PROBING THE ASSOCIATION OF FLUOROBENZENE WITH DISSOLVED ORGANIC-MATTER USING NMR-SPECTROSCOPY

We dissolved and fractionated the commercially available Aldrich humic acid (AHA) into less than 1000 and less than 8000 atomic mass unit (a mu) fractions and used H-1 NMR to analyze the chemical forms of dissol ved organic matter (DOM) of these two AHA fractions in D2O. The range of linewidths of the signals observed in the H-1 NMR spectra of the di fferent molecular mass fractions suggests that DOM in the AHA solution s are mixtures of organic molecules of different molecular sizes. The spectrum of the less than 8000 amu AHA solution exhibited boarder line widths and more pronounced aromatic character than the less than 1000 amu AHA solution. The association of DOM in these two AHA fractions an d fluorobenzene (C6H5F) in D2O solutions as indicated by the translati onal mobility (self-diffusion) of fluorobenzene was examined using F-1 9 pulsed field gradient spin echo NMR. One fraction of the C6H5F molec ules was found to have a diffusion coefficient comparable to that of f luorobenzene dissolved in AHA-free D2O and was assigned to the unbound Ca6H5F. The remainder, fluorobenzene, has a self-diffusion coefficien t considerably smaller than that in AHA-free D2O and was assigned to t he DOM-bound fractions. The magnitude of the self-diffusion coefficien ts of AHA-bound C6H5F decreases with increase in amu of the AHA fracti ons, suggesting sorption as a mechanism of C6H5F-DOM association. (C) 1997 Published by Elsevier Science Ltd.