Spectral Induced Polarization of Goethite Nanoparticles
Friday, 19 December 2014
Goethite nanoparticles are being considered as a tool to enhance in situ remediation of aquifers contaminated with aromatic hydrocarbons. Injection of goethite nanoparticles into the plume is expected to enhance microbial iron reduction and associated beneficial oxidation of hydrocarbons in a cost-effective manner. Amongst others, current challenges associated with this novel approach are the monitoring of nanoparticle delivery and the nanoparticle and contaminant concentration dynamics over time. Obviously, non-invasive monitoring of these properties would be highly useful. In this study, we aim to evaluate whether spectral induced polarization (SIP) measurements of the complex electrical conductivity are suitable for such non-invasive characterization. In principle, this is not unreasonable because the electrical double layers of the goethite nanoparticles are expected to affect electrical polarization and thus the imaginary part of the complex electrical conductivity. In a first set of measurements, we determined the complex electrical conductivity of goethite nanoparticle suspensions with different nanoparticle concentrations, pH, and ionic strength in the mHz to kHz frequency range. In a second set of measurements, mixtures of sand and different concentrations of goethite nanoparticles and variable pH and ionic strengths were analyzed. Finally, flow experiments were monitored with SIP in a 1-m long laboratory column to investigate dynamic effects associated with goethite nanoparticle injection and delivery. The results showed that the imaginary part of the electrical conductivity was only affected in the high frequency range (Hz – kHz), which is expected from the small size of the goethite nanoparticles. Overall, we found that the goethite nanoparticles are associated with a small increase in the imaginary electrical conductivity at 1 kHz that can be measured in situ using recently improved borehole electrical impedance tomography measurement equipment that provides the required accuracy for frequencies above 100 Hz.