analysis of thermal processing applied to contaminated soil for organic pollutants removal

Journal of Geochemical Exploration xxx (2014) xxx–xxx

GEXPLO-05443; No of Pages 8

Contents lists available at ScienceDirect

Journal of Geochemical Exploration

j ourna l homepage: www.e lsev ie r .com/ locate / jgeoexp

Analysis of thermal processing applied to contaminated soil for organicpollutants removal

Cora Bulmău a,⁎, Cosmin Mărculescu a, Shengyong Lu b, Zhifu Qi b

a Department of Energy Production and Use, Power Engineering Faculty, Polytechnic University of Bucharest, 313, Splaiul Independen ei, 060042 Bucharest, Romaniab State Key Laboratory of Clean Energy Utilization, Institute for Thermal Power Engineering, Zhejiang University, Hangzhou 310027, China

⁎ Corresponding author. Tel.: +40 742234586.E-mail address: [email protected] (C. Bulmău).

http://dx.doi.org/10.1016/j.gexplo.2014.08.0050375-6742/© 2014 Elsevier B.V. All rights reserved.

Please cite this article as: Bulmău, C., et al., AnExplor. (2014), http://dx.doi.org/10.1016/j.g

a b s t r a c t
a r t i c l e i n f o
Article history:Received 6 March 2014Revised 23 July 2014Accepted 12 August 2014Available online xxxx

Keywords:PyrolysisSoilThermal treatment for decontaminationPAHs

The paper presents the results of an experimental study conducted to investigate PAHs behavior during non-oxidant thermal decontamination of soil samples contaminated with petroleum products. The study focusedon the assessment of the concentration levels for different PAHs species present in pyrolysis products after thetreatment of contaminated soil samples. Pyrolysis experiments were performed using a horizontal tubularreactor. The contaminated soil samples were treated in inert controlled atmosphere (nitrogen). The treatmentperiod varied between 30 and 60 min at a temperature range of 350 °C–650 °C. Chemical analyses wereperformed to compare the thermal degradation mechanism and the pollutants generation in the flue gases ofthe pyrolysis process. The present research study identifies and quantifies the concentration level of polycyclicaromatic hydrocarbons from the solid and gaseous phases of the pyrolysis products, as well as from the conden-sates produced during the process. The amount of the organic contaminants was determined using a GC/MSanalyzer and Soxhlet extraction method. The results of the experiments revealed that pyrolysis is an efficientprocess that could be used to remove the PAHs from contaminated soil. For 650 °C treatment temperatureperformed for 30min the thermal process registered a decontamination efficiency superior to 80%. The extensionof the treatment period to 60 min increased the decontamination efficiency to more than 90%.

© 2014 Elsevier B.V. All rights reserved.

1. Introduction

Due to the anthropogenic activities related to petroleum processingour environment is exposed to polycyclic aromatic hydrocarbons(PAHs) contamination. These organic compounds attach to solidsediments and are omnipresent: in soil, water and air. PAHs existingin these environments lead to potential hazards to human health(Ukiwe et al., 2013). There are several hundred PAHs types in the envi-ronment and generally they are found as complex mixtures rather thanas individual compounds (HPA, 2008). Only 16 PAHs are classified bythe US EPA as priority pollutants based on toxicity, potential forhuman exposure, frequency of occurrence at hazardous waste sites,and the extent of information available (ATSDR, 2005). Among these,the US EPA considers only seven as probable human carcinogens:benzo(a)anthracene, chrysene, benzo(a)pyrene, benzo(b)fluoranthene,benzo(k)fluoranthene, dibenzo(a,h)anthracene and indeno(1,2,3-cd)pyrene (Bojes and Pope, 2007; NTP 2011).

The polycyclic aromatic hydrocarbons are classified as low molecu-lar weights (LMW) or high molecular weights (HMW). The first groupis formed by two or three benzene rings, while those with four or

alysis of thermal processing aexplo.2014.08.005

more benzene rings represent the class of HMW PAHs. LMW PAHs arefairly soluble in water, but HMW are quite hydrophobic and insoluble(Cerniglia and Heitkamp, 1989). The rate of PAHs absorption into soilorganic matter is proportional with the number of the aromatic rings.Various HMW PAHs are recalcitrant under current conditions in theterrestrial environment; consequently their persistence resides overlong periods of time (Sims and Overcash, 1983; Ukiwe et al., 2013). Soit is more difficult to remove HMW from the soil (Delgado-Balbuenaet al., 2013; Stroud et al., 2007). That explains the necessity to developnew processes and technologies in order to increase the degradationrate of the persistent PAH (Chouychai et al., 2009).

There are many applicable technologies for contaminated soil withpetroleum hydrocarbon (Ram et al., 1993), but the efficiency of thetechnologies closely depends on contaminant and soil characteristicsas well as on cost limitations (Khan et al., 2004; Reddy et al., 1999;Riser-Roberts, 1998). Over the last years, for the remediation of thepetroleum contaminated soils, different processes have been adopted:physical, chemical, biological and thermal methods. Choosing themost appropriate technology for soil decontamination representsa difficult undertaking because many issues must be considered:technical, technological and economical (Kujat, 1999; Reis et al.,2007). Thermal technologies present a scientific and practical interestbecause they proved high efficiency in removing and degrading organicpollutants.

pplied to contaminated soil for organic pollutants removal, J. Geochem.

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Table 1Main properties of the soil from the contaminated site.

Soil property Value Soil pollutant Value

pH (–) 7.7 Pyrene concentration (mg/kgdw) 0.089Humidity (%) 16 B(a)P concentration (mg/kgdw) 0.050Density (g/cm3) 1.5 B(a)A concentration (mg/kgdw) 0.257C organic (%) 13.36 Total PAHs concentration (mg/kgdw) 0.989Humus (%) 23.04 TPH (mg/kgdw) 71,000

dw = dry weight.B(a)P = benzo(a)pyrene.B(a)A = benzo(a)anthracene.

2 C. Bulmău et al. / Journal of Geochemical Exploration xxx (2014) xxx–xxx

According to their operational temperature, thermal treatments canbe classified into desorption and destruction techniques. Pyrolysisis part of the second class by fragmenting the organic molecules.Generally, pyrolysis causes the thermal degradation of the organiccompounds that are converted into primary products such as charcoal,liquids, and flue gas (Bridgewater and Grassi, 1995). When used forcontaminated soils treatment pyrolysis offers the advantages oftreatment temperatures superior to pollutant vaporization tempera-ture. Consequently the pollutant (hydrocarbons in this case) undergoesa phase transformation from liquid to gas and leaves the solid matrix ofthe soil. Therefore pyrolysis can be interesting in this field and requiresfurther study. As for all ex situ thermal treatments, the main advantageof the pyrolysis process is that it requires short times, and there is morecertainty about the uniformity of the treatment, due to the capacityto screen, homogenize, and continuously mix the soils. The majordisadvantage of the process is related to the energy consumption need-ed to bring andmaintain the contaminated soil at a specific temperaturefor a minimum residence time to complete the pollutant removal(Bulmău et al., 2013). The energy consumption is the sum of the energyconsumed for bringing the soil from ambient temperature to pollutantvaporization temperature, plus the energy necessary to heat the pollut-ant up to vaporization and energy for heating of soil to the maximumprocess temperature. Due to this heat demand the pyrolysis of contam-inated soil is an endothermic process. Nevertheless, when applied tohydrocarbons contaminated soil, depending on the contaminationlevel (i.e. quantity of hydrocarbons), the released hydrocarbons can beused to generate heat within the process by controlled combustion.Another limitation of the treatment is given by the excavation of soilsleading to additional costs. If hydrocarbons are used for heat generationby combustion, the flue gas cleaning system configuration can bereduced to minimum. However thermal treatments offer fast cleanupfor heavily contaminated soils with high pollutant removal efficiency(Bulmău et al., 2012a).

The paper provides important information about a thermal processwith high efficiency in polycyclic aromatic carbons removal from soilcontaminated with petroleum products. The experimental studyrevealed that pyrolysis could reach over 90% efficiency in PAHs removalfrom soil.

The main objective of this experimental study is to identify andquantify the concentration levels of different PAHs compounds (pyrene,benzo(a)pyrene, benzo(a)anthracene and total PAHs) existing in a soilcontaminated with petroleum products, and to investigate the removalof these organic pollutants from the soil matrix.We have alsomonitoredthe amount of PAHs compounds generated in gaseous phase during thethermal treatment. Furthermore, the process results at high tempera-tures and low temperatures are compared. This piece of informationcontributed to the assessment of the temperature and the treatmentperiod influences both the efficiency of the contaminants removalfrom the soil matrix during the thermal treatment, and on productsgenerated by these technologies. The experimental research couldprovide fundamental data regarding the PAHs removal during thedecontamination of the polluted soil, thus helping to establish thebehavior of pyrolysis technologies used to remediate contaminated soils.

2. Material and methods

2.1. Samples

The soil used in the study was collected directly from a highlypolluted site as a result of anthropogenic activities related to petroleumprocessing.

The contaminated soil testers were sampled according to STAS7184/1-75, SR ISO 11074-2:2001 and the improved methodologydeveloped by the National Institute of Research–Development forAgrochemistry and Pedology from Romania, according to the EuropeanUnion regulations.

Please cite this article as: Bulmău, C., et al., Analysis of thermal processing aExplor. (2014), http://dx.doi.org/10.1016/j.gexplo.2014.08.005

An experimental campaign was conducted to establish the proper-ties of the soil and to identify the hydrocarbon pollutants within. Themain physical and chemical properties of the soil samples are presentedin Table 1.

2.2. Soxhlet extraction of soil contaminants

The Soxhlet extraction method has the advantage of formingemulsionswithmore rigorous solventmixtures, for an in-depth analysisof soil/waste mixtures. The analytical method designed to determinethe polycyclic aromatic hydrocarbons concentrations was carried outaccording to the current standard methods in force (EPA Method3540:1996), applicable to both soil and solid waste, considering ashes(soils decontaminated by pyrolysis) as hazardous solid waste in termsof environmental quality protection. Before being weighed, the solidsamples passed through a 2-mm opening sieve. Then the sampleswere mixed with anhydrous sodium sulfate, placed in an extractionthimble and extracted using the solvent in a Soxhlet extractor.Sample extractions were carried out by Soxhlet method applied byusing equipment with 6 benches. Approximately 20 g portions of eachcontaminated soil sample were extracted with 250 ml of HPLC gradepetroleum ether solvent. The extract was concentrated to a smallsolvent volume and eluted with hexane using Heidolph rotary evapora-tor. After the concentration step, the samples were cleaned-up, ifnecessary, or they were transferred to a capped and sealed vial forgas-chromatographic analysis. For the analysis of PAHs, each samplewas separately extracted three times and the results are presented asan average of these replicates. In case of the emissions generated bythe thermal treatments, the extractions of PAHs compounds collectedon quartz fiber filters and polyurethane foam filters (PUF) wereperformed by Soxhlet extractor using HPLC grade petroleum ethersolvent. The extraction procedure acquired a time interval that rangedbetween 8 and 10 h and it has the same steps as comparedwith extrac-tion of polycyclic aromatic hydrocarbons from the solid samples.

2.3. Gas chromatography–mass spectrometry (GC/MS) analysis

The qualitative and quantitative analyses were performed by aShimadzu GCMS-QP2010_Plus system gas chromatograph with a massspectrometer detector. Polycyclic aromatic hydrocarbons compoundswere identified in samples by a combination of retention timeand mass spectral match against the calibration standards (AgilentTechnologies, 2008 and 2009). Specific parameters, as resulted fromstandard analyses and individual calibration curves for each of 16 stud-ied compounds (naphthalene, acenaphthylene, acenaphthene, fluorene,phenanthrene, anthracene, fluoranthene, pyrene, benzo(a)anthracene,chrysene, benzo(b)fluoranthene, benzo(k)fluoranthene, benzo(a)pyrene, dibenzo(a,h)anthracene, indeno(1,2,3-c,d)pyrene, benzo(g,h,i)perylene) are presented in Table 2. Calibration curves were obtainedusing a series of standard solutions, prepared by diluting the standardmix (16 PAHs mix from Supelco) with hexane until it reached thedesired concentrations (1; 2; 3 şi 4 ng/μl). Qualitative and quantitativeanalyses of PAHs were conducted according to the standard procedure



Table 2Calibration parameters for polycyclic aromatic hydrocarbons.

PAHs compound Retention time(min)

Quantitative ions(m/z)

Regressioncoefficient

RSD(%)

Anthracene 11.362 178, 176 0.9994259 10.280Benzo(a)anthracene 17.414 228, 114 0.9991368 4.473Benzo(a)pyrene 20.520 252, 250, 126 0.9994745 10.661Benzo(b)fluoranthene 19.864 252, 250, 126 0.9988305 10.532Benzo(k)fluoranthene 19.914 252, 126, 250 0.9999354 3.051Benzo(g,h,i)perylene 23.109 276, 138, 274 0.9995586 12.449Chrysene 17.508 228, 226 0.9987947 5.710Fluoranthene 14.009 202, 101 0.9990474 4.531Indeno(1,2,3-c,d)pyrene 22.708 278, 139, 279 0.9983621 10.242Naphthalene 5.592 128, 127 0.9993591 17.650Phenanthrene 11.263 178, 176 0.9997264 12.310Pyrene 14.516 202, 101 0.9994361 2.772Acenaphthylene 7.926 152, 151 0.9973159 15.830Acenaphthene 8.265 153, 154 0.9995695 16.26Fluorene 9.238 166, 165 0.9999842 15.120Dibenzo(a,h)anthracene 22.654 276, 138, 274 0.9993278 5.386

3C. Bulmău et al. / Journal of Geochemical Exploration xxx (2014) xxx–xxx

(SR EN 14039:2005, EPA 8270:1998; EPA Method 3540:1996), applica-ble to both soil and solidwaste, considering ashes resulted from thermaltreatment of the contaminated soils as hazardous solidwaste in terms ofenvironmental quality protection.

The parameters used for the analytical analysis performed byShimadzu GCMS-QP2010_Plus system are: chromatographic capillarycolumn QPLOT 5MS (cross-linked 5% phenyl-methyl-silicone) of30 m × 0.25 mm × 0.25 μ; interface temperature: 280 °C, injectortemperature: 300 °C; split/split less injection system; ion sourcetemperature: 200 °C; oven temperature program: 50 °C (1 min) upto 240 °C by a rate of 25 °C/min; up to 320 °C by a rate of 10 °C/min; 320 °C (2.40 min); carrier gas— helium (99.99995%); gas pres-sure: 88.2 kPa; linear velocity: 44.4 cm·s−1; purge flow: 30 ml/min;flow rate: 1.5 ml/min; total flow: 39.0 ml/min.

2.4. Experimental installation and thermal treatment

Thermal experiments applied for soil decontamination wereperformed using a laboratory scale experimental set-up available inthe Renewable Source Laboratory from the Power Engineering Faculty,Polytechnic University of Bucharest. Themain device of the experimen-tal set-up consists of a horizontal tubular fixed batch reactorNABERTHERM, type RO 60/750/13 (Fig. 1). In order to investigate thePAHs emissions generated by the thermal treatment, an isokinetic

Fig. 1. Tubular ele


automatic portable sampling system Isostack Basic HV (Fig. 2) wasattached to the gas outlet of the tubular reactor.

The pyrolysis tubular reactor is external electrically heated byresistances and it represents flexible equipment enabling numerousthermal processes of the solid wastes. Different treatment atmospherecould be assured in the thermal reactor depending on the thermo-chemical process applied to the solid material (Mărculescu andStan, 2012): an oxidant atmosphere (incineration and gasification) ora reductive atmosphere (pyrolysis). The furnace is equipped withautomatic integrated control for heating. The active zone, the heatedone, measures about 750 mm and the maximum process temperaturecan rise to approximately 1300 °C. The pyrolysis reactor is working ina discontinuous mode; so small samples (up to 600 g) of soil contami-nated with petroleum products were separately heated. A flow ofnitrogen (0.1–0.2 l/min) was purged into the reactor in order to ensurean inert atmosphere throughout the thermal treatment. Soil sampleswere introduced into the furnace by a crucible of refractory steelW4541 with tubular parallelepiped-shaped (size: 100 cm long, 4 cmwide and 3 cm in height) and they were heated from ambient temper-ature to the maximum pyrolysis process temperature. The soil issubjected to themaximumheating rate determined by the difference be-tween the ambient and the process temperature, 10 °C/min–20 °C/min.By this procedure the industrial treatment conditions are preserved: theproduct is introduced into the reactor when this reaches the operationtemperature. There is no control of the heating rate inside an industrialreactor because it is operated continuously. The heating rate of a stan-dardpyrolysis process applied to a solid organic product usually inducesdifferences on the pyrolysis products distribution and properties.Whenapplied to hydrocarbon contaminated soils, we expect this influence tobe minimum and related only to hydrocarbons vaporization rate. Thisresearch did not focus on such impacts. Further experiments are inprogress.

In order to decontaminate the polluted soil, three different temper-atures at 350 °C, 500 °C and 650 °C were assured into the reactor. Thetreatment time was 30 min and 60 min.

The laboratory scale experiments were conducted under similarpyrolysis conditions, summarized in Table 3.

2.5. Flue gas analysis

Flue gas released from the process was sampled using an isokineticautomatic portable sampling system of particulate emissions (ISO9096, EN 13284-1) — Isostack basic HV from Tecora (Fig. 2) having aheated inlet probe for emissions sampling, temperature measuringsensors, and gas inlet flow and volume monitoring module. Particle

ctric reactor.



Pump of the sampling system

Flue gases sampling

Cooling systemISOFROST 2

Trap for condensation and adsorption MCS 2

Pyrolysis tubular reactor

Gases outlet

Tar outlet

Fig. 2. Schematic presentation of the automatic isokinetic sampling system.


and gas phases were iso-kinetically sampled on quartz fiber filters,respectively on polyurethane foam filters. The system measured auto-matically the flow of the pyrolysis flue gases and registered the gasestemperature, the pressure and the total sampled volume. To ensuretheminimum gas flow rate required by the Isokinetic sampling system,the reactor was continuously purged with nitrogen. This procedure alsoensures a constant, non-turbulent gasmovement inside the reactor andan efficient gaseous phase exhaust. Taking into account that PAHsvolatilization and degradation have different rates and the flue gasescomposition varies in time during the pyrolysis treatment, the time ofthe sampling was correlated with the sampling flow in order to assurethe representative recommended sample volume. The sampled volume(normalized under standard conditions: 20 °C temperature and 1 atmpressure) was used to calculate the pollutant concentration levelas a ratio from the total amount captured during the sampling andmeasured by GC technique and the total normal sampled volume ofemission, according to standards (SR ISO 12884:2008, SR EN15549:2009). Moreover, it must be specified that the sampling of theflue gases was made before they underwent treatment by flue gascleaning system.

9.01

Participation of PAHs compounds [%]

pyrene benzo(a)anthracene benzo(a)pyrene others PAHs

3. Results and discussions

3.1. Pollutant content in samples before treatment

In the laboratory experimental study we considered pyrene,benzo(a)anthracene, benzo(a)pyrene and total PAHs as indicatorsbecause they represent the most common PAHs, especially in PAHmixtures (according to ATSDR — Agency for Toxic Substances andDisease Registry), with important carcinogenic potential. Benzo(a)pyrene and benzo(a)anthracene occupy the top two positions in PAHslisted in the Second Annual Report on Carcinogens from 1981 (NTP2011). Another reason for our opinion is that the highest concentrationsof these compoundswere identified as compared to other hydrocarbons

Table 3Pyrolysis process conditions.

Process parameters Value

Weight of the soil sample (g) 600Process temperature (°C) 350–650Process time (min) 30–60Nitrogen flow (l/min) 0.1–0.2


present in the initial contaminated soil. Their proportions of the totalPAHs determined in the soil are illustrated in Fig. 3.

Consequently, the pyrolysis experiments were conducted to assessthe influence of the thermal treatments operational conditions onremoval of pyrene, benzo(a)anthracene, benzo(a)pyrene and totalPAHs from contaminated soils.

3.2. Pollutant content in sample after thermal treatment

The results achieved from non-oxidant thermal treatments appliedto the petroleum contaminated soil refer to the concentrations ofpyrene, benzo(a)anthracene, benzo(a)pyrene and total PAHs (all 16EPA's priority pollutant polycyclic aromatic hydrocarbons) from initialcontaminated soils, pyrolysis ash samples (decontaminated soils) andpyrolysis emissions. Decontaminated soils were obtained as solidresidues produced by thermal treatments applied to the contaminatedsoil at three different process temperatures.

Table 4 summarizes the concentrations of several organic pollutantcompounds [mg/kgdw] from the PAHs class identified using bothSoxhlet extraction and GC–MS analysis in the initial soil and in theremediated soil.

3.3. PAHs concentration in decontaminated soil

Aiming to investigate the influence of the temperature and time ofthe thermal remediation on pollutants degradation and their remainingconcentrations in the decontaminated soils generated by the non-

26.02

5.02

59.96

Fig. 3. Proportion of pyrene, B(a)P, and B(a)A from the total PAHs identified in the soil.



Table 4Concentrations of the PAHs compounds (mg/kgdw) in the initial soil and in the decontaminated soil, obtained at different process temperatures for 30 min.

Pollutant Concentration from initial soil Decontaminated soil (T = 350 °C) Decontaminated soil (T = 500 °C) Decontaminated soil (T = 650 °C)

Benzo(a)anthracene 0.257 0.105 0.089 0.020Benzo(a)pyrene 0.050 0.020 0.010 0.002Pyrene 0.089 0.066 0.056 0.014Total PAHs 0.989 0.938 0.830 0.201


oxidant thermal-treatment, the concentration values of the analyzedhydrocarbons at all temperatures applied are discussed below.

The concentration evolution of the three PAHs compounds depend-ing on temperature is presented in Fig. 4 and Fig. 5 for the pyrolysistreatment applied for two treatment times: 30 min (Fig. 4) and60 min (Fig. 5). Consequently, Fig. 4 illustrates the variation of pyrene,benzo(a)anthracene and benzo(a)pyrene concentrations remaining inthe soil treated at temperatures in range of 350 °C–650 °C for 30 mintreatment period. As shown in the graph, the increase of the processtemperature from 350 °C to 500 °C has no effect on B(a)P removalfrom the soil, the value of its concentration being the same. Only theconcentration of pyrene and benzo(a)anthracene proved a slightreduction of about 16% between the value determined in the ashgenerated at 350 °C and the decontaminated soil obtained at 500 °C.The figure highlights that the extent of pyrene removal increaseswith temperature increase and reaches completion after 500 °C.Consequently the pyrene removal by pyrolysis treatment cannot becompleted before 500 °C. This tendencywas also observed in the exper-iments performed by Saito et al. (1998). When heating the contaminat-ed soil sample at 650 °C the concentrations of these hydrocarbonsachieve the highest decrease rate as compared to the initial value: forpyrene (84%), for B(a)A (93%) and for benzo(a)pyrene (96%). The highlevel of soil decontamination at 650 °C could be due to these polycyclicaromatic hydrocarbons or to their reaction products that enhance thesoil volatilization if treatment temperature reaches a certain value thatin our experiments seems to be over 500 °C (Saito et al., 1998).

Fig. 4 presents the variation of PAHs residual concentration functionof pyrolysis temperature and the duration of 60 min treatment time.Trends of the illustrated curves indicate that once the pyrolysis temper-ature increases from 350 °C to 650 °C a considerable decrease of B(a)Aconcentration from decontaminated soils (38%) occurs (initial concen-tration = 0.257 mg/kgdw). Another evolution for the rate of the pollut-ant removal is determined for pyrene and B(a)P. The experimentalresults demonstrated that it is still difficult to eliminate these hydrocar-bons from contaminated soil in case of lower temperatures even for alonger treatment period. Only at 650 °C the treated soil analysisrevealed slight traces of pyrene (0.002 mg/kgdw). These values foldwith results obtained in the pyrolysis experiments performed by

0.00

0.02

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300 400 500 600 700Co

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mp

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sfr

om

th

e d

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nta

min

ated

so

il [m

g/k

gd

w]

Treatment temperature [°C]

t = 30min

Pyrene

B(a)A

B(a)P

Fig. 4. Concentration of pyrene, B(a)P and B(a)A from the decontaminated soil function ofthe treatment temperature for a pyrolysis applied for 30 min.


Richter et al. (2000); their tests demonstrated that only one tempera-ture, 750 °C, cleaned the soil of pyrene. In addition to this previousexperiments that we conducted for non-oxidant thermal treatmentapplied to a different contaminated soil also revealed that low temper-atures did not induce a substantial removal of benzo(a)pyrene fromthe contaminated soil (Bulmău et al., 2012b). A modest decrease ofB(a)P was noticed when the residence time was increased. Similarstudies on the influence of the heating time on benzo(a)pyreneand benzo(a)anthracene demonstrated the same effect (Chen andChen, 2001).

As shown in Fig. 5 a higher temperature of the thermal remediation(more than 500 °C) favors total PAHs removal from the polluted soil. Ifthe process temperature increases from 350 °C to 500 °C only 10% oftotal PAHs are released in the pyrolysis flue gases. The major amountof PAHs is released if soil is heated at a temperature in range of500 °C–650 °C. This is due to the fact that most PAHs (US EPA's 16priority-pollutant PAHs) have the boiling point above 300 °C. Fig. 6proves that this tendency remains valid even if the time of the thermaldecontamination is 30 min, or 60 min. The experimental resultspresented above (Figs. 4 and 5) revealed that pyrolysis thermal treat-ment removes more easily B(a)A as compared to B(a)P and pyrene.

3.4. Efficiency of the pyrolysis treatment in removal of polycyclic aromatichydrocarbons

Based on the experimental results we determined the efficiency ofthe thermal remediation applied to the analyzed soil. The efficiencyof the process was calculated through Eq. (1) considering the con-centration of the pollutant from the contaminated soil (initial pollutantconcentration) and the concentration of the pollutant from thedecontaminated soil (final pollutant concentration).

ε ¼ initial pollutant concentration−final pollutant concentrationinitial pollutant concentration

� 100 %½ �

ð1Þ

0.00

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inat

ed s

oil

[mg

/kg

dw

]


t = 60 min

PyreneB(a)AB(a)P

Fig. 5. Concentration of pyrene, B(a)P and B(a)A from decontaminated soil function of thetreatment temperature for a pyrolysis applied for 60 min.



0.00

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tal P

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s fr

om

dec

on

tam

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oil

[mg

/kg

dw

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30min60min

Fig. 6. Concentration of total PAHs from decontaminated soil function of the pyrolysistemperature and time.

0

20

40

60

80

100

350 500 650

Eff

icie

ncy

of

the

pyr

oly

sis

trea

tmen

t [%

]

Process temperature [°C]

Removal of benzo(a)anthracene

30min60min

Fig. 8. Variation of the pyrolysis efficiency in benzo(a)anthracene removal.


With the purpose to evaluate the efficiency of pyrolysis in removal ofeach PAHs compound from soils, we compared the results generated bythermal treatments performed at three temperatures (350 °C, 500 °Cand 650 °C) firstly for 30 min, and secondly for 60 min. These arepresented below.

The experimental results demonstrated that the removal of pyrenefrom the contaminated soil matrix could reach an efficiency of about84% if pyrolysis is performed at 650 °C. The results also indicated animportant increase of pyrolysis efficiency in removal of pyrene fromthe polluted soil matrix if process temperature increases from 350 °Cto 650 °C in case of the shorter treatment time (58%), and for the longerinterval respectively (45%).

Fig. 7 reflects that the removal of B(a)P from the soil is enabled bythermal treatment conducted at 650 °C, especially in case of the soilsample remediated for 30 min. The same figure shows that pyrolysisperformed for all three temperatures has the same efficiency, nomatterhow long the treatment time is (60% for 350 °C, 80% for 500 °C and 96%for 650 °C). So, in terms of energy consumption for the pyrolysis processand for cleaning B(a)P from the soil we can conclude that the treatmentshould be performed only for 30min. The variation of pyrolysis efficien-cy as function of process temperature and time demonstrates thatthe optimum pyrolysis temperature for total PAHs removal fromcontaminated soil is 650 °C.

In case of benzo(a)anthracene removal from contaminated soil bypyrolysis treatment the conclusions resulted from Fig. 8 are clearer.Both process parameters have the same influence; once they increase,pyrolysis efficiency reaches higher values (from about 35% for 350 °Cto 95% for 650 °C).

As compared to the initial total PAHs concentration determinedin the initial soil, the experimental results indicate an importantdecrease of the total PAHs concentration in the decontaminated soilsample if the process temperature increases. Fig. 9 clearly reflects thatthe optimum pyrolysis temperature for total PAHs removal from

0

20

40

60

80

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350 500 650

Eff

icie

ncy

of

the

pyr

oly

sis

trea

tmen

t [%

]


Removal of benzo(a)pyrene

30min60min

Fig. 7. Variation of pyrolysis efficiency in benzo(a)pyrene removal.


contaminated soil is higher than 500 °C (Bulmău et al., 2013; Richteret al., 2000). For a temperature of 650 °C applied to the soil sample for30 min the efficiency of the process reaches 80%, while for 60 minalmost 96% is achieved.

The results of the experimental study demonstrate the fundamentalthermodynamic principle of the temperature effect: high temperatureswill conduct to a high rate of PAHs degradation. That is due to theincrease of the energy content that helps break the bonds from thepolycyclic aromatic hydrocarbons molecules (Pakpahan et al., 2009;Sun, 2004).

3.5. PAHs concentration from the pyrolysis flue gases

For the assessment of the impact generated by the thermal treat-ment, the monitoring of the contaminants concentration from thepyrolysis flue gas is very important. In order to investigate in moredetail the temperature influence on the evolution of pyrene, B(a)P,B(a)A and total PAHs concentration from the pyrolysis emissions anisokinetic sampling using Isostack basic HV was carried out on the fluegases released from the tubular reactor during the thermal process.Gases collected on a PUF from the gas sampling and the condensedwere analyzed by Soxhlet extraction and gas chromatography. Conse-quently, concentrations of B(a)P and PAHs were determined for eachexperiment. Next figures illustrate the evolution of the analyzed PAHsconcentrations from pyrolysis emission generated during every singleexperiment.

All treatment temperatures generated flue gases wherein eachchemical species analyzed in the present experimental study wasidentified. These organic compounds were detected in low concentra-tions from pyrolysis flue gases when the contaminated soil was heatedat 350 °C and at 500 °C, and a higher concentration for 650 °C (Figs. 10and 11) in case of two treatment times. From these figures it is obviousthat increasing the pyrolysis temperature from 500 °C to 650 °C, thequantity of each PAHs species from the pyrolysis flue gases increases

0

20

40

60

80

100

350 500 650

Eff

icie

ncy

of

the

pyr

oly

sis

trea

tmen

t [%

]


Removal of total PAHs

30min60min

Fig. 9. Variation of pyrolysis efficiency in total PAHs removal.



0.00

0.02

0.04

0.06

0.08

0.10

300 400 500 600 700Co

nce

ntr

atio

n o

f th

e P

AH

s co

mp

ou

nd

sfr

om

dec

on

tam

inat

ed s

oil

[µg

/m3]


t = 30 min

PyreneB(a)AB(a)P

Fig. 10. Concentration level of pyrene, B(a)P and B(a)A from the flue gases of the pyrolysisapplied for 30 min to the contaminated soil.

0.00

0.50

1.00

1.50

2.00

2.50

3.00

3.50

4.00

4.50

5.00

300 400 500 600 700

Co

nce

ntr

atio

n o

f th

e to

tal P

AH

s fr

om

th

ep

yro

lisis

flu

e g

ases

[µ

g/m

3]


30min

60min

Fig. 12. Concentration level of total PAHs from the pyrolysis flue gases.


significantly. The time of the thermal remediation influences consider-ably the amounts of all PAHs compounds, especially in case of pyreneand benzo(a)anthracene.

Fig. 12 illustrates that, in pyrolysis processes at 500 °C and 650 °C, ahigh concentration of total PAHs removal can be obtained. For example,if the process temperature is rising from 350 °C to 650 °C the percent-age of total PAHs concentration increases from 1.502 to 4.832 in caseof the pyrolysis applied for 30 min, and from 1.615 to 4.111 in case ofsoil decontaminated for 60 min. So, the concentration of each PAHscompound determined in the pyrolysis flue gases strongly depends onthe process temperature. Thus their formation, retention or presencein the gases phases must be studied in future research. Taking intoaccount that where the higher efficiencies were obtained, higherconcentrations of pyrene, B(a)P, BA and total PAHs were determinedin the flue gas, it is necessary to pay special attention to the flue gascleaning system in order to minimize the impact on the environmentand on human health.

4. Conclusions

The experimental results revealed that pyrene, benzo(a)pyrene,benzo(a)anthracene and total PAHs can be removed almost completelyfrom the soil polluted with petroleum products using non-oxidantthermal processing.

From the results it can be concluded that the PAHs concentrationlevel from the pyrolysis products is influenced by the process condi-tions. The removal of each PAH from soil is more dependent on

0.00

0.02

0.04

0.06

0.08

0.10

0.12

0.14

0.16

0.18

300 400 500 600 700

Co

nce

ntr

atio

ns

of

the

PA

Hs

com

po

un

ds

fro

m t

he

dec

on

tam

inat

ed s

oil

[µg

/m3]


t = 60 min PyreneB(a)AB(a)P

Fig. 11. Concentration level of pyrene, B(a)P and B(a)A from the flue gases of the pyrolysisapplied to the contaminated soil for 60 min.


temperature and less on treatment time. High temperatures conductto a high rate of PAHs degradation. Irrespective of the PHAs species,the trend is the same: once the pyrolysis temperature increases the or-ganic pollutant concentration from decontaminated soil decreases,while their amounts in the flue gases becomemore abundant. With re-spect to the thermal process temperature, our results demonstratedthat pyrolysis is an efficient process for removal of the analyzed PAHsfrom soil. In case of 650 °C the thermal process efficiency is more than80% if the treatment is applied for 30 min and it is above 90% if thermaltreatment is conducted for 60 min. Considering the PAHs compounds,the experiments showed that it is more difficult to remove pyrenefrom the petroleum contaminated soil as compared to benzo(a)pyrene,benzo(a)anthracene or total PAHs.

Concerning the PAHs from the pyrolysis flue gases, the resultsrevealed that the concentration level of each organic compounddepends a lot on the process temperature. Hence their formation,retention or presence in the gas phases must be studied in futureresearch. Furthermore, additional studies must be done in the field ofthe flue gas cleaning system generating a reduced impact on theenvironment and human health. From the economic point of view,future research related to the correlation between the process parame-ters and the energy consumption must be considered.

Acknowledgments

The authorswould like to thank the UEFISCDI - Executive Agency forHigher Education, Research, Development and Innovation Funding,PN II — CAPACITIES (Module III) — Bilateral Cooperation Romania–China, SOTREAT, Contract no. 614/01.01.2013.

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analysis of thermal processing applied to contaminated soil for organic pollutants removal

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