The distribution of humic substances between the solid and aqueous phases of acid organic soils; a description based on humic heterogeneity and charge‐dependent sorption equilibria

Abstract

SUMMARYA description of the partitioning of humic substances (HS) between the solid and solution phases of acid organic soils is proposed, based on the opposition of hydrophobic and hydrophilic properties. Soil HS are considered to be of two types, mobile, i.e. potentially transferable to the soil solution (HSM), and immobile (HSI). The HSM consist of a series of fractions, each with the same ion‐binding properties, but differing in hydrophobicity. Partitioning of HSM between the solid and solution phases depends upon the net humic charge (Z), which is determined by the binding of protons and other cationic species. For each fraction, the extent of transfer to the solution phase increases as Z becomes more negative. The most hydrophilic HSM require only a small negative charge for transfer to the solution phase to be favoured, while the most hydrophobic require the development of a large negative charge. A mathematical version of the model, combined with an existing model of ion‐binding by HS, can explain reasonably well the release of HS from agitated aqueous suspensions of organic soil samples. The model also explains the release of HS when the solution phases of organic soil suspensions were repeatedly replaced with NaCl solution.