RESEARCH ARTICLE

Effect of temperature and particle size on the thermaldesorption of PCBs from contaminated soil

Zhifu Qi & Tong Chen & Sihong Bai & Mi Yan &

Shengyong Lu & Alfons Buekens & Jianhua Yan &

Cora Bulmău & Xiaodong Li

Received: 29 August 2013 /Accepted: 22 November 2013# Springer-Verlag Berlin Heidelberg 2013

Abstract Thermal desorption is widely used for remediationof soil contaminated with volatiles, such as solvents anddistillates. In this study, a soil contaminated with semivolatilepolychlorinated biphenyls (PCBs) was sampled at an interimstorage point for waste PCB transformers and heated to tem-peratures from 300 to 600 °C in a flow of nitrogen to inves-tigate the effect of temperature and particle size on thermaldesorption. Two size fractions were tested: coarse soil of 420–841 μm and fine soil with particles <250 μm. A PCB removalefficiency of 98.0 % was attained after 1 h of thermal treat-ment at 600 °C. The residual amount of PCBs in this soildecreased with rising thermal treatment temperature while theamount transferred to the gas phase increased up to 550 °C; at600 °C, destruction of PCBs became more obvious. At lowtemperature, the thermally treated soil still had a similar PCBhomologue distribution as raw soil, indicating thermal desorp-tion as a main mechanism in removal. Dechlorination anddecomposition increasingly occurred at high temperature,since shifts in average chlorination level were observed, from3.34 in the raw soil to 2.75 in soil treated at 600 °C. Fine soilparticles showed higher removal efficiency and destructionefficiency than coarse particles, suggesting that desorptionfrom coarse particles is influenced by mass transfer.

Keywords PCBs . Thermal desorption . Dechlorination .

Destruction . Particle size . Soil remediation

Introduction

Polychlorinated biphenyls (PCBs) once were important indus-trial chemicals, mainly produced in the USA (1929–1979) andfeaturing remarkable qualities, such as thermal and chemicalstability and low vapor pressure and flammability. Due to theirtoxicity, the US Congress banned PCB production in 1979.PCBs may cause cancer in animals as well as serious effectson the immune, reproductive, nervous, and endocrine systems(USEPA 2012). Historically, China produced about 10,000tons of PCBs, with 9,000 tons primarily as trichlorobiphenyl(TrCB) and 1,000 tons as pentachlorobiphenyl (PeCB) (Xinget al. 2005). TrCBs were mainly used as dielectric fluid intransformers and capacitors. Due to spills, evaporation losses,and inappropriate management of contaminated materials,contamination has spread at those sites where PCBs werehandled. Some areas even became grossly PCB contaminated,reaching soil levels of several thousands of parts per millionPCBs. These contaminated areas need to be identified, docu-mented, and remediated in order to minimize future environ-mental and human exposure (Weber et al. 2008).

Thermal desorption is a soil remediation technique thatutilizes heat to volatize contaminants for recovery or removalof contaminants. Compared with incineration, low-temperature thermal desorption (LTTD) allows a cost savingof roughly 75 %, and the treated soil retains its essentialcharacteristics for reuse (Norris et al. 1999). For a particularcontaminated soil problem, a thermal desorption treatmentsystem must be selected based on projected technical perfor-mance and cost, regulatory permitting, site characteristics/location, and the presence of other contamination problemsat the site (Fox et al. 1991).

Responsible editor: Leif Kronberg

Z. Qi : T. Chen : S. Bai :M. Yan : S. Lu :A. Buekens : J. Yan :X. Li (*)State Key Laboratory of Clean Energy Utilization, Institute forThermal Power Engineering, Zhejiang University,Hangzhou 310027, Chinae-mail: lixd@zju.edu.cn

C. BulmăuPower Engineering Faculty, University Politehnica of Bucharest,Splaiul Independentei No. 313, Bucharest 060042, Romania

Environ Sci Pollut ResDOI 10.1007/s11356-013-2392-4

The effects of final temperature, heating rate and duration,and contamination level on the extent and apparent rate ofcontaminant removal were studied on organics-contaminatedsoil. At a heating rate of 1000 °C/s, almost all fuel oil wasremoved in about 0.7 s and at a final temperature of 700 °C(Bucalá et al. 1994). Merino and Bucalá (2007) investigatedthe effect of temperature on the release of hexadecane fromsoil. More than 80–88 % of the initial hexadecane content inthe soil matrix was recovered by thermal treatment without anychemical transformation, indicating evaporation as the mainmechanism for hexadecane removal. For the soil tested of arelatively low surface area, removal efficiencies (REs) higherthan 99.9 % were reached at about 300 °C. Falciglia et al.(2011) showed that a temperature of 175 °C is sufficient to treatdiesel-polluted sandy and silty soil, yet a temperature of 250 °Cwas needed for clay. Thermal desorption of diesel-polluted soilallows its recovery and is governed by first-order kinetics.

Matrix effects were also studied for semivolatiles. In oxy-gen deficient conditions, Gao et al. (2008) investigated thedechlorination of hexachlorobenzene (HCB) on different solidsupports (SiO2, CaO, CaSiO3, cement, and treated fly ash).All tested supports showed HCB dechlorination potential,except for SiO2. The dechlorination efficiency at 350 °C for4 h varied from 32.2 % (cement) over 64.6 % (CaO), 76.15 %(CaSiO3) to approximately 80 % (treated fly ash).

Lundin and Marklund (2007) described a procedure forreducing the load of mono- to octa-chlorinated polychlorinateddibenzo-p -dioxins and polychlorinated dibenzofurans(PCDD/Fs), PCBs, and HCB on fly ash from municipal solidwaste incineration (MSW). Thermal treatment at 500 °C for60 min in a closed system providing low oxygen conditionsresulted in 97 % and 99 % reduction of the total and interna-tional toxic equivalents (I-TEQ) concentrations of themonochlorinated to octa-chlorinated PCDD/Fs. Similar effectswere observed for HCB and PCBs.

Risoul et al. (2002) conducted a laboratory study on thethermal decontamination of soils polluted by PCBs and com-pared the results with thermogravimetric analysis (TGA). TGAand laboratory scale desorption experiments appear to lead tocomparable effects of the operating parameters. Only slightdifferences were observed between the effects of pressure, initialsample mass, gas flow rate, and initial contamination level. Asignificant effect of the nature of the soil was pointed out.

Desorption of semivolatile compounds (PCDD/Fs; PCBs)under anaerobic conditions is accompanied by dechlorinationand decomposition (Hagenmaier et al. 1987; Weber et al.2002; Misaka et al. 2006). Conversely, PCBs may also actas precursors of polychlorinated dibenzofurans (PCDFs) inthe presence of oxygen, since oxygen insertion could formPCDFs (Sato et al. 2010;Weber and Sakurai 2001). Zhao et al.(2012) investigated the PCDF formation pathways, includingthe loss of ortho -Cl, ortho -H, or HCl, involving a 2,3-chlo-rine shift and dechlorination of PCBs.

Although some studies were carried out on the thermaldesorption of PCB-contaminated soil, they usually focusedon an artificially contaminated soil. In addition, PCBs in thegas phase were seldom studied during thermal desorption. Themain goals of this study were the following: (a) to investigateeffect of temperature on thermal desorption of real PCB-contaminated soil, (b) to obtain a more complete picture ofthermal treatment by analyzing some 150 PCB congeners forsoil samples and gas phase, and (c) to investigate the effect ofparticle size on thermal desorption.

Materials and methods

Materials

A loamy soil was collected from one of the waste transformerconservation points in Shaoxin City, Zhejiang Province,China. This soil was heavily contaminated by PCBs becauseof oil leakage from waste PCB transformers. The PCB con-centration in soil can reach 500 mg/kg, with TrCB andtetrachlorbiphenyl (TeCB) dominating the homologues ofPCBs. Table 1 shows some physicochemical properties ofthe tested contaminated soil. All these properties correspondto the whole soil. Soil texture was determined by pipet meth-od. PH value, organic matter content, water content, bulkdensity, and porosity were measured according to the hand-book for testing soil physical and chemical properties (Qiao2011). Chlorine content wasmeasured by ion chromatographyand heavy metal by inductively coupled plasma atomic emis-sion spectroscopy (ICP-AES).

The raw soil was dried by air in an extraction hood andsieved to two fractions: coarse particles (420–841 μm) andfine particles (<250 μm), respectively, and it was refrigerateduntil analysis.

Table 1 Physicochemical properties of contaminated soil

Property Value

Soil texture Sand (32.6 %); silt (38.8 %);clay (28.6 %)

pH value 6.83

Chlorine content, % 0.898

Organic matter content, % 3.02

Bulk density, g/cm3 1.65

Porosity, % 59.1

Heavy metal (ppm, dw) Cr (47.5), Ni (23.5), Cu (33.9),Zn (56.3), Hg (0.49),Pb (23.0), V (6.55)

Water content, % 12.61

dw dry weight

Environ Sci Pollut Res

Thermal treatment of soil contaminated with PCBs

PCB-contaminated soil samples were treated using a benchscale apparatus (Fig. 1), which consisted of an input nitrogentransport section, a tubular electric furnace (maximum power6.0 kW), a quartz cylindrical tube (inner diameter: 70 mm,length: 800 mm), and a flue gas collection system.Contaminated soil (1.00 g) was placed in a ceramic boat andpushed into the center of the quartz cylindrical tube andtreated in a nitrogen gas flow of 400 ml/min. The furnaceheating temperatures were set at 300, 400, 450, 500, 550, and600 °C. Before the thermal treatment was carried out, thequartz cylindrical tube was purged with nitrogen for 20 min.

After 1 h of thermal treatment, the ceramic boat wasremoved with a crucible tong and quenched in watercontained in a brown glass bottle. After quenching, the boatswere washed with toluene. During the experiment, PCBs inthe carrier gas were absorbed on XAD-2 resin and then intoluene in an ice bath. After sampling, residual PCB concen-trations on soil and those carried along in gas were measured.Experiments were performed in duplicate. Figure 2 shows thetemperature profile of the soil in ceramic boats at given setpoint temperatures of the oven. The temperature of the soilrises quickly, almost attaining set point in 300 s. After about500 s, set point temperature is reached.

PCB analysis

After Soxhlet extraction and acid/base treatment, the sampleswere cleaned up sequentially with multisilica gel and Florisilcolumns, as described in EPA method 1668 (USEPA 2008).The resulting solution was analyzed for PCBs byHRGC/HRMS (JEOL JMS-800D, Japan) with a DB-5MScolumn (60 m×0.25 mm×0.25 μm). All 209 PCB isomersfrom monochlorinated (MoCB) to decachlorinated biphenyls(DeCB) were detected, and over 150 congeners were separat-ed and quantified, as described by Chen et al. (2009). Thesecongeners were quantified by adding a mixture of internal,cleanup, and injection standard solutions, applied before ex-traction, purification, and analysis, respectively. The recoveryrate of each internal standard was between 64 and 128 %, andthose of each cleanup standard were between 78 and 124 %;all values are in accordance with the analytical requirements.

Removal efficiency (RE), destruction efficiency (DE),and weight average chlorination degree

PCB RE is defined and calculated by

removal efficiency REð Þ ¼ PCBs on raw soilð Þ−PCBs on soilð ÞPCBs on raw soilð Þ

PCB DE was defined and calculated by

destruction efficiency DEð Þ ¼ PCBs on raw soilð Þ−PCBs on soilð Þ−PCBs in gasð ÞPCBs on raw soilð Þ

The weight average chlorination degree was defined andcalculated by

weight average chlorination degree ¼X10

i¼0C C12HiCl10−ið Þ � 10−ið Þ

X10

i¼0C C12HiCl10−ið Þ

Nitrogen

Flowmeter

Flowmetercontroller

Heatcontroller

Furnace

Ice bathNitrogen

Heatcontroller

Furnace

Ice bath

XAD-2

Fig. 1 Experimental apparatus

0 600 1200 1800 2400 3000 3600

100

200

300

400

500

600

Tem

pera

ture

, o C

300oC 400

oC 500

oC 600

oC

Time, s

Fig. 2 Temperature profile of the soil in ceramic boats at given set pointtemperatures

Environ Sci Pollut Res

All statistical analyses were performed using SPSS soft-ware version 15.0.

Results and discussion

Total PCB concentration in soil

Figure 3 shows the RE of the PCBs, as well as their weightaverage chlorination degree in raw soil and thermally treatedsoil (coarse particles). The total amount of residual PCBsdecreased with rising thermal treatment temperature. After1 h at 600 °C, the total amount of PCBs in soil decreasedfrom 524 μg to 20.4 μg in 1 g of untreated soil, or a total PCBRE of 96.1 %. Other values of RE are the following: 64.2 % ata furnace temperature of 300 °C, 92.2% at 500 °C, and 96.1%at 600 °C (Fig. 3). Thus, thermal desorption effectivelyremoves PCBs from soil at a suitable combination of treat-ment time (1 h) and temperature.

The RE of PCBs rises quickly between 300 and 400 °Cwhile it increases slowly once the temperature rises above400 °C. This trend suggests a sequence of two distinct phasesin the thermal removal of semivolatile organics from this soil.During the first step, rapid evaporation of contaminant occursfrom the soil particle surface while afterward, the evaporationrate is increasingly limited by internal diffusion inside theparticle pores (Keyes and Silcox 1994).

The RE of PCBs in this study is less than that in Risoulet al. (2002). In that case, higher RE, close to 99.9 %, wasreached after a thermal treatment at 450 °C for only 30 min.There may be two reasons for this difference: (a) the soil usedin this study was real PCB-contaminated soil while the soilthat Risoul et al. (2002) utilized was artificially contaminatedsoil, and (b) the experiment devices and operating conditionswere different.

PCB isomer distribution in raw and treated soil

Figure 4 shows the decrease in PCBs at different thermaltreatment temperatures, split per isomer group. In raw soil,TrCB and TeCB dominate the homologues of PCBs, account-ing for 51.6 % and 37.3 % of the total weight of PCBs. Octa-chlorinated and higher-chlorinated PCB homologues repre-sent less than 0.5 % of the PCB amount.

Figure 5 presents the composition of PCB isomer groupson raw and thermally treated soil. The PCB homologuesdistribution at 300 °C was similar with that in raw soil.Generally, with rising temperature, low-chlorinated PCB ho-mologues increased while high-chlorinated PCB homologuesdecreased. After 1 h of heating, the ratio of MoCB increasedfrom 0.14 % in raw soil to 2.74 % in thermally treated soil at500 °C. Moreover, the ratio of dichlorobiphenyl (DiCB) in-creased from 8.36 % in raw soil to 36.0 % in thermally treated

soil at 600 °C. PCB 11 (3,3′-dichlorobiphenyl) yielded amaximum increment, and the ratio increased from 0.24 % inraw soil to 21.5 % in thermally treated soil at 550 °C, thendecreased to 11.4 % in soil at 600 °C. The ratio of TeCBdecreased from 37.3 % in raw soil to 12.4 % in thermallytreated soil at 600 °C. The ratio of PeCB decreased from2.38 % in raw soil to 1.10 % in thermally treated soil at600 °C. These facts suggested the possibility of the decom-position of given PCB homologues while they are adsorbedon the soil, rather than the decomposition after they aredesorbed.

During thermal desorption of complex hydrocarbon mix-tures, the desorption rate is a strong function of soil type andtemperature; the lighter components will be selectivelydesorbed first without substantial reaction (Lighty 1988;Lighty et al. 1989). For PCBs, a shift in the weight averagechlorination degree was simultaneously observed. The weightaverage number of chlorine atoms also decreased, from 3.34in raw soil to 2.75 % at 600 °C (Fig. 3). Dechlorination wouldoccur before the contaminant is desorbed from soil surface.The concentration of MoCB in thermally treated soil wasequal to or higher than that in the raw soil. Dechlorinationpathways of PCBs were studied by Gryglewicz and Piechocki(2011), and Murena and Schioppa (2000). They both studiedcatalytic hydrodechlorination under hydrogen atmosphere andfound that the Cl atoms in metaposition are preferentiallyattacked. Ortho -substitution is slower than meta-substitutionand para-substitution.

dl-PCB distribution in raw and treated soil

Table 2 shows the concentration of the 12 dioxin-like PCBs(dl-PCBs, also coplanar PCBs). The concentration of dl-PCBswas 7125 ng in 1 g of untreated soil and decreased with risingtreatment temperature. As for total PCBs, the RE of dl-PCBs

2.7

2.8

2.9

3.0

3.1

3.2

3.3

3.4

600oC550oC450oC 500oC400oC

RE

of

PC

Bs,

%

Chl

orin

atio

n D

egre

e

Conditions

Chlorination Degree

Raw S 300oC0

20

40

60

80

100RE of PCBs

Fig. 3 Removal efficiencies and weight average chlorination degree ofPCBs for raw and thermally treated soil

Environ Sci Pollut Res

increased quickly among the temperature range of 300 to400 °C, yet it increased more slowly when temperature ishigher than 400 °C. The RE of dl-PCBs reached 99.3 % at600 °C, markedly higher than the RE of total PCBs at 96.1 %.This is concordant with the findings above. The molecule of dl-PCBs contains four or more chlorine atoms. Compared withless chlorinated PCB homologues, dl-PCBs yield higher RE.

The homologue distribution of dl-PCBs was similar inthermally treated soil. Congeners 3,3′,4,4′-TeCB (#77),2,3′,4,4′,5-TeCB (#118) and 2,3,3′,4,4′-PeCB (#105) led thedl-PCBs. The toxic equivalence factors (TEFs) of the WorldHealth Organization (WHO) were used for calculating thecontributions for PCBs (Van den Berg et al. 2006). With risingtemperature, the TEQ of dl-PCBs decreased from 2.71 ngWHO-TEQ on raw soil to 0.05 ng WHO-TEQ at 600 °C.For several dl-like PCB congeners, the concentration evenincreased with rising temperature, partly because of the

dechlorination that took place during degradation of PCBs,in accordance with the findings of Weber and Sakurai (2001).They also found some slight formations of higher-chlorinatedheptachlorobiphenyl (HpCB) (<1 %) during the destruction ofindividual hexachlorobiphenyl (HxCB) isomers. Besides, noobvious difference was found between the RE of non-orthosubstituted PCBs and mono-ortho substituted PCBs.

PCB concentration in the gas phase

Figure 6 shows the amount of PCBs in the gas phase after theirtransfer to the carrier gas. TrCB and TeCB still predominate,as in raw soil. The residual PCB concentration in this soildecreased with rising thermal treatment temperature while itsconcentration in the gas phase increased; yet at 600 °C, gasphase destruction of PCBs became obvious. The PCB amountin the gas phase increased from 34.0μg at 300 °C to 95.1 μg at550 °C and then slightly decreased to 84.0 μg at 600 °C. Thisphenomenon can be attributed to combined desorption, dechlo-rination, and destruction during thermal treatment. Higherheating temperatures make more PCBs desorb from soil andyield a higher concentration of PCBs in gas while high temper-atures can also conversely enhance the PCB destruction anddecomposition. Most of the decrease in PCBs on soil wasrecorded among the temperature range from 300 to 400 °C.

Compared with raw soil, no obvious difference was foundin the total PCB homologue distribution. In general, the ratioof MoCB increased while TeCB decreased with increasingtemperature. As thermally treated soil, the ratio of PCB 11 alsoincreased from 0.24 % in raw soil to about 2.0 % in the carriergas. The average weight chlorination levels were similar atdifferent temperatures, ranging from 3.19 at 450 °C to 3.31 at600 °C with an average of 3.24, slightly less than that in raw

0

40000

80000

120000

200000

250000

600oC550

oC500

oC450

oC400

oC

gn,srenegnocsBCPfotnuomA

Conditions

MoCB

DiCB

TrCB

TeCB

PeCB

HxCB

HpCB

Raw S 300oC

Fig. 4 Amount of PCBs (innanograms) on raw and thermallytreated soil. Basis=1.00 g of rawsoil

0

10

20

30

60

80

100

600oC550oC500oC450oC400oC

Com

posi

tion,

%

Conditions

MoCB DiCB TrCB TeCB PeCB

HxCB HpCB

Raw S 300oC

Fig. 5 Composition of PCB isomer groups on raw and thermally treatedsoil

Environ Sci Pollut Res

soil. It indicates that dechlorination reactions may not pre-dominate in the gas phase.

The amounts of PCB homologues with different chlorinenumber in carrier gas are shown in Fig. 7. Generally speaking,MoCB, HxCB, and HpCB increased at higher temperature.The amount of DiCB and TeCB increased from 300 °C to500 °C and decreased from 500 °C to 600 °C. TrCB increasedfrom 300 °C to 550 °C and decreased from 550 °C to 600 °C.There may be several reasons: (a) some given PCB homo-logues were more stable and once desorbed, they could not bedecomposed easily in gas phase and (b) the competitionbetween destruction and formation. Dechlorination of somegiven PCB homologues will lead to decrease of these givenPCBs while it will increase the amount of less-chlorinatedPCB homologues.

Total PCBs in soil and gas

With rising temperature, the residual total PCBs on soil de-creased during all the tests, yet the PCBs evolving in gasincreased up to 550 °C (Fig. 8). The DE of PCBs increasedquickly from 57.7 % at 300 °C to 75.1 % at 400 °C. Then DEof PCBs varied rather little from 400 to 500 °C. The DE ofPCBs continued to increase from 74.9 % at 500 °C to 80.1 %at 600 °C.

The DE of WHO-TEQ showed a similar trend (Fig. 9).With rising temperature, TEQ on soil decreased throughoutthe entire test temperature range while the TEQ reporting tothe gas increased. The DE of TEQ increased quickly from68.7 % at 300 °C to 86.8 % at 450 °C. No observable increaseof DE was found at higher temperature.

Table 2 Amount of dl-PCBs onraw and thermally treated soil

All amounts are given in nano-grams for dl-PCBs, except for theabsolute value of WHO-TEQ(nanogam WHO-TEQ)

Raw 300 °C 400 °C 450 °C 500 °C 550 °C 600 °C

3,3′,4,4′-TeCB(#77) 2.70×103 1.01×103 260 175 118 34.4 15.7

3, 4,4′,5-TeCB(#81) 146 63.3 25.8 8.73 5.2 1.95 0.915

3,3′,4,4′,5-PeCB(#126) 19.5 6.35 3.86 1.95 1.12 0.685 0.45

3,3′,4,4′,5,5′-HxCB(#169) 10.5 0.615 0.985 0.425 0.365 0.4 0.12

2′,3,4,4′,5-PeCB(#123) 96.9 16.9 5.09 5.9 4.52 2.5 1.07

2,3′, 4,4′,5-TeCB(#118) 2.45×103 711 203 105 108 30.6 17.3

2,3, 4,4′,5-TeCB(#114) 132 49.5 12.9 13.7 5.72 3.83 2.25

2,3,3',4,4′-PeCB(#105) 1.49×103 324 85.5 99.0 45.7 21.8 9.33

2,3,4,4′,5,5′-HxCB(#167) 17.9 6.41 4.68 0.97 0.655 0.49 0.41

2,3,3′,4,4′,5-HxCB(#156) 38.7 8.42 4.80 4.24 0.995 0.855 0.355

2,3,3′,4,4′,5′-HxCB(#157) 11.9 2.46 3.06 0.925 0.66 0.255 0.18

2,3,3′,4,4′,5,5′-HpCB(#189) 5.64 0.495 0.535 0.4 0.605 0.135 0.155

Sum 7.13×103 2.20×103 611 416 291 97.8 48.2

WHO-TEQ 2.71 0.805 0.46 0.235 0.14 0.085 0.05

0

20000

40000

60000

600oC550oC500oC450oC400oC

Chl

orin

atio

n D

egre

e

MoCB DiCB TrCB TeCB PeCB HxCB HpCB

Am

ount

of

PCB

s co

ngen

ers

in c

arri

er g

as,

ng

Conditions300oC

3.1

3.2

3.3

3.4

3.5

Chlorination Degree

Fig. 6 Amount of PCBs and its average chlorination level in the carriergas

10

100

1000

10000

100000

600oC550oC500oC450oC400oC

gn,sagreirrac

nis

BC

Pfotnu o

mA

Conditions

MoCB DiCB TrCB TeCB PeCB HxCB HpCB

300oC

Fig. 7 Amounts of PCB homologues in the carrier gas after a test of 1 h

Environ Sci Pollut Res

DE and RE of PCBs in soil with different particle size

Soil with smaller particle size (<250 μm, S2) showed similartrends as coarse soil (420–841 μm, S1). Table 3 lists the DEand RE of PCBs for soil with different particle size. At thesame temperature, smaller soil particles (S2) had higher REand DE than these with large particle size (S1).

The total PCB REs of soil at temperature of 400, 500,600 °C were markedly higher for fine soil particles: 89.3 %and 94.3 %, 92.2 % and 97.4 %, 96.1 % and 98.0 % for thecoarse particles and fine particles, respectively. Fine particlesare decontaminated more quickly than large particles. Thereare two possible reasons for this. First, smaller particles mayheat up faster (Falciglia et al. 2011). Second, if the rate-controlling mass transport mechanism is diffusion, smallerparticles should desorb faster than large particles given theidentical heating profiles (Keyes and Silcox 1994).

These results were also concordant with previous study(Falciglia et al. 2011). With mineral oil fractions, a tempera-ture of 175 °C is sufficient to remediate diesel-polluted sandyand silty soils whereas a higher temperature (250 °C) isneeded for clays. While fine sand (75–250 μm) had a higherdiesel RE than coarse sand (500–840 μm) under all testedtemperature range.

The total PCB DEs at temperature of 400 °C, 500 °C, and600 °C were 75.1 % and 77.4 %, 74.9 % and 78.8 %, 80.1 %and 81.4 % for the coarse particles and fine particles, respec-tively. Smaller particles, which had larger specific surfacearea, should desorb faster than large particles when the rate-controlling mass transport mechanism is internal (pore) diffu-sion. In the same manner, the dechlorination and destructionof PCBs in fine particles were stronger than in coarse particles.

Conclusion

Thermal treatment at high temperature is an effective methodto detoxify PCB-contaminated soil. The temperature and par-ticle size dependence of the decontamination of real PCB-contaminated soil were observed during thermal treatment attemperatures ranging between 300 and 600 °C. A PCB RE of98.0 % was attained after 1 hr of thermal treatment at 600 °C.Higher treatment temperature increases the RE of PCBs onsoil while it seems less effective in increasing the DE of PCBs.Dechlorination and destruction also occurred during the ther-mal treatment of PCB-laden soil. At low temperature, thethermally treated soil still had a similar PCB homologuedistribution as raw soil, indicating thermal desorption as themain mechanism in removal. Dechlorination and decomposi-tion increasingly occurred at high temperature. Besides, de-chlorination would occur before the contaminant is desorbedfrom soil surface. This assumption could be verified by thefact that the concentration of MoCB and DiCB in thermallytreated soil was higher than the one in raw soil and others.

Among the tested particle size range, the soil with smallerparticle size had higher RE and DE than that with larger size.These could be attributed to the difference of specific surfacearea, internal pore size, physicochemical property, etc. Further

0

50000

100000

150000

200000

250000

600o

C550o

C500o

C450o

C400o

C

DE

of

PCB

s, %

gn,s

BCPlato

Tfo tnuo

malaudi seR

Conditions

Soil Gas S+G

300o

C

40

50

60

70

80

90 DE of PCBs

Fig. 8 Residual amount in soil, evolving in carrier gas, and both com-bined, and destruction efficiency of PCBs at different temperature levels(weight units)

0.0

0.2

0.4

0.6

0.8

1.0

DE

of

TE

Q,

%

QE

Tgn,

BCP

QE

T-O

HW

fotnuomalaudise

R

Conditions

Soil Gas S+G

300oC 400

oC 450

oC 500

oC 550

oC 600

oC

65

70

75

80

85

90 DE of TEQ

Fig. 9 Residual amount in soil, evolving in carrier gas, and both com-bined, and destruction efficiency of PCBs at different temperature levels(WHO-TEQ units). Destruction efficiencies of WHO-TEQ at differenttemperatures

Table 3 Destruction efficiency (DE) and removal efficiency (RE) ofPCBs in soil with different particle size

300 °C 400 °C 450 °C 500 °C 550 °C 600 °C

RE for S1 64.2 89.3 90.4 92.2 94.9 96.1

DE for S1 57.7 75.1 75.2 74.9 76.8 80.1

RE for S2 NM 94.3 NM 97.4 NM 98.0

DE for S2 NM 77.4 NM 78.8 NM 81.4

NM not measured

Environ Sci Pollut Res

study should focus on the effect of these factors on RE and DEof PCBs.

Acknowledgments This research work was financially supported byMajor State Basic Research Development Program of China (973 Pro-gram) (No.2011CB201500), the National High Technology Research andDevelopment Program of China (No.2009AA061304), and the Sino-Romania Scientific and Technological Cooperative Project (41–6).

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