Sorption characteristics of pesticides on matrix substrates used in biopurification systems.

On-farm biopurification systems were developed to remove pesticides from contaminated water generated at the farmyard. An important process in the system's efficiency is the sorption of pesticides to the substrates used in the biopurification systems. The composition and type of material present in the biobed are crucial for retention of chemicals. This study investigated the sorption of linuron, isoproturon, metalaxyl, isoxaben, bentazon and lenacil on substrates commonly used in a biopurification system, i.e. cow manure, straw, willow chopping, soil, coconut chips, garden waste compost, and peat mix. Linear, Freundlich, and Langmuir sorption isotherms were fitted to the obtained data. The best fit was obtained with the Freundlich model. More immobile pesticides (i.e. linuron and isoxaben) tended to associate with the organic substrate, while more mobile pesticides partition in the water (i.e. bentazon). According to sorption capacity, the substrates could be classified as peat mix > compost, coco chips, straw > cow manure, willow chopping > sandy loam soil. Sorption capacity was positively correlated with the organic carbon content, CaO and the cation exchange capacity. Furthermore, no significant differences in sorption could be found between technical and formulated isoproturon and bentazon. Moreover, the individual sorption coefficient K(d) was additive, which means that individual sorption coefficients can be used to calculate the sorption coefficients of a mixture of substrates. What concerns the mutual interaction of pesticides it could be observed that the sorption of linuron and metalaxyl was significantly lower in combination with isoproturon and bentazon, while the latter pesticides were not influenced by the presence of linuron and metalaxyl. As guidelines, firstly, it could be stated that using the most sorbing materials such as peat mix, might significantly increase the biopurification systems efficiency. Secondly, the treatment of very mobile pesticides, such as bentazon, should be taken with care as these will easily leach through the system. Additional chemical treatment might be necessary for these type of pesticides.