Membrane-assisted removal of hydrocarbons fromcontaminated soils * laboratory test results

Wojciech Kujawski*, Izabela Koter, Stanisław Koter

Faculty of Chemistry, Nicolaus Copernicus University,ul. Gagarina 7, 87-100 Toru�n, Poland

Tel. +48 56 611 43 15; Fax +48 56 654 24 77; email: kujawski@chem.uni.torun.pl

Received 27 August 2007; revised 4 February 2008; accepted 11 February 2008

Abstract

The effectiveness of soil remediation process, consisting of the soil washing, microfiltration and pervaporationprocess to remove iso-octane (VOC) was investigated.

The several different parameters influencing the washing process were investigated: type of the soil (sand andsandy clay), time of the washing process as well as an addition of surfactants to the washing solution. The MF-enhanced soil washing allowed for the removal of iso-octane from the contaminated soil. Efficiency of iso-octaneremoval depended on the type of soil and was worse for the sandy clay, because of its higher stickiness andplasticity. The addition of a surfactant enhanced iso-octane removal; however, its optimal concentration inwashing solution should not be higher than 0.1�0.3wt.%. A substantial excess of a washing solution to the soil (atleast 100:1) was needed to obtain a good removal of volatile organic compounds (VOCs) from contaminated soil.

Iso-octane extracted from the contaminated soil was subsequently removed by pervaporation process withseveral types of hydrophobic membranes. Prior to pervaporation process, the washing solutions underwentmicrofiltration to remove all suspended particles from the feed. The best pervaporative properties were found forthe PDMS-PAN membrane. An addition of the surfactant increased the solubility of iso-octane in water during thewashing step of remediation process but simultaneously dramatically decreased the efficiency of the final removalof VOCs from water by pervaporation.

Keywords: Soil remediation; Microfiltration; Pervaporation; Iso-octane

Presented at the Third Membrane Science and Technology Conference of Visegrad Countries (PERMEA), Siofok,Hungary, 2–6 September 2007.

*Corresponding author.

Desalination 241 (2009) 218�226

0011-9164/09/$– See front matter # 2008 Elsevier B.V. All rights reserved.doi:10.1016/j.desal. 00 .0 .02 8 2 32

1. Introduction

The problem of soil and groundwater pollu-tion was widely recognized in recent years [1�5].The major organic chemical waste categoriesinclude organic aqueous waste (e.g. pesticides),organic liquids (e.g. chlorinated solvents), oils(e.g. different fuels and fuel additives) andsludges or solids containing organic compounds.

The most common local source of soil andwater contamination by petroleum hydrocarbonsare industrial plants, land disposal sites of dangerresidues, petrol stations, car service stations andvehicle accidents. The total petroleum hydro-carbons include saturated alkanes, aromatichydrocarbons, fuel oxygenated additives (e.g.methyl t-butyl ether * MTBE, ethanol, buta-nol) and other compounds containing sulfur ornitrogen. These compounds are harmful or eventoxic to the growth and development of plantsand animals, being a source of long-term waterand air pollution. They are also dangerous to thehuman health.

There are several methods of soil remediationused for the clean-up of contaminated soils:pulsed air sparging, ozonation, natural attenua-tion, bioremediation, incineration, off-site dis-posal, soil flushing, soil washing or soil vaporextraction [1�10]. The transfer of an organiccontamination from soil into aqueous phase isthe most often applied remediation technique.Different types of surfactants and biosurfactantsare also applied to enhance the extraction ofVOCs from soil [11�14]. The washing solutionhas to be further treated to remove VOCs,surfactants and soil fine particles, for instance,by the adsorption on carbon. Membrane separa-tion techniques offer an alternative and veryefficient method for the removal of contaminantsfrom aqueous phase [15�20].

In this work we presented results concerningthe membrane assisted removal of iso-octane(petroleum hydrocarbon) from two types of soil(sand and sandy clay soil). Microfiltration and

hydrophobic pervaporation were applied asmembrane separation techniques. The enhance-ment of soil washing by ionic and nonionicsurfactants was also discussed.

Microfiltration (MF) is a pressure drivenfiltration process that uses porous membranesto separate suspended particles with diametersbetween 0.1 and 10mm. MF separates the feedstream into two fractions on the basis ofparticulate size. Membrane permits smaller spe-cies to pass through a membrane while largerones are retained [21].

Pervaporation (PV) is recognized as a separa-tion process in which a binary or multicompo-nent liquid mixture is separated by its partialvaporization through a dense membrane. Duringhydrophobic pervaporation, the feed mixture isin direct contact with one side of the organo-philic membrane whereas permeate enriched inthe organic component is removed in a vaporstate from the opposite side into a vacuum andthen condensed [22].

2. Experimental

2.1. Soil remediation by washing

Two types of different texture soil werechosen for investigations * sand soil and sandyclay soil. Sand soils are coarse in texture andconsist of particles between 50 mm and 2mmdiameter. Sand is formed of loose grains, whichare not coherent when wet. Clay soil is plastic,consisting mainly of hydrous silicate of alumi-num. At the microscopic level, clay is composedof fine particles (diameterB/2 mm), adheringeasily to one another. The sandy clay soil wascomposed of clay particles (up to 55wt.%) andsand particles (up to 45wt.%) with a smallamount of silt [23]. Samples of soils werecontaminated with iso-octane (5�20wt.%) andthen the washing process was applied for 0.5�4 h, using the water to soil ratio in the range from5:1 to 200:1. Additionally, the influence of the

W. Kujawski et al. / Desalination 241 (2009) 218�226 219

surfactant addition on the remediation efficiencywas also measured. The following surfactantswere used: sodium dodecyl sulfate * NaDS(C12H25SO4Na) and Triton TX-100 (C14H22O(C2H4O)n , n�/9,5). The amount of iso-octaneremaining in the soil after washing process wasdetermined by using a head-space technique andVarian 3800 gas chromatograph (equipped withFID detector).

2.2. Microfiltration

Prior to pervaporation process, microfiltrationcan be applied to remove suspended fine soilparticulates from the washing solution (water orwater with surfactant addition). The scheme ofthe experimental setup for the MF aided soilwashing system is presented in Fig. 1. Polypro-pylene filters (PALL Star Profile) of the averagepore diameter 1 and 5 mm were used. The feedwas recirculated while the permeate was col-lected on the other side of the membrane. Theeffectiveness of the filtration was estimated bydetermining the permeate turbidity.

2.3. Pervaporation

Pervaporation experiments were carried outin the laboratory-scale pervaporation system

presented in Fig. 2. The system was composedof a temperature controlled feed vessel. Feedsolution was circulating over the membrane. Thepermeate was collected in cold traps cooled byliquid nitrogen. During experiments the upstreampressure was maintained at the atmosphericpressure, while the downstream pressure waskept below 0.1 hPa (1mbar) by using a vacuumpump. The permeate flux was determined byweight, whereas the feed and permeate composi-tions were determined chromatographically (Var-ian 3300 gas chromatograph, equipped with TCDdetector).

Performance properties of a given pervapora-tion membrane were defined by the separationfactor a (Eq. (1)) and permeate fluxes J [22].

�VOC=water ¼ðxVOC=xwaterÞPERMEATE

ðxVOC=xwaterÞFEEDð1Þ

where x denotes the weight ratio of VOC andwater in the binary permeate and feed mixture.

The trade-off between selectivity and flux is awell-known phenomenon in pervaporation, so tocompare the efficiency of a given membrane inthe VOCs removal from water the pervaporationseparation index (PSI) was additionally used.PSI was calculated according to the followingequation:

PSI ¼ J�ðgm�2 h�1Þ ð2ÞFour types of hydrophobic membranes were usedin this study: PERVAP-1060, PERVAP-1070

Fig. 1. The scheme of microfiltration-enhanced soilwashing process.

FEED/RETENTATE

PERMEATE

FEEDMEMBRANE

VACUUM

Thermostatedfeed tank

Pervaporationcell

Recirculationpump

Coldfingers

Fig. 2. Scheme of pervaporation experimental set-up.

220 W. Kujawski et al. / Desalination 241 (2009) 218�226

(Sulzer, Germany), PDMS-PAN (Pervatech,the Netherlands) and PEBA-4033 (GKSS,Germany) * Table 1. The PERVAP-1060 andPDMS-PAN membranes possessed a selectivelayer made of polydimethylsiloxane (PDMS).The PERVAP-1070membrane was composed ofthe thin PDMS layer with hydrophobic ZSM-5zeolite particles incorporated into it to increaseselectivity. The selective top layer of PEBA-4033membrane was made of the polyether�polyamide block copolymer (PEBA). PEBA is athermoplastic elastomer which consists of rigidpolyamide linear blocks and flexible polyetherlinear blocks.

The effect of feed composition on pervapora-tion flux and selectivity was determined for eachmembrane in contact with water�iso-octane (0�0.6wt.%) and water�iso-octane�surfactant (0�1.5wt.% iso-octane, 0.3wt.% of Triton TX-100)mixtures. PV measurements were made at 323K.

3. Results and discussion

3.1. Soil remediation by washing

The influence of the water to soil ratio on theremediation efficiency is presented in Fig. 3. Itwas found that increasing this ratio over 100 had

no significant effect on the effectiveness of 2 hwashing. That fact results probable from thesurface adsorption of hydrocarbon molecules onthe soil particulates. Thus, all further experi-ments were performed at the aqueous phase tosoil ratio equal to 100.

It was also found that the efficiency ofremediation depended on the type of the soil andit was much higher for the sand than for the sandyclay soil (Fig. 4). Depending on the washing timefor the soils contaminated by 5wt.% of iso-octane, the VOC removal was up to 90% in thecase of sand and no more than 80% in the case of

Table 1Characteristics of pervaporation membranes investigated

Membrane Material of selective layer Producer Thickness of skin layer (mm)

PERVAP-1060 PDMDSa SULZER Chemtech (Germany) 8PERVAP-1070 PDMSa filled with ZSM-5b zeolite SULZER Chemtech (Germany) 10PDMS-PAN PDMSa Pervatech (The Netherlands) 2PEBA-4033 PEBAc GKSS (Germany) 80

aPDMS*polydimethylsiloxane

CH3

CH3

Si O

n

0B@

1CA

bZSM-5 zeolite (/Nan½AlnSið96�nÞO192� � 16H2O; n< 27)

cPEBA*poly(ether block amide)CCHO PA PEO

OO

O H

n

0@

1A

0

20

40

60

80

100

Water/soil weight ratio

Iso-

octa

ne r

emov

al (

%)

0 20015010050

Fig. 3. Iso-octane removal (%) from sandy soil vs.washing water to soil ratio (initial soil contamination5wt.%, washing time 2 h).

W. Kujawski et al. / Desalination 241 (2009) 218�226 221

sandy clay soil. This difference can be explainedby much looser structure of the soil particulates inthe sandy soil and much higher stickiness andplasticity of clay [23]. It was observed that thewashing time over 2 h had only a moderateinfluence on the iso-octane removal.

Fig. 5 presents the results of remediationefficiency for soils contaminated to the differentlevel. It was found that for the same remediationconditions, the VOC residue in the soil ispractically proportional to its initial level. Theremaining amount of contaminant can be furtherreduced by using bioremediation [10].

As it could be expected, the addition of smallamount of a surfactant resulted in the increasedremoval of the iso-octane from the soil [12�14].This was observed for both ionic (NaDS) andnonionic (Triton TX-100) surfactants (Fig. 6).

Fig. 7 presents the influence of the surfactantconcentration in the washing solution on theremediation efficiency. It was found that the

70

80

90

100

0 0.1 0.2 0.3 0.4 0.5

Surfactant concentration (%)

Iso-

octa

ne r

emov

al (

%)

Sand Sandy clay

Fig. 7. Iso-octane removal from contaminated soils vs.Triton TX-100 concentration (initial soil contamination5wt.%, washing time 2 h).

0

10

20

30

40

50

0Soil contamination (% w/w)

Res

idua

l am

ount

of i

so-o

ctan

e in

the

soil

afte

r w

ashi

ng (

mg/

g of

soi

l)

Sand Sandy clay

252015105

Fig. 5. Influence of the soil contamination level on theamount of iso-octane remaining in the soil after 2 hwashing.

78.5

82.5

87.1

97.8

91.6 93.5

70

75

80

85

90

95

100

Water Sodium dodecylsulfate

Triton TX-100

Iso-

octa

ne r

emov

al (

%)

Sand Sandy clay

Fig. 6. Iso-octane removal from different soils bywashing with water and water containing surfactants(initial soil contamination 5wt.%, concentration of thesurfactant 0.1wt.%, washing time 2 h).

40

60

80

100

Washing time (h)

Iso-

octa

ne r

emov

al (

%)

Sandy clay

Sand

0 54321

Fig. 4. Influence of the washing time on the remediationefficiency (various soil types, initial soil contamination5wt.%).

222 W. Kujawski et al. / Desalination 241 (2009) 218�226

optimum concentration of the surfactant additionshould be in the range 0.1�0.3wt.%.

3.2. Microfiltration

The main purpose of the use of microfiltra-tion was to filtrate solid soil particles from thewater after the washing process to avoid clog-ging of the pervaporation membrane used in thefurther step to remove hydrocarbons from water/organic mixture. The microfiltration was per-formed using two polypropylene membranefilters varying in pore size diameter. To avoidthe membrane fouling, the backflush was appliedevery several minutes. The effectiveness of thefiltration was estimated by determination of thepermeate turbidity. The results showed that 1 mmmembrane filters could effectively remove soilparticulates. The permeate contained practicallyno solid particulates (Fig. 8) so it could bedirectly used as a feed for pervaporation.

3.3. Pervaporation

Fig. 9 compares the values of the separationfactor a of investigated membranes in contactwith 0.2wt.% iso-octane solution. It can be seen

that the unfilled PDMSmembranes (Pervap-1060and PDMS-PAN) showed very similar selectiv-ity; however, the highest selectivity was obtainedwith the zeolite filled Pervap-1070membrane.This means that comparing to the unfilledPervap-1060membrane, the hydrophobic ZSM-5 zeolite filling enhanced the flux of iso-octaneand restricted the flux of water. The selectivity ofPEBA membrane was also high (a$/250) whatsuggests that the soft polyether segments showhigh affinity to the hydrocarbon molecules.Similar behavior of the PEBA membrane was

0

50

100

150

200

250

300

350

PV-1070

Sep

arac

tion

fact

or α

PEBAPDMSPV-1060

Selectivity of hydrophobic membranes in contactwith 0.2 wt% aqueous solution

of iso-octane

Fig. 9. Separation factor a of hydrophobic membranesin contact with 0.2wt.% aqueous solution of iso-octane.

0

0.2

0.4

0.6

0.8

1

Tur

bidi

ty (

a.u.

)

Sandy clay Sand

Without filtration 1 µm 5 µm

MF membrane filter

Fig. 8. Pretreatment of the washing solution by micro-filtration * the influence of the membrane porediameter on the permeate turbidity.

0.00

0.05

0.10

0.15

0.20

0.25

0Iso-octane content in the feed (wt%)

Iso-

octa

ne p

erm

eate

flux

(kg

m–2

h–1

)

PV-1070 PV-1060 PDMS-PAN PEBA 4033

0.60.40.2

Fig. 10. Iso-octane permeate flux through hydrophobicmembranes as a function of the feed mixture.

W. Kujawski et al. / Desalination 241 (2009) 218�226 223

found in contact with aqueous phenol solutions[24]. The permeate flux of iso-octane (Fig. 10) isdependent both on the affinity of iso-octane to themembrane material and on the membrane thick-ness. As it was found for different membranesystems, the permeate flux is inversely propor-tional to the membrane thickness (J�/1/d). ThePDMS-PAN membrane was the thinnest one andthe flux of iso-octane through this membrane wasthus the highest (Table 1, Fig. 10). Although theselectivity of PEBAwas high, this membrane wasthe thickest one and the resulted hydrocarbon fluxwas the lowest. The thicknesses of the selectivelayer of the Pervap-1060membrane and thePervap-1070 one were very similar, so the higher

flux of iso-octane observed with the Pervap-1070membrane resulted from the presence of zeolitefilling that enhanced the transport of VOCmolecules [25].

The comparison of the pervaporation separa-tion index (PSI; Fig. 11) for the investigatedmembranes in contact with 0.2wt.% iso-octaneproved that the PDMS-PAN membrane is themost efficient one in the removal of the hydro-carbons from water. Similar results were foundfor the water�ethanol mixture and differentPDMS based membranes [26].

Results of PSI obtained for the system water�iso-octane were compared with PSI for the samesystem with an addition of 0.3wt.% surfactant(Triton TX-100) * Fig. 11. Results showed thedramatic drop of pervaporation efficiency in thislatter case. When membranes contacted iso-octane solution with the surfactant addition, thePSI values were 8�24 times lower than for thepervaporation performed in the absence of TritonTX-100 [16]. It can be explained by the fact thatiso-octane and surfactant molecules formed verystable micelles. These micelles were not trans-ported through the hydrophobic membranes. Thisfact must be taken into account in the practicalapplications of the suggested remediation system.

4. Conclusions

MF-enhanced soil washing allowed for theremoval of hydrocarbons from the contaminatedsoils. Efficiency of iso-octane removal dependedon the type of the soil and was worse for thesandy clay soils. The addition of a surfactantenhanced iso-octane removal; however, its opti-mal concentration in washing solution shouldnot be higher than 0.1�0.3wt.%. A substantialexcess of washing solution to soil (i.e. 100:1)was needed during remediation to obtain a goodremoval of VOCs from the soil.

VOCs extracted from the contaminated soilsby washing process can be subsequently removed

0

20000

40000

60000

80000

100000

Per

vapo

ratio

n S

epar

atio

nIn

dex

PS

I (gm

–2 h

–1)

PV-1070

Without surfactant 0.3 wt% Triton TX-100

PEBAPDMSPV-1060

The influence of surfactant onPSI of hydrophobic membranes

(feed: 0.2 wt% aqueous solution of Iso-octane)

Fig. 11. Comparison of PSI for membranes in contactwith iso-octane�water mixture without any surfactantand in the presence of 0.3wt.% of Triton TX-100.

Water

Soilwashing

MF

Phaseseparator

Organic phase

Aqueous phase

VOCs depletedretentate

VOCs enrichedpermeate

PhaseseparatorPV

Fig. 12. Scheme of MF�PV membrane enhancedintegrated system for the remediation of soils contami-nated with VOCs.

224 W. Kujawski et al. / Desalination 241 (2009) 218�226

by pervaporation with hydrophobic membranes.Prior to pervaporation process, the washingsolutions must undergo microfiltration to removeall suspended particles from the feed. It was foundthat the PDMS-PAN membrane was the mostefficient in pervaporation removal of VOCs fromwater. An addition of the surfactant increased thesolubility of iso-octane in water during thewashing step of remediation process but simulta-neously dramatically decreased the efficiency ofthe final removal of VOCs from water bypervaporation.

This fact indicates that the soil washing step ofthe remediation procedure should be performedwithout any surfactant addition, as its removalfrom the system for further recycling seems to bevery complicated and could create another pollu-tion problem. Fig. 12 presents the layout of thesuggestedMF�PV integrated remediation systemto treat soils contaminated with such VOCs likepetroleum hydrocarbons.

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