Time-lapse 3D imaging by positron emission tomography of Cu mobilized in a soil column by the herbicide MCPA

Abstract

Phenoxyalkanoic acids like the 4-chloro-2-methylphenoxyacetic acid (MCPA) are the second highest used xenobiotic herbicides worldwide after glyphosate because of their apparently favorable environmental properties. Experimental batch equilibration data suggested a reduced Cu adsorption efficiency with the soil mineral goethite below pH 6 in presence of MCPA. This has been verified by advanced surface complexation adsorption modelling involving dissolved Cu-MCPA complexation constants. Positron emission tomography is a non-invasive molecular imaging method for time-resolved three-dimensional information commonly applied on non-retarded tracers in soil core scale experiments. Mineral surface reactive tracers like Cu-64 are too immobile for the relatively short observation times available with this advanced imaging technique. However, Cu-64 radiolabeled Cu-MCPA complex migration could be observed in as long as 10-cm artificial soil test columns where break-through occurred within a few days. For the first time, time-lapse movies of Cu migration in the opaque soil columns were recorded using this novel reactive transport process tomography approach.

From the Outside in: An Overview of Positron Imaging of Plant and Soil Processes

“…A later study also focused on Cu transport in porous media, using a model soil column composed of sand-and silt-sized quartz coated with goethite (as a reactive solid phase) and illite (Figure 9B). 120 A radiolabelled [ 64 Cu]Cu-MCPA complex showed slowed movement relative to a [ 18 F]F À tracer, suggestive of Cu-MCPA complex interaction with goethite. Despite this interaction with the goethite-coated packing material, breakthrough of Cu-MCPA complexes was observed within 66 hours.…”

“… 115 B) PET imaging of 64 Cu and 18 F in porous media consisting of sand- and silt-sized quartz coated with goethite and illite. 120 [ 64 Cu]Cu-MCPA complex moved slowly relative to a [ 18 F]F − , which suggests Cu-MCPA interaction with goethite. A, B) Flow was driven from the bottom-up to reduce preferential flow channels and flow fingering and to allow escape of any gases formed during reactive flow.…”

“…The graphics in B were adapted from Ref. 120 under Creative Commons Attribution International License CC BY 4.0 ( https://creativecommons.org/licenses/by/4.0/) .…”

“…A) Flow within a sand column with and without a cylindrical obstruction was imaged using positron emission tomography (PET) of [ 18 F]FDG 115. B) PET imaging of64 Cu and18 F in porous media consisting of sand-and silt-sized quartz coated with goethite and illite 120. [ 64 Cu]Cu-MCPA complex moved slowly relative to a [ 18 F]F À , which suggests Cu-MCPA interaction with goethite.…”

“…Isotopes Used in Studies of Soils, Sediments, and Model Porous Media, With Half-Lives and Forms Used. Cu 2þ , 116 [ 64 Cu]Cu 2þ -2-methyl-4-chlorophenoxyacetic acid (MCPA)120 …”