The micellar model of humic acid: Evidence from pyrene fluorescence measurements

A series of studies has shown that the fluorescence intensity of pyrene dis solved in certain humic acid (HA) solutions increased significantly when sa lts were added, even those of highly quenching anions, The phenomenon was f ound to depend on the cation of the salt, generally varying with its charge and quenching characteristics. The average molecular size of the HA in que stion, and probably its flexibility, were also important parameters: smalle r aquatic HAs, or those that were photolytically cleaved, did not produce n otable probe fluorescence enhancement. Increased emissions were also observ ed when HA/pyrene solutions were heated, indicating a parallel with the clo uding phenomenon found in solutions of nonionic synthetic surfactants, The I-1/I-3 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 exten ts, 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 ad dition of salt suggested that the metal ions migrated within the humic stru cture 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 disso lved HAs in which the humic polymers form largely intramolecular domains of relatively low polarity that have the ability to sequester the small hydro phobic species that partition into them.