A brief history of electrokinetic phenomena and their application in contaminated land remediation.

The first thing that my dad said to me when I explained that I was doing some work for a Finnish Company who are using electricity and electrokinetic phenomena to clean up contaminated land was: “Aren’t they electrocuting the worms?”

Contamination of land and groundwater is a global environmental issue and there are a multitude of technologies and methods for addressing the problem. Different approaches are required for different types of contamination and even the same contaminants may need different remediation approaches on different sites based on specifics of the soil and location. There are armies of contaminated-land specialist (of which the author is not one) who assess and develop programs which best fit the needs of each case. Electrokinetics, specifically the method described in this article, does not yet have a prominent place in the contaminated land specialist toolboxes but this could be set to change in the years ahead.

What are electrokinetic phenomena? We have to head back to Moscow in 1807 when a German scientist by the name of Ferdinand Friedrich Reuss was playing around applying an electric current generated by the recently invented voltaic pile to two compartments of water separated by a clay barrier. He observed that water moved from the anode to the cathode through the barrier. This finding didn’t generate too much excitement in the scientific community at the time but, several years later, Robert Porret Jr, an English chemist, conducted some similar experiments and observed ‘electrically driven water flow’. He called this phenomenon electro-filtration which later became known as ‘electrical endosmose’ and is now referred to as electro-osmosis.

At around the same time, the French chemist Nicolas Gautherot, using the Voltaic pile as a new toy, placed water droplets between two electrically connected metal plates. He observed colloids (microscopically dispersed particles in suspension) moving through the water in response to the electrical current – a phenomenon we now refer to as electrophoresis.

Fast forward 200 years and electrokinetic phenomena now have applications in many disciplines.  These include DNA sequencing, protein fractionation, immunoassay and in the cleaning up all sorts of nasty chemicals that have found their way into the soil.

Back in the late 1980’s and 1990’s trials were conducted with using electrokinetic phenomena to clean up contaminated sites. Electrodes were inserted into the soil and when a current was applied, contaminants, including heavy metals migrated towards the cathode.  These could then, with varying degrees of effectiveness, be collected and removed. Although there was some success with the process of migrating contaminants for collection, there were also some negative issues such the soil becoming acidic when subjected to extended treatment. While these methods are still occasionally used, they are not mainstream treatment systems for contaminated land.

In the late 1980’s a system using electrokinetic phenomena to dry cement was established under the trade name X-Cel.  It was found that if pulses of direct current with continually changing polarity were used, rather than applying a continuous current in one direction, the electrokinetic effects were greatly increased. To explain how this works, we can use a simple analogy.  Think about sieving flour when making bread.  If you pour the flour into the sieve and just push the flour in one direction, it would pass through the sieve but not very quickly and it would require a lot of pushing. However, if you tap the sieve every now and again, the flour will pass through the sieve much more quickly and with less effort. Using this analogy, a long pulse in one direction followed by a short pulse (tap of the sieve) in the other direction greatly increases the electro-osmotic effects of moving cations & water (the flour) through the cement or soil (sieve).

Alternating a long pulse in one direction and short pulses in the other, enhances electro-osmosis and can be used, amongst other applications, for distributing additives through the soil.  However, if the pulses applied are symmetrical (say 40 milliseconds one way and then flipping to 40 milliseconds the other way), the electrochemical and electrokinetic processes will take place in the soil but there will be no net movement of ions in any one direction (and no build-up of hydrogen ions to cause soil acidity problems). What results is a continual micro-scale mixing of the particles throughout the soil.  This mixing, combined with electrolysis, electric current induced desorption and other electrokinetic and electrochemical processes, provides a perfect cocktail of reactive particles and oxygen and greatly increases the bioavailability of contaminants for microbes in the soil to break down. The enhanced bioremediation can be so effective that the soil nutrient content becomes the limiting factor as the microbes use up nutrients as they metabolize the contaminants.  When this occurs, the system can be switched to the long pulse, short pulse setup for a brief period in order to spread nutrient additives (and sometimes extra microbes) through the soil.

The question must be asked –  if this process is so effective, why isn’t it being used to address soil pollution problems all over the world? The short answer is that it should be. There are probably half a dozen PhDs needed to understand the precise mechanisms of degradation and enhanced bioremediation which are induced by the pulsed electric current but the bottom line is that it really works. It is cheap to run (only about 500 kWh/month of electricity to treat 10 000m³ of soil), doesn’t require the application of any chemicals and can be easily installed in-situ so is, ultimately, a very cost-effective and sustainable remediation solution.

The Finnish company I mentioned (EKOGRID) has been developing and refining the patented system over the past 10 years and they have trialled and used it in commercial applications in different situations in many parts of the world treating more than 200 000 m³ of contaminated soils.   Time and again it has proved to be highly effective in breaking down a wide range of organic contaminants. The system is fully functional and commercially available but, as with all things that are new, big companies like to see other big companies using new technology before they are prepared to adopt it themselves. Pulsed Electrokinetic Enhanced Bioremediation is still in the ‘early adopter phase’ but – watch this space. In 5 years’ time it could well be sitting right at the top of those contaminated-land specialists’ toolboxes.

Oh, and as far as the worms go…. my dad was pleased to hear that they don’t seem to be bothered by the tiny electrical currents – but I am sure there is another PhD in there somewhere.

Author:  Richard Lewis, Senior Environmental Advisor, EKOGRID Oy