Transformation processes of metals associated with urban road dust: A critical review

The fate of metals in stormwater is primarily influenced by the transformation processes that they undergo during build-up on impervious surfaces. These processes result in changes to the bioavailability, mobility, toxicity and persistence levels of metals which can significantly influence stormwater quality. Accordingly, this article critically evaluates the transformation processes of metals accumulated on road surfaces, namely, adsorption, surface precipitation and oxidation-reduction, and their likely influence on stormwater quality to highlight the key knowledge gaps, and to identify future research needs. Based on the review outcomes, transformation processes of metals are found to be influenced by the fine particle size of road dust, predominantly mu m and physio-chemical properties of solids and environmental factors. The review identified knowledge gaps where further research is needed such as in relation to: mobilization of metal-nitrogen complexes and their impact on stormwater quality, surface precipitation of metals on road dust, conditions favorable for changes in the redox status of road dust, time-dependent transformation characteristics of metals on road surfaces and enhancement of risk assessments by taking into consideration the bioavailability of metals. Research in these areas would benefit decision makers in evaluating the potential environmental risk and for implementing effective stormwater pollution mitigation strategies.