Effects of some dicarboxylic acids on the association of dissolved humic substances

Abstract

Changes in molecular size distributions of four different humic materials were evaluated through high performance size-exclusion chromatography (HPSEC), before and after treating humic solutions with naturally occurring dicarboxylic (oxalic, malonic, succinic and glutaric) acids. Chromatograms of dissolved humic substances showed a decrease in peak absorbance as well as an increase in peak elution volume when the solution pH was lowered from 7 to 3.5 by addition of dicarboxylic acids before HPSEC analysis. The resulting reduction in molecular size is explained by the disruption of unstable humic superstructures into smaller-sized associations stabilized by the formation of strong intra- and inter-molecular hydrogen bonds among humic molecules. The extent of humic size variation was in the order: oxalic<malonic<succinic<glutaric acid. Dicarboxylic acids with an increasing number of C atoms and pKas, had progressively larger degrees of protonation which increasingly reduced the electrostatic repulsion from humic molecules. The less dissociated dicarboxylic acids enabled a closer contact with the hydrophobic domains of weakly bound humic superstructures and produced more extensive conformational rearrangements. Recognition of a process controlling the conformational structure of humic molecules by organic acids exuded in the soil solution by either plant roots or soil microbes may be important in understanding the role of humus in soil-plant interactions.