Study of Chemcatcher® kinetic uptake for herbicide: batch experiment and flow laboratory calibration
Résumé
The widespread use of pesticides in agriculture activities leads to the contamination of surface and ground waters caused by drift, runoff, drainage, and leaching of soils. The pollution of water by pesticides is an issue of environmental concern, owing to the increasing number of pesticides detected in water and to the establishment of directives such as Directive 2000/60/EC which defined a framework for the protection of waters bodies. A number of pesticides belonging to the classes of phenylureas, triazines and chloroacetanilides, are mentioned as priority substances. Passive sampling techniques, which rely on the diffusion of pollutants from the sampled medium to a receiving phase, have gained in popularity because they can provide time-weighted average (TWA) concentrations of pollutants when used in integrative mode. Since these devices accumulate pollutants over a period of time, they can, depending on the compound-dependent sampling rate, provide a preconcentration of analytes, thus enabling the detection of compounds present in the water at very low levels that are not always detected in grab samples of water. In order to calculate the TWA concentration of a pollutant in water, it is necessary to measure the mass of analyte accumulated in the sampler, the deployment time, and to know the corresponding sampling rate Rs. The latter is determined, in particular for Chemcatcher®, in laboratory scale calibration experiments, where the samplers are exposed for a defined time to a constant analyte concentration in water. This work aims to compare the sampling rate Rs obtained through flow pilot calibration and batch experiments for 20 polar herbicides. The uptake of herbicides by the Chemcatcher® (C18 as receiving phase and PES membrane) has been determined by the both methods. Based on a kinetic approach two methods of calculating Rs are proposed; one based on batch experiments, and the other on through flow laboratory calibration. The results of the batch experiment are in good agreement to those of flow laboratory calibration and the former approach has the advantage of simplifying the calibration procedure for determining Rs. However, since hydrodynamic conditions can vary widely in the field, caution is needed when using laboratory based calibration data in deployments.