Effect of pH on the mobility of the herbicide MCPA in an artificial soil matrix: Reactive transport modelling on the basis of 3D flow patterns visualized by Positron Emission Tomography

Adsorption and transport of the herbicide 2-methyl-4-chlorophenoxyacetic acid (MCPA) in a sand-goethite matrix were investigated as a function of pH. Compared to adsorbed amounts reported for freshly prepared goethite, adsorption onto the commercial product used in this study was found to be considerably lower. Under acidic conditions, transport of MCPA was, however, significantly retarded referring to [³H]H₂O as a conservative tracer. Interaction as a function of pH was geochemically modelled using the charge distribution multisite complexation (CD-MUSIC) approach. Based on this calibrated surface complexation model, breakthrough curves were calculated according to the 1D advection-dispersion-reaction equation. Retardation was slightly underestimated at low pH.
As a new approach, this study demonstrates quantitatively that discrepancies between batch and column systems can be caused by peripheral flow, i.e., not necessarily by non-equilibrium conditions, which are commonly taken into account. By means of Positron Emission Tomography (PET) using [¹⁸F]F^– as a radiotracer, flow patterns in the sand-goethite matrix were visualized in 3D. A pseudo-3D flow and transport model was aligned to the images. The observed flow profile was successfully simulated by assuming a peripheral zone with increased permeability and porosity. With this flow model, reactive transport of MCPA was predicted more precisely compared to the 1D calculations with the same parameter values.