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 (amu) fractions and used 1H NMR to analyze the chemical forms of dissolved organic matter (DOM) of these two AHA fractions in D2O. The range of linewidths of the signals observed in the 1H NMR spectra of the different molecular mass fractions suggests that DOM in the AHA solutions are mixtures of organic molecules of different molecular sizes. The spectrum of the less than 8000 amu AHA solution exhibited boarder linewidths and more pronounced aromatic character than the less than 1000 amu AHA solution. The association of DOM in these two AHA fractions and fluorobenzene (C6H5F) in D2O solutions as indicated by the translational mobility (self-diffusion) of fluorobenzene was examined using 19F pulsed field gradient spin echo NMR. One fraction of the C6H5F molecules was found to have a diffusion coefficient comparable to that of fluorobenzene dissolved in AHA-free D2O and was assigned to the unbound C6H5F. The remainder, fluorobenzene, has a self-diffusion coefficient considerably smaller than that in AHA-free D2O and was assigned to the DOM-bound fractions. The magnitude of the self-diffusion coefficients of AHA-bound C6H5F decreases with increase in amu of the AHA fractions, suggesting sorption as a mechanism of C6H5H-DOM association.