An integrated XRF techniques approach for the study of Pb bioavailability in a polluted firing range soil

Lead is among the most hazardous elements on earth, non-essential neither beneficial for biota. Firing ranges are one of the major anthropogenic sources of Pb contamination in soils worldwide. The bullet residues that initially spread over the soil when firing in the form of metallic fragments, give rise over time to the formation of different Pb weathering products. The stability of such Pb-bearing phases is crucial for Pb accessibility to soil biota; therefore, the assessment of the element distribution and speciation is of great importance to determine the actual risks for both humans and the environment. In this study, the Pb availability in a former polluted firing range soil has been investigated integrating conventional soil analyses, X-ray microanalyses and assays with the bio-indicator earthworm Eisenia andrei. In particular, chemical extractions evidenced a rather scarce mobility of soil Pb while micro X-ray fluorescence spectroscopy (µXRF) performed on soil thin sections (30 μm), showed an abundant presence of Pb slivers, ranging from hundreds of microns up to few millimeters. The hyperspectral analysis of µXRF data and further observation with scanning electron microscopy coupled with microanalysis (SEM-EDX) allowed identifying weathering products at the borders of the metallic Pb-particles consisting of a mixture of Pb and Ca phosphates, including the highly insoluble Cl-pyromorphite. Besides, no acute-toxicity effects and low Pb uptake in earthworms tissues and fluids was quantified through total-reflection X-ray fluorescence (TXRF) spectroscopy after 28 days exposure to the polluted soil. Based on these results, it can be assumed that most of the Pb in the firing range soil underwent stabilization processes over time, which strongly limit the accessibility to the hazardous element. These processes have been likely promoted by phosphatic fertilization, since the soil has been used for agriculture for several years after being dismissed for firing activities. The XRF based approach proposed in this study can be applied to investigate Pb bioavailability in other shooting ranges or, more generally, in soils heavily polluted by Pb.