Application of a newly developed AC-DGT for predicting neonicotinoid insecticide (NNI) bioavailability in soils

Neonicotinoid insecticides (NNIs) represent one of the most extensively used classes of pesticides worldwide. However, their off-target risks have raised significant global concerns in recent years. We have developed a passive sampling technique for organic diffusive gradients in thin films using activated carbon as the binding gel (AC-DGT) for in situ monitoring of NNI bioavailability in soils. Five representative NNIs-imidacloprid, acetamiprid, thiamethoxam, dinotefuran, and thiacloprid-were systematically evaluated. The AC-DGT device exhibited excellent sampling capacity and was resistant to the variations in several key environmental parameters, specifically pH, ionic strength, and dissolved organic matter (DOM) concentration, which were determined to be in the ranges of 4-9, 0.001-0.5 M, and 0-20 mg L^-1, respectively. The results demonstrated a significant linear correlation between the DGT-measured concentration (C_DGT) and the root concentration (C_root), indicating that DGT can serve as a reliable tool for predicting the plant uptake of NNIs. This finding highlights the superior stability of the AC-DGT approach compared to the conventional solvent extraction methods. Using the DGT-Induced Fluxes in Soil (DIFS) model, the soil-solution partition coefficients (K_d) of the five NNIs were determined to range from 0.10 to 44.97 mL g^-1, with response times (T_c) of 0.14-3.80 h. These results reveal distinct differences in the mobility and desorption kinetics of NNIs across various soil matrices. Notably, these compounds exhibited the highest environmental activity and potential risk in southern red soils, which have a lower cation exchange capacity (CEC) and clay content than yellow and black soils. This study establishes a novel DGT methodology for assessing the bioavailability and in situ risk of NNIs in soil, providing key evidence of their environmental fate.