Reductive splitting of humic substances with dry hydrogen iodide

The procedure of mild hydrogen iodide for reductive splitting, developed earlier for selective splitting of etheric bonds in lignin and lignin–carbohydrate complex, has been applied successfully to the depolymerization of humic and fulvic acids of both soil and aquatic origin. Molecular mass distribution patterns analyzed by gel-permeation chromatography showed that the aromatic products were similar to lignins except for the absence of monomeric products. The oligomeric units can be treated as branching points from lignin where phenyl-propane units are connected mainly by non-broken carbon–carbon bonds. Such homopolymer segments are slowly degraded during humification processes and can play a role in the overall structure of humic substances. These lignin-derived fragments comprise the major part of aromatic structures in humic substances. The experiments support the idea of the preservation of lignin structural features during humification.