How electron flow controls contaminant dynamics Anthropogenic and natural sources of pollution contribute large amounts of inorganic and organic compounds to the global environment every day. As a result, public health issues and ecosystem deterioration arising from environmental pollution are growing concerns worldwide. Ecosystem services are influenced by intimately coupled relationships between humans and the biological, chemical, and physical characteristics of our natural environment. The rapid growth in the global population and the increasing pressure of understanding and addressing the sustainability of air, soil, and water resources in response to global change and human activities will require transdisciplinary research efforts resulting in the development of science-based solution strategies. The interfaces between oxic and anoxic environments are often of particular importance for element cycling and the fate of contaminants. Such environments include shallow aquifers, river floodplains, rice paddy soils, wetlands, thawing permafrost soils, hydrothermal hot springs, lake sediments, and many others. These environments are true “hotspots” of biogeochemical activity that control element cycles. The redox cycling of organic carbon and nitrogen not only drive the micro- and macro-biological communities, but also have implications for global nutrient balances and climate change. For example, methane emission from wetlands, rice paddies, and thawing permafrost soils contributes significantly to the overall greenhouse gas budget on Earth. Riparian floodplain soils can act either as sinks or sources of nutrients and contaminants, and important contaminant transformations take place in such environments (e.g., Hg-methylation, sulfide formation, or sulfide oxidation). Biogeochemical redox processes occurring in these environments play a key role in controlling the behavior of inorganic and organic contaminants. For example, bacteria can reduce or oxidize many metals (e.g., Fe, Mn, Cu, Hg), metalloids (e.g., As), and nonmetals (e.g., Se) with important consequences for their chemical speciation, mobility, and toxicity. Similarly, the migration behavior of radionuclides (e.g., U, Pu) is often directly affected by abiotic or biotic redox processes. Biogeochemical redox processes also play an important role in the degradation of organic pollutants in the environment. The redox cycling of Fe and Mn mineral phases has been shown to be of particular importance for understanding contaminant fate and transport in a wide variety of environmental systems. For example, redox cycling of contaminants and host soil mineral phases can facilitate transport in surface water and groundwater posing a threat to water quality, as has been shown 10.1021/es903264z
2010 American Chemical Society
Published on Web 12/30/2009
for a variety of different compounds (e.g., As, U, Cr, P). Because of their ubiquity and redox reactivity, reactions involving Fe- and Mn-minerals can be the key to understanding contaminant mobility in soil/sediment systems. Research into biogeochemical redox processes is more than a purely intellectual pursuit; a profound understanding of redox processes can be utilized for developing novel remediation strategies. Of notable success is zerovalent Fe permeable reactive barriers as a cost-effective method for treating groundwater for a variety of (chloro)organic compounds, nitrate, and reductive precipitation of certain trace metals and radionuclides (e.g., Cr and U). There has also been considerable interest in field-scale in situ biostimulation for sequestration of trace metals and radionuclides from groundwater. Native metal-reducing microbial communities are stimulated by amending groundwater with an electron donor such as ethanol or acetate, resulting in reductive precipitation of elements which are immobile when reduced, such as U and Cr. In addition, understanding biogeochemical redox conditions that lead to As mobilization in groundwater, or that can be used for As removal from drinking water, is helping to mitigate the health crisis in southeast Asia, where millions of people in Bangladesh, India, Cambodia, and Vietnam are exposed to high concentrations of naturally occurring As. The complexity inherent in environmental systems has and will continue to pose major challenges, but recent developments in analytical techniques have yielded new opportunities for a greater depth of understanding of biogeochemical redox processes. The role of spatial heterogeneity, the kinetics of coupled redox processes, the dynamics of microbial communities, electron transfer mechanisms, and redox-induced mineral transformations are only some of the future research needs. The availability of synchrotron-based spectroscopy and mapping techniques has revolutionized our ability to observe redox transformations at the micrometer to molecular scale. Various novel in situ electrode and gel probe techniques allow for direct, field-scale observation of redox processes with minimal disturbance of the system of interest. In addition, advances in instrumentation and sample preservation have increased our ability to detect redox-sensitive or ultratrace quantities of important species in the environment. A combination of field studies, novel experimental approaches, analytical techniques, and advanced computational tools will lead to exciting new findings over the coming years. The enormous progress made in many aspects of redox biogeochemistry in recent years has motivated us to proJanuary 1, 2010 / ENVIRONMENTAL SCIENCE & TECHNOLOGY 9 3
pose a Focus Issue in Environmental Science & Technology. Here then, this January 1, 2010 issue presents a series of experimental and theoretical research articles covering multiple scales that illustrate why biogeochemical redox processes are so important in controlling the fate and transport of both organic and inorganic contaminants in the environments. The research articles are accompanied by two critical reviews: one providing a more general overview of the field, and the other addressing energetic constraints on terminal electron accepting processes in anoxic environments. The papers presented herein cover a wide range of environmentally important topics. For instance, it is clearly illustrated how pure minerals (e.g., Fe- and Mnminerals) and anoxic sediments can cause redox transformations of As, Cr, U, and nitrobenzene resulting in a change in their mobility, stability, and/or toxicity. Research also shows how humic substances and nutrients (i.e., nitrate) can impact As mobilization and Tc bioremediation, respectively. Another study reports the importance of why we need to recognize the impact of diffusion-limited systems (e.g., aggregates) with respect to Fe reduction and ultimately contaminant fate. Laboratory-based experiments frequently do not account for the complexity of natural systems, since they often target a single component (e.g., Fe[III] as the only electron acceptor) resulting in an overestimation of the bioreduction kinetics, as reported in this Focus Issue. Finally, several papers demonstrate advances in our ability to measure in situ reductive dissolution rates and microbial activity during bioremediation as well as a new quick scanning X-ray absorption spectroscopy technique for determining redox kinetics and a novel nuclear imaging approach for probing the biogeochemical behavior of Tc. From photochemistry to metal-reducing bacteria, the flow of electrons unites the wide variety of processes addressed in this issue. Given the diversity and importance of biogeochemical redox processes, it is clear that this will continue to be an active, exciting area of research.
the Department of Soil Science at North Carolina State University, where he specialized in soil chemistry and mineralogy. His research group (www. soilchem.ethz.ch) is primarily concerned with the chemical behavior and cycling of trace elements in soils and sediments. Some past and current research topics include colloid aggregation and colloid-facilitated transport in natural porous media, competitive sorption of trace metals to minerals and humic substances, speciation of trace elements using synchrotron X-ray absorption spectroscopy, and dissolution of oxide and silicate minerals in the presence of organic ligands. Currently, the group strongly focuses on the biogeochemistry of metals and metalloids in periodically anoxic soils, such as contaminated river floodplains and irrigated rice paddies. Kretzschmar has been an associate editor of ES&T since September 2007.
Ruben Kretzschmar ETH Zurich.
Thomas Borch is an assistant professor of environmental chemistry and biogeochemistry at Colorado State University (CSU) in the Department of Soil and Crop Sciences. Borch, who also holds a joint appointment to the Department of Chemistry at Colorado State, is a member of CSU’s School of Global Environmental Sustainability. He earned his Ph.D. degree in 2004 in the Department of Land Resources and Environmental Sciences at Montana State University. Following his graduate studies, he worked as a postdoctoral fellow in the Soil and Environmental Biogeochemistry group at Stanford University. His research group is primarily concerned with biogeochemical Fe cycling, C sequestration, and determination of contaminant fate and transport. Some current research topics include climate change impacts on the interrelationship between Fe cycling and organic matter, Fe-induced redox transformations of As, U, and explosives using synchrotron radiation-based techniques, quantum chemical prediction of degradation pathways for new contaminants, and (bio)degradation of steroid hormones. (For more information, visit www.soilcrop.colostate.edu/borch/home.html). Borch won the prestigious NSF CAREER Award in 2009.
Ruben Kretzschmar is a full professor of soil chemistry in the Institute of Biogeochemistry and Pollutant Dynamics, Department of Environmental Sciences at ETH Zurich. He earned his Ph.D. degree in 1994 in
Kate Campbell is currently a research biogeochemist at the U.S. Geological Survey (USGS) in Boulder, CO. She received her Ph.D. in environmental science and engineering at the California Institute of Technol-
Thomas Borch* Colorado State University, Fort Collins, Colorado
[email protected] Kate Campbell U.S. Geological Survey, Boulder, Colorado
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ogy in Pasadena, investigating sediment diagenesis, biogeochemical redox cycling, and sorption controls of As mobilization in sediment porewaters. As a National Research Council Postdoctoral Scholar at the USGS in Menlo Park, CA she was involved in a collaborative study of U bioremediation in contaminated groundwater. Her current research focuses on understanding the roles of microbiology and geochemistry in the redox cycling of trace elements, especially As in geothermal systems and acid mine drainage, while specializing in novel, in situ, field-based techniques.
Andreas Voegelin is a senior researcher at the Swiss Federal Institute of Aquatic Science and Technology (Eawag), and heads the new research group “Molecular Environmental Geochemistry”. He received his Ph.D. in environmental sciences from the Swiss Federal Institute of Technology (ETH) Zurich in 2001. Before his appointment to Eawag in 2009, he worked as a research scientist at the Institute of Biogeochemistry and Pollutant Dynamics at ETH Zurich. His current research addresses various aspects of major and trace element biogeochemistry, such as long-term changes in metal speciation and reactivity in contaminated soils, accumulation and impact of As in paddy fields in southeast Asia, or colloid formation and trace element fate in redoxdynamic environments. Voegelin specialized in synchrotron-based X-ray techniques for the characterization of molecular-level element speciation to elucidate mechanisms controlling trace element behavior in terrestrial and aquatic systems. He has published more than 25 peer-reviewed articles in journals such as ES&T, Geochimica et Cosmochimica Acta, and Nature Geoscience.
Matthew Ginder-Vogel is an associate scientist in the Delaware Environmental Institute at the University of Delaware. Before joining the Institute, GinderVogel worked as postdoctoral researcher with Dr. Donald Sparks at the University of Delaware. He received his Ph.D. in soil and environmental biogeochemistry from Stanford University. Ginder-Vogel’s research seeks to define the fundamental biogeochemical processes controlling the dynamics of nutrients and contaminants within the complex media we denote as soils.
His research combines field-based experiments with simplified lab-based experiments to reveal the dominant biogeochemical mechanisms affecting element mobility in environmental systems. He uses traditional spectroscopy and microscopy methods, as well as state-of-theart methods like synchrotron radiation-based X-ray microscopy, spectroscopy, and diffraction techniques. Ginder-Vogel recently published details of a new X-ray spectroscopy technique used to measure rapid environmental redox reactions in the Proceedings of the National Academy of Sciences of the United States of America and continues to push the boundaries of environmental biogeochemistry.
Kai Uwe Totsche is full professor of hydrogeology at the Institute for Geosciences, Friedrich-SchillerUniversity Jena, Germany. After studying environmental sciences at the University of Bayreuth, Germany, and spending one year as a visiting researcher at the Wageningen Agricultural University in the Netherlands, he earned his Ph.D. in natural sciences from the University of Bayreuth. During his graduate studies, he specialized in the physical chemistry of soils and mathematical models of flow, transport, and transformation processes in natural porous media. His early research investigated the effect of dissolved and colloidal organic matter on flow and transport of nutrients and contaminants in the vadose zone and aquifers. Later research focused on organic contaminant sorption to (mobile) heterogeneous natural sorbents and understanding the mutual effect of mobile organo-mineral geosorbents on flow and transport in natural porous media. At present, he is speaker of the priority program Biogeochemical Interfaces in Soil (www.spp1315.uni-jena.de).
Johannes A. C. Barth is a full professor and chair of applied geology at the Friedrich-Alexander-University Erlangen-Nu ¨rnberg, Germany. After completing his Ph.D. in 1998 at the University of Ottawa he held a twoyear postdoctoral position at the Queens University of Belfast and subsequently a four-year lectureship in environmental geology at the Scottish Universities Environmental Research Centre in Glasgow. He then coordinated the EU project AquaTerra that hosted 45 partner January 1, 2010 / ENVIRONMENTAL SCIENCE & TECHNOLOGY 9 5
institutions (www.eu-aquaterra.de/). Selected awards include an invitation to the DFG Conference “Earth, Fire, Water, Air and Life” for leading young scientists and the Don Rennie Memorial Award for excellence in research and presentation. His work focuses on river geochemistry, groundwater-surface water interaction,
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deep aquifers, and turnover of organic pollutants with geochemical and stable isotope techniques. This led to basin-wide evaluation of transpirational water loss, quantification of turnover rates of pollutants, as well as C and oxygen balances in surface and groundwater systems.
