Solution-state NMR evaluation of molecular interaction between monoaromatic carboxylic acids and dissolved humic acid

Understanding the nature of interactions between the aromatic organic pollutants with dissolved humic acid (HA) is fundamental for the prediction of their environmental fate and subsequent development of efficient remediation methods. The present study employs solution-state ¹H/¹⁹F NMR methods to investigate the non-covalent interaction between aqueous peat humic acid (Aldrich HA) and monoaromatic carboxylic acids (CA), viz., 2, 6 diflourobenzoic acid (DFBA) and its non-fluorinated analog, benzoic acid (BA). NMR self-diffusion measurement of HA protons confirmed micellar nature indicating possibility of encapsulation of small molecules through host-guest interaction. ¹⁹F-¹H and ¹H-¹H saturation transfer difference (STD) experiments reveal the mode of insertion of CA into HA superstructure. The strength of interaction has been evaluated by analyzing T₁/T₂ relaxation times and self-diffusion coefficients of CA as a function of HA concentration. Association constants extracted for CA-HA complexes from NMR diffusion experiments reflected that the association between DFBA-HA (2.34 mM^-1) is significantly higher than that of BA-HA (0.97 mM^-1). The experimental outcome reiterated that substitution of -H with halogen atoms (-F in specific) to aromatic ring plays a dominant role in modulating the strength of association and mode of insertion of organic pollutants into HA superstructure. The present study emphasizes that AHA can be a potential remediating agent for organic contaminants due to its superior binding affinity compared to less humified extracted HA (EHA) from Karwar, Rajasthan, India.