In situ chemical oxidation (ISCO) using permanganate is an approach to organic contaminant remediation increasingly being applied at hazardous waste sites throughout the United States. Manganese dioxide (MnO2) particles are products of the reaction of permanganate with organic contaminants and naturally reduced subsurface materials. These particles are of interest because they have the potential to deposit in the subsurface and impact the flow regime in and around permanganate injection, including the well screen, filter pack, and the surrounding subsurface formation.
The goal of this research was to understand the genesis and control of MnO2 particles and to identify particle stabilization aids that would allow for their transport in groundwater through porous media under a variety of reaction conditions. Control of these particles can lead to improved oxidant injection, oxidant transport, and contact between the oxidant and contaminants of concern. Specific objectives of this project were to determine (1) if MnO2 particles can be stabilized/controlled in an aqueous phase to allow for transport through a solids phase, thereby inhibiting subsurface deposition and (2) the dependence of stabilization and control of MnO2 particles on porous media and groundwater characteristics.
Bench-scale batch experiments to initially study important chemical interactions, followed by column studies to incorporate transport phenomena, were conducted to study particle stabilization aids under varied reaction matrix conditions. Variations included particle and stabilization aid concentrations, groundwater ionic content, pH, porous media type, and oxidation/reduction potential (ORP) conditions.
In the batch experiments, four stabilization aids were evaluated for their ability to stabilize particles in solution over time and a range of groundwater conditions. The stabilization aid sodium hexametaphosphate (HMP) demonstrated the most promising results based on spectrophotometric studies of particle behavior, particle filtration results at varied pore sizes, and optical measurements of particle size and zeta potential. HMP inhibited particle settling, provided for greater particle stability, and resulted in particles of a smaller average size over a range of pH, particle concentration, ionic content/strength, and ORP conditions compared to results for systems that did not include HMP. These results indicate that the inclusion of HMP in a permanganate oxidation system improves conditions that may facilitate particle transport.
Based on the favorable results in the batch experiments, one-dimensional experimental transport studies were conducted to evaluate the impact of including HMP with delivery of permanganate to a nonaqueous phase liquid (NAPL) source zone within four different media types. Media types included sand only, sand and 20% montmorillonite clay, sand and 1% goethite (FeO(OH)), and sand and 0.5% organic carbon. Particle transport through the media and retention of MnO2 particles within the media were characterized following permanganate delivery with and without HMP. While particle retention and transport varied with specific media type, HMP consistently provided for significantly decreased particle retention and improved flow. With HMP, particle retention directly in the NAPL source zone decreased by 25% in sand media, 53% in sand and clay media, 85% in sand and goethite media, and 47% in sand and organic carbon media.
Decreased particle retention with the use of HMP can allow for improved oxidant injection and transport, as well as contact between the oxidant and contaminants of concern. Improved oxidant delivery and flow translates to more efficient ISCO treatment, decreased potential for post-treatment contaminant rebound, and less reliance on invasive or expensive post-ISCO processes for treating contaminant residual. (Project Completed – 2008)