Engineering on the Nanoscale: Enrichment and Separation of Elements Using Membrane-Based Technologies

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1 Engineering on the Nanoscale: Enrichment and Separation of Elements Using Membrane-Based Technologies Kurt E. Geckeler Abstract Membrane-based technologies for the separation of elements using polymer reagents are of great importance and have attracted considerable interest during the last decades. Thus, the development of such separation technologies in the aqueous homogeneous phase is presented and outlined. The concept of separation based on the retention of polymer complexes in aqueous solution, which are separated according to their molecular size, is explained. These hybrid processes are combining the principle complexing agents with the method of ultra- or nanofiltration. The basics of the element interaction in relation to the separation procedures are discussed and the role of the polymer reagent is studied. Typical examples are discussed and the major factors influencing the processes are investigated. In addition, recent developments using this method are also highlighted. E Keywords Elements, Enrichment, Membrane, Separation. I. INTRODUCTION NVIRONMENTAL remediation of municipal and industrial wastewaters is based on the enrichment and separation of hazardous elements in aqueous media. Among many separation techniques, membrane separation is an efficient and widely applied separation process that is comparable to other separation techniques in terms of technical and economical feasibility. Many commercial separation problems are being solved by membrane processes, which can be successfully used to treat industrial effluents. Thus, liquid liquid extraction, sorption, precipitation, ion exchange, and others are classic separation methods for elements in geological, biological, environmental, and industrial fluids. Heterogeneous methods have been employed for the separation of inorganic ions contained in natural waters, industrial fluids, or solid materials. In contrast to homogeneous reactions, these two-phase distribution methods or heterogeneous reactions with diffusion-controlled processes have significant disadvantages, e.g., long contact times. Two-phase systems can be avoided by using separation methods based on Manuscript received October 6, This work was supported in part by the World-Class University (WCU) program at the Gwangju Institute of Science and Technology (GIST) through a grant provided by the Ministry of Education, Science and Technology (MEST) of Korea (Project No. R ). K. E. Geckeler is with the Department of Nanobio Materials and Electronics (DNE), World-Class University (WCU), and the Department of Materials Science and Engineering, Gwangju Institute of Science and Technology (GIST), 1 Oryong-dong, Buk-gu, Gwangju , South Korea (phone: ; fax ; keg@gist.ac.kr). pressure-driven membrane processes in the aqueous homogeneous phase. The selective separation of elements can be achieved by using water-soluble, polymeric reagents in combination with membrane filtration [1]. This technique, termed Liquid-phase Polymer-based Retention (LPR), is based on the separation of ions bound to water-soluble polymers with chelating groups (polychelatogens) from noncomplexed ions [2 4]. This process has found many applications in the recovery of metals from diluted solutions. Major advancements of this method are compiled in Table 1. II. BASICS Nanoscale membrane separation processes are based on semipermeable membranes, which separate certain solution components through a membrane, forming the permeate or filtrate, and also the retentate or concentrate. The general function of the polymeric complexing agents in the LPR process is illustrated in Fig. 1. The retention R of a component is the most important parameter for membrane separation and is defined for a metal ion (R Y ) in Eq. 1: R Y = 1 (c Y,P c Y,F 1 ), (1) where c Y,P and c Y,F are the concentrations of the target metal ion in the permeate and in the feed solution. When the target metal ion (Y) and non-target elements (Z) are separated through the membrane, the separation factor (S Y/Z ) is expressed as: S Y/Z = (c Y,P c Y,F 1 ) (c Z,P c Z,F 1 ) 1, (2) where c Z,P and c Z,F are the concentrations of the non-target elements in the permeate and in the feed solution; R Z is the retention of Z. If Z passes through the membrane freely (R Z = 0), the efficiency of the separation can be characterized by the retention of the target metal ion: S Y/Z = 1 R Y (3) Many parameters influence the retention of elements by membranes such as the type, solution composition, ph, temperature, membrane material, pore size, hydrodynamics, etc. III. LIQUID-PHASE RETENTION Liquid-phase retention is based on the interaction of water-soluble polymeric binding agents present in a multi-component solution, so that these agents will form macromolecular compounds with the target elements. Thus, the ISBN:

2 size of the metal ion would be increased significantly whereas the size of the non-target species would remain unchanged. If such a solution is then passed through a suitable membrane, the membrane would separate the target elements from the non-target species. Conventional membrane filtration systems with stirred cells are used on a laboratory scale, A typical experimental set-up is depicted in Fig. 2. For large-scale applications, however, hollow-fiber systems with pumps have been shown to be more advantageous [5]. The membrane represents a nanoscale barrier that has to retain all ions bound to polymer agents in the liquid-phase polymer-based retention (LPR) process, and to allow to permeate all unbound ions. Thus, the membrane pore size, but not the material or its shape, is the most important feature. On the other hand, the surface properties of the membrane material can influence the separation process, if an interaction between the soluble polymer and the membrane can occur. Mostly polysulfone, polyamide, or cellulose membranes with a molecular mass cut-off (MMCO) of 10 kg mol 1 in connection with polymeric reagents of molar masses in the range between 20 and 100 kg mol 1 are employed. Pressure differences are typically varying in the range from 0.1 to 1 MPa (ca. 15 to 150 psi). In our laboratory a standard pressure of 300 kpa is employed for stirred-cell systems. The experimental details have been described previously in detail [1 4,6]. IV. POLYMER REAGENTS The polymer reagents have to fulfill the following requirements for the separation process [1,4]:. high affinity towards the target element, inactivity towards the non-target element, high molecular mass, possibility of regeneration, chemical and mechanical stability, low toxicity, and low cost. Such soluble, hydrophilic polymers with complex-forming moieties (basis polymers; see Table 3) in aqueous solutions were termed polychelatogens [2 4]. Many different polychelatogens have been prepared and investigated in conjunction with LPR. Derivatives of poly(ethylenimine), poly(vinylamine), and polychelatogens based on polyurethanes, poly- (vinyl alcohol), as well as different other copolymers [4,8,10 18] are encompassed. Poly(ethylenimine) is one of the polymeric binding agents most commonly used and studied in the LPR process to date [3,4,7 9,19 21]. By introducing special ligands the selectivity of the polychelatogens can be significantly enhanced [4,7,9,12-16]. The criteria for selecting the appropriate polymeric ligands, the binding conditions, and several factors influencing the separation of the target elements have been studied and discussed in detail [4,8,10]. Hydroxy-functional polymers based on aziridine such as poly[1-(2-hydroxyethyl) aziridine] containing hydroxyl groups in the side chain have been shown to be able to bind a number of elements [20]. Additional studies in our laboratory include the synthesis of environmentally degradable chelating polymers based on ethylenediamine tetraacetic acid (EDTA) and others [22, 23]. The employment of environmentally degradable polymers as functional reagents in conjunction with the liquid-phase separation procedure has been also studied and also the general extension to environmentally relevant applications [5,22 24]. For example, polymer reagents based on water-soluble polyesters with pendant carboxylic groups were synthesized by polycondensation of ethylenediaminetetraacetic acid (EDTA) dianhydride and diethylenetriaminepentaacetic acid (DTPA) dianhydride with poly(ethylene glycol)s of different chain-lengths [22]. The structure of the DTPA-polyester is given in Fig. 3. Generally, the large number of the elements studied include many transition metals, alkali and alkaline earth metals, and elements from the lanthanide and actinide series [3,4,8,10,17]. LPR studies are typically documented in profiles, which are plots of the retention R vs. the filtration factor Z. The retention of elements in solution by polymeric reagents can be calculated as follows [3,4]: R (%) = c r c o 1 100, (4) where c r is the metal concentration in the retentate after a filtrate volume of V f has been passed, and c o is the initial metal concentration. The filtration factor Z is defined as the ratio of the volume of filtrate V f and the volume of cell solution V o : Z = V f V o 1. V. APPICATIONS Many applications that have been developed so far are related to the removal of heavy metals from liquid wastes or spiked solutions. For example, metals such as copper, nickel, lead, chromium, mercury, and arsenic, among others, can be selectively removed from multi-component solutions of different origins such as electroplating wastewaters, mining and chemical effluents, and wastes from soil washing processes [5]. Also, metals were removed from natural waters, e.g., groundwater and seawater. It was demonstrated that the selectivity of the process is high, so that target transition elements can be separated from each other or from alkali and alkaline-earth metals ions, even if the latter are present in much higher concentrations. New approaches include the application of environmentally degradable polymers as functional reagents in conjunction with the liquid-phase separation procedure and also the general extension to environmentally relevant applications [5,22 24]. Polymer reagents based on water-soluble polyesters with pendant carboxylic groups were synthesized by polycondensation of ethylenediaminetetraacetic acid (EDTA) dianhydride and diethylenetriaminepentaacetic acid (DTPA) dianhydride with poly(ethylene glycol)s of different chain-lengths [22]. Other polymers recently designed for LPR include the poly[(2-hydroxyethyl)-dl-aspartamide], synthesized by polyreaction of aspartic acid and subsequent polymer-analogous functionalization with ethanolamine [23]. After the initial studies considering the fundamental properties and processes a series of applications have been studied and are ISBN:

3 still underway. For example, the separation and enrichment of actinides based on the use of polymer reagents has been studied. For example, poly(ethyleneimine methylphosphonic acid) was used for the interaction with actinide ions in the homogeneous phase. In conjunction with membrane filtration this reagent allows both the separation and the enrichment of actinides from dilute solutions. To this end, the retention profiles for the series of actinides U(VI), Np(V), Pu(IV), Am(III), Cm(III), Cf(III) were investigated at different ph. Significant retention differences between 0% and 100% were found depending on the actinide element and ph. Thus, these elements can be separated and enriched form aqueous solutions of binary or multi-component mixtures. The interactions between Pu(IV) and Pu(VI) and some synthetic water-soluble polymers were also investigated by absorption spectroscopy and ultrafiltration. Depending on the type of polymer plutonium can be bound by different mechanisms. Poly(acrylic acid) binds Pu(IV) and Pu(VI) by stepwise complexation, which can be observed by absorption spectroscopy. Due to chelation, some of the formation constants are greater than those from corresponding complexes with low-molecular carboxylic acids. By poly(styrenesulfonic acid) and poly(vinylsulfonic acid), Pu(IV) is bound via counterion condensation. In nitrate and chloride media, anionic Pu(IV) complexes may be bound by quaternary ammonium polymers like poly(diallydimethylammonium chloride) and permethylated poly(ethyleneimine). In general, the membrane-based methods are characterized by a significantly higher speed compared to conventional methods [24]. One example is given in Table 2, where a comparison of different procedures for the determination of plutonium in environmental samples is given. In view of the development of further experimental methods [25] and processes [26] as well as the availability of new materials [27,28] further advancement in this area can be expected VI. CONCLUSION Nanoscale membrane-based separation technologies such as the Liquid-phase Polymer-based Retention (LPR) process can be effective methods for the selective separation and enrichment of elementss in the homogeneous phase. By tailoring the functional groups it is possible to achieve high selectivities and capacities for many elements. The process has been shown to be applicable both on an analytical scale, especially in conjunction with hybrid techniques, and on a preparative scale for the enrichment an removal of elements from industrial and waste waters. REFERENCES [1] K. E. Geckeler, G. Lange, H. Eberhardt, E. Bayer., Preparation and application of water-soluble polymer-metal complexes in Pure Appl. Chem., vol.52, 1980, pp.1883 [2] B. Ya. Spivakov, K. Geckeler, E. Baye., Liquid-phase polymer-based retention the separation of metals by ultrafiltration on polychelatogens in Nature., vol.315, 1985, pp [3] K. E. Geckeler, E. Bayer, B. Ya. Spivakov, V. M. Shkinev, G. A. Vorobeva. Anal. Chim. Acta 189, 285 (1986). [4] K. E. Geckeler, V. M. Shkinev, B. Ya Spivakov., Liquid-phase polymer-based retention (LPR) - a new method for selective ion separation in Sep. Purif. Methods, vol.17, 1988, pp [5] K. E. Geckeler, K. Volchek., Removal of hazardous substances from water using ultrafiltration in conjunction with soluble polymers in Env. Sci. Tech., vol.30, 2006, pp [6] K. E. Geckeler and H. Eckstein. Analytische und Praparative Labormethoden, Vieweg, Wiesbaden, Germany, [7] K. E. Geckeler, K. Volchek., Synthesis and metal complexation of poly(ethyleneimine) and derivatives in Adv. Polym. Sci., vol.102, 1992, pp [8] K. E. Geckeler. In The Polymeric Materials Encyclopedia, Vol. 6, p CRC Press, Boca Raton (1996). [9] K. E. Geckeler, V. M. Shkinev, B. Ya. Spivakov. Angew. Makromol. Chem. 155, 151 (1987). [10] K. Geckeler, V. N. R. Pillai, M. Mutter, Applications of Soluble Polymeric Supports. in Adv. Polym. Sci., vol.39, 1981, pp [11] K. E. Geckeler, E. Bayer, G. A. Vorobeva, B. YA. Spivako., Water-soluble quinolin-8-ol polymer for liquid-phase separation of elements in Anal. Chim. Acta., vol.230, 1990, pp [12] K. E. Geckeler, B. L. Rivas, R. Zhou. Angew. Makromol. Chem. 193, 195 (1991). [13] K. E. Geckeler, R. Zhou, B. L. Rivas. Angew. Makromol. Chem. 197, 107 (1992). [14] R. Zhou and K. E. Geckeler. Z. Naturforsch. 47b, 1300 (1992). [15] K. E. Geckeler, Synthesis and properties of hydrophilic polymers I. Preparation and metal complexation of poly(4-vinylpyridine-co-nallylthiourea) in Polym. J., vol.25, 1993, pp [16] V. Palmer, R. Zhou, K. E. Geckeler, Angew. Makromol. Chem. 215, 175 (1994). [17] A. Novikov, S. Korpusov, R. Zhou, K. E. Geckeler. Chem. Tech. 45, 464 (1993). [18] K. E. Geckeler, A new method for anion exchange using soluble polymers in Naturwissenschaften, vol.75, 1988, pp [19] I. V. Markova, V. M. Shkinev, G. A. Vorobeva, K. E. Geckeler. Zh. Anal. Khim. 46, 182 (1991). [20] K. E. Geckeler, R. Zhou, A. Fink, B. L. Rivas., Synthesis and properties of hydrophilic polymers. III. Ligand effects of the side chains of polyaziridines on metal complexation in aqueous solution in J. Appl. Polym. Sci., vol.60, 1996, pp [21] M. Tuelue and K. E. Geckeler, Synthesis and properties of hydrophilic polymers. Part 7. Preparation, characterization and metal complexation of carboxy-functional polyesters based on poly(ethylene glycol) in Polym.. Int, vol.48, 1999, pp [22] B. L. Rivas, S. A. Pooley, M. Soto, K. E. Geckeler, Water-Soluble Copolymers of 1-Vinyl-2-pyrrolidone and Acrylamide Derivatives: Synthesis, Characterization, and Metal Binding Capability Studied by Liquid-Phase Polymer-Based Retention Technique in J. Appl. Polym. Sci, vol.72, 1999, pp [23] S.-J. Choi and K. E. Geckeler, Synthesis and properties of hydrophilic polymers. Part 8. Preparation, characterization and metal complexing of poly[(2-hydroxyethyl)- DL-aspartamide] in aqueous solution in Polym. Int, vol.49, 2000, pp [24] B. Ya. Spivakov, V. M. Shkinev, K. E. Geckeler, Separation and preconcentration of trace elements and their physico-chemical forms in aqueous media using inert solid membranes in Pure Appl. Chem, vol.66, 1994, pp.631. [25] K. E. Geckeler and H. Eckstein (Eds.), Bioanalytische und Biochemische Labormethoden, Vieweg, Wiesbaden, Germany, [26] K. E. Geckeler (Ed.), Advanced Macromolecular and Supramolecular Materials and Processes, New York: Kluwer Academic/Plenum Publ., USA, 2003 [27] K. E. Geckeler, E. Rosenberg, (Eds.), Functional Nanomaterials, American Scientific Publ, Valencia, USA, [28] K.E. Geckeler and H. Nishide (Eds.), Advanced Nanomaterials, Wiley-VCH Publishers, Weinheim, Germany, ISBN:

4 TABLE 1 ADVANCEMENTS OF THE LIQUID-PHASE POLYMER-BASED RETENTION METHOD Advancement Year Reference Water-soluble polymer-metal complexes 1980 Pure Appl. Chem. 52, 1883 Liquid-phase Polymer-based Retention (LPR) 1985 Nature 315, 313 Anion exchange in homogeneous liquid-phase 1988 Naturwissenschaften 75, 198 First review on LPR 1988 Sep. Purif. Methods 17, 105 Selective polymer ligands 1990 Anal. Chim. Acta 230, 171 System design for analysis 1992 Internat. Lab. 17, 47 Actinide separation 1993 Chem. Tech. 45, 464 LPR for analysis 1994 Pure Appl. Chem. 66, 631 Environmental applications 1996 Env. Sci. Technol. 30, 725 Mathematical modeling 1996 Macromol. Theor. Simul. 5, 357 Environmentally degradable polymers 1999 Polym. Internat. 48, 909 Renewable resource-based polycondensates Pu application Chemoremediation for environmental protection Polym. Internat. 49, 1519 Radiochim. Acta, 89, 377, Pure Appl. Chem.(IUPAC) 73, 129 TABLE 2 COMPARISON OF DIFFERENT PROCEDURES FOR THE DETERMINATION OF PLUTONIUM IN ENVIRONMENTAL SAMPLES USING THE LPR METHOD+ ISBN:

5 TABLE 3 DIFFERENT EXAMPLES OF BASIS POLYMERS AND FUNCTIONAL GROUPS No. Basis polymer Functional Group 1 Poly(acrylic acid) -COOH 2 Poly(ethylenimine) -(CH 2 ) 2 -N(CH 2 COOH) 2 3 Poly(vinylamine) -(CH 2 ) 2 -N(CH 2 COOH) 2 4 Poly(vinylpyrrolidone) - NH-(CH 2 ) 3 -COOH 5 Poly(vinylalcohol) -PO(OH) 2 6 Poly(vinylsulfonic acid) -SO 3 H 7 Poly(vinylamine) -NH 2 8 Poly(ethylenimine) -N + (CH 3 ) 3 Figures Fig. 1. Concept of the Liquid-phase Polymer-based Retention (LPR) ISBN:

6 Fig. 2. Experimental set-up of the Liquid-phase Polymer-based Retention (LPR) Fig. 3. Structural formula of the DTPA-polyester ISBN:

Polymer metal complexes for environmental protection. Chemoremediation in the aqueous homogeneous phase*

Pure Appl. Chem., Vol. 73, No. 1, pp. 129 136, 2001. 2001 IUPAC Polymer metal complexes for environmental protection. Chemoremediation in the aqueous homogeneous phase* Kurt E. Geckeler Laboratory of Applied