Thematic Articles

Manganese oxides produced by microorganisms are abundant environ- mental nanoparticles whose high retention capacity for toxic trace metals, especially lead, is well established. Until very recently, our knowledge of the molecular-scale structure and reactivity of these biogenic Mn4+ oxide minerals was inferred from studies of synthetic analogues pre- pared in the laboratory. However, biogenic Mn oxides and their reactions with trace metals now can be investigated directly using X-ray absorption spectroscopy, thus bringing new insights into the molecular mechanisms behind the very high scavenging efficiency of these minerals. This new knowledge has important implications for the remediation of trace metal contamination.

Interactions between microbes and minerals can play an important role in metal transformations (i.e. changes to an element’s valence state, coordination chemistry, or both), which can ultimately affect that ele- ment’s mobility. Mineralogy affects microbial metabolism and ecology in a system; microbes, in turn, can affect the system’s mineralogy. Increasingly, synchrotron-based X-ray experiments are in routine use for determining an element’s valence state and coordination chemistry, as well as for examining the role of microbes in metal transformations.

Elucidating the speciation of heavy metals in the environment is para- mount to understanding their potential mobility and bioavailability. Cutting-edge synchrotron-based techniques such as microfocused X-ray absorption fine-structure (XAFS) and X-ray fluorescence (XRF) spectroscopy and microtomography have revolutionized the way metal reactions and processes in natural systems are studied. In this article, we apply these intense-light tools to decipher metal forms (species) and associations in contaminated soils and metal-hyperaccumulating plants.

Many potentially toxic trace metals and radionuclides are strongly adsorbed onto surfaces of mineral and organic compounds in soils and sediments, limiting their mobility in the environment. However, recent studies have shown that trace metals in soils, groundwater, rivers, and lakes can be carried by mobile colloidal particles. Understanding the release, transport, aggregation, and deposition of natural colloidal particles is there- fore of utmost importance for developing quantitative models of contami- nant transport and the biogeochemical cycling of trace metals.

Nanoscale materials, both inorganic and organic, are ubiquitous in the environment. Recent investigations into the nanoscale chemistry and mineralogy of toxic metal distribution in nature have revealed novel and unexpected insights. Additionally, corresponding advances in the field of nanoscience have demonstrated that the physical properties and reactivity of nanomaterials vary dramatically as a function of material size. Geoscientists are uncovering a fascinating story of how the immense surface area, unusual properties, and widespread distribution of natural nanomaterials often affect the fate of toxic metals in surprising ways.

Metals are prevalent in the environment. They are derived from both natural and anthropogenic sources. Certain metals are essential for plant growth and for animal and human health. However, if present in excessive concentrations they become toxic. Metals undergo an array of biogeochemical processes at reactive natural surfaces, including surfaces of clay minerals, metal oxides and oxyhydroxides, humic substances, plant roots, and microbes. These processes control the solubility, mobility, bioavailability, and toxicity of metals in the environment. The use of advanced analytical techniques has furthered our understanding of the reactivity and mobility of metals in the near-surface environment.