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Scaling-up parameters for site restoration processusing surfactant-enhanced soil washing coupled withwastewater treatment by Fenton and Fenton-likeprocessesErick R. Bandala a , Horacio Cossio a , Adriana D. Sánchez-Lopez a , Felipe Córdova a , JuanM. Peralta-Herández b & Luis G. Torres ca Grupo de Investigación en Energía y Ambiente, Universidad de Las Américas, Puebla,Mexicob Centro de Innovación Aplicada en Tecnologías Competitivas, Dirección de InvestigaciónAmbiental, León, Mexicoc Unidad Profesional Interdisciplinaria de Biotecnologia Instituto Politecnico Nacional,Mexico City, Mexic

Accepted author version posted online: 24 May 2012. Version of record first published: 02Jul 2012

To cite this article: Erick R. Bandala, Horacio Cossio, Adriana D. Sánchez-Lopez, Felipe Córdova, Juan M. Peralta-Herández &Luis G. Torres (2012): Scaling-up parameters for site restoration process using surfactant-enhanced soil washing coupled withwastewater treatment by Fenton and Fenton-like processes, Environmental Technology, DOI:10.1080/09593330.2012.696721

To link to this article: http://dx.doi.org/10.1080/09593330.2012.696721

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Environmental TechnologyiFirst, 2012, 1–9

Scaling-up parameters for site restoration process using surfactant-enhanced soil washingcoupled with wastewater treatment by Fenton and Fenton-like processes

Erick R. Bandalaa∗, Horacio Cossioa, Adriana D. Sánchez-Lopeza, Felipe Córdovaa, Juan M. Peralta-Herándezb

and Luis G. Torresc

aGrupo de Investigación en Energía y Ambiente, Universidad de Las Américas, Puebla, Mexico; bCentro de Innovación Aplicada enTecnologías Competitivas, Dirección de Investigación Ambiental, León, Mexico; cUnidad Profesional Interdisciplinaria de

Biotecnologia Instituto Politecnico Nacional, Mexico City, Mexico

Estimation of scaling-up parameters for a site restoration process using a surfactant-enhanced soil washing (SESW) processfollowed by the application of advanced oxidation processes (Fenton and photo-Fenton) was performed. For the SESW, differ-ent parameters were varied and the soil washing efficiency for pesticide (2,4-D) removal assessed. The resulting wastewaterwas treated using the Fenton reaction in the absence and presence of ultraviolet (UV) radiation for pesticide removal. Resultsshowed that agitation speed of 1550 rpm was preferable for the best pesticide removal from contaminated soil. It was possibleto wash contaminated soils with different soil concentrations; however the power drawn was higher as the soil concentrationincreased. Complete removal of the pesticide and the remaining surfactant was achieved using different reaction conditions.The best degradation conditions were for the photo-Fenton process using [Fe(II)] = 0.3 mM; [H2O2] = 4.0 mM where com-plete 2,4-D and sodium dodecylsulfate (SDS) removal was observed after 8 and 10 minutes of reaction, respectively. Furtherincrease in the hydrogen peroxide or iron salt concentration did not show any improvement in the reaction rate. Kineticparameters, i.e. reaction rate constant and scaling-up parameters, were determined. It was shown that, by coupling bothprocesses (SESW and AOPs), it is possible the restoration of contaminated sites.

Keywords: contaminated soils; 2,4-D, mixing of solids; AOPs; power drawn; Fenton reaction; site restoration

1. IntroductionAs a result of pesticide use and abuse in the last halfcentury, residues of these chemicals are spread widely inair, water and soil [1]. It is now well-known that thesepesticides and their residues are related to many healthand environmental effects [2]. A number of studies havebeen reported dealing with the worldwide distributionof pesticides, particularly in developing countries whereimpressive amounts of pesticides are released annually tothe environment [3].

Among the developing countries, Mexico is charac-terized by its long history on pesticide use. Accordingto a recent report [4], Mexico ranked among the top 10countries in its overall use of DDT (dichlorodiphenyl-trichloroethane). In general, pesticide use in Mexico hasrisen continuously over the past 15 years [5]. Specifi-cally, 2,4-dichlorophenoxyacetic acid (2,4-D) is among themost widely used pesticides in Mexico. 2,4-D has beenapplied since the early 1990s for weed control in differ-ent crops and aquatic weed control in reservoirs around thecountry [6]. This widespread use may have led to 2,4-Dbeing one of the most extensively distributed pesticidesin soil with the subsequent potential risk of groundwatercontamination [7].

∗Corresponding author. Email: erick.bandala@udlap.mx

Many different restoration technologies have beentested for pesticide-contaminated soil [8]. Among them,surfactant-enhanced soil washing (SESW) processes haveemerged as a highly efficient methodology for the removalof hydrophobic organic compounds such as crude oil, oilderivatives and pesticides [6,9,10]. SESW processes are anex situ soil restoration procedure which includes removalof the contaminated soil from the site and placing the soilin contact with the washing solution in a tank where animpeller or a set of impellers provides the necessary energyto maintain the solids in suspension during the operationand for the soil washing itself [10]. After mixing, the sus-pension is allowed to settle to allow the washed soil and thewastewater effluent to separate.

The main components in the washing process are a cylin-drical tank and the impeller(s). Regarding with the latter,several basic issues remain unknown, i.e. mixing perfor-mance as a function of the impeller(s) configuration, theagitation speed necessary to maintain complete solid sus-pension and the effect of increasing solid load in the suspen-sion on the overall power consumption and/or the removalefficiency [10]. Among these issues, power consumptionis considered very important for SESW process design asa scaling-up parameter [11]. Murugesan [12] proposed an

ISSN 0959-3330 print/ISSN 1479-487X online© 2012 Taylor & Francishttp://dx.doi.org/10.1080/09593330.2012.696721http://www.tandfonline.com

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equation for the calculation of critical impeller speed for asolid suspension in mechanically agitated contactors whichconsiders parameters such as tank and impeller diame-ter, impeller clearance, particle diameter, soil loading anddensities. By considering all these parameters for use inprediction models, the best approach is probably to carryout assessments on power consumption on real soil wash-ing operations and correlate this with the actual efficiencyof the washing process for the removal of the contaminantsin soil. However, relatively few studies dealing with thesetasks have so far been reported.

Once the SESW process is complete, the wastewatergenerated is an important issue that requires consider-ation. SESW wastewater contains the added surfactantand the extracted pollutants, and it may be consid-ered as highly toxic, including very hazardous materialsthat may jeopardize the environment if the wastewaterreleased without proper and effective treatment. Giventhis, it is clear that the SESW process must includethe further removal of the pollutants from the producedwastewater.

A number of different conventional processes havebeen tested for SESW wastewater treatment includingmass transfer processes (i.e. adsorption, coagulation–flocculation) and biodegradation. Both types of method-ology possess important disadvantages, i.e. the generationof toxic wastes and the inhibition of microbial activ-ity related to the presence of highly toxic contaminants,respectively [6]. An interesting alternative, proposed to becoupled with a conventional process, is an advanced oxi-dation process (AOP). AOPs have been demonstrated asa cost-effective technology for application in wastewatertreatment for effluents with low flow and high toxicityincluding a wide variety of organic pollutants [13–15].Among AOPs, Fenton and Fenton-like processes are highlyoxidising methodologies considered as a very interestingalternative for their application to complex wastewatermatrices and particularly efficient in pesticide degrada-tion [16]. In the past, some approaches for the application ofFenton-like processes to SESW wastewater treatment havebeen carried out with successful results by our researchgroup [17,18]; however, there are relatively few studiesdealing with the complete analysis of the coupled process(i.e. SESW followed by AOP for wastewater treatment)have been reported in the past.

The aim of this work is to estimate the main scaling-up considerations for site restoration through couplingsurfactant-enhanced soil washing and advanced oxidationprocesses (Fenton and photo-Fenton) for the treatment ofthe wastewater effluent generated. Another goal was toestimate the power consumption as function of severaldifferent experimental conditions and its correlation withthe removal efficiency of 2,4-D in an artificially contam-inated soil submitted to the SESW process as well as afigure-of-merit related to the energy consumption for theAOP.

2. Methodology2.1. ReagentsAll the reagents used in this work: sodium dodecylsulfate(SDS, Aldrich), sodium chloride (NaCl, Merck), sulfuricacid (H2SO4, Sigma), sodium hydroxide (NaOH, Sigma),ferrous sulfate (FeSO4, Sigma) and hydrogen peroxide(H2O2, 50% stabilized, Sigma) were purchased as reagentgrade (A.C.S.) and used as received without any furtherpurification. The primary standard used for 2,4-D quantifi-cation (purity 98%) was obtained from ChemService.

2.2. Soil characterizationIn order to determine the actual characteristics of the soilsample, different methodologies for determining humid-ity content, particle size, and uniformity and curvaturecoefficients were carried out.

Moisture content was determined in the soil sam-ple using the standard procedure specified in the ASTMD2216-98 standard test method [19]. Briefly, soil sampleswere transferred to trays and placed in an oven at 100 ◦Cfor 24 hours. After drying, soil samples were weighed todetermine their moisture content. Moisture content wasdetermined as proposed in Equation (1)

Moisture content, % = Wwat

Wdry soil× 100 (1)

where Wwat is the amount of water determined in soil asthe difference in weight among the soil before and after thedrying procedure, and Wdry soil is the weight of dry soil.

To determine soil particle size, the sample was analysedby screening, i.e. shaking the sample material through a setof meshes with openings that gradually decrease in compli-ance with ASTM D422-63 standard test method [20]. Afterobtaining the screening for soil particle size, the uniformity(Cu) and curvature (Cc) coefficients were determined asproposed in Equations (2) and (3):

Cu = D60

D10(2)

Cc = D230

D10 · D60(3)

where D60, D30 and D10 are the particle diameter in which60, 30 and 10%, respectively, of particles are retained bythe sieve.

2.3. Synthetic contaminated soil sample preparationSynthetic contaminated soil was produced in the followingway: 4 kg of lime soil (US Mesh 80) was spiked with therequired quantity of 2,4-D solution (Hierbamina®) in orderto obtain a final pesticide concentration of 20 mg/kg of soil.The spiked soil was allowed to equilibrate for three weeksin the dark at 2 ◦C to avoid modification of the pesticideconcentration.

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2.4. SESW assessmentsThe equipment used for mixing in all the SESW exper-iments was a Lightnin Lab Master®™ mixer BK223116and the experimental setup is detailed elsewhere [21]. Thesystem includes a 60 cm steel arrow and a mixing tank(acrylic) of 10 cm in diameter with four baffles of 1 cm thickplaced at every 90◦ within the circumference of the tank.The impeller was a four-inclined blade 45◦ propeller 4IB45(diameter = 5 cm), which was located in the central part ofthe mixing tank to avoid contact with the baffles and 5 cmfrom the bottom. The liquid height (H )/vessel diameter (T )

ratio was about one, while the impeller diameter (D)/tankdiameter (T ) was about 0.5. Figure 1 depicts a view of theemployed experimental setup.

2.4.1. Optimal mixing time and soil concentrationSix different mixing times (15, 30, 60, 90, 120 and 150 min)were tested using the procedure described in Section 2.4.2 at1400 rpm. Once the best time and mixing speed conditionshad been established, four different soil loads (10, 20, 30and 40% w/v) were tested to determine the effect of soilconcentration on the different parameters. As in previousexperiments, power consumption (W ) was determined forthese experiments every 15 min from the mixer’s display

Figure 1. SESW experimental setup.

screen. In every case, samples were obtained as detailedin Section 2.4.2 and then treated and stored following theprocedure described therein.

2.4.2. Optimal mixing speedFor these experiments 150 g of the spiked soil were putin the mixing tank and 450 mL of the surfactant solution(SDS at 0.5% (v/v)) were added. The SDS concentrationwas established as the one with the highest efficiency for2,4-D removal in SESW as reported previously [6]. Fourdifferent mixing speeds (1200, 1400, 1550 and 1700 rpm)were tested during 120 min. By the end of the 120 min ofmixing, the mixer was stopped and samples were taken fromthe resulting water. The samples were transferred into acentrifuge container and centrifuged at 104 g for 10 min. Theresulting supernatant was filtered using a 0.45 μm nylonfilter (Waters AcroPrep filter), transferred to an amber glassvial and stored at 4 ◦C until analysis.

2.4.3. Power consumption and power number estimationThe impeller’s power consumption (W ) was registered fromthe mixer’s main display screen every 15 minutes duringthe first 90 min of the process and the power number wasobtained using Equation (4) [22]:

NP = PρN 3D5 (4)

where P is the impeller’s power (W ), ρ is the fluid density(kg/m3), N is the impeller’s speed (s−1) and D is theimpeller’s diameter (m).

2.4.4. Sample analysisThe filtered samples obtained for the different experimen-tal assessments were analysed for 2,4-D concentrationby liquid chromatography. The analysis of 2,4-D wascarried out without any extraction procedure by high-performance liquid chromatography (HPLC) with ultravi-olet (UV) detection using a Waters liquid chromatographequipped with a UV diode array detector. The analysiswas performed by injecting 25 μL of the sample underan isocratic regime using a Hypersil-GreenEnv, 5 μm ×250 mm × 4.6 mm column (Thermo Hypersil-Keystone).The mobile phase was 75% (v/v) 2% acetic acid and25% (v/v) acetonitrile. The flow rate was 1 mL/minand the ultraviolet–visible (UV-Vis) detector wavelengthwas set at 280 nm. The amount of anionic surfactantin each sample was determined using a spectrophotome-ter (UV-Vis Hewlett Packard 8452 A) using the methy-lene blue active substances (MBAS) analysis. Statisticalanalysis of the obtained data for 2,4-D and SDS analy-sis determined a maximum standard deviation of 5% ofthe data.

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2.5. Pesticide degradation assessmentsDark iron/hydrogen peroxide (Fe/H2O2) degradationexperiments were carried out in 1000 mL glass reactorvials with 500 mL of SESW wastewater containing 2,4-D(50 mg/L), surfactant and organic matter extracted for thesoil. Four iron concentrations (0, 0.1, 0.2 and 0.3 mmol L−1)and four hydrogen peroxide concentrations (0, 3.0, 4.0 and6.0 mmol L−1) were tested.

The procedure for the degradation experiments was asfollows. Once the SESW wastewater was added to the reac-tor, the pH was adjusted using H2SO4 or NaOH (0.1 N)depending on the initial pH value and the appropriate vol-ume of Fe(II) stock solution was added to achieve thedesired initial concentration. The solution was shaken usinga vortex mixer. Then the required oxidant solution (H2O2)

was added to the mixture until the desired concentrationwas achieved and a sample was analysed considering thisas the initiation point (t0) of the dark reaction. The reac-tion mixture was kept in the dark for the entire duration ofeach experimental run. A sample of 25 μL was obtainedevery 12 min and analysed for pesticide concentration inthe HPLC system as described above. The reaction was fol-lowed until HPLC analysis showed no significant furthervariation in pesticide concentration or until the achievedpesticide concentration was below the method detectionlimit.

The photo-Fenton reaction was carried in exactly in thesame way. In this case, all the experiments were performedusing a UV lamp (365 nm, 75 mW), hydrogen peroxide wasadded simultaneously with turning on the lamp and thestart of the degradation process was considered to be thismoment.

2.6. Estimation of scaling-up parameters2.6.1. SESW wastewater treatmentIn the case of the application of AOPs for wastewatertreatment, identification of scaling-up parameters can be acomplex task. Relatively few studies dealing with the identi-fication and estimation of such indexes have been published.The number of factors to be considered is wide and mayinclude local regulations, effluent quality goals and econ-omy of scale, operation and robustness. Although all theseconsiderations are important, the economics of the processremain the most significant. Since many AOPs are liableto be driven through radiant energy (i.e. solar radiation orlamps), Bolton et al. [23] proposed some figures-of-meritfor the electric energy consumption or the solar collectingsurface area, depending on the case, as interesting scaling-up parameters able to estimate the major fraction of theoperating cost or the main initial investment cost, respec-tively. In this work, we have chosen the electric energy permass (EEM ), the electric energy in kilowatt-hours (kWh)required to bring about the degradation of a unit mass (i.e.one kilogram) of a contaminant C in polluted water as

shown in Equation (5) [23]:

EEM = Pt106

V (ci − cf )(5)

where EEM is the electric energy per mass (kWh g−1); P isthe rated power (kW) of the AOP system; V is the volume(L) of water treated in the time t (h); ci and cf are the initialand final concentrations (mg L−1) and the factor of 106

converts mg to kg. Higher EEM values correspond to lowerremoval efficiencies.

3. Results and discussion3.1. SESW process3.1.1. Soil characterizationUsing the results of granulometric analysis, particle sizedistribution curves in half log graphs were created. Thediameters of the particles are plotted in logarithmic scaleand the percentages of material passing through each meshin arithmetic scale. Figure 2 shows the average resultsobtained for soil condition. The moisture content of thesoil was estimated at 10.25% and the uniformity and curva-ture coefficients were determined as 4 and 1, respectively.In agreement with the Unified Soil Classification System(USCS), the soil used in this work corresponds to poorlygraduated sand [24].

3.1.2. Optimal mixing speedIn order to determine the best mixing conditions for the pes-ticide extraction using the proposed SESW process, differ-ent mixing speeds were tested as described in Section 2.4.2.Figure 3 shows the final 2,4-D concentration (C/C0) asfunction of the different speeds. A significant increase inthe pesticide extraction yield can be observed when mixingspeed values increased from 1200 to 1550 rpm (removal of2,4-D went from 68 to 89%). A further increase in the speeddid not produce a further increase in the pesticide extractionyield but equal or lower results.

Figure 2. Granulometric characterization of the contaminatedsoil.

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Figure 3. Final pesticide concentration as a function of mixingspeed.

The power number for every experimental run is shownin Figure 4. It is noticeable that, whereas 2,4-D removal onlyshowed a slight variation with increasing mixing speed,the effort required to the impeller to overdue the generatedfriction among the fluid and the walls and the bottom ofthe tank as well as the resistance generated by the bafflesincreased linearly with the agitation speed. The power num-ber at 1200 rpm was determined as 504; by increasing thespeed to 1400 rpm, the power number reached 756. Whenthe mixing speed was 1550 rpm, the value of the powernumber was 1008 and, finally, for 1700 rpm the NP valuewas 1260.

With these results in mind, it was decided use 1400 rpmas the mixing speed in further determinations, given thatunder these conditions, the system was able to carry outthe best possible pesticide extraction from the soil with-out demanding a high power consumption from the mixingsystem.

These results are in agreement with previous results [21]where it was found that using 30% (w/v) soil/surfactantsolution and a mixture speed of 1400 rpm was good enough

Figure 4. Final pesticide concentration as a function of soil load.

to achieve reasonable extraction yields (i.e. 50%) forhydrocarbon derivatives in soil using an SESW process.

3.1.3. Optimal mixing time and soil concentrationOnce the mixing speed had been established, different soil–surfactant solution ratios (w/v) were tested to identify theeffect of soil concentration on the extraction procedure. Theresults of 2,4-D extraction yields obtained for the differentsoil concentrations tested are depicted in Figure 4. It is inter-esting to note that increasing the soil concentration did notnecessarily mean an increase in the pesticide extraction.Pesticide removal at a soil concentration of 10% (w/v) was73%; when the soil concentration doubled from 10 to 20%(w/v), the pesticide extraction yield went down from 73 to65% (Figure 4).

Further increases in soil load produced only a slightimprovement in the pesticide extraction yield, which couldalso be explained by the variability of the analytical method.A different trend was observed for power number values(Figure 4). It was clear that with increasing soil load in theSESW process, the power required to carry out the mixingof the soil–surfactant system increases significantly, at leastin the first three soil loads. Power number (NP) values wentfrom 378 to 630 when soil load increased from 10 to 20%(w/v); a further increase in the soil load to 40% produceda NP value of about 756. It is noticeable that no variationin the power number was determined by increasing the soilload from 30 to 40% (w/v); NP was determined to be 756in both cases.

Figure 5 shows the removal of 2,4-D as a function oftime for the described mixing conditions (1400 rpm, 30%(w/v) soil–surfactant solution). As shown, 2,4-D extrac-tion is significant as an extraction yield of almost 70% wasreached for the sample after only 10 min of mixing. Practi-cally, no improvement in the final pesticide extraction yieldwas achieved by continued mixing (Figure 4). In Figure 5,the power number (NP) determined for the different mix-ing time values under the same conditions is also shown.

Figure 5. Final pesticide concentration for the 30% w/v contam-inated soil as a function of time.

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It is worth noting how the effort required by the impeller inthe SESW process increased up to 1260 for the first 15 minof mixing. After an initial increase, the power number fellrapidly by almost 25% to a new value of about 1000 after39 min and remained unchanged for the following 30 min.After the start of mixing, NP fell again to 756 and remainedconstant until 90 min.

3.2. SESW wastewater treatmentFigure 6 depicts the behaviour of 2,4-D concentrationduring oxidation experiments under the optimal reactionconditions for different experimental runs. As shown, thepoorest results were obtained for experiments using hydro-gen peroxide in the absence of UV radiation where almostno decrease in the pesticide concentration was observedafter 80 min of reaction. Low pesticide degradation (<5and 18%), was achieved using UV radiation alone (pho-tolysis) and for the use of hydrogen peroxide combinedwith UV radiation, respectively. In the latter, 25% of 2,4-Dwas removed after 80 min of reaction as result of the well-known decomposition reaction of H2O2 by UV radiation atthe wavelength used in these experiments [13,25]. It is prob-able that the poor performance of the H2O2/UV process isrelated to the low amount of hydrogen peroxide availablefor the experiment (4.0 mmol L−1) since further increasesin the oxidant concentration lead to improvements in thefinal pesticide degradation achieved (data not shown).

Very different results were obtained when a transitionmetal catalyst was used. Almost 40% of the initial pes-ticide in the wastewater was degraded in the first 50 minof reaction for the dark Fenton reaction. It is noteworthythat, after initial degradation, no further decrease in pes-ticide concentration was observed in the last 30 min ofreaction. This behaviour may be rationalized as result ofcomplete oxidation of Fe2+ to Fe3+ and the subsequentdecrease in the reaction rate as it has been proposed previ-ously in literature [26]. This effect was not observed during

Figure 6. Typical profile for pesticide degradation in the soilwashing wastewater using Fenton and photo-assisted Fentonreaction.

the photo-assisted Fenton reaction as it is also shown inFigure 6. The photo-Fenton process was capable of almostcomplete pesticide removal (>98%) in only three minutesof reaction using 0.2 mmol L−1 of Fe(II) and 3 mM of H2O2.In previous works carried out by our research group [17,18]we found that sodium dodecyl sulfate, used also in this workas the surfactant for the SESW assessments, may be com-pletely degraded under reaction conditions similar to thosereported in this work but using the Sun as the radiationsource. The same trend as previously reported was observedin this case for the non-photocatalysed processes and notrace of the surfactant was determined when the photo-assisted process was tested (Figure 7). Further increasesin iron or hydrogen peroxide concentrations did not yieldimprovements in pesticide degradation.

Table 1 shows, for comparative purposes, the obtainedvalues for the oxidation rate (OR, mmol min−1), the amountof oxidized 2,4-D per reaction time unit, in every experi-mental run for the different tests performed in this work. Itis interesting to note that, in general, OR values decreasedas the hydrogen peroxide concentration increased for theFenton reaction using the same iron concentration. Thismay be related with the well know effect of excess hydro-gen peroxide as radical scavenger [26]. This trend may beconfirmed when comparing the increase of iron salt in thereaction mixture. As iron concentration increased, the esti-mated OR values became higher for the lowest hydrogenperoxide concentration.

However, when the photo-assisted process was tested itwas observed that the higher the hydrogen peroxide con-centration, the higher the OR values. In the same way,as the Fe(II) concentration increased in the reaction mix-ture, higher values of OR were estimated. The OR reachedthe highest value when [Fe(II)] = 0.2 mM and [H2O2] =3.0 mM; further increases in both iron salt or hydrogenperoxide concentration did not produce any variation inthe OR values. This trend may be related to the rate ofreactions involved in the whole oxidation process. When

Figure 7. Typical profile for surfactant degradation in the soilwashing wastewater using Fenton and photo-assisted Fentonreaction.

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Table 1. Oxidation rate (OR) and electric energy per mass (EEM ) values for 2,4-D under the different Fentonreaction conditions tested.

Reagent concentration (mM) OR (mmol min−1) EEM (kW kg−1)

Experiment No. [Fe2+] [H2O2] Fenton Photo-Fenton Photo-Fenton

1 0.1 3.0 1.99 7.54 1.502 4.0 1.21 11.3 0.993 6.0 1.12 22.6 0.484 0.2 3.0 2.37 75.3 0.155 4.0 1.78 75.3 0.156 6.0 1.27 75.3 0.157 0.3 3.0 2.40 75.3 0.158 4.0 1.86 75.3 0.159 6.0 1.49 75.3 0.15

the non-irradiated Fenton process is used, the main reac-tion occurring in the mixture is between ferrous iron andhydrogen peroxide to produce hydroxyl radicals as statedin Equation (6):

Fe2+ + H2O2 → Fe3+ + HO• + HO− k = 55 M−1 s−1

(6)

Once formed, hydroxyl radicals may react with organicmolecules, i.e. 2,4-D, with a rate constant of 109 to1010 M−1 s−1 [27]. From Equation (6) is noticeable that thereaction for the oxidation of Fe2+ with hydrogen perox-ide will occur until all the ferrous iron has been oxidizedto ferric iron, at this moment the Fenton process will pro-ceed considerably more slowly or stop completely. Whenexcess hydrogen peroxide is supplied to the reaction mix-ture, hydroxyl radical scavenging by the H2O2 will occuras stated in Equation (7):

HO• + H2O2 → Fe3+ + HO•2 + H2O

k = 2.7 × 107 M−1 s−1 (7)

Indeed, when comparing the rate constant for Equations (6)and (7) it is clear that Equation (7) will proceed slightly lessthan six orders of magnitude faster than Equation (6). Asresult, excess hydrogen peroxide will compete for hydroxylradicals against other organic molecules in the reaction mix-ture, producing the observed decrease in the overall ORobserved in Table 1. When UV radiation is used to assistthe process, photo-reduction of ferric iron to ferrous occursas proposed in Equation (8):

Fe(OH)2+ + hϑ → Fe2+ + HO• k = 2 × 10−3 M−1 s−1

(8)

As shown in Equation (8), photo-production of Fe2+ byradiation absorption proceeded slowly compared with reac-tions from Equations (6) and (7). However, it has a prepon-derant role in the process improvement since the regener-ation of ferrous iron in the reaction shown in Equation (6)may keep going. In this case, since availability of Fe2+ is

not limited by its amount in the reaction mixture, furtheraddition of hydrogen peroxide will increase hydroxyl radi-cal generation and the observed improvement in the OR asshown in Table 1. Of course, excess hydroxyl radicals in thereaction mixture will lead to the plateau observed in the ORdata when pesticide molecules become the limiting reagentin the system and, as observed for non-irradiated Fentonexperiments, further increases in H2O2 concentration willlead to a decrease in the process efficiency.

3.3. Scaling-up parameters estimation3.3.1. SESW processAs stated earlier, mixing is probably among the most impor-tant unitary operation involved in the soil washing processand, if scaling-up is being considered, power consump-tion during the mixing procedure may be an importantconsideration. Table 2 depicts some interesting results forscaling-up parameters such as the volumetric power con-sumption (W/L) and volumetric energy requirement (kJ/L)for the different conditions tested and its correlation to 2,4-Dremoval efficiency (%). It is interesting to observe that,despite mixing speed and volumetric power values remain-ing unchanged for the soil load from 10 to 40%, volumetricenergy vary increasing from 21.5 kJ/L for soil load 10%(w/v) to 33.2 kJ/L at soil load 20% (w/v). Then, for further

Table 2. Mixing values, volumetric power, volumetric energyand pesticide removal.

Mixing Volumetric Volumetric 2,4-DSoil load speed power energy removal(%, w/v) (rpm) (W/L) (kJ/L) (%)

10 1400 6.66 21.5 7320 1400 6.66 33.2 6530 1400 6.66 32.2 6440 1400 5.55 30.5 6440 1200 4.44 13.2 6740 1550 10.0 47.2 8940 1700 15.0 93.3 77

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8 E.R. Bandala et al.

increases in soil load, volumetric energy continued decreas-ing to 32.2 and 30.5 kJ/L for soil loads 30 and 40% (w/v),respectively. Nevertheless, 2,4-D removals did not showthe same trend. In this case, the highest removal was deter-mined for a soil load of 10% (w/v) where pesticide removalreached 73%; further increases in soil load were associatedwith lower 2,4-D removal efficiencies with the lowest valuedetermined for soil load 40% (w/v) with 64%.

It is difficult to compared these results with other workspublished in the literature, since the reported works are verydifferent from the present study. For example, Ozkan-Taskinand Wei [28] studied the effect of impeller tank diameterratio on drawdown of solids. They investigated the devel-opment of pitched blade turbine and LE20 impeller, whensuspending up to 1% of solids in water. They investigatedthe effect of the impeller submergence (Cs) to tank ratios(T ) at different impeller clearances for fully baffled devices.For Cs/T ratios between 0.3 and 0.7, values of power pervolume unit (P/V ) were between 10 and 350 W m−3 (forD = T/2).

For experiments where different mixing speeds weretested for the same soil load (40%, w/v), it is notice-able that higher mixing speeds can be associated withhigher volumetric power and energy but not necessarilywith higher 2,4-D removal efficiencies. Maximum 2,4-Dremoval (89%) was reached at 1550 rpm (volumetric power10 W/L and volumetric energy 47.2 kJ/L). Lower or highermixing speeds resulted in lower pesticide efficiency asshown also in Table 2. Nevertheless, it is important to notethat pesticide extraction was higher by using the mixingprocess than those determined in previous work for a hand-shaken system where the highest 2,4-D removal in the firstsoil washing process was 50% [6,29].

3.3.2. SESW wastewater treatmentTable 1 depicts the estimated EEM values for the experi-mental assessments included in this work. As expected, thetrend for EEM values matches the trend previously discussedfor OR. The lowest EEM (the highest efficiency) was deter-mined for the process carried out using [Fe(II)] = 0.2 mMand 3 mM of hydrogen peroxide; under these reaction con-ditions only 0.15 kWh of electric energy will be required fordegradation of one kilogram of pesticide whereas 1.5 kWhwill be required for operating the system if the worst reac-tion conditions are selected (0.1 mM of iron and 3.0 mM ofH2O2). It is interesting to note that the energy requirementfor operating the wastewater treatment increased by oneorder of magnitude only by a slight change in the reactionconditions.

The attractive of such a figure-of-merit like the EEMis related to its applicability for estimating the operatingcosts of the process. Once the EEM values have been esti-mated, the operating costs can easily being determined ifthe volume of wastewater, pesticide concentration within,the electric energy tariff and the cost of reagent are known.

4. ConclusionsThe surfactant-enhanced soil washing (SESW) process wasdemonstrated as very effective for pesticide removal incontaminated soils. The agitation speed of 1550 rpm wasdetermined as the optimum extraction condition since thisachieved the highest washing of the pesticide-contaminatedsoil. However, it is important to bear in mind that powerdrawn is higher as the agitation speed is higher. It waspossible to wash contaminated soils with 10, 20, 30 and40% (w/v) of soil concentrations. The power drawn require-ments were higher as the soil concentration increased.

Wastewater generation is a key aspect of the SESW pro-cess. For 10% (w/v) of soil load, up to 90 kg of wastewaterwill be produced in every washing batch for every 10 kgof soil (it is frequently necessary to wash the soil twice andthrice in order to obtain a desired contaminant concentrationand a rinsing step is often included in the washing proce-dure). On the contrary, if a 40% (w/v) of soil is used, 60litres of wastewaters will be produced for every 40 kg soilbatch.

For the maximum soil load (40% w/v), the maxi-mum agitation speed assessed gave a volumetric powerof 10 W/L, which gave the best washing efficiency ofabout 90%. It was determined that 10 minutes could bea reasonable mixing time, since for the rest of the elapsedtime, the washing efficiency did not show any additionalimprovement.

The photo-Fenton process was determined as very effec-tive for the removal of contaminants (i.e. 2,4-D and SDS) inthe wastewater. Electric energy per mass (EEM ) was foundto be a very useful figure-of-merit for scaling-up the SESWwastewater treatment using AOPs. The reaction conditionswere estimated with an important influence on the EEM val-ues, so this result should be borne in mind. Despite thelow values of EEM determined, it is possible to reduce thecost involved in wastewater treatment by, for example, cou-pling the AOP system with biological treatment or otherconventional wastewater treatment processes.

AcknowledgementsThis work was partially financed by the National Council ofScience and Technology, Mexico (grants 95753 and 168285).

References[1] E.R. Bandala and L.G. Torres, Pesticide removal from water

using advanced oxidation technologies: Challenges and per-spectives, in Pesticide Research Trends, A.B. Tennefy, ed.,Nova Science Publishers, New York, 2008, pp. 49–100.

[2] E.R. Bandala, J. Andres-Octaviano, P. Pastrana, and L.G.Torres, Removal of aldrin, dieldrin, heptachlor and hep-tachlor epoxide using activated carbon and/or Pseudomonasfluorescens free cultures, J. Environ. Sci. Health B 41 (2006),pp. 551–569.

[3] F. Wong, H. Alegria, T. Bidleman, V. Alvarado, F.Angeles, A. Avila, E.R. Bandala, I. Galindo-Estrada,G. Galindo-Reyes, G. Gold-Bouchot, J. Macias-Zamora,

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J. Murguía-González, and E. Ramírez-Espinoza, Passive airsampling of organochloride pesticides in Mexico, Environ.Sci. Technol. 43 (2009), pp. 704–710.

[4] F. Li and R.W. MacDonald, Sources and pathways of selectedorganochlorine pesticides in the Arctic and the effect of path-way divergence on HCH trend in biota: A review, Sci. TotalEnviron. 342 (2005), pp. 87–106.

[5] G. Santacruz, E.R. Bandala, and L.G.Torres, Chlorinatedpesticides (2,4-D and DDT) biodegradation at high con-centrations using immobilized Pseudomonas fluorescens, J.Environ. Sci. Health B 40 (2005), pp. 571–583.

[6] E.R. Bandala and L.G. Torres, Soil washing wastewatertreatment using Advanced Oxidation Processes, in Reme-diation of Soils and Aquifers, L.G. Torres and E.R. Bandala,eds., Nova Science Publishers, New York, 2009, pp. 25–36.

[7] J. Zobrist, S.R. Müller, A. Ammann, T.D. Bucheli, V. Mot-tier, and M. Ochs, Quality of roof runoff from groundwaterinfiltration, Water Res. 34 (2000), pp. 1455–1462.

[8] J. Mata, J. Karns, and A. Torrents, Influence of rhamnolipidsand Triton X-100 on the desorption of pesticides from soil,Environ. Sci. Technol. 36 (2002), pp. 4669–4675.

[9] L.G. Torres, M. Climent, J. Saquelares, E.R. Bandala, G.Urquiza, and R. Iturbe, Characterization and treatabilityof a contaminated soil from an oil exploration zone, Int. J.Environ. Sci. Technol. 4 (2007), pp. 311–322.

[10] L.G. Torres and E.R. Bandala, Soil washing for remediationof soils contaminated with petroleum hydrocarbons and/ormetals, in Remediation of Soils and Aquifers, L.G. Torresand E.R. Bandala, eds., Nova Science Publishers, New York,2009, pp. 231–244.

[11] V.W. Uhl and J.B. Gray, Mixing Theory and Practice,Academic Press, New York, 1986.

[12] T. Murugesan, Critical impeller speed for solids suspensionin mechanically agitated contactors, J. Chem. Eng. Jpn. 34(2001), pp. 423–429.

[13] E.R. Bandala, L. González, F. De la Hoz, M.A. Pelaez,D.D. Dionysiou, P.S.M. Dunlop, A. Byrne, and J.L.Sánchez-Salas, Application of azo dyes as dosimetricindicators for enhanced photocatalytic solar disinfection(ENPHOSODIS), J. Photochem. Photobiol. A: Chem. 218(2011), pp. 185–191.

[14] E.R. Bandala, J.H. Castillo, L. González, and J.L. Sanchez-Salas, Solar driven advanced oxidation processes for inacti-vation of pathogenic microorganisms in water, Recent Res.Develop. Photochem. Photobiol. 8 (2011), pp. 1–16.

[15] J.H. Castillo-Ledezma, J.L. Sánchez-Salas, A. López-Malo,and E.R. Bandala, Effect of pH, solar irradiation and semi-conductor concentration on the photocatalytic disinfectionof Escherichia coli in water using nitrogen-doped TiO2. Eur.Food Res. Technol. 233 (2011), pp. 825–834.

[16] M.A. Quiroz, E.R. Bandala, and C. Martínez-Huitle,Advanced oxidation processes (AOPs) for removal of

pesticides from aqueous media, in Pesticides- Formulation,Effects, Fate, M. Stoycheva, ed., InTech Press, New York,2011, pp. 685–730.

[17] E.R. Bandala, M.A. Pelaez, M.J. Salgado, and L.G. Torres,Degradation of sodium dodecyl sulfate in water using solardriven Fenton-like advanced oxidation processes, J. Hazard.Mat. 151 (2008), pp. 578–584.

[18] E.R. Bandala, Y. Velasco, and L.G. Torres, Decontam-ination of soil washing wastewater using solar drivenadvanced oxidation processes, J. Hazard. Mat. 160 (2008),pp. 402–407.

[19] ASTM Standard D 2216-98, Laboratory Determinationof Moisture Content of Soil and Rock by Mass, ASTMInternational, West Conshohocken, PA, 1998.

[20] ASTM Standard D 422-63, Particle-Size Analysis of Soils,ASTM International, West Conshohocken, PA. 2002.

[21] B. Vazquez, E.R. Bandala, R. Reyes, and L.G. Torres,Variation of mechanical and hydraulic properties ofoil-contaminated soil due to a surfactant-enhancedwashing process, Soil. Sed. Contam. 19 (2010), pp. 531–546.

[22] J. Buchs, V. Maier, C. Milbradt, and B. Zoels,Power consumption in shaking flasks on rotary shakingmachines: I. Power consumption measurements in unbaf-fled flasks at low viscosity, Biotechnol. Bioeng. 68 (2000),pp. 589–593.

[23] J.R. Bolton, K.G. Bircher, W. Tumas, and C.A. Toldman,Figures-of-merit for the technical development and applica-tion of advanced oxidation technologies for both electric-and solar-driven systems, Pure App. Chem. 73 (2001),pp. 627–637.

[24] R.W. Day, Soil Testing Manual, McGraw-Hill, New York,2001.

[25] E.R. Bandala, L. González, J.L. Sánchez-Salas, and J.H.Castillo, Inactivation of Ascaris eggs in water using sequen-tial solar driven photo-Fenton and free chlorine, J. WaterHealth 10 (2012), pp. 20–30.

[26] S.L. Orozco, E.R. Bandala, C. Arancibia-Bulnes, B. Ser-rano, R. Suarez-Parra, and I. Hernández-Pérez, Effect ofiron salt on the color removal of water containing the azo-dye reactive blue 69 using photo-assisted Fe(II)/H 2O2 andFe(III)/H 2O2 systems, J. Photochem. Photobiol. A: Chem.198 (2008), pp. 144–149.

[27] S.A. Mabury and D.G. Crosby, Pesticide reactivity towardhydroxyl and its relationship to field persistence, J. Agric.Food Chem. 44 (1996), pp. 1920–1924.

[28] G. Ozkan-Taskin and H. Wei, The effect of impeller-to-tankdiameter ratio on draw down of solids, Chem. Eng. Sci. 58(2003), pp. 2011–2022.

[29] E.R. Bandala, F. Aguilar, and L.G. Torres, Surfactantenhanced soil washing for the remediation of sites contam-inated with pesticides, Land Contam. Reclam. 18 (2010),pp. 151–159.

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