MRI measurements of colloid transport and adsorption in porous medium

The accuracy to predict transport and retention of colloidal particles is a major environmental concern as such particles can carry adsorbed pollutants or be pollutants themselves. The methods currently used to predict the fate of colloids in soils are based on studies of breakthrough curves (evolution of concentration as a function of pore volume or time) after injection of particles into a column of porous media. For this reason, the porous media with regards to colloid transport is a kind of a "black box" system.

We carried out Magnetic Resonance Imaging experiments which provide spatial distribution of colloidal particles in time along the sample axis during a transport experiment through a model porous medium (sand column). The decrease of NMR signal comes from adsorbed particles and suspended particles.
Measures were performed on a vertical imaging spectrometer DBX 24/80 from Bruker operating at 0.5T (20MHz proton), which is equipped with a proton birdcage radio frequency coil of 20cm inner diameter for experiment on column, and with a minispec Bruker with 10mm diameter tube for agent contrast analysis effect on signal and for evaluating adsorption. We used global relaxation measurements to calibrate effect of particles on NMR signal, and double spin echo profile sequence to observe and calculate the contrast agent concentration for transport experiments, and finally, spin echo images.

The effect of each kind of particle on the signal was calibrated. Two extreme column experiments were followed by MRI and breakthrough curves were measured. The first experiment was an injection of negatively charged particles, so adsorption was neglected. The particles were recovered at the outlet up to 95%. MRI measures show that the pulse follows water flow. The other experiment was an injection of positively charged particles. The measures show a very fast adsorption, with zero recovery at the outlet. At the end of the experiment, we can observe a final profile of only adsorbed particles in the material. In fact, our method can be used to compare different models with the actual concentrations of suspended and adsorbed particles during a transport experiment.