Porous polymer monolithic columns to investigate the interaction of humic substances with herbicides and emerging...

Understanding the interaction mechanisms and the relevant binding constants between humic acids and emerging or regulated pollutants is of utmost importance in predicting their geochemical mobility, bioavailability, and degradation. Fluorescence spectroscopy, UV-vis spectroscopy, equilibrium dialysis, and solid-phase extraction combined with liquid chromatography-mass spectrometry have been employed to elucidate interactions of humic acids with organic micropollutants, especially pharmaceutical drugs. These methods demand large sample volumes, long equilibration times, and laborious extraction steps which may imply analytical errors. Monolithic high-performance affinity chromatography is an alternative and simpler method to investigate these interactions and determine the binding constants.

Polymer monoliths based on aminated glycidyl methacrylate and ethylene glycol dimethacrylate served to immobilize Cu(II) and then humic acid to produce monolithic affinity chromatography columns with humic acid as the active interaction phase. About 86.5 mg of humic acid was immobilized per gram of polymer. The columns enabled a comparison of the binding strength of humic acid with herbicides and emerging pollutants at 25 °C and pH 6.0 ± 0.1. Paracetamol, acetylsalicylic acid, and salicylic acid did not retain. Among the compounds that interacted with humic acid, the order of increasing affinity, estimated by the global affinity constant (nK_a) or partition coefficient (K_D) was: caffeine < simazine < atrazine ∼ propazine < benzophenone. The nK_a (L mol^-1) values ranged from (4.9 ± 0.3) × 10² for caffeine to (1.9 ± 0.3) × 10³ for benzophenone, whereas K_D (L kg^-1) varied from 14 ± 1 to 56 ± 8 for the same compounds.

To our knowledge, this is the first paper demonstrating the use of a monolithic platform to immobilize supramolecular structures of humic acids exploiting immobilized metal affinity to comparatively evaluate their affinity towards emerging pollutants exploiting the concepts of high-performance affinity chromatography. The proposed approach needs only small amounts of humic acid, which is a relevant feature in preparing columns with humic substances isolated and purified from remote areas.