Structure of humin and humic acid from an acid soil as revealed by phase transfer catalyzed hydrolysis

A multistep alkaline hydrolysis under phase transfer catalysis conditions (PTC) was carried out on humic acid and humin fractions from an acid soil sample (anmoor soil) from the Plateau de Millevaches (France). A large part (87%) of the initially insoluble humin became soluble after a first series of hydrolysis. The main fraction obtained, soluble high molecular-weight products, was still reactive when submitted again to a second series of hydrolysis. Humic acid showed a lower reactivity. Linear dicarboxylic acids were produced in significant amounts from humin, they were short-chained (mainly C9) in the first series and long-chained (C16–C28) with an even carbon number preference indicating a plant origin in the second series. Linear C12–C32 monocarboxylic acids and alkanols were freed from humin and humic acids. The short linear fatty acids and alkanols, accompanied with iso- and anteiso-C15 and C17 members, denote a bacterial input while long even components, as well as various aromatic acids, originate from plants. The results indicate that ester groups are involved in the cross-linking of macromolecular chains forming the matrix of humin. Aliphatic dicarboxylic acids act as bridges between alkyl chains in the matrix, while fatty acids, alcohols and aromatic acids correspond mainly to monosubstituents of the matrix (bound by ester groups). It is interesting to note that this model resembles the kerogen of many immature sediments.