Use of EPR To Monitor the Distribution and Availability of Organic Xenobiotics in Model Soil Systems ALAIN DUMESTRE, †,‡ MURRAY MCBRIDE, § AND PHILIPPE BAVEYE* ,† Laboratory of Environmental Geophysics, Department of Crop and Soil Sciences, Bradfield Hall, Cornell University, Ithaca, New York 14853, Centre de Pe ´dologie Biologique, CNRS, UPR n °6831 associe ´e a ` l’Universite ´ Henri Poincare ´ (Nancy I), 54501 Vandoeuvre-le `s-Nancy, France, and Departm ent of Crop and Soil Sciences, Bradfield Hall, Cornell University, Ithaca, New York 14853 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. Introduction Projected costs associated with the reclamation of contami- nated subsurface environments are rising at a vertiginous rate in the U.S. and in most other industrial countries. As a result, the question of “how clean is clean enough?” in polluted soils and sediments has become in recent years the object of considerable attention among scientists and practitioners as well as the cause of significant concern for the public at large (1-4). In the ongoing debate on the determination of “environmentally acceptable endpoints” for remediation efforts (2, 3), some researchers argue that a portion of the toxic compounds present in contaminated soils is sequestered by the soil matrix, is not available to organisms,smallorbig,and istherefore “safe”,forallpractical purposes. Other investigators contend that, even though contaminants seem partially accessible to the microflora under present conditions, they could still be harmful if left in place, in particular since they might slowly leach from the soil and pollute groundwater, or because some organism might come along at some point in the future, which will be able to dislodge the contaminants from the soil matrix. To assess the validity ofeither one ofthese two divergent viewpoints, information is needed on the mechanisms responsible for the alleged sequestration of xenobiotics in natural porous media and on the time frame in which these mechanisms are reversible. Progress in this direction is hampered by the wide variety ofchemicals to be considered (5) as well as by the large variability of soils and other environmentspolluted byxenobiotics (6).Arguablythemost severe impediment, however, arises from the inadequacy of the various experimental techniques presently available to researchers.Even when recoveryrates are good and artifacts are successfully avoided, physical separation methods and extractions with organic solvents (7, 8) provide data only on the average status of organic contaminants in soils. Experi- ments designed to provide information on specific seques- tration mechanisms are generally carried out on dispersed soilsuspensionsin batch experiments.The kineticsand phase partitioning observed under these conditions may not have much in common with those in undisturbed soils (9). To make sense ofthe maze ofprocesses that controls the distribution and fate oforganic pollutants in naturalporous media (10), a measurement technique is needed that can discriminate between xenobioticmoleculesin theirdifferent molecular-scale environments, without disturbing the mi- crostructure of the solid matrix. Within some constraints, electron paramagnetic resonance (EPR) spectroscopy is ideally suited to meet these requirements. In model soils designed to be asfree aspossible ofparamagneticfunctional groups or elements (e.g., Fe, Mn), EPR can monitor the immediate environment oftargeted paramagneticmolecules. These compounds may be common xenobiotics (e.g., pesticides) that are labeled with a paramagnetic group. Alternatively, they can be paramagnetic (spin) probes, with a stable free-radical center and which may be modified to mimic common organic chemicals. Some of the most popular kinds of spin probes contain paramagnetic nitroxyl radicals and are known as nitroxide spin probes (11). They are characterized by high chemical stability. In addition, commercially available nitroxide spin probesexhibit a wide range ofchemicalproperties(12),such as polarity and charge, and thus can represent the main categories of organic chemicals found in soils (5). Further- more,the fact that these moleculescan be degraded byoxido- reduction reactions is of primary importance to study their dynamics in complex systems. For instance, chemical transformation of the nitroxide probes can be achieved by addition of sodium ascorbate, which is able to reduce the nitroxyl paramagnetic group into a hydroxylamine diamag- netic group, nondetectable by EPR. The disappearance of the EPRsignalgives a direct measurement ofthe degradation of the probe (13, 14). Therefore, besides providing useful information on the interaction of organic compounds with surfaces, the spin probe technique also makes it possible to monitor the kinetics of organics transformation in a given environment. *Correspondingauthor phone: (607)539-6456;fax: (607)255-8615; e-mail: pcb2@cornell.edu. † Laboratory of Environmental Geophysics, Department of Crop and Soil Sciences, Cornell University. ‡ Centre de Pe ´dologie Biologique, CNRS, UPR n °6831 associe ´e a ` l’Universite ´ Henri Poincare ´ (Nancy I). § Department of Crop and Soil Sciences, Cornell University. Environ. Sci. Technol. 2000, 34, 1259-1264 10.1021/es990824k CCC: $19.00  2000 American Chemical Society VOL. 34, NO. 7, 2000 / ENVIRONMENTAL SCIENCE & TECHNOLOGY 9 1259 Published on Web 03/03/2000