Magnetic resonance imaging methods to reveal the real-time distribution of nickel in porous media

Direct and non-destructive measurement of the sorption, diffusion and mobility of ions and molecules in porous media has applications in industry and environmental science. We used magnetic resonance imaging (MRI) to visualize the dynamic distribution of paramagnetic nickel (Ni+2) ions in porous media. Various MRI sequences were tested to image Ni2+ at small concentrations. Noisy gradient echo images had poor contrast between samples containing various Ni2+ concentrations. Turbo spin echo and spoiled gradient echo images showed a linear relation between Ni2+ concentrations and signal intensity over a wide range of concentrations. Spoiled gradient echo images resolved Ni2+ concentrations (down to 30 mg litre(-1)) better than turbo spin echo images. However, for smaller concentrations, uncertainty in intensity values increased. A T-1 measurement, obtained using an inversion recovery sequence, showed a linear correlation between T-1 and Ni2+ concentration down to 1.5 mg litre(-1). In a glass bead medium with an ion exchange resin as an Ni sink, the real-time development of the Ni2+ depletion zone around the resin, as Ni was sorbed into the resin, was imaged by T-1 mapping. A spatial resolution of 0.58 mm and a temporal resolution of less than a minute were achieved. The two-dimensional Ni2+ gradient that was determined from MRI agreed well with geochemical modelling results. The results of this study showed that MRI, in particular T-1 mapping, can quantify microscale behaviour of paramagnetic species in porous media. However, ferromagnetic components that naturally occur in most soils can easily disturb the MRI signal.