Investigation of Iron(III)-Release in the Pore Water of Natural Sands by NMR Relaxometry

Abstract

Proton nuclear magnetic resonance (NMR) relaxometry offers a non-invasive and non-destructive measurement method to observe and to visualise changes in the iron(III)-ion concentration in aqueous solutions. This is possible due to its paramagnetic influence on the relaxation times. In the context of mineral dissolution processes in natural sediments, the effect of the presence of dissolved iron(III) on the NMR relaxation times of the pore water was investigated. The decrease in the relaxation times T1 and T2 corresponding to an increase in the dissolved iron(III) concentration was quantified. This relation was used to monitor relative changes in the concentration of dissolved iron(III)-ions in natural sands. Experiments were conducted to calibrate the iron(III) concentration from the measured relaxation times. These were done in bulk water as well as in iron(III)-mineral bearing sands to take the effect of surface relaxation into account. It was shown that for relatively coarse-grained sand fractions the effect of iron(III)-ions in solution dominates. This allows the determination of the dissolved iron(III) concentration in natural sands by NMR. The method also enables us to capture the changes in the iron(III) concentration with high temporal resolution. This was demonstrated in column experiments, in which an acid (hydrochloric or sulphuric acid) was applied from the top on the sands to dissolve the mineralogical bound iron(III) and were the dissolution of iron(III) can be captured with sufficient temporal resolution.