Depolymerization and degradation of humic acids with sodium perborate

Abstract

Two humic acids of different origin (peat and soil) were degraded with a 5% sodium perborate solution (140°C). This degradation process consists mainly of a stoichiometric production of hydrogen peroxide while the perborate is reacting with carboxyl groups of the oxidized polymers. A single perborate treatment degraded more than 40% of the humic acids to soluble products, but a 5-step oxidation was necessary for total degradation, the sample being transformed into soluble oligomers with properties similar to those of fulvic acids. The oligomeric fractions with lowest molecular weights, including individual molecules (soluble in ethyl acetate), were purified by adsorption chromatography and studied by GC-MS after methylation. The higher molecular weight fractions of oligomers were recovered over polyvinylpyrrolidone, eluted by alkali, and purified by ion-exchange chromatography (47% peat HA; 25% soil HA). Degradation products included alkanes, fatty acids and dicarboxylic acids. Aromatic compounds (mainly phenolic, benzenecarboxylic and cinnamic acids), amounted to 24-50% of the total volatile degradation products. There were striking differences between peat and soil humic acids, the former yielding typical lignin degradation products. Independently checked, the perborate degradation products were not the same as those obtained by mild treatment with hydrogen peroxide under alkaline conditions.