Studies of interactions between humic acid and pharmaceuticals as emerging contaminants using ultrafast affinity...

Pharmaceuticals are increasingly found as emerging environmental contaminants in water. Many of these compounds may interact with humic acid and other forms of natural dissolved organic matter, which affects properties such as their bioavailability and persistence. Ultrafast affinity extraction (UAE) has been previously applied to study the binding of drugs to proteins for biomedical applications. In this study, UAE was adapted and used to examine humic acid binding to ciprofloxacin and norfloxacin, two antibiotics that have been found in the environment. Microscale columns containing entrapped humic acid within porous silica were prepared and used to capture the non-bound and free fractions of these drugs in solution-phase mixtures containing soluble Aldrich or Leonardite humic acid. The results were used to acquire both the apparent dissociation rate constants and binding constants for the drugs with these types of humic acid. The distribution equilibrium constants measured for this binding at pH 7.0 in 0.10 M potassium phosphate buffer and at 25 °C were 3.6-5.6 × 10⁴ L/kg for Aldrich humic acid with ciprofloxacin or norfloxacin and 7.4-8.3 × 10⁴ L/kg for Leonardite humic acid. These binding constants fit well with those determined using other techniques. The apparent dissociation rate constants measured by UAE in the same pH 7.0 buffer and at 25 °C were 0.10-0.17 s^-1 for Aldrich humic acid with ciprofloxacin or norfloxacin and 0.052-0.16 s^-1 for Leonardite humic acid. Advantages of this method included its speed, need for only small amounts of soluble drugs and binding agents, and ability to provide information on both binding strength and dissociation rates for these systems. This approach is a general one that could also be applied for binding and rate studies with other emerging contaminants and soluble binding agents for environmental analysis and research.