Interactions between rotenone and humic acids by means of FT-IR and fluorescence spectroscopies

The aim of this work was to ascertain, on a comparative basis, the compositional, structural and functional differences occurring between three humic acids (HAs), HA S1 (isolated from a Mediterranean brown soil), HA S2 (isolated from a Bavarian brown soil), and HA SR (a Suwannee River standard aquatic HA, purchased from IHSS), and to investigate the influence of their intrinsic properties on the types of binding mechanisms toward the pesticide rotenone. Original HAs and their corresponding HA-rotenone products, obtained by two different interaction protocols, were analyzed for elemental and functional group composition, and spectroscopic techniques, such as Fourier-transform infrared (FT IR) with Fourier self-deconvolution (FSD) and fluorescence both in the single-scan and in three-dimensional modes. The HA S1 sample appeared to be characterized by a greater aromaticity degree and lower polarity with respect to the HA S2, featured by a mixed aromatic/aliphatic character, whereas mainly aliphatic and acidic resulted the HA SR. The data obtained suggested that the low water-soluble, non-polar pesticide rotenone resulted preferentially adsorbed onto HAs by hydrophobic interaction, that was the prevailing mechanism in the order HA S1 > HA S2 >>> HA SR, whereas hydrogen bonds resulted predominant in the opposite order.