treatment of fuel-oil contaminated soils by biodegradable surfactant washing followed by fenton-like...

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Treatment of Fuel-Oil Contaminated Soils by BiodegradableSurfactant Washing Followed by Fenton-Like Oxidation

T. T. Tsai1; C. M. Kao2; Rao Y. Surampalli3; and S. H. Liang4

Abstract: Among petroleum-hydrocarbon pollutants, fuel-oil is more difficult to treat compared to gasoline and diesel fuel. The objec-tives of this bench-scale study were to: �1� develop a two-stage remedial system consisting of surfactant washing followed by Fenton-likeoxidation process to remediate fuel-oil contaminated soils; �2� evaluate the effects of residual surfactant and soil organic matter �SOM� onthe efficiency of Fenton-like oxidation; �3� evaluate the effect of potassium dihydrogen phosphate �KH2PO4� addition on the stability ofH2O2 and oxidation efficiency; and �4� evaluate the possible oxidation products after the oxidation process. In the surfactant washingstage, biodegradable surfactant, Simple Green �SG� �50 g L−1�, was applied to flush fuel-oil contaminated soils with initial totalpetroleum-hydrocarbons �TPHs� concentration of 50,000 mg kg−1. Results show that approximately 90% of TPH could be removed afterwashing with 45 pore volumes �PVs� of SG followed by 25 PVs of deionized water, while the soil TPH concentration dropped from50,000 to 4 ,950 mg kg−1. In the Fenton-like oxidation stage with initial soil TPH concentration was approximately 4 ,950 mg kg−1, TPHremoval efficiency can be significantly increased with increased H2O2 concentrations. Results also reveal that residual SG and SOMwould compete with TPH for oxidants and cause the decrease in oxidation efficiency. An “oxidation-sorption-desorption-oxidation”scheme for soil TPH was observed in this experiment due to the initial sorption of TPH on SOM. Results show that an addition of 2.2 mMof KH2PO4 could increase the stability and half-life of H2O2, but caused the decrease in TPH removal efficiency. The oxidation potentialof Fenton-like process was not capable of completely oxidizing fuel-oil to nontoxic end products. The observed by-products afteroxidation process contained carboxyl groups with molecular weights similar to their parent compounds. Results from this study indicatethat the two-stage remedial system is a promising technology for fuel-oil contaminated soil treatment.

DOI: 10.1061/�ASCE�EE.1943-7870.0000052

CE Database subject headings: Soil pollution; Soil treatment; Fuels; Oils; Petroleum; Biodegradation; Hydrocarbons; Oxidation;Remediation.

Introduction

Soil at many existing and former industrial areas and disposalsites is contaminated by petroleum-hydrocarbons that were re-leased into the environment �Ferguson et al. 2004; Sarkar et al.2005�. Among petroleum-hydrocarbon pollutants, fuel-oil is oneof the most widely used petroleum-hydrocarbon in society today.Fuel-oil is more difficult to treat compared to gasoline and dieseloil due to its characteristics of low volatility, low biodegradabil-ity, and low mobility �Gallego et al. 2001�. Thus, fuel-oil con-taminated soils have been considered a major environmentalproblem �Boopathy 2004; Agence de l’Environnement et de laMaîtrise de l’Energie 2006�.

1Postdoctoral Fellow, Institute of Environmental Engineering,National Sun Yat-Sen Univ., Kaohsiung 80424, Taiwan.

2Professor and Director, Institute of Environmental Engineering,National Sun Yat-Sen Univ., Kaohsiung 80424, Taiwan �correspondingauthor�. E-mail: [email protected]

3Engineer Director, U.S. Environmental Protection Agency, KansasCity 66101, KS.

4Ph.D. Candidate, Institute of Environmental Engineering, NationalSun Yat-Sen Univ., Kaohsiung 80424, Taiwan.

Note. This manuscript was submitted on June 16, 2008; approved onJanuary 26, 2009; published online on April 3, 2009. Discussion periodopen until March 1, 2010; separate discussions must be submitted forindividual papers. This paper is part of the Journal of EnvironmentalEngineering, Vol. 135, No. 10, October 1, 2009. ©ASCE, ISSN 0733-
9372/2009/10-1015–1024/$25.00.
JOURNAL OF

J. Environ. Eng. 2009.1

The most common conventional method for the remediation ofcontaminated soils was excavation followed by landfilling or in-cineration. However, excavation and landfilling could not destroycontaminants, and incineration could be costly and might alsocause a secondary pollution such as formation of volatile organiccompounds. With the growing interest in environmental remedia-tion, various approaches have been proposed for treatingpetroleum-hydrocarbon contaminated sites. Up to now, variousremediation techniques �e.g., phytoremediation, natural attenua-tion, bioremediation, biosparging� have been developed �Bento etal. 2005; Huang et al. 2005; Interstate Technology and RegulatoryCouncil 2005; Menendez-Vega et al. 2007; Kao et al. 2008�.

Given that it is often not possible to remove the released oil orremediate the site completely using a single remedial technology,the concept of “treatment train” should be applied for site reme-diation. Thus, two or more innovative and established technolo-gies may be used together in treatment trains, which are eitherintegrated processes or a series of treatments that are combined insequence to provide the necessary treatment �U.S. EPA 2004�.Some treatment trains are employed when no single technology iscapable of treating all the contaminants in a particular medium.When in situ technologies are used in a treatment train, a moreaggressive technology may be applied to remediate areas withhigh contaminant concentrations �hot spots�, followed by applica-tion of a less aggressive technology to remediate a larger area thatincludes the former hot spot area.

The U.S. EPA have proposed various technological approaches
�physical, chemical, biological, and thermal� for treating land
ENVIRONMENTAL ENGINEERING © ASCE / OCTOBER 2009 / 1015

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contaminated by petroleum-hydrocarbons. Among these treatmentmethods, soil washing and in situ chemical oxidation have beenconsidered as promising remedial technologies due to its potentialfor treating petroleum-hydrocarbon contaminated soils �Fergusonet al. 2004; Conte et al. 2005�. Soil washing is less time consum-ing compared with bioremediation and natural treatment systems�e.g., phytoremediation, natural attenuation�, which are largelyaffected by climatic factors. It is based on the desorption of pol-lutants from contaminated soils through the action of simplewater, or organic surfactants which may be nonionic or anionic.Surfactants are interesting remediation agents because they haveunique characteristics that change the surface property of liquids.Above a specific concentration, the critical micelle concentration�CMC�, the surfactant molecules aggregate to form micelles witha hydrophobic head occupying the core and a hydrophilic taildirected at the exterior surface. Since micelles provide an organicmicrosphere in an aqueous solution, contaminants such aspetroleum-hydrocarbons can be easily solubilized within their mi-cellar phase �Harwell et al. 1999; Urum et al. 2004; Mulligan2005�.

One of the in situ chemical oxidation process is the iron-catalyzed hydrogen peroxide �H2O2� oxidation system to producethe hydroxyl radical �·OH�, which is a very strong and nonspe-cific oxidant. There are two types of iron-catalyzed H2O2 oxida-tion processes: the Fenton oxidation, which utilizes soluble ironsuch as Fe2+, and the Fenton-like process which uses iron oxyhy-droxide such as goethite ��-FeOOH�. A significant portion of con-taminants in soils were found to be oxidized by H2O2 without anyaddition of soluble iron, and the mineral-catalyzed Fenton-likereaction was proposed to describe the oxidation occurring in thenatural soils �Huang et al. 2001; Interstate Technology and Regu-latory Council 2001; Quan et al. 2003; Kulik et al. 2006; Barreiroet al. 2007; Ferraz et al. 2007�.

Recently, researchers suggested that injection of H2O2 mayprovide a more economical remediation design for highly reactiveand impermeable soils �Ferguson et al. 2004; Baciocchi et al.2003,2004; Mecozzi et al. 2006; Hao et al. 2007�. Fenton-likereactions have been investigated specifically for the treatment ofpetroleum fractions and related compounds in soils. Watts andDilly �1996� applied the monobasic potassium phosphate to en-hance the mineral-catalyzed Fenton-like remediation of dieselcontaminated soils, but resulted in only 40% diesel mass removal.Goi et al. �2006� indicated that the oil can be degraded with theaddition of H2O2 when natural iron presented in soil to catalyzethe reaction �Fenton-like treatment�. This has been identified as apotentially effective technology in the treatment of petroleum-hydrocarbon pollutants. However, there is a lack of informationabout its effectiveness for the indirect treatment of fuel-oil in thesubsurface.

In this study, an in situ two-stage remedial system consistingof surfactant washing followed by Fenton-like oxidation processhas been investigated to treat fuel-oil contaminated soils. Theobjectives of this study were to: �1� develop an in situ two-stageremedial system consisting of surfactant washing followed byFenton-like oxidation process to remediate fuel-oil contaminatedsoils; �2� evaluate the effects of residual surfactant and soil or-ganic matter �SOM� on the efficiency of Fenton-like oxidation;�3� evaluate the effect of potassium dihydrogen phosphate�KH2PO4� addition on the stability of H2O2 and oxidation effi-ciency; and �4� evaluate the possible oxidation products after the
oxidation process.
1016 / JOURNAL OF ENVIRONMENTAL ENGINEERING © ASCE / OCTOBE


Materials and Methods

Chemicals and Soil

The fuel-oil �Number 6 fuel-oil� was purchased from ChinesePetroleum Corp. �Taiwan� and used as received. The fuel-oil wascharacterized by the presence of hydrocarbons ranging from C12

to C32 �Taiwan Environmental Protection Administration 2003�.This study employed a biodegradable and nonionic surfactant,Simple Green �SG� �manufactured by Sunshine Makers, Inc.,U.S.A.�, which has a CMC value of 1 g L−1 and a molecularformula as HOCH2H2O– �CH2�3CH3 �Tsai et al. 2008�. The H2O2

�35% �10,294 mM�� and KH2PO4 were purchased from Merckand used as received. The tested soil was sampled from a back-ground and uncontaminated area of a petroleum-hydrocarbon�fuel-oil� spill site in Taiwan. The collected soils were air-dried,passed through a 2-mm-sieve, and kept refrigerated at 4°C untilanalyzed. Soil samples were analyzed to determine their charac-teristics including parameters of pH, particle size, SOM, and ex-tractable iron. The soil pH was measured using a mixture of soiland deionized �DI� water �w/v, 1:1� with a glass electrode. Par-ticle size distribution was determined by the pipette methods �Geeand Bauder 1986�. The Walkley-Black wet oxidation method wasused to measure the organic matter in soil �Nielson and Sommers1986�. Extractable Fe determination, using hydroxylamine chlo-ride solution as an extract, followed the procedures described byHesse �1971�. Amorphous iron concentration was quantified byammonium oxalate extraction �Mckeague and Day 1966�.

Stage 1 of Surfactant Simple Green Washing

Collected soils were mechanically homogenized in a stainlesssteel container. The soil was spiked with fuel-oil dispersed in 1.5L of a 1:1% �v/v� n-hexane/acetone solution. The soil was thenfurther homogenized. The solvents were allowed to evaporatefrom the soil by placing the container of spiked soil in a fumehood, thus leaving behind the fuel-oil in the soil at an initial totalpetroleum-hydrocarbon �TPH� concentration of approximately50,000 mg kg−1 of soil. The initial TPH concentration was con-firmed by the analyses of triplicate soil samples. Less than 1% ofvariation was observed from the triplicates analyses �data notshown�.

A batch experiment was conducted to evaluate the effective-ness of applying the two-stage remedial system scheme on theremediation of fuel-oil contaminated soil. The first stage of thetreatment train system was the washing process applying biode-gradable surfactant SG followed by DI water washing. In thisexperiment, an enclosed reactor �100-mL-glass bottle� contained50 g of fuel-oil contaminated soils and 30 mL �one pore volume�PV�� of SG solution. After washing with a certain amount of SG�50 g L−1�, the soil permeability might decrease due to clogging,and the TPH removal efficiency might also decrease. Thus, thesecond phase of the washing stage was the DI water washingprocess, which was operated after the significant decrease of TPHremoval was observed through SG washing. In this DI waterwashing phase, DI water was used to replace the SG for TPHremoval. SG and DI water washing was carried out at room tem-perature, and the reactor was shaken in a shaker at 200 strokesmin−1. The experiments were conducted at room temperature, andduplicate bottles were sacrificed at each PV for the analysis ofTPH and SG during the washing process.

Samples were spun on a Hettich-Zentrifugen EBA 21 Centri-
fuge for 30 min at 10,000 rpm, and stored until analysis at 4°C in
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the dark. Removal of TPH was monitored by shake-extracting thevial contents for 24 h with mixture of n-hexane/acetone �v/v, 1:1�.Analysis of TPH extract was performed using a Hewlett-Packard6890 gas chromatography �GC� equipped with a flame ionizationdetector and a DB-1 �HT� capillary column �0.32 mm�30 m�.The injector temperature was 300°C and the detector temperaturewas 350°C. The oven temperature was programmed to increasefrom 50°C �5 min� to 350°C �10 min� at 10°C min−1. Analysisof SG extract was performed using a 6850 N GC equipped with aflame ionization detector and a HP-1 capillary column�0.53 mm�30 mm�. A second column experiment �control ex-periment� was operated for comparison.

Stage 2 of Fenton-Like Oxidation

Similar to the soil spiking method described in the previous sec-tion, collected soils were mechanically homogenized in a stainlesssteel container, and spiked with fuel-oil. The spiked soils hadinitial TPH concentration of 4 ,950 mg kg−1. In the Fenton-likeoxidation experiments, four groups of batch experiments wereconducted, and three different H2O2 concentrations �15�0.05 vol %�, 882 �3 vol %�, and 1,765 mM �6 vol %�� wereused. Table 1 shows the characteristics of each batch experiment.The first batch experiment �SG addition group� was performed toevaluate the effect of residual SG after the surfactant washingstage on the efficiency of Fenton-like reaction. The second batchexperiment �H2O2 only group� was performed to evaluate the ef-fect of H2O2 concentrations on the efficiency of Fenton-like re-action. The third batch experiment �SOM removal group� wasperformed to evaluate the effect of SOM on the efficiency ofFenton-like reaction. The SOM removal process was determinedby charring the dried soil samples at 550°C for 5 h �Tiessen et al.1981�. The fourth batch experiment �KH2PO4 addition group� wasperformed to evaluate the effect of KH2PO4 addition on the sta-bility of H2O2 and the efficiency of Fenton-like reaction. The fourgroups of batch experiments were conducted in 100-mL-glasstubes with Teflon caps batch containing 50 g of fuel-oil contami-nated soils with initial TPH concentration of 4 ,950 mg kg−1, 30mL of H2O2 solution, and other additives. The experiment wasconducted at room temperature, and soil samples were collectedat different time points �e.g., 0, 20, 40, 80, and 160 min� for theanalysis of TPH, H2O2, and temperature during the reaction. Theanalysis of TPH was described in the previous section. Hydrogenperoxide concentrations were determined by iodometric titrationmethod �Watts et al. 2002�. The first-order reaction rates �min−1�

Table 1. Characteristics of Batch Experiments

Batch�group� Reagent mixture

Initial H2O2

concentration�mM�

Initial fuel-concentrati

�mg kg−1

1 SG addition 15 4,950

1,765 4,950

2 H2O2 only 15 4,950

882 4,950

1,765 4,950

3 SOM removal 882 4,950

1,765 4,950

4 KH2PO4 addition 15 4,950

882 4,950

1,765 4,950

and half-life of H2O2 were calculated in groups of 1 and 4 to

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evaluate the effect of KH2PO4 addition on the stability of H2O2 inthe Fenton-like reaction. The following equation was applied forH2O2 decay rate �kobs ,min−1� and half-life calculation:

− d�H2O2�dt

= kobs � �H2O2�

half-life of H2O2 =0.693

kobs

kobs:kinetic constant

The oxidation by-products were analyzed using a GC/MS �Agi-lent 6890N GC/MSD� with a flame ionization detector and aDB-1 HT capillary column �0.32 mm�30 m� �Domingues et al.2003; Torrades et al. 2003; Bremner et al. 2006�. The injectortemperature was 300°C and the detector temperature was 350°C.The oven temperature was programmed to increase from 50°C �5min� to 350°C �10 min� at 10°C min−1. In each experiment,temperature variation during the oxidation process was monitoredby a thermometer.

Results and Discussion

Soil Characteristics

Results reveal that the soils had a sandy loam texture �60% sand,35% silt, and 5% clay�. The SOM content and soil pH were ap-proximately 2.1% and 5.3, respectively. The measured extractableiron and amorphous iron in soil were 22.3 g kg−1 and 3.5 g kg−1,respectively. Results indicate that the soils contained significantamount of iron to enhance the reaction of Fenton-like oxidation�Watts and Dilly 1996; Interstate Technology and RegulatoryCouncil 2001; Yeh et al. 2003�.

Stage 1 of Surfactant Simple Green Washing

Fig. 1�a� presents the percentage of TPH removal versus the num-ber of PVs of SG �with the initial SG concentration was50 g L−1� and DI water used. Results show that more than 82%of the TPH can be removed after 45 PVs of SG washing. Resultsalso reveal that the TPH removal efficiency decreased after wash-ing with 45 PVs of SG. This might be due to the decreased soilpermeability after the application of SG. Thus, DI water washing

Components

Addition of SG �1, 3, 5, 7, 10, 30, and 50 g L−1� +H2O2 solution

Fuel-oil contaminated soils+H2O2 solution

Fuel-oil contaminated soils �SOM removed� +H2O2 solution

Fuel-oil contaminated soils+2.2 mM KH2PO4+H2O2 solution

oilon�

was used to replace SG for the second phase of washing. Ap-


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proximately 8% of the remaining TPH could be further removedin the second phase through 25 PVs of DI water washing �from45 to 70 PV�. A TPH “decrease and rebound” phenomenon wasobserved from 45 to 70 PV of DI water washing. The observeddecrease and rebound phenomenon might be due to the occur-rence of SOM in soils. Results from the batch experiment indicatethat approximately 90% of TPH could be removed after surfactant�50 g L−1 of SG� followed by DI water washing, while the TPHconcentrations dropped from 50,000 to 4 ,950 mg kg−1 at the endof the washing experiment. The SG concentration was below1 g L−1 at the end of the experiment �Fig. 1�b��. Results indicatethat SG washing is a promising technology to remediate fuel-oilcontaminated soils containing high TPH concentrations. More-over, application of DI water washing �or extraction� after the SGwashing could further improve the efficiency of TPH removal,and also prevent the possible clogging problem due to the usageof surfactant.

Effect of Biodegradable Surfactant Simple Greenon Total Petroleum-Hydrocarbon Removal

In this batch experiment �SG addition group�, effect of residualSG after the surfactant washing stage on the efficiency of Fenton-like reaction was evaluated. Figs. 2�a and b� show the TPH con-centrations after the Fenton-like oxidation process with the initialH2O2 concentrations of 15 and 1,765 mM, respectively �with theinitial soil TPH concentration was 4 ,950 mg kg−1, reactiontime=160 min�. Results indicate that the addition of SG would

Pore volume

0 5 10 15 20 25 30 35 40 45 50 55 60 65 70 75

SGconc.(mgL-1 )

0

10

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30

40

50

60Pore volume

0 5 10 15 20 25 30 35 40 45 50 55 60 65 70 75

TPH(C/C0,%)

0

10

20

30

40

50

60

70

80

90

100Surfactant SG washing DI water washing

Fig. 1. �a� Percentage of TPH removal vs the PVs of SG and DIwater used �error bars show standard deviation in duplicate samples�;�b� concentration of SG vs the PVs of SG and DI water used �errorbars show standard deviation in duplicate samples�

cause the decrease in oxidation efficiencies and increase in re-



maining TPH concentrations after oxidation. In the experimentswith low H2O2 concentration �15 mM�, significant increase inremaining TPH concentrations was observed after oxidation inexperiments with SG concentrations higher than 1 g L−1. In theexperiments with higher H2O2 concentration �1,765 mM�, lessincrease in remaining TPH concentrations was observed after oxi-dation when SG concentrations were lower than 10 g L−1. Thismight be due to the fact that the supplied SG became an alterna-tive reluctant, which caused the consumption of significantamount of H2O2, and oxidation power. Addition of SG might alsohinder H2O2 from contact with fuel-oil, and thus, caused the de-crease in oxidation efficiency of TPH. Results indicate that theresidual SG after the surfactant washing stage would cause ad-verse effect on the following oxidation reaction. Therefore, froman engineering and economy point of view, application of DIwater/groundwater washing after surfactant washing processmight be necessary to improve the oxidation efficiency.

Effects of H2O2 Concentrations on TotalPetroleum-Hydrocarbon Removal

Fig. 3 shows the efficiencies of TPH removal versus reaction timewith three different initial H2O2 concentrations �15, 882, and1,765 mM� �with the initial soil TPH concentration was4 ,950 mg kg−1�. Results show that TPH removal can be signifi-cantly increased with increased H2O2 concentration with initialTPH concentration of 4 ,950 mg kg−1. Less than 33% of TPHremoval efficiency was observed in experiment with low H2O2

RemainingTPHconc.(mgkg

-1)

0

500

1000

1500

2000

2500

3000

3500No SG addition

SG conc. (g L-1)

0

500

1000

1500

2000

2500

1 3 5 7 10 30 50

( a )

( b )No SG addition

H2O2 conc.: 15 mM

H2O2 conc.: 1,765 mM

Fig. 2. �a� Remaining TPH concentrations after the Fenton-like oxi-dation process vs SG concentrations with the initial H2O2 concentra-tions of 15 mM �initial TPH concentration=4,950 mg kg−1, reactiontime=160 min�; �b� remaining TPH concentrations after the Fenton-like oxidation process vs SG concentrations with the initial H2O2

concentrations of 1,765 mM �initial TPH concentration=4,950 mg kg−1, reaction time=160 min�

concentration �15 mM�. The observed first-order decay rates for

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H2O2 were 1.1�10−3, 1.7�10−3, and 2.0�10−3 min−1 when thereaction time was 160 min and initial H2O2 concentrations were15, 882, and 1,765 mM, respectively. A TPH decrease and re-bound phenomenon was observed after 40 min of reaction, andthis rebound was more significant in the experiment with lowH2O2 concentration �15 mM�. The observed decrease and reboundphenomenon might be due to the occurrence of SOM in soils. Thefollowing SOM removal experiment might further confirm thehypothesis. In the experiment with 15 mM of H2O2, the TPHconcentrations dropped from 4,950 to 3 ,317 mg kg−1 �33% ofTPH removal� after 160 min of reaction. Although H2O2 can beused as an oxidant alone, the oxidation reaction is not kineticallyfast enough to degrade many hazardous organic contaminants be-fore decomposition occurs at low H2O2 concentration��30 mm� �Interstate Technology and Regulatory Council2001,2005�. Results also show that approximately 54% and 66%of TPH were removed in experiments with H2O2 concentrationsof 882 and 1,765 mM, respectively, after 160 min of oxidation.This indicates that the TPH oxidation can be enhanced underconditions of higher H2O2 concentration. The overall TPH re-moval via the Fenton-like oxidation process was approximately7% after 160 min of oxidation �total TPH removal efficiency oftwo-stage remedial system�, while the TPH concentrationsdropped from 4,950 to 1 ,683 mg kg−1 at the end of the oxidationprocess.

Researchers have reported that high concentrations of H2O2

�generally above 294 mM� are required for site application be-cause various organic matters in soil would quench some part ofsupplied H2O2 �Kong et al. 1998; Yeh et al. 2002�. Organic con-taminants might exist in soil as particulate and sorbed conditions,and thus, a strong oxidizing condition is required to solubilize andoxidize contaminant efficiently. In a future practical application, asequential addition of high concentration of H2O2 can be con-ducted to provide a stronger oxidizing power.

Effect of Soil Organic Matter on TotalPetroleum-Hydrocarbon Removal

In this batch experiment �SOM removal group�, the effect ofSOM on the efficiency of Fenton-like reaction was evaluated.Figs. 4�a–c� present the TPH removal efficiency versus reactiontime with and without SOM removal with H2O2 concentrations of15, 882, and 1,765 mM, respectively �with the initial soil TPH

−1

( a )

Time (min)0 20 40 60 80 100 120 140 160

TPHremovalefficiency(%)

0

10

20

30

40

50

60

70

80

90

100

15 mM882 mM1,765 mM

H2O2 initial conc.

Fig. 3. Efficiencies of TPH removal vs reaction time with three dif-ferent initial H2O2 concentrations �15, 882, and 1,765 mM� �initialTPH concentration=4,950 mg kg−1�

concentration was 4 ,950 mg kg �. Results show that the natural

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occurring organic matter would affect the oxidation efficiency �9to 41% lower� under the low H2O2 concentration condition �withthe initial H2O2 concentration was 15 mM�. This indicates theSOM would compete with TPH for oxidants and consume somepart of oxidants. However, in experiments with higher H2O2 con-centrations �882 and 1,765 mM�, the SOM effect on TPH removalefficiency was less significant �varied from 0.1 to 14.5%�. Theoverall TPH removal via the Fenton-like oxidation process wasapproximately 7% after 160 min of oxidation �total TPH removalefficiency of two-stage remedial system�, while the TPH concen-trations dropped from 4,950 to 1 ,535 mg kg−1 at the end of theoxidation process.

In the Fenton-like treatments, SOM was found to consume and

Time (min)0 20 40 60 80 100 120 140 160


0

10

20

30

40

50

60

70

80

90

100


0 20 40 60 80 100 120 140 1600

10

20

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100

Time (min)0 20 40 60 80 100 120 140 160

Time (min)


0

10

20

30

40

50

60

70

80

90

100

H2O2

H2O2 (desorption)

H2O2

H2O2 (desorption)

H2O2 initial conc. : 882 mM

H2O2

H2O2 (desorption)

H2O2 initial conc. : 1,765 mM

H2O2 initial conc. : 15 mM

( a )

( b )

( c )

Fig. 4. �a� TPH removal efficiency vs reaction time with and withoutSOM removal with H2O2 concentration of 15 mM �initial TPHconcentration=4,950 mg kg−1�; �b� TPH removal efficiency vs reac-tion time with and without SOM removal with H2O2 concentration of882 mM �initial TPH concentration=4,950 mg kg−1�; and �c� TPHremoval efficiency vs reaction time with and without SOM removalwith H2O2 concentration of 1,765 mM �initial TPH concentration=4,950 mg kg−1�

compete for the oxidant and excess amount of H2O2 were usually


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required. Moreover, the TPH decrease and rebound phenomenonwas also observed after 40 min of reaction, and this rebound wasmore significant in the experiment with low H2O2 concentration�15 mM�. The observed decrease and rebound phenomenon couldbe caused by the occurrence of SOM in soils. The sorption offuel-oil on organic matter would cause the decrease in TPH con-centration at the earlier reaction period. However, TPH would bereleased �or desorbed� after the oxidation of SOM by oxidants.This might cause the increase in TPH concentration after 40 minof reaction. The released TPH was further oxidized by oxidants,which caused the decrease in TPH concentrations. Thus, an“oxidation-sorption-desorption-oxidation” scheme for TPH wasobserved in this experiment. This scheme can be used to describethe mechanisms among the oxidant, SOM, and TPH during oxi-dation.

Because the applied oxidant would oxidize the SOM in soilsand cause the release of the organic contaminants into groundwa-ter or the pore spaces in soils, there is a potential that the organiccontaminant concentrations would increase during the in situchemical oxidation process. This phenomenon is highly depen-dent on the transport properties of the soil. The more permeablethe soil, the greater chance for release to groundwater because theoxidant has less time for reacting with the contaminants �Inter-state Technology and Regulatory Council 2005�. Accordingly,compounds with various hydrophobic characteristics exhibitedsimilar low oxidation efficiency in soil of high organic content.However, the effects of SOM on Fenton-like oxidation were notconsistent in all cases. SOM has a high oxidant demand, andtherefore, can be important when estimating the required chemi-cal dosage �Yeh et al. 2002�.

Effect of Monobasic Potassium Phosphate on TotalPetroleum-Hydrocarbon Removal

Due to the instability characteristics of H2O2 in the environment,H2O2 concentration might reduce dramatically at increasing soildepths unless a proper stabilizing substance is mixed with H2O2.The decomposition of H2O2 in subsurface environments was alsostudied, even if in model systems �Baciocchi et al. 2003�. In thisbatch experiment �KH2PO4 addition group�, the effect of KH2PO4

�a phosphate salt� addition on the stability of H2O2 and efficiencyof Fenton-like reaction was evaluated. Fig. 5�a� presents the TPHremoval efficiency versus reaction time with and without KH2PO4

addition �2.2 mM� with H2O2 concentrations of 882 and 1,765mM, respectively �with the initial soil TPH concentration was4 ,950 mg kg−1�. The overall TPH removal via the Fenton-likeoxidation process was approximately 3% after 160 min of oxida-tion �total TPH removal efficiency of two-stage remedial system�,while the TPH concentrations dropped from 4,950 to3 ,515 mg kg−1 at the end of the oxidation process.

Fig. 5�b� presents the calculated first-order decay rates forH2O2 under the experimental conditions previously described.Results show that the observed first-order H2O2 decay rates weresignificantly decreased with the addition of KH2PO4 into the re-actor. However, addition of 2.2 mM of KH2PO4 also caused thedecrease in the efficiency of TPH removal. Results indicate thatthe TPH removal efficiencies were approximately 38 and 41%higher in experiments with 882 and 1,765 mM of H2O2 in theabsence of KH2PO4, respectively, compared with the efficienciesof 882 and 1,765 mM H2O2 that is amended with 2.2 mMKH2PO4. The calculated first-order decay rates for H2O2 areshown in Table 2. The estimated efficiency curve matched very
well with the observed data. Moreover, the calculated reaction


rates were close to the kobs values reported by other researchers�Baciocchi et al. 2003�. Results in Table 2 also indicate thatKH2PO4 addition decreased the H2O2 decomposition. Resultsshow that the half-life of H2O2 increased from 110 to 495 min,and the decay rates dropped from 6.3�10−3 to 1.4�10−3 min−1

after 640 min of reaction with the H2O2 concentration of 882mM. In experiments with H2O2 concentration of 1,765 mM, thehalf-life of H2O2 increased from 96 to 365 min, the decay ratesdropped from 7.2�10−3 to 1.9�10−3 min−1 after 640 min ofreaction.

The KH2PO4 stabilizer lowers dissolved iron and other transi-tion metals concentrations by precipitating them or creating com-plexes with them. Their activity as Fenton’s catalysts decreasesbecause they become nonreactive with H2O2 �Watts and Dilly1996�. Watts and Stanton �1999� reported that the KH2PO4 stabi-lizer formed complexes with the easy exchangeable metal ions,which is one of the most active elements in catalyzing H2O2

decomposition. Lu et al. �1997� also indicates that ferric ionsundergo a complex reaction with H2PO4

−, causing ferric ions tolose the power to catalyze H2O2 decomposition. However,FeH2PO4

+ possibly reacts with hydrogen peroxide and producesradicals. The main reason for the suppression of phosphate ions isthat phosphate ions will produce a complex reaction together withferrous ions and ferric ions, which then lowers its ability to cata-lyze H2O2 decomposition. Results from this study indicate that

Time (min)

0 80 160 240 320 400 480 560 6400

1

2

3

4

5

6

7

-ln([H

2 O2 ]/[H2 O

2 ]0)

kobs = 7.2∗10-3 (min-1)

Time(min)

0 80 160 240 320 400 480 560 640


0

10

20

30

40

50

60

70

80

90

100

882 2.21765 2.2882 -1765 -

H2O2 conc. (mM) KH2PO4 conc. (mM) ( a )

( b )

kobs = 6.3∗10-3 (min-1)

kobs = 1.9∗10-3 (min-1)

kobs = 1.4∗10-3 (min-1)

H2O2 conc. (mM) KH2PO4 conc. (mM)

882 2.21765 2.2882 -1765 -

Fig. 5. �a� TPH removal efficiency vs reaction time with and withoutKH2PO4 addition �2.2 mM� with H2O2 concentrations of 882 and1,765 mM, respectively �initial TPH concentration=4,950 mg kg−1�;�b� calculated first-order decay rates for H2O2 under different experi-mental conditions: �1� with and without KH2PO4 addition �2.2 mM�and �2� H2O2 concentrations of 882 and 1,765 mM �initial TPHconcentration=4,950 mg kg−1�.

the concentration of KH2PO4 supplied into the subsurface due to

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the fact that ferrous ions and ferric ions in soils would be precipi-tated. Thus, the efficiency of Fenton-like reactions would be af-fected. In the practical application, the parameters including ironconcentration, method of KH2PO4 addition, and concentration ofKH2PO4 provided need to be further evaluated to effectively andefficiently apply the Fenton-like oxidation in situ.

Effect of Fenton-Like Reaction on TemperatureVariation

Figs. 6�a–c� illustrate the variations in temperature measurementsversus time during the Fenton-like reaction in experiments ofGroup 2 �H2O2 addition only�, Group 3 �SOM removal�, andGroup 4 �KH2PO4 addition�, respectively. Results indicate that asignificant temperature increase was observed in Group 2 �H2O2

addition only� experiment with higher H2O2 addition but withoutH2O2 stabilization. This might be due to the fact that SOM re-acted with H2O2 and caused the increase in temperature �up to39°C during the early 20 min of reaction in experiment with1,765 mM of H2O2�. In Group 3 experiments with SOM removal,increase in temperature was also observed in experiments withhigher H2O2 addition. However, the increased temperature ob-served in Group 3 was slightly lower than the observed tempera-ture increase in Group 2. This could be due to the fact that SOMwas removed in Group 3, and thus, less organic compounds wereoxidized by Fenton-like reaction. When H2O2 was stabilized�Group 4�, a more stabilized temperature increase was observedfrom 20 to 160 min. The pattern of temperature variation mightalso confirm the decreased decay rate of H2O2 due to the KH2PO4

addition.Temperature increases in soil and groundwater are often de-

tected immediately after injection by H2O2 solution. The optimaltemperature for ISCO treatments with H2O2 is generally withinthe interval of 35 to 41°C �Interstate Technology and RegulatoryCouncil 2001�. Fenton-like reactions in the subsurface are in facthighly exothermic and can lead to a significant increase in soiltemperature. Excessive heat generation and gas formation aredangerous because they can lead to undesired soil sterilization,instability of reactants, and explosions. Thus, for safety reasons,site temperatures should be closely monitored in the field appli-cation.

By-Products of Fuel-Oil Oxidation

In this experiment, possible oxidation by-products after theFenton-like oxidation process were evaluated. This is to observeif there is a preferential degradation of certain compounds in fuel-oil applying Fenton-like oxidation. GC/MS method was applied

Table 2. Calculated First-Order Decay Rates and Half-Life for H2O2 un

Operating condition

Reagent mixture

Initial H2O2

concentration�mM�

Initial fuel-oilconcentration

�mg kg−1�With H2O2 only 15 4,950

882 4,950

1,765 4,950

882 4,950

1,765 4,950

Addition of KH2PO4 �2.2 mM� 882 4,950

1,765 4,950

der Different Experimental Conditions

H2O2

residual�mM�

H2O2 kineticconstant�min−1�

H2O2

half-life�min�

TPH removalefficiency

�%� R2Time�min�

160 2 1.1�10−2 63 33 0.96

160 45 1.7�10−2 41 54 0.92

160 56 2.0�10−2 35 66 0.90

640 9 6.3�10−3 110 57 0.92

640 11 7.2�10−3 96 71 0.94

640 329 1.4�10−3 495 19 0.88

640 392 1.9�10−3 365 30 0.92

for the analysis of oil/solvent extracts. Table 3 shows the possible

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Time (min)

Temp.(oC)

20

25

30

35

40

8821,765

0 20 40 80 160 320 640

Background temp. (25oC)

KH2PO4 conc. : 2.2 mM

H2O2 conc. (mM)

Time (min)

Temp.(oC)

20

25

30

35

40

158821,765

0 20 40 80 160

Background temp. (25oC)

H2O2 conc. (mM)

Time (min)

Temp.(oC)

20

25

30

35

40

158821,765

0 20 40 80 160

H2O2 conc. (mM)

- - Background temp. (23oC)( a )

( b )

( c )

Fig. 6. Variations in temperature measurements in experiments with:�a� H2O2 addition only; �b� SOM removal; and �c� KH2PO4 addition�2.2 mM�.


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detected compounds �C12 to C32� in fuel-oil contaminated soilsand possible oxidation by-products in Group 2 experiment �1,765mM H2O2+4,950 mg kg−1 TPH� after 160 min of reaction. Re-sults from the by-product analysis show that the oxidation powerof Fenton-like process is not strong enough to completely destroythe fuel-oil to nontoxic or inorganic end products. The oxidationprocess produced by-products containing carboxyl groups withmolecular weights similar to their parent compounds. Results alsoindicate that the major petroleum-hydrocarbons in fuel-oil �C12 toC32� were mainly straight-chain organic compounds, however, theacids compounds were produced after Fenton-like oxidation. Thedetected oxidation by-products include some straight-chain com-pounds �e.g., tricosane, baccharane�, and acid compound �e.g.,1-acetyl-6-methyl-acetate-3-piperidinol, pimaric acid, 6-amino-5-cyano-4-�5-cyano-2,4-dimethyl-1H-pyrrol-3-yl�-2-methyl-4H-pyran-3-carboxylic acid ethyl ester, 2,6,-dimethyl-pyridine-3,4-carboxylic acid�. Domingues et al. �2003� also reported that

Table 3. Possible Detected Compounds �C12 to C32� in Fuel-Oil ContammM H2O2+4,950 mg kg−1 TPH� after 160 min of Reaction �Reliability

Treatment Observe

No H2O2 addition�fuel-oil contaminated soil�

1,6-dimethyl-Naphthalen

Hexadecane

Heptadecane

Octadecane

Nonadecane

Eicosane

Heneicosane

Docosane

1-�2-hydroxyphenyl�-, �

Tetracosane

Pentacosane

Triacontane

Dotriacontane

Group 2�1,765 mM H2O2+4,950 mg kg−1 TPH�

2,6,11-trimethyl dodeca

Tricosane

2,6,10,14,18-pentamethy

19-nor-5.beta.-con-9-eni

Baccharane

1-acetyl-6-methyl-acetat

Pimaric acid

6-amino-5-cyano-4-�5-c2-methyl-4h-pyran-3-ca

2,6,-dimethyl-pyridine-3

Table 4. Calculated Percentage of Total TPH Removal after the Applica

Stage Reaction mechanism

1 Desorption/solubilization/washing

2 Desorption/oxidation �with the initial H2O2 concentrationwas 1,765 mM�

Note: �1� Total TPH removal efficiency: ��TPH�1− �TPH�2� / �TPH�total��

−1
�TPH�total=50,000 mg kg .


oxidation of side chain occurred if the free radical derived fromFenton’s reaction was not able to destroy structures of organiccompounds completely. Therefore, the products of the side chainoxidation would not cause significant variation of the molecularweight.

Effectiveness of the Two-Stage Remedial System onTotal Petroleum-Hydrocarbon Removal

Table 4 presents the percentage of total TPH removal after theapplication of the two-stage remedial system. Results indicate thatapproximately 90% of TPH could be removed after SG�50 g L−1� and DI water washing �Stage 1�. The Fenton-like oxi-dation process �Stage 2� was able to remove another 7% of theremaining TPH without the presence of surfactant after 160 min.The proposed treatment scheme would be expected to provide amore efficient alternative to remediate fuel-oil contaminated sites.

Soils and Possible Oxidation By-Products in Group 2 Experiment �1,765/MS Chromatograms Analytic: �95%�

oducts Molecular formulaMW

�g mole−1�

C12H12 156.1

C16H34 226.3

C17H36 240.3

C18H36 254.3

C19H40 268.3

C20H42 282.3

C21H44 296.3

C22H46 310.4

rophenyl� hydrazone ethanone C14H12N4O5 316.1

C24H50 338.4

C25H52 352.4

C30H62 422.5

C32H66 450.5

C15H32 212.3

C23H48 324.6

ane C25H52 352.4

ta.-ol,3.beta.-�dimethylamino�-5-methyl C24H40N2O 372.3

C30H54 414.4

eridinol C8H15NO2 157.0

C20H30O2 302.4

,4-dimethyl-1H-pyrrol-3-yl�-c acid ethyl ester

C17H18N4O3 287.4

oxylic acid C9H9NO4 195.0

the Two-Stage Treatment Train System

TPH concentration�mg kg−1� Total TPH

removal efficiency�%�Initial Final

50,000 4,950 90

Group 2 4,950 1,683 7

Group 3 4,950 1,535 7

Group 4 4,950 3,515 3

2� �TPH�1=4 ,950 mg kg−1; �3� �TPH�2: remaining of Stage 2; and �4�

inatedof GC

d by-pr

e

4-dinit

ne

l eicos

n-6.be

e-3-pip

yano-2rboxyli

,4-carb

tion of

100; �

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Conclusions

In this study, an in situ two-stage remedial system consisting ofsurfactant SG washing followed by Fenton-like oxidation to treatfuel-oil contaminated soils was investigated. Conclusions ob-tained from the batch experiments include the following:1. In the SG washing experiment, 90% of TPH could be re-

moved after washing with 45 PVs of SG �50 g L−1� fol-lowed by washing with 25 PVs of DI water, while the fuel-oil concentration reduced from 50,000 to 4 ,950 mg kg−1.This indicates that SG washing is a promising technology toremediate fuel-oil contaminated soils containing high TPHconcentrations. Moreover, application of DI water washing�or extraction� after the SG washing process could furtherimprove the efficiency of TPH removal, and also prevent thepossible clogging problem due to the usage of surfactant.

2. Addition of SG would cause the decrease in oxidation effi-ciencies and increase in remaining TPH concentrations afteroxidation. Thus, the residual SG after the surfactant washingstage would cause adverse effect on the following oxidationreaction. Therefore, from an engineering and economy pointof view, application of DI water/groundwater washing aftersurfactant washing process might be necessary to improvethe oxidation efficiency.

3. The Fenton-like �1,765 mM of H2O2� oxidation process�Stage 2� was able to remove another 3–7% of the remainingTPH without the presence of surfactant.

4. SOM would compete with TPH for oxidants and cause thedecrease in oxidation efficiency under low H2O2 concentra-tion condition �15 mM�. However, the effect of SOM on theoxidation efficiency was not significant when higher H2O2

concentration �882 or 1,765 mM� was applied. An oxidation-sorption-desorption-oxidation scheme for TPH was observedin this experiment due to the initial sorption of TPH onSOM. This scheme can be used to describe the mechanismsamong the oxidant, SOM, and TPH during oxidation.

5. The half-life of H2O2 increased from 110 to 495 min and thedecay rates dropped from 6.3�10−3 to 1.4�10−3 min−1

after 640 min of reaction with addition of 2.2 mM ofKH2PO4. This indicates that KH2PO4 addition could increasethe stability and half-life of H2O2, but caused the decrease inTPH decay rates. Thus, iron concentration in soils, method ofKH2PO4 addition, and concentration of KH2PO4 providedneed to be further evaluated to optimize the efficiency ofFenton-like oxidation.

6. Results from the by-product analysis indicate that the oxida-tion potential of Fenton-like process might not be capable ofcompletely oxidizing fuel-oil to nontoxic or inorganic endproducts. The observed by-products after oxidation processcontained carboxyl groups with molecular weights similar totheir parent compounds.

Results from this study indicate that the two-stage remedialsystem is a promising technology for fuel-oil contaminated soiltreatment. Results will be useful in designing an in situ treatmentsystem for practical application.

Acknowledgments

This project was funded in part by Environmental Protection Ad-ministration �EPA� in Taiwan �Grant No. EPA-94-U1U1-04–010�.
Additional thanks to the personnel of Guan Cheng Environment
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Technology Protection Co., Ltd., Taiwan for the assistance andsupport throughout this project.

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R 2009

35:1015-1024.

treatment of fuel-oil contaminated soils by biodegradable surfactant washing followed by fenton-like...

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