Adsorption behaviour and mechanism of Avermectin-based pesticides in soil driven by H-bonds formed between the compounds and humus

Adsorption is an essential process in regulating pesticide distribution in soil. However, the relationship between soil adsorption mechanisms and the structure of the compounds is unclear. Different soil samples (S1, S2, S3) were selected for adsorption tests of avermectin B2a (AVB2a), avermectin B1a (AVB1a), and emamectin benzoate (EB) to investigate the influence of adsorbent properties and compound structure on the adsorption. The adsorption of the three chemicals by the three soils was extensive and irreversible. Soil adsorption of AVB2a was strongest, followed by AVB1a and EB. Soil adsorption tests of three avermectin-based pesticides showed that the -OH groups of the compounds were the main adsorption site. The humus fraction, a significant component of the soil organic matter, was extracted for further adsorption tests on the compounds. The humus fraction most strongly adsorbs AVB2a, followed by AVB1a and EB, indicating that -OH is also the main site for the humus fraction to adsorb compounds. Differences in the content of polar groups in the humus fraction by elemental analysis and 13C NMR analysis suggested that the -OH of avermectin-based pesticides interacts with the polar groups of humus. Quantum chemical calculations confirm that the -OH groups of the compound form hydrogen bonds with amino, hydroxyl, carboxyl, and carbonyl groups in the humus, thus demonstrating that hydrogen bonding is the mechanism by which avermectin-based pesticides are adsorbed to the soil.