Characterization of Fe plaque and associated metals on the roots of mine-waste impacted aquatic plants

Iron plaque on aquatic plant roots are ubiquitous and sequester metals in wetland soils; however, the mechanisms of metal sequestration are unresolved. Thus, characterizing the Fe plaque and associated metals will aid in understanding and predicting metal cycling in wetland ecosystems. Accordingly, microscopic and spectroscopic techniques were utilized to identify the spatial distributions, associations, and chemical environments of Fe, Mn, Pb, and Zn on the roots of a common, indigenous wetland plant (Phalaris arundinacea). Iron forms a continuous precipitate on the root surface, which is composed dominantly of ferrihydrite (ca. 63%) with lesser amounts of goethite (32%) and minor levels of siderite (5%). Although Pb is juxtaposed with Fe on the root surface, it is complexed to organic functional groups, consistent with those of bacterial biofilms. In contrast, Mn and Zn exist as discrete, isolated mixed-metal carbonate (rhodochrosite/hydrozincite) nodules on the root surface. Accordingly, the soil-root interface appears to be a complex biochemical environment, containing both reduced and oxidized mineral species, as well as bacterial-induced organic-metal complexes. As such, hydrated iron oxides, bacterial biofilms, and metal carbonates will influence the availability and mobility of metals within the rhizosphere of aquatic plants.