Legacy Pb–Zn mine tailings as persistent sources of bioaccessible potentially toxic elements in urban dust: geochemical signatures, exposure pathways, and human health risks
Abstract
Abandoned mine tailings, rich in potentially toxic elements (PTEs), can be redistributed in urban areas through atmospheric resuspension. This process serves as a dynamic geochemical compartment linking legacy contamination to human exposure. This study investigated the occurrence, oral bioaccessibility, ecological significance, and human health risks of PTEs in urban dust from Boquira, a former Pb–Zn mining district in northeastern Brazil. Eighteen urban dust samples and reference soils from uncontaminated areas were collected and analyzed for pseudo-total concentrations of Pb, Zn, Cd, Cu, Mn, Fe, and Al. Analyses were conducted using microwave-assisted acid digestion and X-ray fluorescence spectrometry (XRF). Geochemical indices, including the geoaccumulation index (Igeo), contamination factor (CF), enrichment factor (EF), and ecological risk index (Er), were used to evaluate anthropogenic enrichment and its ecological implications. Oral bioaccessibility was assessed using a simplified dilute nitric acid extraction method that simulates gastric conditions. Elevated enrichment and contamination indices revealed substantial anthropogenic inputs, particularly for Pb and Zn, indicating the long-term environmental persistence of mining-derived particulates. Pb had the highest mean oral bioaccessibility (~ 75%), followed by Cu (54.8%), Mn (28.1%), and Zn (27.0%), indicating the predominance of highly reactive and physiologically available metal-bearing phases. Human health risk assessment identified oral ingestion as the dominant exposure pathway, compared to dermal exposure, with children presenting substantially higher risks than adults. Non-carcinogenic risk thresholds (HI > 1) were exceeded for children, primarily due to Pb exposure, whereas carcinogenic risk associated with Cd ingestion exceeded acceptable limits for both population groups. The results demonstrate that legacy mine tailings in Boquira remain environmentally active decades after mine closure, sustaining chronic atmospheric redistribution of bioaccessible PTEs into urban areas.