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Remediation of soil contaminated with lubricating oil by extraction


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JIEC-1481; No. of Pages 6
Journal of Industrial and Engineering Chemistry xxx (2013) xxx–xxx

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Journal of Industrial and Engineering Chemistry
journal homepage: www.elsevier.com/locate/jiec

Remediation of soil contaminated with lubricating oil by extraction using subcritical water
Mohammad Nazrul Islam a, Young-Tae Jo a, Jeong-Hun Park a,b,*
a b

Department of Environmental Engineering, Chonnam National University, Gwangju 500-757, Republic of Korea Soil Technology Research Institute, Chonnam National University, Gwangju 500-757, Republic of Korea

A R T I C L E I N F O

A B S T R A C T

Article history: Received 5 March 2013 Received in revised form 26 June 2013 Accepted 27 July 2013 Available online xxx Keywords: Soil remediation Lubricating oil Subcritical water Temperature Removal ef?ciency

The remediation of lubricating oil contaminated soil was investigated by extraction using subcritical water. The effects of temperature and time on extraction ef?ciency were studied by performing eight individual extractions and varying the subcritical water temperature (200, 225, 250, and 275 8C) and extraction time (90, 120, 180, and 240 min) in a dynamic mode. Also, a comparison was carried out of the feasibility of two operational modes, namely, dynamic and static-dynamic mode. Of the 25,088 mg/kg of lubricating oil as the total petroleum hydrocarbon (TPH) concentration in untreated soil, the residual concentration was found to be $500 and 235 mg/kg for after 120 min extraction in a lab-scale apparatus and 150 min extraction in a 30-fold scale-up experiment, respectively, at 275 8C in static-dynamic mode. The result of this study showed the signi?cant effect of the static-dynamic mode on extraction ef?ciency. The time and volume of water needed for the static-dynamic mode were much lower than those needed for the dynamic mode. These results are of practical interest in developing the subcritical water extraction technology for extraction of lubricating oil and, in a broad sense, petroleum hydrocarbons contaminated soil. ? 2013 The Korean Society of Industrial and Engineering Chemistry. Published by Elsevier B.V. All rights reserved.

1. Introduction Subcritical water extraction (SCWE) has become a green extraction method for the remediation of different classes of compounds contaminated soil and is a technique based on the use of superheated water (100 8C T 374 8C and pressure < 22.1 MPa) as a solvent instead of organic chemicals. The unique characteristics of subcritical water have been described in the literature [1–3]. Lagadec et al. [4] were among the ?rst to fully exploit the feasibility of SCWE technology to remediate the organic contaminated sites. Their pilot scale study of hot water extraction of polycyclic aromatic hydrocarbons (PAHs) and pesticides from soils was unique in one respect; hot water extraction was compared to bioremediation and supercritical CO2 extraction. Removal rates during one year of bioremediation were much lower even for low molecular weight PAHs than those obtained after 60 min of hot water extraction. The PAHs and pesticide contaminated soils used in their study could not support plant growth prior

* Corresponding author at: Department of Environmental Engineering, Chonnam National University, Gwangju 500-757, Republic of Korea. Tel.: +82 62 530 1855; fax: +82 62 530 1859. E-mail addresses: mnazrul09@yahoo.com (M.N. Islam), tlmanager@hanmail.net (Y.-T. Jo), parkjeo1@jnu.ac.kr (J.-H. Park).

to treatment with hot water extraction, but both soils were fertile without additional treatment after extraction. In addition, a few research groups have reported that SCWE technology can be used to treat organic contaminants such as PAHs [3,5], polychlorinated biphenyls (PCBs) [6,7], pesticides [8], and explosives [9] and have shown the feasibility of extracting contaminants from contaminated soils. These water based solvents have signi?cant potential for bene?cial use in various pollutant removal applications. Contamination of soils by petroleum hydrocarbons (PHCs) poses a major environmental problem, especially to the soil environment, and has caused critical health defects; increasing attention has therefore been paid for developing innovative technology to clean up this contamination. A recent work by Tang et al. [10] evaluated the eco-toxicity of PHCs contaminated soil and reported that the contaminated soil greatly inhibited seed germination when the concentration of petroleum was higher than 0.1%, and that 0.5% should be considered the critical value for living microorganisms. Similar concluding remarks have been reported by other researchers [11,12]. Soil polluted with PHCs is often observed as a result of leaking fuel storage tanks, crude oil spills, and the disposal of re?nery waste and railroad cars. Railroad soil or the soil near railroad junctions usually becomes contaminated with lubricant because the railroad industry uses diesel oil for fuel, lubricating oil for machinery, and waste-lubricating oil on the railroad. Such sites

1226-086X/$ – see front matter ? 2013 The Korean Society of Industrial and Engineering Chemistry. Published by Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.jiec.2013.07.040

Please cite this article in press as: M.N. Islam, et al., J. Ind. Eng. Chem. (2013), http://dx.doi.org/10.1016/j.jiec.2013.07.040

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2 M.N. Islam et al. / Journal of Industrial and Engineering Chemistry xxx (2013) xxx–xxx Table 1 Physicochemical properties of used silt loam soil. Properties pH Total organic matter (%) Particle distribution (%) Sand Silt Clay Initial concentrationa Lubricating oil (mg/kg) as TPH
a

often contain organic contaminants including benzene, toluene, ethylbenzene, and PAHs originating from PHCs [13]; they thus require remediation via a clean-up to protect human health and the ecosystem. Numerous researches have been conducted on the remediation of PHCs contaminated soil using a bioremediation process [14–18]. However, bioremediation is not suitable in treating lubricating oil contaminated sites because the oil is highly viscous and does not biodegrade easily [13]. In addition, the total petroleum hydrocarbon (TPH) removal is frequently poor because biodegradation in soil can be limited by many factors, such as microorganism types, nutrients, pH, temperature, moisture, oxygen, soil properties, and contaminant presence [19]. While supercritical ethane, supercritical argon, organic solvent, and surfactant remediation processes have proven to be an excellent technique for both extraction and chromatography [20–24], the supercritical water extraction process requires a high temperature of >374 8C and pressure of !22.1 MPa and is corrosive, while other processes require large volumes of organic solvents that are not only ?ammable but often toxic or carcinogenic, expensive, and environmentally unfriendly. Electrokinetic (EK) soil remediation has attracted interest among researchers over the last decade. Since 1996, when the ?rst EK remediation was introduced in Korea for soil contaminated with heavy metals, many researchers have investigated different EK system con?gurations to achieve enhanced performance [25]. In recent, modi?ed EK system, namely EK-Fenton remediation has been reported as a promising technique to remediate diesel contaminated soil [26,27]. Pazos et al. [26] reported that the TPH removal ef?ciency of 90% was obtainable by surfactant EK-Fenton process after 15 days of treatment. However, this technique is not suitable for remediation of high range hydrocarbons (heavy oil) as well as requires a long treatment time. For example, Park et al. [13] documented the feasibility of EK technology on the remediation of railroad soil contaminated by lubricating oil and zinc; and the removal ef?ciency of lubricating oil was only $50% after 17 days of operation. As a result, a suitable remediation method is needed to treat PHCs contaminated soils. A study by Kim and Kweon [28] showed that oily lubricating materials were effectively removed from the parts of a lubricating machine by SCWE at a relatively low operating temperature. But, so far, the remediation of used lubricating oil contaminated soil by extraction using subcritical water has not been studied. Therefore, the main objective of this research was to exploit the possibility of the SCWE of lubricating oil as an excellent remediation technique to treat soils contaminated with PHCs. A secondary aim of this study was to optimize the extracting process using subcritical water for the pilot scale application. 2. Experimental 2.1. Soil characterization The main properties assessed in the soil were the pH, the organic matter content and the particle size distribution. The nonpolluted soil was collected from Hwasun in South Korea and a soil sample was air-dried, homogenized, and sieved using a 10-mesh sieve. The physicochemical properties of the soil are summarized in Table 1. 2.2. Soil contamination procedure A <2 mm fraction of 5 kg of non-polluted soil sample was contaminated by mixing the soil with used lubricating oil (density = 0.8364 g/cm3) that was collected from a car repairing centre nearby our university campus. Lubricating oil was selected

Values 6.62 9.23 38 59 3 25,088

Initial concentration was based on the analysis of three 10-g subsamples of a homogenized soil sample.

since it is known as the most dif?cult contaminant to extract from soil due to its high boiling point fractions (>350 8C). Used lubricating oil was previously dissolved in dichloromethane before it was spiked into the soil. 135 g of used lubricating oil, dissolved in 3 L dichloromethane, was added to 5 kg soil sample. The soil– dichloromethane mixture was shaken overnight and subsequently the dichloromethane was evaporated by air-drying the soil for about 6 h. The contaminated soil was kept in a closed aluminium container at room temperature for 3 week to allow the dispersion and sorption of the contaminant in the soil matrix. The average initial concentration of lubricating oil after contamination was 25,088 mg/kg, in terms of TPH. It should be noted that the hydrocarbon components of lubricating oil are classi?ed within the chromotographable hydrocarbon range of C24–C36. 2.3. SCWE equipment and extraction process One semi-pilot and one lab-scale SCWE apparatus were used in this work to carry out the extractions of the lubricant contaminated soil, in which a lab-scale apparatus previously used for PAHs [5] is described in Fig. 1. In the case of the lab-scale apparatus, the stainless steel tube (1 mm o.d., 0.6 mm i.d.), which is part of the equipment, was used as a heating coil and a water ?ow line through the pre-heater, main-heater, and condenser to the outer chamber. A high pressure pump (Series II, Chrom Tech, Inc.) was used to deliver water to the pre-heater, followed by a 9.5 mL extraction cell (17.0 mm o.d., 10.0 mm i.d., and 108.7 mm long; made of stainless steel) and the cell was capped at the inlet and outlet with a 2 mm stainless steel micro?lter. In the case of scale-up extraction, 254 mL of the extraction cell was constructed from stainless steel (30.0 mm o.d., 70.9 mm i.d., and 359.0 mm long) with end cap (20 mm stainless steel micro?lter). An Annovi Reverberi pump (RC-M.01.10, made in PR China) was used in the dynamic ?ow mode to pump water through the preheating coil of 2.0 mm i.d. (4.1 mm o.d.) stainless steel tubing and extraction cell. For both extractions, the soil sample was weighed and inserted into the extraction cell which was placed into the main heating chamber. The extraction cell was then closed tightly and connected to the water and contaminant mass ?ow tube. Thereafter, the water was pumped at a desired ?ow rate for a desired period through the pre-heating coil and extraction cell. The temperature in the preheating coil and extraction cell was observed by a thermocouple connected to a West 6100+ temperature controller (Ise, Inc., USA). Superheated water has a decreased dielectric constant, surface tension, and viscosity, thus making it an ef?cient solvent for extracting soluble poor or non-soluble organic contaminants while passing through the extraction cell. A system pressure regulator valve was used to control and allow the pressure to build up. The extraction cell was heated to the desired temperature. After the desired extraction, the pump and heater were stopped and the pressure was released to atmospheric pressure. The reactor was left to cool to room

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Fig. 1. Subcritical water extraction apparatus. PR: pressure relief valve; PI: pressure indicator; T: thermocouple.

temperature and opened to collect the soil to determine the residual concentrations of lubricating oil. One duplicate extraction procedure was performed for each of the experimental condition conducted in this study. 2.3.1. Dynamic SCWE procedure A 12 g soil sample was weighed and placed into the extraction cell and then into the extraction chamber in all experiments. Note that the lab-scale apparatus was used for this procedure. After assembling the unit of extraction system, the unit was ?lled with water pumped at a ?ow rate of 1 mL/min by closing the outlet valve (in this way the system was pressurized to 6 MPa) and turned on the heating valve. When the temperature inside the cell was 100 8C, the outlet valve was then opened and 0.3 mL/ min of ?ow-rate was enabled until the inside temperature of the cell raised to the desired level (e.g. 275 8C). After the cell inside temperature reached the set temperature, the counting of extraction time began and dynamic extraction was then performed by pumping the water at 1 mL/min during the desired extraction time. 2.3.2. Static-dynamic SCWE procedure Both a lab-scale and semi-pilot apparatus were used for staticdynamic mode extraction. This extraction mode involved the following steps: (1) the same sample amount (12 g) as the dynamic mode extraction (360 g for 30-fold scale-up extraction) was placed in the extraction cell; (2) the unit was assembled and ?lled with water by closing the outlet valve (in this way the system was pressurized to 6 MPa) and temperature switched on; (3) when the cell inside temperature was reached 100 8C, the outlet valve was then opened and 0.3 mL/min of water ?ow (9 mL/min for 30-fold extraction) was allowed until the inside temperature of the cell raised to the desired value; (4) after the inside temperature of the cell reached 275 8C and the system pressure reached 6 MPa, the static extraction step was developed for 15 min by closing the inlet and outlet valves; (5) after this time had elapsed, both the inlet and outlet valves were opened, and the high pressure pump was switched on, driving the water through the system at a 1 mL/min (30 mL/min in the case of 30-fold scale-up extraction to maintain

the same residence time of water within the extraction cell) ?owrate for 15 min (dynamic extraction step); (6) the static and dynamic steps were developed consecutively for the desired extraction period. 2.4. Analytical procedure The method of determining the amount of lubricating oil in the corresponding soil samples was based on the Korean Standard Test Method: 10 g of soil was mixed with anhydrous sodium sulfate and was then ultrasonically extracted twice with 200 mL of dichloromethane in 100 mL dichloromethane for 3 min each time. The extract was ?ltered and extractant was concentrated using a rotary evaporator and puri?ed with silica gel. The amount of TPH in the ?nal solution was analyzed using gas chromatography ?tted with a ?ame ionization detector (HP-6890, Agilent Tech., USA). 3. Results and discussion Remediation of soil contaminated with PHCs at an industrial scale remains a challenge. Thus, as a preliminary step in evaluating subcritical water as a useful extraction solvent for PHCs and to optimize the extraction process, spiking experiments were conducted with silt loam soil at a laboratory scale. The effects of ?ow rate and pressure in SCWE were not studied in detail. In many studies, the ?ow rate of 1 mL/min for lab-scale extraction (using 8– 12 g of contaminated soil) has been found to be appropriate [4,29]. The purpose of maintaining particularly high pressure in SCWE was to maintain water in a liquid state when the temperature was increased. Chang et al. [30] found that pressure had only a minor effect on the removal ef?ciency at 200 8C and similar ?ndings were reported by Hawthorne et al. [7]. However, previous works from the literature and results from our laboratory concur about the best conditions of temperature and extraction time to reach high extraction ef?ciencies of organic pollutants [5,6,31]. The effects of temperature and time on the removal of lubricating oil were therefore investigated using 1 mL/min of water ?ow rate at a constant pressure of 6 MPa.

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Fig. 2. (a) Effect of temperature on the extraction of lubricating oil from contaminated soil in which the constant extraction time was maintained for 90 min and (b) the effect of time on extraction in which the constant subcritical water temperature was 275 8C. All experiments were conducted in dynamic mode using 12 g of soil at a constant 1 mL/ min ?ow of water and 6 MPa of pressure.

3.1. Effect of temperature and time on lubricating oil removal The operating conditions to be optimized were selected as extraction temperature in the range of 200–275 8C and four levels of time (90, 120, 180 and 240 min) in dynamic mode. Although it is believed that subcritical water above a temperature of !300 8C may facilitate the extraction ef?ciency of lubricating oil, this makes it dif?cult to use at a large scale with high pressure. Fig. 2a shows the lubricating oil removal from the soil according to the water temperature, in which each particular extraction experiment was conducted for 90 min. A positive relationship was shown between temperature and lubricating oil extractability (R2 = 0.97). It is clearly observed that the reduction of TPH concentration was $3.5-fold when the increase in temperature was from 200 to 275 8C (the concentration reduction in percent is equivalent to the extraction or removal ef?ciency). Temperature is an important parameter in SCWE since the decreased dielectric constant and surface tension of subcritical water allows for signi?cant dissolution of nonpolar organic compounds in water at elevated temperature [2], thus, solubility of PHCs is greatly enhanced with increasing temperature. When water at a temperature of 25 8C (the water did not maintain any additional pressure) passed through the extraction cell, <1% of the lubricating oil was removed from the soil (data not shown) after 90 min for a ?ow rate of 1 mL/min. The removal ef?ciency of lubricating oil at a maximum of 275 8C was determined since the water heating unit in the scale-up system was capable of heating to 275 8C. After 90 min extraction using a ?ow rate of 1 mL/min, the TPH concentration of same polluted soil reduced from 25,088 to 12,042 mg/kg, which is only 52% of removal ef?ciency observed. The residual concentration value is so far to meets the current environmental legislation value of the Korean government (2000 mg/kg). An optimum temperature of 275 8C was selected for subsequent extraction with an increase in the extraction time. It is expected that a longer extraction time increases the removal ef?ciency of lubricating oil in soil. According to Richter et al. [32], the dynamic time on the extraction ef?ciency was somewhat important in terms of kinetics and soil/water equilibrium. As shown in Fig. 2a, the maximum removal of lubricating oil was observed at 275 8C since it was not at a satisfactory level even after 90 min extraction. Therefore, additional experiments were performed with the three levels of extraction time of 120, 180, and 240 min (the counting of extraction time started after the reactor temperature reached the set temperature of 275 8C) and other parameters were kept constant (water ?ow rate 1 mL/min and pressure 6 MPa). The results showed that there was an increasing trend in the yield of lubricating oil extraction with the increase of time in the range of 90–180 min (Fig. 2b), but there was no signi?cant increase while the extraction time was extended to

240 min, revealing the high persistence of lubricant in soil. Also, this observation suggests that a considerable amount of time is required for the release of lubricant by the SCWE system. When the extraction temperature was 275 8C and the ?ow rate was 1 mL/ min, the removal of lubricating oil as a TPH was 62% after 180 min. The present results show that a signi?cant part (62%) of the lubricating oil is removed by the SCWE mechanism after 180 min extraction at 275 8C. Clearly, this amount is not suf?cient to fully ‘remediate’ heavily contaminated soil. Also, it is believed that the lubricating oil from aged soils would be more dif?cult, due to the stronger interaction with the soil matrix. 3.2. Kinetic study of lubricating oil removal In order to determine the extraction pattern of lubricating oil and a more quantitative account of removal ef?ciency with increasing time, a kinetic study was carried out in which the experimental data were ?tted with the ?rst-order kinetic desorption model. The applied model was described brie?y in our previous study for SCWE of PAHs [33] and elsewhere [23,34]. The plot of the measured TPH removal concentration according to time using the kinetic model illustrated the lubricating oil desorption from soil. The model used in this study was expressed by the following equation: Ct ? 1 ? e??k;t? C0 where Ct (mg/kg) is the amount of lubricating oil removed at instant t, C0 (mg/kg) is the concentration of TPH in spiked soil (untreated), k (min?1) is the desorption rate constant, and t (min) is the time of extraction. Microsoft excel solver was used to ?t the experimental data with kinetic model to obtain the rate constant k value. The value of k is presented in Table 2. The ?rst order model ?tted the experimental results based on the square regression coef?cient, R2; the SCWE of lubricating oil in the case of soil remediation can thus be described by a ?rst-order kinetic model. As shown in Fig. 3, the predicted value of TPH removal of over 90% could be achieved after 9 h extraction. Indeed, although the extraction conditions became harsher with an increasing dynamic

Table 2 First-order desorption kinetic coef?cient. Parameter k (min R2
?1

Value 0.0045 0.96

)

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Fig. 3. Experimental ?tting of ?rst-order kinetic desorption model and predicted removal percentage of lubricating oil.

time of up to 9 h, there was an increase in the amount of compound extracted in terms of the requirement for a large volume of water and energy. Note that, according to this predicted removal requires a large volume of subcritical water, after 9 h extraction, $570 mL of water is needed for a 12 g sample. 3.3. Effect of static-dynamic mode on TPH removal Two operational modes were used to carry out extraction with subcritical water: the dynamic mode and the static-dynamic mode. In the dynamic mode, the subcritical water ?ows continually through the extraction cell. In the static-dynamic mode, a ?xed volume of water was kept within the extraction cell for 15 min by closing the outlet valve (static mode); after this time had elapsed, the outlet valve was opened and a high pressure pump was switched on, driving the subcritical water through the system for 15 min (dynamic mode) under a working temperature of 275 8C and pressure of 6 MPa. The extraction mechanism of contaminants with static-dynamic cycling in SCWE could be described by a combination of equilibrium and separation steps. During the static soak time, the system was allowed ample time to reach equilibrium, and was then released during the dynamic period as fresh solvent (water) was periodically pumped through the extraction cell. In the static-dynamic mode extraction, a single static extraction for 15 min followed by a 15 min dynamic extraction were combined as one cycle and there were four cycles in each run, resulting in a total extraction time for the static-dynamic mode of 120 min. Table 3 shows the residual concentration of TPH at different extraction conditions for the dynamic mode and

Table 3 Results of SCWE at different conditions. Runa Extraction mode Extraction time (min)/no. of cycle Average residual conc. in soil (mg/kg) 12,042 503b Removal ef?ciency (%) 52 98

Lab-scale extraction (12 g of soil) 1 Dynamic 120/– 2 Static-dynamic 120/4 30-Fold extraction (360 g of soil) 3 Static-dynamic 120/4 4 Static-dynamic 150/5

7661 235b

69 99

a All experiments were conducted at 275 8C (6 MPa). Duplicate extraction was carried out for each of the experimental condition with an experimental error less than 5%. b The residual concentration was much below the pollution limit (2000 mg/kg).

static-dynamic mode. Examining the TPH in soil after 120 min of static-dynamic extraction that met the regulation level of the Korean government; however, the residual concentration was much higher after the same period of dynamic mode extraction. The results in Table 3 shows a signi?cant positive impact of the static-dynamic mode on the extraction of lubricating oil from soil, in which the removal was approximately twice that of dynamic extraction under the same condition, indicating that the extraction ef?ciency depends mostly on contact time rather than water volume. This result concurs with that from various other previous studies as outlined below. Comparative studies have been conducted of the static, dynamic, and static-dynamic modes for the analytical purpose of determining PAHs in solid samples with pressurized hot water extraction [35,36]. Luque-Garcia and Luque de Castro [35] analyzed the nitrated PAHs in both spiked and natural contaminated soils, showing that the static-dynamic mode is the most suitable alternative in terms of quantity extracted (based on quantitative analysis) of PAHs and total extraction time shorten. They reported that the static-dynamic mode only required 25 min for total extraction of the analytes, while the static and the dynamic mode required 30 min and 50 min, respectively. According to Ghoreishi and Shahrestani [37], the residence time (the average time needed for the water to remain in an extraction cell) could be a function of extraction yield in a subcritical water extraction system and a lower extraction can be predicted to decrease the residence time due to continuous and/or higher ?ow rates. Aksu et al. [38] observed that for biosorption of reactive dyes on Rhizopus arrhizus in a continuous ?xed bed reactor, if the residence time of the solute in the column is too short, the total amount of sorbed dye is decreased. In this study, the increase in removal ef?ciency at staticdynamic mode with ample residence time indicates that the removal ef?ciency is controlled by intraparticle diffusion and/or mass transfer. The removal ef?ciency was over 98% after 120 min static-dynamic extraction, indicating that the intraparticle diffusion and mass transfer is well balanced with external mass transfer (advection) rate when employing a periodical ?ow condition. In our study, a signi?cant difference was observed between the dynamic and static-dynamic extraction on TPH removal. In this regard, one possible explanation for the effect of dynamic water ?ow is the formation of a preferential pathway (channelling) in the cell, and parts of the lubricant in the soil do not come in contact with advection ?ow. In other words, different mass transfer zones (advection or diffusion dominant zone) can be formed. Desorbed contaminants can easily be removed in an advection dominant zone where channelling ?ow occurs. However, desorbed contaminants in a diffusion dominant zone cannot be easily removed, as shown by the low removal ef?ciency in the dynamic mode. On the other hand, in the staticdynamic extraction, during the static period, the pressurized stagnant liquid penetrated and dispersed through the entire area, causing diffusion and advection transport from all parts of the soil, resulting in increasing the mass transfer from the internal to the external bulk ?uid. This annotation is consistent with similar observations by Pavlostathis and Jaglal [39] who reported that the slow desorption and/or diffusion rates compared to the advective rate under continuous pumping may account for the slow release of trichloroethane (TCE) from a contaminated soil in a column reactor. In this study, a 50% reduction in the volume of ef?uent (extract) was obtained by static-dynamic extraction compared to dynamic mode extraction. The total removal of lubricating oil was achieved in less time than that required by the dynamic mode, thus a small volume of wastewater was produced.

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3.4. Semi-pilot scale study Based on lab-scale results, a 30-fold extraction of the same contaminated soil (360 g) was performed in static-dynamic mode for 120 min (four cycles) at 275 8C (6 MPa). The ratio of the solvent (water) ?ow rate and soil mass was maintained constant between small and large scales, which represents constant residence time of the solvent inside the extractor [40]. This scale-up criterion was considered since the lubricant extractability was controlled by diffusion mechanisms as mentioned before and this was suggested by [40]. Similar to the lab-scale experiment, the ?lled extraction cell (soil and water) was preheated for $30 min to 100 8C before beginning the water ?ow and extraction was carried out following the procedure as described earlier. As shown in Table 3, the TPH concentration was reduced from the initial concentration of 25,088 to 7661 mg/kg after 120 min extraction at 275 8C and the duplicate 30-fold extraction was also showed similar removal behaviour, as a result, the removal ef?ciency was only 69% whereas the removal ef?ciency was obtained 98% for lab-scale studies at the same operating condition. Therefore, it could be noted that the applied scale-up criterion is not adequate for scale-up of SCWE of lubricating oil probably because the inadequate mass transfer. Note that, a good removal ($99%) was achieved at 275 8C, whereby the number of extraction cycles increased to ?ve (150 min, Table 3). Mass balance (i.e., the sum of lubricating oil as TPH concentration from the extractant water plus any lubricating oil residue in the treated soil) was also determined and $90% was obtained. After treatment, the TPH concentration in the soil sample was below the pollution limit. Speci?cally, the results ful?l the Korean regulations published by the Ministry of Environment regarding the use of soil contaminated with petroleum hydrocarbons (<2000 mg/kg). 4. Conclusion The extraction of lubricating oil was studied rather than the extraction of diesel or other PHCs from spiked soil since lubricating oil is extremely dif?cult to remove due to its strong interaction with the matrix. In this work it was found that subcritical water, as a solvent, is highly feasible in the quantitative remediation of soil contaminated with lubricating oil. The results of the effect of extraction in?uencing parameters showed that the lubricant extraction from the soil is strongly dependent on temperature and time. A ?rst-order desorption kinetic model described well the extraction behaviour of lubricating oil at higher extraction times. A comparison of the two SCWE modes (dynamic and static-dynamic) was performed, showing an extraction ef?ciency of TPH almost 100% in the static-dynamic mode, compared to only 52% under the same condition in the dynamic mode extraction. Therefore, we can remark that >99% of lubricating oil could be extracted from soil by following the process developed in this study, using subcritical water.

Acknowledgement This work was supported by a Grant (No. 173-101-033) from the Korea Environmental Industry and Technology Institute (KEITI) through the GAIA project. References
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Please cite this article in press as: M.N. Islam, et al., J. Ind. Eng. Chem. (2013), http://dx.doi.org/10.1016/j.jiec.2013.07.040


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