A potential mechanism for the interactions of dissolved humic acids wi
th nonpolar micropollutants was assessed by monitoring the fluorescenc
e of pyrene added to aqueous humic acid solutions. The results support
the concept of the cation-assisted formation of intramolecular domain
s in humic acids and a partitioning mechanism for probe sequestration
in these domains. Fluorescence enhancement of pyrene in the presence o
f different humic acids showed significant variations, leading to the
conclusion that large soil humic acids are more effective in isolating
small molecules than smaller aquatic or structurally rigid humic acid
s. Solution variables such as ionic strength, pH, and type of cation p
resent also exerted an influence. Stern-Volmer plots showed that salts
cause humic acid to become less quenching towards pyrene fluorescence
, which is ascribed to a change in the quenching mechanism. Precipitat
ion studies indicated that the charge of the cation is the most import
ant parameter in the salt-promoted pseudomicellization of humic acids.