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Journal of Hazardous Materials 174 (2010) 770–775

Contents lists available at ScienceDirect

Journal of Hazardous Materials

journa l homepage: www.e lsev ier .com/ locate / jhazmat

oil remediation using a coupled process: soil washing with surfactant followedy photo-Fenton oxidation

icardo D. Villa1, Alam G. Trovó2, Raquel F. Pupo Nogueira ∗

NESP – São Paulo State University, Institute of Chemistry of Araraquara, Department of Analytical Chemistry, P.O. Box 355, 14801-970 Araraquara, SP, Brazil

r t i c l e i n f o

rticle history:eceived 16 February 2009eceived in revised form2 September 2009ccepted 23 September 2009vailable online 30 September 2009

eywords:oil

a b s t r a c t

In the present work the use of a coupled process, soil washing and photo-Fenton oxidation, was investi-gated for remediation of a soil contaminated with p,p′-DDT (DDT) and p,p′-DDE (DDE), and a soil artificiallycontaminated with diesel. In the soil washing experiments, Triton X-100 (TX-100) aqueous solutions wereused at different concentrations to obtain wastewaters with different compositions. Removal efficienciesof 66% (DDT), 80% (DDE) and 100% (diesel) were achieved for three sequential washings using a TX-100solution strength equivalent to 12 times the effective critical micelle concentration of the surfactant (12CMCeff). The wastewater obtained was then treated using a solar photo-Fenton process. After 6 h irra-diation, 99, 95 and 100% degradation efficiencies were achieved for DDT, DDE and diesel, respectively.In all experiments, the concentration of dissolved organic carbon decreased by at least 95%, indicating

riton X-100

emediationrganochlorineuelsesticides

that residual concentration of contaminants and/or TX-100 in the wastewater was very low. The co-extraction of metals was also evaluated. Among the metals analyzed (Pb, Cr, Ni, Cu, Cd, Mn and Co), onlyCr and Mn were detected in the wastewater at concentrations above the maximum value permitted bycurrent Brazilian legislation. The effective removal of contaminants from soil by the TX-100 washingprocess, together with the high degradation efficiency of the solar photo-Fenton process, suggests that

usefu

this procedure could be a

. Introduction

Washing is a dynamic physical process used to treat con-aminated soil that involves transfer of the contaminant into

liquid stream [1]. Techniques involving washing have beenontinually enhanced, and successfully applied for soil reme-iation worldwide [2–5]. Advances in the equipment neededor in situ remediation, and development of surfactants capa-le of extracting a greater variety of organic contaminants, haveade washing a feasible alternative for remediation of soils con-

aminated with fuels, and several classes of pesticide [1,3,6,7].lthough the washing process is efficient, the contaminants are

ot destroyed, so that further treatment is necessary to removearget compounds from the wastewater [5]. The development offficient wastewater post-treatment methods is one of the mainequirements for the safe application of soil washing processes.

∗ Corresponding author. Tel.: +55 16 3301 6606; fax: +55 16 3301 6692.E-mail addresses: ricardovilla@ufmt.br (R.D. Villa), alamtrovo@smail.ufsm.br

A.G. Trovó), nogueira@iq.unesp.br (R.F.P. Nogueira).1 Present address: UFMT – Mato Grosso Federal University, Chemistry Depart-ent, 78060-900 Cuiabá, MT, Brazil.2 Present address: UFSM – Santa Maria Federal University (CESNORS), Biology

epartment, P.O. Box 511, 98300-000 Palmeira das Missões, RS, Brazil.

304-3894/$ – see front matter © 2009 Elsevier B.V. All rights reserved.oi:10.1016/j.jhazmat.2009.09.118

l option for soil remediation.© 2009 Elsevier B.V. All rights reserved.

However, there are relatively few reported studies in this area[7].

Advanced oxidation processes (AOPs) are promising techniquesfor the treatment of a wide variety of wastewaters [8,9]. AOPsare based on oxidative reactions mediated by the hydroxyl radical(•OH), a species with a high reduction potential (Eo = 2.73 V versusthe normal hydrogen electrode [10]), able to non-selectively oxi-dize a wide variety of organic contaminants. One simple way ofgenerating hydroxyl radicals is using the Fe2+-catalyzed decompo-sition of H2O2 in an acidic medium, known as the Fenton reactionEq. (1) [11]:

Fe2+ + H2O2 → Fe3+ + •OH + OH−, k = 76 M−1 s−1 (1)

The Fe3+ generated in the Fenton reaction is hydrolyzed, formingan iron–hydroxo complex. Under UV and UV–visible (UV–vis) radi-ation, Fe3+ is reduced to Fe2+ (Eq. (2)), which can react with H2O2in the Fenton reaction (Eq. (1)), generating more hydroxyl radicalsand establishing a cycle.

Fe(OH)2+ + h� → Fe2+ + •OH (2)

Although there have been a large number of studies concerningsurfactant-assisted soil washing, as well as Fenton-based processes,less attention has been paid to the possible coupled application ofthese treatments for soil remediation.

R.D. Villa et al. / Journal of Hazardous

Table 1Main characteristics of the soil studied.

Organic matter (%) 7.5Organic carbon (g kg−1) 4.35Sand (%) 41Silt (%) 47

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solution (85:15) [15]. In summary, 50 mL of the sample were placed

Clay (%) 12pH (H2O) 5.7Density (kg m−3) 1500

The aim of this work is to investigate the potential appli-ation of a coupled soil washing and photo-Fenton oxidationrocess for remediation of a soil contaminated by long-erm exposure to dichlorodiphenyltrichloroethane (DDT) andichlorodiphenyldichloroethylene (DDE), as well as a soil artifi-ially contaminated with diesel in the laboratory. The nonionicurfactant TX-100 was chosen for the soil washing processes, sincet is highly effective for the dissolution of hydrocarbons and a vari-ty of pesticides. The wastewaters obtained from different washingxperiments, with different contaminant and dissolved organic car-on (DOC) concentrations, were submitted to solar photo-Fentonrocesses.

. Materials and methods

.1. Reagents

DDT and DDE standards (99% purity) were purchased fromupelco. The diesel used was a commercial product obtained fromroad fuel filling station. Isooctane (Mallinckrodt) and n-hexane

Baker) were used to prepare the pesticide and diesel working stan-ard solutions, respectively. A neutral, deactivated, 70–290 meshlumina (Vetec) was used in the DDT and DDE extraction process.esticide-grade n-hexane and dichloromethane solvents (Tedia)ere used for the extraction of DDT, DDE and diesel. 29% (w/v)2O2 (Synth) was used in the degradation experiments, and Na2SO4

Baker) was used as desiccant. Fe2+ solutions were prepared byissolving FeSO4·7H2O (Carlo Erba) in 0.1 M H2SO4 (Synth). Solu-ion pH was adjusted by the addition of NaOH or H2SO4 (Synth).aCl (Mallinckrodt) was used to reduce emulsion formation dur-

ng liquid/liquid extractions. All the glassware used was previouslyleaned with 10% (v/v) Extran MA-01 (Merck). TX-100 (Merck) wassed in soil washing experiments.

.2. Sampling and soil contamination

The soil used in this work was collected from a former pesticidearehouse in the State of Mato Grosso, Brazil. The soil was heav-

ly contaminated with DDT 8 years before the start of this study,ue to poor storage conditions following the introduction of legalestrictions on its use [12], and also with DDE, which is one ofhe main DDT degradation products [13]. Another sample of soilas used to study diesel degradation, which was collected near theesticide warehouse (about 30 m distant). This soil was not con-aminated, and possessed the same characteristics as the soil usedn the DDT degradation experiments. After sampling, the soil wasassed through a 3.0 mm sieve and dried in air at 25–30 ◦C for 48 h.he main characteristics of this soil were previously determined byilla et al. [14] and are given in Table 1.

The soil spiked with diesel was prepared by adding 200 mL of aolution of 50 g L−1 diesel in n-hexane to 2.0 kg of soil. The spiked

oil was then vigorously homogenized for 30 min and allowed totand for 24 h to eliminate the solvent, the complete loss of whichas confirmed by weighing. The experiments were then subse-

uently carried out.

Materials 174 (2010) 770–775 771

2.3. Soil washing

The removal of contaminants from the soil was evaluated usingthree different TX-100 concentrations (2.1, 4.1 and 8.3 g L−1), cor-responding to about 3, 6 and 12 times the effective critical micelleconcentration (CMCeff) of the surfactant, respectively. In eachexperiment, 150 g of contaminated soil and 1.0 L of TX-100 solu-tion were used. In all cases the working pH was 5.7. A controlexperiment was also undertaken to evaluate solubilization usingdistilled water alone as washing solution. In all cases, the slurryformed was subjected to mechanical stirring, at 110 rpm for 12 h,in an orbital shaker. Samples of slurry and supernatant solutionwere collected (by decanting) after allowing the mixture to settlefor 1 h. The supernatant was filtered through a 0.45 �m membraneprior to analysis, and the soil was washed sequentially 2 more timesunder the same conditions described previously.

2.4. Treatment of wastewater

Wastewaters from soils contaminated with diesel oil, or withDDT and DDE, were subjected to similar treatments. Experimentswere carried out using 250 mL of each wastewater, in the pres-ence of 12 mM FeSO4, in Pyrex beakers fitted with magnetic stirrers.The sample pH was adjusted to 2.8 using 3 M H2SO4 solution. Theexperiments started with the sequential addition of H2O2, usinga peristaltic pump calibrated to add 2.0 mL of 10 M H2O2 solution(0.68 g), every 20 min. The beakers were covered with a transpar-ent PVC film to prevent losses of H2O2 and diesel components byevaporation.

The experiments were carried out during spring (October andNovember) in Araraquara, Brazil (22◦S 48◦W), under clear sky con-ditions between 9:30 and 15:30 h (local time). 12.24 g of H2O2 wereadded during this period. The energy dose accumulated duringirradiation was measured by a radiometer (2100 PMA, Solar LightCo.) fitted with a UVA sensor (320–400 nm), positioned horizon-tally. The average irradiance measured during the experiments was12.5 ± 2.5 W m−2, and the UV dose (6 h) was 30.8 ± 5.8 J cm−2.

Samples were collected every hour for dissolved organic car-bon (DOC) determinations. Concentrations of contaminant in thewastewater were determined at the beginning and at the end ofeach 6 h experiment.

2.5. Chemical analysis

Extraction of DDT and DDE from the soil was carried out usingthe method developed by Villa et al. [12]. In summary, after dry-ing, grinding and homogenizing the soil, triplicate 0.50 g sampleswere mixed with 1.0 g of deactivated alumina. This mixture wastransferred to glass columns (44 cm length × 1.0 cm internal diam-eter), containing 2.0 g of neutral alumina, where the pesticideswere eluted with 210 mL of n-hexane/dichloromethane (7:3) solu-tion. The extracts were concentrated and then dissolved in 6.0 mLn-hexane. DDT and DDE were quantified by gas chromatographywith an electron capture detector, using a Varian 3300 chromato-graph fitted with a DB-5 capillary column (30 m × 0.32 mm). Theanalyses were carried out under the following conditions: injec-tor temperature, 280 ◦C; initial furnace temperature, 110 ◦C; finalfurnace temperature, 250 ◦C; heating rate, 10 ◦C min−1; detectortemperature, 330 ◦C; N2 as carrier gas at a flow rate of 5.0 mL min−1.

The DDT and DDE present in the wastewater were extractedusing liquid–liquid extraction with n-hexane/dichloromethane

in a 100 mL separation funnel and subjected to three sequentialextractions with 10 mL of the solvent. The extracts were then com-bined, and analyzed under the same conditions as those used forthe soil extracts. All determinations were carried out in triplicate.

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72 R.D. Villa et al. / Journal of Haza

Extraction of diesel from the soil was carried out by adding.0 mL of n-hexane/dichloromethane solution (1:1), and 1.0 g ofodium sulfate, to 3.0 g of spiked soil in an airtight sealed glassube (15 cm × 2.0 cm I.D.). The mixture was mechanically stirredt 240 rpm for 2 h, then centrifuged and the extract collected. Thisxtraction procedure was repeated twice. The soil extracts werenalyzed by gas chromatography using a Shimadzu 14B chromato-raph equipped with a DB-5 capillary column (30 m × 0.32 mm)nd a flame ionization detector. The following chromatographiconditions were used: injector temperature, 280 ◦C; initial furnaceemperature, 45 ◦C; final furnace temperature, 250 ◦C; heating rate,2 ◦C min−1; detector temperature, 330 ◦C; H2 as carrier gas at aow rate of 10 mL min−1. The diesel concentration was measured

n terms of total petroleum hydrocarbons (TPH) [16].Extraction of diesel from the wastewater was carried out

n a similar way as for DDT and DDE, but using 200 mL ofhe sample and three sequential extractions with 25 mL of-hexane/dichloromethane (85:15). The diesel analyses werendertaken using the same conditions employed for the soilxtracts. All determinations were carried out in triplicate.

Soil samples used for determination of metal content were pre-ared according to the standard digestion method 6010 [17]. Metalnalyses were performed using flame atomic absorption spec-rometry (AAnalyst 300, Perkin Elmer), after filtration of solutionshrough a 0.45 �m membrane. All determinations were carried outn triplicate, followed by an analytical blank.

DOC in the wastewater was determined using a total organicarbon analyzer (TOC-5000A, Shimadzu), after filtration through a.45 �m membrane.

. Results and discussion

.1. Soil washing

When nonionic surfactants such as TX-100 are added to aoil/aqueous system, some of the surfactant can be sorbed onto soilomponents, and consequently the solubilization process needsigher doses of surfactant than those required for water alone [18].he surfactant concentration at which solubilization initiates, inhe presence of soil, is known as the “effective critical micelle con-entration” (CMCeff). The CMCeff for TX-100 was reported to be.4 × 10−4 M at a soil/water ratio of 1:10 (w/v) [7]. In the presenttudy, experiments were carried out using a soil/water ratio of.5:10 (w/v), and washing solutions possessing surfactant concen-rations of approximately 3, 6 and 12 CMCeff. The objective of thesexperiments was to generate solutions with different contaminantnd DOC concentrations for photo-Fenton treatment experiments,nd demonstrate the versatility of the soil washing processes.

It has been previously suggested that the solubilization ofydrophobic hydrocarbons increases linearly with surfactanticelle concentration [3,19]. In this work, with solutions equiva-

ent to 3 CMCeff, about 36% of the DDT and 49% of the diesel initiallyresent in the soil were removed in the first washing (Fig. 1).

ncreasing to 12 CMCeff, the removal percentage increased to 60%or DDT while in the case of diesel 87% removal was achieved.

No improvement in DDE removal from soil was obtained whenX-100 concentrations were increased from 6 to 12 CMCeff (Fig. 1B).his different behavior, compared with that observed for DDT andiesel, could have been due to the lower initial soil DDE concentra-ion, which could have influenced the kinetics of the extraction. In

ddition, DDE might be generated during the washing procedure,ince it is the main DDT degradation product [13].

In order to increase the removal of contaminants, the soil wasequentially washed 2 more times, under the same conditions usedn the first washing (Fig. 1). The sequential washings had a smaller

Materials 174 (2010) 770–775

effect on DDT and DDE removal, compared to diesel. After threewashings, DDT and DDE removal increased less than 10%, for allsurfactant concentrations evaluated, which was only slightly abovethe relative standard deviation of the extraction method (8%), whilein the case of diesel the removal increased by between 13 and 25%.

Several factors may affect the removal efficiency of DDT, DDEand diesel. Among them are the chemical nature of the contami-nants, the different initial concentrations in soil, and the duration ofsoil contamination. There is considerable evidence that increasedcontact time between contaminant and soil intensifies the adsorp-tion and inhibits removal [20,21]. Considering that contaminationof the soil with DDT and DDE had occurred 8 years prior to thepresent study, it is feasible that the long period of interactionincreased adsorption of the contaminants, hence explaining thepoor removal rates.

The strength of the adsorption of DDT onto the soil used waspreviously evaluated by Villa and Nogueira [22], who studied DDTpartitioning in a soil/n-hexane mixture. In these earlier experi-ments, the soil was subjected to an ultrasound-assisted sequentialextraction with n-hexane. It was observed that 48 ± 4% of DDT wasremoved from the soil during the first extraction. However, theamount extracted in subsequent extractions was less than 10% ofthe amount of DDT remaining in the soil. These results suggest theexistence of a strongly adsorbed fraction of DDT, associated witheither the clay or the organic matter of the soil, which is extremelydifficult to extract.

3.2. Photo-Fenton oxidation of the contaminants in wastewater

After the soil washing experiments, the wastewaters from thesequential extractions (at each surfactant concentration) werecombined and further treated by the solar photo-Fenton process.Degradation percentages higher than 98% were obtained for allcontaminants (Table 2).

A high removal efficiency of DOC from the wastewater was alsoobserved, reaching at least 96% after 6 h (Fig. 2), indicating that thecontaminants and TX-100 had been mineralized. It is importantto mention that more than 98% of the DOC in the washing waterwas due to TX-100, since in a control experiment where the soilsuspension was stirred in the absence of surfactant, only 29 mg L−1

DOC was present. An experiment conducted in the dark for the sametime period using a 12 CMCeff solution resulted in a DOC removalefficiency of 50%. These results confirm the higher efficiency of thephoto-Fenton process, compared to the Fenton process.

The total removal of contaminants and high removal of DOCdemonstrates that combining the soil washing with the photo-Fenton process takes advantages of favorable aspects of eachindividual method. Soil washing, as mentioned previously, can effi-ciently remove several contaminants from soil. However, it is onlya phase transfer process, which does not destroy the contaminantsand which generates a great amount of wastewater, limiting itsapplication [3,4,6]. The direct application of the Fenton process tosoil rarely oxidizes contaminants completely, due to mass trans-fer limitations for hydroxyl radical attack on adsorbed compounds[22,23]. However, the photo-Fenton process in aqueous mediumpresents high efficiency for the total degradation of organic con-taminants. High degradation percentages can be achieved (as inthe present study) when the contaminants are dissolved, whichfacilitates the action of •OH. Furthermore, there is also evidencethat the direct application of the Fenton process is very aggres-sive to the soil, so that in situ remediation imposes substantial

environmental risk [14,24,25]. In aqueous medium, UV–vis radi-ation can be used in the photo-Fenton process (Eq. (2)), drasticallyincreasing the overall efficiency of the process, which is not feasi-ble in soil due to the very limited light penetration [26]. Therefore,the coupling of these processes offers great advantages, since con-

R.D. Villa et al. / Journal of Hazardous Materials 174 (2010) 770–775 773

Fig. 1. (A) DDT, (B) DDE and (C) diesel concentrations in soil after three sequential washings using different concentrations of TX-100.

Table 2DDT, DDE and diesel concentrations in wastewater, before and after application of the solar photo-Fenton process.

TX-100 solution Concentration in wastewater (mg L−1) Degradation (%)

DDT DDE Diesel DDT DDE Diesel

Initial Final Initial Final Initial Final

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3 CMCeff 19 ± 1.4 0.21 ± 0.0 9.0 ± 0.9 06 CMCeff 27 ± 2.0 0.42 ± 0.0 16 ± 1.7 012 CMCeff 38 ± 2.8 0.51 ± 0.0 17 ± 2.2 0

a LOD.

aminants are efficiently removed in soil washing processes andan be further efficiently degraded in the solar photo-Fenton pro-ess.

.3. Co-extraction of metals during soil washing

Some surfactants have been used for desorption of heavy met-ls from soils, under acidic or alkaline conditions, through directomplexation followed by solubilization [27–29]. It was previouslyeported that TX-100 and TX-301 were able to dissolve metalsncluding Cu, Cd, Cr and Ni from soil [30]. Thus, the use of theseurfactants for the removal of organic contaminants may result ino-extraction of metals, raising an additional concern since met-

ls in solution have high mobility and could represent a seriousisk of groundwater contamination during in situ remediation (inn ex situ remediation, dissolved metals can be adequately treatedo avoid any possible environmental contamination). Therefore,lthough the removal of metals from the soil was not the objec-

ig. 2. DOC reductions in wastewater containing DDT and DDE (A) and diesel (B) durin0 min (total amount = 12.24 g). Typical relative standard deviations are <2%.

± 0.0 230 ± 21 ≤41.3 98.9 99.9 100± 0.0 290 ± 28 ≤41.3 98.4 99.2 100± 0.0 350 ± 40 ≤41.3 98.7 99.9 100

tive of the present work, the co-solubilization of metals by TX-100during removal of diesel from soil was also evaluated. Table 3 listswastewater metal concentrations obtained at different surfactantconcentrations, and in a control experiment using only water aswashing solution.

Manganese solubilization was independent of the TX-100 con-centration, since no significant difference was observed in thepresence of TX-100, compared to the control experiment. How-ever, the solubilization of chromium increased about 3-fold using3 and 6 CMCeff solutions, and about 4-fold using 12 CMCeff solu-tion, in comparison to the control. Although there was no record ofmetal contamination of the soil studied, wastewater manganeseand chromium concentrations were higher than the limits for

drinking water allowed by Brazilian legislation [31].

These results suggest that Mn and Cr present in soil can be solu-bilized by surfactants during the remediation process, which meansthat additional treatment of wastewater may be needed to avoidpossible metal contamination. A simple and effective treatment

g the solar photo-Fenton process. [Fe2+] = 12 mM, addition of 0.68 g of H2O2 every

774 R.D. Villa et al. / Journal of Hazardous Materials 174 (2010) 770–775

Table 3Metal concentrations in the soil and wastewaters.

Metal [Metals] in soil (mg kg−1) [Metals] in wastewater (mg L−1) MAVa LODb

Control 3 CMCeff 6 CMCeff 12 CMCeff

Mn 492 2.7 2.6 2.7 2.8 0.1 0.013Cr 64 0.036 0.12 0.089 0.14 0.05 0.019Pb 23 ≤0.080 ≤0.080 ≤0.080 ≤0.080 0.01 0.080Ni 16 ≤0.04 ≤0.04 ≤0.04 ≤0.04 0.02 0.04Cu 27 ≤6.4 × 10−3 ≤6.4 × 10−3 ≤6.4 × 10−3 ≤6.4 × 10−3 2 6.4 × 10−3

−3 −3 × 10−3 −3 −3 −3

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a Maximum allowed value for metals in potable water according to Brazilian legib LOD determined according to IUPAC [32].

ould be precipitation of the metals as hydroxides, by increasinghe pH of the wastewater.

Metal solubilization is affected by the soil type and its horizon,rganic matter content, cation exchange capacity, age, pH, and thenterference effects of other inorganic species present in soil [33].he mechanisms governing the performance of surfactants in sol-bilization of metals from soils are complex, due to the nature ofhe soil system and the variety of factors involved. Consequently,he role of surfactants in metal solubilization is not completelynderstood [34].

. Conclusions

The results obtained in this work indicate that the combinationf soil washing and application of the solar photo-Fenton processan be a useful option for soil remediation. Diesel oil was com-letely extracted from soil, and was not detected in wastewaterfter application of the photo-Fenton process. DDT and DDE wereot fully extracted from soil, even after successive washings, proba-ly due to intensification of pesticide/soil interactions during a longontamination period. However, the pesticide fractions remainingn soil are strongly adsorbed, and therefore probably pose a lownvironmental risk after washings. After 6 h of irradiation, the solarhoto-Fenton process degraded more than 99% of the DDT and DDEesidues present in the wastewater.

The high removal of DOC from the treated wastewater is indica-ive of low concentrations of residual contaminants and/or TX-100.his allows the reuse of treated wastewater for other washings,educing water consumption. Another advantage of the proposedethod is the ability to use solar radiation, abundant in tropi-

al countries such as Brazil, in order to increase the efficiency ofegradation of organic contaminants.

Special attention must be given to the co-extraction of metalsresent in soil. Even when metals are not the target contaminant,hey can be extracted during the washing process. In such cases,he wastewater may require additional treatment, prior to reuse orafe discharge.

Finally, considering that only batch scale experiments were per-ormed in this work, large scale experiments such as continuousolumn or pilot scale field experiments are necessary for morenvestigation of removal efficiencies.

cknowledgments

The authors thank FAPESP (05/00172-0) for the scholarshipwarded to A.G. Trovó. The authors also thank Prof. Dr. M.R. dearchi and Prof. Dr. M.L. Ribeiro for allowing use of the GC-FID andC-ECD equipment, and Prof. Dr. J.C. Rocha for the metal determi-ations.

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≤6.2 × 10 ≤6.2 × 10 0.005 6.2 × 10≤0.078 ≤0.078 0.2 0.078

[31].

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