Phenoxyalkanoic acid herbicide sorption and the effect of co-application in a Haplic Cambisol with contrasting management Agnieszka A. Piwowarczyk ⇑ , Nicholas M. Holden 1 UCD School of Biosystems Engineering, University College Dublin, Belfield, Dublin 4, Ireland highlights " MCPA and mecoprop-p adsorption in a Haplic Cambisol was generally linear. " Mecoprop-p adsorption had less extent that MCPA adsorption. " Adsorption process could not be fully reversed. " Simultaneous presence of both herbicide showed competition for available soil sorption sites. article info Article history: Received 29 May 2012 Received in revised form 25 July 2012 Accepted 7 August 2012 Available online 6 September 2012 Keywords: Phenoxyalkanoic acid herbicides Freundlich isotherm Adsorption Desorption Soil Management abstract 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 24 h 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 phenoxy- alkanoic 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 Fre- undlich 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. Ó 2012 Elsevier Ltd. All rights reserved. 1. Introduction Phenoxyalkanoic acid herbicides are suitable model substances to study adsorption and desorption processes in soils since they have one polar carboxylic group and one lipophilic phenyl moiety (Bolan and Baskaran, 1996; Celis et al., 1999; Haberhauer et al., 2000). MCPA (4-chloro-2-methylphenoxy) acetic acid is an aryl- oxyalkanoic acid, systemic, polar herbicide, used for post-emer- gence control of annual and perennial broad-leaved weeds in a range of crops, in grasslands and on roadside verges. It is also used to control aquatic broad-leaved weeds (Roberts, 1998). Unlike mecoprop-p, MCPA does not possess a propionic acid side chain and thus is more susceptible to microbial degradation (Alexander and Aleem, 1961). Mecoprop-p (R)-2-(4-Chloro-2-methylophen- oxy) propanoic acid is an aryloxyalkanoic acid and is the herbicidal stereoisomer of mecoprop-p (racemic form). It is also a polar her- bicide and is used for post-emergence control of broad-leaved weeds in wheat, barley, oats, herbage, seed crops and grassland (Roberts, 1998). Field applications of MCPA and mecoprop-p are in the form of salts or ester and often exist as co-formulation prod- ucts. An average maximum amount of MCPA that can be applied to 0045-6535/$ - see front matter Ó 2012 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.chemosphere.2012.08.023 ⇑ Corresponding author. Tel.: +353 1 716 7395. E-mail addresses: agnieszka.piwowarczyk@ucd.ie (A.A. Piwowarczyk), nick. holden@ucd.ie (N.M. Holden). 1 Tel.: +353 1 716 7460; fax: +353 1 716 7415. Chemosphere 90 (2013) 535–541 Contents lists available at SciVerse ScienceDirect Chemosphere journal homepage: www.elsevier.com/locate/chemosphere