Noncovalent interactions between monoaromatic compounds and dissolved humic acids:: A deuterium NMR T1 relaxation study

Deuterium nuclear magnetic resonance spectroscopy (2H NMR) spin-lattice relaxation (T-1) experiments were used to examine solution-phase noncovalent interactions between deuterated monoaromatic compounds (phenol-d(5), pyridine-d(5), benzene-d(6)) and Suwannee River, soil, and peat humic acids. Noncovalent interactions, in aqueous solution, were examined as a function of solution pH, monoaromatic hydrocarbon functional groups, and humic acid identity. Benzene interacted with dissolved humic acids at all pH values; however, these interactions increased with decreasing pH and generally were proportional with the humic acid percent aromaticity. Pyridine behaved similarly as benzene; however, two modes of interaction between pyridine and humic acids were detected as a function of pH and humic acid type: bonding with the lone pair of electrons of pyridine's nitrogen and pi-pi interactions between the aromatic ring of pyridine and aromatic components of humic acid. The latter interaction was favored by:increasing humic acid percent aromaticity and decreasing solution pH. On the other hand, because of its strong capacity for hydrogen bonding,phenol interacted preferentially with water, except at pH values 5 or lower, and with humic acids with 45% or greater aromaticity Under these conditions, strong interactions between phenol and humic acids were observed. These results demonstrate that solution-phase, noncovalent interactions between monoaromatic compounds and humic acids are a function of solution pH percent aromaticity, and the monoaromatic functional group.