A Spectral Induced Polarization (SIP) Lab-Scale Framework for Monitoring Controlled Ammonium Desorption from Municipal Solid Waste

The presence of ammonium in leachate from municipal solid waste (MSW) landfills poses a long-term pollution hazard to the environment. Controlled (cat)ion exchange using e.g., potassium has shown promise as a potentially suitable method for in situ pretreatment and removal of ammonium from landfills. However, the spatial heterogeneity of MSW renders tracking the effectiveness of controlled ion exchange in situ an operational challenge. Spectral induced polarization (SIP), a noninvasive geophysical technique sensitive to changes in the electrical properties of subsurface matrices and their interactions with contaminants offers a potential monitoring solution. In the present study, the controlled desorption behavior of ammonium and its relationship with the SIP-derived imaginary conductivity was monitored in flow-through columns filled with MSW. Experiments were carried out under different salinities and leachate concentrations. Ion breakthrough curves were simulated with a reactive transport model that accounts for ion activities at highly saline conditions, enabling the quantification of sorbed ion concentrations on the MSW. For the low salinity injection, imaginary conductivity changes correlate strongly with changes in the sorbed concentrations of cations, closely following variations in sorbed ammonium. At high salinities, the ionic strength contribution dominates changes in electrical signals. While a quantitative assessment of sorbed concentrations is less likely at high salinity, the results show promise in capturing the movement of the ion-exchange front even in high ionic strength environments such as landfills. The laboratory experiments herein highlight the potential and limitations of SIP as a solution for monitoring controlled ammonium release and support its eventual upscaling and field application.