Geochemical occurrence of Pb in soil and road dust from an industrialized urban environment: Fractionation, bioaccessibility and isotopic composition

Anthropogenic Pb originated from leaded petrol and high temperature industrial activities is still a major contamination issue to various compartments in the urban environment, including soil and road dust. Human exposure to Pb near Pb-contaminated areas involves incidental ingestion of soil and road dust and inhalation of resuspended solid particles. Volos is a medium-sized industrialized city, located in central Greece, surrounded by steel and cement factories. The geochemical reactivity, fractionation and bioaccessibility (both oral and inhalable) of Pb in selected soil (n=10) and road dust (n=10) samples were investigated through a variety of laboratory chemical methods comprising the dilute HNO₃ extraction, the BCR sequential extraction procedure, the simple bioaccessibility extraction test (SBET) and a simulated lung fluid (SLF solution, artificially lysosomal fluid). In addition, the Pb isotopic composition of both total and bioaccessible Pb was determined to identify the Pb sources and examine potential differences of Pb solubilization in terms of its origin when interacting with simulated human stomach and lung fluids.

Lead was found in highly reactive forms in both road dust and soil (medians 83% and 69% of total contents, respectively). The majority of Pb was found to be associated with the sum of the acid soluble (F1), reducible (F2) and oxidizable (F3) fractions (median 72% of total contents for both soil and road dust), indicating that Pb has a high remobilization potential from the solid matrix. The oral bioaccessibility (%) of Pb was higher than the inhalation one (medians 49% and 37% of the total content), respectively), highlighting the soil and dust ingestion as the primary route of Pb exposure. The isotopic analyses of total Pb in soil and road dust (²⁰⁶Pb/²⁰⁷Pb = 1.144 to 1.192) suggest that the predominant anthropogenic Pb source is industrial Pb from the steel plant, with minor contributions of Pb derived from vehicular emissions. Interestingly, we found significant differences in the isotopic ratios between total and bioaccessible Pb (²⁰⁶Pb/²⁰⁷Pb = 1.130 to 1.152), demonstrating that Pb solubilized by the simulated gastric and lung extractions is principally anthropogenic. High Pb bioaccessibilities (%) accompanied a shift towards lower ²⁰⁶Pb/²⁰⁷Pb ratios. Moreover, ²⁰⁶Pb/²⁰⁷Pb ratios of both total and bioaccessible Pb exhibited significantly negative correlations with Pb reactive fractions (%), showing that natural Pb is linked to low Pb release from the soil and road dust matrix. Overall, Pb isotopes provide deep insights into the connection between Pb bioaccessibility and reactivity with Pb sources in soil and road dust from industrial environments.

References

Kelepertzis, E., Chrastný, V., Botsou, F., Sigala, E., Kypritidou, Z., Komárek, M., Skordas, K., Argyraki, A. 2021. Tracing the sources of bioaccessible metal(loids) in urban environments: A multidisciplinary approach. Science of the Total Environment 771: 144827.