Interaction of pesticides with a surfactant-modified soil interface: Effect of soil properties

Adsorption of four organophosphorous insecticides, diazinon, dimethoate, malathion and methidathion, on nine soils representative of the Mediterranean area was studied. The sorption of the organophosphorous pesticides was slightly influenced by soil texture and organic matter content. Pesticide sorption was ranked according to their hydrophobicity, except for the pair diazinon/malathion. Pesticide sorption was also examined in aqueous solutions in the presence of surfactants. Three different surfactants were selected: a cationic (hexadecyl trimethyl ammonium bromide), a non-ionic (Tween 80) and an anionic surfactant (Aerosol 22), to verify the effect of their presence on pesticide retention by the different soils. The addition of Tween 80 and Aerosol did not modify the pesticide sorption and no correlation was found between soil organic matter and clay content and pesticides sorption coefficients. The presence of the cationic surfactant increased pesticide retention, especially when the surfactant was present at a concentration between 100 and 200% of the soil cation exchange capacity. In the presence of the cationic surfactant, the distribution coefficients were significantly correlated with soil exchange capacity. The competition between the weakly retained dimethoate and the other insecticides was tested in the presence or absence of the cationic surfactant. Dimethoate has showed a higher retention when examined alone than when the rest of the insecticides competed for sorption sites.