structure and Physical-Chemical Character of Resin SD3 matrix composition

matrix active group

total exchange capacity >(eq/L)

water retention capacity (%)

swelling degree (%)

epoxy chloropropane

-NH2dNHtN

9.0

6070

e30

Figure 2. Theoretical distribution coefficients of the different forms of UMP in the solution.

Figure 3. The effect of solution pH value for adsorption capacity of UMP on SD3 resin at 293.15 K. Condition: mass of SD3 resin, 2 g; solution volume, 100 mL; initial concentration of UMP: 10 g/L; temperature, 293.15 K.

UMP increases with the pH value and reaches the maximum value when the pH is between 3 and 4. As a result, the adsorption capacity increases from pH 2 to 3, and the change between 3 and 4 is not obvious. The second part is from pH 4 to 8. In this part, the distribution coefficient of UMP decreases by the removal of the hydrogen ion to form UMP2-. The concentration of UMP2- is on the increase as the pH value goes up, resulting in the larger adsorption capacity. When the pH value is at 8.12, the concentration of UMP2- reaches the maximum value and so does the adsorption capacity. This is due to the stronger interaction between bivalent UMP2- and resin particle than that between monovalent radical UMP and the resin.

Figure 4. Effect of temperature on the adsorption of UMP on SD3 resin. Condition: mass of SD3 resin, 2 g; solution volume, 100 mL; temperature, 293.15 K; initial concentration of UMP: O, 10 g/L; (, 8 g/L; Δ, 5 g/L.

In this study, UMPNa2, the main preservation form of UMP, was used to prepared the solution with the pH at about 8.1 ((0.1), just as the optimum condition. So the solution pH was not adjusted by hydrochloric acid or sodium hydroxide in the subsequent sections. 3.1.2. Effect of Contact Time and Initial Concentrations of UMP on the Adsorption. The equilibrium adsorption time of UMP on SD3 resin was investigated within 90 min at 293.15 K with three initial concentrations (5 g/L, 8 g/L, and 10 g/L). As shown in Figure 4, it is evident that the adsorption process proceeded in two distinct phases. At the beginning of the adsorption, the rate of uptake was high due to the large quantities of vacant adsorption sites. Subsequently, as most of the adsorption sites were occupied, the rate of uptake became slow. As the initial concentration increased from 5 to 10 g/L, the adsorption capacity increased, but the time of transitional phase was also increased. Based on these experimental results, the equilibrium time and the initial concentration of UMP for the following experiments were fixed at 60 min and 10 g/L, respectively. 3.2. Adsorption Isotherms. Adsorption isotherm, as the basic requirements for the design of adsorption process, is the relationship between the adsorption capacity and the concentrations of absorbate in the equilibrium solution at constant temperature. It is of great significance to obtain the parameters from different isotherm models which could provide different information to disclose the mechanism in adsorption process and the properties of adsorbents and adsorbates.14 In this work, the equilibrium experimental data of adsorption isotherm collected at different concentrations of UMP and various temperatures were fitted with three commonly used adsorption models: Langmuir, Freundlich, and Sips isotherm models. The residual root-mean-square error (RMSE) was used to determine the best fitting isotherm. 9272



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Table 2. Isotherm Constants of Each Isotherm Model for UMP on SD3 Resin at Different Temperatures

be encountered in biosorption process.22 The equation is often expressed as follows

temperature (K) isotherm models Langmuir

Freundlich

SIPS

293.15

303.15

313.15

KL

0.3384

0.2112

0.1488

qm

0.3787

0.4031

0.4455

RL 102RMSE

0.8864 0.5733

0.9215 1.2030

0.9378 0.9635

KF

0.1036

0.0798

0.0648

NF

0.4960

0.5668

0.6502

102RMSE

1.4442

2.1104

1.5271

KS

0.3268

0.2357

0.1798

qm

0.3917

0.2976

0.3181

nS

0.9641

1.4743

1.3450

102RMSE

0.5656

0.7213

0.7423

Langmuir (eq 7) and Freundlich isotherm (eq 8)15 were chosen to interpret the experimental data. The Langmuir equation is based on the assumptions that the adsorption is monolayer and the adsorption sites are distributed uniformly on the surface of adsorbent and that there is not an interacting force among the molecules adsorbed.16 For Freundlich isotherm, it is assumed that the adsorption energy of the adsorbate binding to the adsorbent depends on the availability of adsorption sites. The two isotherms are often expressed as the following equations qe ¼

KL qm Ce 1 + KL C e

qe ¼ KF Cne F

ð7Þ

ð8Þ

The coefficient of Freundlich isotherm, nF, reflects the adsorption affinity of the adsorbent. When the value of nF is below 1, it means that the isotherm is normal, while the value above 1 indicated the cooperative adsorption. In this study the value of nF was between 0 and 1, which was a measure of adsorption intensity or surface heterogeneity, and as the value got closer to 0, the adsorption became more heterogeneous.17 However, lacking a fundamental thermodynamic basis is the limitation of Freundlich isotherm which does not abide by the Henry’s law at vanishing concentrations.18 For Langmuir isotherm, the trend could be indicated by a dimensionless constant separation factor RL which was defined by eq 9. All coefficients for the two isotherms were calculated and shown in Table 2 RL ¼

1 1 + KL q m

ð9Þ

The values of RL which lie between 0 and 1 (Table 2) indicated that the adsorption process of UMP on resin SD3 is favorable.19 The value of KL decreased with the increase of temperature, which meant that low temperature was more favorable for the adsorption process.20 Sips isotherm21 which was also called LangmuirFreundlich (L-F) isotherm (eq 10) has been widely employed as an empirical isotherm. It is derived from the equilibrium of a chemical process and has been improved in accounting for the limitations that may

qe ¼

qm KS Ce ns 1 + K S Ce n s

ð10Þ

where ns and KS are the fitting coefficients of Sips isotherm, thereinto, ns is the dissociation parameter. When the value of ns equals 1, the Sips isotherm reduces to Langmuir isotherm. Experimental adsorption equilibrium of UMP on resin SD3 at 293.15, 303.15, and 313.15 K and Curve Fit to three isotherm models have been shown in Figure 5. As can be seen, the adsorption equilibrium obtained for SD3 resin is quite favorable, and the adsorption capacity of UMP increased slightly when the temperature decreased. In order to judge the fitting of each isotherm to the experimental data better, the residual-root-meansquare error (RMSE) was used. The smaller values of RMSE indicated the better fitting for the model with the experimental data.23 The RMSE is defined as the following equation sffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi 1 N RMSE ¼ ðqe qe, cal Þ2 n 2 i¼1

∑

ð11Þ

According to the data listed in Table 2, it was obvious that the Langmuir and the Sips isotherms both had a good fitting with the experimental data at 293.15 K since the values of 102RMSE were both less than 0.6. While at high temperatures, Sips isotherm model with the lowest values of 102RMSE showed a better fitting than others. Although the Langmuir isotherm could give a good description of adsorption for UMP at 293.15 K, the Sips isotherm could well predict the adsorption process at a wider temperature range. Therefore, the Sips isotherm was chosen to interpret the equilibrium for the concentration of UMP on SD3 resin in the following investigation for adsorption kinetics of UMP. 3.2. Adsorption Kinetics. The adsorption kinetics of UMP on the gel-type anion-exchange resin was investigated at 293.15 K, 303.15, and 313.15 K, and the adsorption rate was investigated with Fick model as well as kinetic model. The adsorption process could be interpreted with three steps: external mass transport, intraparticle diffusion, and adsorption rate on the active site at the surface of adsorbent, while the rate of adsorption on an active site was assumed instantaneous.24 3.2.1. Fick Model. In Figure 1, it is clear that the SD3 resin has a whole, spherical shape and the dispersion of the resin particle is quite homogeneous, and these accord with the assumption of the Fick model. In the Fick model, the assumptions were made to interpret the ion exchange adsorption: (1) the resin has been treated as a quasi-homogeneous particle; (2) the adsorption rate was controlled by intraparticle diffusion; and (3) the effects of pressure gradients and activity are negligible25 The Fick equation is written as N ¼ -De

∂q ∂r

ð12Þ

The mass balance for a sphere is used ∂q 1 ∂ ¼ 2 ðr 2 NÞ ∂t r ∂r 9273

ð13Þ



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Figure 5. Langmuir (a), Freundlich (b), and Sips (c) isotherm models for adsorption equilibrium of UMP on SD3 resin at different temperature: 2, T = 293.15 K; b, T = 303.15 K; 9, T = 313.15 K; solid line, isotherm equation; point, experimental value.

Equation 12 is combined with eq 13 to give ∂q ∂2 q 2 ∂q ¼ De + ∂t ∂r 2 r ∂r

! ð14Þ

where the average mass adsorbed by per unit mass resin can be calculated by Z 3 RP 2 qave ¼ 3 qr dr ð16Þ Rp 0 The boundary conditions can be written as

The initial conditions are given as

t ¼ 0,

9 8 > = < q ¼ qave ¼ 0; > > : C ¼ C0

> ;

∂q ¼0 ∂r r ¼ Rp , q ¼ qe r ¼ 0,

ð15Þ

ð17Þ ð18Þ

In this adsorption process, the rate on the active site was assumed instantaneous, so that the concentration equilibrium 9274



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between liquid and solid phase could be represented by the adsorption isotherm, and in this study, the Sips isotherm which has a best fitting result with the experimental data was chosen to interpret the equilibrium between Ce and qe with eq 10. The effective diffusion coefficient De was estimated by the comparison of residual concentration in the solution between experimental data Ce and the predicted data Cpred, and the best value could be obtained by the following function Minimum ¼

N

∑

i¼1

Ce Cpred Ce

∑

2 ð19Þ

For comparing with the diffusion of UMP in the particle, the solution diffusivity of UMP was estimated by the Wilke-Chang equation26 which could be given as the following equation DU, w

fitting of each model. In this study, qe was not treated as an adjustable parameter since the values of qe and Ce can be calculated by solving the adsorption equation (eq 10) and the mass balance equation (eq 2). The average absolute percentage deviation (%D) could be presented as follows 1 N C Ccal ð25Þ %D ¼ 100% N i ¼ 1 Ccal

pffiffiffiffiffiffiffiffiffiffiffiffi ψ w Mw ¼ 7:4 10 T ηw, T VU0:6 8

ð20Þ

where Ψw is a constant which accounts for solutesolvent interactions, and the value is 2.6 for water; ηw,T is the viscosity of water at different temperature (cP); MW is the molecular weight of the solvent (g); VU is the molar volume (cm3/mole) of the liquid solute at its normal boiling point, and the value can be calculated from group contributions. The errors for eq 20 can be predicted to be less than 10% when water is the solvent.26 The diffusion coefficient of UMP in water (DU,w) and the ratio of De/DU,w were all calculated and listed in Table 3. From the results, it is shown that the diffusion rate in the resin particle is slower than in the solution. The intraparticle diffusion could therefore be assumed to be the rate-limiting step. 3.2.2. Pseudo First- and Second-Order Kinetic Models. Within the scope of the literature review, the pseudo first- and secondorder models27 have been widely used to describe biosorption processes under nonequilibrium conditions. However, the adsorption rate in the kinetic models is only represented as the rate of a chemical reaction without a further theoretical description of biosorption equilibrium.28 The pseudo first- and second-order models could be expressed as the following equations, respectively q ¼ qe ð1 ek1 t Þ

ð21Þ

q ¼ qe ð1 ek2 t Þ2

ð22Þ

Equations 21 and eq 22 could be combined with eq 2 to give C ¼ Ce + ðC0 Ce Þek1 t

ð23Þ

2 V k2 ðC0 Ce Þ t m m C ¼ C0 V V 1 + k2 ðC0 Ce Þ t m

ð24Þ

Concentration decay data of UMP at 293.15, 303.15, and 313.15 K and Curve fit to three models have been shown in Figure 6. The values of k1 and k2 for each model which have been listed in Table 3 were calculated by fitting the model to the experimental data, and the average absolute is used to judge the

According to Figure 6, it was shown that the Fick model fitted best with the experimental data at different temperature. Besides, the average absolute percentage deviations (%D) of the Fick model are lower than the other two kinetic models (Table 3), indicating that the Fick model is applicable to describe the adsorption process of UMP on the SD3 resin. 3.3. The Influence Factors of the Adsorption Rate for UMP on SD3 Resin. 3.3.1. Effect of Temperature on the Adsorption Rate. In Table 3, it could be concluded that the effective diffusivity (De) was increased with the temperature, which meant that as the temperature went up, the diffusion of UMP in the resin particles had been intensified. The concentration decay curve for UMP adsorption on SD3 resin at different temperature was shown in Figure 6. It could be obtained that temperature played an important role in the adsorption of UMP. As the increase of temperature, the rate of initial uptake became higher, and the time to achieve equilibrium was shorter, 15, 25, and 35 min for 313.15, 303.15, and 293.15 K, respectively. When the adsorption achieved equilibrium, the adsorption capacity at high temperature (0.258 g/g resin at 313.15 K) was smaller than that at low temperature (0.286 g/g resin at 293.15 K). The results might be due to the reason that high temperature intensified the movement of molecules which lead to the high rate at the initial adsorption process, but as the adsorption proceeded, more and more molecules were adsorbed which hindered the subsequent adsorption.29 It could be further illustrated in Figure 7, which is the intraparticle two-dimensional profile of SD3 resin during the adsorption process at 0, 1, 4, 15, and 25 min for 293.15, 303.15, and 313.15 K simulated with Matlab using the Fick model. The phenomenon may attribute to another reason that the quickly adsorption rate in the initial phase of adsorption could lead to the decline in the impulsive force which was caused by the concentration change between resin and solution. 3.3.2. Effect of the Average Particle Radius on the Adsorption Rate. The effect of the average particle radius on the adsorption rate was investigated by two kinds of SD3 resin with different particle size (Rp = 3.35 104 m and 5.57 104 m), and the rest of the experimental conditions were kept the same. The experimental data and the concentration decay curves fitted with the Fick model are shown in Figure 8. It was noticed that the rate of adsorption equilibrium became faster when the particle radius of resin was diminished. This result might be due to that the intraparticle diffusion is the rate-limiting step. When the particle radius of absorbent was smaller, the specific surface area per unit mass of absorbent became larger, leading to the larger contact possibility of absorbent and absorbate. Meanwhile, the smaller size of the resin shortened the diffusion distance traveled by absorbate in the absorbent. Both of the two characters contributed to the faster adsorption rate of the smaller resin. 3.4. Adsorption Thermodynamics. The thermodynamic parameters for the adsorption including the free energy change ΔG°, the enthalpy change ΔH°, and the entropy change ΔS° had 9275



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Table 3. Parameters of the Fick Model and the Kinetic Model at Different Temperatures first-order

Fick model

second-order

T (K)

De (1010 m2 s1)

DU (1010 m2 s1)

De/DU

%D

k1

%D

k2

%D

293.15

1.64

5.84

0.2808

1.59

0.1300

2.27

0.6331

2.71

303.15

1.88

7.56

0.2487

2.56

0.1336

3.05

0.9439

4.47

313.15

3.68

9.53

0.3861

1.73

0.2235

1.83

1.8100

5.32

Figure 6. The concentration decay curve for UMP adsorption on SD3 resin at different temperature. The points represent the experimental data, and the lines represent the model predictions. Initial concentration of UMP: 10 g/L; mass of SD3 resin: 2 g; solution volume: 100 mL.

been calculated, respectively. The Gibbs free energy change ΔG° was determined by the following equation ΔGo ¼ RTln K0

ð26Þ

where the thermodynamic equilibrium constant K0 was calculated from the intercept of ln(Ce/Q e) versus Ce plot, in which Q e (g/g resin) is the adsorption capacity on solid at equilibrium, and Ce (g/L) is the equilibrium concentration.30 The values of K0 and 9276



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Figure 7. Intraparticle two-dimensional profile of SD3 resin during the adsorption process. Condition: mass of SD3 resin, 2 g; solution volume, 100 mL; initial concentration of UMP: 10 g/L; the radius particle of SD3 resin, 5.57 104 m.

Table 4. Thermodynamic Parameters for Adsorption of UMP on SD3 Resin at Different Temperatures T (K)

K0

ΔG° (kJ mol1) ΔH° (kJ mol1) ΔS° (J mol1 K1)

293.15 2.4374

2.1714

303.15 2.6798

2.4845

313.15 2.8411

2.7186

5.8617

27.44

ΔG° were listed in Table 4. The negative value of ΔG° indicated that the adsorption for UMP on the resin is spontaneous, and the absolute values less than 20 kJ/mol indicated that the adsorption process was mainly physical. The enthalpy change ΔH° and the entropy change ΔS° were obtained from the slope and intercept of the plot of ln K0 against 1/T according to the Van’t Hoff equation31 ln K0 ¼

ΔH o 1 ΔSo + R T R

ð27Þ

The positive value of ΔH° (Table 4) showed that the adsorption process for UMP on the resin SD3 was endothermic. On the other hand, the value of ΔS° (Table 4) was calculated to be positive, which indicated that irreversible adsorption increased randomness at the solid-solution interface and that the ion replacement reactions also occurred.32

Figure 8. Effect of the radius particle on the UMP concentration decay curves. The points represent the experimental data, and the lines represent the Fick model predictions. Condition: mass of SD3 resin, 2 g; solution volume, 100 mL; initial concentration of UMP, 10 g/L; temperature, 293.15 K, 0, Rp = 3.35 104 m;b, Rp = 5.57 104 m.

4. CONCLUSION In this work, the characters of adsorption of uridine 50 monophosphate (UMP) on a gel-type anion exchange resin were investigated with the batch method. In the study of adsorption isotherm of UMP, three different adsorption isotherm models were employed to fit the experimental data. The result of the nonlinear regression analysis showed that Langmuir and Sips isotherms both have a good fitting with the

experimental data at 293.15 K, and the latter was found to be better within a wide range of temperatures. The dissociation equilibrium of UMP in aqueous solution and the effect of solution pH on the adsorption have been discussed. The optimum pH is observed at about 8.1 ((0.1). For adsorption kinetics, the Fick model and two other kinetic models were used to fit the experimental data. The Fick model was found to describe the experimental data best than other 9277


Industrial & Engineering Chemistry Research models. The diffusivity of UMP both in the solution and the resin had been studied, and the intraparticle diffusion was found to be the rate-limiting step. After the investigation of both the temperature and the particle radius, it could be concluded that the adsorption rate could be accelerated by using smaller size resin at higher temperature. The thermodynamic parameters such as Gibbs free energy and enthalpy and entropy changes were calculated, and the values indicated that the adsorption process of UMP on SD3 resin was spontaneous and endothermic.

’ AUTHOR INFORMATION Corresponding Author

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qave r R RL Rp ΔS° t T V VU

*Fax: +86-25-86990001. E-mail: [email protected].

’ ACKNOWLEDGMENT This work was supported by National Outstanding Youth Foundation of China (Grant No.: 21025625), Program for Changjiang Scholars and Innovative Research Team in University, Project Funded by the Priority Academic Program Development of Jiangsu Higher Education Institutions (PAPD) and the Major Basic R&D Program of China (2007CB714305). ’ NOMENCLATURE C concentration of UMP in the solution (g/L) initial concentration of UMP in the solution (g/L) C0 residual concentration of UMP in the solution at Ce equilibrium (g/L) residual concentration of UMP predicted by diffusion Cpred model (g/L) the effective diffusion coefficient (m2/s) De the diffusion coefficient of UMP in water (m2/s) DU,w ΔG° the free energy change (kJ/mol) ΔH° the enthalpy change (kJ/mol) Langmuir isotherm constant (L/mg) KL Freundlich isotherm constant (L1/nF/(mg1/nF1 g)) KF Sips isotherm constant KS the rate constant of pseudo first-order kinetic model k1 (min1) the rate constant of pseudo second-order kinetic model k2 (min1) dissociation constant of UMP in aqueous solution Ka (mol/L) m mass of the adsorbent (g) mUMP mass of UMP (g) mUMPNa2 mass of UMPNa2 (g) the molecular weight of the solvent (g) Mw MUMP the molecular weight of UMP (g/mol) N the number of the experimental data the coefficient of Freundlich isotherm nF the dissociation parameter of Sips isotherm ns the viscosity of water at different temperature (cP) ηw,T a constant which accounts for solutesolvent interΨw actions q mass of UMP adsorbed per unit mass of adsorbent (g/g wet resin) mass of UMP adsorbed per unit mass of adsorbent at qe equilibrium (g/g wet resin) calculated value from the isotherm (g/g wet resin) qe,cal maximum mass of UMP adsorbed per unit mass of qm adsorbent (g/g wet resin)

average mass of UMP adsorbed per unit mass of adsorbent (g/g wet resin) distance in radial direction of a resin particle (cm) the gas constant (J/(mol K)) dimensionless constant separation factor of Langmuir isotherm radius of a resin particle (m) the entropy change (J/(mol K)) time (min or s) temperature (K) solution volume (L) the molar volume of UMP at its normal boiling point (m3/mol)

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