Technical Report 123

Soil adsorption and metal chelation

There is a discrete class of organic compounds that have the ability to chelate metals, by ligand binding of the metal ions. The so-formed chelate metal complexes can sometimes play an important role in the mode of action of a pesticide, for example HPPD inhibition. In such examples, the same metal chelating properties that drive pesticide efficacy can also result in increased soil adsorption through chelation of metals located on/in the surface of the soil matrix. In the case of mesotrione (Section 5.2.2), a simple comparison of the measured KOC of mesotrione with an estimate derived from the relationship between log KOW and soil adsorption (such as that proposed by Briggs, 1981) will reveal that the actual soil adsorption is much higher than if it were determined by hydrophobicity alone. Therefore, estimations of soil adsorption for compounds with the ability to chelate metals are often inaccurate.