Separation and Concentration of Trace Metals from Natural Waters Partition Chromatographic Technique D.4YTON E. CARRITT Chesapeake Bay Institute, T h e Johns Hopkins University, Annapolis, .Md,
determination of many of the dissolved constituents of processing of relatively large volumes of sample by t,echniques which concentrate the test subitances and separate them from interfering elements. This processing is required because many of the biologically and catal>-tically active elements are so extremely diluted. In addition, the presence of other substances in much higher concentrations often interferes with the analytical processes. Sea water, for example, which is grossly a 3% salt solution, is composed of at least 44 dissolved constituents (4).However, the six elements, vhlorine, sodium, magnesium, sulfur, calcium, and potassium, niake up 94.2% by weight of the dissolved solids. Of the remainto 10-6%, 17 are in the ing 38 elements, 13 are in the range range 10-6 to lo-*%, and 8 are less than 10-8’%. Forty-three elements have not, been detected in sea water. Thus, all dissolved substances, with the exception of the six major constituent’s, are iii concentrations below O.Ol%, a figure suggested by Saridell ( 3 ) as “indicating the approximate upper limit of a t,race constituex1t .” The direct determination of minor constituents in sea water is possible in only a few cases. Methods have been devised for the iletermination of dissolved nitrate, nitrite, and inorganic phosphate, substances which have received considerable attention bevause of their nutrient role during photosynthesis, and direct spectrophotometric methods for several trace metals have also Ileen reported. Nevertheless, our knowledge of the distribution of’most of the trace elements and of their importance in biological :1ritl geological processes is extremely limited and is likely to remain so until concentrating and separating techniques and :rnalytical procedures are developed t h a t can be made to function efficiently under the severe conditions normally encountered in the field operations in oceanography, limnology, and sanitary rngineering. r TIIE
1natural waters requires the
Recovery tests xere made by passing standard solutions and natural water samples containing known added quantities oi test substance through the column. Recovery TT as considered “complete” when the analysis of the concentrate agreed with the analyses of standard solutions prepared with the same quantity of test substances as in the column influent but in a volume equal to that of the concentrate. Column. The construction and operation of the extraction column are shown in Figure 1. The dimensions are those of the column used to obtain the results reported below. N o attempt was made to study the effect? produced by major dimensional changes. I,I
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eluting solutions column
REQUIREMENTS
The following general requirements for a satisfactory concentrating technique will be common to many field uses. Exposition of detailed requirements will depend upon the nature of specific studies. l~~quipment must be simple in both construction and operation. Test substances must be concentrated by factors up to 10,000 with a precision that will not limit the precision and accuracy of modern analytical methods. For many studies, especially in oceanography, specificity of the technique should be limited to a separation of as many of the minor elements as possible from all of the major elements-that is, the concentrate should contain none of the major constituents initially present in the sample and a1 elements as possible. I n some cases sitnte further separation prior to an EXPERI3IENTA L
The operation of the extraction column described here depends upon the reactivity of metallic ions in aqueous solution with dithizone in carbon tetrachloride solution. I n principle, the device operates as a partition chromatographic column in which the carbon tetrachloride-dithixone solution is the immobile pllase, being held in the column on a cellulose acetate support, :iiitl tlie aqueous solution (the sample) is the mobile phase, Seventeen elements are reported to form dithizonates ( 3 ) . Of t h c ~ e leati, , zinc, manganese, cadmium, cobalt, and copper have been studied and are completely removed from solution by the column when operated under the conditions described hererith.
U
vacuum fittrotor
Figure 1.
Extraction Columm
Column Packing Material. The column packing consisted of a granular cellulose acetate support carrying a carbon tetrachloride solution of dithizone. The cellulose acetate was prepared hy crumbling and sieving a commercial product (Fisher Scientific Co., No. C-215) through a set of well cleaned standard sieves. Two fractions gave satisfactory results-that passed by sieve S o . 18 (1000-micron opening) and retained on S o . 25 (750 microns), and that passed by N o . 25 and retained on S o . 35 (500 microns). An attempt to use powdered filter paper (Khatman, cellulose powder, standard grade) was unsuccessful, as the dithizonecarbon tetrachloride was rapidly stripped from the support by passage of the sample. Ten grams of sized cellulose acetate were treated with 100 nil. of a carbon tetrachloride solution of dithizone (0.5 gram per liter) and the slurry was heated with constant agitation in a 500-nil. beaker on a hot plate to evaporate the carbon tetrachloride. Heating was discontinued when the bulk of the material had pasTed from a deep blackish green to a light green, a transition which is marked, and which produces an essentially dry product that can be readily poured and packed into a column. X column m s prepared by adding and tamping firmly into
1927
1928 place approximately 2 grams of untreated cellulose acetate followed by 3 grams of treated cellulose acetate. Glass wool plugs at both ends of the column served to hold the packing in the column. Three milliliters of carbon tetrachloride were then added to the top of the column and drawn into the packing by momentary application of vacuum. This “wet” approximately three quarters of the length of the treated column packing. A column to which carbon tetrachloride had not been added gave practically no recovery and permitted rapid stripping of the dithizone from the support. Excessive wetting with carbon tetrachloride permitted migration of the immobile phase out of the column along with the sample and eluting solutions. The column was washed with 100 ml. of Ji hydrochloric acid and 250 ml. of metal-free water and was then ready for use. Sample Pretreatment. Solutions from which the recovery of lead, zinc, cadmium, cobalt, and copper were to be studied were adjusted to pH 7.0 + 0.1, thip being the only pre-extraction treatment required. The recovery of manganese m s studied using the divalent element. Complete recovery as obtained in experiments in which air oxidation of the manganese was minimized. I n several experiments poor recovery was obtained and in each case the test solutions inadvertently had been made alkaline while adjusting the acid test solution to p H 7.0. As the air oxidation of manganese(I1) in alkaline solution proceeds rapidly and, in fact, is the controlling reaction in the Kinkler determination of dissolved oxygen, the observed behavior on the column suggests that only manganese(I1) reacts on the column or that dithizonates of higher oxidation states cannot be removed from the column by the methods described below. The reduction of manganese prior to column extraction has not been studied. Column Operation. After pretreatment, the sample was drawn through the column with vacuum. In the tests described here the flow rate has been controlled solely by the pressure drop of the packing material. Flow rates up to 6 liters per hour have had no effect on recovery. Most tests, however, were a t a flow rate of approximately 2 liters per hour. Rapid fluctuations in pressure, such as those produced by a slow-speed vacuum pump, tend to strip the dithizone-carbon tetrachloride from the support. This can be prevented by inserting a large free volume between the pump and column. Collection of reactive substance can be observed by the color change produced in the column as the sample is drawn through. The capacity of the column, as estimated by the color change, is approximately 3 mg. of dithizone reactive elements. This has not yet been confirmed by breakthrough studies. Elution from Column. Dithizonates were removed from the column and partial separation of the reactive elements was achieved by elution with the proper reagents. Of the six elements tested, lead, zinc, cadmium, and manganese( 11) were completely removed with 50 ml. of hydrochloric acid X . Copper and cobalt were removed with 50 ml. of concentrated ammonia. Elution with ammonia destroyed the column, whereas an acid-washed column could be re-used, provided sufficient dithizone-carbon tetrachloride remained in the column. Analysis of Elutants. In the studies reported here, all elements except manganese were determined by the polarographic method, using a Sargent Model X X I recording polarograph. Manganese was determined spectrophotometrically using the persulfate oxidation ( 3 ) to permanganate, and a Beckman Model DU spectrophotometer with 10-em. Corex cells. The acid elutant was prepared for analysis by adding 0.5 ml. of sulfuric acid, evaporating to fumes of sulfuric acid, adding 1 ml. of nitric acid, and evaporating to dryness. For the simultaneous analysis of lead, cadmium, and zinc, 5 to 7 ml. of Ji potassium chloride containing 0.001% gelatin as a maximum suppressor were added to the residue, warmed to ensure solution of the residue, cooled, washed into a volumetric flask, and diluted to volume nith A’ potassium chloride. The solution was then polarogramed. RIolar sodium hydrovide containing 0.001% gelatin was used as supporting electrolyte for the polarographic determination of lead and zinc when subsequent spectrophotometric analysis for manganese was to be performed on the same sample. When this was to be done, the sample was removed from the cell after
ANALYTICAL CHEMISTRY polarograming and neutralized with sulfuric acid, and the persulfate oxidation to permanganate was performed without further treatment. The ammonia elutant was evaporated to near dryness to remove the ammonia, 0.5 ml. of sulfuric acid was added and evaporated to fumes of sulfuric, 2.0 ml. of nitric acid were added, and the solution was evaporated to dryness. For the polarographic determination of copper and cobalt in the residue a supporting electrolyte, 1M in ammonia and ammonium chloride and containing 0.001% gelatin, was added, warmed, washed into a volumetric flask, and diluted to volume, and the solution was polarogramed. Recovery Tests. Column efficiency was determined by passing solutions containing singly, and in combinations, known quantities of zinc, lead, cadmium, manganese(II), cobalt, and copper through the column. Sample volumes were 0.5, 1.0, or 10 liters Column elutants, after treatment, were made up to 10 ml. for polarographic analysis. All six substances were tested individually by passing 1 litel of solution containing 1 mg. of test element through the column. Recovery was complete in all cases. Combinations of zinc, cadmium, and lead were studied b~ passing 0.5-liter quantities of solution containing not over 1 mg of total test elements through the column. Complete recovery was obtained. A 10-liter sample containing 10 y each of zinc and copper wai passed through the column. The recovery of zinc was 114% and of copper 102’%. Recovery from a sample of Chesapeake Bay water, salinity = 1 2 ° / ~ ~was , determined by a comparison of zinc analyses of an aliauot to which 10 Y of zinc had been added, with one to which no iinc had been added. Liter samples were’used in both cases. Recovery of the added zinc w-as 93 %. Blank corrections were made for all analyses and residual current correction applied to all polarograms. DISCUSSION
The use of a “solid” extraction column has several advantages over liquid-liquid techniques, especially over batchwise liquidliquid extraction. It permits the use of extremely simple apparatus which requires little or no attention during operation and in the form described here is \vel1 suited for many field studies, thus satisfying one of the requirements mentioned. In addition, the column technique provides a means of attaining much greater efficiency of operation, for its performance approaches that of ‘L system composed of a series of theoretical plates in which equilibrium is maintained between the mobile and immobile phases. The recovery of manganese(I1j is perhaps the best indication of the efficiency of the column, for the dithizonate of this element is reported to be unstable and only 50% recovery is attainable hy batchwise liquid-liquid extraction (3). The dithizone system complies with the specificity requirements, as none of the major constituents appear in the concentrate. Concentration factors up to 1000 have been achieved. An increase to 10,000 would undoubtedly be possible if the residue fromdigestion of the elutantwere to be diluted to 1 ml. rather than to 10 ml. as reported here. Polarographic analysis of samples of 1-ml. volume and less has been described ( 1 ) and absorption c ~ l k with 5-cm. path and volume approvimately 0.5 ml. for the Beckman DU spectrophotometer are available from Microchemiccd Specialties Co., Berkeley, Calif. LITERATURE CITED
Kolthoff, I. PI., and Lingane, J. J., “Polarography,” 2nd ed.. New York, Interscience Publishers, 1952. Martell, A. E., and Calvin, M,, “Chemistry of the Metal Chelate Compounds,” Xew York, Prentice-Hall, Inc., 1952. (3) Sandell, E. B., “Colorimetric Determination of Traces of Metals,” 2nd ed., New York, Interscience Publishers, 1950. (4) Sverdrup, H. U., Johnson, M. W., and Fleming, R. H., “The Oceans,” New York, Prentice-Hall, Inc., 1946. RECEIVEDfor review M a y 15, 1953. Accepted .4ugust 18, 1953. Contribution S o . 11 from the Chesapeake Bay Institute. Results of work carried out for the Office of Naval Research of the Navy Department. the State of Maryland (Department of Research a n d Education), a n d the Commonwealth of Virginia (Virginia Fisheries Laboratories).
