Abstract
Henry's law constants (HLC) play a key role in the environmental fate of pesticides and their distribution between the different phases, i.e., between air, water and soil. For certain compounds such as fenpropidin and pyrimethanil, HLC values are poorly documented in the literature and in particular their temperature dependence. This work reports the experimental HLC values of two pesticides, namely fenpropidin and pyrimethanil, determined by means of a dynamic equilibrium system coupled to an off-line analysis performed by Gas Chromatography–Photoionisation Detection (GC–PID). The measurements were conducted over the range 278–293 K. In pure water, the experimental average values of HLC at 293 K were: HLC293K(fenpropidin) = (10.0 ± 3.1) × 104 M atm–1, HLC293K(pyrimethanil) = (8.2 ± 0.7) × 104 M atm–1. The obtained data were used to derive the following Arrhenius expressions where the quoted errors represent 2σ: ln HLC (fenpropidin) = (6060 ± 2420)/T – (9.1 ± 8.3); ln HLC (pyrimethanil) = (14570 ± 1800)/T – (38.4 ± 6.2). The environmental implications are then discussed in terms of lifetime or partitionning between the different atmospheric compartments.
Highlights
The resulting relative uncertainties quoted on Henry's law constant (HLC) values vary in the ranges 22 – 26 % and 12 – 25% for fenpropidin and pyrimethanil, respectively
3.1 Gas/liquid equilibrium state at the reactor outlet Given that Henry's law constant describes an equilibrium state between gas and liquid phases, it is necessary to check that this equilibrium is reached at the outlet of the microporous tube by varying the gas flow rate at a constant temperature
The environmental fates of Fenpropidin and Pyrimethanil have been widely studied in the literature either in air (Córdoba Gamboa et al, 2020 [22]; Désert et al, 2018 [21]; Villiot et al, 2018 [3]) or in soil and water (Agüera et al, 2000 [51]; Vanni et al, 2003 [13], 2000 [52]), this works reports the first experimental determination of their Henry’s law constants, a key parameter to understand their environmental behavior
Summary
In equilibrium between the gas and liquid phases, the water/air partitioning coefficient corresponds to the Henry’s Law Constant, usually defined as HLC = [X] / PX where [X] is the aqueous concentration of X (mol L–1) and PX is its atmospheric partial pressure (atm). For compounds of low volatility and poorly soluble in water, such as pesticides targeted in this work (see Table 1), the uncertainties in vapor pressure and solubility are often very high, which leads to unreliable estimates made by SAR. This reinforces the need of experimental measurements. Experiments were carried out with an air flow rate of: a 0.1 L min–1; b 0.3 L min–1; c 0.4 L min–1; d 0.5 L min–1
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