1. Research Article
In Situ Radio-Frequency Heating for Soil
Remediation at a Former Service Station:
Case Study and General Aspects
In situ radio-frequency heating (ISRFH) was successfully applied during remedia-
tion of a former petrol station. Using a three-electrode array in combination with
extraction wells for soil vapor extraction (SVE), pollution consisting mainly of
benzene, toluene, ethylbenzene, xylenes, and mineral oil hydrocarbons (in total
about 1100 kg) was eliminated from a chalk soil in the unsaturated zone. Specially
designed rod electrodes allowed selective heating of a volume of approximately
480 m3
, at a defined depth, to a mean temperature of about 50 °C. The heating
drastically increased the extraction rates. After switching off ISRFH, SVE re-
mained highly efficient for some weeks due to the heat-retaining properties of the
soil. Comparison of an optimized regime of ISRFH/SVE with conventional “cold”
SVE showed a reduction of remediation time by about 80 % while keeping the
total energy consumption almost constant.
Keywords: Energy efficiency, Mineral oil hydrocarbons, Radio-frequency heating, Soil vapor
extraction
Received: January 12, 2012; revised: March 23, 2012; accepted: April 10, 2012
DOI: 10.1002/ceat.201200027
1 Introduction
In situ thermal remediation methods [1–19] have the potential
to enhance efficiency and cost-effectiveness of soil remediation
processes. The spectrum of these in situ methods is practically
restricted to heating lances (thermal wells operated with elec-
tricity or gas), injection of hot air or steam, resistive heating
using power-line-frequency energy (usually realized as six-
phase heating with 50 or 60 Hz), and dielectric radio-fre-
quency (RF) heating (applying frequencies in the MHz range).
Studies on soil decontamination by thermal desorption, strip-
ping or chemical conversion of pollutants have also been car-
ried out using microwave heating. However, due to the small
penetration depths at frequencies of some GHz, these investi-
gations were limited to the laboratory scale and the corre-
sponding small volumes [20, 21].
Among the currently available technologies, direct RF heat-
ing [4, 10–18] has some unique advantages as it can be applied
to a variety of soils and achieve a wide temperature range
(potentially up to more than 300 °C). Based on a working
principle similar to that applied in household microwave
ovens, RF heating is characterized by direct heat formation in
the soil volume without requiring a heat transfer medium such
as hot air or steam or overheated surfaces such as heating
lances. In contrast to resistive heating [1, 3, 9, 18], it is not lim-
ited by the requirement of soil moisture ensuring a sufficiently
high electrical conductivity. Additionally, the heating principle
does not depend on the permeability of soil for gas streams (as
in the case of steam injection) or thermal conductivity (as for
thermal lances). However, both parameters are nevertheless
relevant for combination with soil vapor extraction (SVE) or
for the uniformity of temperature profiles especially when
using rod electrodes [10, 16, 17].
As with all thermal methods, RF heating supports the re-
moval of contaminants by enhancing mobility and water solu-
bility, increasing vapor pressures, and decreasing the surface
tension. Adsorption equilibriums in the soil matrix are shifted
towards desorption. All these effects can positively influence
the bioavailability of pollutants as a prerequisite for bio-
degradation. Tests have shown that RF heating methods can be
used to successfully maintain an optimal temperature range
for biodegradation. Additionally, it was proven that the
electromagnetic fields do not inhibit microbial activity
[10, 11, 19, 22, 23]. However, the greatest potential for RF heat-
ing in combination with SVE is seen in the increase of vapor
pressure and stripping effects related to the evaporation of soil
moisture at 100 °C for extracting organic pollutants from soils
[6–8, 24].
www.cet-journal.com © 2012 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim Chem. Eng. Technol. 2012, 35, No. 8, 1534–1544
Gurvan Huon1
Tom Simpson1
Frank Holzer2
Giacomo Maini1
Fraser Will3
Frank-Dieter Kopinke2
Ulf Roland2
1
Ecologia Environmental
Solutions Ltd., Sittingbourne,
Kent, United Kingdom.
2
Helmholtz Centre for
Environmental Research –
UFZ, Department of
Environmental Engineering,
Leipzig, Germany.
3
Total UK, Watford,
United Kingdom.
–
Correspondence: Dr. U. Roland (ulf.roland@ufz.de), Helmholtz Centre
for Environmental Research – UFZ, Department of Environmental
Engineering, Permoserstraße 15, 04318 Leipzig, Germany.
1534 U. Roland et al.

2. In remediation practice, the cost-efficiency of
the method depends among other conditions on
the specific energy consumption and, therefore,
also on the conversion efficiency of high-grade
electrical primary energy into heat in the desired
soil volume. The RF unit has two specific tools de-
signed to optimize efficiency. Firstly, an electronic
matching network (so-called matchbox) fits vari-
able electrical properties of the soil (impedance) to
the generator avoiding reflection of energy from
the electrodes. Secondly, specially designed rod
electrodes, with an air gap, focus energy absorp-
tion to the contaminated soil at discrete depths
[10, 15–17, 25]. Perforated electrodes can simulta-
neously be used as extraction wells for soil vapor,
thus reducing the number of boreholes in the re-
mediation area [17]. This design is significantly
different from that used in the US in the frame of
the Superfund Technology development program
carried out in the 1990s [12–14, 26, 27].
The present study describes a demonstration
project for in situ radio-frequency heating
(ISRFH) in the unsaturated zone at a decommis-
sioned petrol station near Manston, Kent, UK.
Several options of SVE in a chalk soil were tested
in order to optimize the process with respect to
removal efficiency and energy consumption.
2 Description of the
Demonstration Site
The ground underlying the site was mainly classified as cretac-
eous upper chalk (mean natural moisture content 26.5 wt-%,
dry density 1.65 g cm–3
, total porosity 39 %, negligible organic
carbon content). The first 0.75 m consisted of sandy clayey
flint and brick gravel (natural moisture content 11.5 wt-%, dry
density 1.14 g cm–3
, total organic carbon content 1.5 wt-%).
Further details on geology, hydrogeology, and hydrology can
be obtained from [15] and references therein.
The site was a former petrol station owned by Total UK with
the ground impacted by volatile organic compounds (VOC),
including benzene, toluene, ethylbenzene, xylenes (BTEX), and
other petroleum hydrocarbons (reported as total petroleum
hydrocarbons (TPH), mostly with less than 21 carbon atoms
(< C21)). The initial contaminant concentrations are summa-
rized in Tab. 1. For reference, the concentrations have been
compared with risk-based target concentrations, generic as-
sessment criteria (GAC), applicable to human health risk at a
commercial site.
The contamination of the soil was restricted to a relatively
small area with a diameter of less than 20 m and was vertically
contained within an unsaturated chalk stratum to a maximum
depth of 10 m below ground level (bgl.). The highest concen-
trations of contaminants were identified at depths between 2
and 3 m bgl. as illustrated by the distribution of contaminants
in the treatment area, presented in Tab. 2. The underlying
groundwater (below 45 m bgl.) was not contaminated. Under-
ground storage tanks had previously been removed and the ex-
cavation had been backfilled. Thus, the site was a typical
example of a decommissioned medium-sized petrol station.
Voluntary remediation was required in order to reduce risks
associated with environmental liabilities subject to the site
being divested or redeveloped and to reduce the potential for
the significant environmental harm.
3 Design of the ISRFH/SVE System
3.1 Equipment for RF Heating
The employed RF 20-ft container system housed an RF genera-
tor IS 30 (maximum RF power 30 kW) operating at a constant
high frequency of 13.56 MHz, a matchbox PFM 30000A (both
from Hüttinger Elektronik, Freiburg/Brsg., Germany), a water-
cooler (Hyfra, Karst), a fiber-optical temperature measurement
system (OPTOcon, Dresden [28]), and a personal computer
with the controlling software for automatic operation and re-
mote control of the system [15, 16, 29–31]. The electronic
matching network (matchbox, Fig. 1a, connected to the gen-
erator with a coaxial cable) was used to fit the variable imped-
ance of the soil to the generator, thus avoiding reflection of en-
ergy from the electrode system. Under the conditions of the
test, complete fitting could be permanently reached. Therefore,
the effective power was the same as the applied power as stated
in the following.
Chem. Eng. Technol. 2012, 35, No. 8, 1534–1544 © 2012 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim www.cet-journal.com
Table 1. Comparison of pollutant concentrations in soil with GAC for commercial
land-use at 1 % soil organic matter and pH 7.0.
Contaminants Mean
[mg kg–1
]
Maximum
[mg kg–1
]
US95
a
[mg kg–1
]
GAC b
[mg kg–1
]
US95/GAC
Benzene 117 1 360 901 28 c
32
Toluene 1 563 20 400 5 810 870 c
7
Ethylbenzene 451 5 810 2 069 520 4
Xylenes 3 320 38 900 14 246 480 d
30
TPH aromatic C9, C10 1 781 11 025 6 683 3 700 2
TPH aromatic C11, C12 1 408 6 096 5 120 17 000 (< 1)
TPH aromatic C13 – C16 277 1 069 932 36 000 (< 1)
TPH aromatic C17 – C21 24 72 63 28 000 (< 1)
TPH aromatic > C21 39 269 91 28 000 (< 1)
TPH aliphatic C9, C10 594 3 675 2 228 2 100 ≈ 1
TPH aliphatic C11, C12 447 1 934 1 625 10 000 (< 1)
TPH aliphatic C13 – C16 156 601 525 61 000 (< 1)
TPH aliphatic > C16 80 243 224 1 600 000 (< 1)
a
Upper 95th percentile bound of sample mean. b
Generic Assessment Criteria
(GAC), 2nd ed., values published by the Chartered Institute of Environmental
Health in 2009 assuming 1 % of soil organic matter. c
Soil Guideline Values pub-
lished by the UK Environment Agency 2009 assuming 1 % soil organic matter.
d
Total xylenes (o-, m-, and p-isomers) compared to the most conservative Soil
Guideline Value derived for o-xylene.
Energy efficiency 1535

3. For this site, specially designed partially perforated ISRFH
electrodes (Fig. 1b) with an active electromagnetic radiation
range allowing selective heating in a depth between 3 and 6 m
bgl. [10, 15–17, 25] were deployed in a triangular array (Fig. 1c).
The upper ends of the electrodes, extending above ground level,
were enclosed by Faraday cages for electromagnetic shielding
and connected to the matchbox via coaxial steel tubes. The elec-
trodes with the perforated part (Fig. 1b) for ex-
tracting the soil vapor were connected to the SVE
system (see later). In this case, the inner volume of
the electrodes was empty. However, it is also possi-
ble to place a catalyst inside in order to remove
high concentrations of hydrocarbons in the soil
vapor by in situ total oxidation [17].
Risks of exposure to unauthorized personnel
during operation were mitigated by the use of
hoardings and the deployment of an infrared fence
that shuts down the system automatically. Further
safety options to limit maximum temperatures in
the soil and to guard against unusual operating
conditions (too high fields strengths) were also im-
plemented utilizing specific software modules
[29, 30] The electromagnetic field intensity outside
the Faraday cage as monitored by EMR-300 and
ESM-20 radiation sensors (Wandel & Goltermann)
was lower than that generated by computer
monitors or mobile phones. Electromagnetic field
values remained below guidance levels for time-
varying electromagnetic fields for the general pub-
lic published in the most recent International
Commission on Non-Ionizing Radiation Protec-
tion Guidelines (ICNIRP, 1998, [32]) which is the
most restrictive guidance currently available. These
guidelines were more conservative than those
outlined in the European Parliament Directive
2004/40/EC.
3.2 SVE Equipment
The SVE system was equipped with a vacuum blower (6 kW)
providing a maximum flow rate of 95 m3
h–1
. Eight multilevel
SVE wells (screened depth 2.5 to 5 and 5.5 to 7.5 m bgl.)
were deployed in the field trial. A detailed layout of the
electrodes, the monitoring, and the extraction wells is pre-
sented in Fig. 2.
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Table 2. Distribution of contaminants and depths with highest pollution in the treatment area (N/A = not applicable).
TPH
C8 – C35
Benzene Toluene Ethylbenzene Xylenes
Content
[mg kg–1
]
Depth
[m]
Content
[mg kg–1
]
Depth
[m]
Content
[mg kg–1
]
Depth
[m]
Content
[mg kg–1
]
Depth
[m]
Content
[mg kg–1
]
Depth
[m]
Min 17 6 0 3 ... 6 0 6 0 3 ... 6 0 6
Avg 6 437 N/A 206 N/A 2 691 N/A 790 N/A 5 590 N/A
Max 23 500 2 1 360 2 20 400 2 5 810 2 38 900 2
TPH
C9, C10
TPH
C11, C12
TPH
C13 – C16
TPH
C17 – C21
TPH
C22 – C35
Content
[mg kg–1
]
Depth
[m]
Content
[mg kg–1
]
Depth
[m]
Content
[mg kg–1
]
Depth
[m]
Content
[mg kg–1
]
Depth
[m]
Content
[mg kg–1
]
Depth
[m]
Min 0 3 ... 6 0 3 ... 6 3 6 3 6 6 5
Avg 3 182 N/A 2 407 N/A 553 N/A 147 N/A 118 N/A
Max 14 700 2 8 030 2 1 670 2 315 2 498 3
a) b)
c)
Figure 1. Equipment for thermally enhanced soil vapor extraction using RFH at a
decommissioned petrol station: (a) matching network (matchbox) with connec-
tion to the electrodes and coaxial cable to the RF generator, (b) partially perfo-
rated rod electrodes for selective heating in certain depths (from [17]), (c) cages
for shielding the electrodes above the ground with shielded RF energy transfer
lines (coaxial steel tubes).
1536 U. Roland et al.

4. The SVE system was controlled by two programmed logic
controllers set up to apply vacuum extraction (420 mbar gauge
pressure) to each of the eight extraction wells in a cyclical se-
quence for a period of 45 min per well. Gas flow and pressure
were monitored by respective sensors. After condensation and
phase separation, the extracted air was cleaned by using a gran-
ular activated carbon filter (approx. 20 m3
of 2 mm Ecosorb
GXC from Jacobi, UK, were used throughout the project).
3.3 Analytical Tools
Online monitoring of the extraction wells was undertaken
using a Total VOC (TVOC) sensor to quantify the total organic
carbon content of the extracted air. Gas analysis involved purg-
ing and sampling of each SVE line using a small bladder pump.
The sensor was considered accurate to an upper limit of
1000 ppmv. Therefore, in order to quantify higher concentra-
tions (the maximum content during the project was in the
range of 50 000 ppmv), the extracted air had to be diluted with
fresh air which was checked daily (details in [15]). In the be-
ginning of the trial, supplementary measurements were also
made with a photo-ionization detector.
TENAX® thermal desorption tubes were additionally used
to validate the TVOC data. Samples were collected on desorp-
tion tubes at various intervals throughout the trial. In addition
to the eight fiber-optical sensors placed at various depths in
six monitoring wells, thermocouples were discontinuously
introduced into monitoring wells and used to measure the
temperature (after temporary switching off RF power) at
different positions in the soil. Mean soil temperatures were
estimated using fixed point measurements and by con-
sidering the soil volumes represented by the corresponding
sensors [15].
3.4 Soil Sampling
Soil samples were taken before and after treatment from 17
boreholes throughout the field site from three different depths
(approx. 3 m, 5 m, and 7 m, details in [15]). The validation
samples were collected at the end of the project at a maximum
distance of 0.2 m from the initial sample locations in order to
produce comparable results. These locations included the three
electrodes and the extraction wells with the highest and the
lowest extraction rates at the end of the trial. It was not always
possible to retrieve samples from the same depths as the initial
samples for all the validation sampling, as this was dependent
upon the sample recovery from the drilling works. The varia-
tion in depth was sometimes up to 1 m.
Chem. Eng. Technol. 2012, 35, No. 8, 1534–1544 © 2012 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim www.cet-journal.com
to avoid unauthorized access)to avoid unauthorized access)to avoid unauthorized access)to avoid unauthorized access)
Welfare
facilities
RF
unit SVE
unit
Fuel
tank
Diesel
generator
Match-
box
to avoid unauthorized access)to avoid unauthorized access)to avoid unauthorized access)
Welfare
facilities
RF
unit SVE
unit
Fuel
tank
Diesel
generator
Match-
box
Operational boundary
Extraction well
Monitoring well
Electrode
Shielding box
(fencing to avoid unauthorized access)
Figure 2. Arrangement for combined ISRFH/SVE decontamination of a chalk soil at a former petrol station.
Energy efficiency 1537

5. 4 Program and Results of the Remediation
Operation
4.1 Stages of the Demonstration Project
The remediation was performed in six stages in order to dem-
onstrate the added benefits of coupling ISRFH to a traditional
SVE system. Continuous operation of the RF system for sever-
al days could be realized without difficulty by using an appro-
priate software control tool [29, 30] Usually, a duration of
about ten days (10 d) was found to be sufficient to adequately
test one operation option. However, due to operational and
measuring reasons, downtimes occurred during the stages of
the experiment.
During Stage 1 (15 d of operation without downtimes), the
SVE system was run alone in order to obtain the reference
baseline data. The ISRFH system was then turned on and oper-
ated without SVE until the soil around the electrodes was
heated to approximately 45 °C (Stage 2 for 10 d, value in 3.5 m
bgl.). In Stage 3 (11 d), ISRFH and SVE were operated in com-
bination. During Stage 4 (operation lasting 4 d), the ISRFH
system was shut down while SVE continued to work at the ob-
tained temperature level. In the following 10 d, the combina-
tion of both techniques was tested again (Stage 5). Finally, SVE
was operated again alone until completion of the demonstra-
tion program after 30 d (Stage 6). The project was completed
also considering deadline constraints and economic conditions
when an acceptably low extraction rate was detected and,
therefore, residual concentrations below risk assessment targets
were expected (and finally validated).
4.2 Soil Temperatures and Extraction Rates
A summary of the temperatures at a depth of 3.5 m bgl., the
TVOC concentrations in the extracted air, and the extraction
rates as well as the variation of these values, for the duration of
the whole demonstration project, is provided in Tab. 3. The
cold SVE at ambient temperature led to a mean extraction rate
of 2.9 kg d–1
(Stage 1). After 10 d of ISRFH, which established a
mean temperature of 36.1 °C in the balance volume corre-
sponding to an area of 57 m2
(see Fig. 1) and a maximum tem-
perature of about 89 °C in the vicinity of the electrodes (Stages
2 and 3), the mean extraction rate increased by a factor of ap-
prox. 12. In Stage 4, RF heating was switched off and the conti-
nuing SVE benefited from the higher level of soil temperature
compared to the initial cold SVE (Stage 1). The Stage 4 extrac-
tion rate of 16.5 kg d–1
compares favorably with the initial rate
of approximately 3 kg d–1
. This positive memory effect is due
to only moderate soil cooling as a consequence of continued
SVE operation (maximum value –0.374 K d–1
, average
–0.098 K d–1
). It is notable that during Stage 4, the mean as
well as the minimum temperatures further increased, while the
maximum temperature decreased, indicating an efficient heat
transfer within the soil volume caused by SVE. Thermal con-
ductivity of the soil was shown to play a minor role for heat
transfer as proven by comparison without SVE. However, this
effect also shows an uncertainty of the estimation of the mean
temperature for the whole volume because no energy was in-
troduced into the system when operated in the SVE-only
mode. The combined application of ISRFH and SVE in Stage 5
again led to an increase of the extraction rate to 24.8 kg d–1
and
to enhanced mean (45.4 °C) and maximum (96.4 °C) tempera-
tures. The final SVE without further heating in Stage 6 corre-
sponded with moderate cooling of the soil (mean temperature
39.7 °C) and decreased efficiency (extraction rate 17.5 kg d–1
),
similar to that achieved during the SVE-only operation ob-
served in Stage 4. Of course, the reduction of the extraction
rates during the overall trial is an expected phenomenon due
to declining concentrations of contaminants in the soil. The
temperature distributions in the soil at depths of 3.5, 5.5, and
7.5 m bgl. after Stage 5 are illustrated in Fig. 3. Additionally,
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Table 3. Summary of process parameters (soil temperature at 3.5 m depth, hydrocarbon concentrations in the extracted air, daily extrac-
tion rates) for the different stages of the ISRFH/SVE demonstration project (minimal (Min), average (Avg), and maximum (Max) values,
N/A – not applicable).
Parameter Stage 1 Stage 2 Stage 3 Stage 4 Stage 5 Stage 6
SVE on off on on on on
RF heating off on on off on off
Temperature at 3.5 m bgl. [°C] Min 10.5 11.1 12.1 20.8 20.8 18.0
Avg 11.8 15.6 36.1 41.2 45.4 39.7
Max 12.8 35.0 88.9 84.1 96.4 94.7
TVOC concentration [mg m–3
] Min 7 N/A 210 1 422 230 70
Avg 1480 N/A 17 250 7110 12 410 7080
Max 5450 N/A 76 400 14 400 29 700 25 200
Extraction rate of hydrocarbons [kg d–1
] Min 0.7 N/A 5.0 12.5 13.9 6.7
Avg 2.9 N/A 37.4 16.5 24.8 17.5
Max 4.8 N/A 86.3 19.4 37.8 30.2
1538 U. Roland et al.

6. data on the radius of influence of RFH can be obtained from
Tab. 4. The actual extraction rates, the cumulative total VOC
removal, and the average soil temperature as a function of time
for the duration of the whole trial period are illustrated in
Fig. 4.
Chem. Eng. Technol. 2012, 35, No. 8, 1534–1544 © 2012 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim www.cet-journal.com
a)
0 1 2 3 4 5 6 7 8 9 10 11
0
1
2
3
4
5
7
6
8
9
10
11
Length / m
Width/m
Temperature / °C
10
40
70
100
0 1 2 3 4 5 6 7 8 9 10 110 1 2 3 4 5 6 7 8 9 10 110 1 2 3 4 5 6 7 8 9 10 11
0
1
2
3
4
5
7
6
8
9
10
11
0
1
2
3
4
5
7
6
8
9
10
11
0
1
2
3
4
5
7
6
8
9
10
11
Length / m
Width/m
Temperature / °C
10
40
70
100
Temperature / °C
10
40
70
100
10
40
70
100
b)
0 1 2 3 4 5 6 7 8 9 10 11
0
1
2
3
4
5
7
6
8
9
10
11
Length / m
Width/m
Temperature / °C
10
40
70
100
0 1 2 3 4 5 6 7 8 9 10 110 1 2 3 4 5 6 7 8 9 10 110 1 2 3 4 5 6 7 8 9 10 11
0
1
2
3
4
5
7
6
8
9
10
11
0
1
2
3
4
5
7
6
8
9
10
11
0
1
2
3
4
5
7
6
8
9
10
11
Length / m
Width/m
Temperature / °C
10
40
70
100
Temperature / °C
10
40
70
100
10
40
70
100
c)
0 1 2 3 4 5 6 7 8 9 10 11
0
1
2
3
4
5
7
6
8
9
10
11
Length / m
Width/m
Temperature / °C
10
40
70
100
0 1 2 3 4 5 6 7 8 9 10 110 1 2 3 4 5 6 7 8 9 10 110 1 2 3 4 5 6 7 8 9 10 11
0
1
2
3
4
5
7
6
8
9
10
11
0
1
2
3
4
5
7
6
8
9
10
11
0
1
2
3
4
5
7
6
8
9
10
11
Length / m
Width/m
Temperature / °C
10
40
70
100
Temperature / °C
10
40
70
100
10
40
70
100
Figure 3. Temperature distributions after finishing Stage 5 at a
depth of (a) 3.5 m bgl. (mean value 49.1 °C), (b) 5.5 m bgl.
(30.9 °C), (c) 7.5 m bgl. (18.6 °C).
0
20
40
60
80
100
0 500 1000 1500 2000 2500 3000
Time / h
Extractionrate/kg/d
0
10
20
30
40
50
Temperature/°C
Stage2
Stage3
Stage4
Stage5
Stage6
Stage1
0
200
400
600
800
1000
0 500 1000 1500 2000 2500 3000
Time / h
CumulativeHCremoval/kg
0
10
20
30
40
50
60
Temperature/°C
Figure 4. Actual extraction rates vs. average soil temperature
(top) and cumulative total VOC removal vs. average soil temper-
ature (bottom) as a function of time for the whole trial period.
Table 4. Estimated radius of influence around the electrodes ob-
tained for ISRFH in chalk soil.
Tempe-
rature
[°C]
Total
heated
area
[m2
]
Heated
area
per elec-
trode
[m2
]
Total
heated
volume
[m3
]
Heated
volume
per elec-
trode
[m3
]
Radius of
influence a
[m]
20 69 23 208 69 4.7
30 45 15 136 45 3.8
40 33 11 99 33 3.2
50 25 8 75 25 2.8
60 20 7 59 20 2.5
70 15 5 46 15 2.2
80 12 4 35 12 1.9
90 8 3 25 8 1.6
100 3 1 10 3 1.0
a
The radius of influence is an operationally defined measure of
the steepness of the temperature profile around an electrode. As
an example, a temperature of 50 °C can be achieved in a distance
of about 2.8 m from the electrode at the end of the heating
period.
Energy efficiency 1539

7. 4.3 Correlation of Extraction Rates and Vapor
Pressure of Contaminants
Generally, it would be useful to have a tool available to esti-
mate the influence of temperature on the overall extraction
rates. A simple but plausible approach in this context is to cor-
relate extraction rates with vapor pressures of volatile contami-
nants. In this approach, measured extraction rates of specific
compounds or classes of compounds at ambient temperature,
i.e., during cold SVE, are multiplied by ratios of vapor pres-
sures (pT/p10 °C) [33]. This consideration necessitates a number
of assumptions, including: constant conditions of SVE opera-
tion, a homogeneous distribution of contamination, a con-
stant contaminant source (i.e., no depletion), the concentra-
tion of contaminant as vapor phase within the soil pore being
directly related to contaminant extraction, and the mean
temperature being representative of the whole treatment vol-
ume. All these conditions are certainly not perfectly fulfilled at
any real site. Nevertheless, it can be useful to compare
measured ratios of extraction rates ( _mT/ _m10 °C) with ratios
calculated on the basis of vapor pressures according to
( _mT,/ _m10 °C)calc = (pT/p10 °C). Such a correlation would imply
that overall extraction rates (i) are controlled by transport
through the gas flow as the rate-determining process and (ii)
liquid-vapor and adsorption equilibriums are established at a
local scale. A summary of the results will be given in the
following (for details see [15]).
The average measured rates for benzene (bp 80.0 °C, all boil-
ing points obtained from [33]) and toluene (bp 110.6 °C) ex-
traction were significantly lower than predicted (by 85 % and
36 %, respectively). This can be explained by a relatively fast
depletion of these highly volatile compounds during the reme-
diation. The mean extraction rates for many of the other com-
pounds are higher than predicted, namely for ethylbenzene
(bp 136.2 °C, by 47 %), for xylenes (bp ≈ 140 °C, by 58 %), for
aliphatic TPH C9 and C10 (bp > 150.8 °C, by 58 %), for aro-
matic TPH C9 and C10 (e.g., naphthalene bp 217.9 °C, by
71 %), and for aromatic TPH C11 and C12 (by 96 %). This re-
sult can possibly be explained by the fact that the mean tem-
perature T as basis of the calculation was determined as the ar-
ithmetic mean of the distinct temperatures weighted by the
represented volumes. The vapor pressure, however, increases
exponentially with an increase in temperature. Therefore, a
temperature distribution around the mean value always leads
to higher partial pressures than calculated on the basis of the
mean temperature. This was found for all the less volatile
(and, therefore, less depleted) compounds. The tendency to
underestimate extraction rates is higher for higher boiling
compounds because the exponential factor, which is related to
the adsorption or vaporization enthalpy, is larger in this case.
In general, the comparison of predicted and measured data
shows that the control of soil temperature allows exploiting
the positive impact of temperature on vapor pressures and,
therefore, significant faster removal of pollutants. Although
the quantitative performance of this simple approach is mod-
erate, it gives a reasonable estimate (within a factor of 2) of the
removal rates of pollutants at elevated temperatures. There is
another reason restricting the use of mean temperatures as a
measure of SVE enhancement: pollutants in the colder (exter-
nal) zones of the treated soil volume are less affected by the
temperature increase. In case of strict remediation require-
ments, their depletion may control the overall remediation
success and time frame.
4.4 Analysis of Soil Samples after Trial
The validation sampling undertaken at the end of the trial con-
firmed that volatile compounds (BTEX and TPH fraction
< C10) were almost completely removed from the soil (> 95 %
reduction). For example, initial peak concentrations of toluene
(22 400 mg kg–1
) and xylenes (38 900 mg kg–1
) were reduced by
over 99.99 % to 0.56 and 0.72 mg kg–1
, respectively, at the end
of the remediation. Less volatile hydrocarbons (TPH fraction
> C10) were also significantly reduced (approximately 80 % on
average) but showed some concentration build-up near the
SVE wells where the soil temperature was likely to be lower
and, therefore, the mobility of the hydrocarbons was reduced
by partial re-adsorption. However, this effect demonstrates the
mobilization of these compounds in the warmer soil zones.
Similar behavior was observed for the TPH fractions with even
lower volatility (> C16), despite the fact that the initial con-
taminant concentrations recorded were two orders of magni-
tude lower than those of the volatile compounds. The concen-
trations observed across all the validation samples at the end
of the trial was markedly lower (by 80 %) than at the begin-
ning, demonstrating the effectiveness of thermal mobilization
even for compounds with very high boiling points (namely for
the fractions > C16 to C35). Stripping effects [6–8, 16, 20, 24]
may play a significant role in supporting thermal desorption
and SVE. In soil volumes where the temperatures reached 70–
80 °C, steam was produced in situ and this resulted in the pro-
duction of an aqueous (about 1300 L) and a hydrocarbon con-
densate phase (approximately 200 L corresponding to 172 kg)
following the cooling of extracted vapors.
Beside volatilization, three other removal mechanisms may
in principle contribute to the reduction of pollutants: biode-
gradation, hydrolysis ([34], less relevant at this site), and auto-
xidation. Occurrence of accelerated biodegradation at temper-
atures below 50 °C which was achieved in the course of the
demonstration project has been shown in various experiments
in both laboratory and field scale [10, 11, 16, 19, 22, 23]. Dur-
ing the present field trial, it was not possible to collect suffi-
cient analytical data to demonstrate the potential significance
of biodegradation. The dual porosity (matrix and fractures)
and the low permeability of chalk at the site prevented repre-
sentative and reliable measurements of CO2 formation and O2
consumption. Additionally, quantification of CO2 is compli-
cated by the chalk matrix, especially in the presence of moisture.
Hydrocarbons without heteroatoms such as oxygen, nitro-
gen, or chlorine, and, therefore, typical petrol station contami-
nants, are not susceptible to hydrolysis reactions. However,
they may be subject of abiotic autoxidation reactions. It is like-
ly that this removal mechanism contributes more significantly
to the reduction of the less volatile hydrocarbons (> C16) than
their evaporation. Soil validation sampling confirmed that,
with the exception of one single data point in the proximity of
one extraction well (SVE 2, see Fig. 2), the residual values re-
corded in the soil at the end of the treatment were below the
www.cet-journal.com © 2012 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim Chem. Eng. Technol. 2012, 35, No. 8, 1534–1544
1540 U. Roland et al.

8. GAC values for commercial and industrial use. A total of
945 kg of organic contaminants were extracted during the trial,
mostly associated with BTEX and the light hydrocarbon frac-
tion (< C10). In addition, approximately 200 L (corresponding
to 172 kg) of hydrocarbon phase (mostly BTEX and < C10)
were recovered from the SVE system’s air/water separator
along with 1300 L aqueous condensate. The initial and final
concentration profiles of TPH and BTEX contaminants at the
site at three different depths (2 to 3 m, 4 to 5 m, and 5 to 6 m
bgl.) are summarized in Fig. 5.
5 Economic Aspects of the ISRFH/SVE
Remediation
5.1 Energy Consumption and Related Costs
The specific primary energy consumption to heat the soil was
determined for the heating process up to about 100 °C. With
increasing temperature, this value also increased due to en-
hanced evaporation of water requiring additional energy and
intensifying heat transfer processes to the environment. The
obtained specific primary energy consumption increased from
about 3.3 kWh m–3
K–1
at 30 °C to 3.8 kWh m–3
K–1
at 60 °C and,
finally, to about 8 kWh m–3
K–1
at 80 °C. The corresponding
cumulative energy costs for soil heating on the basis of a tariff
of 0.12 £ kWh–1
(at the site, corresponding to 0.20 $ kWh–1
or
0.13 u kWh–1
, exchange rates in May 2011) increased from
11.7 £ m–3
(19.3 $ m–3
or 13.1 u m–3
) for heating to 50 °C over
20.7 £ m–3
(34.1 $ m–3
or 23.1 u m–3
) for heating to 70 °C to
approximately 40 £ m–3
(66 $ m–3
or 45 u m–3
) for heating to
90 °C.
An average cooling rate of –0.098 K d–1
was measured after
switching off RFH. This corresponds to 102 days of continued
SVE operation with only a 10 K fall in soil temperature. This
measured cooling rate is rather low and probably reduced by
internal heat flow from warmer soil regions. If the average
cooling rate of 0.374 K d–1
, measured at 3.5 m bgl., is used, this
equates to approximately 27 days of continuous SVE operation
before a 10 K drop in soil temperature
would be observed. This fits well with
the observed data and it is similar to
previous observations during trials at
Ecologia’s headquarters [15, 35]
5.2 Comparison of Alternatives
for Soil Remediation
A comparison on the basis of energy
consumption has been made between
the effectiveness of SVE during differ-
ent operational modes applied during
the trials. Data obtained during differ-
ent regimes (“cold” SVE, combination
of ISRFH and SVE, SVE post ISRFH)
has been extrapolated to predict time
and energy required to achieve removal
of the same hydrocarbon mass as ob-
tained during the real test from the chalk soil. Additionally, a
fictive optimized regime is evaluated.
Tab. 5 presents an overview of the remediation duration nec-
essary to extract 945 kg hydrocarbons (as in the test), the daily
energy consumptions, and the total energy consumption. The
comparison reveals that the application of ISRFH + SVE, oper-
ated intermittently, reduces the time required for remediation
by 81 % (60 days vs. 325 days) with the total energy input com-
parable to traditional SVE without heating (line 4 in Tab. 5;
operation with 12 days of RFH without SVE followed by
12 days with combined RFH and SVE and subsequently oper-
ating SVE at elevated temperature for 36 days).
The costs of combining ISRFH with SVE and operating the
system intermittently were compared to SVE alone (“cold”
SVE without heating) and excavation and disposal to landfill.
The calculated treatment cost for ISRFH + SVE was 194 £ m–3
(about 320 $ m–3
or 217 u m–3
) corresponding to 97 £ t–1
(160 $ t–1
or 108 u t–1
) with a program duration of approxi-
mately tenweeks. Disposal to landfill of hazardous soil in-
cluding sheet piling and backfilling operations was 398 £ m–3
(approximately 660 $ m–3
or 445 u m–3
) with a program dura-
tion of about five weeks. For SVE alone, unit costs of 174 £ m–3
(about 287 $ m–3
or 195 u m–3
) and a program duration of one
year were estimated. These calculations are based on the vol-
ume of soil treated during the remediation of 480 m3
(area
about 160 m2
, depth 3 m; compare Tab. 4) and they include all
energy costs, maintenance, and monitoring costs. SVE perfor-
mance is predicted on the basis of the extraction rates observed
at the commencement of the remedial operations, which is a
conservative evaluation with respect to ISRFH because it is un-
likely that the “cold” SVE process would continue with the
same efficiency throughout the whole trial. Beyond specific en-
ergy consumption, investment costs play a major role for any
remediation technology. Initial investment costs of ISRFH are
relatively high compared to “cold” SVE, but cannot be ser-
iously determined on the basis of this first field trial. The val-
ues given here can, therefore, be only an argument for a good
chance of this technology to compete with other cleanup op-
tions including thermal technologies [3, 4, 36].
Chem. Eng. Technol. 2012, 35, No. 8, 1534–1544 © 2012 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim www.cet-journal.com
Table 5. Calculation of time and energy requirements to extract 945 kg of volatile compounds
of concern using the different operational modes: SVE alone, SVE + ISRFH, SVE alone after
soil heating with ISRFH, and a fictive combination of both thermal modes.
Mode of operation VOC removal
rate
[kg d–1
]
Time to extract
945 kg c
[d]
Daily energy
consumption
[kWh d–1
]
Total energy
consumption
[MWh]
SVE alone 2.9 325 144 46.924
ISRFH + SVE a
31 30 1539 46.749
SVE post ISRFH b
17 55 264 14.675
Combination of ISRFH with
SVE + SVE post ISRFH
N/A d
60 N/A d
46.254
a
Average RF value is calculated assuming that RF is operated both at full power (24 kW) and
reduced power (10 kW). b
Average post RF is calculated assuming that a chiller may be
required if off-gases have a temperature above 40 °C. c
Does not include hydrocarbon recov-
ery in the knock-out tank. d
N/A = not applicable as VOC removal rate and/or daily energy
consumption is the sum of operating modes ISRFH + SVE (12 days) and SVE post ISRFH
(34 days) multiplied by their respective removal and consumption rates.
Energy efficiency 1541

9. 5.3 Potential and Perspectives of ISRFH Combined
with SVE
The trial study of ISRFH technology in combination with SVE
has demonstrated the efficiency of thermally supporting the
removal of volatile and semivolatile contaminants from chalk
in the unsaturated zone. The duration of remediation work at
the site were significantly reduced by use of the ISRFH, com-
pared to the duration that would have been required by cold
SVE only.
The perception that in situ heating is expensive and not sus-
tainable is erroneous. Heating of soil requires significant ener-
gy input, but soil also has the capacity to retain heat as it has a
relatively large specific heat capacity and a low thermal con-
ductivity reducing the heat flow to the environment. When the
ambient temperature is low. e.g., under cold-climate condi-
tions, heating may lead to a significant increase of the naturally
low elimination rate. However, appropriate thermal insulation
of the heated soil is then essential to reduce undesired heat
flows. Chalk in particular can retain this heat for days or weeks
thus allowing on-going SVE at elevated temperature. Soil tem-
peratures of 50–60 °C are sufficient to markedly increase the
extraction rates, thus avoiding very high energy costs normally
observed near the boiling point of water. Nevertheless, under
certain conditions, stripping at 100 °C to enhance removal of
compounds with lower volatility [24] can be a good option of
ISRFH where other methods fail. The very significant reduc-
tion of program timescales when ISRFH is applied is the prin-
cipal reason for the overall reduction in energy requirements
as demonstrated by this project.
www.cet-journal.com © 2012 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim Chem. Eng. Technol. 2012, 35, No. 8, 1534–1544
a)
b) 70000
62500
55000
47500
40000
32500
25000
17500
10000
2500
mg/kg
70000
62500
55000
47500
40000
32500
25000
17500
10000
2500
mg/kg
Figure 5. Initial (left) and final (right) concentration profiles of (a) TPH and (b) BTEX contaminations for the site in three different depths
(2 to 3 m bgl., 4 to 5 m bgl., 5 to 6 m bgl.).
1542 U. Roland et al.

10. A key advantage of ISRFH is that heat delivery into the
ground can be pinpointed by installing electrodes at prede-
fined depths [10, 15–17, 25]. This allows efficient and targeted
heating of highly contaminated source areas, often at larger
depths, without the need to heat the entire soil profile. The
cost assessment for this project suggests that ISRFH is likely to
be competitive when compared to traditional excavation and
disposal to landfill and also to traditional SVE.
5.4 General Aspects and Assignability to other Sites
The relevance of the study presented consists in the demon-
stration of ISRFH in full scale solving a real remediation prob-
lem. Although typical constraints of a field study (inhomo-
geneity of soil and contamination, limited number of samples,
economic constraints) do not allow data evaluation and mod-
eling in the same quality as in the laboratory scale, some gener-
al conclusions with high relevance for other soil remediation
sites can be drawn: (i) The effect of RF energy application can
be explained solely by thermal effects, i.e., the increase of tem-
perature in the soil. In combination with SVE, the increase of
vapor pressures and, therefore, the concentrations of contami-
nants in the extracted air were found to be the main effect
caused by heating. (ii) RF heating also facilitates other
processes like biodegradation of pollutants or transport by in-
ternally generated steam, as also shown by separate studies
[4, 10, 23, 24, 37]. (iii) The observed temperature effect on va-
por pressures can be generalized to all volatile pollutants. (iv)
The remediation technique ISRFH/SVE can, therefore, be ap-
plied at all sites where SVE itself is applicable and thermal en-
hancement is reasonable. (v) Enhancement of bioremediation
is especially observed for easily biodegradable substances such
as mineral oil hydrocarbons. Relevant for all conditions is the
increase of bioavailability caused by moderate heating [10, 23]
(vi) For contaminants that can be eliminated by autoxidation
(i.e., oxidation with O2) or hydrolysis (e.g., some chlorinated
compounds), these processes are supported by ISRFH as by
other heating methods. Generally, the temperature effects are
well known and frequently demonstrated in different scales.
With respect to the soil properties, ISRFH is a highly flexible
heating method because it is widely applicable independent of
the moisture content and the permeability of the matrix. For
soil with large content of soil organic matter, special effects of
matrix softening leading to higher mobility of pollutants [37]
can be used. The combination of RFH and SVE as described in
this study requires of course certain permeability for the
extracted air flow.
6 Conclusions
Thermally enhanced SVE using RFH was successfully tested at
a former petrol station. Although this method used electro-
magnetic waves for soil heating, safety risks and interference
with RF radiation could be easily eliminated by avoiding access
to the electrode field during operation and by using appropri-
ate Faraday shielding (e.g., using copper gauze).
The contamination mainly consisting of BTEX aromatics
and other mineral oil hydrocarbons were removed from a
chalk soil reaching the cleanup goals. By establishing a mean
soil temperature of more than 50 °C in the remediation vol-
ume, the remediation time could be reduced by more than
80 % when compared to conventional cold SVE. The energy in-
put was comparable or even lower.
The technical and engineering aspects of the described dem-
onstration project are typical for combination of ISRFH with
SVE and can, therefore, be analogously applied at other sites.
A detailed description of different electrode designs of RF heat-
ing has been made elsewhere [17]. With respect to energy and
total costs, preliminary considerations indicate that the ISRFH
method has a good chance to compete with alternatives. This
holds especially for cleanup of highly contaminated source
areas with limited size under time pressure.
The option of heating various materials with RF energy can
also be exploited for other fields in environmental technology
such as decontamination and/or drying of brickwork, steriliza-
tion, or pest control [38, 39]
Acknowledgment
Financial support from Total UK Ltd. is gratefully acknowl-
edged.
The authors have declared no conflict of interest.
Symbols used
bp [°C] boiling point
T [°C] temperature
pT [Pa, mbar] vapor pressure at temperature T
p10°C [Pa, mbar] vapor pressure at T = 10 °C
_mT [g h–1
, kg d–1
] extraction rate at temperature T
_m10°C [g h–1
, kg d–1
] extraction rate at T = 10 °C
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