Use of EPR To Monitor the Distribution and Availability of Organic Xenobiotics in Model Soil Systems

An array of processes appears to control the bioavailability of xenobiotics in soils and sediments. To untangle them, it would be useful to have direct information about the molecular-scale environment of xenobiotics in natural porous media. This article presents a preliminary investigation of the extent to which electron paramagnetic resonance (EPR) spectroscopy can provide this information. The fate and spatial distribution of two nitroxide spin probes (Tempol and Tempamine + ) are monitored in batch experiments involving Ca-hectorite suspensions and pastes. In these systems, EPR is able to discriminate between probe molecules in different environments (e.g., adsorbed, in bulk solution or in large intersticial pores). Addition of sodium ascorbate causes the chemical degradation of the probes in the bulk solution and allows the kinetics of release of the probes from the clay aggregates and/or paste to be monitored in time. In all cases, the release proceeds to completion in less than a day, indicating that the probes are not durably sequestered by the hectorite. Prospects for further study of the applicability of EPR spectroscopy to more complex systems are briefly outlined.