Soil Clay Modification with Spermine and Its Effect on the Behavior of the Herbicide Fluometuron

Research on organoclays as sorbents of pesticides has shown the usefulness of these materials both as soil amendments and as pesticide supports, to prolong the efficacy and reduce the environmental impact of soil-applied pesticides. Organoclays can also be formed in situ by direct modification of smectite-rich soil clays through treatment with appropriate organic cations. In this work, laboratory experiments were conducted to investigate the effect of the addition to soil of a natural organic cation, spermine, on sorption (batch equilibration procedure), persistence (incubation experiments), and leaching (column experiments) of the herbicide fluometuron. Four soils differing in their clay contents and mineralogy were used. A sevenfold increase in the fluometuron sorption distribution coefficient, Kd, by the presence of spermine was observed for a clay soil rich in smectites (Soil 1), whereas the effect on fluometuron sorption was negligible for a soil with similar clay texture but lacking smectites (Soil 4) and for a loamy sand soil with very low content in smectites (Soil 3). A sandy loam soil (Soil 2) with moderate smectite content had an intermediate behavior. The increase in fluometuron sorption was thus related to the smectite content of the soils, strongly indicating that reaction of spermine with soil smectites formed in situ an organoclay complex with high affinity for fluometuron. The simultaneous application of fluometuron and spermine to the smectite-rich soil retarded the leaching of the herbicide but, at the same time, increased the total amount of fluometuron leached from 25 to 52%. This was attributed to the fact that spermine, apart from increasing sorption, also prolonged the persistence of fluometuron in the smectite-rich soil. In contrast, the presence of spermine had no effect on the leaching pattern of fluometuron in the clay soil lacking smectites. The results indicated that the application of suitable organic cations to smectitic soils can be a useful strategy to control pesticide retention and persistence in the soil environment.