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TECHNICAL REPORT
Organic Compounds in the Environment

Adsorption and Degradation of the Weak Acid Mesotrione in Soil and Environmental Fate Implications

^a Syngenta, Jealott's Hill International Research Station, Bracknell, Berkshire RG42 6ET, UK
^b Soil Survey and Land Research Centre, Cranfield University, Silsoe, Bedford MK45 4DT, UK

* Corresponding author (jeremy.dyson{at}syngenta.com)

Received for publication January 22, 2001. The ability of soils to adsorb and degrade pesticides strongly influences their environmental fate. This paper examines the adsorption and degradation of a weak acid, a new herbicide mesotrione [2-[4-(methylsulfonyl)-2-nitrobenzoyl]-1,3-cyclohexanedione], in 15 different soils from Europe and the USA. Experiments were conducted to understand the influence of soil properties, covering a wide range of soil textures, soil pH values (4.4 to 7.5), and organic carbon contents (0.6 to 3.35%). Mesotrione adsorption (K_d values ranged from 0.13 to 5.0 L/kg) was primarily related to soil pH, and to a lesser extent by percent organic carbon (%OC). As soil pH rose, mesotrione K_d values got smaller as mesotrione dissociated from the molecular to anionic form. Mesotrione degradation (half-lives ranged from 4.5 to 32 d) was also related to soil pH, getting shorter as soil pH rose. Simple regression of mesotrione adsorption against soil pH and %OC and against degradation provided a close fit to the data. The correlation between mesotrione adsorption and degradation means that K_d and half-life values are only relevant for use in environmental fate assessment if these values are "paired" for the same soil pH and %OC. The implications were as illustrated for leaching, raising important issues about combining pesticide adsorption and degradation behavior in environmental fate assessments.

Abbreviations: %OC, percent organic carbon