Organic compounds exhibit specific isotopic compositions that can be used as their “fingerprint”. Environmental chemists nowadays exploit changes of isotopic compositions to identify the origin of organic pollutants and to assess their (bio)degradation in the environment by compound-specific stable isotope analysis.
Researcher of ETH Zurich have recently compiled a focus issue in the journal “Environmental Science and Technology”, which documents the significant progress made in this area in recent years.
Human activity results in the production and use of enormous amounts of chemicals which – intentionally or not – can be released into the environment. Examples include various pesticides applied in agriculture to combat fungi, insects or weeds, groundwater pollution with fuel constituents from leaking underground tanks, and pharmaceuticals entering aquatic environments via wastewater treatment plants effluents or from farm fields where antibiotic-containing manure is spread.
Dilution versus degradation
Environmental chemists deal with the fate of such chemicals in the environment. They address important questions such as the rates of pollutant transformation and the identity of the resulting breakdown products. However, assessing transformation processes in complex environments is a challenging task. Pollutant concentrations not only decrease due to degradation but also due to dilution or adsorption to the solid matrix. Such an apparent disappearance of a pollutant from at given system might be mistaken for transformation and, additionally, make it very difficult to quantify how much of the contamination is effectively gone.
In an feature article in the focus issue on “Stable Isotopes” of the journal “Environmental Science & Technology”, Thomas Hofstetter, René Schwarzenbach and Ruben Kretzschmar from the Institute of Biogeochemistry and Pollutant Dynamics and Stefano Bernasconi from the Geological Institute summarize the potential of compound-specific isotope analysis as a tool for the identification of contaminant sources and for assessing reactions of organic chemicals in the environment. In additional 16 research papers, a broad variety of authors present the current state of the art in this area.
Stable isotope fingerprints
Analytical methods for compound-specific isotope analysis have emerged from Geosciences and are now widely used in various disciplines including Environmental Sciences. This method is based on an element’s specific isotopic signature. For example, carbon has two stable isotopes, 12C and 13C, that do not decay like radioactive 14C. At natural abundance, these isotopes exist in a typical ratio. Any persistent compound carries a typical isotopic composition (fingerprint) that originates from the precursor materials from which it was synthesised. This allows one, for example, to distinguish between natural and man-made contaminants or enables the allocation of pollutant sources to individual manufacturers.
One particularly important feature of isotopic analysis from an environmental chemist’s perspective is that variations of isotopic compositions (the so-called isotope fractionation) in individual contaminants enable the identification of degradation processes. The latter can even be quantified – regardless of whether a compound’s concentration is diluted. Depending on the magnitude of the isotope fractionation, conclusions might be drawn about the underlying degradation reaction and the resulting degradation products. For example, typical shifts in isotope signatures make it possible to differentiate between microbial vs. photochemical transformation of pesticides such as atrazine at plant surfaces or in rivers and lakes.
Broad application stimulates dialogue
For Thomas Hofstetter, senior lecturer at the Institute of Biogeochemistry and Pollutant Dynamics and one of the guest editors of the focus issue, compound-specific isotope analysis is a key technique for studying important processes in many research areas. Hofstetter also thinks that, “the scientific discussion about the origins of isotope effects and its applications encourages dialogue between ‘pure’ chemists, environmental and earth scientists, and ecologists. There is much to learn from each another.”
Scientists from different fields laid the foundations for the focus issue of ES&T during an interdisciplinary workshop on the Monte Verita in November 2007. At this occasion, the abovementioned “mixture” of scientists recognized how isotope analysis and insights from isotope fractionation can be used in various disciplines. The focus issue, however, contains mainly contributions from environmental chemistry and geochemistry. Hofstetter explains that, “The basic principles of these methods are the same. We differ predominantly in the time horizon and the level of sophistication in which we need to understand our processes. In environmental chemistry, this means hours to decades, in chemistry it is often only fractions of a second, and questions from earth sciences can also cover considerably longer time periods.”
Literature reference: Focus issue of ES&T, Vol. 42 Issue 21: http://pubs.acs.org/toc/esthag/42/21 Edited by Thomas B. Hofstetter, Stefano M. Bernasconi, René P. Schwarzenbach and Ruben Kretzschmar (DOI: 10.1021/es8027742)
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