Journal of Industrial and Engineering Chemistry 18 (2012) 1689–1693

Remediation of PAHs contaminated soil by extraction using subcritical water

Mohammad Nazrul Islam, Young-Tae Jo, Jeong-Hun Park *

Department of Environmental Engineering, Chonnam National University, Buk-ku, Gwang-ju 500-757, Republic of Korea

A R T I C L E I N F O

Article history:

Received 20 January 2012

Accepted 7 March 2012

Available online 15 March 2012

Keywords:

Subcritical water

PAHs

Extraction

Temperature

A B S T R A C T

The remediation of polycyclic aromatic hydrocarbons (PAHs) contaminated soil has been investigated by

extraction using continuous flowing subcritical water. Water temperature ranging from 100 to 300 8C,

extraction time ranging from 15 to 60 min, and flow rate ranging from 0.5 to 2.0 mL/min were

investigated to determine their effect on the removal efficiencies of target PAHs. More than 95%

extraction of the phenanthrene, fluoranthene, and pyrene from contaminated soil was observed at 300 8Cfor 30 min and 250 8C for 60 min at a constant pressure of 100 bar. However, naphthalene was almost

completely extracted only at a comparatively low temperature of 150 8C and extraction time of up to

30 min and a pressure of 100 bar. The subcritical water flow rate of 0.5 mL/min was recommendable in

this study. The extraction efficiency of PAHs was extremely dependent on water temperature, since the

dielectric constant (polarity) of water could be dramatically lowered by raising the water temperature.

These results suggest that soils contaminated by persistent organic chemicals such as PAHs can be easily

remediated by extraction using pure water under a high temperature without any modification.

� 2012 The Korean Society of Industrial and Engineering Chemistry. Published by Elsevier B.V. All rights

reserved.

Contents lists available at SciVerse ScienceDirect

Journal of Industrial and Engineering Chemistry

jou r n al h o mep ag e: w ww .e lsev ier . co m / loc ate / j iec

1. Introduction

The presence of polycyclic aromatic hydrocarbons (PAHs) asenvironmental contaminants has created concern about passaround and transport in natural water, sediments and soils. PAHsare carcinogenic micro-pollutants which are resistant to environ-mental degradation due to their highly hydrophobic nature [1].They are also problematic chemicals due to their toxicity andbecause they remain in the environment for a long period of time.Due to their persistence in the environment through contamina-tion of water, sediments or soil, remediating these compounds isimportant.

A number of researchers have studied the remediation of PAHscontaminated soil using the bioremediation process [2–4] andphytoremediation process [5–7]. Unfortunately, the removal ofPAHs (especially higher molecular weight) is frequently poor, eventhough the bioremediation process is very cost effective. Cofieldet al. [8] reported that phytoremediation with tall fescue (Festuca

arundinacea) are capable of PAHs remediation with only a 40%removal efficiency. A similar result was found by Lee et al. [9] withthe native Korean grass species Panicum bisulcatum after 80 days oftreatment. Moreover, these processes require a long treatmenttime.

* Corresponding author. 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).

1226-086X/$ – see front matter � 2012 The Korean Society of Industrial and Engineer

doi:10.1016/j.jiec.2012.03.013

The use of supercritical fluid extraction (SFE) has becomeincreasingly apparent when cleaning organic contaminated soilssuch as polychlorinated biphenyls (PCBs) [10,11], PAHs [12,13],and pesticides [14–16]. Unfortunately, supercritical water requiresa temperature of 374 8C and a pressure of >221 bar and is corrosive[17].

Subcritical water extraction (SCWE) is one of the most recenttechniques developed based on the use of superheated water(temperature 100–374 8C and pressure <221 bar) as a solventinstead of organic solvent. Subcritical water has unique character-istics; more specifically, high temperature and pressure stronglyreduces its dielectric constant, surface tension, and viscosity andtherefore the hydrogen bonding network of water molecules isweakened [18]. The dielectric constant of water decreases from 73to 2 by increasing the temperature from 25 8C to 350 8C at apressure of 100 bar. Therefore, the solubilities of nonpolarcompounds increase as the temperature increases in this range.For example, the dielectric constant (e) of superheated water is 27at a temperature of 250 8C and pressure of 100 bar. These valuesare between those of organic solvent ethanol (e = 24 at 25 8C) andmethanol (e = 33 at 25 8C) (Table 1). This indicates that superheat-ed water acts as an organic solvent [19,20] and therefore SCWEcould be categorized as a solvent extraction process [13].Moreover, superheated water is readily available, non-toxic,reusable and very low in cost as well as environmentally friendly.Therefore, SCWE has been suggested as an alternative cleaningtechnology, instead of using organic solvents or toxic and strongaqueous liquid media [21–23]. SCWE has been reported for the

ing Chemistry. Published by Elsevier B.V. All rights reserved.

Table 1Dielectric constant (e) of subcritical water and common organic solvent.

e (at subcritical water, 8C) e of common organic solvent at 25 8C

44 (150) 1.9 (n-hexane)

35 (200) 21 (acetone)

27 (250) 24 (ethanol)

20 (300) 33 (methanol)

2 (350) 39 (acetonitrile)

Table 2Soil properties.

Heukeumgi soil

TOC (%) pH Sand (%) Silt (%) Clay (%)

13.40 5.1 3.30 76.80 19.90

M.N. Islam et al. / Journal of Industrial and Engineering Chemistry 18 (2012) 1689–16931690

extraction of dioxins [24], pesticides [17], PCBs [25], and PAHs[17,18,26] from contaminated soil. However, the extraction ofPAHs in contaminated soil for commercial or practical applicationshas not been sufficiently carried out. Lagadec et al. [17] reportedthat the optimum subcritical water extraction is at 275 8C in35 min for all low and high molecular weight PAHs fromcontaminated manufactured gas plant (MGP) soil. However, theoptimum condition for subcritical water extraction was reportedby Hawthorne et al. [27] to be at 250 8C in 60 min for MGP soil.Another researcher suggested that a longer subcritical waterextraction time induces a higher possibility of thermal degradationat over 200 8C for PAHs [28]. Moreover, SCWE has been used todetermine a superior instant analytical technique (using GC ovenas heater) using organic solvent [18,19,26], but not in terms ofindustrial purification. However, as shown in the above discussion,a complete extraction technology with shorter extraction time at arange of temperature (from 100 to 300 8C) using subcritical waterfor industrial application has not been determined; therefore, anadditional study is necessary to obtain a rapid and efficientextraction condition.

In this study, a lab-scale continuous flowing subcritical waterextraction process was carried out for soil PAHs remediationwithout any chemical reagent at different factors (temperature,extraction time and flow rate) to investigate their effect on removalefficiency and to determine the optimization of experimentalvariables with the aim of maximizing the extraction in theminimum time possible.

2. Materials and methods

2.1. Chemicals

All of the chemicals used in this experiment were HPLC grade.n-Hexane was used to prepare spiking solutions and purchasedfrom J.T. Baker Ltd., USA. Naphthalene, phenanthrene, fluoranthene

Fig. 1. Schematic of subcritical water extraction system. (1) Water tank, (2) high pressu

separator, and (7) back press regulator.

and pyrene (purity > 98%) were purchased from Sigma AldrichLtd., Germany. Methanol (purity > 99.9%) was used to preparesamples and standard for HPLC analysis. Water was deionized to18 MV using an aqua MAXTM purification unit (Younglin Instru-ment, Korea) and was used throughout the experiment.

2.2. Subcritical water extractor

An SCW (subcritical water) extractor was modified and used forsubcritical water examination. Fig. 1 is an illustration of theapparatus. It has a 10 mL reactor filled with the sample and iscomposed of a water tank, high pressure pump, pre-heater reactor,thermometer, back pressure regulator, and solid–water separator.

2.3. Contaminated soil

Heukeumgi soil, which was collected from Jeju Island in Korea,has been used in this study. Heukeumgi soil properties are shownin Table 2. Naphthalene, phenanthrene, fluoranthene, and pyrene(the contaminants purity of which was more than 98%) have beenspiked into Heukeumgi soil to prepare a contaminated soil.Contaminant properties are presented in Table 3. 0.3 g of eachcontaminant was inserted into a 200 g n-hexane to make the spikesolution. The spiking solution was added into the soil and mixed byrotating the mixture for 24 h. Subsequently, the contaminant soilwas dried for 12 h and maintained for 14 days.

2.4. Subcritical water extraction process

Distilled water was used for the extraction solvent. Before usingthe distilled water, it was purged with helium gas for 30 min toremove dissolved oxygen because it has been reported that thedissolved oxygen in water induces the oxidation of PAHs insubcritical water [18]. 8 g of highly contaminated Heukeumgi soilwas packed into the reactor and the purged distilled water wasflowed through the pre-heater and reactor with 0.5–2.0 mL/min offlow rate. The pressure was allowed to build from 50 bar to 100 bar

re pump, (3) pre-heater, (4) main heater with a reactor, (5) chiller, (6) solid/water

Table 3Contaminant properties.

PAHs Molecular weight (g) Melting point (8C) Boiling point (8C) Solubility in water at 25 8C (mg/L)a

Naphthalene 128.2 80.3 218 32

Phenanthrene 178.2 99.2 340 1.3

Fluoranthene 202.3 110.8 375 0.2

Pyrene 202.1 145 404 0.1

a [25].

M.N. Islam et al. / Journal of Industrial and Engineering Chemistry 18 (2012) 1689–1693 1691

during the temperature was reached from room temperature todesired temperature and then maintained at a constant pressure of100 bar. The water was passed vertically through the extractionreactor for up-flow extraction to prevent the clogging of the filterand to extract materials lighter than water. At high temperature,water acts as an organic solvent that can make PAHs soluble andmobile. To test the effect of water temperature, flow rate, andextraction time, experiments were performed according to the setof experimental designs presented in Table 4. The counting ofextraction time was started after the reactor temperature reachedthe set temperature. After the desired extraction, the pump andheater were stopped and the pressure was released to atmosphericpressure. The reactor was left to cool to room temperature and wasremoved from the main heater and packed soil was collected toanalyze the remaining concentrations of contaminant.

2.5. Sample preparation and HPLC analysis

Soil was placed into a 20 mL vial and 10 mL of methanol wasadded to determine the remaining contaminant concentrations bymethanol extraction. The vial was mixed at 200 rpm for 24 h in ashaking incubator. After 24 h, the mixture was separated usingcentrifugally at 2000 rpm for 10 min. Supernatant was taken for theanalysis of contaminants using high performance liquid chroma-tography (HPLC).

The quantification of target PAHs of all extracts was analyzedusing HPLC witha mass selective detector of fluorescent and UV254nm.Compound separation was carried out using an LC-PAH column(25 cm � 4.6 mm � 5 mm). Identification of PAHs was made byintegrating the peak areas using Auto-Chro2000 software providedwith the instrument and by a comparison of known standardconcentrations. The HPLC was operated under the followingconditions; sample injection – 30 mL, flowing liquid – acetonitrilesolution (acetonitrile:water – 8:2), and flow rate – 2 mL/min.

3. Results and discussion

Parameters that affect the extraction efficiency of subcriticalwater extraction include the extraction temperature, time, andflow rate used.

Table 4Experimental design for extraction of representative PAHs.

Experiments Water temperature (8C) Flow rate

Temperature effect 100 2

150

200

250

300

Flow rate effect 250 0.5

1

1.5

2

Time effect 250 1.5

3.1. Influence of water temperature on removal efficiencies

Temperature affects the solubility, distribution coefficient andmolecular diffusion coefficient of chemicals, and this may affectthe extraction efficiency of chemicals in soils. In this study, toexamine the influence of temperature on extraction efficiency,temperature was increased in steps of 50 8C from 100 8C to 300 8Cwith increasing a pressure from 50 bar to 100 bar and at a flow rateof 2.0 mL/min (Table 4). The remaining concentrations ofnaphthalene, phenanthrene, fluoranthene, and pyrene of theextracted soil at different temperatures are summarized in Table5. The concentration of target PAHs in soil decreased withincreasing temperature, although water has a small capabilityfor extraction at room temperature (25 8C). As would be expectedon the basis of the high dielectric constant of water, extractions at100 8C were not sufficient to obtain the satisfactory yield for anyPAHs (Table 5). 51.33, 66.68, and 61.58% of removal efficiencieswere obtained at a temperature of 200 8C for phenanthrene,fluoranthene, and pyrene, respectively (Fig. 2). As shown in Fig. 2,the efficiencies of extractions of phenanthrene, fluoranthene, andpyrene increased by up to 30, 17, and 18%, respectively, at 250 8Cfor 30 min compared with the values at 200 8C for 30 min and thenleveled off at a maximum yield of 98.12, 96.24, and 94.05% forphenanthrene, fluoranthene, and pyrene, respectively, as thetemperature increased at 300 8C. However, 99.61% of naphthalenewas extracted only at up to 150 8C for 30 min (Fig. 2). Thiscondition was comparable to (in terms of both temperature andtime) that of the applied pressurized hot water extraction of lowermolecular weight PAHs from contaminated sediment and asufficient condition was reported for the extraction of naphthaleneat 150 8C for 15 min [29]. This demonstrates that naphthalene iseasier to extract than phenanthrene, fluoranthene, and pyrene;this might be due to the comparatively lower molecular weightand higher solubility properties [26]. In addition, this experimentshowed no significant difference in extraction among of phenan-threne, fluoranthene and pyrene from 100 8C to 300 8C. Phenan-threne, fluoranthene and pyrene were removed at a highertemperature; it was possible to reach the higher water solubilitywithin the reactor, i.e., the polarity of water was sufficientlyreduced [26,30]. A higher temperature results in a decrease of the

(mL/min) Extraction time (min) Pressure (bar)

30 100

30 100

15 100

30

45

60

Table 5Concentration (mg kg�1) of contaminants in initial soil and extracted soil.

PAHs Initial soil conc. Water temperaturea (8C) Water flow rateb (mL/min) Extraction timec (min)

100 150 200 250 300 0.5 1.0 1.5 2.0 15 30 45 60

Naphthalene 380.2 186.0 1.5 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0

Phenanthrene 354.9 260.2 212.9 172.7 63.9 6.7 66.2 101.4 44.6 63.9 123.6 44.6 36.1 13.4

Fluoranthene 494.4 266.5 224.4 164.2 85.1 18.6 81.1 113.7 76.0 85.1 164.5 76.0 72.8 21.2

Pyrene 392.5 248.3 208.1 150.8 86.8 23.3 91.4 114.7 88.9 86.8 60.0 88.9 55.2 8.2

a Experiments (temperature factor) were carried out for 30 min at a pressure of 100 bar and flow rate of 2.0 mL/min.b Experiments (flow factor) were carried out for 30 min at a water temperature of 250 8C and pressure of 100 bar.c Experiments (time factor) were carried out at a water temperature of 250 8C, pressure of 100 bar, and flow rate of 1.5 mL/min.

M.N. Islam et al. / Journal of Industrial and Engineering Chemistry 18 (2012) 1689–16931692

surface tension of the liquid solvent [18] and in its cohesive energydensity, thereby reducing its viscosity and increasing the diffusioncoefficient, i.e., increasing the internal mass transfer within thesoils. From the results, it appears that temperature is one of theimportant factors for extraction of naphthalene, phenanthrene,fluoranthene, and pyrene contaminated soil.

3.2. Influence of water flow rate on removal efficiencies

Water flow rate is related to external mass transfer. Anekpankuland Goto [31] suggested that extraction is controlled by intra-particle diffusion which is related to water flow rate. Therefore, it isexpected that a high flow rate reduces external mass transferresistance and increases extraction efficiency [32]. In this study,several flow rate conditions, 0.5, 1.0, 1.5, and 2.0 mL/min, wereexamined at the constant temperature of 250 8C and extractiontime of 30 min (Table 4). The effect of flow rate on extractionefficiency is shown in Fig. 3. The removal efficiencies of PAHs werea little fluctuated as the flow rate increased from 0.5 to 2.0 mL/minand there was no significant difference of naphthalene, phenan-threne, fluoranthene, and pyrene on their removal efficiencies atdifferent flow rates, i.e., the contaminants extraction was notgreatly affected by water flow rate, suggesting that the extractionrate is not limited by a given retention time at a temperature of250 8C and a pressure of 100 bar. Even when the water retentiontime during the extraction at a flow rate of 0.5 mL/min is higher,the extraction shows an almost similar behavior during theextraction at a flow rate of 2.0 mL/min. This means that the masstransfers of the contaminants from the soil into the liquid phase islimited by the internal diffusion rate or desorption per se and is notregulated by external mass transfer resistance. However, thesefindings partly contradict those of the study conducted byHawthorne et al. [26] who maintained the flow rate of 0.3, 0.5,0.7, 1.1 mL/min and demonstrated that the flow rate had very littleor no significant effect on the extraction of lower molecular weight

Fig. 2. Effect of extraction temperature on the extraction efficiencies of

representative PAHs from a highly contaminated spiked soil. All experiments

were performed at a pressure of 100 bar and flow rate of 2.0 mL/min for 30 min

extraction.

PAHs (e.g., naphthalene), while the extraction of higher molecularweight PAHs (e.g., benzo[a]pyrene) increased substantially with anincreasing flow rate of up to 1.1 mL/min. However, we demon-strated herein among the different flow rates of 0.5, 1.0, 1.5, and2.0 mL/min, 0.5 mL/min was the recommendable flow rate. Furtherresearch is suggested to determine the optimum flow rate in termsof extraction time and desired final extract concentration. Ashorter extraction time and more concentrated final extract will bepreferable.

3.3. Influence of extraction time on removal efficiencies

Extraction time is related to the kinetics and equilibrium timeof the system. It is expected that a longer extraction time increasesthe extraction efficiency of chemicals in soils. In this study, toexamine the influence of extraction time on extraction efficiency,the time was increased in steps of 15 min from 15 min to 60 min ata pressure of 100 bar, temperature of 250 8C, and flow rate of1.5 mL/min (Table 4). The remaining concentrations of contami-nants at different extraction times are presented in Table 5 and theextraction efficiencies are shown in Fig. 4. The extraction of thenaphthalene was essentially completed by up to 15 min at 250 8C(Fig. 4). However, as the time of extraction increased from 15 minto 60 min, there was an increase in the phenanthrene, fluor-anthene, and pyrene extraction from contaminated soil, i.e., thelower molecular weight PAHs extracted at the fastest rate, whilethe higher molecular weight PAHs required longer extraction timedue to a slow desorption and diffusion rate [26]. Fig. 4 alsodemonstrates that 60–70% of removal efficiency for 3 PAHs wasobtained during the first 15 min. A similar study of PAHs extractionfrom contaminated soil at a temperature ranging from 50 to 300 8C(subcritical condition) and 400 8C (supercritical condition) wasconducted by Hawthorne et al. [26] using a GC oven as a heater.They also reported that the largest quantities of the PAHs wereextracted during the first 15 min at 250 8C. In this study, there was

Fig. 3. Effect of extraction flow rate on the extraction efficiencies of representative

PAHs from a highly contaminated spiked soil. All experiments were performed at a

temperature of 250 8C and pressure of 100 bar for 30 min extraction.

Fig. 4. Effect of extraction time on the extraction efficiencies of representative PAHs

from a highly contaminated spiked soil. All experiments were performed at a

temperature of 250 8C, pressure of 100 bar and flow rate of 1.5 mL/min.

M.N. Islam et al. / Journal of Industrial and Engineering Chemistry 18 (2012) 1689–1693 1693

an effect with time of phenanthrene, fluoranthene, and pyrene onextraction as time increased from 15 min to 60 min at 250 8C(Fig. 4). A higher (similar for fluoranthene) extraction was obtainedat 250 8C in 15 min than at 200 8C in 30 min (Table 5). Thus, anincrease in extraction temperature would shorten the extractiontime for quantitative extraction. It appears that the extraction timeis notably influenced by extraction temperature and type ofchemical components.

3.4. Feasibility of subcritical water extraction

Currently, some remediation techniques available includeprocesses such as incineration, thermal desorption, chemicaloxidation, or disposal at hazardous landfills for solid matricesincluding soils, sediments and sludge that are contaminated withheavy organic compounds such as PAHs. Although each of thesetechniques has some advantages, in terms of environmental oreconomical aspects, there are some concerns regarding their usage.Expensive solvents are required for the chemical solvent extractionprocess as follows. Air pollutant from thermal desorption is anecessary secondary treatment of gases. Incineration is the onlydestruction technology that completely degrades the toxic residuein soil, but it totally changes the soil characteristics and isextremely expensive [33].

A study on the remediation of PAHs contaminated soil with asolid–liquid two phase portioning bioreactor (bioremediation) wascarried out by Rehmann and co-workers [34]. After 14 daysoperation period, approximately 78%, 36% and 62% of phenanthrene,fluoranthene, and pyrene, respectively, had been desorbed whereasin our study approximately 96%, 94% and 98% of phenanthrene,fluoranthene, and pyrene, respectively, has been remediated in60 min of hot water (250 8C) extraction. A similar result wasdemonstrated by Lagadec and co-workers with subcritical waterextraction at 275 8C in 35 min for PAHs contaminated MGP soil [17].Also, they compared hot water extraction to bioremediation.Although the removal efficiencies through a one year bioremedia-tion process were fairly high for low molecular weight PAHs(naphthalene, acenaphthalene, fluorine, methylnaphthalene, phen-anthrene), little or no removal was observed for high molecularweight PAHs (pyrene, chrysene, benzo[a]pyrene, etc.). However,even for low molecular weight PAHs, removal efficiencies during 1year of bioremediation were significantly lower than those obtainedin 60 min of subcritical water extraction.

A general comparison of the operation cost of the supercriticalfluid extraction process with other alternative technologies fortreatment of PAHs contaminated soil reported by Montero et al.[35] that exclude any costs on capital. They reported that the costper m3 of treated soil using supercritical water oxidation was at$250–$733, bio-clean at $191–$370, Acurex solvent wash at $196–

$549, methanol extraction at $400–$514, incineration at $1713–$1826, and supercritical fluid extraction at $170–$200. Using thesame assumption as Montero applied to subcritical waterextraction, Lagadec et al. obtained a cost of $150 per m3 of soilwhich successfully treated the PAHs [17]. Considering that theoperational costs are based only on treatment, the subcriticalwater extraction process seems technically attractive; also itcompares favorably both economically and ecologically with thealternatives.

4. Conclusions

This study has demonstrated that extraction efficienciesdepend primarily on extraction temperature and time. Also, therewas significant dependence on flow rate. The results have shownthat the extraction rate of phenanthrene, fluoranthene, and pyreneincreased by increasing the water temperature. Although naph-thalene is the easiest among the PAHs to extract, more than 95%extraction results of phenanthrene, fluoranthene, and pyrene werefound at a temperature of 300 8C in 30 min and a temperature of250 8C in 60 min extraction and at a pressure of 100 bar. Also, itwas observed that there was no significant effect of water flow rateon the removal efficiencies of PAHs. Conclusively, this experimentstudy presented the feasibility of subcritical water extraction fornaphthalene, phenanthrene, fluoranthene, and pyrene contami-nated soil, and its potential benefit such as removal efficiency, andmost importantly use of water as an environmentally friendlysolvent.

Acknowledgment

This study was financially supported by Korea EnvironmentalIndustry and Technology Institute (KEITI) through the GAIAproject.

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