DETERMINATION OF COVALENT AND NONCOVALENT BINDING INTERACTIONS BETWEEN XENOBIOTIC CHEMICALS AND SOIL

Knowledge of the fate of xenobiotics in terrestrial systems is an impo rtant aspect of soil science, This paper reviews experimental approach es that have enhanced our understanding of binding interactions betwee n xenobiotic chemicals and soil, First attempts to evaluate the nature of binding focused on the identification of covalently bound or physi cally sequestered chemicals upon their removal from the soil matrix by alkaline or acid hydrolysis and other release techniques (e.g., high- temperature distillation, supercritical fluid extraction or microwave extraction), The covalent nature of bonds formed was confirmed by mode l studies in which xenobiotic chemicals, such as phenols or anilines, were allowed to interact with monomeric constituents of humus (e.g., s yringic acid or guaiacol). Further studies involved C-13 or N-15 NMR a nalysis of C-13- or N-15-labeled xenobiotics that were bound to natura l humic acid or soil, The resolution of NMR spectra was greatly improv ed by silylation of the soil samples and application of C-13-depleted humic materials, NMR spectroscopy in combination with silylation was a lso instrumental in the evaluation of physically sequestered chemicals , A coherent theory of sequestration (dual-mode sorption model) was de veloped based on adsorption isotherms obtained in experiments involvin g long contact times between xenobiotics and soil. The future of resea rch on binding appears to depend largely on NMR spectroscopy and furth er progress in our knowledge of humus.