The natural nanomineral ferrihydrite is an important component of many environmental and soil systems and has been implicated as the inorganic core of ferritin in biological systems. Knowledge of its ...basic structure, composition, and extent of structural disorder is essential for understanding its reactivity, stability, and magnetic behavior, as well as changes in these properties during aging. Here we investigate compositional, structural, and magnetic changes that occur upon aging of "2-line" ferrihydrite in the presence of adsorbed citrate at elevated temperature. Whereas aging under these conditions ultimately results in the formation of hematite, analysis of the atomic pair distribution function and complementary physicochemical and magnetic data indicate formation of an intermediate ferrihydrite phase of larger particle size with few defects, more structural relaxation and electron spin ordering, and pronounced ferrimagnetism relative to its disordered ferrihydrite precursor. Our results represent an important conceptual advance in understanding the nature of structural disorder in ferrihydrite and its relation to the magnetic structure and also serve to validate a controversial, recently proposed structural model for this phase. In addition, the pathway we identify for forming ferrimagnetic ferrihydrite potentially explains the magnetic enhancement that typically precedes formation of hematite in aerobic soil and weathering environments. Such magnetic enhancement has been attributed to the formation of poorly understood, nano-sized ferrimagnets from a ferrihydrite precursor. Whereas elevated temperatures drive the transformation on timescales feasible for laboratory studies, our results also suggest that ferrimagnetic ferrihydrite could form naturally at ambient temperature given sufficient time.
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Silver nanoparticles (Ag-NPs) readily transform in the environment, which modifies their properties and alters their transport, fate, and toxicity. It is essential to consider such transformations ...when assessing the potential environmental impact of Ag-NPs. This review discusses the major transformation processes of Ag-NPs in various aqueous environments, particularly transformations of the metallic Ag cores caused by reactions with (in)organic ligands, and the effects of such transformations on physical and chemical stability and toxicity. Thermodynamic arguments are used to predict what forms of oxidized silver will predominate in various environmental scenarios. Silver binds strongly to sulfur (both organic and inorganic) in natural systems (fresh and sea waters) as well as in wastewater treatment plants, where most Ag-NPs are expected to be concentrated and then released. Sulfidation of Ag-NPs results in a significant decrease in their toxicity due to the lower solubility of silver sulfide, potentially limiting their short-term environmental impact. This review also discusses some of the major unanswered questions about Ag-NPs, which, when answered, will improve predictions about their potential environmental impacts. Research needed to address these questions includes fundamental molecular-level studies of Ag-NPs and their transformation products, particularly Ag2S-NPs, in simplified model systems containing common (in)organic ligands, as well as under more realistic environmental conditions using microcosm/mesocosm-type experiments. Toxicology studies of Ag-NP transformation products, including different states of aggregation and sulfidation, are also required. In addition, there is the need to characterize the surface structures, compositions, and morphologies of Ag-NPs and Ag2S-NPs to the extent possible because they control properties such as solubility and reactivity.
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4.
Environmental Speciation of Actinides Maher, Kate; Bargar, John R; Brown, Gordon E
Inorganic chemistry,
04/2013, Volume:
52, Issue:
7
Journal Article
Peer reviewed
Although minor in abundance in Earth’s crust (U, 2–4 ppm; Th, 10–15 ppm) and in seawater (U, 0.003 ppm; Th, 0.0007 ppm), light actinides (Th, Pa, U, Np, Pu, Am, and Cm) are important environmental ...contaminants associated with anthropogenic activities such as the mining and milling of uranium ores, generation of nuclear energy, and storage of legacy waste resulting from the manufacturing and testing of nuclear weapons. In this review, we discuss the abundance, production, and environmental sources of naturally occurring and some man-made light actinides. As is the case with other environmental contaminants, the solubility, transport properties, bioavailability, and toxicity of actinides are dependent on their speciation (composition, oxidation state, molecular-level structure, and nature of the phase in which the contaminant element or molecule occurs). We review the aqueous speciation of U, Np, and Pu as a function of pH and Eh, their interaction with common inorganic and organic ligands in natural waters, and some of the common U-containing minerals. We also discuss the interaction of U, Np, Pu, and Am solution complexes with common Earth materials, including minerals, colloids, gels, natural organic matter (NOM), and microbial organisms, based on simplified model system studies. These surface interactions can inhibit (e.g., sorption to mineral surfaces, formation of insoluble biominerals) or enhance (e.g., colloid-facilitated transport) the dispersal of light actinides in the biosphere and in some cases (e.g., interaction with dissimilatory metal-reducing bacteria, NOM, or Mn- and Fe-containing minerals) can modify the oxidation states and, consequently, the behavior of redox-sensitive light actinides (U, Np, and Pu). Finally, we review the speciation of U and Pu, their chemical transformations, and cleanup histories at several U.S. Department of Energy field sites that have been used to mill U ores, produce fissile materials for reactors and weapons, and store high-level nuclear waste from both civilian and defense operations, including Hanford, WA; Rifle, CO; Oak Ridge, TN; Fernald, OH; Fry Canyon, UT; and Rocky Flats, CO.
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Hexavalent chromium is a water-soluble pollutant, the mobility of which can be controlled by reduction of Cr(VI) to less soluble, environmentally benign Cr(III). Iron(II/III)-bearing clay minerals ...are widespread potential reductants of Cr(VI), but the kinetics and pathways of Cr(VI) reduction by such clay minerals are poorly understood. We reacted aqueous Cr(VI) with two abiotically reduced clay minerals: an Fe-poor montmorillonite and an Fe-rich nontronite. The effects of ionic strength, pH, total Fe content, and the fraction of reduced structural Fe(II) Fe(II)/Fe(total) were examined. The last variable had the largest effect on Cr(VI) reduction kinetics: for both clay minerals, the rate constant of Cr(VI) reduction varies by more than 3 orders of magnitude with Fe(II)/Fe(total) and is described by a linear free energy relationship. Under all conditions examined, Cr and Fe K-edge X-ray absorption near-edge structure spectra show that the main Cr-bearing product is a Cr(III)-hydroxide and that Fe remains in the clay structure after reacting with Cr(VI). This study helps to quantify our understanding of the kinetics of Cr(VI) reduction by Fe(II/III)-bearing clay minerals and may improve predictions of Cr(VI) behavior in subsurface environments.
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Geotourism spans a range of visitor interests, from the specialist geotourist to the more general visitor. As well as supporting geoconservation outcomes, it provides economic, cultural, relational ...and social benefits for both visitors and host communities. The interconnections between geoheritage and the cultural components of the landscape have antecedents in concepts of landscape aesthetics in different cultures. These interconnections provide a range of opportunities for enhancing the geotourist experience and promoting geoconservation and geoeducation by means of activities that involve aesthetic and emotional experiences and interpretation through different cultural filters that encourage the rediscovery of a sense of wonder both about the geological stories in the landscape and the human interactions. A cultural ecosystem services framework provides a holistic approach for informing conservation policy, management and planning for geotourism, enabling assessment of multiple benefits and trade-offs for visitors and communities based on the values of the geoheritage assets. Geotourism studies could also benefit from integration of existing theory, conceptual analysis and practice from broader heritage and nature-based tourism and closer collaboration with relevant social sciences. Adhering to sound geoethical practice is an essential part of geotourism, which can also play a role in the promotion of geoethics among the public and professionals.
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The frontier beneath our feet Grant, Gordon E.; Dietrich, William E.
Water resources research,
April 2017, 2017-04-00, 20170401, Volume:
53, Issue:
4
Journal Article
Peer reviewed
Open access
Following the simple question as to where water goes when it rains leads to one of the most exciting frontiers in earth science: the critical zone—Earth's dynamic skin. The critical zone extends from ...the top of the vegetation canopy through the soil and down to fresh bedrock and the bottom of the groundwater. Only recently recognized as a distinct zone, it is challenging to study because it is hard to observe directly, and varies widely across biogeoclimatic regions. Yet new ideas, instruments, and observations are revealing surprising and sometimes paradoxical insights, underscoring the value of field campaigns and long‐term observatories. These insights bear directly on some of the most pressing societal problems today: maintaining healthy forests, sustaining streamflow during droughts, and restoring productive terrestrial and aquatic ecosystems. The critical zone is critical because it supports all terrestrial life; it is the nexus where water and carbon is cycled, vegetation (hence food) grows, soil develops, landscapes evolve, and we live. No other frontier is so close to home.
Plain Language Summary
One of the newest and most exciting frontiers in earth sciences is the critical zone: the thin, dynamic skin of the terrestrial Earth that extends from the top of the vegetation canopy through the soil and down to fresh bedrock and the bottom of groundwater. The critical zone is where water moves, vegetation grows, roots spread, organic matter decomposes, soil develops, and rock weathers. It's also where we, and most life, lives, and is therefore “critical” to our survival. Studying the critical zone has rapidly become an international and interdisciplinary science effort utilizing field studies, long term observatories, and new geophysical measurement techniques. These studies are revealing insights into a broad range of previously unexplored topics: where do trees get their water, how does rock weather, and where does water go when it rains. Understanding the critical zone is vital to addressing key environmental and social problems: maintaining soil productivity in intensively managed landscapes, ensuring that forests don't die during droughts, and improving landscape resilience to wildfires, floods, and hurricanes. Today, the term “critical zone” provides an essential organizing principle for the earth and biological sciences just as “ecosystem” did for ecology half a century ago.
Key Points
The critical zone is a new and exciting frontier in earth and biological sciences
Critical zone science is transforming our understanding of linkages among vegetation, soils, bedrock, and the movement of water
Critical zone science contributes to understanding key social problems: forest health, soil productivity, and effects of natural disasters
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Pristine silver nanoparticles (AgNPs) are not chemically stable in the environment and react strongly with inorganic ligands such as sulfide and chloride once the silver is oxidized. Understanding ...the environmental transformations of AgNPs in the presence of specific inorganic ligands is crucial to determining their fate and toxicity in the environment. Chloride (Cl–) is a ubiquitous ligand with a strong affinity for oxidized silver and is often present in natural waters and in bacterial growth media. Though chloride can strongly affect toxicity results for AgNPs, their interaction is rarely considered and is challenging to study because of the numerous soluble and solid Ag–Cl species that can form depending on the Cl/Ag ratio. Consequently, little is known about the stability and dissolution kinetics of AgNPs in the presence of chloride ions. Our study focuses on the dissolution behavior of AgNPs in chloride-containing systems and also investigates the effect of chloride on the growth inhibition of E.coli (ATCC strain 33876) caused by Ag toxicity. Our results suggest that the kinetics of dissolution are strongly dependent on the Cl/Ag ratio and can be interpreted using the thermodynamically expected speciation of Ag in the presence of chloride. We also show that the toxicity of AgNPs to E.coli at various Cl– concentrations is governed by the amount of dissolved AgCl x (x–1)– species suggesting an ion effect rather than a nanoparticle effect.
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10.
The end of skill mix? Gordon, E
British dental journal,
05/2020, Volume:
228, Issue:
9
Journal Article
Peer reviewed
Open access
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EMUNI, FIS, FZAB, GEOZS, GIS, IJS, IMTLJ, KILJ, KISLJ, MFDPS, NLZOH, NUK, OILJ, PNG, SAZU, SBCE, SBJE, SBMB, SBNM, UKNU, UL, UM, UPUK, VKSCE, ZAGLJ
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