Characterizing reaction and transport processes in an electrolytic reactor for in situ groundwater treatment

Abstract

Groundwater contamination poses a long-term threat to subsurface water resources. Effective mitigation of the risk posed by migrating contaminant plumes requires sustainable strategies and technologies due to the expected longevity of contaminant source and plume lifetimes. Electrochemical-based plume control systems have the potential to meet niche applications in a sustainable manner. A commonly proposed technology format for in situ plume control is a permeable reactive barrier (PRB). Effectively deploying an electrolytic reactor in this format requires a fundamental understanding of reaction and transport processes that occur over multiple length scales to properly design and assess system performance. The governing processes in an electrolytic PRB were characterized using an integrated experimental and modeling approach. The PRB utilized thin expanded mesh electrodes consisting of a titanium substrate coated with a binary mixture of TiO2 and IrO2 (Ti/MMO). The first objective of this research was to address the effectiveness of Ti/MMO as a cathode substrate in terms of the ability to degrade model contaminant mixtures and material stability. Nitroaromatic and heterocyclic nitramines were the model contaminants. Nitro-group reduction at the metal oxide surface was found to occur via an adsorbed hydrogen atom by a mechanism that did not include rate inhibition when multiple species were present. Prolonged cell operation led to depletion of surface roughness on the convex front face of the mesh geometry. Reaction rates increased on the smoother aged surface, indicating that mass transfer limitations to active sites on the surface were diminished. The second objective of this research was to elucidate the distribution of reaction conditions over the mesh electrode surface. Using a computational fluid dynamics model to explore the implications of mesh geometry on current density distribution, new electrode geometries were evaluated. The current density distribution was predicted to flatten, resulting in larger overall flux reduction rates when less accessible regions within the mesh aperture were deemphasized. The third overall research objective was to examine and quantify the effect of gas-induced mixing within the electrode assembly on system performance. Gas evolution that normally occurs concomitant with contaminant degradation served to mix the fluid within the electrode assembly effectively enough that reactor performance resembled a continuously-stirred tank reactor at very low flowrates. These results illustrate the effectiveness of metal oxide electrodes to reduce organic contaminants, a substrate not previously explored in this type of application, in a thin cross-flow reactor. Additionally, this study lays a foundation for further development of the electrochemical-based remediation systems, and exploring new cathode materials and deployment platforms for environmental technologies.