XAFS and XSW study of the distribution of Pb(II) sorbed to biofilms on alpha-Al2O3 and alpha-FeOOH surfaces

Oxide- and Silicate-Water Interfaces and Their Roles in Technology and the Environment

Interfacial reactions drive all elemental cycling on Earth and play pivotal roles in human activities such as agriculture, water purification, energy production and storage, environmental contaminant remediation, and nuclear waste repository management. The onset of the 21st century marked the beginning of a more detailed understanding of mineral aqueous interfaces enabled by advances in techniques that use tunable high-flux focused ultrafast laser and X-ray sources to provide near-atomic measurement resolution, as well as by nanofabrication approaches that enable transmission electron microscopy in a liquid cell. This leap into atomic- and nanometer-scale measurements has uncovered scale-dependent phenomena whose reaction thermodynamics, kinetics, and pathways deviate from previous observations made on larger systems. A second key advance is new experimental evidence for what scientists hypothesized but could not test previously, namely, interfacial chemical reactions are frequently driven by "anomalies" or "non-idealities" such as defects, nanoconfinement, and other nontypical chemical structures. Third, progress in computational chemistry has yielded new insights that allow a move beyond simple schematics, leading to a molecular model of these complex interfaces. In combination with surface-sensitive measurements, we have gained knowledge of the interfacial structure and dynamics, including the underlying solid surface and the immediately adjacent water and aqueous ions, enabling a better definition of what constitutes the oxide- and silicate-water interfaces. This critical review discusses how science progresses from understanding ideal solid-water interfaces to more realistic systems, focusing on accomplishments in the last 20 years and identifying challenges and future opportunities for the community to address. We anticipate that the next 20 years will focus on understanding and predicting dynamic transient and reactive structures over greater spatial and temporal ranges as well as systems of greater structural and chemical complexity. Closer collaborations of theoretical and experimental experts across disciplines will continue to be critical to achieving this great aspiration.

Cu, Pb, and Zn Sorption to Biogenic Iron (Oxyhydr)Oxides Formed in Circumneutral Environments

The transportation and immobilization of potentially toxic metals in near-surface environments may be partially controlled by sorption processes at the solid-water interface. Myriad studies have shown that iron (oxyhydr)oxides have large sorption capacities and form strong surface complexes with metal ions. Biogenic iron (oxyhydr)oxides (BIOS) form at redox gradients where dissolved ferrous iron encounters oxygenated conditions, allowing bacteria to outcompete abiotic Fe oxidation. This process produces biominerals with distinct surface and structural properties (incorporation of cell-derived organic matter, poor crystallinity, and small particle sizes) that may alter their metal-binding affinity and sorption processes. To better understand metal binding by BIOS, Cu, Pb, and Zn, sorption rate and isotherm studies were conducted with synthetic two-line ferrihydrite and BIOS. Additionally, X-ray absorption spectroscopy and total scattering were used to elucidate the BIOS mineral structure and metal ion surface structures. On a mass normalization basis, BIOS sorbed approximately 8, 4, and 2 times more Cu, Pb, and Zn, respectively, than 2LFh over similar dissolved concentrations. Spectroscopic analyses revealed poorly crystalline structures and small coherent scattering domain sizes for BIOS. Additionally, extended X-ray absorption fine-structure spectroscopy revealed Cu, Pb, and Zn sorbed to BIOS via inner-sphere complexes, similar to 2LFh. These results suggest that, in metal contaminated environments, BIOS are more efficient in metal binding than their synthetic counterparts.

Lead accumulation and association with Fe on Typha latifolia root from an urban brownfield site

Synchrotron X-ray microfluorescence and X-ray absorption near-edge microstructure spectroscopy techniques were applied to Typha latifolia (cattail) root sections and rhizosphere soils collected from a brownfield site in New Jersey to investigate lead (Pb) accumulation in T. latifolia roots and the role of iron (Fe) plaque in controlling Pb uptake. We found that Pb and Fe spatial distribution patterns in the root tissues are similar with both metals present at high concentrations mainly in the epidermis and at low concentrations in the vascular tissue (xylem and phloem), and the major Pb and Fe species in T. latifolia root are Pb(II) and Fe(III) regardless of concentration levels. The sequestration of Pb by T. latifolia roots suggests a potential low-cost remediation method (phytostabilization) to manage Pb-contaminated sediments for brownfield remediation while performing wetland rehabilitation.

XAS speciation of arsenic in a hyper-accumulating fern

The coordination environment and the redox speciation of arsenic in a newly discovered arsenic hyper-accumulating fern (Pteris vittata L) were investigated by X-ray absorption spectroscopy. This method allowed us to probe arsenic directly, i.e., with minimal sample preparation. The results indicate that arsenic is predominantly accumulated as As(III) in the leaves. XANES and EXAFS results show that As(III) in the leaves is primarily present as aqueous arsenite species. The plant actively maintains arsenic in this reduced oxidation state, because after sample collection and subsequent aging and drying of the plant material, As(III) is gradually oxidized to As(V). We think that these arsenite species are sequestered in vacuoles. At extremely high As concentrations (ca. 1% As per dry weight) arsenic in the fern leaves is coordinated to a significant degree by sulfur in addition to oxygen. This spectral signature indicates that thiol-rich compounds are implicated in the biochemical transformations of arsenic within the plant.

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.