The Effects of Road Transport on Freshwater and Marine Ecosystems

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crystalline species present were sulphates and carbonates along with oxides and elemental Pb.
Overall, metals introduced to road surfaces from vehicles will undergo a range <strong>of</strong> changes including:
Oxidation due to exposure to air.
Breakdown due to physical abrasion.
Chemical reactions due to change in pH and exposure to sulphate, carbonates and other
chemical species in the atmosphere or rain.
De-sorption <strong>of</strong> adsorbed chemical species.
Acidic rainwater (e.g., pH 5-6 in New Zealand) can release metals to solution through two processes.
<strong>The</strong> first is dissolving inorganic compounds, releasing the metal into the dissolved phase. <strong>The</strong> second
occurs through the release <strong>of</strong> metal species that are attached to the particulate surfaces through the
saturation <strong>of</strong> the surface by hydrogen ions. Typically however, the road surface buffers the pH
increasing it towards neutral.
Studies <strong>of</strong> the chemistry <strong>of</strong> street surface particulate material have been undertaken using different
extractions by a number <strong>of</strong> authors. <strong>The</strong> extraction <strong>of</strong> metals using a sequence <strong>of</strong> extractants
endeavours to distinguish between exchangeable, carbonate, reducible (iron, manganese oxides),
oxidisable and residual phases in the material. <strong>The</strong> oxidisale phase includes organic matter and
sulphides. Given the number <strong>of</strong> emission sources and the different forms that the elements are
present in the environmental mobility and the phase distribution will vary between the different sources
<strong>of</strong> the elements. Extraction and speciation approaches are evaluated by authors such as de Mora &
Harrison (1984) and Forstner (1986).
A number <strong>of</strong> studies have examined the geochemical phase distribution <strong>of</strong> trace elements in road
surface particulates (Wilber & Hunter 1979; Harrison 1979; Lau & Wong 1983). Table 5.2 provides a
summary <strong>of</strong> data from Laxen et al. (1981) and two other studies that have used similar extraction
schemes (Fergusson & Ryan 1984; Banerjee 2003). Fergusson & Ryan (1984) undertook sequential
extraction <strong>of</strong> the various road surface particulate samples they collected from various cities including
Christchurch. <strong>The</strong> proportion <strong>of</strong> each element in the five fractions examined was very similar to the
results obtained by Banerjee (2003). No significant differences in proportion extracted in the different
phases were found between three particle size fractions examined by Fergusson & Ryan (1984).
However, extractability in dilute acid (0.002 N HCl) tends to increase exponentially with decreasing
grain size <strong>of</strong> the particulates present in street dusts (Kennedy unpublished). <strong>The</strong> results presented in
Table 5.2 are similar to those found using similar extraction schemes in other studies such as Li et al.
(2001).
Other studies have utilised 0.5 M HCl extractions to identify a bioavailable fraction in street particulates
(e.g., Sutherland & Tolosa 2000). <strong>The</strong> proportion <strong>of</strong> element extractable by 0.5 M HCL increased from
a low <strong>of</strong> 80% for Pb and Zn (refer Fig. 5.1).
<strong>The</strong> key outcomes <strong>of</strong> the various studies are:
<strong>The</strong> predominance <strong>of</strong> Cd in exchangeable, carbonate and Fe/Mn oxide phases.
<strong>The</strong> predominance <strong>of</strong> Cu in organ phases.
<strong>The</strong> predominance <strong>of</strong> Pb and Zn in carbonate and oxide phases.
<strong>The</strong> predominance <strong>of</strong> Ni and Cr in more stable forms with most <strong>of</strong> the NI in oxide and residual
forms and most Cr in residual forms.
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