Electro-osmotic transport of nano zero-valent iron in Boom Clay

Abstract

One of the many problems that impede the successful remediation of soil and groundwater contaminated with chlorinated hydrocarbons occurs when pollutants diffuse into low permeability zones, and become difficultly accessible. Remediation by reductive dechlorination using nanoscale zero-valent iron (nZVI) is possible, but its delivery into contaminated hot spots is difficult using solely hydraulic gradients. Recently, electrokinetics has been proposed to transport nZVI, with the assumption that both electrophoresis and electro-osmosis are involved. However, so far no study has focused solely on transport driven by electro-osmotic flow. This driving force could significantly improve field remediation, allowing nZVI to reach low permeability hotspots. This work describes the experiments carried out to evaluate if nanoscale zero-valent iron can be transported by electro-osmotic advection in a dense clay using a specifically designed laboratory apparatus. It explores how working parameters (such as voltage, injection point and pH) influence transport, and provides an insight into the subsurface behaviour of nanoparticles using micro-XRF. The results confirm the transport of nZVI by electro-osmotic advection, with an electro-osmotic conductivity in the range (0.5-1.0)·10−10 m2V−1s−1. The presence of nZVI decreases electro-osmotic conductivities by approximately half and affects geochemical conditions of the system, inducing sharper pH profiles and enhancing cation exchange in the clay.