The cation-enhanced formation of hydrophobic domains in aqueous humic acids has been shown to be a slow process, consistent with the evolution and disintegration of humic acid configurations over periods lasting from days to weeks. After theaddition of a magnesium salt to a humic acid solution containing pyrene, the probe fluorescence gradually increased and reached a maximum in 2-10 h. The trend then reversed, and the emission intensity decreased until it came close to its original valueafter an extended period. In the model proposed here, this self-reversing process is ascribed to a slow migration of metal ions within the humic structure. When the salt was added, the cations first associated with anionic groups on the humic acidpolymer, partially neutralizing them and engaging in bridging interactions between different parts of the chain. This drew the humic acid molecule together and enhanced the formation of hydrophobic pseudomicelles. These structures were however of atransitory nature, and they disintegrated as the metal ions gradually migrated to locations where they formed stable inner-sphere complexes with functional groups on the humic acid molecule.
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