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(29) McAuliffe, C. Chem. Technol. 1971,1,46-51. (30) Lincoff, A. H.; Gossett, J. M. In Gas Transfer at Water Surfaces; Brutsaert, W., Jirka, G. H., Eds.; D. Reidel: Boston, MA, 1984. (31) Grob, K.; Habich, A. J. High Resolut. Chromatogr. Chromatogr. Commun. 1983,6, 11-15. (32) Grob, K. J. Chromatogr. 1984, 299, 1-11. (33) Ioffe, B. V.; Vitenberg, A. G. Head-Space Analysis and Related Methods i n Gas Chromatography; Wiley: New York, 1984. (34) Horvath, A. L. Halogenated Hydrocarbons, SolubilityMiscibility with Water; Marcel Dekker: New York, 1982. (35) Nkedi-Kizza, P.; Rao, P. S. C.; Hornsby, A. G. Environ. Sci. Technol. 1985, 19, 975-979. (36) Tokunaga, J. J. Chem. Eng. Data 1975,20,41-46. (37) Alessi, P.; Kikic, I.; Fredenslund, A.; Rasmussen, P. Can. J. Chem. Eng. 1982,60, 300-304. (38) Kikic, I.; Alessi, P.; Rasmussen, P.; Fredenslund, A. Can. J. Chem. Eng. 1980,58, 253-258. (39) Fredenslund, A.; Rasmussen, P. SEP 8419, Instituttet for Kemiteknik, The Technical University of Denmark,
Lyngby, Denmark, 1984. (40) Kehiaian, H. V. Fluid Phase Equilib. 1983, 13, 243-252. (41) Arbuckle, W. B. Environ. Sci. Technol. 1981,15,812-819. (42) Belfort, G. In Chemistry in Water Reuse; Cooper, W. J., Ed.; Ann Arbor Science: Ann Arbor, MI, 1981; Vol. 2, pp 207-241. (43) Leighton, D. T., Jr.; Calo, J. M. J. Chem. Eng. Data 1981, 26, 382-385. (44) McConnell, G.; Ferguson, D. M.; Pearson, C. R. Endeavour 1975,34, 13-18. (45) Hunter-Smith, R. J.; Balls, P. W.; Liss, P. S. Tellus 1983, 25B, 170-176.
Received for review November 18,1985. Accepted April 1,1986. This research was funded in part by the US.Environmental Protection Agency under Assistance Agreement CR-808851 to Stanford University. However, this publication has not been subjected to the Agengy's peer and administrative review and therefore does not necessarily reflect the views of the Agency, and no official endorsement should be inferred.
NOTES Potential Artifacts in the Determination of Metal Partitioning in Sediments by a Sequential Extraction Procedure Franqois Rapln,+Andri Tessier," Peter G. C. Campbell, and Richard Carlgnan
Universit6 du Quebec, INRS-Eau, C.P. 7500, Sainte-Foy, Qugbec, Canada G1V 4C7 The partitioning of trace metals in sediments) as obtained with a sequential extraction procedure, may be affected by (i) the techniques used to preserve the sediments before analysis and (ii) the presencelabsence of atmospheric oxygen during the extraction steps. No storage method tested completely preserved the initial chemical and physical characteristics of the sediments. Drying of the sediment (freeze-drying;oven-drying) should be avoided; acceptable preservation techniques include freezing or short-term wet storage (1-2 "C). Among the different metals (Cd, Co, Cr, Cu, Ni, Pb, Zn, Fe, and Mn), copper, iron, and zinc were particularly sensitive to sample pretreatment. For anoxic sediments) the maintenance of oxygen-free conditions during the extractions is of critical importance. Introduction
In principle, the partitioning of sediment-bound metals could be evaluated both by thermodynamic calculations (provided equilibrium conditions prevail) and by experimental techniques. Although the former approach holds considerable promise in this regard ( I , 2)) it is of limited application at the present time since the thermodynamic data needed for handling the complex sediment-water systems are as yet incomplete. Direct determination of specific sediment-trace metal associations is also difficult, 'Present address: Institut F. A. Forel, Universitg de GenBve, 1290 Versoix, Suisse. 838
Environ. Sci. Technoi., Vol. 20, No. 8, 1986
if not impossible, because of the great variety of solid phases that can bind trace metals, their amorphous character, and the low trace metal concentrations involved. As an alternative to metal partitioning models, methods have been suggested for fractionating the sediment chemically (3-8). The partitioning obtained by such procedures is, however, influenced by factors such as the choice of reagents used for the various extractions and the extraction sequence, the time of extraction, the ratio of extractant to sediment and by inherent analytical problems such as incomplete selectivity and readsorption (7,9-11); it follows that the distribution of a metal among various fractions does not necessarily reflect its association with discrete sediment phases, but rather should be considered as operationally defined by the methods of extraction. In addition to these inherent analytical problems, there is also a potential difficulty in preserving sample integrity between the time of sample collection and extraction; sample preservation is of considerable practical importance as there are often unavoidable delays between the two operations. In effect, the importance of maintaining sample integrity has been alluded to in the literature (12,13), but with the exception of the studies reported by Thompson et al. (14) for estuarine sediments) published experimental data are virtually nonexistent. As a result, there exist numerous published reports in which little or no heed has been paid to the possible effects of sample pretreatment on metal partitioning. In this paper we examine the effects of preservation techniques (wet storage, freezing, freeze-drying,oven-drying) on metal partitioning and evaluate the importance of performing the extractions
0013-936X/86/0920-0836$0 1.50/0
0 1986 American Chemical Society
under an oxygen-free atmosphere.
at room temperature with 3.2 M NH,OAc in 20% (v/v) “03.
Experimental Section
Experimental Procedure. The sediments used for the experiments were highly contaminated with several trace metals. They were collected from three sites: Lakes Nepahwin (grab sampling) and Clearwater (core sampling by a diver with a plexiglass tube) located in the Sudbury area (Ontario) and from the Maskinonge River, a small inflow to Lake Aylmer (Qugbec). In this latter case a large quantity ( 125 L) of the sediments was returned to the laboratory, thoroughly mixed, and then left undisturbed for 3 months in a large plastic tub under -30 cm of lake water. An oxic surface layer developed (a few millimeters), but the underlying sediment was anoxic. A core sample of this “experimental” sediment was collected with a plexiglass tube. The oxic and anoxic samples from Lake Nepahwin were separated in the field and placed in sealed polypropylene containers. The cores were extruded in a glovebox under a nitrogen atmosphere, and oxic (Clearwater; experimental sediment) and anoxic (experimental sediment) samples were retained for the experiments. Before the trials described below were initiated, each sediment sample was introduced, under a N2 atmosphere, into a 50-mL polycarbonate tube; the tubes were then tightly closed, removed from the glovebox, and centrifuged at 16000g for 20 min. After centrifugation, the tubes were returned to the glovebox, the excess water removed by a pipet was discarded, and an aliquot of the sediment was taken for a dry weight determination. To evaluate the effect of oxygen contact with the sediment during the extraction steps, each sediment sample (Nepahwin, anoxic and oxic; experimental sediment, anoxic and oxic) was split in the glovebox into two series of three subsamples to be submitted to the sequential extraction procedure described below. Manipulations in the first series were all done under a N2atmosphere; i.e., extracting solutions for steps a-c were deaerated with N2, and centrifugation was performed under a N2 atmosphere. For the second series, no precautions were taken to minimize contact of the sediment with oxygen. To evaluate the effect of sample pretreatment and storage, each sediment sample (Clearwater, oxic; experimental, anoxic) was split into five portions. The first was extracted immediately in triplicate to serve as a control; the other ones were either stored at 4 OC, frozen and stored at -30 “C, freeze-dried and stored in a desiccator, or dried under air in a convection oven (105 “C) and stored in a desiccator. After a 20-day storage period, each portion was extracted in triplicate. Manipulations of the subsamples were done under a N2 atmosphere as described earlier. Analyses. The various subsamples (2-3 g wet weight; equivalent to -1 g dry weight) from the two experiments described earlier were subjected to a sequential leaching procedure designed to partition the particulate trace metals (M) into the following five fractions: (a)M(F1): exchangeable metals. The sediment sample was extracted for 30 min with 1 N MgClz initially at pH 7.0. N
( b )M(F2): metals bound to carbonates or specifically adsorbed. The residue from (a) was leached for 5 h with 1 M sodium acetate (NaOAc) adjusted to pH 5.0 with
acetic acid (HOAc). (c) M(F3): metals bound to Fe-Mn oxides. The residue from (b) was extracted at 96 “C for 6 h with 0.04 M NH20H.HC1 in 25% (v/v) HOAc. ( d ) M(F4): metals bound to organic matter and sulfides. The residue from (c) was extracted at 85 “C for 5 h with 30% H202 adjusted to pH 2 with HNOBand then
( e ) M(F5): residual metals. The residue from (d) was digested with a 5:l mixture of hydrofluoric and perchloric acids. The details of the experimental procedure and an evaluation of its precision and accuracy have been published elsewhere (7). Owing to the inherent lack of selectivity of the extraction procedure, the five fractions have henceforth been assigned numerical designations (M(Fl), M(F2), ... M(F5)) rather than precise geochemical descriptions. Metal concentrations (Cd, Co, Cr, Cu, Ni, Pb, Zn, Fe, and Mn) in the leachates were fir$ determined by flame atomic absorption spectrophotometry (Varian Techtron, Model 575 ABQ);in some cases, when metal concentrations were below or close to the detection limit (e.g., in F1, F3, or F4), flameless atomic absorption spectrophotometry was used (Varian Techtron, Model 1275; GTA-95). Quantitation was achieved with appropriate calibration curves obtained with the metals added to solutions containing the reactants used for the extractions. Concentrations of acid volatile sulfide (AVS) were determined for the two anoxic sediment samples (Nephawin; experimental sediment) at various steps to evaluate the effects, on sulfides, of sample pretreatment and contact with oxygen. The AVS levels were obtained by adding -50 mL of 10% HC1 to the sediment sample (4-6 g wet weight; equivalent to -2 g dry weight), thus generating H2Sthat was swept out with N2 and bubbled into a sulfur antioxidant buffer solution (SAOB; 15). The measurement of the sulfide concentration was effected with a sulfideselective electrode (Orion 94-16A). Quantitative recovery was obtained when 100 mL of M NazS were treated instead of the sediment samples. Results and Discussion
Exposure to Air during Extractions. In the absence of precautions taken to prevent exposure of sediments to atmospheric oxygen, several statistically significant decreases or increases in trace metal concentrations (level of significance, P, < 0.01) were observed in fractions 1-4 for the anoxic sediment samples; a few examples are given in Figure 1 (complete material is provided in Table A of the supplementary material (see paragraph at end of paper regarding supplementary material)). The significant differences observed can be ascribed to the contact of the sediment with air rather than to experimental artifacts such as inhomogeneity of the sediments or variations in experimental procedures. Indeed, no such differences were obtained for oxic sediments, and the sums of the metal concentrations in the individual fractions of both series agree within 5%. For some metals in a given fraction, the two anoxic sediment samples exhibited contrasting behavior (e.g., compare parts C and D of Figure l). This illustrates the fact that it generally will not be possible to predict the consequences of exposure to oxygen during extraction; i.e., it will be impossible to “correct” retroactively data that have been obtained without adequate precautions. If oxygen was not rigorously excluded during sediment extraction, AVS concentrations dropped markedly during the first two extractions (Table I); 11-39% remained after treatment with MgClz and 1-22% after reaction with the NaOAc/HOAc (pH 5) buffer. If a working atmosphere of nitrogen was maintained, the AVS levels were preserved through the first two extractions (87-100%) but were reduced to
