Environ. Sci. Technol. 2000, 34, 1030-1035
Sequential Extraction of Metal Contaminated Soils with Radiochemical Assessment of Readsorption Effects MARK D. HO AND GREG J. EVANS* Department of Chemical Engineering and Applied Chemistry, University of Toronto, 200 College Street, Toronto, Ontario M5S 3E5 Canada
The operational speciation of selected trace metals in several contaminated Toronto area soils was determined by a modified BCR (Community Bureau of Reference) sequential extraction procedure. This type of environmental analysis is now increasingly used, but its reliability is questioned due to concerns regarding redistribution of extracted metals occurring during the procedure. Using radiotracer techniques, the redistribution of Cd and Zn during the sequential extraction was directly quantified. For a fill material taken from downtown Toronto, metal partitioning behavior conformed with the theory and anecdotal evidence in the soil chemistry literature: Cd was highly mobile, while Cu and Pb were primarily associated with oxidizable organic matter, and Zn was distributed across all soil fractions. Soil from an abandoned metals recycling operation had unusual physical characteristics, and the trace metals within were largely nonextractable. In the soils examined, some 20-30% of the Cd and Zn released from the acid-soluble fraction of the soil was readsorbed to the reducible mineral oxide fraction. The extent of the phenomenon is less than previously suspected and does not invalidate sequential extraction results for these metals.
Introduction The primary goal of this study was to quantify and to characterize trace metal contamination in selected soils and sediments collected from the Toronto region, each representing different contamination problems and remediation challenges. To more clearly identify where and how contaminants were bound, a modified BCR (Community Bureau of Reference, now the Standards, Measurements and Testing Program of the European Commission) sequential extraction procedure was used, to estimate the partitioning and retention behavior of metals within the soil. There is some controversy in the literature over the use of this approach, of which the method developed by Tessier et al. (1), is the most widely used and the best known. The two main criticisms have been the nonselectivity of reagents (2, 3) and the redistribution of extracted metals during the multistage process (4). Keeping these limitations in mind, “operational speciation” results from sequential extractions still provide useful information on metal partitioning, beyond the simple elemental concentrations which are conventionally mea* Corresponding author phone: (416)978-1821; fax: (416)978-8605; e-mail:
[email protected]. 1030
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ENVIRONMENTAL SCIENCE & TECHNOLOGY / VOL. 34, NO. 6, 2000
sured. These additional details aid in evaluating the potential ecological significance of metal contamination, and the feasibility of remediation. To address growing concerns over quality assurance, the BCR protocol (5) emerged out of several years of directed development. It nominally differentiates between (i) “acidextractable” (0.11 M acetic acid), (ii) “reducible,” (0.1 M hydroxylamine hydrochloride acidified to pH 2.0 with nitric acid), and (iii) “oxidizable” (1.0 M ammonium acetate extraction after oxidation by 8.8 M hydrogen peroxide) soil fractions. It is also simpler to perform and to reproduce and thus is better suited for standardization. Certified reference soils of known composition can be used to gauge analytical accuracy and completeness of extraction, and to this end, a reference sediment (CRM 601) certified for BCR-extractable contents of selected metals is now available from the European Commission (5, 6). However, such measures are very recent developments, and rigorous validation of extraction results remains difficult, if not unfeasible (7). Secondary objectives of this work were to verify the obtained extraction results and to evaluate the extent of metal readsorption, which to date, has not been unequivocally addressed by standard analytical schemes. The most likely manifestation of this phenomenon is the case where metal extracted by one reagent is immediately readsorbed to remaining solid substrates, decreasing the apparent amount of metal extracted in that stage. The readsorbed metal is then freed by subsequent stages in the procedure, leading to overestimation in those stages. These effects have been most extensively tested by synthesizing model soils from pure components, and alternately doping a single phase with contaminant metal. Kheboian and Bauer (3) processed such a soil by the Tessier method and found that carbonate-bound Pb was significantly readsorbed to iron oxides and oxidebound Cu to residual silicates, while sulfide-bound Zn was prematurely extracted in the oxide fraction. Raksasataya et al. (4) determined that redistribution of Pb was a major problem for both the Tessier and the newer BCR techniques. Tu et al. (8) found high Cu and Pb binding affinity for humic acid and pyrolusite (β-MnO2), and that metal readsorption was dependent on the content of these two materials. However, the composition and metal distribution patterns of model sediments have been dismissed as unrealistic and artificially constrained, compared to the stable equilibrium state found in natural soils (9, 10). A better method of testing would be to use natural sediments but to add and recover small spikes of known metal quantity, as evaluated by Belzile et al. (11). It was determined that as long as a standard addition of trace metal did not exceed the native concentration in the soil, the existing soil equilibria would not be significantly perturbed, and that losses due to readsorption were negligible. Radiochemical techniques may be even more suitable for evaluating a lengthy, multistep sequential extraction procedure. Radioisotope spikes are easily and directly measurable, yet detectable at far lower concentrations. By adding radioisotope tracers of Cd and Zn into the extraction reagents, redistribution effects for these metals could be directly quantified and corrected for. Schultz et al. (12) used a double spiking technique, where one radioisotope was used to track the readsorption of an actinide during a sequential extraction, and a second radioisotope was added to determine the efficiency of a subsequent chromatographic separation technique. However, it must be stressed that it is not possible to incorporate tracer metal into soil systems in precisely the 10.1021/es981251z CCC: $19.00
2000 American Chemical Society Published on Web 02/12/2000
same form and fashion as it naturally occurs, and the resulting partitioning of tracer will not accurately reproduce the actual metal partitioning. The radiotracer is an indicator only of the metal in solution and not within the soil. Tracer added at the beginning of the first BCR stage follows only the acidextractable metal. Any amount retained on the soil after the first leaching is indicative of readsorption to remaining solid phases. An implicit assumption is that rapid and complete isotopic mixing will occur within that amount of mobilized metal in solution. A further assumption, the validity of which will be shown, is that adsorbed Cd and Zn tracer will be almost entirely remobilized when the second extractant is added. The tracer will then mix with the metal now liberated from Fe and Mn oxides, permitting estimates of the proportion of that oxide bound metal, extracted by the hydroxylamine, which becomes readsorbed to subsequent organic and silicate phases. The radiotracer technique will still be subject to external effects such as the incomplete extraction efficiency of a reagent. Previous studies employing radiotracers in soil studies have focused more on adsorption characteristics of the solid phase, rather than the extraction of metal from the soil, including the processes of solubilization and readsorption. Belzile et al. (11) chose not to employ radiotracers, citing the uncertainty of reaching solid-liquid equilibrium and differential isotopic diffusion rates. However, Riise et al. (13) demonstrated that isotopic exchange is very rapid, and a steady-state partitioning of 109Cd tracer among the various fractions of soil was reached within 30 min of addition. Moreover, this partitioning did not change significantly over periods as long as 221 days. Again, the partitioning of contaminant Cd, as determined by sequential extraction, did not correspond to the tracer fractionation. The equilibration and desorption processes for 65Zn tracer in soil are similarly rapid (14). Fujiyoshi et al. (15) performed 65Zn sorption experiments on a sediment sample, at various points during a sequential extraction procedure. Adsorption followed Langmuir isotherms, except in the residual silicate phase, where minimal tracer was retained.
Method and Materials Soil Preparation. When in operation, the Bristol Metals Recycling facility recovered metal from foil paper used in packaging materials. Residual ash from the combustion of this foil paper was then mixed with the soil on the property. Sample volumes of 15 L were removed from several locations at a depth of approximately 15 cm below surface. Two soils with contrasting metal contents, Bristol 1 and 3, are presented here. For comparison, a fine earth (
