Contamination Of Road Salt Essay, Research Paper

IL 3; Experiment 1

October 31, 1996

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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