Foam-enhanced
removal
of
adsorbed
metal
ions
from
packed
sands
with
biosurfactant
solution
flushing
Bode
Haryanto,
Chien-Hsiang
Chang
*
DepartmentofChemicalEngineering,NationalChengKungUniversity,1Ta-HsuehRoad,Tainan701,Taiwan
1. Introduction
Metalionsinasolutionflowingoversandparticlesmayadsorb ontotheparticle surfaceswith variousinteractionmechanisms [1,2].Theouter-spheretypeistheinteractionbetweenmetalions andthesandsurfacesinvolvingtheliquidphase.Theinner-sphere typeistheinteractionbetweenmetalionsandfunctionalgroups onthesurfaces,whichdoesnotinvolvea liquidphaseorwater moleculesbetweentheionsandthesurfaces[3–6].Naturalsands mayhavemacroporesandmesopores,andtheporosityismostly influencedbyparticlesize,grainshape,androcktype[7,8].The porositycanbe classifiedintointer-particleporosity and intra-particleporosity.Porescausenotonlygreatsurfacearea,butalso highselectivityinadsorption[9].Theinteractionbetweenmetal ionsandsandsurfaceswillaffectthedesorptionbehaviorofthe metalionsinaremediationprocess.
Several remediation technologies have been developed for removing heavy metal ions from contaminated soils. A foam-enhancedsolutionflushingtechniquehasbeenappliedto remediatesoilscontainingmetalioncontaminants,andonewas
abletoimprovethemigrationofsurfactantsolutions withthe presenceoffoamduringthesolutionflushingprocess[10–12]. Thefoamcouldinhibitthechannelingeffectofsolutionflowby increasingtheresistanceofthesolutionflowandthusbyforcing the solution to homogeneously flowthroughout the medium. Thiswouldenhancetheremovalefficiencyofthesoil remedia-tion [10,13,14]. By using a foam-enhanced solution flushing technique, the removal efficiency was increased even in a heterogeneousporous medium[15].The effectivenessoffoam generation is influenced by surfactant concentration [11,16], and this technique is attractive due to the low usage of surfactants[10,11].
Applications of biosurfactants in the field of environmental protectionhavereceivedmuchattentionduetotheir biodegrad-ability, low toxicity, effectiveness in enhancing biodegradation, and ability to solubilize hydrophobic compounds [17–20]. SurfactinisabiosurfactantproducedbyvariousstrainsofBacillus subtiliswithnegativelychargedcharacteristicinrodmicelleform [21,22]andexcellentfoamstability[23].Rhamnolipidisproduced by Pseudomonasaeruginosa [24,25]and hasexcellentquality of foam[26].Rhamnolipidinwaterpossessesanegativelycharged characteristic and may be particularly effectivein remediating soilscontaminatedwithmetalionsthatarelesssensitivetoion exchangeprocesses[25].
ARTICLE INFO
Articlehistory:
Received13February2014
Receivedinrevisedform23April2014
Accepted26April2014
Availableonline2June2014
Keywords:
Biosurfactant
Foam-enhancedsolutionflushing
Surfactantsolutionflushing
ABSTRACT
Thisstudydemonstratedtheabilitiesofnegativelychargedbiosurfactants,surfactinandrhamnolipid,to removeadsorbed copper andcadmiumions from sandsurfaces withthefoam-enhancedsolution flushingtechnique.Apopularanionicsurfactant,sodiumdodecylsulfate(SDS),wasusedforthepurpose ofremovalefficiencycomparison.Theroleofsurfactantfoamingabilityintheflushingapproachwas thenidentified.Itwasfoundthatthesurfactantsolutionflushingcouldonlyresultinlimitedremoval efficiencyof3–10%and13–36%forcopperionandcadmiumion,respectively,after24-porevolume(PV) flushingdue to the channeling effect. As compared to the surfactinsolution, a less pronounced channelingeffectwasdetectedfortherhamnolipidorSDSsolution.Withthepresenceoffoaminthe flushingapproach,thechannelingeffectcouldbeinhibited,andonecouldobtainimprovedremoval efficiencyof10–30%and20–46%forcopperionandcadmiumion,respectively,after24-PVflushing.The removalefficiencywashigherforcadmiumionsthanforcopperions,whichcouldbeexplainedbythe significantadsorptionofthecadmiumionsintheinter-particleporeregions.Moreover,thecumulative removalefficiencyvariationswiththefoam-enhancedsolutionflushingcouldbecorrelatedwiththe dynamicfoamcapacityofthesurfactantsolutions.
ß2014TaiwanInstituteofChemicalEngineers.PublishedbyElsevierB.V.Allrightsreserved.
* Correspondingauthor.Tel.:+88662757575x62671;fax:+88662344496.
E-mailaddress:changch@mail.ncku.edu.tw(C.-H.Chang).
ContentslistsavailableatScienceDirect
Journal
of
the
Taiwan
Institute
of
Chemical
Engineers
j o urn a lhom e pa g e :ww w . e l se v i e r. c om / l oca t e / j t i ce
http://dx.doi.org/10.1016/j.jtice.2014.04.026
Theobjectiveofthisstudyistodemonstrateandcomparethe abilities of two biosurfactants, surfactin and rhamnolipid, to removestronglyadsorbedmetalionsfromsandsurfaceswiththe foam-enhanced solution flushing technique. The key role of surfactant foaming ability in the flushing approach is then identified.Sandswithadsorbedmetalionswerepreparedbyan adsorptionprocessandthenweredriedtoallowthemetalionsto interactwiththesandsurfacesmainlythroughtheinner-sphere interaction. Physical properties of the biosurfactant solutions werecharacterized,andtheremovalefficiencyofthebiosurfactant solutions without or with foam for the adsorbed metal ions from the sand surfaces was evaluated. The removal efficiency obtainedbyusingapopularanionicsurfactant,sodium dodecyl-sulfate(SDS),solutionwasalso determinedforthecomparison purpose,andtheimportanceofthesurfactantfoamingabilityon the removal efficiency of the foam-enhanced solution flushing approachwasdiscussed.
2. Materialsandmethods
SurfactinwasproducedfromBacillussubtilisATCC21332[27] withpurity about90%, and rhamnolipidwas producedfrom P. aeruginosaJ4withpurityabout63%[24].Sodiumdodecylsulfate (SDS),apopularanionicsurfactant,(purity99.0%)waspurchased from Sigma–Aldrich, Japan. Research-grade copper (II) sulfate pentahydrate (Cu2SO45H2O) and cadmium chloride (CdCl2) purchasedfromShowaChemicalCo. Ltd.,Japan,werechosenas thesources for adsorbedmetal ions on sand surfaces. Purified waterwitharesistivityof18.2M
V
cmwasobtainedfromaMilli-Q plus purification system (Millipore, USA) and was used in all experiments.The sands were cleansed with purified water and were adsorbed by copper and cadmium ions through mixing the cleansed sands with the metal ion solutions. 100-g sand was mixedwith100mLof50-ppmmetalionsolution,andthemixture wasshakenfor24-hwith150rotationsperminute.Sandswith adsorbedmetalionsweredriedbyintroducingN2gastoremove theaqueous phase with the metalions adsorbing through the inner-sphereinteraction[5,6].After thesands were completely dried,themetalionconcentrationinthesolutionwasanalyzedby using an atomic absorption spectrometer (model SensAA Dual, GBCScientificEquipmentPtyLtd.,Australia)andtheadsorption densityofthemetalionsonthesandsurfaceswasdetermined.
Forafoam-enhancedsurfactantsolutionflushingoperation,a glasscolumnwitha lengthof 5cmand anoutsidediameterof 3.5cmwasusedasthefoam-generatorwithinletsforsurfactant solution and N2 gas, respectively [12]. The continuous flow of surfactant solution into the foam generator wascontrolled by using a peristaltic pump, and the dynamic foam capacity of a surfactantsolutionwasassessedwhenthevolumeoffoaminthe foam generator was constant. The dynamic foam capacity was determinedbydividingtheconstantvolumeoffoam(mL)bythe flow rateof N2 gas(mL/min). Aglass column witha lengthof 7.5cmandanoutsidediameterof1.5cmwasusedinthe sand-packed column experiments to simulate the soil remediation condition. Sand particles with an average diameter of 320
m
m wereusedastheporousmediuminthecolumn.All experiments were performed at room temperature. Surfactinsolutionwas preparedin a 10 3Mphosphate buffer withapHvalueof8.0[23,28,29].Rhamnolipidwasdissolvedin purewaterwithapHof5.6topreparetheaqueoussolution[30]. TheSDSsolutionwasalsopreparedwithpurewaterofpH=5.6. Flowrateswerefixedat2mL/minforsurfactantsolutionandat 20mL/minforN2gas.Biosurfactantsolutionswitha concentra-tionof5criticalmicelleconcentration(cmc)andSDSsolution withaconcentrationof2.5cmcwereusedtoflushthemetal
ion-adsorbedsand-packedcolumn.Theeffluentfromthepacked columnduringtheflushingoperationwascollectedevery4 pore-volumes (PVs, one PV is about 2.2mL), and the metal ion concentrationintheeffluentwasanalyzedbyatomicabsorption spectrometry.
Imagesofthecleansedsandsurfacewereobtainedbyusinga scanning electron microscope (SEM) (JOEL JSM-6700F, Japan). Thesurfacetension-loweringabilitiesofsurfactinand rhamno-lipidin aqueousphase wereevaluated with aWilhelmy plate tensiometer(CBVP-A3.KyowaInterfaceScienceCo.Ltd.,Japan). The zeta potentials of surfactin and rhamnolipid micelles or aggregates were measured by usinga zeta potential analyzer (model3000HS,MalvernInstrument,UK).
3. Resultsanddiscussion
3.1. Sandswithadsorbedmetalions
The sand surface morphology was observed by SEM and a typicalimageisshowninFig.1.Theporouscharacteristicofthe sand surfaces with the presence of intra-particle regions was demonstratedintheSEMimageandwasexpectedtoaffectthe metalionadsorptiondensity.
Whena100-mLaqueoussolutioncontaining50-ppmmetal ion was mixedwith100-gsand, the metalions wouldadsorb ontothesandsurfacesandtheadsorptiondensitywasincreased withtimeuntiltheadsorptionequilibriumwasreached[2].The metalionsinthesolutionsmayinteractwiththesandsurfaces withouter-sphereandinner-spheretypes[3].Afterdryingthe sand particles with N2 gas, adsorption densities of cadmium andcopperionsonthesandsurfaceswerefoundtobe5.85and 13.45mg/kg,respectively(Fig.2).WiththeN2dryingtreatment, the adsorbed metal ions with the outer-sphere interaction type on inter-particle sand surfaces would be removed, and theadsorbedmetalionswiththeinner-sphereinteractiontype wereexpectedtoremainonthesandsurfaces.
3.2. Physicalpropertiesofsurfactantsolutions
Theabilitiesoftwobiosurfactants,surfactinandrhamnolipid, tolowersurface tensionof aqueousphasearedemonstrated in Fig. 3. Based on the surface tension data, the critical micelle concentrationsofsurfactinandrhamnolipidwereestimatedtobe about20mg/Land40mg/L,respectively.Thesurfacetensionsof surfactin and rhamnolipid aqueous solutions at corresponding critical micelle concentrations were 31mN/m and 35mN/m,
respectively.Witha higher concentration of5criticalmicelle concentration (cmc), the surface tensions of surfactin and rhamnolipidaqueoussolutionswereaboutthesamewithavalue of29mN/m.The cmc of thepopularanionic surfactantSDS in waterat228Cwas8.0mMwithasurfacetensionof38mN/m[31], andwefoundthatthesurfacetensionwasdecreasedto32mN/m ataconcentrationof5cmc.
The charged characteristics of surfactin and rhamnolipid micellesoraggregatesinaqueousphasewerethenevaluatedby zetapotentialmeasurements.Surfactinmicellesataconcentration of5cmcandpH8possessedzetapotentialsintherangeof 60to 90mV.Forrhamnolipidmoleculesinaqueousphase,avarietyof aggregatesincluding micelles and vesiclesmight exist [32–34], andthezetapotentialofrhamnolipidmicellesoraggregatesata concentration of 5 cmc was found to vary between 17 to 56mV.ThenegativelychargedcharacteristicofSDSmicelleshas beengenerallyaccepted[35],andthezetapotentialofSDSmicelles wasabout 18mV[36].
It hasbeen proposed that anionic surfactantswith tension-loweringabilityandnegativelychargedcharacteristicinaqueous phasecouldfacilitatemetalionremovalfromcontaminatedsands inaremediationprocess[10].Thenegativelycharged character-istics of micelles or aggregates of the two biosurfactants were clearlydemonstratedfromthezetapotentialdata,andthus the biosurfactantshavethepotentialofremovingadsorbedmetalions fromsandsurfacesinasolutionflushingprocess.
The foaming ability of thebiosurfactant solutions was then evaluatedwithacontinuousfoamgenerationsystem[12,37],and thedynamic foam capacity wasdetermined [38]. The dynamic
foamcapacityofthebiosurfactantsolutionswasdeterminedwith solution and gas flow rates of 2mL/min and 20mL/min, respectively. Fig. 4 shows the dynamic foam capacity of the biosurfactant solutions at a concentration of 5 cmc. For the comparisonpurpose,thedynamicfoamcapacityofSDSsolutionat a concentration of 2.5 cmc is shown in Fig. 4. The SDS concentrationwassetat2.5cmcbecauseanSDSsolutionwith aconcentrationof5cmcwouldproducefoamoverthecapacity ofthefoamgenerator.Ithasbeenreportedthatsurfactinwasable to produce foam at a low concentration with excellent foam stability [23]. However, rhamnolipid and SDS, especially SDS, possessed better foaming ability in comparison with surfactin undertheexperimentalconditions.
For surfactin,a phosphatebufferwasusedasthesolventto controltheaqueousphaseat pH8. Thepresenceofbufferions wouldaffectthemicellerigidityofananionicsurfactant[39,40]. Whenmicellesarerigid,theymaynotquicklydissociatetosupply monomers, limiting the ability of the surfactant molecules to adsorbontothegas–waterinterfaceoffoamandresultinginlow foaming ability[39]. This may explain the low dynamic foam capacityofsurfactinsolutionobservedinFig.4.However,inview ofthelowerconcentrationofsurfactinthanrhamnolipidorSDS adoptedfordeterminingthedynamicfoamcapacity,theexcellent foamingabilityofsurfactincouldbeassured.
3.3. Removalefficiencyformetalions
Sandswithadsorbedcopperandcadmiumionsmainlythrough the inner-sphere interaction were packed into a column. The potentialofbiosurfactants,surfactinandrhamnolipid,on remov-ing the adsorbed metal ions from the sand surfaces wasthen investigatedandwascomparedwiththatofSDS,apopularanionic surfactant.Thesand-packedcolumnwasflushedwith biosurfac-tant solutions at a concentration of 5 cmc, and the removal efficiencyforthemetalionswascomparedwiththatobtainedby usingaSDSsolution.Figs.5and6showtheremovalefficiencyfor themetalionsbyusingthesolutionflushingtechniquewithout foamandwithfoam,respectively.
3.3.1. Solutionflushingwithoutfoam
Theremovalefficiencyforadsorbedcopperionsfromthesand surfacesbyusingasolutionflushingapproachisplottedinFig.5a. Itwasfoundthatthesurfactinsolutioncouldremovetheadsorbed copperionsfromthesandsurfacesonlywithinitial4-PVsolution flushingandwiththeefficiencyof3%.However,rhamnolipidor SDSsolutioncouldremovethecopperionscontinuouslyatleastup
Adsorption time (hour)
60 50 40 30 20 10 0
Adsorption density (mg/kg)
0 5 10 15 20
Fig.2.Adsorptiondensitiesofcopper(*)andcadmium(*)ionsonthesand
surfaces.
Biosurfactant concentration (mg/L)
240 200 160 120 80 40 0
Surface tension (mN/m)
0 10 20 30 40 50 60 70 80
Fig.3.Surfacetensionsofsurfactin(*)andrhamnolipid(*)aqueoussolutions.
5x cmc 5x cmc 2.5x cmc SDS Rhamnolipid Surfactin
Dynamic foaming capacity (min)
0.0
0.2
0.4
0.6
0.8
1.0
1.2
1.4
Fig.4.Dynamicfoamcapacityofsurfactin,rhamnolipid,andSDSaqueoussolutions
to24-PVsolutionflushingwiththecumulativeremovalefficiency ofabout10%.
The ability for biosurfactant solution flushing to remove adsorbedcadmium ionsfrom the sand surfacesis depicted in Fig. 5b. With 24-PV solution flushing,one could find thatthe removalefficiencyof surfactinsolutionfor adsorbedcadmium ionswas about13% andhigherremoval efficiencyof 23%was foundforrhamnolipid solution.ForSDSsolution,muchhigher removalefficiencyof36%wasdetected.Incomparisonwiththe removalefficiencyforcopperion,higherremovalefficiencywas generallyfoundforcadmiumionswithsolutionflushing.Itwas noted that after 4-PV flushing, cadmium ions could be still removedwithfurthersolutionflushingeven withthesurfactin solution.
Theporouscharacteristicofthesandsurfaceswiththepresence ofinner-particleporeregions,asdemonstratedintheSEMimage (Fig.1),couldaffecttheflushingefficiency.Thelocationofmetal ion adsorption would affect the removal efficiency of the surfactants.Asignificantnumberofcadmiumionsmightadsorb only in the inter-particle pore regions, as judged from the comparatively low adsorption density (Fig. 2). The adsorbed cadmiumionsintheinter-particleporeregionswereexpectedto easilyinteractwiththenegativelychargedsurfactantmonomers ormicellesduringtheflushingoperation.However,copper ions seemedtoadsorbnotonlyintheinter-particleporeregionsbut alsointheintra-particleporeregionswithacomparativelyhigh adsorptiondensity(Fig.2).Theadsorbedcopperionsinthe intra-particle pore regions would be difficult to interact with the surfactantmicellesoraggregatesduringtheflushingoperation.It isnotedthatwateralonecouldonlyremove5and12%ofcopper ionandcadmiumion,respectively,after24-PVflushing.
During the flushing by surfactant aqueous solutions, the channeling effectofthe surfactantflow wasexpectedtooccur, thatis,thesolutionscouldnothomogeneouslyspreadthroughout thesand-packedcolumn[13,14].Thechannelingeffectwouldlimit the efficiency of the solution flushing approach because the solutiononlyflowedthroughcertainchannelsinasand-packed mediumwithalowcontactareabetweenthesolutionandsand surfaces.Inviewofthecumulativeremovalefficiencyvariations during the solution flushing operations, it seemed that the channelingeffectofthesurfactinsolutionwasmorepronounced thanthatoftherhamnolipidorSDSsolution.
Itshould benotedthattheeffectof surfactinsolutionflow rateontheremovalefficiencyforadsorbedcopperionsfromsand surfaces in a solution flushing operation with the similar operation conditions has been investigated [41], and it was found that for solution flow rates of 2–6mL/min, a similar channelingeffectwasdetected.
3.3.2. Solutionflushingwithfoam
Theremovalefficiencyforadsorbedmetalionsfromthesand surfaces by using solution flushing approach with foam is demonstrated in Fig. 6. The efficiencyof the surfactinsolution flushing withfoam forremovingtheadsorbedcopperionswas about 10%, which was muchhigher than that obtained by the surfactinsolutionflushingwithoutfoam.However,ascomparedto rhamnolipidandSDS,surfactincouldonlyremoveacomparatively smallamountofcopperionswith24-PVsolutionflushing(Fig.6a). Forremovingadsorbedcopperionsfromthesandsurfacesbyusing therhamnolipidsolutionflushing withfoam,theefficiencywas increasedfrom15%with4-PVeffluentto23%after24-PVsolution flushing.AsfortheSDSsolutionflushingwithfoam,theremoval
Amount of effluent (pore volume)
24
Cumulative removal efficiency (%)
010
Amount of effluent (pore volume)
24
Cumulative removal efficiency (%)
010
Fig. 5. Cumulative removal efficiency of solution flushing without foam for
adsorbed(a)copperand(b)cadmiumions.
Amount of effluent (pore volume)
24
Cumulative removal efficiency (%)
010
Amount of effluent (pore volume)
24
Cumulative removal efficiency (%)
010
Fig.6.Cumulativeremovalefficiencyofsolutionflushingwithfoamforadsorbed
efficiencywasincreasedfrom23%with4-PVeffluentto30%with 24-PVeffluent. Apparently,the presence of foam in a solution flushing approach could increase the ability of the surfactant solutions to remove the adsorbed metal ions from the sand surfacesinasand-packedcolumn(Figs.5and6).
Foradsorbedcadmiumions,incomparisonwiththesolution flushingwithoutfoam,solutionflushing withfoamcould also increasetheremovalefficiencyofthethreesurfactantsolutions. For the cumulative removal efficiency after 24-PV solution flushing,20%,40%, and46%werefound forsurfactin, rhamno-lipid, and SDS solutions, correspondingly (Fig. 6b). SDS still demonstratedthe highestremoval efficiencyamong thethree surfactantsfortheadsorbedmetalions.Theremovalefficiencyof rhamnolipid was improved in the presence of foam but was slightlylowerthanthatofSDS.
Applyingsurfactantsolutionflushingwithfoamcouldinhibit the channeling effect of the solution flow by increasing the resistanceofthesolutionflowandthusbyforcingthesolutionto homogeneously flow throughout the sand-packed medium [11,13].The presenceoffoamwouldthus improvethesolution spreading inthe sand-packed column and increase thecontact betweenthenegativelychargedsurfactantmicellesoraggregates andtheadsorbedmetalions,resultinginenhanceddesorptionof the metal ions from the sand surfaces. The metal ions were interactedwiththesurfactantsbyformingcomplexesonthesand surfaces and then were moved into the aqueous phase by associatingwiththesurfactantmicellesoraggregates[37].
In view of the removal efficiency data, the removal of adsorbed metal ions by using a surfactant solution flushing approachwasapparentlyenhancedwiththepresenceoffoam. Moreover, the cumulativeremoval efficiency variation for the adsorbedmetalionswasstronglyaffectedbythefoamingability of the surfactants. A high foam capacity could inhibit the channelingeffectofthesolutionflowandincreasethepossibility ofthesurfactantaqueoussolutiontopenetrateintotheporous regionsof thesands, thusimprovingthe contactbetween the surfactant solutions and adsorbed metal ions on the sand surfaces.Amongthethreesurfactantsolutions,theSDSsolution possessedthehighestdynamicfoamcapacity, followedbythe rhamnolipidsolutionandsurfactinsolution,under the experi-mentalconditions.Thebiosurfactantsurfactinmoleculescould form micelles with the most pronounced negatively charged characteristic,butthelowestdynamicfoamcapacitywasfound for the solution, leading to the lowest removal efficiency enhancementbyfoamfortheadsorbedmetalions.
In comparisonwithsurfactin, rhamnolipidmoleculesformed micellesoraggregateswithalessnegativelycharged characteris-tic,buthigherdynamicfoamcapacityoftherhamnolipidsolution was detected. With the comparatively high dynamic foam capacity, rhamnolipid micelles or aggregates would be ease to reachnotonlytheinter-particleporeregionsbutalsothe intra-particleporeregionstointeractwithmoreadsorbedmetalions.As aresult,higherremovalefficiencyenhancementfortheadsorbed copperionsbyfoamwasfoundforrhamnolipidthanforsurfactin. ForSDSwithasmallmicellestructureandtheSDSsolutionwith the highest dynamic foam capacity, the possibility for SDS monomersormicellestointeractwiththeadsorbedmetalions evenintheintra-particleporeregionswouldbeincreased.
Thehighdynamicfoamcapacityoftherhamnolipidsolutionor SDS solution may explain the continuous increase in the cumulative removal efficiency of rhamnolipid or SDS up at least to 24-PV solution flushing for the adsorbed metal ions during the flushing process. The solution flushing with foam demonstrated the good performance of the biosurfactants for removingadsorbedmetalions fromthesand surfaces,and the foam-enhancedbiosurfactantsolutionapproachwascostefficient
duetoalowusageofbiosurfactantandshorttreatmenttime[10]. Moreover,thechannelingeffectisexpectedmoresignificantwith smallersandparticlesizeandhigherinnerporosity.Undersuch conditions,theadvantageofapplyingthefoam-enhancedsolution flushing operation to remove adsorbed metal ions from sand surfaceswouldbemorepronounced.
4. Conclusions
This study demonstrates the potential of applying two biosurfactants,surfactinandrhamnolipid,on removingstrongly adsorbedcopper and cadmium ions fromsand surfaceswitha foam-enhancedsolutionflushingapproach.Theimportantroleof surfactantfoamingabilityintheefficiencyoftheflushingapproach is then identified. The results indicated that surfactin and rhamnolipid with negatively charged characteristics had the ability to interact with the adsorbed metal ions on the sand surfacesand couldbeadoptedinthesolutionflushing process. However,onlylimitedremovalefficiencyfortheadsorbedmetal ions wasfoundby using the surfactinor rhamnolipid solution flushingduetoalowcontactareabetweenthesolutionandthe ions,resultingfromthechannelingeffectofthesolutionflowina sand-packedmedium.Withthefoam-enhancedsurfactant solu-tionflushingapproach,thechannelingeffectofthesolutionflow couldbeinhibitedwiththepresenceof foambyincreasingthe solution flow resistance and thus forcing the solution to homogeneously flow throughout the sand-packed medium, resulting in significantly improved removal efficiency for the adsorbedmetalions.Itisnotedthattheremovalefficiencywas higher for cadmium ions than copper ions, which could be explainedbythesignificantadsorptionofthecadmiumionsinthe inter-particle pore regions. Moreover, the cumulative removal efficiencyvariationsfortheadsorbedmetalionsbyusingthe foam-enhancedsolutionflushingapproachcouldbecorrelatedwiththe dynamicfoamcapacityofthesurfactantsolutionsratherthanthe tension-loweringabilityandaggregatechargecharacteristicofthe surfactants.
Acknowledgements
TheauthorswishtoexpresssinceregratitudetoProfessor Yao-Hui Huang for the help on using the atomic absorption spectrometer and would like to thank Dr. Wei-Ta Li and Mr.An-TsungKuoforeditingthemanuscript.
References
[1]AwanMA,QaziIA,KhalidI.Removalofheavymetalsthroughadsorptionusing sand.JEnvironSci2003;15:413–6.
[2]ArgunME, DursunS,Ozdemir C, KaratasM.Heavymetal adsorptionby modified oak sawdust: thermodynamics and kinetics. J Hazard Mater 2007;141:77–85.
[3]KoretskyC.Thesignificanceofsurfacecomplexationreactionsinhydrologic systems:ageochemist’sperspective.JHydrol2000;230:127–71.
[4]BradlHB.Adsorptionofheavymetalionsonsoilsandsoilsconstituents.J ColloidInterfaceSci2004;277:1–18.
[5]GoldbergS,JohnstonCT.Mechanismsofarsenicadsorptiononamorphous oxidesevaluatedusingmacroscopicmeasurements,vibrationalspectroscopy, andsurfacecomplexationmodeling.JColloidInterfaceSci2001;234:204–16.
[6]SchlegelML,CharletL,ManceauA.Sorptionofmetalionsonclayminerals:II. MechanismofCosorptiononhectoriteathighandlowIonicstrengthand impactonthesorbentstability.JColloidInterfaceSci1999;220:392–405.
[7]FetterCW.Appliedhydrogeology.3rded. NewYork:MaxwellMacmillan International;1994.
[8]ChenH,WangJ,RahmanZ,WordenJ,LiuX,DaiQ,etal.Beachsandfrom CancunMexico: anatural macro-and mesoporousmaterial.J Mater Sci 2007;42:6018–26.
[9]KanekoK.Determinationofporesizeandporesizedistribution:1.Adsorbents andcatalysts.JMembrSci1994;96:59–89.
[11]WangS,MulliganC.Rhamnolipidfoamenhancedremediationofcadmium andnickelcontaminatedsoil.WaterAirSoilPollut2004;157:315–30.
[12]HuangC-W,ChangC-H.Alaboratorystudyonfoam-enhancedsurfactant solutionfloodinginremovingn-pentadecanefromcontaminatedcolumns. ColloidsSurfA–PhysicochemEngAspects2000;173:171–9.
[13]RothmelRK,PetersRW,St.MartinE,DeFlaunMF.Surfactant foam/bioaug-mentationtechnology forin situ treatment of TCE-DNAPLs.EnvironSci Technol1998;32:1667–75.
[14]ChowdiahP,MisraBR,KilbaneIiJJ,SrivastavaVJ,HayesTD.Foampropagation throughsoilsforenhancedin-situremediation.JHazardMater1998;62:265–80.
[15]JeongS-W,CorapciogluMY,RooseveltSE.Micromodelstudyofsurfactant foam remediation of residual trichloroethylene. Environ Sci Technol 2000;34:3456–61.
[16]VikingstadAK,AarraMG,SkaugeA.Effectofsurfactantstructureonfoam–oil interactions:comparingfluorinatedsurfactantandalphaolefinsulfonatein staticfoamtests.ColloidsSurfA–PhysicochemEngAspects2006;279:105–12.
[17]CooperDG,GoldenbergBG.Surface-activeagentsfromtwoBacillusspecies. ApplEnvironMicrobiol1987;53:224–9.
[18]Maget-DanaR,PtakM.Interfacialpropertiesofsurfactin.JColloidInterfaceSci 1992;153:285–91.
[19]BanatIM.Biosurfactantsproductionandpossibleusesinmicrobialenhanced oilrecovery andoil pollution remediation:a review.Bioresour Technol 1995;51:1–12.
[20]PeypouxF,BonmatinJM,WallachJ.Recenttrendsinthebiochemistryof surfactin.ApplMicrobiolBiotechnol1999;51:553–63.
[21]IshigamiY,OsmanM,NakaharaH,SanoY,IshiguroR,MatsumotoM. Signifi-canceofb-sheetformationformicellizationandsurfaceadsorptionof sur-factin.ColloidsSurfB–Biointerfaces1995;4:341–8.
[22]HeerklotzH,SeeligJ.Detergent-likeactionoftheantibioticpeptidesurfactin onlipidmembranes.BiophysJ2001;81:1547–54.
[23]RazafindralamboH,PaquotM,BanielA,PopineauY,HbidC,JacquesP,etal. Foamingpropertiesofsurfactin,alipopeptidebiosurfactantfrombacillus subtilis.JAmOilChemSoc1996;73:149–51.
[24]WeiY-H,ChouC-L,ChangJ-S.Rhamnolipidproductionbyindigenous Pseudo-monasaeruginosaJ4originatingfrompetrochemicalwastewater.BiochemEng J2005;27:146–54.
[25]HermanDC,ArtiolaJF,MillerRM.Removalofcadmium,lead,andzincfrom soilbyarhamnolipidbiosurfactant.EnvironSciTechnol1995;29:2280–5.
[26]MulliganCN.Environmentalapplicationsforbiosurfactants.EnvironPollut 2005;133:183–98.
[27]YehM-S,WeiY-H,ChangJ-S.Enhancedproductionofsurfactinfrombacillus subtilisbyadditionofsolidcarriers.BiotechnolProg2005;21:1329–34.
[28] RazafindralamboH,PopineauY,DeleuM,HbidC,JacquesP,ThonartP, etal.Foamingproperties oflipopeptidesproduced bybacillus subtilis: effect of lipid and peptide structural attributes. J Agric Food Chem 1998;46:911–6.
[29]MulliganCN,YongRN,GibbsBF,JamesS,BennettHPJ.Metalremovalfrom contaminatedsoilandsedimentsbythebiosurfactantsurfactin.EnvironSci Technol1999;33:3812–20.
[30]DahrazmaB,MulliganCN.Investigationoftheremovalofheavymetalsfrom sedimentsusingrhamnolipidinacontinuousflowconfiguration. Chemo-sphere2007;69:705–11.
[31]StokesRJ, EvansDF.Fundamentalsofinterfacial engineering.NewYork: Wiley-VCH;1997.
[32]ZhangY,MillerRM.Enhancedoctadecanedispersionandbiodegradationbya
Pseudomonasrhamnolipidsurfactant(biosurfactant).ApplEnvironMicrobiol 1992;58:3276–82.
[33]Lebron-PalerA. Solution and Interfacial Characterizationof Rhamnolipid Biosurfactant from P.aeruginosa ATCC 9027. Arizona: The University of Arizona;2008.
[34]DahrazmaB,MulliganCN,NiehM-P.Effectsofadditivesonthestructureof rhamnolipid(biosurfactant):asmall-angleneutronscattering(SANS)study.J ColloidInterfaceSci2008;319:590–3.
[35]MalhotraA,CouplandJN.Theeffectofsurfactantsonthesolubility,zeta potential, and viscosity of soy protein isolates. Food Hydrocolloids 2004;18:101–8.
[36]OkazakiM,PolyakovNE,ToriyamaK.DynamicpropertiesofSDS micelle detected with ‘‘radical-pair-probe’’ using the pulse-mode product-yield-detectedESRtechnique.JPhysChem1995;99:6452–6.
[37]MulliganCN,WangS.Remediationofaheavymetal-contaminatedsoilbya rhamnolipidfoam.EngGeol2006;85:75–81.
[38]RaymundoA,EmpisJ,SousaI.Methodtoevaluatefoamingperformance.J FoodEng1998;36:445–52.
[39]PandeyS,BagweRP,ShahDO.Effectofcounterionsonsurfaceandfoaming propertiesofdodecylsulfate.JColloidInterfaceSci2003;267:160–6.
[40]ThomasJK.Effectofstructureandchargeonradiation-inducedreactionsin micellarsystems.AccChemRes1977;10:133–8.
