Electrochemically-aided biodigestion of organic materials

An integrated electrochemical soil remediation method and apparatus for treating contaminated soils, especially those contaminated with mixtures of nonvolatile organic contaminants, ionic contaminants and volatile organic compounds are disclosed. Remediation may be achieved by electrochemically enhancing biodigestion of organic contaminants (using microorganisms present in or added to soil), electrochemically removing ionic contaminants and electrochemically removing volatilized organic contaminants by applying a vacuum over the soil being treated, as dictated by the nature of contamination. Physicochemical conditions of the electrolyte and the soil are managed by monitoring and adjusting the electrolyte. Nutritional needs of microorganisms for biodigestion are adjusted as necessary through the electrolyte.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
The present invention relates to a method of remediation of soil containing 
organic compounds and ionic contaminants and more particularly to a method 
of electrochemically-enhanced remediation of soil contaminated with 
organic compounds and ionic species. 
2. Description of the Related Art 
Across the globe, soils are polluted with inorganic materials such as heavy 
metals, arsenical compounds, cyanides, selenides and radioisotopes. 
Additional contamination arises from organic compounds such as petroleum 
refinery products, coal tar and wood chemicals, solvents from the 
chlor-alkali industry, utilities producing gas and electricity, pesticide 
use and manufacture, residues from metal and munitions manufacturing, 
storage and disposal operations. Most of these activities have taken place 
close to major waterways and ports where land is either clayey or the 
ground water table is close to the surface or both. Clean up costs of 
contaminated soil and sediment in the U.S. alone is estimated to be $1.7 
trillion for using conventional treatments such as soil washing, physical 
removal to land fill operations or various incineration options. Novel in 
situ processes for remediation of soil would therefore be of great benefit 
to society. 
Of particular significance is polluted soils' threat to the earth's 
sub-surface waters which may lead to a potable water crisis on a scale 
reminiscent of the energy crisis of the 1970s. A recent Stamford 
University study points out that global drinking water resources are 
almost completely spoken for already. Yet, world population growth and 
continuing and widespread contamination by industrial, agricultural and 
military activities virtually ensure crisis in the next decade. 
The breakdown of organic compounds in the soil by naturally occurring or 
cultivated bacteria and fungi is well known. Naturally occurring 
"biodigestion" has been used to decompose wastes since biblical times. The 
key elements to successful decomposition are heat, aeration, water and 
nutrients. An inadequate supply of one or more of these elements causes 
decomposition to slow and eventually stop. 
The spectrum of biological species surviving in and around contaminated 
sites must be those selected naturally to survive in the presence of 
contamination. Some improvement in the local environmental conditions 
actually stimulates species growth and activity to such an extent that the 
contamination can be reduced to acceptable background levels and species 
actually thrive. 
The application of electrochemical techniques for enhancing bioremediation 
is described by Kinsel and Umbreit, J. Bacteriol., 87, 1234 (1964) and 
studied by Denisov, et al. in Microbiologiya, 47(3), 400 (1978). Kinsel 
and Umbreit used 90 mA electrical currents to enhance Ferrobacillus 
sulfooxidans yields sixfold compared to conventional cultivation 
processes. 
Acar, et al. in U.S. Pat. No. 5,137,608 mention the introduction of 
bacteria and nutrients into soil to enhance electrochemical degradation of 
organic contaminants, suggesting that bacterial movement in the soil is 
achieved by an electroosmotic mechanism. Brodsky, et al. suggest the use 
of bacterial agents in carbon and other substrates in wells in 
contaminated soil. Brodsky, et al. teach that organic contaminants are 
transported in an electroosmotically-driven water front moving towards a 
cathode by an advection mechanism into bacteria-rich wells surrounding the 
electrodes for biodigestion. 
Pool, in U.S. Pat. No. 5,433,829, incorporated herein in its entirety by 
reference, describes the deployment of electrodes in porous wells and a 
circulating electrolyte management system to provide nutrients, oxygen 
carriers, and a transport mechanism to biologically active species already 
present in or added through the wells to the soil. 
One major obstacle to the advancement of electrochemical techniques for the 
removal of toxic substances from soil is the availability of rugged, 
nontoxic and inexpensive anodes, particularly for in situ processes where 
acidic conditions are present. Conventional electrodes are susceptible to 
attack by deleterious substances present or formed in the environment 
being treated. For example, when electrodes carry current via an aqueous 
electrolyte such as ground water, sea water, brines, mud, sewage sludge, 
wet sand or concrete, the environment around the anode acidifies (becomes 
a proton source) and that around the cathode becomes alkaline due to the 
presence of hydroxyl ions. Water moves toward the cathode by 
electro-osmotic pressure. If a strongly acidic or basic electrolyte is 
used or the electrolyte is well-stirred, the effect of these local changes 
in pH is minimized. However, where thorough mixing is difficult, as in 
electrolytes loaded with a relatively high solids content such as soils, 
muds, wet sands, industrial sludges, the effects of the local changes in 
pH can be very significant. The anodes can be attacked under local 
oxidative conditions and insoluble metal deposits may form at the 
cathodes, both of which effects can adversely affect the conductivity of 
the electrodes and thereby seriously diminish the efficacy of an 
electrochemical process. 
In particular, the electrodes may malfunction or corrode and thereby act as 
a source of species that interfere with soil remediation or are 
contaminants in and of themselves. Therefore, a successful process must 
take into account the dynamics of the physicochemical conditions of the 
soil as electrochemical remediation is being carried out. 
Industrial anodes developed for electrochemical processes are not 
necessarily suitable for electrochemical soil remediation techniques. Such 
anodes include precious metal and precious metal oxide-coated titanium or 
niobium, silicon, iron, carbon, lead and lead alloys and sacrificial 
anodes such as zinc, aluminum and ferrous alloys. Sacrificial electrodes 
and impressed current anodes made from lead and iron are unsuitable for 
electrochemical soil remediation due to their tendency to add toxic ions 
to the environment. Moreover, precious metal-coated electrodes are 
susceptible to attack by chloride and fluoride ions and some organic 
compounds such as carboxylic acids. Platinum coatings can be lost as 
soluble coordination compounds formed in the presence of specific 
reactants found in the contaminated area being treated. Fluoride as low as 
500 ppm can be disastrous to titanium-based electrodes. 
Electrodes used for cathodic protection include carbon granules surrounding 
a precious metal-coated titanium current collector. In these electrodes, 
the carbon granules serve to reduce current density and are consumed. 
However, the wear rate is severe as the carbon is oxidized to carbon 
dioxide and contact to the current collector is uneven at best. 
Large carbon anodes and silicon-iron anodes suffer from "necking": 
accelerated wear around the electrical contact end of the anode. Though 
relatively inexpensive, use of these materials can in practice be a very 
expensive mistake. 
Conventionally, a direct current voltage source is used to set up the 
driving current, thereby creating a constant flux of ionic contaminants 
through the soil. Direct current is also useful for vacuum extraction. For 
a given electrochemical technique, precious metal oxide-coated titanium 
and other conventional electrodes are designed to function as either an 
anode or a cathode, but not both. Indeed, such electrodes would be 
destroyed in an attempt to carry a fluctuating current, i.e., an 
alternating current, because a given electrode designed to serve as an 
anode when current flowed in a certain direction would not function as a 
cathode in response to the fluctuation of current direction and would 
instead dissolve or passivate. 
When a.c. currents are used, metal oxides and hydroxides tend to deposit on 
conventional electrodes, interfering with sustained current. For example, 
sufficiently high temperatures can exist in the soil area adjacent to the 
electrodes that bicarbonate salts are decomposed. 
One approach to eliminating build up of such insoluble metal deposits on 
the electrodes would be to simply use a reversible d.c. source in place of 
the a.c. source. Thus, during each portion of d.c. operation, current is 
switched so that each electrode spends a portion operating as an anode. 
The deposits that form around the electrodes during a.c. operation can be 
dissolved off during this period. However, as discussed above, the 
inability of conventional electrodes to function with current reversal 
prevents the use of alternating currents alone to avoid electrode 
corrosion. 
Another significant issue in the advancement of electrochemical methods of 
soil remediation is electrolyte management. Generally, the purpose of the 
electrolyte is to enable collection of species removed from the 
contaminated environment, support electrokinetic flow through the soil 
while maintaining the physicochemical conditions of the soil. For example, 
it may desirable to control the pH in the soil and to replenish moisture 
in the soil being treated. 
In electrochemical methods generally, ions migrate under the influence of 
the applied driving current. Thus, positively charged ions migrate as an 
acidic "front" through the contaminated medium toward the cathode while an 
alkaline "front" of negatively charged ions migrates in an opposing 
direction toward the anode. These fronts typically can meet within the 
contaminated soil as well as on the electrode surface, whereupon salts or 
alkaline hydroxide form. Precipitates disturb maintenance of the driving 
current supporting ion migration, so that ionic contaminants can no longer 
be removed effectively from the soil. The electrochemical process stops. 
Buildup of these precipitates can bring electrochemical remediation to a 
catastrophic halt. 
Given the nature and extent of soil pollution around the world, it would be 
advantageous for an electrochemical soil remediation process to be capable 
of removing a wide variety of contaminants. For organic contaminants, 
structure, molecular size, water solubility and volatility are the most 
important characteristics to consider in designing and carrying out 
remediation techniques on soil and sediments. 
Volatile organic compounds can be removed selectively from soils by vacuum 
extraction; heated vacuum extraction is economical and widely useful. 
Soluble organic compounds, especially those that are capable of existing 
as solubilized ionic species, such as water soluble dyestuffs, herbicides 
such as paraquat and diquat, phenolic compounds and ionic detergents, can 
be removed by electromigration. 
Organic compounds that are neither water-soluble nor volatile respond to 
neither technique and therefore biodigestion may be useful. For example, 
some polymeric materials such as cellulose may be digested to carbon 
dioxide and water by common soil microorganisms. These organisms can also 
consume organic pollutants such as trinitrotoluene (TNT), a component of 
high explosives, polycyclic aromatic hydrocarbons found in coal tar 
residues, chlorinated hydrocarbons such as dichlorobenzene and some 
polychlorinated biphenyls (PCBs). 
Some organic pollutants are either non-polar or too large to move at a 
reasonable rate electrokinetically. In situ remediation techniques for 
soils containing these pollutants exist and are compatible, if not 
synergistic, with electrochemical techniques. These in situ techniques 
include vacuum extraction of volatile solvents, digestion with bacteria, 
and use of sequestering agents that can be driven through the soil 
electrochemically. Electrochemical processing aids these techniques by 
providing heat generated by the voltage drop as a current passes through 
soil, so-called Joule heating. However, none has been successfully 
exploited because of the difficulties in removing contaminants having 
distinct and widely varying characteristics, inadequate electrodes and 
electrolyte management, as discussed above. 
Accordingly, it is an object of the present invention to efficiently remove 
ionic and organic contaminants from soil, in in situ, continuous or batch 
modes, using microorganisms to digest nonvolatile organic contaminants, 
electrochemically stripping ionic contaminants, and vacuum extracting 
volatile organic compounds in various technique combinations as 
necessitated by the type(s) of contamination present. 
SUMMARY OF THE INVENTION 
The present invention achieves these and other objects by providing methods 
and apparatuses for treating contaminated soils, especially those 
contaminated with "mixed wastes": nonvolatile organic contaminants, ionic 
contaminants and volatile organic compounds. 
Remediation may be achieved by electrochemically enhancing biodigestion of 
organic contaminants (using microorganisms present in or added to soil), 
electrochemically removing ionic contaminants and electrochemically 
removing volatilized organic contaminants by applying a vacuum over the 
soil being treated; one or more of these electrochemical techniques being 
used as dictated by the nature of the soil contamination. Physicochemical 
conditions of the electrolyte and the soil are managed by monitoring and 
adjusting the electrolyte. Nutritional needs of microorganisms for 
biodigestion are adjusted as necessary through the electrolyte. 
One embodiment of the present invention relates to an electrochemical 
method for removing heavy metal or organic contaminants from soil using 
microorganisms in which an anode and a cathode are enclosed in wells in 
the contaminated soil. The wells are permeable to ions, water and 
microorganisms that can consume the contaminants. A circulating 
electrolyte is supplied to the contaminated soil via the electrodes for 
maintaining physicochemical conditions, such as pH and moisture, in the 
soil and to remove contaminants accumulating in the soil adjacent to the 
electrodes. A potential difference is established between the anode and 
the cathode by an applied d.c. current. The current induces transport 
through the soil of ions according to their charge and of microorganisms 
by electrophoresis and heats the contaminated soil to promote 
decomposition of the contaminants by the microorganisms. For aerobic 
decomposition processes, oxygen sources may be provided. 
According to another embodiment of the present invention, when treating 
soil contaminated with ionic contaminants, volatile and nonvolatile 
organics, in addition to the above-described steps, the soil is heated as 
a result of the resistance of the soil to the applied current and a vacuum 
is applied adjacent to the soil to extract volatilized compounds. Soil 
heating may be used to enhance biodigestion. 
In a further embodiment of the present invention, the polarity of the 
current applied to the electrodes initially is reversed to solubilize 
salts or precipitates accumulating at said electrodes when treating 
ionically contaminated soils. 
In carrying out the present invention, natural biodigestion by 
microorganisms is stimulated using electrochemical techniques resulting in 
enhanced utility, efficiency and control. 
Naturally occurring microorganisms or selectively cultivated species can be 
used where present or injected into another environment where 
bioremediation is desired. Also, it is possible to inject and 
electrochemically transport into the soil supporting nutrients or enzymes 
that aid biodigestion processes.

DESCRIPTION OF THE PREFERRED EMBODIMENTS 
Methods according to the present invention include the following 
combinations of techniques: 1) electrochemically enhanced biodigestion 
(i.e., consumption or decomposition of organic contaminants by 
microorganisms) and electrochemical remediation (i.e, removal of ionic 
contaminants electrochemically) with electrolyte management; 2) 
vacuum-assisted electrochemical remediation (i.e., electrochemical 
remediation to remove volatile organics and ionic contaminants) with 
electrolyte management; and 3) electrochemically enhanced biodigestion, 
electrolyte management and vacuum-assisted electrochemical remediation. As 
a result, methods according to the present invention can utilize the 
various techniques to successfully treat contaminated soils, including 
those several distinct types of contaminants, for which prior art 
techniques are ill-equipped, considerably less efficient or otherwise 
unsuitable. The various aspects of these methods are discussed more fully 
below. 
The phrase "ionic species" is used herein to denote charged or polarizable 
particles, such as metal cations--including heavy metals--, anionic 
complexes or radicals. The ionic species may also be organic or inorganic 
compounds. Water-soluble ions or organic contaminants that can be 
converted to soluble ions by the passage of protons or hydroxyl ions from 
the electrodes are also considered ionic species for purposes of the 
present invention. 
Organic contaminants sought to be removed in methods of the present 
invention include volatile organic compounds such as conventional solvents 
and relatively nonvolatile compounds such as monocyclic or polycyclic 
aromatic hydrocarbons or halocarbons, such as dichlorobenzene. 
Ionic contaminants physically adsorbed, i.e., ionically bonded, or 
solubilized in pockets of water or moisture accommodated within the 
lattice structure of the contaminated medium can also be removed from soil 
by methods according to the present invention. 
Methods according to the present invention are capable of treating soils 
contaminated with "mixed wastes": nonvolatile organic contaminants, ionic 
contaminants including radionuclides, and volatile organic compounds. 
In general, contaminated soils suitable for treatment according to the 
present invention include porous soils and may be in bulk or particulate, 
e.g., clods of soil. Contaminated soil suitable for treatment according to 
the present invention also includes sand, mud, dredgings, industrial 
sludges and the like. 
While undergoing treatment, the soil may remain in situ so that its 
physical disposition need not be changed in the course of treatment 
according to the present invention. Alternatively, the soil may be put 
into a reaction vessel or other container for treatment. 
Techniques for in situ treatment of contaminated soil will be described 
first with reference to FIG. 1. 
In situ treatment according to the present invention generally involves 
setting up and maintaining a driving current of sufficient magnitude 
across contaminated soil 10 to cause migration of anionic and cationic 
species to a desired location, e.g., an electrolyte. This migration may be 
accomplished by creating an electrical circuit which includes the 
contaminated soil. 
The actual configuration of the circuit depends in large part on the 
physical disposition on the contaminated soil. Exemplary anode 12 and 
cathode 14 are positioned inside wells 16, 18 dug into the contaminated 
soil to be treated. These electrodes may be rods, tubes, cables, panels or 
other forms known in the art. The electrodes are connected to a power 
supply 20. Power supply 20 connects anode 12 and cathode 14 by 
conventional means and establishes a driving current across the 
contaminated soil. Conventional voltage- or current-regulated power mains 
or locally generated power supplies and any number of current or voltage 
control systems may be utilized for this purpose. Power supplies may be 
controlled remotely to provide the desired driving current in the 
contaminated soil. 
To carry out a method for treating contaminated soil in a batch mode, as 
shown in FIG. 2, contaminated soil 10 is held in a reactor or tank 36 
lined with polyethylene 38 and a course sand base 40 and fitted with 
electrolyte drainage pipes 42. Electrodes 12, 14 are arranged in wells 16, 
18 formed in the batch of soil. Power supply, electrolyte management 
system and pumps are not shown. Preferred electrode spacing for batch mode 
operation is that which permits rapid decontamination of the soil, 
therefore relatively close spacings (15-30 cm) are suitable. 
If multiple anodes and cathodes are used for either batch or in situ 
processing, more than one power supply may be used to connect all of the 
anodes and cathodes in order to establish a uniform electrical field of 
sufficient strength across the contaminated soil being treated at a given 
time. 
Non-corroding electrodes are especially preferred for use in the methods 
according to the present invention as they may remain in the soil for 
extended periods of time without contaminating the soil. Also, desirably, 
the anodes and cathodes can sustain a sufficiently high current density to 
carry out remediation without excessive heat generation. In a preferred 
embodiment, anode 12 and cathode 14 are cables having a conductive core 
coated by an acid-resistant polymeric or ceramic material. An example is 
an aluminum or a copper cable having a Ti.sub.n O.sub.2n-1 (e.g., Ti.sub.4 
O.sub.7) outer coating, such as those sold under the trademark EBONEX, 
commercially available from CBC Electrodes of Orinda, Calif. Other 
examples of suitable conductive materials are mild steel, carbon or 
titanium. The coating serves as the active electrode surface, through 
which microorganisms, nutrients and mobilized contaminants may pass. 
FIG. 3 is an enlarged cross-sectional view of an electrode suitable for use 
in methods according to the present invention for treatment of soils that 
tend to form nonconductive deposits at the electrodes. Typically, the 
length of current collector 42 is surrounded by a particulate material 44 
in inert casing 46. Particulate material 44 is used to increase the 
surface area of the electrode in order to reduce the effect of deposits of 
insoluble metal compounds such as calcium bicarbonate forming on the 
electrodes. A suitable particulate material is coke granules (20 mesh to 
about 1/4" diameter), also known as coke "breeze." 
Another benefit of utilizing electrodes based on the preferred Ti.sub.4 
O.sub.7 composition is that a single electrode may function as an anode or 
as a cathode as needed during soil treatment. For example, advantage may 
taken of this ability by applying an alternating current so that for 
periods of time, current flow is in a direction reverse to that applied to 
support soil decontamination. As a result of the reversal of the current, 
electrodes are cleaned of salt buildup without dissolving the electrode 
material in its place, as would otherwise happen. A dc current, e.g. where 
several minutes, hours or days pass before the polarity is adjusted can 
accomplish this purpose. 
Although several figures herein show a single anode and a single cathode, 
it is possible and may be particularly desirable to carry out the present 
invention utilizing several electrodes, i.e., an electrode array. For 
example, multiple anodes and cathodes may be arranged to establish a 
uniform field of sufficient strength through the soil. Tetragonal and 
hexagonal electrode arrays can be effective in this regard. Suitable 
spacing between electrodes is that which will promote an adequate rate of 
remediation without requiring so many electrodes as to be cost 
prohibitive. In a hexagonal array of six electrodes, a spacing of about 
1.5 m-2.5 m is adequate for an in situ operation. Where the treatment is 
vacuum assisted, a vacuum well may be located adjacent to the electrode 
array serving as a site for withdrawal of volatilized organic 
contaminants. For example, the vacuum well can be positioned at the center 
of the array. 
For electrochemical biodigestion of organic compounds and electrochemical 
remediation to remove ionic contaminants, applied current is desirably 
between about 2 A/m.sup.2 and about 20 A/m.sup.2 ; preferably, the applied 
current is about 8-10 A/m.sup.2. The potential difference established 
between the electrodes generally should be at least about 20 volts/m in 
order to support ionic transport and electrophoretic transport of the 
microorganisms in the soil, but the magnitude of the potential difference 
is not a significant factor on the cultivation or activity of the 
microorganisms. Potential differences as high as 100 volt/m were not 
observed to be adverse to the performance of the microorganisms. 
Methods according to the present invention may be carried out using an 
alternating current for vacuum assisted electrochemical remediation, with 
or without biodigestion. A direct current mode is needed to remove ionic 
contaminants from soil, enables electrode cleaning and enhances 
microorganism cultivation and activity. 
A suitable electrolyte is a liquid, such as water, that will support 
electrochemical processes in the soil being treated, provide a means to 
replenish moisture in the soil and an electrophoretic mechanism by which 
the microorganisms are dispersed into the contaminated soil, enhance 
electrical conductivity of the soil, solubilize ionic contaminants, 
provide nutrients to the microorganisms and conditioning agents as 
necessary into the soil. Water can be directed to the electrolyte 
reservoir tanks or pumped directly at the electrode wells. 
Typically, contaminated soil has some level of moisture, since some water 
penetrates from the surrounding environment. Water and any contaminants 
solubilized therein migrate through the contaminated soil from the area 
surrounding the anode as hydrated hydrogen ion and appearing at the 
cathode as hydrogen gas. Because water facilitates the migration of the 
ionic species through the contaminated soil and helps control the 
increasing acidity therein, especially near the anode, it is desirable to 
replenish the water in the contaminated soil over time. Also, moisture is 
essential to the growth and sustenance of microorganisms utilized in the 
biodigestion techniques according to the present invention. Replenishment 
with water substantially free of the ionic species sought to be removed 
according to the present invention is especially desirable. The water 
added for replenishment need not be completely "deionized", since the 
presence of certain ions may assist in balancing pH and balancing 
conduction. 
As the present invention is carried out, concentration of ionic species 
increases over time both in the electrolytic material and at its interface 
with the cathode. The "loaded" electrolyte may be disposed of or, 
preferably, is regenerated to permit recycling back to the electrodes. 
Flow of water within the contaminated soil provides an effective mechanism 
by which the ionic contaminants may be downloaded into a form that is much 
more conveniently handled and disposed of than the originally contaminated 
soil. Once downloaded, these ionic contaminants may provide feedstocks for 
processes. 
The pH of the electrolyte (and the soil being treated) may be adjusted 
depending on the characteristics of the ionic species being removed. 
Neutral or acidic pH is generally suitable. Where anions such as cyanide 
are contaminants, the electrolyte should be maintained sufficiently 
alkaline to avoid liberation of hydrogen cyanide gas during treatment 
according to the present invention. Likewise, where species such as phenol 
are contaminants, a relatively acidic pH in the electrolyte is preferred. 
In methods according to the present invention, adjustment of the pH is 
achieved simply and efficiently by the addition or removal of acid or base 
as necessary. Adjustment of pH may be accomplished sequentially, for 
example, first, to allow for removal of certain ionic species under 
relatively acidic and then, removal of other ionic species under basic 
conditions, as desired. Certain microorganisms thrive at pH levels of 1. 
Referring again to FIG. 1, electrolyte management obtaining all of the 
above-described functions may be achieved directly and easily by an 
electrolyte management system which typically includes one or more 
electrochemical ion exchange units 24, 26 and may include one or more 
pumps 28, 30 to assist with electrolyte flow therein and to the 
electrodes. Such an electrolyte management system permits regeneration of 
the electrolyte by separating accumulated ionic contamination therefrom, 
which contamination may be recovered in a stream 32. The regenerated 
electrolyte may be recycled back to each of the electrodes for additional 
soil decontamination via stream 34. The electrolyte management system also 
provides a convenient point in the apparatus to adjust pH and soil 
moisture (via line 35) and add nutrients (via nutrient reservoir 37) as 
desired during treatment. 
When current flows, ionic species will migrate according to their charges 
and the soil will be heated gently. Ionic species will migrate under the 
influence of the driving current through the contaminated soil into the 
electrolyte. The driving current creates positively and negatively charged 
streams or "bands" moving through the soil. Water-solubilized ionic 
contaminants are swept up in the charged streams and are ultimately 
dissolved in the electrolyte. Levels of ionic contamination are thus 
reduced in the soil at large and may be collected in a form much more 
easily disposed of than the contaminated soil. Another possible use for 
the recovered contaminants is as a feedstock to other processes. 
The soil is gently heated as a result of its resistance to the flow of the 
applied current, i.e., Joule heating. Heating in this manner provides a 
useful but simple means to promote the activity and growth of 
microorganisms and decomposition of organic compounds not otherwise being 
removed. Operating temperatures are easy to control by adjustment of the 
applied current. This is in distinction to conventional processes using RF 
heaters or steam injection by which the soil (including the organisms 
which accomplish biodigestion) is actually sterilized due to the high 
temperatures achieved. Generally, for the present invention, soil 
temperatures achieved as a result of heating should be no more than those 
at which the microorganisms being used thrive. Typically, the soil 
temperatures can be between about 30.degree. C. and about 70.degree. C. As 
a further benefit, Joule heating is also adequate to volatilize certain 
organic contaminants. 
In vacuum-assisted electrochemical remediation techniques according to the 
present invention, vacuum may be applied adjacent to the soil to draw off 
organics volatilized as a result of the Joule heating. For example, a 
vacuum extraction well located centrally in the electrode array may be 
used for this purpose. The magnitude of the vacuum utilized need be only 
that which is sufficient to draw off volatilized organics, e.g., as low as 
15 in Hg to about 30 in Hg is adequate. The vacuum need not be so strong 
as to extract organics from the soil. Addition of the vacuum does not 
inhibit the decomposition activity of microorganisms, but rather enhances 
such activity by promoting aeration of the soil. Vacuum may be applied 
through use of conventional equipment. 
For methods according to the present invention in which organic compounds 
are decomposed by microorganisms, either naturally occurring levels of 
bacteria may be enhanced or reinstated into contaminated soils or special 
bacterial strains may be introduced into the soil. Bacteria that feed on 
organic molecules need support typically in the form of oxygen, water, 
nutrients and essential elements such as nitrates, phosphates or sulfates. 
Growth of colonies is also encouraged by raising the temperature, e.g., to 
around 40.degree. C., and adding some easily digested organic materials 
such as starches and polysaccharides and other plant residues. In some 
cases, small molecules such as 3-6 carbon carbonyl compounds such as 
chlorinated hydrocarbons may be used. Following the progress of 
bioactivity is accomplished by sampling the bacterial count, measuring the 
release of carbon dioxide, and monitoring pollutant profiles and 
temperature gradients. 
Suitable microorganisms for such methods according to the present invention 
include aerobic bacteria such as Thiobacillus ferrooxidans (this species 
is acidophilic) or Staphylococcus cerevisiae. 
Nutrients for such microorganisms include water-soluble nitrates, 
phosphates and oxy anions (such as peroxides) that can move through the 
soil as the electrolyte flows through the soil. A preferred phosphate is 
sodium hexametaphosphate since it is not readily adsorbed onto the soil. 
These substances can also serve as oxygen sources for the microorganisms. 
Generally, at least 1 ppm oxygen in water is desirable for carrying out 
aerobic biodigestion. Nutrients at about 3-100 ppm level is suitable. 
Various aspects of the methods according to the present invention are 
further illustrated in the following examples, none of which is intended 
to limit the scope of the invention. 
EXAMPLE 1 
Three thousand cubic meters of soil contaminated with a mixture of mineral 
oils, naphthalene and volatile monocyclic aromatic solvents was treated 
using vacuum-assisted electrochemical remediation to remove volatile 
organic species and biodigestion to decompose other organic species. This 
example s hows the compatibility of these techniques. 
Six iron rebar/coke breeze electrodes were arranged in a hexagonal array, 
separated from each other by two meters. At the center of the array, a 
vacuum well (5-10 cm diameter) was located. The well was the same depth (9 
meters) as the electrodes. A 10 mA ac current was applied for three 
months. Vacuum was about 28 in Hg. The results after three months measured 
by gas chromatograph and FTIR spectroscopy are shown below in Table 1. 
TABLE 1 
______________________________________ 
Initial Concentration 
Final Concentration 
______________________________________ 
Organic Contaminant 
(.mu.g/l) (.mu.g/l) 
Benzene 610 &lt;0.20 
Toluene 1,900 &lt;0.20 
Ethylbenzene 2,400 &lt;0.20 
Xylenes 8,500 &lt;0.20 
Total monocyclic aromatics 
13,410 na 
Naphthalene 310 &lt;0.20 
Mineral oil 7,300 &lt;50 
______________________________________ 
As can be seen, the concentration of relatively volatile organic compounds 
(benzene, toluene, xylenes) was dramatically reduced. Likewise, mineral 
oil and naphthalene, nonvolatile organics, were also removed. 
EXAMPLE 2 
Samples of soil from a munitions site that had a devastating explosion in 
1918 were treated with an electrochemically assisted 
biodigestion/electrolyte management technique according to the present 
invention. The soil was contaminated with heavy metals, organic arsenic 
and trinitrotoluene and its breakdown products as shown in Table 2. 
TABLE 2 
______________________________________ 
Toxin As Cd Cr Cu Hg Ni Pb Zi 
______________________________________ 
mg/kg 270- 7-17 30 63-250 
0.14- 
37-54 88- 37- 
780 0.3 12,000 
580 
______________________________________ 
The soil samples were sieved into coarse, medium and fine fractions, most 
of the metals except lead and arsenic were naturally occurring minerals 
that were removed by wet sieving and gravitational separation. Also 
removed were the large pieces of TNT that made the original samples of 
soil inhomogeneous. As a result, the average contaminant concentration was 
less than &lt;500 mg TNT/kg soil or equivalents. 
The wet sieved materials (i.e., those essentially free of heavy metal ores 
and large pieces of TNT but still containing the leachable organic arsenic 
compounds) was treated in a batch reactor similar to that shown in FIG. 2. 
The pretreated fine soil material was fed into a steel vessel (6 
m.times.2.5 m.times.2 m) that was lined with wood and polyethylene sheets. 
Anode and cathode compartments were fitted with filter medium and filled 
with water. The anodes were made from activated titanium and the cathodes 
from stainless steel. Both anode and cathode compartments (porous 
polyethylene) were fitted to anolyte and catholyte circulation loops 
enabling the electrolytes to be continuously treated. 
Resistivity during the period was between 10 to 30.OMEGA., current density 
was 1-2 A/m.sup.2 and voltage was between 20-50 v/m. The electrical power 
supply was rated at 10 kVA. 
During the treatment, the ionic contaminants migrated under the influence 
of the electrical field and were captured in the electrolytes in the anode 
and cathode compartments. Treatment of the electrolytes consisted of 
removal of arsenic and heavy metals by selective electrical ion exchange 
using several different ion exchange resins. The pH of the electrolytes 
was maintained at 7. 
Soil was heated to 25.degree.-30.degree. C. as the result of Joule heating 
(from the passage of current via the electrodes) sufficient to enhance 
biodigestion but conservative enough not to threaten the TNT. 
Periodically, sodium hexametaphosphate and nitrate were added to the 
electrolytes and transported through the soil under the influence of the 
electric field as nutrients for the microorganisms naturally present in 
the contaminated area. After three months, the following results (Table 3) 
were obtained: 
TABLE 3 
______________________________________ 
Applied Organic 
Energy TNT DNT DNB PAH As 
kWh/m.sup.3 
mg/kg mg/kg mg/kg mg/kg mg/kg 
______________________________________ 
0 49 188 553 40 11 
31 70 10 2.7 nd nm 
49 10.1 3.3 6.8 nd 0.11 
______________________________________ 
TNT = trinitrotoluene 
DNT = dinitrotoluene 
DNB = dinitrobenzene 
PAH = polycyclic aromatic hydrocarbons 
nd = not detected 
nm = not measured 
As can be seen from these results, vacuum-assisted electrochemical 
remediation in combination with biodigestion were not only compatible, but 
effective means for handling such mixed wastes. Vacuum was provided via a 
buried porous pipe positioned between the electrodes in the soil. 
EXAMPLE 3 
A test site contaminated with diesel was heated with 10 mA ac current from 
wells arranged in a hexagonal electrode pattern inserted to a depth of 9 
meters, a centrally-located vacuum well was inserted in the center of the 
electrode array. Electrode spacing was about two meters. Vacuum was about 
28-30 inches Hg. The results of the process on the concentrations of 
diesel at 1-, 2- and 3 m depths in the soil before and after treatment and 
corresponding soil temperatures achieved are shown below in Table 4. 
TABLE 4 
______________________________________ 
Initial Final 
Depth Concentration 
Concentration 
Removal 
(m) (mg/kg) (mg/kg) Efficiency 
Temp .degree.C. 
______________________________________ 
1 9000 220 97.6% 40 
2 9000 9 99.9% 55 
3 9000 18 99.8% 70 
______________________________________ 
The results indicate the value of gentle soil heating achieved in the 
present invention: although relatively more contamination was left at the 
lowest temperature, the amount of contamination in the soil was 
dramatically reduced (97.6% removal efficiency). 
EXAMPLE 4 
Samples of soil from a gas-producing site were contaminated with Prussian 
blue dye (potassium ferrous ferricyanide), cadmium, arsenic, phenols and a 
mixture of polycyclic aromatic hydrocarbons and tar from the coking of 
coal. 
Electrochemical remediation with electrolyte management was used to remove 
cyanide in the Prussian blue component, cadmium, arsenic and the phenols 
which, in an alkaline environment, exist as phenate ion. 
Twenty kilogram soil samples were treated. The soil was placed in a small 
batch reactor as shown in FIG. 2. 
Prussian blue was hydrolyzed under alkaline conditions to form CN.sup.- 
anions, iron and potassium cations which electromigrate toward the 
appropriate electrodes. The blue color of the contaminated soil changed to 
a normal brown color as the alkaline front from the catholyte moved 
through the soil. Some metals and arsenic accumulate at the electrodes and 
were removed to the electrolyte compartments. Treatment continued for 400 
hours. The power used was equivalent to 696 kWh/m.sup.3. The results are 
shown below in Table 5. 
TABLE 5 
______________________________________ 
Initial Concentration 
Final Concentration 
Removal 
Contaminant 
(ppm) (ppm) Efficiency 
______________________________________ 
Phenol 340 93 73% 
Cyanide 32,000 1200 96% 
As 15 9.3 38% 
Cd 0.9 0.4 56% 
______________________________________ 
Although the conditions are not optimized for heavy metal removal (which 
would work better under more acidic conditions), the decontamination of 
the soil was significant and rapid. 
EXAMPLE 5 
Soil samples as described in Example 4 were treated under the same 
conditions in that example, except the anolyte and catholyte solutions 
were maintained in an acid condition to improve removal efficiency for the 
cadmium and arsenic. A vacuum well was formed in the center of the soil 
compartment, and a vacuum applied from the laboratory vacuum pump to trap 
any free HCN or cyanogen liberated from residual cyanide left in the soil. 
The soil was heated to 40.degree. C. by Joule heating. 
The residual cyanide and phenol were removed by vacuum-assisted 
electrochemical remediation. Residual metals were also removed. Levels of 
both arsenic and cadmium were less than 1 ppm after 100 hours of 
treatment. 
EXAMPLE 6 
This example illustrates electrochemically enhanced biodigestion, 
electrolyte management and vacuum-assisted electrochemical remediation 
achieved in a single treatment. 
Soil contaminated with polycyclic hydrocarbons and tar residues is added to 
the soil described in Example 4. The added soil contains microorganisms. 
Five grams of CALGON detergent (potassium hexametaphosphate) is added per 
kilogram of soil. The experiment is run for 200 hours under vacuum and the 
temperature is maintained at 30.degree.-40.degree. C. The pH of the soil 
is maintained at 6-7 using electrolyte conditioning units. Table 6 shows 
exemplary results. 
TABLE 6 
______________________________________ 
Contaminant 
Phenol As Cd PAH cyanide 
______________________________________ 
Initial 93 9.3 0.4 300 1200 
concentration 
(ppm) 
Final ND 0.1 0.2 20 12 
concentration 
(ppm) 
______________________________________ 
The experiment although not optimized indicates that mixed contamination 
can be removed using a combination of the electrolyte management, heated 
vacuum and electrochemically supported bioremediation. 
While the present invention is disclosed by reference to the preferred 
embodiments and examples set forth above, it is to be understood that 
these examples are intended in an illustrative rather than a limiting 
sense. It is contemplated that modifications will readily occur to those 
skilled in the art, which modifications will be within the spirit of the 
invention and with scope of the appended claims.