Influence of chlorination on chromophores and fluorophores in humic substances

Three types of fluorophores (decay times 1.2, 4.3, and 10.7 ns, respectively) participate in the fluorescence of Suwannee River hydrophobic acid (HPOA). The emission of the fast-decaying fluorophores (FDF) is red-shifted compared with that of longer-decaying groups. Upon chlorination, the FDF contribution (I-FDF) to the emission declines, the specific absorbance at 254 nm (SUVA(254)) decreases, and the electron-transfer absorbance band in the UV spectra of HPOA contracts. On the basis of the literature data and experimental evidence, the width of the electron-transfer absorbance band (Delta(ET)) is found to be proportional to the gyration radii (R-g) of the humic molecules. The R-g values of chlorinated HPOA are also well-correlated with the energy of the emission band maximum (E-max) and emission bandwidth (Delta(em)). These consistent changes reflect the destruction of aromatic halogen attack sites accompanied by the breakdown of the humic molecules into smaller fragments.