Abstract
BackgroundHumic substances can be considered as polyelectrolytes with supramolecular character and complicated behaviour in water environment. The fractions of humic substances dissolved in water are the most active ones and determinative for their functioning in nature, where the proton-binding and dissociation ability play a crucial role. The dissociation behaviour of humic and fulvic acids can be affected by different circumstances including their concentration which is directly connected with the molecular organization of humic particles in solution and the accessibility of their ionizable functional groups. This study is focused just on these active fractions and their dissociation behaviour in the dependence on their content in studied system.ResultsStandards and reference samples of International Humic Substances Society were used. Flow-through coulometry was used to determine the total content of acidic functional groups in fulvic solutions and humic leachates. The amount of dissociated acidic groups was determined on the basis of potentiometry. Several differences between the behaviour of humic and fulvic acids were found. While whole samples of fulvic acids including the weakest functional groups were analysed, only the active dissolved humic fractions containing stronger acidic functional groups were characterized. The fractions containing higher amounts of the weakest functional groups remained insoluble. The dissociation degree of fulvic acids decreased with their increasing content, but a maximum on its concentration dependence obtained for humic acids was observed.ConclusionsTwo different values of dissociation constants were determined for each sample. The first was determined on the basis of the extrapolation of infinite dilution, second was determined as their average value in the region of high concentrations in which it was constant. Obtained values characterize the functioning of dissolved humic substances from point of view of their dissociation ability. The results obtained by this approach can help to predict the acid–base behaviour of dissolved organic carbon in soil and generally in nature. The acid–base behaviour of dissolved organic carbon depends on its ratio to water. It behaves differently in soil which is dry and in soil which is wet or saturated by water.
Highlights
Humic substances can be considered as polyelectrolytes with supramolecular character and complicated behaviour in water environment
Amounts of acidic functional groups determined on the basis of flow-through coulometry involved both protonated and dissociated groups in studied systems
While the concentration dependence of dissociated acidic functional groups in humic leachates had a linear character in logarithmical coordinates, that of total amounts of acidic functional groups could be divided into two (NLHA, Elliot soil humic acids (ESHA), Suwannee river humic acids (SRHA) and Pahokee peat humic acids (PPHA)) or three (LEHA) parts
Summary
Humic substances can be considered as polyelectrolytes with supramolecular character and complicated behaviour in water environment. Humic substances are heterogeneous mixtures of organic materials formed by the humification of animals, plants and microbes [1, 2] They are the most widespread natural complexing agents occurring in soils, sediments and water. Due to their colloidal and poly-functional character, these substances play important roles in the mobility and bioavailability of nutrients and contaminants in the environment [3, 4]. A study of the interactions of humic substances cannot disregard a knowledge of their proton exchange capacity and the strength of their functional groups which is not easy to acquire— in the case of natural polyelectrolytes, whose heterogeneous structure makes the modelling of protonation equilibria very complicated [11,12,13]. As a consequence of such pronounced heterogeneity with respect to humic structure and composition, a strong influence on the acid–base properties and on binding capacities of humic substances and their functioning in nature must be expected [14, 15]
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