Is magnetic susceptibility a supporting tool in assessing the toxicity of urban road dust?

Abstract

Urban road dust (URD) is a complex matrix of geogenic and anthropogenic particles, posing significant ecotoxicological and human health risks. It contains heavy metals (Ba, Cd, Co, Cr, Cu, Mo, Ni, Pd, Zn, etc.), polycyclic aromatic hydrocarbons (PAHs), phthalates and microplastics, originating from vehicular emissions, industrial activities, pavement abrasion and atmospheric deposition. This study collected road dust and slag samples from five locations representing conventional roads and without tramlines, highways with high traffic, residential areas and a waste slag heap from a ferrochromium industry in Wroclaw, Poland. Acute toxicity of the road samples was assessed using Daphnia magna (LC50) which indicates the concentration of a pollutant that causes 50% mortality in tested population. Although a few studies have examined magnetic susceptibility (χ) and its spatial and seasonal variations in URD, no study has investigated the association between toxicity and χ. This study was conducted to determine whether the χ can be used as a tool for toxicity assessment. According to the results, both the highest χ and the highest LC50 were detected in road dust samples from the same high-traffic highway, despite the sample containing significantly low concentrations of typical magnetic metals such as Fe, Ni and Co. This may be due to nanoscale magnetite (Fe₃O₄) generated from brake and tire wear, which contributes to high χ while its minimal mass fraction can lead to underestimation of total Fe content in bulk chemical analysis. Pearson’s correlation analysis revealed no statistically significant correlation between LC50 and χ (r = 0.24, p = 0.697). The former industrial slag heap, was characterized by the highest concentrations of Cr, Sb and Hg compared to other sites, likely originating from ferrochromium slag, explaining its high toxicity despite low magnetic susceptibility. The road with tramlines showed the elevated levels of Fe, Cd, Cu and Ni, and the presence of these metals was possibly caused by wear of tram rails, in addition to traffic-derived emissions. These findings indicate that magnetic susceptibility is not a supporting tool for interpreting the toxicity in URD, while highlighting the significance of heavy metals and organic pollutants (ex: poly aromatic hydrocarbons, phthalates, microplastics, etc.).