Presentation on theme: "Megan Carmony and David Harvey Department of Chemistry and Biochemistry DePauw University Greencastle, Indiana Abstract Sediments and soils are complex."— Presentation transcript:
Megan Carmony and David Harvey Department of Chemistry and Biochemistry DePauw University Greencastle, Indiana Abstract Sediments and soils are complex mixtures of inorganic and organic materials. Trace metals may bind with these materials in various ways or phases, including surface adsorption, complexation, and co-precipitation. These different types of binding affect the lability of the metal ions. Sequential extractions are designed to release only one phase at a time. They are typically a series of increasingly reactive reagents. The metals that are released by a specific reagent are grouped into operationally defined phases. The value of operationally defined phases has been questioned as well as issues with reproducibility of results. In response to this issue, the European Community Bureau of Reference created the BCR extraction procedure so that different labs could achieve similar results using the same sediment. The BCR procedure operates under a standard ratio of solid to reagent and consists of three different steps. These steps are designed to attack different reactivities of trace metals and to thereby estimate the concentrations of metal release by various changes in the environment. The following table outlines the BCR procedure and the phases each step is designed to attack. Megan Carmony would like to thank DePauw University, the Science Research Fellows program, and Dr. David Harvey for making this project possible. Funding for the Varian 220FS atomic absorption spectrometer is from NSF Grant 0125835. Evaluating the Effect of Calcium Carbonate on the BCR Extraction Procedure Introduction Effect of Changing Concentrations Effect of Acetic Acid Concentration on pH Kinetic Analysis of Lake Sediment Effect of Carbonate on Contaminated Soil pH of Carbonates Future Work References Acknowledgements Procedure Previous work in our lab explored the kinetics of the first step of the BCR procedure using an Indiana lake sediment. As expected, the release of manganese steadily increased during the extraction, reaching a steady-state value after approximately 8-12 hours. Iron and zinc, however, showed an initial rapid release followed by a steady decrease as the metals reentered the sediment via precipitation or readsorption. Additional experiments showed that the pH of the extracting solution increased during the extraction. This rise in pH, which correlates with the reincorporation of iron and zinc into the sediment and suggests a possible relationship between pH and the release of metal, is attributed to the high percentage of carbonate minerals in this sediment. One possible solution to this problem is to use a higher concentration of acetic acid. This research investigates the use of higher concentrations of acetic acid and the possible relationship between concentrations of released of metal and changes in pH. StepExtraction ReagentReaction TimeProposed Phase(s) Attacked 10.11 M Acetic Acid16 hrCarbonates and Surface Bound Metals 20.1 M NH 2 OHHCl (pH 2)16 hrIron and Manganese Oxides 3(a) 30% H 2 O 2 ; (b) 30% H 2 O 2 ; (c) 1 M NH 4 OAc (a) 1 hr; (b) heat to reduce volume; (c)16 hr Organics Table 1: BCR Procedure Figure 1. Release of manganese as a function of time with varying concentrations of acetic acid (modeled after the Step 1 extraction) Figure 2. Release of zinc as a function of time with varying concentrations of acetic acid (modeled after the Step 1 extraction) Kinetic studies of the lake sediment, using 0.11 M, 0.22 M, 0.33 M, 0.44 M, and 0.55 M acetic acid, were conducted over the course of 16 hours with each data point representing a separate sample. Results for manganese and zinc are shown in Figures 1 and 2. The graphs show the change in the concentration of metal release with each concentration of acetic acid during the duration of the 16 hour extraction. According to the BCR procedure, the concentration of metal should increase over the course of the extraction time. The decrease in zinc concentration with time for the lower concentrations of acetic acid indicates that the procedure is not correctly evaluating the amount of metal present in the sample. The large increase in the amount of metal released when using higher concentrations of acetic acid also indicates that the BCR procedure may underestimate the amount of metal present for sediments with high percentages of carbonates. To evaluate the influence of pH on the release of metals, the changes in pH during the extraction of the lake sediment was studied using 0.11 M, 0.22 M, 0.33 M, 0.44 M, and 0.55 M acetic acid (Figure 3). Figure 3. pH of lake sediment in acetic acid as a function of time. To test the effect of the different concentrations of acetic acid on both the lake sediment and contaminated soil, 16 hr extractions were set up using 0.11 M, 0.22 M, 0.33 M, 0.44 M, and 0.55 M acetic acid. The sample were analyzed for various metals and the concentrations recorded. The total change in metal concentration for each metal and each sediment/soil was determined by simply subtracting the 0.11 M concentration from the 0.55 M concentration. This difference was then divided by the 0.11 M concentration and multiplied by 100 to determine the percent increase. These values are outlined in the table below. Kinetic studies using the contaminated soil were done in the same manner as that described earlier for the lake sediment. This soil, however, was also analyzed for cadmium and lead. Results for manganese and zinc are shown in Figures 5 and 6. Figure 6. Concentration of zinc as a function of time. Figure 5. Concentration of manganese as a function of time Table : Change and percent increase of metal concentrations for 16 hr. extraction as acetic acid concentration changes from 0.11 M to 0.55 M To test the effect of carbonates on the release of metals from the contaminated soil, samples were prepared with varying percentages of carbonate mixed with sand and contaminated soil. Each mixture was prepared in triplicate and extracted for 16 hours with 0.11 M acetic acid. The results for each mixture are shown in Figure 7, along with the final pH. Figure 7. Extraction of contaminated soil spiked with carbonate with metal concentration and pH graphed as a function of % CaCO3. To determine minimum concentration of acetic acid needed for the effective extraction of sediments with high percentages of calcium carbonate, kinetic trials were completed using 50% calcium carbonate and 50% sand with 0.11 M, 0.22 M, 0.33 M, 0.44 M, and 0.55 M acetic acid (Figure 8). Figure 8. pH of 0.5 g CaCO3 and 0.5 g sand in 40 mL acetic acid plotted as a function of time. Further research on this project will explore extraction kinetics for samples consisting of the contaminated soil in the presence of calcium carbonate using different concentrations of acetic acid. Research will also include the preparation of model sediments with and without known concentrations of metals that can be used in further evaluations of the BCR procedure. 1.D’Amore, J; Al-Abed, S; spiking, K; Ryan, J. Journal of Environmental Quality. 2005, 34, 1707-1745. 2.Thomas, R; Ure, A.M.; Davidson, C.M.; Littlejohn, D. Analytica Chimica Acta. 1994, 286, 423-429. 3.Turner, A; Harvey, D. Unpublished Research, Summer 2005, DePauw University 4.Manouchehri, N; Besancon, S; Bermond, A. Analytica Chimica Acta, 2006, 559, 105- 112. 5.Cappuyns, V; Swennen, R; Verhulst, J. Soil & Sediment Contamination. 2006, 15, 169- 186. 6.Slavek, J; Pickering, W.F. Water, Air and Soil Pollution. 1986, 28, 151-162. 7.Presutti, K; Harvey, D. Unpublished Research, Summer 2006, Depauw University. Kinetic studies and pH tests on an Indiana lake sediment and a contaminated soil suggest that the first step of the BCR sequential extraction procedure may not provide sufficient acetic acid to accurately assess the release of trace metals from calcareous sediments and soils. Higher concentrations of acetic acid seem to improve the release of metals, but may also attack other phases in the sediment, confounding the meaning of operational phases. Kinetic Analysis of Contaminated Soil CuMnFeCdPbZn Percent Increase in Lake Sediment317%4774% 1163% Percent Increase in Contaminated Soil56%46%662%21%103%21% Lake Sediment Contaminated Soil pH Measurements Sediment samples were obtained from core samples collected from Lake Wawasee and Lake Tippecanoe. The sediment was dried, ground to decrease particle size, and combined into a single homogeneous sample. Samples for these studies (and all others) maintained the BCR ratio of 25 mg solid per 1 mL of extraction solution. Samples were prepared in acid-washed plastic bottles, shaken on a shaker table for the specified time, and filtered by gravity filtration. The concentrations of metals in the solution were then measured by flame atomic absorption spectroscopy using an air/acetylene flame. The soil for all studies involving “contaminated soil” was SRM 2710 Montana soil. These samples were evaluated using the same method as used for the lake sediment. All pH measurements were made with a Vernier pH probe and LabPro interface using LoggerPro software. For kinetic studies of pH, the probe was calibrated and placed into a beaker containing 40 mL of acetic acid. After several minutes 1 g of sediment or soil was added to the beaker and the sample was allowed to sit. All samples were placed on a stir plate and continuously stirred. As shown by the graph, the pH continues to increase for 0.11 M and 0.22 M while it levels out for 0.33 M, 0.44 M, and 0.55 M. This indicates that 0.33 M acetic acid is necessary to neutralize all of the carbonates contained in this sediment. The final pH of the 0.11 M and 0.22 M acetic acid are over 6, suggesting the presence of unreacted carbonate. The lower pH values of the other trials suggest excess acetic acid. These results show that varying the concentration of acetic acid has a much larger affect on the lake sediment than on the contaminated sediment. Also, the largest percentages appear for iron, suggesting that the acetic acid may be dissolving some iron oxides. Increasing the concentration of acetic acid provides a much smaller increase in the concentrations of released metal than found with the lake sediment. This can probably be attributed to fewer carbonates in the contaminated soil, as evidenced by a final pH of 3.1 when using 0.11 M acetic acid. Therefore it can be assumed that the BCR procedure is effective for this soil. As shown by the data, the concentration of released metal decreased as the percentage of calcium carbonate increased, leveling out at about 30% calcium carbonate. The pH increased as the percentage of calcium carbonate increased, also leveling out at about 30% calcium carbonate. This indicates that step one of the BCR procedure is best suited to sediments that are less than 30% calcium carbonate, and that results for sediments that have a higher percentage of calcium carbonate are uncertain. The data shows that at least 0.33 M acetic acid is required for sediments that are 50%. Previous Work
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