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A series of studies has shown that the fluorescence intensity of pyrene dissolved in certain humic acid (HA) solutions increased significantly when salts were added, even those of highly quenching anions. The phenomenon was found to depend on the cation of the salt, generally varying with its charge and quenching characteristics. The average molecular size of the HA in question, and probably its flexibility, were also important parameters: smaller aquatic HAs, or those that were photolytically cleaved, did not produce notable probe fluorescence enhancement. Increased emissions were also observed when HA/pyrene solutions were heated, indicating a parallel with the clouding phenomenon found in solutions of nonionic synthetic surfactants. The I1/I3 ratio of the pyrene emission spectrum showed that the probe entered a more hydrophobic environment when salt was added if a suitable HA solute was present. HAs immobilized pyrene in aqueous solution to different extents, again depending on the nature of the HA and the presence of metal ions. The evolution of pyrene fluorescence profiles in HA solutions after the addition of salt suggested that the metal ions migrated within the humic structure and that the integrity of the enhanced humic micelles was compromised when the ions formed stable inner sphere complexes with the organic matter. All observations were consistent with a pseudomicellar structure of dissolved HAs in which the humic polymers form largely intramolecular domains of relatively low polarity that have the ability to sequester the small hydrophobic species that partition into them.

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