Pb(II) binding to humic substances: an equilibrium and spectroscopic study

Area of Science:

• Environmental Chemistry
• Spectroscopy
• Biogeochemistry

Background:

• Humic acids are crucial soil organic matter components.
• Understanding metal-humic acid interactions is vital for environmental remediation and contaminant transport.
• Lead (Pb(II)) is a toxic heavy metal with significant environmental implications.

Purpose of the Study:

• To elucidate the binding mechanisms of Pb(II) to humic acids.
• To characterize the structural and spectroscopic changes in humic acids upon Pb(II) complexation.
• To correlate binding affinities with humic acid structural properties.

Main Methods:

• Potentiometric and fluorometric titrations.
• Fluorescence excitation-emission matrices (EEM) with parallel factor analysis.
• Infrared (IR) spectroscopy.
• Analysis using NICA and electrostatic models.

Main Results:

• Potentiometry suggests bimodal Pb(II) affinity distributions, while fluorometry indicates monomodal.
• EEM analysis identified three fluorescent components sensitive to Pb(II) concentration.
• Pb(II) binding induced aggregation of humic molecules, affecting fluorescence and consistent with IR data.
• Strong binding sites involve multidentate complexation, promoting aggregation; weaker sites involve single ligand interactions.

Conclusions:

• Pb(II) binding to humic acids is complex, involving both strong and weak interactions.
• Metal-induced aggregation plays a significant role in Pb(II) binding and humic acid structural changes.
• The study provides insights into the environmental fate and transport of lead in soil and aquatic systems.