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

Polycyclic aromatic compounds (PACs) are widespread environmental pollutants with a high potential to act as human carcinogens and mutagens. The behavior of PACs is significantly affected by their interactions with dissolved organic matter (DOM), such as their transport, solubility, bioavailability, and bioaccumulation in the aquatic environment. Being a basic PAC, benzo(h)quinoline (BQ) is the dominant species, as the solution’s pH value is higher than BQ’s pK_a (pK_a of BQ = 4.2). In contrast, benzo(h)quinolinium (BQH⁺) is the major species, as the solution’s pH value is lower than its pK_a. The binding constant (K_DOC), measured by fluorescence quenching, between BQ/BQH⁺ and Leonardite humic acid (LHA) would decrease 70 to 95 % and 20 to 90 % when increasing the ionic strength in acidic and neutral to basic conditions, respectively. The results can be attributed to the added cation (Na⁺ and Mg²⁺), which forms a bridge with LHA and enhances the intramolecular reaction among these functional groups, therefore inducing the coiling up within the LHA molecule. In addition, the decrease of the K_DOC with added MgCl₂/MgSO₄ (75–95 %) is higher than that with added NaCl/Na₂SO₄ (20–75 %), indicating that the K_DOC was affected by the charge density of cations. The fluorescence intensity of BQH⁺ in the absence of LHA (F₀) was found to decay only in the acidic solution with Cl⁻, suggesting that Cl⁻ might be a heavy atom serving as a quencher in an acidic solution.