Spatiotemporal quantitative imaging of leaching processes with positron emission tomography (PET), improving process understanding and modelling performance

A current application that needs improved understanding of coupled reactive transport processes and fluid-rock interactions is in situ leaching of ore minerals. These processes are largely controlled by the chemical and hydrological heterogeneity of the material and the respective transport process.
We apply GeoPET as method for quantitative spatiotemporal imaging of tracer concentrations, which combines molecular sensitivity (picomolar) with suitable spatial resolution (ca. 1 mm) for elucidating heterogeneous effects on the core scale. This laboratory imaging method is virtually unique for crossing the scale from molecular processes to the macrosphere with its typical heterogeneous geoscienctific characteristics. It requires labelling the relevant substance with an appropriate positron-emitting radionuclide, like 18F or 64Cu. PET then makes use of the space-resolved detection of the decay radiation for deriving the tracer concentration.
From the time series of 3D frames of the tracer concentration of conservative flow experiments we compute the distribution of both the process-dependent effective volume and the local velocity distribution of the tracer. These experimental data are the basis of exceptionally efficient finite-element reactive transport models on the millimetre scale with a COMSOL-PhreeqC-coupling.
Our study was conducted within the framework of the French-German project “EcoMetals” which aims at the development of innovative processes for copper (and associated metals) extraction by means of biotechnology. In our showcase experiment we leached copper from pebbles that had been artificially coated with [64Cu]covellite. The PET-experiment has two stages. At first we observed the depletion of the 64Cu activity concentration in the surface layer during leaching with glutamic acid. From this, we derived local covellite leaching rates. After decay of the radionuclide we conducted a conservative flow experiment with pore water labelled with 18F. This experiment identifies strongly localized preferential flow zones. From this, we compute porosity and velocity distributions, which serve as input parameters for the numerical simulation. The simulation results are validated in comparison with observed leaching rates.