New methods for the environmental chemist’s toolbox Assessing transformation processes of organic and inorganic chemicals in complex environments is a difficult task. Take, for example, organic soil and groundwater contaminants. Nowadays, even though the subtle variations of the contaminants’ concentrations can be measured, it is often unclear whether an observed concentration decrease is due to transformation or just a consequence of dilution or hydrodynamic dispersion. In addition, in the case of transformation, one needs to identify the underlying processes to be able to quantitatively assess the risks associated with environmental contamination and to take the appropriate remediation measures. As far as inorganic compounds are concerned, similar questions arise when one tries to elucidate whether and to what extent processes such as sorption, precipitation, and oxidation-reduction reactions affect the fate of metals or govern the metals’ biogeochemical cycling. This focus issue of ES&T presents a series of disparate experimental and theoretical papers that illustrate the usefulness of stable-isotope-based approaches in tackling at least some of the abovementioned questions. The analysis of changes in stable-isotope ratios in individual compounds offers an exceptionally powerful avenue by which to study whether and how biotic and abiotic reactions happen. The outline of the essentials of this approach for organic compounds is given in an accompanying feature article. Because analytical methods for isotopic analysis of various compounds and different elements are developing rapidly, the evaluation of isotope effects might soon become one of the key methods in the environmental chemist’s toolbox. The papers presented in this issue follow this pathsfrom studies on traditional compounds of the early days of isotope analysis, such as aromatic hydrocarbonssto the most recent developments, which illustrate that stable H, C, N, and O isotopes might become commonly used to pin down the transformation of structurally more complex compounds, such as pesticides or explosives. Moreover, isotope effects allow the deciphering not only of degradation but also of the formation pathways of undesirable compounds as they happen, such as during the treatment of drinking water.
In fact, stable-isotope tools are very versatile and thus widely used in scientific research across many disciplines from physical chemistry to ecology. The level of sophistication with which isotopic tools are applied, of course, varies greatly. The underlying principles leading to the observed isotope effect, however, do not. This insight was one of many outcomessalong with this focus issuesof an interdisciplinary, international workshop on stable-isotope tools held in November 2007 in southern Switzerland. Although the language used to document an isotope effect can be quite distinct in different disciplines, there were many more essential things to share. First, there is something like an isotopic food chain! Physical chemists’ knowledge about isotope tools can be applied to problems tackled by environmental chemists, and the work of the former might explain puzzling results found by the latter. This insight as such is nothing new. But in the case of isotope effects, the common denominator is large enough that there can be a mutual exchange of knowledge. Second, we can learn from one another by going up and down the isotopic food chain. Knowledge transfer occurs not only from basic to more applied research but also in the opposite direction. Did you know that for chemists, the elucidation of reaction mechanisms with isotope effects often implies a comparison between reactions of isotopically labeled versus nonlabeled substances? However, as environmental and earth scientists are forced to do, one can learn a lot about reaction mechanisms by studying isotope effects at the naturalabundance isotope concentrations. There seem to be numerous unexplored possibilities to further the understanding of isotope tools and to answer some burning questions on transformation processes. Or, as one workshop participant said, “Working together will make us rich and famous!” That’s an ambitious goal, though. Maybe publishing additional interdisciplinary papers on this topic in ES&T is a first step in this direction!
Because organic compounds do not necessarily occur as contaminants, there is little difference between studying isotope effects of hydrocarbons as contaminants during diffusion or degradation versus studying hydrocarbons as organic material that fuels biogeochemical processes. Examples covered in this issue are the coupling of benzene or acetate oxidation to biogeochemical sulfur cycling and the oxidation of methane in pristine and contaminated environments.
Stefano M. Bernasconi Geological Institute, ETH Zurich
10.1021/es802771t
2008 American Chemical Society
Published on Web 10/30/2008
Thomas B. Hofstetter Institute of Biogeochemistry and Pollutant Dynamics, Swiss Federal Institute of Technology (ETH) Zurich
Rene´ P. Schwarzenbach Institute of Biogeochemistry and Pollutant Dynamics, ETH Zurich Ruben Kretzschmar Department of Environmental Sciences, ETH Zurich November 1, 2008 / ENVIRONMENTAL SCIENCE & TECHNOLOGY 9 7727
