pH and ionic strength effects on the binding constant between a nitrogen-containing polycyclic aromatic compound and...

Area of Science:

• Environmental Chemistry
• Aquatic Chemistry
• Organic Geochemistry

Background:

• Polycyclic aromatic compounds (PACs) are prevalent environmental pollutants with carcinogenic and mutagenic potential.
• The interaction of PACs with dissolved organic matter (DOM) critically influences their environmental behavior, including transport, solubility, bioavailability, and bioaccumulation in aquatic systems.
• Benzo(h)quinoline (BQ), a basic PAC, exists as BQ in neutral to basic conditions (pH > pK a) and as benzo(h)quinolinium (BQH+) in acidic conditions (pH < pK a).

Purpose of the Study:

• To investigate the influence of ionic strength and cation charge density on the binding of benzo(h)quinoline (BQ) and its protonated form (BQH+) to Leonardite humic acid (LHA).
• To elucidate the role of cations (Na+, Mg2+) and anions (Cl-, SO42-) in modifying PAC-DOM interactions.
• To assess the quenching effect of chloride ions on BQH+ fluorescence in acidic solutions.

Main Methods:

• Fluorescence quenching was employed to measure the binding constant (K DOC) between BQ/BQH+ and LHA under varying ionic strengths and solution chemistries.
• Experiments were conducted across a range of pH conditions (acidic, neutral, basic) with the addition of different salts (NaCl, Na2SO4, MgCl2, MgSO4).
• Fluorescence intensity decay of BQH+ was analyzed in the presence and absence of LHA, particularly in acidic solutions with chloride.

Main Results:

• The binding constant (K DOC) between BQ/BQH+ and LHA decreased significantly with increasing ionic strength, with reductions ranging from 70-95% in acidic conditions and 20-90% in neutral to basic conditions.
• The reduction in K DOC was more pronounced with divalent cations (Mg2+) compared to monovalent cations (Na+), indicating a stronger influence of cation charge density.
• Chloride ions (Cl-) in acidic solutions exhibited a quenching effect on BQH+ fluorescence, suggesting a potential heavy atom effect.

Conclusions:

• Ionic strength and cation charge density are critical factors modulating the binding of benzo(h)quinoline to humic substances in aquatic environments.
• The observed decrease in binding is attributed to cation bridging and enhanced intramolecular reactions within the LHA structure, leading to molecular coiling.
• Chloride ions may act as quenchers for protonated benzo(h)quinoline in acidic aquatic media, further influencing its environmental fate.