Essay/Term paper: The study of akali metal contamination in road side soil

Essay, term paper, research paper:  Science Research Papers

The Study of Akali Metal Contamination in Road Side Soil


IL 3; Experiment 1
October 31, 1996

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 y¹0, 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.