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 direc t information about the molecular-scale environment of xenobiotics in natur al porous media. This article presents a preliminary investigation of the e xtent to which electron paramagnetic resonance (EPR) spectroscopy can provi de this information. The fate and spatial distribution of two nitroxide spi n probes (Tempol and Tempamine(+)) are monitored in batch experiments invol ving Ca-hectorite suspensions and pastes. In these systems, EPR is able to discriminate between probe molecules in different environments (e.g., adsor bed, in bulk solution or in large intersticial pores). Addition of sodium a scorbate 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 applicabil ity of EPR spectroscopy to more complex systems are briefly outlined.