Analysis of interactions between pharmaceuticals and humic acid: Characterization using entrapment and high-performance affinity microcolumns

The presence of pharmaceuticals as microcontaminants in the environment has become of particular concern given the growing increase in water reuse and recycling to promote global sustainability of this resource. Pharmaceuticals can often undergo reversible interactions with soluble dissolved organic material such as humic acid, which may be an important factor in determining the bioavailability and effects of these compounds in the environment. In this study, high-performance affinity microcolumns containing non-covalently entrapped and immobilized humic acid are used to examine the binding strength and interactions of this agent for tetracycline, carbamazepine, ciprofloxacin, and norfloxacin, all common pharmaceutical microcontaminants known to bind humic acid. The binding constants, as measured with Aldrich humic acid, have good agreement with values reported in the literature. In addition, the effects of temperature, ionic strength, and pH on these interactions are examined with the humic acid microcolumns. This technique makes it possible to determine the relative importance of electrostatic interactions vs non-polar interactions or hydrogen bonding on these binding processes. This study illustrates how affinity microcolumns can be used to screen and uniformly quantify binding by pharmaceuticals with humic acid, as well as to study the mechanisms of these interactions, with this information often being acquired in minutes and with small amounts of binding agent (∼10 mg per microcolumn, which could be used over 200-300 experiments). Use of entrapment and affinity microcolumns can support similar research for a wide range of other microcontaminants with humic acid or alternative binding agents found in water and the environment.