Determination of Primary and Secondary Structures of a Purified Lignite Humic Acid by Infrared and Nuclear Magnetic Resonance Spectrometry

Infrared spectrometry of a purified lignite humic acid fraction, confirmed the removal of inorganic and non-HA impurities. A broad peak near 2500 cm-1 was indicative of hydrogen-bonded acid groups, but the carbonyl of carboxyl groups was not hydrogenbonded as indicated by its unshifted frequency of 1710 cm-1. Quantitative 13C multi-rampedamplitude cross polarization magic angle spinning nuclear magnetic resonance (multiCPMAS NMR) spectra indicated oxidized phenyl propane structures derived from lignin. MultiCMPAS experiments, conducted on potassium HA salts titrated to pH 10 and pH 12, revealed shifts consistent with carboxylate and phenolate formation, but structural changes associated with enolate formation from aromatic beta keto acids were not detected. Two-dimensional 1H-13C heteronuclear correlation NMR (2D HETCOR) indicated aryl-aliphatic ketones, aliphatic and aromatic carboxyl groups, phenol, and methoxy phenyl ethers. Acidic hydrogen from carboxyl and phenol groups in both the lignite HA and a synthetic HA polymer was hydrogen-bonded with electron-rich aromatic rings.