Abiotic Formation of Humic-Like Substances through Freezing-Accelerated Reaction of Phenolic Compounds and Nitrite

Abstract

A novel humification pathway was investigated, along with the effect of freeze-concentration on humification. Polycondensation of phenolic compounds was initiated by nitrite ions. The concentration of humic-like acids (HLA) and fulvic-like acids (FLA) from the frozen phase were ~36.7 and ~16.1 times higher than under aqueous conditions after six hours reaction time. The enhanced reaction rate was attributed to the freeze-concentration effect, whereby nitrite ions in the ice grain boundary layer are transformed into nitrosonium ions (NO+) and phenols are oxidized to phenolic radicals as a result of humification. One humification product, FLA, demonstrated a unique fluorescence peak, which was identified from dissolved organic matter (DOM) in natural ice samples. In addition, FLA showed a high protein composition, which was also observed in natural ice DOM. This study identifies new humic substance formation pathways in nature that can be used to explain the unique DOM properties of natural ice.