Subsurface contaminants, like petroleum hydrocarbons, have been shown to biodegrade, or breakdown in place via naturally occurring microorganisms. However, the process of biodegradation is often limited by two primary factors: 1) the degree of physical contact between microorganisms and materials needed for contaminant biodegradation – also called contaminant bioavailability, and 2) the degree to which these materials are in a metabolically accessible form to microorganisms – also called bioaccessibility. Bioremediation is a term used to describe remediation techniques that take advantage of these naturally occurring biological processes by creating environmentally favourable conditions in an effort to stimulate microbial activity, and thereby increase the rate of biodegradation. Although not strictly considered a form of bioremediation, electrokinetics, or EK, is a technique that can be used to help overcome the limitations associated with biodegradation, thereby enhancing traditional forms of bioremediation (e.g. bioaugmentation, bioattenuation, biostimulation, etc).
How Does it Work?
Using the same basic principles as a typical battery, EK involves installing an array of anodes and cathodes in subsurface boreholes that are either filled with special electrolytic fluids or use the fluid in the pore space of the soil as an electrolyte. A direct current ranging between ~0.3 and 1 mA/cm2 is then applied to the system to induce several electrical transport phenomena.
The electrical transport phenomena induced by EK include electroosmosis, electromigration and electrophoresis. Electroosmosis describes the flow of fluids through pore space in the soil in response to an applied electric field. Under this transport mechanism, contaminants are moved through the soil by advective processes – meaning they are carried through the pore space by the flow of surrounding fluids. Electromigration involves the movement of ions through the soil pore solution toward the electrodes. Generally, electromigration occurs at a faster rate than electroosmosis and is believed to be the dominant transport mechanism in EK remediation. Electrophoresis is similar to electromigration, except that is involves the movement of dissolved or suspended charged particles called colloids.
In turn, these electrical transport phenomena result in several physical and chemical processes that promote bioavailability and bioaccessibility by enhancing the transport of contaminants, nutrients and microorganisms in the system. The electrical field created by applying current to the system results in electrolysis – or the decomposition of water – which occurs at the anode and cathode.
Oxidation occurs at the anode, according to the following reaction:
And similarly, reduction occurs at the cathode via the reaction:
These reactions facilitate electroosmotic flow of soil pore fluids, which generally flows from the anode to the cathode due to the net negative charge of most clay, silt and organic particles. Electromigration and electrophoresis will occur in both directions, with cations migrating toward the anode and anions migrating toward the cathode. The flow of charged particles between the anode and cathode as well as the electroosmotic flow of contaminants helps supply microorganisms with the electron acceptors and nutrients required for them to optimally biodegrade contaminants.
Benefits of EK
There are two important benefits of EK as a remediation method.
First, it is an in-situ remediation method, which means there is minimal land disturbance because the contaminated soil is treated in place, rather than being removed and trucked to a landfill. This is particularly beneficial for sites that are large or sites in remote locations where traditional “dig-and-dump” methods are logistically and economically infeasible. Additionally, in-situ remediation methods have the potential to provide indirect environmental benefits through avoided carbon emissions associated with the transportation and landfilling of contaminated soils.
Second, because clays and silts consist of extremely fine particles, they have low fluid permeability and are particularly difficult to treat using other types of in-situ remediation. However, these fine-grained clay and silt particles also have a high net surface charge density, which means they are actually beneficial when using EK because they enhance electroosmotic permeability. This means that EK provides an effective alternative to treat clayey or silty soils that are traditionally challenging to remediate.
How Can Eosense Help Optimize EK?
As discussed in a previous blog post, when certain types of microorganisms come into contact with contaminants and nutrients in the subsurface, they are able to incorporate them into their metabolic processes and break them down. This progressive breakdown is referred to as methanogenesis, and eventually results in emissions of carbon dioxide (CO2) at the soil surface. By continuously monitoring these CO2 emissions, Eosense can help site professionals monitor the health and activity levels of microorganisms degrading contaminants in the subsurface as well as determining the rate of biodegradation of these contaminants.
Using the eosFD, site professionals can autonomously collect up to 3 orders of magnitude more data than traditional monitoring methods. This method is also surface-based, meaning it doesn’t require installation of any additional infrastructure on site, and because the CO2 emissions are measured directly via an internal sensor, there’s no need to collect samples or wait for analytical results from a third party lab. Thanks to its low power consumption and rugged design, the eosFD can operate optimally in remote locations – even in the harshest of environments – for long periods of time, making it the most robust system for monitoring EK remediation in real-time.