Environ. Sci. Technol. 1996, 30, 2270-2277
Determination of Cu and Cd Content of Groundwater Colloids by Solid Sampling Graphite Furnace Atomic Absorption Spectrometry YURI E. FREEDMAN,† D A N I E L R O N E N , * ,† A N D G A R Y L . L O N G †,‡ Department of Environmental Science and Energy Research, The Weizmann Institute of Science, Rehovot 76100, Israel, and Department of Chemistry, Virginia Polytechnic Institute and State University, Blacksburg, Virginia 24061-0212
Cu and Cd content of standard reference materials (SRMs), minerals (quartz, calcite, kaolinite, and bentonite), and samples of aquifer colloids, obtained under natural gradient flow conditions, were analyzed by solid sampling graphite furnace atomic absorption spectrometry (GFAAS). Pyrolytically coated tubes, designed for the analysis of liquid samples, were successfully used. Recovery of analytes by solid sampling was 106-112% of the certified values of SRMs. Cu concentration in clay minerals, obtained by solid sampling, was about 9-fold (in kaolinite) and 3-fold (in bentonite) higher than that obtained by nitric acid extraction, indicating that Cu is partially incorporated into the crystal lattice of the minerals. Analysis of aquifer colloids shows that within a 6-month period the average content of Cu and Cd decreased by about 87%. Solid sampling GFAAS was demonstrated to be a convenient method for the study of metal content in small (5 mg) samples of groundwater colloids.
Introduction The study of groundwater colloids has recently become a subject of considerable interest (1). Because of their small size (micron or submicron), aquifer colloids have a ratio of surface area to mass significantly larger than the coarsegrained aquifer matrix. As a result, substantial amounts of compounds, including contaminants, may be sorbed at their surface (2), and the migration of sorbed contaminants may be governed by the movement of colloids in the aquifer. The concentration of contaminants, in particular, trace metal species, associated with the colloids varies according to different system variables such as the size of colloids, the amount and composition of organic matter bound by colloids (or itself present as colloids), the abundance and * Corresponding author e-mail address: cidaniel@weizmann. weizmann.ac.il; fax: (972) 8 344124. † The Weizmann Institute of Science. ‡ Virginia Polytechnic Institute and State University.
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speciation of the compounds of interest in groundwater, and the pH and the redox potential within the aquifer. This abundance of parameters, coupled with a wide range of changes of each variable, makes numerical estimation of the distribution of contaminants between groundwater and colloids extremely complicated. The modeling of the distribution of contaminants between dissolved and particulate phases, while being successfully performed for a single element solution, encounters numerous problems when applied to environmental situations (3). Hence, the measurement of the content of contaminants associated with groundwater and aquifer colloids is often the major source of reliable information about the distribution of contaminants. Passive groundwater sampling is an adequate method for the characterization of colloids transported under natural gradient flow conditions (4). Since the concentration of colloids in natural waters is of the order of tens of milligrams per liter (5-7) and natural groundwater flow velocity can be as low as several meters per year (8), collection of adequate amounts of colloids for laboratory analysis by existing passive sampling methods can take weeks. Moreover, microscale vertical heterogeneity in aquifers (9) puts a strong limitation on the range of the depth interval of sampling (few tens of centimeters) if a set of samples that represent discrete natural conditions is to be obtained. Several methods are available for qualitative study of the chemical composition of discrete colloidal particles. Metal composition of groundwater colloids was directly studied (10) by synchrotron X-ray fluorescence spectroscopy (SXRF) and energy dispersive X-ray analysis (EDX). SXRF was shown to detect trace amounts of metals bound by colloids, using only about 10 mg of a sample for analysis. Other methods that can be applied to the study of chemical composition of groundwater colloids are X-ray photoelectron spectroscopy (XPS) and Auger electron spectroscopy (AES). However, all these methods do not provide quantitative information on the metal content of the studied samples. Bulk chemical composition of groundwater colloids can be determined through the analysis of samples by graphite furnace atomic absorption spectroscopy (GFAAS) and inductively coupled plasma (ICP) coupled either with atomic emission spectroscopy (ICP-AES) or mass spectroscopic detection (ICP-MS) (11). The traditional and most commonly used application of these methods requires the transfer of an analyte into a liquid phase, which makes digestion of a solid sample unavoidable. Hot nitric acid extraction (12), total digestion (13), and sequential extraction (14) are traditional procedures for the determination of metal content of a solid sample. Each procedure requires at least a sample amount in the order of 0.2 g for reliable determination. Moreover, sample digestion poses several distinct problems for the determination of species of trace metals, for example, risk of sample contamination from reagents or laboratory environment and loss of sample in transfer steps associated with digestion and dilution of sample. Such systematic errors can seriously degrade the accuracy of the analytical determination (15). The digestion procedure is also time-consuming.
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1996 American Chemical Society
In the mid-1980s, introduction of the direct analysis of solid materials by means of graphite furnace atomic absorption spectrometry (GFAAS) allowed researchers to determine quantitatively the metal content of the samples, avoiding the procedure of sample digestion. Solid sampling was first applied to the study of geological materials by atomizing a sample in a small graphite crucible (16). Langmuhr et al. (17) showed a solid sampling technique to be applicable for the study of geological materials. Since then, the method has been successfully applied to biological materials (18, 19), alloys (20), marine samples (21), soils (22-24), and various geological materials (25-27). The obvious advantages of this methodsa small amount needed for determination, minimal risk of contamination, and simplicity of sample preparationsmake it a method of choice for the study of groundwater colloids. However, to the best of the authors’ knowledge, the method has never been utilized for such studies. This paper reports on the application of solid sampling GFAAS technique for the determination of Cu and Cd in groundwater colloids. Analysis was performed in standard graphite tubes manufactured for the analysis of liquid samples. Reliability of the method for the determination of Cu and Cd is demonstrated by comparison to certified values of standard reference materials (SRMs) and analysis of minerals after hot nitric acid extraction. The reported method allows the determination of the concentration of Cu and Cd in samples having a weight as small as 5 mg.
Methodology Field Sampling Site. Colloids were sampled in the Coastal Plain aquifer of Israel, 15 km north of Tel Aviv and 2 km inland of the Mediterranean Sea. The dominant minerals in the aquifer are quartz, calcite, aragonite, feldspar, iron oxides, and clay minerals (mostly montmorillonite). Depth to the water table is about 30 m. The groundwater is near saturation with respect to calcite, the concentration of HCO3- ranging from 250 to 270 mg/L. Groundwater pH is from 6.7 to 7.2. The chemistry of groundwater is characterized by abrupt temporal and spatial changes in the concentration of major ions. The concentration of NO3- ions varies from 60 to 180 mg/L, SO42- varies from 90 to 130 mg/L, and Cl- varies from 190 to 270 mg/L. Such changes in the concentration of major ions reflect microscale chemical heterogeneity of the aquifer, i.e., the presence of water parcels of the size ∼1 m, which have substantially different chemical composition (9). Heterogeneity of the aquifer results from agricultural activity and the utilization of sewage effluents for irrigation. Aquifer colloids consist mainly of calcite, clay minerals, and quartz, and their grain size varies from 150 to 3000 nm, with most of the colloids being in the size range of 200-1000 nm (6). The research well, from which colloid samples were obtained, was drilled using a bucket-auger, without any addition of water. Polyvinyl chloride screens were installed to a depth of 16 m below the water table (6). Field Sampling Method. A multi-layer sampler (MLS) was used for the sampling of colloids of the aquifer under natural gradient flow conditions. The MLS system (Figure 1) utilizes dialysis cells and seals for the collection of samples of in-situ filtered groundwater and colloids migrating in the aquifer, respectively (6). By utilizing passive filtration under the natural gradient flow field, the dialysis cell membranes ( 7.5 m), Cd content also increases with depth from 5630 to 8330 ng/g in profile 1, from 926 to 1180 ng/g in profile 2, and from 1320 to 1630 ng/g in profile 3 (Figure 5). Cu content near the water table decreases with depth from 1060 to 828 µg/g in profile 1 and increases with depth from 435 to 793 µg/g in profile 2 (Figure 4). In the lower part of the profile, Cu content increases with depth in all three profiles. The Cu and Cd content of colloids decreases significantly with time from profile 1 to profile 3 (Figures 4 and 5). Average Cu concentration decreases from 919 (profile 1; Table 4) to 123 µg/g (profile 3). Similarly, average Cd concentration decreases from 7890 (profile 1) to 1070 ng/g (profile 3). These results indicate that the concentration of the compounds of studied metals in aquifer colloids can change by 1 order of magnitude within 6 months. Since Cu and Cd in groundwater were consistently below 0.1 µg/L in all three profiles, it is reasonable to assume that changes in the concentration of species of these metals in colloids reflect the natural influx of colloids having a variable content of the compounds Cu and Cd. Considering the 6-month period of the field study and horizontal water flow velocities in the aquifer under natural gradient flow conditions (from 13 to 66 m/yr; 31), it may be estimated that observed variability occurs within water parcels having an horizontal
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dimension scale in the order of 7 m only in the upper part of the profile and 33 m in the lower one. The metal compounds variability within each profile as well as between profiles detected in this highly dynamic and heterogeneous aquifer system clearly demonstrates that, in some cases, it may be necessary to obtain frequent samples of colloidal material over discrete vertical intervals. In such cases, the application of the solid sampling technique, combined with the MLS methodology allows a substantial reduction of the duration of the sampling period. For example, if we assume the concentration of colloids in groundwater to be 20 mg/L, which is the average concentration of colloids in groundwater at the study site (6), groundwater flow velocity to be 10 m/yr (31), the sampling vertical interval to be 30 cm, and the well diameter to be 5 cm, then the time for collection of 200 mg of colloids under natural gradient flow conditions should be 31 days, assuming a 100% collection efficiency. Under the same conditions, the time needed for collecting colloids in a sufficient amount for GFAAS analysis of Cu and Cd by solid sampling (10 mg) is only 2 days.
Acknowledgments This work was supported by Grant 90-00387 from the United States-Israel Binational Foundation (BSF), Jerusalem, Israel. This study is intended to be incorporated as a part of the forthcoming Ph.D. thesis of Y. E. F. in the Feinberg Graduate School at the Weizmann Institute of Science.
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Received for review August 29, 1995. Revised manuscript received January 25, 1996. Accepted March 14, 1996.X ES950637W X
Abstract published in Advance ACS Abstracts, May 1, 1996.
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