Laboratory studies of the adsorption of two pesticides (diuron and tebuconazole) using a batch design and an experimental flume: influence of contact conditions

The adsorption of two commonly used pesticides, diuron and tebuconazole, on an organic substrate (hemp), which was chosen as an analogue for natural substrates often found in agricultural ditches, has been studied using three different contact schemes: 1) contact in a beaker using a modified batch method; 2) contact in an experimental flume, with a dry start substrate; and 3) same as 2) but with the substrate initially saturated with water. Changes in pesticides concentrations as a function of time as well as adsorption after seven hours have been determined for each case. The highest adsorption is observed for the experiments with a flume initially containing dry hemp. In that case, both the initial condition (hemp initially without water) and the contact conditions between the pesticide solution and the substrate appear to favour adsorption. The lowest adsorption is obtained for the flume containing hemp initially saturated with water. In that case, samples that were obtained at different depths inside the hemp using capillary tubes showed that only the superficial part of the hemp was in equilibrium with the surface water. The presence of an initial water table appears to be a limiting factor for pesticides penetration and further adsorption onto hemp. An intermediate adsorption is obtained for the modified batch method. These results highlight the pronounced influence on adsorption of both the initial hemp humidity conditions and the contact conditions between the pesticides solution and the substrate. This influence should be studied further to assess the potential of agricultural ditches to mitigate pesticides contamination in surface water.