Analysis of Copper and Zinc Retention in Urban versus Potting Soil
Authors: Max Mantkowski, Brian Davis, Kaeli Parcel and Kori Goldberg
Jerry Casbohm, Chemistry 1220 Spring 2013, The Ohio State University
ABSTRACT
Both urban and organic soils contain an array of organic matter and metal cations that
come from various sources. The concentration of organic matter in each soil sample,
furthermore, is directly related to the retention of the zinc and copper cations that contaminated
each soil sample. Various soil samples were analyzed using Ion Chromatography, X-Ray
Fluorescence, Flame Atomic Absorbance Spectrophotometry and a Soil Organic Matter test to
determine the relative retention and compositions of Ocean Forest Organic Soil and Urban Soil
samples. For five of the six contaminations, copper was retained more than zinc. The organic soil
was determined to have a lower organic matter content and retention of cations than the urban
soil sample. There is also a positive correlation between organic matter content and clay content,
which is believed to explain the retention percentages of each cation.
RESEARCH HYPOTHESIS
Soil possesses a negative charge which will become more negative as organic matter is
added. A higher negative charge results in higher metal cation retention. If the urban soil
contains less organic matter than potting soil, as expected, it will retain fewer cations than the
Ocean Forest Potting Soil. Several tests, using a variety of instruments and methods, will be
performed to determine composition, retention of zinc and copper cations and percent organic
matter of the soil.
PROCEDURE AND METHODS
Flame Atomic Absorption Spectrophotometer
The first step taken to test this hypothesis was to find whether urban or Ocean Forest
potting soil would better retain the zinc or copper cations. This was accomplished by finding the
amount of cations retained in a sample of soil contaminated with 1, 12.5 and 25 ppm copper and
zinc ions using a Flame Atomic Absorption Spectrophotometer (FAAS) to measure the
absorbance of the supernatant liquid from each sample. Absorption vs. concentration graphs
were then created using the FAAS results for the standard solutions of copper and zinc. The
absorbance values for the supernatant soil solutions were then applied to these graphs and the
lines on best fit in order to find the concentrations of copper and zinc cations in the supernatant
solution. Knowing the concentrations of the cations in solution allows for the calculation of the
cations retained by the soil.
X-Ray Fluorescence
Using X-ray fluorescence it was possible to conduct elemental analysis. Samples in clear
plastic bags were placed in the instrument which were subjected to radiation. X-ray fluorescence
spectrometers bombard samples with X-rays and the emission lines recorded correspond to
specific elements. Because every element has a unique energetic emission, it is possible to
discover the elements present in a sample and the quantities they are present in.
Soil Organic Matter
Next, a Soil Organic Matter (SOM) test was used to find the percent organic matter in
each sample. Both the urban and potting soil were combined with NaOH and EDTA disodium
salt and were shook then filtered. By comparing the color of the samples to the color of standard
solutions, it was possible to calculate the percent organic matter in the samples.
Ion Chromatography
Finally, Ion Chromatography was used to measure the ions that were best retained by the
soil. The soil samples were put through a chromatograph where they were washed with water
causing the ions most loosely retained by the soil to reach the detector first. Standard samples of
sodium, ammonium, potassium, calcium and magnesium were measured and graphed based on
the area under their curve in comparison to their concentration in parts per million. Once the
area under the curve was calculated for each cation in both soil samples, these areas were applied
to standard graphs of area vs concentration based on data obtained by The Ohio State University
Department of Chemistry. Using these calibration curves, the concentrations (in ppm) of the
cations in the tested soils were calculated.
RESULTS
Flame Atomic Absorbance Spectrophotometer
At high contaminations copper was absorbed more strongly for both the Ocean Forest
and urban soil samples as compared to zinc. At lower contaminations, however, the Ocean
Forest soil still absorbed copper more strongly while the urban soil sample absorbed zinc more
strongly. At high contaminations, the Ocean Forest absorbed significantly more zinc than the
urban soil sample did. These results can be found in Table 1 below:
Table 1: Flame Atomic Absorbance Spectrophotometry data for Urban and Ocean Forest Potting Soil Samples..
Metal: Zinc
Metal: Copper
In
Metal
Initial
Solution
On Soil
Initial
In Soln.
On Soil
Strongly
Total Metal
Total Metal
Test
(ppm)
(ppm)
(ppm)
(ppm)
(ppm)
(ppm)
Absorbed
(ppm)
(%)
Urban
1
0.08183
0.9182
1
0.1007
0.8993
Zn
1.818
90.9
Urban
12.5
2.005
10.5
12.5
0.4221
12.08
Cu
22.58
90.32
Urban
25
16.47
8.53
25
1.775
23.23
Cu
31.76
63.52
Ocean
1
0.2314
0.7686
1
0.06325
0.9377
Cu
1.706
85.3
Ocean
12.5
0.8911
11.61
12.5
0.08633
12.41
Cu
24.02
96.08
Ocean
25
3.097
21.9
15
0.1391
24.86
Cu
46.76
93.52
The samples showed much more similar retention rates for copper at all levels of contamination.
X-Ray Fluorescence
For the ocean soil, the concentration of zinc was 122 ppm while the concentration of
copper was 59 ppm. The concentrations of zinc for the urban soil was 209 ppm while the
concentration of copper was only 39 ppm. In the urban sample, the cation in the highest
concentration was iron whereas the ocean soil sample has the highest concentration of calcium.
Both samples had the smallest concentration of the same cation, molybdenum . Titanium,
chlorine, arsenic, cadmium, and mercury were present in the urban sample but they were not
present in the Ocean Forest sample. The results from the X-Ray Fluorescence can be found in
Table 2 on the following page.
Table 2: X-Ray Fluorescence Data for Urban Soil vs. Ocean Forest Soil.
Urban Soil Composition
Ocean Soil Composition
Concentration Concentration
Element Concentration (Group 75)
Concentration Concentration
(Group 87) Element Concentration
(Group 65)
(Group 115)
Fe
5.29%
2.57%
5.53%
Ca
4.30%
3.83%
4.59%
K
1.10%
1.02%
0%
Fe
8952 ppm
.7637%
1.18%
Ca
7792 ppm
2.1%
1.89%
K
4502 ppm
3363 ppm
3666 ppm
Ti
3502 ppm
2914 ppm
3095 ppm
S
3307 ppm
0 ppm
0 ppm
Cl
871 ppm
0 ppm
0 ppm
I
647 ppm
912 ppm
856 ppm
Mn
713 ppm
796 ppm
955 ppm
Mn
407 ppm
282 ppm
220 ppm
Ba
351 ppm
345 ppm
395 ppm
Zn
122 ppm
133 ppm
125 ppm
Zr
210 ppm
186 ppm
200 ppm
Ba
105 ppm
0 ppm
80 ppm
Zn
209 ppm
553 ppm
228 ppm
Cu
59 ppm
71 ppm
61 ppm
Rb
102 ppm
69 ppm
63 ppm
Sr
54 ppm
74 ppm
154 ppm
Sr
101 ppm
149 ppm
130 ppm
Rb
34 ppm
26.7 ppm
35.2 ppm
Cr
45 ppm
49 ppm
40 ppm
Cr
34 ppm
14 ppm
15 ppm
Cu
39 ppm
0 ppm
0 ppm
Pb
20 ppm
23 ppm
32 ppm
Pb
38 ppm
144 ppm
63 ppm
Zr
7.9 ppm
18.9 ppm
6 ppm
As
34 ppm
0 ppm
20 ppm
Mo
6.8 ppm
7.2 ppm
0 ppm
Cd
24 ppm
0 ppm
0 ppm
Hg
15 ppm
0 ppm
0 ppm
Mo
10.2 ppm
0 ppm
0 ppm
The Ocean Forest sample contained iodine and sulfur but the urban sample did not. Similar to
our results, most groups found iron to be the element of the highest concentration in the urban
soil. Also similar to our results, the majority of groups found calcium to be of the highest
concentration out of all of the organic soils.
Soil Organic Matter
The Ocean Forest organic soil contained between 3 and 4% organic matter while the
urban soil contained greater than 4% organic matter. In general the class data agreed with the
findings concerning the potting soil; most groups found the potting soils to contain at least 3%
organic matter. Across the class results the urban soils, however, had much more variation with a
range from 1-4%. More of these values fell in the lower end of this range, making our urban
SOM an outlier.
Ion Chromatography
The Ocean Forest soil had higher concentrations of each individual cation than the urban
soil. The ammonium cation was in the highest concentration for the Ocean Forest soil; however,
it was in the lowest concentration for the urban soil. Instead, calcium was in the highest
concentration for the urban soil, while magnesium was in the lowest concentration for the Ocean
Forest Soil. The results obtained by the ion chromatography test can be seen in Table 3.
Table 3: Ion Chromatography data for Urban Soil and Ocean Forest Soil samples.
Urban Soil Composition
Ocean Soil Composition
Element
Concentration (ppm)/Percent
Element
Concentration (ppm)/Percent
Fe
5.29%
Ca
4.30%
K
1.10%
Fe
8952
Ca
7792
K
4502
Ti
3502
S
3307
Cl
871
I
647
Mn
713
Mn
407
Ba
351
Zn
122
Zr
210
Ba
105
Zn
209
Cu
59
Rb
102
Sr
54
Sr
101
Rb
34
Cr
45
Cr
34
Cu
39
Pb
20
Pb
38
Zr
7.9
As
34
Mo
6.8
Cd
24
Hg
15
Mo
10.2
Discussion
Copper was retained by the soil in greater quantities than zinc in five of six tests. This
finding is generally consistent with the Simultaneous Competitive Adsorption of Heavy Metals by
the Mineral Matrix of Tropical Soils (Fontes, 2003 p. 799). In addition, the urban soil retained a
higher level of both cations when compared to the Ocean Forest Potting Soil. While it was
hypothesized that the Ocean Forest Potting Soil would contain a higher percent of organic
matter, therefore increasing the metal cation retention, the urban soil actually contained a higher
SOM percent than the potting soil. In addition, it was hypothesized that the soil containing a
higher SOM content would retain the most metal cations. This proved to be true since the
potting soil, which contained the higher SOM percent, retained the greatest percent of cations.
The increased retention of metal cations by soil containing organic matter is due to the fact that
soil with high amounts of organic matter possesses a higher negative charge which causes the
cations to be more strongly attached to the soil.
Another possible explanation for the increased copper and zinc retention in the urban soil
could be due to the fact that the urban soil contains a larger amount of clay compared to the
potting soil. The amount of rubidium in the soil is directly proportional to the amount of clay in
the soil. Since the urban soil had a higher concentration of rubidium, it can be determined that
this soil contained more clay than the potting soil. The more clay a soil sample contains, the
more likely the soil is to retain metal cations (Clark).
Both the presence of clay and organic matter increase the cation exchange capacity
(CEC) which is why the urban soil had a higher cation retention percentage. Since urban soil has
a higher CEC, it can be determined that cations in urban soil will be less mobile than cations in
the environment because the environment will have a soil composition similar to that of the
potting soil. This means that if harmful cations, such as mercury, were introduced to soil in the
environment they could easily be absorbed by plants and introduced into the food chain. This
could negatively affect an ecosystem and the food or water supply a community relies on.
Conclusion
In this project, the retention the of zinc and copper in two different types of soil (urban
soil and Ocean Forest) was analyzed. Using Flame Atomic Absorbance Spectrophotometer, it
was found that at high contaminations, copper had higher concentrations for both soils than zinc.
Also, at high contaminations, Ocean Forest had higher concentrations of zinc. This was also true
for Ocean Forest at low contaminations, but urban soil had higher concentrations of zinc at low
contaminations. Using X-Ray Fluorescence, it was found that the urban soil had higher
concentrations of zinc than Ocean Forest, but Ocean Forest had higher concentrations of copper
than urban soil. By analyzing Soil Organic Matter, it was found that urban soil contained greater
than 4 percent organic matter and Ocean Forest contained between 3 and 4 percent. Lastly, using
ion chromatography, it was determined that Ocean Forest soil had higher concentrations of each
individual cation than the urban soil. However, the proportion of these cations in each soil
varied. It was determined that generally copper is more retained than zinc. The amount of clay as
well as the amount of organic matter were higher in the urban soil than the Ocean Forest soil.
These properties support the results from the previous section.
Future Directions
For future experiments, the number of cations in the soil to be analyzed could be
increased to determine further relationships between cation characteristics and retention rates.
Another useful application involving this experiment could be the analyzation of cations in
vitamins, fruits, and vegetables. Many people look for the body’s essential nutrients in these
foods, so they are concerned with the elemental breakdown. By liquefying and performing
similar experiments on vitamins, fruits, and vegetables instead of soil, an analysis of the
concentrations of cations in these foods can be performed. For example, the concentration of iron
and zinc in bananas could accurately be found using these experiments and the results could have
significant importance in the field of nutrition.
References
Clark, Ted. “REEL 3 (Environmental) Results.” Online video clip. YouTube. YouTube, 11 Apr.
2013. Web. 14 Apr. 2013.
Fontes, Maurício Paulo F. and Gomes, Paulo César, “Simultaneous Competitive Adsorption of
Heavy Metals by the Mineral Matrix of Tropical Soils”. Applied Geochemistry, 18, 795804, (2003).