Humic acid nature and compound structure together determine the capacity of soil to sorb Avermectin B1a and its derivatives

Sorption is the key process that regulates the dispersion of pesticides in the environment once they are introduced into the soil. However, the relationship between the molecular structures of these compounds and the sorption mechanism needs to be further clarified. Our objectives were to uncover sorption drivers in distinct soil samples for the parent avermectin B1a (-OH:3) and pharmacophore-altered derivatives [derivative-1 (-OH:2) and derivative-2 (-OH:1) series]. Avermectin B1a sorption to soil was extensive and essentially irreversible, with avermectin B1a exhibiting significantly greater sorption than the derivatives. The sorption difference between avermectin B1a and derivatives in soil suggested that -OH pharmacophore was the main sorption site for avermectin B1a in soil. Further sorption tests on avermectin B1a and derivatives were carried out on humic acid fractions (HA1 and HA2) that we extracted from the soil. Avermectin B1a sorbed most extensively to humic acid fractions, followed by derivative-1 and derivative-2, thereby indicating the -OH pharmacophore remains the main site for the sorption of avermectin B1a and derivatives by the humic acid fractions. The HA1 fraction showed significantly greater sorption of the compound than the HA2 fraction. Elemental analysis and 13C NMR analysis demonstrated that the HA1 fraction contains more polar functional groups than the HA2 fraction. This suggests that the sorption of avermectin B1a and derivatives by the soil is due to the interaction between the -OH pharmacophore and the polar functional groups in the humic acid, implying that hydrogen bonding may be the main sorption mechanism. This study highlighted the potential sorption mechanisms of pharmacophores in pesticide compounds with humic acid fractions.

Automated summary

This study aimed to uncover the sorption drivers of avermectin B1a and its derivatives in distinct soil samples. Avermectin B1a exhibited extensive and irreversible sorption in soil, with significantly stronger sorption than the derivatives. The main sorption site for avermectin B1a in soil was determined to be the -OH pharmacophore. Further tests on humic acid fractions showed that avermectin B1a sorbed most extensively, indicating that the -OH pharmacophore remains the main site for sorption by humic acid fractions. Hydrogen bonding was identified as the main sorption mechanism. This study highlights the potential sorption mechanisms of pharmacophores in pesticide compounds with humic acid fractions.