Determination of covalent and noncovalent binding interactions between xenobiotic chemicals and soil

Knowledge of the fate of xenobiotics in terrestrial systems is an important aspect of soil science, This paper reviews experimental approaches that have enhanced our understanding of binding interactions between xenobiotic chemicals and soil, First attempts to evaluate the nature of binding focused on the identification of covalently bound or physically 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 model studies in which xenobiotic chemicals, such as phenols or anilines, were allowed to interact with monomeric constituents of humus (e.g., syringic acid or guaiacol). Further studies involved C-13 or N-15 NMR analysis of C-13- or N-15-labeled xenobiotics that were bound to natural humic acid or soil, The resolution of NMR spectra was greatly improved by silylation of the soil samples and application of C-13-depleted humic materials, NMR spectroscopy in combination with silylation was also instrumental in the evaluation of physically sequestered chemicals, A coherent theory of sequestration (dual-mode sorption model) was developed based on adsorption isotherms obtained in experiments involving long contact times between xenobiotics and soil. The future of research on binding appears to depend largely on NMR spectroscopy and further progress in our knowledge of humus.