In our experience, the most applied of the in-situ technologies in Canada is In-Situ Chemical Oxidation (ISCO). Put simply, ISCO chemically oxidizes compounds of concern within the subsurface. The process transfers electrons from the contaminants to the oxidant, degrading them into less-toxic compounds and harmless by-products.
Numerous factors need to be considered when designing an effective ISCO program, including the oxidant itself, injection method, reaction time, hydrogeology, geology, geochemistry of the soil and groundwater, and treatment objective.
Common oxidants include:
- Hydrogen Peroxide
Oxidant Forms & Chemical Structures
Oxidants can come in various forms–including gas (i.e. ozone), liquid (i.e. hydrogen peroxide), or solid (i.e. persulphate)–with various chemical structures, which can affect their chemical and physical properties, such as solubility, reaction rate, and density.
Limitations of Chemical Oxidants
All chemical oxidants have limitations, including their limited application for free-phase liquids, diffusion-limited remediation, and challenges associated with delivery and distribution of the oxidant itself.
In general, the presence of free-phase NAPL limits the potential of chemical oxidants. However, work being done by various groups and researchers suggests that chemical oxidants may be somewhat effective in helping to address source zones. Hence, the potential effectiveness of oxidants should be considered for each site individually before ruling out the option.
Challenges with Chemical Oxidants
Perhaps the biggest challenge associated with oxidants in general, is the plateauing or “rebound” of contaminant levels above the remedial objective for the site. This situation is especially common in chlorinated hydrocarbon sites with heterogenetic soil or fractured rock conditions. Often referred to as rebound, it usually results from diffusion of the contaminants from finer-grained layers and lenses, or the rock matrix, back into zones of higher hydraulic conductivity. This process can be addressed somewhat by the choice of oxidant, along with delivery method and design; however, it is nearly always an issue with oxidants. IRSL has completed numerous field, laboratory and numerical modeling studies to better understand and overcome this challenge with our approaches showing excellent results.
Another major challenge with using oxidants is delivering the oxidant to the compound of concern. In order for chemical oxidation to be effective the two must come into contact. This can be very challenging in heterogenetic geologic environments. IRSL has developed a variety of techniques to enhance the distribution of oxidants and has conducted numerous peer-reviewed studies to demonstrate the effectiveness of these techniques.
Chemical Oxidant Example: Permanganate (MnO4)
One of the most applied oxidants, permanganate has been well studied and tested for treating a variety of organic compounds, in both wastewater and groundwater. It is typically applied in either its sodium or potassium forms:
- Sodium Permanganate (NaMnO4) often comes as a liquid with a solubility of approximately 40 wt. %. This high solubility allows for the injection of high-strength solutions into impacted areas, theoretically reducing the volumes required. While this increased solubility can be attractive for some sites, caution needs to be applied when injecting at sites where sodium can be an issue, or if density may affect the distribution of the permanganate.
- Potassium Permanganate (KMnO4) usually comes as a solid and has a solubility of approximately 3 wt. %. The main advantages of using potassium permanganate are its lower cost and fewer health and safety risks, in comparison to Sodium Permanganate.
Historical Permanganate Groundwater Studies
Completed by: Mike Schnarr, Clayton Traux and Eric Hood at the University of Waterloo
Date: early to mid 1990’s
A series of studies completed in the field to demonstrate the potential of permanganate for groundwater remediation, this study used potassium permanganate to effectively treat a sand aquifer impacted by PCE and TCE.
Contaminants Treated with Permanganate
While permanganate can be used to treat some aromatic and polycyclic aromatic hydrocarbons, it is generally considered most effective for treating chlorinated ethenes such as PCE and TCE. The reaction rate between permanganate and chlorinate ethenes can be as fast as seconds; hence, it can theoretically remediate an impacted aquifer in a very short time-frame.
Permanganate is generally applied as a solution at a concentration of between 1 wt. % and 10 wt. % but it can be applied as a solid to the base of an excavation, to address residue impacts following the excavation of a source. Studies have shown that caution should be used when deciding on what concentration to inject: more is not necessarily better.
Unlike many oxidants, permanganate can be relatively long-lasting within the subsurface, being present for weeks to a few months following injection. This persistence can be advantageous in finer-grained aquifers where diffusion of contaminants from fine-grained lens/layers can result in rebound.
Concerns with Permanganate
- The precipitation of manganese dioxide can potentially clog the aquifer, especially in bedrock aquifers. If successful remediation requires multiple injections, the project must often incorporate a well rehabilitation program.
- Uncontrolled release of the purple-coloured solution into sewers and surface water bodies can also cause concerns that must be addressed in the program.
In Situ Chemical Oxidation using Permanganate can be a powerful tool for the destruction of chlorinated ethenes and thus should be considered when completing remedial option reviews. However, realistic remedial goals need to be set out, along with a practical injection program. In our extensive experience, we have only successfully addressed a handful of chlorinated-ethene impacted sites using permanganate injections only. Most often, a multi-phased approach is required.