Sorption–desorption of imidacloprid onto a lacustrine Egyptian soil and its clay and humic acid fractions

Abstract

Sorption-desorption of the insecticide imidacloprid 1-[(6-chloro-3-pyridinyl)-methyl]-N-nitro-2-imidazolidinimine onto a lacustrine sandy clay loam Egyptian soil and its clay and humic acid (HA) fractions was investigated in 24-h batch equilibrium experiments. Imidacloprid (IMDA) sorption-desorption isotherms onto the three sorbents were found to belong to a non-linear L-type and were best described by the Freundlich model. The value of the IMDA adsorption distribution coefficient, Kd(ads), varied according to its initial concentration and was ranged 40-84 for HA, 14-58 for clay and 1.85-4.15 for bulk soil. Freundlich sorption coefficient, Kf(ads), values were 63.0, 39.7 and 4.0 for HA, clay and bulk soil, respectively. The normalized soil Koc value for imidacloprid sorption was ∼800 indicating its slight mobility in soils. Nonlinear sorption isotherms were indicated by 1/n(ads) values <1 for all sorbents. Values of the hysteresis index (H) were <1, indicating the irreversibility of imidacloprid sorption process with all tested sorbents. Gibbs free energy (ΔG) values indicated a spontaneous and physicosorption process for IMDA and a more favorable sorption to HA than clay and soil. In conclusion, although the humic acid fraction showed the highest capacity and affinity for imidacloprid sorption, the clay fraction contributed to approximately 95% of soil-sorbed insecticide. Clay and humic acid fractions were found to be the major two factors controlling IMDA sorption in soils. The slight mobility of IMDA in soils and the hysteresis phenomenon associated with the irreversibility of its sorption onto, mainly, clay and organic matter of soils make its leachability unlikely to occur.

Sorption hysteresis on soils and sediments: Using single‐point desorption isotherms to obtain characteristic free energy values

“…Sorption (a, c, e) and single-point desorption (b, d, f) isotherms refer to the left y-axis and show log sorbed concentrations versus log solution concentrations for naphthalene, phenanthrene and pyrene, respectively. Molar extra Gibbs free energy values associated with the formation of metastable states are plotted against the solution concentration from which desorption started; they are shown as horizontal lines and refer to the right y-axis 3.2 | Free energies associated with SDH: Different soil fractions and soils Kandil et al (2015) examined sorption and desorption of imidacloprid, a neonicotinoid insecticide, on a lacustrine soil collected from the surface layer (0-20 cm) and its humic acid and clay fractions. With pK a values of 1.56 and 11.12 (HSDB; Toxnet, 2019), imidacloprid is practically non-ionizable in soil solutions.…”

“…In kinetics tests ranging from 15 min to 48 hr in duration, the equilibration time was established and SIs, as well as single-point Dis, were obtained after 24 hr. Based on the half-life times, which were indicated to be between 40 days and greater than a year (Kandil et al, 2015), biodegradation was considered minimal. Hence, the molar extra Gibbs free energy g ΔG ext S values associated with SDH were computed using the parameters of the reported SIs and DIs, as well as the solid-to-solution ratios used, and plotted against the imidacloprid concentration in equilibrated solutions from which desorption started (Figure 4).…”

“…Molar extra Gibbs free energy associated with sorption-desorption hysteresis (SDH) and obtained from SIs and the single-point DIs for imidacloprid on the bulk soil and its clay and humic acid fractions (Kandil et al, 2015) plotted against compound concentration in the equilibrated solutions from which desorption started composition of metalaxyl was not affected by sorption/ desorption, which is also strong evidence of the lack of biological transformations of this chemical. In their data analysis, the authors of that work used linear isotherms to calculate the Gibbs free energies of sorption and desorption.…”

“…Free energies associated with SDH: different soil fractions and soils Kandil et al (2015) examined sorption and desorption of imidacloprid, a neonicotinoid insecticide, on lacustrine soil and its humic acid and clay fractions. With the pKa values of 1.56 and 11.12 (HSDB, Toxnet, 2019), imidacloprid is practically non-ionizable in soil solutions.…”

“…The approach described earlier (Borisover, 2019) was focused on the use of successive DIs. However, in multiple studies, desorption has been examined with the single-step remove/refill started at different initial sorbed concentrations across an SI (Zhang, Lin, Wang, & Gan, 2010;Zhang, Ding, Boyd, Teppen, & Li, 2010, Zhang et al, 2014Martins, Melo, Bohone, & Abate, 2018;Mosquera-Vivas et al, 2016;Piwowarczyk & Holden, 2012;Gaonkar, Nambi, & Govindarajan, 2019;Kandil, El-Aswad, & Koskinen, 2015;Kim et al, 2003;Ran et al, 2002 and many others). Then, the determined sorbed concentrations are combined in one curve, which is also called a DI (specifically a "single-point" or sometimes a "single-step" DI).…”