Structural Characterization of a Fulvic Acid and a Humic Acid Using Solid-State Ramp-CP-MAS

This study uses the analysis of 13C nuclear magnetic resonance (NMR) spectra and NMR relaxation parameters to study the structure and functional group distribution of a fulvic acid and a humic acid. The spectra and relaxation parameters are obtained with a new cross polarization magic angle spinning (CP-MAS) 13C NMR technique that gives qualitatively and quantitatively accurate results at higher fields. The analysis of the spectra and relaxation parameters indicates that carbohydrate moieties play a major structural role in the fulvic acid studied and contain the major chemical functionality. This analysis also suggested that the majority of the weakly acid protons may be aliphatic/carbohydrate -OH and not phenolic alcohols, as assigned earlier. Models for the fulvic acid and the humic acid are proposed based on spectra and relaxation parameters obtained. The fulvic acid model consists of three units: (i) large relatively immobile units that are mainly aliphatic in nature and largely unfunctionalized; (ii) relatively unfunctionalized more mobile units that are mainly aromatic in nature; and (iii) and more mobile functionalized units that are mainly carbohydrate in nature. The humic acid model consists of one major immobile unit associated with slightly more mobile functionalized units that are mainly aromatic in nature.