Uptake and transport of Pb across the iron plaque of waterlogged dropwort (Oenanthe javanica DC.) based on micro-XRF and XANES

Abstract

Iron plaque at the soil-root interface is a key position for uptake of heavy metals by plants. Exploring iron plaque’s role in Pb uptake by edible vegetable roots aids understanding Pb uptake mechanisms and developing methods to reduce Pb accumulation. Soil and plant Pb contents were determined. Micro X-ray fluorescence (micro-XRF) determined Fe and Pb distributions in waterlogged and terrestrial Oenanthe javanica DC. roots, and X-ray absorption near-edge spectroscopy (XANES) identified Pb speciation in bulk soil, rhizosphere soil and plant tissues. Waterlogged O. javanica accumulated more Pb and exhibited a higher Pb transfer factor than terrestrial O. javanica. In waterlogged O. javanica, the iron plaque and epidermis contained the most Fe, while the root vasculature contained the most Pb. In terrestrial O. javanica roots, Fe and Pb had similar distributions. Bulk and rhizosphere soils contained different Pb species, and rhizosphere soil had Pb-humate. For iron plaque, a new Pb complex, Pb-ferrihydrite, was identified. Biologically important groups bound (-S, -COO) and precipitated (-PO4) Pb were identified in plants. Waterlogged O. javanica root iron plaque and humic acid increase Pb uptake and accumulation. Thus, avoiding O. javanica root iron plaque formation (dry land growth) and growing in low-humic soil reduce Pb uptake and entry into the food chain.