Chapter 20
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Water-Soluble Metal-Binding Polymers with Ultrafiltration A Technology for the Removal, Concentration, and Recovery of Metal Ions from Aqueous Streams 1
1
2
Barbara F. Smith , Thomas W. Robison , and Gordon D. Jarvinen 1
2
Chemical Science and Technology Division and Nuclear Materials and Technology Division, Los Alamos National Laboratory, Los Alamos, NM 87544 The use of water-soluble metal-binding polymers coupled with ultrafiltration (UF) is a technology under development to selectively concentrate and recover valuable or regulated metal-ions from dilute process or waste waters. The polymers have a sufficiently large molecular size that they can be separated and concentrated using commercially available UF technology. The polymers can then be reused by changing the solution conditions to release the metal-ions, which are recovered in a concentrated form for recycle or disposal. Pilot-scale demonstrations have been completed for a variety of waste streams containing low concentrations of metal ions including electroplating wastes (zinc and nickel) and nuclear waste streams (plutonium and americium). Many other potential commercial applications exist including remediation of contaminated solids. An overview of both the pilot-scale demonstrated applications and small scale testing of this technology are presented. Regulatory limits for discharge of radioactive metal-ions from the United States Department of Energy (DOE) nuclear facilities have become markedly lower in recent years, and older technologies for treatment of waste streams such as carrier precipitation have become much less efficient from an overall systems engineering perspective. In the late 1980s our separations team at Los Alamos National Laboratory began evaluating the use of water-soluble metal-binding polymers in combination with UF as a more cost-effective way of meeting the increasingly stringent regulatory requirements for removal of actinidesfromwaste waters. The concept of using water-soluble metal-binding polymers with UF as a process was first proposed in the late 1960s by Michaels (i). Relevant actinide work had been done by Bayer and Geckeler (2) who, in collaboration with 294
©1999 American Chemical Society
Bond et al.; Metal-Ion Separation and Preconcentration ACS Symposium Series; American Chemical Society: Washington, DC, 1999.
295 Myasoedov's group (5), tested actinide binding with a water-soluble metal-binding polymer containing the 8-hydroxyquinoline ligand. We tested the concept with the same laboratory-prepared polymer along with a number of other commercially available polymers (4) and found that though some polymers did indeed concentrate americium (III) and plutonium (III)/(IV) (the major alpha-active contaminants in our waste waters), our goal of reaching ultra-low discharge levels could not be met with these polymer systems. Consequently, we began designing polymers that would have higher binding constants for the actinides, particularly americium and plutonium, and would have overall better physical properties for use in the UF process. This approach coincided with the development in our laboratory of rapid survey techniques for evaluation of new polymers. The concept was developed into a preconcentration procedure for analysis of actinides in very dilute solutions (5). From the analyticalscale the process evolved to the bench- and pilot-scale for actinide waste water treatment. Shortly after our work with actinides began, we had the opportunity to collaborate with the Boeing Space and Defense Group, Seattle, WA, on a joint project for electroplating waste minimization. A key issue for the electroplating industry was the removal of valuable or hazardous metal-ions from dilute waste streams without generating sludge that requires disposal in landfills. The goal became recycling in a near-closed loop process. After considering a number of technologies in a best available technology review (6), it was decided that watersoluble metal-binding polymers with UF had the potential to meet the needed goals of this project for dilute rinse water treatment and metal-ion recycling. From the analytical-scale studies a process was developed and taken to the bench- and pilotscale for an electroplatingrinsewater recycling process (7). The first metal recovery systems targeted for commercialization for the electroplating industry were nickel (bright nickel, nickel strike), zinc, copper (copper strike), zinc/nickel alloy, and nickel/tungsten alloy (8). The need in the electroplating and the nuclear industries for the recovery and removal of metals that exist as oxyanions has led to many studies using watersoluble polymers for the removal of a variety of oxyanions. These oxyanions include chromate, tungstate, molybdate, selenate, arsenate, and pertechnetate. The removal of some oxyanions from aqueous solutions using water-soluble metalbinding polymers with UF has been reported (9). A logical extension of this technology, once it became commercially available, was further evaluation for removal and recovery applications for other transition and main-group metals (toxic, valuable, or nuisance) from other aqueous process and waste solutions such as acid mine drainage (10) and from solid surfaces. We have been studying the separations chemistry of the elements highlighted in periodic chart format in Figure 1.
Bond et al.; Metal-Ion Separation and Preconcentration ACS Symposium Series; American Chemical Society: Washington, DC, 1999.
296 The Concept of Water-Soluble Metal-Binding Polymers with UF (Polymer Filtration) Polymer Filtration (PF) technology uses water-soluble polymers prepared with chelating or ion-exchange sites to sequester metal-ions in dilute aqueous solutions. The water-soluble polymers have a sufficiently large molecular size that they can be separated and concentrated using commercial UF technology. Water and smaller unbound components of the solution pass freely through the UF membrane allowing for the concentration of the polymer/metal complex. By adjusting the solution conditions, the metal-ions are released and are recovered in a concentrated form for recycling or disposal using a diafiltration process. The water-soluble polymer can be regenerated for further waste-stream processing. The relative efficiency with which an UF membrane retains or rejects a metal species can be determined experimentally with each species assigned a numerical value between 0 and 1 called the rejection coefficient (σ). A rejection coefficient of 0 means that the species freely passes through an UF membrane (permeate) while species with a rejection coefficient of 1 are completely retained (retentate). Small metal-ions will passfreelythrough the membrane (σ = 0) unless the effective size is temporarily increased by binding to the polymer (σ = 1). In the case of a polymer/metal-ion complex in which the polymer (P) is physically too large to pass through the UF membrane, the rejection coefficient of the metal-ions (M ) in the presence of a complexing polymer (P) is a reflection of the equilibrium or stability constant (Kg) of the complex, which is a measure of the affinity of the polymer for a metal-ion. n+
ρ+Μ
η +
^ = ^ PM
n +
(1)
n+
_ [PM ]
Concentration Mode. Generally, there are two modes of operation in PF (77). The first is the concentration mode, schematically shown in Figure 2, where the volume in the retentate is reduced by simple UF. The final concentration of any species in solution can be determined by: V
Λ
(3)
c =c .|^ f
0
where C is the final concentration of the species, C the initial concentration, V the initial volume of solution, V the final volume, and V the permeate volume. If the rejection coefficient of the species is 1, as would be the case for the water-soluble metal-binding polymers, then equation 3 simplifies to: f
0
f
0
p
Bond et al.; Metal-Ion Separation and Preconcentration ACS Symposium Series; American Chemical Society: Washington, DC, 1999.
Figure 1. A Periodic Table Summarizing Elements Under Study for Application of Polymer Filtration Technology.
Figure 2. Schematic of Concentration Process by UF.
Bond et al.; Metal-Ion Separation and Preconcentration ACS Symposium Series; American Chemical Society: Washington, DC, 1999.
298
C =C ^ f
(4)
0
For two species in solution, a polymeric/metal-ion species (PM) and a molecular impurity (A), where σΡΜ )) σΛ the UF of the solution should result in the concentration and enrichment of Ρ based on:
Ί f c Ί fv Ί 0 c f 0
(c
v
(5)
UF is the basis for a significant degree of purification during concentration of polymer/metal-ion complexes in solution. Diafiltration Mode. The second mode of operation in PF is diafiltration (see Figure 3). Wash water ( V ) is added to the retentate at the same rate that the permeate is generated so as to maintain a constant retentate volume. In the diafiltration mode, the lower molecular weight species in solution are removed at a maximum rate when the rejection coefficient equals 0. The retentate is, in effect, washed free of smaller solute. Theoretically, the percent solute (any dissolved species) remaining in the retentate can be calculated by using equation 6: w
C =C .e f
V o
(6)
0
where V is the volume of solute free liquid (volume-equivalents) added, which also equals the amount of permeate produced ( V ). w
p
The effects of various rejection coefficients on the percent solute retained during a diafiltration process are shown in Figure 4. It can be seen that theoretically, after 5 volume equivalents of processed solution, >99% of the lower molecular weight species with a rejection coefficient of 0.0 should be removed. Experimentally, however, rejection coefficients of 0.0 are not ordinarily observed. Even weak interactions between the solute and the water-soluble polymer or the UF membrane can yield a small retention value. The curves for low retention coefficients follow an exponential decay with each additional volume equivalent giving diniinishing returns in percent solute removed, while higher rejection coefficients approach a linear response to solute removal. Methods of Metal Release. The polymer-bound metal-ion can be released from the polymer by a variety of processes including those shown in the following equations: M
n +
+
(P)+nH H P+M
n +
n
Bond et al.; Metal-Ion Separation and Preconcentration ACS Symposium Series; American Chemical Society: Washington, DC, 1999.
7
()
Filter
Figure 3. Schematic of the Diafiltration Process.
100H H
1
1
1
H
Volume Equivalents
Figure 4. Plot of Solute Retained as a Function of Volume Equivalents for Various Rejection Coefficients During Diafiltration
Bond et al.; Metal-Ion Separation and Preconcentration ACS Symposium Series; American Chemical Society: Washington, DC, 1999.
300 M ( P ) + L ^ = ^ ML+(P) n
(n
l)+
M ( P ) + e - ^ = ^ M " +(P)
(8) (9)
where M is the metal-ion, (P) is the water-soluble polymer, L is a competing molecular complexant, η is the oxidation state of the metal-ion, and the reduction reaction can be either chemically or electrically driven. When the metal is released by a proton (equation 7) or by a complexant (equation 8), the polymer-free metalion is recovered by a diafiltration process. In some unusual instances, the metal-ion may be so tightly bound to the polymer that destruction of the polymer (incineration, hot acid digestion, smelting, etc.) is required to recover the metal. Optionally, for waste management purposes it may be most economic to solidify the polymer-bound metal, for example, in a grout or cement material, such that it passes Environmental Protection Agency (EPA) toxicity characteristic leaching procedure (TCLP). General Process Conditions. Generally, the concentrationrangefor the watersoluble polymer in solution ranges from 0.001 weight/volume percent to 20 weight/volume percent of final concentrated solution. It is sufficient, and in some cases desirable, to have only enough polymer in solution such that the polymer's metal-ion loading approaches 90 to 100%. Using higher concentrations of the watersoluble polymer will result in lower flux rates through the membrane during the concentration stage. The use of a high initial polymer concentration can sometimes cause aggregation of the polymer and reduced metal-ion binding capability. In this case, operation at lower initial polymer concentrations can allow more complete metal binding and the polymer can then be concentrated to higher final concentrations with overall improved performance. During the concentration stage for analytical applications, the polymer and metal-loaded polymer concentration can often become quite high and, in the case where the solution goes to near dryness, it can approach 90% of the weight of the concentrate. For a semi-continuous process it is necessary to work at low polymer concentrations to maintain high permeate flux across the membrane during the concentration stage of the process. During the diafiltration stage the polymer concentrations will always be higher, but at this juncture the volumes being treated are small. The flux is dependent on the transmembrane pressure which is commonly in the range of 25 to 50 psi. However, the increase in flux with transmembrane pressure is limited by concentration polarization and the flux gains are often small beyond 50 psi for typical tangential-flow, hollow-fiber UF units (11). Polymer Leakage Through the Membrane. Ideally, there is no polymer permeating through the UF membrane. If there is any polymer breakthrough (σ < 1.0), it will ultimately be lost from the system (12). This result is unacceptable
Bond et al.; Metal-Ion Separation and Preconcentration ACS Symposium Series; American Chemical Society: Washington, DC, 1999.
301 from a number of process perspectives. First, the polymer must remain in the system to maintain its working concentration. Second, polymer contamination in the permeate can create further problems downstream. Third, loss of metal-loaded polymer that would carry bound metal-ions into the permeate can result in failure to achieve target discharge limits. If there were only 1 ppm loss of polymer from a system that contains 1000 ppm polymer (which might be environmentally acceptable), a 50% loss of material in approximately a million volume equivalents would occur. At «2% polymer breakthrough, as has been reported for one experimental system (75), 50% polymer loss would occur in approximately 35 volume equivalents. This amount of polymer loss is unacceptable for a viable process. Measurement of polymer breakthrough has required the use of a variety of methods because each polymer has different functional groups that require different techniques to determine their presence in low concentration. For example, we have used UV-Vis absorption spectra of highly colored metal-ion complexes and total organic carbon analysis to detect polymer in the permeate. However, the absorption spectra are generally limited to levels of 1 ppm or higher and the presence of other organic compounds in the feed solution can interfere with the carbon analysis. We are developing tags for the polymer backbones that will detect breakthrough at the ppb level or less and will be useful for a wide variety of polymers. Concentration Factors and Low Level Metal Ion Removal. When the goal of a process involves the removal of metal-ions to very low levels, it can be useful to think in terms of the resulting metal-ion concentration in the permeate solution as opposed to percent metal-ion retained in the retentate. At 99% retention of metalions from a solution with an initial concentration of 1000 ppm, 10 ppm remain in the permeate. For most RCRA or toxic metals this amount still represents an unacceptable discharge level and the resulting aqueous stream could not be discharged to any Publicly Owned Treatment Works (POTW). In aqueous streams containing radioactive metal-ions the decontamination factors must often be even greater. A solution containing 1 xlO pCi/L of plutonium-239 will require 99.999% removal to reach 10 pCi/L. Thus, in evaluating a process it may be more practical to take into account the final waste stream concentration than to consider the percent of metal removed. Calculation of a concentration factor (CF) for a PF process cycle will require knowledge of the process system which includes: the initial feed metal-ion concentration and volume, the final metal-ion concentrate and volume, the size of the reactor, and the initial polymer concentration (72). The polymer concentrations do not exceed 20% in a continuous process because of the reduction in flux rates. Thus, if the reactor size is 20 L (this includes the holdup volume of the system) and the initial polymer concentration is 1% w/v, we can concentrate the reactor volume from 20 L to 1 L at the end of the concentration phase. If the metal-ion concentration in the feed going to the reactor is 100 ppm (e.g., Cu, AW 63.5) and a 1% w/v polymer solution has a capacity of 0.25 g Cu per g of polymer and a 20 L reactor has 200 g of 6
Bond et al.; Metal-Ion Separation and Preconcentration ACS Symposium Series; American Chemical Society: Washington, DC, 1999.
302 polymer, we can bind 50 g of copper which represents 500 L of feed. Thus, the concentration of 500 L to 1 L is a CF of 500 (CF = Vo/V ). The majority of this process will thus be run at a polymer concentration of 1% w/v or less to maintain the high flux rates. Only at the end of the batch will flux rates be reduced substantially as the polymer concentration is increased from 1 to 20% w/v. In actual single-stage practice
