Raman spectroscopy assisted with XRF and chemical simulation to assess the synergic impacts of guardrails and traffic pollutants on urban soils

Abstract

Urban soils are potential reservoirs of toxic metals as a consequence of traffic emissions. Sources like brake linings, tyres, road pavement, exhaust fumes, guardrail, traffic signals and other galvanised steel structures are used in a large variety of external constructions in the modern urban areas. Their beneficial properties from a corrosion and oxidation perspective are well-known but less is known about their contribution to the environmental fate of corrosion-induced released zinc. In this work, the impact of guardrails and other traffic pollutants on urban soils has been studied by means of Raman spectroscopy (molecular speciation) and thermodynamic speciation to understand the mechanisms of metal release and uptake by the soils. Hydrozincite, Zn 5(CO 3) 2(OH) 6, was identified by means of Raman spectroscopy as the degradation compound of the galvanised zinc layer from guardrails which leads to the formation of soluble zinc, by acidic attack of the urban atmosphere, that drops and accumulate (zinc nitrate was identified) in soils. This fact shows the environmental risk of zinc release from the guardrails because zinc nitrate can be easily mobilised by water runoff, affecting the surrounding areas or groundwater. Other traffic pollutant that reaches guardrail and soil by atmospheric deposition, such as barium, was also identified in soil as well as in the guardrail in its carbonate form, BaCO 3. Because of its low solubility, barium will accumulate in urban soils.