Anal. Chem. 1980, 52,
termination are compared with both the initial weight of potassium chlorate and with the modified iodometric method (14). In either case, the results are in agreement with the present work as noted in Table 111. The synthetic mixtures were analyzed in both buffered and unbuffered systems. Besides the bicarbonate buffer reported in detail in Table 111, it was observed that acetate and borate buffers were also equally satisfactory. The possible interference from As(II1) or the products of its oxidation, or the OsOl catalyst, or the buffer are functions of the appropriate solubility products with silver ion and/or the pH. Under the conditions reported here, no interference is observed. Finally, it should be noted in Table 111, that the present study provides not only a simple and direct way t o determine chloride, chlorite, and chlorate ions but also a method for examination of the total chlorine balance.
1433
1433-1439
LITERATURE CITED Prince, L . A. Anal. Chem. 1964, 36, 613-616. Chen, T. H. Anal. Chem 1967, 39, 804-812. Barney, J. E. 11; Bertolacini, R. J. Anal. Chem. 1957, 29, 1187-1188. Hong, C. C.; Rapson, W. H. Can. J. Chem. 1968. 46, 2061-2064. Kolthoff, I. M.;Belcher, R. “Volumetric Analysis”, Vol. 111; John Wlley 8 Sons: New York, 1957; p 234. Norkus, P. Zh. Anal. Khim. 1965, 20, 612-614; Chem. Abstr. 1965, 63. 10681h. Noikus, P. Zh. Anal. Khim 1965, 20, 496-500. Yamasaki, S.;Ohura, H.; Nakamori, I. Bunseki Kagaku 1973, 22, 843-849. Keiffer, R. G.; Gordon, G. Inorg Chem. 1988, 7 , 235-239. Silverman, R. A.; Gordon, G. J . Chem. Educ. 1973, 50, 654-655. Heunisch, G. W. Anal. Chim. Acta 1976, 701, 221-224. Havacek, H. T.; Swann, W. H. Chem. Anal. 1967. 56, 16-17. Selig, W. Microchem. J. 1976, 27, 291-301. Tang. T. F. M.S. Thesis, Miami university, Oxford, Ohio, 1980.
RECEIVED for review February 4,1980. Accepted May 16,1980.
Determination of Labilities of Soluble Trace Metal Species in Aqueous Environmental Samples by Anodic Stripping Voltammetry and Chelex Column and Batch Methods Paul Figural and Bruce McDuffie” Laboratory for Trace Methods and Environmental Analysis, Department of Chemistry, State University of New York a t Binghamton, Binghamton, New York 13907
A method for differentiating trace metal species on the basis of relative lability has been developed for solutions of Cd, Cu, Pb, and Zn utilizing anodic stripping voltammetry and CaCheiex resin in successive column and batch procedures. Species are classified as being “very labile”, “moderately labile”, “slowly labile”, or “inert”, depending on the characteristic time scale of the measuring technique. A range of dissociation rate constants can be estimated for species in each category. Preconcentration of the various fractions leads to precise determinations at the ppb and sub-ppb levels. The method has been applied to St. Lawrence and Susquehanna River water, a Hudson River estuary sample, and secondary sewage effluent. Reasonably distinctive patterns of speclatlon emerged: Cd and Zn were almost entirely in the “very” and “moderately” labile fractions; Cu, primarily in the “moderately” and “slowly” labile fraction; and Pb, 20-70% “slowly labile”, with a significant “inert” fraction in several cases.
Environmental analytical chemists are becoming increasingly aware of the need for methods yielding detailed information on the various chemical forms of soluble trace metals (TM’s) present in both polluted and unpolluted waters. The total concentration of a metal may not correlate well with its impact on aquatic life forms; rather, the speciation of the metal may determine its ecological significance and, in particular, the availability of the metal to organisms (1-4). Analytical methods such as dialysis, ultrafiltration, solvent extraction, gel filtration chromatography, and polarography P r e s e n t address: S t a u f f e r Research Center,
Dobbs F e r r y , N.Y.
10522. 0003-2700/80/0352-1433$01 .OO/O
have been used to differentiate among the various chemical forms of TM’s ( 5 ) . Of the electrochemical techniques, anodic stripping voltammetry (ASV) has been used extensively for speciation studies (6-11). The simplest approach divides soluble metal species into “labile” and “nonlabile” categories based on their ASV characteristics. The “labile” fraction includes free metal ions plus complexed metal which can dissociate very rapidly to the free electroreducible form, yielding an ASV signal. The “nonlabile” fraction includes metal bound in complexes (or adsorbed on colloidal matter) with a rate of dissociation to the free metal that is slow compared to the time scale of the ASV measuring technique. Attempts are often made to determine the labile fractions for several metals at once, i.e., an initial plating potential (E,) is chosen that reduces several metals simultaneously, the subsequent stripping operation yielding the respective metal peaks (6-10). However, the use of too negative an E , value may cause the direct reduction of complexes that are normally nonlabile (12); in an acetate medium, Cu-EDTA, considered to be nonlabile to the ASV method (13),gives a full stripping signal using an E , of -0.6 V and is partially plated even a t E,’s as low as -0.2 V vs. SCE. Thus the E, chosen is important in evaluating the lability of metal species by the ASV method. Florence and Batley ( 5 , 14, 15) used ASV in conjunction with Chelex-100, a chelating ion-exchange resin, to classify the various chemical forms of soluble metal in both seawater and natural water samples. The aqueous sample and its Chelex-column effluent were analyzed for TM’s by ASV directly and after pretreatment with UV irradiation or acid digestion. The combination of these techniques leads to seven categories of operationally defined species. Their categories, however, are not mutually exclusive; most fractions are obtained by difference or using several differences, and for samples with total TM’s < 0.2 Kg/L, the relatively high blanks 0 1980 American Chemical Society
1434
ANALYTICAL CHEMISTRY, VOL. 52, NO. 9, AUGUST 1980
(from acid digestion) make the concentrations of four fractions relatively uncertain. We present here the development and application of a recently suggested method (12)wherein soluble TM’s (Cd, Cu, Pb, and Zn) in environmental water samples are divided into four categories based on their relative degrees of lability using ASV, Ca-Chelex in sequential column and batch procedures, and acid digestion for the remainder. Thus soluble T M species are classified as “very labile” (which includes free or hydrated metal ions), “moderately labile”, “slowly labile”, or “inert” based on the operational definitions of the measuring technique. Direct reduction of metal complexes in the ASV procedure is minimized by plating each metal separately a t its minimum effective plating potential. Further, the last three classes of species are preconcentrated prior to final measurement, improving the precision of the concentration measurements. The proposed scheme is applied t o samples of St. Lawrence and Susquehanna River water, a sample of Hudson River (estuary) water, and two samples of secondary sewage effluent from the Binghamton-Johnson City (N.Y.) treatment plant.
I
I
INEU’
Figure 1. Flow chart of TM
speciation scheme for aqueous samples
in the vessel. At designated times, 5-mL aliquots of the solution phase were taken, acidified (0.05 mL of concentrated HN03),and analyzed for Cd by ASV. Collection of Water Samples. River samples were taken 20 EXPERIMENTAL cm below the surface from the following sites on the dates indicated: Susquehanna River, Vestal, N.Y., at the Binghamton Reagents. Analytical grade Chelex-100, 100-200 mesh, (lot city line (7/26/78); St. Lawrence River, Wellesley Island, Canada no. 14930) was obtained from Bio-Rad Laboratories, Richmond, (7/26/78); Hudson River, Alpine Park, near Alpine, N.J. (across Calif. Redistilled concentrated HNO, and doubly-distilled 70% from Yonkers, N.Y.) (4/24/78). The secondary sewage effluents HC104 (G. F. Smith Chemical Co., Columbus, Ohio), and isowere grab samples from the Binghamton-Johnson City Joint thermally-distilled 6 M NH40H were used to obtain low TM Sewage Treatment Plant, Binghamton, N.Y.: 2 1 on 3/22/78 and blanks (12). Nitrilotriacetic acid (NTA) used in the batch ex$2 on 4/13/78. Samples were collected in pretreated 2-L polyperiments was recrystallized four times prior to use (12, 16). All ethylene containers, brought back to the laboratory, some of each other chemicals used were certified ACS reagent grade. To ensure sample saved for total TM analysis, and the balance filtered low T M blanks, resin columns were always washed with doubly immediately (or after refrigeration in the case of the St. Lawrence distilled water that had been passed through a column of and Hudson River samples) through pretreated 0.40-Wm Nuclepore NH,-Chelex. filters. The filtered samples were analyzed immediately, except Chelex Column Parameters. Na-Chelex (1.3 g), direct from that soluble organic carbon was determined later on frozen bottle, was slurried into 0.8-cm i.d. glass columns containing portions. Standard methods were used to obtain typical water Styrofoam frits, and converted to the Ca-form as reported prequality data (19). viously (12). A flow rate of 2.7 & 0.2 mL/min was used throughout, Speciation Procedure. A flow chart for the speciation method giving a contact time of 7 s. is presented in Figure 1. All samples were analyzed in triplicate. Apparatus. Differential pulse anodic stripping voltammetry was performed with a PAR Model 174 Polarographic Analyzer The detailed procedures were as follows: (a) Total Metal in Unfiltered or Filtered Samples. To 100-mL using a hanging mercury drop electrode (HMDE) as reported portions of unfiltered or filtered sample, 0.6 mL of concentrated previously (17). The plating current in a Cd2+solution correHNOBand 0.25 mL of 70% HCIOI were added. The samples were sponded to a Nernst layer thickness of 2 X cm (12). Two then evaporated in covered beakers in a HC104 fume hood until methods of electroplating were employed in the initial work. In evolution of white HC104 fumes, diluted to a final volume of 25 the first method, designated the “simultaneous” method, Zn, Cd, mL, and ASV analysis was performed after addition of 0.4 mL Pb, and Cu were reduced simultaneously at an E, of -1.2 V, of 2.5 M NaOAc/HOAc pH 6.3 medium to 10 mL of sample, the subsequent stripping operations yielding the respective metal final pH being -4. Zinc was plated at -1.2 V, while Cd, Pb, and peaks. In the second method, designated the “individual” method, Cu were determined simultaneously at an E , of -0.8 V. Coneach metal was determined separately by plating at the least centrations were determined by the method of standard addition. negative potential (E (-,) which gave maximum plating efficiency for the uncomplexecf metal (12). For these experiments Ep(mm)’~ Reagent blanks subjected to this procedure gave the following typical adjustments in sample concentrations: Cd, 0.08; Pb, 0.18; were determined by constructing pseudo-polarograms (18)of the Cu, 0.60; Zn, 1.1 pg/L. free metals in the relatively noncomplexing buffer system, 0.10 (b) “Very Labile” Fraction. After filtration, direct ASV found were -1.2 V for M NaOAc/HOAc at pH 6.3. The Ep(mml’~ analyses of the samples were performed immediately after addition Zn, -0.80 V for Cd, -0.60 V for Pb, and -0.20 V for Cu (all vs. of 0.4 mL of 2.5 M NaOAc/HOAc pH 6.3 buffer to 10 mL of SCE). for each metal in turn. In cases where the sample, using Ep(min) Pretreatment of Glassware. Glassware and polyethylene Cu peak was unresolved, due to high C1- concentration, medium containers were leached free of TM’s by soaking in 2 M HN03 exchange (18)was utilized, stripping in 0.01 M HNOBmedium. for two days, and then rinsed thoroughly with the Chelex-treated For Susquehanna River water, the ASV-labile fractions of the four doubly distilled water. To minimize adsorptive losses of TM’s metals were also determined “simultaneously”, by plating at an from water samples, the acid-leached glassware and plastic conE, of -1.2 V. In all cases, concentrations were determined by the tainers, as well as the Nuclepore membranes, were equilibrated method of standard addition, employing at least 10-fold conwith purified 2 mM CaC12 in 1 mM tris(hydroxymethy1)aminocentration increases to minimize the extent of possible complexing methane (Tris)/HCl a t pH 7.8 overnight before use, then rinsed of the added metals. thoroughly with the doubly distilled water. (c) “Moderately Labile” and “Slowly Labile” Fractions. A Rate of Uptake of Cd by Ca-Chelex in Batch Technique. 250-mL portion of the filtered sample was passed through a and 5 X were added to NTA in concentrations of 0, column of Ca-Chelex to collect the “very labile” plus “moderately duplicate 400-mL beakers containing 250-mL portions of a solution labile” TM fractions. The effluent was collected in pretreated containing 25 pg/L Cd, M Ca(NO&, and 0.01 M Tris/N03 400-mL beakers, 2 g of Ca-Chelex resin added, and the mixture pH 7.8 buffer. After allowing the solutions to equilibrate overin the beakers capped with Parafilm and stirred for 72 h, after night, 2 g of Na-Chelex converted to the Ca form was added and which the resin containing the “slowly labile” fraction was septhe samples were continuously stirred at lo3 rpm, using a 2.5 cm arated through a fritted column, the solution phase being saved X 0.75 cm 0.d. Teflon-covered stir bar (with spin ring) centered
ANALYTICAL CHEMISTRY, VOL. 52, NO. 9, AUGUST 1980
Table I. ASV-Labile Metal Concentrations in a Susquehanna River Water Sample, Comparing “Individual” and “Simultaneous” Methods of Plating plating method individual simultaneous
> 0.10 s-l,
ANALYTICAL CHEMISTRY, VOL. 52,
NO. 9,
AUGUST 1980
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Table 111. Water Quality Data for Samples Studied suspended solids, mg/L
samples Susquehanna River St. Lawrence River Hudson River (estuarya) SSE # I b SSE +2
laboratory PH
spec. cond., pmho/cm
hardness, mg CaCO,/L
sol. org. C , mg/L
10
8.10
0.5 75 2.5 9.5
8.30 7.84 7.80 7.78
164 277 7600 570 8 20
112 136 940 162
2.3 1.5 5.5 4.9 6.9
180
From a Cl‘ determination, this sample had a salinity of 3.2 ppt, about one-tenth that of seawater. sewage effluent. a
Table IV. Metal Speciation Results, pg/L (av. (1)
sample
total TM’s
Cadmium Susquehanna R. St. Lawrence R. Hudson R. SSE #lc SSE *2
i
(2) total insol.“
SSE = secondary
std. dev.) (3) total sol.
(4) AVS-labile
(5) Chelex col.
(7)
inert
ND ND ND ND 0.01 ND
