On the transport and fate of heavy metals in calcareous soils – a spectral induced polarization study

The fate and transport of heavy metals in the soil have been intensively studied over the last decades due to its implications on public health and the environment. The mobility of heavy metals in the soil depends on the surface characteristics of the soil minerals and other solid components such as organic matter, the pore water pH and its composition, among others. Specifically, in calcareous soils, the introduction of heavy metals has been shown to induce dissolution of the calcite and formation of metal-based carbonate minerals. The study of such processes traditionally involves intensive sample collection and chemical analysis of multiple species. In this study, we use spectral induced polarization (SIP) to in-situ monitor the transport three heavy metals (Pb, Zn and Cu) through soil that contain calcite to different extents. In SIP, an alternating current in wide range of frequencies is injected, and the phase and amplitude difference between the injected and induced potential are measured and translated into a complex conductivity spectrum. These measurements are sensitive to both pore water characteristics and to surface processes. Our experimental setup involves flow-through columns packed with different types of soil, through which the inflow solution is passed. Electrical potentials are recorded at three locations along the column. The analyzed SIP measurements allow not only non-invasive, non-destructive monitoring of the metal progression through the soil but also deduction of its fate through combination with elemental analysis.  The results show that both the real and the imaginary components of the complex conductivity are sensitive to the minerals’ dissolution/precipitation. The conductivity values at the peak polarization frequency over time depict the progression of the dissolution/precipitation ‘front’ along the soil profile.