A new insight into a problem of mediating effects of humic acids on binding and biological effects of organic pollutants in natural soil

Abstract

A major focus of this manuscript is to investigate the molecular environment of soil where organic pollutants including an active functional group, such as amine, are bound to humic acids (HA). Close consideration has been taken to binding and biological effects of organic pollutants in natural soil mediated by HA in dependence on the amine base. The investigation is carried out using a method of spin labeling electron paramagnetic resonance (SL EPR). Spin labels (SL) 2,5,5-trimethyl-2-(3-aminophenyl)pyrrolidin-1-yloxy (Anilino-SL) and 4-Amino-2,2,6,6-tetramethylpiperidine-1-oxyl (4-Amino-TEMPO), as well as extracellular fungal laccase from Trametes versicolor, are applied to samples of chernozem, its HA and standard HA, such as Elliott Soil HA, Pahokee Peat HA, leonardite. It is shown that HA mediate the interaction of amines with soil through the partitioning of them among the different compartments of soil in dependence on the amine base. Both aliphatic and aromatic amines become immediately bound to soil organic matter but via different mechanisms. HA bind only the aromatic amines. Their binding sites are located in the hydrophobic and anaerobic compartments of soil. Moreover, spectroscopic analysis evidenced that a number of binding sites are limited, and aromatic amines attracted to hydrophobic compartments in the soil environment form here the ordered structure. Kinetic analysis of a temporal change in the concentration of bound and non-bound aromatic amines points at a pronounced diamagnetic effect of compartments where aromatic amines become bound. In contrast to amine with a weak base, the aliphatic amines are not influenced by HA and remain in an active soil part. Manuscript concludes that mediating effects of HA are first realized through their electrochemical property. They define and isolate pollutants with a definitive base that can be described using the Octanol-Water Partition Coefficient. This binding in turn defines a biological effect of organic pollutants including amine with a weak base via the oxygen factor that results in a decrease in the magnitude of aerobic soil biota and disbalance of the soil microorganism community.