Reduction-induced molecular signature of humic substances: structural evidence for optical changes

The redox behavior of humic substances (HS) has drawn significant attention. It has been recently reported that aquatic HS after NaBH4 reduction exhibit a loss in UV absorption but enhanced and blue-shifted fluorescence emission. This unique property was proven to also apply to terrestrial HS in the current study. To further understand the underlying relationship between the molecular structural changes and the optical changes, multiple techniques were employed to obtain solid spectroscopic evidence for Aldrich humic acid. Attenuated total reflectance infrared spectroscopy, Raman spectroscopy and solid-state C-13 nuclear magnetic resonance spectroscopy were used together to demonstrate that the alcohol-like moieties increased while the carbonyl moieties decreased in HS molecules during reduction, which is responsible for optical properties. In addition, the obtained results could explain the change in molecular size as evidenced by dynamic light scattering. The integrated results presented in this study unambiguously provide solid spectroscopic evidence for the charge transfer model, which is a relatively new concept framework for understanding HS.