Anal. Chem. 1980, 52, 2347-2350
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Poly(acry1amidoxime) Resin for Determination of Trace Metals in Natural Waters Michael B. Colella,' Sidney Siggia," and Ramon M. Barnes Department of Chemistry, GRC Tower I, University of Massachusetts, Amherst, Massachusetts 0 1003
Fe(III), Mn( 11), Cu(II), Zn(II), Co(II), Ni(II), Hg(II), Pb(II), Cd( 11), and Ag( I ) were sequestered on a poly(acrylamidoxime) resin from laboratory prepared solutions. Recovery of the sequestered metals was achieved by equillbratlon of the resin matrix with either a 1 : l HNO,/water mixture, a 1:1 HCVwater mixture, or a 1 M thiosulfate solutlon. Sequestered metals were at least 90 YO recovered with a standard devlatlon of 1 5 %. Regeneration of the resin was achieved by equllisolution after the acid bration of the resin with a 3 M ",OH equilibration. The resin was applied for the separation and simultaneous concentration of Fe( HI), Cu( 11), Cd( 11), Pb( 11), and Zn(I1) from seawater and pond water. Metals were removed from the resin by equlllbratlng with a 1 : l HCVwater mixture and their concentrations determined by atomic absorption spectrometry. Metal concentrations as determined by the resin method are in good agreement with the values determined directly on samples by either differential pulse polarography or dmerential pulse anodic stripping voltammetry.
water with Chelex-100 followed by determination by anodic stripping voltammetry was described (20). A disadvantage of Chelex-100 is t h e affinity i t exhibits for the alkali a n d alkaline earth metals. Van Grieken et al. (21) demonstrated that the presence of alkali and alkaline earth metals decreased t h e collection efficiency of Chelex-100 filter membranes for several trace metals. Even when t h e optimum collection efficiency is attained, unless a differential elution technique such as t h a t employed by Kingston a n d co-workers (22) is used, the eluate will contain high concentrations of alkali and alkaline earth metals. Recently, Berman et al. (23) stated that using the differential elution technique of Kingston (22)resulted in the loss of some of the metals of interest during the alkali-metal elution step. T h e application of a poly(acry1amidoxime) resin to t h e determination of Fe(III), Cu(II), Cd(II), Pb(II), and Zn(I1) to seawater and pond water is reported here. The synthesis and characterization of this resin have been described elsewhere (24). As shown previously this resin had no affiiity for sodium, potassium, calcium, or magnesium cations (24).
T h e determination of trace metals in natural waters is important from two different points of view. T h e first is the inorganic pollution of these waters in which metals have reached concentrations which may be deleterious t o aquatic life and ultimately man. T h e second consideration is that of micronutrient requirements for various life forms. As pointed out by Hume ( I ) , essential trace elements such as copper, molybdenum, iron, a n d manganese are present in seawater at very low levels. These are readily depleted resulting in large areas of the ocean which are aquatic equivalents of barren deserts. T h e trace metal content of natural waters must therefore be monitored to determine if the concentrations have reached toxic proportions or have been depleted below t h e micronutrient requirement levels. T h e determination of these nanogram-per-gram or sub-nonagram-per-gram levels of metals requires some method of concentration prior to their determinations by existing instrumental methods of analysis ( 2 ) . Metal chelating resins have found widespread application for t h e concentration of trace metals from natural waters. Muzzarelli a n d co-workers chemically modified the natural polymer, chitosan, which contains glucosamine functional groups after deacylation (3-6). Fritz et al. synthesized resins containing propylenediaminetetraacetic acid groups (7),nbutylamide groups (8),a n d hexylthioglycolate groups (9). Vernon a n d Eccles synthesized and applied N-substituted hydroxylamine (IO) a n d 8-hydroxyquinoline resins (11). Leyden et al. immobilized alkylamines, dithiocarbamates, and xanthates onto silica gel or controlled-pore glass beads by silylation reactions (12-15). T h e iminodiacetate containing resin, Chelex-100, is one of the more commonly employed metal chelating resins. Riley and others (16-19) have characterized and applied Chelex-100. Concentration of copper, cadmium, lead, and zinc from sea-
Reagents. The purest available metals and metal salts were used to prepare 1000 Mg/mL stock metal ion solutions. The appropriate quantities of silver nitrate, cadmium sulfate, and lead nitrate (Fisher) were dissolved in 250 mL of distilled deionized water (DDW),50 mL of concentrated nitric acid was added, and the solution was diluted to 1.0 L with DDW. Similarly, cobalt acetate, nickel acetate (Fischer), and mercuric acetate (J.T. Baker) were dissolved in 50 mL of glacial acetic acid and diluted to 1.0 L with DDW. Also the metal powders of copper (E. H. Sargent), zinc (J. T. Baker), iron, and manganese (Fisher) were dissolved in 50 mL of a 1:l nitric acid/water mixture and diluted to 1.0 L with DDW. Three buffers were used in the electrochemical demonstrations of several of the metals. An ammonium tartrate buffer, pH 9.0, was prepared by dissolving 30.0 g of tartaric acid (Fisher) in 500 mL of DDW. The pH was adjusted to 9.0 by the addition of ammonium hydroxide, followed by dilution to 1.0 L with DDW. An ammonium citrate buffer, pH 3.0, was prepared by dissolving 21.0 g of anhydrous citric acid (Fisher) in 400 mL of DDW. Adjustment of the pH to 3.0 was achieved by the addition of ammonium hydroxide, followed by dilution to 500 mL with DDW. A sodium buffer, pH 6.0, was prepared by dissolving 32.68 g of anhydrous sodium acetate (Fisher) in 100 mL of DDW with the addition of 1.3 mL of glacial acetic acid (Fisher). A persulfate oxidizing reagent was prepared by dissolving 1.5 g of potassium persulfate (Fisher) in 100 mL of DDW which was 10% by volume concentrated sulfuric acid. Apparatus. Metal determinations were carried out either by atomic absorption spectrometry with a Perkin-Elmer Model 403 atomic absorption spectrophotometer or by electrochemical means with the PAR Model 174A polarographic analyzer (Princeton Applied Research). Samples. Seawater and pond water samples were collected in either 1-L or 2-L Nalgene polyethylene sample bottles over the persulfate oxidizing reagent, 20 mL of reagent L-' of sample. This reagent was added to preserve the samples by lowering the pH to 2.0, thus minimizing the loss of analyte due to absorption on the container walls. The persulfate reagent was also added because it was used as a digestion reagent for the samples prior to their analyses by the electrochemical methods (25). All seawater
EXPERIMENTAL SECTION
'Present address: General Electric Co., Lynn, MA 01910. 0003-2700/80/0352-2347$01 .OO/O
0 1980 American Chemical Society
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ANALYTICAL CHEMISTRY, VOL. 52, NO. 14, DECEMBER 1980
samples were collected from a breakwater approximately 150 ft from shore a t Pilot's Point Marina, Clinton, CT. Pond water samples were collected from either Puffer's Pond in Amherst, MA, or from Sloper's Pond in Southington, CT. Resin Preconditioning. Preconditioning of all SO/ 100 mesh poly(acry1amidoxime) resin samples was carried out to remove any trace metal impurities contained in the resin. The resin was first equilibrated with 50 mL of a 1:l nitric acid/water mixture for 3 h. The resin was then collected by filtration and washed with 200 mL of DDW, followed by equilibration with 50 mL of 3 M ammonium hydroxide solution for 3 h. After collection by filtration and washing with 200 mL of DDW, the resin was ready for use. Metal Ion Recovery Studies. The recovery of sequestered metals from the resin was evaluated by a column method. Packed columns were prepared by slurrying 75 mg of SO/lOO mesh poly(acry1amidoxime) resin in 25 mL of DDW followed by addition of the resin slurry to Pasteur pipettes which were 8.5 cm long by 0.5 cm i.d. A small plug of silanized glass wool was inserted into all columns to hold the resin in place during the packing and sampling procedures. The columns were soaked in 1:l nitric acid/water for a t least 1 h prior to packing. Through each column was passed 1.00 mL of a stock metal ion solution followed by 4.00 mL of DDW. The effluent from each column was collected. The Mn(II), Co(II), Ni(II), Cu(II), Zn(II), Ag(I), and Cd(I1) laboratory prepared test solutions were a t an initial concentration of 10 rg/mL at pH 5.0. The 10 pg/mL Fe(1II) test solution was used a t pH 2.0 and the 50 rg/mL Hg(I1) and Pb(I1) test solutions were employed a t pH 5.0. The columns were then eluted with 5.00 mL of either a 1:l nitric acid/water mixture, a 1:l hydrochloric acid/water mixture, or a 1 M thiosulfate solution. Both the effluent and the eluate from each column were analyzed by atomic absorption spectrometry. In this fashion, both the uptake of metal by the resin and the recovery of the metals sequestered by the resin were established. Reagent and DDW blanks were run in an identical manner for each analysis. All analyses were carried out in triplicate. Analysis of Seawater. Metal-free seawater samples were prepared by equilibrating 1.0 L of seawater, adjusted to pH 6.0 with ammonium hydroxide, with 2.0 g of 80/100mesh resin on a mechanical shaker for 24 h. The resin was removed from solution by filtration and the filtrate employed to establish the recovery of trace metal spikes. Metal-free seawater samples, 1.0 L spiked to contain 5,10, or 20 ng/mL each of Cu(II), Fe(III), Zn(II), and Cd(I1) and 50, 100, or 200 ng/mL of Pb(I1) were passed through packed columns. The columns were eluted with 5.00 mL of 1:l HCl/water and the recoveries of the spiked metals determined by atomic absorption spectrometry. The feasibility of the simultaneous recovery of these metals from spiked seawater was also studied by means of a batch equilibration. Metal-free seawater samples, 1.0 L, spiked with 10 ng/mL each of Cu(II), Fe(III), Cd(II), and Zn(I1) and 100 ng/mL of Pb(I1) were equilibrated with 0.5 g of 80/100mesh resin on a mechanical shaker for 24 h. The resin was then collected by filtration and equilibrated with 10.00 mL of 1:l hydrochloric acid/water for 3 h and centrifuged. The concentration of the metals in the eluant was determined by atomic absorption spectrtrometry. Seawater samples were analyzed by using the batch equilibration technique. Acidified seawater samples, 1.0 L, were adjusted to pH 6.0 with ammonium hydroxide and then equilibrated with 0.5 g of SO/lOO mesh poly(acry1amidoxime) resin for 24 h. The metals were stripped from the resin by equilibration of the resin with 5.00 mL of 1:l HCl/water for 3 h and their concentrations in the eluate determined by atomic absorption spectrometry. The Cd(I1) and Pb(I1) concentrations in the eluate from one set of seawater samples were determined by differential pulse polarography. A 1.00-mL aliquot of eluate was added to 5.00 mL of citrate buffer and approximately 10 mL of DDW. The pH of the solution was adjusted to pH 3.0 with ammonium hydroxide, followed by dilution to 25.00 mL with DDW. Polarograms for the determination of both Pb(I1) and Cd(I1) were obtained as described for the determination of total iron in water (26). The initial potential was set a t -0.30 V instead of -1.2 V. Lead was
determined by standard additions with the Cd(I1) serving as an internal reference. The Cd(I1) was then determined in an identical way with the Pb(I1) as the internal reference. Iron was determined directly in seawater by differential pulse polarography. Digestion of 100.00 mL of seawater was carried out by boiling in a 250-mL Erlenmeyer flask on a hot plate until the crystallization of salt began. Approximately 30 mL of DDW and 5.00 mL of tartrate buffer were then added. The pH of the solution was adjusted to pH 9.0 with 15% NaOH followed by dilution to 50.00 mL with DDW. A 25.00-mL aliquot was taken and the iron concentration determined by differential pulse polarography (26). The direct determination of Cu(II), Cd(II),Pb(II), and Zn(I1) in seawater was performed by differential pulse anodic stripping voltammetry. Acidified seawater samples were digested by heating 50.00 mL of seawater in a 125-mL Erlenmeyer flask on a hot plate. The solution was evaporated until the crystallization of salt began and then diluted to 35 mL with DDW. Acetate buffer, 1.00 mL, and 2 drops of 10% hydroxylamine hydrochloride solution were added. The pH was adjusted to 6.0 with 15% NaOH, followed by dilution to 50.00 mL with DDW. A 25.00mL aliquot was taken, and voltammograms were obtained as described by Chau and Lum-Shue-Chan (25). Zinc, cadmium, and copper were determined by standard additions. The lead oxidation peak served as an internal reference. Similarly, zinc, copper, and lead were determined by standard additions with the cadmium oxidation peak as the internal reference. All metal determinations in seawater were carried out in triplicate. Analysis of Pond Water. Metal-free pond water samples were prepared in the same manner as metal-free seawater samples. These were then spiked with 10 ng/mL each of Fe(III), Cu(II), Cd(II), and Zn(I1) or 50 ng/mL each of Fe(III), Cu(II), Cd(II), Zn(II), and Pb(I1). The simultaneous recovery of these spikes was then studied by the batch equilibration procedure. Metals were determined directly in pond water by the batch technique. Acidified pond water samples, 2.0 L, were adjusted to pH 6.0 with ammonium hydroxide and then equilibrated for 24 h on a mechanical shaker with 1.0 g of SO/lOO mesh poly(acrylamidoxime) resin. The metals were stripped from the resin as described earlier and their concentrations in the eluate determined by atomic absorption spectrometry. One set of pond water samples was collected with no persulfate reagent added. Resin was added directly to the samples a t the sampling site, and the remainder of the procedure was followed. Copper, cadmium, lead, and zinc were determined directly in pond water by differential pulse anodic stripping voltammetry (25),and iron was determined directly by differential pulse polarography (26). All metal determinations in pond water were carried out in triplicate. Also, DDW and reagent blanks were carried out for each determination in both the seawater and the pond water analyses.
RESULTS AND DISCUSSION Recovery of Sequestered Metals. Since atomic absorption spectrometry was employed as the analytical method for this resin study, removal of the metals from the resin matrix prior t o their determinations was necessary. A significant preconcentration of metals from natural waters can only be effected if t h e volume of the eluant is small relative t o the initial sample volume. For this reason, small eluant volumes, 5.00 mL, were used in this recovery study. T h e uptake of metals in this study was 95% or better for all the metals tested except for Mn(I1) which was only 90% sequestered. T h e recovery of the metals was then calculated on the basis of the quantity of metal sequestered. The first eluant tested was a 1:l "OB water/mixture. T h e recovery of sequestered metals was 1 9 5 % for all the metals tested except Zn(II), Ag(I), and Cd(II),which were 88%, 8 l % , and 78% recovered, respectively. In an effort to boost the recovery of these metals, a 1:l HCl/water mixture was tested as a n eluant. T h e increased affinity of chloride relative t o nitrate for the metals, coupled with the acidity of the eluant
ANALYTICAL CHEMISTRY, VOL. 52, NO. 14. DECEMBER 1980
Table I. Percentage Recovery of Trace Metals from Spiked Seawatera
Table 11. Determination of Trace Metals in Seawater Sample 1a
spikes, ng/mL A b = 5 Ab=10 Ab=20 B b = 50 B b = 100 B b = 200 trial no, 1, column method
Fe (111) Cu(I1) Zn (11) Cd(I1) Pb(I1) Fe (111) Cu (11) Zn (11)
98 ( 2 ) 93(3)
84 ( 2 )
metal Fe( 111) Cu( 11) Cd( 11) Pb( 11) Zn( 11)
74 ( 5 )
95(0) 93(5) 71 ( 2 ) 56(3) 42(5) 1 7 ( 1 ) 14 (1) 8(1) 98(2) 97(10) 94(0) trial no. 2, 93 (4) batch 97 ( 2 ) method 92 ( 3 ) Cd (11) 88 ( 2 ) Pb (11) 91 ( 5 ) a Standard deviation of three determinations in parentheses. Row A is Fe(III), Cu(II), Cd(II), and Zn(I1) spikes. Row B is Pb(I1) spikes.
-
increased the recoveries of Zn(II), Ag(I), and Cd(I1) to 295%. However, the HC1 based eluant was not as effective as the HN03-water mixture for the recovery of Co(I1) or Hg(I1) which were only 76% and 79% recovered with the 1:l HC1. T h e final eluant tested was a 1 M thiosulfate solution for the recovery of Ag(1). Thiosulfate forms a very stable complex with Ag(I), with a three-step formation constant of 1.4 X 1014 (27). T h e use of thiosulfate as an eluant effected the quantitative removal of sequestered Ag(1) from the resin matrix. T h e quantatitive recovery of sequestered Hg(I1) using thiosulfate as an eluant would also be expected since Hg(I1) has an overall formation constant of 1.0 X loa for its complex with thiosulfate (27). T h e standard deviation for three determinations was 5570 for every metal tested in the recovery study. The resin was regenerated after the acid elution by passing 5 mL of 3 M ",OH through the column followed by 5 mL of DDW. This procedure was carried out 10 times on the same column with no loss in efficiency of metal uptake or removal. S e a w a t e r Analysis. T h e recoveries of trace metals from spiked seawater are presented in Table I. Trial one was carried out by the packed column method. Only Fe(III), Cu(II), and Pb(I1) were 290% recovered a t the 5 ng/mL level (50 ng/mL for Pb(I1)). As the concentration of the spikes was increased, only Cu(I1) and Pb(I1) were quantatively recovered. T h e poor recoveries of some of the metals was due probably to a combination of poor selectivity, slow kinetics, and adsorption of the metals onto the sample reservoir (24). A batch technique permits equilibration of the sample with the resin over a longer period of time. In this manner, the selectivity of kinetics of the complexation will not have as significant an influence on the sequestering of metals in a column technique. Adsorption on the container walls is also minimized because the entire sample volume is in intimate contact with the resin phase a t one time. T h e nature of a column technique dictates that only a small fraction of the total sample volume can be in contact with the resin phase a t any point in time. Trial two (Table I) shows the marked increase in the recovery of trace metal spikes from seawater when a batch rather than a column technique is employed. For this reason, the batch approach was chosen in the actual sample analyses. The simultaneous determination of Fe(III), Cu(II), Zn(II), Cd(II), and Pb(I1) in seawater was achieved by a batch equilibration of 0.5 g of 80/ 100 mesh poly(acry1amidoxime) resin with 1.0 L of seawater a t pH 6.0. T h e results of this determination are shown in Table 11. The concentrations of Pb(I1) and Cd(I1) in the eluate were too low to be determined by atomic absorption spectrometry and were determined by
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determinatn by resin preconcn,b ng/mL 92 (5.0) 63 (6.0) 2.8 (0.30) 3.2 ( 0 . 6 0 ) 7.0 (0.70)
direct deterrninatqc nglmL 62 (4.0) 3 (0.5) 3 (0.4) 10 (0.60)
a Standard deviation of three determinations in parenCd(I1) and Pb(I1) determined by differential theses. pulse polarography, others by atomic absorption spectroDetermined by differential pulse anodic stripmetry. ping voltammetry.
Table 111. Determination of Trace Metals in Seawater Sample 2 =
metal Fe(II1) Cu( 11) Cd( 11) Pb( 11) Zn( 11)
equilibrn no. 1 equilibrn no. 2 with 0.5 g of with 0.5 g of resin, ng/mL resin, ng/mL 289 (17.0) 39 (6.0) 1 0 (1.0) 2 (1) 1.4 (0.20) 1.5 < 5.0 < 5.0 1 3 (0.30) 2 (0.2)
equili brn with 1.0 g of resin, ng/mL 338 (17.0) 9.1 (1.1) 1.6 (0.10)
