Effect of different fractions of organic carbon on adsorption of metalaxyl in waste organic substrates and soils

A laboratory experiment was carried out to determine the organic carbon content and quality of different organic residues (urban and green compost, moist olive oil husks) and soils of different origin (forest and agricultural) to be used as biofilter in the biodepuration of pesticide contaminated water. An adsorption/desorption experiment was also performed to assess the retention capacity of substrates tested against fungicide metalaxyl. The goal of the work was to determine the extent of the adsorption/desorption of the mobile fungicide metalaxyl in substrates containing different amounts and quality of organic carbon and to characterize the relationship be-tween the carbon fractions and the adsorption/desorption parameters determined.The Kads values found in the organic substrates turned out to be higher than those reported for soils, indicating the highest sorption capability of these materials and their suitability to be used as bio-filters for decontaminating water. The quality of organic carbon strongly influenced the entity and the reversibility of bonds between substrates and fungicide. The influence of different organic fractions on the adsorption of metalaxyl can be summarized in the order of humic acids carbon > total humified extractable carbon > total organic carbon > total carbon, leading to the conclusion that the presence of the humified fraction in the organic substrates is essential for a good performance of biofilters. Indeed, the desorption experiment evidenced that metalaxyl is less desorbed in the most humified substrates.Therefore, in bio-depuration systems such as biobed it is convenient to use mature substrates, which have to undergo a composting process, leading to high humus content and quality, responsible for irreversible bonds with pesticides.