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Equilibrium and kinetic modeling of adsorption of urea nitrogen onto chitosan coated dialdehyde cellulose
Process Biochemistry 40 (2005) 3218–3224 www.elsevier.com/locate/procbio
Equilibrium and kinetic modeling of adsorption of urea nitrogen onto chitosan coated dialdehyde cellulose Zu Pei Liang a,b, Ya Qing Feng a,*, Shu Xian Meng a, Zhi Yan Liang a b
a Department of Fine Chemicals, Tianjin University, Tianjin, China Department of Applied Chemistry, Qingdao University of Science and Technology, Qingdao, China
Received 30 April 2004; received in revised form 24 June 2004; accepted 2 March 2005
Abstract The adsorption of urea nitrogen onto chitosan coated dialdehyde cellulose (CDAC) under urease catalysis was studied in a batch system. The equilibrium isotherm of urea nitrogen adsorption onto CDACs with different degree of oxidation (DO) and the kinetics of adsorption with respect to the DO of CDAC (41, 60 and 74%), the initial urea nitrogen concentration (544, 635 and 785 mg L1), temperature (37, 42 and 47 8C) and CDAC/urease weight ratio (100:3, 100:2 and 100:1) were investigated. Langmuir and Freundlich adsorption models were applied to describe the experimental isotherm and isotherm constants. Equilibrium data fitted very well to the Langmuir model in the entire saturation concentration range (152–756 mg L1). The maximum monolayer adsorption capacities obtained from the Langmuir model are 50.0, 60.6 and 69.2 mg g1 for the CDACs with DO 41, 60 and 74%, respectively, at 37 8C and CDAC/urease weight ratio 50:1. Pseudo first-order and second-order kinetic models were used to describe the kinetic data and the rate constants were evaluated. The experimental data fitted well to the second-order kinetic model, which indicates that chemical adsorption is the rate-limiting step, instead of mass transfer. The DO of CDAC, initial urea nitrogen concentration, temperature and CDAC/urease weight ratio affected the adsorption capacity significantly. The activation energy is 11.64 kJ mol1 for the adsorption of the urea nitrogen onto the CDAC under urease catalysation at DO 74% of CDAC, initial urea nitrogen concentration 635 mg L1 and CDAC/urease weight ratio 50:1. # 2005 Elsevier Ltd. All rights reserved. Keywords: Adsorption; Equilibrium; Kinetic; Urea nitrogen; Chitosan coated dialdehyde cellulose; Urease
1. Introduction Urea is one of the major metabolic end products and the removal of its excess is a major problem for patients suffering from chronic renal failure (CRF). Haemodialysis is the conventional treatment for CRF. Approximately, half a million patients worldwide are being supported by haemodialysis [1]. Although haemodialysis has been effective for the treatment of CRF, there are a number of related problems. It is bulky, cumbersome, expensive, timeconsuming, and difficult to handle, limiting the mobility of * Corresponding author. Tel.: +86 22 27404769; fax: +86 22 27401824. E-mail address: [email protected] (Y.Q. Feng). 1359-5113/$ – see front matter # 2005 Elsevier Ltd. All rights reserved. doi:10.1016/j.procbio.2005.03.041
the patient [2] and some uraemic patients are reported to have suffered from the so-called haemodialysis-resistant uraemic syndrome [3]. Oral urea nitrogen adsorbents could potentially delay the onset of haemodialysis therapy in patients who still have some renal function and reduce haemodialysis treatment times. Active charcoal [4], oxystarch [5], oxycellulose [6] and zirconium phosphate [7] have been used for the removal of urea through oral administration. In this work, a complex type urea nitrogen adsorbent, which consisted of chitosan coated dialdehyde cellulose (CDAC) and urease, was prepared. Urea was hydrolysed to ammonia and carbon dioxide by urease catalysis, and the ammonia was chemically adsorbed onto CDAC through a
Z.P. Liang et al. / Process Biochemistry 40 (2005) 3218–3224
Schiff reaction between the ammonia and the aldehyde group of CDAC. The aim of this paper was to study the adsorption behaviour of urea nitrogen using CDAC–urease complex type adsorbent. Equations were used to fit the equilibrium isotherm. The adsorption rates were measured and determined quantitatively in correlation with initial urea nitrogen concentration, temperature, degree of oxidation (DO) of CDAC and CDAC/urease weight ratio. These results will be useful for further applications in urea nitrogen removal by oral administration.
2. Materials and experimental 2.1. Chemicals a-Cellulose (degree of polymerisation, 500 and molecular weight, 243,000) was supplied by Huludao Chemical Co., Liaoning, China. Chitosan (degree of deacetylation, 88% and molecular weight, 210,000) was supplied by Hengsheng Biochemical Co., Qingdao, China. Sodium metaperiodate was obtained from Tianhe Chemical Reagent Co., Tianjin, China. Urease (EC 3.5.1.5, from jackbean) was obtained from BDH Chemicals Ltd., Poole, England. Urea was obtained from Kewei Co., Tianjin, China. 2.2. Preparation of 2,3-dialdehyde cellulose by periodate oxidation Periodate oxidation of cellulose leads to breaking of C2–C3 bond of the glucose and formation of 2,3dialdehyde cellulose (DAC) [8]. Preparation of DAC and determination of its DO followed the methods previous reported [9]. Sodium metaperiodate (21.4–35.6 g) was dissolved in 400 mL of deionized water and the solution was adjusted to pH 2–2.5 by concentrated H2SO4, and 22.5 g of a-cellulose powder was added to the solution. The mixture was stirred at 43 8C for 3.5 h, and the products were then filtered and washed with H2SO4 solution (pH 4.5–5.5) till all iodic compounds were removed. The resulting DAC was used for the next step without drying. The DACs with the different DO 41, 60 and 74% based on glucose monomer units were obtained by adjusting the quantity of oxidizing agents.
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resulting CDAC was filtered and washed with deionized water, and then dried at 40 8C. 2.4. Batch equilibrium The urea was dissolved in deionized water to the required urea nitrogen concentration. In experiments of equilibrium adsorption isotherm, the CDAC (0.5 g), urease (10 mg) and urea nitrogen solution (75 mL) were placed in a 150 mL Erlenmeyer flask and shaken for 24 h by a shaker in a water bath to control temperature at 37 8C. The supernatant liquid 2 mL was taken out and placed in a flask containing desired amount of urease and residual urea were hydrolysed to ammonia and carbon dioxide by urease catalysing for 12 h. The concentration of unadsorbed urea nitrogen was analyzed by measuring the amount of ammonia liberated from the urease-catalysed hydrolysis of urea. The amount of adsorption at equilibrium qe (mg g1) was calculated as follows: qe ¼
ðC0 Ce Þ V W
(1)
where C0 and Ce (mg L1) are the initial and equilibrium urea nitrogen concentration, respectively, V (L) the volume of the solution and W (g) is the weight of CDAC used. 2.5. Batch kinetic In experiments of batch kinetic adsorption, CDAC (0.5 g), urease (10 mg) and urea nitrogen solution (75 mL) were placed in a 150 mL Erlenmeyer flask and shaken by a shaker in a water bath to control temperature. Every other period of time, the urea nitrogen concentration was determined as above-mentioned measuring method. The experimental parameters included DO of CDAC, initial urea nitrogen concentration, temperature and CDAC/urease weight ratio.
2.3. Preparation of chitosan coated dialdehyde cellulose Chitosan (0.4 g) was dissolved in 1% (v/v) aqueous acetic acid solution (40 mL) with stirring for 1 h at 25 8C and then diluted to 200 mL with deionized water. The abovementioned DAC was ground into paste in 100 mL deionized water and then immersed in the chitosan solution with constant stirring for 1 h at 25 8C. The solution was adjusted to about pH 7.0–7.5 with 2% (w/v) sodium hydroxide solution, and then was gelized for 0.5 h at 25 8C. The
Fig. 1. Equilibrium adsorption of urea nitrogen onto CDACs with different DO.
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Z.P. Liang et al. / Process Biochemistry 40 (2005) 3218–3224 Table 1 Langmuir and Freundlich isotherm constants for CDACs with different DO at 37 8C and CDAC/urease weight ratio 50:1
3. Results and discussion 3.1. Equilibrium adsorption
DO (%) Langmuir
Fig. 1 shows the equilibrium adsorption of urea nitrogen onto the CDACs with three different DO at 37 8C, CDAC/ urease ratio 50:1. It is seen that the saturation adsorption capacity increases with increasing DO of CDAC. This might suggest that the adsorption took place mainly by a Schiff reaction between ammonia and aldehyde group of CDAC. 3.1.1. Langmuir isotherm The equilibrium adsorption isotherm is fundamental in describing the interactive behaviour between solutes and adsorbent and is important in the design of adsorption system. The widely used Langmuir isotherm [10] has found successful application in many real adsorption processes and expressed as: qe ¼
QbCe 1 þ bCe
(2)
which can be rearranged to obtain a linear form Ce 1 Ce þ ¼ qe Qb Q
(3)
Freundlich
Q (mg g1) b (L mg1) R2 41 60 74
50.00 60.57 69.20
6.584E3 8.408E3 9.309E3
Qf (mg g1) n
0.9974 6.88 0.9986 8.76 0.9992 10.93
R2
3.653 0.9762 3.626 0.9342 3.766 0.9497
and intercepts of different straight lines representing CDACs with different DO. Table 1 lists the calculated results. The fits are good for all three DACs with different DO under the urea nitrogen concentration ranges studied (correlation coefficient, R2 > 0.9974). Table 1 indicates that the computed maximum monolayer capacity Q of urea nitrogen onto the CDACs with DO 41, 60 and 74% are 50.0, 60.6 and 69.2 mg g1, respectively. Similar trends are observed for b, indicating that the affinity of the CDAC for urea nitrogen increases as the DO of CDAC increases. 3.1.2. Freundlich isotherm The Freundlich isotherm [11], used for isothermal adsorption, is a special case for heterogeneous surface energy in which the energy term in the Langmuir equation varies as a function of surface coverage strictly due to variation of the adsorption. The Freundlich equation is given as:
where Q (mg g1) is the maximum amount of the urea nitrogen per unit weight of CDAC to form a complete monolayer coverage on the surface bound at high equilibrium urea nitrogen concentration Ce and b (L mg1) is the Langmuir constant related to the affinity of binding sites. Q represents a practical limiting adsorption capacity when the surface is fully covered with urea and ammonia molecules and assist in the comparison of adsorption performance. A linearized plot of Ce/qe versus Ce is obtained from the model shown in Fig. 2. Q and b are computed from the slopes
where Qf and n are indicators of adsorption capacity and adsorption intensity, respectively. Qf and n can be determined from the linear plot of ln qe versus ln Ce (Fig. 3).
Fig. 2. Langmuir plot for CDAC with different DO.
Fig. 3. Freundlich plot for CDAC with different DO.
qe ¼ Qf Ce1=n
(4)
which can be rearranged to obtain a linear form ln qe ¼ lnQf þ
1 ln Ce n
(5)
Z.P. Liang et al. / Process Biochemistry 40 (2005) 3218–3224
Fig. 4. Adsorption kinetics of urea nitrogen onto CDACs with different DO.
Table 1 lists the calculated results. The magnitude of the exponent 1/n gives an indication of the favorability of adsorption. Values, n > 1 represent favourable adsorption condition [12]. From Table 1, the exponent n is larger than 3 for adsorption of urea nitrogen onto the CDACs with different DO. However, the low correlation coefficients (R2 < 0.9762) show poor agreement of Freundlich isotherm with the experimental data. 3.2. Kinetics of adsorption 3.2.1. Effect of DO of CDAC Fig. 4 shows the effect of DO of CDAC on adsorption of urea nitrogen onto CDAC at initial urea nitrogen 635 mg L1, CDAC/urease weight ratio 50:1 and 37 8C. It is seen that the adsorption capacity increases significantly with increasing DO of CDAC. This might suggest that the adsorption took place mainly by a Schiff reaction between ammonia and aldehyde group of CDAC. 3.2.2. Effect of CDAC/urease weight ratio Fig. 5 shows the effect of CDAC/urease weight ratio on adsorption of urea nitrogen onto CDAC with DO 74% at initial urea nitrogen concentration 635 mg L1 and 37 8C. It can be seen from Fig. 5 that the adsorption capacity increases significantly on increasing the CDAC/urease weight ratio. After adsorption for 24 h, the adsorption density q (46.9 mg g1) at CDAC/urease weight ratio 100:3 is more than 1.38 times of that (34.1 mg g1) at CDAC/urease weight ratio 100:1. It can be seen that the ratio plays an important role in the adsorption of urea nitrogen onto CDAC. These results suggest that the mechanism of adsorption of urea nitrogen onto CDAC might be mainly Schiff reaction between ammonia, which was product of urea hydrolysed by urease catalysing, and aldehyde group of CDAC. Increasing the urease amount leads to increasing ammonia concentration and enhances
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Fig. 5. Adsorption kinetics of urea nitrogen onto CDAC at different CDAC/ urease weight ratio.
the Schiff reaction between ammonia and binding sites of CDAC. 3.2.3. Effect of initial urea nitrogen concentration Fig. 6 shows that the effect of initial urea nitrogen concentration on the adsorption of the urea nitrogen onto CDAC with DO 74% at CDAC/urease weight ratio 50:1 and 37 8C. An increase in the initial urea nitrogen concentration leads to an increase in the adsorption capacity of the urea nitrogen onto CDAC. This is due to the increase in the driving force of the concentration gradient, as an increase in the initial urea nitrogen concentration. Fig. 6 also shows that most of urea nitrogen is adsorbed to achieve adsorption equilibrium in about 24 h. The adsorption density is 52.8 mg g1 at an initial urea nitrogen concentration of 785 mg L1, while it is 44.8 mg g1 at an initial urea nitrogen concentration of 544 mg L1.
Fig. 6. Adsorption kinetics of urea nitrogen onto CDAC with DO 74% at different initial concentration.
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3.3. Rate constant In order to investigate the mechanism of adsorption, the pseudo first-order and second-order equations were used to test the experimental data of initial urea nitrogen concentration, temperature, DO of CDAC and CDAC/urease weight ratio. The first-order rate expression of Lagergren [13] is given as: logðqe qÞ ¼ log qe
Fig. 7. Adsorption kinetics of urea nitrogen onto CDAC at different temperature.
3.2.4. Effect of temperature Fig. 7 shows the effect of temperature on adsorption of urea nitrogen onto CDAC with DO 74% at initial urea nitrogen concentration 635 mg L1 and CDAC/urease weight ratio 50:1. It indicates that an increase in the temperature leads to an increase in urea nitrogen adsorption rate dq/dt and adsorption capacity q, which indicates a kinetically controlling process. It can be seen from Fig. 7 that these effects of temperature on adsorption rate and adsorption capacity are significant and after adsorption for 24 h, the adsorption capacity is 45.2, 46.2 and 52.9 mg g1 at 37, 42 and 47 8C, respectively. This suggests that higher temperature enhances the hydrolysis of urea to ammonia and carbon dioxide by urease catalysis and increases the reaction rate between ammonia and aldehyde group of CDAC.
k1 t 2:303
(6)
where qe and q (mg g1) are the amounts of urea nitrogen adsorbed onto adsorbent at equilibrium and at time t, respectively, and k1 (min1) is the rate constant of first-order adsorption. A straight line of log (qe q) versus t suggests the applicability of this kinetic model to fit the experimental data. The equilibrium adsorption density qe is required to fit the data, but in many cases qe remains unknown due to slow adsorption processes. Also, in many cases, the first-order equation of Lagergren does not fit well to the whole range of contact time and is generally applicable over the initial stage of the adsorption processes [13,14]. The second-order kinetic model [15] is expressed as: t 1 t ¼ þ q k2 q2e qe
(7)
where k2 (g mg1 min1) is the rate constant of secondorder adsorption. If second-order kinetics is applicable, the plot of t/q versus t should show a linear relationship. There is no need to know any parameter beforehand and the more likely to predict the behaviour over the whole range of adsorption and is in agreement with chemical adsorption being the rate-controlling step [13,14]. The slopes and intercepts of plots of log (qe q) versus t were used to determine the first-order rate constant k1 and equilibrium adsorption qe. The straight lines in plot of t/q
Table 2 Comparison of the first-order and second-order adsorption rate constants, and calculated and experimental qe values at different initial urea nitrogen concentration, temperature, DO of CD AC and CDAC/urease weight ratio Parameters
qe,exp (mg g1)
First-order kinetic model k1 (min1)
Second-order kinetic model
qe,cal (mg g1)
R2
k2 (g mg1 min1)
qe,cal (mg g1)
R2
Initial urea nitrogen concentration (mg L1) 544 42.08 3.501E3 635 45.21 2.695E3 785 52.79 2.441E3
19.38 25.11 27.66
0.9379 0.9781 0.9730
2.305E4 1.729E4 1.334E4
44.23 48.69 57.69
0.9993 0.9997 0.9993
DO of CDAC (%) 41 35.38 60 42.08 74 45.21
3.639E3 2.787E3 2.695E3
28.99 28.02 25.11
0.8915 0.9936 0.9781
2.127E4 1.525E4 1.729E4
38.42 46.84 48.69
0.9993 0.9996 0.9997
CDAC/urease weight ratio 100/1 34.06 100/2 45.21 100/3 46.86
4.122E3 2.695E3 2.695E3
30.86 25.11 25.60
0.9791 0.9781 0.9441
1.817E4 1.729E4 1.756E4
37.84 48.69 50.28
0.9996 0.9997 0.9990
Temperature (8C) 37 45.21 42 48.27 47 52.89
2.695E3 2.741E3 2.672E3
25.11 26.54 25.61
0.9781 0.9844 0.9860
1.729E4 1.861E4 1.988E4
48.69 51.28 55.65
0.9997 0.9995 0.9994
Z.P. Liang et al. / Process Biochemistry 40 (2005) 3218–3224
Fig. 8. Plot of pseudo first-order model at different initial concentration.
versus t (Fig. 8) show a poor agreement of experimental data with a first-order kinetic model for different initial urea nitrogen concentrations at DO 74% of CDAC, CDAC/urease weight ratio 50:1 and 37 8C. Table 2 lists the calculated results and a comparison of results with the correlation coefficients (R2). The correlation coefficients for the firstorder model were low. The calculated qe values obtained from the first-order kinetic model do not give a reasonable value, which are too low compared with experimental qe values. This suggests that the adsorption of urea nitrogen onto CDAC is not a first-order reaction. The slopes and intercepts of plots of t/q versus t were used to calculate the second-order rate constant k2 and qe. The straight lines in plot of t/q versus t (Fig. 9) show a good agreement of experimental data with second-order kinetic model for different initial urea nitrogen concentrations at DO 74% of CDAC, CDAC/urease weight ratio 50:1 and 37 8C. The similar straight line agreements are also observed for data at different temperature, CDAC/urease weight ratio
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Fig. 10. Plot of ln k vs. reciprocal temperature for urea nitrogen onto CDAC with DO 74% at initial urea nitrogen concentration 635 mg L1 and CDAC/ urease weight ratio 50:1.
and DO of CDAC, although their plots are not shown in this paper. Table 2 lists the computed results obtained from the second-order kinetic model. The correlation coefficients for the second-order kinetic model, which are also presented in Table 2, are higher than 0.9990 for all the cases. These suggest that the adsorption of urea nitrogen onto CDAC follows the second-order kinetic model and chemical adsorption might be the rate-limiting step. The rate constant k2 at different temperature (37, 42 and 47 8C) listed in Table 2 was applied to estimate the activation energy of the adsorption of urea nitrogen onto CDAC. Assume that the correlation among the rate constant k2, temperature T and activation energy Ea follows the Arrhenius equation: ln k2 ¼ ln k0
Ea RT
(8)
where R (= 8.314 kJ mol1 K1) is the gas constant, k0 (g mg1 min1) the temperature independent factor, Ea (kJ mol1) the activation energy of adsorption and T(K) is the solution temperature. Fig. 10 shows the corresponding linear plot of ln k2 versus 103/T. The activation energy for the adsorption system of urea nitrogen onto the CDAC with DO 74% was found as 11.64 kJ mol1 from the slope of this plot. This activation energy (>0) indicates that urea nitrogen adsorption system is thermo-negative. Since adsorption system is thermonegative, it would be expected that an increase in solution temperature would result in an increase in adsorption capacity.
4. Conclusions
Fig. 9. Plot of pseudo second-order model at different initial concentration.
This paper investigated the adsorption of urea nitrogen onto CDAC under urease catalysation, including the
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equilibrium isotherm of adsorption, and the effects of DO of CDAC, initial urea nitrogen concentration, temperature and DAC/urease weight ratio on the kinetics of the adsorption systems. The Langmuir equation agrees very well with the equilibrium isotherm for the adsorption of urea nitrogen onto CDAC with different DO under the entire studied concentration range. However, fitting the isotherm data by the Freundlich equation gives a low correlation coefficient. The pseudo second-order kinetic model agrees very well with the dynamical behaviour for the adsorption of urea nitrogen onto the CDAC at different DO of CDACs, initial urea nitrogen concentration, temperature and CDAC/urease weight ratio in the whole ranges studied. On the contrary, the pseudo first-order kinetic model fits poorly the experimental data for the entire ranges studied. These suggest that the rate-limiting step might be chemical adsorption but not mass transfer.
Acknowledgment This work was supported by the Natural Science Foundation of Tianjin City (No. 03380211).
Appendix A. Nomenclature b Ce C0 Ea k0 k1 k2 n q qe Q
Langmuir constant defined in Eqs. (2) and (3) (L mg1) equilibrium urea nitrogen concentration (mg L1) initial urea nitrogen concentration (mg L1) activation energy of the adsorption system (kJ mol1) temperature independent factor (g mg1 min1) rate constant of pseudo first-order model defined in Eq. (6) (min1) rate constant of pseudo second-order model defined in Eq. (7) (g mg1 min1) Freundlich constant defined in Eqs. (4) and (5) amount of adsorption at time t (mg g1) amount of adsorption at equilibrium (mg g1) maximum monolayer amount of adsorption (mg g1)
Qf R R2 t T V W
parameter of Freundlich equation defined in Eqs. (4) and (5) gas constant (= 8.314 kJ mol1 K1) correlation coefficient time (min) solution Kelvin temperature (K) volume of the solution (L) amount of the dialdehyde cellulose (g)
References [1] Nose´ Y. Artificial kidney research, is it really not necessary? Atrif Organs 1990;14:254–6. [2] Krajewska B, Leszko M, Zaborska W. Immobilization of urease fordialyste regeneration system of artificial kidney. Post Fiz Med 1988;23:115–30. [3] Stefoni S, Feliciangeli G, Coli L, Prandini R, Bonomini V. Use of combined hemodialysis/hemoperfusion in chronic uremia. Contrib Nephrol 1982;29:123–30. [4] Lee CJ, Hsu ST. Preparation and spherical encapsulation of activated carbons and their adsorption capacity of typical uremic toxins. J Biomed Mater Res 1990;24:243–58. [5] Shimizu T, Fujishige S. Anewly prepared surface-treated oxystarch for removal of urea. J Biomed Mater Res 1983;17:597–612. [6] Yagi M, Kato S, Nishitoba T. Effects of chitosan-coated dialdehyde cellulose, a newly developed oral adsorbent, on glomerulonephritis induced by anti-Thy-1 antibody in rats. Nephrology 1998;78:433–9. [7] Wolfe EA, Chang TMS. Orally ingested microencapsulated urease and an adsorbent, zirconium phosphate, to remove urea in kidney failure. Int J Artif Organs 1987;10:269–74. [8] Nevell TP. In: Whistler RL, editor. Methods in carbohydrate chemistry, vol. 3. New York: Academic Press; 1963. p. 164. [9] Feng YQ, Liu Y, Zhang XD, Li XF. Preparation of of dialdehyde cellulose and its adsorption property. J Tianjin Univ 2002;35:766–8. [10] Langmuir I. Adsorption of gases on plain surfaces of glass mica platinum. J Am Chem Soc 1918;40:1361–403. [11] Freundlich H. Adsorption solution. Z Phys Chem 1906;57:384–470. [12] Mckay G, Blair HS, Gardner JR. Adsorption of dye on chitin. I Equilibrium studies. J Appl Polym Sci 1982;27:3043–57. [13] Lagergren, S. Zur theorie der sogenannten adsorption geloster, Kungliga Svenska vetenskapsakademiens, handlinggar 1898; 24:1–39. [14] McKay G, Ho YS. The sorption of lead(II) on peat. Water Res 1999;33:578–84. [15] McKay G, Ho YS. Pseudo-second order model for sorption processes. Process Biochem 1999;34:451–65.
Equilibrium and kinetic modeling of adsorption of urea nitrogen onto chitosan coated dialdehyde cellulose Zu Pei Liang a,b, Ya Qing Feng a,*, Shu Xian Meng a, Zhi Yan Liang a b
a Department of Fine Chemicals, Tianjin University, Tianjin, China Department of Applied Chemistry, Qingdao University of Science and Technology, Qingdao, China
Received 30 April 2004; received in revised form 24 June 2004; accepted 2 March 2005
Abstract The adsorption of urea nitrogen onto chitosan coated dialdehyde cellulose (CDAC) under urease catalysis was studied in a batch system. The equilibrium isotherm of urea nitrogen adsorption onto CDACs with different degree of oxidation (DO) and the kinetics of adsorption with respect to the DO of CDAC (41, 60 and 74%), the initial urea nitrogen concentration (544, 635 and 785 mg L1), temperature (37, 42 and 47 8C) and CDAC/urease weight ratio (100:3, 100:2 and 100:1) were investigated. Langmuir and Freundlich adsorption models were applied to describe the experimental isotherm and isotherm constants. Equilibrium data fitted very well to the Langmuir model in the entire saturation concentration range (152–756 mg L1). The maximum monolayer adsorption capacities obtained from the Langmuir model are 50.0, 60.6 and 69.2 mg g1 for the CDACs with DO 41, 60 and 74%, respectively, at 37 8C and CDAC/urease weight ratio 50:1. Pseudo first-order and second-order kinetic models were used to describe the kinetic data and the rate constants were evaluated. The experimental data fitted well to the second-order kinetic model, which indicates that chemical adsorption is the rate-limiting step, instead of mass transfer. The DO of CDAC, initial urea nitrogen concentration, temperature and CDAC/urease weight ratio affected the adsorption capacity significantly. The activation energy is 11.64 kJ mol1 for the adsorption of the urea nitrogen onto the CDAC under urease catalysation at DO 74% of CDAC, initial urea nitrogen concentration 635 mg L1 and CDAC/urease weight ratio 50:1. # 2005 Elsevier Ltd. All rights reserved. Keywords: Adsorption; Equilibrium; Kinetic; Urea nitrogen; Chitosan coated dialdehyde cellulose; Urease
1. Introduction Urea is one of the major metabolic end products and the removal of its excess is a major problem for patients suffering from chronic renal failure (CRF). Haemodialysis is the conventional treatment for CRF. Approximately, half a million patients worldwide are being supported by haemodialysis [1]. Although haemodialysis has been effective for the treatment of CRF, there are a number of related problems. It is bulky, cumbersome, expensive, timeconsuming, and difficult to handle, limiting the mobility of * Corresponding author. Tel.: +86 22 27404769; fax: +86 22 27401824. E-mail address: [email protected] (Y.Q. Feng). 1359-5113/$ – see front matter # 2005 Elsevier Ltd. All rights reserved. doi:10.1016/j.procbio.2005.03.041
the patient [2] and some uraemic patients are reported to have suffered from the so-called haemodialysis-resistant uraemic syndrome [3]. Oral urea nitrogen adsorbents could potentially delay the onset of haemodialysis therapy in patients who still have some renal function and reduce haemodialysis treatment times. Active charcoal [4], oxystarch [5], oxycellulose [6] and zirconium phosphate [7] have been used for the removal of urea through oral administration. In this work, a complex type urea nitrogen adsorbent, which consisted of chitosan coated dialdehyde cellulose (CDAC) and urease, was prepared. Urea was hydrolysed to ammonia and carbon dioxide by urease catalysis, and the ammonia was chemically adsorbed onto CDAC through a
Z.P. Liang et al. / Process Biochemistry 40 (2005) 3218–3224
Schiff reaction between the ammonia and the aldehyde group of CDAC. The aim of this paper was to study the adsorption behaviour of urea nitrogen using CDAC–urease complex type adsorbent. Equations were used to fit the equilibrium isotherm. The adsorption rates were measured and determined quantitatively in correlation with initial urea nitrogen concentration, temperature, degree of oxidation (DO) of CDAC and CDAC/urease weight ratio. These results will be useful for further applications in urea nitrogen removal by oral administration.
2. Materials and experimental 2.1. Chemicals a-Cellulose (degree of polymerisation, 500 and molecular weight, 243,000) was supplied by Huludao Chemical Co., Liaoning, China. Chitosan (degree of deacetylation, 88% and molecular weight, 210,000) was supplied by Hengsheng Biochemical Co., Qingdao, China. Sodium metaperiodate was obtained from Tianhe Chemical Reagent Co., Tianjin, China. Urease (EC 3.5.1.5, from jackbean) was obtained from BDH Chemicals Ltd., Poole, England. Urea was obtained from Kewei Co., Tianjin, China. 2.2. Preparation of 2,3-dialdehyde cellulose by periodate oxidation Periodate oxidation of cellulose leads to breaking of C2–C3 bond of the glucose and formation of 2,3dialdehyde cellulose (DAC) [8]. Preparation of DAC and determination of its DO followed the methods previous reported [9]. Sodium metaperiodate (21.4–35.6 g) was dissolved in 400 mL of deionized water and the solution was adjusted to pH 2–2.5 by concentrated H2SO4, and 22.5 g of a-cellulose powder was added to the solution. The mixture was stirred at 43 8C for 3.5 h, and the products were then filtered and washed with H2SO4 solution (pH 4.5–5.5) till all iodic compounds were removed. The resulting DAC was used for the next step without drying. The DACs with the different DO 41, 60 and 74% based on glucose monomer units were obtained by adjusting the quantity of oxidizing agents.
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resulting CDAC was filtered and washed with deionized water, and then dried at 40 8C. 2.4. Batch equilibrium The urea was dissolved in deionized water to the required urea nitrogen concentration. In experiments of equilibrium adsorption isotherm, the CDAC (0.5 g), urease (10 mg) and urea nitrogen solution (75 mL) were placed in a 150 mL Erlenmeyer flask and shaken for 24 h by a shaker in a water bath to control temperature at 37 8C. The supernatant liquid 2 mL was taken out and placed in a flask containing desired amount of urease and residual urea were hydrolysed to ammonia and carbon dioxide by urease catalysing for 12 h. The concentration of unadsorbed urea nitrogen was analyzed by measuring the amount of ammonia liberated from the urease-catalysed hydrolysis of urea. The amount of adsorption at equilibrium qe (mg g1) was calculated as follows: qe ¼
ðC0 Ce Þ V W
(1)
where C0 and Ce (mg L1) are the initial and equilibrium urea nitrogen concentration, respectively, V (L) the volume of the solution and W (g) is the weight of CDAC used. 2.5. Batch kinetic In experiments of batch kinetic adsorption, CDAC (0.5 g), urease (10 mg) and urea nitrogen solution (75 mL) were placed in a 150 mL Erlenmeyer flask and shaken by a shaker in a water bath to control temperature. Every other period of time, the urea nitrogen concentration was determined as above-mentioned measuring method. The experimental parameters included DO of CDAC, initial urea nitrogen concentration, temperature and CDAC/urease weight ratio.
2.3. Preparation of chitosan coated dialdehyde cellulose Chitosan (0.4 g) was dissolved in 1% (v/v) aqueous acetic acid solution (40 mL) with stirring for 1 h at 25 8C and then diluted to 200 mL with deionized water. The abovementioned DAC was ground into paste in 100 mL deionized water and then immersed in the chitosan solution with constant stirring for 1 h at 25 8C. The solution was adjusted to about pH 7.0–7.5 with 2% (w/v) sodium hydroxide solution, and then was gelized for 0.5 h at 25 8C. The
Fig. 1. Equilibrium adsorption of urea nitrogen onto CDACs with different DO.
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Z.P. Liang et al. / Process Biochemistry 40 (2005) 3218–3224 Table 1 Langmuir and Freundlich isotherm constants for CDACs with different DO at 37 8C and CDAC/urease weight ratio 50:1
3. Results and discussion 3.1. Equilibrium adsorption
DO (%) Langmuir
Fig. 1 shows the equilibrium adsorption of urea nitrogen onto the CDACs with three different DO at 37 8C, CDAC/ urease ratio 50:1. It is seen that the saturation adsorption capacity increases with increasing DO of CDAC. This might suggest that the adsorption took place mainly by a Schiff reaction between ammonia and aldehyde group of CDAC. 3.1.1. Langmuir isotherm The equilibrium adsorption isotherm is fundamental in describing the interactive behaviour between solutes and adsorbent and is important in the design of adsorption system. The widely used Langmuir isotherm [10] has found successful application in many real adsorption processes and expressed as: qe ¼
QbCe 1 þ bCe
(2)
which can be rearranged to obtain a linear form Ce 1 Ce þ ¼ qe Qb Q
(3)
Freundlich
Q (mg g1) b (L mg1) R2 41 60 74
50.00 60.57 69.20
6.584E3 8.408E3 9.309E3
Qf (mg g1) n
0.9974 6.88 0.9986 8.76 0.9992 10.93
R2
3.653 0.9762 3.626 0.9342 3.766 0.9497
and intercepts of different straight lines representing CDACs with different DO. Table 1 lists the calculated results. The fits are good for all three DACs with different DO under the urea nitrogen concentration ranges studied (correlation coefficient, R2 > 0.9974). Table 1 indicates that the computed maximum monolayer capacity Q of urea nitrogen onto the CDACs with DO 41, 60 and 74% are 50.0, 60.6 and 69.2 mg g1, respectively. Similar trends are observed for b, indicating that the affinity of the CDAC for urea nitrogen increases as the DO of CDAC increases. 3.1.2. Freundlich isotherm The Freundlich isotherm [11], used for isothermal adsorption, is a special case for heterogeneous surface energy in which the energy term in the Langmuir equation varies as a function of surface coverage strictly due to variation of the adsorption. The Freundlich equation is given as:
where Q (mg g1) is the maximum amount of the urea nitrogen per unit weight of CDAC to form a complete monolayer coverage on the surface bound at high equilibrium urea nitrogen concentration Ce and b (L mg1) is the Langmuir constant related to the affinity of binding sites. Q represents a practical limiting adsorption capacity when the surface is fully covered with urea and ammonia molecules and assist in the comparison of adsorption performance. A linearized plot of Ce/qe versus Ce is obtained from the model shown in Fig. 2. Q and b are computed from the slopes
where Qf and n are indicators of adsorption capacity and adsorption intensity, respectively. Qf and n can be determined from the linear plot of ln qe versus ln Ce (Fig. 3).
Fig. 2. Langmuir plot for CDAC with different DO.
Fig. 3. Freundlich plot for CDAC with different DO.
qe ¼ Qf Ce1=n
(4)
which can be rearranged to obtain a linear form ln qe ¼ lnQf þ
1 ln Ce n
(5)
Z.P. Liang et al. / Process Biochemistry 40 (2005) 3218–3224
Fig. 4. Adsorption kinetics of urea nitrogen onto CDACs with different DO.
Table 1 lists the calculated results. The magnitude of the exponent 1/n gives an indication of the favorability of adsorption. Values, n > 1 represent favourable adsorption condition [12]. From Table 1, the exponent n is larger than 3 for adsorption of urea nitrogen onto the CDACs with different DO. However, the low correlation coefficients (R2 < 0.9762) show poor agreement of Freundlich isotherm with the experimental data. 3.2. Kinetics of adsorption 3.2.1. Effect of DO of CDAC Fig. 4 shows the effect of DO of CDAC on adsorption of urea nitrogen onto CDAC at initial urea nitrogen 635 mg L1, CDAC/urease weight ratio 50:1 and 37 8C. It is seen that the adsorption capacity increases significantly with increasing DO of CDAC. This might suggest that the adsorption took place mainly by a Schiff reaction between ammonia and aldehyde group of CDAC. 3.2.2. Effect of CDAC/urease weight ratio Fig. 5 shows the effect of CDAC/urease weight ratio on adsorption of urea nitrogen onto CDAC with DO 74% at initial urea nitrogen concentration 635 mg L1 and 37 8C. It can be seen from Fig. 5 that the adsorption capacity increases significantly on increasing the CDAC/urease weight ratio. After adsorption for 24 h, the adsorption density q (46.9 mg g1) at CDAC/urease weight ratio 100:3 is more than 1.38 times of that (34.1 mg g1) at CDAC/urease weight ratio 100:1. It can be seen that the ratio plays an important role in the adsorption of urea nitrogen onto CDAC. These results suggest that the mechanism of adsorption of urea nitrogen onto CDAC might be mainly Schiff reaction between ammonia, which was product of urea hydrolysed by urease catalysing, and aldehyde group of CDAC. Increasing the urease amount leads to increasing ammonia concentration and enhances
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Fig. 5. Adsorption kinetics of urea nitrogen onto CDAC at different CDAC/ urease weight ratio.
the Schiff reaction between ammonia and binding sites of CDAC. 3.2.3. Effect of initial urea nitrogen concentration Fig. 6 shows that the effect of initial urea nitrogen concentration on the adsorption of the urea nitrogen onto CDAC with DO 74% at CDAC/urease weight ratio 50:1 and 37 8C. An increase in the initial urea nitrogen concentration leads to an increase in the adsorption capacity of the urea nitrogen onto CDAC. This is due to the increase in the driving force of the concentration gradient, as an increase in the initial urea nitrogen concentration. Fig. 6 also shows that most of urea nitrogen is adsorbed to achieve adsorption equilibrium in about 24 h. The adsorption density is 52.8 mg g1 at an initial urea nitrogen concentration of 785 mg L1, while it is 44.8 mg g1 at an initial urea nitrogen concentration of 544 mg L1.
Fig. 6. Adsorption kinetics of urea nitrogen onto CDAC with DO 74% at different initial concentration.
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3.3. Rate constant In order to investigate the mechanism of adsorption, the pseudo first-order and second-order equations were used to test the experimental data of initial urea nitrogen concentration, temperature, DO of CDAC and CDAC/urease weight ratio. The first-order rate expression of Lagergren [13] is given as: logðqe qÞ ¼ log qe
Fig. 7. Adsorption kinetics of urea nitrogen onto CDAC at different temperature.
3.2.4. Effect of temperature Fig. 7 shows the effect of temperature on adsorption of urea nitrogen onto CDAC with DO 74% at initial urea nitrogen concentration 635 mg L1 and CDAC/urease weight ratio 50:1. It indicates that an increase in the temperature leads to an increase in urea nitrogen adsorption rate dq/dt and adsorption capacity q, which indicates a kinetically controlling process. It can be seen from Fig. 7 that these effects of temperature on adsorption rate and adsorption capacity are significant and after adsorption for 24 h, the adsorption capacity is 45.2, 46.2 and 52.9 mg g1 at 37, 42 and 47 8C, respectively. This suggests that higher temperature enhances the hydrolysis of urea to ammonia and carbon dioxide by urease catalysis and increases the reaction rate between ammonia and aldehyde group of CDAC.
k1 t 2:303
(6)
where qe and q (mg g1) are the amounts of urea nitrogen adsorbed onto adsorbent at equilibrium and at time t, respectively, and k1 (min1) is the rate constant of first-order adsorption. A straight line of log (qe q) versus t suggests the applicability of this kinetic model to fit the experimental data. The equilibrium adsorption density qe is required to fit the data, but in many cases qe remains unknown due to slow adsorption processes. Also, in many cases, the first-order equation of Lagergren does not fit well to the whole range of contact time and is generally applicable over the initial stage of the adsorption processes [13,14]. The second-order kinetic model [15] is expressed as: t 1 t ¼ þ q k2 q2e qe
(7)
where k2 (g mg1 min1) is the rate constant of secondorder adsorption. If second-order kinetics is applicable, the plot of t/q versus t should show a linear relationship. There is no need to know any parameter beforehand and the more likely to predict the behaviour over the whole range of adsorption and is in agreement with chemical adsorption being the rate-controlling step [13,14]. The slopes and intercepts of plots of log (qe q) versus t were used to determine the first-order rate constant k1 and equilibrium adsorption qe. The straight lines in plot of t/q
Table 2 Comparison of the first-order and second-order adsorption rate constants, and calculated and experimental qe values at different initial urea nitrogen concentration, temperature, DO of CD AC and CDAC/urease weight ratio Parameters
qe,exp (mg g1)
First-order kinetic model k1 (min1)
Second-order kinetic model
qe,cal (mg g1)
R2
k2 (g mg1 min1)
qe,cal (mg g1)
R2
Initial urea nitrogen concentration (mg L1) 544 42.08 3.501E3 635 45.21 2.695E3 785 52.79 2.441E3
19.38 25.11 27.66
0.9379 0.9781 0.9730
2.305E4 1.729E4 1.334E4
44.23 48.69 57.69
0.9993 0.9997 0.9993
DO of CDAC (%) 41 35.38 60 42.08 74 45.21
3.639E3 2.787E3 2.695E3
28.99 28.02 25.11
0.8915 0.9936 0.9781
2.127E4 1.525E4 1.729E4
38.42 46.84 48.69
0.9993 0.9996 0.9997
CDAC/urease weight ratio 100/1 34.06 100/2 45.21 100/3 46.86
4.122E3 2.695E3 2.695E3
30.86 25.11 25.60
0.9791 0.9781 0.9441
1.817E4 1.729E4 1.756E4
37.84 48.69 50.28
0.9996 0.9997 0.9990
Temperature (8C) 37 45.21 42 48.27 47 52.89
2.695E3 2.741E3 2.672E3
25.11 26.54 25.61
0.9781 0.9844 0.9860
1.729E4 1.861E4 1.988E4
48.69 51.28 55.65
0.9997 0.9995 0.9994
Z.P. Liang et al. / Process Biochemistry 40 (2005) 3218–3224
Fig. 8. Plot of pseudo first-order model at different initial concentration.
versus t (Fig. 8) show a poor agreement of experimental data with a first-order kinetic model for different initial urea nitrogen concentrations at DO 74% of CDAC, CDAC/urease weight ratio 50:1 and 37 8C. Table 2 lists the calculated results and a comparison of results with the correlation coefficients (R2). The correlation coefficients for the firstorder model were low. The calculated qe values obtained from the first-order kinetic model do not give a reasonable value, which are too low compared with experimental qe values. This suggests that the adsorption of urea nitrogen onto CDAC is not a first-order reaction. The slopes and intercepts of plots of t/q versus t were used to calculate the second-order rate constant k2 and qe. The straight lines in plot of t/q versus t (Fig. 9) show a good agreement of experimental data with second-order kinetic model for different initial urea nitrogen concentrations at DO 74% of CDAC, CDAC/urease weight ratio 50:1 and 37 8C. The similar straight line agreements are also observed for data at different temperature, CDAC/urease weight ratio
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Fig. 10. Plot of ln k vs. reciprocal temperature for urea nitrogen onto CDAC with DO 74% at initial urea nitrogen concentration 635 mg L1 and CDAC/ urease weight ratio 50:1.
and DO of CDAC, although their plots are not shown in this paper. Table 2 lists the computed results obtained from the second-order kinetic model. The correlation coefficients for the second-order kinetic model, which are also presented in Table 2, are higher than 0.9990 for all the cases. These suggest that the adsorption of urea nitrogen onto CDAC follows the second-order kinetic model and chemical adsorption might be the rate-limiting step. The rate constant k2 at different temperature (37, 42 and 47 8C) listed in Table 2 was applied to estimate the activation energy of the adsorption of urea nitrogen onto CDAC. Assume that the correlation among the rate constant k2, temperature T and activation energy Ea follows the Arrhenius equation: ln k2 ¼ ln k0
Ea RT
(8)
where R (= 8.314 kJ mol1 K1) is the gas constant, k0 (g mg1 min1) the temperature independent factor, Ea (kJ mol1) the activation energy of adsorption and T(K) is the solution temperature. Fig. 10 shows the corresponding linear plot of ln k2 versus 103/T. The activation energy for the adsorption system of urea nitrogen onto the CDAC with DO 74% was found as 11.64 kJ mol1 from the slope of this plot. This activation energy (>0) indicates that urea nitrogen adsorption system is thermo-negative. Since adsorption system is thermonegative, it would be expected that an increase in solution temperature would result in an increase in adsorption capacity.
4. Conclusions
Fig. 9. Plot of pseudo second-order model at different initial concentration.
This paper investigated the adsorption of urea nitrogen onto CDAC under urease catalysation, including the
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equilibrium isotherm of adsorption, and the effects of DO of CDAC, initial urea nitrogen concentration, temperature and DAC/urease weight ratio on the kinetics of the adsorption systems. The Langmuir equation agrees very well with the equilibrium isotherm for the adsorption of urea nitrogen onto CDAC with different DO under the entire studied concentration range. However, fitting the isotherm data by the Freundlich equation gives a low correlation coefficient. The pseudo second-order kinetic model agrees very well with the dynamical behaviour for the adsorption of urea nitrogen onto the CDAC at different DO of CDACs, initial urea nitrogen concentration, temperature and CDAC/urease weight ratio in the whole ranges studied. On the contrary, the pseudo first-order kinetic model fits poorly the experimental data for the entire ranges studied. These suggest that the rate-limiting step might be chemical adsorption but not mass transfer.
Acknowledgment This work was supported by the Natural Science Foundation of Tianjin City (No. 03380211).
Appendix A. Nomenclature b Ce C0 Ea k0 k1 k2 n q qe Q
Langmuir constant defined in Eqs. (2) and (3) (L mg1) equilibrium urea nitrogen concentration (mg L1) initial urea nitrogen concentration (mg L1) activation energy of the adsorption system (kJ mol1) temperature independent factor (g mg1 min1) rate constant of pseudo first-order model defined in Eq. (6) (min1) rate constant of pseudo second-order model defined in Eq. (7) (g mg1 min1) Freundlich constant defined in Eqs. (4) and (5) amount of adsorption at time t (mg g1) amount of adsorption at equilibrium (mg g1) maximum monolayer amount of adsorption (mg g1)
Qf R R2 t T V W
parameter of Freundlich equation defined in Eqs. (4) and (5) gas constant (= 8.314 kJ mol1 K1) correlation coefficient time (min) solution Kelvin temperature (K) volume of the solution (L) amount of the dialdehyde cellulose (g)
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