Environmental Engineering Reference
In-Depth Information
Sampling was performed on asphalt covered roadside. Road dust was collected
using a brush and placed in ziplocked PE bags for transport and storage.
Samples were dried overnight at 105°C. Dry samples were then sieved and the
<125 mm fraction was retained for analysis. Samples were prepared by microwave-
assisted acid digestion using closed Teflon vessels (Mars5, CEM, USA).
Approximately 0.5 g of dry dust was placed in microwave digestion vessels.
Digestion was performed in closed vessels after addition of 8 ml
Aqua regia
(6 ml
HCl + 2 ml HNO
3
) using 2-step temperature increase to 185°C and a maximum
allowed pressure of 200 psi. It is important to note that the procedure does not
provide a full digestion, but a strong leach usually suitable for the study of anthro-
pogenic metals in road dust. The leachate was then slowly taken to dryness on a hot
plate and redissolved in 5% HNO
3
.
Prepared samples were analyzed by inductively coupled plasma-mass spectrometry
(ICP-MS) using a quadrupole system (Elan 6000, Perkin Elmer, USA) and standard
operating conditions. Calibration was performed by the analysis of multi-element
standard solution, except for Pt for which a single element solution was used.
Interferences on Pt analysis were corrected mathematically
[9, 10]
and reference
material BCR-723 was analyzed to assess the accuracy of Pt concentrations.
Results and Discussion
Average Pb and Pt concentrations in road dust in South African cities are presented
in Table
1
.
Lead Concentrations
Average Pb concentrations were found to range from 345 mg/g in Rustenberg to
775 mg/g in Pretoria (Table
1
), with a minimum concentration of 103 mg/g at
Joubert Street in Rustenburg and a maximum concentration of 1,928 mg/g at
Church Street in Pretoria (Fig.
2
). For comparison, urban concentrations in
Accra, Ghana were reported to be 365 ± 93 mg/g
[11]
. The higher concentrations
in South Africa are likely the result of the larger consumption of Pb gasoline
additives
[2]
.
Large concentration variations were observed in individual cities; Pb concentra-
tions in Pretoria range for instance from 186 mg/g at Schueman Street to 1,928 mg/g
at Church Street (Fig.
2
). Although automobile traffic is expected to be the main
source of Pb, there is no direct correlation between Pb concentrations and traffic
density. The absence of correlation may be the result of factors other than Pb emis-
sion from automobile traffic and subsequent deposition. Because leaded gasoline is
now banned in South Africa, it is possible that removal efficiency play a more
important role than Pb deposition.
