Spectral Induced Polarization for Evaluating Cr(VI) Removal by ZVI-AC Composites with Effects of Carbon Structure

Spectral-induced polarization (SIP) is a pivotal technique for characterizing the remediation performance of permeable reactive barriers (PRBs). Using two experimental cases, this study aimed to elucidate the regulatory effect of the carbon structure on the remediation of Cr(VI) in groundwater by zerovalent iron-activated carbon (ZVI-AC) composites. In case 2, the average particle diameter of AC is 3.5 times that in Case 1, with a 10-fold higher specific surface area. The large-sized, high-surface-area AC composite achieved stable Cr(VI) removal of 95.3% across 94 pore volumes. AC adsorbed reaction precipitates, alleviated ZVI surface passivation, and promoted the reduction of Cr(VI) to Cr(III). SIP parameters captured the dynamic remediation evolution: conductivity was positively correlated with treatment performance, while chargeability was negatively correlated with cumulative Cr(VI) removal. The relaxation time exhibited a strong linear correlation with the cumulative Cr(VI) removal amount (CRA), and its variation was dominated by the component with the most significant particle-size change. Specifically, ZVI dominated in Case 1, while AC played a leading role in Case 2. This study reveals SIP responses to composite media changes during Cr(VI) remediation and supports PRB filler optimization and real-time groundwater remediation monitoring.