Environ. Sci. Technol. 1998, 32, 3622-3627
Acid Leaching of Metals from Environmental Particles: Expressing Results as a Concentration within the Leachable Fraction Y V E S G EÄ L I N A S , * , † , ‡ , ⊥ R A M O N M . B A R N E S , ‡ DIETER FLORIAN,§ AND JEAN-PIERRE SCHMIT† Chemistry Department, Universite´ du Que´bec a` Montre´al, C.P. 8888, Succ. Centre-Ville, Montre´al, Que´bec, Canada H3C 3P8, Department of Chemistry, Lederle Graduate Research Center, University of Massachusetts, Box 34510, Amherst, Massachusetts 01003-4510, and Technical University Graz, Institute for Analytical Chemistry, Micro- and Radiochemistry, Technikerstrasse 4, A-8010 Graz, Austria
The merits of expressing the results obtained following an acid extraction (EPA leaching method 3051) of environmental particles as the metal concentration within the leachable fraction only were examined for samples collected in a small agricultural watershed (southern Que´ bec, Canada). The concentration of Fe, Al, Cr, Ni, Cu, Zn, Cd, and Pb in sediments, suspended solids, and soil was calculated using the leached mass instead of the total mass of the starting material to eliminate bias introduced by the variable amount of nonreactive residual material. A mass balance calculation was performed on a sediment sample, and four different applications are given to illustrate the advantages of this method compared to the conventional way of reporting leach data. Acid leaching methods will be used more extensively as leach information data for reference materials are published because they fulfill an important need, in particular for environmental monitoring programs. However, we believe that the approach reported here generates results that have more scientific meaning and usefulness. It is not affected by grain size or composition, it does not need data correction, it provides the true metal concentrations of the leachable fraction, and more importantly, it offers a direct link between environmental particles of different nature (sediment, soil, and suspended solids).
Introduction Acid leaching methodologies for the metal extraction from environmental particles are increasingly superseding the traditional total mineralization method and have been widely adopted in many countries to assess metal pollution in soils and sediments (1-6). They offer a better contrast between contaminated and background samples than does the determination of the total concentration (7, 8), and they are †
Universite´ du Que´bec a` Montre´al. University of Massachusetts. § Technical University Graz. ⊥ Present address: School of Oceanography, Box 357940, University of Washington, Seattle, WA 98195-7940. * Corresponding author, phone: (206)543-6790; fax: (206)5430275; e-mail:
[email protected]. ‡
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ENVIRONMENTAL SCIENCE & TECHNOLOGY / VOL. 32, NO. 22, 1998
faster, simpler, and safer to perform since no perchloric/ hydrofluoric acid or tedious fusion dissolution methods are used. They also permit better instrumental precision and accuracy by reducing the matrix content of the solution to analyze, and they provide a tentative assessment of the metal fraction that is not immobilized in inert minerals at the human time scale (2). There has been a growing effort in recent years to standardize the different leaching protocols in order to establish standard methods yielding comparable results from sample to sample and to introduce a series of information leach values for certified reference materials (1, 2, 9). The rationale underlying leaching methodologies to extract metals from environmental particles is based on “the worst case environmental scenario where the potentially labile components of the sample become mobile” (2). They are designed to solubilize the fraction of a sample composed of a mixture of authigenic, hydrogenic, and biogenic materials, without attacking the highly resistant detritial fraction composed mainly of primary Si and Al oxide minerals. This detritial fraction is believed to act mainly as a nonreactive substrate on which are adsorbed metal-accumulating components such as organic matter, sulfides, and iron and manganese oxyhydroxides (10-12). Usually accounting for more than half of the mass of typical soil, sediment, or surface water suspended particles, it thus acts as a nonreactive diluant component that remains unaffected by short-term natural or anthropogenic changes (13). Leaching methods suffer from the same drawback as total concentration determinations. They are affected by grain size and composition, which makes difficult the direct comparison of samples with different mineralogy and granulometry (1, 10). Because of the very strong correlation between decreasing grain size and increasing surface/volume ratio, the smaller grain-size fractions contain relatively higher amounts of surface coating, metal-accumulating components. A strong empirical correlation is thus usually found between decreasing particle size and increasing metal concentration, e.g., the grain-size effect (11, 12, 14, 15). Hence, results at close sites for leached metals and for total concentrations are highly influenced by granulometry. To overcome the variability introduced by grain-size effect, physical separation of the smaller grains or mathematical corrections based on grain-size distribution or surface area are usually performed (10, 16). However, physical separation procedures are tedious, may lead to sample contamination or losses of sorbed metals, and are at best operationally defined since comparisons between various separation techniques usually display marked differences and fail crossvalidation (10). Published data for riverine suspended solids also indicate that in some cases contribution from larger grain fractions (i.e., >63 µm) to the overall metal concentration in the samples should not be ignored (17-19), and a similar conclusion was reached for some organic pollutants in marine sediments (20). Finally, mathematical corrections based on grain-size distribution generate data that do not reflect true chemical concentrations and also require an assessment of the grain size distribution for each sample (10). While data reported in leaching studies should provide an accurate status solely of the metal fraction not immobilized in the inert detritial matrix, a highly significant bias is introduced with the use of the total (i.e., leachable + detritial fractions) amount of material as the reference mass (13). Although only a small fraction of the leachable metals is bioavailable in most natural environments, organisms in any 10.1021/es980092g CCC: $15.00
1998 American Chemical Society Published on Web 09/22/1998
environment can only be affected by the leachable fraction of the particles, which represents a highly variable percentage of the total mass. Therefore, it is the relative concentration and chemical form of the essential and toxic metals within this fraction that determine the potential toxicity of the particulate material. To be consistent with the rationale of the acid leaching approach, the leached mass, measured by differential weighing before and after the leaching step, should be used as the reference mass to report leached concentrations. Reporting the actual metal concentrations within the leached fraction alone seems a more logical way to eliminate the need for mathematical corrections or physical separations and to circumvent the bias introduced by the varying amount of nonmetal bearing, nonreactive detritial material. This study explores the merits of this approach for soil, sediment, and suspended particles collected between May 1993 and September 1995 in the Des Hurons River watershed (Que´bec, Canada). It compares the environmental relevance of data measured in this way to the commonly reported “total” or “percent leachable” values. Fe, Al, Cr, Ni, Cu, Zn, Cd, and Pb were analyzed.
Experimental Section Study Site and Sampling. The Des Hurons River watershed is located at 30 km east of Montre´al in the St. Lawrence Valley plain (Que´bec, Canada). About 50% of the catchment area (334 km2) is occupied by agricultural activities. The Des Hurons River is a typical agricultural stream with a mean annual discharge of 4.76 m3 s-1, high peak runoff rates, and massive losses of suspended solids owing to improved drainage of crop lands and high surface soil erosion throughout the watershed (21). The complete details for the sampling step and the material used can be found in ref 22. Leaching Procedure. The slightly modified Environmental Protection Agency leaching procedure EPA 3051 was used throughout (23). An aliquot of 200-250 mg (dry weight equivalent) was placed in the PTFE container of a highpressure Parr microwave oven digesting bomb (Moline, IL). Five milliliters of sub-boiled 16 N HNO3 and 2 mL of H2O2 30% were added sequentially. The container was capped but unsealed, gently agitated to obtain a homogeneous slurry, and left to react for 60 min in a clean room. Since the Parr digestion bombs do not allow temperature or pressure control, the heating program was designed by trial/error to achieve maximum pressure without breaking the pressurerelieve seal. Acknowledging the difficulty of evaluating the exact pressure and temperature inside the bomb owing to reaction products, it is roughly estimated by the manufacturer from vapor pressure data that the leaching temperature and pressure were about 190 °C and 800 psi, respectively. The bombs were heated one by one at maximum power in two steps of 45 s separated by a waiting time of 2 min in a 750 W domestic microwave oven. They were then left to stand for an additional 15 min and allowed to cool at 4 °C for 60 min. The solution was decanted into a 25-mL volumetric flask, and the residue was rinsed several times with aliquots of Milli-Q water that were added to the flask before completing the volume to the mark with water. The solid residue was then collected by filtration on a preweighted 0.45 µm polycarbonate filter, dried in a clean room, and weighed. The bomb containing the sample was weighed before and after the heating cycle to verify the integrity of the O-ring seal. Total mineralizations were done in PFA containers on a hot plate with sequential heating and evaporating to almost dryness with 16 N HNO3 and 30% H2O2, twice with concentrated HF, and then dissolution of the residue with hot HNO3 followed by dilution with Milli-Q water.
It must be noted that this leaching method does not entirely satisfy the requirements of the EPA 3051 leaching procedure (i.e., heating ramp between 20 and 175 °C lasting no more than 5.5 min, then controlled temperature at 175 to 180 °C for the remainder of 10 min (23), compared to a ramp of 3.5 min to an estimated 190 °C, followed by a waiting time of 15 min for the method used in this work) and that leach recoveries may vary with leaching conditions (1). The best compromise between reproducibility of the leaching conditions and compliance to the EPA procedure requirements were sought with the equipment available in the Montre´al laboratory. However, the conclusions reached in this study are not affected by these differences. To estimate the potential differences between the leaching protocol used in this study and the EPA 3051 Leach method, six replicate samples of the SRM2704 Buffalo River Sediment were leached in the PFA vessels of a CEM 2100 microwave oven (Matthews, NC) using the EPA 3051 temperature-controlled heating program and 10 mL of concentrated HNO3. No significant differences at the 95% confidence level were found between the two methods (results not shown). Instrumentation and Analysis. Metals were analyzed with an Elan 250 inductively coupled plasma mass spectrometer from Sciex/Perkin-Elmer (Norwalk, CT). The standard additions calibration method with internal standardization was used throughout. The complete methodological details are given elsewhere (24). Precision and accuracy were verified using the SRM 2704 Buffalo River Sediment standard reference material (National Institute of Standards and Technology, Gaithersburg, MD) and the complete dissolution method. Strict quality control checks were carried out every day to verify the reliability of the results. Procedural blanks revealed no significant contamination. Organic carbon in sediments was estimated from the weight loss after combustion at 450 °C for 4 h.
Results and Discussion Representativity of the sample is probably the greatest source of error in environmental metal analysis. The main concern for representativity when sampling environmental particles is grain-size distribution owing to the strong relation between metal concentration and grain size. Proper comparison of samples asks for homogeneity within and between samples with comparable grain-size distribution. As a result, particulate samples are usually sieved to keep the smaller grainsize fractions and to reduce the grain-size effect. A wide variety of grain-size fractions was reported to be useful when dealing with grain-size effect, including
