Environ. Sci. Technol. 1994, 28,2074-2079
Field Screening of Chromium, Cadmium, Zinc, Copper, and Lead in Sediments by Stripping Analysis Khris 5. Olsen"
Pacific Northwest Laboratory, Richland, Washington 99352 Joseph Wang,' Rossl Setladji, and Jianmin Lu
Department of Chemistry, New Mexico State University, Las Cruces, New Mexico 88003 Stripping analysis (SA) was successfully employed for field verification of metals contaminants in soils and sediments at hazardous waste sites. Microwave digestion procedures were tailored to meet the needs of field activities and electrochemical measurements. An adsorptive stripping voltammetric (AdSV) scheme was used for monitoring total chromium and for chromium speciation, while conventional anodic stripping voltammetry (ASV) and potentiometric stripping analysis (PSA) were used for measuring cadmium, zinc, copper, and lead. The results demonstrate that SA is capable of on-site identification of contaminated layers in soils and sediments. Concentration values measured by SA correlated well with those obtained by U S . Environmental Protection Agency (EPA)-approved atomic or mass spectroscopy methods. The remarkable sensitivity, portability, low-power need, and low cost of SA make it an attractive choice for on-site analysis of selected metals during site characterization and remediation activities.
Introduction The U.S. Department of Energy (DOE) is the governing agency for the nuclear weapons complex, consisting of the national laboratories and facilities related to the research, development, production, and testing of nuclear weapons. Fifty years of activities has resulted in the production of tens of thousands of warheads and millions of cubic meters of hazardous and/or radioactive waste. The urgency of production, lack of environmental regulations, and lack of regulatory oversight led to numerous environmental releases of those wastes. Those wastes have resulted in widespread contamination of soil and groundwater a t numerous sites throughout the United States. Cognizant of the public concern and fully aware of the enormous expenditures required to support characterization and remediation activities, DOE's Office of Technology Development established a program to support innovative technology. DOE's goal for this program is to develop characterization or remediation technologies capable of providing results faster, cheaper, safer, or better than conventional methods. One such technology being evaluated by Pacific Northwest Laboratory for DOE's Mixed Waste Landfill Integrated Demonstration Project at Sandia National Laboratories (Sandia) in Albuquerque, NM, is SA. At these sites, SA is being used to measure the concentrations of chromium, cadmium, zinc, copper, and lead in soil and sediment samples collected during characterization activities. Stripping analysis is a powerful electroanalytical technique for trace metal measurements ( I , 2). The remarkable
* Corresponding authors. 2074
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sensitivity of SA is attributed to the deposition step in which the target m e t a b ) is(are) preconcentrated onto the working electrode. The accumulated metal is measured in the second (stripping) step by applying an anodic potential scan and monitoring the resulting peak current+). Alternately, in a version known as PSA (3), a constant current or chemical oxidant is used to strip the accumulated metal(@, and the stripping time serves as the analytical signal. Approximately 30 metals can be measured by using electrolytic deposition (amalgam formation) or adsorptive accumulation of the suitable chelate onto the electrode surface. The inherent sensitivity, high precision, speciation capability, portable/compact instrumentation, low-power needs, and low cost of the SA system make it very attractive for field measurement of trace metals and for environmental analysis in general. For example, an early work by the U.S. Navy described an automated SA system for trace metals surveys in San Diego Bay ( 4 ) . An automated flow system for on-line monitoring of trace uranium was developed recently at New Mexico State University (5). These and other field and on-line monitoring opportunities have been summarized recently (6). Other environmental applications of SA have been reviewed in this journal (2). Stripping analysis was applied previously for assays of soils in centralized laboratories (7, B), and its utility for on-site field screening of metals in sediments has not been previously reported. Field demonstration of SA was conducted at Sandia's Chemical Waste Landfill during the summer of 1992 and the spring of 1993. Characterization activities during the summer of 1992 included drilling three vertical boreholes in and around an unlined pit used for the disposal of chromic acid (Unlined Chromic Acid Pit [UCAP]). Chromium was the specific element of concern during the 1992 field exercise. Characterization activities during the spring of 1993 included slant directional drilling of two boreholes (north and south boreholes) under several pits used for waste disposal of various chemical wastes during the 1960s (1960s pits). Chromium, cadmium, zinc, copper, and lead were the constituents of concern during the spring 1993 field exercise. During both of these characterization exercises, a mobile SA laboratory was present to provide near real-time information of the extent of metals contamination in the subsurface environment. Our findings are reported in the following sections.
Experimental Section Analyses for elements of interest most often require treatment of samples before they are introduced to the analytical instrument. Current analytical technologies operate and give superior results when samples are in an aqueous matrix. However, a significant number of en0013-936X/94/0928-2074$04.50/0
@ 1994 American Chemical Society
vironmental samples are solids (Le., soils and sediments). The traditional EPA-approved digestion method (Le., Method 3050) (9) requires refluxing a waste with either HN03 or HC1 and subsequent analysis via inductively coupled argon plasma (ICAP), ICAP-mass spectroscopy (ICAP-MS), or graphite furnace atomic absorption (GFAA). However, the EPA digestion method (9) is cumbersome, requiring hot plates, beakers, etc., and labor intensive; thus, it is not amenable for field-screening activities. An innovative method was first introduced in the 1970s that was based on the microwave heating of samples. The clear advantages of this method include rapid drying of sediment samples, shorter digestion times, potential for digesting multiple samples simultaneously, higher digestion temperatures for more efficient extractions, and increased safety because laboratory personnel are not exposed to hot acid solutions. Therefore, the use of a microwave digestion system in the field appeared quite suitable for drying and digesting sediment samples for subsequent SA. Sample Preparation. Sediment samples were collected during characterization activities conducted during the summer of 1992 and the spring of 1993. Sediment samples in 250-mL, wide-mouth, glass jars were delivered to the field laboratory during drilling operations. Those samples were subsequently dried, digested, and analyzed by SA either in a mobile field laboratory or in the centralized laboratory (i.e., New Mexico State University and/or Pacific Northwest Laboratory) according to the methods described below. A CEM Corp. (Westlake, OH) MDS 81D microwave digestion apparatus, rated at 650 W, was used for drying and digesting the sediment samples. For drying, approximately 8-12 100-g samples were weighed into 100mL glass beakers and dried simultaneously. The MDS 81D was programmed to heat at 50% power for 5 min. The samples were assumed to be dry after all visible water had dissipated from the inside wall of the beaker, usually requiring 4-5 5-min drying cycles at 50% power. The dried samples were coned and quartered. An aliquot from one of the quarters was used for digestion and subsequent SA. Microwave digestion was conducted in sets of 12 samples (9 samples, 1duplicate, 1blank, and 1standard), samples were digested using 10 mL of Ultrex HN03, heating for 2.5 min a t 100% power and 10 min a t 80% power. Samples were diluted to 100 mL, then transferred into 125-mL acid-cleaned bottles. During initial methods development for chromium, a distilled water extraction and an HNOdhydrogen peroxide (Hz02) digestion were also investigated. For the distilled water leach, 10 mL of distilled water replaced "03 as the leachate; for the HN03/H202 leach, the sediment was digested using 10 mL of Ultrex HN03, followed by the addition of 5 mL of 30 % HzO2. Leach solutions from the deionized (DI) water extraction were filtered, acidified with 1 mL of Ultrex "03, and diluted to 100 mL. Measurement Procedure. Cadmium, zinc, copper, and lead in solution were determined by PSA and/or ASV. Solution concentrations of chromium, zinc, and copper were verified by EPA Method 6010 (9);cadmium by EPA Method 7131 (9);and lead by EPA Method 6020M, with thallium as an internal standard. Anodic stripping voltammetry was carried out with an EG&G/Princeton Applied Research (Princeton, NJ) PAR Model 264A voltammetric analyzer, a PAR 303A static
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Flgure 1. Stripping voltammograms for Cr in contaminated soil: a = blank solution containing the DTPA complexing agent: b = sample spike at 200 000-fold dilution; c-e = standard additions of 0.25 ppb Cr(VI), 15-s preconcentration.
mercury drop electrode (SMDE), and a PAR Model 0073 X-Y recorder. PSA was performed with a TraceLab system (PSU 20, Radiometer America, Inc., (Westlake, OH),with a SAM 20 sample station (containing a glassy carbon disk) and an IBM PS/2 55SX as system controller and for data acquisition. Adsorptive stripping voltammetry measurements of chromium were based on the procedure of Golimowski et al. (IO),where 9.9 mL of the supporting electrolyte solution, containing 0.05 M diethylenetriaminepentaacetic acid (DTPA), was injected by a pipet into the cell and purged with nitrogen for 4 min. An accumulation potential of -0.8 V was applied to a fresh mercury drop, while the solution was stirred (usually for 10-30 s). The stirring was then stopped, and the voltammogram was recorded by applying a negative-going differential pulse potential scan terminating a t -1.65 V. A known volume of the unfiltered leachate (usually 10-100 pL) was then added, and the accumulation/stripping cycle was repeated with a new mercury drop. Subsequent standard additions (0.25-1.0 pg/L chromium) were used for quantifying the original chromium level in each sample. Typical voltammograms resulting from this procedure are shown in Figure 1. The remarkable sensitivity of this procedure is indicated from the well-defined peak for the 0.25-pgfL additions of chromium following a 15-saccumulation, and the substantial (200 000-fold) sample dilution. The total chromium was determined in the presence of 1 X 10-3 M potassium permanganate (KMn04). Chromium speciation was achieved without KMn04 by measuring the first peak (corresponding to the total chromium) and waiting 10min for the peak to decrease to a value corresponding only to Environ. Sci. Technol., Vol. 28, No. 12, 1994 2075
the Cr(V1) specie in solution. The Cr(II1) concentration was then calculated by the difference. Anodic stripping voltammetric measurements of cadmium, copper, zinc, copper, and lead were conducted by diluting the leachate 10-fold in the acetate buffer electrolyte, purging with nitrogen for 4 min, depositing the metals at -1.10 V for 2 min, stopping the stirring, and scanning to +0.1 V in a differential pulse mode. Zinc was determined in a similar manner, using a 50-fold dilution and a 30-s deposition at -1.4 V. Potentiometric stripping analysis for cadmium, zinc, and lead was performed with a preplated mercury film electrode by diluting the filtered leachate 10-foldin a nonoxygenated HC1 solution (0.1 M), depositing the metals at -1.10 V for 2 min from the stirred solution, and stripping them chemically in a quiescent solution (using oxygen as the oxidant). The potential was then switched to -0.1 V for a 1-min “cleaning” period. All reagents were prepared with double-distilled water. Stock solutions of the various metals (1000 mg/L) were purchased from Aldrich Chemical Co. (Milwaukee WI) and diluted daily, as required. Chemicals used were of analytical grade. The supporting electrolytes were a 0.2 M sodium acetate (CH3COONa)-2.5 M sodium nitrate (NaN03) solution, adjusted to pH 6.0 with sodium hydroxide (NaOH) (for chromium) and a pH 4.0 acetate buffer (for cadmium, zinc, copper, and lead). PSA of cadmium and lead employed a 0.1 M HC1 solution. Results and Discussion The selection of screening methods for chromium, cadmium, zinc, copper, and lead was based on a review of disposal records for Sandia’s Chemical Waste Landfill during the preliminary assessment phase of a Comprehensive Environmental Response, Compensation and Liability Act (CERCLA) of 1980 investigation. Of the five metals selected for method development, chromium exhibits the most diverse geochemical behavior in the environment because it can exist in environmental systems as either Cr(II1) or Cr(V1). The remaining four metals exist almost exclusively in the +2 oxidation state. Furthermore, the concentration of cadmium, zinc, copper, and lead potentially can be measured simultaneously in solution by conventional ASV or PSA. The remarkable sensitivity of SA procedures permits significant (10-1000) dilution of contaminated soil samples; hence, a substantial minimization of matrix effects. Chromium. When chromium contamination is present in sediments, it is important to determine its oxidation state. Cr(V1) is highly mobile in the environment and can be easily mobilized by water. Conversely, Cr(II1) is highly insoluble in water at the pH range of normal groundwater (pH 6-8); therefore, it is much less mobile in the subsurface environment and less of an environmental consideration. Because of the concern for the redox state in sediment samples, several leachate solutions were evaluated during the initial method development. A Sandia soil contaminated with chromium was leached using HN03/H202, HN03, and DI water. The leachate solutions were then analyzed by AdSV and ICAP (Table 1). Measured by ICAP, the HN03/H202 and HN03 extractions give comparable total chromium concentration values. This result was expected because the digestion procedure is identical, and the HzOz is a postdigestion 2076
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Table 1. Chromium Concentrations in Leachate Solution from Chromium-Contaminated Soil as a Function of Digestion and Analytical Method AdSV Cr(V1) (ppm)
AdSV Cr(II1) (ppm)
AdSV total Cr (ppm)
ICAP total Cr (PPm)
HN03/Hz02 HN03iHzOz HNO3
