Electrokinetic techniques in subsurface applications

Electrokinetic (EK) techniques involve the establishment of electric fields to enhance solute transport and species mobility with many applications in different fields of science and engineering. In the context of remediation of contaminated sites, electrokinetics is considered a promising technology for in situ interventions due to its potential to overcome limitations regarding the delivery of amendments and the mobilization of contaminants in low-permeability zones.

Several studies confirmed the effectiveness of EK techniques for the remediation of different contaminants both at laboratory and at pilot field scales. However, the mechanistic understanding of the governing processes and their interactions is still limited due to the complexity of the underlying physical and biogeochemical phenomena.

This thesis presents an investigation on the interplay of the major processes during electrokinetics with the goal of expanding the current understanding of the fundamental mechanisms of EK transport and of developing modeling frameworks necessary for interpretation of experimental observations and for optimal design of in situ interventions.

In a first experimental study, it was demonstrated that changes in chemical conditions of a background electrolyte in the pore water of a saturated porous medium, exert a key control on the macroscopic transport of charged solutes through electromigration. Successively, experiments on EK transport were also performed in multidimensional heterogeneous settings. The results of this investigation show that physical heterogeneity and pore water chemistry greatly impact transport patterns, with radically different mixing dynamics with respect to transport driven by natural water flow and advective-dispersive mass transfer.

A process-based modeling framework was developed to take into account the transport of charged and uncharged species in heterogeneous domains, with the aim of interpreting the experimental observations and assessing the suitability of the model to simulate electrokinetic processes in different conditions. The insights gained from the fundamental studies were applied for the process-based modeling of in situ applications of electrokinetics in subsurface environments. In the context of groundwater remediation, process-based modeling of electrokinetic bioremediation was conducted to identify the controlling processes of amendments delivery and contaminant degradation, and to develop quantitative metrics for the assessment of the remediation performance. In another study, a machine learning surrogate model was developed to overcome the long computational times of the process-based models. With such approach it is possible to obtain model results in the order of milliseconds thus allowing the possibility to perform extensive model exploration, sensitivity analysis and uncertainty analysis of EK applications. Finally, the last study presents the proof of concept of electrokinetic in situ leaching (EK-ISL), a promising technique for metals recovery, applied for the leaching of copper from intact ores.

In conclusion, the research performed in this PhD has: (i) furthered the understanding of fundamental processes occurring during electrokinetic transport of solutes in homogeneous and heterogeneous porous media, (ii) demonstrated the suitability of both process-based and machine learning modeling approaches for quantitative description of EK processes in subsurface applications, and (iii) investigated the potential of EK in the field of remediation of contaminated sites and for the sustainable recovery of key metals for the green transition.