Adsorption of UO22+ on tannins immobilized collagen fiber membrane

Journal of Membrane Science 243 (2004) 235–241

Adsorption of UO22+ on tannins immobilized collagen fiber membrane Xuepin Liao, Hewei Ma, Ru Wang, Bi Shi∗ The Key Laboratory of Leather Chemistry and Engineering of Ministry of Education, Sichuan University, Chengdu 610065, PR China Received 18 February 2004; accepted 19 June 2004 Available online 17 August 2004

Abstract A novel adsorption membrane was prepared by immobilizing condensed vegetable tannins onto collagen fiber membrane. The tannins immobilized onto the membrane keep their ability of chelating metal ions and can withstand the exposure to water and organic agents. The adsorption capacities of bayberry tannin and black wattle tannin immobilized membranes to UO2 2+ were 56.8 mg U/g and 53.0 mg U/g, respectively, at 303 K when the initial concentration of UO2 2+ was 263 mg U/L.The adsorption isotherm of tannins immobilized membrane to UO2 2+ can be described by the Freundlich model. The adsorption kinetics of the membrane to UO2 2+ can be well described by the pseudo-second-order rate model. The adsorption capacity calculated by the pseudo-second-order rate model was closed to that determined by actually measurement. The continuous adsorption–desorption studies indicated that this kind of adsorption membrane has excellent adsorption–desorption properties. For monolayer membrane, more than 90% UO2 2+ was adsorbed when 500 mL UO2 2+ solution (conc. 263 mg U/L) passed through it. When three layers of membrane were employed, no UO2 2+ was detected until 1000 mL the effluent was collected. The membrane can be easily regenerated by a small quantity of 0.1 M HNO3 after adsorption, and therefore UO2 2+ can be greatly concentrated. The repeated adsorption–desorption experiments confirmed that the tannins immobilized membrane could be used for at least 10 times without considerable decrease of adsorption capacity. © 2004 Elsevier B.V. All rights reserved. Keywords: Tannins; Immobilization; Collagen fiber; Membrane; Adsorption; UO2 2+

1. Introduction It is well known that uranium is one of the most vital elements for human beings, and that the contamination caused by uranium is a serious environmental problem. Solvent extraction, ion-exchange and adsorption are the possible ways of removing uranium from wastewater, among which the adsorption is the most effective approach for dilute solution. Activated carbon has long been used as an adsorbent for adsorption of heavy metal ions in the situation of relatively low content [1,2]. Up to now, numerous experimental studies on UO2 2+ adsorption by minerals [3–6], phosphates [7,8], resins [9,10] and microorganisms [11,12] adsorbents have been published.

∗ Corresponding author. Tel.: +86 28 85405508; fax: +86 28 85405237. E-mail address: [email protected] (B. Shi).

0376-7388/$ – see front matter © 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.memsci.2004.06.025

Tannins have multiple adjacent phenolic hydroxyls and exhibit specific affinity to metal ions. Thus they promise to be a versatile agent for the treatment of metal ions containing wastewater. However, tannins are water-soluble compounds, which restricts their practical application in this field. To overcome this disadvantage, attempts had been made to immobilize tannins onto various water-insoluble matrices. The immobilization of tannic acid onto cellulose by successive reactions of epichlorohydrin activation, diaminohexane expanding chain length and epichlorohydrin re-activation has been reported [13]. Tannic acid immobilized on agarose [14], viscose rayon fiber [15] and the matrices containing amino groups such as albumin, gelatin, aminopolystyrene and 2-vinyl-4,6-diamino-s-triazine [16] were also studied. However, the immobilization procedures are complicated and furthermore, the tannins are easily leached out by water during the adsorption process, due to the weak association between tannins and matrices. As a result, even though

236

X. Liao et al. / Journal of Membrane Science 243 (2004) 235–241

tannins immobilized by these matrices exhibit excellent adsorption capacity to UO2 2+ and other metal ions, the disadvantage still restricts their practical uses. In addition, it was reported [17,18] that the tannins resin prepared from aldehyde and tannins had significant adsorption capacity to Fe(III), Cr(VI), Th(IV), UO2 2+ and Cd(II) [18–20], but it has no more advantage than the immobilized tannins and the disadvantage of leaching by water still remains. Tannins have been traditionally used as tanning agent in leather making due to their highly reactive activity with skin collagen fibers. In this study, the natural collagen fiber membrane was used as the matrices for the immobilization of tannins to simulate the leather making process. Considering that the interaction of collagen and tannins is primarily based on hydrogen bond, aldehydic cross-linking agent was used to improve the association of collagen and tannins. As a result, a novel affinity membrane system on the basis of condensed tannins and natural collagen fiber membrane was prepared in this study and the adsorption–desorption behaviors of UO2 2+ on the membrane were investigated in detail.

2. Experimental 2.1. Preparation of collagen fiber membrane Goatskin was used as raw material for preparing collagen fiber membrane. Goatskin was cleaned, limed, splitted, delimed, bated and pickled according to the procedures of leather processing, so that the non-collagen components in the skin could be removed. Then the skin was treated with 150% aqueous solution of acetic acid (conc. 16 g/L) for three times to remove mineral substances, rinsed by water and stored at 4 ◦ C in wet state. The thickness of collagen fiber membrane was 0.7 mm in wet state. 2.2. Preparation of bayberry tannin and black wattle tannin Bark of Myrica rubra of 200 g was extracted by using 2000 mL 70% (v/v) acetone–water solution for three times. The extract solutions were combined together. After recovery of acetone, the solution was extracted by using 500 mL petroleum ether and 600 mL ethyl acetate, respectively, to remove grease and low molecular weight polyphenols. Then the residual solution was spray dried and 31.2 g bayberry tannin extract with tannin content of 75.4% was obtained. Black wattle tannin extract was obtained from bark of Acacia mearnsii in the same procedures as above and the tannin content was 72.5%. 2.3. Immobilization of tannins onto collagen fiber membrane Collagen fiber membrane of 400 g was depickled to pH = 5.0–5.2 by using 5% Na2 S2 O3 and reacted with tannins in an

experimental drum with 900 mL tannins solution at 25 ◦ C. The original content of tannin extract in the solution was 66.7 g/L. The reaction was processed until full penetration was achieved. The tannin content in the residual solution was analyzed by UV-spectrophotometry at 280 nm so that the amount of tannin fixed on collagen fiber membrane can be calculated. Then the membrane was fully washed with distilled water, immersed in 900 mL oxazolidine (aldehydic cross-linking agent) solution (conc. 12.0 g/L), and reacted at 55 ◦ C for 3 h. The resultant membrane was rinsed with distilled water and dried at 30 ◦ C in vacuum. The tannin immobilized membrane was cut into circular pieces (9.0 cm in diameter) with a perforator. 2.4. Membrane characterization The thickness of the tannin immobilized membrane was measured with thickness gauge. In order to determine the swelling behavior, the membrane pieces were put into distilled water and stilled for 48 h at 25 ◦ C. After the surface water of swollen membranes was removed by filter paper, swollen membranes were weighed by an electronic balance. The water content soaked by the membranes was calculated as (Ws − Wo ), where Wo and Ws are the masses of membrane before and after swelling. The mechanical strengths of the membrane were determined in standard methods as for leather samples [21]. The heat denaturation temperature of the membrane was determined by Differential Scanning Calorimetry (PC 200DSC, NETZSH Company, German) with heating rate 5 ◦ C/min. 2.5. Adsorption capacity of tannin immobilized membrane to UO2 2+ Adsorption capacity of tannin immobilized membrane to UO2 2+ was performed by batch adsorption equilibrium experiments. The stock solution of 10 mmol/L UO2 2+ was prepared with uranyl nitrate hexahydrate and further diluted to control concentrations for practical uses. Tannin immobilized membrane (2 cm × 2 cm) of 0.296 g (dry base) was immersed in distilled water for 48 h. After surface water was removed by filter paper, the membrane was suspended in 100 mL of UO2 2+ solution in which the concentrations of UO2 2+ were 50.8, 101, 159, 213 and 263 mg U/L, respectively. The initial pH of the UO2 2+ solutions was adjusted to 5.0 with 0.1 M NaOH and 0.1 M HNO3 . The adsorption experiments were conducted by constant stirring at controlled temperature for 24 h. The concentrations of UO2 2+ in residual solutions were analyzed by Arsenazo III spectrophotometry. The amount of UO2 2+ adsorbed onto membrane at equilibrium was calculated as: [(Ci − Ce )V ] qe = Wo where qe is the amount of UO2 2+ adsorbed onto unit gram of tannin immobilized membrane (mg U/g), Ci and Ce are

X. Liao et al. / Journal of Membrane Science 243 (2004) 235–241

the concentrations of UO2 2+ in initial solution and residual solution (mg U/L), V the volume of solution (L), Wo the mass of membrane (g). As control experiment, the adsorption capacity of collagen fiber membrane without immobilization of tannins to UO2 2+ was also tested at 303 K in the same procedures as above. 2.6. Adsorption kinetics of UO2 2+ onto tannin immobilized membrane Bayberry tannin immobilized membrane was used for this research. The procedures were similar to those of adsorption isotherms study as above. But the concentration of UO2 2+ in the solution during adsorption process was analyzed in a regular interval. 2.7. Study of the continuous adsorption–desorption of UO2 2+ Bayberry tannin immobilized membrane was used for this research. The membrane of 90 mm in diameter and 0.65 mm in thickness was soaked in distilled water for 48 h, and then installed on the continuous adsorption equipment. The trials using monolayer, two layers and three layers of membrane were undertaken. For multi-layers experiments, the interval distance between membranes was 50 mm. The adsorption process was proceeded at atmospheric pressure and 303 K, and UO2 2+ solution with concentration of 263 mg U/L

237

and pH 5.0 was continuously pumped into adsorption equipment with constant volume velocity of 12.0 mL/min. The effluent was collected by an automatic collector and the concentration of UO2 2+ in effluent was analyzed by Arsenazo III spectrophotometry. After adsorption, the membrane was regenerated by 0.1 mol/L HNO3 solution with constant volume velocity of 12.0 mL/min. Eluent was collected by automatic collector and the content of UO2 2+ in the eluent was also analyzed by means of Arsenazo III spectrophotometry. 3. Results and discussion 3.1. The principle of preparing tannin immobilized collagen fiber membrane Many affinity membranes used for metal ions adsorption have been documented [22,23], but none of them referred to tannin immobilized collagen membrane. The skin collagen fiber membrane is insoluble in water. The collagen making up the membrane is mainly of type I collagen which contains three polypeptide ␣-chains, each consisting of more than 1000 amino acids. Its primary sequence is basically a tri-peptide repeat, (Gly-X-Y)100–400 , where X is often proline and Y sometimes is hydroxyproline [24]. Collagen has abundant functional groups ready to react with other chemicals, like OH, COOH, CONH2 and NH2 , as shown in Fig. 1. Tannins, which are widely distributed in plants, are the

Fig. 1. The illustration of chemical structures of tannins and primary amino acid sequence of skin collagen.

238

X. Liao et al. / Journal of Membrane Science 243 (2004) 235–241

Table 1 Physical and mechanical properties of the tannins immobilized membranes Properties

3.2. Physical and mechanical properties of tannins immobilized membranes

Values

Density (in dry state) Thickness (in dry state) Tannin content (dry base) Water absorptivity Transmembrane flow of water (at atmospheric pressure) Tensile strength (in dry state) Tear strength (in dry state) Denaturation temparaturea

Black wattle tannin immobilized membrane

Bayberry tannin immobilized membrane

0.55 g/cm3 0.65 mm 23.1 g/g 1.88 gH2 O/g 112 L/m2 h

0.62 g/cm3 0.65 mm 22.9 g/g 1.73 gH2 O/g 93.9 L/m2 h

1.02–1.09 MPa 245 ± 20.0 N/mm 98–108 ◦ C

1.07–1.87 MPa 343 ± 20.0 N/mm 95–105 ◦ C

a

The denaturation temperature of raw collagen fiber membrane is 60–65 ◦ C.

polyphenols with molecular weight 500–3000 Da. According to the chemical structures of tannins, they are usually 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 flavan-3-ols and/or flavan-3,4diols. The bayberry tannin and black wattle tannin employed in this study are typical condensed tannins. Unlike hydrolyzable tannins, condensed tannins possess highly nucleophilic sites (C-6 and C-8 of A ring) in molecules (see Fig. 1), so they can be firmly fixed onto collagen fibers through Mannich reaction in the presence of aldehydic agent. Therefore, the tannins immobilized membranes prepared according to the method described in Section 2 are capable of withstanding water extraction and in fact, no leaked tannin was detected in all the adsorption and desorption experiments. A particular advantage of the membranes is of their highly mechanical strengths and convenient to handing, as referred to the properties of leather. Meanwhile, the porosity of collagen fiber membrane permits well mass-transfer performance, thus providing a high adsorption rate of metal ions. In addition, large surface area, short diffusion path, low pressure drop and very short residence time for adsorption and desorption are also the potential advantages of tannin immobilized membranes.

The main physical and mechanical properties of tannins immobilized membranes were summarized in Table 1. It is indicated that the mechanical strengths and thermal stability of the membranes perfectly satisfy the property of membrane materials, which is due to the so-called vegetable tanning effect. 3.3. Adsorption capacity of tannins immobilized membranes to UO2 2+ The adsorption capacities of UO2 2+ on tannins immobilized membranes were presented in Table 2. It can be found that the adsorption capacities of the two membranes were nearly same and increased with the rise of temperature. In comparison with raw collagen fiber membrane, the enhancement of tannins in adsorbing UO2 2+ is remarkable due to their chelating ability with metal ions. The data determined were further analyzed by the Langmuir and the Freundlich equations, and it was observed that the data fit well to the classical Freundlich equation (1), rather than the Langmuir equation for the studied system: 1 log ce + log k (1) n where qe and ce are the amounts adsorbed (mg U/g) and left in bulk solution (mg U/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 obtained by plotting log qe versus log ce at 303, 313 and 323 K (Fig. 2), which give the values of k and 1/n by intercept and slope of these lines, respectively. The data of k and 1/n were listed in Table 3. log qe =

3.4. Adsorption rate of bayberry tannin immobilized membrane to UO2 2+ Considering that the difference of adsorption capacities of the two tannins immobilized membranes is not significant, all the following studies were focused on bayberry tannin immobilized membrane. Fig. 3 shows the adsorption rate of UO2 2+ on the membrane is a function of time. High adsorp-

Table 2 The adsorption capacities of tannins immobilized membranes and raw collagen fiber membrane to UO2 2+ at equilibrium (mg U/g) Initial concentration (mg U/L)

Black wattle tannin immobilized membrane

Bayberry tannin immobilized membrane

303 K

313 K

323 K

303 K

313 K

323 K

50.8 101 159 213 263

11.6 22.8 31.6 42.8 53.1

12.4 24.7 31.6 46.4 56.4

14.0 32.6 39.8 53.6 66.6

11.8 22.9 31.9 43.5 56.8

12.5 24.9 34.9 48.8 57.8

13.9 32.0 40.2 53.7 67.5

Collagen fiber membrane (303 K) 2.2 3.6 4.3 6.7 7.1

X. Liao et al. / Journal of Membrane Science 243 (2004) 235–241

239

Fig. 2. The Freundlich isotherms of UO2 2+ adsorbed on tannins immobilized membranes: (a) black wattle tannin immobilized membrane; (b) bayberry tannin immobilized membrane.

Table 3 Freundlich parameters of UO2 2+ adsorbed on tannins immobilized membranes Freundlich parameters

Black wattle tannin immobilized membrane 303 K

313 K

323 K

303 K

313 K

323 K

1/n k (mg U/g) Correlation coefficient, R2

0.779 1.35 0.994

0.713 2.11 0.968

0.760 2.53 0.974

0.809 1.27 0.987

0.763 1.83 0.994

0.801 2.18 0.985

tion rate is observed at the beginning of adsorption, and then the adsorption rate slows down and adsorption equilibrium is approached. It can be also observed that the higher concentration of UO2 2+ leads to faster adsorption rate. This should be due to the fact that higher concentration difference of UO2 2+ between the liquid and solid phases produces higher driving force for diffusion and adsorption of UO2 2+ onto the membrane.

Bayberry tannin immobilized membrane

It was also found that the experimental data of adsorption rates could be described by pseudo-second-order rate model [25]: dqt = k2 (qe − qt )2 dt

(2)

where k2 is the constant of pseudo-second-order rate, g/mg min; qe the adsorption capacity at equilibrium, mg U/g and qt the adsorption capacity at time t, mg U/g. Separating the variables in Eq. (2) and integrating give: t 1 1 = + t 2 qt qe k2 q e

(3)

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. Fig. 4 is the fitting of experimental data by pseudo-secondorder model, and the parameters of the fitting are listed in Table 4. It is illustrated that the pseudo-second-order model gives a perfect fit to all of the experimental data. Table 4 The parameters of adsorption rate fitted by pseudo-second-order model

Fig. 3. The adsorption rate of bayberry tannin immobilized membrane to UO2 2+ at 303 K.

Initial concentration (mg U/L)

k2 × 104 g/mg min

Calc. qe (mg U/g)

R2

Exp. qe (mg U/g)

50.8 159 263

2.54 2.77 4.32

18.8 41.0 55.3

0.991 0.987 0.993

12.7 39.2 60.2

240

X. Liao et al. / Journal of Membrane Science 243 (2004) 235–241

Fig. 4. The fitting of experimental data of adsorption rate by pseudo-secondorder model at 303 K.

The equilibrium adsorption capacity calculated by pseudosecond-order model and that determined by actual measurement are very closed to each other. 3.5. Continuous adsorption–desorption of UO2 2+ on bayberry tannin immobilized membrane Fig. 5(a) and (b) are the adsorption and desorption curves of monolayer bayberry tannin immobilized membrane. When the volume of effluent was 500 mL, the concentration of UO2 2+ in the effluent was around 20 mg U/L compared to 263 mg U/L of inlet solution, indicating that more than 90% UO2 2+ had been retained by the membrane. The adsorption curves nearly overlapped, and the same phenomenon was also observed for the desorption curves. This fact indicates that the adsorption property of the membrane was almost unchanged for three repeated adsorption–desorption cycles.

Fig. 6. Adsorption curves of UO2 2+ on bayberry tannin immobilized membrane.

It can be found that about 100 mL of 0.1 M HNO3 solution was capable of regenerating the membrane, as shown in Fig. 5(b). The maximum concentration of UO2 2+ in eluent was 4500–5000 mg U/L, much higher than inlet concentration of UO2 2+ , which indicates that the tannin immobilized membrane could greatly concentrate UO2 2+ . Our further experiments confirmed that this kind of membrane could be repeatedly used for at least 10 times. In general, the tannin immobilized membrane has excellent adsorption–desorption properties. Fig. 6 shows the adsorption curves of monolayer, two layers and three layers of membrane, respectively. It was found that, for three layers of membrane, there is no UO2 2+ came out when the effluent volume was 1000 mL, which represents the effectiveness and practicability of this novel adsorption membrane for the recovery of UO2 2+ .

Fig. 5. Adsorption and desorption curves of UO2 2+ on monolayer bayberry tannin immobilized membrane: (a) adsorption curves and (b) desorption curves.

X. Liao et al. / Journal of Membrane Science 243 (2004) 235–241

4. Conclusion The chelating ability of tannins to metal ions originates from its adjacent phenolic hydroxyls. This potential is still kept when immobilized onto collagen fiber membrane by cross-linking reaction. Therefore, a novel adsorption membrane for recovery of metal ions could be prepared on the basis of tannins and collagen fiber membrane. This paper showed that the tannins immobilized membranes possess proper physical properties being used as membrane material and have excellent adsorption and desorption characteristics for UO2 2+ . The effectiveness and practicability of tannins immobilized membrane in the recovery of UO2 2+ from aqueous solution are obvious. On the basis of this study, wider investigations of the membranes in adsorption of metal ions should be undertaken.

Acknowledgments This research was financially supported by National Science Fund for Distinguished Young Scholars (20325619) and The Key Science and Technology Research Project of Sichuan Province, China (01ZQ052-52).

References [1] K. Kadirvelu, C. Faur-Brasquet, P. Le Cloirec, Removal of Cu(II), Pb(II), and Ni(II) by adsorption onto activated carbon cloths, Langmuir 16 (2000) 8404. [2] S. Biniak, M. Pakuta, G.S. Szymanski, A. Swiatkowski, Effect of activated carbon surface oxygen- and/or nitrogen-containing groups on adsorption of copper(II) ions from aqueous solution, Langmuir 15 (1999) 6117. [3] D.E. Giammer, J.G. Hering, Time scales for sorption–desorption and surface precipitation of uranyl on goethite, Environ. Sci. Technol. 35 (2001) 3332. [4] M. Villalobos, M.A. Trotz, J.O. Leckie, Surface complexation modeling of carbonate effects on the adsorption of Cr(VI), Pb(II), and U(VI) on goethite, Environ. Sci. Technol. 35 (2001) 3849. [5] B.Y. Kornilovich, G.N. Pshinko, I.A. Koval’chuk, Effect of fulvic acids on sorption of U(VI) on clay minerals of soils, Radiochemistry 43 (2001) 464. [6] D.I. Kaplan, S.M. Serkiz, Quantification of thorium and uranium sorption to contaminated sediments, J. Radioanal. Nucl. Chem. 248 (2001) 529.

241

[7] A.V. Savenko, Sorption of UO2 2+ on calcium carbonate, Radiochemistry 43 (2001) 174. [8] L. Baraniak, G. Bernhard, H. Nitsche, Influence of hydrothermal wood degradation products on the uranium adsorption onto metamorphic rocks and sediments, J. Radioanal. Nucl. Chem. 253 (2002) 185. [9] J.A. Greathouse, R.J. O’Brien, G. Bemis, R.T. Pabalan, Molecular dynamics study of aqueous uranyl interactions with quartz(0 1 0), J. Phys. Chem. B 106 (2002) 1646. [10] L. Al-Attar, A. Dyer, R. Blackburn, Uptake of uranium on ETS10 microporous titanosilicate, J. Radioanal. Nucl. Chem. 246 (2002) 451. [11] J. Rakovan, R.J. Reeder, E.J. Elzinga, D.J. Cherniak, C.D. Tait, D.E. Morris, Structural characterization of U(VI) in apatite by X-ray adsorption spectroscopy, Environ. Sci. Technol. 36 (2002) 3114. [12] V.A. Borovinskii, E.V. Lyzlova, L.M. Ramazanov, Sorption of uranium on zirconium phosphate inorganic cation exchanger, Radiochemistry 43 (2001) 84. [13] I. Chibata, T. Tosa, T. Mori, T. Watanabe, N. Sakata, Immobilized tannins: a novel adsorbent for protein and metal ion, Enzyme Microb. Technol. 8 (1986) 129. [14] A. Nakajima, T. Sakaguchi, Recovery of uranium by tannins immobilized on agarose, J. Chem. Tech. Biotechnol. 40 (1987) 223. [15] T. Sakaguchi, A. Nakajima, Recovery of uranium from seawater by immobilized tannins, Sep. Sci. Technol. 22 (1987) 1609. [16] A. Nakajima, T. Sakaguchi, Recovery of uranium by tannins immobilized on matrices which have amino group, J. Chem. Tech. Biotechnol. 47 (1990) 31. [17] H. Yammguchi, M. Higuchi, I. Sakata, Methods for preparation of absorbent microspherical tannins resin, J. Appl. Polym. Sci. 45 (1992) 1455. [18] H. Yammguchi, R. Higasida, M. Higuchi, I. Sakata, Adsorption mechanism of heavy-metal ion by microspherical tannins resin, J. Appl. Polym. Sci. 45 (1992) 1463. [19] A. Nakajima, T. Sakaguchi, Uptake and removal of iron by immobilized tannin, J. Chem. Technol. Biotechnol. 75 (2000) 977. [20] J.L. Santana, L. Lima, J. Torres, F. Martinez, S. Olivares, Simultaneous metal adsorption on tannin resins, J. Radioanal. Nucl. Chem. 251 (2002) 467. [21] IULTCS, IULTCS (IUP) test methods, J. Soc. Leather Technolog. Chemist 84 (2002) 317. [22] D. Bhattacharyya, J.A. Hestekin, P. Brushaber, L. Cullen, L.G. Bachas, S.K. Sikdar, Novel poly-glutamic acid functionalized microfiltration membranes for sorption of heavy metals at high capacity, J. Membr. Sci. 141 (1998) 121. [23] L.N. Zhang, J.P. Zhou, D.C. Zhou, Y.R. Tang, Adsorption of cadmium and strontium on cellulose/alginic acid ion-exchange membrane, J. Membr. Sci. 162 (1999) 103. [24] W. Friess, Collagen-biomaterial for drug delivery, Eur. J. Pharmaceut. Biopharmaceut. 45 (1998) 113. [25] Y.S. Ho, G. Mckay, Sorption of dye from aqueous solution by peat, Chem. Eng. J. 70 (1998) 115.