Impact of the soil structure and organic matter contents on the photodegradation of the insecticide carbaryl

Abstract

The aim of the present work is to study the photodegradation of the pesticide carbaryl at the surface of soil and its fractions in order to have a better insight for the effect of the particle size and the organic matter content in its photochemical behavior. Moreover, the photoinductive properties of the soil and its fractions, in terms of reactive oxygen species formation, were also explored. A soil sample from Orange region (south of France) was fractionated to several fractions with different particle sizes. Some of these samples were characterized and treated to partially remove organic matter. The pesticide carbaryl was deposited at the surface, and the mixture was irradiated using a solar light simulator. The degradation of the pollutant was followed by HPLC analyses. Similar solid supports were studied for their photoinductive properties by using chemical probes for hydroxyl radical and singlet oxygen formation. The experimental results indicated that carbaryl degradation at the surface of soil and its fractions were efficient and appeared to follow pseudo first-order kinetics. The particle size was found to be an important parameter for the transformation efficiency, the large particles improved carbaryl degradation. Furthermore, natural organic matter showed a beneficial effect at low concentration (<2 % in weight) while inhibition effect was clearly present at higher concentrations. The fractionation procedure and the organic matter removal allowed us to show that the major parameters that control the degradation efficiency are the particle size and organic matter concentration. Moreover, experiments with chemical probes proved the involvement of photoinduced processes implying not only singlet oxygen and hydroxyl radicals produced by the mineral and/or organic part of the soil but also the direct degradation of carbaryl.