The Study of Akali Metal Contamination in Road Side Soil
Abstract
Six soil samples were taken from a roadside that was expected to exhibit characteristic
of road salt contamination. This contamination is characterized by the presence of
magnesium, calcium and sodium. The relationship between akali metal concentration and
distance from the pavement was examined and determined to be nonexistent. Additionally,
atomic absorbtion and atomic emission spectroscopy were compared and and atomic
absorbtion was found to be 1.89 times as sensitive as atomic emission.
Introduction
A common technique in snow and ice removal on roadways is the application of magnesium,
calcium, and sodium chloride salts to the surface of the road. When the ice melts it
dissolves these salts and causes them to migrate into soil that is adjacent to the
pavement. Over time, the accumulation akali metal salts can change the chemical profile
of the soil which can lead to detrimental biological effects. Flame atomic spectroscopy
provides a technique that can quantify metal concentrations in the extracts of the soil
samples and consequently examine the relationship between distance from the point of road
salt application and akali metal concentrations.
Experimental
Soil preparation: Six surface soil samples were collected at the intersection of Cold
Spring Lane and the exit ramp of Interstate 83, in northwest Baltimore city. These
samples were collected at distances from the roadway of 0m, 2m, 4m, 6m, 10m, and 20m.
These samples were dried in a convection oven at 110?C for over 24 hours then crushed.
Aliquots of approximately one gram were weighed and then extracted with 10.0 mL of 1M
ammonium acetate. The extract was filtered with an inline filter disc with a pore size of
5mm and then diluted to 100.0 mL.
Instrumental: The extracts were analyzed for Ca, Na, and Mg using a Varian model AA-3
flame atomization spectrophotometer with a diffraction grating monochromator. Data was
collected with a Houston Instrument chart recorder. An acetylene/air reducing flame was
used for all determinations (10 psi acetylene/7 psi air). Two replicates of each sample
were made and averaged for both AA and AE. The analysis was seperated into two methods;
atomic absorbtion (AA) and atomic emission (AE). The emission intensities and absorbances
were determined from the measured peak height obtained from the chart recordings.
Atomic Emission: Na and Ca concentrations in the soil were determined using AE. The
spectrophotometer was calibrated using the standard series method for both elements.
Regression analysis was performed on the calibration data to provide a functional
relationship between emision intensity and concentration.
Results and Conclusions:
Sodium: The atomic line used in the analysis for sodium was at 589.0 nm. The relationship
between emision intensity and concentration was found to be quadriatic, as depicted in
the below chart. The equation that describes intensity (I) as a function of concentration
(C) is as follows:
eq (1): I=(-0.0207?0.0004)C2+(0.814?0.0168)C+(0.894?0.0242)
The fact that the relationship is quadriatic shows the effects of self absorbtion at
higher concentrations, which suggests that the linear dynamic range is smaller than 20
ppm.
Chart 1:
Calcium: The atomic line used in the analysis of calcium was at 422.6 nm .The
relationship between emision intensity and concentration was found to be linear, as
depicted in the below chart. The equation that describes intensity (I) as a function of
concentration (C) is as follows:
eq(2): I=((0.243?0.0117)C)+(0.570?0.0430)
Chart 2:
Atomic Absorbance: Mg and Ca concentrations in the soil were determined using AA. The
source used was a Varian multielement (Mg/Ca) hollow cathode lamp running at 25
milliamperes. The spectrophotometer was calibrated using the standard series method for
both elements. Regression analysis was performed on the calibration data to provide a
functional relationship between absorbance and concentration.
Calcium: The atomic line used in the analysis of calcium was at 422.6 nm. The
relationship between absorbance and concentration was found to be linear, as depicted in
the below chart. The equation that describes atomic absorbtion (A) as a function of
concentration (C) is as follows:
eq(3): A=((0.459?0.0152)C)+(0.100?0.0181)
Chart 3:
Magnesium: The atomic line used in the analysis of magnesium was at 285.2 nm. The
relationship between absorbance and concentration was found to be linear, as depicted in
the below chart. The equation that describes atomic absorbtion (A) as a function of
concentration (C) is as follows:
eq(4): A=((10.4?0.420)C)+(0.238?0.0478)
Chart 4:
Soil Samples: The soil extracts were analyzed for Na, Ca, and Mg at the aforementioned
wavelengths. To determine the unknown concentrations of the soils from the known emission
intensities or absorbances rearangement of equations 1-4 was required and each new
equation is denoted by the suffix A following the original equation number.
?Na Emission:
eq (1): I=(-0.0207)C2+(0.814)C+(0.894)
eq(1A):C=
Note: This is a result of the fact that equation 1 is a quadriatic equation of the
general form:
y=ax2+bx+c, with y'0, where a, b, and c are constants. At any point in the domain of x, y
takes on a constant value and the following equation can be written: 0= ax2+bx+(c-y). Let
(=(c-y).The difference of two constants is certainly a constant, thus, 0= ax2+bx+(. The
quadriatic formula can be written as x= . Only the solution obtained from adding the
discriminant was used in subsequent calculations.
?Ca Emission: ?Ca Absorbtion:
eq(2): I=(0.243)C+(0.570) eq(3): A=(0.459)C+(0.100)
eq(2A) C=(I-0.570)/0.243 eq(3A): C=(A-0.100)/0.459
?Mg Absorbtion
eq(4): A=(10.4)C+(0.238)
eq(4A): C=(A-0.238)/10.4
Solutions of the previous equations are tabulated as follows:
Table 1:
Distance (m) Na Conc.(mg/kg) Mg Conc.(mg/kg) Ca Conc. by AA(mg/kg) Ca Conc. by AE(mg/kg)
0 427 17.7 344 627
2 536 50.6 1840 2520
4 448 80.5 1590 2340
6 166 47.1 1080 4070
10 337 47.2 1020 1720
20 62.4 76.4 1940 2070
It would appear that there is no relationship between akali metal concentration and
distance from the roadway at the particular location that the samples were obtained from.
The following charts illustrate this graphically.
Atomic Emission vs. Atomic Absorbtion in calcium determination: The did not appear to be
much correlation between AA and AE for the soil samples, which is demonstrated in Table
2.O
On average, the AA values were -88.1% lower than AE values, with a sample standard
deviation of 87.8% and a relative standard deviation of -99.7%.
Table 2:
Distance (m) Ca Conc. by AA(mg/kg) Ca Conc. by AE(mg/kg) % difference
0 344 627 -82.3
2 1840 2520 -37.0
4 1590 2340 -57.0
6 1080 4070 -277
10 1020 1720 -68.6
20 1940 2070 -6.7
Average N/A N/A -88.1
Std. Dev N/A N/A 87.8
%RSD N/A N/A -99.7%
The sensitivities of the two methods were compared using the parameter defined as
calibration sensitivity, which is the slope of the calibration curve. Analytical
sensitivity was not determined because it is concentration dependent and the signal
standard deviations were often zero due to the fact that only two replicates per standard
were made. The ratio of the slopes (AA:AE) of the curves is 1.89, indicating that atomic
absorbtion is almost twice as sensitive as atomic emission.
In conclusion, the dry weight concentrations of magnesium, calcium, and sodium in
roadside soil samples were determined by atomic spectroscopy and no relationship between
distance from the road and concentration was observed. Atomic absorbtion spectroscopy was
compared to atomic emission spectroscopy and emission spectroscopy was found to be
0.529 times as sensitive atomic absorbtion. When actual concentrations that were
determined by the two techniques were compared, AA values were, on average, -88% lower.
This could be a result of matrix effects or spectral interferences in the soil extracts
used for AE.
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