Noncovalent interactions between aromatic compounds and dissolved humic acid examined by nuclear magnetic resonance spectroscopy.
We examined the molecular-level interactions of aromatic compounds with a humic acid that was extensively characterized with one- and two-dimensional nuclear magnetic resonance (NMR) spectroscopy. Interactions of target compounds were evaluated by monitoring their NMR relaxation properties. Solid-state NMR revealed that the humic sample contains paraffinic carbon (31%), substituted aliphatic carbon (30%), and aromatic carbon (28%). The liquid-state experiments further identified amino acids, a range of carbohydrates, methoxylated aromatics (likely derived from lignin), and a series of aliphatic chains. The 13C spin-lattice relaxation time (T1) of site-specifically labeled naphthalene, 1-naphthol, and quinoline was found to decrease with increasing additions of humic acid. The 1H T1 values also were measured and demonstrated that the association with the humic acid was not specific because all the protons acquired the T1 value of the humic acid at the same rate. The lack of a chemical shift change and an increase in signal line broadening indicates that the interaction between these compounds and humic acid is noncovalent. These interactions were detected at low humic concentrations (5 mg C/L) and suggest that low concentrations of humic material, which are prevalent in both aquatic and terrestrial systems, will significantly affect the fate and transport of contaminants in the environment.