Binding of Phenol and Differently Halogenated Phenols to Dissolved Humic Matter As Measured by NMR Spectroscopy

H-1- and F-19-NMR measurements of spin-lattice (T-1) and spin-spin (T-2) relaxation times and diffusion ordered spectroscopy (DOSY) were applied to investigate the association of nonsubstituted (phenol (P)) and halogen-substituted (2,4-dichlorophenol (DCP); 2,4,6-trichlorophenol (TCP), and 2,4,6-trifluorophenol (TFP)) phenols with a dissolved humic acid (HA). T-1 and T-2 values for both H-1 and F-19 in phenols decreased with enhancing HA concentration, indicating reduction in molecular mobility due to formation of noncovalent interactions. Moreover, correlation times (tau(C)) for different hydrogen and fluorine atoms in phenols showed that anisotropic mobility turned into isotropic motion with HA additions, Changes in relaxation times suggested that DCP and TCP were more extensively bound to HA than P and TFP. This was confirmed by diffusion measurements which showed full association of DCP and TCP to a less amount of HA than that required for entire complexation of P and TFP, Calculated values of binding constants (Ka) reflected the overall NMR behavior, being significantly larger for DCP- and TCP-HA (10.04 +/- 1.32 and 4.47 +/- 0.35 M-1, respectively) than for P- and TFP-HA complexes (0.57 +/- 0.03 and 0.28 +/- 0.01 M-1, respectively). Binding increased with decreasing solution pH, thus indicating a dependence on the fraction of protonated form (alpha) of phenols in solution. However, it was found that the hydrophobicity conferred to phenols by chlorine atoms on aromatic rings is a stronger drive than alpha for the phenols repartition within the HA hydrophobic domains.