Is the traditional alkali extraction method valid in isolating chemically distinct humic acid?

Abstract

The validity of traditional humic acid (HA) extraction methods and the existence of distinct chemical structures of HA have been recently intensively debated because some researchers believe that HA only forms (humification) due to secondary synthesis during the extraction process. In this study, HAs were synthesized through abiotic humification of a polyphenol and an amino acid with and without MnO2 as the catalyst. The synthesized HAs were first analyzed by Fourier-transform infrared spectroscopy and matrix-assisted laser desorption/ionization-time of flight mass spectrometry, and the results showed that the basic functional groups and molecular weight distribution of the synthesized HAs are similar to those of a standard peat HA from the International Humic Substance Society (IHSS). The products of homogenous abiotic humification (without MnO2) both before and after alkali extraction were then examined by 1D 1H and 2D 13C-1H nuclear magnetic resonance. Interestingly, the dominant features in the NMR spectra of the HAs before and after alkali extraction were largely comparable and no new structural signals were observed after the extraction, indicating that major structural features were not altered. This finding refuted the notion that the extraction method caused secondary synthesis and validated the extraction method of HA.