The electrokinetic bioremediation of hydrocarbons from the sub-surface

Hydrocarbon contamination of soil from accidental spills and negligence represents a major cause of sub-surface contamination. There are numerous physical, chemical and biological techniques available for contaminant remediation. However, many of these methods can be difficult to implement at sites where access to soils, or delivery of treatments, requires substantial excavation works and disruption (for example where contamination is located beneath or adjacent to building structures and underground services). An innovative technology that may provide a solution for the remediation of subsurface contamination without causing interim damage is that of electrokinetic remediation. Electrokinetic remediation is the application of a low voltage, direct current (of the order of mA/cm2) between electrodes placed in a contaminated soil system. The production of an electric field causes a number of electrically-induced processes to occur that can promote soil remediation, principally: electromigration, electroosmosis, electrophoresis, and electrolysis. This study examined the use of electrokinetics coupled with biodegradation to remediate kerosene and transformer oil from sub-surface soils. The principal investigations carried out within this study included: the use of electroosmosis as an in situ flushing technique; use of electrokinetics to deliver surfactants into the soil to promote mobilisation of contaminants within the sub-surface; and the use of electrokinetics to create optimum conditions for biodegradation by the delivery of nutrients and electron acceptors, the removal of toxic metabolites, and by controlling the pH and moisture content of the soil. As an integral part of the research, a field trial was designed and set up to determine whether electrokinetics could be used to promote the biodegradation of transformer oil from the subsurface. This was one of the first field trials in the UK for the electrokinetic remediation of hydrocarbon contaminants. The results of the study support the use of electrokinetics as a field tool for the enhancement of remediation in the sub-surface. Laboratory-scale studies showed the potential to mobilise the more soluble hydrocarbons (such as toluene and xylene) in the soil, to deliver enhancing agents (e.g. surfactants and nutrients) and to remove toxic metabolites. The field-scale experiment demonstrated the concept of a relatively low cost treatment cell on-site and provided some evidence that biodegradation of the transformer oil was promoted. Consequently, it is suggested that electrokinetics, with the correct engineering controls in place, provides a valuable method of optimising the biodegradation of hydrocarbons in the sub-surface.