Chemical and spectroscopic characterization of insoluble and soluble humic acid fractions at different pH values

A soil humic acid was extracted by NaOH following the procedure established by the IHSS. Aliquots of the Na-humate solution were acidified at different pH (1, 3, 5, and 7). At each pH, the precipitate and the soluble fractions were separated and characterized by elemental analysis, total acidity and carboxylic group content, InfraRed, 13C NMR and 1H NMR spectroscopy. No attempt was made to eliminate the inorganic material.The acid humic fraction insoluble at pH 1 and 3 had a high acidic group content and aromaticity, but a low content of O- and N-containing groups. At these pHs the inorganic phase bound to the insoluble humic material was constituted by clay minerals and some Al and Fe hydroxides. The soluble fractions at pHs 1 and 3 were very poor and they were composed of a silica gel-like phase associated with polar organic material rich in carboxylic and metal-carboxylate groups. At pHs 5 and 7 only a small fraction of the Na-humates precipitated. The fractions remaining in the solution were mainly composed of organic material particularly rich in aromatic and aliphatic groups, while the inorganic phase contained the phyllosilicates, as well as the fractions insoluble at pHs 5 and 7. Our results indicated that humic acids were composed of heterogeneous molecules that randomly associated according to size and that the structural characterization of single molecules was limited by the strong intermolecular interactions, brought about by the addition of hydrochloric acid, that stabilize their associations. The soluble fractions obtained at pHs 5 and 7 represent the humic component that, in environmental situations, would be dissolved in the soil solution.