Metal-Ion Separation and Preconcentration - American Chemical Society

Chapter 17

Metal-Ion Separations Using SuperLig or AnaLig Materials Encased in Empore Cartridges and Disks Downloaded by UCSF LIB CKM RSCS MGMT on November 30, 2014 | http://pubs.acs.org Publication Date: February 11, 1999 | doi: 10.1021/bk-1999-0716.ch017

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Garold L. Goken, Ronald L. Bruening, Krzysztof E. Krakowiak, and Reed M Izatt IBC Advanced Technologies, Inc., P. O. Box 98, 856 East Utah Valley Drive, American Fork, UT 84003

By combining two state-of-the-art technologies, a promising membrane approach has been found to have broad utility in the separation of metal ions from solution. Molecular recognition technology consists of selective separation chemistries in the form of macrocycles or chelating ligands and solid phase extraction particles. The products resulting from the combination are referred to as SuperLig or Analig materials. Loaded Empore membranes are very densely packed with small high surface area SuperLig or Analig particles, yet achieve exceptional flow rates. IBC's selective chemistry ignores most non-target species, therefore membranes containing these materials achieve very high separation efficiencies. The membranes can be fabricated into filter-shaped disks or cartridges ranging from analytical-scale to process-size components. These combined characteristics have been incorporated into analytical, processing, and final polishing products that have had a major impact on current processes. This paper will present the application of these technologies to selective analytical separations involving mercury, lead, strontium, and radium.

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Current address: Department of Chemistry and Biochemistry, Brigham Young University, Provo, UT 84602. ©1999 American Chemical Society

In Metal-Ion Separation and Preconcentration; Bond, A., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1999.

251

252 For the past decade, IBC Advanced Technologies, Inc. (IBC) has been a provider of selective separation materials (1-9) in the forms of macrocycles and solid phase extraction (SPE) particles. Key to the chemistry is the development of macrocyclic and chelating ligands that selectively recognize the shape and electrical charge of a target ion, then preferentially complex with this ion. These ligands show little affinity for nontarget species, even those with similar electric charges, ionic radii, molecular shapes, or other target-like attributes. These ligands are highly selective for a single ionic species, even when other similar species are present at concentrations many times (i.e., 10 -10 ) greater than those of the target ion. Sophisticated reactions to covalently bond ligands to solid phase supports such as silica gel (2) and polymeric resin particles have been developed by IBC. In these particles, the ligand and solid phase support are connected by a spacer. This spacer is attached by stable covalent bonds to the ligand and the solid phase support. The spacer has the important function of allowing the ligand to be immersed in the aqueous phase (6). The solid phase in the SPE system has an important role in the separation procedure. This phase must be compatible with the solution from which the separation is to be made and should not react with or dissolve in the medium used. A variety of supports have been used allowing effective separations to be made in highly acidic, highly basic, and HF-containing solutions. The support used in the studies reported here is silica gel. This support material is one of the most effective for use in SPE separations. It can be obtained in pure form, is hydrophilic making it particularly effective for use in aqueous systems, has a high concentration of active sites for chemical bonding, and is readily converted to an SPE material by synthesis (6). Families of highly selective SPE materials for use in ion separation applications resultfromthis procedure. Unlike ion-exchange resins, which attract almost all similarly charged species present, SPE systems based on molecular recognition technology (MRT) are highly discriminating. High selectivity is an essential criterion for many applications such as concentrating and purifying analytical samples, removing residual process catalysts or toxic compounds, removing interfering species present in other separation or analytical processes, and recovering precious metals. IBC's SPE particles are called AnaLig for analytical applications and SuperLig for the process industry. Empore membranes were used initially in a process for preparing water samples for analysis (10). These thin membranes (0.5 mm) are composed of reactive particles enmeshed in an inert, fibrous matrix without the use of binders or adhesives. The particles can compose up to 95% of the membrane's weight and are very small compared to particles used in typical SPE columns. Even though the particles are small and closely packed, flow rates can be 10 to 100 times faster than those in columns, yet equal extraction efficiencies can be achieved. Channeling or wall effects typical of columns are absent. The combination, in the early 1990s, of IBC's selective separation particles and 3M's membrane technology produced a novel and effective separation procedure. The resulting Empore disk may be considered to be a short column that is much more compact than a normal column. The enclosed SuperLig or AnaLig material has a mesh size of less than ΙΟμπι (compared to 100 μιη in the column mode) allowing a very large

Downloaded by UCSF LIB CKM RSCS MGMT on November 30, 2014 | http://pubs.acs.org Publication Date: February 11, 1999 | doi: 10.1021/bk-1999-0716.ch017

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253 concentration of active sites to be present. These effects result in a system capable of much more rapid separations than are possible with a fixed bed column or with ionexchange (6-8). For example, flow rates in the Empore system can be up to 400 times those of a column system and 4000 times those of ion-exchange. The selectivity and removal ability of the ligands employed in the SPE systems are maintained at these flow rates making separations of trace amounts of metal ions from large volumes of solution feasible. This highly efficient use of particle capacity has been employed for a number of unique process and analytical applications (4-9). One of the most interesting applications is a series of Rapid Analysis Products (RAP) which could bring about a fundamental change in the way sample preparation is performed. Four of these RAP products have been commercialized, Pb and Hg test kits (Hach, Loveland, Colorado) and Sr and Ra Rad Disks (3M, St. Paul, Minnesota). RAP Membranes The ability of these SPE materials to distinguish and separate target ions in one easy step eliminates the numerous chemical manipulations one would normally encounter in a purification process. Since sample preparation is often a purification process, streamlined procedures to improve the analytical process appeared. The name Rapid Analysis Products became associated with numerous developmental products resulting from the combination of selective particle and membrane technologies. If the selectivity for a target species is high, purification and isolation of this species can be accomplished in a one-step process. The need for other chemical manipulations to separate the target ion, such as extraction, precipitation,filtration,or crystallization are not needed. Moreover, even though the selective ligands exhibit several orders of magnitude more affinity for a specific species, selective elution and recovery chemistries have also been developed. In most cases, the target species can be removed and retrieved in quantitative amounts by applying a variety of conditions such as pH change, water wash, or treatment with a complexing agent. A prime example of the RAP target is the published EPA Method 905 (11) for radioactive strontium determination where over fifty steps are listed in the process. Traditional determinations of Sr in aqueous matrices have involved the application of tedious, time-consuming, and labor-intensive separation schemes in order to analyze radiostrontium via its radioactive progeny, (12). The published Sr Rad Disk method requires only six steps and approximately twenty minutes to complete the analysis (12,13). There is a similar reduction in steps for analysis using the Ra Rad Disk (14). These improvements amount to considerable savings in costs and time. Other RAP procedures are also simpler and can be accomplished in fewer steps. For example, the Hg and Pb test kits eliminate the need for advanced instrumentation, such as graphite furnace operation for lead and cold vapor atomic absorption for mercury. These test kits for Hg and Pb extraction also eliminate the need for several chemicals that are associated with health risks. Reducing the time spent in preparing samples increases the time available for method development and analysis. 90


254 An important objective for RAP is to develop a broad-based sample preparation technique that is applicable to diverse analytes including inorganic cations and anions, and even select organic molecules. IBC and 3M provide most of the materials science and technologies to develop RAP and the products introduced for Pb and Hg * resulted from joint efforts of IBC and 3M with Hach scientists. The Rad Disk methods and results for Sr * and Ra came from joint efforts with scientists at Argonne National Laboratory and DOE's Environmental Measurements Laboratory. 2+


2

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Applications Hach Test Kits. Hach has developed test kits (15-17) for mercury and lead thatfitunder the RAP objectives for time, labor, and material savings. The membranes incorporate AnaLig PbOl for lead and AnaLig HgOl for mercury. It is necessary to convert all mercury present to Hg prior to analysis. The 25 mm diameter membrane disk is sealed in a syringe barrel type lead or mercury extractor that can befittedto a hand-operated vacuum pump. A measured acidified or digested sample is drawn through the appropriate extractor. The absorbed target is washed with deionized water and then eluted. A color producing porphyrin indicator is added to the eluate and the pH adjusted to above pH 13. Color measurement is done with the Hach DR/4000, or equivalent, UVvisible spectrophotometer. Color absorption is measured at 412 nm for Hg and at 477 nm for Pb. A complexing reagent is added to the eluate to produce a zero blank and color absorption is measured again. The difference in absorbances represents the amount of the target ion present. Hach has found that the capture efficiency for lead as Pb is 90 to 95% from several standard solutions. Estimated detection limits at the 99% confidence level were 10 μg·L" for a 10 mL sample. Precision for a 250 μg·L" standard at the 95% confidence interval was ±5 μg·L" Pb . The high selectivity of the particles is evident even in the presence of numerous potential interferences. Table I summarizes the results of an interference study for the lead test kit. The results in Table I and in the preceeding discussion demonstrate that the system is capable of accurate and precise determination of Pb even in the presence of high concentrations of a wide range of possible interferring ions. The ions with the lowest tolerable interference levels are those where even traces (low μ^Ε" or less) present in the porphyrin-basedspectrophotmetric analyses (following Pb concentration and separation) can interfere with the final spectral analysis. Ba and Sr * have chemistry similar to that of Pb . However, the interference levels for both Ba and Sr * are high, thereby providing evidence for the high selectivity of the AnaLig material. Mercury, as Hg , was found to interfere with the test at a concentration of 50 μg·L" Hg . However, sodium thiosulfate can be used to mask mercury interference up to 10 mg-L" Hg in the final eluate solution. The common ubiquitous environmental ions, Na , Mg , Ca , and Fe , are seen to offer little interference. 2+

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Test Kit (Id) Table I. Interference Levels' for Various Ions iniLead \ Interference Levels, Species Species Interference Levels, (mg-L") (mg-L") 1

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500 Mn 50 Ag Al Mo(VI) 100 100 Ba Na 20000 100 Br 50000 NaCl 1000 20000 Na S 0 Ca 1000 500 NH Cd 10 Ni 300 3000 ciCo 500 N0 50 500 Cr(III/VI) N0 10 Cu 1000 100 PO F40 Sn(IV) 10 Fe 1000 250 so Fe 250 Sr * 100 10 Γ V(V) 75 K 750 W(VI) 100 Li 75 Zn 250 Mg 500 ^Defined as the levels at which the Pb recoveries are 90% or greater. ^Concentrations of Pb were either 50 μ ^ " or 200 μ^" . Recoveries >90% for Pb were found for each solution. 2+

+

3+

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3

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Strontium Rad Disks. The procedure for analysis with the Sr Rad Disk (12,13), which contains AnaLig SrOl, involves passing a sample in 2 Μ HN0 through a 47 mm disk positioned on a vacuum filter apparatus at a rate of 50 mL-min" . The radioactive strontium may then be quantified using a variety of techniques. For proportional counting, the disk is dried with 2 mL of an acetone wash and placed in a planchet. Counting may then be done with a low background gas proportional counter. For liquid scintillation counting, the Rad Disk is placed in a scintillation vial with liquid scintillation cocktail such as Ultima Gold (Canberra, Meridan, CT). Some investigators prefer methods that use an added tracer to monitor the separation of the strontium. Considerable work has been done to validate results using tracers with the Sr Rad Disk. For example, Sr tracer was added to a sample being tested for Sr, prior to the sample being passed through a Sr Rad Disk. The disk was counted using a liquid scintillation counter capable of evaluating dual labeled samples. A 0-12 KeV window was used for Sr and a 12-500 KeV window for Sr measurement. Corrected tracer results showed >95% recoveries. An alternate technique was used for gamma spectroscopy. The original sample was processed through a disk and a second sample, spiked with Sr, was processed through a second disk. The original sample was analyzed by either gas proportional or 3

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liquid scintillation counting. The second disk with the Sr was analyzed by gamma spectroscopy. The corrected tracer results again showed >95% recoveries. The results of interference studies for the Sr Rad Disks are given in Table II. Quantitative strontium recoveries with 1 L samples were achieved in the presence of the indicated interferences. 1

Table II. Ion Interferences' for the Strontium Rad Disk* (13,18) Ion Concentration Concentration Ion Mg 1 mg-L' 10,000 mg-L" Pb Ca 1000 mg-L500 mg-LNa Ba 10 mg-L" 0.1 mg-L" K Ra 300 pCi-L" "Defined as the levels at which the Sr recoveries are 95% or greater. One liter solutions were spiked with 3 mg Sr"". Recoveries >95% for Sr * were found for each solution. 2+


1

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1

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The data in Table II demonstrate the effectiveness of the Sr Rad Disk in separating Sr * from the indicated ions. Some sample solutions with high Ca concentrations can create a competition for sorbent sites between Sr * and Ca . Solutions containing Ca at levels as high as 500 mg-L" have been tested for the simultaneous uptake of Sr (13-18). Quantitative recoveries were obtained from one liter solutions containing one mg Sr . Typical environmental samples may contain an assortment of these cationic species. It is of particular interest that separations can be achieved at high Mg , Ca , and Na levels since these ions are usually common in these samples. Typical potable water levels of K are less than that shown in Table II. However, levels higher than 10 mg-L" would interfere with Sr analysis (as may be found in some ground water samples). 2

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Radium Rad Disks. Use of the Ra Rad Disk, which contains AnaLig RaOl, allows simultaneous quantitation of Ra and Ra. Due to their origin in the naturally occurring U and Th decay chains, these two radium isotopes, with half-lives of 1600 and 5.76 years, respectively, represent the most significant health hazards and concern for accurate detection. The sample preparation process is the same as for the Sr Rad Disk; a sample is acidified to 2 M with concentrated nitric acid and then passed through a 47 mm disk positioned on afilterapparatus. Once isolated on the disk, the investigator has a number of quantification options. Radiation can be measured directly from the disk but interpretation is difficult because of the multiple ingrowth paths occurring. Where EPA methods are required, Smith et al. (14) describe how Ra may be determined by eluting the radium with 15 to 20 mL of basic 0.25 M EDTA solution and then transferring this solution to a radon bubbler for determination by the radon emanation technique, EPA Method 903.1. Radium-228 may be determined by allowing the ingrowth of Ac and the decay of Ra to occur and then eluting actinium with 15 to 20 mL of 0.5 Μ HN0 . The eluate solution is processed for gasflowproportional 226

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257 counting by evaporation on a planehet, or by coprecipitation on yttrium oxalate as described in EPA Method 904.0. A new method described by Seely and Osterheim (18) demonstrates the RAP objectives by providing a simultaneous measurement of Ra and Ra. After the acidified sample (2 M with HN0 ) is drawn through the Ra Rad Disk, it is washed with 20 mL of nitric acid, dried with a 20 mL wash of acetone, and heated at 60 °C for 30 min. The dried disk is heat sealed in a 3.5 mm Mylar (polyethyleneterephthalate) envelope and set aside for 21 days to allow for equilibrium to be approached between radium and its progeny. The envelope is then placed directly on a gamma spectrometer detector endcap for counting. A multichannel analyzer is used to identify Pb peaks at 352 KeV and 295 KeV for the quantification of Ra and Ac. Peaks at 338 KeV, 911 KeV, and 969 KeV are used to quantify Ra. Scarpitta and Miller (19) have reported on the viability of using Ra Rad Disks in DOE's Environmental Measurements Laboratory's (EML) Quality Assessment Program (QAP). They conclude that of twenty potential nuclide interferences tested, only Pb, Sr, and Ba might interfere with Ra determination. Since these three interferents are primarily β or ^ emitters, their presence would only interfere if Ra determination is by liquid scintillation analysis (LSA). Their presence is of little consequence if one uses an appropriate counting method that measures α-radiation. For QAP purposes, stripping with EDTA or liquid scintillation counting of the disk itself gave alpha efficiencies of 98%. Their summary states that the "Radium disk is effective at separating Rafromother elements and was found to be both a time and reagent saving product suitable for EML's QAP." Ion interferences for the Ra Rad Disk reported by Seely and Osterheim (18) are given in Table III. 226

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Table III. Ion Interferences for the Radium Rad Disk (18) Ion Concentration, mg*L Mg 10,000 10,000 Ca Sr * 100 10 Pb Na 10,000 K 1,000 10 Ba NH 100 ^Defined as the levels at which the Ra recoveries are 95% or greater. One liter solutions were spiked with 10.9 Bq-L Ra . Recoveries >95% for Ra were found for each solution. _1

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In Table III, it is seen that the Ra Rad Disk effectively removes the Ra isotopes present in a one liter sample containing cations that might be present in an environmental sample (i.e., Mg , Ca , Na , and K ). As would be expected, selective removal is most difficult from solutions containing Sr"", Ba , and Pb . The properties (e.g., ionic radii) of these three cations are most similar to those of Ra . However, as noted by Scarpitta 2+

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and Miller (19) their presence may not be a problem in Ra analysis since their interferents are primarily β or $ emitters.


Conclusions In conclusion, it has been shown that the RAP concept is possible using state-of-the-art selective particle and membrane technologies. The purification and isolation of a target ion can be accomplished in one step with the appropriate solid phase extraction membranes. Elution can be employed as a secondary purification step though the particles need to be rugged and neither bleed nor alter the chemistry of the eluent. In some cases, direct quantification of the absorbed target ion from the disk is possible. Time, accuracy, and costs inherent in elemental analysis can all be improved by using RAP. Acknowledgments We greatly appreciate the cooperative work done in testing these materials by personnel at 3M, Hach, Argonne National Laboratory, and the DOE Environmental Measurements Laboratory. SuperLig and AnaLig are registered trademarks of IBC Advanced Technolo gies, Inc. and Empore is a trademark of 3M.

Literature Cited (1) (2) (3)

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Izatt, R. M.; Bruening, R. L.; Bruening, M. L.; Tarbet, B. J.; Krakowiak, Κ. E.; Bradshaw, J. S.; Christensen, J. J.Anal.Chem. 1988, 60, 1825-1826. Bradshaw, J. S.; Bruening, R. L.; Krakowiak, Κ. E.; Tarbet, B. J.; Bruening, M . L.; Izatt, R. M.; Christensen, J. J. J. Chem.Soc.,Chem. Commun. 1988, 812-814. Izatt, R. M.; Bradshaw, J. S.; Bruening R. L.; Tarbet, B. J.; Krakowiak, Κ. E. In Emerging Separation Technologies for Metals and Fuels; Lakshmanan, V. I., Bautista, R. G.; Somasundaran, P.; Eds.; TMS: Warrendale, PA, 1993; pp 67-75. Izatt, Ν. E.; Bruening, R. L.; Anthian, L.; Griffin, L. D.; Tarbet, B. J.; Izatt, R. M.; Bradshaw, J. S. In Proceedings: Metallurgical Processes for Early 21 Century; Sohn, H. Y., Ed.; TMS: Warrendale, PA, 1994; pp 1001-1018. Izatt, R. M.; Bradshaw, J. S.; Bruening, R. L.; Tarbet, B. J.; Bruening, M. L. Pure Appl. Chem. 1995, 67, 1069-1074. Izatt, R. M.; Bradshaw, J. S.; Bruening, R. L. PureAppl.Chem. 1996, 68, 12371241. Izatt, R. M.; Bradshaw, J. S. In Comprehensive Supramolecular Chemistry: Supramolecular Technology; Reinhoudt, D. N., Ed.; Vol. 10; Elsevier: New York; 1996, pp 1-11. Izatt, R.M.;Bradshaw, J.S.;Bruening, R. L. In Perspectives in Supramolecular Chemistry, Reinhoudt, D. N., Ed.; John Wiley & Sons: New York; 1998. Izatt, R. M.; Bradshaw, J. S.; Bruening, R. L.; Bruening, M . L. Am. Lab. 1994, 26, no. 18, 28c-28m. Hagen, D. F.; Markell, C.; Schmitt, G. Anal. Chim. Acta 1990, 236, 157-164. st

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259 (11)

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(14) (15) (16) (17) (18)

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"Prescribed Procedures for Measurement of Radioactivity in Drinking Water, Method 905.0; EPA-600/4-80-032; U.S. Environmental Protection Agency; 1980. Smith, L. L.; Orlandini, Κ. Α.; Alvarado, J. S.; Hoffmann, Κ. M.; Seely, D. C.; Shannon, R.T. Radiochim. Acta 1996, 71, 165-170. Empore™ Strontium Rad Disks, Strontium Interference Summary, 3M: St. Paul, MN; 1997. Smith, L. L.; Alvarado, J. S.; Markun, F. J.; Hoffman, Κ. M.; Seely, D. C.; Shannon, R. T. Radioact. Radiochem. 1997, 8, 30-37. Products for Analysis Catalog and Technical Data, Hach; Loveland, CO, 1997, p 35. Heinzig, M.W. "Lead Determination with the New AnaLig/Pb Ex Method"; Hach, Loveland, CO, 1996. "Analytical Procedures: Mercury (0 to 250 μg·L )"; Hach, Loveland, CO, 1997. Seely, D. C.; Osterheim, J. Α., "Radiochemical Analyses Using Empore™ Disk Technology", Fourth Conference on Methods and Applications of RadioAnalytical Chemistry; Kona, HI, April 6-11, 1997. Scarpitta, S. C.; Miller, P. W., "Evaluation of 3M Empore™ Rad Disks for Radium Determination in Water", 42 Annual Conference on BioAssay, Analytical and EnvironmentalRadiochemistry; San Francisco, CA, October 1317, 1996. -1

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Metal-Ion Separation and Preconcentration - American Chemical Society

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