Investigation of the Electrostatic Properties of Humic Substances by FluorescenceQuenching

摘要

A fluorescence quenching technique was employed to explore the electrostaticproperties of fulvic acid (FA) and humic acid (HA). Cationic nitroxides were found to be up to 16 times more effective than neutral analogues in quenching the fluorescence of humic materials. This result is attributed to the enhanced Coulombic attraction of cations to the anionic FA or HA surface and is interpreted as an estimate of surface potential. Reduction of molecular charge at low pH and shielding of charge at high ionic strength produced diminished enhancements, consistent with this interpretation. High molecular weight fractions of HA have a higher apparent surface potential than lower molecular weight fractions, indicating that larger humic molecules may have an enhanced ability to bind metal ions.