Multiple polar components in poorly-humified humic acids stabilizing free radicals: Carboxyl and nitrogen-containing carbons

Abstract

To reveal the structural factors associated with free radicals in humic substances (HSs) with lower degrees of humification that are comparable to Type Rp, correlations between free radical content and chemical properties of 10 humic acids (HAs) and 2 fulvic acids (FAs) from different sources were investigated. Free radical contents of the HSs were analyzed using electron spin resonance spectroscopy (ESR) and ranged from 7.0 × 10 15–5.7 × 10 17spins g −1 carbon. The relative abundances for aromatic and/or olefin, alkyl, and carboxylic carbon species in the HSs were determined using solid state cross polarization magic angle spinning (CPMAS) 13C nuclear magnetic resonance spectroscopy (NMR). Aromaticity of the HSs estimated from 13C NMR varied slightly (30–40%) and no correlation was observed between aromaticity and free radical content. In contrast, a positive exponential relationship was observed between relative abundance of carboxyl groups and free radical content in HAs, while a negative exponential relationship was observed between relative abundance of alkyl carbon and free radical content. Two FAs were outliers in the correlations, probably due to the different chemical properties between HAs and FAs. Analysis of structural fragments using pyrolysis–gas chromatography–mass spectrometry (GC/MS) showed that additional chemical properties were also positively correlated with free radical content in HAs: two or more acidic functional groups-substituted phenolic compounds, short chain dicarboxylic acids, hydroxyacetic acid, nitrogen-containing compounds and heterocyclic carbonyl compounds. Possible semiquinone precursors, such as methoxyphenols, were not related to free radical content. These results show that free radicals are stabilized by combinations of multiple polar components such as carboxyl and nitrogen-containing carbons within HA structures during the early stages of humification.