A Water Soluble Polymer as a Working Structural Model for Humic Acids: H-binding and Spectroscopic Properties

HA occur ubiquitously in terrestrial and aquatic environments, indicating that many precursors and several formation pathways exist. Distinguishing among these pathways has been difficult because individual scientists have tended to concentrate their efforts on specific types of samples (e.g. soil, sediment or water) for which different isolation or fractionation methods are often used. Humic acids ultimately result from abiotic and biotic polymerization and polycondensation reactions involving such small precursors. The relative importance of abiotic and biotic processes in humification in soils and sediments still remains obscure even though much research has been devoted to them. For example the oxidative polymerization of phenols, which is an important precursor reaction in the formation of HA, is catalysed both by enzymes and abiotic (inorganic) catalysts. Polyphenols have been shown to be important precursors in the abiotic formation of HA. More particularly, the oxidative polymerization of polyphenols in soils is thought to be one of the major processes of formation of humic substances. According to the polyphenolic theory, humic acids are formed from simple phenols and phenolic acids via the formation of a semiquinone radical. Coupling of semiquinones originating from diphenols can lead to the formation of stable humic polymers under the catalytic action of Mn (IV) oxides. In the present case, we have focused our work on molecules bearing both carboxy and phenolic moieties as well as radical activity. In this model, protocatechuic acid (3,4-dihydroxybenzoic acid), and gallic acid (3,4,5-trihydroxybenzoic acid) are among the main low molecular weight aromatic acids formed after lignin degradation. As we have reported recently polymerization of the mono-, di-, and trihydroxyphenolic compounds may occur in alkaline media i.e. under ambient O2. i.e. under conditions where radicals are formed. In the present work, we have investigated the oxidative co-polymerization of some simple hydrobenzoic molecules which have been shown to be good models certain physicochemical properties of humic acids. As we show by controlling the pH, redox potential and the ratio of the polyphenolics, a water soluble polymer is produced that mimics certain key- physicochemical and spectroscopic properties of humic acids.