Those That Remain: Sorption/Desorption Behaviour and Kinetics of the Neonicotinoids Still in Use

Abstract

In January 2023, the derogation loophole was closed on “emergency authorisations” for the use of three out of five neonicotinoids in all EU states. In this study, we analysed the sorption/desorption behaviour and kinetic parameters of acetamiprid and thiacloprid, the two neonicotinoids that are still approved for use, either regularly or under emergency authorisations in the EU, and widely used worldwide. Sorption and desorption curves in four soils with different organic matter content were analysed using four kinetic models, namely, Lagergren’s pseudo first-order model, two-site model (TSM), Weber–Morris intraparticle diffusion model and Elovich’s model. Kinetic parameters were correlated to soil physico-chemical characteristics. To determine the mutual influence of soil characteristics and sorption/desorption parameters in the analysed soils, a factor analysis based on principal component analysis (PCA) was performed. Even though the two insecticides are very similar in size and chemical structure, the results showed different sorption/desorption kinetics. The model that best fits the experimental data was TSM. Thiacloprid showed a more rapid sorption compared to acetamiprid, and, in all soils, a higher proportion sorbed at equilibrium. Intra-particle diffusion seemed to be a relevant process in acetamiprid sorption, but not for thiacloprid. Desorption results showed that acetamiprid is more easily and more thoroughly desorbed than thiacloprid, in all soils. The kinetic behaviour differences stem from variations in molecular structure, causing disparate water solubility, lipophilicity, and acid–base properties.

Those That Remain Caught in the “Organic Matter Trap”: Sorption/Desorption Study for Levelling the Fate of Selected Neonicotinoids

“…To estimate the time needed to achieve sorption and desorption equilibrium, the sorption and desorption kinetics of acetamiprid, imidacloprid, and thiacloprid (30 mg/L) were monitored at 20 (±1) °C for a time frame of 96 h. The results of the sorption/desorption study are presented in our previous publication [ 53 ]. In our studies, the sorption kinetics showed rapid initial insecticidal sorption within the first few hours, depending on the insecticide, and the equilibrium was reached within 47 h, while the equilibrium for the desorption reactions was reached within 94 h. The sorption and desorption processes were conducted for 96 h, and this time was sufficient to reach equilibrium conditions.…”

“…A discrepancy between the sorption and desorption isotherms for imidacloprid in Croatian soils was found in our previous publication. For all analyzed soils, the H values were lower than 1, and we found that the hysteresis was more pronounced in soils with a higher content of OM, primarily humic acids [ 53 ]. Cox et al [ 66 ] point out the important fact that soil with a stronger sorption capacity at higher imidacloprid concentrations showed higher desorption compared to other soils, which was also confirmed by a higher hysteresis coefficient.…”

“…In alkaline conditions, its dissociation begins, resulting in increased soil mobility. Imidacloprid undergoes very weak dissociation at soil pH values of 5–8 [ 53 , 84 ], while at very high pH values, dissociation increases, which may affect its bioavailability and interactions with soil constituents. Furthermore, neonicotinoids also contain nitrogen atoms in a similar environment: one sp -hybridized (nitrile group—N in acetamiprid and thiacloprid), two/three sp 2 -hybridized (one of which is a pyridine N in all neonicotinoids), and one sp 3 -hybridized N. The most pronounced difference can be observed for sp 3 N, since it is located inside the ring structure and near S or N in thiacloprid and imidacloprid but not in acetamiprid.…”

“…Another partially negative part of the molecule is the nitrogen atom inside the pyridine ring, which has a lone electron pair capable of creating π-π or p-π electron donor–acceptor interactions (EDA) with aromatic parts of organic matter, which are rich in humic substances [ 71 ]. The oxygen in the –NO 2 group in imidacloprid can bind with soil organic matter by forming H-bonds with functional groups of organic matter (carboxyl, hydroxyl, amide) [ 53 , 77 ]. Furthermore, N, S, and Cl heteroatoms in neonicotinoid molecules can act as hydrogen (H–) bond acceptors and form H-bonds with H-donating functional groups in the soil [ 85 ].…”

“…The availability of pesticides sorbed in soil may progressively decline with time, affecting the insecticides’ uptake into plants, their leaching and runoff in the soil system, and biotic/abiotic degradation, as well as their control of weeds and pests. Recently, we demonstrated the kinetic behavior differences between acetamiprid and thiacloprid, suggesting that intra-particle diffusion is a relevant process in acetamiprid sorption, while thiacloprid was likely sorbed externally, at sites closely associated with organic matter [ 53 ]. Adding to the existing knowledge, this study aims to further clarify if sorption, and to which extent, can control the availability of acetamiprid, imidacloprid, and thiacloprid to other fate-determining processes [ 54 ] in the soil environment.…”