The remediation of contaminated land is a major challenge for the construction industry in preparing brownfield sites for redevelopment.

1
The Use of Electrokinetics to Enhance the
Degradation of Organic Contaminants in Soils
Michael Harbottle and Dr Gilliane Sills,
Department of Engineering Science, University of
Oxford, Parks Road, Oxford, OX1 3PJ. Tel. 01865
273165 Email michael.harbottle_at_eng.ox.ac.uk Dr
Ian Thompson, Centre for Ecology and Hydrology,
Mansfield Road, Oxford, OX1 3SR
Introduction The remediation of contaminated
land is a major challenge for the construction
industry in preparing brownfield sites for
redevelopment. These sites have often previously
been used for industrial purposes and so can
contain a cocktail of undesirable chemicals that
have to be removed. This work is investigating
the combination of two well-established
remediation techniques, electrokinetics and
bioremediation. The goals of this project are a
better understanding of the fundamental
processes, for example how an applied electric
field can affect bioavailability of chemicals,
how bacteria respond to electric fields and what
conditions give the best results and why.
The Combination of Electrokinetics and
Bioremediation Bioremediation
harnesses the degradative powers of bacteria to
metabolize or otherwise help remove a wide range
of organic and inorganic pollutants in
soils. Depending on the site and the
contaminant, naturally occurring microbes may be
able to degrade the undesirable chemical(s),
especially with the addition of nutrients
or surfactants. Otherwise, suitable bacteria can
be inoculated into the site soil. The
efficacy of bioremediation can be limited by the
problem of bioavailability - chemicals can become
adsorbed onto soil particles and trapped in
very small pores where bacteria cannot
access them. The term electrokinetics
refers to the phenomena that arise when an
electric field is applied to a sample of
soil. The diagram (right) illustrates the effect
of electroosmosis, the phenomenon that leads to a
flow of pore fluid towards an electrode (usually
the cathode) in soils containing clay particles.
A range of forces are experienced by both
pollutant (in this case, pentachlorophenol, or
PCP) and bacteria. This leads to relative motion
between the two. The increased contact that
results will lead to increased degradation. The
electric field may assist in the desorption of
contaminant molecules from the soil matter, and
the electroosmotic flow moves them to where they
may be degraded. This therefore overcomes the
problems of bioavailability. Water is
electrolysed at the electrodes, leading to oxygen
and hydrogen production, as well as the
production of hydrogen and hydroxyl ions in the
electrolyte. These ions move through the soil due
to the field, resulting in changes in pH
throughout the soil which can drastically affect
the efficiency of the electrokinetic technique,
and may also affect bioremediation. Oxygen and
heat, produced as a result of the electrolysis of
water, provide extra impetus to the actions of
bacteria. Even a temperature increase of a few
degrees can increase degradation rates, and the
presence of oxygen is necessary for aerobic
degradation to occur.
The model pollutant is
pentachlorophenol (PCP), a highly toxic organic
chemical used primarily in the wood preservation
industry. Soil samples
are prepared unsaturated, ensuring there is
oxygen in the pores (to allow the aerobic
degradation of the PCP), and, after the
pollutant is added, the sample may either be
tested immediately or stored to investigate the
effect of aging of the soil. The electrodes
are constructed from graphite, which minimises
any electrode degradation. Control of pH
changes is important in maximising the effects of
this technique, and so either suitable chemicals
will be added to the electrode chambers, or the
electrolyte can be cycled between anode and
cathode. Current experiments are looking at the
effects of pH change in the soil and how
electroosmosis is controlled by these events.