Influence of Adjuvants on Pesticide Soil–Air Partition Coefficients: Laboratory Measurements and Predicted Effects on Volatilization

Abstract

A solid-phase fugacity meter was used to measure the soil−air partition coefficients of three semivolatile pesticides (chlorpyrifos, pyrimethanil, and trifluralin) in the absence of additional adjuvants (K soil−air,AI ), as part of commercial formulations (K soil−air,formulation ), and as formulation mixtures with an additional spray adjuvant added (K soil−air,formulation+spray adjuvant ). Chlorpyrifos K soil−air,formulation values were also measured over 15−30 °C, allowing for the change in internal energy of the phase transfer reaction (Δ soil−air U) to be calculated and compared to the Δ soil−air U for K soil−air,AI from the literature. Measured K soil−air values were then used as input parameters in a pesticide volatilization model to understand how their variability affects pesticide volatilization rates under different conditions. Initial experiments conducted at ∼24 °C indicated that all pesticides volatilized more readily in the presence of adjuvants than in their absence and that the additional spray adjuvant had minimal impact. The Δ soil−air U values were 328 and 90 kJ/mol for chlorpyrifos in the absence and presence of formulation adjuvants, respectively, suggesting that adjuvants may weaken or disrupt intermolecular attractions between pesticide molecules and soil. At temperatures below 24.5 °C, modeled chlorpyrifos volatilization rates were higher in the presence of adjuvants than in their absence; however, the opposite occurred at temperatures above 24.5 °C.

Investigating the Effects of Temperature, Relative Humidity, Leaf Collection Date, and Foliar Penetration on Leaf–Air Partitioning of Chlorpyrifos

“…The mean log K leaf‑air,AI values ranged from 5.1 to 5.7 for the tested temperatures. These values are lower than the log K soil‑air,AI values (∼6–9) and log K soil‑air,formulation values (∼7) previously reported for chlorpyrifos using a similar experimental setup . These results align well with those from field studies, indicating that pesticide volatilization from plants is generally higher than from soil .…”

“…This highlights the need to experimentally measure Δ leaf‑air U values for more compounds of interest and to not rely on predictions from PCB data. Our Δ leaf‑air U values were also substantially lower than the chlorpyrifos Δ soil‑air U AI and Δ soil‑air U formulation values (328 and 90 kJ mol –1 , respectively) reported previously, indicating that the K leaf‑air values are less sensitive to temperature changes than K soil‑air values.…”

“…These values are lower than the log K soil-air,AI values (∼6−9) and log K soil-air,formulation values (∼7) previously reported for chlorpyrifos using a similar experimental setup. 38 These results align well with those from field studies, indicating that pesticide volatilization from plants is generally higher than from soil. 43 The K leaf-air,AI values for chlorpyrifos decreased with temperature, indicating increasing volatility with temperature, although the correlation was not significant (p > 0.05) (Figure 1).…”

“…K leaf-air values were measured using a solid-phase fugacity meter similar to that described previously. 38 A schematic and a photograph of the fugacity meter are provided in Figures S1 and S2, respectively. The same setup was used for all parameters tested.…”

Foliar Photodegradation in Pesticide Fate Modeling: Development and Evaluation of the Pesticide Dissipation from Agricultural Land (PeDAL) Model

“…Another limitation is that the K plant‑air values and photodegradation rates we used in the model were measured for pure active ingredients rather than formulations. The adjuvants in formulations can effect pesticide volatilization rates from soils, glass, and other surfaces, − and formulated epoxiconazole has been shown to penetrate into the leaf tissue more readily than pure epoxiconazole . However, data concerning the potential effects of formulation adjuvants on pesticide volatilization from leaf surfaces are extremely limited in the literature .…”