FACTORS AFFECTING THE SURFACE TENSION OF SOIL SOLUTIONS AND SOLUTIONS OF HUMIC ACIDS

Abstract

Humic and fulvic acids and other naturally derived organic compounds such as proteins and fatty acids are important constituents of soil solutions and surface waters and groundwater. The presence of both hydrophilic polar or ionic functional groups and hydrophobic structural units on these natural organic compounds can be expected to promote accumulation at the air-water interface, which would thereby influence the solution surface tension. This study evaluated the influence of organic C concentration, pH, and temperature on the surface tension of soil solutions extracted from forest soils and solutions of International Humic Substances Society (IHSS) reference humic acids. The surface tensions of the soil solutions were substantially lower than those of IHSS humic acids at equivalent organic C concentrations and solution conditions. Differences were also observed in the temperature dependence of surface tension for the solutions. All solutions exhibited a strong dependence of surface tension on solution pH. Characterization of the soil solutions and humic acid solutions by fluorescence and absorption spectroscopy and titration analysis confirmed that the soil solutions varied markedly from the humic acid solutions. The soil solutions bore substantially higher acidity and yielded qualitatively different fluorescence spectra when compared with the humic acid solutions under equivalent conditions.