Investigation into irreversible sorption of pesticides to soil

The importance of extremely slow retention and release has superseded the notion that sorption of pesticides to soil is an instantaneous and reversible process. A fraction of sorbed pesticide is also often reported to bind irreversibly to the soil matrix. This has important implications for pesticide mobility and bioavailability. It is essential to understand sorption phenomena to allow accurate prediction of pesticide fate within the soil environment.
This thesis describes the result of applying a sequential extraction procedure, based on the principles of isotope or “self-exchange”, to nine pesticide/soil systems. The significance of irreversible sorption in controlling pesticide mobility was assessed using isotope exchange (12C and 14C) to characterise pesticide exchange kinetics in-situ over protracted time-scales. Sequential extraction increased in harshness in the order: isotope exchange < forced isotope exchange < solvent extraction. Three pesticides (one neutral, one basic, one acidic) and three temperate, arable soils (ranging in texture and pH) were studied. A three-site sorption model was developed to further interpret the data obtained. Although results showed the experimental design of the isotope exchange technique was not powerful enough to identify whether remaining sorbed pesticide was participating in slowly reversible or irreversible sorption, the forced isotope exchange procedure was able to provide an indication of amounts of pesticide not participating in exchange between the soil and solution. Under abiotic conditions, only minimal amounts of initial-applied pesticide were found to take part in irreversible binding. Soil combustion quantified irreversible sorption in the order: chlorotoluron (≤ 2.27 ± 0.36%) > prometryn (≤ 1.35 ± 0.60%) > hexaconazole (≤ 0.50 ± 0.06%). Varying the soil composition had little effect on amounts of irreversibly sorbed pesticide, probably due to the small amounts of irreversible sorption observed overall. These results suggest that the vast majority of sorbed chlorotoluron, prometryn and hexaconazole (in the parent form) participated in very slow but reversible binding, a result also confirmed by the three-site sorption model.
Pesticide sorption behaviour is a complex process. Although sorption phenomena are still not fully understood, these results provide a greater insight into the significance of irreversible binding for predicting pesticide fate.