Since the industrial revolution man has been contaminating soil and water with poisons from a multitude of sources. The extent of worldwide environmental contamination is staggering and as pressure upon resources increases there is no time for the natural processes of remediation and soil recovery to occur.
In 1780, the managers of a silver factory in Nieuwpoort, Holland, gave no thought to the future as they disposed of its scouring agents. More than two centuries later Eric Foppen, an engineer working for the Province of South Holland, was faced with the problem of cleaning up the site.
In a 1997 investigation, Foppen found that the concentration of chlorinated hydrocarbons was more than 100 times permissible levels. Over time, pollution had spread downwards as a deep plume. Groundwater quality was threatened but removing contaminated soil for offsite disposal was ruled out because of the close proximity of the historic Arsenal building and modern residential properties.
Site geology comprises layers of peat, clay and sand, making remediation difficult. At first, Foppen considered an insitu containment solution, but this was soon ruled out because of the extent of the plume and high projected costs.
'We were after a definitive solution and found it in electro-reclamation,' says Foppen.
An innovative remediation technique invented by geologists Reinout Lageman and Wiebe Pool, electro-reclamation had already proven itself in the removal of heavy metals but not for chlorinated hydrocarbons.
'Direct current is used to remove heavy metals, but Lageman and Pool's approach was to heat up the soil with alternating current,' explains Aldert van der Kooij, project adviser and head of consultant DHV's environment and infrastructure department.
He compares it with a light bulb. 'The resistance in the filament is high, which causes it to glow.' In the ground, because of the high temperature, 'the chlorinated hydrocarbons dissolve more quickly and easily in the groundwater and can be pumped away using air or water. The heating process also provides a stimulus for the biological cleansing process in which bacteria eat up the pollutants.'
The method was tested on a 200m3 pilot project costing £65,000. Electrodes were placed 12m deep in the ground. As the soil temperature rose, concentrations of the chlorinated hydrocarbons also increased as expected and enabled the levels of released pollutants to be measured.
Results were promising but equally it was seen that because of the nature of the ground, subsidence might occur as a result of heating the peat. However, the risk was found to be small and no foundation damage was detected. In June 1997, power was connected for a three-year, 7,700m3 project costing £880,000 to ensure that the whole site could be declared 'power cleansed' and that the bacteria had 'licked the plates clean'. 'We anticipate the results will prove positive, then the method will be applied to other locations,' says van der Kooij.
Electrochemical techniques and the use of electricity for remediation are not commonly thought to be able to compete with traditional methods of site clean-up. Soil as an electrolyte or as a resistor suggests very high voltages between electrodes and dispersion of electrical currents to earth would weaken the focus or there are insufficient reactants to provide reasonable current efficiency. It was thought that putting enough non-corroding electrodes in the ground for the process to work would be too expensive. To overcome these concerns Lageman, Pool and Geert Seffinga set up Geokinetics in 1985. In the early 1990s they teamed up with A Hak, a piping and construction manufacturer.
Known in Europe as A Hak Milieutechnik/Geokinetics, and for worldwide operations as Geokinetics International, this group of geologists, hydrogeologists, electrochemists and electrical engineers recognised that most contaminated sites contain a mixture of both inorganic and organic compounds. The common denominator to this challenge was the use of electricity.
In Holland alone over the past 14 years, £11M worth of electro-reclamation projects have been carried out using a combination of field and laboratory techniques. These include electrokinetic technology - a widely patented approach of deploying electrodes in contaminated conditions; electrolyte management and contaminant recovery; and electrodes and electrode systems (electrokinetic ground remediation). The latter requires stable and cost- effective electrodes. Conditions inside electrode chambers are particularly aggressive in terms of environment and duration, which has led to extensive evaluation of different materials. Ebonex, a patented conductive ceramic material based on a sub-oxide of titanium, gives several important operating advantages. Significantly, it is the only material which has proved to be durable enough to withstand continuous operation.
The first electrical experiments in soil were carried out in Moscow in 1807 but it was not until 70 years ago that the first successful laboratory work was undertaken in Leipzig. In the mid 1940s, crude electrodes based on steel pipes and railway lines were used in England with some effect, while better technology evolved by stabilising clay with electro-osmosis. Russian workers also used electromigration in metal prospecting in the 1960s. Most of the early work was inconclusive due to the failure to manage the pH of the soil around the electrodes and a neglect of the ion exchange capacity of real soils compared to laboratory models.
The breakthrough came with the work of Lageman, Pool and Seffinga in 1987, who focused on electromigration and patented the use of circulating electrolytes. Work included the use of ion permeable wells to hold the anolyte and catholyte and managing the pH and other electrolyte conditions within the electrode casings, considered to be the critical element in controlling system performance. The success of electro-remediation as a means of remediating land contaminated with heavy metals, cyanides, arsenic and many other toxic pollutants is based on these principles.
Electrokinetic remediation is currently deployed in three ways. Insitu methods involve electrodes placed directly on the ground. Remediation is achieved with minimal disturbance to the site, as at Nieuwpoort. In batch remediation, contaminated material is transported to a batch facility and treated off site. Finally, an electrokinetic ring fence (EKRF) uses a chain of electrode pairs in the ground to stop migration of contaminated groundwater from a source point.
Lageman appreciates that electrokenetics is not the 'be all and end all' for soil remediation, but 'with the enthusiasm of my colleagues, we have developed a set of powerful tools that can remediate land completely for any future use. Resources recovery is the fast track to having our technology accepted by commercial organisations.'
But he says that 'departments of the environment and other agencies in many countries are still on a slow learning curve due to a few false starts, set-backs and unrealistic expectations.'
'If we don't grasp the nettle of toxic contamination then it will not just be the politicians who will be shouting. We all will,' he warns.
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