Ind. Eng. Chem. Res. 1994,33, 904-906
904
Concentration and/or Removal of Metal Ions Using a Water-Soluble Chelating Polymer and a Microporous Hollow Fiber Membrane Tahei Tomida,' Tatsuya Inoue, Katsumi Tsuchiya, and Seizo Masuda Department of Chemical Science and Technology, The University of Tokushima, Tokushima 770, Japan
Methods for concentrating and/or removing metal ions from dilute solutions using a water-soluble chelating polymer and a microporous hollow fiber were studied. Experiments were made with a model system of polyacrylic acid and Cu(11). In the once-through mode, where the metal solution flowed inside the fiber immersed in the polymer solution, concentrations of metal ions of as high as 80% in the sample solutions were deionized by use of a fiber of 1.2-m length and 0.8-mm inner diameter. In the circulating mode, where the polymer solution was circulated through the fiber immersed in the sample solution, the metal ions could be recovered from dilute solutions even when concentrationswere much lower than those on the recovery side. The overall mass transfer coefficient obtained in this experiment was 10-6-10-6 m/s. The methods reported here were demonstrated to be effective for concentrating and/or removing metal ions from dilute solutions. 1. Introduction
In a previous paper (Tomida et al., 19931, we proposed a new method using a water-soluble chelating polymer and a dialysis membrane for recovering metal ions. The effectiveness of this method was demonstrated by results in a batch process. As microporous hollow fibers have a much higher surface area per unit volume of fiber, the efficiencies of concentrating and/or removing metal ions should be improved by replacing the dialysis membrane by a microporous hollow fiber membrane. In addition, the use of a microporous hollow fiber coupled with a watersoluble chelating polymer should result in diversity of the process configurations. The unique feature of the present method is that the metal ions permeate through a microporous fiber membrane to reach the chelating polymer solution on the recovery side and react with the polymer to form metalchelating polymer complexes. The membrane allows permeation of metal ions but not water-soluble chelating polymer or metal-polymer complexes. Therefore, in this method,there is no aqueous-organic interface that causes unfavorable mass-transfer resistances, unlike in the method of membrane solvent extraction (Yun et al., 1992) or in extraction using an oil-soluble chelating polymer (Goto et al., 1992). In addition, this method does not need any equipment for filtration, such as that required for membrane filtration of a water-soluble chelatingpolymer-metal complex (Geckeler et al., 1980). In this study, we examined the concentration and/or removal of metal ions in a model system of Cu(I1) and polyacrylic acid (a water-soluble chelating polymer). Polyacrylic acid is known to form a stable complex with Cu(1I) (Gregor et al., 1955). Experiments were carried out in two modes: (1) a circulating mode in which the chelating polymer solution circulated inside a hollow fiber immersed in a sample solution of metal ions, and (2) a once-through mode, in which the metal ion solution passed inside the hollow fiber immersed in the polymer solution. 2. Experimental Section
Materials. The water-soluble chelating polymer used was commercially available polyacrylic acid, PAA, with a molecular weight of 90 000 (Aldrich Chemical Co.). PAA solutions were adjusted to pH 5 with NaOH solution. The microporous hollow fiber used was a polyacrylonitrile fiber with a molecular cutoff of 13 000 of 0.3-mm thickness, 0888-588519412633-0904$04.50/0
(A)
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(6) Once-through mode
Figure 1. Diagrams of experimental modes: (A) circulating mode and (B)once-throughmode. (1) Feed solution, (2) chelatingpolymer solution, (3) microporow hollow fiber, (4) peristaltic pump, and (6) magnetic stirrer.
1.2-m length, and 0.8-mm inner diameter (Asahikasei Kogyo Co.). Thus, the volume of the inner cylinder, the surface area, and the specific surface area (surface area per unit volume) based on the inner diameter of the hollow fiber were calculated to be 0.603 cm3, 30.16 cm2, and 50 cm-l, respectively. All the chemicals used were analytical or special grade. Sample solutions were prepared by dissolving copper chloride in demineralized, distilled water. Experimental Procedure. Figure 1 shows the two modes used for metal ion treatmenta. In the circulating mode (Figure lA), the water-soluble chelating polymer, PAA, was passed through the hollow fiber immersed in a metal solution at a fixed flow rate obtained with a peristaltic pump. In the once-through mode (Figure lB), the metal solution was passed through the hollow fiber membrane immersed in PAA solution. In both modes, the solution in the cell was stirred with a magnetic stirrer at 500 rpm. Experiments were carried out at a room temperature of about 25 "C. For determination of the change of metal ion concentration with time, samples of 0.5 cm3of aqueous solution 1994 American Chemical Society
Ind. Eng. Chem. Res., Vol. 33, No. 4, 1994 905 1.5
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Concentration and/or Removal of Metal Ions. Figure 2 depicts the time courses of changes in concentration of Cu(I1) in the feed cell and in the recovery cell (containingthe polymer solution)for the circulatingmode. The concentration in the recovery cell, C2, shown by open circles, was estimated using the experimental values of C1 from eq 1:
where VI and V2 are the volumes of the sample solution and the polymer solution, respectively. As seen in the figure, it was possible to recover metal ions even when their concentration was much lower than that in the recovery cell. The figure shows that the concentration of metal ions recovered (or concentrated) in about 3 h was about 15-fold their initial concentration in the feed solution. The concentration factor depended on the volume ratio of the feed solution to the chelating polymer solution, provided there was a sufficient stoichiometric amount of the chelating polymer. Figure 3 shows results for the once-through mode. The outlet concentration of Cu(I1) is plotted against the space time, t,. As much as 80%of the metal ions were removed within about 3 min of the space time using a hollow fiber of 1.2-m length and 0.8-mm inner diameter. These results demonstrate that this method is effectivefor deionization. Overall Mass-Transfer Coefficient. In evaluation of the overall mass transfer coefficient, the following assumptions were made: (1)plug flow occurred with no axial diffusion in the microporous hollow fiber tube; (2) there were no radial gradients in the concentration of the solution flowing in the hollow fiber; (3) the concentration of free metal ions in the chelating polymer solution was negligiblewhen the concentrationof the chelatingpolymer solution was much higher than that of the metal ions. As seen in Figure 4, the concentration variations in the feed cell in the circulating mode, were well fitted to eq 2: ln(C,/C,) = -K&/V,
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where A is the surface area based on the inner diameter of the hollow fiber. Therefore, the overall mass-transfer coefficient, Kf, was calculated from the slopes of these lines. For the once-throughmode, Kf was calculated from eq 3:
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(3)
where a and U,are the interfacial area per unit volume based on the inner diameter of the microporous hollow fiber and the space velocity, respectively. Figure 5 showsthe relationshipbetween Kf and the space velocity of the solution in the fiber. In the once-through mode, the value of Kf increased with an increase in the U, of the solution. The extent of the dependence of Kf on U, decreased with an increase in the metal ion concentration. However,in the circulating mode, the change in Kf (shown by dashed lines) was less with respect to that of either the flow rate of chelating polymer solution or the metal ion concentration in the sample solution. The values of Kf obtained in this experiment are of the same order of magnitude as the mass-transfer coefficient obtained for gas absorption in a microporous hollow fiber device (Sujatha and Sirkar, 1993) and about 10 times the permeability of metal ions on extractionwith the oil-soluble chelating agents using hollow fiber membranes (Goto et al., 1992). The high Kfvalue obtained by this method is consideredto be due to the absence of an aqueous-organic interface, which is a major cause of resistance to mass transfer across the interface. From the present results, we conclude that the methods proposed here are simple and effective for concentration
906 Ind. Eng. Chem. Res., Vol. 33, No. 4, 1994
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Literature Cited 1
and/or removal of metal ions from dilute solution. In addition, this method is available for treating a large quantity of metal ion solution because it involves no processes for mixing the chelating polymer with the metal solutions and/or filtration of the solutions.
Nomenclature A = interfacial area of hollow fiber based on inner diameter, m2 a = interfacial area per unit volume of hollow fiber based on inner diameter, l / m C = concentration, m0Vm3 CO= initial concentration, m0l/m3 C, = concentration of water-solublechelating polymer, mol/ m3
Geckeler, K.; Lange, G.; Eberhardt, H.; Bayer, E. Preparation and Application of Water-Soluble Polymer-Metal Complexes. Pure Appl. Chem. 1980,52,1883. Goto, M.; Miyata, T.; Kubota, F.; Nakashio F. Selective Separation of Rare Earth Metals with New Oil-Soluble Complexing Agents. J. Chem. Eng. Jpn. 1992,25,349. Gregor, H. P.;Luttinger, L. B.; Loebl, E. M. Metal-Polyelectrolyte Complexes. I. The Polyacrylic Acid-Copper Complex. J. Phys. Chem. 1955,59,34. Sujatha, K.; Sirkar, K. K. Gas Absorption Studies in Microporous Hollow Fiber Membrane Modules. Znd.Eng. Chem. Res. 1993,32,
674. Tomida, T.; Ikawa, T.; Masuda, S. Recoveryof Metal Ions from Dilute Solutions Using a Water-Soluble Chelating Polymer and Dialysis Membranes. J. Chem. Eng. Jpn. 1993,26,575. Yun,C. H.;Prasad, R.; Sirkar, K. K. Membrane Solvent Extraction Removal of Priority Organic Pollutants from Aqueous Waste Streams. Ind. Eng. Chem. Res. 1992,31, 1709. Received for review August 30, 1993 Revised manuscript received December 13, 1993 Accepted December 22,1993.
* Abstract published in Advance ACS Abstracts, February 15,1994.
