Objective. The biological activity of humic acids (HA), considering them as open non-equilibrium systems, in which properties are determined by the state or level of organization of the system.
Methods. The dependence of physicochemical properties of HA on their concentration (optical and sorption properties, ζ-potential and electrical conductivity, average size of optical homogeneities) were analyzed by several methods: optical microscopy; low-angle laser light scattering (LALLS) method on a Zetasizer Nano ZS analyzer; absorption on a Shimadzu UV-3600 spectrophotometer; chemical analysis on an ICP-QMS Agilent 7500CE spectrometer. Biological activity of HA was studied on the crustacean Daphnia magna and the monocotyledonous plants Allium cepa L.
Results. It was established that the nature of the HA effect on biological objects is non-linear and has an extremum at 1 mg•L^-1. It was shown that the mineralization of solutions affects the biological activity of HA. So in the presence of the addition of mineral salts, for D. magna the effect of HA is insensible, while for A. cepa L. it changes its direction. In the absence of mineral salts at HA concentrations < 5-10 mg•L^-1, a sharp drop in the survival of D. magna and suppression of root growth of A. cepa L. was observed, and in saline solution – stimulation of root growth. It has been established that the concentration range of 5-10 mg•L^-1 was also critical for concentration changes in a number of physicochemical parameters of aqueous solutions of HA, which makes it possible to consider it a phase transition characterized by new properties.
Findings. It was shown that changes in various properties of aqueous solutions of HA, including biological activity, were characterized by non-linear concentration dependencies with the presence of intermittent changes indicating qualitative rearrangements in the system.