Science of the Total Environment 419 (2012) 240–249

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Modified Fenton oxidation of polycyclic aromatic hydrocarbon (PAH)-contaminatedsoils and the potential of bioremediation as post-treatment

Venny a, Suyin Gan a,⁎, Hoon Kiat Ng b

a Department of Chemical and Environmental Engineering, The University of Nottingham Malaysia Campus, Jalan Broga, 43500 Semenyih, Selangor Darul Ehsan, Malaysiab Department of Mechanical, Materials and Manufacturing Engineering, The University of Nottingham Malaysia Campus, Jalan Broga, 43500 Semenyih, Selangor Darul Ehsan, Malaysia

⁎ Corresponding author. Tel.: +60 3 89248162; fax:E-mail address: suyin.gan@nottingham.edu.my (S. G

0048-9697/$ – see front matter © 2012 Elsevier B.V. Alldoi:10.1016/j.scitotenv.2011.12.053

a b s t r a c t

a r t i c l e i n f o

Article history:Received 5 October 2011Received in revised form 20 December 2011Accepted 22 December 2011Available online 28 January 2012

Keywords:BioremediationChelating agentFentonOptimisationPAHResponse surface methodology

This work focuses on the remediation of polycyclic aromatic hydrocarbon (PAH)-contaminated soil usingmodified Fenton (MF) treatment coupled with a novel chelating agent (CA), a more effective techniqueamong currently available technologies. The performance of MF treatment to promote PAH oxidation in arti-ficially contaminated soil was investigated in a packed columnwith a hydrogen peroxide (H2O2) delivery sys-tem simulating in-situ soil flushing which is more representative of field conditions. The effectiveness ofprocess parameters H2O2/soil, Fe3+/soil, CA/soil weight ratios and reaction time were studied using a 24

three level factorial design experiments. An optimised operating condition of the MF treatment was observedat H2O2/soil 0.05, Fe

3+/soil 0.025, CA/soil 0.04 and 3 h reaction time with 79.42% and 68.08% PAH removalsattainable for the upper and lower parts of the soil column respectively. The effects of natural attenuationand biostimulation process as post-treatment in the remediation of the PAH-contaminated soil were alsostudied. In all cases, 3-aromatic ring PAH (phenanthrene) was more readily degraded than 4-aromatic ringPAH (fluoranthene) regardless of the bioremediation approach. The results revealed that both natural atten-uation and biostimulation could offer remarkable enhancement of up to 6.34% and 9.38% in PAH removals re-spectively after 8 weeks of incubation period. Overall, the results demonstrated that combined inorganic CA-enhanced MF treatment and bioremediation serves as a suitable strategy to enhance soil quality particularlyto remediate soils heavily contaminated with mixtures of PAHs.

© 2012 Elsevier B.V. All rights reserved.

1. Introduction

Polycyclic aromatic hydrocarbons (PAHs) are ubiquitous organiccompounds that consist of more than two aromatic rings. The distin-guishing features of these highly toxic pollutants are their hydropho-bicity and recalcitrance causing serious environmental concern andsubsequent challenging soil clean-up procedures. Several remediationtechnologies for treating PAH-contaminated soils have been devel-oped depending on site characteristics, remedial objectives, operatingcost and time constraints. In the past decades, Fenton process has re-ceived much attention as an attractive remediation technology inview of its effectiveness to rapidly oxidise refractory organic con-taminants and the ease of implementation for transforming non-biodegradable and recalcitrant pollutants into innocuous end prod-ucts (Huling and Pivetz, 2006; Palmroth et al., 2006; Silva et al., 2009).

In modified Fenton (MF) treatments, chelating agent (CA) isadded to prevent iron precipitation at pH regimes of 5–7 by formingcomplexes with the iron catalyst and minimising non-specific lossesof soluble iron (Kang and Hua, 2005). Contaminant degradation and

+60 3 89248017.an).

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free radical generation without acid adjustment using MF oxidation,which is more practical from in-situ application standpoint, has alsobeen suggested by Kakarla et al. (2002). Other distinct advantagesof the chelation process in soil systems include the prevention ofmineral nutrients from forming immobile precipitates and reductionof toxicity from some metal ions to the soil ecosystem (Seol andJavandel, 2008). Despite the advantages, most reported works havefocussed on organic CAs where the formation of toxic byproducts issuspected (e.g. ethylene diamine tetraacetic acid and nitrilo triaceticacid) causing reduced applicability for environmental applications(Sillanpaa and Pirkanniemi, 2001).

Nam et al. (2001) examined the efficacy of Fenton reaction forremediating manufactured gas plant soils. The results revealed thatonly 20–40% of high molecular weight (HMW) were degraded byconventional Fenton in the aged soils. In an attempt to overcomethe limitations of conventional Fenton oxidation, MF treatmentsusing ferric ions and organic CAs such as catechol and gallic acidwere also conducted. The combined MF and biodegradation resultedin >98% removal of lowmolecular weight (LMW) PAHs and >70% re-moval of HMW PAHs. These findings reflect the significance of apply-ing CA for PAH removal from aged soils which in general are moredifficult to treat than spiked soils. Nevertheless, the results reportedso far vary greatly on soil types and history of contamination.

241Venny et al. / Science of the Total Environment 419 (2012) 240–249

To the best knowledge of the authors, the use of inorganic CA hasyet to be reported for treatment of contaminated soils, particularly incolumn experiments in which the results are more representative offield conditions and thus more directly useful. The application ofinorganic CA pyrophosphate to activate peroxymonosulphate inaqueous solution, however, has been evaluated for its effectivenessin the degradation of chlorophenols (Rastogi et al., 2009). The authorshighlighted several advantages of using the inorganic CA: (1) lesserlikelihood to compete for generated active radicals responsible forpollutant oxidation and (2) total organic carbon is not increased byinorganic CAs throughout the course of pollutant degradation.

Owing to the fact that soil remediation is always site-specific, totaloxidant demand which measures the amount of reagents required istherefore an imperative aspect to be taken into consideration forprocess design. In addition, reaction duration is also of importanceas it has been reported that depending on treatment duration, whenresidual non-aqueous phase liquid (NAPL) sorbed or otherwise,immobilised contaminants are sometimes re-dissolved into the sys-tem causing contaminant rebound effect after treatment (Land AndWater Quality, 2003; Pinto and Moore, 2000). Hence, optimisationof process variables including the amount of reactants required andreaction time is crucial in developing an efficient and economic reme-diation approach. Many studies on Fenton oxidation to date havebeen conducted by changing one variable at a time to study theeffects on the responses. Nonetheless, many reactions are simulta-neously influenced by more than one variable. Apart from that,conventional optimisation approach particularly for multivariablesystems could lead to misinterpretation of the effects of interactionsbetween the variables. To overcome the limitation of conventionaloptimisation process, statistical design of experiments is necessaryto understand the effects of process parameters and to obtain statisti-cally significant models for the respective responses. With statisticalmethodologies such as Response Surface Methodology (RSM), the in-teractions between process parameters can be fully addressed andused to optimise the operating conditions. RSM is a powerful andwidely developed mathematical model broadly used to analyse de-sign parameters with fewer experimental trials.

The selection of suitable techniques for soil remediation rarelyconsiders the impact on soil qualitywhich is of paramount importancefor soil development. In line with Fenton treatment, biodegradationtreatment which exploits the natural capacity of microorganisms tometabolise organic substances such PAHs has gained interest as oneof the most environmentally friendly and versatile approaches suc-cessfully used at laboratory-scale (Prasanna et al., 2008; VenkataMohan et al., 2008; Wang et al., 2004) and pilot-scale experiments(Hansen et al., 2004; Juhasz et al., 2005; Pradhan et al., 1997). Biore-mediation as post-treatment has been proven not only to improvePAH removal but also to reduce soil toxicity after chemical treatmentwhich meet soil revegetation goals (Sirguey et al., 2008).

In most cases, natural bioremediation of contaminants in soils relieson indigenous microorganisms (natural attenuation/intrinsic bioreme-diation), additional nutrients and oxygen into soil (biostimulation)or inoculation of enriched microbial consortium into soil (bioaugmen-tation). Integrated treatments of chemical oxidation followed bybioremediation of PAH-contaminated soils have received much atten-tion due to satisfying results in enhancing contaminant removal(Nadarajah et al., 2002; Nam et al., 2001). The factors favouring such acombined treatment include: (1) PAH degrading bacteria and fungiare common in natural environment, (2) partially oxidised fractions ofPAHs are more water soluble and thus more bioavailable, (3) oxygenis released from H2O2 decomposition fromMF treatment thus providesaeration for metabolic activity within the soil matrix. Nam et al. (2001)studied the effect of combinedMF oxidation and bioremediation for theremediation of manufactured gas plant soils contaminated with PAHs.The active bacterial consortium integrated treatment destroyed >98%of LMWPAHs and>70% of HMWPAHs. In contrast, a recentmicrocosm

study of bacterial dynamics demonstrated that bioaugmentationof creosote-contaminated soil did not influence PAH degradation(Vinas et al., 2005). In another similar work, it was found that PAH-degrading (Mycobacterium) strain did not favour PAH degradationcompared to creosote-contaminated soil treated by nitrogen (N), phos-phorus (P) and potassium (K) (Juhasz et al., 2005).

The objectives of this work are to evaluate the application of sodi-um pyrophosphate (SP) as an inorganic CA for PAH oxidation inpacked column experiments simulating in-situ soil flushing, to opti-mise the MF treatment of PAH-contaminated soil using RSM andfinally, to assess the compatibility of bioremediation (natural attenu-ation and biostimulation) with the MF treatment. In this work, phen-anthrene (PHE) and fluoranthene (FLUT) were selected as the modelcontaminants representing LMW and HMW PAHs respectively withregard to their elevated concentrations in contaminated lands suchas wood treatment sites (Bates et al., 2000) and occurrence withinboth combustion and unburned fossil fuel sources (Mosisch andArthington, 2004).

2. Material and methods

2.1. Chemicals

PHE (97%) and FLUT (98%) were purchased fromMerck and FisherScientific respectively. Hydrogen peroxide (H2O2, 35%), ferric sul-phate (Fe2(SO4)3·xH2O, 76%) and SP (99+%) were purchased fromR&M Chemicals. Dichloromethane (DCM, 99.5%, AR analysis) and cal-cium chloride dehydrate (CaCl2, 99+%, ACS grade) were purchasedfrom Merck. Acetonitrile (ACN, 99.8%, HPLC grade) and n-pentane(99%, R&M) were purchased from Rank Synergy. Anhydrous sodiumsulphate (Na2SO4, 99+%) was purchased from Fisher Scientific. Am-monium sulphate ((NH4)2SO4, ≥99.5%) and dihydrogen phosphate(KH2PO4, 99.81%) were purchased from Systerm and Fisher Scientificrespectively.

2.2. Soil characterisation

Uncontaminated surface soil samples (0–10 cm) were collectedfrom Selangor, Malaysia. Gummy and fibrous materials not amenablefor grinding were removed to allow maximum exposure of thesample surface for subsequent treatment. The soil samples were air-dried and passed through 2 mm mesh using a laboratory sieve shaker(BSE, NL 1015).

The particle size analysiswas determined according to the Buoyoucoshydrometer method. Analyses for soil bulk density, moisture content,pHH2O, pHCaCl2, loss-on-ignition (LOI) and total iron were also carriedout and are detailed in a previous work (Venny et al., 2011). Meanwhile,the surface area and pore volumeof the soil sampleswere determined byusing liquid nitrogen in a porosimeter (Micromeritics, ASAP 2020). Adegassing vacuum condition of 10 μm Hg for 3 h and a degassing tem-perature of 150 °C were applied.

The majority of the soil particles present were within the range of0.3–0.6 mm (46.8 wt.%), followed by 0.6–1.18 mm (26.2 wt.%), 0.15–0.3 mm (17.6 wt.%), 1.18–2 mm (5.4 wt.%), b0.15 mm (3.6 wt.%)and >2 mm (0.4 wt.%). Table 1 presents the physicochemical proper-ties of the soil samples used in the present study. It is worth notingthat the soil samples used are acidic (pH 4.07–5.41). Soil pH plays avital role as it can affect the efficacy of Fenton treatment becauseH2O2 is inherently stable at pHb4.5 but its stability deteriorates rap-idly at pH>5 (Vicente et al., 2011).

2.3. Soil spiking

The soil spiking procedure was a modified method adapted fromNorthcott and Jones (2000). PAHs–DCM stock solutions (500 μl per5 g of soil sample) resulting in 500 mg PAH/kg soil each of PHE and

Table 2Coded and actual levels of independent variables for the design of MF experiment.

Symbol Independentvariable

Unit Coded level

−1 0 +1

X1 H2O2/soil w/w 0.050 0.075 0.100X2 Fe3+/soil w/w 0.000 0.020 0.040X3 SP/soil w/w 0.000 0.020 0.040X4 Reaction time h 3.000 13.50 24.00

Table 1Physicochemical properties of soil samples.

Characteristic Loamy sand

Sand (wt.%) 87.40Silt (wt.%) 1.38Clay (wt.%) 11.22Bulk density (g/ml)a 1.30±0.01Porosity (%)a 50.84±0.36Adsorption cumulative surface area (m2/g) 0.28Desorption cumulative surface area (m2/g) 0.55Adsorption pore volume (m3/g) 0.005982Desorption pore volume (m3/g) 0.006342Moisture content (%)a 1.14±0.18pHH2O at 23.8±0.06 °C a 5.41±0.33pHCaCl2 at 23.8±0.06 °C a 4.07±0.01LOI (%)a 0.03±0.01Total iron (mg/g)a 45.82±1.23

a Average of three replicate determinations.

242 Venny et al. / Science of the Total Environment 419 (2012) 240–249

FLUT were carefully added onto the soil surface to avoid PAH loses onthe recipient walls or to the base of the containers. Mixing was per-formed thoroughly using a spatula. After that, the solvent was

Table 3Experimental design matrix for MF treatment.

Runno.

Pointtype

Coded values Real value

x1 x2 x3 x4 X1

1 Factorial −1 −1 −1 −1 0.0502 Factorial 1 −1 −1 −1 0.1003 Factorial −1 1 −1 −1 0.0504 Factorial 1 1 −1 −1 0.1005 Factorial −1 −1 1 −1 0.0506 Factorial 1 −1 1 −1 0.1007 Factorial −1 1 1 −1 0.0508 Factorial 1 1 1 −1 0.1009 Factorial −1 −1 −1 1 0.05010 Factorial 1 −1 −1 1 0.10011 Factorial −1 1 −1 1 0.05012 Factorial 1 1 −1 1 0.10013 Factorial −1 −1 1 1 0.05014 Factorial 1 −1 1 1 0.10015 Factorial −1 1 1 1 0.05016 Factorial 1 1 1 1 0.10017 Axial −1 0 0 0 0.05018 Axial 1 0 0 0 0.10019 Axial 0 −1 0 0 0.07520 Axial 0 1 0 0 0.07521 Axial 0 0 −1 0 0.07522 Axial 0 0 1 0 0.07523 Axial 0 0 0 −1 0.07524 Axial 0 0 0 1 0.07525 Centre 0 0 0 0 0.07526 Centre 0 0 0 0 0.07527 Centre 0 0 0 0 0.07528 Centre 0 0 0 0 0.07529 Centre 0 0 0 0 0.07530 Centre 0 0 0 0 0.075

X1 (H2O2/soil, w/w) is calculated as: X1=0.075+x1 (0.025).X2 (Fe3+/soil, w/w) is calculated as: X2=0.020+x2 (0.020).X3 (SP/soil, w/w) is calculated as: X3=0.020+x3 (0.020).X4 (reaction time, h) is calculated as: X4=13.50+x4 (10.50).

allowed to evaporate for 3 h in a fume hood. The spiked soils weresubsequently stored at 15–18 °C in order to prevent soil microbes todegrade labile compounds.

2.4. Modified Fenton oxidation

The MF treatment of PAH-contaminated soil was carried out with-out pH adjustment on a laboratory-scale glass column operated atambient temperature of 28 °C. The system was installed with a H2O2

delivery system simulating in-situ injection as illustrated in Fig. 1.The spiked soil (1.2 kg, resulting soil height in column was 8 cm)

was packed into the glass column (packing density of 1.28 kg/dm3)which was fabricated with an inner diameter of 12.2 cm and totalheight of 16 cm. During the MF experiments, the following mecha-nisms are expected to take place: (1) soil column flushing with MFreagents (in-situ injection), (2) PAH desorption from soil matrix(NAPLs and other organic matters) followed by PAH degradationwhen reacting with MF reagents, (3) PAHs oxidation by •OH radicals,transformation of oxidised products and H2O2 decomposition intogases such as oxygen and water and (4) heat release from the catalyt-ic decomposition of H2O2. Taking into account the heat and oxygenreleased, only 50% working volume (approximately 935.19 cm3)was utilised to avoid excessive temperature fluctuation at the soilsurface.

The Fenton's reagent was delivered to the soil column via perfo-rated glass tube (consists of 4 holes at each left and right, 6 mminner diameter, 400 mm length, 2 mm holes diameter) using a peri-staltic pump (BT100-1F with YZ1515 pump head) operated at a rateof 100 ml/min for the injection of Fe3+ solution and 160 ml/minfor the injection of both H2O2 and SP solution. Stainless steel mesh(b1 mm, outer diameter 38 mm) was used as the base of the soil

s Observed responses

X2 X3 X4 Y1 Y2 Y3

0.000 0.000 3.000 42.73 40.40 3.830.000 0.000 3.000 61.42 51.00 3.870.040 0.000 3.000 62.46 60.71 2.180.040 0.000 3.000 72.48 61.98 2.390.000 0.040 3.000 42.03 40.04 6.640.000 0.040 3.000 45.35 41.81 5.700.040 0.040 3.000 70.77 77.95 2.800.040 0.040 3.000 80.22 76.82 2.240.000 0.000 24.000 69.98 67.90 3.700.000 0.000 24.000 59.81 61.95 3.200.040 0.000 24.000 73.70 69.66 2.230.040 0.000 24.000 78.37 77.05 2.180.000 0.040 24.000 19.49 16.11 6.200.000 0.040 24.000 23.76 11.70 5.830.040 0.040 24.000 53.52 52.56 2.920.040 0.040 24.000 50.01 47.71 2.800.020 0.020 13.500 82.45 80.27 4.060.020 0.020 13.500 72.03 77.35 3.910.000 0.020 13.500 26.54 15.82 5.920.040 0.020 13.500 80.19 78.17 2.480.020 0.000 13.500 54.44 48.50 3.580.020 0.040 13.500 67.84 73.43 3.260.020 0.020 3.000 73.81 72.86 3.840.020 0.020 24.000 72.19 70.77 3.350.020 0.020 13.500 68.24 63.33 3.360.020 0.020 13.500 59.44 56.95 2.940.020 0.020 13.500 62.64 59.36 3.470.020 0.020 13.500 56.27 59.80 3.290.020 0.020 13.500 70.60 69.30 2.780.020 0.020 13.500 71.15 71.45 3.14

Fig. 1. Column experiment set-up for MF treatment.

243Venny et al. / Science of the Total Environment 419 (2012) 240–249

column to retain the solid phase. H2O2 was diluted using distilledwater to 15% in order to avoid highly exothermic reaction near the in-jection point. The reaction started upon the addition of H2O2,followed by the addition of Fe3+ and SP solution. After certain reac-tion time (3, 13.5 or 24 h), the solid phase retained in the soil columnwas divided into two equivalent parts (upper and lower) and subse-quently analysed for PAH residual concentration, after homogenisingand mixing the soil of each layer. The column leachate was collectedand analysed for pH using a BP3001 (Trans Instruments) pH meter.

2.5. Process optimisation analysis

The most commonly used design of experiment under RSM is theCentral Composite Design (CCD). It is a user-friendly, flexible and ef-ficient tool for providing useful information on the effects and inter-actions of process parameters with tremendous reduction in totalnumber of experiments (Bianco et al., 2011; Mohajeri et al., 2010).Design Expert 7.1.6 software (Stat-Ease Inc., Minneapolis, USA) wasused for the design, mathematical modelling, regression analysisand optimisation of process parameters involved in the MF treatment.

The low, centre and high levels of each independent factor aredesignated according to face centred CCD as−1, 0 and 1 level respec-tively. The coded and actual values of the independent variables atvarious levels are listed in Table 2. All variables at zero level consti-tute the centre points and the combinations of each variable at eitherlowest (−1) or highest (+1) level constitute the axial points. Thiswork aimed to investigate the responses on PAH removal for upper(y1) and lower part (y2) of the soil column and leachate pH (y3) atambient temperature of 28 °C. For statistical calculations, the inde-pendent variables Xi have been transformed into xi in order to allowcomparison of factors with different natures and units according toEq. (1).

xi ¼Xi e Xo

ΔXwhere i ¼ 1;2;…;k: ð1Þ

where xi is the dimensionless coded value of the ith independentvariable Xi, Xo is the actual value of Xi at the centre point and ΔX isthe step change.

The complete design matrix consisted of 30 runs (2k=24=16factorial points, 2k=8 axial points and a centre point with 6 replica-tions to give better estimation of the experimental error and theprobability of curvature). The experimental design matrix for the de-termination of predicted responses in the MF treatment of the PAH-contaminated soil is given in Table 3.

An empirical second order polynomial model describing the inter-actions between the process dependent variables (responses) and in-dependent variables is expressed in Eq. (2).

y ¼ βo þXki¼1

βxi þXki¼1

βiix2i þ∑

i¼1

Xki≠j¼1

βijxixij þ ε ð2Þ

where y represents the predicted response (i.e. for PAH removal orleachate pH), i , j are linear and quadratic coefficients respectively,while β indicates regression coefficient, k stands for the number offactors (x) considered and ε signifies the random error (Stat-Ease,2009). This work aimed to optimise four reaction parameters includ-ing weight ratios of H2O2/soil (0.05–0.1), Fe3+/soil (0–0.04), SP/soil(0–0.04) and reaction time (3–24 h). The selection of the rangeswas carried out on the basis of results obtained from preliminarysoil slurry experiments (Venny et al., 2011), considering limits forthe experiment set-up (maximum capacity of the glass column) andpotential experimental hazards from the exothermic Fenton reaction(Huling and Pivetz, 2006).

Analysis of variance (ANOVA) was employed for statistical ana-lyses of the results. The quality of fit of the predicted models was in-dicated by the coefficient of determination R2 while its statisticalsignificance was examined by the student t-test. The significance ofeach model term was evaluated by the P-value (probability) with95% confidence level. Noteworthy, the statistical analysis assumesequal variance of the data. The response surface equations were

244 Venny et al. / Science of the Total Environment 419 (2012) 240–249

optimised using numerical evaluation for maximum PAH removalwith leachate pH in the range of 4 to 6.

2.6. Bioremediation treatments

High dosage of H2O2 often limits the application of Fenton oxida-tion due to the toxicity of H2O2 towards several microorganismsthat may lead to deterioration of the ecosystem (Winterbourn,1995). Due to that reason, the bioremediation treatments were per-formed on two selected runs (nos. 8 and 16 in Table 3) that repre-sented the highest H2O2 dosage employed for the PAH oxidation.These biological treatments were sequenced after the MF oxidationdue to the fact that it has proved feasible for treating lower contami-nation sites (Valderrama et al., 2009).

After the MF oxidation, the soil samples were divided into bothupper and lower parts. For natural attenuation experiments, approx-imately 100 g of pre-treated soil samples (each for upper and lowerpart) for each experimental setting were placed in a 250 ml beaker(covered with aluminium foil). The soil samples were kept in an incu-bator operated at constant temperature of 30 °C for a certain period(1, 2, 4, 6 and 8 weeks). The soil samples were weekly mixed usingspatula and moistened with 4.5 ml of distilled water per 100 g ofsamples.

Identical conditions provided in the natural attenuation were ap-plied in the biostimulation experiments, with the addition of200 mg/kg of (NH4)2SO4 (3.040 ml at 0.05 M) and 200 mg/kg ofKH2PO4 (2.940 ml at 0.05 M) at the beginning of the bioremediationtest. The control was sterilised 3 times by autoclaving at 105 °C for60 min with 2-day intervals.

2.7. Analytical methods

PAHs in the soil samples were extracted using automated Soxhletextraction (Gerhardt Soxtherm) according to the US EPA Method3540C. In brief, 5 g of soil sample was placed in a thimble andmixed with Na2SO4 at a ratio of 2:1 w/w. The mixture was extractedwith 140 ml of n-pentane for 3 h. The remaining solvent was evapo-rated to dryness using a rotary evaporator (Heidolph) and subse-quently solvent-exchanged using 1 ml of ACN.

Subsequently, the PAHs were analysed using a gas chromato-graph (GC, Perkin Elmer Clarus 500), equipped with a flame ionisa-tion detector (FID) and fused silica capillary column (DB-5MS,30 m×0.25 mm×0.25 μm, splitless injection), according to the USEPA Method 8100. Helium gas (16 psi) was used as the carrier gas.The temperatures of the injector and the detector were set at 290 °Cand 300 °C respectively. The temperature in the oven was set at100 °C for 1 min, ramped at 25 °C/min to 310 °C and held for 2 min.The concentration of individual PAHs in the solvent was quantifiedusing standard calibration method (all R2>0.98). Individual PAHswere identified by retention times of approximately 8.20 min and9.60 min for PHE and FLUT respectively. The limits of detectionwere found to be 1.24 and 0.9 mg/kg for PHE and FLUT respectively.

3. Results and discussion

3.1. MF oxidation

3.1.1. Efficacy of chelated iron in MF oxidationIn an attempt to evaluate the efficacy of MF treatment in the pres-

ence of chelated iron, three different scenarios were comparedincluding (a) experimental runs with only H2O2 (run nos. 1, 2, 9,10), (b) runs with H2O2 and Fe3+, (run nos. 3, 4, 11 and 12) and (c)runs with H2O2 and Fe3+ as well as SP (run nos. 7, 8, 15, and 16). Itwas obvious that H2O2 alone resulted in PAH removals in the rangeof 40.40% to 61.42% within 3 h of reaction. When Fe3+ was intro-duced, the PAH removals were further improved to between 60.71%

and 72.48% again within 3 h of reaction. The results revealed that in-deed Fe3+ serves as an efficient catalyst for H2O2 decomposition intreating PAH-contaminated soils. In contrast, the addition of SP toH2O2 system without iron (run no. 9 compared to no. 13) dramatical-ly reduced the PAH removal to 16.11%. In separate cases with the ad-dition of SP into H2O2 and Fe3+ system (run nos. 7 and 8), PAHremoval was enhanced to 80.22% within 3 h of reaction, most proba-bly through the formation of iron complexes which caused increasediron availability for the catalytic decomposition.

The increase in reaction time, however, did not necessarily resultin a positive influence on PAH removal efficiency. Unlike in scenario(c) where SP was applied, reaction time was positively correlatedwith PAH removal by compromising the leachate pH in scenarios(a) and (b) which might render the process to be incompatible withbiological treatment. It was evident that there was a trade-off be-tween the reactants dosage, reaction time and PAH removal indicat-ing the complexity in determining the optimum operating conditionwithin the ranges studied. The constraints in determining the opti-mum operating conditions are discussed in the following sections.

3.1.2. Regression modelsRegression equations were developed to quantitatively describe

the MF treatments using a least square analysis (Eqs. (3)–(5)). Itcan be seen that the upper and lower PAH removals (y1 and y2)increased with H2O2 and Fe3+. This finding is expected since bothH2O2 and Fe3+ are the key reagents for the formation of reactive•OH radicals. In contrast, the negative influence of SP on PAH removal(y1 and y2) was unlike the results observed in aqueous treatmentswhich showed positive impact of inorganic pyrophosphate in treatingorganic pollutants (Rastogi et al., 2009; Venny et al., 2011). This dis-crepancy could be attributed to the different type of targeted contam-inant, operating conditions and/or pH regime. It is plausible that theSP solution caused the overall pH to increase slightly when it wasfirst introduced resulting in an environment with potential iron pre-cipitation and impediment of stable H2O2 decomposition. Soil clog-ging and/or channelling, at the same time, might have occurred as aresult of potential iron precipitation which in turn reduced perme-ability and led to ineffectiveness of the Fenton treatment. The resultssuggested that pH adjustment of SP solution would be necessary forbetter PAH removal. On the other hand, the decrease in PAH removalefficiency with reaction time could be justified in terms of the lifetimeof •OH radicals within the soil matrices. The •OH radicals generatedfrom the Fenton reaction act as the principal oxidising agents forthe destruction of PAHs. As decomposition of H2O2 to generate •OHradicals is known to be initially rapid, Fenton reaction is often con-strained by the limited lifespan of •OH radicals in prolonged treat-ments. Moreover, many complex reactions could also be involved asthe radicals might react with other naturally occurring minerals andorganic matters non-selectively at diffusion controlled rates within107–1010 M−1 s−1 (Nam et al., 2001; Kang and Hua, 2005).

Meanwhile, the leachate pH (y3, linear equation) was negativelycorrelated with H2O2, Fe3+ and reaction time. This phenomenon isexpected since Fenton-driven oxidation is acid-generating (the solu-tions of H2O2 and Fe3+ are acidic in nature) which contributes toacidification (Huling and Pivetz, 2006). The slight decrease of pHwith time was also likely due to the presence of sulphate ions fromFe2(SO4)3·xH2O dissociation and reaction with water moleculesnear the end of the reaction. Despite the fact that SP solution usedwas relatively alkaline (pH>7.5), a decrease in pH could be attribut-ed to the enormous buffering capacity of the model soil (Kakarla et al.,2002).

y1 ¼ 67:59þ 1:42x1 þ 12:77x2−6:76x3−2:84x4−8:44x3x4−11:29x22

ð3Þ

Table 4ANOVA for response surface model for upper PAH removal.

Source SS DF Mean square F-value Prob>F Remark

Model 6794.49 14 485.32 5.43 0.0012 Significantx1 36.49 1 36.49 0.41 0.5324x2 2936.71 1 2936.71 32.87 b0.0001x3 823.02 1 823.02 9.21 0.0084x4 145.20 1 145.20 1.63 0.2218x1x2 1.71 1 1.71 0.02 0.8917x1x3 6.71 1 6.71 0.08 0.7877x1x4 129.61 1 129.61 1.45 0.2471x2x3 307.26 1 307.26 3.44 0.0834x2x4 7.76 1 7.76 0.09 0.7722x3x4 1140.34 1 1140.34 12.76 0.0028x12 192.33 1 192.33 2.15 0.1630x22 603.40 1 603.40 6.75 0.0202x32 145.25 1 145.25 1.63 0.2217x42 49.61 1 49.61 0.56 0.4677Residual 1340.25 15 89.35Lack of fit 1148.33 10 114.83 2.99 0.1191 InsignificantPure error 191.91 5 38.38R2 0.84AP 9.46

SS: sum of squares; DF: degree of freedom; AP: adequate precision.

Table 6ANOVA for response surface model for leachate pH.

Source SS DF Mean square F-value Prob>F Remark

Model 40.50 10 4.05 15.97 b0.0001 Significantx1 0.33 1 0.33 1.30 0.2676 Insignificantx2 28.55 1 28.55 112.60 b0.0001 Significantx3 7.01 1 7.01 27.63 b0.0001 Significantx4 0.06 1 0.06 0.26 0.6190 Insignificantx1x2 0.10 1 0.10 0.39 0.5422 Insignificantx1x3 0.18 1 0.18 0.70 0.4119 Insignificantx1x4 0.00 1 0.00 0.01 0.9181 Insignificantx2x3 3.99 1 3.99 15.74 0.0008 Significantx2x4 0.17 1 0.17 0.65 0.4284 Insignificantx3x4 0.11 1 0.11 0.44 0.5170 InsignificantResidual 4.82 19 0.25Lack of fit 4.47 14 0.32 4.61 0.0505 InsignificantPure error 0.35 5 0.07R2 0.89AP 14.04

SS: sum of squares; DF: degree of freedom; AP: adequate precision.

245Venny et al. / Science of the Total Environment 419 (2012) 240–249

y2 ¼ 66:95þ 0:077x1 þ 14:20x2−5:59x3−2:70x4þ 6:05x2x3−10:71x3x4−14:19x22 ð4Þ

y3 ¼ 3:60� 0:14x1−1:26x2 þ 0:62x3−0:06x4−0:50x2x3 ð5Þ

A remarkable interaction effect among the iron and SP (x2x3) aswell as SP and reaction time (x3x4) was also observed, particularlyfor lower PAH removal (y2). The behaviour of interactions reflectedthat sufficient time is required for SP to form complexes with iron cat-alyst and reach equilibrium state. This can be described by the stu-dent t-test and P-values listed in Tables 4–6.

The Prob>F values less than 0.05 indicated statistically significantmodels for predicting the upper and lower PAH removal as well as theleachate pH at which there is only a 0.01–0.12% chance that the F-values this large could occur due to noise. The lack of fit (LOF) F-values for each response were insignificant relative to the pureerror (PE) implying good fitting to the proposed models. The analysisalso demonstrated the significant terms of x2, x3, x3x4 and x22 forupper PAH removal; x2, x3, x2x3, x3x4 and x22 for lower PAH removal

Table 5ANOVA for response surface model for lower PAH removal.

Source SS DF Mean square F-value Prob>F Remark

Model 8750.45 14 625.03 5.49 0.0011 Significantx1 0.11 1 0.11 0.00 0.9759x2 3627.41 1 3627.41 31.86 b0.0001x3 563.04 1 563.04 4.94 0.0420x4 130.85 1 130.85 1.15 0.3007x1x2 0.07 1 0.07 0.00 0.9809x1x3 31.12 1 31.12 0.27 0.6088x1x4 24.93 1 24.93 0.22 0.6466x2x3 586.53 1 586.53 5.15 0.0384x2x4 13.19 1 13.19 0.12 0.7383x3x4 1835.42 1 1835.42 16.12 0.0011x12 326.70 1 326.70 2.87 0.1110x22 1097.36 1 1097.36 9.64 0.0073x32 113.37 1 113.37 1.00 0.3342x42 46.56 1 46.56 0.41 0.5322Residual 1708.03 15 113.87Lack of fit 1537.54 10 153.75 4.51 0.0551 InsignificantPure error 170.50 5 34.10R2 0.84AP 9.27

SS: sum of squares; DF: degree of freedom; AP: adequate precision.Fig. 2. Predicted versus actual plot for (a) upper PAH removal, (b) lower PAH removaland (c) leachate pH.

246 Venny et al. / Science of the Total Environment 419 (2012) 240–249

and x2, x3, x2x3 for leachate pH. Surprisingly, the weight ratio of H2O2/soil (x1) and reaction time (x4) were found to be statistically insignif-icant for the three responses. This result could be ascribed to rapid re-actions of the H2O2 that occurred within the first few minutes (Kanelet al., 2003). In addition, the PAH removal in the lower part of thepacked soil column was also found to be lower than the removal inthe upper part (differed by 18–144%) although the same contact

Fig. 3. Three-dimensional surface plot for upper PAH removal.

area was provided (four dispensing holes at each layer). This observa-tion was presumably due to (1) the closed-end structure of the glasstube that led to partially clogged end (filled with soil samples prior tothe treatment) and reduced distribution of the MF reactants at thelower part and (2) more densely packed soil layering in the lower

Fig. 4. Three-dimensional surface plot for lower PAH removal.

Table 7Model validation.

Response X1 X2 X3 X4 Observed Predicted

Upper PAH removal (%) 0.05 0.025 0.04 3 79.42 85.95Lower PAH removal (%) 0.05 0.025 0.04 3 68.06 74.47Leachate pH 0.05 0.025 0.04 3 4.42 4.00

247Venny et al. / Science of the Total Environment 419 (2012) 240–249

part of the column causing the flow behaviour of the MF reactants toencounter channelling (limited penetration) and lateral spreading(Illangasekare, 1998).

Despite their insignificance, the terms x1 and x4 were included inthe analyses for maintaining a hierarchical structure of the models.Satisfactory correlation coefficients (R2) of 0.84, 0.84 and 0.89 forthe upper and lower PAH removals as well as the leachate pH wereobtained from the regression. Adequate precision (AP) values whichmeasure the signal to noise ratio were found to be higher than 4 con-firmed that adequate signal for the models could be used to navigatethe design space developed by the CCD. The predicted (obtained fromthe models) versus actual (obtained from laboratory experiments)plots are illustrated in Fig. 2. The data distributed well about they=x line specifying good agreement between the predicted and ob-served data. Furthermore, the normality assumption was also foundto be fulfilled for all responses as the normal plot of residual corre-sponded closely to a straight line (data not shown).

3.1.3. Three-dimensional (3D) surface plots and optimisation analysisDue to the linear correlation in leachate pH (Eq. (5)), it is mean-

ingless to present the responses in 3D surface plot. Figs. 3 and 4 dem-onstrated the 3D surface plots for upper and lower PAH removalsrespectively. Clear peaks and curvature confirmed pronounced inter-actions among the tested variables.

For any Fenton oxidation process, the ratio of Fe2+:H2O2:soil forthe removal of PAHs has been demonstrated to be contaminant-specific and plays a major role during the course of Fenton treatment(Kulik et al., 2006; Sun and Yan, 2008). This is likely due to the differ-ent characteristics of the model soils and the occurrence of scaveng-ing effect towards the generated •OH radicals. The latter is mainlycontributed by the presence of naturally occurring organic matter insoil, excess iron catalyst and H2O2 competing for available •OH radi-cals to destruct the PAHs, as illustrated in Eqs. (6) and (7).

Fe2þ þ •OH→Fe

3þ þ OH− ð6Þ

H2O2 þ •OH→HO2• þ H2O ð7Þ

The results from the present study elucidated that an increase ofthe oxidant (H2O2/soil) from 0.05 to 0.10 being the least significanton PAH removal. There are two possibilities for this observation. First-ly, it could be due to the presence of side reactions as described inEqs. (6) and (7) that led to undesirable scavenging effects. Secondly,

Table 8Upper part PAH removal after MF treatment followed by bioremediation (Run no. 8).

Week Upper part

PHE removal (%) FLUT removal (%)

Initial (MF) Attenuation Biostimulation Initial (MF)

87.22±3.01 73.22±3.261 90.13±0.06 92.34±0.112 91.06±2.87 94.08±2.724 91.66±9.40 94.21±7.096 92.14±3.12 95.33±5.218 93.70±4.11 95.28±3.90Increase (%) 6.48 8.06

the amount of reactant implemented resulted in excessive oxidantH2O2 for the loamy sand used which contained very little organicfraction. The excess H2O2 then caused self decomposition of H2O2

into •OH radicals that likely resulted the recombination of •OH radi-cals and generation of more H2O2 thereby decreasing the amountof •OH radicals (Eq. (8)). Similar phenomenon of undesired sidereactions have also been reported in the literature. Flotron et al.(2005) who studied the removal of sequestered PAHs from soilshighlighted the competition between PAHs and natural organic mat-ter with •OH radicals. The excessive loss of oxidant due to reactivitytowards non-targeted compounds is one of the disadvantages ofFenton based oxidation. The presence of oxidant scavengers, particu-larly with concentrated H2O2 may reduce system efficiency becauseof an increase in quenching reactions (Ferrarese et al., 2008).

•OH þ •OH→H2O2 ð8Þ

Aside from the aforementioned, pH also plays an important role.In general, an increase in pH reduced the efficiency of the MF treat-ment. It was obvious that when the leachate pH reached a value with-in 5.70–6.64 (run nos. 5, 6, 13, 14 and 19), maximum removals ofb46% were obtained for both the upper and lower parts. The overalloptimum condition, however, first estimated from the hump in thecontour plots (Figs. 3 and 4), was approximately at Fe3+/soil 0.028,and SP/soil 0.02 with little influence from reaction time. However,further refinement on the optimum condition was applied using nu-merical optimisation method (Design Expert) for maximum PAH re-moval and leachate pH targeted in the range of 4 to 6, taking intoaccount the soil acidity, as well as the fact that soil fertility oftentakes place near this range (Pidwirny and Jones, 1999) and that athigh pH, soil nutrients such as P becomes insoluble and unavailablefor uptake. Under these constraints, the optimum condition underthe highest desirability condition of 0.76 at H2O2/soil 0.05, Fe3+/soil0.025, SP/soil 0.04 and reaction time 3 h resulted in 85.95% and74.47% upper and lower PAH removal with a resulting leachate pHof 4. Model validation was then performed by conducting anotherset of laboratory experiment at the optimum condition. As shown inTable 7, the observed experimental result of 79.42% and 68.08%upper and lower PAH removal and leachate pH 4.42 fitted well tothe predicted values (differenceb10%). On the whole, the applicationof RSM in this study could also minimise potential risks associatedwith inappropriate use of oxidant (Haselow et al., 2003; ITRC, 2005).

3.2. Bioremediation treatments

The effects of natural attenuation and biostimulation as bioreme-diation approach after the MF treatment of the PAH-contaminatedsoils have been investigated as listed in Tables 8–11. In all cases,both natural attenuation and biostimulation have facilitated continu-ous PAH removal in the upper and lower parts. However, naturaldegradation of PAHs in the soil was slightly less efficient comparedto biostimulation in which soil nutrients such as N, P and K were

Average (%)

Initial Attenuation Biostimulation

Attenuation Biostimulation

80.2273.08±1.30 75.81±0.85 81.61 84.0874.25±2.16 76.32±2.29 82.66 85.2075.39±7.34 77.09±4.97 83.53 85.6575.97±2.88 77.20±5.49 84.06 86.2777.08±4.26 78.41±4.13 85.39 86.853.86 5.19 5.17 6.63

Table 9Lower part PAH removal after MF treatment followed by bioremediation (Run no. 8).

Week Lower part Average (%)

PHE removal (%) FLUT removal (%) Initial Attenuation Biostimulation

Initial (MF) Attenuation Biostimulation Initial (MF) Attenuation Biostimulation

78.06±2.88 75.58±3.47 76.821 80.86±3.22 83.09±3.97 75.72±2.85 77.18±4.06 78.29 80.142 82.31±4.94 84.17±2.33 76.33±5.40 78.36±3.25 79.32 81.274 82.94±3.83 84.08±4.29 76.08±4.06 78.24±4.04 79.51 81.166 83.06±7.03 85.38±6.41 76.85±6.49 79.30±6.88 79.96 82.348 83.31±6.28 85.64±3.08 77.16±5.36 79.87±2.83 80.24 82.76Increase (%) 5.25 7.58 1.58 4.29 3.42 5.94

248 Venny et al. / Science of the Total Environment 419 (2012) 240–249

added to stimulate naturally occurring microbial populations. The en-hanced degradation of PAHs in the post-treatment has shown that theuse of SP as CA is not likely to pose environmental concern and theecological impacts after the MF oxidation due to the presence ofH2O2 can be reduced by biological treatment. This is further sup-ported by Nam et al. (2001) who reported the advantage of imple-menting bioremediation in conjunction with MF oxidation insteadof either treatment alone in destroying a mixture of PAHs. In the pre-sent study where inorganic SP was utilised as the CA, it was likely thatSP also contributed to the addition of phosphate ions to the soil sys-tem during incubation.

Regardless of the type of the bioremediation treatment, the resultsshowed that PHE as a LMW PAH was more susceptible to bioremedi-ation treatment than FLUT (Tables 8–11). The observation was ingood agreement with literature reporting that HMW PAHs are moreresistant to biodegradation due to their more hydrophobic natureand stronger sorption properties onto micropores of particulates insoil matrices (Rivas, 2006; Gan et al., 2009).

As shown in Tables 8–11, the effect of the MF reaction time did notsignificantly influence the steady rate of increase in PAH removal dur-ing bioremediation. It was discovered that the efficiency of the biore-mediation methods decreased after the soil samples have beenincubated for 4 weeks. Apart from that, greater enhancement attain-able from experiment run no. 16 (Tables 10 and 11) was observedin which more PAHs were available and remained in the soil samples(greater driving force/concentration gradient).

For the upper parts, PAH removal increased in the range of 6.48%to 10.09% and 3.86% to 7.86% for PHE and FLUT respectively over anincubation period of 8 weeks. Meanwhile for the lower parts, an in-crement in PAH removal varied from 5.25% to 7.58% and 1.58% to7.19% for PHE and FLUT respectively. On average, the effect of nutrientaddition (biostimulation) to the model soil promoted notable PAHdegradation of up to 9.38% enhancement in PAH degradation overan incubation period of 8 weeks. In comparison, the highest PAH re-moval obtained by using natural attenuation was 6.34%.

The results in general showed that SP-enhanced MF treatment ofthe PAH-contaminated soil was compatible with bioremediationthus this serves as a viable integrated strategy for PAH oxidation

Table 10Upper part PAH removal after MF treatment followed by bioremediation (Run no. 16).

Week Upper part

PHE removal (%) FLUT removal (%)

Initial (MF) Attenuation Biostimulation Initial (MF)

56.80±3.17 43.22±4.811 59.29±4.03 61.86±3.642 61.06±3.22 63.19±5.264 62.41±6.07 64.28±4.066 62.80±2.46 65.94±3.148 64.09±2.88 67.70±3.87Increase (%) 7.29 10.90

and soil quality enhancement after MF oxidation process. Soil proper-ties and microbial population also affect the efficacy of this integratedremediation approach hence detailed site characterisation studies areneeded to ascertain the most cost-effective way to implement themethod.

4. Conclusions

In-situ remediation can eliminate the need for excavating con-taminated soils thus allowing rapid site closure although it is chal-lenged by site complexities. While MF technologies are currentlybeing employed to facilitate successful field application by mitigatingthe mobility and longevity concerns associated with conventionaltreatments (Kakarla et al., 2002), the implementation of inorganicCA for remediation of PAH-contaminated soils is still in the earlystages of development. Therefore, the use of SP in MF soil treatmentwas investigated using soil column experiments simulating in-situsoil flushing. Unlike the majority of Fenton studies utilising soil slurryexperiments, the approach used in this present study is more repre-sentative of field conditions and serves as a platform to observe in-situ Fenton oxidation.

Under the studied range of operating variables, SP at optimisedcondition favoured PAH oxidation and resulted in >68% PAH removalfor both upper and lower parts of the soil column. The negative influ-ence of SP addition in some of the experimental runs, however, couldbe attributed to the unbuffered SP solution and the existence ofscavenging effects towards •OH radicals during prolonged treatmentduration. As reaction time increases, abundant reactive species willreact with H2O2 including, but not limited to, microbial enzymes, or-ganic matter and naturally occurring metal ions (Huling and Pivetz,2006). In this case, it is plausible that SP act as a sink for these •OHradical scavengers.

Overall, this work has demonstrated that MF oxidation using SP-chelated iron is compatible with biological treatment in promotingPAH oxidation in soils. Optimum operating condition for the MF oxi-dation was achieved at H2O2/soil 0.05, Fe3+/soil 0.025, SP/soil 0.04and reaction time 3 h with PAH removals of 79.42% and 68.08% attain-able for the upper and lower parts of the soil column respectively. The

Average (%)

Initial Attenuation Biostimulation

Attenuation Biostimulation

50.0145.10±3.74 46.30±3.05 52.20 54.0846.86±3.04 48.72±5.81 53.96 55.9646.96±5.69 50.11±3.86 54.69 57.2048.12±5.45 50.98±3.44 55.46 58.4648.60±4.29 51.08±4.37 56.35 59.395.38 7.86 6.34 9.38

Table 11Lower part PAH removal after MF treatment followed by bioremediation (Run no. 16).

Week Lower part Average (%)

PHE removal (%) FLUT removal (%) Initial Attenuation Biostimulation

Initial (MF) Attenuation Biostimulation Initial (MF) Attenuation Biostimulation

49.34±5.04 46.08±3.16 47.711 51.73±2.09 53.49±4.12 46.13±4.41 49.22±3.84 48.93 51.362 52.89±0.84 54.64±1.30 47.38±1.28 51.38±1.67 50.14 53.014 53.04±3.77 55.82±3.05 48.07±3.06 52.85±4.13 50.56 54.346 54.26±2.46 56.30±2.66 49.94±3.32 53.09±1.94 52.10 54.708 54.80±4.14 56.91±3.87 50.31±4.58 53.27±4.08 52.56 55.09Increase (%) 5.46 7.57 4.23 7.19 4.85 7.38

249Venny et al. / Science of the Total Environment 419 (2012) 240–249

results also showed that MF oxidation followed by biodegradation(natural attenuation or biostimulation) is a promising and inex-pensive technique to remediate soils contaminated with LMW andHMW PAHs. Biostimulation as a post-treatment step was a more ef-fective solution compared to natural attenuation in which up to9.38% and 6.34% increases in PAH removal respectively could beobtained after 8 weeks of incubation period. Detailed studies on re-duced permeability due to potential iron immobilisation and kineticmodelling as a function of column depth are recommended for futurework.

Acknowledgements

This work was supported by the Ministry of Science, Technologyand Innovation (MOSTI), Malaysia under the eScienceFund 03-02-12-SF0078. The Faculty of Engineering at the University of Notting-ham Malaysia Campus is also acknowledged for its support towardsthis project.

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