Transcript Document

Abstract
Effect of Carbonate on Contaminated Soil
Evaluating the Effect of Calcium Carbonate on the BCR Extraction Procedure
Megan Carmony and David Harvey
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.
0.33M
0.44M
1000
Extraction Reagent
Reaction Time
16 hr
0.11 M Acetic Acid
2
0.1 M NH2OH•HCl (pH 2) 16 hr
Iron and Manganese Oxides
(a) 30% H2O2; (b) 30%
H2O2; (c) 1 M NH4OAc
Organics
(a) 1 hr; (b) heat to
reduce volume;
(c)16 hr
0.55M
0.55 M
Proposed Phase(s) Attacked
1
3
0.11M
Carbonates and Surface
Bound Metals
500
Cu
Percent Increase in Contaminated Soil
Mn
Fe
Cd
317%
4774%
46%
662%
Pb
Zn
1163%
56%
21%
103%
21%
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
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.
Kinetic Analysis of Contaminated Soil
0
0
2
4
6
8
10
12
14
16
1200
6
1000
5
800
4
Cu
600
3
400
2
200
1
0
Mn
pH
0
10
20
30
40
50
60
% CaCO3
2000
0.22M
7
0
Percent Increase in Lake Sediment
[Mn]
Table 1: BCR Procedure
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.
2500
1500
Step
Effect of Changing Concentrations
Kinetic Analysis of Lake Sediment
1400
pH
Department of Chemistry and Biochemistry  DePauw University  Greencastle, Indiana
Introduction
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.
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.
Concentration
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.
Figure 7. Extraction of contaminated soil spiked with carbonate with metal
concentration and pH graphed as a function of % CaCO3.
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.
pH of Carbonates
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).
8
18
Time (hrs)
Figure 1. Release of manganese as a function of time with varying concentrations
of acetic acid (modeled after the Step 1 extraction)
50
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.
7
6
5
45
0.22 M
pH
Previous Work
0.11 M
2000
40
0.33 M
4
0.44 M
1800
Lake Sediment
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.
3
0.11 M
0.33 M
25
1400
2
0.44 M
1200
0.55 M
20
0.55 M
15
0.11 M
1000
0.33 M
0.44 M
10
800
5
600
0
400
0
2
4
6
8
10
12
14
16
0.55 M
Figure 2. Release of zinc as a function of time with varying concentrations of
acetic acid (modeled after the Step 1 extraction)
Effect of Acetic Acid Concentration on pH
2
4
6
8
10
12
14
16
18
Figure 5. Concentration of manganese as a function of time
1800
1600
7.2
1200
0.11 M
1000
6
0.22 M
0.33 M
0.44 M
800
5.3
0.55 M
4.8
0.11 M
4.5
0.22 M
4
600
0.33 M
0.44 M
400
200
2
0
0
2
4
6
8
10
12
14
16
18
1
Time (hrs)
Figure 6. Concentration of zinc as a function of time.
0
150
200
250
300
350
250
300
350
400
References
6.8
100
200
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.
Time (hrs)
1400
50
150
Future Work
0
8
0
100
Figure 8. pH of 0.5 g CaCO3 and 0.5 g sand in 40 mL acetic acid plotted as a
function of time.
The data shows that at least 0.33 M acetic acid is required for sediments that are 50%.
3
pH Measurements
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.
50
0
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).
0.55 M
Contaminated Soil
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.
0
Time (min)
200
5
0
18
Time (hrs)
7
1
0.22 M
[Mn]
[Zn]
30
[Zn]
Procedure
0.22 M
1600
pH
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.
0.55 M
35
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, 105112.
5. Cappuyns, V; Swennen, R; Verhulst, J. Soil & Sediment Contamination. 2006, 15, 169186.
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.
400
Time (min)
Figure 3. pH of lake sediment in acetic acid as a function of time.
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.
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.
Acknowledgements
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.