Noninvasive Investigation of Gold Nanoparticle Transport and Fate in Soil

This work uses spectral-induced polarization (SIP) to monitor the transport of citrate-coated gold nanoparticles (AuNPs) in soil. The experimental setup includes batch experiments with different NP-to-soil ratios, designed to determine the NP concentration at which electrodic polarization is significant, as well as a flow-through experiment in a natural soil column. The analyzed SIP measurements allow not only the noninvasive monitoring of NP progression through the soil but also the deduction of NP-soil interactions through combination with elemental analysis. The real conductivity increased in a breakthrough pattern in response to the AuNP transport due to CaCO₃ dissolution and subsequent Ca²⁺ release. The imaginary conductivity presented a more complex pattern and reflected a combination of NP-soil interactions (NP retention and accumulation, CaCO₃ dissolution with subsequent Ca²⁺ release, and exchange processes). Overall, we show that the progression of NPs through soil can be tracked using SIP even at low concentrations due to their effect on pore water, sediments, and NP-soil and NP-CaCO₃ interactions. Nevertheless, many parts of the observed phenomena require further research to gain better understanding and quantification.