Environmental lead

Labguide
Back to List of Experiments

Determination of Lead in the environment

Introduction

A recent survey of heavy metals in roadside dust (Malsingh 1999) has shown that lead, copper, and zinc are consistently present. The major source of lead along local roadways is leaded gasoline combustion products in automobile exhaust emissions (Benard 1979). On the other hand, copper appears to originate from electricity and telephone cables, while zinc is likely to be from galvanized iron sheets that are used on the roofs of most buildings.
Samples will be dried, sieved and extracted with conc. nitric acid. Heavy metal concentrations in sample extracts will be determined by flame atomic absorption spectrometry, after calibrating the instrument with standard solutions of each metal. The % of each metal in the smallest particle size will also be determined relative to the whole sample to assess the distribution of the metals in the dust.

Experimental

The sampling plan to be used will be determined by the Lecturer in charge

Plastic dustpans, brushes, new plastic bags and 500ml beakers will be required for collection and drying of the samples.

When samples are being collected, the following observations must be made:

(a) Location of sample (Street name, area, distance from and landmark etc)
(b) Distance of sample from the edge of the road, windward or leeward side
(c)Form of sample taken (sweepings vs scraped sample)
(d) Weather conditions prior to and during sampling
(e) Contour of terrain over which sample is taken (whether flat or sloping; top, middle or bottom of slope; in a depression etc)
(f) Estimated traffic density (heavy, medium or low)
(g) Population density (heavy, medium or low)

If possible, make a sketch of the site sampled
Collect each sample by sweeping (or shoveling if wet) the roadside dust into the dustpan. Collect from an area approximately 1m square about 200g of material, discarding large objects( plastic objects, sticks and stones, glass fragments etc). Transfer each sample into a clean plastic bag, label and close with a bag tie. Place in a container for transport back to the laboratory.

In addition, collect from an area remote from any roadway, a control sample for comparison with your roadside samples.

Preparation of samples

Transfer each sample into a clean dry beaker and place it in a drying oven set at 105°C for 12 to 15 hrs. After drying, cool to room temperature and sieve through a 600um mesh size stainless sieve using a circular action, and collect the sieved material. Take care not to press or force any material through the sieve.

Weigh about half of the sieved material and sieve again through a 53um stainless sieve. Collect and weigh the sieved fraction and determine its percentage relative to the weighed sample. Discard the >53 um fraction left on the sieve.

For each sample, weigh triplicate 1g aliquots of the <600 um and <53 um materials respectively into boiling tubes. To each, add 10 ml analytical grade nitric acid and swirl to ensure that the samples are throughly wetted by the adid. Prepare an acid blank as well. Cover the tubes with a clean glass plate and allow to stand in a fume hood for 12 to 15 hrs.

Place the tubes on a dry-block heater set at 135 to 140°C and digest for four hours, swirling about once every hour. Allow to cool to room temperature. add 10 ml distilled water to each tube and mix well. Filter through Whatman #1 filter paper into 50 ml volumetric flasks. Make up to the mark with distilled water rinses of the tube and filter. Mix each solution throughly.

Atomic Absorption Spectrophotometry

Prepare lead, copper and zinc standards for calibration of the AAS (see demonstrator for concentration range for each metal)

Set up and calibrate the instrument for each element and determine the concentrations in each sample solution. Where sample absorption values exceed that of the highest concentration standard, dilute the samples and then analyze them.

Results and Discussion

Express you results as ug/g of dried material. In addition. for the <53 um fraction, calculate the % of each metal per sample relative to the whole <600 um sample as follows:

If (a) concentration of metal in X g of <600 um sample = A ug/g
  (b) concentration in the <53 um fraction = B ug/g
  (c) % by mass of the <53 um fraction in the <600 um sample = C

Then % of metal in the < um fraction relative to the 600 um fraction
= (C x B) ÷ (A x X) %

Determine whether significant differences (p < 05) exist between the individual concentrations of the metals with respect to:

(1) The control sample
(2) The side of the roadway sampled
(3) The estimated traffic densities of the roadways sampled
(4) The <600 um vs the < um fractions of each sample.

References

Malsingh,J.I. The determination of Heavy Metal Concentrations in Roadside Dust Particles in Trinidad. C31F Advanced Analytical Chemistry Project Report, April 1999.

Bernard G.I. A Study Lead Pollution Resulting from Automotive Exhaust Emissions.



delloyd infolab

---