Sorption of pendimethalin by humin fraction isolated from the mollic horizon of Chernozems

The aim of the research was to investigate interactions between pendimethalin (PE), an active substance of plant protection products, and the humin fraction (HM), which is the most recalcitrant component of soil organic matter. The soil material used for the study was collected from the mollic horizon of two Chernozems different in physicochemical properties. HM was obtained in the solid state after discarding humic acid (HA) and fulvic acid (FA) and digesting the mineral fraction in an HF-HCl mixture. Pure PE and two commercial products containing the substance (Penfox 330 EC and Stomp Aqua 455 SC), dissolved in hexane and added to the HM, were used in the research. The batch sorption experiment was carried out in accordance with OECD Guidelines for the Testing of Chemicals No.106. Spectroscopic methods (UV-Vis-nearIR and fluorescence) were used to assess the molecular characteristics of HM exposed to pesticides. The study showed significant differences in the molecular structures of the tested HM as a result of interactions with the pesticides used. The results revealed that PE had a different effect than the commercial products, and proved that the composition of commercial products significantly modified the strength and ability of organic matter to bind pesticides. This is probably related to the presence of adjuvants that are more competitive in forming bonds with the active sorption sites of HM, and thus strengthen or weaken the process of forming pesticide-HM complexes. As a result, the diversity of the molecular structure of HM and the presence of accompanying substances in commercial pesticide products interact, leading to different sorption mechanisms and durability of the resulting products.