Ind. Eng. Chem. Res. 2005, 44, 4221-4226
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Adsorption Behaviors of Pt(II) and Pd(II) on Collagen Fiber Immobilized Bayberry Tannin Ru Wang, Xuepin Liao, and Bi Shi* The Key Laboratory of Leather Chemistry and Engineering of Ministry of Education, Sichuan University, Chengdu 610065 China
It was found that the collagen fiber immobilized bayberry tannin exhibited high adsorption capacity to Pt(II) and Pd(II). The adsorption isotherms of Pt(II) and Pd(II) could be well described by the Langmuir equation, and their adsorption capacities increased with the rise of temperature. The adsorption kinetics investigations indicated that the adsorption rate of Pt(II) and Pd(II) on the immobilized bayberry tannin could be well described by the pseudo-second-order rate model, and the adsorption capacities calculated by the pseudo-second-order rate model were close to those determined by actual measurement. The adsorption column could be easily regenerated by dilute HCl solution after adsorption of Pd(II), but it was somewhat difficult to regenerate the adsorption column after adsorption of Pt(II). The influence of pH value on the adsorption of Pd(II) and Pt(II) was not significant, and the immobilized bayberry tannin kept high adsorption capacity for these two precious metal ions even at acidic pH. In consideration of this fact, the studies on selective adsorption of Pt(II) and Pd(II) from the mixture solution containing Fe(III), Cu(II), Ni(II), and Zn(II) were carried out by using immobilized bayberry tannin. The experimental results indicated that the adsorption selectivity of the immobilized bayberry tannin to Pt(II) and Pd(II) was remarkable at pH 2.0. Introduction Platinum-group metals (PGM) are precious metals and are widely used in industries because of their specific physical and chemical properties. Therefore, PGM should be recovered not only from primary sources such as hydrochloric acid solution originated from leaching step of hydrometallurgical processes, but also from secondary sources such as industrial and nuclear wastes, from the viewpoint of full utilization of resources. Solvent extraction, ion-exchange and adsorption, and membrane separation processes have been developed for the recovery of Pt(II) and Pd(II) from aqueous solutions.1-5 The industrial application of solvent extraction for separation of PGM has been operated for some years. However, the adsorption had been proved to be an effective approach for the recovery of Pt(II) and Pd(II) from dilute solutions. Therefore, a series of adsorbents for this purpose, like synthetic resins, microorganisms, and biomaterials, have been reported.6-10 Vegetable tannins contain multiple adjacent hydroxyl groups and exhibit specific affinity for many metal ions.11 Thus, they promise to be proper biomass materials to be used as effective and efficient adsorbent for metal ions. However, tannins are water soluble compounds and they have to be chemically modified or immobilized onto water-insoluble matrixes.12 The immobilizations of tannic acid and other tannins have been reported, and the syntheses and characterizations of water insoluble tannin resins have been also described.13-16 Our previous work had developed a novel tanninimmobilization method by using collagen fibers as matrixes.17 The studies indicated that bayberry tannin * To whom correspondence should be addressed. Fax: (86)028-85405237. E-mail:
[email protected].
immobilized onto collagen fibers exhibited excellent adsorption properties to some metal ions, such as Au(III) and UO22+.18-20 In this paper, this kind of adsorbent was used for the recovery of Pt(II) and Pd(II) from aqueous solutions. The adsorption isotherms, adsorption kinetics, and column adsorption kinetics of Pt(II) and Pd(II) on the immobilized bayberry tannin were investigated. In addition, the adsorption selectivity of the immobilized tannin to Pt(II) and Pd(II) in the mixture solution containing Fe(III), Cu(II), Ni(II), and Zn(II) was also investigated. Experimental Section Reagents. The preparations of collagen fiber and bayberry tannin (from the bark of Myrica rubra tree), and the immobilization of the tannin onto collagen fiber were the same as our previous work.18,19 K2PtCl4 and PdCl2 were purchased from Sigma Co. Fe(NO3)3‚6H2O, CuCl2‚2H2O, NiCl2‚6H2O, ZnCl2, and all other reagents were analytical gradet. Determination of Metal Ions. The content of Pd(II) in solutions was determined by inductively coupled plasma (ICP, ARL-3410 ICP-AES, Fison Instruments). The contents of Pt(II), Fe(III), Cu(II), Ni(II), and Zn(II) in solutions were analyzed by atomic absorption spectrometry (AAS, SpetrAA-300,Varian Instruments, Australia). Adsorption Isotherms. The stock solutions of Pt(II) and Pd(II) were prepared by dissolving K2PtCl4 and PdCl2, respectively, in 0.1 mol/L HCl and they were further diluted with distilled water to control concentration for practical uses. A 50-mg portion of immobilized bayberry tannin was suspended in 50 mL of Pt(II) solutions in which the concentration of Pt(II) was 0.103, 0.206, 0.308, 0.410, 0.513, 0.564, and 0.615 mmol/L, respectively. The initial pH of each solution was adjusted to 5.6 with 0.1 mol/L HCl or 0.1 mol/L NaOH. The adsorption experiments
10.1021/ie049069w CCC: $30.25 © 2005 American Chemical Society Published on Web 05/13/2005
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Ind. Eng. Chem. Res., Vol. 44, No. 12, 2005
Figure 1. Adsorption isotherms of (a) Pt(II) and (b) Pd(II) on collagen fiber immobilized bayberry tannin. Table 1. Experimental Conditions for the Study of the Effect of pH on Adsorption Capacity condition solution volume (mL) initial concn. (mmol/L) adsorbent dose (mg) adsorption time (h) adsorption temp. (K) pH
adsorption of Pt(II) 100 0.513 50 48 303 2,3,4,5,6
adsorption of Pd(II) 100 1.88 100 48 303 2,3,4,5,6
Table 2. Experimental Conditions for the Study of Adsorption Kinetics condition
adsorption of Pt(II)
adsorption of Pd(II)
solution volume (mL) initial concn. (mmol/L) adsorbent dose (mg) adsorption temp. (K) pH
100 1.03 50 303 5.6
100 0.940 30 303 5.6
were conducted by constant stirring for 48 h at control temperature. The procedures of making Pd(II) adsorption isotherms were the same as those of Pt(II) study except the concentration of Pd(II) was 0.188, 0.376, 0.564, 0.752, and 0.940 mmol/L, respectively, and the amount of adsorbent was 30 mg. The concentrations of Pt(II) or Pd(II) in residual solution were analyzed by AAS or ICP, respectively. Effect of pH on Adsorption Capacity. The procedures were similar to those of adsorption isotherms study, and the experimental conditions are listed in Table 1. Adsorption Kinetics. The procedures were similar to those of adsorption isotherms study. The concentration of Pt(II) and Pd(II) during adsorption process was analyzed at regular intervals. The experimental conditions are listed in Table 2. Column Adsorption Kinetics. A 2.5-g portion of immobilized bayberry tannin adsorbent was soaked in distilled water for 12 h. Then the adsorbent was filled into a column with diameter of 11 mm. The height of the bed was 215 mm. The 0.256 mmol/L Pt(II) or 0.470 mmol/L Pd(II) solution with initial pH ) 5.6 was pumped into the column with a constant volume velocity of 1.76 BV/h (BV ) bed volume). Effluent was collected by automatic collector and the concentration of Pt(II) or Pd(II) in the effluent was analyzed. At the end of adsorption, 0.1 mol/L HCl solution was used as eluant for desorption with constant volume velocity 1.76 BV/ h, and the concentration of Pt(II) or Pd(II) in the eluate was also determined. The column after desorption was reused for adsorption-desorption recycle experiments. Separation of Pt(II) and Pd(II) from the Mixture Solution of Metal Ions. The solutions of Pt(II), Pd(II),
Fe(III), Cu(II), Ni(II), and Zn(II) were mixed. The concentrations of Pt(II), Pd(II), Fe(III), Cu(II), Ni(II), and Zn(II) in the mixture solution were 0.256, 0.470, 0.895, 0.787, 0.852, and 0.765 mmol/L, respectively, and the pH of the solution was adjusted to 2.0 with 0.1 mol/L HCl. The adsorption dose was 100 mg, and the adsorption experiments were similar to those of the adsorption isotherms study. Results and Discussion Adsorption Isotherms. The adsorption isotherms of Pt(II) and Pd(II) indicated that the collagen fiber immobilized bayberry tannin has high adsorption capacity for these two precious metal ions, as shown in Figure 1. For example, when the initial concentrations of Pt(II) and Pd(II) were 0.513 and 0.940 mmol/L, respectively, the adsorption capacities of immobilized bayberry tannin to Pt(II) and Pd(II) were 0.372 and 0.755 mmol/g, respectively, at 303 K. Adsorption equilibrium data were analyzed by the Langmuir and Freundlich equations. It was observed that the data fit well to the Langmuir equation (eq 1) rather than the Freundlich equation for the studied system.
Ce Ce 1 + ) qe qmaxb qmax
(1)
where qe and Ce are the amounts adsorbed (mmol/g) and bulk concentration (mmol/g), respectively, at equilibrium; qmax is the maximum adsorption amount (mmol/g); and b is the coefficient related to the strength of adsorption, b ) ka/kd (ka is the rate constant of adsorption, kd is the rate constant of desorption).The straight lines were found by plotting Ce/qe vs Ce, as shown in Figure 2, which give the values of b and qmax according to the intercept and slope of these lines, respectively. The Langmuir parameters listed in Table 3 indicate that the qmax increases with increasing temperature, implying that the adsorption of Pt(II) and Pd(II) on the immobilized bayberry tannin should be chemical adsorption. Effect of pH on Adsorption Capacity. The adsorption capacities of Pt(II) and Pd(II) slightly increased with increasing pH, as shown in Figure 3. Considering the fact that Pt(II) and Pd(II) in solution would precipitate when pH is higher than 6.0, the suitable pH for the adsorption of Pt(II) and Pd(II) should be 2.0-6.0. An interesting phenomenon is that the adsorption capacities of Pt(II) and Pd(II) were as high as 0.903 and 0.416 mmol/g, respectively, at pH 2.0. That is to say, the immobilized bayberry tannin keeps significant
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Figure 2. Langmuir isotherms of (a) Pt(II) and (b) Pd(II) absorbed on immobilized bayberry tannin.
Adsorption Kinetics. The experimental data (Figure 4) indicated that the adsorption rates of Pt(II) and Pd(II) on the immobilized bayberry tannin were fast at the beginning and then slowed as equilibrium approached. The adsorption equilibrium of Pt(II) was attained in 200 min, while it took 2000 min for the adsorption of Pd(II). It was found that the adsorption kinetics of Pt(II) and Pd(II) on immobilized bayberry tannin could be well described by the pseudo-second-order rate model (eq 2)21
dqt ) k2(qe - qt)2 dt
Figure 3. Effect of pH on adsorption capacity of Pt(II) and Pd(II). Table 3. Langmuir Parameters of Pt(II) and Pd(II) Adsorbed on Immobilized Bayberry Tannin metal ion Pt(II) Pd(II)
T (K)
qmax (mmol/g)
b
correlation coeff. R2
303 313 323 303 313 323
0.495 0.554 0.599 0.800 0.914 1.29
28.5 27.9 49.9 25.3 14.0 10.9
0.984 0.988 0.999 0.9978 0.9915 0.9837
adsorption capacity for these two metal ions even at acidic pH value. Our previous studies have shown that the adsorption capacity of some common metal ions, such as Cu(II), on the immobilized bayberry tannin was very small at acidic pH.20 Therefore, it might be valuable to use this immobilized bayberry tannin as selective adsorbent for recovery of Pt(II) and Pd(II) from the mixture solutions containing other metal ions in proper pH conditions.
(2)
where k2 is the constant of pseudo-second-order rate, g/mmol‚min; qe is the adsorption capacity at equilibrium, mmol M(II)/g; and qt is the adsorption capacity at time t, mmol M(II)/g. Separating the variables in eq 2 and integrating gives
t 1 1 ) + t qt k q 2 qe
(3)
2 e
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 t/qt against t. Figure 5a and b illustrate that the pseudo-secondorder model perfectly fits to the experimental data of adsorption processes of Pt(II) and Pd(II), and equilibrium adsorption capacities calculated by pseudo-secondorder model and those determined by actual measurement are very close (error
