Modelling of isotope effects measured by quantitative NMR in the monitoring of soil contamination by gasoline-derived pollutants – ISOTO-POL

Better methodologies appear to be necessary for detecting the origins of given pollutants, and for understanding the natural processes involved in their migration routes, biodegradation and eventual disappearance. In addition, it is essential to check whether treatment leads to complete mineralization or if cleaning of the polluted area is effective. A fundamental tool that can be used in the management of a pollution to provide indicators as to containment, spread and bioremediation possibilities is proposed in the project. As a complete illustration of this principle, ethers used as octane enhancers in gasoline are given as examples.
The frequent detection of the ethers used as octane enhancers in gasoline (methyl and ethyl tert-butyl ethers (MTBE; ETBE) and tert-amyl methyl ether (TAME)) in surface and ground water is a major concern. Isotopes have been used successfully to demonstrate the natural or enhanced biodegradation of petroleum hydrocarbons, solvents, and gasoline additives in soil and groundwater. So far, overall isotopic fractionation in 2H and 13C have been exploited, the isotope content being measured by isotope ratio mass spectrometry (irm-MS), but this leads to the loss of valuable site-related information. In contrast, NMR spectrometry offers the capability to measure site-by-site both the 2H/1H and the 13C/12C ratios, giving new information. NMR has successfully been applied to 2H at natural abundance for a wide range of chemicals over the last 25 years. An equivalent technique for 13C has been very challenging to develop but a sufficiently robust and precise method is now available. This new tool has already proved its capability to detect unexpected effects, such as non-covalent isotope effects, and to study bio transformations.
ISOTO-POL will associate this technically and methodologically new isotopic approach with model construction to provide a unique means to allow detailed detection and tracing of a given pollutant. To develop the necessary tool, three capabilities brought by the partners are required: isotope ratio measurements by NMR; modelling abiotic degradation of ethers; modelling biotic transformation of ethers. The work has been divided into four main tasks: (1) Measurement of the site-specific isotope abundance of 13C and 2H by quantitative NMR and how this is influenced by different purification techniques, manufacturing methods and precursors; (2) Influence on the site-specific isotope abundance of 13C fractionation in the ethers and their products due to physical interactions (gas-phase diffusion, volatilisation, partitioning between a solid matrix and a fluid phase) and chemical transformation processes (acid hydrolysis, chemical oxidation); (3) Assessment of the site-specific isotope abundance of 13C and/or 2H in the products of specific microbial degradation of the ethers and the relation between microbial environment and the resulting isotopic signatures; (4) Design of a reactive transport model that is capable of predicting the isotopic changes in ethers along a contaminant plume of gasoline in groundwater or in soil.
The innovative results are expected to be the following: (i) database of site-specific isotope composition of ethers from different manufacturing processes; (ii) fundamental understanding of the processes that are known to lead to fractionation: biodegradation, hydrolysis, sorption, volatilisation, and whether they act on isotopes at specific sites of the ethers; (iii) database on site-specific fractionation factors; (iv) substantial contribution to the assessment, comprehension and modelling of non-covalent isotope effects; (v) reactive transport- and fate model capable of predicting isotope ratios as a function of time, space, and processes acting on the contaminant.