Surface activity of humic substances depending on their origin

Humic substances are a general category of naturally occurring, biogenic, heterogeneousorganic substances that can be characterised as being yellow to black in colour, of highmolecular weight and refractory. Surface tension measurement defined humic substances assurface active substances. Unless micellar structural model of humic substances has beensuggested, in the same time there are only a few studies about the factors that affect thesurface activity of humic solutions. The objective of this study was to study changes insurface activity depending on the origin and properties of humic substances.The surface tension-pH curves of humic substances featured a minimum for all solutions,declining steeply from higher and lower pH values. The decrease in surface tension withdecreasing pH reflects the gradual neutralization of acidic sites, which created amphiphilicspecies that migrated to the surface. Surface tension of solutions of humic substancesdecreased with increasing concentration, as well as in presence of metal salts. Manyindustrially produced humic materials demonstrated no or insignificant impact on surfacetension of their solutions, but humic substances isolated from natural environments (water,soil, peat, sediments) demonstrated significant impact of surface tension of their solutions.Thus there exist direct links between origin, structure of humic substances and their ability toinfluence surface tension of aquatic solutions.