Environ. Sci. Technol. 2004, 38, 324-328
Collagen Fiber Immobilized Myrica rubra Tannin and Its Adsorption to UO2+ 2 XUEPIN LIAO, ZHONGBI LU, XIAO DU, XIN LIU, AND BI SHI* The Key Laboratory of Leather Chemistry and Engineering of Ministry of Education, Sichuan University, Chengdu, 610065, P. R. China
Tannins, which are rich in ortho-hydroxyl groups, have a high affinity for UO22+. In this paper, Myrica rubra tannin was immobilized on collagen fiber by an aldehydic crosslinking reaction to prepare a novel adsorbent for uranium (UO2+ 2 ) recovery from wastewater. The adsorption equilibrium, the adsorption kinetics, and the effects of temperature and pH on the adsorption equilibrium were investigated in detail. It was found that the Myrica rubra tannin immobilized on collagen fiber exhibits an excellent adsorption capacity for UO2+ 2 . The adsorption capacity at 293 K and pH 5.0 was as high as 1.19 mmol UO2+ 2 /g (283.3 in solution mgU/g) when the initial concentration of UO2+ 2 was 7.5 mmol/L. The adsorption isotherms could be described by the Freundlich equation, and the increase of temperature promoted the adsorption to UO2+ 2 . The adsorption kinetics data were fitted very well by the pseudosecond-order rate model, and the equilibrium adsorption capacity calculated by the pseudo-second-order rate model was almost the same as that determined by the actual measurement with the error e4%. The pH has a significant effect on the adsorption process. According to our experiments, the suitable pH scope should be 5-8.
Introduction Uranium is a major contaminant in soils and surface or groundwater. It comes from mining, refining of uranium ores and various processes relating to the production of nuclear reactor fuel as well as the manufacturing of nuclear weapons. It is well-known that uranium is one of the most lethal elements for human beings and that the contamination caused by uranium is a serious environmental problem. Moreover, the uranium released into the environment is often in the dissolved state in wastewater and is predominantly in the hexavalent form as UO2+ 2 . Therefore, the removal of uranium from wastewater is indispensable to uranium contamination control. The possible ways of removing uranium from wastewater include solvent extract, ion-exchange and adsorption, among which the adsorption approach is the most efficient way. Up to now, numerous experimental studies on UO2+ 2 adsorption by minerals (1-4), phosphates (5, 6), resins (7, 8) and microorganisms (9, 10) have been published. Tannins, which are widely distributed in plants, are the polyphenols with molecular weight between 500 and 3000 * Corresponding author fax: (86)028-85405237; e-mail: Shibi@ pridns.scu.edu.cn. 324
9
ENVIRONMENTAL SCIENCE & TECHNOLOGY / VOL. 38, NO. 1, 2004
Dalton. According to the chemical structures of tannins, they can usually be classified into hydrolyzable tannins and condensed tannins. Hydrolyzable tannins yield gallic acid or ellagic acid when hydrolyzed by acid, base or some enzymes. Tannic acid is a representative of hydrolyzable tannins. Condensed tannins are the polymerized products of flavan3-ols and/or flavan-3,4-diols. Black wattle tannin is a representative of condensed tannins. Tannins have multiple adjacent hydroxyl groups and exhibit specific affinity toward metal ions. Thus they can be probably used as an alternative, effective and efficient adsorbent in UO2+ 2 adsorption. However, tannins are water soluble compounds. When they are used directly as an adsorbent for uranium recovery from an aqueous system, they have the disadvantage that they are leached by water. To overcome this disadvantage, attempts had been made to immobilize tannins onto various waterinsoluble matrices. For example, tannic acid was immobilized on cellulose by means of epichlorohydrin activation, diaminohexane expanding chain length and epichlorohydrin reactivation (11). Tannins immobilized on agarose (12), viscose rayon fiber (13) and on the matrices containing amino groups such as albumin, gelatin, aminopolystyrene and 2-vinyl-4,6-diamino-s-triazine (14) were also reported. However, not only all of the immobilization procedures are complicated but also the tannins on the matrices are still easily leached out by water. As a result, even though tannins immobilized by these matrices exhibit excellent adsorption capacity to UO2+ 2 and to other metal ions, the disadvantages still restrict their practical uses. In addition, it was reported (15, 16) that the tannin resin prepared from aldehyde and tannins had significant adsorption capacity to Cr(VI), Cu(II) and Cd(III), but they have no more advantages than immobilized tannins and the disadvantage of leaching by water still remains. Collagen fiber has abundant functional groups ready to react with other chemicals; these functional groups include -OH, -COOH, -CONH2 and -NH2. Tannins have been traditionally used as a tanning agent in leather making due to their highly reactive activity with hide collagen fibers. The immobilized tannins prepared on the basis of collagen fiber have high mechanical strength and are suitable for column adsorption operation. Compared with synthetic polymers, the rapid adsorption rate and the easier regeneration may be expected for the collagen-immobilized tannins due to little porous in these adsorbents. In addition, the approach of tannin immobilization is simple and cost effective as compared with other immobilization methods. In this study, Myrica rubra tannin, a condensed tannin, was covalentbinding immobilized onto cattle collagen fibers by aldehydic cross-linking reaction, and its adsorption capacity, adsorption isotherms, and adsorption kinetics to UO2+ 2 were investigated in detail.
Experimental Procedures Preparation of Collagen Fibers. The cattle hide powder prepared according to standard procedure (17) was used as collagen fibers. Cattle pelt was cleaned, limed, splitted and delimed according to the approaches of leather processing, so that the noncollagen components could be removed. Then the pelt was treated with 150% aqueous solution of acetic acid (concentration 16 g/L) three times to remove mineral substances. After the pH of the pelt was adjusted to 4.8-5.0 with acetic acid-sodium acetate buffer solution, the pelt was dehydrated by absolute ethyl alcohol, dried in a vacuum to moisture content e10%, ground and sieved. 10-20 mesh 10.1021/es034369a CCC: $27.50
2004 American Chemical Society Published on Web 11/21/2003
FIGURE 1. Scheme of chemical structure of Myrica rubra tannin and primary amino acid sequence of hide collagen. hide powder with moisture e 12%, ash content e 0.3%, and pH ) 5.0-5.5 was obtained. Preparation of Myrica rubra Tannin. 200 g of Myrica rubra bark was extracted by using 2000 mL of 70%(V/V) acetone-water solution three times. The extract solutions were combined together. After the recovery of acetone, the residual solution was extracted by using 500 mL of petroleum ether and by using 600 mL of ethyl acetate respectively to remove grease and low molecular polyphenols. Then the residual solution was spray dried, and 31.2 g of Myrica rubra tannin extract with a tannin content of 75.4% was obtained. The amount of tannin extract used in the following experiments was based on pure tannin. Preparation of Immobilized Myrica rubra Tannin. 9 g of Myrica rubra tannin was dissolved in 300 mL of distilled water and mixed with 15 g of collagen fiber. The mixture was stirred at 25 °C for 24 h. After the intermediate product was collected by filtration and washed with distilled water, 300 mL of 2%(wt %) aldehydic cross-linking agent solution with pH ) 6.5 was added. The mixture was first stirred at 25 °C for 1 h and then stirred at 50 °C for 4 h. When the reaction was completed, the product was washed with 2000 mL of distilled water and vacuum-dried at 60 °C for 12 h, and then the immobilized tannin was obtained. DSC (Differential Scanning Calorimetry, DSC 200PC, NETZSCH, German) determination indicated that the deconstruction temperature of the collagen fibers after an immobilization reaction was increased to 90-95 °C, compared to 60-65 °C of raw collagen fibers, which shows an additional advantage of this approach. The higher thermal stability comes from the so-called vegetable tanning effect. Study of Adsorption Isotherms. The stock solution of 10 mmol/L UO2+ 2 was prepared with uranyl nitrate hexahydrate and further diluted to control concentration for practical uses. 0.5 g of immobilized tannin adsorbent was sealed in filter cloth and was suspended in 100 mL of UO2+ 2 solution in which the concentrations of UO2+ 2 were 0.5, 1.0, 1.5, 2.0, 2.5 mmol/L, respectively. The initial pH of the UO2+ 2 solution was adjusted to 5.0 with 0.1 M NaOH or 0.1 M HNO3. The adsorption experiments were conducted by constant stirring at controlling temperature for 24 h. The concentration of UO2+ 2 in residual solution was analyzed by 5-[(5-bromo-2pyridyl)azo]-5-diethylaminophenol (5-Br-PADAP) spectrophotometry. Study of Adsorption Kinetics. The procedures were similar to those of adsorption isotherms study, and the concentration of UO2+ 2 during adsorption process was analyzed in a regular interval. Study of the Effect of pH on Adsorption Capacity. The procedures were similar to those of adsorption isotherms study, and the initial concentration of UO2+ 2 was 2.5 mmol/L with initial pH equal to 2, 3, 4, 5, 6, 7, 8 and 9, respectively.
Study of the Immobilized Tannin to Withstand Organic Solvent and Urea Solution Extraction Ability. 0.5 g of immobilized tannin prepared by different approaches was suspended in 100 mL of 95% (v/v) ethanol solution and 1 M, 3 M and 8 M urea solutions, respectively. The extraction experiments were conducted by constant stirring at room temperature for 4 h. The concentration of tannin in the extraction solutions was analyzed by UV-vis spectrophotometer (UV-2501PC SHMADZU) at 280 nm.
Results and Discussion Immobilization of Myrica rubra Tannin by Collagen Fibers. Tannins, having a molecular weight between 500 and 3000 dalton, are soluble in polar solution and are traditionally used as a tanning agent in leather manufacturing because of their unique abilities to precipitate proteins and metal ions. The chemical structure of Myrica rubra tannin is illustrated in Figure 1. Its basic structure is flavan-3-ols, and it is often attached with a galloyl group on a pyrane ring (C ring) (18). The cattle hide collagen fibers are insoluble in water and are much more stable than gelatin. The collagen of cattle hide is type I collagen which contains three polypeptide R-chains, each consisting of more than 1000 amino acids. Its primary sequence is basically a tripeptide repeat, (Gly-X-Y)100-400, where X is often proline and Y sometimes is hydroxyproline (19). The collagen fibers with abundant functional groups are very suitable to be used as matrices for immobilizing tannins compared with most synthetic polymers and other biomasses. Because the interaction between tannins and collagen fibers is through hydrogen bond and hydrophobic bond associations and can be easily broken by organic solvent and urea solution, tannin will be easily leaked out in practical application. The C-6 and C-8 positions of A ring of a Myrica rubra tannin molecule have highly nucleophilic reaction activity; therefore, it can be covalently bonded to amino groups of collagen molecules by reaction with aldehyde (18). The immobilized tannin prepared according to the described method in the Experimental Section can withstand organic solvent and urea solution extraction, as shown in Table 1. Analyzing the content of residual solution after the tannin-collagen interaction, it can be calculated that the ratio of tannins in the adsorbent was approximately 48.0%, that is, 1 g of collagen fiber can bind to 0.48 g of Myrica rubra tannin. It was reported that the coupled amount of tannic acid immobilized on cellulose was 0.158-0.286 mmol/g adsorbent, that was 410.8-743.6 mg/g adsorbent (in this calculation, molecular weight of tannic acid was taken as 2600) (11), but the stability of the immobilization was not reported. Adsorption Isotherms of Immobilized Tannin to UO2+ 2 . Adsorption equilibrium data were analyzed for Langmuir VOL. 38, NO. 1, 2004 / ENVIRONMENTAL SCIENCE & TECHNOLOGY
9
325
TABLE 1. Concentration of Tannin in Extract Solutiona concn of tannin in extract solution (mg/L) 95% 1M 3M 8M (v/v) urea urea urea water ethanol solution solution solution
immobilization method tannin + collagen fibers + aldehyde tannin + collagen fibers
b 44.2
8.1
8.2
9.0
9.0
260.2
245.3
355.4
534.7
a Immobilized tannin: 0.5 g; organic solvent and urea solution: 100 mL; extract conditions: 4 h with stirring at room temperature; detection: UV spectrophotometry. b No tannin was determined.
Adsorption Kinetics of Immobilized Tannin to UO2+ 2 . In general, the ortho-phenolic hydroxyl groups of tannins can chelate with metal ions to form a five-member ring (21, 22). Therefore, the reaction of tannin and UO2+ 2 can be described as follows: 2+ 2(-OH ) + UO2+ 2 ) UO2 (-OH)2
(2)
The reaction rate equation can be described by a pseudosecond-order rate model
d(P)t ) k2[(P)0 - (P)t]2 dt
(3)
where (P)t represents occupied adsorption sites at time t and (P)0 represents total adsorption sites. The kinetics equation can be rewritten as
dqt ) k2(qe - qt)2 dt
(4)
where k2 is the constant of pseudo-second-order rate, g/mmol‚h; qe is the adsorption capacity at equilibrium, mmol UO2+ 2 /g; and qt is the adsorption capacity at time t, mmol UO2+ 2 /g. Separating the variables in eq 4 and integrating give
t 1 1 ) + qt k q 2 qe
(5)
2 e
FIGURE 2. Freundlich isotherms of tannin.
UO2+ 2
adsorbed on immobilized
TABLE 2. Freundlich Parameters of UO2+ 2 Adsorbed on Immobilized Tannin temp (K)
1/n
k (mmol/g)
correl coeff R2
293 303 313 323
0.4391 0.3322 0.2171 0.1973
0.6580 0.6573 0.6406 0.6804
0.9984 0.9937 0.9999 0.9993
and Freundlich adsorption isotherms. It was observed that the data fit well with the classical Freundlich equation, eq 1, rather than the Langmuir equation for the studied system
log qe ) 1/nlog ce + log k
(1)
where qe and ce are the amounts adsorbed (mmol UO2+ 2 /g) and bulk concentration (mmol UO2+ 2 /L) at equilibrium and k and 1/n are the Freundlich constants referring to adsorption capacity and intensity of adsorption, respectively. The straight lines were found for these systems by plotting log qe vs log ce (Figure 2) at 293 K to 323 K with an interval of 10 K, which give the values of k and 1/n by intercept and slope of these lines, respectively. The parameters are listed in Table 2. The value of 1/n is smaller than 1, indicating that the surface of the adsorbent is heterogeneous in nature. As the temperature increased, the 1/n decreased, but k almost remained unchanged. It implies that the energy of adsorption sites on an adsorbent has an exponential distribution (20). This can be further inferred from the molecular structure of Myrica rubra tannin in Figure 1. The ortho-phenolic hydroxyl group of B ring and the galloyl group should have different activation energies when Myrica rubra tannin is chelated with UO2+ 2 . 326
9
ENVIRONMENTAL SCIENCE & TECHNOLOGY / VOL. 38, NO. 1, 2004
The equilibrium adsorption capacity qe and the pseudosecond-order rate constant k2 can be experimentally determined from the slope and the intercept of the plot of t/qt against t. Figure 3 is the fitting experimental data and pseudosecond-order model, and the parameters of the fitting are listed in Table 3. It can be found from Figure 3 and Table 3 that the pseudosecond-order model gives a perfect fit to all of the experimental data. The equilibrium adsorption capacity calculated by the pseudo-second-order model and that determined by actual measurement are almost the same (error 7 may be due to the fact that the existing state of UO2+ 2 does not favor the adsorption process. As it is known that when the pH is higher than 6.0, UO2+ 2 in the solution will be precipitated after standing for a certain time and will change the existing state of UO2+ 2 in the solution (23). Therefore, it seems that the pH of the solution used for adsorption should be lower than 6.0. But our experiments showed that, even though the pH of the solution was 8.0, no precipitation was observed at the end of the adsorption process. At pH ) 1+ 4.0-5.0, uranyl is mainly in the state of UO2+ 2 and UO2OH , and little UO2(OH02) exists (23). The UO2(OH02) should be the precipitation of UO2+ 2 . In fact, the pH of the solution after adsorption was about 5.0 when the initial concentration of UO2+ 2 was 2.5 mmol/L and the original pH was 8.0. It should be due to the fact that the pH will be decreased because of the release of H+ as the proceeding adsorption process. So the suitable pH for the adsorption should be in the range of 5.0-8.0.
Literature Cited (1) Giammer, D. E.; Hering, J. G. Environ. Sci. Technol. 2001, 35, 3332. (2) Villalobos, M.; Trotz, M. A.; Leckie, J. O. Environ. Sci. Technol. 2001, 35, 3849. (3) Kornilovich, B. Y.; Pshinko, G. N.; Koval’chuk, I. A. Radiochemistry 2001, 43 (5), 464. (4) Kaplan, D. I.; Serkiz, S. M. J. Radioanal. Nucl. Chem. 2001, 248 (3), 529. (5) Rakovan, J.; Reeder, R. J.; Elzinga, E. J.; Cherniak, D. J.; Tait, C. D.; Morris, D. E. Environ. Sci. Technol. 2002, 36, 3114. (6) Borovinskii, V. A.; Lyzlova, E. V.; Ramazanov, L. M. Radiochemistry 2001, 43 (1), 84. (7) Xiong, C. H.; Mo, J. J.; Lin, F. Uranium Mining and Metallurgy (Chinese). 1994, 13 (4), 264. (8) Merdivan, M.; Duz, M. Z.; Hamamci, C. Talanta 2001, 55 (3), 639. (9) Malekzadeh, F.; latifi, A. M.; Shahamat, M.; Levin, M.; Colwell, R. R. World J. Microb. Biotechnol. 2002, 18, 599. (10) Kedari, C. S.; Das, S. K.; Ghosh, S. World J. Microb. Biotechnol. 2001, 17, 789. (11) Chibata, I.; Tosa, T.; Mori, T.; Watanabe, T.; Sakata, N. Enzyme Microb. Technol. 1986, 8, 129. (12) Nakajima, A.; Sakaguchi, T. J. Chem. Technol. Biotechnol. 1987, 40, 223. (13) Sakaguchi, T.; Nakajima, A. Sep. Sci. Technol. 1987, 22 (6), 1609. (14) Nakajima, A.; Sakaguchi, T. J. Chem. Technol. Biotechnol. 1990, 47, 31. VOL. 38, NO. 1, 2004 / ENVIRONMENTAL SCIENCE & TECHNOLOGY
9
327
(15) Yamguchi, H.; Higuchi, M.; Sakata, I. J. Appl. Polym. Sci. 1992, 45, 1455. (16) Yamguchi, H.; Higasida, R.; Higuchi, M.; Sakata, I. J. Appl. Polym. Sci. 1992, 45, 1463. (17) Lu, X. Y. Chem. Ind. Forest Products (Chinese) 2000, 20 (1), 71. (18) Shi, B.; Di, Y. Plant Polyphenols; Science Press (Chinese): Beijing, 2000. (19) Friess, W. Eur. J. Pharmaceutics Biopharmaceutics 1998, 45, 113. (20) Tiwari, D.; Mishra, S. P.; Mishra, M.; Dubey, R. S. Appl. Radiat. Isotopes 1999, 50, 631.
328
9
ENVIRONMENTAL SCIENCE & TECHNOLOGY / VOL. 38, NO. 1, 2004
(21) Hynes, M. J.; Coinceanainn, M. O. J. Inorg. Biochem. 2001, 85, 131-142. (22) Ross, A. R. S.; Ikonomou, M. G.; Orians, K. J. Anal. Chim. Acta 2000, 411, 91. (23) Vodolazov, L. I.; Shatalov, V. V.; Molchanova, T. V.; Peganov, V. A. Atomic Energy 2001, 90 (3), 213.
Received for review April 20, 2003. Revised manuscript received September 29, 2003. Accepted October 6, 2003. ES034369A
