Spatiotemporal process monitoring of conservative and reactive tracer transport in a synthetic soil column

Transport and retardation of chemical species in soils as observed by input-output approaches are commonly interpreted by process simulations and break-through curve (BTC) fitting. Positron emission tomography (PET) provides a direct quantitative spatiotemporal (4D) visualization method for the propagation of compounds labelled with a PET-tracer at intermediate resolution and molecular sensitivity (Kulenkampff et al. 2013).
In the framework of SPP 1315, we conducted transport experiments on an artificial soil column with both reactive and conservative tracers, which were monitored with sequential PET imaging. The soil column used (l: 94.5 mm, d: 40 mm; composition: 94% sand, 5% illite, 1% goethite; porosity: 29%) was prepared under CO2-atmosphere and structurally characterized by µCT imaging as widely homogeneous. For the conservative tracer experiment, 5 mL 0.001 M NaNO3 + 0.01 M [18F]KF was flown through the equilibrated column. For the reactive species experiment, 64Cu was produced at the Leipzig cyclotron by the nuclear reaction 64Ni(p,n)64Cu and separation by ion exchange. 5 mL of 0.0008 M [64Cu]Cu(MCPA)2 was produced from 2 mL 64Cu2+ in 0.1 M HNO3, 1 mg Cu(NO3)2•3H2O and 2 mg MCPA in synthetic pore water. The labeled solution was adjusted to pH 5 and flown through the column, which had no former contact with MCPA and had been preconditioned for 4 days with synthetic pore water at pH 5. In both experiments the flow rate was 0.1 ml/min.
In the conservative experiment, the break-through occurred after 140 min, and – in spite of the homogeneous packing of the column – the tracer propagation observed with PET showed a preferential flow field towards the rim of the sample. The reactive [64Cu]Cu(MCPA)2 pulse was strongly retarded with a break-through of the activity after 66 h. Fig. 1 shows a snapshot of both experiments after 110 min.
Preferential and superficial transport, commonly ignored in input-output approaches, controls the effective volume and reactive internal surface area, and thus impacts interpretation and inverse numerical modelling of BTCs. Such effects can be assessed and quantified with PET process tomography, especially when the pore structure is heterogeneously altered by microbial activity.

Reference
Kulenkampff, J., Gründig, M., Korn, N., Zakhnini, A., Barth, T., Lippmann-Pipke, J., 2013. Application of high-resolution positron-emission-tomography for quantitative spatiotemporal process monitoring in dense material. http://www.isipt.org/world-congress/7/902.html.