Discovery and potential ramifications of reduced iron-bearing nanoparticles—magnetite, wüstite, and zero-valent iron—in wildland–urban interface fire ashes

Abstract

The increase in fires at the wildland–urban interface has raised concerns about the potential environmental impact of ash remaining after burning. Here, we examined the concentrations and speciation of iron-bearing nanoparticles in wildland–urban interface ash. Total iron concentrations in ash varied between 4 and 66 mg g⁻¹. Synchrotron X-ray absorption near-edge structure (XANES) spectroscopy of bulk ash samples was used to quantify the relative abundance of major Fe phases, which were corroborated by transmission electron microscopy measurements. Maghemite (γ-(Fe³⁺)₂O₃) and magnetite (γ-Fe²⁺(Fe³⁺)₂O₄) were detected in most ashes and accounted for 0–90 and 0–81% of the spectral weight, respectively. Ferrihydrite (amorphous Fe(III)–hydroxide, (Fe³⁺)₅HO₈·4H₂O), goethite (α-Fe³⁺OOH), and hematite (α-Fe³⁺₂O₃) were identified less frequently in ashes than maghemite and magnetite and accounted for 0–65, 0–54, and 0–50% of spectral weight, respectively. Other iron phases identified in ashes include wüstite (Fe²⁺O), zerovalent iron, FeS, FeCl₂, FeCl₃, FeSO₄, Fe₂(SO₄)₃, and Fe(NO₃)₃. Our findings demonstrate the impact of fires at the wildland–urban interface on iron speciation; that is, fires can convert iron oxides (e.g., maghemite, hematite, and goethite) to reduced iron phases such as magnetite, wüstite, and zerovalent iron. Magnetite concentrations (e.g., up to 25 mg g⁻¹) decreased from black to gray to white ashes. Based on transmission electron microscopy (TEM) analyses, most of the magnetite nanoparticles were less than 500 nm in size, although larger particles were identified. Magnetite nanoparticles have been linked to neurodegenerative diseases as well as climate change. This study provides important information for understanding the potential environmental impacts of fires at the wildland–urban interface, which are currently poorly understood.