A Comparative Study of the Removal of Heavy Metal Ions from Water

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SEPARATIONS A Comparative Study of the Removal of Heavy Metal Ions from Water Using a Silica-Polyamine Composite and a Polystyrene Chelator Resin Susan T. Beatty, Robert J. Fischer, Dana L. Hagers, and Edward Rosenberg* Department of Chemistry, The University of Montana, Missoula, Montana 59812

The maximum Cu(II), Ni(II), and Co(II) ion capacities of a silica-poly(ethyleneimine) composite (WP-1) are compared with those of the commercially available iminodiacetic acid chelator resin Amberlite IRC-718. Under batch (static) conditions, IRC-718 exhibits better capacities for these metals than WP-1. Dynamic studies, however, revealed that WP-1 possessed a much higher capacity for all three divalent metals than IRC-718, with relative metal capacities in the order Cu(II) > Co(II) ≈ Ni(II). In the presence of the competing chelator ethylenediaminetetraacetic acid, the Cu(II) capacities of WP-1 and IRC-718 lost 48% and 45%, respectively, of their original adsorption values. Even with this decrease, however, WP-1 maintained a higher Cu(II) capacity than IRC-718. Repeated cycle testing, using Cu(II) solutions at both room temperature and 97 °C, was conducted to compare the long-term stability of each material. WP-1 maintained 94% of its original Cu(II) capacity and maintained structural integrity after 3000 cycles using room temperature copper solutions, while IRC-718 compressed and dropped to 64% of its original capacity. When boiling copper solutions were used, the capacity of WP-1 increased slightly over 1500 cycles, while IRC-718 lost 13% of its original copper capacity and again became compressed, indicating degradation of the polystyrene beads. Introduction Water contaminated with heavy metal ions is a byproduct of many industries, including those involving metal plating, mining, pickling and tanning. Because of stricter environmental regulations and economics, many companies are actively seeking inexpensive, efficient means to remove and recover these metals from their waste streams with high throughput processes. Ion-exchange (chelator) resins, based on cross-linked polystyrene, are used widely in industry for metal ion removal. Chelator resins, however, often suffer from low selectivity between heavy metals, and their elastic nature limits their use in packed column systems because of shrinking and swelling during pump-and-treat cycles. As such, most chelator resins used industrially are utilized in batch systems, where large vats of wastewater are stirred with the chelator resin beads before the beads are filtered off and regenerated, or under very slow flow conditions. Several research groups are working to modify chelator resins through the bonding of dyes or other chelating agents onto polymer microspheres or hydrophobic polymer resins.1-5 Another area of research focuses on developing new biopolymer ionexchange resins from inexpensive natural materials,6 such as crustacean shells,7 seaweed,8 or corn.9 We previously reported the synthesis, kinetics, and pH profile of a poly(ethyleneimine) (PEI)-silica com* Corresponding author. Telephone: (406) 243-2592. Fax: (406) 243-4227. E-mail: [email protected].

posite material, WP-1 (Figure 1).10,11 Additionally, the metal ion removal and recovery properties of WP-1 were compared against similar silica-polyamine composite materials.11 The syntheses of two WP-1 modified materials, WP-2 (silica-PEI-CH2-COOH, Figure 1) and WP-3 (silica-PEI-EtSH), were also reported, as were the abilities of all three WP materials to remove low levels of mercury and lead or to separate transitionmetal ions from abandoned mine water.12 These materials potentially offer considerable advantages over competing technologies, including faster capture kinetics at rapid flow rates, acid and alkaline resistance, long-term usage durability, and low production costs. Significantly, the recovered metal solutions are highly concentrated and ready for reuse or recovery. The work presented here was performed in order to compare the binding capacities of our silica-polyamine heavy metal ion chelating agent, WP-1, vs the widely used chelator resin, Amberlite IRC-718. Amberlite IRC718 was devised specifically as a heavy metal ion removal resin, and its selectivity for heavy metals over alkali or alkaline-earth cations is achieved by an iminodiacetic acid (IDA) functionality bonded to a macroreticular cross-linked polystyrene resin matrix (Figure 1). A comparison of the physical characteristics of IRC718 and WP-1 is provided in Table 1. As one can see, the work represented here compares two materials that are very different physically. In addition, the mechanisms of metal uptake for WP-1 and IRC-718 are also very different and are represented in eqs 1 and 2. Based on our prior work, each Cu(II) adsorbed by WP-1 is

10.1021/ie9903386 CCC: $18.00 © 1999 American Chemical Society Published on Web 10/14/1999

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Figure 1. Structures of the highly branched poly(ethyleneimine) material, WP-1, Amberlite IRC-718, EDTA, and WP-2 (the acetic acid derivative of WP-1). Table 1. Comparison of WP-1 and IRC-718 WP-1

particle size density cation-exchange capacity a

1

white granular particles 90-105 µm 0.486 g/mL 0.93 mmol/g

Amberlite IRC-718a hydrated, opaque beads 16-50 mesh (297-1190 µm) 42 lb/ft3 1.1 mequiv/mL of wet resin

Data from manufacturer.

WP-1:

M2+ + nRxN+Hy f nRx(NHy-1)Cu + nH+ 1

(1)

where x ) 1-3, y ) 3 - x, n(average) ) 3 1

IRC-718:

M2+ + PCH2N(CH2COOH)2 f PCH2N(CH2COO)2Cu + 2H+

1

(2)

bound to an average of three amine nitrogens, with the fourth position occupied by water or sulfate in the expected square-planar geometry.12 In the case of IRC718, the dications are most likely bonded in a tridentate fashion through the amine and two carboxylic acid ligands of the iminodiacetic acid groups. Coordination numbers for cobalt and nickel would be expected to be four and six, respectively, with these ligands. Experiments were conducted to determine the metal dication removal capacities of WP-1 and IRC-718 from aqueous solutions under both static (batch) and dynamic (flow) conditions. Additional flow studies were performed to determine the effects of repeated adsorption/ desorption cycles on the copper capacity and physical integrity of WP-1 and IRC-718 at room temperature. Another application for these materials is the remediation of power plant cooling water. The cooling water of many power plants is circulated through copper pipes, resulting in hot dilute aqueous copper solutions. Typi-

cally, these solutions are cooled to ambient temperature before the copper can be removed using ion-exchange or chelator resins.13 However, removal of the copper directly at elevated temperatures would expedite wastewater cleanup, reduce energy costs, and be much more cost-effective than disposal. To determine if either WP-1 or IRC-718 could be used in an industrial setting to remediate hot power plant water, flow tests were performed with these materials using 97 °C copper solutions. Another factor to be considered in the testing of heavy metal chelating materials for industrial use is the utilization of metal ion chelating agents such as ethylenediamine derivatives to stabilize the metal ions in solution. These agents may compete with the binding sites of metal ion removal materials, thereby decreasing the effectiveness of these agents. The effects of the competing chelating agent ethylenediaminetetraacetic acid (EDTA) on the copper ion removal capacities of WP-1 and IRC-718 were tested under flow conditions. Experimental Section Materials. WP-1 was synthesized as described previously.10,12 Amberlite IRC-718 in the sodium salt form was furnished by Kinetico Engineered Systems, Inc. (Newbury, OH), and used as received. Regeneration of IRC-718 converts it to the ammonium salt form. Amberlite IRC-718 is manufactured by Rohm and Haas (Philadelphia, PA). Metal solutions were prepared from reagent-grade metal salts (CuSO4‚5H2O, NiCl2‚6H2O or CoCl2‚6H2O). 2 M sulfuric acid and 4 M ammonium hydroxide solutions were prepared from reagent-grade concentrated H2SO4 and ammonium hydroxide, respectively. Trace-metal-free concentrated nitric acid was used to preserve all samples. Deionized water was used for the preparation of all solutions and for all rinses unless otherwise noted.

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Apparatus. Dynamic experiments were carried out using a column fashioned from a 5 mL disposable syringe fitted with frits at both ends and filled with the material being tested. The column was attached to a variable-flow FMI Lab Pump model QG150 (Fluid Metering Inc., Syosset, NY). Solution intakes were controlled by computer-activated solenoid valves (ColeParmer) using a PC with software developed for this application by Gamble and Associates Ltd., LLC (Pasadena, CA). Equipment. Flame atomic absorption data were measured on Unicam 969 flame atomic absorption (FAA) spectrometer. Kinetic Studies. Kinetic studies of WP-1 were previously reported12 and were performed as batch tests (vide infra), with the samples removed from the shaker between 1 min and 24 h before filtration and analysis. The same procedure was performed for IRC-718. The copper solutions were at an initial concentration of 1 mM, pH of 3.4, and temperature of 25 °C. The general equation 1/C ) 1/C0 + kt was used to calculate the second-order rate constant k, where C is the concentration of the solution at time t, and C0 the concentration at t ) 0. The slope of the plot of 1/C vs time yields the rate constant k. Batch Tests. WP-1 or IRC-718 (0.2000 ( 0.0001 g) was weighed into a glass vial with a screw top. A total of 10 mL of the appropriate metal ion solution was added to each vial via a volumetric pipet, and the vials were capped. The initial solution concentrations were 0.003 125, 0.006 25, 0.0125, 0.0250, 0.0500, 0.100, and 0.200 M. Each sample was shaken for 24 h to ensure that equilibrium had been reached. Each sample was then filtered, preserved with 1 drop of trace-metalquality HNO3 (Fisher Scientific), and tested for metal ion concentration. Dynamic Capacity Tests. For each test, a column containing ∼5.5 cm3 of material (2.50 g of WP-1 or 3.50 g of Amberlite IRC-718) was assembled. The column was attached to the pump set at a flow rate of 10 mL/ min, which provided a constant backpressure of 30 psi. Water (50 mL) was pumped through the column to wet the gel. The appropriate metal ion solution (50 mM, 40 mL) was pumped through column, followed by a 40 mL H2O rinse. The total flowthrough volume was collected for analysis. The metal on the column was then eluted with 5 mL of 2 M H2SO4, followed by 15 mL of H2O to create a total elution volume of 20 mL, which was also collected for analysis. The column was then rinsed with an additional 85 mL of H2O. Regeneration with 10 mL of 4 M NH4OH was followed by a final H2O rinse (100 mL). This cycle, excluding the first (wetting) step, was repeated five times for each column. EDTA Competition Studies. Batch tests and flow tests were performed as previously described for copper solutions containing a 2:1 ratio of CuSO4‚5H2O/Na2H2EDTA‚2H2O. Material Lifetime Tests. The initial capacity of WP-1 or IRC-718 for copper was determined in a fashion similar to that for the dynamic capacity tests. The capacity of each material was determined after 50, 100, 250, 500, 1000, 1500, 2000, 2250, 2500, 2750, and 3000 cycles. Between capacity measurements the cycle was modified to the following protocol: the pump flow rate was increased to 50 mL/min, none of the flowthrough was collected, and the volumes of solutions were changed to 5.0 mL of copper sulfate, 2.5 mL of water rinse, 5.0

Table 2. Maximum Flow and Batch Test Adsorption Capacities (mmol/g) of M(II) by WP-1 and IRC-718 batch tests

flow tests

metal

WP-1

IRC-718

WP-1

IRC-718

Cu(II) Ni(II) Co(II)

0.93 0.51 0.44

1.26 1.65 0.67

0.84 0.40 0.37

0.27 0.28 0.26

mL of 2 M H2SO4, 12.5 mL of water rinse, 8.5 mL of regeneration with 4 M NH4OH, and a final 7.0 mL of water rinse. High-Temperature Lifetime Tests. The initial copper capacity of WP-1 or IRC-718 was determined in a fashion similar to that for the room-temperature tests, using ∼4.4 cm3 of each material (3.48 g of WP-1 and 3.13 g of IRC-718). A 50 mM copper solution was heated to boiling14 in a large round-bottom flask with a heating mantle. The pump was attached and set to a constant flow rate of 50 mL/min which maintained the temperature of the copper solution as it passed through the column. There was virtually no difference between the inflow and exit temperatures of the copper solution. The rinse, acid, and base solutions were at ambient temperature. The capacity of each material was determined after 25, 75, 150, 250, 400, 650, 800, 1000, and 1500 cycles. For capacity measurements the cycle was as follows: 70 mL of copper sulfate, 70 mL of water rinse, 8 mL of 2 M H2SO4, 37 mL of water rinse, 8 mL of regeneration with 4 M NH4OH, and a final 25 mL of water rinse. Between capacity measurements the cycle was modified to the following volumes: 4.8 mL of copper sulfate, 12 mL of water rinse, 8 mL of 2 M H2SO4, 24 mL of water rinse, 8 mL of regeneration with 4 M NH4OH, and a final 20 mL of water rinse. The copper adsorption data for these tests were calculated using only the acid elutions of copper from the materials (vs averaging the capture and elution data). This method was used to avoid the problem of incorrect capacity data based on decreases in elution volume upon cooling. Determination of Capacity. Capacities were calculated as a fraction of the M(II) ions adsorbed by the material being tested out of the total M(II) ions introduced or

fraction adsorbed ) (C0 - Ce)/C0

(3)

where C0 is the initial M(II) concentration and Ce represents the concentration of M(II) in solution at equilibrium. However, it must be pointed out that these fractions are only useful in comparisons against other materials where the same initial concentration of the metal solution is used. For this reason, we have also provided a mmole per gram capacity of each material in Table 2. For flow data, Ce represents the concentration of M(II) in the effluent of each test. Langmuir Isotherms. The derivation of the Langmuir equation for the adsorption of a metal ion onto a solid adsorption surface has been previously published.15 In the rearranged Langmuir equation

Ce Ce 1 ) + R Rmax KadsRmax

(4)

Ce represents the concentration of metal ions in solution at equilibrium (mol/L) and R is the number of moles of adsorption sites per gram of the adsorbent, also referred

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Figure 2. Adsorption isotherms of Cu(II) onto WP-1 and IRC718 at pH 3.4 and 25 °C as a function of time.

Figure 3. Batch test adsorption isotherms of Cu(II), Ni(II), and Co(II) ions by WP-1 and IRC-718.

to as the surface concentration. Rmax, the maximum surface concentration value, is calculated from the slope (1/Rmax) of the straight line resulting from a plot of Ce vs Ce/R. The equilibrium constant for the coordination reaction of a metal ion upon a solid surface, Kads (mol/ L), is obtained from Rmax and the y-intercept of the same plot (1/KadsRmax). Results and Discussion Kinetic Studies. To compare the performance of WP-1 and IRC-718 in kinetic terms, the adsorption of Cu(II) onto these materials was monitored with time. Equilibrium adsorption of copper onto IRC-718 is reached in approximately 15 min, versus 4 h for WP-1 (Figure 2). This faster copper adsorption is reflected in the higher (pseudo)-second-order rate constant of 2.39 M-1 s-1 for IRC-718, which was determined from a linear plot of 1/C vs t. This value is approximately 6 times the rate constant of WP-1 (0.437 M-1 s-1) under identical conditions.12 This experiment verifies that IRC-718 presents most of its chelating groups on or near the surface of the beads. Indeed, in the product description, it is stated that IRC-718 has a “short ion diffusion path...(which) improves the kinetics of ion exchange.”16 Therefore, under static conditions, the chelating groups rapidly come into contact with the copper ions that are then adsorbed onto IRC-718. WP-1, however, requires more time under batch conditions to reach equilibrium than IRC-718, indicating that it has a longer ion diffusion path, and so the metal ions must penetrate more deeply into the polyamine surface of WP-1 in order to reach its chelating ligands. The differences in ion diffusion path lengths explain why WP-1 has a slower equilibrium rate constant than IRC-718, and this will come into play later when we examine the differences in flow capacities of these materials. Batch Tests. A summary of all batch tests performed is represented in Figure 3, and the highest per gram adsorption values are listed in Table 2. In general, IRC718 exhibits a higher M(II) ion adsorption capacity than WP-1 over the entire range of concentrations. This result is not surprising and can be explained by the same rationale as the previously discussed kinetic studies: WP-1 possesses a longer ion diffusion path, and it is therefore harder for the metal ions to reach all of the available adsorption sites under batch conditions.

Figure 4. Flow test adsorption of Cu(II), Ni(II), and Co(II) ions by WP-1 and IRC-718.

Figure 3 also shows that both materials adsorb a larger fraction of available M(II) ions at lower concentrations, with a maximum fraction of adsorption occurring at a M(II) concentration of 12.5 mM. This is not surprising, because the same amount of M(II) adsorbed from less concentrated solutions represents a higher fraction of metal ion removal. Flow Tests. The fraction of metal ions removed during flow tests is relatively similar for both materials (Figure 4). A comparison of the per gram capacities of the tested materials, however, revealed that under flow conditions of 10 mL/min WP-1 has roughly 3 times the adsorption capacity for copper than IRC-718 (Table 2). When nickel and cobalt are tested, WP-1 has approximately 1.5 times the per gram capacity of IRC-718. To obtain a rough estimate of the metal binding abilities of WP-1 and IRC-718, the formation constants of two metal ligand complexes of the general formula M(II)L were compared. The formation constants of MIIen (en ) ethylenediamine) were used to approximate the metal ion binding to the PEI ligand of WP-1, and those of MIIIDA were used to approximate the metal chelation with the IDA ligand of IRC-718. Because the chelator in WP-1 is a polymer, it is very difficult to find an exact small-molecule analogue for the comparison of formation constants; however, ethylenediamine is the closest analogue. Divalent Ni, Co, and Cu have slightly lower formation constants for complexes with the en ligand than with the IDA ligand,16 suggesting that these metals should preferentially chelate to IRC-718 in all cases, and in the order Cu > Ni > Co for both ligands. While it is true

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that IRC-718 has a higher capacity for all three metals in batch tests, the order of metal preference is Ni > Cu > Co. For flow tests, WP-1 has higher metal removal capacities, and the order of metal preference for chelation with both materials is Cu > Ni ≈ Co. The relative metal capacity values of WP-1 and IRC-718 are therefore not based solely on metal-ligand binding affinities, because neither batch nor flow tests correlate directly with the solution equilibrium data for the formation of complexes of en and IDA.18 It must be the case, therefore, that the physical diffusion of the metal ion solutions through the WP-1 and IRC-718 chelators must play a greater role than metal ligand binding affinities when comparing the capture kinetics of these two materials. It is therefore important to discuss how the metal ion solution comes into contact with materials during batch and flow studies. We have already seen from our kinetic experiments that it takes longer for aqueous metal ion solutions to reach all of the available binding sites on WP-1. In batch tests, WP-1 consistently adsorbs fewer metal ions than IRC-718, verifying that it is more difficult for metal ions to fully penetrate the interior of the polyamine surface of WP-1 under static conditions. Flow tests, on the other hand, tell a different story. All of the WP-1 flow test adsorption values are greater than those of IRC-718, which are, in turn, lower than their corresponding batch test values. This suggests that, first, the metal ion solutions are more effectively reaching the available binding sites of WP-1 than those of IRC-718 during flow tests and, second, the metal ions cannot reach the chelating sites of IRC-718 as well in flow tests. In flow tests, the aqueous metal ion solution is forced through the hydrophilic polyamine chelators which coat the surface and line the inside of the 150 Å pores of WP1.17 More of the metal ions come into contact with the chelators in a shorter amount of time, resulting in higher capacities for WP-1 under flow conditions. During the same tests, most of the metal solution flows around the IRC-718 beads, preventing the chelators below the surface to remove any metal ions from the solution. Variations in the flow rate were studied by increasing the flow rate of all solutions to 50 mL/min.17 This increase in the solution flow rate reduces the copper ion capacity of WP-1 but not that of IRC-718; however, WP-1 still maintains a higher capacity than IRC-718. This reinforces the argument that the IRC-718 chelators bind metal ions quickly (in order to chelate the same amount of Cu(II) at 50 and 10 mL/min) and that only the outer surface chelators are able to bind metal ions under flow conditions. It is therefore a combination of the hydrophilic nature of PEI vs polystyrene and the higher porosity of the silica that most likely accounts for the superior metal ion capabilities of WP-1. Langmuir Isotherms. Langmuir isotherms were plotted for the batch test data as described above. A representative Langmuir isotherm, for the batch tests of copper with WP-1, is shown in Figure 5. The calculated values of Rmax and Kads are presented in Table 3. In general, the values of Rmax and Kads reflect the metal ion removal preferences as seen in the flow tests. For WP-1, Rmax for Cu is greater than that for Ni and Co, and for IRC-718, the order of chelating preference is Ni

Figure 5. Langmuir plot of the batch test adsorption of Cu(II) onto WP-1. Table 3. Values of Rmax and Kads Derived from the Langmuir Isotherms of Batch Test Data Rmaxa

Kadsb

metal

WP-1

IRC-718

WP-1

IRC-718

Cu(II) Ni(II) Co(II)

1000 625 588

1250 1667 714

33 20 17

80 233 67

a

mol of M(II)/g of adsorbent. b M-1.

Figure 6. Batch test adsorption isotherms of Cu(II) in the presence of EDTA by WP-1 and IRC-718. The ratio of Cu(II)/EDTA concentration in all tests was 2:1.

> Cu > Co. That Rmax is higher for IRC-718 verifies that IRC-718 has more possible binding sites than WP-1. The lower Rmax for Ni and Co adsorption onto WP-1 shows that fewer binding sites are available for these metal ions and that they require more amines per metal for chelation. That the predicted Rmax for all three metal dications is far greater than the highest observed per gram capacities suggests either that it is very difficult for the copper solution to access all of the potential binding sites (perhaps due to cross-linking within the polymer system) or possibly that the secondary and tertiary amines are less likely to bind the Cu(II) ions because of steric hindrance. Batch Tests in the Presence of EDTA. In general, the fraction of Cu(II) adsorbed decreases dramatically for both WP-1 and IRC-718 in the presence of the competing chelator EDTA (Figure 6 and Table 4). At lower concentrations of copper, WP-1 shows a lower capacity than IRC-718, while the situation is reversed at higher concentrations. In the case of WP-1, secondary and tertiary amine binding becomes more prevalent at higher concentrations.

Ind. Eng. Chem. Res., Vol. 38, No. 11, 1999 4407 Table 4. Fraction of Cu(II) Adsorbed onto WP-1 and IRC-718 in the Absence and Presence of EDTA during Flow Testsa Cu(II) Cu(II) + EDTA

WP-1

IRC-718

0.70 0.37

0.57 0.21

a The solutions had initial concentrations of 50 mM Cu(II) and (where present) 25 mM EDTA.

Figure 7. Copper adsorption isotherms of WP-1 and Amberlite IRC-718 during lifetime cycle tests at room temperature (shown with 10% esd error bars).

The fraction of Cu(II) adsorbed onto IRC-718 in the presence of 2:1 Cu(II) to EDTA varies little with Cu(II) concentration. This probably reflects the more similar binding constants for IDA residues on both EDTA and IRC-718. Flow Tests in the Presence of EDTA. In the presence of 25 mM EDTA, the copper adsorption drops by 48% for WP-1 and 45% for IRC-718 in the presence of EDTA (Table 4). However, the resultant WP-1 copper adsorption remains more than double that of IRC-718. On the basis of the strong affinity of divalent Cu, Co, and Ni ions for the hexadentate EDTA ligand,18 it was anticipated that the presence of EDTA would reduce the capacity of WP-1 in both flow and batch tests. What was not expected, however, was that in the presence of EDTA WP-1 would still have a much larger capacity for copper than IRC-718 during flow tests. This indicates once again that the faster diffusion of metal ion solution through the accessible WP-1 polyamine plays a greater role in the metal ion capture kinetics than does the chelator binding constant. Lifetime Studies. Industrial and environmental remediation applications require long material lifetimes. The useable lifetime test was designed to measure how repeated copper capture-and-release cycles affect the capacity of the materials. Figure 7 shows that WP-1 maintains nearly its full capacity after 3000 cycles of testing. The capacity of IRC-718, however, gradually decreased to 64% of its initial capacity after 3000 cycles. Significantly, during the testing, the IRC-718 resin shrank and produced voids in the column (Figure 8), resulting in compaction of the resin and requiring backflushing of the column after 1000 cycles and every 250 cycles thereafter because high backpressures that developed within the column. The combination of volume loss and a moderate decrease in capacity suggests a combination of slow degradation (leaching of the chelate) and compression of the IRC-718 resin occurs over many cycles. Either process would limit the use of IRC-718 in long-term, high-volume aqueous processes

Figure 8. Columns of WP-1 (top) and IRC-718 (bottom) after 3000 cycles.

Figure 9. Copper adsorption isotherms of WP-1 and Amberlite IRC-718 during lifetime cycle tests using boiling copper solutions (shown with 10% esd error bars).

involving large changes in pH. Part of the deterioration of IRC-718 may be due to its repeated interconversion between the free acid and the ammonium form of the IDA ligand. High-Temperature Lifetime Studies. When a boiling copper solution is utilized during flow tests for 1500 cycles, WP-1 shows a slight increase in its copper ion capacity (Figure 9). During identical tests, however, IRC-718 lost 13% of its initial copper capacity over 1500 cycles and had to be backflushed after the 150th cycle to relieve backpressure. In addition, between the 150th and 650th cycles, IRC-718 developed a 0.6 cm3 void space. That the capacity of IRC-718 decreases during longevity testing at higher temperatures further demonstrates the limits of the IRC-718 resin. When considering the metal ion capacities of WP-1 and IRC-718, we appreciate that we are comparing materials that bear two different chelating groups-poly(ethyleneamine) for WP-1 and iminodiacetic acid for IRC-718. Because our intent was to compare the performance of our novel WP-1 material to one that is used commercially, we were more concerned with ion removal abilities than a comparison of ligands vs metal ion

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removal. This comparison will provide us, however, with an increased understanding of metal ion-ligand interactions and may assist us in selecting future modifications of the WP-1 materials. We have also synthesized a new material, WP-2, in which the amine groups of WP-1 have been functionalized with acetic acid ligands. WP-2 possesses a basic chelating system very similar to that of EDTA and IRC-718 (Figure 1). The tests performed to date show that WP-2 has a similar capacity for Cu(II) (approximately 95% that of WP-1) but does not coordinate Al(III) as does WP-1.12 This difference may prove useful for Al(III)/Cu(II) separation. Lifetime and batch tests have not yet been performed on WP-2. Regeneration of WP-1 produces little change in its performance characteristics, even after being subjected to pH extremes (-1 to +10) for 3000 adsorption/ regeneration cycles. Stronger basic conditions (1 M NaOH), however, do result in decreased performance, but under stronger acid conditions (concentrated H2SO4) WP-1 stands up well. Conclusions WP-1 exhibits better metal ion capture kinetics under rapid flow conditions and maintains its copper capacity and structural integrity better than the polystyrenebased chelator resin IRC-718 after 3000 testing cycles at ambient temperature and 1500 cycles at high temperatures. The chelator resin IRC-718 does, however, exhibit higher capacities for all the metal ions tested under static conditions. Comparison of WP-1 and IRC718 in the presence of high levels of EDTA, reveals that WP-1 possesses a higher metal dication capacity than IRC-718 under rapid flow conditions, and in higher concentration batch testing. WP-1 can be modified chemically to improve the selectivity and binding efficiencies toward specific heavy metal ions. Further studies of competitive metal ion binding in the presence of other chelators present in some waste streams (citrate, oxalate, and tartrate) will be performed to determine if and how these competing chelators affect the capacities of the silica-polyamine materials or cause chelator leaching. Acknowledgment This research was supported by MONTS-NSF-EPSCOR and PDAB-TECHLINK program (a division of Montana Science and Technology Alliance). The authors acknowledge Kinetico Engineered Systems, Inc. (Newbury, OH), for supplying a research sample of Amberlite IRC-718. A special thanks goes to Purity Systems Inc., our partner in this endeavor. Literature Cited (1) (a) Salih, B.; Denizli, A.; Engin, B.; Tuncel, A.; Piskin, E. Congo Red Attached Poly(EGDMA-HEMA) Microspheres as Specific Sorbents for Removal of Cadmium Ions. J. Appl. Polym.

Sci. 1996, 60, 871. (b) Salih, B.; Denizli, A.; Engin, B.; Piskin, E. Congo Red-Attached Poly(EGDMA-HEMA) Microbeads for Removal of Heavy Metal Ions. Sep. Sci. Technol. 1996, 31, 715. (2) Salih, B.; Denizli, A.; Piskin, E. Removal of Cadmium(II) Ions by Using Alkali Blue 6B Attached Poly(EGDMA-HEMA) Microspheres. React. Funct. Polym. 1995, 27, 199. (3) Brajter, K.; Dabek-Zlotorzynska, E. On-line Separation of Silver With the Chelate-forming Resin Amberlite XAD-2-PAR Prior to Its Determination by Flame Atomic Absorption Spectrometry. Analyst 1988, 113, 1571. (4) Huang, S.-P.; Franz, K. S.; Albright, R. L.; Fish, R. H. Polymer Pendant Chemistry. 3. A Biomimetic Approach to Selective Metal Ion Removal and Recovery from Aqueous Solution with Polymer-Supported Sulfonated Catechol and Linear Catechol Amide Ligands. Inorg. Chem. 1995, 34, 2813 and references therein. (5) Kaybay, N.; Egawa, H. Kinetic Behavior of Lightly Cross linked Chelating Resins Containing Amidoxime Groups for Batchwise Adsorption of UO22+. Sep. Sci. Technol. 1993, 28, 1985. (6) Pehlivan, E.; Ersoz, M.; Yildiz, S.; Duncan, H. J. Sorption of Heavy Metal Ions on New Metal-Ligand Complexes Chemically derived from Lycopodium clavatum. Sep. Sci. Technol. 1994, 29, 1757. (7) Ullah, S. S.; Ahmad, J. U.; Kabir, A.; Azam, T. A.; Islam, R. Low Cost Water Purification Columns Based on Chitosan and Easily Available Materials. J. Bangladesh Acad. Sci. 1996, 20, 167. (8) Konishi, Y.; Asai, S.; Midoh, Y.; Oku, M. Recovery of Zinc, Cadmium, and Lanthanum by Biopolymer Gel Particles of Alginic Acid. Sep. Sci. Technol. 1993, 28, 1691. (9) Chaudhari, S.; Tare, V. Analysis and Evaluation of Heavy Metal Uptake and Release by Insoluble Starch Xanthate in Aqueous Environment. Water Sci. Technol. 1996, 34, 161. (10) Rosenberg, E.; Pang, D.; Gamble, R.; Torp, G. System for Extracting Soluble Heavy Metals from Liquid Solutions. U.S. Patent 5,695,882,1997. (11) Beatty, S. T.; Fischer, R. J.; Rosenberg, E.; Pang, D. Comparison of Novel and Patented Silica-Polyamine Composite Materials as Aqueous Heavy Metal Ion Recovery Materials. Sep. Sci. Technol., in press. (12) Fischer, R. J.; Pang, D.; Beatty, S. T.; Rosenberg, E. Applications of Silica-Polyamine Composites. II. Metal Ion Separations from Mine Wastewater and Soft Metal Ion Extraction Efficiency. Sep. Sci. Technol., in press. (13) Ion Exchange Resins: Amberlite IRC-718, IE-313; Rohm and Haas: Philadelphia, PA, 1993. (14) Approximately 97 °C at Missoula’s elevation of 3000 ft. (15) (a) Langmuir, I. J. Am. Chem. Soc. 1918, 40, 1361. (b) Adamson, A. W.; Gast, A. P. Physical Chemistry of Surfaces, 6th ed.; John Wiley & Sons: New York, 1997. (c) Reed, B. E.; Matsumoto, M. R. Sep. Sci. Technol. 1993, 28, 2179. (16) Ethylenediamine: Smith, R. M.; Martell, A. E. Critical Stability Constants; Plenum Press: New York, 1975; Vol. 2. IDA: Ibid. 1989; Vol. 6. (17) Hagers, D. L. Performance Evaluation for Heavy Metal Ion Removal Using Silica-Polyamine Composite Materials Made with Different Silica Gels and Polyamines. M.S. Thesis, University of Montana, Missoula, MT, 1999. (18) EDTA: Smith, R. M.; Martell, A. E. Critical Stability Constants; Plenum Press: New York, 1989, Vol. 6.

Received for review May 14, 1999 Revised manuscript received August 23, 1999 Accepted August 25, 1999 IE9903386