Unveiling Spectral Induced Polarization Responses of ZVI-AC-Sand Mixtures in Groundwater Remediation

The long-term performance of permeable reactive barriers may diminish due to oxidation, adsorption on the particle surfaces, and pore space clogging. Offering warnings about performance reduction through geophysical techniques enables timely remedial measures. The impact of volume content of conductive particles, particle size, and pore water conductivity on the spectral induced polarization (SIP) response of mixtures comprising zero-valent iron (ZVI), activated carbon (AC), and silica sand was revealed through column experiments. The electrical conductivity and the chargeability are correlated with the surface area of ZVI and AC per unit pore volume. The relaxation time is positively correlated with particle sizes and electrolyte conductivity. The polarization response resulting from the mixture of two types of filled particles exhibits a superposition relationship governed by the particle size distribution. SIP monitoring for 720 hr was conducted on a sand-ZVI-AC column saturated with NaCl or NaNO₃ solutions. Compared to the NaCl solution, under NO₃⁻ contamination, a thicker adsorption layer (∼0.05 mm) was observed on the ZVI surface in the microstructure. This resulted in a doubling of the parameters related to the relaxation time. Chargeability indicates changes in the mixture's active surface area, reflecting the reactivity of ZVI and the adsorption capacity of AC. Our findings illustrate that the induced polarization parameters record the changes in the reactive surface and particle size of the conductive particles in the sand, ZVI, and AC mixture, indicating the potential for long-term monitoring of the reaction barrier.