Sorption and degradation of four nitroaromatic herbicides in mono and multi-solute saturated/unsaturated soil batch systems

This paper presents a study on sorption and degradation processes involved in the fate of nitroaromatic herbicides in an alluvial sandy loam. Particular attention was given to a competitive sorption process and its impact on herbicide biodegradation through bioavailability modification. The main question addressed was the occurrence of antagonistic or synergistic effects in herbicide mixtures. Approaching the problem by using a herbicide combination, it was demonstrated that the more soluble herbicides strongly decreased the sorption of the more hydrophobic ones on the soil organic fraction. Conversely, ionic strength was shown to increase sorption levels dramatically. These results prove that soil solution chemistry is a relevant factor to be taken into account in pesticide behaviour studies. Herbicide biodegradation was studied with the same approach, and the results revealed that degradation of a particular dinitrophenol is affected by the presence of similar molecules. In well-dispersed soil suspensions where herbicide/micro-organism contact is optimal, toxicity was shown to increase herbicide persistence and to be the controlling factor of biodegradation. Conversely, persistence in repacked unsaturated soil batches was strongly decreased in both mono and multi-solute systems. In such solid batches, the soil structure imposed mass transfer kinetics which modify micro-organism/herbicide contact and decreased toxicity effects. Furthermore, the competitive sorption observed in multi-solute systems was supposed to be responsible for the observed increase of herbicide biodegradation presumably by keeping molecules bioavailable for microbial attack. These results support the assumption that soils are divided into two compartments presenting different capacities in regard to chemical sorption and biodegradation, which could be a basis for explaining chemical aging in soil. This study providing new information on physico–chemical control of pollutant biodegradation in soils, contributes to the understanding of the reasons for chemical persistence and mobility, and opens some perspectives to improvements of soil bioremediation.