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Ind. Eng. Chem. Res. 2002, 41, 6436-6442
Polymeric Ion-Exchange Fibers James Economy* and Lourdes Dominguez University of Illinois, Department of Materials Science, 1304 West Green Street, Urbana, Illinois 61801
Christian L. Mangun EKOS Materials Corporation, 101 Tomaras Avenue, Savoy, Illinois 61874
This work explores the design of new ion-exchange materials in the form of fibers that yield a number of important advantages over conventional ion-exchange beads. In this approach, ionexchange fibers are prepared by (1) coating low-cost glass fiber substrates with an appropriate oligomer, (2) cross-linking, and (3) functionalizing the coating to produce either anionic or cationic capability. As a result of the thin coatings, the use of solvents prior to both functionalization and preswelling of the finished product prior to end-use was eliminated, representing a significant simplification of current synthesis methods. Kinetic experiments showed that the contact efficiencies of these systems were greatly improved over the traditional beads because of the higher surface-to-volume ratio and shorter diffusion path lengths. This improvement translated into an order of magnitude increase in both ion-exchange and regeneration rates. Another advantage is the excellent resistance of the fibers to osmotic shock even after multiple regenerations. Finally, these systems were shown to remove heavy metal contaminants effectively to well below part per billion concentrations. Introduction and Background Historical Review. Early references to ion-exchange resins date back to ancient times, particularly in relation to soils and clays. However, two chemists, Thompson and Way, were the first to establish the mechanism of the ion-exchange reaction in the 1900s. Treating soft coals with fuming sulfuric acid produced the first organic ion exchangers, which were termed carbonaceous zeolites.1 The first completely organic ion exchanger was prepared in 1935 by Adams and Holmes. It was synthesized through a condensation polymerization reaction, producing a phenol-formaldehyde cationexchange resin.2 Later, in 1944, D’Alelio patented the now well-known and more stable styrene-based cation exchanger.3 Such a system can be used to generate both cationic and anionic exchange resins. In 1947, McBurney contributed to the advancement of anion-exchange resins with the development of chloromethylation and amination resulting in the desired functionalization. In the chloromethylation step, a chloromethyl functional group is introduced into the ethenylbenzene nuclei. The copolymer can then be functionalized with various alkyl substituted aliphatic amines in the amination step.1 Over the past 70 years, a number of important advances have been made in the area of ion-exchange systems of various types, such as inorganic clays, zeolites, and specialized polymeric systems designed for the “selective” removal of unwanted ions. Ion exchange today has a wide variety of important applications in industries such as pharmaceutical, food processing, chemical synthesis, biomedical, hydrometallurgy, water treatment, synthetic fiber production, and chromatography.4 The impetus for work in this field continues to grow as our water resources become reduced and as the * To whom correspondence should be addressed. E-mail:
[email protected]. Phone: 217-333-9260. Fax: 217-3332736.
U.S. Environmental Protection Agency (EPA) regulations become more stringent. Safe drinking water is becoming a precious and limited commodity, and in many communities, it can be obtained only with great effort and cost because of either scarcity or contamination. Slightly more than half of the U.S. population receives its drinking water from groundwater sources,5 and water usage has increased steadily during the past several decades. This has resulted in widespread declines in water table levels by 40 ft or more.6 Some of the contaminants of increasing concern are heavy metals, pesticides, and toxic chemicals, which have found their way into underground aquifers through anthropogenic as well as natural sources. For example, in 1995, more than 270 million lb of toxic waste (cadmium, lead, arsenic, dioxins, etc.) was recycled in fertilizer factories and applied at farms.7 These toxic contaminants have the potential to continue into the food chain through crop uptake, as well as percolating into underground aquifers as a result of agricultural runoff. Current State of the Art: Ion-Exchange Beads. Conventional ion-exchange beads (IEBs) consist of three-dimensional covalent networks to which ionexchange groups are bonded. The network preserves the structural integrity of the material, while the bound ions provide either cationic or anionic exchange sites. For example, the typical styrene-based matrix is prepared by the suspension polymerization of styrene with varying ratios of divinylbenzene. The beads are then swollen with organic solvents, either sulfonated or chloromethylated, and subsequently aminated to prepare strong cationic or anionic exchange systems, respectively.1,8,9 IEBs are used extensively for the purification and demineralization of water. Although ion-exchange beads can be very effective, it should be noted that they have a number of drawbacks. During the functionalization stage, solvents must be used to facilitate reaction rates and maintain the spherical form of the beads. Examples of solvents used include but are not limited to toluene,
10.1021/ie0204641 CCC: $22.00 © 2002 American Chemical Society Published on Web 10/19/2002
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Figure 1. SEM images of (a) Purolite cationic exchange bead displaying the effect of osmotic shock and (b) ion-exchange fibers.
methylene chloride, perchloroethylene, carbon tetrachloride, ether, and dioxane.1,8 These toxic solvents also impart strong odors or tastes that remain in the finished product. The beads are very susceptible to fracture and breakage due to osmotic shock and must be kept wet at all times (Figure 1a). In addition, the beads frequently require costly service containment systems (because the prevailing shape of the final product is generally limited to small particles) and the added expense of EPArequired disposal of spent beads. More importantly, there are a number of major needs with respect to environmental pollution, where greatly improved ionexchange materials are required. These needs include materials with better contact efficiencies, capabilities for rapid regeneration, much longer service lifetimes, and selectivities to the removal of specific ions (such as heavy metals to acceptable levels of less than 1 ppb). Polymeric Ion-Exchange Fibers The concept of developing ion-exchange materials in the form of fibers was first reported by Economy et al. 30 years ago.10 Fibrous ion-exchange materials have several advantages over the conventional ion-exchange units. These advantages include the ability to be fabricated in the form of felts or fabrics where the contact efficiency is greatly improved. A fibrous form eliminates the need for currently used retainers such as canisters, screens, etc. Thus, the concept of designing cationic and anionic exchange materials by coating glass fiber substrates with a polymeric ion-exchange resin was developed.11
Important parallels can be drawn from recent work on a family of activated carbon coatings prepared on a glass fiber substrate.12,13 In earlier work, issues of environmental concern were addressed with the development of improved activated carbon fibers (ACFs) with tailored pore sizes and pore surface chemistries.14-16 These systems established that activated carbon coated on glass fiber substrates could be produced more costeffectively than commercially available ACFs. This was the consequence of using much lower-cost starting materials and a simplified synthesis route. It was shown that such systems were far more efficient in removing trace contaminants (such as benzene, toluene, ethylbenzene, xylene) to below the part per billion range than granular activated carbon.17 In addition, the carboncoated fibers displayed a 7-fold improvement in rates of adsorption and desorption. This essentially nullified the disadvantage of having a glass core, which tended to decrease the total overall capacity. More recently, research was undertaken to extend many of the above concepts to the design of polymeric based ion-exchange fibers (IEFs) for the removal of undesirable aqueous contaminants. Application of IEFs. Ion-exchange materials coated on glass fiber substrates should demonstrate a number of advantages over the conventional ion-exchange beads. These include simplification of the overall synthesis procedures, including more efficient functionalization and elimination of toxic solvents. Other benefits include the ability to be easily fabricated in the form of felts, papers, or fabrics that will improve media contact efficiency. This will enhance the rates of both reaction and regeneration. In addition, physical and mechanical requirements of strength and dimensional stability should be achieved by the use of glass fiber substrates.11,18 The research outlined in this paper has application to the cleaning of waters contaminated with toxic metal ions from the metal plating industry; tap water that contains excessive lead and copper owing to corrosion of premises plumbing; waters that contain excessive concentrations of species such as arsenic, mercury, and fluoride; and treated drinking water that contains excessive concentrations of the suspected carcinogen bromate. There is a need for new materials that can selectively and cost-effectively remove such contaminants from the matrix of other benign substances. Polymeric Cationic Exchange Fibers. In this approach, cationic exchange fibers were prepared by (1) coating glass fiber substrates with a polystyrene/divinylbenzene oligomer with loadings in the range of 3060%, (2) curing, and (3) sulfonating. Through improved polymerization techniques, the use of solvents prior to functionalization and end-use was eliminated, thus greatly simplifying the overall preparation procedure (Figure 2).11 The use of relatively thin coatings on glass fiber substrates appeared to ensure longer life by eliminating long-term fatigue and cracking caused by residual strains and swelling of the beads (Figure 1b). By lowering the oligomerization temperature to 80 °C and slowing the gelation period (coating at room temperature), the effects of shrinkage forces that can produce internal strains were minimized. In 1952, Wheaton established that the change in volume that the cross-linked beads undergo during sulfonation in concentrated sulfuric acid is 20%.8 In addition, styrene undergoes approximately 14% shrinkage in going from
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Figure 2. Comparison of the synthetic steps required for ion-exchange bead versus fiber.
Figure 4. Batch rates of exchange at two saline concentrations, illustrating a 10-fold increase in reaction rates for the IEF. Figure 3. Capacities of cationic exchange fibers resulting from various sulfonation treatments.
monomer to polymer.19 These shrinkage forces during polymerization can result in internal strains that are frozen into the molecule and are strong enough to rupture the material during the early curing stage. A slow gelation period tends to minimize this effect by allowing the gel to accommodate itself to those shrinkage forces with a minimum of strain frozen into the polymer mass.20 In the spherical cationic beads, the osmotic pressures can be strong enough to destroy the resinous structure and have been calculated to be 143 atm for a 10% cross-linked resin.21 The sulfonation process can be carried out with sulfuric acid, sulfur trioxide, oleum, chlorosulfonic acid, or mixtures thereof.22 Achieving close molecular contact between aromatic sites and the sulfonating agent becomes problematic for the beads, as the surface of the polymer must swell before the next layer can be exposed to the reagent.8 It follows that uniformity of sulfonation might not occur throughout the copolymer. Typically, swelling agents are used to access the bead core and achieve higher levels of functionalization. Fortunately, the ease of sulfonation with thin resin coatings eliminates this problem by circumventing the need for solvents while still attaining homogeneous distributions of exchange groups. Cation-Exchange Properties. A range of cationexchange capacities (CECs) was achieved as a function of the sulfonation time and temperature, with values
reaching as high as 5 mequiv/g based on resin loading (see Figure 3). This is comparable to the best CEC values for the commercial beads. However, kinetic experiments showed that the contact efficiencies of the new systems were greatly improved over those of the traditional beads because of the greater surface/volume ratio associated with the film morphology and the shorter diffusion lengths. This translated into an order of magnitude increase in the ion-exchange rate, as illustrated by Figure 4. (Two concentrations of sodium chloride were evaluated.) The rate of exchange also increased when the ion concentration was higher. The solution closely in contact with the resin was rapidly depleted of ions by proton exchange from the resin. Further exchange was delayed until more ions from the bulk solution could diffuse into the surface film. Diffusion was easier with increasing ionic concentration in bulk solution, making the exchange faster. This supports the view that at low concentrations, the rate is controlled by “film” diffusion and, at higher concentrations, the rate is controlled by “particle” diffusion. In addition, the usefulness of the ion-exchange process resides in the ability to regenerate quickly with no loss in capacity. The IEF were successfully regenerated for 10 cycles with no observable changes in capacity or stability. (A corresponding improvement in the rate of regeneration was also observed.) Although water softening is an important commercial use for ion-exchange materials, a much more serious concern is the removal of toxic inorganics such as heavy
Ind. Eng. Chem. Res., Vol. 41, No. 25, 2002 6439 Table 1. Characteristics of Cationic Filters for Breakthrough Experiments filter type
degree of crosslinking (%)
resin wt %
dry resin weight (g)
filter dimensions (diameter × length)
capacity (mequiv/g of resin)
C-100H beads cationic fibers
5 8
100 57
1.51 1.80
1.3 cm × 5.2 cm 2.54 cm × 3.4 cm
5.0 4.0
Figure 5. Dynamic-mode kinetics of cationic fibers versus beads at 150 ppm lead influent concentration.
metals that remain in wastewaters from the plating industry, tap water that contains excessive lead and copper owing to corrosion, well water that contains species such as arsenic, etc. Recent dynamic mode testing of IEF filters (Table 1) with lead and mercury has demonstrated the superiority of the fibrous substrate over the beads. In the high-concentration (150 ppm lead) regime, the beads’ breakthrough point is evident at 50 ppm (see Figure 5) and then gradually increases. In comparison, the cationic fiber filter was capable of decreasing the lead concentration to approximately 5 ppb. Thus, the fiber form greatly outperformed the cationic beads and, even at this elevated concentration, was able to achieve values below the current EPA maximum containment level (MCL) of 15 ppb. The fiber retains this low effluent concentration until a relatively sharp breakthrough occurs, which is indicative of a short mass transfer zone and fast reaction kinetics. The increased surface-to-volume ratio of the fibers is the apparent reason behind these contrasting results. After regeneration, these same filters were then tested with a 9.6 ppm lead influent concentration, and the breakpoint dropped to 1.8 ppm for the beads and 1 ppb for the ion-exchange fibers, over a 1000-fold difference (Figure 6). For the mercury kinetic experiments at a starting concentration of 98 ppm, the beads’ breakthrough point was observed at 45 ppm, and the fibers’ breakthrough point was observed at 0.4 ppm (see Figure 7). At a 7.2 ppm influent concentration, the two systems dropped to 3.5 and 0.2 ppm respectively (see Figure 8). Finally, at a starting concentration of 0.75 ppm, the beads’ breakthrough point dropped to 0.33 ppm, and the ion-
Figure 6. Dynamic-mode kinetics of cationic fibers versus beads at 9.6 ppm lead influent concentration.
Figure 7. Dynamic-mode kinetics of cationic fibers versus beads at 98 ppm mercury influent concentration.
exchange fibers’ dropped to
