Phenoxyalkanoic acid herbicide sorption and the effect of co-application in a Haplic Cambisol with contrasting management.

The adsorption and desorption behaviour of two phenoxyalkanoic acid herbicides (MCPA and mecoprop-p) in a Haplic Cambisol with tillage and grassland management was examined using a batch equilibrium method. Additionally, the effect on adsorption of the simultaneous presence of the two herbicides was also studied. The sorption equilibrium was reached within 24h for adsorption and desorption in both soils. The experimental sorption data for MCPA and mecoprop-p fitted the Freundlich and the linear adsorption isotherms very well (R(2)>0.99). The Freundlich exponent values of the adsorption isotherm ranged from 0.91 to 0.98 indicating a non-linear and a linear adsorption of the two chemicals studied. Generally, mecoprop-p showed lower adsorption than MCPA, although the adsorption of both phenoxyalkanoic acid herbicides was low. The adsorption process was not fully reversed during one washing cycle, as the K(f) values for desorption were greater than corresponding K(f) values for adsorption. The Freundlich exponent (1/n) of the MCPA adsorption isotherm was affected by the simultaneous presence of both herbicides and tended more towards non-linearity, whilst the mecoprop-p adsorption exponent remained unaltered. The Freundlich and the linear adsorption coefficients calculated in the mix study were not that different from the adsorption coefficients calculated in the single compound study, and therefore single compound adsorption data can be used when modelling the fate of simultaneously applied phenoxyalkanoic acid herbicides. The study also indicated that both herbicides were poorly sorbed by the soils studied, and therefore may pose a risk of surface and/or groundwater pollution in Ireland.