Environ. Sci. Technol. 2010, 44, 9502–9508
Thermal desorption of a wide spectrum of organic contaminants, initiated by radio frequency (RF) heating, was studied at laboratory and pilot-plant scales for an artificially contaminated soil and for an originally contaminated soil from an industrial site. Up to 100 °C, moderate desorption rates were observed for light aromatics such as toluene, chlorobenzene, and ethylbenzene. Desorption of the less volatile contaminants was greatly enhanced above 100 °C, when fast evaporation of soil-water produced steam for hydrocarbon stripping (steam-distillation, desorption rates increased by more than 1 order of magnitude). For hydrocarbons with low water solubility (e.g., aliphatic hydrocarbons), the temperature increase above 100 °C after desiccation of soil again led to a significant increase of the removal rates, thus showing the impact of hydrocarbon partial pressure. RF heating was shown to be an appropriate option for thermally enhanced soil vapor extraction, leading to efficient cleaning of contaminated soils.
than 400 °C if desired. Compared to heating lances, the RF energy transfer into the soil is not bound to thermal conductivity and is thus able to realize more homogeneous temperature profiles. In contrast, the heat profiles are mainly determined by the electrode geometry and the corresponding shape of the electric field. The potential (e.g., flexibility, remediation speed) and the limitations (e.g., applicable only for source removal, significant investment costs) of RF heating have been discussed in detail elsewhere (3, 4). This paper is focused on the influence of soil moisture content on the thermal desorption at elevated temperature, especially in the range near the boiling point of water. Since many organic contaminants such as mineral oil hydrocarbons (HCs) or chlorinated solvents exhibit boiling points significantly above 100 °C, the role of steam formation for the release of these hazardous compounds is of great importance for soil remediation (5–7). Steam injection uses steam as the energy carrier, leading to water saturation of soil at the steam front. Heating with thermal wells is connected with steep temperature profiles and desiccation of soil in the vicinity of the lances. Resistive heating inherently requires certain soil-water content for operation, thus practically not allowing temperatures above 100 °C. RF heating, however, has the potential for utilizing an efficient combination of temperature and moisture control. Dielectric volumetric heating enables formation of steam from pore water throughout the whole remediation volume, thus providing an effective transport medium for volatilized contaminants out of the porous soil matrix. In contrast to other thermal remediation techniques, in case of RF heating the evaporation of water is directly initiated by dielectric energy absorption and not by, e.g., due to the thermal conductivity of the soil or steam injection (5–7). The aim of this study was to reveal the influence of heating on the release of various HCs with different boiling points and water solubilities for a wide temperature range (up to about 200 °C). The thermodesorption from an artificially contaminated sandy soil was compared with that from an original polluted soil from a former industrial site. Laboratory and technical-plant scale experiments were carried out, thus clarifying the potential of soil vapor extraction (SVE) supported by RF heating.
Introduction
Experimental Section
Thermal methods for soil decontamination are considered as an option to enhance remediation processes especially applicable for recalcitrant pollution, for remediation under pressure of time and for source removal in combination with other cleanup methods, e.g. enhanced natural attenuation. Under in situ conditions, the spectrum of available thermal methods is practically restricted to heating lances (thermal wells), hot air or steam injection, resistive heating using power-line-frequency (PLF) energy (e.g., realized as six-phase heating with 50 or 60 Hz), and dielectric heating using radio frequency (RF) energy. All these methods have some characteristic advantages and disadvantages (1–4). However, with respect to the appropriate temperature range and soil properties (moisture content and soil type), RF heating can be considered as the most flexible technique because different types of soil (sandy or tenacious, i.e. clay with low permeability) can be heated to final temperatures of up to more
Artificial Contamination of Sand. As a model for the contaminated original soil, a sand sample (fraction 0.315 to 0.63 mm; specific surface area approximately 1 m2 g-1; main inorganic components SiO2 [84.2 wt.-%], Al2O3 [5.9 wt.-%], Fe2O3 [2.3 wt.-%] and K2O [2.0 wt.-%]; carbon content less than 0.03 wt.-%; specific heat capacity 0.8 J g-1 K-1; apparent bed density 1.52 g cm-3) was spiked with hexane as a solvent for a wide spectrum of organic probe compounds. After removal of hexane under vacuum in a rotary evaporator, the residual content of these compounds was determined by microwave extraction (MLS Mega 1000, 750 W, 15 min, 5 g sample) with dichloromethane and GC analysis of the extracts (using a GC HP 5890 equipped with FID). The initial concentrations, the boiling points, and the water solubilities are given in Table 1 (data taken from ref 8). Prior to thermodesorption experiments using RF heating, water was added to the samples, thus adjusting the soil moisture content to 15 wt.-%. Originally Contaminated Samples from an Industrial Site. Contaminated sandy soil samples originating from a former industrial site (lignite pyrolysis factory in Espenhain near Leipzig, Germany) were taken from about 1 m depth.
Influence of in Situ Steam Formation by Radio Frequency Heating on Thermodesorption of Hydrocarbons from Contaminated Soil U L F R O L A N D , * ,† S A B I N E B E R G M A N N , ‡ FRANK HOLZER,† AND FRANK-DIETER KOPINKE† Department of Environmental Engineering, Helmholtz Centre for Environmental Research - UFZ, Permoserstrasse 15, 04318 Leipzig, Germany, and Institute Dr. Appelt, Ta¨ubchenweg 28, 04317 Leipzig, Germany
Received September 7, 2010. Revised manuscript received November 12, 2010. Accepted November 15, 2010.
* Corresponding author: phone ++49 (341) 235 1762; fax: + +49 (341) 235 45 1762; e-mail:
[email protected]. † Helmholtz Centre for Environmental Research - UFZ. ‡ Institute Dr. Appelt. 9502
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ENVIRONMENTAL SCIENCE & TECHNOLOGY / VOL. 44, NO. 24, 2010
10.1021/es1027772
2010 American Chemical Society
Published on Web 11/24/2010
TABLE 1. Initial Concentrations, Boiling Points, and Water Solubilities of the Probe Compounds in an Artificially Contaminated Sandy Soil
compound toluene chlorobenzene ethylbenzene 1,3,5-trimethylbenzene n-decane (n-C10) n-dodecane (n-C12) naphthalene n-tetradecane (n-C14) 1-chloronaphthalene 2,3-dimethylnaphthalene n-hexadecane (n-C16) fluorene phenanthrene anthracene n-eicosane (n-C20) fluoranthene n-pentacosane (n-C25)
concentration boiling (mg kg-1 point dry soil) (°C) 7.8 5.4 8.9 32.1 44.0 74.9 70.7 82.4 114.4 76.4 81.8 79.6 91.3 44.8 82.4 86.9 77.6
110.6 131.7 136.2 164.7 174.2 216 218 254 259 268 287 295 340 340 343 384 402
water solubility (mg L-1) 531 (25 °C) 484 (25 °C) 161 (25 °C) 50 (25 °C) 0.05 (0 °C) 3.7 · 10-3 31.6 (25 °C) 2.2 · 10-4 22.4 (25 °C) 2.5 (25 °C) 1.3 · 10-5 1.9 (25 °C) 1.1 (25 °C) 0.045 (25 °C) 3 · 10-8 0.26 (25 °C) 2 · 10-11
The samples exhibited a natural water content of about 15 wt.-%. The dichloromethane extract obtained via the same microwave-supported procedure was analyzed by means of GC/MS without further purification. Scattering in GC results (about 10%) was much lower than variance due to soil inhomogeneities. Additional data on this soil are given in ref 9. The main HC classes and components were identified as n-alkanes (particularly C10 to C14), BTEX (benzene, toluene, ethylbenzene, and xylenes) and less volatile aromatics with side chains up to five carbon atoms as well as naphthalene and alkylnaphthalenes. For the thermodesorption experiments in the laboratory and in the pilot-plant scales described below, the components summarized in Table 2a to c were analyzed. Due to storage of the soil before the experiments, a loss of highly volatile compounds is hardly avoidable, and some inhomogeneity of the samples has to be taken into account when comparing the results from different scales. Therefore, in Tables 2a and b the values for the HC contents before and after RF treatment (up to final temperatures of 160 and 110 °C, respectively) are compared for the same soil samples. Additionally, HC concentrations were determined in the thermodesorption effluent, separately for the liquid condensate fraction and the effluent gas. Thermodesorption Experiments Using RF Heating at the Laboratory Scale. The contaminated samples were heated in a thermally insulated, cylindrical soil reactor with a total volume of 3.2 L (see Supporting Information, Figure S1). Two RF electrodes are placed in a coaxial arrangement. The inner electrode, acting as the so-called “hot” electrode, had an outer diameter of 52 mm, whereas the reactor wall acting as the “cold”, i.e. grounded, electrode had an inner diameter of 148 mm. The sample was filled into the space between these electrodes, thus forming a lossy dielectric of the cylindrical capacitor. The electrodes were connected with an RF generator (PFG 1000RF with a maximum power of 1 kW) via an electronic network (matchbox PFM 3000A, both from Hu ¨ttinger Elektronik, Freiburg/Brsg., Germany) allowing optimal energy transfer into the sample. For the experiments, a constant RF power of 150 W was applied. Temperature was either discontinuously (after switching off RF power) or continuously (during heating) measured by thermocouples or fiber-optical sensors, respectively. Using vertical quartz tubes in the center of the reactor, the temperature sensors could be placed in various depths of the soil bed (see SI,
Figure S1). The error of the temperature measurement was below 0.5 K (smaller than the size of the symbols in the diagrams). During heating, the reactor was continually purged with nitrogen (100 mL min-1) to remove HCs and water. Downstream of the reactor an additional N2 flow (at least 500 mL min-1) was added to the purge flow in order to avoid HC condensation. The concentration of water in the gas phase was measured by a humidity sensor (Hygromer A2, Rotronic). The content of HCs was determined after temporal adsorption on two different types of carbon black (Carbotrap and Carbotrap C from Supelco) and thermodesorption (TDS 2 Gerstel) by GCFID analysis. These data were quantified after optimizing the procedure on the basis of calibration measurements. Thermally Enhanced SVE at the Pilot-Plant Scale. For SVE supported by RF heating, a larger soil sample from the same industrial site in Espenhain was used. The soil was taken from a depth between 60 and 150 cm below ground. A standardized analysis (DIN H18) based on infrared spectroscopy provided a mean HC content of 1300 mg kg-1. The spectrum of contaminants was comparable to that stated already above, with benzene (59 mg kg-1), toluene (82 mg kg-1), xylenes (100 mg kg-1 m), n-decane (78 mg kg-1), n-dodecane (115 mg kg-1), naphthalene (50 mg kg-1), and n-tetradecane (130 mg kg-1) as key compounds according to GC analysis. The soil reactor for pilot-scale treatment had a volume of 1.4 m3. The two opposite side walls represented two “cold” electrodes connected to the ground. In the middle, a set of four “hot” rod electrodes was installed, thus forming a capacitor-like geometry. The soil vapor was extracted from the surface of the soil bed. The system was equipped with an RF generator PFG 5000RF (maximum power 5 kW) and a matchbox PFM 6000A (both from Hu ¨ ttinger; experimental setup and positions of the temperature sensors presented in SI, Figures S2 and S3). During heating over 13 days, the mean RF power was 2.5 kW leading to a final temperature of about 230 °C in the middle and about 100 °C near the reactor walls. During soil cleaning, the effluent (pump rate approximately 10 m3 h-1) passed a cooler where a liquid condensate (water and HCs) was collected. The composition of this condensate was analyzed (every 2 h, after extraction with hexane). Finally, soil samples were taken in order to quantify the residual HC concentrations in the same manner as described before. Temperature profiles for various treatment times in the mean horizontal layer are given in SI, Figure S4.
Results and Discussion Laboratory Tests on RF Heating of Artificially Contaminated Sand. During heating with an RF power of 150 W, the temperature was monitored at five different depths within the cylindrical soil reactor as shown in Figure 1a. Differences in the heating rates may be due to some inhomogeneity of the moisture content but also of the heat flow into the surrounding. A significant desorption of water was observed after the soil temperature reached 100 °C (after about 150 min; see Figure 1a). The temperature was stabilized at this value for a long time (in this case about 300 min) during evaporation of water. After evaporation of most of the water, the temperature started to increase again. Before the boiling point of water was reached, only the volatile aromatic compounds toluene, chlorobenzene, and ethylbenzene (to a much smaller extent also trimethylbenzene) could be detected in the gas phase (Figure 1b). Because of their higher water solubility, their permeation through still-existing water films is least retarded. Almost no desorption was observed for the n-alkanes (>C10) below 100 °C, as shown in the bottom part of Figure 1b. VOL. 44, NO. 24, 2010 / ENVIRONMENTAL SCIENCE & TECHNOLOGY
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TABLE 2. Initial and Residual Concentrations of the Main Contaminants in an Original Polluted Soil Studied in the Laboratory Thermodesorption Experiments with (a) Dry and (b) Humid Purge Gas and (c) Cleaning Efficiency and Boiling Points of PAH in an Original Polluted Soil Studied in the Pilot-Plant Scale Thermodesorption Experiments with RF Heating compound
concentration before heating (mg kg-1)
benzene toluene ethylbenzene n-decane (n-C10) n-dodecane (n-C12) naphthalene n-tetradecane (n-C14)
0.1 1.1 0.9 12.1 26.7 8.2 32.5
benzene toluene ethylbenzene n-decane (n-C10) n-dodecane (n-C12) naphthalene n-tetradecane (n-C14)
0.1 0.5
